JP4621083B2 - Multi-carrier light source - Google Patents

Description

  The present invention relates to a multicarrier light source capable of generating a plurality of optical carriers at once, and more particularly to a technique for generating a large number of optical carriers at regular intervals, which is used in a WDM (wavelength division multiplexing) system.

In recent years, as a multicarrier light source, as shown in FIG. 6, research and development of a light source having a configuration in which a pulse light source 601 having a constant pulse repetition frequency f _rep and a nonlinear optical medium 603 are combined (Non-Patent Document 1). ~ 3). In an optical pulse train having a repetition frequency f _rep , optical carriers (line spectra) are arranged at intervals f _rep on the frequency axis in the spectrum. When this optical pulse train generated from the pulse light source 601 enters the nonlinear optical medium 603, the spectrum width is expanded due to the nonlinear effect, and the number of optical carriers is increased. As a result, the light output from the nonlinear optical medium 603 can be applied as multicarrier light having a frequency interval f _rep .

  Conventionally, a mode-locked LD (laser diode) has been mainly used as the pulse light source 601 for a multicarrier light source (Non-Patent Documents 1 and 2). The mode-locked LD alone has a narrow spectral width of about several nanometers at most, so the number of optical carriers is as small as about 10 to 30 (when the carrier interval is about 25 to 50 GHz), but light is applied to the nonlinear optical medium 603 such as an optical fiber. By entering a pulse train and utilizing a nonlinear optical effect such as self-phase modulation or four-wave mixing, the spectrum width is expanded (several tens to several hundreds of nm), and the number of optical carriers is increased to 100 to 1,000.

  Such a multi-carrier light source is promising for the application of a light source for a multi-wavelength WDM system because it can collectively generate 100 or more optical carriers having a uniform optical carrier frequency interval (Non-patent Document 3). In the case of a WDM light source, it is necessary to generate an optical carrier having a high SNR (signal-to-noise ratio) that is precisely arranged at a prescribed optical frequency interval.

K. Sato, A. Hirano, and H. Ishii, “Chirp-compensated 40-GHz mode-locked lasers integrated with electroabsorption modulators and chirped gratings”, IEEE J. Selected Topics in Quantum Electron. 5 (1999) 590 .: Mode Literature on synchronous LD.

E. Yamada, H. Takara, T. Ohara, K. Sato, T. Morioka, K. Jinguji, M. Itoh, and M. Ishii, “150 channel Supercontinuum CW optical source with high SNR and precise 25 GHz spacing for 10 Gbit / s DWDM systems ", Electron. Lett. 37 (2001) 304 .: Literature on multi-carrier light sources (LD).

H. Takara, E. Yamada, T. Ohara, K. Sato, K. Jinguji, Y. Inoue, T. Shibata, and T. Morioka, “106 channel x 10 Gbit / s, 640km DWDM transmission with 25 GHz spacing with Supercontinuum multi-carrier source ", Electron. Lett. 37 (2001) 1534.: Literature on WDM transmission experiments using multiple light sources.

M. Suzuki, H. Tanaka, N. Edagawa, K. Utaka, and Y. Matsushima, “Transform-limited optical pulse generation up to 20-GHz repetition rate by a sinusoidally driven InGaAsP electroabsorption modulator”, J. Lightwave Technol., vol.11, pp.468-473 .: Literature on CW light source + intensity modulation.

T. Kobayashi, H. Yao, K. Amano, Y. Fukushima, A. Morimoto, and T. Sueta, “Optical pulse compression using high-frequency electrooptic phase modulation”, IEEE J. of Quantum Electron., 24, (1988 pp. 382-387. Literature on CW light source + phase modulation + chromatic dispersion.

  However, in mode-locked LD, the pulse repetition frequency is determined by the resonator length, and the variable range is as narrow as about 10 MHz. Therefore, the mode-locked LD is precisely at the optical frequency interval (12.5 GHz, 25 GHz, 50 GHz, 100 GHz) defined by the WDM system. It is not easy to match. Driving a mode-locked LD with a shifted resonator frequency at a specified frequency leads to degradation of SNR and the like of output light. In addition, mode-synchronized LD may deteriorate the SNR of the optical carrier due to mode distribution noise. In addition, the mode-locked LD generally has an output optical power of about several mW, and the optical power of each optical carrier included in the mode-locked LD is 1 mW or less of a fraction, and the SNR is also determined by this optical power. It was restricted.

