JP3311451B2 - Optical amplifier - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplifier used in an optical multi-relay transmission system, and more particularly to a configuration of a control unit for controlling an output.

[0002]

2. Description of the Related Art With the digitization of information networks and the spread of data communication, optical fiber transmission has been used in trunk transmission networks. Although optical fibers have low loss, they can be transmitted over long distances. However, recently emerging erbium-doped optical fibers (EDFs) can be amplified as optical signals by optical amplifiers, and the regenerative repeat interval is reduced. It became possible to extend dramatically. When an optical amplifier is used in a multi-repeater system, it is necessary to stabilize the output. ALC (Automatic Le
vel Contorol) is discussed in "1992 IEICE Spring Conference, B-936".

[0003]

However, the actual A
In LC, it is necessary to consider a response to a change in input light or a pump light source. For example, when the input light level fluctuates, the ALC operates to keep the output light level constant with the fluctuation. As a DC response, the output light level is basically constant by ALC. However, transiently, due to the speed and magnitude of the fluctuation of the input light level, A
The LC cannot always control the output light level to be constant. As described above, to improve the transient ALC tracking characteristic when the input light level changes, the loop gain of the ALC may be increased, but the input / output of the optical amplifier is reduced due to the low-frequency cutoff characteristic of the EDF. Peaking occurs in the low frequency range. That is, the EDF has a phase delay in the modulation characteristic of the pump light-light output near the low cutoff frequency. The ALC loop includes an integral element to keep the output light level constant irrespective of the operation state of the EDF. Due to the phase delay of EDF and the phase delay of 90 ° due to the integral element, E
ALC becomes positive feedback near the low cutoff frequency of DF,
When the loop gain is increased, the operation becomes unstable, and peaking occurs in the input / output frequency characteristics. In an optical amplifier having peaking in input / output characteristics, when a fluctuation in the peaking frequency is input, the fluctuation is amplified and output.
When optical amplifiers with the same characteristics are used in multi-relay transmission, fluctuations at the above peaking frequency are amplified in all optical amplifiers, so even small peaking results in large fluctuations after multi-relay, degrading transmission characteristics. Let it.

An object of the present invention is to provide an optical amplifier for an optical multi-relay transmission system capable of stably controlling the output.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical amplifier having a function of controlling an output by amplifying or attenuating an input optical signal using a rare earth element-doped optical fiber. This is achieved by having elements. That is, in the present invention, the phase lag caused by the EDF and the integration element is compensated by inserting the phase lead element into the ALC loop. In particular, the frequency at which the phase of the phase advance element starts to advance is set to be equal to or lower than the low cutoff frequency of the EDF. Further, the loop gain of the ALC loop is made variable according to the error signal amount which is the difference between the observed output light level and the set output light level.

[0006]

The phase delay element inserted in the ALC loop compensates for the phase delay occurring above the low cut-off frequency of the EDF, and the phase delay in the entire loop becomes 180 ° or less. At this time, since the gain margin of the control loop is increased as compared with the case where the phase lead element is not inserted, stable control is possible even if the loop gain of the ALC loop is increased, and the ALC tracking characteristic when the input fluctuates. Can be improved. In addition, since the phase delay is reduced, positive feedback is not likely to occur, and the frequency characteristics are flattened and peaking does not occur.

If the ALC loop gain is controlled in accordance with the error signal amount, which is the difference between the observed output light level and the set output light level, the error signal increases with a large change in the input light level. By increasing the loop gain accordingly, the output fluctuation can be suppressed to a certain amount or less. When the fluctuation of the input light level is small, the loop gain can be reduced, so that a stable operation is possible in a normal operation range of the optical amplifier.

