Optical modulator

Abstract

This optical modulator includes a light absorbing layer (102) formed on a substrate (101). The light absorbing layer (102) is formed in a core shape extending in a predetermined direction on the substrate (101). The light absorbing layer (102) has a multiple quantum well structure made of a compound semiconductor. The multiple quantum well structure of the light absorbing layer (102) gradually changes such that the photoluminescence wavelength of the light absorbing layer (102) gradually increases from the light incident side toward the light emission side. The spacing between the photoluminescence wavelength on the light incident side and that on the light emission side of the light absorbing layer (102) can be 10 nm or more.

Description

Optical Modulator

[0001] The present invention relates to an optical modulator.

[0002] Electro-absorption modulators (EAMs) are widely used in applications with relatively short transmission distances of 100 km or less. EAMs modulate light using a light absorption process caused by the application of voltage. The operating principle of EAMs is explained using Figure 5. When a voltage is applied to an EAM, the wavelength of light input to the EAM, λ0, and the peak wavelength of light absorption caused by the light absorption layer of the EAM, λ, are modulated. EA The wavelength difference Δλ changes from the state shown in FIG. 5(a) to the state shown in FIG. 5(b), creating an ON-OFF state of the light, and optical modulation occurs.

[0003] The detuning amount Δλ, which indicates the difference between the absorption peak and the wavelength of the input light when no bias is applied, is determined by the design (material, layer thickness, composition ratio) of the quantum well structure constituting the light absorption layer of the EAM, and is an important parameter that determines the absorption characteristics of the EAM. The quantum well structure of the light absorption layer of the EAM is generally evaluated and determined using the photoluminescence (PL) wavelength.

[0004] In optical communication systems using optical fiber and EAM, the transmission distance is limited by the chromatic dispersion of the optical fiber and the chirp characteristics of the EAM. Chirp is a phenomenon in which wavelength fluctuations occur due to changes in refractive index when light is turned on and off. As signal light transmits through the optical fiber, the effects of chirp become apparent in the transmitted waveform due to the dispersion characteristics of the optical fiber, hindering high-speed operation and long transmission distances.

[0005] The chirp parameter (hereinafter defined as chirp: α), which serves as an index for quantitatively evaluating chirp characteristics, can be expressed as the ratio of the refractive index change k (imaginary part: optical absorption) to the refractive index change n (real part) of the light absorption layer of the EAM, as shown in "α = dn / dk (1)" (Non-Patent Document 1).

[0006] F. Dorgeuille and F. Devaux, "On the transmission performances and the chirp parameter of a multiple-quantum-well electroabsorption modulator", IEEE Journal of Quantum Electronics, vol. 30, no. 11, pp. 2565-2572, 1994.W. Kobayashi et al., "Design and Fabrication of 10- / 40-Gb / s, Uncooled Electroabsorption Modulator" "Integrated DFB Laser With Butt-Joint Structure", Journal of Lightwave Technology, vol. 28, no. 1, pp. 164-171, 2010.

[0007] Next, the light absorption layer of an EAM in the prior art will be described with reference to Fig. 6. In the prior art, the light absorption layer of an EAM has a uniform quantum well structure in the direction of light propagation. Therefore, the PL wavelength λ is proportional to the length of the light absorption layer of the EAM (EAM length). PL is uniform. Therefore, as shown in formula (1), once the desired refractive index change k is determined, the refractive index change n is determined from the Kramerth-Kronig relation, and the chirp parameter α is uniquely determined (Non-Patent Document 2). Note that the Kramerth-Kronig relation is a relationship in which the refractive index and the absorption coefficient are linked by a function related to the electric field strength. Due to this relationship, the refractive index and the absorption coefficient cannot be changed independently in principle.

[0008] That is, in the prior art, although it is possible to control the light absorption characteristics, there is a problem in that it is not possible to control the chirp.

[0009] The present invention has been made to solve the above problems, and has as its object to make it possible to control chirp in an electroabsorption optical modulator.

