JP2016194655A - Optical waveguide device - Google Patents

Abstract

The present invention provides an optical waveguide element in which stress of a protective film for an optical waveguide multiplexer / demultiplexer is relaxed to suppress deterioration of signal quality. An optical waveguide element having an optical waveguide layer formed on a semiconductor substrate 401 and laminated in order of a lower clad layer 403, a core layer 404, and an upper clad layer 405, the optical waveguide layer on the semiconductor substrate 401 An optical waveguide multiplexer / demultiplexer formed as a part of the optical waveguide layer, a first protective film 411 formed to cover the optical waveguide layer and the optical waveguide multiplexer / demultiplexer, and the first protective film 411 And second protective films 410a and 410b formed only on the portion where the optical waveguide layer is formed. [Selection] Figure 5

Description

  The present invention relates to an optical waveguide device, and more particularly to an optical waveguide device including an optical waveguide type multiplexer / demultiplexer used in an optical integrated circuit.

  Conventionally, a DP-QPSK (Dual Polarization-Quadrature Phase Shift Keying) method is known as an optical modulation method for large-capacity optical communication. As a DP-QPSK optical modulator, research and development of an optical modulator using an optical waveguide made of an InP-based compound semiconductor has been actively conducted (for example, see Non-Patent Document 1).

  FIG. 1 shows a configuration of a conventional QPSK optical modulator. In the QPSK optical modulator, a Mach-Zehnder Interferometer (MZI) has a two-layer nesting structure. It consists of one parent MZI 101 included in the waveguide. This QPSK optical modulator modulates one polarization of the input optical signal, and another set of QPSK optical modulators modulates the other polarization to form a DP-QPSK optical modulator. .

  The child MZI 102 includes two 1 × 2 multimode interferometers (MMI) 121 and 122 on the input side and the output side, and arm waveguides 123 and 124 connecting them. Similarly, the child MZI 103 includes two 1 × 2 MMIs 131 and 132 and arm waveguides 133 and 134 that connect them. The parent MZI 101 includes 1 × 2 MMI 111 and 2 × 2 MMI 112 and arm waveguides 113 and 114 including children MZIs 102 and 103 connecting them.

  In each of the arm waveguides of the child MZIs 102 and 103, an electrode for applying an electric field to the optical waveguide layer is formed. By applying a voltage to these electrodes, the refractive index of the optical waveguide layer is changed via the Pockels effect and the quantum confined Stark effect (QCSE). Due to this refractive index change, the child MZIs 102 and 103 generate phase states of 0 and π, respectively. The 2 × 2 MMI 112 of the parent MZI 101 synthesizes the two phase states generated by the child MZIs 102 and 103 in an orthogonal state. Specifically, four phase states are generated by synthesizing by giving a phase difference of π / 2 to one output of the child MZI.

  FIG. 2 is a cross-sectional view of an electric field application region in a conventional optical modulator. The optical modulator includes an SI (Semi-Insulating) -InP substrate 201, and in that order from the substrate surface, an n-type InP lower cladding layer 202, a non-doped multi-quantum well (MQW) core layer 203, A non-doped InP clad layer 204, a p-type clad layer 205, and an n-type InP clad layer 206 are stacked (for example, see Non-Patent Document 2). The optical waveguide layer has a high mesa structure above the n-type InP lower cladding layer 202 to realize light confinement in the horizontal direction of the substrate. The high mesa structure has a width of about 2 μm and a height of about 3 μm.

  Furthermore, in order to protect the high mesa structure which is an optical waveguide layer, benzocyclobutene (BCB: Benzocyclobutene) layers 209a and 209b are formed so as to cover the high mesa structure (for example, see Non-Patent Document 3). The thickness of the BCB layer 209 is about 0.3 μm on the upper surface of the high mesa structure and about 4 μm on the side surface. The electrode 207 for applying an electric field along the light guiding direction is formed so as to be electrically connected to the n-type InP cladding layer 206 by partially removing the BCB layer 209 on the n-type InP cladding layer 206. Has been. In addition, ground electrodes 208a and 208b are formed on the n-type InP lower clad layer 202 on both sides of the high mesa structure.

  FIG. 3 is a cross-sectional view of a non-electric field application region in a conventional optical modulator. The structure of the optical waveguide layer is the same as the cross section of the electric field application region shown in FIG. The high mesa structure is completely covered by the BCB layer 209 on both the side surface and the upper surface.

