Coherent light generators – Particular temperature control
Patent
1985-09-24
1987-07-28
Scott, Jr., Leon
Coherent light generators
Particular temperature control
372 29, 372 38, 372 33, H01S 304
Patent
active
046835733
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention pertains to apparatus for stabilizing the temperature, and thereby the output wavelength, of injection lasers.
BACKGROUND OF THE INVENTION
Optical communications systems utilizing lasers, and having high-bit rate modulation and/or wavelength division multiplexing, require accurate control of laser wavelength. Such accurate control is difficult to implement in present transmission systems because the laser wavelength is monitored by using elaborate and complex optical components and/or circuit elements. Some examples of the prior art include: U.S. Pat. Nos. 3,588,253, 4,387,462, and Japanese Patent Disclosure No. 1983-77272.
U.S. Pat. No. 3,588,253 discloses a spectrograph which includes a temperature tunable laser diode. The laser diode is placed in a Dewar flask and liquid helium is used to control the temperature of the environment in which the laser diode is positioned. Since the wavelength of single mode emission from the laser diode depends upon the temperature thereof, the wavelength of the single mode emission from the diode is varied by varying the temperature of the diode.
U.S. Pat. No. 4,387,462 discloses a temperature stabilization apparatus which stabilizes a single frequency laser. The laser is disposed in a hard-sill glass laser tube having a resistance coating on an exterior surface thereof. A power supply is coupled to the resistance coating for heating the glass tube. Temperature sensors are mounted at various locations to monitor the temperature of the laser tube. The temperature sensors are coupled to a temperature comtroller, which in turn, is connected to the power supply for controlling the temperature of the tube.
Japanese Pat. No. 1983-77272 discloses an apparatus for temperature control of a semiconductor laser. The apparatus uses a photodiode, having a predetermined voltage-temperature characteristic, to sense the temperature of a heat sink upon which the photodiode and the laser are disposed. A thermo-module adjusts the temperature of the laser in response to variations in the photodiode output voltage.
If the large bandwidthe capability of presently available single-mode fibers is to be used to its full capacity, it is necessary to develop a compact and integrated wavelength stabilization apparatus for laser transmitters other than those disclosed in the art.
SUMMARY OF THE INVENTION
Apparatus in accordance with the present invention stabilizes the temperature, and thereby the output wavelength, of an injection laser by monitoring the laser terminal voltage and deriving a voltage therefrom which varies with the temperature of the laser. The temperature of the laser is then varied in response to this derived voltage. Thus, by monitoring the voltage, the inventive apparatus can more quickly and accurately stabilize the laser's temperature than apparatus that monitor laser heat sink temperatures and whose operation depends on the thermal lag of the heat sink as well as apparatus whose operation depends on the position of a temperature sensor.
In one embodiment, a circuit senses the laser's terminal voltage and derives a voltage therefrom which is proportional to the laser's junction voltage. Further circuitry compares the voltage to a reference voltage and generates an output signal whose value depends on the comparison. The output signal serves as an input into a temperature controller, for example a thermoelectric cooler, in order to adjust the laser's temperature. The thermoelectric cooler is a conventionally known device which has the property of getting colder when electric current flows in one direction and getting hotter when the direction of the current is reversed.
The above-described embodiment depends upon the fact that the junction voltage of an injection laser saturates above laser current threshold at a value--referred to as the "saturation voltage"--which is proportional to the energy of the semiconductor bandgap, and thereby to the laser output frequency. Further, the energy of the semiconductor bandgap, and thereby the l
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Wittke et al; "Stabilization of CW Injection Lasers"; RCA Tech. notes, TN No. 1005, Apr. 9, 1975.
A. Albanese, "An Automatic Bias Control (ABC) Circuit for Injection Lasers," The Bell System Technical Journal, vol. 57, No. 5, May.-Jun. 1978, pp. 1533-1544.
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Bell Communications Research Inc.
Falk James W.
Jr. Leon Scott
Peoples John T.
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