  When this mode-locked LD is used as a pulsed light source and the spectrum is expanded with a nonlinear optical medium, the SNR of the optical carrier included in this spectrum may also deteriorate due to the difference between the resonance frequency and the drive frequency and the mode distribution noise. . Even without these deterioration factors, the SNR of the optical carrier in the expanded optical spectrum is limited to the SNR of the optical carrier immediately after the mode-locking LD.

  In addition, in order to cause sufficient spectrum expansion with a normal nonlinear optical medium, it is necessary to make an optical pulse train having a pulse width of 10 ps (picoseconds) or less incident on the nonlinear optical medium. However, since the duty of the mode-locked LD is about 1/5 to 1/10, the repetition frequency at which an optical pulse train having a pulse width of 10 ps or less can be generated is 10 GHz or more. Therefore, it has been difficult to make the optical carrier frequency interval narrower than 10 GHz.

  On the other hand, as a pulse light source having a variable pulse repetition frequency, there is a pulse light source that combines a CW (continuous wave, continuous wave) light source and external light modulation means (Non-Patent Documents 4 and 5). 7 and 8 show the basic configuration of these light sources.

  In the case of the conventional pulse light source including the CW light source 701 and the intensity light modulation means 703 shown in FIG. 7, an electroabsorption modulator is mainly used as the intensity light modulation means 703 (Non-Patent Document 4). This light source can control the pulse repetition frequency by the drive frequency to the intensity modulation means 703. However, this electroabsorption modulator 703 has a low incident light power limit with no damage of 10 mW or less and a large modulator loss of 10 dB or more. Therefore, the optical power of each optical carrier contained therein is 0.1 mW or less. , The SNR is only lower than that of the mode-locked LD.

  In the conventional pulse light source including the CW light source 801, the phase modulation unit 803, and the chromatic dispersion applying unit 805 shown in FIG. 8, the drive frequency to the external phase modulating unit 803 and the chromatic dispersion value of the chromatic dispersion applying unit 805 are determined. The pulse repetition frequency can be controlled (Non-patent Document 5). The generation of pulsed light by this method is based on the following operation principle. By applying sinusoidal phase modulation to the CW light generated from the CW light source 801 by the phase modulation means 803, chirped light whose intensity is constant and remains CW light but whose instantaneous frequency changes sinusoidally is obtained. . By passing this chirped light through the chromatic dispersion providing means 805, a different delay is given for each optical frequency component. As a result, part of the energy of the CW light is periodically concentrated at one point on the time axis. As a result, the CW light changes to an optical pulse train having a repetition frequency equal to the phase modulation frequency.

When a LiNbO ₃ phase modulator is used as the phase modulation means 803, this pulsed light source can receive an optical power of 100 mW or more, and the loss is 3 dB or less, so that an output light of 50 mW or more can be obtained. Further, this pulse light source can generate an optical pulse of 10 ps or less even when the repetition frequency is 10 GHz or less. However, as shown in FIGS. 9A and 9B, the output optical carrier from the pulse light source has a large level difference and a large SNR wavelength dependency. That is, in the output of the phase modulator 803, the SNR is as shown in FIG. 9 due to the variation in the intensity level of each optical carrier (5 dB or more between the five central optical carriers) as shown in FIG. (B) has a wavelength dependency as shown in FIG.

  For the above reasons, a pulse light source combining a CW light source and an external light modulation means cannot be applied to a WDM light source. Therefore, conventionally, the use of a combination of the pulse light source using the phase modulation means 803 and the nonlinear optical medium 703 as a WDM multi-carrier light source has not been studied.

  In order to solve the above-described problems, an object of the present invention is to provide a multicarrier light source that has a high SNR and can easily adjust an optical carrier frequency interval in a WDM light source having 100 wavelengths or more.

