Optical transmitter

Abstract

Provided is an optical transmitter that has high manufacturing stability and reflection suppressing effect. This optical transmitter comprises a monolithic integration, on a single substrate, of: a distributed-feedback (DFB) laser which includes an active region formed of a multiple quantum well that produces an optical gain by current injection, and a diffraction grating; an electroabsorption (EA) modulator having an absorption region formed of a multiple quantum well of a composition different than that of the DFB laser; a semiconductor amplifier (SOA) having an active region of the same composition as that of the DFB laser; a bending waveguide which rotates a light propagation direction only by an angle θwg; and a passive waveguide which is connected to the SOA and has a core having a bandgap wavelength shorter than the oscillation wavelength of the DFB laser. The passive waveguide includes a tapering region and a narrow waveguide region. The tapering region is configured to convert a width W1 of the passive waveguide connecting to the SOA into a width W2 of the narrow waveguide region. The passive waveguide adjoins an end surface of the substrate at an angle θwg with respect to the normal to the end surface of the substrate.

Description

Technical Field

[0001] The present disclosure relates to an optical transmitter, and more particularly to a semiconductor laser device in which an electro-absorption (EA) optical modulator is integrated on an InP substrate. More particularly, the present disclosure relates to an optical transmitter including an EA modulator, a semiconductor optical amplifier (SOA), and a distributed feedback (DFB) laser.

Background Art

[0002] With the recent spread of video distribution services and the increase in mobile traffic demand, network traffic has been explosively increasing. In optical transmission lines that carry networks, there is a trend towards reducing the cost of networks by increasing the transmission rate, reducing power consumption, and extending the transmission distance. For semiconductor modulation light sources used in networks, it is required to achieve high speed and high output while suppressing an increase in excessive power consumption.

[0003] Electro-absorption modulator integrated DFB (EADFB) lasers have been used in a wide range of applications so far because they have high extinction characteristics and excellent chirp characteristics compared to directly modulated lasers.

[0004] Furthermore, an EADFB laser integrated with a semiconductor optical amplifier (SOA) (SOA Assisted Extended Reach EADFB Laser: AXEL) has been proposed (see, for example, Non-Patent Document 1). AXEL can be used as an optical transmitter in optical communication.

Prior Art Documents

Non-Patent Documents

[0005]

Non-Patent Document 1

[0006] There is a demand for improved manufacturing stability of AXEL and for sufficient anti-reflection effect at the end face of a semiconductor chip on which AXEL is formed.

[0007] The present disclosure has been made in view of the above circumstances, and an object thereof is to provide an optical transmitter with high manufacturing stability and high reflection suppression effect.

[0008] To achieve this object, an optical transmitter according to one embodiment of the present invention is an optical transmitter monolithically integrated on a single substrate: a distributed feedback (DFB) laser having an active region formed of multiple quantum wells and a diffraction grating that generates optical gain when current is injected; an electroabsorption (EA) modulator having an absorption region formed of multiple quantum wells with a different composition from that of the DFB laser; a semiconductor amplifier (SOA) having an active region with the same composition as that of the DFB laser; a bent waveguide that rotates the propagation direction of light by an angle θwg; and a passive waveguide connected to the SOA and having a core with a bandgap wavelength shorter than the oscillation wavelength of the DFB laser, wherein the passive waveguide includes a tapered region and a narrow waveguide region, and the tapered region is configured to convert a width W1 of the passive waveguide connected to the SOA to a width W2 of the narrow waveguide region, and the passive waveguide is in contact with the end face of the substrate at an angle θwg with a normal to the end face of the substrate.

[0009] As described above, according to one embodiment of the present invention, it is possible to provide an optical transmitter with high manufacturing stability and high reflection suppression effect. [Brief explanation of the drawings]

[0010]

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[0011] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. The same or similar reference numerals indicate the same or similar elements, and redundant explanations may be omitted. The numerical values or materials in the following description are examples, and other numerical values or materials may be used to implement the present disclosure without departing from the spirit of the present disclosure.

[0012]

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[0013] Before describing the embodiments of the present disclosure, an AXEL, which is an EADFB laser integrated with an SOA, will be described.

[0014] FIG. 1 is a cross-sectional view showing the schematic configuration of a typical EADFB laser. The typical EADFB laser 10 shown in FIG. 1 has a structure in which a DFB laser 11 and an EA modulator 12 are integrated on the same chip. The DFB laser 11 has an active layer 13 formed of a multiple quantum well (MQW) and oscillates at a single wavelength using a diffraction grating 14 formed in the resonator. The EA modulator 12 has an optical absorption layer formed of a multiple quantum well (MQW) with a different composition from that of the DFB laser 11, and the amount of optical absorption can be changed by voltage control. The EA modulator 12 blinks the light by driving it under conditions that allow it to transmit and absorb the output light from the DFB laser 11, thereby converting the electrical signal into an optical signal.

[0015] The EADFB laser 10 has a problem in that it is difficult to achieve high output power because the EA modulator is accompanied by a large optical loss. The above-mentioned AXEL has been proposed as a solution to this problem.

[0016] Figure 2 is a cross-sectional view showing the schematic configuration of AXEL. The AXEL 20 shown in Figure 2 modulates light from a DFB laser 11 into signal light using an EA modulator 12 and then amplifies the signal light using an SOA 16, thereby enabling increased output power. AXEL achieves approximately twice the output power of a conventional EADFB laser. Furthermore, because AXEL enables highly efficient operation due to the SOA integration effect, power consumption can be reduced by approximately 40% when driven under operating conditions that achieve the same optical output power as a conventional EADFB laser. Furthermore, in AXEL, the SOA 16 and the active layer 13 of the DFB laser 11 are formed using the same MQW structure. Therefore, AXEL 20 can be fabricated using the same manufacturing process as a conventional EADFB laser 10, without the need for an additional regrowth process for integrating the SOA 16.

[0017] AXEL, with its high optical output power due to the integration of SOA16, faces the challenge of significant degradation of operating characteristics due to reflected light. In optical transmitters such as conventional semiconductor lasers, reflected light reflected from the semiconductor chip facets and returning to the chip interior is known to adversely affect the device's operating characteristics. Therefore, semiconductor optical transmitters typically employ anti-reflection (AR) coating on the chip facets to suppress reflected light from the chip facets to less than 0.1%. However, in the case of an EADFB laser (AXEL) integrated with an SOA, even a small amount of reflected light significantly affects operating characteristics due to its high output power. If the optical amplification effect of the SOA in the AXEL is +3 dB compared to a conventional EADFB laser, the average optical output power increases by +3 dB, and the intensity of the reflected light also increases by 3 dB. Furthermore, because the reflected light from the AXEL facets is amplified again within the SOA, the intensity of the reflected light reaching the DFB laser in the AXEL increases by +6 dB compared to an EADFB laser. For this reason, AXEL uses an AR coating to prevent reflected light, as well as a structure that combines a window structure and a curved waveguide.

[0018] FIG. 3 is a top view showing the schematic configuration of the output end face of AXEL. A schematic diagram (top view) of this configuration is shown in [fig. 3]. FIG. 3 shows a portion of a waveguide 32 connected to an SOA of AXEL20, which has been made into a chip. Signal light from the SOA of AXEL20 in a semiconductor chip (also simply referred to as a chip in this specification) 31 is guided through the waveguide 32 and output from the output end face 33. Typically, the output end face of a semiconductor chip is a crystal plane formed by cleavage, and the waveguide in the semiconductor chip is formed at a perpendicular angle to this output end face. Therefore, light propagating through the waveguide is incident perpendicularly on the output face and is output from the semiconductor chip. In contrast, in the AXEL20 shown in FIG. 3, a bend is provided in the waveguide 32 in the semiconductor chip 31. The signal light guided through the waveguide 32 is incident on the output end face 33 at an incident angle of θwg. This makes it difficult for light reflected at the output end face 33 to be recoupled to the waveguide 32. That is, it is possible to suppress the reflected returning light from the output end face 33. Generally, the incident angle θwg of the signal light from the waveguide 32 to the output end face 33 to suppress reflection is set to 4 to 8°.

