WO1994021997A1

WO1994021997A1 - Optical responder

Info

Publication number: WO1994021997A1
Application number: PCT/GB1994/000571
Authority: WO
Inventors: Timothy John Simmons; Andrew Thomas Harker; Michael Fake
Original assignee: Bt & D Technologies Limited
Priority date: 1993-03-24
Filing date: 1994-03-21
Publication date: 1994-09-29
Also published as: GB9306090D0; EP0642659A1; JPH08501157A; AU6261694A

Abstract

High powered optoelectronic devices are continuously monitored to detect potentially dangerous default conditions in the transmission channel to which the signal generators are connected. The monitoring comprises continuously measuring the level of signals reflected from the optical transmission channel and performing corrective action when the level of said reflected signals suddenly changes. Preferably the monitoring comprises comparing the absolute level of a proportion of the reflected signals with the absolute level of a portion of the output of the signal generator. The absolute level of reflected signals will depend upon the power input into the channel and the level of reflected signals will rise if the input power rises. It is therefore desirable to take the input power into account in assessing whether or not a default condition exists.

Description

OFΠCAL RESPONDER

This invention relates to optical responders and more specifically to responders which are adapted to detect faults in optical transmission systems utilising high optical power densities, e.g. systems using optoelectronic lasers and components such as optical amplifiers.

In absolute terms the power of an optical system is usually only a few milliwatts. For example, powers of 5 milliwatts are common but few systems achieve powers above 50 milliwatts. However the waveguides produce beams which have small diameters, e.g. below 10 microns. Therefore the power density is high since 50 milliwatts concentrated into a circle of radius 5μm gives a power loading of 4 x 10⁸ watts/m² or 40,000 watts/cm². Devices with powers above 50 milliwatts will, of course, give even higher power densities.

An optical telecommunications system usually comprises an optical signal generator or an optical amplifier which is connected to an output fibre. After a short run, e.g. less than 10 metres, this fibre is joined to a connector to facilitate linkage to a further optical system, e.g. telecommunications transmission fibre. If this connector becomes separated an optical beam with the power densities mentioned above will emerge. This beam would be dangerous if it impinged on an engineer, especially if it impinged on an eye. This becomes a substantial risk when it is necessary to separate a connector, e.g. for maintenance or servicing.

Good safety practice suggests that engineers should ensure that optical sources are disabled before they separate connectors but errors can happen and the engineers will be near (and probably looking at) the connector so that there is a substantial risk of injury.

In addition the engineers often move so that the injury may be more extensive than a spot of 10 μ diameter.

It is an object of this invention to detect a potentially dangerous default, e.g. a disconnection as mentioned above. In preferred embodiments automatic remedial action follows the detection of a default

It is to be understood that, in this specification, "remedial action" implies reducing potential for danger, e.g. by disabling the optical source or by reducing its output power and/or by initiating an alarm, e.g. an audible alarm, to warn engineers. It is emphasised that this invention is primarily concerned with recognising the default state because this recognition must precede any attempt at remedial action. This invention, which is defined in the claims, is based on the recognition that all optical channels contained reflected signals and a default state, e.g. a potentially dangerous default state, affects the level of these reflected signals. Therefore monitoring the level of the reflected signals so as to detect changes of level provides recognition of a default state.

The reflected signals are, of course, derived from the input signals and it is to be expected that the power of the reflected signals will be closely related to the power of the input signals. In many cases the power of the reflected signals will be proportional to the power of the input signals from which they are derived. Therefore it is preferred to measure the level of the reflected signals as a ratio with the input signals. When this ratio changes, this is taken as an indication of the default. In the case of modulated signals, it is appropriate to take a time average over a few periods of modulation to reduce the possibility that variations due to modulation are mistaken for changes due to a default.

This invention monitors the level of reflections returned from the system. These reflections may arise from all pans of the system. However, the path from the optoelectronic device to the first connector is usually a continuous fibre so, if this path produces any reflections at all, they will have a very low level. In any case defaults which occur outside this path are unlikely to affect the level of reflections occurring therein. However, the downstream system is larger and more complicated and this part of the system will usually produce a low level of reflections which are returned to the input (and monitored in accordance with this invention). In addition there is also a possibility of reflections occurring from the connector whether this is linked or separated. It is now appropriate to comment on the nature of these reflections but it is necessary to distinguish between two types of reflector, e.g. oblique connectors and simple connectors.

