JP2897076B2 - Optical fiber amplifier - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Overview] Regarding an optical fiber amplifier comprising an amplification optical fiber, for the purpose of reducing noise components due to spontaneous emission light, signal light and pump light are supplied to an amplification optical fiber having a predetermined length. In the optical fiber amplifier that propagates and amplifies the signal light, in each of a plurality of sections of the amplification optical fiber,
A filter is provided to pass the signal light and the excitation light.

[Industrial applications]

The present invention relates to an optical fiber amplifier including an amplification optical fiber.

The optical fiber amplifier is composed of a rare earth element erbium ion (Er ³⁺ ) doped amplifying optical fiber and a wavelength 1.
The signal light is amplified and output by propagating pump light having a wavelength of 1.48 μm or 980 nm together with the signal light in the 5 μm band. Using such an optical fiber amplifier, 10
Experiments have been conducted to amplify Gb / s optical signals and transmit them over 100 km.

[Conventional technology]

For example, as shown in FIG. 6, a conventional example of an optical fiber amplifier is configured so that an amplifying optical fiber 31 doped with erbium ions is supplied from an excitation semiconductor laser 33 to a wavelength of, for example, 1.48.
μm pump light is incident through a multiplexer 34,
The signal light having a wavelength of 1.55 μm is made incident via 34, whereby the signal light is amplified, and only the signal light amplified through the band-pass filter 32 having the center wavelength of 1.55 μm is output.

The length L of the amplification optical fiber 31 is selected depending on the erbium doping concentration, the light intensity of the excitation light source, and the like. For example, when the concentration is 30 ppm, the length L is 60 m or 90 m.
A configuration in which the length L is 10 m when the concentration is 330 ppm is known, and a gain in this case is 16.5 dB.

[Problems to be solved by the invention]

The main noise of the optical fiber amplifier is the amplification optical fiber 31.
These are beat noise between the signal light and the spontaneous emission light due to the spontaneous emission light generated in the light source. When the input intensity of the signal light is low, the latter noise becomes dominant. Although a bandpass filter 32 is provided to remove such noise components due to spontaneous emission light, the characteristics required for the bandpass filter 32 are narrow, which allows only the wavelength of the signal light to pass. It is a band characteristic, is not easy to realize, and has the disadvantage of being expensive.

An object of the present invention is to reduce a noise component due to spontaneous emission light.

[Means for solving the problem]

The optical fiber amplifier according to the present invention divides the entire length of the amplification optical fiber into a plurality of sections and provides a filter for each section. This will be described with reference to FIG.

In an optical fiber amplifier for amplifying signal light by transmitting signal light and excitation light from an excitation light source 3 to the amplification optical fiber 1, filters 2 are provided in a plurality of sections of the amplification optical fiber 1. The filter 2 is constituted by an optical band-pass filter that passes one of the signal light and the pump light, and a multiplexer / demultiplexer that passes the other.

[Action]

The total length of the amplifying optical fiber 1 is set to a predetermined length. The total length is divided into a plurality of sections, filters 2 are provided in each section, and the signal light is split by a wavelength-selective multiplexer / demultiplexer. Alternatively, one of the pump lights is demultiplexed, and the other is passed through an optical bandpass filter, and then multiplexed, thereby passing the signal light and the pump light, suppressing the spontaneous emission light, and amplifying the signal light. be able to.

〔Example〕

This will be described below with reference to the drawings. FIG. 2 is an explanatory view in the case where a fusion type optical coupler is provided.
Is, for example, doped with erbium (Er) ions, and its length L is selected depending on the doping concentration, the intensity of the excitation light, etc., and the lengths L ₁ , L ₂ ,.
.. Divided into L _n sections, and a fusion-type optical coupler 12 using an amplification optical fiber is provided in each section. It is desirable that the lengths L ₁ , L ₂ ,..., L _n of the sections are the same, but they may be different lengths. Reference numeral 13 denotes an excitation light source made of a semiconductor laser, 14 denotes a multiplexer, and 15 denotes an optical bandpass filter for removing the excitation light.

The excitation light having a wavelength of 1.48 μm from the excitation light source 13 is incident on the amplification optical fiber 11 via the multiplexer 14, and the signal light having the wavelength of 1.55 μm is incident on the amplification optical fiber 11 via the multiplexer 14. In each section of the amplifying optical fiber 11, the pump light is removed by the optical band-pass filter 15, and only the amplified signal light is transmitted.

