JP2001318397A - Light noise suppressing circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light noise suppressing circuit which can suppress signal fluctuations without performing discrimination nor reproduction at an electric stage and responds to a very fast signal. SOLUTION: This circuit is equipped with an optical fiber 6 as an optical nonlinear medium, an optical parametric amplifier having a pump light source 2 as a means for inputting pump light to the optical fiber 6, a means for inputting signal light to the optical parametric amplifier so that gain saturation is caused, and an optical filter 7 as a means for outputting signal light, i.e., output light 8 from the optical parametric amplifier.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical noise suppressing circuit for suppressing signal fluctuation due to noise light superimposed on signal light while propagating through an optical fiber transmission line, for example.

[0002]

2. Description of the Related Art In optical fiber transmission, noise light such as spontaneous emission light of an optical amplifier inserted into an optical fiber transmission line and four-wave mixing light generated in the optical fiber transmission line during wavelength division multiplexing transmission is generated. The signal fluctuation caused by the superposition of the noise light on the signal light is a factor limiting the transmission distance. Therefore, when performing long-distance transmission, it is necessary to remove signal fluctuation due to noise light for each predetermined section.

[0003] A first conventional technique for removing signal fluctuation is as follows.
The optical signal is once converted to an electrical signal, and the electrical stage identifies and identifies the signal.
This is a method of converting the optical signal again after performing the reproduction.

[0004] A second conventional technique for removing signal fluctuation is as follows.
This is a method for removing signal fluctuation without converting an optical signal into an electric signal. In this regard, several methods have been proposed, but the most actively studied at present are those using an interferometer with a built-in optical nonlinear medium.

FIG. 12 is a diagram showing a configuration example of a conventional interferometer incorporating an optical nonlinear medium.

Reference numeral 61 denotes a signal light having a wavelength λ _s , and 62 denotes a signal light having a wavelength λ _c
, 63, 64 are paths, 65 is an optical nonlinear medium, 6
6 optical filters, 67 is the output light of the wavelength lambda _s.

[0007] As shown in FIG. 12, the interferometer, after a steady light 62 with a wavelength lambda _c is branched into two by a route 63 and 64, and the basic structure of a Mach-Zehnder interferometer are combined again. An optical nonlinear medium 65 is inserted into one path 63 of the Mach-Zehnder interferometer. And
Signal light 61 having a wavelength lambda _s is has a configuration which is incident on the optical nonlinear medium 65.

FIG. 13 is a diagram showing the principle of operation of the interferometer of FIG.

The output light 67 from the Mach-Zehnder interferometer
Changes sinusoidally according to the optical length difference between the two paths 63 and 64. In FIG. 12, when the signal light 61 is incident on the optical nonlinear medium 65, the refractive index of the optical nonlinear medium 65 changes according to the power of the signal light 61, whereby the optical length difference of the interferometer changes. Therefore, the power of the output light 67 of the stationary light 62 from the interferometer changes sinusoidally according to the input power of the signal light 61 as shown in FIG. Here, it is assumed that the signal light 61 has its light intensity modulated on / off. Then, the on-level of the signal light 61 reaches the peak of the transmission characteristic with respect to the transmission characteristic of FIG.
It is assumed that the off level of the signal light 61 is set to be at the bottom of the transmission characteristic.

In the above configuration, a signal light 61 having a waveform 71 whose power level fluctuates due to noise as shown in FIG. 13 is input. Then, since the on / off level of the signal light 61 is located at the peak / bottom of the sine wave characteristic, the output waveform 72 of the stationary light 62 has a level fluctuation suppressed as shown in FIG. Becomes Therefore, if taken out only the output light 67 having a wavelength lambda _s by the optical filter 66, it is possible to obtain an optical signal noise is suppressed.

[0011]

In the first prior art, since the signal is identified and reproduced in the electric stage, the configuration becomes complicated. Further, when processing high-speed signals, an identification / reproduction circuit that operates at high speed is required.

In the second prior art, it is necessary to integrate the elements into one chip in order to operate the interferometer stably. Therefore, it is usually possible to make one chip, and
A semiconductor optical amplifier having a large nonlinearity is used as an optical nonlinear medium. However, the response time of the semiconductor optical amplifier is limited, and cannot follow an ultra-high-speed signal.

An object of the present invention is to provide a second conventional technique which can suppress signal fluctuation without performing identification and reproduction in an electric stage as in the first prior art, and cannot follow an ultra-high-speed signal. It is another object of the present invention to provide an optical noise suppression circuit that responds to an ultra-high-speed signal.

