WO2001035135A1

WO2001035135A1 - Method and apparatus for stabilizing attenuators in optical networks

Info

Publication number: WO2001035135A1
Application number: PCT/US2000/027032
Authority: WO
Inventors: Janet Jackel
Original assignee: Telcordia Technologies, Inc.
Priority date: 1999-11-10
Filing date: 2000-09-29
Publication date: 2001-05-17
Also published as: US6207949B1; AU7743300A

Abstract

A methodology and concomitant circuitry wherein an optical attenuator (420), having a range of settings including a minimum attenuation, is set to a pre-selected value less than the minimum attenuation whenever a loss of incoming signal power is detected in an optical path (402) coupled to the attenuator (420).

Description

METHOD AND APPARATUS FOR STABILIZING ATTENUATORS LN OPTICAL NETWORKS

BACKGROUND OF THE DISCLOSURE

1. Field of the Invention

This invention relates generally to optical communications networks, and,

more particularly, to methodologies and concomitant circuitry for mitigating transient

effects in the networks caused by attenuator adjustments which compensate for signal

power changes.

2. Descnption of the Background

Recent research advances in optical Wavelength Division Multiplexing

(WDM) technology have fostered the exploratory development of optical networks that

are orders of magnitude higher in transmission bandwidth than existing commercial

networks. While such an increase in throughput is impressive on its own, a

corresponding decrease in network latency can also be achieved in the same networks

Thus, it is clear that the Next Generation Internet (NGI) vision of providing ultra high¬

speed networks that can meet the requirements for supporting new applications, including

national initiatives, is indeed feasible However, in both commercial networks and exploratory networks,

network reconfigurations, failures, protection switching, and even the fact that not all

signals ong ate at the same point in the optical network (that is, a network element may

drop an incoming signal for delivery to a destination device, or may add an incoming

signal from a source device onto the optical network) may cause abrupt changes of the power levels of the signals propagating in such optical networks; to fully realize the

benefits of the NGI applications, there are potentially deletenous effects to overcome

because of such power-changing mechanisms

First, since network elements contain optical amplifiers, it is known that if

the power in a given wavelength serving as the input to an amplifier is large relative to

the power in other incoming wavelengths, the dominant wavelength is emitted with more

power than the other wavelengths and the power in each of the other wavelengths is

reduced. This dominance by the given wavelength causes unequal signal-to-noise ratios

for the signals propagated by the wavelengths which, in turn, can cause system

degradation. To compensate for such incoming power vanations in a conventional

arrangement, a servo-controlled attenuator is inserted before each amplifier to serve as a

power equalizer. In particular, an optical attenuator is interposed in the path of the

incoming signal for each wavelength, and the attenuator's setting is a value that is based

upon the history of the optical power that has entered the attenuator In normal operation,

the attenuator settles to an equilibnum state wherein the setting is typically a mid-range

value (in the range between a maximum attenuation and a minimum attenuation) based

upon desired network operating charactenstics, such as the necessary signal-to-noise

ratio To achieve the equilibnum state, the power m the incoming signal is measured and

then compared to a "companson value", which is also selected in view of the network

operating charactenstics. Then, if the incoming power is too high relative to the

companson value, the attenuation can be increased to offset the high power signal,

conversely, the attenuation can be decreased to increase the signal serving as the network element's input. In the extreme case of no measurable input power, the attenuator is set

to a mode whereby no attenuation ("no attenuation" mode for later reference) is provided

by the attenuator.

Power fluctuations are typically measurable at the input or output of the

servo-controlled attenuators. Servo-controlled attenuators exhibit transient settling times

before compensating for the power fluctuations and reaching equilibrium; moreover,

depending upon their design, such settling times can be long relative to the time constants

of other components in the optical network. During the settling time, system performance

may be degraded, so an objective in the provision of a power-correcting attenuator

network is the minimization of such settling time.

Second, it is also known that the activity of compensating for power

fluctuations in a given wavelength by an upstream attenuator impacts on the operation of

downstream attenuators and can induce transient settling times in the downstream

attenuators. As before, during periods of adjustment, a given wavelength may

predominate at a downstream amplifier, and S/N can be degraded. Thus, power fluctuations in an upstream link can cause a "rippling effect" in downstream network

elements, and must be mitigated to maintain system performance.

