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

Compact high-power 320-nm continuous-wave single-frequency laser based on power scaling of a diode-pumped Pr:LiYF4 ring laser at red

Single-frequency (SF) lasers in the visible spectral region are usually obtained through an indirect method, i.e., frequency doubling of near-infrared SF lasers. In this work, we report on the direct generation of a high-power continuous-wave (CW) SF laser in red based on a diode-pumped Pr:LiYF4 (YLF) ring cavity technology. A maximum output power is scaled to 3.98 W at 640 nm with a linewidth of about 17.2 MHz and a power stability of 0.6%. Moreover, by inserting a LBO crystal into the ring cavity for intracavity frequency doubling of the 640 nm SF laser, we have also successfully demonstrated an ultraviolet (UV) SF laser at 320 nm, for the first time to the best of our knowledge, with a maximum power of 670 mW. This work provides a promising route for the development of simple, compact, and high-power SF lasers operating in visible and UV spectral regions.

589 nm yellow laser pumped Kerr-lens mode-locked Alexandrite laser producing sub-50 fs pulses

We report a femtosecond Kerr-lens mode-locked (KLM) Alexandrite laser resonantly pumped by a 589 nm yellow laser. The 4 nJ pulses as short as 42 fs were obtained corresponding to a peak power of 100 kW. With the repetition rate of 104 MHz, the average power of 420 mW was attained. The time-bandwidth product of generated laser pulse was measured to be 0.324 with a beam quality factor of M2 ≤ 1.13. The exceptional performance of visible femtosecond laser may find potential applications in various fields.

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 4.6 mJ and a maximum average power of 2.7 W were achieved for a repetition rate range from 500 to 750 Hz 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 3 MHz is measured. Measurements of atmospheric winds and temperatures were conducted during several field campaigns from summer 2022 to spring 2023.

Comparative study of lidars for measuring atmospheric temperature and wind

The uncertainty of lidar measured atmospheric temperature T or line-of-sight (LOS) wind V is inversely proportional to the signal-to-noise ratio (SNR) of the received photocounts. We term the proportionality constant, which depicts the efficacy of the measurement method, the single-photon (or unity SNR) measurement uncertainty for T and/or V measurement. In this study, we use the single-photon measurement uncertainty as the figure of merit to compare and understand the practical differences between Cabannes scattering (CS), Rayleigh inversion (RI), rotational Raman (RR), and laser induced fluorescence (LIF) lidars for atmospheric temperature and wind measurements, and to optimize the choice and receiver design of a lidar system for a potential application.

Alexandrite lasers with blue-diode-pumping

The availability of high-power and high-brightness blue diode lasers makes them attractive as low-cost pump sources for broadly tunable Alexandrite lasers. In this paper we investigate the performance of an Alexandrite laser pumped by a high-power fiber-delivered blue diode module. Output power 1.84 W is achieved, the highest power from blue diode pumped Alexandrite to date. Excellent pump absorption is demonstrated of scrambled pump polarization on both a-axis and b-axis of Alexandrite crystal. Wavelength tuning and dual wavelength operation is produced using the self-birefringent filtering of the Brewster-cut Alexandrite crystal. An analysis is made of laser efficiency and mode formation including the creation of higher-order Laguerre-Gaussian vortex modes (LG₀₁ and LG₀₂). Performance is compared to red diode pumping and prospects for further optimization and power-scaling are discussed.

High-performance cavity-dumped Q-switched Alexandrite laser CW diode-pumped in double-pass configuration

We present a high-performance Alexandrite laser for LIDAR applications with repetition rates up to 20 kHz in cavity-dumped Q-switched operation continuous-wave diode-pumped in the red spectral region. With a double-pass pump configuration, short pulses with 2.8 ns duration at repetition frequencies ranging from 1 kHz to 20 kHz could be demonstrated. At 5 kHz a - to our knowledge - record pulse energy of over 500 µJ could be achieved at 755 nm in TEM₀₀. Furthermore, a stability measurement at an energy of around 350 µJ with 5 kHz showed no degradation over 150 Mega-shots. The influence of the crystal temperature on the laser performance is also investigated, first in continuous-wave and secondly in cavity-dumped Q-switched operation.

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.

