JPH05283787A - Optical amplifier - Google Patents

Abstract

PURPOSE:To provide an optical amplifier which is made stable in amplification factor based on the quantity of spontaneous emission light separated from light signal generated in light amplification medium, where a rare earth doped optical fiber or a semiconductor laser is used as light amplification medium, and induced emission phenomenon is utilized. CONSTITUTION:A current is fed to a semiconductor laser 1 from a constant current source 2 corresponding to a required amplification factor to amplify signal lights, where the spontaneous emission light is generated. The signal light and the spontaneous emission light outputted from the semiconductor laser 1 are inputted into a light branching device 3 and separated from each other, and the signal light and the spontaneous emission light separated from each other through the light branching device 3 are inputted into a photodetector 4. The photodetector 4 converts the spontaneous emission light inputted from the branching device 3 into electrical signals, being inputted into a controller 5. The controller 5 controls a constant current source 2 corresponding to the size of the electrical signal to form a negative feedback loop.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplifier utilizing the stimulated emission phenomenon used in the field of optical communication or optical measurement, and more particularly to an optical amplifier having a stable amplification factor.

[0002]

2. Description of the Related Art As means for amplifying light propagating in an optical fiber, there are a rare earth-doped fiber type optical amplifier using a rare earth-doped optical fiber as an optical amplification medium and a semiconductor laser type optical amplifier using a semiconductor laser. These optical amplifiers amplify light by the phenomenon of stimulated emission of light. This stimulated emission phenomenon of light is described in the literature (Yonezu: Optical Communication Device Engineering (2nd Edition), P26-30, Engineering Book Co., 1984.
2), the basic principle is shown in FIG.
It demonstrates using (e). Regarding an isolated atom having an energy level as shown in FIG. 3A, focusing on two specific energy levels (1 and 2), electrons are in a low energy level in a thermal equilibrium state (see FIG. b)).
When excitation energy such as light is applied in this state, the electrons are pushed up to higher energy levels (FIG. 3 (c)). The excitation energy at this time is the amount of light in the rare earth-doped fiber amplifier, and the injection current in the semiconductor laser amplifier.

Then, in this state, the excited electrons try to return to the original stable energy level, and at that time, extra energy is consumed by light emission (FIG. 3 (d)). This phenomenon is called spontaneous emission. At this time, when light having the same energy level difference is input from the outside, light having the same phase and the same wavelength as the input light is emitted to perform optical amplification (FIG. 3).
(E)). This phenomenon is called stimulated emission. In such an optical amplifier, the light amount and the amplification factor of the spontaneous emission light (the light spontaneously emitted in FIG. 3D above) have a positive correlation with the amount of electrons existing in the higher energy level. is there. The amount of the electrons fluctuates due to changes in the excitation energy supplied from the outside and changes in the ambient temperature, and as a result, the amplification factor fluctuates depending on these conditions.

Further, when the level of signal light input from the outside is high, the amount of electrons consumed by stimulated emission is smaller than the amount of electrons excited to the upper energy level by the energy supply from the outside. Since it cannot be ignored, there is a problem in that the amount of electrons existing in the upper energy level decreases and the amplification factor decreases. The conventional techniques for solving such problems are roughly divided into the following two.

Prior Art 1 As for the rare earth-doped fiber type optical amplifier, as shown in FIG. 4, from the output light amplified and output by the rare earth-doped optical fiber 12, the pumping light (in the pumping light generator 16) The residual light (generated and incident on the rare earth-doped optical fiber 12 through the multiplexing unit 13) is separated by the demultiplexer 17, detected by the photodetector 14, and the controller 15 determines based on this detection output. A method of controlling the excitation energy (injection current) of the excitation light generator 16 is disclosed in JP-A-3-100527. In this method, even if the amplification factor fluctuates due to changes in ambient temperature, disturbances such as mechanical vibration, etc., fluctuations in the amplification factor are detected as fluctuations in the residual pump light, and a negative feedback loop is formed based on this. Therefore, the amplification factor is stabilized. However, this method cannot be applied to a semiconductor laser type optical amplifier that does not use pumping light because it detects residual pumping light as described above.

