JP5786934B2 - Optical communication lens and semiconductor module - Google Patents

Description

  The present invention relates to an optical communication lens and a semiconductor module which are used for optical communication or the like, for example, for coupling laser light of a semiconductor laser to an optical fiber.

  In optical communication or the like, it is required to efficiently couple a semiconductor laser or a light receiving element and an optical fiber. For this reason, an optical coupling lens is used to collect the light beam from the semiconductor laser. By the way, although the glass lens is mainly used in the conventional optical coupling lens, the glass lens having an aspheric surface is generally expensive, and there is a problem that the cost is increased. Accordingly, plastic optical coupling lenses that can be easily molded with high accuracy and can be mass-produced have been developed. However, plastic has a problem that the coefficient of thermal expansion is larger than that of glass, and the refractive index of the plastic material changes with temperature. For this reason, when the plastic lens is used, when the distance between the semiconductor laser and the lens is fixed, the focal length, that is, the imaging position in the direction of the optical fiber changes according to the change in the environmental temperature, so that the coupling efficiency is lowered. On the other hand, it is conceivable to use an actuator that moves the lens in the direction of the optical axis. However, an increase in the number of movable parts increases the cost, and there is also a problem of how to supply power.

  On the other hand, Patent Document 1 discloses a semiconductor module for optical communication in which diffractive structures are provided on both surfaces of a plastic lens. According to the technique of Patent Document 1, by providing a diffractive structure on both surfaces of a plastic lens, it is possible to suppress the diffraction pitch from becoming too small and to easily mold the lens.

  Further, in Patent Document 2, by providing a relief type diffraction structure on one side of a plastic lens, it is possible to suppress the variation of the focal position due to temperature while suppressing the distribution of diffraction efficiency from the center to the periphery of the lens. Yes.

Japanese Patent Laid-Open No. 11-274646 International Publication No. 00/17691 Pamphlet

  Incidentally, one problem with the lens of Patent Document 1 is that, as a result of using a diffractive structure on two surfaces, errors in the diffractive shape of each surface are affected by products, and unnecessary diffracted light is generated. Is likely to occur and is not preferable. Further, the problem of the lens in Patent Document 2 is that the diffraction efficiency is low because it has a relief-shaped diffractive structure, so that the coupling efficiency to the optical fiber is low, and unnecessary diffracted light is generated much, which is not necessary for a pin pole or the like. Since it is necessary to shield the luminous flux, it is not preferable in use.

  The present invention has been made in view of such problems, and the semiconductor module is made compact, and the fluctuation of the coupling efficiency is suppressed by suppressing sensitivity such as a temperature change to a small size while having a simple configuration. An object of the present invention is to provide a lens for optical communication that can be used and a semiconductor module using the same.

The lens for optical communication according to claim 1 is a lens for optical communication that collects a light beam having a wavelength λ emitted from a semiconductor laser, and the lens is made of plastic, and the optical surface of the lens is An optical path difference providing structure that suppresses variation in the focal position during temperature changes is formed only on the surface from which the light beam from the semiconductor laser is emitted, and there are a plurality of optical path difference providing structures centered on the optical axis of the lens. The cross-sectional shape of the annular zone cut along the plane passing through the optical axis of the lens is a blazed shape, and the NA on the side on which the light beam from the semiconductor laser is incident is expressed by the conditional expression (2) The filling,
0.35 ≦ NA ≦ 0.85 (2)
According to the optical path difference providing structure, the pupil transmittance distribution of the light beam that has passed through the lens satisfies the conditional expression (1).
0.5 ≦ T1 / T0 ≦ 0.85 (1)
Where T0: transmittance in the vicinity of the optical axis of the lens, and T1: transmittance in the vicinity of the lens.

  Note that “near the optical axis” is preferably in the range of 0 to 0.05, where 0 is the optical axis and 1 is the outer periphery of the lens optical surface, and “periphery” is 0.95 to 1 It is preferable to be within the range.

  As a result of diligent research, the inventor has been able to increase the diffraction efficiency by using a blazed optical path difference providing structure, and by providing the optical path difference providing structure only on one surface of the optical surface, the coupling efficiency can be changed. I found it to suppress. First, the blaze-type diffractive structure has advantages in that unnecessary diffracted light is reduced and diffraction efficiency can be secured higher than the relief-shaped diffractive structure provided in the lens of Patent Document 2. That is, in the blazed-type optical path difference providing structure, the influence of shadows is particularly likely to occur, and the diffraction efficiency can be easily controlled by controlling the step between the annular zones.

