JP2014095780A - Optical amplifier - Google Patents

Abstract

In a phase sensitive optical amplifier of the prior art, an individual optical component connected to a substrate including an optical waveguide and a phase synchronization circuit between signal light and pump light are required. An optical isolator and an optical circulator are also required, which increases the number of parts and complicates the configuration of the entire amplifier. Since the reflected light of the signal light that could not be converted when the excitation light was generated enters the PPA PPLN as return light, there is a problem that the parametric amplification efficiency is lowered and the amplification characteristics are deteriorated.
A phase-sensitive optical amplifier according to the present invention has a second-order nonlinear optical device and two MMIs fabricated by a direct bonding method integrated on a single substrate, and the integrated substrate is end-face processed. The reflective end face of the MMI having a filter function forms part of the end face of the substrate. It is possible to further integrate a modulator for phase synchronization between the signal light and the excitation light on the same substrate. Furthermore, the structure which performs phase modulation with respect to the 2nd harmonic which is not only fundamental wave light but excitation light is also employ | adopted.
[Selection] Figure 3

Description

  The present invention relates to an optical amplifying device, and more specifically, to a phase sensitive optical amplifier used in an optical communication system and an optical measurement system.

  In the optical transmission system of the prior art, in order to regenerate a signal attenuated by propagating through an optical fiber, an identification reproduction optical repeater that converts an optical signal into an electric signal and regenerates the optical signal after identifying the digital signal is provided. It was used. This identification and reproduction optical repeater has problems such as a limitation in response / processing speed of an electronic component that converts an optical signal into an electric signal, and an increase in power consumption as the speed of a signal to be transmitted increases.

  As a signal amplifying means for solving this problem, a fiber laser amplifier or a semiconductor laser amplifier that amplifies signal light by making excitation light incident on an optical fiber doped with a rare earth element such as erbium or praseodymium is used. Since the fiber laser amplifier and the semiconductor laser amplifier can amplify the signal light as it is, there is no limitation on the electrical response and processing speed which has been a problem in the identification reproduction optical repeater. In addition, fiber laser amplifiers and semiconductor laser amplifiers have the advantage that their device configuration is relatively simple. However, these laser amplifiers do not have a function of shaping a deteriorated signal light pulse waveform. Further, in these laser amplifiers, unavoidably and randomly generated spontaneous emission light is mixed regardless of the signal component. For this reason, the S / N ratio of the signal light decreases by at least 3 dB before and after amplification. These lead to an increase in transmission code error rate at the time of digital signal transmission, which is a factor of reducing transmission quality.

  As means for overcoming the limitations of the conventional laser amplifier, a phase sensitive optical amplifier (PSA) has been studied. This phase-sensitive optical amplifier has a function for shaping a signal light pulse waveform that has deteriorated due to the influence of dispersion in the transmission fiber. Further, since spontaneous emission light having a quadrature phase irrelevant to the signal light can be suppressed, in principle, the S / N ratio of the signal light can be kept the same before and after amplification.

  The implementation method of the phase sensitive amplifier can be roughly divided into a configuration using a third-order nonlinear optical medium such as an optical fiber and a configuration using a second-order nonlinear optical medium. The configuration using the second-order nonlinear optical medium is less mixed with GAWBS (guided acoustics wave Brillouin scattering) and ASE than the configuration using the third-order nonlinear optical medium such as an optical fiber, and can be amplified with low noise. .

  FIG. 1 is a diagram showing a basic configuration of a phase-sensitive optical amplifier of prior art 1. The phase sensitive optical amplifier 100 shows a basic configuration of a phase sensitive optical amplifier using a periodically poled waveguide as a second-order nonlinear optical medium. In all the drawings after FIG. 1, the dotted arrow indicates the second harmonic light (wavelength 0.77 μm), and the solid arrow indicates the fundamental light (wavelength 1.54 μm). In addition, light with a thick arrow line indicates a higher level than light with a thin arrow line.

  In this configuration, in order to obtain sufficient power to obtain a nonlinear optical effect from weak laser light used for optical communication, a part of signal light is extracted by the optical branching unit 101, and this is used as pumping light to obtain a fiber laser. The fundamental light is amplified using the amplifier 104. The amplified fundamental wave light is incident on the first second-order nonlinear optical element 105 to generate a second harmonic. Further, the signal light and the second harmonic 113 generated by the first second-order nonlinear optical element 105 are incident on the second second-order nonlinear optical element 107 and degenerate parametric amplification is performed, so that phase-sensitive amplification is achieved. Done. The first second-order nonlinear optical element 105 includes an optical waveguide 121 made of periodically polarized lithium niobate (PPLN), and the second second-order nonlinear optical element 107 has an optical waveguide 122 made of PPLN. Prepare.

  In this phase sensitive optical amplifier 100, when the phase of the signal light and the phase of the pumping light in the phase sensitive light amplification unit 107 coincide with each other, the input signal light 112 is amplified, and when the phase of both becomes 90 degrees shifted, The input signal light 112 has a characteristic to attenuate. Utilizing this characteristic, if the phase of the pumping light and the phase of the signal light are matched so that the amplification gain is maximized, the spontaneous emission light having the quadrature phase with the signal light is not generated, that is, the SN ratio is deteriorated. Signal light can be amplified.

  Specifically, in the phase sensitive optical amplifier 100 shown in the figure, a part of the output amplified signal light is branched by the optical branching unit 108 and received by the photodetector 110, and then the phase locked loop circuit (PLL) 111. The phase synchronization is performed. Using the phase modulator 102 disposed on the upstream side of the EDFA 104, weak phase modulation is applied to the input excitation fundamental wave light extracted by the optical branching unit 101 using a sine wave. The photodetector 110 and the PLL circuit 111 detect the phase shift of the phase modulation, and this error signal is used to drive the optical fiber stretcher 103 and the bias of the phase modulator 102 by the PZT disposed on the front side of the EDFA 104. Feedback to voltage. This feedback operation absorbs fluctuations in the optical phase due to vibrations of the optical fiber components and temperature fluctuations, thereby enabling stable phase sensitive amplification.

  In the phase-sensitive optical amplifier of Prior Art 1 shown in FIG. 1, the individual phase modulator 102, the fiber amplifier 104, the second-order nonlinear optical element 105 for generating the second harmonic, and the secondary that performs phase-sensitive amplification (OPA). Various optical elements such as the nonlinear optical element 107 and the multiplexers / demultiplexers 124, 125, and 126 provided in the second-order nonlinear optical elements 105 and 107 are connected by an optical fiber. For this reason, there has been a drawback that the SNR of the amplified light 114 is lowered due to the loss caused by the connection of each part. In addition, since a plurality of second-order nonlinear optical devices 105 and 107 are required and the two phases of the excitation light source and the signal light need to be synchronized, components for phase synchronization (phase modulation) Device 102, optical branching unit 110, and detector 111) are required. Compared to an optical amplifier that does not use phase-sensitive amplification, the number of parts required for the entire optical amplifier 100 is increased, and the configuration is further complicated.

  For these problems of the phase sensitive optical amplifier, by adopting a configuration in which the above-mentioned elements are integrated on the same substrate, the connection loss between the elements is eliminated in principle, and the amplified light deteriorates due to the connection loss. The SN ratio (SNR) can be improved. However, although the SNR degradation can be improved in the integrated configuration, an increase in the substrate size after integration cannot be avoided. As a method for solving such a problem, the configuration of the prior art 2 using the MMI as shown in FIG. 2 has been proposed.

  FIG. 2 is a diagram showing a configuration of a phase sensitive optical amplifier using the second-order nonlinear optical effect of Conventional Technique 2. The configuration of the phase sensitive optical amplifier of the prior art 2 shown in FIG. 2 corresponds to the increase in the substrate size accompanying the integration, which was a problem in the configuration shown in FIG. In the configuration of the prior art 1 shown in FIG. 1, when the PPLN 105 for generating the second harmonic and the PPLN 107 for degenerate parametric amplification are simply integrated on the same substrate, it is inevitable to increase the substrate size.

  In the configuration shown in FIG. 2, a single PPLN 209 that performs both second harmonic generation (SHG) and degenerate parametric amplification (DPA), and a multimode interferometer (MMI) 212 as a multiplexer / demultiplexer. One LN substrate 205 is integrated. The signal light 214 having a wavelength of 1.54 μm input through the optical isolator 214 is input to the input waveguide 224 and amplified by the PPLN 209 via the MMI 212. The integrated MMI 212 couples signal light having a wavelength of 1.54 μm to the DPA function portion of the PPLN 209 with low loss by optimizing the waveguide width, waveguide length and input / output port position.

  A portion 226 of the input signal light is branched by the branching unit 201 and input to the EDFA 204 and amplified by the EDFA 204 to obtain an amplified light 215 of the fundamental light. The amplified fundamental light 215 is input from the right end of the substrate 205 via the optical circulator 206 and used as excitation light. The excitation light input from the right end of the substrate 205 propagates through the waveguide of the SHG function portion of the PPLN waveguide 209 having the functions of SHG and DPA and reaches almost all the second harmonic component ( (Dotted arrow).

