WO2024024086A1

WO2024024086A1 - Optical transmitter

Info

Publication number: WO2024024086A1
Application number: PCT/JP2022/029294
Authority: WO
Inventors: 明晨陳; 隆彦進藤; 慈金澤
Original assignee: 日本電信電話株式会社
Priority date: 2022-07-29
Filing date: 2022-07-29
Publication date: 2024-02-01

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

optical transmitter

The present invention relates to an optical transmitter.

With the spread of video distribution services and the increase in demand for mobile traffic in recent years, network traffic is increasing explosively. In the optical transmission lines that play a role in networks, trends are toward higher transmission rates, lower power consumption, and lower network costs by extending transmission distances, and the semiconductor modulated light sources used here are also becoming faster.・Requirements for higher output are increasing.

Electroabsorption modulator integrated DFB (EADFB) lasers have higher extinction characteristics and better chirp characteristics than directly modulated lasers, so they have been used in a wide range of applications including light sources for access networks. I've been exposed to it. FIG. 7 shows the configuration of a general EADFB laser. In a typical EADFB laser, a DFB laser 221 and an EA modulator 222 are integrated into the same chip. The DFB laser 221 includes an active layer 203 made of a multiple quantum well (MQW), and oscillates at a single wavelength using a diffraction grating 207 formed within a resonator. Furthermore, the EA modulator 222 has a light absorption layer 204 made of MQW.

The active layer 203 and the light absorption layer 204 are sandwiched between the light confinement layer 202 and the light confinement layer 205, and have a Separate Confined Heterostructure (SCH) structure. Further, these are sandwiched between a lower cladding layer 201 and an upper cladding layer 206. An electrode 211 is formed on the upper cladding layer 206 of the DFB laser 221. Further, an electrode 212 is formed on the upper cladding layer 206 of the EA modulator 222.

In the light absorption layer 204 of the EA modulator 222, the output light from the DFB laser 221 is quenched by light absorption, thereby converting the electrical signal into an optical signal. The electrical signal is converted into an optical signal by driving the EA modulator 222 under transmission/absorption conditions to flicker the output light from the DFB laser 221.

A problem with this EADFB laser is that the EA modulator involves large optical loss, making it difficult to increase the output. As a solution to this problem, AXEL (SOA Assisted Extended Reach EADFB Laser), which is an EADFB laser that further integrates a semiconductor optical amplifier (SOA) at the light emitting end of the EADFB laser, has been proposed (Non-Patent Document 1).

The outline of AXEL will be explained using FIG. 8. In AXEL, the signal light oscillated by the DFB laser 221 and modulated by the EA modulator 222 is amplified by the integrated SOA 224, so high output is possible. Note that the SOA 224 also has an SCH structure in which the active layer 209 is sandwiched between the optical confinement layer 202 and the optical confinement layer 205. Further, these are 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 .

In this way, according to the AXEL provided with the SOA 224, high output characteristics approximately twice as high as those of a general EADFB laser can be obtained. In addition, since AXEL is capable of highly efficient operation due to the integration effect of SOA, it is possible to reduce power consumption by approximately 40% when driven under operating conditions that provide the same optical output as a general EADFB laser. be. Furthermore, AXEL can use the same MQW structure as DFB laser for the active layer of SOA. Therefore, the device can be manufactured using the same manufacturing process as a conventional EADFB laser without adding a regrowth process for integrating the SOA region.

Incidentally, in a laser used as a transmitter for optical communication, the injection current is controlled in order 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 general EADFB without SOA integration, the light-emitting device that contributes to the output power is the laser section, so to monitor this output, you can monitor the front output, and also monitor the rear leakage light. You may do so.

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. Therefore, in the case of AXEL, the forward output is monitored for the above-mentioned control. However, since many optical components such as lenses and isolators are generally arranged to couple the output light of the laser chip to the optical fiber, the space for arranging the monitoring photodiode (PD) is limited. . For this reason, in the case of a laser integrated with a modulator and SOA, there are restrictions on the design of providing a PD for monitoring the power of the forward output.

The present invention has been made to solve the above-mentioned problems, and provides a PD for monitoring the forward output of a laser integrated with a modulator and a semiconductor optical amplifier without being restricted by design. The purpose is to make it possible.

