Abstract
We demonstrate, for the first time, optical sampling by repetition-rate tuning (OSBERT) at record megahertz scan rates. A low-cost, tunable and extremely compact 2-section passively mode-locked laser diode (MLLD) is used as the pulsed laser source, whose repetition rate can be modulated electronically through biasing of the saturable absorber section. The pulsed output is split into two arms comparable to an imbalanced Michelson interferometer, where one arm is significantly longer than the other (a passive delay line, or PDL). The resulting electronic detuning of the repetition rate gives rise to a temporal delay between pulse pairs at a detector; the basis for time-resolved spectroscopy. Through impedance-matching, we developed a new system whereby a sinusoidal electrical bias could be applied to the absorber section of the MLLD via a signal generator, whose frequency could be instantly increased from sub-hertz through to megahertz modulation frequencies, corresponding to a ground-breaking megahertz optical sampling scan rate, which was experimentally demonstrated by the real-time acquisition of a cross-correlation trace of two ultrashort optical pulses within just 1 microsecond of real time. This represents scan rates which are three orders of magnitude greater than the recorded demonstrations of OSBERT to date, and paves the way for highly competitive scan rates across the field of time-resolved spectroscopy and applications therein which range from pump probe spectroscopy to metrology.
Highlights
We demonstrate, for the first time, optical sampling by repetition-rate tuning (OSBERT) at record megahertz scan rates
The crucial difference between OSBERT and these techniques is that we take advantage of entirely electronically driven scanning, which is enabled by applying an electronic bias directly to the metal contacts of our unique choice of laser: the 2-section quantum dot semiconductor mode-locked laser diode (MLLD)
For the first time, optical sampling was conducted over just 1 μs using the OSBERT technique, where the cross-correlation of picosecond pulses was acquired at a scan rate of 1 MHz
Summary
For the first time, optical sampling by repetition-rate tuning (OSBERT) at record megahertz scan rates. The crucial difference between OSBERT and these techniques is that we take advantage of entirely electronically driven scanning, which is enabled by applying an electronic bias directly to the metal contacts of our unique choice of laser: the 2-section quantum dot semiconductor mode-locked laser diode (MLLD) Since these are diodes, they are entirely electronically pumped, meaning there is no requirement for optical pumping by a second laser which is commonplace for the solid state and fibre-laser alternatives. Passive mode-locking, which leads to the generation of ultrashort pulses, may be achieved by forward biasing a gain section which is electrically isolated from a reverse-biased saturable absorber section, see Fig. 1
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