PRIORITY

The present application claims priority from U.S. Provisional Patent Application No. 60/267,879, which was filed on Feb. 9, 2001, and is hereby incorporated herein by reference in its entirety.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application may be related to the following commonly owned U.S. patent applications, which are hereby incorporated herein by reference in their entireties:

U.S. patent application Ser. No. 09/846,886 entitled OPTICAL LIMITER BASED ON NONLINEAR REFRACTION, filed on May 1, 2001 in the names of Edward H. Sargent and Lukasz Brzozowski; and

U.S. patent application Ser. No. 09/933,315 entitled OPTICAL LOGIC DEVICES BASED ON STABLE, NON-ABSORBING OPTICAL HARD LIMITERS, filed on even date herewith in the names of Erik V. Johnson and Edward H. Sargent.

FIELD OF THE INVENTION

The present invention relates generally to optical information processing, and more particularly to optical automatic gain control using stable, non-absorbing optical hard limiters.

BACKGROUND OF THE INVENTION

In today's information age, optical communication technologies are being used more and more frequently for transmitting information at very high speeds. Traditionally, information processing equipment (such as switches, routers, and computers) process information electronically. Therefore, optical communications are often converted into electronic form for processing by the information processing equipment. This electronic processing is slow relative to the speed of the optical communications themselves, and thus often becomes a “bottleneck” of optical communication and processing systems.

A communication channel can be used more efficiently to transmit information if an encoding scheme is used to assign binary values to discrete intensity levels. This is difficult in an optical communication system due to the difficulty in controlling the intensity of the signal due to attenuation in the optical fiber. Therefore, it is difficult to establish a reference intensity level for optical communications over the optical fiber.

Automatic gain control can be used to normalize packets of varying intensities. Automatic gain control for optical communications is often accomplished by detecting the optical signal, transforming the optical signal in an electronic signal, processing the signal electronically, converting the processed electronic signal back into an optical form, and retransmitting the converted optical signal. Unfortunately, this process is limited by the speed of the electronics.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, optical automatic gain control (AGC) is accomplished using stable, non-absorbing optical hard limiters and various optical logic gates derived therefrom. The AGC mechanism preserves the ratios between signal levels and provides an adjustable amount of gain.

An optical automatic gain controller typically includes a number of AGC stages, where, in each AGC stage, a threshold input signal derived from an optical input signal is compared against a predetermined threshold for the AGC stage, and a gain input signal also derived from the optical input signal is amplified if and only if the threshold input signal is below the predetermined threshold. The threshold is reduced in each successive AGC stage.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1

is a block diagram showing an exemplary optical automatic gain controller in accordance with an embodiment of the present invention;

FIG. 2

is a block diagram showing the relevant logic blocks of an exemplary optical automatic gain controller in accordance with an embodiment of the present invention;

FIG. 3

is a schematic block diagram showing the relevant components of exemplary initialization stage logic in accordance with an embodiment of the present invention;

FIG. 4

is a schematic block diagram showing the relevant components of exemplary AGC stage logic in accordance with an embodiment of the present invention;

FIG. 5

is a schematic block diagram showing the relevant components of exemplary threshold logic in accordance with an embodiment of the present invention;

FIG. 6

is a schematic block diagram showing the relevant components of exemplary gain logic in accordance with an embodiment of the present invention;

FIG. 7

is a schematic block diagram showing the relevant components of exemplary gain select logic in accordance with an embodiment of the present invention;

FIG. 8

is a block diagram showing an optical automatic gain control system including an optical automatic gain controller coupled in series to a linear amplifier in accordance with an embodiment of the present invention; and

FIG. 9

is a schematic block diagram showing the relevant components of an exemplary single-stage optical automatic gain controller in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

In an embodiment of the present invention, optical automatic gain control (AGC) is accomplished using stable, non-absorbing optical hard limiters and various optical logic gates derived therefrom, as described in the related application entitled OPTICAL LOGIC DEVICES BASED ON STABLE, NON-ABSORBING OPTICAL HARD LIMITERS incorporated by reference above. The described AGC mechanism preserves the ratios between signal levels and provides an adjustable amount of gain.

In a typical embodiment of the present invention, AGC is accomplished by processing an optical input signal in one or more stages. In each stage, the output signal from the previous stage is amplified by a predetermined amount if and only if the intensity of the input signal is below a predetermined threshold. The threshold decreases in each successive stage, so that lower intensity input signals are amplified more than higher intensity input signals. This tends to reduce the dynamic range of the input signal. The optical output signal from the last stage may be amplified through a linear amplifier in order to compensate for signal losses in the various stages, which is primarily from signal splitting.

