Design of a fibre optic coupled spectrometer
AstraNet Systems Based in the high‐tech region of Cambridge, UK was
formed in 2001 to market the StellarNet Inc (Tampa,Florida, USA) range of fibre optic coupled spectrometers in the UK.
During the past 6 years we have built up a team of more than a dozen self‐employed technical experts with skills in the following areas: ‐ optical design, electronics, fibreoptics, mechanical design, firmware and software.
Many successful installations have involved the development of special accessories including customisedsoftware and electronics to create turnkey systems.
The Technology
To my knowledge, there are no manufacturers of these products in the UK except AstraNet Systems Ltd.
Miniature fibre optic‐coupled spectrometers haverevolutionised UV/Visible spectroscopy during the past decade.
Traditional laboratory spectrometers are large and have several moving parts.
Over the coming decade, these miniature spectrometers with no moving parts, high reliability and low maintenancewill take an ever larger share of the spectroscopy market.
SpecificationsFor the optics, we decided to base our specifications around a simple flat‐field
grating spectrograph with good signal/noise, low stray light and reasonable optical resolution.
The wavelength range was set to be from around 220 to 850nm with an optical resolution (FWHM) of 2nm.
A second version of the instrument would have a wavelength range from 350 to 1050nm with 4nm resolution.
We also required fibre optic coupling and of course an array detector.
We chose to use the industry‐standard multimode fibres with 0.22NA and with SMA 905 coupling.
This instrument would ideally be simple to assemble and have no moving parts.
Design Elements
We started with the Optics
Progressed to Electronics
Then to Mechanical
Finally, software
Optics DesignAllan Newton, Optical systems designer, was strongly
recommended to us and much of the following information comes from him.Small compact designs will generally use only few components, the
simplest designs generally being the most attractive.
The simplest of all designs are those based on concave reflecting gratings. Such designs can be implemented with just one component, the grating itself.
Having decided to go ahead with this approach, we approached Headwall Photonics in the USA and they provided us with details of their standard substrates for concave aberration‐corrected holographic gratings.
Allan collaborated with Headwall to create two customized gratings for our instruments.
Linear spaced grating design
Classic curved grating assembly with some disadvantages
Aberration‐corrected
This picture shows the general layout for our spectrometers
The source, typically the end of a fibre, is at the extreme left
Factors affecting optical resolution
Aberration from grating
Our customised grating is almost aberration‐ free, allowing us to use quite narrow slits,
certainly down to 50 microns
Diffraction of grating
Our grating provides approximately 600nm over a 25mm detector, giving us approx
24nm/mm
Optical ResolutionSize of detector elements
Our current detector has 2048 pixels, each 14 microns wide
Slit widthWe typically install a 150 micron slit, which provides
24 x 0.150 = 3.6 nm resolution
A 50 micron slit will give a theoretical resolution of 1.2nm
Obviously, narrow slits will impact on the S/N ratio and our choice of slits has been influenced by all of the above factors
Stray LightStray light is minimised by good optical design
Once light has passed through the slit, the minimal number of optical components reduces the surface scattering which causes stray light
The only light that gets through the slit and does not fall on the grating comes from the extremities of the fibre polar diagram where intensity is low
Careful design of diffraction grating produces minimal stray light.
Each grating is a master holographic and controlled manufacture in clean rooms ensures gratings are delivered in excellent condition
Electronics DesignTo provide versatility we chose to use a Field Programmable
Gate Array (FPGA) device.
This would allow us to drive a wide variety of detectors by simply changing firmware.
We also decided to have a microprocessor on board, with a good amount of memory
We chose a high speed 14‐bit Successive Approximation Register (SAR) type ADC
We included digital I/O and analogue outputs
Flash Memory was included to store calibration data and spectra
Mechanical DesignPhysical size of polychromator body
Location and mounting of components
Integration of optics, electronics
Cover design
DetectorPrism
Grating
Polychromator assembly
Assembled Instrument
AstraNet Spectrometer
FOC ApplicationsThe concept of
FOC’s
opens up a huge range of ‘new’
applications
The main advantages include:‐
Speed of measurement:
Remote measurements: measure reactions directly in containers
Ambient light rejection: take measurements in open air
Small footprint: portability, convenient size
Versatility: one instrument for several applications
No moving parts: high reliability ; low cost of ownership
follow reactions/changes in real time
SpectroradiometryColour measurementFluorescenceAbsorption spectroscopyProcess control
Applications
AccessoriesApplication‐specificCosine response diffuserCuvette holdersImmersion probesFlow CellReflectance Probes
PC SoftwarePC SoftwareCalibrations
Wavelength calibration
Radiation Measurement
Calibration using Bentham CL6 calibrated light source
LUX Measurement
Software has been created using Microsoft .NET
Graphics driven by hardware‐accelerated DirectX
Printing with preview and scheduled saving supported
Application‐specific software includes radiometry, spectroscopy, colour measurement
Applications Software
Slide Number 1AstraNet Systems The TechnologySpecificationsDesign ElementsOptics DesignLinear spaced grating designAberration-correctedFactors affecting optical resolutionOptical ResolutionStray LightElectronics DesignMechanical DesignPolychromator assemblyAssembled InstrumentAstraNet SpectrometerFOC ApplicationsApplicationsAccessoriesPC SoftwareWavelength calibrationRadiation MeasurementLUX MeasurementApplications Software