TEM00 Mode Operation Research Articles

Multi-Fresnel-Lens Pumping Approach for Simultaneous Emission of Seven TEM00-Mode Beams with 3.73% Conversion Efficiency

TEM00-mode operation is a requirement in many laser-based applications due to the small divergence and high-power density of the emitted laser beam. A solar laser scheme was designed and numerically studied with the goal of increasing the solar-to-laser power conversion efficiency in the TEM00-mode operation. The collection and primary concentration of sunlight was performed via twelve sets of folding mirrors and Fresnel lenses, toward a laser head composed of a fused silica torus volume and seven Ce:Nd:YAG rods, in a side-pump configuration. With this scheme, TEM00-mode laser power totaling 212.39 W could potentially be produced from seven beams, with six of them being 32.60 W each and with Mx2 = 1.00, My2 = 1.01 quality factors. Notably, 35.40 W/m2 collection efficiency and, most importantly, 3.73% solar-to-laser power conversion efficiency were numerically achieved. The latter efficiency value represents a 1.81-time improvement over the experimental record, established with a prototype that had a single Ce:Nd:YAG rod in an end-side pump configuration.

Self-mixing interference in a thin-slice solid-state laser with few feedback photons per observation period

Formation of a peculiar lasing pattern leading to a TEM00-like mode accompanied by a nonzero bright outer-ring emission (namely, a modified TEM00 mode) was observed in a 300-\mu m-thick LiNdP4O12 (LNP) laser with reflective flat end mirrors that was pumped with a laser diode whose focused spot size was larger than the lasing beam spot size determined by the thermal lens effect. The photon lifetimes of the laser oscillations were found to be greatly shortened to \tau_{p} = 6.74 ps as compared with those in pure TEM00 mode operation, while the fluorescence lifetime was \tau = 130 \mu s. The formation of the peculiar transverse mode was theoretically treated in terms of the pump-intensity-dependent thermal lens effect, which yields the lasing beam spot sizes in the resultant optical cavity. In addition, transverse spatial hole burning of population inversions due to the preceding transverse eigenmode intensity and the associated anti-guidance of the pure TEM00 field leading to formation of the TEM00 with an emergent outer ring emission was shown to appear with increasing pump power through the electric lens (i.e., population lens) effect, in which almost all of the population inversion contributes to lasing with a pronounced effective modal gain. Self-mixing laser Doppler velocimetry (LDV) experiments and numerical simulations revealed that spontaneous emission factors in the peculiar mode oscillations were greatly decreased in comparison with those in pure TEM00 mode operation. The present thin-slice solid-state laser possessing a large fluorescence-to-photon lifetime and decreased spontaneous emission factor enabled us to detect LDV signals in the regime of few feedback photons per observation period.

Fibre-coupled red diode-pumped Alexandrite TEM00 laser with single and double-pass end-pumping

We report the investigation of an Alexandrite laser end-pumped by a fibre-coupled red diode laser module. Power, efficiency, spatial, spectral, and wavelength tuning performance are studied as a function of pump and laser cavity parameters. It is the first demonstration, to our knowledge, of greater than 1 W power and also highest laser slope efficiency (44.2%) in a diode-pumped Alexandrite laser with diffraction-limited TEM00 mode operation. Spatial quality was excellent with beam propagation parameter M 2 ~ 1.05. Wavelength tuning from 737–796 nm was demonstrated using an intracavity birefringent tuning filter. Using a novel double pass end-pumping scheme to get efficient absorption of both polarisation states of the scrambled fibre-delivered diode pump, a total output coupled power of 1.66 W is produced in TEM00 mode with 40% slope efficiency.

