JP2012156336A - Optical integrated circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform miniaturization and to reduce power consumption in an optical integrated circuit for which a semiconductor laser and a semiconductor Mach-Zehnder modulator are integrated.SOLUTION: A first optical waveguide is connected to one end of a wavelength variable DFB laser array, a second optical waveguide is connected to the other end, a first semiconductor Mach-Zehnder modulator is connected to the first optical waveguide, and a second semiconductor Mach-Zehnder modulator is connected to the second optical waveguide. The arrangement direction of a mesa structure of the semiconductor laser is a direction optimum for regrowth to a side face of the mesa structure, and the arrangement direction of two arm waveguides provided in the semiconductor Mach-Zehnder modulator is a direction for maximizing Pockels effect to the two arm waveguides. In addition, in the optical integrated circuit related to this embodiment, the resonator structure of the semiconductor laser is configured symmetrically so as to equalize optical outputs at both ends thereof, and they are both inputted to the semiconductor Mach-Zehnder modulator without abandoning the optical output from one part.

Description

  The present invention relates to an optical integrated circuit, and more particularly to an optical integrated circuit in which a semiconductor laser and a semiconductor Mach-Zehnder modulator are integrated.

  With the recent spread of optical communication networks, adaptation to backbone optical communication systems having a transmission capacity exceeding 40 Gbit / sec (Gigabit per second) has been actively promoted. These ultra-high-speed optical communication systems have higher frequency utilization efficiency and optical signal-to-noise ratio (Optical Signal-to-To--) compared to NRZ (Non Return to Zero) modulation schemes that have been applied in conventional optical communication systems. Adoption of various light modulation schemes such as Noise Ratio (OSNR) proof strength and non-linear proof strength has been continued. In such a background, a method of transmitting multi-bit information in one symbol time is considered as the most effective means.

For example, in the case of differential quadrature phase shift keying (DQPSK), which is a method capable of allocating 2-bit data to four modulated phases, at least four high-speed transmitters are used. A device configuration in which a phase modulator is integrated is required. At present, these high-speed phase modulators are widely used in which an LN modulator is provided on an LN waveguide made of lithium niobate (LiNbO ₃ : LN) (hereinafter referred to as an LN modulator). The entire LN waveguide is referred to as “LN waveguide”). By using the LN waveguide, there is an advantage that these optical modulators can be realized with low loss. However, when dealing with a more complicated information transmission format using the LN waveguide, there is a demerit that the device size is increased. For example, in order to realize 100 Gbit / sec, when adopting DP-QPSK (Dual Polarization Quadrature Phase Shift Keying) in which 4-bit data including polarization multiplexing is included in each phase, the transmitter is provided for each polarization. It becomes the form provided with the structure of DQPSK. When this is formed using an LN waveguide, the number of required couplers increases, and the number of phase modulators is at least eight, which is twice that of DQPSK, resulting in a problem that the element size becomes very large. In the future, if an attempt is made to cope with a more complicated information transmission format than DP-QPSK, the device size will become very large in the LN waveguide.

  Also, in the module containing the LN waveguide, a light source such as a semiconductor laser is separately prepared and connected by a fiber. Therefore, not only the module size is inevitably increased by the amount of the light source and the fiber, but also a space is required for storing the extra length fiber. As a method for solving these problems, it is conceivable that the light source and the LN waveguide are housed in the same package. However, since the materials constituting the LN waveguide and the light source are different, it is not always easy due to a difference in thermal expansion coefficient.

  Taking DP-QPSK as an example, it is necessary to input light having the same wavelength and non-random phase to a Mach-Zehnder interferometer for both polarizations. Usually, a semiconductor single mode oscillation laser input to an LN waveguide The output light from is divided into two equal parts by a power splitter provided on the LN waveguide, and is incident on each Mach-Zehnder interferometer.

  As a semiconductor single mode oscillation laser used as a light source, a distributed feedback laser (DFB laser) having a narrow oscillation spectrum linewidth is typically used. In particular, a λ / 4 shift DFB laser is often used. In principle, when the λ / 4 shift is arranged at the center of the resonator structure, the light intensities output to both ends of the laser of the non-reflective structure are equal. Usually, by slightly shifting the position of the λ / 4 shift, the light intensity in the resonator structure is asymmetrical, and the light output from the front is adjusted to be strong. However, the asymmetry obtained by adjusting the position of the λ / 4 shift is small. In other words, considering the case where it is arranged near the center of the resonator structure, it is a structure in which the rear light output, which is almost half, is discarded.