[0010] The optical modulator according to the present invention comprises a light absorption layer of a multiple quantum well structure made of compound semiconductors formed in a core shape extending in a predetermined direction on a substrate, and a voltage application structure for applying a voltage to the light absorption layer, in which the multiple quantum well structure of the light absorption layer gradually changes from the light incident side to the light output side so that the photoluminescence wavelength of the light absorption layer becomes longer from the light incident side toward the light output side.

[0011] As described above, according to the present invention, the multiple quantum well structure of the optical absorption layer is gradually changed from the light incident side to the light exit side so that the photoluminescence wavelength of the optical absorption layer becomes longer from the light incident side to the light exit side, thereby making it possible to control chirp in the electroabsorption optical modulator.

[0012] FIG. 1 is a configuration diagram showing the configuration of an optical modulator according to an embodiment of the present invention. FIG. 2 is a plan view (a) showing the configuration of the optical modulator according to the embodiment of the present invention and a characteristic diagram (b) showing the characteristics of the optical modulator. FIG. 3 is a characteristic diagram showing the relationship between the chirp parameter and the voltage applied to the optical absorption layer 102 of the optical modulator according to the embodiment of the present invention. FIG. 4A is a cross-sectional view showing the state of the optical modulator in an intermediate process for explaining a manufacturing method of the optical modulator according to the embodiment of the present invention. FIG. 4B is a cross-sectional view showing the state of the optical modulator in an intermediate process for explaining a manufacturing method of the optical modulator according to the embodiment of the present invention. FIG. 4C is a cross-sectional view showing the state of the optical modulator in an intermediate process for explaining a manufacturing method of the optical modulator according to the embodiment of the present invention. FIG. 4D is a cross-sectional view showing the state of the optical modulator in an intermediate process for explaining a manufacturing method of the optical modulator according to the embodiment of the present invention. FIG. 4E is a cross-sectional view showing the state of the optical modulator in an intermediate process for explaining a manufacturing method of the optical modulator according to the embodiment of the present invention. FIG. 4F is a cross-sectional view showing the state of the optical modulator in an intermediate process for explaining a manufacturing method of the optical modulator according to the embodiment of the present invention. FIG. 4G is a cross-sectional view showing the state of the optical modulator in an intermediate process for explaining a manufacturing method of the optical modulator according to the embodiment of the present invention. Fig. 5 is an explanatory diagram illustrating the operating principle of the EAM, and Fig. 6 is a characteristic diagram showing the change in PL wavelength in the light absorption layer of a conventional EAM.

[0013] An optical modulator according to an embodiment of the present invention will be described below with reference to Fig. 1 and Fig. 2. Fig. 1 schematically shows a cross section perpendicular to the waveguide direction of light, and Fig. 2(a) shows a cross section parallel to the plane of a substrate 101. The optical modulator according to the embodiment is an electro-absorption modulator (EAM).

[0014] This optical modulator includes a light absorbing layer 102 formed on a substrate 101. The light absorbing layer 102 is formed in a core shape extending in a predetermined direction on the substrate 101. The light absorbing layer 102 has a multiple quantum well structure made of a compound semiconductor. For example, the light absorbing layer 102 can be made of InGaAlAs. A first high-resistance layer 104a and a second high-resistance layer 104b made of a high-resistance compound semiconductor are formed on both sides of the light absorbing layer 102. The first high-resistance layer 104a and the second high-resistance layer 104b function as cladding for an optical waveguide with the light absorbing layer 102 as the core.

[0015] This optical modulator also includes a voltage application structure that applies a voltage to the light absorption layer 102. The voltage application structure can be composed of a substrate 101, which is a layer made of an n-type compound semiconductor, and a contact layer 103, which is a layer made of a p-type compound semiconductor and formed on the light absorption layer 102. The substrate 101 can be composed of, for example, n-type InP. The contact layer 103 can be composed of, for example, p-type InP.

[0016] The voltage application structure also includes an n-electrode (not shown) formed in connection with the substrate 101, and a p-electrode 111 formed in connection with the contact layer 103. The light absorption layer 102 and the contact layer 103 are disposed between the first high-resistance layer 104a and the second high-resistance layer 104b, and the p-electrode 111 is formed on the contact layer 103, the first high-resistance layer 104a, and the second high-resistance layer 104b.

