JPH0921985A - Mach-zehunder modulator and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a Mach-Zehunder modulator which has resistance to the deviation in optical coupling to optical fibers and exhibits a stable extinction characteristic. SOLUTION: An incident light waveguide 111 in front of a branching part 113 is partly provided with a filter structure 112 for waveform shaping of incident guided light to shape the waveform of the incomplete waveguide light generated by the deviation in the optical coupling to the optical fibers before the light arrives at the branching part. As a result, the differences in the light intensity, initial phase, etc., between the two branched light rays are decreased and the deterioration of the extinction characteristic is suppressed. The allowance in a packaging stage can be enhanced and the yield can be improved by adopting such structure.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an important element in an optical communication system or an optical information processing system.
In particular, the present invention relates to a structure and manufacturing method of a Mach-Zehnder modulator having various performances such as extinction ratio and operating voltage that are stable against misalignment with an optical fiber.

[0002]

2. Description of the Related Art A waveguide type optical device such as a semiconductor laser and an optical modulator is considered to be one of the key elements of a high speed optical communication system and an optical information processing system, and research and development have been activated in various places.

In recent years, as optical communication systems have become faster and longer distances, the problems of the conventional direct modulation system using semiconductor lasers have become apparent. That is, in the semiconductor laser direct modulation method, wavelength chirping occurs during modulation, which deteriorates the waveform after fiber transmission. This phenomenon becomes more remarkable as the signal transmission speed increases and the transmission distance increases. In particular, this problem is serious in the existing system using 1.3 μm zero-dispersion fiber, and even if an attempt is made to extend the transmission distance using a light source in the wavelength 1.55 μm band with a small fiber propagation loss, it is caused by chirping. The dispersion distance limits the transmission distance.

This problem can be solved by causing the semiconductor laser to emit light with a constant light output and adopting an external modulation method in which the emitted light of the semiconductor laser is modulated by an optical modulator different from the semiconductor laser. Therefore, in recent years, the development of the external optical modulator has been activated.

As the external optical modulator, one using a dielectric such as LiNbO ₃ and one using a semiconductor such as InP or GaAs can be considered, but other optical elements such as an optical amplifier and electronic elements such as FET. In recent years, expectations have been rising for a semiconductor optical modulator that can be integrated with a circuit and can be easily downsized and reduced in voltage.

As a semiconductor optical modulator, the Franz-Keldysh Effect of a bulk semiconductor is used.
Absorption type optical modulators that utilize the effect of shifting the absorption edge to the long-wave side by applying an electric field, such as the quantum confined Stark effect in multiple quantum well structures, and the electro-optic effect (Pockels effect) of bulk semiconductors and A typical Mach-Zehnder type modulator uses the effect of changing the refractive index by applying an electric field like the quantum confined Stark effect in a multiple quantum well structure.

By the way, the absorption modulator has a wavelength chirp that is much smaller than that of the semiconductor laser direct modulation system, but it is still not zero.

On the other hand, the Mach-Zehnder modulator can make the chirping zero or negative in principle, and is expected to be a modulator for future ultrahigh-speed long-distance optical communication.

As an example of the semiconductor Mach-Zehnder modulator, a high-mesa-type Mach-Zehnder modulator using an InGaAs / InAlAs multiple quantum well as a waveguiding layer is described by Sano et al.
Proceedings of the 93rd IEICE Spring Conference, Volumes 4, 4
-186, Lecture No. C-150).

In this report example, for an incident light wavelength of 1.55 μm, an MQA of 6.5 nm and an InAlAs barrier of 6.0 nm are used for MQ.
W (Multi-Quantum Well) is configured and its bandgap wavelength is 1.45 μm.

The waveguide layer is composed of this InGaAs / InAlAs MQW30 period, the element length is 1.2 mm, and the two phase modulator sections to which an electric field is applied have the same length (0.5 mm) and the same structure. There is.

