JP2002141592A - Optical fiber amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ALC operation or AGC operation optical fiber amplifier applicable even to a case where the wavelength of a signal light varies with time in an optical transmission system, or a case where the signal light has a wide line width and an ASE component contained within the line width is not negligible as compared with the signal light component. SOLUTION: The optical fiber amplifier employing a rare earth added optical fiber 1 as an amplification medium comprises a pumping light source 7, a pumping light source drive circuit 6, a liquid mixing coupler 12 for pumping light and signal light, a tap 11 for branching a part of amplified light exiting the optical fiber, and a monitor photodetector 2 for subjecting the branched outgoing light to OE conversion. The optical fiber amplifier further comprises an electric filter 3 for passing only a specified frequency component of an electric signal from the monitor photodetector 2, an operating circuit 4 for calculating the difference between an electric signal passed through the electric filter 3 and an electric signal of preset value and outputting the calculation results, and a light source drive circuit 6 for regulating the quantity of pumping light based on the output from the operating circuit 4 thus ensuring a constant quantity of the output light.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention is used in the field of optical fiber amplifiers used in optical transmission systems using optical fibers as transmission media.

[0002]

2. Description of the Related Art In the field of optical fiber transmission using an optical fiber amplifier, in order to maintain a certain level of communication quality,
The light quantity of the amplified signal light emitted from the optical fiber amplifier or the gain of the optical amplifier needs to be controlled so as to match the design value calculated from the transmission path loss and the receiving sensitivity of the receiver. The former is a constant output control operation (ALC operation type)
An optical fiber amplifier is called, and the latter is a constant gain operation type (AG
C operation type) called an optical fiber amplifier.

In order to realize the ALC or AGC operation, an optical fiber amplifier generally includes an output light monitor for branching a part of the output light from the optical fiber amplifier and receiving the light at a PD, and an optical fiber amplifier. It incorporates both, or at least the former, an incident light monitor that branches off a part of the incident signal light and receives it at the PD. In order to keep the level of the signal light emitted from the optical fiber amplifier constant, feedback control is performed on the light quantity of the pumping light source in the optical fiber amplifier so that the monitor signal level from the emitted light monitor matches the set value.

In order to keep the gain of the optical fiber amplifier constant, a circuit for calculating the ratio between the monitor signal level from the incident light monitor and the monitor signal level from the output light monitor is added, and the ratio is set. Feedback control is performed on the amount of light of the excitation light source in the optical fiber amplifier so as to match the value. The light emitted from the optical fiber amplifier includes amplified spontaneous emission light (ASE) in addition to the amplified signal light. The ratio between the signal light and the ASE of the emitted light varies depending on the operating conditions of the optical fiber amplifier.

As a result, when performing the above-described control, it is common practice to provide an optical bandpass filter for optically separating only the signal light component of the emitted light. In many cases,
In an optical transmission system using an optical fiber amplifier, DF
B, LD, etc. are used as signal light sources, the wavelength of the signal light is temporally constant, and the line width is sufficiently narrow. Therefore, by passing through a narrow band optical bandpass filter, the ASE component was effectively removed, and only the signal light component could be extracted.

[0006]

According to the present invention, when the wavelength of the signal light changes with time in the optical transmission system, and when the line width of the signal light is wide, the ASE component included in the line width is reduced. An object of the present invention is to provide an AGC operation optical fiber amplifier which can be applied even in a case where it cannot be neglected as compared with a signal light component.

A problem with the transmission system is that an optical narrow bandpass filter placed in front of a monitor photodetector has a problem in that the line width of the signal light is wide or the wavelength thereof fluctuates with time. It is difficult to extract only the optically amplified signal light component.

[0008]

According to a first aspect of the present invention, there is provided an optical fiber amplifier using a rare earth-doped optical fiber as an amplification medium, a light source for excitation, and a light source drive circuit for excitation. A coupler for combining a pump light and a signal light, a tap for branching a part of the amplified outgoing light from the optical fiber, and a light detection for outgoing light monitoring for OE conversion of the branched outgoing light. An optical filter that passes only a specific frequency component of the electric signal from the output light monitoring photodetector, and an electric signal that has passed through the electric filter and a preset value. And an arithmetic circuit for calculating the difference between them and outputting the result, and a light source drive circuit for adjusting the amount of excitation light based on the output of the arithmetic circuit, to obtain a constant output light amount. And wherein the door.

