JPS61256320A - Waveguide type optical modulator - Google Patents

Abstract

PURPOSE:To obtain an optical switch (optical modulator) which is driven by a low voltage in a waveguide type optical modulator for which a semiconductor waveguide is used by providing a trap for capturing electrons or holes into a semiconductor which acts as the optical waveguide. CONSTITUTION:The n-type GaAs for an n-type GaAs substrate 1 and an optical waveguide layer 2 is formed of an LPE method. A ridge type optical waveguide 3 is the waveguide layer which is formed by the LPE method and is worked by a photolithographic method and dry etching method. Signal light 9 is made incident on the ridge type optical waveguide. Exit light 10 is from the optical waveguide. The release of the electrons from the electron trap is proportional to the number of photons of the incident excitation light when the relation between the irradiation time and the attenuation quantity of the signal light on the incident side is obtd. and therefore the quantity of the electrons to be released from the trap is larger and the attenuation quantity of the signal light on the exit side is larger as the power of the excitation light is higher. The electrons released from the electron trap and the electrons captured again in the trap are balanced with each other and the stationary stage is attained when the excitation light is irradiated for the specified time.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to an optical switch (optical modulator) used in the fields of optical communications and optical information processing, and in particular to an optical switch that can operate at low voltage. It's about.

[Background of the invention]

BACKGROUND OF THE INVENTION As optical communications and optical information processing systems become more complex, sophisticated, and diversified, there is a strong demand for smaller, faster, and more reliable optical components. In recent years, research has been actively conducted on waveguide type optical devices using ■-■ group compound semiconductors as materials for optical devices in the wavelength range of 1.0 to 1.5 μm. For example, "QAAs directional coupler type optical switch" by Kawaguchi (IEICE Technical Report 0QB78-27, 1978 P.
9) We will now discuss conventionally developed waveguide optical modulators. FIG. 1 schematically shows a perspective view of a GaAs directional coupler type optical modulator (optical switch). In this figure, 1 is an n''-GaAS substrate doped with Si and having a carrier concentration of I x 10 ``cm-''.
S single crystal, 2 is an optical waveguide layer, which is an n-type high resistance GaAS thin film grown on a GaAS substrate 1 by liquid phase epitaxial method (LPE method), and has a carrier concentration I x 10111.
cm-'', thickness 3 μm, 3 is the above-mentioned high resistance '(2) fabricated on a QaAs thin film by photolithography.
IJ Tsuji type optical waveguide, the width of the ridge is 7μm3 the height is 1
．． The size is 5 μm and a single mode for light with a wavelength of 1.3 μm. Further, the two ridges are parallel to each other with an interval of 3 μm, and have a length of 8 nm. On the upper part of the ridge, Schottky electrodes 4 and 5 made of At, and on the back side of the substrate 1, an ohmic electrode 6 made of Au (3eNI) are formed by vacuum evaporation. These are an external drive power source and a lead wire, respectively.In addition, the optical switch shown in Fig. 1 has two ridge-type optical waveguides that have a ridge length so that they have a perfect coupling length for light with a wavelength of 1.3 μm. Width, spacing, length, etc. are selected.

Light switching is performed by applying a reverse bias voltage to the kA Schottky electrode provided above the ridge, and
This is done by increasing the refractive index of the S optical waveguide and changing the coupling state between the two ridge-type optical waveguides. In the optical switch shown in Figure 1, optical switching is achieved at a reverse bias voltage of 20 to 30 V, and the extinction ratio at that time is usually 1.
It is 0-20d13. At the voltage that realizes optical switching, the electric field strength directly under the Atshosotogie electrode is 106
2/m, and there is a very high risk of damage to the element due to dielectric breakdown. This is true not only for optical switches using Schottky junctions but also for pn junction type optical switches, and is one of the reasons why optical switches driven at low voltage are desired.

[Purpose of the invention]

An object of the present invention is to provide an optical switch (optical modulator) that is driven at low voltage.

[Summary of the invention]

The structure of the present invention for achieving the above object is to add impurities that capture free electrons (or free holes) to the semiconductor material used for the optical waveguide, and to add light of a wavelength different from the signal light wavelength to the optical waveguide. The purpose is to modulate the signal light by making the light incident on the optical waveguide, exciting the electrons captured by the impurities into the conduction band, and changing the refractive index of the optical waveguide.

FIG. 2 is a schematic diagram of a state near a metal-semiconductor Schottky junction for illustrating the principle of the present invention.

