Abstract:
A laser diode module has a laser diode, a lens for condensing the laser beam from the laser diode, an optical fiber for receiving the laser beam from the lens at the end face thereof, and support assembly for supporting the laser diode, the lens, and optical fiber, the support assembly positions the end face of the optical fiber at a deviation position apart from the focal point of the lens by an allowable distance when a temperature of the laser diode module is room temperature and moves the end face of the optical fiber at a close position relatively closer to the focal point than the deviation position when the temperature of the laser diode module rises from the room temperature.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a laser diode module which is suitably used in the field of optical communications and optically couples a laser diode to an optical fiber.  
           [0003]    2. Description of the Related Art  
           [0004]    A laser diode module (hereinafter called “LD module”) in which a laser diode (hereinafter abbreviated as “LD”) and an optical fiber are optically coupled to each other in a manner of condensing a laser beam from the LD on an inlet end face of the optical fiber through a lens. The optical fiber delivers the received laser beam from an outlet end face thereof. At the time of manufacturing an LD module, alignment including optical axis alignment and focus point adjustment among the LD, the lens and the optical fiber is carried out with a precision of 1 μm or less in order to acquire the desired optical output. The alignment is executed while monitoring the optical output of the LD module. That is, to acquire the maximum amount of light from the outlet end face of the optical fiber, the inlet end face of the optical fiber is positioned at the focal point of the lens where the laser beam is condensed. The aim of such alignment is to minimize the adverse influence of a variation in the working precision of the constituting members with respect to the performance of the LD module.  
           [0005]    When the support members that support the constituting elements of the LD module, such as the LD, the lens and the optical fiber, are heated due to the heat generation of the LD itself or a rise in the environmental temperature of the LD module, those support members thermally expand, deviating the relative positions of the LD, the lens and the optical fiber that have been aligned in the order of microns. The deviation in the relative positions lowers the optical coupling efficiency between the LD and the optical fiber (hereinafter simply called “optical coupling efficiency”) and results in reduction of the optical output of the LD module. Specifically, the temperature of the support members comes in a range of 60° C. to 90° C., for example, the optical output of the LD module declines prominently. The reduction in the optical output of the LD module may disable optical communications using the LD module.  
           [0006]    To cope with the problem, a typical LD module is equipped with a temperature control device, such as a Peltier device. The temperature control device discharges the thermal energy originated from the heat generation of the LD or a rise in environmental temperature out of the LD module.  
           [0007]    However, the LD module equipped with a temperature control device has following shortcomings. First, because of the large size, this type of LD module cannot be housed in a package for a communication transceiver which demands miniaturization. Further, the LD module needs a power source for driving the temperature control device or the Peltier device and an external control circuit for controlling the operation of the Peltier device. The power source and external control circuit result in an increase in the cost of the LD module.  
           [0008]    Moreover, when the performance of the temperature control device is insufficient, the temperature of the LD module, i.e., the temperature of the support members inevitably rises.  
           [0009]    Accordingly, it is an object of the present invention to provide an LD module which suppresses a reduction in optical output originated from the heat generation of an LD or a rise in ambient temperature and whose cost reduction and miniaturization can be achieved.  
         SUMMARY OF THE INVENTION  
         [0010]    A laser diode module according to the present invention comprises a laser diode having an end face from which a laser beam is emitted; a lens which has an input-side principal plane and an output-side principal plane, and condenses the laser beam from the laser diode at a focal point thereof; an optical fiber having an end face for receiving the laser beam condensed by the lens and delivering the received laser beam as an output of said module; and support means for supporting the laser diode, the lens and the optical fiber and securing an inlet distance between the end face of the laser diode and the input-side principal plane of the lens and an outlet distance between the output-side principal plane of the lens and the end face of the optical fiber. The support means positions the end face of the optical fiber at a deviation position apart from the focal point of the lens by an allowable distance with respect to the output of said module when a temperature of the laser diode module is room temperature and moves the end face of the optical fiber at a close position relatively closer to the focal point than the deviation position when the temperature of the laser diode module rises from the room temperature. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitative of the present invention, and wherein:  
         [0012]    [0012]FIG. 1 is a longitudinal cross-sectional view of an LD module  6 A according to one embodiment of the present invention;  
         [0013]    [0013]FIG. 2 is a diagram for explaining a change in the focal point of the LD module  6 A in FIG. 1, which is caused by a rise in environmental temperature;  
         [0014]    [0014]FIG. 3 is a diagram for explaining relative positions of individual members that constitute the LD module  6 A in FIG. 1;  
         [0015]    [0015]FIG. 4 is a longitudinal cross-sectional view of an LD module  6 B according to another embodiment of the invention;  
         [0016]    [0016]FIG. 5 is a diagram for explaining a change in the focal point of the LD module  6 B in FIG. 4, which is caused by a rise in environmental temperature;  
         [0017]    [0017]FIG. 6 is a diagram for explaining relative positions of individual members that constitute the LD module  6 B in FIG. 4;  
         [0018]    [0018]FIG. 7 is a longitudinal cross-sectional view of an LD module  6 C according to a further embodiment of the present invention;  
         [0019]    [0019]FIG. 8 is a diagram for explaining a change in the focal point of the LD module  6 C in FIG. 7, which is caused by a rise in environmental temperature; and  
         [0020]    [0020]FIG. 9 is a diagram for explaining relative positions of individual members that constitute the LD module  6 C in FIG. 7. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    In this description, an “inlet distance” indicates a distance between an end face of an LD from which a laser beam is emitted and an input-side principal plane of a lens. An “outlet distance” indicates a distance between an output-side principal plane of the lens and an end face of an optical fiber which receives the laser beam that is condensed by the lens.  
