⇦Chinese Physics Letters, 2017, Vol. 34, No. 2, Article code 024201 Passively Q-Switched Ho:SSO Laser by Use of a Cr$^{2+}$:ZnSe Saturable Absorber * Xiao-Tao Yang(杨晓涛), Long Liu(刘龙)**, Wen-Qiang Xie(谢文强) Affiliations College of Power and Energy Engineering, Harbin Engineering University, Harbin 150001 Received 8 November 2016 *Supported by the National Natural Science Foundation of China under Grant No 61405046, and the Natural Science Foundation of Heilongjiang Province under Grant No 51305089.
**Corresponding author. Email: yangxiaotao1985@163.com Citation Text: Yang X T, Liu L and Xie W Q 2017 Chin. Phys. Lett. 34 024201 Abstract A cw operation and a passively Q-switched (PQS) Ho:SSO laser (Cr$^{2+}$:ZnSe as a saturable absorber) end-pumped by a Tm:YAP laser operating at near room temperature are reported. It is the first time to report a PQS Ho:SSO laser. For the cw mode, a maximum cw output power of 3.0 W is obtained, corresponding to a slope efficiency of 31.4%. For the PQS mode, a Cr$^{2+}$:ZnSe is used as the saturable absorber, with transmission of 88.4% at 2112 nm. A maximum pulse energy of 1.29 mJ is obtained, corresponding to the pulse repetition frequency of 2.42 kHz. In this study, we change the distance between Cr$^{2+}$:ZnSe and the output mirror to research the pulse characteristic of the PQS Ho:SSO laser. The minimum pulse width of 73.5 ns is obtained, corresponding to the pulse energy of 0.9 mJ and the pulse repetition frequency of 2.65 kHz. DOI:10.1088/0256-307X/34/2/024201 PACS:42.55.Xi, 42.60.Pk, 42.60.Gd, 42.65.Yj © 2017 Chinese Physics Society Article Text The 2 μm lasers are in the eye-safety wavelength regions, which include strong absorption lines of water and weak absorption lines of atmosphere. Therefore, it is expected to have potential applications in a wide range of fields, such as atmospheric remote sensing (Doppler radar wind sensing and water vapour profiling by differential absorption radar), spectroscopy and basic research.[1,2] Furthermore, high-power quasi-continuous wave (QCW) 2 μm lasers with high peak power are effective pump sources of optical parametrical oscillators and solid-state lasers further in the mid-infrared region.[3] Laser media with Tm$^{3+}$ and Ho$^{3+}$ active ions doped are the main way to obtain 2 μm lasers.[4,5] Actively Q-switched Tm and Ho lasers were developed during the early 1990s.[6] Compared with the actively Q-switched lasers, there are several advantages for the saturable absorber Q-switch, which are well known and consist primarily of simplicity, reliability, and economy.[7] The saturable absorbers are significant to the passively Q-switched lasers (PQSs). Ho$^{3+}$doped crystals have been used for Q-switched lasers. Some good repeatable Q-switching results have been obtained by Ho:YLF. However, intracavity focusing was required for these Q-switched lasers.[8,9] SWCNT,[10] graphene-[11] and Cr$^{2+}$-doped chalcogenide crystals (ZnS, ZnSe and so on) have been used as saturable absorbers also.[12] Compared with SWCNT and graphene, Cr$^{2+}$-doped chalcogenide crystals with a non-centrosymmetric structure are isotropic medium with a larger absorption cross section, higher absorption threshold and larger crystal hardness.[13] In this work, we use Cr$^{2+}$:ZnSe as a saturable absorber. The $^{5}\!E$ and $^{5}\!T_{2}$ energy levels of Cr$^{2+}$:ZnSe crystal provide absorption around 2 μm region. The decay lifetime of Cr$^{2+}$ in ZnSe is as short as 8 μs,[14] which can provide a fast repetition rate for Q-switched pulses. Holmium-doped Sc$_{2}$SiO$_{5}$(SSO) is chosen as the laser gain medium, which is monoclinic crystal with low symmetry. The change of the Ho:SSO crystal refractive index with temperature $dn/dT$ (the derivative of the refractive index with temperature) is a negative amount, which can limit the thermal lens effects, distortion of crystallographic sites and birefringent effects.[15] Figure 1 shows the room-temperature absorption and emission spectra of Ho:SSO.
cpl-34-2-024201-fig1.png
Fig. 1. Room-temperature absorption and emission spectrum of Ho:SSO.
