JPH0823631B2 - Light modulator - Google Patents

Description

TECHNICAL FIELD The present invention relates to an optical modulator for high speed modulation used in optical communication and the like.

[Conventional technology and its problems]

In optical communication or the like, there are roughly two types of methods for changing the output intensity and phase of a semiconductor laser used as a light source at high speed. There are a method of directly changing the current for driving the semiconductor laser and a method of modulating the light output from the light source by passing it through an optical modulator which is a passive element. Both of these have advantages and disadvantages.

The former does not use an optical modulator, so there is no insertion loss due to the optical modulator, but during high-speed modulation of several hundreds of megahertz or more, distortion of the modulation waveform due to relaxation oscillation of carriers in the semiconductor laser and temporal change of the oscillation wavelength ( (Chirping) occurs, and it becomes difficult to detect the signal light. Further, this modulation speed is limited by the carrier life, and direct modulation of about 4 gigabits per second or more is theoretically difficult.

On the other hand, the latter is capable of high-speed modulation of about 10 gigabits per second, and has little chirping even at high-speed modulation, but it has a large insertion loss in ordinary optical modulators, and is particularly disadvantageous for long-distance transmission. . In addition, it is necessary to drive at a high voltage in order to obtain modulation with a high extinction ratio.

Therefore, the latter type of optical modulator using a multi-layer thin film semiconductor capable of high-speed modulation with low loss has been proposed. An example is Yamanishi et al.'S Japanese Journal of Applied Physics.
Applied Physics) 1983, Vol. 22, L22, by applying a voltage to the multilayer thin film semiconductor,
The absorption edge is shifted to a long wavelength, which is
At the same time, electrons and holes are spatially separated, and the absorption probability is reduced. Another method proposed by Dhler is to modulate light using a superlattice structure formed by spatially varying the doping type and concentration [2nd International MBE and related cleansing]. Surface Technology Symposium, Proceedings page 21 (Co
llected Papers of 2nd International Symposium on M
olecular Beam Epitaxy and Related Clean Surface Te
chniques, p21)]. However, in this structure, the shape of the band of the semiconductor is changed by performing high-concentration impurity doping, so that the interface between differently doped regions cannot be sharply manufactured. Therefore, electrons and holes exist spatially separated from each other, and it is necessary to drive at a high voltage in order to cause a change in absorption probability to the extent that light can be modulated.

[Object of the Invention]

An object of the present invention is to provide an optical modulator capable of high-speed modulation, which has a low driving voltage and a low insertion loss, and a high extinction ratio.

[Structure of Invention]

The optical modulator has a semiconductor multi-layer structure in which first semiconductor layers and second semiconductor layers are alternately stacked, and a lower end of a conduction band of the second semiconductor is lower than a lower end of a conduction band of the first semiconductor. The upper end of the valence band of the second semiconductor is lower than the upper end of the valence band of the first semiconductor, and an electrode for applying a bias voltage is provided to the semiconductor multilayer structure, and when no bias voltage is applied, The hole wave function of the first semiconductor and the electron wave function of the second semiconductor are spatially separated, and the wave functions spatially overlap when a bias voltage is applied.

[Operation and principle of invention]

The principle of the present invention will be described below with reference to the drawings. FIG. 1 (a) is a wave function of an electron in a semiconductor multilayer structure according to the present invention when a bias voltage is not applied in the stacking direction.
FIG. 3 is a band diagram showing a wave function 12 of holes and holes. In the absence of this bias voltage, electrons are confined and distributed in the second semiconductor layer 13 and holes are distributed in the first semiconductor layer 14.
At this time, assuming that the difference 15 between the lower end of the conduction band of the second semiconductor and the upper end of the valence band of the first semiconductor is εl, even if light having a photon energy of about εl is applied to this laminated structure, it is positive with the electrons. Since the wavefunctions of the holes are spatially separated, the absorption probability given by quantum mechanics near the interface becomes very small. Further, the energy εl is equal to the first and second
Since the forbidden band width of each semiconductor is smaller than the forbidden band width, light having a photon energy of about εl is not absorbed by each semiconductor layer. Therefore, in this state, the transmittance of light having a photon energy of about ε1 to the semiconductor multilayer structure is very close to 1.

Now, FIG. 1 (b) schematically shows the wave function 11 of electrons and the wave function 12 of holes when a bias voltage is applied in the stacking direction in the semiconductor multilayer structure according to the present invention. In this case, due to the bias voltage, the potential shape becomes triangular near the semiconductor interface, and the potential causes the wave functions 11 and 12 of the electrons and holes to be localized near the interface. At this time, the energy levels of electrons and holes are discretized. In this state, the wavefunctions of electrons and holes exist in close spatial contact, so the absorption probability given by quantum mechanics near the interface increases,
As a result, the transmittance for light having a photon energy of about εl becomes very small.

