JP2011258785A - Optical waveguide and optical semiconductor device using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical waveguide which ensures efficient optical joint as compared with an optical waveguide of conventional taper structure when optical waveguides of different types having significantly different mode fields are connected.SOLUTION: In the optical waveguide where two waveguides 41 and 42 of different structure join through a discontinuous taper structure joint body 50 having a predetermined length L, and lateral spread of light guided by one waveguide 41 is wider than lateral spread of light guided by the other waveguide 42, width of the discontinuous taper structure joint body 50 becomes narrower gradually from one end 51 touching one waveguide 41 toward the other end 52 touching the other waveguide 42, and the width Wof one end 51 in the discontinuous taper structure joint body 50 is made wider than the width Wof one waveguide 41.

Description

  The present invention relates to an optical waveguide and an optical semiconductor device using the same, and more particularly to an optical semiconductor device such as an electroabsorption optical modulator integrated semiconductor laser having a strained quantum well layer.

  Due to the explosive spread of the current Internet, the opticalization of networks has expanded from backbone networks between large cities to access networks represented by FTTH (Fiber To The Home). It has begun to cover a wide range from applications to data communications. In particular, the opticalization of Ethernet (registered trademark) has been discussed recently, and standardization of various forms and performances of the transceiver is being promoted.

  Among them, a transceiver for 100 Gigabit Ethernet (100 GbE), which is being discussed as one of the next generation networks, has been studied extensively as a future ultra-high speed network. In 100 GbE, various standards are discussed according to the transmission distance, but for transmission over medium distance (10 km) and long distance (40 km) using a single mode fiber (SMF) as a transmission medium, The names are defined as 100GBASE-LR4 (hereinafter referred to as LR4) and 100GBASE-ER4 (hereinafter referred to as ER4), respectively, 1294.53 nm to 1296.59 nm (lane 1), 1299.02 nm to 1301.09 nm (lane 2) ), 1303.54 nm to 1305.63 nm (lane 3), 130.09 nm to 1310.19 nm (lane 4), 4.5 nm intervals, and a total of 15 nm transmitters for four waves are prepared. Each is operated at 25 Gbit / s, and each transmitter Idomodo suppression ratio (Side Mode Suppression Ratio: SMSR) is more than 30 dB, the dynamic extinction ratio, etc. must be a least 8dB relative to 4dB, ER4 respect LR4, required performance is finely set.

  As a light source in a transmitter for 100 GbE, an electroabsorption modulator integrated laser (Electrobsorption Modulator integrated with DFB laser: EADFB laser) is currently promising. The EADFB laser is known to be very advantageous for high-speed modulation because the semiconductor laser and the modulator can be optimized independently, and the dynamic extinction ratio is 8 dB or more particularly for ER4 of 100 GbE. An EADFB laser that is necessary and easy to obtain a high extinction ratio is almost essential.

  Currently, a standard package called CFP MSA transceiver has been proposed as a transceiver for 100 GbE, in which four optical transmitters (Transmitter Optical SubAssembly: TOSA) are arranged, separately from this, a multiplexer Is installed. There are four other receivers, duplexers, and electrical signal processing units in the transceiver, and the size thereof is very large at 75 mm × 140 mm. Each TOSA includes, in addition to a light source operating at 25 Gbit / s, a Peltier element for temperature control, a lens for focusing on an optical fiber, an isolator for preventing return light, and an optical fiber. In addition to its large size, a lens, an isolator, a Peltier element, and an optical fiber are required for every four wavelengths, and both the member cost and power consumption increase.

T. Fujisawa, M. Arai, N. Fujiwara, W. Kobayashi, T. Tadokoro, K. Tsuzuki, Y. Akage, R. Iga, T. Yamanaka and F. Kano, "25 Gbit / s 1.3 μm InGaAlAs-based electroabsorption modulator integrated with DFB laser for metro-area (40km) 100 Gbit / s Ethernet system ", IEE Electronics Letters, vol.45, no.17, pp.900-901, August 13, 2009

  In order to solve these problems, it is effective to integrate four transmitters and a multiplexer. There are two types of integration methods: hybrid integration using different materials and monolithic integration on one substrate. In the hybrid integration, for example, a method of integrating a light source made of another material system into a multiplexer made of a certain material system by some method is used, but between the four transmitters and the multiplexer. Active alignment is almost indispensable to reduce the coupling loss in the process, and the mounting process is inevitably complicated. In addition, since the transmitter and the multiplexer are separately manufactured, there is a limit to downsizing the size even if it is integrated, and the manufacturing and mounting processes are complicated.

