Sanjay Kher

Turnaround-Point Long-Period Fiber Gratings (TAP-LPGs) as High-Radiation-Dose Sensors

This letter reports the application of turnaround-point (TAP) long-period gratings (LPGs) for radiation dosimetry. The LPGs were realized using the laser-writing technique. After a gamma dose of 6 kGy, the TAP-LPGs show a radiation-induced shift of more than 35 nm for each of the dual resonance dips, which is the highest shift reported to date for any type of fiber gratings. These LPGs can therefore be used as radiation sensors down to doses of a few Gy with modest wavelength-resolving instrumentation. The LPGs show significant sensitivity of wavelength shifts up to 80 nm for a dose of 65 kGy. We attribute the very high radiation sensitivity to near-TAP operation and doping of Boron in the fiber. This should open up applications of LPGs to radiation dosimetry.

Detection of Fuel Adulteration With High Sensitivity Using Turnaround Point Long Period Fiber Gratings in B/Ge Doped Fibers

We report for the first time the application of turnaround-point long period fiber gratings for wavelength encoded detection of automobile fuel adulteration. The demonstrated high sensitivity of 0.96 nm/% change of kerosene in petrol up to 10% adulteration is significantly high compared with previously published values. These specialty gratings can easily detect the presence of 1% contamination of kerosene in petrol and thus provide opportunities for development of portable fuel adulteration sensors.

Measurement of

We propose a novel approach to measure in-situ refractive index changes induced by gamma radiation in single mode fibers. The changes are derived from wavelength shifts of resonances of a CO2 laser written long period fiber grating. Using this approach, we estimate the radiation induced refractive index increase in the core of B/Ge co-doped fiber as 2.1×10-5 at 10 kGy and 1.85×10-4 at 1.54 MGy. At doses , the index changes are considered to be governed by glass polarizability changes. Numerical simulations show that saturation of the index change can be observed at doses . Irradiation has a negligible effect on the resonance dip strengths and bandwidths, with the exception of the turnaround point mode.

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Turn around point (TAP) in long period fiber grating (LPG) provides extremely high sensitivity to external environmental parameters like temperature, strain and refractive index. It also opens up new dimensions for development of photonic devices. Higher order cladding mode coupling in a LPG requires relatively lower grating period (< 300 µm), which can be achieved by improving the spot size in CO2 laser based grating inscription methods. But the inscription of LPG with resonance exactly at TAP within 950–1700 nm wavelength band requires a period precision better than 0.5 µm. Since, our fabrication system based on automated CO2 laser has limited translational resolution, we have used exposure to gamma radiation and tapering of fiber as tools to tailor the spectral characteristics of our gratings. Total Gamma radiation exposure of about 6 kGy resulted in ± 35nm wavelength shift of dual resonant loss peaks in a 206 µm period near TAP-LPG, almost merging the two peaks. This exposed LPG was then tapered using CO2 laser for exact TAP operation. Amplitude based strain sensitivity of 1.13dB/me was obtained with tuned TAP LPG.

-Radiation Induced Refractive Index Changes in B/Ge Doped Fiber Using LPGs

The calibration of Raman scattering-based optical fiber distributed temperature sensor (OFDTS) is performed using temperature of the integrated reference (calibration) loop located at the start of the sensing fiber. OFDTS measures distributed temperature profile assuming that sensing fiber is free from any discontinuity, so that anti-Stokes (AS) and Stokes (St) lights have uniform decay. However, in real cases, the fiber loss may get affected by the bend in fiber which causes discontinuity in AS and St signals. If the distributed temperature profile is still calibrated by using the same calibration loop, temperature profile of the fiber zone that exists after the bend will be highly erroneous. Therefore, detection of the bend, temperature error caused by that bend, and compensation of this error are of utmost importance. It is difficult for the user to visually identify the presence and location of the bend from AS and St signals. This paper presents the empirical mode decomposition-based automatic technique to dynamically detect the presence of the bend and its location using area parameter of analytic intrinsic mode functions (IMFs). We demonstrate that the measure of area parameter for the analytic IMFs of St signal can serve as a feature for automatic detection of bend. The utilization of second calibration loop after the detected bend makes it possible to use rest of the fiber for correct temperature profiling.

