JPS623221A - Optical modulator - Google Patents

Abstract

PURPOSE:To obtain a high quenching ratio with a low driving voltage by constituting the titled modulator in such a manner that the forbidden band width of a semiconductor layer having the narrow forbidden band width in a thin semiconductor film has the max. value near the central part with respect to the lamination direction and decreases toward both ends. CONSTITUTION:Thin film structure is formed by laminating an Si-doped n-type GaAs buffer layer 22 and an Si-doped n-type Al0.4Ga0.6As clad layer 23 on an Si-doped n-type GaAs substrate 21 and alternately laminating non-doped AlxGa1-xAs quantum well layers 24 in which the Al compsn. ratio (x) is continuously incerased from 0 to 0.15 and is again continuously decreased down to 0 and non-doped Al0.4Ga0.6As barrier layers 25 to 30 periods. A Be-doped p-type Al0.4Ga0.6As clad layer 26 and a Be-doped p-type GaAs contact layer 27 are grown thereto to make the multi-layered structure. Electrodes 28 are formed on the top and bottom surfaces and thereafter the layers are removed by selective etching to a circular shape up to and down to the GaAs layers on the top and bottom surfaces. The high quenching ratio is thus obtd. with the low voltage and intensity modulation of several tens GHz is made possible in principle.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical modulator capable of high-speed modulation and capable of obtaining a high extinction ratio with a low driving voltage.

(Prior Art and its Problems) When rapidly changing the output intensity and phase of a semiconductor laser used as a light source in optical communications and the like, there are broadly two types of methods. There are two methods: one is to directly change the current that drives the semiconductor laser, and the other is to modulate the optical output from the light source by passing it through an optical modulator, which is a passive element. Both have their own advantages and disadvantages. The former does not use an optical modulator, so there is no insertion loss caused by the optical modulator, but during high-speed modulation of several hundred megahertz or higher, distortion of the modulation waveform due to relaxation oscillation of carriers in the semiconductor laser and time change in the oscillation wavelength ( Chirping) occurs, making it difficult to detect the signal light. Further, this modulation speed is limited by the carrier lifetime, and direct modulation of more than about 4 gigabits per second is difficult in principle.

On the other hand, the latter is capable of high-speed modulation of about 10 gigabits per second and has little chirping even during high-speed modulation, but ordinary optical modulators have large insertion loss, which is particularly disadvantageous for long-distance transmission. .

Furthermore, in order to obtain modulation with a high extinction ratio, it is necessary to drive at a high voltage.

Therefore, an optical modulator of the latter type using a multilayer thin film semiconductor has been proposed, which is capable of high-speed modulation with low loss. An example is the Japanese Journal of Applied Physics by Yamanishi et al.
As published in Vol. 22, L22 of Applied Physics (1983), applying an electric field to a multilayer thin film semiconductor shifts the absorption edge to longer wavelengths, but this also means that electrons and This method has the disadvantage that the holes are spatially separated and the probability of absorption is reduced. Furthermore, the driving voltage required to obtain modulation with a high extinction ratio is still relatively high in practice.

(Means for Solving the Problems) An optical modulator according to the present invention has a single or multilayer semiconductor thin film whose film thickness is equal to or less than the mean free path of electrons, and an electric field is applied to the semiconductor thin film in the stacking direction. The optical modulator is characterized in that the forbidden band width of a semiconductor layer having a narrow forbidden band width in the semiconductor thin film has a maximum value near the center in the stacking direction and decreases as it approaches both ends. There is.

(Operation/Principle of the Invention) The operation/principle of the present invention will be explained below with reference to the drawings. First, the semiconductor thin film structure of the optical modulator according to the present invention “□
The band structure is schematically shown in FIG. 1(a) when no electric field is applied, and in FIG. 1(b) when an electric field is applied in the stacking direction. Here, the deformation of the electron wave function 11 and the hole wave function 12 due to the electric field will be considered. In the absence of an electric field, that is, in the case shown in Figure 1(a), both wave functions exist near the heterointerface on both sides of the narrow bandgap layer (hereinafter referred to as quantum well layer), and moreover, the left and right wave functions are approximately parallel to each other. It can be seen that it has a symmetrical shape. Therefore, the value of the overlap integral of the two wave functions is approximately 1. However, when the band structure in the quantum well layer is deformed by applying an electric field, the wave function in the structure according to the present invention is near the hetero-interface where electrons and holes are on the opposite side, as shown in FIG. 1(b). Therefore, the value of the overlap integral of the two wave functions becomes 0 because they do not overlap in the region where the wave functions decrease exponentially.
The value is close to . The value of this multiple column integral is approximately proportional to the absorption coefficient, and depending on the electric field applied, the electron energy level 1
Since the absorption edge energy 15 determined by the difference between the energy level 14 of the hole and the energy level 14 of the hole decreases,
The change in the absorption coefficient spectrum due to the application of this electric field is as shown in FIG. 1(C). The solid line is the absorption coefficient spectrum 16. without electric field. The broken line is the absorption coefficient spectrum 17 when an electric field is applied.

Therefore, if we consider light with energy 18, which is slightly larger than the absorption edge in the absence of an electric field, the absorption coefficient is large in the absence of an electric field, so it is greatly absorbed by this quantum well layer, but when an electric field is applied, the absorption coefficient It can be seen that since the value of 0 is close to O, almost no absorption occurs in this quantum well layer. Therefore, it can be seen that the intensity modulation of the light having energy 18 can be performed with a high extinction ratio by turning on and off the application of the electric field.

