JP5458964B2 - Semiconductor Mach-Zehnder optical modulator, optical transmission device, method for manufacturing semiconductor Mach-Zehnder optical modulator, and method for driving semiconductor Mach-Zehnder optical modulator - Google Patents

Description

  The present invention relates to a semiconductor Mach-Zehnder type optical modulator, an optical transmission device including the semiconductor Mach-Zehnder type optical modulator, a method for manufacturing the semiconductor Mach-Zehnder type optical modulator, and a method for driving the semiconductor Mach-Zehnder type optical modulator. .

Most existing optical transmission devices (optical transmitters and optical repeaters) use Mach-Zehnder type light that uses lithium niobate (LiNbO ₃ : hereinafter referred to as LN) as an optical modulator. It has a modulator.

  Here, as shown in FIG. 8, the Mach-Zehnder type optical modulator (hereinafter referred to as an MZ optical modulator) includes two arms (branch waveguides) and two waves of input light to each arm. An optical device including an optical distributor that divides the light into two, an optical coupler that interferes and multiplexes light from each arm, and an electrode that applies a voltage to each arm. An MZ optical modulator using LN (hereinafter referred to as an LN-MZ optical modulator) is an LN electro-optical device manufactured using an LN substrate and having a refractive index of each arm as a dielectric. It is an MZ optical modulator that changes depending on the effect. The MZ optical modulator shown in FIG. 8 uses a Y-branch waveguide as an optical distributor / optical coupler, but a directional coupler or an MMI (Multiplexer) is used as the optical distributor / optical coupler. -Mode Interference: An MZ optical modulator using a coupler is also known.

The general optical transmission apparatus, as a drive circuit of the LN-MZ light modulator, each arm of the LN-MZ modulator (electrode), and the modulation voltages _{V 1} changes from _{V M} to _{V M} + [Delta] V, from V _M - [Delta] V to _{V M,} which is assumed having a circuit for applying a modulation voltage _{V 2 dV 2 / dt = -dV 1 /} dt is changed to stand.

  The modulation voltage is applied to the LN-MZ optical modulator (hereinafter referred to as push-pull drive) when the push-pull drive is performed. Is capable of functioning as a modulator with small optical frequency fluctuation (chirp).

Specifically, the LN-MZ optical modulator is an optical device in which the following relationship is substantially established between the electric field intensity E _O of the output light and the electric field intensity E _i of the input light.

In the equation (1) and the following equations, κ is an intensity ratio of light propagating through both arms, and φ ₁ and φ ₂ are light phase change amounts in the arms.

  Therefore, the light output from the LN-MZ optical modulator is light of intensity I and phase φ expressed by the following equations (2) and (3).

From these equations, the α parameter (chirp amount index) of the LN-MZ optical modulator can be expressed as follows by φ ₁ ′ (= dφ ₁ / dt), φ ₂ ′ (= dφ ₂ / dt), and the like. It will be.

This equation (4) can be modified as follows when φ ₁ ′ = −φ ₂ ′.

Accordingly, in each arm of the LN-MZ optical modulator where κ = 1 (the light splitting ratio to both arms is 1: 1), φ ₁ ′ = −φ ₂ ′. If a voltage is applied, chirp can be suppressed ideally (α = 0).

  As shown in FIGS. 9A and 9B, since the refractive index of each arm of the LN-MZ optical modulator changes substantially linearly with respect to the voltage applied to each arm, LN− The amount of phase change in each arm of the MZ optical modulator also changes substantially linearly with respect to the voltage applied to each arm.

Therefore, as shown in FIGS. 9B and 9C, the modulation voltage V _{1 that} changes from V _M to V _H (= V _M + ΔV) where dV ₂ / dt = −dV ₁ / dt is established. and _V L modulated voltage _{V 2} changes from _{(= V} M - [Delta] V) to _{V M,} do it is applied to each arm of the LN-MZ modulator, the LN-MZ modulator, small modulator chirp ( It is made to function as a modulator whose output optical phase is constant during modulation.

  Thus, the LN-MZ optical modulator can suppress chirp. However, the LN-MZ optical modulator uses a dielectric electro-optic effect (the amount of change in the refractive index of each arm when a voltage is applied is relatively small), so the module size cannot be reduced. It has become.

If a semiconductor having a larger amount of change in the refractive index when a voltage is applied is used, a smaller MZ optical modulator with lower power consumption can be realized. Therefore, MZ optical modulators using semiconductors (hereinafter referred to as semiconductor MZ optical modulators) are being developed. Semiconductor MZ optical modulators are chirped by the above-described push-pull drive. Cannot be made sufficiently small.

Specifically, Pockels effect, Franz-Keldysh effect (hereinafter referred to as FK effect), Stark effect, and the like are known as effects contributing to a change in the refractive index of a semiconductor when an electric field is applied. The Pockels effect is a refractive index change phenomenon (effect) caused by birefringence that occurs when an electric field is applied to an isotropic crystal. The FK effect is a phenomenon in which a refractive index change is caused by a change in the fundamental absorption edge due to the penetration of the wave function of electrons and holes into the forbidden band. The Stark effect is a phenomenon in which a change in refractive index is caused by a change in exciton absorption wavelength due to interaction between excitons and an electric field. In particular, in the quantum well structure, the dissociation of excitons is hindered, thereby enhancing the Stark effect and providing a dominant refractive index change mechanism (see Non-Patent Document 1).

