JPWO2005060058A1 - Semiconductor laser and manufacturing method thereof - Google Patents

Abstract

In a semiconductor laser in which an active waveguide is composed of a 1 × 1-MMI waveguide region 111 and single mode waveguide regions 112 and 113 connected to both ends of the 1 × 1-MMI waveguide 111, a single mode is used. A grating for selecting a single wavelength of the oscillation light propagating through the active waveguide is provided in the waveguide region 112.

Description

  The present invention relates to a semiconductor laser, and more particularly to a dynamic multimode optical interference (active MMI) semiconductor laser and a method for manufacturing the same.

  With the recent increase in communication demand, optical communication technology that enables high-speed and large-capacity communication is not limited to backbone systems (pointing between large cities), but also so-called metros (pointing to urban areas) and access systems (homes and homes). It has come to apply to areas such as between buildings). The metro / access system is expected to be used in large quantities (for example, it is expected that the number of subscribers will increase due to the penetration of optical communication technologies such as “Fiber to the Home (FTTH)”). Therefore, a transmission light source with low power consumption is required. On the other hand, a transmission light source with high light output performance is also required. For example, in the case of a metro system, by realizing high optical output performance, it is not necessary to install a repeater in the middle of the transmission path, and the entire system can be constructed at a low cost.

  By the way, in optical communication in which optical fiber transmission is performed, when a semiconductor laser including a plurality of wavelengths in the oscillation wavelength, such as a Fabry-Perot semiconductor laser (FP-LD), is used as a transmission light source, The resulting chromatic dispersion occurs, and high-speed signal communication becomes impossible. In order to avoid such influence of chromatic dispersion of the optical fiber, a semiconductor laser capable of oscillation at a single wavelength (single axis mode) is usually used as a transmission light source.

  A distributed feedback semiconductor laser (DFB-LD) is known as a semiconductor laser capable of oscillation at a single wavelength. For example, in non-patent literature (Govind P. Agrawal, “Fiber-Optic Communication Systems, WILEY-INTERSCIENCE, p. 105, etc.), a grating layer is provided in the vicinity (directly or directly above) of the light emitting layer (active layer). However, a DFB-LD having low power consumption characteristics and high optical output performance required for a transmission light source in a metro / access system has not yet been realized.

  On the other hand, although it is FP-LD, an active MMI type semiconductor laser has been proposed by Hamamoto et al., The inventor of the present application, as a new semiconductor laser capable of realizing low power consumption characteristics and high light output performance. 11-68241). This active MMI type semiconductor laser is a semiconductor laser that outputs single mode light, and an active waveguide including an active layer is a 1 × 1-MMI waveguide and a pair of single modes connected to both ends thereof. A waveguide (a single mode waveguide designed to propagate only the fundamental mode). The 1 × 1-MMI waveguide is designed to perform “1 × 1 operation” based on the MMI theory. Hereinafter, the MMI theory will be briefly described.

  The MMI theory is known as a theory for designing a 1 × N or N × N branching / merging passive optical waveguide (for example, “Lucas B. Soldano”, “Journal of Lightware Technology”, Vol. .13, No. 4, pp. 615-627, 1995). The MMI length Lπ derived from this MMI theory is given by the following equation.

ここで、ＬはＭＭＩ領域の長さ、Ｗ１はＭＭＩ領域の幅、Ｎｒは導波領域の屈折率、Ｎｃはクラッド領域の屈折率、λ0は入射光波長である。σは、ＴＥモードのとき「０」、軸モードのとき「１」である。

Here, L is the length of the MMI region, W1 is the width of the MMI region, Nr is the refractive index of the waveguide region, Nc is the refractive index of the cladding region, and λ 0 is the wavelength of incident light. σ is “0” in the TE mode and “1” in the axial mode.

  According to MMI theory,

という条件を満たすとき、ＭＭＩ領域は１×Ｎ光導波路として動作する。また、

When this condition is satisfied, the MMI region operates as a 1 × N optical waveguide. Also,

という条件を満たすとき、ＭＭＩ領域はＮ×Ｎ光導波路として動作する。この原理に基づき、両端部においてシングルモード光となるような１×１−ＭＭＩ導波路を設計することが可能である。

When the condition is satisfied, the MMI region operates as an N × N optical waveguide. Based on this principle, it is possible to design a 1 × 1-MMI waveguide that is single-mode light at both ends.

  In the active MMI type semiconductor laser in which a part of the active waveguide is an MMI waveguide as described above, the waveguide width can be increased (the area of the active layer can be increased). Compared with an existing semiconductor laser having the same element length composed only of the element, the element resistance can be reduced and low power consumption characteristics can be realized. For this reason, if a structure capable of obtaining a stable oscillation at a single wavelength can be realized with an active MMI semiconductor laser, it is considered promising as a future transmission light source of a metro / access system.

