Sandile Ngcobo

A digital laser for on-demand laser modes

Customizing the output beam shape from a laser invariably involves specialized optical elements in the form of apertures, diffractive optics and free-form mirrors. Such optics require considerable design and fabrication effort and suffer from the further disadvantage of being immutably connected to the selection of a particular spatial mode. Here we overcome these limitations with the first digital laser comprising an electrically addressed reflective phase-only spatial light modulator as an intra-cavity digitally addressed holographic mirror. The phase and amplitude of the holographic mirror may be controlled simply by writing a computer-generated hologram in the form of a grey-scale image to the device, for on-demand laser modes. We show that we can digitally control the laser modes with ease, and demonstrate real-time switching between spatial modes in an otherwise standard solid-state laser resonator. Our work opens new possibilities for the customizing of laser modes at source.

Doughnut laser beam as an incoherent superposition of two petal beams

Laguerre-Gaussian beams with a nonzero azimuthal index are known to carry orbital angular momentum (OAM), and are routinely created external to laser cavities. The few reports of obtaining such beams from laser cavities suffer from inconclusive evidence of the real electromagnetic field. In this Letter we revisit this question and show that an observed doughnut beam from a laser cavity may not be a pure Laguerre-Gaussian azimuthal mode but can be an incoherent sum of petal modes, which do not carry OAM. We point out the requirements for future analysis of such fields from laser resonators.

Modal decomposition without a priori scale information

The modal decomposition of an arbitrary optical field may be done without regard to the spatial scale of the chosen basis functions, but this generally leads to a large number of modes in the expansion. While this may be considered as mathematically correct, it is not efficient and not physically representative of the underlying field. Here we demonstrate a modal decomposition approach that requires no a priori knowledge of the spatial scale of the modes, but nevertheless leads to an optimised modal expansion. We illustrate the power of the method by successfully decomposing beams from a diode-pumped solid state laser resonator into an optimised Laguerre-Gaussian mode set. Our experimental results, which are in agreement with theory, illustrate the versatility of the approach.

Exciting higher-order radial Laguerre–Gaussian modes in a diode-pumped solid-state laser resonator

In this paper we experimentally demonstrate the intracavity generation of selected Laguerre-Gaussian modes of variable radial order, from 0 to 5. Our technique requires only an amplitude mask made up of absorbing rings to be placed inside the cavity, with the ring radii selected to coincide with the zeros of the desired Laguerre-Gaussian mode. We demonstrate high mode purity and a mode volume proportional to the order of the mode. Our results suggest a possible route to high brightness diode-pumped solid-state laser sources.

Tuneable Gaussian to flat-top resonator by amplitude beam shaping

We outline a simple laser cavity comprising an opaque ring and a circular aperture that is capable of producing spatially tuneable laser modes, from a Gaussian beam to a Flat-top beam. The tuneability is achieved by varying the diameter of the aperture and thus requires no realignment of the cavity. We demonstrate this principle using a digital laser with an intra-cavity spatial light modulator, and confirm the predicted properties of the resonator experimentally.

Implementation of a spatial light modulator for intracavity beam shaping

In this paper we outline the steps necessary to create a laser with an intra-cavity spatial light modulator (SLM) for transverse mode control. We employ a commercial SLM as the back reflector in an otherwise conventional diode-pumped solid state laser. We show that the geometry of the liquid crystal (LC) arrangement strongly influences the operating regime of the laser, from nominally amplitude-only mode control for twisted nematic LCs to nominally phase-only mode control for parallel-aligned LCs. We demonstrate both operating regimes experimentally and discuss the potential advantages of and improvements to this new technology.

Direct fiber excitation with a digitally controlled solid state laser source.

Mode division multiplexing has been mooted as a future technology to address the impending data crunch of existing fiber networks. Present demonstrations delineate the light source from the mode creation steps, potentially inhibiting integrated solutions. Here we create fiber modes on demand with a digital laser and couple them directly into a few-mode fiber, where after transmission they are decoupled by modal decomposition. This is the first demonstration of an integrated source for encoding information into the spatial modes of light.

Laser beam propagation characteristics of incoherently added diode bars

The second moment method of laser beam propagation allows for the calculation of the beam quality factor for any laser beam, or combination of laser beams. When several laser beams are added, their effective beam quality factor is not simply the sum of the individual beam quality factors, that is, it does not act as a linear operator. In this paper we derive an analytical expression for the beam quality factor of incoherently added laser beams whose centroids are not collinear. We illustrate the versatility of the final result by showing how this may be applied to the problem of the laser beam propagation characteristics of high power diode bar stacks.

Graphene for improved femtosecond laser based pluripotent stem cell transfection

Pluripotent stem cells are hugely attractive in the tissue engineering research field as they can self-renew and be selectively differentiated into various cell types. For stem cell and tissue engineering research it is important to develop new, biocompatible scaffold materials and graphene has emerged as a promising material in this area as it does not compromise cell proliferation and accelerates specific cell differentiation. Previous studies have shown a non-invasive optical technique for mouse embryonic stem (mES) cell differentiation and transfection using femtosecond (fs) laser pulses. To investigate cellular responses to the influence of graphene and laser irradiation, here we present for the first time a study of mES cell fs laser transfection on graphene coated substrates. First we studied the impact of graphene on Chinese Hamster Ovary (CHO-K1) cell viability and cell cytotoxicity in the absence of laser exposure. These were tested via evaluating the mitochondrial activity through adenosine triphosphates (ATP) luminescence and breakages on the cell plasma membrane assessed using cytosolic lactate dehydrogenase (LDH) screening. Secondly, the effects of fs laser irradiation on cell viability and cytotoxicity at 1064 and 532 nm for cells plated and grown on graphene and pure glass were assessed. Finally, optical transfection of CHO-K1 and mES cells was performed on graphene coated versus plain glass substrates. Our results show graphene stimulated cell viability whilst triggering a mild release of intracellular LDH. We also observed that compared to pure glass substrates; laser irradiation at 1064 nm on graphene plates was less cytotoxic. Finally, in mES cells efficient optical transfection at 1064 (82%) and 532 (25%) nm was obtained due to the presence of a graphene support as compared to pristine glass. Here we hypothesize an up-regulation of cell adhesion promoting peptides or laminin-related receptors of the extracellular matrix (ECM) in cell samples grown and irradiated on graphene substrates. By bringing together advances in optics and nanomaterial sciences we demonstrate pathways for enhancement of pluripotent stem cell biology.

Rapid measurement of the fiber’s transmission matrix

A novel fiber characterization technique is introduced that combines two holographic procedures for selective fiber-mode excitation and complete fiber-mode analysis. The fiber’s transmission matrix is constructed by directly measuring the response of the excited modes in terms of the entire guided mode spectrum. By applying a spatial multiplexing scheme for the modal decomposition, the transmission matrix is rapidly determinable, whereas the amount of needed measurements is proportional to the number of guided modes. Our technique characterizes the impact of the fiber on the mode signals and enables the investigation of underlying physical effects as well as signal correction schemes.