JPH05129681A - Optical loss compensation circuit - Google Patents

Abstract

PURPOSE:To automatically remove influence of time-variant optical loss over a wide frequency range. CONSTITUTION:An amplification medium and excitation source 5 connected to the output side of an optical loss source 3 on an optical transfer path and a feedback optical path which feeds a part of the output from the amplification medium and excitation source 5 back to the optical transfer path on the input side of the optical loss source are provided. An optical ring resonator consisting of the feedback optical path including the optical loss source 3 and the amplification medium and excitation source 5 is laser-oscillated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical loss compensating circuit for automatically eliminating the influence of a variable optical loss source localized in an optical transmission line.

[0002]

2. Description of the Related Art A local optical loss which fluctuates with time may occur in an optical transmission line, particularly an optical fiber optical line, due to temperature change, mechanical deformation, vibration, bending and other causes. In order to automatically eliminate the effect of such time-varying optical loss using an optical amplifier, the fluctuation of the signal power due to the loss fluctuation is electrically detected, and the optical amplifier is excited based on this detected signal. It is conceivable to control the power or the pumping light, thereby controlling the gain of the optical amplifier and compensating the influence of optical loss.

[0003]

However, there is still no example in which the above-described function is specifically verified.
In addition, in order to actually construct the above-mentioned circuit, there is a drawback that a photodetector and electric circuits and parts are required, and the configuration is complicated and expensive. Moreover, the response speed of the control system is limited due to the speed limitation of the electric circuit.

The present invention has been made in view of such circumstances, and an object thereof is to provide an optical loss compensating circuit capable of eliminating the influence of optical loss with a simple structure over a wide frequency range. ..

[0005]

To achieve the above object, in claim 1, an amplifying medium and a pumping source connected to an output side of an optical loss source on an optical transmission line, and a part of an output of the amplifying medium. And a feedback optical path for returning the light to the optical transmission path on the input side with respect to the optical loss source, and oscillating the optical ring resonator constituted by the feedback optical path including the optical loss source, the amplification medium, and the pump source. ..

According to a second aspect of the present invention, in the optical loss compensation circuit according to the first aspect, an optical attenuator is arranged on the feedback optical path.

[0007]

According to the first aspect of the present invention, the light propagated through the optical transmission line is lost in the optical loss source. The light that has received this loss passes through the amplification medium, a part of the light is taken out, is guided to the return optical path, and is again coupled to the optical transmission path. Here, an optical ring resonator constituted by a feedback optical path including an optical loss source, an amplification medium and a pumping source is lased. At this time, the amplification medium is excited by the excitation source.

In the optical ring that is lasing as described above, the gain generated by the amplifying medium with respect to the light that matches the laser oscillation wavelength is just compensated for the optical loss in all rounds in the optical ring. The gain of the amplification medium is also fixed at that value. Therefore, the propagation light of the optical transmission line is amplified with the gain.

According to the second aspect, the loss value is adjusted by the optical attenuator and the amplification gain for the signal light is controlled.

[0010]

1 is a block diagram showing an embodiment of an optical loss compensation circuit according to the present invention inserted in an optical fiber transmission line. In FIG. 1, 1 is an optical input, 2 is a first optical fiber coupler, and one input port is an optical input 1 and the other port is
From the optical feedback circuit 8 which will be described later. Reference numeral 3 denotes an optical loss source, which is located on the output side of the first optical fiber coupler 2 on the optical fiber transmission line and is subject to temporal fluctuation due to the disturbance 4. Reference numeral 5 denotes an optical amplifier as an amplification medium and a pumping source, which is connected to the output of the optical loss source 3 and is composed of, for example, an optical amplifier using a fiber doped with a rare earth element (erbium, neodymium, etc.) or a semiconductor optical amplifier. To be done. An optical isolator 6 is connected to the output side of the optical amplifier 5. A second optical fiber coupler 7 is connected to the output side of the optical isolator 6 and has one output port as an optical output 11. Reference numeral 8 denotes an optical feedback circuit, which is formed by connecting another output port of the second optical fiber coupler 7 and another input port of the first optical fiber coupler 2 with an optical fiber. A part of the output is fed back to the input side of the optical loss source 3. 9 is an optical filter,
It is arranged on the optical feedback circuit 8. Reference numeral 10 denotes a variable optical attenuator, which is arranged on the output side of the optical filter 9 on the optical feedback circuit 8 and attenuates the light returning from the optical feedback circuit 8 by an arbitrary amount.

Next, the operation of the above configuration will be described.

The optical input 1 is a first optical fiber coupler 2
To the optical loss source 3, where it is subject to a time-varying loss due to the disturbance 4. The light that has received this loss fluctuation is
A part of the light is extracted from the optical amplifier 5 through the optical isolator 6 and the second optical fiber coupler 7, and the optical filter 9,
It is guided to the optical feedback circuit 8 including the optical attenuator 10 and is coupled again to the path of the optical input 1 by the first optical fiber coupler 2.
Here, the optical ring constituted by the feedback optical path including the optical loss source 3, the optical amplifier 5, the variable optical attenuator 9, etc. is lased.
For this purpose, it is necessary to sufficiently excite the optical amplifier 5.

