The development of a 20.6 W radially polarized Ho:YAG laser is reported, which has been actively Q-switched in order to generate 0.52 mJ pulses with a 210 ns full-width half-maximum duration. By utilizing a laser-written spatially-variant birefringent waveplate (S-waveplate) with ultra-low scattering losses inside the cavity, the linearly polarized beam exiting the acousto-optic modulator is transformed into radial polarization with very high conversion efficiency. The excellent radial polarization purity (49:1) and beam quality (M2 of 2.14) highlight this approach as a very suitable platform for high power operation, assisted by the large damage threshold and customizability of the S-waveplate.
High power amplification of a 2090 nm radially polarized laser beam has been demonstrated using a double-pass Ho:YAG thin-slab crystal, generating up to 33.7 W of output power for a 13.2 W radially polarized seed laser. The amplifier crystal was end-pumped with a 95 W single-mode 1907 nm thulium fiber laser, tailored for suitable overlap with the radially polarized beam. By compensation of depolarizing effects such as thermally-induced stress birefringence and the Gouy phase shift, the degradation of the radially polarized beam is minimized, with the chosen amplifier architecture being readily applicable to much higher seed and pump powers.
Double-clad silica fibres used in high power lasers typically comprise a core doped with a laser active ion, a silica inner-cladding pump guide and a low refractive index outer polymer coating for protection and low loss pump guidance. For efficient pump absorption in the active-ion doped core, the inner-cladding must be shaped in order to scramble the pump radiation to maintain overlap with the core. This shaping is traditionally undertaken via diamond, or ultrasonic, milling of the fibre preform into an octagon or hexagon, leaving a rough surface which is subsequently fire polished before drawing into a fiber. We report on our developments of an alternative approach for shaping the inner-cladding using a 10.6µm pulsed CO2 laser to machine the fibre preform. This process is shown to allow fabrication of N sided polygon shaped fiber claddings as well as novel cladding structures, which include concave and convex surfaces, as well as core to cladding area ratio adjustment. Processing speed is significantly increased whilst maintaining improved surface qualities that remove the requirement of further fire polishing prior to fibre drawing. We will discuss recent developments of novel cladding geometries such as Reuleaux polygon cladding shapes that allow use of automated x-y profilometers on the draw tower whilst maintaining a shaped cladding for pump scrambling.
A route to high power continuous-wave generation around 1.7 µm has been explored using an Er-only doped LMA fiber laser for high brightness core pumping of a thulium-doped fiber laser (TDFL). The maximum output power of 47 W with a slope efficiency of 80% was achieved at 1726 nm which, to the author’s best knowledge, is the highest recorded power in this wavelength region from a TDFL. The motivation for an Er-only pump source is scalability; these results show that this configuration has significant potential for further power scaling towards hundred-watt class systems at 1.7 µm and beyond.
Output power levels from 1907 nm Tm fiber laser sources are restricted by low lasing efficiency and challenging thermal management. Here we develop a novel fiber core structure where the Tm dopant material is confined to a ring within the larger undoped glass core. This evolution of dopant profiling has tripled the fiber device lengths and produced 131 W of 1907 nm emission with 64.5% absorbed slope efficiency. This, to the best of our knowledge, is the most efficient 1907 nm Tm fiber laser that has been demonstrated with the capability to perform beyond 100 W output power.
The automated selection and stabilization of the transverse mode of a radially polarized Ho:YAG laser is reported. A convolutional neural network (CNN) was developed to analyze the modal composition of the laser output in real-time. Calculated error signals from the CNN are compared to the desired mode, allowing a PID control algorithm to dynamically optimize the position of an intracavity lens and therefore maintain desired modal content over pump power changes. This CNN based diagnostic system provides a fast method for selection and stabilization of transverse modes in order to advance radially polarized sources for applications such as laser processing.
We have investigated the effect of the frequency shift associated with an acousto-optic (AO) interaction on the spectral quality of acousto-optically tuned external cavity semiconductor lasers. A ring-cavity was built and the wavelength controlled by a frequency compensating acousto-optic tunable filter pair, which allowed user control over the frequency shift of the interacting light. Laser spectra were recorded for a range of frequency shifts, and a 30 kHz driving frequency gave the optimal spectral response where the output spectral shape did not vary as a function of operating wavelength and resulted in <0.1 nm linewidths.
