High-power free-running single-longitudinal-mode diamond Raman laser enabled by suppressing parasitic stimulated Brillouin scattering
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2023-01-01 |
| Journal | High Power Laser Science and Engineering |
| Authors | Yuxuan Liu, Chengjie Zhu, Yuxiang Sun, Richard P. Mildren, Zhenxu Bai |
| Institutions | University of Chinese Academy of Sciences, Shanghai Institute of Optics and Fine Mechanics |
| Citations | 13 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research details the successful engineering and demonstration of a high-power, continuous-wave (CW) single-longitudinal-mode (SLM) diamond Raman laser (DRL) operating without complex axial-mode selection elements.
- Core Achievement: Demonstrated 20.6 W CW SLM Stokes output at 1240 nm, achieving a 30% optical-to-optical conversion efficiency from a 1064 nm pump.
- SLM Mechanism: Stable SLM operation was achieved by leveraging the spatial-hole-burning free nature of Raman gain, which inherently favors single-mode output.
- Parasitic Suppression: SLM stability at high power was maintained by suppressing parasitic Stimulated Brillouin Scattering (SBS), identified as the primary destabilizing factor.
- Engineering Solution: SBS suppression was accomplished through precise cavity length adjustment and using the diamond crystal edge as an effective aperture to block higher-order SBS spatial modes.
- Stability Metrics: The V-shaped cavity design minimized pump resonance effects, resulting in excellent power stability (1.8% Root Mean Square (RMS) fluctuation over 1 hour).
- Coherence Measurement: For the first time, the spectral linewidth of a CW SLM DRL was resolved using the delayed self-heterodyne interferometric (DSHI) method, measuring 105 kHz at 20 W output.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Maximum Stokes Output Power | 20.6 | W | Achieved at 69 W pump power |
| Stokes Wavelength | 1240 | nm | Single-Longitudinal Mode (SLM) |
| Pump Wavelength | 1064 | nm | SLM DFB seed laser source |
| Optical-to-Optical Efficiency | 30 | % | Conversion from 1064 nm to 1240 nm |
| Slope Efficiency | 51.3 | % | Above 31.5 W threshold |
| Stokes Linewidth (SLM) | 105 | kHz | Measured via DSHI at 20 W output |
| Pump Linewidth | 60 | kHz | Input pump source linewidth |
| Stokes Power Stability | 1.8 | % RMS | Measured over 1 hour |
| Pump Power Stability | 0.55 | % RMS | Measured over 1 hour |
| Raman Medium | CVD Single-Crystal Diamond (Type IIa) | N/A | Element Six, 7 mm x 2 mm x 2 mm |
| Diamond Orientation | <110> | N/A | Propagation direction |
| Diamond Temperature | 20 | °C | Stabilized using water-cooled copper holder |
| Cavity Type | V-shaped Standing-Wave | N/A | Comprised of two plane-concave mirrors (M1, M2) and one plane mirror (M3) |
| Cavity FSR (Free Spectral Range) | 531.6 | MHz | Corresponds to 564.33 mm round-trip optical length |
| Pump Beam Waist (w0) | 15 | ”m | In the diamond crystal |
| Stokes Beam Waist (w0) | 31 | ”m | In the diamond crystal |
Key Methodologies
Section titled âKey MethodologiesâThe high-power SLM DRL was realized through specific cavity design and precise alignment to manage parasitic effects:
- V-Shaped Cavity Architecture: A V-shaped standing-wave resonator was implemented. This design minimizes the perturbing effect of resonances at the pump frequency, leading to weak intracavity pump and Stokes intensity modulation.
- Pump Source and Polarization: A CW SLM Yb fiber amplifier (M2 1.15) provided the 1064 nm pump. A half-wave plate (HWP) ensured the pump polarization was parallel to the <111> axis of the diamond to maximize Raman gain.
- Diamond Medium: A Type IIa CVD single-crystal diamond (7 mm long) was used as the gain medium, featuring anti-reflective coatings for both 1064 nm and 1240 nm. The crystal temperature was stabilized at 20 °C.
- Mode Matching: A telescope and focusing lens (f = 75 mm) were used to achieve optimal mode matching between the pump (15 ”m waist) and Stokes (31 ”m waist) beams within the diamond.
- Parasitic SBS Suppression: Stable SLM operation at high power required suppression of parasitic SBS, achieved by:
- Cavity Length Tuning: Delicately adjusting the cavity length (via M3) to avoid resonances that favor SBS oscillation.
- Spatial Mode Selection: Moving the pump-Raman interaction region to the edge of the diamond crystal. This acts as an aperture, blocking higher-order spatial modes that typically facilitate SBS oscillation.
- Linewidth Characterization: The spectral linewidth was measured using the long-delayed self-heterodyne interferometric (DSHI) technique, employing a 23-km delay fiber to achieve the necessary resolution (minimum measurable linewidth of 50 kHz).
Commercial Applications
Section titled âCommercial ApplicationsâThe development of high-power, narrow-linewidth, wavelength-flexible lasers based on diamond Raman shifting is critical for several advanced engineering and scientific fields:
- Coherent Detection and Lidar: The 105 kHz linewidth at 20 W output provides the high coherence and power necessary for long-range, high-resolution coherent Lidar systems and Doppler velocimetry.
- Quantum Technology: SLM lasers with high coherence are essential for precision control and manipulation in quantum computing, quantum sensing, and atomic clocks.
- Astronomical Observation: Narrow-linewidth, frequency-stable sources are required for high-resolution spectroscopy and calibration in advanced astronomical instruments.
- Wavelength Extension: Raman lasers provide an effective method to generate high-power SLM output in spectral regions (e.g., 1240 nm) that are difficult to access directly with conventional fiber or diode lasers, enabling new applications in medical imaging and telecommunications.
- High-Power Laser Systems: The successful use of CVD diamond (Type IIa) confirms its superior performance as a gain medium in high-power laser architectures, leveraging its exceptional thermal conductivity and high damage threshold.
View Original Abstract
Abstract A continuous-wave (CW) single-longitudinal-mode (SLM) Raman laser at 1240 nm with power of up to 20.6 W was demonstrated in a free-running diamond Raman oscillator without any axial-mode selection elements. The SLM operation was achieved due to the spatial-hole-burning free nature of Raman gain and was maintained at the highest available pump power by suppressing the parasitic stimulated Brillouin scattering (SBS). A folded-cavity design was employed for reducing the perturbing effect of resonances at the pump frequency. At a pump power of 69 W, the maximum Stokes output reached 20.6 W, corresponding to a 30% optical-to-optical conversion efficiency from 1064 to 1240 nm. The result shows that parasitic SBS is the main physical process disturbing the SLM operation of Raman oscillator at higher power. In addition, for the first time, the spectral linewidth of a CW SLM diamond Raman laser was resolved using the long-delayed self-heterodyne interferometric method, which is 105 kHz at 20 W.