Unstable Resonator Design Research Articles

电激励重频HF激光器非稳腔设计及实验研究

电激励重频HF激光器非稳腔设计及实验研究

Detailed performance modeling of a pulsed high-power single-frequency Ti:sapphire laser

Differential absorption lidar (DIAL) is a unique technique for profiling water vapor from the ground up to the lower stratosphere. For accurate measurements, the DIAL laser transmitter has to meet stringent requirements. These include high average power (up to 10 W) and high single-shot pulse energy, a spectral purity >99.9%, a frequency instability <60 MHz rms, and narrow spectral bandwidth (single-mode, <160 MHz). We describe extensive modeling efforts to optimize the resonator design of a Ti:sapphire ring laser in these respects. The simulations were made for the wavelength range of 820 nm, which is optimum for ground-based observations, and for both stable and unstable resonator configurations. The simulator consists of four modules: (1) a thermal module for determining the thermal lensing of the Brewster-cut Ti:sapphire crystal collinear pumped from both ends with a high-power, frequency-doubled Nd:YAG laser; (2) a module for calculating the in-cavity beam propagations for stable and unstable resonators; (3) a performance module for simulating the pumping efficiency and the laser pulse energy; and (4) a spectral module for simulating injection seeding and the spectral properties of the laser radiation including spectral impurity. Both a stable and an unstable Ti:sapphire laser resonator were designed for delivering an average power of 10 W at a pulse repetition frequency of 250 Hz with a pulse length of approximately 40 ns, satisfying all spectral requirements. Although the unstable resonator design is more complex to align and has a higher lasing threshold, it yields similar efficiency and higher spectral purity at higher overall mode volume, which is promising for long-term routine operations.

1 W tunable near diffraction limited light from a broad area laser diode in an external cavity with a line width of 1.7 MHz

A novel unstable external cavity for a broad area laser diode is presented. The cavity is based on a V-shaped setup that improves the slow axis beam quality by coupling the internal modes of a gain guided laser diode. The novelty here is the compact unstable resonator design without lenses in direction of the slow axis. For frequency stabilisation and to narrow the line width of the laser diode emission a diffraction grating in a Littrow configuration is used. With this setup up to 1W of near diffraction limited light with a beam quality of M2⩽1.3 and a line width of 1.7MHz could be achieved. The external cavity laser was tunable over a range of 35nm (FWHM) around the center wavelength of 976nm.

Efficient, reliable, long-lifetime, diode-pumped Nd:YAG laser for space-based vegetation topographical altimetry.

A highly efficient, diode-pumped, Nd:YAG laser is described. The oscillator utilizes an unstable resonator design with a Gaussian reflectivity output coupler and a side-pumped zigzag slab gain medium. The laser produces 18-mJ, 10-ns pulses at a repetition rate of 242 Hz in a near-TEM00 mode with an optical efficiency of up to 14%. An extended performance test was recently concluded in which the transmitter operated at reduced output for more than 4.8 x 10(9) shots with no optical damage. Design criteria, beam quality, and lifetime data are presented.

Laser light sources for space-based gravitational wave detectors

This contribution summarizes recent progress in the development of diode-pumped Nd:YAG lasers suited for applications in laser-based metrology. Applying an unstable resonator design to a monolithic ring laser, 2 W single-frequency output power in a diffraction-limited beam is generated. Injection locking was used to increase the available power even further. Finally, techniques to improve the free-running lasers intensity and frequency stability are demonstrated. Low intensity noise is accomplished by application of an electronic feedback loop. Low frequency noise is achieved by stabilization onto a hyperfine transition of molecular iodine.

Semiconductor unstable resonators with laterally finite mirrors

The design of semiconductor-based unstable resonators with laterally finite mirrors is investigated. A time-independent model used for tapered cavity semiconductor lasers is implemented to analyze this cavity design. Initial investigations show a significant improvement in external quantum efficiency, as well as a decrease in threshold current density as compared to the traditional tapered lasers with laterally infinite mirrors. The advantages and disadvantages of the proposed design will be discussed.

Design of unstable laser resonators with user-defined mode shape

By introducing surface-structured mirrors into stable laser resonators, the generation of user-defined mode profiles and properties has become possible. On a wave-optical basis, we show that there is a general connection between the property of the modes to be phase-conjugated during reflection at the mirrors and their diffraction losses at apertures. This connection makes the design method known for stable resonators with predefined modes unsuitable for the design of such resonators of unstable geometry. An alternative method for the calculation of unstable resonators with user-defined output beam shape is presented and demonstrated by an example design.

Design of hybrid waveguide resonators for the ENEA compact far-infrared FEL

Novel resonator schemes to be tested on the ENEA compact far-infrared FEL are described in this paper. The design of a negative branch confocal unstable resonator is reported, which shows low sensitivity to angular misalignment and employs all-metal optics providing broad band output coupling.

Formation of output in copper vapor lasers

Detailed mechanisms that control the formation of output from copper vapor laser (CVL) oscillators are investigated. Measurements of the spatial and temporal evolution of gain in a CVL amplifier and a CVL oscillator show that a short period of high gain that occurs at the beginning of the inversion period is the dominant feature. This leads to the formation of a burst of amplified spontaneous emission (ASE), whose subsequent propagation and amplification leads to all observable CVL output. The spatial characteristics of this initial burst of ASE are shown to be strongly dependent on the operating conditions of the laser. The implications of this description of CVL output for the design of unstable resonators and oscillator-amplifier systems is discussed.

