research activities in 2007 - CSEM

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TUGON – Compact MEMS-based Spectrometers for Infra-Red Spectroscopy M. Tormen, R. Lockhart, J-M, Mayor, R. P. Stanley Deformable MEMS diffraction gratings have great promise as tuning elements for external cavity lasers and for compact spectrometers. The challenge is to make high efficiency tunable MEMS gratings and incorporate them into practical devices. CSEM has successfully designed, fabricated and tested MEMS gratings. Their spectral response has been tested and the potential to design ultra-compact spectrometers based around this technology has been shown. In Optical MEMS, the family of diffractive MEMS is interesting for a wide range of applications because they can be compact, fast and their narrow spectra response can be used in spectrometers and for tunable lasers [1] . Commercially available diffractive MEMS are used in displays, in spectroscopy and optical telecommunications [2, 3] . Optical grating Comb drives Figure 1: Overview of a tunable MEMS grating. The white dotted region denotes the optical grating which is actuated by four sets of electrostatic comb drives. CSEM has been developing a tunable MEMS grating technology, and this report demonstrates how it could be incorporated into a compact spectrometer. In the spectrometer, the MEMS grating is stretched like an accordion. The change in the size of the grating changes directly the period of the grating and hence the wavelength tuning of the grating. This method of tuning is completely different from normal spectrometers where the grating is rotated. The advantage of MEMS grating is that the complex mechanics for controlling the rotation of the grating in the standard configuration is replaced by simple electrostatic comb drives which stretch the MEMS grating. Figure 1 shows a processed MEMS device [4] which comprises the tunable optical grating and the electrostatic comb drives which stretch the grating. The device has been fabricated using standard MEMS manufacturing techniques. The complete die measures 6 x 3 mm, with a 1 mm x 1 mm grating. The grating itself is formed from free-standing beams with a 12 µm period and a 50% duty-cycle. The beams are attached to each other using leaf springs. So that it can be stretched in its plane, the grating is free standing. One of the challenges is to make a MEMS grating that has a high diffraction efficiency. A grating with a square profile is the easiest structure to manufacture but has only about 40% diffraction efficiency in the first order. In contrast, blazed gratings can have efficiencies close to unity. In order to achieve this, the gratings have been blazed using anisotropic KOH etching. This technique is widely used to make V-grooves. It yields smooth angled surfaces while maintaining the mechanical properties of the grating beams. Extending this technology to a MEMS device has been a challenge. The MEMS grating shown in Figure 1 is actually a blazed grating. Figure 2 : A minature monochromator in the lab. The light is coupled into and out of the grating (bottom centre) using a pair of fibres and collimating lenses. For scale the MEMS chip is 12 x 6 mm. The spectral response of the MEMS grating was measured using an optical spectrum analyser and a collimated white light source in a configuration shown in Figure 2. The optical system is extremely compact. The resulting spectra are shown for different drive voltages in Figure 3. A tuning range of 3% and subnanometer linewidths have been achieved. Potentially 10% is achievable with the improved mechanical design. The spectral response can be better appreciated in Figure 4 taken for a 1.5 mm long fixed grating made with the same MEMS process technology. A 25 db rejection has been achieved experimentally. Efficiency (Normalized) 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 5 mm 1505 1510 1515 1520 Wavelength (nm) 1525 1530 1535 Figure 3: The spectral response of the MEMS grating shown in Figure 1 for several different drive voltages ranging from 0 to 55 volts. A unique property of these MEMS gratings is that the efficiency of the gratings is high at all wavelengths. Although, the efficiency is strongly angular dependence, as the angle is never varied, in contrast to standard scanning monochromator, it remains constant. This means that the same device can be used for a wide range of spectral regions from the UV to the mid-IR. This versatility is very promising for 0V 5V 10V 15V 20V 25V 30V 35V 40V 45V 50V 55V 19
spectrometer applications so CSEM has targeted to miniature monochromators as a demonstrator of this technology. The next stage of this work will be to move towards a fully packaged compact spectrometer. The near-IR has been chosen to demonstrate the technology, although main potential applications are excepted to be in the mid IR. Several challenges remain, such as increasing the tuning range, control of stray light, etc. In addition all the system related issues such as drive electronics and the mechanical housing need to be finalized. 20 1.0 0.8 0.6 0.4 0.2 0.0 Measured (Normalized) Theory ‐ NA = 0.0011 810 812 814 816 818 820 822 824 Wavelength (nm) Figure 4: Optical characterization and simulation. Optical characterization has proven the expected high optical performance: a linewidth of 0.4 nm at 800 nm has been demonstrated for a 1.5 mm long blazed grating. The MEMS gratings can also be used as the tuning element for external cavity lasers. They have many advantages over standard gratings because the mechanics to tune the grating is built-in so the total module can be compact, rapid and eventually low cost. The gratings are currently being tested with external cavity tunable Quantum Cascade Lasers [5] . A success here will lead to the very compact tunable Mid-IR source. CSEM thanks NCCR Quantum Photonics that partly funded this work. [1] H. Schenk, et al., “Photonic microsystems: an enabling technology for light deflection and modulation”, SPIE, vol.5348, Nr 1, (2004), 7-21 [2] www.siliconlight.com [3] www.polychromix.com/html/products.htm [4] M. Tormen, et al., “Deformable MEMS grating for wide tunability and high operating speed”, SPIE, vol. 6114, (2006), 61140C [5] C. Sirtori, J. Faist, F. Capasso, “The quantum cascade laser. A device based on two-dimensional electronic subbands”, Pure and Applied Optics, vol. 7, Nr. 2, (March 1998), 373-81
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Page 4 and 5: CONTENTS PREFACE 5 RESEARCH ACTIVIT
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