213.4 W Continuous-Wave Diamond Raman Laser at 1240 nm with Polarization-Combined Pumping
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2025-10-24 |
| Journal | High Power Laser Science and Engineering |
| Authors | Muye Li, Yuxiang Sun, Huawei Jiang, Xuezong Yang, Yan Feng |
| Institutions | University of Chinese Academy of Sciences |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis study reports a new world record for continuous-wave (CW) diamond Raman laser (DRL) output power, significantly surpassing the decade-old benchmark of 154 W.
- Record Output Power: Achieved 213.4 W CW Stokes output at 1240 nm, representing the highest reported CW DRL performance to date.
- High Efficiency: Demonstrated a 64.1% slope efficiency and 39.2% optical-to-Stokes conversion efficiency.
- Pumping Strategy: Implemented polarization beam combining (PBC) using dual fiber lasers (350 W vertical + 200 W horizontal) to overcome individual pump power limitations and achieve a total input power of 544 W.
- Cavity Design: Utilized a quasi-Z-shaped resonator with single-pass pumping and tilted optics to effectively suppress back-reflection and reduce intracavity power density at the output coupler (OC).
- Thermal Management: Employed active thermal control (water-cooled mounts, TEC module) to stabilize cavity optics and maintain the diamond crystal temperature at 30°C, mitigating thermal lens effects.
- Stability and Loss Mitigation: Successfully suppressed parasitic Stimulated Brillouin Scattering (SBS) up to 75 W Stokes power via cavity length tuning and spatial filtering, a critical factor for high-power scaling.
- Polarization Invariance: Confirmed that the orthogonally polarized secondary pump amplified the existing Stokes field without altering its polarization, achieving a polarization-insensitive gain regime.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Maximum CW Stokes Output | 213.4 | W | Operating at 1240 nm |
| Maximum Pump Power | 544 | W | Combined primary (350 W) and secondary (200 W) pumps |
| Slope Efficiency | 64.1 | % | Under combined pumping conditions |
| Optical-to-Stokes Conversion | 39.2 | % | Total conversion efficiency |
| Stokes Threshold Power | 211 | W | Using only the primary pump |
| Primary Pump Power | Up to 350 | W | Vertically polarized, 20 GHz linewidth |
| Secondary Pump Power | 200 | W | Horizontally polarized, 20 GHz linewidth |
| Pump Wavelength | 1064 | nm | Standard fiber laser output |
| Raman Shift (Diamond) | 1332.3 | cm-1 | Intrinsic property of diamond |
| Diamond Thermal Conductivity | 2000 | W/(m·K) | Exceptional thermal property |
| Diamond Thermo-Optic Coeff. | 20 x 10-6 | /K | Mitigates thermal lensing |
| Diamond Dimensions | 2x2x7 | mm3 | Type IIa single-crystal gain medium |
| Diamond Operating Temp. | 30 | °C | Maintained by TEC module |
| Pump Beam Waist (in diamond) | 26 | ”m | Calculated radius |
| Stokes Beam Waist (in diamond) | 50 | ”m | Calculated radius |
| Output Coupler (OC) Transmissivity | 3.2 | % | At 1240 nm (Stokes wavelength) |
| Optical Isolator Extinction Ratio | 28 | dB | Limited capability to mitigate back-reflection |
| SBS Suppression Threshold | 75 | W | Stokes power level below which SBS was suppressed |
| SRS Linewidth Broadening | 6.44 to 10 | GHz | Observed between 60 W and >100 W Stokes power |
Key Methodologies
Section titled âKey MethodologiesâThe record-breaking performance was achieved through a combination of advanced pumping, cavity, and thermal management strategies:
-
Polarization Beam Combining (PBC):
- Two high-power fiber lasers (Primary: 350 W, Vertical Polarization; Secondary: 200 W, Horizontal Polarization) were combined using a polarizer (Pol 2).
- This technique met the high pump power requirements while distributing intensity across two channels, reducing power density on individual optical isolators and enhancing resilience to optical feedback.
-
Cavity and Back-Reflection Mitigation:
- A four-mirror, single-pass quasi-Z-shaped resonator (M1, M3 planar; M2, M4 curved, 150 mm) was implemented.
- The quasi-Z configuration produced larger beam sizes at the output coupler (M4) compared to a true Z-cavity, reducing power density and coating stress.
- All optics in the pump path were tilted to eliminate back-reflection, compensating for the optical isolatorâs limited 28 dB extinction ratio.
-
Thermal Management and Component Survivability:
- Cavity mirrors (M1-M4) were mounted on water-cooled mounts to mitigate thermo-optic instabilities.
- The Type IIa diamond crystal was secured in a copper holder and maintained at 30°C using a thermoelectric cooler (TEC) module.
- High damage threshold coatings were applied to all cavity optics.
-
Stimulated Brillouin Scattering (SBS) Suppression:
- Cavity length tuning was employed to confine parasitic SBS oscillation to higher-order transverse modes.
- A variable aperture was positioned between mirrors M2 and M3 to spatially filter and suppress these higher-order transverse SBS modes.
- This combined approach enabled complete SBS suppression until Stokes power exceeded 75 W.
-
Gain Optimization and Polarization Control:
- The pump and Stokes beams were mode-matched using a plano-convex lens (200 mm focal length).
- The use of orthogonally polarized pumps confirmed polarization-independent gain enhancement, allowing the secondary pump to amplify the existing Stokes field without seeding new, destabilizing oscillations.
Commercial Applications
Section titled âCommercial ApplicationsâDiamond Raman Lasers (DRLs) operating in the 1.2 ”m band, especially at high CW power, are critical for applications requiring high beam quality and specific wavelength conversion.
- Remote Sensing and Lidar: The 1240 nm output can be used as a source for subsequent frequency conversion (e.g., to 620 nm or 607 nm) for atmospheric monitoring, trace gas detection (like methane or CO2), and sodium guide star lasers (as referenced in the paper).
- Material Processing: High-power CW lasers are essential for advanced material processing, including cutting, welding, and selective laser sintering (SLS) for additive manufacturing. Diamondâs superior thermal properties enable reliable kilowatt-class operation necessary for industrial throughput.
- Medical and Biomedical Imaging: High-power sources in the near-infrared (NIR) region are valuable for clinical applications such as laser speckle contrast imaging and other therapeutic or diagnostic procedures.
- Advanced Laser Systems: DRLs serve as robust frequency converters for high-power fiber and solid-state lasers, enabling the development of next-generation laser architectures that require specific, non-standard wavelengths.
- Defense and Security: High-power, high-beam-quality lasers are fundamental components in directed energy systems and advanced optical countermeasures.