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Abstract
We investigate high-energy mJ-class diode-pumped acousto-optic (AO) Q-switched alexandrite lasers with broad tunability at both the fundamental near-IR wavelength range and second harmonic generation (SHG) in the UV wavelength range. An AO Q-switched alexandrite laser with continuous-wave diode-pumping has been operated at up to 10 kHz and producing pulse energy of 700 µJ at repetition rate of 1 kHz. With pulsed double-pass diode-pumping, we demonstrate higher pulse energy of 2.6 mJ. With wavelength tuning, this laser system demonstrated broad fundamental tuning range from 719 to 787 nm with diffraction-limited beam quality (M2 = 1.05). By external cavity second harmonic generation in a Type-I LBO crystal, pulse energy of 0.66 mJ is generated at 375 nm and with UV tuning range from 361 to 391 nm. To our knowledge, this is the first demonstration of AO Q-switched alexandrite laser with broad wavelength tuning in the near-IR and UV wavelength ranges and shows its excellent potential as a pulsed source for future applications.
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1. Introduction
Pulsed lasers are valuable sources for many applications requiring high peak power and short duration including lidar-based remote sensing [1,2], photoacoustic imaging [3], biophotonics [4], and material processing [5]. Many well-established laser systems such as the solid-state Nd:YAG laser at 1064 nm have fixed wavelength with almost no tunability and while harmonic generation can produce new wavelengths these also are discrete. Several applications would be enhanced if the laser source has wavelength tunability such as to match to a narrow molecular transition in atmospheric sensing (e.g., differential absorption lidar, DIAL), or to select absorption features of chromophores in photoacoustic imaging (e.g., to detect and distinguish oxygenated and deoxygenated haemaglobin, Hb). It is also desirable in real-world applications for the laser source to be compact, efficient, and robust with low size, weight, and power (SWaP) for system integration and ideally at low cost. In this latter aspect, whilst Ti: sapphire lasers are broadly tunable their short upper-state lifetime ∼ 3 µs necessitates complex and inefficient pulsed pump sources (e.g., frequency-doubled Q-switched neodymium lasers) to obtain high pulse energy.
Alexandrite, chromium doped chrysoberyl (Cr3+: BeAl2O3), is an interesting solid-state laser gain medium as it has broad wavelength tunability with demonstrated operation from ∼ 701 to 858 nm [6,7]. The second harmonic generation (SHG) of Alexandrite’s fundamental wavelength band can directly generate broadly tunable ultraviolet (UV) and blue laser radiation (∼ 350–430 nm). The third harmonic generation (THG) and fourth harmonic generation (FHG) provide tunability in the deep UV (233 - 287 nm and 175–215 nm, respectively). Its long upper-state fluorescence lifetime of 260 µs at 293 K [6] is two orders of magnitude longer than Ti: sapphire and provides good energy storage for Q-switching operation. Together with high thermal conductivity and fracture resistance allows the possibly of high energy operation [6]. Alexandrite can be pumped by red diode lasers at ∼ 638 nm, with high Stokes efficiency and low quantum defect heating.
Several prior demonstrations of Q-switched diode-pumped Alexandrite lasers have been performed over the past ten years. Teppitaksak et al presented the first diode-pumped Q-switched Alexandrite laser in 2014 producing 0.7 mJ pulse energy and 58 ns pulse duration at 100 Hz repetition rate [8]. In 2016, Thomas et al demonstrated CW diode-pumped Q-switched Alexandrite lasers with 170 µJ at 10 kHz pulse rate and high pulse energy of 3 mJ at 500 Hz repetition rate in TEM00 mode [9]. In this same work, they also operated the first cavity-dumped Q-switching of diode-pumped Alexandrite demonstrating pulse energy of 510 µJ at 3 kHz pulse rate with short pulse duration 3 ns (peak power of 170 kW) in TEM00 mode. This high peak power cavity-dumped Q-switched system was used to demonstrate second harmonic generation (SHG) in a BBO crystal producing pulse energy of 184 µJ at UV wavelength 379 nm with 47% conversion efficiency. In 2019, Munk et al presented a diode-pumped Alexandrite laser in Q-switching operation with 1.7 mJ pulse energy at pulsed pumping of 18 mJ at 500 Hz in TEM00 mode (M2 < 1.1) and high pulse-to-pulse stability of 0.2% (rms) [10]. In 2020, Coney et al achieved record highest pulse energy of 3.8 mJ at 100 Hz based on a diode-pumped Q-switched Alexandrite oscillator [11]. In 2023, Unland et al reported a CW diode-pumped cavity-dumped Q-switched Alexandrite laser with pulse energy greater than 500 µJ and pulse duration of 2.8 ns at repetition rate of 5 kHz [12]. These prior Q-switched diode-pumped Alexandrite lasers were run at fixed wavelength and were based on electro-optic (EO) Q-switching technique. The electro-optic modulator (EOM) employed in these systems was BBO which provides a large aperture, is compatible with high power and pulse energy, and has a fast rise and fall time and a high contrast ratio in Q-switching.