  On the other hand, in monolithic integration, since a transmitter and a multiplexer are built on one substrate, alignment is not necessary in the first place, and mounting is easy, and mounting of four elements is completed in one mounting. . Further, since the two elements are directly coupled, the size can be easily reduced. Furthermore, since the number of Peltier elements, lenses, and isolators necessary for manufacturing the TOSA is only one, it is possible to significantly reduce the member cost and power consumption. However, the currently proposed EADFB laser employs a ridge structure in its optical waveguide in order to enable high-speed modulation of 25 Gbit / s. In this structure, the optical confinement is weak, the bending loss of the optical waveguide is large, the bending radius cannot be reduced, the multiplexer is large, and there is a limit to downsizing. This problem can be solved to some extent by using, for example, a high-mesa waveguide having a higher optical confinement at the multiplexing part, but the optical electric field distribution in the ridge-type waveguide spreads widely in the lateral direction. On the other hand, since the optical electric field distribution in the high-mesa waveguide is strongly confined in the lateral direction, mode matching cannot be achieved with a normal taper structure, and the coupling efficiency is reduced.

  Accordingly, the present invention has been made to solve the above-described problems, and is more efficient than a conventional tapered optical waveguide when connecting different types of optical waveguides having greatly different mode fields. An object of the present invention is to provide an optical waveguide capable of joining light and an optical semiconductor device using the same.

The optical waveguide according to the first invention for solving the above-described problem is as follows.
Two waveguides having different structures are in contact with each other through a junction having a predetermined length, and the lateral spread of the light guided in one waveguide is the lateral extension of the light guided in the other waveguide. In an optical waveguide wider than the spread,
The width of the joint gradually decreases from one end in contact with the one waveguide toward the other end in contact with the other waveguide,
The width of the one end of the joint is wider than the width of the one waveguide.

The optical waveguide according to the second invention for solving the above-described problem is
An optical waveguide according to the first invention,
The one waveguide and the other waveguide each have a mesa;
The mesa height of the one waveguide is lower than the mesa height of the other waveguide.

An optical waveguide according to a third invention for solving the above-described problem is
An optical waveguide according to a second invention,
The joint has a mesa;
The mesa height of the joint is the same as the mesa height of the other waveguide.

An optical waveguide according to a fourth invention for solving the above-described problem is
An optical waveguide according to any one of the first to third inventions,
A connection region having a predetermined length and a width wider than the width of the junction and having no structure for confining light in the lateral direction is provided between the one waveguide and the junction. It is characterized by that.

An optical semiconductor device according to a fifth invention for solving the above-described problem is
An optical semiconductor device having a ridge structure and an optical semiconductor device having a high mesa structure are connected via an optical waveguide according to any one of the first to fourth inventions.

An optical semiconductor device according to a sixth invention for solving the above-described problem is
A plurality of light sources, a plurality of modulators that respectively modulate light incident from the plurality of light sources, and a multiplexer that multiplexes light emitted from the plurality of modulators via the optical waveguide A part of the optical waveguide connected on the substrate and between the modulator and the multiplexer is the optical waveguide according to any one of the first to fourth inventions.

  According to the optical waveguide and the optical semiconductor device using the same according to the present invention, when connecting different types of optical waveguides having greatly different mode fields, it is possible to join light more efficiently than the conventional optical waveguide having a tapered structure. By using such an optical waveguide, it becomes possible to monolithically integrate one and the other waveguide on one chip while maintaining high coupling efficiency, and to optimize each integrated element independently. Is possible.

1 is a cross-sectional view showing a basic structure of an optical semiconductor device according to the present invention. It is the perspective view which showed the structure of the optical semiconductor device based on the 1st, 2nd Example of this invention. FIGS. 3A and 3B are diagrams schematically showing the structure of an optical waveguide. FIG. 3A shows a buried type, FIG. 3B shows a ridge type, and FIG. 3C shows a high mesa type. Show the case. FIGS. 4A and 4B are diagrams showing an electric field distribution of an optical waveguide used in the optical semiconductor device according to the first embodiment of the present invention. FIG. 4A shows a case of a ridge type optical waveguide, and FIG. The case of a type optical waveguide is shown. It is explanatory drawing of the conventional taper structure junction part which joins a different kind of optical waveguide, Comprising: FIG. 5 (a) shows the plane, FIG.5 (b) shows the perspective view of the example, FIG.5 (b) The perspective of another example is shown in FIG. It is explanatory drawing of the discontinuous taper structure junction part in the optical waveguide used with the optical semiconductor device which concerns on 1st Example of this invention, Comprising: The plane is shown to Fig.6 (a), A perspective view is shown. It is a graph which shows the relationship between the coupling efficiency by a discontinuous taper structure junction part, and the magnitude | size of the width direction of a high mesa type | mold optical waveguide. It is a graph which shows the relationship between the coupling efficiency by a discontinuous taper structure junction part, and the taper length of a discontinuous taper structure junction part. It is explanatory drawing of a discontinuous taper structure junction part. It is a top view of the discontinuous taper structure junction part in the optical waveguide used with the optical semiconductor device which concerns on the 2nd Example of this invention.

