Optical Transmitter

Abstract

This optical transmitter comprises a waveguide type laser (102) formed on a substrate (101), a waveguide type EA modulator (103), and a waveguide type semiconductor optical amplifier (104) and comprises a condensing unit (106) that uses multi-mode interference to condense reflected light that has been output by the semiconductor optical amplifier (104), has been made incident obliquely to an exit end (105), and has been reflected by the exit end (105) toward the substrate (101) side. The condensing unit (106) is formed from an optical waveguide made of a flat core having the core width larger than the core thickness when viewed in cross section.

Description

[Technical Field]

[0001] The present invention relates to an optical transmitter. [Background technology]

[0002] In recent years, the spread of video streaming services and the growing demand for mobile traffic have led to an explosive increase in network traffic. In the optical transmission lines that support these networks, there is a trend toward lowering costs by increasing transmission rates, lowering power consumption, and extending transmission distances. This has led to increasing demand for higher speeds and higher output power for the semiconductor modulated light sources used in these networks.

[0003] Electroabsorption modulator integrated DFB (EADFB) lasers have superior extinction characteristics and chirp characteristics compared to directly modulated lasers, and have been used in a wide range of applications, including as light sources for access networks. Figure 7 shows the configuration of a typical EADFB laser. A typical EADFB laser integrates a DFB laser 221 and an EA modulator 222 on the same chip. The DFB laser 221 has an active layer 203 made of a multiple quantum well (MQW) and oscillates at a single wavelength using a diffraction grating 207 formed in the resonator. The EA modulator 222 also has an optical absorption layer 204 made of an MQW.

[0004] The active layer 203 and the optical absorption layer 204 are sandwiched between the optical confinement layers 202 and 205 to form a separate confined heterostructure (SCH). These are also sandwiched between the lower cladding layer 201 and the upper cladding layer 206. An electrode 211 is formed on the upper cladding layer 206 of the DFB laser 221. An electrode 212 is formed on the upper cladding layer 206 of the EA modulator 222.

[0005] The electrical signal is converted into an optical signal by quenching the output light from the DFB laser 221 through optical absorption in the optical absorption layer 204 of the EA modulator 222. The electrical signal is converted into an optical signal by driving the EA modulator 222 under conditions of transmission and absorption to cause the output light from the DFB laser 221 to blink.

[0006] One issue with EADFB lasers is that they are difficult to achieve high output power due to the large optical loss of the EA modulator. To solve this issue, AXEL (SOA Assisted Extended Reach EADFB Laser) has been proposed, which is an EADFB laser that further integrates a semiconductor optical amplifier (SOA) at the light output end of the EADFB laser (Non-Patent Document 1).

[0007] An overview of AXEL will be explained using Figure 8. In AXEL, signal light oscillated by a DFB laser 221 and modulated by an EA modulator 222 is amplified by an integrated SOA 224, making it possible to achieve high output power. The SOA 224 also has an active layer 209 sandwiched between optical confinement layers 202 and 205, forming an SCH structure. These are also sandwiched between a lower cladding layer 201 and an upper cladding layer 206. An electrode 213 is formed on the upper cladding layer 206 of the SOA 224.

[0008] As such, AXEL equipped with SOA224 can achieve approximately twice the output power of conventional EADFB lasers. In addition, because AXEL is capable of highly efficient operation due to the SOA integration effect, it is possible to reduce power consumption by approximately 40% when driven under operating conditions that produce the same optical output as conventional EADFB lasers. Furthermore, AXEL can use the same MQW structure as DFB lasers for the SOA active layer. This means that devices can be fabricated using the same manufacturing process as conventional EADFB lasers, without the need for an additional regrowth process for integrating the SOA region. [Prior art documents] [Non-patent literature]

[0009] [Non-Patent Document 1] W. Kobayashi et al., "Novel approach for chirp and output power compensation applied to a 40-Gbit / s EADFB laser integrated with a short SOA", Optics Express, vol. 23, no. 7, pp. 9533-9542, 2015. Summary of the Invention [Problem to be solved by the invention]

[0010] In lasers used as transmitters in optical communications, the injection current is controlled to prevent the output power from changing over time. This control is achieved by monitoring part of the output power. In the case of a typical EADFB without an integrated SOA, the light-emitting device that contributes to the output power is the laser part, so this output can be monitored by monitoring either the forward output or the backward leakage light.