  Further, for example, in Patent Document 1, diffractive structures are provided on both surfaces of a lens in order to widen the diffraction pitch. On the other hand, the present inventor tried to make the diffraction pitch finer by providing an optical path difference providing structure only on one surface of the lens. If the diffraction pitch is made fine, as in the conditional expression (1), particularly in the periphery of the lens, the influence of the shadow of the light flux increases due to the combination with the refractive surface. Such influence will be described.

With reference to FIG. 1, here, an example in which a blazed diffraction structure is formed on a lens as an optical path difference providing structure will be described. FIG. 1A is an enlarged cross-sectional view of a lens provided with a diffractive structure on the semiconductor laser side, and FIG. 1B is an enlarged cross-sectional view of a lens provided with a diffractive structure on the semiconductor laser side. In FIG. 1, a ring-shaped diffractive structure DS connects a step surface ST facing an optical axis (not shown in FIG. 1) and an outer end and an inner end in the optical axis direction of an adjacent step surface ST. It has a slope CP. Here, the light beam passing through the inclined surface CP is the sum of the refractive power of the refracting surface of the lens and the diffractive power of the diffractive structure DS, and is used for condensing, but passes through the step surface ST. The luminous flux to be used is not used for condensing and causes a decrease in transmittance. This is called the influence of shadows.
However, near the optical axis of the lens, the light beam is emitted parallel to the optical axis, so the light loss due to the influence of the shadow is small, but the refractive surface as the mother aspheric surface falls down at the periphery of the lens. A light beam inclined with respect to the axis is likely to be incident, and combined with the inclination of the step surface ST due to the influence of the draft angle of the mold, the light amount loss due to the influence of the shadow increases as shown in FIG. Here, in the condensing spot, when the amount of light at the peripheral portion is lower than the amount of light that has passed near the optical axis of the lens, the spot becomes thick, that is, the effective NA decreases. If the effective NA can be reduced, the variation in coupling efficiency when a temperature change occurs can be reduced. In other words, a lens for optical communication with small fluctuations in coupling efficiency can be provided by intentionally providing a distribution at the center and the periphery of the lens with a shadow effect and diffraction efficiency distribution in the diffractive structure. In addition, if the efficiency of the peripheral part cannot be reduced only by the shadow effect, the diffraction efficiency is adjusted by finely adjusting the step between each ring zone, and the distribution of the center part and the peripheral part that is insufficient due to the shadow effect It becomes possible to have. However, if the value of T1 / T0 exceeds the lower limit of the expression (1), the decrease in NA becomes too large, which is not preferable. More preferably, it satisfies 0.6 ≦ T1 / T0 ≦ 0.85.

  By satisfying conditional expression (2), it is possible to secure an appropriate diameter of the focused spot while securing the amount of light taken from the semiconductor laser.

According to a second aspect of the present invention, in the optical communication lens according to the first aspect , the optical path difference providing structure has a step that faces the optical axis of the lens. This is because, in such an optical path difference providing structure, it is possible to suppress the focal position at the time of temperature change, and in particular, the influence of shadows easily occurs.

According to a third aspect of the present invention, there is provided the optical communication lens according to the first or second aspect , wherein the optical path difference providing structure imparts an optical path difference that is substantially an integral multiple of the wavelength λ to the light flux that passes therethrough. It is characterized by. Thereby, it is possible to suppress a focal position shift due to a change in the refractive index of the lens when the temperature changes while having a high diffraction efficiency.

According to a fourth aspect of the present invention, in the optical communication lens according to the third aspect , the optical path difference providing structure has an optical path difference that is one time the wavelength λ over the entire effective diameter range of the lens. It is characterized by being imparted to the light beam passing through. By providing as small an optical path difference as possible, the light transmittance can be improved by suppressing the height of the step of the optical path difference providing structure and suppressing molding errors and the like.

The optical communication lens according to claim 5 is the optical communication device according to any one of claims 1 to 4 , wherein the optical path difference providing structure has an X of the wavelength λ in a central region near the optical axis of the lens. Double the optical path difference, and in the peripheral area outside the central area of the lens, an optical path difference of Y times the wavelength λ is given to the passing light beam, and the decimal point of X is rounded off to an integer. The value is characterized in that it is smaller than the value obtained by rounding off the decimal point of Y to an integer. Thereby, it is possible to easily process the mold for transferring the optical path difference providing structure in the peripheral region where the pitch tends to be narrowed.

The lens for optical communication according to claim 6 is the invention according to claim 5 , wherein the value obtained by rounding the decimal point of X to an integer is 1, and the decimal point of Y is rounded off. The integer value is 2.

