JPH11186639A - Pumping light generator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the redundancy and pumping power of a pumping light generator. SOLUTION: Lasers 40a, 40b, 40c and 40d respectively oscillate laser beams at wavelengths λ1, λ2, λ3 and λ4. The wavelengths λ1 and λ2 are slightly shorter than the center wavelengths of the 980 nm absorbing wavelength bands of fiber amplifiers 20a and 20b and the wavelengths λ3 and λ4 are slightly longer than the center wavelengths. The laser beams outputted from the lasers 40a and 40b are made incident to a polarization beam splitter 42a in orthogonal polarization directions, polarized, and multiplexed. The laser beams outputted from the lasers 40c and 40d are made incident to a polarization beam splitter 42b in orthogonal polarization directions, polarized, and multiplexed. A 3-dB coupler 44 demultiplexes the laser beam from the beam splitters 42a and 42b to two laser beams after demultiplexing the laser beams. One output light of the coupler 44 is supplied to the fiber amplifier 20a through a WDM coupler 16a and the other output light of the coupler 44 is supplied to the fiber amplifier 20b through a WDM coupler 16b.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pump light generator, and more particularly, to a pump light generator that simultaneously excites the optical amplification media of two optical amplifiers.

[0002]

2. Description of the Related Art In an optical fiber communication system, particularly a long-distance optical fiber communication system using an optical submarine cable,
The reliability of an optical amplifier, especially the reliability of a pump light source that excites the optical amplification medium, is important. For that purpose, at first, the optical amplification medium with two pump laser diodes,
For example, a configuration capable of supplying excitation light to an erbium-doped optical fiber has been proposed and put to practical use. That is, a pump laser for supplying pump light from the front to the optical amplification fiber.
A diode and a pump laser diode to be supplied from the rear are provided, one pump laser diode is used as the current and the other is used as a spare, and the output light of the two pump laser diodes is multiplexed. There has been proposed a configuration in which an optical beam splitter that outputs light toward an amplification fiber is provided, one of the pump laser diodes is in use, and the other is a standby laser diode. In these configurations, two pump laser diodes are provided for one optical amplification fiber, and two pump laser diodes are provided.
When both diodes fail to emit light, the optical amplifier fails completely. In these configurations, a total of four pump laser diodes are required for both the upstream system and the downstream system.

[0003] After that, as a configuration that can obtain the same or higher reliability with a smaller number of laser diodes for pumps,
A configuration in which two pump laser diodes are multiplexed / demultiplexed by a 3 dB coupler and supplied to upstream and downstream optical amplification media has been proposed and put to practical use. FIG. 5 shows a schematic configuration diagram of a conventional optical amplification repeater. Reference numerals 10 and 12 denote laser diodes that generate pump light, and their output light is 3 dB.
After being mixed by the fiber coupler 14 and divided,
It is supplied to the DM couplers 16a and 16b. For example, W
The DM coupler 16a couples the pump light from one output of the fiber coupler 14 to the upstream optical transmission line 18a so as to propagate in a direction opposite to the signal light propagating through the upstream optical transmission line 18a. Couples the pump light from the other output of the fiber coupler 14 to the downstream optical transmission line 18b so as to propagate in the opposite propagation direction to the signal light propagating through the downstream optical transmission line 18b.

In the upstream optical transmission lines 18a and 18b, erbium-doped optical amplification fibers 20a and 20b are disposed upstream of the WDM couplers 16a and 16b, respectively, and viewed downstream of the WDM couplers 16a and 16b in the propagation direction of the signal light. On the side, optical isolators 22a and 22b for blocking return light propagating in the direction opposite to the signal light propagation direction are arranged, respectively. That is, the pumping light generated by the laser diodes 10 and 12 is supplied from the WDM couplers 16a and 16b to the optical amplifying fibers 20a and 20b, respectively, so that the signal light can be optically amplified.

In the upstream optical transmission line 18a, the signal light is
The signal light propagates from right to left in the downstream optical transmission line 18b. In the upstream optical transmission line 18a, the signal light propagating through the transmission optical fiber 24a enters the optical amplification fiber 20a and is optically amplified there, and then propagates through the WDM coupler 16a, the optical isolator 22a, and the transmission optical fiber 24a. . The same applies to the downstream optical transmission line 18b, in which the signal light propagating through the transmission optical fiber 24b enters the optical amplification fiber 20b and is optically amplified there. Thereafter, the WDM coupler 16b, the optical isolator 22b, and the transmission optical fiber 24b Is propagated.

