Christophe Moser

Focusing and scanning light through a multimode optical fiber using digital phase conjugation

We demonstrate for the first time to our knowledge a digital phase conjugation technique for generating a sharp focus point at the end of a multimode optical fiber. A sharp focus with a contrast of 1800 is experimentally obtained at the tip of a 105 μm core multimode fiber. Scanning of the focal point is also demonstrated by digital means. Effects from illumination and fiber bending are addressed.

High-resolution, lensless endoscope based on digital scanning through a multimode optical fiber

We propose and experimentally demonstrate an ultra-thin rigid endoscope (450 μm diameter) based on a passive multimode optical fiber. We use digital phase conjugation to overcome the modal scrambling of the fiber to tightly focus and scan the laser light at its distal end. By exploiting the maximum number of modes available, sub-micron resolution, high quality fluorescence images of neuronal cells were acquired. The imaging system is evaluated in terms of fluorescence collection efficiency, resolution and field of view. The small diameter of the proposed endoscope, along with its high quality images offer an opportunity for minimally invasive medical endoscopic imaging and diagnosis based on cellular phenotype via direct tissue penetration.

Design and cost considerations for practical solar-hydrogen generators

Solar-hydrogen generation represents a promising alternative to fossil fuels for the large-scale implementation of a clean-fuel transportation infrastructure. A significant amount of research resources has been allocated to the development of photoelectrochemical components (i.e. photovoltaic and water splitting catalysts) that are able to spontaneously split water in the presence of solar irradiation, which has led to major advances in the solar-fuels field. At the same time, only limited attention has been given to understanding the key aspects that drive economically viable solar-fuel generators. This study presents a generalized approach to understand the economic factors behind the design of solar-hydrogen generators composed of photovoltaic components integrated with water electrolyzers. It evaluates the underpinning effects of the material selection for the light absorption and water splitting components on the cost of the generated fuel (

Volume Holographic Grating Wavelength Stabilized Laser Diodes

per Kg of H2). The results presented in this work provide insights into important engineering aspects related to the sizing of devices and the use of light concentration components that, when optimized, can lead to costs below

Volume holographic grating-based continuously tunable optical filter

2.90 per kilogram of hydrogen after compression and distribution. Most significantly, the analysis demonstrates that the cost of hydrogen is defined primarily by the light-absorbing component (up to 97% of the cost) while the material selection for the electrolysis components has, to a large extent, minor effects. The findings presented here can help direct research and development efforts towards the fabrication of deployable solar-hydrogen generators that are cost competitive with commercial energy sources.

Dynamic bending compensation while focusing through a multimode fiber

Volume holographic gratings (VHGs) are the key components for producing laser diodes (LDs) with a temperature-stabilized wavelength and narrowed linewidth. We review the unique characteristics of these gratings that make them useful for this application as well as various alternative approaches of stabilizing LDs and their performance.

Delivery of focused short pulses through a multimode fiber.

We propose and demonstrate a widely tunable optical filter, realized by angle tuning a volume holographic grating. The volume holographic grating selectively drops a narrow portion of the signal bandwidth into a fiber while passing through the rest of the signals. The demonstrated 1510- to 1590-nm tuning range covers the entire erbium-doped fiber amplifier (EDFA) C band, with small bandwidth variation and low insertion loss (<1 dB). Group delay, polarization-dependent loss, and polarization mode dispersion are measured and investigated for optimizing the filter characteristics.

Dual wavelength full field imaging in low coherence digital holographic microscopy.

Multimode fiber endoscopes have recently been shown to provide sub-micrometer resolution, however, imaging through a multimode fiber is highly sensitive to bending. Here we describe the implementation of a coherent beacon source placed at the distal tip of the multimode fiber, which can be used to compensate for the effects of bending. In the first part of this paper, we show that a diffraction limited focused spot can be generated at the distal tip of the multimode fiber using the beacon. In the second part, we demonstrate focusing even when the fiber is bent by dynamically compensating for it. The speckle pattern at the proximal fiber end, generated by the beacon source placed at its distal end, is highly dependent on the fiber conformation. We experimentally show that by intensity correlation, it is possible to identify the fiber conformation and maintain a focus spot while the fiber is bent over a certain range. Once the fiber configuration is determined, previously calibrated phase patterns could be stored for each fiber conformation and used to scan the distal spot and perform imaging.

Two-photon imaging through a multimode fiber

Light propagation through multimode fibers suffers from spatial distortions that lead to a scrambled intensity profile. In previous work, the correction of such distortions using various wavefront control methods has been demonstrated in the continuous wave case. However, in the ultra-fast pulse regime, modal dispersion temporally broadens a pulse after propagation. Here, we present a method that compensates for spatial distortions and mitigates temporal broadening due to modal dispersion by a selective phase conjugation process in which only modes of similar group velocities are excited. The selectively excited modes are forced to follow certain paths through the multimode fiber and interfere constructively at the distal tip to form a focused spot with minimal temporal broadening. We demonstrate the delivery of focused 500 fs pulses through a 30 cm long step-index multimode fiber. The achieved pulse duration corresponds to approximately 1/30th of the duration obtained if modal dispersion was not controlled. Moreover, we measured a detailed two-dimensional map of the pulse duration at the output of the fiber and confirmed that the focused spot produces a two-photon absorption effect. This work opens new possibilities for ultra-thin multiphoton imaging through multimode fibers.

Proof of principle demonstration of a self-tracking concentrator

A diffractive optical element (DOE) is presented to simultaneously manipulate the coherence plane tilt of a beam containing a plurality of discrete wavelengths. The DOE is inserted into the reference arm of an off-axis dual wavelength low coherence digital holographic microscope (DHM) to provide a coherence plane tilt so that interference with the object beam generates fringes over the full detector area. The DOE maintains the propagation direction of the reference beam and thus it can be inserted in-line in existing DHM set-ups. We demonstrate full field imaging in a reflection commercial DHM with two wavelengths, 685 nm and 794 nm, resulting in an unambiguous range of 2.494 micrometers.