Negative-branch Unstable Resonator Research Articles

A 1350W Nd: YAG Unstable Ring Laser With Efficient Reverse Wave Suppression

A breakthrough result of a kW-level Nd: YAG slab laser with a unidirectional unstable ring resonator is presented. The reverse wave is efficiently suppressed by a diaphragm placed at the focal point of the negative-branch unstable ring resonator. With specifically developed multi-function sensor and precision placement of the diaphragm, we have achieved the maximum output power of 1350 W with reverse suppression ratio of 54:1 corresponding to an optical-optical efficiency of 21.6% and a slope efficiency of 33.7%. The output power is currently limited by the available pump power. To the best of our knowledge, this is the highest power demonstrated from an Nd: YAG slab laser based on an unstable ring resonator with efficient reverse wave suppression, which is a paramount step to the further development of high power solid-state unidirectional ring lasers.

The approximate analytic solution for the mode of strip negative branch unstable resonators

In this paper, the formula of the edge wave (EW) which is applicable for strip negative branch unstable resonators is derived. It is shown that the lowest-loss mode can be represented as the superposition of these EWs in an explicit form, the validity is verified by numerical comparison with the classic power method.

Off-axis negative-branch unstable resonator in rectangular geometry

The application of an off-axis negative-branch unstable resonator to an active medium of rectangular geometry is examined. The presented unstable resonator consists of spherical mirrors and a scraper mirror. The adaptation to the rectangular cross section is performed by the scraper, which takes two different shapes. One shape resembles a rectangular bracket "[" and the other resembles the letter "L." The [ and L configurations correspond to a shift of the optical axis away from the center of the cross section, toward one of the edges or toward one of the corners, respectively. Both scraper setups are examined numerically and experimentally. Experiments are performed with a multikilowatt class chemical oxygen iodine laser. The active medium is characterized by a low amplification coefficient. Measured results of the intensity distribution in the far field and of the phase distribution in the near field are shown for both resonator configurations. Using the same resonator magnification, the setup with the L-shaped scraper has a lower output coupling and, therefore, a higher output power and a slightly higher beam divergence. The L-shaped scraper configuration is able to cover the gain medium completely.

Analysis of lasing in chemical oxygen-iodine lasers with unstable resonators using a geometric-optics model

A simple geometric-optics model is developed that describes the power extraction in chemical oxygen-iodine lasers (COILs) with unstable resonators. The positive and negative branch unstable resonators with cylindrical mirrors that were recently used in COILs are studied theoretically. The optical extraction efficiency, spatial distributions of the intracavity radiation intensity in the flow direction, and the intensity in the far field are calculated for both kinds of resonator as a function of both the resonator and the COIL parameters. The optimal resonator magnifications that correspond to the maximum intensity in the far field are found.

A Planar Waveguide Nd:YAG Laser Using Active Q-Switching of a Hybrid Unstable Resonator

A planar waveguide laser operating in a negative branch unstable resonator is Q-switched by an acoustooptic modulator in a new configuration, providing effective, high-speed switching. The laser using a 200-mum Nd:YAG core, face pumped by 10 laser diode bars, has produced 100-W output in a good beam quality at 100-kHz pulse rate, and 4.5 mJ at lower frequency with 15-ns pulse duration.

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.

Numerical studies on hybrid resonators for a medium-sized chemical oxygen iodine laser

Off-axis negative-branch and off-axis positive-branch hybrid resonators, suitable for a laser with large medium cross section and a small output coupling, are explored numerically. The basis of our theory is the Fresnel-Kirchhoff integral equation, and the calculations describe a passive resonator. With respect to mirror tilting, the calculations confirm that in the unstable direction, the off-axis negative-branch resonator is less sensitive to mirror misalignments than the off-axis positive-branch one. In the stable direction, the sensitivity to mirror misalignment is larger than in the unstable direction and is dependent on the mirror curvature. The fabrication of cylindrical mirrors for such hybrid resonators is difficult, and deviations in mirror radius of curvature are possible. The effects of such deviations are addressed. Near the nominal mirror parameters, the off-axis negative-branch unstable resonator is insensitive to mirror curvature variations if the resonator length is appropriately adapted to the mirror curvatures.

RF generator for compact CO 2 waveguide lasers

This letter reports the fabrication and testing of a compact Radio Frequency Generator (Oscillator-Amplifier) for its integration in a CO 2 waveguide laser. The circuit uses a common source MOSFET transistor that has been modified for self-sustained oscillations impedance-matched to 50 Ω. RF power is supplied to the laser resonant cavity made of a ceramic–metal slab which is placed between a negative branch unstable optical resonator.

