JPH05273504A - Optical modulator - Google Patents

Abstract

PURPOSE:To provide the optical modulator which can be more drastically reduced in size and weight than the conventional bulk type optical modulators, is small in driving power, enables high-speed modulation and is suitable for integration. CONSTITUTION:Incident light 1 from a photodetecting window 14 is passed through a multiple quantum well layer (MQW layer) 7, is reflected by a semiconductor multilayered reflection mirror 10; is again passed through the multiple quantum well layer (MQW layer) 7 and is emitted from the photodetecting window 14. The peak of absorption spectra is shifted, by which the coefft. of absorption to light is changed when a modulation electric signal is applied to a p side electrode 3 and an n side electrode 8. Exit light 2 is thus modulated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical modulator used in fields such as optical communication and optical signal processing.

[0002]

2. Description of the Related Art As a conventional optical modulator, an optical modulator utilizing the interaction between ultrasonic waves and light is known. In principle, ultrasonic wave modulators used in this light modulator include those applying Raman-Nass diffraction and those applying Bragg diffraction. FIG. 5 is an explanatory diagram of the principle. (A) Figure is an application of Raman-Nass diffraction,
(B) is an application of Bragg diffraction. In the figure,
21 is an ultrasonic medium, 22 is a transducer, 23 is an ultrasonic absorber, 24 is a high frequency signal source, 25 is incident light, 26
Is the output light.

Raman-Nass diffraction will be described.
As shown in (A), the transducer 22 is provided on one end surface of the ultrasonic medium 21, and the ultrasonic absorber 23 is provided on the other end surface. When the transducer 22 is driven by the high frequency signal source 24, ultrasonic waves propagate in the ultrasonic medium 21 and are absorbed by the ultrasonic absorber 23. The energy of the propagating ultrasonic waves causes a photoelastic effect to create a compressional wave,
The refractive index in the ultrasonic medium 21 periodically changes corresponding to the frequency of the ultrasonic wave, and a diffraction grating is formed. In the case of Raman-Nass diffraction, the direction parallel to this diffraction grating, that is,
Incident light 25 of a plane wave is made to enter perpendicularly to the traveling direction of the ultrasonic wave. The m-th order diffracted light in the emitted light 26 propagates in the direction of the angle θ _m given by sin θ _m = −mλ _O / λ _A. Where λ _O ,
λ _A is the wavelength of light waves and sound waves, respectively. Therefore, the incident light is incident parallel to the surface of the diffraction grating formed by the compressional wave, whereas the emitted light is vertically +1, +2, +3, ... -2, -3, ...
・ Each order is diffracted.

For the ultrasonic medium 21, a material is selected such that a diffraction grating having a high light transmittance is formed, the attenuation of ultrasonic waves is small, and a high diffraction efficiency can be obtained by a small ultrasonic output. It becomes the condition of. In addition, regarding the transducer that is the ultrasonic wave oscillating source,
iNbO ₃ is often used.

The Bragg diffraction shown in FIG. 5B is also shown in FIG.
The diffraction grating by the ultrasonic wave generated by the high frequency signal 24 described in (A) is used. The incident light 25 is made incident by the diffraction grating at an angle θ that satisfies the Bragg condition, 2d sin θ = nλ _O. d is the distance between the lattice planes. When this condition is satisfied, diffracted light by Bragg reflection is obtained in the θ direction. That is, the first-order light is obtained symmetrically with the zero-order light. If the Bragg condition is not satisfied, reflection by the lattice plane does not occur.

An ultrasonic optical modulator (AOM) is constructed using the ultrasonic optical modulator as described above. Ultrasonic light modulators can be used for both analog and digital modulation. FIG. 6 is a schematic configuration diagram of an analog modulation ultrasonic optical modulator. In the figure, 30 is an ultrasonic light modulator, 31
Is a laser light source, 32 is a transducer, 33 is an aperture, 34 is a half mirror, 35 is a high frequency amplifier, 36
Is a balanced modulator, 37 is a carrier oscillator, 38 is a photodetector, 39 is an error amplifier, 40 is output modulated light, and 41 is an input analog signal. A PIN diode, a photomultiplier, or the like is used for the photodetector 38. When no modulation signal is applied to the ultrasonic light modulator 30 via the high frequency amplifier 35, the first-order diffracted light from the ultrasonic light modulator 30 is blocked by the aperture 33. When an ultrasonic signal is applied from the transducer 32 to the ultrasonic light modulation element 30 by the input analog signal 41, the light from the laser light source 31 is diffracted while passing through the ultrasonic light modulation element 30 and is emitted from the aperture 33. The modulated light 40 having a frequency corresponding to the modulation frequency is emitted.

