FI108581B - Electrically adjustable optical filter - Google Patents

Description

]! 1 i Electrically adjustable optical filter

The invention relates to an electrically adjustable optical bandpass filter according to the preamble of claim 1.

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The invention is intended to be used as an electrically modulated optical bandpass filter in applications where high contrast or 1-2 adjustable bandwidths are required.

10 Fabry-Perot interferometers are used in optical analysis and modulators as optical bandpass filters. The surface micro-mechanics of silicon provide a practical opportunity to produce Visible Infrared (VIS-IR) quality filters. The interferometers made by this technique are so called. short interferometers, which means that the optical resonator has a length of 1-3 half waves. The width of the passband 15 depends on the reflection coefficients of the mirrors. Performance can be described by the bandwidth (Full Width at Half Maximum, FWHM) and contrast, which is the ratio of the maximum throughput intensity to the throughput intensity next to the pass band. The contrast is typically about 200-300. Electrostatic adjustment 20 provides an adjustment range of about 25% relative to the wavelength corresponding to the zero voltage.

Optical band-pass filters are generally sandwich interference filters whose pass band cannot be adjusted. Adjustable filters are usually of the Fabry-Perot type, the most recent versions of which are made of silicon surface micromechanics. These 25 techniques are described e.g. in M. Blomberg, M. Orpana, A Lehto, "Electrically Adjustable Surface Micromechanical Fabry-Perot Interferometer for Use in Optical Material Analysis," U.S. Patent Application No. 08 / 386,773. These typically have three-layer mirrors, alternately made of polycrystalline silicon and silica. The layers of the mirror have an optical thickness of λ / 4: η multiplied by 30, typically one λ / 4. Bandwidth can be affected by the number of layers of mirrors, but the contrast cannot be significantly increased in this way and the passband cannot be split.

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The invention is based on the fact that the optical bandpass filter has two electrically adjustable surface micromechanical Fabry-Perot filters fixedly superimposed on the same optical axis.

More specifically, the optical filter according to the invention is characterized in what is stated in the characterizing part of claim 1.

The invention provides significant advantages 10

With the solution of the invention, the contrast can be increased to several tens of thousands, even hundreds of thousands, and alternatively the pass band can be divided into two bands. If the refractive index of the fitting layer differs from the first, the bandwidth will be slightly twofold. If the central mirror is identical to the 15 mirror mirrors, two transmission bands of the order of 10,000 silicon-silica mirrors are obtained. This enables two wavelengths to be analyzed at the same time, which is impossible with a filter having only one transmission band. The position of the peaks can also be adjusted.

In the following, the invention will be further explored by means of the following exemplary embodiments.

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Fig. 1 is a sectional side elevational view of an optical filter of the invention.

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Fig. 2 is a graph showing the pass band of a filter according to the invention where: • t *;,; is the air fitting between the mirrors.

Fig. 3 graphically shows the pass band of a filter according to the invention in which. 30 is a silica fitting between mirrors.

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Figure 4 is a graph showing the pass band of a filter according to the invention having a silicon nitride fit between the mirrors.

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Figure 5 graphically illustrates the pass band of a filter according to the invention at three different control voltages, with corresponding air gaps of 420 nm, 450 nm and 480 nm.

Figure 6a is a sectional side view of a filter of the invention for wavelengths smaller than 1.1 µηι.

Fig. 6b is a sectional side elevation view of a filter of the invention for wavelengths greater than 1.1 µm.

Fig. 7 is a sectional side elevation view of a filter according to the invention in which the optical fit is performed with an air layer.

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Figure 8 is a top view of a filter according to the invention.

Figure 9 is a sectional side elevation view of a filter according to the invention in which the air gaps are 450 nm long and the central mirror is common.

Fig. 10 graphically shows the pass band of the filter of Fig. 9 when the air gaps i * 1 / * are 450 nm long.

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Thus, according to Figure 1, the structure according to the invention consists of a first resonator 15, 10, 12 formed on the substrate 1 ·· »* i * * 25 and a second resonator 20, 10, 16 formed thereon. a silicon substrate 1 with an etched opening 5 at the filter. Over the opening 5 is

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t three-layer mirror 15, which according to the manufacturing orientation can be called t 'the first mirror. Mirror 15 consists of alternating polysilicon layers 2 and »» 30 silicon layers 3. A similar mirror 16 is also present in the structure,

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as the third mirror. The middle layer structure 17, consists of two »'1' similar triple-layer mirrors 12 and 20, between which there is a fitting layer 18 having a optical thickness λ / 4 of 4 'HB ^ öl. The wavelength λ is the passband wavelength of the filter. The material of the fitting layer 18 preferably has a refractive index of 1 (air or vacuum) if a single high contrast penetration band is desired. In practice, a good result is obtained even if the layer 18 is silica with a refractive index of about 1.46. Above and below the central mirror structure 17 are cavities 10 which act as optical resonators. The length of the cavity 10 in the direction of the optical axis 11 is typically n * λ / 2, where = 1.2.3.

The lengths of the cavities 10 can be independently adjusted by the voltages V1 and V2 applied to the conductive regions 10 13 and 14, which cause an electrical force between the outermost mirrors 15 and 16 and the central mirror structure 17. The central mirror 17 can be used as a common voltage terminal. Under the influence of electrostatic forces, the outer mirrors 15 and 16 are inclined towards the central mirror 17. The position of the passband can be adjusted to about 25% of the rest wavelength. The control voltage required is typically from a few 15 volts to a few tens of volts, completely depending on the resting wavelength, i.e. the height of the cavities and the internal tension of the mirrors. Both DC and AC voltages can be used for control.

In the IR region (λ> 1.1 µm), the aperture 5 under the mirrors is not required as shown in Fig. 6b, since the low-doped silicon is transparent.

Fig. 2 shows the pass-through band of the filter of Fig. 6a when the refractive index of the fitting layer 18 is one (air), in Fig. 3 when it is silica and in Fig. 4 when it; 'Ton of silicon nitride. Figure 5 shows the effect of the control voltage on the location of the transmission band, *,;,: 25 when the voltages of the two interferometers are equal. In this case, the fitting layer 18 is silica.

: 'The more detailed layer structure of the interferometer using a silicon substrate is shown in Figures 6a and 6b. The interferometer is made by growing alternating layers of polysilicon «<· · ... · 30 2 and silica layers 3 on a planar substrate 1. The substrate 1 can be: t 1 monocrystalline silicon, germanium, metal oxide or nitride, lithium niobate, glass, or »·: V a compound semiconductor, e.g., GaAs. The metal oxide may be e.g. alumina and t 1 1 f 5! O 8 S δ 1 nitride eg titanium nitride. In principle, the substrate 1 can be any material on which the mirror layers can be applied, and whose optical properties are suitable for the purpose. If the substrate 1 is transparent in the selected wavelength range, the structure may be as shown in Figure 6b. If the substrate 1 is silicon, then at a wavelength 5 greater than about 1.1 µm, aperture 5 is not required. The oxide can be removed through the interferometer mirrors 10 through the openings 4 with e.g. hydrofluoric acid. The walls of the apertures 4 are polycrystalline silicon. Mirrors in the interferometer typically have a diameter of 1 to 2 mm and an optical thickness of λ / 4 of layers 2 and 3 of the mirrors. The openings 4 can be very small in diameter, a few micrometers is sufficient. The aperture 5 is etched into silicon, e.g. KOH or TMAH, if necessary, whereby layer 6, typically silicon nitride, acts as an etching stop. If the aperture 5 is not etched, an anti-reflection film 7, typically λ / 4, of silicon nitride must be applied to the underside of the silicon substrate. In this case, the layer 6 is most preferably silica. The optical thickness of the anti-reflection film 7 is λ / 4 for any substrate and its preferred refractive index is the square root of the refractive index of the substrate 15.

If an air layer (or vacuum) 18 is used between the middle mirrors, the structure of the interferometer is as shown in Figure 7.

20 As the mirror layers are raised planarly on top of each other, the middle mirrors are fixed to each other. When viewed from above, the interferometer looks like in Figure 8. Upper mirror: Like other mirrors, the upper mirror is most preferably circular and the holes 4 are located around the periphery of the circle. The dark squares 20 are contact areas for power supply.

: 25 'i *:, ·' · * The middle mirror 17 need not be a matched combination of two mirrors, but may have a smaller number of layers. Figure 9 shows a structure in which the central mirror 17 is similar to other mirrors. Figure 10 shows a response corresponding to this structure having two transmission bands. In contrast, this structure 30 provides about 10,000 silicon-silica-mirror structures.

Claims (11)

1 U 8 5 8 1 5 1. An electrically adjustable optical filter made of a silicon micromechanical sandwich structure comprising: - a substantially planar substrate (1), a first mirror (15) formed on a substrate (10) and a second mirror (17) formed thereon. ), and - an optical resonator chamber (10) formed between the mirrors (15, 17) having an optical length of approximately n * λ / 2, where n = 1,2,3,15, characterized in that - on the second mirror (17) a third mirror structure (16) is formed, and a second optical resonator (10) having an optical length of approximately n * λ / 2 is formed between the third mirror (16) and the second mirror (17), in which: ··! n = l, 2,3. » Optical filter according to claim 1, characterized in that the optical thicknesses of the layer structures (2, 3) of the filter are substantially one quarter of the measuring wavelength. Optical filter according to claim 1, characterized in that the second mirror (17) comprises two mirror structures (12, 20) between which an optical matching layer (18) is formed. > 08581 Optical filter according to claim 1, characterized in that the filter comprises electrical contacts (13, 14) for independently adjusting the length of each optical resonator (10) by means of an electric force. j Optical filter according to claim 1, characterized in that the substrate (1) is monocrystalline silicon, monocrystalline germanium, lithium niobate, glass, metal oxide or nitride or a compound semiconductor. Optical filter according to Claim 1, characterized in that the mirrors (15, 10 16, 17) are made of alternating layers of silicon (2) and silica (3). Optical filter according to claim 1, characterized in that the optical matching layer (18) of the second mirror structure (17) is silicon, germanium, lithium niobate, glass, metal oxide, silica or silicon nitride or a compound semiconductor. ! 15 Optical filter according to claim 1, characterized in that the optical matching layer (18) of the second mirror structure (17) is air. Optical filter according to Claim 1, characterized in that an anti-reflective film (7) is provided on the underside of the substrate (1). Optical filter according to Claim 1, characterized in that it consists of two closely overlapping Fabry-Perot types, ·. ·. an interferometer having a common mirror (17). | 25 An optical filter according to claim 1, characterized in that all mirrors (15, 17, 16) of the structure are at least approximately identical.