Diode-pumped Alexandrite Laser Research Articles

Energy-scaling of a diode-pumped Alexandrite laser and prototype development for a compact general-purpose Doppler lidar.

We present design and performance data of an energy-scaled diode-pumped Alexandrite laser in single longitudinal mode operation developed as a beam source in a mobile general-purpose Doppler lidar. A maximum pulse energy in Q-switched operation of 4.6mJ and a maximum average power of 2.7W were achieved for a repetition rate range from 500 to 750Hz with excellent beam quality of M 2=1.1. Two rugged and compact demonstrator lasers were built and integrated into mobile lidar systems, where a bandwidth of approximately 3MHz is measured. Measurements of atmospheric winds and temperatures were conducted during several field campaigns from summer 2022 to spring 2023.

Power-scaled CW Alexandrite lasers

We present several strategies for increasing the output laser power of near-diffraction limited continuous-wave Alexandrite lasers pumped by fibre-coupled red laser diodes. Laser mode size control using a convex mirror with the varying thermal lens is shown to be an efficient method for generating record levels of output power from a diode-pumped Alexandrite laser. 8.6 W is obtained with M^{2}<1.1 and an optical efficiency of 27%. 735–790 nm wavelength tuning is demonstrated with > {2.5},hbox {W} and a record dual-wavelength output power of 4.2 W. In a ring-cavity geometry, 5.1 W is obtained with M^{2}<1.1 showing promising potential for multi-watt single-longitudinal-mode operation. Detailed laser cavity modelling including thermal lens analysis is also provided.

High repetition rate diode-pumped self-pulsed few-cycle alexandrite laser

We reported a high repetition rate diode-pumped alexandrite femtosecond laser using a simple linear cavity. Laser emission spectrum was as broad as ∼20 nm by inserting a 0.5-mm-thick quartz birefringent plate as a spectral modulation component. Pulse duration as short as few-cycle 36 fs with the repetition rates of 2.48 GHz was measured at pump power of 17 W. To the best of our knowledge, this is the shortest pulse duration achieved in the alexandrite lasers.

2.55 W continuous-wave 378 nm laser by intracavity frequency doubling of a diode-pumped Alexandrite laser.

A high-power continuous-wave (CW) ultraviolet (UV) laser at 378 nm from an intracavity frequency-doubled Alexandrite laser has been demonstrated with 638 nm fiber-coupled laser diodes as the pump source. A maximum output power of 2.55 W was obtained, which is the highest power for CW frequency-doubled Alexandrite lasers, to the best of our knowledge, corresponding to the optical-to-optical conversion efficiency of 7.9% from 638 nm pump laser to 378 nm UV laser. The beam quality factors M2 were measured to be 2.19 and 2.47 in x and y directions at UV output power of 1 W, respectively.

VAHCOLI, a new concept for lidars: technical setup, science applications, and first measurements

Abstract. A new concept for a cluster of compact lidar systems named VAHCOLI (Vertical And Horizontal COverage by LIdars) is presented, which allows for the measurement of temperatures, winds, and aerosols in the middle atmosphere (∼ 10–110 km) with high temporal and vertical resolution of minutes and some tens of meters, respectively, simultaneously covering horizontal scales from a few hundred meters to several hundred kilometers (“four-dimensional coverage”). The individual lidars (“units”) being used in VAHCOLI are based on a diode-pumped alexandrite laser, which is currently designed to detect potassium (λ=770 nm), and on sophisticated laser spectroscopy measuring all relevant frequencies (seeder laser, power laser, backscattered light) with high temporal resolution (2 ms) and high spectral resolution applying Doppler-free spectroscopy. The frequency of the lasers and the narrowband filter in the receiving system are stabilized to typically 10–100 kHz, which is a factor of roughly 10−5 smaller than the Doppler-broadened Rayleigh signal. Narrowband filtering allows for the measurement of Rayleigh and/or resonance scattering separately from the aerosol (Mie) signal during both night and day. Lidars used for VAHCOLI are compact (volume: ∼ 1 m3) and multi-purpose systems which employ contemporary electronic, optical, and mechanical components. The units are designed to autonomously operate under harsh field conditions in remote locations. An error analysis with parameters of the anticipated system demonstrates that temperatures and line-of-sight winds can be measured from the lower stratosphere to the upper mesosphere with an accuracy of ±(0.1–5) K and ±(0.1–10) m s−1, respectively, increasing with altitude. We demonstrate that some crucial dynamical processes in the middle atmosphere, such as gravity waves and stratified turbulence, can be covered by VAHCOLI with sufficient temporal, vertical, and horizontal sampling and resolution. The four-dimensional capabilities of VAHCOLI allow for the performance of time-dependent analysis of the flow field, for example by employing Helmholtz decomposition, and for carrying out statistical tests regarding, for example, intermittency and helicity. The first test measurements under field conditions with a prototype lidar were performed in January 2020. The lidar operated successfully during a 6-week period (night and day) without any adjustment. The observations covered a height range of ∼ 5–100 km and demonstrated the capability and applicability of this unit for the VAHCOLI concept.

