JP2003066389A - Optical clock regeneration device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fast optical clock reproducing device with a simple configuration. SOLUTION: A portion of an light output of semiconductor mode synchronous laser 1 having a light transmittance modulating part 5 is branched, an optical amplifier 2 amplifies the branched optical signal, a photodetector 3 subsequently converts the amplified optical signal into an electric signal, the electric signal is supplied to the optical transmittance modulating part 5 through a band transmitting filter 4, and thereby, a phase locked loop for driving the optical transmittance modulating part 5 with an optical output signal subjected to photoelectric conversion by the photodetector 3 is constituted.

Description

Description: BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to an optical clock regenerating apparatus, and more particularly to a clock regenerating technique generally used in communication technology. It is useful as a device that extracts and outputs a continuous optical pulse train corresponding to this clock signal. 2. Description of the Related Art In a regenerative repeater used in optical communication, a function for extracting a clock frequency of an encoded input signal is required in order to receive and retransmit a transmitted optical signal without error. It is. FIG. 5 shows an example of the configuration of a conventional optical repeater. In the figure, 30 is an optical demultiplexer, 31 and 3
2 is a light receiving element, 33 is a clock extraction circuit, 34 is an optical pulse train generator for continuous light, 35 is an encoding device, and 36 is an optical amplifier. The input optical signal split by the optical splitter 30 is
The continuous optical pulse train which is converted into an electric signal by the light receiving element 32 and generated by the optical pulse train generator 34 is re-encoded by using the encoder 35, amplified by the optical amplifier 36, and retransmitted. This is an operation called a 3R function (re-timing, re-shaping, re-genetating) of the repeater. Here, in order for the encoder 35 to perform encoding without error, the continuous optical pulse train generated by the optical pulse train generator 34 needs to be synchronized with the input signal electrically converted by the light receiving element 32. Therefore, it is necessary to supply a "clock signal" to the optical pulse train generator 34. This is because the other optical input signal demultiplexed by the optical demultiplexer 30 is converted into an electric signal by the light receiving element 31, and the clock frequency of the input signal is extracted from the electric signal by the clock extracting circuit 33. This is achieved by adjusting the phase difference and inputting it to the clock signal input terminal of the optical pulse train generator 34. In such regenerative repeaters, demands for higher bit rates have been increasing with the recent increase in communication capacity. There has been a problem that it is necessary to further increase the speed of element devices and elements. In addition, a receiving / signal separating (DEMUX) device used in optical communication also needs a function of extracting a clock frequency of an input signal in order to receive and separate a transmitted optical signal without error. It is. FIG. 6 shows a configuration of a conventional receiver. In the figure, 40 is an optical demultiplexer, 41 and 42 are light receiving elements, 43 is a clock extraction circuit, and 44 is a circuit for lowering an input signal to a lower bit rate.
This is a so-called DEMUX circuit. Here, 4: 1 D
The EMUX circuit 44 is illustrated. The input optical signal demultiplexed by the optical demultiplexer 40 is converted into an electric signal by the light receiving element 42 and is divided by the DEMUX circuit 44 into a signal having a lower bit rate. At this time, the other optical input signal split by the optical splitter 40 is converted into an electric signal by the light receiving element 41, the clock frequency of the input signal is extracted by the clock extracting circuit 43, and the phase difference is adjusted. And the DEMUX circuit 44
Clock signal input terminal. Even with such a configuration, there has been a problem that in order to respond to a higher-speed signal, it is necessary to further increase the speed of each of the element devices and elements. [0005] These element elements and devices are stable and compact,
Further, it is desired that the adjustment is easy, and therefore, it is required to configure the semiconductor-based element using an industrially important semiconductor-based element. With the recent advance in semiconductor integrated circuit technology, the performance of the clock extraction circuit has already been increased to 40 Gbit.