Third, transient conditions caused by power variations of one wavelength

channel can even be coupled to other wavelength channels due to the cross-saturation

effects of an amplifier; this is especially true if the amplifier is an Erbium-doped fiber amplifier (EDFA) which is not gain-clamped - such EDFAs are typically used in present-

day optical networks. This mechanism can be responsible for sustained power

fluctuations in large scale optical networks composed of closed loops. Such a network

transient response depends upon the magnitude of the initial power perturbation, the

speed of the servo-controlled attenuators, the design of the EDFAs, the network topology, and the add/drop characteristics of the network elements, as well as the interactions of the

foregoing mechanisms and components.

It is now understood in the art that elimination of coupling between

wavelength channels can be achieved by using gain-clamped EDFAs or fast servo-

controlled attenuators, that is, attenuators that have response times which are an order of

magnitude faster (in the range of 10-100 microseconds) than the corresponding amplifiers

(about 1 millisecond).

An article fully discussing the effects of transients induced by the

operation of conventional servo-controlled attenuators is published in the Conference

Proceedings of the Optical Fiber Communication Conference (OFC) and International

Conference on Integrated Optics and Optical Fiber Communication (IOOC), TuRl-1, pgs.

246-248, 1999, and is entitled "Transient Effects in Wavelength Add-Drop Multiplexer

Chains".

However, because of the high bit-rate signals in an optical network, even

fast-operating attenuators operating in aforementioned speed range will not preclude

degraded S/N ratios during the adjustment time, either in a given attenuator or in the downstream attenuators impacted by the transient effects of the given upstream

attenuator. The prior art is devoid of teachings or suggestions relating to mitigation of transient oscillations caused by attenuators during periods in which an attenuator is

adjusting for shifts in incoming power.

SUMMARY OF THE INVENTION

Shortcomings and limitations of the prior art are obviated, in accordance

with the present invention, by a methodology and concomitant circuitry wherein an attenuator is set to a pre-selected value, rather than the "no attenuation" mode, whenever

a loss of incoming signal power is detected.

Broadly, in accordance with one method aspect of the present invention, a

method for controlling an optical attenuator disposed in optical path propagating an

optical signal, the attenuator having a range of settings including a minimum attenuation,

wherein the method includes: (a) measuring energy in the optical signal at the attenuator,

and (b) setting the attenuator to a pre-selected value whenever the energy is below a pre¬

determined threshold indicative no optical signal, the pre-selected value being less than

the minimum attenuation.

Broadly, in accordance with one system aspect of the present invention,

circuitry for controlling an optical attenuator disposed in optical path propagating an

optical signal, the attenuator having settings having a range of settings including a

minimum attenuation, wherein the circuitry includes: (a) a detector for detecting energy in the optical signal at the attenuator, and (b) a signal processor, responsive to the detector,

for setting the attenuator to a pre-selected value whenever the energy is below a predetermined threshold indicative no optical signal, the pre-selected value being less than

the minimum attenuation.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings of the present invention can be readily understood by

considering the following detailed description in conjunction with the accompanying

drawings, in which:

FIG. 1 depicts, in high-level block diagram form, a sub-network of a conventional

optical system, the sub-network including a cascade of a first attenuator, an associated amplifier, and a downstream attenuator;

FIGS. 2A-2D are time plots of certain signals in the sub-network of FIG. 1 based

upon the conventional operation of the attenuators;

FIGS. 3A-3D are time plots, corresponding to the time plots of FIGS. 2A-2D,

depicting the same signals of FIG. 2 but based upon the operation of the attenuators in accordance with the present invention;

FIG. 4 is illustrative of an attenuator arrangement wherein an tapped optical signal

is processed to produce the control signal for the attenuator; and

FIG. 5 is a flow diagram for the attenuator processor of FIG. 4.

