Overview of Thesis - 以高功率摻釹晶體雷射實現有效非線性轉換: 從人眼安全波段至紫外及深紫外波段

Chapter 1...................................................................................................... 1

1.4 Overview of Thesis

This thesis is organized as follows. Chapter 1 gives the general introduction to the property and development of diode-pumped solid-state laser. Operation principles for Q-switching and nonlinear frequency conversion are also addressed.

Then, the features of commonly used Nd:YVO4 and Nd:YLF crystals are briefly discussed in 2.1 and 2.4, respectively. For the Nd:YVO4 crystal, we take into account of the thermal-lensing effect and second threshold condition to design a high-peak-power passively Q-switched (PQS) laser at 1064 nm with the Cr⁴⁺:YAG crystal as a saturable absorber. We also consider the parasitic lasing effect and thermal-lensing effect to optimize a high-peak-power actively Q-switched (AQS) Nd:YVO4 laser at 1064 nm. As for the Nd:YLF crystal, a novel method is proposed to efficiently select the σ-polarization at 1053 nm in the a-cut crystal, and the PQS performance with the Cr⁴⁺:YAG crystal is systematically investigated for various output couplings. We further present a practical tactic to scale up the pulse energy of the continuously pumped PQS laser at the ⁴F3/2 → ⁴I11/2 transition with the c-cut Nd:YLF crystal. Pulsed pumping is subsequently utilized to reduce the thermal effect and improve the timing jitter of a mJ- and ns-level PQS Nd:YLF/Cr⁴⁺:YAG laser. AQS operation in the Nd:YLF crystal is also realized to provide a sequence of giant pulses with continuously adjustable pulse repetition rate. It is worthwhile to mention that all of above-mentioned achievements are optimized on the basis of extremely simple and compact linear cavity configurations.

In previous studies on the eye-safe radiations obtained from the Nd-doped crystal lasers, shared and coupled cavity configurations for intracavity optical parametric oscillator (IOPO) are adopted. However, the fundamental IR and eye-safe cavities can not be optimized independently, which in turn restricts the optical conversion efficiency and long-term stability. In Chap. 3, we develop a separable monolithic IOPO cavity to remarkably improve the performance of the PQS Nd:YVO4/Cr⁴⁺:YAG/KTP eye-safe laser at 1572 nm. With the same concept, we demonstrate a compact and efficient high-energy AQS Nd:YLF eye-safe laser at 1552 nm. In addition, it is found that the thermally induced birefringence can lead the mutually orthogonal polarization states of the fundamental IR pulses to be effectively switched for accomplishing an efficient IOPO operation without any extra polarization control.

In Chap. 4, the optimized Q-switched Nd-doped crystal lasers designed in Chap. 2 are employed to produce green, UV, and DUV radiations via extracavity harmonic generations. Firstly, the output performance between the extracavity and intracavity SHGs (ESHG and ISHG) at 532 nm under a similar operated condition is thoroughly explored with the type-I LBO crystal as a frequency doubler. Secondly, with the type-I and type-II LBO crystals as a frequency doubler and a frequency tripler, highly efficient UV emissions at 355 and 351 nm are obtained from the high-power Nd:YVO4 lasers and high-energy Nd:YLF laser, respectively. Thirdly, the green lasers based on ESHG and ISHG are utilized as pump sources to make a comparison of the conversion efficiencies for producing 266-nm radiations, where the BBO crystal is exploited in the process of extracavity fourth harmonic generation (EFHG). Efficient nonlinear frequency conversions demonstrated in Chaps. 3 and 4 not only enable us to extend the emission lines from IR to eye-safe, green, UV, and DUV regimes, but also validate the usefulness of our cavity design for Q-switched IR lasers developed in Chap. 2.

Finally, a summary is given in Chap. 5 to conclude this thesis. Future plans and prospects are also described in this chapter with the aim for completing this doctoral thesis.

References

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Berlin, 2005), Chap. 1.

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[19] Y. Sato, T. Taira, N. Pavel, and V. Lupei, “Laser operation with near quantum-defect slope efficiency in Nd:YVO4 under direct pumping into the emitting level,” Appl. Phys. Lett. 82, 844-846 (2003).

