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Abstract
An eye-safe 1567 nm continuous-wave laser with a maximum output power of 50 mW and a slope efficiency of 21.1% was demonstrated in an Er:Yb:Ba3Gd(PO4)3 crystal. By using a Co2+:MgAl2O4 crystal with an initial transmission of 95% as a saturable absorber, a stable passively Q-switched pulsed laser was also realized in the crystal. The effects of the output coupler transmission and cavity length on pulsed performance were investigated. At an absorbed pump power of 350 mW, a 1541 nm Er:Yb:Ba3Gd(PO4)3 pulsed laser with a repetition frequency of 0.86 kHz, duration of 38 ns, energy of 21.2 µJ, and peak output power of 0.56 kW was obtained.
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1. Introduction
Er:Yb:phosphate glass has been widely investigated and become the commercial gain medium for eye-safe 1.5 µm laser at present, which can be used in many fields, such as lidar, laser rangefinder, and remote sensing [1–3]. However, the low thermal conductivity (approximately 0.85 Wm-1K-1) and laser damage threshold of Er:Yb:phosphate glass limit its average output power and pulsed laser performance [3]. Compared with glass, crystals generally have the higher thermal conductivity and laser damage threshold. Therefore, some Er3+/Yb3+ co-doped borate and silicate crystals have also been investigated as the gain media for 1.5 µm laser [4–10]. However, the short fluorescence lifetime (lower than 1.0 ms [4–6]) of the 4I13/2 multiplet of Er3+ in the Er3+/Yb3+ co-doped borate crystals, and low Yb3+ → Er3+ energy transfer efficiency (lower than 85% [8–10]) in the Er3+/Yb3+ co-doped silicate crystals also limit their 1.5 µm laser performances.
M3R(PO4)3 (MRP, M = Sr and Ba, R = Y and Gd) crystals belong to the cubic system with a space group of $I\overline 4 3d$ and can be grown easily by the Czochralski method. The thermal conductivities of Sr3Y(PO4)3 (SYP) and Ba3Gd(PO4)3 (BGP) crystals are 1.53 and 1.39 Wm-1K-1 at room temperature, respectively [11], which are higher than that of the phosphate glass [3]. When the temperature of the crystal approaches 400 °C, the above values increase to 3.08 and 3.27 Wm-1K-1, respectively, due to the glasslike behavior of these crystals [11]. Furthermore, the laser damage threshold (1150 and 680 MW/cm2 for SYP and BGP crystals, respectively [11]) of MRP crystals are higher than that (430 MW/cm2 [3]) of phosphate glass. Therefore, MRP crystals may be suitable as laser hosts. Up to now, a 1036 nm continuous-wave (CW) laser with a maximum output power of 2.72 W and a slope efficiency of 59.8%, as well as a 1060 nm CW laser with a maximum output power of 714 mW and a slope efficiency of 12.9% have been obtained in the Yb:SYP and Nd:SYP crystals, respectively [12,13]. Recently, spectroscopic and CW laser properties of the Er:Yb:BGP crystal have also been studied [14]. The investigation has shown that the fluorescence lifetime (9.91 ms) of the 4I13/2 multiplet of Er3+ and Yb3+ → Er3+ energy transfer efficiency (about 90%) in the Er:Yb:BGP crystal are close to those (7.9 ms and about 90%, respectively) of the Er:Yb:phosphate glass [3,14–16]. A 1567 nm CW laser with a maximum output power of 73.9 mW and a slope efficiency of 16.3% has been reported in an Er:Yb:BGP crystal [14]. However, to the best of our knowledge, the passively Q-switched pulsed laser operation has still not been demonstrated in the Er3+/Yb3+ co-doped phosphate crystal up to now.
In this work, CW and passively Q-switched pulsed lasers were demonstrated in an Er:Yb:BGP crystal. The effects of output coupler transmission and cavity length on pulsed performance were investigated in detail.