         [0022]    In this description, a “first link member” is a member that supports the LD and the lens and determines a relative distance of the end face of the LD and the input-side principal plane of the lens.  
         [0023]    In this description, a “second link member” is a member that supports the lens and the optical fiber and determines a relative distance of the output-side principal plane of the lens and the end face of the optical fiber.  
         [0024]    Each of the first and second link members may comprise a plurality of constituting elements.  
         [0025]    In this description, an “environmental temperature” is a temperature of a local ambient at which the individual members constituting the LD module are exposed. A “standard temperature” indicates a reference temperature at the time the LD module is assembled. For example, room temperature of 18° C., 20° C., 25° C. or the like can be selected as the reference temperature. When the environmental temperature is the standard temperature, the LD can keep a high optical output. The upper limit of temperature at which the LD module is used is restricted within a temperature range of 60 to 90° C.  
         [0026]    An LD module  6 A in FIG. 1 is a receptacle module provided with an LD 1 . The LD  1  has an end face  1   a  from which a laser beam is emitted.  
         [0027]    As shown in FIG. 1, the LD 1  is mounted on a pedestal  12  via a heat sink  14 . For example, the pedestal  12  and the heat sink  14  are made of an iron-based alloy and copper, respectively. The heat sink  14  facilitates heat discharge from the LD  1 . The heat sink  14  may be made of aluminum or diamond.  
         [0028]    The pedestal  12  is secured to the inner surface of an end plate  11  which has a disk-like shape. A photodiode (hereinafter abbreviated as “PD”)  90  is attached onto the inner surface of the end plate  11 . The PD  90  monitors a light emission state of the LD  1 . Specifically, the PD  90  detects a backward radiation from the LD  1  toward the end plate  11 . The backward radiation is significantly smaller than the laser beam emitted from the end face  1   a  of the LD  1 . A plurality of lead pins  91  to  93  are protruded outward from the outer surface of the end plate  11 , and are electrically connected to the LD  1  and the PD  90 .  
         [0029]    The LD  1  and the PD  90  are enclosed by a cap  15  of Fe—Ni—Co alloy called KOVAR(Trade Mark). The cap  15  has a hollow cylindrical shape with its one end secured to the inner surface of the end plate  11 . An inner flange  15   b  is integrally formed at the other end of the cap  15 . The inner flange  15   b  defines a circular window  15   c  at the other end of the cap  15 . Glass  7   a  is adhered to the window  15   c  to permit the transmission of the laser beam from the LD  1 . The end plate  11 , the cap  15  and the glass  7   a  define an airtight chamber, which is filled with an inactive gas.  
         [0030]    A hollow cylindrical lens holder  35  is coupled to the other end of the cap  15  via the inner flange  15   b . The lens holder  35  is made of a stainless steel material, for example, SUS 304 that is defined by JIS (Japanese Industrial Standard). A through hole  35   f  of the lens holder  35  is open to both end faces  35   d  and  35   e  thereof. The lengthwise center portion of the hole through hole  35   f  is defined as a small-diameter portion  35   a . A sphere lens  3  is partly fitted in one end of the small-diameter portion  35   a  on the LD  1  side or the end face  35   e  side. The sphere lens  3  is fixed to the lens holder  35  by cement  7   b  of low-melting point glass. The other opening end of the through hole  35   f  communicates with a ferrule sleeve  42 . The ferrule sleeve  42  is mounted on a small-diameter portion of a ferrule holder  41  which will be discussed later. A chamber  35   c  is defined between the other end of the small-diameter portion  35   a  and the ferrule sleeve  42  in the lens holder  35 . The chamber  35   c  is needed for alignment operation of the ferrule holder  41 .  
         [0031]    The lens  3  has a focal distance or a focal point. The output-side principal plane and input-side principal plane of the lens  3  are both denoted by symbol “ 3   a ”, and those principal planes  3   a  lie in the same plane. The principal planes  3   a  are perpendicular to an optical axis  8  indicated by a one-dot chain line. The optical axis  8  extends from the LD  1  and passes through the center of the lens  3 . The lens  3  may be a spherical lens, an aspherical lens, a rod lens or a combination of those lenses. The lens  3  may be secured to the holder  35  via a fixing member instead of the cement  7   b . In this case, the shape of the fixing member is not particularly limited.  
         [0032]    The ferrule sleeve  42  made of, for example, SUS 304 is coupled to the holder  35  so that the other end face  35   d  of the holder  35  and an end face  42   b  of the ferrule sleeve  42  keep in contact with each other. More specifically, the holder  35  and the ferrule sleeve  42  are coupled to each other by laser-welding the peripheries of the butted end faces  35   d  and  42   b  using a YAG laser after alignment to be discussed later is performed. The ferrule sleeve  42  is a nearly circular cylinder with a flange  42   a  for securing the end face  42   b  at the one end. The ferrule holder  41  is fitted in the ferrule sleeve  42 . One end of the ferrule holder  41  slightly protrudes into holder  35  while the other end of the ferrule holder  41  significantly sticks out of the sleeve  42 .  
