Coherent light generators – Particular temperature control
Reexamination Certificate
1999-01-25
2001-06-05
Arroyo, Teresa M. (Department: 2881)
Coherent light generators
Particular temperature control
C372S029020
Reexamination Certificate
active
06243404
ABSTRACT:
TECHNICAL FIELD
The present invention relates to optical modules and, more particularly, to laser modules used in lightwave transmission systems.
BACKGROUND OF THE INVENTION
Typically, optical modules house optical components hermetically in a box, such as in a so-called “14-pin butterfly” housing or package. For example, laser modules used in lightwave transmission systems include a semiconductor laser configured to emit coherent radiation for communication purposes. Although the laser resonates over a range of frequencies, the laser is typically confined to operate at, or is so-called “locked” to a single desired wavelength, even with variations in temperature, such as by using an external fiber grating. This locking mechanism, however, only works over a particular range in temperatures.
Accordingly, such laser modules are specifically manufactured so that the laser properly locks over the span in temperatures for the desired application. That is, the laser's so-called “locking range” is nominally designed to match the desired operating temperature range. Unfortunately, for various manufacturing reasons, the laser's locking temperature range may not match the span in temperatures for the desired application. Although the laser's temperature can be maintained to fall within its locking range using thermal electric coolers (TECs) or resistive heaters, it may not be practical to do so for certain applications because of reliability and maintenance considerations, such as for so-called “submarine” applications.
In the prior art, the prevailing wisdom is either to discard or rework the laser module, which in either case is usually cost prohibitive. It would therefore be desirable to provide for an improved laser module wherein its operating temperature range is readily adjustable so as to substantially cover or be coextensive with the span in temperatures for the desired application.
SUMMARY OF THE INVENTION
The present invention is directed to a method for adjusting the operating temperature range of a laser module such that it substantially covers or is coextensive with the span in temperatures for the desired application. In accordance with the principles of the invention, it has been found that the laser module's operating temperature range can be readily adjusted by judiciously increasing its thermal resistance so as to elevate, and thereby offset the laser's temperature from its ambient or surrounding temperature. As such, by judiciously impeding the laser's heat flow, the laser module's operating temperature range can be made to be cover the span in temperatures for the desired application. Preferably, washers of known thermal resistance are employed to increase the laser module's thermal resistance to accordingly impede heat flow from the laser to its surroundings, and thereby offset the laser's temperature.
In an exemplary embodiment, the inventive method is applied to a laser module comprising a “14-pin butterfly” housing configured to contain a semiconductor laser chip that emits coherent light. To stabilize lasing with variations in temperature, a fiber grating is employed which consists of a periodic variation in the refractive index of the fiber's core. Although the laser is designed to lock over the span in temperatures for the desired application, its locking range may not cover or be coextensive with the span in temperatures for the desired application. The laser module is modified, however, to include thermal element(s) capable of increasing the laser module's thermal resistance to accordingly impede heat flow to its surroundings. Each thermal element is formed to comprise a thickness t, having a known thermal resistance R, defined as that quantity which when multiplied by the heat flux, H, entering into the thermal element yields the change in temperature.
Suitable materials for the thermal elements include, Kovar, BeO, Si, and plastics, with the thermal elements preferably fabricated as a washer. Neglecting other power sources in the laser module, the laser's temperature is approximately elevated by an amount &Dgr;T=&eegr;(VI)×R
T
, where V and I are the laser's operating voltage and current, respectively, &eegr; is the dissipation factor, and R
T
is the total thermal resistance of the thermal elements.
In accordance with the teachings of the present invention, the laser module's operating temperature range can readily be made substantially coextensive with the desired operating temperature range by simply elevating the laser's temperature by an amount, &Dgr;T, corresponding to the offset between the laser's locking range and the span in temperature for the desired application. Inasmuch as the laser's temperature, however, can only be elevated to a higher temperature, the laser should be preferably be designed to operate at a temperature range slightly above the desired span in temperatures. In this manner, the laser's temperature can always be elevated from the ambient temperature so that the laser module operates properly within the span of temperatures for the desired application.
In certain instances, the laser's locking range may not be simply offset from the span in temperatures for the desired application, but also may be narrower. In another aspect of the present invention, the laser's module's operating temperature range may be centered with the span in temperatures for the desired application, improving the laser module's centering over the desired span in temperature. Of course, in those instances where the laser's locking range is broader than, but offset from the desired temperature range, the laser's operating temperature range may be adjusted anywhere within the desired temperature span, but should preferably be centered.
In accordance with another aspect of the present invention, it is contemplated that the operating temperature range of the laser module may be extended by purposely designing the gain peak and grating wavelengths to be offset, and then elevating the laser's temperature from its surroundings. In this latter manner, the laser module's operating temperature range can be extended as well as adjusted so as to substantially operate within the span in temperatures for the desired application.
REFERENCES:
patent: 4786132 (1988-11-01), Gordon
patent: 4849719 (1989-07-01), Belek et al.
patent: 4912715 (1990-03-01), Aoki et al.
patent: 5717804 (1998-02-01), Pan et al.
patent: 5724377 (1998-03-01), Huang
patent: 5805621 (1998-09-01), Grubb et al.
patent: 6056447 (2000-05-01), Caras
W.B. Joyce et al., “Thermal Resistance of Heterostructure Lasers,”Journal of Applied Physics, vol. 46, vol. 2, Feb. 1975, pp. 855-862.
Arroyo Teresa M.
De La Rosa J.
Inzirillo Gioacchino
Lucent Technologies - Inc.
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