Theoretical analysis of acoustic-optic frequency shifter by dual mode fiber

The frequency shifting between two modes in an optical fiber resulting from acoustic wave propagation has been studied theoretically. Our theoretical results confirm the earlier experimental work. The polarization effect introduced by acoustic waves has been examined as well. The analysis shows that the polarization characteristic and symmetry property of the modes are not preserved in the coupling process of acoustic-optic frequency shifting for arbitrary ratio of the fiber cladding radius to the acoustic wavelengthd/ _a, in contrast to stable mechanical microbending, and that for the givend/ _a = 0.397, the polarized light signal at frequency ₂, shifted from that at frequency ₁, is quasi-linearly polarized for any acoustic-optic interaction length. However, the polarization effect can be neglected when the fiber cladding radius is much smaller than the acoustic wavelength.     

High-energy handling capabilities of optical fibers: Application to pulse compression and direct generation of megawatt picosecond pulses

The compression of the output of a Q-switched, mode-locked Nd:YAG (130 ps, 1.06 m) is reported for a number of different core-size single mode, multimode and polarization preserving fibers at power levels near the damage threshold. In maximizing the energy throughput, the main limitations are dielectric breakdown and nonlinear induced birefringence. Compressed pulse stability is found to depend critically on core size at this power level. An optimized compressor has been obtained capable of compression factors up to 35 (3.7 ps) with peak power in excess of 2 MW. Second-harmonic generation has been observed in polarization preserving fibers.     

Wavelength tunable subpicosecond pulse generation with a fiber grating Raman laser

Operation of a wavelength tunable fiber grating Raman laser working in the 1.1 m regime is demonstrated. The pumping mechanism utilizes the process of stimulated perpendicular Raman scattering, possible in fibers for which birefringence can compensate dispersion. A total tuning range of 40 nm with a minimum output pulse duration of 0.8 ps is achieved by using two optical fibers and controlling their birefringence thermally.Dedicated to Prof. Dr. Herbert Welling on the occasion of his 60th birthday     

Active phase stabilization in a two-fiber interferometer

We report on the frequency spectrum of phase fluctuations of a two fiber interferometer in a normal laboratory environment. A feedback system in connection with a piezo regulator is used for compensation of the fluctuations. The resulting stability of the compensator has been measured.     

Laser amplification in an optical fiber by evanescent field coupling

A laser signal propagating in an optical fiber is amplified by the evanescent field interacting with the excited Rhodamine 6 G solution surrounding the fiber. A small signal gain of 430 at 600 nm has been measured in a long fiber with a 2 cm long fiber optic amplifier.     

Fast local intensity fluctuations in a pulsed untuned dye laser

Intensity fluctuations in a pulsed dye laser are studied. Fluctuations are local and very fast. The onset of the fluctuating regime coincides with the laser threshold. The average Fourier spectrum of fluctuations shows three broad peaks at frequencies v ₁=55 MHz, v ₂=110 MHz, v ₃=165 MHz. The three frequencies remain unchanged when resonator geometrical and optical parameters are changed. Nevertheless the amplitude of fluctuations is easily affected by such changes and by the solution temperature. The application of the Grassberger-Procaccia algorithm to the experimental fluctuations gives slope convergence (between 3 and 5) in some cases. This seems to indicate that the origin of fluctuations is chaotic-deterministic.     

A comparison measurement of nonlinear ultrasonic waves in tubes by a microphone and by an optical interferometric probe

This paper deals with the analysis of ultrasonic fields inside waveguides generated by ultrasonic waves of high amplitude. These waves behave nonlinearly, so it is not possible to use standard linear equations to describe their behaviour. Therefore, we started with an experimental determination of the acoustic pressure of air in glass tubes. We chose two methods of measurement--by a microphone and by an optical interferometric probe. The conventional method by a microphone creates numerous problems, which can be avoided by using an optical method, a heterodyne laser interferometer.     

Longitudinal structure formation in a nonlinear resonator

A fiber ring resonator, synchronously driven by picosecond pulses, exhibits a spatiotemporal instability. A rapid modulation in the pulse envelope is spontaneously formed and evolves dynamically. This structure may be called one-dimensional optical turbulence. Single-shot detection of these structures is not viable. We report how to obtain experimental information which, when combined with numerical simulation, provides an adequate picture of the processes at work. Intrapulse correlations are studied; they reveal clear differences between dispersion of either sign. Also, they reveal the formation of soliton-like structures in the turbulent regime. A criterion is given that allows the prediction of the degree of complexity.     

Propagation in unstable and plane-mirror optical resonators

It is shown that propagation in an unstable resonator can be expressed in a form similar to that applicable to a plane-mirror resonator, but with the magnification appearing as a scaling parameter.     

Saturation stabilization of the CO fundamental-band laser

We recently reported the extension of CO laser operation to a variety of single-line transitions in the fundamental band (v=1 0). Now saturation stabilization of these lines could be achieved using an intracavity absorption cell with CO at low pressure. A 3f-detection technique allowed reliable frequency locking on transitions from P(8) to P(14). A frequency stability of better than 10kHz was indicated. The frequency of the P(14) transition was measured in a heterodyne experiment to be 62, 546, 358.324 (50) MHz.     

Phase-matched four-photon mixing processes in birefringent fibers

We describe several new phase-matching processes for four-photon mixing in polarization-preserving birefringent fibers. The frequency shifts and polarization conditions of the waves involved in this nonlinear interaction are specified for a single-mode fused silica birefringent fiber. Our calculations include the effect of dispersion in the birefringence of the fibers.     

