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We report on an acousto-optic Q-switched 1645 nm Er:YAG ceramic laser resonantly pumped by using an Er,Yb fiber laser at 1532 nm. Maximum continuous wave output powers of 2.1 W and 2.4 W were obtained for 10% and 20% transmission OCs under 10.5 W of incident pump power, respectively. In Q-switched mode, the laser produced pulses with ~3.7 mJ energy and 82 ns width at 200 Hz repetition rate for 20% transmission OC under 8.6 W of incident pump power, corresponding to a peak power of ~45 kW.
© 2014 Optical Society of America
1. Introduction
High-energy repetitively pulsed lasers operating in the eye-safe wavelength regime around 1.5-1.7 μm have wide applications including remote sensing, wind-field velocity mapping, and the generation of 3-5 μm mid-infrared lasers via optical frequency conversion, among others. Efforts have focused on the development of diode or erbium-doped fiber lasers resonantly pumped Q-switched Er:YAG single-crystal lasers for these applications [1–11], effectively minimizing the waste heat from the pumping process and hence reducing the detrimental effects such as thermal lensing and birefringence.
Rare-earth doped transparent ceramics have drawn much attention as new laser gain media since first high-efficiency Nd:YAG ceramic laser was demonstrated in 1995 because they have some specific advantages, e.g., rapid and large scale fabrication, flexibility in producing ceramics with composite structures, possibility of fabricating laser materials difficult to be prepared with melt-growth process, such as sesquioxide ceramics (Sc2O3, Y2O3, Lu2O3), with optical quality. As fabrication technology advances, Nd- and Yb- doped YAG ceramics are now routinely available near 1 μm wavelength region with essentially the same laser performances as that of single crystals [12]. In recent years, ceramic lasers at ~1.6 μm have also been reported for Er3+-doped polycrystalline sesquioxide Sc2O3 [13], Y2O3 [14] and YAG ceramics [15–20]. However, those sesquioxide ceramic erbium lasers have been demonstrated mostly in liquid nitrogen cooling condition. First room-temperature operation of erbium ceramic laser was demonstrated for a composite ceramic Er:YAG with 56.9% of slope efficiency with respect to the absorbed pump power [15]. In 2013, more than 16 W of CW output power was reported for 1645 nm and 1617 nm Er:YAG ceramic laser resonantly pumped by an Er,Yb fiber laser [19]. Recently, Q-switched operation of Er:YAG ceramic has been also demonstrated using multilayer graphene as saturable absorber with several μJ pulse energy and dozens of kHz repetition rate [20].
In this paper, we report on an AO Q-switched 1645 nm polycrystalline Er:YAG ceramic laser resonantly pumped by using an Er, Yb fiber laser at 1532 nm. Maximum continuous wave (CW) output powers of 2.1 W and 2.4 W were obtained for 10% and 20% transmission OCs under 10.5 W of incident pump power, respectively. In Q-switched mode, using a 20% transmission OC, the laser produced pulses with ~3.7 mJ energy and 82 ns width at 200 Hz repetition rate for 8.6 W of incident pump power, corresponding to a peak power of ~45 kW. To our best knowledge, this is the first actively Q-switched Er:YAG ceramic laser at 1.6 μm.
2. Experimental setup
The schematic of the Er:YAG ceramic laser is shown in Fig. 1. The pump source used in our experiments was a homemade Er,Yb fiber laser comprising a double-clad geometry with a 30 μm diameter (0.2 NA) Er, Yb-doped phosphor-silicate core surrounded by a 350 μm diameter pure silica inner-cladding with a nominal NA = 0.49. The active fiber was pumped through opposite ends by a high power 976 nm diode laser source that was split into two beams of roughly equal power. A volume Bragg grating (VBG) with reflectivity of higher than 99% at center wavelength of 1570 nm was employed to implement wavelength selection. By adjusting the incident angle of VBG, the emission wavelength of the Er, Yb fiber laser was tuned to match the absorption peak of the Er:YAG ceramic at 1532 nm. The pump laser was collimated by a 30 mm focal length plano-convex lens and sequentially focused using a 200 mm focal length lens to a beam diameter of ~200 μm in the center of Er:YAG ceramic. The confocal parameter (2πnwp2/λM2) of the pump beam inside Er:YAG ceramic was estimated to be ~36 mm with measured M2 factor of 2.1 for Er, Yb fiber laser.
