Performance of Nd:YAG lasers in coupled generalized self-filtering and positive-branch unstable resonators

A new optical resonator based on the combination of a generalized self-filtering unstable resonator (GSFUR) and a positive-branch unstable resonator (PBUR) in a three-mirror scheme is reported. It is shown both theoretically and experimentally that a nearly diffraction-limited Gaussian-output laser beam with a large mode volume can be obtained with this cavity design. The laser cavity is particularly interesting for use in high-threshold pumped gain media and eliminates some disadvantages of the SFUR and GSFUR designs. This resonator, with an effective magnification of -6.16, was applied to a pulsed Nd:YAG laser in free-running and in Q-switched modes of operation. The output energy was ~70 mJ, 5.5 times greater than when a single GSFUR design was used. The output beam had a pulse duration of ~30 ns in the Q-switched mode of operation and a beam divergence of 0.26 mrad. The required relations for the GSFUR-PBUR optical design and the output energy were derived and verified experimentally.     

Efficient diode side-pumping configuration of a Nd:YAG rod laser with a diffusive cavity

We propose an efficient, simple, and scalable diode side-pumping configuration of a solid-state rod laser. The rod is excited in a diffusive cavity and diode powers are transferred by a thin plate into the cavity. In multitransverse-mode cw operation, 40 W of laser power was generated with an optical-to-optical slope efficiency of 52.2% at a diode output power of 135.1 W and an electrical input power of 327.3 W into the diodes. These values correspond to an optical-to-optical total efficiency of 29.6% and an electrical-to-optical total efficiency of 12.2%. The electrical-to-optical slope efficiency was 30.3%.     

Two-dimensional simulation of a high-gain, generalized self-filtering, unstable resonator

The performance of a high-power excimer laser, generalized self-filtering, unstable resonator has been modeled by means of a numerical code. The spectral method and the Rigrod equations are basic to the numerical procedure, which is quite general because it results from an appropriate combination of independent propagation algorithms. The code can be applied to arbitrary resonator geometry and can be used to take account of gain medium inhomogeneities and instability phenomena.     

Design of a high-power Nd:YAG Q-switched laser cavity

An electro-optically Q-switched Nd:YAG laser resonator that uses two end prisms placed orthogonally perpendicular to each other has been designed. This configuration improves the stability of the resonator and does not alter the characteristics of the electro-optical Q switch. The outcoupling ratio of the cavity is optimized by a change in the azimuthal angle of a phase-matched Porro prism placed at one end of the cavity. The prism placed at the other end of the cavity is designed so that it introduces a phase change of II, regardless of its orientation and index of refraction, resulting in a more efficient and stable cavity.     

Unidirectional single-mode Nd:YAG laser with a planar semimonolithic ring cavity

Unidirectional single-mode operation of a diode-pumped Nd:YAG laser with a planar semimonolithic ring cavity has been demonstrated at 1064 nm. The semimonolithic cavity consists of a laser active medium placed in a magnetic field, a crystal quartz plate, and an output coupling mirror, which form an optical diode by acting as a Faraday rotator, a reciprocal polarization rotator, and a partial polarizer, respectively. A single-mode output power of 155 mW and a slope efficiency of 17% were obtained with a 1.2-W diode laser at 809 nm. A laser linewidth of less than 100 kHz is inferred from a beat note frequency spectrum between two identical laser systems and continuous tuning to greater than 2 GHz was observed.     

Nanosecond pulsed Bessel--Gauss beam generated directly from a Nd:YAG axicon-based resonator

We present what we believe to be a novel, simple, and compact axicon-based resonator Nd:YAG laser in which a nanosecond pulsed Bessel-Gauss beam is generated directly for the first time. Using the theory of the Bessel-Gauss beam, theoretical analysis and numerical simulation are consistent with the experimental results.     

Generation of flattened Gaussian beam profiles in a Nd:YAG laser with a Gaussian mirror resonator

We report the experimental generation of a family of flattened Gaussian beams with bell-shaped, flattened, and annular intensity profiles in an electro-optically Q-switched Nd:YAG laser with a variable reflectivity mirror of a Gaussian reflectivity profile as an output coupler. The laser beams of different profiles were generated by modifying the resonator magnification. The propagation characteristics of the experimentally generated flat Gaussian beams were found to be in agreement with theory. To the best of our knowledge this is the first time such a family of flattened Gaussian beams is experimentally generated intracavity using a single variable reflectivity mirror.     

