JP5952685B2 - Apparatus and method for stabilizing the frequency of a plurality of semiconductor lasers in an ultra high density frequency multiplex transmission system - Google Patents

Description

  The present invention relates to an apparatus and method for stabilizing a frequency of a plurality of semiconductor lasers in an optical fiber transmission system, and more particularly, in an ultra-high density frequency multiplex transmission system that effectively utilizes frequency utilization efficiency to the limit.

  In a frequency demultiplexing optical fiber transmission system in which a plurality of optical signals are arranged on the frequency axis, the usable optical band is limited, and therefore it is important to improve frequency utilization efficiency. In an ultra-high density frequency multiplex transmission system that improves frequency utilization efficiency to the limit, as shown in FIG. 1 (a), it is required to arrange signals with the frequency gap between signals as small as possible. The main limitation to this is the time fluctuation of the center frequency of the continuous light generator used in the optical transmitter. Generally, a semiconductor laser is used in a continuous light generator of an optical transmitter. When the center frequency of the continuous light drifts with time, spectrum overlap occurs between the frequency-multiplexed signals as shown in FIG. 1B, and crosstalk occurs between the signals. In order to suppress this crosstalk, it is essential to stabilize the frequency intervals of a plurality of semiconductor lasers in a continuous light generator.

  A general method for stabilizing the frequency of a semiconductor laser is to use an optical interferometer described in Non-Patent Document 1. The configuration is shown in FIG. The continuous light from the semiconductor laser is passed through the optical interferometer and its output power is measured. As shown in FIG. 2 (b), when the frequency of continuous light is close to the transmission frequency of the optical interferometer, a part of the power of continuous light passes through the optical interferometer. Since the optical interferometer output power depends on the frequency of the continuous light, the frequency fluctuation of the continuous light is converted into the optical interferometer output power fluctuation. Frequency stabilization is realized by controlling the frequency of the semiconductor laser using this power as an error signal.

  Another approach is to use a single optical interferometer. In Patent Document 1, each laser output light passes through a Fabry-Perot optical interferometer, the output light is monitored, and frequency multiplex separation of lock-in detection is used for separation of each laser power change. Patent Document 2 uses the above technique for an optical frequency division multiplex transmission system. Patent Document 3 uses a Fabry-Perot optical interferometer to monitor frequency fluctuations of a plurality of lasers, and uses time multiplexing to separate the power of each laser.

Japanese Patent Laid-Open No. 64-15992 JP-A-1-164135 Japanese Patent Laid-Open No. 9-298511

T. Okoshi and K. Kikuchi, “FrequencyStabilisation of Semiconductor Lasers for Heterodyne-Type Optical CommunicationSysytems,” Electron. Lett., Vol.16, No.5, 179 (1980).

  However, the method of Non-Patent Document 1 is limited by the temperature stability of the optical interferometer. In the temperature stability of ± 0.1 K that can be realized by a general temperature adjustment circuit, a frequency fluctuation of about 1 GHz occurs. Therefore, it is extremely difficult to stabilize the fluctuation of the frequency interval of the plurality of semiconductor lasers to 1 GHz or less.

  In general, when a high frequency component increases in a signal for controlling the laser frequency, the laser frequency purity tends to deteriorate. In the techniques of Patent Documents 1 to 3 that use lock-in detection and time demultiplexing that require the laser to be controlled by a high-frequency component, there is a high possibility that the FM noise of the laser will deteriorate.

  Therefore, the present invention is not limited to the temperature stability of the optical interferometer, and can be applied to a system that is frequency-multiplexed at a very high density without a frequency gap without using lock-in detection or time demultiplexing. An object of the present invention is to provide an apparatus and method for stabilizing a frequency interval of a plurality of semiconductor lasers.

