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Abstract
We report on developing three flashlamp-pumped electro-optically Q-switched Cr:Er:YSGG lasers with the Q-switch based on a La3Ga5SiO14 crystal. The “short” laser cavity was optimized for high peak power applications. In this cavity, 300 mJ output energy in 15 ns pulses at a 3 Hz repetition rate was demonstrated with pump energy below 52 J. However, several applications, such as Fe:ZnSe pumping in a gain-switched regime, require longer (∼ 100 ns) pump pulse duration. We developed a 2.9 m long laser cavity that delivers 190 mJ of output energy in 85 ns pulses for these applications. We also demonstrated the Cr:Er:YSGG MOPA system producing 350 mJ output energy at 90 ns pulse duration and 47.5 J of pumping, corresponding to an amplification factor of 3.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
High energy 3-µm laser systems are attractive for many scientific, medical, and biological applications due to strong water absorption around this wavelength [1,2]. Another application of 3µm Q-switched lasers is pumping optical parametric oscillators/generators [3]. Finally, in recent years, there has been increasing interest in these laser systems as a pump source for room temperature (RT) mid-IR solid-state lasers based on iron-doped II-VI crystals [4] featuring tunability over the 3.5-6.9 µm spectral range.
However, despite the strong demand for many research and industrial applications and the long history of 3 µm Er3+ lasers at the 4I11/2 → 4I13/2 transition, their reliable operation in the actively Q-switched regime is still under development. The laser systems’ major limitation is the lack of adequate and high optical damage middle-infrared electro-optical (EO) or acousto-optical (AO) materials. In addition, operation in the mid-IR spectral range requires higher applied voltage to EO Q-switch or higher AO Q-switch operational RF power than for near-IR lasers. The large Stokes shift between the flashlamp pump and oscillation wavelengths results in the propagating beams’ strong thermal lens formation and depolarization. The overlap of the lasing wavelength with the intracavity elements’ hydroxyl group absorption makes the requirements for the intracavity materials and antireflection coatings more challenging. The small emission cross-section of erbium ions at this transition limits the application of these gain elements in single-pass amplifiers. As a result, the development of high energy Q-switched Er3+ lasers at 4I11/2 → 4I13/2 transition is still in progress, and requirements for high optical damage threshold of intracavity optical elements represent significant challenges. In addition to these issues, the cross-relaxation and excited state absorption processes in Er3+ ions, being strongly dependent on ion concentration and crystalline hosts, make optimization of Q-switched Er3+ lasers a multi-parameter task with an optimum solution strongly dependent on the required output characteristics.
Recently, we reported a flashlamp-pumped mechanically Q-switched 2.94 µm Er:YAG laser based on a spinning mirror with the highest output energy of 0.8 J at a pulse duration of 61 ns [5]. This approach significantly mitigates the influence of thermal depolarization and problems with optical damage. However, many practical applications require external triggering with a small timing jitter which could be difficult to achieve with mechanically based approaches such as spinning mirrors or frustrated total internal reflection. Therefore, developing laser systems with active EO and AO Q-switching is still of great interest. Unfortunately, few EO materials are capable of operating around the 3 µm spectral range. Previous advances with EO Q-switched 3 µm Er3+ systems were mainly based on lithium niobate LiNbO3 (LNO) crystals which have a relatively low optical damage threshold [6]. The output energy of 226 mJ in pulses with 62 ns duration was reported for an Er:YAG laser with LNO Q- switch [6]. However, the lifetime of high-output energy laser systems based on LNO Pockels Cells is limited due to LNO's optical damage accumulation effect [7,8]. Recently, a langasite La3Ga5SiO14 (LGS) crystal was suggested as a promising alternative for EO Q-switched 2.79 µm Cr:Er:YSGG lasers [9,10]. The spectroscopic parameters of LNO and LGS materials are summarized in Table 1. Comparative damage thresholds of LNO and LGS were measured in [11] under 1.064 µm, 10 ns pulsed excitation.
