Bindhu, C V; Harilal, S S; Nampoori, V P N; Vallabhan, C P G(Springer-Verlag, 2000)
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Abstract:
Optical limiting and thermal lensing studies are
carried out in C70–toluene solutions. The measurements are
performed using 9-ns pulses generated from a frequencydoubled
Nd:YAG laser at 532 nm. Optical limiting studies in
fullerene molecules lead to the conclusion that reverse saturable
absorption is the major mechanism for limiting. Analysis
of thermal lensing measurements showed a quadratic
dependence of thermal lens signal on incident laser energy,
which also supports the view that optical limiting in C70 arises
due to sequential two-photon absorption via excited triplet
state (reverse saturable absorption).
Rajesh, Mandamparambil; Sheeba, Mavila; Geetha, Karinjamanna; Vallabhan, Chakkalakkal P G; Radhakrishnan, Padmanabhan; Nampoori, Vadakkedathu P N(Optical Society of America, January , 2007)
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Abstract:
The fabrication and characterization of a Rhodamine 6G-doped polymer optical fiber amplifier have been carried out. Two different schemes were employed to characterize the optical fiber: the stripe illumination technique to study the fiber as a gain medium and another technique to study its performance as an amplifier. We observed a spectral narrowing from 42 to 7 nm when the pump energy was increased to 6 mJ in the stripe illumination geometry. A gain of 18 dB was obtained in the amplifier configuration. The effects of pump power and dye concentration on the performance of the fiber as an amplifier were also studied.
Sheeba, M; Rajesh, M; Nampoori, V P N; Radhakrishnan, P(Optical Society of America, 2008)
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Abstract:
Rhodamine 6G and Rhodamine B dye mixture doped polymer optical fiber amplifier (POFA), which can operate in a broad wavelength region (60 nm), has been successfully fabricated and tested. Tunable operation of the amplifier over a broad wavelength region is achieved by mixing different ratios of the dyes. The dye doped POFA is pumped axially using 532 nm, 10 ns laser pulses from a frequency doubled Q-switched Nd: YAG laser and the signals are taken from an optical parametric oscillator. A maximum gain of 22.3 dB at 617 nm wavelength has been obtained for a 7 cm long dye mixture doped POFA. The effects of pump energy and length of the fiber on the performance of the fiber amplifier are also studied. There exists an optimum length for which the amplifier gain is at a maximum value.
Rajesh, M; Geetha, K; Sheeba, M; Radhakrishnan, P; Vallabhan, C P G; Nampoori, V P N(Elsevier, May , 2006)
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Abstract:
A simple, effective and inexpensive fiber optic sensor for investigating the setting characteristics of various grades of cement is described. A finite length of unsheathed multimode optical fiber laid inside the cement mix, is subjected to stress during the setting process. The microbends created on the fiber due to this stress directly influence the intensity of light propagating through the fiber. Continuous monitoring of such variations in the light output transmitted through the fiber gives a clear measure of the setting characteristics of the cement mix, thus providing a simple and elegant technique of great practical importance in the field of civil engineering. The smart fiber optic sensor described above can be incorporated into a building during the construction process itself so that continuous monitoring of the deterioration process for the entire life time of the building can be carried out.
Girijavallabhan, C P; Nampoori, V P N; Radhakrishnan, P; Sheeba, M; Rajesh, M(Measurement Science and Technology, Institute of Physics Publishing, October 4, 2005)
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Abstract:
The design and development of a fibre optic evanescent wave refractometer
for the detection of trace amounts of paraffin oil and palm oil in coconut oil
is presented. This sensor is based on a side-polished plastic optical fibre. At
the sensing region, the cladding and a small portion of the core are removed
and the fibre nicely polished. The sensing region is fabricated in such a
manner that it sits perfectly within a bent mould. This bending of the
sensing region enhances its sensitivity. The oil mixture of different mix
ratios is introduced into the sensing region and we observed a sharp decrease
in the output intensity. The observed variation in the intensity is found to be
linear and the detection limit is 2% (by volume) paraffin oil/palm oil in
coconut oil. The resolution of this refractometric sensor is of the order of
10−3. Since coconut oil is consumed in large volumes as edible oil in south
India, this fibre optic sensor finds great relevance for the detection of
adulterants such as paraffin oil or palm oil which are readily miscible in
coconut oil. The advantage of this type of sensor is that it is inexpensive and
easy to set up. Another attraction of the side-polished fibre is that only a
very small amount of analyte is needed and its response time is only 7 s.
Harilal, S S; Issac, Riju C; Bindhu, C V; Nampoori, V P N; Vallabhan, C P G(Pramana, September , 1997)
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Abstract:
Time resolved optical emission spectroscopy is employed to study the expansion
dynamics of C2 species in a graphite plasma produced during the Nd : YAG ablation. At low laser
fluences a single peak distribution with low kinetic energy is observed. At higher fluences a twin
peak distribution is found. It has been noted that these double peak time of flight distribution splits
into a triple peak structure at distances >_ 17mm from the target surface. The reason for the
occurrence of multiple peak is due to different formation mechanisms of C2 species
Sathy, P; Reji, Philip; Nampoori, V P N; Vallabhan, C P G(Springer, June , 1990)
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Abstract:
Experimental method for measuring photoacoustic(PA) signals generated by a pulsed laser beam in liquids is described. The pulsed PA technique is found to be a convenient and accurate method for determination of quantum yield in fluorescent dye solutions. Concentration dependence of quantum yield of rhodamine 6G in water is studied using the above method. The results indicate that the quantum yield decreases with increase in concentration in the quenching region in agreement with the existing reports based on radiometric measurements.