[0014]

In order to solve the above-mentioned problems, an optical noise suppressing circuit according to the present invention comprises: an optical parametric amplifier having an optical nonlinear medium; and means for inputting pump light to the optical nonlinear medium. The optical parametric amplifier includes means for inputting signal light in a state where gain saturation occurs, and means for outputting the signal light from the optical parametric amplifier.

Further, the optical nonlinear medium is an optical fiber.

According to the present invention, with the above configuration, it is possible to provide an optical noise suppressing circuit capable of suppressing the level fluctuation superimposed on the high-speed signal light in the light state.

Further, the optical noise suppression circuit of the present invention, an optical fiber, and means for inputting the pumping light of said light Faibahe wavelength lambda _p, amplifies the signal light of wavelength lambda _s, 1 / lambda _out = wavelength λ satisfying 2 / λ _s −1 / λ _p
an optical parametric amplifier which generates light _out, with respect to the optical parametric amplifier, and means a state in which gain saturation occurs inputs the signal light of the wavelength lambda _s, the output light of the wavelength lambda _out from the optical parametric amplifier Means for performing the operation.

According to the present invention, it is possible to provide an optical noise suppressing circuit capable of suppressing a level fluctuation superimposed on a high-speed signal light in a light state without deteriorating a signal extinction ratio by the above configuration.

Further, the apparatus is characterized in that it comprises means for expanding the spectrum of the pump light.

Further, the spectrum expanding means is a phase modulator.

Further, the pump light is oscillation light of a semiconductor laser, and the spectrum expanding means is a modulation circuit of an injection current to the semiconductor laser.

[0022]

Embodiments of the present invention will be described below in detail with reference to the drawings. In the drawings described below, those having the same functions are denoted by the same reference numerals, and the repeated description thereof will be omitted.

The present invention utilizes a phenomenon called optical parametric amplification that occurs in an optical nonlinear medium. First, this phenomenon will be described.

When light propagates through a certain medium, the polarization field P induced by the light is approximately proportional to the electric field E, but a very small distortion component is generated. This is usually expressed as the developed form of the electric field E as follows.

P = χ ₁ : E + χ ₂ : EE + χ ₃ : EEE + (1) In the equation (1), the first term is a linear term, the second term is a second-order nonlinear term,
The third term is a third-order nonlinear term. When a plurality of light waves propagate in this medium, mixing between the light waves occurs via a nonlinear term.

For example, three light waves having different light frequencies are the same.
It propagates in one direction. E = (1/2) A_pexp [-i (ω_ptk_pz)] + cc. + (1/2) A_sexp [-i (ω_stk_sz)] + cc. + (1/2) A_iexp [-i (ω_itk_iz)] + c.c. (2), and by substituting into equation (1), various terms can be calculated from the nonlinear term.
A polarization field of the wave number component occurs. Where ω_p, Ω_s, Ω_i
Is the angular frequency of each light wave, A_p, A_s, A_iIs
Elementary amplitude, k_p, K_s, K_iAre the respective propagation constants, t is
Time, z is the propagation coordinates, and cc is the complex conjugate. Polarization field
Generates light of that frequency component and adds it to the original light.
Is done. For example, from the third-order nonlinear term, ω_p+ Ω_p−ω_p
= Ω_p, Ω _p+ Ω_s−ω_s= Ω_p, Ω_p+ Ω_i−ω_i=
ω_pNon-linear light with frequency components such as
A_pIs added to Furthermore, three frequencies are ω_s−ω_p
= Ω _p−ω_iIf they are equally spaced like this (ω_p
Gamanchu), -ω_p+ Ω_s+ Ω_i= Ω_pA component is also A
_pIs added to A similar addition is A_s, A _i
Also happens about. Maxwell equation
When described by an expansion expression based on

DA_p/ Dz = iγ [｛| A_p|²+2 | A_s|²+2 | A_i|²｝ A_p+2 A_p ^*A_sA_ie^iΔkz(3a) dA_s/ Dz = iγ [｛| A_s|²+2 | A_p|²+2 | A_i|²｝ A_s+ A _p ² A_i ^*e^−iΔkz(3b) dA_i/ Dz = iγ [｛| A_i|²+2 | A_p|²+2 | A_s|²｝ A_i+ A _p ² A_s ^*e^−iΔkz(3c) γ is a constant representing coupling between light waves, Δk = k_s+ K
_i-2k_pIt is. No propagation loss for simplicity
I watched.