  As a semiconductor laser having an MMI waveguide and capable of oscillation at a single wavelength, for example, there is a semiconductor laser described in Japanese Patent Application Laid-Open No. 2003-46190. This semiconductor laser constitutes an active region (light emitting region) only with an MMI waveguide, and a grating is provided in a passive region behind the active region. As another form, Japanese Patent Laying-Open No. 2003-46190 also describes a structure in which an external grating is provided behind the active region.

  The problems to be solved by the invention are the following problems that the above-described prior art has.

  The DFB-LD described in the above-mentioned non-patent document can oscillate at a single wavelength, but has the following problems because the waveguide structure is composed of a narrow single mode waveguide. .

  In order to reduce the power consumption, it is necessary to reduce the driving resistance by reducing the element resistance. Although the element resistance can be reduced by increasing the area of the active layer (light emitting layer), in the DFB-LD in which the waveguide structure is a narrow single-mode waveguide, the area of such an active layer is increased. It is difficult. Although the element resistance can be reduced by increasing the element length, an increase in the element length causes a decrease in element yield (production amount), which increases the cost. As described above, the DFB-LD has a problem that it cannot achieve low power consumption characteristics and high light output performance required for a transmission light source of a metro / access system.

  In the active MMI type semiconductor laser described in Japanese Patent Laid-Open No. 11-68241, since the active layer area can be increased, the low power consumption characteristic and high light output performance required for a metro / access transmission light source are achieved. Although it can be achieved, there is a problem that stable oscillation at a single wavelength cannot be obtained.

  In an active MMI type semiconductor laser, it is conceivable to provide a grating in the vicinity of the active layer like DFB-LD in order to perform single wavelength oscillation. However, in a structure in which the main light emitting region is composed of an MMI waveguide, there are a plurality of different propagation modes in the MMI waveguide, so that stable single wavelength oscillation is realized even if a grating is simply provided near the active layer. It is difficult.

  In the semiconductor laser described in Japanese Patent Laid-Open No. 2003-46190, since a grating is provided in a passive waveguide region connected to an MMI waveguide, it is necessary to newly integrate a passive region in addition to an active region for light emission. . For this reason, optical loss in the passive region is inevitable, and it cannot be said that the structure is suitable for high output. In addition, the passive region and the active region must be formed in the same element, which is disadvantageous in terms of cost reduction. A structure having an external grating has the same problem.

  An object of the present invention is a semiconductor that can solve the above-described problems, obtain low power consumption characteristics and high light output performance required for a transmission light source of a metro / access system, and perform stable single wavelength oscillation. To provide a laser.

  Another object of the present invention is to provide a semiconductor laser manufacturing method capable of manufacturing such a semiconductor laser with a high yield and a low cost.

  Another object of the present invention is to provide an optical communication module comprising such a semiconductor laser.

  The semiconductor laser according to the present invention is characterized in that an active waveguide is composed of at least one multimode interference waveguide and at least one single mode waveguide connected to both ends or one end of the multimode interference waveguide. In the constructed semiconductor laser, a grating for selecting a single wavelength of the oscillation light propagating through the active waveguide is provided in a part of the active waveguide.

  According to the above configuration, since the single wavelength (single axis mode) of the oscillation light propagating through the active waveguide is selected by the grating, stable laser oscillation at a single wavelength is possible. Therefore, by applying this structure, an active MMI semiconductor laser capable of single wavelength oscillation can be realized.

  In an active waveguide having a multimode interference waveguide, the active layer area can be increased to reduce the element resistance, so that an existing semiconductor laser having only the single mode waveguide and having the same element length can be used. The power consumption characteristics can be greatly improved compared to the above. Therefore, it is possible to achieve the low power consumption characteristics required for the transmission light source of the metro / access system.

  In addition, since the amount of heat generated by current injection generated in the active layer is reduced by reducing the element resistance, the current that is the cause of DFB-LD kinks (nonlinearity in optical output-operating current characteristics) The center wavelength shift of the active layer itself due to the implantation hardly occurs. For this reason, stable single-wavelength oscillation can be achieved even with a high injection current, thereby realizing high output characteristics.

  In the semiconductor laser of the present invention described above, a grating may be provided in the single mode waveguide. According to this configuration, a single wavelength is selected in the single mode waveguide, and stimulated emission occurs in the multimode interference waveguide with respect to the selected single wavelength. Therefore, a high output with a more stable single wavelength oscillation is possible.

  Further, the grating may be provided in the multimode interference waveguide, and the grating width may be within twice the width of the single mode waveguide. According to this configuration, since a single transverse mode can be reflected with respect to the grating center wavelength among a plurality of transverse modes propagating in the multimode interference waveguide, stable single wavelength operation is realized. . When the grating width exceeds twice the width of the single mode waveguide, grating reflection occurs for each transverse mode propagating in the multimode interference waveguide, so stable single wavelength oscillation It is relatively difficult to obtain.

