Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-06-09
2001-04-24
Chan, Jason (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
Reexamination Certificate
active
06222660
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to avalanche photodiode (APD) gain control circuits, and more specifically to a method using an adaptive power supply for avalanche photodiodes that provides a means for determining an optimum bias voltage for the APD.
Avalanche photodiodes are used in optical receivers for converting an optical signal into an electrical signal. The electrical signal output from the APD is coupled to an amplifier for amplification. One of the most important parameters of an APD is the reverse bias voltage associated with breakdown. When operated below the reverse breakdown voltage, increases in reverse bias results in amplification. This is the region of normal APD operation. However at the breakdown voltage dark currents increase exponentially, causing the receiver to be saturated with noise and possibly damaging or destroying the APD.
Therefore the APD generally is biased close to its breakdown voltage to achieve maximum sensitivity. Typically the bias voltage for an APD is set at a specified number of volts, such as five volts, below the specified breakdown voltage. However since each APD is different, the breakdown voltage of each APD is different. This requires different bias voltages from APD to APD.
A measure of the reliability of an APD is the ability to pass optical digital data signals with a virtually zero bit error rate (BER)—for SONET systems the virtually zero BER is specified to be 10
−10
or less. For different optical power levels, the bias voltage at which the bit error rate becomes non-zero differs. Especially at larger voltages and/or larger optical power levels the APD generates more noise which causes the bit error rate to be non-zero.
What is desired is a method using an adaptive power supply for an avalanche photodiode for determining an optimum bias voltage for the avalanche photodiode.
BRIEF SUMMARY OF THE INVENTION
Accordingly the present invention provides an adaptive power supply for an avalanche photo-diode that may be part of the APD circuit board. The adaptive power supply is coupled to the APD via a current sense circuit and a load resistor. The bias voltage provided by the adaptive power supply to the APD is determined by a digital control voltage from a controller that is part of the APD circuit board. The APD current sensed by the current sense circuit is fed back to the controller. The measure of the actual breakdown voltage for the APD is achieved without optical input to the APD by gradually increasing the bias voltage from a voltage significantly below the specified breakdown voltage from the adaptive power supply while sensing the APD current. When the sensed APD current indicates breakdown, the voltage at that point is stored as the breakdown voltage. The controller then establishes an optimum bias voltage as being a specified number of volts below the actual measured breakdown voltage. Where the APD is used in a receiver that processes an optical data signal, such as a SONET/ATM signal having an overhead with parity checking, the optical data signal source is coupled to the APD via a programmable optical attenuator for varying the input optical power level. The electrical signal output from the APD is processed by an amplifier. The output from the amplifier is input to a clock and data recovery circuit, and the recovered data is input to an overhead bit error rate counter. For each of a plurality of optical power level settings determined by the programmable optical attenuator, the power supply voltage is varied to develop a family of current-voltage curves. The point on each curve where the bit error rate becomes non-zero is indicated to generate an envelope defining a region within which the bit error rate is zero. The point on the envelop for the expected optical power level closest to the breakdown voltage is determined to be the optimum bias point for the avalanche photodiode for digital data applications.
The objects, advantages and other novel features of the present invention are apparent from the following detailed description when read in conjunction with the appended claims and attached drawing.
REFERENCES:
patent: 5808760 (1998-09-01), Gfeller
patent: 5953690 (1999-09-01), Lemon et al.
patent: 6072609 (2000-06-01), Masuda
Chan Jason
Gray Francis I.
Leung Christina Y
Tektronix Inc.
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