Sub-5-pm linewidth, 130-nm-tuning of a coupled-cavity Ti:sapphire oscillator via volume Bragg grating-based feedback

A novel coupled-cavity Ti:sapphire oscillator architecture featuring a volume Bragg grating as a feedback element is presented. The oscillator provides continuous wave lasing within a spectral linewidth as narrow as 5 pm. The output can be wavelength-tuned over an ultrabroad spectral range of 130 nm, extending from 714 to 842 nm. This unique combination of narrow spectral linewidth and wide tuning range makes the laser suitable for applications such as sensing and Raman and absorption spectroscopy. The laser also displays ideal TEM00 mode operation throughout its tuning range with output powers beyond 300 mW. Detailed studies of the cw lasing dynamics across the wide tuning range are described. The general architecture of this design can be implemented for high resolution tuning across the broad spectral emission bands of other solid state lasers with single mode operation.

Scaling the output power of quantum-cascade lasers with a number of cascades

A series of quantum-cascade lasers emitting at λ≈11μm with different numbers of cascades in the active region – between 25 and 200 –grown by gas-source molecular beam epitaxy, is compared in terms of their room-temperature performance in pulsed mode as a function of the number of cascades. Within the given range of cascade number we observe a superlinear increase in both slope efficiency and differential quantum efficiency as functions of cascade number. The differential quantum efficiency scaled per number of cascades, however, shows a sublinear increase. A differential quantum efficiency of 17% per cascade and pulse power exceeding 7W per facet, both at room temperature, was demonstrated for quantum-cascade lasers with 200 cascades. All lasers with stripe width below 30μm are operated in the fundamental TEM00 spatial mode. This combination of high pulse power and TEM00 mode operation for room-temperature mid-infrared laser emission can be attractive for a number of applications.

A birefringence-compensated two-rod Nd:YAG laser operating in TEM00 mode with a CW 61W output power

Two identically pumped Nd:YAG rods with a imaging system and a polarization rotator are used in experiments to achieve a complete birefringence compensation. The measured depolarization ratio is from 1.0 to 2.5%, which increases with the increasing of pump power. With a TEM00-mode operation, the depolarization ratio achieved is less than 1%. A dynamically stable resonator is performed using two flat mirrors and an intra-cavity lens. 61 W linearly polarized output was achieved with a M 2 factor of 1.6, which is the state of the art for a lamp pumped laser.

A 15.1 W continuous wave TEM00 mode laser using a YVO4/Nd:YVO4 composite crystal

A effective continuous-wave (CW), high power laser generated using a YVO4/Nd:YVO4 composite crystal is presented. 18.8 W output power in multi-mode has been achieved with a maximum absorbed pump power of 31.2 W, corresponding to an optical-to-optical efficiency of 60.26%. In TEM00 mode operation, 15.1 W output power also has been achieved with the maximum absorbed pump power of 31.2 W, corresponding to an optical-to-optical efficiency of 47.69%. With a 200 mm focal-length positive lens and using the moving knife-edge method, the beam quality factor is measured to be M2 = 1.2 for TEM00 mode beam.

Laser performance study of mixed vanadate crystals <inline-formula><math display="inline" overflow="scroll"><mrow><mi mathvariant="normal">Nd</mi><mo>:</mo><msub><mi mathvariant="normal">Y</mi><mi>x</mi></msub><msub><mi mathvariant="normal">Gd</mi><mrow><mn>1</mn><mo>−</mo><mi>x</mi></mrow></msub><mi mathvariant="normal">V</mi><msub><mi mathvariant="normal">O</mi><mn>4</mn></msub></mrow></math></inline-formula> with direct and indirect pumping

We present, for the first time, comparative studies of laser performance in the mixed vanadate crystals Nd:YxGd1−xVO4 with direct and indirect pumping. A series of mixed vanadate crystals Nd:YxGd1−xVO4 with different yttrium and gadolinium composition ratios but the same Nd3+ dopant level is prepared under the same fabrication processes and their laser performance in the TEM00 mode operation is studied using the same experimental configuration under direct pumping with laser diodes at 880 nm as well as indirect pumping at 808 nm. Remarkable improvement in laser efficiency and maximum achievable output power is achieved under direct 880-nm excitation over indirect 808-nm excitation. The maximum optical slope efficiency in the TEM00 mode operation is 74% with direct 880-nm pumping, which also produces the maximum overall optical-to-optical conversion efficiency of 59%. The maximum attainable laser power is up to 50% higher under direct 880-nm pumping over the more conventional 808-nm pumping. These achieved results provide valuable insight into crystal composition selection and pump schemes for the mixed vanadate crystals, which have the potential to tailor laser parameters through crystal composition engineering.