In the above description, the energy levels of electrons and holes are discretized only when a bias voltage is applied. However, due to the thinning of the first and second semiconductor layers 14 and 13, even when a bias voltage is not applied, electrons and holes are not generated. It does not matter even if the energy level of is discretized. In particular, each semiconductor layer is made into a thin film, and the energy level shift due to the quantum confinement effect caused by this is utilized to make the energy level of the electrons existing in the second semiconductor layer 13 in the state where no bias voltage is applied. And the photon energy of the light of a specific wavelength that is desired to be modulated, the difference in the energy level of the holes existing in the first semiconductor layer 14 is made equal to the desired wavelength without being limited by the material system. The optical modulator of can be realized. FIG. 2 (a) shows the conduction band lower end 21, the valence band upper end 22, and the ground energy level of the electron in the material system of the semiconductor made into such a thin film in the state where the bias voltage is not applied. 23 is a band diagram showing a ground level 24 in the hole energy level 23. FIG. The film thickness is designed so that the energy difference between the levels 23 and 24 matches the photon energy of the light to be modulated. When a bias voltage is applied to this structure, each band edge and energy level change as shown in FIG. 2 (b). As a result of this change, it can be understood that both the electron wave function 25 and the hole wave function 26 are localized near the interface, and the absorption probability increases by applying a bias voltage, as in the case of FIG.
Therefore, the transmittance of the light to be modulated is large when the bias voltage is not applied, and the transmittance is small when the bias voltage is applied. In this case as well, the same effect as that considered using FIG. 1 is obtained. You can see the effect.

As described above, the optical modulator according to the present invention changes the shape of the wave function of electrons and holes in the semiconductor multilayer structure by applying an electric field, thereby changing the transmittance of light to be modulated. The principle is used. In principle, the modulation speed in this case can be as high as the limit given by quantum mechanics. In addition, the electric field required to sufficiently change the shape of the wave function requires a smaller voltage because the interface is steep. The change in the transmittance due to the application of the bias voltage is large, and a high extinction ratio can be obtained.

〔Example〕

FIG. 3 shows a sectional view of the optical modulator of the first embodiment of the present invention.

This optical modulator comprises a n-type GaAs layer 32 with a thickness of 1.0 μm formed on a n-type GaAs substrate 31 by a molecular beam epitaxy method (MBE method).
N-type In with gradually added In as a Group III element with a thickness of 3.0 μm
_x Ga _1-x As (0 ≦ x ≦ 0.15) Graded layer 33, thickness 1.0
An n-type In _0.15 Ga _0.85 As layer 34 having a thickness of 500 μm is sequentially grown, and a non-doped high-purity Al _0.5 Ga _0.5 Sb layer 35 having a film thickness of 500 Å and a non-doped high-purity In _0.15 Ga _0.85 As layer 36 having a film thickness of 500 Å are formed on the n-type In _0.15 Ga _0.85 As layer 36. 20 layers are alternately laminated to form a multi-layered thin film structure 3A, and a p-type In _0.15 Ga _0.85 As layer 37 having a thickness of 1.5 μm is grown thereon.
Next, the thickness of the substrate 31 is reduced to 30 μm, and the electrodes 38 are attached to the upper and lower surfaces. The band structure from the substrate 31 of this embodiment is schematically shown in FIG. The photon energy of the modulated light is Al _0.5 Ga _0.5 which is the first semiconductor layer.
The upper end of the valence band of the Sb layer 35 and the second semiconductor layer In
Energy difference between the bottom of the conduction band of _0.15 Ga _0.85 As layer 36 and 41
Shall be close to. The energy difference 41 in this example is
It is 0.8 eV, which shows that it is suitable for modulating light with a wavelength of 1.55 μm. Further, in this embodiment, the absorption edge 42 of GaAs is about 0.86 μm.
m, In _0.15 Ga _0.85 As absorption edge 43 is about 0.98 μm, Al _0.5 Ga _0.5 Sb
Has an absorption edge 44 of about 1.07 μm, and light with a wavelength of 1.55 μm
It is not absorbed except in the vicinity of the interface between the Al _0.5 Ga _0.5 Sb layer 35 and the In _0.15 Ga _0.85 As layer 36. Actually, in the structure of FIG. 3, when light of 1.55 μm was transmitted in the direction perpendicular to the laminated surface through the openings 39 provided in the electrodes 38 on the upper and lower surfaces, the transmittance was 90%. This corresponds to a loss of about 0.5 dB, and it can be seen that the absorption coefficient α at 3A is about 5 × 10 ² cm ^-1 , assuming that this absorption is due to the multilayer thin film structure 3A having 20 layers.

