JPH03274525A - Multiple quantum well optical modulating element - Google Patents

Abstract

PURPOSE:To allow the execution of a modulation operation without entailing the deterioration of an extinction ratio even in an ultra-high speed modulation or strong input operation by determining a bias in such a manner that a resonance tunnel phenomenon arises between adjacent quantum wells in the transmission state or absorption state of a light absorption layer. CONSTITUTION:The cause for a sharp increase in the absolute value of a photoinduced current at an impressed voltage V10 in the case of the reverse bias applied to the multiple quantum wells lies in that the resonance tunnel phenomenon arises between the adjacent quantum wells. Namely, the base state E1 of the quantum well 42 on the right side and the 1st excitation state E2 of the quantum well 44 on the left side coincide in energy and, therefore, the tunnel phenomenon eventually takes place. Photoexcited carriers can be rapidly removed from the multiple quantum wells by setting the bias of the element in the transmission state or absorption state of the light absorption layer at the value at which the resonance tunnel phenomenon takes place. The optical modulation operation to obviate the generation of the piling up of the carriers in the quantum well absorption layer and to prevent the degradation in the extinction ratio, etc., is possible in this way.

Description

[Detailed Description of the Invention] [Objective of the Invention] (Industrial Application Field) The present invention relates to an external modulation type multi-quantum well optical modulation device used in optical fiber communication, etc., and particularly relates to an improvement in modulation drive means. This invention relates to a multi-quantum well optical modulator.

(Prior art) In optical fiber communications, in order to minimize signal distortion when transmitting optical signals over long distances,
It is necessary that the spectral linewidth of the optical signal is sufficiently narrow.

When direct eclipse dimming of a semiconductor laser is used for optical signals, the spectral linewidth widens due to a phenomenon called chirping, in which the oscillation frequency fluctuates in response to carrier fluctuations within the semiconductor laser, resulting in spectral narrowing. It becomes a hindrance. In order to improve this, an external modulation method in which a semiconductor laser is continuously oscillated and the light is modulated by an external modulator is considered to be promising.

As an external modulator, a multi-quantum well optical modulator that utilizes the quantum-confined Stark effect of a semiconductor multi-quantum well structure has been proposed (for example, D, A, B, Mlll
er et at, IEEE J, Quantus
Electron, QE-22, 1818 (198B
) The operating principle of this device is shown in FIG.

! ! The fss diagram (a) shows the band diagram and wave function when a vertical electric field is applied to a quantum well structure made of GaAs/AlGaAs. In the figure, 51 is the quantum well, and 52 is the fundamental electron. wave function of state, 53
indicates the energy level of the ground electronic state, 54 indicates the wave function of the ground hole state, and 55 indicates the energy level of the ground hole state. Since the electrons and holes are confined by the barrier layer, the electrons and holes are not separated by electric field,
Excitons are present even at high electric fields.

Furthermore, the energy between quantum levels moves to the lower energy side due to the Stark effect. FIG. 5(b) is a diagram showing this situation as a change in the absorption coefficient. In the figure, 56 indicates the absorption spectrum of the multiple quantum well under no electric field, and 57 indicates the absorption spectrum of the multiple quantum well under a finite limit. There is. Since the absorption peak shifts to the longer wavelength side (lower energy side) as the electric field applied to the quantum well increases, if the incident wavelength is appropriately set near the band edge (λjn in Figure 5(b))
, optical modulation (absorption loss type in this figure) using an external electric field becomes possible. Usually, a quantum well structure is formed in an i-layer (non-doped layer) of a pin structure, and application of an electric field to the quantum well is realized by reverse bias. The ON state (transmissive state!g) of the modulation element of , = can be changed to the OFF state by applying no bias to the element! @ (absorption state) can be obtained by reverse biasing.

However, this type of element has the following problems. That is, when performing ultrahigh-speed modulation or strong light input operation using a multi-quantum well light absorption layer, even if the electron-hole pairs generated by absorbing light in the absorption state become transparent, the i-layer ( The carrier pile up occurs without cutting from the m-well structure region). When pile-up of carriers occurs, the absorption coefficient of the multiple quantum well decreases as shown by solid lines 56' and 57' in FIG. 5(e) due to its optical nonlinearity. This has caused problems such as a reduction in the extinction ratio (1i1, which is the optical output in the ON state divided by the optical output in the OFF state).

