JPH05107573A - Optical amplifier - Google Patents

Abstract

PURPOSE:To enable easy gain constant control over a wide signal light level range by eliminating the need for a photodetector and a complicate electric control circuit. CONSTITUTION:An epsilon rbium(Er)-doped fiber 11 as an optical amplifying medium and a saturable absorbing medium (e.g. epsilon rbium(Er)-doped fiber 12) are longitudinally connected. Input signal light and exciting light from an exciting light source 14 after being multiplexed by a multiplexing module 13 are inputted to the Er-doped fiber 11. The signal light which is amplified while traveling in the Er-doped fiber 11 is inputted to the Er-doped fiber 12 and then outputted while part of it is absorbed. The Er-doped fiber 12 is so constituted as to compensate the input signal light dependency of the gain characteristics of the Er-doped fiber 11, so the gain is controlled to a constant value over a wide signal light level range. Neither the photodetector nor the complicate electric control circuit is needed.

Description

Detailed Description of the Invention

[0001]

[Industrial application] In recent years, research and development of optical communication systems have been vigorously pursued, and the importance of booster amplifiers, repeaters, and preamplifiers using optical amplification technology has become clear. The present invention relates to these optical amplifiers, and more particularly, to an optical amplifier in which an optical amplification medium and a saturable absorption medium are connected in cascade so that constant gain control is performed at a wide signal light level.

[0002]

2. Description of the Related Art In an optical communication system, when an optical amplifier is applied as a booster amplifier, a repeater or a preamplifier, control of the optical amplifier is indispensable. A conventional optical amplifier using an Erbium (hereinafter referred to as Er) doped fiber as an optical amplification medium is shown in FIG.

As shown in the figure, a part of the input signal light is branched by the optical coupler 21, and the remaining input signal light is multiplexed by the multiplexer 22.
Then, it is multiplexed with the pumping light output from the pumping light source 24 such as a laser diode under the control of the control circuit 23, and is input to the Er-doped fiber 25. Here, the excitation light source 24
Is provided to energize the Er ions in the Er-doped fiber 25 to excite electrons.

A part of the signal light that is input to the Er-doped fiber 25 and is output by being amplified while traveling through the fiber is branched by the optical coupler 26 and the rest is output to the outside. The signal lights branched by the optical couplers 21 and 26 are respectively received by the photodetectors 27 and 28 such as photodiodes, converted into electric signals and output to the control circuit 23.

The control circuit 23 keeps the output signal light power (that is, the gain) relative to the input signal power constant.
For example, the drive current of the pumping light source 24 is changed to control the pumping light power to perform the constant gain control (AGC). Further, the control circuit 23 performs control (APC) for keeping the output power constant.

[0006]

By the way, as in the above-mentioned conventional example, in a system of controlling an optical amplifier after converting an optical signal into an electric signal by using an electric control circuit, the response is slow and the control is performed. There was a problem that the system became complicated.

An object of the present invention is to eliminate the need for a photodetector and a complicated electric control circuit, and to easily perform constant gain control at a wide signal light level.

[0008]

In the optical amplifier of the present invention, by connecting the optical amplification medium 1 (see the principle block diagram of FIG. 1, the same hereinafter) and the saturable absorption medium 2 in cascade, a wide signal can be obtained. It is characterized in that constant gain control is performed at the optical level.

[0009]

First, the principle of the present invention will be described. When, for example, an Er-doped fiber is used as the optical amplification medium, the gain vs. input signal light power characteristic of the optical amplification medium alone is
It becomes as shown in FIG. As shown in the figure, as the input signal light becomes larger, the gain typically sharply decreases from a certain point P ₁ (gain saturation phenomenon).

The loss vs. input signal light power characteristic of the saturable absorbing medium alone is as shown in FIG. 2 (b). As shown in the figure, it shows a certain loss characteristic with respect to the input signal light power up to a certain degree, but when the input signal light is made larger than that, saturation of the medium that absorbs the signal light starts, and from a certain point P ₂ Losses decrease sharply.

This is because in a saturable absorption medium, light is incident and electrons in the absorption medium are excited to cause light absorption. Therefore, when a large signal light is input, the medium that contributes to the absorption of light is finite. Therefore, all of the electrons that contribute to absorption in the absorption medium are excited and go up to the upper energy level, so there is nothing that absorbs light, and the absorption characteristics of light are saturated ( Saturation phenomenon).

Therefore, the gain characteristic shown in FIG.
Based on the loss characteristics shown in (b), an optical amplification medium and a saturable absorption medium configured to have characteristics such that points P ₁ and P ₂ match for a certain input signal light power value are selected. The gain-to-input signal light power characteristics of the optical amplifiers connected in cascade and combined are as shown in FIG. The characteristics of this saturable absorbing medium are as follows: changing the temperature of the medium or exciting the medium so that gain is not generated, and the length of the saturable absorbing medium in the propagation direction of the signal light (in the case of a fiber structure). Can be adjusted by adjusting the fiber length).

