JP2018066888A - Optical module - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical module capable of reducing variations of intensity of light being incident on a light-receiving layer of a semiconductor light-receiving element caused by a change in temperature and a wavelength.SOLUTION: An optical module 1A comprises: a first lens 12 for converting a laser beam L1 outputted from an LD 11 into collimated light L2; a BS (Beam Splitter) 13 for branching a part of the collimated light L2; and a PD 14 for receiving collimated light L5 branched by the BS 13. The BS 13 has a multilayer film filter 13c sandwiched between a first glass member 13a and a second glass member 13b. The PD 14 has a semiconductor layered portion including a light-receiving layer, provided on a semiconductor substrate. A rear surface of the semiconductor substrate is fixed on a surface of one glass member 13b, and optically coupled to the multilayer film filter 13c via the one glass member 13b. An antireflection film for making a refractive index of the one glass member 13b match a refractive index of the semiconductor substrate is provided on the rear surface of the semiconductor substrate.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an optical module.

  Patent Document 1 discloses a technique related to a semiconductor light receiving element and an optical module. FIG. 10 is a schematic diagram showing a stacked configuration of the semiconductor light receiving element 130 described in Patent Document 1. As shown in FIG. The semiconductor light receiving element 130 includes an n-type InP substrate 132, an n-type InGaAs layer 134, and an n-type InP layer 136. The n-type InP substrate 132 has a front surface and a back surface that face each other. The n-type InGaAs layer 134 is provided on the surface of the n-type InP substrate 132. The band gap of the n-type InGaAs layer 134 is smaller than the band gap of the n-type InP substrate 132. The n-type InP layer 136 is provided on the n-type InGaAs layer 134. P-type regions 138 and 140 are provided in part of the n-type InP layer 136. A cathode electrode 142 is connected to the n-type InP layer 136 and the p-type region 138, and an anode electrode 144 is connected to the p-type region 140. A low reflection film 146 is provided on the back surface of the n-type InP substrate 132. The back surface of the n-type InP substrate 132 is a light receiving surface for incident light. The back surface of the n-type InP substrate 132 is not provided with a substance or structure having a higher reflectance with respect to incident light than the low reflective film 146. In the optical module, the semiconductor light receiving element 130 is flip-chip mounted on a pedestal, and the back light La emitted from the semiconductor laser element enters the back surface of the n-type InP substrate 132.

JP 2011-253987 A

  In the optical module described in Patent Document 1, the semiconductor light receiving element is flip-chip mounted on a pedestal, and the back light of the semiconductor laser element is received through air. However, the light receiving form of the semiconductor light receiving element is not limited to such a form. For example, there is a form in which part of the laser light emitted from the front surface of the semiconductor laser element is branched and the branched light is received. As one of such forms, a semiconductor light receiving element is mounted on the surface of a beam splitter for branching laser light, wire bonding is performed, and the light branched by the beam splitter is transferred from the surface of the beam splitter to the semiconductor light receiving element. The form which injects into the back surface of this can be considered.

  However, the present inventor has found that the following problems exist in such a form. That is, there is a non-negligible refractive index difference between the beam splitter and the semiconductor light receiving element. For example, when the back surface of the semiconductor light receiving element is made of InP, the refractive index of InP is 3.20. On the other hand, when the beam splitter is made of glass, the refractive index of the glass is 1.50. In addition, there is a non-negligible refractive index difference between the semiconductor substrate constituting the back surface of the semiconductor light receiving element and the semiconductor stacked portion including the light receiving layer stacked on the semiconductor substrate. For example, when the semiconductor stacked portion is made of InGaAs, the refractive index of InGaAs is 3.92. As a result of experiments by the present inventor, it has been found that multiple reflection of light occurs between the interface on the beam splitter side of the semiconductor substrate and the interface on the semiconductor laminated portion side, resulting in interference. Then, it was found that the reflection characteristics at these interfaces change with temperature and wavelength, and as a result, the intensity of light incident on the light receiving layer varies with changes in temperature and wavelength.

  An object of this invention is to provide the optical module which can reduce the fluctuation | variation of the incident light intensity to the light reception layer of the semiconductor light receiving element accompanying the change of temperature and a wavelength.