To facilitate understanding, identical reference numerals have been used,

where possible, to designate identical elements that are common to the figures. DETAILED DESCRIPTION

To fully appreciate the import of the signal processing system of the present invention, as well as to gain an appreciation for the underlying operational

principles of the present invention, it is instructive to first present, in quantitative fashion,

two heuristic examples of the effects of attenuator settling times on power levels within a

segment of an optical network. This overview also serves to introduce terminology so as to facilitate the more detailed description of an illustrative embodiment in accordance

with the present invention. Following these motivating examples, a description of the

illustrative embodiment is then elucidated.

Heuristic Examples

Example 1 - Conventional Operation of Attenuators

With reference to FIG. 1, there is shown sub-network 100 of an optical

network, with sub-network 100 being composed, in cascade, of: (a) first attenuator 110

having as its input an optical signal at a given wavelength propagating on fiber 101; (b)

amplifier 120 being coupled to attenuator 110 and outputting an optical signal at the

given wavelength on fiber 102; and (c) second, downstream attenuator 130 having as its

input an optical signal at the given wavelength propagating on optical fiber 102. For

purposes of the immediate discussion, it is assumed that a measure of the optical power at

the input to attenuators 110 and 130, respectively, is available in terms of an electrical

voltage; in FIG. 1, this measure is shown as VI for attenuator 110 and as V2 for attenuator 130. (Such a measure may be effected, for example, by tapping the optical

signal from the input of each attenuator, and passing the tapped signal through an optical- to-electrical converter).

With reference to FIGS. 2A-2D, there is shown a sequence of four related

figures. To arrive at the following figures, certain assumptions with respect to response

times have been made for sake of clarity, but without loss of generality. In particular, rise

times and fall times of signals have been idealized. Also, because any practical attenuator

has a finite response time, the response cannot be instantaneous — the response time of

the attenuator is assumed to be a constant, designated Tr. Thus, once an event is detected

requiring a change in the setting of an attenuator, the time for the attenuator to respond is

Tr. Such a response time is due to, for instance, mechanical movement of certain types of servo-controlled attenuators or time required for the generation of a servo-control signal.

The plots of FIGS. 2A-2D are described as follows:

(i) in FIG. 2A, the voltage VI is plotted versus time for a signal propagation

scenario as follows: the signal at the given wavelength propagates at a normalized value

of 1.0 (VI = 1.0) up to time Toff; at Toff, the propagating signal disappears (VI = 0.0),

say due to a reconfiguration activity in the network, and the signal remains off until the

time Ton; at Ton, signal power restored at a normalized value VI = 1.0.

(ii) in FIG. 2B, the settings for attenuator 110 are plotted versus time for the time

events of FIG. 2A, as follows: at time t = 0, the setting on attenuator 110 has achieved an equilibrium state, with a concomitant attenuator setting which a normalized basis is

presumed to be 0.5. Attenuator 110 fully attenuates when the setting is 0.0, and provides

no attenuation for a setting of 1.0. At time Toff, VI drops to 0.0, and attenuator 110 must respond to this change, with the response time Tr in FIG. 2B. Because there is no

measurable power in the incoming optical signal (i.e., VI = 0.0), the attenuator is

adjusted for minimum attenuation, or a setting of 1.0. At time Ton, power restored in the

incoming optical signal at the normalized level, so attenuator 110 readjusts to the

equilibrium state having a setting of 0.5. However, a response time of Tr is again

required before attenuator 110 fully compensates for the increased power. During the

time period (Ton, Ton + Tr), the power to amplifier 120 is larger than desired, and the

effects discussed the Background can occur, including degraded S/N ratio and reduced

power in other incoming wavelengths.

(iii) in FIG. 2C, V2 is plotted versus time for the time periods of FIGS. 2A and

2B. To arrive at the plot of this figure, it is assumed that amplifier 120 provides the

incoming signal with a 3 dB gain. Thus, for the time interval up to Toff, V2 has a

normalized value of 1.0, as expected for the system operating properly in steady-state.