  FIG. 1 schematically shows an approximate optical power output in DP-QPSK using an LN waveguide. Although it is impossible in practice, it is assumed that the loss on the LN waveguide and the connection loss connecting each optical component are zero. If 3 dBm is required for the output of the TE polarization and TM polarization LN waveguides, the light intensity incident on the LN waveguide is 6 dBm. If the coupling loss is assumed to be zero, the same amount of output from the semiconductor laser is required. However, assuming a laser light source such as a DFB laser, the energy output efficiency is low because the light output discarded behind is about 6 dBm. Actually, an insertion loss of an LN modulator of about 5 dB and a coupling loss between a laser and a fiber of about 1.5 dB are added, so that a higher output is required, and the amount of backward discard increases accordingly. become.

  In FIG. 1, the light emitted from the LN waveguide is shown as an output for both polarizations, but only one polarization is actually guided in the LN waveguide and has the same polarization. One of them passes through the half-wave plate and rotates the polarization, and then combines and outputs the polarization.

  In order to avoid the problem that the size of the modulator composed of the LN waveguide is large, an optical waveguide is composed of a semiconductor, and input is performed using the refractive index change mainly caused by the Pockels effect when an electric field is applied to the semiconductor element. Mach-Zehnder type optical modulators (hereinafter referred to as “semiconductor Mach-Zehnder modulators”) that convert electrical signals into light phase changes have attracted attention. Since the semiconductor Mach-Zehnder modulator has a larger relative refractive index difference between materials constituting the waveguide than the LN waveguide, the bending radius can be reduced, and a small circuit layout can be achieved. A high-speed modulator corresponding to a multilevel transmission format such as DQPSK has already been reported (see Non-Patent Document 1).

  Another advantage of the semiconductor Mach-Zehnder modulator is that it can be integrated on a single chip with the laser light source. By integrating, not only the connection loss between the semiconductor laser and the semiconductor Mach-Zehnder modulator can be reduced, but also the extra length fiber processing is unnecessary, and the advantage of small size can be obtained.

  As a specific example of an optical integrated circuit in which a semiconductor laser and a semiconductor Mach-Zehnder modulator are integrated, an example using an InP substrate will be described in more detail. The following explanation is applicable to all zinc blende type crystals to which InP also belongs, and the same applies to GaAs and the like which are the same crystal system. When fabricating a semiconductor laser on an InP substrate having a (100) plane, it is generally desirable to fabricate an active layer waveguide of the semiconductor laser in the [011] direction. There are various laser structures, but when fabricating a buried semiconductor laser capable of low threshold and high output, it is difficult to bury by re-growth unless a mesa structure having an active layer is fabricated in the [011] direction. is there. On the other hand, a semiconductor Mach-Zehnder modulation having a multiple quantum well layer (MQW) in the active region, using both a refractive index change due to the Pockels effect and a refractive index change due to the quantum confined Stark effect (QCSE). When the device is manufactured in the [01-1] direction, the refractive index change due to the Pockels effect when an electric field is applied changes so as to increase the refractive index change due to the QCSE, whereas when the device is manufactured in the [011] direction, the refractive index changes. To do. Therefore, the semiconductor Mach-Zehnder modulator is better manufactured in the [01-1] direction because the modulation efficiency is improved, and the electric field strength can be lowered correspondingly, and the element length of the modulator can be shortened. become. That is, there are a plane orientation suitable for a semiconductor laser and a plane orientation suitable for a semiconductor Mach-Zehnder modulator, respectively, and integration requires a design that also considers the plane orientation.

  Therefore, in an optical integrated circuit in which a semiconductor laser and a semiconductor Mach-Zehnder modulator are integrated, the arrangement direction of the mesa structure of the semiconductor laser is set to a direction different from the arrangement direction of the two arm waveguides of the semiconductor Mach-Zehnder modulator. (See Patent Document 1). For example, when an InP substrate is used as the semiconductor substrate, the arrangement direction of the mesa structure of the semiconductor laser is the [011] direction, and the arrangement direction of the arm waveguide of the semiconductor Mach-Zehnder modulator is the [01-1] direction. Thereby, an optical integrated circuit having excellent modulation characteristics can be obtained as compared with the case where the semiconductor laser and the semiconductor Mach-Zehnder modulator are linearly arranged.