Fast optical shuttering and wavelength selection in copper vapor lasers and copper bromide lasers by intracavity polychromatic acoustooptic modulation

We propose a fast effective intracavity technique for optical shuttering and wavelength selection in copper vapor lasers and copper bromide lasers by inserting a polychromatic acoustooptic modulator in a low-intensity section of a classical negative branch unstable resonator. Single-pulse or multiple-pulse operation in one- or two-wavelength emission was achieved in a CuBr laser system. The spatial distributions of the output laser beam as recorded by a CCD camera showed neither power losses nor worsening of the beam quality in comparison with the classical scheme without the modulator. The laser output was characterized with a high degree of linear polarization.

Focusing properties of semiconductor lasers with broad-area and shaped unstable resonator

Negative branch unstable resonators, with different magnifications, have been fabricated and analyzed. Investigating the focusing properties of our designs, we show that variations in the astigmatism of the beam are dependent on the magnification of the resonator and thermal effects. Given this, we have produced devices with stable astigmatism and good focus points. Numerical simulations were also carried out showing good agreement with experimental results.

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.

Analysis of lateral-mode behavior in broad-area InGaN quantum-well lasers

A wave-optical model that is coupled to a microscopic gain theory is used to investigate lateral mode behavior in group-III nitride quantum-well lasers. Beam filamentation due to self-focusing in the gain medium is found to limit fundamental-mode output to narrow stripe lasers or to operation close to lasing threshold. Differences between nitride and conventional near-infrared semiconductor lasers arise because of band structure differences, in particular, the presence of a strong quantum-confined Stark effect in the former. Increasing mirror reflectivities in plane-plane resonators to reduce lasing threshold current tends to exacerbate the filamentation problem. On the other hand, a negative-branch unstable resonator is found to mitigate filament effects, enabling fundamental-mode operation far above threshold in broad-area lasers.

Diffraction losses reduction in multiapertured non-Hermitian laser resonators

The behavior of diffraction losses in multiapertured laser resonators is theoretically and experimentally investigated. In the case of geometrically unstable resonators, it is shown that the combination of several apertures whose diameters have been independently optimized to reduce the laser diffraction losses leads to a still lower value for these losses. This puzzling fact is verified experimentally for a negative branch unstable resonator laser containing up to five intracavity apertures. In the case of geometrically stable resonators, it is shown that the introduction of a first intracavity aperture that selects the fundamental mode leads to modifications of the fundamental mode shape which, besides being relatively small, are sufficient to make the diffraction losses oscillate with the diameters of the following extra intracavity apertures. In particular, the diameters of these apertures can be optimized to reduce the diffraction losses below the value obtained with the first aperture only. These predictions are experimentally verified in the case of a dynamically stable cavity high power Nd:YAG (yttrium aluminum garnet) laser. In all cases, the calculations based on the Huygens-Fresnel principle exhibit a good agreement with the experimental results.

Hybrid stable–unstable resonators for diffusion-cooled CO_2 slab lasers

We present the numerical and experimental study that we carried out to compare the performances of two hybrid stable-unstable resonators for diffusion-cooled CO(2) slab lasers. The two resonators are designed to fit a 320 mm × 60 mm ×2 mm rf-discharge channel and are both guided in the narrow transverse direction. They differ in the other transverse direction, consisting of a positive- or a negative-branch unstable resonator scheme. The two solutions have been characterized in terms of modal structure, power extraction, stability, and quality of the extracted beam.

Numerical modeling of short-pulse excimer lasers with negative branch unstable cavities

A one-dimensional code for the numerical simulation of negative branch unstable resonators with an intracavity aperture that are applied to high-gain, short-pulse XeCl lasers is described. The model predicts near- and far-field performance of the output laser beams. The intracavity aperture size is shown as an important parameter for control of the output beam energy and divergence. A comparison with experimental measurements is presented.