(Problems to be Solved by the Invention) Conventionally, when a multi-quantum well optical modulator is operated at ultra-high speed or with a strong input, problems such as carrier pile-up are caused and the extinction ratio is deteriorated. there were.

The present invention has been made in consideration of the above circumstances, and its purpose is to provide a multiple quantum well that can perform modulation operations without deteriorating the extinction ratio even in ultra-high-speed modulation or strong input operations. An object of the present invention is to provide a light modulation element.

[Structure of the Invention] (Means for Solving the Three Problems) The gist of the present invention is to create a resonant tunneling phenomenon between adjacent quantum wells in the transmission state or absorption state of the light absorption layer, thereby reducing the pile-up of carriers. The goal is to prevent uploads.

That is, in the present invention, the light absorption layer into which light of a predetermined wavelength is incident is made up of a plurality of quantum wells, and the light absorption layer is placed in a transmission state or an absorption state for the wavelength of the incident light depending on the magnitude of the bias applied to the quantum wells. By selecting, in a multi-quantum well light modulation element that modulates light obtained through a light absorption layer, resonant tunneling occurs between adjacent quantum wells in at least one of the transmission state and absorption state of the light absorption layer. The bias is set as follows.

(Function) According to the present invention, by setting a bias so that a resonant tunneling phenomenon occurs between adjacent quantum wells in the transmission state or absorption state of the light absorption layer, electrons and holes generated in the absorption state are Since the pairs quickly escape from the quantum well structure region, it is possible to prevent pile-up of carriers from occurring in the light absorption layer.

The reason for this will be explained below.

FIG. 4(a) is a characteristic diagram showing the photo-induced current when a reverse bias is applied to the multiple quantum well.

Applied voltage V1. ), the absolute value of the photoinduced current increases rapidly at vl. This is because resonant tunneling occurs between adjacent quantum wells. This situation is shown in FIG. 4(b). In addition, 41, 43, 45 in the figure is Al
GaAs layer, 42° 44 indicates a GaAs layer (quantum well).

At applied voltages v and o, the ground state E1 of the quantum well 42 on the right side and the first excited state E2 of the quantum well 44 on the left side coincide in energy, so that a tunneling phenomenon occurs resonantly (FuruLa eL al, Jp
n, J, ^[l], Phys, 25(2) L151(
198B)).

As described above, in the present invention, by setting the bias of the element in the transmission state or absorption state of the light absorption layer to a value that causes the resonant tunneling phenomenon, it is possible to quickly exclude photoexcited carriers from the multiple quantum well. Therefore, pile-up of carriers does not occur in the quantum well absorption layer, and optical modulation operation that prevents deterioration of extinction ratio etc. is possible.

(Example) Hereinafter, details of the present invention will be explained with reference to illustrated embodiments.

FIG. 1 is a perspective view showing the device structure of a multi-quantum well light modulation device according to an embodiment of the present invention, and FIG. 2 is a band diagram of the element. The multiple quantum well region 20 is an i-layer, and the AlGaAs layer 21 (21+ to 21N+1)
and GaAs layers 22 (221 to 22N) are alternately stacked. A p-type GaAs layer 11 is formed on the upper side sandwiching this multiple quantum well region 20, and an n-type GaAs layer 11 is formed on the lower side.
A type GaAs layer 12 is formed. And G
Electrodes 13.14 are applied to the surfaces of the aAs layers 11.12, respectively. In addition, in the figure, 15 is a circular window for introducing incident light, 16 is a circular window for leading out output light,
30 is a driving power source for optical modulation.

In the multiple quantum well region 20, AIo, 4Gao, .
, the width of the As layer (barrier layer) 21 is 5n■, GaA
The width of each s-layer (quantum well) 22 was set to IQn*.

Therefore, one period LP of the quantum well structure is 15n■.

Further, the number of quantum wells was set to 5o. Note that the outermost AlGaAs layer 211.21N+1 of the multiple quantum well region 20 was formed thicker than the other portions in consideration of etching for opening the window. The built-in potential of this device was approximately 2 eV. The exciton absorption peak of this device without bias was 0.859 μm, and by applying a reverse bias of 7 eV (corresponding to 1,2×10′ V/ell), the peak wavelength was shifted to 0.873 μm.

On the other hand, the energy difference ΔEIO between the first excited state and the ground state of the quantum well is calculated to be 110 seV, which causes the internal electric field to be ΔE + o / L p -7,3X 10' V /
(It is calculated that resonance tunneling occurs when the value is 1.

