Complex lasers with controllable coherence

Abstract

The invention of lasers 60 years ago is one of the greatest breakthroughs in modern optics. Throughout the years, lasers have enabled major scientific and technological advancements, and have been exploited in numerous applications due to their advantages such as high brightness and high coherence. However, the high spatial coherence of laser illumination is not always desirable, as it can cause adverse artifacts such as speckle noise in imaging applications. To reduce the spatial coherence of a laser, novel cavity geometries and alternative feedback mechanisms have been developed. By tailoring the spatial and spectral properties of cavity resonances, the number of lasing modes, the emission profiles and the coherence properties can be controlled. This technical review presents an overview of such unconventional, complex lasers, with a focus on their spatial coherence properties. Laser coherence control not only provides an efficient means for eliminating coherent artifacts, but also enables new applications.

Water-resistant perovskite nanodots enable robust two-photon lasing in aqueous environment

“…Different from the WGM laser made by incorporating the wr-PNDs in the microcavity as demonstrated above, the powder sample itself can work as a random laser without the need of optical coupling with an external cavity resonator 38 . In general, random lasing does not require a well-confined laser cavity but relies on multiple light scattering in a disordered scattering medium [39][40][41] . As a result, random lasers have lower coherence than the lasers based on optical cavities, thus ensuring their application in speckle-free imaging and bioimaging 42 .…”

Determining random lasing action

“…Clear assessment of random lasing action, as given by the evidence of g (2) reducing to unity, has, to date, been very challenging and expensive, requiring picosecond or femtosecond lasers and singlephoton detectors. However, assessing random lasing -and similarly other kinds of complex lasers -is essential for the progress of the field, which is already reaching technological applications, ranging from lowcoherence imaging 56 to superresolution spectroscopy (A. Boschetti, A. Taschin, P. Bartolini, A. K. Tiwari, L. Pattelli, R. Torre and D. Wiersma, personal communication) to sensing 10 and many others 6 .…”

Fast laser speckle suppression with an intracavity diffuser

“…It can be achieved by using multimode lasers with low and tunable spatial coherence [13][14][15][16][17][18][19]. The decoherence time of such lasers, critical for fast speckle suppression in short integration times, is determined by the frequency spacing and linewidth of the individual lasing modes, as well as the total width of the emission spectrum ΔΩ [20]. Let us consider N transverse modes lasing simultaneously and assume that the linewidth of each individual transverse mode is smaller than the typical frequency spacing Δω t of neighboring modes.…”