JP2015060944A - Wavelength control filter and wavelength variable laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably output high-quality light by suppressing fluctuations of an internal loss that accompanies an operation of controlling wavelength.SOLUTION: A wavelength control filter 10 includes: filter means 20 that has a flat-top light transmittance characteristic with respect to wavelengths and transmits light at a predetermined mode interval; phase adjustment means 30 that adjusts a phase of light by changing an optical length; reflection means 40 that totally reflects light input via the filter means and the phase adjustment means; and phase control means 50 that detects a phase of the light output from the filter means 20 and controls the phase adjustment means 30 based on the detection result.

Description

  The present invention relates to a wavelength control filter capable of outputting light having a desired wavelength and a wavelength tunable laser capable of selecting a desired laser oscillation wavelength.

  One of the technologies for increasing the capacity of an optical communication system is an optical wavelength division multiplexing (WDM) system. In the WDM system, a plurality of optical signals having different carrier wavelengths are transmitted simultaneously on one optical fiber. For example, if modulation is performed at 10 gigabits / second per channel and 100 channels are transmitted by one common optical fiber, the communication capacity reaches 1 terabit / second. However, in the WDM system, it is necessary to prepare laser devices for each carrier wavelength, and when 100 types of laser devices are prepared for 100 channels, the cost increases.

  On the other hand, in the medium and long distance optical communication in recent years, a communication band of C band (1530 to 1570 nm) that can be amplified by EDFA (Erbium Doped Fiber Amplifiers) is widely used. Depending on the type of optical fiber used, a communication band of L band (1570 to 1610 nm) is used. Therefore, in a general WDM system for medium and long distance optical communication, a wavelength tunable laser that can handle the entire wavelength in the C band (or L band) with a single laser is used.

  One of tunable lasers applied to a general WDM system for medium and long distance optical communication is an external resonator type tunable laser. An external resonator type tunable laser forms a resonator using a semiconductor optical amplifier (SOA) and an external reflecting mirror, and selects a wavelength by inserting a tunable filter in the resonator. The external resonator type wavelength tunable laser is disclosed in, for example, Patent Documents 1 and 2, and can handle the entire wavelength band of the C band (or L band) relatively easily.

  FIG. 8 shows a schematic configuration diagram of the wavelength tunable laser disclosed in Patent Document 1. In FIG. A wavelength tunable laser 900 disclosed in Patent Document 1 includes a semiconductor optical amplifier 910, a wavelength tunable filter 920, and a highly reflective coating film 930 attached to one end of the wavelength tunable filter 920. The tunable filter 920 is provided with a multiple ring filter 921 in which ring filters 922 and 923 having different optical path lengths are connected via an optical coupling means. The first port 924 of the wavelength tunable filter 920 has an antireflection coating 926 on its end surface, and is optically coupled to a semiconductor optical amplifier 910 disposed outside. On the other hand, a highly reflective coating film 930 is applied to the end face of the second port 925 of the wavelength tunable filter 920 to totally reflect light from the wavelength tunable filter 920.

  The wavelength tunable laser 900 of Patent Document 1 shown in FIG. 8 resonates the ring filters 922 and 923 by tuning such as thermal change, and performs wavelength control by multiplexing and demultiplexing light having resonance wavelengths. Further, the wavelength tunable laser 900 is made to coincide with the peak wavelengths of the ring filters 922 and 923 by changing the phase within a certain range and controlling the phase to obtain a local maximum output.

  The wavelength tunable laser 900 of FIG. 8 configured as described above can obtain a large free spectral range (FSR) due to the vernier effect. Here, the vernier effect is a phenomenon in which when a plurality of resonators having different optical path lengths are combined, the resonance frequency becomes the frequency of the least common multiple of each resonator. Multiple optical resonators combining a plurality of resonators use this vernier effect to make the FSR appear to have a resonant frequency (the least common multiple of the resonant frequency of each resonator) and use a single resonator. In comparison, a wide range of frequency characteristic control can be performed with a slight change in refractive index.

  In addition, since most of the basic characteristics of this type of wavelength tunable laser are determined by the wavelength tunable filter, various wavelength tunable filters having excellent characteristics have been developed. For example, Patent Document 3 discloses a filter for rotating an etalon, Patent Document 4 discloses a filter for rotating a diffraction grating, Patent Document 5 discloses an acoustic engineering filter and a dielectric filter, and Patent Document 6 describes a Mach-Zehnder MZ structure. Patent Document 7 discloses a filter with a ring resonator in which a plurality of ring filters are combined.

