Nandakumaran, V M; Bindu M, Krishna; Manu P, John(Cochin University of Science & Technology, 2004)
[+]
[-]
Abstract:
Chaotic dynamics of directly modulated semiconductor lasers have been
studied extensively over the last two decades because of their application in secure
optical communication. However, chaos is generally suppressed in such systems when
the nonlinear gain reduction factor is above 0.01 which is very much smaller than
the reported values in semiconductors like InGaAsP. In this paper we show that by
giving an optoelectronic feedback with appropriate delay one can increase the range
of the values of the gain reduction factor for which chaos can be observed. Numerical
studies show that negative feedback is more efficient in producing chaotic dynamics.
Ritty J, Nedumpara; Binoy, Paul; Santhi, A; Radhakrishnan, P; Nampoori, V P N(Elsevier, January , 2004)
[+]
[-]
Abstract:
In this paper, we present a laser-induced photoacoustic study on the photostability of laser dye Coumarin 540 doped in PMMA matrix and modified by the incorporation of low-molecular weight additives. The dependence of photostability of the dye on various experimental conditions, such as nature of solvents, incident optical power and dye concentration, is investigated in detail. The activation rates for the bleaching process are calculated for different concentrations and they suggest the possibility of two distinct mechanisms responsible for photodegradation. Further, analysis of the data confirms the linear dependence of photodegradation on the intensity of incident radiation. The role of different externally influencing parameters, such as wavelength and modulation frequency of incident radiation, is also discussed.
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)
[+]
[-]
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
Aneeshkumar, B; Pramod, Gopinath; Jayan,Thomas; Vallabhan, C P G; Nampoori, V P N; Radhakrishnan, P(Elsevier, June , 2004)
[+]
[-]
Abstract:
The nanosecond optical limiting characteristics of sandwich-type neodymium diphthalocyanine in a co-polymer matrix of polymethyl methacrylate (PMMA) and methyl-2-cyanoacrylate have been studied for the first time. The measurements were performed using 9 ns laser pulses generated from a frequency-doubled Nd:YAG laser at 532 nm wavelength. The optical limiting performance of neodymium diphthalocyanine in co-polymer host was studied at different linear transmission. Laser damage threshold was also measured for the doped and undoped co-polymer samples. The optical limiting response is attributed to reverse saturable absorption which is due to excited-state absorption.
Girijavallabhan, C P; Radhakrishnan, P; Nampoori, V P N; Sajan, D George; Saravanan, S; Venkatachalam, S(Physical Review, American Physical Society, June 8, 2004)
[+]
[-]
Abstract:
Thermal diffusivity of the composites of camphor sulphonic acid (CSA) doped polyaniline (PANI) and its
composites with cobalt phthalocyanine (CoPc) has been measured using open cell photoacoustic technique.
Analysis of the data shows that the effective thermal diffusivity value can be tuned by varying the relative
volume fraction of the constituents. It is seen that polaron assisted heat transfer mechanism is dominant in CSA
doped PANI and these composites exhibit a thermal diffusivity value which is intermediate to that of CSA
doped PANI and CoPc. The results obtained are correlated with the electrical conductivity and hardness
measurements carried out on the samples.
Annieta, Philip K; Lyjo, Joseph K; Litty, Mathew Irimpan; Radhakrishnan, P; Nampoori, V P N(IOP, 2005)
[+]
[-]
Abstract:
The photosensitivity of dye mixture-doped polymethyl methacrylate (PMMA) films are investigated as a function of laser power, concentration of the dyes, modulation frequency and the irradiation wavelength. Energy transfer from a donor molecule to an acceptor molecule affects the emission output of the dye mixture system. Photosensitivity is found to change with changes in donor–acceptor concentrations. PMMA samples doped with the dye mixture are found to be more photosensitive when the dyes are mixed in the same proportion.
Nibu, A George; Jyotsna, R; Thomas Lee, S; Aneeshkumar, B; Jayan,Thomas; Nampoori, V P N; Radhakrishnan, P(Elsevier, 2005)
[+]
[-]
Abstract:
Photoacoustic spectrum of samarium phthalocyanine powder is recorded and compared with previously reported UV–vis absorption spectra of the same dissolved in different liquid and solid host media. The Davidov splitting of Q band is observed in the PA spectrum but the two bands are overlapped considerably and the shorter wavelength band is more intense and dominating one in the powder spectrum.
Achamma, Kurian; Sajan, D George; Nampoori, V P N; Vallabhan, C P G(Elsevier, 2005)
[+]
[-]
Abstract:
A sensitive method based on the principle of photothermal phenomena to study the energy transfer processes in organic dye mixtures is presented. A dual beam thermal lens method can be very effectively used as an alternate technique to determine the molecular distance between donor and acceptor in fluorescein–rhodamine B mixture using optical parametric oscillator.
Bindu, Krishnan; Nampoori, V P N(Indian Academy of Sciences, June , 2005)
[+]
[-]
Abstract:
Nanosized ZnO was prepared by polyol synthesis. Fluorescence spectrum of the ZnO colloid at varying pump intensities was studied. The powder was extracted and characterized by XRD and BET. The extracted powder was screen printed on glass substrates using ethyl cellulose as binder and turpinol as solvent. Coherent back scattering studies were performed on the screen printed sample which showed evidence of weak localization. The screen printed pattern showed strong UV emission.
Girijavallabhan, C P; Nampoori, V P N; Radhakrishnan, P; Sheeba, M; Rajesh, M(Measurement Science and Technology, Institute of Physics Publishing, October 4, 2005)
[+]
[-]
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.
