Micro-mirror

Micro-mirror

Supervisor: Prof. KämperTeam member:Chun Wa George YoungIssis ContrerasRamanathan SwaminathanSpandan Shroff10-10-2011

Micromechatronics SystemFH AACHENUniversity of Applied Science

small sizelow cost,fast response – filter unwanted wavelengthshigh signal to noise ratioreduced complexity and low power consumption

Advantage of MEMS Spectrometer over conventional

Key features of micromirror array

a 100% fill-factorminimum surface roughness and high reflectivity (ideally 100%) in the wavelength range of interestzero insertion losszero noisesimple controlidentical deflection versus input energy response

A unique application of micromachined mirrors can be performance enhancement of a spectrometer by selectively focusing a particular spectral component on a detector thereby reducing the number of detectors required.

improves the signal to noise ratio of the spectrometer device

IR RANGE SPECTROMETER

Source: FhG-IZMLittrow configuration

PORTABLE UNIVERSIAL SPECTROMETER

Small mirror – reducing bandpass - higher resolutionBut low light intensity

Variable band pass filter

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DIFFRACTION GRATING

Monochromatic lightd sina + d sinb = wavelengthSo leading to constructive interference

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DIFFRACTION GRATING

When a diffraction grating is used, care must be taken in the design of broadband monochromators because the diffraction pattern has overlapping orders.

In-plane diffraction refers to a diffraction technique in which both the incident and diffracted beams are nearly parallel to the sample surface.

The spectrum is scanned by rotating the grating; this moves the grating normal relative to the incident and diffracted beams, which (by Eq. (2-1)) changes the wavelength diffracted toward the second mirror.

The grating diffracts the light, which converges toward the exit slit; the spectrum is scanned by rotating the grating to bring different wavelengths into focus at or near the exit slit.

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GRAT SCANNING

The spectrum is scanned by rotating the grating; this moves the grating normal relative to the incident and diffracted beams, which (by Eq. (2-1)) changes the wavelength diffracted toward the second mirror.

The grating diffracts the light, which converges toward the exit slit; the spectrum is scanned by rotating the grating to bring different wavelengths into focus at or near the exit slit.

the grating is rotated to bring different wavelengths into focus at the (stationary) exit slit.

The efficiency is maximal when the grating is used in the Littrow configuration

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PLAN GRATING AND MOUNTS

The Czerny-Turner monochromator

the light incident on and diffracted by the grating is collimated, the spectrum remains at focus at the exit slit for each wavelength,

The Ebert-Fastie monochromator

This design is a special case of a Czerny-Turner mount

DisadvantageThis can be seen by recognizing that the Ebert-Fastie monochromator is a special case of the Czerny-Turner monochromator in which both concave mirror radii are the same, and for which their centers of curvature coincide.AdvantageHowever, an advantage that the Ebert-Fastie mount provides is the avoidance of relative misalignment of the two mirrors

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PLAN GRATING AND MOUNTS

The Monk-Gillieson monochromator

AdvantageOften the angles of reflection (from the primary mirror), incidence and diffraction are small (measured from the appropriate surface normals), which keeps aberrations (especially off-axis astigmatism) to a minimum.DisadvantageConsequently the spectrum cannot remain in focus at a fixed exit slit when the grating is rotated (unless this rotation is about an axis displaced from the central groove of the grating42).

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PLAN GRATING AND MOUNTS

The Littrow monochromator

The same auxiliary optics can be used as both collimator and camera, since the diffracted rays retrace the incident rays.

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DEFINITIONIn-plane diffraction refers to a diffraction technique in which both the incident and diffracted beams are nearly parallel to the sample surface.

The blaze wavelengthis defined as that wavelength, in a given diffraction order m, for which the efficiency curve reaches its maximum.

ml = d (sina + sinb)

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FABRICATION

polysilicon (poly-Si) and gold micromirror is fabricated by a surface -micromachining process

Drawback: residual stressResult: curvature of the mirror surface which results in high insertion loss and crosstalk

So monocrystalline silicon

best results and closely resemblesan ideal reflector surface.

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1 DIMENSION ARRAY MICRO MIRROR

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TYPE OF MICRO MIRROR

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QUESTIONThe proposal is workable?Mirror size?

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OPTICS DEFINITIONAberrations are departures of the performance of an optical system from the predictions of paraxial optics

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SLIT

The entrance slit is placed at the effective focus of the collimator so that the light from the slit reflected from the mirror is collimated (focused at infinity)

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PORTABLE MONOCHROMATOR

300nm – 5um

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PORTABLE MONOCHROMATOR

Light source

Incident ray

reflected ray

InGaAs Detector

HgCdTe Detector

Linear motion

Rotational motion

Diffraction grating with different Blaze wavelength

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