Thermal-electrically cooled photodetector

Details Thermal-electrically cooled photodetector Thermal-electrically cooled photodetector

2001-03-16

2003-04-08

Le, Que T. (Department: 2878)

Radiant energy

Photocells; circuits and apparatus

Photocell controlled circuit

C250S231120, C250S238000

Reexamination Certificate

active

06545257

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to interferometric fiber optic gyros (IFOG), and more particularly to improving the total gyro linearity in an IFOG.
2. Description of the Related Art
There is a growing demand for high accuracy gyros for satellite pointing applications. In order to improve gyro sensitivity, it is necessary to lower the total gyro noise. In typical satellite pointing applications, an interferometric fiber optic gyro (IFOG) is employed. In the IFOG, noise elements arise from both the optical and electrical elements.
A closed loop IFOG is illustrated in FIG.
1
. An IFOG generally includes a light source
10
, a coupler
20
, an integrated optics chip (IOC)
30
, and a fiber coil
40
, which comprise the optical circuit
5
. The fiber coil
40
provides the rotation-sensitive interferometer. The processing electronics
45
of the IFOG generally comprise a photodetector
50
, an amplifier/filter
60
, an analog-to-digital converter (A/D)
70
, a digital signal processor (DSP)
80
, a digital to analog converter (D/A)
90
and amplifier
95
.
The processing electronics
45
function to provide a feedback phase shift in the optical circuit
5
which effectively compensates a rotation-induced phase shift sensed in the fiber coil
40
. The magnitude of the feedback phase shift is an indication of the rotation rate. The photodetector
50
converts an optical power output by optical coupler
20
to a corresponding voltage. The corresponding voltage is processed by amplifier/filter
60
and converted to a digital signal by A/D converter
70
. A corresponding feedback signal is calculated in DSP
80
, and fed back into the gyro via D/A converter
90
and amplifier
95
.
A rotation about a rate input axis
41
of the fiber coil
40
produces input signals at the photodetector
50
, which are denoted as “A” and “B” in
FIG. 1A. A
difference A-B corresponds to the sensed input rotation rate and is designated D
error
. The input signal characteristics result from interference patterns of a counter within the light source
10
, which propagates square wave modulated signals that travel in the fiber optic coil
40
. The sharp spikes present in the waveform are a result of the modulated signals driving the interference patterns through the peak of the interference curves.
An accurate measurement of the magnitude of the A and B levels of the photodetected signals is crucial to the overall performance of the IFOG. However, this measurement accuracy is compromised by the presence of noise and optical spikes. A high gain is required to maintain closed loop performance. However, the high gain results in signal distortion due to saturation effects from the optical spikes in the signal processing electronics
45
, most notably in the photodetector
50
and amplifier/filter
60
.
Noise generated within the photodetector
50
also makes detection of the A and B levels difficult, which results in degraded performance within the IFOG.
A conventional photodetector
50
is further illustrated in FIG.
2
. Referring to
FIG. 2
, the photodetector
50
is comprised of the photodiode
52
, amplifier
54
, and feedback resistor
55
. The photodiode
52
is reverse biased to operate in a photoconductive mode, thereby optimizing the bandwidth and speed of the photodetector
50
while minimizing signal distortion.
In operation, the photodiode
52
converts optical power, received from the coupler
20
, into a corresponding electrical current denoted as I
ph
. The current flows through feedback resistor
55
. In this configuration, the amplifier
54
provides an output voltage V
out
, which is calculated using Equation 1 below:
V
OUT
≡−I
ph
·R
f
  Equation 1
where R
f
is the value of the feedback resistor.
The amplifier
54
provides amplification while maintaining a high input impedance with respect to the photodiode
52
. The photodiode
52
and amplifier are selected based on parameters, such as bandwidth, amount of detected power, optical wavelength, available gain, etc. The photodetector
50
circuitry is typically realized in a hermetically sealed microcircuit package for ruggedness and shielding from external electric fields, as is commonly known in the art.
A disadvantage of the prior art photodetector
50
, however, is the susceptibility of the photodiode
52
and feedback resistor
55
to thermal noise. As discussed above, a relatively high gain is required to maintain closed loop performance. Consequently, large resistance values are required in the photodiode
52
(equivalent resistance) and feedback resistor
55
. This results in the generation of thermal noise in the photodiode
52
and feedback resistor
55
. Thermal noise, also known as Johnson noise, is a well-documented phenomenon in which electronic noise signals are produced by the random thermal motion of charges in circuit elements. Thermal noise varies as a function of resistance and temperature. Thermal noise is a major contributor to the noise generated within the photodetector
50
, which results in a degradation of IFOG performance, as discussed above.
Therefore, there is a need for a photodetector with reduced thermal noise generation.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a photodetector having reduced thermal noise generation.
In accordance with an aspect of our invention the photodetector of an Interferrometric Fiber Optic Gyro (IFOG) includes a photodiode that converts an optical power signal received from a coupler of the IFOG to an electrical compensation signal and which photodiode is in mechanical contact with one or more thermal electrical coolers (TEC) to lower an operating temperature of the photodiode. The photodetector also includes an amplifier circuit to amplify the electrical compensation signal. The amplifier circuit includes an operational amplifier (OP-AMP) having an input and an output, with a feedback resistor interposed between the input and output. The feedback resistor is also in mechanical contact with a TEC to lower an operating temperature of the feedback resistor. In accordance with our invention by reducing the operating temperature of the feedback resistor and photodiode the thermal noise of the IFOG is also reduced.


REFERENCES:
patent: 5229831 (1993-07-01), Carroll et al.
patent: 5686990 (1997-11-01), Laznicka, Jr.
patent: 6359918 (2002-03-01), Bielas
Patent Abstracts of Japan, vol. 012, No. 427 (P-784), Nov. 11, 1988, & JP 63 159712 A (Tokyo Keiki Co Ltd), Jul. 2, 1988 abstract.

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