Modelling of fibre laser cutting via deep learning.

Laser cutting is a materials processing technique used throughout academia and industry. However, defects such as striations can be formed while cutting, which can negatively affect the final quality of the cut. As the light-matter interactions that occur during laser machining are highly non-linear and difficult to model mathematically, there is interest in developing novel simulation methods for studying these interactions. Deep learning enables a data-driven approach to the modelling of complex systems. Here, we show that deep learning can be used to determine the scanning speed used for laser cutting, directly from microscope images of the cut surface. Furthermore, we demonstrate that a trained neural network can generate realistic predictions of the visual appearance of the laser cut surface, and hence can be used as a predictive visualisation tool.

Enhanced responsivity with skew ray excitation of reflection- and transmission-type refractometric sensors.

The responsivity of optical fibers to refractive index can be enhanced using high-order skew rays compared with using meridional rays. Skew rays can have a much higher number of reflections with increased interaction length along the core-cladding interface, which gives rise to stronger interactions with the external medium. Reflection/transmission-type refractometric sensors based on twin-coupled-core and multimode fibers showed one/two orders of magnitude increase in responsivity with skew ray excitation. The responsivity and sensitivity for the two types are ~2000%/RIU, ~1400%/RIU, and 4.9×10⁻5 RIU, 7.0×10⁻5 RIU, respectively.

Broadband third harmonic generation in tapered silica fibres.

Optical microfibres have recently attracted much attention for nonlinear applications, due to their tight modal confinement. Here, we report broadband third harmonic generation based on the intermodal phase matching technique in silica microfibres of several centimetres. The third harmonic signal is predominantly generated from the taper transition regions (rather than the waist), wherein the range of diameters permits phase matching over a wide bandwidth. Microfibres up to 4.5 cm long were fabricated with waist diameters below 2.5 µm to allow a λ = 1.55 µm pump to phase match with several higher order third harmonic modes; conversion rates up to 3 × 10⁻4 were recorded when pumped with 4 ns pulses at a peak power of 1.25 kW. Analysis of the third harmonic frequencies generated from the nonlinearly broadened pump components indicate a 5 dB conversion bandwidth of at least 36 nm, with harmonic power detected over a 150 nm range.

Spectral gain control using shaped pump pulses in a counter-pumped cascaded fiber Raman amplifier.

We investigate the Raman gain spectra produced from pulse-pumping a highly nonlinear fiber with shaped optical pulses delivered from a Yb-doped fiber MOPA pump source. Cascaded Raman wavelength shifting up to seven Stokes orders is demonstrated and the counter-propagating gain is measured across all seven Stokes orders. Step-shaped optical pulses with varying instantaneous powers are then used to pump the highly nonlinear fiber, generating a controllable gain spectrum across multiple Stokes orders. Furthermore, we extend this work by using multiple pump wavelengths along with step-shaped pulses to increase the bandwidth of the Raman gain spectrum.

Excitation of individual Raman Stokes lines in the visible regime using rectangular-shaped nanosecond optical pulses at 530 nm.

We demonstrate the selective excitation of individual Raman Stokes lines of up to the ninth order by pumping with rectangular-shaped optical pulses at 530 nm from a frequency-doubled adaptively pulse shaped fiber master oscillator power amplifier. We achieve extinction ratios of up to 15 dB between selected and adjacent Raman orders in a 1-km-long fiber.

Picosecond fiber MOPA pumped supercontinuum source with 39 W output power.

We report a picosecond fiber MOPA pumped supercontinuum source with 39 W output, spanning at least 0.4-2.25 microm at a repetition rate of 114.8 MHz. The 2m long PCF had a large, 4.4 microm diameter core and a high-delta design which led to an 80% coupling efficiency, high damage threshold and rapid generation of visible continuum generation from the picosecond input pulses. The high and relatively uniform power density across the visible spectral region was approximately 31.7 mW/nm corresponding to peak power density of approximately 12.5 W/nm for the 21 ps input pulses. The peak power density was increased to 26.9 W/nm by reducing the repetition rate to 28 MHz. This represents an increase in both average and peak power compared to previously reported visible supercontinuum sources from either CW pumped or pulsed-systems.

Multi-watts narrow-linewidth all fiber Yb-doped laser operating at 1179 nm.

An all-fiber, narrow-linewidth, high power Yb-doped silica fiber laser at 1179 nm has been demonstrated. More than 12 W output power has been obtained, corresponding to a slope efficiency of 43% with respect to launched pump power, by core-pumping at 1090 nm. In order to increase the pump absorption, the Yb-doped fiber was heated up to 125 degrees C. At the maximum output power, the suppression of amplified spontaneous emission was more than 50 dB. Furthermore, theoretical work confirms that the proposed laser architecture can be easily scaled to higher power.

Analysis and optimization of acoustic speed profiles with large transverse variations for mitigation of stimulated Brillouin scattering in optical fibers.

We derive and numerically verify a simple analytical approximation for the calculation of Brillouin gain spectra (BGS) in acoustically antiguiding optical fibers. We use our approximation to find acoustic speed profiles that minimize the peak Brillouin gain. The optimized BGS are top-hat-shaped, regardless of the optical mode distribution, with peak value inversely proportional to the effective area of the optical mode. We further investigate acoustically guiding speed profiles, in which acoustic waveguide modes lead to peaks in the Brillouin gain spectrum. However, for large cores with diameters down to 50 microm, and often even smaller, these Brillouin gain peaks are spectrally dense, and can be smeared out due to broadening in many practical systems. The Brillouin gain spectrum is then similar for waveguiding and antiguiding acoustic profiles.

Guided-wave second-harmonic generation in a LiNbO3 nonlinear photonic crystal.

We demonstrate twin-beam second-harmonic generation from telecommunications wavelengths in an optimized buried reverse proton exchanged planar waveguide made in 2D hexagonally poled LiNbO3. Experiments carried out with a nanosecond narrow-bandwidth, high-power fiber source thoroughly explored the response of the nonlinear photonic crystal device in terms of its power, wavelength, and angle tunability.

High-energy in-fiber pulse amplification for coherent lidar applications.

An Er:Yb codoped fiber amplifier chain for the generation of pulses for coherent lidar applications at a wavelength near 1.5 microm is reported. The final 1.8-m-long power amplification stage had a 50-microm core diameter and yielded a 23-dB energy gain, resulting in 0.29-mJ, 100-ns pulses at a repetition rate of 4 kHz with no Brillouin scattering and an M2 of 2.1.

Modal power decomposition of beam intensity profiles into linearly polarized modes of multimode optical fibers.

We calculate the modal power distribution of a randomly and linearly polarized (LP) multimode beam inside a cylindrical fiber core from knowledge of spatial-intensity profiles of a beam emitted from the fiber. We provide an exact analysis with rigorous proofs that forms the basis for our calculations. The beam from the fiber end is collimated by a spherical lens with a specific focal length. The original LP-mode basis is transformed by the spherical lens and forms another orthogonal basis that describes the free-space beam. By using this basis, we calculate the modal power distribution from the mutual-intensity profile. This is acquired by adopting a well-known mutual-intensity-profile-retrieving technique based on measurements of the intensity patterns several times after two orthogonal cylindrical lenses with varying separation. The feasibility of our decomposition algorithm is demonstrated with simulations.