[0011] However, in the case of an AXEL with an integrated SOA, the amplification effect of the SOA affects the output power, so the method of monitoring the leakage light from the laser from the rear cannot be used for the above-mentioned control. For this reason, in the case of an AXEL, the forward output must be monitored for the above-mentioned control. However, since many optical components such as lenses and isolators are generally required to couple the output light from the laser chip to the optical fiber, the space for arranging a monitoring photodiode (PD) is limited. For this reason, in the case of a laser with an integrated modulator and SOA, there are design constraints on installing a PD to monitor the forward output power.

[0012] The present invention has been made to solve the above problems, and its object is to make it possible to install a PD for monitoring the forward output of a laser that integrates a modulator and a semiconductor optical amplifier without being subject to design restrictions. [Means for solving the problem]

[0013] The optical transmitter according to the present invention comprises a waveguide-type laser formed on a substrate, a waveguide-type electro-absorption modulator formed on the substrate continuous with the laser in the waveguiding direction, a waveguide-type semiconductor optical amplifier formed on the substrate and amplifying the output light output from the electro-absorption modulator, an output end formed on one end of the substrate from which the output light output from the semiconductor optical amplifier is emitted, a focusing section formed on the substrate and focusing, by multimode interference, light reflected by the output end toward the substrate, and a photodiode for monitoring the focused light focused by the focusing section, wherein the output light output from the semiconductor optical amplifier is incident obliquely on the output end. [Effects of the Invention]

[0014] As described above, according to the present invention, a focusing section is provided that focuses light that is output from a semiconductor optical amplifier and reflected toward the substrate at the output end, so that a PD for monitoring the forward output of a laser that integrates a modulator and a semiconductor optical amplifier can be provided without being subject to design restrictions. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a plan view showing the configuration of an optical transmitter according to a first embodiment of the present invention. [Figure 2A] FIG. 2A is an explanatory diagram showing the state of an equivalent linear optical system in which light 131 emitted from semiconductor optical amplifier 104, reflected at output end 105, and incident on light-collecting section 106 is replaced with light traveling along the optical axis. [Figure 2B] FIG. 2B is an explanatory diagram showing the state of an equivalent linear optical system in which light 131 emitted from semiconductor optical amplifier 104, reflected at output end 105, and incident on light-collecting section 106 is replaced with light traveling along the optical axis. [Figure 3] FIG. 3 is a characteristic diagram showing the results of an optical waveguide simulation of the coupling efficiency from the SM optical waveguide 132 to the SM optical waveguide 133 shown in a linear optical system. [Figure 4] FIG. 4 is a characteristic diagram showing the results of an optical propagation simulation when the light collecting section 106 is introduced. [Figure 5] FIG. 5 is a plan view showing a configuration of an optical transmitter according to a second embodiment of the present invention. [Figure 6] FIG. 6 is a plan view showing the configuration of an optical transmitter according to a third embodiment of the present invention. [Figure 7] FIG. 7 is a cross-sectional view showing the configuration of a typical EADFB laser. [Figure 8] FIG. 8 is a cross-sectional view showing the configuration of AXEL. DETAILED DESCRIPTION OF THE INVENTION

[0016] An optical transmitter according to an embodiment of the present invention will now be described.

[0017] [Embodiment 1] First, an optical transmitter according to a first embodiment of the present invention will be described with reference to Fig. 1. This optical transmitter includes a waveguide-type laser 102, a waveguide-type electro-absorption modulator (EA modulator) 103, and a waveguide-type semiconductor optical amplifier 104, which are formed on a substrate 101.