In order to achieve the above object, a multicarrier light source of the present invention includes a CW light source that generates CW light, phase modulation means that phase-modulates the CW light at a pulse repetition frequency f _rep , and wavelength of the phase-modulated CW light. As chromatic dispersion providing means for applying dispersion and converting it into an optical pulse train having a pulse repetition frequency f _rep , and as a nonlinear optical medium for injecting the optical pulse train to expand the spectrum width, the chromatic dispersion in the longitudinal direction is changed from anomalous dispersion to normal dispersion. It possesses a decreasing optical fiber, the optical power threshold for optical spectral broadening occurs in the optical fiber as the nonlinear optical medium, lower than the peak power of the optical pulse train entering the optical fiber, and optical pulse train It is characterized by higher than the power of the pedestal .

  Here, an optical fiber, a fiber grating, or a planar lightwave circuit can be used as the chromatic dispersion providing means.

  In addition, an optical amplifier and an optical bandpass filter may be disposed between the chromatic dispersion providing unit and the optical fiber as the nonlinear optical medium.

  Further, an optical amplifier and an optical bandpass filter may be disposed between the phase modulation unit and the chromatic dispersion providing unit.

  With the above configuration, according to the present invention, a carrier light source can be provided in a wide wavelength region. Furthermore, the multi-carrier light source of the present invention can generate a large number of optical carriers with little variation in a wide wavelength range and a high SNR. In addition, the multicarrier light source of the present invention can generate a large number of optical carriers with uniform and narrow frequency intervals.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
(First embodiment)
FIG. 1 shows a configuration of a first embodiment of a multicarrier light source of the present invention. As shown in this figure, the multicarrier light source of this embodiment includes a CW light source 101 that generates CW light as a pulse light source, phase modulation means 103 that phase-modulates the CW light at a frequency f _rep , and phase-modulated light. A chromatic dispersion providing means 105 for applying chromatic dispersion to the CW light and converting it into an optical pulse train having a pulse repetition frequency f _rep , and an optical fiber (nonlinear light) that reduces the chromatic dispersion from anomalous dispersion to normal dispersion in the longitudinal direction as a nonlinear optical medium. Fiber) 111. If necessary, an optical amplifier 107 that amplifies the optical pulse train and an optical bandpass filter 109 that filters the amplified optical pulse train are connected between the chromatic dispersion providing means 105 and the nonlinear optical fiber 111. Is also preferable. As the chromatic dispersion providing means 105, an optical fiber, a fiber grating, or a planar lightwave circuit can be used.

  As described above, the conventional pulse light source including the CW light source, the phase modulation unit, and the chromatic dispersion providing unit has a problem that the level and SNR vary greatly between the output optical carriers, and is applied to the WDM light source. I could not. On the other hand, in the present invention, the pulse light source having the same configurations 101, 103, and 105 as the conventional one is combined with the optical fiber 111 in which the chromatic dispersion decreases from the anomalous dispersion to the normal dispersion in the longitudinal direction. Thus, the above-described problems of the prior art can be solved, and a high SNR optical carrier can be generated in a wide wavelength range. Hereinafter, this operation principle of the present invention will be described.

  FIG. 2 shows a pulse waveform in the time domain of a pulse light source composed of the CW light source 101, the phase modulation means 103, and the chromatic dispersion imparting means 105. In the case of a pulse light source composed of the CW light source 101, the phase modulation means 103, and the chromatic dispersion imparting means 105, unlike a mode-locked LD or CW light source + electroabsorption modulator, there is a CW light component between optical pulses. Thus, a small mountain (pedestal) is formed at the base of each light pulse.

On the other hand, in the optical fiber 111 in which the chromatic dispersion decreases in the longitudinal direction from anomalous dispersion to normal dispersion, as shown in FIG. 3A, the relationship between the peak power of the incident optical pulse and the expanded spectrum width has a nonlinear characteristic. Have. (A) threshold P _th shown in the peak power of the optical pulses is 3, to the spectral hardly spread (see (B) in FIG. 3), the peak power of the optical pulse threshold P _th If it exceeds, the spectrum expands rapidly (see FIG. 3C).