In this device, the modulation voltage (half-wavelength voltage) for incident light with a wavelength of 1.55 μm is 4.2 V, the extinction ratio at that time is 13 dB, and the interfiber insertion loss is 12 dB.

As described above, when a Mach-Zehnder modulator is constructed by using a semiconductor material, in particular, a multi-quantum well structure as a refractive index changing medium, the size is several tens of mm when a dielectric material such as LiNbO ₃ is used. The advantage is that a very small modulator with a size of 1 mm can be realized.

By the way, these single Mach-Zehnder modulators are usually installed between optical fibers, and their input and output ends are optically spatially coupled with the optical fibers.

However, in this structure, the optical coupling is apt to be displaced due to the disturbance in the mounting process and other factors,
There is concern that the extinction characteristic of the modulator may deteriorate.

Therefore, in an actual optical communication system, it is desired to realize a modulator having resistance to the optical coupling deviation.

[0017]

As described above, in the conventional semiconductor Mach-Zehnder modulator, the modulator is installed between the optical fibers and the optical fibers are optically spatially coupled to each other. There is concern about the deterioration of various characteristics due to the positional deviation between the modulator and the input / output waveguide end.

A light wave incident in a misaligned state propagates while meandering through an input waveguide and is branched in an asymmetrical state at a branching portion. That is, the two branched lights have different light intensities and initial phases.

The difference in the light intensity between the branched lights reduces the interference effect and causes the extinction ratio to deteriorate.

Further, the initial phase difference between the branched lights shifts the operating point for the modulated signal, and similarly deteriorates the extinction characteristic.

Therefore, in the actual application to the optical communication system, there is a problem that the optical coupling shift due to the disturbance or other factors significantly lowers the reliability of the system.

According to the present invention, in order to solve the above problem, if the inconsistent evanescent component caused by the deviation of the optical coupling between the optical fiber and the input waveguide is removed in front of the branch portion, the meandering light wave is shaped. The present invention has been made on the basis of the finding that it is good, and an object of the present invention is to have resistance to the coupling deviation by adding a function of shaping the light wave generated by the optical coupling deviation in front of the branching portion. It is to provide a structure and a manufacturing method of a Mach-Zehnder modulator.

[0023]

In order to achieve the above object, the present invention provides, on a substrate, an incident optical waveguide for guiding incident light and a branching portion for distributing the guided light into two waveguides. A phase modulator section connected to each of the two waveguides branched from the branch section, a multiplexer section for combining the outputs of the two phase modulators, and an output optical waveguide for outputting the combined light are formed. A waveguide type Mach-Zehnder modulator, which has an optical filter section between the incident optical waveguide and the branch section in order to remove the incomplete waveguide component in front of the branch section and shape the waveform of the guided light. A Mach-Zehnder modulator is provided.

Further, according to the present invention, an incident optical waveguide for guiding incident light, a branch portion for distributing the guided light into two waveguides, and two waveguides branched from the branch portion are provided on the substrate. A waveguide type Mach-Zehnder modulator comprising a phase modulator section connected to each of the two and a combiner section for combining the outputs of two phase modulators and an output optical waveguide for outputting the combined light, Provided is a Mach-Zehnder modulator in which an optical waveguide having a width different from that of the optical waveguide is inserted between the optical waveguide and the branch portion.

Further, according to the present invention, an incident optical waveguide for guiding incident light, a branch portion for distributing the guided light into two waveguides, and two waveguides branched from the branch portion are provided on the substrate. A waveguide type Mach-Zehnder modulator comprising a phase modulator section connected to each of the two and a combiner section for combining the outputs of two phase modulators and an output optical waveguide for outputting the combined light, Provided is a Mach-Zehnder modulator characterized in that an optical waveguide having a width narrower than the width of the incident optical waveguide is inserted between the incident optical waveguide and the front of the branch portion.

The present invention is preferably characterized in that lithium niobate is used as the substrate.

Further, in the present invention, preferably, a semiconductor material is used as the substrate.