Embodiments of an optical waveguide according to the present invention and an optical semiconductor device using the optical waveguide will be specifically described below.
First, the basic structure of the optical semiconductor device according to the present invention will be described with reference to FIG.

  As shown in FIG. 1, the optical semiconductor device according to the present invention is a monolithic integrated light source, and on a substrate 1 made of a semiconductor mixed crystal, a semiconductor laser active part (Laser diode: LD part) 21 and a passive layer ( An optical waveguide unit 22, an electroabsorption modulator unit (Electroabsorption modulator unit: EAM unit) 23, and a multiplexing unit 24 are formed.

  The LD portion 21 has an active portion 2 having a multiple quantum well structure including a quantum well layer, a barrier layer, and a diffraction grating formation layer, and cladding portions 3 and 4 provided above and below the active portion 2.

  The EAM portion 23 includes an active portion 5 having a multiple quantum well structure including a quantum well layer, a barrier layer, and an optical confinement layer, and clad portions 6 and 7 provided above and below the active portion 5.

  The passive layer 22 has a core portion 8 and clad portions 9 and 10 provided above and below the core portion 8.

  The multiplexing unit 24 includes a core unit 11 and clad units 12 and 13 provided above and below the core unit 11.

  A lower electrode 14 is provided on the lower surface of the substrate 1. An upper electrode 16 is provided on the upper surface of the upper cladding portion 3 in the LD portion 21 and the upper surface of the upper cladding portion 6 in the EAM portion 23 via a contact layer 15. On the other hand, neither the contact layer nor the upper electrode is provided on the upper surface of the upper cladding portion 9 in the passive layer 22, and the passive layer 22 electrically insulates the LD portion 21 from the EAM portion 23. Neither the contact layer nor the upper electrode is provided on the upper surface of the upper cladding portion 12 in the multiplexing portion 24.

  An optical waveguide and an optical semiconductor device using the same according to a first embodiment of the present invention will be described with reference to FIG.

  As shown in FIG. 2, the 100 GbE monolithic integrated light source, which is an optical semiconductor device according to the present embodiment, has a configuration in which four lanes 31 A, 31 B, 31 C, and 31 D are provided on a substrate 1 made of a semiconductor mixed crystal. ing. Each of the lanes 31A to 31D is provided with a photodiode portion (PD portion) 25, an LD portion 21, and an EAM portion 23. The PD portion 25, the LD portion 21, and the EAM portion 23 are disposed on the same substrate 1 via an optical waveguide. Connected on. In the four lanes 31 </ b> A to 31 </ b> D, the light guided through the lanes 31 </ b> A to 31 </ b> D is combined into one by a multimode interference coupler (MMI coupler) which is the multiplexing unit 24.

  The LD portion 21 is formed on the n-type InP substrate 1 and includes, in order from the InP substrate 1, the lower n-InP clad portion 4, an InAlGaAs barrier layer, an InAlGaAs quantum well layer, and an InAlGaAs diffraction grating formation layer. It has an active portion 2 having a multiple quantum well structure and an upper p-InP clad portion 3.

  The EAM portion 23 is formed on the n-type InP substrate 1 and is a multiple quantum including the lower n-InP cladding portion 7, an InGaAsP guide layer, an InAlGaAs barrier layer, and an InAlGaAs quantum well layer in order from the InP substrate 1 side. It has an active portion 5 having a well structure and an upper p-InP clad portion 6.

  The core portion 8 and the upper and lower cladding portions 9 and 10 of the passive layer 22 have the same configuration as the EAM portion 23. That is, the passive layer 22 is formed on the n-type InP substrate 1, and includes a lower n-InP cladding portion 10, an InGaAsP guide layer, an InAlGaAs barrier layer, and an InAlGaAs quantum well layer in order from the InP substrate 1 side. It has a core portion 8 made of an active portion having a multiple quantum well structure and an upper p-InP clad portion 9. However, since the PD unit 25, the LD unit 21, and the EAM unit 23 are electrically insulated by the passive layer 22, the passive layer 22 is not provided with an upper electrode.

  The PD unit 25 is provided on one end side of each of the lanes 31A to 31D. The PD unit 25 has substantially the same layer structure as that of the LD unit 21 and is formed on the n-type InP substrate 1, and in order from the InP substrate 1 side, a lower n-InP clad unit, an InAlGaAs barrier layer, and an InAlGaAs It has an active part of a multiple quantum well structure including a quantum well layer and an upper p-InP cladding part.