Here, as an initial state, a high power ω _p
It is assumed that light and ω _s light having relatively low power enter the medium. Hereinafter, the former is called pump light, and the latter is called signal light (signal light). Then, from the fourth term on the right side of the equation (3c), ω _i
Light is generated (this light is called idler light), and the three light waves propagate through the medium while interacting according to the equations (3a) to (3c). Here, if Δk ≒ 0, the added nonlinear term acts to reduce the optical power for the pump light and increase the optical power for the signal light and the idler light due to the phase relationship with the original light. I do.
The manner of interaction depends on the phase relationship between the lights, and in the case of the above-mentioned initial state, power transfer from pump light to signal light and idler light occurs. That is, the signal light power is amplified. This is a phenomenon called optical parametric amplification.

It is known that the interaction between the three light waves occurs efficiently when the phase matching is satisfied. Intuitively, this is a condition under which light generated by nonlinear polarization is added to the original light in the same phase.

The source of the power received by the signal light is the pump light. Therefore, when the amplified signal light power is large, the pump light decreases accordingly, and the amplification rate of the signal light decreases accordingly. This is called gain saturation.

FIG. 3 is a diagram showing the gain saturation characteristic of the optical parametric amplifier, in which the amplification gain with respect to the input signal light power is obtained by calculation.

In the calculation, a communication optical fiber is assumed as an optical nonlinear medium, γ = 2.4 / km · W, fiber length =
The values of 2.5 km and input pump light power = 0.5 W were used.

As shown in FIG. 3, in the region where the input signal light power is small, the amplification gain is almost constant because the amplification light power is small and the amount of reduction in the pump light power is small.
If the input signal light power is large, gain saturation occurs. When this relationship is rewritten as input signal light power versus output signal light power, the result is as shown in FIG.

FIG. 4 is a diagram showing the signal light input / output characteristics of the optical parametric amplifier.

The decrease in the gain means that the change rate of the output signal light power becomes smaller with respect to the change of the input signal light power. Therefore, as shown in FIG. 4, there is a region where the amount of change in the output signal light power is small even if the input signal light power changes.

Using the gain saturation characteristics described above,
Noise suppression of signal light becomes possible.

Embodiment 1 FIG. 1 is a diagram showing a configuration of an optical noise suppression circuit according to Embodiment 1 of the present invention.

1 is a signal light of wavelength λ _s , 2 is a pump light source,
3 the pump light of wavelength lambda _p, the 4 polarization control circuit, the fifth optical multiplexer, an optical fiber, 7 is an optical nonlinear medium 6 optical filter, 8 is the output light of the wavelength lambda _s.

The optical noise suppressing circuit according to the present embodiment includes an optical parametric amplifier having an optical fiber 6 which is an optical nonlinear medium, and a pump light source 2 which is a means for inputting the pump light 3 to the optical fiber 6. The parametric amplifier includes means for inputting the signal light 1 in a state where gain saturation occurs, and an optical filter 7 for outputting the signal light 1, that is, the output light 8 from the optical parametric amplifier.

[0040] For example the optical fiber transmission line wavelength lambda _s signal light 1 that has been transmitted to can be multiplexed by the optical multiplexer 5 with pump light 3 of wavelength lambda _p, is configured to be input to the optical fiber 6 ing. Then, the output stage of the optical fiber 6 is provided with an optical filter 7 for transmitting light of wavelength lambda _s. The optical fiber 6 is used as an optical nonlinear medium, and the configuration of FIG. 1 functions as an optical parametric amplifier for the signal light 1. In general, the optical parametric characteristics depend on the polarization state of the input signal light 1. The polarization control circuit 4 at the output stage of the pump light source 2 is provided to match the polarization state of the pump light 3 with the signal light 1.

As described in the section of the prior art, the signal light transmitted through the optical fiber transmission line is composed of spontaneous emission light generated by an optical amplifier and four-wave mixing generated in an optical fiber during wavelength division multiplex transmission. The signal level fluctuates due to waves. The main cause of the level fluctuation is due to the interference effect between the signal light and the noise light. When noise light of the same wavelength is superimposed on the signal light, they interfere with each other, but the phase relationship between the signal light and the noise light fluctuates randomly with time, so that the power level resulting from the interference also fluctuates randomly. become. As a result, the signal light level fluctuates. Since this level fluctuation occurs due to the interference effect between the signal light and the noise light, it occurs only when the signal light exists. When the signal light is off, the level fluctuation due to the power fluctuation of the noise light occurs. However, since the noise light is usually smaller than the signal light,
The level fluctuation is smaller than the fluctuation when the signal light is on. That is, the signal light transmitted through the optical fiber transmission line mainly fluctuates in on-level.