In the configuration shown in FIG. 5, even if one of the laser diodes 10 and 12 is broken, the output light of the remaining laser diode can excite the optical amplification fibers 20a and 20b to a considerable extent. , 20b are less likely to be in a non-excited state entirely. However, in this configuration, the pumping power of each of the optical amplification fibers 20a and 20b is almost the same as when pumping by a single laser diode.

There is also known a pump light generator that further increases the redundancy and combines the output lights of the four laser diodes to divide them into two to excite two optical amplification fibers.
FIG. 6 shows a schematic block diagram of the conventional example. The output lights of the laser diodes 30a and 30b are multiplexed by a 3dB fiber coupler 32a, and the output lights of the laser diodes 30c and 30d are multiplexed by a 3dB fiber coupler 32b, so that each of the fiber couplers 32a and 32b is multiplexed. The output is multiplexed / demultiplexed by the 3 dB fiber coupler 34, and the two output lights are supplied to the optical amplification fiber 20a of the upstream transmission line and the optical amplification fiber 20b of the downstream transmission line, respectively. Thereby, the laser diodes 30a to 30d
Is supplied to the optical amplification fiber 20a and also to the optical amplification fiber 20b.

In a relay optical amplifier, an optical component such as an optical isolator is not arranged on the input side in order to reduce an effective noise figure, and a transmission optical fiber and an optical amplification fiber are directly connected in many cases. In this case, the pump light not absorbed by the optical amplification fiber is input to the transmission optical fiber,
A part thereof is radiated toward the optical amplification fiber by Rayleigh scattering and Brillouin scattering generated in the transmission optical fiber, and is consequently returned to the pump light source. The oscillation wavelength of the pump light source may change due to the injection locking phenomenon caused by the feedback light.

Therefore, in the conventional example shown in FIG.
Each of the optical amplifying fibers 20 is locked by injection locking to the diodes 10 and 12 and interference by the 3 dB fiber coupler 14.
There is a problem that the excitation power for the a and 20b becomes unstable. This can basically occur in the conventional example shown in FIG. The problem is that the laser diode 10,
The problem was solved by inserting an optical isolator into the twelve output stages. Conventionally, in the signal band used in optical communication, the wavelength of the pump light is the 1480 nm band. In this wavelength band, there is a low-loss optical isolator, and the laser diode itself has a high reliability with respect to an increase in output. is there.

[0010]

In the conventional example shown in FIG. 5, one laser diode (for example, diode 1) is used.
Even if 0) is broken, the other laser diode 12 contributes, so that the optical amplification fibers 20a and 20b do not become entirely non-excited. Further, in the conventional example shown in FIG. 6, the influence of a fault of one laser diode is 1/4.
And the reliability increases accordingly.

However, in the conventional example, the future 100 gbi
In consideration of high-speed transmission exceeding t / s, there is a concern that pump power may be insufficient. In the conventional example shown in FIG. 5, the pumping power of each of the optical amplification fibers 20 and 20b is almost the same as that in the case where only one of the laser diodes 10 and 12 is used. In the conventional example shown in FIG. Fiber 20
The pumping powers of a and 20b are almost the same as when pumped by one laser diode, and it cannot be said that the output of the laser diode is effectively utilized.

In recent years, low noise operation can be expected and a method of exciting an erbium-doped optical fiber in the 980 nm band is considered to be promising (for example, Japanese Patent Application No. 310508, 1996,
And 1996 Patent Application No. 329960). However, 98
Since the 0-nm-band laser diode is high-power and easily deteriorated, a configuration using both a 980-nm-band laser diode and a 1480-nm-band laser diode that is relatively noisy but can operate at a high output is considered promising. (See FIG. 6 of Japanese Patent Application No. 329960).

However, in the 980 nm band, the insertion loss of the optical isolator is 1 dB or more.
It is said that the life of a band laser diode is shortened in proportion to the fourth power of the output power, and an optical isolator having a large insertion loss cannot be used. Therefore, measures to avoid injection locking are required.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a pump light generating device which not only has a high degree of redundancy but also can effectively utilize optical power.

