Cascaded Raman lasing in a lithium tetraborate whispering gallery mode resonator

Chengcai Tian; Jervee Punzalan; Petra Becker; Ladislav Bohatý; Keith C. Gordon; Richard Blaikie; Harald G. L. Schwefel; Florian Sedlmeir

doi:10.1364/PRJ.560671

Photonics Research, Volume. 13, Issue 8, 2232(2025)

Cascaded Raman lasing in a lithium tetraborate whispering gallery mode resonator Editors' Pick

Chengcai Tian^1,2,3, Jervee Punzalan^1,3,4, Petra Becker⁵, Ladislav Bohatý⁵, Keith C. Gordon^1,3,4, Richard Blaikie^1,2,3, Harald G. L. Schwefel^1,2, and Florian Sedlmeir^1,2、*

Author Affiliations

¹Dodd-Walls Centre for Photonic and Quantum Technologies, Dunedin 9016, New Zealand

²Department of Physics, University of Otago, Dunedin 9016, New Zealand

³MacDiarmid Institute for Advanced Materials and Nanotechnology, Wellington 6012, New Zealand

⁴Department of Chemistry, University of Otago, Dunedin 9016, New Zealand

⁵Institute of Geology and Mineralogy, Section Crystallography, 50674 Köln, Germany

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Figures & Tables(6)

Fig. 1. Schematic of experimental setup. A green laser, generated using the third harmonic of a telecom seed laser, goes through a phase modulator and a polarization controller (PC) and is coupled into the LB4 resonator via a fiber pigtailed ferrule, a gradient-index (GRIN) lens, and a diamond prism. An incoupling angle for the LB4 resonator, denoted as $ϕ = 39 °$ , is illustrated. After the prism, both pump and signal modes are observed using two silicon photodiodes (Si PD 1 and Si PD 2) and an oscilloscope. An optical grating is used to spatially separate the generated cascaded signal from the pump. Green curves: pump; yellow curves: Raman signal. The inset displays the LB4 resonator with a major radius of $2.97 mm \pm 0.01 mm$ and a minor radius of $2.30 mm \pm 0.01 mm$ .

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Fig. 2. One transverse magnetic (TM, electric field vertical to the symmetry axis of our disc resonator) mode at undercoupling state when the resonator is excited at 517 nm. The linewidth $Δ ν$ is determined to be $289 \pm 1 kHz$ from the Lorentzian fitting to an undercoupled mode corresponding to $Q = 2.0 \times 10^{9}$ . The sidebands are for frequency calibration. Blue dots: data acquired on the photodiode using the oscilloscope; red curve: Lorentzian fitting.

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Fig. 3. Cascaded SRS recorded by Ocean Optics in LB4 resonator. The leftmost peak is the pump laser at 517 nm. These SRS peaks are located at 537.1 nm, 558.8 nm, 582.4 nm, and 608.2 nm. Note that the peak intensities cannot be directly compared since we measured the Raman peaks at different positions after the grating and at different pump powers. This was necessary to avoid saturation of the spectrometer. Each measurement is distinguished by different colors. Inset: a photograph of the cascaded SRS signal captured with a USB camera after the optical grating. This is a single shot measurement at a fixed pump power.

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Fig. 4. First-order SRS intensity versus the pump power coupled to the fundamental mode. The solid line represents a linear fit within the gray region, from which the threshold $P_{thr} = 0.71 mW$ and the slope efficiency of SRS of 0.072 are derived. In the light orange region, higher-order Raman lasing emerges at 2 mW, causing the first-order output power to deviate from the linear trend.

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Table 1. Some Optic-Related Material Parameters of Lithium Tetraborate

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Table 1. Some Optic-Related Material Parameters of Lithium Tetraborate