[0018] The EA modulator 103 is formed on the substrate 101, continuous with the laser 102 in the waveguiding direction. The semiconductor optical amplifier 104 amplifies the output light from the EA modulator 103. The output light from the semiconductor optical amplifier 104 is emitted to the outside from an output end 105 formed on one end side of the substrate 101. The output light from the semiconductor optical amplifier 104 is incident on the output end 105 at an angle.

[0019] For example, the substrate 101 is rectangular in plan view, and one of the opposing short sides serves as an output end 105. The laser 102 and the EA modulator 103 are formed so that their waveguide structures extend in a direction parallel to the long sides of the rectangular substrate 101 in plan view, and the waveguiding direction is perpendicular to the output end 105.

[0020] The laser 102 may be a distributed feedback (DFB) laser. The laser 102 and the EA modulator 103 have the same structure as the EADFB laser described with reference to Figures 7 and 8. For example, although not shown, the laser 102 has an active layer made of a multiple quantum well (MQW) and oscillates at a single wavelength using a diffraction grating formed in the resonator. Furthermore, although not shown, the EA modulator 103 has a light absorption layer 204 made of an MQW.

[0021] The active layer of laser 102 and the optical absorption layer of EA modulator 103 are sandwiched between two optical confinement layers (not shown) on the top and bottom as viewed from substrate 101, forming a separate confinement heterostructure. These are also sandwiched between a lower cladding layer and an upper cladding layer (not shown). Electrodes (not shown) are formed on the upper cladding layer of laser 102 and the upper cladding layer of EA modulator 103.

[0022] Although not shown, the semiconductor optical amplifier 104 also has an active layer, which is sandwiched between two optical confinement layers on the top and bottom to form a separate confinement heterostructure. These are also sandwiched between a lower cladding layer and an upper cladding layer, and an electrode is formed on the upper cladding layer, although not shown.

[0023] This example includes a curved optical waveguide 108 that optically connects the EA modulator 103 and the semiconductor optical amplifier 104. The laser 102 and the EA modulator 103 have their waveguiding directions perpendicular to the output end 105, but the curved optical waveguide 108 causes the waveguiding direction of the semiconductor optical amplifier 104 to be oblique, not perpendicular, to the output end 105.

[0024] This optical transmitter also includes a focusing unit 106 formed on the substrate 101, which focuses light reflected by the output end 105 toward the substrate 101 by multimode interference. The focusing unit 106 focuses the light output from the semiconductor optical amplifier 104, incident obliquely on the output end 105, and reflected by the output end 105 toward the substrate 101. The focusing unit 106 is configured from an optical waveguide with a flat core whose core width is greater than its thickness in cross section. A window structure 111 in which no waveguide structure is formed is provided between the output end 105 and the output end of the semiconductor optical amplifier 104.

[0025] This optical transmitter also includes a photodiode 107 that monitors the light collected by the light collecting unit 106. In this example, the photodiode 107 is a waveguide type and is formed on the substrate 101 in an area opposite the output end 105 as viewed from the light collecting unit 106. The photodiode 107 is optically connected to the output optical waveguide 106a of the light collecting unit 106.

[0026] Moreover, in the first embodiment, there is provided a notch 109 formed in the substrate 101 on the light emission side of the photodiode 107. Between the notch 109 and the emission end of the photodiode 107, there is provided a window structure 112 in which no waveguide structure is formed.

[0027] For example, substrate 101 can be formed into a rectangular shape in a plan view, and laser 102, EA modulator 103, and semiconductor optical amplifier 104 can be arranged on one side (the upper side in FIG. 1 ) of substrate 101 that is in contact with one end side (emission end 105) of rectangular substrate 101 in a plan view from the center of substrate 101. Also, light collecting unit 106 can be arranged on the other side (the lower side in FIG. 1 ) of substrate 101 that is in contact with one end side (emission end 105) of rectangular substrate 101 in a plan view from the center of substrate 101. In this configuration, notch 109 can be formed at the end of the other side of substrate 101, at a location that will be the light emission side of photodiode 107.

