Skip to content
6CCVD Research

Analysis of Thermal Effects in Kilowatt High Power Diamond Raman Lasers

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2022-12-14
JournalCrystals
AuthorsQiaoxia Gong, Mengxin Zhang, Chaonan Lin, Xun Yang, Xihong Fu
InstitutionsChangchun Institute of Optics, Fine Mechanics and Physics, Zhengzhou University
Citations5
AnalysisFull AI Review Included

Analysis of Thermal Effects in Kilowatt High Power Diamond Raman Lasers

Section titled “Analysis of Thermal Effects in Kilowatt High Power Diamond Raman Lasers”

Executive Summary

Section titled “Executive Summary”

This study establishes a thermal-structural coupling model to analyze the fundamental limits imposed by thermal effects on high-power Chemical Vapor Deposition (CVD) Diamond Raman Lasers (DRLs).

  • Performance Limit: Under ideal single-side cooling (298 K heat sink), the maximum temperature rise in the diamond crystal is limited to ~23.4 K, corresponding to an output power of ~2.8 kW.
  • Thermal Dynamics: The device exhibits extremely fast thermal response times, with heating and cooling reaching a steady state in only ~1.5 ms and ~2.5 ms, respectively, making the system suitable for high-repetition-rate pulsed operation (~250 Hz).
  • Cavity Optimization: A symmetrical concentric cavity structure demonstrates significantly less thermal impact compared to an asymmetrical concentric cavity.
  • Design Recommendations (Cavity): Increasing the radius of curvature of the cavity mirror reduces thermal lensing strength (e.g., from 163.6 m-1 to 103.5 m-1 at 5.3 kW pump power).
  • Design Recommendations (Crystal Size): Alleviating thermal effects requires increasing the crystal length and width, or decreasing the crystal thickness (e.g., reducing thickness minimizes the distance between the heat source and the heat sink).
  • Thermal Stress/Deformation: Maximum thermal stress reached 137.4 MPa, and maximum deformation was 0.13 ”m at 5.3 kW pump power, significantly lower than in other common laser crystals (e.g., Tm:YAP or Tm:LuAG).

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
Thermal Conductivity (Diamond)2000W/mKKey material property
Thermal Expansion Coefficient (αT)1.1 x 10-6K-1Key material property
Absorption Coefficient (α)0.375m-1@ 1 ”m (Pump/Stokes assumed equal)
Thermal Conversion Coefficient (Ο)0.142-Efficiency loss factor
Crystal Size (Thickness x Width x Length)1.2 x 4 x 8.6mmReference DRL system
Young’s Modulus (E)1100GPaMechanical property
Pump Wavelength (λp)1064nmNd:YAG source
Stokes Wavelength (λs)1240nmFirst-order Stokes
Reference Output Power (Experimental)1.2kWReported result [27]
Optical-to-Optical Conversion Efficiency52.2%At 2.3 kW pump power
Maximum Temperature Rise (Ideal Cooling)23.4KAt ~2.8 kW output power (5.3 kW pump)
Thermal Constant Time (Heating)~1.5msTime to reach 99% steady state
Thermal Constant Time (Cooling)~2.5msTime to reach equilibrium after pump stop
Maximum Thermal Stress (Simulated)137.4MPaAt 5.3 kW pump power
Maximum Thermal Deformation (Simulated)0.13”mAt 5.3 kW pump power
Thermal Lensing Strength (f-1) Range103.5 to 163.6m-1At 5.3 kW pump power, depending on cavity radius

Key Methodologies

Section titled “Key Methodologies”

The analysis utilized a thermal-structural coupling model based on the reference DRL system (1.2 kW output power).

  1. System Configuration: The DRL was modeled with a nearly concentric cavity (R1 = 150 mm, R2 = 92 mm) using a 1.2 mm x 4 mm x 8.6 mm CVD diamond crystal.
  2. Heat Source Modeling: A Gaussian heat source function (Qv) was employed, which is characteristic of the actual pump beam. The model assumed the pump beam waist radius was equal to the Stokes beam waist radius to satisfy good mode matching requirements.
  3. Thermal Analysis (Ideal Cooling): The three-dimensional heat conduction equation was solved. The ideal heat dissipation condition was assumed: the bottom surface of the diamond crystal (in contact with the copper heat sink) was fixed at a constant temperature of 298 K (25 °C).
  4. Transient Simulation: Transient analysis was performed to determine the thermal constant time (time required for the temperature gradient to reach 99% of the steady-state value) during both pulsed heating and cooling cycles.
  5. Thermo-Elasticity Modeling: Mechanical analysis was coupled with the thermal results. The stress and deformation state were determined by solving a set of thermoelastic equations (geometric, physical, and equilibrium differential equations) based on the calculated uneven temperature distribution.
  6. Thermal Lensing Calculation: The thermal lensing intensity (f-1) was calculated using a simplified model focusing primarily on the thermo-refractive index change (dn/dt term), as this effect is several orders of magnitude higher than the photo-elastic effect in diamond.

Commercial Applications

Section titled “Commercial Applications”

High-power, high-beam-quality DRLs utilizing CVD diamond are critical enabling technologies for demanding applications where high energy density and excellent beam quality are required.

  • Industrial Processing: High-precision material processing, cutting, and welding requiring kilowatt-level power.
  • Remote Sensing and Lidar: Development of eye-safe lidar systems, often requiring 1.5 ”m wavelengths achievable via Raman conversion.
  • Defense and Military Countermeasures: High-power directed energy systems and laser weapons.
  • Space and Satellite Communications: High-efficiency, high-power lasers for space exploration and satellite communication links.
  • Laser Medicine: Advanced surgical and therapeutic laser systems.
  • Scientific Research: Generation of new wavelengths and high-brightness sources for fundamental physics experiments.
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

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 2014 - Toward directed energy planetary defense [Crossref]
  2. 2015 - Military technology: Laser weapons get real [Crossref]
  3. 2018 - Laser technology for direct IR countermeasure system [Crossref]
  4. 2017 - High-power continuous-wave Raman frequency conversion from 1.06 ”m to 1.49 ”m in diamond [Crossref]
  5. 2018 - Large brightness enhancement for quasi-continuous beams by diamond Raman laser conversion [Crossref]
  6. 2020 - Analysis of a thermal lens in a diamond Raman laser operating at 1.1 kW output power [Crossref]
  7. 2012 - High average power (11 W) eye-safe diamond Raman laser
  8. 2018 - Wavelength diversification of high-power external cavity diamond Raman lasers using intracavity harmonic generation [Crossref]
  9. 2020 - Eye-safe diamond Raman laser [Crossref]
  10. 2020 - Diamond sodium guide star laser [Crossref]

Reads Similar to This

Channel waveguide lasers in bulk Tm -LiYF4 produced by deep diamond-saw dicing Diamond on aluminum nitride as a platform for integrated photonic circuits (Invited) Switching Characteristics of GaN Power Transistors