FIG. 1

shows an exemplary optical automatic gain controller (AGC)

100

. The optical AGC

100

receives as inputs an optical input signal

110

and a bias signal

120

and generates optical output signal

130

. The optical input signal

110

has a first intensity range and the optical output signal

130

has a second intensity range less than the first intensity range. The bias signal

120

is used to set the thresholds for the various AGC stages.

FIG. 2

is a block diagram showing the relevant logic blocks of the optical AGC

100

. Among other things, the optical AGC

100

includes an initialization stage

210

and a number of AGC stages

220

1

-

220

N

. The initialization stage

210

processes the optical input signal

110

in order to provide the necessary inputs to the first AGC stage

220

1

, as described below. The outputs from each AGC stage are fed as inputs to the next AGC stage. Each AGC stage amplifies a received signal if and only if the optical input signal

110

is below a predetermined threshold for that AGC stage, which is set using the bias signal

120

. The threshold decreases in each successive AGC stage, so that lower intensity input signals are amplified more than higher intensity input signals.

In one exemplary embodiment of the present invention, the threshold for the first AGC stage is set to roughly one half of a predetermined maximum signal intensity, and the thresholds are reduced by roughly one half in each successive AGC stage. In each AGC stage, the incoming signal is amplified by roughly 3 dB (i.e., doubled) if the incoming signal is below the threshold for the AGC stage. Thus, in an optical AGC having N AGC stages, an optical input signal below the first stage threshold is amplified once by 3 dB (i.e., doubled), an optical input signal below the second stage threshold is amplified twice by 3 dB (i.e., quadrupled), and so on, such that an optical input signal below the Nth stage threshold is amplified N times by 3 dB.

In this exemplary embodiment, each AGC stage

220

receives as inputs a threshold input signal and a gain input signal and outputs a threshold output signal and a gain output signal. The threshold input signal is used to determine whether the optical input signal

110

is above or below the threshold for the AGC stage. The threshold output signal is typically one half of the threshold input signal intensity. The gain output signal is equal to the gain input signal, if the optical input signal

110

is above the threshold for the AGC stage, or to the gain input signal amplified by 3 dB, if the optical input signal

110

is below the threshold for the AGC stage.

The initialization stage

210

separates the optical input signal

110

into a threshold input signal and a gain input signal for the first AGC stage

220

1

. The threshold input signal is typically equal in intensity to the optical input signal

110

, and the gain input signal is typically one fourth the intensity of the optical input signal

110

.

FIG. 3

is a schematic block diagram showing the relevant components of an exemplary initialization stage

210

. Among other things, the initialization stage

210

includes optical splitters

310

and

350

and 3 dB amplifier

340

.

The optical input signal

110

is fed into the optical splitter

310

. The optical splitter

310

splits the optical input signal

110

into two signals

320

and

330

, each having half the intensity of the optical input signal

110

.

The signal

320

is fed into the 3 dB amplifier

330

. The 3 dB amplifier

330

amplifies the signal

320

to produce output signal

360

with an intensity substantially equal to the intensity of the optical input signal

110

.

The signal

330

is fed into the optical splitter

350

. The optical splitter

350

splits the signal

330

to produce output signal

370

with an intensity substantially equal to one fourth the intensity of the optical input signal

110

.

The output signals

360

and

370

are fed to the first AGC stage

220

1

as the threshold input signal and gain input signal, respectively.

FIG. 4

is a schematic block diagram showing the relevant components of an exemplary AGC stage

220

. Among other things, the AGC stage

220

includes optical splitter

406

, gain logic

412

, threshold logic

418

, and gain select logic

423

.

The threshold input signal

402

is fed into the optical splitter

406

. The optical splitter

406

splits the threshold input signal

402

into two signals

408

and

410

, each having half the intensity of the threshold input signal

402

.

The signal

408

is output as the threshold output signal.

The signal

410

is fed as an input into the threshold logic

418

, as is the bias signal

120

. The threshold logic

418

outputs an above-threshold signal

420

and a below-threshold signal

422

. If the signal

410

is above the threshold for the AGC stage as set by the bias signal

120

, then the above-threshold signal

420

is typically output at a “high” signal level and the below-threshold signal

422

is typically output at a “low” signal level. If the signal

410

is below the threshold for the AGC stage as set by the bias signal

120

, then the below-threshold signal

422

is typically output at a “high” signal level and the above-threshold signal

420

is typically output at a “low” signal level.

The gain input signal

404

is fed as an input into the gain logic

412

. The gain logic

412

outputs two signals

414

and

416

. The signal

414

is substantially equal in intensity to the gain input signal

404

. The signal

416

is substantially equal in intensity to the gain input signal

404

amplified by 3 dB.