  Furthermore, a grating may be provided over the entire length of the active waveguide, and a phase adjustment region may be provided in a portion of the grating located in the middle of the active waveguide. According to this configuration, the phase of the grating is inverted before and after the phase adjustment region, so that the gain difference between the single transverse mode selected by the grating and the other modes is sufficiently large. It becomes possible. Therefore, single wavelength oscillation with high yield can be realized.

  The optical communication module of the present invention is characterized in that any one of the above-described semiconductor lasers and a circuit for driving the semiconductor lasers are accommodated. By providing the above-described semiconductor laser, an optical communication module having low power consumption characteristics and high optical output performance required for a transmission light source of a metro / access system is realized.

  The method of manufacturing a semiconductor laser according to the present invention includes a step of forming a grating region having a width wider than the width of the single mode waveguide in a region where a single mode waveguide is formed on a semiconductor substrate, and the formed grating Etching the region into the shape of the active waveguide. According to this manufacturing method, it is possible to manufacture the semiconductor laser having the structure in which the grating is provided in the single mode waveguide among the above-described semiconductor lasers of the present invention with high yield and low cost.

  In the semiconductor laser manufacturing method of the present invention, a grating having a width equal to or larger than the width of the single mode waveguide and within twice the width of the single mode waveguide in the region where the active waveguide is formed on the semiconductor substrate. Forming the region over the entire length of the active waveguide, and etching the formed grating region into the shape of the active waveguide. According to this manufacturing method, it is possible to manufacture a semiconductor laser having a structure in which a grating is provided in a multimode interference waveguide among the above-described semiconductor lasers of the present invention with high yield and low cost.

  As described above, according to the present invention, an active MMI type semiconductor laser capable of stable single wavelength oscillation is realized, and low power consumption characteristics and high output characteristics required for a transmission light source in a metro / access system are achieved. There is an effect that can be done.

  In addition, the achievement of high output characteristics eliminates the need to install a repeater on the transmission path in a metro / access communication system, particularly a metro communication system, so that the entire system can be constructed at low cost. There is an effect that it is possible.

It is a schematic diagram when the active MMI type semiconductor laser which is the 1st Embodiment of this invention is seen from the upper surface. It is sectional drawing in the dashed-dotted line AA 'of FIG. 1A. It is sectional drawing in the dashed-dotted line BB 'of FIG. 1A. It is a schematic diagram which shows the cross-section of the active layer of the active MMI type | mold semiconductor laser shown in FIG. It is a figure for demonstrating the manufacturing process of the active MMI type | mold semiconductor laser shown to FIG. 1A-FIG. 1C, Comprising: It is a top view of a grating formation area. It is a figure for demonstrating the manufacturing process of the active MMI type | mold semiconductor laser shown to FIG. 1A-FIG. 1C, Comprising: It is sectional drawing after a MO-VPE process. It is a figure for demonstrating the manufacturing process of the active MMI type | mold semiconductor laser shown to FIG. 1A-FIG. 1C, Comprising: It is a top view after mask formation for mesa preparation. It is a figure for demonstrating the manufacturing process of the active MMI type | mold semiconductor laser shown to FIG. 1A-FIG. 1C, Comprising: It is sectional drawing after mesa preparation. It is a figure for demonstrating the manufacturing process of the active MMI type | mold semiconductor laser shown to FIG. 1A-FIG. 1C, Comprising: It is sectional drawing after a MO-VPE recrystallization growth process. It is a figure for demonstrating the manufacturing process of the active MMI type | mold semiconductor laser shown to FIG. 1A-FIG. 1C, Comprising: It is sectional drawing after electrode formation. It is a schematic diagram at the time of seeing the active MMI type | mold semiconductor laser which is the 2nd Embodiment of this invention from the upper surface. It is sectional drawing in the dashed-dotted line AA 'of FIG. 4A. It is sectional drawing in the dashed-dotted line BB 'of FIG. 4A. FIG. 5 is a schematic diagram for explaining a grating region width of the active MMI type semiconductor laser shown in FIGS. 4A to 4C. It is a schematic diagram at the time of seeing the active MMI type | mold semiconductor laser which is the 3rd Embodiment of this invention from the upper surface. It is sectional drawing in the dashed-dotted line AA 'of FIG. 6A. It is sectional drawing in the dashed-dotted line BB 'of FIG. 6A. FIG. 7 is a schematic diagram for explaining a grating region width of the active MMI semiconductor laser shown in FIGS. 6A to 6C.

Explanation of symbols

101 n-InP semiconductor substrate 102 n-InGaAsP guide layer 103 n-InP cladding layer 104 active layer 105 p-InP cladding layer 106 p-InP cladding layer 107 p-InGaAs contact layer 108 InGaAsP-SCH layer 109 InGaAsP / InGaAsP-MQW Layer 111 1x1-MMI waveguide region 112 to 114 single mode waveguide region 120 grating 121 λ / 4 phase adjusting region 121 SiO ₂ film 131 p-InP current blocking layer 132 n-InP current blocking layer 135, 136 electrode

  Next, embodiments of the present invention will be described in detail with reference to the drawings.

