JPH02178635A - Optical resonance type filter - Google Patents

Abstract

PURPOSE:To provide a filter characteristic which allows the transmission of only the single frequency by using a light transparent structural body which generates Bragg reflection to form at least one of a pair of translucent mirrors which constitute a Fabry-Perot interferometer. CONSTITUTION:InGaAs layers 2 and InP layers 3 are alternately laminated on a substrate consisting of an InP single crystal 1. The respective thicknesses of the layers are 1/4 the wavelength of the light to be transmitted, i.e. lambda/4. The refractive index distribution of the structural body is periodic and the respective thicknesses is lambda/4; therefore, the Bragg reflection takes place in the same manner as with a distribution feedback type semiconductor laser. Namely, of the incident light from the surface 4 side, only the light of the wavelength satisfying the Bragg condition with respect to the periodic structure reflects in the incident direction. If the surface 4 is finished to a smooth specular surface, the light arriving there from the InP side reflects on this surface and is made incident again to the periodic structural part. The repetition of such reflections takes place only with the specific wavelength light. The characteristic which resonates only to the light of the single wavelength is obtd. in this way.

Description

[Detailed Description of the Invention] [Summary] The present invention relates to a Fabry-Perot interference needle (etalon) type filter that transmits light of a specific wavelength using optical resonance, and provides an easy-to-form interferometer structure. At least one of the pair of semi-transparent mirrors that make up the Fabry-Perot interferometer is designed to realize filtering characteristics that transmit only a single frequency, variable frequency characteristics, or to realize an integrated structure that can operate independently. The basic configuration is an optical resonant filter that is a transparent structure that causes Bragg reflection, and in another invention of the present invention, an electrode is provided on the peripheral surface of the optical input/output surface of the optical resonant filter, and a voltage is applied to the electrode. By applying , the refractive index of the standing wave generation space of the interferometer is changed and the resonant frequency can be changed. The filters are arranged in an array on a substrate, and a voltage is independently applied to each of the filters to vary the resonance frequency.

(Industrial Application Field) The present invention relates to an optical filter having a shape that can be arranged in a plane, and particularly to a solid optical filter based on the principle of Fabry-Perot interferometer.

As an optical filter that only transmits light of a specific wavelength,
The Fabry-Perot interferometer is known.

This is also called an etalon, and has a structure in which a pair of semi-transparent mirrors are arranged precisely in parallel with a certain distance apart.

The light that enters the etalon through one half mirror is
The light is repeatedly reflected between the two half mirrors and interferes with each other, producing a standing wave of light of a specific wavelength. As a result, light with a wavelength that causes a standing wave within the etalon is transmitted through the etalon without being reflected, whereas light with other wavelengths is reflected and does not pass through the etalon. That is, the etalon functions as a wavelength selective filter with steep characteristics that transmits only specific wavelengths.

On the other hand, when information is transmitted using optical signals such as optical communication,
Wavelength multiplexing systems and processing of information using optical signals as they are, rather than converting them into electrical signals as in the past, are becoming increasingly popular.

In response to these technological trends, the development of elements with various functions is progressing, but in terms of filters, there is a need for filters with excellent characteristics that selectively transmit specific wavelengths, and optical integrated A circuit that can integrate multiple elements into a circuit.
There is also a need for filters with possible structures in two-dimensional arrays.

The filter of the present invention is, in principle, an advanced version of an etalon, and has a structure that is easy to integrate.

[Prior Art and Problems to be Solved by the Invention] Conventionally, the above etalon has been widely used as a wavelength selective filter. In order to improve the resonance characteristics of the etalon, a pair of half mirrors must be placed accurately in parallel at a predetermined distance, but such strict mechanical precision simply reduces productivity. Moreover, it is not suitable for precise control of specifications in semiconductor technology, and it is not desirable to require this kind of precision in the formation of optical integrated circuits.

Furthermore, since the etalon has Fabry-Perot resonance characteristics, there are many resonance peaks, and all light of wavelengths that fall outside of one of them will be transmitted, so only light of a single wavelength will be transmitted. This does not meet the requirement to do so.

In response to the demand for varying the wavelength of transmitted light, the refractive index of a semiconductor, which serves as an optical interference space, is changed by applying a voltage, thereby changing the resonant wavelength. FIG. 5(a) is a schematic cross-sectional view showing the structure of such an etalon, in which electrodes 6 and 6' are provided on a GaAfAs substrate 11 whose front and back surfaces are mirror-finished and accurately parallel. Light enters and exits through the window portions where these electrodes are not provided.