The lens for optical communication according to claim 7 is the optical coupling according to any one of claims 1 to 6 , wherein the lens condenses the light beam emitted from the semiconductor laser on an end face of the optical fiber. It is a lens.

According to an eighth aspect of the present invention, in the optical communication lens according to the seventh aspect , the coupling lens has an optical system magnification M that satisfies the following conditional expression (3).
1.0 ≦ M ≦ 4.0 (3)

This makes it possible to provide optimum efficiency from NA 0.1 to NA 0.12 that can be taken into a general single mode fiber. Particularly preferably, the following formula is satisfied.
2.0 ≦ M ≦ 4.0 (3 ′)

  The optical communication lens according to claim 10 is the invention according to any one of claims 1 to 9, wherein the optical path difference providing structure is provided on a surface from which a light beam from the semiconductor laser is emitted. It is characterized by.

  In FIG. 1A, since the diffractive structure DS is provided on the semiconductor laser side, the ratio of the light beam incident on the step surface ST increases, and the influence of the shadow increases. On the other hand, in FIG. 1B, since the diffractive structure DS is provided on the side opposite to the semiconductor laser, the light flux tends to travel in the direction along the step surface ST of the annular zone, that is, the step surface ST. The ratio of the light beam incident on the lens becomes relatively low, and the influence of the shadow becomes small. In other words, by having a diffractive structure on the surface from which the light beam from the semiconductor laser is emitted, even if the stepped surface ST of the annular zone is inclined due to manufacturing errors, the coupling efficiency is not easily lowered, and a lens that is resistant to manufacturing errors can be realized. The optical surface on the optical fiber side has a relatively large area compared to the laser side, so the area where the diffractive structure is provided is widened, so the ring pitch can be increased accordingly, and the decrease in transmittance is suppressed. It becomes easy to do.

A semiconductor module according to a ninth aspect is characterized in that the optical communication lens according to any one of the first to eighth aspects and a semiconductor laser are assembled together.

According to a tenth aspect of the present invention, in the semiconductor module according to the ninth aspect , the lens is attached to a housing that seals the semiconductor laser.

According to an eleventh aspect of the present invention, in the semiconductor module according to the ninth aspect , the lens is integrated with a housing that seals the semiconductor laser.

  A “semiconductor laser” is a laser that utilizes recombination emission of a semiconductor. In general, the oscillation wavelength becomes longer as the environmental temperature increases, and the oscillation wavelength becomes shorter as the environmental temperature decreases.

  The lens is a lens that condenses the light beam emitted from the semiconductor laser, a lens that condenses the light beam emitted from the end surface of the optical fiber, and in particular, the light that condenses the light beam emitted from the semiconductor laser on the end surface of the optical fiber. It includes a coupling lens and an optical coupling lens that condenses the light beam emitted from the end face of the optical fiber onto the light receiving surface of the light receiving element. The lens is made of plastic.

  The optical path difference providing structure referred to in this specification is a general term for structures that add an optical path difference to an incident light beam. The optical path difference providing structure also includes a phase difference providing structure for providing a phase difference. The phase difference providing structure includes a diffractive structure. The optical path difference providing structure of the present invention is preferably a diffractive structure. The optical path difference providing structure has a step, preferably a plurality of steps. This step adds an optical path difference and / or phase difference to the incident light flux. The optical path difference added by the optical path difference providing structure may be an integer multiple of the wavelength of the incident light beam or a non-integer multiple of the wavelength of the incident light beam. The steps may be arranged with a periodic interval in the direction perpendicular to the optical axis, or may be arranged with a non-periodic interval in the direction perpendicular to the optical axis. When the lens provided with the optical path difference providing structure is a single aspherical lens, the incident angle of the light flux to the lens differs depending on the height from the optical axis. It will be slightly different. For example, when the lens is a single lens aspherical convex lens, even if it is an optical path difference providing structure that provides the same optical path difference, generally the distance from the optical axis tends to increase.

  In addition, the diffractive structure referred to in this specification is a general term for structures that have a step and have an action of converging or diverging a light beam by diffraction. For example, a plurality of unit shapes are arranged around the optical axis, and a light beam is incident on each unit shape, and the wavefront of the transmitted light is shifted between adjacent annular zones, resulting in new It includes a structure that converges or diverges light by forming a simple wavefront. The diffractive structure may have a plurality of steps, and the steps may be arranged at periodic intervals in the direction perpendicular to the optical axis, or may be arranged at non-periodic intervals in the direction perpendicular to the optical axis. In addition, when the lens provided with the diffractive structure is a single aspherical lens, the incident angle of the light flux to the lens differs depending on the height from the optical axis, and therefore the step amount of the diffractive structure is slightly different for each annular zone. Become. For example, when the lens is a single aspherical convex lens, even if it is a diffractive structure that generates diffracted light of the same diffraction order, generally, the distance from the optical axis tends to increase.