From Toff to Ton, V2 = 0.0, since power in the signal is lost upstream, and even the

removal of attenuation by attenuator 110 and amplification by amplifier 120 cannot

overcome no incoming power. During the period (Ton, Ton + Tr), the signal emitted by

attenuator 110 is 1.0, and after amplification by amplifier 120, the normalized signal is

such that V2 = 2.0. Once attenuator 110 responds, then the normalized signal V2 reduces to 1.0 at Ton + Tr. (iv) in FIG. 2D, the settings for attenuator 130 are plotted versus time for the time

events of FIG. 2A-2C, as follows: up to Toff, the setting is the equilibrium state having a

concomitant attenuator setting of 0.5. Once V2 drops to zero, attenuator 130 responds

with a setting of 1.0, but with a response time of Tr. During the period (Ton, Ton + Tr),

the signal emitted by attenuator 110 is 1.0, and after amplification by amplifier 120, the

signal V2 = 2.0. Because this signal emitted by attenuator 130 should be 0.5, attenuator

130 adjusts to a setting of 0.25. However, the response time is Tr, so the effect of the

adjustment does not occur until Ton + Tr. By this time, attenuator 110 has reduced the

incoming power to the equilibrium state, so attenuator 130 is overcompensating at time

Ton + Tr. Attenuator 130 effects a final adjustment by setting a value of 0.5, but the full effect of the adjustment does not take place until Ton + 2Tr. During the time interval

(Ton, Ton + 2Tr), the deleterious transient effects of unequal power on incoming signals

exist.

Example 2 - Operation of Attenuators in Accordance With Present Invention

Again by way of motivation, the example discussed with respect to FIGS.

2A-2D is now recast in FIGS. 3A-3D to elucidate the principles of the present invention.

The same assumptions regarding idealizations of time waveforms used to present the

plots of FIGS. 2A-2D are used for FIGS. 3A-3D. The essential difference between the

plots is as follows: whenever no signal is present as measured by VI or V2, rather that set

attenuators 110 or 130 to the "no attenuation" setting (i.e., 1.0), each attenuator is set to a pre-selected value. The pre-selected value is based upon system considerations and engineering judgments.

The plots of FIG. 3A-3D are described as follows:

(i) FIG. 3A is FIG. 2A repeated for reference purposes.

(ii) in FIG. 3B, the settings for attenuator 110 are plotted versus time for the time

events of FIG. 3A, as follows: at time t = 0, attenuator 110 is set to the equilibrium state

having an associated attenuator setting which on a normalized basis is presumed to be

0.5. At time Toff, VI drops to 0.0, and the attenuator must respond to this change, with

the response time Tr in FIG. 3B. Because there is no measurable power in the incoming optical signal (i.e., VI = 0.0), attenuator 110 is adjusted for the "no incoming signal"

condition - in this case, however, a pre-selected value of 0.75 is chosen for the attenuator

setting. At time Ton, power is restored in the incoming optical signal at the normalized

level, so attenuator 110 is readjusts to the equilibrium state wherein the attenuator is set to a value of 0.5. However, a response time of Tr is again required before attenuator 110

fully compensates for the increased power. During the time period (Ton, Ton + Tr), the

power to amplifier 120 is larger than desired, but the normalized signal from attenuator

110 is 0.75, not the 1.0 value of FIG. 2B, so the dominance of the given wavelength is

less-pronounced.

(iii) in FIG. 3C, V2 is plotted versus time for the time periods of FIGS. 3A and

3B. To obtain this plot, it is assumed that amplifier 120 amplifies the provides a 3 dB

gain to the incoming signal. Thus, for the time interval up to Toff, V2 has a normalized value of 1.0, as expected steady-state system operation. From Toff to Ton, V2 = 0.0,

since power in the signal is lost upstream, and even a setting of 0.75 for attenuator 110

and amplification by amplifier 120 cannot overcome no incoming power. During the

period (Ton, Ton + Tr), the signal emitted by attenuator 110 is 0.75, and after

amplification by amplifier 120, the normalized signal is such that V2 = 1.5. Once attenuator 110 completes its compensation, then V2 = 1.0 for time > (Ton + Tr).

