JPH11186641A - Optical amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent pumping light from returning to a pumping light source when the pumping light source is excited in a 980 nm band. SOLUTION: A 980 nm band rejection filter 34 which absorbs and diffuses 980 nm band light, an erbium-doped optical fiber (fiber amplifier) 36, a WDM coupler 38, and an optical isolator 40 are connected in series between optical fibers 30 and 32 for transmission. The 980 nm band pumping light outputted from a laser diode 42 for pumping is multiplexed on an optical transmission line by means of the coupler 38 and made incident to the erbium-doped optical fiber 36 in the direction opposite to the propagating direction of signal light. The filter 34 is composed of, for example, a leak type fiber grating which radiates 980 nm band light to the outside or a WDM coupler which absorbs the 980 nm band light by discharging the light from the optical transmission line.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an optical amplifier.

[0002]

2. Description of the Related Art In an optical fiber communication system, particularly a long-distance optical fiber communication system, a large number of optical amplification repeaters are used on an optical transmission line.

FIG. 2 is a schematic block diagram of a conventional optical amplification repeater. Between the transmission optical fibers 10 and 12,
Erbium-doped optical fiber (optical amplification fiber) 14, W
The DM coupler 16 and the optical isolator 18 are serially connected. Pump light output from the pump laser diode 20 is transmitted through the optical isolator 22 to the WDM
The light enters the coupler 16, is multiplexed on the optical transmission line by the WDM coupler 16, and enters the erbium-doped optical fiber 14 in a direction opposite to the propagation direction of the signal light. This allows
The erbium-doped optical fiber 14 is excited and optically amplifies the signal light from the transmission optical fiber 10.

The pump light that has passed without being absorbed by the erbium-doped optical fiber 14 is reflected by the transmission optical fiber 10 by Rayleigh scattering or Brillouin scattering, and returns to the laser diode 20 via the erbium-doped optical fiber 14 and the WDM coupler 16. I'm trying to make an optical isolator 2
2 prevents the return light from being incident on the laser diode 20. This is because when the return light is incident on the laser diode 20, the operation of the laser diode becomes unstable due to injection locking or the like.

[0005] In order to reduce the effective noise figure, back pumping is generally employed.

Usually, in order to improve the noise figure of the erbium-doped optical fiber 14, a large pump power is required not only at the pump light input side but also at the pump light output side of the erbium-doped optical fiber 14. Power pump light is incident on the erbium-doped optical fiber 14. For example, it is 40 mW on the pump light incident side, and about 10 mW remains on the pump light output side.

[0007]

In recent years, attention has been paid to a configuration using 980 nm band pump light because low noise can be obtained (for example, Japanese Patent Application No. 310508/1996).
issue). As mentioned earlier, erbium-doped optical fiber 1
The pump light not absorbed in 4 enters the upstream transmission optical fiber 10, where it is reflected by Rayleigh scattering or Brillouin scattering, propagates in the same direction as the signal light, and again enters the erbium-doped optical fiber 14. I do. Return light due to Rayleigh scattering in the transmission optical fiber 10 is 980 nm.
It is about -29 dB (actual value) in the band.

Although this return light is partially absorbed by the erbium-doped optical fiber 14, it is partially transmitted, and
The light is incident on the pump laser diode 20 by the coupler 16. Return light incident on the pump laser diode 20 is about -35 dB. The operation of the laser diode 20 due to this return light becomes unstable.

A laser diode 2 for such a return light
Although an optical isolator 22 is provided for the purpose of preventing incidence to zero, the optical isolator 22 cannot be used for 980 nm band pump light. This is because it has been found that the life of the 980 nm laser diode becomes extremely short as the light output increases. The reason is that the insertion loss of the 980 nm band optical isolator is as large as 1 dB or more. Therefore, if the optical isolator is used, the life of the laser diode 20 is extremely shortened.

It is an object of the present invention to provide an optical amplifier which can prevent return light from entering a pump light source without using an optical isolator.

[0011]

According to the present invention, there is provided a removing means for removing, without reflecting, the pump light which has passed through the optical amplifying medium. As a result, it is possible to prevent the return light of the pump light from re-entering the optical amplifying medium, or it is possible to greatly reduce the re-incident intensity of the return light. As a result, the return light to the pump light source can be greatly reduced, and the operation of the pump light source and, consequently, the operation of the optical amplification medium are stabilized.

