JP2005257859A - Optical power limiter circuit and method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical power limiter circuit and method for performing satisfactory operation with low optical power without using an optical fiber and a passive component. <P>SOLUTION: The optical power limiter circuit is so constituted that an optical signal 25 from a signal processing system and a probe beam 26 having a wavelength different from the wavelength of the optical signal 25 from a probe beam generator 23 are inputted to a semiconductor optical amplifier 22. The probe beam 26 is amplified by optical gain of the semiconductor optical amplifier 22, but gain to the probe beam 26 is reduced only for the time at which the optical signal 25 is inputted since carrier density is reduced in an induction and emission process in which the optical signal 25 is amplified. Thereby, a signal having intensity of a high level/a low level reverse to that of the optical signal 25 is converted to the wavelength of the probe beam 26. A wavelength filter 24 selectively takes out only an optical signal 28 corresponding to the optical signal 25 among the optical signals 27 to 29 outputted from the semiconductor optical amplifier 22. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical power limiter circuit and method for outputting optical signals input with various optical powers as optical signals having the same optical power and outputting them.

  Conventionally, as an optical power limiter circuit, a circuit using gain saturation of a semiconductor optical amplifier (for example, see Non-Patent Document 1), or a circuit using an intensity suppression phenomenon due to four-wave mixing in an optical fiber (for example, Non-Patent Document) 2), using a directional coupler having nonlinear characteristics (for example, see Non-Patent Document 3), using a self-phase modulation (SPM) in an optical fiber (for example, see Non-Patent Document 4), In addition, the one using a non-linear material (for example, see Non-Patent Document 5) has been proposed.

  In the case of using the gain saturation of the semiconductor optical amplifier, as shown in FIG. 1A, when a high level current is injected into the semiconductor optical amplifier 1 to amplify the optical signal, the optical gain decreases due to the carrier density decrease by stimulated emission. Gain saturation occurs. As a result, even if the input light is increased, the optical power of the output signal does not increase linearly, and the increase is suppressed.

  As shown in FIG. 1B, an optical signal (pump light) is generated from the signal generator 2 and assist light having a wavelength different from that of the optical signal is used in the case of using the intensity suppression phenomenon due to the four-wave mixing in the optical fiber. These signals are generated by the assist light generator 3 and these signals are combined by a coupler to generate four-wave mixing which is a nonlinear effect in the optical waveguide (optical fiber) 4. Thereafter, only the optical signal (pump light) is output from the narrow-band wavelength filter 5. Since the intensity suddenly increases when the input light intensity exceeds a certain value, conversely, the power of the signal light is taken away, so that the output light power is limited.

  In the case of using a directional coupler having nonlinear characteristics, as shown in FIG. 1C, the optical branching characteristics of the directional coupler 8 in which the two optical waveguides 6 and 7 are brought close to each other are made nonlinear. A structure is manufactured, and a characteristic in which the output changes rapidly from a constant value as the input light increases is realized.

  In the case of using self-phase modulation in an optical fiber, as shown in FIG. 1D, when a high-intensity optical signal is input to an optical waveguide (optical fiber) 10 through an amplifier 9, the refractive index changes in a strong time. An SPM phenomenon occurs and the signal light wavelength changes. Since the amount of wavelength change increases as the light intensity increases, the wavelength spectrum spreads and the peak power does not increase even if the signal light intensity increases. Therefore, when a part of the output signal is cut out by the narrow band wavelength filter 11, the optical power is held at a constant value.