International Publication No. 2009/022969

Nobuhiro Kikuchi, Yasuo Shibata, Ken Tsuzuki, Hiroaki Sanjoh, Tomonari Sato, Eiichi Yamada, Tadao Ishibashi, and Hiroshi Yasaka, “80-Gb / s Low-Driving-Voltage InP DQPSK Modulator With an npin Structure,” IEEE Photonics Technology Letters, 2009, Vol. 21, No. 12, pp. 787-789 H. Oohashi, Y. Shibata, H. Ishii, Y. Kawaguchi, Y. Kondo, Y. Yoshikuni, and Y. Tohmori, “46.9-nm Wavelength-Selectable Arrayed DFB lasers with Integrated MMI Coupler and SOA,” 2001 Intemational Conference on Indium Phosphide and Related Materials (IPRM 2001) Conference Proceedings, 2001, pp. 575-578 Hiroyuki Ishii, Hiromi Oohashi, Kazuo Kasaya, Ken Tsuzuki, and Yuichi Tohmori, “High-power (40 mW) L-band tunable DFB laser array module using current tuning,” 2005 Optical Fiber Communication Conference (OFC / NFOEC) Technical Digest, 2005, Vol. 2 Kenichi Iga, "Applied physics series semiconductor laser", Ohm, 1994 N. Nunoya, H. Ishii, Y. Kawaguchi, Y. Kondo and H. Oohashi, “Wideband tuning of tunable distributed amplification distributed feedback laser array,” ELECTRONICS LETTERS, 2008, Vol. 44, No. 3, pp. 205- 207

  An optical integrated circuit in which a semiconductor laser and a semiconductor Mach-Zehnder modulator are integrated is promising in terms of integration, but further performance improvement is desired.

  The present invention has been made in view of such problems, and an object thereof is to reduce the size and power consumption of an optical integrated circuit in which a semiconductor laser and a semiconductor Mach-Zehnder modulator are integrated. .

  In order to achieve such an object, according to a first aspect of the present invention, an optical integrated circuit in which a semiconductor laser and a semiconductor Mach-Zehnder type modulator are integrated on a zincblende type semiconductor substrate has a mesa structure. A semiconductor laser, a first semiconductor Mach-Zehnder modulator connected to one end of the semiconductor laser by a first optical waveguide, an input-side demultiplexer, and an optical signal demultiplexed by the demultiplexer A first semiconductor Mach-Zehnder modulator having two arm waveguides for phase modulation with respect to the two arm waveguides and a multiplexer connected to the two arm waveguides; 2 is a second semiconductor Mach-Zehnder modulator connected by two optical waveguides, the input side duplexer and two for performing phase modulation on the optical signal demultiplexed by the duplexer Arm waveguide, and 2 A second semiconductor Mach-Zehnder modulator having a multiplexer connected to the arm waveguide, wherein the arrangement direction of the mesa structure of the semiconductor laser is optimal for regrowth on the side surface of the mesa structure The active layers of the two arm waveguides included in the first and second semiconductor Mach-Zehnder type modulators have a multiple quantum well structure, and the first and second semiconductor Mach-Zehnder type modulators include the active layer. The arrangement direction of the two arm waveguides is such that when the electric field for optical modulation is applied to the two arm waveguides, the refractive index change due to the Pockels effect intensifies the refractive index change due to the quantum confined Stark effect. It is characterized by being.

  According to a second aspect of the present invention, in the first aspect, the semiconductor substrate is an InP substrate, the semiconductor laser and the semiconductor Mach-Zehnder modulator are integrated on a (100) plane of the InP substrate, The arrangement direction of the mesa structure of the semiconductor laser is the [011] direction of the InP substrate, and the arrangement direction of the two arm waveguides of the Mach-Zehnder modulator is [01-1] of the InP substrate. ] Direction.

  A third aspect of the present invention is the semiconductor device according to the first or second aspect, wherein the semiconductor laser is a DFB laser, and a λ / 4 shift is arranged at the center of the DFB laser, so that the both ends of the DFB laser Are equal in light output.

  According to a fourth aspect of the present invention, in the first or second aspect, the semiconductor laser is a wavelength tunable DFB laser array, and a λ / 4 shift is arranged at the center of the wavelength tunable DFB laser array. The optical outputs from both ends of the wavelength tunable DFB laser array are equal.

  According to a fifth aspect of the present invention, in the fourth aspect, the multiplexer in the wavelength tunable DFB laser array is an N × M funnel multiplexer or mode interferometer multiplexer (N is an output side). The number of channels is 2 or more, and M is the number of DFB lasers and is 2 or more).

  According to a sixth aspect of the present invention, in any one of the first to fifth aspects, a polarization rotator configured by a semiconductor waveguide connected to the output of the first semiconductor Mach-Zehnder modulator is further provided. It is characterized by providing.

  Further, a seventh aspect of the present invention is the polarization according to the sixth aspect, comprising a semiconductor waveguide connected to the output of the polarization rotator and the output of the second semiconductor Mach-Zehnder modulator. Further comprising a multiplexer.

  According to an eighth aspect of the present invention, in any one of the first to fifth aspects, the cube mirror optically coupled to the output of the first semiconductor Mach-Zehnder modulator through a lens; A half-wave plate bonded to one surface of the cube mirror; the half-wave plate; and a polarization multiplexer bonded to the surface facing the cube mirror; and output from the first semiconductor Mach-Zehnder modulator The optical signal is converted by the cube mirror in the direction of the half-wave plate by 90 degrees, passes through the half-wave plate, is introduced into the polarization multiplexer, and the second semiconductor Mach-Zehnder modulation is performed. And an optical signal output from the optical multiplexer and multiplexed in the polarization multiplexer.