Characterization of a diode double-end pumped <inline-formula><math altimg="none" display="inline" overflow="scroll"><mrow><mi>Nd</mi><mo>:</mo><msub><mi>YVO</mi><mn>4</mn></msub></mrow></math></inline-formula> laser for high-bit-rate free-space and intersatellite communications

Interorbit links, intersatellite links, and deep space missions are certain key applications of free-space optical communication links. For efficient and better signal propagation ensuring minimum possible bit error rate (BER) and maximize signal-to-noise ratio (SNR), the laser source used for carrying information should be one in which the laser power and beam quality do not degrade due to vibrations, shock, and thermal cycling. As part of our attempt to design an efficient and suitable laser source for optical free-space communication, a diode double-end pumped and Q-switched Nd:YVO4 laser is designed and fabricated. The thermally induced lens in the laser crystal brings the flat-convex cavity into geometric stability. A slope efficiency of 58.38% is observed in continuous wave (CW) multimode and 40.49% is observed in CW TEM00 mode operation, which is sufficiently high for free-space optical communication. A minimum pulse width of 7.3 ns is observed corresponding to a peak power of 63.01 kW at 20 kHz. It can be seen that at lower Q-switch frequencies, the pulse width is short and the energy per pulse is high, whereas at higher Q-switch frequencies, the energy per pulse is low and the pulse width is long but the average power is high. The peak power increases with the absorbed pump power, similar to a lamp-pumped Nd:YAG laser. The maximum pulse energy observed is 430 µJ at 10 kHz. Below 20 kHz, no further increase in pulse energy is observed due to the lower lifetime of the upper laser level (~90 to 100 µs) of Nd:YVO4 crystal. Changes in the laser beam profile with pump power have been studied. The major advantage of the Nd:YVO4 crystal is the ability to retain short pulse width even at higher Q-switch frequencies like 100 kHz, which makes this laser suitable for ultrahard material processing at higher Q-switch frequencies.At full pump power, the pulse width varies from 7.3 ns at 20 kHz to 19.5 ns at 100 kHz.(partial abstract)

Improving the output beam quality of multimode laser resonators

Multimode laser operation is usually characterized by high output power, yet its beam quality is inferior to that of a laser with single TEM00 mode operation. Here we present an efficient approach for improving the beam quality of multimode laser resonators. The approach is based on splitting the intra-cavity multimode beam into an array of smaller beams, each with a high quality beam distribution, which are coherently added within the resonator. The coupling between the beams in the array and their coherent addition is achieved with planar interferometric beam combiners. Experimental verification, where the intra-cavity multimode beam in a pulsed Nd:YAG laser resonator is split into four Gaussian beams that are then coherently added, provides a total increase in brightness of one order of magnitude. Additional spectral measurements indicate that scaling to larger coherent arrays is possible.

Operation of a helium-free TEA CO2 laser

A novel and inexpensive excitation circuit based on a single switch and a single source drives helium-free TEA CO2 lasers, mini to conventional types. By varying the partial pressure of either CO2 or N2, the duration (FWHM) and the peak power of the laser pulse (for a system with active volume ∼70 cc) is made to vary by a factor of ∼7 and ∼5, respectively. Although, the multi-mode operation efficiency of this laser is marginally smaller than that of the same laser when operated with conventional gas mixture, its efficiency in TEM00 mode operation has always been found to be higher. This laser can be used wherever a TEA CO2 laser with conventional gas mixture containing helium is used.

Compact diode-side-pumped Nd:YVO 4 laser in grazing-incidence configuration

The analysis of compact CW diode-side-pumped grazing-incidence-geometry Nd:YVO4 laser designs is presented. An output power of 5 W (λ=1064 nm) was produced at 17 W of diode pump (conversion efficiency of 30%) in single transverse TEM00 mode operation at high laser beam quality (Mx2≈1.05 and My2≈1.01). The resonator geometry was analyzed by applying generalized 4×4 matrix modeling of the spatial mode size, including the impact on the laser operation of cavity astigmatism and a thermal lens in the laser slab. The simplicity and compactness of the laser cavities allow their use for technological applications.