According to a second aspect of the present invention, there is provided an optical fiber amplifier using a rare earth-doped optical fiber as an amplification medium, a pumping light source, a pumping light source driving circuit, a pumping light and a signal light. A multiplexing coupler, a tap for branching a part of the incident signal light to the optical fiber, an incident signal light monitoring photodetector for OE conversion of the branched optical signal, and amplification from the optical fiber amplifier A tap for branching a part of the split outgoing light and an O
An optical fiber amplifier comprising an output light monitoring photodetector for E-conversion, an electric filter for passing only a specific frequency component of an electric signal from the output light monitoring photodetector, and the electric filter And an arithmetic circuit that calculates the ratio of the electrical signals and outputs the result, and adjusts the amount of excitation light based on the output of the arithmetic circuit. And a light source driving circuit for obtaining a constant output light quantity with respect to the signal light.

According to a third aspect of the present invention, there is provided an optical fiber amplifier according to the first or second aspect, wherein the frequency characteristic of the electric filter is:
The method is characterized in that a high-frequency component having a frequency equal to or higher than a frequency determined by the reciprocal of the lifetime of the starting level of stimulated emission transition involved in the amplification of rare earth ions in the rare earth-doped optical fiber is passed.

According to a fourth aspect of the present invention, there is provided an optical fiber amplifier according to the first or second aspect, wherein the frequency characteristic of the electric filter is:
It has a bandpass characteristic centered on the transmission speed of the amplified signal light.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, as a method for separating an ASE component and a signal light component, after a monitor light receiver receives light without separating the two, an obtained electric signal is subjected to an electric frequency discrimination filter. To remove the ASE component and extract only the signal light component. Use a band-pass filter with the transmission bit rate of the optical transmission system used as the center frequency as the electrical discrimination filter, or determine from the reciprocal of the lifetime of the starting level of the stimulated emission transition of rare earth ions used in the optical fiber amplifier. Alternatively, a bypass filter that passes a high-frequency component higher than the frequency to be used may be used.

The main points of the present invention will be described below with reference to the drawings. As shown in FIG. 1, the incident signal light passes through the signal light input photometric isolator 8 and enters the rare earth-doped optical fiber 1. The signal light amplified in the rare earth-doped optical fiber 1 passes through the signal light output photometric isolator 9, and a part of the signal light enters the output light monitoring photodetector 2 by the signal light output monitoring tap 11. The pumping light is multiplexed with the signal light by the pumping light / signal light multiplexing / demultiplexing coupler 12 as forward pumping or backward pumping (both may be used as needed).

In a normal ALC operation type optical fiber amplifier, an electric signal from the output light monitoring photodetector 2 is compared with a signal from an output level setting signal generation circuit 5 by a direct comparison circuit 4 (differential amplification). Then, an error signal corresponding to the set level is extracted, and the error signal is input to the excitation light source drive circuit 6, and the excitation light amount is adjusted so that the error signal becomes zero.
A feature of the present invention is that the electric signal from the output light monitoring photodetector 2 is passed through the electric filter circuit 3 before being input to the comparison circuit 4. The effect of this filter will be described in detail with reference to FIGS.

FIG. 3 shows the spectrum of an optical signal incident on the output light monitoring photodetector 2. In this example, the rare-earth-doped optical fiber is a Pr (praseodymium) -doped optical fiber, but other types of rare-earth-doped optical fibers may of course be used. The spectrum of the light emitted from the optical fiber amplifier includes an ASE (amplified spontaneous emission light) component in a form overlapping with the amplified signal light component in addition to the amplified signal light component. FIG. 4 illustrates the relationship between the amplified signal light component and the ASE component on the frequency axis of the electric signal in more detail.

