arXiv subjects

Hui Cao

Publications and source records attributed to Hui Cao.

At least 109 records · Page 6Linked to original sources

arXiv2016

Principal modes in multimode fibers: exploring the crossover from weak to strong mode coupling

We present experimental and numerical studies on principal modes in a multimode fiber with mode coupling. By applying external stress to the fiber and gradually adjusting the stress, we have realized a transition from weak to strong mode coupling, which corresponds to the transition from single scattering to multiple scattering in mode space. Our experiments show that principal modes have distinct spatial and spectral characteristic in the weak and strong mode coupling regimes. We also investigate the bandwidth of the principal modes, in particular, the dependence of the bandwidth on the delay time, and the effects of the mode-dependent loss. By analyzing the path-length distributions, we discover two distinct mechanisms that are responsible for the bandwidth of principal modes in weak and strong mode coupling regimes. Taking into account the mode-dependent loss in the fiber, our numerical results are in good agreement with our experimental observations. Our study paves the way for exploring potential applications of principal modes in communication, imaging and spectroscopy.

physics.optics↗

arXiv2016

Controlling mode competition by tailoring the spatial pump distribution in a laser: A resonance-based approach

We introduce a simplified version of the steady-state ab initio laser theory for calculating the effects of mode competition in continuous wave lasers using the passive cavity resonances. This new theory harnesses widely available numerical methods that can efficiently calculate the passive cavity resonances, with negligible additional computational overhead. Using this theory, we demonstrate that the pump profile of the laser cavity can be optimized both for highly multi-mode and single-mode emission. An open source implementation of this method has been made available.

physics.optics↗

arXiv2016

Condensation of Thresholds in Multimode Microlasers

We show from ab initio laser theory that by choosing an appropriate spatial pump profile, many different spatial modes of a typical microlaser can be turned on at the same pump energy, substantially increasing the number, N, of simultaneous lasing modes. The optimal pump profile can be obtained simply from knowledge of the space-dependent saturated gain profile when the system is uniformly pumped up to the Nth modal threshold. We test this general result by applying it to a two-dimensional diffusive random laser and a microdisk laser. Achieving highly multimode lasing at reasonable pump powers is useful for reducing the spatial coherence of laser sources, making them suitable for use in speckle-free imaging and other applications.

physics.optics↗

arXiv2016

Evanescently coupled multimode spiral spectrometer

We designed a high-resolution compact spectrometer based on an evanescently-coupled multimode spiral waveguide. Interference between the modes in the waveguide forms a wavelength-dependent speckle pattern which is used as a fingerprint to identify the input wavelength after calibration. Evanescent coupling between neighboring arms of the spiral results in a non-resonant broad-band enhancement of the spectral resolution. Experimentally, we demonstrated a resolution of 0.01 nm at a wavelength of 1520 nm using a 250 μm radius spiral structure. Spectra containing 40 independent spectral channels are recovered simultaneously and the operation bandwidth is significantly increased by applying compressive sensing to sparse spectra reconstruction.

physics.ins-det↗

arXiv2016

Control of energy density inside disordered medium by coupling to open or closed channels

We demonstrate experimentally an efficient control of light intensity distribution inside a random scattering system. The adaptive wavefront shaping technique is applied to a silicon waveguide containing scattering nanostructures, and the on-chip coupling scheme enables access to all input spatial modes. By selectively coupling the incident light to open or closed channels of the disordered system, we not only vary the total energy stored inside the system by 7.4 times, but also change the energy density distribution from an exponential decay to a linear decay and to a profile peaked near the center. This work provides an on-chip platform for controlling light-matter interactions in turbid media.

physics.optics↗

arXiv2016

Broadband multimode fiber spectrometer

A general-purpose all-fiber spectrometer is demonstrated to overcome the trade-off between spectral resolution and bandwidth. By integrating a wavelength division multiplexer with five multimode optical fibers, we have achieved 100 nm bandwidth with 0.03 nm resolution at wavelength 1500 nm. An efficient algorithm is developed to reconstruct the spectrum from the speckle pattern produced by interference of guided modes in the multimode fibers. Such algorithm enables a rapid, accurate reconstruction of both sparse and dense spectra in the presence of noise.

physics.ins-det↗

arXiv2016

Coherent control of photocurrent in a strongly scattering photoelectrochemical system