  By the way, it is preferable that the optical path difference providing structure has a plurality of concentric annular zones centered on the optical axis. The optical path difference providing structure can generally have various cross-sectional shapes (cross-sectional shapes on the plane including the optical axis), and the cross-sectional shapes including the optical axis are roughly classified into a blazed structure and a staircase structure.

    As shown in FIGS. 2A and 2B, the blazed structure means that the cross-sectional shape including the optical axis of an optical element having an optical path difference providing structure is a sawtooth shape. In the example of FIG. 2, it is assumed that the upper side is the light source side and the lower side is the optical disk side, and the optical path difference providing structure is formed on a plane as a mother aspherical surface. In the blazed structure, the length of one blazed unit in the direction perpendicular to the optical axis is referred to as a pitch P (see FIGS. 2A and 2B). The length of the step in the direction parallel to the optical axis of the blaze is referred to as a step amount B (see FIG. 2A). In the blaze mold shape, as shown in FIG. 1, for example, the stepped surface may be slightly inclined without being parallel to the optical axis in order to make it easy to cut the mold with a cutting tool. The position of the apex of one blaze is preferably within a range of 0 to 25% of the optical axis orthogonal direction pitch.

  In addition, as shown in FIGS. 2C and 2D, the staircase structure has a small staircase shape in cross section including the optical axis of an optical element having an optical path difference providing structure (referred to as a staircase unit). ). In the present specification, “V level” means a ring-shaped surface (hereinafter also referred to as a terrace surface) corresponding to (or facing) the vertical direction of the optical axis in one step unit of the step structure. In other words, it is divided by V steps and divided into V ring zones. Particularly, a three-level or higher staircase structure has a small step and a large step.

  For example, the optical path difference providing structure illustrated in FIG. 2C is referred to as a five-level step structure, and the optical path difference providing structure illustrated in FIG. 2D is referred to as a two-level step structure (also referred to as a binary structure). . A two-level staircase structure is described below. A plurality of annular zones including a plurality of concentric annular zones around the optical axis, and a plurality of annular zones including the optical axis of the objective lens have a plurality of stepped surfaces Pa and Pb extending in parallel to the optical axis, The light source side terrace surface Pc for connecting the light source side ends of the adjacent step surfaces Pa and Pb and the optical disk side terrace surface Pd for connecting the optical disk side ends of the adjacent step surfaces Pa and Pb are formed. The surface Pc and the optical disc side terrace surface Pd are alternately arranged along the direction intersecting the optical axis.

  The optical path difference providing structure is preferably a structure in which a certain unit shape is periodically repeated. As used herein, “unit shape is periodically repeated” naturally includes shapes in which the same shape is repeated in the same cycle. In addition, the unit shape that is one unit of the cycle has regularity, and the shape in which the cycle gradually increases or decreases gradually is also included in the “unit shape is periodically repeated”. Suppose that

  When the optical path difference providing structure has a blazed structure, the sawtooth shape as a unit shape is repeated. As shown in FIG. 2 (a), the same serrated shape may be repeated, or as shown in FIG. 2 (b), the serrated shape gradually increases as it moves away from the optical axis. A shape in which the pitch (also referred to as a diffraction pitch) becomes longer or a shape in which the pitch becomes shorter may be used. In addition, in some areas, the blazed structure has a step opposite to the optical axis (center) side, and in other areas, the blazed structure has a step toward the optical axis (center). It is good also as a shape in which the transition area | region required in order to switch the direction of the level | step difference of a blaze | braze type | mold structure is provided in the meantime. In addition, when it is set as the structure which switches the direction of the level | step difference of a blaze | braze type | mold in this way, it becomes possible to widen an annular zone pitch and it can suppress the transmittance | permeability fall by the manufacturing error of an optical path difference providing structure.

  In the case where the optical path difference providing structure has a staircase structure, there may be a shape such that a 5-level stair unit as shown in FIG. 2C is repeated. Furthermore, it may be a shape in which the pitch of the staircase unit gradually increases as it moves away from the optical axis, or a shape in which the pitch of the staircase unit gradually decreases. When the blazed shape is approximated by a staircase shape, the same effect as that of the present invention can be obtained although the diffraction efficiency is lowered.

