In the field of near-infrared (NIR) spectroscopy, a system that combines portability with the accuracy and functionality of a high-performance laboratory system will greatly improve real-time analysis. The development of a small handheld power spectrum analyzer powered by a battery enables more efficient monitoring of industrial processes or food maturity on site.
Most dispersion spectroscopy measurements are performed in the same way at the outset. The analyzed light passes through a small slit; this slit is combined with a grating to control the resolution of the instrument. This diffraction grating is specifically designed to reflect light of different wavelengths at a known angle. The spatial separation of this wavelength allows other systems to measure light intensity based on wavelength.
The main difference in traditional spectral measurement architectures is the way in which scattered light is measured. Two common methods are (1) a single-element (or single-point) detector combined with a physical scan of scattered light, and (2) imaging scattered light over a set of detectors.
Method of using MEMS technologyMany new limitations in traditional spectral analysis methods can be overcome using a new approach based on optical microelectromechanical systems (MEMS) array technology with a single point detector. In a single point detector based system, a solid state optical MEMS array replaces a conventional electric grating with a simple, spatial wavelength filter. This approach takes advantage of the performance advantages of single-point detectors while eliminating problems in fine-grained control of electric systems. In recent years, such systems have been put into production in which scanning gratings are replaced and MEMS devices filter each specific wavelength into a single point detector. This approach also delivers high performance while achieving a smaller, robust and robust spectrum analyzer.
The use of optical MEMS arrays has several advantages over linear array detector architectures. First, larger single-element detectors can be used to increase daylighting and greatly reduce system cost and complexity, especially for infrared systems. In addition, since no array detectors are used, pixel-to-pixel noise is eliminated, which can greatly improve signal-to-noise ratio (SNR) performance. The improved SNR performance allows for more accurate measurements in less time.
In a spectroscopic analysis system using MEMS technology, the diffraction grating and focusing elements function the same as before, but the light from the focusing elements is imaged on the MEMS array. To select a wavelength for analysis, a specific spectral response band is activated so that light can be introduced into a single point detector for acquisition and measurement.
These advantages can be achieved if the MEMS device is highly reliable, capable of generating predictable filter responses, and being constant at different times and temperatures.
Using a DLP® chip or digital micromirror device (DMD) as a spatial light modulator and using it as a MEMS device in a spectrum analyzer system architecture can overcome several challenges. First, a set of aluminum micromirrors is used to turn the light entering the single point detector on and off, which is optically efficient over a wide range of wavelengths. Second, the open and closed states of the digital micromirror are controlled by mechanical latching devices and latching circuits of complementary metal oxide semiconductor (CMOS) static random access memory (SRAM) cells to provide fixed voltage mirror control. This fixed voltage, standstill control means that this system does not require mechanical scanning or analog control loops and simplifies calibration. It also makes the spectrum analyzer design more immune to sources of error such as temperature, aging or vibration.
DMD's programmable properties have many advantages. One of these advantages will appear when designing the spectrum analyzer architecture - if based on the addressing column of the micromirror used as the filter. Since the DMD resolution is usually higher than the desired spectrum, the DMD area will be underfilled and the spectrum will be oversampled. This makes the wavelength selection fully programmable and uses an additional micromirror as a recalibration column in the event of extreme mechanical displacement of the light engine.
In addition, DMD is a two-dimensional programmable array that provides users with a high degree of flexibility. Resolution and throughput can be adjusted by selecting a different number of columns. The scan time can be dynamically adjusted so that the user can perform longer, more detailed checks on the desired wavelength for better instrument time and functionality. In addition, a high degree of flexibility and higher performance can be achieved with respect to the fixed filter device 1, such as the advanced aperture coding technique such as the Hadamard pattern employed.
In summary, spectral analysis devices using DMDs enable higher resolution, greater flexibility, greater robustness, smaller form factor and lower cost compared to current spectral analysis systems, making them available for a wide range of applications. Commercial and industrial applications are more attractive.
Single detector architecture eliminates noiseAt present, linear array-based optical spectrum analyzers are mainly limited by two factors. First, the wavelength selection of the detector is limited by the pixel aperture. The size of the detector determines the amount of light collected, which affects the SNR. A typical GaAs indium gallium arsenide (InGaAs) 256 pixel linear array such as Hamamatsu G9203-256 has a size of 50 microns x 500 microns. Conversely, a digital micromirror array is a fully programmable matrix that can be configured for the number of columns and scanning techniques for the application. This allows a larger signal to be presented to a larger 1 mm or 2 mm single point detector that is typically used with the DMD. Filtering narrowband light into a linear array—usually 50 micron wide pixels—may cause crosstalk problems. Pixel-to-pixel interference can be a major cause of noise during reading. These disturbances can be eliminated by a single detector architecture. In addition, by using digital micromirror scanning speeds from 1 kHz to 4 kHz, single-point detectors can achieve similar dwell times as parallel multi-point sampling. For MEMS-based or DMD-based compact spectrum analyzer engines, the results show a range of SNRs greater than 10,000:1.
Small, high-resolution 2D MEMS array critical for super mobile spectrum analyzers
In order to maximize performance, the user needs to consider the total area of ​​the MEMS that can be used to reflect light to the detector. This area is then carefully matched to the available single point detector aperture size.
A DMD with 5.4 micron micromirrors has more than 400,000 available pixels and can be optimized for wavelengths from 700 nm to 2500 nm. The DMD is the DLP2010NIR, which uses a new pixel architecture called TRP. As seen in Figure 1, this pixel provides a 17 degree tilt angle. The DLP2010NIR runs in an evaluation module; this evaluation module provides a unique optical architecture for the spectrum analysis application scenario. An optical path that utilizes 17 degree turn-on and turn-off angles enables high-performance sensing resolution with a compact engine that minimizes scattered light.
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