  In order to solve the above-described problems, an optical module according to an embodiment of the present invention includes a semiconductor laser element and an optically coupled semiconductor laser element, and converts laser light output from the semiconductor laser element into collimated light. And a first lens that is optically coupled to the first lens, collects collimated light and guides the collimated light to the optical waveguide, and is provided on an optical path between the first lens and the second lens. A beam splitter that branches a part of the light, and a semiconductor light receiving element that receives the collimated light branched by the beam splitter. The beam splitter has a multilayer filter sandwiched between a first glass member and a second glass member. The semiconductor light receiving element includes a semiconductor substrate having a main surface and a back surface, and a semiconductor stacked portion including a light receiving layer provided on the main surface. The back surface of the semiconductor substrate is fixed to the surface of one glass member, and is optically coupled to the multilayer filter through the one glass member. An antireflection film for matching the refractive index of one glass member with the refractive index of the semiconductor substrate is provided on the back surface of the semiconductor substrate.

  According to the optical module of the present invention, it is possible to reduce fluctuations in incident light intensity to the light receiving layer of the semiconductor light receiving element due to changes in temperature and wavelength.

FIG. 1 is a cutaway perspective view showing an internal structure of an optical module according to an embodiment. FIG. 2 is a plan view schematically showing the internal structure of the optical module. FIG. 3 is a diagram schematically showing a partial configuration of the optical module. FIG. 4 is a cross-sectional view showing the configuration in the vicinity of the PD in detail. FIG. 5 is a diagram schematically showing an optical system as a comparative example. FIG. 6 is a graph showing the relationship between the output value, wavelength, and temperature of the PD when the light intensity of the collimated light incident on the PD is constant. FIG. 7 is a diagram schematically showing a laminated structure including a transparent resin film, an antireflection film, a semiconductor substrate, and a semiconductor laminated portion. FIG. 8A and FIG. 8B are diagrams showing the transmission characteristics from the transparent resin film to the semiconductor laminated portion. FIG. 9 is a graph in which the relationship between the refractive index of the antireflection film and the ratio between the maximum value and the minimum value of the transmittance from the transparent resin film to the semiconductor laminated portion is estimated. FIG. 10 is a schematic diagram showing a stacked configuration of the semiconductor light receiving elements described in Patent Document 1. As shown in FIG.

  Specific examples of the optical module according to the embodiment of the present invention will be described below with reference to the drawings. In addition, this invention is not limited to these illustrations, is shown by the claim, and intends that all the changes within the meaning and range equivalent to the claim are included. In the following description, the same reference numerals are given to the same elements in the description of the drawings, and redundant descriptions are omitted.

  FIG. 1 is a cutaway perspective view showing an internal structure of an optical module 1A according to an embodiment. FIG. 2 is a plan view schematically showing the internal structure of the optical module 1A. As shown in FIGS. 1 and 2, the optical module 1 </ b> A is a light emitting module (TOSA; Transmitter Optical SubAssembly) including a rectangular parallelepiped housing 2 and a cylindrical optical coupling portion 3 having a flange. The optical module 1A includes a plurality of semiconductor laser elements (LD) 11a to 11d, a plurality of first lenses 12a to 12d, a beam splitter (BS) 13, and a plurality of semiconductor light receiving elements (photodiodes, PD) 14a. ˜14d and the multiplexing optical system 18 are provided. In an example, the optical module 1A is a four-channel light emitting module including four LDs 11a to 11d. The LDs 11 a to 11 d, the first lenses 12 a to 12 d, the BS 13, the PDs 14 a to 14 d, and the multiplexing optical system 18 are disposed on the flat main surface of the base member 7 provided inside the housing 2.

  In the optical module 1A, the LDs 11a to 11d functioning as light sources are independently driven, and the LDs 11a to 11d individually output the laser beams L1a to L1d. Drive signals to the LDs 11a to 11d are provided from the outside of the optical module 1A. The laser beams L1a to L1d are modulated according to the drive signal. The LDs 11a to 11d are, for example, distributed feedback (DFB) lasers. Each wavelength of the laser beams L1a to L1d is, for example, a 1.3 μm band and is different from each other. The first lenses 12a to 12d are optically coupled to the LDs 11a to 11d, respectively. The laser beams L1a to L1d output from the LDs 11a to 11d are input to the first lenses 12a to 12d, respectively. Each LD 11a to 11d is disposed at the focal point of the corresponding first lens 12a to 12d. The first lenses 12a to 12d convert the laser beams L1a to L1d that are divergent light into collimated beams L2a to L2d, respectively.

