We report on the output performances of a fiber-coupled diode pumped Nd:LGSB laser passively Q-switched by a Cr4+:YAG saturable absorber (SA), with emissions at 1.06-μm near-infrared wavelength, as well as at 0.53-μm visible green spectrum through the self-frequency doubling process. The 2.3-at.% Nd:LGSB crystal delivered 1.43 W continuous-wave power at 1.06 μm with an overall optical-to-optical efficiency (with respect to the absorbed pump power) of 0.50. Green light with 3.3-mW power was emitted in a free-generation regime at 1.95-W absorbed pump power. By employing a Cr4+:YAG SA crystal with initial transmission Ti = 0.89, laser pulses at 1.06 μm with 13.4-μJ energy at 17.1-kHz repetition rate were obtained for ∼1.8 W absorbed pump power. The green laser pulse energy was 0.21 μJ at a 37.7-kHz repetition rate. The pulse duration amounted to 4.8 ns for both wavelengths of emission, indicating pulse peak power of 2.8 kW and 43.7 W for operation at 1.06 μm and 0.53 μm, respectively.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Nowadays, numerous applications including information processing and displays, medicine and biology, gas-trace analysis, underwater communications, as well as lighting, pointing or alignment, require compact and efficient visible (VIS) laser sources [1–4]. Often, a VIS laser is obtained by the use of nonlinear crystals that convert the near infrared (NIR) laser emission of different Nd3+ or Yb3+ doped crystals by means of second order nonlinear processes. Higher compactness of the laser device can be achieved by using Nd3+ or Yb3+ activated crystalline hosts possessing second order nonlinearities, thus providing both NIR fundamental emission and VIS radiation from a single bifunctional medium [5–7]. The advantages of using bifunctional crystals rely on reduced number of surfaces inside the resonator, simplified alignment procedure, or lower cost. Along with these benefits, a bifunctional crystal has to be highly efficient and should be easily to be manufactured at large scale, in order to become commercially available.
Various researches were focused on finding the best bifunctional crystal, simultaneously satisfying the requirements for efficient laser action in the NIR range and in the corresponding VIS range generated through self-frequency conversion process. First self-frequency-doubling process, at 926.6 nm, was observed in Tm:LiNbO3 . Since then, much work has been done especially on Nd- or Yb-doped self-frequency doubling (SFD) crystals. In particular, for green spectral region the most well-known Nd-doped SFD crystals are Nd:MgO:LiNbO3 [9–11], NdxY1-xAl3(BO3)4 (Nd:YAB) [1,12–15] and Nd,Lu:YAl3(BO3)4 (Nd,Lu:YAB) , as well as Nd:YCa4O(BO3)3 (Nd:YCOB) [2,17] and Nd3+:GdCa4O(BO3)3 (Nd:GdCOB) [3,4,18–20]. Thus, a diode-pumped SFD Nd:YAB laser emitting 50 mW output power at 531 nm with nearly 4% optical-to-optical efficiency was realized by Hemmati in 1992 . Furthermore, a SFD Nd:YAB laser generating 225-mW output power in green, at 531 nm wavelength, under the pump with 1.6 W at 807 nm, was reported by Bartschke et al. in 1997 . Wang et al. have developed, in 2000, a SFD Nd:GdCOB laser emitting 225 mW power at 530.5 nm with 14.4% optical conversion efficiency . In addition, a SFD Nd:GdCOB laser yielding 3 W power at 545 nm with 20.7% optical efficiency was reported by Yu et al. in 2011 . Recently, in 2020, Du et al. have obtained a record output power of 17.9 W at 545.5 nm from a slab Nd:GdCOB crystal, end-pumped by laser diodes at 803 nm . On the other hand, despite its good SFD properties, the growth of Nd:YAB can be achieved only by the top-seeded solution method; thus, flux inclusions often appear in the grown crystals and their size is limited. In addition, considerable thermal problems were observed in Nd:YAB crystals . In order to overcome these issues, other investigations have focused on doping YAB with Yb ions to achieve efficient green SFD operation [22,23]. Dekker et al. have realized, in 2001, a diode end-pumped SFD Yb:YAB laser emitting 1.1 W continuous-wave green power with 10% diode-to-green optical conversion efficiency . The same research group reported, in 2005, an Yb:YAB laser Q-switched by acousto-optic modulator, with 2.27 W average green power at 520-522 nm . Furthermore, Lu et al. have obtained, in 2019, high 6.2-W output power at 513 nm from a SFD Yb:YCOB crystal that was pumped at 976 nm with a fiber-coupled laser diode . A study of thermal lensing induced in Yb:YCOB crystals under lasing conditions was performed by P. Loiko et al. . Advances in the growth of self-frequency doubling laser crystals and the laser emission obtained from these crystals are available in a review paper authored by Yu et al. .