Laguerre-Gaussian (LG) modes have properties that make them well suited to many applications, particularly laser processing when scaled to high-power. Here we present an approach for generating high-purity LG01 vortex beams in a Nd:YVO4 laser which overcomes the common problems of degenerate handedness and low damage thresholds in previous methods. The obtained modes are scaled in power by application of a novel Thermally-Guiding Fiber-Rod Amplifier (TGFRA).
Our approach is based on a novel end-pumping arrangement for efficiently generating the Hermite-Gaussian TEM01 mode in a 1064 nm Nd:YVO4 laser. A fiber-coupled laser diode was spliced to a 50:50 fiber splitter to give two equal outputs. These outputs were bonded to a bulk optic with a small separation for spatially matching the TEM01 mode. An astigmatic mode converter made using two concave mirrors was used to obtain a LG01 mode with controlled handedness.
The obtained LG01 mode was propagated through the 300 µm core of a 10 cm long sample of triple-clad Yb-doped silica fiber by utilizing thermal lensing as a waveguiding mechanism. The fiber was pumped using a high-power 915 nm diode laser. This amplifier geometry ensures preservation of the mode while the inheriting good thermal management from a fiber geometry and the large mode area common in rod geometries.
The 0.89 W LG01 seed-source was amplified with gain of 2.7 dB. The gain was limited by available pump power and the emission cross-section at 1064 nm. This result provides an avenue to high-power LG01 modes.
There are a number of spectral features around 1700 nm which are very attractive to the laser community. Strong C-H bond absorption and moderate water absorption lend this region to many applications such as polymer processing and laser surgery. Despite being a growing area of interest, development of high-power, laser sources in this region is quite challenging. Here we present preliminary results of in-band pumping a thulium-doped fiber laser (TDFL) with erbium-only doped fiber laser (EDFL) to generate 1700 nm
Erbium-only was chosen to avoid the power scaling limitations of co-doping with ytterbium. Parasitic lasing on the ytterbium band at 1030 nm results in efficiency roll-off and self-pulsing. Because of this, single-mode Er/Yb laser systems are generally limited to moderate powers of ~20W and reliability remains an issue.
An in-house double-clad large mode-area fiber, with a fundamental mode-field diameter of 20 um, was cladding pumped at 975 nm. Maximum power at 90 W launched was 31 W and the slope efficiency was 44.2% (35.4%) with respect to absorbed (launched) pump power. Suitability for high-brightness core pumping of a TDFL was confirmed with an M2 measurement of 1.1 ± 0.1.
For 1700 nm generation, the 1580 nm pump light was free-space coupled into an in-house TDF (0.2 wt.% dopant concentration). For a maximum launched power of 23 W, 15 W of 1726 nm output was generated with a slope efficiency of 67.1% (64.7%) with respect to absorbed (launched). To the author’s best knowledge, this is the highest recorded power in this wavelength region from a TDFL.
Laser and optical amplifier geometries may be split into categories such as rod and fiber. Rod gain media are susceptible to thermal effects at high power, whereas fiber suffer from detrimental non-linear effects due to their long length and small mode areas. Here we present an application of a hybrid architecture between the two geometries – the Thermally-Guiding Fiber-Rod (TGFR). The TGFR inherits the large mode area of the rod amplifier, the high surface area of a fiber, and exploits thermal lensing to guide modes.
We present a successful demonstration of amplification of a radially polarized mode using the TGFR. A 1030 nm continuous-wave radially polarized seed source of high purity and beam quality (M2=1.9±0.1) was constructed using thermal bifocussing in a Yb:YAG crystal to provide mode selection. This seed source was carefully focussed into the 300 µm core of a 10 cm long sample of commercially available triple-clad Yb-doped silica fiber in order to satisfy the thermal guidance condition and avoid waveguiding due to the refractive index step. The TGFR was pumped using a high power 915 nm diode laser.
The radially polarized mode was preserved through transmission of the TGFR. The output beam polarization was maintained at 99.1% purity while the M2 factor was measured to be 2.1±0.1. The maximum output power was 12.6 W of radially polarized light, corresponding to a gain of 7.0 dB limited by available pump power. This promising geometry the potential for further power scaling of radially-polarized beams for application in laser processing.
