KR100201009B1 - Oscillating loop for optical control of erbium doped optical fiber amplifier gain - Google Patents

Abstract

The present invention relates to an oscillation loop for use in a gain controlled erbium-doped fiber amplifier. The oscillation loop of the present invention is an erbium-doped optical fiber; Reflective filter means for returning a part of the natural emission light amplified by the erbium-doped optical fiber to the erbium-doped optical fiber and performing both functions of preventing the oscillated light from coming out of the output terminal together with the signal light; A first optical fiber combiner at the signal light output end side for dividing the oscillation light reflected by the reflective filter means and returned; Loop paths; And a second optical fiber coupler at the signal light input end side for injecting the oscillated light returned along the loop path back into the erbium-doped optical fiber. The oscillation loop of the present invention can not only stably control the amplification gain of an erbium-doped fiber amplifier with a simple and inexpensive configuration at a constant value irrespective of power change of input signal light or power change of pump light, and amplification of a low speed modulated signal. Waveform distortion can be minimized at time.

Description

Oscillation loop for optical control of erbium-doped fiber amplifier gain

1 is a basic configuration diagram of a conventional optical fiber amplifier without an oscillation loop.

2 is a basic block diagram of a gain control optical fiber amplifier including the oscillation loop of the present invention.

3 is a graph showing the reflection and transmission characteristics of the optical fiber grating or dielectric thin film filter constituting the oscillation loop of the present invention.

4 is a basic configuration of the oscillation loop according to the present invention.

5 is a graph showing a change in gain according to a change in power of an input signal light in an erbium-doped fiber amplifier.

6 is a photograph showing an output waveform when the erbium-doped optical fiber amplifier amplifies a slow modulated optical signal.

* Explanation of symbols for main parts of the drawings

101,104,202,205,402,403: Optical Isolator

102,204: wavelength division multiple device 103,203: pump laser diode

206,404: first optical fiber combiner 201,401: second optical fiber combiner

207,405: fiber optic grating or dielectric thin film filter

208: variable optical attenuator 209,406: loop path

The present invention relates to an oscillation loop for optically controlling the gain of an erbium-doped fiber amplifier. More specifically, an object of the present invention is to maintain and control the amplification gain constant in an erbium-doped fiber amplifier in which an erbium-doped fiber is an optical gain medium, irrespective of the change in the input signal optical power. The present invention relates to a novel oscillation loop configuration in which one optical fiber grating or a dielectric thin film filter having the same transmission / reflection characteristics is attached to the signal light output end side.

In general, an erbium-doped fiber amplifier is for directly amplifying a weak optical signal in optical communication of 1.55 占 퐉 wavelength band to obtain a high power optical signal output. A typical basic configuration is shown in FIG.

Conventional erbium-doped fiber amplifiers, as shown in FIG. 1, include an optical amplification medium, an erbium-doped optical fiber, a laser diode 103 for an optical pump for excitation of the added erbium ions, and a signal light. A wavelength division multiplexer (102) for separating or combining light of a wavelength and a pump light wavelength, and an optical isolator (101, which passes light in a forward direction and blocks light in a reverse direction); When the pump light enters the erbium-doped optical fiber from the pump laser diode through the wavelength division multiplexed element, the erbium ions in the optical fiber are excited, while the input signal light is excited along the erbium-doped optical fiber. As the erbium ions transition to a low energy state, amplification of the signal light is performed. Therefore, in the conventional erbium-doped fiber amplifier, in general, as the power of the input signal light increases, the gain of the fiber amplifier decreases. As the power level of the input signal light increases, the rate of reduction of the gain for the power of the input signal light increases.

