1. Field of the Invention
The present invention relates to a method and device for generation of phase conjugate light and wavelength conversion, and a system having the device.
2. Description of the Related Art
Owing to the development of a low-loss silica optical fiber, a number of optical fiber communication systems each using the optical fiber as a transmission line have been put to practical use. The optical fiber itself has a very wide band. However, a transmission capacity by the optical fiber is actually limited by a system design. The most important limitation is due to waveform distortion by chromatic dispersion occurring in the optical fiber. Further, the optical fiber attenuates an optical signal in a proportion of about 0.2 dB/km, for example; however, loss by the attenuation has been compensated by adopting an optical amplifier such as typically, an erbium-doped fiber amplifier (EDFA).
The chromatic dispersion frequently simply called dispersion is a phenomenon that the group velocity of an optical signal in an optical fiber changes as a function of wavelength (frequency) of the optical signal. In a standard single-mode fiber, for example, an optical signal having a longer wavelength propagates faster than an optical signal having a shorter wavelength, for wavelengths shorter than 1.3 .mu.m, and the resultant dispersion is usually called normal dispersion. For wavelengths longer than 1.3 .mu.m, an optical signal having a shorter wavelength propagates faster than an optical signal having a longer wavelength, and the resultant dispersion is called anomalous dispersion.
In recent years, attention has been paid to nonlinearities due to an increase in optical signal power by the use of an EDFA. The most important nonlinearity limiting a transmission capacity is an optical Kerr effect. The optical Kerr effect is a phenomenon that the refractive index of an optical fiber changes with the intensity of an optical signal. A change in refractive index modulates the phase of an optical signal propagating in the optical fiber, and as a result there occurs frequency chirping that changes a signal spectrum. This phenomenon is known as self-phase modulation (SPM). The spectrum is broadened by SPM, causing a further increase in waveform distortion by chromatic dispersion.
Thus, the chromatic dispersion and the Kerr effect cause waveform distortion to an optical signal with an increase in transmission distance. Accordingly, to allow a long-haul transmission by using an optical fiber, the chromatic dispersion and the nonlinearity must be controlled, compensated, or suppressed.
As a technique for controlling the chromatic dispersion and the nonlinearity, the use of a regenerative repeater including an electronic circuit for a main signal is known. For example, a plurality of regenerative repeaters are provided on a transmission line, and each regenerative repeater performs photo-electric conversion, regeneration, and electro-photo conversion in this order before the waveform distortion of an optical signal becomes excessive. However, this method has problems that an expensive complicated regenerative repeater is required, and that the electronic circuit included in the regenerative repeater limits a bit rate of the main signal.
As a technique for compensating the chromatic dispersion and the nonlinearity, an optical soliton is known. An optical signal pulse having an amplitude, pulse width, and peak power exactly specified to a given anomalous dispersion is generated to thereby balance pulse compression by both SPM due to optical Kerr effect and anomalous dispersion and pulse expansion by dispersion. As a result, the optical soliton propagates without waveform changes.
As another technique for compensating the chromatic dispersion and the nonlinearity, application of optical phase conjugation is known. For example, a method for compensating chromatic dispersion of a transmission line has been proposed by Yariv et al. (A. Yariv, D. Fekete, and D. M. Pepper, "Compensation for channel dispersion by nonlinear optical phase conjugation" Opt. Lett., vol. 4, pp. 52-54, 1979). An optical signal is converted into phase conjugate light at a middle point in a transmission line, and waveform distortion by chromatic dispersion occurred in a front half of the transmission line is compensated by distortion by chromatic dispersion in a rear half of the transmission line.
Particularly in the case that phase changes of electric fields at two points are caused by the same factor and that an environmental change inviting this factor is gentle in a light propagation time between the two points, the phase changes can be compensated by locating a phase conjugator (phase conjugate light generator) at the middle of the two points. (S. Watanabe, "Compensation of phase fluctuation in a transmission line by optical conjugation" Opt. Lett., vol. 17, pp. 1355-1357, 1992). Accordingly, waveform distortion due to SPM can also be compensated by adopting a phase conjugator. However, in the case that the distribution of optical power is asymmetrical with respect to the position of the phase conjugator, compensation for the nonlinearity becomes incomplete.
The present inventor has proposed a technique for overcoming the incompleteness of compensation due to the asymmetry of optical power distribution in the case of using a phase conjugator. (S. Watanabe and M. Shirasaki, "Exact compensation for both chromatic dispersion and Kerr effect in a transmission fiber using optical phase conjugation" J. Lightwave Technol., vol. 14, pp. 243-248, 1996). The phase conjugator is located in the vicinity of a point in a transmission line where the total dispersions or the total nonlinear effects in front and rear parts of the transmission line with respect to this point are equal to each other, and various parameters are set in each small section of the front and rear parts.
Regarding a phase conjugator and its application to optical fiber communication, the present inventor has already filed applications (Japanese Patent Application Nos. 6-509844, 7-44574, and 7-304229, and Japanese Patent Laid-open Nos. 7-98464 and 7-301830).
