Continuous-Wave and Passively Q-Switched Er : Y 2 O 3 Ceramic Laser at 2 . 7 μ m

We report on a continuous-wave (CW) and passively Q-switched Er:Y2O3 ceramic laser in mid-infrared spectral region. In the CW regime, a maximum output power of 2.07W is achieved at 2717.3 nm with a slope efficiency of 13.5%. Stable passive Q-switching of the Er:Y2O3 ceramic laser is demonstrated based on semiconductor saturable absorber mirror. Under an absorbed pump power of 12.4W, a maximum average output power of 223mW is generated with a pulse energy of 1.7μJ and a pulse width of 350 ns at 2709.3 nm.


Introduction
Mid-infrared coherent resources in the 2∼5 m spectral range have attracted more and more attention due to the potential applications in spectroscopy, medical treatment, security and defense, free-space communication, and advanced sensing technologies for trace gas detection and greenhouse gas monitoring [1][2][3][4][5].Rare-earth-doped laser is a general way to generate high-power and high-beam-quality mid-infrared coherent resource [6][7][8].
Er-doped solid-state lasers can be emitted in the spectral range of 2.7∼3.0 m by electronic transition ( 4  11/2 → 4  13/2 ).So far, Er:YAG has been widely used for commercial 2.94 m laser.For mid-infrared Er:YAG lasers, high Er-doping concentration (50%) is required to overcome self-terminating phenomenon which is caused by a long lifetime of lower laser level [9,10].Although high doping concentration can reduce the population in lower laser level by enhancing the energy transfer upconversion process ( 4  13/2 + 4  13/2 → 4  9/2 + 4  15/2 ), it will bring serious thermal effects due to heat deposited in a small region, which will limit the available output power and beam quality.Er:Y 2 O 3 is an excellent alternative for mid-infrared solid-state laser materials.For Er:Y 2 O 3 gain medium, a much lower doping concentration (e.g., 2%∼ 7%) is feasible for laser emission [11][12][13][14], which significantly alleviates the thermal effects.Besides, Er:Y 2 O 3 has a larger product of emission cross section (∼1 × 10 −19 cm 2 ) and upper laser level lifetime (2.4 ms) in comparison with Er:YAG [14], which is favorable to achieve a high laser gain.Due to the high melting point, the growth of Er:Y 2 O 3 single crystal is challenging.Er:Y 2 O 3 ceramic has similar optical performances to the Er:Y 2 O 3 single crystal, and the ceramic possesses the advantages of short fabrication period, large size, and low cost.So far, CW operation of Er:Y 2 O 3 ceramic laser at room temperature has been reported with an output power of 320 mW and a slope efficiency of 6.5% [12].At cryogenic temperature of 77 K, a CW output power of 14 W was generated [13].For passive Q-switching operation, only an average power of 6 mW was generated in Er:Y 2 O 3 ceramic laser [11].Besides, passively Q-switched lasers in Er:CaF 2 , Er:SrF 2 , Er:Lu 2 O 3 , and Er:Pr:GGG crystals were also reported (Table 1) [15][16][17][18][19].
Here we demonstrate a passively Q-switched Er:Y 2 O 3 ceramic laser at the wavelength of 2.7 m.In the CW regime, an output power of 2.07 W is obtained with a slope efficiency of 13.5%.The Q-switched operation is realized based on

Experimental Setup
The experimental setup schematics of CW and passively Qswitched Er:Y 2 O 3 ceramic laser are shown in Figure 1.A commercial 970 nm laser diode (LD) was adopted as the pump source, delivering a maximum power of 30 W from a multimode fiber with a core diameter of 105 m and a numerical aperture of 0.22.After collimating and focusing by a pair of lenses  1 and  2 ( 1 = 18 mm and  2 = 75 mm), the pump light was imaged into the gain medium.The gain medium of Er:Y 2 O 3 ceramic had a length of 6.6 mm, a cross section of 2 × 3 mm 2 , and an Er-doping concentration of 7%.In order to remove the generated heat while pumping, the Er:Y 2 O 3 ceramic was wrapped in indium foil and tightly fixed in a copper holder which was cooled by a thermoelectric cooler.
The gain medium was antireflectively coated for both pump and laser wavelengths (R < 0.5%).For CW operation, a linear cavity structure was adopted (Figure 1(a)), which consisted of a plane-concave input mirror  1 (ROC = 50 mm) and a plane-plane output coupler (OC).Figure 1(b) shows the experimental setup schematic of SESAM Q-switched laser.Er:Y 2 O 3 ceramic laser had a total cavity length of 25 mm.The employed SESAM (SAM-2400-1-10ps-e, BATOP GmbH) has total absorbance of 5% and 18% at 2700 nm and 2717 nm, respectively.