Another object of the present invention is to provide a pump light generator which does not cause injection locking.

Another object of the present invention is to provide a more reliable pump light generator with a small number of laser elements.

[0017]

A pump light generating apparatus according to the present invention is a pump light generating apparatus for supplying excitation light to two optical amplifying media, and having a wavelength belonging to an absorption wavelength band of the optical amplifying medium. First, second, third, and fourth laser devices that output laser light; first multiplexing means that combines output light from the first and second laser devices with polarizations orthogonal to each other;
A second multiplexing means for polarizing and combining the output lights of the third and fourth laser devices with orthogonal polarizations, and a multiplexed output light of the first and second multiplexing means for splitting. The output wavelengths of the first and second laser devices and the output wavelengths of the third and fourth laser devices are different from each other.

With this configuration, each optical amplifying medium is
Since the pumping is performed by the output lights of the two laser devices, a high redundancy configuration is achieved, and the reliability is improved. Since polarization combining is used for multiplexing the output lights of the two laser devices, the loss is small. Further, by making the wavelengths of the pump lights from the two multiplexing means different at the time of multiplexing by the multiplexing / demultiplexing means, it is possible to prevent the output ratio from fluctuating due to interference. Although four laser devices are used, practically only two wavelengths are required. Therefore, the laser devices are arranged near the center of the absorption wavelength band of the optical amplifying medium, so that good amplification characteristics can be realized.

When the first, second, third, and fourth laser devices have stabilized output wavelengths, injection locking becomes difficult.

By setting the output wavelengths of the first and second laser devices to be equal to each other and the output wavelengths of the third and fourth laser devices to be equal to each other, the output light of each laser device can be reduced to a narrow absorption wavelength band. Can be placed efficiently within
Optical amplification characteristics, particularly noise figure, can be improved.

By setting the output wavelengths of the first, second, third and fourth laser devices in the 980 nm band, a low-noise optical amplifier for a long-distance optical communication system can be realized.

[0022]

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. However, the same components as those in FIG. 5 are denoted by the same reference numerals.

The lasers 40a, 40b, 40c and 40d oscillate at wavelengths λ1, λ2, λ3 and λ4, respectively. Oscillation wavelengths λ1, λ2, λ of lasers 40a to 40d
3, .lambda.4 are stabilized by means described later.

The output lights of the lasers 40a and 40b are incident on the polarization beam splitter 42a in polarization directions orthogonal to each other and are polarized and synthesized. The output lights of the lasers 40c and 40d enter the polarization beam splitter 42b in polarization directions orthogonal to each other, and are polarization-combined. Since multiplexing is performed using orthogonal polarization, multiplexing loss is small. The 3 dB fiber coupler 44 multiplexes the laser beams from the polarization beam splitters 42a and 42b and splits them into two. 3dB fiber
One output light of the coupler 44 is the WDM coupler 16a
The other output light is supplied to the optical amplification fiber 20a through the WDM coupler 16b.
b.

Wavelength λ1 and wavelength λ2, wavelength λ3 and wavelength λ4
Are combined by the polarization beam splitters 42a and 42b, and therefore may be equal. However, the wavelength λ
1 and λ2 need to be different from the wavelengths λ3 and λ4. 3d
This is to avoid interference in the B fiber coupler 44. As shown in FIG. 2, the wavelengths λ1, λ2, λ3, and λ4 are the optical amplification fibers 20a and 20b (usually,
Erbium-doped fiber) in the 980 nm absorption wavelength band, the wavelength λ1 (and λ2) is slightly shorter than the center of the absorption wavelength band, and the wavelength λ3 (and λ4) is slightly longer than the center of the absorption wavelength band. Set to wavelength.

With such a configuration, the optical amplification fiber 2
0a is supplied with output light of four lasers 40a to 40d, and the optical amplification fiber 20b is also supplied with four lasers 40a to 40a.
-40d of output light will be supplied, resulting in a highly redundant light source configuration. That is, two lasers are provided for each of the optical amplification fibers 20a and 20b. In the present embodiment, the two lasers 40a, 40a
b; Since the output lights of 40c and 40d are polarization-combined by the polarization beam splitters 42a and 42b, the multiplexing loss can be reduced. As a result, the output of the lasers 40a to 40d becomes
It may be relatively low.