In this paper, we present the first study of acousto-optic (AO) Q-switched diode-pumped Alexandrite lasers with wide wavelength tunability of a high-energy mJ-class oscillator, and further show its conversion using second harmonic generation (SHG) into a tunable UV source. This work employs an acousto-optic modulator (AOM) to explore its potential as an alternative to EOM as a Q-switch device for Alexandrite. One significant advantage of an AOM is that it eliminates the high voltage driver needed for Q-switching with an EOM that can cause problems with electromagnetic interference (EMI) due to high-speed switching of high voltages, lifetime issues of the EOM, and safety issues associated with use of high voltages. An AOM can have lower insertion loss since it has just two AR-coated end faces, whereas a BBO EOM is often operated with additional cell windows as BBO is hygroscopic. The BBO may contain a pair of crystals to operate at lower quarter-wave voltages and need an additional cavity polarising element to provide the loss mechanism. The AOM cell can be smaller size and does not need additional polarising cavity element(s), allowing integration into a more compact laser device. The AOM can be lower cost and operated at very high repetition rates with a simple RF driver. The AOM has lower contrast ratio and switching speed than an EOM, but since Alexandrite is a low gain medium, high extinction ratios are not critical and low cavity insertion loss of the Q-switching system is a much more important issue for the efficiency of the laser.
In the study of this paper, we employ a low insertion loss acousto-optic modulator (AOM) for Q-switching of diode-pumped Alexandrite lasers. With continuous-wave (CW) diode-pumping, AO Q-switching of Alexandrite has been investigated up to 10 kHz repetition rate and we demonstrate pulse energy of 700 µJ at 1 kHz. The laser was wavelength tuned from 734 to 783 nm at repetition rate of 5 kHz (limited by the birefringent tuning element employed). With pulsed diode-pumping in a double-pass configuration, higher pulse energy of 2.6 mJ was achieved. An extended wavelength tuning of this system was demonstrated from 719 to 787 nm. Using nonlinear conversion of this fundamental Alexandrite laser cavity in an LBO crystal, UV pulse energy of 0.66 mJ was generated with 33% SHG conversion efficiency, and with UV tuning from 361 to 391 nm. To our knowledge, this is the first demonstration of a widely wavelength tunable, mJ-class diode-pumped Alexandrite laser at both its fundamental near-IR band and its second harmonic UV band.
2. Acousto-optic (AO) Q-switched alexandrite lasers
In this study, we employed two designs of acousto-optic (AO) Q-switched Alexandrite lasers: the first used single end-pumping with linear-polarisation from a diode-pump operating in continuous-wave (CW) mode; the second used a design to enable double-pass pumping with an unpolarised diode-pump operating in pulsed quasi continuous (QCW) mode. Initially the simple linear cavity with single-end CW pumping showed the basic AO Q-switched operation on the Alexandrite laser. Later, the double-pass pulsed diode pumped Alexandrite laser demonstrated further scaling of the laser output energy. Both systems used the same pump source, Alexandrite crystal, and acousto-optic modulator (AOM) for Q-switching.
The pump source is a fibre-delivered red-diode laser module (core diameter 200 µm / 0.22 NA) with 35 W output power centred at ∼636 nm and unpolarised. Output of the fibre was collimated with an f = 35 mm lens and subsequently focused by a second lens into the Alexandrite laser crystal. The laser gain medium is a 0.2 at.% chromium-doped Alexandrite crystal with dimension of 4 × 4 × 6 mm3. It was embedded in a copper heat sink with water-cooling for heat removal and temperature control. The AOM used for Q-switching is a crystal quartz cell with 20 mm length. It had > 99.5% transmission at 760 nm and operating with an RF driver with 80 MHz frequency and 15 W RF power. The insertion loss of the AOM is < 1% in double-pass. We measured the diffraction efficiency of the AOM module for both vertical polarisation and horizontal polarisation to be 66% and 29%, respectively at the 15 W RF power. The quoted diffraction efficiency of the AOM cell at 20 W RF power was 91% for vertical polarisation.