(Embodiment 1)
FIG. 1A is a schematic view of an active MMI semiconductor laser according to a first embodiment of the present invention as viewed from above. In this active MMI type semiconductor laser, an active waveguide structure including an active layer is composed of a 1 × 1-MMI waveguide region 111 and a pair of single mode waveguide regions 112 and 113 provided at both ends thereof. ing.

  The end surface of the single mode waveguide region 112 opposite to the side connected to the 1 × 1-MMI waveguide region 111 is an end surface on the front side of the device (hereinafter simply referred to as a front end surface), from which laser Light is emitted. An antireflection film is provided on the front end face (cleavage face). On the other hand, the end surface opposite to the side connected to the 1 × 1-MMI waveguide region 111 of the single mode waveguide region 113 is an end surface on the rear side of the element (hereinafter simply referred to as a rear end surface). A highly reflective film is provided on the rear end face. The front end face provided with the antireflection film and the rear end face provided with the high reflection film constitute a reflector before and after the laser resonator. The element length is about 600 μm, the length of the single mode waveguide region 112 is about 300 μm, and the length of the 1 × 1-MMI waveguide region 111 is about 230 μm. The waveguide width of the 1 × 1-MMI waveguide region 111 is about 9 μm, and the waveguide widths of the single mode waveguide regions 112 and 113 are both about 2 μm.

  FIG. 1B schematically shows a cross-sectional structure taken along one-dot chain line AA ′ in FIG. 1A. This cross section is obtained by cutting the single mode waveguide region 112 in a direction crossing the longitudinal direction of the element. The waveguide portion of the single mode waveguide region 112 has an n-InGaAsP guide layer 102, an n-InP clad layer 103, an active layer (light emitting layer) 104, and a p-InP clad layer 105 on an n-InP semiconductor substrate 101. A current block layer is formed by sequentially stacking p-InP current blocking layer 131 and n-InP current blocking layer 132 on both sides of the mesa structure. ing. The active layer 104 has an existing structure well known for semiconductor lasers. For example, as shown in FIG. 2, an active layer 104 includes an InGaAsP / InGaAsP-MQW (multiple quantum well) layer 109 in which quantum wells are stacked in multiple layers. The structure is sandwiched between InGaAsP-SCH (separate confinement heterostructure) layers 108 from above and below. A p-InP clad layer 106, a p-InGaAs contact layer 107, and an electrode 135 are sequentially stacked on the surface of the upper layer of the mesa structure portion and the upper layer of the current blocking layer. An electrode 136 is formed on the back side of the n-InP semiconductor substrate 101.

  FIG. 1C schematically shows a cross-sectional structure taken along one-dot chain line BB ′ in FIG. 1A. This cross section is obtained by cutting the waveguide portions of the 1 × 1-MMI waveguide region 111 and the single mode waveguide regions 112 and 113 along the longitudinal direction of the element. In the waveguide portion of the single mode waveguide region 112, a grating 120 having periodic irregularities is formed at the interface between the n-InP semiconductor substrate 101 and the n-InGaAsP guide layer 102 over the longitudinal direction. The normalized coupling constant (kL) by the grating 120 is about 2. On the other hand, the waveguide portions of the 1 × 1-MMI waveguide region 111 and the single mode waveguide region 113 have the same stacked structure as the single mode waveguide region 112, but the grating 120 is not formed.

  In the active MMI type semiconductor laser of the present embodiment configured as described above, a predetermined bias voltage is applied between the electrodes 135 and 136 so that the central portion of the mesa structure portion limited by the current blocking layer is formed. A current flows through the active layer 104. Below the threshold current, spontaneous emission and absorption occur, and when the threshold current is exceeded (stimulated emission exceeds absorption), laser oscillation is possible.

  When laser oscillation is enabled, the light amplified by stimulated emission propagates as multimode light in the 1 × 1-MMI waveguide region 111 according to the MMI theory, but single mode waveguide regions 112 at both ends thereof. , 113 propagate as single mode light. In the process in which single mode light propagates through the single mode waveguide region 112, a single wavelength is selected by the grating 120, and laser oscillation occurs at the selected single wavelength. The single wavelength to be selected is a wavelength at which the reflectance is maximum in the grating 120, and can be arbitrarily set by adjusting the interval of the grating 120.

  A part of the light laser-oscillated with a single wavelength propagates in the rear 1 × 1-MMI waveguide region 111 and further propagates in the rear single-mode waveguide region 113 to reach the rear end face. The single-mode light having a single wavelength that has reached the rear end face is reflected there and propagates again in the single-mode waveguide region 113 and the 1 × 1-MMI waveguide region 111, and then reaches the single-mode waveguide region 112. The laser beam is emitted from the front end face. In this propagation process, stimulated emission occurs in the 1 × 1-MMI waveguide region 111 with respect to light having a single wavelength selected by the single mode waveguide 112. Therefore, stable laser oscillation operation at a single wavelength is possible. Is realized.