As semiconductor MZ optical modulators, those using the quantum confined Stark effect (hereinafter referred to as QCSE) are being studied. However, such semiconductor MZ optical modulation is being studied. It is known that the refractive index change Δn of the arm in the container can be expressed by the following equation (6) (see Non-Patent Documents 1 and 2). In this equation (6) and the following equations, n ₀ is the refractive index in the absence of electric field (voltage) application, and E is the electric field strength of the electric field applied to the quantum well. Further, s is a coefficient (constant) determined by the semiconductor material.

Field intensity E of the electric field applied to the quantum well, and the built-in potential E _B due to carrier diffusion in the semiconductor, the sum of the field strength E _D of the electric field caused by the voltage application from the outside. Further, the electric field strength E _D of the electric field caused by the voltage application from the outside, the applied voltage V, the value obtained by dividing the active layer thickness d.

Thus, _{(6), E = E B + E D} , _and, from E D = V / d, the refractive index variation caused by application of the voltage V [Delta] n '(= voltage V applied during the refractive index - Voltage Not applied during (Refractive index) can be expressed by the following equation (7).

The phase change amount in each arm is a value obtained by multiplying the refractive index change amount Δn ′ by the electrode length and the optical confinement coefficient Γ in the active layer (that is, a value proportional to Δn ′). . Therefore, in the semiconductor MZ optical modulator, the light phase change amounts φ ₁ and φ ₂ in each arm change nonlinearly with respect to V ₁ and V ₂ as shown in the following equations (8) and (9). Will do. Note that “r” in these equations is “2sE _B ” (see equation (7)).

As described above, in the semiconductor MZ optical modulator, the light phase change amounts φ ₁ and φ ₂ in each arm change nonlinearly with respect to V ₁ and V ₂ . When φ ₁ and φ ₂ change nonlinearly with respect to V ₁ and V ₂ , as schematically shown in FIGS. 10A and 10B, dV ₂ / dt = −dV ₁ / Even if V ₁ and V ₂ satisfying dt are applied to each arm, φ ₁ ′ cannot be set to −φ ₂ ′. Therefore, the chirp of the semiconductor MZ optical modulator cannot be made sufficiently small by the push-pull drive.

  Further, in the semiconductor MZ optical modulator, the change amount Δα of the extinction coefficient with respect to light having a wavelength in the vicinity of the interband transition wavelength of the semiconductor (that is, the wavelength at which the refractive index change due to QCSE can be effectively used) is expressed by the following equation (10): It is known that there is a voltage dependence approximated by (see Non-Patent Document 3). In this equation (10), K and R are constants determined by the semiconductor material and the quantum well structure.

  That is, as shown in FIG. 11, the loss (propagation loss) of each arm in the LN-MZ optical modulator hardly changes depending on the applied voltage, whereas the semiconductor MZ optical modulator has a loss in each arm. Is relatively large depending on the applied voltage.

  For this reason, when chirp suppression of the semiconductor MZ optical modulator is desired, it is desired that deterioration of the extinction ratio due to this change in loss can also be suppressed. However, at present, a technology capable of modulating light with a semiconductor MZ optical modulator having a high extinction ratio and a small amount of chirp has not been developed yet.

JP 2006-309124 A JP 7-191290 A JP-A-8-146365 JP-A-9-49935

J. S. Weiner, et al, Appl. Phys. Lett., Vol.50, pp.842-844 (1987) M. Glick, et al, Appl. Phys. Lett., Vol. 48, pp. 989-991 (1986) Nishimura, et al, IEEE Photon. Technol. Lett., Vol.4, pp.1123-1126 (1992) L. B. Soldano, et al, J. Lightwave Technol., Vol.13, pp.615-627 (1995)

  Accordingly, the problem of the disclosed technology is that an optical transmission device capable of modulating light with a high extinction ratio and low chirp, a semiconductor MZ optical modulator capable of realizing such an optical transmission device, and such a semiconductor MZ light The object is to provide a “method of manufacturing a semiconductor MZ optical modulator” capable of manufacturing a modulator.

  Another object of the disclosed technique is to provide a “semiconductor MZ optical modulator driving method” that can drive the semiconductor MZ optical modulator with less chirp.

  In order to solve the above problems, a semiconductor Mach-Zehnder optical modulator according to an aspect of the disclosed technology includes a first and second optical waveguides having a semiconductor quantum well structure, which form a Mach-Zehnder interferometer, and the first optical waveguide. And a first electrode for applying a voltage to the second optical waveguide, and a second electrode for applying a voltage to the second optical waveguide, the first optical waveguide and the second optical waveguide. A voltage that minimizes the intensity of output light is applied to the first electrode and the voltage at a part of at least one of the optical waveguides, the loss adjusting unit having a light absorption coefficient larger than that of the other parts of the optical waveguide. A configuration in which a loss adjusting portion whose shape is determined so that the propagation loss of the first optical waveguide and the propagation loss of the second optical waveguide coincide with each other when applied to the second electrode is provided Have

  Further, an optical transmission device according to an aspect of the disclosed technology provides a semiconductor Mach-Zehnder optical modulator having the above-described configuration, and the first electrode and the second electrode of the semiconductor Mach-Zehnder optical modulator at the same time. And a drive circuit for driving the semiconductor Mach-Zehnder type optical modulator by applying a modulation voltage having opposite amplitudes and equal amplitudes.

