Actively Q-Switched Nd:YLF Laser at 1053 nm

I. Introduction

As discussed in Sec. 2.3, the AO Q-switch can offer the convenience of converting from CW to repetitively Q-switched operations simply by transmitting the RF drive power. In this section, we utilize the AO Q-switch to obtain a high-energy AQS Nd:YLF laser with the pulse repetition rate tunable from 5 to 40 kHz. We exhaustively explore the influences of the thermal effect and the anisotropic property of the AO Q-switch on the polarization characteristics of the c-cut Nd:YLF laser in the CW and Q-switched operation, respectively. Moreover, under an incident pump power of 12.7 W, the maximum output power of 4.5 W at 5 kHz and the largest pulse energy of 800 μJ at 40 kHz are accomplished, respectively.

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II. Experimental setup

The experimental setup for the AQS Nd:YLF laser is schematically shown in Fig.

2.8.1. The input mirror was a concave mirror with the ROC of 300 mm. It was AR coated at 806 nm on the entrance face, and was coated at 806 nm for high transmission as well as 1053 nm for high reflection on the second surface. The gain medium was a 0.8 at. % c-cut Nd:YLF crystal (CASTECH) with the diameter of 4 mm and the length of 15 mm. Both facets of the laser crystal were AR coated at 806 and 1053 nm. Note that although it is an uniaxial crystal with the highly anisotropic property, the Nd:YLF crystal effectively exhibits the optically isotropic characteristics in the transverse plane when it is cut along the crystallographic c axis. The orientation of the rod axis of the present c-cut crystal to the crystallographic c axis was within 1 degree. The Nd:YLF crystals was wrapped with indium foil and mounted in a water-cooled copper heat sink at 18 °C. A 20-mm-long AO Q-switch (Gooch & Housego) was AR coated at 1053 nm on both surfaces. It was placed in the center of the laser cavity, and was driven at a central frequency of 41 MHz with a RF power of 25 W. A flat mirror with a reflectivity of 80 % at 1053 nm was utilized as the output coupler during the experiment. The pump source was an 806-nm fiber-coupled laser diode with a core diameter of 600 μm and a numerical aperture of 0.2, respectively. The polarization state emitted from the fiber-coupled laser diode was measured to be randomly polarized. The pump beam was reimaged into the laser crystal with a lens set that has a focal length of 25 mm with a magnification of unity and a coupling efficiency of 90 %. The cavity length was set to be Lcav = 115 mm for the construction of a compact AQS laser. The pulse temporal behaviors were recorded by a LeCroy digital oscilloscope (Wavepro 7100, 10 G samples/s, 1 GHz bandwidth) with a fast InGaAs photodiode.

Fig. 2.8.1. Experimental setup for the AQS Nd:YLF laser.

AO Q-switch

L_cav= 115 mm

Output coupler R_OC= 80 % @ 1053 nm

0.8 % Nd:YLF Laser diode

_p= 806 nm, = 600 m

Input mirror

R₁= 300 mm AO Q-switch

L_cav= 115 mm

Output coupler R_OC= 80 % @ 1053 nm

0.8 % Nd:YLF Laser diode

_p= 806 nm, = 600 m

Input mirror R₁= 300 mm

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III. Performance of CW and AQS operations

First of all, the AO Q-switch was removed from the laser cavity to investigate the CW performance of the c-cut Nd:YLF laser. We utilized an intracavity polarizer to make a comparative study of the output characteristics between the linearly and randomly polarized states, respectively. Figure 2.8.2(a) illustrates the output powers at 1053 nm with and without an intracavity polarizer versus the incident pump power at 806 nm. The maximum output power and slope efficiency without an intracavity polarizer are found to be up to 4.7 W and 43.1 %, respectively, as depicted by the red curve in Fig. 2.8.2(a). However, the output power and slope efficiency obtained with an intracavity polarizer are found to be remarkably lower than those obtained without an intracavity polarizer, as revealed by the green curve in Fig. 2.8.2(a). Moreover, the roll-over phenomenon in the linearly polarized state was experimentally observed at an incident pump power of 10.4 W. In the early researches on the solid-state laser, it was found that the thermally induced birefringence of the optically isotropic material brings in the coupling of the power between the mutually orthogonal polarization components.

Consequently, the forbidden polarization state would be removed with the introduction of a polarizer inside the laser cavity [21]. This so-called thermal depolarization loss undoubtedly explains why substantially decreased output power and considerably poorer slope efficiency are obtained in the present linearly polarized c-cut Nd:YLF laser.

