Alexandrite: an attractive thin-disk laser material alternative to Yb:YAG?

Umit Demirbas; Franz X. Kärtner

doi:10.1364/JOSAB.380140

Journal of the Optical Society of America B
Vol. 37,
Issue 2,
pp. 459-472
(2020)
•https://doi.org/10.1364/JOSAB.380140

Alexandrite: an attractive thin-disk laser material alternative to Yb:YAG?

Umit Demirbas and Franz X. Kärtner

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Umit Demirbas and Franz X. Kärtner, "Alexandrite: an attractive thin-disk laser material alternative to Yb:YAG?," J. Opt. Soc. Am. B 37, 459-472 (2020)

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Abstract

Yb:YAG thin-disk (TD) technology has enabled construction of laser/amplifier systems with unprecedented average/peak power levels, and has become the workhorse of many scientific investigations. On the other hand, for some applications, the narrow emission bandwidth of Yb:YAG limits its potential, and the search for alternative broadband TD gain media with suitable thermo-optomechanical parameters is ongoing. The alexandrite gain medium has a broad emission spectrum centered around 750 nm, possesses thermomechanical strength that even outperforms Yb:YAG, and has unique spectroscopic properties enabling efficient laser operation even at elevated temperatures. In this work, we have numerically investigated the power scaling potential of continuous-wave (cw) alexandrite lasers in TD geometry for the first time. Using a detailed laser model, we have compared the potential cw laser performance of Yb:YAG, Ti:Sapphire, Cr:LiSAF, Cr:LiCAF, and alexandrite thin-disk lasers under similar conditions and show that among the investigated transition metal-doped gain media, alexandrite is the best alternative to Yb:YAG in power scaling studies at room temperature. Our analysis further demonstrates that potentially Ti:Sapphire is also a good alternative TD material, but only at cryogenic temperatures. However, in comparison with Yb:YAG, the achievable laser gain is relatively low for both alexandrite and Ti:Sapphire, which then requires usage of low-loss cavities with small output coupling for efficient cw operation.

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Gain Medium	Ti:Sapphire	Alexandrite	Yb:YAG
Dopant site	$({T i}_{x} {A l}_{1 - x})_{2} O_{3}$	$B e ({A l}_{1 - x} {C r}_{x})_{2} O_{4}$	$({Y b}_{x} Y_{1 - x})_{3} {A l}_{5} O_{12}$
Dopant density at 1% doping [ ${\times 10}^{20} i o n s / {c m}^{3}$ ]	4.7 [45]	3.51 [48]	1.38 [45]
Birefringence	negative uniaxial	biaxial	isotropic
Mass density [ $g / {c m}^{3}$ ], ρ	3.98	3.69 [49]	4.56 [48]
Melting point [°C]	2040	1870	1970
Specific heat capacity [J/g°C], Cp	0.761	1.05 [46]	0.59 [48]
Knoop hardness [ $k g / {m m}^{2}$ ]	1800 (//c), 2200 (//a)	1600–2300 [50]	1320 [48]
Thermal conductivity [W/K.m], $[W / K \cdot m] κ$	30.3 (//a), 32.5(//c) [51] 1000 (@ 77 K) [52]	28 [44]	10 [53]
Thermal expansion coefficient $[{\times 10}^{- 6} / K], α$	4.8 and 5.3 [53], 0.34 (@ 77 K) [52]	5.9 (//a), 6.1 (//b), 6.7(//c) [54]	6.7 [53]
Temperature dependence of refractive index [ ${\times 10}^{- 6} / K$ ], dn/dT	13 [45], 1.9 (@ 77 K) [52]	5.9 (//a), 6.9 (//b), 15.2 (//c) [54]	9.9 [45]
Young modulus [ ${\times 10}^{9} P a$ ], E	335	469 [49]	310 [48]
Poisson’s ratio, $ν$	0.29	0.3	0.3 [48]
Fracture toughness [ ${\times 10}^{6} P a m^{1 / 2}$ ], $K_{1 c}$	2.2 [53]	2.6 [46]	1.4 [53]
Tensile (fracture) strength [ ${\times 10}^{6} P a$ ], $σ_{f}$	440	520	280
Thermal FOM [ $W / m^{1 / 2}$ ], $R_{T}^{n}$	22 [53]	14 [46]	5.1 [53]
Thermal shock resistance parameter [W/cm], $R_{T}$	44	28	10.2
Maximum temperature difference before cracking [°C], $Δ T_{m a x, d i s k}$	740	460 (//a and //b), 400 (//c)	380
Quantum defect (%), $q_{d}$	35	11 (681 nm), 17 (640 nm)	9
Emission cross section [ ${\times 10}^{- 20} {c m}^{2}$ ], $σ_{e m}$	15 (//a), 41 (//c) [14]	$\sim 0.55 @ 25^{\circ} C$ , $\sim 1.3 @ 100^{\circ} C$ (//b)	2.1 [48]
Fluorescence lifetime [µs], $τ_{f}$	3.2 [55]	262 [56]	940 [45]
Gain product [ ${\times 10}^{- 26} {c m}^{2} s$ ] $σ_{e m} τ_{f}$	48 (//a), 131 (//c)	130 @ 25°C (//b), 163 @ 100°C (//b)	1975
$T_{1 / 2}$ [°C]	$\sim 80$ [14,38]	530	–
Intrinsic slope efficiency [%], $η^{\circ}$	64 [57]	65 [41]	$> 85$ [58]
Relative strength of laser ESA, $f_{l} = σ_{l, e s a /} σ_{e m}$	$\sim 0$ [14]	$\sim 0.07 @ 25^{\circ} C$ , $\sim 0.17 @ 100^{\circ} C$ [59]	$\sim 0$ [60]
Relative strength of pump ESA, $f_{p} = σ_{p, e s a /} σ_{a b s}$	$\sim 0$	$\sim 0.7 @ 640 n m$ , $\sim 0.01 @ 681 n m$ [61]	$\sim 0$
Passive losses [%/cm]	2 [62]	0.06 [41]	0.3
Crystal FOM	150 [62]	3000 [43]	3000
Nonlinear refractive index [ ${\times 10}^{- 16} {c m}^{2} / W$ ], $n_{2}$	3.2 [27]	2 [49], 3.54 [63]	6.9 [64]
Group velocity dispersion ( ${f s}^{2} / m m$ ), GVD	56.6 [65]	60.7 [66]	66.6 [67]
Third-order dispersion ( ${f s}^{3} / m m$ ), TOD	41.4 [65]	39.5 [66]	66.7 [67]
Gain bandwidth, FWHM (nm)	260	55	8
Tuning range [nm]	660–1180 [12]	701–858 [15–19]	1016–1108
Minimum theoretical pulse duration [fs]	3.5	11.9	26
Demonstrated shortest pulse duration [fs]	$\sim 5$ [13]	70 [31]	35 [68]
Polarizability difference [ $\times 10^{- 26} {c m}^{3}$ ]	$\sim - 100$ [69]	$\sim 25$ [70]	1.95 [71]

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Journal of the Optical Society of America B