Table 1. Spectroscopic properties of LiNbO3 and La3Ga5SiO14 nonlinear materials
The voltage required for operation in $\lambda /4$ regime depends on the electro-optic coefficient and refractive index as [12]
where λ is oscillation wavelength, n is the refractive index, ${r_{ij}}$ is the electro-optic coefficient, and $L/d\; $ is the crystal aspect ratio. While LNO crystals feature better electro-optical parameters, LGS crystals feature a high optical damage threshold (∼750 MW/cm2 at 2.79 µm [13]), a crucial advantage of LGS material. LGS transmission spectrum is reported in [13]. LGS features high transparency over the 0.5-5 µm spectral range with absorption peaks at 1.85 and 3 µm due to oxygen defects and Ga-O bond vibrations, respectively. We measured the absorption of AR coated 6 × 6x45 mm LGS (DIAN Tech, PRC), which was ∼5% at 2.79 µm. It was demonstrated that LGS EO Q-switch in Cr:Er:YSGG laser cavities can operate in the pulse-off mode at voltages below 5 kV. A Cr:Er:YSGG laser oscillation with an LGS EO Q-switch was reported with an output energy of 216 mJ in 14.4 ns pulses with a flashlamp pumping with the energy of 150J [9]. This pump energy level requires using an intracavity quarter-wave plate to compensate for thermally induced birefringence in Cr:Er:YSGG crystals.The most commonly used crystalline hosts for 3-µm Er3+ lasers are YAG, YSGG, and YAP crystals. A comparison between some spectroscopic properties of these materials hosts reported in [14–17] is summarized in Table 2.
Table 2. Spectroscopic properties of Er3+ ions at 4I11/2 → 4I13/2 transition in YSGG and YAG hosts
Compared to Er:YAG and Er:YAP, Er:YSGG crystals feature better energy storage capabilities due to a much longer Er3+ upper laser level lifetime and a comparable lifetime of the lower laser level. In addition, LGS crystals have an absorption band of around 3.0 µm; therefore, the laser media operating at shorter wavelengths (i.e., Er:YSGG and Er:YAP) are preferable. YAP crystal is biaxial and has strong, difficult-to-compensate thermal distortion under flashlamp pumping. Chromium co-doped Er:YSGG lasers feature a low threshold and high efficiency of lasing under flashlamp excitation due to a good overlap of the flashlamp emission and broad absorption bands of Cr3+ as well as effective Cr3+-Er3+ energy transfer [13]. High efficiency of Cr:Er:YSGG lasing is reinforced by using an efficient MegaWatt Lasers commercial pump chamber that enables the use of much lower pump energies than for Er:YAG and Er:YAP, which results in potentially manageable thermal birefringence in Cr:Er:YSGG crystals. Therefore, the combination of Cr:Er:YSGG and LGS crystals for flashlamp-pumped Q-switched lasers is the most attractive and was studied in our work.
2. Experimental results and discussion
The experimental setup is depicted in Fig. 1. The plane-plane cavity of a 36 cm length consisted of a high reflector (HR), 40% reflective output coupler (OC), and AR coated Cr:Er:YSGG rod (Ø 4 × 100 mm, DIEN Tech, PRC) doped with 3 at% Cr3+ and 30 at% Er3+ mounted in a MegaWatt Lasers commercial elliptical pump chamber model 4X100C2. This pump chamber utilizes a close-coupled, BaSO4 diffuse reflector design for pumping uniformity and high efficiency. The flow tube is UV-fused silica, and the rod-to-flashlamp centerline spacing is 7.5 mm. The flashlamp is a M201 model with a 4 mm bore, 90 mm arclength, and 450 torr Xenon fill. A z-cut LGS EO Q-switch (6 × 6x45 mm, DIAN Tech, PRC) operated in a pulse-off mode, and the applied voltage was ∼5 kV (smaller than∼ 6 kV quarter-wave voltage). Three 1-mm-thick Al2O3 plates placed at Brewster's angle between the LGS and Cr:Er:YSGG crystals formed a polarizer. The KALD flashlamp driver provided pump energy up to 60 J at a pulse duration of 300 µs. The experiments were performed at a 3 Hz repetition rate. Thermal depolarization in the gain medium was compensated by rotation of the LGS EO Q-switch about its axis. A single Q-switch pulse was obtained with up to 32.5 J of pump energy using three Al2O3 Brewster plates. We used an additional 1-mm-thick YAG Brewster plate placed between Cr:Er:YSGG rod and the OC to enable a single Q-switch pulse with pump energies up to a maximum of 52.5 J.
