JP2007298765A - High-density multi-wavelength light source - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-density multi-wavelength light source from which a preferable SNR can be obtained in a wide range. <P>SOLUTION: The multi-wavelength light source includes a plurality of SC (supercontinuum) light generating means that generate super continuum light and a light-multiplexing means that multiplexes the generated SC light, and is set in such a manner that the longitudinal modes of the SC light differ by a predetermined light frequency, for each of the plurality of SC light generating means. Otherwise, the multi-wavelength light source is equipped with: a plurality of light pulse generating means that generate light pulses at predetermined repetition frequencies, the light pulse generating means having longitudinal modes with a frequency interval equal to the repetition frequencies of light pulses; a light-multiplexing means that multiplexes the generated light pulses; and a SC light generating means that allows the multiplexed light pulses to enter a nonlinear medium to generate SC light. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a multi-wavelength light source. More particularly, the present invention relates to a multi-wavelength light source that can be used for wavelength division multiplexing optical transmission systems, optical signal processing, optical frequency reference, and the like.

  As communication traffic increases, wavelength division multiplexing optical transmission systems are used to increase transmission capacity in optical communication systems. In order to perform wavelength division multiplexing optical transmission, light sources corresponding to the number of wavelengths are required. For this reason, it is expensive as many as the number of light sources, and precise adjustment of the wavelengths of these light sources is required. Therefore, multi-wavelength light sources that collectively generate multi-wavelength light from a single light source have been studied.

  Various multi-wavelength light sources that generate multi-wavelength light using supercontinuum (SC) light generation technology have been reported as multi-wavelength light sources (for example, Patent Documents 1 and 2). FIG. 16 shows a configuration example of a conventional multi-wavelength light source and the optical spectrum of each part of the multi-wavelength light source.

  The short pulse light source 10 generates an optical pulse configured in a longitudinal mode having a frequency interval equal to the repetition frequency of multi-wavelength light. When this optical pulse is amplified by the optical amplifier 20 and incident on the nonlinear optical fiber 30 with strong intensity, the optical spectrum of the original optical pulse is propagated through the nonlinear optical fiber 30 by the nonlinear optical effect between the optical spectra. New light spectra are generated on the short wavelength side and the long wavelength side. When further propagating through the nonlinear optical fiber 30, new optical spectra are generated on the shorter wavelength side and the longer wavelength side of the generated optical spectrum. By repeating this, the incident optical spectrum becomes a broadband optical spectrum while propagating through the nonlinear optical fiber 30 (see the optical spectrum in FIG. 16). This technology is called supercontinuum light generation technology. By extracting each longitudinal mode of such supercontinuum light with an optical filter, continuous light in a single longitudinal mode is obtained and used as a light source for wavelength multiplexing transmission.

JP-A-8-29815 Japanese Patent Application No. 2005-201327 K. Mori, and K. Sato, “Supercontunuum Lightwave Generation Employing a Mode-Locked Laser Diode With Injection Locking for a Highly Coherent Optical Multicarrier Source,” IEEE Photon. Technol. Lett., Vol.17, no.2, pp480- 482, Feb. 2005. Takara, H., Ohara, T., Yamamoto, T., Masuda, H., Abe. M., Takahashi, H., Morioka, T. “Field demonstration of over 1000-chanel DWDM transMission with supercontinuuM Multi-carrier source , ”Electron. Lett., 2005, 41, (5).

  In supercontinuum light, the energy of the short pulse light source for excitation is distributed to all longitudinal modes, and broadband wavelength division multiplexed light is obtained. Therefore, when trying to generate many multi-wavelength lights that are wavelength-multiplexed with high density, there is a problem that energy per wavelength is reduced and a signal-to-noise ratio (SNR) is lowered. Further, in order to obtain a good SNR, there is a problem that the wavelength band of multi-wavelength light has to be narrowed.

  The present invention has been made in view of such a problem, and an object thereof is to realize a high-density multi-wavelength light source capable of obtaining a good SNR over a wide band.

  In order to achieve the above object, the present invention provides a multi-wavelength light source for generating multi-wavelength light, and a plurality of SC lights for generating supercontinuum (SC) light. Generating means and optical multiplexing means for multiplexing the SC light from the plurality of SC light generating means, and the optical frequency of the longitudinal mode of the output light from the plurality of SC light generating means is the plurality of SC light generating means It is characterized in that it is set to be different by a predetermined frequency every time.

  According to a second aspect of the present invention, there is provided a multi-wavelength light source for generating multi-wavelength light, and a plurality of optical pulse generating means for generating optical pulses, and optical pulses from the plurality of optical pulse generating means are multiplexed. And a non-linear optical medium that receives the multiplexed optical pulses and generates supercontinuum (SC) light, and the plurality of optical pulse generating means includes optical pulses at a predetermined repetition rate. The optical pulse has a longitudinal mode with a frequency interval equal to the repetition frequency, and the optical frequency of the longitudinal mode of the optical pulse output from the plurality of optical pulse generating means is the plurality of optical pulse generating means. It is characterized in that it is set to be different by a predetermined frequency every time.

