Mirrors for Multipass Cells (MPC)

Why MPCs?

Increasing demand for higher pulse energies with pulse durations of tens of femtoseconds is driven by emerging requirements in material processing and various ultrafast research & development fields including ultrafast pump-probe spectroscopy, attosecond pulse generation and OPA pumping. Although Ti:Sapphire lasers can meet the pulse duration criteria, they are constrained to low repetition rates and moderate average power due to quantum defects and cooling. On the other hand, Yb-doped systems have shown high average power and high pulse energies, though additional spectral broadening is needed to reach the ultrafast pulse durations of tens of femtoseconds. In order to expand the spectral bandwidth of systems with high energy and average power, as well as to produce shorter pulse durations, one has to leverage nonlinear pulse compression. For this, OPTOMAN views Multipass Cells as a prominent solution.

In MPCs, the spectrum of an ultrafast pulse is broadened by self-phase modulation in the non-linear medium. Using MPCs, pulses can be compressed down to the sub-50 fs durations while still maintaining high quality of the laser beam. By shortening the pulse duration, MPCs help to increase the intensity which is desirable in attosecond physics, laser-driven particle accelerators, pump-probe experiments, lightwave electronics and more. Laser manufacturers are considering MPCs as candidates to become integral parts of their laser systems in the next cycle of laser improvement and development.

Figure 1 – Multipass Cell schematic. [1]

What’s up with mirrors for MPCs?

We all know that any laser, no matter how powerful and fast it may be, is only as strong as its weakest link. The same applies to Multipass Cells. Since MPCs involve multiple reflections of light on the mirror surfaces, the efficiency of the cell scales exponentially with the quality of its mirrors. The main requirements for an MPC mirror are large reflection bandwidth, high reflectivity, high LIDT and low group delay dispersion (GDD).

Firstly, the minimum achievable duration of the compressed pulse is proportional to the inverse of its bandwidth. A few-cycle optical pulse may require a mirror as broadband as a couple hundreds of nanometers.
Secondly, an optical pulse in an MPC will make many bounces until its spectrum becomes wide enough for compression. Twenty reflections of the mirror with 98% reflectivity will reduce the available energy by around one-third. In addition, the input of MPC typically receives a high-energy pulse, which is why the mirrors must be of high reflectivity and LIDT.
Lastly, as the laser pulse propagates through the cell, the GDD induced by non-linear medium must be compensated. If the compensation is not constant across the spectrum after many reflections, the laser pulse spectral phase could end up unusable afterwards, so low, even and precisely according to the cell design optimised GDD of mirrors is also vital.

What is OPTOMAN offering?

OPTOMAN sees a big potential for MPCs as their adoption to laser systems seems inevitable. For this reason, OPTOMAN developed dielectric mirrors optimised specifically for MPC applications. OPTOMAN offers flat, concave, and convex broadband mirrors that possess all the features mentioned before: high reflectivity (R>99.98%), high LIDT (>0.69 J/cm² @ 1030 nm, 180 fs), and low and spectrally uniform GDD.

Moreover OPTOMAN has developed non-degrading mirrors optimised for Multipass Cells. This advancement means that OPTOMAN mirrors for MPCs are now more reliable and durable than ever before, ensuring your ultrafast laser systems perform at their highest potential even after many pulses.

Any options?

OPTOMAN offers two options of mirrors for MPC application that you can choose based on your preferences and requirements: Enhanced LIDT and Enhanced Spectral Bandwidth. But remember – any optical component can be customised to your needs and “in-between” options are possible.

OPTOMAN standard MPC mirror, that fits standard MPC design, can be found at OPTOSHOP.

Measurement of LIDT

Measurement of Enhanced Spectral bandwidth option.

The graph shows no evidence of color-change of laser-induced damage.

Tested to withstand >0.2 J/cm^² at 1030 nm, 190 fs.

Design Examples

Reflectivity and Group Delay Dispersion (GDD) graphs of Ion Beam Sputtered coating of the Enhanced LIDT option.

Reflectivity and Group Delay Dispersion (GDD) graphs of Ion Beam Sputtered coating of the Enhanced Spectral Bandwidth option.

OPTOMAN also has a capability of optimising the Mirrors for negative Group Delay Dispersion.

Resources

[1] Hanna, M., Guichard, F., Daher, N., Bournet, Q., Délen, X., Georges, P., Nonlinear Optics in Multipass Cells. Laser & Photonics Reviews 2021, 15, 2100220.