[0028] To briefly explain the fabrication of the above-mentioned laser 102, EA modulator 103, and semiconductor optical amplifier 104, first, a lower optical confinement layer is formed on a substrate 101. For example, the substrate 101 can be made of n-type InP. For example, the lower optical confinement layer can be formed by depositing (crystal growing) undoped i-InGaAsP using a known metalorganic chemical vapor deposition (MOCVD) method.

[0029] Next, an MQW is formed on the lower optical confinement layer to serve as the active layer for the laser 102 and the semiconductor optical amplifier 104. For example, the MQW can be formed by growing multiple layers of undoped i-InGaAsP as well layers and InGaAsP as barrier layers alternately using the MOCVD method. The well layers can be approximately 15 nm thick, and the barrier layers can be approximately 8 nm thick.

[0030] Next, a diffraction grating is formed on the active layer of the laser 102. Generally, a DFB laser can be made by forming a diffraction grating directly on the active layer and using this region as a resonator. Note that a distributed Bragg reflector (DBR) can be made by forming diffraction gratings at two positions on either side of the active layer region in the waveguiding direction.

[0031] Next, the active layer in the region that will become the EA modulator 103 and the curved optical waveguide 108 is removed, and an MQW light absorption layer is formed by crystal regrowth in the region that will become the EA modulator 103. The MQW light absorption layer can be formed by crystal growing multiple layers of undoped i-InGaAsP well layers and InGaAsP barrier layers, with a composition ratio different from that of the active layer. The MQW light absorption layer and the active layer in the already formed laser 102 region are butt-jointed. Furthermore, an MQW light absorption layer is also formed in the region that will become the photodiode 107.

[0032] Furthermore, an InGaAsP core layer is formed by crystal regrowth in the region that will become the curved optical waveguide 108. The core layer is formed so as to be butt-jointed with the active layer and light absorption layer in the region of the semiconductor optical amplifier 104 that have already been formed. Furthermore, a core layer made of InGaAsP is also formed in the region that will become the light-collecting section 106 (output optical waveguide 106a).

[0033] Next, an upper optical confinement layer made of i-InGaAsP and an upper cladding layer made of p-InP are formed to realize optical confinement in the up and down (vertical) direction as viewed from the substrate 101. In addition, for example, p + A contact layer made of InGaAsP can also be formed.

[0034] Next, each layer crystal-grown on the substrate 101 is selectively removed using a selective etching mask made of SiO2 or the like, thereby forming an optical waveguide structure (channel-type optical waveguide structure) with a high mesa structure of a predetermined width consisting of a lower cladding layer, a lower optical confinement layer, an active layer, an optical absorption layer, a core layer, an upper optical confinement layer, an upper cladding layer, and a contact layer in the areas to be the laser 102, the EA modulator 103, and the semiconductor optical amplifier 104.

[0035] At this time, the above-mentioned selective etching mask is used to form a mesa-structured optical waveguide structure in the region that will become the photodiode 107. On the other hand, in the region that will become the light-collecting portion 106 (output optical waveguide 106a), a core layer made of InGaAsP is left in an area that is wider than the area that will become the light-collecting portion 106 in plan view.

[0036] Next, the above-mentioned selective etching mask is used as a selective growth mask to bury and re-grow InP, for example, doped with Fe to make it highly resistive, on the substrate 101 around the formed mesa structure, thereby burying the high mesa structure with a buried insulating layer. Next, the selective etching mask used as the selective growth mask is removed, and a new selective etching mask is formed to selectively remove the core layer remaining in the region that will become the light-collecting portion 106, thereby forming the mesa shape of the light-collecting portion 106 (output optical waveguide 106a). In addition, a protective layer made of an insulating material such as SiO2 is formed on the surface of the mesa shape of the formed light-collecting portion 106 (output optical waveguide 106a). A protective layer is formed on the mesa shape of the light-collecting portion 106 (output optical waveguide 106a), but the mesa shape of the light-collecting portion 106 (output optical waveguide 106a) is not buried with a buried insulating layer.