Next, a voltage is applied to the structure of this example in the stacking direction,
Similar measurements were made. First, ground the n-type GaAs substrate 31,
A voltage of −5 V was applied to the p-type In _0.15 Ga _0.85 As layer 37, thereby applying an electric field of about 3 × 10 ⁴ V / cm to the multilayer thin film structure 3A.
In this state, the transmittance was 20%. It has a loss of about 7 dB and an absorption coefficient of 8 × 10 ³ cm ^-1 . This change in transmittance due to ON / OFF of the bias voltage is sufficiently practical. Furthermore, good modulation characteristics were obtained even when modulation was performed by applying an AC bias voltage of 800 MHz, and the effectiveness of using this structure as an optical modulator was confirmed.

In fact, even in a simulation experiment using a DFB (distributed feedback type) laser that oscillates at a wavelength of 1.55 μm with a spectrum width of 10 MHz, it was possible to perform optical intensity modulation of 500 Mbit / S without causing waveform deterioration. It was also understood that the upper limit of this modulation speed is due to the electric circuit, and higher speed modulation is possible by lowering the bias voltage.

Next, an optical modulator according to the second embodiment of the present invention will be described with reference to FIG. In this figure, a 1.0 μm thick n-type GaSb layer 52 is grown on an n-type GaSb substrate 51 by a molecular beam epitaxy method, and a non-doped high-purity InAs layer having a film thickness of 10 Å is grown on the n-type GaSb layer 52.
Similar to 53, 100 high-purity GaSb layers 54 each having a thickness of 15 Å are alternately laminated to form a multi-layer ultra-thin film structure 5A, on which a 0.5 μm-thick p-type GaSb layer 55 is sequentially grown, Board 51
FIG. 9 is a cross-sectional view of an example in which the electrode is thinned to 10 μm and electrodes 56 are attached to the upper and lower surfaces. The band structure from the substrate 51 of this embodiment is shown in FIG. In this band structure, the conduction band and the valence band in the InAs layer 53, which is the second semiconductor, and the GaSb layer 54, which is the second semiconductor, overlap in terms of energy. Although it becomes metallic, since the InAs layer 53 and the GaSb layer 54 are made ultrathin as described above, holes can exist in the conduction band at the ground energy level 61 where electrons can exist. The ground energy level is the level indicated by 62, and it can be considered that this embodiment also has a band structure similar to that of the first embodiment. The wavelength suitable for the modulation in this case is the energy of the difference between the electron energy level 61 and the hole energy level 62, which is about 0.8 eV, and the wavelength is about 1.55 μm in the above structure.

In this example, first, light having a wavelength of 1.55 μm was passed in the stacking direction through the openings 57 provided in the upper and lower electrodes 56, and a bias voltage was applied in the stacking direction to examine changes in the absorption coefficient. The transmittance is 70% without bias voltage, and the loss is about 1.5 dB.
Met. This is because the substrate and the upper GaSb layer have a wavelength of 1.55 μm.
It is considered that the transmittance is lower than that of the first embodiment because it is not transparent to the light of (1). Next, the n-type GaSb substrate 51 was grounded, a voltage of −1 V was applied to the p-type GaSb layer 55, and an electric field of 4 × 10 ⁴ V / cm was applied to the multilayer ultrathin film structure 5A. It was 10% and the loss was 10 dB. This modulation characteristic was sufficient. Furthermore, when a simulation experiment was performed using a DFB laser that oscillates at a wavelength of 1.55 μm, it was 2.0 Gb.
Modulation of it / S light intensity was realized by applying an AC voltage signal. Moreover, as in the first embodiment, there was no distortion due to the modulation of the laser output waveform. The upper limit of the modulation speed of 2.0 Gbit / S was also due to the electric circuit.

Next, in order to eliminate absorption of light by the substrate and the upper GaSb layer of the optical modulator of this structure, light is made incident in a direction parallel to the multilayer ultrathin film structure 5A, that is, a direction parallel to the stacking plane, and The characteristics were investigated. This was investigated by performing the same measurement as above with the element length set to 200 μm. In this case, it was understood that a high extinction ratio was obtained, with the transmittance changing to 90% without applying a bias voltage and to 5% with applying a bias voltage.

