Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2000-06-22
2003-02-11
Kim, Robert H. (Department: 2882)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S2140RC
Reexamination Certificate
active
06518563
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to optical components and networks, and in particular to a method and circuit for detecting aging of such components.
BACKGROUND OF THE INVENTION
Optical networks are becoming increasingly important in telecommunications primarily due to their huge information handling capacity. One of the key elements of such a network is the light emitting device, which is usually in the form of a semiconductor laser, such as a Distributed Bragg Reflected (DBR) laser, which is capable of emitting light of several different wavelengths and therefore desirable in Dense Wavelength Division Multiplexing (DWDM) systems. For this type of application, the DBR laser is set to operate at these wavelengths by a selection of tuning currents at an initial temperature (typically 25 deg. C.). The selected wavelengths are typically stabilized by detecting a portion of the light emitted by the laser, usually from the backface of the laser, and transmitting the detected light through a filter whose output is a function of wavelength. The light is detected and then received by a wavelength control circuit which generates a signal in response thereto for either changing the bias supplied to the tuning section of the laser or the temperature of the laser. The wavelengths are then considered “locked” onto the desired value by the feedback circuitry. The desired wavelengths correspond to the transmission channels of the network also known as “ITU grid channels”.
One of the problems with present networks is the fact that the laser and the optical filter will tend to age over a period of time. This aging can make it difficult to correlate the laser wavelengths with the proper channels. Compounding the problem is the fact that it may not be apparent whether the laser or the filter has aged, and therefore, proper correction is thwarted.
It is desirable, therefore, to provide a way of determining when the light emitting device and/or the optical filter has aged. It is further desirable to distinguish between the aging of the optical device and filter.
SUMMARY OF THE INVENTION
The invention in accordance with one aspect is a method of determining aging of a light emitting device which is capable of selectively emitting light having different wavelengths corresponding to different channels for light transmission as a function of electrical bias supplied to the device. A bias is applied to the device which results in a wavelength corresponding to a desired channel at an initial temperature. The slope of a curve which is light power from the device as a function of bias is determined at the applied tuning current. The determined slope is compared with a prior determined value in order to detect any change from the prior value.
REFERENCES:
patent: 4785454 (1988-11-01), Olsson et al.
patent: 5019769 (1991-05-01), Levinson
patent: 5742418 (1998-04-01), Mizutani et al.
Broutin Scott L.
Plourde James Kevin
Stayt, Jr. John William
Agere Systems Inc.
Birnbaum Lester H.
Kim Robert H.
Thomas Courtney
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