An oblique connectors has surfaces which probably give low levels of reflection but the surfaces are angled so that even these weak reflections are not acquired by the waveguide. Hence, whether linked or separated, such a connector returns virtually no optical power back to the input. When such a connector becomes separated during use the apparent level of reflected signals falls because the low level of reflections from downstream vanishes. Thus the detected reflection drops from a low level to an ultra low level when a default condition occurs. It is convenient to provide a threshold between "low" and "ultra low". Reflections above this threshold are accepted as normal whereas levels below the threshold are taken as a default. Thus, when the level falls below the threshold, remedial action as indicated above is initiated.

The surfaces of the simple connector are so configured that reflections, if any, will be acquired by the waveguide and returned to the input. When linked, a glass-to-glass interface is formed and the connector will pass optical signals without substantial reflection but, as mentioned above, there will be a low level of reflections from downstream. When separated a simple connector will produce substantial reflections which are acquired by the waveguide and returned to the input. Hence the level of reflected signals will rise on separation. For use with a simple connector the threshold is set between lower and higher and levels above the threshold indicated a default state.

It is also possible to combine both systems, e.g. to provide upper and lower thresholds. The normal state is assumed when the level of reflections is between the lower and upper thresholds and a default is assumed either when the level of reflections is below the lower threshold or the level of reflections is above the higher threshold. This invention includes both monitors and optical devices which comprise an optoelectronic means for generating optical signals and the monitor. Preferably such a monitor is functionally connected to the optoelectronic device so as to disable it or reduce its power when a default condition is recognised.

The optoelectronic device may be a signal generator or an optical amplifier. Both semiconductor devices and fibre amplifiers are included.

The invention also includes the method of monitoring an optical transmission channel for an indication of a potentially dangerous situation, which method comprises monitoring the level of reflections returned from said channel, preferably by comparing said reflections to the input to the channel, and initiating corrective action when the monitored level changes.

The invention will now be described by way of example with reference to the accompanying drawing which illustrates an optical device which includes a high power optical source and a monitor in accordance with the invention.

The device shown in the Figure comprises a high power optical source 10 which is provided with electrical power by a unit 11. The source 10 is connected to an output 13, e.g. a fibre tail, via a coupler 12. The invention is applicable to a wide range of high power sources and the source 10 may be a semiconductor laser which is a primary generator of an optical signal. Alternatively the high power source 10 may be an optical amplifier such as a semiconductor amplifier or a fibre amplifier. In any case, when the soturce 10 is an amplifier it is necessary to provide an input port 14, e.g. a fibre tail, to provide attenuated optical signals which are amplified in the amplifier 10 and then provided to the output port 13. Whatever its nature, the important fact is that source 10 provides a high power, e.g. 50-200 mW optical signal to output port 13.

The device shown in the Figure includes a monitor generally indicated by the numeral 20. During use, the monitor 20 receives reflected signals from the input port 13 (i.e. from an optical channel to which the input port 13 is connected). The purpose of the monitor 20 is to disable or reduce the power output of the source 10 on detecting a fault as described below. In addition the monitor 20 may issue an alarm but this optional extra is not illustrated in the Figure.

The monitor 20 comprises detectors 15A and 15B each of which is connected to the coupler 12. The coupler 12 separates returned signals from transmitted signals and it provides a proportion of the returned signals to detector 15A and a proportion of the high power output source 10 to detector 15B. The monitor 20 also comprises two logarithmic amplifiers namely amplifier 16A connected to detector 15A and amplifier 16B connected to detector 15B. The outputs of the two logarithmic amplifiers are connected to a differential amplifier 17.

The effect of the circuitry just described is as follows. The logarithmic amplifiers 16A and 16B effectively take the logarithms of the two detected optical levels and the differential amplifier 17 subtracts these two logarithmic signals. Since the subtraction of logarithms is equivalent to division the output of differential amplifier 17 represents the ratio of the reflected signals received in the output port 13 and the output of the high power source 10. The reflections from any optical channel to which the output port 13 is connected depend upon the state of the channel. Under normal operation the reflections should have low intensity because this is the normal operational state of a well designed transmission channel. However, if a connector (such the connector 21 schematically illustrated in the Figure) becomes separated the nature of the optical channel changes and hence the level of reflected signals also changes.