Prior Art 2 On the other hand, as a method that can be used for both rare earth-doped fiber type and semiconductor laser type optical amplifiers, an output side (or an input side and an output side) of an optical amplification medium (rare earth-doped optical fiber, semiconductor laser, etc.) JP-A-2-27.
It is disclosed in Japanese Patent Publication No. 3976 and Japanese Patent Laid-Open No. 3731/1993. However, in this method, since the signal light must be branched, there is a problem that the signal light is lost by the optical demultiplexer or the like.

[0007]

The problems to be solved by the conventional techniques are as follows, and an object of the present invention is to provide an optical amplifier which solves these problems. The method of detecting the residual pumping light and controlling the amplification factor cannot be applied to the semiconductor laser type optical amplifier. The method of branching a part of the signal light to control the amplification factor is
Inefficiency due to loss of signal light. Further, in the method of detecting the residual pumping light and the method of branching a part of the signal light, an optical hybrid circuit such as an optical demultiplexer for branching a part of the residual pumping light and the signal light is Since it must be provided on the output side, in the case of an optical amplifier intended for high power light, there is a risk that the high power light output from the optical amplifier may damage the optical hybrid circuit.

[0008]

In order to solve the above problems, in the present invention, the magnitude of the amplification factor is the magnitude of the spontaneous emission light generated in the optical amplification medium (especially on the shorter wavelength side than the signal light wavelength). We paid attention to the fact that it is accurately proportional to. Spontaneous emission light is always generated in the case of an optical amplifier utilizing the stimulated emission phenomenon, such as a semiconductor laser type or rare earth-doped fiber type optical amplifier. Since the wavelength band of the spontaneous emission light is wide, the wavelength of the part different from the wavelength of the signal light and accurately proportional to the magnitude of the amplification factor is extracted from the band, and the amplification factor Can be used for control. That is, the spontaneous emission light emitted from the optical amplification medium is detected, and the excitation energy is controlled so as to keep the detected output constant, thereby stabilizing the amplification factor.

[0009]

The relationship between the spontaneous emission light and the amplification factor and the method of separating the spontaneous emission light will be described below. Relationship between spontaneous emission light and amplification factor First, spontaneous emission light will be described. As an example, FIG.
When the signal light having the optical spectrum as shown in (a) is input to the optical amplifier as the input light, the output light having the optical spectrum as shown in FIG. 5B is output from the optical amplifier. That is, in the output light of the optical amplifier, in addition to the amplified signal light, light called spontaneous emission light is distributed around the amplified signal light. The spontaneous emission light is the light emitted by the optical amplification medium itself, and the half-valued wavelength bandwidth Δλspon is several tens.
nm. This spontaneous emission light is generated regardless of whether it is a semiconductor laser type or a rare earth-doped fiber type optical amplifier, as long as it is an optical amplifier utilizing the stimulated emission phenomenon.

As described above, the amount of spontaneous emission light is proportional to the number of electrons existing in the upper energy level,
Since the same is true for stimulated emission light, the amplification factor is accurately proportional to the amount of spontaneous emission light. This tendency is particularly remarkable in spontaneous emission light on the shorter wavelength side (high energy side) than the signal light.

Next, the relationship between the spontaneous emission light power and the amplification factor will be described. FIG. 6 shows an example of the characteristics of the amplification factor Gain and the spontaneous emission light power Pspon when the input light power Pin is changed in the Er-doped fiber optical amplifier (a kind of rare earth-doped fiber type). In a region where the input optical power is high, the internal population inversion is reduced, so that the amplification factor is reduced, and at the same time, the spontaneous emission optical power is also reduced. As a result, it is found that the characteristics of the amplification factor and the spontaneous emission light power are proportional to the input light power.

FIG. 7 shows an example of the amplification factor Gain and the spontaneous emission light power Pspon characteristic when the ambient temperature Temp is changed in the semiconductor laser type optical amplifier. It can be seen that the characteristics of the amplification factor and the spontaneous emission power are also proportional to the temperature. Separation Method of Spontaneous Emission Light Generally, the wavelength of signal light is known, and the half-valued wavelength bandwidth Δλsig (shown in FIG. 3 (a)) is 0.1 nm or less in the case of distributed feedback laser. On the other hand, since the half-valued wavelength bandwidth Δλspon of spontaneous emission light is several tens of nm, signal light and spontaneous emission light can be separated by ordinary optical demultiplexers using dielectric multilayer films, optical directional couplers, etc. The optical hybrid circuit of is sufficient.

