Challenges in power scaling of Cr:LiCAF lasers: effect of passive losses

Zekican Erturk; Zekican Erturk; Muharrem Kilinc; Muharrem Kilinc; Serdar Okuyucu; Serdar Okuyucu; Serdar Okuyucu; Yusuf Ozturk; Umit Demirbas; Umit Demirbas

doi:10.1364/AO.513510

Applied Optics
Vol. 63,
Issue 4,
pp. 1144-1152
(2024)
•https://doi.org/10.1364/AO.513510

Challenges in power scaling of Cr:LiCAF lasers: effect of passive losses

Zekican Erturk, Muharrem Kilinc, Serdar Okuyucu, Yusuf Ozturk, and Umit Demirbas

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Zekican Erturk, Muharrem Kilinc, Serdar Okuyucu, Yusuf Ozturk, and Umit Demirbas, "Challenges in power scaling of Cr:LiCAF lasers: effect of passive losses," Appl. Opt. 63, 1144-1152 (2024)

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Table of Contents Category
- Lasers, Optical Amplifiers, and Laser Optics
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About this Article
History
- Original Manuscript: November 16, 2023
- Revised Manuscript: January 4, 2024
- Manuscript Accepted: January 4, 2024
- Published: January 29, 2024

Abstract

In this work, we have investigated the continuous-wave (cw) lasing potential of thin slab-shaped Cr:LiCAF crystals with a low chromium doping level of around 1% and various lengths of 1 to 2 cm. These relatively long crystals with low Cr-doping facilitate the distribution of heat load in a larger volume and could enable power scaling of Cr:LiCAF lasers. However, long crystals tend to have larger passive losses, and it is also more challenging to achieve efficient mode-matching to the low-brightness pump mode in a longer gain element, which could hinder laser performance. To explore the issue, we have performed detailed cw lasing experiments in single- and multimode diode-pumped Cr:LiCAF laser systems employing crystals with different doping and length. Our results showed that current state-of-the-art crystal growth methods provide Cr:LiCAF crystals with low enough passive losses to enable cw laser efficiencies of up to 50%, even in these long samples. The pump powers available in this study (5.35 W) limited the cw powers we could achieve experimentally to 2.25 W level; however, our simulations indicate that thin slab-type Cr:LiCAF crystals with low Cr-doping have the potential to achieve cw powers above 10 W level.

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Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

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Output Power ( $W$ )	Vertical Axis $M^{2}$ ( $x$ )	Horizontal Axis $M^{2}$ ( $y$ )
0.5	4.67	1.19
1	5.86	1.33
1.5	6.03	1.27
2.2	8.69	1.29

Crystal Cr Doping (%)	Crystal Length (mm)	Crystal Absorption at 665 nm (%)	Crystal Loss at 800 nm (%)	Crystal Loss per cm (%)	Figure of Merit
0.45	20	83	0.6	0.3	300
1.25	10	91.5	0.36	0.36	690
			0.17	0.17	1465^a
1.4	12.5	96.9	0.54	0.43	645
1.25	15	97.6	0.53	0.35	705
1.25	15	97.5	0.44	0.29	850
1	20	98.1	0.69	0.35	580
3.5 [30]	5	—	$\sim 0.28$	$\sim 0.56$	$\sim 1250$
5 [6]	4	$\sim 97$	$\sim 0.05$	$\sim 0.125$	$\sim 6000$ ^a
7 [32]	4	$\sim 99$	$\sim 0.7$	$\sim 1.75$	$\sim 800$
10 [31]	2	$\sim 97$	$\sim 0.125$	$\sim 0.625$	$\sim 3200$ ^a

Parameter	Symbol	Value/Function
Total absorbed pump power	$P_{{a b}_{l}} + P_{{a b}_{r}}$	0 to 100 W ( $p$ -polarized, parallel to $E / / c$ axis)
Pump beam waist	${w_{T}}_{p 0}$ , ${w_{S}}_{p 0}$	70 µm, 25 µm
Pump beam quality factor	$M_{T}^{2}$ , $M_{S}^{2}$	10, 1.5
Pump wavelength	$λ_{p}$	660 nm
Laser wavelength	$λ_{l}$	795 nm
Absorption cross-section at pump wavelength	$α$	$2.1 \times 10^{- 20} {c m}^{2}$ ( $E / / c$ ) [2]
Peak emission cross-section	$σ_{e m}$	$1.3 \times 10^{- 20} {c m}^{2}$ ( $E / / c$ ) [2]
Excited state absorption cross-section	$σ_{e s a}$	0.18 × $σ_{e m}$ [5]
Auger rate	$γ$	$1 + 0.2 * N_{d o p} \cdot 10^{- 16} {c m}^{3} / s$ [39]
Doping concentration	$N_{d o p}$	1.25% to 10%
Crystal length	$L$	2 to 16 mm
Refractive index	$n$	1.39 [5]
Mode matching factor	$M M F$	0.8
Density	$R h o$	$2.988 {g / m}^{3}$ [14]
Specific heat capacity	$C p$	$0.935 J / (g \cdot K)$ [14]
Young modulus	$E$	96 GPa [14]
Poisson’s ratio	$V$	0.25 [14]
Thermal conductivity	$E / / a$ ,	4.58 W/(K.m),
Thermal conductivity	$E / / c$	5.14 W/(K.m) [19]
Ambient temperature	$T_{r o o m}$	293 K
Temperature of cooling water	$T_{w a t e r}$	288 K
Low temperature limit of radiative lifetime	$τ_{r_{0}} (T)$	196 µs [2]
Empirical fit coefficient	$α_{f i t}$	$447 \times 10^{- 6} K^{- 1}$ [2]
High-temperature limit of the nonradiative lifetime	$τ_{{n r}_{0}}$	113 fs [2]
Activation energy	$Δ E$	$8235 {c m}^{- 1}$ [2]

Output Power ( $W$ )	Vertical Axis $M^{2}$ ( $x$ )	Horizontal Axis $M^{2}$ ( $y$ )
0.5	4.67	1.19
1	5.86	1.33
1.5	6.03	1.27
2.2	8.69	1.29

Crystal Cr Doping (%)	Crystal Length (mm)	Crystal Absorption at 665 nm (%)	Crystal Loss at 800 nm (%)	Crystal Loss per cm (%)	Figure of Merit
0.45	20	83	0.6	0.3	300
1.25	10	91.5	0.36	0.36	690
			0.17	0.17	1465^a
1.4	12.5	96.9	0.54	0.43	645
1.25	15	97.6	0.53	0.35	705
1.25	15	97.5	0.44	0.29	850
1	20	98.1	0.69	0.35	580
3.5 [30]	5	—	$\sim 0.28$	$\sim 0.56$	$\sim 1250$
5 [6]	4	$\sim 97$	$\sim 0.05$	$\sim 0.125$	$\sim 6000$ ^a
7 [32]	4	$\sim 99$	$\sim 0.7$	$\sim 1.75$	$\sim 800$
10 [31]	2	$\sim 97$	$\sim 0.125$	$\sim 0.625$	$\sim 3200$ ^a

Abstract

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Equations (9)

Applied Optics