FIG. 4 is a diagram showing a signal received by a photodetector displayed on a frequency axis by an RF spectrum analyzer. The signal format of the optical signal input to the optical amplifier is
156Mb / s NRZ (Non Return to Zero code)
Modulation. In the figure, the low-frequency component of 10 Mz or less is a component that depends on the temporal variation of the amplified output light.
This occurs because the average optical power of the signal light input to the optical fiber amplifier fluctuates with time.

On the other hand, a peak is observed near 156 MHz, which is a peak corresponding to the modulation component of the signal. FIG. 5 shows the output light level (thin solid line) output when the input level of the optical signal to the optical amplifier is changed when the 156 Mb / s optical signal described in FIG. 4 is amplified. The relationship between the optical power (thick solid line) of the signal light component included in the output light is shown. At the same time, FIG. 5 shows all the spectral components (-dot chain line) of the electric signal obtained from the output monitor before passing through the frequency filter and the like,
An electric signal component (dotted line) after passing through an RF band-pass filter having a center frequency of 156 Mb / s, and an electric signal component passing through an RF high-pass filter that passes a high-frequency component of 50 MHz or more instead of the band-pass filter (Two-dot chain line) are also shown.

Output light level from optical amplifier (thin solid line)
It can be seen that, even if is maintained at a constant value, the signal light level (thick solid line) in the outgoing light decreases when the incident signal light level decreases. All the spectral components (dashed lines) of the electric signal from the output light monitor in the optical amplifier have a substantially constant value irrespective of the incident signal light level. On the other hand, RF
It can be seen that the components (dotted line and two-dot chain line) passed through the band-pass filter or the high-pass filter have almost the same output characteristics as the actual signal light output. As is apparent from the above description, if the components (dotted line and two-dot chain line) passed through the RF band-pass filter or the high-pass filter are used as the monitoring electric signal, the monitor output proportional to the amplified optical signal light can be obtained. Thus, a constant output operation type optical amplifier or a constant gain type optical amplifier can be realized.

[0019]

Embodiment 1 In this embodiment, an optical amplifier for the 1.3 μm band was studied based on the basic configuration 1 shown in FIG.
The format of the optical signal was a Fabry-Perot semiconductor LD with a wavelength of 1.33 μm, and the signal modulation format was a binary scrambled NRZ of 156 Mb / s. Pr (praseodymium) is used as the rare earth-doped optical fiber 1.
An additive Zr-based fluoride fiber was used, and an oblique V-groove connection was used for connection between the fluoride fiber and the quartz fiber.

The specifications of the Pr-doped fluoride fiber are as follows. Pr-added concentration 1000 ppm Cut-off wavelength 0.95 μm Relative refractive index difference 2.5% As the coupler 12 for multiplexing / demultiplexing the excitation light / signal light, a fusion drawn fiber coupler is used, and the light source for excitation is 1020 n.
An m oscillation semiconductor LD was used.

The output light monitoring photodetector 2 is a PI
Using an N-PD, the electrical signal after OE conversion has a center frequency of 156 MHz and a spectral half width at the time of 3 dB reduction is 20.
MHz. The signal after passing through the RF band-pass filter 3 was amplified by an OP amplifier and then input to a comparison circuit 4 together with a set level signal to generate an error signal.

This error signal was input to the drive circuit 6 of the 1020 nm LD for excitation, and the drive current of the LD was controlled so that the error signal became zero. With this configuration, it was possible to control the output of only the signal light component from the optical amplifier to 6 ± 0.2 dBm in the signal light input range of −10 Bm to +2 dBm. When only the RF bandpass filter 3 was removed with the same configuration, the output of only the signal light component from the optical amplifier changed from -3 dBm to +6 dBm in the signal light input range of -10 Bm to +2 dBm.

[0023]

Embodiment 2 This embodiment has the same configuration as that of Embodiment 1 except for the RF filter. As the RF filter 3,
A filter having a cutoff frequency of 10 MHz and having a bypass characteristic was used. With this configuration, it was possible to control the output of only the signal light component from the optical amplifier to 6 ± 0.2 dBm in the signal light input range of −10 Bm to +2 dBm.