[20] P. Zhu, D. Li, P. Hu, A. Schell, P. Shi, C. R. Haas, N. Wu, and K. Du, “High efficiency 165 W near-diffraction-limited Nd:YVO4 slab oscillator pumped at 880 nm,” Opt. Lett. 33, 1930-1932 (2008).

[21] N. Pavel, T. Dascalu, N. Vasile, and V. Lupei, “Efficient 1.34-μm laser emission of Nd-doped vanadates under in-band pumping with diode lasers,” Laser Phys.

Lett. 6, 38-43 (2009).

[22] X. Ding, R. Wang, H. Zhang, X. Y. Yu, W. Q. Wen, P. Wang, and J. Q. Yao,

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Opt. Commun. 282, 981-984 (2009).

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[24] Y. F. Lü, X. H. Zhang, X. D. Yin, J. Xia, A. F. Zhang, and J. Q. Lin, “Highly efficient continuous-wave intracavity frequency-doubled Nd:YVO4-LBO laser at 457 nm under diode pumping into the emitting level ⁴F3/2,” Appl. Phys. B 99, 115-119 (2010).

[25] L. Cui, H. L. Zhang, L. Xu, J. Li, Y. Yan, P. F. Sha, L. P. Fang, H. J. Zhang, J. L.

He, and J. G. Xin, “880 nm laser-diode end-pumped Nd:YVO4 slab laser at 1342 nm,” Laser Phys. 21, 105-107 (2011).

[26] W. Koechner, Solid-State Laser Engineering, 6^th edn. (Springer, Berlin, 2006), Chap. 8.

[27] R. Paschotta, Field Guide to Lasers, (SPIE, Bellingham, Washington, 2007).

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[30] W. Koechner, Solid-State Laser Engineering, 6^th edn. (Springer, Berlin, 2006), Chap. 10.

[31] R. W. Boyd, Nonlinear optics, 3^rd edn. (Elsevier, London, 2008).

[32] M. J. Weber, Handbook of laser wavelengths, (CRC, New York, 1999).

[33] http://en.wikipedia.org/wiki/List_of_laser_types [34] http://www.laserfest.org/lasers/innovations.cfm [35] http://www.rp-photonics.com/laser_applications.html

[36] http://www.photonics.com/LinearCharts/Default.aspx?ChartID=1

[37] http://www.coherent.com/Products/index.cfm?868/AVIA-Family-of-DPSS-Lase rs

[38] http://www.jdsu.com/en-us/Lasers/Products/A-Z-Product-List/Pages/laser-solid-state-q-switched-355-532-q-series.aspx

Chapter 2

Fundamental IR Lasers with Nd-doped Crystals

2.1 Properties of Nd:YVO

₄

Crystal

Nd-doped yttrium vanadate (Nd:YVO4) is an important laser material which was first recognized in 1966 [1], and efficient diode-pumped Nd:YVO4 laser was successfully demonstrated in 1987 [2]. Basic properties of the Nd:YVO4 crystal are illustrated in Fig. 2.1.1, which is obtained from CASTECH [3]. The Nd:YVO4 crystal belongs to the tetragonal group in crystal structure. The natural birefringence of this uniaxial crystal dominates the thermally induced birefringence, and subsequently the linearly polarized output can eliminate the undesirable thermal depolarization loss in the high-power operation, which is frequently observed in optically isotropic laser crystals such as the Nd:YAG crystal. Optical properties of the Nd:YVO4 crystal are strongly polarization dependent, which can be classified as π-polarization (extraordinary beam) and σ-polarization (ordinary beam). The π- and σ-polarizations are defined as the oscillated polarizations of the light to be parallel and perpendicular to the crystallographic c axis, respectively. Unlike the Nd:YAG crystal, the Stark splitting in the Nd:YVO4 crystal is small and the multiple transitions are more compact. These lead the Nd:YVO4 crystal to possess the outstanding features over other Nd-doped crystals;

that is, the large absorption coefficient around 808 nm and high stimulated emission cross section at 1064 nm. The former property allows the use of short crystal to efficiently absorb the incident pump light for the construction of a compact microchip laser, while the latter one is inherently suitable for developing a high-repetition-rate pulsed laser.