E. Yamada et al., “112-Gb / s InP DP-QPSK Modulator Integrated with a Silica-PLC Polarization Multiplexing Circuit”, PDP5A. 9, Mar., OFC 2012, Los Angeles N. Kikuchi et al., “80-Gb / s low-driving-voltage InP DQPSK modulator with an n-p-i-n Structure”, IEEE Photonics Technology letters, Vol. 21, No. 12, June 2009, pp. 787-789. Hideki Yagi et al., “1.3Gam AlGaInAs / InP Ridge Waveguide Laser Using BCB Planarization Process”, July 2009 SEI Technical Review No. 175, pp. 120-123

  In the conventional optical modulator, the arm waveguide, which is the optical waveguide layer, 1 × 2 MMI, and 2 × 2 MMI are all covered with the BCB layer. That is, the BCB layer 209 having the same height as the high mesa waveguide was also formed around the 2 × 2 MMI 112 (see FIG. 1).

  BCB has a thermal expansion coefficient of about 42 ppm / ° C., whereas InP has a thermal expansion coefficient of about 4.5 ppm / ° C., and BCB has a thermal expansion coefficient that is an order of magnitude greater than that of InP. As described above, the dimension of the high mesa structure and the thickness of the BCB layer are approximately the same, and an extremely large stress is generated inside the InP high mesa structure covered with the BCB layer due to the difference in thermal expansion coefficient between InP and BCB. . As a result, the equivalent refractive index in the waveguide layer having the high mesa structure changes via the photoelastic effect. This equivalent refractive index change has a problem in that the interference condition between a plurality of modes inside the 2 × 2 MMI is shifted from the design value and a coupling rate shift of 2 × 2 MMI is generated.

  In the DP-QPSK optical modulator, when the two phase states generated by the child MZIs 102 and 103 are combined by the parent MZI 101, if a coupling ratio shift of 2 × 2 MMI occurs, the two phase states are combined with an equal power ratio. It will not be possible. As a result, the phase difference from the child MZIs 102 and 103 deviates from the orthogonal state (90 degrees), which is a factor that degrades the signal quality during modulation.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an optical waveguide device in which stress of a protective film for an optical waveguide multiplexer / demultiplexer is relaxed to suppress deterioration of optical characteristics.

  In order to achieve such an object, an embodiment of the present invention provides an optical waveguide element having an optical waveguide layer formed on a semiconductor substrate and laminated in the order of a lower cladding layer, a core layer, and an upper cladding layer. An optical waveguide multiplexer / demultiplexer formed as a part of the optical waveguide layer on the semiconductor substrate, and a first formed so as to cover the optical waveguide layer and the optical waveguide multiplexer / demultiplexer. And a second protective film formed on the first protective film, the second protective film being formed only on the portion where the optical waveguide layer is formed. It is characterized by.

  According to the present invention, since the portion where the optical waveguide multiplexer / demultiplexer is formed is not covered with the second protective film, the stress on the multiplexer / demultiplexer can be reduced, and the optical modulator In the optical waveguide type multiplexer / demultiplexer, the two phase states of the input optical signal can be synthesized with an equal power ratio.

It is a figure which shows the structure of the conventional QPSK optical modulator. It is sectional drawing of the electric field application area | region in the conventional optical modulator. It is sectional drawing of the non-electric field application area | region in the conventional optical modulator. It is a figure which shows the structure of the QPSK optical modulator concerning one Embodiment of this invention. It is sectional drawing of the electric field application area | region in the optical modulator of this embodiment. It is sectional drawing of the non-electric field application area | region in the optical modulator of this embodiment. It is sectional drawing of 2x2 MMI in the optical modulator of this embodiment. It is a figure which shows the structure of 2x2 MMI in the optical modulator of this embodiment.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In this embodiment, a DP-QPSK optical modulator will be described as an example of an optical waveguide element including an optical waveguide multiplexer / demultiplexer used in an optical integrated circuit. However, an optical integrated circuit including an optical waveguide multiplexer / demultiplexer Obviously, it can be applied to any optical waveguide device.

  FIG. 4 shows a configuration of a QPSK optical modulator according to an embodiment of the present invention. The QPSK optical modulator has a Mach-Zehnder Interferometer (MZI: Mach-Zehnder Interferometer) with a two-layered nesting structure. It consists of one parent MZI 301 included in the waveguide.