Rugged diode-pumped Alexandrite laser as an emitter in a compact mobile lidar system for atmospheric measurements.

We present design and performance data of two diode-pumped Alexandrite lasers developed explicitly as laser emitters in mobile potassium resonance lidar systems. The lasers yield an output power of up to 1.75 mJ at a repetition rate of 500 Hz with a beam quality of $M{{^2}} \lt {1.1}$ in both spatial directions. Reliable single longitudinal mode operation with a unrivaled narrow linewidth of 3.3 MHz at a potassium resonance line at 769.898 nm is achieved. The wavelength can be switched from pulse to pulse in a range of several gigahertz so the potassium line can be scanned. The lasers are finally integrated in highly efficient lidar systems with a power consumption of 500 W for the whole lidar system. The extremely high spectral requirements are investigated and the performance for different working points regarding repetition rates and pump durations is investigated. Several weeks of remotely controlled operation of the prototype in a field campaign were conducted without changes of the output parameters. Approximately 1000 h of reliable single longitudinal mode operation was achieved during the campaign and measurements of Doppler-Mie wind observations in the stratosphere and of the potassium layer in the mesopause were conducted simultaneously even at daytime.

High-energy diode-pumped alexandrite amplifier development with applications in satellite-based lidar

Efficient, wavelength-tunable diode-pumped alexandrite laser systems offer the potential for a more versatile, satellite-based lidar source compared to fixed wavelength Nd:YAG systems and non-space compliant lamp-pumped alexandrite. In this paper, we develop a strategy to enable the high-energy operation required for atmospheric lidar based on an efficient diode-pumped master oscillator power amplifier (MOPA) system design. A novel multipass “diamond” slab amplifier geometry is introduced alongside the experimental results of the world’s first diode-pumped alexandrite amplifier producing a gain of 2.13 in a demonstration system. A diode-pumped Q -switched alexandrite oscillator is presented with a record-highest pulse energy of 3.80 mJ. Detailed optimization of a two-stage amplifier design is studied numerically and maximized with temperature, wavelength, and pump pulse duration to produce 50 mJ pulse energy. This forms part of an optimized alexandrite MOPA design capable of high pulse energy, showing the future potential of diode-pumped alexandrite for satellite-based atmospheric lidar.

Diode-pumped cw Alexandrite laser with temporally stable 6.5 W in TEM00 operation with prospect of power scaling.

We present the design of a longitudinally diode-pumped Alexandrite laser in continuous-wave operation and resulting performance data. A laser power of 6.5 W in fundamental mode operation was measured, which is, to the best of our knowledge, the highest laser power in fundamental mode operation yet reported. The laser crystal was pumped by two diode modules emitting at 637 nm. The pump radiation was polarization-combined and spatially symmetrized. The laser operates at an output power of 6.5 W with an optical-to-optical efficiency of 26%, temporally stable output with stability of 8% on ms timescale, a beam quality of M2 = 1.1 in both spatial directions and emission of an output wavelength of 752 nm. Measurements of the thermal dioptric power at pumping intensities up to 9.5 kW/cm2 support the appropriate approach of the design. Based on our results, we estimate the potential and show our concept for future scaling of the output power.

Pump-induced lensing effects in diode pumped Alexandrite lasers.

It is essential to understand the pump-induced lensing and aberration effects in solid-state lasers, such as Alexandrite, since these set limits on laser power scaling whilst maintaining high spatial TEM00 beam quality. In this work, we present direct wavefront measurements of pump-induced lensing and spherical aberration using a Shack-Hartmann wavefront sensor, for the first time, in a diode-pumped Alexandrite laser, and under both non-lasing and lasing conditions. The lens dioptric power is found to be weakly sub-linear with respect to the absorbed pump power, and under lasing, the lensing power is observed to decrease to 60 % of its non-lasing value. The results are inconsistent with a thermal lens model but a fuller theoretical formulation is made of a combined thermal and population lens model giving good quantitative agreement to the observed pump power dependence of the induced-lensing under non-lasing conditions and the reduced lensing under lasing conditions. The deduced value for the difference in excited to ground state polarizability is consistent with prior measurement estimates for other chromium-doped gain media. The finding of this paper provide new insight into pump-induced lensing in Alexandrite and also provides a basis for a fast saturable population lens mechanism to account for self-Q-switching observed recently in Alexandrite laser systems.