[0008]

Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing a configuration of an optical amplifier according to an embodiment of the present invention, FIG. 2 is a diagram showing a modulation characteristic of an output by an EDF pumping light, FIG. 3 is a diagram showing a characteristic of an integral element, and FIG.
FIG. 5 is a diagram showing characteristics of a phase lead element, FIG. 6 is a diagram showing characteristics compensated by a phase lead element, and FIG. 7 is a diagram showing a configuration of an integral element having a phase lead element. FIG. 8 is a diagram illustrating gain control characteristics, and FIG. 9 is a diagram illustrating a configuration example of a gain control unit.

FIG. 1 shows the configuration of an optical amplifier according to the present invention.
Wherein 1 is a multiplexer for multiplexing the signal and the pump light,
2 is an erbium-doped fiber (EDF), 3 is a duplexer for monitoring output power, 4 is a photodiode,
5 is a transimpedance amplifier that converts a photocurrent into a voltage signal, 6 is an error circuit that generates a difference between the actual output power and the set power, 7 is a gain control unit having nonlinear characteristics,
8 is a phase lead element, 9 is an integration element, 10 is an excitation light source drive current source, and 11 is an excitation light source. The configuration shown in FIG. 1 is forward pumping in which the traveling direction of the signal coincides with the traveling direction of the pumping light. However, even in the case of backward pumping in the opposite direction and bidirectional pumping using both, the pumping light source driving current source 10 The control system has the same configuration, except that the pump light source 11 and the multiplexer 1 are changed or added. Further, the order of the gain control unit 7, the phase advance element 8, and the integration element 9 may be interchanged.

Next, the DC operation of control in the optical amplifier shown in FIG. 1 will be described. The optical input signal and the pump light from the pump light source 11 are multiplexed by the multiplexer 1 and propagate through the EDF 2. The wavelength of an optical signal amplified in the case of EDF is 1.5.
Light having a wavelength of 1.48 μm or 0.98 μm is used as the excitation light in the μm band. The excitation light is E
The 1.5 μm optical signal that excites and passes through DF2 is amplified by stimulated emission. Part of optical output is splitter 3
, And the output power is monitored by the photodiode 4 as a photocurrent. Although omitted in FIG. 1, an error occurs if the excitation light is included in the light separated by the demultiplexer 3. Therefore, an optical filter for removing the excitation light may be inserted before the photodiode 4. is there. The photocurrent monitored by the photodiode 4 is converted into a voltage by a transimpedance amplifier 5 and then compared with an output set value by an error circuit 6 to become an error signal.
Here, the output set value is adjusted so that the output of the optical amplifier becomes a correct value in consideration of the loss of the duplexer 3, the efficiency of the photodiode 4, and the transimpedance of the transimpedance amplifier 5. The error signal is converted by the gain control unit 7 in accordance with the amount of the error signal. However, for the sake of explanation of the basic operation, the error signal is regarded as a simple amplifier. The operation of the phase lead element 8 is related to the transient characteristics, and will be described later in detail. Further, the error signal is integrated by the integration element 9, and the current of the excitation light source drive current source 10 is determined by the integrated value, and the excitation light power of the excitation light source 1 is determined accordingly. In the control loop shown in FIG. 1, the polarity of the loop gain is determined so that when the optical output exceeds a set value, the pumping light power is reduced. Since the gain of the EDF 2 decreases as the pumping light power decreases, it becomes negative feedback. Therefore, the control loop operates so that the optical output of the optical amplifier becomes the set value.