  The child MZI 302 includes two 1 × 2 multimode interferometers (MMI) 321 and 322 on the input side and the output side, and arm waveguides 323 and 324 connecting them. Similarly, the child MZI 303 includes two 1 × 2 MMIs 331 and 332 and arm waveguides 333 and 334 that connect them. The parent MZI 301 includes arm waveguides 313 and 314 including 1 × 2 MMI 311 and 2 × 2 MMI 312 and children MZIs 302 and 303 connecting them.

  In each arm waveguide of the child MZIs 302 and 303, an electrode for applying an electric field to the optical waveguide layer is formed. The electrode of the child MZI includes a modulation electrode 325 for modulating an optical signal with a high frequency signal of several tens of GHz and a phase adjustment electrode 326 for compensating for a phase shift between the arm waveguides of the child MZI. In each arm waveguide of the parent MZI 301, the output signal from the child MZI 303 is shifted by π / 2 with respect to the output signal from the child MZI 302. Is formed.

  The modulation operation of the DP-QPSK optical modulator will be briefly described. An optical signal incident from one input waveguide 341 is branched into two by the 1 × 2 MMI 311 of the parent MZI 301 and enters the I-channel child MZI 302 and the Q-channel child MZI 303. The two children MZI are formed with a modulation electrode and a phase adjustment electrode, and a voltage can be applied to the optical waveguide layer using these electrodes. Through the Pockels effect and the quantum confined Stark effect (QCSE) generated by applying a voltage to the optical waveguide layer, the equivalent refractive index of the optical waveguide layer changes, and the phase of the optical signal propagating through the optical waveguide layer changes.

  When the voltage is not applied to the modulation electrode, the voltage is applied to the phase adjustment electrode of the child MZI so that the light output from the child MZI is in the light OFF state (extinction state). Then, the voltage applied to each modulation electrode of the two arm waveguides is switched at high speed so that the phase difference of the optical signal propagating between the two arm waveguides becomes + π or −π. As a result, when a voltage is applied to the modulation electrode, the light output from the child MZI is in the light ON state (light output state), and the phase is rapidly switched between + π and −π. In this way, the two optical signals from the phase-modulated child MZI are set so that the phase difference between the arm waveguides of the parent MZI 301 becomes π / 2 via the π / 2 phase adjustment electrode of the parent MZI 301. Driven. Then, by the 2 × 2 MMI 312 of the parent MZI 301, the two optical signals from the child MZI are combined to generate a modulated signal (QPSK signal) having four phase states and emitted from the output waveguides 342 and 343.

  FIG. 5 is a cross-sectional view of the electric field application region in the optical modulator of this embodiment. A semi-insulating (SI) -InP substrate 401 is provided, and an n-type InP lower clad layer 402, a non-doped InP clad layer 403, a multiple quantum well (MQW) core layer 404, a non-doped, in that order from the substrate surface. An InP cladding layer 405, a p-type InAlAs layer 406, and an n-type InP cladding layer 407 are stacked. The optical waveguide layer has a high mesa structure above the n-type InP lower cladding layer 402 to realize light confinement in the substrate horizontal direction. The high mesa structure has a width of about 2 μm and a height of about 3 μm.

Further, in order to protect the high mesa structure which is an optical waveguide layer formed on the SI-InP substrate 401 and other optical integrated circuits, a protective film is formed so as to cover the semiconductor structure on the SI-InP substrate 401. The SiO ₂ film 411 (first protective film) is deposited. Further, BCB layers 410a and 410b (second protective films) are formed so as to cover the high mesa structure and the SiO ₂ film 411, thereby protecting the high mesa structure and planarizing the surface of the semiconductor structure. The thickness of the SiO ₂ film 411 is about 0.4 μm, and the thickness of the BCB layer 410 is about 0.3 μm on the upper surface of the high mesa structure and about 4 μm on the side surface.

The electrode 408 for applying an electric field along the light guiding direction is obtained by removing a part of the SiO ₂ film 411 and the BCB layer 410 on the n-type InP cladding layer 407 to thereby remove the n-type InP cladding layer 407. It is formed so as to be electrically connected. In addition, ground electrodes 409a and 409b are formed on the upper side of the n-type InP lower cladding layer 402 on both sides of the high mesa structure.