JP 2006-245344 A WO2009 / 119284 Japanese Patent Laid-Open No. 04-069987 JP 05-48220 A Japanese Patent Laid-Open No. 2000-261086 Special table 2009-545016 gazette JP 2000-298215 A

  In the tunable laser 900 shown in FIG. 8, in addition to selecting light of a desired wavelength using the multiple ring filter 921, the selected light is oscillated by changing the phase within a certain range. The wavelength is fine tuned to satisfy the conditions. However, in the wavelength tunable laser 900, since the ring filters 922 and 923 constituting the multiple ring filter 921 have a Lorentzian light transmission characteristic, the internal loss of the laser fluctuates by changing the phase.

  When the internal loss of the laser fluctuates, the threshold carrier density of the laser fluctuates, the light output output from the wavelength tunable laser 900 fluctuates, and the basic characteristics deteriorate. Furthermore, an increase in the threshold carrier density of the laser leads to a decrease in differential gain, which is a basic characteristic of the laser, and the resistance to reflected return light is weakened. Since it is necessary to finely adjust the wavelength frequently, it is difficult for the tunable laser 900 shown in FIG. 8 to stably output a high-quality laser beam with a constant output.

  The present invention has been made in view of the above problems, and provides a wavelength control filter and a wavelength tunable laser capable of stably outputting high-quality light by suppressing fluctuations in internal loss associated with wavelength control operation. For the purpose.

  In order to achieve the above object, the wavelength control filter according to the present invention has a light transmission characteristic that is flat-topped with respect to the wavelength, and by changing the optical length by filter means that transmits light of a predetermined mode interval, A phase adjusting means for adjusting the phase of the light, a reflecting means for totally reflecting the light input through the filter means and the phase adjusting means, and a phase of the light output from the filter means, and based on the detection result Phase control means for controlling the phase adjustment means.

  In order to achieve the above object, a wavelength tunable laser according to the present invention includes an optical amplifying unit that amplifies input light, and the wavelength control filter that receives the amplified light and transmits light having a predetermined mode interval. And comprising.

  According to the above-described aspect of the present invention, it is possible to provide a wavelength control filter and a wavelength tunable laser capable of stably outputting high-quality light by suppressing fluctuations in internal loss accompanying the wavelength control operation.

1A is a block configuration diagram of a wavelength control filter 10 according to the first embodiment, and FIG. 2B is a block configuration diagram of a wavelength tunable laser 60. It is a block diagram of the wavelength tunable laser 100 which concerns on 2nd Embodiment. (A) The light transmission characteristic of the multistage ring filter which concerns on 2nd Embodiment, (b) The light transmission characteristic of the ring filter of patent document 1. FIG. (A) A state in which only one laser mode exists in the 1 dB transmission band, and (b) a state in which a plurality of laser modes exist in the 1 dB transmission band. It is a block block diagram of the wavelength tunable laser 100B which concerns on 3rd Embodiment. It is a block diagram of the wavelength variable laser 100C which concerns on 4th Embodiment. It is a block diagram of wavelength variable laser 100D which concerns on 5th Embodiment. 1 is a configuration diagram of a wavelength tunable laser 900 disclosed in Patent Document 1. FIG.

(First embodiment)
A first embodiment of the present invention will be described. A block diagram of the wavelength control filter according to the present embodiment is shown in FIG. In FIG. 1A, the wavelength control filter 10 includes a filter unit 20, a phase adjustment unit 30, a reflection unit 40, and a phase control unit 50.

  The filter means 20 has a light transmission characteristic that is flat-topped with respect to the wavelength, and transmits light having a predetermined wavelength. The wavelength control filter 10 according to the present embodiment further includes wavelength changing means not shown in FIG. The wavelength changing unit changes the wavelength of light transmitted through the filter unit 20 by changing the temperature of the filter unit 20.

  The filter means 20 can be formed, for example, by arranging two multistage filters having light transmission characteristics that are flat-topped with respect to the wavelength and having characteristics different from each other and connecting them with an optical coupler. In this case, each of the two multistage filters is configured by arranging the ring filters having the same transmission band period in multiple stages. Since the filter means 20 has a flat-top light transmission characteristic, the loss is maintained within a certain range even when the wavelength control operation is performed.