  The optical pulse train output from the pulse light source composed of the CW light source 101, the phase modulation means 103, and the chromatic dispersion providing means 105 is incident on the optical fiber 111 in which the chromatic dispersion decreases from anomalous dispersion to normal dispersion in the longitudinal direction. By setting the peak power of the pedestal lower than the threshold value Pth, the CW light component between the light pulses and the pedestal of each light pulse do not contribute to the spectrum expansion, and the desired spectrum expansion is performed. Can do.

  As a result, as shown in FIG. 3C, the output of the nonlinear optical fiber 111 has some level variations only in the pumping light wavelength region (about several nm) originally possessed by the pulse light source. In a wide wavelength range (100 nm or more), the spectrum has very little level variation between optical carriers.

  As already described in the conventional example, the pulse light source including the CW light source, the phase modulation unit, and the chromatic dispersion imparting unit can generate a high-power optical pulse train as compared with the mode-locked LD or CW light source + electroabsorption modulator. There are advantages. Therefore, the configuration of the present invention in which the nonlinear optical fiber 111 is added to the pulse light source can eliminate the difficulty (variation in level and SNR) of the pulse light source and take advantage of the advantage (high SNR due to high power). Therefore, there is little variation in a wide wavelength range other than the excitation light wavelength range, and generation of a multi-optical carrier having a high SNR can be realized.

  Furthermore, since the power light source composed of the CW light source, the phase modulation means, and the chromatic dispersion imparting means can generate a short optical pulse train even at a pulse repetition frequency of 10 GHz or less, as already described in the conventional example, The multi-carrier light source can generate multi-optical carriers at low frequency intervals (10 GHz or less).

  Output optical carrier in the present invention (CW light source + phase modulation means + wavelength dispersion providing means + optical fiber in which chromatic dispersion is reduced from anomalous dispersion to normal dispersion in the longitudinal direction) and the prior art (mode-locked LD in FIG. 6 + wavelength in the longitudinal direction) FIG. 4 shows the result of comparison of the SNR of the output optical carrier in an optical fiber whose dispersion decreases from anomalous dispersion to normal dispersion. In this case, the optical carrier frequency interval is set to 6.25 GHz. In the case of the present invention, the multi-optical carrier having a frequency interval of 6.25 GHz is directly generated by the configuration of FIG.

  On the other hand, in the case of the prior art, since an optical pulse train having a pulse repetition frequency of 6.25 GHz cannot be generated directly from the mode-synchronized LD 601, an optical gate 501 is arranged immediately after the mode-synchronized LD 601 as shown in FIG. By thinning the repetition frequency from 25 GHz to ¼ 6.25 GHz, a multi-optical carrier having a frequency interval of 6.25 GHz is generated.

  FIG. 4 shows that the SNR of the present invention is superior to that of the prior art because the SNR of the present invention is about 1 to 4 dB higher than the SNR of the prior art.

(Second Embodiment)
FIG. 10 shows the configuration of the second embodiment of the multicarrier light source of the present invention. Here, the same reference numerals are used for the same components as in FIG. As shown in FIG. 10, the multi-carrier light source of this embodiment includes a CW light source 101 that generates CW light as a pulse light source, phase modulation means 103 that phase-modulates the CW light at a frequency f _rep , and phase-modulated light. As an optical amplifier 107 that amplifies CW light, chromatic dispersion providing means 105 that converts chromatic dispersion into an optical pulse train having a pulse repetition frequency f _rep , and a nonlinear optical medium, the chromatic dispersion is changed from anomalous dispersion to normal dispersion in the longitudinal direction. And a decreasing optical fiber (non-linear optical fiber) 111. If necessary, an optical bandpass filter 109 that removes unnecessary wavelength components from the amplified light may be connected to the subsequent stage of the optical amplifier 107. Here, the optical amplifier 107 is inserted between the phase modulation means 103 and the chromatic dispersion imparting means 105. By inserting the optical amplifier 107 at this position, the CW light is amplified before being pulsed. As a result, the peak power of light in the optical amplifier 107 becomes lower than when the optical amplifier 107 is inserted in the subsequent stage of the chromatic dispersion providing means 105. As a result, it is possible to prevent an unnecessary optical spectrum change from occurring due to a nonlinear phenomenon in the optical amplifier.