Fibre lasers operating in the 2μm region are of increasing interest for a range of applications, including laser machining and biomedical systems. The large mode area compared to 1μm fibre lasers combined with operation in an “eye-safe” region of the spectrum makes them particularly attractive. When developing fibre lasers at 1μm and 1·5μm manufacturers were able to call upon enabling technologies used by the telecoms industry, but at longer wavelengths, including 2μm, many such components are either unavailable or immature. We report on recent developments of Acousto-Optic Modulators and Tunable Filters that are specifically optimised for use with fibre systems operating at or around 2μm. AO devices are interesting due to their ability to conserve spatial-coherence, making them appropriate for use with single-mode optical fibres. We describe how the choice of interaction medium is an important consideration, particularly affecting the drive power and the polarisation behaviour of the device – the latter being an important parameter when used in a fibre system. We also describe two designs of AO Tunable Filter intended for laser tuning. Both designs have been demonstrated intracavity in 2μm fibre lasers. The first gives exceptionally narrow resolution (δλ/λ<0·1%). The second design is of a novel type of AOTF where a matched pair of AOTFs is configured to give a substantially net zero frequency-shift with little or no loss of pointing stability, any minor deviations in manufacture being self-compensated. Furthermore, small controlled frequency-shifts (up to about 10kHz) may be introduced with little or no detriment to the alignment of the system.
In this paper, an overview of the EU FP7 project ISLA (Integrated disruptive componentS for 2 μm fibre Lasers) is given. The aim of ISLA was to develop a set of “building block” components and a “tool-kit” of processes to define an integrated modular common platform for two micron fibre lasers consisting of compatible and self-consistent active and passive fibres, fused fibre couplers and combiners, fibre-coupled isolators, modulators and high power pump laser diodes. We also present results from our work on developing passive components for 2 μm fibre lasers. This includes high power pump combiners that have been tested up to 0.5 kW and combiners for in-band pumping of holmium lasers. Couplers for use as splitters, power monitors and wavelength division multiplexers have also been demonstrated. Wideband couplers, with a coupling ratio that only varies ± 12% over 400 nm, have also been developed to exploit the wide tuning range possible with thulium fibre lasers. Research into different isolator materials was also conducted to find materials with large Verdet constants to be used in 2 μm isolators. Fibre-coupled isolators were then manufactured using a selection of these materials. Isolators that had insertion losses of < 1 dB and isolation of > 35 dB were demonstrated using PM and non-PM fibres. In the PM isolators, PER > 23 dB was achieved.
We report on the development and testing of optical isolators for use in 2-micron fiber laser systems. A variety of potential Faraday rotator materials were characterised to identify the most suitable materials for use in the 1700-2100nm wavelength range. Isolators based on the three best performing materials were then developed and packaged as fiber-in, fiber-out and fiber-in, beam-out devices. The isolators were then tested in CW, pulsed and ultrafast laser systems. The three different designs produced different performance characteristics, but all designs demonstrated isolation >25dB and insertion losses of <1.2 dB.
Since the conception of the Global Positioning System (GPS) its applications have grown to provide not only a navigation, but also a geodetic, and more recently a meteorological tool. This latter application has become termed GPS Meteorology. Atmospheric research in GPS initially centered around the premise that the atmosphere was a nuisance parameter (NP). To obtain the highest precisions (particularly in height) this NP has to be modeled, or estimated, in addition to the geodetic parameters. One of the most successful techniques for dealing with this NP has been to stochastically estimate the NP using a Kalman filter. As a by-product to the precise geodetic parameters obtained, tropospheric wet delays (the NP) can also be output. These tropospheric wet delays (TWD) can be related to integrated precipitable water vapor (IPWV) through the simplified equation: TWD equals k(DOT)IPWV. The dimensionless value of k is dependent on the weighted mean temperature of the atmosphere and can be predicted from surface temperature alone (but typically ranges from 6.0 to 6.5). The expansion of the International GPS Geodynamics Service (IGS) network provides the possibility of near-continuous measurements of IPWV anywhere on the Earth by using GPS as a passive remote sensing tool. Such a possibility provides applications for numerical weather prediction (NWP) models and climate and global change research. This paper outlines the current status of atmospheric research in GPS. A project aimed at validating the use of GPS for meteorology at Nottingham is discussed using observations from ground-based water vapor radiometers (WVRs) for validation.
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