Santhi, A; Vinu V, Namboodiri; Radhakrishnan, P; Nampoori, V P N(American Institute of Physics, 2006)
[+]
[-]
Abstract:
Wavelength dependence of saturable absorption (SA) and reverse saturable absorption (RSA) of zinc phthalocyanine was studied using 10 Hz, 8 ns pulses from a tunable laser, in the wavelength range of 520–686 nm, which includes the rising edge of the Q band in the electronic absorption spectrum. The nonlinear response is wavelength dependent and switching from RSA to SA has been observed as the excitation wavelength changes from the low absorption window region to higher absorption regime near the Q band. The SA again changes back to RSA when we further move over to the infrared region. Values of the imaginary part of third order susceptibility are calculated for various wavelengths in this range. This study is important in identifying the spectral range over which the nonlinear material acts as RSA based optical limiter.
Santhi, A; Vinu V, Namboodiri; Radhakrishnan, P; Nampoori, V P N(American Institute of Physics, 2006)
[+]
[-]
Abstract:
Wavelength dependence of saturable absorption (SA) and reverse saturable absorption (RSA) of zinc phthalocyanine was studied using 10 Hz, 8 ns pulses from a tunable laser, in the wavelength range of 520–686 nm, which includes the rising edge of the Q band in the electronic absorption spectrum. The nonlinear response is wavelength dependent and switching from RSA to SA has been observed as the excitation wavelength changes from the low absorption window region to higher absorption regime near the Q band. The SA again changes back to RSA when we further move over to the infrared region. Values of the imaginary part of third order susceptibility are calculated for various wavelengths in this range. This study is important in identifying the spectral range over which the nonlinear material acts as RSA based optical limiter.
Bindu, Krishnan; Deepthy, A; Litty, Irimpan; Dann, V J; Nampoori, V P N(Elsevier, 2006)
[+]
[-]
Abstract:
We report enhanced back scattering in nanometer-sized ZnO colloids prepared in two different media, by different methods. The FWHM of the back scattered cone and hence the mean free path varied with concentration of ZnO as well as particle size. The Lorentzian profile of backscattered cone indicates the presence of coherence.
Swapna, Nair S; Rajesh, S; Abraham, V S; Anantharaman, M R; Nampoori, V P N(Elsevier, 2006)
[+]
[-]
Abstract:
Fine magnetic particles (size≅100 Å) belonging to the series ZnxFe1−xFe2O4 were synthesized by cold co-precipitation methods and their structural properties were evaluated using X-ray diffraction. Magnetization studies have been carried out using vibrating sample magnetometry (VSM) showing near-zero loss loop characteristics. Ferrofluids were then prepared employing these fine magnetic powders using oleic acid as surfactant and kerosene as carrier liquid by modifying the usually reported synthesis technique in order to induce anisotropy and enhance the magneto-optical signals. Liquid thin films of these fluids were prepared and field-induced laser transmission through these films was studied. The transmitted light intensity decreases at the centre with applied magnetic field in a linear fashion when subjected to low magnetic fields and saturate at higher fields. This is in accordance with the saturation in cluster formation. The pattern exhibited by these films in the presence of different magnetic fields was observed with the help of a CCD camera and was recorded photographically.
Santhi, A; Vinu V, Namboodiri; Radhakrishnan, P; Nampoori, V P N(American Institute of Physics, 2006)
[+]
[-]
Abstract:
The authors apply the theory of photothermal lens formation and also that of pure optical nonlinearity to account for the phase modulation in a beam as it traverses a nonlinear medium. It is used to simultaneously determine the nonlinear optical refraction and the thermo-optic coefficient. They demonstrate this technique using some metal phthalocyanines dissolved in dimethyl sulfoxide, irradiated by a Q-switched Nd:YAG laser with 10 Hz repetition rate and a pulse width of 8 ns. The mechanism for reverse saturable absorption in these materials is also discussed.
Geetha, K; Rajesh, M; Nampoori, V P N; Vallabhan, C P G; Radhakrishnan, P(Optical Society of America, 2006)
[+]
[-]
Abstract:
The propagation characteristics of amplified spontaneous emission (ASE) through a rhodamine 6 G-doped polymethyl methacrylate freestanding film waveguide were studied. This was done by shifting the excitation stripe horizontally along a transversely pumped waveguide. By this method, we could tune the ASE wavelength. The maximum tunability thus obtained was ~18 nm with a pump stripe length of 6 mm.
Rajesh, S; Nandakumaran, V M(Elsevier, January , 2006)
[+]
[-]
Abstract:
We show numerically that direct delayed optoelectronic feedback can suppress hysteresis and bistability in a directly modulated semiconductor laser. The simulation of a laser with feedback is performed for a considerable range of feedback strengths and delays and the corresponding values for the areas of the hysteresis loops are calculated. It is shown that the hysteresis loop completely vanishes for certain combinations of these parameters. The regimes for the disappearance of bistability are classified globally. Different dynamical states of the laser are characterized using bifurcation diagrams and time series plots.
Radhakrishnan, P; Nampoori, V P N; Girijavallabhan, C P; Geetha, K; Rajesh, M(Journal of Optics A: Pure and Applied Optics, Institute of Physics Publishing, January 13, 2006)
[+]
[-]
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.
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)
[+]
[-]
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.
Nampoori, V P N; Radhakrishnan, P; Girijavallabhan, C P; Geetha, K; Rajesh, M(Journal of Optics A: Pure and Applied Optics, Institute of Physics Publishing, January 13, 2006)
[+]
[-]
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.