JPH05313215A - Low polarizing light signal transmitter - Google Patents

Abstract

PURPOSE:To stabilize the output power of an optical repeater without complicating the constitution of the optical repeater by providing a low polarizing means which controls the degree of polarization of the light signal outputted from a light source means to a certain specified degree or bellow and is equipped behind this light source means. CONSTITUTION:A light signal S1 outputted from a transmission laser diode light source (transmission LD light source) 1 has generally >=0.9 degree of polarization. The degree of polarization of the light signal S1 is decreased by a depolarizer 2 and is outputted from a light signal output terminal 4 through an external modulator 3. The degree of polarization of the outputted light signal S3 is equal to the degree of polarization of the light signal S2 outputted by the depolarizer 2. Namely, the degree of polarization of the light signal S1 is decreased by inserting the depolarizer 2 to lower the degree of polarization of the output light signal S1 of the transmission LD light source 1 and this signal is outputted as the light signal S2, by which the influence of the fluctuation of the gain by the dependency of the optical amplifier in the optical repeater on polarization and the influence of the four photon mixing by the nonlinearity of optical fibers are substantially prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical signal transmitting apparatus in an optical amplification repeater communication system composed of an optical transmission line including an optical amplifier such as an optical fiber amplifier as an optical repeater and an optical transmitting / receiving terminal station. ..

[0002]

2. Description of the Related Art An optical transmission line using an optical amplifier such as an optical fiber amplifier as an optical repeater can very flexibly cope with the transmission speed of an optical signal and has been put into practical use as a long-distance backbone transmission line. I'm trying. However, in general, an optical amplifier has a polarization dependency of optical gain, and as an example thereof, an optical fiber amplifier has a polarization dependency derived from optical components that compose it.

[0003]

In order to stabilize the optical signal power and the signal reception, a circuit for gain stabilization has been conventionally added to each optical amplifier. There is a problem that the repeater becomes complicated and the cost increases. As a method of stabilizing the optical signal power without an additional circuit in the optical repeater, there is a technique of using a polarization scrambler in the optical transmitter.

However, in the case of using the polarization scrambler, since it is not effective unless the polarization state is randomized at a speed higher than the bit rate of the optical signal, an optical device or an electric circuit which responds at a very high speed is required. Therefore, it is very difficult to support a high bit rate.

Further, in an optical amplification repeater system in which an optical fiber having a small wavelength dispersion is used as a transmission line, four-photon mixing is caused between an optical signal and optical noise generated by an optical amplifier due to the nonlinearity of the optical fiber. It occurs and causes signal deterioration, which is the main limiting factor for the performance of the optical amplification repeater system. Four-photon mixing is a type of non-linear optical phenomenon caused by the non-linearity of an optical fiber, and a new optical signal is generated at a distance corresponding to the wavelength difference between the two optical waves due to the interaction of the two optical waves having different wavelengths. The phenomenon that occurs. The generation efficiency is higher as the power of the two light waves having different wavelengths is higher, and is higher as the dispersion of the optical fiber is closer to zero.

The generation efficiency of four-photon mixing is proportional to the power of the related light waves and is highest when all the related light waves have the same polarization state and almost zero when the polarization states are orthogonal. When four-photon mixing is generated between non-polarized light and an optical signal, such as optical noise generated from an optical amplifier, even if a polarization scrambler is used, its generation efficiency cannot be reduced. This signal deterioration is unavoidable.

In order to overcome the above problems of the prior art, the present invention eliminates the fluctuation of the optical signal power due to the polarization dependence of the optical amplifier without adding a complicated circuit to the optical repeater, It is possible to provide an optical signal transmission device that enables stable signal reception, and at the same time, it is possible to minimize signal deterioration due to four-photon mixing that occurs between an optical signal and optical noise generated from an optical amplifier. It is an object of the present invention to provide such a low polarization optical signal transmission device.

