# US 4,399,565 A · Device for triggering an alarm system in the event of insufficiency of the transmission level, for the receiver module of an optical-fiber transmission system

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- Number: US 4,399,565 A
- Document ID: US4399565A
- Status: Granted
- Kind: A
- Application number: 06/232,966
- Filed: 1981-02-09
- Granted: 1983-08-16
- Earliest claimed: 1980-02-12
- Assignees: Lignes Telegraphiques et Telephoniques
- Inventors: Bertrand Jarret; Rose Cordier
- Web page: https://patentgrep.com/patent/US4399565A

## CPC

- H04B 10/6911: {Photodiode bias control, e.g. for compensating temperature variations}
- H04B: TRANSMISSION

## Abstract

An alarm-triggering device for the receiver of an optical-fiber transmission system including in series a filter which has a passband located externally with respect to the band of frequencies employed for optical-fiber transmission and the input of which is connected to an avalanche photodiode, an amplifier, a rectifier, a voltage comparator circuit, and alarm-triggering circuit.

## Claims

### Claim 1

A device for triggering an alarm system in the event of insufficiency of the transmission level for the receiver of a transmission system which utilizes an optical fiber, comprising:
a photodetector having an automatic gain regulation and producing noise in the event of said insufficiency of transmission level;
a filter connected to said photodetector for passing said noise, said filter having a passband which is external to the frequency band employed for optical fiber transmission;
means for amplifying said noise present at the output of said filter;
means for rectifying said noise present at the output of said amplifying means;
a circuit for comparing the amplitude of said noise to a threshold amplitude value, said comparing circuit being connected to the output of said rectifying means; and
alarm-triggering means connected to the output of said comparing circuit for producing an alarm signal when said noise amplitude is greater than said threshold value.

### Claim 2 (depends on 1)

A device according to claim 1, wherein the input of said filter is connected to the output of an amplifier which is supplied by said photodetector.

### Claim 3 (depends on 2)

A device according to claim 2, further including means for automatically regulating the gain of said photo detector in accordance with said alarm signal.

### Claim 4 (depends on 1, 2)

A device according to claim 1 or claim 2, wherein the mean frequency of said filter is lower than the bottom limit of the frequency band employed for optical-fiber transmission.

### Claim 5 (depends on 1, 2)

A device according to claim 1 or claim 2, wherein the mean frequency of said filter is higher than the top limit of the frequency band employed for optical-fiber transmission.

### Claim 6 (depends on 1)

A device according to claim 1, wherein said amplifying means are constituted by a first low-noise amplifier and a second ampifier connected in series.

### Claim 7 (depends on 1)

A device according to claim 1, wherein said photodetector is an avalanche photodiode.