Numerical optimization of the extra-cavity diamond Raman laser in the multi-phonon absorption band

At a Glance

Metadata	Details
Publication Date	2022-10-12
Journal	Frontiers in Physics
Authors	Zhenhua Shao, Bei Li, Hongzhi Chen, Jun Cao
Institutions	Shanghai Power Equipment Research Institute
Analysis	Full AI Review Included

Executive Summary

This research presents a numerical optimization study for an extra-cavity diamond Raman laser (DRL) operating in the challenging mid-infrared (Mid-IR) multi-phonon absorption band (2.5-3 µm).

Core Challenge Addressed: Mitigating the significant linear absorption losses inherent to diamond in the 2.5-6.5 µm multi-phonon region to achieve practical conversion efficiencies.
Methodology: A theoretical model based on Raman coupled-wave equations and boundary conditions was developed and numerically solved to simulate DRL performance.
Key Optimization Strategy: Optimization focuses on balancing the diamond length (L) and the output coupler transmittance (T) to minimize the lasing threshold and maximize Stokes conversion.
Critical Finding: To overcome strong absorption losses in the Mid-IR, it is necessary to appropriately increase the output coupling (T) of the cavity, favoring higher output over multiple internal reflections.
Predicted Performance (3 µm Example): A conversion efficiency of 10% is predicted using a 1 cm diamond length, an optimal transmittance of 69%, and a high pump intensity of 1.2 GWcm^-2.
Design Recommendation: Due to the high threshold intensity (GWcm² magnitude), adopting Brewster-cut diamond is recommended to eliminate Fresnel reflection losses, rather than relying on antireflection coatings.
Temporal Behavior: The model confirms significant pulse-shortening, predicting a Stokes pulse duration of 4.4 ns from a 10 ns pump pulse.

Technical Specifications

Parameter	Value	Unit	Context
Target Stokes Wavelength Range	2.5-3	µm	Operating within the multi-phonon absorption band.
Optimized Stokes Wavelength (Example)	3	µm	Corresponds to a pump wavelength of 2.14 µm.
Maximum Conversion Efficiency (Predicted)	10	%	Achieved at optimal L and T, I_p = 1.2 GWcm^-2.
Optimal Diamond Length (L)	1	cm	For 10% CE at 3 µm.
Optimal Output Transmittance (T)	69	%	For 10% CE at 3 µm.
Pump Intensity (I_p) for 10% CE	1.2	GWcm^-2	Required input intensity for maximum efficiency.
Stokes Absorption Coefficient (α_s)	1.58	cm^-1	Measured loss at 3 µm.
Pump Absorption Coefficient (α_p)	0.11	cm^-1	Measured loss at 2.14 µm.
Effective Raman Gain Coefficient (g)	1.59	cmGW^-1	Calculated effective gain at 3 µm.
Diamond Raman Gain Coefficient (Intrinsic)	17	cmGW^-1	Record high intrinsic value.
Diamond Thermal Conductivity (k)	2,000	W m^-1K^-1	Record high value for heat dissipation.
Diamond Optical Transmission Window	0.23-2.5 and >6.5	µm	The 2.5-6.5 µm range is the high-loss multi-phonon band.
Diamond Damage Threshold	3-4	GWcm^-2	High resistance to optical damage.

Key Methodologies

The study relied entirely on numerical simulation and theoretical modeling to optimize the extra-cavity diamond Raman laser performance:

Model Formulation: The theoretical framework was established using the time-dependent Raman coupled-wave equations, describing the evolution of pump (I_p) and Stokes (I_s) intensities with forward (+) and backward (-) propagation.
Boundary Conditions: Standard boundary conditions were applied, incorporating input pump intensity (I_in) and the reflectivities (R) of the resonator mirrors for both pump and Stokes wavelengths.
Loss Integration: The model explicitly incorporated linear absorption loss coefficients (α_p and α_s) derived from Fourier transform infrared spectrometer measurements of the diamond material, accurately accounting for the high losses in the multi-phonon band.
Parameter Sweep and Optimization: Extensive numerical simulations were performed by systematically varying the key design parameters:
- Diamond length (L) (0.4 cm to 3.0 cm).
- Output coupler transmittance (T) (20% to 80%).
- Input pump intensity (I_p) (up to 1.2 GWcm^-2).
Linewidth Analysis: The influence of the pump linewidth (Δv_p) on the effective Raman gain (g) and threshold intensity was analyzed, confirming that narrower linewidths are necessary to reduce the lasing threshold.
Optimal Design Determination: Optimal combinations of L and T were identified that yielded the minimum lasing threshold and maximum output intensity for a given pump power, specifically targeting the 3 µm Stokes output.

Commercial Applications

The optimization of Mid-IR diamond Raman lasers is crucial for applications requiring high-power, high-brightness sources in the 2.5-3 µm band:

Mid-Infrared Spectroscopy and Sensing: Providing high-intensity, narrow-linewidth sources for detecting trace gases (e.g., atmospheric monitoring, industrial process control).
Directed Energy and Countermeasures: High-power pulsed Mid-IR lasers are essential for defense applications, including infrared countermeasures (IRCM).
Materials Processing: Utilizing high-energy pulsed Mid-IR output for specialized cutting, drilling, or surface modification of materials that strongly absorb in this band.
Nonlinear Frequency Conversion Modules: Serving as robust, high-thermal-conductivity frequency shifters for existing solid-state pump lasers (e.g., Tm-doped or Ho-doped systems).
Medical and Surgical Lasers: Mid-IR wavelengths are highly absorbed by water, making them ideal for precise soft-tissue ablation and surgery.

View Original Abstract

The physical process of stimulated Raman scattering (SRS) in the diamond and the performance of the Raman laser in the multi-phonon absorption band of 2.5-3 μm were theoretically studied. A theoretical model for the external-cavity diamond Raman laser emitting at the waveband was built based on the Raman coupled-wave equation and boundary conditions. Raman laser output characteristics such as lasing threshold, input-output, and temporal behavior of Stokes conversion were investigated and theoretically simulated by varying the values of the length of the diamond and the transmittance of the output coupler. The numerical modeling shows that to reduce the impact of the multi-phonon absorption and obtain a higher conversion efficiency, it is necessary to appropriately increase the output coupling of the cavity. Taking the 3 μm diamond Raman laser optimization as an example, it is predicted that the conversion efficiency of 10% could be obtained with a diamond length of 1 cm, a transmittance of 69%, and a pump intensity of 1.2 GWcm −2 . The theoretical model also could be used to investigate other wavelengths of the external-cavity diamond Raman laser and be helpful for the optimum design of diamond Raman lasers in the mid-infrared band.

Tech Support

Original Source

DOI: https://doi.org/10.3389/fphy.2022.1027998

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