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).

Unstable resonator modes for lasers with circular mirrors and high Fresnel numbers

The design of unstable resonators for large lasers with high Fresnel numbers and circular mirrors requires an ability to calculate their mode structures. Four methods for obtaining mode structure by solving the complex integral equation are analyzed. Included are a numerical method, two hybrid methods, and a virtual-source method. The hybrid methods are basically analytical methods with special numerical integration of analytical solutions (over the feedback mirror) to obtain improved solutions in the output annulus. The hybrid methods are designed for use with high-Fresnel-number resonators. However, their applicability extends into the low-Fresnel-number regime, where a comparison shows one of the hybrid methods agreeing exceptionally well with the numerical method. For analysis at high Fresnel numbers, the hybrid and virtual-source methods are compared with each other. The two hybrid methods are expected to differ from each other in the central core region when the Fresnel number is low, but they are expected to agree with each other when the Fresnel number is high. For the hybrid comparison at a high Fresnel number, the next to lowest loss modes show a similar structure. However, lack of agreement for the lowest loss mode shows that approximations in the development of the second hybrid method cause the selection of the wrong geometrical mode.

Design of aberrationless aspheric unstable resonators

We present a method for designing aspheric mirrors for unstable-resonator lasers to yield an arbitrary laser mode inside the resonator. Both analytical and numerical examples are given of aberration-compensated resonators, including solid resonators whose modes on refraction through the output coupler have identical zero spherical aberration.

Characterization of an ultrahigh peak power XeF(C to A) excimer laser system

The gain characteristics of an electron-beam pumped XeF(C to A) excimer amplifier operating in the blue-green spectral region were investigated for several laser pulse lengths. Saturation energy densities of 50 and 80 mJ/cm/sup 2/ were measured for injected laser pulse durations of 250 fs and approximately 100 ps, respectively. A gain bandwidth of 60 nm was observed with approximately 100-ps pulse injection. Using an optimized unstable resonator design, the laser amplifier has produced 275-mJ pulses with a pulse duration of 250 fs and a 2.5 times diffraction-limited beam quality, making the XeF(C to A) amplifier the first compact laser system in the visible spectral region to reach peak powers at the terawatt level.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High power, nearly diffraction-limited output from a semiconductor laser with an unstable resonator

The authors describe an on-the-chip design of a nearly diffraction-limited broad-area semiconductor diode laser. The device achieved single lateral mode operation as unstable resonators with magnifications between two and three. The unstable resonators were realized by focused ion beam (FIB) micromachining a diverging mirror at one of the outcoupling facets. The modeling efforts agree well with experimental data and show that an optimum device design exists in which stable nearly diffraction-limited operation is predicted for up to six times threshold. This unstable resonator design has achieved, experimentally, the highest diffraction limited power and best external differential efficiency ever reported for any broad-area device with a curved facet.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Performance comparison of positive branch unstable resonator cavities for excimer lasers

The near- and far-field performance of three practical positive branch unstable resonator designs was compared for a commercial excimer laser operating on ArF and KrF. Atmospheric attenuation at 193 and 248 nm was measured.

Unstable laser resonators with super-Gaussian mirrors

A new class of tapered reflectivity mirrors with a super-Gaussian profile R infinity exp(-kr(n)) is introduced, and a geometrical-optics approach for analysis and design of unstable resonators made with these mirrors is presented. A super-Gaussian mirror, built by a special evaporation technique, has been tested in an unstable resonator of a pulsed Nd:YAG laser, demonstrating its effectiveness in generating diffraction-limited beams.

Diode-laser-pumped Nd:YAG laser injection seeding system

We have designed and tested a compact injection seeding system consisting of a diode-laser-pumped Nd:YAG master oscillator and a permanent-magnet Faraday isolator. With active resonator frequency stabilization, this system permits highly reliable single-axial-mode operation of a Q-switched Nd:YAG laser over a period of hours. The system is capable of injection seeding both stable and unstable resonator designs and is suitable for injection seeding commercial lasers with only minor modifications.

Unstable resonator and output-coupling design study for diffraction radiation generator near-mm sources

Configurations of unstable resonators that could be used for diffraction radiation generators (DRG's) are presented. The output coupling of a negative branch unstable resonator exhibits an extreme sensitivity to mirror radius and appears to be unsuited to DRG use. The positive branch resonator does not exhibit this sensitivity to mirror radius, and the mirror radii that are needed to give output couplings of a few percent appear practical.

Mode control and performance studies of a pulsed unstable resonator HF/DF laser

In this paper we consider the design of unstable resonators for use on pulsed HF/DF chemical lasers. The limits on the resonator magnification, as set by mode-growth time requirements and parasitic oscillation, are evaluated and the predictions compared with experimental results obtained using a fast pumped transverse discharge HF laser. A detailed experimental study of the laser performance with a 2.2 magnification unstable resonator has been carried out for both HF and DF operation and optimum operating conditions for energy and peak power extraction defined. Time resolved studies of the multiline HF and DF laser emission spectra are reported. The system described should prove useful in experiments which require a high intensity focused beam.