Fabry-­‐Perot  Interferometer  Background    An  optical  Fabry-­‐Perot  filter  consists  of  two  mirrors  separated  by  a  distance  (Figure  1).    In  its  simplest  form  it  is  only  a  slab,  or  an  etalon,  where  incident  light  is  reflected  back  and  forth  between  the  etalon  faces,  thus  forming  constructive  and  destructive  standing  wave  conditions  for  a  certain  wavelength.  In  effect,  the  Fabry-­‐Perot  filter  transmits  certain  narrow  wavelength  bands  while  blocks  the  others.  A  tunable  filter  can  be  constructed  by  placing  two  mirrors  close  to  each  others  while  adjusting  the  gap  between  them.  That  is  where  the  parallelism  of  the  optical  gap  and  the  precision  of  gap  adjustment  become  critical.  The  spectral  resolution  of  a  Fabry-­‐Perot  interferometer  depends  on  the  reflectivity  of  the  mirrors  and  the  gap  between  the  mirrors  while  the  passed  wavelengths  depend  only  on  the  gap.  In  an  FPI  the  resolution  and  the  Free  Spectral  Range,  i.e.  the  distance  between  two  multiple  order  pass  bands,  are  inversely  related  –  having  a  very  high  resolution  results  in  a  very  small  working  range  and  vice  versa.    

 Figure  1  -­‐  Working  principle  of  Fabry-­‐Perot  Interferometer  (FPI).  

 

Piezo-­‐actuated  Fabry-­‐Perot  Interferometers  (PFPI)  Fabry-Perot interferometers using piezo-elements for actuation is a well-known technology. VTT has been developing piezo-actuator–based Fabry-Perot interferometers for spectral measurement applications since 2006. These devices follow in many parts conventional FPI manufacturing techniques where three piezo actuators are used to move the mirrors and three plate capacitances are used for closed loop feedback control of the device. Such a principle has been presented e.g. by Rees et al. in the 80’s [1] but also VTT has had their first activities during the 90’s [2]. VTT’s piezo-actuated Fabry-Perot Interferometers (PFPIs) have been constructed using silica substrates and Ti-Ag-SiO2

mirrors. The novelty is in the integrated structure: capacitors are an integral part of the mirrors, not separate components.

 

Figure 2. Three types of PFPI devices. a) A small version with an aperture of 2.8 mm dedicated for

miniature tunable LED IR source; b) a 7-mm version for VIS/NIR imaging spectrometer; c) a 19-mm version for chemical imaging and multipoint measurements.

A complete camera system utilizing the piezo-FPI tuneable filter component has been made for Unmanned Aerial Vehicles [3] for the spectral range of 500 – 900 nm. The complete device weighed less than 350 g including batteries and it was used to identify diseases in pear trees. Another system that has been built is a prototype of a chemical imaging spectrometer operating in the wavelength range of 1000 – 2500 nm [4]. The concentration distribution for caffeine, aspirin and acetaminophen in an Excedrin™ tablet was successfully measured. Figure 2 shows the developed three different piezo-FPI filter versions. These components enable development of spectrometers and spectral imagers at much lower cost and size than before.

Microelectromechanical  Fabry-­‐Perot  Interferometers  (MFPI)  VTT started the development of microelectromechanical Fabry-Perot interferometers (MFPIs) in the 90’s. The original scientific paper by Blomberg et al. was published in 1997, explaining the construction and usage of a surface micromachined Fabry-Perot Interferometer [5]. This component was applied to a CO2 ensor, which is still produced by Vaisala Oyj under the product name Carbocap®. Another version of the component was made for the European Space Agency, where miniaturized spectrometers were constructed for measuring C2H4 and H2O simultaneously [6].

Figure 3. MEMS-FPI spectrometer for C2H4 measurements [6].

  Significant research effort has been put on the MEMS FPI technology development over the recent years, both at VTT as well as in other institutes. Correia et al. [7] presented a visible range FPI using bulk-micromachining, Weidong et al. [8] analyzed a MEMS FPI filter with dielectric mirrors for telecom applications and Calaza et al. [9] developed a process and a component for Mid Infrared (MIR) FPI. Recently Dell et al. presented a technology for fabricating an MEMS FPI –based spectrometer for real-time testing of soils for agricultural applications [10] and Milne et al. realized a structure for allowing a wider usable wavelength range [11]. In visible wavelength range, VTT’s MEMS FPI platform offers very high transmission [12] compared to bulk-micromachined or metal layer alternatives. The tensioned membrane structure also allows realization of large optical apertures for MEMS based hyperspectral imagers [13-15]. Due to low temperature processing based on atomic layer deposition (ALD) [16], this technology has also been utilized to create monolithical visible light microspectrometer chips consisting of integrated filter on a silicon photodiode [17]. In thermal IR (TIR) wavelength range, VTT has demonstrated a unique MEMS FPI platform which enables realization of high-resolution mirrors based on polysilicon-air structure [18].

Compared  to  alternative  Fabry-­‐Perot  filter  technologies,  the  VTT  FPI  technology  has  some  key  technical  and  cost  benefits:    

• Large optical apertures up to several millimeters in diameter • Intergrated capacitor structures • Superior mirror flatness • Very high nominal frequencies, ~ 1 MHz (insensitive to vibrations) • No gravitational effects • Low operation voltages • Low cost in high quantities (monolithic construction) • Fast MEMS ramp-up time • Wide set of wavelength ranges

 For  more  information,  please  contact  Open  Photonics  Inc,  or          Jarkko  Antila

Jarkko.antila@vtt.fi Tel.:  +358  20  722  6819

   

References:  [1] Rees, D., McWhirter, I., Hays P. B. and Dines, T., “A stable, rugged, capacitance

stabilised piezoelectric scanned Fabry-Perot etalon”, J. Phys. E: Sci. Instrum. 14, pp. 1320-1325 (1981).

[2] Niemelä, Pentti; Sensor, PCT Patent Application, PCT-FI-9200294 (1991). [3] Saari, H., Aallos, V., Akujärvi, A., Antila, T., Holmlund, C., Kantojärvi, U.,

Mäkynen, J. and Ollila, J., ”Novel miniaturized hyperspectral sensor for UAV and space applications”, Proc. SPIE, Vol. 7474, 74741M (2009).

[4] Malinen, Jouko; Saari, Heikki; Kemeny, G.; Shi, Z.; Anderson, C., “Comparative performance studies between tunable filter and push-broom chemical imaging systems “, Next-Generation Spectroscopic Technologies III. Orlando, FL, USA, 5 - 6 April 2010 Proceedings of SPIE - The International Society for Optical Engineering. Vol. 7680 (2010).

[5] Blomberg, M., Torkkeli, A., Lehto, A., Helenelund, Ch. and Viitasalo, M., “Electrically Tuneable Micromachined Fabry-Perot Interferometer for Gas Analysis”, Physica Scripta. Vol. T69, pp. 119-121 (1997).

[6] Saari, H., Mannila, R., Antila, J., Blomberg, M., Rusanen, O. and Tenhunen, J., “Miniaturised gas sensor on the basis of micromachined Fabry–Perot interferometer”, Proceedings of the Third Round Table on Micro/Nano Technologies for Space, pp. 307–313, (2000).

[7] Correia, J.H., Bartek, M., Wolffenbuttel, R.F., “Bulk-micromachined tunable Fabry-Perot microinterferometer for the visible range”, Sensors and Actuators 76, pp. 191-196 (1999).

[8] Weidong, S., Xiangdong, L., Biqin, H., Yong, Z., Xu, L. and Peifu, G., “Analysis on the tunable optical properties of MOEMS filter based on Fabry-Perot Cavity”, Optics Communications 239, pp. 153-160 (2004).

[9] Calaza, C., Fonseca, L., Moreno, M., Marco, S., Cané, C. and Gracia, I., “A surface micromachining process for the development of a medium-infrared tuneable Fabry-Perot interferometer”, Sensors and Actuators A 113, pp. 39-47 (2004).

[10] Dell, J.M., Keating, A.J., Milne, J., Antoszewski, J., Musca, C.A., Faraone, L., Murphy, D. and Samardzic, O., “Micro-electromechanical systems-based microspectrometers covering wavelengths from 1500nm to 5000nm”, Proc. of SPIE, Vol. 6765 (2007).

[11] Milne, J. S., Dell, J. M, Keating, A. J. and Faraone, L., “Widely Tunable MEMS-Based Fabry–Perot Filter”, Journal of microelectromechanical systems, Vol. 18, no. 4 (2009).

[12] Blomberg M., Kattelus, H., Miranto, A., “Electrically tunable surface micromachined Fabry–Perot interferometer for visible light”, Sens. Act. A: Physical, Vol. 162 (2), pp. 184-188 (2010)

[13] Rissanen, A., Akujärvi, A., Antila, J., Blomberg, M., Saari, H.; “Moems miniature spectrometers using tuneable fabry-perot interferometers”. J. Micro/Nanolith. MEMS MOEMS. 0001;11(2):023003-1-023003-6.

  doi:10.1117/1.JMM.11.2.023003.

[14] Rissanen, A., Mannila, R, Antila, J., “Bragg reflectors for large optical aperture MEMS Fabry-Perot interferometers”. Proc. SPIE 8373, Micro- and Nanotechnology Sensors, Systems, and Applications IV, 83732R (May 1, 2012); doi:10.1117/12.920578.

[15] Antila, J., Mannila, R., Kantojärvi, U., Holmlund, C., Rissanen, A., Näkki, I., Ollila, J., Saari, H., “Spectral imaging device based on a tuneable MEMS Fabry-Perot interferometer”. Proc. SPIE 8374, Next-Generation Spectroscopic Technologies V, 83740F (May 1, 2012); doi:10.1117/12.919271.

[16] Rissanen, A., Puurunen, R.L., “Use of ALD thin film Bragg mirror stacks in tuneable visible light MEMS Fabry-Perot interferometers”. Proc. SPIE 8249, Advanced Fabrication Technologies for Micro/Nano Optics and Photonics V, 82491A (February 9, 2012); doi:10.1117/12.905084.

[17] Rissanen, A., Kantojärvi, U., Blomberg, M., Antila, J., Eränen, S., “Monolithically integrated microspectrometer-on-chip based on tunable visible light MEMS FPI”, Sens. Act. A: Physical, Vol. 182, pp. 130-135 (2012)

[18] Tuohiniemi, M., Blomberg, M., Akujärvi, A., Antila, J. and Saari, H., “Optical transmission performance of a surface-micromachined Fabry-Pérot interferometer for thermal infrared”, J. Micromech. Microeng., Vol. 22 (2012) No: 11