Observation of laser pulse propagation in optical fibers with a SPAD camera

Abstract

Recording processes and events that occur on sub-nanosecond timescales poses a difficult challenge. Conventional ultrafast imaging techniques often rely on long data collection times, which can be due to limited device sensitivity and/or the requirement of scanning the detection system to form an image. In this work, we use a single-photon avalanche detector array camera with pico-second timing accuracy to detect photons scattered by the cladding in optical fibers. We use this method to film supercontinuum generation and track a GHz pulse train in optical fibers. We also show how the limited spatial resolution of the array can be improved with computational imaging. The single-photon sensitivity of the camera and the absence of scanning the detection system results in short total acquisition times, as low as a few seconds depending on light levels. Our results allow us to calculate the group index of different wavelength bands within the supercontinuum generation process. This technology can be applied to a range of applications, e.g., the characterization of ultrafast processes, time-resolved fluorescence imaging, three-dimensional depth imaging, and tracking hidden objects around a corner.One of the first slow motion events captured on film was a galloping horse. "Sallie Gardner at a Gallop" is a series of 24 photographs taken in rapid succession to analyse the gait of a horse 1 . It demonstrated that all four feet were simultaneously off the ground during the gallop. Ever since, there has been a fascination with slow-motion video, and by the 1930s, speeds of 1000 frames per second (fps) were achievable on 16 mm film with a built-in shutter/ correction plate 2 . By the early 1960s, a 10,000 fps rotating prism camera was demonstrated 3 . Technological developments have led to cameras capable of realtime monochromatic filming at several million fps 4 and the observation of femtosecond pulses of light using a streak camera combined with a stroboscopic approach 5 . Most recently, compressed single-shot photography at 100 billion frames per second was reported using a streak camera 6 without relying on stroboscopic illumination. The main limitations of this work were the limit of 350 frames per acquisition (3.5 ns) and the need for computational reconstruction techniques to realise a final video.In photon-starved applications, single-photon detection and, more specifically, time-correlated single-photon counting (TCSPC), offers extreme sensitivity and picosecond timing resolution 7 . For many applications, single-photon avalanche diodes (SPADs) are the favoured choice of detector due to their relatively compact nature, ease of integration, high efficiency and low noise characteristics, combining to give ultra-high sensitivity devices. It is common to use silicon SPADs for visible wavelengths 8 and InGaAs/InP SPADs for NIR and telecommunications wavelengths 9 . When coupled to an optical system, usually with optical fiber, SPADs offer single-point detection. While useful in many applications, such as time-resolved p...

Ballistic and snake photon imaging for locating optical endomicroscopy fibres

“…2(c)) to aid understanding of image reconstruction and later results. A pulse of 785 nm light travelling along an optical fibre undergoes scattering, a limited number of the scattered photons are detected by the camera and can be used to observe the passage of the pulse along the fibre length [20]. A series of images were reconstructed such that the time axis of the data becomes the time axis of a video with photon count intensity as the z-axis.…”

“…A variety of related low photon number imaging techniques use advanced computation to improve image quality [20,[23][24][25]. Expected temporal correlations between photon arrivals at neighbouring image pixels can be exploited to improve image quality [23] or overcome detector array timing limitations [24].…”

A trillion frames per second: the techniques and applications of light-in-flight photography

“…In Ref. [100] a few meters of curled photonic crystal fibre were imaged with a SPAD camera. A series of measurements were performed, each time with a different interference filter placed in front of the camera so as to observe the LiF of different spectral components in the pulse.…”