  Here, the flat top property can be controlled to a desired characteristic by appropriately designing the number of stages of the ring filter and the optical coupling coefficient between the rings. Further, by appropriately designing the ring length of the ring filter and the group refractive index of the optical path, the interval between the excited modes can be controlled to a desired value. In the present embodiment, in order to stably maintain the single mode oscillation, the flat top property and the mode interval are designed so that there is only one excitation mode in the frequency band that is a flat top.

  The phase adjusting unit 30 adjusts the phase of the light transmitted through the filter unit 20 by changing the optical length of the wavelength control filter 10. The phase adjusting means 30 according to this embodiment adjusts the phase by changing the refractive index (optical length) of the waveguide by injecting current or applying a voltage to the heater electrode arranged in the vicinity of the optical path through which the light passes. To do.

  The reflection unit 40 totally reflects the light input through the filter unit 20 and the phase adjustment unit 30 toward the filter unit 20 and the phase adjustment unit 30. By repeating the reflection of light by the reflection means 40, the wavelength control filter 10 functions as a resonator.

  The phase control unit 50 detects the phase of the light output from the filter unit 20 or the phase adjustment unit 30 and feedback-controls the phase adjustment unit 30 based on the phase detection result. The phase control unit 50 according to the present embodiment detects the phase of the light output from the filter unit 20. When the phase of the transmitted light from the filter unit 20 coincides with the phase of the laser output light of the wavelength control filter 10, the maximum output laser light is output from the wavelength control filter 10. Here, as the phase control means 50, a ring filter, an etalon filter, or the like can be applied.

  The wavelength control filter 10 configured as described above has a flat top range even if any phase adjustment is performed in the wavelength control filter 10 because the filter means 20 has a flat top light transmission characteristic with respect to the wavelength. If it is within, the light transmission power is maintained almost constant. Therefore, a stable light output can be obtained.

  When applying the filter means 20 having a flat top light transmission characteristic with respect to the wavelength, the phase cannot be finely adjusted based on the power of the light output from the filter means 20. In the present embodiment, the phase control means 50 is arranged, and the phase is finely adjusted by detecting the phase of the light output from the filter means 20 or the phase adjustment means 30 in the phase control means 50.

  Therefore, the wavelength control filter 10 according to the present embodiment can stably output high-quality light while suppressing fluctuations in internal loss accompanying the wavelength control operation.

  In addition, a wavelength tunable laser can also be comprised using the wavelength control filter 10 of Fig.1 (a). A block diagram of the wavelength tunable laser in this case is shown in FIG. The wavelength tunable laser 60 shown in FIG. 1 (b) has an optical amplification means 70 for amplifying input light and the wavelength of FIG. 1 (a) through which the amplified light is input and transmits light of a predetermined mode interval. It consists of a control filter 10.

  A wavelength tunable laser 60 shown in FIG. 1B includes a filter means 20 having a flat-top light transmission characteristic with respect to the wavelength, and a wavelength control filter 10 in which the flat-top property and the excitation mode interval are appropriately designed. Has been. Therefore, in the wavelength tunable laser 60, the laser light output does not fluctuate during the wavelength control operation, and the single mode oscillation is stably maintained.

(Second Embodiment)
A second embodiment will be described. FIG. 2 shows a configuration diagram of the external resonator type wavelength tunable laser according to the present embodiment. 2, the wavelength tunable laser 100 according to the present embodiment includes a semiconductor optical amplifier 200, a wavelength tunable filter substrate 300, two optical monitor PDs (Photodiodes) 410 and 420, and a DSP not illustrated in FIG. (Digital Signal Processor) 500 is provided.

  The semiconductor optical amplifier 200 is composed of, for example, a multiple quantum well (MQW), and generates light when current is injected, and amplifies the light. On the side of the external resonator of the semiconductor optical amplifier 200, a wavelength tunable filter substrate 300 is attached and coupled. Usually, the distance between the semiconductor optical amplifier 200 and the wavelength tunable filter substrate 300 is several μm to several tens of μm. The end face on the light output side of the semiconductor optical amplifier 200 is provided with a low reflection coating 210 (1% to 10% reflectivity). On the other hand, an antireflection coating 220 (1% or less) is applied to the end face of the semiconductor optical amplifier 200 on the external resonator side.

