Ramakant Srivastava

Search of soft modes in crystals having KDP structure

Abstract The low-frequency Raman scattering in KDP, RDP and RDA in Y(XY)X configuration has been reinvestigated as a function of temperature with particular emphasis near the transition temperature Tc. The spectra in the region of the overdamped and A-BO4 stretching modes have been fitted to the response function of two coupled damped harmonic oscillators and that of lattice oscillators due to Silverman.8 In both the formalisms, assuming purely real or purely imaginary coupling the overdamped mode softens with temperature well above T c but attains a “hard-core” frequency near the phase transition. These results do not indicate existence of a mode approaching zero frequency in contrast to the earlier work by various authors.

Picosecond duration laser pulse generation by simultaneous active-passive mode-locking in Ar+ ion and dye lasers

We use a rate equation model to analyse a technique for the generation of picosecond duration laser pulses in argon pumped dye lasers. The system is based on passive mode-locking of the Ar+ laser by saturable absorption of the dye which is inserted in the Ar+ laser cavity. Simultaneously, the dye is forced to oscillate in a regime of synchronously pumped mode-locking. The system is relatively simple, does not require the use of an acousto-optical light modulator and can be easily implemented in commercially available lasers. Pulses shorter than 100 ps and having an average power of 20 mW were obtained at a repetition rate of 110 MHz. Good agreement between the model and experimental results was also obtained.

WKB analysis of graded-index multimode fibers with a central-index dip

We have extended the Gloge-Marcatili analysis to include the effect of a refractive-index dip in a multimode fiber characterized a square-law profile. The analysis gives analytical expressions for the propagation characteristics and shows that the dip has a very significant effect on the impulse response.

Geometrical characterization of liquid core fibers by measurement of thermally induced mode cutoffs and interference

Transmitted and scattered intensity in liquid core fibers have been measured as a function of temperature and v values of cutoff points obtained which are in good agreement with weakly guiding mode theory. We also report observation of oscillations in the transmitted intensity which we interpret to be caused by interference between modes. We show that measurement of the period of these oscillations can be used to make high-precision nondestructive measurements of core radius and core ellipticity along the fiber length.

Low-frequency optical phonons in silica

Abstract Measurements of low_frequency Raman spectra of silica fibers under longitudinal tensions as a function of temperature have been correlated with the measurements of FIR spectra of bulk silica in reflection and transmission. The results indicate existence of an LO-TO pair of a low-frequency optical phonon branch in amorphous silica.