[0019] Furthermore, in the structure shown in Figure 3, the waveguide 32 terminates inside the semiconductor chip 31. The signal light emitted from the waveguide 32 propagates through a bulk semiconductor called the window region 35, reaches the output end face 33, and is emitted from the semiconductor chip 31 to the outside. At this time, the signal light propagates while expanding its beam diameter due to the diffraction effect in the window region 35. This reduces the proportion of the optical field reflected by the output end face 33 and overlapping with the waveguide 32 inside the semiconductor chip 31, further reducing the reflected back light that couples with the waveguide 32. Generally, the length of the window region 35 in the light propagation direction (X direction) is made to be about 10 μm. With AXEL20, unless sufficient measures are taken to prevent reflected back light, it is not possible to obtain high-quality transmission characteristics as well as high-output characteristics.

[0020] As mentioned above, AXEL uses a curved waveguide and window structure at the output end face to minimize reflected light at the end face of the chip. Here, we will explain the challenges of this structure.

[0021] First, referring to FIG. 4, the problems in the case of adopting a bent waveguide will be described. FIG. 4 is a top view showing a plurality of semiconductor chips 31 arranged in the X direction and the Y direction on a wafer. In this specification, a semiconductor substrate on which AXEL is formed and chipped is also referred to as an AXEL chip. and In FIG. 4, two semiconductor chips 31 on which AXEL having a waveguide 32 is formed are opposed to each other in the light propagation direction (X direction). The waveguide 32 includes a straight waveguide portion 32a and a bent waveguide portion 312b. FIG. 4 illustrates an AXEL chip having no window region 35.

[0022] Normally, in an optical semiconductor device such as a semiconductor laser, a plurality of chips having the same emission end face are cleaved in a bar shape to form an emission end face. Also, the BARs adjacent in the light propagation direction are arranged such that their emission end faces face each other, and the opposed chips are arranged in a form that shares the emission end face formed by cleavage.

[0023] As shown in FIG. 4, in the plurality of chips 31, a bent waveguide portion 32b is formed such that the optical axis forms an angle θwg with respect to the emission end face, and the bent waveguides 32b in two adjacent and opposed chips 31 in the X direction are connected. That is, two adjacent and opposed chips are arranged with the same optical axis. In other words, the two chips are continuously created on the same waveguide. The adjacent two Bars 1 and 2 are cleaved by cleavage to form an emission end face 33 at which the waveguide 32 terminates. In the actual cleavage process of the semiconductor chip, there is a limit to the accuracy of the cleavage position, and a positional deviation of about ±10 μm usually occurs. As described above, by arranging the optical axes of the opposed chips to be aligned and connecting the waveguides, even when an error occurs in the cleavage position, a structure in which the waveguide terminates at the emission end face in both chips can be formed, so that the yield reduction in the cleavage process can be suppressed. However, this arrangement has a disadvantage that the chip size increases.

[0024] The DFB laser 11, the EA modulator 12, and the SOA 16 in the AXEL are formed on a waveguide having an optical axis in a direction (X direction) perpendicular to the emission end face 33. Here, in two opposing chips, the optical axes of the DFB lasers 11 do not coincide and are arranged at positions parallel to each other and separated by a distance ΔY. This ΔY is called the waveguide offset. As is clear from FIG. 4, the size of the semiconductor chip 31 in the Y direction needs to be equal to or larger than this waveguide offset. Also, it is desirable that the optical element portion such as the DFB laser 11 and the light emission position of the cleavage end face (the end position of the waveguide 32) be at the center of the chip. For this reason, the size of the chip in the Y direction is designed to have a sufficient margin with respect to the waveguide offset. Here, the bending waveguide portion 32b of a normal AXEL requires a length of at least about 400 μm. Assuming that the bending angle θwg of the waveguide is 5°, the waveguide offset is about ΔY = 70 μm. In this case, when a margin of 100 μm is secured on both sides of the straight waveguide portion 32a of each chip 31 in Bar1 and Bar2 (margin 1 and margin 2 in FIG. 4), a chip width (Y reported length) of at least 270 μm is required. Thus, in a conventional AXEL, it has been difficult to reduce the width (size in the Y direction) of the AXEL chip to 250 μm or less in order to secure a sufficient margin (distance) in the chip with respect to the waveguide position from the viewpoints of manufacturing margin and long-term reliability. In addition, the chip yield during wafer manufacturing also becomes a problem in the arrangement of a conventional AXEL.

[0025] Referring to FIG. 5, the chip yield during wafer manufacturing will be described. FIG. 5(a) schematically shows the arrangement of four chips (Chip1 to 4) continuously formed on the same optical waveguide 32 on a semiconductor wafer in the manufacturing process of the AXEL chip. As described above, after the manufacturing process of each element in the chip, each chip is separated by a dicing process. Chip 1 and Chip 2 are arranged and connected opposite to each other so that the SOA 16 for obtaining an optical output is close, and Chip 2 and Chip 3 are connected so that the DFB laser is adjacent. As described above, the emission end face of the chip and the end of the waveguide are determined by dicing. For this reason, adjacent chips in the x direction are arranged with the waveguide 32 connected, taking into account the dicing position deviation error in advance. This is the same not only on the SOA side (the boundary between Chip 1 and Chip 2) which is the emission end side but also on the DFB laser side (the boundary between Chip 2 and Chip 3).

[0026] Here, due to the above-described waveguide offset, a positional deviation in the Y direction occurs between Chip 2 and Chip 3. FIG. 5(a) shows only four chips continuously arranged in the X direction, but in fact, more than 10 chips are formed on the same optical waveguide 32 on the wafer. Therefore, the larger the number of consecutive chips, the more the positional deviation in the Y direction due to the waveguide offset accumulates. When 10 chips are arranged on the wafer at once, if the waveguide offset ΔY is 40 μm, the deviation in the Y direction between Chip 1 and Chip 10 reaches 400 μm. Such a positional deviation on the wafer has an adverse effect on the chip arrangement.

[0027] Figure 5(b) is a schematic diagram of a wafer manufactured with 10 x 10 AXEL chips arranged on it. Normally, in the optical semiconductor manufacturing process, a basic unit called a shot, which is a rectangular (or square) array of multiple chips, is divided and arranged on the wafer. As shown in Figure 5(b), if a waveguide offset occurs, a dead zone (blank space) will be created within the shot where no chips can be placed, which inevitably reduces the number of chips manufactured within the same wafer and reduces yield. In Figure 5(b), if the length (length in the X direction) of the AXEL chip is 1000 μm, the chip width (length in the Y direction) is 300 μm, and the waveguide offset is 70 μm, the dead zone will be 700,000 μm within the shot. 2 The size of the AXEL chip is 300,000 μm 2 This leads to a decrease in the overall yield of about 2 to 3%.

[0028] Next, referring to Figure 6, we will discuss the issues that arise when a window region is employed. Figure 6 is a top view showing a schematic configuration of the vicinity of the emission section of an AXEL having a window region. To show the arrangement within the wafer, Figure 6 shows adjacent chips within the same bar (Bar1) and adjacent chips within an opposing bar (Bar2). As described above with reference to Figure 4, in Figure 6, even in an AXEL having a window region, the optical axis of the waveguide 32 at the emission end is arranged to coincide with the optical axis of the waveguide 32 of the opposing chip.

[0029] Furthermore, the two opposing chips each have a window region 35, and the end of the waveguide 32 is located at a position separated from the planned cleavage position 44 by the length of the window region 35 (length in the X direction). The length of a window region is typically designed to be about 10 μm. As mentioned above, a positional deviation error of about ±10 μm occurs in the cleavage process. Therefore, if an error in the cleavage position occurs of 10 μm or more, the window region 35 in one of the two bars shown in FIG. 6 will disappear, and sufficient reflection suppression effect will not be achieved.

[0030] Figure 7 shows the results of evaluating the optical waveform quality of multiple modules equipped with AXEL chips manufactured using the same process. The AXEL chips here all have the same structure and emit light in the 1.3 μm band. The only difference between the chips is the length of the window region caused by manufacturing errors. sa The only difference is the length of the window region for each chip. After measuring the length of the window region for each chip in advance, each chip was mounted in a common butterfly-type semiconductor module consisting of a two-lens system. The butterfly-type package also has a high-frequency connector, and each manufactured module was modulated with a 25 Gbit / s NRZ signal to evaluate the optical waveform (EYE waveform). The vertical axis of Fig. 7 is an index called mask margin, which indicates the quality of the optical waveform, and the larger the margin, the clearer the eye opening. The horizontal axis of Fig. 7 is the length of the window region for each manufactured AXEL chip, with the design value for the length of the window region being 10 μm. sa This shows the deviation from the design value. It can be seen that the mask margin tends to deteriorate as the length of the window region becomes smaller. This is because as the length of the window region becomes smaller, the intensity of the light reflected back from the chip edge into the chip interior increases, causing instability in the operation of AXEL. fixed 7, it can be seen that the length of the window region must be at least 5 μm or more. Also, if the window region 35 is too long, it will have a negative effect on the characteristics of AXEL.

