Eye-safe intra-cavity diamond cascaded Raman laser with high peak-power and narrow linewidth

At a Glance

Metadata	Details
Publication Date	2024-01-01
Journal	Chinese Optics Letters
Authors	Xiaobo Mi, Chaonan Lin, Yongsheng Hu, Houjie Ma, Jiuru He
Institutions	Zhengzhou University, Yangzhou University
Citations	8
Analysis	Full AI Review Included

Executive Summary

This research details the successful development of a high-performance, eye-safe (1485 nm) cascaded Raman laser utilizing an intra-cavity CVD diamond crystal.

Record Peak Power: The system achieved a peak power of 40 kW, the highest reported for an eye-safe diamond Raman laser, enabled by significant pulse compression (from 60 ns to 1.1 ns).
High Efficiency: A diode-to-second-Stokes conversion efficiency of 8.1% was obtained, yielding 2.2 W of average output power at a 50 kHz pulse repetition frequency (PRF).
Optimized Cavity Design: A V-shaped folded cavity was implemented to ensure superior mode matching and thermal stability, effectively reducing the fundamental laser spot size on the diamond crystal (72 µm x 80 µm).
Narrow Linewidth: The device operated in a single longitudinal mode, exhibiting an ultra-narrow spectral width of less than 0.02 nm without requiring additional mode selection components.
Thermal Management: The use of a composite Nd:YVO₄ crystal and 880 nm in-band pumping successfully mitigated thermal effects, allowing stable operation at high pump levels.

Technical Specifications

Parameter	Value	Unit	Context
Output Wavelength	1485.9	nm	Second-Stokes (Eye-safe)
Maximum Average Output Power	2.2	W	At 50 kHz PRF
Maximum Peak Power	40	kW	Single pulse peak power
Diode-to-Stokes Conversion Efficiency	8.1	%	Diode input to 1485 nm output
Pulse Width (Second-Stokes)	1.1	ns	Compressed pulse width
Pulse Width (Fundamental)	60	ns	1064 nm fundamental laser
Spectral Linewidth	< 0.02	nm	Resolution-limited, single longitudinal mode
Fundamental Wavelength	1064	nm	Nd:YVO₄ emission
Pump Wavelength	880	nm	Fiber-coupled LD (In-band pumping)
Raman Crystal Material	CVD Diamond	-	Uncoated, 2 mm x 2 mm x 7 mm
Diamond Raman Shift	1332.3	cm^-1	-
Diamond Thermal Conductivity	2200	W/mK	Key material property
Fundamental Beam Size on Diamond	72 µm x 80 µm	-	Tangential (T) x Sagittal (S) planes
Beam Quality (M²)	1.33 (H), 1.76 (V)	-	Horizontal (H) and Vertical (V) directions
Cavity Type	V-shaped Folded	-	Total length 265 mm
Cooling Temperature	18	°C	Water cooling for crystals

Key Methodologies

The high-performance eye-safe laser was achieved through optimized material selection, cavity design, and pumping schemes:

In-Band Pumping: A 65 W, 880 nm laser diode was used for in-band pumping the Nd:YVO₄ gain medium. This approach, combined with a composite crystal, effectively reduced the thermal load on the gain medium.
Composite Gain Medium: The Nd:YVO₄ crystal (4 mm x 4 mm x 20 mm) featured a 16 mm core of 0.3% Nd³⁺-doped YVO₄ flanked by 2 mm end caps of pure YVO₄. This structure further mitigated thermal effects.
V-Shaped Folded Cavity: A V-shaped resonator was designed to optimize mode matching between the pump and fundamental beams and maintain high cavity stability, even under high thermal load (stable down to 170 mm thermal lens focal length).
Short Raman Cavity: The cascaded Raman cavity (formed by M3 and M4) was kept extremely short (10 mm). This short length contributed to faster Stokes pulse build-up and enhanced pulse compression.
CVD Diamond Utilization: An uncoated CVD diamond crystal (2 mm x 2 mm x 7 mm) was used as the Raman medium. Its high thermal conductivity (2200 W/mK) and large Raman gain coefficient (17 cm/GW @1 µm) were critical for high-power operation.
Polarization Alignment: The fundamental laser polarization was carefully adjusted to match the (111) direction of the diamond crystal, maximizing the Raman gain coefficient.
Cascaded Conversion: The output coupler (M4) was designed with high reflectivity (HR) at the fundamental (1064 nm) and first-Stokes (1240 nm) wavelengths, and high transmission (HT) at the second-Stokes (1485 nm), ensuring efficient cascaded conversion.

Commercial Applications

The combination of eye-safe operation, extremely high peak power, and narrow linewidth makes this technology highly valuable for demanding industrial and defense applications.

High-Resolution LIDAR (Laser Detection and Ranging): The 40 kW peak power and narrow linewidth (< 0.02 nm) are ideal for long-range, high-resolution atmospheric sensing and ranging systems, particularly in eye-safe environments.
Laser Medicine and Surgery: The 1.4-1.8 µm spectral region is highly relevant for medical procedures due to strong water absorption, while the eye-safe nature simplifies regulatory requirements.
Nonlinear Frequency Conversion: The high peak power (40 kW) provides an excellent pump source for driving subsequent nonlinear optical processes, such as optical parametric oscillators (OPOs) or harmonic generation, to access new spectral regions.
High-Precision Spectroscopy: The single longitudinal mode operation and narrow spectral width enable high-fidelity measurements in scientific and industrial spectroscopy.
CVD Diamond Material Supply: The performance relies directly on the quality of the CVD diamond crystal, linking this technology to manufacturers specializing in high-purity, low-loss optical-grade diamond materials (e.g., 6ccvd.com).

View Original Abstract

The 1.4-1.8 µm eye-safe lasers have been widely used in the fields of laser medicine and laser detection and ranging. The diamond Raman lasers are capable of delivering excellent characteristics, such as good beam quality concomitantly with high output power. The intra-cavity diamond Raman lasers have the advantages of compactness and low Raman thresholds compared to the external-cavity Raman lasers. However, to date, the intra-cavity diamond cascaded Raman lasers in the spectral region of the eye-safe laser have an output power of only a few hundred milliwatts. A 1485 nm Nd:YVO4/diamond intra-cavity cascaded Raman laser is reported in this paper. The mode matching and stability of the cavity were optimally designed by a V-shaped folded cavity, which yielded an average output power of up to 2.2 W at a pulse repetition frequency of 50 kHz with a diode to second-Stokes conversion efficiency of 8.1%. Meanwhile, the pulse width of the second-Stokes laser was drastically reduced from 60 ns of the fundamental laser to 1.1 ns, which resulted in a high peak power of 40 kW. The device also exhibited single longitudinal mode with a narrow spectral width of < 0.02 nm.

Tech Support

Original Source

DOI: https://doi.org/10.3788/col202422.041402

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