  Since the light sources (LD11a to 11d) are not ideal point light sources, the laser light that has passed through the first lenses 12a to 12d is not strictly collimated light. In other words, the collimated lights L2a to L2d are quasi-collimated lights whose light diameter is reduced to the minimum at the beam waist (neck generated in the field pattern along the optical axis) and then turned into divergent light.

  The BS 13 extends in a direction intersecting with each optical axis of the first lenses 12a to 12d, and more specifically, on the optical path between the first lenses 12a to 12d and the second lens 4 described later, more specifically, the first lens. They are arranged on the optical path between 12a to 12d and the multiplexing optical system 18. The BS 13 includes a dielectric multilayer film that is inclined with respect to the optical axes of the first lenses 12a to 12d. When collimated light L2a to L2d passes through the dielectric multilayer film, the collimated light L2a. -L2d (for example, 5 to 10% of the amount of collimated light L2a to L2d) is branched. The PDs 14a to 14d are mounted on the surface of the BS 13 and receive a part of each of the branched collimated lights L2a to L2d.

  The multiplexing optical system 18 is optically coupled to the first lenses 12a to 12d via the BS 13, and multiplexes the collimated lights L2a to L2d. The multiplexing optical system 18 of this embodiment includes a first WDM filter 15, a second WDM filter 16, a mirror 17, and a polarization beam combiner 19.

  The mirror 17 is optically coupled to the first lenses 12a and 12b via the BS13. The light reflecting surface of the mirror 17 is located on the optical axes of the first lenses 12a and 12b, and is inclined with respect to these optical axes. The mirror 17 reflects the collimated lights L2a and L2b toward the direction intersecting with these optical axes.

  The first WDM filter 15 is optically coupled to the first lens 12 c via the BS 13. The wavelength selection surface of the first WDM filter 15 is located on the optical axis of the first lens 12c and is inclined with respect to the optical axis. The first WDM filter 15 transmits the collimated light L2c from the first lens 12c and reflects the collimated light L2a reflected by the mirror 17. As a result, the optical paths of the collimated lights L2a and L2c coincide with each other, and the collimated lights L2a and L2c are combined with each other to become the collimated light L3a.

  The second WDM filter 16 is optically coupled to the first lens 12d via the BS 13. The wavelength selection surface of the second WDM filter 16 is located on the optical axis of the first lens 12d and is inclined with respect to the optical axis. The second WDM filter 16 transmits the collimated light L2d from the first lens 12d and reflects the collimated light L2b reflected by the mirror 17. As a result, the optical paths of the collimated lights L2b and L2d coincide with each other, and the collimated lights L2b and L2d are combined with each other to become the collimated light L3b.

  The polarization beam combiner 19 is a plate-like member having a first surface 19a and a second surface 19b. An antireflection film 19c and a polarization filter film 19d are formed on the first surface 19a, and a reflection film 19e and an antireflection film 19f are formed on the second surface 19b. Collimated light L3a is input to the antireflection film 19c on the first surface 19a. The collimated light L3a passes through the inside of the polarization beam combiner 19, reaches the reflection film 19e on the second surface 19b, is reflected by the reflection film 19e, and then reaches the polarization filter film 19d on the first surface 19a. On the other hand, collimated light L3b is input to the polarization filter film 19d on the first surface 19a. One of the collimated lights L3a and L3b is rotated by 90 ° by a half-wave plate (not shown), so that the collimated light L3a is reflected by the polarization filter film 19d, and the collimated light L3b passes through the polarization filter film 19d. To Penetrate. As a result, the collimated lights L3a and L3b are combined with each other to become collimated light L4. The collimated light L4 is output from the polarization beam combiner 19 through the antireflection film 19f on the second surface 19b, and is output to the outside of the housing 2 through the window 2a provided on the side wall 2A of the housing 2. .

