JP4468877B2 - Multi-wavelength light source - Google Patents

Description

  The present invention relates to a multi-wavelength light source. More specifically, the present invention relates to a multi-wavelength light source capable of generating a plurality of optical carriers in a wavelength multiplexing system.

  The most common method of generating a plurality of optical carriers in a wavelength division multiplexing system is to prepare CW (continuous-wave) light sources having the required number of wavelengths. In this case, in order to keep the frequency interval between the optical carriers constant, an apparatus for monitoring the wavelength and stabilizing the temperature of the CW light source is required. Further, in this method, it is necessary to adjust the drive current of the CW light source for each optical carrier in order to reduce the intensity deviation between the carriers.

  On the other hand, a method has been studied in which intensity modulation and phase modulation by a sine wave signal is performed on CW light, and a modulation sideband having a flat intensity distribution obtained as a result is used as an optical carrier (Patent Document 1 and Non-patent document 1). In this method, the frequency interval between the carriers is fixed to a value equal to the modulation frequency, and the intensity deviation between the carriers can be within a range of about several dB.

FIG. 1 shows a configuration example of such a multi-wavelength light source. In the multi-wavelength light source 100, CW light from the CW laser source 110 is incident on the phase modulator 120, where it is phase-modulated by a sine wave having a frequency _{f m} from the signal generator 140. Subsequently, the phase modulated CW light, the intensity modulator 130 of the Mach-Zehnder type, the sine wave of frequency f _m from the signal generator 140, is intensity modulated by the modulation index [pi / 2 through a phase shifter 150 .

  FIG. 2 shows an example of the calculation result of the optical spectrum at the output of the intensity modulator 130 in the configuration of FIG. This calculation result is obtained when the modulation frequency is 25 GHz and the phase modulation index Δθ is 5.0π. The vertical axis of this figure shows the relative power of each line spectrum with the power of the original CW light being 0 dB, and the horizontal axis shows the frequency shift of each optical carrier from the wavelength of the original CW light. Yes. In this case, as seen in the figure, it can be seen that the intensity deviation between the optical carriers is within the range of ± 2 dB over the range of about ± 300 GHz.

JP 2004-310138 A M. Sugiyama et al., “A low drive voltage LiNbO3 phase and intensity integrated modulator for optical frequency comb generation and short pulse generation,” 30th European Conference and Exhibition on Optical Communication (ECOC2004), 2004.

  When CW light is externally modulated to generate multi-wavelength light, loss of light in the modulator leads to degradation of the signal-to-noise ratio (SNR) of the multi-wavelength light. Therefore, it is desirable that the loss of the modulator is as small as possible.

  However, in the generation of multi-wavelength light that applies phase modulation and intensity modulation to CW light, the intensity of light is reduced by the intensity modulator, so that essential light loss is inevitable. Further, in the Mach-Zehnder type intensity modulator, it is necessary to provide a feedback mechanism in order to suppress a change in the output light spectrum due to DC drift. In this feedback mechanism, since it is necessary to branch and monitor a part of the output light of the intensity modulator, light loss due to this branching is inevitable.

  The present invention has been made in view of such a problem, and an object of the present invention is to realize a multi-wavelength light source excellent in SNR by reducing loss in a modulator.

In order to achieve such an object, the present invention is a multi-wavelength light source that generates multi-wavelength light from continuous-wave laser light, and provides chirp to the continuous-wave laser light. A first phase modulation unit, a group velocity dispersion applying unit that applies group velocity dispersion to the light subjected to the chirp, and a second phase modulation unit that applies chirp to the light subjected to the group velocity dispersion , The first and second phase modulation means modulate incident light at the same modulation frequency .

The invention according to claim 2 is the multi-wavelength light source according to claim 1, wherein the first phase modulation means performs chirp by applying phase modulation by a sine wave with a modulation index of π / 4. imparting to said group velocity dispersion providing unit, the modulation frequency of the first and second phase modulation means as f _m, and wherein the amount of dispersion will give a group velocity dispersion of ± 1 / (4πf m ^₂₎ To do.