FIG. 6 is a cross-sectional view of the non-electric field application region in the optical modulator of this embodiment. The structure of the optical waveguide layer is the same as the cross section of the electric field application region shown in FIG. The high mesa structure is completely covered by the SiO ₂ film 411 and the BCB layer 410 on both the side surface and the upper surface.

FIG. 7 is a cross-sectional view of 2 × 2 MMI in the optical modulator of the present embodiment. The 2 × 2 MMI 312 is an optical waveguide multiplexer / demultiplexer and has the same high mesa structure as the optical waveguide layer. The structure of the optical waveguide layer is the same as the cross section of the electric field application region shown in FIG. Here, the high mesa structure is different from the above-described cross section of the electric field application region and the non-electric field application region in that it is covered only with the SiO ₂ film 411.

That is, in the QPSK optical modulator of this embodiment, the SiO ₂ film is formed so as to cover the high mesa structure that is the optical waveguide layer, 2 × 2 MMI, and other semiconductor structures, and the high mesa structure that is the optical waveguide layer A BCB layer is formed on the SiO ₂ film only in the portion where the semiconductor structure is formed.

FIG. 8 shows a 2 × 2 MMI configuration in the optical modulator of the present embodiment. An example of the dimensions of the 2 × 2 MMI 312 is that the core thickness (vertical direction in the figure) of the MQW core layer in FIG. 7 is 0.4 μm, the width (W2) of the slab waveguide portion is 12 μm, and the length (L) is 205 μm. It is. The width (W1) of the input / output waveguide connected to the slab waveguide is 2 μm, and the center-to-center distance (d) of the input / output waveguide is 4 μm. In the 2 × 2 MMI dimensions of this embodiment, the design value of the coupling rate (branch ratio) is 50% at a wavelength of 1.55 μm. As shown in FIG. 7, when the high mesa structure of 2 × 2 MMI 312 is covered only with the SiO ₂ film 411, the coupling rate is 50% with respect to the wavelength of 1.55 μm, which is a designed value. That is, the stress generated by the SiO ₂ film 411 is relaxed, and the coupling rate as designed is obtained. The reason for this is that the thermal expansion coefficient of SiO ₂ is about 0.5 ppm / ° C. and has a thermal expansion coefficient difference from InP (about 4.5 ppm / ° C.), but the film thickness of the SiO ₂ film 411 is This is because the stress applied to the high mesa structure by the SiO ₂ film is small and negligible because it is about an order of magnitude smaller than the dimensions of the high mesa structure.

  On the other hand, as shown in FIG. 6, when the high mesa structure of 2 × 2 MMI 312 is covered with the BCB layer 410 in the same manner as the non-electric field application region, the coupling rate is 55% with respect to the wavelength of 1.55 μm. A deviation from the design value was observed.

(Optical waveguide type multiplexer / demultiplexer)
As described above, since the 2 × 2 MMI synthesizes two phase states, the coupling rate shift due to the stress generated by the difference in thermal expansion coefficient greatly affects the deterioration of signal quality. On the other hand, since 1 × 2 MMI only branches and combines optical power, the stress generated by the difference in coefficient of thermal expansion has little influence on signal quality degradation. However, even in 1 × 2 MMI, if the influence of stress can be suppressed, signal quality can be maintained, and therefore, a configuration similar to 2 × 2 MMI is desirable.

(Protective film)
In the present embodiment, a SiO ₂ film is used as a protective film that is in close contact with the high mesa structure that is the optical waveguide layer and other optical integrated circuits. However, other insulating films such as a SiN film and a SiON film can be applied as a protective film as long as a barrier effect that can prevent moisture from entering can be expected. As described above, these protective films can have a film thickness of 1 μm or less, are smaller than the dimensions of the high mesa structure, and have little stress on the optical waveguide layer. Therefore, in 2 × 2 MMI, by covering only with the SiO ₂ film 411 and removing the BCB layer, the effect of stress relaxation can be obtained, and the two phase states can be synthesized with the same power ratio.

Further, instead of the SiO ₂ film, a semi-insulating InP film that is the same kind of material as the optical waveguide layer can be used. Since the thermal expansion coefficients are equal, an effect of stress relaxation can be obtained, and deterioration of signal quality during modulation can be suppressed.

Also in this embodiment, BCB is used as a protective film. This is for the purpose of flattening the surface of the substrate on which the high mesa structure or the like is formed and for preventing damage to the optical waveguide element due to handling mistakes in the manufacturing process. is there. Such a protective film is not limited to BCB but may be other organic materials. According to the present embodiment, since 2 × 2 MMI is covered only with the SiO ₂ film, it may be a protective film having a large difference in thermal expansion coefficient from InP.