  The optical path difference providing structure preferably has a step that faces the optical axis of the lens. “The level difference faces the optical axis” means a state as shown in FIG.

  It is preferable that the optical path difference providing structure provides an optical path difference that is approximately an integral multiple of the wavelength λ to the passing light beam. The term “substantially integer multiple” refers to (N−0.4) λ or more and (N−0.4) λ or less, where N is an integer other than 0. The optical path difference providing structure gives an optical path difference of 1 times the wavelength λ in the entire effective diameter range of the lens, or an optical path difference of 1 time of the wavelength λ in the central region near the optical axis of the lens. In the peripheral region outside the central region of the lens, it is preferable to provide an optical path difference that is twice the wavelength λ. The boundary between the central region and the peripheral region is preferably, for example, a height from the optical axis that is 2/3 of the effective diameter.

It is preferable that the pupil transmittance distribution of the light beam that has passed through the lens satisfies the conditional expression (1). The pupil transmittance distribution is a transmittance distribution on the exit pupil plane, and corresponds to a brightness distribution on the exit pupil plane with respect to an incident light beam having a uniform intensity. The vicinity of the optical axis of the lens is, for example, within a height from the optical axis that is 1/3 of the effective diameter, more preferably within 0.2, and the periphery of the lens is, for example, 2/2 of the effective diameter. 3 is outside the height from the optical axis of 3, more preferably outside 0.8.
0.4 ≦ T1 / T0 ≦ 0.8 (1)
However,
T0: transmittance in the vicinity of the optical axis of the lens T1: transmittance in the vicinity of the lens

The NA on the side on which the light beam from the semiconductor laser is incident preferably satisfies the conditional expression (2).
0.30 ≦ NA ≦ 0.85 (2)
It is more preferable that conditional expression (4) is satisfied.
0.35 ≦ NA ≦ 0.65 (4)

  According to the present invention, a semiconductor module is made compact, and furthermore, a lens for optical communication capable of suppressing fluctuations in coupling efficiency by suppressing the effective NA of a focused spot on the optical fiber side, and a semiconductor module using the same. Can be provided.

(A) is an enlarged sectional view of a lens provided with a diffractive structure on the semiconductor laser side, and (b) is an enlarged sectional view of a lens provided with a diffractive structure on the semiconductor laser side. It is an expanded sectional view which shows the example of an optical path difference providing structure, (a), (b) shows the example of a blaze type structure, (c), (d) is a figure which shows the example of a staircase type structure. (A) shows a state in which the step is directed in the direction of the optical axis, and (b) is a diagram showing a state in which the step is directed in a direction opposite to the optical axis. (A) is sectional drawing of the optical axis direction of the semiconductor module LM concerning 1st Embodiment, (b) is a figure which expands and shows the lens surface shown by arrow IVB. (A) is sectional drawing of the optical axis direction of the semiconductor module LM concerning 2nd Embodiment, (b) is a figure which expands and shows the lens surface shown by arrow VB. (A) is an optical axis direction sectional view of optical coupling lens CL concerning a 3rd embodiment, and (b) is a figure expanding and showing the lens surface shown by arrow VIB. It is a graph of the coupling efficiency in Examples 1-4 and a comparative example. It is a graph of the coupling efficiency in Examples 5-8 and a comparative example. 10 is a graph of coupling efficiency in Example 9. It is a figure which shows the relationship between the phase difference provided by the level | step difference of a diffraction structure, and diffraction efficiency.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 4 is a cross-sectional view in the optical axis direction of the semiconductor module LM according to the first embodiment. A substrate of a semiconductor laser LD is attached to a base BS having three legs LG for feeding, and the semiconductor laser LD that emits a light beam having a wavelength λ is covered by a cover (housing) CV having an aperture AP. It has been broken. The opening AP is shielded from the inside by a transparent cover glass CG, and seals the semiconductor laser LD from the outside air.

  The flange portion FL of the optical coupling lens CL is bonded to the cover CV so as to cover the outside of the opening AP. The plastic optical coupling lens CL has an optical surface S1 which is a refractive surface on the semiconductor laser side and an optical surface S2 on the opposite side, and the optical surface S2 has a diffractive structure DS formed on the refractive surface. Yes. The curvature radius of the optical surface S1 is larger than the curvature radius of the optical surface S2. The blazed diffraction structure DS provides an optical path difference that is an integral multiple of the wavelength λ when a light beam having the wavelength λ is incident.