Fig. 2.1.1. Basic properties of the Nd:YVO4 crystal.

2.2 Passively Q-Switched Nd:YVO

₄

Laser at 1064 nm

I. Introduction

Passive Q-switching of a solid-state laser with a saturable absorber is a convenient way to provide a reliable pulsed operation with the benefits of high stability, inherent compactness, and low cost. As a promising saturable absorber near the IR region, the Cr⁴⁺:YAG crystal has been widely investigated on the PQS performance thanks to its good chemical and mechanical stability, long lifetime, excellent optical quality, high damage threshold, high thermal conductivity, and large absorption cross section [4-12].

However, the stimulated emission cross section of the Nd:YVO4 crystal at 1064 nm is too large to achieve a good PQS operation when the Cr⁴⁺:YAG crystal is used as a saturable absorber. Several methods have been proposed to overcome the well-known condition for good passive Q-switching, including the intracavity focusing obtained from the three-element resonator [13-15] and the employment of a c-cut crystal as a gain medium [16-18]. Nevertheless, the peak powers with the above-mentioned reports are not high enough for some practical applications, especially for efficient extracavity nonlinear frequency conversion. Therefore, it is highly desirable to develop a high-peak-power PQS Nd:YVO4 laser with the Cr⁴⁺:YAG saturable absorber.

In this section, we take into account the second threshold and thermal-lensing effect to design and realize a compact reliable PQS Nd:YVO4 laser with the Cr⁴⁺:YAG crystal as a saturable absorber. At an incident pump power of 16.3 W, the output power at 1064 nm reaches 6.16 W with a pulse width of 7 ns and a pulse repetition rate of 56 kHz. The corresponding pulse energy and peak power are evaluated to be 111 μJ and 16 kW, respectively.

II. Cavity analysis

It is well known that the absorption saturation in the saturable absorber should occur before the gain saturation in the laser crystal for good PQS operation. The so-called second threshold condition has been analytically derived from the coupled rate equation, which can be mathematically expressed as [13,19]:

where T0 is the initial transmission of the saturable absorber, ROC is the reflectivity of the output coupler, Ls is the nonsaturable round trip dissipative loss of the resonator, σgsa

is the ground-state absorption of the saturable absorber, σ is the emission cross section of the laser crystal, A/As is the ratio of the laser mode area in the laser crystal to that in the saturable absorber, γ is the population inversion reduction factor, which is equal to one for the ideal four-level laser and two for the three-level laser, and β = σesa/σgsa is the ratio of the excite excited-state absorption cross section to the ground-state absorption cross section of the saturable absorber. With the following parameters: T0 = 0.7, ROC = 0.5, Ls = 0.03, σgsa = 2 × 10^-18 cm² [12], σ = 2.5 × 10^-18 cm², γ = 1, and β = 0.06 [12], the second threshold condition can be deduced to be A/As > 2.68 in the case of the Nd:YVO4 and Cr⁴⁺:YAG crystals as a gain medium and a saturable absorber, respectively. As a consequence, the ratio of the laser mode radius in the laser crystal to that in the saturable absorber needs to be larger than 1.64 for achieving a high-quality PQS operation.

The configuration for a simple plano-concave resonator with the thermal-lensing effect is schematically shown in Fig. 2.2.1(a). In the present experiment, the laser crystal and saturable absorber are aimed to be as close as possible to the input concave mirror and flat output coupler, respectively. An optical resonator with an internal thermal lens between the resonator mirrors can be replaced by an empty cavity with the equivalent g-parameters g^* and the equivalent cavity length Lcav*, which are given by [20]:

i, j = 1, 2; i ≠ j, where fth is the effective thermal focal length, d1 and d2 are the optical path lengths from the center of the gain medium to the input mirror and output coupler, R1 and R2 are the radius of curvature (ROC) of the input mirror and output coupler. In terms of the equivalent cavity parameters, the cavity mode radii at the input mirror (ω1) and at the output coupler (ω2) are given by [20]:

As a result, we can calculate the variations of the cavity mode radius ω1 and ω2 with respect to the effective thermal focal length. The effective focal length of thermal lens in the end-pumped laser crystal can be estimated with the following equation [21]:

where ξ is the fraction of the incident pump power that results in heat, Pin is the incident pump power, Kc is the thermal conductivity, α is the absorption coefficient at the pump wavelength λp, lcry is the crystal length, dn/dT is the thermal-optic coefficient, n is the refractive index, and αT is the thermal expansion coefficient, M² is the pump beam quality factor, and ωp(z) is the variation of the pump radius, where the pump beam waist ωpo is assumed a distance z0 from the entrance of the laser crystal. With the following parameters: ξ = 0.24, Kc = 5.23 W/m K, α = 0.2 mm^-1, λp = 808 nm, lcry = 12 mm, dn/dT

= 3 × 10^-6 K^-1, n = 2.1652, αT = 4.43 × 10^-6 K^-1, M² = 230, ωpo = 300 μm, and z0 = 1 mm, the effective thermal focal length can be calculated as a function of the incident pump power. To be brief, the dependence of the ratio ω1/ω2 on the incident pump power can be generated to design and realize a high-quality PQS laser. Figure 2.2.1(b) depicts the calculated results for the cases of Lcav = 90, 80, 70, 60 and 50 mm, where the Lcav stands for the cavity length and the other parameters used in calculation are as follows: R1 = 100 mm, R2 → ∞, d1 = 6 mm, d2 = (Lcav – 6) mm. From the Fig. 2.2.1(b), it is obvious that the thermal lensing effect will make the cavity to be unstable when the cavity

length is too long; whereas the PQS laser can not well operate in a high-quality state when the cavity length is too short. Comparative speaking, we chose a resonator with Lcav = 70 mm to simultaneously satisfy the second threshold criterion and cavity stability condition to realize a compact reliable PQS laser.

Fig. 2.2.1. (a) The configuration for a simple plano-concave cavity with the thermal lensing effect; (b) Calculated results for the ratio of the cavity mode size in the gain medium to that in the saturable absorber as a function of the incident pump power for the cases of Lcav = 90, 80, 70, 60, and 50 mm when the ROC of the input mirror is chosen to be R1 = 100 mm.

Incident pump power at 808 nm (W)

0 2 4 6 8 10 12 14 16 18

Ratio of mode radius  1/ 2

1 2 3 4 5 6 7

L_cav = 80 mm

L_cav = 60 mm L_cav = 70 mm

L_cav = 50 mm L_cav = 90 mm

L_cav f_th

(b)

(a)

III. Experimental setup

The experimental setup is schematically shown in Fig. 2.2.2. The input mirror was a concave mirror with the ROC of 100 mm. It was antireflection (AR) coated at 808 nm on the entrance face, and was coated at 808 nm for high transmission as well as 1064 nm for high reflection on the second surface. The gain medium was a 0.1 at. % a-cut Nd:YVO4 crystal with dimensions of 3 × 3 × 12 mm³, and it was placed as close as possible to the input mirror. Both facets of the laser crystal were AR coated at 808 and 1064 nm. The Cr⁴⁺:YAG saturable absorber with an initial transmission of 70 % was AR coated at 1064 nm on both surfaces, and it was placed near to the output coupler.

The laser crystal and saturable absorber were wrapped with indium foil and mounted in water-cooled copper heat sinks at 20 °C. The pump source was an 18-W 808-nm fiber-coupled laser diode with a core diameter of 600 μm and a numerical aperture of 0.2. 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 91 %.

Therefore, the maximum incident pump power in our experiment was approximately 16.3 W. The flat output coupler with 50-% transmission was employed during the experiment. As designed in subsection II, the cavity length was set to be 70 mm for the construction of a compact high-power PQS laser. The pulse temporal behaviors were recorded by a LeCroy digital oscilloscope (Wavepro 7100, 10 G samples/s, 1 GHz bandwidth) with a fast Si photodiode.

