Coherent light generators – Particular temperature control
Reexamination Certificate
1999-05-20
2001-05-22
Scott, Jr., Leon (Department: 2881)
Coherent light generators
Particular temperature control
C372S038060, C372S036000, C372S069000
Reexamination Certificate
active
06236668
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a semiconductor laser apparatus and a pumping circuit therefor capable of providing a stable light output therefrom.
The light output from a semiconductor laser apparatus usually fluctuates because the threshold current intensity for laser excitation depends on temperature. It has therefore been known to make use of a so-called automatic power control (APC) circuit in order to obtain a stable light output without regard to changes in temperature. Such an APC circuit may comprise a photodetector and serve to detect a portion of the light output from a semiconductor laser apparatus with the photodetector and to automatically adjust the pump current for the semiconductor laser according to the level of output from the photodetector. This method, however, has several disadvantages. Firstly, it is difficult to obtain a stable light output from the moment immediately after the pumping of the semiconductor laser is started because the stabilization of the light output is obtained by a feedback circuit and there are delay elements due to the time constant of the feedback loop. The method may be suited for the CW operation but in the case of pulse pumping, the on-off control of the semiconductor laser may be carried out, for example, by integrating the output pulse signals from the photodetector and detecting changes in the integrated output. Thus, the control circuit inclusive of the photodetector will have a complicated structure as a whole. Secondly, an excessively large current may flow into the semiconductor if there is an abnormality in any of the elements of the feedback loop but there is no adequate means to protect the semiconductor laser therefrom. Thirdly, the circuit structure as a whole becomes complicated. Not only does this adversely affect the cost, but it will also be difficult to miniaturize the apparatus.
In view of the first of the problems described above, Japanese Patent Publications Tokkai 59-17291 and Tokkai 8-316560 disclosed circuits adapted to carry out temperature compensation by means of an open loop and to thereby avoid a time delay associated with the detection of light and the feedback circuit. In view of the second of the problems, Japanese Patent Publication Tokkai 3-145171 disclosed a protective circuit which detects the pump current of the semiconductor laser by means of a current-detecting resistor and switches off the transistor which controls the current to the semiconductor laser when the detected value increases past a specified level. The circuit disclosed in Tokkai 8-316560, however, includes a large number of components such as differential amplifiers, control transistors, resistors of all kinds, diodes and thermistors and hence cannot overcome the third of the problems described above. Neither is the protective circuit disclosed in Tokkai 3-145171 capable of solving the third problem described above because it must be used together with a circuit forming a feedback loop by using a prior art photodetector. As for the circuit disclosed in Tokkai 59-17291, it has no protective means against abnormal heating of the semiconductor laser due to a rise in the environmental temperature or an overcurrent.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide a pumping circuit for a semiconductor laser capable of suppressing variations in the light output due to temperature changes and eliminating at the same time the problems of overheating of the semiconductor laser without making the circuit structure overly complicated or increasing the size of the apparatus.
It is another object of this invention to provide a semiconductor laser apparatus incorporating such a pumping circuit.
A pumping circuit for a semiconductor laser embodying this invention, with which the above and other objects can be accomplished, may be characterized as having a positive temperature coefficient (PTC) thermistor element with temperature coefficient in the range between −1.5%/° C. and −0.5%/° C. within the temperature range between −20° C. and +70° C. and the temperature coefficient becoming positive at a temperature over +70° C. connected in series in a current route to the semiconductor laser.
The ratio of temperature change to change in light output for small and medium sized semiconductor lasers of less than 100 mW is usually −1%/° C. within the range of temperatures under normal use between −20° C. and +70° C. Thus, the temperature coefficient of a PTC thermistor as described above is sufficient to cancel the change in light output from such a semiconductor laser due to temperature changes and hence the light output under normal temperatures during operation can be stabilized. When the environmental temperature becomes higher than the normal operating temperature, a prior art APC circuit of the kind discussed above will tend to increase the current passing to the semiconductor laser, but the PTC thermistor described above will change its temperature coefficient to a positive value at a temperature higher than 70° C. and its resistance increases further as temperature increases more. Thus, abnormal heating of a semiconductor apparatus due to an excessively strong current can be prevented according to this invention.
The temperature coefficient of a PTC thermistor in the temperature range between −20° C. and +70° C. can be made approximately equal to the temperature coefficient of light output from an ordinary semiconductor laser within the same temperature range but when the characteristic of change in resistance against change in temperature is set, the resistor may be connected either in series or in parallel with the PTC thermistor element.
If the current passing to the semiconductor laser becomes excessively large for whatever reason such as an abnormal rise in the source voltage, the semiconductor laser may be destroyed by a large current. If a PTC thermistor element is set near the semiconductor laser so as to be thermally coupled with it, the temperature coefficient of the PTC thermistor element becomes positive at a specified temperature above 70° C. and becomes higher in the higher temperature region, thereby rapidly suppressing the current passing to the semiconductor laser and controlling the generation of heat therefrom. Destruction of the expensive semiconductor laser can thus be averted.
A semiconductor laser apparatus embodying this invention may be characterized wherein the semiconductor laser forming the semiconductor laser circuit described above and one PTC thermistor together form one semiconductor laser apparatus. In other words, a complete circuit is formed only with passive elements, unlike the prior art APC circuits or protective circuits, such that compact apparatus can be obtained as semiconductor laser with a PTC thermistor element.
REFERENCES:
patent: 4243952 (1981-01-01), Patterson
patent: 5740195 (1998-04-01), Murray et al.
patent: 5796568 (1998-08-01), Baiatu
patent: 5999551 (1999-12-01), Yoshizawa
patent: 6023053 (2000-02-01), Nakayama et al.
patent: 6094129 (2000-07-01), Baiatu
patent: 59-017291 (1984-01-01), None
patent: 3-145171 (1991-06-01), None
patent: 8-255704 (1996-10-01), None
patent: 8-316560 (1996-11-01), None
Osada Shinichi
Yamanouchi Tomozo
Coudert Brothers
Jr. Leon Scott
Murata Manufacturing Co. Ltd.
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