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Thorlabs' Herriott cell mirrors— a common type of concave mirror with hole— feature 1" or 2" outer diameters (OD) with centered or off-axis holes (3 mm to 4 mm diameters) and mid-infrared (mid-IR) enhanced gold coatings. These coatings provide >98% average reflectance from 2-20 µm, a wavelength range critical for gas spectroscopy (e.g., detecting CO₂, methane) and high-power mid-IR lasers. The mirror’s concave surface has a precise radius of curvature (ROC), typically 1 m to 5 m for Herriott cells, ensuring that beams reflect multiple times within the cavity while maintaining alignment .
Optimized Mid-IR Reflectivity: Enhanced gold coatings (with a 5 nm chromium adhesion layer and 100 nm gold layer) deliver >95% absolute reflectance across 2-20 µm—significantly higher than standard gold coatings (which drop to 90% at 20 µm). The coatings are deposited via thermal evaporation in a high-vacuum environment (<10⁻⁶ Torr) to ensure uniformity and minimize absorption (absorption <2% at 10.6 µm), critical for multi-pass systems where each reflection contributes to signal loss .
Precision Hole Placement and Machining: Available with two hole configurations: centered holes (ø3 mm for 1" mirrors, ø4 mm for 2" mirrors) for simple beam injection, and off-axis holes (offset by 5-10 mm from the center) for maximizing path length in Herriott cells. The holes are drilled using laser machining (for glass substrates) or ultrasonic drilling (for metal substrates), resulting in clean edges (burr <5 µm) and perpendicularity to the mirror surface (<0.1° deviation)—prevents beam deflection at the hole .
Thermal Stability for High-Power Applications: Constructed with UV fused silica substrates, which have a low coefficient of thermal expansion (0.55 × 10⁻⁶ /°C) and high thermal conductivity (1.4 W/m·K). This stability minimizes changes in the radius of curvature (ROC variation <0.1% over -40°C to +80°C), ensuring consistent beam focusing even in high-power systems (e.g., 100 W CO₂ lasers). For extreme temperatures, sapphire substrates (thermal expansion 5.0 × 10⁻⁶ /°C) are available .
Protective Overcoat for Environmental Durability: The gold coating is capped with a 10 nm SiO₂ protective overcoat, which improves resistance to humidity (95% relative humidity for 1000 hours without tarnishing) and mechanical abrasion (Mohs hardness increased from 2.5 to 5). This overcoat also reduces scattering at the coating surface (scattering loss <0.5% at 10.6 µm), improving signal-to-noise ratio in spectroscopy .
Large Clear Aperture and ROC Tolerances: 1" diameter models have a clear aperture of >ø22 mm (88% of OD), ensuring that beams utilize most of the reflective surface. The radius of curvature (ROC) is machined to ±0.5% tolerance (e.g., 1 m ROC ±5 mm), which is critical for Herriott cells—small ROC variations can reduce the number of reflections (and thus path length) by 10-20%. Surface flatness of the concave surface is <λ/4 at 633 nm, minimizing wavefront distortion .
Herriott Cells and Gas Sensing: Enable long optical path lengths (up to 100 m) in compact cavities (volume <1 L) for gas spectroscopy. In environmental monitoring, Herriott cells with concave mirrors with hole detect trace gases (e.g., methane at concentrations as low as 1 ppm) by measuring absorption of mid-IR light. The hole allows the laser beam to enter the cell, reflect 50-100 times off the concave mirrors, and exit for detection .
Laser Cavities and Resonators: Facilitate beam injection and extraction in high-finesse laser resonators (e.g., diode-pumped solid-state lasers, DPSSLs). In a DPSSL cavity, the mirror’s hole lets the pump beam enter the gain medium (e.g., Nd:YAG crystal) while the concave surface reflects the laser beam (1064 nm) to form the resonator. This design eliminates the need for separate beam splitters, reducing cavity losses .
Raman Spectroscopy: Enhance signal collection in Raman spectroscopy systems, which detect molecular vibrations by measuring scattered light. The concave mirror with hole focuses the excitation laser (e.g., 532 nm) onto the sample via the hole, then collects and reflects the Raman-scattered light (shifted wavelengths) to a detector. This configuration increases signal intensity by 50-100% compared to flat mirrors .
Telecom Testing and Optical Delay Lines: Create controlled optical delay lines for testing fiber optic components (e.g., modulators, amplifiers). By adjusting the distance between two concave mirrors with hole, the beam path length (and thus delay) can be tuned from 10 cm to 10 m—critical for testing signal propagation in long-haul telecom networks (e.g., 10 Gbps systems) .
Material Processing with High-Power Lasers: Focus and redirect high-power laser beams in drilling, welding, and marking applications. For example, in laser drilling of aerospace components (e.g., turbine blades), the mirror’s concave surface focuses the beam (e.g., 1 kW fiber laser) to a 50 µm spot, while the hole allows coolants to flow through, preventing thermal damage to the mirror .
Off-axis holes allow significantly longer path lengths by utilizing more of the mirror’s reflective surface. In a Herriott cell with center holes, beams reflect in a linear pattern (back and forth between mirrors), limiting the number of reflections (typically 20-30). With off-axis holes, beams follow an elliptical path, reflecting 50-100 times—doubling or tripling the path length (e.g., 50 m vs. 20 m for a 1 L cell). This longer path length improves gas detection sensitivity (lower detection limits by 2-3x) but requires more precise alignment (±0.01° angular tolerance) .
The back-side of the hole (opposite the reflective surface) features 60° chamfers with a diameter of 6.5-8.0 mm. Chamfers serve two key purposes: first, they prevent beam scattering from sharp hole edges (which would introduce noise in spectroscopy), and second, they guide the beam into/out of the mirror without additional optics (e.g., collimators). The chamfer surface is polished to 60-40 scratch-dig quality, reducing scattering loss to <0.1%. Without chamfers, sharp edges can cause up to 5% beam loss and distort the beam profile .
With proper cooling, these mirrors handle CW power densities up to 50 W/cm² in the 2-20 µm range (e.g., 500 W CO₂ laser with a 3.5 mm diameter beam). Cooling is critical because gold coatings absorb ~2% of incident light, which generates heat. For low-power systems (<10 W), passive cooling (aluminum heat sink with thermal grease) is sufficient. For high-power systems, active cooling (water-cooled heat sink with flow rate 1 L/min) keeps the mirror temperature <50°C, preventing coating degradation (gold tarnishes at >150°C) and ROC changes .
Yes, manufacturers offer extensive customization to match specific system requirements. Custom hole sizes range from 0.5 mm to 10 mm (±0.1 mm tolerance), with shapes including circular, square, or rectangular (for specialized beam shapes). Hole positions can be offset by 0-15 mm from the center (±0.05 mm tolerance). Custom ROC values range from 0.5 m to 10 m (±0.5% tolerance). Lead times for custom mirrors are typically 2-4 weeks for small quantities (1-10 units) and 4-6 weeks for large quantities (>10 units). Prototyping (1-2 units) can be completed in 1 week for urgent projects .