The output of the differential amplifier 17 is provided to a comparator 19 which receives a threshold signal from reference 18. There are two main embodiments, e.g. one embodiment for an oblique connector and a different embodiment for a simple connector as described above.

For an oblique connector the reference 18 provides a low level signal as the threshold to the comparator 19. When the signal from differential amplifier 17 falls below this threshold the comparator 19 produces an output signal indicative of die default state.

For a simple connector the requirements are similar but reversed. Thus the reference 18 provides a high level signal and the comparator 19 indicates a default state when the signal from differential amplifier rises above the threshold. Both of these embodiments correspond to the Figure but there is a third embodiment which is not separately illustrated. In this embodiment the comparator 19 has two thresholds and it provides a default signal when the output from differential amplifier falls below the lower threshold or rises above the higher threshold. It is a common feature of all three cases that the comparator 19 produces a default signal when the reflections change to an abnormal state.

Conveniently the output of the comparator 19 is returned to the power unit 11 so that the power to the source 10 is terminated whereby the optical of the source is also terminated. Thus the output 13 receives no more optical power and any dangers associated therewith are eliminated.

As an alternative, the output from the comparator 19 may simply reduce the power provided to the source 10 so that the optical power is reduced to safe, but non zero, levels.

If the device is designed to terminate the power supply to source 10 then it is appropriate that the trigger is such that the power remains off until it is restored by an engineer. Otherwise, since no signals will be detected in either detector 15A or 15B once the power is off, there would still be a potential danger of optical power being inadvertently restored.

In the case where the power is reduced to safe levels detectors 15A and 15B will still receive signals. The absolute level of both signals will, of course, be reduced but the ratio will remain constant and hence the device will be held in the default state.

Claims

1. An optical responder comprising a monitor adapted to monitor the level of optical signals reflected from an optical transmission channel and to initiate corrective action on detecting a significant change in said level.

2. An optical responder according to claim 1, wherein said corrective action comprises initiating an alarm and/or terminating the optical signal or reducing the power thereof.

3. An optical responder according to either claim 1 or claim 2, wherein the monitor is also adapted to monitor the level of optical signals provided in the said optical channel and said significant change is the change in the ratio of said input signals and said reflected signals.

4. An optical device including a responder according to claim 3, wherein said device comprises:-

(a) an output port adapted for connection to an optical transmission channel, (b) optical source means for providing an optical signal to said output port, and

(c) monitor means also connected to said output port so as to receive optical signals received at said output port, wherein said monitor means is also connected to receive a portion of the optical output of said optical source means whereby the level of the reflected signals is monitored as a ratio of the reflected signals to the signals supplied to the output port.

5. An optical device according to claim 4, wherein the monitor comprises optical detectors for converting a sample of the reflected optical signals and a sample of the output of the optical source means into electrical signals, logarithmic amplifiers for amplifying said electrical signals and a differential amplifier to providing a difference between said amplified signals whereby said difference represents the ratio between the output of the optical source means and the reflective signals.

6. An optical device according to either claim 4 or claim 5 wherein the optical source means is selected from semiconductor lasers, semiconductor laser amplifiers, optical fibre laser, and optical fibre amplifiers.

7. An optical device according to any one of claims 4 - 6, wherein the significant change occurs when the ratio falls below a preset lower threshold.

8. An optical device according to any one of claims 4 - 6, wherein a significant change occurs when the ratio rises above a preset upper threshold.

9. An optical device according to any of claims 4 - 6, wherein the significant change occurs when either the ratio falls below a preset lower threshold or rises above a preset higher threshold.

10. A method of operating an optical source means so as to provide an optical signal to an output port which method comprises:-

(i) continuously measuring the level of a proportion of the signals provided to said output port, (ii) continuously measuring the level of a proportion of the optical signals returned to said output port,

(iii) continuously measuring the ratio of signals as measured in (i) and (ii) and producing a default signal from said ratio,

(iv) providing said default signal to said opto-electronic device so as to reduce the optical power output thereof when said default signal is produced from (iii); said default signal being produced either when the ratio produced in (iii)

(a) falls below a preset threshold, or

(b) rises above a preset threshold or

(c) falls below a lower preset threshold or rises above an upper preset threshold.