(iv) in FIG. 3D, the settings for attenuator 130 are plotted versus time for the

events of FIG. 3A-3C, as follows: at Toff, attenuator 130 adjusts to the "no signal"

condition with the pre-selected setting of 0.75. At Ton, attenuator 130 adjusts to a setting

of 1/3 to reduce the value of V2 to 0.5, but the response time is Tr. At Ton + Tr, V2 =

1.0, so attenuator 130 adjusts to a final value of 0.5, with compensation being completed

by Ton + 2Tr. Again, during the time interval (Ton, Ton + 2Tr), the deleterious transient

effects of unequal power exist. But, the variations are not as pronounced as in the

example conveyed by FIGS. 2A-2D, so system degradation is reduced.

In the motivating example of FIGS. 3A-3D, it can be readily appreciated

that as the pre-selected value converges to the value representative of the equilibrium

state, power variations due to changing settings are mitigated, Thus, as the pre-selected

value approaches, in the limit, the value representative of the equilibrium state from

above or below, there are no deleterious effects because no changes are required in the

settings on the attenuators, and hence no required settling times. Illustrative Embodiment

As readily discerned from the foregoing examples, the subject matter in accordance with the present invention covers a modification to the arrangement and

operation of a conventional optical attenuator such as attenuator 110 or 130 of FIG. 1. In

the following descπption, those aspects of the conventional operation that are pertinent to

the inventive subject matter are reviewed so as to convey a complete understanding of the

principles of the present invention. To this end, a high-level block diagram which

encompasses both a conventional attenuator-adjusting arrangement as well as the

inventive adjusting-arrangement is shown in FIG. 4; the pπmary difference between the conventional arrangement and the arrangement in accordance with the present invention

is the signal processing earned out by attenuator processor 440 of FIG. 4, as discussed in

detail below.

With reference to FIG. 4, there is shown sub-network 400 of an optical

network composed of: (a) attenuator 420; (b) fiber tap 410 for tapping off a portion of the energy from optical fiber 401, with the main portion of the energy being delivered to

attenuator 420 via path 402, and the tapped portion of the energy being emitted on path

411; (c) optical/electrical converter 430 having path 411 as an input; (d) attenuator

processor 440 being coupled to converter 430 via path 431; and (e) setting controller 450

having the output of processor 440 as its input, via path 441, and providing its output to

attenuator 420 via path 451. Converter 430 has the two-fold function of: (I) converting

optical energy at a given wavelength to electπcal energy; and (n) converting the electπcal energy to a quantity indicative of the energy in the given wavelength, that is, providing a

measurable quantity to processor 440. For example, such a measurable quantity may be a voltage, which is referred to as Vt and appears on path 431.

Conventionally, processor 440 utilizes a comparison voltage, designated

Vc, for comparison to Vt (Vc is, for example, the value of voltage associated with the

equilibrium state for attenuator 420). If Vt < Vc, then the power level in the optical signal on fiber 401 must be increased at the output of attenuator 420, so the setting of

attenuator 420 is adjusted to decrease the attenuation. In operation, the result of the

comparison is delivered to controller 450, and an attenuator control signal commensurate

with the comparison is delivered by controller 450 to attenuator 420. In a similar manner,

if Vt > Vc, then the power level in the signal on fiber 401 is decreased via the operation of processor 440 in conjunction with controller 450. In the extreme case of loss of signal

power, then the former case applies, that is, Vt < Vc, and controller 450 removes all

attenuation from attenuator 420. The plots of FIGS. 2A-2D exemplified the operation of sub-network 400 in this extreme condition and, moreover, covered the various other

operating alternatives of: steady-state operation (up to Toff); loss of power (Toff to Ton);

and recovery after loss of power to re-capture steady-state (time greater than Ton). By

way of terminology, the foregoing operation of circuitry 400 of FIG. 4 is called the

"conventional operation" of attenuator 420. As already alluded to, the operation of circuitry 400 in accordance with the

present invention is engendered by the signal processing of processor 440, which is now discussed with respect to flow diagram 500 of FIG. 5. In FIG. 5, processing by block 510

provides the measurable quantity Vt. Next, decision block 520 is invoked to determine if

