JP4735574B2 - Semiconductor laser and semiconductor laser module - Google Patents

Description

  The present invention relates to a semiconductor laser and a semiconductor laser module used in an optical communication system, and more particularly to a semiconductor laser and a semiconductor laser module that suppress performance deterioration with respect to reflected return light.

  In an optical communication system, a single mode laser beam having a specific wavelength is often used. One such laser light source is a distributed feedback semiconductor laser (DFB laser) in which a diffraction grating is formed in or near an active layer. In the DFB laser, only light having a specific wavelength determined by the pitch of the diffraction grating can be generated, so that single mode oscillation can be stably obtained. For this reason, DFB lasers are widely used as light sources for today's optical fiber communication systems.

  However, in a semiconductor laser used in a conventional optical communication system, light that is emitted and then partially reflected at an external optical component or a connection point of an optical fiber is returned to the inside of the element (hereinafter referred to as reflected return light). There has been a problem of a decrease in performance due to reflected return light, such as an increase in the oscillation line width due to, and an increase in relative noise intensity (RIN). In particular, in a laser having a high coherence of emitted laser light such as a DFB laser, an increase in oscillation line width or RIN is likely to occur due to reflected return light.

  In order to avoid the influence of the reflected return light, an optical isolator is often arranged at the output end of the laser. Since the optical isolator has a property of transmitting light only in one direction, it is possible to prevent reflected return light from entering the element.

Non-Patent Document 1 shows a DFB laser in which a diffraction grating is formed only on a part of the emission end side in a resonator, and the intensity of reflected return light is 0.01% (10 ⁻⁴ ) or less. On the other hand, it was shown that RIN is reduced compared with a normal DFB laser (FIG. 3).

  Patent Document 1 discloses that in a semiconductor laser having a quantum well structure composed of a well layer and a barrier layer, the well layer can be a tensile strained quantum well layer, and the fluctuation in oscillation wavelength can be suppressed by reducing the line width increase coefficient. It has been shown.

Y. Huang, et al. "External optical feedback resistant charactaristics in partially correlated laser diodes", ELECTRONICS LETTERS, UK, May 23, 1996. 32, no. 11, pp1008-1009 JP 2000-277851 A

  In a conventional semiconductor laser for optical communication, when an optical isolator is disposed on the output end side, the manufacturing cost increases by the amount of the optical isolator as a new component, and the configuration of the semiconductor laser module is complicated.

Further, in a laser structure in which a diffraction grating is formed only at a part on the exit end side in the resonator as in the DFB laser of Non-Patent Document 1, strong reflection with an intensity of 0.1% (10 ⁻³ ) or more. Since a part of the return light enters the active layer and is amplified, the RIN is not significantly lowered as compared with a normal DFB laser in which the diffraction grating is formed over the entire area, and therefore, the return light is caused by a relatively strong reflected return light. The effect of suppressing performance degradation is not sufficient.

  In addition, the semiconductor laser disclosed in Patent Document 1 has an effect of reducing fluctuations in the oscillation wavelength due to reflected return light entering the active layer, but has no effect of reducing light entering the active layer. not enough.

  Accordingly, the present invention is a semiconductor laser and semiconductor capable of suppressing performance degradation such as RIN even when there is relatively strong reflected return light, and that can be used in an optical communication system without an optical isolator. An object is to provide a laser module.

  The semiconductor laser of the present invention is a semiconductor laser having a window region made of a semiconductor having a band gap larger than that of the active layer between the emission end surface of the laser beam and the active layer, and the window region is on the emission end surface side of the active layer. A first semiconductor layer extending from the vicinity of the end face to the emission end face, and a second semiconductor layer and a third semiconductor layer sandwiching the first semiconductor layer in the thickness direction, and the refractive index of the first semiconductor layer is The refractive index of the second semiconductor layer and the third semiconductor layer is lower, the thickness of the first semiconductor layer is greater than or equal to the thickness of the active layer, and less than or equal to the thickness of the second semiconductor layer. A semiconductor laser having a thickness less than that of the layer was obtained.

Further, the semiconductor laser module of the present invention includes the above-described semiconductor laser of the present invention, a lens for collecting the emitted light of the semiconductor laser, an optical fiber on which the collected emitted light is incident,
The semiconductor laser module includes a polarizer between the semiconductor laser and the optical fiber.

  As described above, the semiconductor laser of the present invention includes a first semiconductor layer extending from the vicinity of the end face on the emitting end face side of the active layer to the emitting end face, a second semiconductor layer sandwiching the first semiconductor layer in the thickness direction, and A refractive index of the first semiconductor layer is lower than that of the second semiconductor layer and the third semiconductor layer, and the thickness of the first semiconductor layer is the thickness of the active layer. As described above, since the window region having a thickness equal to or smaller than the thickness of the second semiconductor layer and equal to or smaller than the thickness of the third semiconductor layer is provided, the reflected return light does not directly enter the active layer but passes through the window region. It reaches the active layer after being attenuated. As a result, performance degradation can be suppressed even when there is relatively strong reflected return light.

  In addition, since the semiconductor laser module of the present invention includes a polarizer between the semiconductor laser strong against reflected return light and the optical fiber as described above, it suppresses performance degradation even when there is stronger reflected return light. Can do.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The same components are denoted by the same reference numerals, and the description thereof will not be repeated.

(Embodiment 1)
FIG. 1 is a cross-sectional view showing a basic configuration of the semiconductor laser according to the first embodiment. On a p-InP substrate 8 having a length L, a p-InP lower cladding layer 14, an active layer 3 thereon, an n-InP upper cladding layer 13 thereon, and an n-InGaAsP contact layer 12 thereon. . Here, the active layer 3 has a multiple quantum well structure in which layers having different compositions of InGaAsP are stacked. A p-side electrode 2 is formed below the p-InP substrate 8, and an n-side electrode 1 is formed on the n-InGaAsP contact layer 12.