[0031] FIG. 8 is a diagram schematically showing the optical beam shape in the vicinity of the end face when the length L of the window region 35 becomes longer than the design due to a manufacturing error in the open position 44. FIG. 8(a) is a top view and (b) is a cross-sectional view. In the case of the communication wavelength bands in the 1.55 μm band and 1.3 μm band, the thickness of the core layer is about 200 nm to 300 nm. Usually, in an InP-based optical semiconductor device, optical confinement in the waveguide vertical direction (Z direction) is stronger than in the waveguide horizontal direction (XY direction). Therefore, the vertical beam divergence angle in the window region 35 becomes larger than the horizontal beam divergence angle. From this, when the window region length is long, the upper end of the beam reaches the interface between the cladding layer and the outside of the semiconductor (air or electrode) within the window region, causing beam shape defects (keration) and optical losses (FIG. 8(b)). In a general InP-based optical semiconductor device, the cladding layer on the upper part of the waveguide is formed by regrowth, and the upper cladding layer thickness is about 2 μm. When providing a window structure, it is necessary to design the cladding layer thickness above the core layer in consideration of beam divergence due to the diffraction effect of the optical beam.

[0032] Since the InP thickness in the window region 35 tends to be thinner than the upper cladding thickness of the waveguide portion, it is particularly likely to affect the optical field. The reason for this will be explained with reference to the following Fig. 9. The normal window region 35 is an InP region where the waveguide terminates as described above. The InP of the window region 35 is formed simultaneously during the upper cladding layer growth process or the buried growth process. Here, the case of forming the window region during the buried growth will be described. Fig. 9 is a cross-sectional view showing a part of the manufacturing process of an AXEL having a buried hetero structure (BH structure). The DFB laser 11, the EA modulator 12, and the SOA 16 have a waveguide with a common buried structure. As shown in Fig. 9(a), first, after forming an insulating film on the surfaces of the lower cladding layer, the core layer 83, and the upper cladding layer 84 laminated on the InP substrate 81, patterning is performed to form an insulating layer mask 85 having a waveguide shape. Then, as shown in Fig. 9(b), a mesa formation process is performed by a dry or wet etching process to form a waveguide including the waveguide core 83a. Subsequently, as shown in Fig. 9(c), the waveguide is buried by regrowing the InP layer 86 which is a current blocking layer, and then the insulating layer mask 85 is removed to complete the BH structure. The aforementioned window region 35 is also formed simultaneously during the fabrication of this BH structure.

[0033] Fig. 10 is a schematic diagram showing the schematic configuration of an AXEL having a window region formed by buried regrowth. Fig. 10(a) is a schematic diagram seen from the upper surface direction of the substrate of the window region of the AXEL. Fig. 10(b) is a cross-sectional view of the waveguide of the AXEL, Fig. 10(c) is a cross-sectional view of the optical waveguide, and Fig. 10(d) is a cross-sectional view of the window region. Here, for simplicity of explanation, a straight waveguide without a bent waveguide is illustrated. As shown in Fig. 10(b), InP 86 which is a current blocking layer is formed by buried regrowth on both sides of the mesa-shaped waveguide, and the waveguide core 83a has a structure buried by InP 86. Also, as shown in Figs. 10(c) and (d), the portion that becomes the window region at the time of fabricating the mesa shape of the waveguide is also etched simultaneously, and the same InP layer as the current blocking layer InP is buried in the buried regrowth process.

[0034] In normal embedded regrowth, the thickness of the InP layer to be regrown is set so that the waveguide is completely embedded according to the height of the mesa-shaped waveguide. If the InP layer to be embedded and regrown is made too thick compared to the waveguide, the InP layer will ride up and deposit on top of the insulating layer mask 85 shown in FIG. 9(c). On the other hand, as shown in the top view of FIG. 10(a), since the window region is the part where the mesa shape terminates, the opening area to be embedded during regrowth is relatively wider compared to other parts. From this, when the mesa-shaped waveguide and the window region are simultaneously embedded with InP by regrowth, as shown in FIG. 10(c), a portion where the InP thickness in the window region is thinner than that in the mesa-shaped waveguide portion will occur. As described above, in the window region, the optical field propagates while spreading due to the diffraction phenomenon. Therefore, when the InP film thickness is thin, the optical field is likely to contact the upper air interface and optical loss is likely to occur.

[0035] FIG. 11 is a diagram showing the result of evaluating the relationship between the variation in the window region length of the optical loss in a module, which is obtained by fabricating a plurality of modules implementing an AXEL having a window region fabricated using a normal embedding and regrowth process. Here, it is the result of evaluation using the same modules as the data shown in FIG. 7. As described above, each module has an AXEL chip with the same structure, and the designed value of the length of the window region is 10 μm. Before mounting each AXEL chip on the module, the optical output characteristics were evaluated using a large-diameter photodetector. After that, after mounting the AXEL chip on the module and coupling it to an optical fiber to evaluate the light intensity, the optical loss caused by mounting on the module was estimated. The module used here has a two-lens system and was mounted on the module by an active alignment process. As is clear from FIG. 11, it can be confirmed that as the length of the window region increases, the loss inside the module tends to increase. This is because as the length of the window region increases, the upper end of the emitted beam reaches the boundary between the cladding layer and the outside of the chip, and a defect occurs in the beam shape, resulting in a decrease in the coupling efficiency to the optical fiber. Here, in order to suppress the optical loss in the module to 3.0 dB or less and create a stable optical module, the window region length of the AXEL chip needs to be 15 μm or less, that is, the deviation amount of the cleavage position needs to be 5 μm or less. However, in the actual cleavage process of a semiconductor chip, a cleavage position error of about ±10 μm generally occurs. In an AXEL having a window structure, in manufacturing an optical transmitter having sufficient characteristics, since a sufficient margin cannot be provided when forming the emission end face by cleavage, a certain number of defective products (chips in which the window region length deviates from the allowable value) are inevitably generated due to the manufacturing error of the cleavage position, which has been a cause of reducing the manufacturing yield.

[0036] According to an embodiment of the present disclosure, it is possible to provide an AXEL, that is, an optical transmitter, having high manufacturing stability and a reflection suppression effect. Hereinafter, the AXEL according to an embodiment of the present disclosure will be described.

[0037] (Embodiment) As mentioned above, the optical beam propagates through the window region due to diffraction. This reduces the proportion of the optical field distribution that recouples into the waveguide even when reflected at the end face, thereby reducing the amount of returning light. Therefore, in this disclosure, the optical beam shape is expanded by narrowing the waveguide using a tapered shape. In principle, if the waveguide width becomes sufficiently small, cutoff occurs, and the light is not confined within the waveguide and is emitted. In this state, the optical beam propagates while expanding, achieving the same reflection suppression effect as in the window region. However, it is difficult to consistently manufacture a waveguide narrow enough to cut off propagating light in the communication wavelength band (1.3 μm or 1.55 μm). In this disclosure, it is not necessary to use a waveguide narrow enough to achieve the cutoff condition; sufficient effectiveness can be achieved by fabricating a waveguide width within the manufacturing margin that sufficiently increases the beam diameter of the guided light.

[0038] This is shown in Figure 12. Figure 12 is a diagram illustrating the vicinity of the output end of the SOA in the rear stage of an AXEL device according to one embodiment of the present disclosure. In this disclosure, a tapered region 32c and a narrow waveguide region 34d are provided to narrow the waveguide width before the output end of the curved waveguide portion 32b of the waveguide 32, through which light from the SOA is guided. The narrow waveguide region 32d has a fixed length and is positioned beyond the designed cleavage position 44. Here, the narrow waveguide region 32d is long enough to tolerate misalignment errors in the cleavage process. Therefore, even if the cleavage position is misaligned, the end face is always formed within the narrow waveguide region 32d, so the quality of the output beam is not affected. In addition, the waveguide termination 32e is positioned outside the intended cleavage position, i.e., outside the chip. This arrangement prevents the propagation light from being affected by the thinning of the InP86 film thickness or deterioration of the surface flatness in the window region of the waveguide termination 32e, as described above with reference to Figure 10.

