Power Scaling of CW Crystalline OPOs and Raman Lasers

At a Glance

Metadata	Details
Publication Date	2021-12-10
Journal	Photonics
Authors	Soumya Sarang, Martin Richardson
Institutions	University of Central Florida
Citations	4
Analysis	Full AI Review Included

Executive Summary

Spectral Gap Filling: Optical Parametric Oscillators (OPOs) and Raman lasers are essential nonlinear technologies used to generate wavelengths in spectral gaps (blue, yellow-red, mid-IR) inaccessible to conventional inversion lasers.
Thermal Limitation: Power scaling in CW OPOs and non-diamond crystalline Raman lasers is severely limited (typically to tens of watts) by thermal lensing, stress, and photorefractive damage in the gain media (e.g., PPLN, KGW).
Diamond Superiority: Diamond Raman Lasers (DRLs) exhibit the highest power scalability due to diamond’s extraordinary thermal conductivity (2000 W/mK), which minimizes thermal lensing effects by factors of 1000 compared to materials like LiNbO₃.
Performance Benchmark: DRLs have demonstrated quasi-CW output powers up to 1.2 kW (1240 nm) with 53% conversion efficiency, a level unattainable by other crystalline nonlinear converters.
Brightness Enhancement: DRLs effectively perform “Raman Beam Cleanup” (RBC), converting low-quality pump beams (M² = 15) into high-quality Stokes beams (M² = 2.95), achieving a record Brightness Enhancement Factor (BEF) of 56.
Future Outlook: The technology shows strong potential for multi-kilowatt CW operation, provided advanced thermal management strategies (e.g., cryogenics, optimized cooling mounts) are implemented.

Technical Specifications

Parameter	Value	Unit	Context
Peak DRL Output Power (Quasi-CW)	1.2	kW	1240 nm, 53% conversion efficiency
Maximum CW OPO Output Power	30	W	Signal (1.55 µm) + Idler (3.4 µm)
Maximum DRL Brightness Enhancement Factor (BEF)	56	N/A	Converting M²=15 pump to M²=2.95 Stokes
Diamond Thermal Conductivity	2000	W/mK	Highest among all known crystals
LiNbO₃ (LN) Thermal Conductivity	5	W/mK	Typical OPO crystal (for comparison)
Diamond Raman Gain Coefficient (@1064 nm)	10	cm/GW	Highest among reviewed crystals
Diamond Raman Shift	1332.3	cm^-1	Large shift, enabling wide spectral steps
Diamond Optical Transparency Range	0.23-100	µm	Ultra-wide range (UV to mid-IR)
KTP Coercive Field	2	kV/mm	Allows poling of thick crystals (up to 3 mm)
PPLN Coercive Field	21	kV/mm	Limits crystal thickness to approximately 0.5 mm

Key Methodologies

Comparison of Gain Mechanisms

OPOs (Parametric Amplification):
- Relies on χ⁽²⁾ nonlinearity.
- Requires stringent phase-matching (angle/temperature tuning or quasi-phase matching) to conserve momentum, complicating cavity design.
- Theoretically, zero quantum defect (no heat deposited by the conversion process itself), but high intracavity power leads to heating via parasitic absorption.
Raman Lasers (Stimulated Scattering):
- Relies on χ⁽³⁾ nonlinearity.
- Phase-matching is inherently satisfied by the medium (non-parametric), simplifying cavity design.
- Involves a quantum defect (energy difference transferred to optical phonons), leading to intrinsic heating of the crystal.

Cavity Architectures for Power Scaling

OPOs: Singly Resonant Oscillators (SROs) are preferred for high-power CW operation due to stability and relaxed mirror specifications, but necessitate extremely high intracavity signal power (up to kW level) to reach threshold, exacerbating thermal load.
Raman Lasers: External cavity configurations are favored for high-power CW DRLs, allowing the pump and Raman cavities to be placed separately, which simplifies thermal management and avoids coupling instabilities seen in intracavity designs.

Thermal Management and Brightness

Thermal Limiter: Thermal lensing (focal length f) is the primary constraint, inversely proportional to the thermo-optic coefficient (dn/dT) and directly proportional to thermal conductivity (K).
Material Selection: Diamond is the optimal material because its high K (2000 W/mK) drastically increases the thermal lens focal length, making it highly resistant to thermal distortion compared to LN or KGW.
Brightness Enhancement (RBC): Raman lasers inherently utilize the Raman Beam Cleanup (RBC) mechanism, which allows the conversion of low-quality, multi-mode pump beams into high-quality, near-diffraction-limited Stokes beams (M² ≈ 1).

Commercial Applications

The power and brightness scalability of DRLs, combined with their ability to access new spectral regions, makes them highly relevant for:

Defense and Directed Energy: Generating multi-kW output powers with high brightness (low divergence) for long-range applications.
Remote Sensing and LIDAR: Producing eye-safe (1.4 µm to 1.8 µm) and mid-IR wavelengths via Raman cascading for probing atmospheric molecules and long-distance detection.
Advanced Material Processing: High-precision applications (engraving, micromachining, drilling fine holes) requiring high power density and excellent beam quality (high brightness).
Spectroscopy and Scientific Research: Providing narrow-linewidth, single-longitudinal-mode (SLM) sources across a wide tunable range.
Medical and Biomedical: Generating novel, high-power, good beam-quality wavelengths in the visible spectrum (e.g., 589 nm, 620 nm via frequency doubling) for medical laser therapy and diagnostics.
Free-Space Optical Communications: Utilizing high-power, low-divergence beams for high signal-to-noise ratio over large distances.

View Original Abstract

Optical parametric oscillators (OPOs) and Raman lasers are two nonlinear-based laser technologies that extend the spectral range of conventional inversion lasers. Power and brightness scaling of lasers are significant for many applications in industry, medicine, and defense. Considerable advances have been made to enhance the power and brightness of inversion lasers. However, research around the power scaling of nonlinear lasers is lacking. This paper reviews the development and progress of output power of continuous-wave (CW) crystalline OPOs and Raman lasers. We further evaluate the power scalability of these two laser technologies by analyzing the cavity architectures and gain materials used in these lasers. This paper also discusses why diamond Raman lasers (DRLs) show tremendous potential as a single laser source for generating exceedingly high output powers and high brightness.

Tech Support

Original Source

DOI: https://doi.org/10.3390/photonics8120565

References

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