  Also, a method of manufacturing a semiconductor Mach-Zehnder optical modulator according to an aspect of the disclosed technology is an optical waveguide in which first and second optical waveguides having a semiconductor quantum well structure, which form a Mach-Zehnder interferometer, are formed on a semiconductor substrate. A part of at least one of the first optical waveguide and the second optical waveguide, wherein the light absorption coefficient is larger than that of the other part of the optical waveguide. The propagation loss of the first optical waveguide and the propagation loss of the second optical waveguide when the voltage that minimizes the intensity of the output light is applied to the first electrode and the second electrode are matched. And a loss adjusting portion forming step of forming a loss adjusting portion whose shape is determined.

  Further, in the driving method of the semiconductor Mach-Zehnder type optical modulator according to one aspect of the disclosed technology, the semiconductor Mach-Zehnder type optical modulator is configured to constitute a Mach-Zehnder interferometer 2 in order to drive the semiconductor Mach-Zehnder type optical modulator. Two optical waveguides are applied with modulated voltages having the same amplitude but with opposite signs of time change rate at the same time.

If the said structure is employ | adopted, the optical transmission apparatus which can modulate light in a form with a high extinction ratio and few chirps can be obtained. In addition, if a semiconductor Mach-Zehnder type optical modulator having the above-described configuration is used, an optical transmission device that can modulate light with a high extinction ratio and a small amount of chirp can be realized. A semiconductor Mach-Zehnder type optical modulator can be manufactured. According to the above driving method, the semiconductor Mach-Zehnder optical modulator can be driven with less chirp.

1 is a schematic configuration diagram of an optical transmission apparatus according to an embodiment. Explanatory drawing of the drive principle of the semiconductor MZ optical modulator in the optical transmission apparatus which concerns on embodiment. The figure for demonstrating the malfunction which arises when the semiconductor MZ light modulator of the existing structure is used. Explanatory drawing of the loss amount of both arms at the time of the modulation | alteration of the semiconductor MZ optical modulator which concerns on embodiment. Sectional drawing for demonstrating the manufacturing procedure of the semiconductor MZ optical modulator which concerns on embodiment. Sectional drawing for demonstrating the manufacturing procedure of the semiconductor MZ optical modulator which concerns on embodiment. The top view corresponding to the sectional view of Drawing 5B for explaining the manufacture procedure of the semiconductor MZ light modulator concerning an embodiment. Sectional drawing for demonstrating the manufacturing procedure of the semiconductor MZ optical modulator which concerns on embodiment. Sectional drawing for demonstrating the manufacturing procedure of the semiconductor MZ optical modulator which concerns on embodiment. Sectional drawing for demonstrating the manufacturing procedure of the semiconductor MZ optical modulator which concerns on embodiment. Sectional drawing for demonstrating the manufacturing procedure of the semiconductor MZ optical modulator which concerns on embodiment. The top view corresponding to the sectional view of Drawing 5F for explaining the manufacture procedure of the semiconductor MZ light modulator concerning an embodiment. Sectional drawing for demonstrating the manufacturing procedure of the semiconductor MZ optical modulator which concerns on embodiment. Sectional drawing for demonstrating the manufacturing procedure of the semiconductor MZ optical modulator which concerns on embodiment. The top view for demonstrating the manufacturing procedure of the semiconductor MZ optical modulator which concerns on embodiment. Sectional drawing for demonstrating the manufacturing procedure of the semiconductor MZ optical modulator which concerns on embodiment. FIG. 5B is a plan view corresponding to the cross-sectional view of FIG. 5L for explaining the manufacturing procedure of the semiconductor MZ optical modulator according to the embodiment. Sectional drawing for demonstrating the manufacturing procedure of the semiconductor MZ optical modulator which concerns on embodiment. Sectional drawing for demonstrating the manufacturing procedure of the semiconductor MZ optical modulator which concerns on embodiment. The top view corresponding to the sectional view of Drawing 5O for explaining the manufacture procedure of the semiconductor MZ light modulator concerning an embodiment. Sectional drawing for demonstrating the manufacturing procedure of the semiconductor MZ optical modulator which concerns on embodiment. The top view for demonstrating the manufacturing procedure of the semiconductor MZ optical modulator which concerns on embodiment. Sectional drawing for demonstrating the manufacturing procedure of the semiconductor MZ optical modulator which concerns on embodiment. Explanatory drawing of the deformation | transformation form of the semiconductor MZ optical modulator which concerns on embodiment. Explanatory drawing of the other deformation | transformation form of the semiconductor MZ optical modulator which concerns on embodiment. The block diagram of a Mach-Zehnder type optical modulator. The figure for demonstrating the operation | movement content at the time of the push pull drive of an LN-MZ optical modulator. Explanatory drawing of the problem which arises at the time of the drive of a semiconductor MZ light modulator. Explanatory drawing of the applied voltage dependence of the loss about a LN-MZ optical modulator and a semiconductor MZ optical modulator.

  Hereinafter, an example of an optical transmission device developed by the inventors (hereinafter referred to as an optical transmission device according to an embodiment) will be described in detail with reference to the drawings.

  First, the configuration of the optical transmission apparatus according to the embodiment will be described with reference to FIG.

  The optical transmission apparatus according to the embodiment is an optical transmitter including the semiconductor MZ optical modulator 10, the light source 17, and the drive circuit 18 as main components.

  The light source 17 provided in this optical transmission device is a semiconductor laser (diode laser) that outputs laser light. The semiconductor MZ light modulator 10 is an optical modulator (details will be described later) for modulating (pulsing) the laser light from the light source 17.

