ABSTRACT
We propose a compact setup to determine the size and concentration of spherical particles in a turbid medium. A pair of plane mirrors is used to multifold the undeviated laser beam, and measure it at a detector placed close to the sample, to determine the interaction coefficient. The size of particles is uniquely determined by comparison of the scattered light from the medium, measured at two separate detectors placed at two different angular positions, with that from Monte Carlo simulations. The methodology is verified using measurements with turbid samples comprising polystyrene spheres.
ABSTRACT
We present a method to determine the anisotropy parameter g of a scattering liquid medium. A simple experimental setup is used to measure the scattered light from turbid samples, and a calibration curve is used to determine g. The interaction coefficient of the medium is experimentally determined, and the calibration curve is obtained through Monte Carlo simulations. The method is tested for nonabsorbing solutions of polystyrene spheres, and the simulation results are found to agree with the experiments.
ABSTRACT
We present numerical results of an efficient scheme for mid-infrared generation using simultaneous quasi-phase matched optical parametric oscillators (OPO) and difference-frequency generation (DFG) processes in a singly resonant cavity with double-pass pump configuration. Considering an appropriately poled grating structure in MgO doped congruent lithium niobate, we have shown that it is possible to realize the double-pass pump singly resonant (DPSR) cavity configuration for the simultaneous OPO+DFG process, unlike the cascaded interaction scheme, leading to efficient mid-IR generation. Our numerical results are in agreement with the recently reported experimental results. We also present optimum parameters for the DPSR cavity configuration that could provide maximum conversion efficiency.
ABSTRACT
We present numerical results for optimization of the overall idler conversion efficiency of a nanosecond optical parametric oscillator (OPO), wherein the signal generated in the OPO process is also used as the pump for a difference frequency generation (DFG) process in a quasi-periodic MgO:LiNbO(3) crystal. The phase-matching conditions are considered such that the generated idler frequencies in both the processes (i.e., OPO and DFG) coincide. Optimization for the idler generation has been performed with respect to the different parameters, such as input pump power, pump pulse duration, and the output coupler reflectivity, for quasi-phase-matched interaction in MgO:LiNbO(3). Wavelength of the pump, signal, and idler waves considered in the optimization are 1.064 µm, 1.456 µm, and 3.95 µm, respectively. A maximum overall idler generation efficiency of ≈33% could be obtained in the simultaneous OPO+DFG process for a pump pulse duration of 72 ns and output coupler reflectivity (R(s)) of 90%, whereas for the stand-alone OPO process, the maximum idler generation efficiency was found to be ≈15%. The optimization has been illustrated for an average pump power of 8 W at a pulse repetition frequency (PRF) of 10 kHz. This approach of simultaneous OPO+DFG process can be employed to significantly enhance the idler generation efficiency of nanosecond OPOs.
ABSTRACT
The output pulse characteristics of Q-switched Yb-doped fiber lasers have been investigated experimentally. It has been observed that for any typical modulation frequency, the pump power and the modulator OFF-time govern the shape of the output Q-switched pulse. At a fixed modulation frequency, with a fine adjustment of acousto-optic modulation window ON-time, pump power and cavity mirror position, it was possible to obtain modulation free single-peak pulse, multi-peak pulse, mode-locked resembling pulse and multi-pulse structured pulse shapes in a Q-switched fiber laser output. These observations have been analyzed and explained. Our investigations show that multi-peak pulse output is due to onset of nonlinear phenomena like SBS and SRS. Similarly, we have found that the mode-locked resembling periodically modulated output pulse shape is due to mode beating between the zeroeth order and the first order diffracted beams of the intra-cavity acousto-optic Q-switch.
