JP3151892B2 - Optical fiber dispersion compensation method and apparatus - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical fiber dispersion compensation method using non-degenerate four-wave mixing and an optical fiber dispersion compensator using non-degenerate four-wave mixing of a semiconductor laser.

[0002]

2. Description of the Related Art In optical fiber communication, when a signal propagation speed or a transmission distance increases, a signal waveform deteriorates due to frequency dispersion of an optical fiber as a transmission line. For this reason, the transmission speed or distance is limited. For example, 1.55 μm using a normal 1.3 μm zero-dispersion fiber
When transmitting a 10 Gb / s optical signal of μm, the limit of the transmission distance due to second-order frequency dispersion is several tens km or less.

As a method of compensating for this frequency dispersion, a method described in detail in Japanese Patent Application No. Hei. That is, in this method, the nonlinear optical element is placed at the midpoint of the transmission path, and the nonlinear effect of the nonlinear optical element is used to perform frequency conversion on the spectrum of the signal light that has been deteriorated due to the influence of frequency dispersion in the first half of the transmission path. Then, a signal light having a spectrum folded on the frequency axis is obtained. This signal light is sent to the latter half transmission line on the receiving side, whereby the influence of the frequency dispersion in the first half of the transmission line is compensated for by the frequency dispersion in the second half. As the nonlinear optical effect, non-degenerate four-wave mixing or optical parametric amplification can be used. In the case of non-degenerate four-wave mixing, when pump light and probe light having a different frequency from the pump light are simultaneously incident on the nonlinear optical medium, the probe is placed at a position symmetrical with respect to the frequency of the pump light on the frequency axis. The fact that four-wave mixing output signal light having a spectrum obtained by inverting the spectrum of the light on the frequency axis is used for dispersion compensation. With respect to the dispersion compensator utilizing non-degenerate four-wave mixing, an apparatus utilizing non-degenerate four-wave mixing of a semiconductor laser amplifier has been filed with Japanese Patent Application Laid-Open No. 3-125124 (Japanese Patent Application No. 1-263678). I have. As an example of the development by the Company, an apparatus utilizing the non-degenerate four-wave mixing of the above-mentioned Fabry-Perot type semiconductor laser has been filed by Murata (Japanese Patent Application No. 3-024233).

A mechanism for generating non-degenerate four-wave mixing in a semiconductor laser or a semiconductor laser amplifier is disclosed in Japanese Patent Application No. 3-0242.
As also shown in No. 33, it is believed that two different mechanisms exist. One is due to the variation in carrier density. Although the frequency conversion efficiency is high, the frequency response is limited by the carrier lifetime (about 1 ns). Therefore, the transmission speed of an optical signal that can be converted using this conversion mechanism is at most 1 Gb. / S. Another mechanism is due to the in-band nonlinear process represented by hole burning.
Although the conversion efficiency is low, the response frequency is limited by a high-speed in-band relaxation process, and is considered to be several hundred GHz or more. Therefore, if this mechanism is used, a signal of 100 Gb / s or more can be converted in principle. Since the conversion efficiency of the latter is usually very small, a contrivance has been made to increase the conversion efficiency by using a Fabry-Perot type semiconductor laser as a nonlinear element and using a resonance effect of a resonator.

[0005]

In these conventional dispersion compensating elements utilizing four-wave mixing, the pump light, the input signal light, and the output signal light are spatially and spatially displaced due to restrictions on conversion efficiency.
It is difficult to separate them in terms of frequency, and in particular, there is a problem that a strong pump light is mixed with the output signal light, resulting in deterioration of the signal. To avoid this, it was necessary to separate the output signal light from the pump light by using a filter having an extremely narrow pass frequency band. In addition, as described above, if the optical nonlinearity due to the in-band nonlinear process is used, a high-speed transmission signal can be frequency-converted in principle, but the converted output signal is actually caused by a low-speed carrier density change. There is a problem that there is contribution from optical non-linearity having a slow response speed, which causes signal distortion.

An object of the present invention is to solve these problems, compensate for waveform deterioration due to frequency dispersion of a high-speed signal of several Gb / s or more, and generate a waveform resulting from interference with pump light and low-speed optical nonlinearity. An object of the present invention is to provide an optical fiber dispersion compensation method and apparatus capable of obtaining an output signal with little distortion.