Further, the invention according to claim 3 is the multi-wavelength light source according to claim 1, wherein the group velocity dispersion providing means sets the modulation frequency of the first and second phase modulation means as f _m , A group velocity dispersion having a dispersion amount of ± 1 / (4πf _m ² ) is given, and the second phase modulation means gives a chirp by giving a phase modulation by a sine wave with a modulation index of π / 4. To do.

  The invention according to claim 4 is the multi-wavelength light source according to any one of claims 1 to 3, wherein the group velocity dispersion providing means includes an optical circulator and an optical fiber grating. To do.

  The invention according to claim 5 is the multi-wavelength light source according to any one of claims 1 to 4, wherein the first or second phase modulation means includes a plurality of phase modulators. It is characterized by.

  According to the present invention, it is possible to generate multi-wavelength light by performing external modulation on CW light without using lossy intensity modulation. Thereby, a multiwavelength light source device having an excellent signal-to-noise ratio (SNR) can be realized.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 3 shows an example of the configuration of the multi-wavelength light source according to the first embodiment of the present invention. As shown in the figure, a multi-wavelength light source 300 includes a CW laser light source 310 that generates continuous wave light (CW light), phase modulators 322, 324, and 326 that perform phase modulation, and a signal generator that generates a modulation signal. 340, phase shifters 352 and 354 that adjust the phase of the modulation signal, and a group velocity dispersion medium 360 that provides group velocity dispersion.

In this configuration, the CW light generated from the CW laser light source 310 is incident on the phase modulator 322, where it is phase-modulated with a modulation index π / 4 by a sine wave having a modulation frequency f _m [Hz] from the signal generator 340. And a chirp is given. The chirped CW light passes through the group velocity dispersion medium 360 and is given a group velocity dispersion B (= ± 1 / (4πf _m ² )) [second ² ]. As a group velocity dispersion medium, an optical fiber having a large group velocity dispersion is generally used, but by using a combination of a chirped fiber grating and an optical circulator, the influence of propagation delay due to temperature change can be reduced. .

CW light received group velocity dispersion in the phase modulator 324, by a sine wave of a frequency f _m from the signal generator 340 is phase-modulated by the modulation index [Delta] [theta] ₁ through the phase shifter 352. Furthermore, CW light received phase modulation, the phase modulator 326, by a sine wave of a frequency f _m from the signal generator 340 is phase-modulated by the modulation index [Delta] [theta] ₂ through the phase shifter 354.

FIG. 4 shows an example of the calculation result of the optical spectrum at the output of each phase modulator in the configuration of FIG. 4A shows the output of the phase modulator 322, FIG. 4B shows the output of the phase modulator 324, and FIG. 4C shows the optical spectrum at the output of the phase modulator 326, respectively. This calculation result is obtained when the modulation frequency f _m = 25 GHz and the modulation index Δθ ₁ = Δθ ₂ = 2.5π. By increasing the phase modulation index in the subsequent stage of the group velocity dispersion medium in this way, as shown in FIGS. 4B and 4C, the optical spectrum waveform remains flat and the frequency is expanded. I understand.

  In the optical spectrum waveform of FIG. 4C, the intensity deviation between the optical carriers is within the range of ± 2 dB over the range of about ± 300 GHz as in the case of FIG. However, in FIG. 4C, the light intensity of the flat portion of the optical spectrum waveform is increased by about 3 dB compared to the case of FIG. Unlike the configuration of FIG. 1, the configuration of this embodiment does not include an intensity modulator, so that the loss in the modulator is small, and the resulting degradation of the signal-to-noise ratio can be reduced.