An optical transmitter according to the present invention includes: a waveguide type laser formed on a substrate; a waveguide type electroabsorption modulator formed on the substrate continuously from the laser in the waveguide direction; A waveguide-type semiconductor optical amplifier is formed on the substrate and amplifies the output light output from the electro-absorption modulator, and a waveguide-type semiconductor optical amplifier is formed on one end of the substrate and outputs the output light from the semiconductor optical amplifier. a condensing section that is formed on the substrate and uses multimode interference to condense the reflected light reflected toward the substrate at the output end, and a photodiode that monitors the condensed light condensed by the condensing section. The output light output from the semiconductor optical amplifier is incident obliquely to the output end.

As described above, according to the present invention, the modulator and the semiconductor optical amplifier are integrated, since the present invention includes a condensing section that condenses the reflected light that is output from the semiconductor optical amplifier and reflected on the substrate side at the output end. A PD for monitoring the forward output of the laser can be provided without being restricted by the design.

FIG. 1 is a plan view showing the configuration of an optical transmitter according to Embodiment 1 of the present invention. FIG. 2A shows a state of an equivalent linear optical system in which light 131 emitted from the semiconductor optical amplifier 104, reflected at the output end 105, and incident on the condenser 106 is replaced by light traveling along the optical axis. It is a diagram. FIG. 2B is an explanation showing the state of an equivalent linear optical system in which light 131 emitted from the semiconductor optical amplifier 104, reflected at the output end 105, and incident on the condenser 106 is replaced by light traveling along the optical axis. It is a diagram. 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. FIG. 4 is a characteristic diagram showing the results of an optical propagation simulation when the light condensing section 106 is introduced. FIG. 5 is a plan view showing the configuration of an optical transmitter according to Embodiment 2 of the present invention. FIG. 6 is a plan view showing the configuration of an optical transmitter according to Embodiment 3 of the present invention. FIG. 7 is a cross-sectional view showing the configuration of a general EADFB laser. FIG. 8 is a cross-sectional view showing the configuration of AXEL.

Hereinafter, an optical transmitter according to an embodiment of the present invention will be described.

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

The EA modulator 103 is formed on the substrate 101 so as to be continuous with the laser 102 in the waveguide direction. The semiconductor optical amplifier 104 amplifies the output light output from the EA modulator 103. The output light output from the semiconductor optical amplifier 104 is output to the outside from an output end 105 formed at one end of the substrate 101. The output light output from the semiconductor optical amplifier 104 enters the output end 105 obliquely.

For example, the substrate 101 is rectangular in plan view, and one of the short sides facing each other is the output end 105. Further, the laser 102 and the EA modulator 103 are formed in a state in which a waveguide structure extends in a direction parallel to the long side of the substrate 101 which is rectangular in plan view, and the waveguide direction is directed toward the output end 105. It is in a vertical state.

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 using FIGS. 7 and 8. For example, although not shown, the laser 102 includes an active layer made of a multiple quantum well (MQW), and oscillates at a single wavelength using a diffraction grating formed within a resonator. Furthermore, although not shown, the EA modulator 103 has a light absorption layer 204 made of MQW.

The active layer of the laser 102 and the light absorption layer of the EA modulator 103 are sandwiched between two optical confinement layers (not shown) on the upper and lower sides when viewed from the substrate 101, forming a separate confinement heterostructure. Although not shown, these are sandwiched between a lower cladding layer and an upper cladding layer. Electrodes (not shown) are formed on the upper cladding layer of the laser 102 and on the upper cladding layer of the EA modulator 103, respectively.

Although not shown, the semiconductor optical amplifier 104 also includes an active layer, and the upper and lower parts of the active layer are sandwiched between two optical confinement layers, forming a separate confinement heterostructure. Further, although not shown, these are sandwiched between a lower cladding layer and an upper cladding layer, and an electrode is formed on the upper cladding layer.

This example includes a bent 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 waveguide directions perpendicular to the output end 105 , but the bent optical waveguide 108 makes the waveguide direction of the semiconductor optical amplifier 104 not perpendicular to the output end 105 . It is in a diagonal state.

This optical transmitter also includes a condensing section 106 that is formed on the substrate 101 and condenses the reflected light reflected toward the substrate 101 at the output end 105 by multimode interference. The condensing unit 106 condenses the reflected light that is output from the semiconductor optical amplifier 104 and obliquely enters the output end 105 and is reflected at the output end 105 toward the substrate 101 side. The light condensing section 106 is constituted by an optical waveguide having a flat core, the core width of which is larger than the core thickness when viewed in cross section. Here, a window structure 111 in which a waveguide structure is not formed is provided between the output end 105 and the output end of the semiconductor optical amplifier 104.

