Confocal Unstable Resonator Research Articles

Analyzing Output Phase Properties of Confocal Positive-Branch Laser Resonators by Use of Hartmann-Shack Sensor

The effect of intracavity aberrated perturbation on output mode structure properties of passive confocal unstable resonator is been experimentally researched by adopting H-S wavefront sensor and Zernike modal wavefront reconstruction on the basis of numerical simulation. Results show that intracavity tilted perturbation notablely affects outcoupled intensity distribution, and will also increase some high-order aberrations of beam phase properties. However, low-order Zernike tilt aberration is the main component when phase-tilted perturbation is introduced into the resonator. Defocus, astigmatism and coma aberration will all be brought, and also such high-order aberration included in wavefront will directly degrade output beam quality. When correction device of adaptive optics adopted for intracavity aberration correction, correction of tilted aberration should be considered firstly.

Analysis of Maladjustment Sensitivity and Calibration Method for a Positive-Branch Confocal Unstable Resonator

The effect of intracavity aberration perturbation on output mode structure properties of unstable laser resonator is been experimentally researched using Hartmann-Shack wavefront sensor on the basis of numerical analysis. Results show that intracavity tilt aberration notablely affects outcoupled intensity distribution, but only a small intracavity tilt perturbation will not obviously augment the high-order aberrations of beam phase properties. The tilt aberration, coma aberration and astigmatism will all be brought, and also tilt aberration is the main component when intracavity mirrors have a vertical movement along the direction of optic axis. When adaptive optical elements such as deformable mirrors are adopted for intracavity aberration correction, the correction for tilt aberration should be considered firstly.

Numerical investigation of unstable resonator for generating radially or azimuthally polarized beams

We present the calculations of intracavity radiation for a confocal positive-branch unstable resonator with a radially or azimuthally selective mirror. We carried out beam propagation in the resonator by the fox-li iterative method using jones vector calculus for the polarization state. The optimization of the resonator for generating a high-power radially or azimuthally polarized beam is discussed.

Propagation offset characteristics of annular laser beams from confocal unstable resonators through the natural atmosphere

Based on the laser field from a positive confocal unstable resonator (ab initio), the propagation characteristics of the beam through atmosphere are investigated by means of fast Fourier transform (FFT), and it is assumed that the refractive index and absorption of atmosphere are only decided by the air molecules. Considering the thermal effects of the laser beam and wind velocity of air, we evaluate the beam offsets, beam spreading, and beam qualities of the laser field transmitting from the resonator through the natural atmosphere. Meanwhile, the far-field intensity distributions of the laser beam with various intensities in the atmosphere are obtained. It is shown that the far-field diffraction patterns are deformed due to absorption of air and wind velocity. β parameter and Strehl ratio, which are usually used for high-power lasers, are introduced to estimate the beam quality characteristics. With increasing the intensity of the beam, the far-field diagram patterns are outspreaded, the peak intensities reduced, the peak centers shifted, and the beam quality characteristics greatly degraded.

COIL emission of a modified negative branch confocal unstable resonator

A modified negative branch confocal unstable resonator (MNBUR) was coupled to the chemical oxygen-iodine laser (COIL) device of the German Aerospace Center. It consists of two spherical mirrors and a rectangular scraper for power extraction. Experimentally measured distributions of the near- and far-field intensities and the near-field phase were found in close agreement to numerical calculations. The extracted power came up to approximately 90% of the power as expected for a stable resonator coupled to the same volume of the active medium. The output power revealed a considerable insensitivity towards tilts of the resonator mirrors and the ideal arrangement of the scraper was found to be straightforward by monitoring the near-field distributions of intensity and phase. The beam quality achieved with the MNBUR of an extremely low magnification of only 1.04 was rather poor but nevertheless in accordance with theory. The demonstrated consistency between theory and experiment makes the MNBUR an attractive candidate for lasers that allow for higher magnification. In particular, it promises high brilliance in application to 100 kW class COIL devices, superior to the conventional negative branch confocal unstable resonator.

Compact multipass Nd:YVO_4 slab laser amplifier based on a hybrid resonator

The design of a diode stack end-pumped multipass Nd:YVO4 slab laser amplifier based on a stable and off-axis positive-branch confocal unstable hybrid resonator was demonstrated. Without the use of active switching elements, the hybrid resonator of the amplifier permits high multipass amplification of Q-switched laser beams in a compact package. The pulse energy of 3.8mJ and pulse width of 5ns was obtained at a repetition of 1kHz with the amplification factor of 11. In cw operation, the maximum output power of 35.2W was obtained by amplifying the seeder power of 4W. The beam propagation factor M2 in both the cw and pulse models was under 1.3.

