Laser resonator with ultrabroad longitudinal mode spacing

A novel type of laser resonator is proposed, in which the resonator length changes with the wavelength of the laser generation. This results in longitudinal mode spacing three orders of magnitude broader than that of conventional cavities. An elementary analysis of the laser resonator is presented and some features and possible applications are discussed.     

Single longitudinal mode lasing of coumarin-doped sol-gel silica laser

Homogeneity of sol-gel-derived silica was improved by slowing down the drying and aging rate. Single longitudinal mode lasing was achieved in sol-gel silica doped with coumarin 460 dye. The laser linewidth was 1.58 GHz and the output wavelength was at 467 nm. Received: 2 February 1999 / Published online: 28 April 1999     

Active mode locking of a high pressure CW waveguide CO2 laser

In this paper, we report on a new concept for active mode locking of lasers. It has been successfully applied to a cw waveguide CO₂ laser and pulse widths as short as 2 ns have been obtained.     

Single longitudinal mode interaction between a uranium atomic beam and a modified Hänsch-type pulsed dye laser

Laser-induced fluorescence is used as a diagnostic tool for testing the tuning between a longitudinal mode of a nitrogen-pumped dye laser and a uranium spectroscopic level in an atomic beam. According to a simple resonator model, the tuning instabilities can be ascribed to thermal drifts in the dye-laser system. Problems encountered in attempting a single mode scanning are also described.The work described in this paper was carried out at the ENEA Laboratory while P. Benetti was assigned to this Laboratory (1981)     

Additive-pulse mode locking of a diode-pumped Nd3+:YVO4 laser

We demonstrate self-starting additive-pulse mode locking of a diode-pumped Nd³⁺:YVO₄ laser. Pulse durations of 2.7 ps are measured at a repetition frequency of 90.7 MHz and at an average output power of 1.1 W. This corresponds to a peak power of 4.5 kW. Received: 27 June 2001 / Revised version: 10 October 2001 / Published online: 29 November 2001     

Folded diffractive laser resonators with user-defined fundamental mode

The adjustment effort required for the alignment of laser resonators with arbitrarily shaped end mirrors can be significantly reduced by folding the resonator path half-way between the two end mirrors and by slightly tilting the resonator axis. Thus the resulting resonator consists only of one diffractive mirror and a plane mirror, and the separate alignment of both diffractive end mirrors with respect to each other can be avoided. This principle is demonstrated by a Nd:YAG laser with super-Gaussian output beam. Furthermore, it is shown theoretically that the influence of phase quantization effects on the beam quality can be strongly reduced as a consequence of the tilted resonator axis.     

Generation of 49 fs pulses in a CPM laser pumped by a small-frame Ar+ laser

A colliding pulse mode-locked ring dye laser, pumped by an all-line small frame Ar⁺ laser has been used to generate stable pulses as short as 49 fs with pulse duration and stability similar to those obtained with the usual configuration where a single line of the pump is used. The effects of cavity alignment on the absorber saturation are presented and explained in terms of an analysis of stability of the ring cavity.     

Differential variation of laser longitudinal mode spacing induced by small intra-cavity phase anisotropies

Intra-cavity phase anisotropy causes each laser longitudinal mode to split into two orthogonally polarized frequencies. When the frequency difference generated by small anisotropy is about 40 MHz or less, only one frequency of the splitting mode oscillates while the other is always in lock-in state due to strong mode competition. In that case the longitudinal mode spacing deviates from its original value. And the spacing of two adjacent longitudinal modes shows differential variations depending on direction of cavity length tuning and mode polarization. In this paper, the relationship between spacing variation and small intra-cavity anisotropy is demonstrated experimentally and theoretically. The equivalent fast axis of anisotropy is determined additionally by its correlation with laser intensity tuning of the two orthogonally polarized components. The novel measurement method is successfully tested in several applications.     

A continuous-mode model of the mode locking in a pump-modulated laser

The active mode-locking process of the multimode laser with an external pump modulation is theoretically investigated in the frequency domain within the framework of the continuous-mode approximation. Intermode interaction and mode-coupling effects, including both AM and FM modulations, are naturally considered in a hierarchical equation of the mode components derived from the multimode Maxwell-Bloch equations. It is reduced to a continuous-mode equation that can be solved analytically in a stationary case, and used to discuss the spectral line shape and the phase dynamics of mode-components as a function of modulation amplitude and detuning of the modulation frequency. We predict a novel oscillation existing below the threshold of the ordinary complete mode-locking: The intensity of the total electric field yields a stable pulse train but its phase varies irregularly in time. This semi-locked state is characterized by a nonlinear chirping, an asymmetric spectrum, and drifting phases of the field mode-components.     

A tunable,single mode,injection-locked flashlamp pumped dye laser

A tunable, very narrow bandwidth, flashpumped dye laser is described. Publse energies up to 50 mJ at a bandwidth of less than 30 MHz, and with a divergence of 0.4 mrad, are obtained. Spectral narrowing and tuning is achieved by injection locking of the pulsed laser to the radiation, of a tunable cw dye laser. The cavity is designed for single longitudinal and transverse mode operation, with a mode diameter large enough to fill the active medium. This work was supported in part by the Israel commission for Basic Research, and in part by a grant from the United States Israel Binational Science Foundation (BSF), Jerusalem, Israel.     

Passive mode locking of the continuous wave DCM dye laser

The first passive mode locking of the continuous wave (cw) DCM dye laser is reported. Subpicosecond pulses as short as 0.68 ps were obtained over the spectral region from 655 to 673 nm from a simple linear cavity with no dispersion optimisation. The dye 1,3 Diethyl 4,2 quinolythiacarbocyanine iodide (DQTCI) was used as the saturable absorber.     