[0008]

The above-mentioned problems can be solved by adopting the following novel characteristic construction means of the present invention. That is, a feature of the present invention is that, in an optical signal transmitter used as a transmission terminal station of an optical repeater system including an optical amplifier, the light source means for emitting an optical signal and the polarization degree of the optical signal output from the light source means are controlled. A low polarization optical signal transmission device comprising: a polarization reducing unit that is provided at a stage subsequent to the light source unit and is suppressed to a certain level or less.

[0009]

As a light source of an optical transmitter for long distance transmission which is currently put into practical use, a light source having a high degree of polarization (generally, a degree of polarization of 0.9
Since the above is used, the fluctuation of the gain is caused by the polarization state of the optical signal, and when the long-distance multi-amplification relay is transmitted, the optical signal power fluctuates and the fluctuation of the optical signal S / N ratio is caused. It has been experimentally proved that good signal reception as shown in CCITT recommendation cannot be performed. As an application related to the clarification of this, there is Japanese Patent Application No. 4-98195 by the present applicant.

Since the present invention employs the above-mentioned construction means to suppress the polarization degree of the light source, the fluctuation of the optical signal power due to the polarization dependence of the optical amplifier of the optical repeater is eliminated, and stable signal reception is achieved. It is possible and suppresses signal deterioration due to four-photon mixing to a minimum.

[0011]

Embodiments of the present invention will be described with reference to the drawings. Figure 1
Is a block diagram showing the present embodiment. In the figure,
α is a low polarization optical signal transmission device of this embodiment, 1 is a laser diode light source for transmission (hereinafter, referred to as a transmission LD light source) that outputs light with a high degree of polarization, 2 is a depolarizer, and 3 is an external modulator 4 is an optical signal output terminal.

Optical signal S1 output from the transmission LD light source 1
Generally has a polarization degree of 0.9 or more. Optical signal S
The degree of polarization of 1 is reduced by the depolarizer 2 (for example, the degree of polarization of 0.3 or less), passes through the external modulator 3, and is output from the optical signal output terminal 4. The polarization degree of the optical signal S3 output from the optical signal output terminal 4 is determined by the depolarizer 2
Is equal to the degree of polarization of the optical signal S2 output by.

The low polarization optical signal transmitter α of this embodiment is
A means (so-called depolarizer) 2 for reducing the degree of polarization of the output optical signal S1 of the transmission LD light source 1, for example, a degree of polarization of 0.3 or less is inserted to reduce the degree of polarization of the optical signal S1 and output as the optical signal S2. By doing so, the influence of the gain fluctuation due to the polarization dependence of the optical amplifier existing in the optical repeater and the influence of the four-photon mixing due to the nonlinearity of the optical fiber are suppressed.

There is a "Poincare sphere" as a form for expressing the polarization state. The depolarizer 2 used in the present embodiment means that the incident polarized light at one point on the Poincare sphere or in a very small area is entirely covered by the Poincare sphere. It is an element or device that disperses evenly over the surface. The principle of the depolarizer 2 is to use a statistical phenomenon called scattering. An electro-optic crystal is used to rapidly change the phase difference between two orthogonal polarization components from 0 to 2π.
To change the incident light into two incoherent eigenpolarizations and combine them.

As a concrete example of the depolarizer 2,
An example is a combination of two polarization-maintaining fibers that are longer than the coherent length of the optical signal from the transmitting LD light source 1 with their axes displaced by 45 °. This has already been described in various documents as the Riot depolarizer. Also,
By splitting the light output from one light source into two by a demultiplexer, and making the two split light uncorrelated (incoherent) with each other, the polarized light is combined and returned to one light again. Is also possible. As a method of decorrelating the split two lights, for example, a device in which one of the lights is passed through an optical fiber longer than the coherent length can be considered.

The present embodiment presents a concrete embodiment as described above, and the operation in the optical transmission terminal station will be described below. An optical signal suppressed with low polarization degree is hardly affected by the gain fluctuation due to the polarization dependence of the optical amplifier as compared with the case where the polarization degree remains high, so the fluctuation of the optical repeater output is suppressed. As a result, fluctuations in the optical signal S / N ratio do not occur and good reception becomes possible.