  The wavelength tunable filter substrate 300 transmits light having a predetermined wavelength. The wavelength tunable filter substrate 300 according to the present embodiment includes a silica waveguide (refractive index n = 1.5), and includes three ports 311, 312, 313, a multiple filter 320, a phase adjustment region 330, a reflecting mirror 340, and light. A duplexer 350 and a ring-type wavelength locker 360 are provided.

  The ports 311, 312, and 313 input and output light to and from the outside of the wavelength tunable filter substrate 300. In the present embodiment, the first port 311 is between the external resonator side end face of the semiconductor optical amplifier 200, and the second port 312 is between the incident side end face of the first optical monitor PD 410 and the third port. The port 313 inputs / outputs light to / from the incident side end face of the second optical monitor PD 420. The light input to the first port 311 is sequentially input to the multiple filter 320, the phase adjustment region 330, and the reflecting mirror 340. A part of the light output from the multiplex filter 320 is input to the ring-type wavelength locker 360 side via the optical demultiplexer 350, and a part thereof is sent from the second port 312 to the first optical monitoring PD 410. The remaining light is emitted from the third port 313 to the second optical monitor PD 420 via the ring-type wavelength locker 360.

  The multiplex filter 320 is configured by multiplexing two multistage ring filters 321 and 322 having different flat top characteristics, and functions as a wavelength tunable filter.

  In each of the multistage ring filters 321, 322, ring filters having the same transmission band period (FSR: Free Spectral Range) are arranged in multiple stages, and the ring filters are connected by an optical coupler that couples light at a certain ratio. Consists of. Note that the finesse of the ring filter (ratio of the transmission peak band to the FSR) may be a value in the range of 2 or 3 to several tens that is normally used, and is not particularly defined in the present embodiment.

  Here, the FSR of the ring filter is determined by the circumference of the ring, and is expressed by Expression (1) where n is the group index of the waveguide and C is the speed of light.

FSR = C / (n × L) (1)
In the present embodiment, assuming that the refractive index n = 1.5, the first multistage ring filter 321 sets FSR1 to 50.0 GHz by forming the circumferences of the two ring filters to 4 mm. On the other hand, in the second multistage ring filter 322, FSR2 was set to 50.5 GHz by forming the circumferences of the two ring filters to be 3.96 mm, respectively. In this case, the FSR of the multiple filter 320 configured by the two multistage ring filters 321 and 322 becomes the lowest common multiple of 5050 GHz (about 40 nm) due to the vernier effect. By designing each loop length so that the greatest common multiple of FSR1 and FSR2 is increased, the FSR of the multiple filter 320 can be set to a sufficiently large value.

  Moreover, the multistage ring filters 321 and 322 configured by multistage ring filters have a flat top light transmission characteristic with respect to the wavelength. The light transmission characteristics of the multistage ring filters 321 and 322 according to this embodiment are shown in FIG. 3A, and as a comparison, a single stage ring filter 931 disposed in the wavelength tunable laser 900 of FIG. 8 described in the background art. The transmission characteristics of 932 are shown in FIG. The multistage ring filters 321 and 322 shown in FIG. 3A have flat top light transmission characteristics, whereas the single stage ring filters 931 and 932 shown in FIG. 3B are Lorentzian type. Light transmission characteristics.

  In the case of having a Lorentzian light transmission characteristic, as shown in FIG. 3B, by performing some phase adjustment in the wavelength tunable laser 900 of FIG. 8, the power of the light transmitted through the ring filter 930 is the light of the filter. Along with the transmission characteristics, the laser internal loss changes accordingly, and the laser light output itself rises and falls.

  On the other hand, when the light transmission characteristic is a flat top, as shown in FIG. 3A, even if any phase adjustment is performed in the wavelength tunable laser 100 of FIG. The change is maintained within 1 dB (almost constant). The influence of the change of 1 dB on the fluctuation of the laser beam output is about 0.5 dB, which is about half of that. That is, the wavelength tunable laser 100 according to the present embodiment can obtain a stable laser light output. In addition, flat top property can be made high by designing appropriately the number of steps and the optical coupling coefficient between rings.

  The phase adjustment region 330 includes a heater electrode (not shown in FIG. 2) disposed in the vicinity of the waveguide, and adjusts the phase of the input light. The phase adjustment region 330 heats the waveguide by injecting current or applying a voltage to the heater electrode, and adjusts the phase of light using the refractive index change of the waveguide due to heat.