  The base BS of the semiconductor module LM is connected to the optical fiber OF at a predetermined distance via a housing (not shown). Note that the optical fiber OF is covered with an outer shell SL.

  The operation of the semiconductor module LM of the present embodiment will be described. When power is supplied through the leg LG, the semiconductor laser LD emits light, and the emitted light beam passes through the cover glass CG and the aperture AP and enters the optical coupling lens CL. The light beam incident on the optical coupling lens CL is refracted on the refracting surface, passes through the diffractive structure DS, exits, is condensed on the end surface of the optical fiber OF, and then propagates in the optical fiber OF.

  When the ambient temperature increases (or decreases), the oscillation wavelength of the semiconductor laser LD increases (or decreases), and accordingly, the diffraction angle of the diffracted light generated from the diffractive structure DS changes accordingly. It is possible to correct a focal position shift caused by a change in the refractive index of the lens CL. Further, according to the present embodiment, since the diffractive structure DS is provided only on one surface of the optical coupling lens CL, the diffraction pitch is reduced overall, and the light is condensed on the end surface of the optical fiber OF by the optical coupling lens CL. In the spot, the amount of light at the peripheral portion is lower than the amount of light that has passed through the vicinity of the optical axis of the lens, so that the spot becomes thick and the effective NA is reduced. Thereby, the fluctuation of the coupling efficiency when the temperature change occurs is reduced.

  FIG. 5 is a cross-sectional view in the optical axis direction of the semiconductor module LM according to the second embodiment. A substrate of a semiconductor laser LD is attached to a base BS having three legs LG for power feeding. In the present embodiment, the cover (housing) CV that hermetically covers the semiconductor laser LD that emits the light beam having the wavelength λ is a plastic cup shape, and the optical coupling lens portion CL is integrally formed on the shielding end side thereof. Is formed.

  The optical coupling lens CL has an optical surface S1 that is a refractive surface on the semiconductor laser side and an optical surface S2 that is a refractive surface on the opposite side, and a diffractive structure DS is formed on the refractive surface in the optical surface S2. Has been. The curvature radius of the optical surface S1 is larger than the curvature radius of the optical surface S2. The blazed diffraction structure DS provides an optical path difference that is an integral multiple of the wavelength λ when a light beam having the wavelength λ is incident.

  The base BS of the semiconductor module LM is connected to the optical fiber OF at a predetermined distance via a housing (not shown). Note that the optical fiber OF is covered with an outer shell SL.

  The operation of the semiconductor module LM of the present embodiment will be described. When power is supplied via the leg LG, the semiconductor laser LD emits light, and the emitted light beam is directly incident on the optical coupling lens portion CL. The light beam incident on the optical coupling lens portion CL is refracted by the refracting surface, passes through the diffractive structure DS, exits, is condensed on the end surface of the optical fiber OF, and then propagates in the optical fiber OF.

  When the ambient temperature increases (or decreases), the oscillation wavelength of the semiconductor laser LD increases (or decreases), and accordingly, the diffraction angle of the diffracted light generated from the diffractive structure DS changes accordingly. It is possible to correct a focus position shift caused by a change in the refractive index of the lens portion CL. Further, the lens position expands (or contracts) due to the temperature increase (or decrease), so that the focal position shift can be further corrected. According to the present embodiment, since the diffractive structure DS is provided only on the outer surface of the optical coupling lens portion CL, the diffraction pitch is narrowed as a whole and is condensed on the end surface of the optical fiber OF by the optical coupling lens portion CL. In the spot, the amount of light in the peripheral portion is lower than the amount of light that has passed near the optical axis of the lens, so that the spot becomes thick and the effective NA is reduced. Thereby, the fluctuation of the coupling efficiency when the temperature change occurs is reduced.

  FIG. 6 is a cross-sectional view in the optical axis direction of the optical coupling lens CL according to the third embodiment. In the present embodiment, a light beam propagated from an unshown semiconductor laser through an optical fiber OF and emitted from its end face is condensed on a light receiving surface PDa of the photodiode PD through an optical coupling lens CL. It is like that. On the refractive surface of one optical surface (optical surface on the optical fiber OF side) S1 of the optical coupling lens CL, a blazed diffractive structure DS for correcting a focal position shift at the time of temperature change is provided. The other optical surface S2 comprises only a refractive surface. The radius of curvature of the optical surface S1 is smaller than the radius of curvature of the optical surface S2. Note that CG is a cover glass of a photodiode.

