Single-step phase identification and phase locking for coherent beam combination using deep learning.

Coherent beam combination offers a solution to the challenges associated with the power handling capacity of individual fibres, however, the combined intensity profile strongly depends on the relative phase of each fibre. Optimal combination necessitates precise control over the phase of each fibre channel, however, determining the required phase compensations is challenging because phase information is typically not available. Additionally, the presence of continuously varying phase noise in fibre laser systems means that a single-step and high-speed correction process is required. In this work, we use a spatial light modulator to demonstrate coherent combination in a seven-beam system. Deep learning is used to identify the relative phase offsets for each beam directly from the combined intensity pattern, allowing real-time correction. Furthermore, we demonstrate that the deep learning agent can calculate the phase corrections needed to achieve user-specified target intensity profiles thus simultaneously achieving both beam combination and beam shaping.

Multiframe-based non-local means denoising for Raman spectra.

A method for denoising Raman spectra is presented in this paper. The approach is based on the principle that the original signal can be restored by averaging pixels based on structure similarity. Similarity searching and averaging are not limited to the neighbouring pixels but extended throughout the entire signal range across different frames. This approach is distinguished from the conventional single-frame neighbour pixel-based filtering. The effectiveness and robustness of the proposed method are demonstrated through denoising simulated and experimental Raman data sets with fixed denoising parameters. Several denoised results and statistical indicators are presented for the simulated data. Recovery of the experimental Raman spectrum from our newly developed cost-effective waveguide-enhanced Raman spectroscopy system is also presented and compared to the spectrum from a conventional expensive Raman microscope for the same analyte.

Live imaging of laser machining via plasma deep learning.

Real-time imaging of laser materials processing can be challenging as the laser generated plasma can prevent direct observation of the sample. However, the spatial structure of the generated plasma is strongly dependent on the surface profile of the sample, and therefore can be interrogated to indirectly provide an image of the sample. In this study, we demonstrate that deep learning can be used to predict the appearance of the surface of silicon before and after the laser pulse, in real-time, when being machined by single femtosecond pulses, directly from camera images of the generated plasma. This demonstration has immediate impact for real-time feedback and monitoring of laser materials processing where direct observation of the sample is not possible.

Acoustic and plasma sensing of laser ablation via deep learning.

Monitoring laser ablation when using high power lasers can be challenging due to plasma obscuring the view of the machined sample. Whilst the appearance of the generated plasma is correlated with the laser ablation conditions, extracting useful information is extremely difficult due to the highly nonlinear processes involved. Here, we show that deep learning can enable the identification of laser pulse energy and a prediction for the appearance of the ablated sample, directly from camera images of the plasma generated during single-pulse femtosecond ablation of silica. We show that this information can also be identified directly from the acoustic signal recorded during this process. This approach has the potential to enhance real-time feedback and monitoring of laser materials processing in situations where the sample is obscured from direct viewing, and hence could be an invaluable diagnostic for laser-based manufacturing.

Grating-incoupled waveguide-enhanced Raman sensor.

We report a waveguide-enhanced Raman spectroscopy (WERS) platform with alignment-tolerant under-chip grating input coupling. The demonstration is based on a 100-nm thick planar (slab) tantalum pentoxide (Ta2O5) waveguide and the use of benzyl alcohol (BnOH) and its deuterated form (d7- BnOH) as reference analytes. The use of grating couplers simplifies the WERS system by providing improved translational alignment tolerance, important for disposable chips, as well as contributing to improved Raman conversion efficiency. The use of non-volatile, non-toxic BnOH and d7-BnOH as chemical analytes results in easily observable shifts in the Raman vibration lines between the two forms, making them good candidates for calibrating Raman systems. The design and fabrication of the waveguide and grating couplers are described, and a discussion of further potential improvements in performance is presented.

Transverse mode instability in fiber laser oscillators.

What we believe to be a first theoretical study of transverse mode instability (TMI) in oscillators based on a stimulated thermal Rayleigh scattering (STRS) model is conducted. Higher order mode (HOM) lasing is found to happen at high powers. Further fundamental mode (FM) growth is limited once HOM lasing takes place, with further increase of pump power mostly going to HOM growth, a fundamentally different phenomenon from that in fiber amplifiers. TMI thresholds defined as when the HOM lasing condition is met is studied. The results are consistent with the measured TMI thresholds and their dependence on pumping configurations and pump wavelengths.

