JPS619606A - Improvement of extinction ratio of optical fiber and communication equipment utilizing said fiber - Google Patents

Abstract

PURPOSE:To improve the extinction ratio of the signal light inputted to an optical fiber having the diffenct refractive indices in orthogonal optical main axis directions in such a manner that the polarization plane thereof coincides with the short axis by making pump light incident on said optica fiber while making the plane of polarization thereof coincident with the long axis. CONSTITUTION:A signal light source 1 outputs polarized signal light to the short axis of a polarization maintaining optical fiber 6 and a pump light source 2 outputs polarized pump light to the long axis of the fiber 6. The mode of the pump light is preferably the waveguide mode of the lowest order propagating in the fiber 6. The wavelengths mode of the lowest order propagating in the fiber 6. The wavelengths of the signal light and pump light may be equal or different. The signal light is transmitted through a dichroic reflecting mirror 4 an is made incident on the fiber 6. The pump light is reflected by a total reflecting mirror 3 and the mirror 4 and is made incident on the fiber 6. The pump light component is separated and removed by a polarizer 7 at the output end of the fiber 6.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to improving the extinction ratio of optical fibers.

In particular, the present invention relates to improvements in the extinction ratio of optical fibers used in coherent optical transmission, sensors using optical fibers, etc., and optical fiber communication devices using the same.

[Conventional technology]

In coherent optical transmission, which is expected to be used in large-capacity optical communication systems, and in sensors using optical fibers such as gyroscopes, phase information is transmitted through optical fibers. Therefore, it is necessary to make the amount of phase change occurring in the optical fiber constant in the length direction of the optical fiber. For this purpose, it is necessary to transmit all information in one waveguide mode. Therefore, it is desirable to use single mode optical fiber to transmit such information.

However, the lowest order mode, so-called single mode, actually has two waveguide modes with mutually orthogonal polarization planes, namely LPo+X mode and LPo+y mode.
Contains modes. Generally, the propagation constants of these two waveguide modes are different, and mode coupling occurs between these waveguide modes due to various mismatches in the optical fiber. Therefore, the amount of phase change (i.e., propagation constant ×
fiber length) is not necessarily constant.

The extinction ratio η is usually used as an index for evaluating the amount of mode coupling. The extinction ratio η is defined as the reciprocal of the ratio of the light intensity at the input end when only one of the LP01X mode or the LPO1' mode is excited and the light intensity of the mode orthogonal to the incident light at the output end. .

The extinction ratio η is the mode coupling coefficient representing the strength of mode coupling, the optical fiber length 2, and η-10' log+o(tanh hz)
-----There is the relationship (1). However, the unit is decibel (dB). As is clear from equation (1), the smaller the mode coupling coefficient is, the higher the extinction ratio is.

Also, the mode coupling coefficients are LP, , X mode and LPo
It has a low-pass characteristic with respect to the propagation constant difference δβ with the 1y mode. Therefore, if the propagation constant difference δβ is made sufficiently large, mode coupling will be reduced and the extinction ratio will be improved.

The propagation constant difference δβ is determined by the birefringence fold s and the wave number in vacuum, as follows: δβ−Bs −k −−−−
−(Represented as 21.

In order to make the propagation constant difference δβ sufficiently large, in conventional polarization-maintaining optical fibers, the core shape is ovalized, stress is applied only in one direction of the core, and birefringence is intentionally caused to increase the propagation constant difference δβ. However, with optical fibers made using this method, 10-
Birefringence of about 4 can be achieved, but the extinction ratio is at most 20 dB.
/km, which is still insufficient, and it is necessary to further improve the extinction ratio.

[Problem that the invention seeks to solve]

An object of the present invention is to provide a method for improving the extinction ratio of a birefringent optical fiber.

Means for Solving Problem C] The method for improving the extinction ratio of an optical fiber of the present invention uses optical fibers with different refractive indexes in the directions of orthogonal optical principal axes, and the plane of polarization matches the short axis of the optical fiber. A signal light is made to enter the optical fiber in such a manner that the pump light is made to enter the optical fiber so that its plane of polarization matches the long axis of the optical fiber.

Further, the optical fiber communication device of the present invention is an optical fiber communication device in which a transmitting device is provided at one end of an optical fiber having different refractive indices in orthogonal optical principal axis directions, and a receiving device is provided at the other end of this optical fiber. The transmitter includes means for inputting an input signal to be transmitted so that its plane of polarization coincides with the short axis of the optical fiber, and means for inputting the pump light so that its plane of polarization coincides with the long axis of the optical fiber. The receiving device is characterized by comprising means for detecting a signal of a plane of polarization in the short axis direction of the optical fiber from the optical fiber.