/ S input signal level. However, there is a physical limitation in improving the performance of a transistor, and there is a problem that it is extremely difficult to further increase the speed of a clock extraction circuit even with the latest technology. On the other hand, there has been proposed an element for performing clock recovery with a light input and light output configuration. FIG. 7 shows an example in which an injection-locked semiconductor mode-locked laser is used for optical clock reproduction. Reference numeral 51 denotes a semiconductor mode-locked laser having an optical input terminal. Consists of Semiconductor mode-locked laser 51
Normally oscillates in a self-excited manner, but has a characteristic that when a modulation signal close to the self-excited oscillation frequency is applied to the light transmittance modulation section 52 from the outside, the self-excited oscillation frequency is pulled to the frequency. Normally, such external modulation is performed by applying an electric signal to the light transmittance modulator 52. The external input light modulates the light transmittance of the light transmittance modulator 52 or the gain in the gain region. The same effect can be obtained by this (light injection mode locking). Utilizing this, an input optical signal is incident from one end face of the semiconductor mode-locked laser 51 to modulate the light transmittance of the light transmittance modulation section 52 which is a saturable absorption region, thereby realizing light injection mode locking. Reproduction of a continuous optical pulse train synchronized with the clock frequency of the signal becomes possible. In the case of performing signal separation, a low frequency (here, 1/1 of the original signal speed) corresponding to the separated signal speed is used.
(where n and n are natural numbers), it is necessary to generate a clock and supply it to the signal separation device. However, if the semiconductor mode-locked laser 51 having a self-excited oscillation frequency close to 1 / n of the clock frequency is used, the clock frequency is increased. A continuous optical pulse train synchronized with the frequency of 1 / n can be directly obtained. However, when synchronizing a clock having a frequency of 1 / n to an input signal, n phases can be obtained depending on which pulse of the input signal is synchronized with the clock when viewed from the clock. If the input signal is interrupted due to continuation, a phase jump may occur. If a phase jump occurs as described above, signal separation of a predetermined channel cannot be performed, so that it cannot be used in a practical system. As an apparatus for solving this problem, an apparatus using two semiconductor mode-locked lasers 61 and 63 and one optical gate element 62 has been proposed as shown in FIG. In this device, a first-stage semiconductor mode-locked laser 61 having a self-excited oscillation frequency close to the signal frequency generates a pulse train synchronized with the signal frequency. Second-stage semiconductor mode-locked laser 63 having self-excited oscillation frequency
Inject into Further, by driving the optical gate element 62 using the optical output pulse train of the second-stage semiconductor mode-locked laser 63, the phase jump is suppressed. Further, in order to drive the optical gate element 62, a high-output optical signal is required as an optical gate signal, so that the optical amplifier 64 is essential. However, in the configuration shown in FIG. 8, a semiconductor mode-locked laser 61 having a self-excited oscillation frequency close to the signal frequency and an optical gate element 62 must be used separately. There are problems with simplification, miniaturization, and mounting costs. An object of the present invention is to provide a high-speed optical clock reproducing apparatus having a simple configuration in view of the above prior art. [0013] The structure of the present invention that achieves the above object has the following features. 1) In an optical clock regenerating apparatus for outputting a continuous optical pulse train synchronized with an encoded optical input signal, a semiconductor mode-locked laser having at least a light transmittance modulator, and an optical output signal of the semiconductor mode-locked laser An optical demultiplexer that amplifies one of the optical output signals split by the optical demultiplexer; a light receiving element that converts the optical signal amplified by the optical amplifier into an electric signal; A band-pass filter electrically connected to a light receiving element and a light transmittance modulator of the semiconductor mode-locked laser, the center frequency of which is substantially equal to the self-excited oscillation frequency of the semiconductor mode-locked laser; Driving the light transmittance modulator with an electric signal corresponding to the light output signal of the laser and photoelectrically converted by the light receiving element. 2) In the optical clock reproducing apparatus described in 1) above, a ring type semiconductor mode-locked laser having at least a light transmittance modulating section using a semiconductor high mesa waveguide, a multimode interference coupler as an optical demultiplexer, The SOA as an optical amplifier and the light receiving region functioning as a light receiving element are monolithically integrated. 3) In an optical clock regenerating apparatus for outputting a continuous optical pulse train synchronized with an encoded optical input signal, a semiconductor mode-locked laser having at least a light transmittance modulator, and an optical output signal of the semiconductor mode-locked laser , A light-receiving element that converts one of the optical output signals branched by the optical splitter into an electric signal, an electric amplifier that amplifies the output of the light-receiving element, and an output of the electric amplifier. A terminal and a band-pass filter electrically connected to a light transmittance converter of the semiconductor mode-locked laser, the center frequency of which is substantially equal to the self-excited oscillation frequency of the semiconductor mode-locked laser; Corresponding to the optical output signal of