  In the present embodiment, in the monolithic integrated light source for 100 GbE having the structure as described above, a multiple quantum well structure having a band gap wavelength of 1.3 μm is used as the active part 2 of the LD part 21, and the active part of the EAM part 23 is used. 5, the band gap wavelength of the well layer is a mixed crystal in which the wavelength between the first quantization levels is 1.23 μm with respect to a given strain so that the detuning with the LD portion 21 is 70 nm at room temperature. Is used. For the multiplexing unit 24, the core unit 11 is InGaAsP with a band gap wavelength of 1.15 μm, and the lower cladding unit 13 is n-InP.

  The composition of the barrier material is assumed to lattice match with the substrate, but there is no problem even if strain is introduced into the barrier layer for the purpose of strain compensation. The composition ratio of the semiconductor mixed crystal is specified by the strain amount, the well width, and the wavelength between the first quantization levels, and is not specifically described unless necessary.

Next, a method for manufacturing a 100 GbE monolithic integrated light source, which is an optical semiconductor device according to this embodiment, will be described.
First, a lower clad portion and an active portion having a multiple quantum well structure are grown on an n-type InP substrate 1 in order from the InP substrate 1 side. Thereby, the lower clad part and active part in LD part 21 and PD part 25 are produced. Further, the lower clad portion in the passive layer 22, the EAM portion 23 and the multiplexing portion 24 is produced.
Next, of the grown active portion of the multiple quantum well structure, portions other than those necessary for the LD portion 21 and the PD portion 25 are scraped off by wet etching, and the active portion of the multiple quantum well structure and the multiple quantum well structure are removed. The butt joint is regrown with the core portion composed of the active portion. Thereby, the active part 5 of the EAM part 23 and the core part 8 of the passive layer 22 are produced.
Of the active portion 5 and the core portion 8 that have been regrown, the portions other than those necessary for the EAM portion 23 and the passive layer 22 are scraped off by wet etching, and the core portion 11 of the combining portion 24 is regrown to the butt joint. Let

  Subsequently, after forming a diffraction grating in the LD part 21, an upper cladding part is grown by 2 μm on each of the LD part 21, the passive layer 22, the EAM part 23, the multiplexing part 24, and the PD part 25, and p By growing the InGaAs contact layer 15, a wafer in which the PD portion 25, the LD portion 21, the passive layer 22, the EAM portion 23, and the multiplexing portion 24 are formed is completed. Here, if the MMI coupler of the multiplexing unit 24 is formed as it is, since the upper clad portion is p-InP, the loss of light is large and the output light power is reduced. Therefore, here, from this state, the upper clad portion of the multiplexing portion 24 is further removed by etching, and non-doped InP is regrown to the butt joint.

  On the completed wafer, the LD portion 21, EAM portion 23, passive layer 22, and PD portion 25 have a ridge-type waveguide mesa formation process, the multiplexing portion 24 has a high-mesa waveguide mesa formation process, and an electrode formation process (however, An upper electrode is not formed on the passive layer 22 and the combining part 24), and after cleavage, a non-reflective coating (not shown) is applied to the front and rear end faces of the chip.

  In the case of a ridge type optical waveguide, the above-mentioned mesa is a portion corresponding to the upper clad portion 114 shown in FIG. 3B, and its height is H1. In the case of a high mesa type optical waveguide, the mesa means a portion of the upper clad portion 124, the active layer 123 having a multiple quantum well (MQW) structure shown in FIG. 3C, and the lower clad portion 122 up to the substrate 121. That is, it is a convex portion made of a semiconductor protruding from the plane of the semiconductor, and its height is H2.

  The completed optical semiconductor device (device) is used in the following control method. A current is injected into each LD section 21 to generate continuous light. The light incident on the EAM unit 23 is driven with a certain voltage amplitude around a certain bias voltage to modulate the light. The light incident on the PD unit 25 is converted into an absorption current, and the oscillation wavelength of the LD unit 21 is obtained from the value of this current, and this is fed back to the injection current of the LD unit 21 so that the oscillation wavelength does not change. Used as

Here, the structure of the optical waveguide used for each element in the integrated element will be described with reference to FIG.
There are mainly three types of optical waveguides used in semiconductor optical devices: buried optical waveguides, ridge optical waveguides, and high-mesa optical waveguides. The schematic diagrams of these are shown in FIGS. 3 (a) and 3 (b). ), As shown in FIG.

  As shown in FIG. 3A, the buried optical waveguide 100 is formed by sequentially forming a lower clad portion 102, an MQW structure active layer 103, and an upper clad portion 104 on a substrate 101 so as to activate the MQW structure. The structure is such that both sides of the layer 103 are etched until reaching the substrate 101, and both sides of the active layer 103 having the MQW structure are embedded and embedded with the semiconductor 105, and the radiation is good and efficient current injection is possible. Further, since the beam shape can be made close to a circle, it can be easily coupled with an optical fiber and has been widely used in semiconductor lasers. However, when considering application to the EAM portion, since the electrode is formed on the top of the semiconductor having a large dielectric constant, there is a problem that parasitic capacitance increases and high-speed modulation necessary for the 100 giga standard is difficult. In addition, since the InAlGaAs-based material is oxidized when exposed to air, the active layer is exposed at the time of etching on both sides of the active layer, so that the quality of the crystal may be deteriorated.