The reason why the integrating element 9 is used in the control mechanism is to control the output to be constant even when the input optical power to the optical amplifier, the IL characteristic of the pump light source 11, and the like change. FIG. 2 shows the modulation characteristics of the output by the EDF excitation light source,
FIG. 3 shows the characteristics of the integral element. In the figure, the horizontal axis is normalized by the cutoff frequency fEDF of the EDF, and the vertical axis is the relative gain. As shown in FIG. 2, the EDF has a cutoff frequency fE
A 45 ° phase delay occurs in the DF. Also, as shown in FIG. 3, the integral element causes a 90 ° phase delay at all frequencies. Therefore, when the control mechanism is constituted only by the integral element, the control loop has a phase delay of 135 ° at the cutoff frequency of the EDF as shown in FIG. This is ED
Considering the phase lag that occurs outside of F2 and the integral element 9,
If the frequency is higher than fEDF, the output becomes unstable and the output cannot be controlled to be constant. Also, if the loop gain is increased to reduce the output error, peaking occurs in the gain due to the phase delay, which adversely affects the transmission characteristics. However, as shown in FIG. 1, in the configuration having the phase lead element 8 having the characteristic shown in FIG. 5 in the control loop, it is possible to compensate for a high-frequency phase delay. In particular, by setting the frequency of the phase lead element to be equal to or lower than the cutoff frequency of the EDF, stable control can be performed in all frequency ranges. FIG. 6 shows a characteristic in which the phase lag is compensated by the phase lead element 8. The solid line shows the case where the frequency fPL of the phase lead element 8 is equal to the cutoff frequency fEDF of the EDF. The phase is delayed by 90 ° at all frequencies, and has a sufficient margin for a phase delay other than the phase lead element 8 and the integral element 9. If the frequency fPL of the phase lead element 8 is set to be equal to or lower than the cutoff frequency fEDF of the EDF, the maximum phase delay is 90 ° as shown by the broken line in FIG. 6, and a sufficient phase margin can be similarly secured. .
FIG. 7 shows a circuit in which the phase delay element 8 and the integration element 9 are combined. Reference numeral 12 denotes an OP amplifier. The gain of the integral element is determined by R1 and C, and the frequency fPL of the phase lead element is 1
/ (2 × π × C × R2). Therefore, 1 /
(2 × π × C × R2) may be set to fEDF or less.

Next, the operation of the loop gain control function will be described.
In general, in closed loop control, the maximum loop gain is designed from around the phase. The application of the optical amplifier is for relaying an optical signal, but the envelope change of the input optical signal power is small under normal use conditions. Therefore, lowering the loop gain enables more stable control. However, if the input signal fluctuates greatly due to an unexpected reason, it is better to increase the loop gain in order to quickly converge the output to the set value. The conflicting operations described above can be achieved by controlling the loop gain according to the difference (error signal) between the output set value and the output observation value. FIG. 8 shows the input / output characteristics of the loop gain control unit. In the normal operating state of the optical amplifier, a stable operation is possible because the gain when the error signal is small is small, but the gain increases when the error becomes larger than a certain value,
Try to quickly converge to the set value. Since it is considered that the input fluctuation of an actual optical amplifier often becomes smaller than the input signal sharply increases, it is sufficient to increase the gain only for the error of the input decrease as shown by the broken line. The effect can be obtained. In this case, oscillation due to transition only in a region where the gain is large can be avoided. FIG.
1 shows an example of the configuration of the gain control circuit. The gain of the circuit is -Rf / Ri for a small signal, but when the input voltage is equal to or higher than the threshold voltage of the diode, the gain becomes -Rf / Rd (Rd is the series resistance of the diode), and the gain can be increased.

Although the above embodiment has been described with reference to the case where the output is kept constant, the configuration of the control unit according to the present invention is effective in terms of stabilizing the operation even when the output set value is controlled internally or externally. . It is also effective when controlling the gain instead of the output power.

[0014]

As described above, the optical amplifier according to the present invention has:
In an optical amplifier having a function of controlling output by amplifying or attenuating an input optical signal using a rare-earth element-doped optical fiber, low frequency cutoff by phase compensation by having an integrating element and a phase advance element in the control mechanism. Stable control is possible even at frequencies higher than the frequency, and the frequency characteristics are flattened and peaking does not occur. Therefore, even in multi-relay transmission in which optical amplifiers are cascaded, stable transmission is possible because only a specific frequency component of an optical signal is not amplified.