         [0033]    After the alignment to be discussed later is executed with the ferrule holder  41  fitted in the ferrule sleeve  42 , the ferrule holder  41  is connected to the sleeve  42  by laser welding using the YAG laser. The holder  41  made of, for example, SUS 304 has a stepped hollow cylindrical shape. More specifically, the holder  41  has a small-diameter portion  41   b  which is fitted in the sleeve  42 , and a large-diameter portion  41   a  which sticks out of the sleeve  42 . The large-diameter portion  41   a  forms a flange. A stepped through hole is formed coaxial to the optical axis  8  in the ferrule holder  41 . The through hole has a small-diameter hole portion  41   c , an intermediate-diameter hole portion  41   d  and a large-diameter hole portion  41   e  in order from the end face side of the small-diameter portion  41   b.    
         [0034]    A ferrule  4   b  is retained concentrically in the ferrule holder  41  and has its one end fitted in the small-diameter hole portion  41   c . A pipe sleeve  45  is fitted between the ferrule  4   b  and the inner surface of the intermediate-diameter hole portion  41   d . The pipe sleeve  45  protrudes from the other end of the holder  41 . A sleeve retainer  47 , i.e., a small-diameter end portion  47   a  of the retainer  47  is fitted between the pipe sleeve  45  and the inner surface of the large-diameter hole portion  41   e . The large-diameter portion of the sleeve retainer  47  encloses the portion of the pipe sleeve  45  that protrudes from the holder  41 .  
         [0035]    The ferrule  4   b , which is made of zirconia, has a through hole in its center. An optical fiber  4  is inserted in the through hole of the ferrule  4   b  and is coupled to the ferrule  4   b  by an adhesive. One end face  4   a  of the optical fiber  4  on the lens  3  side is positioned in approximately the same plane as the other end face  35   d  of the lens holder  35 . Both end faces of the ferrule  4   b  are polished together with the optical fiber  4 . Specifically, one end face  4   a  of the ferrule  4   b  is polished at a bevel to prevent light reflection, and the other end face  4   c  of the ferrule  4   b  is rounded for PC (Physical Contact) connection to another optical fiber  5 .  
         [0036]    The fixing of the ferrule  4   b  and the pipe sleeve  45  to the holder  41  and the fixing of the holder  41  and the sleeve retainer  47  are carried out as follows. First, the optical fiber  4  is connected to the ferrule  4   b  by an adhesive. Then, the pipe sleeve  45  is attached to the ferrule  4   b . Then, the ferrule  4   b  and the pipe sleeve  45  are respectively fitted in the small-diameter hole portion  41   c  and the intermediate-diameter hole portion  41   d  of the holder  41 . Then, the inserting end portion  47   a  of the sleeve retainer  47  is pressed into the large-diameter hole portion  41   e  of the holder  41 .  
         [0037]    The sleeve retainer  47  has a plug hole  47   b  which has the same diameter as that of the small-diameter hole portion  41   c . At the time the LD module  6 A is actually used, a ferrule  5   b  with the optical fiber  5  is inserted into the plug hole  47   b  so that the optical fiber  5  is connected by PC to the optical fiber  4 . For example, the ferrule  5   b  is an SC ferrule, FC ferrule, LC ferrule or the like.  
         [0038]    In manufacturing the LD module  6 A, alignment of the LD module  6 A is carried out in the following procedures.  
         [0039]    First, instead of the ferrule  5   b  with the optical fiber  5 , a connector plug (not shown), which has an optical fiber for monitoring the optical output of the LD module  6 A, is inserted in the plug hole  47   b  of the sleeve retainer  47 . The optical fiber of the connector plug is connected by PC to the optical fiber  4 . Therefore, the laser beam emitted from the LD  1  is delivered from the optical fiber of the connector plug via the lens  3  and the optical fiber  4 .  
         [0040]    In this state, before connection of the ferrule sleeve  42  to the lens holder  35 , the ferrule sleeve  42  is moved in a direction perpendicular to its axial direction while the contact of the end face  42   b  of the ferrule sleeve  42  to the end face  35   d  of the lens holder  35  is maintained. This changes the radial position of the ferrule sleeve  42  with respect to the lens holder  35  or the optical axis. When a maximum of the optical output from the optical fiber  4  is detected based on the output change of the laser beam delivered from the optical fiber of the connector plug, the relative position between the ferrule sleeve  42  and the lens holder  35  is determined. At this time, the lens holder  35  and the ferrule sleeve  42  are connected together by the YAG laser as mentioned earlier. As a result, the optical fiber  4  is accurately positioned on the optical axis  8 , which completes the optical axis alignment.  
         [0041]    Next, positioning of one end face  4   a  of the optical fiber  4  with respect to the focal point  7  of the lens  3  is performed. In this positioning, the ferrule holder  41  is moved in the direction of the optical axis in the ferrule sleeve  42 . When a maximum output of the optical output from the optical fiber  4  is detected based on the output change of the laser beam delivered from the optical fiber of the connector plug, the axial position of one end face  4   a  of the optical fiber  4  or the ferrule holder  41  is located in connection with the focal point  7 . Thereafter the end face  4   a  of the optical fiber  4  is moved by a preset distance c from the located position, the ferrule sleeve  42  and the ferrule holder  41  are connected together by laser welding using the YAG laser. This completes the positioning of the optical fiber  4  to the focal point  7 , i.e., the focus point adjustment.  