  As shown in FIG. 3B, the ridge-type optical waveguide 110 is formed by sequentially forming a lower clad portion 112, an active layer 113 having an MQW structure, and an upper clad portion 114 on a substrate 111. A mesa is formed by etching only the upper clad portion 114 without cutting the layer 113 to form a waveguide. By embedding both sides of the mesa with the organic substance 115 having a small dielectric constant, it is possible to greatly reduce the parasitic capacitance in the EAM portion, which is considered advantageous for high-speed modulation. In addition, when an etching stop layer is sandwiched between the upper clad portion 114 and the active layer 113 having the MQW structure, the MQW structure active layer 113 can be manufactured without being exposed to the air, and an InAlGaAs-based material is used. It is possible to protect the crystal quality of the active layer. As a disadvantage, the beam shape is elliptical, so that the coupling with the fiber is difficult compared to the buried type.

  As shown in FIG. 3C, the high-mesa optical waveguide 120 is formed by sequentially forming a lower clad portion 122, an MQW structure active layer 123, and an upper clad portion 124 on a substrate 121, thereby forming an MQW structure active. Etching is performed on both sides of the layer 123 until it reaches the substrate 121, and the structure is usually embedded with an organic material 125 having a low dielectric constant. This optical waveguide has the advantage that the difference between the refractive index in the lateral direction and the active layer 123 of the MQW structure becomes very large, so that the light is confined strongly, so that it is difficult to emit light even if the optical waveguide is bent. However, since both sides of the MQW structure active layer 123 are etched, deterioration of crystal quality is inevitable. Therefore, for light sources that require high-speed modulation such as 100 GbE, the LD portion 21 and the EAM portion 23 both protect the InAlGaAs-based crystal quality and can secure a large modulation band, so that an organic embedded ridge structure is useful. Conceivable.

  On the other hand, the multiplexing unit 24 naturally requires an optical waveguide structure, but the required performance is different from that of the optical transmission unit and the signal conversion unit. In order to integrate a large number of elements on one chip, it is usually inevitable to bend the optical waveguide when wiring a signal output from one element to the output section. It is known that when an optical waveguide is bent, light is emitted to a region outside the bending, which is called bending loss. This loss is smaller as it is gently bent (larger bend radius) and larger as it is bent more rapidly (lower bend loss). The magnitude of this loss is determined by the difference between the refractive index of the core portion of the optical waveguide and the refractive index of the cladding portion surrounding it. The greater the difference in the refractive index, the better the light is confined in the core part. Therefore, if the bending loss is small, the waveguide can be bent at a steeper angle, which is advantageous for downsizing of the chip.

  Also in the MMI coupler, the radiation loss of light can be suppressed as the difference in refractive index between the core portion and the cladding portion of the waveguide increases. Considering the structure of the optical waveguide used in the integrated portion from these viewpoints, it is considered that the ridge type and buried type optical waveguides have a large bending loss and are not suitable for the optical waveguide for integration. The high-mesa optical waveguide has a very large difference in refractive index between the core portion of the optical waveguide and the cladding portion in the lateral direction, can reduce bending loss, and can also suppress radiation loss in the MMI coupler. However, it is considered that InGaAsP is more suitable for the core portion of the optical waveguide than InAlGaAs, which has oxidation problems.

  The problem here is the problem of connection between the light source including the EAM portion (ridge-type optical waveguide) and the multiplexing portion (high mesa-type optical waveguide). Here, it demonstrates using FIG. 4 which shows the electric field distribution of an optical waveguide. 4A shows the electric field distribution of a typical ridge type optical waveguide, and FIG. 4B shows the electric field distribution of a high mesa type optical waveguide. In FIG. 4A and FIG. 4B, the density of the dots indicates the intensity of light. The mesa width is 1.8 μm for the ridge type optical waveguide and 1.5 μm for the high mesa type optical waveguide so that it becomes a single mode waveguide with respect to the transverse mode. Since the high-mesa optical waveguide has stronger optical confinement, the mesa width that satisfies the single mode condition is smaller. As can be seen from FIG. 4, in the ridge type optical waveguide, since the core layer having a large refractive index spreads in the lateral direction, the electric field is distributed wider than the mesa width. On the other hand, in the high mesa type optical waveguide, most of the electric field distribution is within the mesa width. As apparent from FIG. 4, since the two mode fields are greatly different from each other, simply connecting the butt joints increases the coupling loss, resulting in a reflection point. The coupling efficiency of the two waveguides of FIG. 4 with respect to the butt joint coupling is only 85% (see point A in FIG. 7), and a more efficient coupling is required.