Continuous wave and actively Q-switched Ho:SSO lasers have been reported.[16] To our knowledge, it is the first time to report a passively Q-switched Ho:SSO laser with a Cr$^{2+}$:ZnSe saturable absorber. The experimental setup is schematically shown in Fig. 2, in which a simple U-shaped plane-concave cavity with a physical length of 120 mm is employed. The pump source is a diode-pumped Tm:YAP laser, whose maximum output power is 25 W at the central wavelength of 1936.3 nm. The pump beam is focused into a spot of 0.68 mm in diameter around the Ho:SSO crystal. The dichroic mirrors M1 and M2 are plate mirrors with high-reflection (HR) coating at 2.1 μm and high-transmission (HT) coating at 1.94 μm, respectively. The mirror M3 is a plate mirror, which is coated HR at 2.1 μm. M4 serves as an output coupler, which is a concave mirror with curvature radius of 150 mm. The transmission is 30% at 2.1 μm. The Ho:SSO crystal doped with 0.5 at.% has a dimension of 4 mm$\times$4 mm (in section)$\times$13 mm (in length). Both the end faces of the crystal are anti-reflection (AR) coated at 1.94 μm and 2.1 μm. We measure the actual absorption efficiency of the Ho:SSO crystal. It is found that 52% of the pump power is absorbed. This Ho:SSO crystal is wrapped in indium foil and held in a copper heat-sink bonded on a thermal electric cooler (TEC) for precise temperature control. In this experiment, the temperature of Ho:SSO crystal is held at 20$^{\circ}\!$C. A 3-mm-thick Cr$^{2+}$:ZnSe plate with AR coating ($R < 0.5\%$ at 2.1 μm) is used as saturable absorber (SA), whose transmission is 88.4% at 2112 nm. The SA is placed in the resonator, which is 10 mm away from the output mirror. The beam radius inside the resonator of the Ho:SSO laser is calculated using the well-known ABCD matrix, corresponding to the $|A+D|/2$ value of the resonator lower than 0.2, which indicates that the resonator is always kept stable.
cpl-34-2-024201-fig2.png
Fig. 2. Schematic diagram of the PQS Ho:SSO laser.
cpl-34-2-024201-fig3.png
Fig. 3. (a) The free running output power and (b) the PQS average output power versus the absorbed pump power.
Figure 3 shows the output characteristics of the Ho:SSO laser free running and with the Cr$^{2+}$:ZnSe SA. We can obtain that the laser threshold changes from 2.5 W to 3.5 W with the SA. The maximum output power reaches 3.0 W at the cw mode. The slope efficiency of the free running Ho:SSO laser is 31.4%, which decreases in the PQS Ho:SSO laser. The SA inserted brings optical loss to the Ho:SSO laser. The slope efficiency also changes with the value of $L$ (the distance between SA and the output mirror). In this experiment, we place the SA at 5 mm, 10 mm and 20 mm away from the output mirror. The slope efficiency is 29.4%, when $L$ is 5 mm. We can obtain from Fig. 3 that the slope efficiency decreases with the increase of $L$. The maximum average output power reaches 2.7 W at the PQS mode.
cpl-34-2-024201-fig4.png
Fig. 4. Pulse width of the PQS Ho:SSO laser.
cpl-34-2-024201-fig5.png
Fig. 5. Pulse of the PQS Ho:SSO laser.
cpl-34-2-024201-fig6.png
Fig. 6. (a) Repetition frequency and (b) pulse energy as a function of the absorbed pump power. Here $L$ is the distance between Cr:ZnSe SA and the output mirror.
Figure 4 shows the laser pulse width with the Cr$^{2+}$:ZnSe SA. The pulse width decreases with the increase of the absorbed pump power. However, with the same absorbed pump power, the pulse width varies with the value of $L$. We can obtain that the pulse width becomes narrower when $L$ is larger. The minimum pulse width of 73.5 ns is obtained, when $L$ is 20 mm at the absorbed pump power of 11.5 W. Figure 5 shows the pulse characteristic of the PQS Ho:SSO laser with 10.5 W absorbed pump power when $L$ is 5 mm. With the change of the absorbed pump power, there are some changes of the pulse repetition frequency and the pulse energy also, as shown in Fig. 6.
cpl-34-2-024201-fig7.png
Fig. 7. (a) Wavelength of the free running Ho:SSO laser. (b) Wavelength of the PQS Ho:SSO laser.