Meanwhile, in the optical communication system for subscribers having wavelength-division multiplexing, each wavelength channel is arbitrarily turned on and off independently when signal signals of several wavelength channels are incident together into an optical fiber amplifier. Assuming the case, the level of the input signal optical power (sum of all channel signal optical powers) of the optical fiber amplifier can vary over a significant range over time, in which case the gain of the optical fiber amplifier varies considerably with the on-off states of the channels. There can be a shake over. Therefore, the amplification gain for one particular channel is affected by the on-off of the other channels, there is a problem that such instability of the gain may interfere with the operation of the entire system.

One way to solve this problem is that the optical fiber amplifier oscillates at a specific wavelength other than the signal light wavelength and operates with a kind of laser, so that the gain of the optical fiber amplifier with respect to the signal light wavelength is changed despite the change in the power level of the input signal light or the pump light. It is to control to be stabilized tied to a certain value. In order to oscillate the fiber amplifier as a laser as described above, a feedback structure must be formed. The feedback structure includes a device having a function of selecting a wavelength to oscillate at a specific wavelength desired, and preventing the oscillating light from going to the output terminal together with the amplified signal light. A filter having a function of blocking light of the oscillating light wavelength and transmitting light of the other signal light wavelength band is required.

M. Zirngible et al. And H. Okamura et al. Form a ring resonator that returns a portion of the optical fiber amplifier's output light to its input stage to control the amplification gain by oscillating the fiber amplifier in the form of a ring laser. A method of M. Zirngible et al., Electron. Lett., 27: 560 (1991); And in H. Okamura et al., Electron. Lett., 27: 2155 (1991)]. In this method, a variable optical attenuator was placed at the part of the ring oscillation loop outside the signal amplification path to change the controlled gain of the fiber amplifier by changing the loss of the oscillation light, that is, the laser resonator loss. However, this method is a simple ring structure of the oscillation loop, and requires an optical filter to select the oscillation wavelength inside the ring, and another optical filter to block the oscillation light component at the output stage.

Also, E. Delevaque et al., A method for oscillating an optical fiber amplifier, attaches an optical fiber grating to an end surface of an optical amplifier and an output end of an erbium-doped optical fiber, respectively, to fabricate a Fabry-Perot resonator. A method of oscillation at a specific wavelength has been proposed [E. Delevaque et al., Electron. Lett., 29: 1112 (1993)],

The optical fiber grating is a reflective filter element with a large reflectance at a specific wavelength, and most of the light that is not reflected is transmitted without loss. The optical fiber grating is the same form as general optical fiber, and it is easy to connect with transmission optical fiber or erbium-doped optical fiber and connection loss is very small, so it is very desirable device in optical communication system and apparatus, such as possibility of mass production at low price in the future. Form. In the above-described methods, such as the zigzag and the Okamura, only a transmissive filter can be used inside the ring, and thus a simple and simple optical fiber grating cannot be used.

On the other hand, the deliberate method has an advantage of simplifying the oscillation structure by using an optical fiber grating, but because it is a Fabry-Perot oscillation structure, the oscillation light path and the signal light amplification path in the fiber amplifier are matched. In such a gain-controlled fiber amplifier, it is necessary to adjust the oscillation light loss independently of the signal light loss to control the control gain. In this method, the control gain is structurally difficult because the oscillation light path and the signal light path coincide. .

Accordingly, the present invention is to solve the above problems of the prior art, the inventor modified the conventional simple ring-shaped oscillation loop feedback structure, has a maximum reflectance at Bragg wavelength, the reflectance is almost 0 at other wavelengths If only one reflective filter such as an optical fiber grating having a phosphorous reflection characteristic or a dielectric thin film filter having the same transmission / reflection characteristics as the optical fiber grating is attached to the signal light output end side of the oscillation loop, one optical fiber grating or dielectric thin film filter It can simultaneously perform two functions: selecting the oscillation wavelength and blocking the light of the oscillation light wavelength and transmitting the light of the other signal light wavelength band so that the oscillation light does not go to the output terminal together with the amplified signal light. We also verify that the control gain can be varied by adjusting the loss of the oscillation loop. Leading to the completion of the person.