In the case of using a nonlinear material, as shown in FIG. 1E, an optical power limiter is realized using a nonlinear material 12 such as CS2 or a polymer.
Kyo Ineue et al. “Suppression of Signal Fluctuation Induced by Crosstalk Light in a Gain Saturated Laser Diode Amplifier”, IEEE PHOTNICS TECHNOLOGY LETTERS, VOL. 8, NO.3, MARCH 1996, pp.458-460 (Fig. 1) A. Hirano et al. “All-optical limiter circuit based on four-wave mixing in optical fibers” ELECTRONICS LETTERS 9th July 1998 Vol.34 No.14 (Fig. 1) Byongin Ma et al. “Realization of All-Optical Wavelength Converter Based on Directionally Coupled Semiconductor Optical Amplifiers”, IEEE PHOTNICS TECHNOLOGY LETTERS, VOL. 11, NO.2, FEBRUARY 1999, pp.188-190 (Fig. 1) M. Matsumoto et al. “Analysis of 2R optical regenerator utilizing self-phase modulation in highly nonlinear fiber”, ELECTRONICS LETTERS 6th June 2002 Vol.38 No.12, pp 576-577 (Fig. 1) MJ Soileau, “Optical Power limiter with Picosecond Response Time”, IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. QE-19, NO. 4, APRIL 1983, pp.731-738 (Fig. 1)

  However, when the gain saturation of the semiconductor optical amplifier is utilized, there is an advantage that a small semiconductor optical device can be used, but even if the gain saturation phenomenon occurs, the fluctuation of the optical output is generally small. That is, only one wave of signal light is amplified by the semiconductor optical amplifier, but in the gain saturation phenomenon due to stimulated emission, a transient response exists only for the time required for the reduction of the carrier density, and the rising portion of the signal has a large gain that does not saturate the gain. The falling part of the signal is affected by a small gain due to gain saturation. Therefore, it is difficult to realize an ideal optical power limiter.

  When the intensity suppression phenomenon due to four-wave mixing in an optical fiber is used and when self-phase modulation is used in an optical fiber, a long optical fiber of the order of km is necessary to use the optical fiber, and several tens of Requires a large power of mW.

  When a directional coupler having non-linear characteristics is used, the directional coupler has wavelength dependency because of the passive optical component. Furthermore, when using a nonlinear material, it does not operate unless a large optical power of several W to several hundred kW is input.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an optical power limiter circuit and a method for performing a good operation with a low optical power without using an optical fiber or a passive component.

An optical power limiter circuit according to the present invention includes:
First optical signal generating means for generating first and second optical signals having different wavelengths from each other;
A first semiconductor optical amplifier which receives the first and second optical signals and has a mutual gain modulation characteristic;
And a first optical signal selection means for selectively extracting only the first optical signal from the optical signal output from the first semiconductor optical amplifier.

An optical power limiter method according to the present invention includes:
Generating first and second optical signals having different wavelengths from each other;
Optically amplifying the first and second optical signals using mutual gain modulation characteristics;
Selectively extracting only the first optical signal from the optically amplified first and second optical signals.

  According to the present invention, the mutual gain modulation (XGM) of the first semiconductor optical amplifier is used. A first optical signal and a second optical signal having a wavelength different from the wavelength of the first optical signal are input to the first semiconductor optical amplifier. The second optical signal is amplified by the optical gain of the first semiconductor optical amplifier. However, since the carrier density is reduced by the stimulated emission process for amplifying the first optical signal, the first optical signal is input. Only the time decreases the gain for the second optical signal.

  Therefore, a signal in which the intensity of the low level (0 level) and the high level (1 level) is inverted from that of the first optical signal is converted into the wavelength of the second optical signal. Such conversion is so-called wavelength conversion, but in the present invention, only the first optical signal is selectively extracted from the optical signals output from the first semiconductor optical amplifier.

  At the time of mutual gain modulation, the carrier density in the first semiconductor optical amplifier is reduced in advance by the stimulated emission of the second optical signal, and the gain is saturated. Although the degree of gain saturation is further enhanced by the input of the first optical signal, since the change in carrier density is smaller than the amplification of only the first optical signal, it is possible to suppress the large transient characteristics as already described.

  Further, the intensity of the optical signal input to the first semiconductor optical amplifier is stronger in the case of two (two waves) optical signals than in the case of one (one wave) optical signals, and is due to stimulated emission. The effect of reducing the carrier life is also great. As a result, it is possible to realize an optical power limiter circuit and a method that perform good operation without using an optical fiber or passive components. In addition, since optical power may be about several mW, optical power becomes low compared with the past.

