Time to reflect: The role of mirrors in ultrafast laser performance

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Gemma Church explains how mirrors are improving ultrafast laser performance

Ultrafast lasers are now capable of reaching pulse lengths as short as 10 femtoseconds or less. They are used across a broad range of industrial and research applications, including spectroscopy, distance ranging, terahertz radiation generation, and capturing both biological and physical processes that normally happen across timescales that are simply too fast to measure. 

Due to the unique nature of ultrafast pulses, today’s standard laser components are not suitable for such applications, where dispersion lengthens the pulse duration and reduces its peak power. Laser damage is another fundamental issue and these two factors – pulse dispersion and laser damage – have a direct and detrimental impact on the system performance. 

Ryan McGuigan, a physicist at Manx Precision Optics, explained: ‘Engineers face the challenge of generating and maintaining ultrashort pulse structure and energy with large bandwidth and low dispersion components. Ultrashort pulses incident upon standard optical components will suffer from adverse dispersive effects and therefore require specially designed components.’

Instead, today’s ultrafast laser systems must include specially-designed ultrafast components, which are optimised to operate with ultrafast pulses. McGuigan said: ‘The wide bandwidth, LIDT and dispersion control of ultrafast optics allow access to the femtosecond pulse regime. Generation of ultrashort pulses (and hence the applications of ultrashort pulses) would otherwise be impossible.’

The laser-induced damage threshold (LIDT) is a key parameter and defined within ISO 21254 as the ‘highest quantity of laser radiation incident upon the optical component for which the extrapolated probability of damage is zero.’ Simply put, the purpose of the LIDT is to specify the maximum laser intensity that a laser optic can withstand before damage occurs.

When using ultrafast pulses, different laser damage mechanisms dominate for ultrashort pulses, making it more difficult to fully understand the LIDT and related scaling factors in this pulse duration regime. Tunnel and avalanche ionisation damage mechanisms, for example, are two prevalent and unique issues where the strong electric field generated by ultrashort pulses releases electrons, causing damage to the optical component in question.

Group delay dispersion (GDD) control is another significant challenge for today’s ultra-fast optical systems. This is a measure of the temporal distortion added to a pulse by a specific component.

When an ultrashort pulse propagates through or reflects from a specific medium, the pulse is broadened and it acquires a large GDD. This is essentially the same as the characteristic splitting of a beam of white light passing through a prism into the light’s different, visible frequency components. 

Pulse broadening has a series of knock-on effects, reducing the peak pulse power, limiting the duration of the output pulse and introducing a chirp, which causes frequency variations along the length of the pulse. When the GDD is positive, shorter wavelengths are delayed more than the longer ones and the pulse duration is stretched, causing a positive chirp. If the GDD is negative, the pulse is compressed and a negative chirp results. 

To overcome these issues, ultra-fast systems require low GDD and high LIDT components to access and maintain femtosecond pulse durations.

To achieve this, engineers must take a vast range of factors into account when setting up and using an ultrafast laser system. McGuigan explained: ‘An engineer needs to take into account bandwidth/pulse duration, central wavelength, angle of incidence and LIDT amongst other considerations. Manx Precision Optics has a sellers guide to aid customers in choosing the right optics for them.’ 

Ultrafast mirrors

Whether specially designed for ultrashort pulses or not, every component in an ultrafast laser system possesses dispersive properties that broaden the laser pulse and introduce an unreasonable amount of chirp. 

Ultrafast mirrors are now critical to almost every ultrafast laser application to minimise both this dispersion issue and laser damage. McGuigan explained: ‘Ultrafast mirrors tend to have a very large band-width as well as a very high reflectivity and low GDD (dispersion) across that bandwidth.’

For example, a pulse with a strong positive GDD across its spectrum can have its GDD reduced by reflecting it off a highly negative GDD mirror. Mirrors can also possess a very low, constant GDD across their reflective bandwidths such that the dispersion experienced by a pulse is minimised. It is also important that ultrafast mirrors have as high a reflectivity as possible to reduce losses. 

Manx Precision Optics supplies a specialist line of ultrafast mirrors, which can provide high damage thresholds, minimal dispersion over a broad wavelength range, and are extremely reflective to reduce further losses. 

The company has four lines of ultrafast mirrors referred to as TTS, TTB, TTW and TTMH (standing for tuneable Ti:sapphire, standard, broad, wide and metal hybrid). The first three types of mirror are composed of layers of dielectric material. The TTMH ultrafast mirrors include a thick metal layer, as well as the additional dielectric layers.

When working with ultrafast laser systems, it is essential to choose the right optical components to provide optimal performance. Understanding how an ultrafast mirror will affect your system is an important part of making the right choice.

Dr Helmut Kessler, managing director at Manx Precision Optics, explained: ‘When you go to the femtosecond regime, metal mirrors are far better at achieving a wide reflectivity bandwidth and very low GDD, compared to using dielectric layers alone.’

‘But dielectric mirrors tend to have a higher LIDT than metal mirrors making them more suitable for high power applications.’

A MPO whitepaper provides a brief overview of LIDT in the ultrashort pulse regime, the detrimental impact of dispersion in this regime and explores the design and capabilities of ultrafast mirrors.

Manx Precision Optics continues to innovate in this space, as McGuigan concluded: ‘Ultrafast mirrors are an essential technology for optical systems in research or industry.’

‘In the near-future ultra-fast optical systems will operate over larger and larger bandwidths as pulse durations shrink and ultra-fast mirrors will have to be developed to meet that specification. These mirrors will also require even higher LIDT as peak intensities continue to grow and high rep-rate lasers become more common.’ 

Related article:

Read more about laser induced damage threshold (LIDT) in the ultrafast regime in the following whitepaper: www.electrooptics.com/premium-access/135/ultrafast-mirrors-femtosecond-pulse-duration-laser-systems

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