For example, when a polarization fluctuation causes a gain fluctuation of 0.5 dB when the polarization degree is 0.9, the gain fluctuation is set to 0.3 by setting the polarization degree to 0.3.
Since it can be suppressed to about 1 dB, the optical signal S / N
The effect on the ratio will be greatly reduced. Therefore, generally speaking, a better effect can be obtained by using the low polarization means for suppressing the polarization degree to 0.3 or less.

Further, in an optical amplification repeater system in which an optical fiber having very little wavelength dispersion is used as a transmission line,
Due to the non-linearity of the optical fiber, four-photon mixing occurs between the optical signal and the optical noise generated from the optical amplifier, causing signal deterioration, which is a major limiting factor of the optical amplification repeater system.

The generation efficiency η of four-photon mixing is proportional to the power Pn of the related light waves, and is highest when all the related light waves have the same polarization state, and is almost zero when the polarization states are orthogonal. .. For example, when there are two kinds of light waves (L1, L2) related to four-photon mixing, the power Pfp of the interference light generated by four-photon mixing is represented by the following expression (1).

[Equation 1]

In the above equation (1), P1 is the power of the light wave L1, P2 is the power of the light wave L2, and η (P1, P
2, φ12) is the generation efficiency η of four-photon mixing when the light waves L1, L
2 is a function of the phase relationship φ12 of the light waves L1 and L2 in proportion to the powers P1 and P2 of 2.

As can be seen from the above equation (1), Pfp
Is a quadratic function of the power P of the light wave. If the power of the interference light generated between the signal light Ls (power Ps) and the optical noise Ln (power Pn) when the polarization degree is 0.9 is Psn and the generation efficiency of four-photon mixing is ηsn, the interference light Power Ps
n is shown by the following formula (2).

[Equation 2]

When the degree of polarization of the signal light is 0.3, the power of the entire signal light is dispersed between the orthogonal polarizations, so that the four-photon mixing generation efficiency ηsn 'is suppressed. ηs
When n ′ is represented by ηsn, the following equation (3) is obtained.

[Equation 3]

Therefore, the interference light power Psn 'generated when the degree of polarization is 0.3 is expressed by the following equation (4) in consideration of orthogonal polarization components.

[Equation 4]

As shown in the equation (4), the power Psn 'of the interference light generated by the four-photon mixing is 1.6 dB.
Being suppressed, signal degradation due to four-photon mixing is reduced. Especially, when the polarization degree becomes completely 0, the power is equally divided between the orthogonal polarizations, so that the power of the interference light generated by the four-photon mixing is suppressed by 3 dB, and the nonlinearity of the optical fiber affecting the entire optical repeater system is suppressed. The influence can be reduced.

[0025]

As described above, according to the present invention, in an optical communication system using a polarization-dependent optical amplifier as an optical repeater, without complicating the configuration of the optical repeater,
The output power of the optical repeater can be stabilized, and the effect of stabilizing the optical signal S / N ratio is extremely large. In addition, the received signal deterioration due to the non-linearity of the optical fiber is inevitable as long as the optical fiber is used as the transmission line, so that it is possible to reduce the optical signal deterioration. Demonstrate.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention.

[Explanation of symbols]

 α ... low polarization optical signal transmitter S1, S2, S3 ... optical signal 1 ... transmission LD light source 2 ... depolarizer 3 ... external modulator 4 ... optical signal output terminal

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Hiroharu Wakabayashi 2-3-2 Nishishinjuku, Shinjuku-ku, Tokyo International Telegraph and Telephone Corporation

Claims (1)

[Claims] 1. An optical signal transmitter used as a transmission terminal of an optical repeater system including an optical amplifier, wherein a light source means for emitting an optical signal and a polarization degree of the optical signal output from the light source means are kept constant. A low-polarization optical signal transmission device, which comprises: