# US 5,703,711 A · In-line optical amplifier

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- Number: US 5,703,711 A
- Document ID: US5703711A
- Status: Granted
- Kind: A
- Application number: 08/659,934
- Filed: 1996-06-07
- Granted: 1997-12-30
- Earliest claimed: 1995-06-09
- Assignees: NEC Corporation
- Inventors: Satoshi Hamada
- Web page: https://patentgrep.com/patent/US5703711A

## CPC

- H04B 10/2912: {characterised by the medium used for amplification or processing}
- H01S 3/094003: {the pumped medium being a fibre}
- H01S 3/13013: {by controlling the optical pumping}
- H04B 10/077: using a supervisory or additional signal
- H04B 10/07953: {Monitoring or measuring OSNR, BER or Q}
- H04B 10/07955: {Monitoring or measuring power}
- H04B 10/0797: {Monitoring line amplifier or line repeater equipment}
- H04B 10/2931: {using AGC (H04B10/294 takes precedence)}
- H04B 10/2935: {with a cascade of amplifiers}
- H01S 3/0014: {Monitoring arrangements not otherwise provided for (photometry G01J1/00, e.g. G01J1/4257; radiation pyrometry G01J5/00; measuring coherence of light G01J9/00; measuring wavelength of light G01J9/00, e.g. G01J9/0246; measuring optical pulses G01J11/00; calorimetrically measuring power of laser beams G01K17/003)}
- H01S 3/10015: {by monitoring or controlling, e.g. attenuating, the input signal}
- H01S 2301/02: ASE (amplified spontaneous emission), noise; Reduction thereof
- H04B 2210/08: Shut-down or eye-safety
- H04B: TRANSMISSION
- H01S: DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL

## Abstract

An in-line optical amplifier includes a pumping source, a rare-earth element doped fiber amplifier, a gain regulator and a noise detector. An input power and an output power of the in-line optical amplifier are detected from the input optical signal and the output optical signal, respectively, and then they are used to detect a resultant gain of the in-line optical amplifier. The gain regulator controls the pumping light emitted by a pumping source such that the gain of the rare-earth element doped fiber amplifier is increased and a gain deviation of the resultant gain from a predetermined gain is reduced. The noise detector detects an internal noise or a noise figure based on the gain deviation, for example, by detecting the intensity of the pumping light or the magnitude of drive current of the pumping source.

## Claims

### Claim 1

A circuit for optically amplifying an input optical signal to produce an output optical signal, comprising:
power detecting means for detecting an input power and an output power from the input optical signal and the output optical signal, respectively;
amplifying means for optically amplifying the input optical signal after the input power is detected with a gain varying in accordance with an intensity of pumping light;
gain detecting means for detecting a resultant gain of the circuit based on the input power and the output power;
gain control means for adjusting the intensity of the pumping light such that the resultant gain of the circuit is kept at a predetermined gain; and
detecting means for detecting a physical quantity associated with a deviation of the resultant gain of the circuit from the predetermined gain.

### Claim 2 (depends on 1)

The circuit according to claim 1, wherein the detecting means detects the intensity of the pumping light.

### Claim 3 (depends on 1)

The circuit according to claim 1, wherein the detecting means detects a magnitude of current converted into the pumping light.

### Claim 4 (depends on 1)

The circuit according to claim 1, wherein the detecting means detects the deviation of the resultant gain of the circuit from the predetermined gain.

### Claim 5 (depends on 1)

The circuit according to claim 1, wherein the gain control means comprises:
light-emitting means for emitting the pumping light to the amplifying means, the intensity of the pumping light varying in accordance with a drive current supplied to the light emitting means; and
control means for controlling the drive current such that the resultant gain of the circuit is kept at the predetermined gain.

### Claim 6 (depends on 5)

The circuit according to claim 5, wherein the detecting means comprises a photodetector for detecting the intensity of the pumping light emitted from the light-emitting means.

### Claim 7 (depends on 5)

The circuit according to claim 5, wherein the detecting means comprises a current detector for detecting the drive current supplied to the light-emitting means.

