CdSe passively Q-switched Ho: YAG laser - OSA Publishing

Letter

Vol. 42, No. 13 / July 1 2017 / Optics Letters

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Cr2+: CdSe passively Q-switched Ho: YAG laser ENCAI JI,1,† MINGMING NIE,1,† XING FU,1 ZHOUGUO GUAN,2

AND

QIANG LIU1,*

1

State Key Laboratory of Precision Measurement Technology and Instruments, Department of Precision Instruments, Tsinghua University, Beijing 100084, China 2 Southwest Technical Institute of Physics, Chengdu 610061, China *Corresponding author: [email protected] Received 27 February 2017; revised 31 May 2017; accepted 7 June 2017; posted 7 June 2017 (Doc. ID 287621); published 27 June 2017

We first demonstrate the laser performance of a Cr2 :CdSe passively Q-switched (PQS) 2.09 μm Ho: YAG laser resonantly pumped by a thulium-doped fiber laser. The maximum output pulse energy of 1.766 mJ, corresponding to a repetition frequency of 685 Hz, was obtained with a pulse duration of 15.4 ns. The pulse peak power was 114.7 kW, while the PQS efficiency was 21.45% against the continuous-wave output. The laser wavelength in the PQS operation was precisely measured to be 2090.2 nm. © 2017 Optical Society of America OCIS codes: (140.3070) Infrared and far-infrared lasers; (140.3540) Lasers, Q-switched; (140.3580) Lasers, solid-state. https://doi.org/10.1364/OL.42.002555

2 μm pulsed lasers are of great value for scientific and engineering fields such as medical treatment [1], lidar systems [2], and mid-infrared parametric light source [3]. Apart from the nonlinear frequency conversion technique, the stimulated emission on 5 I 7 → 5 I 8 transition of Ho3 -doped media is always utilized to obtain 2.05 ∼ 2.15 μm lasers [4,5]. Altering the quality factor of resonant cavity periodically, namely, the Q-switching technique, it is possible to achieve a giant laser pulse >2.05. Both the actively Q-switched (AQS) and passively Q-switched (PQS) Ho3 lasers have been reported by a number of groups [6–10]. As to the PQS technique, the great benefits are the simple structure and low cost, but the most challenging issues relate to the Q-switching stability and the damage threshold. Compared to a new type of 2 μm saturable absorber (SA), such as black phosphorus [11,12], single wall carbon nanotubes (SWCNTs) [13], MoS2 [14], and topological insulator [15], crystalline transition-metal-ion-doped II–VI compounds have been applied successfully in high-peak-power 2 μm PQS lasers [16–19], such as Cr2 :ZnSe and Cr2 :ZnS. In 2001, Cr2 :ZnSe was first used as a SA in Ho: YAG and Tm: YAG lasers [10]. A pulse energy of 1.3 mJ was obtained in the flash-lamp-pumped Ho: YAG laser with a pulse duration of 90 ns, corresponding to a peak power of 14.4 kW. In 2015, Yao et al. reported [17] a Cr2 :ZnS PQS Ho: YAG ceramic laser with the current maximum average power. A pulse energy of 0.6 mJ was obtained with the average power of 14.64 W and pulse duration of 90 ns, corresponding to a peak 0146-9592/17/132555-04 Journal © 2017 Optical Society of America

power of 6.7 kW. This group also adopted a Cr2 :ZnS in PQS Ho: YLF laser [18] and a PQS Ho: LuAG laser [19]. Another Cr2 -doped II–VI crystal, Cr2 :CdSe (Cr: CdSe), has been successfully applied as an active medium in the broadband tunable laser [20–22]. The relative short lifetime and large absorption cross section around 2 μm [20] make us believe Cr2 :CdSe should also be a quality SA for a 2 μm PQS laser. Until now, to the best of our knowledge, no work has been reported on the Cr: CdSe performance when it is used as a SA in 2 μm laser. In this Letter, a Cr: CdSe PQS Ho: YAG laser directly pumped by a Tm3 -doped fiber laser (TDFL) was first demonstrated. The pulse performance indicates that a Cr: CdSe crystal can be a promising SA in 2 μm PQS lasers. The experimental setup is depicted in Fig. 1. A homemade 1.91 μm TDFL was used as the pump source with a beam quality factor of 1.8 and maximum output power of 50 W. With two antireflection (AR)-coated lenses, F 1 and F 2 , the pump light was focused into a 60 mm long active medium, keeping the beam waist position located within an Ho: YAG crystal. The pump beam waist radius was about 300 μm. A 45° dichroic mirror was used to further eliminate the residual 793 nm light. Ho3 dopant concentration of 0.7 at.% was adopted for the laser rod. The laser rod was wrapped with indium foil and conductively water cooled through a copper heat-sink at the temperature of 15°C. The U-shaped cavity included a plane mirror M1, two 45° dichroic mirrors DM2 , and a concave output mirror M2. The reflectivity (R 2 ) of M2 was 50% around 2.09 μm. The radius of curvature (R oc ) of M2 was 0.3 m. The SA, placed near M2. Cr: CdSe crystal, as shown in the inset of Fig. 1, was supplied by the Institute for Single Crystals in Ukraine with a dimension of Φ9 mm × 3 mm. The Cr2 dopant concentration is 2 × 1019 cm−3 . The geometric cavity length was about 160 mm. The ideal TEM00 mode radius is verified to be 315 μm, which matches the pump size. The power measurement was finished with a 30 W power meter (30(150)ASV-SH, Ophir Optronics). The laser wavelength was measured by a high-resolution spectral analyzer (AQ6375, Yokogawa). The pulse signal was received by a high-speed InGaAs detector with a bandwidth of 1 GHz. Meanwhile, an oscilloscope (MDO3104, Tektronix) was used to display and analyze the single pulse profile and pulse sequence. Considering the special spectral properties of our TDFL, as shown in Fig. 2, only several pump currents were chosen as the