[0039] Here, the length required for the narrow waveguide region will be explained using FIG. 13. As mentioned above, a positional error of about ±10 μm occurs during the cleavage process. Therefore, the narrow waveguide region 32d must be at least 20 μm long to compensate for this error (M1 in FIG. 13). Furthermore, to obtain a sufficient anti-reflection effect, the narrow waveguide region 32d following the tapered region 32c must remain at least 5 μm long (M2 in FIG. 13). Furthermore, when a cleavage position error occurs, a margin of about 5 μm must be secured for the remaining waveguide 32f remaining in the adjacent chip (M3 in FIG. 13). From these points, the length of the narrow waveguide region 32d must be at least about 30 μm. In this case, taking into account the above-mentioned cleavage position error, the length of the narrow waveguide region 32d in the AXEL chip after cleavage is 5 μm to 25 μm, and the length of the remaining waveguide 32f in the adjacent chip, including the waveguide termination 32e, must be 5 μm to 15 μm.

[0040] In addition, by adjusting the position of the waveguides 32 between adjacent chips, it is possible to eliminate the waveguide offset. Figure 13 shows adjacent chips arranged on a wafer before cleaving. In this disclosure, the waveguide terminations 32e are arranged outside the chips. This means that the waveguide terminations 32e are arranged inside the adjacent chips facing each other. Therefore, by utilizing the waveguide offset, the positions of the waveguide terminations 32e are positioned sufficiently far from the optical axis propagating within each chip, so that the influence of the waveguide terminations 32e on the guided light is not exerted. Therefore, after cleaving, the waveguide terminations 32e of the remaining waveguides 32f of the adjacent chips on the wafer remain inside the chips. The waveguide terminations 32e of the remaining waveguides 32f do not affect chip operation at all.

[0041] In addition, the chip size restriction imposed by the waveguide offset is alleviated by using the arrangement shown in Fig. 13. That is, the waveguides are arranged so that the optical axes of opposing chips do not coincide with each other, which makes it possible to reduce the chip size.

[0042] By adopting the above-mentioned narrow waveguide region 32d, a sufficient reflection suppression effect can be obtained. Here, first, AXEL chips with different widths W perpendicular to the light propagation direction in the narrow waveguide region 32d were fabricated, and the influence of the reflected return light was evaluated. Here, similar to the study described in FIG. 7, a butterfly-type module incorporating a 1.3 μm wavelength band AXEL was fabricated, and the influence of the amount of return light was evaluated from the waveform quality of a 25 Gbit / s optical waveform. AXEL chips having narrow waveguide regions with different widths W were used for the evaluation. AXEL chips having the same structure except for the width W of the narrow waveguide region are used.

[0043] FIG. 14 is a diagram showing the dependence of the width of the narrow waveguide region of the evaluated mask margin. The mask margin is an index representing the waveform quality of the propagating light. A larger value of the mask margin indicates higher waveform quality of the propagating light. The width W of the mesa-shaped narrow waveguide region was changed from 0.25 μm to 1.5 μm for evaluation. Here, W = 1.5 μm is a chip with a constant waveguide width of a normal AXEL, that is, a chip having a waveguide with the same width up to the end face without using a taper near the output end of the waveguide. As is clear from FIG. 14, the narrower the width W of the narrow waveguide region at the output end, the more the end face reflection is suppressed and the waveform quality can be improved. If the width W of the narrow waveguide region is 0.7 μm or less, a mask margin of 30% or more can be ensured, which can be said to have a sufficient reflection suppression effect in practical use. However, when the width W of the narrow waveguide region is 0.25 μm, it is difficult to stably manufacture in the photolithography process, which is a manufacturing process of a general optical semiconductor device. As a structure that achieves both manufacturing stability and a sufficient reflection suppression effect, the width W of the narrow waveguide region exhibits the most effect at about 0.4 to 0.8 μm.

[0044] Next, the influence of the deviation of the cleavage position on the chip characteristics in the AXEL having a narrow waveguide region according to the present disclosure was evaluated. FIG. 15 shows the results of evaluating the optical loss of a plurality of modules mounted with AXEL chips fabricated by a normal cleavage process. In the AXEL chip here, the width W of the narrow waveguide region was set to 0.5 μm. Also, the data shown as the conventional AXEL in FIG. 15 is the same as the data shown in FIG. 11. Further, the evaluation is the same as that described in FIG. 11, that is, the optical loss in the module was calculated from the change in the optical output before and after modularization. There is concern that the loss increases when beam keratosis or the like occurs above the clad layer described above. From FIG. 15, as the deviation of the cleavage position of the conventional AXEL chip increases, the optical loss increases, whereas in the AXEL having a narrow waveguide region according to the present disclosure, the optical loss is almost constant regardless of the deviation of the cleavage position and is generally less than 2.5 dB. From this result, it was confirmed that the introduction of the narrow waveguide region of the present disclosure significantly suppresses the variation in chip performance due to the deviation of the cleavage position, which has been a conventional problem.

[0045] In addition, as an effect of the AXEL having a narrow waveguide region of the present disclosure, there is a point that miniaturization of the chip size and improvement of the chip yield during wafer manufacturing can be achieved. FIG. 16 is a diagram for explaining the miniaturization of the chip size and the improvement of the chip yield by the AXEL having a narrow waveguide region. As shown in FIG. 16, the straight waveguide portion 32a of the waveguide 32 of the AXEL can be arranged at the center of the chip in the Y direction. Therefore, even when the Y-direction length of the chip is reduced, sufficient margins can be ensured on both sides of the waveguide. Here, with the Y-direction size of the chip being 200 μm, by arranging the straight waveguide portion 32a at the center of the chip, sufficient margins of 100 μm each are ensured on the upper and lower sides of the waveguide. When the angle θwg formed by the optical axis of the bent waveguide portion 32b and the normal (X direction) of the cleavage plane (end face) is 5° and the length of the bent waveguide portion 32b is 400 μm, the positional deviation at the chip output end is about 35 μm, which is an arrangement that is sufficiently acceptable in manufacturing.

[0046] In addition, in FIG. 16, the optical axes of the straight waveguide portions 32a of adjacent chips facing each other are aligned. As a result, the waveguide offset ΔY in the Y direction can be set to 0. This can solve the problem of chip yield due to waveguide offset as described with reference to FIG. 5. Here, in a general AXEL chip, the length of the bending waveguide portion 32b in FIG. 16 is about 150 μm to 500 μm. Also, the angle θwg formed between the optical axis of the general bending waveguide portion 32b and the normal of the cleavage plane (end face) is 4° to 8° as described above. Therefore, assuming an angle θwg = 4°, the deviation D / 2 of the emission position in FIG. 16 is 10 μm < D / 2 < 35 μm. Also, assuming an angle θwg = 8°, 21 μm < D / 2 < 70 μm. Therefore, the narrow waveguide regions 32d of adjacent AXEL chips are separated by a distance D on the cleavage plane, and D satisfies 10 μm < D < 140 μm. However, in actual use, since the surface flatness decreases during regrowth at the waveguide end 32e as described above, it is necessary to maintain a distance of at least 30 μm from the bending waveguide portion 32b through which light propagates. Therefore, 30 μm < D < 140 μm is desirable.

[0047] (Example 1) The AXEL of Example 1 will be described with reference to FIG. 17. This example realizes an optical transmitter capable of generating a 25 Gbit / s modulation signal, in which the optical output during modulation is increased to 9 dBm or more in order to cope with a high-loss budget system.

[0048] FIG. 17 is a top view showing a schematic configuration of the AXEL according to this example. The semiconductor chip 31 shown in FIG. 17 includes a waveguide 32, a DFB laser 11, an EA modulator 12, and an SOA 16 connected by the waveguide 32. The waveguide 32 has a bending waveguide portion as described above. The bending waveguide portion has a taper region and a narrow waveguide region. The semiconductor chip 31 is an AXEL chip in which the DFB laser 11, the EA modulator 12, and the SOA 16 are monolithically integrated, and constitutes an optical transmitter. In FIG. 17, another adjacent AXEL chip (semiconductor chip 31) on the wafer is also shown to clarify the process of forming the emission end face by cleavage.