GaAj! Since the refractive index of As is high, the interface with the atmosphere functions as a reflective surface, and if both surfaces of the nuclear layer are parallel, a Fabry-Perot type resonator is constructed, as shown in Figure 5 (b).
As shown by the solid line in , the transmittance of light other than the resonant wavelength light has an extremely reduced characteristic.

Here, since the resonant frequency is also a function of the refractive index of the interference space, changing the refractive index will also change the resonant frequency. The etalon of FIG. 5(a) is constructed as an interference space GaAj! by applying a voltage to the electrodes 6 and 6'. If the refractive index of the As layer is changed, the resonance frequency will change as shown by the dotted line in FIG. 5(b).

Naturally, there is a demand for building multiple filters of this type into an optical integrated circuit and setting the transmission wavelength independently for each filter, but it is difficult to arrange and form two semi-transparent mirrors within an integrated circuit with high precision. , is more difficult than forming alone.

The purpose of the present invention is to provide a filter that utilizes optical interference like an etalon and has the characteristic of resonating only with light of a single wavelength.Another purpose of the present invention is to provide a filter that uses optical interference and has the characteristic of resonating only with light of a single wavelength. Another object of the present invention is to provide an integrated circuit device incorporating a filter having such characteristics, and a common object of the above-mentioned filters is to provide a structure that is easy to form. It is to be.

[Means to solve the problem]

In order to achieve the above object, the resonant filter of the present invention is a Fabry-Perot interferometer type in which a pair of reflectors are arranged facing each other, and a standing wave of light of a specific wavelength is generated by interference of reflected light between the reflectors. In the optical device, one of the pair of reflectors is a translucent structure that causes Blang reflection, or a surface other than the light input/output surface of the optical resonant filter is The interferometer is characterized in that it is configured such that the refractive index of the standing wave generation space of the interferometer can be changed by providing an electrode thereon and applying a voltage to the electrode, or a plurality of the optical resonant filters. It is characterized in that it is constructed so that the refractive index can be changed by applying pressure to the semiconductor substrate.

[For production]

The basic structure of the optical resonant filter of the present invention is shown in the schematic cross-sectional view of FIG. The structure and function of the filter of the present invention will be explained below with reference to the drawings, and it will be explained that the filter meets the above object.

Reference numeral 1 denotes an InP single crystal, which serves as a substrate for epitaxial growth when forming the filter, and its thickness is usually larger than the wavelength of the light to be handled.

InGaAsP layers 2 and 102 layers 3 are alternately laminated thereon, and the thickness of each layer is 174 times the wavelength of the light to be transmitted, that is, λ/4. Note that λ is a value that takes into account the refractive index of each semiconductor material, and the composition of InGaAsP is, for example, 1 no, s++Gao, 4zASo, qPo,
It is +.

The refractive index distribution of such a structure is periodic and has a thickness of λ/4, as shown in the right part of FIG.
Therefore, Bragg reflection occurs in the same way as in distributed feedback semiconductor lasers.

That is, of the light incident from the surface 4 side, only light with a wavelength that satisfies the Bragg condition for the periodic structure is reflected in the direction of incidence.

When the surface 4 is finished as a smooth mirror surface, light that reaches here from the InP side, which has a higher refractive index than the atmosphere, is reflected at the core and enters the periodic structure again. Similar to the Fabry-Perot interferometer, such repeated reflections occur only for light of a specific wavelength. Note that the phase condition for generating a standing wave by Bragg reflection is satisfied by making the outermost layer a low refractive index layer.

The back surface of the filter is also a flat surface, but since the non-reflection coating film 5 is provided, no reflection occurs at the core, and the light that reaches this core is simply transmitted and emitted. Therefore, the thickness of the InP layer 1 does not affect the interference conditions, and no special conditions are set for the thickness of the core layer.

As mentioned above, the structure shown in Figure 1 functions as a filter with steep filtering characteristics that transmits only light of a specific wavelength in the same way as the Fapley-Perot interference needle, but it differs from the Fapley-Perot interference needle in that , there is almost no transmission of high-order resonant wavelength light.

Furthermore, as shown in FIG. 2, if electrodes 6 and 6' are provided on the structure shown in FIG. 1 and the refractive index of the periodic structure is changed by applying a voltage between the electrodes, the wavelength of the transmitted light can be changed. I can do it.

Next, the point that it is easy to form two reflectors in parallel in the structure shown in FIG. 1 or 2 will be explained.

As already mentioned, the part where the refractive index changes periodically is I
The nP layer and InGaAsP layer are formed by epitaxial growth alternately, but in epitaxial growth, the growth surface is parallel to the substrate surface, so the deposition direction of these layers is perpendicular to the final InP growth plane. This allows the two planes of incidence to accurately face each other in a self-aligning manner.