  Further, since the active waveguide has a structure including the 1 × 1-MMI waveguide region 111, in addition to the above features, the following advantages are also obtained.

  (1) Since the device resistance can be reduced by widening the waveguide width (increasing the area of the active layer), compared to existing semiconductor lasers of the same device length composed only of single mode waveguides Excellent low power consumption characteristics.

  (2) It is difficult to cause a center wavelength shift of the active layer itself accompanying current injection, which is a cause of normal DFB-LD kinks (non-linearity in optical output-operating current characteristics). Stable single wavelength oscillation can be achieved and high output characteristics can be realized.

  The advantage (2) will be briefly described. Normally, in the active layer, the refractive index changes due to heat generated by current injection, and the center wavelength shifts due to this refractive index change. According to the waveguide structure including the 1 × 1-MMI waveguide region 111, the amount of heat generation is reduced by increasing the area of the active layer and reducing the electric resistance. As a result, the shift amount of the center wavelength is relatively small. Get smaller.

  In the present embodiment, since there is no need to integrate the passive waveguide region with a grating and the external grating region, a semiconductor laser can be manufactured with high yield and low cost.

  Next, a method for manufacturing the above-described active MMI semiconductor laser of this embodiment will be described. 3A to 3F show a series of manufacturing procedures for the active MMI type semiconductor laser shown in FIGS. 1A to 1C. 3A and 3C are schematic views seen from the upper surface side of the waveguide, and FIGS. 3B and 3D to 3F are cross-sectional views of the waveguide (corresponding to a cross-sectional portion taken along one-dot chain line AA ′ in FIG. 1A). is there.

  First, as shown in FIG. 3A, a grating 120 is formed on a part of the n-InP semiconductor substrate 101 by an electron beam exposure method and a normal wet etching method. The formation range of the grating 120 is a range including the single mode waveguide region 112, and the width (width direction of the waveguide) is wider than the width of the single mode waveguide region 112.

Next, as shown in FIG. 3B, an n-InGaAsP guide layer 102 and an n-InP clad layer are formed on the n-InP semiconductor substrate 101 on which the grating 120 is formed by metal organic chemical vapor deposition (MO-VPE method). 103, an active layer 104, and a p-InP clad layer 105 are sequentially formed. Thereafter, as shown in FIG. 3C, a SiO ₂ film is deposited on the entire surface using a thermal CVD method, and a SiO ₂ mask 130 is formed using a normal photolithography method and a reactive ion etching method (RIE method). To do. At this time, the SiO ₂ mask 130 is formed in a waveguide shape such that the single mode waveguide region 112 is formed on the grating 120.

Next, as shown in FIG. 3D, a mesa is formed by an inductively coupled plasma (ICP) method using the SiO ₂ mask 130. In this mesa formation process, the surface of the n-InP semiconductor substrate 101 is also etched, so that the region of the grating 120 coincides with the single mode waveguide region 112. After the mesa formation, as shown in FIG. 3E, the p-InP current blocking layer 131 and the n-InP current blocking layer 132 are formed around the mesa using the MO-VPE method, and the SiO ₂ mask remaining on the mesa is left. 130 is removed using buffered hydrofluoric acid, and a p-InP clad layer 106 and a p-InGaAs contact layer 107 are sequentially formed on the entire surface. Thereafter, as shown in FIG. 3F, the electrode 135 is formed on the upper surface by electron beam evaporation, and the back surface of the n-InP semiconductor substrate 101 is polished to form the electrode 136.

  A plurality of laser elements are formed on the wafer in accordance with the manufacturing procedure of FIGS. 3A to 3F described above. By cleaving along the boundary between the laser elements, a laser element having a structure as shown in FIGS. 1A to 1C is obtained. By this cleavage, a rear end face and a front end face of the laser element are formed. Finally, an antireflection film is formed on the front end face, and a high reflection film is formed on the rear end face, and the manufacture of the device is completed.

  According to the manufacturing process described above, the grating is directly provided in the active region, and does not include a step of separately integrating the passive region unlike a conventional semiconductor laser (see Japanese Patent Laid-Open No. 2003-46190). The laser element can be manufactured with good yield and low cost.

  The waveguide structure and the manufacturing method thereof according to the present embodiment described above are examples, and the configuration and procedure can be changed as appropriate. For example, in the manufacturing process described above, the MO-VPE method is used as the crystal growth method, but instead, for example, a molecular beam growth method (MBE method) may be used. Also, the mesa forming process is not limited to the ICP method, and an RIE method or the like can also be applied.

