Wavelength stabilized laser

Details Wavelength stabilized laser Wavelength stabilized laser

1999-03-05

2001-04-10

Scott, Jr., Leon (Department: 2881)

Coherent light generators

Particular beam control device

Optical output stabilization

C372S031000, C372S038060, C372S029011, C372S006000

Reexamination Certificate

active

06215801

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to lasers, and more specifically to a system and method for stabilizing the wavelength of a laser or light source.
BACKGROUND OF THE INVENTION
Stabilizing the wavelength of a laser or light source is important in many applications. One such application is a telecommunication system that employs wavelength division multiplexing, in which a plurality of lasers or light sources each emit signals having different wavelengths that are multiplexed over a single fiber-optic cable. In this application, the plurality of signals having different wavelengths are demultiplexed after transmission across the fiber-optic cable and are delivered to corresponding recipients.
Due to the increased demands placed on telecommunication systems, it is increasingly desirable that fiber-optic cables employed in a wavelength division multiplexing system be capable of handling a large number of signals. Unfortunately, increasing the number of signals handled by a fiber-optic cable requires that the wavelengths of the different signals be closer together, which in turn increases the likelihood that the wavelengths of the signals will wander, and interfere with each other.
Additionally, it has been recognized that, over long periods of usage, laser and light sources gradually experience wavelength drift. In the short term, differences in temperature at a source can also cause fluctuations in the wavelength of the emitted light from a laser. Thus, in order to minimize the likelihood of wavelength crosstalk, it is necessary to stabilize the wavelengths of telecommunications transmitters.
Several methods have been devised for measuring and stabilizing wavelengths.
FIG. 1
is a diagram that illustrates a prior art apparatus employing one such method, as disclosed in International Publication No. WO 97/05679 of PCT Application No. PCT/CA96/00416, published Feb. 13, 1997. In the figure, a Fabry-Perot etalon filter
14
a
receives light emitted from source
10
, and based upon the wavelength of the light received, outputs a signal to detector
16
a.
The intensity of the signal detected by detector
16
a
is fed to circuit
11
. Circuit
11
is also fed the intensity of the light detected by detector
16
b,
which receives a reference signal from source
10
via beam splitter
12
. Circuit
11
compares the signals that it receives from detectors
16
a
and
16
b,
and adjusts the wavelength produced by source
10
if the ratio of the intensities of the two received signals changes beyond pre-determined limits.
FIG. 2
is a diagram that illustrates another prior art method, as disclosed in B. Villeneuve, H. Kim, M. Cyr and D. Gariepy,
A Compact Wavelength Stabilization Scheme for Telecommunication Transmitters
, Ontario, Canada. In the figure, source
10
emits light having a frequency which is received by photodetectors
16
a
and
16
b
via Fabry-Perot (hereinafter “FP”) filter
14
. The angular behavior of FP filter
14
(which varies the wavelengths of light received depending on the cosine of its tilt angle) results in differing spectral responses at photodetectors
16
a
and
16
b.
The differing spectral responses are converted into a discrimination signal
18
, which is utilized to stabilize the wavelength.
These and other prior art methods (such as the use of holographic crystals, which are not shown) require the use of optical elements that are neither compact nor simple to implement. Furthermore, these and other prior art methods are not programmable or adjustable for various desired frequency channels.
Thus, there is a need for an improved apparatus for stabilizing the wavelength of a laser.
SUMMARY OF THE INVENTION
The present invention, in accordance with one embodiment, provides an apparatus for stabilizing the wavelength of light emitted by a laser or light source. In one embodiment, the invention comprises a divider for dividing a sample of emitted light into a first and second portion. The invention also comprises a first photodetector configured to measure an intensity of the first portion and a second photodetector configured to measure an intensity of the second portion. The second photodetector is configured to receive the emitted light via an optical frequency discriminator, such as a dielectric coating, wherein the optical frequency discriminator is configured to modify the intensity of the emitted light at the second photodetector as a function of an optical frequency of the emitted light. The invention also comprises a processor configured to maintain a desired level corresponding to a ratio of the intensity measured by the first photodetector and the intensity measured by the second photodetector, so as to provide a feedback control signal to adjust the wavelength of the emitted light. According to one embodiment, the processor is programmable.
According to one embodiment, first and second photodetectors are photodetector diodes having a base electrode, a first electrode and a second electrode. The diode is configured to generate an electrical current between the base electrode and the first electrode corresponding to the intensity of light received by the first photodetector and to generate an electrical current between the base electrode and the second electrode corresponding to the intensity of light received by the second photodetector.
In still another embodiment, the sample is a back-facet emission emitted by the source. In this embodiment, the entire apparatus is disposed on a silicon chip. In another embodiment, the silicon chip has a surface that is mirrored, and a dielectric coating is applied directly to the surface of the mirrored silicon chip so as modify the intensity of the light received at one of the photodetectors.


REFERENCES:
patent: 4914662 (1990-04-01), Nakatani et al.
patent: 5200967 (1993-04-01), Miyata et al.
patent: 5331651 (1994-07-01), Becker et al.
patent: 5825792 (1998-10-01), Villeneuve et al.
patent: 9705679 (1997-02-01), None
Subpicometer Accuracy Laser Wavelenght Sensor Using Multiplexed Bragg Gratings by Feng Zhao et al. IEEE Photonics Technology Letters. 9, No. 11. Nov. 1997.
A Compact Wavelength Stabilization Scheme for Telecommunication Transmitters by Villeneuve et al.

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