A method of generating a phase conjugate wave by using a traveling wave type semiconductor laser amplifier is described in 1! A. MECOZZI ET AL., IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 31, NO. 4, APRIL 1995, PP. 689-699. As shown in FIG. 6 of this literature, pump (excitation) light and probe light (also called signal light) are coupled by a directional coupler. The coupled pump light and probe light are input through a lens and an optical isolator into a traveling wave type semiconductor laser amplifier, thereby generating a phase conjugate wave from the traveling wave type semiconductor laser amplifier. The pump light is given by inputting light output from a color center laser (CCL) through an optical isolator (OI), a Babinet-Soleil compensator, and a lens into the directional coupler. The probe light is given by inputting light output from an external-cavity laser diode (ECLD) through an optical isolator, a .lambda./2 plate, and a .lambda./4 plate into the directional coupler.
A method of generating a phase conjugate wave by using a semiconductor laser instead of the semiconductor laser amplifier is described in 2! PATRICK P. IANNONE ET AL., IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 31, NO. 7, JULY 1995, PP. 1285-1291. This method employs a device having substantially the same mechanism as that in the above-mentioned literature 1! except the use of the semiconductor laser. The semiconductor laser oscillates light having the same wavelength as the wavelength of pump light to be injected from the outside.
The above two methods are common in the point that the pump light and the probe light are input to one end of the semiconductor laser amplifier or the semiconductor laser, and the pump light, the probe light, and the phase conjugate wave are output from the other end.
In contrast, a method of inputting probe light into a semiconductor laser oscillating pump light from its first end face, and outputting a phase conjugate wave from the same first end face is described in 3! S. MURATA ET AL., APPL. PHYS. LETT. 58(14), Apr. 8, 1991, PP. 1458-1460.
In the methods described in the above-mentioned literatures 1! and 2!, it is necessary to use three optical devices, i.e., the light source for generating the probe light, the light source for generating the pump light, and the semiconductor laser amplifier or the semiconductor laser for generating the phase conjugate wave. Accordingly, an optical system for coupling these three optical devices is complicated. In particular, an optical coupler for efficiently coupling the probe light and the pump light is required.
Further, in the method described in the above-mentioned literature 3!, it is necessary to form a reflecting film having high reflectivity on an nonoutput end face of the semiconductor laser for outputting the phase conjugate wave. Therefore, a Fabry-Perot mode exists in the semiconductor laser. Accordingly, as described also in the literature 3!, the wavelength of the phase conjugate wave is limited to a wavelength resonant with the Fabry-Perot mode.
A method for generating phase conjugate light by nondegenerate four-wave mixing (FWM) in a lasing DFB-LD has been recently reported in the following paper.
H. Kuwatsuka, H. Shoji, M. Matsuda, and H. Ishikawa, "THz frequency conversion using nondegenerate four-wave mixing process in a lasing long-cavity .lambda./4-shifted laser", ELECTRONICS LETTERS, Vol. 31, No. 24, pp. 2108-2110, 1995.
This method will now be described in brief. A semiconductor high-gain medium with a highly injected carrier has a large third-order nonlinear sensitivity, so that this medium is one of the optimum materials for four-wave mixing. In a lasing state of a semiconductor laser, high-intensity oscillation light exists in the laser. Therefore, inputting external light into the laser causes four-wave mixing in the laser, thereby generating phase conjugate light. This process is theoretically known; however, when external light is input into the laser in the lasing state, there actually occurs a problem that the oscillation light is pulled to the wavelength of the external light, or the oscillation light becomes unstable. Further, although the phase conjugate light is generated, the generation of the phase conjugate light is allowed only by the light having a wavelength resonant with the cavity constituting the semiconductor laser. Thus, the wavelength cannot be freely converted.
As described in the above paper, the quarter-wave phase-shifted DFB semiconductor laser includes two diffraction gratings for reflecting only light having a wavelength intended to be oscillated. The two diffraction gratings are formed so as to be shifted in phase from each other by a quarter wave. By the two diffraction gratings, the oscillation light is strongly confined in the semiconductor laser. By forming antireflection coatings on the opposite end faces of the semiconductor laser, light having wavelengths different from the wavelength of the oscillation light is passed without internal reflection in the laser. Accordingly, it is possible to generate phase conjugate light corresponding to the external light input into the semiconductor laser by using the oscillation light as pump light. Thus, high-efficient, high-speed, and wide-band conversion is allowed without the use of external pump light.
Although a conversion efficiency in the phase conjugate light generator depends on the conformity of the polarization planes of probe light and pump light, a general optical fiber transmission line has no polarization maintaining ability. Accordingly, to configure an optical system using optical phase conjugation, it is necessary to realize a phase conjugate light generator which can exhibit high-efficient, high-speed, and wide-band conversion, and further has no polarization dependence.