[0049] In the optical transmitter (semiconductor chip 31) of this embodiment, the length of the DFB laser 11 (the length along the waveguide 32, the same applies hereinafter) is 300 μm, the length of the EA modulator 12 is 150 μm, and the length of the SOA 16 is 200 μm. Also, the waveguide 32 employs an embedded hetero structure using semi-insulating InP that provides a high heat dissipation effect and a current confinement effect. As described above, in order to obtain a sufficient reflection suppression effect, the AXEL chip of this embodiment provides a tapered region 32c and a narrow waveguide region 32d in the bent waveguide portion 32b. The narrow waveguide region 32d is located near the output end face. The AXEL chip is formed on the InP substrate (100) surface, and the DFB laser 11 is arranged to output light in the direction of the substrate orientation

[0011] . The light from the DFB laser 11 propagates through the EA modulator 12 on the same optical axis as the DFB laser 11, and then the propagation direction is changed in the bent waveguide portion so as to have an angle θwg with respect to the crystal orientation

[0011] , and is incident on the SOA 16. The light emitted from the SOA 16 maintains the angle θwg and is converted to the width W of the narrow waveguide region in the tapered region. The light that has propagated through the narrow waveguide region passes through the cleavage planned position 44 and then terminates inside the adjacent chip 31. Here, the angle θwg is set to 5° as the bending angle that can obtain a sufficient reflection suppression effect. Also, the width W of the narrow waveguide region, which is within the range where stable manufacturing is possible in terms of the semiconductor processing process to obtain a sufficient reflection suppression effect, is set to 0.5 μm.

[0050] Under this condition, the narrow waveguide region has not reached the cut-off condition. Therefore, light propagates through the narrow waveguide region without radiating. Furthermore, two adjacent AXEL chips in the X direction face each other. Also, within the two AXEL chips, the straight waveguide portions in which the DFB laser 11 and the EA modulator 12 are formed are arranged in parallel and with their optical axes aligned. The bent waveguide portions of the two AXEL chips are arranged in parallel and are separated by a distance D at the planned cleavage position. Here, the length L_bend of the bent waveguide portion of the AXEL chip is 400 μm. Therefore, the waveguide distance D is 70 μm. The two AXEL chips are separated by a cleavage process. As described above, the cleavage process usually generates a positional deviation error of about ±10 μm. Therefore, the final light emission position of the chip is determined when the cleavage process is completed.

[0051] In order to obtain a sufficient reflection suppression effect, it is necessary to ensure a length of 5 μm or more for the narrow waveguide region following the tapered region within the AXEL chip (M2 in FIG. 13). Also, considering the positional deviation error in the cleavage process, it is necessary to ensure a cleavage error compensation region L_scr of 20 μm or more (M1 in FIG. 13) and a remaining waveguide length of 5 μm or more (M3 in FIG. 13) within the adjacent AXEL chips. Therefore, in this embodiment, the length L_nar of the narrow waveguide is designed to be 30 μm. Assuming that the positional deviation in the cleavage process is 0, 15 μm of the narrow waveguide portion remains within the AXEL chip. Also, even when a cleavage position deviation occurs and the narrow waveguide region becomes shorter, as long as the cleavage error is within the range of ±10 μm, a narrow waveguide region of 5 μm or more remains within the chip. Conversely, even when the narrow waveguide region becomes longer due to the cleavage position deviation, the length of the narrow waveguide region within the chip is 25 μm or less, and the length of the remaining waveguide remaining within the oppositely arranged AXEL chips is always 5 μm or more.

[0052] Next, the fabrication process of the optical transmitter (semiconductor chip 31) of this embodiment will be described. The semiconductor chip 31 was fabricated using an initial substrate in which a lower SCH (Separated Confinement Heterostructure) layer, a multiple quantum well active layer (MQW1), and an upper SCH layer were sequentially grown on an n-InP substrate (100) surface. The multiple quantum well layer is configured to have optical gain in the 1.3 μm oscillation wavelength band. Here, the multiple quantum well includes six quantum well layers. The initial substrate has a structure optimized for high-efficiency operation of the DFB laser 11.

[0053] First, the active layer is selectively etched, leaving the portions that will become the DFB laser 11 and SOA 16. Next, a multiple quantum well layer (MQW2) is grown by butt-joint regrowth on the portion that will become the EA modulator 12. The DFB laser 11, EA modulator 12, and SOA 16 are arranged so that light propagates in this order. The SOA 16 also utilizes the layer structure of the core layer formed on the initial substrate. Therefore, the layer structure of the SOA 16 is identical to that of the DFB laser 11. Ultimately, the only difference between the layer structures of the DFB laser 11 and the SOA 16 is the presence or absence of a diffraction grating. This minimizes the number of regrowths, enabling low-cost manufacturing despite the structure integrating multiple regions.

[0054] Next, the boundary region between the DFB laser 11 and the EA modulator 12, the boundary region between the EA modulator 12 and the SOA 16, and the region from the end of the region that will become the SOA 16 to the output facet are selectively etched again, and butt-joint regrowth is performed to grow a bulk semiconductor that will become the core layer of the passive region. The core layer of the passive region has a bandgap wavelength shorter than the oscillation wavelength of the DFB laser. In this specification, a waveguide including a passive region connected to the end of the region that will become the SOA 16 is also referred to as a passive waveguide.

[0055] Subsequently, a diffraction grating 14 (not shown) that operates in the oscillation wavelength band of 1.3 μm was formed in the portion that becomes the DFB laser 11. Here, the diffraction grating 14 is configured such that the resonator of the DFB laser 11 outputs light in the direction of the substrate orientation

[0011] . Thereafter, an InP clad layer and a contact layer were grown on the entire surface of the semiconductor chip 31 by regrowth again. Next, a mesa structure was formed in the portion that becomes the waveguide 32. Here, the waveguide widths of the DFB laser 11, the EA modulator 12, and the SOA 16 are set to 1.5 μm, which is the same as that of a general semiconductor device. In this process, the taper region and the narrow waveguide region of the bent waveguide portion, and the waveguide end are also created collectively. An insulating layer mask 85 was formed in the portion that becomes the waveguide 32, and a mesa structure was formed by dry etching.

[0056] Next, with the insulating layer mask remaining, the mesa structure was embedded with InP 86 by embedded regrowth. The embedded regrowth adjusted the growth amount of InP so that the waveguide 32 of the mesa structure was completely embedded. When the growth amount of semi-insulating InP is too small, the current blocking effect is not sufficiently exerted, and current is not efficiently injected into the DFB laser 11 and the SOA 16, and sufficient characteristics cannot be obtained. Also, when the growth amount of semi-insulating InP is too large, abnormal growth occurs where the InP regrown on the insulating film on the upper part of the waveguide 32 rides up, making it difficult to form electrodes for the DFB laser 11 and the SOA 16. Therefore, in order to ensure a sufficient current constriction effect and flatness on the upper part of the waveguide, the growth amount of InP during the embedded growth was adjusted so that the waveguide 32 of the mesa structure was completely embedded and the semi-insulating InP had the same height as the waveguide 32 of the mesa structure in the waveguide cross-section. In this process, in the normal window region, thinning of the InP film thickness occurs at the waveguide end 32e, which affects the optical field (see FIGS. 10(c) and (d)). However, in the present disclosure, since the waveguide end 32e is arranged outside the chip (adjacent chip), the light propagating in the chip is not affected by the waveguide end.

[0057] Subsequently, after removing the insulating layer mask 85, the contact layer between each region was removed by wet etching in order to electrically isolate the regions of the DFB laser 11, the EA modulator 12, and the SOA 16. Next, a P-side electrode for injecting current was formed through the contact layer on each region of the upper surface of the semiconductor substrate. Thereafter, the InP substrate 81 was polished to about 150 μm, and an electrode was formed on the back surface of the substrate, completing the process on the semiconductor wafer.

[0058] Next, a semiconductor bar including a plurality of chips 31 was fabricated by cleaving along the (011) crystal plane. Here, a general semiconductor chip cleavage process was used, and the cleavage position accuracy was ±10 μm or less as described above. Using the semiconductor bar in which a plurality of chips fabricated by the cleavage process were connected in the Y direction, an AR coating was applied to the emission end face, and a high-reflection coating (HR) was applied to the end face on the DFB laser 11 side.