  The MMI 212 couples the second harmonic to the lower output waveguide 221 with low loss. Thereafter, the second harmonic is reflected at the left end portion of the substrate 205 with high efficiency by the high reflection film (optical multilayer filter) 221 having a high reflectance at the wavelength of the second harmonic of 0.77 μm, and the same waveguide. Propagate 221 in the reverse direction. The second harmonic propagating in the reverse direction is coupled and propagated to the PPLN waveguide 209 that also serves as SHG and DPA via the MMI 212 again. Similarly, the signal light combined by the MMI 212 via the upper waveguide 224 and the second harmonic are mixed in the PPLN waveguide 209, and the signal light is amplified by the DPA function. The function of the phase modulator 102 in FIG. 1 is realized by applying a modulation signal to the electrode 202 formed on the waveguide 221 in the lower stage of the MMI.

  With the configuration shown in FIG. 2, the single phase modulator placed on the front side of the EDFA 204 is omitted, and the second harmonic is modulated, so that the drive voltage is almost half, The phase modulator can be operated with a low drive compared to the phase sensitive amplifier. The configuration shown in FIG. 2 uses an optical fiber amplifier for obtaining sufficient power from the weak optical power used in optical communication to use parametric optical amplification, and the S / N ratio of pumping light generated along with optical amplification. A phase sensitive amplifier can be constructed while suppressing deterioration.

  Further, by integrating the second harmonic generator for generating pumping light in the optical PLL circuit, that is, the PPLN waveguide 209 that also functions as SHG and DPA, and the phase modulator 202 on one substrate, Compared with the case of the configuration of the prior art 1 shown in FIG. 1 in which the separate modulator 102 and the second-order nonlinear optical elements 105 and 107 are connected, the influence of the attenuation of the excitation light of the fundamental light can be reduced. As a result, the signal-to-noise ratio of the signal in the optical fiber can be improved by the phase sensitive amplifier that can be applied to optical communication and can amplify the optical fiber. Compared to the configuration of the prior art 1, it has become possible to transmit a high-speed signal to a long distance with low power. Further, by reflecting the excitation light by the high reflection film 221 on the left end surface of the substrate 205, the PPLN for excitation light generation and the PPLN for DPA are shared by one PPLN 209, thereby reducing the size of the apparatus.

J. A. Levenson, I. Abram, T. Rivera, and P. Grainger, “Reduction of quantum noise in optical parametric amplification,” J. Opt. Soc. Am. B, vol. 10, pp. 2233-2238 (1993). W. Imajuku, and A. Takada, “Gain characteristics of coherent optical amplifiers using a Mach-Zehnder interferometer with Kerr Media,” IEEE J. Quantum Electron., Vol. 35, no. 11, pp. 1657-1665 (1999) . R. Slavik et al., “All-optical phase and amplitude regenerator for next-generation telecommunications system,” Nature Photonics., Vol. 4, pp. 690-695 (2010). T. Umeki, O. Tadanaga, and M. Asobe, “Highly efficient wavelength converter using direct-bonded PPZnLN ridge waveguide,” IEEE J. Quantum Electron., Vol. 46, no. 8, pp. 1003-1008 (2010) . R. Slavik et al., “All-optical phase-regenerative multicasting of 40 Gbit / s DPSK signal in a degenerate phase sensitive amplifier,” In Proceedings of the European Conference and Exhibition on Optical Communication (ECOC 2010, Torino, Italy) MO .1.A.2. Isao Morohashi, Takahide Sakamoto, Hideyuki Tonobayashi, Tetsuya Kawanishi, Satoshi Hirosako, “Mach-Zehnder Modulator-Based Optical Comb Generator and 100fs-class Pulse Generation Using Soliton Compression,” Proceedings of the 72th JSAP Meeting (Applied Physics Society 2011 Autumn Yamagata University) 30a-P3-1 R. Tang et al., “In-line phase-sensitive amplification of multichannel CW signals based on frequency nondegenerate four-wave-mixing in fiber,” Optics Express., Vol. 16, pp. 9046-9053 (2008). Nakagawa Kiyoji and three others, "Optical Amplifiers and Their Applications", Ohmsha, 1992/05, p.26 Nishihara et al., “Optical Integrated Circuits”, Ohm Company

  However, the phase sensitive optical amplifier of the configuration of the prior art 2 shown in FIG. 2 can perform phase sensitive amplification while avoiding noise due to GAWBS or ASE by using a second-order nonlinear optical element. Has occurred. There is still a need for a circuit that synchronizes the phase between the fundamental or second harmonic excitation light labeled Pump (LO) and the signal light labeled signal. In addition, the optical isolator 213 and the optical circulator 206 are required as new single components, and the number of components is increased. Furthermore, there is a problem that the configuration of the entire amplifier becomes rather complicated. Furthermore, there is a problem that the reflected light of the signal light that cannot be completely converted when the excitation light is generated enters the PPA PPLN as return light, thereby reducing the efficiency of parametric amplification and degrading the amplification characteristics.

  In view of the above-described problems of the prior art, an object of the present invention is a phase-sensitive light that can be applied to amplification of weak laser light used for optical communication and can be amplified with low noise. It is to provide a new integrated configuration of the amplification device.

  In order to achieve such an object, the present invention provides a light that amplifies the fundamental wave light in a phase-sensitive optical amplifier that amplifies signal light by optical mixing using a nonlinear optical effect. A first second-order nonlinear optical element comprising a fiber laser amplifier, a second-order nonlinear optical material that is periodically poled and having a light waveguide that generates sum frequency light from the excitation fundamental light; and And coupled to the optical waveguide of a second-order nonlinear optical element, and has an action of reflecting the sum frequency light from the first second-order nonlinear optical element and transmitting the excitation fundamental wave light, and the excitation fundamental wave light and the sum A first filter having a coupled waveguide that separates only the sum frequency light from the frequency light and guides the separated sum frequency light; and an input waveguide to which the signal light is input. Light and front An optical waveguide composed of a multiplexer that multiplexes the sum frequency light from a coupling waveguide and a second-order nonlinear optical material that is periodically poled and that performs parametric amplification of the signal light using the excitation light. A second second-order nonlinear optical element, a second filter that separates the amplified signal light and the sum frequency light that is the excitation light, a phase of the amplified signal light, and an optical fiber laser amplifier Means for synchronizing the phase of the excitation light input to the optical waveguide of the first second-order nonlinear optical element, the first filter, the coupling waveguide, the multiplexer, and the first The optical waveguide of the second-order nonlinear optical element is integrated on the same substrate, and the optical waveguide, the first filter, and the coupling waveguide of the first second-order nonlinear optical element are: On a continuous waveguide The input waveguide, the multiplexer, and the optical waveguide of the second second-order nonlinear optical element are formed adjacent to each other on a continuous waveguide, and the coupling waveguide, the multiplexing The phase sensitive optical amplifying device is characterized in that the optical waveguides of the second and second nonlinear optical elements are formed adjacent to each other on a continuous waveguide.

  The first filter corresponds to, for example, the second MMI 312 of the first embodiment, and the multiplexer corresponds to, for example, the first MMI 330 of the first embodiment. The coupled waveguide corresponds to the coupled optical waveguide 332 of the second MMI 312. Furthermore, the sum frequency light corresponds to the second harmonic.

  A second aspect of the present invention is the phase sensitive optical amplifying device according to the first aspect, wherein the first filter uses the surface perpendicular to the waveguide as a reflection end surface and the sum frequency light on the reflection end surface. A multimode interference multiplexer / demultiplexer (MMI) including one or more thin films that reflect and transmit the excitation fundamental light, wherein the reflection end face of the first filter is a single end face of the substrate. The one or more thin films are formed on the entire end surface of the substrate, and the optical waveguide of the first second-order nonlinear optical element and the second-order nonlinear optical The optical waveguide is a directly bonded optical waveguide manufactured by directly bonding a first substrate having a nonlinear optical effect and a second substrate having a refractive index smaller than that of the first substrate. And One or more thin films correspond to, for example, the reflecting member 321 and the antireflection member 320a of the first embodiment.

  A third aspect of the present invention is the phase sensitive optical amplifying device according to the first or second aspect, wherein the optical fiber manufactured as a part of the synchronizing means is formed on the output side of the optical fiber laser amplifier by a direct bonding method. A phase modulator made of a waveguide is provided.

  A fourth aspect of the present invention is the phase sensitive optical amplifying device according to any one of the first to third aspects, wherein the phase modulator is arranged on the front side of the optical waveguide of the first second-order nonlinear optical element. It is formed adjacent to the optical waveguide of the first second-order nonlinear optical element, and is integrated with the optical waveguide of the first second-order nonlinear optical element on the substrate.

  The invention according to claim 5 is the phase sensitive optical amplifying device according to any one of claims 1 to 3, wherein the phase modulator includes the optical waveguide of the first second-order nonlinear optical element and the first filter. And is formed adjacent to the optical waveguide of the first second-order nonlinear optical element and integrated on the optical waveguide of the first second-order nonlinear optical element on the substrate. Features.

  A sixth aspect of the present invention is the phase sensitive optical amplifying device according to the fifth aspect, wherein the fundamental wave light remaining in the optical waveguide of the first second-order nonlinear optical element without being converted into sum frequency light is removed. Between the modulator and the optical waveguide of the first second-order nonlinear optical element, or at least one on the coupled optical waveguide between the first filter and the multiplexer, It is characterized by being integrated on a substrate.

  The invention according to claim 7 is the phase sensitive optical amplifying device according to claim 6, wherein the sum frequency light is a second harmonic, and the means for removing the fundamental light is the first secondary light. The sum frequency light from the nonlinear optical element is constituted by a bending waveguide or a tapered waveguide that operates so as to be converted into a single mode.

