JPH087356B2 - Optical soliton generation method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for generating an optical soliton generated by balancing the self-phase modulation effect due to the Kerr effect in an optical fiber and the anomalous dispersion in the fiber, and particularly to a semiconductor. A method for generating a wavelength-tunable soliton pulse by amplifying a weak optical pulse of a laser to a level at which a self-phase modulation effect is generated by stimulated Raman scattering in an optical fiber and balancing it with anomalous dispersion Is.

[Prior Art] Possibility of existence of optical solitons is 1973 Bell Lab's A. Ha
segawa and F. Tappert (Appl.Phys.Lett., Vol.23, P.14
2 “Transmission of stationary nonlinear optical pu
Since it was pointed out by "lses in dispersive dielectric fibers"), the research on the propagation of non-linear waves has become active. Among them, the research is progressing drastically after the experiments of Mollenauer who demonstrated solitons (LFMoll).
enauer, RHStolen, JPGordon; Phys.Rev.Lett., Vol.4
5, P.1095,1980 “Experimental observation of picosec
ond pulse narrowing and solitons in optical fiber
s ").

F, which has been used without exception in soliton experiments so far
FIG. 9 shows a block diagram of a center (color center) laser. In the figure, 1 is a mode-locking YAG laser, 2, 2 ', 2 "are laser boundaries, 3 is an F center crystal, 4 is a crystal cooling liquid N ₂ dewar, and 5 is a wavelength tunable birefringent filter. Is a synchronous laser excited by the YAG laser 1, and the output obtained by passing through the laser mirror 2 ″ has a peak output of about 100 W, a width of 8 ps, and a wavelength variable characteristic of 1.48 to 1.60 μm. However, the laser crystal must always be cooled to the liquid N ₂ temperature, and the pulse repetition
The YAG laser has the drawbacks of being fixed and not variable, the life of the crystal being short, and the pulse width being high and not being variable. Therefore, the results of soliton research have been announced by a small number of F-center laser holding groups.

Fig. 10 shows the method of generating solitons and the method of measuring them by the F center laser. The light pulse from the F center laser 6 shown in FIG. 9 is incident on the single mode optical fiber 7. Since the wavelength of the optical pulse is originally set in the anomalous dispersion region with a wavelength longer than 1.30 μm, it balances with the self-phase modulation effect in the single-mode optical fiber, and finally an optical soliton is generated. Semi-transparent mirror 8, solitary pulse from optical fiber, position-changing corner cube 9, objective lens 1
0, the second harmonic generation crystal 11, and the photodetector 12 are incident on the autocorrelator, and the position varying corner cube 9 is moved to obtain the autocorrelation. The soliton pulse width is obtained from the waveform obtained by autocorrelation. The soliton waveform thus obtained is shown in FIG. First
FIG. 11 (A) shows the optical fiber input waveform, and FIG. 11 (B) shows the optical fiber output waveform when the peak power is changed. The laser output waveform and the optical fiber output waveform do not change until the input power reaches about 1.2W. It can be seen that as the power is increased to 5 W, the pulse width begins to narrow and N = 1 solitons are formed. Furthermore, if the input is set to 11.4 W, it can be seen that higher-order solitons are excited.

[Problems to be Solved by the Invention] As described above, although it is possible to sufficiently generate optical solitons by the F center laser, soliton controllability such as repetition of solitons, pulse width variability, wavelength variability, etc. bad. For example, when pursuing the possibility of 1 Gbit / s communication due to the soliton's ultrashort pulse property, 0.1 Gbit / s is the limit as long as the current F center laser is used. Moreover, the indispensability of the cooling device lacks versatility of the laser, and it is difficult to maintain stable pulse operation.
In addition, there is a drawback that the oscillation stops when the reflection from the fiber end face or the optical system is returned to this laser,
Even slight reflection was a factor that made laser oscillation unstable.