[0059] To verify the effectiveness of this example, 20 AXEL chips fabricated using the same process were assembled into modules and evaluated. First, the optical output of the AXEL chips before assembly into the module was evaluated using a large-aperture PD. The input currents to the DFB laser 11 and SOA 16 were set to 80 mA and 40 mA, respectively, and a 0.6 V voltage and a 25 Gbit / s electrical signal were input to the EA modulator 12. As a result, the net optical output of the fabricated AXEL chips was approximately 12 dBm on average in terms of modulated output power (Pavg). Next, the optical output was evaluated after assembly into the module. The module was fabricated using the active alignment process described above. As a result, it was confirmed that all 20 modules had modulated optical output power (Pavg) exceeding 9 dBm during optical coupling. Similar to the evaluation shown in Figure 15, the intra-module loss, obtained by comparing the optical output power before and after assembly into the module, was below 2.5 dB in all cases, demonstrating a significant improvement in module manufacturing yield compared to conventional AXEL chips without window regions. If a conventional AXEL chip were mounted on a module, a loss of 3 dB or more would be expected during mounting, making it difficult to manufacture an optical transmitter that achieves the target optical output of +9 dBm. While the yield of conventional AXEL chips manufactured within a wafer when modularized is around 40%, the module manufacturing yield of the AXEL module according to this embodiment has been improved to 65%.

[0060] Next, a module equipped with the AXEL chip of this embodiment was used to evaluate the modulation characteristics at 25 Gbit / s and confirm the operation quality. The modulation signal was NRZ, PRBS2 31-1 was used. The driving conditions of AXEL are the same as those described above. Here, the EYE waveform quality at 25 Gbit / s was evaluated, and the dynamic extinction ratio was obtained as an average of 9.1 dB. Also, as a result of calculating the mask margin from the EYE waveform, more than 30% was obtained for all modules. In this embodiment, the width of the narrow waveguide region is set to 0.5 μm, and the above results support the results described in FIG. 14. Further, from these results, in the optical transmitter formed by the module according to this embodiment, a stable mask margin > 30% is obtained regardless of the deviation of the cleavage position, which is at the same level as the module in which good quality was obtained in the conventional AXEL chip. In the module equipped with the AXEL chip of this embodiment, there is no deterioration in the waveform quality of the optical signal, and only the manufacturing yield has increased significantly.

[0061] Finally, when the transmission characteristics over 40 km using a single-mode fiber were evaluated using the above-described operating conditions, error-free transmission with a bit error rate below 10 -12 was confirmed. From the above results, it was confirmed that in the AXEL with the dummy mesa structure introduced in the window region, a dramatic improvement in the manufacturing yield of the module was achieved without degrading the performance of the conventional module.

[0062] (Example 2) The AXEL of Example 2 will be described with reference to FIG. 18. In this embodiment, a chip design capable of improving the production quantity within the same wafer surface was adopted for the AXEL chip in the 1.5 μm band. FIG. 18 is a diagram schematically showing an arrangement example of the AXEL chip adopted in this embodiment.

[0063] As previously described with reference to Figure 4, when two opposing AXWL chips (two semiconductor chips 31) are arranged on a wafer so that their optical axes near the output are aligned (i.e., the waveguides are continuous), as in the case of conventional AXEL chips, the Y-axis positions of the DFB laser 11 and EA modulator 12, which have waveguides 32 aligned along the

[0011] substrate crystal orientation, do not match between the two AXEL chips, resulting in a large offset. From the perspective of ensuring a sufficient margin for cleaving the chip sidewalls and sufficient electrode area, it is desirable to position the DFB laser 11 and EA modulator 12 at the center of the chip, far from the chip sidewalls. Therefore, this chip arrangement necessitates a relatively large chip width. When using this type of arrangement with a typical AXEL chip, the chip width (length in the Y-axis direction) must be at least approximately 300 μm.

[0064] In contrast to this, in the AXEL of this embodiment, as shown in Fig. 18, the two opposing AXEL chips are arranged on the wafer so that the optical axes of the curved waveguide portions 32b of the waveguides 32 do not coincide, but the optical axes of the straight waveguide portions 32a of the waveguides 32 coincide. This allows the waveguides of the DFB laser 11 and EA modulator 12, which are oriented in the substrate crystal orientation

[0011] , to be arranged near the center of the AXEL chip, making it possible to reduce the chip width.

[0065] The fabrication process for the AXEL chip according to this embodiment is almost identical to that for the AXEL chip according to the first embodiment. The lengths of the DFB laser 11, EA modulator 12, and SOA 16 within the AXEL chip are 350 μm, 200 μm, and 300 μm, respectively. The diffraction grating 14 (not shown) formed in the DFB laser 11 has a period designed to achieve an oscillation wavelength of 1.55 μm. The length L_bend of the bent waveguide portion, including the 300 μm length of the SOA 16, is 500 μm, and the angle θwg is 5°. Therefore, the waveguide offset ΔY is 44 μm, and the waveguide distance D is 88 μm. The two AXEL chips (two semiconductor chips 31) are separated by a cleaving process. As mentioned above, the cleaving process typically results in a misalignment error of approximately ±10 μm. As in Example 1, in this example, the narrow waveguide L_nar is designed to be 30 μm, taking into account positional misalignment errors during the cleavage process, and the cleavage error compensation region L_scr is secured to be 20 μm or more. Furthermore, in this example, the chip width (length in the Y direction) is reduced to 200 μm, which was difficult with conventional AXEL. As mentioned above, this is because the present disclosure alleviates the chip size constraints imposed by waveguide offset. In the AXEL chip in this example, the straight waveguide portion is positioned at the center of the chip in the Y direction, and 100 μm margins 1 and 2 are secured on both sides of the straight waveguide portion in the Y direction. While the width of conventional AXEL chips was limited to approximately 300 μm, the reduction in chip size achieved by the invention of this example successfully increased the number of AXEL chips that can be fabricated on the same wafer by 50%.

[0066] Subsequently, the basic characteristics of the fabricated AXEL chip were evaluated. At an operating temperature of 55°C, currents of 80 mA and 100 mA were applied to the DFB laser 11 and the SOA 16, respectively. In the EA modulator 12, modulation was performed with a 10 Gbit / s NRZ signal at an applied voltage of -1.5 V and an amplitude voltage Vpp = 1.5 V. As a result, high output characteristics with a maximum optical output of 11 dBm during modulation were confirmed. When evaluating the production yield of the module equipped with the AXEL chip of this example, for a target optical output of 10 dBm or more during modulation, very good results with a module production yield of approximately 60% were confirmed for the AXEL chip having a narrow waveguide region according to this example.

[0067] (Example 3) Example 3 will be described with reference to FIGS. 19, 20, and 21. In this example, in order to improve the manufacturing stability of the window structure portion, a structure in which the width of the waveguide near the end is enlarged was adopted. First, the problems regarding manufacturing stability to be solved by this example will be described. In the AXEL according to this example, the passive waveguide among the waveguides 32 terminates inside the chip. In the manufacturing process of the AXEL, the collapse and disappearance of the passive waveguide occur starting from the termination. The collapse and disappearance of the passive waveguide contribute to the manufacturing yield of the AXEL. Further, the problems caused by the termination of this passive waveguide occur more prominently particularly in a narrow waveguide as used in the present disclosure. This will be explained with reference to FIG. 19.

[0068] FIG. 19(a) is a cross-sectional view showing a mesa shape formed by dry etching in the manufacturing process of the AXEL. The cross-section of FIG. 19(a) is a view of the mesa structure seen from the Y' direction intersecting the Y direction at an angle θwg. This FIG. 19(a) is the same figure as FIG. 9(b). In the actual manufacturing process of the AXEL, a wet etching process is performed to remove the damaged layer on the sidewall of the mesa shape generated by dry etching before buried regrowth and expose a normal semiconductor surface on the sidewall.