According to the present invention, a semiconductor buffer layer, a semiconductor waveguiding layer and a semiconductor clad layer are sequentially laminated on a semiconductor substrate, and then a dielectric mask for selective growth is formed on the semiconductor clad layer. , A semiconductor clad layer and a semiconductor cap layer are selectively formed in the void formed by using the dielectric mask, and an electric field is applied to the semiconductor waveguide layer of the phase modulator section of the Mach-Zehnder modulator on the semiconductor cap layer. An electrode for applying is included, including each step, in the dielectric mask for selective growth, the incident optical waveguide and the width as an optical waveguide between the incident optical waveguide and the branch portion of the Mach-Zehnder modulator. Provided is a method for manufacturing a Mach-Zehnder modulator, which is characterized in that patterns are formed so that different optical waveguides are formed.

Further, the present invention comprises (a) a step of sequentially laminating a semiconductor buffer layer, a semiconductor waveguide layer and a semiconductor clad layer on a semiconductor substrate, and (b) a dielectric mask for selective growth on the clad layer. A step of (c) selectively forming a semiconductor clad layer and a semiconductor cap layer in the void formed by the dielectric mask, and (d) a predetermined dielectric protective film on the entire upper surface of the substrate. And (e) a step of removing the dielectric protective film in a portion corresponding to the phase modulator part of the Mach-Zehnder modulator, and (f) the exposed dielectric protective film being removed. A step of forming an electrode for applying an electric field to the semiconductor waveguide layer of the phase modulator portion on the semiconductor cap layer, and (g) a step of removing the dielectric protective film other than the electrode lower portion, In the dielectric mask, the semiconductor Mach The width of the void portion of the dielectric mask is made different so that an optical waveguide having a different width between the incident optical waveguide and the optical waveguide is formed between the incident optical waveguide and the branch portion of the Der modulator. A method for manufacturing a characteristic Mach-Zehnder modulator is provided.

[0030]

The principle and operation of the semiconductor Mach-Zehnder modulator of the present invention will be described below with reference to FIGS. 5, 6, 7 and 8.

FIGS. 5 and 6 show how a light wave having an optical coupling shift propagates in the conventional Mach-Zehnder modulator and the Mach-Zehnder modulator of the present invention to which a filter function is added, respectively.

7 and 8 show the light intensity ratio and the initial phase difference of two branched lights corresponding to the conventional Mach-Zehnder modulator and the Mach-Zehnder modulator of the present invention to which a filter function is added, respectively. ing. The light intensity ratio (power splitting ratio) of FIG. 7 is 10 log (s) when the powers of the two split lights split by the splitting unit (Y-Splitter) are P1 and P2, respectively, as shown in FIG. P1 / P
Given in 2). Further, the initial phase difference in FIG. 8 is | when the phases of the two branched lights in FIG. 9 are φ1 and φ2, respectively.
It is given by φ1-φ2 |.

In the Mach-Zehnder modulator of the present invention, the guided light is waveform-shaped by irradiating the mismatched evanescent component in the filter portion provided in the preceding stage of the branching portion, and the two light waves generated in the branching portion. The difference between the intensity ratio and the initial phase difference is small, and the symmetry of the branched light is maintained.

On the other hand, in the conventional Mach-Zehnder modulator, the meandering of the guided light causes a large difference in the intensity ratio and the initial phase difference between the two branched lights, resulting in asymmetry between the branched lights.

As described above, the Mach-Zehnder modulator according to the present invention has a filter function for removing a mismatched light wave component in front of the branching portion, and the meandering incomplete guiding caused by the optical coupling deviation. After the wave light is shaped, it is sent to the branch.

As a result, the Mach-Zehnder modulator of the present invention has a structure capable of suppressing the light intensity difference and the initial phase difference generated between the two branched lights and preventing the deterioration of the extinction characteristic caused by the coupling shift. ing.

