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microFTS
Hugh Mortimer, Rutherford Appleton Laboratory (STFC)
Competition sponsored by:
Spectroscopy: What is it?
“The branch of science concerned with the investigation and measurement
of spectra produced when matter interacts with or emits electromagnetic
radiation.”
• Electromagnetic radiation is passed through or reflected from a sample.
• Some of the radiation is absorbed and some is transmitted or reflected by
the sample.
• Spectrometers separate (think glass prism) and measure the intensity of
the emerging radiation as a function of its wavelength – to enable
analysis of the optical, chemical, and physical properties of the
sample.
Spectrometers• Nowadays, two of the most commonly used spectrometer designs are
Michelson Fourier-transform spectrometers (FT-IRs) (high signal-to-
noise, but bulky and sensitive to disturbance) and miniature dispersive
spectrometers (small & rugged, but low signal-to-noise), which are suitable
to very different applications.
• Dispersive spectrometers disperse the incoming light into a frequency
spectrum, which is directly recorded by a detector array.
• Fourier-transform spectrometers split and recombine the incoming light to
generate an interference pattern which is recorded - the frequency spectrum
is subsequently generated by taking the Fourier-transform of the
interference pattern.
Over the next few slides I’ll explain the operating principles of each of
these spectrometer designs and compare them to our microFTS, a new
kind of Fourier-transform spectrometer.
Dispersive Spectrometers
ENTRANCE SLIT
MIRROR 1
GRATING
MIRROR 2
DETECTOR ARRAY
Light enters the spectrometer through a slit and
is reflected from a collimating mirror (MIRROR 1).
The collimated light is reflected from a
diffraction grating to disperse the light into
its separate wavelength components.
The dispersed light is reflected from a second
mirror (MIRROR 2) and refocused onto a detector
array.
The frequency spectrum of the incident radiation
is recorded directly by the detector array.
How they work
wavelength or frequency
inte
nsity
SPECTRUMOptical Bench
(Un-Folded Czerny-Turner)
MIRROR 1
(FIXED)
MIRROR 2
(MOVING)
SINGLE POINT DETECTOR
BEAMSPLITTER
Michelson FT-IR
Light enters the spectrometer and is split into two
perpendicular beams at the beamsplitter.
One beam of light is reflected from a static mirror
(MIRROR 1) and the other beam from a moving mirror
(MIRROR 2). The moving mirror introduces a time
delay to the second beam.
The two beams are brought back together at the
beamsplitter and interfere to form an interference
pattern that is temporally modulated.
Each data point in the interference pattern
corresponds to a time and thus a position of the
scanning mirror – the range and speed of the
scanning mirror determines the number of data
points in the interference pattern.
Taking the Fourier-transform (FT) of the interference
pattern yields the frequency spectrum of the incident
radiation.
FOURIER TRANSFORM
ALGORITHM
How it works
time
inte
nsity
wavelength or frequency
inte
nsity
INTERFERENCE PATTERN
SPECTRUM
Optical Bench
microFTS
MIRROR 1
DETECTOR ARRAY
BEAMSPLITTER
MIRROR 2
How it works
Light enters the spectrometer and is split into
two beams at the beamsplitter.
One beam of light is transmitted by the
beamsplitter and reflected from MIRROR
2, then MIRROR 1, before being again
transmitted by the beamsplitter and striking
the detector array. The other beam of light is
reflected by the beamsplitter and reflected
from MIRROR 1, then MIRROR 2, before
reflecting from the beamsplitter and striking
the detector array. The two beams are
focussed by the curved mirrors to combine
and interfere at the detector array.
An optical path difference between the two
beams is introduced by the different
distances the two beams travel around the
set-up. The resulting spatially modulated
interference pattern is spread across a
detector array.
Taking the Fourier-transform (FT) of the
interference pattern yields the frequency
spectrum of the incident radiation.
FOURIER TRANSFORM
ALGORITHM
distance
inte
nsity
wavelength or frequency
inte
nsity
INTERFERENCE PATTERN
SPECTRUM
Optical Bench
Comparison TableInstrument Format Michelson Dispersive microFTS
Also Known As FT-IR or FT-NIR spectrometersGrating-based, diode array, or
CCD spectrometers
Static-Imaging Fourier
Transform Spectroscopy
(SIFTS)
Moving Parts? Yes No No
Relative Cost High Low Low
Data Acquisition SpeedSlow ~ 10s
(limited by mirror scan speed)
Fast ~ 100ms
(limited by detector read-out
rate)
Fast ~100ms
(limited by detector read-out
rate)
Signal-to-Noise Ratio /
SensitivityHigh Low Medium
Internal Wavelength
Calibration Possible?Yes No Yes
Size And Weight Large and heavy Compact and light Compact and light
Spectral Region Of
OperationMIR to NIR NIR , Vis and UV MIR, NIR, Vis and UV
Examples of Market
Leading Brands
Bruker, PerkinElmer,
ThermoFisher Scientific, ABB,
FOSS
Ocean Optics, Avantes,
ThermoFisher ScientificN/A
Basically, it’s the best of both worlds: Great signal (almost) like the bulky lab setups
we all know but fast, small & rugged like miniature dispersive spectrometers.
Early Prototype microFTS
A an early prototype
instrument, operating in the visible
spectral region.
The instrument is fibre-optic fed
and uses a off-the-shelf optical
components and a CCD detector
array.
35m
m
BEAMSPLITTER
MIRROR 2
MIRROR 1
DETECTOR ARRAY
FIBRE OPTIC INPUT
Early Performance Specifications
Note that these are indicative specifications, based on laboratory-measurements
i. The system will incorporate alternative detector technologies, such as linear PbSe (lead salt) arrays, 2d VOx
microbolometer arrays, and others.
ii. Wavelength range is dependent on beamsplitter material and detector type. For the IR model the ranges quoted
are for ZnSe or KBr beamsplitters.
iii. Instrument resolution is determined by the maximum optical frequency (or minimum wavelength) in the recorded
spectrum. The instrument resolution was measured as the FWHM of a laser source.
iv. Measurement of instrument stability was limited by the experimental set-up. It is anticipated that it will be an
order of magnitude better than the value quoted. A calibration laser can be incorporated into the instrument for
enhanced stability (equivalent to the use of a HeNe calibration laser in a Michelson-interferometer).
v. SNR measurements were made as the ratio of the central peak of a laser line to an average background noise
level.
Spectral Region UV-Vis IR
Detector(i) 2d CCD array Linear PZT pyroelectric array
Wavelength
Range(ii)200 to 1 050 nm
2 to 17 µm
(5 100 to 600 cm-1)
or
2 to 20+ µm
(7 000 to 350 cm-1)
Resolution(iii)
@ 200 nm
(50 000 cm-1)
< 1.2 nm
(300 cm-1)
@ 10 µm
(1 000 cm-1)
0.1 µm
(10 cm-1)
@ 500 nm
(20 000 cm-1)
< 2.9 nm
(120 cm-1)
@ 20 µm
(500 cm-1)
0.2 µm
(5 cm-1)
Stability(iv) Better than 1 part in 25 000 per ⁰CEquivalent to: 0.04 nm per ⁰C @ 1 000 nm (0.4 cm-1 per ⁰C at 10 000 cm-1)
SNR(v) 500:1 100:1
Size ~ 160 x 115 x 45 mm
Mass ~ 0.5 kg
How can you help?
We’ve developed what we think is a really cool instrument, a real step forward in spectrometer design. While it was developed for atmospheric measurements on Mars, we’d like to find out how it could be useful for us Earthlings.
• Do you have some thoughts on how to implement the proposed applications listed in the challenge page below?
• Can you think of any new applications?
• It’s a pretty modular setup – How could we modify the instrument to be even more useful or useful in a different setting?
Thanks a lot guys! Remember to use those marbles…