  The invention of claim 8 is the phase sensitive optical amplifying device according to any one of claims 1 to 7, wherein the second filter includes a first output waveguide that outputs the amplified signal light, and It comprises a multi-mode interference type multiplexer / demultiplexer (MMI) having a second output waveguide that extracts a part of the amplified signal light and outputs it as a detection signal to the means for synchronizing. To do.

  The invention according to claim 9 is the phase sensitive optical amplifying device according to any one of claims 1 to 8, wherein the multiplexer includes the signal light and the sum from the coupling waveguide of the first filter. It is composed of a multimode interference multiplexer / demultiplexer that multiplexes frequency light.

  A tenth aspect of the present invention is the phase sensitive optical amplifying device according to any one of the first to third aspects, wherein the phase modulator is configured on an input waveguide to which the signal light is input, and is formed on the substrate. It is characterized by being integrated.

The invention according to claim 11 is the phase sensitive optical amplifying device according to any one of claims 1 to 10, wherein the second-order nonlinear optical material whose polarization is periodically inverted is LiNbO ₃ , KNbO ₃ , LiTaO ₃ , LiNb. _x Ta _1-x O ₃ (0 ≦ x ≦ 1), KTiOPO ₄ , or at least one selected from the group consisting of Mg, Zn, Fe, Sc, and In as an additive Features.

  As described above, the phase sensitive optical amplifier of the present invention uses the second-order nonlinear optical device and two MMIs fabricated by the direct bonding method in an integrated manner, and the integrated substrate is processed on the end face. Yes. At each end face, the fundamental wave light and the second harmonic are appropriately reflected or non-reflected. Compared to the phase sensitive optical amplifier of the prior art, the substrate size can be reduced by eliminating individual components such as a multiplexer / demultiplexer, and further, an optical isolator, an optical circulator, and the like can be eliminated. According to the present invention, it is possible to suppress degradation of the amplifier SNR caused by connection loss between separate components, and to suppress an increase in the board size.

  Using the MMI (first filter) having the reflection end face, the PPLN can be arranged close to the compact while effectively separating the second harmonic wave and the fundamental wave light. Further, since the reflection end face of the MMI constitutes a part of the end face of the integrated substrate, the affinity for the manufacturing process of the optical circuit integrated on one substrate is high.

  The phase-sensitive optical amplifier of the present invention is a phase-sensitive amplifier that can be applied to optical communications and can be amplified with low noise, and can improve the signal-to-noise ratio of a signal in an optical fiber. Can be transmitted over long distances with low power. In addition, because it is possible to correct and amplify the phase chirp of the incident signal light, it is possible to suppress the influence of signal degradation due to chromatic dispersion of the optical fiber and extend the transmission distance of the amplified signal light Become. Further, according to the phase sensitive optical amplifier of the present invention, an optical signal without chirp is generated even if an inexpensive or simple optical modulator having phase chirp is used as a signal source in an application requiring a long distance transmission. It becomes possible to do.

FIG. 1 is a diagram showing a configuration of a phase sensitive optical amplifier of prior art 1. FIG. 2 is a diagram illustrating a configuration of a phase sensitive optical amplifier according to the related art 2. FIG. 3 is a diagram illustrating a configuration of the integrated phase-sensitive optical amplifier according to the first embodiment. FIG. 4 is a diagram illustrating the configuration of the second MMI of the present invention in comparison with a general MMI of the prior art. FIG. 5 is a graph showing the relationship between the phase difference Δφ between the input signal light and the pump light and the gain (dB) in the phase sensitive optical amplifier according to the first embodiment. FIG. 6 is a diagram illustrating a time waveform of a signal amplified by the phase sensitive optical amplifier according to the first embodiment. FIG. 7 is a diagram illustrating a configuration of the integrated phase-sensitive optical amplifier according to the second embodiment. FIG. 8 is a diagram illustrating a configuration of an integrated phase sensitive optical amplifier according to the third embodiment. FIG. 9 is a diagram illustrating a configuration of an integrated phase-sensitive optical amplifier according to the fourth embodiment. FIG. 10 is a diagram illustrating the configuration of the integrated phase-sensitive optical amplifier according to the fifth embodiment. FIG. 11 is a diagram illustrating the configuration of the integrated phase-sensitive optical amplifier according to the sixth embodiment. FIG. 12 is a diagram illustrating a configuration example of a bending waveguide and a means for removing the fundamental wave of the phase sensitive optical amplifier according to the seventh embodiment. FIG. 13 is a diagram illustrating another configuration example of the means for removing the fundamental wave light of the phase sensitive optical amplifier according to the seventh embodiment.

  The phase-sensitive optical amplifier of the present invention is configured by integrating a second-order nonlinear optical device and two MMIs manufactured by a direct bonding method on a single substrate, and the integrated substrate is end-face processed. Using an MMI type multiplexer / demultiplexer (first filter) having a reflection end face, two PPLNs can be arranged close to each other while effectively separating the second harmonic wave and the fundamental wave light. . In addition, since the above-described reflection end face of the MMI constitutes a part of the end face of the integrated substrate, it has a high affinity with the manufacturing process of the optical circuit integrated on one substrate.

  A phase modulator for synchronization between the signal light and the excitation light may be further integrated on the same substrate. In addition, a configuration in which phase modulation is applied not only to the fundamental wave light but also to the second harmonic wave that is the excitation light can be employed. Furthermore, by appropriately designing the radius of curvature of the bending waveguide so that the higher harmonic modes of the second harmonic are radiated effectively, only the fundamental mode of the second harmonic is phase-modulated, and the signal light wavelength is changed. On the other hand, by making the radius of curvature steep, it is possible to simultaneously remove the excitation signal light that cannot be converted in the PPLN for generating the excitation light and remains.

  The conventional phase-sensitive optical amplifier solves the problem that the number of parts is increased and the configuration of the entire amplifier is rather complicated. Further, it solves the problem that the efficiency of parametric amplification due to the reflected light of the signal light that cannot be completely converted when the excitation light is generated, and the amplification characteristic is deteriorated. Provided is a new integrated configuration of a phase sensitive optical amplifying apparatus which can be applied to amplification of weak laser light used for optical communication and can be amplified with low noise. Hereinafter, the present invention will be described in detail with reference to the drawings.

  FIG. 3 is a diagram showing a configuration of the integrated phase sensitive optical amplifier according to the first embodiment of the present invention. According to the phase sensitive optical amplifier 300 of the present embodiment, (1) degradation of the amplifier SNR due to connection loss between separate parts, (2) second harmonic generation, which is a problem in the configurations of the prior art 1 and the prior art 2. (SHG) PPLN, signal light and second harmonic multiplexer / demultiplexer, and degenerate parametric amplification (DPA) PPLN can be prevented from increasing on the same substrate.

  In the configuration of the phase sensitive optical amplifier 300, a PPLN 305 that generates a second harmonic on the same LN substrate 340, a PPLN 306 that performs degenerate parametric amplification, and two multimode interference types that function as a multiplexer / demultiplexer ( An MMI (Multi Mode Interference) multiplexer / demultiplexer, that is, a first MMI 330 and a second MMI 312 are integrated. The signal light 314 having a wavelength of 1.54 μm is amplified to obtain an amplified signal light 316. Compared to the configuration of FIG. 2 of the prior art, the large individual parts isolator 213 and circulator 206 are eliminated.

  In the following description, for simplicity, a multimode interference type (MMI) multiplexer / demultiplexer is simply referred to as MMI. A part of the signal light is branched by the coupler 301 and input to the EDFA 304 through the phase modulator 302 for synchronizing the phase of the signal light and the phase of the excitation light. After the fundamental wave light is amplified by the EDFA 304, the amplified light 315 is input from the right end of the substrate and used as excitation light. The excitation light input from the right end of the substrate is almost entirely converted into the second harmonic component 317 before reaching the second MMI 312.

  The two MMIs 330 and 312 integrated here will be described in more detail. The first MMI 330 (corresponding to the multiplexer in the claims) is designed to optimize the waveguide width, waveguide length and positions of the input port and the output port, thereby reducing the signal light and the pump light with low loss. Can be coupled to the PPLN 306. Specifically, signal light having a wavelength of 1.54 μm input from the input waveguide 333 could be coupled to the DPL PPLN 306 with an insertion loss of about 1.0 dB. Further, the excitation light having a wavelength of 0.77 μm generated by the second harmonic generation PPLN 305 could be coupled to the DPA PPLN 306 with an insertion loss of about 1.1 dB.

FIG. 4 is a diagram for explaining the configuration of the second MMI 312 used in the phase-sensitive optical amplifier of the present invention in comparison with the general MMI configuration of the prior art. FIG. 4A shows a configuration of a general MMI type multiplexer / demultiplexer. The MMI waveguide 401 has an input waveguide 404 and two output waveguides 402 and 403. For example, input different wavelengths lambda ₁ from the waveguide 401, by entering the two light lambda _2, two different light wavelengths lambda ₁ to one of the output waveguides 402 and 403, the other wavelength lambda ₂ Light is output. That is, the optical signal is demultiplexed to the output port depending on the wavelength of the input light. Needless to say, if light is propagated in the opposite direction, it functions as a multiplexer. There may be a plurality of input ports and output ports.