Thomas, Baby; Sathianandan, K; Nampoori, V P N; Vallabhan, C P G; Ramachandran, T(American Institute of Physics, September , 1991)
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Abstract:
A high power Nz laser of the double-Blumlein type having a modified gas flow system,
electrode configuration, and discharge geometry with minimum inductance is described. By
incorporating a triggere’d-pressurized spark gap switch, arc-free operation was achieved
for a wide E/P range. The device gives a peak power in excess of 700 kW with a FWHM of
3 ns and an efficiency of 0.51%, which is remarkably high for a pulsed nitrogen laser
system. The dependence of output power on parameters such as operating pressure, voltage,
and repetition rate are discussed.
Sasi Kumar, P R; Harilal, S S; Nampoori, V P N; Vallabhan, C P G(Pramana, March , 1994)
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Abstract:
High resolution optogalvanic spectrum of the (11, 7) band in the first positive system
of nitrogen molecule has been recorded from 17179 to 17376 cm- 1. Assignment of 432 rotational
lines belonging to the 27 branches of this band has been carried out.
Subhadra, V K; Syamaprasad, U; Vallabhan, C P G(American Institute of Physics, 1983)
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Abstract:
Results of axiswise measurements of the electrical conductivity (dc and ac) and dielectric constant of NH4H2PO4 confirm the occurrence of the recently suggested high‐temperature phase transition in this crystal (at 133 °C). The corresponding transition in ND4D2PO4 observed here for the first time takes place at 141.5 °C. The mechanism involved in these transitions and those associated with the electrical conduction and dielectric anomalies are explained on the basis of the motional effects of the ammonium ions in these crystals. Conductivity values for deuterated crystals give direct evidence for the predominance of protonic conduction throughout the entire range of temperatures studied (30–260 °C).
Unnikrishna Pillai, P R; Padma, Nambisan; Nampoori, V P N; Vallabhan, C P G(HEREDITAS ( Beijing ), 1997)
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Abstract:
Laser irradiation at wavelength 514 nm was used to study the effect, of lasers in inducing
chromosomal aberrations at mitosis. This study offers a new radiation system which could be used for the
induction of mutations. Results are compared with those obtained from studies using y-rays as irradiation
source.
Harilal, S S; Bindhu, C V; Nampoori, V P N; Vallabhan, C P G(American Institute of Physics, 1998)
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Abstract:
The effect of ambient gas on the dynamics of the plasma generated by laser ablation of a carbon target using 1.06 μm radiation from a Q-switched Nd:YAG laser has been investigated using a spectroscopic technique. The emission characteristics of the carbon plasma produced in argon, helium and air atmospheres are found to depend strongly on the nature and pressure of the surrounding gas. It has been observed that hotter and denser plasmas are formed in an argon atmosphere rather than in helium or air as an ambient.
Bindhu, C V; Harilal, S S; Nampoori, V P N; Vallabhan, C P G(Indian Academy of Sciences, April , 1999)
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Abstract:
Thermal lensing effect was studied in aqueous solutions of rhodamine B using 532 nm,
9 ns pulses from a Nd:YAG laser. A low intensity He-Ne laser beam was used for probing the thermal
lens. Results obtained show that it is appropriate to use this technique for studying nonlinear absorption
processes like two photon absorption or excited state absorption and for analyzing dimerization
equilibria.
Geetha K, Varier; Issac, Riju C; Harilal, S S; Bindhu, C V; Nampoori, V P N; Vallabhan, C P G(Elsevier, 1997)
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Abstract:
The laser produced plasma from the multi-component target YBa2CU3O7 was analyzed using Michelson interferometry and time resolved emission spectroscopy. The interaction of 10 ns pulses of 1.06 mum radiation from a Q-switched Nd:YAG laser at laser power densities ranging from 0.55 GW cm-2 to 1.5 GW cm-2 has been studied. Time resolved spectral measurements of the plasma evolution show distinct features at different points in its temporal history. For a time duration of less than 55 ns after the laser pulse (for a typical laser power density of 0.8 GW cm-2, the emission spectrum is dominated by black-body radiation. During cooling after 55 ns the spectral emission consists mainly of neutral and ionic species. Line averaged electron densities were deduced from interferometric line intensity measurements at various laser power densities. Plasma electron densities are of the order of 1017 cm-3 and the plasma temperature at the core region is about 1 eV. The measurement of plasma emission line intensities of various ions inside the plasma gave evidence of multiphoton ionization of the elements constituting the target at low laser power densities. At higher laser power densities the ionization mechanism is collision dominated. For elements such as nitrogen present outside the target, ionization is due to collisions only.