The Er:YAG ceramic laser was designed to have a z-shaped resonator to minimize the influence of pump light on laser operation. We found that pulse-pulse amplitude fluctuation was abrupt with a simple two-mirror resonator and it was observably relieved when pump light was filtered out with this z-shaped cavity before it impinged onto the AO Q-switch. The cavity comprises a plane input mirror with high reflectivity (>99%) at the lasing wavelength (1600-1700 nm) and high transmission (>96%) at 1532 nm, a concave mirror with high reflectivity (>99.8%) at both the lasing and the pump wavelengths and 200 mm radius of curvature, a plane folding mirror with high reflectivity at the lasing wavelength and high transmission at the pump wavelength to filter out the unabsorbed pump laser, a flat mirror OC with transmission of 10% or 20% at 1645 nm. Polycrystalline Er:YAG ceramic of 0.5 at.% Er3+-doping concentration (developed at Jiangsu Normal university) were cut and polished to have a dimension of 2 × 3 mm2 in cross section and 14 mm in length. Both faces of the sample were antireflection-coated at 1500-1700 nm. The sample was wrapped with indium foil and mounted on a water-cooled copper heat-sink maintained at temperature of 18 °C to allow for effective heat removal. The physical length of the resonator is ~437 mm, including 127 mm distance from the IC to R = 200 mm concave mirror, 60 mm distance from R = 200 mm mirror to the plane folding mirror, 250 mm distance from plane folding mirror to the OC, resulting in a TEM00 beam radius of ~100 μm in the gain media.
3. Results and discussion
The Er:YAG ceramic laser was firstly evaluated in CW mode of operation with AO removed from the cavity. Figure 2 shows the CW output power versus incident pump power for 10% and 20% transmission OCs. As shown in the figure, better laser performance in terms of slope efficiency and maximum output power was obtained for 20% transmission OC. Up to 2.4 W of output power at 1645 nm for 10.5 W of incident pump power was delivered with this OC, corresponding to a slope efficiency of 24% and optical-optical efficiency of 23% with respect to the incident pump power. For T = 10% OC, the maximum output power is 2.1 W and the slope efficiency is 21% with respect to the incident pump power.
Fig. 2 CW output power versus incident pump power. Insert, output spectrum of the Er:YAG ceramic laser for T = 20% under 10.5 W of incident pump power.
Beam quality of 1645 nm laser output was checked with a beam profiler (NanoScan, Photon Inc.) for 20% OC under 10.5 W of pump power. M2 factors along the horizontal and vertical axes were measured to be 1.3 and 1.5, respectively. The asymmetry of beam profile for the two perpendicular directions may come from the astigmatism of folded cavity configuration. The optical spectrum of the laser output was measured using an optical spectrum analyzer (AQ6375, Yokogawa) with a resolution of 0.1 nm. Only 1645 nm wavelength oscillation was inspected for either OC under various incident pump power. The insert of Fig. 2 shows a typical optical spectrum profile for T = 20% OC under 10.5 W of incident pump power. Another emission peak of ~1617 nm haven’t been seen since the lasing threshold of 1617 nm wavelength is higher than that of 1645 nm due to stronger reabsorption loss for 1617 nm wavelength laser emission. Wavelength discrimination component would be required to be inserted into the cavity to force 1617 nm wavelength lasing.