  On the other hand, the second MMI used in the phase sensitive optical amplifier 300 of this embodiment is formed together with the first MMI 330 on the same substrate 340 as shown in FIG. It is composed of a single multimode interference optical waveguide (MMI) 312 having an incident / exit end face 406 that enters and exits and a reflection end face 405. The MMI 312 can be manufactured by performing, for example, cutting and polishing, although not limited to a conventional MMI at a specific position, for example, a half position. Therefore, the left end face 405 of the MMI 312 in FIG. 4B is a reflection end face. Even if this reflection end face 405 is used alone, it has a property of reflecting the second harmonic wave and the fundamental wave light.

  The second MMI 312 of this embodiment further includes a reflection member 321 and an antireflection member 320a on the reflection end surface 405. The reflection member 321 and the antireflection member 320a correspond to the high reflection film 321 and the antireflection film 320a formed on the left end surface of the substrate 340 in FIG. 3, respectively. The second MMI 312 has two optical waveguides connected on the other end face 406, one optical waveguide 331 connected to the first PPLN 305 for generating second harmonics, and the other coupled optical waveguide. 332 is connected to the second PPLN 306 for DPA via the first MMI 330.

  Accordingly, the second MMI 312 is coupled to the optical waveguide 305 of the first second-order nonlinear optical element, reflects the second harmonic (sum frequency light) from the first second-order nonlinear optical element, and is the excitation fundamental wave light. And a coupling waveguide 332 that separates only the front second harmonic from the excitation fundamental wave light and the second harmonic and guides the separated second harmonic. The second MMI 312 corresponds to the first filter recited in the claims.

  Further, the second MMI 312 (first filter) uses a surface perpendicular to the MMI waveguide as a reflection end surface 405, reflects the second harmonic (sum frequency light) on the reflection end surface 405, and excites the fundamental. A multimode interference multiplexer / demultiplexer (MMI) including one or more thin films that transmit wave light. Further, the reflective end face of the second MMI 312 (first filter) constitutes a part of one end face of the substrate 340, and the one or more thin films 320 a and 321 described above are formed on the entire end face of the substrate 340. Formed on top.

  In the phase sensitive amplifier 300 of this embodiment, the second harmonic 317 is taken as an example. However, as long as the sum frequency light based on the basic excitation light that is easily separated by the second MMI 312 is used, the second harmonic 317 is used. It is not limited to harmonics. That is, the sum frequency light based on the basic excitation light generated by the first PPLN waveguide 305 can be used.

  Returning to FIG. 3 again, a high reflection film having a high reflectivity of 99.99% at a wavelength of 0.77 μm is applied to the left end of the substrate 340 including the second MMI 312 as a reflection treatment for the second harmonic. 321 is provided. The highly reflective film 321 can be constituted by an optical multilayer filter, for example. As described with reference to FIG. 4B, the second harmonic is reflected using the highly reflective film 321 such as a dielectric multilayer film provided on one end face 405 of the second MMI 312. The second harmonic input from the first PPLN 305 is reflected and folded at the end face 405 in the middle of the multimode interference in the forward path in the second MMI 312, and propagates while further performing multimode interference in the return path in the MMI 312. To do.

  Within the second MMI 312, guided light (second harmonic) having a wavelength of 0.77 μm is condensed on the end face 406 while periodically repeating image formation. The condensing position on the end face 406 depends on parameters such as the input position of the waveguide 331, the width and length of the MMI waveguide, and the reflection position. Therefore, by optimizing these parameters, the second harmonic excitation light generated by the first PPLN 305 for generating the second harmonic is coupled to the coupled optical waveguide 332 with low loss. The second harmonic excitation light 318 is input to the first MMI 330 via the coupled optical waveguide 332. At the same time, with respect to the second MMI 312, the residual excitation light having the wavelength of the fundamental wave light is designed so as to minimize mixing into the coupled optical waveguide 332, so that the residual excitation light is incident on the coupled optical waveguide 332. Can be very little. In this example, the second harmonic 317 could be coupled to the coupling optical waveguide 332 connected to the second PPLN 306 for DPA with an insertion loss of 1.5 dB.

  The second harmonic 318 is coupled to the second PPLN waveguide 306 for DPA via the first MMI 330. Similarly, the second harmonic 318 is optically mixed with the input signal light 314 combined via the first MMI 330, and the signal light is amplified by the second PPLN waveguide 306 for DPA.

  In FIG. 3 and each diagram of the embodiment described later, a solid line (fundamental wave light) and a broken line (second harmonic wave) reciprocating arrows are drawn on the left side of the second MMI outside the substrate. This conceptually shows that each light is reflected at the end face of the substrate. Therefore, it does not mean that light actually returns outside the substrate. The reciprocating arrow meaning reflection is also drawn for the fundamental wave light in order to show that it is impossible to transmit 100% of the fundamental wave light and it is slightly reflected. Therefore, this does not mean that the reflection end face of the second MMI has a function of actively reflecting the fundamental wave light.

  The signal light that reflects the second harmonic wave at the left end of the above-mentioned second MMI 312 and transmits the residual excitation light and is parametrically amplified by the second PPLN waveguide 306 for DPA becomes PPLN at the right end of the substrate 340. Different end surface treatments were applied to the left and right end surfaces of the substrate 340 so as not to be reflected back to the waveguide 306 and return again. After the left and right end faces of the substrate 340 are processed, the left end face is first coated with a high reflection (HR) film for the second harmonic wave 0.77 μm and further for antireflection for the 0.77 μm fundamental wave light. An (AR) film was formed by sputtering. On the right end surface of the substrate 340, an antireflection (AR) film 320b for the fundamental wave light of 1.54 μm and an antireflection (AR) film 322 for the second harmonic wave of 0.77 μm were formed by the same sputtering method. .

  As can be seen from the film forming procedure described above, the second MMI 312 can be formed simultaneously with the end face processing of the substrate 340. Through the above processing, a waveguide end face having a function of selectively reflecting and / or preventing reflection with respect to light of each desired wavelength was formed.

  With the above-described configuration, the optical waveguide 305 of the first second-order nonlinear optical element, the second MMI 312 (corresponding to the first filter in the claims), and the coupling optical waveguide 332 are adjacent as one continuous waveguide. Will be formed. Further, the input waveguide 333, the first MMI 330 (corresponding to the multiplexer in the claims), and the optical waveguide 306 of the second second-order nonlinear optical element are also formed adjacent to each other as one continuous waveguide. . Further, the coupling optical waveguide 332, the first MMI 330 (corresponding to the multiplexer in the claims) and the optical waveguide 306 of the second second-order nonlinear optical element are also formed adjacent to each other as one continuous waveguide. It will be.

In the phase sensitive amplifier of this embodiment, in order to amplify only the light in phase with the signal light, the phase of the signal light and the phase of the excitation light match as described above, or only π radians It needs to be shifted. That is, when using the second-order nonlinear optical effect, that the phase phi _2Omegaesu of the excitation light is a wavelength corresponding to the second harmonic, and the phases phi _.omega.s of the signal light, satisfies the following relationship (1) is necessary.
_{Δφ = φ 2ωs / 2-φ} ωs = nπ ( where, n is an integer) (1)

  FIG. 5 is a graph showing the relationship between the phase difference Δφ between the input signal light and the pump light and the gain (dB) in the phase sensitive optical amplifier using the second-order nonlinear optical effect of this embodiment. It can be seen that the gain G is maximum when Δφ on the horizontal axis is −π, 0, or π. In order to achieve the phase synchronization between the input signal light and the excitation light as shown in FIG. 5, the phase sensitive amplifier 300 of the present embodiment shown in FIG. 3 has the following configuration.

  In front of the optical fiber laser amplifier 304, a phase modulator 302 for modulating the phase of the excitation light 326 of the fundamental wave light with a pilot signal having a constant frequency and a small amplitude is provided. In a state in which the phase of the excitation light 326 is minutely modulated, the parametrically amplified signal light 316 is branched by the optical branching unit 308 and received and observed by the photodetector (PD: photodiode) 310. As shown in FIG. 5, in the state where the gain is maximized and the phase synchronization is established, the fluctuation of the gain due to the phase modulation is minimized. On the other hand, in a state where the phase synchronization is lost, the gain is modulated by phase modulation as the phase difference between the pumping light and the signal light increases as shown in FIG. The amplified light 316 also generates a modulation component having the same frequency as the pilot signal.

  The phase of the pump light and the signal light are synchronized by applying feedback to the phase of the pump light using PLL (Phase Lock Loop) technology so that the modulation component appearing in the amplified light 316 is minimized. Can be made. In this embodiment, feedback from the PLL circuit 311 to the optical fiber expander 303 using PZT is made possible to absorb phase fluctuations due to expansion and contraction of optical fiber parts and temperature fluctuations.

  In the present embodiment, although not shown in FIG. 3, an LN Mach-Zehnder modulator is used as a data signal modulator for input signal light, and amplification is performed when a 10 Gb / s NRZ signal is input as input signal light. Characteristics were evaluated.

  FIG. 6 is a diagram for explaining the time waveform of the signal amplified by the phase sensitive optical amplifier 300 of the present embodiment. 8A shows the output waveform of the incident signal light when the excitation light does not enter the phase-sensitive optical amplifier, and FIG. 8B shows the phase of the excitation light and the signal light by the PLL according to the equation (1). 8 (c) shows the output waveform when the phase of the excitation light and the phase of the signal light are set so as to be shifted by 90 degrees from the relationship of the expression (1). Each is shown. In the phase sensitive optical amplifier according to the present embodiment, the phase of the pumping light and the phase of the signal light are synchronized with each other so that the phase of the pumping light and the phase of the signal light satisfy the relationship of the expression (1). A gain of about 11 dB could be obtained under the condition that the power of the second harmonic 319 incident on the second PPLN waveguide 306 was 300 mW.

