JPH0774875B2 - Optical soliton generation method and optical repeater amplification method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention uses an induced four-photon mixing effect in generating an optical soliton by balancing the self-phase modulation effect in an optical fiber and the negative group velocity dispersion. The present invention relates to a method for amplifying the Stokes light pulse intensity to energy sufficient for generating an optical soliton to generate a soliton.

[Prior Art] Optical solitons can be generated by balancing the self-phase modulation effect and the negative group velocity dispersion, and are very effective for ultrahigh-speed optical communication in order to realize stable and thin pulses. The methods for generating optical solitons are roughly classified into the following three.

The first method is a method in which a high power optical pulse having a wavelength of negative group velocity dispersion is made incident on an optical fiber to produce an optical soliton. The method is shown in FIG. Here, the light pulse from the high-power light source 1 is guided to the optical fiber 3 for generating an optical soliton via the coupling lens 2. The optical fiber 3 is an ordinary single-mode optical fiber, and the wavelength of the optical pulse is set to a wavelength that gives negative group velocity dispersion in this optical fiber 3. The solitonized output from the optical fiber 3 is measured by the optical pulse measuring device 4.

The second method is the soliton laser method,
The method proposed by enauer and Stolen (LFMollenauer and Stolen
RHStolen “The Soliton Laser” Opt.Lett.vol.9,13 (1
984)) and the Raman soliton laser method (HAHaus and M. Nakazawa “Theory of the
Fiber Raman Soliton Laser "J.Opt.Soc.Amer.B4,652
(1987)). Eighth method of Mollenauer and Stolen
As shown in the figure, the method using the Raman soliton laser is shown in FIG.

In FIG. 8, in the method of Mollenauer and Stolen, a color center laser crystal 7 is used as a mode-locked YAG laser 5.
To generate a light pulse in the laser constituted by the laser mirrors 6 and 6 '. A semi-transmissive mirror 8 is arranged in this laser to guide a part of the light pulse to the optical fiber 3.
As a result, the optical soliton effect is produced by the optical fiber 3, and a narrow and stable pulse is constantly produced in the resonator.

In the Raman soliton laser of FIG. 3, reference numeral 9 is a light source for generating stimulated Raman scattering in the optical fiber 3. If the round-trip propagation time of the Stokes light pulse generated by stimulated Raman scattering in the resonator constituted by the laser mirrors 6 and 6'is set to an integral multiple of the repetition period of the light source 9, the Stokes light is effectively synchronized. It is amplified and eventually becomes solitons. The wavelength of the pumping light needs to be set so that the Stokes pulse has a negative group velocity dispersion in the optical fiber 3.

The third method is a method that uses stimulated Raman scattering, which is a non-linear effect in an optical fiber that does not form a single-pass resonator, and its configuration example is shown in FIG. This case is similar to the example of FIG. 7, but it is a method in which stimulated Raman scattering is first generated and the Stokes light is converted into solitons, instead of directly generating optical solitons. Therefore, the excitation light is cut by the filter 10, only the Stokes soliton is transmitted, and the photodetector 4 detects it. It is the same as the method shown in FIG. 9 in that Raman scattering is used, but in the case of FIG. 10, the resonator is not constructed.

[Problems to be Solved by the Invention] Conventionally, among nonlinear optical effects, there has been a method of generating optical solitons by using stimulated Raman scattering, but there is no method of generating optical solitons by using the stimulated four-photon mixing effect. It was

The following is the stimulated four-photon mixing effect (Stimulated Four Photon M
ixing (hereinafter abbreviated to SFPM)). When a single-mode fiber is excited by a Q-switched mode-locked high-intensity pulse, Stokes light and anti-Stokes light are shifted to a wavelength where the sum of material dispersion ΔK _m and waveguide dispersion ΔK _w becomes zero Occurs. However, since SFPM uses high-intensity pulse which is more than 10 times higher than stimulated Raman scattering, other nonlinear effects are induced, and therefore the controllability is extremely poor and the generation of solitons is difficult.

