Analysis of Thermal Effects in Kilowatt High Power Diamond Raman Lasers

At a Glance

Metadata	Details
Publication Date	2022-12-14
Journal	Crystals
Authors	Qiaoxia Gong, Mengxin Zhang, Chaonan Lin, Xun Yang, Xihong Fu
Institutions	Chinese Academy of Sciences, Changchun Institute of Optics, Fine Mechanics and Physics
Citations	5
Analysis	Full AI Review Included

Technical Documentation: Thermal Management in Kilowatt Diamond Raman Lasers

This documentation analyzes the findings of Gong et al. (2022) regarding thermal effects in high-power Diamond Raman Lasers (DRLs) and connects the material requirements directly to the advanced capabilities of 6CCVD’s Chemical Vapor Deposition (CVD) diamond products.

Executive Summary

The research confirms that CVD Single Crystal Diamond (SCD) is the optimal material platform for next-generation Kilowatt-level DRLs, provided thermal management is optimized.

Material Superiority: CVD diamond’s exceptional thermal conductivity (2000 W m^-1 K^-1) and low thermal expansion coefficient (1.1 x 10^-6 K^-1) are critical for mitigating thermal stress and deformation at high power.
High-Power Robustness: Modeling predicts a maximum temperature rise of only 23.4 K in the diamond crystal at an output power of ~2.8 kW under ideal heat dissipation conditions.
Fast Transient Response: The thermal constant times are extremely short: ~1.5 ms for heating and ~2.5 ms for cooling, enabling high-repetition-rate pulsed operation (speculated at ~250 Hz) without significant heat accumulation.
Geometric Optimization: Alleviating thermal impact requires increasing the crystal length and width (maximizing contact area with the heat sink) and decreasing the crystal thickness (minimizing the thermal path length).
Cavity Design Impact: Symmetric concentric cavity structures are demonstrated to have significantly less thermal impact on the device compared to asymmetric configurations.
Thermal Stress Control: Maximum thermal stress remains significantly lower than in other crystal materials, reaching 137.4 MPa at 5.3 kW pump power.

Technical Specifications

Data extracted from the analysis of the thermal-structural coupling model for high-power DRLs.

Parameter	Value	Unit	Context
Material Platform	CVD Diamond	N/A	High Power Raman Lasers (DRLs)
Thermal Conductivity (K)	2000	W m^-1 K^-1	Diamond Crystal
Thermal Conversion Coefficient (ξ)	0.142	N/A	Conversion of pump energy to heat
Absorption Coefficient (α)	0.375	m^-1	@ 1 µm pump wavelength
Thermal Expansion Coefficient (α_T)	1.1 x 10^-6	K^-1	Low expansion characteristic
Crystal Dimensions (L x W x T)	8.6 x 4 x 1.2	mm	Crystal used in simulation
Maximum Output Power (Modeled)	~2.8	kW	Under ideal heat dissipation
Maximum Temperature Rise (ΔT)	23.4	K	At 5.3 kW pump power (2.8 kW output)
Heating Time to Steady State	~1.5	ms	Thermal constant time
Cooling Time to Equilibrium	~2.5	ms	Thermal constant time
Pump Wavelength (λ_p)	1064	nm	Nd: YAG laser source
Stokes Wavelength (λ_s)	1240	nm	First-order Stokes shift

Key Methodologies

The thermal analysis was conducted using a coupled thermal-structural model to simulate the performance of DRLs based on a previously reported 1.2 kW system.

System Modeling: The DRL cavity structure was based on a nearly concentric design utilizing concave mirrors (R=150 mm and R=92 mm).
Heat Source Definition: A Gaussian heat source model was employed, which is considered closer to actual pump beam characteristics than a simpler point source model.
Mode Matching Assumption: The pump beam waist radius was assumed to be equal to the Stokes beam waist radius to satisfy the requirement for good mode matching.
Ideal Heat Dissipation: The simulation assumed an ideal single-side water cooling scenario, fixing the bottom surface of the diamond crystal (in contact with the copper heat sink) at a constant temperature of 298 K (25 °C).
Coupled Analysis: Thermal analysis (3D heat conduction equation) was performed first, followed by mechanical analysis (thermo-elasticity model) to determine thermal stress and deformation based on the temperature distribution.
Thermal Lensing Simplification: Thermal lensing intensity (f^-1) was primarily calculated based on the thermo-refractive index change (dn/dt), neglecting the smaller photo-elastic effects.
Systematic Variation: The study systematically varied pump power (800 W to 5.3 kW), cavity type (symmetric vs. asymmetric), cavity mirror radius of curvature, and crystal dimensions (length, width, thickness) to determine optimal design parameters.