However, a bulk type optical modulator such as an ultrasonic optical modulator that has been conventionally used has a large external dimension on the order of several tens of mm. The driving power is several hundred mW or more, which is large. The modulation frequency band is not so large, at most 100's tens of MHz. Not suitable for optical integration. There is a problem.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and achieves a significant reduction in size and weight as compared with a conventional bulk type optical modulator, and also has a small driving power and a high speed. It is an object of the present invention to provide an optical modulator capable of modulation and having a structure suitable for integration.

[0009]

According to the present invention, in an optical modulator, a semi-insulating semiconductor substrate is provided, a semiconductor multilayer film reflecting mirror is provided as a lower layer, a buffer layer is provided as an intermediate layer, a multiple quantum well layer is provided thereon, and a cap is provided as an upper layer. It is characterized by having an epitaxial structure in which layers are formed.

[0010]

[Function] The multilayer film reflecting mirror will be described. The reflector can be formed by alternately laminating two kinds of materials having a difference in refractive index and having a thickness of λ / 4 with respect to the wavelength λ of light. Multilayer film mirrors used in semiconductor devices include dielectric multilayer films and semiconductor multilayer films.

Among these, in the semiconductor multilayer film, the refractive index of the semiconductor can be controlled by the semiconductor mixed crystal technique. Consistent with the crystal growth process, the reflector can be formed continuously with the growth of the epitaxial layer. Has the advantage.

An example of a semiconductor multilayer film having a quarter wavelength thickness will be described. InGaAsP (wavelength corresponding to bandgap is λ _g
= 1.3 μm) and InP, their refractive indices are 3.36 and 3.1 at λ = 1.5 μm, respectively.
Since it is 5, its thickness is 1120Å and 11 respectively.
If 90Å is selected, the thickness will be λ / 4 in either case, and a multilayer-film reflective mirror made of a semiconductor multilayer film can be obtained. FIG. 2 shows the relationship between the number of layers and the reflectance in which the high-refractive index layer and the low-refractive index layer are paired when the incident-side layer and the outgoing-side layer are both made of InP. About 35 pairs have a reflectance of about 90%. Also,
FIG. 3 shows the reflection spectrum when the number of layer pairs is 35.
Therefore, it can be seen that the multilayer-film reflective mirror has wavelength selectivity.

The multiple quantum well layer (MQW layer) will be described. A potential structure in which electrons and holes are confined in an extremely thin layer of 200 Å or less is called a quantum well, and a coupling force due to Coulomb force works between confined electrons and holes. This electron-hole pair is called an exciton. A multiple quantum well layer (MQW layer) is formed by interposing a barrier layer between the quantum well layers. Regarding the multiple quantum well layer (MQW layer), when an electric field is applied perpendicularly to the quantum well layer, the so-called quantum confined Stark effect (QCSE) in which the exciton absorption spectrum shifts to the longer wavelength side is observed as shown in FIG. It

This quantum confined Stark effect (QCS
According to E), when voltage is applied to the multiple quantum well layer (MQW layer) for light that is on the longer wavelength side than the peak wavelength of the absorption spectrum shown in FIG. 4 and is transmitted without being absorbed at a voltage of 0 V, Since the peak of the absorption spectrum shifts to the long wavelength side, the absorption coefficient for light of the above wavelength increases, and as a result, the above light is modulated.

[0015]

1 is a sectional view of an embodiment of an optical modulator of the present invention. In the figure, 1 is incident light, 2 is emitted light, 3 is a p-side electrode, 4 is a surface protective film / antireflection film, 5 is a p ⁺ region, 6 is an n-type InP cap layer, and 7 is a multiple quantum well layer ( MQW
Layer), 8 is an n-side electrode, 9 is an n-type InP buffer layer, 10
Is a semiconductor multilayer film reflecting mirror, 11 is a p-electrode bonding pad, 12 is an n-electrode bonding pad, 13 is a semi-insulating InP substrate, and 14 is a light receiving window. In this example, the multiple quantum well layers (MQW layers) 7 were provided with 30 pairs of InP as a barrier layer and InGaAsP or InGaAs as a quantum well layer.