  In the mutual gain modulation, the low level (0 level) and the high level (1 level) of the output optical signal are reversed from those of the input optical signal due to the operation principle. However, the first and third optical signals having different wavelengths are generated, the first and third optical signals are optically amplified using the mutual gain modulation characteristics, and the optically amplified first and third optical signals are generated. By selectively taking out only the first optical signal among the optical signals, the inversion of the optical signal can be avoided. Further, from the viewpoint of all-optical signal processing, if the first optical signal can be converted into a linearly polarized optical signal, the application range can be expected to be expanded.

Embodiments of an optical power limiter circuit and method according to the present invention will be described in detail with reference to the drawings.
FIG. 2 is a block diagram and a schematic diagram of signal waveforms of the first embodiment of the optical power limiter circuit according to the present invention. In FIG. 2A, the optical power limiter circuit 21 includes a semiconductor optical amplifier 22 having a mutual gain modulation characteristic, a probe light generator 23, and a wavelength filter 24.

  In the present embodiment, mutual gain modulation (XGM) of the semiconductor optical amplifier 22 is used. The (CW) probe light 26 from the probe light generator 23 is input to the semiconductor optical amplifier 22 together with the optical signal 25 whose polarization is controlled by a signal processing system (not shown). The probe light 26 has a wavelength different from that of the optical signal 25 and is amplified by the optical gain of the semiconductor optical amplifier 22. However, since the carrier density is reduced by the stimulated emission process for amplifying the optical signal, the optical signal 25 is input. Only for time, the gain for the probe light 26 decreases (FIG. 2B).

  Therefore, a signal in which the intensities of the low level (0 level) and the high level (1 level) are inverted from the optical signal 25 is converted into the wavelength of the probe light 26. Such conversion is so-called wavelength conversion, but in this embodiment, the wavelength filter 24 is polarized by a signal processing system (not shown) of the optical signal 27 (FIG. 2B) output from the semiconductor optical amplifier 22. Only the controlled optical signal 28 (FIG. 2C) is selectively extracted.

  At the time of mutual gain modulation, the carrier density in the semiconductor optical amplifier 22 is reduced in advance by the stimulated emission of the probe light 26, and the gain saturation state is reached. Although the degree of gain saturation is further increased by the input of the optical signal 25, since the change in the carrier density is smaller than the amplification of only the optical signal 25, a large transient characteristic as described above can be suppressed.

  The intensity of the optical signal input to the semiconductor optical amplifier 22 is stronger in the case of two (two waves) optical signals than in the case of one (one wave) optical signals, and the carrier lifetime due to stimulated emission. The reduction effect is also great. As a result, it is possible to realize an optical power limiter circuit and a method that perform good operation without using an optical fiber or passive components. In addition, since optical power may be about several mW, optical power becomes low compared with the past. In addition, the semiconductor optical amplifier generally has a relatively wide gain band of about 50 nm and can operate in that wavelength range.

  FIG. 3 is a block diagram of a second embodiment of the optical power limiter circuit according to the present invention. In FIG. 3, an optical power limiter circuit 31 has a structure in which two optical power limiter circuits 21 (FIG. 2) are combined, a semiconductor optical amplifier 32 having a mutual gain modulation characteristic, a probe light generator 33, A wavelength filter 34, a semiconductor optical amplifier 35 having a mutual gain modulation characteristic, a probe light generator 36, and a wavelength filter 37 are provided.

  In the mutual gain modulation, the low level (0 level) and the high level (1 level) of the output optical signal are reversed from those of the input optical signal due to the operation principle. However, the probe light generator 36 generates probe light having a wavelength different from that of the optical signal output from the wavelength filter 34, and optically amplifies the probe light using the mutual gain modulation characteristic by the semiconductor optical amplifier 35, thereby generating a signal (not shown). Only the optical signal corresponding to the optical signal input to the semiconductor optical amplifier 32 from the processing system is selectively extracted by the wavelength filter 37, thereby avoiding the inversion of the optical signal.

