Multiwavelength fiber laser with ultradense wavelength spacing based on inhomogeneous loss with assistance of nonlinear polarization rotation

We demonstrate a multiwavelength fiber laser with ultradense wavelength spacing and ultrabroad bandwidth based on inhomogeneous loss mechanism with assistance of nonlinear polarization rotation. The inhomogeneous loss, implemented by incorporating a section of highly nonlinear fiber (HNLF) and a Sagnac filter in the laser cavity, can balance mode competition in erbium-doped fiber and result in ultradense multiwavelength generation. The bandwidth of the multiwavelength spectrum is greatly broadened owing to the intensity-dependent loss induced by nonlinear polarization rotation. Stable multiwavelength lasing with wavelength spacing of 0.08 nm and wavelength number up to 254 is achieved at room temperature. Moreover, multiwavelength tuning is realized through modifying polarization-dependent cavity loss.     

The competition between two polarization states in two-dimensional random medium

In a two-dimensional random medium, when the transverse magnetic (TM) polarization state and the transverse electric (TE) polarization state share the same population inversion, there is a competition in the population inversion between the two states. Such competition is analyzed by using the finite difference time domain method to numerically solve Maxwell equations with a set of system parameters. The results indicate that TM state has a larger lasing threshold than TE state does so that TM state is strongly suppressed by TE state in the competition. The larger the size of the medium, the easier the lasing modes in TM state appears.     

Instability and satellite pulse of passively Q-switching Nd:LuVO4 laser with Cr:YAG saturable absorber

This work presents experimental results concerning a passively Q-switching Nd:LuVO₄ laser with a Cr⁴⁺:YAG saturable absorber operated in a three-element cavity. When the pump power exceeded 5.47 W, the system transfers stable pulse train into spatial-temporal instability. Furthermore, the chaotic pulse train accompanied the generation of a satellite pulse. The experimental results reveal that the mechanisms of instability and generation of the satellite pulse are governed by the multitransverse mode competition.     

Morphology Dependence of Power Spectra for Different Polarized States from Two-Dimensional Active Random Media

Spectra for two polarized states with competition on the inverted population in a set of two-dimensional （2D） active random media are calculated, and these random media have the same random constitution but different shapes. Results show that the power spectra for the two polarized states are all morphology dependent in the competition. The appropriate design of the shapes has an obvious effect on the selection of TM polarized modes and can improve the adverse situation of the TM polarized state in the competition. Such a property presents a new mode-selecting technique for each polarized mode in random media.     

Control of transverse mode pattern in a helium-neon laser using the cat’s eye cavity

This paper introduces an innovative method to control and select transverse mode pattern in a He-Ne laser. The cat’s eye cavity is established by using a cat’s eye reflector as the reflecting mirror. Laser transverse mode pattern varies continuously when the distance between the convex lens and the concave mirror is changed, which form the cat’s eye reflector. By Newton-Cotes method, the numerical solution of two-dimensional diffraction integral equation in the cat’s eye cavity is obtained, the laser operating modes under various parameters are analyzed, and the eigenvalue η_mn and diffraction power loss corresponding with every transverse mode are calculated. The parameters assuring fundamental transverse mode output are figured out, which match the experiment results well. Therefore, a new convenient real-time means for the control and selection of the laser transverse mode is realized.     

WG-Like Modes Selectivity in a Square Cavity with Posts

We investigate the characteristics of Whispering-Gallery（WG）-like modes in a square cavity with posts by employing the two-dimensional （2D） finite-difference time-domain （FDTD） technique combined with the effective index method. The results indicate that the posts can result in mode selection in the WG-like modes. The WG-like modes with odd mode numbers are not much sensitive to the sizes of the posts. However, the quality factor （i.e. Q-factor） of the WG-like modes with even mode numbers decreases sharply with the increasing size of the posts. The decreasing Q-factor is attributed to mode leakage and scattering loss due to the presence of the post. The mode selection increases the mode spacing of square cavity twice in an optimized structure.     

