Customized Ruby Laser Rods (Al₂O₃:Cr³⁺) Diameter 2mm/4mm Length
10mm/20mm
Abstract of the Ruby Laser Rod
The ruby laser rod was the first successfully realized laser gain
medium in human history, first demonstrated by Theodore Maiman in
1960. Its core component is an α-alumina (Al₂O₃) single crystal
doped with about 0.05% trivalent chromium ions (Cr³⁺), exhibiting a
characteristic deep red color. This synthetic ruby crystal
uniformly disperses chromium ions as the active medium within the
corundum matrix, where a special energy level structure is formed
through crystal field effects.
The ruby laser operates based on a typical three-level system
principle:
- Ground state energy level ⁴A₂: initial state of chromium ions
- Broad absorption bands ⁴F₁ and ⁴F₂: correspond to green (550 nm)
and violet (400 nm) light absorption
- Metastable energy level ²E: includes two closely spaced levels E
(694.3 nm) and 2Ā (692.9 nm)
Upon strong optical pumping, Cr³⁺ ions are excited from the ground
state to the ⁴F₁/⁴F₂ bands and rapidly relax non-radiatively to the
²E metastable state. Due to the relatively long lifetime (~3 ms) of
the ²E level, population inversion can be achieved, ultimately
producing 694.3 nm red laser output via stimulated emission.
The Attribute Table of Ruby Laser Rods
| Property | Specification | Unit/Notes |
| Material Composition | Al₂O₃ doped with 0.05% Cr₂O₃ | Weight percentage |
| Crystal Structure | Trigonal (α-Al₂O₃) | R3c space group |
| Laser Wavelength | 694.3 nm (R₁ line) | Primary emission line |
| 692.9 nm (R₂ line) | Secondary line (low temp) |
| Physical Dimensions | Diameter: 2-10 mm | Customizable (2mm/4mm shown) |
| Length: 10-200 mm | Standard (10mm/20mm shown) |
| Optical Properties | Refractive Index: 1.763 694nm | Ordinary ray (nₒ) |
| Absorption Coefficient: 0.4-1.2 cm⁻¹ | Depends on Cr³⁺ concentration |
| Thermal Properties | Thermal Conductivity: 40 W/(m·K) | 300K |
| Thermal Expansion: 5×10⁻⁶/K (∥c-axis) | Anisotropic |
| Mechanical Properties | Mohs Hardness: 9 | Second only to diamond |
| Density: 3.98 g/cm³ | |
| Laser Performance | Fluorescence Lifetime: 3 ms | 300K |
| Emission Cross-Section: 2.5×10⁻²⁰ cm² | For R₁ line |
| Thermal Lens Coefficient: 3×10⁻⁶ K⁻¹ | Important for high-power operation |
| Surface Quality | Flatness: λ/10 633nm | Laser-grade polish |
| Surface Roughness: <5 Å RMS | Superpolished finish |
| Coating Options | AR Coating 694nm: R<0.2% | Typical specification |
| HR Coating 694nm: R>99.8% | For laser cavity mirrors |
| Damage Threshold | >500 MW/cm² | For 10ns pulses |
Structural Characteristics of Ruby Laser Rod
1. Crystal Growth and Processing High-quality ruby laser rods are typically grown using the flame fusion
method (Verneuil process). By precisely controlling the ratio of
Al₂O₃ to Cr₂O₃ (around 0.05 wt%) and growth conditions, single
crystals with excellent optical uniformity can be obtained. The
crystal processing requirements include: - Rod diameter usually
between 3–10 mm, length from 50–200 mm - End-face parallelism
better than 10 arc seconds - Surface finish reaching λ/10 quality
level - Usually cut at Brewster’s angle or coated with
anti-reflection films
2.Optical and Thermal Properties
- Refractive index: 1.76 at 694.3 nm
- Thermal conductivity: approximately 40 W/(m·K)
- Thermal expansion coefficient: 5 × 10⁻⁶ K⁻¹ (parallel to c-axis)
- Mohs hardness: 9, second only to diamond
- Damage threshold: >500 MW/cm² (10 ns pulses)
3. Doping Characteristics Chromium ion concentration directly
affects laser performance:
- Optimal doping concentration: 0.03%–0.07% (by weight)
- Higher concentrations cause fluorescence quenching and exacerbate
thermal effects
- Chromium ions substitute for Al³⁺ sites in the crystal, occupying
octahedral coordination
Operating Characteristics of Ruby Laser Rod
