Christophe A. Codemard

Laser cutting is a fast, precise, and noncontact processing technique widely applied throughout industry. However, parameter specific defects can be formed while cutting, negatively impacting the cut quality. While light-matter interactions are highly nonlinear and are, therefore, challenging to model analytically, deep learning offers the capability of modeling these interactions directly from data. Here, we show that deep learning can be used to scale up visual predictions for parameter specific defects produced in cutting as well as for predicting defects for parameters not measured experimentally. Furthermore, visual predictions can be used to model the relationship between laser cutting defects and laser cutting parameters.

There are no changes to the key results or conclusions of the paper, only corrections to display errors.

In this paper we demonstrate a fast fiber mode converter in a short piece of special passive fiber. The switching mechanism relies on selective wavelength resonance between the core and a high index surrounding ring in the fiber. Beam switching at 530 kHz from a Gaussian beam shape to an expanded beam with a ring with corresponding M2 from 1.4 to 3.5 is demonstrated in a modified industrial pulsed fiber laser at power levels in excess of 15 W.

We present a new sensing technique exploiting light-sheet excitation of skew rays in a multimode fiber, which can be applied to enhance the sensitivity of a range of sensing mechanisms such as pump absorption. The underlying principle is that a light sheet (i.e. thin plane of light) can selectively concentrate the optimum ray group, giving rise to enhanced interaction between light and matter (e.g. fluorophores). We compared this excitation method with others in terms of attenuation of pump light through Rhodamine B. It was observed that the attenuation experienced by light-sheet skew rays can be up to one order of magnitude higher than that of collimated skew rays, and three orders of magnitude higher than that of normal-incidence rays.

Transverse mode competition and instabilities in high-power fiber oscillators have been studied experimentally by monitoring the dynamic power exchanges and characteristic frequencies of the transmitted fundamental mode (FM) and scattered high-order modes (HOMs) of the fiber laser cavity under CW and pulsed pumping. The FM and HOM power evolution indicates the presence of two competing effective laser cavities which result in rich output dynamics and full chaotic operation. The thermal and inversion related contributions to the observed instabilities have been identified by monitoring the associated characteristic instability frequencies under pulsed pumping. It is shown that in the transient regime, both inversion and thermal effects contribute successively to the observed power instabilities. Increasing the pump power leads to full chaotic response through an interplay between transverse and longitudinal mode instabilities.

We report on novel approach to switch the optical beam profile in a fibre laser using a short piece of special passive fibre. The switching mechanism relies on selective wavelength resonance between the core and a high index surrounding ring in the fibre. Beam switching at 530 kHz from a Gaussian beam shape to a flat-top beam with a ring with corresponding M² from 1.4 to 3.5 is demonstrated in a modified industrial pulsed fibre laser.

We analyze the requirements and demonstrate good beam quality transportation over several hundred meters in highly multimode step-index fibers. The beam quality factor is preserved over 100m (M²≈1.3) and degrades linearly to M²≈2.0 at 300m.

We explore the properties of a novel stimulated Raman scattering (SRS) filtering fiber for high average or high peak optical power delivery applications. The fiber geometry is based on a series of circularly arranged high index rods embedded in a leaky silica cladding. The operation principle relies on the resonant coupling of the core and rod modes and the wavelength-dependent leaking of the structure. The fabricated fiber demonstrated wide transmission window and filtering of SRS with extinction in excess of 20 dB at the Raman Stokes wavelength, excellent robustness with bending, and high output beam quality. The fiber has been tested as a beam delivery fiber of a commercial pulsed fiber laser system in order to explore the filtering performance and its limitations.

Summary form only given. Transverse modal instability (TMl) [1] is proving to be a major power-scaling limitation in high-power fibre lasers (HPFLs) [2]. The origin of TMI is predominantly thermal and a number of theoretical models have been introduced to explore its characteristics [3-9]. The majority of TMI reports so far deal with fibre amplifiers in MOPA configurations [3-14], and only a small numbers of recent reports refer to TMI in single-cavity HPFLs [15-16]. In addition, most of the experimental TMI work is carried out under steady-state pumping conditions and the transient TMI response is largely overlooked [17]. TMI threshold reduces in the presence of backreflections in amplifiers [18], and, therefore, TMI effects are expected to be more pronounced in single-cavity HPFL oscillators.In this work, we explore for the first time the transient behaviour of power instabilities in single-cavity HPFLs with modulated (ON/OFF) pumping (quasi-CW (QCW) operation). Fig. 1(a) shows the schematic of the tested HPFL. The ends of the Yb-doped signal fibre of a GTWaveTM [19-20] are spliced (S) with singlemode (SM) pigtails with a high reflector (HR) and output coupler (OC) fibre Bragg gratings inscribed in their cores. The fundamental mode (FM) of the active fibre is matched to the SM pigtails. Higher-order modes (HOMs) supported by or thermally induced in the active fibre are scattered into the pigtail cladding at the splices (S), and a cladding-mode stripper (CMS) in the output pigtail is used to strip them out and monitor their power. The combined pump diodes are modulated with variable repetition rates and ON/OFF duty cycles and are launched into the pump fibres of the GTWaveTM. The fundamental mode (FM) at the output and HOMs at the CMS are monitored with high-speed photodetectors.