In order to solve the above problems, a frequency stabilization device for a plurality of semiconductor lasers according to the present invention has a transmission characteristic in which the transmittance periodically changes on the frequency axis, and spatially transmits light waves at a plurality of transmission frequencies in the transmission characteristic. an optical interferometer that be separated into a plurality of semiconductor lasers which are set to one of a plurality of slopes each transmission frequency of each of the oscillation frequency transmission characteristic of the optical interferometer, passing light from the optical interferometer a and a plurality of photodiodes for converting the electrical signals, respectively, by respective fluctuations of the electrical signal, and means you are stabilizing an oscillation frequency of the plurality of semiconductor lasers.

Further, the stabilizing to that means, each of said electrical signals, the respective fluctuations of the electric signal after comparing the direct light to converted electrical signals continuous light from each of the plurality of semiconductor lasers, the preferred you to each stabilizing the oscillation frequency of a plurality of semiconductor lasers.

Further, means you the stabilization using the respective low frequency components of the fluctuation of the electrical signal, Rukoto be respectively stabilize the oscillation frequency of said plurality of semiconductor lasers are also preferred.

In order to solve the above problems, a method for stabilizing a frequency of a plurality of semiconductor lasers according to the present invention has a plurality of semiconductor lasers and a transmission characteristic in which the transmittance periodically changes on the frequency axis. there a plurality semiconductor laser frequency stabilizing method in the transmitter comprising an optical interferometer you spatially separated light waves at frequencies, and a plurality of photodiodes to convert the respective electrical signal passing light from the optical interferometer Te, a step to set the respective oscillation frequencies of said plurality of semiconductor lasers to one of a plurality of slopes each transmission frequency of the transmission characteristic of the optical interferometer by the respective fluctuations of the electrical signals, said plurality of and a step respectively stabilize the oscillation frequency of the semiconductor laser.

  When the frequencies of a plurality of semiconductor lasers are stabilized using individual optical interferometers, the stability is determined by the temperature stability of the optical interferometer transmission frequency. It is about> 1 GHz / K, and it is not easy to stabilize the frequency interval of a plurality of semiconductor lasers on the order of 100 MHz or less. On the other hand, the stability of the present invention is determined by the temperature stability of the FSR of the optical interferometer. This fluctuation is 0.1 GHz / K or less even when estimated to a large extent, and a stability of 10 MHz or less can be easily obtained.

  Further, in the present invention, only low frequency components are fed back so that error signals of high frequency components are not fed back to the semiconductor laser, and FM noise degradation is extremely small.

A spectral waveform (a) of an optical signal frequency-multiplexed with ultra-high density, and crosstalk generation (b) due to frequency drift are shown. The structure (a), the optical interferometer transmittance and the LD spectrum waveform (b) in the oscillation frequency stabilization method of a single LD are shown. The 1st example of a structure of the frequency stabilization apparatus of this invention is shown. The transmission characteristic (a) of the optical interferometer of the present invention and the frequency arrangement (b) of a plurality of semiconductor lasers are shown. The 2nd structural example of the frequency stabilization apparatus of this invention is shown. The structure which implement | achieves the frequency stabilization apparatus of this invention with a planar optical waveguide is shown. The structure which implement | achieves the frequency stabilization apparatus of this invention with a spatial optical system is shown.

  The best mode for carrying out the present invention will be described in detail below with reference to the drawings. FIG. 3 shows a first configuration example of the frequency stabilizing device in the ultra-high density frequency multiplex transmission system of the present invention. Here, one optical interferometer is used to stabilize the interval between the oscillation frequencies of the four semiconductor lasers. In an optical interferometer, light waves from each laser are transmitted at different locations and the outputs are spatially separated.

  The frequency stabilization device includes a plurality of semiconductor lasers 1 (LD1 to LD4), an optical interferometer 2, a plurality of photodiodes 3 (PD1 to PD4), a plurality of amplifiers 4, and a plurality of LPFs (low pass filters) 5. Yes.

  Continuous light from a plurality of semiconductor lasers 1 is passed through a single optical interferometer 2 and the output power is measured by a photodiode 3. The frequency fluctuation of the semiconductor laser 1 is converted into the fluctuation of the output power of the optical interferometer 2 and converted into an electric signal by the photodiode 3. An error signal is output from the fluctuation of the electric signal, and the oscillation frequency of the semiconductor laser 1 is stabilized by controlling the injection current or temperature of the semiconductor laser 1.

