Wavefront shaping to improve beam quality: converting a speckle pattern into a Gaussian spot

Abstract

A perfectly collimated beam can be spread out by multiple scattering, creating a speckle pattern and increasing the étendue of the system. Standard optical systems conserve étendue, and thus are unable to reverse the process by transforming a speckle pattern into a collimated beam or, equivalently, into a sharp focus. Wavefront shaping is a technique that is able to manipulate the amplitude and/or phase of a light beam, thus controlling its propagation through such media. Wavefront shaping can thus break the conservation of étendue and, in principle, reduce it. In this work we study how much of the energy contained in a fully developed speckle pattern can be converted into a high quality (low $M^2$) beam, provide with a theoretical framework and discuss the advantages and limitations of this approach, with special attention given to the inherent variability in the quality of the output due to the multiple scattering.

Effect of core geometry on the channel capacity of a multimode optical fiber

“…Therefore, the overall wavefront shaping fidelity is the product of the fidelities from each source of imperfection. In our experiment, the dominant contributions were phase-only modulation |γ ph | 2 ≈ π/4 [23], discrete phase modulation in Lee holography |γ Lee | 2 ≈ 0.98 [28], and linearly-polarized light illumination at the input |γ a | 2 = 0.5 [29]. Multiplying these individual contributions, we find the overall wavefront shaping fidelity to be |γ| 2 ≈ 0.38, resulting in an estimated focusing enhancement |γ| 2 N seg ≈ 400.…”

Effect of core geometry on the channel capacity of a multimode optical fiber

“…Therefore, the overall wavefront shaping fidelity is the product of the fidelities from each source of imperfection. In our experiment, the dominant contributions were phase-only modulation |γ ph | 2 ≈ π/4 [23], discrete phase modulation in Lee holography |γ Lee | 2 ≈ 0.98 [28], and linearly-polarized light illumination at the input |γ a | 2 = 0.5 [29]. Multiplying these individual contributions, we find the overall wavefront shaping fidelity to be |γ| 2 ≈ 0.38, resulting in an estimated focusing enhancement |γ| 2 N seg ≈ 400.…”

Nonlinear harmonic wave manipulation in nonlinear scattering medium via scattering-matrix method

“…However, we observe that the obtained PBR is lower than that in the linear case for the same input mode. Apart from facts of wavefront distortion due to dead zones of SLM, noise in the measurement, phase-only modulation, discrete phase modulation, and nonuniform illumination, 32 the most significant reasons for this result are as follows: (i) the presence of the reference field increases the background; (ii) the nonlinear conversion efficiency of the nonlinear scattering medium is relatively low, and the nonlinear signal intensity generated by different incident modes may not be uniform, inevitably introducing measurement errors of the SM. It is worth mentioning that the measurement method is not only applicable to backscattering configurations but also to transmission configuration.…”