Vt is less than a threshold voltage, designated Vth. Vth is selected according to network

operating characteristics; for example, Vth may be set to a value that is 10 db below the

lowest expected power in an incoming optical signal. If Vt > Vth, then the "conventional

operation" of attenuator 420 is effected, as evidenced by processing block 540. On the

other hand, if Vt < Vth, then processing by block 530 is invoked to set attenuator 530 to a

pre-scribed value. The plots of FIGS. 3A-3D exemplified the operation of sub-network

400 in this extreme condition with a pre-scribed value for the attenuator, on a normalized basis, of 0.75 (rather than 1.0) and, moreover, covered the various other operating

alternatives of: steady-state operation (up to Toff); loss of power (Toff to Ton); and

recovery after loss of power to re -capture steady-state (time greater than Ton).

It is readily contemplated that other equivalent arrangements are possible

to carry out the functionality of the inventive subject matter. For example, it is possible

to use the output from attenuator 420 as a measure of the optical signal power, so that in

FIG. 4, fiber tap 410 may be placed after attenuator 420 — this arrangement is

characterized as "feedback" measurement, as contrasted to the former arrangement of

"feedforward" measurement. Also, the signal processing by processor 440 may be effected in hardware

to realize process flow diagram 500 of FIG. 5.

Finally, even though flow diagram 500 depicts processing as occurring in

discrete steps, it is clear that such processing is being carried out continuously. Consider,

for example, the "lost power" case: since Vt is being measured continuously, as soon as

power is restored, the test by block 520 immediately invokes processing by block 540.

Although various embodiments which incorporate the teachings of the

present invention have been shown and described in detail herein, those skilled in the art

can readily devise many other varied embodiments that still incorporate these teachings.

Claims

1. A method for controlling an optical attenuator disposed in optical path propagating an optical signal, the attenuator having a range of settings including a

minimum attenuation, the method comprising the steps of

measuring energy in the optical signal at an input to the attenuator, and

setting the attenuator to a pre-selected value whenever the energy is below

a pre-determined threshold indicative no optical signal, the pre-selected value being less than the minimum attenuation.

2. The method as recited in claim 1 wherein the step of setting includes the step

of setting the attenuator with reference to prescribed parameters whenever the energy

exceeds the pre-determined threshold.

3. The method as recited in claim 1 wherein the step of setting includes the step

changes to a level exceeding the pre-determined threshold.

4. A method for controlling an optical attenuator disposed in optical path

propagating an optical signal, the attenuator having a range of settings including a

minimum attenuation, the method comprising the steps of

measuring energy in the optical signal at an input to the attenuator,

if the energy is below a pre-determined threshold, setting the attenuator to

a pre-selected value indicative no optical signal, the pre-selected value being less than the

minimum attenuation, and if the energy exceeds the pre-determined threshold, operating the

attenuator with reference to prescribed parameters.

5. The method as recited in claim 4 wherein the step of operating includes the

step of operating the attenuator with reference to the prescribed parameters if the energy is initially below the pre-determined threshold and changes to a level exceeding the pre¬

determined threshold.

6. A method for controlling an optical attenuator disposed in optical path

propagating an optical signal, the attenuator having a range of settings including a minimum attenuation, the method comprising the steps of

measuring energy in the optical signal at an output of the attenuator, and

setting the attenuator to a pre-selected value whenever the energy is below

a pre-determined threshold indicative no optical signal, the pre-selected value being less

than the minimum attenuation.

7. The method as recited in claim 6 wherein the step of setting includes the step

exceeds the pre-determined threshold.

8. The method as recited in claim 6 wherein the step of setting includes the step

changes to a level exceeding the pre-determined threshold.

9. A method for controlling an optical attenuator disposed in optical path

minimum attenuation, the method comprising the steps of measuring energy in the optical signal at an output of the attenuator,

if the energy is below a pre-determined threshold, setting the attenuator to

a pre-selected value indicative no optical signal, the pre-selected value being less than the minimum attenuation, and

if the energy exceeds the pre-determined threshold, operating the

attenuator with reference to prescribed parameters.