The signals

414

,

416

,

420

, and

422

are fed as inputs into the gain select logic

423

. The gain select logic

423

outputs gain output signal

428

. If the above-threshold signal

420

is input at a “high” signal level and the below-threshold signal

422

is input at a “low” signal level, the gain select logic

423

outputs the signal

414

(equal to the gain input signal

404

) as the gain output signal

428

. If the below-threshold signal

422

is input at a “high” signal level and the above-threshold signal

420

is input at a “low” signal level, the gain select logic

423

outputs the signal

416

(equal to the gain input signal

404

amplified by 3 dB) as the gain output signal

428

.

FIG. 5

is a schematic block diagram showing the relevant components of exemplary threshold logic

418

. Among other things, the threshold logic

418

includes a threshold limiter

510

, an optical splitter

520

, two 3 dB amplifiers

530

and

540

, and an optical NOT gate (inverter)

550

.

The threshold limiter

510

is typically a number of optical hard limiters connected in series. As described in the related application entitled OPTICAL LOGIC DEVICES BASED ON STABLE, NON-ABSORBING OPTICAL HARD LIMITERS incorporated by reference above, the optical hard limiter has three regimes of operation, specifically a low regime in which the transmitted signal is low (zero), a middle regime in which the transmitted signal increases as the input signal increases, and a high regime in which the transmitted signal is high (one). Connecting multiple optical hard limiters in series tends to compress the middle regime such that the multiple optical hard limiters behave as if there is only a low regime below which the output is low (zero) and a high regime above which the output is high (one). This transition point is essentially the threshold of the threshold limiter

510

. The bias signal

120

is fed as an input into the threshold limiter

510

, and more specifically to the various optical hard limiters in the threshold limiter

510

, and essentially sets the threshold point for the threshold limiter

510

.

The signal

410

is fed as an input into the threshold limiter

510

. The threshold limiter

510

outputs a low (zero) if the signal

410

is below a predetermined threshold and outputs a high (one) if the signal

410

is above the predetermined threshold.

The output signal

501

from the threshold limiter

510

is fed as an input into the optical splitter

520

. The optical splitter

520

splits the signal

510

into two signals

502

and

502

, each having half the intensity of the signal

501

.

The signal

502

is amplified by the 3 dB amplifier

530

to produce the above-threshold signal

420

.

The signal

503

is amplified by the 3 dB amplifier

540

to produce signal

504

, which is fed into optical NOT gate

550

to produce the below-threshold signal

422

.

If the signal

410

is above the threshold of the threshold limiter

510

, then the above-threshold signal

420

is output at a “high” signal level and the below-threshold signal

422

is output at a “low signal level”. If, however, the signal

410

is below the threshold of the threshold limiter

510

, then the below-threshold signal

422

is output at a “high” signal level and the above-threshold signal

420

is output at a “low” signal level.

FIG. 6

is a schematic block diagram showing the relevant components of exemplary gain logic

412

. Among other things, the gain logic

412

includes an optical splitter

610

and a 3 dB amplifier

620

.

The signal

404

is fed as an input into the optical splitter

610

. The optical splitter

610

splits the signal

404

into two signals

601

and

414

. The signal

601

is fed as an input into the 3 dB amplifier

620

to produce signal

416

.

FIG. 7

is a schematic block diagram showing the relevant components of exemplary gain select logic

423

. Among other things, the gain select logic

423

includes three optical combiners

710

,

720

, and

750

as well as two optical hard limiters

730

and

740

. Each optical combiner combines two optical inputs in equal proportions.

The above-threshold signal

420

and the non-amplified signal

414

are fed as inputs into the optical combiner

710

to produce signal

701

. Signal

701

is fed as an input into the optical hard limiter

730

. The transmitted signal

703

from the optical hard limiter

730

is fed as one input into the optical combiner

750

.

The below-threshold signal

422

and the amplified signal

416

are fed as inputs into the optical combiner

720

to produce signal

702

. Signal

702

is fed as an input into the optical hard limiter

740

. The transmitted signal

704

from the optical hard limiter

730

is fed as the other input into the optical combiner

750

.

If the signal is above the threshold for the AGC stage, then the above-threshold signal

420

will be high and the below-threshold signal

422

will be low. In this case, the signal

701

will be within the middle regime of the optical hard limiter

730

(i.e., above I1) such that the signal

703

is an analog of the non-amplified signal

414

. The signal

702

, however, will be in the low regime of the optical hard limiter

740

(i.e., below I1) such that the signal

704

is low. Therefore, the non-amplified signal

414

is passed by the combiner

750

as the gain output signal

428

.