  In the phase sensitive optical amplifier 300 of this embodiment, signal light and excitation light are selected using two MMI type multiplexers / demultiplexers 312 and 330 integrated on one substrate 340, and a second PPLN 306 for DPA is selected. Can be incident at a high extinction ratio. As a result, it is possible to suppress a decrease in amplification factor, which is a problem in the configuration using the high reflection film 221 with respect to one conventional MMI 212 and second harmonic excitation light shown in FIG. In the configuration of the phase sensitive optical amplifier shown in FIG. 2, the reflected light of the residual signal light (fundamental wave light) that could not be completely converted to the second harmonic when the pumping light is generated is mixed into the PPLN 209 for DPA to reduce the amplification factor. It was. On the other hand, in the phase sensitive optical amplifier 300 of the present embodiment, the reflected light of the residual signal light to the second PPLN 306 is sufficiently suppressed using the reflection end face of the second MMI and each thin film, A highly efficient optical amplifier could be realized without causing a decrease in amplification factor in the second PPLN 306.

  In addition, since the pumping light and the signal light are separated by using the two MMIs 312 and 330 according to the configuration of the phase sensitive optical amplifier of this embodiment, the optical isolator necessary for the phase sensitive optical amplifier of the prior art 2 in FIG. The optical circulator becomes unnecessary and the influence of the loss caused by the connection of these components can be avoided. Since the light propagating to the signal light side does not exist in principle as compared with the configuration of the prior art 2 shown in FIG. 2, the optical isolator necessary for the signal light side is no longer necessary. By removing the isolator, the loss on the front stage side of the phase sensitive optical amplifier is lowered, so that the noise figure (NF) on the input side is lowered. It is well known that the NF value on the front side of the amplifier contributes to the NF value of the entire amplifier, and eliminating the optical isolator has a great effect on improving the NF value of the entire amplifier.

  Further, both the optical isolator and the optical circulator have a size of 60 mm × φ10 mm. Since the size of the LN substrate in this embodiment is about 60 mm × 4 mm, the effect of reducing the overall size of the apparatus by removing the optical isolator and the optical circulator is great.

  In this embodiment, the wavelength relationship between the signal light and the excitation light in the parametric amplification process uses the degenerate parametric process. However, when the wavelengths are different, that is, in the non-degenerate parametric process, the signal light is similarly amplified with low noise. Is possible.

  As described above, the phase sensitive optical amplifier according to the present embodiment combines the first PPLN 305 for generating the second harmonic generation (SHG) excitation light and the second PPLN 306 for degenerate parametric amplification (DPA). Two multi-mode interference (MMI) multiplexers / demultiplexers that function as demultiplexers, that is, a first MMI 330 and a second MMI 312 are integrated. As a result, it is possible to reduce the substrate size by eliminating the multiplexer / demultiplexer of individual components such as the dichroic mirror that existed in the substrate as in prior art 1 (FIG. 1). The dichroic mirror has a size of about 6 mm square, and the removal of the dichroic mirror has an effect of suppressing an increase in the substrate size. It is possible to suppress the deterioration of the amplifier SNR caused by the connection loss between the separate components and the optical circuit board, and to suppress the increase in the board size.

  Further, in the configuration of the phase sensitive optical amplifier of the present invention, the second MMI having the reflection end face 405 is used. Therefore, the first PPLN 505 for generating the second harmonic and the second PMI for degenerate parametric amplification are used. It is possible to take an arrangement in which PPLNs 306 are arranged substantially in parallel in the vertical direction. It should be noted that this arrangement can provide an effect of preventing a decrease in amplification factor as compared with a configuration in which two PPLNs are arranged on a straight line. When two PPLNs are arranged in a straight line, the phase matching wavelengths of the two PPLNs may be different due to non-uniform material properties in the substrate surface. In such a case, the phase matching condition indispensable for parametric light amplification cannot be satisfied, and the amplification factor of the entire phase sensitive optical amplifier is lowered.

  If the reflection end face of the second MMI 312 is used as in the present invention, the two PPLNs 305 and 306 can be arranged close to a distance of about 100 μm, for example, if the optical path is reversed. A decrease in rate can be avoided. Although it is possible to fold the optical path without using the MMI, in order to reduce the loss in the bending waveguide, it is necessary to increase the radius of curvature, and there is a drawback that the entire circuit becomes large. Further, the fundamental light and the second harmonic can be separated by the reflection end face of the second MMI 312 as described above.

  As described above, in the phase sensitive optical amplifier according to the present embodiment, the second harmonic, which is the excitation light, is applied to the reflection end face of the MMI waveguide having a configuration in which the conventional MMI waveguide is cut at the center portion. It is reflected by the provided high reflection film and can be coupled to the second PPLN for parametric amplification. At the same time, the residual fundamental wave component that has not been converted to the second harmonic can be efficiently removed. Since the reflection end face of the second MMI is partly formed on the end face of the substrate, it is possible to use a configuration having high affinity for the manufacturing process of the optical circuit integrated on one substrate. Since the reflection configuration is used, the substrate size can be significantly reduced as compared with the case where the PPLN having the SHG function and the DPA function is simply integrated.

In this embodiment, lithium niobate (LiNbO ₃ ) added with Zn is used as the second-order nonlinear optical material whose polarization is periodically inverted. However, the present invention is not limited to this. Lithium tantalate (LiTaO ₃ ), mixed crystal of lithium niobate and lithium tantalate (LiNb _(x) Ta _(1-x) O ₃ (0 ≦ x ≦ 1)), potassium niobate (KNbO ₃ ), titanyl phosphate Similar effects can be obtained with a second-order nonlinear optical material typified by potassium (KTiOPO ₄ ) or the like. Further, the additive of the second-order nonlinear optical material is not limited to Zn, and Mg, Zn, Sc, In, and Fe may be used instead of Zn. Moreover, it is not necessary to add an additive.

  In the above description of the configuration of the phase sensitive optical amplifier according to the present embodiment, the case where phase sensitive amplification by degenerate parametric amplification using signal light having a single wavelength (1.54 μm) as input signal light has been described. The configurations of the light source and the modulator that generate the input signal light can also be applied to the case of performing phase sensitive amplification by non-degenerate parametric amplification using a plurality of carrier waves having different wavelengths as input signals.

  The phase sensitive optical amplifier having the configuration of the first embodiment described above can further improve the problems of the prior art from another viewpoint. The parametric amplification operation itself by the nonlinear optical medium in the phase sensitive amplifier can essentially perform low-noise optical amplification. However, in actual operation, the influence of incidental noise as described below cannot be ignored. For example, it is necessary to consider that noise included in the excitation light itself is converted into noise of amplified light by a parametric amplification process.

  In the configuration of the phase sensitive optical amplifier 300 according to the first embodiment illustrated in FIG. 3, the phase modulator 302 used for phase synchronization is disposed on the upstream side of the EDFA 304. For this reason, the incident power level to the EDFA 304 is reduced by the insertion loss of the phase modulator 302. As shown in Non-Patent Document 3, it is known that in a laser amplifier such as an EDFA, if there is a loss on the front side of the amplifier, the SN ratio is degraded by the loss. If the SN ratio of the pumping light 326 deteriorates due to the insertion loss of the phase modulator 302 in this way, the noise component is converted into the noise of the amplified light in the parametric amplification process, and low-noise amplification is performed. Can not be. In the phase sensitive optical amplifier according to the second embodiment described below, the problem of noise of the excitation light is solved.

  FIG. 7 is a diagram showing a configuration of an integrated phase sensitive optical amplifier according to the second embodiment of the present invention. The present embodiment shows a configuration that prevents deterioration of the S / N ratio in the optical fiber laser amplifier due to the insertion loss of the phase modulator 302, which is a problem in the first embodiment. As shown in Non-Patent Document 3, in the laser amplifier, when there is a loss on the front side of the laser amplifier, the SN ratio is degraded by the loss. On the other hand, when there is a loss on the rear stage side of the laser amplifier, the output level is reduced by the amount of the loss, but the SN ratio does not deteriorate. In this embodiment, using this property, the phase modulator 702 is arranged on the output side of the optical fiber laser amplifier 704 as shown in FIG.

Here, it should be noted that the phase-sensitive optical amplifier according to the prior art cannot take the phase modulator arrangement as shown in FIG. The reason is that many of the existing phase modulators are made of an optical waveguide in which Ti is diffused in a LiNbO ₃ (LN) crystal. Since optical damage is significant in the Ti diffusion waveguide, a change in refractive index due to the photorefractive effect occurs when a large optical power is incident. Since the phase change is caused by the refractive index change of the Ti diffusion waveguide, a so-called drift phenomenon is caused in which the voltage applied to the phase modulator for obtaining the same phase condition is changed.

  In consideration of this drift phenomenon, assuming that the laser amplifier is disposed on the rear stage side, the optical power that can be input to the phase modulator is limited to about 20 dBm (100 mW). Under such circumstances, the power of the pumping light after amplification is reduced by arranging a phase modulator having a large insertion loss at the rear stage side of the laser amplifier. Eventually, the pumping light power necessary and sufficient for producing the optical parametric effect cannot be obtained, and phase sensitive amplification with a large amplification factor cannot be realized. Therefore, the configuration shown in FIG. 7 cannot be adopted in the configuration of the prior art.

