JP2626197B2 - Frequency converter - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a frequency converter using four-wave mixing of a semiconductor laser amplifier.

(Conventional technology) A frequency conversion device that converts an optical signal of a certain frequency into an optical signal of another frequency is one of the most important devices in an application system using frequency multiplexing technology such as frequency division optical conversion. Conventionally, several frequency conversion methods are known. Among them, a frequency conversion method using four-wave mixing of a semiconductor laser or a semiconductor laser amplifier has a high conversion efficiency and can reduce the size of an apparatus using the same. There are features. In frequency conversion using four-wave mixing of a semiconductor laser amplifier, pump light and probe light are simultaneously injected from an end face of the semiconductor laser amplifier. The probe light is modulated, and a signal is placed here. The frequencies of the pump light and the probe light are slightly different. At this time, phase conjugate light is generated at a frequency position symmetrical to the probe light with respect to the frequency of the pump light. Since this phase conjugate light carries the same signal as the probe light, if only the phase conjugate light is extracted by a frequency filter, the frequency conversion from the frequency of the probe light to the frequency of the phase conjugate light can be performed. If the frequency of the pump light is changed when the frequency of the incident probe light is fixed, the frequency of the phase conjugate light generated at the corresponding frequency position also changes. Thus, it can be converted to a desired frequency. Semiconductor laser amplifier 4
For frequency conversion using light wave mixing, for example, G. Gr
obkopf et al., Optical and Quantum Electronics, vol. 21, S59-S74 (G. Grobkopf et. al., Op.
tical and Quantum Electronics, vol. 21 (1989) S59-S
74). There, an InGaAsP / InP traveling-wave semiconductor laser amplifier is used as a frequency conversion medium. In addition, there has been reported an example in which the oscillation light itself of a semiconductor laser is used as pump light.

(Problem to be Solved by the Invention) The above-described conventional example has the following problems. In other words, phase conjugate light is obtained only in a narrow range of at most about 10 GHz or less in frequency difference (detuning) between pump light and probe light. This is because the conversion efficiency of the phase conjugate light (the ratio of the output phase conjugate light to the incident probe light) sharply decreases as the detuning increases. For this reason, frequency conversion can be performed only in a narrow frequency range of about 10 GHz near the pump light. Moreover, the speed of a signal that can be converted is at most several hundred Mb / s or less, and it is difficult to convert a signal having a higher speed than this. Further, since the frequency difference is small, it becomes difficult to separate the pump light and the phase conjugate light.

To improve this and expand the range of frequency conversion,
The above-mentioned G. Grobkopf et al.
The frequency range was expanded to 1 THz or more using the pump light of (1). Therefore, a traveling-wave semiconductor laser amplifier is used as a frequency conversion medium. Since phase conjugate light is also generated near the second pump light, frequency conversion can be performed by extracting this phase conjugate light. This method is also described in the above-mentioned document. However, in this case, there is a disadvantage that two pump lights are required and the apparatus becomes complicated. Also the second
Since the frequency difference between the pump light and the phase conjugate light is also about 10 GHz or less, the signal speed that can be converted is low and the difficulty in separating the second pump light and the phase conjugate light remains unchanged.