Girijavallabhan, C P; Radhakrishnan, P; Nampoori, V P N; Geetha, K; Rajesh, M(Journal of Optics A: Pure and Applied Optics, Institute of Physics Publishing, January 13, 2006)
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Abstract:
We present a compact solid-state laser based on leaky mode propagation
from a dye-doped polymer free-standing film waveguide. The edge emitted
spectrum clearly indicated the existence of periodic resonant modes. The
reflections from the lateral faces of the free-standing film provided the
optical feedback thus giving rise to a Fabry–Perot like optical cavity. This
together with the guidance through the gain medium gave rise to intense
narrow emission lines. For a pump energy of 1.82 mJ/pulse, an intense line
with FWHM ∼0.4 nmwas observed at 576.5 nm.
Nibu, A George; Vallabhan, C P G; Nampoori, V P N; George, A K; Radhakrishnan, P(Taylor & Francis, 2001)
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Abstract:
In this paper we report the use of a laser induced phoroacoustic technique for the detection of multiple phase transitions in heptyl-oxy-cyanobiphenyl (70CB) and octyl-oxy-cyanobiphenyl (80CB) liquid crystals. The observed photoacoustic signal amplitude profile carries clear signatures of two tirst order transitions in 70CB and two tirst order and a second order transitions in 80CB. Analysis of the experimental data using Rosencwaig-Gersho theory shows that the sudden decrease in the photoacoustic (PA) signal amplitude during phase transitions is due to a sharp increase in the heat capacity of the samples near the transition temperatures.
Girijavallabhan, C P; Radhakrishnan, P; Nampoori, V P N; Sajan, D George; Aji, A Anapara; K G K Warrier(Proceedings of SPIE, 2003)
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Abstract:
In this paper, we report the measurements of thermal diffusivity of nano Ag metal dispersed ceramic alumina
matrix sintered at different temperatures using laser induced non-destructive photoacoustic technique. Measurements of
thermal diffusivity also have been carried out on specimens with various concentration of nano metal. Analysis of the
data is done on the basis of one-dimensional model of Rosencwaig and Gersho. The present measurements on the
thermal diffusivity of nano metal dispersed ceramic alumina shows that porosity has a great influence on the heat
transport and the thermal diffusivity value. The present analysis also shows that the inclusion of nano metal into
ceramic matrix increases its interconnectivity and hence the thermal diffusivity value. The present study on the samples
sintered at different temperature shows that the porosity of the ceramics varies considerably with the change in sintering
temperature. The results are interpreted in terms of phonon assisted heat transfer mechanism and the exclusion of pores
with the increase in sintering temperature.
Ritty J, Nedumpara; Geetha, K; Dann, V J; Vallabhan, C P G; Nampoori, V P N; Radhakrishnan, P(IOP Publishing, 2007)
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Abstract:
We report unusual spectral narrowing and laser emission from polymer thin
films doped with Coumarin 540 dye. The laser emission from the polymer
films is found to be highly dependent upon the excitation length of the
medium. Even a short length of 1.75 mm of the dye doped film gave rise to
laser emission with FWHM of 0.3 nm for a pump intensity of 825 kW cm−2.
The partial reflections from the broad lateral surfaces of the free standing
films provided the optical feedback for the laser emission. Occurrence of
well-resolved equally spaced resonant modes confirmed the effect of a
Fabry–Perot-like optical cavity between the film surfaces.
Thomas Lee, S; Dinesh Kumar, S; Suresh, Kumar P; Radhakrishnan, P; Vallabhan, C P G; Nampoori, V P N(Elsevier, 2003)
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Abstract:
We propose and demonstrate a new technique for evanescent wave chemical sensing by writing long period gratings in a bare multimode plastic clad silica fiber. The sensing length of the present sensor is only 10 mm, but is as sensitive as a conventional unclad evanescent wave sensor having about 100 mm sensing length. The minimum measurable concentration of the sensor reported here is 10 nmol/l and the operating range is more than 4 orders of magnitude. Moreover, the detection is carried out in two independent detection configurations viz., bright field detection scheme that detects the core-mode power and dark field detection scheme that detects the cladding mode power. The use of such a double detection scheme definitely enhances the reliability and accuracy of the results. Furthermore, the cladding of the present fiber need not be removed as done in conventional evanescent wave fiber sensors.
Girijavallabhan, C P; Radhakrishnan, P; Nampoori, V P N; Geetha, K; Rajesh, M(Journal of Optics A: Pure and Applied Optics, Institute of Physics Publishing, February 24, 2004)
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Abstract:
We report the position dependent tuning of fluorescence emission from
rhodamine 6G doped polymethylmethacrylate film waveguide using a side
illumination technique. The transmitted fluorescence as a function of the
distance from the point of illumination is measured by translating the
waveguide horizontally across a monochromatic light source. This technique
has been utilized to characterize the optical loss in dye doped waveguides.
We observe that the optical loss coefficients for shorter and longer distances
of propagation through the dye doped waveguide are different. At longer
distance of propagation a decrease in optical loss coefficient is observed