Laser diode module

A laser diode module in which an optical system that magnifies NFP of the laser diode by twice or more and ten times or less is disposed along an optical axis z of a laser light from a laser diode, an elliptic core GI type optical fiber of a predetermined length and an elliptic core cross section with the major radius direction and the minor radius direction being aligned with the direction x and the direction y of the NFP respectively is disposed at an image formation position of the optical system, and a single mode optical fiber connected is connected to a light emitting end of the elliptic core GI type optical fiber, the diode module providing an advantage that the design and manufacture for the optical system are simple, production cost is reduced and a laser light even at a high aspect ratio can be introduced into a single mode optical fiber at a high coupling efficiency of 60% or more.

DESCRIPTION OF PREFERRED EMBODIMENTS This invention is to be explained by way of a preferred embodiment with reference to the drawings. A laser diode module 1 shown in FIG. 1 and FIG. 2 is adapted to output a laser light emitted from a laser diode 2 by way of a single mode optical fiber 3 disposed coaxially with an optical axis z thereof. Assuming the major radial direction and the minor radial direction of NFP, which is an optical spot pattern of a laser on a light emission face 2 out of the laser diode 2 as the direction x and the direction y, an axis-symmetrical convex range (optical system) 4 for magnifying the NFP by twice or more and ten times or less is disposed along the optical axis z of a laser light. An elliptic core GI type optical fiber 5 having an elliptic core cross section with the major radial direction and the minor radial direction of the core being aligned with the direction x and the direction y respectively of NFP is located at an image formation position of the light. A single mode optical fiber 3 is connected to a light emitting end 5 out of the fiber 5 . The laser diode 2 is tightly sealed in a can type housing 6 and the laser light is adapted to be emitted to the outside through a sealing glass portion 7 . A cylindrical lens holder 8 containing an axis-symmetrical convex lens disposed to the inside is YAG-welded to the housing 6 . Further, a single mode optical fiber 3 having an elliptic core GI type optical fiber 5 fused at the top end is inserted being aligned through a ferrule 9 , and a sleeve 10 in which the ferrule 9 is inserted and secured is YAG-welded to the lens holder 8 . The elliptic core GI type optical fiber 5 is formed into a laterally long ellipsis similar with NFP such that the elliptic ratio at the core cross section is substantially equal with the aspect ratio of NFP as shown in FIG. 3 . Further, a graded index optical fibers (GI type optical fiber) having a parabolic distribution shape both in the direction x and direction y with the gradient for the distribution of the refractive index being different with each other is used as shown in FIG. 4 . Generally, a single mode light that propagates through an optical fiber periodically changes its optical spot radius in accordance with the propagation distance. In the single mode light propagating through the elliptic core GI type optical fiber 5 , the change in period and the change in the amplitude of the optical spot radius is different between the direction x and direction y in which both of changes are greater in the direction x than in the direction y. Then, when a single mode light at a wavelength identical with that of the laser light is entered from one end of this optical fiber, a light of an elliptic or circular optical spot shape determined depending on the length of the elliptic core GI type optical fiber 5 is emitted from the other end as shown in FIG. 5 . Accordingly, by properly selecting the length of the elliptic core GI type optical fiber, when a single mode light is entered, the elliptic optical spot shape of a light emitted from the other end can be made substantially equal with the NFP image, regarding the minor radius and the major radius. Since the light propagates reversibly, when an NFP image is focused at the light incident end 5 in of the elliptic core GI type optical fiber 5 , it is outputted from an light emitting end 5 out as a single mode light of a circular optical spot shape, which is then introduced into the single mode optical fiber 3 . An example for the constitution of the laser diode module 1 according to this invention is as has been described above and the operation thereof is to be explained below. Explanation is to be made for a case of introducing a laser light emitted from the laser diode 2 at a wavelength of 980 nm, with an aspect ratio of 3.5 and with a radius in the direction x: &ohgr; x0 &equals;2.43 &mgr;m and a radius direction y: &ohgr; y0 &equals;0.7 &mgr;m in NFP into a single mode optical fiber 3 having a core radius of 6.2 &mgr;m. In this specification, the extent of the optical spot means that within a range having a light intensity of 1/e 2 for the light intensity at the center. Assuming the magnifying ratio of the axis-symmetrical convex lens 4 as: m&equals;10, the distance from the light emitting face 2 a of the laser diode 2 to the axis-symmetrical convex lens 4 as d 0 and the distance from the axis-symmetrical convex lens 4 to the image formation position as d 1 , the following equation is established. d 1 &equals;md 0 Since d 1 &equals;20 mm when the d 0 &equals;2 mm, the core GI type optical fiber 5 is disposed at a position where the distance from the axis-symmetrical convex lens 4 to the light incident 5 in is 20 mm. Since the magnifying ratio is 10, the size of the NFP real image at the image formation position is such that the radius in the direction x: &ohgr; x &equals;24.3 &mgr;m and radius in the direction y: &ohgr; y &equals;7.0 &mgr;m. On