ABSTRACT
We report a novel idea for achieving highly efficient dispersion-compensating Bragg fiber by exploiting a modified quarter-wave stack condition. Our Bragg fiber yielded an average dispersion of approximately -1800 ps/(nm km) across the C band for the fundamental TE mode and an ultrahigh figure of merit of approximately 180,000 ps/(nm dB), which is at least 2 orders of magnitude higher than that of conventional dispersion-compensating fibers. The proposed methodology could be adopted for the design of a dispersion compensator across any desired wavelength range.
ABSTRACT
We present what is to our knowledge the first experimental observation by fluorescent visualization, of the evolution of the mode field profile in a periodically segmented waveguide. The experimental observations are then compared with the numerical results obtained by a finite-difference beam propagation method. Good agreement between experimental and numerical results is observed.
ABSTRACT
A ray analysis of periodically segmented waveguides with parabolic-index variation in the high-index region is presented. We carried out the analysis using ray transfer matrices, which is convenient to implement and which can be extended to study different types of graded-index segmented waveguide. Results of this ray tracing approach clearly illustrate the waveguiding properties and the existence of stable and unstable regions of operation in segmented waveguides. We also illustrate the tapering action exhibited by segmented waveguides in which the duty cycle varies along the length of the waveguide. This analysis, although restricted to multimode structures, provides a clear visualization of the waveguiding properties in terms of ray propagation in segmented waveguides.
ABSTRACT
We report the spectral characterization of proton-exchanged lithium niobate (PE:LiNbO(3)) waveguides in terms of the variation of the refractive-index difference between the waveguiding layer and the substrate. The dispersion of the extraordinary refractive-index increase (deltan(e)) is measured from 405 to 1319 nm with several light sources. Two types of proton-exchanged waveguide, prepared under different conditions, are studied. These measurements should be of use in the optimization of PE:LiNbO(3) waveguides for nonlinear optical applications, particularly in second-harmonic generation in the blue-green wavelength region.
ABSTRACT
We present theoretical modeling of a novel configuration for quasi-phase-matched parametric amplification in which the pump and the signal fields form guided waves of a planar waveguide while the idler is radiated into the substrate. We follow a coupled-mode approach to study the parametric amplification in periodically domain-reversed proton-exchanged LiNbO(3) waveguides upon a Z-cut substrate and present numerical results relevant to the amplification process. In particular, for given pump and signal wavelengths we have studied the dependence of the gain coefficient on the periodicity of a grating formed by domain inversion. This configuration, which is referred to as the Cerenkov-idler configuration, is shown to provide a large signal gain bandwidth and is more tolerant to variations in pump frequency.
ABSTRACT
We have analyzed the linear propagation characteristics of planar periodic segmented waveguides with infinitely extended parabolic refractive-index segments in the transverse direction, using the matrix formulation of Gaussian beam propagation. Modal propagation and the variation of effective index as a function of duty cycle and the period of segmentation have been studied. Conditions for the existence of guided modes in such a segmented waveguide are presented.
ABSTRACT
We present a simple and general approach for analysis of absorbing and leaky waveguides. The real and imaginary parts of the propagation constant beta of a planar optical waveguide are obtained by evaluating a function, defined through the eigenvalue equation, in real beta. The applicability of the method is demonstrated for simple structures. To apply the method for arbitrarily graded inhomogeneous or multilayered structures we use a simple matrix approach to obtain the eigenvalue equation. The method is straightforward, accurate, and requires no iteration in the complex beta plane.
ABSTRACT
In the weakly guiding approximation, we present an analysis describing the propagation in bent waveguides characterized by a separable profile (which may be graded) in the transverse direction. We then show how the recently developed matrix method can be used to calculate accurately the bending loss in such waveguides. We then suggest a novel refractive-index profile for which the bending losses can be made extremely small.
ABSTRACT
We present a novel technique for the measurement of the beat length of birefringent single-mode fibers based on differential attenuation induced by a metal. The proposed technique is particularly suited for the measurement of local birefringence in polished-fiber semiplates. The technique should also find application in the characterization of polished fiber semiplates used in the fabrication of fiber directional couplers.