Additionally, this optical transmitter includes a photodiode 107 that monitors the condensed light condensed by the condensing section 106. In this example, the photodiode 107 is of a waveguide type, and is formed on the substrate 101 in a region on the opposite side of the output end 105 when viewed from the light condensing section 106. The photodiode 107 is optically connected to the output optical waveguide 106a of the light condensing section 106.

Further, in the first embodiment, a notch 109 is provided in the substrate 101 on the light emission side of the photodiode 107. A window structure 112 in which no waveguide structure is formed is provided between the notch 109 and the output end of the photodiode 107.

For example, the substrate 101 can be formed into a rectangular shape in plan view, with one side (on the upper side of the paper in FIG. A laser 102, an EA modulator 103, and a semiconductor optical amplifier 104 can be arranged on the side (side). Further, the light condensing unit 106 is arranged on the other side (the lower side of the paper in FIG. 1) that is in contact with the side of one end (output end 105) of the rectangular substrate 101 in plan view from the center of the substrate 101. be able to. In this configuration, a notch 109 can be formed at the end of the substrate 101 on the other side at a location on the light output side of the photodiode 107.

To briefly explain the production of the laser 102, EA modulator 103, and semiconductor optical amplifier 104 described above, first, a lower optical confinement layer is formed on the 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 growth) non-doped i-InGaAsP using a known metal organic chemical vapor deposition (MOCVD) method.

Next, an MQW that will become the active layer of the laser 102 and the semiconductor optical amplifier 104 is formed on the lower optical confinement layer. For example, an MQW can be formed by crystal-growing multiple layers of non-doped i-InGaAsP as well layers and InGaAsP as barrier layers alternately by MOCVD. The well layer can have a thickness of about 15 nm, and the barrier layer can have a thickness of about 8 nm, for example.

Next, a diffraction grating is formed on the active layer of the laser 102. Generally, a DFB laser can be obtained by forming a diffraction grating directly above the active layer and using this region as a resonator. Note that by forming diffraction gratings at two positions sandwiching the active layer region in the waveguide direction, a distributed reflection type (Distributed Bragg Reflector: DBR) can be achieved.

Next, the active layer in the region that will become the EA modulator 103 and the bent optical waveguide 108 is removed, and an MQW that will become the light absorption layer is formed in the region that will become the EA modulator 103 by crystal regrowth. An MQW as a light absorption layer can be formed by alternately growing multiple layers of undoped i-InGaAsP as a well layer and InGaAsP as a barrier layer in a composition ratio different from that of the active layer. The MQW serving as the light absorption layer and the active layer in the region of the laser 102 that has already been formed are formed in a butt-joint state. Further, an MQW layer serving as a light absorption layer is also formed in a region to be used as a photodiode 107.

Furthermore, InGaAsP, which is to be used as a core layer, is formed by crystal regrowth in the region to be the bent optical waveguide 108. The core layer is formed so as to be butt-jointed with the active layer and the 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 a region to be the light condensing section 106 (output optical waveguide 106a).

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 vertical (vertical) direction when viewed from the substrate 101. Further, a contact layer made of, for example, p ⁺ -InGaAsP can also be formed on the upper cladding layer.

Next, by selectively removing each layer crystal-grown on the substrate 101 using a selective etching mask made of SiO ₂ or the like, the lower part is removed in the regions where the laser 102, EA modulator 103, and semiconductor optical amplifier 104 are to be formed. Optical waveguide structure (channel type optical waveguide structure) with a high mesa structure of a predetermined width, consisting of a 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. form.

Furthermore, at this time, an optical waveguide structure having a mesa structure is also formed in the region to be the photodiode 107 using the above-mentioned selective etching mask. On the other hand, the core layer made of InGaAsP remains in the region that is to be the light condensing section 106 (output optical waveguide 106a), which is wider than the region that is to be the light condensing section 106 in plan view.

Next, using the above-described selective etching mask as a selective growth mask, for example, InP doped with Fe and made to have high resistance is buried and regrown on the substrate 101 around the formed mesa structure. Embed the high mesa structure with a buried insulating layer. Next, the selective etching mask used as the selective growth mask is removed, a new selective etching mask is formed, and the core layer remaining in the region to be the light condensing part 106 is selectively removed, thereby forming the light condensing part 106. A mesa shape (output optical waveguide 106a) is formed. Further, a protective layer made of an insulating material such as SiO ₂ is formed on the mesa-shaped surface of the formed light condensing section 106 (output optical waveguide 106a). Although the mesa shape of the light condensing section 106 (output optical waveguide 106a) forms a protective layer, it is not embedded with a buried insulating layer.