In such a case, a method of adiabatically converting the electric field distribution using a tapered optical waveguide 130 as shown in FIG. The optical waveguide 130 is connected to the ridge type optical waveguide 141 and to the high mesa type optical waveguide 142. The width of one end 131 of the optical waveguide 130 is formed to be the same as the width W _ridge of the ridge type optical waveguide 141. The width of the other end 132 of the optical waveguide 130 is formed to be the same as the width W _hm of the high mesa optical waveguide 142. The width of the optical waveguide 130 is formed so as to gradually narrow from one end 131 toward the other end 132. Possible configurations in this case include an optical waveguide 130A in which the mesa of the ridge optical waveguide 141 has a taper structure as shown in FIG. 5B, and a high mesa optical waveguide as shown in FIG. 5C. The case of the optical waveguide 130B in which the waveguide 142 has a tapered structure is conceivable. In the case of FIG. 5B, a ridge type optical waveguide having a mesa width of 1.5 μm is connected to the high mesa type optical waveguide at the junction of the ridge type optical waveguide 141 and the high mesa type optical waveguide 142. In the case of the type optical waveguide 141, when the mesa width is reduced, the electric field distribution in the core is further oozed out in the lateral direction, so that the coupling efficiency becomes even worse than in the case of butt joint without using the taper structure. . In the case of FIG. 5C, the ridge type optical waveguide 141 and the high mesa type optical waveguide 142 having the same mesa width at the connection point are connected, but the coupling efficiency in this case is 93% (point B in FIG. 7). However, the loss is still large. 5 (b) and 5 (c), the mesas involved in the light guiding in the waveguides 141 and 142 and the optical waveguides 130A and 130B are illustrated, and the other portions are not shown. ing.

Therefore, in this embodiment, a discontinuous taper structure junction as shown in FIG. 6 is used. As shown in FIG. 6, the discontinuous tapered structure junction is a ridge-type optical waveguide 41 and a high-mesa optical waveguide 42, that is, an optical waveguide in contact with two waveguides 41 and 42 having different structures, and has a predetermined length. It consists of a discontinuous taper structure joint body 50 having a _length L _taper . One end 51 of the discontinuous tapered structure joint body 50 is in contact with one waveguide 41, and the other end 52 of the discontinuous taper structure joint body 50 is in contact with the other waveguide 42. One waveguide 41 has a structure in which the lateral spread of light guided through the one waveguide 41 is wider than the lateral spread of light guided through the other waveguide 42. have. The mesa height of one optical waveguide 41 is formed lower than the mesa height of the other optical waveguide. The width of the discontinuous taper structure joint body 50 is formed so as to gradually narrow from one end 51 toward the other end 52. The width W _taper of one end 51 in the discontinuous tapered structure joint body 50 is formed wider than the width W _ridge of one waveguide 41. With such a discontinuous tapered structure joint body 50, the width of the high mesa optical waveguide is widened at the joint of one waveguide 41, which is a ridge optical waveguide, and the other waveguide 42, which is a high mesa optical waveguide. By doing so, the light distributed outside the mesa in the ridge structure can also be taken into the high mesa optical waveguide. The discontinuous taper structure junction is manufactured by a mesa formation process for a high mesa waveguide, and the mesa height of the discontinuous taper structure main body 50 is the same as the mesa height of the other waveguide 42. is there. In FIG. 6, the mesas involved in the light guiding in the waveguides 41 and 42 and the discontinuous tapered structure joint body 50 are illustrated, and the other portions are not illustrated.

FIG. 7 shows the taper width W _taper dependence of the coupling efficiency of the ridge type optical waveguide and the high mesa type optical waveguide at the discontinuous tapered structure junction. As shown in FIG. 7, the coupling efficiency increases as the taper width W _taper , that is, the width of one end 51 of the discontinuous tapered structure joint body 50 increases, and the coupling efficiency decreases beyond a certain width. . From FIG. 7, it is understood that the taper width W _taper may be set to 2.0 μm or more and 2.8 μm or less in order to obtain a coupling efficiency of 95% or more. FIG. 8 shows the dependency of the coupling efficiency on the taper length L _taper when the _taper width W _taper = 2.5 μm. When the taper length L _taper is 40 μm or more, the coupling efficiency is saturated. When the taper length L _taper is 20 μm, a coupling efficiency of 95% or more is obtained, and when the taper length L _taper is 40 μm or more, the coupling efficiency is 98% or more. However, since there is a loss of the optical waveguide itself, if it is too long, the waveguide loss becomes large. Therefore, it is preferable to set the thickness to about 100 μm at most.