  The optical coupling unit 3 is a coaxial module having a second lens 4 and a fiber stub 6. The second lens 4 is optically coupled to the multiplexing optical system 18 and further optically coupled to the first lenses 12 a to 12 d via the multiplexing optical system 18 and the BS 13. The fiber stub 6 holds an optical fiber (optical waveguide) 5. The second lens 4 collects the collimated light L4 and guides it to the end face of the optical fiber 5. The optical coupling unit 3 is aligned with the optical axis of the collimated light L4 and then fixed to the side wall 2A of the housing 2 by welding. The optical coupling unit 3 may further include an optical isolator that blocks light from the outside in addition to the second lens 4 and the fiber stub 6.

  In the present embodiment, temperature adjustment is not performed for all components including the LDs 11a to 11d and the PDs 14a to 14d. That is, the optical module 1A does not include a TEC (Thermo-Electric Controller) such as a Peltier element that electrically controls the temperatures of the LDs 11a to 11d, the PDs 14a to 14d, and the like. In a communication system (for example, CWDM: Coarse Wavelength Division Multiplexing) in which the wavelength band allocated to each of the four signals is relatively wide, even if the oscillation wavelength of a certain LD is shifted due to temperature change, the optical signal output from the LD The optical signals output from other LDs can be sufficiently distinguished. Therefore, the manufacturing cost of the optical module 1A can be reduced by omitting the TEC in such a communication method.

  FIG. 3 is a diagram schematically showing a partial configuration of the above-described optical module 1A. 3 shows the LD 11, the first lens 12, and the PD 14, which correspond to the LDs 11a to 11d, the first lenses 12a to 12d, and the PDs 14a to 14d, respectively.

  As shown in FIG. 3, the laser light L1 output from the LD 11 (corresponding to the laser light L1a to L1d in FIG. 2) becomes collimated light L2 (corresponding to the collimated light L2a to L2d in FIG. 2) in the first lens 12. Converted. After passing through the BS 13, the collimated light L <b> 2 is combined with other collimated light to become collimated light L <b> 4, collected by the second lens 4, and incident on the end face of the optical fiber 5. A part of the collimated light L2 is branched at the BS13. Part of the branched collimated light L5 enters the PD 14 mounted on the surface of the BS 13 from the back side.

  The BS 13 includes a first glass member 13a, a second glass member 13b, and a multilayer filter 13c. The glass members 13a and 13b are made of, for example, a glass material (for example, BK-7) that is transparent with respect to the wavelength of the collimated light L2. The first glass member 13a has a lower surface 13d that is fixed to the bottom surface of the housing 2 and a slope 13f that is inclined with respect to the optical path of the collimated light L2. The second glass member 13b has a surface 13e on which the PD 14 is mounted and an inclined surface 13g that is inclined with respect to the optical path of the collimated light L2. The slope 13f of the first glass member 13a and the slope 13g of the second glass member 13b are arranged to face each other. The multilayer filter 13c is sandwiched and fixed between the slope 13f and the slope 13g. The number of layers, the refractive index, and the thickness of the multilayer filter 13c determine the branching ratio of the collimated light L2. The angles of the inclined surfaces 13f and 13g with respect to the optical path of the collimated light L2 are slightly larger than 45 °. Thereby, the incident angle of the collimated light L5 to the surface 13e formed substantially parallel to the optical path of the collimated light L2 can be made larger than 0 °, and reflected return light to the LD 11 due to Fresnel reflection can be reduced.

  The PD 14 is mounted on the BS 13 with its back surface and the front surface 13e of the BS 13 facing each other. The PD 14 receives the collimated light L5 branched by the multilayer filter 13c on the back surface. FIG. 4 is a cross-sectional view showing the configuration in the vicinity of the PD 14 in detail. As shown in FIG. 4, the PD 14 includes a semiconductor substrate 21 and a semiconductor stacked portion 22 formed on the semiconductor substrate 21. The semiconductor substrate 21 is, for example, an InP substrate. The semiconductor substrate 21 has a main surface 21a and a back surface 21b facing each other. The semiconductor stacked portion 22 is formed on the main surface 21a, and the back surface 21b faces the surface 13e of the second glass member 13b of the BS 13.