  On the other hand, in the phase sensitive optical amplifier 700 of this embodiment, the phase modulator 702 can be arranged on the output side from the EDFA 704. The configuration of the present embodiment is the same as that of the first embodiment except that the position of the phase modulator 702 is different. Therefore, detailed description is omitted except for the portion related to the phase modulator 702. In the first PPLN waveguide 705 that generates the second harmonic, in order to handle the second harmonic in which the optical damage becomes more significant, a method of forming a waveguide that is more resistant to optical damage than the Ti diffusion method is used. Is common. In the present embodiment, similarly to the first PPLN waveguide 705 that generates the second harmonic, the phase modulator 702 is configured by using an optical waveguide manufactured by a direct bonding method. It becomes possible to use a larger excitation light power. Specifically, in the phase modulation unit 702, a direct junction waveguide using, as a core, lithium niobate added with Zn having excellent optical damage resistance was used.

  In the phase sensitive optical amplifier 700 of this embodiment, since the phase modulator 702 is not arranged on the input stage side of the EDFA 704, the SN ratio of the pump light 717 before being amplified by the EDFA 704 is compared with the configuration of the first embodiment. The improvement was about 5 dB.

  A gain of about 11 dB could be obtained under the condition that the power of the second harmonic wave incident on the second PPLN waveguide 706 was 300 mW. At this time, the output power of the EDFA 704 was about 1 W, and the input power to the direct junction waveguide formed in the phase modulator 702 was 630 mW. Even when such high power level light is incident, the phase modulator 702 operates without causing a drift phenomenon of the applied voltage, and can realize a stable phase synchronization operation.

  The phase sensitive optical amplifier of the present embodiment can improve the S / N ratio of the pumping light in addition to the effect of the configuration of the first embodiment, so that the noise contained in the pumping light itself is reduced by the parametric amplification process. It is possible to suppress the effect of being converted into As a result, optical signals can be amplified with lower noise. By integrating the phase modulator 702, the number of parts can be further reduced and the loss can be reduced as in the following embodiment.

  FIG. 8 is a diagram showing a configuration of an integrated phase sensitive optical amplifier according to the third embodiment of the present invention. The configuration of this example uses a second-order nonlinear optical device and two MMIs fabricated by a direct bonding method in an integrated manner, and the integrated substrate is processed in the same manner as in the first and second examples. It is. The difference between the present embodiment and the second embodiment is that the phase modulator 802 for synchronizing the signal light and the pump light is the same as the substrate 840 in which two PPLNs 805 and 806 and two MMIs 812 and 813 are integrated. This is the point of further integration on the nonlinear optical crystal substrate. That is, it is characterized in that a phase modulation unit 802 formed on the most input side of the integrated substrate 840 is provided. That is, the phase modulator 802 is formed adjacent to the optical waveguide 805 of the first second-order nonlinear optical element on the front stage side of the optical waveguide 805 of the first second-order nonlinear optical element. 1 is integrated with the optical waveguide of the second-order nonlinear optical element.

  On the same substrate 840 on which the PPLN waveguides 805 and 806 are formed, a phase modulation unit 802 without a periodically poled structure is integrated and formed by the same waveguide formation method as the PPLN waveguide. In the phase modulation unit 802, an electric field application electrode 819 is formed on the waveguide, and an electric signal is applied from an oscillator 823 to enable phase modulation by an electro-optic (EO) effect. As described above, this waveguide formation method is excellent in optical damage resistance. Therefore, even when the power level of the excitation fundamental wave light 815 amplified by the EDFA 804 is increased, a drift phenomenon of the operating voltage of the phase modulation unit 802 occurs. There is nothing. Phase modulation by a pilot signal for optical PLL operation can be performed on the fundamental light 815 that is pumping light.

  Since a plurality of phase modulators are integrated in this embodiment, the connection loss is reduced as compared with the configuration in which a single phase modulator is placed outside the integrated substrate as in Embodiment 2. The The S / N ratio of the amplified signal light 816 that can supply pump light with a higher power level to the first PPLN 805 that generates the second harmonic is improved.

In this embodiment, the third MMI 818 integrated on the substrate 840 is used for the signal light amplified in the second PPLN 806 and the second harmonic excitation light remaining in the second PPLN. It was used as an excitation light blocking filter for preventing residual excitation light from entering the amplified signal light output port 825. At this time, a part of the amplified signal light is extracted from the output port 825, input to the detector 810, and used as a feedback signal to the PLL circuit 811. With the third MMI 818, the optical branching portions 308 and 708 in the first and second embodiments can be taken into the substrate 840 to increase the degree of integration.

  That is, in this embodiment, the optical branching unit (second filter) synchronizes with the first output waveguide 825 that outputs the amplified signal light by extracting a part of the amplified signal light. It comprises a multimode interference type multiplexer / demultiplexer (MMI) 818 having a second output waveguide 824 that outputs a detection signal to the means 811. By optimizing the width, length, input position and output position of the multimode waveguide section of the third MMI 818, for example, 90% of the amplified light is used as the main output and 10% is used as the monitor can do. At the same time, it becomes possible not to mix the residual excitation light into the amplified light output port 825 and the monitor port 824.

  Compared with the configuration of the second embodiment, the connection loss generated in the phase modulator can be significantly reduced by the phase sensitive optical amplifier of the present embodiment. Thus, optical amplification can be performed with lower noise. By integrating the PPLN for generating the second harmonic for generating the excitation light and the phase modulator, it is possible to reduce the influence of the attenuation of the excitation light compared to the case where a single phase modulator is connected. High-quality optical signal amplification is possible while preventing degradation of the signal-to-noise ratio. As a result, the signal-to-noise ratio of the signal in the optical fiber can be improved by the phase sensitive amplifier that can be applied to optical communication and can be amplified with low noise. It becomes possible to transmit an optical signal to a long distance with a lower power than a conventional high-speed signal.

  The phase sensitive optical amplifier of the present embodiment can be more effectively integrated as in the following embodiment by further configuring the phase modulation section.

  FIG. 9 is a diagram showing a configuration of an integrated phase sensitive optical amplifier according to the fourth embodiment of the present invention. In this embodiment, a second-order nonlinear optical device and two MMIs manufactured by a direct bonding method are integrated on one substrate, and the integrated substrate is processed with an end face, and signal light and The point that the phase modulator for synchronization with the excitation light is further integrated on the same substrate is the same as the configuration of the third embodiment. The difference in configuration between the present embodiment and the third embodiment is that the phase modulation unit 902 is arranged so that phase modulation is performed not on the fundamental wave light but on the second harmonic wave that is the excitation light. Is a point. That is, the phase modulator 902 is adjacent to the optical waveguide 905 of the first second-order nonlinear optical element between the optical waveguide 905 of the first second-order nonlinear optical element and the second MMI 912 (first filter). And integrated on the optical waveguide 905 of the first second-order nonlinear optical element on the substrate 940. Accordingly, the phase sensitive optical amplifier 900 of FIG. 9 is the same as the configuration of the third embodiment except for the portion related to the phase modulation unit 902, and therefore detailed description is omitted except for the portion related to the phase modulator 902. To do.

The effect produced by disposing the phase modulation unit 902 on the second harmonic waveguide instead of on the fundamental wave waveguide can be explained as follows. When an external force such as an electric field or stress is applied to the optical material from the outside, a refractive index change occurs in the optical material. When the electro-optic effect of the LN crystal is used as the phase modulator, the phase is modulated by applying an electric field to the crystal. As shown in Non-Patent Document 5, the half-wave drive voltage Vπ indicating the performance of the modulator depends on the electro-optic coefficient, refractive index, applied electric field, wavelength, and the like of the LN crystal. In particular, when attention is paid to the wavelength used, the relationship shown in the following equation (2) is established.
Vπ∝λ Formula (2)

  Compared with the arrangements shown in the first to third embodiments, the arrangement employed in this embodiment reduces the half-wave driving voltage, that is, the voltage required for phase modulation by half compared to the configuration of the third embodiment. Thus, the required voltage from the voltage application device 923 can be greatly reduced. By reducing the required voltage, the peripheral devices of the phase sensitive optical amplifier can be reduced in size and power consumption. If the phase modulation voltage is constant, the required optical path length, that is, the length of the phase modulation section, can be greatly reduced to ½ in principle compared to the configuration of the third embodiment. Actually, in order to obtain a feedback signal to the PLL circuit 911 of the same level, the length of the actual electric field application electrode 819 in the phase modulation unit 802 used in the third embodiment of the present invention is required to be about 50 mm. On the other hand, the electric field applying electrode 919 of this example could be significantly shortened to about 28 mm.

  Also in this embodiment, as in the other embodiments, the phase modulation section 902 is a phase modulation section using a direct junction waveguide using lithium niobate doped with Zn having excellent optical damage resistance as a core. It was possible to suppress the operating voltage drift.

  A substrate that is integrated by further reducing the electrode area compared to the configuration of the third embodiment by configuring the phase modulator portion on the second harmonic waveguide by the phase sensitive optical amplifier of the present embodiment. An increase in the area can be suppressed.

