Unidirectional Ring Laser Research Articles

Ultrashort-pulse fiber ring lasers

This paper reviews recent progress on ultrashort pulse generation with erbium-doped fiber ring lasers. The passive mode-locking technique of polarization additive pulse mode-locking (P-APM) is used to generate stable, self- starting, sub-500 fs pulses at the fundamental repetition rate from a unidirectional fiber ring laser operating in the soliton regime. Saturation of the APM, spectral sideband genera- tion, and intracavity filtering are discussed. Harmonic mode- locking of fiber ring lasers with soliton pulse compression is addressed, and stability regions for the solitons are mapped and compared with theoretical predictions. The stretched- pulse laser, which incorporates segments of positive- and negative-dispersion fiber into the P-APM fiber ring, gen- erates shorter (sub-100 fs) pulses with broader bandwidths (> 65 nm) and higher pulse energies (up to 2: 7n J). We dis- cuss optimization of the net dispersion of the stretched-pulse laser, use of the APM rejection port as the laser output port, and frequency doubling for amplifier seed applications. We also review the analytical theory of the stretched-pulse laser as well as discuss the excellent noise characteristics of both the soliton and stretched-pulse lasers.

Polarization spectroscopy applied to the frequency stabilization of rare-earth-doped fiber lasers: a numerical and experimental demonstration

The frequency response of a unidirectional ring laser containing a linear polarizer and a weakly birefringent etalon is analyzed numerically. The data obtained indicate that by sensing the changes in the polarization state of the light that is coupled from the etalon, a suitable error signal can be derived that can be used to prevent longitudinal mode hops. This idea is verified experimentally by configuration of an erbium ring fiber laser with a nonplanar 3×3 fused-fiber coupler to form the intracavity etalon. When the longitudinal mode hops are inhibited and the oscillating frequency of the fiber laser is locked to a reference confocal etalon, the resulting radiation has a linewidth of 14 kHz with an associated rms frequency jitter of 2 kHz.

Steady-state emission and stability of a single-mode two-level Fabry-Perot cavity laser

An analytical steady-state solution for a single-mode homogeneously-broadened two-level Fabry-Perot laser, valid for any field intensity, cavity detuning and level-population decay rates, is obtained. A power-series expansion of this solution allows to perform a linear stability analysis which reveals the existence of two Hopf bifurcations instead of one as in unidirectional ring lasers. These bifurcations delimit the domain of unstable emission of the laser with respect to small perturbations. The instability threshold for hard-mode excitation is higher than in a ring laser, although introduction of a small definite detuning makes them similar. The time-dependent behaviour above the instability threshold is somewhat more complex than in a ring laser.

Increased single-frequency power from end-pumped ring lasers by pump beam displacement

End-pumped unidirectional ring lasers often suffer some degree of residual spatial hole burning, where a standing-wave region overlaps the pumped region. By non-concentric pumping this overlap can be reduced, thus increasing the output power level at which single frequency operation can be maintained. Experimental results show a factor of 2 to 3 improvement in single-frequency performance for only a small drop in output power.

Fast polarization dynamics of an erbium-doped fiber ring laser

The polarization dynamics of a unidirectional erbium-doped fiber ring laser has been observed for individual round trips in the cavity. A rich variety of dynamic states, including square-wave pulses and irregular temporal patterns, was observed as operating parameters were changed. A model with coupled delay and differential equations is used to interpret the dynamics.

Single-mode operation of an injection-seeded alexandrite ring laser for application in water-vapor and temperature differential absorption lidar

A major improvement of a differential absorption lidar (DIAL) system for measurements of tropospheric water vapor and temperature is introduced. A Q-switched unidirectional alexandrite ring laser is injection seeded by a cw Ti:sapphire ring laser. Using an especially developed single-mode electronic, one starts the Q switch when the slave resonator is in resonance with the frequency of the Ti:sapphire laser. Long-term single-mode operation of the alexandrite laser is achieved. A single-shot spectral linewidth of <40 MHz and a frequency stability of 15 MHz rms can be specified. Thus what is to our knowledge the first single-mode DIAL system in the near infrared is presented.

Rotating and oscillating transverse patterns in an inhomogeneously broadened laser operating in a pair of doughnut modes

A detailed numerical investigation of an inhomogeneously broadened unidirectional ring laser operating in a pair of doughnut modes was performed for the parameter region corresponding to a transient from the standing wave to the travelling wave operation. It was shown that the dynamical transverse patterns arising in the transient region can either oscillate back and forth or continuously rotate around the cavity optical axis.

Limitations imposed by spatial hole burning on the single-frequency performance of unidirectional ring lasers

Residual spatial-hole-burning in a unidirectional ring laser can limit the maximum power available in a single-frequency. These effects have been modelled numerically and verified experimentally in Nd:YAG and Nd:YLF lasers. For convenience, an analytical expression for maximum single-frequency power has also been developed for use in designing ring lasers.

10 GHz 3 ps actively mode-locked ring laser incorporating a semiconductor laser amplifier and an electroabsorption modulator

The authors report a novel actively mode-locked unidirectional semiconductor ring laser constructed from discrete fibre pigtailed components. When driven at 10 GHz, the laser produces near transform-limited pulses of 3 ps duration.

