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Unlike lenses that focus light to a single point, axicons deflect light rays at a constant angle relative to the optical axis (following Snell’s law), creating a conical beam that maintains its intensity distribution over long distances (non-diffracting behavior). Thorlabs' zinc selenide (ZnSe) axicons excel in mid-infrared (mid-IR) applications (7-12 µm), a wavelength range critical for CO₂ lasers and thermal imaging. These axicons offer precise angle control (0.1° to 2.0° physical apex angles) and exceptional transmission (>97% average) across the 7-12 µm wavelength range, thanks to ZnSe’s high mid-IR transmittance (>90% at 10.6 µm) and broadband antireflection coatings .
Precision Apex Angle Control: Available with physical apex angles from 0.1° to 2.0° (the angle at the tip of the conical surface) with ±0.01° tolerance—this level of precision directly determines the beam’s divergence angle (β), which is calculated using Snell’s law: β = arcsin(n × sin(α/2)) - α/2, where n is the substrate refractive index (2.4 for ZnSe at 10.6 µm). For example, a 1.0° apex angle produces a ~0.7° divergence angle, creating a ring with a 14 mm diameter at 1 m working distance .
Broadband AR Coatings on Both Surfaces: Both the plano and conical surfaces feature anti-reflection coatings optimized for 7-12 µm, providing <1% average reflectance per surface. This minimizes reflection losses (which would reduce ring intensity) and prevents back-reflections that can damage the laser source (e.g., CO₂ lasers). The coatings are deposited via electron-beam evaporation and tested for uniformity using Fourier-transform infrared (FTIR) spectroscopy .
High-Quality ZnSe Substrates: Constructed from high-purity ZnSe (99.99% purity) to ensure minimal absorption in the mid-IR (absorption coefficient <0.005 cm⁻⊃1; at 10.6 µm)—critical for high-power laser applications, as absorption leads to heating and potential damage. ZnSe also transmits visible light (400-700 nm), allowing alignment with HeNe lasers (633 nm) without additional alignment tools .
Tight Manufacturing Tolerances: Fabricated with ultra-precise dimensional and surface specifications, including surface flatness <λ/2 at 633 nm (for the plano surface), surface roughness <20 Å RMS (root mean square, measured via atomic force microscopy), and diameter tolerance +0.0/-0.05 mm. These tolerances ensure that the conical surface is symmetric, preventing ring distortion (e.g., elliptical rings caused by asymmetric machining) .
Large Clear Aperture and Robust Design: 1" diameter models feature a clear aperture >ø22.86 mm (90% of total diameter), ensuring that even large collimated beams (up to 20 mm diameter) are fully utilized. The axicons are also designed with a 2 mm thick plano base, providing mechanical stability during mounting and reducing the risk of breakage (ZnSe is relatively brittle, with a Mohs hardness of 4) .
Laser Drilling and Cutting: Create precise ring-shaped beam profiles for material perforation (e.g., drilling holes in aerospace components) and cutting (e.g., slicing ceramic substrates). The annular beam delivers energy to the edge of the target, reducing heat-affected zones (HAZ)—for example, when drilling 1 mm holes in titanium, an axicon-based system produces a HAZ of <50 µm, compared to 100 µm with a Gaussian beam .
Bessel Beam Generation: Produce non-diffracting Bessel beams for long-distance propagation in free-space communication (e.g., satellite-to-ground laser links) and particle trapping (e.g., optical tweezers). Bessel beams maintain their intensity over kilometers (vs. Gaussian beams, which diverge significantly), making them ideal for deep-space communication where signal loss is a major challenge .
CO₂ Laser Systems: Ideal for integration into mid-IR laser processing equipment (e.g., CO₂ lasers for plastic welding and engraving). In plastic welding, the annular beam creates a strong, uniform weld seam (vs. a Gaussian beam’s uneven seam), improving the strength of medical device components (e.g., plastic syringes) .
3D Sensing and Structured Light Imaging: Generate annular or Bessel-shaped structured light patterns for depth mapping (e.g., facial recognition systems) and surface profilometry (e.g., measuring semiconductor wafer topography). In facial recognition, the non-diffracting Bessel pattern ensures that features are detected even at varying distances (0.5-2 m), improving accuracy .
Medical Devices: Enable specialized laser delivery in dermatology (e.g., laser resurfacing) and ophthalmology (e.g., glaucoma treatment). In dermatology, the annular beam targets the outer layer of skin (epidermis) without damaging the underlying dermis, reducing recovery time. In glaucoma treatment, it creates precise drainage channels in the eye with minimal tissue damage .
The output ring diameter (D) depends on three factors: the axicon’s apex angle (α), the working distance (L, distance from the axicon’s plano surface to the target), and the substrate’s refractive index (n). The formula is: D = 2L × tan(β), where β is the beam divergence angle (β = arcsin(n × sin(α/2)) - α/2). For example, using a ZnSe axicon (n=2.4) with α=1.0° and L=100 mm: β = arcsin(2.4 × sin(0.5°)) - 0.5° ≈ 0.7°, so D = 2×100×tan(0.7°) ≈ 2.44 mm. Factors that alter diameter include temperature (changes n slightly) and input beam collimation (uncollimated beams increase diameter variation) .
ZnSe axicons handle continuous wave (CW) power densities up to 10 W/cm² in the 7-12 µm range (e.g., 100 W CO₂ laser with a 10 mm diameter beam). To avoid damage, ensure proper cooling—use a heat sink (e.g., aluminum with thermal grease, thermal resistance <0.5°C/W) attached to the axicon’s plano base, as ZnSe has low thermal conductivity (18 W/m·K). Also, avoid operating near the laser’s wavelength edges (e.g., <7 µm or >12 µm for mid-IR axicons), as absorption increases outside the optimal range, leading to overheating .
While ZnSe axicons are optimized for mid-IR, axicons are available for visible and UV wavelengths using appropriate substrates. For visible lasers (400-700 nm), use N-BK7 or fused silica axicons (with AR coatings for 400-700 nm); these deliver >95% transmission and produce Bessel beams with similar non-diffracting properties. For UV lasers (190-380 nm), use UV-grade fused silica axicons (which resist UV-induced degradation) with UV AR coatings. For example, a UV axicon (α=0.5°) at 355 nm produces a ring diameter of 1.2 mm at L=100 mm—ideal for UV laser micromachining .