  When viewed in the direction along the substrate 8, there is an emission end face 9 from which one of these layers emits laser light 17 and a reflection end face 7 on the other side. A low reflection film 16 is coated on the emission end face 9 side, and a high reflection film 15 is coated on the opposite reflection end face 7.

  As shown in FIG. 1, the active layer 3 is not directly in contact with the emission end face 9 among the above layers, and a semiconductor having a larger band gap than the active layer 3 between the emission end face 9 of the laser beam and the active layer 3. It has a window region 11 made of InP. The window region 11 includes a first semiconductor layer 21 extending from the vicinity of the end face on the emission end face side of the active layer 11 to the emission end face 9. On the opposite side of the first semiconductor layer 21 from the p-InP substrate 8, there is a second semiconductor layer including a part of the n-InP upper cladding layer 13. Further, on the p-InP substrate side of the first semiconductor layer 21, there is a third semiconductor layer 23 including a p-InP lower cladding layer 14 and a part of the p-InP substrate 8. Accordingly, the first semiconductor layer is sandwiched between the second semiconductor layer 22 and the third semiconductor layer 23 in the thickness direction. The refractive index of one semiconductor layer 21 is lower than the refractive indexes of the second semiconductor layer 22 and the third semiconductor layer 23. Further, the thickness of the first semiconductor layer is not less than the thickness of the active layer 3, not more than the thickness of the second semiconductor layer 22, and not more than the thickness of the third semiconductor layer 23.

  FIG. 2 is a cross-sectional view showing the configuration of the semiconductor laser according to the first embodiment in more detail. There is a current confinement layer 10 in the window region 11 between the vicinity of the side surface on the emission end face 9 side of the active layer 3 and the emission end face 9. The current confinement layer 10 is sandwiched between the lower p-InP lower cladding layer 14 and the upper n-InP upper cladding layer 13 in the thickness direction of the layer. Here, the current confinement layer 10 is a p-InP / n-InP / p-InP in which semiconductor layers having different conductivity types of a p-InP layer 10c, an n-InP layer 10b, and a p-InP layer 10a are sequentially stacked from the substrate side. It has a structure. The first semiconductor layer 21 in FIG. 1 is the n-InP layer 10b in FIG. 2, and the second semiconductor layer 22 in FIG. 1 is a layer in which the p-InP layer 10a and the n-InP upper cladding layer 13 in FIG. The third semiconductor layer 23 in FIG. 1 corresponds to the p-InP layer 10c, the p-InP lower cladding layer 14, and the p-InP substrate 8 layered in FIG.

  There is an n-InP buried layer 4 between the active layer 3 and the n-InP upper cladding layer 13, and a length Lg close to the emission end face 9 side at a position close to the active layer 3 in the n-InP buried layer 4. In the extending diffraction grating region 6, there is a diffraction grating 5 made of an InGaAsP guide layer along the active layer 3.

  Here, the impurity concentration of the n-InP layer 10b of the current confinement layer 10 is higher than the impurity concentration of the p-InP layer 10c and the p-InP layer 10a sandwiching the n-InP layer 10b from above and below, and the current confinement layer 10 is higher than the impurity concentration of the p-InP substrate 8, the p-InP lower cladding layer 14, and the n-InP upper cladding layer 13 located above and below the thickness direction 10.

  Since the refractive index is lowered due to the plasma effect by increasing the impurity concentration, the n-InP layer 10b in the current confinement layer 10 is composed of the p-InP layer 10c and the p-InP layer 10a sandwiched from above and below, and further the p-InP layer 10b. The refractive index is lower than that of the InP substrate 8, the p-InP lower cladding layer 14, and the n-InP upper cladding layer 13. Accordingly, the semiconductor layer extending from the vicinity of the end face on the emission end face side of the active layer 3 to the emission end face has a lower refractive index than the upper and lower semiconductor layers sandwiching the semiconductor layer.

For example, when the impurity concentration of the n-InP layer 10b is set to a value in the range of 7 × 10 ¹⁸ to 2 × 10 ¹⁹ cm ⁻³ , the p-InP layer 10c and the p-InP layer 10a sandwiching the n-InP layer 10b from above and below, The impurity concentration of the p-InP substrate 8, the p-InP lower cladding layer 14, and the n-InP upper cladding layer 13 is set to a value in the range of 1 × 10 ^{18 to} 5 × 10 ¹⁸ cm ⁻³ . As a result, the refractive index of the n-InP layer 10b is about 0.1% to several percent lower than the refractive index of the semiconductor layer sandwiching the layer from above and below in the thickness direction.

  The thickness of the n-InP layer 10b is equal to or greater than the thickness of the active layer 3, and is equal to or less than the p-InP substrate 8 and the n-InP upper cladding layer 13. Therefore, the thickness of the n-InP layer 10b is equal to or less than the thickness of the semiconductor layer sandwiching the n-InP layer 10b from above and below in the thickness direction. Therefore, the semiconductor layer having a low refractive index extending from the vicinity of the end face on the emission end face side of the active layer 3 toward the emission end face is not less than the thickness of the active layer 3 and not more than the thickness of the semiconductor layer sandwiching the semiconductor layer.

  For example, the active layer 3 has a thickness of 0.1 to 0.3 μm, the n-InP layer 10 b has a thickness of 0.4 to 1 μm, the n-InP upper cladding layer 13 has a thickness of 1 to 3 μm, p− The thickness of the InP substrate 8 is set to 30 to 100 μm or the like. The total length of the substrate, that is, the distance L between the emission end face 9 and the reflection end face 7 is 250 to 400 μm, the length Lg of the diffraction grating region 6 is 50 to 150 μm, and the length of the window region 11 is 20 to 50 μm.