[0069] Figure 19(b) is a cross-sectional view of the mesa structure after wet etching. Similar to Figure 19(a), the cross section in Figure 19(b) is a view of the mesa structure from the Y' direction, which intersects with the Y direction at an angle θwg. This wet etching process typically involves the use of multiple etching solutions, one of which is typically a solution that selectively etches the core layer. This selective etching not only removes the passive waveguide, but also simultaneously removes the core layer of the DFB laser 11, the core layer (MQW1) of the SOA 16, and the core layer (MQW2) of the EA modulator 12. As a result, particularly in the light-emitting region of the DFB laser 11, the damaged layer on the sidewall of the mesa structure is removed, suppressing non-radiative recombination that does not contribute to oscillation during current injection. This improves light-emitting efficiency, optical output characteristics, and long-term reliability. As shown in Figure 19(b), even in the passive waveguide, the waveguide core 83a is selectively etched by this wet etching process. As a result, side etching progresses from both sides of the waveguide core 83a, narrowing the core width (length in the Y' direction) relative to the width W1 of the top of the mesa structure. When a narrower-than-normal waveguide is used as in the present disclosure, the remaining waveguide core 83a after wet etching is even narrower. This makes the waveguide prone to defects during the AXEL fabrication process. Figure 19(c) shows the waveguide termination break near the end face. Figure 19(c) shows the mesa structure viewed from the light propagation direction (the X' direction perpendicular to Y'). The aforementioned side etching also progresses at the waveguide termination. Therefore, near the waveguide termination, the etching solution advances from three directions: both sides of the waveguide and the termination face, making side etching more likely to progress. This results in the waveguide core 83a being significantly retracted toward the SOA 16 relative to its original waveguide termination position. This phenomenon is particularly pronounced when a narrower-than-normal waveguide width is used as in the present disclosure. The phenomenon occurs in which the mesa structure waveguide is damaged or lost starting from the waveguide termination where the termination position of the waveguide core 83a is set back. Damage or loss of the mesa structure waveguide leads to a decrease in the manufacturing yield of AXEL chips.Fundamentally, in order to prevent the waveguide from being damaged and disappearing, a method of shortening the wet etching treatment time before embedded regrowth can be considered. However, since the wet etching process is an essential process to ensure the high output and high reliability of the DFB laser section described above, it is difficult to solve this problem only by adjusting the manufacturing conditions.

[0070] Therefore, in this embodiment, a structure is adopted in which the narrow width of the narrow waveguide at the cleavage position 34 of the AXEL chip is converted back to a wide width in the taper region provided near the waveguide end. This structure can suppress the damage and disappearance of the waveguide due to side etching during the wet etching process and enables stable manufacturing.

[0071] FIG. 20 is a diagram showing a schematic configuration of the AXEL chip according to this embodiment. In this embodiment, it is possible to improve the yield of the AXEL chip having broadband characteristics enabling high-speed modulation of 50 Gbaud class and high output characteristics of 10 dBm or more. However, the improvement in manufacturing yield, which is an effect of this embodiment, does not depend on the operating speed or optical output characteristics of the AXEL chip, and can be introduced into AXEL chips of various specifications. As described above, in order to obtain a sufficient reflection suppression effect, the width W2 of the waveguide 32 is narrowed in the vicinity of the cleavage position 34. Thus, the same reflection suppression effect as in the above-described first embodiment can be expected. To narrow the waveguide 32, a taper region is used to gradually widen the width of the waveguide along the optical waveguide direction. The AXEL chip 31 of this embodiment constituting the optical transmitter is a semiconductor chip in which a DFB laser 11 having a length of 300 μm, an EA modulator 12 having a length of 125 μm, and an SOA having a length of 150 μm are monolithically integrated. Further, the waveguide 32 adopts an embedded hetero structure using semi-insulating InP capable of obtaining a high heat dissipation effect and a current constriction effect.

[0072] In addition, in this embodiment, in order to support high-speed modulation operation of 50 Gbaud, the quantum well structure and the length of the EA modulator 12 are optimized. As described above, in order to obtain a sufficient reflection suppression effect, the AXEL chip of this embodiment is provided with a taper region 32c and a narrow waveguide region 32d in the bent waveguide portion 32b. The narrow waveguide region 32d is located near the output end face. The AXEL chip is formed on the InP substrate (100) surface, and the DFB laser 11 is arranged so as to output light in the direction of the substrate orientation

[0011] . The light from the DFB laser 11 passes through the EA modulator 12 on the same optical axis as the DFB laser 11, and then the propagation direction is changed by the bent waveguide portion so as to have an angle θwg with respect to the direction of the crystal orientation

[0011] , and is incident on the SOA 16. The light emitted from the SOA 16 is converted to the width W2 of the narrow waveguide in the taper region while maintaining the angle θwg, passes through the cleavage planned position 44, and then is expanded along the propagation direction of the light by another taper region arranged in an adjacent chip, and is converted to a wide waveguide with a width of W3, and then reaches the waveguide end of the extended waveguide portion 32g. Here, the angle θwg is set to 5° as the bending angle that can obtain a sufficient reflection suppression effect. Also, the width W2 of the narrow waveguide region is set to 0.5 μm within the range where stable manufacturing is possible in the semiconductor processing process in order to obtain a sufficient reflection suppression effect.

[0073] Under this condition, the narrow waveguide region has not reached the cut-off condition. Therefore, the light propagates through the narrow waveguide region without radiating. Furthermore, two adjacent AXEL chips in the X direction face each other. Also, in the two AXEL chips, the linear waveguide portions in which the DFB laser 11 and the EA modulator 12 are formed are arranged in parallel and with their optical axes coinciding. The bent waveguide portions of the two AXEL chips are arranged in parallel and are separated by a distance D at the cleavage planned position. Here, the length L_bend of the bent waveguide portion of the AXEL chip is 400 μm. Therefore, the waveguide distance D is 70 μm. The two AXEL chips are separated by a cleavage process. As described above, the cleavage process usually generates a positional deviation error of about ±10 μm. Therefore, the final chip light output position is determined when the cleavage process is completed.

[0074] It is necessary to ensure that the length of the narrow waveguide region following the taper region in the AXEL chip is 5 μm or more (M2 in FIG. 13). Also, considering the positional deviation error in the cleavage process, it is necessary to ensure that the cleavage error compensation region L_scr is 20 μm or more (M1 in FIG. 13) and the length of the remaining waveguide is 5 μm or more (M3 in FIG. 13) in adjacent AXEL chips. Therefore, in this embodiment, the length L_nar of the narrow waveguide is designed to be 30 μm. Assuming that the positional deviation in the cleavage process is 0, 15 μm of the narrow waveguide part remains in the AXEL chip. Also, even when a cleavage position deviation occurs and the narrow waveguide region becomes shorter, as long as the cleavage error is in the range of ±10 μm, a narrow waveguide region of 5 μm or more remains in the chip. Conversely, even when the narrow waveguide region becomes longer due to the cleavage position deviation, the length of the narrow waveguide region in the chip is 25 μm or less, and the length of the remaining waveguide remaining in the oppositely arranged AXEL chip is necessarily 5 μm or more.

[0075] Also, the extended waveguide part 32g of the waveguide 32 is provided at a position sufficiently separated from the planned cleavage position, and the light propagating in the AXEL chip is not affected. The size of the extended waveguide part 32g is such that the length L_wide is 5 μm and the width W3 is 4 μm. Regarding L_wide and W3, since light does not propagate, it is not necessary to consider optical characteristics during design. Only from the perspective of the manufacturing stability of the AXEL chip, a mesa structure is adopted in which side etching by wet etching is difficult to progress and collapse and disappearance are difficult to occur in the processing process. Also, the change in the structure of the waveguide end according to this embodiment can be achieved by changing the insulating layer mask 85 (FIG. 19) during mesa formation, and has the feature that almost no additional manufacturing process or introduction load of cost increase is required.

[0076] In this prototype, in order to compare the effects, in the same lot and the same process, when the width of the waveguide end is narrow (W3 = W2 = 0.5 μm) and when the width is wide (W3 = 4 μm) were fabricated and compared. First, in the mesa processing process during the fabrication process, the side etching amount after wet etching treatment was compared. In the normal fabrication process, in the cross-section of the waveguide in the direction (Figure 19(b)), side etching of about 0.25 μm proceeds on one side. Therefore, in order to realize the waveguide width W2 = 0.5 μm, it is necessary to set the mesa width to 1.0 μm during the dry etching process for mesa formation. When the waveguide end is this narrow waveguide, when checking the side etching amount in the cross-section with respect to the traveling direction of the light at the end, it was 0.6 μm. Since etching proceeds from three directions at the waveguide end, side etching goes deep, and there is a high possibility of causing the collapse and disappearance of the waveguide core. On the other hand, when the width W3 of the waveguide was expanded to 4 μm, the side etching amount in the cross-section with respect to the traveling direction of the light at the end was reduced to 0.3 μm. By expanding the width W3 of the waveguide, in addition to the reduction of the side etching amount at the waveguide end, the region of the remaining waveguide core 83a also becomes larger, so it has a physically stable structure. For this reason, it becomes difficult for the waveguide to break starting from the waveguide end.

[0077] Figure 21 is a diagram showing the result of examining the relationship between the width W3 of the waveguide end and the side etching amount in the samples fabricated on the same wafer. In this embodiment, the width W3 of the waveguide end portion is set to 4 μm, but according to Figure 21, it was confirmed that if the width W3 of the waveguide end portion is 3 μm or more, the side etching amount is sufficiently suppressed.