Design of variable reflectivity mirrors and unstable resonators for Nd:YAG lasers with high average power

Positive branch as well as negative-branch unstable resonators in different quadrants of the stability diagram were examined both theoretically and experimentally. Variable reflectivity mirrors (VRM) Were used as outcoupling elements. The resonators comprised up to three Nd:YAG rods (6* 3/8 inch) with a maximum input power of 12 kW each. This system provided a maximum output power of 550 W per rod using a symmetric flat resonator. A novel design of unstable resonator (near-concentric unstable resonator NCUR), showed to be quite insensitive to input power changes within the complete range: the round trip magnification only changed by a factor of 3, resulting in a nearly constant slope efficiency and a very much reduced shift of the focal spot. With one rod, 420 W maximum laser power with a beam parameter product of 2.3 mm mrad (two rods: 680 W, 7 mm mrad) was obtained. Kirchhoff-integral based numerical calculations indicate smaller beam parameter products. This deterioration is caused by spherical aberration of the thermal lens and by differences of refractive power for r- and phi -polarizations. The VRMS Were produced at FLI's coating laboratory. An industrial evaporation machine equipped with the two electron-beam heated crucibles and a planetary drive was used. Thin-film designs with only one shaped layer yield reflectivity profiles of nearly any super-Gaussian shape with peak reflectivities of up to 90%. If an additional layer is inserted, one obtains apodized apertures. The phase shift of the reflected beam is similar to that of single-layer VRM. With a geometric model, the production parameters of the fixed mask method, such as aperture diameter and distance to the substrate, can be estimated as before. The damage threshold is 10-16 3 cm-2 (15 ns).

Multirod unstable resonators for high-power solid-state lasers

The properties of positive-branch and negative-branch unstable resonators with variable reflectivity mirrors and several variable internal lenses were investigated both theoretically and experimentally. Design rules for optimized unstable resonators for one or more active elements are derived on the basis of the ABCD matrix formalism. Experiments were performed with a pulsed Nd:YAG system consisting of three 6 in. × 3/8 in. (15.24 cm × 0.95 cm) rods. This system provided a maximum output power of 550 W per rod when a symmetric flat-flat resonator was used. Unstable resonators achieved up to 75% of this maximum value with beam-parameter products between 2 and 10 mm mrad. The beam quality becomes worse as more active elements are used inside the resonator. This deterioration of focusability is caused by spherical aberration in combination with differences of refractive power for r and Φ polarizations.

A 5 kW CW CO/sub 2/ laser using a novel negative-branch unstable resonator with a phase-unifying output coupler

A 5 kW CW CO/sub 2/ laser using a negative-branch unstable resonator (M=-1.5) is proposed and characterized experimentally. The resonator consists of a stepwise variable reflecting output coupler, called the phase-unifying output coupler, and a total reflector. A 5 kW CW laser beam with diffraction limited quality (2 theta =0.45 mrad) is obtained and affected by the focal point in the resonator. The misalignment angle to reduce the laser power to 95% is improved by a factor of 19 compared with a positive-branch unstable resonator (M=1.5) with a phase-unifying output coupler at the same resonator length. >

Mode properties and characteristics of negative branch unstable ring resonators

Laboratory-scale negative-branch unstable ring resonators were designed to simulate large Fresnel number, large-mode cross section positive-branch high-power ring lasers. These laboratory-scale lasers can be built having small-mode cross sections, long lengths, and large Fresnel numbers by incorporating imaging systems within the resonators. Large variations of Fresnel numbers can also be obtained by simple positioning of the output aperture. To verify that the diffractive modes of the high-power lasers are accu rately si mulated, ba re cavity mode discri mi nation properties of these ring resonator designs were calculated by various 3-D diffractive models. Modeling was completed for resonators with equivalent Fresnel numbers up to 1 1 and with equivalent Fresnel numbers of 10 and above using a fast Fourier transform (FFT) and a virtualsource code, respectively. Comparison of the FFT and the virtual source code results, at moderate Fresnel numbers, indicated good agreement between the different modeling techniques. Good mode discrimination was observed at half-integral equivalent Fresnel numbers, whereas poor mode discrimination was observed at integral equivalent Fresnel numbers. These results show that less computational intensive codes, such as the virtual source code, can quickly and cheaply model a resonator having a wide range of Fresnel numbers. These results also show that the equivalent Fresnel number of a near-imaging negative-branch ring resonator is a reasonably valid parameter for determining resonator diffractive phase and irradiance profiles even for resonators having large Fresnel numbers. The modeling results show that the modes of positive- and negative-branch resonators, having the same Fresnel number, behave the same to within a complex conjugate.

High brightness operation of a XeCl laser with negative branch unstable resonators

Confocal and non-confocal negative branch unstable resonators with an intracavity spatial filter have been applied to a high-gain short-pulse UV preionized XeCl laser. The near- and far-field radiation characteristics have been investigated in both configurations. Laser beams of larger brightness have been obtained with non-confocal schemes. A maximum beam brightness of 1.3×1014 Wcm−2Sr−1 has been achieved.