Next, in order to electrically insulate each region of the laser 102, EA modulator 103, semiconductor optical amplifier 104, and photodiode 107, the contact layer between the regions is removed, and then electrodes are formed in each region. . Thereafter, chipping by cleavage, formation of an anti-reflection (AR) film 114, and formation of a high reflection (HR) film 115 are performed. The HR film 115 is formed on the end (end surface) of the substrate 101 opposite to the output end 105 . Note that, hereinafter, the end of the substrate 101 on which the HR film 115 is formed, on the opposite side from the output end 105, will be referred to as the rear end.

In this optical transmitter, a laser 102 is driven by a constant current to emit continuous wave (CW) oscillation, and by modulating the voltage applied to an EA modulator 103, the light absorption is modulated and the intensity of the light is modulated. Achieves modulation. The semiconductor optical amplifier 104 amplifies the output light of the EA modulator 103 in order to compensate for the insertion loss of the absorption type EA modulator 103. In the semiconductor optical amplifier 104, the reflected light from the output end 105 is also amplified, so it is said that a general AXEL is easily affected by the reflected return light.

On the other hand, in the first embodiment, a bent optical waveguide 108 is inserted between the EA modulator 103 and the semiconductor optical amplifier 104, and end face incidence (oblique incidence) with an angle at the output end 105 is realized. . With this configuration, the reflected return light at the output end 105 is suppressed more than in the case of only the AR film 114. In addition, a window structure 111 is provided immediately in front of the output end 105 in which the optical waveguide structure disappears for a certain period in both the horizontal and vertical directions, and by utilizing light diffusion, return light can be further suppressed.

The main output light of this optical transmitter passes through a lens focusing optical system and an isolator placed in front of the output end 105, and is coupled to an optical fiber. In order to monitor the change over time of this output light, an optical element for partially extracting the light is inserted within the section of the condensing optical system. In recent years, packages that accommodate modulated lasers have become smaller, and the physical length of fiber-coupled systems has become shorter. Furthermore, as the optical path length increases, the output light having a high NA spreads accordingly, resulting in a decrease in coupling efficiency. For this reason, a technique of monitoring inside the substrate 101 or behind the substrate 101 where there is no other optical system is preferable to a technique of monitoring on the side of the substrate 101 from which the output light is output, as described above.

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 slight amount of reflected light from the AR film 114 is reflected into the substrate 101 in a direction different from that of the semiconductor optical amplifier 104. Furthermore, since the light is diffused by the window structure 111, for example, when a light absorption layer of a photodiode (PD) is provided in the substrate 101 and the photocurrent in the light absorption layer is monitored, the light is diffused vertically. In this direction, there is little overlap between the light absorption layer and the diffused light, resulting in low absorption efficiency. On the other hand, since light is also diffused in the horizontal direction, the width of the light absorption layer is increased in order to absorb more of this diffused light. A reverse bias is always applied to the monitor PD, and dark current is generated as noise. When the area of the monitor PD becomes large, the resistance at the pn junction in the light absorption layer decreases, so the dark current increases, and if the intensity of the target light to be monitored is low, it will be buried in noise.

In contrast, in the present invention, a condensing section 106 is introduced to condense horizontally diverging light. In the vertical direction, when attempting to introduce a special structure, it is necessary to introduce an additional layer structure above and below the light absorption layer of the PD, which is constrained by crystal growth, so the degree of freedom is low. On the other hand, it is easier to change the shape in the horizontal direction because it is possible to change the shape through a process. In the present invention, optical monitoring within the substrate 101 is realized by condensing light by the condenser 106 using multimode interference (MMI).

As shown in FIG. 1, light 131 emitted from the semiconductor optical amplifier 104 and reflected by the AR film 114 (outgoing end 105) enters the light condensing section 106. If this is considered as light traveling along the optical axis, it can be rewritten into 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, light 131 is diffused in the XY axis directions. In the YZ plane of FIG. 2A, a semiconductor optical amplifier 104 serving as a single mode (SM) optical waveguide is placed ahead of the window structure 111. For example, this also has the structure of an SM optical waveguide in the XZ plane. In other words, consider optical coupling when two SM

optical waveguides

132 and 133 face each other with the window structure 111 in between.