1. Output Characteristics
- Wavelength: primarily R₁ line (694.3 nm) at room temperature; at
low temperature (77 K), R₂ line (692.9 nm) can also oscillate
- Linewidth: 0.53 nm (fluorescence), narrowing down to 0.001 nm
(single longitudinal mode)
- Typical pulse energy: 0.1–10 J (Q-switched)
- Peak power: up to several hundred megawatts (mode-locked)
- Divergence angle: 0.5–5 mrad (depending on resonator design)
2. Temperature Effects Ruby lasers exhibit significant temperature
dependence:
- Wavelength temperature coefficient: 0.065 Å/K
- Efficiency decreases with temperature increase (due to changes in
energy level splitting)
- Thermal lensing and stress-induced birefringence need to be
considered at high operating temperatures
3. Polarization Characteristics Ruby lasers naturally output
linearly polarized light due to:
- Anisotropic emission characteristics of Cr³⁺ ions
- Higher gain for E∥c-axis polarization compared to E⊥c-axis -
Polarization ratio can exceed 100:1
Application Fields of Ruby Laser Rod
1. Scientific Research
- Laser plasma studies: used in inertial confinement fusion
experiments
- Nonlinear optics: pump source for OPOs and Raman lasers
- Spectroscopy: high-resolution absorption and fluorescence
spectroscopy
- Atmospheric sensing: laser radar (LIDAR) systems
2. Industrial Processing
- Precision drilling: gemstone bearings for watches, fuel injector
nozzles
- Material marking: marking special materials such as ceramics and
sapphires
- Welding and cutting: thin metal material processing
3. Medical Applications
- Dermatology: treatment of pigmented lesions and tattoo removal
- Ophthalmology: irisectomy (gradually replaced by other lasers)
- Dentistry: hard tissue treatment
4. Military and Aerospace
- Range finding and target designation: early military laser
rangefinders
- Laser guidance: target illumination and designation
- Space communication: experimental laser links between satellites
Advantages and Limitations of Ruby Laser Rod
Main Advantages:
- High pulse energy: strong energy storage, suitable for
high-energy pulse output
- Excellent optical quality: few crystal defects, high beam quality
- Mechanical stability: high hardness, strong resistance to
environmental impacts
- Long lifetime: no aging issues, long service life
- Polarized output: simplifies optical system design
Main Limitations:
- Low efficiency due to three-level system: high threshold, typical
efficiency only 0.1%–1%
- Significant thermal effects: unsuitable for high repetition rate
operation (usually <1 Hz)
- Fixed wavelength: difficult to tune
- Requires strong optical pumping: generally pumped by flash lamps
- Higher cost: growing high-quality crystals is challenging
Technological Developments of Ruby Laser Rod
1. Cooling Technology Improvements - Conductive cooling: use of
copper heat sinks with high thermal conductivity - Liquid cooling:
circulation of deionized water or fluorinated liquids -
Low-temperature operation: efficiency improves by 3–5 times at 77 K
2. Pumping Method Innovations - Solar pumping: early space
application schemes - Laser diode pumping: improves efficiency and
reduces thermal load - Side pumping: improves energy distribution
uniformity
3. New Structural Designs - Slab structures: reduce thermal lensing
effects - Multiple rod tandem: increase total output energy -
Miniaturization: for special application scenarios
Q&A
Q:What is the primary laser wavelength of a ruby laser rod, and why
does it emit red light?
A:The ruby laser emits at 694.3 nm (R₁ line) due to Cr³⁺ ion
transitions in the Al₂O₃ crystal. The red color comes from the
electronic transition between the ²E excited state and the ⁴A₂
ground state of Cr³⁺. At low temperatures (~77K), a secondary line
at 692.9 nm (R₂ line) may also lase.
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