Modified negative-branch confocal unstable resonator

A new type of unstable resonator, suitable for a laser with a large medium cross section and a small or median output coupling, is presented. The resonator configuration, a modification of a negative-branch confocal unstable resonator, is numerically investigated. The basis of the theory is the Fresnel-Kirchhoff integral equation, and the calculations describe a passive resonator. With respect to output mirror tilting, the calculations confirm that the modified negative-branch confocal unstable resonator is less sensitive to mirror misalignments than the conventional negative-branch confocal unstable resonator. Furthermore, the modified resonator improves the beam quality in comparison with the conventional unstable resonator.

불안정형 레이저 공진기 정렬을 위한 null 렌즈 설계

공초점 불안정형 공진기의 광학 정렬을 정밀하게 수행할 수 있는 새로운 개념의 null 렌즈 광학 정렬 방법을 제안하였다. 제안된 정렬 방법은 3.5 m 공진기에 적용하여 민감도 분석을 통해 실용성 여부를 판단하였으며, 공진기의 길이 변화가 1.0 mm 발생하였을 때와 반사경이 광축에 대하여 1.0 mrad 기울었을 때에 대한 pupil map 분석 데이터를 제시하였다. The use of null optics is proposed as a new concept for the precise alignment of a confocal unstable resonator. The characteristics of the proposed null optics are investigated and analysed with the designed null lens for a real confocal unstable resonator of which the length is 3.5 m. As a result of the analysis, the pupil map data are shown about the despace error of 1.0 mm and tilt error of 1.0 mrad.

Integral plug-in RF module in a CO 2 hybrid-waveguide laser: Its performance and overall evaluation

This work describes the performance of a compact “in-house built” radio–frequency (RF)-excited CO 2 slab waveguide laser which has the innovation of having a plugged-in RF generator–amplifier module directly connected into the positive electrode of the laser head. The design circuit parameters include a matching circuit and a feed-through element as a whole. The overall laser performance takes into account the waveguide dimension ( y-axis) as approximately one-tenth of the free space transverse dimension ( x-axis). The optical resonator is calculated to be in the regime of the negative branch for unstable confocal resonators, having focal lengths of f 1 = 21.49 cm and f 2 = 19.39 cm with geometrical amplification of 1.108. Optical output coupling mirror was set to ∼9.7%. The calculated waveguide length is 37.73 cm whilst the total resonator length was adjusted to 42 cm to allow coupling losses less than 1%. The laser operational efficiency was about 12% and the output beam quality of 1.13 which is close to the ideal Gaussian beam. The optical output power was accomplished by playing with different gas compositions to have a final optimized gas proportion of 1:1:2.7:0.3 correspondingly to CO 2, N 2, He and Xe as admixture.

Off-Axis Confocal Unstable Resonator for Large-Bore High-Power Laser Chemical Pumped Oxygen–Iodine Energy Transfer Laser

An exotic resonator called the off-axis confocal unstable resonator is analyzed for large-bore low-gain high-saturation-intensity high-power lasers such as chemical pumped oxygen–iodine energy transfer laser (COIL), for which suitable resonators with high beam quality have not existed until now. The result of the calculations clearly showed that the off-axis confocal unstable resonator could produce multi-kW output with excellent beam quality for COIL.

Output Beam Quality of Edge Pumped Planar Waveguide Lasers with Confocal Unstable Resonators

Output beam quality of edge pumped planar waveguide lasers with confocal unstable resonators is investigated by diffraction methods, taking into account gain saturation, asymmetric pumping, and beam interaction. The influences of pumping uniformity, doping concentration, cavity length and effective Fresnel number are analyzed with respect to output beam quality and pumping efficiency. It is found that good beam quality and high efficiency can be obtained with asymmetric pumping and optimized negative branch confocal unstable resonators.

Coupled mode competition in unstable resonators using the exact cavity equations of motion with dynamic gain

The dynamic coupled mode (DCM) method has been applied to study the transverse mode competition in optical resonators. In this work a differential equation for the homogeneously saturating dynamic gain is included in the original dynamic coupled mode method, thus increasing its physical resemblance and allowing the retrieval of gain temporal evolution at every point within the lasing medium. This new model provides a realistic temporal evolution of the mode competition and gain saturation within the resonator, which can give further information on spatial coherence properties. The new general formulation does not assume that lasing medium fills the optical cavity and is suitable for modelling the dependence of small-signal gain and power gain on the transverse and longitudinal coordinates. The application of the method to a typical CO2 unstable confocal resonator is fully described; results and their connection to relevant physical properties of gas lasers, such as spiking and relaxation oscillations, are discussed. Results of the numerical implementation of the DCM method with dynamic gain are in very good agreement with experimental measurements reported previously.