The present invention solves the above-mentioned drawbacks by amplifying a weak optical pulse from a semiconductor laser by stimulated Raman scattering and compressing the pulse by anomalous dispersion, and provides a stable soliton pulse train with a narrow width. The purpose is to Another object of the present invention is to provide an optical soliton pulse train having a variable wavelength with a variable repetition period and variable wavelength, focusing on the variability of the repetition frequency of semiconductor laser light and the broadband property of stimulated Raman scattering.

[Means for Solving the Problems] In order to achieve such an object, the first mode of the optical soliton generating method of the present invention is to balance the self-phase modulation effect and the group velocity dispersion in the optical fiber. In generating the generated optical soliton, the pumping light from the stimulated Raman scattering pumping light source set to a wavelength longer than the zero-dispersion wavelength of the single-mode optical fiber and the optical pulse from the semiconductor laser operating in the anomalous dispersion wavelength range are described above. It is characterized in that a wavelength-variable optical soliton is generated by simultaneously incident on a single mode optical fiber and directly amplifying the optical pulse by stimulated Raman scattering.

A second aspect of the method for generating an optical soliton of the present invention is a method for generating an optical soliton generated by balancing the self-phase modulation effect and group velocity dispersion in an optical fiber, in which two continuous wave signal lights are simultaneously generated. Injected into an optical fiber and using stimulated Raman scattering and modulation instability, 2
It is characterized in that a soliton train that returns in the same way as the frequency difference between two signal lights is created.

[Operation] In the present invention, a semiconductor laser that generates a weak light pulse to generate an optical soliton is combined with a high-gain stimulated Raman scattering.

This method enables the generation of solitons by means other than the F-center laser, which has been considered impossible in the past, by the semiconductor laser and stimulated Raman scattering. In addition, since the pulse width of the semiconductor laser can be changed arbitrarily, the width of the soliton can also be changed. .

Further, the signal light that repeatedly oscillates sinusoidally due to the modulation instability can be converted into an optical soliton train, and the signal light that oscillates two continuous waves has a repetition of several THz by using stimulated Raman scattering and modulation instability. Can be converted to soliton pulse trains.

Example An example of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of an embodiment of the present invention. In the figure, 7 is a single mode optical fiber, 13 is a light source for stimulated Raman scattering excitation, 14 is a dichroic mirror, and 15 is a semiconductor laser light source for signal light.

To operate this, a light source on the longer wavelength side than the zero dispersion wavelength of 1.30 μm of the silica single mode optical fiber is used as the light source 13 for exciting Raman scattering excitation. Wavelength 1.32μm, 1.34
A YAG laser, an Er ³⁺ laser, or an F center laser having an oscillation line of μm or 1.44 μm is suitable as a light source.
Among them, the YAG laser has a high output, and the first Stokes wavelength due to Raman amplification has a shift amount of 460 cm ⁻¹ , so that the solitons are 1.41 μm, 1.43 μm, and 1.54 μm, respectively, and solitons are formed at any wavelength.

Now, this excitation light is superposed on the signal light 15 from the semiconductor laser by the dichroic mirror 14, and is guided to the single mode optical fiber 7. The dichroic mirror 14 is a mirror having wavelength selectivity, and the pumping light is totally transmitted, the signal light is totally reflected, and is efficiently coupled to the fiber. The signal light wavelength is set to the Stokes wavelength that maximizes the Raman gain. As the two waves propagate through the single mode optical fiber 7, the energy of the excitation light is transferred to the signal light and amplified by stimulated Raman scattering. The characteristics of stimulated Raman amplification when using a polarization-maintaining fiber as the amplification medium are described by M. Nakazawa, “Highly Effici
ent Raman Amplification in a Polarization-Preserv
ing Optical Fiber "(Appl.phys.Lett., Vol.46 P.628−
630, 1985), it is as shown in FIGS.
As shown in Fig. 2, the Raman gain is 20 dB or more for a 1 W continuous wave or pulse excitation input. Therefore, when the signal light input is 1 mW, the Stokes light output is 100 mW or more. Also, since pulse excitation of about 10 W can be easily performed, it is possible to obtain Stokes output of 1 W or more.