2.1 AO Q-switched alexandrite laser with single-end CW diode pumping
Figure 1 shows the Alexandrite laser with continuous-wave (CW) single-end diode-pumping in a compact linear cavity. A polariser placed in the path of the unpolarised pump beam was used to split the pump into s- and p-polarised beams. The transmitted p-polarised red diode pump light was used to pump the Alexandrite crystal. To match the high absorption b-axis of the Alexandrite crystal, the pump polarisation was optimised with a half-wave plate (HWP). An fp = 79 mm lens was used to focus the pump beam onto the laser crystal. The linear laser cavity was comprised of a back mirror (BM) with high reflection at laser wavelength (∼ 760 nm) and high transmission at pump wavelength (∼636 nm) and an output coupler (OC) with 98.75% reflectivity at 760 nm. The Alexandrite crystal was located next to the back mirror BM followed by the AOM and a 1-mm thick quartz birefringent filter (BiFi) plate for wavelength tuning. Initial work was with a compact cavity with 39 mm length without BiFi and later with its addition in a slightly extended cavity length.
Fig. 1. Schematic of compact AO Q-switched laser with single-end diode pumping with a linearly polarised CW pump beam.
In the initial compact cavity, the temperature of the Alexandrite crystal was controlled at 30 °C. In this diode-pumping configuration, AO Q-switching performance was studied between 1 - 10 kHz repetition rate. Figure 2 shows laser output pulse energy against absorbed pump power at repetition rates of 1 kHz, 5 kHz, and 10 kHz. The best pulse energy performance was at 1 kHz pulse rate where 660µJ of pulse energy was achieved and with pulse duration 81 ns for 16.3 W absorbed pump power.
Fig. 2. Pulse energy as function of absorbed pump power, at 1 kHz, 5kHz and 10 kHz repetition rate. Alexandrite crystal temperature = 30 °C.
Improved performance in the output pulse energy at 1kHz was achieved by reducing the temperature of the Alexandrite crystal to 16 °C. Figure 3(a) shows output pulse energy and pulse duration against absorbed pump power at this lower temperature. Output pulse energy increased to 700 µJ while maintain similar pulse duration at 1kHz. This is in good agreement with previously observed behaviour from operating Alexandrite lasers at lower temperatures at low pulse rates below the inverse upper-state lifetime, taking advantage of the longer upper-state lifetime in Alexandrite as temperature is decreased for better energy storage at the lower pulse rate of 1 kHz [13]. Figure 3 (b) shows the spectrum of the AO Q-switched laser. It is seen to have a very broad bandwidth ∼20 nm. The beam quality was measured as Mx2= 3.2, My2 = 4.1 but was not optimised for spatial quality in this compact cavity.
Fig. 3. (a) Output pulse energy and pulse duration as functions of absorbed pump power at 1 kHz with CW diode-pumping and Alexandrite crystal at 16 °C. (b) Output laser spectrum.
The broad bandwidth of the Q-switched Alexandrite laser is not beneficial for second harmonic generation. To narrow the laser spectrum and tune the output wavelength, the compact linear laser cavity was extended, and a 1 mm-thick BiFi plate was inserted in the laser cavity between the AOM and the OC. Figure 4(a) shows the laser linewidth with the BiFi was reduced to ∼1.4 nm, as measured with a spectrometer with ∼ 1.0 nm resolution. Figure 4(b) shows the wavelength tuning curve of the Q-switched laser oscillator at 5 kHz extending from 734 to 783 nm. The tuning range was solely limited by the free-spectral range of the BiFi used, as can be seen by the abrupt cut-off of tuning at 783 nm, despite the high output pulse energy.
Fig. 4. (a) Laser spectrum and (b) wavelength tuning of CW-pumped AO Q-switched Alexandrite laser at 5 kHz repetition rate.
2.2 AO Q-switched alexandrite laser with double-pass pulsed diode pumping
In this section, the single-end-pumped AO Q-switched Alexandrite laser with CW pumping is converted into a pulsed pumped Alexandrite laser with dual-end double-pass diode-pumping of the full unpolarised power of the pump laser to generate higher pulse energy.