Parameters	Values
Point group	4 mm
Sellmeier equations [5]^a	$n_{o} = 2.56431 + \frac{0.012337}{λ^{2} - 0.013103} - 0.019075 λ^{2}$ $n_{e} = 2.38651 + \frac{0.010664}{λ^{2} - 0.012878} - 0.012813 λ^{2}$
Transparency range [2,5]	0.16–3.5 μm
$χ^{(2)}$ nonlinear coefficients [13,15]	$d_{32} = d_{15} = d_{24} = d_{31} = 0.5 pm / V$ $d_{33} = 3 pm / V$
Pockels electro-optic coefficients [15–17]	$r_{13} = r_{23} = 4.03 pm / V$ , $r_{33} = 3.96 pm / V$ $r_{42} = r_{51} = - 0.11 pm / V$
Raman scattering [18]	Raman shifts: $720 {cm}^{- 1}$ , $160 {cm}^{- 1}$
	Raman gain: $> 1.8 cm / GW$
	Raman linewidth: $8 - 10 {cm}^{- 1}$
Brillouin scattering [19]^b	Brillouin shifts: $\approx 22 GHz$ and $\approx 30 GHz$
Laser damage threshold [3]	$40 {GW / cm}^{2}$ at 1.064 μm
Piezo-optic coefficients ( $\times 10^{- 12} {Pa}^{- 1}$ ) [20]	$π_{11} = - 0.307$ , $π_{12} = 0.769$ , $π_{13} = 3.878$ , $π_{31} = 1.150$ , $π_{33} = 1.639$ , $π_{44} = - 0.131$ , $π_{66} = - 1.387$
Piezoelectric coefficients [21]	$d_{15} = 8.07 pm / V$ , $d_{33} = 19.4 pm / V$ , $d_{31} = - 2.58 pm / V$
Temperature derivative of refractive indices ( $\times 10^{- 6} K^{- 1}$ ) [5]^c	$\frac{d n_{o}}{d T} = 1.2$ , $\frac{d n_{e}}{d T} = 3$
Relative dielectric constants [22]^d	$ϵ_{11} = 7.4$ , $ϵ_{33} = 9.7$
Thermal expansion coefficients ( $\times 10^{- 6} K^{- 1}$ ) [21]^e	$α_{11} = 11.1$ , $α_{33} = - 3.7$
Mohs hardness [3]	6

Table 2. Best $Q$ Factors of LB4 WGM Resonator and Inferred Upper Limit of Absorption Coefficients of LB4 at Different Wavelengths
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Table 2. Best $Q$ Factors of LB4 WGM Resonator and Inferred Upper Limit of Absorption Coefficients of LB4 at Different Wavelengths
Wavelength 517 nm 795 nm 1550 nm
TM Q factor $2.0 \times 10^{9}$ $1.2 \times 10^{9}$ $6.1 \times 10^{7}$
TE Q factor $3.3 \times 10^{8}$ $1.0 \times 10^{9}$ $6.8 \times 10^{7}$
$α$ ( $m^{- 1}$ ) 0.010 0.011 0.095

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Chengcai Tian, Jervee Punzalan, Petra Becker, Ladislav Bohatý, Keith C. Gordon, Richard Blaikie, Harald G. L. Schwefel, Florian Sedlmeir, "Cascaded Raman lasing in a lithium tetraborate whispering gallery mode resonator," Photonics Res. 13, 2232 (2025)

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Paper Information

Category: Nonlinear Optics

Received: Mar. 25, 2025

Accepted: May. 23, 2025

Published Online: Jul. 25, 2025

The Author Email: Florian Sedlmeir (florian.sedlmeir@otago.ac.nz)

DOI:10.1364/PRJ.560671

CSTR:32188.14.PRJ.560671

Topics

laser devices and laser physics

Lasers and Laser Optics

Laser physics

laser manufacturing

Instrumentation, Measurement and Metrology

Table 1. Some Optic-Related Material Parameters of Lithium Tetraborate

Table 1. Some Optic-Related Material Parameters of Lithium Tetraborate

Table 2. Best Q Factors of LB4 WGM Resonator and Inferred Upper Limit of Absorption Coefficients of LB4 at Different Wavelengths

Table 2. Best Q Factors of LB4 WGM Resonator and Inferred Upper Limit of Absorption Coefficients of LB4 at Different Wavelengths

Table 2. Best $Q$ Factors of LB4 WGM Resonator and Inferred Upper Limit of Absorption Coefficients of LB4 at Different Wavelengths

Table 2. Best $Q$ Factors of LB4 WGM Resonator and Inferred Upper Limit of Absorption Coefficients of LB4 at Different Wavelengths