It is, therefore, an object of the present invention to provide a simplified oscillation loop of an erbium-doped fiber amplifier, in which the amplification gain in the erbium-doped fiber amplifier can be kept constant and controlled irrespective of the change in the input signal optical power.

Other objects and advantages of the present invention will become apparent from the following detailed description.

The oscillation loop of the gain-controlled erbium-doped fiber amplifier for achieving the object of the present invention,

Erbium-doped optical fiber;

Reflective filter means for simultaneously returning a portion of the natural emission light amplified by the erbium-doped optical fiber to the erbium-doped optical fiber and simultaneously performing two functions of preventing the oscillated light from coming out of the output terminal together with the signal light;

A first optical fiber coupler at a signal light output end side for dividing the oscillation light reflected by the reflective filter means;

Loop paths; And,

A second optical fiber coupler at the signal light input end side for injecting the oscillated light returned from the reflective filter means back into the erbium-doped optical fiber along the loop path;

Characterized in that it comprises a.

In the oscillation loop of the present invention, the reflective filter means is positioned between the signal light output terminal and the first optical fiber coupler, and the first optical fiber coupler and the second optical fiber coupler are connected by the loop path to form an oscillation loop structure.

In the present specification, the erbium-doped optical fiber means an optical fiber in which erbium is added alone or erbium as a main component and a small amount of other metal is added thereto as a gain medium.

Only one reflective filter means is required to construct the oscillation loop of the present invention, and as such a reflective filter means, a fiber grating or a dielectric thin film having the same reflection and transmission characteristics in the same wavelength band as this fiber grating Dielectric thin film filters may be used, preferably optical fiber gratings.

The optical fiber grating constituting the oscillation loop of the present invention is a device in the form of an optical fiber in which the refractive index of the optical fiber core is periodically modulated in the axial direction of the optical fiber, and the Bragg wavelength determined by the refractive index modulation period, that is, the grating period. (* In FIG. 3) has the maximum reflectance, and the reflection characteristic at the wavelength above that is almost zero.

The loop path of the oscillation loop according to the present invention may include a loop loss adjusting means, and the loop loss adjusting means may include a variable optical attenuator or the first optical amplifier without using a variable optical attenuator. And a tunable fiber coupler capable of changing the optical power splitting ratio of any one of the second optical fiber couplers.

Hereinafter, a preferred embodiment of the oscillation loop according to the present invention used in the gain control erbium-doped fiber amplifier will be described in more detail with reference to the accompanying drawings.

First, the technical principle of the gain-controlled erbium-doped optical fiber amplifier including the oscillation loop of the present invention will be described with reference to FIGS.

2 is a basic configuration diagram of a gain controlled erbium-doped fiber amplifier including an oscillation loop according to the present invention. As shown in FIG. 2, the oscillation loop according to the present invention has a first optical fiber coupler 206 and a second optical fiber coupler 201 at the signal optical output end side and the input end side of the erbium-doped optical fiber amplifier, respectively. The optical fiber terminal 3 of the optical fiber combiner 201 and the third terminal of the first optical fiber combiner 206 are configured to be connected by a loop path through the variable optical attenuator 208 which is a loop loss control means.

Further, the second terminal of the first optical fiber coupler 206 on the signal light output end side has an optical fiber grating 207, which is a reflective filter having reflection and transmission characteristics as shown in FIG. 3, or a dielectric having the same reflection / transmission characteristics. The thin film filter is connected.

Looking at the path of the signal light with reference to Figure 2, the input signal light is incident to the input terminal is incident to the erbium-doped optical fiber via the second optical fiber coupler 201 and the optical isolator 202 on the input end side. The signal light amplified in the erbium-doped optical fiber passes through another optical isolator 205 and the first optical fiber coupler 206 on the signal light output end side, and then passes through the optical fiber grating 207 to the output end. Since the optical fiber grating 207 has a high transmittance for a signal light wavelength other than the Bragg wavelength, most of the amplified signal light incident on the optical fiber grating comes out to the output terminal without reflection.

