JP2004109556A - Wavelength variable optical filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wavelength variable optical filter applicable to many wavelengths used in the optical communication of a WDM system with less crosstalk between close channels. <P>SOLUTION: This wavelength variable optical filter has a structure in which multilayer film reflection mirrors made to face each other at prescribed intervals so as to constitute two or more sets of optical resonators are each supported by a cantilever. By deforming the cantilever by an electric means and changing the interval of the multilayer reflection mirrors, the wavelength that the optical resonator passes through is changed. By realizing the structure by using the compound semiconductor of GaAs/AlGaAg system or the like, integration with another optical element is facilitated. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a wavelength tunable optical filter used in the field of optical communication and the like, and more particularly to an optical resonator type wavelength tunable optical filter having a means for controlling a resonator length.
[0002]
[Prior art]
In order to cope with an increase in information transmission capacity due to the rapid spread of the Internet, development of a wavelength division multiplexing (WDM) system in which optical signals of a plurality of wavelengths are multiplexed and propagated on one optical fiber has been advanced. Since this WDM system uses a large number of light beams having close wavelengths, an optical filter for multiplexing / demultiplexing them is an important optical element. However, when the number of wavelengths (the number of channels) to be used is large, it is necessary to prepare an optical filter having transmission or reflection characteristics corresponding to each wavelength, which complicates the system configuration. In order to solve such a problem, a tunable optical filter capable of controlling and changing transmission or reflection characteristics has been demanded.
[0003]
A tunable optical filter based on several principles is known. There is one in which the transmission wavelength is changed by changing the optical thickness of the multilayer film by changing the incident angle of light to the multilayer filter (see, for example, Patent Document 1).
[0004]
It is also known that the transmission wavelength is changed by changing the interval (resonator length) between the reflecting mirrors of a Fabry-Perot optical resonator formed by a pair of reflecting mirrors. In recent years, in order to reduce the size of such a filter and enable external control using an electric signal, development of an optical micromachine optical filter using a microelectromechanical system (MEMS) has been promoted (for example, non-electromechanical filters). Patent Document 1).
[0005]
In this example, one of the pair of multilayer mirrors is supported by a cantilever to form an optical resonator. There are various means for changing the resonator length with this structure. By applying a voltage between the two multilayer films, the cantilever can be elastically deformed by electrostatic force. In addition, the cantilever can be deformed by forming a structure in which layers having different coefficients of thermal expansion are laminated on the cantilever and generating heat by applying a current to this portion.
[0006]
In the configuration described in Non-Patent Document 1 described above, since the multilayer mirror and the cantilever are formed of a compound semiconductor (GaAs / AlGaAs) layer, a process for manufacturing a semiconductor optical element such as a semiconductor laser is performed. It has excellent features such as being able to be manufactured, being able to integrate a plurality of filters, and being able to be integrated with other semiconductor optical elements.
[0007]
[Patent Document 1]
Japanese Patent Application Laid-Open No. 2000-292715 [Non-Patent Document 1]
F. Koyama et al., "Japanese Journal of Applied Physics," Vol. 39, Part 1, No. 3B, 2000, pp. 1542-1545.
[Problems to be solved by the invention]
However, an optical filter using an optical resonator has a certain transmission bandwidth. This is no exception in the above-mentioned optical micro-machine filter, and when applied to WDM optical communication, it causes crosstalk between channels having wavelengths close to each other.
[0009]
The present invention has been made to solve such a problem, and an object of the present invention is to provide a tunable optical filter which has a small transmission wavelength band and is applicable to WDM optical communication.
[0010]
[Means for Solving the Problems]
The wavelength tunable optical filter according to the present invention has a structure in which multilayer film mirrors facing each other at a predetermined interval so as to form an optical resonator are supported by cantilever beams. The cantilever is deformed by electrical means to change the distance between the multilayer mirrors. In such a wavelength tunable optical filter, it is desirable that three or more multilayer film reflecting mirrors are formed, and that the distance between the reflecting mirrors is maintained even when the cantilever is deformed and its absolute value changes.
[0011]
By arranging three or more multilayer film reflectors at equal intervals, a structure in which two or more sets of equivalent optical resonators are cascaded can be realized, and the length of the resonator is changed by a reflector supported by a cantilever. In this case, if the intervals are maintained to be equal, a tunable optical filter with a narrow transmission bandwidth can be realized.
[0012]
In particular, it is preferable that three multilayer mirrors have the same shape. In this case, the electric means is a voltage applying means, and it is desirable to apply a voltage so that the potentials of the two reflecting mirrors with respect to the central reflecting mirror are equal to each other. Further, it is desirable that the reflectance of the central reflecting mirror is higher than the reflectance of the reflecting mirrors on both sides.
