KR20050013013A - Optical amplifier and gain control method using optical delay - Google Patents

Abstract

PURPOSE: An optical amplifier and a gain control method using optical delay are provided to improve transient response characteristics and output characteristics by using an optical gain control and an optical delay. CONSTITUTION: A first coupler(202) is used for branching an inputted optical signal into a first optical signal and a second optical signal. An optical delay line part(204) is formed with an optical fiber having a preset length and delays an output of the first optical signal. An optical detector(206) is used for generating an electric signal according to a variation of the second optical signal. An optical amplifier(220) is used for amplifying the delayed signal of the first signal by using a pump beam generated corresponding to the electric signal. A feedback controller(240) is used for branching a part of the amplified optical signals and outputting the branched signals to the optical amplifier.

Description

Optical amplifier and gain control method using optical delay

The present invention relates to an optical amplifier and a gain control method, and more particularly, to an optical amplifier and a gain control method capable of reducing an instantaneous change in output due to an input channel change of an optical amplifier.

The basic structure of the optical amplifier used in the optical transmission device is gained by injecting the pump light into the gain medium having the gain of the optical amplifier. The optical amplifier is located in the middle of the optical path in the optical transmission device for a long distance transmission so that the optical signal It compensates for the losses that occur as it progresses, allowing the optical signal to reach long distances.

As an optical amplifier, an erbium-doped fiber amplifier (EDFA) that converts and amplifies the energy of a light source pumped into an erbium-doped fiber (EDF) into the energy of an input optical signal is widely used. . EDFA usually has a small output change with respect to the change in input intensity when operating in a high saturation region. For example, if the output for 40 channels is 100 mW and the intensity of each channel is uniform, then each channel has an intensity of 2.5 mW. If 20 channels are input to the optical amplifier and the optical amplifier operates in the saturation region, the intensity per channel is doubled to 5mW if there is no change in the overall output. As such, when the number of input channels changes, there is no change in the overall output, but the output per channel changes. Therefore, when passing through a plurality of amplifiers, the optical signal may be out of the range of the appropriate reception level that can be received at the final receiver by the accumulation of the output change.

Wavelength Division Multiplexing (WDM) optical communication systems transmit multiple channels simultaneously but do not always use the maximum number of channels that can be accommodated. It must be kept constant. Particularly, in recent years, WDM transmissions transmitted through different lines have evolved from linear point-to-point transmission methods to optical cross connect systems that bind multiple WDM systems. Divergence and coupling of the signal at each node where the channels meet causes a change in the number of input channels of the optical amplifier. In addition, there may be a case where the loss of the transmission line is changed, and the signals are branched by the reconfiguration of the network or the optical branch coupler to change the number of input channels of the optical amplifier. Therefore, there is a need for optical gain control to maintain a constant output for each channel regardless of the change in the number of channels.

Conventional gain control methods can be divided into electrical control methods and optical control methods.

The conventional electrical control method is a method of controlling by adjusting the magnitude of the current driving the intensity of the pump light when the input changes. Qualitatively, when the intensity of the pump is high when the optical signal of constant size is input, the gain (amplification factor) is large. On the contrary, when the strength of the pump is weak, the gain is small. Therefore, if the input is small, reducing the pump reduces the output. Therefore, a constant output intensity can be obtained regardless of the change in the input of each channel. However, in electrical control, the degree of instantaneous gain change depends on the speed of the electric circuit which detects the input change of the optical signal and changes the pump. Therefore, the speed of the controlling electric circuit should be as fast as possible, but the speed is limited.

An electrical control (pump gain control) method for amplifying a signal in the 1550 nm wavelength band using a 980 nm pump and adjusting the strength of the 980 nm pump according to the input size is disclosed in US Patent No. 5,923,462.

Conventional optical control method is a method to modify the path of the light propagation in the optical amplifier so that the laser oscillation occurs in a portion or near the wavelength band of the optical signal. Such optical control is disclosed in US Pat. No. 6,094,298. In the case of the optical control method, since the current of the pump is a fixed value, a high speed control circuit for adjusting the strength of the pump is not required. However, there is a problem in that a relaxation oscillation (RO) phenomenon occurs in which a vibration is carried on each channel when a channel is changed.

FIG. 1 is a diagram illustrating the output of one channel when the number of input channels decreases by half and then increases in a conventional optical amplifier without a gain control function.

