Electric heating – Metal heating – By arc
Reexamination Certificate
1998-04-22
2001-04-17
Evans, Geoffrey S. (Department: 1725)
Electric heating
Metal heating
By arc
C385S092000
Reexamination Certificate
active
06218641
ABSTRACT:
TECHNICAL FIELD
The present invention relates to optical modules and, more particularly, to a method for compensating for stress induced in optical components contained within such optical modules.
BACKGROUND OF THE INVENTION
Optical modules, such as transmitters, typically house optical components hermetically in a box, such as a so-called “14-pin butterfly” housing or package. Generally, such optical modules include a laser configured to emit polarized radiation for communications purposes. Preferably, an optical isolator is disposed within or near the laser so as to suppress optical feedback caused by unwanted radiation reflected or scattered back into the laser. Optical isolators are well known in the art and typically comprise a Faraday rotator disposed between two crossed polarizers. See, for example, S. Makio et al.,
Electronics and Communications in Japan
, Part 2, Vol. 74(2), p. 323 (1991).
Recently developed optical isolators, however, employ a distributed configuration so as to advantageously adjust the optical isolation performance for a given radiation wavelength. See, for example, U.S. Pat. No. 5,737,349, entitled “Optical Isolator and Alignment Method,” which is commonly assigned and incorporated herein by reference. In this latter instance, the output polarizer or so-called “analyzer” is disposed within an external retainer that is welded to an opening in an end sidewall of the housing. A lens or window disposed within the opening, in part, hermetically seals the module and couples the light to the exterior of the housing. Alternatively, the analyzer is disposed within the sidewall, such as in a tube.
During assembly, the transmission axis of the output polarizer is rotated so as to maximize the optical isolation, with the polarizer then secured in place by welding the retainer to the housing. Any induced stress caused from such welding, or from any other source, however, can be transmitted to the lens or window so as to induce a birefringence. This stress-induced birefringence is typically the most dominant depolarization mechanism responsible for limiting the achievable optical isolation in such optical modules.
It would therefore be desirable to provide an optical module wherein stress induced in the optical components is substantially eliminated or reduced, particularly for optical modules employing distributed optical isolators.
SUMMARY OF THE INVENTION
The present invention relates to a method for compensating for stress induced in optical components used to couple egressing light to the exterior of the housing of an optical module employing, for example, a distributed optical isolator. It has been discovered that the induced stress may be counteracted by transmitting a balancing or compensating force or stress to the optical component. The method uses circumferential laser welds judiciously formed at a distance from the holder housing the optical component so as to control the magnitude and direction of the applied compensating force. Preferably, the effects of the induced stress are monitored during welding, and based upon any changes therein pairs of laser welds are formed around the circumferential surface of the housing of the optical component until the induced stress is substantially reduced or eliminated. Alternatively, a predetermined pattern of laser welds can be formed around the surface, which pattern has been determined empirically to reduce or eliminate the induced stress for the particular type of optical module.
In an exemplary embodiment, the inventive method is applied to a laser module comprising a housing configured to contain a laser that emits polarized radiation. The housing includes a base wall and sidewalls. An optical isolator is employed to suppress optical feedback caused by any unwanted radiation being reflected back into the laser. The optical isolator employs a distributed configuration comprising an input polarizer and a Faraday rotator, defined as a “semi-isolator.” Also, the optical isolator comprises an output polarizer or “analyzer” held in an external retainer aligned radially with a tube brazed to the front wall of the housing. A lens is hermetically sealed within the front end of the tube so as to couple the light egressing from the laser to the exterior of the housing. In order to ensure that the relative position of the output polarizer remains fixed, the retainer is welded to the tube along flush points where the edges coincide.
Induced stress, such as from joining the lens to the tube, brazing the tube to the housing, or welding the retainer to the tube can be transmitted to the lens vis-a-vis a radial force transmitted along the tube, causing a stress-induced birefringence. This stress-induced birefringence may be counteracted by transmitting a balancing or compensating force or stress to the lens using laser welds formed around the tube of the optical component. In forming such laser welds, the outer portion of the tube is heated to a molten state forming a weld pool which is then allowed to cool. The weld pool, however, contracts unevenly with respect to the underlying material, transmitting along the tube a balancing force or counter-stress to the lens.
That is, the inventive method transmits a compensating or opposing force to compensate for the undesired induced stress. To do so in this latter embodiment, the magnetization field in the Faraday rotator is first reversed. With the laser turned on, the amount of light passing through the output polarizer is then measured and recorded. Next, pairs of laser welds, preferably 180° radially apart, are formed along the circumferential seam of the retainer and tube so as to apply a compensating force to the lens. Each subsequent pair of laser welds is radially offset from the last pair until the amount of power transmitted through the output polarizer no longer decreases. In this manner, the magnitude and direction of the compensating force transmitted to the lens is controlled, thereby substantially reducing the amount of or eliminating the stress-induced birefringence.
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Gaebe Carl Edward
Schweizer Rudolph Conrad
Shakespeare Walter Jeffrey
Young Christopher Eugene
De La Rosa J.
Evans Geoffrey S.
Lucent Technologies - Inc.
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