Fast optical sampling by electronic repetition-rate tuning using a single mode-locked laser diode

Abstract

This paper demonstrates optical sampling by electronic repetition-rate tuning (OSBERT): a single-laser optical sampling technique capable of fast scan rates and customisable scan ranges. The method has no moving parts and is based on the electronic modulation of the repetition rate of a passively mode-locked laser diode, simply by varying the reverse bias applied directly to the saturable absorber section of the laser. Varying the repetition rate in a system built as a highly imbalanced interferometer results in pairs of (pump, probe) pulses with successive increasing delay. The resulting scan range is proportional to the magnitude of the repetition rate modulation and is scaled by the chosen length of the imbalance. As a first proof of concept, we apply the method to distance measurement, where the displacement of a target across 13.0 mm was detected with ∼0.1 mm standard deviation from an equivalent free-space distance of 36 m and at a real-time scan rate of 1 kHz. The customizable scan range and competitive scan rate of the method paves the way for single ultrafast semiconductor laser diodes to be deployed as fast, low-cost, and compact optical sampling systems in metrology, biomedical microscopy, and sensing applications.

Megahertz scan rates enabled by optical sampling by repetition-rate tuning

“…It is well understood from the literature and our previous reports 15 , 16 that small variations in the driving conditions (temperature, forward bias and reverse bias) allow for the tuning of the laser output, notably the repetition rate 17 – 19 . Varying the biasing levels of the far shorter absorber section in order to tune the repetition rate is a principle OSBERT takes advantage of in order to deliver optical sampling, which is similar to that of OSCAT, but without the need for any moving parts whatsoever.…”

“…The experimental setup is shown in Fig. 2 , and is based on that found in our previous work 16 , including the same MLLD whose fabrication and characteristics may be found there. The forward current supplied to the gain section of the MLLD was held constant throughout all experiments at 210 mA, whilst the entire device temperature was held constant at 20 °C.…”

“…The saturable absorber bias modulation which gives rise to repetition rate tunability is achieved using a signal generator ( Keysight Technologies 33612A Waveform Generator, 80 MHz ), a copy of which is used to trigger the oscilloscope. A sinusoidal signal drives the continuous repetition rate tuning of the MLLD by setting a voltage offset ( V off = − 6.80 V) and voltage amplitude ( V pp = 1.20 V), which were previously found to give rise to ~ 3.4 MHz repetition rate tunability 16 . Using expression 1 , this gives a scan range of ~ 16.3 picoseconds, which was more than sufficient to fully capture the duration of a cross-correlation trace of the two pulse copies interfering at the detector.…”

“…As such, a δ-step increment in , which is achieved by a δ-step increment in the voltage supplied to the absorber section, in this configure gives a δ-step increment within the scan range. Our previous work 16 showcased the first acquisition of optical pulse cross-correlation traces at 1 kHz scan rates using the OSBERT technique. This was then utilised in a metrology application as a distance measurement, where target’s motion across 13.0 mm was detected with ~ 0.1 mm standard deviation from an equivalent free-space distance of 36 m.…”

Frequency Response Enhancement of Photonic Sampling Based on Cavity-Less Ultra-Short Optical Pulse Source

“…The parameters of the ultrashort optical pulse train are of great importance to the sampling performance such as sampling rate, operation bandwidth and accuracy. So far, many efforts have been made to employ various optical pulse sources to achieve high-performance sampling [11]- [19]. Thereinto, passively mode-locked lasers (PMLLs) are characterized by extremely low time jitter down to several femtoseconds and narrow pulse width below 1 ps, which are favorable for improving the sampling accuracy and enhancing the available operation bandwidth.…”