A phase-locked loop is formed by driving the light transmittance modulator with the electric signal photoelectrically converted by the light receiving element. 4) An optical clock regenerating apparatus for outputting a continuous optical pulse train synchronized with an encoded optical input signal, comprising: a semiconductor mode-locked laser having at least a light transmittance modulation unit and a light receiving region serving as a photoelectric conversion unit; An electric amplifier for amplifying an electric signal corresponding to the optical output signal of the semiconductor mode-locked laser converted in the light receiving region; and an output terminal of the electric amplifier and a light transmittance modulation unit of the semiconductor mode-locked laser. A band-pass filter whose center frequency is substantially equal to the self-excited oscillation frequency of the semiconductor mode-locked laser, and corresponds to the optical output signal of the semiconductor mode-locked laser, and is photoelectrically converted in the light receiving region. A phase lock loop is formed by driving the light transmittance modulator with the electric signal. Embodiments of the present invention will be described below in detail with reference to the drawings. <First Embodiment> FIG. 1 shows a first embodiment of the present invention.
It is a block diagram which shows the optical clock reproduction | regeneration apparatus which concerns on embodiment. In the figure, 1 is InP / InGaAsP.
Semiconductor mode-locked laser, 2 is an optical amplifier constituted by a fiber amplifier, 3 is a light receiving element, 4 is a band-pass filter, 5 is a light transmittance modulator of the semiconductor mode-locked laser 1, and is an electroabsorption modulator. And has an electrode that is electrically separated from the gain region 6 of the semiconductor mode-locked laser 1. Here, a part of the output light of the semiconductor mode-locked laser 1 is branched by the optical demultiplexer 7, amplified by the optical amplifier 2, and input to the light receiving element 3. In the present embodiment, as a light receiving element, an InP / InGaAs UTC-PD (Uni-Trav
A photodiode called "eling-Carrier Photodiode" (JP-A-9-275224) was used. The output electrode of the light receiving element 3 is electrically connected to the light transmittance modulator 5 via the band transmission filter 4. The self-excited oscillation frequency of the semiconductor mode-locked laser 1 substantially matches the center frequency of the band-pass filter 4 within a range where the frequency can be pulled in. In such an optical clock recovery apparatus, when an input optical signal is incident on the semiconductor mode-locked laser 1,
The transmittance of the light transmittance modulator 5 or the gain of the gain region 6 is modulated according to the input optical signal. Here, the self-excited oscillation frequency of the semiconductor mode-locked laser 1 is designed to be substantially equal to 1/4 of the repetition frequency of the input optical signal.
By the injection light mode locking, an optical pulse train having a repetition frequency exactly １ ／ of the repetition frequency of the input optical signal is output. Needless to say, when the input optical signal is a continuous pulse train, the same light injection mode locking occurs in the case of a return-to-zero (RZ) signal modulated by random data, and clock optical pulse regeneration is achieved. You.
However, this alone is no different from the configuration using the conventional injection-locked semiconductor mode-locked laser for optical clock regeneration.
A phase jump occurs when the input signal is interrupted. Therefore, in this embodiment, a part of the optical output of the semiconductor mode-locked laser 1 is cut out by the optical demultiplexer 7, amplified by the optical amplifier 2, and converted into an electric signal by the light receiving element 3. . The output electrode of the light receiving element 3 is electrically connected to the light transmittance modulator 5 via the band pass filter 4. Here, the center frequency of the band-pass filter 4 is the self-excited oscillation frequency of the semiconductor mode-locked laser 1,
They are almost the same as far as the frequency can be pulled. Further, by using UTC-PD for the light receiving element 3, it is possible to generate a high-output electric pulse. Since it is possible to directly drive the light transmittance modulator 5 without using an electric amplifier, as described above, the light transmittance modulator 5 of the semiconductor mode-locked laser 1 uses the photoelectrically converted optical output signal as described above. , A phase lock loop is formed. The band-pass filter 4 is used to eliminate noise in the loop and selectively feed back only a frequency component close to the self-excited oscillation frequency of the semiconductor mode-locked laser 1. The difference between the optical clock reproducing apparatus according to the present embodiment and the conventional apparatus is that when a clock having a frequency of 1 / n is synchronized with an input signal, the phase of the optical output pulse train jumps when the input signal is interrupted. Is a point that can be suppressed.
