Academic literature on the topic 'Thulium-holmium fibers'

Laser sources emitting in the infrared range at around 2 µm are attracting great interest for a variety of applications like processing of transparent thermoplastic polymers in industry as well as plenty of applications in medicine, spectroscopy, gas sensing, nonlinear frequency conversion to the mid-infrared, to mention a few. Of late, fiber lasers compared to other kinds of lasers benefit from their all-fiber design, leading to a compact, robust, and well thermally manageable device. Particularly, thulium- and holmium-doped fiber lasers are the first choice in fiber lasers emitting light around 2 µm. In this paper, we give an overview of our recent results in the research on thulium- and holmium-doped optical fibers, fiber lasers, and related research topics in the 2-µm spectral range. In particular, we present, to our knowledge, the first results of improvement of pump absorption in double-clad fibers thanks to the fiber twist frozen during drawing. Finally, a brief demonstration of material processing by thulium all-fiber laser operating at 2 µm is presented.

In recent years, huge progress has been made in the development of rare-earth ion doped tellurite glass laser sources, ranging from watt- and multiwatt-level fiber lasers to nanowatt level microsphere lasers. Significant success has been achieved in extending the spectral range of tellurite fiber lasers generating at wavelengths beyond 2 μm as well as in theoretical understanding. This review is aimed at discussing the state of the art of neodymium-, erbium-, thulium-, and holmium-doped tellurite glass fiber and microsphere lasers.

(1) Objective: To support the efficacy and safety of a range of thulium fiber laser (TFL) pre-set parameters for laser lithotripsy: the efficiency is compared against the Holmium:YAG (Ho:YAG) laser in the hands of juniors and experienced urologists using an in vitro ureteral model; the ureteral damage of both lasers is evaluated in an in vivo porcine model. (2) Materials and Methods: Ho:YAG laser technology and TFL technology, with a 200 µm core-diameter laser fibers in an in vitro saline ureteral model were used. Each participant performed 12 laser sessions. Each session included a 3-min lasering of stone phantoms (Begostone) with each laser technology in six different pre-settings retained from the Coloplast TFL Drive user interface pre-settings, for stone dusting: 0.5 J/10 Hz, 0.5 J/20 Hz, 0.7 J/10 Hz, 0.7 J/20 Hz, 1 J/12 Hz and 1 J/20 Hz. Both lasers were also used in three in vivo porcine models, lasering up to 20 W and 12 W in the renal pelvis and the ureter, respectively. Temperature was continuously recorded. After 3 weeks, a second look was done to verify the integrity of the ureters and kidney and an anatomopathological analysis was performed. (3) Results: Regarding laser lithotripsy efficiency, after 3 min of continuous lasering, the overall ablation rate (AR) percentage was 27% greater with the TFL technology (p < 0.0001). The energy per ablated mass [J/mg] was 24% lower when using the TFL (p < 0.0001). While junior urologists performed worse than seniors in all tests, they performed better when using the TFL than Ho:YAG technology (36% more AR and 36% fewer J/mg). In the in vivo porcine model, no urothelial damage was observed for both laser technologies, neither endoscopically during lasering, three weeks later, nor in the pathological test. (4) Conclusions: By using Coloplast TFL Drive GUI pre-set, TFL lithotripsy efficiency is higher than Ho:YAG laser, even in unexperienced hands. Concerning urothelial damage, both laser technologies with low power present no lesions.

Purpose: Laser Lithotripsy has remained the cornerstone for the management of urolithiasis for more than thirty years. Miniaturization of endoscopic equipment, digital vision, improvement of laser lithotripters, laser fibers has made endourology a field of growing interest, immersed in a technologic revolution. The aim of this article is to do an extense review on laser lithotripsy starting from the physics of the lasers, to translational science apply to lithotripsy fundamentals in order to make lithotripsy safer and more efficient. Methods: We performed a review of the literature in four different databases (PubMed, Embase, Ovid®, and Scopus®,) on any information concerning laser lithotripsy in February 2020 independently by three authors, a total of 186 articles were reviewed and 38 of the most influential articles were selected and a detailed reviewed on this topic is presented. Results: We aim to make a reference paper for all urologists and health personal involved in laser lithotripsy, starting from the physics to answer practical questions as how to set the parameters in my laser system, how to improve lithotripsy efficiency, should we dust or bust? and finally discussing new technologies such as the Holmium: Ytrium-aluminium-garnet (Ho:YAG) Moses technology, the revolutionary thulium laser fiber (TLF) and discussing the future of laser lithotripsy. Conclusions: Laser lithotripsy must offer higher ablative efficiency, wider range of laser parameters and comprehensive combinations, reduce retropulsion and fiber burn-back, scope miniaturization capabilities, smaller fiber sizes, increased safety, lower environmental impact, reduced maintenance costs Ho: YAG has remained the unquestioned gold standard for laser lithotripsy, but the recently launched Thulium fiber laser has all the above mentioned features and outruns without no doubt the current gold standard and is set to gradually replace.