  The semiconductor stacked unit 22 includes a first contact layer 23, a light receiving layer 24, and a second contact layer 25, which are epitaxially grown in order on the main surface 21a. In the present embodiment, the first contact layer 23 is in contact with the semiconductor substrate 21, the light receiving layer 24 is in contact with the first contact layer 23, and the second contact layer 25 is in contact with the light receiving layer 24. The first contact layer 23, the light receiving layer 24, and the second contact layer 25 mainly contain, for example, InGaAs. The first contact layer 23 is n-type and is in ohmic contact with the n-side electrode 26. The light receiving layer 24 is i-type. The second contact layer 25 is p-type and is in ohmic contact with the p-side electrode 27. The semiconductor stacked portion 22 is covered with an insulating film 28, and the n-side electrode 26 and the p-side electrode 27 are respectively connected to the first contact layer 23 and the second contact layer 25 through openings formed in the insulating film 28. In contact. The n-side electrode 26 and the p-side electrode 27 are electrically connected to a wiring board disposed inside the housing 2 via a bonding wire (not shown).

  It is also conceivable to mount the PD 14 on the BS 13 by flip chip bonding. However, flip chip bonding is disadvantageous in cost compared to wire bonding. This is because it is necessary to form a metal electrode pattern on the surface of the glass BS 13. Further, when the PD 14 is arranged for the purpose of detecting the average light intensity of the collimated light L2 as in this embodiment, it is not necessary to perform flip chip bonding in consideration of frequency characteristics. Therefore, in this embodiment, the back surface side of PD14 is adhere | attached on BS13, and it is set as the structure which performs the wire bonding of the electrodes 26 and 27 located in the surface side of PD14. Therefore, the collimated light L5 is incident on the back surface of the PD 14.

  An antireflection film 29 is provided on the back surface 21 b of the semiconductor substrate 21. The antireflection film 29 is a dielectric multilayer film for matching the refractive index of the second glass member 13b with the refractive index of the semiconductor substrate 21, and is, for example, a SiN film.

  An antireflection film that is effective when light is incident from the surface of the PD 14 may be formed on the second contact layer 25. This antireflection film is also effective when light is incident from the back side of the PD 14 as in the present embodiment, and light that has passed through the light receiving layer 24 can be prevented from returning to the light receiving layer 24 again.

  A transparent resin film 31 is provided between the antireflection film 29 and the surface 13e of the second glass member 13b. The back surface 21b of the semiconductor substrate 21 is fixed to the front surface 13e of the second glass member 13b via the transparent resin film 31. In one example, the transparent resin film 31 is formed by filling a resin between the antireflection film 29 and the surface 13e, and substances and voids other than the transparent resin film 31 are interposed between the antireflection film 29 and the surface 13e. do not do. The back surface 21b of the semiconductor substrate 21 is optically coupled to the multilayer filter 13c (see FIG. 3) via the transparent resin film 31 and the second glass member 13b. The transparent resin film 31 is, for example, an ultraviolet curable resin film. The refractive index of the transparent resin film 31 is substantially equal to the refractive index of the second glass member 13b (in the case of BK-7, 1.50).

  Here, problems to be solved by the optical module 1A of the present embodiment will be described. FIG. 5 is a diagram schematically showing an optical system 100 as a comparative example, in which a lens 103 is provided between a lens 102 and a lens 104. The distance between the LD 101 and the lens 102 is set to be longer than the focal position of the lens 102, and the laser light L output from the LD 101 once converges between the lens 102 and the lens 103 to form a beam waist. The lens 103 is collimated. The BS 113 is disposed between the lens 102 and the lens 103 and branches a part of the converged laser beam L to provide it to the PD 114. A part of the laser light L is incident on the PD 114 while being converged.

  In the optical system as shown in FIG. 5, three lenses 102 to 104 are necessary for propagating the laser light L, and a reduction in the number of lenses is desired in order to reduce the size and cost of the optical module. Therefore, a two-lens system as shown in FIG. 3 can be considered. That is, the lens 103 in FIG. 5 is removed, and the lens 102 converts the laser light L into collimated light. Compared with the configuration shown in FIG. 5, one lens can be reduced for each LD, which can contribute to miniaturization and cost reduction of the optical module.