[0013]
A configuration using three reflecting mirrors can achieve the object of the present invention, and can realize a configuration in which two equivalent optical resonators having the simplest structure are cascaded. By giving equal potentials to the central mirror and the mirrors on both sides, the absolute value of the interval can be changed while maintaining the same interval. That is, even if the length of the resonator changes, the state where the same optical resonator is cascaded is maintained. Also, by increasing the reflectance of the central reflecting mirror, resonance between the reflecting mirrors on both sides can be suppressed.
[0014]
It is desirable that the multilayer film constituting the reflecting mirror be composed of a GaAs layer and an AlGaAs layer. The manufacturing process of the GaAs / AlGaAs system is established for semiconductor lasers and the like, and by using this, the structure of the wavelength tunable optical filter of the present invention can be easily manufactured. Further, integration with a semiconductor laser and other optical elements is also possible.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
It is known to cascade a plurality of resonators as a means for reducing the spread of the transmission wavelength band of a filter using an optical resonator. The present invention has been made based on the idea that the cascaded optical resonator is constituted by a semiconductor multilayer film supported by a cantilever. Hereinafter, a specific embodiment will be described with reference to the drawings.
[0016]
FIG. 1 shows the structure of the wavelength tunable optical filter of the present invention. Three semiconductor multilayer mirrors 22, 24 and 26 are supported by a cantilever 30 with a gap from the substrate 10. A first optical resonator is formed by the upper reflecting mirror 22 and the central reflecting mirror 24, and a second optical resonator is formed by the central reflecting mirror 24 and the lower reflecting mirror 26, and the first optical resonator and the second optical resonator are formed. The two optical resonators are cascaded, sharing the central reflector 24. Although each reflecting mirror is shown as having a circular shape, it may have another shape such as a rectangular shape. However, it is preferable that all three sheets have the same shape. As for light incident from above, only light having a wavelength resonated in the gaps 42 and 44 between the reflecting mirrors is selectively transmitted, and this structure functions as an optical filter.
[0017]
A variable DC voltage source 50 is connected to the central reflecting mirror 24 as shown in the figure, and an equal voltage is applied to the upper and lower reflecting mirrors 22 and 26, so that the cantilever portion is bent. Can be changed. That is, the two cascaded resonators change in length under the control of the applied voltage while the resonator lengths are kept equal to each other. This makes it possible to selectively transmit the incident light 1 of various wavelengths.
[0018]
Next, a method of manufacturing the structure of FIG. 1 will be described with reference to FIG. On the GaAs substrate 110, the reflection layers 122, 124, and 126 having a pair of GaAs / Al _0.8 Ga _0.2 As are grown. The layers 122 and 126 corresponding to the upper and lower reflecting mirrors are doped with Se to make the conductivity type n-type. These layers were 13.5 pairs of stacked layers in which the uppermost layer and the lowermost layer were GaAs layers. On the other hand, the layer 124 corresponding to the central reflector was doped with Zn to be p-type, and 31.5 pairs were stacked. Sacrifice layers 142, 144, and 146, which are finally voids, are provided between the reflection layers and between the lower reflection mirror and the substrate. The composition of these sacrificial layers is Al _0.98 Ga _0.02 As (FIG. 2A).
[0019]
When the GaAs substrate 10 is below the reflecting mirror as shown in FIG. 1 and the incident light 1 is incident as shown in FIG. 1, the transmitted light of the optical filter is incident on the surface of the GaAs substrate 10 and the reflected light is Occurs. In the case of normal incidence, the reflected light returns to the direction of the incident light, and when a semiconductor laser is used as a light source, the operation of the semiconductor laser becomes unstable, which is not preferable. Therefore, it is desirable to provide the antireflection film 12 on both surfaces of the GaAs substrate in advance.
[0020]
Next, a mask having the shape of the reflector and the cantilever portion is formed by photolithography, and the reflector, the cantilever portion, and the cantilever base 60 (see FIG. 1) are removed by inductively coupled plasma (ICP) etching. Is removed. Next, a void is formed. Nitrogen in which pure water is bubbled is introduced into an annealing furnace, and the semiconductor layer is heated at 480 ° C. As a result, the Al _0.98 Ga _0.02 As layer is selectively oxidized to become an Al oxide substantially similar to Al ₂ O ₃ . This oxide layer can be selectively removed by etching with a buffered hydrofluoric acid aqueous solution. As described above, a basic structure of three multilayer mirrors supported by the cantilever can be formed (FIG. 2B).
[0021]
By leaving the original semiconductor layer as it is when forming the cantilever, the pedestal portion 160 supporting the cantilever can be formed at the same time. Part of the pedestal is removed so that the surface of the multilayer film corresponding to the central reflecting mirror is exposed. Au layers 172, 174, and 176 to be the electrodes 72 and 74 are formed on the exposed surface, the uppermost layer, and the back surface of the GaAs substrate (not shown in FIG. 1) by vapor deposition, respectively (FIG. 2C).