         [0042]    The distance C is determined in such a way that the optical output of the LD module  6 A at the standard temperature does not get lower than the value defined by the specification of the LD module  6 A in consideration of the influence of a rise in environmental temperature on the LD module  6 A.  
         [0043]    More specifically, when the environmental temperature becomes higher than the standard temperature by ΔT, as shown in FIG. 2, the inlet distance a between the output-side end face  1   a  of the LD  1  and the input-side principal plane  3   a  of the lens  3  changes by a change Δa with the input-side principal plane  3   a  as a reference due to the thermal expansions of the pedestal  12 , the cap  15  and an area A (see FIG. 1) of the lens holder  35  between the input-side principal plane  3   a  of the lens  3  and the one end  35   e  of the lens holder  35  (where Δa≧0). The change Δa is obtained by the following equations (1), (2) and (3) for a model shown in FIG. 3 which is a simplified illustration of the LD module  6 A in FIG. 1.  
               Δ                 a                =       (       L15   ×   α15     +     L35A   ×   α35     -     L12   ×   α12       )     ×   Δ                 T             (   1   )                            =     L0   ×   α0   ×   Δ                 T               (   2   )                            =     k1   ×   Δ                 T               (   3   )                               
 
         [0044]    In the equation (1), L12, L15 and L35A respectively indicate the lengths of the pedestal  12 , the cap  15  and the area A of the lens holder  35  along the optical axis as shown in FIG. 3, and α12, α15 and α35 respectively indicate the coefficients of linear expansion of the pedestal  12 , the cap  15  and the lens holder  35 . L0 and α0 in the equation (2) are respectively an equivalent length and an equivalent coefficient of linear expansion of the pedestal  12 , the cap  15  and the area A of the lens holder  35  when these three parts are considered as a single member. Given that L0×α0 is k1, k1 indicates a unit change (mm/° C.) in the inlet distance a or a thermal expansion rate when the environmental temperature rises by 1° C. Therefore, k1=L15×α15+L35A×α35−L12×α12.  
         [0045]    That is, the change Δa is proportional to a change ΔT in environmental temperature in accordance with the thermal expansion rate k1 as a proportional coefficient, which is determined by the lengths and the coefficients of linear expansion of the pedestal  12 , the cap  15  and the area A of the lens holder  35 . In the LD module  6 A, therefore, the pedestal  12 , the cap  15  and the area A of the lens holder  35  and the end plate  11  constitute the first link member, wherein the pedestal  12 , the cap  15  and the area A determine the relative distance between the end face  1   a  of the LD  1  and the input-side principal plane  3   a  of the lens  3 . In this case, L0 and α0 correspond to the length and the coefficient of linear expansion of the first link member, respectively.  
         [0046]    When a change Δa occurs, a distance b between the focal point  7  of the lens  3  and the output-side principal plane  3   a  of the lens  3  changes by a change Δb as shown in FIG. 2 in accordance with the formula of the lens given by the following equation 4 (where Δb&gt;0).  
         1 /f =1/( a+Δa )+1/( b−Δb )  (4)  
         [0047]    where f is the focal distance of the lens  3 . The focal distance f is determined by the diameter and the refractive index or the like of the lens  3  and is considered not to be changed by a change in environmental temperature. When the environmental temperature of the LD module  6 A changes, therefore, the change Δb depends on the change Δa alone.  
         [0048]    The distance between one end face  4   a  of the optical fiber  4  and the focal point  7  that are respectively shown by broken lines in FIG. 2 is equal to the distance c at the standard temperature (25° C.) as mentioned earlier. Suppose that the environmental temperature has risen by ΔT and the distance between the end face  4   a  of the optical fiber  4  and the focal point  7  that are respectively shown by solid lines becomes c−Δc (where Δc≧0), the change Δc is acquired by the following equation (5) based on the thermal expansion of an area B (see FIG. 1) between the output-side principal plane  3   a  of the lens  3  and the other end face  35   d  of the lens holder  35  and the change Δb according to the change Δa.  
               Δ                 c     =       L35B   ×   α35   ×   Δ                 T     +     Δ                 b               (   5   )                            =       k2   ×   Δ                 T     +     Δ                 b                 (   6   )                               
 
         [0049]    where L35B is the length of the area B of the lens holder  35  as shown in FIG. 1. Given that k2=L35B×α35, k2 indicates a unit change (mm/° C.) in the relative distance between the one end face  4   a  of the optical fiber  4  and the output-side principal plane  3   a  of the lens  3  or a thermal expansion rate with respect to a rise in temperature by 1° C.  
         [0050]    In the LD module  6 A, therefore, the area B of the lens holder  35  constitutes the second link member, wherein the area B defines the relative distance between the one end face  4   a  of the optical fiber  4  and the output-side principal plane  3   a  of the lens  3 . In this case, L35B and α35 correspond to the length and the coefficient of linear expansion of the second link member, respectively.  
         [0051]    The feature of the LD module  6 A lies in that a decline of the optical output of the LD  1  when the environmental temperature rises is compensated for by improving the optical coupling efficiency between the LD  1  and the optical fiber  4 . This requires that the one end face  4   a  of the optical fiber  4  should approach the focal point  7  as the environmental temperature rises.  
