The present invention relates to a light source-optical fiber coupler for coupling light emitted from a light source onto an end surface of an optical fiber by a gradient index rod lens and particularly to a light source-optical fiber coupler in which a light source and a gradient index rod lens having a planar end surface facing the light source and a spherical end surface facing an optical fiber are arranged closely to each other to thereby make it possible to reduce the size of the coupler. 
In an optical communication system or the like, a lens is used so that a diffused luminous flux emitted from a light source such as a semiconductor laser or the like is transmitted to an optical fiber core by the lens.  Heretofore, a spherical lens, an aspherical lens, a gradient index rod lens, or the like, has been used as the lens to perform optical coupling. 
The spherical lens can be produced most inexpensively but is insufficient from the point of view of a characteristic to optically couple a semiconductor laser to a single-mode optical fiber with low loss.  This is because the core diameter of the single-mode optical fiber is so small that strict performance is required of the aberration of the lens in order to improve optical coupling efficiency Therefore, an aspherical lens is generally used as a highly coupled lens. 
On the other hand, when a gradient index rod lens is used, the rod lens is used in the condition a semiconductor laser side end surface of the rod lens is provided as a convex spherical surface and an optical fiber side end surface of the rod lens is provided as a planar surface because a high numerical aperture (NA) is required for coupling of light emitted from the semiconductor laser (see JP-A-61-107207).  This is based on the idea that light-condensing power is obtained by the convex spherical surface while the aberration generated in the convex spherical surface is cancelled by the refractive-index distribution shape of the rod lens. 
In order to produce an aspherical lens, however, a mold was required in accordance with the kind of the aspherical lens.  A heat-resistant material and superfine machining were required for the production.  Hence, there was a problem that the cost of the lens increased particularly in the case of multikind and small quantity production. 
On the other hand, in the case of a gradient index rod lens having a convex spherical surface on a semiconductor laser side, it was difficult to produce a high numerical aperture lens because the controllable range of the refractive-index distribution was narrow.  Moreover, the lens length of the rod lens became long, so that the material cost for the lens increased. 
An object of the present invention is to provide a light source-optical fiber coupler which can couple a semiconductor laser to a single-mode optical fiber with low loss and which can be produced as a small-size coupler inexpensively and easily. 
A subject of the present invention is a light source-optical fiber coupler using a gradient index rod lens for coupling a diffused luminous flux emitted from a light source onto an end surface of an optical fiber.  Here, the gradient index rod lens has a planar end surface on the light source side, and a convex spherical end surface on the optical fiber side.  The gradient index rod lens has a light source side numerical aperture NA2 in a range of from 0.40 to 0.75, an effective lens radius r0 in a range of from 0.3 to 1.0 mm, and a spherical curvature radius R1 in a range of from 1.2 to 2.0 mm. 
In the present invention, a semiconductor laser is generally used as the light source.  When a single-mode optical fiber having a small diameter is used as the optical fiber which is a partner of optical coupling, the highly efficient optical coupling effect is particularly remarkable.  In the present invention, the planar surface is used on the semiconductor laser side and the convex spherical surface is used on the optical fiber, so that high numerical aperture is attained and the aberration of light can be corrected by the refractive-index distribution shape of the rod lens.  Hence, reduction in size of an optical module can be achieved when such a gradient index rod lens having a small lens diameter is used. 
Preferably, the gradient index rod lens used in the present invention has a center axis refractive index n0 in a range of from 1.5 to 1.8, a value of n0xc2x7gxc2x7r0 in a range of from 0.40 to 0.65 (in which a is a secondary refractive-index distribution coefficient), and a value of Z/R1 in a range of from 1.1 to 2.5 (in which Z is the lens length of the rod lens) .  Preferably, the gradient index rod lens has a value of g in a range of from 0.38 to 0.50, a value of h4 in a range of from xe2x88x920.2 to 1.8, a value of h6 in a range of from xe2x88x920.5 to 10, and a value of h8 in a range of from xe2x88x9225 to 45 (in which h4, h6 and h8 are higher-order refractive-index distribution coefficients respectively). 
A practical optical module structure may be preferably configured as follows.  A semiconductor laser is used as the light source.  A chip of the semiconductor laser and the gradient index rod lens are retained in a housing in the condition that they are arranged closely to each other.  The housing is provided so that a ferrule of an optical plug which is used as a mating partner and in which a single-mode optical fiber is retained can be fitted and retained in the housing. 
The present disclosure relates to the subject matter contained in Japanese patent application No.  2000-326471 (filed on Oct. 26, 2000), which is expressly incorporated herein by reference in its entirety.