  FIG. 3 is a perspective view showing the configuration of the semiconductor laser according to the first embodiment. In FIG. 3, the shape cut at the cross section 20 corresponds to FIG. In FIG. 3, the low reflection film 16 on the emission end face and the high reflection film 15 on the reflection end face are omitted. The active layer 3 has a stripe shape with a width of about 1-2 microns, and its longitudinal direction is along the direction connecting the emission end face and the reflection end face. The side surface of the active layer 3 is buried with a current confinement layer 10 made of p-InP / n-InP / p-InP.

  Next, a method for manufacturing the semiconductor laser according to the first embodiment will be briefly described. On the p-InP substrate 8, a p-InP lower cladding layer 14, an active layer 3 thereon, and an n-InP buried layer 4 thereon are sequentially laminated using a semiconductor epitaxial growth method. In the middle of stacking the n-InP buried layer 4, the diffraction grating 5 is formed in the vicinity of the active layer 3 by stacking the InGaAsP guide layer and etching. Here, the diffraction grating 5 is set so that its Bragg reflection wavelength is within a wavelength range in which gain can be obtained by laser oscillation of the active layer 3.

  For example, when the wavelength range in which gain is obtained by laser oscillation of the active layer 3 is 1.4 to 1.6 μm, the Bragg reflection wavelength of the diffraction grating 5 is set to an appropriate wavelength in the range of 1.45 to 1.55 μm.

Next, a SiO ₂ film or a SiN _X film is formed thereon, and a stripe-shaped resist pattern having a width of about 1-2 microns is created by photolithography. Using this resist pattern as a mask, the SiO ₂ film or the SiN _X film is formed. Etch. After removing the resist, the patterned SiO ₂ film or SiN _X film is used as a mask to further etch the n-InP buried layer 4, the active layer 3, and the p-InP lower cladding layer 14. In FIG. 2, etching is performed up to the position of the bottom surface of the p-InP layer 10 c at the bottom of the current confinement layer 10. As a result, an etching region having a step portion higher than the thickness of the active layer 3 is formed outside the side surface of the active layer 3. Therefore, this etching region is a space extending from the end face on the emission end face side of the active layer 3 in the emission end face direction and thicker than the thickness of the active layer 3.

Thereafter, a current confinement layer 10 made of p-InP / n-InP / p-InP is formed by using a patterned SiO ₂ or SiN _X film as a mask for selective growth using MOCVD, LPE, or the like. The etching region outside the side surface of the active layer 3 is buried until it becomes almost the same height as the n-InP buried layer 4. At this time, the thicknesses of the p-InP layer 10c, the n-InP layer 10b, and the p-InP layer 10a can be adjusted by the film formation speed and the film formation time. Here, the thickness of the n-InP layer 10 b is set to be equal to or greater than the thickness of the active layer 3.

  Note that the p-InP / n-InP / p-InP layers of the current confinement layer 10 in the vicinity of the end face on the emission end face side of the active layer 3 are as shown in FIG. 2 depending on the shape of the etching region, film formation conditions, and the like. Often, it becomes a tilted layer. Even in such a case, these layers can be regarded as extending from the end face of the active layer 3 on the exit end face side in the exit end face direction.

  For example, if the height of the step portion outside the side surface of the active layer 3 is set to three times or more the thickness of the active layer 3 during etching, the p-InP layer 10c, the n-InP layer 10b, and the p-InP layer 10a Even if each layer has the same thickness, the thickness of the n-InP layer 10b is equal to or greater than the thickness of the active layer 3.

  Further, the conductivity type and impurity concentration of each layer of the current confinement layer 10 can be adjusted by the concentration of impurities added during film formation. In the case of InP, an impurity such as Zn is added to make it P type, and an impurity such as S or Se is added to make it n type. Here, the impurity concentration of the n-InP layer 10 b is set higher than the impurity concentration of the p-InP layer 10 c and p-InP layer 10 a of the current confinement layer 10, the p-InP substrate 8, and the p-InP lower cladding layer 14. Thereby, the refractive index of the n-InP layer 10b can be lowered.

Thereafter, the SiO ₂ or SiN _X film is removed, and an n-InP upper cladding layer 13 having an impurity concentration lower than that of the n-InP layer 10b and equal to or greater than the thickness of the n-InP layer 10b is laminated to embed the entire upper surface. . Further, the n-InGaAsP contact layer 12 is laminated, the n-side electrode 1 is formed, and the low-reflection film 16, the reflection is formed after cutting at the output end surface 9 and the reflection end surface 7 that form the p-side electrode 2 on the back surface of the p-InP substrate 8. A highly reflective film 15 is formed on the end face 7 side.

  By the procedure as described above, a window region made of a semiconductor having a band gap larger than that of the active layer is provided between the emission end surface and the active layer, and the window region extends from the vicinity of the end surface on the emission end surface side of the active layer to the emission end surface. A first semiconductor layer extending to the thickness direction, a second semiconductor layer sandwiching the semiconductor layer in the thickness direction, and a third semiconductor layer, and the refractive index of the first semiconductor layer is the second semiconductor layer and the third semiconductor layer A semiconductor having a refractive index lower than the refractive index of the semiconductor layer, the thickness of the first semiconductor layer being equal to or greater than the thickness of the active layer, equal to or less than the thickness of the second semiconductor layer, and equal to or less than the thickness of the third semiconductor layer. A laser can be produced.