The drive circuit 18 is based on data given from the outside, and outputs each of the arms 11 _X (X = X =) of the semiconductor MZ optical modulator 10 so that an optical signal having contents corresponding to the data is output from the semiconductor MZ optical modulator 10. 1, 2) A circuit for applying a modulation voltage to the electrode 15 on the top. This drive circuit 18 is a circuit that applies to each arm 11 _X (electrode 15) a modulation voltage having the same amplitude (voltage change range) with the time change rate being reversed at each time. The driving circuit 18, has also become circuit for applying a modulation voltage to be a phase change of approximately - [pi] / 4 radians ~ + [pi / 4 radians light propagating through each arm 11 _X occurs in each arm 11 _X.

The semiconductor MZ optical modulator 10 is an optical modulator that includes two arms 11 ₁ and 11 ₂ , an optical distributor 12, an optical coupler 13, and two electrodes 15.

Optical distributor 12, the laser beam from the light source 17 (hereinafter referred to as input light), and a device for separating the two waves to each arm 11 _X. The light distributor 12 is a device that divides input light into two waves having equal intensities and supplies them to each arm 11 _X (X = 1, 2) (in this embodiment, an MMI coupler).

Each of the arms 11 ₁ and 11 ₂ is an optical waveguide having a semiconductor quantum well structure. As is shown, the arm 11 ₁ provided in the semiconductor MZ optical modulator 10 has, on its part, the loss adjuster 14 (described in detail later) has a those with. In addition, each optical waveguide (each optical waveguide other than the arm 11 _X and the arm 11 _X ) provided in the semiconductor MZ optical modulator 10 has a width in which the waveguide mode is single at the modulated light wavelength.

The optical coupler 13 is a device that causes the light from the arm 11 ₁ and the light from the arm 11 ₂ to interfere and multiplex, and outputs the multiplex result to the upper optical waveguide in FIG. The semiconductor MZ optical modulator 10 employs an MMI coupler as the optical coupler 13.

Electrode 15 is an electrode for applying a voltage to a portion of the region of the arm 11 _X. As is shown, each electrode 15 of the semiconductor MZ optical modulator 10, the rectangular electrode covering a portion of the region of the arm 11 _X, was connected to pads 15a for connecting the signal line from the drive circuit 18 It has a shape.

Loss adjusting unit 14, than the other portion (portion other than the loss adjuster 14 of the arm 11 _1), which is formed by the portion as the layer thickness of the quantum well is increased. When the layer thickness of the quantum well is increased, the exciton absorption light wavelength becomes longer, and as a result, the light absorption coefficient increases. Therefore, the loss adjusting unit 14 has a larger light absorption coefficient than the other parts.

The semiconductor MZ optical modulator 10 is configured such that the loss adjustment unit 14 has a shape (mainly the length in the length direction of the arm 11 ₁ ) of the arm 11 ₁ (the entire arm 11 ₁ including the loss adjustment unit 14) in the off state. It has become one (propagation loss) and the loss of the arm 11 ₂ is determined to match. The OFF state is a state in which a voltage that minimizes the intensity of output light is applied to each arm 11 _X (electrode 15).

The semiconductor MZ optical modulator 10, the phase difference of light to which the voltage is outputted from the arm 11 ₁ and 11 ₂ in a state not applied (hereinafter, referred to as the initial phase difference), "π / 2 + 2nπ" It is also designed and manufactured to be radians (n is an integer).

  Next, the reason why the above configuration is adopted in the optical transmission apparatus according to the embodiment will be described.

As described above, in the optical transmission device according to the embodiment, the drive circuit 18 that applies the modulation voltage having the same amplitude and the opposite polarity of the time change rate at each time to each arm 11 _X of the semiconductor MZ optical modulator 10. It has.

Such a drive circuit 18 is employed in the optical transmission device because semiconductor MZ optical modulation is performed by applying to each arm 11 _{X a} modulation voltage having the same time amplitude of each time and opposite amplitude and equal amplitude. This is because the device 10 can function as an optical modulator with little chirp.

That is, when the modulation voltage changing as described above is applied to each arm 11 _X of the semiconductor MZ modulator 10, the maximum phase change amounts Δφ ₁ and Δφ ₂ in each arm 11 _X as shown in FIG. The absolute values of match. The maximum phase variation and is caused to light propagating through the arm 11 _X by applying a voltage of the output light intensity becomes maximum (voltage on the tip side of the arrow in FIG. 2), the voltage output light intensity becomes minimum ( The voltage at the base side of the arrow in FIG.

Thus, the positive and negative reverse time rate of change at each time, by applying an amplitude equal modulation voltages to the respective arm 11 _X (if adopted the drive circuit 18), the chirp semiconductor MZ optical modulator 10 It can function as a small number of optical modulators.

However, as already described, the semiconductor MZ modulator is such that the loss of each arm changes relatively greatly depending on the applied voltage (see FIG. 11). Therefore, when the above-described control is performed on the semiconductor MZ modulator having the existing configuration, as shown in FIG. 3, the losses α ₁ and α ₂ of each arm in the off state are greatly different. As a result, the intensity of the output light in the off state does not decrease so much.

The arm 11 ₁ of the semiconductor MZ modulator 10, what is provided with the loss adjuster 14, is so that this problem does not occur.

That is, the arm 11 ₁ has a “loss adjusting unit 14 whose shape is determined so that the loss of the arm 11 ₁ in the off state and the loss of the arm 11 ₂ in the off state coincide with each other”. 4, as schematically shown in FIG. 4, the losses α ₁ and α ₂ of the arms 11 _X (arm _{X in the} figure) in the off state can be matched.