Let us consider n-type GaAs as a semiconductor.

In the figure, Eme Ell, ET, and Eo are the energy gap between the conduction band and valence band, the Fermini Nerky, the energy measured from the lower end of the conduction band of the donor-type electron trap, and the energy of the donor, respectively. b, v , represents the reverse bias voltage applied to the Schottky junction. When a reverse bias voltage is applied to the Schottky junction, the depletion layer extends from the junction interface to a depth W, υ, while the electron traps that exist up to a depth yT have a lower Fermi energy. can also rise to the top and emit electrons. Ejection of electrons from an electron trap located above the Fermi energy usually takes a time on the order of seconds when carried out thermally. However, if we do this now using light, we can emit electrons at the speed of light. In the case of light, electrons can also be emitted from electron traps below the Fermi energy. When an electron is released from an electron trap, a current flows through the junction,
Junction capacitance increases.

Therefore, if the region where electron traps exist is an optical waveguide, the refractive index decreases as the junction capacitance increases. This can be expressed as follows. When the energy of light propagating through the optical waveguide is smaller than the energy gap of the semiconductor, the dielectric constant can be approximately expressed as:

Here, n, m'', and el el '0 are the free electron concentration, the effective mass of free electrons, the angular frequency of light, the charge of electrons, and the permittivity of vacuum, respectively. Also, ε(0) is the dielectric constant of the lattice. Represents dielectric constant.

Since the refractive index N is given by N=V7 (2), the change ΔN in the refractive index due to a change in the free electron concentration n by Δn can be found as follows.

The energy of light propagating through the optical waveguide is smaller than the energy ET of the electron trap, and the energy of light larger than ET and smaller than El is used to emit electrons from the electron trap. The number n of incident photons when the optical waveguide under consideration is irradiated with excitation light having a power of P [W] for 1 second.

teeth, is given by Here, hv is the energy of light.

In addition, the concentration n4 of electron traps existing in the optical waveguide where the excitation light is incident is n, =N t VQ (
5) Therefore, Nt is the electron trap concentration in unit volume, v
(1 is the volume of the optical waveguide into which the excitation light is incident.

If n (1≧nl), all the electrons present in the electron traps in the optical waveguide can be emitted by the excitation light.

[Embodiments of the invention]

The present invention will be explained in detail below.

This will be explained using Example 1 and FIG. 3.

FIG. 3 schematically shows a perspective view of a waveguide type optical modulator using a linear ridge type optical waveguide. l is an n-type QaAs substrate with a carrier concentration of 1×10 ``cm-'', 2
is an optical waveguide layer formed by an n-type GaAstl-LPE method with a carrier concentration of 5 x 10''cm-3.

Reference numeral 3 denotes a ridge type optical waveguide, in which a 5 μm thick optical waveguide layer formed by LPE is processed into a height of 2 μm and a width of 7 μm by photolithography and dry etching. Reference numeral 9 indicates a signal light input to the ridge type optical waveguide, and reference numeral 10 indicates light emitted from the optical waveguide.

Now, as an electron trap in the optical waveguide layer, the energy is E T = 0.8 e VO per unit volume, N
? = 10 "cm-" was formed. As the signal light wavelength, light with λ = 2.0 μm is used, and as the excitation light wavelength,
Using light with a wavelength of λ = 1.55 μm, the excitation light was made to enter the optical waveguide from the same location as the signal light, and only the signal light wavelength was selectively detected on the output side.

Now, excitation light was irradiated with a power of 1 [W] for 10 nanoseconds. At this time, all the electrons captured in the electron traps in the optical waveguide are released into the conduction band, and the signal light is absorbed due to the increase in the concentration of free electrons in the conduction band, causing the signal light on the output side to increase by 10 dB. Attenuated. When the excitation light was blocked, the electrons excited in the conduction band fell into the empty electron trap, and the signal light on the output side returned to the same intensity as before the excitation light was irradiated. Next, we changed the excitation light power and found the relationship between the irradiation time and the output side signal light attenuation.
Various graphs shown in the figure were obtained. Since the emission of electrons from an electron trap is proportional to the number of photons of the incident excitation light,
The stronger the power of the excitation light, the greater the amount of electrons emitted from the trap, and the greater the amount of attenuation of the output side signal light. When the excitation light is irradiated for a certain period of time, the electrons emitted from the electron trap and the electrons captured by the trap again are balanced, and a steady state is reached. This is the fourth reason why the amount of output signal light attenuation becomes saturated after a certain period of time for each pumping power.

