AM and high-harmonic FM laser mode locking

Abstract

We demonstrate a new technique of active mode locking that combines amplitude-modulated (AM) mode locking at the cavity fundamental repetition rate with frequency-modulated (FM) mode locking at a high harmonic. This method combines the advantages of pulse shortening by high-harmonic mode locking while preserving the higher peak powers available at the fundamental repetition rate. We demonstrate this technique using a Nd:YAG laser that is simultaneously AM mode locked at 80 MHz and FM mode locked at the 22nd harmonic (1.76 GHz). Pulses as short as 16 ps with a peak power of 6.25 kW were measured.

Theory of combined AM and high-harmonic FM mode locked laser

“…The product varies between 0.44 and 0.626 which correspond to the pure AM and FM cases, respectively. The predicted values of the pulsewidth and pulsewidth-bandwidth products match very well with the recently reported experimental values for the Nd:YAG laser system [11].…”

“…The frequency of modulation fern for the FM modulator is chosen such that it is less than (Wam/2A0) where A0 is the phase angle satisfying the size of the window in radians during which the net gain exceeds the loss inside the laser cavity. This ensures that only one mode locked pulse exists per round trip inside the laser cavity and for the modulation frequencies above this value multiple pulsing occur leading to the unstable operation of the laser [11].…”

“…Hence many efforts have been made to enhance pulse compression efficiency of this technique while retaining its high stability [7][8][9]. Based on earlier proposals [10] recently a new approach to overcome the inherent limitations of the active mode locking has been reported [11]. It is shown that by combining the AM mode locking at the cavity fundamental repetition rate with FM mode locking at a high-harmonic, better pulse compression (characterised by high-harmonic mode locking) with higher peak powers (characterised by the mode locking at fundamental repetition rate) can be simultaneously achieved in a laser system.…”

Variable Repetition Rate Picosecond Master Oscillator for Photoelectron Gun

“…Much higher energy pulses needed for photoelectron gun operation (10-100 nJ at the wavelength of 1 µm) can be achieved using solid-state bulk lasers pumped by an array of laser diodes [20]. A frequently occurring method of achieving HML in such types of lasers is the implementation of electro-optic modulation [21][22][23]. Electro-optic phase modulation based on LiNbO 3 crystal was successfully used in Nd:YAG laser to obtain quite short picosecond pulses at the fundamental frequency of 235 MHz [24].…”

Phase-Modulation-Mode-Locked Continuous-Wave MgO:PPLN Optical Parametric Oscillator

“…As known, the active mode-locking technique can be achieved using amplitude or frequency modulation. Based on acousto-optic or electro-optic elements, mode-locking has been demonstrated in various earlier reports on lasers [14]- [17].…”