  FIG. 4A shows a waveform of a polarization-controlled optical signal having 10101 data and 111001 data, and is input to the optical power limiter circuit 21 (FIG. 2). FIG. 4B shows the waveform of the optical signal output from the wavelength filter 34. The waveform shown in FIG. 4A corresponds to the input optical signal, the waveform shown in FIG. 4B corresponds to the output optical signal, and power fluctuation is limited.

  FIG. 5 is a diagram illustrating a polarization control circuit that separates an optical signal into linearly polarized waves orthogonal to each other. In FIG. 5, the polarization control circuit 41 includes a polarization beam splitter 42, two photodiodes 43 and 44, a comparator 45, and a 2 × 2 optical switch 46.

  When an optical signal having a polarization plane as shown in FIG. 5 is input to the polarization beam splitter 42, the optical signal is separated into two optical signals having linearly polarized waves orthogonal to each other. The optical powers of these two optical signals are monitored by the photodiodes 43 and 44, respectively, and the light intensities detected by the photodiodes 43 and 44 are input to the comparator 45, respectively. The comparison result of the comparator 45 is input to the 2 × 2 optical switch 46, and the 2 × 2 optical switch 46 is based on the comparison result of the comparator 45 and is out of the two optical signals separated by the polarization beam splitter 42. The optical signal having the higher optical power is output to an optical power limiter circuit (not shown).

The present invention is not limited to the above-described embodiment, and many changes and modifications can be made.
For example, the case where the polarization control of the optical signal input to the optical power limiter circuit is performed using the polarization control circuit shown in FIG. 5 has been described. An optical signal can also be input to the optical power limiter circuit.

It is a block diagram of various conventional optical power limiter circuits. 1 is a block diagram and a schematic diagram of signal waveforms of a first embodiment of an optical power limiter circuit according to the present invention. It is a block diagram of 2nd Embodiment of the optical power limiter circuit by this invention. It is a figure which shows the signal waveform relevant to the optical power limiter circuit of FIG. It is a figure which shows the polarization control circuit which isolate | separates an optical signal into the linearly polarized wave which mutually orthogonally crosses.

Explanation of symbols

1, 22, 32, 35 Semiconductor optical amplifier 2 Signal generator 3 Assist light generator 4, 6, 7, 10 Optical waveguide 5, 11, 24, 34, 37 Wavelength filter 8 Directional coupler 9 Amplifier 12 Non-linear material 21 , 31 Optical power limiter circuit 23, 33, 36 Probe light generator 25, 27, 28 Optical signal 26 Probe light 41 Polarization control circuit 42 Polarization beam splitter 43, 44 Photodiode 45 Comparator 46 2 × 2 optical switch

Claims (6)

First optical signal generating means for generating first and second optical signals having different wavelengths from each other;
A first semiconductor optical amplifier which receives the first and second optical signals and has a mutual gain modulation characteristic;
An optical power limiter circuit comprising: first optical signal selection means for selectively extracting only the first optical signal from the optical signal output from the first semiconductor optical amplifier. Second optical signal generating means for generating a third optical signal having a wavelength different from the wavelength of the first optical signal;
A second semiconductor optical amplifier in which the first optical signal output from the optical signal selection means is input together with the third optical signal and has a mutual gain modulation characteristic;
2. The light according to claim 1, further comprising second optical signal selection means for selectively extracting only the first optical signal from the optical signal output from the second semiconductor optical amplifier. Power limiter circuit.   3. The optical power limiter circuit according to claim 1, wherein the first optical signal is a linearly polarized optical signal. Generating first and second optical signals having different wavelengths from each other;
Optically amplifying the first and second optical signals using mutual gain modulation characteristics;
An optical power limiter method comprising: selectively extracting only the first optical signal from the optically amplified first and second optical signals. Generating a third optical signal having a wavelength different from the wavelength of the first optical signal;
Optically amplifying the selectively output first optical signal and the third optical signal using mutual gain modulation characteristics;
5. The optical power limiter method according to claim 4, further comprising the step of selectively extracting only the first optical signal from the optically amplified first and third optical signals.   6. The optical power limiter method according to claim 4, wherein the first optical signal is a linearly polarized optical signal.

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