  FIG. 10 is a diagram showing a configuration of an integrated phase sensitive optical amplifier according to the fifth embodiment of the present invention. In this example, a second-order nonlinear optical device and two MMIs manufactured by a direct bonding method are integrated on one substrate, and the integrated substrate is end-face processed, and signal light and Similar to the third embodiment, a phase modulator for synchronization between pumping lights is further integrated on the same substrate. The difference in configuration between the present embodiment and the third embodiment is that phase modulation is performed on the signal light to the phase-sensitive optical amplifier, not on the fundamental light or the second harmonic excitation light. The phase modulator 1002 is disposed at the point. Therefore, the phase-sensitive optical amplifier 1000 in FIG. 10 is the same as the configuration of the third embodiment except for the portion relating to the phase modulator 1002, and therefore detailed description thereof is omitted except for the portion related to the phase modulator 1002.

  In the embodiments described so far, the arrangement for causing the phase modulation in the phase-locking optical PLL to function with respect to the pumping light has been shown. Even if an arrangement is made to make this phase modulation function for signal light, phase synchronization can be realized in exactly the same way. When a generally available single phase modulator is inserted on the signal light side to form a phase sensitive amplifier, the influence of the insertion loss of the phase modulator is relatively large, and the degenerate parametric amplification (DPA) section first The signal light is attenuated before reaching the second PPLN waveguide 1006. Therefore, the deterioration of the SNR of the entire phase sensitive optical amplifier 1000 cannot be avoided. In order to solve this problem, a phase modulation unit 1002 that modulates the signal light 1014 as shown in FIG. 10 is further integrated in an integrated substrate 1040. With the configuration of this example, it was possible to improve the SNR by 3 dB compared to the case where a separate single phase modulator was inserted on the signal light side outside the substrate.

  With the phase sensitive optical amplifier according to the present embodiment, the connection loss generated in the phase modulator can be significantly reduced as in the configuration of the third embodiment. Thus, optical amplification can be performed with lower noise. High-quality optical signal amplification is possible while preventing degradation of the signal-to-noise ratio. As a result, the signal-to-noise ratio of the signal in the optical fiber can be improved by the phase sensitive amplifier that can be applied to optical communication and can be amplified with low noise. It becomes possible to transmit an optical signal to a long distance with a lower power than a conventional high-speed signal.

  In the phase-sensitive amplifier according to the fourth embodiment that modulates the second harmonic described above, the waveguide of the phase modulation unit 902 has a fundamental light wavelength of 1.54 μm, similar to the first PPLN waveguide 905. Designed to be single mode. For this reason, in the second harmonic wavelength of 0.77 μm, the multimode is set. The second harmonic generated in the second PPLN waveguide 905 propagates basically only in the fundamental mode due to restrictions due to the phase matching condition. However, there is a problem in that higher-order modes are excited by non-uniformity due to fluctuations in the waveguide width in the waveguide on the rear stage side of the second harmonic PPLN in relation to fluctuations in the waveguide processing process accuracy. There is. Therefore, in the configuration of the phase sensitive amplifier according to the fourth embodiment, although the electrode can be reduced in size, there is a problem that the PLL operation becomes unstable because modulation is applied to the second harmonic including the higher order mode. In this embodiment, problems caused by this higher order mode are improved.

  FIG. 11 is a diagram showing a configuration of an integrated phase sensitive optical amplifier according to the sixth embodiment of the present invention. In this embodiment, (1) a second-order nonlinear optical device and two MMIs manufactured by a direct bonding method are integrated on one substrate, and the integrated substrate is processed with an end face. (2 The point that the phase modulator for synchronization between the signal light and the pumping light is further integrated on the same substrate, and (3) the phase modulation is performed on the second harmonic wave that is the pumping light is an example. This is the same as the configuration of FIG. The difference in configuration between the present embodiment and the fourth embodiment is that the waveguide 1124 between the first PPLN 1105 for generating the excitation light and the phase modulator 1102 is bent. The phase-sensitive optical amplifier 1100 in FIG. 11 is the same as the configuration of the fourth embodiment except for the portion related to the phase modulation unit 1102, and thus detailed description is omitted except for the portion related to the phase modulator 1102.

  In the phase sensitive amplifier having the configuration of the fourth embodiment shown in FIG. 8, phase modulation is possible with a low voltage because phase modulation is applied to the second harmonic. However, although the first PPLN waveguide 1105 operates in a single mode with respect to the wavelength of the signal light that is the fundamental light, a higher-order mode may be excited with respect to the second harmonic. When the phase modulation is applied to the second harmonic guided light including the higher order mode from the first PPLN waveguide 1105, the phases of the plurality of propagation modes are modulated. For this reason, it becomes unstable and difficult to synchronize the phase with the amplified signal light 1116 which is a single mode.

  Therefore, in order to remove unnecessary higher-order modes from the generated second harmonic and to make a single mode, as shown in FIG. 10, the first PPLN 1105 for generating the excitation light and the phase modulation unit 1102 The waveguide 1124 between them was bent. By appropriately designing the radius of curvature of the bending waveguide 1124 so as to effectively radiate the higher harmonic mode of the second harmonic out of the waveguide, only the fundamental mode of the second harmonic can be phase-modulated. It becomes possible. The phase synchronization between the signal light and the pump light can be realized stably. Further, since this bending waveguide 1124 has a steep radius of curvature with respect to the wavelength of the signal light, it is used for exciting the wavelength of the fundamental wave light that remains in the first PPLN 1105 for generating the excitation light without being completely converted to the second harmonic light. Signal light can also be removed simultaneously.

  With the configuration of the phase sensitive amplifier of this embodiment, highly efficient and stable phase sensitive amplification was possible. It has been confirmed that the same effect can be obtained even when a tapered waveguide is applied instead of the bent waveguide 1124, and that a highly efficient and stable phase sensitive amplification can be realized.

  In the configuration including the bending waveguide shown in the sixth embodiment, a phase sensitive optical amplifier in which the phase synchronization operation is further stabilized can be realized, but there is room for improvement in terms of suppressing an increase in the substrate size. In this embodiment, a configuration in which the substrate size is further suppressed and the degree of integration is increased is presented.

  FIG. 12 is a diagram showing a configuration of an integrated phase sensitive optical amplifier according to the seventh embodiment of the present invention. In this embodiment, (1) a second-order nonlinear optical device and two MMIs manufactured by a direct bonding method are integrated on a single substrate, and the integrated substrate is end-face processed, (2) A point where a phase modulator for synchronization between the signal light and the pumping light is further integrated on the same substrate, (3) a point where phase modulation is applied to the second harmonic wave which is the pumping light, and (4) By appropriately designing the radius of curvature of the bending waveguide to effectively radiate higher-order modes of the second harmonic, only the fundamental mode of the second harmonic is phase-modulated, and with respect to the signal light wavelength Similar to the configuration of the sixth embodiment, the steep radius of curvature allows simultaneous removal of the remaining excitation signal light that could not be converted by the PPLN for generating the excitation light. The difference in configuration between the present embodiment and the sixth embodiment is that the waveguide connecting the two MMIs is not bent between the first PPLN for generating the excitation light and the phase modulation unit. This is an added point.

  In the phase-sensitive optical amplifier according to the sixth embodiment shown in FIG. 11, a configuration in which a bending waveguide 1124 is used immediately after generating pumping light to remove higher-order modes of the residual fundamental wave and the second harmonic of the pumping light. I was trying. In the configuration of FIG. 11, the interval between the excitation light optical waveguide 1124 and the PPLN optical waveguide 1106 for parametric amplification is narrowed, and the residual fundamental wave light component radiated by bending is easily propagated to the parametric amplification region 1106, which is an unnecessary component. Light easily enters the parametric amplification region 1106. Therefore, in order to prevent such interference between the bending waveguide 1124 and the parametric amplification optical waveguide 1106, it is necessary to sufficiently widen the interval between these optical waveguides. For this reason, there is a drawback that circuit elements cannot be densely integrated on the substrate 1140 and the element size becomes large.

  In this embodiment, a bending waveguide is provided between a filter that reflects and separates excitation light, that is, the second MMI 1112, and a multiplexer, that is, the first MMI 1113. It is difficult to arrange the second MMI 1112 and the first MMI 1113 in the same straight line from the flow of the input light and the output light in each phase sensitive optical amplifier of the first to sixth embodiments. Therefore, as shown in FIGS. 3 and 7 to 11, two MMIs are inevitably connected by a bent waveguide or the like. By optimizing the waveguide width and the radius of curvature of the bent waveguide, it is possible to remove the higher order modes of the residual fundamental wave light and the second harmonic of the excitation light.

  FIG. 12 is a diagram illustrating a configuration example of a bending waveguide of the phase sensitive optical amplifier according to the seventh embodiment. FIG. 12 shows only the vicinity of the second MMI 1212 and the first MMI 1213. The second harmonic 1219 reflected by the second MMI 1212 travels through the bending waveguide 1224 toward the first MMI 1213, but the fundamental wave lights 1216a and 1216b are radiated out of the waveguide. Higher order modes of the second harmonic of the excitation light are also removed during propagation through the bending waveguide 1224. When this configuration is used, the first PPLN for generating the excitation light and the second PPLN for parametric amplification can be integrated sufficiently close to each other, so that the substrate can be compared with the configuration of the sixth embodiment shown in FIG. It was confirmed that the layout can be reduced by reducing the size to about 1/3.

  Therefore, as shown in the sixth and seventh embodiments, the means for removing the fundamental wave light that is not converted into the second harmonic (sum frequency light) in the optical waveguides 1105 and 1205 of the first second-order nonlinear optical element. (Bending waveguide) between the modulators 1102 and 1202 and the optical waveguides 1105 and 1205 of the first second-order nonlinear optical element, or the second MMI 1112 and 1212 (first filter) and the first MMI 1113 , 1213 (multiplexer) on at least one of the coupled optical waveguides.