Transverse-mode astigmatism in a diode-pumped unstable resonator Nd:YVO(4) laser

We have observed a sizable astigmatism in the output beam from a diode-pumped unstable resonator Nd:YVO(4) laser operating in a single polarization and a single-longitudinal and transverse mode. The anisotropic index of refraction of the vanadate crystal has been identified as the source of this astigmatism. A theoretical prediction of the eigenmode astigmatism based on this index anisotropy is consistent with our experimental measurements.     

Self-mixing interference effects of microchip Nd:YAG laser with a wave plate in the external cavity

We present the optical feedback characteristics of a single-mode Nd:YAG laser with a wave plate in the external cavity. The laser intensities of the two orthogonal directions, which are both modulated by the change of external cavity length, have a phase difference due to the birefringence effect of the wave plate. When threshold intensity is introduced, a period of intensity fringe can be divided into four equal zones. Each zone corresponds to lambda/8 displacement of the external feedback reflector. The direction of displacement can be discriminated by the sequence of these four zones. This phenomenon provides a potential displacement sensor with directional discrimination and high resolution of eighth wavelength compared with the traditional optical feedback.     

Optico-acoustic characterization of the fractal structure of radiation from a laser with unstable resonator

The thermooptical excitation of sound in a liquid by fluctuating laser radiation with sine-modulated intensity is studied theoretically. The intensity is randomly distributed over the beam cross section. It is assumed that the random processes are homogeneous and that the spatial spectrum of intensity fluctuations obeys a power fractal law. Possibilities for the optico-acoustic characterization of the fractal radiation structure of a laser with unstable resonator are discussed.     

Tunability of a 946-nm Nd:YAG microchip laser by use of a double-cavity configuration

We observed the tunability of a 946-nm Nd:YAG microchip laser by using a double-cavity configuration. We shifted the lasers wavelength from 938 to 946 nm by changing the thickness of the air gap. In addition, differences in reflectivity of the output mirror yielded the tunable range of the 946-nm band, with the center oscillation wavelength maintained at 946.1 nm.     

Absolute frequency stability of a diode-laser-pumped Nd:YAG laser stabilized to a high-finesse optical cavity

We report the frequency stabilization of a diode-laser-pumped monolithic ring Nd:YAG laser locked to a high-finesse optical cavity. With an independent cavity as a frequency discriminator, the absolute frequency noise was measured to be as low as 2 × 10(-2) Hz/Hz(1/2) at the Fourier frequency of approximately 3 kHz. We also measured the heterodyne beat note between two lasers locked to the independent cavities. The beat linewidth is narrower than 30 Hz and the minimum root Allan variance is approximately 6 × 10(-14).     

Numerical simulation of a high-average-power diode-pumped ytterbium-doped YAG laser with an unstable cavity and a super-Gaussian mirror

A numerical technique with which to compute the output characteristics of a solid-state laser with an unstable cavity and a super-Gaussian coupling mirror is proposed. This technique is applied to an Yb:YAG actively Q-switched laser. With this formalism, the mode formation for the fundamental mode is analyzed and the performance achievable by such a laser for various cavity parameters is determined. Then the results obtained with such a cavity are compared with those given for a stable cavity with graded phase output mirror that is also used for obtaining super-Gaussian mode.     

Output power and polarization characteristics for a diode-side-pumped Nd:YAG rod laser with a diffusive optical pump cavity

We fabricated and analyzed the output power and polarization characteristics of an efficient diode-side-pumped Nd:YAG rod laser with a diffusive optical cavity. The resonator stability conditions are analyzed graphically in the symmetric and asymmetric configurations for a plane-parallel resonator. On the basis of an analysis of the stability condition and mode size for the r and theta polarizations, we clarify how the stable laser operation is possible for various resonator configurations. In particular, we show that the critical stability region of around g1*g2* = 0 provides a stable resonator in the symmetric resonator, even with a slight asymmetry. Experimentally, the output power and polarization characteristics are confirmed in association with the resonator stability condition.     