In the waveguide structure of this embodiment, the grating 120 is provided on the surface of the n-InP semiconductor substrate 101 below the active layer 104. However, the present invention is not limited to this structure. If stable single wavelength oscillation can be obtained, the grating 120 may be provided in another portion. As a specific formation location of the grating 120, for example,
a) Between n-InGaAsP guide layer 102 and n-InP cladding layer 103 b) Between n-InP cladding layer 103 and active layer 104 c) Between active layer 104 and p-InP cladding layer 105 d) p-InP Examples include between the cladding layer 106 and the p-InGaAs contact layer 107. In addition, an n-InGaAs guide layer is provided between the p-InP clad layer 105 and the p-InP clad layer 106, and the n-InGaAs guide layer and the p-InP clad layer 105 or the p-InP clad layer 106 are provided. A structure in which a grating 120 is provided is also conceivable.

  The grating 120 may be formed over the entire length of the single mode waveguide region 112 or may be formed in a part of the single mode waveguide region 112.

  Further, the grating 120 may be formed in the single mode waveguide region 113 instead of the single mode waveguide region 112 or may be formed in both the single mode waveguide regions 112 and 113.

(Embodiment 2)
FIG. 4A is a schematic view of the active MMI semiconductor laser according to the second embodiment of the present invention as viewed from above. 4B is a cross-sectional view taken along one-dot chain line AA ′ in FIG. 4A, and FIG. 4C is a cross-sectional view taken along one-dot chain line BB ′ in FIG. 4A.

  As shown in FIGS. 4A to 4C, the active MMI semiconductor laser of this embodiment also includes an active waveguide including an active layer, a 1 × 1-MMI waveguide region 111 and a pair of ends provided at both ends thereof. The single-mode waveguide regions 112 and 113 are basically the same as those in the first embodiment described above except that the formation portion of the grating 120 is different. The element length is about 600 μm, and the single mode waveguide regions 112 and 113 both have a waveguide length of about 185 μm and a width of about 2 μm. The length of the 1 × 1-MMI waveguide region 111 is about 230 μm and the width is about 9 μm.

  In the present embodiment, the grating 120 is formed on the surface of the n-InP semiconductor substrate 101 below the active layer 104 from the rear end surface to the front end surface. A λ / 4 phase adjustment region 121 is formed between the rear end surface and the front end surface of the grating 120. The λ / 4 phase adjustment region 121 is obtained by shifting the pitch of the grating 120 by λ / 4, and the phase of the grating 120 is inverted across the λ / 4 phase adjustment region 121. The width of the grating 120 is the same as the width of these waveguides (about 2 μm) in the single mode waveguide regions 112 and 113, and the single mode waveguide regions 112 and 113 in the 1 × 1-MMI waveguide region 111. The width is within twice the waveguide width (about 4 μm). Here, the width of the grating 120 is about 3 μm, and the normalized coupling constant (kL) is about 1. An antireflection film is provided on both the rear end face and the front end face.

  If a grating is provided in the 1 × 1-MMI waveguide region 111 without any restrictions, grating reflection usually occurs not only in the fundamental mode but also in a higher-order mode. Cannot get oscillation. From the results of experiments conducted so far by the present inventors, it has been found that since the propagation constants of the fundamental mode and the higher-order mode are different, single wavelength oscillation cannot be obtained as a result. In the present embodiment, the grating 120 is formed at the waveguide center of the 1 × 1-MMI waveguide region 111, and the grating width is the single mode waveguides 112 and 113 (these are single transverse modes). The width is equal to or less than twice the width of the waveguide (corresponding to the waveguide). According to this structure, since the single transverse mode among the transverse modes propagating in the 1 × 1-MMI waveguide region 111 can be reflected with respect to the grating center wavelength, stable single wavelength operation is achieved. Realized.

  In addition, a λ / 4 phase adjustment region 121 is formed in the resonator, and the phase of the grating 120 is inverted across the λ / 4 phase adjustment region 121. The gain difference between the main and sub modes can be made sufficiently large. Therefore, single wavelength oscillation with high yield can be realized. Other advantages are the same as those of the first embodiment described above.

  Next, a method for manufacturing the above-described active MMI semiconductor laser of this embodiment will be described.

  The active MMI semiconductor laser of this embodiment can also be basically manufactured by the steps shown in FIGS. 3A to 3F, but the method of forming the grating 120 is the case of the first embodiment described above ( This is different from the process of FIG. 3A. That is, in this embodiment, as shown in FIG. 5, the grating 120 is formed over the entire length of the active waveguide in the region of the surface of the n-InP semiconductor substrate 101 where the active waveguide is formed. . The width A of the grating 120 is greater than or equal to the waveguide width B of the single mode waveguide 112 (which is the same as the waveguide width of the single mode waveguide 113) formed in a later step, and within twice the waveguide width B. . Here, the waveguide width B of the single mode waveguide 112 is about 2 μm, and the width A of the grating 120 is about 3 μm. However, considering the positional accuracy in the mesa formation process (positional accuracy between the grating 120 and the single mode waveguides 112 and 113), it is desirable that the width A of the grating 120 be sufficiently larger than the waveguide width B.

