Ocean Optics MMS Raman - Manual (Page 21)

B: Introduction to Multimodal Sampling
Coded Aperture
The coded aperture is fabricated in such a manner that it has
transmissive and blocking apertures. MMS allows for the
flexible use of these aperture codes and custom codes can be
implemented to optimize the performance for any specific
application. To estimate a spectrum, place the source at the
input to illuminate the whole aperture. For ease and
convenience, a fiber bundle can
also be attached to the aperture
if the sample is physically remote from the spectrometer. By
the use of collimating and focusing lenses, the input aperture is
spectrally imaged onto the CCD. The CCD measures
multiplexed patterns of the aperture depending on the spectral
content of the source. The CCD measurements are then inverted
by the use of appropriate algorithms and the source spectrum is
reconstructed. It is important to know the particular aperture code that is implemented in the system for
the inversion algorithms. Figure 3 illustrates the spectrum from an Argon lamp obtained with a MMS
spectrometer. Some key differentiators for this technique include the following:
Figure 3: Spectrum of Argon lamp
obtained with an MMS
spectrometer
Multiplex (SNR) Advantage
A slit/fiber spectrometer measures each spectral channel separately onto a pixel in the detector. However
in MMS, each pixel in the detector measures several spectral channels of the source in tandem. This is the
multiplex advantage of MMS; resulting in a substantial improvement in signal-to-noise ratio (SNR) (see
Signal to Noise Ratio ).
Multimodal (Etendue) Advantage
MMS spectrometers offer an etendue advantage (see Etendue ). Every spectral source radiates into
numerous spatial modes. For fiber or slit based systems, only a few modes can be coupled into the
system. Since the aperture area of the MMS input is comparatively large, many modes of the source can
be simultaneously coupled into the optical system.
Imaging Aspect of MMS
Inherently, MMS is an imaging spectrometer design. So this technique can be easily adapted to
simultaneously measuring many spectral inputs using a single detector array.
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