Optical Systems Design for Ultrashort Pulse Lasers
by Daniel Csati - January 06, 2026
We have been involved in numerous ultrashort pulse laser optical system designs in the past and it’s time to share some of the essential knowledge on the topic.
People often have the impression that setting up a laser material processing system is simple: just mount a galvo or microscope objective on front of the laser and we can start processing. Well, yes, sometimes that’s enough. But as soon as you start diving into required beam diameters, working distance, fine-tuning, polarization ... you will come across some potential issues.
Ultrashort pulse lasers work a bit differently than other lasers. The shorter the pulses get, the more it matters what materials are in use, how the coatings are designed and fine details like how large the beam diameter is.
Standard optics that work perfectly for nanosecond lasers might fail, distort, or even self-destruct (!) when faced with a 300-femtosecond pulse.
0. The Complexity of the Femtosecond Regime
The shorter the pulse, the more complex the management. This is due to three primary factors:
Broad Spectral Bandwidth: According to the uncertainty principle, a short pulse in time must be broad in the frequency domain. Optics must perform consistently across this entire bandwidth. We are talking of 10nm bandwidth at 300fs and 100nm spectral bandwidth at 30fs.
Dispersion Sensitivity: Even a few millimeters of glass can introduce Group Delay Dispersion (GDD), which will be stretching the pulse in time and ruining its peak power. Passing through gratings, waveplates and multilayer waveplates (coatings) can have major effect.
Pulse-Length Dependent Damage: Generally, the shorter the pulse the smaller the damage threshold. The high peak power triggers non-linear absorption, meaning the damage mechanisms are entirely different from long-pulse thermal melting. Multiphoton absorption can lead to for instance thermal lensing.
1. Why Fused Silica is the Gold Standard
When selecting a substrate, Fused Silica is the best choice for USP applications.
Kerr Lensing: Fused silica has a lower non-linear refractive index compared to other glasses like BK7, reducing the risk of unwanted beam distortion. The higher the index, the higher the nonlinear refractive index.
Minimal GDD: It offers predictable and relatively low dispersion. This is a common property of glasses: the lower the index, the lower the refractive index difference between shorter and longer wavelength components of the pulse.
High LIDT: It handles the extreme electric fields of USP lasers better than almost any other optical glass.
2. No Glue Allowed: Air-Spaced vs. Cemented
In traditional optics, achromats or beam splitters are often "cemented" using optical adhesives. In the USP world, optical cement is a liability. The intense peak power will cause the adhesive to darken, bubble, or even delaminate. Instead, systems must use:
Air-Spaced Designs: Elements are held apart by precision spacers. Some air spaced doublets are available even off the shelf. Microscope objectives are often made of many cemented elements – you need one which is especially designed for ultrashort pulse laser applications.
Optical Contacting: A glue-free bonding method where surfaces are so flat they are held together by Van der Waals forces. Optically contacted beam splitters or plate beam splitters are the best choice.
3. The Science of Coatings
Coatings are usually the "weakest link" in an ultrafast system. Because most USP coatings are Multilayer Dielectric (MLD) stacks, they introduce two major challenges:
High-Order Dispersion: The layers can "trap" different wavelengths for different amounts of time, distorting the pulse shape. Some of the mirrors are specifically designed for distorting the beam (GTI or Chirped mirrors)
Electric Field Enhancement: If not designed correctly, the electric field can peak inside the coating layers rather than at the surface, causing internal failure. Keep in mind that the multilayer dielectric coatings function on interference. Interference can be destructive (AR coatings) but it can quickly turn constructive.
Over time, even "safe" power levels can cause the material structure to shift slightly, leading to darkening (increased absorption) and eventual catastrophic failure. The darkened coating (layer) will have increased absorption and the increased absorption will cause stress and distortion even on a simple flat surface.
4. Practical Engineering Advice
Building a USP beamline requires a well-thought through design regarding non-linear effects:
Go Big: Keep the beam diameter large. This lowers the intensity and prevents Kerr lensing and self-focusing, which can collapse the beam and damage optics. Kerr lensing is non-spherical which means the wavefront distortions it causes will be not just hard to compensate but also intensity dependent!
Watch the stray light focus: Always trace stray light paths. A partial reflection from a back surface can inadvertently focus inside another optical element or on a mechanical housing. F-theta lenses for ultrashort pulses are specifically designed to have zero focus inside the lens (assuming the user uses the right lens-galvo mirror distance).
No Intermediate Focus: When using beam expanders, avoid "Keplerian" designs that focus the beam in the air. The peak power is often high enough to ionize air, causing a plasma breakdown that distorts the beam profile. If an intermediate focus is unavoidable (like with afocal relays) then use vacuum to avoid creating plasma.
The Plasma Mirror Effect: Be prepared for strong back-reflections from the target material. These reflections can travel back and cause damage at an unexpected spot. Plasma mirrors created in focus are relatively efficient reflectors (10s of %).
X-Ray Safety: When etching or cutting materials with USP lasers, the plasma generated can emit X-rays. The x-rays originate from the electrons which are first stripped off then accelerated by the pulse and finally smashed into unionized material.
Sourcing and Cost
Specialized optics are a significant investment. The price premium comes from the need for IBS (Ion Beam Sputtering) coating methods, specialty materials, high surface quality .. which provide precision required for performance.
For those looking for specialized suppliers:
Coatings: Companies like Laseroptik and Optoman are industry leaders in complex, high-LIDT coatings tailored for ultrafast pulses.
Components: Edmund Optics and EKSMA Physics offer excellent selections of fused silica optics and optics optimized for specific femtosecond wavelengths (like 1030nm or 800nm).