Radiation and taper tuning of Long Period Grating for high sensitivity strain measurement

This article discusses a digital filtering technique to improve the temperature resolution of a Raman fiber optic distributed temperature sensor. A frequency-domain digital filtering algorithm has been used for this typically time-domain task to show that by reducing the effect of noise introduced by analog electronics and the quantization noise of the analog-to-digital converter in the detection and data acquisition stages, superior temperature resolution is achievable. An important feature of the filtering scheme used is that no spatial inaccuracy is introduced into the ranging of hot zones, despite the highly nonlinear phase response of the filter. Digital filtering used for postprocessing of data shows clearly and unambiguously a temperature resolution of 5 K in the present setting of our experiment, with the possibility of much greater improvement. Merely using analog signal conditioning does not provide the same clarity and uniqueness in temperature resolution and spatial location that digital filtering offers. This digital filtering scheme offers greater flexibility and versatility than mere signal-averaging approaches to improve the SNR of such systems. The filters ability to reject deterministic interfering frequencies of appreciable energy content is also demonstrated by simulating such frequencies of much greater energy than the actual signal.

Empirical Mode Decomposition-Based Detection of Bend-Induced Error and Its Correction in a Raman Optical Fiber Distributed Temperature Sensor

GaN Schottky photodetectors are fabricated on heavily doped n-type GaN epitaxial layers grown by the hydride vapour phase epitaxy technique. The effect of 60Co γ-radiation on the electronic transport in GaN epilayers and Schottky detectors is studied. In contrast to earlier observations, a steady rise in the carrier concentration with increasing irradiation dose is clearly seen. By considering a two layer model, the contribution of interfacial dislocations in carrier transport is isolated from that of the bulk layer for both the pristine and irradiated samples. The bulk carrier concentration is fitted by using the charge balance equation which indicates that no new electrically active defects are generated by γ-radiation even at 500 kGy dose. The irradiation induced rise in the bulk carrier concentration is attributed to the activation of native Si impurities that are already present in an electrically inert form in the pristine sample. Further, the rise in interfacial contribution in the carrier concentr...

Bidirectional frequency-domain digital filtering to simultaneously improve temperature resolution and eliminate spatial inaccuracy of a distributed temperature sensor

Error-free temperature measurement by Raman scattering based optical fiber distributed temperature sensor (OFDTS) is possible if the sensing fiber has no sharp bend. However, in real life applications, the sensing fiber might have sharp bend that may cause discontinuity in the profiles of Raman anti-Stokes (AS) and Stokes (St) signals resulting in erroneous distributed temperature measurement by OFDTS. In such a case, it is necessary to detect the presence and location of the bend. For a human operator using OFDTS, it is difficult to detect the presence of bend and locate it by visually observing AS and St signals. In this article, an online automated technique based on measurement of a new parameter called the instantaneous area of analytic intrinsic mode functions (IMFs) of St signals is presented to serve the purpose. The proposed technique is more automation friendly than earlier reported technique where area corresponding to IMF of complete duration of St signal was considered. The instantaneous area parameter alone is able to detect both the bend induced malfunctioning of OFDTS and the location of bend.

Effect of 60Co γ-irradiation on the nature of electronic transport in heavily doped n-type GaN based Schottky photodetectors

This is the first report, to the best of our knowledge, of synchrotron X-ray beam induced writing of long-period fiber gratings in photosensitive silica optical fibers. We demonstrate efficient writing and significant index changes in optical fiber. These gratings were found to be highly resistant to gamma radiation and can be used for temperature sensing applications in high dose nuclear radiation environment.

Instantaneous Area Based Online Detection of Bend Generated Error in a Raman Optical Fiber Distributed Temperature Sensor

We have experimentally studied in real time the effect of gamma radiation on the characteristics of long period fibre gratings written in different types of fibres. The grating parameters and fibre composition have been optimized. Prototype packaged sensors have been developed for field application.