[0078] Next, the actual AXEL manufacturing process was completed, and the appearance inspection of AXEL chips fabricated from the same wafer was performed, and the chip yields were compared. Here, a normal appearance inspection was carried out, and the ratio of defective chips due to waveguide breakage starting from the waveguide termination was compared. As a result, it was confirmed that in the AXEL chip according to this example (width W3 of the waveguide termination = 4 μm), the yield of defective chips due to waveguide breakage was improved by about 15% compared to the AXEL chip according to Example 1 (width W3 = W2 = 0.5 μm of the waveguide termination).

[0079] Subsequently, the basic characteristics of the fabricated AXEL chips were evaluated. At an operating temperature of 55°C, currents of 60 mA and 100 mA were applied to the DFB laser 11 and the SOA 16, respectively, and in the EA modulator 12, modulation was performed with an applied voltage of -1.5 V and an amplitude voltage Vpp = 1.5 V using a 50 Gbit / s NRZ signal. As a result, high output characteristics reaching a maximum of 10.5 dBm were confirmed as the optical output during modulation. Even when the width of the waveguide termination was expanded, no influence on the basic characteristics of the AXEL chip was observed, and performance comparable to the conventional case was confirmed.

[0080] (Example 4) Example 4 will be described with reference to FIG. 22. In this example, by adjusting the crystal plane orientation at the waveguide termination, the side etching amount was suppressed and the manufacturing stability was improved. As described above, in the AXEL chip according to the present disclosure, the passive waveguide terminates inside the chip, and the collapse and disappearance of the waveguide occurring starting from this termination part during the process steps contribute to the reduction in yield. The cause of this waveguide collapse and disappearance is that side etching progresses in the waveguide core 83a due to the wet etching process described with reference to FIG. 19(b), and the progress of side etching is particularly remarkable at the waveguide termination part.

[0081] Therefore, in this example, in addition to expanding the width of the waveguide termination shown in Example 3 above, the surface of the waveguide termination was set to the (011) crystal plane. The basic structure of the AXEL chip is the same as that of Example 3 above, and in this example, the surface of the waveguide termination is changed.

[0082] Figure 22 is a top view showing the schematic configuration near the end of the waveguide in the AXEL chip according to this embodiment. To achieve sufficient reflection suppression, a narrow waveguide region 32d with a narrow width W2 is provided in the waveguide 32 near the cleavage position. This allows for the same reflection suppression effect as in the first or third embodiment. A tapered region is also provided in the waveguide 32, gradually narrowing the width W2 along the light propagation direction. The AXEL chip is formed on the InP substrate (100) surface, and a DFB laser 11 is positioned so that light is output in the substrate orientation

[0011] . Light from the DFB laser 11 passes through an EA modulator 12 on the same optical axis as the DFB laser 11, then changes its propagation direction by a bent waveguide section at an angle θwg relative to the crystal orientation

[0011] , and enters the SOA 16. The light emitted from the SOA 16 is converted into a narrow waveguide with width W2 in the tapered region while maintaining the angle θwg. After passing through the planned cleavage position 44, the light is widened along the light propagation direction by another tapered region disposed in the adjacent chip, converted into a wide waveguide with width W3, and then reaches the waveguide termination of the extended waveguide section 32g. Here, the termination facet of the extended waveguide section 32g of the waveguide 32 is one of the sidewall faces formed by dry etching, and the waveguide termination facet is usually formed as a face perpendicular to the optical axis. Therefore, if the waveguide itself forms an angle θwg with respect to the crystal orientation

[0011] , the termination facet will be a face tilted θwg from the crystal plane (011).

[0083] In this embodiment, the end face of the extended waveguide portion 32g is not a plane perpendicular to the optical axis, but a plane that coincides with the crystal orientation (011) plane. The change in the end face of the extended waveguide portion 32g can be achieved by changing the insulating layer mask 85 (FIG. 19) during mesa formation, and it has the characteristic that it hardly requires an additional manufacturing process or an introduction load of cost increase. The side etching amount of the waveguide core 83a by the wet etching described above depends on the plane orientation. That is, the side etching amount on the (011) plane is the smallest, and the side etching amount tends to increase as the angle formed with respect to the (011) plane increases. As described above, at the waveguide end, since side etching proceeds from three directions, the side etching proceeds most significantly. In this embodiment, by setting the end face of the extended waveguide portion 32g corresponding to the waveguide end face to the (011) plane, the side etching of the waveguide core 83a is suppressed, preventing the collapse and disappearance of the waveguide during the manufacturing process, and enabling stable manufacturing and improvement of the yield.

[0084] The AXEL chip according to this embodiment was fabricated and its basic characteristics were evaluated. The structure and manufacturing process of the AXEL chip are the same as those of the AXEL chip shown in Embodiment 3 except for the structure of the waveguide end described above. Also, in the basic characteristics of the fabricated AXEL chip, high output performance and modulation characteristics comparable to those of the AXEL chip of Embodiment 3 described above were confirmed. In addition, it was confirmed that the AXEL chip according to this embodiment has a chip defect rate reduced by about 5% due to abnormalities in the waveguide end portion compared to the AXEL chip according to Embodiment 3, and has a sufficient effect on improving the manufacturing yield.

Industrial Applicability

[0085] It becomes possible to improve the manufacturing stability of the optical transmitter and the reflection suppression effect.

Explanation of Reference Numerals

[0086] 10 EADFB laser 11 DFB laser 12 EA modulator 13 Active layer 14 Diffraction grating 15 Absorbing layer 16 SOA 20 AXEL 31 Semiconductor chips 32 Waveguide 32a Straight waveguide section 32b Bent waveguide section 32c Tapered area 32d narrow waveguide region 32e waveguide termination 32f remaining waveguide 32g Extended waveguide section 33 Output end face 34 Cleavage position 35 Window Area 44 Planned cleavage position 81 InP substrate 82 Lower cladding layer 83 Core layer 83a Waveguide core 84 Upper cladding layer 85 Insulation layer mask 86 Current blocking (InP) layer

Claims

1. A distributed feedback (DFB) laser having an active region formed of multiple quantum wells that generate optical gain by current injection and a diffraction grating, An electro-absorption (EA) modulator having an absorption region formed of multiple quantum wells with a composition different from that of the DFB laser, A semiconductor optical amplifier (SOA) having an active region with the same composition as the DFB laser, A bent waveguide that rotates the optical propagation direction by an angle θwg, A passive waveguide connected to the SOA and having a core with a bandgap wavelength shorter than the oscillation wavelength of the DFB laser Is an optical transmitter monolithically integrated on one substrate, The passive waveguide includes a tapered region and a narrow waveguide region, The tapered region is configured to convert the width W1 of the passive waveguide connected to the SOA to the width W2 of the narrow waveguide region, where W1 > W2, The passive waveguide forms an angle θwg with the normal of the end face of the substrate and is in contact with the end face of the substrate, an optical transmitter.

2. Further comprising a second passive waveguide, The second passive waveguide, Is located at a distance D from the passive waveguide, Has a core with the same composition as the passive waveguide, Makes an angle θwg with respect to the normal of the end face of the substrate and is in contact with the end face of the substrate, Ends within the substrate, Has a width of W2, Has a length of L_d, the optical transmitter according to claim 1.

3. The substrate is an InP substrate, The DFB laser, the EA modulator, and the SOA are formed on the (100) plane of the InP substrate, The optical axis of the DFB laser is in the direction of the substrate crystal orientation [011], The passive waveguide forms an angle θw with the substrate crystal orientation [011] and is in contact with the end face of the substrate, the optical transmitter according to claim 2.

4. The second passive waveguide, Has a width of W2 at the position where it contacts the end face of the substrate, Has an extended waveguide portion at the end, The width of the extended waveguide portion is W3, Where W2 < W3, the optical transmitter according to claim 3.

5. W3 > 3 μm, the optical transmitter according to claim 4.

6. The end face of the extended waveguide portion coincides with the substrate crystal plane (011), the optical transmitter according to claim 5.

7. 5 μm < L_d < 25 μm, the optical transmitter according to claim 2.

8. 0.4 μm < W2 < 0.8 μm, the optical transmitter according to claim 1.

9. 4° < θwg < 8°, the optical transmitter according to claim 1.

10. The optical transmitter according to claim 2, wherein 30 μm < D < 140 μm.

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