300-W cw diode-laser side-pumped Nd:YAG rod laser

We report on the laser performance and power scaling properties of diode-laser side-pumped Nd:YAG rod lasers operating at high cw output powers. Applying a pump power of 1100 W cw in a ninefold pump geometry, we obtain an output power of more than 300 W in multimode operation. For a comparable pump scheme an output power of more than 45 W in TEM(00) mode operation is realized with an optical-to-optical efficiency of more than 11%.

40-W cw, TEM_00-mode, diode-laser-pumped, Nd:YAG miniature-slab laser

We have built a diode-laser-pumped Nd:YAG slab laser that emits 40 W of cw power in a TEM(00) mode and 72 W of power in multimode operation when pumped with 235 W. The slope efficiencies are 22% for TEM(00)-mode operation and 36% for multimode operation. The laser uses a zigzag slab geometry to reduce thermally induced distortions and operates at less than one wave of distortion at the full pump power. A significant advantage of our design over those of previous slab lasers is a new Teflon AF protective coating on the slab total-internal-reflection surfaces, which greatly simplifies the mounting and cooling of the slab laser medium.

High Power CO2 Laser Using a Gauss-core Resonator for 6 kW Large-volume TEM00 Mode Operation.

High Power CO2 Laser Using a Gauss-core Resonator for 6 kW Large-volume TEM00 Mode Operation.

Diode-pumped Nd:YAG laser using reflective pump optics

We have examined the performance of a diode-laser side-pumped Nd:YAG laser using elliptical mirrors to focus the output of 6 × 10 W laser-diode arrays into the Nd: YAG rod. The multimode cw output power was 14 W with an optical to optical efficiency of 29%. With a resonator designed for TEM00 mode operation 12 W of output was achieved.

76 W of continuous-wave radiation in a TEM_00 mode from a laser-diode end-pumped Nd:YAG laser

We have demonstrated 7.6 W of continuous-wave radiation at λ = 1.06 μm in a TEM00 mode from a diode end-pumped Nd:YAG laser pumped by 38 laser diodes that are coupled to a bundle of 19 fibers. TEM00-mode operation was enforced by using an intracavity aperture. The absorbed power in the Nd:YAG rod was 16.4 W when the incident power upon the laser rod was 21.1 W. The optical-to-optical conversion efficiency was 46%. In multimode operation, 11 W of output power and a slope efficiency of 69% were obtained. The thermal analysis was in good agreement with interferometric measurements.

A Flashlamp-pumped Dye Laser in a Telescopic Resonator Configuration

We describe a flashlamp-pumped dye laser in a telescopic resonator configuration. Theory and experiment are used to show that such a resonator allows high energy, TEM00 mode operation of this type of laser. Also demonstrated for the first time is the tunability of the telescopic resonator, a feature which should allow diffraction-limited coverage of the entire visible region of the spectrum merely by changing the dye.

The effect of thermal blooming in a liquid filter on the emission of a Nd: Glass laser

The spiking behaviour of a Nd: glass laser operating reproducibly in TEM 00-mode operation was investigated with a liquid filter inside the cavity. It was found that thermal blooming in the liquid causes time-dependent losses of the order of 1%, and therefore a substantial decrease of spike intensity.

Convex-Concave Resonators for TEM00 Operation of Solid-State Ion Lasers

Optical resonators with one convex and one concave mirror offer significant advantages for TEM00 mode operation of solid-state lasers. High efficiency and low sensitivity to perturbations and compactness have been obtained in comparison to other resonator types. The resonators described are characterized by a stability product equal to one-half, with one g parameter greater than unity. An equivalent resonator incorporating an intracavity lens is also described which offers a focused light beam of a selected radius useful for intracavity modulation or nonlinear effects. Experimental results are given for continuously pumped Nd: YAG lasers.