Referring to FIG. 1, when the number of input channels is cut in half, the overall output is constant, and thus the intensity of each channel increases by 3 dB (twice). However, the changing aspect is not simple and grows very instantaneously and gradually approaches steady state. That is, a momentary change, denoted by A, and a change in steady state, denoted by B, appear. Conversely, if the half channel disappeared after a certain period of time, it returns to its original strength but then instantly changes to a smaller value and then gradually returns to a normal state.

FIG. 2 is a diagram illustrating the output of one channel when the number of input channels decreases by half and increases in a conventional optical amplifier having an electric gain control function. Referring to FIG. 2, compared with FIG. 1, an effect of reducing the change of the portions indicated by A and B all decreases. However, there is still a great change in transient transient response due to channel changes.

FIG. 3 is a diagram illustrating an output change when the number of input channels is changed from 40 to 1 and then to 40 in a conventional optical amplifier having an optical gain control function. Referring to FIG. 3, when the number of input channels changes, RO (relaxation oscillation) occurs instantaneously. The intensity of the output is smaller than in the case of FIGS.

SUMMARY OF THE INVENTION The present invention has been made in an effort to provide an optical amplifier and a gain control method for maintaining a constant output for each channel regardless of a change in the number of input channels in an optical amplifier and improving transient transient response characteristics. have.

Figure 1a is a view showing the output change according to the channel change of the conventional optical amplifier without a gain control function,

Figure 1b is a view showing the output according to the input channel change of the conventional optical amplifier with an electric gain control function,

Figure 1c is a view showing the output according to the input channel change of the conventional optical amplifier with the optical gain control function,

2 is a block diagram illustrating an example of an optical amplifier according to the present invention;

3A and 3B illustrate an output according to an input channel change of an optical amplifier, and

4 is a flowchart illustrating a gain control method according to the present invention.

In order to achieve the above technical problem, an embodiment of an optical amplifier using an optical delay according to the present invention, the first coupler for splitting the input optical signal into a first optical signal and a second optical signal; An optical delay line unit configured to delay an output of the first optical signal by being composed of an optical fiber having a predetermined length; A light detector for generating an electric signal according to the change amount of the second optical signal; An optical amplifier for amplifying the delayed first optical signal using pump light generated in response to the electrical signal; And a feedback controller which branches a part of the optical signal amplified by the optical amplifier and outputs the optical signal to the optical amplifier.

Preferably, the optical amplifier comprises an electronic control unit for generating a current based on the electrical signal generated by the photodetector; A pumping unit for outputting pump light corresponding to the intensity of the current generated by the electronic controller; A wavelength multiplexer for integrating the pump light and the delayed first optical signal; An erbium amplifier which amplifies the optical signal integrated and output by the wavelength multiplexer using an optical fiber doped with rare earth elements; And an advertisement part outputting the optical signal amplified by the erbium amplification part in a predetermined direction.

Preferably, the feedback control unit includes a second coupler for branching a part of the optical signal output by the advertising unit; A filter unit for passing a specific wavelength of the branched optical signal; An attenuation unit for reducing the intensity of the optical signal filtered by the filter unit; And a third coupler for integrating the optical signal attenuated by the attenuation unit with the optical signal output by the optical delay line unit.

In order to achieve the above technical problem, a gain control method using an optical delay according to the present invention comprises the steps of: branching an input optical signal into a first optical signal and a second optical signal; Delaying and outputting the first optical signal for a predetermined time; Generating an electrical signal according to the change amount of the second optical signal; Amplifying the delayed first optical signal using a pump light generated in response to the electrical signal; And branching a part of the amplified optical signal and outputting the amplified step.

According to the present invention, since the electric gain control method using the intensity of the pump light and the optical gain control method using the feedback are used, the output characteristics are improved compared with the case of performing only one gain control. In particular, the optical delay is used to solve the limitation of the processing speed of the electronic circuit of the electrical gain control, thereby improving the output characteristics.

Hereinafter, with reference to the accompanying drawings will be described in detail an optical amplifier and a gain control method using the optical delay according to the present invention.

2 is a block diagram of an optical amplifier using an optical delay according to the present invention.

Referring to FIG. 2, the optical amplifier according to the present invention includes an optical delay unit 200, an optical amplifier 220, and a feedback controller 240.

The optical delay unit 200 splits the input optical signal, delays and outputs the branched optical signal for a predetermined time, and uses the remaining branched optical signal as a control signal for amplifying the optical signal. The photo delay unit 200 includes a first coupler 202, an optical delay line unit 204, and a photo detector 206.