the other hand, when a single mode light at a wavelength &lgr;is entered to the elliptic core GI type optical fiber 5 , the radius in the direction x: &ohgr; x and the radius in the direction y: &ohgr; y change along with propagation in the core as shown in FIG. 5 . Assuming the change of period in the direction x as Cx, and the max value and the Min value for radii of the spots as &ohgr; x1 and &ohgr; x2 , the following equations are established: Cx &equals;2&pgr; *a x /{square root}(2&Dgr;) &ohgr; x1 *&ohgr; x2 &equals;(&lgr;/ n 1 &pgr;)* a x /{square root}(2&Dgr;) where &Dgr;&equals;( n 1 −n )/ n 2 a x :core radius n 1 :refractive index at the center core n 2 :refractive index at the clad &lgr;:wavelength The same equations as direction x are established about values of direction y. In this embodiment, when the single mode light at a wavelength of 980 nm, with a spot radius &ohgr; x1 &equals;&ohgr; y1 &equals;3.10 &mgr;m is entered into the elliptic core GI type optical fiber 5 having &Dgr;&equals;1.0%, and an elliptic spot with radius in the direction x:&ohgr; x2 &equals;24.3 &mgr;m and radius in the direction y:&ohgr; y2 &equals;7.0 &mgr;is intended to the outputted, the core radius a x and a y and the core length L can be determined based on the above equations as: a x &equals;49.6 &mgr;m a y &equals;14.3 &mgr;m L &equals;550.9 &mgr;m Accordingly, when an elliptic core GI type optical fiber 5 having a core with the radius in the direction x of 99.2 &mgr;m, the radius in the direction y of 28.6 &mgr;m and a length of 550.9 &mgr;m is fused to the top end of a single mode optical fiber 3 and a laser light of an NFP real image is entered to the light incident end 5 in of the elliptic core GI type optical fiber 5 , the light propagates reversibly and a single mode light at a 980 nm is emitted from the light emitting end of the fiber 5 and then introduced into a single mode fiber 3 of 6.2 &mgr;m core radius. In case where the laser light emitted from the laser diode 2 diverges at a radiation angle of 30°, the radius in the direction y is about 2.3 mm at the position for the lens 4 . Since the image formation position is spaced apart by 20 mm from the lens 4 , the angle of condensed light is about 6.5°, which is smaller than the critical incident angle of 11° determined by the number of aperture NA of 0.2 of the quartz fiber. Further, since the radiation angle in the direction x is smaller than that in the direction y, the angle of condensed light is naturally 6.5° or less which is also smaller than the critical incident angle of 11°. Therefore, most of the light after transmitting the lens is incident to the elliptic core GI type optical fiber 5 with scarce coupling loss. As described above, an overall coupling efficiency as high as 80% can be obtained that exceeds the theoretical limit of 60% in a case of using the axis symmetrical lens system. When the outer profile of the elliptic core GI type optical fiber 5 is made identical with the outer profile of the single mode optical fiber 3 , they can be fused more easily to each other. In this case, when the magnification ratio of the axis-symmetrical convex lens 4 is made greater than ten times, the distance from the axis-symmetrical convex lens 4 to the light incident end 5 in of the elliptic core GI type optical fiber 5 exceeds 20 mm in practical use and the module is enlarged in the size and increased in the cost. In addition, an allowable angle of inclination of the elliptic core GI type optical fiber 5 is narrowed as ±0.3° within a range of the coupling loss of 1 dB, which worsens the yield upon assembling the laser module. On the other hand, in a case of using an axis-symmetrical convex lens 4 having a magnifying ratio of less than twice and having a radius capable of receiving 90% of the laser light emitted from the laser diode, since the angle of condensed light is enlarged, it may sometimes cause a trouble that light can not be taken into the elliptic core GI type optical fiber 5 having NA&equals;0.2. In addition, an allowable tolerance for the positional displacement in the orthogonal direction of the elliptic core GI type optical fiber 5 is narrowed as ±0.7 &mgr;m within the coupling loss of 1 dB, which worsens the yield upon assembling. Accordingly, the magnifying ratio of the lens is limited within 2 to 10 times and, preferably, 3 to 5 times. In the foregoings, although explanations have been made to a case where the axis-symmetrical convex lens 4 is disposed to the outside of the housing 6 , it may be disposed within the housing 6 . Further, although the axis-symmetrical convex lens 4 is explained as a separate member, it may be integrally formed with the sealing glass portion 7 of the housing 6 . Furthermore, enlarging optical system is not limited to a single lens system of the axis-symmetrical convex lens 4 but plural lens may be used in combination. As has been described above, this invention can provide an excellent effect, capable of converting most of laser light irradiated in an elliptic optical spot shape into a circular spot shape and introducing the same at an extremely high coupling efficiency into the single mode optical fiber even in a case where the lens is inevitably spaced from the laser diode as in a laser diode tightly sealed in a housing. Further, since a lens, a single mode optical fiber having an elliptic core GI type optical fiber coupled at the top end may be fixed successively to the outside of the housing relative to the laser diode sealed within the housing, it can also provide an advantage that the design is easy and labors of manufacture can be moderated, to decrease the manufacturing cost. The present disclosure relates to subject matter contained in priority Japanese Patent Application No. 2000-377,679 filed on Dec. 12, 2000, the contents of which is herein expressly incorporated by reference in its entirety.