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Fig. 1. Layout of a TDFL-pumped Cr: CdSe PQS Ho: YAG laser. (DM1 , HT @ 0.79 μm & HR @ 1.91 μm; DM2 , HT @ 1.91 μm & HR @ 2.09 μm; M1 , HR @ 1.9 ∼ 2.15 μm; M2 , T 50%@ 1.9 ∼ 2.15 μm).

Fig. 3. (a) Nonlinear bleaching property of a Cr: CdSe crystal measured with a pulsed 2.09 μm laser. (b) Experimental absorption spectra of Cr: CdSe. Fig. 2. Typical output spectra of a TDFL when the pump currents were set at 1.45, 1.65, and 1.98 A.

Fig. 3(a) shows the experimental data and the fitting curve (the red solid line), which follows the equation [23] working points. When the pump currents of the TDFL were set at 1.60 A, 1.90 A, and 2.20 A, the common feature of the output spectra for them is that a severe side mode, around 1905.8 nm, exits on the left of the expected fundamental mode, around 1907.4 nm. This feature mainly resulted from the current bad quality FBGs and resulted in the almost zero output of an Ho: YAG laser. The Ho: YAG laser output came back to normal only with a little offset of the pump currents. Figure 2 shows three typical spectra of the TDFL at 1.65 (10.5 W), 1.95 (15.3 W), and 1.98 A (24.4 W). The side mode becomes very weak. The corresponding continuous-wave (CW) output powers were 1.67 W, 3.66 W, and 5.64 W, respectively. Thus, the PQS experiment was mainly operated at the above three points. The saturable absorption property of Cr: CdSe is critical for investigating the possibility for a PQS laser, as well as precisely estimating some important spectral data. In our experiment, an AQS Ho: YAG laser at 2090 nm with the pulse energy of 1.8 mJ and pulse duration of 30 ns was used as the pulse source. A simple 30 mm focal-length lens was adopted to focalize the beam spot radius to about 76 μm. To obtain the pulse fluence in the 0.05–8.6 J∕cm2 range, the distance between the SA and the focal lens was changed gradually from 150 to 34 mm. It should be noted that the SA was not AR coated just for this operation. The SA was damaged when it was exactly placed at the focal point of lens. Thus, the laser-induced damage threshold of uncoated Cr: CdSe is around 9.92 J∕cm2 .

T F e 1 − T ns − ΔT · exp−F e ∕F e;sat ;

(1)

where F e is the incident pulse fluence on the SA, T is the total transmission, ΔT is the modulation depth, T ns is the nonsaturation loss, and F e;sat is the pulse saturation fluence. With our measurement, the maximum transmission was about 78%, and the minimum transmission was about 42%. Thus, ΔT was estimated as 36%, and T ns was estimated as 22%. When F e;sat 1.06 J∕cm2 , the fitting curve right coincides with the experimental data, as shown in Fig. 3(a). The corresponding saturation peak power intensity was about 35.3 MW∕cm2 . The large T ns is mostly due to the current low surface polishing quality, the surface Fresnel reflections, and the intrinsic crystal absorption. It is hopeful to further reduce T ns to about 10% when the polishing process for soft crystals is improved, and the surface is AR coated. The ideal bleaching curve of Cr: CdSe crystal is predicted as the black dotted curve in Fig. 3(a). The bleaching curve can also be roughly simulated with the modified Avizonis– Grotbeck equation [10] ∂F e z∕∂z −hvs N a0 ∕2f1 − exp−2σ a F e z∕hvs g − αa F e z;