It has been theoretically shown by H. Hasegawa et al. That the solitons in an optical fiber are given by the nonlinear Schrodinger equation (H. Hasegawa and F. Tappert; the aforementioned article). It was The power required to generate N = 1 solitons in a single-mode optical fiber P _{N = 1} is the core cross-sectional area A _eff , the Stokes pulse width τ _s , the group velocity dispersion D, the nonlinear refractive index n ₂ , and the wavelength. Λ
_s (Masaka Nakazawa "Soliton Laser" Solid State Physics, Volume 21, 9 P.47, 1986). Therefore, in the case of an ordinary single mode optical fiber, | D | = 16ps / km ・ nm, τ _s = 7ps, λ
_{When s} = 1.55 μm and A _eff = 5 to 10 ^-7 cm ² , P _{N = 1} becomes P _{N = 1} = 0.5 to 1.0 W. Stokes output in stimulated Raman scattering is 0.1-1 W
It can be seen from the above calculation that an optical soliton can be easily formed because of the above. Figure 3 shows the Raman wavelength band, which has a width of 130 cm ^-1 , and when the speed of light is applied to it,
The band is 3.9 THz. That is, it can be seen that the band is wide enough to amplify a 1 ps pulse. Further, FIG. 4 shows the signal light level, which shows that an S / N ratio of about 24 dB can be obtained from the difference with the noise level, which shows that low noise soliton amplification is possible.

FIG. 5 shows a method for facilitating the generation of optical solitons. In the figure, 16 is a single-mode optical fiber for high-gain Raman amplification, for example, a single-mode optical fiber in which GeO ₂ is heavily doped in the core is used. Other configurations are the same as those of the embodiment of FIG. The optical fiber 16 has a Raman gain of about 9 times that of a normal silica fiber. Therefore, it is possible to increase the Stokes power and easily form a soliton.

FIG. 6 shows that the pumping light from the pumping light source 13 and the signal light from the signal light source 15 are made incident on the single-mode optical fiber 16 for high gain Raman amplification by the optical fiber coupler 17, and the optical soliton is connected to the optical fiber coupler 17 '. It is designed to be emitted from.

In any of the methods shown in FIGS. 5 and 6, it is possible to generate a soliton train of arbitrary repetition by changing the repetition of the semiconductor laser.

As the pump light input increases, the gain of the first Stokes light due to stimulated Raman scattering increases, but when the pump input power exceeds a certain value, the second Stokes light is generated, and the gain of the first Stokes light further increases. Absent. Therefore, the upper limit of the soliton amplitude is limited, which can be solved by the following method. When deuterium is filled in the core of a single-mode optical fiber and its Raman scattering is used, the wavelength is 1.06.
The first Stokes light becomes 1.56 μm with respect to the light of μm. Two
The next Stokes light is in the vicinity of 2 μm, but at this wavelength, the silica optical fiber does not grow because the loss is very large. Using a fiber with P ₂ O ₅ as the core material, a wavelength of 1.32
Or the first Stokes wavelength is 1.5 to 1.6 with 1.34 μm excitation.
The secondary Stokes light is suppressed in the same manner as in (1). For example, it is used that the optical loss caused by winding the fiber into a small diameter becomes extremely large in the second-order Stokes.

7 and 8 show other embodiments of the present invention. Both are two semiconductor laser light sources for signal light 15,15 '
Other configurations are the same as the example shown in FIG. 5 for the example of FIG. 7 and the example shown in FIG. 6 for the example of FIG. Here, signal light from two semiconductor laser light sources for signal light is input, and stimulated Raman scattering and modulation instability (A.Hase
gawa, Generation of a train of soliton pulses by in
duced modulational instability in optical fibers "O
pt.Lett., Vol.9, P.288 1984) and a repetitive soliton pulse train equal to the oscillation frequency difference of two signal light sources is obtained. Therefore, if the oscillation frequency difference is several THz, several THz
An ultrafast repeating soliton pulse train of is obtained. Modulation instability is a phenomenon in which a sinusoidal continuous wave with slight modulation eventually becomes a soliton due to the dispersion of the fiber and the self-phase modulation effect. The beat components of the signals from the light sources 15 and 15 ', which were weak at the fiber entrance end, are Raman amplified, and due to modulation instability, the sine wave becomes soliton. Therefore, by changing the beat frequency, a pulse train of arbitrary repetition can be output.