In this case, as actually shown in FIG. 2 (c), the gain itself is lower than that in the case of the optical amplification medium alone (shown by the dotted line), but a wider signal than the characteristics of the optical amplification medium alone. The characteristics become flat at the light level, and it is possible to keep the gain constant or suppress the gain fluctuation. That is, a saturable absorption medium having a characteristic of compensating the dependence of the gain of the single optical amplification medium on the input signal light power with respect to the gain of the single optical amplification medium is cascaded to obtain a gain at a wide signal light level. Control is realized.

The present invention is made on the basis of the above principle, and as shown in FIG. 1, the optical amplification medium 1 and the saturable absorption medium 2 are connected in cascade. Here, the optical amplification medium 1
Of the gain vs. input signal light power characteristic and the loss vs. input signal light power characteristic of the saturable absorbing medium 2 are the descending points P ₁ , P respectively.
₂ (see FIGS. 2A and 2B) are configured to match as much as possible.

The optical amplifying medium 1 is placed in a state of amplifying an input signal, and the signal light amplified by passing through the optical amplifying medium 1 passes through the saturable absorbing medium 2 and a part thereof is absorbed. Is output. The overall characteristic of the gain of the optical amplifier of the present invention is as shown in FIG. 2C, and the gain becomes constant over a wide signal light level.

In this case, as described above, in the optical amplifier of the present invention, a feedback loop is not particularly formed, an electric control circuit is not required, and constant gain control can be easily realized at a wide signal light level. it can.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment will be described below with reference to the drawings. FIG. 3 is a block diagram showing the configuration of an embodiment of the optical amplifier of the present invention.

As shown in the figure, in this embodiment,
The traveling wave type optical amplifying medium 11 and the traveling wave type saturable absorbing medium 12 are connected in cascade, and both are made of Er-doped fiber.

The input signal light having a wavelength of 1.55 μm, which is output from a light source (not shown) on the signal light transmitting side and contains, for example, blinking light information of “1” or “0”, is generated by the multiplexing module 13.
And the wavelength output from the pumping light source 14 is 0.98 μm or
After being multiplexed with the 1.48 μm excitation light, it is output to the _first isolator (ISO ₁ ) 15.

Here, the pumping light source 14 is composed of, for example, a laser diode, and externally applies energy to Er ions in the Er-doped fiber 11 serving as an optical amplification medium to pump electrons and amplify the input signal light. ..

The Er-doped fiber 11 includes a pump light source 14
The light that is excited by the input signal light is amplified, and the light input through the isolator (ISO ₁ ) 15 is amplified as it travels through the Er-doped fiber 11, and the second isolator (ISO 1 ₂ ) Output to 16. The Er-doped fiber 11 is configured so that the gain-to-input signal light power characteristic has a characteristic as shown in FIG. That is, the gain decreases as the input signal light power increases.

The respective isolators 15 and 16 arranged so as to sandwich the Er-doped fiber 11 are provided so that the optical amplifier does not operate unstable. That is, when an optical amplification medium is provided in the optical path, the quality and design of the optical member are not good, and if a small number of reflecting mirrors are formed on both sides, reflected light will appear unintentionally, resulting in unstable characteristics. It is provided to suppress this problem.

Further, the amplified signal light that has passed through the isolator (ISO ₂ ) 16 is further passed through a band pass filter (BPF) 17 to be signal light in a predetermined wavelength range, and Er as a saturable absorbing medium is obtained. It is input to the doped fiber 12 and part of it is absorbed. The Er-doped fiber 12 is not pumped and is configured to exhibit a loss-to-input signal light power characteristic as shown in FIG. 2B, that is, an absorption characteristic for a constant input signal light. That is, up to a certain power level of the input signal light, the loss characteristic is substantially constant, but when the power level is further increased, saturation of the medium that absorbs the signal light starts and the loss decreases.

Here, the Er-doped fiber 11 as the optical amplifying medium and the Er-doped fiber 12 as the saturable absorbing medium have the gain-input signal optical power characteristic drop point P _{1 in the same} manner as described above. The medium is selected such that the loss (see FIG. 2A) and the drop point P _{2 of the} loss-to-input signal light power characteristic (see FIG. 2B) match. The method of adjusting the characteristics of the Er-doped fiber is as described above.

As described above, with respect to the gain vs. input signal light power characteristic of the Er-doped fiber 11 as the optical amplification medium,
An Er-doped fiber 12 as a saturable absorbing medium having a characteristic of compensating the dependence of the gain on the input signal light is cascade-connected. Therefore, the overall characteristic of the gain of the optical amplifier of this embodiment is flatter than the characteristic of the optical amplifying medium alone as shown in FIG.