JGProvost et al. Use the amplification effect at the resonance frequency of an AlGaAs / GaAs Fabry-Perot semiconductor laser as a method of directly extracting phase conjugate light in a large detuning region. About Applied Physics Letters 55
519 (1989). (JGProvost et.
al., Apll. Phys. Lett., vol. 55 (1989) 519). They are,
Using the oscillation light of the semiconductor laser itself as the pump light,
The probe light was injected at the position of the resonance frequency next to the oscillation light. At this time, the semiconductor laser oscillates in the single axis mode. In the Fabry-Perot semiconductor laser, the resonance frequencies are arranged at equal intervals, so that the phase conjugate light is generated at a position of the resonance frequency symmetrical to the probe light with respect to the oscillation light. At this resonance frequency, the phase conjugate light is amplified, and a large signal can be extracted. using this method,
Phase conjugate light is obtained in the detuning region of 100 GHz or more. However, in this method using a normal Fabry-Perot semiconductor laser, the frequency of the pump light cannot be freely controlled to change the frequency of the phase conjugate light. Further, since the amplification bandwidth at the resonance frequency is very narrow, 100 MHz, it is difficult to control the frequency of the probe light to be injected, and it is not possible to convert a high-speed signal having a large bandwidth. Further, an InGaAsP / InP Fabry-Perot semiconductor laser, which is important for optical communication, has a disadvantage that oscillation light cannot be used as pump light because it is difficult to oscillate in a single axis mode.

An object of the present invention is to improve the above-described disadvantages of the conventional example, to obtain a wide frequency conversion range, and to convert a high-speed signal.
It is to provide a frequency conversion device.

(Means for Solving the Problems) A frequency conversion device using a semiconductor laser amplifier according to the present invention comprises: (1) a resonance type semiconductor laser amplifier; a means for injecting pump light and probe light into the resonance type semiconductor laser amplifier; Means for extracting phase conjugate light from the resonant semiconductor laser amplifier, wherein the frequency of the probe light is substantially equal to one of the resonant frequencies of the resonant semiconductor laser amplifier, and the frequency of the pump light is ,
The frequency of the probe light is variable and substantially coincides with one of the resonance frequencies of the resonance type semiconductor laser amplifier.

(2) a resonance type semiconductor laser amplifier, means for feeding back feedback light of a specific frequency to the resonance type semiconductor laser amplifier, means for injecting probe light into the resonance type semiconductor laser amplifier, and the resonance type semiconductor laser amplifier Means for extracting phase conjugate light from the probe light, the frequency of the probe light is substantially equal to one of the resonance frequencies of the resonance type semiconductor laser amplifier, the frequency of the feedback light is variable and the probe It is characterized in that it substantially coincides with one of the resonance frequencies of the resonance type semiconductor laser amplifier different from the frequency of light.

(Example) Next, the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram of a first embodiment of the present invention, and shows a frequency converter using four-wave mixing of a semiconductor laser amplifier described in claim 1. The modulated probe light (input light) is injected from one end face of the resonant semiconductor laser amplifier 10 through the semi-transparent mirror 50. Pump light is injected from the pump laser 20 through the optical isolator 40 from the other end face of the resonant semiconductor laser amplifier 10. The polarization planes of the probe light and the pump light need to match. At this time, due to four-wave mixing in the resonance type semiconductor laser amplifier 10, phase conjugate light carrying the same signal as the probe light is generated. The phase conjugate light (output light) is output through the semi-transparent mirror 50. Here, a frequency filter 30 is introduced to separate the phase conjugate light from the pump light and the probe light.