Further, the spontaneous emission light generated in the optical amplification medium is emitted toward both the input side and the output side of the optical amplification medium. Therefore, when the signal light input to the optical amplifier and the spontaneous emission light are separated at the input side of the optical amplification medium, an optical demultiplexer, an optical directional coupler, an optical duplexer, an optical circulator, or the like is used. Further, when the signal light output from the optical amplifier and the spontaneous emission light are separated on the output side of the optical amplification medium, an optical demultiplexer is used. The optical demultiplexer can be used on both the input side and the output side of the optical amplification medium, as described above. Some optical circulators are reported in, for example, "1991 Autumn Meeting of the Institute of Electronics, Information and Communication Engineers, B-665, high isolation characteristics in polarization independent circulator type optical circuits".

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. (First Embodiment) FIG. 1 is a block diagram showing a schematic configuration of a semiconductor laser type optical amplifier according to the first embodiment of the present invention. Signal light input from the input end (for example, wavelength 1552
nm) is input to the semiconductor laser 1 (optical amplification medium) by an optical coupling means such as a spherical fiber. Semiconductor laser 1
A current (excitation energy) according to a desired amplification factor is supplied from the constant current source 2 (excitation device) to amplify the signal light and generate spontaneous emission light. The signal light and the spontaneous emission light output from the semiconductor laser 1 are extracted by the optical coupling means and input to the optical demultiplexer 3.

The optical demultiplexer 3 is composed of, for example, a bandpass filter having a bandwidth of 1 nm and has terminals a, b and c. The signal light and spontaneous emission light input from the port a are
The signal light is separated by the wavelength, and the signal light is output to the output end from port b through a 1 nm bandwidth filter. The spontaneous emission light on the shorter wavelength side (higher energy side) than the signal light is extracted with a predetermined bandwidth and then port. It is output from c to the photodetector 4. The photodetector 4 converts the spontaneous emission light output from the optical demultiplexer 3 into an electric signal, and the controller 5 (negative feedback circuit)
Output to. The controller 5 controls the constant current source 2 according to the magnitude of the electric signal from the photodetector 4, and forms a negative feedback loop.

In the optical amplifier thus constructed,
Even if the amplification factor of the semiconductor laser 1 fluctuates due to changes in various conditions, the fluctuation of the amplification factor is converted into an electric signal by the photodetector 4 as a fluctuation of the amount of spontaneous emission light, and this electric signal is always constant. The constant current source 2 is controlled by the controller 5 so that the amplification factor can be stabilized. Although the spontaneous emission light is extracted on the output side of the semiconductor laser 1 in this embodiment, the optical demultiplexer 3 may be provided on the input side and extracted on the input side. In that case, in particular, In the optical amplifier for high power light, there is an effect that there is no possibility of damaging the optical demultiplexer 3 by the high power light output from the optical amplifier. Further, the optical demultiplexer 3 is not limited to the band pass filter, and may be a high-pass blocking filter or a low-pass blocking filter.

(Second Embodiment) FIG. 2 is a block diagram showing a schematic structure of a rare earth-doped fiber type optical amplifier according to a second embodiment of the present invention. The Er-doped fiber 6 (optical amplification medium) is a kind of rare-earth-doped optical fiber, and is an optical fiber doped with a rare-earth element erbium (element symbol: Er). Excitation light (excitation energy) having a desired amplification factor is supplied to the Er-doped fiber 6 by the excitation light source 8
From the optical multiplexer 7 and the optical duplexer 9. The excitation light source 8 is composed of, for example, a semiconductor laser and a constant current source (not shown), and the amount of excitation light is controlled by the current supplied from the constant current source to the semiconductor laser.

The signal light (for example, wavelength 1552 nm) input from the input end is multiplexed with the pump light by the optical multiplexer 7.
The combined light passes through the port A of the optical duplexer 9 and enters the Er-doped fiber 6 connected to the port B. The Er-doped fiber 6 has a desired amplification factor due to the excitation light in the combined light, amplifies the signal light and outputs it to the output end, and also generates spontaneous emission light. The spontaneous emission light generated by the Er-doped fiber 6 is
The light passes through the port B of the optical duplexer 9 and is input to the photodetector 10 connected to the port C. The photodetector 10 converts the spontaneous emission light output from the optical duplexer 9 into an electric signal and outputs the electric signal to the controller 11 (negative feedback circuit). The controller 11 controls a constant current source (not shown) of the excitation light source 8 according to the magnitude of the electric signal from the photodetector 10,
Form a negative feedback loop.