Further, according to the loop gain control shown in the present invention, it is possible to quickly converge the output to a set value with respect to a large input fluctuation, and to achieve stable transmission even on a transmission line having fluctuation. Will be possible.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration in an embodiment of an optical amplifier according to the present invention.

FIG. 2 is a diagram showing a modulation characteristic of an output by an EDF pumping light.

FIG. 3 is a diagram illustrating characteristics of an integral element.

FIG. 4 is a diagram showing a combined characteristic of an EDF and an integral element.

FIG. 5 is a diagram illustrating characteristics of a phase lead element.

FIG. 6 is a diagram showing characteristics compensated by a phase lead element.

FIG. 7 is a diagram showing a configuration of an integral element having a phase lead element.

FIG. 8 is a diagram illustrating gain control characteristics.

FIG. 9 is a diagram illustrating a configuration example of a gain control unit;

[Explanation of symbols]

2 ... Rare earth element doped optical fiber (erbium doped fiber (EDF)) 6 ... Error circuit 8 ... Phase lead element 9 ... Integral element

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Junya Kosaka 216 Totsuka-cho, Totsuka-ku, Yokohama-shi, Kanagawa Prefecture Optical Technology Development Promotion Division, Hitachi, Ltd. (72) Inventor Kazuo Aida 1-1-1, Uchisaiwaicho, Chiyoda-ku, Tokyo No. 6 Nippon Telegraph and Telephone Corporation (72) Yoshiaki Sato Inventor Nippon Telegraph and Telephone Corporation 1-6, Uchisaiwaicho, Chiyoda-ku, Tokyo (56) References JP-A-5-75198 (JP, A) JP JP-A-5-198877 (JP, A) JP-A-6-333258 (JP, A) JP-A-64-5085 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01S 3 / 00-3/30 G02F 1/35 501 JICST file (JOIS)

Claims (4)

(57) [Claims] 1. Use of a rare earth element-doped optical fiber (2) ,
Regardless of the magnitude of the optical signal input to the optical fiber (2).
The optical output from the optical fiber (2) becomes a predetermined value
An optical amplifier having a function of controlling so, the output of the fiber (2) the optical fiber (2)
A splitter (3) for monitoring the output power of the
The output light of the splitter (3) is close to the splitter (3).
Photocurrent by the photodiode (4) provided in contact
And the photocurrent is transimpedance
(5) converts the voltage into a voltage,
(6) the error circuit is inputted to the first input terminal and
The voltage input to the second input terminal of (6) is
A value such that the light output from the fiber (2) becomes the predetermined value.
And the error circuit (6) sets the first and second
Error signal that compares and outputs the input voltage value of the input terminal of
Is a phase lead provided on the output side of the error circuit (6).
Element (8) and integral element for integrating the error signal
Through (9), the integral value of the error signal is
Input to the dynamic current source (10), and is determined by the integral value.
An excitation light source driving current source, the excitation light source driving current source being
The excitation light source is output from the
(11) is driven, and the output light of the excitation light source (11) is
The optical signal is multiplexed with the input signal by the multiplexer (1).
An optical amplifier, which is input to a fiber (2) . Wherein said phase advance the integral element and element (8)
2. The optical amplifier according to claim 1, wherein the optical amplifier is combined with (9) to form an integrated configuration . 3. An output of said error circuit (6) and said phase lead.
A gain control unit (7) is provided between the control unit and the element (8).
The gain control section (7) is an output of the error circuit (6).
According to the magnitude of the error signal, the multiplexer (1)
Optical fiber (2), the splitter (3), and the pump light
A loop returning to said multiplexer (1) via a source (11)
2. The method according to claim 1, wherein the gain is changed.
Or the optical amplifier according to 2 . 4. The gain control section (7) further comprises:
It is assumed that the absolute value of the magnitude of the error signal is not less than a certain value.
Larger than when the absolute value is less than the fixed value.
4. The optical amplifier according to claim 3, wherein the optical amplifier is configured to: