ELECTRICALLY ADJUSTABLE OPTICAL FILTER

Abstract: The invention relates to an electrically controllable optical interferometer filter with a layered structure made using silicon micromechanical techniques. The filter comprises an essentially planar substrate (1), a first mirror element (15) deposited on said substrate (1) and a second mirror element (12, 17) superimposed on said first mirror element, and an optical resonator cavity (10) formed between said mirror elements (15, 17) with an optical length of about n. lambda /2, where n = 1, 2, 3. According to the invention, above said second mirror element (12, 17) is further made a third mirror element (16), and between said third mirror element (16) and said second mirror element (12, 17) is formed a second optical resonator cavity (10) with an optical length of about n. lambda /2, where n = 1, 2, 3.

Description: Electrically adjustable optical filter The present invention relates to an electrically adjustable optical bandpass filter according to the preamble of claim 1. The invention is intended for use as an electrically modulatable optical bandpass filter in applications requiring a high contrast ratio or 1-2 adjustable bandwidths. Fabry-Perot interferometers are used in optical analysis and modulators as optical bandpass filters. Surface micromachining of silicon offers a practicable approach to the manufacture of high-quality interferometer-type filters for the VIS-IR (Visible Infrared) range. Interferometers manufactured using this technique are so-called short interferometers, which means that the length of the optical resonator is in the range 1-3 half- wavelengths. The bandwidth shape of the filter passband is determined by the reflection coefficients of the mirrors. The filter performance can be characterized by the width of its transmittance curve (Full Width at Half Maximum, FWHM) and the contrast ratio of the filter, which is defined as d e ratio of the filter maximum passband transmittance to the filter transmittance just adjacent to the passband. In a silicon-silicon dioxide-silicon three-layer mirror, the bandwidth can be made as small as about 2 % of the passband center wavelength. The contrast ratio of the filter is typically about 200-300. Using electrostatic control techniques, a control range of approx. 25 % about the zero-control-voltage wavelength can be obtained. Optical bandpass filters are generally implemented as multilayer interference filters lacking any means of controlling the passband. Typically, controllable filters are of the Fabry-Perot- type, of which the latest versions are implemented by silicon micromechanical techniques. This technology is described in, e.g. , the US Pat. Appl. No. 08/386,773 "An electrically controllable silicon surface micro- mechanical Fabry-Perot interferometer for use in optical material analysis" , filed by the inventors M. Blomberg, M. Orpana and A. Lehto. These devices typically have three-layer mirrors made from alternate layers of polysilicon and silicon dioxide. The optical thickness of the mirror layers are made an odd multiple of a quarterwave λ/4, typically one λ/4. While the filter bandwidth shape may be affected by the number of mirror layers, no significant increase of the filter contrast ratio or division of filter bandwidth into separate bandwidths cannot be attained in this way. The present invention is based on constructing an optical bandpass filter by virtue of placing two electrically controllable, surface micromechanically fabricated Fabry-Perot resonator filters in a superimposedly fixed manner on the same optical axis. More specifically, the optical filter according to the invention is characterized by what is stated in the characterizing part of claim 1. The invention offers significant benefits. The arrangement according to the invention permits an increase of the filter con- trast ratio to a level of tens of thousands, even up to hundred thousand, or alternatively, splitting of the filter bandwidth into two separate bandwidths. In the case that the refractive index of the optical matching layer is made different from one, the filter transmittance curve can be made slightly dual-peaked. Provided that the center mirror is made identical with the outer mirrors, two bandwidths are obtained having a contrast ratio in the order of 10,000 using a silicon-silicon dioxide-silicon mirror structure. This feature facilitates analysis simultaneously at two wavelengths, which is not possible by means of single-bandwidth filter. Moreover, the present filter construction permits separate center wavelength control of the filter passbands. In the following, the invention will be examined in greater detail with the help of exemplifying embodiments illustrated in the appended drawings, in which Figure 1 shows a longitudinally sectional side view of an optical filter according to the invention; Figure 2 shows the transmittance curve of a bandpass filter according to the invention having the outer mirrors matched with an air layer; Figure 3 shows the transmittance curve of a bandpass filter according to the invention having the outer mirrors matched with a silicon dioxide layer; Figure 4 shows the transmittance curve of a bandpass filter according to the invention having the outer mirrors matched with a silicon nitride layer; Figure 5 shows the transmittance curve of a bandpass filter according to the invention at three different control voltage levels corresponding to air layer thicknesses 420 nm, 450 nm and 480 nm; Figure 6a shows a longitudinally sectional side view of a filter structure according to the invention suited for wavelengths shorter than 1.1 μm; Figure 6b shows a longitudinally sectional side view of a filter structure according to the invention suited for wavelengths longer than 1.1 μm; Figure 7 shows a longitudinally sectional side view of a filter structure according to the invention having the optical matching of the outer mirrors implemented by means of an intermirror air layer; Figure 8 shows a top view of a filter according to the invention; Figure 9 shows a longitudinally sectional side view of a filter structure according to the invention with a common center mirror and the air layer gaps adjusted to 450 nm; and Figure 10 shows a bandwidth curve of the filter structure of Fig. 9 with the air gaps adjusted to 450 nm. Now referring to Fig. 1, the interferometer-type filter structure according to the invention shown therein comprises a first optical resonator 15, 10, 12 formed on a substrate 1 and a second optical resonator 20, 10, 16 located above the first resonator. The lowermost element of the filter structure is the silicon substrate 1 having an opening 5 etched at the filter cavity. Above the opening 5 is formed a three-layer mirror 15, herein denominated as the first mirror analogously to its manufacturing order. The mirror 15 is formed by alternating polysilicon layers 2 and silicon dioxide layers 3. A similar mirror 16 is located as the topmost member of the filter structure acting as the third mirror. The center element formed by a layered element 17 comprises two identical three-layer mirrors 12 and 20 having a index-matching layer 18 with an optical thickness of λ/4 provided thereinbetween. The wavelength λ herein denotes the center wavelength of the filter passband. The refractive index of the material used in the matching layer 18 is most advantageously 1 (i.e. , that of air or a vacuum) when a single bandwidth of high contrast ratio is desired. In practice a good result is also obtained by making the matching layer 18 from silicon dioxide having a refractive index of about 1.46. Above and below the center mirror structure 17 are provided cavities 10 acting as optical resonators. Typically, the length of the cavity 10 in die direction of its optical axis 11 is n-λ/2, where n = 1, 2, 3. The gap of the cavities 10 can be adjusted independently from each other by means of control voltages V, and V 2 applied to conducting areas 13 and 14, whereby the applied voltages impose an electrical force between the center mirror structure 17 and outer mirrors 15 and 16. Herein, the center mirror structure 17 can be used as a common terminal for the control voltages. Owing to the electrostatic force, the outer mirrors 15 and 16 are flexed toward the center mirror 17. Thus, the center wavelength(s) of the resonators can be adjusted by about 25 % of dieir rest wavelength. The required control voltage varies typically from a few volts to a few tens of volts, depending on the rest wavelength determined by the resonator gap and the internal tension of the mirror structures. The wavelength adjustment can be accomplished using either DC or AC as the control voltage. In interferometer filters designed for the IR range (λ > 1.1 μm), the opening 5 of Fig. 6b under the mirrors is redundant, because at these wavelengths lightly doped silicon is transparent. In Fig. 2 is shown the bandwidth curve of the interferometer filter illustrated in Fig. 6a when the matching layer 18 has the refractive index made equal to one (using air as the medium), in Fig. 3 when the matching layer is of silicon dioxide and in Fig. 4 when the layer is of silicon nitride. In Fig. 5 is shown the effect of the control voltage on the filter passband center wavelength when the control voltages applied to either interferometer structure are equal. Herein, the optical matching layer 18 is in both structures made of silicon dioxide. Figs. 6a and 6b illustrate in more detail the manufacture of the layered interferometer structure on a silicon substrate. The interferometer filter is fabricated by growing alternately polysilicon layers 2 and silicon dioxide layers 3 on a planar substrate 1. The substrate 1 may be selected from the group of single-crystal silicon, germanium, a metal oxide or nitride, lithium niobate, glass or any combination compound semiconductor such as GaAs. The metal oxide can be, e.g. , aluminium oxide and the metal nitride can be, e.g. , titanium nitride. In principle, the substrate 1 can be of any material on which the deposition of the mirror layers can be performed and which has optical properties compatible with the specifications of the interferometer filter. If the material of the substrate 1 is transparent over the selected wavelength range, its structure can be such as shown in Fig. 6b. In applications using silicon as the substrate 1 for wavelengths longer than 1.1 μm, the opening 5 will be redundant. The oxide from the gap 10 between the mirrors of the interferometer filter can be removed via openings 4 using, e.g., hydrofluoric acid as the etchant. The walls of the openings 4 may be of polysilicon, for instance. Typically, the diameter of the mirrors in the interferometer filter is in the order of 1-2 mm, whereby the optical thickness of the mirror layers 2 and 3 is λ/4. The openings 4 can have a very small diameter, e.g. , a diameter of a few micrometers will be sufficient. When required, the opening 5 is etched in silicon using, e.g. , KOH or TMAH as the etchant, whereby the layer 6, typically made of silicon nitride, acts as the etching barrier. In the case that the opening 5 is not etched in the structure, the lower surface of the silicon substrate must be provided with an antireflective layer 7, typically made from a λ/4 layer of silicon nitride. Herein, the layer 6 is advantageously of silicon dioxide. The optical thickness of the antireflective layer 7 is λ/4 for any substrate, and most advantageously the refractive index of the layer is adjusted to be the square root of the substrate refractive index. If the gap between the center mirrors is made using air (or a vacuum) as the filling medium, the structure of the interferometer filter will be such as shown in Fig. 7. Owing to the superimposed deposition of the mirror layers, the center mirrors are located tightly adjacent to each other. When viewed from above, the interferometer filter will have the structure shown in Fig. 8. The upper mirror 16 as well as the other mirrors are advantageously made circular, and also the openings 4 are placed about the perimeter of a circle. The square areas 20 marked dark indicate contact pad areas for electrical connections. While the center mirror 17 is typically a combination of two mirrors, it may be made using a smaller number of layers. In Fig. 9 is shown a structure having the center mirror 17 made identical to the other mirrors. Fig. 10 shows a bandwidth curve of two bandwidths corresponding to such a structure. The contrast ratio of this structure will be about 10,000 when the interferometer filter is implemented as a silicon-silicon dioxide-silicon mirror construction.

Claims: Claims: 1. An electrically controllable optical interferometer filter with a layered structure made using silicon micromechanical techniques, said filter comprising - an essentially planar substrate (1), - a first mirror element (15) deposited on said substrate (1) and a second mirror element (17) superimposed on said first mirror element, and - an optical resonator cavity (10) formed between said mirror elements (15, 17) with an optical length of about n-λ/2, where n= 1,2,3, characterized in that - above said second mirror element (17) is further made a third mirror element (16), and - between said third mirror element (16) and said second mirror element (17) is formed a second optical resonator cavity (10) with an optical length of about n-λ/2, where n = 1, 2, 3. 2. An optical filter as defined in claim 1, characterized in that the optical thicknesses of said layered structures (2, 3) of the filter are equal to a quarterwave of the measurement wavelength using the filter. 3. An optical filter as defined in claim 1, characterized in that said second mirror element (17) comprises two mirror members (12, 20) having an optical matching layer (18) adapted there inbetween. 4. An optical filter as defined in claim 1, characterized in that said filter includes electrical contacts (13, 14) for the purpose of adjusting the length of either optical resonator (10) by means of an electrically applied control force. 5. An optical filter as defined in claim 1, characterized in that said substrate (1) is of single-crystal silicon, single-crystal germanium, lithium niobate, glass, a metal oxide or nitride or any combination compound semiconductor. 6. An optical filter as defined in claim 1, characterized in that said mirror elements (15, 16, 17) are made from alternate layers of silicon (2) and silicon dioxide (3). 7. An optical filter as defined in claim 1, characterized in that the medium of the optical matching layer (18) of said second mirror element (17) is of silicon, germanium, lithium niobate, glass, a metal oxide, silicon dioxide or nitride or any combination compound semiconductor. 8. An optical filter as defined in claim 1, characterized in that die medium in the optical matching layer (18) of said second mirror element (17) is air. 9. An optical filter as defined in claim 1, characterized in that the lower surface of said substrate (1) is provided with an antireflective layer (7). 10. An optical filter as defined in claim 1, characterized in that said filter comprises two Fabry-Perot-type interferometers connected superimposedly to each other in a fixed manner and having a common mirror element (17). 11. An optical filter as defined in claim 1 , characterized in that all the mirror elements (15, 17, 16) of the filter structure are made at least essentially identical.

Inventors: BLOMBERG MARTTI, LEHTO ARI, TORKKELI ALTTI, BLOMBERG, MARTTI, LEHTO, ARI, TORKKELI, ALTTI
Applicants: BLOMBERG MARTTI, LEHTO ARI, TORKKELI ALTTI, VALTION TEKNILLINEN, BLOMBERG, MARTTI, LEHTO, ARI, TORKKELI, ALTTI, VALTION TEKNILLINEN TUTKIMUSKESKUS
Application year: 1997
Main classification: G02B5
IPC codes: G01J3, G02B5, G02B26