  Next, the operation of this semiconductor laser will be described. Between the n-side electrode 1 and the p-side electrode 2 of this semiconductor laser, a current flows in the forward direction via the n-InGaAsP contact layer 12, the n-InP upper cladding layer 13, and the p-InP lower cladding layer 14, A laser beam is generated in the active layer 3 by injecting current into the active layer 3 composed of multiple quantum wells. Since the active layer 3 made of InGaAsP has a higher refractive index than the n-InP upper cladding layer 13 and the p-InP lower cladding layer 14, the generated light is almost confined in the active layer 3 and propagates. Since the diffraction grating 5 is optically coupled to the adjacent active layer 3, the light propagating through the active layer 3 having the Bragg reflection wavelength of the diffraction grating 5 is reflected in the active layer 3. When the gain exceeds the loss due to the feedback of light from the diffraction grating 5 and the reflection end face 7 covered with the high reflection film 15, laser oscillation occurs. The laser light from the active layer 3 is emitted from the emission end face 9 through the window region 11, but the window region 11 is made of a semiconductor material having a band gap larger than that of the active layer 3, and thus transmits the laser light from the active layer 3. Most of the output light 17 is extracted from the emission end face 9 covered with the low reflection film 16. The wavelength of the emitted laser output light is determined by the Bragg reflection wavelength of the diffraction grating 5 provided at a position close to the active layer 3.

  It is output by adjusting the optical coupling strength (coupling coefficient) between the active layer 3 and the diffraction grating 5, the length Lg of the diffraction grating region 6, the total length of the active layer 3, its refractive index, and the like. The laser beam can be a single mode.

  In addition, between the vicinity of the end face of the active region 3 on the emission end face 9 side of the window region 11 and the exit end face 9 is formed by a current confinement layer 10 made of p-InP / n-InP / p-InP. The current confinement layer 10 is a layer whose resistance is increased so that a current flows mainly only in the active layer 3 when a current is supplied to the laser. In the first embodiment, by stacking layers of different conductivity types such as p-InP / n-InP / p-InP, current is blocked by a depletion layer having a very high resistance generated at the pn junction at the interface between them. Thus, the current flows only in the active layer 3 without the current confinement layer 10. As a result, the efficiency of laser light oscillation can be increased.

  Next, the effect | action of the window area | region 11 in this Embodiment 1 is demonstrated. FIG. 8 is a cross-sectional view schematically showing a configuration of a conventional semiconductor laser which is a comparative example of the first embodiment. The semiconductor laser has a diffraction grating, and the active layer extends over the entire length of the emission end face and the reflection end face. This is a formed semiconductor laser. Compared with FIG. 2 of the structure of the first embodiment, the window area 11 is not provided.

  The laser light emitted from the emission end face of the semiconductor laser is partially reflected by an external optical component or the like, and a part thereof returns to the emission end face 9 side of the semiconductor laser. When light incident from the emission end face 9 enters the active layer 3, stimulated emission occurs using the light as a seed, and another signal is added to the original laser light signal to increase RIN. Further, when the refractive index changes due to the influence of light entering the active layer 3, performance degradation such as expansion of the oscillation wavelength band occurs.

  In an optical waveguide structure in which a region having a high refractive index is surrounded by a region having a low refractive index, light propagates through the region having a high refractive index. In the structure having no window region as shown in FIG. 8, the active layer 3 having a higher refractive index than the n-InP upper cladding layer 13 and the p-InP lower cladding layer 14 is provided up to the emission end face 9, and thus the optical waveguide up to the emission end face 9. It has the structure of. Light entering the vicinity of the active layer 3 on the emission end face 9 is more easily propagated in the active layer 3 having a higher refractive index than in the clad layer, so that most of the light is coupled into the active layer 3. Therefore, in the conventional structure having no window region, the reflected return light entering the active layer 3 cannot be reduced.

  On the other hand, in the structure having the window region 11 as shown in FIG. 2 of the first embodiment, since the optical waveguide structure is not formed in the window region 11, the reflected return light passes through the window region 11 and is coupled to the active layer. Mode conversion occurs until the n-InP upper clad layer 13, the p-InP lower clad layer 14, and the p-InP substrate 8 are widely dispersed. The dispersed light, for example, is absorbed by the electrodes above and below them and reaches the active layer 3 and attenuates before being coupled. Thus, since the reflected return light couple | bonded with the active layer 3 by the window area | region 11 can be reduced, the performance degradation by reflected return light can be suppressed.

  Furthermore, in the semiconductor laser of the first embodiment, a semiconductor layer extending in the window region 11 from the vicinity of the end face on the exit end face side of the active layer toward the exit end face has a higher refractive index than the semiconductor layer in the thickness direction. The low refractive index layer is sandwiched between the layers. In FIG. 2, the low refractive index layer corresponds to the n-InP layer 10b in the current confinement layer 10, and the n-InP layer 10b has a p-InP layer 10c, a p-InP lower cladding layer 14, and a p-InP substrate on the lower side. 8 is sandwiched between a semiconductor layer having a higher refractive index than that of the n-InP layer 10b, and a semiconductor layer having a higher refractive index than that of the n-InP layer 10b including the p-InP layer 10a and the p-InP upper cladding layer 13. It is.

  In the window region 11, the thickness of the low refractive index n-InP layer 10 b is equal to or greater than the thickness of the active layer 3, and each semiconductor layer has a higher refractive index than the n-InP layer 10 b sandwiching the n-InP layer 10 b. The thickness is below.

  Note that the thickness of the p-InP layer 10a and the thickness of the p-InP layer 10c may be smaller than the thickness of the n-InP layer 10b having a low refractive index. In that case, the n-InP upper cladding layer 13 having a higher refractive index and a thicker thickness than the n-InP layer 10b is provided on the opposite side of the p-InP layer 10a to the n-InP layer 10b. The p-InP lower cladding layer 14 and the p-InP substrate 8 that have a higher refractive index and a larger thickness than the n-InP layer 10b may be provided on the side opposite to the -InP layer 10b. In addition, the layer having a low refractive index in the current confinement layer 10 is not necessarily the n-InP layer 10b. In the window region 11, the active layer 3 has a layer thicker than the active layer 3 and a lower refractive index than the upper and lower semiconductor layers on the emission end face 9 side. It may be configured to be thinner than the semiconductor layer.