Single step phase optimisation for coherent beam combination using deep learning.

Coherent beam combination of multiple fibres can be used to overcome limitations such as the power handling capability of single fibre configurations. In such a scheme, the focal intensity profile is critically dependent upon the relative phase of each fibre and so precise control over the phase of each fibre channel is essential. Determining the required phase compensations from the focal intensity profile alone (as measured via a camera) is extremely challenging with a large number of fibres as the phase information is obfuscated. Whilst iterative methods exist for phase retrieval, in practice, due to phase noise within a fibre laser amplification system, a single step process with computational time on the scale of milliseconds is needed. Here, we show how a neural network can be used to identify the phases of each fibre from the focal intensity profile, in a single step of ~ 10 ms, for a simulated 3-ring hexagonal close-packed arrangement, containing 19 separate fibres and subsequently how this enables bespoke beam shaping. In addition, we show that deep learning can be used to determine whether a desired intensity profile is physically possible within the simulation. This, coupled with the demonstrated resilience against simulated experimental noise, indicates a strong potential for the application of deep learning for coherent beam combination.

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.

Waveguide Enhanced Raman Spectroscopy for Biosensing: A Review.

Waveguide enhanced Raman spectroscopy (WERS) utilizes simple, robust, high-index contrast dielectric waveguides to generate a strong evanescent field, through which laser light interacts with analytes residing on the surface of the waveguide. It offers a powerful tool for the direct identification and reproducible quantification of biochemical species and an alternative to surface enhanced Raman spectroscopy (SERS) without reliance on fragile noble metal nanostructures. The advent of low-cost laser diodes, compact spectrometers, and recent progress in material engineering, nanofabrication techniques, and software modeling tools have made realizing portable and cheap WERS Raman systems with high sensitivity a realistic possibility. This review highlights the latest progress in WERS technology and summarizes recent demonstrations and applications. Following an introduction to the fundamentals of WERS, the theoretical framework that underpins the WERS principles is presented. The main WERS design considerations are then discussed, and a review of the available approaches for the modification of waveguide surfaces for the attachment of different biorecognition elements is provided. The review concludes by discussing and contrasting the performance of recent WERS implementations, thereby providing a future roadmap of WERS technology where the key opportunities and challenges are highlighted.

Optimized design for grating-coupled waveguide-enhanced Raman spectroscopy.

We report a new design optimization process for planar photonic waveguides applied to waveguide-enhanced Raman spectroscopy (WERS) that combines the optimization of both the surface intensity performance and the grating coupling efficiency. We consider the impact of film thickness on the grating coupling efficiency of two materials with different refractive indices, namely tantalum pentoxide (Ta2O5) and silicon (Si). We propose a new figure-of-merit (FOM) that takes into account both the coupling efficiency and surface intensity dependence for Raman excitation on the film thickness. Our study shows that the optimum surface-sensitive waveguide thickness is thinner than the optimum coupling efficiency thickness for both material systems. As an example, for a tantalum pentoxide waveguide operating at 785 nm, our optimization strategy proposes a 20% increase in waveguide core thickness relative to the optimum surface-sensitive thickness to achieve the best performance in WERS applications.

Reconfigurable structured light generation in a multicore fibre amplifier.

Structured light, with spatially varying phase or polarization distributions, has given rise to many novel applications in fields ranging from optical communication to laser-based material processing. However the efficient and flexible generation of such beams from a compact laser source at practical output powers still remains a great challenge. Here we describe an approach capable of addressing this need based on the coherent combination of multiple tailored Gaussian beams emitted from a multicore fibre (MCF) amplifier. We report a proof-of-concept structured light generation experiment, using a cladding-pumped 7-core MCF amplifier as an integrated parallel amplifier array and a spatial light modulator (SLM) to actively control the amplitude, polarization and phase of the signal light input to each fibre core. We report the successful generation of various structured light beams including high-order linearly polarized spatial fibre modes, cylindrical vector (CV) beams and helical phase front optical vortex (OV) beams.