(Other embodiments)
In the above, the preferred embodiment of the present invention has been described by way of example. However, the embodiment of the present invention is not limited to the above-described example, and the constituent members thereof are within the scope of the claims. Various modifications such as replacement, change, addition, increase / decrease in number, change in shape design, etc. are all included in the embodiments of the present invention.

It is a block diagram which shows the structure of 1st Embodiment of the multicarrier light source of this invention. It is a figure which shows the pulse waveform in the time domain of the pulse light source which consists of a CW light source of FIG. 1, a phase modulation means, and a wavelength dispersion provision means. FIG. 1 shows the characteristics of an optical fiber in which chromatic dispersion decreases from anomalous dispersion to normal dispersion in the longitudinal direction of FIG. 1, (A) shows the relationship between the peak power of the incident optical pulse and the expanded spectrum width, and (B) The figure which shows the relationship between the optical frequency and optical intensity until the peak power of an optical pulse reaches threshold value _Pth , (C) is the optical frequency and optical intensity when the peak power of an optical pulse exceeds threshold value _Pth . It is a figure which shows a relationship. It is a figure which shows the result of having compared SNR of the output optical carrier in this invention, and the output optical carrier in a prior art. It is a block diagram which shows the detailed structure of the multicarrier light source which consists of a mode synchronous LD + nonlinear optical fiber of a prior art. It is a figure which shows the structure of the pulse light source which consists of a pulse light source of a prior art, and a nonlinear optical medium, and its output characteristic. It is a figure which shows the structure of the pulse light source which consists of a conventional CW light source, and an intensity modulator, and its output characteristic. It is a figure which shows the structure of the pulse light source which consists of a CW light source of a prior art, a phase modulation means, and a wavelength dispersion provision means, and its output characteristic. The characteristics of the pulsed light source of FIG. 8 are shown, (A) is a diagram showing the relationship between the optical frequency and optical intensity of the output optical carrier from the pulsed light source, and (B) is a diagram showing the relationship between the optical frequency and SNR. . It is a block diagram which shows the structure of 2nd Embodiment of the multicarrier light source of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 101 CW light source 103 Phase modulation means 105 Wavelength dispersion provision means 107 Optical amplifier 109 Optical filter 111 Nonlinear optical fiber (Optical fiber in which chromatic dispersion decreases from anomalous dispersion to normal dispersion in the longitudinal direction)
501 Optical gate 503 Optical amplifier 507 Optical filter 601 Optical pulse light source 603 Nonlinear optical medium (nonlinear optical fiber)
701 CW light source 703 Intensity modulator 801 CW light source 803 Phase modulation means 805 Wavelength dispersion providing means

Claims (4)

A CW light source that generates CW light;
Phase modulation means for phase modulating the CW light at a pulse repetition frequency f _rep ;
_Chromatic dispersion providing means for applying chromatic dispersion to the phase-modulated CW light and converting it to an optical pulse train having a pulse repetition frequency f _rep ;
As the nonlinear optical medium to expand the spectral width enters the optical pulse train, it possesses an optical fiber whose wavelength dispersion in the longitudinal direction decreases from anomalous dispersion to normal dispersion,
The optical power threshold at which optical spectrum expansion occurs in the optical fiber as the nonlinear optical medium is set lower than the peak power of the optical pulse train incident on the optical fiber and higher than the power of the pedestal of the optical pulse train. Multi-carrier light source characterized by   The multicarrier light source according to claim 1, wherein an optical fiber, fiber grating, or a planar lightwave circuit is used as the chromatic dispersion imparting means.   3. The multicarrier light source according to claim 1, wherein an optical amplifier and an optical bandpass filter are disposed between the chromatic dispersion imparting unit and the optical fiber as the nonlinear optical medium.   The multicarrier light source according to claim 1, wherein an optical amplifier and an optical bandpass filter are disposed between the phase modulation unit and the chromatic dispersion providing unit.

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