Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1998-10-15
2001-04-17
Lee, John R. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S214100
Reexamination Certificate
active
06218657
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of optics. More particularly, the present invention relates to a circuit for detecting optical pulses that contain a small number of photons.
2. Description of the Related Art
Detection of optical pulses at the level of a single photon is important for many scientific and engineering applications, such as optical communications, quantum cryptography, time-resolved spectroscopy and quantum optics.
A semiconductor device known as an avalanche photodiode (APD) can be used for single-photon counting and/or for triggering a sequence of events that are time-dependent upon the incidence of a photon. See, for example, S. Cova et al., Review of photon counting with APD's, Appl. Opt., 35(12), p. 1956, 1996. A silicon APD provides detection of visible and near-infrared light to about 1 &mgr;m wavelength. Both Ge- and InGaAs-type APD's are used for detecting light at 1.3 &mgr;m, one of the two preferred telecommunications wavelengths, while only InGaAs-type APD's are suitable for detecting light at 1.55 &mgr;m, the other preferred telecommunications wavelength. Photomultipliers (PMTs) and multichannel plate (MCP) detectors are also used for detecting single photons and/or for triggering a sequence of events that are time-dependent upon the incidence of a photon. Time gating the gain of any of these devices can provide discrimination with respect to scattered light and information about the time variation of the light intensity.
When an APD is biased above its reverse breakdown voltage V
br
in the so-called “Geiger mode”, a single photon that is absorbed by the APD excites a single conduction electron, which gains sufficient energy due to the bias voltage to excite secondary conduction electrons by collision cascade. The secondary conduction electrons can, in turn, excite more electrons and so forth, resulting in a current avalanche that can provide an electronic gain on the order of 10
5
-10
8
.
Such an enormous gain can be used for generating charge pulses on the order of picoCoulombs (pC) within a few nanoseconds, producing corresponding voltages across a 50 Ohm load resistor on the order of millivolts to tens of millivolts that can readily be sensed by conventional electronic circuitry. Generally, the voltage pulses are amplified and applied to a discriminator that outputs a pulse only when an input voltage pulse exceeds a predetermined threshold level. Discrimination helps eliminate spurious counts, and provides an output pulse having fixed characteristics suitable for triggering counters or other electronic apparatus.
After an electron avalanche has been triggered by an incident photon, the current must be turned off, or quenched, and the APD restored to a state in which it is again sensitive to an incident photon. Several quenching schemes have been proposed. For example, active quenching techniques are disclosed by R. G. W. Brown et al., Active quenching of Si APD's, Appl. Opt., 26(12), p. 2383, 1987. Passive quenching techniques are disclosed by, for example, R. G. W. Brown et al., Passive quenching of Si APD's, Appl. Opt., 25(22), p. 4122, 1986, and by P. C. M. Owens et al., Passive quenching of Ge APD's, Appl. Opt., 33(30), p. 6895, 1994.
In active quenching, additional electronic circuitry monitors the onset of an avalanche, and terminates the avalanche as quickly as possible by dropping the bias voltage below the reverse breakdown voltage V
br
of the APD. In passive quenching, the current resulting from an avalanche flows through a quenching resistor, causing a voltage drop that momentarily reduces the bias voltage below the reverse breakdown voltage V
br
of the APD. Once the avalanche current stops flowing, both types of quenching schemes restore the bias to a level above V
br
as quickly as possible so that the APD is in a state in which it is sensitive to an incident photon.
When photon arrival time is arbitrary, restoring the bias level of the APD as quickly as possible minimizes the possibility that an incident photon will be missed. On the other hand, maintaining a constant bias above the reverse breakdown voltage makes an APD susceptible to avalanches triggered by thermally generated carriers, giving rise to a “dark count” rate. This rate can be reduced by cooling the device. Maintaining constant bias also makes the APD sensitive to spurious photons that may be incident upon the APD. Applying a continuous bias also limits how far above breakdown the device can be biased. A higher bias voltage leads to a higher quantum efficiency and faster response time, but with the drawback of a higher dark count rate.
In some applications, the potential arrival times of photons are accurately predictable, or a narrow time window of sensitivity is desired. In such cases, it is advantageous to use a “pulsed biasing” technique for detecting ultra-weak optical pulses. For pulsed biasing, the bias voltage of an APD detector is only raised above V
br
during the intervals of time when photon arrival is anticipated. Between these intervals the detector bias is reduced, suppressing both generation of dark counts and triggering caused by spurious photons. Pulsed biasing has significant advantages for clocked applications such as quantum cryptographic systems, for applications where one wishes to avoid detection of a strong flash of light that may precede a signal of interest, or for applications where it is desired to obtain accurate time-variation data.
An important consideration when an APD is used for detecting a single photon is that a fraction of the electrons that flow through the APD during an avalanche becomes trapped in defect or impurity states lying within the bandgap of the semiconductor. The trapped electrons can later be excited into the conduction band by thermal fluctuations, thus triggering spurious avalanches. To minimize the amount of trapped charge, it is highly desirable to minimize the total charge passing through the APD due to both photodetection events and dark counts. Pulse biasing inherently suppresses dark counts between the bias pulses by reducing the bias below V
br
, and is particularly suited for InGaAs-type APD detectors because these type of detectors exhibit very high dark count rates when operated with constant bias.
A pulsed-bias technique for detecting a single photon at 1.3 &mgr;m using a Ge APD is disclosed by, for example, B. F. Levine et al., Pulse biased APD's, Appl. Phys. Lett., 44(5), p. 553, 1984. Relatively long bias pulses (~10 ns) were used with a maximum pulse repetition rate of 1 MHz. Subsequently, a short bias pulse (~1 ns) and a pulse repetition rate of 10 MHz is disclosed by B. F. Levine et al., Pulse biased APD's, Appl. Phys. Lett., 44(6), p. 581, 1984. Eventually, the pulse repetition rate was increased to 45 MHz, as disclosed by B. F. Levine et al., Pulse biased APD's, Electron. Lett., 20(6), p. 270, 1984. As another example, U.S. Pat. No. 4,754,131 issued Jun. 28, 1988, to Bethea et al. relates to use of APD's for detection of small numbers of photons using APD's.
FIG. 1
shows a voltage waveform diagram for biasing an avalanche photodiode for pulsed-biased single-photon counting.
FIG. 2
shows a schematic diagram of a conventional APD detector circuit
20
that provides pulse-biasing for an avalanche photodiode. APD detector circuit
20
includes a coupling capacitor C
1
and a resistor R
1
that are both connected to the cathode of an avalanche photodiode APD
1
. The anode of avalanche photodiode APD
1
is connected to a signal common through a load resistor R
L
. A DC bias voltage V
DC
is applied to the cathode of avalanche photodiode APD
1
through a resistor R
1
so that avalanche photodiode APD
1
is reverse-biased below the reverse breakdown voltage V
br
of avalanche photodiode APD
1
. A pulse bias voltage V
pulse
is applied through coupling capacitor C
1
. When a photon
21
is incident on avalanche photodiode APD
1
, the output signal of APD detector circuit
20
appear
Bethune Donald S.
Devoe Ralph G.
Kurtsiefer Christian
Rettner Charles T.
Risk William P.
Banner & Witcoff , Ltd.
Berthold Thomas R.
International Business Machines - Corporation
Lee John R.
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