FIG. 2 is a diagram showing the principle of operation of the first embodiment of the present invention.

Such a signal light is input to an optical noise suppression circuit having an optical parametric amplifier configured as shown in FIG. Here, using an ordinary optical fiber amplifier or the like, as shown in FIG. 2, the ON level of the input signal light 1 is set so that the change rate of the output light 8 is small with respect to the change of the input signal light 1. Set. Then, even if the on-level of the input signal light 1 fluctuates as shown by the waveform 21, the signal light 8 whose level fluctuation is suppressed is output at the output stage as shown by the waveform 22. That is, the signal light 8 in which noise is suppressed is output.

According to the above principle, an optical noise suppressing circuit can be realized. The response speed of the optical noise suppression circuit is determined by the response speed of the optical fiber 6 used as the optical nonlinear medium. It is known that the response time of the nonlinearity of an optical fiber is very fast, on the order of femtoseconds. Therefore, it can be operated with respect to the signal light having a higher speed than the conventional technology.

The above is the optical noise suppressing circuit according to the first embodiment of the present invention. In the first embodiment, the optical noise suppressing effect is obtained, but the extinction ratio of the output signal light is lower than that of the input signal. There is a problem that. Embodiment 2 of the present invention described below
Is to solve this problem.

Prior to the description of the second embodiment, the problem of the first embodiment will be described based on experimental results.

FIG. 5 is a diagram showing a measurement result of optical parametric amplification actually obtained by the present inventor in an experiment.

In this experiment, a dispersion-shifted fiber having a length of 2.5 km and a zero dispersion wavelength of 1550.7 nm was used as an optical nonlinear medium.
m, pulse pump light having a peak power of 1.28 W and stationary signal light having a wavelength of 1544.1 nm were input, and the signal light input power versus the signal light output power were measured. This figure shows how the signal light is amplified and output by the parametric amplification operation.

In a region where the signal light input power is small, the amplification factor is constant at about 45 dB, and the signal light output power increases in proportion to the signal light input power. However, the amplification rate decreases as the signal light input power increases, and the increase rate of the signal light output power decreases. And the signal light input power =
The signal light output power becomes maximum around −14 dBm, and thereafter starts decreasing.

The measurement results of FIG. 5 basically show the same characteristics as the calculation results of FIG. However, there is a difference between the plotting up to the region where the signal light output power starts to decrease and the point of dB display (linear display in FIG. 2) for the following description.

As described in the first embodiment, the noise of the input signal light can be suppressed by using the characteristics shown in FIG. That is, as shown in FIG. 6, the on-level of the input signal light is set to the level at which the output becomes maximum, and is input to the optical parametric amplifier. In the vicinity of the maximum output, the change in the signal light output power is substantially flat with respect to the change in the signal light input power. Due to this characteristic, a signal light whose level fluctuation is suppressed is output. That is, the noise of the input signal light can be suppressed.

The above is the operation principle of the first embodiment. However, in the first embodiment, there is a problem that the ON / OFF level ratio of the signal light, that is, the extinction ratio is deteriorated due to the noise suppressing action. is there.

The off-level of a digital optical signal is ideally zero, but actually has a finite value. For signal transmission, it is desirable that the extinction ratio be large. In the first embodiment, the gain saturation characteristic of the optical parametric amplifier is used. At the time of the ON state input, the amplifier is in a gain saturated state. On the other hand, when the input is in the OFF state, the amplifier is in an unsaturated state because the input level is low. Therefore, the signal gain for the on state is smaller than the signal gain for the off state. As a result, the extinction ratio of the output signal light deteriorates more than the input signal.

For example, it is assumed that a signal light having an on-level of -14 dBm and an off-level of -26 dBm is input to the optical parametric amplifier having the characteristics shown in FIG. Then, from FIG. 5, the ON level of the output light is 24 dB.
m, and the off level is 17 dBm. That is, the extinction ratio of the output light is 7 dB with respect to the extinction ratio of the input light of 12 dB, and the extinction ratio is degraded by 5 dB.

