Semiconductor laser and light source

Details Semiconductor laser and light source Semiconductor laser and light source

1997-08-26

2001-05-01

Arroyo, Teresa M. (Department: 2874)

Coherent light generators

Particular active media

Semiconductor

C372S046012

Reexamination Certificate

active

06226309

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor laser, and to a light source incorporating a semiconductor laser.
Due to their small size and low cost, semiconductor lasers are widely employed as light sources in fiber-optic communication systems, optical measurement systems, and the like. A typical semiconductor laser operates as a Fabry-Perot resonator, having cleaved facets at both ends that create an optical cavity in which lasing oscillation takes place. If the two facets are uncoated, light is emitted equally through both of them. Light emitted through the front facet is used for the intended application, e.g. communication or measurement. Light emitted through the rear facet is not used, or is used for monitoring the output power of the laser.
To increase the useful optical output of the laser, it is common to coat the end facets so that the front facet has a lower reflectivity than the rear facet. This increases the proportion of the optical power emitted through the front facet, but there is an unwanted consequence: more returning light, that has been reflected outside the laser, re-enters the laser cavity through the front facet. This returning light interferes with the operation of the laser, causing problems such as relative intensity noise and intermodulation distortion. Further details will be given later.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to reduce the problems caused by returning light in a semiconductor laser.
Another object is to enhance the accuracy with which the optical output power of a semiconductor-laser light source can be monitored.
A further object is to enhance the accuracy with which the optical output power of a semiconductor laser can be controlled.
The invented semiconductor laser has a front facet and a rear facet, the front facet being optically coupled to an application system such as a communication or measurement system. The reflectivity of at least one of the two facets is modified so that the front facet has a higher reflectivity than the rear facet. Consequently, more light is emitted through the rear facet than through the front facet, and the amount of returning light that re-enters the laser cavity through the front facet is reduced. The linearity of the response of the semiconductor laser to applied voltage and current is thereby improved, enhancing the accuracy with which the output of the semiconductor laser can be controlled.
The invented light source comprises the invented semiconductor laser, an optical detector for converting the optical power emitted through the rear facet to an electrical signal, and a control circuit for controlling the semiconductor laser according to this electrical signal. The increased optical power emitted through the rear facet enhances the accuracy of the electrical signal produced by the optical detector.


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