2. Experimental arrangement
A linear plano-concave cavity was adopted and the experimental setup is shown in Fig. 1. A 970 nm CW fiber-coupled laser diode (LD) with a core diameter of 105 µm and a numerical aperture of 0.22 was used as the pumping source. An Er(1.85 at.%):Yb(23.95 at.%):BGP crystal with a cross section of 3.0 × 3.0 mm2 and a thickness of 2.6 mm was used as a gain medium. Room-temperature absorption coefficient spectrum of the crystal in 800–1100 nm was recorded by a spectrophotometer (Lambda 950, Perkin Elmer) and is shown in Fig. 2. The peak absorption coefficient of the crystal is 6.34 cm-1 at 975 nm and the full width at half the maximum (FWHM) of this absorption band is 6.5 nm. The emission spectrum of the used LD is also shown in Fig. 2. The absorption coefficient of the crystal is 2.29 cm-1 at pump wavelength of 970 nm. Then, about 50% of incident pump power can be absorbed by the 2.6-mm-thick crystal in a single pass. Using a telescopic lens system (TLS) consisting of two convex lenses, the pump beam with a waist diameter of about 150 µm was focused into the Er:Yb:BGP crystal. An input mirror (IM) film with a transmission of 90% at 970 nm and a reflectivity of 99.7% in 1.5–1.6 µm was directly deposited onto the input surface of a 1.0-mm-thick sapphire crystal, which has a high thermal conductivity of 42 Wm-1K-1 [17] and was used as an efficient heat sink to reduce the thermal effect of the gain medium. Three plano-concave output couplers (OCs) with the same curvature radius of 100 mm but different transmissions (0.6%, 2.0%, and 3.0%) in 1.5–1.6 µm were used in the plano-concave cavity with a cavity length of 100 mm. For Q-switched laser operation, an uncoated 1.2-mm-thick Co2+:MgAl2O4 crystal with a cross section of 3 × 3 mm2 and an initial transmission of 95% at 1541 nm was inserted into the cavity and used as a saturable absorber. Another two OCs with the same transmission of 2.0% but different curvature radii (52 and 28 mm) were used to investigate the pulsed laser performance in the plano-concave cavity with different cavity lengths (52 and 28 mm). All the crystals were optically contacted and then placed into a Cu-holder cooled by water at about 20 °C. Without the sapphire crystal, laser performance of the Er:Yb:BGP crystal will be decreased and the crystal is easy to crack. There is a hole with a 1.2 mm diameter in the center of the Cu-holder to permit the passing of the laser beams. Power was recorded by a power meter (LP-3B, Physcience Optic-electronics Co., Ltd.). Spatial profile of laser beam was recorded by a Pyrocam III camera from Ophir Optronic Ltd. Laser spectrum was recorded by a spectrometer (waveScan, APE). Pulsed profile was recorded by a 5 GHz InGaAs photodiode (DET08C, Thorlabs Inc) connected to a digital oscilloscope with a bandwidth of 1 GHz (DSO6102A, Agilent).
Fig. 2. Room-temperature absorption coefficient spectrum of the Er:Yb:BGP crystal in 800–1100 nm. The emission spectrum of the used LD is also shown.
3. Results and discussion
CW laser performance of the Er:Yb:BGP crystal was firstly investigated for different OC transmissions at a cavity length of 100 mm and the results are shown in Fig. 3(a). The output power was increased with the increment of OC transmission from 0.6% to 2.0%. For 2.0% OC transmission, a CW laser with a maximum output power of 50 mW and a slope efficiency of 21.1% was obtained at an absorbed pump power of 350 mW. However, when the OC transmission was further increased, CW laser performance of the Er:Yb:BGP crystal became worse. The laser threshold was increased and then the crystal was easy to crack in the experiment. Therefore, laser performance of the Er:Yb:BGP crystal was not investigated when the OC transmission was higher than 3.0%. As shown in the inset of Fig. 3(a), laser oscillating wavelength was 1567 nm at the OC transmissions of 0.6% and 2.0%. When the OC transmission was increased to 3.0%, laser wavelength blue-shifted to 1542 nm. This change can be explained by the blue-shift of the peak gain wavelength with the increment of inversion ratio β in the gain cross section spectrum of the Er:Yb:BGP crystal [14]. Due to the optimization of mode matching between pump and fundamental laser beams, the slope efficiency obtained in this work is higher than that (16.3%) reported previously in the Er:Yb:BGP crystal [14]. Furthermore, the maximum output power and slope efficiency obtained presently in the Er:Yb:BGP laser are close to those (20–90 mW and 