[0037]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a perspective view for explaining the structure of a semiconductor Mach-Zehnder modulator having an InP multiple quantum well as a guide layer as a Mach-Zehnder modulator according to an embodiment of the present invention.

First, a method of manufacturing the semiconductor Mach-Zehnder modulator shown in FIG. 1 will be described with reference to FIGS. 2A to 3F schematically show cross-sections along the line AA ′ shown in FIG. 1 in the order of manufacturing steps.

First, the (100) plane oriented n-InP substrate 10
N-Inp buffer layer 102 (film thickness 0.5 μm,
Carrier concentration 5 × 10 ¹⁷ cm ^-3 ), i-InGaAs PMQW waveguide layer (guide layer) 103 (InGaAsP (10 nm) / InP (5 nm) 20 periods: film thickness 0.3 μm, carrier concentration 5 × 10 ¹⁵ cm ^-3 ), I-InP
Clad layer 104 (layer thickness 0.05 μm, carrier concentration 5 × 10
¹⁵ cm ⁻³ ) are sequentially laminated (see FIG. 2 (A)).

Next, a SiO ₂ film having a film thickness of 1200 Å (120 nm) to be a dielectric mask for selective growth is formed on the entire surface of the i-InP clad layer 104. Then, the SiO ₂ film is patterned by using a normal photolithography technique to form a SiO ₂ mask 201 for selective growth (FIG. 2).
(B)).

FIG. 4 shows a pattern of the selective growth SiO ₂ mask 201 seen from the upper surface of the wafer.

In order to form the filter portion 112 having a narrow waveguide width in front of the branch portion 113 (see FIG. 1), as shown in FIG. 4, the shape of the SiO ₂ mask 201 for selective growth has a corresponding position. The mask opening is narrowed.

Next, using the SiO ₂ film as a mask, p-InP cladding layer 10 is formed in the voids by selective MOVPE growth.
5, p-InGaAs cap layer 106 is sequentially laminated (see FIG. 2).
(C)).

Subsequently, a polyimide film is applied to the entire surface of the wafer, and the polyimide film is uniformly removed until the p-InGaAs cap layer 106 is exposed (see FIG. 2D).

On the exposed p-InGaAs cap layer 106,
The p-side electrode 109 made of Cr / Au is formed, and the p-side electrode 10 is formed.
9. The polyimide film except the lower part is removed (FIG. 3 (E)).
By removing the polyimide film other than the lower part of the p-side electrode 109, the subsequent cleaving process of the element is facilitated.

Next, the n-side electrode 108 made of Cr / Au is formed on the back surface of the n-InP substrate 101 (see FIG. 3F).

Finally, the device is cleaved, and the cleaved end face is coated with a non-reflective coating to complete the device fabrication.

FIG. 5 shows the calculation result of the optical waveguide when the fundamental mode of the light wave is shifted and incident on the end of the incident waveguide in the semiconductor Mach-Zehnder modulator shown in FIG. 1 manufactured by the above manufacturing method. Show.

FIG. 6 shows how light waves propagate when no filter structure is introduced.

From the comparison between FIG. 5 and FIG. 6, according to the present embodiment in which the filter structure is introduced, the meandering of the light wave is suppressed in the front of the branching portion, and the asymmetry of the two guided light intensities after the branching is improved. There is.

Further, FIGS. 7 and 8 show the calculation results of the intensity ratio and the phase difference between the two guided lights immediately after the branching portion, respectively.

In the structure of this embodiment having the filter function, the difference in both the light intensity ratio and the phase difference is reduced as compared with the case where the filter function is not introduced, and the symmetry is maintained.

As described above, the Mach-Zehnder modulator of the present embodiment having the filter structure in which a part of the width of the waveguide in the front part (front side) of the branching part is narrowed is the optical coupling deviation with the optical fiber. It is possible to secure reliability when constructing an optical communication system.