The means for detecting the signal of the polarization plane in the short axis direction includes a polarizer that separates the signal according to the polarization direction, but if the wavelength of the input signal to be transmitted and the wavelength of the pump light are different, the polarizer Instead, it may include a demultiplexer that separates signals by wavelength.

[Effect]

The method of the present invention involves injecting pump light into optical fibers with different refractive indices in the directions of orthogonal optical principal axes so that the plane of polarization matches the plane of polarization along the long axis. This improves the extinction ratio of the input signal light.

〔Example〕

FIG. 1 is a block diagram of an optical fiber communication device according to a first embodiment of the present invention.

The transmitter includes a signal light source 1, a pump light source 2, and a total reflection mirror 3.
, a dichroic reflecting mirror 4, and a lens 5 for collecting light. The receiving device includes a polarizer 7.

The signal light source 1 outputs signal light polarized along the short axis of the polarization-maintaining optical fiber 6 . On the other hand, the pump light source 2 outputs pump light polarized along the long axis of the polarization maintaining optical fiber 6. The signal light passes through the dichroic reflector 4, is focused by the lens 5, and enters the polarization-maintaining optical fiber 6. The pump light is reflected by the total reflection mirror 3 and the dichroic ink reflection mirror 4, is converged by the lens 5, and enters the polarization maintaining optical fiber 6. At the output end of the polarization maintaining optical fiber 6, the signal light and the pump light are separated by a polarizer 7.

The polarization maintaining optical fiber 6 is an optical fiber having different refractive indices in orthogonal optical principal axis directions.

The mode of the pump light is preferably the lowest-order waveguide mode propagating through the polarization-maintaining optical fiber 6. Also,
The wavelengths of the signal light and the pump light may be the same or different.

FIG. 2 is a block diagram of an optical fiber communication device according to a second embodiment of the present invention.

In this embodiment, the configuration of the receiving device is different from that in the first embodiment. In the case of this embodiment, the wavelength λS of the signal light output from the signal light source 1 and the wavelength λp of the pump light output from the pump light source 2 are different.

Therefore, in order to separate the two wavelengths, the polarizer 7 is not necessary, and a wavelength demultiplexer 8 composed of a monochromator or a prism is sufficient.

Next, improvement of the extinction ratio by pump light will be explained.

In the presence of strong pump light, the birefringence s of the polarization-maintaining optical fiber is expressed as a function of the pump light intensity by the Kerr effect of the optical fiber: Bs = Bso+ −(Px −Py) −−− −
[Represented as 31. Here, Bso is the initial birefringence when there is no pump light, χ is the self-focusing coefficient due to the Kerr effect, and Px and Py are the X component and the X component of the light intensity of the pump light, respectively.

FIG. 3 shows the birefringence s relative to the pump light intensity in a typical silica-based optical fiber.

In the example of Fig. 3, the χΦ value is 1.1 xio-' (e, s
, u, ). When Py=00, the value of birefringence s increases linearly as the pump light increases.

The optical intensity of the pump light of 10' (W) is a value widely used for conventional nonlinear optical experiments, and is a value easily obtained with a mode-locked and Q-switched Nd:YAG laser. With respect to this change in light intensity, the birefringence s changes by about 1.5 Xl0-5.

FIG. 4 is a diagram showing the value of the mode coupling coefficient with respect to the birefringent fold s.

Obviously, in order to increase the extinction ratio η, the mode coupling ratio must be decreased. In order to reduce the mode coupling ratio, as is clear from FIG. 4, the birefringence fold s is increased. In order to increase the birefringence s, as is clear from FIG. 3, the X component (short axis direction component) of the intensity of the pump light is decreased and the X component is increased. Therefore, the polarization direction of the pump light needs to be in the X-axis (long axis) direction.

FIG. 5 is a diagram showing the mode coupling coefficient with respect to the light intensity Px in the X direction.

The relationship shown in FIG. 5 is derived from FIGS. 3 and 4. The mode coupling coefficient decreases as the X component Px of the pump light intensity increases. For example, the change in mode coupling coefficient from no pump light input to 10' (W) input is from 3.9 to 0.01, approximately 1/40.
It has decreased to 0. Conversely, when the polarization direction of the pump light is set to the minor axis (y-axis) direction, the case is Px = 0 as shown in Fig. 3, and as the pump light increases, the birefringence s decreases, thereby increasing the mode coupling coefficient and deteriorating the extinction ratio η.