A fundamental issue that limits the efficiency of many photoelectrochemical systems is that the photon absorption length is typically much longer than the electron diffusion length. Various photon management schemes have been developed to enhance light absorption; one simple approach is to use randomly scattering media to enable broadband and wide-angle enhancement. However, such systems are often opaque, making it difficult to probe photo-induced processes. Here we use wave interference effects to modify the spatial distribution of light inside a highly-scattering dye-sensitized solar cell to control photon absorption in a space-dependent manner. By shaping the incident wavefront of a laser beam, we enhance or suppress photocurrent by increasing or decreasing light concentration on the front side of the mesoporous photoanode where the collection efficiency of photoelectrons is maximal. Enhanced light absorption is achieved by reducing reflection through the open boundary of the photoanode via destructive interference, leading to a factor of two increase in photocurrent. This approach opens the door to probing and manipulating photoelectrochemical processes in specific regions inside nominally opaque media.

physics.optics↗

arXiv2016

Interaction-induced mode switching in steady-state microlasers

We demonstrate that due to strong modal interactions through cross-gain saturation, the onset of a new lasing mode can switch off an existing mode via a negative power slope. In this process of interaction-induced mode switching (IMS) the two involved modes maintain their identities, i.e. they do not change their spatial field patterns or lasing frequencies. For a fixed pump profile, a simple analytic criterion for the occurrence of IMS is given in terms of their self- and cross-interaction coefficients and non-interacting thresholds, which is verified for the example of a two-dimensional microdisk laser. When the spatial pump profile is varied as the pump power is increased, IMS can be induced even when it would not occur with a fixed pump profile, as we show for two coupled laser cavities. Our findings apply to steady-state lasing and are hence different from dynamical mode switching or hopping. IMS may have potential applications in robust and flexible all-optical switching.

physics.optics↗

arXiv2016

Spatio-temporal Control of Light Transmission through a Multimode Fiber with Strong Mode Coupling

We experimentally generate and characterize the eigenstates of the Wigner-Smith time-delay matrix, called principal modes, in a multimode fiber with strong mode coupling. The unique spectral and temporal properties of principal modes enable a global control of the temporal dynamics of optical pulses transmitted through the fiber, despite random mode mixing. Our analysis reveals that the well-defined delay time of the eigenstates are formed by multi-path interference, which can be effectively manipulated by the spatial degrees of freedom of the input wavefront. This study is essential to controlling the dynamics of wave scattering, paving the way for coherent control of pulse propagation through complex media.

physics.optics↗

arXiv2015

Coherence switching of a degenerate VECSEL for multimodality imaging

We demonstrate a VECSEL (vertical external cavity surface emitting laser) based degenerate source with an adjustable degree of spatial coherence that is electrically pumped, mechanically compact and supports continuous-wave emission. The laser operation can be switched between a large number of mutually incoherent spatial modes and few-mode operation at little power loss. This technology allows multimodality imaging, where low spatial coherence illumination is used for traditional high-speed video-microscopy and high spatial coherence illumination is used to extract dynamic information of flow processes. The initial demonstration is performed on imaging embryo heart function in Xenopus, which is an important animal model for human heart disease.

physics.optics↗

arXiv2015

Control of mesoscopic transport by modifying transmission channels in opaque media

While controlling particle diffusion in a confined geometry is a popular approach taken by both natural and artificial systems, it has not been widely adopted for controlling light transport in random media, where wave interference effects play a critical role. The transmission eigenchannels determine not only light propagation through the disordered system but also the energy concentrated inside. Here we propose and demonstrate an effective approach to modify these channels, whose structures are considered to be universal in conventional diffusive waveguides. By adjusting the waveguide geometry, we are able to alter the spatial profiles of the transmission eigenchannels significantly and deterministically from the universal ones. In addition, propagating channels may be converted to evanescent channels or vice versa by tapering the waveguide cross-section. Our approach allows to control not only the transmitted and reflected light, but also the depth profile of energy density inside the scattering system. In particular geometries perfect reflection channels are created, and their large penetration depth into the turbid medium as well as the complete return of probe light to the input end would greatly benefit sensing and imaging applications. Absorption along with geometry can be further employed for tuning the decay length of energy flux inside the random system, which cannot be achieved in a common waveguide with uniform cross-section. Our approach relies solely on confined geometry and does not require any modification of intrinsic disorder, thus it is applicable to a variety of systems and also to other types of waves.

physics.optics↗

arXiv2015

Broadband Coherent Enhancement of Transmission and Absorption in Disordered Media

We study the optimal diffusive transmission and absorption of broadband or polychromatic light in a disordered medium. By introducing matrices describing broadband transmission and reflection, we formulate an extremal eigenvalue problem where the optimal input wavefront is given by the corresponding eigenvector. We show analytically that a single wavefront can exhibit strongly enhanced total transmission or total absorption across a bandwidth that is orders of magnitude broader than the spectral correlation width of the medium, due to long-range correlations in coherent diffusion. We find excellent agreement between the analytic theory and numerical simulations.

