Ultra-Short Pulse Modulation with Electro-Optic Modulators

Multiphoton microscopy (MPM) is a promising technology for intravital imaging, providing deep tissue penetration, high 3D resolution and low photobleaching [1]. To realize MPM, it is crucial to make maximum use of the nonlinearity of the excitation probability by using high intensity laser illumination. Most often, this is achieved by using femtosecond pulses from a mode-locked laser, however, these pulses suffer from chromatic dispersion and unwanted nonlinearities. Recent research endeavours are exploiting picosecond pulses as pulse-on-demand alternatives [2], [3]. Here, we utilize a Mach-Zehnder-based intensity electro-optic modulator (EOM), which splits an optical beam in two partial beams and induces a phase modulation in one of the partial beams by means of an applied voltage [4]. Being waveguide-based, a small driving voltage of 5V (TTL levels) are required to achieve a $(V_{pi})$ full modulation between constructive and destructive interference. To achieve short picosecond pulses expensive electrical pulse generators are required to provide the short picosecond electrical pulses. In this work we report on a driving signal employing twice the $V_{pi}$ voltage to generate ultra short optical pulses. As shown in Fig. 1, using a voltage of $2V_{pi}$ causes the EOM to jump between two states of maximal suppression in the short time of the rising or falling edge (80/20-times of 35 ps shown) with a very short open state of the EOM in between.