  As a result of the above, the light incident on the n-InP layer 10b is coupled to the upper and lower semiconductor layers having a higher refractive index so as to propagate through the semiconductor layers. 10b is a layer in which light does not easily propagate. Therefore, the light propagating through the low refractive index n-InP layer 10b in the window region 11 becomes weak. Since the n-InP layer 10b extends from the vicinity of the end face of the active layer 3 on the exit end face 9 side in the direction of the exit end face 9, the reflected return light returning from the exit end face 9 side is coupled to the active layer 3 Will decrease. As a result, performance degradation can be suppressed even when there is relatively strong reflected return light.

  Further, the thickness of the n-InP layer 10b having a low refractive index in the window region 11 is equal to or greater than the thickness of the active layer 3, and a semiconductor layer having a refractive index higher than that of the n-InP layer 10b sandwiching the n-InP layer 10b. Since the thickness is not more than each thickness, light easily propagates through a semiconductor layer having a higher refractive index than that of the n-InP layer 10b, and light that propagates through a semiconductor layer having a higher refractive index is greatly combined with the active layer 3. Can be reduced. As a result, performance degradation due to reflected return light can be further suppressed.

  The low refractive index layer extending in the direction of the emission end face from the vicinity of the end face on the emission end face 9 side of the active layer 3 does not necessarily have to cover the entire area from the active layer 3 to the emission end face 9 in the window region 11 as shown in FIG. 3 is effective if it is in a region extending from the vicinity of the end face on the exit end face 9 side in the direction of the exit end face.

  Even if the active layer 3 is not located in the region extended to the emission end face 9 side, it is effective if it is in the vicinity of the extended region as shown in FIG. 2 of the first embodiment.

  The low refractive index layer may be formed at the position of the region where the active layer 3 is extended to the emission end face 9 side. In this case, for example, before processing the shape of the active layer 3 into a stripe shape by etching, n-InP A height adjusting InP clad layer is formed on the buried layer 4, and a layer having a low refractive index in the current confinement layer 10 emits the active layer 3 from the relationship between the etching depth and the thickness direction of the current confinement layer 10. You may make it locate in the area | region extended to the end surface 9 side.

  Further, the layer portion of the current confinement layer 10 does not necessarily have a stacked structure of p-InP / n-InP / p-InP, and may be a structure composed of two or four or more layers, for example. . The layer having a low refractive index may not be an n-InP layer, and the refractive index of any layer or all layers may be low.

  The method of causing the refractive index change of the low refractive index layer or the semiconductor layer sandwiching the low refractive index layer may be a method of changing the semiconductor composition of either the low refractive index layer or the semiconductor layer sandwiching the low refractive index layer, or both, A method of applying strain to these layers or a method of adding impurities or causing defects by ion implantation or the like may be used.

  Further, a part or the whole of the current confinement layer 10 may be formed of a semiconductor material having another composition having a band gap larger than that of the active layer 3.

  In the above-described semiconductor laser, the case where the p-InP lower cladding layer 14 is thick and remains in the window region 11 after etching of the active layer 8 has been described. However, the p-InP lower cladding layer 14 is thin, A structure in which the p-InP lower cladding layer 14 in the window region 11 is eliminated during etching may be used. FIG. 4 is a sectional view showing the configuration of a semiconductor laser according to another embodiment of the first embodiment. Compared with FIG. 2, the p-InP lower cladding layer 14 is thinner, and the window region 11 does not have the p-InP lower cladding layer 14. Even in this configuration, since the refractive index of the p-InP substrate 8 is higher than the refractive index of the n-InP layer 10b and the thickness of the p-InP substrate 8 is equal to or greater than the n-InP layer 10b, the effect of the present invention is obtained. .

  In the first embodiment, the DFB laser has the diffraction grating 5 in the vicinity of the active layer 3. However, even the semiconductor laser without the diffraction grating 5 includes the window region 11 as in the first embodiment. If it has, it has the effect of this invention.

  As described above, in the first embodiment, the first semiconductor layer extending in the direction of the emission end face from the vicinity of the end face on the emission end face side of the active layer, and the second semiconductor layer sandwiching the first semiconductor layer in the thickness direction And the third semiconductor layer, the refractive index of the first semiconductor layer is smaller than that of the second semiconductor layer and the third semiconductor layer, the thickness of the first semiconductor layer is equal to or greater than the thickness of the active layer, A window region having a thickness equal to or smaller than the thickness of the second semiconductor layer and equal to or smaller than the thickness of the third semiconductor layer is provided. For this reason, the reflected return light does not directly enter the active layer, but reaches the active layer after being attenuated while passing through the window region. As a result, performance degradation can be suppressed even when there is relatively strong reflected return light.

  Further, as described above, since the influence of the reflected return light can be suppressed by the window region 11, not the return light of the semiconductor laser itself of the present invention but, for example, light having a different wavelength is input from the emission end face side. Even if it is a case, the influence can be suppressed.

(Embodiment 2)
FIG. 5 is a cross-sectional view showing the configuration of the semiconductor laser according to the second embodiment. In addition to the configuration of the first embodiment, the semiconductor laser of the second embodiment emits light by providing a diffraction grating 5b having a Bragg reflection wavelength shorter than the gain peak wavelength of the active layer along the active layer. The wavelength of the laser beam is shorter than the gain peak wavelength of the active layer. Basically, it has the same configuration as that of FIG. 2 and is characterized by the period of the diffraction grating.

  When the reflected return light is absorbed by the active layer 3, the electric field and carrier density inside the laser fluctuate and the equivalent refractive index fluctuates. The equivalent refractive index fluctuation causes dynamic light output fluctuation and oscillation wavelength fluctuation, and contributes to an increase in RIN. A proportional coefficient between the carrier density fluctuation and the refractive index fluctuation is represented by a line width increase coefficient α. Therefore, in principle, reducing α is effective in suppressing performance degradation due to reflected return light.