The laser system is shown in Fig. 5. The AO Q-switched Alexandrite laser has an L-shaped laser cavity with a 45° turning mirror (TM) that was highly reflective (HR) for 760 nm and highly transmitted (HT) for 636 nm, to facilitate the double-pass pumping. In the first pass, the pump polarisation parallel to the Alexandrite b-axis is predominantly absorbed. The unabsorbed orthogonal pump polarisation is flipped to the high absorbing b-axis of the Alexandrite by double passing a quarter waveplate (QWP) and is refocused onto the rear face of the laser crystal with a curved mirror (CM) with radius of curvature of 100 mm. The double-pass pumping provides pumping with the full available pump power using both states of pump polarisation. The double-pass dual end-pumping also divides the pump-induced inversion between the two ends of the Alexandrite crystal. This distributes the inversion in the laser gain medium and diminishes the strength of excited state absorption (ESA) that depends on inversion [14], thereby reducing ESA loss and enhancing laser efficiency [15].
Fig. 5. Schematic of AO Q-switched Alexandrite laser with double-pass end pumping of unpolarized pump beam.
The L-shaped laser cavity had an overall physical length 67 mm, longer than the linear cavity in section 2.1, which enlarged fundamental laser mode size to ∼ 219 µm. In addition, the pump lens was changed from fp =79 mm to 125.4 mm. The weaker pump lens provided a larger pump radius ∼ 358 µm. Cavity design or pump lenses with different focal lengths could be investigated to provide a better matching between the laser mode and the pump size. To increase the pump energy the diode pump laser was operated in a quasi-CW pumped mode rather than the CW mode allowing use of higher currents to the diode module. The Alexandrite crystal was kept at 16 °C. The OC had 97.6% reflectivity at ∼760 nm, which provided higher output coupling than the CW cavity for optimised efficiency and reducing the risk of laser-induced damage to cavity optics at the higher pulse energies produced in this laser cavity.
Figure 6 shows pulse energy under free-running conditions as a function of pump energy with the pump repetition rate of 100 Hz and pulse width of 500 µs. A free-running output pulse energy of 6.57 mJ was obtained at pump energy of 27.2 mJ. The laser slope efficiency was 33.4%.
Figure 7(a) shows the results of output pulse energy and pulse duration under AO Q-switching operation without inclusion of the BiFi tuning element. Pulse energy of 2.6 mJ and pulse duration of 75 ns have been achieved at absorbed pump energy of 27.2 mJ with beam quality of Mx2 = 2.32 and My2 = 2.16. Figure 7(b) shows the output pulse spectrum at maximum pumping. It had broad ∼10 nm laser bandwidth under Q-switched operation.
Fig. 7. (a) Pulse energy and duration as functions of pump energy. (b) Laser spectrum at maximum pump energy.
The L-shaped laser cavity was extended slightly to insert a 0.5 mm-thick BiFi plate in the cavity to narrow the laser spectrum and achieve wavelength tuning. Figure 8(a) shows pulse energy and pulse duration against pump energy. The maximum output pulse energy was 2.01 mJ and output pulse duration 115 ns. The inset in the figure shows the excellent spatial output mode quality at maximum energy with beam propagation factor M2∼1.05. Figure 8(b) shows the narrowed laser spectrum with bandwidth < 1.5 nm (at resolution-limit of spectrometer used). Figure 8(c) shows wavelength tuning of pulse energy as a function of wavelength. Wavelength tuning range across 68 nm was achieved from 719 to 787 nm. To our knowledge, this is the widest tunability of range demonstrated in Q-switched diode-pump Alexandrite lasers, exceeded the 61 nm tuning range demonstrated in EO Q-switched Alexandrite lasers [9].
Fig. 8. Performance of pulse pumped AO Q-switched Alexandrite laser with BiFi tuning plate: (a) Pulse energy and duration as a function of pump energy. Inset: Output beam spatial profile; (b) Laser spectrum at 27.2 mJ absorbed pump energy; and (c) Wavelength tuning.
2.3 Second harmonic generation (SHG) to produce tunable UV output
Good wavelength tuning range, good energy storage, and direct second harmonic conversion to the ultraviolet (UV) make diode-pumped Q-switched Alexandrite lasers attractive for applications requiring tunable UV high energy pulses. An investigation was made of converting the tunable Q-switch fundamental Alexandrite laser into a tunable UV source.
Figure 9 shows the investigated UV conversion system using the fundamental tunable Q-switched Alexandrite laser to generate tunable UV by extra-cavity second harmonic generation. The output of the fundamental AO Q-switched laser was focusing with a plano-convex lens with focal length f = 30 mm into an LBO second harmonic generation (SHG) crystal. The LBO crystal had dimensions of 4 × 4 × 10 mm3 and its crystal cut was θ = 90°, φ = 35.9° for Type-I phase matching. The LBO crystal was at room temperature without temperature control, and during the scanning of the fundamental wavelength, angle-tuning of the LBO crystal was used to maintain phase-matching. The generated UV beam was separated from the fundamental beam using a dichroic mirror and sent to diagnostics for measuring pulse energy, spatial profile, and spectrum.
