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Indium Gallium Arsenide NIR Photodiode Array Spectroscopy

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Indium Gallium Arsenide NIR Photodiode Array Spectroscopy MOLECULAR SPECTROSCOPY Indium Gallium Arsenide NIR Photodiode Array Spectroscopy G EORGE A. GASPARIAN AND H ARTMUT L UCHT pectrometers measure the light en- plifier. Longer arrays having hundreds to ergy absorbed or reflected by a ma- thousands of pixels are primarily oper- Electronic terial. Prism spectrometers are the interface ated in a serial-out arrangement, in oldest (~ 2000 years) instruments which each pixel is amplified off-chip Swhereby color measurements were and sequentially read-out. The photodi- made using the eye as the photoreceiver. ode operates in the integrating mode Diode array William Herschel used a prism spec- Grating rather than direct detection, wherein a trometer in the 18th century to examine capacitive transimpedance amplifier is Optical light beyond the red with the first pho- fiber input the first-stage amplification circuit. Fig- ton detector: a blackened thermometer, ure 2 illustrates the detection format. now known as a bolometer. Since Her- These self-scanned linear arrays in- schel and Newton’s time, spectroscopy Slit clude on-board signal analysis electron- has evolved as a separate field of sci- ics that perform operations such as cor- ence. An essential factor to this growth related double sampling, a method to has been the advancements in photore- Figure 1. Diagram of a reflection-grating improve noise performance. Figure 3 il- ceivers. This paper concentrates on the spectrometer with a photodiode array. lustrates an InGaAs photodiode array hy- near-infrared (near-IR) region of the bridized to a silicon readout integrated spectrum between 800 and 2200 nm (ap- circuit. Hybridization is the principal ar- proximately where silica-based optics be- ray fabrication technique for nonsilicon come opaque). Indium gallium arsenide materials such as indium antimonide, (InGaAs) is the best material for a solid- VSS mercury cadmium telluride, indium ar- state detector, offering the highest sensi- senide, and InGaAs. tivity in the near-IR. It has become the Light The array operation is straightfor- Reset detector of choice for near-IR spec- ward: a predetermined exposure time is S/H1 troscopy. specified via a clock pulse, and the inci- Scanning monochrometers (prisms dent photons produce electron-hole and diffraction gratings) with single- S/H2 pairs in the InGaAs absorption layer. element photodetectors have been the VREF These photocarriers generate a pho- mainstay in spectroscopy. Using silver- tocurrent that is processed by the off- emulsion film technology, a complete chip capacitive transimpedance ampli- spectrum (visible–short wave–near-IR) Figure 2. Schematic of an integrating fier into a measurable voltage. After was obtained without the need of a mov- transimpedance amplifier. exposure, the array circuit serially out- ing dispersive element (hence, the name puts each element. A detailed descrip- spectrograph was coined). This allowed tion is beyond the scope of this note. Ju- the spectrometer to be moved into re- in agricultural, food, pharmaceutical, and dicious selection of the source light and mote locations and initiated what be- petrochemical firms. Figure 1 illustrates spectrometer (specifically, diffraction came known as multichannel spectral a reflection grating spectrometer with a grating and optics) is necessary to opti- analysis. With photodetector technology photodiode array. mize the detection process. The photo- advances, linear and matrix photodiode Linear detector arrays are available diode array’s mechanical, optical, and arrays are becoming the detector of with thousands of individual photodiode electrical parameters are interdepen- choice for remote monitoring. Real-time elements (pixels) in a convenient, small dent and affect system performance. measurements of key on-line parameters package. Arrays having less than 64 ele- The remainder of this article will high- are performed in process control, using ments can be operated in the parallel out light the performance parameters and diode-array spectroscopy. Uses of re- configuration, in which each pixel is indi- operating techniques for a grating- mote diode-array spectroscopy are found vidually addressable with a separate am- based InGaAs photodiode array. ........................ MOLECULAR SPECTROSCOPY WORKBENCH ........................ tinction must be made between linear eight (relative to 20 °C) and the dark response and saturated response. As the noise is reduced by a factor of 2.83 (or photoelement approaches saturation, it 883-e noise level). However, if the readout can produce a nonlinear response noise is 2000 e, the root-sum-square noise (supraresponsivity). The preferred mode is 2186 e, a 9% noise reduction. The extra of operation of the array is radiometric, effort required to cool the array for a 9% common to most spectrometers. improvement might not be cost effective. The lowest signal level is defined by Techniques are available to reduce the ef- the silicon multiplexer’s readout noise. fects of dark signal such as offset correc- Neither the photon shot noise nor dark tion, dark field subtraction, and multiple Figure 3. An InGaAs photodiode array hy- signal shot noise is considered here. scans at shorter integration times. bridized to a silicon readout integrated The latter exclusion may seem unusual. The charge capacity in a capacitive circuit. Normally, the dark signal limits the per- transimpedance amplifier is determined formance; cooling the array enhances from the integrating capacitance output the dynamic range. Sometimes this voltage product. Diode arrays are avail- The principal mechanical parameters helps, but there is no point in cooling an able with multiple capacitors selectable are array length, center-to-center spac- array such that the dark noise is much to the specific application (for example, ing, element geometry, fill factor, and below the silicon multiplexer’s read high sensitivity and high dynamic operability (number of nonworking pix- noise. The noise is uncorrelated, hence, range). Consider the following example: els). Photodiode-array lengths are the root-sum-square defines the total the amplifier output is 2 V with an inte- paired to the length (and availability) of noise (see the following example). Per- grating capacitor equal to 10.4 pF, the silicon readout integrated circuit (or forming at levels much below the read hence, the maximum charge capacity is multiplexer). Presently, commercial In- noise (temperature independent) does 20.8 pC, corresponding to 130 Me. Be- GaAs photodiode arrays are available not help. Meanwhile, much money can cause we are measuring voltages, the that range in size from 128 elements to be spent on a cooling system. Dynamic transimpedance gain is calculated to be 512 range is also used to determine the level 2 V/130 Me, which equals 15.4 nV/e or elements. The pitch or center-to-center of digitization and whether the operation 11 nV/photon. Thus, the array’s signal- spacing has been standardized to 50 is optimized for high sensitivity or high to-noise ratio is simply the maximum ␮m; hence, array lengths are between dynamic range. Selection will be based charge capacity (sometimes called “full 6.4 mm and 25.6 mm. The pixel geome- on the spectrometer and application. well”) and shot noise level quotient. try is rectangular with available heights The pixel dark signal is defined in units The photodetector spectral response ranging from 50 ␮m to 1 mm. Tradeoffs of volts per second to rapidly determine defines the maximum spectrometer free- between size (area) and signal-to-noise allowable exposure times as well as the spectral range. A single photodiode ar- ratio will determine the element selec- more familiar definition of dark current in ray covering the 800–2200 nm spectrum tion. amperes. It is due to the nonzero input (allowable transmission band for silica- InGaAs photodiode arrays have a 100% offset voltage of the multiplexer’s capaci- based lenses and optical fiber) is not fill factor (ratio of active photoresponse tive transimpedance amplifier. This will ei- available. Two InGaAs compositions area to element area). Thus, maximized ther forward or reverse bias the photodi- have been commercially developed that light collection efficiency is realized ode producing a dark current. cover the 800–1700 nm and 1100–2200 compared with arrays having less than Additionally, this bias level can be differ- nm ranges. Selection of a particular unity fill factor. Zero dropout arrays are ent for each element, which produces wavelength range is dependent on the common with 128 and 256 elements. Sys- dark signal nonuniformity — a compo- application and tradeoffs associated with tem performance depends on the coordi- nent in fixed pattern noise (defined later the instrument performance. nation of the components to match the in this article). In most cases, the dark Exposure time and readout time for array of the spectrometer, based on the signal is the dominant noise generator, es- diode arrays are temporally separated. application. Figure 4 is a photograph of pecially during long exposures. An In- The photodiode array, including the the InGaAs photodiode array illustrating GaAs photodiode element can have a multiplexer, is first exposed (integration a 0.5-mm pixel on a 50-␮m pitch. dark current of 1 pA, corresponding to time), then deactivated and serially The linear array is characterized by 6.25 Me/s. Photodiodes follow Poisson read-out. Commercially available photo- its opto-electronic performance parame- statistics and the rms noise value is the diode arrays can have ␮second expo- ters: dynamic range, dark signal, maxi- square root of the measured output. sure and readout times. Exposure times mum output voltage (transimpedance Thus, a 1-s exposure will produce a 6.25 are electrically generated (no mechani- gain), spectral responsivity, and maxi- Me dark signal having an rms noise level cal shutter) via the control electronics. mum exposure and minimum (fastest) of 2500 e. Cooling the photodiode array Readout times are specified by the num- readout time. Dynamic range can be de- will decrease the dark current and the ber of samples read per second. Thus, a fined as the maximum usable output noise.
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