[0024]

Embodiment 3 In this embodiment, a 1.5 m band optical amplifier was studied based on the basic configuration 2 shown in FIG. The format of the optical signal is a DFB semiconductor L with a wavelength of 1.55.
D was used, and the signal modulation format was 156 Mb / s binary scrambled NRZ. As the rare earth-doped optical fiber, an Er (erbium) -doped Al co-doped quartz fiber was used.

The specifications of the Er-doped fiber are as follows. Er added concentration 1000 ppm Cut-off wavelength 0.95 μm Relative refractive index difference 1.8% As the coupler 12 for the multiplexing / demultiplexing of pumping light / signal light, a bulk type coupler constituted by a fiber collimator using a dielectric multilayer filter is used. Light source for 1480nm
An oscillating semiconductor LD was used.

The output light monitoring photodetector 2 includes a PI
Using an N-PD, the electrical signal after OE conversion has a center frequency of 156 MHz and a spectral half width at the time of 3 dB reduction is 20.
MHz. The signal after passing through the RF bandpass filter 3 was amplified by an OP amplifier and then input to a division circuit 14. At the same time, the electric signal from the input light monitoring photodetector 15 is appropriately amplified by an OP amplifier, and then input to a division circuit 14, where the signal is divided by the signal after passing through the above-mentioned RF band-pass filter 3, and divided. And a comparison error signal together with the set level signal from the gain setting signal generation circuit 13.

This error signal was input to the drive circuit 6 of the excitation 1480 nm LD, and the LD drive current was controlled so that the error signal became zero. When the gain is adjusted to be 20 dB by this configuration, the signal light input range is -15.
In the range from Bm to 0 dBm, it was possible to control the gain of only the signal light component from the optical amplifier to 20 ± 0.2 dB. When only the RF bandpass filter was removed with the same configuration, the gain of only the signal light component from the optical amplifier was changed from 25 dB to 20 dB in the signal light input range from -15 sB to 0 Bm.

[0028]

[Embodiment 4] In this embodiment, an example in which a bypass type is used only for an RF filter will be described in comparison with Embodiment 3. The life of the stimulated emission transition of Er in the used Er-doped fiber is about 9 msec, and is about 1 msec even under the condition where the saturation signal is input. Therefore, a high-pass filter (cut-off frequency 0.5 KHz) that passes a frequency lower than the frequency calculated by the reciprocal of the stimulated emission lifetime,
2 of a high-pass filter using a high-pass filter having a cut-off wavelength at a frequency sufficiently higher than the frequency calculated by the reciprocal of the stimulated emission lifetime (the cut-off frequency is 10 MHz)
The types were compared and examined.

First, a case of a filter having a cut-off frequency of 10 MHz will be described. In this case, when the signal light input is adjusted so that the gain becomes 0 dBm and the gain becomes 20 dB,
The signal light input range is -15 Bm to 0 dBm, and the gain of only the signal light component from the optical amplifier is 20 ±
It could be controlled to 0.2 dB. Next, 0.5mz
The case of a filter having a cutoff frequency of
In this case, when the signal light input is adjusted to 0 dBm and the gain is set to 20 dB, the signal light input range is -15 B
In the range from m to 0 dBm, the gain of only the signal light component from the optical amplifier was changed from 25 dB to 20 dB. The life of the stimulated emission transition of Er in the used Er-doped fiber is about 9 msec, and is about 1 msec even under the condition where the saturation signal is input.

As described above, according to the present invention, a monitor light is converted into an electric signal in an optical fiber amplifier that controls the output intensity by feeding back a part of the output light (monitor light) to the excitation light source light amount adjusting unit. And separates the signal from noise and feeds it back to the light amount adjustment unit. Since the line width of the signal light is wide or its wavelength fluctuates with time, an optical band-pass filter extracts only the signal component. Even when it is difficult, only signal components can be extracted by electrical processing, so that an amplifier with stable output can be realized.

