JP2001305594A - Optical parametric amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical parmetric amplifier in which amplification gain is improved. SOLUTION: In the optical parametric amplifier, pump light beams having a wavelength λp and signal light beams having a wavelength λs are inputted into a third order optically nonlinear medium and the signal light beams having the wavelength λs are amplified and outputted by an optical parametric process. The amplifier is provided with an optical amplifier which amplifies the pump light beams and a means which eliminates light beams having the wavelengths other than the wavelength of the pump light beams from the light beams outputted from the optical amplifier and inputs the pump light beams into the medium.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical parametric amplifier, and more particularly to a technique effective when directly amplifying signal light in an optical fiber transmission system.

[0002]

2. Description of the Related Art There are several types of optical amplifiers for optical fiber systems. Among them, an optical parametric amplifier utilizing the third-order nonlinearity of an optical fiber has a gain response speed. It is fast and can be applied to various optical function circuits such as optical limiters. FIG.
FIG. 1 is a block diagram showing a basic configuration of a conventional optical parametric amplifier. As shown in FIG. 1, in the conventional optical parametric amplifier, a pump light having a wavelength λp and a signal light having a wavelength λs are input to an optical fiber 1 which is a tertiary optical nonlinear medium via an optical multiplexer 11. At this time, the wavelength λp and the wavelength λ
s, and the zero dispersion wavelength of the optical fiber 1 satisfies a certain relationship, the optical power shift from the pump light to the signal light occurs due to the optical mixing phenomenon via the optical nonlinearity,
The signal light having the wavelength λs is amplified and output. Therefore, if the output stage of the fiber 1 is provided with an optical filter 12 that transmits light having a wavelength of λs, it functions as an optical amplifier for signal light.

[0003]

Generally, in an optical parametric amplifier, a higher amplification gain is obtained as the optical power of the pump light increases. Therefore, it is effective to increase the optical power of the pump light by using an optical amplifier in order to obtain a high gain. Such a configuration is also proposed in, for example, the following document (a). (B) MEMarhic, N, Kagi, T.-K.Chiang, and LGKazovs
ky, “Broadband fiber optical parametric amplifier
s, "Optics Letters, Vo1.21, pp. 573-575, 1996. However, in an optical amplifier, light called amplified ASE (Amplified Spontaneous Emission, hereinafter referred to as ASE) is provided in an amplification wavelength region. This A occurs.
SE light is light that occurs spontaneously even when no signal light is input, and is amplified and output in an optical amplifier.

Therefore, in order to increase the optical power of the pump light by using an optical amplifier in order to obtain a high gain, ASE light is generated from the optical amplifier and is incident on the optical fiber 1 together with the pump light. In the conventional optical parametric amplifier, there is a problem that the amplification efficiency is reduced and the amplification gain is reduced by the ASE light. SUMMARY OF THE INVENTION The present invention has been made to solve the problems of the conventional technique, and an object of the present invention is to provide a technique that can improve an amplification gain in an optical parametric amplifier. The above and other objects and novel features of the present invention will become apparent from the description of the present specification and the accompanying drawings.

[0005]

SUMMARY OF THE INVENTION Among the inventions disclosed in the present application, the outline of a representative one will be briefly described.
It is as follows. That is, the present invention provides an optical parametric amplifier which inputs a pump light having a wavelength λp and a signal light having a wavelength λs into a third-order optical nonlinear medium and amplifies and outputs the signal light having a wavelength λs by an optical parametric process. An optical amplifier for amplifying light, and means for removing wavelength light other than pump light from output light output from the optical amplifier and inputting the light to the optical nonlinear medium.

Further, the present invention is characterized in that the optical nonlinear medium is an optical fiber and includes means for expanding the spectrum of the pump light. In a preferred embodiment of the present invention, the spectrum expanding means is a phase modulator. In a more preferred embodiment of the present invention, the pump light is oscillation light of a semiconductor laser, and the spectrum expanding means is a modulation circuit of a current injected into the semiconductor laser.