The pump light is multiplexed on the optical transmission line by, for example, a WDM coupler disposed on the optical transmission line and supplied to the optical amplification medium.

When the pump light source is a 980-nm band laser device, no optical isolator is required, so that the output power can be relatively reduced, and a longer life and improved reliability can be measured.

The removing means is, for example, a leaky fiber grating that selectively leaks the wavelength band of the pump light, or a WDM that selectively splits the wavelength band of the pump light.
Consists of a coupler. Thereby, the return light of the pump light can be efficiently removed, and the insertion loss to the signal light can be suppressed low. Moreover, it can be realized at low cost and easily.

[0015]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a schematic block diagram of one embodiment of the present invention. Between the transmission optical fibers 30 and 32, a 980 nm band removal filter 34 for absorbing and dispersing 980 band light, an erbium-doped optical fiber (optical amplification fiber) 36, a WDM coupler 38, and an optical isolator 40 are serially connected. The 980 nm band pump light output from the pump laser diode 42 is multiplexed on the optical transmission line by the WDM coupler 38 and enters the erbium-doped optical fiber 36 in the direction opposite to the signal light propagation direction.

The 980-nm band removal filter 34 is, specifically, a leak-type fiber grating that radiates the 980-nm band to the outside or a WDM coupler that emits and absorbs the 980-nm band light out of the optical transmission line. In these optical elements, the loss in the 1550 nm band, which is the signal light wavelength band, can be reduced to about 0.1 to 0.2 dB, so that the deterioration of the noise figure of the entire optical repeater is slight.

The 980 nm band pump light output from the pump laser diode 42 is multiplexed on the optical transmission line by the WDM coupler 38, and the erbium-doped optical fiber 36
, The light beam is incident in a direction opposite to the propagation direction of the signal light. Most of the pump light is absorbed by the erbium-doped optical fiber 36, whereby the signal light is optically amplified. The pump light that is not absorbed by the erbium-doped optical fiber 36 enters the 980 nm band removal filter 34 from the erbium-doped optical fiber 36. The 980 nm band removal filter 34
The pump light from the erbium-doped optical fiber 36 is emitted or emitted to the outside. As a result, the return light power of the pump light re-entering the erbium-doped optical fiber 36 is greatly reduced as compared with the conventional example, and as a result, the intensity of the return light to the laser diode 42 does not need to include an optical isolator. To a large extent.

Although the description has been given of the case of backward pumping, the same applies to forward pumping. A 980 nm band elimination filter may be arranged downstream of the erbium-doped optical fiber.

[0020]

As can be easily understood from the above description, according to the present invention, the intensity of the return light to the pump light source can be greatly reduced, so that the operation of the pump light source and the operation of the optical amplifier can be stabilized. be able to. Since no optical isolator is required, the output power of the pump light source can be reduced accordingly, leading to a longer life of the pump light source and an improvement in reliability.

[Brief description of the drawings]

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

FIG. 2 is a schematic block diagram of a conventional example.

[Explanation of symbols]

 10, 12: Transmission optical fiber 14: Erbium-doped optical fiber (optical amplification fiber) 16: WDM coupler 18: Optical isolator 20: Pump laser diode 22: Optical isolator 30, 32: Transmission optical fiber 34: 980 nm band Removal filter 36: Erbium-doped optical fiber (optical amplification fiber) 38: WDM coupler 40: Optical isolator 42: Laser diode for pump

Claims (5)

[Claims] An optical amplification medium disposed on an optical transmission line; a pump light source; coupling means for supplying pump light output from the pump light source to the optical amplification medium; An optical amplifier, comprising: a removing unit that removes a pump light without reflecting it. 2. The optical amplifier according to claim 1, wherein said coupling means is a WDM coupler disposed on said optical transmission line. 3. The optical amplifier according to claim 1, wherein said pump light source comprises a 980 nm band laser device. 4. The optical amplifier according to claim 1, wherein said removing means comprises a leaky fiber grating that selectively leaks a wavelength band of said pump light. 5. The optical amplifier according to claim 1, wherein said removing means comprises a WDM coupler for selectively splitting a wavelength band of said pump light.