[Effects of the Invention] As described above, according to the present invention, a soliton pulse train having an arbitrary repetition period can be easily produced for the first time, and the width and wavelength of the soliton can be freely changed within the band of stimulated Raman scattering. There is an advantage that can be. Further, the signal light that repeatedly oscillates sinusoidally due to the modulation instability can be converted into an optical soliton train, and the signal light that oscillates two continuous waves has a repetition of several THz by using stimulated Raman scattering and modulation instability. There is an advantage that it can be converted into a soliton pulse train.

[Brief description of drawings]

1 is a configuration diagram of an embodiment of the present invention, FIG. 2 is a characteristic diagram showing a relationship between pump input and Raman gain, FIG. 3 is a characteristic diagram showing a relationship between Raman wavelength band and gain, FIG. Is a characteristic diagram showing the relationship between the pumping input and the signal light and noise level, FIGS. 5 and 6 are configuration diagrams of another embodiment of the present invention, and FIGS. 7 and 8 are further diagrams of the present invention. FIG. 9 is a configuration diagram of another embodiment, FIG. 9 is a configuration diagram of an F center laser, FIG. 10 is a configuration diagram of a conventional optical soliton generation and measurement method, and FIGS. 11 (A) and 11 (B) are each an F center. It is a laser output waveform diagram and a soliton waveform diagram. 1 ... Mode-locking YAG laser light source, 2,2 ', 2 "... F center laser resonator laser mirror, 3 ... F center laser crystal, 4 ... cooling dewar, 5 ... birefringence filter, 6 ... … F center laser, 7 …… single mode optical fiber, 8 …… semitransparent mirror, 9 …… corner cube, 10 …… objective lens, 11 …… second harmonic generation crystal, 12 …… photodetector , 13 …… Stimulated Raman scattering excitation light source, 14 …… Dichroic mirror, 15,15 ′ …… Semiconductor laser light source for signal light, 16 …… Single mode optical fiber for high gain Raman amplification, 17 …… Optical fiber coupler .

 ─────────────────────────────────────────────────── --- Continuation of the front page (72) Inventor Takashi Nakajima 162 Shirahane, Shikata, Tokai-mura, Naka-gun, Ibaraki Prefecture Nippon Telegraph and Telephone Corporation Ibaraki Telecommunications Research Institute (72) Inventor Yasuro Kimura Naka-gun, Ibaraki Prefecture Tokai-mura, Shirahoji 162, Shirane, Nippon Telegraph and Telephone Corporation, Ibaraki Research Institute of Electrical Communication (56) References JP 60-186085 (JP, A) JP 59-65828 (JP, A)

Claims (3)

[Claims] 1. A method for generating an optical soliton generated by balancing self-phase modulation effect and group velocity dispersion in an optical fiber, wherein stimulated Raman scattering is set to a wavelength longer than a zero dispersion wavelength of a single mode optical fiber. A pumping light from a pumping light source and a light pulse from a semiconductor laser operating in an anomalous dispersion wavelength region are simultaneously incident on the single-mode optical fiber, and the light pulse is tunable by directly amplifying the light pulse by stimulated Raman scattering. A method for generating an optical soliton, which comprises generating an optical soliton. 2. A method for generating an optical soliton generated by balancing self-phase modulation effect and group velocity dispersion in an optical fiber, wherein stimulated Raman scattering is set to a wavelength longer than a zero dispersion wavelength of a single mode optical fiber. Pumping light from a pumping light source and signal lights from two semiconductor lasers that continuously oscillate in an anomalous dispersion wavelength range are simultaneously incident on the single mode optical fiber, and stimulated Raman scattering and modulation instability are used,
A method for generating an optical soliton, characterized in that a soliton train that returns in the same manner as the frequency difference between the two signal lights is created. 3. The optical soliton generating method according to claim 2, wherein the frequency difference is several THz.