Since the 3 dB coupler 44 is used, the output light of the lasers 40a and 40b is transmitted to the optical transmission lines 18a and 18b.
Without being absorbed by the optical amplification fibers 20a and 20b,
The light may be reflected by Rayleigh scattering or Brillouin scattering by the transmission optical fibers 24a and 24b at the preceding stage, and may enter the lasers 40a to 40d via the WDM couplers 16a and 16b and the 3 dB fiber coupler 44. The same applies to the output light of the lasers 40c and 40d. The incidence of such external light may cause injection locking in the lasers 40a to 40d. Also, in the 3 dB coupler, when light of the same wavelength enters each port, interference occurs, and the output ratio of the 3 dB coupler fluctuates due to the phase relationship between the interference lights, which is also a problem.

In this embodiment, the polarization beam splitter 4
The oscillation wavelengths λ1 and λ2 of the lasers 42a and 42b that supply the laser light to the laser beam 2a, and the oscillation wavelengths λ of the lasers 42c and 42d that supply the laser light to the polarization beam splitter 42b.
Such an adverse effect can be prevented by setting the wavelength interval between 3, 3 and λ4 to such an extent that injection locking does not occur. Wavelength λ
1 and the wavelength λ2, and the wavelength λ3 and the wavelength λ4 may be equal, so that, in effect, the two wavelengths λ1 (λ2) and λ3 (λ
4) only needs to be put in the narrow absorption wavelength band of the 980 nm band. As a result, these wavelengths λ1 (λ2) and λ3 (λ
4) can be located near the center of the absorption wavelength band,
Therefore, the amplification characteristics (gain and noise figure) of the optical amplification fibers 20a and 20b can be improved.

From the viewpoint of preventing injection locking, it is needless to say that the wavelength λ1 and the wavelength λ2, and the wavelength λ3 and the wavelength λ4 are preferably greatly different. However, if each wavelength difference is too large, the wavelength band of the pump light wavelength becomes too wide with respect to the narrow absorption wavelength characteristic of the optical amplification fiber, and the optical amplification characteristics of the optical amplification fibers 20a and 20b deteriorate, that is,
The gain becomes small and the noise figure deteriorates, which is not preferable. Further, if the characteristics of the pump laser differ greatly, the cost will increase.

Each laser 40a, 40b, 40c, 40d
Uses a reflective element provided outside the laser diode to convert a specific wavelength component of the output light of the laser diode
The oscillation wavelength is stabilized by feedback to the diode. FIG. 3 is a schematic configuration diagram of a wavelength stabilized laser. The wavelength-stabilized laser has the specified wavelengths λ1, λ2, λ
3, a laser diode 46 manufactured to oscillate around λ4, and designated wavelengths λ1, λ2, λ3, λ
It is composed of a polarization-maintaining fiber grating 48 that selectively reflects about four percent of the four components. Laser diode 4
The output light of 6 is a polarization-maintaining fiber grating 4
8 and a part of the specified wavelength λ1, λ2, λ3, λ4 component is partially reflected and returned to the laser diode 46.
As a result, the oscillation wavelength of the laser diode 46 is forced by the reflection wavelength of the polarization maintaining fiber grating 48 and is stabilized. Fiber grating 48
The laser light transmitted through becomes the output light of the lasers 40a to 40d.

By stabilizing the wavelength by the fiber grating 48 and sufficiently separating the wavelengths λ1 (λ2) and λ3 (λ4) so as not to cause injection locking, light incident from outside (the coupler 42 and the beam splitter) 44a,
44b etc., and the transmission paths 18a, 18b
Of the signal light and the scattered light transmitted therethrough) can be reduced or prevented.

With such a configuration, each of the optical amplification fibers 20a and 20b outputs the output light (wavelength λ1) of the lasers 40a and 40b and the output light (wavelength λ) of the lasers 40c and 40d.
Excited by 2). Each optical amplification fiber 20a, 20
Since b is excited by the output lights of the four lasers 40a to 40d, the redundancy of the light source is sufficiently increased, and the reliability is improved.