  Note that it is also possible to feed back only the low frequency component so that the LPF 5 is used in the stabilization electronic circuit of the semiconductor laser 1 to suppress the FM noise degradation of the semiconductor laser 1 and the error signal of the high frequency component is not fed back. In this case, it is possible to cope with a slow frequency fluctuation of the semiconductor laser 1.

  FIG. 4 shows the transmission characteristics (a) of the optical interferometer and the frequency arrangement (b) of a plurality of semiconductor lasers. Here, FSR is a signal code rate frequency. As shown in FIG. 4A, the optical interferometer 2 has a transmission characteristic in which a spectrum waveform repeats periodically and has a plurality of transmission frequencies. As shown in FIG. 4B, the oscillation frequency of the semiconductor laser 1 is set near one of the transmission frequencies of the optical interferometer 2 and a different transmission frequency is assigned to another semiconductor laser 1. The single optical interferometer 2 can collectively stabilize the frequencies of the plurality of semiconductor lasers 1 with the slope of the single optical interferometer.

  FIG. 5 shows a second configuration example of the frequency stabilization apparatus of the present invention. In this configuration example, continuous light from the semiconductor laser 1 is split into two, one is directly received by the photodiode 3 and the other is received by the photodiode 3 after passing through the optical interferometer 2.

となる。半導体レーザ１の光パワーがＰ_ＬＤ１＋ΔＰ_ＬＤ１に変化すると

これより光ダイオード３で変換された電気信号電圧をＶ_ＰＤ１の揺らぎΔＶ_ＰＤ１は

となり、光パワーの揺らぎ（ΔＰ_ＬＤ１）が、光干渉計出力パワー揺らぎ（ΔＶ_ＰＤ１）に変換され、光パワーが揺らぐと周波数に揺らぎが発生したと誤認識される。このため、第１の構成例の場合、半導体レーザ１の光パワーの揺らぎにより安定度が制限される。 In the case of the first configuration example of FIG. 4, the optical power of the output light (frequency ν ₁ ) of the semiconductor laser 1 is P _LD1 , and the optical power of the transmitted light after passing through the optical interferometer 2 is T (ν ₁ ) P. _When the electric signal voltage converted by _LD1 and the photodiode 3 is _VPD1 ,

It becomes. When the optical power of the semiconductor laser 1 changes to P _LD1 + ΔP _LD1

From this fluctuation [Delta] V _PD1 of the converted electrical signal voltage at the photodiode 3 _{V PD1} is

Thus, the optical power fluctuation (ΔP _LD1 ) is converted into the optical interferometer output power fluctuation (ΔV _PD1 ), and when the optical power fluctuates, it is erroneously recognized that the frequency fluctuates. For this reason, in the case of the first configuration example, the stability is limited by fluctuations in the optical power of the semiconductor laser 1.

である。αは構成によって決定される値であり、αＰ_ＬＤ１＝Ｔ（ν_１）Ｐ_ＬＤ１と調整することで、半導体レーザ１のパワーの揺らぎによる影響を排除できる。これにより、第２の構成例の場合、半導体レーザ１の光パワーの揺らぎの影響を除去することができる。 For the second configuration example of FIG. 5, and an electric signal V _PD1a the power P _LD1 continuous light is directly received by converting from the semiconductor laser 1, electric signals obtained by converting the transmitted light from the optical interferometer 2 V _PD1b Comparing

It is. α is a value determined by the configuration. By adjusting αP _LD1 = T (ν ₁ ) P _LD1 , the influence of fluctuations in the power of the semiconductor laser 1 can be eliminated. Thereby, in the case of the 2nd example of composition, the influence of fluctuation of the optical power of semiconductor laser 1 can be removed.