  Here, in the configuration of the two lens system, the present inventor has found the following problem. In the two-lens configuration, the collimated light is incident on the PD, unlike the configuration shown in FIG. In that case, multiple reflections are remarkably generated inside the PD, and the influence of interference occurs on the intensity of light incident on the light receiving layer. As a result, when the temperature or wavelength varies, the output value of the PD varies. Usually, in an apparatus including an optical module such as an optical transmitter, the average light intensity of laser light is controlled to be constant by controlling the magnitude of the drive current to the LD based on the output value from the PD (Auto Power Control). Therefore, the fluctuation of the output value of the PD becomes a factor that fluctuates the average light intensity of the laser light.

  The inventor conducted an experiment on the interference. FIG. 6 is a graph showing the relationship between the output value, wavelength, and temperature of the PD when the light intensity of the collimated light incident on the PD is constant. The horizontal axis in FIG. 6 represents the wavelength of light, and the vertical axis represents the output value (relative value) of the PD. Graphs G11 to G16 in the figure show PD output values when the PD temperature is 25 ° C, 40 ° C, 50 ° C, 60 ° C, 70 ° C, and 80 ° C, respectively. The PD used in this experiment is provided with an antireflection film (having a transmission characteristic optimized for air (refractive index 1.0), for example, see Patent Document 1) on the back surface of the semiconductor substrate. It has been.

  Referring to FIG. 6, it can be understood that the output value of the PD fluctuates in a sine wave with a large amplitude (for example, about 0.6 dB) according to a change in wavelength, and fluctuates up and down according to a change in temperature. . This suggests that interference due to multiple reflection occurs inside the PD. The fluctuation period shown in FIG. 6 is estimated to be about 1.3 nm. Based on this fluctuation period, the refractive index of InP, and the wavelength of collimated light, the distance between the end faces of multiple reflections was calculated to be 203.1 μm, which almost coincided with the thickness of the semiconductor substrate of PD (200 μm). Therefore, one end face of the multiple reflection is an interface between the semiconductor substrate and the BS (or an interface between the semiconductor substrate and the transparent resin film), and the other end face is an interface between the semiconductor substrate and the semiconductor stacked portion. I guess that.

  When the semiconductor substrate is an InP substrate, the refractive index of the semiconductor substrate is 3.20. When the BS is made of glass, the refractive index of the BS is 1.50. When the semiconductor multilayer portion is InGaAs, the refractive index of the semiconductor multilayer portion is 3.92. Thus, there is a large refractive index difference that cannot be ignored between the semiconductor substrate and the BS and between the semiconductor substrate and the semiconductor stacked portion. From this, it can be said that the certainty of the above estimation is extremely high.

  In the optical system shown in FIG. 6, the laser light is incident on the PD while being converged. In this case, since the incident angle to the PD varies within the laser beam, it is considered that the above-described problem due to interference does not easily occur.

  In order to reduce the multiple reflection, it is desirable to reduce the reflectivity of any of the interface between the semiconductor substrate and the BS and the interface between the semiconductor substrate and the semiconductor stacked portion. However, in order to reduce the reflectivity at the interface between the semiconductor substrate and the semiconductor stacked portion, it is necessary to change the semiconductor material of the semiconductor stacked portion, but it is difficult to change the semiconductor material in order to obtain desired photoelectric conversion characteristics. is there. Further, the reflectance given by this interface is 1.0% or less and is relatively small. Therefore, the present inventor has focused on reducing the reflectance at the interface between the semiconductor substrate and the BS.

  That is, in the present embodiment, the antireflection film 29 is provided on the back surface 21 b of the semiconductor substrate 21 of the PD 14. The antireflection film 29 is a transmission suitable for reducing reflection at the interface between the semiconductor substrate 21 and the transparent resin film 31 (or the second glass member 13b of the BS 13 having the same refractive index as the transparent resin film 31). It has characteristics.