In addition, the magnitude of the electric field required at this time can also be determined by the present invention.
Since the essential point of modulation is to change the shape of the wave function by an electric field, modulation of □ up to several tens of GHz or more is possible in principle.

(Embodiment) FIG. 2(a) shows a perspective view of an optical modulator according to a first embodiment of the present invention, and FIG. 2(b) shows its band diagram. This example was manufactured using the molecular beam epitaxy (MBE) method. First, a Si-doped n-type GaAs doped layer 22. Si-doped n-type A10.4G with a thickness of 2.0 layers m
An ao6As cladding layer 23 was laminated. Next, the A1 composition ratio X was continuously increased from O to 0.15, and then continuously decreased to O again.
80 thin-doped Alo, 4GaO, and 6As barrier layers 25 were alternately stacked in 30 cycles to form a thin film structure.

On top of this, Be-doped p-type Al with a thickness of 2.0 m, 4Ga
A multilayer structure was fabricated by growing a 6As cladding layer 26 and a Be-doped P-type GaAs contact layer 27 with a thickness of 0.5 m. Next, add this to 5
The layer has a size of about 5 mm, and electrodes 28 are placed on the top and bottom surfaces.
After fabrication, the GaAs layers on the top and bottom surfaces were selectively etched away in a circular pattern.

When light was incident perpendicularly into this circular "window" and a voltage was applied between the electrodes, the voltage dependence of the absorption coefficient spectrum was investigated, and a tendency as shown in Figure 1 (C) was clearly observed. appeared. The transmittance of light having energy above the absorption edge (wavelength: 800 nm) in the absence of an electric field is approximately 3% in the absence of an electric field, and approximately 80% when a reverse bias voltage of 5V is applied.
A very good extinction ratio of about 14 dB was obtained.

As for high-speed modulation characteristics, good intensity modulation was applied up to about 300 MHz. This upper limit is due to the parasitic capacitance between the electrodes.

Next, a second embodiment of the present invention will be described. FIG. 3 shows a perspective view of this embodiment. This is the same as the first embodiment except that the thin film structure has eight stacking periods of quantum well layers and barrier layers. Next, electrodes 31 are fabricated on the upper and lower surfaces of the substrate, and after a Sio2 film is attached to the upper surface of the substrate by CVD method, 1
．． The 8102 film was left in a stripe shape with a width of 5 μm and the other parts were removed, and then the part to which the SiO2 film was not attached was removed by etching up to the n-type A10.4GaO,6As cladding layer 23, and the remaining part was removed. A waveguide structure is formed by removing SiO2.

When this waveguide length was set to 200 pm and a laser beam with a wavelength of 800 nm was incident, the transmittance by applying an electric field was measured, and the extinction ratio was approximately 0.5% when no electric field was applied, and approximately 40% when a reverse bias voltage of IV was applied. A very good value of about 20 dB was obtained. Approximately 3GH high speed modulation characteristics
Good intensity modulation characteristics were obtained up to z and above.

Moreover, this was determined by the parasitic capacitance of the element.

Although two embodiments have been described here, the present invention is characterized in that the forbidden band width of the quantum well layer initially widens in the stacking direction and narrows midway through. It is clear that there are no limitations to the material system, semiconductor growth method, etc. The manner in which the forbidden band width changes may be spatially quadratic as shown in FIG. 4(a), or stepwise as shown in FIG. 4(b). The effect is similar.

(Effects of the Invention) The optical modulator according to the present invention is characterized in that it can obtain a high extinction ratio with a low voltage and can theoretically perform intensity modulation at several tens of GHz.

[Brief explanation of drawings]

FIGS. 1(a) and (b) are the band diagrams of the multilayer thin film structure of the optical modulator according to the present invention when no electric field is applied, and when an electric field is applied, respectively, and FIG. 2
FIG. 3 is a diagram showing absorption coefficient spectra in two cases. FIGS. 2(a) and 2(b) are a perspective view and a band diagram of the first embodiment, respectively. FIG. 3 is a perspective view of the second embodiment. FIGS. 4(a) and 4(b) are band diagrams showing modified examples of the band structure within the quantum well. In the figure, 11... Wave function of electron 12... Wave function of hole 13... Energy level of electron 14... Energy level of hole 1-1-...111v%/7-1 〒 ÷ 11, 7-
16-0, Absorption coefficient spectrum when no electric field 17...Absorption coefficient spectrum when electric field is applied 1800. Energy slightly larger than absorption edge 21...n-type GaAs substrate 2
2... n-type GaAs buffer layer 23, n-type A1o,
4GaO, 6As cladding layer 24-0. Non-doped A1
xGat-xAs (x; 0-0.15-0) quantum well layer 25--non-doped AI. , 4Gao6AsGa type layer 26..., p-type A1o, 4Gao,s As cladding layer 27...p-type GaAs contact layer 28...electrode 31...electrode tree Figure 1 (C), Figure 1-2 (a ) 2nd O 2 Figure (b) ,? 3 Figure O 4 Figure (b)

Claims (1)

[Claims] A semiconductor thin film structure comprising one or more laminated semiconductor layers having a film thickness equal to or less than the mean free path of an electron, and means for applying an electric field to the semiconductor thin film structure in the lamination direction, further comprising the semiconductor thin film structure. An optical modulator characterized in that the forbidden band width of a semiconductor layer having a narrow forbidden band width among the semiconductor layers takes a maximum value near the center in the stacking direction and decreases as it approaches both ends.