Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-01-21
2001-10-23
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C455S073000, C455S130000
Reexamination Certificate
active
06307660
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to semiconductor amplifier circuits. In particular, the invention relates to a receiver circuit coupled to the output of a semiconductor photodiode.
BACKGROUND ART
Modem telecommunications networks are increasingly relying upon optical fibers and wavelength-division multiplexing (WDM). Silica optical fibers can transmit optical signals in optical bands near 1300 nm and 1550 nm over long distances with low loss and low chromatic dispersion. The digital transmission rate is primarily limited by the electronics at the transmitter and receiver ends, currently about 10 gigabits per second (Gb/s). The net data rate can be multiplied by wavelength division multiplexing in which the fiber carries multiple optical carrier wavelengths in one or the other of the previously mentioned bands. At the transmitter end, multiple laser diodes emitting light at respective ones of the multiple WDM wavelengths of number M have their outputs modulated by separate electronic data signals. The multiple optical signals are optically multiplexed together and then coupled onto the optical fiber. At the receiver end, the multiple WDM signals of number Mare optically demultiplexed to their constituent wavelengths. The individual optical signals are detected by a photodetector, typically a photodiode and most usually a PIN diode. A PIN diode has a large intrinsic region between its p- and n-type layers. The photodetector output is then conditioned and amplified for further processing as an electronic signal. The combination of photodetector and associated electronic circuitry is often referred to as an optical receiver. For a WDM receiver, the receiver function needs to be replicated for all the WDM wavelengths in what is referred to as a receiver array. WDM systems are being fielded with four wavelengths, that is, M=4. Sixteen wavelengths are being planned. The number M is expected to increase to up to about forty.
Great strides have been made in integrating the optical demultiplexer and photodetectors on a single integrated circuit chip, usually formed in compound semiconductors compatible with InP, for example, InGaAs. The receiver circuitry can also be integrated on the same chip, thus resulting in a moderately inexpensive, highly integrated receiver system.
However, the electronic circuitry required in a receiver array must satisfy several difficult requirements. To achieve long propagation spans, the optical signal level at the photodetector may be very small. The detection process may involve only a few photons. Thus, the receiver must accept low electronic input signals. The receiver circuitry must introduce a minimum of its own noise since low-level photodetector signals are already noisy. The receivers should operate at high data rates, a minimum of 2 Gb/s and preferably at least 10 Gb/s. For multiple receivers to be integrated onto a chip and using the same power supply lines, the cross talk between receivers, corresponding to different data channels, must be small.
Hence, in order to meet system requirements, the receiver array circuitry requires an advanced design.
SUMMARY OF THE INVENTION
The invention may be summarized as an optical receiver, especially appropriate for integration into a receiver array for use in a multi-wavelength communication system. A basic configuration includes a biased photodiode feeding its photocurrent into a circuit which can be characterized as a transimpedance amplifier designed as an operational amplifier including a feedback loop incorporating a feedback resistance. The operational amplifier includes a first bipolar input transistor into the base of which the photocurrent is input. The collector is connected to the feedback loop including a second bipolar output transistor, the biased base of which is connected to the collector of the input transistor. The emitter of the output transistor drives the voltage output and is also connected to the feedback loop through the feedback resistor to the base of the input transistor.
In one aspect of the invention, a cascode transistor has its principal current terminals interposed between the collector of the input transistor and the base of the output transistor.
In another aspect of the invention, the biasing of the base of the cascode transistor is referenced to a local ground, preferably through a diode string, thus providing isolation from cross talk from the first power supply line.
In yet another aspect of the invention, the biasing of the photodiode is referenced to a local ground, preferably through a diode string. Even more preferably, the diode strings for the cascode and photodiode biasing is accomplished with separate diode strings, allowing separate tuning of the two biasing schemes to optimize high-frequency performance.
In a progressive aspect of the invention, the output transistor is configured as an emitter follower and one or more additional emitter followers may be coupled on the output path.
In one more aspect of the invention, a second power supply line, preferably of opposed polarity from that of the first power supply line biases one or more output transistors.
In yet one more aspect of the invention, non-resistive elements are put in parallel to the feedback resistor in the feedback path. A shunt capacitor improves high-frequency stability. A further resistor placed in series with the shunt capacitor provides a finite feedback impedance even at very high frequencies. The shunt capacitance may be coupled to an earlier emitter follower than is the feedback transistor to provide a better combination of bandwidth and stability. A clamp diode or transistor in parallel with the feedback resistor prevents the output from going negative in the presence of a very large detected optical signal.
REFERENCES:
patent: 4241455 (1980-12-01), Eibner
patent: 4257125 (1981-03-01), Theall, Jr.
patent: 4713841 (1987-12-01), Porter et al.
patent: 5257285 (1993-10-01), Thorp
patent: 5455703 (1995-10-01), Duncan et al.
patent: 5636048 (1997-06-01), Kogure et al.
Cordell Robert Roger
Grabbe Paul
Ringo James John
Morgan & Lewis & Bockius, LLP
Pascal Leslie
Sedighian M. R.
Tellium Inc.
LandOfFree
Optical receiver particularly useful for a multi-wavelength... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical receiver particularly useful for a multi-wavelength..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical receiver particularly useful for a multi-wavelength... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2615596