となる。ｃは光速、ｎは屈折率である。一方、光干渉計２を用いて、各半導体レーザ１の発振周波数を単一干渉計のスロープで安定させる本発明の場合、ＦＳＲ周波数の揺らぎΔＦＳＲは、

となる。 The effects of the present invention are shown below. In the method of stabilizing to the absolute frequency of the conventional optical interferometer, the relationship between the fluctuation Δf _{m of the mth} transmission frequency fm counted from 0 Hz and the fluctuation Δl of the interferometer length l is

It becomes. c is the speed of light and n is the refractive index. On the other hand, in the case of the present invention in which the oscillation frequency of each semiconductor laser 1 is stabilized by the slope of a single interferometer using the optical interferometer 2, the FSR frequency fluctuation ΔFSR is:

It becomes.

For example, in the case of f _{m to} 200 THz, FSR to ₁₀ GHz, and m = 20000, Δf _{m to 1} GHz, while ΔFSR to 0.05 MHz, the present invention that stabilizes to FSR is an absolute frequency of a conventional optical interferometer. The stability is 10,000 times higher than that to be stabilized.

  As a method for realizing the frequency stabilization device of the present invention, a method using a planar optical waveguide (PLC) in addition to the configuration in which the in-line optical components described in FIGS. 3 and 5 are connected, and a method realized by a spatial optical system. There is.

  FIG. 6 shows a configuration for realizing the frequency stabilization device of the present invention with a planar optical waveguide. The optical waveguide realizes not only the optical interferometer but also the connection of the output light from the semiconductor laser. FIG. 7 shows a configuration for realizing the frequency stabilization apparatus of the present invention with a spatial optical system. The arrow indicated by → in the square is spatial propagation, and frequency separation is performed in space. 6 and 7 are examples corresponding to the second configuration example, but can also be applied to the first configuration example.

  Moreover, all the embodiments described above are illustrative of the present invention and are not intended to limit the present invention, and the present invention can be implemented in other various modifications and changes. Therefore, the scope of the present invention is defined only by the claims and their equivalents.

1 Semiconductor Laser 2 Optical Interferometer 3 Photodiode 4 Amplifier 5 LPF

Claims (4)

Has a transmission characteristic in which the transmittance changes cyclically on the frequency axis, and the optical interferometer you spatially separated light waves at a plurality of transmission frequencies in the transmission characteristics,
A plurality of semiconductor lasers each of the oscillation frequency is set to one of a plurality of slopes each transmission frequency of the transmission characteristic of the optical interferometer,
A plurality of photodiodes each converting the passing light from the optical interferometer into an electrical signal;
By the respective fluctuations of the electrical signal, and means you are stabilizing an oscillation frequency of the plurality of semiconductor lasers,
An apparatus for stabilizing the frequency of a plurality of semiconductor lasers. Wherein the stabilizing to that means, each of said electrical signals, the respective fluctuations of the plurality of semiconductor laser continuous light from each received directly converted electric signals electric signals after comparing with the plurality of frequency stabilization device of the plurality semiconductor laser according to claim 1, characterized that you are stabilizing the oscillation frequency of the semiconductor laser. Said means you stabilization uses low frequency components of the respective fluctuations of the electrical signals, described the oscillation frequency of said plurality of semiconductor lasers to claim 1 or 2, respectively, characterized that you stabilize Frequency stabilizer for multiple semiconductor lasers. A plurality of semiconductor lasers, has a transmission characteristic in which the transmittance changes cyclically on a frequency axis, and the optical interferometer you spatially separated light waves at a plurality of transmission frequencies in the transmission characteristics, the optical interferometer A method for stabilizing the frequency of a plurality of semiconductor lasers in a transmitter comprising a plurality of photodiodes that respectively convert the light passing from each into an electrical signal ,
A step to set the respective oscillation frequencies of said plurality of semiconductor lasers to one of a plurality of slopes each transmission frequency of the transmission characteristic of the optical interferometer,
By the respective fluctuations of the electrical signal; it is stabilizing an oscillation frequency of the plurality of semiconductor lasers,
A method for stabilizing the frequency of a plurality of semiconductor lasers, comprising:

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