  The reflecting mirror 340 totally reflects the light input from the phase adjustment region 330. That is, light is resonated between the semiconductor optical amplifier 200 and the reflecting mirror 340.

  The optical demultiplexer 350 is disposed between the multiple filter 320 and the phase adjustment region 330 and branches a part of the light from the second multistage ring filter 322 to the ring-type wavelength locker 360 side. A part of the light branched from the optical demultiplexer 350 to the ring-type wavelength locker 360 side is emitted as it is from the second port 312, and the light intensity is monitored by the first light monitoring PD (photodiode) 410. . On the other hand, the remaining light branched from the optical demultiplexer 350 to the ring-type wavelength locker 360 side is emitted from the third port 313 via the ring-type wavelength locker 360, and the light intensity is output in the second optical monitor PD 420. Is monitored.

  The ring wavelength locker 360 has a periodic wavelength (frequency) dependency. As the ring-type wavelength locker 360, a filter that circulates light such as a ring or an etalon can be applied.

  The light monitoring PD 410 monitors the intensity of light output from the multiple filter 320 and emitted outside without passing through the ring-type wavelength locker 360. On the other hand, the optical monitor PD 420 monitors the intensity of light output from the multiple filter 320 and emitted to the outside via the ring-type wavelength locker 360.

  The DSP 500 (not shown) acquires the wavelength of the light output from the multiple filter 320 based on the ratio of the light intensity monitored by the light monitoring PDs 410 and 420. For example, the DSP 500 calculates the light intensity ratio between the light that has passed through the ring-type wavelength locker 360 and the light that has not passed through, and compares it with the wavelength dependence of the ring-type wavelength locker 360 that has been measured in advance. Get the wavelength of the emitted light.

  Here, the wavelength tunable laser 900 in FIG. 8 described in the background art monitors the intensity of light output from the multiple ring filter 930 at the second port 940 as it is, and finely adjusts the wavelength so that the monitor value becomes maximum. By doing so, Formula (2) can be satisfied. On the other hand, in the wavelength tunable laser 100 of FIG. 2 according to the present embodiment, the output from the multiple filter 320 has a flat top characteristic, so that the wavelength cannot be finely adjusted based on a change in light intensity. Therefore, in the present embodiment, the DSP 500 acquires the wavelength of the light output from the multiple filter 320 and finely adjusts the wavelength of the light output from the multiple filter 320 based on the acquired wavelength.

  As the light travels a distance of one wavelength, the phase of 2π rotates, but an integral multiple thereof needs to match the distance of one round trip of the laser resonator. That is, when laser oscillation is performed in the wavelength tunable laser 100, if the optical path length for one way of the laser is L, the effective refractive index is n, and the optical wavelength is λ, the formula (2) needs to be satisfied.

2L = m × 2πn / λ (m is an integer) Formula (2)
The DSP 500 performs feedback control of the phase adjustment region 330 by calculating a current value for changing the effective refractive index n of the substrate to a desired value based on the acquired wavelength. The wavelength tunable laser 100 according to the present embodiment finely adjusts the wavelength to match the transmission peak wavelength of the ring filter, thereby suppressing laser modes other than the desired mode (λ) and transmitting only the desired mode. .

  The operation of the wavelength tunable laser 100 configured as described above will be described. In the wavelength tunable laser 100, the components of the semiconductor optical amplifier 200 and the wavelength tunable filter substrate 300 are mounted on the same temperature controller (TEC, Thermo-Electric Cooler), and the same temperature is provided by a temperature monitor thermistor (not shown). Is controlled.

  When the multiplex filter 320 is operated as a wavelength tunable laser, the transmission spectrum is changed by changing the temperature of at least one multistage ring filter for each ring filter. In the present embodiment, a micro heater is disposed in the vicinity of the second multistage ring filter 322, and the temperature is changed with the second multistage ring filter 322 as one unit.

  When the temperature of the second multistage ring filter 322 changes, the least common multiple of the frequencies of the multistage ring filters 321 and 322 changes, and the multiple filter 320 functions as a wavelength tunable laser. That is, the multiple filter 320 functions as a wavelength tunable filter by changing the wavelength band of the second multistage ring filter 322 using the first multistage ring filter 321 with FSR1 = 50.0 GHz as a reference. It is desirable to monitor the temperature of each ring individually for highly accurate control.