[0037] Next, to electrically insulate the laser 102, EA modulator 103, semiconductor optical amplifier 104, and photodiode 107 regions, the contact layers between the regions are removed, and then electrodes are formed in each region. This is followed by chipping by cleavage, formation of an anti-reflection (AR) film 114, and formation of a high reflection (HR) film 115. The HR film 115 is formed on the end (end face) of the substrate 101 opposite the emission end 105. Hereinafter, the end of the substrate 101 opposite the emission end 105 on which the HR film 115 is formed will be referred to as the rear end.

[0038] In this optical transmitter, laser 102 is driven by a constant current to generate continuous wave (CW) oscillation, and the voltage applied to EA modulator 103 is modulated to modulate optical absorption and thereby achieve intensity modulation, which modulates the intensity of the light. Semiconductor optical amplifier 104 amplifies the output light of EA modulator 103 to compensate for the insertion loss of absorption-type EA modulator 103. Semiconductor optical amplifier 104 also amplifies the reflected light from output end 105, so general AXEL is said to be susceptible to the effects of reflected return light.

[0039] In contrast, in the first embodiment, a curved optical waveguide 108 is inserted between the EA modulator 103 and the semiconductor optical amplifier 104, thereby realizing angled end face incidence (oblique incidence) at the output end 105. With this configuration, the reflected returning light at the output end 105 is further suppressed than in the case of using only the AR film 114. In addition, just before the output end 105, a window structure 111 is formed in which a certain section of the optical waveguide structure disappears in both the horizontal and vertical directions, and by utilizing the diffusion of light, the returning light is further suppressed.

[0040] The main output light of this optical transmitter passes through a lens focusing optical system and an isolator located in front of the output end 105 and is coupled to an optical fiber. To monitor the change over time of this output light, an optical element for extracting a portion of the light is inserted within the focusing optical system. In recent years, the packages that house modulated lasers have become smaller, shortening the physical length of the fiber coupling system. Furthermore, as the optical path length increases, the high-NA output light spreads accordingly, resulting in a decrease in coupling efficiency. For this reason, rather than the above-mentioned monitoring technology on the side where the output light from the substrate 101 is output, a monitoring technology that monitors the inside of the substrate 101 or behind the substrate 101 where no other optical systems are present is preferable.

[0041] The optical transmitter according to the first embodiment includes a photodiode 107 on a substrate 101. Due to oblique incidence at the output end 105, a small amount of light reflected by the antireflection film 114 is reflected back into the substrate 101 in a direction different from that of the semiconductor optical amplifier 104. Furthermore, the window structure 111 diffuses light. For example, if a light absorption layer of a photodiode (PD) is provided within the substrate 101 and photocurrent in the light absorption layer is monitored, the light diffusion results in a small overlap between the light absorption layer and the diffused light in the vertical direction, resulting in low absorption efficiency. On the other hand, light also diffuses in the horizontal direction, so the width of the light absorption layer must be expanded to absorb more of this diffused light. A reverse bias is always applied to the monitor PD, generating dark current as noise. As the area of ​​the monitor PD increases, the resistance of the pn junction in the light absorption layer decreases, increasing the dark current. If the intensity of the light being monitored is weak, the dark current will be drowned out by the noise.

[0042] In contrast, the present invention introduces a focusing unit 106 to focus the light diverging in the horizontal direction. When introducing a special structure in the vertical direction, additional layer structures must be introduced above and below the light absorption layer of the PD, which limits the degree of freedom due to restrictions imposed by crystal growth. In contrast, the horizontal direction is easy because the shape can be changed through processing. In the present invention, optical monitoring within the substrate 101 is achieved by focusing light using the focusing unit 106, which utilizes multimode interference (MMI).