Atomic vapor filter revisited: a Cabannes scattering temperature/wind lidar at 770 nm

Using an atomic/molecular vapor as an aerosol blocking filter for atmospheric temperature measurements with a Cabannes lidar is revisited. Different problems in previously used barium and iodine filters prevented them from delivering the 78 times signal advantage (8.8 times less uncertainty) over rotational Raman lidar. We conclude that, despite the sensitivity optimization in rotational Raman lidar, a proposed Cabannes lidar utilizing potassium vapor filters can have 6.1 times less temperature uncertainty. By tuning the laser frequency cyclically to above and below the potassium D₁ transition, the lidar system can measure temperature and wind simultaneously.

Broadband tunable single-longitudinal-mode erbium-doped fiber ring laser based on a microfiber knot resonator

We propose and demonstrate a broadband tunable single-longitudinal-mode (SLM) erbium-doped fiber laser based on a microfiber knot resonator (MKR). The MKR is made from a double-ended fiber taper and employed for SLM filtering based on the Vernier effect. An unpumped erbium-doped fiber is used in the fiber laser cavity to suppress mode-hopping for a stabilized SLM laser operation. When combined with an optical fiber filter, widely tunable SLM laser generation is achieved. The proposed SLM laser can be tuned from 1545 to 1565 nm with a high side-mode suppression ratio of about 55 dB and a high stability.

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.

Tunable, dual wavelength and self-Q-switched Alexandrite laser using crystal birefringence control

We present a red-diode-pumped Alexandrite laser with continuous wavelength tunability, dual wavelength and self-Q-switching in an ultra-compact resonator containing only the gain medium. Wavelength tuning is obtained by varying the geometrical path length and birefringence by tilting a Brewster-cut Alexandrite crystal. Two crystals from independent suppliers are used to demonstrate and compare the performance. Wavelength tuning between 750 and 764 nm is demonstrated in the first crystal and between 747 and 768 nm in the second crystal. Stable dual wavelength operation is also obtained in both crystals with wavelength separation determined by the crystal free spectral range. Temperature tuning was also demonstrated to provide finer wavelength tuning at a rate of -0.07 nm K ^-1. Over a narrow tuning range, stable self-Q-switching is observed with a pulse duration of 660 ns at 135 kHz, which we believe is the highest Q-switched pulse rate in Alexandrite to date. Theoretical modelling is performed showing good agreement with the wavelength tuning and dual wavelength results.

Unidirectional single-frequency operation of a continuous-wave Alexandrite ring laser with wavelength tunability

High resolution spectroscopy, metrology and quantum technologies (e.g. trapping and cooling) require precision laser sources with narrow linewidth and wavelength tunability. The widespread use of these lasers will be promoted if they are cost-effective, compact and efficient. Alexandrite lasers with a broad tuning band pumped efficiently by low-cost red diodes are a potential candidate, but full performance as a precision light source has not been fully achieved. We present in this work the first continuous-wave (CW) and single-frequency operation of a unidirectional diode-end-pumped Alexandrite ring laser with wavelength tunability. An ultra-compact bow-tie ring cavity is developed with astigmatic compensation and a novel 'displaced mode' design producing CW output power > 1 W in excellent TEM₀₀ mode (M² < 1.2) when using a low brightness pump (M² ≥ 30). Wavelength tuning from 727 - 792 nm is demonstrated using a birefringent filter plate. This successful operation opens the prospects of precision light source applications.

Diode-pumped Q-switched Alexandrite laser in single longitudinal mode operation with Watt-level output power

We present significantly improved performance data of a diode-pumped Q-switched Alexandrite laser in single longitudinal mode operation developed as a beam source for resonance lidar systems. The average output power of the laser-operating at the potassium resonance at 770 nm with a linewidth below 10 MHz-could be increased by a factor of five to the Watt-level by means of an optimized resonator design and pump scheme. The pulse energy is 1.7 mJ with a beam quality of M²≤1.1 in both spatial directions at a repetition rate of 500 Hz.

Graphene mode-locked femtosecond Alexandrite laser

We report for the first time, to the best of our knowledge, graphene mode-locked operation of a femtosecond Alexandrite laser at 750 nm. A multipass-cavity configuration was employed to scale the output energy and to eliminate spectral/Q-switching instabilities. By using a monolayer graphene saturable absorber, mode locking could be obtained. With 5 W of pump at 532 nm, nearly transform-limited, 65 fs pulses with a time-bandwidth product of 0.319 were generated. The mode-locked laser operated at a pulse repetition rate of 5.56 MHz and produced 8 mW output power, corresponding to a pulse energy and peak power of 1.4 nJ and 22 kW, respectively. These experiments further show that graphene can be used to initiate mode locking at wavelengths as low as 750 nm.