Therefore, an object of the present invention is to provide stimulated four-photon mixing (Stimulated Four Ph
The objective is to provide a method for efficiently generating optical solitons with a low pumping input by setting the pumping wavelength near the zero-dispersion wavelength of the optical fiber by utilizing a nonlinear optical effect called oton mixing (SFPM).

[Means for Solving the Problems] In order to achieve such an object, the present invention provides an optical fiber including an excitation light pulse, a Stokes light pulse, and an anti-Stokes light pulse, which is made to enter an optical fiber. In the process of stimulated four-photon mixing, the Stokes light pulse is induced in the negative group velocity dispersion region and the anti-Stokes light pulse is made positive by setting the excitation light pulse at or near the zero dispersion wavelength of the optical fiber. Induction in the group velocity dispersion region, and at that time, the interaction delay is increased by reducing the group delay between the pump light, the Stokes light pulse, and the anti-Stokes light pulse, and the energy of the Stokes light pulse is increased. By so doing, the self-phase modulation effect and the negative group velocity dispersion are balanced to make the Stokes light pulse an optical soliton. To do.

In another aspect of the present invention, the method according to claim 1 is characterized in that the wavelength of the generated optical soliton is made variable by changing the light intensity of the pumping light pulse.

In still another aspect of the present invention, in performing optical repeater amplification between each of the sequential optical fiber transmission lines by using the method according to claim 1 or 2, one of the sequential optical fiber transmission lines is used. A certain one optical fiber transmission line is used as the optical fiber according to claim 1, and an optical soliton obtained by causing a pumping light pulse and a signal light to enter the optical fiber in synchronism with each other is used as a second signal light. A light obtained from an optical fiber that constitutes a fiber transmission line and is made incident on the second optical fiber used as the optical fiber according to claim 1 and also an excitation light pulse corresponding to the optical fiber is made incident on the optical fiber. All-optical soliton transmission is performed by further injecting a soliton into the next optical fiber transmission line as a signal light pulse.

[Operation] In the present invention, in the pulsed operation of induced four-photon mixing in the optical fiber, the pumping light wavelength is set to the zero dispersion wavelength of the optical fiber or in the vicinity thereof, and the negative group velocity dispersion occurs on the long wavelength side. Stokes Make light solitons.

In the present invention, by setting the wavelength of the pumping light to the zero dispersion wavelength of the optical fiber or in the vicinity thereof, Stokes light,
The group delay time difference between the anti-Stokes light and the pump light is made small, and as a result, the interaction length is made long and the SFPM is efficiently generated. As the light source, a mode-locked pulse having a good characteristic, which has been conventionally used to generate stimulated Raman scattering, is used. In that case, since the interaction length can be long, there is an advantage that the SFPM can be generated even with a low output mode-locked pulse without Q switching.

In addition, the wavelength of the soliton generated could not be tuned by a laser other than Raman soliton laser.
In the present invention, the wavelength of soliton generation can be changed by changing the incident light intensity through the nonlinear phase change caused by the optical Kerr effect.

Embodiments Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a diagram showing the configuration of the first embodiment of the present invention.
This embodiment is similar in configuration to the conventional soliton generation method by stimulated Raman scattering shown in FIG. 10, but is completely different in principle in that SFPM is used. In FIG. 1, 9 is a light source for generating a pumping light pulse, 11 is an optical isolator,
The excitation light pulse from the light source 9 is guided to the optical fiber 12 by the coupling lens 2 through the optical isolator 11. Here, the optical fiber 12 is an optical fiber whose zero dispersion wavelength is within the variable wavelength range of the light source 9. This optical fiber
Optical filter for output light from 12 through coupling lens 2 '
Take out from 10. The optical filter 10 is an optical filter that cuts anti-Stokes light and excitation light of SFPM and allows only Stokes optical solitons to pass through.

To operate this, the wavelength of the light source 9 is set at or near the zero dispersion wavelength of the optical fiber 12 and the optical fiber 12 is excited. In this way, the group velocity difference between the pump light pulse and the Stokes light pulse and the anti-Stokes light pulse is small, so that the interaction occurs over a long distance, and the Stokes light pulse is greatly amplified into an optical soliton. To go.