[0007]

An optical fiber dispersion compensating method according to the present invention for solving the above-mentioned problems is provided by an input signal light having waveform distortion generated during optical fiber transmission due to the dispersion characteristics of the optical fiber. And pump light are simultaneously injected into the nonlinear optical medium to obtain dispersion-compensated signal light as frequency-converted output signal light generated by the non-degenerate four-wave mixing process in the nonlinear optical medium. The polarization directions of the light and the input signal light are made orthogonal to each other, and the separation of the output signal light and the pump light is performed using a polarization selection element.

Further, the optical fiber dispersion compensator of the present invention comprises a nonlinear optical medium, a pump light source for generating pump light, means for injecting input signal light and pump light into the nonlinear medium, Means for extracting a frequency-converted output signal light generated by a non-degenerate four-wave mixing process from the non-degenerate four-wave mixing process, wherein the nonlinear optical medium is a surface-emitting type semiconductor laser, and the frequency of the pump light and the input Means for making the frequency of the signal light substantially coincide with one of the different resonance modes of the surface-emitting type semiconductor laser, and making the polarization directions of the pump light and the input signal light orthogonal to each other; and A polarization selecting element for separating output signal light from the pump light is provided.

Further, the optical fiber dispersion compensator of the present invention comprises a nonlinear optical medium, means for injecting an input signal light into the nonlinear medium, and frequency conversion generated by the non-degenerate four-wave mixing process from the nonlinear optical medium. Means for extracting the output signal light, wherein the nonlinear optical medium is a surface-emitting type semiconductor laser having a plurality of resonance modes and oscillating in a single axis mode, and the frequency of the input signal light is higher than the surface light emission. Means for causing the laser oscillation light from the semiconductor laser to substantially coincide with one of the resonance modes different from the oscillation frequency of the light-emitting type semiconductor laser, and for making the polarization directions of the laser oscillation light and the input signal light orthogonal to each other; It is characterized by having a polarization selection element for separating output signal light from the laser oscillation light.

[0010]

The method and apparatus according to the present invention utilize only a high-speed in-band nonlinear process among non-degenerate four-wave mixing processes in a semiconductor laser or a semiconductor laser amplifier. Although it is difficult to describe all four-wave mixing processes that occur in a nonlinear optical medium in words, generally, four-wave mixing processes are caused by the third-order optical nonlinearity of the medium and are caused by two of the four light waves. Some kind of diffraction grating is generated in the medium, whereby one light wave is coherently scattered (diffracted) (this process is a third-order nonlinear optical process), and finally one light wave which is four-wave mixing output light is formed. It is understood as occurring. There is a detailed explanation of this image in Junichi Sakai's "Phase Conjugation Optics" (Asakura Shoten, Advanced Science and Technology Series B1). According to this, as a diffraction grating generated, for example, the spatial modulation of a higher level distribution density (population = carrier density in the case of a semiconductor) resulting from spatial modulation of light intensity due to optical interference of two light waves, A spatial diffraction grating having a refractive index distribution and a spatial diffraction grating having an absorptivity distribution (collectively referred to as a spatial diffraction grating having a complex refractive index), which are felt by the third light wave, are given.

When these viewpoints are applied to the in-band nonlinear process, four-wave mixing by the in-band nonlinear process becomes 2
Interaction between the light wave and the medium causes spatial modulation of carrier energy or momentum distribution due to hole burning, non-thermal equilibrium carrier distribution, and the like, and the third light wave is coherently scattered by the resulting optical spatial diffraction grating. It can be described as a result.

As described above, the four-wave mixing process by the two different mechanisms described above depends not only on the magnitude of the response speed due to the speed of relaxation to the equilibrium state, but also on the dependence of the light wave on the polarization direction. There are significant differences. This will be described below. It was mentioned earlier that four-wave mixing caused by a change in carrier density is due to spatial modulation of light intensity due to optical interference of the two light waves involved.
Therefore, four-wave mixing based on this mechanism cannot occur unless interference between two light waves occurs. It is well known that light waves are transverse waves and have two independent degrees of freedom, which appear as two orthogonal polarization directions. Light waves having polarization directions orthogonal to each other are completely independent, and no interference phenomenon occurs. Therefore, in a situation where two light waves involved in four-wave mixing have polarization directions orthogonal to each other, four-wave mixing caused by a change in carrier density does not occur. this is,
This corresponds to a case where pump light and probe light having polarization directions orthogonal to each other are injected into the semiconductor laser.

