JPH0616145B2 - Optical A / D converter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Industrial Application Field In the rapidly developing optical-related technology, integration of optical circuits is desired. The present invention relates to an optical analog / digital (A / D) conversion element in a single optical thin film circuit element that can be developed for future integration of optical circuits. According to the present invention, since the branch portion of the Mach-Zehnder interferometer is configured by the laminated waveguide, it is possible to reduce the size and the insertion loss,
Furthermore, it is easy to increase the number of bits simply by changing the planar electrode structure.

These features can be applied to various devices and systems in the technological flow in which the digitization and speeding up of signals are progressing with the development of computers in recent years.
It is used in a wide range of industrial fields such as information, medical care, disaster prevention, crime prevention, transportation, and communication.

Configuration of Conventional Example and Problems Thereof In principle, an optical A / D conversion element using a phase modulator has already been proposed. However, it has not been experimentally attempted, and it is only possible to obtain 2-bit A / D in LiNbO ₃ crystal.
The converter is only being prototyped. Here future optical IC
It is considered that miniaturization of such elements, reduction of operating voltage, and monolithicization with other optical elements will be unavoidable as the number of optical elements increases. Considered difficult.

OBJECT OF THE INVENTION In order to solve the problems described above, the present invention aims to realize a compact and high-performance optical A / D converter.

According to the present invention, a first optical waveguide sandwiched by a low refractive index clad layer is provided on a compound semiconductor substrate, and a second optical waveguide is formed by sharing a part of the clad layer on the first optical waveguide. The first optical waveguide and the second optical waveguide are formed of a compound semiconductor having a composition different from that of the clad layer, and the second optical waveguide and the second optical waveguide have opposite conductivity to the second optical waveguide. Type compound semiconductor layer is formed, an electrode for applying a voltage to the compound semiconductor layer is provided to form an optical phase modulation unit, and the laminated optical branching unit, the phase modulation unit, and the optical coupling unit are integrally configured. Mach-Zehnder interferometer type optical A / characterized by
It is a D converter.

That is, the present invention can be expected as an element that enables integration with a semiconductor laser and a light receiving element as an optical A / D conversion element, and is composed of a branch portion, a phase changing portion, and a coupling portion in a laminated waveguide structure.

Description of Embodiments First, the basic principle of the present invention is shown in FIG.

FIG. 1 shows a Mach-Zehnder interferometer in which an incident light beam 1 is split into two by a beam splitter 2. One of the divided light rays is reflected by the reflection mirror 3 and passes through the phase conversion unit 6.
On the other hand, the other split light beams interfere with the light that has passed through the phase conversion unit by the coupling mirror 4, and the output light beam 7 appears as a change in output intensity corresponding to the amount of the phase modulated by the phase conversion unit.

Here, the relationship of Pn / Po∝ [1 + cos (Δφ)] (1) is established between the optical input Po and the output Pn depending on the amount of phase change Δφ by the phase changing unit 6. Further, the cubic compound semiconductor causes a Pockels effect when a voltage is applied from the outside, and causes a change in the refractive index according to the voltage. The amount of phase change based on that change is It is represented by. Here, λ: wavelength of light in the medium, n:
Refractive index of medium, γ ₄₁ : Pockels constant, E: applied voltage,
l: Crystal length.

Therefore, the output Pn oscillates at COS (Δφ). That is, the output Pn oscillates at the voltage E and the crystal length cos (Δφ) with respect to the externally applied voltage E, and its period changes depending on the crystal length l.

A cross-sectional view of the structure of the device used in the present invention is shown in FIG.

The optical waveguides 12 and 14 are provided on the substrate 16 with the cladding layers 11 and 1.
It is configured on the laminated body through 3,15. Reference numerals 10 and 8 are light branching or coupling electrodes, 9 is a phase modulation electrode, and 17 is a common electrode. Now, the light li that has entered the optical waveguide 14 is branched into the optical waveguides 12 and 14 as l ₁ and l ₂ by the voltage applied between the electrodes 10 and 17. Of the branched light, the light traveling in either the waveguide 12 or 14 is phase-modulated by the voltage applied between the electrodes 9 and 17. The modulated light is coupled again to the optical waveguide 14 by the voltage between the electrodes 8 and 17, and is emitted as output light l ₀ having intensity change.