         [0052]    To improve the optical coupling efficiency proportionally with respect to a rise in temperature, it is preferable that the change Δc should not exceed the distance c during the temperature rise and the change Δc becomes equal to the distance c at the upper limit of temperature for the usage of the LD module  6 A. This is because as the change Δc changes this way, the optimal optical coupling efficiency can be achieved at the upper limit of temperature for the usage of the LD module  6 A, e.g., 85° C.  
         [0053]    It is therefore desirable that the distance c and the change Δc have the relationship that is given by the following equation (7).  
         0&lt;Δ c≦c   (7)  
         [0054]    Combining the equations (6) and (7) derive the following equation (8).  
         0 &lt;Δc=k 2 ×ΔT+Δb≦c   (8)  
         [0055]    It is therefore clear that the lengths and the coefficients of linear expansion of the first and second link members should be selected in such a way as to satisfy the equation (8) in manufacturing the LD module  6 A. In the LD module  6 A set this way, the distance k is longer than the outlet distance by the distance c when the environmental temperature is room temperature. As the environmental temperature rises, the distance k becomes shorter by the change Δb and the outlet distance becomes longer at the same time. As a result, the focal point  7  of the lens  3  and the one end face  4   a  of the optical fiber  4  come closer to each other by the change Δc. It is needless to say that the distance c is determined in such a way that the optical output of the LD module  6 A obtained from the optical fiber at the standard temperature does not go below the value defined by the specification of the LD module  6 A.  
         [0056]    In case where the distance k changes greatly with respect to a slight change in the inlet distance a due to a rise in environmental temperature, the LD module  6 A has a difficulty to provide a stable optical coupling efficiency. To achieve the stable optical coupling efficiency by avoiding such a large change in distance b, it is preferable to set a ratio of the distance a to the focal distance f of the lens at a value from 1.03 to 3, and more preferably, the ratio is set at two. With this setting, if Δa is very small, it is apparent from the equation (4) that Δb becomes approximately equal to Δa (Δa≈Δb), Δb also becomes very small and does not become large.  
         [0057]    When Δa≈Δb is satisfied, replacing Δb in the equation (8) with Δa in the equation (3) derives the following equations (9) and (10).  
           k 2 ×ΔT+k 1 ×ΔT≦c   (9)  
           k 2&gt;(−1)× k 1  (10)  
         [0058]    With the equation (8) satisfied, if Δa, thus Δb, is so small as to be negligible with respect to Δc, the following equation (11) is derived from the equation (8).  
         Δ c=k 2 ×ΔT≦c   (11)  
         [0059]    As apparent from the equation (1), when L15×α15+L35A×α35=L12×α12 is set, k1 and Δa can be rendered 0. If the change Δa is 0 when the environmental temperature changes, the change Δb of the distance b between the focal point  7  and the output-side principal plane  3   a  of the lens  3  becomes 0. In this case, the change Δb is sufficiently small with respect to the thermal expansion rate k2, so that the distance Δc by which the one end face  4   a  of the optical fiber  4  approaches the focal point  7  depends only on the thermal expansion rate k2 as seen from the equation (6). In the LD module  6 A whose the inlet distance a does not depend on the temperature, the optical coupling efficiency is controlled only by the thermal expansion rate k2 or the outlet distance. This LD module  6 A can therefore provide the stable optical coupling efficiency.  
         [0060]    In the LD module  6 A, as mentioned earlier, KOVAR (having a coefficient of linear expansion of 5×10 −6 ) can be used for the cap  15 , SUS 304 of stainless steel (having a coefficient of linear expansion of 16×10 −6 ) can be used for the lens holder  35 , and an iron-based alloy (having a coefficient of linear expansion of 11×10 −6 ) can be used for the pedestal  12 . The change Δb can be minimized by adequately setting the lengths of the pedestal  12 , the cap  15  and the area A of the lens holder  35  based on the coefficients of linear expansion of those members in such a way that k1 becomes 0 as mentioned above.  
         [0061]    Table 1 shows the focal distance of the sphere lens  3  of the LD module  6 A and the materials and coefficients of linear expansion of the members (excluding the end plate  11 ) that constitute the first link member and the second link member.  
                                           TABLE 1                           Focal distance f of sphere lens 3: 1.46712 mm (25° C.)                Coefficient of linear           expansion (1/° C.)                        Upper limit of           Standard   temperature for           temperature   use       Name of member   Material   (20° C.)   (85° C.)               Pedestal 12   Iron-based alloy   11 × 10 −6     11 × 10 −6         Cap 15   KOVAR    5 × 10 −6      5 × 10 −6         Lens holder 35   SUS 304   16 × 10 −6     16 × 10 −6                    
 
         [0062]    Table 2 shows the sizes of the members (excluding the end plate  11 ) that constitute the first and second link members of the LD module  6 A at the standard temperature of 20° C. and at 85° C. which is the upper limit of temperature for the use of the LD module  6 A.  
                                     TABLE 2                                       Length                    Upper limit of           Standard   temperature for           temperature   use       Name of member or the like   (20° C.)   (85° C.)               First link member               Pedestal 12: L12   1.27 mm   1.270908 mm       Cap 15: L15    2.3 mm   2.300748 mm       Area A of lens holder 35: L35A   1.12 mm   1.121165 mm       Second link member       Area B of lens holder 35: L35B   5.03 mm   5.035231 mm                  
 
         [0063]    Table 3 shows the thermal expansion rates k1 and k2 of the first and second link members that are acquired based on the coefficients of linear expansion and the lengths of the members (excluding the end plate  11 ) constituting the first and second link members, which are given in Tables 1 and 2.  