10. The method as recited in claim 9 wherein the step of operating includes the

step of operating the attenuator with reference to the prescribed parameters if the energy

is initially below the pre-determined threshold and changes to a level exceeding the pre-

determined threshold.

11. A method for controlling an optical attenuator disposed in optical path

measuring energy in the optical signal at the attenuator, and

setting the attenuator to a pre-selected value whenever the energy is below

than the minimum attenuation.

12. Circuitry for controlling an optical attenuator disposed in optical path

minimum attenuation, the circuitry comprising

means for measuring energy in the optical signal at the input to the

attenuator, and means, responsive to the means for measuring, for setting the attenuator to a pre¬

selected value whenever the energy is below a pre-determined threshold indicative no optical signal, the pre-selected value being less than the minimum attenuation.

13. The circuitry as recited in claim 12 wherein the means for setting further

includes means for setting the attenuator with reference to prescribed parameters

whenever the energy exceeds the pre-determined threshold.

14. The circuitry as recited in claim 12 wherein the means for setting includes

means for setting the attenuator with reference to prescribed parameters whenever the

energy changes to a level exceeding the pre-determined threshold.

15. Circuitry for controlling an optical attenuator disposed in optical path

minimum attenuation, the circuitry comprising means for measuring energy in the optical signal at an input to the

attenuator, and

means, responsive to the means for measuring, for setting the attenuator to

a pre-selected value if the energy is below a pre-determined threshold indicative no

optical signal, the pre-selected value being less than the minimum attenuation, and for

setting the attenuator with reference to prescribed parameters if the energy exceeds the

pre-determined threshold.

16. Circuitry as recited in claim 15 wherein the means for setting includes the

step of setting the attenuator with reference to the prescribed parameters if the energy is initially below the pre-determined threshold and changes to a level exceeding the predetermined threshold.

17. Circuitry for controlling an optical attenuator disposed in optical path

minimum attenuation, the circuitry comprising

means for measuring energy in the optical signal at the output of the

attenuator, and

means, responsive to the means for measuring, for setting the attenuator to

a pre-selected value whenever the energy is below a pre-determined threshold indicative

no optical signal, the pre-selected value being less than the minimum attenuation.

18. The circuitry as recited in claim 17 wherein means for setting includes means

for setting the attenuator with reference to prescribed parameters whenever the energy

exceeds the pre-determined threshold.

19. The circuitry as recited in claim 17 wherein the means for setting includes

energy changes to a level exceeding the pre-determined threshold.

20. Circuitry for controlling an optical attenuator disposed in optical path

minimum attenuation, the circuitry comprising

means for measuring energy in the optical signal at an output of the

attenuator, and means, responsive to the means for measuring, for setting the attenuator to

a pre-selected value if the energy is below a pre-determined threshold indicative no optical signal, the pre-selected value being less than the minimum attenuation, and for

pre-determined threshold.

21. The circuitry as recited in claim 20 wherein means for setting includes means

for setting the attenuator with reference to the prescribed parameters if the energy is

initially below the pre-determined threshold and changes to a level exceeding the pre¬

determined threshold.

22. Circuitry for controlling an optical attenuator disposed in optical path

minimum attenuation, the circuitry comprising

a detector for detecting energy in the optical signal at an input to the

attenuator, and

a signal processor, responsive to the detector, for setting the attenuator to a pre¬

23. Circuitry for controlling an optical attenuator disposed in optical path

minimum attenuation, the circuitry comprising

a converter for detecting energy in the optical signal at an output of the

attenuator, and a signal processor, responsive to the converter, for setting the attenuator to a pre¬

selected value whenever the energy is below a pre-determined threshold indicative no

optical signal, the pre-selected value being less than the minimum attenuation.

24. Circuitry for controlling an optical attenuator disposed in optical path

minimum attenuation, the circuitry comprising

a detector for detecting energy in the optical signal at the attenuator, and a signal processor, responsive to the detector, for setting the attenuator to a

pre-selected value whenever the energy is below a pre-determined threshold indicative no

optical signal, the pre-selected value being less than the minimum attenuation.