In the optical amplifier thus constructed,
Even if the amplification factor of the Er-doped fiber 6 fluctuates due to changes in various conditions, the fluctuation of the amplification factor is converted into an electric signal by the photodetector 10 as a fluctuation of the amount of spontaneous emission light, and this electric signal is always Since the pumping light source 8 is controlled by the controller 11 so as to be constant, the amplification factor can be stabilized.

The optical duplexer 9 also functions as an optical isolator and contributes to improving the stability of the amplification function. Moreover, Er is connected to the port C of the optical duplexer 9.
The signal light reflected from the doped fiber 6 may appear, or the light due to nonlinear optics may appear. In such a case, an optical filter may be provided between the port C and the photodetector 10 to remove unnecessary light other than spontaneous emission light. In this embodiment, the optical duplexer 9 is used for extracting the spontaneous emission light, but the present invention is not limited to this, and an optical demultiplexer, an optical directional coupler, or an optical circulator may be used.

[0021]

As described above, in the optical amplifier of the present invention, the amplification factor thereof is such that the amount of spontaneous emission light generated in the optical amplification medium (especially on the shorter wavelength side than the signal light wavelength).
By paying attention to the fact that they are accurately proportional to each other, and separating and extracting this spontaneous emission light from the signal light and using it for controlling the amplification factor, the following effects could be achieved. Since the spontaneous emission light always exists in the optical amplifier with stimulated emission, it can be applied to both the semiconductor laser type and the rare earth-doped fiber type optical amplifiers, and the amplification factor can be stabilized.
In particular, when the spontaneous emission light on the short wavelength side of the signal light is used, it is possible to perform control with higher accuracy. Since the spontaneous emission light can be extracted at a wavelength different from that of the signal light, no loss occurs in the signal light. Since an optical hybrid circuit such as an optical demultiplexer can be provided on the input side of the optical amplifier, especially in an optical amplifier for high power light, the high power light output from the optical amplifier damages the optical hybrid circuit. There is no fear of doing it.

[Brief description of drawings]

FIG. 1 is a block diagram showing a schematic configuration of a semiconductor laser type optical amplifier according to a first embodiment of the present invention,

FIG. 2 is a block diagram showing a schematic configuration of a rare earth-doped fiber optical amplifier according to a second embodiment of the present invention,

FIG. 3 is a diagram for explaining a stimulated emission phenomenon,

FIG. 4 is a block diagram of a conventional rare earth-doped fiber optical amplifier,

FIG. 5 is a spectrum example of input light input to an optical amplifier and output light amplified and output,

FIG. 6 shows characteristics of input optical power versus amplification factor / spontaneous emission optical power in an Er-doped fiber type optical amplifier,

FIG. 7 shows the ambient temperature vs. amplification factor / spontaneous emission light power characteristics in a semiconductor laser type optical amplifier.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Semiconductor laser, 2 ... Constant current source, 3 ... Optical demultiplexer, 4, 10 ... Photodetector, 5, 11 ... Controller, 6 ... Er-doped fiber, 7 ... Optical multiplexer, 8 ... Excitation light source, 9 ... Optical duplexer.

Claims (1)

[Claims] 1. An optical amplifying medium (1) which gives an amplifying action by a stimulated emission phenomenon to a wavelength of a desired signal light by giving an exciting energy, and a pumping device (2) which supplies the exciting energy to the optical amplifying medium. ) And amplifying and outputting the input signal light, the spontaneous emission light emitted from the optical amplification medium, which is provided on the input side or the output side of the optical amplification medium, is used as the signal light. An optical hybrid circuit (3) that separates and extracts, a photodetector (4) that receives the spontaneous emission light extracted by the optical hybrid circuit and converts it into an electrical signal, and the magnitude of the output signal of the photodetector And a negative feedback circuit (5) for controlling the pumping device so that the amplification factor of the optical amplification medium changes in accordance with the above.