An investigation of a solid–liquid phase-change-cooled mirror

This article details a new design for a laser-reflecting mirror utilizing phase-change-cooling technology, which is innovative in using spiral cooling ducts filled with solid-liquid phase-change material to maintain a constant temperature inside the reflecting mirror in a positive-branch telescopic, virtually confocal unstable resonator. The structure no longer needs the 45° scraper plate that is always utilized in a positive-branch telescopic, virtually confocal unstable resonator for coupling the output and provides a high-quality coaxial laser output. The numerical calculation results show, that when phase-change cooling is utilized, the localized maximum temperature dropped from 44 to 33°C. Experiments show that the total surface deformation of a copper mirror dropped from 1.4 µm to 0.27 µm when using phase-change cooling, after continued laser exposure for 10 s and, under the same conditions, that of a silicon mirror dropped from 1.1 µm to 0.4 µm.

Efficient, high power, Q-switched Nd:YLF slab laser end-pumped by diode stack

A high power diode stack end-pumped electro-optically Q-switched Nd:YLF slab laser with a stable and off-axis negative-branch confocal unstable hybrid resonator was demonstrated. By using a cylindrical lens in the stable direction the thermal lens effect was compensated. Pulse energy of 25mJ was obtained with a pulse width of 22.4ns at repetition rates of 500Hz and a conversion efficiency of 22%. The stability was better than 0.8% and the beam propagation M2 factor was about 1.2.

Two-dimensional time-resolved modelling of copper HyBrID laser beam quality

A numerical model is used to describe the spatio-temporal evolution of the beam delivered by a pulsed copper HyBrID laser. The model takes diffraction, transient gain saturation and the cavity geometry into account. The time evolution of experimentally accessible beam parameters, like M 2, power-in-the-bucket or brightness, is deduced. The model is applied to the description of a copper HyBrID flat–flat cavity and to the optimization of an unstable confocal positive branch copper HyBrID resonator.

Numerical methods for solving the eigenvalue problem for a positive branch confocal unstable resonator

The resonator problem for a positive branch confocal unstable resonator reduces to a Fredholm homogeneous integral equation of the second kind, whose numerical solution here is based on a sequence of algebraic eigenvalue problems. We compare two algorithms for the solution of an optical resonator problem. These are obtained by (i) successive degenerate kernel approximation by Taylor polynomials of the Fredholm kernel and (ii) Nyström’s method with Simpson’s rule as the subordinate numerical integration method. The numerical results arising from these routines compare well with other published results, and have the added advantage of simplicity and easy adaptability to other resonator problems.

Unstable resonator optical parametric oscillator based on quasi-phase-matched RbTiOAsO_4

We demonstrate improved signal and idler-beam quality of a 3-mm-aperture quasi-phase-matched RbTiOAsO(4) optical parametric oscillator through use of a confocal unstable resonator as compared with a plane-parallel resonator. Both oscillators were singly resonant, and the periodically poled RbTiOAsO(4) crystal generated a signal at 1.56 mum and an idler at 3.33 mum when pumped at 1.064 mum. We compared the beam quality produced by the 1.2-magnification confocal unstable resonator with the beam quality produced by the plane-parallel resonator by measuring the signal and the idler beam M(2) value. We also investigated the effect of pump-beam intensity distribution by comparing the result of a Gaussian and a top-hat intensity profile pump beam. We generated a signal beam of M(2) approximately 7 and an idler beam of M(2) approximately 2.5 through use of an unstable resonator and a Gaussian intensity profile pump beam. This corresponds to an increase of a factor of approximately 2 in beam quality for the signal and a factor of 3 for the idler, compared with the beam quality of the plane-parallel resonator optical parametric oscillator.

Extension of the self-filtering unstable resonator to a ring cavity

We present a numerical and experimental study of a confocal unstable ring resonator that performs similarly to a self-filtering unstable resonator. The resonator presents two apertures: one of them is the output coupler and the other provides spatial filtering. A simple rule for the choice of the aperture size enables us to expand the output beam diameter without increasing the magnification or the total length. The resonator is tested in a TEA CO2 laser in which the output beam diameter, the output energy, and the effective reflectivity are measured. The energy ratio of the forward and reverse waves scales approximately as the square of the magnification. The results show good agreement with the theoretical values.

Advantages of negative-branch compared with positive-branch one-dimensional unstable resonators

Effects of small misalignments in positive- and negative-branch strip confocal unstable resonators have been compared at the same absolute values of collimated Fresnel numbers and at the same Fresnel numbers. We show that positions of beam modes in negative-branch unstable resonators are far less sensitive to misalignment because of both geometric features and diffraction effects of beam flipping in the resonators.

Intracavity beam behavior in hybrid resonator planar-waveguide CO_2 lasers

We describe a combined computer simulation and experimental investigation of the intracavity spatial beam profile characteristics of a planar-waveguide rf-excited CO(2) laser that incorporates a hybrid waveguide confocal unstable negative-branch resonator. The study includes results for the intracavity lateral beam intensity profile and output power of the laser as a function of resonator mirror misalignment. In addition, the behavior of the unstable resonator, observed experimentally and predicted by the simulation, in generating localized high intensity hot-spots when it is subjected to relatively large misalignment angles is reported.