Laser Mode Locking in diode pumped solid state lasers
Laser Mode Locking is a regime of operation of laser oscillators where lasers generate so called ultrashort pulses. Depending on the active medium, method of locking the modes, duration of the laser pulses can vary form few picoseconds to few femtoseconds (10-12 – 10 -15 [s]). Usually, the energy in a single pulse is of the order of (10-12 – 10 -15 [J]) that is insufficient for most of the applications.
Goals of the project
- Characterization of new laser materials – capabilities of newly emerging laser materials for ultrashort pulse generation are being studied by different techniques for Mode Locking.
- Development of new techniques for laser Mode Locking or using standard techniques in new spectral regions.
- Development of a system appropriate for some industrial and scientific applications, that is based only on laser osculator without additional amplifier.
Basic idea here is to use diode pumped Nd+3 dopped laser materials, to generate pulses with high peak power and excellent beam quality.
The time structure of the output beam will be a train of picosecond pulses with duration less than 10 ps, grouped in macro pulse with controllable duration and amplitude profile. The system will combine the advantages of both lase regimes of operation: high energy in a single macro pulse typical for the systems working in Q-switching and short pulse duration of micropulse typical for the systems operating in mode locking regime.
Results
Work on 1 and 2 is constant and the results can be seen in the section with scientific publication or on my profile in Reserch Gate.
Most important results up to now are reported at different scientific forums in Bulgaria as well as at international level. Summarized results are published in few full text papers in international journals.
Work on 3 is in progress at the moment with some promising preliminary results. Major effort are focused on the pulsed pumping and the system for negative feedback control. A train of microparsecs grouped in a macro pulse with tunable duration down to 100 [µs] was already achieved. The length of picosecond micropulses was measureto be of the order 10 [ps] with an energy of a single pulse 500 [nJ]. This is two orders of magnitude higher than the energy achieved in typical systems working in CW mode locking regime. Figure below sows an example of the train of pulse (left) with an inset showing small time interval of the macro pulse containing only few micro pulses. Measured autocorrelation and its corresponding autocorrelation trace is shown on the right.
Described system shows excellent spatial characteristics of the output beam with low divergence and ellipticity. Figure below shows 3D plot of the laser beam taken with a CCD camera (left) and one cross section on the right. Black dots correspond to experimentally measured data, while the red curve show M2 fit of the data.
Another possibility, that we are exploring at the moment is an oscillator working in mixed Q-switching Mode Locking regime. This regime is typically considered as dangerous and unwanted because it is hard to control and can lead to irreversable damage of optical components.
The preliminary results here are summarized below. We design and test an oscillator wit CW pumping mode locked by second harmonic generation. This is a technique showing capabilities to work in systems with high average power and wide spectral range. For more information you can visit the section with my scientific publications as well as my profile in Reserch Gate, where you can find full text of some of the publications.
Schematic of the laser oscillator is shown on the figure above, while the figure blow shows areal picture of the oscilator working in CW mode locking regime. Greene light is coming from the crystal for second harmonic generation that we use for laser Mode Locking. Additionally, when we use second harmonic generation for laser mode locking, a train of picosecond pulses at second harmonic are achieved colinear with the fundamental.
The same experimental setup ca be used in mixed operation mode: Q-switching plus mode locking. By optimization of laser cavity and pace matching conditions stable, mixed mode of operation was already achieved with average output power of > 2 W. Figure below shows the signal at the output of fast PIN photodiode seen on standard digital oscilloscope.
Future work includes optimization of this regime for controllable pulse length and repetition rate of the macro pulses. This will allow the energy of the single micropulse to exceed substantially the energy of the pulse in the case of CW mode locking.