Note that FIG. 4 shows the calculation result when Δθ ₁ = Δθ ₂ , but even if Δθ ₁ ≠ Δθ ₂ , the waveform of the obtained optical spectrum is as long as Δθ ₁ + Δθ ₂ is constant. does not change. Further, by adding a phase modulator downstream of the group velocity dispersion medium 360 and further increasing the phase modulation index, the frequency range in which the waveform of the optical spectrum is flat can be further expanded. Here, even when the phase modulator in the subsequent stage of the group velocity dispersion medium 360 is only one stage and phase modulation with a modulation index of 5.0π is given by this phase modulator, the same optical spectrum waveform as in FIG. Is obtained. However, in an actual apparatus, there is an upper limit to the modulation index that can be realized by a single-stage phase modulator due to the upper limit of the output of the RF amplifier for driving the modulator. Therefore, in order to realize a large phase modulation index, it is preferable to connect a plurality of phase modulators in the subsequent stage of the group velocity dispersion medium as shown in FIG.

FIG. 5 shows an example of an experimental result of an optical spectrum at the output of each phase modulator in the configuration of FIG. 5A shows the output of the phase modulator 322, FIG. 5B shows the output of the phase modulator 324, and FIG. 5C shows the optical spectrum at the output of the phase modulator 326. This experimental result is obtained when the modulation frequency f _m = 25 GHz, the modulation index Δθ ₁ = 1.9π, and Δθ ₂ = 0.7π. The group velocity dispersion medium 360 is a normal dispersion optical fiber having a length of 5.0 km. Even in the case of the experimental result, as shown in FIGS. 5 (b) and 5 (c), the frequency is expanded while the waveform of the optical spectrum remains flat, as shown in FIGS. I understand. More specifically, in the optical spectrum waveform of FIG. 5C, the intensity deviation between the carriers of the 17 line spectra near the center is within a range of ± 2 dB.

In this embodiment, the modulation index of the phase modulator 322 in the previous stage of the group velocity dispersion medium is π / 4, and the dispersion amount of the group velocity dispersion medium 360 is ± 1 / (4πf _m ² ). There is a possibility that the flat frequency range of the optical spectrum can be further expanded by adjusting the value.

  FIG. 6 shows an example of the configuration of a multi-wavelength light source according to the second embodiment of the present invention. As shown in the figure, a multi-wavelength light source 600 includes a CW laser light source 610 that generates continuous wave light (CW light), phase modulators 622, 624, and 626 that perform phase modulation, and a signal generator that generates a modulation signal. 640, phase shifters 652 and 654 for adjusting the phase of the modulation signal, and a group velocity dispersion medium 660 for providing group velocity dispersion.

In this configuration, as compared with the first embodiment, the CW light is first subjected to large phase modulation, then the group velocity dispersion is given, and finally the phase modulation is given with a modulation index of π / 4. Specifically, CW light generated from the CW laser light source 610 is incident on the phase modulator 622, where it is phase-modulated with a modulation index Δθ1 by a sine wave of the modulation frequency fm [Hz] from the signal generator 640. . Further, the phase-modulated CW light is phase-modulated by the phase modulator 624 with the modulation index Δθ2 through the phase shifter 652 by the sine wave of the frequency fm from the signal generator 640 .

Next, the phase-modulated CW light passes through the group velocity dispersion medium 660 and is given group velocity dispersion (= ± 1 / (4πf _m ² )) [second ² ]. As the group velocity dispersion medium, the same medium as in the first embodiment can be used. The CW light subjected to the group velocity dispersion is phase-modulated by the phase modulator 626 with the modulation index π / 4 via the phase shifter 654 by the sine wave of the modulation frequency f _m [Hz] from the signal generator 640. .

FIG. 7 shows an example of the calculation result of the optical spectrum at the output of each phase modulator in the configuration of FIG. 7A shows the output of the phase modulator 622, FIG. 7B shows the output of the phase modulator 624, and FIG. 7C shows the optical spectrum at the output of the phase modulator 626. This calculation result is obtained when the modulation frequency f _m = 25 GHz and the modulation index Δθ ₁ = Δθ ₂ = 2.5π. When the phase modulation index is increased in the preceding stage of the group velocity dispersion medium as described above, an optical spectrum waveform having a large intensity deviation is obtained as seen in FIGS. 7 (a) and 7 (b). However, this phase-modulated light passes through the group velocity medium and is given a modulation index of π / 4, so that a flat optical spectrum waveform can be obtained over a wide frequency range as seen in FIG. It has been. Further, by adding the phase modulator in the previous stage of the group velocity dispersion medium 660 and further increasing the phase modulation index, the frequency range in which the waveform of the optical spectrum is flat can be further expanded.

