A femtosecond laser is a type of laser whose pulse width in the time domain is on the order of a femtosecond (1 fs = 10^-15 seconds). Therefore, femtosecond lasers are categorized under ultrafast lasers (or ultra-short pulse lasers). Unlike monochromatic light, femtosecond laser light is a combination of wavelengths that vary continuously around a central wavelength. By utilizing the spatial coherence of this continuous range of wavelengths, the light pulses are significantly compressed in time, allowing for the generation of pulses at the femtosecond level.