FIG. 4 shows the wavelength of the prototype pump light generator and the loss of each part. 980n for lasers 40a and 40b
m, and lasers 40c and 40d are 978 nm
Laser oscillation. Between the lasers 40a, 40b and the polarizing beam splitter 42a, and
0c, 40d and the polarization beam splitter 42b are connected by a polarization maintaining fiber for 980 nm, and the polarization beam splitters 42a, 42b and the 3 dB coupler 4b are connected.
4, 3 dB coupler 44 and WDM coupler 1
6a and 16b were connected by an optical fiber for 980 nm band. The loss of each part is shown in FIG.

The fusion loss at the portion where the polarization maintaining fiber is used is about 0.3 dB, and the fusion loss at the general optical fiber portion is about 0.1 dB. Polarized beam splitter 42
a, the loss at the 42b is 0.7 dB, the loss at the 3 dB coupler 44 is 3 + 0.3 dB, and the WDM coupler 16
a, the loss at 16b is 0.2 dB, and the optical amplification fiber 20
The loss at the connection point between a and 20b is 0.2 dB. As a result, the loss from the outputs of the lasers 40a and 40d to the inputs of the optical amplification fibers 20a and 20b is about 4.7 dB.
And can be kept fairly low. For example, laser 4
Assuming that the output from 0a to 40d is 60 mW, each of the optical amplifying fibers 20a and 20b can be pumped at about 81 mW.
a and 20b. In other words, since the intermediate loss is small, the output power of the lasers 40a and 40d may be low, and as a result, the life is prolonged and the reliability is improved.

[0036]

As can be easily understood from the above description, according to the present invention, since one optical amplifying medium is excited by four laser devices, a high degree of redundancy can be secured. The optical output power of each laser device can be efficiently supplied to the optical amplification medium,
Since pump power of two laser devices can be realized per optical amplification medium, an economical optical amplifier can be realized. Since the wavelengths can be sufficiently separated between laser devices that may be injection-locked, injection locking can be avoided.

[Brief description of the drawings]

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

FIG. 2 shows laser oscillation wavelengths λ1, λ2, λ of the present embodiment.
It is a figure which shows the relationship of 3, (lambda) 4.

FIG. 3 is a schematic configuration diagram of lasers 40a to 40d.

FIG. 4 is a diagram showing losses of respective parts of a prototype using the present embodiment.

FIG. 5 is a schematic configuration diagram of a conventional example.

FIG. 6 is a schematic configuration diagram of another conventional example.

[Explanation of symbols]

 10, 12: laser diode 14: 3dB fiber coupler 16a, 16b: WDM coupler 18a: upstream optical transmission line 18b: downstream optical transmission line 20a, 20b: erbium-doped optical amplification fiber 22a, 22b: optical isolator 24a, 24b: transmission Optical fibers 30a, 30b, 30c, 30d: laser diodes 32a, 32b: 3dB fiber coupler 34: 3dB fiber couplers 40a, 40b, 40c, 40d: lasers 42a, 42b: polarization beam splitter 44: 3dB fiber coupler 46 ： Laser diode 48 ： Polarization maintaining fiber grating

Claims (5)

[Claims] 1. A pump light generator for supplying pump light to two optical amplifying media, comprising: a first, a second, a third, and a second laser for outputting laser light having a wavelength belonging to an absorption wavelength band of the optical amplifying medium. A fourth laser device, first multiplexing means for combining the output lights of the first and second laser devices with orthogonal polarizations, and orthogonalizing the output lights of the third and fourth laser devices. A second multiplexing unit that performs polarization combining with the polarized light to be combined, and a multiplexing / demultiplexing unit that multiplexes and demultiplexes the multiplexed output light of the first and second multiplexing units. An output wavelength of the second laser device and output wavelengths of the third and fourth laser devices are different from each other. 2. The pump light generator according to claim 1, wherein said multiplexing / demultiplexing means is a 3 dB coupler. 3. The pump light generator according to claim 1, wherein the first, second, third, and fourth laser devices have stabilized output wavelengths. 4. The pump according to claim 1, wherein output wavelengths of the first and second laser devices are equal to each other, and output wavelengths of the third and fourth laser devices are equal to each other. Light generator. 5. An output wavelength of the first, second, third and fourth laser devices belongs to a 980 nm band.
The pump light generator according to any one of claims 1 to 4.