A general model for taper coupling of multiple modes of whispering gallery resonators and application to analysis of coupling-induced Fano interference in a single cavity.

In this paper, we give a general model for analysis of multimode Whispering Gallery Mode (WGM) resonators coupled to multimode tapered fibers based on the coupled-mode theory. Such formulation takes into account the asymmetry of the taper-resonator coupling. Simulations for a microsphere show that the tapered fiber coupling mechanism induces cross-coupling between coherent orthonormal WGMs. We show that the degree of such cross-coupling depends basically on the fiber diameter, air-gap between the taper and resonator, intrinsic losses and eccentricity. The WGM cross-coupling affects the total transmission and spectral line-shape of the internal powers resulting in a controllable transformation of the line-shape to non-Lorentzian spectra. This analysis can be utilized to precisely determine the output and intra-cavity intensity of multimode microresonators, which is important in lasers, nonlinear optical signal generation and realization of optical delays.

Transverse mode instability, thermal lensing and power scaling in Yb3+-doped high-power fiber amplifiers.

Transverse mode instability (TMI) is compared to thermal lensing (TL) power threshold and used to derive power scaling limits in high-power fiber amplifiers. The TMI power threshold is shown to be ~65% of the TL one and dominates power scaling. In addition to commonly used limiting effects, we introduce a bend-induced mechanical reliability criterion, which limits the maximum allowable cladding diameter to ~600µm. This also results in the introduction of a critical pump brightness, the minimum required pump brightness at which the maximum signal power is achieved. The maximum achievable power depends primarily on the choice of pumping wavelength, amplifier gain and heat coefficient. Maximum signal powers of ~28kW to ~38kW, for diode pumping (λp = 976nm), and ~35kW to ~52kW, for tandem pumping (λp = 1018nm), are predicted for single-mode fiber amplifiers operating at signal wavelength λs = 1070nm, when the amplifier gain is increased from 10dB to 20dB. For an amplifier gain of 10dB, the maximum achievable signal power varies from 85kW to 25kW for tandem pumping and 35kW to 20kW for diode pumping, when the heat coefficient varies from 1% to 15% and 5.5% to 20%, respectively. The corresponding critical pump brightness varies from ~0.50 W/(µm2 sr) to ~0.14 W/(µm2 sr) for tandem pumping and ~0.25 W/(µm2 sr) to ~0.13 W/(µm2 sr) for diode pumping.

Design of rare-earth-doped microbottle lasers.

Coupling strength in taper-coupled microbottle resonators can be tuned by offsetting the taper along the resonator profile, similar to controlling the air-gap in microsphere excitation, and hence, achieve desired coupling characteristics for a specific mode. Such flexibility makes microbottles attractive and adaptable laser cavities. In this paper, lasing characteristics of Yb3+-doped microbottle laser (MBL) coupled to tapered fiber are theoretically investigated. It is demonstrated that desired lasing characteristics for a particular mode are achievable by controlling the taper-resonator coupling, intrinsic quality factor (Q) and dopant concentration. Although, high Q whispering gallery cavities provide high internal powers, which is favorable especially for low gain materials, they lack high output powers. Hence, care should be taken in designing MBLs to attain the highest possible output power. Here, we address such issues, and optimized the required resonator parameters (for both pump and signal) for a low threshold pump power, high efficiency and desired lasing wavelength.

Non-destructive characterization of rare-earth-doped optical fiber preforms.

We present a non-destructive optical technique for rare-earth-doped optical fiber preform inspection, which combines luminescence spectroscopy measurements, analyzed through an optical tomography technique, and ray-deflection measurements for calculating the refractive-index profile (RIP) of the sample. We demonstrate the technique on an optical fiber preform sample with a Yb3+-doped aluminosilicate core. The spatial distribution of the photoluminescence signals originating from Yb3+-single ions and from Yb3+-Yb3+ cluster sites were obtained inside the core. By modifying the characterization system, we were able to concurrently evaluate the RIP of the core and, thus, establish with good accuracy the dopant distribution within the core region. This technique will be useful for quality evaluation and optimization of optical fiber preforms.

5.6 kW peak power, nanosecond pulses at 274 nm from a frequency quadrupled Yb-doped fiber MOPA.