The first coupler 202 branches the optical signal received from the optical transmission system. A part of the branched optical signal is output to the optical delay line unit 204 for the optical signal delay, and the remaining branched optical signal is output to the photodetector 206 for detecting the variation of the optical signal.

The optical delay line unit 204 delays the optical signal output from the first coupler 202 for a predetermined time. The optical delay line portion 204 is composed of an optical fiber of a predetermined length. The speed at which light is transmitted in the medium is (speed of light in the vacuum) / (refractive index of the medium), so when the refractive index of the transmission optical fiber is approximately 1.5, the speed of the optical signal in the optical fiber is approximately 2 * 10 ⁸ m / s. . Thus, a 1 Km optical fiber provides an optical delay time of 5 ms.

Since the optical amplifier 220 detects a change in the input optical signal and requires a process of generating a control current according to the change of the optical signal and generating a pump light according to the control current, a finite time for amplification control is required. Therefore, when the optical signal is input to the optical amplifier 220 without a time delay, the output of the pump light for amplification is slower than the input of the optical signal due to the time limit for electrical control of the optical amplifier 220. The larger the time difference between the input of the optical signal and the output of the pump light, the worse the output characteristics of the amplification. Therefore, the delay time of the optical delay line unit 204 is preferably delayed by a time necessary for the electrical control of the optical amplifier 220.

The light detector 206 detects an optical signal branched from the first coupler 202 and outputs an electric signal corresponding to the detected optical signal. The light detector 206 outputs a weak electric signal when the intensity of the input optical signal is weak, and outputs a strong electric signal when the intensity of the optical signal is strong. The photodetector may be configured by a photo diode.

The optical amplifier 220 amplifies the optical signal output from the optical delay unit. The optical amplifier 220 includes an electronic controller 222, a pumping unit 224, a wavelength multiplexing unit 226, an erbium amplifier 228, and an advertising unit 230.

The electronic controller 222 generates a current in response to the electrical signal output from the photodetector 206. The intensity of the current by the electronic controller 222 is preferably optimized by an experimental or theoretical investigation result on the correlation between the electrical signal generated by the photodetector 206 and the intensity of the pump light according to the electrical signal.

The pumping unit 224 generates pump light in response to the current generated by the electronic control unit 222. The pump light generates a population inversion of the erbium amplifier 228. The wavelength of the pump light of the pumping unit 224 is constant and the intensity of the pump light changes according to the intensity of the current. The wavelength of the pump light is mainly 980 nm or 1480 nm. The intensity of the current which determines the intensity of the pump light is determined and output by the electronic controller. Since the electronic control unit 222 needs time for controlling the strength of the current, the optical signal delay by the optical delay line unit 204 results in an increase in the efficiency of the electronic control.

The wavelength multiplexer 226 integrates the optical signal output from the optical delay unit 200 and the pump light output from the pumping unit 224 for optical amplification. Since the wavelength multiplexer 226 has different wavelengths between the optical signal and the pump light, the wavelength multiplexer 226 is integrated to output a single optical fiber and does not affect the intensity of each other.

The erbium amplifier 228 amplifies the optical signal using the pump light output from the wavelength multiplexer. The erbium amplification part 228 is composed of an optical fiber doped with a rare earth element, and mainly uses an erbium doped optical fiber among rare earth elements. That is, as the erbium amplifier 228, an erbium-doped fiber amplifier (EDFA) is mainly used. In the erbium-doped optical fiber amplifier, erbium ions are excited by the input pump light, and the excited erbium ions are amplified by a process of inducing and releasing the excited erbium ions. The erbium amplifier 228 may use an optical amplifier other than the erbium-doped fiber amplifier by amplifying the optical signal through a process called excitation by the pump light and induced emission by the optical signal. The erbium amplifier 228 may be configured using a C-band EDF that amplifies a wavelength band of 1,530 to 1,560 nm, or an L-band EDF that amplifies a wavelength band of 1,570 to 1600 nm. Since the C-band EDF and L-band EDF are related to the amount of doped erbium, the amplification unit for the C-band and L-band can be easily implemented by increasing the doping rate or increasing the length of the EDF.

The advertising unit 230 outputs the optical signal amplified by the erbium amplifying unit 228 in one direction. The advertising unit 230 is used to remove noise caused by the reflection of the optical signal.