  Then, the DSP 500 feedback-controls the phase adjustment region 330 so that the transmission peak wavelength matches the laser oscillation wavelength based on the measurement result using the ring-type wavelength locker 360, so that the maximum output laser beam is output. The

  Here, with the above configuration alone, unnecessary laser oscillation may occur at the wavelength peak adjacent to the transmission wavelength peak of the ring. In order to suppress unnecessary laser oscillation, it is desirable to design so that only one laser mode exists within the wavelength range that is the flat top of the filter. The mode interval design will be described below.

  When multistage ring filters 321 and 322 having a ring structure in which light circulates are used, the laser mode interval (Fabry-Perot mode interval) Δf is C for the velocity of light, n for the group refractive index of the waveguide, and L for the laser resonator length. Is expressed by equation (3).

Δf = C / (2 nL) (3)
For example, in the case of the wavelength tunable laser 900 shown in FIG. 8 described in the background art, the waveguide length of the multiple ring filter 930 is 15 millimeters or more, so Δf is about 4 to 5 gigahertz and adjacent modes are close to each other. When adjacent modes are close to each other, the laser easily oscillates in a multimode, and the laser oscillation stability deteriorates. When the multistage ring filters 321 and 322 having flat top characteristics are applied, this feature appears more remarkably.

  FIG. 4A shows a case where only one laser mode exists within the wavelength range of the flat top, and FIG. 4B shows a case where a plurality of laser modes exist. Here, as inserted in FIG. 4 (a), the portion from the peak of the transmission band to 1 decibel (dB) lower is defined as the flat top, and the bandwidth from the top to 1 decibel is defined as the flat top band. And is described as “1 dB transmission band”.

  The 1 dB transmission band in FIG. 4A is in a frequency range corresponding to 8 GHz, and the laser mode interval Δf is equivalent to 10 GHz. That is, only zero or one laser mode exists within the flat top range. On the other hand, although the 1 dB transmission band in FIG. 4B is also in the range corresponding to 8 GHz, the laser mode interval Δf is about 4 GHz, so that there are a plurality of laser modes in the flat top range. End up.

  That is, by increasing the laser mode interval Δf, the number of laser modes existing in the 1 dB transmission band can be limited. In the present embodiment, the laser mode interval Δf is designed based on the equation (3) so that only one laser mode exists within the wavelength range that is a flat top.

  As described above, the wavelength tunable laser 100 according to the present embodiment multiplexes the multistage ring filters 321 and 322 having flat top characteristics to form the multiplex filter 320, so that wavelength control is performed while suppressing fluctuations in optical power. can do. In addition, the wavelength tunable laser 100 according to the present embodiment can perform the phase control of light with high accuracy by acquiring the transmission wavelength using the ring-type wavelength locker 360. Further, in the wavelength tunable laser 100 according to the present embodiment, the laser mode interval Δf is designed so that only one laser mode exists within the flat top wavelength range. In this case, the laser mode interval Δf increases to such an extent that the laser does not become multimode, and single mode oscillation is stably maintained during the wavelength control operation.

  Therefore, the wavelength tunable laser 100 according to the present embodiment can stably output high-quality light by suppressing fluctuations in internal loss associated with the wavelength control operation.

  Here, in the above-described embodiment, the ring lengths of the multistage ring filters 321 and 322 are set to 4 mm and 3.96 mm, respectively, so that FSR1 = 50.0 GHz and FSR2 = 50.5 GHz are set. However, the present invention is not limited to this. For example, it is possible to set FSR1 = 500 GHz and FSR2 = 505 GHz by forming the circumferences of the ring filters of the multistage ring filters 321 and 322 to be 0.4 mm and 0.396 mm, respectively.

  When such a small ring is used, the bending loss increases because the difference in refractive index between the waveguide core layer and the cladding layer is small and the optical confinement is weak in the silica waveguide. Accordingly, it is desirable to use a silicon waveguide on an SOI (Silicon on Insulator) that can increase the optical confinement ratio as the wavelength tunable filter substrate 300. By using the silicon waveguide on the SOI, the circumference of the ring filter can be reduced to 0.2 mm or less, and the wavelength tunable filter can be reduced in size.

  When applied to a system with a channel spacing of 50 GHz with FSR1 = 500 GHz and FSR2 = 505 GHz, the temperatures of both rings are adjusted.