  FIG. 13 is a diagram illustrating another configuration example of the means for removing the fundamental wave light of the phase sensitive optical amplifier according to the seventh embodiment. FIG. 13 shows only the vicinity of the second MMI 1312 and the first MMI 1313. The second harmonic wave 1319 reflected by the second MMI 1312 travels through the tapered waveguide 1324 toward the first MMI 1313, but the fundamental wave lights 1316a and 1316b are radiated out of the waveguide. Similarly, the higher order mode of the second harmonic of the excitation light is also removed during propagation through the tapered waveguide 1324. In place of the bending waveguide shown in FIG. 12, a taper-type optical waveguide 1324 appropriately designed and manufactured can be replaced with an effect of removing higher fundamental modes of the residual fundamental wave and the second harmonic. It is done.

  According to the present embodiment, the high-order mode of the second harmonic of the excitation light can be removed, and the high-efficiency and stable phase sensitive amplification similar to that of the sixth embodiment can be realized, and the substrate size can be further reduced to increase the degree of integration. The raised phase sensitive optical amplifier can be configured.

  As described above in detail, the phase-sensitive optical amplifier of the present invention uses a second-order nonlinear optical device and two MMIs fabricated by a direct bonding method in an integrated manner, and the integrated substrate performs end face processing. It has been subjected. At each end face, the fundamental wave light and the second harmonic are appropriately reflected or non-reflected. Compared with the phase sensitive optical amplifier of the prior art, the substrate size is reduced by eliminating individual components such as a multiplexer / demultiplexer, and further, an optical isolator and an optical circulator are not required. As a result, it is possible to suppress degradation of the amplifier SNR caused by connection loss between separate components, and to suppress an increase in the board size. Further, since the phase modulator can be arranged on the rear stage side of the laser amplifier and the S / N ratio of the pumping light can be improved, the noise included in the pumping light itself is converted into the noise of the amplified light by the parametric amplification process. SNR degradation of the entire amplifier can be suppressed.

  Furthermore, as a configuration in which the phase modulator is integrated on the substrate and phase modulation is performed on the fundamental wave light, connection loss that occurs when an individual phase modulator is provided can be significantly reduced. In addition, by applying phase modulation to the second harmonic, the effective modulation optical path length is doubled, or the length of the drive electrode for optical PLL phase modulation is changed to phase modulation with respect to the fundamental light. Compared to the conventional configuration to be applied, the length can be significantly shortened.

  The waveguide between the two MMIs removes unnecessary higher-order modes from the second harmonic, and further pumps the wavelength of the fundamental wave light that remains without being completely converted into the second harmonic light in the PPLN for generating the excitation light. The signal light for use can also be removed at the same time. It is possible to effectively suppress deterioration of amplification characteristics caused by mixing of excitation light of fundamental light having the same wavelength as that of signal light.

  The phase sensitive optical amplifier of the present invention can be applied to optical communication, can be amplified with low noise, and can improve the SN ratio of a signal in an optical fiber. It is possible to transmit up to a long distance. In addition, since the phase chirp of the incident signal light can be corrected and amplified, it is possible to suppress the influence of signal deterioration due to the wavelength dispersion of the optical fiber and to extend the transmission distance of the amplified signal light. . Further, according to the phase sensitive optical amplifier of the present invention, it is possible to generate an optical signal without a chirp even if an inexpensive or simple optical modulator having a phase chirp is used in an application requiring a long distance transmission. become.

  The present invention is generally applicable to an optical communication system.

100, 200, 300, 700, 800, 900, 1000, 1100 Phase sensitive optical amplifier 101, 108, 201, 208, 301, 308, 701, 708, 801, 808, 901, 908, 1001, 1008, 1101, 1108 Couplers 102, 202, 302, 702, 802, 902, 1002, 1102 Phase modulators 104, 204, 304, 704, 804, 904, 1004, 1104 Fiber laser amplifiers 105, 107, 209, 305, 306, 705, 706 805, 806, 905, 906, 1005, 1006, 1105, 1106 PPLN waveguide 212, 312, 330, 712, 730, 812, 813, 912, 913, 1012, 1013, 1112, 1113 MMI
320a, 320b, 720a, 720b, 820a, 820b, 920a, 920b, 1020a, 1020b, 1120a, 1120b High reflection (HR) film 1124, 1224 Bending waveguide 1324 Tapered waveguide

Claims (11)

In a phase-sensitive optical amplifier that amplifies signal light by optical mixing using a nonlinear optical effect,
An optical fiber laser amplifier for amplifying the excitation fundamental light;
A first second-order nonlinear optical element comprising a second-order nonlinear optical material periodically poled and having an optical waveguide for generating sum frequency light from the excitation fundamental wave light;
The pump is coupled to the optical waveguide of the first second-order nonlinear optical element, has a function of reflecting the sum frequency light from the first second-order nonlinear optical element and transmitting the excitation fundamental wave light, and A first filter having a coupling waveguide for separating only the sum frequency light from the fundamental wave light and the sum frequency light, and guiding the separated sum frequency light;
A multiplexer having an input waveguide to which the signal light is input, and combining the signal light and the sum frequency light from the coupling waveguide;
A second-order nonlinear optical element comprising a second-order nonlinear optical material periodically poled and having an optical waveguide that performs parametric amplification of the signal light using the excitation light;
A second filter that separates the amplified signal light and the sum frequency light that is the excitation light;
Means for synchronizing the phase of the amplified signal light and the phase of the pumping light input to the optical fiber laser amplifier,
The optical waveguide of the first second-order nonlinear optical element, the first filter, the coupling waveguide, the multiplexer, and the optical waveguide of the second second-order nonlinear optical element are on the same substrate Integrated on top,
The optical waveguide, the first filter, and the coupling waveguide of the first second-order nonlinear optical element are formed adjacent to each other on a continuous waveguide;
The input waveguide, the multiplexer and the optical waveguide of the second second-order nonlinear optical element are formed adjacent to each other on a continuous waveguide;
The phase sensitive optical amplifying apparatus, wherein the coupling waveguide, the multiplexer, and the optical waveguide of the second second-order nonlinear optical element are formed adjacent to each other on a continuous waveguide. The first filter includes a multi-mode interference having one or more thin films that reflect the sum frequency light and transmit the excitation fundamental wave light on the reflection end surface with a surface perpendicular to the waveguide as a reflection end surface. MMI) type multiplexer / demultiplexer,
The reflective end face of the first filter constitutes a part of one end face of the substrate, and the one or more thin films are formed on the entire end face of the substrate;
The optical waveguide of the first second-order nonlinear optical element and the optical waveguide of the second second-order nonlinear optical element have a refractive index smaller than that of the first substrate having a nonlinear optical effect and the first substrate. The phase sensitive optical amplifying device according to claim 1, wherein the phase sensitive optical amplifying device is a directly bonded optical waveguide manufactured by directly bonding to a second substrate.   3. The phase sensitive device according to claim 1, further comprising a phase modulator made of an optical waveguide manufactured by a direct bonding method on an output side of the optical fiber laser amplifier as a part of the synchronizing means. Type optical amplifier.   The phase modulator is formed adjacent to the optical waveguide of the first second-order nonlinear optical element on the front side of the optical waveguide of the first second-order nonlinear optical element, and on the substrate, 4. The phase sensitive optical amplifying device according to claim 1, wherein the phase sensitive optical amplifying device is integrated with the optical waveguide of the first second-order nonlinear optical element.   The phase modulator is formed between the optical waveguide of the first second-order nonlinear optical element and the first filter and adjacent to the optical waveguide of the first second-order nonlinear optical element, 4. The phase sensitive optical amplifying device according to claim 1, wherein the phase sensitive optical amplifying device is integrated on the optical waveguide of the first second-order nonlinear optical element on a substrate.   Means for removing the fundamental light remaining in the optical waveguide of the first second-order nonlinear optical element that is not converted into sum frequency light is provided between the modulator and the optical waveguide of the first second-order nonlinear optical element. Or at least one on the coupled optical waveguide between the first filter and the multiplexer and integrated on the substrate. Type optical amplifier. The sum frequency light is a second harmonic,
The means for removing the fundamental wave light is constituted by a bent waveguide or a tapered waveguide that operates so as to convert the sum frequency light from the first second-order nonlinear optical element into a single mode. Item 7. The phase-sensitive optical amplifier according to Item 6.   The second filter extracts a part of the amplified signal light from the first output waveguide that outputs the amplified signal light, and outputs it as a detection signal to the synchronizing means. 8. The phase sensitive optical amplifying device according to claim 1, comprising a multimode interference multiplexer / demultiplexer (MMI) having a plurality of output waveguides.   2. The multi-mode interference multiplexer / demultiplexer configured to multiplex the signal light and the sum frequency light from the coupling waveguide of the first filter. The phase sensitive optical amplifying device according to any one of 1 to 8.   4. The phase sensitive optical amplification according to claim 1, wherein the phase modulator is configured on an input waveguide to which the signal light is input and integrated on the substrate. apparatus. The periodically poled second-order nonlinear optical material is LiNbO ₃ , KNbO ₃ , LiTaO ₃ , LiNb _x Ta _1-x O ₃ (0 ≦ x ≦ 1), KTiOPO ₄ , or Mg, Zn The phase sensitive optical amplifying device according to claim 1, wherein at least one selected from the group consisting of Fe, Sc, and In is contained as an additive.

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