  FIG. 7 is a diagram schematically showing a laminated structure including the transparent resin film 31, the antireflection film 29, the semiconductor substrate 21, and the semiconductor laminated portion 22. The collimated light L5 having a wavelength of 1.30 to 1.32 μm incident on the PD 14 enters the antireflection film 29 at an incident angle of 10 °, for example, and then passes through the antireflection film 29 and the semiconductor substrate 21, and then the semiconductor stacked portion 22 To reach. In this structure, if the case where the antireflection film 29 does not exist is calculated, the transmission characteristics from the transparent resin film 31 to the semiconductor laminated portion 22 are as shown in FIG. In FIG. 8A, the maximum value of the transmittance is -0.343 dB, the minimum value of the transmittance is -0.991 dB, the ratio between the maximum value and the minimum value is 0.648 dB, and the period is 1.35 nm. Further, if a case where an antireflection film optimized for air (refractive index n = 1.789, thickness d = 0.183 μm) is present instead of the antireflection film 29 is calculated, the transparent resin film 31 is calculated. The transmission characteristics from the semiconductor layer 22 to the semiconductor laminated portion 22 are the characteristics shown in FIG. In FIG. 8B, the maximum value of the transmittance is -0.045 dB, the minimum value of the transmittance is -0.401 dB, the ratio between the maximum value and the minimum value is 0.356 dB, and the period is 1.35 nm.

  When there is no antireflection film, the variation in transmittance due to the change in wavelength is as large as 0.648 dB. However, even when the antireflection film is provided, it is optimized for air. In such a case, the change in transmittance due to the change in wavelength is as large as 0.356 dB. In order to bring the fluctuation close to zero, the refractive index n of the antireflection film 29 is preferably increased to 2.191. This results in a non-reflective condition and no interference occurs. As a trial calculation, if the refractive index of the antireflection film 29 is 2.07, the change in transmittance due to interference can be suppressed to 0.1 dB or less. Furthermore, if the refractive index of the antireflection film 29 is set to 2.14 to 2.26, the change in transmittance due to interference can be suppressed to 0.05 dB or less.

  FIG. 9 is a graph in which the relationship between the refractive index of the antireflection film 29 and the ratio between the maximum value and the minimum value of the transmittance from the transparent resin film 31 to the semiconductor laminated portion 22 is estimated. As shown in FIG. 9, in order to make this ratio 0.1 dB or less, the refractive index of the antireflection film 29 is preferably 2.07 or more. When the refractive index of the antireflection film 29 is increased to 2.2, there is substantially no reflection interface between the semiconductor substrate 21 (InP) and the transparent resin film 31, and almost no fluctuation in transmitted light intensity occurs. Disappear.

  Note that the thickness of the antireflection film 29 also affects the variation in transmittance. According to the calculation of the present inventor, when the ratio of the maximum value to the minimum value of the transmittance is less than 0.1 dB, the thickness of the antireflection film 29 is 0.15 μm or more and 0.16 μm or less. , The transmittance variation is substantially eliminated.

In order to realize the above-described refractive index with respect to the antireflection film 29, the antireflection film 29 is preferably a SiN film. This is because the refractive index of SiN can take a value in the range of about 1.6 to 2.5, depending on the Si content and thickness of SiN. The upper limit of the refractive index of SiON is about 2.0, and the upper limit of the refractive index of SiO ₂ is about 1.45.

  For example, in an optical module used in a system having a short wavelength interval between LDs such as DWDM (Dense Wavelength Division Multiplexing), an optical system including each LD is mounted on the TEC in order to suppress fluctuation of each wavelength due to a temperature change. Thus, the temperature is controlled to be constant. Also in this embodiment, if the temperature control by TEC is performed, the fluctuation of the wavelength and the temperature change of the PD are suppressed, and as a result, the fluctuation of the transmittance can be avoided. However, since the optical module 1A of this embodiment does not include a TEC, it is very effective to dispose the antireflection film 29 described above.

  As described above, according to the optical module 1A of the present embodiment, it is possible to reduce fluctuations in the incident light intensity on the light receiving layer of the PD due to changes in temperature and wavelength. Therefore, the average light emission intensity of the LD can be controlled stably. In addition, since the optical system can be constituted by a two-lens system, and the TEC can be made unnecessary, the optical module 1A can be reduced in size and the cost can be reduced.

  The optical module according to the present invention is not limited to the above-described embodiment, and various other modifications are possible. For example, in the above embodiment, an antireflection film is formed on a photodiode having an InGaAs stacked portion on an InP substrate. However, in the present invention, an antireflection film may be formed on photodiodes of various other semiconductor materials. Good. In the above embodiment, the transparent resin film is interposed between the antireflection film and the second glass member, but the antireflection film and the second glass member may be in direct contact with each other.