                           TABLE 3                                   Name of member   Thermal expansion rate                           First link member   1.545 × 10 −5  mm/° C.           (Thermal expansion rate k1)           Second link member    8.05 × 10 −5  mm/° C.           (Thermal expansion rate k2)                      
 
         [0064]    Table 4 shows the distances a and c, and changes Δa, Δb and Δc that are obtained from the equations 3, 4 and 6 based on the distances a and c and the thermal expansion rates k1 and k2 given in Table 3.  
                                                       TABLE 4                                       Length                        Upper limit of               Standard   temperature for               temperature   use           Distance or change   (20° C.)   (85° C.)                       Distance  a     2.15 mm   2.151 mm           Distance  b     4.62 mm   4.615 mm           Distance  c     0.88 mm   0.885 mm                Change Δa   1.17 × 10 −3  mm           Change Δb   5.32 × 10 −3  mm           Change Δc   0.011 mm                      
 
         [0065]    As apparent from Table 4, the change Δc when the environmental temperature changes from 20° C. to 85° C. is smaller than the distance c at 20° C., which satisfies the condition of Δc (25° C.-85° C.)≦c (20° C.) given in the equation (7). As apparent from the value of the change Δc, the one end face  4   a  of the optical fiber  4  comes closer to the focal point  7  by 0.011 mm at 85° C. compared to at 20° C. That is, the optical coupling efficiency increases in accordance with a temperature rise (ΔT=65° C. in this case). The increase in optical coupling efficiency compensates for the decline of the output of the LD  1  caused by the temperature rise thereof. This suppresses a change in the optical output from the optical fiber  4  caused by the temperature rise of the LD  1 . Specifically, the optical outputs at 85° C. and 20° C. are respectively 0.12 mW and 0.125 mW and the difference in optical output between both temperatures is very small.  
         [0066]    Although the LD module  6 A has been described as a receptacle module, the invention can be adapted to other LD modules, such as a pig tale module.  
         [0067]    An LD module  6 B in FIG. 4 outputs a laser beam from the optical fiber  4 . To avoid the redundant description, same symbols are given to those components of the LD module  6 B in FIG. 4 which have substantially the same functions and shapes as the those of the corresponding components of the LD module  6 A in FIG. 1.  
         [0068]    The LD module  6 B has a lens holder  61  of, for example, KOVAR, which has a hollow cylindrical shape. The lens holder  61  has one end connected to the cap  15 . A hollow cylindrical link member  62  of, for example, KOVAR, is fitted over the other end of the lens holder  61 , and a disc-shaped member  53  is secured to the end of the link member  62  which is remote from the holder  61 . A ferrule sleeve  52  is supported on the member  53 .  
         [0069]    In the optical axis alignment of the LD module  6 B, the ferrule sleeve  52  is moved in a direction perpendicular to the optical axis  8  with respect to the member  53 , so that the relative position between the ferrule sleeve  52  and the member  53  is determined. As a result, the optical fiber  4  is positioned accurately on the optical axis  8 . Thereafter, the ferrule sleeve  52  and the member  53  are coupled together. In the focus point adjustment of the LD module  6 B, as schematically illustrated in FIGS. 5 and 6, the ferrule sleeve  52  and a ferrule holder  51  are connected together in such a way that one end face  4   a  of the optical fiber  4  is positioned apart by a predetermined distance c2 along the optical axis from the focal point  7  of the lens  3  to the opposite side of the lens  3  in consideration of the influence of a rise in environmental temperature on the LD module  6 B.  
         [0070]    The following equations (20), (21) and (22) respectively correspond to the equations (1), (2) and (3) that have been given in the foregoing description of the LD module  6 A.  
               Δ                 a2                =       (       (       L15   ×   α15     +     L61A   ×   α61A       )     -     L12   ×   α12       )     ×   Δ                 T             (   20   )                            =     L10   ×   α10   ×   Δ                 T               (   21   )                            =     k10   ×   Δ                 T               (   22   )                               
 
         [0071]    In the equation (20), L61A is the length of area A of the lens holder  61  and α61 is the coefficient of linear expansion of the lens holder  61 . In the equation (21), L10 and α10 are respectively an equivalent length and an equivalent coefficient of linear expansion of the pedestal  12 , the cap  15  and the area A of the lens holder  61  all considered as a single member. Given that L10×α10 is k10, k10 indicates a unit change (mm/° C.) in inlet distance a2 or a thermal expansion rate when the environmental temperature rises by 1° C. Therefore, k10=(L15×α15+L35A×α35)−L12×α12.  
         [0072]    In the LD module  6 B, the first link member that defines the relative distance between the end face  1   a  of the LD  1  and the input-side principal plane  3   a  of the lens  3  comprises the pedestal  12 , the cap  15 , the area A of the lens holder  61  and the end plate  11  to which the pedestal  12  and the cap  15  are secured. It can be denoted that the length and the coefficient of linear expansion of the first link member are respectively L10 and α10.  
         [0073]    The following equations (23) and (25) respectively correspond to the equations (5) and (6) that have been given in the foregoing description of the LD module  6 A.  