Here, the relationship between the frequencies of the respective lights and the principle of the present invention will be described with reference to FIG. Basically, utilizing the large amplification factor at the resonance frequency of the resonance type semiconductor laser amplifier 10, the phase conjugate light in the case of large detuning is extracted,
Performs frequency conversion. The upper part of FIG. 2 schematically shows the relationship between the amplification degree (gain) and the frequency of the resonance type semiconductor laser amplifier 10. As is well known, the resonant semiconductor laser amplifier 10 has a periodic gain characteristic with respect to the frequency of the injected light. The resonance frequencies are arranged at regular intervals on the frequency axis, and the gain becomes maximum at this resonance frequency. The bandwidth of the gain at one resonance frequency is determined by the internal gain, the end face reflectivity, and the like. In order to widen the gain bandwidth without lowering the gain, the end face reflectivity may be reduced to some extent (several percent or less). However, when the end face reflectance is close to zero, the effect of resonance is lost and a so-called traveling-wave semiconductor laser amplifier is obtained. As described in the conventional example, in four-wave mixing using a traveling-wave type semiconductor laser amplifier, the internal light intensity cannot be increased, so that the conversion efficiency of phase conjugate light in a region where detuning is large is extremely small. This is the reason for using the resonant semiconductor laser amplifier 10 in the present invention. Mukai et al. Provide a detailed report on the resonant semiconductor laser amplifier 10 (Mukai et al., IEICE Transactions, J69C (1986) 421). Now, the modulated probe light is applied to one resonance frequency of the resonance type semiconductor laser amplifier 10 (here, this position is set to No. 0 and the other resonance frequency is set to the second resonance frequency).
(It will be numbered as shown in the figure). At this time, the pump light is changed to the nth (n = ± 1, ± 2,
...) near the resonance frequency. Then, due to four-wave mixing of the resonance type semiconductor laser amplifier 10, phase conjugate light modulated in the same manner as the probe light is generated near the 2nth resonance frequency. Since this phase conjugate light is greatly amplified by resonance, high frequency conversion efficiency can be obtained. If the frequency of the pump light is changed sequentially at the resonance frequency interval,
The frequency of the phase conjugate light changes at twice the resonance frequency interval. In this way, frequency conversion can be performed over a wide frequency range. In this case, the frequencies that can be converted are discontinuous. However, if the length of the resonant semiconductor laser amplifier 10 is increased, the frequency interval becomes narrower, so that the number of usable frequencies can be increased. For example, if the length is 800 μm in the resonance type semiconductor laser amplifier 10 in the 1.55 μm band, the resonance frequency interval is about 50 GHz. Since the gain width of the resonance type semiconductor laser amplifier 10 is several THz, the number of usable frequencies is several tens or more.

In the first embodiment, a 1.55 μm band InGaAsPs / InP embedded semiconductor laser laser amplifier is used as the resonance type semiconductor laser amplifier 10. The length of the element is 800 μm, and both end faces are coated with a low reflection coating of about 2%. The gain at the resonance frequency is adjusted to about 20 dB by adjusting the drive current.
The gain band is about 30 GHz. The frequency of the probe light (wavelength: 1.55 μm) is adjusted to one of the resonance frequencies by adjusting the ambient temperature of the resonance type semiconductor laser amplifier.
As the pump laser 20, a so-called external resonator type semiconductor laser having a frequency variable function is used. The wavelength of the pump light is 1.5 μm by rotating the diffraction grating of the pump laser 20 and changing the ambient temperature of the laser.
It can be changed almost continuously from m to 1.6 μm. By adjusting the frequency of the pump light to a resonance frequency near the probe light, phase conjugate light is generated at a position symmetrical with respect to the pump light. The generated frequency range is 1 THz or more centered at 1.55 μm. Further, since the gain band at the resonance frequency is wide, a high-speed signal of 1 Gb / s or more can be converted. As the frequency filter 30 for separation, a normal diffraction grating is used.

In this embodiment, an integrated frequency-variable laser or the like may be used as the pump laser 20.
Further, the pump light and the probe light may be injected from either end face of the resonance type semiconductor laser amplifier 10, and the effect is not changed even if two lights are injected from the same end face. In these cases, of course, a slight change in the optical system is required with respect to the configuration shown in FIG. Further, the phase conjugate light can be extracted from either end face.

FIG. 3 is a block diagram of a second embodiment of the present invention, and shows a frequency conversion device described in claim 2. First
The difference from the embodiment is that the pump laser is used as the pump light.
The point is that instead of using the light of 20, the return light of the resonance type semiconductor laser amplifier 10 itself is used instead. If the light of one resonance frequency of the resonance type semiconductor laser amplifier 10 is selectively fed back, only the light of that frequency is greatly amplified,
The same effect as in the case where the pump light is injected from the outside in the first embodiment can be obtained. Here, a diffraction grating 60 is used to selectively feed back a specific frequency. Diffraction grating 60
By rotating, the frequency of the returning light can be changed. However, here, the frequency of the feedback light is selected so as to substantially match the resonance frequency of the resonance type semiconductor laser amplifier 10. Resonant semiconductor laser amplifier 10 and diffraction grating
The combination of 60 is the same as the configuration of the so-called external cavity semiconductor laser. Other configurations are the same as those of the first embodiment. This embodiment is simpler in configuration than the first embodiment.