This internal electric field corresponds to applying a reverse bias of 3.5 V in the device of this example. In fact, an increase in photoexcitation current originating from resonant tunneling was observed at this reverse bias. Furthermore, the exciton absorption peak wavelength when a reverse bias of 3.5 μm was applied was 0.865 μm. Note that the peak of the absorption coefficient when reverse bias is applied to L5V shifts to the lower energy side compared to when no bias is applied;
Exciton absorption peak wavelength when reverse bias is applied is 0.873
It was sufficiently small in μm.

From the above, using a continuous laser beam of 0.87 μm as the incident light wavelength and setting the bias in the transmission (ON) state, Te Vl −
-3,5V-1 Also, V, -7V was selected as the bias for the absorption (OFF) state (Fig. 3). This has made it possible to perform ultra-high-speed modulation of tens of GHz or more without carrier pile-up.

Thus, according to this embodiment, since the bias of the element in the transmission state of the quantum well light absorption layer 20 is set to a value that causes the resonant tunneling phenomenon, the photoexcited carriers generated in the absorption state are quickly absorbed by light in the transmission state. layer 2
It is possible to exclude from 0. Therefore, even in ultra-high speed modulation or strong input operation, a good optical modulation operation can be performed without causing a pile-up of carriers in the quantum well absorption layer 20. Another advantage is that it can be easily realized by simply setting the bias voltage without changing the conventional element structure.

Note that the present invention is not limited to the embodiments described above. In the above embodiment, the element was driven so that the resonant tunneling phenomenon occurs with a bias in the transmission state, but a reverse bias may be set so that it occurs in the absorption state or in both states. Furthermore, although the resonant tunneling phenomenon has been described as occurring between the ground state and the first excited state, it is also possible to utilize the resonant tunneling phenomenon that utilizes a higher-order excited state. As for the material system, GaAs/Al
Not limited to GaAs type! nP/I
It can be similarly applied to other material systems such as nGaAsP system, GaAs/1nGaAs system, and InGaAs/InAlAs system. The element structure is not limited to cases in which light is incident perpendicularly to the laminated surfaces, but can also be applied to so-called waveguide type optical modulation elements in which incident light is incident parallel to the laminated surfaces. Furthermore, although the embodiments have been described using the simplest box-like potential as a quantum well structure, the invention can be similarly applied to an asymmetric quantum well or a quantum well having a potential other than a box-like potential. In addition, various modifications can be made without departing from the gist of the present invention.

[Effects of the Invention] As detailed above, according to the present invention, the bias is set to cause a resonant tunneling phenomenon between adjacent quantum wells in the transmission state or absorption state of the light absorption layer. Pile-up can be prevented in advance, and a multi-quantum well region 2 element can be realized in which performance such as extinction ratio does not deteriorate even under high-speed modulation or strong input light intensity.

[Brief explanation of drawings]

Fig. 1 is a perspective view showing the device structure of a multi-quantum well light modulation device according to an embodiment of the present invention, Fig. 2 is a schematic diagram showing the band structure of the device, and Fig. 3 explains the operation of the device. FIG. 4 is a schematic diagram for explaining the resonant tunneling phenomenon, and FIG. 5 is a schematic diagram for explaining the operating principle of a conventional multi-quantum well optical modulator and its problems. 11-p-type GaAs layer, 12-n-type Ga1As layer, 13.14...electrode, 15.16...window, 20...multiple quantum well region, 21-i! 11 AIGaAs layer (barrier layer), 22-i type GaAs layer (quantum well), 30... drive power source. Irimura Kouta Ill

Claims (2)

[Claims] (1) The light absorption layer into which light of a predetermined wavelength is incident is made up of a plurality of quantum wells, and the light absorption layer is selected to be in a transmitting state or an absorbing state for the wavelength of the incident light depending on the magnitude of the bias for the quantum wells. In a multi-quantum well optical modulator that modulates light obtained through a light absorption layer, a bias is set so that a resonant tunneling phenomenon occurs between adjacent quantum wells in at least one of the transmission state and absorption state. A multi-quantum well optical modulation device characterized by comprising: (2) The multi-quantum well optical modulator according to claim 1, wherein the bias is set so as to cause a resonant tunneling phenomenon between the ground state and the first excited state in adjacent quantum wells.