In the optical resonant filter of the present invention, since the orientation of the mirror surface is aligned in a self-aligned manner, it is easy to form a large number of filters arranged on the same substrate. By applying a voltage, a device that integrates filters with different characteristics can be realized.

〔Example〕

FIG. 3 is a schematic diagram showing the structure of a filter according to the first embodiment of the present invention. Figure (a) shows a cross section of a structure equipped with a refractive index control electrode, in which a Bragg reflection layer is formed on the surface side of an n-type InP substrate 1, and the surface of the light input window is finished with a mirror finish. It is being

A 510 g layer is applied as a non-reflective coating film 5 to the light output window on the back side. The roles of these periodic structures, mirror finishing, and non-reflection coating film have already been explained.

The filter of this embodiment is provided with electrodes 6,6'. It is appropriate that the electrode be provided so as to surround the light input/output window. The shape of the window is usually circular or square, but is not limited to this.

As for the overall configuration of the filter of this embodiment, only the negative reflecting surface is a Bragg reflecting surface, and the other is a normal mirror surface.

Details of the periodic structure are shown in FIG. 3(b).

An InP layer not doped with impurities and an In6. The s+Gao, 4zASo, qPo, and + layers are all stacked alternately in 30 layers each with a thickness of 120 nm, and the p-InP layer is also 120 nm thick as the top layer.
It is formed to a thickness of nm. The refractive index distribution of this laminate is as drawn on the right side of the figure.

When a voltage of 15 V is applied to the element having the above conductivity type configuration through the refractive index control electrode, the refractive index of the InGaAs P layer changes from 3.50 to 3.49, and that of the InP layer changes from 3.16. 3.15, respectively, and accordingly, the resonance wavelength shifts by 4 nm from 1.598 μm to 1.594 μm.

FIG. 4 is a schematic cross-sectional view showing a main part of a second embodiment of the present invention. In the filter of this embodiment, both of the two reflecting surfaces are Bragg reflecting surfaces. In order to make the Bragg reflective surfaces face each other and resonate the reflected light from both sides, the high refractive index layers are butted against each other. It will be easily understood by reference. In this example, the optical input window,
A non-reflective coating film 5 is provided on both output windows.

When integrating the filters of these embodiments, it is conceivable to arrange independent refractive index control electrodes in a matrix or hexagonal dense arrangement, but since the above filters can be formed by epitaxial growth, , such integration is also easy.

〔Effect of the invention〕

As explained above, the filter of the present invention has a basic function similar to an etalon, but by adopting a novel structure, the facing of the re-reflection surfaces is achieved in a self-aligned manner. Therefore, it is easy to realize an integrated structure in which a large number of these are arranged, and in terms of optical characteristics, it has the characteristic of transmitting only light of a single wavelength.

[Brief explanation of the drawing]

Fig. 1 is a schematic cross-sectional diagram showing the basic structure of the filter of the present invention, Fig. 2 is a schematic diagram showing the wavelength control structure of the filter of the present invention, and Fig. 3 is a schematic diagram showing the structure of the filter of the first embodiment. 4 is a schematic diagram showing the structure of the filter of the second embodiment, and FIG. 5 is a schematic cross-sectional diagram showing a known voltage application type etalon, in which l is an InP substrate, 1' 2 is an n-1nP substrate, and 2 is InGaAsP. 3 is InP, 3' is pInP. 4 Surface InP 4 is a surface that becomes one of the reflective surfaces, 5 is a non-reflective coating film, and 6.6' is an electrode. Schematic cross-sectional diagram showing the basic structure of the filter of the present invention. Schematic diagram 1 showing the wavelength control structure of the filter of the present invention. (b) Schematic diagram 1 showing the structure of the filter of the first embodiment. Known voltage application. Diagram 1 showing the type etalon Schematic 1 diagram showing the structure of the filter of the second embodiment

Claims (3)

[Claims] (1) In a Fabry-Perot interferometer type optical device in which a pair of reflectors are arranged facing each other and a standing wave of light of a specific wavelength is generated by interference of reflected light between the reflectors, the pair of reflection An optical resonant filter characterized in that at least one of its bodies is a translucent structure that causes Bragg reflection. (2) By providing an electrode on a surface other than the light input/output surface of the optical resonant filter according to claim (1) and applying a voltage to the electrode, the refractive index of the standing wave generation space of the interferometer is changed. An optical resonant filter characterized by being configured as desired. (3) A plurality of optical resonant filters according to claim (2) are arranged and formed on the same semiconductor substrate, and the refractive index can be changed by applying a voltage to each of the filters independently. Characteristic optical resonant filter.