  FIG. 6 is a graph schematically showing the relationship between the gain and line width increase coefficient α and the wavelength of the semiconductor laser according to the second embodiment. Here, the active layer 3 of the semiconductor laser has a light emission wavelength in the 1.5 micron band. As seen from the gain graph 52 in the upper graph of FIG. 6, the gain of the active layer 3 has a peak with respect to a specific wavelength depending on the composition and structure of the active layer. On the other hand, as shown in the lower graph of FIG. 6, α increases monotonously with respect to the wavelength, and tends to increase more rapidly when the wavelength 51 of the gain peak of the active layer is exceeded. Accordingly, when the oscillation wavelength 54 is detuned 58 to a shorter wavelength side than the gain peak wavelength 51 of the active layer, the gain 55 at the oscillation wavelength decreases from the gain peak 52, but at the oscillation wavelength. α56 is smaller than α52 at the gain peak wavelength. In the case of a DFB laser, the oscillation wavelength is determined by the Bragg reflection wavelength of the diffraction grating 5b. In the second embodiment, a diffraction grating 5b having a Bragg reflection wavelength shorter than the gain peak wavelength of the active layer is provided along the active layer 3. It was. As a result, the wavelength of the emitted laser light becomes shorter than the wavelength 51 of the gain peak of the active layer 3, and α is correspondingly reduced. Therefore, even when the reflected return light returns to the active layer 3, performance degradation due to the reflected return light is reduced. Can be suppressed.

  For example, if the detuning is performed on the short wavelength side by 10 nanometers or more, more preferably 20 nanometers or more from the gain peak wavelength 51 of the active layer, the equivalent refractive index due to the reflected return light reaching the active layer 3 can be reduced. Since dynamic fluctuations are greatly suppressed, the effect of suppressing performance degradation is great.

  Further, as a method of reducing α, in addition to the method of the second embodiment, application of compression or tensile strain to the multiple quantum well layer constituting the active layer 3 or only the barrier layer of the multiple quantum well layer is performed. It is also effective to use a method such as modulation doping that performs p-type doping.

  FIG. 7 is a graph showing the relationship between the amount of reflected return light and the RIN relative noise intensity of the semiconductor laser of the second embodiment. FIG. 7 shows three RIN measurement values 62 of the laser of the second embodiment and measurement values 64 of a distributed feedback type conventional semiconductor laser having a diffraction grating 5c along the entire surface of the active layer as comparison data. Show. In the measurement of the reflected return light and RIN, the polarization plane of the reflected return light is adjusted by the polarization controller so that RIN takes the maximum value. Here, the used laser has an interval L between the emission end face 9 and the reflection end face 7 of 300 μm, a length of the window region 3 of 20 μm, a detuning amount detuned to a shorter wavelength side than the gain peak wavelength, and a diffraction of 10 nm. In this laser, the product κ × Lg of the coupling coefficient κ between the grating 5b and the active layer 3 and the diffraction grating length Lg of the diffraction grating region 6 is set to 0.9. As shown in FIG. 7, compared with the conventional semiconductor laser, the RIN of the semiconductor laser of the second embodiment maintains a low value for all the reflected return light amounts from the weak reflected return light amount to the relatively strong -15 dB reflected return light amount. ing. Although not shown in the figure, even when compared with a semiconductor laser having a diffraction grating locally as in Non-Patent Document 1, RIN is extremely high even for a relatively strong reflected return light of −30 dB or more. The second embodiment has a remarkable effect.

  Further, as a result of fabricating semiconductor lasers having various structural parameters and evaluating the dependence of RIN on the amount of reflected return light, when the distance L between the emission end face and the reflection end face is constant, the values of κ and Lg are obtained. It has been found that performance deterioration due to reflected return light can be remarkably suppressed by selecting appropriately. When L is 250 to 400 μm, the window region length is 20 to 50 μm, and the amount of detuning between the Bragg wavelength and the gain peak wavelength of the diffraction grating is 10 nm or more, the product value of κ and Lg is 0.6 to When it is in the range of 1.1, it is possible to remarkably suppress performance deterioration due to reflected return light.

  As described above, in the second embodiment, the return light coupled to the active layer can be reduced by the window region of the first embodiment, and the Bragg reflection wavelength of the diffraction grating that determines the oscillation wavelength of the DFB laser is active. Since it is detuned (detuned) so as to be shorter than the gain peak wavelength 51 of the layer 3, the influence of the dynamic fluctuation of the equivalent refractive index caused by the reflected return light reaching the active layer 3 is reduced. It is possible to suppress performance degradation.

  Moreover, in the structure of the DFB laser having the window region 11 as in the first and second embodiments, the reflected return light is reflected by the diffraction grating installed in the vicinity of the active layer 3 and is emitted from the emission end face again. It also has the effect of reducing light. The reflected return light reflected by the diffraction grating is known to cause a crosstalk with the original output light on the light receiving side, but the present invention is also effective in reducing the crosstalk. .

  In the first and second embodiments described above, the case where an element is formed on a p-InP substrate is shown. However, the conductivity of each layer on the n-InP substrate is set within a range not departing from the gist of the present invention. A structure in which the structure is reversed or a structure in which a diffraction grating is provided below or in the active layer is also possible. Furthermore, in the first and second embodiments, the case where the highly reflective coating film is formed on the end surface opposite to the emission side has been described. However, the cleaved end surface is used as it is without forming the highly reflective coating film. May be. Further, the window region may be located not only on the emission end face side but also on the reflection end face side.

  If the semiconductor lasers as in the first and second embodiments are used, the semiconductor laser has a small performance deterioration with respect to the reflected return light that can be used in the optical communication system even without an isolator. By using it as a laser light source for a general purpose system or as a laser light source module, an isolator becomes unnecessary, and an optical communication system or a laser light source module with a simpler configuration can be realized.