Theory of nonlinear optical response of gauge invariant currents to linear and nonlinear couplings

When a gauge field interacts with a quantum condensed matter system, at first order of the gauge field it couples to the current operator of the electrons. Higher orders of the gauge field couple to electrons through other operators such as the stress tensor, etc. On the other hand, when one performs a measurement on a quantum system, not only the current operator, but also stress tensor operator of the electrons, etc. are hidden in the measurement, as they contribute to the gauge invariant current. We formulate a general problem of nonlinear optical response of the gauge invariant currents in presence of nonlinear couplings. We show that the new couplings along with new responses arising from field current have a very simple structure which can be formulated as time ordered multi-particle correlation functions. We also obtain their Lehman representation and thereby show that one need not use non-equilibrium formulations to deal with them. These new correlation functions suggest that in nonlinear optical response many new processes are possible. The experimental detection of the new terms in the current operator, and application corresponding multi-photon processes needs further theoretical and experimental investigations.     

A dense collection of two-level atoms embedded in a dielectric medium in a ring cavity

The hysteresis behavior of a dense collection of two-level atoms embedded in a linear dielectric medium in an optical cavity is investigated theoretically in terms of a parametric formulation method under the thin-sample approximation. The relation of the output intensity to the input intensity is obtained in an explicit formula. The numerical results show that the hysteresis behavior and the cavity transmission are different from those in vacuum because of the local-field enhancement and the local cooperative decay effect. Received: 27 June 2000 / Revised version: 11 September 2000 / Published online: 10 January 2001     

Optical and spectroscopic characterization of Nd-doped aluminosilicate fiber preforms made by the MCVD method using chelate delivery

4 F_3/2 level of Nd³⁺ in aluminosilicate optical fiber preforms after 807-nm excitation are reported in the temperature range from 15 °C to 500 °C for Nd₂O₃ concentrations in the range from 0.2 to [%wt]0.9 and Al₂O₃ concentrations of between [%wt]3.8 and [%wt]8. At room temperature it is observed that for concentrations of less than [%wt]0.6 Nd₂O₃ the fluorescence decay is approximately exponential; for the higher Nd₂O₃ concentrations the decay is increasingly bi-exponential. We have identified a “slow” (≈500 μs) and a “fast” (≈100 μs) decay time; the slow decay is attributed to isolated ions and the faster decay to energy transfer between homogeneously dispersed ions. The measured 1/e folding fluorescence decay time τ decreases slowly at a constant rate of -0.08±0.007 μs/K for [%wt]0.9 Nd₂O₃. It is shown that increasing the Al₂O₃ concentration tends to suppress this temperature quenching effect. A radiative decay time of 640 μs is obtained by Judd–Ofelt analysis. Received: 24 October 1997/Revised version: 2 February 1998     

Study of a carbon dioxide ring laser gyroscope

We have experimentally investigated a basic ring laser gyroscope (RLG) with carbon dioxide gain, and studied the prospects of developing a practical CO₂ RLG. Rotation sensing was demonstrated on a number of transitions in the 9.4 m and 10.4 m vibration-rotation bands. Gyro response is discussed with regard to lock-in, bias, homogeneous broadening effects, and high power operation. We show that such a system may offer important advantages over standard helium-neon RLGs, including reduced quantum limit and backscattering. The prospects and possible approaches for developing a practical high power CO₂ RLG are discussed, and a method of eliminating cross-saturation at high pressure is proposed and analyzed.     

Multiple pulses with broadband spectrum generation in passively mode-locked fiber ring laser incorporating a highly nonlinear fiber

Multiple pulses with broadband spectrum generation are experimentally obtained in passively mode-locked erbium-doped fiber ring lasers based on nonlinear polarization rotation, which is achieved by incorporating a length of highly nonlinear dispersion-shifted fiber in the laser cavity. The temporal and spectral dynamic behaviors of multipulse bunching states are demonstrated. The maximum spectral width and flatness are attained when the continuous-wave component is suppressed under appropriate polarization states and pump powers.     

Two-photon emission competing with cooperative and nonlinear processes

Stimulated non-degenerate two-photon emission from a pencil of lithium vapour is demonstrated on three decay cascades of the Li atom, namely accompanied with transitions from the 3s, 3d and 4s levels to the 2s level, upon pulsed two-photon excitation of the Li 4f and 4d levels. It competes with superfluorescence and parametric emission which show up at slightly shifted wavelengths. In the cascade via the 3d level, which is strongest under the experimental conditions, also two-photon Stokes scattering of the excitation light is identified.     

Variance squeezing and information entropy squeezing via Bloch coherent states in two-level nonlinear spin models

The nonclassical squeezing effect emerging from a nonlinear coupling model (generalized Jaynes–Cummings model) of a two-level atom interacting resonantly with a bimodal cavity field via two-photon transitions is investigated in the rotating wave approximation. Various Bloch coherent initial states (rotated states) for the atomic system are assumed, i.e., (i) ground state, (ii) excited state, and (iii) linear superposition of both states. Initially, the atomic system and the field are in a disentangled state, where the field modes are in Glauber coherent states via Poisson distribution. The model is numerically tested against simulations of time evolution of the based Heisenberg uncertainty relation variance and Shannon information entropy squeezing factors. The quantum state purity is computed for the three possible initial states and used as a criterion to get information about the entanglement of the components of the system. Analytical expression of the total density operator matrix elements at t > 0 shows, in fact, the present nonlinear model to be strongly entangled, where each of the definite initial Bloch coherent states is reduced to statistical mixtures. Thus, the present model does not preserve the modulus of the Bloch vector.