[0043] As shown in FIG. 1, light 131 emitted from the semiconductor optical amplifier 104 and reflected by the AR film 114 (output end 105) enters the light-collecting section 106. If we consider this as light traveling along the optical axis, it can be rewritten as an equivalent linear optical system as shown in FIGS. 2A and 2B. FIG. 2A shows a side view, and FIG. 2B shows a top view. In the window structure 111, the light 131 is diffused in the X and Y axis directions. In the YZ plane of FIG. 2A, the semiconductor optical amplifier 104, which serves as a single-mode (SM) optical waveguide, is located beyond the window structure 111. For example, this also has an SM optical waveguide structure in the XZ plane. That is, consider optical coupling when two SM optical waveguides 132 and 133 face each other across the window structure 111.

[0044] For example, assuming that the reflectance at output end 105 is 100%, the coupling efficiency from SM optical waveguide 132, via window structure 111, to the opposing SM optical waveguide 133 is 9% when the length of window structure 111 (window structure length) is 10 μm, as shown by the black squares in Fig. 3, which is the result of an optical waveguide simulation. In contrast, in the present invention, as shown in Fig. 2B, light collecting section 106 is arranged in place of a single-mode optical waveguide in the XZ plane, and in the X-axis direction, light is collected by reflection by the sidewalls of light collecting section 106 and optical interference, thereby achieving efficient coupling to SM optical waveguide 133.

[0045] Fig. 4 shows the results of an optical propagation simulation when the light collecting section 106 is introduced. According to the optical propagation shown in this simulation result, as shown by the black circles in Fig. 3, when the window structure length is the same as 10 µm, it is possible to improve the coupling efficiency up to 14%.

[0046] In this example, the laser wavelength is 1310 nm, and the focusing unit 106 is a passive optical waveguide whose core layer is made of InGaAsP material with a refractive index of 3.4. The core layer is 300 nm thick. Meanwhile, the cladding material is InP with a refractive index of 3.2. The designed physical length of the window structure 111 is 10 μm, and the distance from the output end of the semiconductor optical amplifier 104 to the start (input end) of the focusing unit 106 is 20 μm, which is twice the length of the window structure 111.

[0047] The width of the core layer of the focusing portion 106 is 8 μm and the length is 300 μm. The mesa width of the waveguide structure of the laser 102, the EA modulator 103, and the semiconductor optical amplifier 104 is 1 μm. The inclination of the waveguiding direction of the semiconductor optical amplifier 104 relative to the plane of the output end 105 is 5 degrees. In contrast, the inclination of the waveguiding direction of the focusing portion 106 relative to the plane of the output end 105 is -10 degrees.

[0048] Because the length of window structure 111 is 10 μm, the displacement of the optical axis in the X direction when propagating from the output end of semiconductor optical amplifier 104 to output end 105 is approximately 1 μm. To prevent interference between light collecting unit 106 and the optical waveguide of semiconductor optical amplifier 104 from this point, the center of the optical waveguide of light collecting unit 106 is offset by a further 4 μm in the X direction. For this reason, light enters light collecting unit 106 from a location displaced from the center of light collecting unit 106. The core width of output optical waveguide 106a connected to light collecting unit 106 is 2.6 μm.

[0049] Generally, the length of the window structure 111 during manufacturing is determined by the accuracy of the cleavage inclination. In reality, the length of the window structure 111 varies by several μm. If the window structure 111 is shorter than designed as a result of manufacturing, the result shown in FIG. 4 shows that the coupling efficiency improves when the SM optical waveguide is opposed. If the window structure length is shorter than 8 μm, the coupling efficiency is higher when the SM optical waveguide is opposed than in the present invention. However, a shorter window structure 111 is not preferable for the semiconductor optical amplifier 104 because it weakens the suppression of feedback light.