  After the formation of the grating 120, the steps shown in FIGS. 3B to 3F are performed. Then, the rear end face and the front end face of the laser element are formed by cleavage, and antireflection films are formed on both end faces, thereby completing the element manufacture. In this way, a laser device having the structure shown in FIGS. 4A to 4C is obtained.

  Also in the above manufacturing method, as in the manufacturing method in the first embodiment described above, it is possible to manufacture an element with a high yield and a low cost.

  The waveguide structure and the manufacturing method thereof according to the present embodiment described above are examples, and the configuration and procedure can be changed as appropriate. For example, in the manufacturing process described above, for example, a molecular beam growth method (MBE method) can be used instead of the MO-VPE method, or an RIE method can be used instead of the ICP method.

  The formation position of the grating 120 can also be changed within a range where stable single wavelength oscillation can be obtained. For example, the grating 120 may be provided at any of the positions a) to d) described in the first embodiment. Further, the grating 120 may be provided only in the 1 × 1-MMI waveguide region 111.

(Embodiment 3)
FIG. 6A is a schematic view of an active MMI semiconductor laser according to a third embodiment of the present invention as viewed from above. 6B is a cross-sectional view taken along one-dot chain line AA ′ in FIG. 6A, and FIG. 6C is a cross-sectional view taken along one-dot chain line BB ′ in FIG. 6A.

  As shown in FIGS. 6A to 6C, the active MMI type semiconductor laser of the present embodiment is divided into two 1 × 1-MMI waveguide regions 111 in the structure of the second embodiment described above. The MMI waveguide regions 111a and 111b are replaced with single-mode waveguide regions 114 that connect them. The other parts are basically the same as the structure described in FIGS. 4A to 4C. The element length is about 600 μm. The single mode waveguide regions 112 and 113 both have a waveguide length of about 40 μm and a width of about 2 μm. Both the 1 × 1-MMI waveguide regions 111a and 111b have a waveguide length of about 230 μm and a width of about 9 μm. The single mode waveguide region 114 has a length of about 60 μm and a width of about 2 μm.

  The grating 120 is formed on the surface of the n-InP semiconductor substrate 101 below the active layer 104 from the rear end surface to the front end surface, and a λ / 4 phase adjustment region 121 is formed in the resonator of the grating 120. Has been. The λ / 4 phase adjustment region 121 is obtained by shifting the pitch of the grating 120 by λ / 4, and the phase of the grating 120 is inverted across the λ / 4 phase adjustment region 121. The width of the grating 120 is the same as the width of these waveguides (about 2 μm) in the single mode waveguide regions 112 and 113, and the single mode waveguide regions 112 and 113 in the 1 × 1-MMI waveguide region 111. The width is within twice the waveguide width (about 4 μm). Here, the width of the grating 120 is about 3 μm, and the normalized coupling constant (kL) is about 1. An antireflection film is provided on both the rear end face and the front end face.

  Also in this embodiment, the grating 120 is formed at the waveguide center in the 1 × 1-MMI waveguide regions 111a and 111b, and the grating width is the waveguide width of the single mode waveguides 112 to 114. It is assumed that the width is within twice. Therefore, since the single transverse mode can be reflected with respect to the grating center wavelength among the transverse modes propagating in the 1 × 1-MMI waveguide regions 111a and 111b, stable single wavelength operation is realized. The Other advantages are the same as those of the first and second embodiments described above.

  Next, a method for manufacturing the above-described active MMI semiconductor laser of this embodiment will be described.

  The active MMI type semiconductor laser of this embodiment can also be basically manufactured by the steps shown in FIGS. 3A to 3F, but the 1 × 1-MMI waveguide region and the region where the grating is formed are as described above. This is different from the case of the first and second embodiments (steps of FIGS. 3A and 5a). That is, in the present embodiment, as shown in FIG. 7, the grating 120 includes 1 × 1-MMI waveguide regions 111 a and 111 b and single mode waveguide regions 112 to 114 on the surface of the n-InP semiconductor substrate 101. In the region where the active waveguide is formed, it is formed over the entire length of the active waveguide. The width of the grating 120 is set to be equal to or larger than the waveguide width of the single mode waveguides 112 to 114 formed in a later process and within twice the waveguide width. Here, the waveguide width of the single mode waveguides 112 to 114 is about 2 μm, and the width of the grating 120 is about 3 μm.

  After the formation of the grating 120, an active waveguide including the 1 × 1-MMI waveguide regions 111a and 111b and the single mode waveguide regions 112 to 114 is formed. Then, the rear end face and the front end face of the laser element are formed by cleavage, and antireflection films are formed on both end faces, thereby completing the element manufacture. Thus, a laser element having the structure shown in FIGS. 6A to 6C is obtained.

  Also in the above manufacturing method, as in the manufacturing method in the first embodiment described above, it is possible to manufacture an element with a high yield and a low cost.

