Pulse shaping is a technique used to control the temporal shape and duration of light pulses by removing or changing the phase of certain frequencies of light. Pulse shaping has been successfully applied to ultrashort pulses, pulses of broadband light with a duration of femtoseconds to a few tens of picoseconds, to pre-compensate for optical dispersion and increase the efficiency of multiphoton imaging.
The time domain and frequency domain of a light pulse are related by a Fourier transform. Pulse shaping is typically applied in the frequency domain as it is more practical than in the time domain. Typically, the pulse is shaped in the following way:
- The frequencies of the pulse are angularly dispersed by a diffraction grating.
- The dispersed light is collimated with a lens placed at one focal length from the grating.
- The lens spatially maps and focusses each frequency onto the Fourier plane.
- The pulse shape is modified by modulating the amplitude and/or phase of each frequency in the Fourier plane by means of a two-dimensional spatial light modulator (SLM).
- The spectrum is collected by a second lens and then gathered by a second diffraction grating. The frequencies interfere to create a shaped pulse.
The spatial light modulator at the Fourier plane can block certain frequencies entirely, preventing them from combining with the rest of the spectrum. Alternatively, the spatial light modulator adjusts the spectral phase of certain frequencies, creating an optical delay. Both of these operations are used to change the temporal shape of the pulse.
An ultrashort pulse comprises many frequencies of light which sum coherently to create a temporal pulse shape with a time duration of femtoseconds to a few tens of picoseconds.
The contrast in multiphoton images arises from nonlinear optical processes such as two-photon absorption fluorescence (TPAF), second harmonic generation (SHG), or third harmonic generation (THG). In each process, the imaging efficiency depends on the duration of the excitation pulse. As a result, pulse shaping can be used to optimise the duration of ultrashort pulses and improve imaging efficiency.
Coherent anti-Stokes Raman scattering (CARS)
Coherent anti-Stokes Raman scattering (CARS) is used to enhance weak Raman signals. Such Raman signals can be split into resonant and non-resonant signals. The resonant Raman signal provides the unique spectral fingerprint of a sample. Pulse shaping is applied in CARS to maximise the resonant Raman signal and/or minimise the non-resonant Raman signal.