A. A. Kapralov

Low Voltage CO

This study presents a comparative analysis of parameters of infrared radiation source and detector hardware that are most important for the creation of portable optical nondispersive infrared (NDIR) gas sensors. An analytical model of the optical gas sensor is proposed. The notions of an instrument and transfer functions of the sensor as a measurement device are introduced. Capabilities of the model have been demonstrated by the example of carbon dioxide sensor based on immersion diode optopairs. Validity of the model is proved by good coincidence of calculated and experimental data. Analytical description of the transfer function of a NDIR carbon dioxide sensor allowed us to carry out the comparative analysis of potential sensitivity of sensors based on different hardware produced by basic fabricators of IR components.

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Recently developed mid-IR LED non-dispersive gas analyzers can replace conventional infrared gas monitors. The advantages of novel analyzers are compact size, fast response and low power consumption. InAsSb(P)- and InGaAs(Sb)-based LEDs developed and manufactured at Ioffe Physico-Technical Institute suit for optical gas sensing well because of narrow emission band and the possibility of making LED emission peak spectrally matched with the gas absorption maximum. We present estimations of gas detection limits for LED based optical sensors, sensor head constructions and some experimental results for CO2 and CH4 detectors based on LED-PbSe photoresistor and LED - InAs photodiode optopairs.

-Gas Sensor Based on III–V Mid-IR Immersion Lens Diode Optopairs: Where we Are and How Far we Can Go?

The dielectric and pyroelectric properties of a typical relaxer ferroelectric, 0.9PbMg1/3Nb2/3O3-0.1PbTiO3 (PMN-PT), are studied experimentally. Based on the results obtained, the pyroelectric constant and figure of merit of the material when used in IR detectors are calculated. These parameters are presented as a function of temperature and external electric field. The current and voltage sensitivities and the detectivity of PMN-PT-based IR detectors are evaluated. They are compared with the same properties of pyroelectric detectors and dielectric bolometers that use traditional pyroelectric materials as the active element and also of other uncooled photodetectors.

Portable optoelectronic gas sensors operating in the mid-IR spectral range (λ=3 5 μm)

An optical pyrometer designed for precision measurement of the GaAs substrate temperature during MBE growth is considered. The pyrometer can be calibrated against a certain characteristic absolute temperature that is visually determined from a change in the RHEED pattern. This enables one to calculate the absolute temperature of the substrate with regard to its radiant emissivity and minimize the inaccuracy of radiation temperature measurement. The inaccuracy is associated with the deposition of growth products on the pyrometer window.

Relaxer ferroelectrics as promising materials for IR detectors

An analytic model is proposed for an optical gas sensor based on diode optopairs, which takes into account the line spectral structure of gases being analyzed, as well as peculiarities of spectral characteristics of immersion-type light-emitting diodes and photodiodes operating in the mid-IR spectral range. The model makes it possible to calculate the transfer characteristic of the sensor and to estimate the measurement error for gas analyzers operating on the basis of these sensors. The experiments demonstrate bright prospects of application of sensors based on immersion-type diode optopairs in small-size gas analyzers; the expected values of the threshold sensitivity of a CO2 sensor at a level of 10 ppm and the absolute error of measurements below 0.1% (reduced error is 1%) in a range of up to 10 vol % at a speed of up to 10 counts per second exceed the parameters of available portable CO2 gas analyzers. The validity of the model is confirmed by conformity between the calculated data and the experimental results obtained on a prototype of a CO2 diode sensor.

Pyrometer unit for GaAs substrate temperature control in an MBE system

A simple method for determining heat-transfer coefficients of solid objects is proposed that is based on direct measurement of the sample surface temperature dynamics. The object is probed by a laser beam with preset temporal variation of the radiation power, and the thermal response is detected by photodiodes operating in the mid-IR spectral range without forced cooling.

Simulation of characteristics of optical gas sensors based on diode optopairs operating in the mid-IR spectral range

The dependence of the sensitivity of photodetectors based on AIII–BV photodiodes on accidental variations of the temperature of its elements is analyzed. It is shown that the temperature drift of the bias level in input circuits of op-amps strongly contributes to the resulting photodetector noise up to frequencies on the order of 1 MHz. To reach the limiting sensitivities of the sensors, it is necessary to stabilize the temperature of not only the photodiode chip, but also the integrated circuit of the first amplifier stage. For most of applications, the required stabilization accuracy does not exceed ±0.1°C. As a result of the analysis, prototype high-sensitivity medium-wavelength (2–5 μm) sensors were developed that operate without forced cooling and have a detection threshold of tens of nanowatts at a detection bandwidth of 0–1 MHz.

Determining heat-transfer coefficients of solid objects by laser photothermal IR radiometry

We present model for determination of performance of optical sensor that takes into account fine structure of gas absorption band and specific features of immersion lens LEDs and photodiodes operating in the mid-IR spectral range (3-5 mum). The model incorporates an instrument and transfer functions that enables evaluation of accuracy, sensitivity and limit of detection (LOD) of various optical sensors. The predictions of the model were compared with the experimental values obtained with the CO2 analyzer based on immersion lens LEDs and photodiodes operating near 4.2 mum spectral band.

The effect of temperature instability on the threshold sensitivity of photodetectors based on AIII–BV photodiodes

The design of multichannel data acquisition computer controlled setup for the pyroelectric measurement in a wide range of DC bias and ambient temperatures (280-350 K) under different conditions of pyroelectric currents is presented. The ceramic samples of 0.9 Pb Mg1/3Nb2/3O3 - 0.1 PbTiO3 (PMN-PT) solid solutions prepared in Ioffe Physico-Technical Institute were selected for measurements. The figures of merit of this ceramics calculated using the measured dielectric and pyroelectric data are presented in comparison with other pyroelectric materials and IR detectors.

Modeling of performance of mid-infrared gas sensors based on immersion lens diode optopairs

We have studied the electrocaloric effect in 0.68BaTiO3–0.32SrTiO3 solid solution, as well as the dielectric, pyroelectric, and piezoelectric properties of this compound in the vicinity of the first-order phase transition in a bias electric field. The dielectric and electromechanical contributions to the pyroelectric and electrocaloric effects are discussed.