I am reading a review paper by D. S. Wiersma recently.1 In this paper, he mentioned that the spatial profile of the modes of random lasers is dominated by a speckle pattern with gradually varying envelope. But I have read several articles in which researchers directly claimed that random lasers are speckle-free.2, 3 So are all random lasers speckle-free?
1Wiersma, D. The physics and applications of random lasers. Nature Phys 4, 359–367 (2008).
https://doi.org/10.1038/nphys971
2Redding, B., Choma, M. & Cao, H. Speckle-free laser imaging using random laser illumination. Nature Photon 6, 355–359 (2012).
https://doi.org/10.1038/nphoton.2012.90
3Brandon Redding, Hui Cao, and Michael A. Choma, "Speckle-Free Laser Imaging with Random Laser Illumination," Optics & Photonics News 23(12), 30-30 (2012)
Its a question of scale. Random lasers tend to have very little speckle. However, it is still a coherent sum. At some small enough size scale there will still be modulation from diffraction. There is a transverse coherence length. The more disordered the gain process, the smaller the transverse coherence length, the smaller the scale you have to look to see speckle. If the coherence length is small enough that it doesn't show up in your application (say the size of a pixel in an imager) then you can say it is speckle free. However, if you somehow looked closer, you would still find speckle.
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