The feedback controller 240 is used to improve the output characteristic of the optical signal. The feedback controller 240 includes a second coupler 248, a third coupler 242, a filter unit 244, and an attenuation unit 246.

The third coupler 242 branches the optical signal amplified and output by the optical amplifier 220. A part of the branched optical signal is output to the feedback controller 240 for feedback control. The filter unit 244 is a band pass filter (BPF) that passes only a specific wavelength of the optical signal output from the third coupler 242. The wavelength passed is a wavelength that does not overlap the wavelength of the optical signal but is near the wavelength band of the optical signal. The attenuation part 246 reduces the intensity of the specific wavelength selectively selected by the filter part 244. The second coupler 248 integrates the optical signal for controlling the feedback of the feedback path with the optical signal output from the optical delay unit.

An optical signal having a specific wavelength selected by the filter unit 244, that is, a laser, forms a loop that undergoes amplification and attenuation by a feedback path. Since the laser has a characteristic in which the gain and the loss at the laser wavelength coincide, the loss generated in the optical path of the feedback controller 240 and the gain generated by the optical amplifier 220 coincide. That is, the amplification factor at the laser wavelength is determined by the passive element which is a loss component present on the loop path of the laser. The attenuator 246 serves to determine the loss on the path of the laser. The loss determination on the path by the attenuator 246 has the effect of determining the amplification factor of the optical amplifier 220.

Erbium-added optical fiber (EDF), which is a gain medium of the optical amplifier 220, has a similar characteristic in the band near the laser wavelength of the feedback controller 240 because the homogeneity is relatively large. Therefore, the feedback controller 220 has the effect of fixing the gain of the optical signal in the vicinity of the laser wavelength band.

According to the above technical configuration, the present invention can improve the characteristics of the gain control by the pump light by the optical delay and further improve the output characteristics by the feedback control.

FIG. 3A illustrates an output according to an input channel change of an optical amplifier without an optical delay, and FIG. 3B illustrates an output according to an input channel change of an optical amplifier using an optical delay according to the present invention.

3B illustrates a case in which the input channel changes from 40 to 1 and then back to 40, and there is an optical delay. The optical delay time is about 7 ms. Comparing FIGS. 3A and 3B, the maximum minus the instantaneous change minus the minimum value is reduced to 0.8 dB without optical delay, but to 0.5 dB with optical delay.

4 is a flowchart illustrating a gain control method using an optical delay according to the present invention.

Referring to FIG. 4, the first coupler 202 branches an input optical signal and outputs a part of the branched optical signal to the optical delay line unit 204, and outputs the remaining branched optical signal to the photodetector 206. (S400). The branching ratio is better when the magnitude of the optical signal input to the optical delay line unit 204 is larger than the magnitude of the optical signal input to the photodetector 206. The branching ratio is mainly used as 99: 1, 98: 2, 95: 5, 90:10.

In the case of the optical signal output to the optical delay line unit 204 (S405), the optical delay line unit 204 composed of optical fibers of a predetermined length delays the entry time of the optical signal to the optical amplifier 220 (S425). ). The length of the optical fiber for the optical signal delay is set to be delayed by the electronic control processing time for the amplification control of the optical amplifier 220. Since the speed of the optical signal is related to the refractive index of the optical fiber, the length of the optical fiber according to the required delay time can be calculated based on the speed of the light and the refractive index.

The optical signal delayed and output by the optical delay line unit 204 is integrated with the optical signal for feedback control by the feedback controller 240 by the second coupler 248 (S430). Feedback control improves the output characteristics of the optical signal.

When a part of the optical signal branched to the first coupler 202 is output to the photodetector 206 (S405), the photodetector 206 detects a change in the input optical signal and reacts to the detected change in the optical signal. The electrical signal is output (S410). The output electrical signal is mainly in the form of voltage.

The electrical signal output to the photodetector 206 is input to the electronic controller 222 of the optical amplifier 220. The electronic controller 222 receiving the electric signal generates a current for controlling the pump light based on the electric signal (S415). The pumping unit 224 outputs the pump light according to the strength of the current generated by the electronic controller 222. The wavelength of the pump light is fixed and the intensity of the pump light changes according to the intensity of the current.

The wavelength multiplexer 226 integrates and outputs the pump light into the optical signal integrated with the optical signal for feedback control. The integrated and output optical signal is amplified according to the intensity of the pump light in the erbium amplifier 228 (S435). The advertising unit 230 does not reflect the amplified optical signal and outputs it in a predetermined direction.