Several passively Q-switched SFD lasers were reported to date. For example, Zhao et al. have obtained, in 1998, green pulses at 0.53 μm from a SFD Nd:YAB laser that was passively Q-switched by Cr4+:YAG SA . Also, Zhang et al. demonstrated a flash-lamp-pumped SFD Nd:GdCOB green laser that was passively Q-switched with Cr4+:YAG SA , as well as by GaAs SA . More recently, in 2017, Zhang et al. have achieved pulsed 0.53-μm green emission from a SFD Nd:Ca3NbGa3Si2O14 silicate medium, employing also Cr4+:YAG SA as Q-switch . The average output power amounted to 16.2 mW; laser pulse energy and pulse duration were 7.2 μJ and 13.7 ns, respectively. Zhou et al. have developed, in 2019, a passively Q-switched SFD Nd:Ca3TaGa3Si2O14/Cr4+:YAG composite laser, delivering green pulses at 532 nm of 1.0-kW peak power (7.81-μJ pulse energy, 7.12-ns pulse duration) . These results indicate that a SFD medium in combination with a SA crystal could be a way to obtain efficient laser pulses into green visible spectrum.
Recently, Nd:LGSB bifunctional medium was proposed by our group for the green spectral range [31–33]. Nd:LGSB is isostructural to Nd:YAB, but it can be grown by the Czochralski method, having some important advantages. Thus, the Nd:LGSB nonlinear properties, such as effective nonlinear coefficient, deff∼1.1 pm/V, angular acceptance of Δθ×L = 0.03°×cm, spectral acceptance of Δλ×L = 0.79 nm × cm, or walk-off angle of ρ = 2.6° for type I second-harmonic generation (SHG) at 1.06-μm wavelength, are comparable with those of Nd:YAB [32,33]. Compared to Nd:GdCOB, the Nd:LGSB crystal presents higher absorption and emission cross-sections and two times wider emission bandwidth, which is a prerequisite for achieving tunability and short pulses in mode-locking operation. In addition, Nd:LGSB has higher symmetry than that of Nd:GdCOB (uniaxial negative as compared to biaxial negative). Also, Nd:LGSB has a wider transparency range, from 200 nm to 2500 nm as compared to that of Nd:GdCOB, with 320 nm to 2600 nm range . Furthermore, Nd:LGSB can incorporate high Nd-doping levels, being thus attractive for microchip laser devices, similar to Nd:LSB crystal . Up to now, efficient NIR laser operation at 1.06 μm was obtained from Nd:LGSB, for various laser operation regimes, including free running, passive Q-switching and passive mode-locking by using SESAM device [31,33].
In this work we report on laser operation of Nd:LGSB in free-generation regime and in pulsed operation by passive Q-switching with Cr4+:YAG SA. The experimental set-up is presented in Section 2. The characteristics of NIR at 1.06-μm emission are described in Section 3.1. The Nd:LGSB laser crystal (2.3-at.% Nd3+, cut for type I phase matching condition at 300 K) yielded 1.43 W output power with optical-to-optical efficiency (with respect to the absorbed pump power) of 0.50; the slope efficiency amounted to 0.55. Employing a Cr4+:YAG SA with initial transmission Ti = 0.89, laser pulses with energy of 13.4 μJ at 17.1-kHz repetition rate and 4.8-ns duration have been obtained. The results on VIS green at 0.53-μm light generation through SFD are given in Section 3.2. Continuous wave (CW) power of 3.3 mW for 1.95-W absorbed pump power was obtained. In Q-switch regime, green laser pulses with 0.21-μJ energy at 37.7-kHz repetition rate and 4.8-ns duration were determined. Section 4 concludes this work.