Single-frequency erbium fibre ring laser with intracavity phase-shifted fibre Bragg grating narrowband filter

The authors report a single-frequency unidirectional erbium fibre ring laser operating at 1.55 /spl mu/m which uses an intracavity phase-shifted fibre Bragg grating as an ultranarrow frequency bandpass filter. The linewidth of the laser was measured to be <2.0 kHz.

Tunable, single frequency erbium fiber laser using an overlay bandpass filter

We describe a single frequency, unidirectional erbium fiber ring laser. Wavelength selection is performed by a ‘‘continuous fiber’’ filter of the fiber to planar waveguide coupler type. Varying the refractive index experienced by the exposed surface of the filter allowed the single longitudinal mode output from the laser to be tuned, discretely, from 1556–1561 nm with a resolution limited linewidth of 52 kHz.

Patterned chaos in a system of three coupled identical unidirectional ring lasers

Three coupled identical unidirectional B-type ring lasers are considered and the dynamics for varying coupling parameter investigated. For given system parameters like pumping, cavity damping etc. the system is characterized by three distinct types of steady states. The linear stability of these steady states is investigated in detail which are unstable beyond a critical value of the coupling parameter. For a certain range of the coupling parameter chaotic behaviour of the system is demonstrated. The dynamical variables are mapped on to a complex variable reflecting the symmetry of the system. The plot of the real vs. the imaginary part of this complex variable exhibits patterns with threefold symmetry. The largest Lyapunov exponent as well as the power spectra corresponding to the chaotic attractors are also calculated.

A unidirectional Er/sup 3+/-doped fiber ring laser without isolator

An Er/sup 3+/-doped fiber ring laser with unidirectional operation without optical isolator has been investigated for different cavity conditions. The fiber ring laser cavity is built in such a way that the optical fields propagating in the two directions suffer different losses. As a consequence, the laser oscillation appears in a quasi-unidirectional form. By incorporating a fiber pigtailed bandpass filter to enhance mode competition, a purely unidirectional tunable fiber ring laser is obtained with high efficiency and broad tunability. >

Strong optical asymmetry of an interference wedge with unequal-reflectivity mirrors and its use in unidirectional ring laser designs

On the basis of theory and experiment we discuss the strong optical asymmetry in reflection exhibited by an interference wedge when it is constructed of unequal-reflectivity mirrors and show how this feature can be used to develop attractive unidirectional ring lasers. In the proposed ring cavity schemes a single interference wedge is employed simultaneously as a tuning element, as an element introducing asymmetry, and as a laser output coupler.

Understanding the bifurcation to traveling waves in a class-B laser using a degenerate Ginzburg-Landau equation.

We analyze the Risken-Nummendal--Graham-Haken instability occurring in a homogeneously broadened two-level unidirectional ring laser modeled by the Maxwell-Bloch equations. We investigate the class-B limit of these equations and derive partial differential equations describing the evolution of traveling waves. In particular, we obtain a degenerate Ginzburg-Landau equation with higher-order nonlinearities. We then investigate this equation and determine periodic traveling wave solutions. We discuss their stability in terms of the laser parameters and predict unusual properties for the long-time behavior of the laser.

Origin of an lf maximum in the noise spectrum of a unidirectional solid-state ring laser

It is shown theoretically that an lf maximum in the noise spectrum of travelling-wave ring solid-state lasers is a manifestation of beats between the wave generated and a regeneratively amplified spontaneous radiation propagating in a direction along which there is no lasing.

Active-passive modelocking in a single-polarisation erbium fibre laser

Subpicosecond pulses have been generated in an actively modelocked, polarisation-maintaining Er-doped fibre ring laser. A single-polarisation fibre amplifier and an uncompensated polarisation bias section are integrated into the laser. This laser combines the nonlinear pulse-shortening mechanism of polarisation-rotation lasers and the single-polarisation operation, environmental insensitivity, and capacity for active modelocking characteristic of unidirectional single-polarisation ring lasers.

Unidirectional semiconductor ring lasers with racetrack cavities

We report the development of racetrack-cavity semiconductor ring lasers that operate unidirectionally by means of an active crossover waveguide. These racetrack-cavity lasers show improved performance over previous circular-cavity unidirectional ring lasers with continuous-wave single-frequency output powers of ≥10 mW, and with up to 99% of the lasing output emitted from the preferred ring direction.

Collisional effects in gas lasers.

The effects of phase-interrupting and velocity-changing collisions between active and perturber atoms on the spectral profile of a gas laser are studied. Exact analytical results for the density matrix are presented for the case of strong collisions in a unidirectional ring laser, and the laser intensity is then evaluated in the Doppler limit under the condition of self-consistency. The washout of the hole burning in the steady-state population difference and the broadening of the laser line shape are discussed. The results of the third-order perturbation theory are compared with the exact calculations valid for an arbitrarily intense pump.

Mode-locking dynamics of synchronously pumped color-center lasers: Fabry-Pérot and ring geometries.

Numerical simulations of mode locking in synchronously pumped KCl:${\mathrm{Tl}}^{0}$(1) and LiF:${\mathrm{F}}_{2}^{+}$ color-center lasers in Fabry-P\'erot and ring configurations are presented. The presence of spontaneous-emission noise leads to large-scale fluctuations in the resulting pulses, and conditions for pulse optimization and stabilization are described. There exists a close correspondence between regimes of quasistationary and fluctuating pulses in the Fabry-P\'erot and unidirectional ring lasers, with regimes of quasistationary, spontaneously unidirectional and stochastically driven, competitive, bidirectional pulsing in the bidirectional lasers.