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
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.
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.
Near infrared (NIR) spectroscopy can provide inexpensive, rapid and contact-free chemical content measurements for
on-line, hand-held and laboratory applications. Traditionally multiwavelength NIR analyzers are based on incandescent
lamp light sources with rotating filter wheels, even though designs relying on lamp technology and moving parts mean
larger size, require frequent maintenance and eventually limit measurement speed of the system. Today, optical power
and available wavelength range of LEDs enable their use in chemical content analyzers. In this publication, a paper
moisture meter with high speed LED techniques is presented. A prototype developed at VTT utilizes an extended
InGaAs detector to measure diffuse reflection at four NIR wavelengths ranging from 1.2 to 2.1 μm. Source LED currents
are amplitude modulated with fixed sinusoidal frequencies. Optical signals at each wavelength are demodulated from the
detector signal using real-time digital lock-in detection method on an FPGA. Moisture content is calculated and
displayed on the embedded platform. The design allows very high speed operation, where the result is updated every 1
ms. Performance of the prototype system was studied by measuring a set of known sealed paper samples. Paper moisture
measurement accuracy was 0.14, repeatability 0.01 and 2σ noise 0.04 moisture percent. Laboratory tests showed that
channel crosstalk after detection is below background noise level. The measured signal-to-noise ratios per channel were
70 - 85 dB when all LEDs were on. The overall performance equals the level of incandescent lamp based on-line
moisture meters currently in use in paper mill and process automation. The developed system forms a good basis also for
other content measurements.
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.
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.
This paper will discuss recent results obtained when applying a photoconductive linear MCT array in a demonstration spectrometer designed for the NIR wavelength range from 1300 to 2500 nm. A new 128x1 element MCT sensor was developed specifically for spectroscopy, i.e. with "tall", rectangular pixels in order to optimize both wavelength resolution and optical throughput. Also new read-out electronics was developed using multilayer LTCC (Low Temperature Co-Fired Ceramics) techniques, which is integrated into the package and realizes synchronous ("lock in") detection for each of the 128 channels. Advantages of this current-detection scheme include compatibility with chopped light sources (insensitivity to ambient stray light) and elimination of read-out noise (affecting charge-detection amplifiers). The first test results reported here confirm spectrometer operation and present encouraging performance, even though the system is not yet optimized. The spectrometer is very fast, with minimum integration time of 1.2 ms, while photometric noise will reduce with longer integration times. There is no fundamental limit in the maximal length of the integration time. Testing with integration times of 1.2, 12, 120 and 1200 ms resulted in absorbance noise levels of approximately 2500, 330, 94 and 49 μA units. Demonstration spectra were measured from lactose and copying paper samples. Thanks to high speed and parallel spectral recording of 128 wavelengths, MCT array technology appears highly potential for developing powerful on-line spectrometers for process analytical applications not only in the near infrared (NIR) but also for the lower mid-IR wavelengths, up to approximately 6 μm.
The optical analyzers used in on-line and off-line process measurements set a large variety of special demands for optical detectors, i.e. customized detectors are needed. Both the detector (array) and its read-out circuit are affected. The typical volume for process analyzers is so low that fully customized ASICs easily become too expensive for signal recovery solutions in optical detectors. This is due to high fixed costs in ASIC development.
Low Temperature Co-fired Ceramics (LTCC) substrates allow high integration grade and the smart packaging solutions needed in optical detectors. The ceramic substrate is suitable for hermetic packaging. The high integration grade is possible thanks to multilayer capability, with narrow metal strips as well as blind and buried vias which can be placed directly underneath the solder pads. Standard connection methods can be used, i.e. soldering, gluing and wire bonding for components and detector chip assembly. By using small passive components and bare chip or chip scale packaged active components, one is able to integrate the read-out functions in a small enough space.
This paper briefly presents three different cases where infrared photoconductive detector arrays are attached to read-out circuitry which is made on LTCC substrate. The detector read-out hybrids are packaged in hermetic metal packages. The packages have been either non-cooled or thermoelectrically cooled. 24 element PbS, 4 by 4 element PbS and 128 element MCT cases are handled. Also an example of an integrated infrared light emitting diode array for an LED-spectrometer is presented. General feasibility analysis, including some electrical test results and the management of substrate dimensional tolerances, is given.
The use of optical spectroscopic methods for quantitative composition measurements in the field of process control is increasing rapidly. Various optical configurations are already in use or are being developed, with the aim of accomplishing the wavelength selectivity needed in spectroscopic measurement. The development of compact and rugged spectrometers for process monitoring applications, has been one of the major tasks for the optical measurements research team at VTT Electronics. A new PbS detector array- based spectrometer unit has now been developed for use in process analyzers, providing 24-wavelengths ranging from 1350 to 2400 nm. Extensive testing has been carried out to examine the performance of the developed units, concerning performance in normal operating conditions, characteristics vs. temperature, unit-to-unit variation and preliminary environmental testing. The main performance characteristics of the developed spectrometer unit include stable output, a band center wavelength (CW) unit-to-unit tracking better than -+ 1 nm, a band CW draft vs. operating temperature less than 1.8 nm in the temperature range +10 degree(s)C...+50 degree(s)C, and optical stray light below 0.1 percent. The combination of technical performance, small size, rugged construction, and potential for medium manufacturing cost (4000-5000 dollars in quantities) make the developed unit a promising alternative in developing competitive high-performance analyzers for various NIR applications.
This paper discusses the possibilities of the light-emitting diode technology in view of application as an instrument light source, especially for on-line process analyzers and hand-held optical sensors. The results of comparative measurements carried out on the spectral power output of selected NIR LEDs and incandescent lamps are summarized. Some aspects concerning the technical optimization of LED- based instruments are dealt with, and the availability of commercial LED devices with a wavelength range from visible to mid-IR wavelengths, are briefly covered. Finally, some examples of LED-based multiwavelength light source modules developed at VT T Electronics over the past few years are briefly reviewed. These comprise a multiwavelength LED source for pulp and paper applications, a prototype 7- wavelength LED spectrometer for the 3.4 micrometers region, and a second generation 32-wavelength SWNIR spectrometer unit for high performance analyzer applications.
The trends in optoelectronic products are towards higher integration level of optics, electronics and mechanics. It means smaller dimensions and tighter packaging density. The precisions in component manufacturing and accuracies in module assemblings typically are in 10 to 50 micrometer range. Due to demands of the production in series of tens of thousands it means new type of know-how in production and assembling technologies.
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.
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.
Optical analysis techniques, infrared spectroscopy in the front end, are rapidly achieving new applications in process control. This progress is accelerated by the development of more rugged instrument constructions. This paper describes two analyzer techniques especially developed for use in demanding environments. First, the integrated multichannel detector techniques is suitable for applications where the measurement can be accomplished by using 2 to 4 wavelengths. This technique has been used to construct several compact, portable and battery-operated IR analyzers, and process analyzers which measure exactly simultaneously at each wavelength resulting in very high tolerance against rapid changes and flow of the process stream. Secondly, a miniaturized Fourier transform infrared (FTIR) spectrometer is being developed for use as an OEM module in specific process and laboratory instruments. Special attention has been paid to increase the resistance of FTIR technique to ambient vibrations. The module contains an integrated digital signal processing electronics for intelligent control of the spectrometer and for fast real time spectral data treatment. Application studies include on line measurement of the concentrations of diluted and colloidal organic detrimental substances, especially pitch components, in the circulating waters in paper machine wet end.
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.
This paper discusses the requirements for a portable infrared analyzer and presents the design of a four wavelength NIR analyzer utilizing the integrated multichannel detector technique for wavelength separation and detection. This technique, together with an electrically modulated miniature tungsten filament source and surface mount electronics manufacturing, provides a compact, rugged handheld instrument construction without any moving parts. Improved accuracy and short stabilization time is achieved through a combination of thermoelectric temperature stabilization in the four channel detector and a calculated signal compensation for the residual temperature error. Parallel analog phase sensitive detection of the detector signal maximizes the S/N ratio and maintains the simultaneity of the measurement. All composition calculations are performed in a microcomputer built inside the analyzer. The weight of the prototype analyzer is about 1 kg and a NiCd battery pack provides capacity for hundreds of single measurements or about 3 hours of continuous operation. Two prototype instruments have been fabricated with optimized NIR wavelengths for the moisture measurement of milled fuel peat on production fields. The accuracy of digitized two-wavelength signal ratios were tested in the laboratory over time and against temperature. Full accuracy is achieved in 10 seconds after switch-on and the maximum short time peak-to-peak variation in the signal ratios is 0.2%. The errors due to temperature fluctuations in the range from +2 to +50 degree(s)C are between -0.4 to +0.6%. The instruments were calibrated using 102 samples of Finnish milled fuel peat. the cross-validation testing of calibration gave a standard deviation of 1.6% (moisture by weight) compared to the reference method. Other applications for the analyzer are being planned in wood processing and chemical industries as well as in agriculture.
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.