The third coupler 242 branches the amplified and output optical signal and outputs a part of it, and outputs a part of it to the feedback control unit for feedback control (S445). The filter unit 244 filters and selects a specific wavelength of the optical signal output to the feedback controller 240 (S450). The specific wavelength selected is a wavelength that exists near the bandwidth of the optical signal without overlapping the wavelength of the optical signal. The optical signal of the specific wavelength selected by filtering is used as the optical signal for feedback control.

The intensity of the feedback control optical signal selected by the filter unit 244 is attenuated by the attenuation unit 246 (S455). An optical signal having a specific wavelength selected by the filter unit 244, that is, a laser, forms a loop through amplification and attenuation. Since the laser has a characteristic in which the gain and the loss at the laser wavelength coincide, the loss generated in the optical path by the feedback controller and the gain generated by the optical amplifier are coincident. The attenuator 246 determines the loss on the path of the laser and thus may determine the amplification factor of the optical amplifier.

The feedback control optical signal attenuated by the attenuator 246 is integrated with the optical signal output by the optical delay unit 200 and input to the optical amplifier 220.

The foregoing descriptions are merely illustrative of preferred embodiments, and the present invention is not limited to the above-described embodiments and can be variously changed within the scope of the appended claims. For example, the shape and structure of each component specifically shown in the embodiment of the present invention may be modified.

According to the present invention, the electric gain control using the intensity of the pump light and the optical gain control using the feedback are used, so that the output characteristics are improved compared with the case of performing only one gain control. In particular, the optical delay is used to solve the limitation of the processing speed of the electronic circuit for the electrical gain control, thereby improving the output characteristics. In addition, in the case of using the optical delay according to the present invention, the characteristics of the transient response according to the channel change are improved than in the case of not using the optical delay.

Claims (8)

A first coupler for branching an input optical signal into a first optical signal and a second optical signal; An optical delay line unit configured to delay an output of the first optical signal by being composed of an optical fiber having a predetermined length; A light detector for generating an electric signal according to the change amount of the second optical signal; An optical amplifier for amplifying the delayed first optical signal using pump light generated in response to the electrical signal; And And a feedback controller for dividing a portion of the amplified optical signal and outputting the amplified optical signal to the optical amplifier. The method of claim 1, The optical amplifier using an optical delay, characterized in that the length of the optical fiber is set to a length that delays the optical signal by the time required from the generation of the electrical signal to the generation of the pump light. The method of claim 1, The optical amplifier, An electronic controller which generates a current based on the electrical signal generated by the photodetector; A pumping unit configured to output pump light corresponding to the strength of the current; A wavelength multiplexer for integrating the pump light and the delayed first optical signal; An erbium amplifier which amplifies the optical signal integrated and output by the wavelength multiplexer using an optical fiber doped with rare earth elements; And And an advertising unit for outputting the optical signal amplified by the erbium amplifier in a predetermined direction. The method of claim 1, The feedback control unit, A second coupler for branching a part of the optical signal output by the advertising part; A filter unit for passing a specific wavelength of the branched optical signal; An attenuation unit for reducing the intensity of the optical signal filtered by the filter unit; And An optical amplifier using an optical delay, comprising: a third coupler for integrating the attenuated optical signal with the first optical signal delayed by the optical delay line unit; (a) dividing the input optical signal into a first optical signal and a second optical signal; (b) delaying and outputting the first optical signal for a predetermined time; (c) generating an electrical signal according to the change amount of the second optical signal; (d) amplifying the delayed first optical signal using the pump light generated in response to the electrical signal; And (e) branching a portion of the amplified optical signal and outputting the amplified step; gain control method using an optical delay comprising a. The method of claim 5, And the predetermined delay time is set to a time corresponding to the time taken from the generation of the electrical signal to the generation of the pump light. The method of claim 5, In step (d), (d1) generating a current based on the generated electrical signal; (d2) outputting pump light corresponding to the intensity of the generated current; (d3) integrating the pump light with the delayed first light signal; (d4) amplifying the integrated optical signal by using an optical fiber doped with rare earth elements; And (d5) outputting the amplified optical signal in a predetermined direction; optical gain control method using an optical delay comprising a. The method of claim 5, In step (e), (e1) branching a part of the amplified and output optical signal; (e2) passing a specific wavelength of the branched optical signal; (e3) attenuating the intensity of the optical signal of the specific wavelength; And (e4) integrating the attenuated optical signal with the delayed optical signal.