Further, in the method of manufacturing the semiconductor Mach-Zehnder modulator of the present embodiment, the ridge waveguide including the filter portion is collectively formed by using the selective MOVPE growth method, and the etching step is unnecessary (that is, the cladding layer). Since 105, the cap layer 106 and the like are formed by selective growth and an etching process is not necessary), the Mach-Zehnder modulator of this embodiment can be realized uniformly and with good reproducibility on the same wafer.

In the Mach-Zehnder modulator shown in FIG. 1, since the polyimide film 601 is thickly laminated under the pad electrode in order to reduce the electrode capacitance, it has a structure capable of high speed modulation. .

The present invention is not limited to the above embodiment. In the above embodiment, the Mach-Zehnder modulator having an InP-based multiple quantum well structure has been described as an example.
The present invention is not limited to this, but Al-based InGaAs / In
It can also be applied to AlAs multiple quantum wells. Further, the present invention can be similarly applied to a Mach-Zehnder modulator having a bulk type waveguide structure as well as a multiple quantum well type waveguide structure. Further, it is needless to say that the present invention is not limited to the element shape shown in the above-described embodiment, that is, the thickness of each layer, the composition of each layer, the waveguide size, and the like.

[0058]

As described above, according to the present invention,
There is an effect that it is possible to realize a Mach-Zehnder modulator having a highly reliable modulation operation by suppressing the deterioration of the extinction characteristic due to the optical coupling deviation with the optical fiber.

In particular, in the manufacture of a semiconductor Mach-Zehnder modulator, the present invention does not use a semiconductor etching process, but patterns a thin dielectric film (for example, SiO ₂ ) serving as a mask for the difference in selective growth by etching and selects it. A waveguide structure is formed by dynamic crystal growth. Therefore, fine patterning of a thin dielectric film over a large area with good reproducibility is much easier than etching a semiconductor to a depth of the order of μm. Therefore, according to the manufacturing method of the present invention, a Mach-Zehnder modulator having resistance to deterioration of extinction characteristics against optical coupling deviation can be manufactured with good reproducibility and over a wide area on a wafer.

[Brief description of drawings]

FIG. 1 is a perspective view showing the structure of an InGaAsP / InP multiple quantum well Mach-Zehnder modulator which is an embodiment of the present invention.

FIG. 2 is a diagram for explaining the manufacturing method of the InGaAsP / InP multiple quantum well Mach-Zehnder modulator according to the embodiment of the present invention in the order of manufacturing steps.

FIG. 3 is a diagram for explaining the manufacturing method of the InGaAsP / InP multiple quantum well Mach-Zehnder modulator according to the embodiment of the present invention in the order of manufacturing steps.

FIG. 4 is a plan view showing an example of a mask pattern for selective growth when manufacturing an InGaAsP / InP multiple quantum well Mach-Zehnder modulator according to an embodiment of the present invention.

FIG. 5 is a perspective view of a calculation result showing a state of light wave propagation in the Mach-Zehnder modulator of one embodiment of the present invention having a filter structure.

FIG. 6 is a perspective view of a calculation result showing a state of light wave propagation in a conventional Mach-Zehnder modulator.

FIG. 7 is a diagram showing a calculation result of a light intensity ratio between split lights immediately after a split portion in a Mach-Zehnder modulator according to an embodiment of the present invention and a conventional Mach-Zehnder modulator.

FIG. 8 is a diagram showing a calculation result of a phase difference between branched lights immediately after a branching portion in a Mach-Zehnder modulator according to an embodiment of the present invention and a conventional Mach-Zehnder modulator.

FIG. 9 is a diagram for schematically explaining the relationship between the power and the phase difference between the split lights immediately after the split unit (Y-Splitter).