That is, by forming the phase-locked loop as described above, the optical output signals of the semiconductor mode-locked laser 1 are integrated in the loop, and the effect of the input fluctuation is greatly reduced. As a result, the phase jump of the optical output pulse train is reduced. Be suppressed. Further, intensity noise generated as a result of light injection mode locking by a random signal can be largely removed by the effect of the filter in the loop. In the present embodiment, in order to adjust the phase of the signal fed back to the input optical signal, a method of adjusting the optical path length between the optical demultiplexer 7 and the light receiving element 3 and the method of adjusting the optical path length from the light receiving element 3 There is a method of adjusting the length of the electric line up to the light transmittance modulating unit 5, which can be realized by appropriately using one or both of them. Further, by providing a variable optical delay line in the optical path between the optical demultiplexer 7 and the light receiving element 3, the phase can be adjusted after the device is manufactured, and a large margin for design and element manufacture can be obtained. . A similar effect can be obtained by providing a phase shifter in an electric line between the light receiving element 3 and the light transmittance modulator 5 instead of the optical delay line. In FIG. 1 showing the above-described embodiment, parts such as a capacitor, an inductor, a resistor, an electrode, and a wiring for a DC bias which are usually used are not shown in order to avoid complicating the drawing. In the above embodiment, the light receiving element 3 is a type of photodiode U
Although the TC-PD is used, other light receiving elements such as a pin photodiode and a phototransistor can be used.
As the form of the photodiode, an end face type is used, but another form such as a back side type can be used. Further, in the above embodiment, the electro-absorption modulator is used as the light transmittance modulator 5, but a saturable absorption region formed by separating a part of the gain region from the electrode may be used. In the above embodiment, the band-pass filter 4
Center frequency of the input optical signal is 1 /
Although it is set to 4, it may be another natural number other than 1, and a signal separation function of an order corresponding to each natural number is realized. Also,
The optical amplifier 2 can be suitably configured by a fiber amplifier, but is not limited thereto, and another amplifier such as a semiconductor optical amplifier (SOA) can be used. Further, the branched light waveguide can be suitably formed of an optical fiber, but can also be formed of an optical waveguide manufactured using a semiconductor, glass, or a polymer on a semiconductor substrate. Furthermore, these are monolithic on the same substrate,
Alternatively, it can be implemented in a hybrid. <Second Embodiment> FIG. 2 shows a second embodiment of the present invention.
It is a top view which shows embodiment. As shown in the figure,
In this embodiment, a ring type semiconductor mode-locked laser 12 using a semiconductor high mesa waveguide 11 on a semiconductor substrate 10,
A multimode interference coupler as an optical demultiplexer 13, an SOA 14 as an optical amplifier, and a light receiving area 15 are monolithically integrated. In general, when a mode-locked laser is monolithically integrated with another optical element, there are two methods of using a ring resonator and a method using a Bragg reflector. When used, the band in the optical spectrum is limited, which is disadvantageous for generating short pulses. Therefore, in this embodiment, a ring-type semiconductor mode-locked laser in which a gain region 16, an electro-absorption modulator as an optical transmittance modulator 17, and a multi-mode interference coupler as an optical multiplexer / demultiplexer 18 are connected by a semiconductor high-mesa waveguide. 12 were used. Also, the band-pass filter 1
As 9, a short stub using a capacitor that can be easily integrated on a semiconductor substrate is used, but a filter composed of a capacitor and an inductor and a λ / 2 composed of a planar circuit are used.