(2)

where vs is the light frequency, N a0 is the Cr2 dopant concentration, σ a is the ground-state absorption cross section, and αa is the nonsaturable loss coefficient in SA. σ a is determined by


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Table 1. Output Characteristics of a Cr: CdSe PQS Ho: YAG Lasera P p W P C W P Q W f s Hz E s mJ τ s ns P peak kW 10.5 15.3 24.4

1.67 3.66 5.64

0.377 0.884 1.21

510 595.5 685

0.739 1.484 1.766

11.37 13.34 15.4

64.6 111.2 114.7

a (I p is the pump current, P p is the 1.91 μm pump power, P C is the 2.09 μm output power in the CW operation, P Q is the 2.09 μm output power in the PQS operation, f s is the pulse repetition frequency, E s is the single pulse energy, τs is the pulse duration, and P peak is the pulse peak power.)

σ a hvs ∕F e;sat :

(3)

The theoretical bleaching curve, shown as the blue dotted line in Fig. 3(a), was then obtained with these parameters: N a0 2 × 1019 cm−3 , F e;sat 1.06 J∕cm2 , and αa 0.1512 cm−1 . Since the excited-state absorption is neglected, the theoretical result here is consistent with our predicted ideal bleaching curve. With Eq. (3), σ a is estimated as 8.97 × 10−20 cm2 , which is very close to the experimental value, 9.384 × 10−20 cm2 , at 2090 nm, which is shown in Fig. 3(b). The broad absorption band (1.5 ∼ 2.3 μm) is related to the transition 5 T 2 5 D → 5 E5 D of Cr2 . Cr: CdSe was AR coated in 1.9 ∼ 2.15 μm range for the PQS experiment. Table 1 presents the detailed output characteristics of a PQS Ho:YAG laser at the above three working points. The pulse energies were calculated as 0.739, 1.484, and 1.766 mJ at 1.45, 1.65, and 1.98 A, respectively. The PQS efficiencies, P Q ∕P C , were 22.57%, 24.15%, and 21.45%, respectively. The corresponding pulse peak powers

Fig. 4. (a) Pulse train at 1.45 A, (b) pulse train at 1.65 A, (c) pulse train at 1.98 A, and (d) typical pulse envelope at the above three working points.

Fig. 5. Output laser spectra in the CW operation and PQS operation at the pump current of 1.98 A.

were 64.6, 111.2, and 114.7 kW, respectively. Another higher pump current, 2.79 A, was also a qualified working point. The 2.09 μm output power was about 8.9 W in the CW operation, but it resulted in the coating damage of the SA in the PQS operation, as shown in the inset of Fig. 4(d). Currently, the 2 μm coating damage threshold of the SA is close to 30 MW∕cm2 . The laser spot radius at the position of the SA was around 500 μm. It can be predicted that the circulating pulse energy at the position of the SA is not more than 4.712 mJ, assuming the pulse duration of 20 ns at 2.79 A. Thus, the output pulse energy should be less than 2.356 mJ. Assuming the PQS efficiency is 22% at 2.79 A, the pulse repetition frequency is predicted to be about 830 Hz, which basically conforms to the experimental trend. Finally, the estimated laser-induced damage threshold of the coated Cr: CdSe SA is around 0.6 J∕cm2 , which is far less than the value of uncoated Cr: CdSe, around 9.92 J∕cm2 . Figures 4(a)–4(c) depict the pulse sequences at 1.45, 1.65, and 1.98 A, respectively. The pulse stability can be described by the standard deviation, σ p , of the pulse peak intensity. Here σ p were worked out as 4.44%, 6.11%, and 1.19% at the above three working points, respectively. Three typical pulse profiles are shown in Fig. 4(d), corresponding to the pulse duration of 11.37, 13.34, and 15.4 ns, respectively. The output laser spectra were precisely analyzed at 1.98 A, as shown in Fig. 5. The peak wavelength in the PQS operation was 2090.2 nm, a little longer than that in the CW operation as 2090.1 nm. In summary, we have first presented a high-peak-power Cr: CdSe PQS Ho: YAG laser directly excited by the TDFL. The maximum pulse peak power was 114.7 kW, corresponding to a stable output pulse energy of 1.77 mJ and a short pulse width of 15.4 ns. We believe that the pulse energy can be clamped around 2.4 mJ. Meanwhile the pulse repetition frequency can be gradually increased with higher pump power, which should be useful for certain applications. The laser performance indicates that Cr: CdSe crystal is an effective candidate SA for a 2 μm PQS laser. Funding. National Natural Science Foundation of China (NSFC) (61275146). † These authors contributed equally to this Letter.

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