Andrew T. Ryan

Optical-feedback-induced chaos and its control in multimode semiconductor lasers

The effects of optical feedback in multilongitudinal mode semiconductor lasers are studied through computer simulations. Two separate regimes are found based on the length of the external cavity. For long external cavities (external-cavity mode spacing larger than the relaxation-oscillation frequency), the laser follows a quasi-periodic route to chaos as feedback is increased. For short external cavities, the laser can undergo both quasi-periodic and period doubling routes to chaos. When the laser output becomes chaotic, the relative-intensity noise is greatly increased (by more than 20 dB) from its solitary-laser value. Considerable attention is paid to the effects of optical feedback on the longitudinal-mode spectrum. The stabilization of the mode spectrum and the reduction of the feedback-induced noise through current modulation are studied and compared with experimental results. Current modulation eliminates feedback-induced chaos when the modulation frequency and depth are suitably optimized. This technique of chaos control has applications in optical data recording. >

Control of optical-feedback-induced laser intensity noise in optical data recording

The usefulness of semiconductor lasers can be greatly limited when the laser is subjected to uncontrolled optical feedback (OFB). In particular, the laser intensity noise can be severely degraded when OFB is greater than 0.1%. Although the technique of high-frequency injection (HFI) can solve this problem, the proper modulation frequency and depth must be chosen empirically. We investigate this problem through cornputer simulations of the multimode stochastic rate equations, modified to include OFB and HFI. By providing the program with measurable laser and system parameters, the simulations predict the HFI modulation frequency and depth that optimize the laser behavior. The results of the simulations are compared with experiment, and good agreement is obtained.

Spectroscopic detection of methane by use of guided-wave diode-pumped difference-frequency generation

We report spectroscopic gas detection by the use of mid-infrared difference-frequency mixing of two diode lasers in a channel waveguide. The waveguide was fabricated by annealed proton exchange in periodically poled lithium niobate. We generated 3.43-3.73-microm tunable radiation in a single waveguide at room temperature by mixing diode lasers near 780 and 1010 nm. High-resolution spectra of methane were obtained in 2 s with electronically controlled frequency scans of 45 GHz. The use of highly efficient waveguide frequency converters pumped by fiber-coupled diode lasers will permit construction of compact, solid-state, room-temperature mid-infrared sources for use in trace-gas detection.

Pulse compression and spatial phase modulation in normally dispersive nonlinear Kerr media

Numerical simulations show that, because of the spatiotemporal coupling implied by the multidimensional nonlinear Schrödinger equation, self-focusing of ultrashort optical pulses can lead to pulse compression even in the normal-dispersion regime of a nonlinear Kerr medium. We show how this coupling can be further exploited to control the compression by use of spatial phase modulation. Both the compression factor and the position at which the minimum pulse width is realized change with the amplitude of the phase modulation.

Steering of optical beams in nonlinear Kerr media by spatial phase modulation

A simple scheme to steer optical beams is proposed. The basic idea is to impose a sinusoidal phase modulation on the optical beam and then propagate it in a nonlinear Kerr medium. Spatial phase modulation splits the input beam into multiple subbeams, while the nonlinear medium shapes a particular subbeam into a spatial soliton in such a way that most of the beam power appears in a narrow beam whose direction can be controlled by changing the modulation parameters. We present numerical results showing how spatial phase modulation can be used to alter the path of an optical beam propagating in a nonlinear Kerr medium.

Optical-feedback-induced chaos and its control in semiconductor lasers

In this paper we describe some of the effects of external optical feedback (OFB) on semiconductor lasers by simulation of the stochastic rate equations. Particular attention is paid to the lasers transition to optical chaos. In addition, we describe three techniques for avoiding this chaotic regime. The technique of high frequency injection, used in optical recording, can delay the onset of chaos till very high values of OFB. Experimental results are given and are in excellent agreement with the theory. A second technique called occasional proportional feedback can be used with some success to stabilize the chaotic output of semiconductor lasers. The final technique for controlling chaos consists of the optimization of various system and laser parameters so that the laser is least susceptible to OFB.

Spatiotemporal coupling in dispersive nonlinear planar waveguides

The multidimensional nonlinear Schrodinger equation governs the spatial and temporal evolution of an optical field inside a nonlinear dispersive medium. Although spatial (diffractive) and temporal (dispersive) effects can be studied independently in a linear medium, they become mutually coupled in a nonlinear medium. We present the results of numerical simulations showing this spatiotemporal coupling for ultrashort pulses propagating in dispersive Kerr media. We investigate how spatiotemporal coupling affects the behavior of the optical field in each of the four regimes defined by the type of group-velocity dispersion (normal or anomalous) and the type of nonlinearity (focusing or defocusing). We show that dispersion, through spatiotemporal coupling, can either enhance or suppress self-focusing and self-defocusing. Similarly, we demonstrate that diffraction can either enhance or suppress pulse compression or broadening. We also discuss how these effects can be controlled with optical phase modulation, such as that provided by a lens (spatial phase modulation) or frequency chirping (temporal phase modulation).

Mid-infrared laser source for gas sensing based on waveguide difference frequency generation

Compact, room-temperature mid-infrared laser sources have a variety of applications in gas detection. We have developed a room-temperature mid-IR laser source based on the frequency mixing of two diode lasers in periodically poled lithium niobate waveguides. Output powers greater than 1 /spl mu/W have been obtained. This laser source has been used to measure strong absorption lines of water vapor at 2.7 /spl mu/m.

Ultrashort Pulse Propagation in Nonlinear Planar Optical Waveguides

For many years, optical pulse propagation in fibers has been an area of intense investigation.1 Apart from the obvious communication applications, optical fibers are important because they provide a simplified environment for studying the nonlinear effects. Two important consequences of the guiding nature of fibers are that diffractive effects can be eliminated from consideration and, perhaps more importantly, nearly constant pulse energies can be maintained over relatively long propagation distances due to the low loss in fibers. Thus, optical fibers are an excellent tool for studying the interplay of chromatic dispersion and optical nonlinearities, in particular the intensity-dependent refractive index or Kerr nonlinearity.1