It is also possible to use a known filter such as a side-coupled band-pass filter. By monolithically integrating a semiconductor waveguide using a semiconductor waveguide as in this embodiment, a very compact optical clock reproducing apparatus can be constructed. Further, there is an advantage that mounting cost can be significantly reduced because alignment is not required when optically connecting components as in the case of using an optical fiber. In the above embodiment, the light receiving area 15
Although a UTC-PD, which is a kind of photodiode, is used, other light receiving regions such as a pin photodiode and a phototransistor can be used. The form of the photodiode is not limited to the waveguide type, and other forms such as an end face incident type can be used. Although the electro-absorption modulator is used as the light transmittance modulator 17 in the above embodiment, a saturable absorption region formed by separating a part of the gain region 16 from the electrode may be used. In the above embodiment, the components are monolithically integrated on a semiconductor substrate.
Hybrid integration on an LC (Planar Lightwave Circuit) substrate is also possible. <Third Embodiment> FIG. 3 shows a third embodiment of the present invention.
It is a block diagram showing an embodiment. The first shown in FIG.
In the first embodiment, the high-power electric signal is generated by photoelectrically converting the signal amplified by the optical amplifier 2 by the high-output light receiving element 3. In the present embodiment, as shown in FIG. After the signal is photoelectrically converted by the light receiving element 3, the signal is amplified using the electric amplifier 28. However, since the frequency of the electric signal is 1 / n of the frequency of the input signal, the electric circuit is appropriately designed so as not to limit the high speed. <Fourth Embodiment> FIG. 4 shows a fourth embodiment of the present invention.
It is a block diagram showing an embodiment. As shown in the figure, the optical clock reproducing device according to the present embodiment differs from the optical clock reproducing device according to the third embodiment shown in FIG. However, in this embodiment, it is monolithically integrated as the light receiving region 29 in the semiconductor mode-locked laser 1. As a result, according to the present embodiment, the optical demultiplexer 7 according to the third embodiment,
A fiber or a waveguide connecting the optical splitter 7 and the light receiving element 3 can be omitted. Here, the light receiving region 29 is designed to absorb a part of the optical signal circulating in the resonator of the mode-locked laser while applying a sufficient reverse bias, and transmit the rest. Here, a waveguide type UTC-PD excellent in high speed is used as the light receiving region 29, but another light receiving element such as a pin photodiode can be used. In the case where an electro-absorption modulator is used as the light transmittance modulator 5, the light receiving region may have the same layer structure as the electro-absorption modulator, although the characteristics are slightly inferior. The center frequency of the band-pass filter 4 substantially matches the self-excited oscillation frequency of the mode-locked laser 1 within a range where the frequency can be pulled. In this embodiment, the phase of the input optical signal and the phase of the signal fed back to the light transmittance modulator 5 are adjusted by adjusting the electric line length from the light receiving region 29 to the light transmittance modulator 5. Can be realized by Further, the same can be realized by providing a phase shifter in an electric line between the light receiving region 29 and the light transmittance modulator 5, and in this case, adjusting the phase after manufacturing the optical clock reproducing device. It is possible to take a large margin for design and device fabrication. As described in detail with the above embodiments, according to the present invention, a high-speed continuous transmission synchronized with the clock frequency of the incident signal light can be realized with a simple configuration without using the conventional clock extraction circuit. There is an effect that an apparatus for reproducing an optical pulse train can be realized. Furthermore, using this device,
It is possible to realize a regenerative repeater and a reception / signal separation device having a high-speed and simple configuration required for a broadband optical communication system.