According to the above means, the means (optical filter) for transmitting only the pump light wavelength is arranged at the output stage of the optical amplifier for amplifying the pump light so as to block the ASE light in the signal light wavelength range. ASE from optical amplifier for pump light amplification
Light can be prevented from being incident on the optical nonlinear medium (optical fiber), whereby the AS in the optical nonlinear medium can be prevented.
E light generation can be reduced, and signal light can be efficiently amplified. According to the above-mentioned means, since the apparatus further includes means for expanding the spectrum of the pump light to suppress stimulated Brillouin scattering, it is possible to amplify the signal light more efficiently.

[0008]

Embodiments of the present invention will be described below in detail with reference to the drawings. In all the drawings for describing the embodiments, components having the same functions are denoted by the same reference numerals, and repeated description thereof will be omitted. [First Embodiment] FIG. 1 is a block diagram showing a basic configuration of an optical parametric amplifier according to a first embodiment of the present invention. In the figure, 1 is an optical fiber as an optical nonlinear medium, 2 is a pump light source, 3 is a polarization control circuit, 4 is an optical amplifier, 5 and 12 are optical filters, and 11 is an optical multiplexer. The optical parametric amplifier of the present embodiment has the same basic configuration as the conventional optical parametric amplifier shown in FIG. 3, and a pump light having a wavelength λp and a signal light having a wavelength λs are input to an optical fiber 1 which is a nonlinear medium. Then, it is configured to take out light having a wavelength of λs from the optical filter 12 at the output stage.

As the optical amplifier 4, for example, a rare earth ion-doped optical fiber amplifier is used. The feature of this embodiment is that an optical amplifier 4 and an optical filter 5 that transmits only the pump light are provided at the output stage of the pump light source 2 that generates the pump light. As described above, in the optical parametric amplifier, a higher amplification gain can be obtained as the optical power of the pump light is larger. However, in the present embodiment, simply using the optical amplifier 4 to increase the optical power of the pump light is sufficient. In addition, the output stage is different from the conventional optical parametric amplifier in that an optical filter 5 that transmits only the pump light of the wavelength λp is provided. Thereby, the optical parametric amplifier of the present embodiment can obtain a higher signal gain than the conventional optical parametric amplifier. The reason is as follows.

As described above, in an optical amplifier, A
Although the SE light is generated, the ASE light is generated spontaneously even when no signal light is input, and is light that is amplified and output in the optical amplifier. Optical parametric amplifiers are no exception, and optical mixing occurs spontaneously.
SE light is generated. The ASE light not only acts as noise light on the signal light, but also hinders efficient signal amplification. This is because both ASE light generation and signal light amplification use pump light as their source. When the optical power of the pump light is consumed for the generation of the ASE light, the optical power of the pump light for amplifying the signal is reduced accordingly, and the amplification efficiency is reduced. Therefore, in order to efficiently amplify the signal light, it is important to suppress the generation of the ASE light as much as possible.

If the optical amplifier 4 is used to increase the optical power of the pump light, ASE light is generated from the optical amplifier 4 and is incident on the optical fiber 1 as an optical nonlinear medium together with the pump light. Then, the ASE light is amplified in the optical fiber, and a large parametric ASE light is generated by the optical amplifier 4 for amplifying the pump light, which is larger than the optical power of the pump light. The generation of ASE light causes a reduction in amplification efficiency.
Therefore, in the present embodiment, the optical filter 5 that transmits only the pump light of the wavelength λp is arranged at the output stage of the optical amplifier 4 for amplifying the pump light. As a result, the ASE in the signal light wavelength range
The light is blocked, and ASE is output from the optical amplifier 4 for amplifying the pump light.
Light can be prevented from being input to the optical fiber 1. As a result, ASE light generation in the optical fiber can be reduced, and the signal light can be efficiently amplified.

Although FIG. 1 shows a configuration in which the amplified pump light is passed through the optical filter 5 and then multiplexed with the signal light by the optical multiplexer 11, an optical multiplexer 11 having a wavelength filtering characteristic is used. The optical multiplexer 11 may be configured to prevent the signal wavelength band ASE light from being input to the optical fiber 1 from the optical amplifier 4 for amplifying the pump light. The efficiency of the optical parametric amplifier depends on the polarization state of the signal light and the pump light. In FIG. 1, a pump light source 2
The polarization control circuit 3 disposed at the output stage of (1) is provided for maximizing the amplification efficiency of the polarization relationship between them.
It is not essential to the invention.