Figure 10 shows output UV pulse energy and conversion efficiency against input fundamental pulse energy. High energy tunable UV of 0.66 mJ with 32.9% conversion efficiency at 375 nm has been achieved.
Figure 11(a) shows UV spectrum with ∼1.2 nm bandwidth, which is at the resolution-limit of the spectrometer used. Figure 11(b) shows pulse energy as a function of the UV tuned wavelength. For the frequency-doubled laser, the tuning wavelength was from 361 to 391 nm with ∼30 nm wavelength range. The figure also shows the corresponding pulse energy of the fundamental beam tuned from 719 to 787 nm with ∼68 nm tuning range.
Fig. 11. (a) UV spectrum. (b) Tuning of the UV second harmonic pulse energy as function of tuning wavelength. For comparison the fundamental wavelength tuning of the AO Q-switched Alexandrite laser is also shown.
The broad wavelength tuning of the UV pulse has been achieved with the widely tunable fundamental beam. However, it is noted that the system is unoptimised for spectral coverage due to the non-optimal spectrum of the optical coatings for AR-coated surfaces and cavity mirror reflectance which were optimised near ∼ 760 nm, but performance will degrade at edges of Alexandrite spectrum. Furthermore, increasing Alexandrite crystal temperature increases gain cross-section and favours operation at the longer wavelength to beyond 800 nm, whilst lower temperature reduces ground-state absorption to enhance performance at shorter wavelengths approaching 700 nm [21]. Hence further improvements can be expected for extending spectral coverage of fundamental tuning range (and correspondingly at second harmonic UV tuning range) with better optimised coatings of cavity elements and suitable use of crystal temperature tuning.
3. Conclusions
AO Q-switched Alexandrite lasers with CW and pulsed pumping have been demonstrated. We have produced AO Q-switched lasers with CW pumping at repetition rates up to 10 kHz. The best energy performance of Q-switched laser of 700 µJ pulse energy and 82 ns pulse duration has been obtained at 1 kHz. The CW-pumped AO Q-switched Alexandrite has been tuned from 734 to 783 nm, with tuning range limited by the thickness of the BiFi tuning plate used. A laser design with double-pass QCW diode pumping with unpolarised pump light was employed for pulse energy scaling. The QCW-pumped AO Q-switched laser has achieved pulse energy of 2.6 mJ and pulse duration of 75 ns at 100 Hz repetition rate. With BiFi tuning element the broad 10 nm output laser spectrum of the cavity was narrowed to ∼ 1.5 nm, > 2 mJ pulse energy and tuning range from 719 to 787 nm with high spatial beam quality ${\textrm{M}^2} = 1.05$. Using this fundamental pulsed laser system, high energy tunable UV of 0.66 mJ has been produced in a Type-I LBO second harmonic generation (SHG) crystal, with 32.9% conversion efficiency. Wavelength tuning of the UV output was from 361 to 391 nm with tuning range of 30 nm. The efficiency of the UV Alexandrite laser is competitive with UV lasers generated by the more mature technology of Q-switched Nd:YAG lasers with third harmonic generation at 355 nm where conversion is usually much less than 30% [16–20], but a key advantage of Alexandrite UV is its wavelength tunability.
This demonstration of a high energy tunable diode-pumped Alexandrite laser shows its excellent potential as a wavelength tunable laser source in the near-IR and UV wavelength ranges. The investigation of the use of an AOM shows it is a practical device for high energy Q-switching of the Alexandrite laser, with compact size, lower insertion loss, and elimination of high voltages compared to using EOM devices for Q-switching. Broader tuning range of diode-pumped Alexandrite is expected with more control of cavity optic coatings and the appropriate use of temperature tuning of the Alexandrite crystal. The performance of high pulse energy AO Q-switched Alexandrite laser is suitable for photoacoustic deep-body imaging of the vascular system. By using dual or multiple wavelengths across the fundamental Alexandrite wavelength range would allow discrimination of oxy- and de-oxy Hb providing information on blood oxygenation, tissue health and early diagnosis of cancer [21,22]. The high energy tunable fundamental and UV output can be a general source for other applications including remote sensing for altimetry and vegetation and atmospheric monitoring [23]. Using both UV and residual fundamental output simultaneously gives the opportunity for dual-wavelength lidar systems providing enhanced sensing information.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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