  According to the present embodiment, since the blazed diffractive structure DS is provided only on one surface S1 of the optical coupling lens CL, the diffraction pitch is reduced overall, and the optical coupling lens CL causes the light receiving surface PDa of the photodiode PD to be formed. In the collected spot, the amount of light at the peripheral portion is lower than the amount of light that has passed through the vicinity of the optical axis of the lens, so that the spot becomes thick and the effective NA increases. Thereby, the fluctuation of the coupling efficiency when the temperature change occurs is reduced. Other effects are the same as those of the above-described embodiment.

(Example)
Examples that can be used in the above-described embodiment will be described below. Examples 1 to 4 are all resin lenses, and are optical coupling lenses suitable for the semiconductor module shown in FIG. As a comparative example, a glass lens and a resin lens without a diffractive structure are referred to. Table 1 shows the specifications of the semiconductor modules of Examples 1 to 4, Table 2 shows the specifications of the semiconductor modules of Examples 5 to 8, Table 3 shows the specifications of the semiconductor module of Example 9, 4 shows the refractive index of each material of Examples 1-4, Table 5 shows the refractive index of each material of Examples 5-8, and Table 6 shows the refractive index of each material of Example 9. .

  The optical surface of the optical coupling lens is formed as an aspherical surface that is symmetric about the optical axis and is defined by a mathematical formula obtained by substituting the coefficients shown in Table 1 into Formula 1.

  Here, X (h) is an axis in the optical axis direction (with the light traveling direction being positive), κ is a conical coefficient, Ai is an aspherical coefficient, h is a height from the optical axis, and r is a paraxial radius of curvature. It is.

  Further, in the case of the embodiment using the diffractive structure, the optical path difference given to the light flux of each wavelength by the diffractive structure is defined by an equation in which the coefficient shown in the table is substituted into the optical path difference function of Formula 2. .

(Equation 2)
Φ (h) = B ₂ h ² × λ × m / λB
Here, λ: wavelength used, m: diffraction order, λB: blazed wavelength, h: distance in the direction perpendicular to the optical axis from the optical axis.

  Table 7 shows lens data of Examples 1 to 4 and Comparative Example, Table 8 shows lens data of Examples 5 to 8, and Table 9 shows lens data of Example 9. The pitches and steps of the diffractive structures in the examples are optimized by the optical path difference function, diffraction order, and blazed wavelength shown in Tables 7, 8, and 9.

As described with reference to FIG. 1, a lens having a diffractive structure has a decrease in coupling efficiency due to the influence of a shadow that increases in accordance with a step manufacturing error of the diffraction zone. The reduction in coupling efficiency tends to increase as the annular zone pitch decreases. In this embodiment, when the transmittance at the center is 100%, the degree of decrease in the vicinity of the lens effective diameter (T1 / T0) is reduced. Shown in Tables 10-12.
It can be seen that the glass lens and the resin lens without the diffractive structure theoretically show no reduction in transmittance near the lens effective diameter, but in Examples 1 to 9, the transmittance reduction is at least 15% or more.

  Next, FIGS. 7 to 9 show graphs of coupling efficiency in the examples and comparative examples. 7 to 9, the vertical axis represents the coupling efficiency and the horizontal axis represents the environmental temperature. Referring to FIGS. 7 to 9, it can be seen that the glass lens has no refractive index variation due to temperature, and therefore the efficiency variation is small. However, since it is expensive, there is a problem of increasing the cost of the semiconductor module. Next, it can be seen that the resin lens without the diffractive structure has a large efficiency fluctuation due to the influence of the refractive index fluctuation due to the environmental temperature. For example, the coupling efficiency is less than 10% at the environmental temperature of 80 ° C.

  On the other hand, although Examples 1-9 which have a diffractive structure are all resin lenses which have a diffractive structure, it turns out that an efficiency fluctuation | variation can be suppressed small compared with the resin lens without a diffractive structure. In particular, in Example 2, considering only the efficiency variation, it is suppressed to be equivalent to a glass lens. In Example 4, when the environmental temperature is 0 ° C., the coupling efficiency is 20% or less, but the variation of the coupling rate is still smaller than that of the resin lens without the diffractive structure. Furthermore, in Example 4, the coupling efficiency is higher than that of the other examples at around 60 ° C. on the high temperature side. That is, Example 4 is effective in an environment where the temperature of the semiconductor module rises due to light emission of the semiconductor laser LD and is always used at around 60 ° C. Further, Example 9 is a lens that can realize a coupling efficiency of 10% when used at around 60 ° C. while suppressing a coupling efficiency change due to an environmental temperature change. Incidentally, by manufacturing the lens in consideration of not only the use of the influence of the shadow but also the depth of the blaze, the T1 / T0 is about 80% with the effect of the shadow alone, as shown in FIG. From the relationship between the phase difference and the diffraction efficiency, the phase difference is adjusted so as to be a step of 0.6λ or 1.4λ, which is a phase difference having an efficiency of about 50% near the effective diameter, and T1 / T0 = 80% × 50. % = 40% is realized. At this time, the vicinity of the optical axis is set to 1λ, and the step difference up to the effective diameter is gradually adjusted to 0.6λ or 1.4λ by gradually adjusting the phase difference.