[0050] On the other hand, as the window structure 111 becomes longer, the coupling efficiency decreases exponentially in the SM optical waveguide facing direction, whereas in the structure of the present invention in which the light-collecting portion 106 is introduced, the coupling efficiency changes parabolically. This shows that the structure including the light-collecting portion 106 has a higher tolerance to manufacturing variations. A photodiode 107 having an optical waveguide structure is optically connected to the end of the output optical waveguide 106a of the light-collecting portion 106.

[0051] The light absorption layer of the photodiode 107 can be fabricated without any additional processes by leaving the light absorption layer of the EA modulator 103 or the active layers of the laser 102 and semiconductor optical amplifier 104 remaining during the mesa processing process.

[0052] The optical waveguide structure (mesa structure) in the light-collecting portion 106 can be formed by performing a separate, additional dry etching process after the high mesa burying process of the laser 102, EA modulator 103, and semiconductor optical amplifier 104. The optical waveguide structure (mesa structure) in the light-collecting portion 106 does not require burying, and achieves light confinement in the horizontal direction by the interface between air and the semiconductor via a protective layer. Note that the photodiode 107 can also have a so-called ridge optical waveguide structure.

[0053] Ideally, all of the light collected by the light collecting section 106 would be absorbed by the photodiode 107. However, for residual light, reflected light is suppressed by a window structure 112 provided on the output side of the photodiode 107 and the slanted side surfaces of the notch 109 formed by etching. As shown in Figure 1, the notch 109 is preferably located away from the rear end of the substrate 101 where the HR film 115 is to be formed. This is to prevent the HR film from being formed on the slanted side surfaces of the notch 109.

[0054] [Embodiment 2] Next, an optical transmitter according to a second embodiment of the present invention will be described with reference to Fig. 5. This optical transmitter includes a waveguide-type laser 102, an EA modulator 103, and a waveguide-type semiconductor optical amplifier 104 formed on a substrate 101. The optical transmitter also includes a curved optical waveguide 108 that optically connects the EA modulator 103 and the semiconductor optical amplifier 104. An AR film 114 is formed on an output end 105 of the substrate 101, an HR film 115 is formed on a rear end of the substrate 101, and a window structure 111 is provided just before the output end 105 of the substrate 101. The optical transmitter also includes a light-collecting section 106 (output optical waveguide 106a) and a notch 109. These are the same as those in the first embodiment described above.

[0055] The second embodiment includes a photodiode 107' that monitors external output light 134 that is collected by light collecting section 106, propagates through output optical waveguide 106a, passes through window structure 112, and is emitted from the oblique side surface of notch 109.

[0056] For example, the substrate 101 can be formed into a rectangular shape in a plan view, and the laser 102, the EA modulator 103, and the semiconductor optical amplifier 104 can be arranged on one side (the upper side in FIG. 1 ) of the substrate 101 that is in contact with one end side (the output end 105) of the rectangular substrate 101 in a plan view from the center of the substrate 101. The light collecting unit 106 can be arranged on the other side (the lower side in FIG. 1 ) of the substrate 101 that is in contact with the one end side (the output end 105) of the rectangular substrate 101 in a plan view from the center of the substrate 101. In this configuration, a notch 109 can be formed at the end of the other side of the substrate 101, at a location that will be the light output side of the light collecting unit 106 (output optical waveguide 106 a).

[0057] [Embodiment 3] Next, an optical transmitter according to a third embodiment of the present invention will be described with reference to Fig. 6. This optical transmitter includes a waveguide-type laser 102, an EA modulator 103, and a waveguide-type semiconductor optical amplifier 104 formed on a substrate 101. The optical transmitter also includes a curved optical waveguide 108 that optically connects the EA modulator 103 and the semiconductor optical amplifier 104. An AR film 114 is formed on an output end 105 of the substrate 101, an HR film 115 is formed on a rear end of the substrate 101, and a window structure 111 is provided just before the output end 105 of the substrate 101. The optical transmitter also includes a light-collecting section 106 (output optical waveguide 106a). These are the same as those in the first embodiment described above.