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing an optical clock reproducing device according to a first embodiment of the present invention. FIG. 2 is a plan view showing an optical clock reproducing device according to a second embodiment of the present invention. FIG. 3 is a block diagram showing an optical clock reproducing device according to a third embodiment of the present invention. FIG. 4 is a block diagram showing an optical clock reproducing device according to a fourth embodiment of the present invention. FIG. 5 is a block diagram showing a configuration of a conventional optical signal repeater. FIG. 6 is a block diagram showing a configuration of a conventional optical signal DEMUX device. FIG. 7 is a block diagram showing a conventional optical clock reproducing device. FIG. 8 is a block diagram showing a conventional optical clock reproducing device. [Description of Signs] 1 semiconductor mode-locked laser 2 optical amplifier 3 light-receiving element 4 band-pass filter 5 light transmittance modulator 6 gain region 7 optical demultiplexer 10 substrate 11 semiconductor high-mesa waveguide 12 ring-type semiconductor mode-locked laser 13 light Demultiplexer 14 SOA 15 Light receiving area 16 Gain area 17 Light transmittance modulator 18 Optical multiplexer / demultiplexer 19 Band transmission filter 28 Electrical amplifier 29 Light receiving area

   ────────────────────────────────────────────────── ─── Continuation of front page    (72) Inventor Tomoda Furuta             2-3-1 Otemachi, Chiyoda-ku, Tokyo Sun             Nippon Telegraph and Telephone Corporation (72) Inventor Kiyoto Takahata             2-3-1 Otemachi, Chiyoda-ku, Tokyo Sun             Nippon Telegraph and Telephone Corporation F-term (reference) 2H079 AA02 AA13 BA01 CA09 CA24                       DA16 EA03 EA07 HA08 KA19                 5F072 AB13 HH07 JJ20 MM03 SS06                       YY17                 5F073 AA61 AB22 BA03 EA29 GA12                       GA24                 5K002 BA07 BA13 CA16 DA05

Claims (1)

1. An optical clock regenerating apparatus for outputting a continuous optical pulse train synchronized with an encoded optical input signal, comprising: a semiconductor mode-locked laser having at least a light transmittance modulator; An optical splitter that splits the optical output signal of the synchronous laser, an optical amplifier that amplifies one of the optical output signals split by the optical splitter, and converts the optical signal amplified by the optical amplifier into an electric signal And a band-pass filter electrically connected to the light-receiving element and the light transmittance modulator of the semiconductor mode-locked laser, and having a center frequency substantially equal to the self-excited oscillation frequency of the semiconductor mode-locked laser. The light transmittance change corresponding to the light output signal of the semiconductor mode-locked laser and the electric signal photoelectrically converted by the light receiving element. Optical clock recovery apparatus characterized by being configured to phase-locked loop by driving the parts. 2. The optical clock reproducing apparatus according to claim 1, wherein a ring-type semiconductor mode-locked laser having at least a light transmittance modulator using a semiconductor high-mesa waveguide, and multimode interference as an optical demultiplexer. SO as coupler and optical amplifier
An optical clock regenerating device, wherein A and a light receiving area functioning as a light receiving element are monolithically integrated. 3. An optical clock regenerating apparatus for outputting a continuous optical pulse train synchronized with an encoded optical input signal, comprising: a semiconductor mode-locked laser having at least a light transmittance modulation unit; and an optical output signal of the semiconductor mode-locked laser. An optical demultiplexer that converts one of the optical output signals branched by the optical demultiplexer into an electric signal; an electric amplifier that amplifies the output of the light receiving element; and an output of the electric amplifier A band-pass filter electrically connected to a terminal and a light transmittance converter of the semiconductor mode-locked laser, the center frequency of which is substantially equal to a self-excited oscillation frequency of the semiconductor mode-locked laser; Driving the light transmittance modulation unit with an electric signal photoelectrically converted by the light receiving element while corresponding to the light output signal. An optical clock regenerating device characterized by comprising a phase locked loop. 4. An optical clock regenerating apparatus for outputting a continuous optical pulse train synchronized with an encoded optical input signal, comprising: a semiconductor mode-locked laser having at least a light transmittance modulation unit and a light receiving region serving as a photoelectric conversion unit; An electrical amplifier for amplifying an electrical signal corresponding to the optical output signal of the semiconductor mode-locked laser converted in the light receiving region; and an output terminal of the electrical amplifier and a light transmittance modulation unit of the semiconductor mode-locked laser. A band-pass filter whose center frequency is substantially equal to the self-excited oscillation frequency of the semiconductor mode-locked laser, and corresponds to the optical output signal of the semiconductor mode-locked laser, and is photoelectrically converted in the light receiving region. Characterized in that a phase-locked loop was formed by driving the light transmittance modulator with the applied electric signal. Optical clock reproducing apparatus that.