[0017] Furthermore, the multiple quantum well structure of the light absorption layer 102 gradually changes so that the photoluminescence wavelength of the light absorption layer 102 gradually increases from the light incident side 131 toward the light emitting side 132. The interval between the photoluminescence wavelengths of the light absorption layer 102 on the light incident side 131 and the light emitting side 132 can be 10 nm or more.

[0018] 2B, by gradually changing the PL wavelength of the light absorption layer 102 having a quantum well structure from the light input side 131 to the light output side 132 of the optical modulation region, the refractive index change (real part) n and the refractive index change k (imaginary part: light absorption) in the optical waveguide having the light absorption layer 102 as its core can be controlled, thereby making it possible to control the chirp characteristics of the optical modulator. For example, as will be described later, the PL wavelength of the light absorption layer 102 can be changed by changing the state of the multiple quantum well structure in the waveguiding direction during selective growth to form the light absorption layer 102.

[0019] For example, the wavelength range of the input wavelength λ0 to the optical modulator (light absorption layer 102) is the O band (1.26 μm≦λ0≦1.36 μm) within the optical communication wavelength band. In this wavelength range, the chirp characteristics can be controlled by changing the PL wavelength by changing the quantum well structure of the light absorption layer 102. The results of calculations on this subject are shown below. The calculations were performed using the calculation software "Harold" (manufactured by Photon Design). The calculation results are shown in Figure 3.

[0020] As shown in FIG. 3, when the voltage applied to the light absorption layer 102 is within a predetermined range, the PL wavelength λ PL It can be seen that the α parameter can be arbitrarily controlled by controlling the PL wavelength at the light input side 131 of the optical modulator as λ PL1 = 1.22 μm, and the PL wavelength on the light output side 132 is λ PL2 = 1.23 μm. In this setting, when the voltage applied to the light absorption layer 102 is in the range of 0.8 to 1.2 [-V], the amount of change in the α parameter is |Δα| < 1.8. From this, the interval Δλ of the PL wavelength from the light incident side 131 to the light emitting side 132 of the optical modulator is PL By setting the value to 10 nm or more, the amount of change in the α parameter can be set to |Δα|>1.8.

[0021] The optical modulator according to the above-described embodiment can be formed by integrating it on the substrate 101 with a semiconductor laser such as a distributed feedback (DFB) laser. For example, it can be formed by integrating it on the same substrate as a semiconductor laser having an active layer with a multi-quantum well structure (with a uniform PL wavelength) similar to the light absorption layer. Also, it can be formed by integrating it on the same substrate as a semiconductor laser having an active layer with a multi-quantum well structure (with a uniform PL wavelength) different from the light absorption layer.

[0022] Next, a method for manufacturing an optical modulator according to an embodiment will be described with reference to Figures 4A to 4G, which show cross sections perpendicular to the waveguide direction.

[0023] 4A, a first semiconductor layer 201 is grown by crystal growth on a substrate 101 made of n-type InP. The first semiconductor layer 201 can be made of, for example, InGaAsP. For example, the first semiconductor layer 201 can have a portion, not shown, that will become the active layer (multiple quantum well) of an integrated semiconductor laser.

[0024] 4B , a selective growth mask 202 is formed on the first semiconductor layer 201. The selective growth mask 202 has an opening 202′ in a region where the light absorption layer 102 is to be formed. For example, the selective growth mask 202 can be formed by depositing an insulating material such as silicon oxide on the first semiconductor layer 201 to form an insulating layer, and then patterning this insulating layer by known lithography and etching techniques.

[0025] Furthermore, the first semiconductor layer 201 in the region of the opening 202′ is removed, and an opening 201a is formed in the first semiconductor layer 201, as shown in FIG. MASK The width W MASK The width of the opening 202' can be varied from 5 μm to 40 μm from one end to the other in the waveguide direction. The width of the opening 202' is uniform in the waveguide direction.