Passive mode locking of a diode-pumped Nd:Gd 0.64 Y 0.36 VO 4 laser with a GaAs saturable absorber mirror

We report a diode-pumped Nd:Gd_0.64Y_0.36VO₄ laser passively mode locked by using a GaAs saturable absorber mirror. Both the Q-switched and continuous-wave (CW) mode locking were experimentally realized. The CW mode-locked pulses have a pulse width of about 8.8 ps at a repetition rate of 161.3 MHz. Limited by the available pump power, a maximum output power of 2.47 W was obtained for the CW mode-locked pulses with a slope efficiency of about 26.6%.     

A new mode to excite a gas-discharge XeCl laser

The charge mode is a new mode to excite a discharge XeCl laser based on the dynamics of a spiker-sustainer circuit with a magnetic pulse compressor. The breakdown voltage is higher than in any other mode, due to a very fast rise time. The higher breakdown voltage provides a wider discharge and we found that more energy can be deposited before discharge instabilities terminate the optical output.     

Comparison of pulsed and continuous operation of the Cu II laser

The peak output power of Cu II laser emission in the uv at 260 nm and in the ir at 780 nm was measured for different operating conditions in the pulsed and the cw mode. In contrast to the ir laser output the laser power in the uv is higher in the cw mode than in the quasi cw mode. This property depends on contaminations of the discharge gas.     

Feedback coupling loss in single mode fiber lasers

Based on the expansion of the fundamental mode LP₀₁ in single mode fiber in terms of Laguerre-Gaussian free space modes, the feedback coupling losses for two different types of cavity mirror, i.e., a curvature mirror and a combing mirror of a lens and a plane mirror, are numerically calculated for the first time. The results show that, for the curvature mirror, the lowest coupling loss is obtained when its curvature radius matches the wavefront curvature. In particular, if a plane mirror is used as the cavity mirror, it has to be placed close to the fiber end to obtain the low coupling loss. For the combing mirror, the lowest coupling loss can be obtained when the plane mirror is placed at the back focal plane of the lens, and the variation of the coupling loss is insensitive to the mirror positions for the lens with longer focal length. Finally, the plane mirror and the combing mirror of a lens and a plane mirror are suggested to be the cavity mirror in the practical construction of the high power fiber lasers.     

Propagation of Single-Mode Fibre Laser Beams through an Optical ABCD System with Circular Aperture at the Fibre Output End

Based on the expansion expression of the fundamental mode of a single-mode fibre in terms of Laguerr-Gauss modes, the propagation of a beam of a weakly guiding fibre laser through an opticM ABCD system with a circular aperture at the fibre end is studied. The results show that there is much difference between the propagation of the laser beam described by the expansion expression and by the Gaussian mode approximation. The depth of focus of the laser beam is longer than that of the Gaussian modes.     

Ring-down spectroscopy for studying properties of CW Raman lasers

We describe theoretically a simple technique for analyzing the conversion efficiency and the oscillation threshold of all-resonant intracavity Raman lasers. The method is based on a dependence of the ring-down time of the pump cavity mode on the energy accumulated in the cavity.     

The research on polarization states with local pumping in two-dimensional active random medium

The polarization-dependent competition on inversed population with local pumping in two-dimensional (2D) active random media is investigated by simultaneously solving Maxwell’s equations and rate equations of electronic population for both transverse magnetic (TM) and transverse electric (TE) modes. The threshold property of the two polarization states with local pumping is analyzed. Results show that the lasing threshold of TM modes is lower than that of TE ones when the radius of the local pumping region is suitably selected, and the case would be contrary when the selected radius of the local pumping region is excessive.     

Magnetic-spiker excitation of gas-discharge lasers

A review is presented on the use of magnetic-spiker circuits to electrically excite rare-gas-halide and CO₂ gas-discharge lasers. The design and operation of the diode, switch, overshoot, and charge modes of magnetic-spiker excitation will be described. Special emphasis has been placed upon understanding the dynamical behaviour and saturation properties of the magnetic core material at high frequencies. The effect of voltage risetime, preionization, pulse-forming network design, and prefire on laser performance is also described. Experimental data are presented on a new and very simple magnetic-spiker circuit called the modified overshoot mode. A relatively long optical pulse magnetic-spiker KrF laser is also desribed which uses self-timed corona preionization initiated by the spiker circuit. The use of magnetic-spiker excitation to produce high output laser energies, high repetition rates, and long optical pulses is discussed. Applications of magnetic-spiker lasers in the field of medicine, microelectronics and materials processing will be reviewed.     