2. Experimental set-up
The experimental set-up is presented in Fig. 1. The Nd-doped La0.64Gd0.41Sc2.95(BO3)4 (Nd:LGSB) laser crystal was grown by Czochralski pulling technique, by using the procedure reported elsewhere [31–33]. The Nd:LGSB doping level was 2.3-at.% Nd3+ and its length was 3.7 mm; the crystal was cut according to the type I phase-matching condition at 300 K for SHG of 1.06-μm laser emission (θ = 35.3o, φ = 60o) and it was polished at laser grade, being used without any coatings. The laser crystal was fixed in a copper mount and kept at 22°C with re-circulating water and Peltier element.
The optical pump was done at λp = 807 nm with a fiber-coupled laser diode (LIMO GmbH, Germany); the fiber diameter was 100 μm with numerical aperture NA = 0.22. The resonator length was 10 mm. The rear mirror (HRM) was plane, being coated with high reflectivity (HR, reflectivity R>99.9%) at both laser wavelength, λω = 1.06 μm and its second harmonic, λ2ω = 0.53 μm, and with high transmission (HT, transmission T>90%) at the pump wavelength, λp. Based on availability, for NIR emission we used a plane out-coupling mirror (OCM) with transmission T = 2.4% at λω (plane-plane resonator), as well as an OCM with radius of 100 mm and T = 5% at λω (plane-concave resonator). In the case of green-light generation, the dichroic OCM was plane, being coated HR (R>99.9%) at λω and HT (T>99%) at λ2ω. Two Cr4+:YAG SA crystals with initial transmission Ti of 0.89 and 0.84 were employed for passive Q-switching; these media were coated antireflection, AR (T>99.7%) at 1.06 μm. The Nd:LGSB crystal was positioned as near as possible of HRM whereas each Cr4+:YAG SA was placed close of OCM. The pump beam was focused into the Nd:LGSB crystal by two achromatic lenses, L1 and L2. In the experiments we used two diameters of the pump beam, of 2ωp = 100 μm and of 2ωp = 150 μm, in order to obtain superior performances for the investigated configuration. Two dichroic mirrors, M1 and M2 in Fig. 1, were placed after the optical resonator, to select only the laser emission in NIR (at 1.06 μm) and the one in VIS spectrum (at 0.53 μm), respectively.
3. Experimental results and discussion
3.1 NIR laser emission characteristics
In order to investigate the laser emission characteristics at λω = 1.06 μm, the Cr4+:YAG SA (Fig. 1) was removed from the resonator. Figure 2 presents the CW output power, Pout versus the absorbed pump power, Pabs at 807 nm. In the case of the pump with a strongly focused beam (2ωp = 100 μm) and the OCM with T = 5%, the Nd:LGSB crystal emitted the power Pout = 0.67 W at Pabs = 2.3 W, which corresponds to an overall optical-to-optical efficiency (with respect to Pabs) of ηoa = 0.29. The slope efficiency amounted to ηsa = 0.33. The pump beam absorption efficiency in Nd:LGSB was ηoa = 0.64. For the OCM with T = 2.4%, an improvement of the slope was obtained, ηoa = 0.35, but a saturation of emission for Pabs in excess of ∼2.0 W is observed, most probably due to the thermal effects induced in Nd:LGSB.
The pump done with a beam of diameter 2ωp = 150 μm enabled improvements of the laser power. Thus, for the OCM with T = 2.4%, the output power increased to Pout = 1.15 W (for Pabs = 2.83 W); the slope efficiency was ηoa = 0.44. The OCM with T = 5% allowed to further rise the power at Pout = 1.43 W, with optical efficiency ηoa = 0.50, while the laser operated with an improved slope of ηoa = 0.55. A Findlay-Clay analysis  of the pump power at threshold was performed, considering several plane OCM with T between 1% and 5%. The round-trip resonator residual loss was determined to be Li∼2%, thus proving the good optical quality of Nd:LGSB crystal. A Glan-Taylor calcite polarizer with 100.000:1 extinction ratio was used to determine the laser beam polarization. It was found that the beam was linearly polarized, with a polarization ratio better than 85:1 on the entire output power range.
The differences between the laser performances obtained with the two pump beams are related to the thermal effects induced in Nd:LGSB and the resonator used in the experiments. The focal length, fth of the thermal lens induced by optical pumping in Nd:LGSB was estimated based on the model developed by Innocenzi et al. . In calculus, the thermal conductivity, Kc = 0.028 Wcm-1K-1 and the temperature dependent coefficient of the index of refraction, dn/dT = 4.4 × 10−6 K-1 were considered similarly to Nd:LSB . It was concluded that for the pump with the beam focussed at 2ωp = 100 μm, the Nd:LGSB thermal lens has fth∼21 mm focal length at Pabs = 2 W. For the same Pabs, calculus indicated that the pump beam with 2ωp = 150 μm induces in Nd:LGSB a thermal lens with longer, fth∼47 mm focal length. Next, the laser beam size in the laser crystal was evaluated using ABCD resonator modelling of Paraxia Plus Software. The simulations showed that in the plane-plane resonator the diameter of the laser beam reduces from 160 μm for fth = 47 mm to 140 μm for fth = 21 mm. On the other hand, in the plane-concave resonator, the diameter of the laser beam in Nd:LGSB was determined to be 150 μm for fth = 47 mm and 130 μm for fth = 21 mm. These data confirm a better overlap between the laser beam and the pump beam with 2ωp = 150 μm, for both types of resonator, thus partially explaining the experimental results (Fig. 2).
Pulsed laser operation was achieved by inserting a Cr4+:YAG SA in the resonator, close of OCM (Fig. 1). The laser pulse characteristics obtained with the Cr4+:YAG SA having initial transmission Ti = 0.89 and the OCM with T = 2.4% are given in Fig. 3. When the pump was done with the beam focused at 2ωp = 100 μm, the laser delivered the average output power Pave = 116 mW for absorbed pump power Pabs = 2.3 W [Fig. 3(a)]. As shown in Fig. 3(b), the laser pulse repetition rate increased linearly with Pabs; however, small deviations from this relationship were observed at the maximum Pabs, these being attributed mainly to the thermal effects induced in the Nd:LGSB crystal. For Pabs = 2.3 W, the laser runs at 34.7-kHz repetition rate, indicating laser pulse energy of Ep = 3.3 μJ [Fig. 3(c)]. An ultrafast InGaAs photodiode (Alphalas GmbH; 35-ps rise time), connected to a digital Tektronix oscilloscope (2.5 GHz bandwidth, 40 GS/s sample rate) was used to measure the laser pulse duration. As illustrated in Fig. 3(d), pulse duration was quite constant (or varied slightly) on the entire range of operation. Thus, pulse duration was τp = 4.4 ns (FWHM definition) at Ep = 3.3 μJ, hence the peak power of the laser pulse being evaluated as Pp = 0.75 kW.
For the pump beam with 2ωp = 150 μm, the passively Q-switched Nd:LGSB laser yielded power Pave = 122 mW for Pabs = 2.8 W [Fig. 3(a)]. The device runs at 34.5-kHz repetition rate [Fig. 3(b)], delivering pulses with energy Ep = 3.5 μJ [Fig. 3(c)] and shorter duration τp = 3.6 ns [Fig. 3(d)]; consequently, the laser pulse peak power amounted to Pp = 0.97 kW.
When the OCM was changed to T = 5% and the pump was done at 2ωp = 100 μm, the Nd:LGSB laser with Cr4+:YAG SA of Ti = 0.89 delivered increased average output power Pave = 229 mW [Fig. 4(a)]; the absorbed pump power was limited at Pabs = 1.78 W, as a rapid decrease in Pout was noticed above this level of pumping. As shown in the inset of Fig. 4(a), the emission spectrum was centered at 1062.2 nm, with a bandwidth (FWHM definition) of 1.56 nm. The laser run at the highest repetition rate of 17.1 kHz, thus delivering pulses with energy Ep = 13.4 μJ [Fig. 4(b)]. The pulse duration was τp = 4.8 ns, indicating pulse peak power Pp∼2.8 kW. A Mode Master PC (Coherent Inc.) device was employed to measure the laser beam M2 factor. The beam distribution was close of a Gaussian shape (M2<1.1) for pump levels up to Pabs = 1.5 W [Fig. 4(c)]. Close of the maximum pump level, the beam quality was characterized by an increased M2 factor, M2∼3, thus indicating multi-mode beam operation.
This configuration was also operated with a Cr4+:YAG SA of Ti = 0.84. Laser pulses with duration τp = 3.8 ns were obtained. However, due to the thermal effects, absorbed pump power Pabs was limited to 1.7 W, for which the laser emitted an average power, Pave = 91 mW; the pulse peak power reached Pp = 1.7 kW.