Also in the second embodiment, a 1.55 μm band InGaAsPs / InP embedded semiconductor laser amplifier is used as the resonance type semiconductor laser amplifier 10. By adjusting the diffraction grating 60, the frequency of the feedback light (that is, the pump light) is controlled. The probe light is injected from the end face opposite to the diffraction grating 60. Other operation methods are the same as those described in the first embodiment. In this embodiment, effects similar to those of the first embodiment can be obtained.

In the second embodiment, the diffraction grating 60 is used for controlling the frequency of the feedback light.
It is also possible to use an integrated one of the semiconductor laser amplifier 10 and the resonance type semiconductor laser amplifier 10. Such an integrated device is known as a variable frequency distributed Bragg reflection semiconductor laser. When this integrated element is used, the frequency variable range is narrower than in the second embodiment, but there is a feature that the stability of the optical system is good. Regarding the frequency-variable distributed Bragg reflection type semiconductor laser, for example, S. Murata et al. Report in Electronics Letters (S. Murata et.al, Electro.
n. Lett., vol. 23 (1987) 403).

Although the two embodiments have been described in detail, some supplements will be made below. First, if the resonance type semiconductor laser amplifier 10 is appropriately selected, the same frequency conversion can be performed in a wavelength band other than the above-mentioned wavelength band, for example, in the 1.3 μm band. Also, a quantum well semiconductor laser amplifier can be used as the resonance type semiconductor laser amplifier. Further, since the frequency conversion device of the present invention can perform frequency conversion irrespective of the modulation method of the probe light, the frequency filter 30 is unnecessary if the heterodyne detection method is applied to modulation and demodulation.

(Effect of the Invention) As described above, according to the present invention, it is possible to realize a frequency conversion device having a wide frequency conversion range of 1 THz or more and capable of converting a high-speed signal of 1 Gb / s or more.

[Brief description of the drawings]

FIG. 1 is a block diagram of a first embodiment of the present invention, FIG. 2 is a diagram for explaining the principle of operation, and FIG. 3 is a block diagram of a second embodiment. In the figure, 10 is a resonance type semiconductor laser amplifier, 20 is a pump laser, 30 is a frequency filter, 40 is an optical isolator, 50 is a semi-transparent mirror, and 60 is a diffraction grating.

Claims (2)

(57) [Claims] 1. A semiconductor device comprising: a resonance type semiconductor laser amplifier; means for injecting pump light and probe light into the resonance type semiconductor laser amplifier; and means for extracting phase conjugate light from the resonance type semiconductor laser amplifier; The frequency of the probe light is substantially coincident with one of the resonance frequencies of the resonance type semiconductor laser amplifier, and the frequency of the pump light is variable and the frequency of the resonance type semiconductor laser amplifier is different from the frequency of the probe light. A frequency converter characterized by being substantially coincident with one of the resonance frequencies. 2. A resonance type semiconductor laser amplifier, means for feeding back feedback light of a specific frequency to said resonance type semiconductor laser amplifier, means for injecting probe light into said resonance type semiconductor laser amplifier, and said resonance type semiconductor laser amplifier Means for extracting phase conjugate light from a laser amplifier, wherein the frequency of the probe light is substantially coincident with one of the resonance frequencies of the resonance type semiconductor laser amplifier, the frequency of the feedback light is variable, and A frequency converter, wherein the frequency is substantially equal to one of the resonance frequencies of the resonance type semiconductor laser amplifier different from the frequency of the probe light.