Growth and Laser Performance of Tm∶CaGdAlO4 Single-Crystal Fiber

doi:10.16553/j.cnki.issn1000-985x.2026.0071

Abstract

Abstract: In this study， a Tm³⁺-doped CaGdAlO₄ （Tm∶CALGO） disordered single-crystal fiber with a doping concentration of 4% （molar fraction） was grown by the micro-pulling-down method. The crystal structure， optical properties， continuous-wave laser performance， and passively Q-switched pulsed laser characteristics integrated with BiOX （X=Cl， Br， I） layered crystal saturable absorbers were systematically investigated. The crystal belongs to the tetragonal system with the space group I4/mmm. X-ray diffraction and Raman spectroscopy show that the as-grown crystal exhibits a pure CALGO phase with good crystallinity. The absorption spectrum reveals a main absorption peak at 793 nm with a full width at half maximum of 19.4 nm， which is attributed to the inhomogeneous broadening induced by the disordered structure. The highest phonon energy measured by Raman spectroscopy is 614 cm^-1， which is lower than that of YAG crystal and beneficial for suppressing non-radiative relaxation. Fourier-transform infrared transmission spectroscopy shows that the crystal exhibits a transmittance exceeding 80% in the 2~6 μm mid-infrared region， with an infrared cut-off edge extending beyond 10 μm. In a straight cavity configuration， a maximum continuous-wave output power of 3.338 W is achieved from the Tm∶CALGO single-crystal fiber， with a slope efficiency of 27.3% and a central wavelength of 1 982.8 nm. When BiOX crystals were integrated into the laser cavity as saturable absorbers， stable passively Q-switched pulsed laser outputs were obtained under an output coupler transmittance of 5%. With BiOCl， BiOBr， and BiOI as saturable absorbers， the maximum single-pulse energies obtained are 18.564， 20.335， and 17.599 μJ， the shortest pulse widths are 420， 570， and 506 ns， and the highest peak powers are 44.199， 35.675， and 34.781 W， respectively. This work demonstrates the excellent performance of Tm∶CALGO disordered single-crystal fibers as gain media for 2 μm lasers and provides a feasibility validation for the integration of single-crystal fibers with novel saturable absorbers.

Key words: Tm∶CALGO single-crystal fiber; micro-pulling-down method; disordered structure; 2 μm; continuous-wave laser; passive Q-switching; bismuth oxyhalide crystal

CLC Number:

O799

MA Xiaofei, ZHU Xiangfei, ZHANG Mingji, GAO Chenxin, WANG Tao, ZHANG Jian, JIA Zhitai, WANG Zefeng. Growth and Laser Performance of Tm∶CaGdAlO₄ Single-Crystal Fiber[J]. Journal of Synthetic Crystals, 2026, 55(7): 1100-1110.

Figures/Tables 11

Fig.1 （a） Schematic diagram of the micro-pulling-down setup for Tm∶CALGO single-crystal fiber growth；（b） crystal structure illustration of CALGO；（c） photograph of the as-grown Tm∶CALGO single-crystal fiber； photographs of BiOCl （d）， BiOBr （e）， and BiOI （f） crystals， respectively， prepared by the chemical vapor transport method

Fig.2 Schematic diagram of the open-aperture Z-scan experimental setup

Fig.3 Schematic diagram of the passively Q-switched Tm∶CALGO single-crystal fiber pulsed laser setup （inset： typical output pulse train and waveform）

Fig.4 Crystal quality characterization of Tm∶CALGO single-crystal fiber. （a） Powder XRD pattern；（b） Laue back-reflection diffraction pattern；（c） Raman spectrum

Fig.5 Spectral properties of Tm∶CALGO single-crystal fiber. （a） FTIR transmission spectrum；（b） absorption spectrum

Fig.6 Continuous-wave laser performance of Tm∶CALGO single-crystal fiber under different output coupler transmittances. （a） Output power versus absorbed pump power；（b） output spectra

Fig.7 Optical properties of BiOX saturable absorbers. （a） FTIR transmission spectra；（b） open-aperture Z-scan curves under 2 μm laser excitation

Table 1 Nonlinear optical parameters of BiOX crystals at 2 μm

SA	α₀/mm^-1	β/（cm·MW^-1）	ΔR/%
BiOCl	0.023	-0.204	8.8
BiOBr	0.027	-0.322	12.3
BiOI	0.033	-0.471	16.2

Fig.8 Passively Q-switched laser output power and spectra of Tm∶CALGO single-crystal fiber with BiOX saturable absorbers. （a） Average output power versus absorbed pump power；（b） output spectra

Fig.9 Passively Q-switched laser performance of Tm∶CALGO single-crystal fiber with BiOX saturable absorbers. （a） Pulse repetition rate and pulse duration versus absorbed pump power；（b） single-pulse energy and peak power versus absorbed pump power

Table 2 Results of passively Q-switched pulsed lasers realized by combining Tm3+-doped gain media with different saturable absorbers

SA	Gain medium	λ/nm	Mean power/W	Single pulse energy/μJ	Pulse duration/ns	Repetition rate/kHz	Reference
MoS₂	Tm∶CaYAlO₄	1 850	0.490	4.87	480	102.6	［39］
MoS₂	Tm∶YAG	2 330	0.204	1.07	150	190	［40］
Graphdiyne	Tm∶YAG	2 007.6	1.044		895	180.5	［41］
MoS₂	Tm∶CLNGG	1 977	0.062	0.72		110	［42］
Cr²⁺∶ZnS	Tm，Ho∶LLF	2 055	0.074	13	12 000	5.8	［43］
BiOCl	Tm∶CALGO	1 978.5	1.352	18.564	420	72.83	This work
BiOBr	Tm∶CALGO	1 979.6	1.421	20.335	570	69.88	This work
BiOI	Tm∶CALGO	1 981.7	1.532	17.599	506	87.05	This work

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Growth and Laser Performance of Tm∶CaGdAlO₄ Single-Crystal Fiber

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