(Embodiment 3)
FIG. 9 is a schematic diagram showing the configuration of the semiconductor laser module of the third embodiment. The case 88 includes a semiconductor laser 81, a plurality of lenses 82 a, 82 b, 82 c that condense the emitted light 83 of the semiconductor laser 81, and a polarizer 85. Here, as the semiconductor laser 81, the semiconductor laser described in the first or second embodiment is used.

  First, an electrical signal to be transmitted from the outside of the case 88 is input to the semiconductor laser 81 through the wiring 89. The outgoing light 83 emitted from the semiconductor laser 81 in response to this electric signal is converted into a substantially parallel beam by a lens 82a which is a spherical lens, and then passes through a polarizer 85. The polarizer 85 transmits linearly polarized light in a specific direction (transmission axis direction) and absorbs polarized light that deviates from that direction. In a semiconductor laser having a quantum well structure as described in the first or second embodiment, TE (Transverse Electric) light whose electric field oscillation direction is generally parallel to the substrate is a main component of outgoing light. In some cases, TM (Transverse Magnetic) light perpendicular to the substrate may be the main component. Therefore, the direction of the transmission axis of the polarizer 85 is made to coincide with the direction of the main polarization component of the outgoing light 83 emitted from the semiconductor laser 81. As a result, the intensity of the outgoing light 83 passing through the polarizer 85 is maximized.

  As the polarizer 85, for example, a plate-like glass containing metal nanoparticles such as silver and the like having a polarization characteristic by extending and arranging in one direction in the plane can be used.

  The linearly polarized outgoing light 83 that has passed through the polarizer 85 is collected by the lenses 82 b and 83 c and enters the end face of the single-mode optical fiber 87. As these lenses 82b and 83c, GRIN (GRaded-INdex, refractive index gradient type) rod lenses can be used. The electric signal input as described above is converted into an optical signal and propagates through the optical fiber 87.

  In the above configuration, the polarization direction of the outgoing light 83 is maintained without being rotated from the semiconductor laser 81 to the incident end face of the optical fiber 87. The light propagating through the optical fiber 87 is reflected when there is a reflection point such as a discontinuous portion of the refractive index in the middle, and it returns to the semiconductor laser 81 side in the reverse order again. However, usually, the polarization direction is not maintained during propagation through the optical fiber 87 or at the reflection point, and the polarization direction of the return light is deviated from the polarization direction immediately after the original emission. In the semiconductor laser module of the third embodiment, since the polarizer 85 is installed between the semiconductor laser 81 and the incident end face of the optical fiber 87 as described above, the return light does not coincide with the transmission axis direction of the polarizer 85. The polarization component is absorbed. For this reason, many components of the return light are absorbed by the polarizer 85, and the reflected return light returning to the semiconductor laser 81 is reduced. By reducing the reflected return light, it is possible to make it difficult to increase the oscillation line width of the semiconductor laser 81 and increase the relative noise intensity RIN.

  In the reflected return light, the polarization component having the same polarization direction of the semiconductor laser 81 returns directly to the emission end face of the semiconductor laser 81 without being attenuated by the action of the polarizer 85. Since the semiconductor laser 81 having high return light resistance described is used, the RIN value satisfying the specifications required by the system can be maintained. Therefore, the performance of the optical communication system can be improved even with a semiconductor laser module that does not use an optical isolator with the above configuration.

  Note that the polarizer 85 and the lenses 82a, 82b, and 83c are formed on the incident side or the outgoing side of the light, and on both sides thereof, an antireflection film is formed so that the light having the wavelength of the outgoing light 81 has low reflection. This is preferable because it prevents generation of reflected return light inside the laser module. Further, the reflection loss from the module can be sufficiently increased by the polarizer 85 having the antireflection film formed thereon.

  Further, as shown in FIG. 9, the polarizer 85 may be disposed so that the incident surface thereof is perpendicular to the optical axis that is the propagation direction of the outgoing light 83, but may be disposed so as to be inclined. When the incident surface is inclined, the light reflected on the surface is difficult to return to the semiconductor laser 81, so that the generation of reflected return light inside the module can be further reduced. Instead of inclining, the polarizer 85 may be curved so that the incident surface on the semiconductor laser 81 side becomes a convex surface. Further, when the polarizer 85 is disposed in an inclined or curved manner, the above reflection is performed so that the reflection of light is low at the wavelength of the outgoing light 83, for example, the reflectance is 1% or less even when the polarizer 85 is incident obliquely with respect to the incident surface. The prevention film is preferably a multilayer filter.

  The case 88 preferably has a structure in which light does not enter from the outside, and the inner wall may be coated with a light absorbing material or paint.

  In the third embodiment, a plurality of lenses are used to form a so-called second lens division type confocal system. However, it is sufficient that the lens has at least one lens that collects light on the optical fiber 87. The polarizer 85 may be between the semiconductor laser 81 and the optical fiber 87 and the incident end face.

(Embodiment 4)
FIG. 10 is a schematic diagram showing the configuration of the semiconductor laser module according to the fourth embodiment. In FIG. 9 of the third embodiment, a plurality of lenses are used, and the incident end face of the optical fiber 87 is connected to a lens 82c made of a GRIN lens. However, in the fourth embodiment, the lens is a single lens 82d. The lens and the incident end face of the optical fiber 87 are separated. The incident end face of the optical fiber 87 is inclined with respect to the optical axis of the outgoing light 83, and the incident end face portion is held by the ferrule 90. A polarizer 81 with an antireflection coating is provided on the end face of the ferrule 90. Thus, since the incident surface of the polarizer 81 and the incident end surface of the optical fiber 87 are inclined with respect to the optical axis of the outgoing light 83 of the semiconductor laser 81, the light reflected by the polarizer 81 and these surfaces is reflected by the semiconductor laser 81. Can be prevented from returning to. The effect of reducing the reflected return light by the polarizer 81 is the same as in the third embodiment.