On the other hand, in the case of four-wave mixing caused by an in-band nonlinear process, it is known that four-wave mixing occurs even when the polarization directions of two light waves involved are made orthogonal. For example, it is described in IEEE Journal of Quantum Electronics QE19, No. 4, pp. 680-689. (BS Wherrett et al.
“Theory of Degenerate Fou
r-Wave Mixing in Picoseco
nd Excitation-Probe Explorer
imments "IEEE Journal of Qu
nature Electronics, Vol. QE-
19, no. 4, pp. 680-689) In this case, there is no spatial intensity modulation due to interference between the two light waves involved, and the light intensity in the medium is uniform regardless of location. However, coherent superposition of two lightwaves having orthogonal polarization directions results in spatial modulation in the direction of the synthesis of the electric field vectors. Due to the electric field-dipole interaction, there is usually a large correlation between the direction of the electric field vector and the direction of the excited dipole. Further, the direction of the dipole and the direction of the momentum vector of the free carrier in the semiconductor have a correlation with each other due to the symmetry determined by the atomic bonding mode of the crystal. As a result, anisotropy occurs in the distribution of the carriers in the momentum space, which is determined by the direction of the electric field vector. This results in some spatial modulation of the optical properties in the medium, and the third lightwave is scattered by the resulting spatial diffraction grating, resulting in four-wave mixing.

According to the above description, when pump light and probe light having polarization directions orthogonal to each other are injected into a medium,
Four-wave mixing with a slow response speed due to a change in carrier density does not occur, and four-wave mixing due to in-band nonlinearity with a fast response occurs.
It can be seen that only light wave mixing occurs. According to a detailed analysis, the polarization direction of the generated four-wave mixing signal light is the same as the polarization direction of the probe light, that is, orthogonal to the polarization direction of the pump light. III commonly used in semiconductor lasers
In the case of a group V semiconductor, a four-wave mixing signal light intensity obtained when the polarization directions of the pump light and the probe light are made parallel and a four-wave mixing signal obtained when the polarization directions of the pump light and the probe light are made vertical. It has been found that the magnitude of the light intensity does not differ significantly. In the above description, a semiconductor material is mainly considered as a non-linear medium.However, a material having an energy level structure similar to a band structure and having a sufficiently high transition probability between the energy levels, such as an organic dye, is also used. The above is valid. Therefore, it can be said that such materials are common.

Using the above principle, in the present invention, the polarization directions of the pump light and the probe light to be injected into the nonlinear medium are made orthogonal to each other, and a four-wave mixing process using only the in-band nonlinear process with a fast response speed is used. By using the fact that there is no waveform distortion caused by four-wave mixing with a slow response speed due to a change in carrier density, and that the polarization directions of the converted output light and the pump light are orthogonal to each other, An optical fiber dispersion compensation method and apparatus for separating converted output light from pump light using a polarization selection element having higher selection efficiency is provided. Also, in this method, the separation between the converted output light and the pump light is not separated in the frequency space using a frequency filter, but is separated in the real space using polarized light. It also has the advantage of being able to choose.

[0016]

FIG. 1 is a diagram showing a first embodiment of the present invention.
FIG. 3 conceptually shows a basic configuration of the optical fiber compensation method described in claim 1. This method uses the nonlinear optical medium 1
0, a pump light source 20, polarization selection elements 30 and 31, an optical fiber amplifier 40, a polarization rotation element 50, an optical isolator 60, and an optical system connecting them. The spectrum of the input signal light deteriorated by the influence of the frequency dispersion from the transmission line 200 on the transmission side is inverted on the frequency axis by using non-degenerate four-wave mixing of the nonlinear optical medium 10 (that is, the frequency conversion is performed). T) As an output signal light, send it out to the transmission line 300 on the receiving side. The polarization directions of the pump light and the input signal light are adjusted to be orthogonal by the polarization selection element 30 and the polarization rotation element 50, and the transmission polarization plane of the polarization selection element 31 is made to coincide with the polarization direction of the input signal light.