               Δ                 c2     =         (       L51   ×   α51     -     (       L61B   ×   α61     +     L62   ×   α62       )       )     ×   Δ                 T     -     Δ                 b2               (   23   )                            =       L20   ×   α20   ×   Δ                 T     -     Δ                 b2                 (   24   )                            =       k20   ×   Δ                 T     +     Δ                 b2                 (   25   )                               
 
         [0074]    In the equation (23), the length L61B is the length of area B of the lens holder  61 , and the length L61B and the length L62, as schematically shown in FIG. 6, are values measured without duplication over the overlap position where the holder  61  is fitted in the link member  62 . In the equation (24), L20 and α20 are respectively an equivalent length and an equivalent coefficient of linear expansion of an assembly of the ferrule holder  51 , the area B of the lens holder  61  and the link member  62 . Given that L20×α20 being k20, k20 indicates a unit change (mm/° C.) in the relative distance between the one end face  4   a  of the optical fiber  4  and the output-side principal plane  3   a  of the lens  3  or a thermal expansion rate with respect to a rise in temperature by 1° C. Therefore, k20=L51×α51−(L61B×α 61 +L62×α62).  
         [0075]    In the LD module  6 B, the second link member that defines the relative distance between the one end face  4   a  of the optical fiber  4  and the output-side principal plane  3   a  of the lens  3  comprises the ferrule holder  51 , the area B of the lens holder  61 , the link member  62  and the member  53  to which the ferrule holder  51  and the link member  62  are secured. It can be denoted that L20 and α20 are the length and the coefficient of linear expansion of the second link member, respectively.  
         [0076]    Because of the same reason given in the case of the LD module  6 A, it is necessary that the distance c2 and the change Δc2 have the relationship that is given by the following equation (26).  
         0&lt;Δ c 2≦ c 2  (26)  
         [0077]    That is, the distance c2 and the change Δc2 should meet the following equation (27).  
         0 &lt;Δc 2 =k 20×Δ T−Δb 2≦ c 2  (27)  
         [0078]    It is therefore apparent that the lengths and the coefficients of linear expansion of the pedestal  12 , the cap  15 , the lens holder  61 , the link member  62  and the ferrule holder  51  should be so selected as to satisfy the equation (27) in manufacturing the LD module  6 B. In the LD module  6 B set this way, the distance b2 is shorter than the outlet distance by the distance c2 when the environmental temperature is room temperature. As the environmental temperature rises, the distance b2 becomes shorter by the change Δb2. As the outlet distance becomes shorter by more than the change Δb2, however, the focal point  7  of the lens  3  and the one end face  4   a  of the optical fiber  4  come closer to each other by the change Δc2.  
         [0079]    The value of the change Δc2 is set to increase in accordance with the increase ΔT in environmental temperature by selecting the material for the ferrule holder  51  whose coefficient of linear expansion α51 is sufficiently larger than the coefficients of linear expansion α15, α61 and α62 of the cap  15 , the lens holder  61  and the link member  62 . In other words, the one end face  4   a  of the optical fiber  4  can be made closer to the focal point  7  of the lens  3  in accordance with the increased temperature ΔT. When the environmental temperature rises, therefore, the optical coupling efficiency increases so that the decline of the optical output of the LD  1  caused by the temperature rise thereof can be compensated for by the improved optical coupling efficiency. This compensation can suppress the temperature dependency of the optical output from the optical fiber  4  of the LD module  6 B.  
         [0080]    Because of the same reason given in the case of the LD module  6 A, the following equation (28) corresponding to the equation (10) is met when Δb2 is made approximately equal to Δa2 (Δa2≈Δb2) in the LD module  6 B.  
           k 20×Δ T&lt;k 10 ×ΔT    
         k20&gt;k10  (28)  
         [0081]    As in the case of the LD module  6 A, when Δa2, thus Δb2, is so small as to be negligible with respect to Δc2, the following equation (29) similar to the equation (11) is satisfied.  
         Δ c 2 =k 20 ×ΔT≦c 2  (29)  
         [0082]    In the LD module  6 B, it is possible to use material having a small coefficient of linear expansion, such as KOVAR or Invar, for the cap  15 , the lens holder  61  and the link member  62  and to use material having a large coefficient of linear expansion, such as an aluminum alloy, for the ferrule holder  51 . Tables 5 to 8, like Tables 1 to 4, show various parameters of the LD module  6 B.  
         [0083]    As apparent from the positional change Δc2 given in Table 8, the one end face  4   a  of the optical fiber  4  at 85° C. is closer to the focal point  7  than that at 25° C., thereby improving the optical coupling efficiency of the LD module  6 B.  