Transverse mode dynamics in a free-electron laser

We derive the most general equations of motion for the electrons and the electromagnetic field in a free-electron laser including the effects of diffraction and pulse propagation. The field evolution is expressed in terms of the amplitudes and phases of a complete set of transverse modes. The analytic solution is given in the small-signal regime, where the theory is shown to be in excellent agreement with a recent experiment at Orsay.     

Grating feedback in a 810 nm broad-area diode laser

Narrow linewidth, single spectral mode operation has been obtained in a high power, 810 nm broad-area diode laser in an extended cavity configuration with a grating as external reflector (grating feedback). For stable operation it was necessary to misalign the feedback slightly in the plane of the laser junction. Characteristics of the thus obtained laser system are a linewidth below 5 MHz, an output intensity of about 50% of the free running power, a large-scale tuning range of 15 nm and continuous scanning over 4 GHz. In the spatial domain, the laser remains multimode and astigmatic. To show the practical applicability of this system, saturated absorption of a krypton line is demonstrated.     

Controlled two-pulse generation in Q-switch mode from a single laser using Q-modulator with frustrated total internal reflection

In this work a cavity design for double-pulse generation in Q-switched mode from a single laser is proposed, based on the construction of a second laser channel using a FTIR (frustrated total internal reflection) Q-modulator. A time interval between the two pulses from 500 ns to 120 μs is obtained in a Nd:YAG laser. A comparison with other methods and cavity designs for double-pulse generation is presented. The case when this technique is applied on a tunable laser with metastable upper laser level (Cr:LiCAF, Cr:LiSAF, Alexandrite, Co:MgF₂ or other) is also considered. Even though the method presented in the paper does not rely only on the cavity configuration proposed, some advantages can be obtained – both polarization and wavelength-independent tuning without polarization and wavelength restrictions in combination with the possibility of different wavelengths and polarizations in each pulse. Moreover, by using an active medium generating wavelengths around and up to 3 μm, the Pockels-cell-Q-switch optical transmission problems can be avoided. Received: 9 May 2001 / Revised version: 2 August 2001 / Published online: 15 October 2001     

Double-ring cavity configuration of actively mode-locked multi-wavelength fiber laser with equally tunable wavelength spacing

A novel actively mode-locked multi-wavelength tunable fiber laser with equally increased or decreased wavelength spacing is presented. It is constructed using a double-ring cavity combined with an amplitude modulator and cascaded fiber Bragg gratings (FBGs). The cavity lengths for all FBGs are intrinsically identical due to the double-ring configuration, and simultaneous mode locking of multiple wavelengths is therefore achieved by applying only one mode-locking signal to the modulator. The FBGs are mounted on a organic plate with angles between adjacent gratings such that the forces applied to the FBGs have an equal force increment; thus, stretching or compressing the cascaded FBGs can achieve a wavelength tuning with equally increased or decreased wavelength spacing.This revised version was published online in March 2005. In the previous version, the published online date was missing     

High-power broad-area diode lasers for laser cooling

Received: 1 April 1996/Revised version: 24 September 1996     

96 mW longitudinal single mode red-emitting distributed Bragg reflector ridge waveguide laser with tenth order surface gratings

Red-emitting ridge waveguide lasers with integrated tenth order surface distributed Bragg reflector gratings were developed. The grating was implemented by the use of a BCl3-Ar-plasma, while the shape of the grating trench was controlled by additional He-backside cooling of the wafer. The devices exhibit longitudinal single mode operation up to 96 mW at 635.3 nm with a side mode suppression ratio of 18 dB and a good beam quality of M2<3. The spectrum is free of mode hops for a span of more than 55 pm.     

All-fiber actively Q-switched fiber laser tuned by a pair of temperature controlled fiber Bragg gratings

We report an all-fiber actively Q-switched erbium-doped fiber laser, where the linear laser cavity mirrors are composed of two fiber Bragg gratings (FBGs). The laser oscillation wavelength could be tuned by this pair of temperature controlled FBGs. The Q-switching is achieved by an all-fiber phase modulation device. Using this system, we could obtain stable Q-switched laser pulses output, which could be optimized by tuning the reflection wavelengths of the two FBGs to be adjacent to each other. Instead of being modulated by the FBG filter in high-speed oscillation, this fiber laser system is operating in the Q-switched regime using an all-fiber phase modulator, producing a more stable laser output spectrum.     

Mode selection in an unstable-resonator TEA CO2 laser by injection from a waveguide laser

The effect of injecting radiation from a cw waveguide CO₂ laser into a TEA laser through a hole in one mirror of its unstable resonator has been studied experimentally. High-power single longitudinal mode operation of the TEA laser is achieved over a wide but finite range of injection frequencies, the frequency of the single-mode pulse being that of the TEA laser cavity mode lying closest to the injected frequency. Although a simple theoretical model shows good qualitative agreement with observations it underestimates the range of injection frequencies which result in single-mode pulses unless a fast chirping of the cavity mode frequency is postulated.     

Doughnut-like beam generation of Laguerre-Gaussian mode with extremely high mode purity

An apparatus to achieve a single doughnut-like Laguerre-Gaussian (LG) mode with extremely high mode purity higher than 99.6% is reported. The proposed apparatus is a conceptual combination of a high-order HG mode selector and an astigmatic mode converter, which consists of only four simple and easily-made optical elements: three lens elements and one binary phase element. Theoretical simulations show that the apparatus has a good tolerance for both system production and alignment errors. With such high mode purity, the doughnut-like LG beam generated by the apparatus carrying an orbital angular momentum will be beneficial to numbers of studies including the trapping and rotating of micro and nano-particles, non-linear optics, and atom-light interaction.