Embodiment 2 In order to solve this problem, in Embodiment 2 of the present invention,
Utilizes higher-order optical parametric processes. First, the higher-order optical parametric process will be described.

The interaction between the three frequency lights ω _p , ω _s , and ω _i , which is the basic process of the optical parametric amplification, has been described above. However, when the signal light and the idler light become sufficiently large by the parametric amplification, From these sources, further new frequency light can be generated. Returning to the original equation, substituting equation (2) into equation (1),
It can be seen that there is a frequency component ω _p + ω _s −ω _i ≡ω _F in addition to the three frequency lights. That is, light having a frequency omega _F (hereinafter, referred to as omega _F frequency light) is generated. However, in order to generate new light efficiently, it is necessary that the phase matching is satisfied in a parametric process for generating the new light. In fact, using an optical fiber as the optical nonlinear medium, the pump light power and the pump wavelength and the signal light wavelength appropriately set, the phase matching of the parametric process of generating the frequency light almost satisfied, omega _F frequency light Generated efficiently.

FIGS. 7 (a) and 7 (b) show, in an actual experiment by the inventor, a dispersion-shifted optical fiber having a zero dispersion wavelength of 1550.7 nm and a length of 2.5 km, and a wavelength of 155.
It is a figure which shows the output light spectrum at the time of inputting the pump light of 4.1 nm and the signal light of wavelength 1549.5 nm. The pump light input power is 1.3 W and the signal light input power is FIG.
In FIG. 7A, it was 13 nW, and in FIG. 7B, it was 1.1 μW.

When the signal light input power is small (FIG. 7)
In (a)), a basic parametric process mainly occurs, and pump light, signal light, and idler light are mainly output. On the other hand, when the signal light input power is large (FIG. 7B), a new frequency light is output on the short wavelength side of the signal light in addition to the three light waves. This is the light of a frequency omega _F mentioned above. generation of omega _F frequency light, the center wavelength of the pump light and the signal light, matches the zero dispersion wavelength of the optical fiber, or, effectively occurred when it from a somewhat wavelength position shifted to the long wavelength side Was.

FIG. 8 is a diagram showing the results of measuring the output light power of each frequency light when the signal light input power is changed in the above experimental configuration.

In the region where the signal light input power is small, the output power of the signal light and the idler light increases in proportion to the signal light input power. When the output power of the signal light and the idler light becomes larger, omega _F frequency light comes increased.
When the ω _F- frequency light increases, the output characteristic exhibits a different behavior from the ordinary optical parametric amplification because of the parametric process that generates this light. That is, in the ordinary optical parametric amplification, the signal light and the idler light have the same output characteristics, but in FIG. 8, the signal light starts to saturate faster than the idler light as the signal light input power increases.
Then, the output becomes maximum around the input power = −15 dBm, and thereafter starts to decrease. On the other hand, the idler light is increasing as it is, although the inclination is gentle.

Here, attention is paid to the ω _F frequency light. FIG.
FIG. 9 is a diagram in which data of the ω _F frequency light is re-plotted from the above experimental results.

As the signal light input power increases, the signal light output power increases to a certain input level (、 −15).
(dBm), and then starts decreasing. It should be noted here that the slope of the increase in the increase region (small signal region) is steeper than the proportional line. (For reference, a proportional straight line, that is, a dashed line with a slope of 1 is shown.) In this region, the output light power is
It increases in proportion to the square of the input light power. By utilizing this characteristic, it is possible to obtain an optical noise suppressing operation without deterioration of the extinction ratio, as described below.

FIG. 10 is a diagram showing a configuration of the optical noise suppressing circuit according to the second embodiment of the present invention.

[0064] 1 signal light of wavelength lambda _s, 2 pump light source,
3 the wavelength lambda _p pumping light, the polarization control circuit 4, 5 optical multiplexer, 6 optical fiber is an optical nonlinear medium, the 13 optical filter, 14 is the output light of the wavelength lambda _out.

[0065] Light noise suppression circuit of the present embodiment, the optical fiber 6, and a pump light source 2 is means for inputting a pump light 3 of the optical fiber 6 F wavelength lambda _p, the signal light of wavelength lambda _s 1 and 1 / λ _out = 2 / λ _s −1
An optical parametric amplifier that generates light of wavelength λ _out that satisfies / λ _p ,
Means for inputting the signal light 1 having the wavelength λ _{s in} a state where the gain saturation occurs, and the wavelength λ _out
And an optical filter 13 which is a means for outputting the output light 14.