                                           TABLE 5                           Focal distance f of sphere lens 3: 1.46712 mm (25° C.)                Coefficient of linear           expansion (1/° C.)                        Upper limit of               Standard   temperature for               temperature   use       Name of member   Material   (25° C.)   (85° C.)               Cap 15   KOVAR   5 × 10 −6     5 × 10 −6         Lens holder 61   KOVAR   5 × 10 −6     5 × 10 −6         Link member 62   KOVAR   5 × 10 −6     5 × 10 −6         Ferrule holder 51   Aluminum   23 × 10 −6      23 × 10 −6              alloy                  
 
         [0084]    [0084]                                     TABLE 6                                       Length                    Upper limit of           Standard   temperature for           temperature   use       Name of member or the like   (25° C.)   (85° C.)               First link member               Equivalent length: L10   3.0000 mm    3.0009 mm        Second link member       (length L61B of area B of lens   9.900 mm   9.903 mm       holder 61) + (length L62 of link       member 62)       Ferrule holder 51: L51   7.000 mm   7.010 mm                    
         [0085]    [0085]                           TABLE 7                                   Name of member   Thermal expansion rate                           First link member     5 × 10 −6  mm/° C.           (Thermal expansion rate k10)           Second link member   1.1 × 10 −4  mm/° C.           (Thermal expansion rate k20)                        
         [0086]    [0086]                                                       TABLE 8                                       Length                        Upper limit of               Standard   temperature for               temperature   use           Distance or change   (25° C.)   (85° C.)                       Distance a2   3.00 mm   3.009 mm           Distance c2   0.03 mm    0.02 mm                Change Δa2   9.0 × 10 −4 mm             Change Δb2   8.2 × 10 −4 mm             Change Δc2   0.01 mm                        
         [0087]    An LD module  6 C in FIG. 7 outputs a laser beam from the optical fiber  4 . To avoid the redundant description, same symbols are given to those components of the LD module  6 C which have substantially the same functions and shapes as those of the corresponding components of the LD module  6 A in FIG. 1 or the LD module  6 B in FIG. 4.  
         [0088]    Although the LD modules  6 A and  6 B have been described with the positions of the principal planes  3   a  of the lens  3  in the direction of the optical axis taken as reference points, the position of the end face  1   a  of the LD 1  in the direction of the optical axis is taken as a reference point for the sake of convenience.  
         [0089]    The LD module  6 C has a cap  75  in which a window  75   c  is formed. The lens  3  is fitted in the window  75   c . The lens  3 , the end plate  11  and the cap  75  define an airtight chamber which is filled with an inactive gas.  
         [0090]    A link member  76  having a hollow cylindrical shape is connected to the end plate  11 . The link member  76  is longer than the cap  15  in axial direction and is larger in diameter. The cap  15  is concentrically arranged in the link member  76 .  
         [0091]    In the optical axis alignment of the LD module  6 C, as in the case of the LD module  6 B, the ferrule sleeve  52  and the member  53  are fixed together. In the focus point adjustment of the LD module  6 C, as schematically illustrated in FIGS. 8 and 9, the ferrule sleeve  52  and the ferrule holder  51  are connected together in such a way that the one end face  4   a  of the optical fiber  4  is positioned apart by a predetermined distance c3 along the optical axis from the focal point  7  to the opposite side of the lens  3 .  
         [0092]    As easily inferred from the foregoing description of the LD module  6 B, one end face  4   a  of the optical fiber  4  can be made closer to the focal point  7  in accordance with the temperature increase ΔT by setting the coefficient of linear expansion of the ferrule holder  51  sufficiently larger than the coefficient of linear expansion of each of the cap  75  and the link member  76  in the LD module  6 C having the above-described structure. Specifically, the link member  76  should be made of, for example, KOVAR having a small coefficient of linear expansion and the ferrule holder  51  should be made of, for example, an aluminum alloy having a large coefficient of linear expansion. Those settings can increase the optical coupling efficiency in accordance with the temperature increase ΔT and the increase in optical coupling efficiency can compensate for the drop of the optical output of the LD 1  caused by the temperature rise thereof.  
         [0093]    In the LD module  6 C, the first link member that defines the relative distance between the end face  1   a  of the LD 1  and the input-side principal plane  3   a  of the lens  3  comprises the pedestal  12 , the cap  75  and the face of the end plate  11  to which the pedestal  12  and the cap  75  are secured.  
         [0094]    In the LD module  6 C, the second link member that defines the relative position between one end face  4   a  of the optical fiber  4  and the output-side principal plane  3   a  of the lens  3  comprises the ferrule holder  51 , the link member  76  and the member  53  to which the ferrule holder  51  and the link member  76  are secured.  
         [0095]    In the LD module  6 C, a change Δa3 of an inlet distance a3 between the output-side end face  1   a  of the LD 1  and the input-side principal plane  3   a  of the lens  3 , which is caused by a temperature rise, can be suppressed to a minute value or substantially 0 by adequately setting the lengths of the pedestal  12  of, for example, an iron-based alloy having a predetermined coefficient of linear expansion and the cap  75  of, for example, KOVAR.  
         [0096]    Although the length of the second link member or the outlet distance changes in accordance with a rise in environmental temperature in the LD modules  6 A,  6 B and  6 C, the lengths and the coefficients of linear expansion of the first link member and the second link member may be set in such a way that only the inlet distance depends on the temperature but the outlet distance does not. In case where the outlet distance does not depend on the temperature, the first link member should be set in such a way that the inlet distance makes a predetermined change as the environmental temperature rises. That is, as the inlet distance changes, the spatial distance between the output-side principal plane of the lens and the focal point of the lens changes in accordance with the formula of the lens given in the equation (4). The use of a change in this distance can move the focal point closer to the position of the end face of the optical fiber relatively, thereby improving the optical coupling efficiency. This results in compensation for the temperature dependency of the optical output of the LD.  
         [0097]    With regard to an LD module whose outlet distance does not depend on the temperature, the optical coupling efficiency depends only on the inlet distance, so that the LD module can achieve a stable optical coupling efficiency.