Design and fabrication of a diode-side-pumped Nd:YAG laser with a diffusive optical cavity for 500-W output power

A ray-tracing code has been developed, and the design parameters of the laser pump head were analyzed in terms of crystal diameter, doping concentration, and optical cavity diameter. According to the numerical analysis, we fabricated an efficient diode-pumped Nd:YAG laser and experimentally obtained 500-W output power. The output power is close to the numerically calculated output power of approximately 450 W and corresponds to an optical-to-optical conversion efficiency of 46.7% and an optical slope efficiency of 49%.     

Influence of external cavity length on multimode hopping in microchip Nd:YAG lasers

The influence of external cavity length on multimode hopping in microchip Nd:YAG lasers is investigated experimentally. With an optical feedback loop, the threshold gain of different longitudinal modes are all modulated by changing the external cavity length; a lambda/2 change in the external cavity length causes a one-period oscillation. The longitudinal modes can be divided into groups according to different initial threshold gain variations and modulation trends corresponding to different external cavity phases. Because of the initial gain difference, only one mode in each group is the dominant potential lasing mode, while others are suppressed. During the 2 pi change of the external cavity phase, mode hopping occurs among these potential lasing modes from different groups. Both the intensity waveforms and the number of hopping modes strongly depend on the external cavity length. Experimental results agree well with the theoretical analysis of the phenomenon of multimode hopping subjected to optical feedback in microchip Nd:YAG lasers.     

Modeling of a Diode-Side-Pumped Nd:YAG Laser

We have developed a unique numerical laser model by use of a commercial physical optics software package. The experimentally measured lasing threshold, slope efficiency, power output distribution, and phase front have been derived. This model is particularly powerful for monitoring the effects caused by thermal distortions encountered in power scaling lasers. Extrapolations have been made through parametric studies to predict changes required in the laser design that would optimize the performance of the laser.     

Novel microwave assisted synthesis of highly doped phase pure Nd:YAG nanopowder

For the first time, the studies on 2 to 10 at.% neodymium (Nd³⁺) ion doped Yttrium Aluminum Garnet (Nd:YAG) nanopowders obtained by microwave assisted citrate nitrate gel combustion synthesis is described in this work. This paper reports on high doping of Nd³⁺ ions with retaining the cubic garnet structure of YAG as evidenced from XRD, except the case of 8 at.% doped Nd:YAG. Phase pure YAG formation with 8 at.% Nd³⁺ doping was explored by using urea and alanine as alternative to citric acid complexing agents. Complete crystallization of YAG as a result of 2 hour thermal treatment at 900 °C under oxygen supply was studied by using Fourier Transform Infra-Red Spectroscopy (FTIR) and X-Ray Diffraction (XRD) techniques. With an increase in the dopant concentration a red shift in the FTIR peaks was observed. Using the XRD data, the cell parameter of Nd³⁺ (2 to 6 and 10 at.%) YAG was found to increase with an increase in the dopant concentration. The average primary particle size calculated using Scherrer’s equation was ～25 nm which was additionally supported by Transmission Electron Microscopy (TEM) results yielding particle sizes in the range of ～25 to 30 nm for all the cases.     

Pulse compression with a high-energy Nd:YAG regenerative amplifier system

We describe a method for the generation of readily synchronizable, near-transform-limited, 1064-nm, 6-mJ pulses with <20-ps duration at a repetition rate of 20 Hz. The method employs chirped pulse amplification of spectrally broadened and temporally stretched pulses from a cw mode-locked Nd:YAG laser in a commercial Nd:YAG regenerative amplifier followed by pulse compression with a grating pair. Linear amplification subsequent to regenerative amplification is not required with this method, although higher energies would be easily obtained.     

Thermal Compensation of a Cw-Pumped Nd:YAG Laser

Thermally induced lensing and birefringence modify the transverse laser profile and may eliminate any global polarization state in systems utilizing Nd:YAG as a gain medium. This creates fundamental difficulties in obtaining a high-power, polarized output beam. Although abundant literature exists regarding thermal lensing, only one birefringence compensation scheme is prevalent in the literature. A modification of this scheme is given that eliminates residual birefringence. Experimental data verify the model's validity. A theoretical model is then presented that modifies the birefringence-compensated amplifier as a single power-dependent lens. After showing that solutions exist for a power-independent resonator consisting of a power-dependent lens between two flat mirrors, this amplifier is inserted into the resonator solution to produce, to first order, a resonator that is insensitive to thermally induced fluctuations in the rod focal length.