Numerical Simulation of Long-Wave Infrared Generation Using an External Cavity Diamond Raman Laser

At a Glance

Metadata	Details
Publication Date	2021-07-05
Journal	Frontiers in Physics
Authors	Hui Chen, Zhenxu Bai, Zhao Chen, Xuezong Yang, Jie Ding
Institutions	Hebei University of Technology, Macquarie University
Citations	4
Analysis	Full AI Review Included

Executive Summary

This research presents a numerical simulation demonstrating a viable path toward high-power, all-solid-state Long-Wave Infrared (LWIR) laser generation using an external-cavity diamond Raman laser (DRL).

Core Achievement: Simulated the generation of 10 µm LWIR output by utilizing the first-order Stokes shift from a 4.3 µm pump laser in a diamond crystal.
Material Advantage: Diamond is selected for its exceptional properties, including the largest known Raman frequency shift (1,332 cm^-1) and extremely high thermal conductivity (>2000 W m^-1 K^-1), enabling high-power operation without significant thermal accumulation.
Performance Prediction: Simulations predict a maximum Stokes peak power output of 123 MW when using a 1x1x1 cm³ diamond crystal with an optimal output coupler transmission of 40%.
Efficiency and Threshold: The maximum conversion efficiency approaches the quantum limit (~43%). The calculated Stokes generation threshold for 0.5% output coupling is 34.8 kW, reflecting the challenges of low Raman gain and high absorption loss in the LWIR band.
Design Optimization: Key cavity parameters were optimized, including a pump waist size of 252 µm and a crystal length of 5 mm, to maximize conversion efficiency and output power under steady-state conditions.
Beam Quality: The Stimulated Raman Scattering (SRS) process inherently provides “beam cleanup,” suggesting the resulting LWIR source will possess high beam quality.

Technical Specifications

Parameter	Value	Unit	Context
Target Output Wavelength (Stokes)	10	µm	Long-Wave Infrared (LWIR)
Pump Wavelength (Input)	4.3	µm	Mid-Wave Infrared (MWIR)
Diamond Raman Frequency Shift	1,332.3	cm^-1	Largest among known Raman crystals
Diamond Thermal Conductivity	>2000	W m^-1 K^-1	Used for stable, high-power operation
Diamond Refractive Index (n)	2.38	-	Constant for wavelengths >2 µm
Crystal Length (L)	5	mm	Brewster-cut single-crystal diamond
Optimal Pump Waist Size (w_p)	252	µm	Focused beam size inside the crystal
Intrinsic Stokes Waist Size (w_s)	251	µm	Resonator mode size
Cavity Total Length	102	mm	External-Cavity DRL structure
Mirror Curvature Radius (IC/OC)	50	mm	Near-concentric cavity design
Stokes Generation Threshold (T=0.5%)	34.8	kW	Required pump power
Predicted Max Stokes Peak Power	123	MW	Achieved at 40% output coupling (T)
Maximum Conversion Efficiency	~43	%	Approaching the theoretical quantum limit
Diamond Absorption Coefficient (α)	0.03	cm^-1	Used in LWIR band simulation
Intrinsic Gain Linewidth (Diamond)	~40	GHz	Requires control of MWIR pump linewidth

Key Methodologies

The study utilized a steady-state numerical model of an external-cavity diamond Raman laser (EC-DRL) to simulate and optimize LWIR generation.