Thus, the semiconductor MZ optical modulator 10 the loss adjusting portion 14 is provided on the arm 11 _1, (in other words, the extinction ratio is high optical modulator) optical modulator which the problem does not occur will function as .

  In addition, the loss adjusting part 14 is formed by adjusting the layer thickness of the quantum well. In that case, the loss adjusting part 14 can be easily realized (manufactured), and between the arms. This is because it becomes possible to add a loss difference depending on the optical wavelength.

Specifically, a method other than adjusting the layer thickness of the quantum well, for example, a method of making the shapes of both arms asymmetric (for example, changing the waveguide width between both arms), or a band gap wavelength becomes a long wave. A loss difference can be provided between both arms of the semiconductor MZ optical modulator also by a method of changing the composition. However, in order to manufacture two arms having a desired loss difference by the former method, the waveguide width and the like must be controlled with extremely high accuracy. Therefore, it is unrealistic to provide a loss difference between both arms of the semiconductor MZ optical modulator by making the shapes of both arms asymmetric.

  Further, in the semiconductor MZ optical modulator, the relationship between the refractive index change and the absorption change itself has wavelength dependency. Therefore, when the semiconductor MZ optical modulator is used over a wide wavelength range, the loss difference in the OFF state is set to zero for each wavelength unless the loss difference between the arms is also wavelength-dependent. I can't do it. However, the former method has a drawback that a loss difference depending on the optical wavelength cannot be provided between the arms.

  On the other hand, according to the latter method, it is possible to add a loss difference depending on the optical wavelength between the arms. However, when this method is adopted, the manufacturing procedure must be complicated. However, if the loss adjusting unit 14 is formed by adjusting the layer thickness of the quantum well, selective growth using a mask (for details, see FIG. As will be described later, the loss adjusting unit 14 can be easily realized. In addition, by adjusting the layer thickness of the quantum well, it is possible to add a loss difference depending on the optical wavelength between both arms. Therefore, the loss adjusting part 14 is formed by adjusting the layer thickness of the quantum well.

Further, if the initial phase difference between the arms 11 ₁ and 11 _{2 is set} to “π / 2 + 2nπ” radians (n is an integer), as shown in FIG. 2, the maximum value and the minimum value of the modulation voltage applied to each arm 11x. Since driving with uniform values can be performed, the amount of chirp can be minimized. Therefore, the semiconductor MZ optical modulator 10 is assumed to have an initial phase difference between the arms 11 ₁ and 11 ₂ of “π / 2 + 2nπ” radians, and the drive circuit 18 uses the light propagating through each arm 11 _X. , Which outputs a modulation voltage that causes a phase change of −π / 4 radians to + π / 4 radians is employed.

  Next, an example of a manufacturing procedure of the semiconductor MZ modulator 10 according to the present embodiment will be described.

  The semiconductor MZ modulator 10 described above can be manufactured, for example, according to the procedure shown in FIGS. 5A to 5S.

That is, when the semiconductor MZ modulator 10 is manufactured by this procedure (see FIG. 5A), first, the SiO ₂ film 22 is formed on the n-InP substrate 21 by the CVD (Chemical Vapor Deposition) method. . Next, a resist having a predetermined shape (details will be described later) is formed on the SiO ₂ film 22 by a procedure such as “applying a photoresist on the SiO ₂ film 22, exposing the applied photoresist, and developing the exposed photoresist”. A pattern 40 is formed.

Then, the SiO ₂ film 22 is formed with BHF (Buffered HF) using the resist pattern 40 as a mask.
) Or the like. Thereafter, by removing the resist pattern 40, two selective growth mask regions 23 (a part of the SiO ₂ film 22) having the shapes as shown in FIGS. 5B and 5C are provided on the n-InP substrate 21. The structure is manufactured.

  The shape and position of each selective growth mask region 23 (hereinafter also referred to as mask region 23) (the shape of the resist pattern 40) are determined in consideration of various things.

Specifically, the width Wm of each mask region 23 and the gap width Wg between the mask regions 23 are active layers in a region sandwiched between two mask regions 23 (hereinafter referred to as “high growth rate region”). The ratio of the growth rate (average: the same applies hereinafter) to the active layer growth rate in other regions is determined to be a desired value (for example, 1.2). The length Lm is in each mask area 23, so that the loss of the arm 11 ₁ is subsequently formed, and the loss of the arm 11 ₂ becomes the length to match in the off state (e.g. 500 [mu] m), the active layer It is determined from the composition, well layer thickness, driving conditions, and the like.

After the mask region 23 is formed, the thickness of the n-InP substrate 21 on the side where the mask region 23 is formed is about 100 nm thick by MOCVD (Metal Organic Chemical Vapor Deposition). The n-type InP buffer layer 24 is stacked (FIG. 5D). Next, an MQW (Multi-Quantum Wells) layer 25 composed of InGaAsP-wells and InP-barriers is formed on the InP buffer layer 24.

In order to obtain a practically sufficient refractive index change by QCSE, it is desirable that the band edge transition wavelength of the MQW layer 25 is close to the input light wavelength. However, if the input light wavelength is too close to the band edge transition wavelength, the influence of absorption increases. Therefore, when the input light wavelength is around 1.55 μm, the composition of the MQW layer 25 (for example, In (0.6) Ga (0.4) is set so that the band edge transition wavelength is about 1.4 μm. As (0.85) P (0.15)) and the layer thickness (for example, 10 nm) are desirably determined in advance. At this time, in the high growth rate region, the MQW layer thickness increases (here, 12 nm) in accordance with the growth rate ratio.