Incident light 1 from the light receiving window 14 passes through the multiple quantum well layer (MQW layer) 7, is reflected by the semiconductor multilayer film reflecting mirror 10, passes through the multiple quantum well layer (MQW layer) 7 again, and receives light. Emitted from 14. At this time, when a modulation electric signal is applied to the p-side electrode 3 and the n-side electrode 8, as described above, the peak of the absorption spectrum shifts, the absorption coefficient for light changes, and the outgoing light 2 is modulated. ..

By adjusting the thickness and composition of the quantum well layer, the first level heavy hole exciton absorption peak can be adapted to the wavelength of the incident signal light. In this way, the incident light 1 is absorbed by the multiple quantum well layer 7 and the emitted light 2 is not generated. When a forward voltage is applied to the p-side electrode 3 and the n-side electrode 8, carriers are injected into the multiple quantum well layer 7, so that the above-mentioned absorption saturation of exciton absorption occurs. Therefore, the incident light 1 is transmitted through the multiple quantum well layer 7, and the emitted light 2 is obtained on the output side. That is, the incident light 1 can be turned on and off by controlling the applied voltage.

An outline of the manufacturing process of the optical modulator described in FIG. 1 will be described. First, on the semi-insulating InP substrate 13, a semiconductor multilayer film reflecting mirror 10, an n-type InP buffer layer 9, a multiple quantum well layer (MQW layer) 7, and an n-type InP are sequentially formed.
The cap layer 6 is crystal-grown.

Next, by the Zn selective diffusion method or the like, n-type I
A P ⁺ region 5 is formed in the nP cap layer 6. And n
To form the electrode 8 on the buffer layer 9, or p
Mesa etching is performed to form the electrode bonding pad 11 and the n-electrode bonding pad 12 on the semi-insulating substrate 13. The surface protection film 4 is formed on it.

In order to apply a voltage to the multiple quantum well layer (MQW layer) 7, a contact hole is formed in the surface protective film 4, a p-side electrode 3 and an n-electrode 8 are further formed, and a wiring metal and a p-electrode are formed. The bonding pad 11 and the n-electrode bonding pad 12 are formed.

In the embodiment described above, the multiple quantum well layer (M
InGaAsP / InP or I as a QW layer)
The nGaAs / InP system was used. This material is for the long wavelength band, but can be applied to the short wavelength band by using the GaAs / AlGaAs system.

[0022]

As is clear from the above description, the optical modulator of the present invention can be made much smaller and lighter than the bulk type optical modulator. Upon modulation, since absorption saturation of excitons in the quantum well layer occurs at a low injected carrier density, driving power is small and high-speed modulation is possible. Since it has an epitaxial structure including a multi-quantum well layer (MQW layer) and a semiconductor multilayer mirror, it is a structure suitable for integration. There is an effect.

[Brief description of drawings]

FIG. 1 is a sectional view of an embodiment of an optical modulator of the present invention.

FIG. 2 is a diagram showing the relationship between the number of layers and the reflectance of a multilayer-film reflective mirror.

FIG. 3 is a diagram showing a reflection spectrum when the number of layer pairs is 35.

FIG. 4 is an explanatory diagram of an operation of a multiple quantum well layer.

FIG. 5 is an explanatory diagram of an operation principle of the ultrasonic light modulator.

FIG. 6 is a schematic configuration diagram of an example of a conventional ultrasonic light modulator.

[Explanation of symbols]

1 incident light 2 emitted light 3 p-side electrode 4 surface protective film 5 p ⁺ region 6 n-type InP cap layer 7 multiple quantum well layer (MQW layer) 8 n-side electrode 9 n-type InP buffer layer 10 semiconductor multilayer film reflector 11 p electrode bonding pad 12 n electrode bonding pad 13 semi-insulating InP substrate 14 light receiving window

Claims (1)

[Claims] 1. A semi-insulating semiconductor substrate having an epitaxial structure in which a semiconductor multilayer film reflecting mirror is formed as a lower layer, a buffer layer is formed as an intermediate layer, a multiple quantum well layer is formed thereon, and a cap layer is formed as an upper layer. Light modulator.