  The invention according to claim 3 is the multi-wavelength light source according to claim 1, wherein the light source that generates continuous light and the continuous light from the light source are frequency (2 · N + 1) · Δf (where N is a natural number), modulating means for outputting modulated light having a plurality of longitudinal modes with an optical frequency of (2 · N + 1) · Δf, and modulating light from the modulating means is demultiplexed into two longitudinal An optical demultiplexing unit for outputting a mode, and each of the two longitudinal modes demultiplexed from the demultiplexing unit is supplied as seed light to the plurality of SC light generating units. To do.

  The invention according to claim 4 is the multi-wavelength light source according to claim 2, wherein the light source that generates continuous light and the continuous light from the light source are frequency (2 · N + 1) · Δf (where N is a natural number), modulating means for outputting modulated light having a plurality of longitudinal modes with an optical frequency of (2 · N + 1) · Δf, and modulating light from the modulating means is demultiplexed into two longitudinal An optical demultiplexing means for outputting a mode, wherein each of the two longitudinal modes demultiplexed from the demultiplexing means is supplied as seed light to the plurality of optical pulse generating means. To do.

  The invention according to claim 5 is the multiwavelength light source according to any one of claims 1 to 4, wherein the optical multiplexing means is an optical polarization multiplexing means.

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

(First embodiment)
FIG. 1 shows a basic configuration of a high-density multi-wavelength light source according to the first embodiment of the present invention. The multi-wavelength light source 100 includes N (N is a natural number of 2 or more) supercontinuum (SC) light generating means 120-1 to 120-N that generate supercontinuum light configured in a longitudinal mode with equal frequency intervals. The optical multiplexing unit 140 multiplexes the SC light from the N SC light generating units. Here, as shown in FIG. 2 (a), each supercontinuum light generating means has a longitudinal mode frequency interval of N · Δf so that the longitudinal mode frequency of each SC light generating means is different by Δf. Has been placed. Then, by combining these SC lights different by Δf by the optical multiplexing means 140, as shown in FIG. 2B, high-density multi-wavelength light over a wide band can be obtained.

  According to the present embodiment, the SC light generating means 120-1 to 120-N may generate a longitudinal mode having a frequency interval N times the longitudinal mode interval Δf of the high-density multi-wavelength light to be finally generated. In order to generate multi-wavelength light having a frequency interval of Δf, it is more preferable to use N SC light generating units having a frequency interval of N · Δf than to generate from one SC light generating unit having a frequency interval of Δf. Since the number of longitudinal modes generated by one SC light generating means may be 1 / N, broadband multi-wavelength light having a good SNR can be easily obtained. Various methods have been devised so far for the supercontinuum light source, any of which can be applied to the present invention.

  Next, a specific example of the high-density multi-wavelength light source according to the first embodiment of the present invention will be described. FIG. 3 shows the configuration. This multi-wavelength light source 100a includes N (N is a natural number of 2 or more) supercontinuum (SC) light generating means 120a-1 to 120a-1N that generate supercontinuum light configured in longitudinal modes with equal frequency intervals. , And an optical multiplexer 140 that multiplexes supercontinuum light generated from these N SC light generating means.

  As shown in FIG. 3, each of the SC light generation means 120a-1 to 120a-1N includes a light source 121 that oscillates in a single longitudinal mode, an optical phase modulator 122 that phase-modulates light from the light source, and an optical phase modulator. An RF oscillator 123 for applying a modulation signal to the optical signal, a dispersion-applying optical fiber 124 for applying wavelength dispersion to the phase-modulated optical signal to convert it into an optical pulse train, and an optical fiber 127 for generating SC light. ing. If the input light intensity to the SC light generating optical fiber 127 is insufficient to generate the SC light, the optical amplifier 125 for amplifying the optical pulse train and the amplified optical pulse train are filtered as shown in the figure. The optical bandpass filter 126 may be inserted in front of the SC light generating optical fiber 127. Further, instead of the chromatic dispersion providing optical fiber 124, a fiber grating or a planar lightwave circuit may be used.

  In the optical fiber 127 for generating SC light, the chromatic dispersion is reduced from the anomalous dispersion to the normal dispersion in the longitudinal direction, and the relationship between the peak power of the incident optical pulse and the output expanded spectral width has a desired nonlinear characteristic. Have. Specifically, the optical power threshold at which optical spectrum expansion occurs in the SC light generating optical fiber 127 is lower than the peak power of the incident optical pulse train and higher than the power of the pedestal of the optical pulse train. Thus, it is possible to generate supercontinuum light having a good SNR with little variation in a wide band (see Patent Document 2).