FIG. 3 shows an example in which a 4-bit optical A / D converter is composed of a compound semiconductor as an embodiment of the present invention. On the n-type InP substrate 26, the n-type InP clad layer 25, the n-type In-Ga-As-P optical waveguide layer 24, the n-type InP clad layer 23, and the n-type In-Ga-As.
The -P optical waveguide layer 22 and the p-type InP cladding layer 21 are sequentially epitaxially grown. Further, optical branching or coupling electrodes 20 and 18 are formed thereon corresponding to each bit, and the phase modulation section electrode 19 is formed so that the length of each bit sequentially becomes half. . The electrode 27 is a common electrode. Reference numerals 28, 29, 30, and 31 denote optical waveguides below each electrode. The light incident on the waveguide layer 24 is modulated under each electrode 19 according to the principle shown in FIG.
The light whose output intensity is changed is output to. FIG. 4 shows the output change that occurs when the same voltage is applied to each electrode 19. 4A, 4B, 4C and 4D show the optical waveguides 31, 30,
It corresponds to the output light of 29, 28.

In FIG. 4, the light output above the origin is 1, with the broken line as the origin,
When the light output below the broken line is set to 0, it is detected as a digital signal of 16 gradations according to the voltage ignited by each electrode 19. In this way, it is possible to convert an electrical analog signal into a digital signal with a simple device structure. The electrodes 18 and 20 can be configured to have a 3 dB branch and a coupler by appropriately selecting the film thickness, length, and refractive index (composition) of the stacking directional coupler portion. it can. It is necessary to select the film thickness, composition and refractive index so that the optical waveguides 22 and 24 are not coupled to each other except for the electrodes 18 and 20, and the waveguide 22 when the electrode 19 is ignited by voltage. Alternatively, it is necessary to configure the electric field to concentrate on only one of 24, and it is also possible to configure the p-type and the n-type in the opposite type arrangement. In addition, it is necessary to dispose an electrical isolation layer between the electrodes 18 and 20 and the electrode 19 as required.

The group of electrodes 20 for splitting the input light is arranged to make coherent and coherent light beams enter the optical waveguides 20 and 24. If possible, it is not necessary to arrange them. Further, the InP / In-Ga-As-P-based material has been described as an example of the present invention. However, GaAs / In-Ga-As-P, GaAs / A
Similar effects can be obtained with other compound semiconductor materials such as l-Ga-As and other II-VI compound semiconductors. Further, the present invention can be integrated with a laser as a light source or a light receiving element.

EFFECTS OF THE INVENTION (1) By adopting a laminated waveguide, the branching portion, the coupling portion, and the phase modulating portion can be integrated at the same time.

(2) By forming a pn junction to form a compound semiconductor, it is possible to apply an electric field to only one of the waveguides even though it is a laminated waveguide, and fine-tune phase modulation, branching, and coupling of only one waveguide. Is possible.

(3) The insertion loss can be reduced because the coupling between each element is not taken out once outside of the space.

(4) Multiple bits can be achieved by simply changing the number of planar electrodes.

(5) A light source and a light receiving element can be integrated, and a high-speed and compact A / D converter can be manufactured.

[Brief description of drawings]

FIG. 1 is a diagram showing the basic principle of an optical A / D converter, FIG. 2 is a sectional view showing the configuration of an optical A / D converter used in the present invention, and FIG. 3 is an optical A / D converter according to an embodiment of the present invention. Fig. 4a-
FIG. 3d is a light output characteristic diagram of the optical A / D converter of the present invention. 22, 24 ... Optical waveguides 21, 23 ... Cladding layers,
26 ... InP substrate, 18, 20 ... Coupling electrode, 19 ...
... Phase modulator electrode.

Claims (1)

[Claims] 1. A first optical waveguide sandwiched by a low-refractive-index cladding layer on a compound semiconductor substrate, and a second optical waveguide sharing a part of the cladding layer on the first optical waveguide. Forming a waveguide, the first optical waveguide and the second optical waveguide are formed of a compound semiconductor having a composition different from that of the clad layer,
A compound semiconductor layer having a conductivity type opposite to that of the waveguide is formed on the second optical waveguide, and an electrode for applying a voltage is provided to the compound semiconductor layer to form an optical phase modulation unit. A Mach-Zehnder interferometer type optical A in which the branching section, the phase modulating section, and the optical coupling section are integrally formed.
/ D converter.