Next, a sectional view of a third embodiment of the present invention is schematically shown as a seventh embodiment.
Shown in the figure. This does not require the cleaved surface of one element as a laser resonator, but it is a distributed feedback type (DF
B: Distributed Feed Back) A semiconductor laser and an optical modulator according to the present invention are monolithically manufactured. This structure will be described below. First, an n-type In _x Ga _1-x As (0 ≦ x ≦ 0.54) graded layer 72 having a thickness of 3.0 μm, an n-type InP layer 73 having a thickness of 4.0 μm, and a thickness of 0.1 are formed on an n-type GaAs substrate 71.
μm InGaAsP active layer 74 with 1.55 μm oscillation composition, thickness 0.15
A p-type InGaAsP optical guide layer 75 of 1.3 μm oscillation composition of μm is sequentially grown by vapor phase epitaxy, and thereafter, a diffraction grating 76 having a pitch of 240 nm is formed on the upper optical guide layer 75 by two-beam interference method and etching. 2 μm thick p-type InP layer 77, thickness
A semiconductor laser substrate having a 0.2 μm p-type InGaAsP cap layer 78 grown thereon was produced. Next, a part of this semiconductor laser substrate is selectively etched to expose the GaAs substrate 71,
An optical modulator having the same structure as that of the embodiment shown in FIG. 3 was formed thereon by the molecular beam epitaxy method (the same elements as those shown in FIG. 3 are designated by the same reference numerals). Here, the active layer of the DFB semiconductor laser section 70 and the multilayer thin film structure 3A of the optical modulator section 30 were manufactured so as to be located on substantially the same plane. Next, in order to electrically separate the DFB semiconductor laser part 70 and the optical modulator part 30, the p-type clad part near the boundary of the above structure was removed by etching, and the respective electrodes 79, 38 were formed by vapor deposition. After that, the length of the DFB semiconductor laser unit 70 was about 300μ.
m, and the element was cut out so that the length of the optical modulator portion 30 was about 50 μm.

In this element, the DFB semiconductor laser section 70 was energized to oscillate and a voltage signal of about 1 V was applied to the optical modulator section 30. The optical output of the optical modulator 30 when the absorption is small is 5 mW, and when the absorption is large is 0.5 μW, and very good modulation characteristics with less waveform deterioration and chirping can be obtained even at the time of 2.0 Gbit / S modulation. Was given.

The three embodiments have been described above, but the present invention is not limited to the first embodiment.
Light having a photon energy which is about the difference in energy between the hole wave function localized in the second semiconductor layer and the electron wave function localized in the second semiconductor layer is absorbed between the two levels. However, the present invention is not limited to the material system, the semiconductor growth method, the incident direction of light, other integrated devices, and the like.

〔The invention's effect〕

As described above, according to the present invention, it is possible to obtain an optical modulator that can be driven by a low voltage, has a high extinction ratio, and can perform high-speed modulation with low insertion loss.

[Brief description of drawings]

1 is a band diagram showing the principle of the present invention, FIG. 2 is a band diagram showing the principle of the present invention in a thin film structure, FIG. 3 is a cross-sectional view of an optical modulator of the first embodiment of the present invention, FIG. 4 is a band diagram in the first embodiment, FIG. 5 is a sectional view of the optical modulator of the second embodiment, FIG. 6 is a band diagram of the second embodiment, and FIG. 7 is a third embodiment. It is sectional drawing of an example. 1,25 …… Electron wave function 12,26 …… Hole wave function 14,35,53 …… First semiconductor layer 13,36,54 …… Second semiconductor layer 15,41 …… Energy difference 21 ...... Lower conduction band 22 ...... Higher valence band 23 …… Basic energy level of electrons 24 …… Basic energy level of holes 30 …… Optical modulator section 70 …… DFB semiconductor laser section 3A …… Multilayer Thin film structure 5A ... Multi-layer ultra-thin film structure

Continuation of the front page (56) References JP-A-60-205421 (JP, A) "Physics and applications of semiconductor superlattices" edited by the Physical Society of Japan (SHO 59-11-10) Baifukan PP. 240 ~ 242

Claims (1)

[Claims] 1. A semiconductor multi-layer structure in which first semiconductor layers and second semiconductor layers are alternately laminated, and a lower end of a conduction band of the second semiconductor is a lower end of a conduction band of the first semiconductor. Lower, the upper end of the valence band of the second semiconductor is lower than the upper end of the valence band of the first semiconductor, and an electrode for applying a bias voltage is provided to the semiconductor multilayer structure, and no bias voltage is applied. The wave function of the hole of the first semiconductor and the wave function of the electron of the second semiconductor are spatially separated at the time, and the wave functions spatially overlap when a bias voltage is applied. A light modulator.