  According to a ninth aspect of the present invention, in the eighth aspect, a delay line is connected to the output of the second semiconductor Mach-Zehnder modulator.

  According to the present invention, the first optical waveguide is connected to one end of the semiconductor laser, the second optical waveguide is connected to the other end, the first semiconductor Mach-Zehnder modulator is connected to the first optical waveguide, and the second optical waveguide is connected to the second optical waveguide. By connecting the two semiconductor Mach-Zehnder modulators, both of them can be input to the semiconductor Mach-Zehnder modulator and used without discarding the optical output from one of them. Since the arrangement directions of the first and second semiconductor Mach-Zehnder modulators are separately determined, the optical integrated circuit can be reduced in size.

It is a figure which shows typically the approximation of the optical power output in DP-QPSK using a LN waveguide. 1 is a schematic diagram of an optical integrated circuit according to a first embodiment of the present invention. It is a figure for demonstrating the manufacturing method of the optical integrated circuit by the 1st Embodiment of this invention. (A) is sectional drawing of the DFB laser along the aa line of FIG. 3, (b) is sectional drawing of the semiconductor Mach-Zehnder modulator along the bb line of FIG. It is a figure for demonstrating the manufacturing method of the optical integrated circuit by the 1st Embodiment of this invention. It is the figure which showed the part replaced in 2nd Embodiment among FIG. 2 with the dotted line. It is a figure which shows the optical integrated circuit which concerns on 3rd Embodiment.

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

(First embodiment)
FIG. 2 is a schematic diagram of an optical integrated circuit according to the first embodiment of the present invention. In the present embodiment, a wavelength tunable DFB laser array will be described as an example of the semiconductor laser. A first optical waveguide is connected to one end of the tunable DFB laser array, a second optical waveguide is connected to the other end, the first semiconductor Mach-Zehnder modulator is connected to the first optical waveguide, and the second semiconductor is connected to the second optical waveguide. A Mach-Zehnder modulator is connected. Here, the active layers of the two arm waveguides included in the first and second semiconductor Mach-Zehnder type modulators have a multiple quantum well structure. In the example of FIG. 2, each semiconductor Mach-Zehnder modulator is a DQPSK optical modulator composed of a two-stage Mach-Zehnder type optical modulator. In this configuration, a DP-QPSK modulator and a wavelength tunable DFB laser array (TLA) are integrated.

  The arrangement direction of the mesa structure of the semiconductor laser is an optimum direction for regrowth on the side surface of the mesa structure, and the arrangement direction of the two arm waveguides included in the semiconductor Mach-Zehnder type modulator is the two arm waveguides. When an electric field for light modulation is applied, the refractive index change due to the Pockels effect is in a direction that intensifies the refractive index change due to QCSE. In addition, in the optical integrated circuit according to the present embodiment, the resonator structure of the semiconductor laser is configured to be symmetrical so that the optical outputs at both ends thereof are equal, and the semiconductor Mach-Zehnder is used together without discarding the optical output from one side. Input to the modulator. When a DFB laser is used, the light intensity output at both ends becomes equal if the λ / 4 shift is arranged at the center of the resonator structure. Here, “configure the resonator structure symmetrically” means more specifically that the resonator structure is configured to have an axis of symmetry with respect to both end faces thereof.

  Although described in relation to the background art, many devices have been made to increase the optical output in the forward direction of the fixed wavelength DFB laser and the variable wavelength DFB laser array (see Non-Patent Document 4). The light intensity (about half) is also in the rear. In the conventional optical integrated circuit in which the semiconductor laser and the semiconductor Mach-Zehnder modulator are integrated, the light emitted behind this makes the laser operation unstable and is discarded by forming a non-reflective structure or absorbing it. Was. Alternatively, it has been only used to monitor the power of the semiconductor laser by utilizing the fact that the light intensity of the rear light is correlated with the light intensity of the front light. In the first embodiment of the present invention, the light that is emitted backward is also used, thereby improving the energy use efficiency and reducing the power consumption of the optical integrated circuit. Assuming that the light intensity discarded backward in the conventional structure (see FIG. 1) is the same as the light intensity emitted forward, the energy utilization is twice as high as that in the conventional structure. That is, it means that the same optical output can be achieved with 50% driving energy, and the power consumption of the optical integrated circuit in which the semiconductor laser and the semiconductor Mach-Zehnder modulator are integrated can be reduced accordingly. This reduction in power consumption of the optical integrated circuit also reduces the heat generation of the optical integrated circuit. Therefore, the burden of TEC (Thermoelectric Controller, generally referred to as a temperature controller using a Peltier element) that is often provided near the circuit to ensure the wavelength accuracy of the optical integrated circuit is reduced, and the power consumption of the TEC is also reduced. Things will be possible.