We then inserted the AO Q-switch into the laser cavity without an intracavity polarizer to explore the polarization characteristics of the c-cut Nd:YLF laser. Figure 2.8.2(b) describes the dependences of the polarization ratio Phorizontal/Pvertical on the incident pump power at a pulse repetition rate of 5, 8, 10, 20, and 40 kHz, where Phorizontal and Pvertical stand for the output power with the oscillated polarization to be parallel and perpendicular to the base of the AO Q-switch, respectively. The polarization ratios for all cases are found to continuously decrease with the increase of the incident pump power. This observation is similar to the works reported in Refs.

[62,63]. The diffractive efficiency of the AO Q-switch operated at the compressional mode is polarization dependent, in which the light with the oscillated polarization that is parallel to the propagation of the acoustic wave experiences lower diffractive loss. In the meantime, the randomly polarized pump beam leads the gain distribution in the c-cut Nd:YLF crystal to be isotropic; that is, the gains for the mutually orthogonal

polarization components of the laser beam are the same. As a consequence, the lower diffractive loss makes the horizontally polarized laser beam to own the larger net gain as compared with the vertically polarized one, which produces a high degree of the linearly polarized operation at a low incident pump power. However, the polarization ratio is experimentally found to decrease with increasing the incident pump power owing to the reduced difference of the net gain between the mutually orthogonal polarization components. Eventually, a nearly random polarization state was acquired at the maximum incident pump power of 12.7 W.

Figure 2.8.3 depicts the dependences of the output power, pulse width, pulse energy, and peak power on the pulse repetition rate at an incident pump power of 12.7 W. When the pulse repetition rate increases from 5 to 40 kHz, the output power varies from 4 to 4.5 W and the pulse width increases linearly from 25 to 180 ns, as shown in Fig. 2.8.3(a). Consequently, it can be found that the pulse energy changes from 800 to 113 μJ and the peak power decreases from 32 to 0.63 kW by increasing the pulse repetition rate from 5 to 40 kHz, as revealed in Fig. 2.8.3(b). Figures 2.8.4(a)-(d) illustrate the pulse trains of the AQS Nd:YLF laser at a pulse repetition rate of 5, 40, 50, and 100 kHz, respectively. It is experimentally found that the pulse-to-pulse amplitude stability is better than ± 8 % when the laser operates at 5-40 kHz, as exhibited in Figs.

2.8.4(a) and (b). Nevertheless, increasing the pulse repetition rate beyond 50 kHz results in an unstable Q-switched operation with the amplitude fluctuation larger than 20 %, as revealed in Fig. 2.8.4(c). Moreover, the phenomenon of the pulse missing shown in Fig.

2.8.4(d) is observed at a pulse repetition rate of 100 kHz owing to the lack of the gain for the Q-switched Nd:YLF laser operated at such high pulse repetition rate. Therefore, it is of crucial importance to design an intricate cavity for the stable pulsed operation if the Q-switched Nd:YLF laser with high pulse repetition rate is required [53].

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Fig. 2.8.2. (a) Output powers at 1053 nm with and without an intracavity polarizer versus the incident pump power at 806 nm in the CW operation; (b) The polarization ratios Phorizontal/Pvertical with respect to the incident pump power at a pulse repetition rate of 5, 8, 10, 20, and 40 kHz, where Phorizontal and Pvertical represent the output powers with the oscillated polarization to be parallel and perpendicular to the base of the AO Q-switch, respectively.

Incident pump power at 806 nm (W)

f = 5 kHz f = 8 kHz f = 10 kHz f = 20 kHz f = 40 kHz

AQS operation without an intracavity polarizer

0 2 4 6 8 10 12 14

Output power at 1053 nm (W)

Incident pump power at 806 nm (W)

CW operation without an intracavity polarizer CW operation with an intracavity polarizer

(b)

(a)

Fig. 2.8.3. Dependences of the (a) output power, pulse width, (b) pulse energy and peak power at 1053 nm on the pulse repetition rate at an incident pump power of 12.7 W.

Output power at 1053 nm (W)

0 50 100 150 200

Pulse width at 1053 nm (ns)

P_in = 12.7 W

Pulse energy at 1053 nm (J)

0.1 1 10 100 P_in = 12.7 W

Peak power at 1053 nm (kW)

(b)

(a)

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Fig. 2.8.4. Pulse trains of the Q-switched Nd:YLF laser at a pulse repetition rate of (a) 5 kHz, (b) 40 kHz, (c) 50 kHz, and (d) 100 kHz. The dashed circle in Fig. 4(d) indicates the phenomena of the pulse missing.

200 s/div

f = 5 kHz f = 40 kHz 25 s/div

20 s/div

f = 50 kHz f = 100 kHz 10 s/div

(a)

(c)

(b)

(d)