Resonator Design: A near-concentric external cavity was modeled, featuring 50 mm radius-of-curvature input (IC) and output (OC) couplers, totaling 102 mm in length.
Gain Medium Integration: A 5 mm long, Brewster-cut (67.2°) single-crystal diamond was placed at the intrinsic Stokes beam waist (251 µm). Brewster cutting was used to avoid crystal coating and film damage, ensuring high transmittance for both pump and Stokes beams.
Pump Focusing and Mode Matching: A focusing lens (F3, 100 mm focal length) was used to focus the 4.3 µm pump beam to an optimal waist size of 252 µm, ensuring good mode matching with the Stokes beam for maximum conversion efficiency.
Steady-State Analysis: The relationships between pump power (P_p), Stokes power (P_s), residual pump power (P_res), and Raman gain (G) were analyzed using established steady-state DRL equations, incorporating parameters like output coupler transmittance (T), absorption coefficient (α), and quantum defect (η).
Parameter Optimization: Simulations were performed to determine the optimal values for output coupler transmission (T), pump waist size (w_p), and crystal length (L) that maximize the output Stokes power for a given pump power.
Damage and Thermal Consideration: The analysis included predicting the maximum output power achievable before potential diamond damage, leveraging diamond’s high thermal conductivity to mitigate thermal lensing effects.
Time-Domain Discussion: The effect of oscillator gain on pulse characteristics was discussed, noting that high gain leads to strong pulse width compression, which is relevant for nanosecond-pulsed LWIR-DRLs.

Commercial Applications

The development of high-power, all-solid-state LWIR sources based on diamond Raman conversion has significant implications across several high-tech sectors:

Defense and Aerospace: High-power LWIR lasers are critical for military applications, including laser remote sensing, target tracking, and infrared countermeasures (IRCM), particularly due to their ability to penetrate atmospheric obscurants like fog and smoke.
Environmental Monitoring: LWIR spectroscopy is essential for detecting and quantifying atmospheric gases (e.g., CO₂, methane) and pollutants, as many fundamental molecular vibrational modes fall within the 8-12 µm window.
Biomedical and Chemical Sensing: The 10 µm region is vital for highly specific biochemical detection and diagnostics, enabling non-invasive medical imaging and material analysis.
Advanced Manufacturing: High-brightness, high-power diamond lasers can be used in precision material processing and cutting applications where LWIR wavelengths offer specific absorption advantages.
Scientific Research: Providing stable, high-quality, and high-power LWIR beams for fundamental research in nonlinear optics, quantum cascade laser development, and frequency comb generation.

View Original Abstract

Diamond has a broad spectral transmission range (&gt;0.2 μm) and the largest Raman frequency shift (1,332 cm −1 ) among known Raman crystals. Hence, the diamond Raman laser has the potential to achieve lasing in the long-wave infrared (LWIR) range, which is difficult to reach via other crystalline lasers. Here, we report a new approach to achieve LWIR output using diamond Raman conversion and provide the corresponding analysis model and simulation results. The conversion efficiency is analyzed as function of the pump waist size, output-coupler transmission, and crystal length, at constant pump power. The maximum output power at which a diamond of relatively large size can be operated without damage is predicted. This study paves a way for high-power LWIR lasing in diamond.

Tech Support

Original Source

DOI: https://doi.org/10.3389/fphy.2021.671559

References

2001 - Mid-Wave IR and Long-Wave IR Laser Potential of Rare-Earth Doped Chalcogenide Glass Fiber [Crossref]
2007 - Wavelength-agile Mid-infrared (5-10 μm) Generation Using a Galvano-Controlled KTiOPO_4 Optical Parametric Oscillator [Crossref]
2005 - Study of Nonlinear-Optical Characteristics of AgGa1-xInxSe2crystals [Crossref]
2003 - Synchronously Pumped CdSe Optical Parametric Oscillator in the 9-10 μm Region [Crossref]
2017 - Recent Progress of Quantum cascade Laser Research from 3 to 12 μm at the Center for Quantum Devices [Invited] [Crossref]
2018 - High Power diamond Raman Lasers [Crossref]
1999 - Intracavity Raman Conversion and Raman Beam Cleanup [Crossref]
2018 - Large Brightness Enhancement for Quasi-Continuous Beams by diamond Raman Laser Conversion [Crossref]
2018 - 302 W Quasi-Continuous Cascaded diamond Raman Laser at 15 Microns with Large Brightness Enhancement [Crossref]
2014 - Investigating diamond Raman Lasers at the 100 W Level Using Quasi-Continuous-Wave Pumping [Crossref]