  Although the present invention has been specifically described based on several embodiments, the embodiments described herein are merely illustrative in view of many possible forms to which the principles of the present invention can be applied. However, it does not limit the scope of the present invention. The embodiments illustrated herein can be modified in configuration and details without departing from the spirit of the present invention. Further, the illustrative components and procedures may be changed, supplemented, or changed in order without departing from the spirit of the invention.

It is a functional block diagram which shows an example of a structure of the conventional multiwavelength light source using an external modulator. 3 is a graph showing an example of a calculation result of an optical spectrum at the output of an intensity modulator in the configuration of FIG. It is a functional block diagram which shows an example of a structure of the multiwavelength light source by 1st Example of this invention. 4 is a graph showing an example of a calculation result of an optical spectrum at the output of each phase modulator in the configuration of FIG. 3, where FIG. 4 (a) is the output of the first-stage phase modulator, and FIG. 4 (b) is the second stage. FIG. 4C shows an optical spectrum at the output of the third-stage phase modulator. FIG. 5 is a graph showing an example of an experimental result of an optical spectrum at the output of each phase modulator in the configuration of FIG. 3, where FIG. 5 (a) is the output of the first phase modulator, and FIG. 5 (b) is the second stage. FIG. 5C shows an optical spectrum at the output of the third-stage phase modulator. It is a functional block diagram which shows an example of a structure of the multiwavelength light source by the 2nd Example of this invention. FIG. 7 is a graph showing an example of a calculation result of an optical spectrum at the output of each phase modulator in the configuration of FIG. 6, where FIG. 7 (a) is the output of the first phase modulator, and FIG. 7 (b) is the second stage. FIG. 7C shows an optical spectrum at the output of the third-stage phase modulator.

Explanation of symbols

100 multi-wavelength light source 110 CW laser light source 120 phase modulator 130 intensity modulator 140 signal generator 150 phase shifter 300 multi-wavelength light source 310 CW laser light source 322, 324, 326 phase modulator 340 signal generator 352, 354 phase shifter 360 group Velocity dispersion medium 600 Multi-wavelength light source 610 CW laser light source 622, 624, 626 Phase modulator 640 Signal generator 652, 654 Phase shifter 660 Group velocity dispersion medium

Claims (5)

A multi-wavelength light source that generates multi-wavelength light from continuous wave laser light,
First phase modulation means for imparting chirp to the continuous wave laser beam;
Group velocity dispersion imparting means for imparting group velocity dispersion to the chirped light;
Second phase modulation means for imparting chirp to the light subjected to the group velocity dispersion , wherein the first and second phase modulation means modulate incident light at the same modulation frequency. Wavelength light source. The multi-wavelength light source according to claim 1,
The first phase modulation means applies chirp by applying phase modulation by a sine wave with a modulation index of π / 4, and the group velocity dispersion applying means is a modulation frequency of the first and second phase modulation means. as the f _m, multi-wavelength light source, wherein the amount of dispersion gives group velocity dispersion of ^{_{± 1 / (4πf m 2)}} . The multi-wavelength light source according to claim 1,
The group velocity dispersion providing unit, wherein the modulation frequency of the first and second phase modulation means as f _m, gives group-velocity dispersion of the dispersion amount ^{_{± 1 / (4πf m 2)}} , the second phase modulation The means provides a chirp by applying phase modulation by a sinusoidal wave with a modulation index of π / 4. The multi-wavelength light source according to any one of claims 1 to 3,
The group velocity dispersion imparting means comprises an optical circulator and an optical fiber grating. The multi-wavelength light source according to any one of claims 1 to 4,
The first or second phase modulation means comprises a plurality of phase modulators.

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