Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2000-05-31
2001-11-06
Lee, John R. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S2140AG, C327S514000
Reexamination Certificate
active
06313459
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to receivers in optical communication systems and, more particularly, to a method for calibrating and operating an uncooled avalanche photodiode optical receiver.
BACKGROUND OF THE INVENTION
Optical communication systems use optical fiber as a communication medium and light as an information carrier. For instance, an optical signal may be a beam of light modulated to represent binary-coded information. When light is used to transmit information, the information may be extracted from the beam of light through the use of a photodetector in a receiver. A photodetector is an electronic component that detects the presence of light radiation through conversion of light energy to electrical energy. A common photodetector is called a photodiode which consists of a semiconductor having a property called photoconductivity, in which the electrical conductance varies depending on the intensity of radiation striking the semiconductor material comprising the photodiode. Essentially, a photodiode is the same as an ordinary diode, except that the package has some transparency that allows light energy to effect junctions between the semiconductor materials inside.
An optical receiver using an avalanche photodiode (APD) features a built-in gain as part of the optical-to-electrical conversion process. This built-in gain makes APD receivers attractive as receivers in high bit-rate optical communication systems. However, the built-in gain requires very careful bias (operating condition) control and temperature compensation to maintain system performance over a wide range of optical input powers. This is especially true when the APD receiver does not contain any active cooling circuitry.
In U.S. Pat. No. 5,953,690, issued to Lemon et al. Sep. 14, 1999, an intelligent fiber-optic receiver is disclosed. During calibration procedures for the receiver, an optical-to-electrical conversion device (avalanche photodiode or PIN photodiode) and its supporting control and monitoring circuits in a receiver module are characterized over a defined operating temperature range. Characteristic data and/or curves defining these operational control and monitoring functions over the range of operating conditions (temperature, power supply) are stored in non-volatile memory. During operation, an embedded micro-controller, together with analog to digital converters, digital to analog converters and other associated circuitry, dynamically controls the operational constants of the module based on the current operating conditions (temperature, power supply). Unfortunately, this approach, which employs a thermal chamber while measuring receiver performance at the extremes of operating temperature, may be very time consuming and hence costly. Further, this approach may not be suited to volume manufacturing of receivers.
SUMMARY OF THE INVENTION
The present invention comprises an operational algorithm, and calibration process, for an avalanche photodiode (ADP) receiver which takes into account an APD behavioral model. In-situ optical and electrical measurements (calibration) of the APD are performed to determine key constants for use in the model. Knowledge of these constants allows for optimum operation of the APD over a wide range of input optical powers. The operational algorithm also gives an estimate of input optical power over a wide range of ambient temperatures.
In accordance with an aspect of the present invention there is provided a method of operating an optical receiver, the optical receiver including an avalanche photodiode, where the method includes receiving an indication of temperature magnitude of the avalanche photodiode and determining breakdown voltage magnitude for the avalanche photodiode corresponding to the received temperature magnitude indication. The method farther includes determining bias voltage magnitude for the avalanche photodiode as a function of the breakdown voltage magnitude and an avalanche photodiode current magnitude, where the avalanche photodiode current magnitude is set at a predetermined constant magnitude, and outputting the bias voltage magnitude. In another aspect of the invention an optical receiver is provided including an avalanche photodiode, a bias control circuit to control a bias voltage supplied to the avalanche photodiode and a bias control processor for performing this method. In a further aspect of the present invention, there is provided a software medium that permits a general purpose computer to carry out this method.
In accordance with another aspect of the present invention there is provided a method of determining magnitude of optical power input to an avalanche photodiode including receiving an indication of temperature magnitude of the avalanche photodiode, determining breakdown voltage magnitude for the avalanche photodiode corresponding to the received temperature magnitude, receiving an indication of avalanche photodiode bias voltage magnitude, receiving an indication of avalanche photodiode current magnitude, determining gain magnitude of the avalanche photodiode as a function of the bias voltage magnitude, the breakdown voltage magnitude and the current magnitude and determining input optical power magnitude from the determined gain magnitude and the current magnitude.
In accordance with a still further aspect of the present invention there is provided a method of calibrating an avalanche photodiode optical receiver including determining a constant relating avalanche photodiode gain to bias voltage at low current levels, determining a constant relating avalanche photodiode gain to avalanche photodiode current and determining a first avalanche photodiode breakdown voltage at a calibration temperature.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.
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patent: 5953690 (1999-09-01), Lemon et al.
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Hoffe Ronald D.
Kelly Collin G.
Nadeau Marc A.
Truscott Ashley A.
Wan Ping W.
Lee John R.
Nortel Networks Limited
Pyo Kevin
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