10–30%, respectively) reported in most of the Er3+/Yb3+ co-doped phosphate glass lasers end-pumped by a CW LD [2,18–20]. However, the obtained maximum output power is still far lower than the highest CW output power (350 mW) reported in the Er3+/Yb3+ co-doped phosphate glass laser [21]. In this work, only 50% of incident pump power is absorbed by the 2.6-mm-thick Er:Yb:BGP crystal in a single pass. Furthermore, limited by the low thermal conductivity of the crystal, the pump power was not further increased for avoiding the fracture of the crystal. Then, by further optimizing the length and cooling technology of the Er:Yb:BGP crystal, the output power can be enhanced in the future. The fundamental laser beam was focused by a convex lens with a 100 mm focal length, and then the spatial profiles of the focused beam at different positions from the focusing lens were recorded along the laser propagation path. The beam radius was calculated by the 4-sigma method and the beam quality factor M2 can be estimated by fitting these data to the Gaussian beam propagation expression. The squared beam radii at different distances from the focusing lens are shown in Fig. 3(b) at an absorbed pump power of 350 mW and OC transmission of 2.0%. It can be seen that a nearly circular symmetric laser beam was observed, and Mx2 and My2 for the horizontal and vertical directions were fitted to be 1.42 and 1.40, respectively.
Fig. 3. (a) CW output power versus absorbed pump power of the Er:Yb:BGP CW laser for different OC transmissions T. The insets show the laser spectra at an absorbed pump power of 350 mW for OC transmissions of 2.0% and 3.0%. (b) Square beam radius ω2 versus measured distance Z from the focusing lens for the Er:Yb:BGP CW laser at an absorbed pump power of 350 mW and OC transmission of 2.0%. The insets show 2D and 3D images of the output laser beam.
When a 1.2-mm-thick Co2+:MgAl2O4 crystal was inserted into the cavity and used as a saturable absorber, passively Q-switched pulsed laser operation was realized in the Er:Yb:BGP crystal, as shown in Fig. 4. To the best of our knowledge, it is the first passively Q-switched pulsed laser operation demonstrated in Er3+/Yb3+ co-doped phosphate crystals up to now. As shown in Fig. 4(a), a pulsed laser with a maximum average output power of 22 mW and a slope efficiency of 11.5% was obtained at an absorbed pump power of 350 mW, when the OC transmission was 2.0% and cavity length was 100 mm. The conversion efficiency from the CW to Q-switched regime of operation was 44%. Compared with that of the CW laser, the oscillating wavelength of the pulsed laser blue-shifted to 1541 nm shown in the inset of Fig. 4(a), due to the higher cavity loss of the pulsed laser caused by the insertion of the saturable absorber [22]. The pulsed characteristics of the passively Q-switched Er:Yb:BGP laser, including repetition frequency, duration, energy, and peak output power, for different absorbed pump powers are shown in Figs. 4(b) and (c). At an absorbed pump power of 350 mW, a 1541 nm pulsed laser with a repetition frequency of 0.9 kHz, duration of 124 ns, energy of 24.3 µJ, and peak output power of 0.2 kW was obtained.
Fig. 4. Pulsed characteristics of the passively Q-switched Er:Yb:BGP laser for different absorbed pump powers at an OC transmission of 2.0% and a cavity length of 100 mm. (a) Average output power. The inset shows the spectrum of the pulsed laser. (b) Repetition frequency and duration. (c) Energy and peak output power.
In order to investigate the effect of the OC transmission on pulsed performance, three OCs with different transmissions were used when the cavity length was kept at 100 mm. The results are shown in Fig. 5 at an absorbed pump power of 350 mW. When the OC transmission was increased from 0.6% to 3.0%, the repetition frequency and duration decreased, while the highest energy and peak output power were obtained at the OC transmission of 2.0%. According to the passively Q-switched theoretical model [23], the OC transmission has an optimal value for achieving the maximum output energy, and the optimal OC transmission is decreased with the increment of the initial transmission of the saturable absorber. In this work, the initial transmission of the used Co2+:MgAl2O4 saturable absorber was 95%. Therefore, combined with the prediction of the theoretical model and the obtained experimental result, the optimal OC transmission of the Er:Yb:BGP crystal pulsed laser may be close to 2.0% under these experimental conditions.