FIG. 6 is a diagram showing the extinction ratio of signal light with respect to the intensity of the X component of pump light.

This figure is obtained from equation (1) and FIG. In this figure, the optical fiber length is 1 (km). The extinction ratio when there is no pump light is extremely low at 4xlo-3 (dB), but when a pump light of 105 [W] is input, the extinction ratio is 20 (dB), which is about 5000 times higher. An extinction ratio of is obtained.

In this way, the extinction ratio of the signal light can be significantly improved by injecting the strong pump light polarized in the long axis direction of the polarization maintaining optical fiber together with the signal light.

Therefore, as in the first and second embodiments, by making the polarization direction of the pump light coincide with the long axis of the polarization-maintaining optical fiber and making the polarization direction of the signal light coincide with the short axis direction, , the extinction ratio of signal light is significantly improved. The pump light component is removed by a polarizer 7 in the first embodiment, and by a wavelength demultiplexer 8 in the second embodiment.

〔Effect of the invention〕

As described above, the optical fiber extinction ratio improvement method of the present invention has the effect of further improving the extinction ratio of polarization maintaining optical fibers, which are currently becoming popular, by simply inputting pump light.

Furthermore, this method is simple and has the advantage that it can be easily applied to optical fibers for coherent transmission and optical fibers for optical sensors.

Furthermore, the present invention is applicable not only to polarization-maintaining optical fibers, but also to ordinary optical fibers whose extinction ratio is one to two orders of magnitude inferior to polarization-maintaining optical fibers. therefore,
This has the great effect of making it possible to realize optical fibers for coherent transmission and optical fibers for optical sensors using ordinary optical fibers that are highly economical.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of an optical fiber communication device according to a first embodiment of the present invention. FIG. 2 is a configuration diagram of an optical fiber communication device according to a second embodiment of the present invention. FIG. 3 is a diagram showing birefringence s with respect to pump light intensity. FIG. 4 is a diagram showing the mode coupling coefficient for birefringence s. FIG. 5 is a diagram showing the mode coupling coefficient versus light intensity. FIG. 6 is a diagram showing the extinction ratio of signal light with respect to the optical intensity of pump light. DESCRIPTION OF SYMBOLS 1... Signal light source, 2... Pump light source, 3... Total reflection mirror, 4... Dichroic ink reflecting mirror, 5... Lens, 6... Polarization maintaining optical fiber, 7...・Polarizer, 8・
...Wavelength demultiplexer.

Claims (7)

[Claims] (1) Using optical fibers with different refractive indexes in the directions of orthogonal optical principal axes, a signal light is input so that its polarization plane matches the short axis of this optical fiber, and its polarization plane matches the long axis of this optical fiber. A method for improving the extinction ratio of an optical fiber by injecting pump light so that the (2) The method for improving the extinction ratio of an optical fiber according to claim (1), wherein the mode of the pump light is the lowest-order waveguide mode propagating through the optical fiber. (3) A method for improving the extinction ratio of an optical fiber according to claim (1), wherein the signal light and the pump light have the same wavelength. (4) A method for improving the extinction ratio of an optical fiber according to claim (1), in which the extinction ratio and the wavelength of the pump light are different. (5) In an optical fiber communication device in which a transmitting device is provided at one end of an optical fiber having different refractive indexes in the directions of orthogonal optical principal axes, and a receiving device is provided at the other end of the optical fiber, the transmitting device has a transmitting device. means for causing an input signal to be incident on the short axis of the optical fiber so that its plane of polarization coincides with the short axis of the optical fiber, and means for making the pump light incident on the long axis of the optical fiber so that its plane of polarization coincides with the long axis of the optical fiber, An optical fiber communication device characterized in that the receiving device includes means for detecting a signal of a polarization plane in the short axis direction of the optical fiber. (6) Claims in which the wavelength of the input signal to be transmitted and the wavelength of the pump light are equal, and the means for detecting the signal of the polarization plane in the short axis direction includes a polarizer that separates the signal according to the polarization direction. No. (
The optical fiber communication device according to item 5). (7) Different from the wavelength of the input signal to be transmitted and the wavelength of the pump light, the means for detecting the signal of the polarization plane in the short axis direction includes a demultiplexer that separates the signal according to the wavelength. (5)
The optical fiber communication device described in .