For pulsed operation, a crystalline quartz AO Q-switch (Gooch & Housego Ltd) was directly inserted into the cavity in front of the OC with all the other components fixed. Maximum average power of 1.73 W and 1.75 W were achieved at 5 kHz PRF for T = 10% and 20% OCs under 10.5 W of incident pump power, corresponding to slope efficiencies of 16.5% and 15.2% with respect to incident pump power, respectively. Reduction of maximum output power and slope efficiency for Q-switched operation compared with that of the CW operation for either OC can be mainly attributed to the additional resonator loss that resulted from the inclusion of the AO Q-switch. For 20% transmission OC, pulse energy increased from 0.35 mJ to 2 mJ with decreasing the PRF from 5 kHz to 500 Hz while keeping the incident pump power at 10.5 W. Go on decreasing the PRF to 200 Hz, mirrors were damaged. 3.7 mJ of maximum pulse energy was achieved with 82 ns pulse width at 200 Hz PRF for T = 20% OC under 8.6 W of incident pump power, corresponding to a peak power of ~45 kW. Figure 3 depicts the dependences of pulse energy and pulse width on the PRF for T = 20% OC under 8.6 W of incident pump power. Pulse width decreased from 151 ns to 82 ns while pulse energy increased from 0.28 mJ to 3.7 mJ when the PRF was decreased from 5 kHz to 200 Hz. For 10% transmission OC, the laser produced pulses with 4 mJ of energy and 88 ns duration at 200 Hz PRF under 10.5 W of incident pump power.
Pulse train and single pulse profile were detected by an InGaAs detector with 10 ns rise time (Thorlab, DET 10C/M) and monitored with a 1 GHz bandwidth oscilloscope (Tektronix, DPO 7104C). Pulse-to-pulse amplitude fluctuations for T = 10% and 20% OCs were estimated to be 5.5% and 4.7%, respectively.
Figure 4 displayed the average output power as a function of PRF under various incident pump power for 20% transmission OC. The average power drops with decreasing PRF for given pump power and the rapidity of drop accelerates at higher pump power. Similar behavior can also be discovered for T = 10% OC. The declines of average output powers at low PRF can be attributed to the decrease of storage efficiency with the decrease of PRF. The storage efficiency for a CW pumped Q-switched laser can be expressed as [21]
whereis the upper level lifetime and equals to the inverse of PRF. When is very large compared with, approaches unity.Figure 5 illustrated relative storage efficiency versus the PRF for various incident pump power according to experimental results in Fig. 4. Relative storage efficiency was defined as the ratio of the average output power at some PRF divided by that at the highest PRF (i.e., 5 kHz in our case) [1]. Theoretical storage efficiency was also plotted with [22] and substituted into Eq. (1) because Er:YAG ceramic had similar optical spectrum as its single-crystal counterpart.
It is obvious that experimental curves apparently differ from theoretical fitting curves ofand. Many other values of have also been tested, but the experimental curves cannot be fitted in the whole PRF region with Eq. (1) for any. Researches based on Er:YAG single-crystal lasers have proposed that excited-state absorption (ESA) [8], energy-transfer upconversion (ETU) [4], and amplified spontaneous emission (ASE) [11] are possible mechanisms responsible for this phenomenon, but there is no consensus with it yet. The situation for Er:YAG ceramic is more complicated compared with Er:YAG single crystal since there exits tendency of grain boundary segregation for Er:YAG ceramic. Further experiments, especially optical spectrum measurements under lasing condition, should be conducted to examine the contribution of each physical mechanism to the rapid decrease of storage efficiency with respect to the decrease of PRF.
4. Conclusion
In summary, we report on an AO Q-switched 1645 nm polycrystalline Er:YAG ceramic laser resonantly pumped by using an Er, Yb fiber laser at 1532 nm. CW operation of the Er:YAG ceramic laser was examined with two OCs of 10% and 20% transmissions. Maximum continuous wave (CW) output powers of 2.1 W and 2.4 W were obtained for 10% and 20% transmission OCs under 10.5 W of incident pump power, respectively. In Q-switched mode, by using the 20% transmission OC, the laser produced pulses with ~3.7 mJ energy and 82 ns width at 200 Hz repetition rate for 8.6 W of incident pump power, corresponding to a peak power of ~45 kW. Further improvement of pulse energy can be realized through expanding the beam spot of pump light and optimizing the cavity design.
Acknowledgments
This work is supported by the National Natural Science Foundation of China (NSFC 61177045, NSAF U1430111), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
References and links
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