3.2 SFD green at 0.53 μm light generation
SFD experiments were performed with a plane dichroic OCM; the free generation emission was obtained without a Cr4+:YAG SA in the resonator. As shown in Fig. 5, when the pump beam diameter was 2ωp = 150 μm, the Nd:LGSB bifunctional crystal delivered 3.3-mW power of green light at 0.53 μm; the absorbed pump power was Pabs = 3.2 W. On the other hand, when the strongly focused pump beam was used, with 2ωp = 100 μm, similar green power Pout = 3.3 mW was measured for Pabs = 1.95 W. However, the pump power could not be increased further, as a roll-over followed by a rapid decrease in emission was observed above this level of pumping. This behavior can be attributed to the thermal effects induced in Nd:LGSB due to the tight pumping. An investigation of these effects and their influence on laser performance will be considered in subsequent experiments.
The operation of the SFD Nd:LGSB laser in Q-switch regime was obtained with the Cr4+:YAG SA of initial transmission Ti = 0.89. For the pump beam with larger diameter, 2ωp = 150 μm, the green light average output power reached Pave = 6 mW for Pabs = 2.5 W [Fig. 6(a)]. The laser runs at 30-kHz repetition rate [Fig. 6(b)], delivering pulses with duration of 4.4 ns. Consequently, the energy of the laser pulse was Ep = 0.2 μJ [Fig. 6(c)], while the pulse peak power reached Pp = 45.5 W. For the pump with the tightly focused beam, 2ωp = 100 μm, the green power emitted by the laser was Pave = 7.8 mW for Pabs = 1.95 W. The repetition rate was 37.7 kHz, so the laser pulse energy was determined to be Ep = 0.21 μJ. The duration of the laser pulse was τp = 4.8 ns, indicating a peak power Pp = 43.7 W. The measurements showed that the emission spectrum was centered at 531 nm with a bandwidth of 0.8 nm.
The main characteristics of laser emission obtained in this work from the 2.3-at.% Nd:LGSB bifunctional crystal operating at 1.06 μm and 0.53 μm, in free-generation mode as well as when passively Q-switched by Cr4+:YAG SA, are summarized in Table 1.
4. Conclusions and prospects
In conclusion, in this work we report on laser emission at 1.06 μm and, for the first time, on green generation at 0.53 μm by self-frequency doubling from a Nd:LGSB bifunctional crystal. The laser medium was cut for type-I SHG of the 1.06-μm laser emission (θ = 35.3o, φ = 60o) at 300 K. Using the pump with a fiber-coupled laser diode, this configuration showed good performance for free generation emission, yielding 1.43-W output power at 1.06 μm with 0.50 optical-to-optical efficiency; slope efficiency amounted at 0.55. The first green generation experiments indicated a power of 3.3 mW at 0.53 μm for 1.95-W absorbed pump power. Passive Q-switching regime was investigated with Cr4+:YAG SA crystals. With a Cr4+:YAG SA of initial transmission Ti = 0.89, the Nd:LGSB laser delivered 229 mW average power at 1.06 μm; pulse energy and duration was 13.4 μJ and 4.8 ns, respectively, indicating a pulse peak power of 2.8 kW. The pulsed self-frequency doubling green emission reached an average power of 7.8 mW. The laser pulse duration was 4.8 ns and the energy was limited to 0.21 μJ, for a laser pulse peak power of 43.7 W. The results obtained for the green generation are modest at the moment, in fact most often common for the research stage of a new laser crystal, but encouraging. Therefore, in the following investigations we will aim to improve these performances, by optimizing the Nd:LGSB characteristics (Nd3+ concentration and crystal length), by using appropriate coatings on the active medium and on Cr4+:YAG SA, as well as by optimizing the laser resonator configuration. In particular, regarding the Q-switch regime of operation, the use of a folded resonator may be considered, so as to avoid changing of Cr4+:YAG properties due to the absorption of green light in the SA crystal. Future plans also include the generation of ultra-short pulses in the green visible spectrum around 0.53 μm.
Ministerul Cercetării şi Inovării Bucureşti, România (Program NUCLEU-LAPLAS VI - 16N/2019, Project PN-III-P4-ID-PCE-2016-0853, Project PN-III-P4-ID-PCE-2020-2203).
Portions of this work were presented at the Advanced Solid State Lasers Conference, 13–16 October 2020, oral presentation AF2A.6, paper title “Self-frequency-doubling Nd:LGSB laser passively Q-switched by Cr4+:YAG saturable absorber”.
The authors declare that there are no conflicts of interest related to this article.
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