This paper presents the results of numerical simulations of a ring-core thulium-doped silica fiber (RC-TDF). Enhanced spontaneous emission (ASE) was generated for a fiber with 4wt.% thulium content. An analysis of the formation of the ASE spectrum parameters (λmax, FWHM, output power) as a function of fiber length is also shown. The modal map is presented as a combination of outer and inner core radii and ∆n. Full Text: PDF References Y. Huang, Q. Xu, S. Peng, C. Xu, T. Liao, "2 μm Laser generation and amplification based on dual Tm3+-doped high-Q silica microsphere using an ASE light source for pumping", Opt Laser Technol , 153, 108282 (2022). CrossRef D. Theisen-Kunde, V. Ott, R. Brinkmann, R. Keller, "Potential of a new cw 2 μm laser scalpel for laparoscopic surgery", Med Laser Appl. 22, 139 (2007). CrossRef N.P. Barnes, B.M. Walsh, D.J. Reichle, R.J. DeYoung, "Tm:fiber lasers for remote sensing", Opt Mater (Amst), 31, 1061 (2009). CrossRef S.W. Henderson, C.P. Hale, J.R. Magee, M.J. Kavaya, A.V. Huffaker, "Eye-safe coherent laser radar system at 2.1 μm using Tm,Ho:YAG lasers", Opt Lett, 16, 773 (1991). CrossRef S.D. Jackson, "Cross relaxation and energy transfer upconversion processes relevant to the functioning of 2 μm Tm3+-doped silica fibre lasers", Opt Commun. 230, 197 (2004). CrossRef J. Wu, S. Jiang, T. Luo, J. Geng, N. Peyghambarian, N.P. Barnes, "Efficient thulium-doped 2-μm germanate fiber laser", IEEE Phot Technol Lett. 18, 334 (2006). CrossRef M.J. Barber, P.C. Shardlow, P. Barua, J.K. Sahu, W.A. Clarkson, "Nested-ring doping for highly efficient 1907nm short-wavelength cladding-pumped thulium fiber lasers", Opt Lett. 45, 5542 (2020). CrossRef P. Miluski, K. Markowski, M. Kochanowicz, M. Łodziński, J. Żmojda, W.A. Pisarski, J. Pisarska, M. Kuwik, M. Leśniak, D. Dorosz, T. Ragiń, V. Askirka, J. Dorosz, "Tm3+/Ho3+ profiled co-doped core area optical fiber for emission in the range of 1.6–2.1 µm", Sci Rep. 13, 13963 (2023). CrossRef H. Ono, T. Hosokawa, K. Ichii, S. Matsuo, H. Nasu, M. Yamada, "2-LP mode few-mode fiber amplifier employing ring-core erbium-doped fiber", Opt Express, 23, 27405 (2015) CrossRef M. Kasahara, K. Saitoh, T. Sakamoto, N. Hanzawa, T. Matsui, K. Tsujikawa, F. Yamamoto, "Design of Three-Spatial-Mode Ring-Core Fiber", J. Lightwave Technol. 32, 1337 (2014). CrossRef P. Sillard, M. Bigot-Astruc, D. Boivin, H. Maerten, L. Provost, "Few-Mode Fiber for Uncoupled Mode-Division Multiplexing Transmissions", in 37th European Conference and Exposition on Optical Communications, (2011). CrossRef S. D. Jackson, S. Mossman, "Laser-induced changes on the complex refractive indices of phase-change thin film", Appl Opt. 42, 2702 (2003). CrossRef S. Unger, A. Schwuchow, J. Dellith, J. Kirchhof, "Optical properties of ytterbium/aluminium doped silica glasses", Opt Mater Express, 10, 907 (2020). CrossRef P. Honzatko, Y. Baravets, I. Kasik, O. Podrazky, "Wideband thulium–holmium-doped fiber source with combined forward and backward amplified spontaneous emission at 1600–2300 nm spectral band", Opt Lett. 39, 3650 (2014). CrossRef P. Miluski, M. Kochanowicz, J. M. Zmojda, A. Baranowska, M. Leśniak, D. Dorosz, K. Markowski, J. Dorosz, "Large mode area fibers for single-mode transmission near 2μm", Proc. SPIE 12142, (2022). CrossRef Y. Jung, Q. Kang, H. Zhou, R. Zhang, S. Chen, H. Wang, Y. Yang, X. Jin, F. P. Payne, S. Alam, D. J. Richardson, "Low-Loss 25.3 km Few-Mode Ring-Core Fiber for Mode-Division Multiplexed Transmission", J. Lightwave Technol. 35, 1363 (2017). CrossRef