FIG. 2 shows a second embodiment, which is a more specific version of the first embodiment, and schematically shows the basic configuration of the optical fiber compensator according to the second embodiment. This device is basically filed by Murata (Japanese Patent Application No. Hei.
No. 024233) is an improvement of the device based on the principle of the present invention. This apparatus is composed of a surface-emitting type semiconductor laser 11 (hereinafter referred to as a surface-emitting laser), which is a nonlinear optical medium, a pump laser 21, a polarization selecting element 30, an optical fiber amplifier 40, a polarization rotating element 50, an optical isolator 60, and an optical connecting element. Consists of a system. The input signal light from the transmission line 200 on the transmission side is converted into an output signal light by the surface emitting semiconductor laser 11 in the same manner as described above, and sent out to the transmission line 300 on the reception side. The adjustment of the polarization selection elements 30 and 31 and the polarization rotation element 50 is the same as described above. The reason for using a surface-emitting type semiconductor laser instead of an edge-emitting type semiconductor laser here is that it is difficult to use the edge-emitting type semiconductor laser because it has a polarization selectivity because of its waveguide structure. It is.

The surface emitting laser 11 is a 1.55 μm band semiconductor laser, and the pump laser 20 is 1.55 μm.
A band distributed reflection type semiconductor laser (hereinafter, DFB laser) is used. Both lasers are temperature-stabilized to ± 0.1 ° C or less. The resonance mode of the surface emitting laser 10 and the frequency of the pump laser 20 are both controlled by temperature control. By changing the frequency of the pump laser 20, the surface emitting laser 1
By changing the oscillation mode 1, the frequency of the output signal light can be changed. Gran Thomson prisms are used as the polarization selection elements 30 and 31 so that the transmission polarization plane can be freely rotated. Extinction ratio of the prism 10 ^{- 5} or less. The polarization rotator 50 is a Gran Thompson prism and has a wavelength of 1.55 μm.
Is used. Gran
The transmission polarization plane of the Thomson prism is adjusted so that the input signal light from the transmission path 200 is transmitted at the maximum, and the polarization plane can be freely rotated by rotating the half-wave plate. The optical fiber amplifier 40 is a device for amplifying the converted output signal light and sending out the converted output signal light to the receiving side transmission line 300, and an erbium-doped optical fiber amplifier pumped by a semiconductor laser can be used.

FIG. 3 is a diagram showing a third embodiment of the present invention, and schematically shows a basic configuration of an optical fiber dispersion compensator according to a third aspect of the present invention. The main difference from the second embodiment is that a surface semiconductor laser 12 having a plurality of resonance modes and oscillating in a single axis mode (hereinafter referred to as a single mode surface emitting laser) is used as a nonlinear medium. Other points are almost the same as those of the first embodiment. In this device, the oscillation light itself of the single mode surface emitting laser 12 can be used as the pump light, so that the pump light source as provided in the second embodiment is not required. Therefore, the device configuration is simplified. Also in this embodiment, the single mode surface emitting laser 1
The frequency of 2 is temperature controlled.

As described in the description of the prior art, the spectrum of the output signal light has a shape obtained by inverting the spectrum of the input signal light around the frequency of the pump light. Therefore, this dispersion compensator is installed at an intermediate point of an optical fiber transmission line having a substantially uniform frequency dispersion characteristic, and the output signal light folded on the spectrum is transmitted to the reception side, so that the transmission line on the transmission side is transmitted. Can be compensated for by the transmission line on the receiving side.