(Third embodiment)
In the second embodiment, an optical demultiplexer 350 is disposed between the multiplex filter 320 and the phase adjustment region 330, and the wavelength of light from the multiplex filter 320 is acquired by the ring-type wavelength locker 360 to obtain the phase adjustment region 330. However, the present invention is not limited to this. For example, an optical demultiplexer 350 may be disposed between the phase adjustment region 330 and the reflecting mirror 340, and the wavelength (phase information) of light from the phase adjustment region 330 may be acquired to feedback control the phase adjustment region 330. it can. FIG. 5 shows a block diagram of the wavelength tunable laser in this case.

  In the wavelength tunable laser 100B of FIG. 5, the semiconductor optical amplifier 200B functions in the same manner as the semiconductor optical amplifier 200 of FIG. 2 described in the second embodiment. The multiple filter 320B is configured in the same manner as the multiple filter 320 in FIG. 2, and functions as a wavelength tunable filter. That is, the multiplex filter 320B is configured by multiplexing two multistage ring filters each having different flat top characteristics. In the present embodiment, the laser resonator 370B is configured by the multiple filter 320B and the phase adjustment region 330B.

  The wavelength tunable laser 100B in FIG. 5 monitors the phase of the light output from the laser resonator 370B by the phase detector 600B. The DSP 500B calculates the phase adjustment amount of the laser resonator 370B based on the monitoring result of the phase detector 600B, and feedback-controls the phase adjustment region 330B.

  The wavelength tunable laser 100B of FIG. 5 can also suppress fluctuations in optical output by configuring the multiple filter 320B with a multi-stage ring filter having flat top characteristics, and output light from the laser resonator 370B using the phase detector 600B. By detecting this phase, the phase adjustment region 330B can be feedback-controlled so that the output light satisfies the excitation condition.

(Fourth embodiment)
In the second embodiment, the ring-type wavelength locker 360 formed on the wavelength tunable filter substrate 300 is used for wavelength acquisition. However, the present invention is not limited to this. For example, wavelength acquisition means can be disposed outside the wavelength tunable filter substrate 300. FIG. 6 shows a block configuration diagram of the wavelength tunable laser in this case.

  In the external resonator type wavelength tunable laser 100C of FIG. 6, an etalon type wavelength locker 700C for wavelength acquisition is disposed outside the wavelength tunable filter substrate 300C. The etalon-type wavelength locker 700C includes a lens 710C that collimates the light emitted from the wavelength tunable filter substrate 300C, and an etalon filter 720C that has periodic transmission characteristics with respect to the wavelength.

  The optical monitor PD 410C is output from the wavelength tunable filter substrate 300C and monitors the intensity of light not passing through the etalon filter 720C. The optical monitor PD 420 is output from the wavelength tunable filter substrate 300C and has passed through the etalon filter 720C. Monitor the light intensity. The DSP 500C (not shown in FIG. 6) obtains the wavelength of the output light from the wavelength tunable filter substrate 300C based on the intensity ratio of the light monitored by the optical monitor PD 410C and the optical monitor PD 420C, and feedback-controls the phase adjustment region 330C. .

(Fifth embodiment)
In the second embodiment, the multiplex filter 320 is configured by multiplexing the two multistage ring filters 321 and 322 having different flat top characteristics, but the present invention is not limited to this. The multiplex filter 320 may be any filter that can realize a flat-top wavelength characteristic. For example, an etalon type, an AWG (Arrayed Waveguide Grating) type, a Mach-Zehnder structure type filter, or the like may be used. FIG. 7 shows a configuration diagram of an external resonator type wavelength tunable laser using a Mach-Zehnder structure type filter.

  The external resonator type wavelength tunable laser 100D in FIG. 7 is different from the external resonator type wavelength tunable laser 100C in FIG. 6 described in the fourth embodiment in that the multiple filter 320C in which the multistage ring filter is multiplexed is two asymmetrical. The Mach-Zehnder interferometers 381D and 382D are replaced with a multiplexed filter 380D.

  Each of the two asymmetric Mach-Zehnder interferometers 381D and 382D has a flat top wavelength characteristic, and the multiple filter 380D functions in the same manner as a multiple filter in which a multistage ring filter is multiplexed. The transmission characteristics of the asymmetric Mach-Zehnder interferometers 381D and 382D can be designed by Expression (2) as in the ring filter, where L is the optical path length difference between the two waveguides of the Mach-Zehnder interferometer.