  Finally, when comparing the losses of various types of junctions and the junctions according to the present embodiment, the ridge type optical waveguide and the high mesa type optical waveguide are directly butt-joined with the respective waveguide widths (see point A in FIG. 7), and the taper structure is obtained. When not provided, the loss is 1.41 dB, the width of the high mesa optical waveguide is the same as that of the ridge optical waveguide, and a butt joint is made (see point B in FIG. 7), and the taper length is about 1 μm. When the coupling loss is 1.02 dB and the taper length is 40 μm or more, the coupling loss is 0.4 dB, and the taper length is 40 μm or more by using the discontinuous taper structure joint portion of this embodiment. The coupling loss is only 0.17 dB, and a great effect can be obtained with respect to the reduction of the loss of the connection between the different types of waveguides.

  In the case of the optical device (optical semiconductor device) in which the light source and the multiplexing unit are integrated as shown in this embodiment, the intensity of the signal output from the light source is considerably due to the loss caused by the optical waveguide and the multiplexing unit in the middle. Since it becomes weak, it is necessary to reduce the loss as much as possible and increase the final light output. When the optical output is 0 dB when the discontinuous taper junction that is a different waveguide connection technique according to the present embodiment is not used, the discontinuous taper junction included in the optical waveguide according to the present embodiment is used. Since the output can be increased to 1.4 dBm, this embodiment can achieve a great effect particularly in an integrated optical device in which a large number of optical devices are integrated.

An optical waveguide and an optical semiconductor device using the same according to a second embodiment of the present invention will be described with reference to FIGS. 2, 9, and 10. FIG.
The optical waveguide according to the present embodiment and the optical semiconductor device using the same are arranged in the transverse direction in the discontinuous tapered structure joint body included in the optical waveguide according to the first embodiment and the optical semiconductor device using the optical waveguide. In this configuration, a connection region having no structure for confining light is added, and the other configuration is the same as that of the optical waveguide according to the first embodiment and the optical semiconductor device using the same.

  The optical semiconductor device according to the present embodiment is manufactured through the same steps as in the first embodiment described above. That is, first, a wafer in which each layer in the LD unit 21, the passive layer 22, the EAM unit 23, the multiplexing unit 24, and the PD unit 25 is formed is manufactured. Subsequently, a ridge-type waveguide mesa formation process is performed on the LD portion 21, the passive layer 22, the EAM portion 23, and the PD portion 25, and a high-mesa waveguide mesa formation process and an electrode formation process are performed on the multiplexing portion 24. (However, the upper electrode is not formed on the passive layer 22 and the combining portion 24), and after cleaving, a non-reflective coating (not shown) is applied to the front and rear end faces of the chip.

  Then, the discontinuous taper structure joint portion included in the optical waveguide according to the present embodiment is similar to the first embodiment described above in the mesa formation process for the high mesa waveguide, and the EAM portion 23 and the multiplexing portion 24. It is fabricated in a part of the optical waveguide between.

  Here, the mesa formation process for the ridge waveguide and the mesa formation process for the high mesa waveguide are performed using a waveguide mask. For the production of the waveguide mask, patterning by photolithography is used. Since the size of the optical waveguide is usually larger than 1 μm, the structure of the discontinuous tapered structure joint body 50 described above can be satisfactorily manufactured by ordinary photolithography. The structural joint is rounded, that is, one end 151 of the discontinuous tapered structural joint body 150 extends in a direction inclined by an angle θ with respect to a direction orthogonal to the extending direction of the one optical waveguide 41. It may become a shape to do. At this time, one waveguide 41 that is a ridge-type optical waveguide and the discontinuous taper structure joint body 150 that is a high-mesa optical waveguide are bonded with the same mesa width in principle, so if the angle θ is sufficiently small There is no problem, but if it exceeds a certain level, the coupling efficiency deteriorates.

In order to avoid this problem, the present embodiment uses a discontinuous tapered structure joint as shown in FIG. As shown in FIG. 10, the discontinuous taper structure joint portion includes the discontinuous taper structure joint body 50 described above, and one end portion 51 of the discontinuous taper structure joint body 50 and one waveguide 41. And a connection region 60 provided in the area. The connection region 60 has the same layer structure as that of the multiplexing unit 24. The connection region 60 is in contact with the end portion of the one waveguide 41 and extends in a direction orthogonal to the extending direction of the one waveguide 41, and one of the discontinuous tapered structure joint body 50. The other end portion 62 is in contact with the one end portion 61 and faces the one end portion 61, and the side end portion 63 connecting the one end portion 61 and both ends of the other end portion 62. The connection region 60 has a width wider than the width W _taper of one end portion 51 of the discontinuous taper structure joint body 50 and a predetermined length G. The connection region 60 is formed in a structure that does not have a mesa for confining light in the lateral direction. Similar to the discontinuous taper structure main body 50, the connection region 60 is manufactured by a mesa formation process for a high mesa waveguide. The height of the mesa in the connection region 60 is the discontinuous taper structure joint main body 50 or the other mesa. The height of the mesa of the waveguide 42 is the same.