Diode-pumped Alexandrite laser for next generation satellite-based earth observation lidar

In this work, the design of a diode-pumped Alexandrite ring laser in Q-switched single-longitudinal-mode (SLM) operation for a spaceborne lidar mission is presented. The laser is pumped by a self-developed fiber-coupled laser diode pump device and yields a pulse energy of 1.7 mJ at a repetition rate of 500 Hz with an excellent beam quality of M2 < 1.1. By seeding the resonator with a narrow band diode laser, SLM operation with a linewidth of approximately 10 MHz is achieved. The electro-optical efficiency of 2% is the highest achieved for all Alexandrite lasers in SLM operation and reasonable for space operation. The performance analysis as well as benchmarking with the space-qualified mounting technology points out the TRL and the remaining effort for the development of the technology. An estimation of the requirements for a spaceborne resonance lidar mission underlines the suitability of such a lidar system with a diode-pumped Alexandrite laser as the beam source.

Diode-pumped alexandrite ring laser in single-longitudinal mode operation for atmospheric lidar measurements.

We present, to the best of our knowledge, design and performance data of the first diode-pumped Alexandrite ring laser in Q-switched single-longitudinal mode (SLM) operation. The laser resonator contains two Alexandrite crystals, which are pumped longitudinally by means of two laser diode-bar modules emitting at 636 nm. Single-longitudinal mode operation is achieved by seeding the laser with a diode laser operating in SLM and actively stabilizing the cavity, yielding a linewidth of < 10 MHz at the potassium resonance line at 770 nm. The pulse energy is 1 mJ at a repetition rate of 150 Hz and 0.65 mJ at 320 Hz. The beam quality of M2 < 1.2 in both directions remains unchanged for the different repetition rates. After characterization in the laboratory, the laser was implemented in a novel mobile lidar system and first atmospheric measurements were conducted successfully.

Optical vortex generation from a diode-pumped alexandrite laser

We present the demonstration of an optical vortex mode directly generated from a diode-pumped alexandrite slab laser, operating in the bounce geometry. This is the first demonstration of an optical vortex mode generated from an alexandrite laser or from any other vibronic laser. An output power of 2 W for a vortex mode with a ‘topological charge’ of 1 was achieved and the laser was made to oscillate with both left- and right-handed vorticity. The laser operated at two distinct wavelengths simultaneously, 755 and 759 nm, due to birefringent filtering in the alexandrite gain medium. The result offers the prospect of broadly wavelength tunable vortex generation directly from a laser.

Diode-pumped Alexandrite laser with passive SESAM Q-switching and wavelength tunability

We report the first experimental demonstration of a wavelength tunable passively Q-switched red-diode-end pumped Alexandrite laser using a semiconductor saturable absorber mirror (SESAM). We present the results of the study of passive SESAM Q-switching and wavelength-tuning in continuous diode-pumped Alexandrite lasers in both linear cavity and X-cavity configurations. In the linear cavity configuration, pulsed operation up to 27 kHz repetition rate in fundamental TEM00 mode was achieved and maximum average power was 41 mW. The shortest pulse generated was 550 ns (FWHM) and the Q-switched wavelength tuning band spanned was between 740 nm and 755 nm. In the X-cavity configuration, a higher average power up to 73 mW, and obtained with higher pulse energy 6.5μJ at 11.2 kHz repetition rate, in fundamental TEM00 mode with excellent spatial quality M2<1.1. The Q-switched wavelength tuning band spanned was between 775 nm and 781 nm.

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.

High efficiency >26 W diode end-pumped Alexandrite laser.

We show for the first time that multi-ten Watt operation of an Alexandrite laser can be achieved with direct red diode-pumping and with high efficiency. An investigation of diode end-pumped Alexandrite rod lasers demonstrates continuous-wave output power in excess of 26W, more than an order of magnitude higher than previous diode end-pumping systems, and slope efficiency 49%, the highest reported for a diode-pumped Alexandrite laser. Wavelength tuning from 730 to 792nm is demonstrated using self-seeding feedback from an external grating. Q-switched laser operation based on polarization-switching to a lower gain axis of Alexandrite has produced ~mJ-pulse energy at 1kHz pulse rate in fundamental TEM(00) mode.