Folded resonator-dual polarization competition laser displacement sensor

The existing so-called “dual polarization competition laser displacement sensor” applies a cat’s eye resonator. The resonator could be disarranged when the cat’s eye reflector (CER) swings slightly with the rod of the slide rail in measuring the procedure and droops due to its deadweight in long-time operation. This influences the stability of the sensor significantly. In this paper, a novel cat’s eye folded resonator was built, based on which our system named “folded resonator-dual polarization competition laser displacement sensor” was set up. It has a high resolution of λ/16 (39.6 nm) and large measurement range of up to 17 mm. The misalignment sensitivities of traditional cat’s eye resonator and the folded one are both analyzed; and comprehensive experiments are carried out, through which a conclusion can be drawn out that the maximal adjusting angle and the maximal cross displacement of the CER in folded resonator are both about 5 times of those in cat’s eye cavity. Furthermore, the measurement results from this new sensor prove its high stability.     

Mode competition in multi-core fiber amplifier

3D beam propagation numerical code and a newly developed optical mode solver were implemented for studies on the recently experimentally realized Yb fiber amplifier with 7-core hexagonal structure. It was found that the launched seven single-frequency beams matched with the cores converge to a wave field with permanently diminishing space beating patterns, but the result is sensitive to phases of separate beams. Namely, at a spread of phases of individual beams within a few tenths of radian, the convergence of the wave field to the in-phase mode is observed, while at phases random spread on the order of π the out-of-phase mode is dominating. Such behavior is observed at strong gain saturation in the cores. Both effects take place even if the guiding index of the cores fluctuates within a limited range. The mode solver predicts that the in-phase mode has the minimal small signal gain, and the gain of the out-of-phase mode is maximal one. This fact proves that there is no strict correlation between modal gains and amplification in strong saturated multi-core fiber. The smaller core index step significantly improves the convergence of the amplified radiation to the in-phase mode. An approach to the waveguide structure tailoring is described which assures the single in-phase mode stability.     

Passive mode locking of thin-disk lasers: effects of spatial hole burning

We recently demonstrated that passive mode locking of a thin-disk Yb:YAG laser is possible and that this concept leads to sources of femtosecond pulses with very high average power. Here we discuss in detail the effect of spatial hole burning on the mode-locking behavior of such lasers. We have developed an efficient numerical model and arrive at quantitative stability criteria which agree well with experimental data. The main result is that stable soliton mode locking can in general be obtained only in a certain range of pulse durations. We use our model to investigate the influence of various cavity parameters and the situation for different gain media. We also consider several methods to reduce the effect of spatial hole burning in order to expand the range of possible pulse durations. Received: 4 September 2000 / Published online: 10 January 2001     

Two-wavelength operation of a tunable KrF excimer laser — A promising technique for combustion diagnostics

A KrF excimer laser was operated on two independently tunable lines. This was achieved by a double-resonator configuration with two gratings. Tuning range and gain competition were investigated. The narrow line width (<1 cm^–1) and the independent tunability make this laser ideal for simultaneous spectroscopic detection of two species and temperature measurements in combustion processes.     

Q-switched and continuous-wave mode-locking of a diode-pumped Nd:Gd0.64Y0.36VO4−Cr4+:YAG laser

We report on a diode-pumped passively mode-locked Nd:Gd_0.64Y_0.36VO₄ laser with a Cr⁴⁺:YAG saturable absorber. Q-switched mode locking (QML) with 90% modulation depth was obtained. The peak power of the mode-locked pulse near the maximum of the Q-switched envelope was estimated to be about 1.7 MW at the pump power of 12 W. Besides QML, continuous-wave mode locking was also experimentally realized, for the first time to our knowledge, in the laser under a strong intracavity pulse energy fluence. The mode-locked pulse width is about 2.96 ps at a repetition rate of 161.3 MHz.