### Claim 8 (depends on 5)

The circuit according to claim 5, wherein the detecting means comprises a gain deviation detector for detecting the deviation of the resultant gain of the circuit from the predetermined gain.

### Claim 9 (depends on 1)

The circuit according to claim 1, further comprising:
determination means for determining whether the physical quantity exceeds a predetermined level.

### Claim 10 (depends on 5)

The circuit according to claim 5, further comprising:
determination means for determining whether the physical quantity exceeds a predetermined level.

### Claim 11 (depends on 10)

The circuit according to claim 10, wherein:
the detecting means comprises a photodetector for detecting the intensity of the pumping light emitted from the light-emitting means; and
the determination circuit determines whether the intensity of the pumping light exceeds a predetermined intensity level.

### Claim 12 (depends on 10)

The circuit according to claim 10, wherein:
the detecting means comprises a current detector for detecting the drive current supplied to the light-emitting means; and
the determination circuit determines whether the drive current exceeds a predetermined current level.

### Claim 13 (depends on 10)

The circuit according to claim 10, wherein:
the detecting means comprises a gain deviation detector for detecting the deviation of the resultant gain of the circuit from the predetermined gain; and
the determination circuit determines whether the gain deviation exceeds a predetermined threshold.

### Claim 14 (depends on 9)

The circuit according to claim 9, wherein the determination circuit produces a noise detection signal when the physical quantity exceeds a predetermined level, the noise detection signal indicating that an optical loss occurs within the circuit.

### Claim 15 (depends on 9)

The circuit according to claim 9, wherein the determination circuit produces an alarm signal when the physical quantity exceeds a predetermined level, the alarm signal indicating that a noise figure of the circuit is deteriorated.

### Claim 16 (depends on 10)

The circuit according to claim 10, wherein the determination circuit produces a noise detection signal when the physical quantity exceeds a predetermined level, the noise detection signal indicating that an optical loss occurs within the circuit.

### Claim 17 (depends on 10)

The circuit according to claim 10, wherein the determination circuit produces an alarm signal when the physical quantity exceeds a predetermined level, the alarm signal indicating that a noise figure of the circuit is deteriorated.

### Claim 18 (depends on 5)

The circuit according to claim 5, wherein the power detecting means comprises:
an input optical element provided at an input end of the circuit;
an input power detecting means for detecting the input power from the input optical signal passing through the input optical element;
an output optical element provided at an output end of the circuit; and
an output power detector for detecting the output power from the output optical signal passing through the output optical element.

### Claim 19 (depends on 10)

The circuit according to claim 10, wherein the power detecting means comprises:
an input optical element provided at an input end of the circuit;
an input power detecting means for detecting the input power from the input optical signal passing through the input optical element;
an output optical element provided at an output end of the circuit; and
an output power detector for detecting the output power from the output optical signal passing through the output optical element.

### Claim 20 (depends on 18)

The circuit according to claim 18, wherein the amplifying means comprises:
a rare-earth element doped fiber for amplifying the input optical signal passing through the input optical element; and
coupling means connected to the rare-earth element doped fiber at an output end, for coupling the pumping light with a signal amplified by the rare-earth element doped fiber.

### Claim 21 (depends on 19)

The circuit according to claim 19, wherein the amplifying means comprises:
a rare-earth element doped fiber for amplifying the input optical signal passing through the input optical element; and
coupling means connected to the rare-earth element doped fiber at an output end, for coupling the pumping light with a signal amplified by the rare-earth element doped fiber.

### Claim 22

An in-line optical amplifier for amplifying an input optical signal to produce an output optical signal, comprising:
an input power detector for detecting the input power from the input optical signal;
a rare-earth element doped fiber for optically amplifying the input optical signal after the input power is detected with a gain varying in accordance with an intensity of pumping light;
a light source for supplying the pumping light to the rare-earth element doped fiber;
an output power detector for detecting the output power from the output optical signal;
a gain detector for detecting an overall gain of the in-line optical amplifier based on the input power and the output power;
a gain regulator for adjusting the intensity of the pumping light such that the overall gain is kept at a predetermined gain; and
a noise level detector for detecting an internal noise level based on a physical quantity associated with a deviation of the overall gain from the predetermined gain.