If the signal is below the threshold for the AGC stage, then the below-threshold signal

422

will be high and the above-threshold signal

420

will be low. In this case, the signal

702

will be within the middle regime of the optical hard limiter

740

(i.e., above I1) such that the signal

704

is an analog of the amplified signal

416

. The signal

701

, however, will be in the low regime of the optical hard limiter

730

(i.e., below I1) such that the signal

703

is low. Therefore, the amplified signal

416

is passed by the combiner

750

as the gain output signal

428

.

In a multiple stage AGC

100

(i.e., N>1) as shown in

FIG. 2

, the threshold output signal

408

and gain output signal

428

from one AGC stage

220

n

are coupled respectively as the threshold input signal

402

and gain input signal

404

of the subsequent stage

220

n+1

. The gain output signal

428

of the last AGC stage

220

N

represents the optical output signal

130

of the AGC

100

.

In the above exemplary embodiment, the optical input signal

110

is split a number of times such that the intensity of the optical output signal

130

is typically well below the intensity of the optical input signal

110

, even if the signal is amplified in various AGC stages. Therefore, it is common to amplify the optical output signal

130

using a linear amplifier in order to compensate for the overall reduction in signal intensity caused by the AGC

100

.

FIG. 8

is a block diagram showing an exemplary AGC system in which the optical output signal

130

is amplified by a linear amplifier

800

to produce an amplified signal

830

.

FIG. 9

is a schematic block diagram showing an exemplary single-stage AGC

900

for coarse AGC control. Among other things, the AGC

900

includes various optical logic devices including optical splitters, optical combiners, optical hard limiters, and various components created from optical hard limiters, including a threshold limiter, various gain (amplifier) elements, and an optical NOT gate (inverter). For convenience, optical splitters and combiners are not shown explicitly, but instead are shown implicitly where two optical signal paths either join or diverge.

The optical input signal X

904

is split with a 90:10 bias, with roughly 90 percent of the signal fed to the gain logic and 10 percent of the signal fed to the threshold logic. This 90:10 bias preserves most of the signal through the gain logic.

In the threshold logic, the 10 percent signal is combined 50:50 at point

906

with a bias signal

902

having an intensity of approximately 3.8 times I1. The resulting signal is fed into the threshold limiter

910

. The output of the threshold limiter

910

is split 50:50 at point

912

. One signal is fed into an amplifier

916

to produce the above-threshold signal. The other signal is fed into an amplifier

918

and then into an inverter

920

to produce the below-threshold signal.

In the gain logic, the 90 percent signal is split 50:50 at point

914

. One of the signals is amplified by amplifier

922

, while the other is left non-amplified.

The non-amplified signal from the gain logic is combined 50:50 at point

924

with the above-threshold signal. The combined signal is fed into optical hard limiter

928

.

The amplified signal from the gain logic is combined 50:50 at point

926

with the below-threshold signal. The combined signal is fed into optical hard limiter

930

.

The outputs from the optical hard limiters

928

and

930

are combined 50:50 at point

932

. The combined signal is amplified by amplifier

934

to produce optical output signal

936

.

It should be noted that the present invention is in no way limited to the specific embodiments described above. The present invention is in no way limited to the logical separation of the AGC

100

into an initialization stage and a number of AGC stages, to the logical separation of each AGC stage into threshold logic, gain logic, and gain select logic, or to any particular configuration of components whether in a stage, logic block, or otherwise. It will be apparent to a skilled artisan that various optical hard limiters and optical components built therefrom can be configured in different ways to construct alternative optical automatic gain controllers.

The threshold limiters are typically constructed of multiple optical hard limiters coupled in series. The number of optical hard limiters essentially determines the “slope” of the middle regime, with the slope increasing as the number of optical hard limiters increases. A typical threshold limiter includes at least four optical hard limiters. With a slope approaching the vertical, the middle regime of the threshold limiter approaches zero such that the threshold limiter outputs a low signal for input signals below approximately I1 and outputs a high signal for input signals above approximately I1. Thus, the threshold point of the threshold limiter is essentially fixed at I1. However, the threshold limiter is used along with the bias signal

120

to effectively set the threshold for the threshold limiter. The bias signal is typically selected so that, when combined with the input signal, the threshold point for the input signal is roughly equal to I1. The bias signal may be different for different AGC stages.

Additional considerations are discussed in E. V. Johnson, ALL-OPTICAL SIGNAL PROCESSING AND PACKET FORWARDING USING NONMONOTONIC INTENSITY TRANSFER CHARACTERISTICS, a thesis submitted in conformity with the requirements for the degree of Master of Applied Science, Graduate Department of Electrical and Computer Engineering, University of Toronto (2001), which is hereby incorporated herein by reference in its entirety.

The present invention may be embodied in other specific forms without departing from the true scope of the invention. The described embodiments are to be considered in all respects only as illustrative and not restrictive.