6CCVD Solutions & Capabilities

The research highlights the critical need for high-purity, precisely dimensioned, and perfectly polished SCD crystals for high-power DRL applications. 6CCVD is uniquely positioned to supply materials that meet or exceed these stringent requirements.

Requirement from Research Paper	6CCVD Solution & Capability	Technical Advantage
High-Purity Material (Minimizing absorption coefficient α)	Optical Grade Single Crystal Diamond (SCD)	Our SCD features ultra-low nitrogen and defect concentrations, minimizing parasitic absorption (α) of pump and Stokes light, thereby ensuring the lowest possible thermal conversion coefficient (ξ = 0.142).
Custom Crystal Geometry (Optimizing L, W, T)	Custom Dimensions up to 125 mm	We supply SCD plates/wafers from 0.1 µm up to 500 µm thick, and substrates up to 10 mm. This allows engineers to implement the key finding: maximizing length/width (contact area) and minimizing thickness (thermal path) for superior heat extraction.
Ideal Thermal Interface (Fixed 298 K bottom)	Precision Polishing (Ra < 1 nm)	To replicate the ideal heat dissipation modeled, the crystal surface must be atomically flat. Our SCD polishing achieves Ra < 1 nm, guaranteeing maximum thermal coupling efficiency with the copper heat sink.
Cavity Mirror Radius Variation (R₁=15 mm to R₁=125 mm)	Custom Laser Cutting & Shaping	6CCVD offers precision laser cutting services to achieve complex geometries and tight tolerances required for integrating crystals into highly sensitive concentric cavity designs.
Thermal Contact Integration (Heat sink bonding)	Custom Metalization Services	We provide in-house metalization (Au, Pt, Pd, Ti, W, Cu) for robust, high-integrity thermal and electrical contacts, essential for mounting the diamond crystal securely to the heat dissipation system.
Design Consultation (Cavity structure, cooling)	Expert Engineering Support	6CCVD’s in-house PhD team can assist with material selection and geometric optimization for similar High Power Diamond Raman Laser projects, ensuring designs maximize thermal performance and beam quality (M²).

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly. We offer global shipping (DDU default, DDP available) to facilitate your research and development worldwide.

View Original Abstract

Chemical vapor deposition (CVD) diamond crystal is considered as an ideal material platform for Raman lasers with both high power and good beam quality due to its excellent Raman and thermal characteristics. With the continuous development of CVD diamond crystal growth technology, diamond Raman lasers (DRLs) have shown significant advantages in achieving wavelength expansion with both high beam quality and high-power operation. However, with the output power of DRLs reaching the kilowatt level, the adverse effect of the thermal impact on the beam quality is progressively worsening. Aiming to enunciate the underlying restrictions of the thermal effects for high-power DRLs (e.g., recently reported 1.2 kW), we here establish a thermal-structural coupling model, based on which the influence of the pump power, cavity structure, and crystal size have been systematically studied. The results show that a symmetrical concentric cavity has less thermal impact on the device than an asymmetrical concentric cavity. Under the ideal heat dissipation condition, the highest temperature rise in the diamond crystal is 23.4 K for an output power of ~2.8 kW. The transient simulation further shows that the heating and cooling process of DRLs is almost unaffected by the pump power, and the times to reach a steady state are only 1.5 ms and 2.5 ms, respectively. In addition, it is also found that increasing the curvature radius of the cavity mirror, the length and width of the crystal, or decreasing the thickness of the crystal is beneficial to alleviating the thermal impact of the device. The findings of this work provide some helpful insights into the design of the cavity structure and heat dissipation system of DRLs, which might facilitate their future development towards a higher power.

Tech Support

Original Source

DOI: https://doi.org/10.3390/cryst12121824

References

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2015 - Military technology: Laser weapons get real [Crossref]
2018 - Laser technology for direct IR countermeasure system [Crossref]
2017 - High-power continuous-wave Raman frequency conversion from 1.06 µm to 1.49 µm in diamond [Crossref]
2018 - Large brightness enhancement for quasi-continuous beams by diamond Raman laser conversion [Crossref]
2020 - Analysis of a thermal lens in a diamond Raman laser operating at 1.1 kW output power [Crossref]
2012 - High average power (11 W) eye-safe diamond Raman laser
2018 - Wavelength diversification of high-power external cavity diamond Raman lasers using intracavity harmonic generation [Crossref]
2020 - Eye-safe diamond Raman laser [Crossref]
2020 - Diamond sodium guide star laser [Crossref]