  In addition, in the asymmetric type DFB laser, there is a problem that the single mode is deteriorated by making it asymmetric. However, when the DFB laser is positively configured symmetrically, in principle, this problem does not occur. An optical output excellent in single mode can be obtained.

  Conventionally, there has been a technique for separately determining the direction of arrangement of a semiconductor laser and a semiconductor Mach-Zehnder modulator in order to improve modulation characteristics. This is compared to the case where the semiconductor laser and the semiconductor Mach-Zehnder modulator are arranged linearly. Therefore, it takes a useless area and the size is increased, which is not necessarily preferable.

  In the present invention, when it is intended to realize a modulator having two semiconductor Mach-Zehnder modulators, that is, an IQ modulator, as an optical integrated circuit, the arrangement directions of the semiconductor laser and the semiconductor Mach-Zehnder modulator are determined separately. On the contrary, downsizing can be achieved.

  In the example of FIG. 2, after the output of the first semiconductor Mach-Zehnder modulator is connected to a polarization rotator configured with a semiconductor waveguide and the polarization is converted from the TE mode to the TM mode, The output of the TE mode of the other second semiconductor Mach-Zehnder modulator is combined with a polarization beam combiner (PBC). The polarization multiplexer is composed of a Mach-Zehnder interferometer (MZI).

  The optical integrated circuit in which the DP-QPSK modulator and the tunable DFB laser array shown in FIG. 2 are integrated has very high energy utilization efficiency. In the conventional structure as described with reference to FIG. 1, light output from one end of a semiconductor laser is branched by a 3 dB power splitter and is incident on a DQPSK optical modulator that is a semiconductor Mach-Zehnder modulator. . However, in the structure of the present embodiment, in addition to using the optical outputs from both ends of the semiconductor laser, the first branching by the 3 dB power splitter is unnecessary. Since light of the same wavelength and the same power is supplied from the wavelength tunable DFB laser through two paths, they may be incident on the DQPSK optical modulator as they are.

  The wavelength tunable DFB laser array (TLA) is a device in which a plurality of DFB lasers, a multimode interference (MMI) type multiplexer, and a semiconductor optical amplifier (SOA) are integrated on one chip. For details, see Non-Patent Documents 2 and 3. The number of DFB lasers to be integrated may be appropriately selected according to the variable wavelength range, and may be 8, 12, or the like. In FIG. 2, the SOA is manufactured in a direction parallel to the arm waveguide of the semiconductor Mach-Zehnder modulator, but it may be parallel to the TLA. In the case of the direction parallel to the TLA, the wet etching method can be used at the time of forming the mesa structure of the TLA, and at the same time, the waveguide mesa structure of the SOA can be produced. When the SOA is manufactured in a direction parallel to the arm waveguide, the manufacturing method described later is different in that the regrowth process is increased in order to integrate the SOA, but basically the same is not changed.

  In the example of the present embodiment, a wavelength tunable DFB laser array in which SOA is integrated is used as the wavelength tunable light source. However, if sufficient light intensity can be obtained with only the wavelength tunable DFB laser array, integration of SOA is not necessary. . In this case, since the number of regrowths can be reduced, the number of process steps can be reduced, and the cost can be reduced.

  Further, instead of the wavelength tunable DFB laser array, a single mode semiconductor laser that emits light with a single wavelength may be used, and a configuration without a light coupler or SOA may be used. In this case, the wavelength tunable function is lost, but the advantages of high energy use efficiency and miniaturization are not lost. Furthermore, the wavelength tunable DFB laser array is not limited to a single mode semiconductor wavelength tunable laser that transmits in both directions at the same wavelength. For example, a TDA-DFB (Tunable distributed amplification fedback) laser (see Non-Patent Document 5), an SSG-DBR laser, or the like can be given.

  In the example of the present embodiment, the configuration in the case of a DP-QPSK multilevel modulation transmitter has been described. However, a single-stage semiconductor Mach-Zehnder modulator may be integrated as polarization multiplexed DPSK. In addition, even a transmitter corresponding to another transmission format can have the same configuration as long as the configuration is polarization multiplexed, and the same effect can be obtained.

  In the following description, the description referring to the plane orientation is also made, but this is merely a representative one, and it is intended to include a crystal engineering equivalent. I want to be.

  Here, examples will be described.

