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How to do the coating on alexandrite laser rod?

Del Mar Photonics, Inc. is an established manufacturer and system integrator of advanced photonics products for scientific and industrial applications. We offer broad range of lasers, optics, optical crystals and other instrumentation.
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Coating an Alexandrite Laser Rod
The coating process for alexandrite laser rods requires precision and specialized knowledge and equipment. Here's how it's typically done:

Preparation
Clean the alexandrite rod thoroughly using appropriate solvents (typically acetone, methanol, or isopropyl alcohol) to remove any contaminants
Ensure the rod is completely dry and free of particles
Handle only with clean gloves and tools to prevent fingerprints or contamination

Coating Methods
The most common methods for coating alexandrite laser rods include:
Electron Beam Evaporation
This method deposits thin dielectric layers with precise thickness control and is commonly used for high-quality optical coatings.
Ion-Assisted Deposition
This enhances the coating density and durability by bombarding the surface with energetic ions during deposition.
Sputtering
Provides excellent adhesion and uniformity for dielectric coatings.

Typical Coating Specifications
Anti-reflection (AR) coatings at the pump wavelength (typically 755nm)
High-reflection (HR) coatings at the lasing wavelength (tunable, typically 700-850nm)
Multi-layer dielectric coatings for specific wavelength operation
Damage threshold typically more than10 J/cm²

Typical Coating Structure and Materials for Alexandrite Laser Rods
Coating Structures

Alexandrite laser rods typically receive one of these coating configurations:
AR/AR Configuration (Anti-reflection on both ends)
Used when the rod is placed between external resonator mirrors
Minimizes losses at both pump and lasing wavelengths
Typical reflectivity: less than 0.25% per surface
HR/AR Configuration (High-reflection on one end, anti-reflection on the other)
One end serves as a cavity mirror
HR end typically has R more than 99.9% at lasing wavelength
AR end has minimal reflection (less than 0.25%)
Brewster-Cut Ends
Alternative to coating for polarized operation
Ends are cut at Brewster's angle (about 61° for alexandrite)
No coating required if properly oriented

Common Coating Materials
Dielectric Materials
Silicon dioxide (SiO₂) - Low refractive index layer (n≈1.46)
Titanium dioxide (TiO₂) - High refractive index layer (n≈2.4)
Hafnium dioxide (HfO₂) - High damage threshold alternative
Aluminum oxide (Al₂O₃) - Used for enhanced durability

Layer Structure
Typically uses quarter-wave optical thickness (QWOT) alternating layers
AR coatings: Usually 2-8 alternating layers optimized for minimal reflection
HR coatings: Usually 20-40 alternating layers for maximum reflection
Total coating thickness: Typically 1-5μm depending on design

Special Considerations
Coatings must handle both pump wavelength (typically 532nm or 755nm) and tunable output (700-850nm)
Must withstand high peak powers (often more than 10 MW/cm²)
Thermal stability needed for operation at elevated temperatures
Enhanced adhesion layers sometimes used between rod and first dielectric layer

The exact materials and layer counts are determined by the specific application requirements, including wavelength range, power handling, and environmental conditions.

AR/PR Configuration for Alexandrite Laser Rods
The AR/PR coating structure features:
One end with anti-reflection (AR) coating: Minimizes reflection losses (typically less than 0.25% reflectivity)
One end with partial-reflection (PR) coating: Acts as an output coupler with controlled reflectivity

Specifications and Design
PR coating typically provides 30-80% reflection at the lasing wavelength
Precise reflectivity is selected based on:

Cavity design requirements
Desired output coupling
Gain characteristics of the specific alexandrite rod
Pump power levels

Advantages
This configuration offers several benefits:
Simplified resonator design (fewer optical components)
Reduced cavity losses
Better control of output characteristics
More compact overall laser design
Can be optimized for specific Q-switching applications

Post-Coating
Allow proper curing time according to coating specifications
Test for reflection/transmission properties using a spectrophotometer
Perform damage threshold testing if needed

This process should be performed in a cleanroom environment with proper temperature and humidity control for optimal results.

http://www.dmphotonics.com/
For additional information and quote email to [email protected]