  The waveguide structure and the manufacturing method thereof according to the present embodiment described above are examples, and the configuration and procedure can be changed as appropriate. For example, in the manufacturing process described above, for example, a molecular beam growth method (MBE method) can be used instead of the MO-VPE method, or an RIE method can be used instead of the ICP method.

  The formation position of the grating 120 can also be changed within a range where stable single wavelength oscillation can be obtained. For example, the grating 120 may be provided at any of the positions a) to d) described in the first embodiment. The grating 120 may be provided only in the 1 × 1-MMI waveguide regions 111a and 111b. Furthermore, the grating 120 may be provided only in one of the 1 × 1-MMI waveguide regions 111a and 111b.

  Furthermore, the number of 1 × 1-MMI waveguide regions can be changed within a range in which the element length does not increase. For example, three or more 1 × 1-MMI waveguide regions may be provided.

  In the first to third embodiments described above, a part of the active waveguide including the active layer is configured by the 1 × 1-MMI waveguide. However, the present invention is not limited to this. For example, instead of the 1 × 1-MMI waveguide, a 1 × N-MMI waveguide or an N × N-MMI waveguide can be used. When the 1 × N-MMI waveguide is used, the “N” side is the rear side, the “1” side is the front side, and the single mode waveguide 113 is provided at a position corresponding to the N branch on the rear side. When the N × N-MMI waveguide is used, the single mode waveguide 112 is provided at a position corresponding to the front N branch, and the single mode waveguide 113 is provided at a position corresponding to the rear N branch.

  In addition, the waveguide structures of the above-described embodiments have a structure in which a single mode waveguide is connected to both ends of the MMI waveguide, but only the front side or the rear side of the MMI waveguide has a single mode waveguide. Such a structure is also possible. In this case, the end face of the MMI waveguide where the single mode waveguide is not provided is the end face of the laser element. For example, when a single mode waveguide is not provided on the rear side, the end face on the rear side of the MMI waveguide becomes the end face on the rear side of the laser element, and a highly reflective film is formed on this end face.

(Module structure)
Using the active MMI semiconductor laser of the present invention described above, an optical communication module such as an optical transmission module or an optical transmission / reception module can be configured. In the case of an optical transmission module, the active MMI type semiconductor laser of the present invention and a circuit for driving the same are mounted. In the case of an optical transceiver module, the active MMI semiconductor laser of the present invention, a circuit for driving the laser, and a light receiving unit for receiving light input from the outside are mounted. In any case, various other circuits (such as a modulation circuit and a waveform shaping circuit) according to use can be mounted. These optical communication modules can be driven at a low voltage by using the active MMI semiconductor laser of the present invention. Therefore, it is possible to provide a module that has an unprecedented low power consumption.

Claims (11)

A semiconductor laser in which an active waveguide is composed of at least one multimode interference waveguide and at least one single mode waveguide connected to both ends or one end of the multimode interference waveguide,
A semiconductor laser having a grating for selecting a single wavelength of oscillation light propagating through the active waveguide at a part of the active waveguide.   The semiconductor laser according to claim 1, wherein the grating is provided in a region of the single mode waveguide. The single mode waveguide is a pair of single mode waveguides provided at both ends of the active waveguide,
The semiconductor laser according to claim 2, wherein the grating is provided in at least one of the pair of single mode waveguides.   The semiconductor laser according to claim 1, wherein the grating is provided in a region of the multimode interference waveguide.   The semiconductor laser according to claim 4, wherein a width of the grating is within twice a width of the single mode waveguide. The single mode waveguide is a pair of single mode waveguides provided at both ends of the active waveguide,
The semiconductor laser according to claim 5, wherein the grating is provided in a region of the multimode interference waveguide on an extension line of the pair of single mode waveguides.   The semiconductor laser according to claim 6, wherein the grating is provided over the entire length of the active waveguide.   The semiconductor laser according to claim 7, wherein a phase adjustment region is provided in the grating.   An optical communication module that houses the semiconductor laser according to claim 1 and a circuit that drives the semiconductor laser. A method of manufacturing a semiconductor laser comprising an active waveguide comprising at least one multimode interference waveguide and at least one single mode waveguide connected to both ends or one end of the multimode interference waveguide There,
Forming a grating region having a width wider than the width of the single mode waveguide in a region on the semiconductor substrate where the single mode waveguide is formed;
And a step of etching the grating region into the shape of the active waveguide. A method of manufacturing a semiconductor laser comprising an active waveguide comprising at least one multimode interference waveguide and at least one single mode waveguide connected to both ends or one end of the multimode interference waveguide There,
In the region where the active waveguide is formed on the semiconductor substrate, a grating region having a width equal to or larger than the width of the single mode waveguide and within twice the width of the single mode waveguide is provided. Forming over the entire length,
And a step of etching the grating region into the shape of the active waveguide.

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