Fig. 2.2.2. Schematic of the cavity setup for a diode-pumped PQS Nd:YVO4 laser with the Cr⁴⁺:YAG saturable absorber.

L_cav Laser diode

_p= 808 nm,  = 600 m

0.1 % Nd:YVO₄ Cr⁴⁺:YAG, T₀= 70 % Output coupler R_OC= 50 % @ 1064nm Input mirror

R₁= 100 mm

IV. Performance of CW and PQS operations

First of all, the continuous-wave (CW) operation without the saturable absorber is studied. The output power as a function of the incident pump power is presented by the red curve in Fig. 2.2.3(a). The pump threshold and slope efficiency are determined to be 2.1 W and 62 %, respectively. At the maximum incident pump power of 16.3 W, the output power of 8.8 W is obtained, corresponding to the optical conversion efficiency from 808 to 1064 nm up to 54 %.

We then inserted the Cr⁴⁺:YAG saturable absorber into the laser cavity to investigate the PQS performance in detail. The dependence of the output power on the incident pump power in the PQS operation is illustrated by the green curve in Fig.

2.2.3(a). The pump threshold and slope efficiency are found to be 3.3 W and 47.4 %, respectively. At the maximum incident pump power of 16.3 W, the output power as high as 6.16 W is obtained, corresponding to the optical conversion efficiency up to 37.8 %. Figures 2.2.3(b) and (c) show the pulse width, pulse repetition rate, pulse energy, and peak power as a function of the incident pump power. When the incident pump power increases from 5 to 16.3 W, the pulse repetition rate varies from 15.5 to 56 kHz and the pulse width changes from 20 to 7 ns, as shown in Fig. 2.2.3(b).

Accordingly, it can be seen that the pulse energy increases from 27 to 111 μJ and the peak power increases from 1.3 to 16 kW by increasing the incident pump power from 5 to 16.3 W, as revealed in Fig. 2.2.3(c). Figures 2.2.4(a) and (b) show the typical oscilloscope traces of the output pulses at 1064 nm under an incident pump power of 16.3 W with the time span of 200 and 2 μs, respectively. Note that the appearance of the satellite pulses following the main Q-switched pulse was frequently observed in the past research [22-25]. This phenomenon inevitably degrades the Q-switched performance, leading to the restriction of the maximum achievable Q-switched pulse energy and peak power. However, we didn’t observe any satellite pulses during the present experiment, indicating the validness of our cavity optimization.

Fig. 2.2.3. (a) Output powers in CW (red curve) and PQS (green curve) operations as a function of the incident pump power; (b) Dependences of the pulse repetition rate (red curve) and pulse width (green curve) on the incident pump power; (c) Dependences of the pulse energy (red curve) and peak power (green curve) on the incident pump power.

Incident pump power at 808 nm (W)

4 6 8 10 12 14 16 18

Incident pump power at 808 nm (W)

0 2 4 6 8 10 12 14 16 18

Incident pump power at 808 nm (W)

4 6 8 10 12 14 16 18

Fig. 2.2.4. Typical oscilloscope traces of the output pulses at 1064 nm under an incident pump power of 16.3 W with the time span of (a) 200 and (b) 2 μs.

20 s/div 20 s/div

200 ns/div 200 ns/div

(b)

(a)

2.3 Actively Q-Switched Nd:YVO

₄

Laser at 1064 nm

I. Introduction

Compared with the PQS laser, AQS solid-state laser can provide a high-stability, low timing jitter and high-peak-power pulsed operation with a continuously adjustable pulse repetition rate. The acousto-optic (AO) Q-switch, which is characterized by its low-insertion loss, is one of promising methods to achieve AQS operation. It can exceptionally offer the convenience of converting from repetitively Q-switched to CW operations simply by removing the RF drive power [26-31].

Nevertheless, it is observed that the parasitic lasing effect is a critical issue for scaling up the output peak powers of the AQS lasers [26,32,33], and this detrimental

在文檔中以高功率摻釹晶體雷射實現有效非線性轉換: 從人眼安全波段至紫外及深紫外波段 (頁 38-0)