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.
High Power Fiber Lasers: A Review Zervas, Michalis N.; Codemard, Christophe A.
IEEE journal of selected topics in quantum electronics,
09/2014, Letnik:
20, Številka:
5
Journal Article
Recenzirano
In this paper, we summarize the fundamental properties and review the latest developments in high power fiber lasers. The review is focused primarily on the most common fiber laser configurations and ...the associated cladding pumping issues. Special attention is placed on pump combination techniques and the parameters that affect the brightness enhancement observed in single-mode and multimode high power fiber lasers. The review includes the major limitations imposed by fiber nonlinearities and other parasitic effects, such as optical damage, transverse modal instabilities and photodarkening. Finally, the paper summarizes the power evolution in continuous-wave and pulsed ytterbium-doped fiber lasers and their impact on industrial applications.
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.
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.
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 (λ
= 976nm), and ~35kW to ~52kW, for tandem pumping (λ
= 1018nm), are predicted for single-mode fiber amplifiers operating at signal wavelength λ
= 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/(μm
sr) to ~0.14 W/(μm
sr) for tandem pumping and ~0.25 W/(μm
sr) to ~0.13 W/(μm
sr) for diode pumping.
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.
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.
Selective excitation of spheroidal whispering gallery modes and bottle modes in a robust bottle microresonator fabricated straightforwardly from a short section of optical fiber is demonstrated. ...Characteristic resonance spectra of long-cavity bottle modes were obtained by using a tapered fiber to excite evanescently bottle microresonator at different points along its axis. Compared to bare-fiber cylindrical resonators, the bottle microresonator results in a 35x increase of the observed Q factor.
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.
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