EXAMPLE FIG. 3 is a diagram for explaining a method of manufacturing an optical integrated circuit according to the first embodiment of the present invention. For simplicity, only one semiconductor Mach-Zehnder modulator is shown. On the (100) plane of the InP substrate, a DFB laser is arranged in the [011] direction, a semiconductor Mach-Zehnder modulator is arranged in the [01-1] direction, and both are connected by an optical waveguide. 4A is a cross-sectional view of the DFB laser along the line aa in FIG. 3, and FIG. 4B is a cross-sectional view of the semiconductor Mach-Zehnder modulator along the line bb in FIG. is there. The manufacturing method will be described with reference to FIGS. 5A and 5B, but some processes that can be easily inferred from the preceding and following descriptions by those skilled in the art are partially omitted.

  First, a method for manufacturing a DFB laser will be described. An n-type InP is grown on a (100) plane of an SI (semi-insulating) -InP substrate, and an unstrained InGaAlAs guide layer having a PL wavelength of 1.15 μm is grown thereon (not shown). An active layer having a multiple quantum well (MQW) structure is grown thereon. The multi-quantum well structure has an InGaAlAs well layer with a PL wavelength of 1.55 μm and a compressive strain of + 1.0% and a thickness of 7 nm, and an InGaAlAs barrier with a PL wavelength of 1.20 μm and a tensile strain of −0.4% and a thickness of 10 nm. It is a structure in which a layer is repeated for 8 periods. A p-InGaAsP layer is formed on this active layer (FIG. 5A).

  Next, a diffraction grating is formed in the p-InGaAsP layer using lithography and etching (FIG. 5B). The grooves of the diffraction grating are manufactured so as to be parallel to the [011] direction perpendicular to the [011] direction in which the mesa structure is manufactured by a process described later.

  Then, a p-InP overclad layer and a p-InGaAsP contact layer are grown in order (FIG. 5C).

  The layer structure of the DFB laser portion is completed through the steps so far. Utilizing this, a semiconductor Mach-Zehnder modulator coupled with a DFB laser is manufactured using a butt joint technique.

  Initially, a SiO2 mask is applied over the layer structure of FIG. 5C and patterned using standard photolithography and etching. The patterning is performed so that an island pattern having a width of 15 μm and a length of 450 μm remains immediately above the region where the diffraction grating is formed. Here, the length direction is the direction in which the grooves of the diffraction grating are arranged, specifically, the [011] direction. Next, the active layer by MQW is removed by etching to form an island-like mesa structure having a width of 15 μm and a length of 450 μm (FIG. 5D). In other words, the MQW active layer is left only in the region having a width of 15 μm and a length of 450 μm. The n-InP layer sandwiched between the active layer and the InP substrate is shared by the DFB laser portion and the semiconductor Mach-Zehnder modulator portion, so that it is not mistakenly removed by etching when the island mesa structure is formed.

Next, the layer structure of the semiconductor Mach-Zehnder modulator is crystal-grown so as to embed the DFB laser portion by selective growth using a SiO 2 mask (FIG. 5E). The structure that grows as the modulator portion is as follows. First, an n-type GaAlAs / InAlAs multi-quantum well active layer having a thickness of 0.4 μm is grown, followed by i-InP having a thickness of 0.5 μm, a thickness of 0.05 μm to be a carrier block layer, a hole concentration. A 1 × 10 ¹⁸ p-InP, n-InP overclad layer, and p-InGaAs contact layer are grown in this order. Thereby, the layer structure of the modulator having the npin structure is formed, and at the same time, it is coupled to the DFB laser portion by the bad joint.

  Next, after removing the SiO2 mask in the DFB laser portion with hydrofluoric acid, SiO2 is deposited again, and a mask having two stripe-shaped openings in a part of the DFB laser portion is formed by photolithography and etching. . Using this as a mask, the active layer of the DFB laser portion is removed by reactive ion etching (RIE). As a result, a high mesa structure with a width of 2 μm is formed (FIG. 5F). The high mesa structure is formed in a direction parallel to the [011] direction so that the next current block layer can be easily regrown.

  Next, an Fe-doped InP layer is grown on both sides of the high mesa structure by a thickness equivalent to the amount etched previously (FIG. 5G). This InP layer functions as a current blocking layer for the DFB laser.

  Next, the SiO2 mask used previously is removed with hydrofluoric acid, and SiO2 is formed again. After masking the portion where the DFB laser and the semiconductor Mach-Zehnder modulator are formed by photolithography and etching, the n-InP is exposed immediately above the InP substrate leaving the high mesa waveguide in the shape of the DFB laser and the semiconductor Mach-Zehnder modulator by the RIE method. Etching is performed until this is done (FIG. 5 (h)). The semiconductor Mach-Zehnder modulator, like a general Mach-Zehnder modulator, includes an input-side duplexer, two arm waveguides for performing phase modulation on the optical signal demultiplexed by the duplexer, A multiplexer connected to two arm waveguides, and both arm waveguides of the semiconductor Mach-Zehnder modulator are parallel to the [01-1] direction in order to maximize the Pockels effect. Form. Through the steps so far, the structure of the DFB laser and the semiconductor Mach-Zehnder modulator is formed.