For example, assuming that the reflectance at the output end 105 is 100%, the coupling efficiency from the SM optical waveguide 132 to the opposing SM optical waveguide 133 via the window structure 111 is the result of an optical waveguide simulation. As shown by the black square in , it is 9% when the length of the window structure 111 (window structure length) is 10 μm. In contrast, in the present invention, as shown in FIG. 2B, in the XZ plane, a light condensing section 106 is arranged instead of a single mode optical waveguide, and in the X-axis direction, reflection by the side wall of the condensing section 106 and optical interference are arranged. This makes it possible to condense light and efficiently couple it to the SM optical waveguide 133.

FIG. 4 shows the results of an optical propagation simulation when the light condensing section 106 is introduced. According to the optical propagation shown in the simulation results, as shown by the black circles in FIG. 3, it is possible to improve the coupling efficiency to 14% for the same window structure length of 10 μm.

Here, the wavelength of the laser is 1310 nm, and the condensing section 106 is a passive optical waveguide whose core layer is made of InGaAsP material and has a refractive index of 3.4. The thickness of the core layer is 300 nm. On the other hand, the cladding material is InP and has 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 portion (input end) of the light condensing section 106 is 20 μm, which is twice the length of the window structure 111. Become.

Furthermore, the width of the core layer of the light condensing section 106 is 8 μm and the length is 300 μm. The mesa width of the waveguide structure of the laser 102, EA modulator 103, and semiconductor optical amplifier 104 is 1 μm. Further, the inclination of the output end 105 of the semiconductor optical amplifier 104 in the waveguide direction with respect to the plane is 5 degrees. On the other hand, the inclination of the output end 105 of the light condensing section 106 in the waveguide direction with respect to the plane was set to -10 degrees.

Since the length of the window structure 111 is 10 μm, the displacement of the optical axis in the X direction when propagating from the output end of the semiconductor optical amplifier 104 to the output end 105 is about 1 μm. In order to prevent the optical waveguides of the light collecting section 106 and the semiconductor optical amplifier 104 from interfering with each other from this point, the center of the optical waveguide of the light collecting section 106 was further offset by 4 μm in the X direction. Therefore, light enters the light condensing section 106 from a location shifted from the center of the light condensing section 106 . The core width of the output optical waveguide 106a connected to the light condensing section 106 is 2.6 μm.

Generally, the manufacturing length of the window structure 111 is determined by the precision of the cleavage inclination. In reality, the length of the window structure 111 varies by about several μm. If the window structure 111 is made shorter than the design as a result of manufacturing, the results shown in FIG. 4 show that the coupling efficiency improves when the SM optical waveguides are opposed to each other. When the window structure length is shorter than 8 μm, the coupling efficiency of the SM optical waveguides facing each other is higher than that of the present invention. However, it is not preferable for the window structure 111 to become short because it weakens the suppression of return light for the semiconductor optical amplifier 104.

On the other hand, when the window structure 111 becomes longer, the coupling efficiency decreases exponentially when facing the SM optical waveguide, whereas in the structure of the present invention in which the light condensing section 106 is introduced, the coupling efficiency decreases parabolically. has been shown to change. Therefore, it can be seen that the provision of the light condensing section 106 has higher tolerance to manufacturing variations. A photodiode 107 having an optical waveguide structure is optically connected to the tip of the output optical waveguide 106a of the light condensing section 106.

The light absorption layer of the photodiode 107 can be formed by leaving the light absorption layer of the EA modulator 103 or the active layer of the laser 102 or semiconductor optical amplifier 104 during these mesa processing processes, thereby eliminating the need for additional processes. It can be made to

The optical waveguide structure (mesa structure) in the condensing section 106 can be formed by performing additional dry etching after the high mesa embedding process of the laser 102, EA modulator 103, and semiconductor optical amplifier 104. can. The optical waveguide structure (mesa structure) in the light condensing section 106 is not buried, and in the horizontal direction, optical confinement is achieved by the interface between air and the semiconductor via the protective layer. Note that the photodiode 107 can also have a so-called ridge optical waveguide structure.