In the optical ring that is lasing as described above, the gain generated by the optical amplifier 5 with respect to the signal light having the same wavelength as the laser oscillation wavelength is equal to the optical loss of all the rounds in the optical ring. It is well known that the value is just fixed to compensate. Therefore, if the total loop optical loss in the optical ring is TdB, the gain of the optical amplifier 5 is also TdB. Here, if the total round optical loss TdB in the optical ring is divided into an optical loss portion Tp dB localized on the optical path for transmitting an optical signal and a portion Tfb dB on the feedback optical path other than the transmission optical path, The effective gain exerted on the signal by the entire system is always Tfb dB and does not depend on the value of Tp. Therefore, even if Tp fluctuates, its influence does not appear in the signal. The above is the basic operation of the loss guarantee circuit. The loss guarantee circuit of course not only guarantees the loss, but also functions as an amplifier circuit that exhibits Tfb gain.

Further, the variable optical attenuator 9 arranged on the optical feedback circuit 8 arbitrarily sets the loss in the feedback optical path.
Thus, for example, if the optical loss portion on the signal transmission optical path is constant, the gain of signal amplification can be controlled by setting the loss value on the feedback optical path. This is because in the conventional amplification of the high-intensity signal light, the gain of the optical amplifier is saturated and the gain is lowered, whereas the system of the present invention, under the condition that the feedback optical path continues the laser oscillation, The gain is determined only by the loss value in the return optical path. That is, the gain is not affected by the signal light intensity. Therefore, a large gain can be obtained by setting the loss of the return optical path to an arbitrary value by the optical attenuator 9 and intentionally giving the loss. Further, this effective gain is not affected by the fluctuation of the pump power of the pump source.

Next, the wavelength band of the optical signal which can compensate the loss in the circuit of the present invention will be considered.

It is natural that the circuit of the present invention can compensate the loss for an optical signal having a wavelength that matches the laser oscillation wavelength of the feedback optical path. However, the circuit of the present invention is also effective for a signal having a wavelength in a range different from this. The spread of the gain of the optical amplifier on the frequency axis is uniform (Homogeneous)
In such a range, the gain saturation of the optical amplifier with respect to the signal is considered to be determined by the total power of the input signals in the wavelength range. Therefore, if the feedback optical path oscillates at some wavelength within the uniform range, the optical loss compensation according to the present invention can be performed on the signal having another wavelength within the uniform range.

As a result of examining such a uniform wavelength range using an erbium-doped fiber optical amplifier as an example, there is a wide uniform range near the gain peak on the long wavelength side of the erbium-doped fiber optical amplifier, which is 1540 nm within this region. From 1553
It was found that the optical loss compensation of the present invention is possible in a wide range of nm.

FIG. 2 is a diagram showing an example in which the effect of the present invention is experimentally confirmed using an erbium-doped fiber optical amplifier. The horizontal axis represents the optical loss on the signal transmission optical path (signal path), and the vertical axis. Indicates the relative value of signal power. The oscillation wavelength and the signal wavelength were selected to be 1553 nm and 1548.5 nm, respectively, within the above uniform range.

From FIG. 2, when the optical feedback is not applied (white circles in the figure), the signal strength is reduced in accordance with the increase of the optical loss on the signal transmission optical path, while the optical feedback is applied (the figure). It can be confirmed that the signal output is kept constant during the laser oscillation of the feedback optical path. However, even when optical feedback is applied (black circles in the figure), when the optical loss becomes large and the laser oscillation stops, the signal output decreases thereafter as the loss increases.

Further, as an example of application of the present invention, application to a fiber sensor using optical interference can be considered. A fiber sensor using optical interference detects a slight phase change due to some physical quantity to be measured in the sensing head part, but due to the influence of the physical quantity applied to the sensing fiber, there is also an optical loss together with the target phase change. It often fluctuated. When the optical loss of the sensor head portion fluctuates, the relationship between the intensities of two lights that cause optical interference fluctuates, so that the scale factor of the interference fluctuates, which eventually causes the fluctuation of the measurement accuracy. In the present invention, the optical loss that occurs together with such a phase change is compensated, but since the information of the phase change is not affected, fluctuations in measurement accuracy can be avoided.

[0021]

As described above, according to the first aspect of the present invention, by using the feedback optical path including the amplification medium and the pumping source, the influence of the optical loss which fluctuates with time is automatically applied over a wide frequency range. It can be removed.

According to the second aspect, the amplification gain for the signal light can be controlled by adjusting the loss value using the optical attenuator. This gain is not affected by the signal light intensity and is not affected by the fluctuation of the pump power of the pump source under the condition that the laser oscillation is involved.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of an optical loss compensation circuit according to the present invention.

FIG. 2 is a diagram showing an experimental example of the present invention.

[Explanation of symbols]

1 ... Optical input, 2 ... 1st optical fiber coupler, 3 ... Optical loss source, 4 ... Disturbance, 5 ... Optical amplifier, 6 ... Optical isolator,
7 ... Second optical fiber coupler, 8 ... Optical feedback circuit, 9 ...
Optical filter, 10 ... Optical attenuator, 11 ... Optical output.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location H01S 3/07 8934-4M

Claims (2)

[Claims] 1. An amplification medium and a pumping source connected to the output side of an optical loss source on an optical transmission line, and a feedback for returning a part of the output of the amplification medium to the optical transmission line on the input side to the optical loss source. And an optical path, wherein an optical ring resonator constituted by a feedback optical path including the optical loss source, the amplification medium and the pumping source is laser-oscillated. 2. The optical loss compensating circuit according to claim 1, wherein an optical attenuator is arranged on the return optical path.