[Embodiment 2] As described above, in the optical parametric amplifier, the higher the optical power of the pump light, the higher the amplification gain. However, it is known that when high-power light is input to the optical fiber 1, an optical nonlinear phenomenon called stimulated Brillouin scattering occurs, and the input light is converted into reflected Brillouin light and returns to the input end. When this phenomenon occurs, the optical power of the pump light of a certain level or more cannot be propagated in the fiber, and a high amplification gain cannot be obtained. Conventionally, several methods have been known as methods for suppressing stimulated Brillouin scattering. Embodiment 2 of the present invention is different from Embodiment 1 in that
This is a combination of a conventionally known induced Brillouin suppression method.

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. The Brillouin threshold has a property of increasing as the spectrum width of input light increases. This is because the effect of Brillouin scattering is dispersed on the spectrum. Therefore, if means for expanding the spectrum width of the pump light is provided, the pump input light power can be increased, and the amplification gain of the optical parametric amplifier can be increased.

FIG. 2 is a block diagram showing a basic configuration of the second embodiment of the present invention. In addition to the configuration of the first embodiment,
Means are provided for expanding the spectral width of the pump light. In this embodiment, two means are used.
One method is to use a semiconductor laser (LD) 6 as the pump light source 2 and modulate the injection current. The semiconductor laser 6 has a property that the oscillation frequency changes according to the injection current. Therefore, when the injection current is modulated, the oscillation frequency is modulated, whereby the pump light spectrum can be broadened, and as a result, the Brillouin threshold can be increased.

Further, in FIG. 2, an optical phase modulator 7 is provided at the output stage of the pump light source 2. When phase modulation is applied to the pump light, a modulation sideband is generated, which broadens the pump light spectrum.
The Brillouin threshold can be increased. As described above, if the induced Brillouin suppressor is used in combination with the optical parametric amplifier of the first embodiment, an optical parametric amplifier with higher gain can be obtained. Note that, here, two spectrum enlarging means, that is, current modulation and external phase modulation of the semiconductor laser (LD) 6 have been described, but either one may be used as necessary. As described above, the invention made by the inventor has been specifically described based on the embodiment. However, the present invention is not limited to the embodiment, and can be variously modified without departing from the gist of the invention. Of course, it is.

[0017]

The effects obtained by typical ones of the inventions disclosed in the present application will be briefly described as follows. According to the optical parametric amplifier of the present invention, it is possible to efficiently amplify signal light.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a basic configuration of an optical parametric amplifier according to a first embodiment of the present invention.

FIG. 2 is a block diagram illustrating a basic configuration of an optical parametric amplifier according to a second embodiment of the present invention.

FIG. 3 is a block diagram showing a basic configuration of a conventional optical parametric amplifier.

[Explanation of symbols]

1. optical fiber (optical nonlinear medium), 2. pump light source,
3: polarization control circuit, 4: optical amplifier, 5, 12: optical filter, 11: optical multiplexer, 6: semiconductor laser, 7: phase modulator.

Claims (4)

[Claims] 1. An optical parametric amplifier which inputs a pump light having a wavelength of λp and a signal light having a wavelength of λs into a third-order optical nonlinear medium and amplifies and outputs the signal light having a wavelength of λs by an optical parametric process. An optical parametric amplifier, comprising: an optical amplifier that amplifies the optical signal; and a unit that removes wavelength light other than pump light from output light output from the optical amplifier and inputs the light to the optical nonlinear medium. 2. The optical parametric amplifier according to claim 1, wherein the optical nonlinear medium is an optical fiber, and further comprises a means for expanding the spectrum of the pump light. 3. The optical parametric amplifier according to claim 2, wherein said spectrum expanding means is a phase modulator. 4. The optical parametric amplifier according to claim 2, further comprising a semiconductor laser for generating said pump light, wherein said spectrum expanding means is a circuit for modulating an injection current to said semiconductor laser.