Fig. 5. Pulsed characteristics of the passively Q-switched Er:Yb:BGP laser for different OC transmissions in a 100 mm plane-concave cavity when the absorbed pump power was 350 mW. (a) Repetition frequency and duration. (b) Energy and peak output power.
Some investigations have shown that the duration of pulsed laser can be reduced with the shortening of cavity length, which can enhance the pulsed laser performance [24–26]. Then, three OCs with the same transmission of 2.0% but different curvature radii (100, 52, and 28 mm) were adopted, and the corresponding cavity lengths were 100, 52, and 28 mm, respectively. The results at an absorbed pump power of 350 mW are shown in Fig. 6. With the shortening of the cavity length from 100 to 28 mm, the repetition frequency kept at about 0.8–0.9 kHz, but the duration and energy decreased. In addition, there was a decrement of energy from 24.1 to 21.2 µJ by changing the cavity length from 52 to 28 mm, while only small change of energy from 24.3 to 24.1 µJ by changing the cavity length from 100 to 52 mm. This discrepancy may be originated from the experimental error caused by the cavity misalignment, because a shorter cavity length is easier to cause a misalignment of laser cavity. Due to the shorter duration, the peak output power increased from 0.2 to 0.56 kW. When the cavity length was 28 mm, a 1541 nm Er:Yb:BGP pulsed laser with a repetition frequency of 0.86 kHz, duration of 38 ns, energy of 21.2 µJ, and peak output power of 0.56 kW was obtained at an absorbed pump power of 350 mW. When the absorbed pump power and cavity length were 350 mW and 28 mm, respectively, the profile of the passively Q-switched Er:Yb:BGP pulsed laser was recorded and is shown in Fig. 7. It can be seen that the pulsed laser operation was stable. The amplitude variation between various pulses was generally kept within ±2%, and the interpulse time jittering was less than ±5%. The energy realized in the Er:Yb:BGP laser is higher than those (about 10-20 µJ) of the Er:Yb:phosphate glass lasers end-pumped by a CW or quasi-CW LD [15,27–29]. Because the fluorescence lifetime (about 9.91 ms) of the 4I13/2 multiplet of Er3+ in the Er:Yb:BGP crystal is longer than that (7.9 ms) of the Er:Yb:phosphate glass, the Er:Yb:BGP crystal has a higher energy storage capacity. However, the peak output power obtained presently in the Er:Yb:BGP laser is still lower than that (2.2 kW) of the Er:Yb:phosphate glass micro-laser with a cavity length of 3 mm [15]. Therefore, the pulsed performance of the Er:Yb:BGP crystal can be further enhanced in the future by using a micro-laser structure.
Fig. 6. Pulsed characteristics of the passively Q-switched Er:Yb:BGP laser for different cavity lengths at an absorbed pump power of 350 mW and an OC transmission of 2.0%. (a) Repetition frequency and duration. (b) Energy and peak output power.
Fig. 7. Profile of the passively Q-switched Er:Yb:BGP pulsed laser at an absorbed pump power of 350 mW, when the OC transmission and the cavity length were 2.0% and 28 mm, respectively. (a) Pulsed train profile at an oscilloscope scanning speed of 1.0 ms/div. (b) Pulsed train profile at an oscilloscope scanning speed of 20 ms/div. (c) Single pulsed profile.
4. Conclusion
A 1567 nm CW Er:Yb:BGP laser with a maximum output power of 50 mW and a slope efficiency of 21.1% was obtained. A 1541 nm passively Q-switched Er:Yb:BGP pulsed laser was also successfully demonstrated, which is the first pulsed laser operation reported in Er3+/Yb3+ co-doped phosphate crystals to the best of our knowledge. Benefitting from efficient Yb3+→Er3+ energy transfer efficiency and the long fluorescence lifetime of upper laser level, the Er:Yb:BGP crystal may be a promising gain medium for high-energy 1.5 µm pulsed laser.
Funding
National Key Research and Development Program of China (2021YFB3601504); National Natural Science Foundation of China (52272010); Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China (2021ZR119, 2021ZZ118); Scientific Instrument Developing Project of the Chinese Academy of Sciences (YZLY202001).
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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