### Claim 23 (depends on 22)

The in-line optical amplifier according to claim 22, wherein the noise level detector comprises:
a photodetector for detecting an intensity of the pumping light emitted from the light source; and
a determination circuit for determining whether the intensity of the pumping light exceeds a predetermined intensity level.

### Claim 24 (depends on 22)

The in-line optical amplifier according to claim 22, wherein the noise level detector comprises:
a current detector for detecting a drive current supplied to the light source; and
a determination circuit for determining whether the drive current exceeds a predetermined current level.

### Claim 25 (depends on 22)

The in-line optical amplifier according to claim 22, wherein the noise level detector comprises:
a gain deviation detector for detecting a deviation of the resultant gain of the circuit from the predetermined gain; and
a determination circuit for determining whether the deviation exceeds a predetermined level.

### Claim 26

An in-line optical amplifier comprising:
an input branching device for branching an input optical signal into a first input signal and a second input signal;
an input detector for detecting an input intensity level from the first input signal;
a rare-earth element doped fiber for optically amplifying the second input signal with a gain varying in accordance with an intensity of pumping light and producing a third signal;
an optical coupler for coupling the third signal with the pumping light;
a pumping source for supplying the pumping light to the rare-earth element doped fiber through the optical coupler;
an output detector for detecting an output intensity level from an output optical signal passing through the optical coupler;
a gain detector for detecting an overall gain of the in-line optical amplifier based on the input intensity level and the output intensity level;
a gain regulator for adjusting the intensity of the pumping light such that the overall gain is kept at a predetermined gain; and
a noise level detector for detecting an internal noise level based on a physical quantity associated with a deviation of the overall gain from the predetermined gain.

### Claim 27 (depends on 26)

The in-line optical amplifier according to claim 26, wherein the noise level detector comprises:
a photodetector for detecting an intensity of the pumping light emitted from the pumping source; and
a determination circuit for determining whether the intensity of the pumping light exceeds a predetermined intensity level.

### Claim 28 (depends on 26)

The in-line optical amplifier according to claim 26, wherein the noise level detector comprises:
a current detector for detecting a drive current supplied to the pumping source; and
a determination circuit for determining whether the drive current exceeds a predetermined current level.

### Claim 29 (depends on 26)

The in-line optical amplifier according to claim 26, wherein the noise level detector comprises:
a gain deviation detector for detecting a deviation of the resultant gain of the circuit from the predetermined gain; and
a determination circuit for determining whether the deviation exceeds a predetermined level.

### Claim 30

A method for detecting an internal noise in an in-line optical amplifier which optically amplifies an input optical signal to produce an output optical signal, the method comprising the steps of:
detecting an input power from the input optical signal;
amplifying the input optical signal after the input power is detected with a gain varying in accordance with an intensity of pumping light;
detecting an output power from the output optical signal;
detecting a resultant gain of the circuit based on the input power and the output power;
adjusting the intensity of the pumping light such that the resultant gain of the in-line optical amplifier is kept at a predetermined gain; and
detecting a physical quantity associated with a deviation of the resultant gain of the in-line optical amplifier from the predetermined gain.

### Claim 31 (depends on 30)

The method according to claim 30, wherein the physical quantity is the intensity of the pumping light.

### Claim 32 (depends on 30)

The method according to claim 30, wherein the physical quantity is a magnitude of current converted into the pumping light.

### Claim 33 (depends on 30)

The method according to claim 30, wherein the physical quantity is the deviation of the resultant gain from the predetermined gain.

### Claim 34 (depends on 30)

The method according to claim 30, further comprising a determination step of determining whether the physical quantity exceeds a predetermined level.

### Claim 35 (depends on 34)

The method according to claim 34, wherein the determination step produces a noise detection signal when the physical quantity exceeds a predetermined level, the noise detection signal indicating that an optical loss occurs within the in-line optical amplifier.

### Claim 36 (depends on 34)

The method according to claim 34, wherein the determination step produces an alarm signal when the physical quantity exceeds a predetermined level, the alarm signal indicating that a noise figure of the in-line optical amplifier is deteriorated.