physics.optics↗

arXiv2015

Fluctuations and Correlations of Emission from Random Lasers

When light travels through strongly scattering media with optical gain, the synergy between diffusive transport and stimulated emission can lead to lasing action. Below the threshold pump power, the emission spectrum is smooth and consistent from shot-to-shot. Above the lasing threshold, the spectrum of emitted light becomes spiky and shows strong fluctuations from shot-to-shot. Recent experiments have reported that emitted intensity resembles a power-law distribution (\emph{i.e.} Lévy statistics). Recent theories have described the emergence of Lévy statistics as an intrinsic property of lasing in random media. To separate intrinsic intensity fluctuations from the motion of scatterers, we compare the statistics of samples with stationary or freely-diffusing scatterers. Consistent with previous reports, we observe Lévy-like statistics when intensity data are pooled across an ensemble of scatterer configurations. For fixed scatterers, we find exponential intensity distributions whose mean intensities vary widely across wavelengths. Lévy-like statistics re-emerges when data are combined across many lasing modes. Additionally, we find strong correlations of lasing peak intensities across wavelengths. A simple mean-field statistical model captures the observed one- and two-point statistics, where correlations in emission intensity arise from competition among all lasing modes for limited gain.

physics.optics↗

arXiv2015

Minimum reflection channel in amplifying random media

We present a numerical study on the minimum reflection channel in a disordered waveguide and its modification by coherent amplification of light. The minimum reflection channel is formed by destructive interference of quasi-normal modes at the front surface of the random medium. While the lowest reflection eigenvalue increases with gain in most random realizations, the minimum reflection channel can adjust its modal composition to enhance the destructive interference and slow down the growth of reflectance with gain. Some of the random realizations display a further reduction of the minimum reflectance by adding optical gain. The differential amplification of the modes can make their destructive interference so effective that it dominates over the amplitude growth of the modes, causing the reflectance to drop with gain. Therefore, the interplay between interference and amplification makes it possible to further minimize light reflection from a strong scattering medium by introducing optical gain.

physics.optics↗

arXiv2015

Using geometry to manipulate long-range correlation of light inside disordered media

We demonstrate experimentally that long-range intensity correlation for light propagating inside random photonic waveguides can be modified by changing the shape of the waveguide. The functional form of spatial correlation is no longer universal in the regime of diffusive transport and becomes shape-dependent due to the non-local nature of wave propagation. The spatial dependence of the correlation may be asymmetric for light incident from opposite ends of the waveguide. This work opens the door to control non-local effects in mesoscopic transport of waves by manipulating the geometry of random systems.

physics.optics↗

arXiv2015

Low-spatial-coherence broadband fiber source for speckle free imaging

We designed and demonstrate a fiber-based amplified spontaneous emission (ASE) source with low spatial coherence, low temporal coherence, and high power per mode. ASE is produced by optically pumping a large gain core multimode fiber while minimizing optical feedback to avoid lasing. The fiber ASE source provides 270 mW of continuous wave emission, centered at λ=1055 nm with a full-width half-maximum bandwidth of 74 nm. The emission is distributed among as many as ~70 spatial modes, enabling efficient speckle suppression when combined with spectral compounding. Finally, we demonstrate speckle-free full field imaging using the fiber ASE source. The fiber ASE source provides a unique combination of high power per mode with both low spatial and low temporal coherence, making it an ideal source for full-field imaging and ranging applications.

physics.optics↗

arXiv2015

Rotating optical microcavities with broken chiral symmetry

We demonstrate in open microcavities with broken chiral symmetry, quasi-degenerate pairs of co-propagating modes in a non-rotating cavity evolve to counter-propagating modes with rotation. The emission patterns change dramatically by rotation, due to distinct output directions of CW and CCW waves. By tuning the degree of spatial chirality, we maximize the sensitivity of microcavity emission to rotation. The rotation-induced change of emission is orders of magnitude larger than the Sagnac effect, pointing to a promising direction for ultrasmall optical gyroscopes.

physics.optics↗

arXiv2015

Remote Key Establishment by Mode Mixing in Multimode Fibres and Optical Reciprocity

Disorder and scattering in photonic systems have long been considered a nuisance that should be circumvented. Recently, disorder has been harnessed for a rapidly growing number of applications, including imaging, sensing and spectroscopy. The chaotic dynamics and extreme sensitivity to external perturbations make random media particularly well-suited for optical cryptography. However, using random media for distribution of secret keys between remote users still remains challenging, since it requires the users have access to the same scattering system. Here we utilize random mode mixing in multimode fibres to generate and distribute keys simultaneously. Fast fluctuations in the fibre mode mixing provide the source of randomness for the key generation, and optical reciprocity guarantees that the keys at the two ends of the fibre are identical. We experimentally demonstrate the scheme using classical light and off-the-shelf components, opening the door for cost effective key establishment at the physical-layer of fibre-optic networks.

physics.optics↗