Ideally, all of the light focused by the light focusing section 106 is absorbed by the photodiode 107. However, as for residual light, reflected light is suppressed by the window structure 112 provided on the output side of the photodiode 107 and the oblique side surfaces of the notch 109 formed by etching. As shown in FIG. 1, the notch 109 is preferably located at a location away from the rear end of the substrate 101 where the HR film 115 is formed. This is to prevent the HR film from being formed on the oblique sides of the notch 109.

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

Embodiment 2 is a photodiode that monitors external output light 134 that is collected by a light collection unit 106, guided through an output optical waveguide 106a, passed through a window structure 112, and exited from an oblique side surface of a notch 109. 107'.

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

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

In the third embodiment, the configuration is such that the light emitted from the photodiode 107 that monitors the condensed light condensed by the condensing section 106 is incident perpendicularly to the side of the rear end (end) of the substrate 101. For example, the output optical waveguide 106a is provided with a bent portion 161, and the waveguide direction of the output optical waveguide 106a is changed in a plane parallel to the plane of the substrate 101 to a direction perpendicular to the rear end side of the substrate 101. By doing so, the above-mentioned configuration can be achieved.

In this configuration, light 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 reflected by the HR film 115 and returned to the photodiode 107 can be absorbed by the photodiode 107 again. Thereby, the absorption length of the photodiode 107 can be halved, for example, and dark current can be suppressed.

As described above, according to the present invention, the light condensing section that condenses the reflected light that is output from the semiconductor optical amplifier and reflected on the substrate side at the output end, allows the modulator and the semiconductor optical amplifier to It becomes possible to provide a PD for monitoring the forward output of the integrated laser without being restricted by the design.

It should be noted that the present invention is not limited to the embodiments described above, and many modifications and combinations can be made within the technical idea of the present invention by those having ordinary knowledge in this field. That is clear.

DESCRIPTION OF SYMBOLS 101... Substrate, 102... Laser, 103... Electro-absorption modulator (EA modulator), 104... Semiconductor optical amplifier, 105... Outgoing end, 106... Condensing part, 106a... Output optical waveguide, 107... Photodiode, 108 ...Bent optical waveguide, 109...Notch, 111...Window structure, 112...Window structure, 114...Anti-reflection (AR) film, 115...Reflection (HR) film 115.

Claims

A waveguide type laser formed on a substrate,
a waveguide-type electroabsorption modulator formed on the substrate in succession to the laser in the waveguide direction;
a waveguide type semiconductor optical amplifier formed on the substrate and amplifying the output light output from the electroabsorption modulator;
an output end formed on one end side of the substrate, from which output light output from the semiconductor optical amplifier is output;
a condensing section formed on the substrate and condensing reflected light reflected toward the substrate at the output end by multimode interference;
and a photodiode for monitoring the condensed light condensed by the condensing section,
An optical transmitter characterized in that the output light output from the semiconductor optical amplifier is incident obliquely to the output end.
The optical transmitter according to claim 1,
The optical transmitter is characterized in that the photodiode is of a waveguide type and is formed on the substrate in a region opposite to the output end when viewed from the light condensing section.
The optical transmitter according to claim 2,
The substrate is formed into a rectangular shape in plan view,
The laser, the electroabsorption modulator, and the semiconductor optical amplifier are arranged on one side of the substrate that is in contact with one end side of the substrate that is rectangular in plan view from the center of the substrate,
The light condensing portion is disposed on the other side of the substrate that is in contact with one end side of the substrate that is rectangular in plan view from the center of the substrate,
An optical transmitter comprising: a notch formed at an end of the substrate on the other side at a location on a light output side of the photodiode.
The optical transmitter according to claim 2,
a reflective film formed on an end of the substrate opposite to the output end;
An optical transmitter, wherein the light emitted from the photodiode is incident perpendicularly to the end.
The optical transmitter according to claim 1,
The substrate is formed into a rectangular shape in plan view,
The laser, the electroabsorption modulator, and the semiconductor optical amplifier are arranged on one side of the substrate that is in contact with one end side of the substrate that is rectangular in plan view from the center of the substrate,
The light condensing portion is arranged on the other side of the substrate that is in contact with one end side of the rectangular substrate in plan view from the center of the substrate,
An optical transmitter comprising: a notch formed at an end of the substrate on the other side at a location on a light output side of the light condensing section.
The optical transmitter according to any one of claims 1 to 5,
It is characterized by comprising a bent optical waveguide for optically connecting the electro-absorption modulator and the semiconductor optical amplifier, and for making the output light output from the semiconductor optical amplifier obliquely incident on the output end. An optical transmitter.