  After the formation of the MQW layer 25, the mask region 23 is wet-etched with BHF or the like. Thereafter, a p-type InP cladding layer 26 and a p-type InGaAs contact layer 27 are formed by MOCVD (FIG. 5E).

Then, the same procedure as when the mask area 23 formed, for forming a waveguide, the mask pattern 41 is formed consisting of Si0 ₂ (Fig. 5F). Then, dry etching by RIE (Reactive Ion Etching) is performed using the formed mask pattern 41 as a mask, thereby forming a mesa stripe on the n-InP substrate 21 (FIG. 5G).

Note that the mask pattern 41 formed in the above process has a shape as shown in FIG. 5H. However, the mask pattern 41 actually formed in the above process can be obtained by the dry etching using the mask pattern 41 as a mask, the optical distributor 12, the arm _11X, etc. having the functions and configurations already described with reference to FIG. As such, the shape is designed.

When the formation of the mesa stripe is completed, the mask pattern 41 is removed after the SI (Semi-Insulated: high resistance) -InP layer 31 (FIG. 5I) is grown around the mesa stripe by MOCVD. (FIG. 5J). Then, through a series of steps including “a step of forming a resist mask on the InGaAs contact layer 27” and the like, the portion of the InGaAs contact layer 27 other than the portion where the Au / Zn / Au electrode 33 ′ is formed is wet etched. (FIG. 5K). The Au / Zn / Au electrode 33 ′ is formed on a portion to which the voltage of each arm 11 _X (stripe-like portion where the InP buffer layer 24, MQW layer 25, etc. are stacked) is applied. This is a part of the electrode (see FIG. 5O).

After patterning the InGaAs contact layer 27, a SiO ₂ film 32 (see FIG. 5L) is formed by CVD on the entire surface of the n-InP substrate 21 on which the SI-InP layer 31 is formed after a series of steps. Is done.

Next, a resist mask 42 (see FIG. 5M) in which a portion where the Au / Zn / Au electrode 33 ′ is to be formed is an opening 42 a is formed on the SiO ₂ film 32.

After the formation of the resist mask 42, the portion of the SiO ₂ film 32 on the portion where the Au / Zn / Au electrode 33 ′ is formed is removed by etching using the resist mask 42 as a mask (FIGS. 5L and 5M). ). Thereafter, an Au / Zn / Au film 33 is formed by vacuum evaporation (FIG. 5N). Then, by removing the resist mask 42 together with the Au / Zn / Au film 33 formed thereon, a structure in which an Au / Zn / Au electrode 33 'exists on each arm 11X (FIG. 5O, FIG. 5P) is manufactured.

  Thereafter, a resist mask 44 for forming the Au electrode 34 having a shape as shown in FIG. 5R is formed (FIG. 5Q). Then, after the Au electrode 34 is formed by plating (after the electrode 15 (see FIG. 1) including the Au electrode 34 and the Au / Zn / Au electrode 33 ′ is formed), the resist mask 44 is removed. (FIG. 5R).

  After removing the resist mask 44, the back electrode 37 is formed (FIG. 5S). The back electrode 37 is usually formed by a procedure in which an AuGe / Au film 35 is deposited and then an Au plating layer 36 is formed.

If such a procedure is employed, the QCSE between the arms 11 ₁ and 11 ₂ in the off state can be driven by the modulation voltage having a modulation voltage center bias of about −3 V and a modulation amplitude (full width) of about 2 V. Since the excess loss difference (α ₂ −α ₁ in FIG. 3) is, for example, the semiconductor MZ optical modulator 10 having 0.5 dB, the loss adjustment unit 14 that generates the same propagation loss is provided. The semiconductor MZ light modulator 10 having a high extinction ratio can be manufactured easily and reliably.

<Deformation>
The optical transmission apparatus described above can be variously modified. For example, the optical transmission device is a device including the semiconductor MZ optical modulator 10 having the configuration shown in FIG. 6, that is, the optical splitter 12 and the optical coupler 13 are not MMI couplers but Y-branch waveguides. The semiconductor MZ optical modulator 10 can be modified into a device. Further, the optical transmission device can be modified as a device including the semiconductor MZ optical modulator 10 in which a directional coupler is used as the optical distributor 12 and the optical coupler 13.

In addition, the arm 11 ₁ and the arm 11 ₂ are not designed so that the initial phase difference is “π / 2 + 2nπ” radians (n is an integer), but the initial position is set in the arm 11 ₂ and / or the arm 11 _1. A phase adjustment mechanism for adjusting (controlling) the phase difference can also be provided. Such a phase adjustment mechanism includes a mechanism that adjusts the phase by using a change in refractive index due to voltage application, a mechanism that adjusts the phase by using a change in refractive index due to a change in free carrier density due to current injection, and a temperature change. A mechanism for adjusting the phase by using a change in refractive index accompanying the above can be considered. However, the mechanism using the voltage / current is difficult to control because a change in absorption occurs with a change in refractive index. Therefore, a semiconductor when implementing the phase adjustment mechanism to the MZ optical modulator 10, as shown in FIG. 7, the arm 11 ₂ above (or, the arm 11 ₁ above) of the semiconductor MZ optical modulator 10 the heater electrode 19 to Is preferably provided.