[Explanation of symbols]

101 n-InP substrate 102 n-InP buffer layer 103 i-InGaAsP / InP MQW guide layer 104 i-InP clad layer 105 p-InP clad layer 106 p-InGaAs cap layer 108 n-side electrode 109 p-side electrode 601 polyimide film 111 Incident optical waveguide 112 Filter section 113 Branching section 114 Phase modulator section 115 Multiplexing section 116 Output optical waveguide section 201 SiO ₂ mask for selective growth

Claims (8)

[Claims] 1. An incident optical waveguide for guiding incident light on a substrate, a branch portion for distributing the guided light into two waveguides, and two waveguides branched from the branch portion, respectively. A waveguide-type Mach-Zehnder modulator including: a phase modulator section, a multiplexing section for combining outputs of the two phase modulators, and an output optical waveguide for outputting combined light. A Mach-Zehnder modulator having, between the incident optical waveguide and the branching section, an optical filter section for removing an incomplete guided wave component in front of the branching section and shaping the waveform of the guided light. 2. An incident optical waveguide for guiding incident light, a branching portion for distributing the guided light into two waveguides, and two waveguides branched from the branching portion are respectively connected on a substrate. A waveguide-type Mach-Zehnder modulator including: a phase modulator section, a multiplexing section for combining outputs of the two phase modulators, and an output optical waveguide for outputting combined light. A Mach-Zehnder modulator, wherein an optical waveguide having a different width between the incident optical waveguide and the optical waveguide is inserted between the incident optical waveguide and the branch portion. 3. An incident optical waveguide for guiding incident light, a branch portion for distributing the guided light into two waveguides, and two waveguides branched from the branch portion are respectively connected on a substrate. A waveguide-type Mach-Zehnder modulator including: a phase modulator section, a multiplexing section for combining outputs of the two phase modulators, and an output optical waveguide for outputting combined light. A Mach-Zehnder modulator characterized in that an optical waveguide having a width narrower than that of the incident optical waveguide is inserted between the incident optical waveguide and the front of the branch portion. 4. The Mach-Zehnder modulator according to claim 1, wherein the substrate is made of lithium niobate. 5. The Mach-Zehnder modulator according to claim 1, wherein the substrate is made of a semiconductor material. 6. A semiconductor buffer layer, a semiconductor waveguide layer, and a semiconductor clad layer are sequentially stacked on a semiconductor substrate, and a dielectric mask for selective growth is formed on the semiconductor clad layer. Second in the void formed using the mask
Selectively forming a semiconductor clad layer and a semiconductor cap layer, and providing an electrode for applying an electric field to the semiconductor waveguide layer of the phase modulator section of the Mach-Zehnder modulator on the semiconductor cap layer. The dielectric mask for selective growth is patterned so that an optical waveguide having a width different from that of the incident optical waveguide is formed as an optical waveguide between the incident optical waveguide and the branch portion of the Mach-Zehnder modulator. A method of manufacturing a Mach-Zehnder modulator characterized by the above. 7. A step of sequentially laminating a semiconductor buffer layer, a semiconductor waveguide layer, and a semiconductor clad layer on a semiconductor substrate, and a step of forming a dielectric mask for selective growth on the clad layer. (C) a step of selectively forming a semiconductor clad layer and a semiconductor cap layer in the void formed by the dielectric mask, and (d) a step of forming a predetermined dielectric protective film on the entire upper surface of the substrate. And (e) a step of removing the dielectric protective film in a portion corresponding to the phase modulator portion of the Mach-Zehnder modulator, and (f) removing the dielectric protective film and exposing the exposed semiconductor cap layer. The step of forming an electrode for applying an electric field to the semiconductor waveguiding layer of the phase modulator section above, and (g) the step of removing the dielectric protective film other than the lower part of the electrode, wherein: In the body mask, semiconductor Mach-Zehnder modulator The Mach is characterized in that the widths of the void portions of the dielectric mask are made different so that optical waveguides having different widths are formed between the incident optical waveguide and the branch portion. Manufacturing method of Zender modulator. 8. The predetermined dielectric protective film in the step (d) includes at least a polyimide film, and in the step (g), the polyimide film other than the lower portion of the electrode is removed. 7. A method for manufacturing a Mach-Zehnder modulator according to 7.