Results and Discussion
3.1.CW Laser Performance.We used different OCs (T = 1%, 3%, and 5%, resp.) to test the CW output powers.Figure 2(a) shows the CW output power versus absorbed pump power with an optimal OC (T = 5%).The laser threshold was ∼1.6 W. The output laser power increased linearly with absorbed pump power with a slope efficiency of 13.5%.A maximum CW power of 2.07 W was generated.The output power rollover was not observed even at the maximum pump power, indicating that CW output power could be further increased with higher pump power.At the CW output power of 1 W, we measured the output spectrum, as shown in Figure 2(b).
The spectrum peak is at 2717.3 nm with a full width at half maximum (FWHM) of 4.6 nm.With the knife-edge method, we measured the beam quality of CW output laser.The measured  2 factor of the beam was 2.84 (Figure 2(c)).We also captured the output beam profile with a thermosensitive card (VRC6S, Thorlabs).The inset of Figure 2(c) shows that the laser beam has a round beam profile.In order to test the power stability of the Er:Y 2 O 3 ceramic laser, we monitored the output power fluctuation for one hour, as shown in Figure 2(d).The output power fluctuation (Δ RMS , the ratio of root-mean-square error to average value) is ∼0.65%, indicating that the output power of Er:Y 2 O 3 ceramic laser is stable.

SESAM Q-Switched Er:Y 2 O 3 Laser. In the Q-switched
Er:Y 2 O 3 ceramic laser, we employed a plane-concave OC with transmission of 1%.After introducing the SESAM in the cavity, laser threshold was significantly increased due to the absorption loss of SESAM.The laser threshold was 4.8 W of absorbed pump power.As the absorbed pump power was increased to 9.5 W, Q-switching operation was initiated.Q-switched pulse train was captured by a midinfrared photodetector (VIGO system, PCI-9) and displayed on a digital oscillator, as shown in Figure 3(a).In order to achieve short Q-switched pulses, we built up a short laser cavity of 25 mm, which emitted the shortest pulse of 350 ns (Figure 3(b)).
Figure 4(a) shows the output power, pulse energy, and peak power evolution with the absorbed pump power.They all increase with the absorbed pump power.At the absorbed pump power of 12.4 W, we obtained a maximum average output power of 223 mW and a maximum pulse energy of 1.7 J, corresponding to the peak power of 4.9 W. Beyond the absorbed pump power of 12.4 W, the laser quenching appeared.The laser quenching phenomenon can be attributed to serious thermal lens effect in the gain medium, which results in formation of instable cavity.The focal length of thermal lens was directly measured to be 35 mm at the absorbed pump power of 12.4 W. We added the thermal lens to ABCD propagation matrix and the formation of instable cavity was verified by the numerical calculation.The Qswitched pulse could be recovered by slightly decreasing the incident pump power.The evolution of repetition rate and pulse width with absorbed pump power is shown Pump OC 2.7 m laser (970 nm)  in Figure 4(b).The pulse repetition rate increased from 105.3 kHz to 130.6 kHz in the absorbed pump power range of 9.5∼12.4W.However, the pulse duration decreased from 434 ns to 350 ns and it tended to saturate beyond the absorbed pump power of 12 W. Figure 5 shows the Q-switched pulse spectrum.The spectral peak is at 2709.3 nm with a FWHM of 2.4 nm.Compared with the CW spectrum, there is a blue shift in Q-switching pulse spectrum, which could be attributed to the larger loss (18%) of the SESAM, measured at the wavelength of 2717 nm.Consequently, the Q-switched Er:Y 2 O 3 ceramic laser was forced to oscillate at shorter wavelength of 2709 nm.At this wavelength, the Q-switched Er:Y 2 O 3 ceramic laser suffered from an reabsorption loss under high pump power [12], which dramatically reduced the average output power.

Conclusions
In conclusion, we experimentally demonstrated a CW and passively Q-switched Er:Y 2 O 3 ceramic laser at 2.7 m wavelength.A maximum CW output power of 2.07 W was achieved with a slope efficiency of 13.5%.In Q-switching operation, the laser emitted stable Q-switched pulses with an average output power of 223 mW, a pulse energy of 1.7 J, a pulse width of 350 ns, and a repetition rate of 130.6 kHz at 2709.3 nm.Our research results show that Er:Y 2 O 3 ceramic is an excellent gain material for CW and Q-switched laser at 2.7 m wavelength.

Figure 2 :
Figure 2: (a) CW output power versus absorbed pump power.(b) Spectrum of CW laser.(c) Measurement of the beam quality.Inset: beam profile.(d) CW output power stability of Er:Y 2 O 3 ceramic laser.

Figure 5 :
Figure 5: Optical spectrum of the Q-switched pulse.

Table 1 :
Passively Q-switched Er-doped bulk lasers near 3 m spectral region. ave , average power;  peak , peak power.SESAM.Under an absorbed pump power of 12.4 W, the Qswitched Er:Y 2 O 3 ceramic laser emits an average power of 223 mW with a pulse width of 350 ns at repetition rate of 130.6 kHz, resulting in a pulse energy of 1.7 J.