  On the other hand, Examples 7 and 8 are suitable for the embodiment shown in FIG. 4 and correspond to the increase (or decrease) in the oscillation wavelength of the semiconductor laser LD when the environmental temperature increases (or decreases). In addition to the action of changing the diffraction angle of the diffracted light generated from the diffractive structure DS, by adjusting the distance between the light source and the optical coupling lens CL due to the thermal expansion of the cover CV, the refractive index of the optical coupling lens CL due to the environmental temperature change It is possible to correct the focal position shift caused by the change in the angle. Here, the linear expansion coefficient of the cover CV is 0.00006 cm / cm ° C.

  As is apparent from FIG. 8, in Examples 7 and 8, when considering only the fluctuation in efficiency, it is suppressed to be equivalent to a glass lens.

  The present invention is not limited to the embodiments described in the specification, and other embodiments and modifications are apparent to those skilled in the art from the embodiments and ideas described in the present specification. It is.

AP opening BS base CG cover glass CL optical coupling lens or optical coupling lens part CV cover DS diffraction structure FL flange part LD semiconductor laser LG leg part LM semiconductor module OF optical fiber PD photodiode PDa light receiving surface S1 semiconductor laser side optical surface S2 Semiconductor laser side optical surface

Claims (11)

A lens for optical communication that collects a light beam having a wavelength λ emitted from a semiconductor laser,
The lens is made of plastic;
On the optical surface of the lens, only on the surface from which the light beam from the semiconductor laser is emitted, an optical path difference providing structure that suppresses focal position fluctuation at the time of temperature change is formed,
The optical path difference providing structure includes a plurality of annular zones centered on the optical axis of the lens, and the sectional shape of the annular zone cut along a plane passing through the optical axis of the lens is a blazed shape,
NA on the side on which the light beam from the semiconductor laser is incident satisfies the conditional expression (2),
0.35 ≦ NA ≦ 0.85 (2)
An optical communication lens characterized in that, by the optical path difference providing structure, a pupil transmittance distribution of a light beam that has passed through the lens satisfies the conditional expression (1).
0.5 ≦ T1 / T0 ≦ 0.85 (1)
However,
T0: transmittance in the vicinity of the optical axis of the lens T1: transmittance in the vicinity of the lens The optical communication lens according to claim 1 , wherein the optical path difference providing structure has a step toward the optical axis of the lens. The optical path difference providing structure, a lens for optical communication according to claim 1 or 2, characterized in applying an optical path difference of approximately an integral multiple of the wavelength lambda, the light flux passing through. 4. The optical communication lens according to claim 3 , wherein the optical path difference providing structure applies an optical path difference that is one time the wavelength λ to a light beam that passes through the entire effective diameter range of the lens. . The optical path difference providing structure provides an optical path difference of X times the wavelength λ in the central region near the optical axis of the lens, and Y times the wavelength λ in a peripheral region outside the central region of the lens. A value obtained by adding an optical path difference to a passing light beam and rounding a decimal point of X to an integer is smaller than a value rounded to a whole number by rounding a decimal point of Y. The lens for optical communication according to any one of 1 to 4 . 6. The optical communication according to claim 5 , wherein a value obtained by rounding the first decimal place of X to an integer is 1, and a value obtained by rounding the first decimal place of Y to an integer is 2. Lens. The lens on the end face of the optical fiber, a lens for optical communication according to any one of claims 1 to 6, characterized in that the light flux emitted is condensed to the optical coupling lens from the semiconductor laser. The optical lens according to claim 7 , wherein the coupling lens has an optical system magnification M that satisfies the following conditional expression (3).
1.0 ≦ M ≦ 4.0 (3) And a lens for optical communication according to any one of claims 1-8, the semiconductor module, characterized by comprising integrally assembled to the semiconductor laser. The semiconductor module according to claim 9 , wherein the lens is attached to a housing that seals the semiconductor laser. The semiconductor module according to claim 9 , wherein the lens is integral with a housing that seals the semiconductor laser.

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