[0031]

As described above in detail with reference to the embodiments, according to the present invention, when configuring an optical amplifier of a constant output or constant gain operation type, only the conventional signal light component is separated. This eliminates the need for an optical bandpass filter, which is necessary for the above, and the same effect can be realized only by electrical processing. This has an advantage that a stable optical amplifier can be realized only by electrical processing even when the wavelength of an optical signal fluctuates with time.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an optical fiber amplifier mainly according to claim 1 of the present invention.

FIG. 2 is a configuration diagram of an optical fiber amplifier mainly according to claim 2 of the present invention.

FIG. 3 is a graph showing a spectrum of output light from an optical amplifier.

FIG. 4 is a graph showing a frequency spectrum of an electric signal subjected to OE conversion by an output-side monitoring photodetector.

FIG. 5 is a graph showing an input optical signal level dependency of an electric signal from an output monitor and an amplified optical signal level.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 rare-earth doped optical fiber 2 output light monitoring photodetector 3 high-frequency pass electric filter (or band-pass electric filter) 4 comparison circuit 5 output level setting signal generation circuit 6 excitation light source drive circuit 7 excitation light source 8 signal light Input photometric isolator 9 Signal light output photometric isolator 10 Signal light input monitor tap 11 Signal light output monitor tap 12 Excitation light / signal light multiplexing / demultiplexing coupler 13 Gain setting signal generation circuit 14 Division circuit 15 Signal light input monitor light detection vessel

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04B 10/04 10/17 10/16 (72) Inventor Tadashi Sakamoto 2-3-3 Otemachi, Chiyoda-ku, Tokyo No. 1 Inside Nippon Telegraph and Telephone Corporation (72) Hirotaka Ono Inventor 2-3-1 Otemachi, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (72) Yoshiki Nishida 1-chome Dogenzaka, Shibuya-ku, Tokyo No. 12-1 NTT Electronics Corporation (72) Inventor Yasutake Oishi 1-12-1 Dogenzaka, Shibuya-ku, Tokyo F-term (Reference) 5F072 AB07 AK06 HH02 JJ05 PP07 YY17 5K002 AA06 BA05 BA13 CA09 CA10 CA13

Claims (4)

[Claims] An optical fiber is used as an amplifying medium. A pumping light source, a pumping light source driving circuit, a coupler for combining a pumping light and a signal light, and an amplified outgoing light from the optical fiber. A tap for branching a part of the optical fiber, and a photodetector for outgoing light monitoring for OE conversion of the branched outgoing light, wherein an electrical signal from the photodetector for outgoing light monitoring is provided. An electric filter that allows only a specific frequency component to pass therethrough, and an electric circuit that receives an electric signal that has passed through the electric filter and an electric signal having a preset value, calculates a difference between them, and outputs a result, An optical fiber amplifier comprising a light source drive circuit for adjusting the amount of excitation light based on the output of the arithmetic circuit, and obtaining a constant amount of output light. 2. A rare earth-doped optical fiber is used as an amplification medium, a pumping light source, a pumping light source driving circuit, a coupler for combining pumping light and signal light, and a part of signal light incident on the optical fiber. And a tap for splitting a part of the amplified output light from the optical fiber amplifier, a tap for splitting the output signal, an optical detector for monitoring the incident signal light for OE conversion of the split optical signal, and a splitter. An optical fiber amplifier comprising an output light monitoring photodetector for performing OE conversion of the output light, wherein an electric filter that passes only a specific frequency component of an electric signal from the output light monitoring photodetector; An electric signal passing through the electric filter and an electric signal from the incident signal light monitoring photodetector are input,
An arithmetic circuit that calculates these ratios and outputs the result, and a light source drive circuit that adjusts the amount of excitation light based on the output of the arithmetic circuit is provided to obtain a constant output light amount with respect to the signal light. Optical fiber amplifier. 3. The optical fiber amplifier according to claim 1, wherein a frequency characteristic of the electric filter is based on a reciprocal of a lifetime of a starting level of a stimulated emission transition related to amplification of rare earth ions in the rare earth doped optical fiber. An optical fiber amplifier for passing a high frequency component higher than a determined frequency. 4. The optical fiber amplifier according to claim 1, wherein a frequency characteristic of the electric filter has a band-pass characteristic centered on a transmission speed of the signal light to be amplified.