  Note that a light intensity modulator may be used as the SC light generating means 120a in place of the phase modulator 122. In this case, the dispersion-providing optical fiber 124 is not necessary. It is also possible to use an injection-locked mode-locked semiconductor laser in place of the optical phase modulator 122 and the dispersion-providing optical fiber 124. Non-patent document 1 describes, for example, SC light generation means using such an injection-locked mode-locked semiconductor laser.

  In the present embodiment, it is necessary to multiplex multi-wavelength light from a plurality of supercontinuum light generating means by an optical multiplexing means to obtain high-density multi-wavelength light with a longitudinal mode interval Δf accurately. For this purpose, the multi-wavelength light to be combined must have a longitudinal mode that is accurately shifted by Δf. That is, it is necessary to synchronize the longitudinal mode optical frequencies of the multi-wavelength light generated by the plurality of SC light generating means. Therefore, the optical frequency synchronization means 111 is used to synchronize the optical frequency of the light source 121 of each SC light generation means. For example, when light from two light sources is received by a photodiode, a beat having a frequency corresponding to the difference between the optical frequencies of the two light sources can be obtained. By controlling the oscillation frequency of the light source so that this becomes Δf, the frequency difference between the light from the two light sources can be set to be accurately shifted by Δf. In the case of three or more, a plurality of light sources whose optical frequencies are shifted by Δf can be obtained using the above principle.

(Second Embodiment)
Next, a description will be given of a configuration for obtaining light sources having different optical frequencies by Δf with a simpler configuration. When continuous light is modulated by phase modulation or intensity modulation, several side modes are established with the optical frequency of the input continuous light as the center. The frequency spacing between the adjacent side modes and between the center mode and the adjacent side modes is exactly equal to the modulation frequency. Since the accuracy of the interval between these longitudinal modes is determined by the frequency accuracy of the electric microwave oscillator used for modulation, the accuracy can be set to the KHz order comparable to the frequency accuracy of the electric microwave oscillator. Therefore, the accuracy of the longitudinal mode interval can be 10 ⁻⁹ or more.

  In this way, a continuous light having a single longitudinal mode is phase-modulated or intensity-modulated at a frequency Δf, and a plurality of side modes having an optical frequency interval Δf centering on the longitudinal mode of the original continuous light can be obtained. If the modulation index is set so that at least N−1 side modes are established, N seed light sources applicable to the present invention can be obtained.

  FIG. 4 shows a specific configuration example for obtaining two SC light sources with synchronized optical frequencies in the longitudinal mode as the second embodiment. The multi-wavelength light source 100b includes a light source 110 that outputs continuous light having a single longitudinal mode, and modulates continuous light with a frequency (2 · M + 1) · Δf (where M is a natural number). Optical modulation means 112 for generating one or more side modes having an optical frequency interval equal to the modulation frequency on both sides of the longitudinal mode as a center, and a central mode and a side mode whose optical frequencies are different by (2 · M + 1) · Δf Optical demultiplexing means 114 for demultiplexing the output light from the optical modulation means, and SC light generation means 120b-1 and 120b-2 for generating SC light with an optical frequency of 2 · Δf using the demultiplexed light as seed light. And optical multiplexing means 140 for multiplexing the multi-wavelength light from the SC light generating means.

  With this configuration, the accuracy of the longitudinal mode interval (2 · M + 1) · Δf of the seed light by the modulation unit 112 can be set to the KHz order which is the frequency accuracy of the electric microwave oscillator 113. Further, since an optical frequency stabilized light source having an output frequency accuracy and stability of ± 1 MHz or less is commercially available as the light source 110, two seed lights having an absolute optical frequency accuracy of about ± 1 MHz or less can be obtained with this configuration. it can. These are input to the SC light generating means 120b-1 and 2 through the optical demultiplexing means 114, and the output SC light is multiplexed, so that multi-wavelength light with Δf intervals having a frequency accuracy of about ± 1 MHz or less is obtained. Can be obtained.

  In this configuration, since the optical frequency interval of the output light from the optical modulation unit 112 is (2 · M + 1) · Δf, the optical demultiplexing unit is compared with the case where the output light of the optical frequency interval Δf is demultiplexed. The demultiplexing characteristics required for 114 can be relaxed. In particular, when M is increased, a cheaper optical filter can be used as the optical demultiplexing means 114.

(Third embodiment)
FIG. 5 shows a basic configuration of a high-density multi-wavelength light source according to the third embodiment of the present invention. The multi-wavelength light source 200 includes two supercontinuum (SC) light generation means 220-1 and 220-2 that generate supercontinuum light configured in a longitudinal mode with equal frequency intervals, and these SC light generation means. Optical polarization multiplexing means 240 for polarization multiplexing the SC light generated from the light. Here, as shown in FIG. 6A, each SC light generating means 220 is arranged so that the longitudinal mode frequency of each SC light generating means is different by Δf, with the longitudinal mode frequency interval being 2 · Δf. Has been. Then, the SC light different by Δf is multiplexed by the optical polarization multiplexing means 240 so that the polarization directions thereof are orthogonal to each other, and as shown in FIG. Light can be obtained.

  According to the present embodiment, the SC light generating means 220 may generate a longitudinal mode having a frequency interval twice as long as the interval Δf of the longitudinal mode of the high-density multi-wavelength light to be finally generated. Therefore, it is possible to easily generate broadband SC light having a good SNR as compared with SC light having a longitudinal mode with a frequency interval of Δf.

  In the first embodiment, the plurality of SC lights to be combined have the same polarization, and some of them are orthogonal, but the SC light combined with the polarization in the same direction (when the longitudinal mode is close) May interfere with each other, and may be superimposed as noise due to nonlinear optical effects or the like, thereby degrading the quality of the optical signal. On the other hand, in this embodiment, supercontinuum light is multiplexed with polarizations orthogonal to each other, so that signal quality degradation can be minimized without causing interference. Various methods have been devised so far for the supercontinuum light source, any of which can be applied to the present invention. Although the case where two SC lights are multiplexed is described here, N SC lights whose longitudinal mode frequencies are shifted by Δf also when N SC lights are multiplexed as in the first embodiment. Is multiplexed with alternately orthogonal polarizations to improve noise characteristics.

  Next, a specific example of a high-density multi-wavelength light source according to the third embodiment of the present invention will be described. FIG. 7 shows the configuration. This multi-wavelength light source 200a is composed of two SC light generating means 220a-1 and 220a-2 and an optical polarization multiplexer 240 that multiplexes the SC light from these SC light generating means with mutually orthogonal polarizations. Has been. Further, the multi-wavelength light source 200a differs in optical frequency by (2 · M + 1) · Δf to the two SC light generating means, and provides a synchronized seed light. A phase modulation unit 212 that phase-modulates continuous light from the light source, an RF oscillator 213 that applies a modulation signal of frequency (2 · M + 1) · Δf (where M is a natural number) to the optical phase modulation unit, And an AWG optical filter 214 that demultiplexes the output light composed of the center mode and the side mode from the optical phase modulation means and provides seed light whose optical frequency is different by (2 · M + 1) · Δf to the SC light generation means. Yes. If the intensity of the output light from the optical frequency stabilized light source 210 is not sufficient, an optical amplifier may be added as shown in the figure.

  As shown in FIG. 7, each of the SC light generating means 220a-1 and 220a-2 includes an optical phase modulator 222 for phase-modulating the seed light from the AWG optical filter 214, and a modulation frequency 2 · An RF oscillator 223 that applies a modulation signal of Δf, a dispersion-applying optical fiber 224 that converts wavelength-modulated optical signals into optical pulse trains, and an optical fiber 227 that generates SC light Has been. If the input light intensity to the SC light generating optical fiber 227 is insufficient to generate SC light, as shown in the figure, the optical amplifier 225 for amplifying the optical pulse train and the amplified optical pulse train are filtered. Alternatively, an optical bandpass filter 226 that removes noise components composed of ASE light from the optical amplifier may be inserted in front of the SC light generating optical fiber 227.

  As the SC light generating means 220a, a light intensity modulator can be used instead of the phase modulator 222. In this case, the dispersion-providing optical fiber 224 is not necessary. It is also possible to use an injection-locked mode-locked semiconductor laser in place of the optical phase modulator 222 and the dispersion-providing optical fiber 224. Non-patent document 1 describes, for example, SC light generation means using such an injection-locked mode-locked semiconductor laser.

  As described above, the frequency interval of the longitudinal mode of the SC light generating means is set to 2 · Δf, and the longitudinal mode frequencies of the SC light generating means are set different from each other by Δf, and the supercontinuum light from each SC light generating means is polarization multiplexed. By doing so, it is possible to obtain high-density supercontinuum light having a longitudinal mode with a frequency interval Δf and good SNR.

  Next, the results of experiments in this embodiment will be described. Various methods have been devised so far for the supercontinuum light source, and any of them can be applied to the present invention. In this experiment, the method shown in Non-Patent Document 2 was used.

  FIG. 8 shows the experimental results. FIG. 8A shows an optical spectrum in the case where multi-wavelength light having a 2.5 GHz interval is generated using a single supercontinuum light source according to the conventional configuration. FIG. 8B shows an optical spectrum when multi-wavelength light with 2.5 GHz intervals is generated using two supercontinuum light sources consisting of multi-wavelength light with 5 GHz intervals according to the configuration of the present invention. FIG. 8C shows the optical frequency stability at a wavelength of 1605 nm among the generated multi-wavelength light.

  8B and 8C, the multi-wavelength light shown in FIGS. 8B and 8C is configured so that the longitudinal modes at the centers of the two supercontinuum lights are shifted by 2.5 GHz according to the configuration of the present invention. It was obtained by multiplexing so as to be orthogonal.

  As can be seen from FIG. 8A, when a conventional single supercontinuum light source is used, wavelength multiplexed light of about 4800 wavelengths is obtained from about 1510 nm to 1605 nm. From FIG. 8 (a), the spread of the spectrum of the SC light is observed up to the wavelength region outside this, but a good longitudinal mode could not be observed. This is probably because the SNR of SC light in this wavelength range is low.

  On the other hand, when two supercontinuum light sources according to the present invention were used, it was confirmed that the longitudinal mode was generated from 1460 nm to 1640 nm as shown in FIG. 9B. Since the measuring range of the measuring instrument used was up to 1640 nm, it was not possible to confirm how much the longitudinal mode appeared at a wavelength longer than that, but the longitudinal mode appeared up to 1660 nm or more from the supercontinuum light source at 5 GHz intervals. Therefore, it is presumed that the longitudinal mode has appeared almost to that extent, and it is considered that multi-wavelength light close to 10,000 waves is generated.

  Compared to the case where one supercontinuum light source is used as described above, when the two supercontinuum light polarizations are combined orthogonally, the bandwidth generated by the wavelength multiplexed light is doubled. You can see that it can be expanded.

  Further, one wavelength was extracted from the generated multi-wavelength light, and the optical frequency stability was measured. A wavelength meter with an accuracy of ± 30 MHz was used for the measurement. It can be seen from FIG. 8C that the stability of ± 30 MHz or less, which is the accuracy of the wavelength meter used for the measurement, is shown.

  As described above, according to the present invention, it is understood that multi-wavelength light having a good SNR can be obtained over a wide wavelength range with a simple configuration, and the frequency stability of each longitudinal mode is good.

(Fourth embodiment)
FIG. 9 shows a basic configuration of a high-density multi-wavelength light source according to the fourth embodiment of the present invention. This multi-wavelength light source includes N (N is a natural number greater than or equal to 2) short pulse light generating means 320-1 to 320-N that generate repetitive light pulses configured in a longitudinal mode with a frequency interval N · Δf, and these N light sources. The optical multiplexing means 340 for multiplexing the optical pulses from the short pulse light generating means, and the nonlinear optical medium 350 for expanding the spectrum width of the multiplexed optical pulses. Here, as shown in FIG. 10A, the short pulse light generating means 320 are arranged so that the longitudinal modes of the light pulses are different from each other by Δf. By combining the optical pulses different by Δf by the optical multiplexing means 340, as shown in FIG. 10B, short pulse light composed of a high-density longitudinal mode is obtained. Then, as shown in FIG. 10 (c), when these optical pulses different by Δf are incident on the nonlinear optical medium 350, high-density multi-wavelength light having an optical frequency interval Δf over a wide band can be obtained.

  Next, a specific example of a high-density multi-wavelength light source according to the fourth embodiment of the present invention will be described. FIG. 11 shows the configuration. This multi-wavelength light source 300a includes N (N is a natural number of 2 or more) short pulse light generating means 320a-1 to 320N that generate optical pulses configured in a longitudinal mode with equal frequency intervals, and these N short light sources. The optical multiplexer 340 multiplexes the optical pulses generated from the pulsed light generating means, and the nonlinear optical medium 350 that expands the spectrum width of the multiplexed optical pulses.

  As shown in FIG. 11, each of the short pulse light generating means 320a-1 to 320a-N includes a light source 321 that oscillates in a single longitudinal mode, an optical phase modulator 322 that phase-modulates light from the light source, and an optical phase modulation. It comprises an RF oscillator 323 for applying a modulation signal to the device, and a dispersion-providing optical fiber 324 for applying wavelength dispersion to the phase-modulated optical signal and converting it to an optical pulse train. When the output light intensity of the short pulse light generation means is insufficient to generate SC light, as shown in the figure, an optical amplifier 325 for amplifying the optical pulse train and an optical bandpass for filtering the amplified optical pulse train A filter 326 may be added to the output of the dispersion-providing optical fiber 324. Further, a fiber grating or a planar lightwave circuit can be used instead of the chromatic dispersion providing optical fiber 324. Further, a light intensity modulator can be used as the short pulse light generating means 320a in place of the phase modulator 322. In this case, the dispersion-providing optical fiber 324 is not necessary. It is also possible to use an injection-locked mode-locked semiconductor laser in place of the optical phase modulator 322 and the dispersion-providing optical fiber 324.

  In the optical fiber 350 for generating SC light, the chromatic dispersion is decreased from the anomalous dispersion to the normal dispersion in the longitudinal direction, and the relationship between the peak power of the incident optical pulse and the output expanded spectral width has a desired nonlinear characteristic. Have. Specifically, the optical power threshold at which optical spectrum expansion occurs in the SC light generating optical fiber 350 is lower than the peak power of the incident optical pulse train and higher than the power of the pedestal of the optical pulse train. Therefore, it is possible to generate super-continuum light having a high SNR with little variation in a wide band (see Patent Document 2).

  In the present embodiment, it is necessary to multiplex optical pulses from a plurality of short pulse light generating means by an optical multiplexing means to accurately obtain optical pulses having a longitudinal mode interval Δf. For this purpose, the optical pulses to be combined must have a longitudinal mode that is accurately shifted by Δf. That is, it is necessary to synchronize the longitudinal mode optical frequencies of the optical pulses generated by the plurality of optical pulse signal generating means. Therefore, the optical frequency synchronization means 311 is used to synchronize the optical frequency of the light source 321 of each short pulse light generation means. For example, when light from two light sources is received by a photodiode, a beat having a frequency corresponding to the difference between the optical frequencies of the two light sources can be obtained. By controlling the oscillation frequency of the light source so that this becomes Δf, the frequency difference between the light from the two light sources can be set to be accurately shifted by Δf. In the case of three or more, a plurality of light sources whose optical frequencies are shifted by Δf can be obtained using the above principle.

(Fifth embodiment)
Next, a configuration example for obtaining two light sources in which the optical frequencies of the longitudinal mode are synchronized based on the same principle as that of the second embodiment in FIG. 4 will be described. FIG. 12 shows a specific configuration example for obtaining two short pulse light sources whose optical frequencies in the longitudinal mode are synchronized. The multi-wavelength light source 300b includes a light source 310 that outputs continuous light having a single longitudinal mode, and modulates the continuous light with a frequency (2 · M + 1) · Δf (where M is a natural number), and the incident longitudinal mode. An optical modulation means 312 for generating one or more side modes having an optical frequency interval equal to the modulation frequency on both sides thereof, and an optical modulation consisting of a central mode and a side mode whose optical frequencies differ by (2 · M + 1) · Δf Optical demultiplexing means 314 for demultiplexing the output light from the means, short pulse light generating means 320b-1 for generating optical pulses having a longitudinal mode with an optical frequency of 2 · Δf using the demultiplexed light as seed light, and 320b-2, an optical multiplexing unit 340 that multiplexes the optical pulses from these short pulse light generation units, and a nonlinear optical medium 350 that expands the spectrum width of the multiplexed optical pulses. It has been.

  With this configuration, the interval (2 · M + 1) · Δf between the longitudinal modes of the seed light by the modulation means 312 can be set to the KHz order which is the frequency accuracy of the electric microwave oscillator 313. Further, since an optical frequency stabilized light source having an output frequency accuracy and stability of ± 1 MHz or less is commercially available as the light source 310, two seed lights having an absolute optical frequency accuracy of about ± 1 MHz or less can be obtained with this configuration. it can. These are input to the short pulse light generating means 320b-1 and 320b-2 through the optical demultiplexing means 314, and the output optical pulses are multiplexed, so that the optical pulses of Δf intervals having a frequency accuracy of about ± 1 MHz or less. Can be obtained.

  With this configuration, the optical frequency interval of the output light from the light modulation means 312 is (2 · M + 1) · Δf, so that the demultiplexing characteristics required for the light demultiplexing means 314 can be relaxed. In particular, when M is increased, a cheaper optical filter can be used as the optical demultiplexing means 314.

  According to the present embodiment, N short pulse light generation means are used as the short pulse light that becomes the seed light for generating the SC light, and therefore, when the SC light is generated using one short pulse light generation means. Compared with this, it is possible to easily generate short pulse light having a good SNR, and to generate a broadband supercontinuum light having a good SNR.

(Sixth embodiment)
FIG. 13 shows a basic configuration of a high-density multi-wavelength light source according to the sixth embodiment of the present invention. This multi-wavelength light source 400 includes short pulse light generating means 420-1 and 420-2 that generate repetitive light pulses composed of longitudinal modes with a frequency interval of 2 · Δf, and light pulses from these short pulse light generating means. Is composed of an optical polarization multiplexing means 440 for optically polarization multiplexing and a nonlinear optical medium 450 for expanding the spectrum width of the multiplexed optical pulse. Here, as shown in FIG. 14A, the short pulse light generating means 420 are arranged so that the longitudinal modes of the light pulses are different from each other by Δf. By combining the optical pulses different by Δf by the optical multiplexing means 440, as shown in FIG. 14B, short pulse light composed of a high-density longitudinal mode is obtained. Then, as shown in FIG. 14C, when these optical pulses different by Δf are incident on the nonlinear optical medium 450, high-density multi-wavelength light having an optical frequency interval Δf over a wide band can be obtained.

