Precision spectral control for high-energy and ultrafast laser cavities
The etalon acts as an energy gatekeeper in a high-power laser system, acting as the primary mechanism for linewidth narrowing and mode selection and effectively governing the energy permitted to amplify within the laser cavity.
Historically, the primary metric for an etalon was its finesse, a dimensionless parameter characterising the ability to resolve closely spaced spectral lines. However, high finesse requires high reflectivity, which leads to intense intracavity energy storage. This intensity enhancement is fundamentally not required in a high-energy laser system where the priorities are dimensional stability under thermal load over traditional spectroscopic resolution.
At MPO, we draw on a deep understanding of optical theory to manufacture etalons that meet the most exacting standards of flatness and finesse. We fabricate coated and uncoated etalons for precision wavelength control, delivering exceptional precision and versatility, selectivity and superior flatness after coating for peak performance.
The impact of high-energy pulses on etalon design
In a resonant cavity, the intensity inside the etalon can be orders of magnitude higher than the incident laser beam. This intensity enhancement transforms the component into a significant thermal load within the laser system. Consequently, modern advice for sourcing high-energy etalons often contradicts traditional spectroscopic wisdom. Rather than maximising 𝐹𝑖𝑛𝑒𝑠𝑠𝑒, the goal is to minimise it to the lowest value that still provides necessary spectral filtering, thereby reducing the thermal load and the risk of failure. Sourcing criteria are based on survivability metrics, i.e. absorption coefficients, thermal conductivity, and laser-induced damage thresholds (LIDT), as well as on optical transfer functions.
H2 Etalon configurations
We design and manufacture both solid and air-spaced etalons, ensuring the correct Free spectral range (FSR) and finesse for your specific application.
- Air-spaced etalons: Consisting of two matched plates separated by a precise spacer, these offer an athermal solution. Because the beam passes primarily through air, which has a negligible thermo-optic coefficient (dn/dT) compared to glass. Air-spaced etalons offer flexibility in tuning and are less sensitive to thermal refractive index changes.
- Solid etalons: Robust and stable, these are formed from a single substrate, with partially reflecting coatings on both sides. Due to their mechanical stability, they are the industry standard for laser system stabilisation and mode selection in environments subject to vibration.
- VIPAs (Virtually Imaged Phased Arrays): Specialised solid etalons designed to introduce angular dispersion rather than simple filtering. They are significant for resolving gigahertz-level frequency shifts in applications such as Brillouin microscopy where standard gratings are insufficient.
Key characteristics of our ETALONS
Building on our commitment to quality, we ensure specific characteristics are met for every component:
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Exceptional optical quality: We utilise Grade 4 homogeneity substrates (or better) according to ISO 10110 Part 4 to prevent wavefront distortion within the bulk material.
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Customisable designs for specific requirements: Because we manufacture our optics in-house, we can grind and polish substrates to precise thickness targets to achieve your specific FSR requirements.
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Flatness after coating: Our optics are characterised by the highest degree of flatness after coating for ultimate wavefront performance; we manufacture to lambda/20 or better to ensure the effective finesse (F_eff) approaches the theoretical limit.
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Substrate purity: For continuous wave (CW) or high-energy pulsed lasers, we specify IR-grade fused silica (Type IV) manufactured in a water-free plasma environment to ensure negligible hydroxyl (OH) content of < 1ppm to avoid thermal runaway.
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Ion beam sputtering (IBS): This is the standard recommendation for high-energy systems, as IBS coatings are virtually defect-free, exhibiting near-zero absorption and negligible scatter.
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Electric field optimisation: Our coating designs shift electric field peaks into the bulk of low-index layers, such as SiO2, which have a higher bandgap and damage threshold.
Specifications
- UV fused silica, Diameter Tolerance: +0/-0.25mm,
- =80% clear aperture
- Thickness Tolerance: +/- 5% of thickness (up to 2mm thickness)
- Better than 1 arc sec parallel,
- λ/20 flatness
- 10-5 scratch-dig
We hold stock of 30mm diameter (20mm clear aperture) and larger etalon plates and also carry a vast selection of ready-made spacers in stock.
Standard etalons are uncoated, however we can readily provide coated products.
With our own in-house software we can model the etalon you need to ensure the finished product meets your requirements. Ask us for further details.
PART NUMBER |
DIAMETER (D) |
THICKNESS (T) |
ET-FS-25.4-0.2 25. |
25.4mm |
0.2mm |
ET-FS-25.4-0.3 25. |
25.4mm |
0.3mm |
ET-FS-25.4-0.5 25. |
25.4mm |
0.5mm |
ET-FS-25.4-1.0 25. |
25.4mm |
1.0mm |
ET-FS-25.4-2.0 25. |
25.4mm |
2.0mm |
Applications
- Laser system stabilisation: Ensuring single-longitudinal-mode operation and enhancing spectral purity in multi-kilowatt systems.
- Ultrafast laser cavities: Utilising IBS coatings to withstand extreme peak field intensities in femtosecond and picosecond regimes.
- Lidar and frequency conversion: Providing the necessary spectral rejection to prevent parasitic mode amplification in high-gain cavities.
Frequently Asked Questions
H3 Why is it significant to avoid maximising finesse in high-power laser systems? High finesse increases internal intensity enhancement, which can de-rate the damage threshold of the material and lead to catastrophic failure. We recommend specifying the lowest finesse that meets the spectral requirement to increase survivability.
H3 What is the difference between a solid and an air-spaced etalon? A solid etalon is a single plane-parallel plate, offering ruggedness for vibrating environments. An air-spaced etalon consists of two mirrors held apart by spacers; it provides a more thermally stable solution because the beam passes primarily through air rather than glass.
H3 How does surface flatness affect etalon performance? Surface flatness is the primary factor limiting the effective finesse (F_eff). Even if coating reflectivity is high, surface irregularities cause the finesse to collapse. We manufacture to λ/20 or better to ensure wavefront integrity.
H3 Why is “low-OH” fused silica specified for high-power lasers? Standard UV-grade fused silica contains hydroxyl impurities that create strong absorption bands in the near-infrared. For high-power NIR systems (such as 1064nm), using IR-grade material with negligible OH content is required to prevent thermal lensing and runaway.
H3 Can you manufacture etalons with custom free spectral ranges (FSR)? Yes. The free spectral range is determined by the cavity thickness or spacing. Because we manufacture in-house, we grind and polish substrates to precise thickness targets to meet specific FSR requirements.