Example 2 Next, a directional coupler type optical modulator was manufactured using the same ridge type optical waveguide as in Example 1. FIG. 5 is a perspective view of this directional coupler type optical modulator.

On an n-type QaAa substrate 1, a high-resistance optical waveguide layer 2 of n-type GaA3 is formed to a thickness of 5 μm by MOCVD, and A4 is vacuum-deposited. Thereafter, eight ridges 3 each having a width of 7 .mu.m and a height of 2 .mu.m and having a length of 3 .mu.m are formed using the photo-IJ sogelafy method and the dry etching method. On the back surface of the substrate 1, an ohmic electrode 6 is formed by vacuum evaporating Au.

Reference numeral 11 denotes a signal light incident on the ridge-type optical waveguide, and 12 and 13 are signal lights that are incident on the ridge-type optical waveguide, and have powers Pl and P3, respectively.

Similar to Example 1, the optical waveguide layer 2 contains energy Et
=0.8eV electron trap has concentration Nt=10''c
m-" is formed.

When the wavelength of the input signal light 11 is λ=2 μm, the output light becomes Pg>Pt, and the extinction ratio (
The intensity ratio) was 15 dBT. Next, A on the incident light 11 side
When a reverse bias voltage was applied to the t-Schottky electrode, Pl gradually increased, and conversely, P8 decreased. When the reverse bias voltage is loVO, Pl-P
2, and P t > P z at 20V. The extinction ratio at this time was 18 dB.

Next, simultaneously with the input signal light, excitation light with a wavelength of 1.5 μm was made to enter the same optical waveguide as the input signal light.

When the reverse bias voltage applied to the At electrode 5 was 0, when IW was irradiated with excitation light power, electrons were emitted from the electron traps and a current flowed through the Schottky junction. While the current is flowing, the capacitance of the Schottky junction increases, and the refractive index of the optical waveguide decreases by 7t according to equation (8), so the coupling state of the two ridge-type optical waveguides changes, and the output signal For light at 12°13, Pl>Pg), and the extinction ratio was 13 dB.

Next, a reverse bias voltage of IOV is applied to the electrode 5, and the Pt
=P1, when excitation light was irradiated with 1) ■, Px<P29, and the extinction ratio was 20 dB.

Note that the refractive index changes during the time when the excitation light is irradiated and electrons are emitted from the electron trap, but once the electron emission is finished, the refractive index returns to the state before the excitation light irradiation. . FIG. 6 shows the extinction ratio of the emitted signal light versus the excitation light irradiation time when the voltage applied to the electrode 5 is O. As the excitation light power becomes weaker, the maximum extinction ratio becomes smaller and the time taken to reach the maximum becomes longer.

As shown in this embodiment, if an electron trap is formed in a semiconductor optical waveguide, it becomes possible to perform pulsed optical modulation even when the applied voltage is 00.

[The moving bed of invention]

As explained in the above embodiments, according to the present invention, it is possible to modulate and switch light with low voltage, suppress element deterioration due to breakdown, and achieve longer life and higher reliability of the element. effective.

(12).

[Brief explanation of the drawing]

FIG. 1 is a perspective view of a directional coupler type optical modulator, FIG. 2 is a schematic diagram of the vicinity of the metal-semiconductor junction interface, and FIG. 3 is a diagram of a waveguide type optical modulator as an embodiment of the present invention. The perspective view, Figure 4, is
FIG. 5 is a perspective view of a directional coupler type optical modulator, and FIG. 6 is a diagram showing the modulation characteristics of signal light. DESCRIPTION OF SYMBOLS 1... Substrate, 2... Optical waveguide layer, 3... Ridge, 4
．． s. 6... Electrode, 7... Power supply, 8... Lead wire, 9.
11...1st item]

Claims (1)

[Claims] 1. In a waveguide optical modulator using a semiconductor optical waveguide,
A waveguide type optical modulator characterized in that a trap for capturing electrons or holes is provided in a semiconductor serving as an optical waveguide. 2. In the waveguide optical modulator as set forth in claim 1, the energy of the signal light is made lower than the activation energy of the electron or hole captured in the trap, and the activation light is used as excitation light. It is characterized by modulating light by emitting electrons or holes captured in traps using light with energy greater than energy and smaller than the energy gap between the conduction band and the valence band. Waveguide optical modulator.