(Other embodiments)
In the second embodiment, the angle of the waveguide end face on the wavelength tunable filter substrate 300 side of the semiconductor optical amplifier 200 is designed to be 90 degrees, but the present invention is not limited to this. The waveguide end face of the semiconductor optical amplifier 200 on the wavelength tunable filter substrate 300 side and the waveguide end face angle of the input port 310 of the wavelength tunable filter substrate 300 can be optically coupled by inclining them from several degrees to several ten degrees. In this case, the residual reflectance of light at the waveguide end faces of the semiconductor optical amplifier 200 and the wavelength tunable filter substrate 300 can be reduced.

  Further, the multiple filter 320 can be formed on a semiconductor, SOI, polymer, or the like instead of being formed on the silica waveguide. When the multiple filter 320 is formed on a semiconductor or SOI, a waveguide having a refractive index higher than that of silica can be formed. Therefore, when a wavelength tunable filter is configured using the waveguide, the size of the wavelength tunable filter can be reduced.

  Further, the phase adjustment region 300 can be built in the semiconductor optical amplifier 200. In this case, the phase adjustment may be performed by current drive, or the entire semiconductor optical amplifying element may be heated using a phase change caused by heat using heater heat.

  The present invention is not limited to the above-described embodiment, and design changes and the like within a range not departing from the gist of the present invention are included in the present invention.

DESCRIPTION OF SYMBOLS 10 Wavelength control filter 20 Filter means 30 Phase adjustment means 40 Reflection means 50 Phase control means 60 Wavelength variable laser 70 Optical amplification means 100, 100B, 100C, 100D Wavelength variable laser 200, 200B, 200C, 200D Semiconductor optical amplifier 300, 300C, 300D wavelength tunable filter substrate 311, 312, 313 port 320, 320B, 320C Multiplex filter 321, 322 Multistage ring filter 330, 330B, 330C, 330D Phase adjustment region 340, 340B, 340C, 340D Reflector 350, 350C, 350D Waveformer 360 Ring type wavelength locker 370B Laser resonator 380D Multiplex filter 381D, 382D Asymmetric Mach-Zehnder interferometer 410, 420, 410C, 420C P for optical monitor
500, 500B DSP
600B Phase detector 700C Etalon type wavelength locker 710C Lens 720C Etalon filter 900 Wavelength tunable laser 910 Semiconductor optical amplifier 920 Wavelength tunable filter 921 Multiple ring filter 922, 923 Ring filter 924 First port 925 Second port 926 Non-reflective coating 930 High reflective coating film

Claims (10)

A filter means having a flat top light transmission characteristic with respect to the wavelength, and transmitting light of a predetermined mode interval;
Phase adjusting means for adjusting the phase of the light by changing the optical length;
Reflection means for totally reflecting light input through the filter means and the phase adjustment means;
A phase control means for detecting the phase of the light output from the filter means and controlling the phase adjustment means based on a detection result;
A wavelength control filter comprising: The wavelength control filter according to claim 1, wherein the predetermined mode interval is larger than a bandwidth at which a light transmission characteristic of the filter unit becomes a flat top. The wavelength control filter according to claim 1, further comprising wavelength changing means for changing a wavelength of transmitted light by changing a temperature of the filter means. The filter means is configured by arranging a first multistage filter and a second multistage filter having different characteristics from each other and connecting them with an optical coupler,
Each of the first multistage filter and the second multistage filter is configured by arranging ring filters having equal transmission band periods in multiple stages.
The wavelength control filter according to any one of claims 1 to 3. The wavelength control filter according to claim 1, wherein the filter member is configured by an asymmetric Mach-Zehnder interferometer. The wavelength control filter according to claim 1, wherein the phase control unit detects a phase of light output from the filter unit using a ring filter or an etalon filter. The phase adjustment means includes a heater electrode disposed in the vicinity of an optical path through which the light passes, and changes the optical length by injecting current or applying voltage to the heater electrode. The wavelength control filter according to the item. The wavelength control filter according to claim 1, wherein the filter unit, the phase adjusting unit, and the reflecting unit are formed on the same substrate. An optical amplification means for amplifying the input light;
The wavelength control filter according to claim 1, wherein the amplified light is input and transmits light having a predetermined mode interval;
A tunable laser comprising: The wavelength tunable laser according to claim 9, wherein a connection surface between the optical amplification unit and the wavelength control filter is inclined with respect to an optical path direction.

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