FIGS. 2A to 2C show the relationship of the group delay with respect to the pumping light having the three kinds of wavelengths. FIG. 2 (a) satisfies the above conditions, and it is difficult to generate optical solitons in FIGS. 2 (b) and 2 (c). Here, V _p is the group velocity of the excitation light, V _s is the group velocity of the Stokes light, and V _as is the group velocity of the anti-Stokes light.

In the SFPM, assuming that the amount of phase change of material dispersion, structural dispersion and nonlinear refractive index is ΔK _m , ΔK _w and ΔK _SPM , respectively, the sum of them, that is, the total amount of phase change is zero (momentum is preserved). Occurs under the conditions of. That is, ΔK _m + ΔK _w + ΔK _SPM = 0 (1) where ΔK _m = n _a ω _a / c + n _s ω _s / c-2n _p ω _p / c (2) ΔK _w = Δn (b _a ω _a / c + b _s ω _s / c-2b _p ω _p / c) (3) Is.

Where n _a is the refractive index of the optical fiber at the wavelength of the anti-Stokes light n _s is the refractive index of the optical fiber at the wavelength of the Stokes light n _p is the refractive index of the optical fiber at the wavelength of the pumping light n _{2 is the} light Nonlinear refractive index in fiber Δn: Refractive index difference between core and clad in optical fiber ω _a : Frequency of anti-Stokes light ω _s : Frequency of Stokes light ω _p : Frequency of pumping light c: Speed of light b _a : Frequency of anti-Stokes light Normalized propagation constant at ω _a b _s : Normalized propagation constant at frequency ω _{s of} Stokes light b _p : Normalized propagation constant at frequency ω _{p of} pump light.

FIG. 3 shows the Stokes shift amount Ω on the horizontal axis and the phase change amount on the vertical axis under these conditions. This is a result when a pumping light wavelength is set equal to the zero dispersion wavelength using a single mode optical fiber having a zero dispersion wavelength of 1.525 μm. In the figure, − ・ − is ΔK _w , ... is ΔK _m , −
Is ΔK _w + ΔK _m , and is −ΔK _SPM . However, the excitation light intensity Ip = 5 W, pulse width 13 ps, effective area 5 × 10 ^-7 cm ² ,
ΔK _SPM was calculated for the case of n ₂ = 3.2 × 10 ⁻¹⁶ cm ² / w.
Therefore, the condition of ΔK _m + ΔK _w + ΔK _SPM = 0 at which phase matching is established is given by ●. Therefore, an optical soliton whose frequency is shifted by about 130 cm ^{−1 with} respect to the excitation light can be generated.

As for the gain of SFPM, the amount of phase mismatch is γ (= ΔK _m + ΔK _w + Δ
K _SPM ), the light intensity is Ip, and the third-order nonlinearity is x ⁽³⁾ (Ω) g = [α ² x ⁽³⁾ (Ω) ² Ip ² − (γ / 2) ² ] ^{1 / It} is expressed as ² (5). However, α is a proportional constant determined by the structure of the optical fiber. As is clear from the equation (5), γ = 0
When (ie, ΔK _m + ΔK _w + ΔK _SPM = 0), the phase is perfectly matched and g = α x ⁽³⁾ (Ω) Ip (6). Therefore, when the pumping light intensity of the light source 9 is increased, the gain of SFPM is increased, and highly efficient soliton amplification is possible. As the optical fiber 12, a normal 1.3 μm or 1.5 μm band zero dispersion fiber can be used. Alternatively, those polarization maintaining fibers may be used.

Further, in the method of the present invention, as shown in equation (4), Δ
Since K _SPM changes according to the light intensity Ip of the excitation light, when Ip is changed, the Stokes shift amount at which ΔK _m + ΔK _w + ΔK _SPM = 0 becomes different. Therefore, a wavelength-variable soliton pulse can be generated by changing the intensity of the pumping input light.

Actually, using a color center laser as the light source 9,
Length 2 with zero dispersion at wavelength 1.525 μm as optical fiber 12
The results of soliton generation when a fiber of 50 m is used are shown in FIGS. 4 (a) and 4 (b).