Figure 2(A) depicts input-output characteristics of the Cr:Er:YSGG laser in free-running (blue triangles, curve i) and Q-switched (black squares curve ii) regimes with Al2O3 polarizers and pump energies up to 32.5 J. The slope efficiency in the Q-switched regime reached 1.2%. The Q-switched-to-free-running-mode extraction efficiency was close to 100%. The maximum energy was 210 mJ at 15 ns pulse duration, corresponding to 14 MW peak power. A further increase in pump energy resulted in the appearance of a combined Q-switched and free-running spiked regime. We added a 1-mm-thick YAG Brewster plate polarizer near the output coupler to avoid this regime. The laser crystal could then be pumped up to 53 J in this cavity without free-running spikes. Figure 2(A) (curve iii) also shows the input-output characteristic of the Cr:Er:YSGG laser in a Q-switched regime measured up to 52.5 J of pump energy with the additional YAG Brewster plate polarizer. Due to minor damage to the Cr:Er:YSGG crystal coating, the slope efficiency decreased to 0.85%. Still, we could pump the crystal up to 52.5 J and obtain an output energy of 300 mJ at 15 ns pulse duration and 20 MW peak power without degradation and roll-off of the output energy.
Fig. 2. A) Output energy as a function of pump energy for the free running regime (curve i), Q-switched regime with three Brewster polarizers (curve ii), and four Brewster polarizers (curve (iii); B) Pulse duration as a function of output energy in the cavity with four Brewster polarizers (curve iv) and three Brewster polarizers (curve v).
Figure 2(B) shows the pulse duration dependence versus output energy in the cavity with three Brewster polarizers (curve iv) and four Brewster polarizers (curve v). As one can see from the plots, the pulse duration decreases with the increase of the pump energy from ∼110 ns near the threshold to ∼ 15 ns at 52.5 J of pumping. The maximum output power of 20 MW in the Q-switched oscillation regime was achieved in 15 ns pulse duration.
Figure 3 depicts the spatial profile of the output beam measured at 210 mJ and 300 mJ output energies. The beam has an almost flat-top profile, practically without hot spots.
As mentioned in the introduction, one of the important applications of Q-switched Cr:Er:YSGG laser is gain-switched Fe:ZnSe lasers’ pumping. This application requires a pulse duration of ∼100 ns. A shorter pump pulse duration reduces Fe:ZnSe laser efficiency and could result in optical damage to the gain element. The effect of laser efficiency growth with the increase of the pulse duration is well known for dye and color center gain-switched lasers [18]. It is due to bleaching of the active absorption and low conversion of pump radiation during initial oscillation build-up time. As it was reported for many transparent materials, as well as our experience in the development of high energy Fe:ZnSe lasers [19,20], the optical threshold fluence [J/cm2] follows τ0.5 scaling factor (where, τ is pulse duration in ns time scale) [21]. Therefore, developing high energy Cr:Er:YSGG lasers with ∼100 ns pulse duration will enable significant energy scaling of mid-IR room temperature Fe:ZnSe lasers with respect to “short” 15 ns pumping. To realize Fe:ZnSe pump system with a long pulse duration, the Cr:Er:YSGG laser cavity with a total cavity length of 291 cm was developed (see Fig. 4). In this experiment, we used the same intracavity elements as described before. In addition, we used two uncoated 50 cm focal length CaF2 lenses placed 50 cm from the HR and OC. We folded the cavity using two flat HR mirrors to reduce the laser footprint.
Figure 5(A) depicts input-output characteristics of the Cr:Er:YSGG laser in free-running (triangles) and Q-switched (circles) regimes with Al2O3 polarizers and pump energies up to 57.5 J. The output energy in the free-running regime reached 350 mJ at 58 J pump energy. The slope efficiency in this cavity was lower than in previous experiments. However, due to the efficient and uniform pump chamber, additional intracavity lenses could compensate for thermal lensing in the Cr:Er:YSGG. The output energy in the Q-switched regime was close to that of the free-running regime at pump energies below ∼35 J and the plot shows roll-off for greater pump energy due to depolarization losses. However, the output energy of 190 mJ in the Q-switched regime was reached at 58 J pump energy. Further increases in the pump energy resulted in the appearance of a combined Q-switched and free-running spiked regime.
Fig. 5. A) Output energy as a function of pump energy for free running (triangles) and Q-switched (circles) regimes; B) Pulse duration as a function of output energy.
Figure 5(B) shows the dependency of the pulse duration on the output energy. As can be seen, the pulse duration in the long cavity decreases with the increase of the pump energy from ∼160 ns near the threshold to ∼ 85 ns at 58 J of pump energy. Figure 6 depicts the spatial profile of the output beam measured at 116 mJ of output energy. Compared to the short cavity, it has a smoother flat-top profile without hot spots. This oscillation regime is more optimal for the pumping of gain-switched lasers in comparison with more efficient oscillation in the “short” cavity.
As a next step, we fixed the output energy of the long-cavity master oscillator at 116 mJ, corresponding to ∼ 90 ns pulse duration, and, as shown in Fig. 4, used it as a seed for Cr:Er:YSGG energy amplifier based on ϕ5 × 100 mm rod which was placed at a distance of 40 cm from the masters’ output coupler. The pulse duration of the MOPA output coincided with the 90 ns seed pulse duration for the used 18-47.5 J range of pump energies. The output radiation has a flat-top profile without hot spots. The plot of output energy of Cr:Er:YSGG amplifier seeded by 116 mJ & 90 ns pulses from 2.9 m long master oscillator as a function of amplifier pump energy is depicted in Fig. 7. The highest output energy was 350 mJ at 47.5 J of pumping, corresponding to the amplification factor of ∼ 3 with respect to the seed energy.
Fig. 7. Output energy of Cr:Er:YSGG amplifier seeded by 116 mJ & 90 ns pulses from 2.9 m long master oscillator as a function of amplifier pump energy.
3. Conclusions
Our major objective was to answer whether electro-optically Q-switched Cr:Er:YSGG is an adequate pump platform for energy-scaled room-temperature Fe:ZnSe gain-switched systems. Our manuscript describes three major steps in answering this question. In the first step, we proved that the use of the modern pump chambers from MegaWatt Lasers enables high output energy Cr:Er:YSGG master oscillator 300 mJ output energy at 15 ns pulse duration at a 3 Hz repetition rate, which is, to the best of our knowledge, the record result documented to date at 2.79 µm. In the second step, we demonstrated the first long cavity master oscillator featuring 190 mJ output energy, the uniform flat-top spatial profile of the beam, and adequate for pumping Fe:ZnSe systems with 85 ns pulse duration. In the third step, we demonstrated a long pulse (90 ns) 350 mJ Cr:Er:YSGG MOPA with an amplification factor of 3, finalizing a positive answer to the major question. Further improvements in the AR coating of the gain element, efficiency of the pump chamber, and laser performance over a wide range of repetition rates are planned for the near future.
Funding
U.S. Department of Energy (DE-SC0018378); National Institute of Environmental Health Sciences (P42ES027723).
Disclosures
DM, VF, and SM declare no conflicts of interest. SH: MegaWatt Lasers (I,E).
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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