Ramakanta Mahakud

Study on the quality of interference fringes from a pulsed UV source for application in a biprism based fiber Bragg grating writing

This paper presents a study on the quality of interference fringes formed from a pulsed UV (255 nm, 5.6 kHz, and 40 ns) source for an application in writing fiber Bragg gratings (FBGs). The interference fringes of separation of about 8 microm, formed by a biprism of apex angle 2 degrees , were studied for their contrast, evolution of contrast, and positional and intensity stability over a period of 5 min (over 10(6) pulses). Second harmonic UV (255 nm) sources of different spatial coherence and pointing stability characteristics were employed as the inputs. It is established that the UV fringes contrast and interference pattern stability with time is largely controlled by the optical resonator geometry of the fundamental wavelength (510 nm) copper vapor laser (CVL) oscillator. In particular, the generalized diffraction filtered resonator (GDFR) CVL produced the highest quality second harmonic beam with the highest fringes contrast and stability. The implications of these results were studied by employing these UV sources for the fabrication of the C-band FBGs by a 24 degrees apex angle biprism.

Study of the nonuniform behavior of temperature sensitivity in bare and embedded fiber Bragg gratings: experimental results and analysis.

This paper presents an experiment and analysis on the factors affecting nonlinear evolution of Bragg wavelength with change in temperature in typical bare and embedded fiber Bragg grating-based (FBG) temperature sensors. The purpose of the study was to find the constants in the function required to evaluate temperature from Bragg wavelength shift. The temperature sensitivity of bare FBGs was found to increase with temperature elevation, and is different for FBGs written in different fiber types. The average temperature sensitivity increased by about 20% when the bare FBG temperature was elevated from 25°C to 525°C. The average temperature sensitivity of the embedded FBG sensor, investigated in the temperature range of 30°C-90°C, was a factor of 2-3 times larger than for bare FBG, depending on its fastened length with the substrate. Analytically, it is shown that the nonuniform behavior of temperature sensitivity in bare FBGs is the result of both the thermal expansion effect of the fiber and the temperature derivatives of the effective refractive index. The strain transfer and temperature coefficients of thermal expansion of the substrate affect the nonuniform behavior of temperature sensitivity in embedded FBG sensors.

Analysis on the saturation of refractive index modulation in fiber Bragg gratings (FBGs) written by partially coherent UV beams.

We present an analysis on the saturation of refractive index modulation of fiber Bragg gratings written in nonhydrogenated Ge-B co-doped single-mode photosensitive optical fiber by partially coherent pulsed UV beams. The UV beams of different spatial coherence properties were generated by second harmonic conversion of high repetition rate, high average power copper vapor laser (CVL) oscillators with different optical resonators. It is observed that for UV beams of higher spatial coherence, the fiber Bragg grating reflectivity growth was faster and saturation of refractive index modulation was higher. The experimental results are explained with the help of a physical model based on exponential decay of defect centers per unit volume on UV absorption in the fiber core. The subsequent increase in the refractive index was attributed to the structural modification and densification of the fiber core.

Cylindrical-lens-based wavefront-reversing shear interferometer for the spatial coherence measurement of UV radiations

We present the design and results of a cylindrical-lens-based reversal shear interferometer for measuring the spatial coherence of partially or fully coherent UV beams. This is a modified Michelson interferometer consisting of only four optical components. The wavefront is reversed in one of the interferometers arm by the combination of a plane mirror located at the focal plane of a cylindrical lens. The selected wavefront can be sheared at any arbitrary angle with respect to the incident wavefront by rotating the cylindrical lens, thus enhancing the capability of this design for a real 3-D wavefront evaluation. This interferometer is used to measure the spatial coherence of 255-nm UV radiations obtained from the second harmonic of a copper vapor laser for pump beams of different spatial coherence characteristics, a result being reported for the first time.

Comparative study on second harmonic conversion and saturation characteristics of three copper vapor laser beams of same average power and different spatial coherence

This paper presents a comparative study on second harmonic conversion and saturation behavior of three copper vapor laser (CVL) beams of same average power up to 11 W (510 nm), however with different spatial coherence width of 17%, 35%, and 100% of the full beam cross-section. The nonlinear crystal employed is beta barium borate. It is observed that the trends in second harmonic (SH) conversion efficiency and its saturation are widely different for the three CVL pump beams. As expected, the conversion efficiency scaled up with increase in pump power. However, very interesting trends are observed in SH saturation characteristics with scaling of pump beams spatial coherence. For lower coherence CVL beam, the saturation of SH efficiency is observed at lower pump flux and at higher average power. Also, the saturation occurred at lower SH efficiency for CVL beam of lower spatial coherence. The fall in the SH efficiency after the peak is more rapid for lower spatial coherence pump beam. The experimental trends are explained in terms of combined effect degree of spatial coherence of pump beams and the thermal dephasing in the nonlinear crystal by high pump average power.

Surface Modified Long Period Fiber Grating Sensor for Rapid Detection of Aspergillus Niger Fungal Spores

ABSTRACT We present development of a compact and label-free sensor based on the surface modification of copper vapor laser fabricated long period fiber gratings for detection of airborne Aspergillus niger (A. niger) fungal spores. Surface of sensors were functionalized with monoclonal glucose oxidases IgG1 for target-specific covalent binding. In process of functionalization and binding of 103 cfu/ml of pathogenic A. niger fungal spores, notable shorter wave transition in resonance wavelength from 1562.93 nm to 1555.97 nm, and significant reduction in peak loss from 61.72 dB to 57.48 dB were recorded. The implementation was cost effective and yielded instantaneous results.

Studies on the stability of regenerated fiber Bragg gratings at 1100 °C

This paper presents studies on the long term thermal stability of regenerated fiber Bragg gratings (FBGs) at 1100 oC. The seed type-I FBGs of almost similar reflectivity (∼ 35 dB) were inscribed in hydrogen loaded fused silica fibers using high repetition rate (5.5 kHz) 255 nm UV radiation. These FBGs were subjected to multi-step annealing process upto 1100 oC spanning over 60 hours for regeneration and stabilisation of regenerated grating. The residual reflectivity of 40% and 10% was obtained for Ge doping of 3 mol% and 18 mol% respectively, which was stable even after ∼10 hours annealing at 1100 oC.

Optimization of FBG writing by biprism interferometer

We report the maximum reflectivity (12 dB, 93.7 %) so far observed for FBGs written by biprism interferometer, in single mode photosensitive fiber without hydrogen loading.

Temperature sustainability of type IIa FBGs written in different Ge doped fiber

The effect of Ge doping concentration on refractive index rollover fluence and thermal annealing characteristics of type IIa FBG are studied. The temperature sustainability was higher for grating written at higher refractive index rollover fluence.