Fig. 4 (a) shows the soliton waveform measured by an autocorrelator, and its full width _at half maximum Δτ _FWHM is 210fs (0.21ps).
Is.

On the other hand, the measurement result of the spectrum by the spectrometer is shown in FIG. Here, the broken line portion is the portion cut by the optical filter 10, and the solid line portion is the Stokes soliton spectrum. This solid line portion has a clean spectrum, and its full width _at half maximum Δν _FWHM is 13.8 nm.
The product of both, Δτ and Δν, is ΔνΔτ = 0.35, which means that Hyperbol
It can be seen that it is a Transform Limited pulse assuming an ic-secant pulse. In this way, it can be demonstrated that optical solitons can be formed by SFPM.

A second embodiment of the present invention is shown in FIG. In the method of the embodiment of the present invention described above, the noise of Spontaneous Emission was used for the rise of the Stokes light. In the case of FIG. 5, the signal light pulse Is corresponding to the Stokes light is generated. Prepare in advance with the vessel 13. That is, the excitation light pulse Ip from the excitation light source 9
An optical fiber 12 which combines the signal light Is of the signal light source 13 and the signal light Is of the signal light source 13 by the optical coupler 14 to generate the output light Ip + Is of SFPM.
Lead to. The weak signal light Is is pulse-amplified by SFPM,
Eventually it becomes a soliton. In this case, in order to have an effective interaction, the light sources 9 and 13 are synchronized by the synchronizer 15, and when the excitation pulse propagates through the optical fiber 12, the signal light is set to overlap with it. is important.

FIG. 6 shows an embodiment in which the second embodiment of the present invention is applied to optical repeater amplification.

In FIG. 6, sequential optical fiber transmission lines are indicated by reference numerals 12-1, 12-2, ..., 12-n corresponding to the optical fiber 12 in FIG. Each of 14 is shown by reference numerals 9-1, 9-2, ... And 14-1, 14-2, ... Corresponding to each transmission path.

That is, 12-1, 12-2, ..., 12-
In performing optical repeater amplification between n intervals, an optical soliton is generated from the optical fiber transmission line 12-1 based on the pump light pulse Ip from the pump light source 9-1 and the signal light Is from the signal light source 13. Let The optical soliton from the optical fiber transmission line 12-1 is used as the second signal light for the next optical fiber transmission line 12-1.
-2, and the pumping light pulse from the pumping light source 9-2 assigned to this optical fiber transmission line 12-2 is also made to enter this optical fiber transmission line 12-2. As a result, the second signal light and the pumping light pulse synchronized with the second signal light are incident on the optical fiber transmission line 12-2, and optical solitons are generated accordingly. This optical soliton, that is, the third signal light and the pumping light pulse from the third pumping light source 9-3 is further incident on the next optical fiber transmission line 12-3, and optical repeater amplification is sequentially performed. To do.

In the example of FIG. 6, in synchronizing each pumping light pulse in the second and subsequent stages with each signal light, the optical couplers 14-1, 1
The input form of the pumping light pulse input to 4-2, ... is different from that in the case of FIG. 5 to synchronize both optical inputs. Instead of doing this, the synchronizing signal from the synchronizer 15 is used. The light sources for excitation 9-1, 9-2, ... May be supplied to be synchronized with each other.

According to this method, a gain that overcomes the loss in the optical fiber is generated, and all-optical soliton transmission can be performed over a long distance.

Furthermore, in this method, by setting different pumping light intensities of the pumping light sources 9-1, 9-2, ... In each repeater, optical amplification at different wavelengths is possible, so It becomes possible to perform soliton amplification.

[Effects of the Invention] As described above, according to the present invention, the SFPM in the optical fiber which has conventionally been poor in controllability
The dispersion has a wavelength at or near zero and is excited by a mode-locked optical pulse train to reduce the group delay time difference between Stokes light, anti-Stokes light, and pump light, and as a result, It takes a long action length to efficiently generate SFPM and generate optical solitons.