The transmitted signal light 1 of wavelength λ _s is multiplexed by the optical multiplexer 5 together with the pump light 3 of wavelength λ _p output from the pump light source 2 via the polarization control circuit 4,
It is configured to be input to the optical fiber 6. The output stage of the optical fiber 6 is provided with an optical filter 13 that transmits light having a wavelength λ _out . Wavelength λ
_out is a wavelength that satisfies 1 / λ _out = 2 / λ _s −1 / λ _p . Here, the zero-dispersion wavelength of the optical fiber 6 and each wavelength light include the pump light 3 as the pump light in the optical parametric process described above, the signal light 1 as the signal light, and the wavelength λ.
the output light 14 is omega _F frequency light _out, is assumed to be set to the corresponding. With such a setting, the signal light 1 is amplified by an optical parametric process in the optical fiber 6, and an output light 14 having a wavelength λ _out is generated. Generally, the optical parametric process depends on the polarization state of the input light. The polarization control circuit 4 at the output stage of the pump light source 2 is provided to match the polarization state of the pump light 3 with the signal light 1.

The operating principle of optical noise suppression by the above configuration is the same as that of the first embodiment. That is, when the signal light 1 is input to the optical parametric amplifier configured as shown in FIG. 10, using a normal optical fiber amplifier or the like,
As shown in FIG. 9, the ON state of the input signal light is λ _out
The output light power of the wavelength light is set so as to be the maximum level. In this input region, the λ _out wavelength optical output power is substantially flat with respect to the change in the input optical power. Therefore, even if the on-level of the input signal light fluctuates, the output stage outputs light with suppressed level fluctuation. That is, as described in the first embodiment, since the signal light transmitted through the optical fiber mainly fluctuates in on-level, light whose noise is suppressed by this is output.

A characteristic of the optical noise suppression circuit of the present embodiment is that noise can be suppressed without deteriorating the extinction ratio. This means that in the small signal input area,
This is because the increase rate of the output light is sharper than the proportional relation.
Due to this characteristic, the output level in the off state is smaller than that of the optical noise suppression circuits of FIGS. 5 and 6 using the gain saturation characteristic of the signal light. As a result, light having a large extinction ratio is output. For example, based on the experimental results of FIG.
Assuming that signal light having an extinction ratio of 12 dB is input in a state where the ON level is near the maximum output, the extinction ratio of the output light is 15 dB.
dB. That is, light with no extinction ratio deterioration is output.

According to the above principle, an optical noise suppressing circuit without extinction ratio degradation can be realized.

Third Embodiment In order to operate the optical noise suppression circuit of the first embodiment, it is necessary to input pump light having a relatively large power (0.5 W in the calculation example of FIG. 2) to the optical fiber 6. is there. However, when high-power light is input to the optical fiber 6, an optical nonlinear phenomenon called stimulated Brillouin scattering occurs.
It is known that input light is converted into reflected Brillouin light and returns to the input end. Therefore, the power of the pump light 3 at a certain level or higher cannot be propagated through the optical fiber 6. In the first embodiment, unless sufficient pump light 3 propagates through the optical fiber 6, the operating conditions shown in FIG. Embodiment 3
Is designed to avoid this problem.

The input light power at which stimulated Brillouin scattering occurs (hereinafter referred to as Brillouin threshold) depends on the spectral width of the input light. It is known that the Brillouin threshold increases as the spectral width of input light increases. This is because the effect of stimulated Brillouin scattering is dispersed on the spectrum. Therefore, the pump light 3
When the means for expanding the spectrum width of the first embodiment is used, the input power of the pump light 3 can be increased, and the first embodiment can be used.
Can be solved.

FIG. 9 is a diagram showing a configuration of an optical noise suppression circuit according to Embodiment 3 of the present invention.

9 is a phase modulator, 10 is an injection current modulation circuit, 11 is a bias current, and 12 is a modulation current.

The basic configuration of this embodiment is the same as that of the first embodiment, except that a means for expanding the spectrum width of the pump light 3 is provided.

Here, two means are used.
As a first means, the pump light source 2 is connected to a semiconductor laser (L
D), and the injection current modulation circuit 1 for modulating the injection current
0 is provided. Semiconductor lasers have the property that the oscillation frequency changes according to the injection current. Therefore, when the injection current is modulated, the oscillation frequency is modulated, thereby broadening the spectrum of the pump light 3. as a result,
The Brillouin threshold can be increased.

