Coherent light generators – Particular beam control device – Optical output stabilization
Reexamination Certificate
1999-06-09
2001-05-15
Evans, F. L. (Department: 2877)
Coherent light generators
Particular beam control device
Optical output stabilization
C372S038020
Reexamination Certificate
active
06233262
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to monitoring and controlling laser wavelength.
BACKGROUND OF THE INVENTION
Lasers have long been known in the art for a wide variety of uses. These sources of light are arranged to function at a certain predetermined wavelength. In some applications, it is unimportant whether the wavelength of the laser remains essentially constant over time. However, in other applications, such as optical telecommunications, and medical lasers, it is crucial that the preset wavelength be maintained throughout the use of the laser. This is necessary due to aging of the laser, as well as the in environmental effect of temperature, two of the major factors affecting laser wavelength.
A number of proposals have been made in the literature as to ways to monitor and control lasers. In particular, much research has been conducted in the area of telecommunications over optical fiber lines. The most common solution is to use an outside wavelength reference from, for example, a synchronized Etalon filter which provides a set of equally spaced references at the standardized wavelengths, or from a frequency stabilized master laser. However, these devices require additional expensive equipment as well as temperature control and stabilization of the Etalon or master laser. Thus, these devices rely on an external reference source to maintain the laser wavelength.
Another solution is to use a reference filter. Yet another suggestion is to lock the wavelength comb of a tunable DBR to a waveguide grating router to enable a laser to track and correct uncontrolled changes in a remotely located WDM device. Another is to use an arrrayed-waveguide grating. Yet another utilizes a narrow-band reflective fiber grating at a temperature sensor at the remote receiver. A further proposal includes a frequency control scheme where the frequency of each transmitter is referenced to a silica based optical frequency discriminator, which in addition is locked to an absolute reference frequency. It has also been suggested to use cascaded fibre Bragg gratings as channel allocators for unequally spaced channel multiplexing, and multiwavelength lasers can be successfully locked to the transmission dips produced from the cascade FBG's by using the dithering technique. These proposed devices are generally complicated mechanically and expensive, requiring a number of filters arranged with high accuracy.
Accordingly, there is a long felt need for a simple device for monitoring and controlling laser wavelength which is relatively simple to build and maintain, highly accurate, and which is substantially unaffected by temperature, and it would be very desirable to have such a device which does not rely on an external reference wavelength,
SUMMARY OF THE INVENTION
According to the present invention, there is provided a device for monitoring and controlling the wavelength of a semiconductor laser having an adjustable wavelength, the device including a wavelength filter including a medium-dense wavelength division demultiplexer (WDM) arranged to receive a portion of the laser power of the laser and divide it between two filter outputs, a photoreceiver arranged to measure the output power at each filter output, a processor coupled to the photoreceivers and arranged to compute the ratio of power at the two filter outputs, apparatus to compare the computed ratio with a predetermined reference ratio, and apparatus to adjust the wavelength of the laser in response to the comparison.
According to one embodiment of the invention, the WDM filter has an optical spectral range of about 40 nm around 1550 nm, and is centered at a wavelength within this range.
According to another embodiment of the invention, the apparatus for adjusting the wavelength includes a temperature controller associated with the laser, and means in said processor to activate said temperature controller until said computed ratio equals said predetermined reference ratio.
There is also provided in accordance with the present invention a method for monitoring and controlling the wavelength of a semiconductor laser, the method including the steps of:
(a) feeding a portion of the output laser power of the laser to a wavelength filter including a wavelength division demultiplexer (WDM) arranged to receive the laser output and divide it between two filter outputs:
(b) measuring the power at each of the two filter outputs;
(c) computing the ratio of the power at the filter outputs;
(d) comparing the computed ratio with a predetermined reference ratio;
(e) adjusting the wavelength of the laser in response to the comparison; and
(f) repeating steps (b) through (e) until the computed ratio equals the predetermined reference ratio.
REFERENCES:
patent: 5299212 (1994-03-01), Koch et al.
Mesh Michael
Weiss Yossi
Browdy and Neimark
ECI Telecom Ltd.
Evans F. L.
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