Basics of ultrafast lasers
Unique features of ultrafast lasers
Thanks to their ultrashort pulse durations, ultrafast lasers possess key features that distinguish them from longer pulse or continuous-wave (CW) lasers. Generating such a short pulse requires a broad spectral bandwidth. The minimum bandwidth to generate a pulse of a particular duration depends on the pulse shape and the central wavelength. In general, this relationship is described by the time-bandwidth product (TBP), which arises from the uncertainty principle. For a Gaussian pulse, the TBP is given by:
where Δτ is the temporal duration of the pulse and Δν is the frequency bandwidth.4 Fundamentally, this equation says there must be a reciprocal relationship between spectral bandwidth and pulse duration—as the pulse gets shorter, the bandwidth required to generate it gets wider. Figure 1 demonstrates the minimum bandwidth required to support a variety of pulse durations.
Beyond broad spectral bandwidths, incredibly high peak powers are another consequence of ultrashort pulse durations. For context, let’s examine the difference in the peak power output of a 10 W CW laser vs. that of a 10 W ultrafast laser with 150 fs pulses and a repetition rate of 80 MHz—properties common for many commercially available ultrafast laser sources.
Selecting ultrafast optical components
The technical challenges associated with ultrafast lasers can create difficulties when selecting the proper optical components for your application. The required spectral coverage, laser damage threshold (LDT), and dispersion specifications may be foreign to users accustomed to continuous-wave (CW) or longer-duration pulsed lasers.
While the coating you select for your optical component may reflect or transmit the entire bandwidth of your ultrafast laser pulse, it does not mean the optic is designed to withstand the enormous peak power associated with these short pulses. Because long-pulse or CW lasers have dominated the commercial laser market for so long, it is common to see LDT ratings determined with nanosecond or CW laser sources. Unfortunately, it is impossible to scale these LDT ratings down to the ultrafast regime because the mechanisms for damage are entirely different across these orders of magnitude in pulse duration (see Fig. 2).
As ultrafast lasers are integrated into a wider variety of application spaces, more users are facing the technical challenges posed by their broad spectral bandwidths, ultrahigh peak powers, and ultrashort pulse durations. No matter the application, ultrafast laser users can follow these practical guidelines to deliver their ultrafast laser pulse to the target material without decreasing their spectral coverage, distorting their temporal profile, or damaging optical components along the way.
Read more: https://www.laserfocusworld.com/lasers-sources/article/14291054/basics-of-ultrafast-lasers-part-1
https://www.laserfocusworld.com/lasers-sources/article/14292432/basics-of-ultrafast-lasers-part-2