As a result, it is possible to provide a large-capacity light source for ultra-high-speed optical communication, which will be the mainstream of future optical communication, and to configure an optical repeater by using the amplification effect of an optical soliton using stimulated four-photon mixing. The signal transmission distance can be dramatically increased.

Furthermore, the present invention has an advantage that the wavelength of the Stokes soliton can be varied by changing the intensity of the excitation light. In the conventional method, the wavelength of the soliton cannot be changed without changing the wavelength of the incident excitation light, but in the present invention, the soliton wavelength can be changed only by changing the intensity of the excitation light. Therefore, according to the present invention, by using this wavelength tunability, there is also an advantage that solitonic amplification of multiple wavelengths can be performed by simply changing the pumping light intensity.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of the present invention, FIG. 2 is an explanatory diagram of the optimum conditions of the operating wavelength of the present invention, and FIG. 3 is an explanatory diagram of the operating principle of the present invention. FIGS. 4 (a) and 4 (b) are explanatory diagrams showing how the wavelength shift amount is determined, and FIG. 4 (a) and FIG. 4 (b) are respectively a waveform diagram and a spectrum diagram showing a soliton waveform and its spectrum, respectively, measured in one embodiment of the present invention. FIG. 5 is a configuration diagram showing a second embodiment of the present invention, FIG. 6 is a configuration diagram showing a third embodiment of the present invention, and FIG. 7 is a configuration showing a simplest conventional soliton generation method. FIGS. 8 and 9 are configuration diagrams showing two examples of conventional optical soliton lasers, and FIG. 10 is a configuration diagram showing a conventional optical soliton generation method by stimulated Raman scattering which is a nonlinear optical effect. 1 ... High-power light source, 2 ... Coupling lens, 3 ... Single-mode optical fiber, 4 ... Optical pulse detector, 5 ... Mode-locking YAG laser, 6,6 '... Laser mirror, 7 ... … Color center laser medium, 8 …… Semi-transparent mirror, 9,9-1,9-
2, ……… Mode-locked pulse light source, 10 …… Optical filter, 11 …… Optical isolator, 12, 12-1, 12-2,… 12-n ……
Dispersion-shifted single-mode fiber or its polarization-maintaining fiber, 13 ... Signal light source, 14, 14-1, 14-2, ...
…… Optical coupler, 15 …… Pump optical signal optical synchronizer.

Claims (3)

[Claims] 1. A pumping light pulse is incident on an optical fiber, and in the process of stimulated four-photon mixing in the optical fiber consisting of the pumping light pulse, the Stokes light pulse, and the anti-Stokes light pulse, the pumping light pulse is supplied to the optical fiber. By setting the zero dispersion wavelength of the fiber or in the vicinity thereof, the Stokes light pulse is induced in the negative group velocity dispersion region, and the anti-Stokes light pulse is induced in the positive group velocity dispersion region, and at that time, By increasing the interaction length by reducing the group delay between the pump light, the Stokes light pulse and the anti-Stokes light pulse, and increasing the energy of the Stokes light pulse,
An optical soliton generating method, wherein the Stokes light pulse is converted into an optical soliton by balancing the self-phase modulation effect and the negative group velocity dispersion. 2. The optical soliton generating method according to claim 1, wherein the wavelength of the optical soliton generated is variable by varying the light intensity of the excitation light pulse. 3. When performing optical repeater amplification between successive optical fiber transmission lines using the method according to claim 1 or 2, one of the sequential optical fiber transmission lines is provided. 2. The optical fiber transmission line according to claim 1 is used as the optical fiber according to claim 1, the excitation light pulse and the signal light are synchronously incident on the optical fiber, and the signal light is optically amplified by the induced four-photon mixing. The optical soliton to be used as the second signal light constitutes the next optical fiber transmission line, and is made incident on the second optical fiber used as the optical fiber according to claim 1 and the excitation light corresponding to the optical fiber. A pulse is also incident on the optical fiber, and an optical soliton obtained from the optical fiber is further incident on the next optical fiber transmission line as the signal light pulse to generate all-optical light. Optical repeater amplifier wherein the performing soliton transmission.