  DESCRIPTION OF SYMBOLS 1A ... Optical module, 2 ... Housing | casing, 2A ... Side wall, 2a ... Window, 3 ... Optical coupling part, 4 ... 2nd lens, 5 ... Optical fiber, 6 ... Fiber stub, 7 ... Base member, 11, 11a-11d Semiconductor laser element (LD), 12, 12a to 12d ... First lens, 13 ... Beam splitter (BS), 13a ... First glass member, 13b ... Second glass member, 13c ... Multilayer filter, 14, 14a to 14d ... PD, 15 ... first WDM filter, 16 ... second WDM filter, 17 ... mirror, 18 ... multiplexing optical system, 19 ... polarization synthesizer, 21 ... semiconductor substrate, 22 ... semiconductor laminate, 23 ... contact Layer, 24 ... light-receiving layer, 25 ... contact layer, 26 ... n-side electrode, 27 ... p-side electrode, 28 ... insulating film, 29 ... antireflection film, 31 ... transparent resin film, 100 ... optical system, 101 ... LD, 102 ~ 04 ... lens, 113 ... BS, 114 ... PD, L1, L1a~L1d ... laser light, L2, L2a to L2d ... collimated light, L3a, L3b ... collimated light, L4, L5 ... collimated light.

Claims (7)

A semiconductor laser element;
A first lens optically coupled to the semiconductor laser element and converting laser light output from the semiconductor laser element into collimated light;
A second lens optically coupled to the first lens, condensing the collimated light and guiding it to an optical waveguide;
A beam splitter provided on an optical path between the first lens and the second lens and branching a part of the collimated light;
A semiconductor light receiving element that receives the collimated light branched by the beam splitter;
With
The beam splitter has a multilayer filter sandwiched between a first glass member and a second glass member,
The semiconductor light receiving element includes a semiconductor substrate having a main surface and a back surface, and a semiconductor stacked portion including a light receiving layer provided on the main surface,
The back surface of the semiconductor substrate is fixed to the surface of one of the glass members, and optically coupled to the multilayer filter through the one glass member,
An optical module, wherein an antireflection film for matching the refractive index of the one glass member and the refractive index of the semiconductor substrate is provided on the back surface of the semiconductor substrate. The semiconductor substrate is an InP substrate;
The optical module according to claim 1, wherein the layer in contact with the semiconductor substrate of the semiconductor stack includes InGaAs.   The optical module according to claim 1, wherein the antireflection film is a SiN film. A resin film provided between the antireflection film and the one glass member;
The optical module according to claim 1, wherein a refractive index of the resin film is equal to a refractive index of the one glass member.   The optical module as described in any one of Claims 1-4 which does not provide the element which electrically controls the temperature of the said semiconductor laser element and the said semiconductor light receiving element. A plurality of semiconductor laser elements that output laser beams of different wavelengths;
A plurality of first lenses that are optically coupled to the plurality of semiconductor laser elements and convert the laser light output from each semiconductor laser element into collimated light;
A multiplexing optical system optically coupled to the plurality of first lenses and configured to multiplex a plurality of the collimated lights with each other;
A second lens optically coupled to the multiplexing optical system, condensing the combined collimated light and guiding it to an optical waveguide;
A beam splitter provided on an optical path between the plurality of first lenses and the multiplexing optical system, and branching a part of each collimated light;
A plurality of semiconductor light receiving elements for receiving each collimated light branched by the beam splitter;
With
The beam splitter has a multilayer filter sandwiched between a first glass member and a second glass member,
Each semiconductor light receiving element includes a semiconductor substrate having a main surface and a back surface, and a semiconductor stacked portion including a light receiving layer provided on the main surface,
The back surface of the semiconductor substrate is fixed to the surface of one of the glass members, and optically coupled to the multilayer filter through the one glass member,
An optical module, wherein an antireflection film for matching the refractive index of the one glass member and the refractive index of the semiconductor substrate is provided on the back surface of the semiconductor substrate.   The optical module according to claim 1, wherein the antireflection film has a refractive index of 2.05 to 2.35.

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