A 2 W deep-ultraviolet (DUV) source at 274 nm with 5.6 kW peak power is demonstrated by frequency quadrupling a diode-seeded, polarization-maintaining (PM), Yb-doped fiber master oscillator power amplifier (MOPA) system delivering 1.8 ns pulses at a repetition rate of 200 kHz. The second harmonic generation (SHG) and the fourth harmonic generation (FHG) are achieved by using Lithium Triborate (LBO) crystal and ß-BaB2O4 (BBO) crystal in sequence, with an IR-to-green and green-to-UV conversion efficiency of up to 65% and 26%, respectively. This is the first kW peak power pulsed UV system reported at 274 nm which has great potential for machining insulators, 2D materials, isotopic separation of Calcium-48, and fluorescence analysis of biological molecules.

Surface and waveguide collection of Raman emission in waveguide-enhanced Raman spectroscopy.

We demonstrate Raman spectroscopy on a high index thin film tantalum pentoxide waveguide and compare collection of Raman emission from the waveguide end with that from the waveguide surface. Toluene was used as a convenient model analyte, and a 40-fold greater signal was collected from the waveguide end. Simulations of angular and spatial Raman emission distributions showed good agreement with experiments, with the enhancement resulting from efficient collection of power from dipoles near the surface into the high-index waveguide film and substrate, combined with long interaction length. The waveguide employed was optimized at the excitation wavelength but not at emission wavelengths, and full optimization is expected to lead to enhancements comparable to surface-enhanced Raman spectroscopy in robust low-cost metal-free and nanostructure-free chips.

Power Budget Analysis for Waveguide-Enhanced Raman Spectroscopy.

Waveguide-enhanced Raman spectroscopy (WERS) is emerging as an attractive alternative to plasmonic surface-enhanced Raman spectroscopy approaches as it can provide more reproducible quantitative spectra on a robust chip without the need for nanostructured plasmonic materials. Realizing portable WERS systems with high sensitivity using low-cost laser diodes and compact spectrometers requires a detailed analysis of the power budget from laser to spectrometer chip. In this paper, we describe theoretical optimization of planar waveguides for maximum Raman excitation efficiency, demonstrate WERS for toluene on a silicon process compatible high index contrast tantalum pentoxide waveguide, measure the absolute conversion efficiency from pump power to received power in an individual Raman line, and compare this with a power budget analysis of the complete system including collection with an optical fiber and interfacing to a compact spectrometer. Optimized 110 nm thick Ta2O5 waveguides on silica substrates excited at a wavelength of 637 nm are shown experimentally to yield overall system power conversion efficiency of ∼0.5 × 10(-12) from the pump power in the waveguide to the collected Raman power in the 1002 cm(-1) Raman line of toluene, in comparison with a calculated efficiency of 3.9 × 10(-12) Collection efficiency is dictated by the numerical and physical apertures of the spectral detection system but may be improved by further engineering the spatial and angular Raman scattering distributions.

Transmittance and surface intensity in 3D composite plasmonic waveguides.

A detailed theoretical study of composite plasmonic waveguide structures is reported. Expressions for modal expansion coefficients, optical transmittance and surface intensity are presented and used to describe the behavior of dielectric channel waveguides containing a short gold-coated section. The superstrate refractive index is shown to control modal beating and modal attenuation in the gold-coated region leading to distinctive features in the surface intensity and device transmittance. The model presented allows detailed prediction of device performance, enabling improved design of highly sensitive miniature devices for evanescent refractometry and vibrational spectroscopy, and can be extended to the design and optimization of composite waveguides structures with nano-patterned overlayers.

Inverse scattering designs of dispersion-engineered planar waveguides.

We have introduced a semi-analytical IS technique suitable for multipole, rational function reflection coefficients, and used it for the design of dispersion-engineered planar waveguides. The technique is used to derive extensive dispersion maps, including higher dispersion coefficients, corresponding to three-, five- and seven-pole reflection coefficients. It is shown that common features of dispersion-engineered waveguides such as refractive-index trenches, rings and oscillations come naturally from this approach when the magnitude of leaky poles in increased. Increasing the number of poles is shown to offer a small but measureable change in higher order dispersion with designs dominated by a three pole design with a leaky pole pair of the smallest modulus.