[0026] Next, by selective growth using a selective growth mask 202, a barrier layer and a well layer made of InGaAlAs are crystal-grown on the substrate 101 exposed at the bottom of the opening 201a, thereby forming a light absorption layer 102 with a multi-quantum well structure as shown in Fig. 4D. The multi-quantum well structure of the light absorption layer 102 is gradually changed by selective growth using the selective growth mask 202 whose width gradually changes in the waveguide direction. This change in the multi-quantum well structure causes, for example, a change in the PL wavelength λ on the light incident side. PL = 1.22 μm, the PL wavelength λ on the light output side PL = 1.23 μm.

[0027] Next, the selective growth mask 202 is removed, and then, as shown in Fig. 4E, a second semiconductor layer 203 made of p-type InP is crystal-grown on the light absorption layer 102. At the same time, a p-type semiconductor layer can be crystal-grown to serve as a contact layer of a semiconductor laser to be formed in a region not shown.

[0028] Next, the second semiconductor layer 203 is patterned to form the contact layer 103 as shown in FIG. 4F, and the first semiconductor layer 201 is patterned to remove the light absorbing layer 102 in a core shape. For example, the light absorbing layer 102 is formed to have a core width of about 1 to 2 μm. At the same time, a contact layer for the semiconductor laser to be formed in a region not shown can be formed, and a core-shaped active layer for the semiconductor laser can be formed.

[0029] Next, on the exposed portion of the substrate 101 on the side of the optical absorption layer 102, InP is doped with Fe to increase its resistance, and crystal regrowth is performed to form a first high-resistance layer 104a and a second high-resistance layer 104b, burying the sides of the optical absorption layer 102 and the contact layer 103, as shown in FIG. 4G. These first and second high-resistance layers 104a and 104b can be used to bury the contact layer and active layer of the semiconductor laser, which will be formed in regions not shown. The first and second high-resistance layers 104a and 104b can be made of a dielectric material such as silicon oxide. After this, a p-electrode 111 and an n-electrode (not shown) are formed, thereby obtaining the optical modulator according to the embodiment shown in FIG. 1. At the same time, electrodes for the semiconductor laser can be formed in regions not shown.

[0030] As described above, by growing the multiple quantum well structure of the light absorption layer 102 by selective growth using the selective growth mask 202 whose width gradually changes in the waveguide direction, the quantum well structure can be gradually changed in the waveguide direction, thereby gradually changing the PL wavelength. By changing the PL wavelength in this way, the chirp parameter α can be controlled.

[0031] As described above, according to the embodiment of the present invention, the multiple quantum well structure of the optical absorption layer is gradually changed from the light incident side to the light exit side so that the photoluminescence wavelength of the optical absorption layer becomes longer from the light incident side to the light exit side, thereby making it possible to control chirp in the electroabsorption optical modulator.

[0032] It should be noted that the present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention.

[0033] 101...substrate, 102...light absorption layer, 103...contact layer, 104a...first high resistance layer, 104b...second high resistance layer, 111...p electrode.

Claims

1. An optical modulator comprising: a light absorption layer of a multiple quantum well structure made of compound semiconductors formed in a core shape extending in a predetermined direction on a substrate; and a voltage application structure for applying voltage to the light absorption layer, wherein the multiple quantum well structure of the light absorption layer gradually changes from the light incident side to the light output side so that the photoluminescence wavelength of the light absorption layer becomes longer from the light incident side to the light output side.

2. An optical modulator according to claim 1, wherein the interval between the photoluminescence wavelengths on the light incident side and the light emitting side of the light absorption layer is 10 nm or more.

3. An optical modulator according to claim 1, wherein said optical absorption layer is made of InGaAlAs.

4. An optical modulator according to any one of claims 1 to 3, wherein the voltage application structure comprises a layer made of a p-type compound semiconductor, a layer made of an n-type compound semiconductor, a p-electrode formed and connected to the layer made of the p-type compound semiconductor, and an n-electrode formed and connected to the layer made of the n-type compound semiconductor.

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