With such a discontinuous taper structure joint, it is possible to avoid in principle the deterioration of the coupling efficiency due to the angle dullness as shown in FIG. 9, and the value of G is one of the ridge type optical waveguides. The distance may be set so that the beam emitted from the waveguide 41 does not spread. If the distance is about 0.5 μm to 5 μm, the spread of the beam is not a problem, and the same effect as in the first embodiment is obtained. be able to. Further, regarding the region without the lateral light confinement structure, the width in the lateral direction is arbitrary as long as it is sufficiently larger than W _taper , but the width of the chip of a normal optical semiconductor device is 200 μm to 400 μm at most. Then, considering that the mesa width of a normal optical waveguide is several μm, it may be set to about 10 μm to 200 μm.

  In the above description, the taper width of the discontinuous taper structure joint body connecting the ridge type optical waveguide 41 and the high mesa type optical waveguide 42 is linearly changed. However, from the end in contact with one optical waveguide to the other Any shape that gradually narrows to the end in contact with the optical waveguide may be used.

  In the above description, the optical semiconductor device is integrated with the light source, the modulation unit, and the multiplexing unit. However, another device such as a semiconductor optical amplifier or a waveguide type diffraction grating may be integrated.

  In the above description, the optical waveguide related to the connection between the ridge structure and the high mesa structure is described. However, in the connection of two optical waveguides having different structures, the lateral spread of light guided through one optical waveguide is as follows. It is sufficient if the width is wider than the lateral spread of the light guided through the other optical waveguide. For example, even when the above-described optical waveguide is applied to the connection between the buried optical waveguide and the high-mesa optical waveguide, which are weakly confined, the same effects as those of the first and second embodiments can be obtained.

  In the above description, an optical semiconductor device using InP as a substrate, InGaAsP as a core portion, and InAlGaAs as an active portion has been described. However, an optical semiconductor device using another compound semiconductor, for example, GaAs or Si, may be used. Good. Moreover, it is applicable also to a silica type optical waveguide and a polymer optical waveguide.

  The present invention relates to an optical waveguide and an optical semiconductor device using the optical waveguide, and when connecting different types of optical waveguides having greatly different mode fields, it is possible to join light more efficiently than a conventional optical waveguide having a tapered structure. When each element device is integrated on the same substrate, it is possible to freely select the waveguide structure of each element device, and each can be optimized independently, so it is very useful in the optical communication industry. can do.

DESCRIPTION OF SYMBOLS 1 Substrate 2 Active part 3 of multiple quantum well structure 3, 4 Clad part 5 Active part 6 of multiple quantum well structure 6, 7 Clad part 8 Core part 9, 10 Clad part 11 Core part 12, 13 Clad part 14 Lower electrode 15 Contact layer 16 Upper electrode 21 Semiconductor laser active part (LD part)
22 Passive layer (optical waveguide)
23 Electroabsorption optical modulator (EAM)
24 Multiplexing section 25 Photodiode section (PD section)
31A to 31D Lane 50 Discontinuous taper structure joint body 51 One end 52 The other end 60 Connection region

Claims (6)

Two waveguides having different structures are in contact with each other through a junction having a predetermined length, and the lateral spread of the light guided in one waveguide is the lateral extension of the light guided in the other waveguide. In an optical waveguide wider than the spread,
The width of the joint gradually decreases from one end in contact with the one waveguide toward the other end in contact with the other waveguide,
An optical waveguide characterized in that a width of the one end portion in the joint is wider than a width of the one waveguide. The one waveguide and the other waveguide each have a mesa;
2. The optical waveguide according to claim 1, wherein the height of the mesa of the one waveguide is lower than the height of the mesa of the other waveguide. The joint has a mesa;
3. The optical waveguide according to claim 2, wherein the height of the mesa of the joint is the same as the height of the mesa of the other waveguide. A connection region having a predetermined length and a width wider than the width of the junction and having no structure for confining light in the lateral direction is provided between the one waveguide and the junction. The optical waveguide according to any one of claims 1 to 3, characterized in that: 5. An optical semiconductor device, wherein an optical semiconductor element having a ridge structure and an optical semiconductor element having a high mesa structure are connected via the optical waveguide according to claim 1. A plurality of light sources, a plurality of modulators that respectively modulate light incident from the plurality of light sources, and a multiplexer that multiplexes light emitted from the plurality of modulators via the optical waveguide 5. An optical semiconductor device connected on a substrate, wherein a part of the optical waveguide between the modulator and the multiplexer is the optical waveguide according to claim 1. .