Although the embodiments have been described in detail, some supplements will be made below. Although only the device corresponding to the 1.55 μm band as the wavelength band of the signal light is shown in the embodiment, other wavelength bands such as the 1.3 μm band can be obtained by changing the wavelengths of the semiconductor laser serving as the nonlinear optical medium and the pump light source. The effect of the present invention does not change even for the signal light. The method and apparatus of the present invention are effective not only in the direct detection method but also in other methods such as the optical heterodyne method as the signal light modulation / demodulation method. The optical fiber amplifier 40 of the embodiment can be omitted depending on the transmission system. When a semiconductor laser having its own amplifying ability is used as a nonlinear medium as in the present invention, output signal light can be extracted from either of two resonator surfaces of the semiconductor laser which is a nonlinear optical medium. Therefore, it is possible to use not only the arrangement as in the embodiment but also an arrangement in which the pump light and the input signal light are injected from the output side resonator surface by appropriately changing the optical system, for example.

Further, in the present invention, a semiconductor laser is mainly described as a nonlinear optical medium. However, as described above, the principle of the present invention is applicable to a medium having an energy level structure similar to a band structure. Therefore, the method of the present invention is also effective.

[0023]

As described above, according to the present invention, waveform degradation due to frequency dispersion of a high-speed signal of several Gb / s or more is compensated, and waveform distortion caused by interference with pump light or low-speed optical nonlinearity is compensated. And an optical fiber dispersion compensation method and apparatus that can obtain an output signal with less noise.

[Brief description of the drawings]

FIG. 1 is a configuration diagram illustrating a first embodiment of an optical fiber dispersion compensation method according to the present invention.

FIG. 2 is a configuration diagram illustrating a second embodiment of the optical fiber dispersion compensator according to the present invention.

FIG. 3 is a configuration diagram illustrating a third embodiment of the optical fiber dispersion compensator of the present invention.

[Explanation of symbols]

 Reference Signs List 10 nonlinear optical medium 11 surface emitting semiconductor laser 12 single mode surface emitting semiconductor laser 20 pump light source 21 pump laser 30 polarization selecting element 31 polarization selecting element 40 optical fiber amplifier 50 polarization rotating element 60 optical isolator 200 transmission side transmission line 300 Receiver transmission line

──────────────────────────────────────────────────続 き Continued on the front page (56) References IEEE Journal of Quantum Electronics, Vol. QE-19, no. 4, April 1983, pp. 147-64. 680-689 IEEE Transactions Photonics Technology Letters, Vol. 3, No. 11, November, 1991, p. 1021-1023 (58) Field surveyed (Int. Cl. ⁷ , DB name) G02F 1/35 H01S 3/10 H01S 3/108 H04B 10/02 H04B 10/18 JICST file (JOIS)

Claims (3)

(57) [Claims] 1. A non-degenerate non-degenerate state in a nonlinear optical medium by simultaneously injecting input signal light and pump light having waveform distortion generated during transmission of the optical fiber due to dispersion characteristics of the optical fiber into the nonlinear optical medium. A method of obtaining dispersion-compensated signal light as frequency-converted output signal light generated by a four-wave mixing process, wherein the pump light and the input signal light have polarization directions orthogonal to each other,
And an optical fiber dispersion compensating method characterized in that the output signal light and the pump light are separated by using a polarization selecting element. 2. A non-linear optical medium, a pump light source for generating pump light, means for injecting input signal light and pump light into the non-linear medium, and non-degenerate from the non-linear optical medium.
Means for extracting a frequency-converted output signal light generated by the light wave mixing process,
The nonlinear optical medium is a surface-emitting type semiconductor laser, the frequency of the pump light and the frequency of the input signal light substantially coincide with one of the different resonance modes of the surface-emitting type semiconductor laser, and An optical fiber dispersion compensator comprising: means for setting the polarization directions of the pump light and the input signal light to be orthogonal to each other; and a polarization selection element for separating the output signal light from the pump light. 3. A nonlinear optical medium, means for injecting input signal light into the nonlinear medium, and means for extracting frequency-converted output signal light generated by a non-degenerate four-wave mixing process from the nonlinear optical medium. An apparatus comprising: a surface-emitting semiconductor laser in which the nonlinear optical medium has a plurality of resonance modes and oscillates in a single axis mode, wherein the frequency of the input signal light is different from the oscillation frequency of the surface-emitting semiconductor laser Means for causing the laser oscillation light from the semiconductor laser and the input signal light to be substantially orthogonal to each other in one of the resonance modes, and separating the output signal light from the laser oscillation light. An optical fiber dispersion compensating device, comprising: