Sandip Maity

A Hybrid MEMS–Fiber Optic Tunable Fabry–Perot Filter

This paper describes a bulk-micromachined electrostatically tunable Fabry-Perot interferometric filter. The device is a hybrid optical filter combining fiber optics and microelectromechanical systems (MEMS) technologies. The static mirror is a Bragg grating mirror built on the end face of an optical fiber that is integrated with a MEMS device, which includes the tunable gold-coated mirror. The MEMS device is fabricated in ¿100¿-oriented silicon wafers using modified and optimized batch MEMS processes. A two-stage approach was used to achieve high finesse (F) and broad tunability simultaneously. In the first stage, actuator issues and mirror structural defects were corrected for by optimizing the fabrication-process parameters. In this stage, near-theoretical performance has been achieved with a pure Si MEMS tunable etalon. In stage two, optical fibers with dielectric stack mirrors from Micron Optics, Inc. have been integrated with the MEMS devices to form tunable cavities. The device insertion loss was below 15 dB and was mostly attributed to absorption losses in the gold coating. We measured a pass bandwidth of around 0.54 nm and a tuning range of nearly 120 nm resulting in an F of over 220.

Development of Steam Quality Measurement and Monitoring Technique Using Absorption Spectroscopy With Diode Lasers

Techniques using a tunable diode laser (TDL) and multiple broadband lasers have been developed and tested for measuring the steam quality or steam wetness fraction. The steam wetness fraction was estimated using a ratiometric technique combining measured absorbance of water and water vapor overtone transitions in the near infrared spectral band. Using these techniques we were able to measure a wide variation in steam quality ranging from saturated steam condition to 80% mass fraction of steam with less than 1% error. Our sensor can be tailor-made to suit cost, sensitivity, and operating conditions as per requirements, and could be used eventually for measuring the quality of steam in the low pressure stage of steam turbines.

Laser Calorimetry Spectroscopy for ppm-level Dissolved Gas Detection and Analysis

In this paper we report a newly developed technique – laser calorimetry spectroscopy (LCS), which is a combination of laser absorption spectroscopy and calorimetry - for the detection of gases dissolved in liquids. The technique involves determination of concentration of a dissolved gas by irradiating the liquid with light of a wavelength where the gas absorbs, and measuring the temperature change caused by the absorbance. Conventionally, detection of dissolved gases with sufficient sensitivity and specificity was done by first extracting the gases from the liquid and then analyzing the gases using techniques such as gas chromatography. Using LCS, we have been able to detect ppm levels of dissolved gases without extracting them from the liquid. In this paper, we show the detection of dissolved acetylene in transformer oil in the mid infrared (MIR) wavelength (3021 nm) region.

Temperature dependence of the reflectance of metals at visible wavelengths

Non-contact temperature measurement is a preferred technique for rotating, hazardous and inaccessible objects. A major challenge for IR thermometry is the dependence of metal emissivity on wavelength and temperature. Optical reflectivity of metals is known to depend on metal temperature, plasma frequency of metal, angle and wavelength of the incident light. A major challenge in reflectance based temperature measurement techniques is the dependence of the reflectance on the surface roughness of the target metal. Sudden change in surface roughness (related or unrelated to temperature) can lead to spurious changes in reflectance irrespective of the temperature. To mitigate the surface roughness effect, we have investigated the speckle pattern emanating from the surface irregularities on the metal. An initial measurement on the speckle pattern also shows an enhanced sensitivity in temperature measurement of the surface that is a function of the inherent surface properties of the metal.

Optical sensors for harsh environment applications

The development of a harsh environment ammonia slip sensor based on tunable diode laser absorption spectroscopy is presented. A hybrid optical sensor design, through combination of wavelength modulation spectroscopy (WMS) and alignment control, is proposed as an approach towards reliable in-situ measurements in misalignment prone harsh environments. 1531.59 nm, 1553.4 nm and 1555.56 nm are suggested as possible absorption lines for trace ammonia measurement (<1ppm at 10m path length at 500K) in gas turbine exhaust conditions. Design and performance of the alignment control system are presented in detail. Effect of misalignment related measurement degradation is investigated and significant improvement in measurement fidelity is demonstrated through the use of the hybrid optical sensor design.

An optical method for measuring Metal Surface temperature in Harsh environment conditions

Commercially available instruments for measuring and monitoring surface temperature of metal parts are very limited and often unsuitable for applications at harsh environment conditions. Another major challenge is to measure temperature of a rotating surface, as it is very difficult to take an electrical signal out from rotating parts. A novel optical reflectance based non-contact temperature measurement technique is discussed which can be used for temperature measurement on metal surfaces. The optical reflectivity of metals is known to depend on metal temperature and wavelength of the incident light. An increase in metal temperature resulted in the change (increase or decrease depending on particular metal properties) of reflected laser power from the metal surface. This also depends on the surface geometries of the metal surface being measured. We have shown that the sensitivity of the temperature measured depends on the angle of incidence, surface topology and surface properties of the object.