(Embodiment 5)
FIG. 11 is a schematic diagram showing the configuration of the semiconductor laser module according to the fifth embodiment. This semiconductor laser module is used in a communication system of an optical access system, and is a single fiber bidirectional optical module in which upstream / downstream optical signals having different wavelengths are transmitted bidirectionally.

  The semiconductor laser and lens having the configuration of the fourth embodiment are housed in the semiconductor laser package 91, a wavelength selection filter 93 is added between the semiconductor laser package 91 and the optical fiber 87, and a photodiode package 94 is further added. It is a configuration. The semiconductor laser package 91, the photodiode package 94, and the ferrule 90 are fixed to the wall surface of the case 88. The semiconductor laser package 91 and the photodiode package 94 have terminals 94 through which electric signals are input and output from the outside of the case 88.

  Inside the semiconductor laser package 91, emitted light from the semiconductor laser is collected by a lens and is incident as an upstream light 97 from the end face of the optical fiber 87. On the other hand, downstream light 98 having a wavelength different from that of upstream light 97 is emitted from the end face of the optical fiber 87 into the case 88.

  Between the semiconductor laser package 91 and the end face of the optical fiber 87, a wavelength selection filter 95 that transmits light having the wavelength of the upstream light 97 and reflects light having the wavelength of the downstream light 98 is installed. The optical axis of the upstream light 97 from the semiconductor laser package 91 is disposed so as to coincide with the optical axis of the optical fiber 87, and the photodiode package 94 is disposed in a direction perpendicular to a straight line connecting the optical axes. A wavelength selection filter 95 is installed at an angle of 45 degrees at the intersection of the straight line connecting the optical axes and the perpendicular line. As a result, the upstream light 97 travels on the optical axis, passes through the wavelength selection filter 95, and enters the end face of the optical fiber 87. On the other hand, the downstream light 98 is reflected and incident on the photodiode package 94, and bidirectional optical communication is possible.

  The wavelength selection filter 95 reflects the light having the wavelength of the upstream light 97 and transmits the light having the wavelength of the downstream light 98, and the arrangement of the semiconductor laser package 91 and the photodiode package 94 is changed. Also good.

  In the fifth embodiment, similarly to the fourth embodiment, since the polarizer 85 is fixed to the ferrule to which the optical fiber 87 having the end surface oblique to the optical axis of the emitted light is fixed, The return light to the laser is reduced. As a result, even if reflection increases in the middle of the optical fiber 87, for example, it is difficult to be affected by this, so that highly reliable optical communication is possible.

1 is a cross-sectional view showing a configuration of a semiconductor laser according to a first embodiment. 1 is a cross-sectional view showing a configuration of a semiconductor laser according to a first embodiment. 1 is a perspective view illustrating a configuration of a semiconductor laser according to a first embodiment. It is sectional drawing which shows the structure of the semiconductor laser of another form of this Embodiment 1. FIG. It is sectional drawing which shows the structure of the semiconductor laser of this Embodiment 2. It is a graph which shows the characteristic of the active layer of the semiconductor laser of this Embodiment 2. It is a graph which shows the characteristic of the semiconductor laser of this Embodiment 2. It is sectional drawing which shows the structure of the conventional semiconductor laser of the comparative example of this Embodiment 1. It is the schematic which shows the structure of the semiconductor laser module of this Embodiment 3. It is the schematic which shows the structure of the semiconductor laser module of this Embodiment 4. It is the schematic which shows the structure of the semiconductor laser module of this Embodiment 5.

Explanation of symbols

  1 n-side electrode, 2 p-side electrode, 3 active layer, 4 n-InP buried layer, 5, 5b diffraction grating, 6 diffraction grating region, 7 reflection end face, 8 p-InP substrate, 9 emission end face, 10 current confinement layer 10a p-InP layer, 10b n-InP layer, 10c p-InP layer, 11 window region, 12 n-InGaAsP contact layer, 13 n-InP upper cladding layer, 14 p-InP lower cladding layer, 15 high reflection film , 16 low reflection film, 17 laser light, 20 cross section, 21 first semiconductor layer, 22 second semiconductor layer, 23 third semiconductor layer, 51 active layer gain peak wavelength, 52 gain peak, 53 gain Α, 54 oscillation wavelength, 55 gain at oscillation wavelength, 56 α at oscillation wavelength, 58 detuning, 62 RIN measured value, 64 conventional semiconductor laser RIN measurement value, 81 semiconductor laser, 82a, 82b, 82c lens, 83 outgoing light, 85 polarizer, 87 optical fiber, 88 case, 89 wiring, 90 ferrule, 92 semiconductor laser package, 93 photodiode package, 94 terminals, 95 wavelength selection filter, 97 upstream light, 98 downstream light

Claims (3)

A semiconductor laser having a window region made of a semiconductor having a larger band gap than the active layer between an emission end face of the laser beam and the active layer;
The window region includes a first semiconductor layer extending from the vicinity of the end face of the active layer on the exit end face side to the exit end face, a second semiconductor layer sandwiching the first semiconductor layer in a thickness direction, and a third semiconductor layer A semiconductor layer,
The refractive index of the first semiconductor layer is lower than the refractive indexes of the second semiconductor layer and the third semiconductor layer, and the thickness of the first semiconductor layer is equal to or greater than the thickness of the active layer, A semiconductor laser having a thickness equal to or less than a thickness of the second semiconductor layer and equal to or less than a thickness of the third semiconductor layer.   2. The semiconductor laser according to claim 1, wherein a diffraction grating having a Bragg reflection wavelength shorter than the gain peak wavelength of the active layer is provided along the active layer.   The semiconductor laser according to claim 1, a lens for collecting the emitted light of the semiconductor laser, an optical fiber on which the collected emitted light is incident, and between the semiconductor laser and the optical fiber A semiconductor laser module comprising: a polarizer.

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