[0058] In the third embodiment, the light emitted from photodiode 107, which monitors the light collected by light collecting unit 106, is incident perpendicularly to the rear end (edge) side of substrate 101. For example, the above-described configuration can be achieved by providing bent portion 161 in output optical waveguide 106a and changing the waveguiding direction of output optical waveguide 106a to a direction perpendicular to the rear end side of substrate 101 within a plane parallel to the plane of substrate 101.

[0059] In this configuration, light that is emitted without being absorbed by the photodiode 107 is reflected by the HR film 115 at the rear end of the substrate 101 and returned to the photodiode 107. The light that is reflected by the HR film 115 and returned to the photodiode 107 can be absorbed again by the photodiode 107. This makes it possible to reduce the absorption length of the photodiode 107, for example, by half, and suppress dark current.

[0060] As described above, according to the present invention, a focusing section is provided that focuses the light that is output from the semiconductor optical amplifier and reflected toward the substrate at the output end, so that a PD for monitoring the forward output of a laser that integrates a modulator and a semiconductor optical amplifier can be provided without being subject to design restrictions.

[0061] It should be noted that the present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention. [Explanation of symbols]

[0062] 101...substrate, 102...laser, 103...electro-absorption modulator (EA modulator), 104...semiconductor optical amplifier, 105...output end, 106...light-collecting section, 106a...output optical waveguide, 107...photodiode, 108...bent optical waveguide, 109...notch, 111...window structure, 112...window structure, 114...anti-reflection (AR) film, 115...reflective (HR) film 115.

Claims

1. a waveguide laser formed on a substrate; a waveguide-type electroabsorption modulator formed on the substrate so as to be continuous with the laser in a waveguiding direction; a waveguide-type semiconductor optical amplifier formed on the substrate for amplifying output light from the electroabsorption modulator; an output end formed on one end side of the substrate, through which output light from the semiconductor optical amplifier is output; a focusing section formed on the substrate, which focuses light reflected by the exit end toward the substrate by multimode interference; a photodiode that monitors the light collected by the light collecting unit; Equipped with The output light from the semiconductor optical amplifier is incident obliquely on the output end.

1. An optical transmitter comprising:

2. 2. The optical transmitter according to claim 1, The optical transmitter is characterized in that the photodiode is a waveguide type and is formed on the substrate in an area opposite the light-collecting portion from the light-emitting end.

3. 3. The optical transmitter according to claim 2, The substrate is formed into a rectangular shape in a plan view, the laser, the electro-absorption modulator, and the semiconductor optical amplifier are arranged on one side of the rectangular substrate in plan view from the center of the substrate, the side being in contact with one end side of the rectangular substrate; the light collecting unit is disposed on the other side of the rectangular substrate in plan view, the other side being in contact with the side on the one end side of the rectangular substrate from the center of the substrate; an optical transmitter comprising an end portion of the substrate on the other side, the end portion having a notch formed at a position on the light emitting side of the photodiode;

4. 3. The optical transmitter according to claim 2, a reflective film formed on an end of the substrate opposite to the emission end, The light emitted from the photodiode is incident perpendicularly to the end portion.

1. An optical transmitter comprising:

5. 2. The optical transmitter according to claim 1, The substrate is formed into a rectangular shape in a plan view, the laser, the electro-absorption modulator, and the semiconductor optical amplifier are arranged on one side of the rectangular substrate in plan view from the center of the substrate, the side being in contact with one end side of the rectangular substrate; the light collecting unit is disposed on the other side of the rectangular substrate in plan view, the other side being in contact with the side on the one end side of the rectangular substrate from the center of the substrate; an optical transmitter comprising an end portion of the substrate on the other side, the end portion having a notch formed at a location on the light-emitting side of the light-collecting portion;

6. 6. The optical transmitter according to claim 1, an optical transmitter comprising a curved optical waveguide that optically connects the electro-absorption modulator and the semiconductor optical amplifier and causes output light from the semiconductor optical amplifier to be incident obliquely on the output end.

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