The spontaneous emission light spectrum of the amplification optical fiber 11 is
An example is shown in FIG. In the figure, λ _s
The wavelength of the signal light, lambda _p represents the wavelength of the excitation light, the case without the fused optical coupler 12 of each section of amplifying optical fiber 11, and the spontaneous emission with the amplified signal light exciting light Is output to the optical bandpass filter 15, and when only the signal light is passed to remove other excitation light and spontaneous emission light, the optical bandpass filter 15 has an extremely narrow passband characteristic. Is required.

In the configuration shown in FIG. 2, a filter using a fusion-type optical coupler 12 is provided for each section to pass signal light and pump light, and an optical bandpass filter 15 filters out the pump light. Things. Further, the fusion type optical coupler 12 has a configuration in which an optical fiber having a short length is fused to the amplification optical fiber 11 over a length to obtain desired characteristics, and the end of the optical fiber is obliquely cut or the like. Has a non-reflective termination configuration. Then, as shown in FIG. 4, the characteristic of the filter is such that a pass band occurs at every predetermined wavelength interval, so that the wavelength λ _{p of the} pump light and the wavelength λ _{s of the} signal light are set to the pass band. By setting to, the amplification optical fiber
Most of the spontaneous emission light in each section 11 is removed, and the excitation light and the signal light are added to the next section.

Therefore, if the characteristics of one fusion-type optical coupler 12 are indicated by solid lines in FIG. 4, the total characteristics for a plurality of sections are indicated by dotted lines, and most of the spontaneous emission light shown in FIG. 3 is removed. it can. Then, the optical bandpass filter 15 includes:
A light component in the vicinity including the wavelength λ _p and the wavelength λ _s in the dotted line in FIG. 4 is incident, and only the excitation light of the wavelength λ _p and the vicinity thereof need to be removed. The pass filter 15 can be realized with a relatively simple configuration.

FIG. 5 is an explanatory view of an embodiment of the present invention. For example, as described above, an amplification optical fiber 21 doped with erbium (Er) is supplied from an excitation light source 23 to, for example, a wavelength 1.48.
The μm pump light is incident via the multiplexer 24, and a signal light having a wavelength of 1.55 μm is incident via the multiplexer 24, and the signal light is amplified. The entire length of the amplification optical fiber 21 is divided into sections of lengths L ₁ , L ₂ ,..., And for each section, an optical bandpass filter 22 that allows only signal light to pass is provided.
The multiplexer / demultiplexer 25 demultiplexes only the short wavelength side including the excitation light,
The signal light from the optical bandpass filter 22 is multiplexed by the multiplexer / demultiplexer 26 via the optical fiber 27, and the signal light and the pump light can be made incident on the next section. That is, by removing the spontaneous emission light for each section and causing the signal light and the excitation light to enter the next space, the noise component can be suppressed and the signal light can be amplified.

In this case, the light incident in each section is branched and
An optical bandpass filter 22 that passes signal light through one of them
To the optical band-pass filter that allows the other excitation light to pass therethrough. That is, an optical bandpass filter that allows excitation light to pass through the optical fiber 27 can be provided. The filter configuration in each section of the amplification optical fiber may be a configuration that allows signal light and pump light to pass therethrough. Therefore, a configuration other than the above-described embodiment may be used.

[Effects of the Invention] As described above, the present invention divides the amplification optical fiber 1 having a predetermined length into a plurality of sections, and transmits the signal band or the excitation light through each section. A filter 2 constituted by a pass filter and a multiplexer / demultiplexer that passes the other filter is provided. Spontaneous emission light is removed for each section, and only signal light and pump light are incident.
Since a low-loss filter 2 using a fused optical coupler or the like can be used, noise components due to spontaneous radiation can be suppressed,
There is an advantage that only the signal light can be effectively amplified and output. That is, with a relatively simple configuration, there is an advantage that the S / N can be improved and the amplification factor can be improved. Therefore, it can be applied as an optical preamplifier having low noise characteristics.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the principle of the present invention, FIG. 2 is a diagram illustrating a case in which a fused optical coupler is provided, FIG. 3 is a diagram illustrating a spontaneous emission light spectrum, FIG. FIG. 5 is an explanatory view of an embodiment of the present invention, and FIG. 6 is an explanatory view of a conventional example. 1 is an amplification optical fiber, 2 is a filter, and 3 is an excitation light source.

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Claims (1)

(57) [Claims] 1. An optical fiber amplifier for amplifying a signal light by propagating a signal light and a pump light through an amplification optical fiber (1) having a predetermined length, wherein a plurality of the amplification optical fibers (1) are provided. , A filter (2) is provided in each of the sections, and the filter (2) is formed by an optical band-pass filter that passes one of the signal light and the pump light and a multiplexer / demultiplexer that passes the other. An optical fiber amplifier characterized by comprising.