Further, the loss adjusting unit 14 is “the shape of which is determined so that the loss of the arm 11 ₁ in the off state and the loss of the arm 112 ₂ match each other when the light absorption coefficient is larger than the other parts”. All you have to do is Therefore, the loss adjuster 14 after the formation (loss adjusting unit 14 is a semiconductor MZ optical modulator 10 provided) is, in fact, as to match the losses of the arm 11 ₁ loss and the arm 11 ₂ in the off state completely It does not have to be.

  Further, the loss adjusting unit 14 can be provided under the electrode 15. However, when such a configuration is adopted, it is difficult to determine the size of the loss adjustment unit 14. Therefore, it is desirable to provide the loss adjusting unit 14 in a portion that is not under the electrode 15 like the semiconductor MZ optical modulator 10 described above.

Semiconductor MZ optical modulator 10 having an active layer other than InGaAsP / InP quantum well (for example, II, III, V, VI group elements such as Zn, Cd, Hg, Al, N, Sb, S, Te, etc.) It is also possible to transform the mixed crystal consisting of In addition, the semiconductor MZ optical modulator 10 is provided with both of the arms 11 ₁ and 11 ₂ that are equivalent to the loss adjusting unit 14 (the loss between the arms 11 ₁ and 11 ₂ in the off state is reduced). as matching, it can also be transformed into one) having the structure loss in each arm 11 _X is adjusted.

  The semiconductor MZ optical modulator 10 includes a waveguide having a structure different from the above structure (SI-BH structure) (for example, a PNPN-type embedded waveguide or a high-mesa waveguide that is not embedded with a semiconductor). It can also be transformed into Further, the optical coupler 13 of the semiconductor MZ optical modulator 10 may output some% of the combined light to the lower optical waveguide in FIG. Of course, it may be transformed into a repeater.

  As described above, the following additional notes are disclosed with respect to the disclosed technology.

(Additional remark 1) The 1st and 2nd optical waveguide which has a semiconductor quantum well structure which comprises a Mach-Zehnder interferometer,
A first electrode for applying a voltage to the first optical waveguide;
A second electrode for applying a voltage to the second optical waveguide;
With
A part of at least one of the first optical waveguide and the second optical waveguide is a loss adjustment unit having a light absorption coefficient larger than that of the other part of the optical waveguide, The shape is such that the propagation loss of the first optical waveguide coincides with the propagation loss of the second optical waveguide when a voltage having a minimum intensity is applied to the first electrode and the second electrode. A semiconductor Mach-Zehnder type optical modulator, characterized in that a predetermined loss adjusting unit is provided.

(Supplementary Note 2) The loss adjustment unit is
The semiconductor Mach-Zehnder optical modulator according to appendix 1, wherein the quantum well has a layer thickness larger than that of the quantum well of the other portion of the optical waveguide.

(Appendix 3) At least one of the first optical waveguide and the second optical waveguide is
The semiconductor Mach-Zehnder optical modulator according to appendix 1, wherein the semiconductor Mach-Zehnder type optical modulator includes a quantum well structure including a portion that functions as the loss adjustment unit, the thickness of which varies continuously along the light propagation direction.

(Supplementary note 4) Any of Supplementary notes 1 to 3, wherein an initial phase difference between the first optical waveguide and the second optical waveguide is π / 2 + 2nπ (n is an integer) radians. A semiconductor Mach-Zehnder optical modulator according to claim 1.

(Supplementary Note 5) Any one of Supplementary Notes 1 to 3, further comprising a phase adjustment mechanism for adjusting an initial phase difference between the first optical waveguide and the second optical waveguide. The semiconductor Mach-Zehnder optical modulator according to the item.

(Additional remark 6) The said loss adjustment part is
Note that the first optical transmission line is provided in a portion that is not covered by the first electrode, or a portion that is not covered by the second electrode in the second optical transmission line. The semiconductor Mach-Zehnder optical modulator according to any one of 1 to Appendix 5.

(Supplementary note 7) The semiconductor Mach-Zehnder optical modulator according to any one of supplementary notes 1 to 6,
The semiconductor Mach-Zehnder optical modulator is applied with a modulation voltage having the same amplitude but opposite amplitude of the time change rate at the same time, to the first electrode and the second electrode of the semiconductor Mach-Zehnder optical modulator. A drive circuit for driving
An optical transmission device comprising:

(Additional remark 8) The optical waveguide formation process which forms the 1st and 2nd optical waveguide which has a semiconductor quantum well structure which comprises a Mach-Zehnder interferometer on a semiconductor substrate,
A part of at least one of the first optical waveguide and the second optical waveguide is a loss adjustment unit having a light absorption coefficient larger than that of the other part of the optical waveguide, The shape is such that the propagation loss of the first optical waveguide coincides with the propagation loss of the second optical waveguide when a voltage having a minimum intensity is applied to the first electrode and the second electrode. A loss adjusting part forming step for forming a defined loss adjusting part;
A method of manufacturing a semiconductor Mach-Zehnder type optical modulator.

(Additional remark 9) The process of forming the mask in which the constituent material of the quantum well of the said 1st and 2nd optical waveguide is not laminated | stacked on the vicinity of the part which should form the said loss adjustment part of the said semiconductor substrate, In addition,
As the optical waveguide forming step and the loss adjusting portion forming step,
By depositing the constituent material of the quantum well, on the semiconductor substrate on which the mask is formed, a part of at least one of the optical waveguides functions as the loss adjusting unit, as compared to the other part of the quantum well. 9. The semiconductor Mach-Zehnder type light according to appendix 8, wherein a step of forming first and second optical waveguides having a semiconductor quantum well structure, which forms a Mach-Zehnder interferometer, in which a thick layer portion exists is performed. Modulator manufacturing method.

