Abstract

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. The high-speed EOM (ML-1550-40-PM-V-HER, Optilab) was driven with square wave signals from an arbitrary waveform generator (AWG7122B, Tektronix), generating optical pulses with a FWHM of 10.9 ps on both edges of the electrical signal. This means that the optical pulses are generated at much shorter pulse length than the electronic bandwidth of the driving signal by using the periodic interference condition as electro-optic gearbox.

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