  Thereafter, in order to electrically insulate between the arm waveguides of the semiconductor Mach-Zehnder modulator and between the DFB laser and the semiconductor Mach-Zehnder modulator, an isolation groove is formed so that the n-InP contact layer is independent in each part. . Next, a p-electrode was formed on the upper surface of the DFB laser, and an n-electrode was formed on the n-InP contact layers of the DFB laser and the Mach-Zehnder modulator. Then, an insulating organic film such as polyimide or BCB is applied to the entire surface, and the organic film is removed leaving only the mesa side of the semiconductor Mach-Zehnder modulator. Various electrodes are formed, and then cleavage is performed after the SI-InP substrate is polished.

  In the description of this example, various materials and processing techniques were mentioned, but these are for fully explaining the method of manufacturing the optical integrated circuit according to the first embodiment of the present invention. The optical integrated circuit is not intended to be limited to those manufactured by these materials and processing techniques. For example, although an InP substrate has been described as an example of a semiconductor substrate, a semiconductor substrate having a zinc blende type (zinc blend type) structure can be used.

  It was confirmed that a 3 dBm optical output and a high-speed modulation characteristic with a 3 dB bandwidth of 30 GHz can be obtained by the optical integrated circuit of this example manufactured in this way. This indicates that a high-speed modulator can be realized by the configuration of this embodiment.

(Second Embodiment)
In the semiconductor Mach-Zehnder modulator in which the tunable DFB laser array with bidirectional optical output of the first embodiment is integrated, bidirectional output is performed via two 12 × 1 MMI type multiplexers. The second embodiment replaces a part of the DQPSK coupler following the latter part with an N × M funnel waveguide from this part.

  FIG. 6 is a diagram showing a part to be replaced in the second embodiment in FIG. 2 by a dotted line. In this case, N is 12 and M is 4. In the configuration of the first embodiment, 12 DFB lasers are divided into two equal parts after passing through a 12 × 1 MMI type multiplexer at both outputs of the tunable DFB laser array with bidirectional optical output, Each is further divided into two equal parts. The light intensity of the driving DFB laser becomes 1/12 by passing through this MMI type multiplexer, and then passes through the SOA to obtain a gain. Furthermore, when passing through the subsequent two-stage coupler, it is distributed to the four arm waveguides, and the same thing occurs in both directions. In this sense, this configuration is such that light is attenuated and then amplified. Of course, if the attenuation can be reduced, the amplification may be small and energy consumption can be suppressed. Further, if sufficient light intensity can be obtained, SOA is not necessary. In addition, when SOA is not required, the process can be simplified and the manufacturing cost can be reduced.

  Therefore, this part is replaced with a 12 × 4 funnel multiplexer. In any case, since it is divided into four parts, the part surrounded by a dotted line in the figure (only the upper part is shown in the figure for the sake of brevity, but the lower symmetrical part is also the same). If it is replaced with a × 4 funnel multiplexer, the rate of unnecessary attenuation can be reduced, and the attenuation of 1/12 can be reduced to 4/12, that is, 1/3. Thus, in addition to the use of bidirectional light output, it is possible to further increase the energy utilization efficiency.

  Here, although a 12 × 4 funnel multiplexer is used in the present embodiment, the same effect can be obtained even if a 12 × 4 MMI is used. In this embodiment, the wavelength tunable DFB laser array is used as a light source. However, if the active portion constitutes the array, is output through a multiplexer, and is tunable so that bidirectional output can be used. Of course, the same effect can be obtained.

(Third embodiment)
In the first embodiment, after passing through the semiconductor Mach-Zehnder modulator, one polarization is converted from the TE mode to the TM mode, and then passed through the PBC to match the output (TE) of the other DQPSK optical modulator. Waved. In an integrated device, there is a problem that unless the completeness and yield of the element devices to be integrated are high, the performance of the entire device cannot be obtained, and the yield is remarkably reduced. In particular, the polarization rotator made of a semiconductor in the first embodiment is difficult to manufacture.

  Therefore, in the present embodiment, the portion to the right of the dotted line in FIG. 6 is configured using a space-based bulk microengineering system.

  FIG. 7 shows an optical integrated circuit according to this embodiment. The TE-polarized light emitted from the bidirectional output of each single-mode semiconductor wavelength tunable laser is collimated by passing through the lens after passing through the semiconductor Mach-Zehnder modulator. One is optically coupled to the cube mirror through a lens and passes through the cube mirror to change the optical path by 90 degrees. Thereafter, the light passes through a half-wave plate attached to one surface of the cube mirror, and the polarization is rotated by TM deflection. Thereafter, the light passes through the half-wave plate and the PBC bonded on the surface facing the cube mirror, and is combined and output. Actually, though not shown, the light passes through the isolator and is again focused on the fiber by the lens and output. In the example of this embodiment, a 1 mm square cube mirror and PBC are used, and a half-wave plate thickness of 20 μm made of polyimide is used.