On the other hand, in the erbium-doped optical fiber, not only the signal light is amplified, but also spontaneous emission generated over a wide wavelength band is also amplified to enter the optical fiber grating 207. In amplified spontaneous emission (ASE), the wavelength component that matches the Bragg wavelength of the fiber grating is reflected from the fiber grating and flows back toward the erbium-doped fiber. The reflected light is split in the first optical fiber coupler 206 on the output side, and the light exiting to terminal 1 is blocked by the optical isolator, and the light exiting to terminal 3 is along the connected loop path as shown in FIG. After entering the terminal 2 of the second optical fiber coupler 201 on the input end side through the variable optical attenuator 208, and exits to terminal 2, it is incident on the erbium-doped optical fiber and amplified again, and this process is repeated.

Therefore, the Bragg wavelength component of the optical fiber grating of the amplified natural emission light is repeatedly amplified while circulating along the loop path. When the amplification gain in the optical fiber amplifier is equal to the loss of the entire loop, oscillation is performed as a laser. Here, the loss of the entire loop refers to the total loss experienced by the oscillating light, including the loss due to the splitting in the two optical fiber couplers, the loss in the variable optical attenuator, and the connection loss of all the optical fiber connection sites.

In general, fiber amplifiers operate in a steady state, in which case the laser oscillation is fixed with the amplification gain clamped to the same magnitude as the loss of the entire loop. Once the fiber amplifier is oscillated as a laser by the Bragg wavelength of the fiber grating, the gain for other wavelengths in the spectral line (signal wavelength) is due to the spectroscopic properties of the element of erbium having a homogeneous spectral lineshape. It is tied to a specific value and fixed. Therefore, when operating in the steady state, even if the input signal light level or the pump field power level changes within the limit of the oscillation of the optical fiber amplifier, the gain with respect to the signal light wavelength does not change and remains constant.

On the other hand, the variable optical attenuator 208 can control the loss of the loop. When the laser oscillation, the gain is tied and fixed at the same size as the loss. Therefore, when the variable optical attenuator is adjusted to increase or decrease the loop loss, the operation gain of the optical fiber amplifier is increased. It increases and decreases together. Accordingly, a variable optical attenuator 208 can control the operating gain of the optical fiber amplifier, and a tunable fiber that can change the optical power split ratio of any one of the two optical fiber couplers 201 and 206 in FIG. By using a coupler, the operating gain of the fiber amplifier can be controlled just as with an optical attenuator.

4 is a basic configuration diagram of an oscillation loop according to the present invention, which is the core of a gain-controlled erbium-doped fiber amplifier.

The oscillating light is reflected by the optical fiber grating 405, and a part of the reflected light enters the second optical fiber coupler 401 via the loop path 406 via the first optical fiber coupler 404 as shown in FIG. It is incident again on the erbium-doped fiber and looped back into the fiber amplifier, contributing to oscillation. On the other hand, since the amplified signal light has almost zero reflectance of the optical fiber grating at the wavelength of the signal light, the amplified signal light passes through the optical fiber grating without reflection and exits to the output terminal.

In the configuration of the feedback loop, the optical fiber grating returns a part of the amplified natural emission light to the optical fiber amplifier to perform oscillation, and at the same time, blocks the oscillated light from coming out to the output terminal together with the signal light.

5 is a graph showing the results of measuring the amplification gain change according to the power change of the input signal light in the erbium-doped fiber amplifier with feedback loop and the conventional erbium-doped fiber amplifier without feedback loop.

In the graph of Figure 5 Denotes a gain change when the optical loss in the variable optical attenuator 208 is adjusted to infinity, that is, when no feedback loop is formed. When the feedback loop is not performed, there is no difference from the conventional optical fiber amplifier, and the gain decreases as the input signal optical power increases.