(Supplementary Note 10) A method of driving a semiconductor Mach-Zehnder optical modulator,
The semiconductor Mach-Zehnder optical modulator is configured to apply a modulation voltage having the same amplitude and opposite amplitudes to the two optical waveguides constituting the Mach-Zehnder interferometer at the same time. A method for driving a semiconductor Mach-Zehnder optical modulator, comprising: driving a type optical modulator.

(Supplementary Note 11) The semiconductor Mach-Zehnder optical modulator includes:
11. The method of driving a semiconductor Mach-Zehnder optical modulator according to appendix 10, wherein the modulator has equal propagation loss in both optical waveguides when a voltage that minimizes the intensity of the output light is applied to each optical waveguide.

DESCRIPTION OF SYMBOLS 10 Semiconductor MZ light modulator 11 ₁ , 11 ₂ arm 12 Optical distributor 13 Optical coupler 14 Loss adjustment part 15 Electrode 15a Pad 17 Light source 18 Drive circuit 19 Heater electrode 21 n-InP substrate 22, 32 SiO ₂ film 23 Selective growth Mask region 24 InP buffer layer 25 MQW layer 26 InP cladding layer 27 InGaAs contact layer 31 SI-InP layer 33 Au / Zn / Au electrode 34 Au electrode 35 AuGe / Au film 36 Au plating layer 37 Back electrode 40 Resist pattern 41 Mask pattern 42,44 resist mask 42a opening

Claims (8)

First and second optical waveguides having a semiconductor quantum well structure forming a Mach-Zehnder interferometer;
A first electrode for applying a voltage to the first optical waveguide;
A second electrode for applying a voltage to the second optical waveguide;
With
A part of at least one of the first optical waveguide and the second optical waveguide is a loss adjustment unit having a light absorption coefficient larger than that of the other part of the optical waveguide, The shape is such that the propagation loss of the first optical waveguide coincides with the propagation loss of the second optical waveguide when a voltage having a minimum intensity is applied to the first electrode and the second electrode. A semiconductor Mach-Zehnder type optical modulator, characterized in that a predetermined loss adjusting unit is provided. The loss adjusting unit is
2. The semiconductor Mach-Zehnder optical modulator according to claim 1, wherein the semiconductor Mach-Zehnder optical modulator has a layer thickness larger than that of the quantum well of the other portion of the optical waveguide. At least one of the first optical waveguide and the second optical waveguide is
2. The semiconductor Mach-Zehnder optical modulator according to claim 1, wherein the semiconductor Mach-Zehnder optical modulator includes a quantum well structure including a portion functioning as the loss adjusting unit and having a thickness that continuously changes along a light propagation direction. 4. The initial phase difference between the first optical waveguide and the second optical waveguide is π / 2 + 2nπ (where n is an integer) radians. 5. The semiconductor Mach-Zehnder optical modulator according to the item. A semiconductor Mach-Zehnder optical modulator according to any one of claims 1 to 4,
The semiconductor Mach-Zehnder optical modulator is applied with a modulation voltage having the same amplitude but opposite amplitude of the time change rate at the same time, to the first electrode and the second electrode of the semiconductor Mach-Zehnder optical modulator. A drive circuit for driving
An optical transmission device comprising: An optical waveguide forming step of forming first and second optical waveguides having a semiconductor quantum well structure, which forms a Mach-Zehnder interferometer on a semiconductor substrate;
A part of at least one of the first optical waveguide and the second optical waveguide is a loss adjustment unit having a light absorption coefficient larger than that of the other part of the optical waveguide, The shape is such that the propagation loss of the first optical waveguide coincides with the propagation loss of the second optical waveguide when a voltage having a minimum intensity is applied to the first electrode and the second electrode. A loss adjusting part forming step for forming a defined loss adjusting part;
A method of manufacturing a semiconductor Mach-Zehnder type optical modulator. Providing a mask in which the constituent material of the quantum well is not stacked thereon in the vicinity of the portion where the loss adjusting portion of the semiconductor substrate is to be formed, and then depositing the constituent material of the quantum well on the semiconductor substrate, The method of manufacturing a semiconductor Mach-Zehnder type optical modulator according to claim 7, further comprising a step of forming the layer so that the layer thickness is larger than that of the quantum well in the portion. First and second optical waveguides having a semiconductor quantum well structure forming a Mach-Zehnder interferometer;
A first electrode for applying a voltage to the first optical waveguide;
A second electrode for applying a voltage to the second optical waveguide;
With
A part of at least one of the first optical waveguide and the second optical waveguide is a loss adjustment unit having a light absorption coefficient larger than that of the other part of the optical waveguide, The shape is such that the propagation loss of the first optical waveguide coincides with the propagation loss of the second optical waveguide when a voltage having a minimum intensity is applied to the first electrode and the second electrode. A specified loss adjustment section is provided.
A method of driving a semiconductor Mach-Zehnder type optical modulator, characterized in that
A modulation voltage having the same amplitude and opposite amplitude of the time change rate at the same time is applied to the first and second optical waveguides constituting the Mach-Zehnder interferometer of the semiconductor Mach-Zehnder type optical modulator. Thus, the semiconductor Mach-Zehnder optical modulator is driven. A method for driving a semiconductor Mach-Zehnder optical modulator.

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