Lasing in disordered media presents both theoretical challenges and practical opportunities.

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The physics and applications of random lasers

I will overview our recent results on ultra-long lasers and will discuss the concept of a fiber laser with an open cavity that operates using random distributed feedback provided by Rayleigh scattering amplified through the Raman effect.

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Random distributed feedback fiber laser

A random laser is a non-conventional laser whose feedback mechanism is based on disorder-induced light scattering. Depending on whether the feedback supplied by scattering is intensity feedback or amplitude feedback, random lasers are classified into two categories: random lasers with incoherent feedback and random lasers with coherent feedback. A brief survey of random lasers with incoherent feedback is presented. It is followed by a review of our recent experimental work on random lasers with coherent feedback, including measurement of the lasing threshold, lasing spectra, emission pattern, dynamical response, photon statistics, speckle pattern and the investigation of relevant length scales. Large disorder leads to spatial confinement of the lasing modes, that is the foundation for the micro random laser. Some theoretical models of random lasers with coherent feedback are briefly introduced. The study of random lasers improves our understanding of the interplay between light localization and c...

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Lasing in random media

A random collection of scatterers in a gain medium can produce coherent laser emission lines dubbed “random lasing.” We show that biological tissues, including human tissues, can support coherent random lasing when infiltrated with a concentrated laser dye solution. To extract a typical random resonator size within the tissue we average the power Fourier transform of random laser spectra collected from many excitation locations in the tissue; we verified this procedure by a computer simulation. Surprisingly, we found that malignant tissues show many more laser lines compared to healthy tissues taken from the same organ. Consequently, the obtained typical random resonator was found to be different for healthy and cancerous tissues, and this may lead to a technique for separating malignant from healthy tissues for diagnostic imaging.

Random lasing in human tissues

In this paper we present calculations on light diffusion with amplification that can explain previous experiments on the spontaneous emission from such a medium. Also we discuss the experimental considerations on realizing a medium that both multiply scatters and amplifies light. In an amplifying random medium different processes can occur. We argue that one can distinguish three regimes depending on the amount of scattering, and discuss these regimes in the context of random laser action. @S1063-651X~96!05109-4#

/pdf/light-diffusion-with-gain-and-random-lasers-3c68c7hu2g.pdf

Light diffusion with gain and random lasers

We have developed mirrorless quasi-monochromatic laser sources made of stoichiometric neodymium compounds (Nd0.75:La0.25P4O15 and NdCl3· 6H2O) pumped by nanosecond laser pulses. We find that short subnanosecond and narrow-bandwidth (0.15-nm) pulses are generated in both compounds when they are pumped at high intensities. This emission is spatially incoherent as shown by a speckle statistics analysis. Its origin is discussed in terms of collective effects. This shows that poor optical materials with strongly quenched emission may be useful for generating incoherent short pulses.

F. Auzel

Papers

Generation of spatially incoherent short pulses in laser-pumped neodymium stoichiometric crystals and powders