[0022]
The same positive potential is applied to the center electrode 74 to the upper and lower electrodes 72 and 76 on the substrate. As a result, the upper and lower reflecting mirrors 22 and 26 are drawn with reference to the central reflecting mirror 24, and the resonator length can be changed.
[0023]
FIG. 3 shows transmission characteristics of the manufactured optical resonator type filter. It can be seen that when the applied potential difference is changed in the range of 0 to 10 V, the center transmission wavelength changes in the range of 1550 nm to 1517 nm. For comparison, the characteristics of a single resonator without the upper or lower reflector are shown. The change of the center transmission wavelength with respect to the voltage is the same in both cases, but the full width at half maximum (3 dB transmission band) of the transmission spectrum is greatly improved by using a double resonator. In the case of 0V, it is 0.8 nm, and in the case of 10V, it is 1 nm. The insertion loss of this optical filter is 1 dB or less.
[0024]
When an optical filter having the above characteristics is used in a WDM system having a channel spacing of 1.6 nm, it means that a form factor of about 0.27 and a crosstalk of 30 dB can be obtained. In the case of a single resonator, the form factor is 0.06 and the crosstalk is about 10 dB, so that the characteristics are greatly improved.
[0025]
Although the above structure shows an example of a double resonator including three reflecting mirrors, multiple resonators may be used. In the above-described manufacturing method, it is possible to form more multilayer mirrors by the same method only by increasing the number of layers to be grown first.
[0026]
In addition, since this optical filter is basically manufactured by growing a multilayer film and etching the multilayer film, it is easy to form a large number of optical filters on the same substrate to form an optical filter array. If electrodes are formed independently for each optical filter and different voltages are applied to each filter, an optical filter array having many different transmission wavelengths can be realized.
[0027]
In the above example, GaAs / AlGaAs is used as the material of the reflection layer, but the material is not limited to this. If the same compound semiconductor is used, an InP / InGaAsP system or the like can be used. Further, a dielectric multilayer film may be used. In the case of a dielectric, a separate transparent electrode layer must be provided.
[0028]
The advantage of forming a multilayer film from a compound semiconductor is that when another optical semiconductor element is manufactured, it can be manufactured in substantially the same steps as the steps. For this reason, for example, this optical filter can be formed simultaneously with the light emitting element and the light receiving element, and can be integrated. In the structure shown in FIG. 1, if a pn junction is formed on the substrate below the reflecting mirror to form a light receiving element, a demultiplexing optical system that detects only light having a wavelength transmitted through the filter can be formed.
[0029]
【The invention's effect】
According to the present invention, a structure in which two or more sets of equivalent optical resonators are cascaded can be realized, and a wavelength tunable optical filter having a narrow transmission bandwidth can be realized. Therefore, when applied to a WDM system or the like, crosstalk can be reduced. Further, by forming the reflecting mirror with a compound semiconductor multilayer film such as a GaAs / AlGaAs system, integration with an optical element such as a light emitting element or a light receiving element is easy.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing a structure of a wavelength tunable optical filter of the present invention.
FIG. 2 is a diagram showing a manufacturing process of the wavelength tunable optical filter of the present invention.
FIG. 3 is a diagram illustrating an example of characteristics of the wavelength tunable optical filter of the present invention.
[Explanation of symbols]
1 Incident light 10, 110 Substrate 22, 24, 26 Multilayer reflector 30 Cantilever 42, 44 Void 50 Variable DC power supply 60 Base 122, 124, 126 Reflective layers 142, 144, 146 Sacrificial layers 172, 174, 176 Au layer

Claims (5)

The multilayer reflectors, which are opposed to each other at a predetermined interval so as to form an optical resonator, are supported by cantilever beams, and the cantilever beams are deformed by electric means to change the intervals between the multilayer reflectors. A wavelength tunable optical filter comprising three or more multilayer film reflecting mirrors, wherein the intervals are maintained equal to each other. 2. The tunable optical filter according to claim 1, wherein the three multilayer mirrors have substantially the same shape. 3. The tunable optical filter according to claim 2, wherein the electric means is a voltage applying means, and applies a voltage such that the potentials of the two reflecting mirrors with respect to the central reflecting mirror are equal to each other. 4. The tunable optical filter according to claim 3, wherein the reflectance of the central mirror is greater than the reflectance of the mirrors on both sides. The tunable optical filter according to any one of claims 1 to 4, wherein the multilayer film forming the reflecting mirror includes a GaAs layer and an AlGaAs layer.

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