  In the fourth embodiment, a plurality of repetitive optical pulses to be combined have the same polarization, and some of them are orthogonal, but a repetitive optical pulse combined with a polarization in the same direction is There is a possibility that interference will occur with each other (especially in the case of nearness) and superimposed as noise due to the nonlinear optical effect or the like, thereby degrading the quality of the optical signal. On the other hand, in the present embodiment, since optical pulses are multiplexed with polarizations orthogonal to each other, interference does not occur and signal quality deterioration can be minimized. Further, here, the case where two short pulse lights are multiplexed has been described. However, when N short pulse lights are multiplexed as in the fourth embodiment, the longitudinal mode frequency is shifted by Δf by N pieces. By multiplexing short pulse light with alternately orthogonal polarization, noise characteristics are improved.

  Next, a specific example of a high-density multi-wavelength light source according to the sixth embodiment of the present invention will be described. FIG. 15 shows the configuration. The multi-wavelength light source 400a includes two short-pulse light generators 420a-1 and 420a-2, and an optical polarization multiplexer 440 that multiplexes optical pulses from these short-pulse light generators with mutually orthogonal polarizations. And a non-linear optical medium 450 that expands the spectrum width of the multiplexed light pulse. Further, the multi-wavelength light source 400a differs in optical frequency by (2 · M + 1) · Δf to the two short-pulse light generating means, and provides an optical light with stabilized output optical frequency in order to provide synchronized seed light. 410, a phase modulation means 412 for phase-modulating continuous light from this light source, and an RF oscillator 413 for applying a modulation signal of frequency (2 · M + 1) · Δf (where M is a natural number) to the optical phase modulation means, And an AWG optical filter 414 for demultiplexing the output light composed of the center mode and the side mode from the optical phase modulation means and providing seed light whose optical frequency is different by (2 · M + 1) · Δf to the SC light generation means. ing. If the intensity of the output light from the optical frequency stabilized light source 410 is not sufficient, an optical amplifier may be added as shown in the figure.

  As shown in FIG. 15, each of the short pulse light generating means 420a-1 and 420a-2 includes an optical phase modulator 422 for phase-modulating the seed light from the AWG optical filter 414, and a modulation frequency 2 for the optical phase modulator. An RF oscillator 423 that applies a modulation signal of Δf and a dispersion-applying optical fiber 424 for converting the phase-modulated optical signal into an optical pulse train by applying chromatic dispersion. When the output light intensity of the short pulse light generating means is insufficient to generate SC light, as shown in the figure, an optical amplifier 425 for amplifying the optical pulse train and an optical bandpass for filtering the amplified optical pulse train A filter 426 may be added to the output of the dispersion-providing optical fiber 424. Further, instead of the chromatic dispersion providing optical fiber 424, a fiber grating or a planar lightwave circuit can be used. Further, as the short pulse light generation means 420a, a light intensity modulator can be used instead of the phase modulator 422. In this case, the dispersion-providing optical fiber 424 is not necessary. It is also possible to use an injection-locked mode-locked semiconductor laser in place of the optical phase modulator 422 and the dispersion-providing optical fiber 424.

  As described above, the longitudinal mode frequency interval of the short pulse light generating means is set to 2 · Δf, and the longitudinal mode frequencies of the short pulse light generating means are set to be different from each other by Δf. Then, by entering the SC generating optical fiber, it is possible to obtain high-density supercontinuum light having a good SNR and having a longitudinal mode with a frequency interval Δf.

