Conventional Optical Feedback Research Articles

Stability and noise properties of diode lasers with phase-conjugate feedback

For a diode laser subjected to filtered feedback from a phase-conjugating mirror, we present the first exact stability analysis and various noise spectra. The stability properties are intermediate between those of the injection laser and the laser with conventional optical feedback. The role of a finite response-time is to drastically enhance the steady-state stability For moderate feedback, the frequency noise is suppressed by several orders of magnitude, and the main relaxation frequency of the laser shows a crossover from the usual relaxation oscillation frequency to a new frequency determined by the amount of feedback. This may be of technological importance since it is expected to improve the modulation bandwidth.

Contrasting conventional optical and phase-conjugate feedback in laser diodes

An experimental comparison of phase conjugate feedback to conventional optical feedback in a semiconductor laser is presented, which contrasts the optical frequency spectra and power noise spectra with varying levels of feedback. These spectra are correlated with the single frequency regimes III and V and the chaotically unstable regime IV. Conventional feedback is derived from a mirror and variable neutral density filter, and the phase conjugate feedback is derived from a self-pumped rhodium doped barium titanate photorefractive crystal. Both systems show two stable, single frequency operation regions analogous to regime III and V operation separated by a single region of unstable operation analogous to regime IV. It is found that phase conjugate feedback leads to distinctive behaviors including: differences in the relative intensity noise spectra; dynamically varying output frequency spectra close to the transition from regime IV to V; systematic power transfer from one laser diode longitudinal mode to a nearest neighbor through regime IV; and a very much larger range of feedback levels leading to unstable output. The level of optical feedback leading to a transition from regime III to IV is the same for conventional optical feedback and phase conjugate feedback when the correction for the different coupling efficiencies is made. The transition from regime IV to V occurs for much higher levels of feedback when PCF is used leading to the larger range of feedback levels giving chaotic behavior with PCF. The results are discussed in the context of existing theoretical models of laser diodes with phase conjugate feedback.

Symmetry properties of lasers subject to optical feedback

We show that (in the description by rate equations) a laser with conventional optical feedback has the symmetry of any rotation of the electric field, whereas a laser with phase-conjugate feedback has the symmetry of only the rotations of the electric field over 0 and π. This has important consequences for dynamics and bifurcations of these two laser systems. Most importantly, a PCF laser shows symmetry breaking and restoring bifurcations, something that does not occur in a COF laser.

Determination of external cavity coupling-coefficientfor diode laser with phase-conjugate feedback

An experimentally determined coupling coefficient for a singlemode diode laser operated with conventional optical feedback is contrasted with that for phase conjugate feedback. Values are determined to be 0.2 ± 0.05, and 0.7 ± 0.1, respectively. These quantitative measurements indicate the enhancement in coupling efficiency that can be achieved using a phase conjugate reflector.

Dynamics of Semiconductor Lasers with Optical Feedback from Photorefractive Phase Conjugate Mirror

The dynamics of semiconductor lasers with photorefractive phase conjugate optical feedback are experimentally studied. Photorefractive fringes considered here are rather static compared with time fluctuations of laser output power. Therefore, it is expected that a semiconductor laser with photorefractive feedback shows similar dynamics to those with conventional optical feedback. We examine relaxation oscillation and external cavity modes of laser output power in the presence of photorefractive phase conjugate feedback. It is proved that the dynamics of photorefractive phase conjugate feedback are fundamentally the same as those of conventional optical feedback.

Stability analysis of semiconductor laser with phase-conjugate feedback

The stability of the output power of a semiconductor laser with phase-conjugate feedback (PCF) is studied by numerical simulations based on the rate equations. We investigated the critical reflectivity at which the stable fixed output power evolves into periodic oscillation. As a result, we found that the critical reflectivity shows a periodic structure against the external cavity length and the stability is much enhanced at the periodic peak positions. The stability and dynamic behavior of a semiconductor laser with PCF are compared with those for conventional optical feedback. It is also found that the periods of the nearby peaks correspond to the frequency of the laser relaxation oscillation frequency and the stable peaks for the PCF are located at external lengths completely out of phase from those for the conventional feedback. Linear stability analysis using the rate equations is performed and the theoretical background for the stability is also given.

Improved control techniques for the reactive magnetron sputtering of silicon to produce silicon oxide and the implications for selected film properties

Silicon oxide films were reactively sputtered to give reproducibly controlled properties with refractive indices between 1,400 and 1,490. They were deposited at rates of up to 0.85 nm s −1 from a 100 mm diameter polycrystalline silicon cathode, sputtered at 200 W, 40 kHz rectified a.c. power in a reactive environment. This frequency, combined with feedback control of the flow of reactive gas, achieved from a d.c. signal derived from the cathode potential, used directly or with a novel gas pulsing unit, has demonstrated that the process can be controlled with at least as much success as conventional optical emission feedback from the plasma. Stress-stoichiometry curves showed a peak in stress at a refractive index of 1,460 indicating both a dense structure and optimized SiO 2. We have demonstrated a reactive-gas pulsing system which allows stable sputtering conditions to be easily established for the duration of the deposition, which gives rapid recovery from arc disruptions and allows various combinations of reactive gas incorporation into the growing film.

Lateral spatial effects of feedback in gain-guided and broad-area semiconductor lasers

Starting from Fresnel diffraction theory, we derive analytic expressions for the lateral spatial dependence of feedback fields for the cases of conventional and phase-conjugate optical feedback. By using numerical simulations we show that for narrow-stripe gain-guided lasers, both types of optical feedback from an external cavity can convert the twin-lobed far fields into a nearly single-lobed far field. We also find that conventional and phase-conjugate feedbacks in broad-area lasers induce a spatial modulation of the lateral field that increases the tendency for filamentation at moderate (-30 dB) feedback levels.

Intensity noise and linewidth characteristics of laser diodes with phase conjugate optical feedback

The intensity noise and linewidth characteristics of semiconductor lasers with phase conjugate optical feedback are investigated numerically. The results are compared to those from conventional mirror optical feedback. The dynamics of semiconductor lasers with phase conjugate optical feedback are also considered. The relevance to phase conjugate optical feedback of the feedback regimes used to describe conventional mirror optical feedback is discussed.

Coherent-state optical amplifier: a proposal

A new kind of phase-insensitive optical amplifier is proposed whose output is in the coherent state (ideal laser light) if the input is in the coherent state. In-phase and quadrature modulations are preserved in absolute values. Both modulations can be tapped off. The amplifier employs conventional optical amplifiers, electrical feedback, and all-pass filters. Remarkably, these properties hold when either linear or nonlinear optical amplifiers are employed.

Waveguide lasers operating at 1084 nm in neodymium-diffused lithium niobate

The authors report the demonstration of waveguide lasers in lithium niobate into which the active element neodymium has been introduced by thermal diffusion. The waveguides were fabricated using conventional Ti indiffusion and optical feedback was provided by the polished endfaces alone. The absorbed power threshold of one waveguide laser was estimated at 13+or-3 mW for pumping at 814 nm, and its slope efficiency in terms of total lasing output as a function of absorbed pump power was estimated to be 55+or-8%. Laser output powers in excess of 100 mW were observed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>