In comparison, in the graph of FIG. Wow Represents the gain change when the feedback loop is formed, which corresponds to the case where the loop loss is increased by using a variable optical attenuator (loop loss of 23 dB) and the case of decreasing (loop loss of 20 dB). As can be seen in FIG. 5 for each case, oscillations are made in the input signal light power range of less than -12 dBm and -8 dBm, respectively. Is fixed on the At this time, even if there is a power change of the pump light, the gain does not change as long as the optical fiber amplifier oscillates. In the case where the loop loss is increased, the loop operation is fixed while the gain is large compared to the case where the loop loss is small. In this case, however, the loss of power of the input signal light, which can reduce the gain as shown in FIG. 5, is reduced.

FIG. 6 compares the output waveform change of the output signal light by inputting a digital input optical signal modulated at a relatively low speed (about 10 kbps) to a gain-controlled optical fiber amplifier including an oscillation loop according to the present invention and a conventional conventional erbium-doped fiber amplifier. The result is shown.

As can be seen in FIG. 6, in a conventional erbium fiber amplifier, when a digital input optical signal modulated at a relatively low speed (about 10 kbps) is input, the gain is large at the time the signal becomes 1, but the signal is '1'. While the phosphorus state persists, the gain gradually decreases due to the reduction of the inversion density of the erbium element, which causes distortion in the amplified output waveform. However, in the gain-controlled loop optical fiber amplifier according to the present invention, since the gain is kept at a constant value every moment, the distortion is not generated in the output waveform.

As described in detail above, the oscillation loop of the present invention has a maximum reflectance at the Bragg wavelength, and at other wavelengths, an optical fiber grating having a reflection characteristic of which the reflectance is almost zero, or a dielectric thin film filter having the same transmission / reflection characteristics. Simple and inexpensive configuration can be achieved by using only one, and the gain-controlled erbium-doped optical fiber amplifier including the oscillation loop of the present invention can stably amplify the gain to a constant value irrespective of power change of input signal light or power change of pump light. In addition to controlling, waveform distortion can be minimized during amplification / depth of a slow modulated signal.

Claims (3)

An erbium-doped optical fiber for amplifying the input signal light incident through the second optical fiber couplers 201 and 401 and the optical isolators 202 and 402 on the input side of the signal light; A portion of the natural emission light amplified in the electric erbium-doped optical fiber and emitted through the optical isolator (205,403) and the first optical fiber coupler (206,404) at the output end is selectively returned to the erbium-doped optical fiber, and the oscillating light is output to the output terminal together with the signal light. As a reflective filter means for performing two functions to block the coming out as an optical fiber grating or dielectric thin film filter (207,405) adjacent to the signal light output stage; First optical fiber couplers 206 and 404 located between the optical isolator 205 and 403 at the signal light output side and the optical fiber grating or dielectric thin film filter 207 and 405 to split the oscillated light reflected back from the optical fiber grating or dielectric thin film filter 207 and 405. ; A loop type loop path 209 and 406 including a variable optical attenuator 208 as a loop loss adjusting means between the first optical fiber coupler 206 and 404 on the signal light output end side and the second optical fiber coupler 201 and 401 on the signal light input end side; And a loop path at a position before the optical isolator 202, 402 on the signal light input end side for injecting the oscillated light returned from the optical fiber grating or the dielectric thin film filter 207, 405 along the loop path 209, 406 back to the erbium-doped fiber. An oscillating loop for an erbium-doped fiber amplifier including a second fiber coupler (201, 401) coupled to (209, 406). The oscillating loop for an erbium-doped optical fiber amplifier according to claim 1, wherein either the electrical first or second optical fiber coupler is replaced with a variable optical fiber coupler capable of changing the optical power splitting ratio. A method of controlling amplification gain using an erbium-doped optical fiber comprising the oscillation loop of claim 1.