  In this configuration, after leaving the semiconductor chip, it passes through a half-wave plate sandwiched between two blocks. The upper optical path in the figure has a longer optical path length than the light passing through the lower part. That is, the transmission timing is greatly shifted due to the polarization. In order to eliminate skew (transmission timing deviation between polarized waves), a delay line is provided in the waveguide that becomes the short optical path to eliminate the skew. The length of the delay line may be determined in a timely manner by the refractive index of the waveguide and the refractive index thickness of the cube mirror and the half-wave plate constituting the waveguide.

  Further, this delay line is not necessarily required, and skew can be avoided by delaying a high-speed electrical signal used for optical modulation.

  According to this embodiment, a low power consumption DP-QPSK using bidirectional output can be manufactured with a high yield by combining with a space system member.

Claims (9)

In an optical integrated circuit in which a semiconductor laser and a semiconductor Mach-Zehnder type modulator are integrated on a zincblende type semiconductor substrate,
A semiconductor laser having a mesa structure;
A first semiconductor Mach-Zehnder modulator connected to one end of the semiconductor laser by a first optical waveguide, the phase splitter for an input-side duplexer and an optical signal demultiplexed by the duplexer A first semiconductor Mach-Zehnder modulator having two arm waveguides for performing the above and a multiplexer connected to the two arm waveguides;
A second semiconductor Mach-Zehnder modulator connected to the other end of the semiconductor laser by a second optical waveguide, the phase difference with respect to an input-side demultiplexer and an optical signal demultiplexed by the demultiplexer A second semiconductor Mach-Zehnder modulator having two arm waveguides for performing modulation and a multiplexer connected to the two arm waveguides;
The arrangement direction of the mesa structure of the semiconductor laser is an optimum direction for regrowth on the side surface of the mesa structure,
The active layers of the two arm waveguides of the first and second semiconductor Mach-Zehnder modulators have a multiple quantum well structure,
Refraction due to Pockels effect when the arrangement direction of the two arm waveguides of the first and second semiconductor Mach-Zehnder type modulators applies an electric field for optical modulation to the two arm waveguides. An optical integrated circuit characterized in that the rate change is in a direction to increase the change in refractive index due to the quantum confined Stark effect. The semiconductor substrate is an InP substrate;
The semiconductor laser and the semiconductor Mach-Zehnder modulator are integrated on the (100) surface of the InP substrate,
The arrangement direction of the mesa structure of the semiconductor laser is the [011] direction of the InP substrate, and the arrangement direction of the two arm waveguides of the Mach-Zehnder modulator is [01-1] of the InP substrate. The optical integrated circuit according to claim 1, wherein The semiconductor laser is a DFB laser,
3. The optical integrated circuit according to claim 1, wherein a λ / 4 shift is arranged in the center of the DFB laser, and the optical outputs from both ends of the DFB laser are equal. 4. The semiconductor laser is a wavelength tunable DFB laser array,
3. The optical integrated circuit according to claim 1, wherein a λ / 4 shift is arranged in the center of the wavelength tunable DFB laser array, and optical outputs from both ends of the wavelength tunable DFB laser array are equal.   The multiplexer in the wavelength tunable DFB laser array is an N × M funnel multiplexer or mode interferometer multiplexer (N is two or more channels on the output side, and M is two or more DFB lasers). The optical integrated circuit according to claim 4, wherein:   6. The optical integrated circuit according to claim 1, further comprising a polarization rotator constituted by a semiconductor waveguide connected to an output of the first semiconductor Mach-Zehnder modulator.   The polarization multiplexer made up of a semiconductor waveguide connected to the output of the polarization rotator and the output of the second semiconductor Mach-Zehnder modulator, further comprising: Optical integrated circuit. A cube mirror optically coupled to the output of the first semiconductor Mach-Zehnder modulator via a lens;
A half-wave plate attached to one surface of the cube mirror;
The half-wave plate and a polarization multiplexer bonded together on a surface facing the cube mirror,
The optical signal output from the first semiconductor Mach-Zehnder modulator is converted by the cube mirror in the direction of the half-wave plate by 90 degrees, passes through the half-wave plate, and enters the polarization multiplexer. The optical integrated circuit according to claim 1, wherein the optical integrated circuit is combined with an optical signal introduced and output from the second semiconductor Mach-Zehnder modulator in the polarization multiplexer. .   9. The optical integrated circuit according to claim 8, wherein a delay line is connected to an output of the second semiconductor Mach-Zehnder modulator.

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