The repetition frequency of the PQS Ho:SSO laser increases with the absorbed pump power. The maximum repetition frequency is 2.65 kHz, when the absorbed pump power is 11.5 W and $L$ is 20 mm, corresponding to a single pulse energy of 0.9 mJ. The single pulse energy of the PQS Ho:SSO laser increases with the absorbed pump power also. At the same time, the laser repetition frequency and pulse energy vary with $L$. When $L$ is 5 mm, the output energy is 0.434 mJ around the laser threshold, which reaches 1.29 mJ with the absorbed pump power of 11.5 W. The increase of the single pulse with the absorbed pump power is not linear, which changes slightly when the absorbed pump power is larger than 10 W. The output pulse energy decreases with the increase of $L$. The output wavelength of the Ho:SSO laser also changes between the free running Ho:SSO laser and the PQS Ho:SSO laser, as shown in Fig. 7. The output wavelength is measured by a WDM1-3 grating monochromator (resolution of 0.2 nm) and a fast response InGaAs detector. When the Cr$^{2+}$:ZnSe is placed into the cavity, the Ho:SSO laser is PQS operation, and the central wavelength of the output laser pulse is changed from 2112 nm to 2104 nm with the absorbed pump power of 10.5 W, when $L$ is 5 mm. In the PQS mode, the loss in the resonator increases due to the Cr$^{2+}$:ZnSe SA, leading to a higher population inversion density when the laser pulse is formed. Thus the gain coefficient changes in relation to emission cross section and emitting wavelength. In summary, the PQS (Cr$^{2+}$:ZnSe as a saturable absorber) Ho:SSO lasers end-pumped by a Tm:YAP laser operating at near room temperature has been reported for the first time. A maximum cw output power of 3.0 W at the absorbed pump power of 11.5 W is obtained, corresponding to a slope efficiency of 31.4%. The PQS operation is obtained by inserting Cr$^{2+}$:ZnSe SA into the laser cavity. Compared with the output wavelength of 2112 nm in the cw operation, the output wavelength of the PQS Ho:SSO laser is 2104 nm. The experimental results are compared when the distances between the Cr$^{2+}$:ZnSe SA and the output mirror are different. The minimum pulse width of 73.5 ns is obtained when $L$ is 20 mm at the absorbed pump power of 11.5 W, corresponding to the pulse repetition rate of 2.65 kHz. The maximum pulse energy of 1.29 mJ is obtained when $L$ is 5 mm at the absorbed pump power of 11.5 W, corresponding to the pulse repetition rate of 2.42 kHz. Under the damage threshold of Cr$^{2+}$:ZnSe SA, we can increase the pump power for obtaining the higher Q-switched output power. References Industrial applications of laser micromachiningHigh average power 2 μm generation using an intracavity PPMgLN optical parametric oscillatorModeling of intracavity-pumped quasi-three-level lasersA 60W Tm:YLF Laser with Triple Tm:YLF RodsA Narrow Linewidth Continuous Wave Ho:YAG Laser Pumped by a Tm:YLF LaserSPIE ProceedingsGeneration of 54 Fs Laser Pulses from a Diode Pumped Kerr-Lens Mode-Locked Yb:LSO LaserSPIE ProceedingsHo:YVO_4 solid-state saturable-absorber Q switch for a 2-μm Tm, Cr:Y_3Al_5O_12 laserPerformance of 2 μm Tm:YAP pulse laser based on a carbon nanotube absorberGraphene on SiC as a Q-switcher for a 2 μm laserFemtosecond SESAM-modelocked Cr:ZnS laserProgress in Cr 2+ and Fe 2+ doped mid-IR laser materialsQ-switched 2-µm lasers by use of a Cr^2+:ZnSe saturable absorberSpectral properties and laser performance of Ho: Sc_2SiO_5 crystal at room temperaturePerformance of acousto-optically Q-switched Ho:SSO laser pumped by a Tm:YAP laser
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