Further, in FIG. 9, as a second means, a phase modulator 9 is provided at the output stage of the pump light source 2.
When phase modulation is applied to the pump light 3, a modulation sideband is generated,
This broadens the spectrum of the pump light 3. This means can also increase the Brillouin threshold.

In this embodiment, by the above means,
Increasing the power of the pump light 3 that can be input to the optical fiber 6 facilitates setting of conditions for obtaining the noise suppression effect described at the beginning of the embodiment.

In this case, two spectrum expanding means, ie, the injection current modulation circuit 10 for performing injection current modulation of the semiconductor laser and the phase modulator 9 for performing external phase modulation have been described. May be provided.

The third embodiment can be similarly applied to the second embodiment. That is, in the second embodiment, by providing a means for expanding the spectrum width of the pump light 3 as in the third embodiment, the same operation and effect as in the third embodiment can be obtained.

Although the present invention has been specifically described based on the embodiments, the present invention is not limited to the above-described embodiments, and it is needless to say that various modifications can be made without departing from the gist of the present invention. It is.

[0081]

As described above, according to the present invention,
It is possible to provide an optical noise suppressing circuit capable of suppressing level fluctuation superimposed on high-speed signal light in a light state.
Further, according to the present invention, it is possible to provide an optical noise suppressing circuit capable of suppressing a level fluctuation superimposed on a high-speed signal light in a light state without deteriorating a signal extinction ratio.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a configuration of an optical noise suppression circuit according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating an operation principle of the first embodiment of the present invention.

FIG. 3 is a diagram illustrating gain saturation characteristics of an optical parametric amplifier.

FIG. 4 is a diagram illustrating signal light input / output characteristics of an optical parametric amplifier.

FIG. 5 is a diagram illustrating a measurement example of input / output characteristics of a conventional optical parametric amplifier.

FIG. 6 is an explanatory diagram of a conventional optical noise suppressing operation.

FIGS. 7A and 7B are diagrams illustrating measurement examples of an output spectrum of an optical parametric amplifier. FIGS.

FIG. 8 is a diagram illustrating a measurement example of input / output characteristics of an optical parametric amplifier.

FIG. 9 is a diagram replotted from FIG. 8 for the ω _F frequency light.

FIG. 10 is a diagram illustrating a configuration of an optical noise suppression circuit according to a second embodiment of the present invention.

FIG. 11 is a diagram illustrating a configuration of an optical noise suppression circuit according to a third embodiment of the present invention.

FIG. 12 is a diagram showing a configuration example of an interferometer incorporating a conventional optical nonlinear medium.

FIG. 13 is a diagram illustrating the operation principle of the interferometer of FIG.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Signal light, 2 ... Pump light source, 3 ... Pump light, 4 ... Polarization control circuit, 5 ... Optical multiplexer, 6 ... Optical fiber (optical nonlinear medium), 7 ... Optical filter, 8 ... Output light (? _S , 13 optical filter, 14 output light (λ _out ), 9 phase modulator, 10 injection current modulation circuit, 11 bias current, 1
2. Modulation current.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04B 10/17 10/16

Claims (6)

[Claims] 1. An optical parametric amplifier comprising: an optical nonlinear medium; and means for inputting pump light to the optical nonlinear medium; and means for inputting signal light to the optical parametric amplifier in a state where gain saturation occurs. Means for outputting the signal light from the optical parametric amplifier. 2. The optical noise suppression circuit according to claim 1, wherein said optical nonlinear medium is an optical fiber. 3. An optical fiber and a wavelength λ _{p to} said optical fiber.
Means for amplifying the signal light having the wavelength λ _s , and 1 / λ _out = 2 / λ _s −1 / λ
an optical parametric amplifier that generates light having a wavelength λ _out that satisfies _p ; a means for inputting the signal light having the wavelength λ _s to the optical parametric amplifier with gain saturation occurring; and a means for outputting _out light. 4. The optical noise suppressing circuit according to claim 1, further comprising a means for expanding the spectrum of said pump light. 5. The optical noise suppression circuit according to claim 4, wherein said spectrum expanding means is a phase modulator. 6. The optical noise suppressing circuit according to claim 4, wherein said pump light is oscillation light of a semiconductor laser, and said spectrum expanding means is a modulation circuit of a current injected into said semiconductor laser.