In the present embodiment, a SiO ₂ film is used as a protective film that is in close contact with the high mesa structure that is the optical waveguide layer and other optical integrated circuits. This is to prevent the semiconductor from changing its characteristics due to moisture. However, in the multimode interferometer (MMI) type wavelength multiplexer / demultiplexer, if there is little change in characteristics due to moisture, the SiO ₂ film in 2 × 2 MMI is also removed, resulting in a difference in thermal expansion coefficient between InP and SiO ₂ Even the influence of slight stress can be eliminated.

(Clad regrowth)
A p-type InAlAs layer 406 and an n-type InP cladding layer 407 are stacked on the MQW core layer 404 in the non-electric field application region of the present embodiment. When the mode field of guided light is distributed in these layers, light absorption occurs and causes an increase in loss. Therefore, by removing the n-type or p-type cladding layer and re-growing the SI-InP layer on the non-doped InP cladding layer 405, a low-loss optical waveguide can be configured. Therefore, the SI-InP layer can be replaced with a cladding layer in a portion other than the electric field application region in the optical modulator of the present embodiment. Even in the case of 2 × 2 MMI in which the SI-InP layer is replaced with a clad layer, since it is not covered with the BCB layer in this embodiment, an effect of stress relaxation is obtained, and signal quality is deteriorated during modulation. Can be suppressed.

(Other optical waveguide elements)
In the present embodiment, the optical modulator having the npn structure has been described as an example. However, the present invention is not limited to such a semiconductor cross-sectional structure. For example, “C. Rolland et al.,“ 10 Gbit / s, 1.56 μm multiquantum well InP / InGaAsP Mach-Zehnder optical modulator, ”Electron, Lett., Vol. 29 , no.5, pp.471-472, 1993 "can also be applied to an optical modulator having a pin structure. When the difference in coefficient of thermal expansion between the organic material that covers the high mesa structure and the semiconductor that forms the high mesa structure is large, the configuration similar to that of this embodiment is used to suppress signal quality degradation in the optical modulator. can do.

  Further, in the optical modulator having the pin structure, the P-type upper cladding layer is removed, and the non-doped InP layer is regrown on the optical waveguide layer, thereby forming a low-loss optical waveguide. be able to. Therefore, the SI-InP layer can be replaced with a cladding layer in a portion other than the electric field application region in the optical modulator. Similar to the present embodiment, since it is not covered with the BCB layer, an effect of stress relaxation can be obtained, and deterioration of signal quality during modulation can be suppressed.

101,301 Parent MZI
102, 103, 302, 303 child MZI
111, 121, 122, 131, 132, 311, 321, 322, 331, 332 1 × 2 MMI
113, 114, 123, 124, 133, 134, 313, 314, 323, 324, 333, 334 Arm waveguide 112, 312 2 × 2 MMI
201, 401 SI-InP substrate 202, 402 n-type InP lower clad layer 203, 404 MQW core layer 204, 403, 405 non-doped InP clad layer 205 p-type clad layer 206, 407 n-type InP clad layer 207, 408 electrode 208, 409 Ground electrode 209,410 BCB layer 406 p-type InAlAs layer 411 SiO ₂ film

Claims (3)

An optical waveguide device having an optical waveguide layer formed on a semiconductor substrate and laminated in order of a lower cladding layer, a core layer, and an upper cladding layer,
An optical waveguide multiplexer / demultiplexer formed as part of the optical waveguide layer on the semiconductor substrate;
A first protective film formed to cover the optical waveguide layer and the optical waveguide multiplexer / demultiplexer;
And a second protective film formed on the first protective film, the second protective film formed only on a portion where the optical waveguide layer is formed. Waveguide element. An optical waveguide device having an optical waveguide layer formed on a semiconductor substrate and laminated in order of a lower cladding layer, a core layer, and an upper cladding layer,
An optical waveguide multiplexer / demultiplexer formed as part of the optical waveguide layer on the semiconductor substrate;
An optical waveguide device comprising: a single-layer or multilayer protective film formed only on a portion where the optical waveguide layer is formed.   The optical waveguide device according to claim 1 or 2, wherein the optical waveguide multiplexer / demultiplexer comprises a multimode interferometer.

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