The operation of the above configuration will be described. In FIG. 3, the input signal light output from the light source (not shown) is combined with the pump light output from the pump light source 14 in the multiplexing module 13, and is passed through the _first isolator (ISO ₁ ) 15. , Is input to the Er-doped fiber 11 in a state of being excited by the pumping light source 14 and amplifying the input signal light.

The input signal light is amplified as it travels through the Er-doped fiber 11, and is output to the _second isolator (ISO ₂ ) 16. Then, the signal light passes through the band-pass filter (BPF) 17 only the signal light in a predetermined wavelength range and spontaneous emission light in the vicinity thereof and is input to the Er-doped fiber 12, and a part of the Er-doped fiber 12 is input. It is output after being absorbed.

Since the Er-doped fiber 12 has a characteristic of compensating the input signal light dependency in the gain-to-input signal light power characteristic of the Er-doped fiber 11, the signal output through the Er-doped fiber 12 is output. The optical gain vs. input signal light power characteristic is constant over a wide signal light level as shown in FIG.

Since this embodiment is constructed as described above, the gain is constant over a wide signal light level. In this case, the Er-doped fiber 11 as the optical amplification medium and the Er-doped fiber 12 as the saturable absorption medium are only connected in series, and a feedback loop is not particularly formed unlike the conventional example. In addition, a light receiver and a complicated electric control circuit are not required, and constant gain control can be easily realized in a wide signal light level. From this, it can be expected to make a great contribution to the optical communication system.

In the above embodiment, the optical amplification medium 1 is used.
The saturable absorbing medium 2 and the saturable absorbing medium 2 have been described by taking the Er-doped fiber as an example, but the medium is not necessarily limited to the Er-doped medium. The medium may be a medium doped with other rare earth elements such as neodymium (Nodymium, Nd) or praseodymium (Praseodymium, Pr).

Further, the optical amplification medium 1 and the saturable absorption medium 2
Is not limited to a medium doped with a rare earth element, but a semiconductor having a characteristic capable of amplifying input light and a saturable absorption characteristic, for example, GaInAsP / InP.
A medium using a crystal material of a semiconductor laser such as a GaAlAs / GaAs system or a GaAlAs / GaAs system may be used.

Further, in the above embodiment, the optical amplifying medium 1 and the saturable absorbing medium 2 were made of Er-doped fibers and both of them had the same quality, but they do not necessarily have to have the same quality. The optical amplifying medium and the saturable absorbing medium may be selected from a medium doped with another kind of rare earth element,
Further, it may be selected from semiconductors made of different kinds of crystalline materials. Then, the optical amplifying medium may be a medium doped with a rare earth element, and the saturable absorbing medium may be selected as a semiconductor having a property capable of amplifying light, or vice versa. In this case, if the medium can be composed of the same quality, that is also an advantage.

Moreover, in the above embodiment, the optical amplifying medium 1 and the saturable absorbing medium 2 are described as media having a fiber structure, but the present invention is not limited to this, and media other than the fiber structure may be used. However, if both of them have a fiber structure, it is easy to connect the both, and it is easy to cause amplification and absorption of the signal light.

[0034]

According to the present invention, a photodetector and a complicated electric control circuit are unnecessary, and constant gain control can be easily realized at a wide signal light level. From this, it can be expected to make a great contribution to the optical communication system.

[Brief description of drawings]

FIG. 1 is a principle block diagram of the present invention.

FIG. 2 is a diagram for explaining the principle of the present invention, in which (a) is a gain body input signal light power characteristic of a single optical amplification medium, (b) is loss vs. input signal light power characteristic of a saturable absorption medium alone, and (c). ) Is a gain vs. input signal light power characteristic of the cascade connection of (a) and (b).

FIG. 3 is a diagram showing a configuration of an exemplary embodiment of the present invention.

FIG. 4 is a diagram showing a configuration of a conventional example.

[Explanation of symbols]

 1 Optical amplification medium 2 Saturable absorption medium

Claims (6)

[Claims] 1. An optical amplifier, wherein a constant gain control is performed in a wide signal light level by connecting an optical amplification medium (1) and a saturable absorption medium (2) in cascade. 2. A medium doped with a rare earth element is used as the optical amplification medium.
The optical amplifier described. 3. The optical amplifier according to claim 1, wherein a semiconductor having a characteristic capable of amplifying light is used as the optical amplification medium. 4. The optical amplifier according to claim 1, wherein a medium doped with a rare earth element is used as the saturable absorbing medium. 5. The optical amplifier according to claim 1, wherein a semiconductor having a saturable absorption characteristic is used as the saturable absorption medium. 6. The light according to claim 2, wherein any one of erbium (Er), neodymium (Nd) and praseodymium (Pr) is selected as the rare earth element to be doped. amplifier.