Jouko Malinen

LED-based NIR spectrometer module for hand-held and process analyser applications

Abstract This paper discusses an electronically scanned, small size 32-wavelength NIR spectrometer unit developed for use as a building block in hand-held composition meters as well as in spectroscopic sensors for process automation. The design relies on a linear LED array light source and a fixed grating monochromator. The benefits of the LED spectrometer unit are a highly accurate wavelength scale, with fixed precalibration for subnanometer tolerances and compatibility with a wide operating temperature range, thanks to internal temperature stabilisation. A series of LED spectrometer modules was manufactured for short wave near infrared (SW-NIR) specifications, covering the range from 832 to 1048 nm and a testing program was arranged to characterise typical performance and the spread of the key optical parameters. Potential applications for this SW-NIR wavelength scale can be seen in food, agricultural and petrochemical analysis. The design can also be modified for other wavelengths, for which LEDs are currently available, i.e. from 400 nm to beyond 2 μm, which will further widen the scope of possible applications in the field of NIR spectroscopy and colour analysis.

MEMS and piezo actuator-based Fabry-Perot interferometer technologies and applications at VTT

Miniaturized spectrometers covering spectral regions from UV to thermal IR are of interest for several applications. For these purposes VTT has for many years been developing tuneable MEMS-based and more recently piezo-actuated Fabry-Perot Interferometers (FPIs). Lately several inventions have been made to enter new wavelengths in the VIS range and enlarge apertures of MEMS devices and also extending the wavelength range of piezo-actuated FPIs. In this paper the background and the latest FPI technologies at VTT are reviewed and new results on components and system level demonstrators are presented. The two FPI technologies are compared from performance and application point of view. Finally insight is given to the further development of next generation devices.

Novel spectroscopic techniques for biomedical applications

This paper describes two new spectroscopic techniques which are utilizing hybrid integrated optoelectronics particularly suitable for field and hand-held use. First, the LED module is based on a linear array of light emitting diodes and a fixed monochromator, and provides a solid-state electrically scanned source for pre-dispersive spectrometers. A prototype module operating from 810 to 1060 nm with resolution of 10 nm scans one spectrum in 19 ms and has a solid glass construction with dimensions of 4 X 4 X 7 cm. Potential applications include miniature, rugged and low cost instruments for transcutaneous blood and tissue spectroscopy in the near infrared (NIR) region.

Nondispersive and multichannel analyzers based on mid-IR LEDs and arrays

This paper briefly describes the fabrication of infrared light emitting diodes by liquid phase epitaxy including both InGaAs LEDs for emission wavelengths from 2.5 to 3.8 micrometers and InAsSbP for wavelengths from 3.8 to 4.7 micrometers . Some of the first applications of these LEDs in spectroscopic instrumentation are described together with the main instrument characteristics. Nondispersive analyzers have been developed for CH4 and CO detection utilizing LED sources emitting at 3.3 and 4.7 micrometers , respectively. A novel infrared spectrometer construction has been developed based on a linear LED array emitting at 3.3 micrometers and a fixed grating monochromator. This miniature construction can be used as an electrically scanned spectrometer module in future IR analyzers for portable and process on- line applications.

Advances in miniature spectrometer and sensor development

Miniaturization and cost reduction of spectrometer and sensor technologies has great potential to open up new applications areas and business opportunities for analytical technology in hand held, mobile and on-line applications. Advances in microfabrication have resulted in high-performance MEMS and MOEMS devices for spectrometer applications. Many other enabling technologies are useful for miniature analytical solutions, such as silicon photonics, nanoimprint lithography (NIL), system-on-chip, system-on-package techniques for integration of electronics and photonics, 3D printing, powerful embedded computing platforms, networked solutions as well as advances in chemometrics modeling. This paper will summarize recent work on spectrometer and sensor miniaturization at VTT Technical Research Centre of Finland. Fabry-Perot interferometer (FPI) tunable filter technology has been developed in two technical versions: Piezoactuated FPIs have been applied in miniature hyperspectral imaging needs in light weight UAV and nanosatellite applications, chemical imaging as well as medical applications. Microfabricated MOEMS FPIs have been developed as cost-effective sensor platforms for visible, NIR and IR applications. Further examples of sensor miniaturization will be discussed, including system-on-package sensor head for mid-IR gas analyzer, roll-to-roll printed Surface Enhanced Raman Scattering (SERS) technology as well as UV imprinted waveguide sensor for formaldehyde detection.

Semiconductor emitter based 32-channel spectrophotometer module for real-time process measurements

A new type of semiconductor emitter based multichannel spectrophotometer has been designed and tested. The spectrophotometer consists of a small electrically conirolled narrow band light source an optical receiver and microprocessor electronics for data processing. The light source is based on a 32-element GaAs and GaAIAs LED chip array which is connected to a diffraction grating and feedback optics. The source is capable of emitting intensity-stabilized single-beam narrow band light pulses. The wavelength of the light pulse can be selected by the electronics without using any moving parts. The optical mechanical and optoelectronic parts of the source have been integrated to form a compact hybrid construction. Main characteristics have been tested with an experimental 32-channel spectrophotometer designed for the wavelength range 810 nm - 1060 nm. Measured wavelength half-power bandwidths are 8 nm and channel separation is 7. 5 nm. A single spectrum scan can be recorded in 8 ms. 64 scans are averaged by the microprocessor electronics and data is transferred to a PC for a multicomponent spectrum analysis program. Output light power level is better than i05 times the averaged detector noise level. The wavelength range used is optimized for near infrared transmittance (NIT) analysis of agricultural products. 1.

Handheld hyperspectral imager

VTT Technical Research Centre of Finland has developed a new low cost hand-held staring hyperspectral imager for applications previously blocked by high cost of the instrumentation. The system is compatible with standard video and microscope lenses. The instrument can record 2D spatial images at several wavelength bands simultaneously. The concept of the hyperspectral imager has been published in SPIE Proc. 7474. The prototype fits in an envelope of 100 mm x 60 mm x 40 mm and its weight is ca. 300 g. The benefits of the new device compared to Acousto-Optic Tunable filter (AOTF) or Liquid Crystal Tunable Filter (LCTF) devices are small size and weight, speed of wavelength tuning, high optical throughput, independence of polarization state of incoming light and capability to record three wavelengths simultaneously. The operational wavelength range with Silicon-based CCD or CMOS sensors is 200 - 1100 nm and spectral resolution is 2 - 10 nm @ FWHM. Similar IR imagers can be built using InGaAs, InSb or MCT imaging sensors. The spatial resolution of the prototype is 480 x 750 pixels. It contains control system and memory for the image data acquisition. It operates either autonomously recording hyperspectral data cubes continuously or controlled by a laptop computer. The prototype was configured as a hyperspectral microscope for the spectral range 400 - 700 nm. The design of the hyperspectral imager, characterization results and sample measurement results are presented.

Thirty-two-channel LED array spectrometer module with compact optomechanical construction

A compact and versatile 32-wavelength spectrometer module has been developed based on a linear LED array and a fixed grating monochromator. The design includes all the optical, mechanical, and optoelectronic parts in a size of approximately 4 x 4 x 7 cu cm. The wavelength bands are scanned electronically without any moving parts. All the optical parts have been assembled to form a cemented solid glass construction, which is mechanically and thermally stable and well protected against water condensation or dust. The developed source module can be easily modified and has obvious advantages for spectroscopic analyzers, especially in process and portable applications.

Comparative performance studies between tunable filter and push-broom chemical imaging systems

This paper reports instrument characterization measurements, which were recently arranged to provide comparative information on different hyperspectral chemical imaging systems. Three different instruments were studied covering both tunable filter and push-broom techniques: The first instrument MatrixNIRTM is based on a LCTF tunable filter and InGaAs camera and covers wavelengths from 1000 to 1700 nm. The second one SisuCHEMATM is based on push-broom technology and MCT camera operating from 1000 to 2500 nm. The third system is an instrument prototype from VTT Technical Research Centre of Finland exploiting high speed Fabry-Perot interferometer and MCT camera, currently calibrated from 1260 to 2500 nm. The characterization procedure was designed to study instrumental noise, signal-to-noise ratio, linearity and spectral as well as spatial resolution. Finally, a pharmaceutical tablet sample was measured with each instrument to demonstrate speed of measurement in a typical application. In spite of differences in wavelength ranges and camera technologies used, the results provide interesting information on relative instrumental advantages and disadvantages, which may be useful for selecting appropriate instrumentation for defined applications. Further, an additional aim of this study is to compare the high speed Fabry-Perot imaging technology under development against the established chemical imaging techniques available on the market today.

Multiplexed read-out electronics implemented on LTCC substrate for PbS array

Infrared detector arrays are very useful for spectroscopy to realize grating spectrometers with the ability for parallel signal detection. The signals from the detector pixels are usually very weak, and they have to be amplified. Because of the large number of pixels, it is often necessary to multiplex the signals to reduce the number of signal lines out from the package. This work presents the preamplifier and multiplexer modules built on low temperature co-fired ceramic (LTCC) substrate. The LTCC material is suitable for hermetic packaging, which is often needed for PbS detectors in process applications. The capacitive cross coupling of the modules is measured from the modules. The modules are implemented into a package next to the PbS array and the nonlinearity and normalized detectivity of the system are measured. The measurements show that special concern is needed in layout design to avoid capacitive cross coupling. The measurements also show that multiplexing does not deteriorate the good detectivity of the PbS detector.