  The present invention has been specifically described above with reference to several embodiments. However, in view of many possible embodiments to which the principle of the present invention can be applied, the embodiments described here are merely examples, It is not intended to limit the scope of the invention. The configuration and details of the embodiment exemplified here can be changed 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 figure which shows the basic composition of the high-density multiwavelength light source which concerns on the 1st Embodiment of this invention. 2A and 2B are diagrams showing optical spectra of respective parts in FIG. 1, FIG. 2A shows an output light spectrum of supercontinuum light generating means, and FIG. 2B shows an output light spectrum of optical multiplexing means. It is a figure which shows the specific example of the high-density multiwavelength light source which concerns on the 1st Embodiment of this invention. It is a figure which shows the specific example of the high-density multiwavelength light source which concerns on the 2nd Embodiment of this invention. It is a figure which shows the basic composition of the high-density multiwavelength light source which concerns on the 3rd Embodiment of this invention. 6A and 6B are diagrams showing the optical spectrum of each part of FIG. 5, in which FIG. 6A shows the output light spectrum of the supercontinuum light generating means, and FIG. 6B shows the output light spectrum of the optical polarization multiplexing means. Yes. It is a figure which shows the specific example of the high-density multiwavelength light source which concerns on the 3rd Embodiment of this invention. It is a figure which shows the experimental result of the high-density multiwavelength light source which concerns on the 3rd Embodiment of this invention, Fig.8 (a) shows the result of the optical spectrum by the conventional supercontinuum light generation means, FIG.8 (b) ) Shows the result of the optical spectrum by the high-density multi-wavelength light source according to the third embodiment of the present invention, and FIG. 8C shows the optical frequency stability of the high-density multi-wavelength light source according to the third embodiment of the present invention. Sexual results are shown. It is a figure which shows the basic composition of the high-density multiwavelength light source which concerns on the 4th Embodiment of this invention. FIG. 10A is a diagram showing an optical spectrum of each part of FIG. 9, FIG. 10A shows an output optical spectrum of a short pulse light source, and FIG. 10B shows an output optical spectrum of an optical multiplexing unit. It is a figure which shows the specific example of the high-density multiwavelength light source which concerns on the 4th Embodiment of this invention. It is a figure which shows another specific example of the high-density multiwavelength light source which concerns on the 4th Embodiment of this invention. It is a figure which shows the basic composition of the high-density multiwavelength light source which concerns on the 6th Embodiment of this invention. FIG. 14A is a diagram showing an optical spectrum of each part of FIG. 13, FIG. 14A shows an output optical spectrum of a short pulse light source, FIG. 14B shows an output optical spectrum of an optical polarization multiplexing unit, and FIG. c) shows an output light spectrum of the nonlinear optical medium. It is a figure which shows the specific example of the high-density multiwavelength light source which concerns on the 6th Embodiment of this invention. It is a figure which shows the basic composition of the conventional multiwavelength light source.

Explanation of symbols

100, 200, 300, 400 Multi-wavelength light source 124, 224, 324 Dispersion imparting optical fiber 125, 225, 325 Optical amplifier 127, 227 SC light generating optical fiber 350, 450 Nonlinear optical medium

Claims (5)

A multi-wavelength light source that generates multi-wavelength light,
A plurality of SC light generating means for generating supercontinuum (SC) light;
Optical multiplexing means for multiplexing SC light from the plurality of SC light generating means, and
A multi-wavelength light source characterized in that an optical frequency of a longitudinal mode of output light from the plurality of SC light generating means is set to be different by a predetermined frequency for each of the plurality of SC light generating means. A multi-wavelength light source that generates multi-wavelength light,
A plurality of optical pulse generating means for generating optical pulses;
Optical multiplexing means for multiplexing optical pulses from the plurality of optical pulse generating means;
A non-linear optical medium that receives the multiplexed light pulses and generates supercontinuum (SC) light, and
The plurality of optical pulse generating means generate optical pulses at a predetermined repetition frequency, and the optical pulses have a longitudinal mode with a frequency interval equal to the repetition frequency,
A multi-wavelength light source characterized in that a longitudinal mode optical frequency of the optical pulse output from the plurality of optical pulse generation means is set to be different by a predetermined frequency for each of the plurality of optical pulse generation means. The multi-wavelength light source according to claim 1,
A light source that generates continuous light;
Modulation that modulates continuous light from the light source with a frequency (2.multidot.N + 1) .multidot..DELTA.f (where N is a natural number) and outputs modulated light having a plurality of longitudinal modes with optical frequency (2.multidot.N + 1) .multidot..DELTA.f intervals. Means,
Optical demultiplexing means for demultiplexing the modulated light from the modulation means and outputting two longitudinal modes;
A multi-wavelength light source configured to supply each of the two longitudinal modes demultiplexed from the demultiplexing means as seed light to the plurality of SC light generating means. The multi-wavelength light source according to claim 2,
A light source that generates continuous light;
Modulation that modulates continuous light from the light source with a frequency (2.multidot.N + 1) .multidot..DELTA.f (where N is a natural number) and outputs modulated light having a plurality of longitudinal modes with optical frequency (2.multidot.N + 1) .multidot..DELTA.f intervals. Means,
Optical demultiplexing means for demultiplexing the modulated light from the modulation means and outputting two longitudinal modes;
A multi-wavelength light source configured to supply each of the two longitudinal modes demultiplexed from the demultiplexing means as seed light to the plurality of optical pulse generating means. The multi-wavelength light source according to any one of claims 1 to 4,
The multi-wavelength light source, wherein the optical multiplexing means is an optical polarization multiplexing means.

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