Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2000-05-17
2003-01-07
Bovernick, Rodney (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S093000
Reexamination Certificate
active
06504975
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a semiconductor laser module and a coupling lens used therein. Particularly, the present invention relates to a low power semiconductor laser module and a coupling lens that are suitable for a short-distance optical communication system.
BACKGROUND ART
In a semiconductor laser module used for optical communication, it is required to couple a semiconductor laser or receiving optics and an optical fiber efficiently.
FIG. 23
shows a configuration of a conventional laser module. A semiconductor laser
2302
and a coupling lens
2303
are fixed to a lens holder
2301
. The lens holder
2301
is inserted into a connecting holder
2304
. To this connecting holder
2304
, a ferrule holder
2307
is fixed. An optical fiber
2305
is fixed removably by a ferrule
2306
. The coupling lens
2303
is made of glass. Abeam of light emitted from the semiconductor laser
2302
is focused on an end face of the optical fiber
2305
by the coupling lens
2303
, thus being coupled to the fiber.
In this case, from a safety aspect in handling, it is necessary to restrict the optical output from the laser module to a certain level or lower. Therefore, in addition to the basic configuration shown in
FIG. 23
, the laser module is provided with: a means for reducing the optical output, such as an attenuation film, a polarizer, or the like, between the semiconductor laser and the optical fiber (for instance, JP 4-97208 A, JP 7-43563 A, or the like); an aperture to control the quantity of light; or a control circuit for stopping emission of the semiconductor laser automatically when the optical fiber comes off.
As a condenser lens, an aspheric lens made of glass has been used conventionally, but for the purpose of cost reduction, a resin lens has come to be used (for instance, JP 5-60952 A, JP 61-245594 A, JP 5-27140 A, JP 5-60940, or the like). In the case of using the resin lens, a refractive index of resin varies with variation in temperature, resulting in variation in the focal length. In addition, its coefficient of thermal expansion is higher than that of a glass material. Therefore, an imaging position varies with variation in temperature, resulting in variation in the coupling efficiency to an optical fiber.
In the semiconductor laser module, an emission wavelength of the semiconductor laser as a light source also varies with variation in temperature. Therefore, it is conceivable that by providing a diffraction lens on the surface of the lens, the variation of focal length of the resin lens with temperature is corrected by the diffraction lens. The reason is that since a focal length of the diffraction lens varies greatly depending on a wavelength of the light source compared to that of a refractive lens, the focal length of the diffraction lens varies more than that of the refractive lens when the wavelength of the light source varies with variation in temperature. In other words, by employing a design enabling the variation in focal length of the diffraction lens to cancels out the variation of focal length with the temperature of the refractive lens, a lens in which the variation in focal length due to variation in temperature has been compensated can be obtained.
In the above-mentioned method of using an attenuation film, a polarizer, or the like for restricting the optical output from the laser module to a certain level or lower, manufacturing cost is high. In the method of controlling the quantity of light by an aperture, the quantity of light varies depending on the processing accuracy of the aperture or variation in flare angle of the laser, and in addition, high assembly accuracy is required. Further, the method of providing a control circuit for stopping emission of the laser automatically when the optical fiber comes off is not preferable, since not only the configuration of a device as a whole becomes complicated but also the manufacturing cost is high.
When a resin lens is used for reducing a lens cost, a diffraction lens is formed on the surface of the lens so as to correct the variation in focal length due to the variation in temperature since the variation in refractive index due to variation in temperature is greater in a resin lens compared to that in a glass lens. As a method of manufacturing such a one-piece lens in which a diffraction lens is integrated, a method of processing a lens or a mold for manufacturing a lens by precision cutting using a diamond bit has been used widely. In this case, since the tip of the diamond bit has finite roundness (a nose radius), a processed relief comes to have a shape with edges rounded due to the nose radius of the bit. In a diffraction lens, generally a sawtooth relief shape is used in many cases. However, when a sawtooth relief is processed by the above-mentioned cutting, the diffraction efficiency at the periphery of the lens deviates from a design value.
This is because a pitch of zones of the diffraction lens becomes shorter at the periphery of the lens, and therefore the influence of deterioration in the relief shape caused by the nose radius of the bit becomes more serious at the periphery compared to the center portion of the lens having a longer pitch. In a lens for optical fiber coupling, when the diffraction efficiency at the periphery of the lens decreases, the same effect as that in the case where an effective NA of the lens is decreased is provided and therefore a spot is enlarged, thus causing the decrease in coupling efficiency to an optical fiber. In order to prevent this, there is a method of using a bit for processing with a sharp tip, but it causes the decrease in productivity, which is therefore not preferable. In order to carry out the processing without impairing the productivity, it is said to be desirable that a bit has a nose radius of at least about 10 &mgr;m. In addition, a diffraction lens for temperature compensation requires higher power (refracting power) than that of a diffraction lens for so-called chromatic-aberration correction and as a result, has a shorter pitch of diffraction zones at the periphery of the lens compared to the diffraction lens for chromatic-aberration correction. Therefore, in a conventional diffraction lens having a sawtooth relief shape, there has been a problem that the compatibility between the productivity of the lens and mold and the diffraction efficiency performance cannot be obtained.
When using a diffraction lens, a plurality of focal spots corresponding to respective diffraction orders are generated on an optical axis. In order to reduce the intensity of light emitted from a module, an aperture is provided at a position of a focal spot corresponding to a diffraction order used for coupling, thus intercepting lights focused on focal spots corresponding to unnecessary diffraction orders. In this case, lights focused on focal spots with longer focal lengths than that of a focal spot corresponding to the diffraction order used for coupling tend to go through the aperture compared to lights focused on focal spots with shorter focal lengths. In the case of a conventional diffraction lens with a sawtooth relief, when its shape is varied by cutting, unwanted lights tend to be focused on the focal spots corresponding to lower orders and cannot be intercepted by the aperture completely, which has been a problem.
DISCLOSURE OF THE INVENTION
In view of the above points, the present invention is intended to provide an inexpensive optical coupler in which the diffraction efficiency of a diffraction lens formed on a surface of a lens is adjusted and therefore an attenuation film or a polarizer for restricting optical output from a laser module to a certain level or lower, or a control circuit for stopping emission automatically when an optical fiber comes off is not required.
In order to achieve the above-mentioned object, the present invention employs the following configurations.
A coupling lens according to a first configuration of the present invention is used for coupling a beam of light emitted from a semiconductor laser to an optic
Asakura Hiroyuki
Sasano Tomohiko
Tanaka Yasuhiro
Yamagata Michihiro
Bovernick Rodney
Matsushita Electric - Industrial Co., Ltd.
Merchant & Gould P.C.
Stahl Mike
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