Ultrafast Mid-IR Optics

Laser optics for this region must feature high spectral performance and low absorption. Although scattering might not be a significant issue when speaking of Mid-IR optics due to the long wavelengths, absorption remains a serious concern, because it leads to reduced Laser-Induced Damage Threshold (LIDT). 

And here‘s where OPTOMAN comes in. OPTOMAN concentrates solely on the most advanced sputtering technology - Ion-Beam Sputtering (IBS), which offers many great capabilities in depositing thin film coatings with great uniformity and bulk-like density.

• Film Stress. It is true that IBS coatings due to high density have higher stress values that can lead to coating cracking or peeling. At OPTOMAN, by introducing specific pre- and post- coating processes, we were able to reduce the stress which leads to fewer of the aforementioned problems. 
  
  
• Adhesion issues. Substrate material properties as well as differences in thermal expansion coefficients typically cause adhesion issues for IBS coatings. OPTOMAN was no exception, but R&D department found the way to make high repeatability optics for Mid-IR. 
  
  
• High cost. Coatings for Mid-IR spectral region are usually quite thick, which sometimes motivates to avoid IBS coatings and sacrifice some of the performance for the better price provided by alternative coating technologies or metal coatings. However, OPTOMAN now offers two alternatives that addresses the cost drivers: 
          1)  Scale. OPTOMAN has one of the largest IBS coating chambers in the World enabling to reduce the cost per piece significantly. 
         2)  Coating thickness. Recent R&D achievement enables OPTOMAN to offer relatively thin coatings that perform like you would expect from IBS coatings but with a significantly reduced price. This technology is suitable for <100 GW/cm2 laser power levels and is a perfect choice for laser communication applications, environmental monitoring, sensing and imaging. By this innovation, you can remain at your target price, but improve your laser system performance by advanced IBS coatings. 
  
  

Due to these challenges, it is not common to manufacture Mid-IR laser optics with IBS technology. However, OPTOMAN has overcome and solved these problems and confidently believes that using IBS is advantageous for the reasons mentioned above. OPTOMAN can make all types of coatings – anti-reflective, mirrors, GTI mirrors, polarizers, partial reflectors, dichroic mirrors, etc. up to 5000nm. 

In ultrafast laser applications, optimizing Group Delay Dispersion (GDD) is crucial for minimizing dispersion effects. With IBS technology, precisely controlling thicknesses and refractive indexes of IR coatings enables minimizing GDD to values below 100 fs².  

Design example

HRp>99.8% @ 1900-2700 nm + HT>97% @ 1550 nm, AOI=0-5°, |GDDr | <100 fs² @ 1900-2700 nm

Usually, the Highly Dispersive Mid-IR technology relies on combining transmissive materials with opposite signs of group velocity dispersion (GVD) (e.g. semiconductors and dielectrics), however strong absorption bands limit the transmittance in this spectral region, which limits the ability to adjust the chirp. Broadband dispersive mirrors, on the other hand, can reduce, or even eliminate the need to use such combinations to compensate dispersion and finally recompress broadband pulses close to Fourier limit duration in the sub-2 cycles regime. Which in the end allows shorter pulse durations and higher peak intensities.

Find out more about IBS-coated Highly Dispersive Mid-IR mirrors in our tech note.