At present, minipercutaneous nephrolithotripsy is a highly effective and safe method for fragmentation of stones with sizes up to 3 cm. The use for lithotripsy of a holmium: yttrium aluminum garnet laser with wavelength 2.1 μm (Ho:YAG) is a common practice today. Of late, however, more attention is paid to a new superpulse thulium fiber laser with wavelength 1.94 μm and maximum output power 40 W. The use of a thulium laser with pulse energy 0.025–6 J and a high repetition rate (to 1600 Hz) permits to obtain (compared with a holmium laser) better results of stone dusting. Objective. Selection of the optimal modes of stone fragmentation in minipercutaneous nephrolithotripsy with the use of a superpulse thulium fiber laser with wavelength 1.94 μm and maximum output power 40 W in patients with nephrolithiasis. Patients and methods. The study included patients (n = 171), who underwent minipercutaneous nephrolithotripsy using a superpulse thulium fiber laser during the period from February 2018 to July 2019. The following parameters of laser radiation were used: pulse energy 0.1-6 J, power 6-40 W, pulse rate 30-300 Hz. The impact of modes on endoscopic visualization and retropulsion was assessed intraoperatively. Results. Statistical analysis of the specificities of the most often used modes was conducted: No 1 – «0.15 J, 200 Hz, 30 W», No 2 – «0.5 J, 30 Hz, 15 W», No 3 – «0.8 J, 31.25 Hz, 25 W», No 4 – «0.8 J, 37.5 Hz, 30 W», No 5 – «1.5 J, 15 Hz, 30 W», No 6 – «1.5 J, 26.6 Hz, 40 W», No 7 – «2 J, 15 Hz, 30 W», No 8 – «2 J, 20 Hz, 40 W», No 9 – «4 J, 10 Hz, 40 W»; their effects on retropulsion and on endoscopic visualization were studies. The stone free rate was assessed on the first post-operative day according to the findings of low-dose CT. Conclusion. Study of the modes of thulium fiber lithotripsy, assessment of retropulsion and endoscopic visualization depending on the choice of emitted radiation parameters of a thulium fiber laser make it possible to improve the outcomes of operative treatment in patients with nephrolithiasis. Key words: minipercutaneous, nephrolithotripsy, nephrolithiasis, MPNL, thulium fiber lithotripsy, retropulsion, visibility, superpulse thulium fiber laser, wavelength 1.94 μm, stone free rate

The factors formation the efficiency and safety of urinary stone crushing using holmium and thulium fiber lasers were selected, substantiated and normalized. A multifactorial integral criterion of urolithiasis treatment efficiency by laser lithotripsy was formed and analyzed. Recommendations are given for the use of lasers for crushing urinary stones.

We demonstrated an all-fiber mode-locked Thulium–Holmium co-doped fiber laser (THDFL) based on molybdenum disulfide (MoS2) tape saturable absorber. The THDFL generates a mode-locked pulse in anomalous regime at 1,979[Formula: see text]nm using 5[Formula: see text]m long Thulium–Holmium co-doped (THDF) as a gain medium. Through mechanical exfoliation method, the MoS2 was mechanically extracted from a commercial MoS2 crystal by using a clear scotch tape. Through balanced twin-detector measurement, the obtained MoS2 tape has a nonlinear absorption of 10% with 100[Formula: see text]MW/cm2 saturation intensity. Under 775[Formula: see text]mW to 852[Formula: see text]mW pump power, a stable pulse train was obtained at 9.12[Formula: see text]MHz repetition rate with a signal-to-noise ratio (SNR) of 45[Formula: see text]dB. The maximum output power and pulse energy were measured about 20[Formula: see text]mW and 2.2[Formula: see text]nJ, respectively. With a 3-dB spectral bandwidth of 2.1[Formula: see text]nm, the minimum possible pulse width was determined as 1.97[Formula: see text]ps.

Made available in DSpace on 2014-10-09T12:25:43Z (GMT). No. of bitstreams: 0
Made available in DSpace on 2014-10-09T14:03:38Z (GMT). No. of bitstreams: 1 06894.pdf: 5004886 bytes, checksum: cd5287c06ff61cc0e61899d7fc6403fa (MD5)
Dissertacao (Mestrado)
IPEN/D
Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

La technique d’amplification à dérive de fréquence "chirped pulse amplification" (CPA)a été développée pour augmenter la puissance des impulsions issues d’oscillateurs à verrouillage de mode, car l’énergie des impulsions n’étaient pas suffisantes pour des applications telles que la physique des champs forts et l’accélération de particules. Depuis son développement en 1985, elle a été utilisée dans une grande variété de systèmes laser industriels et dans des installations laser à ultra-haute puissance. Cette technique permet d’une part de s’affranchir de l’accumulation de phase non linéaire qui entrave la compression d’impulsions et d’autre part de maintenir la fluence des impulsions en dessous des seuils de dommages induits par laser aux composants. Dans cette thèse, nous développons des systèmes laser CPA à la longueurs d’onde de 2.05μm avec une puissance moyenne élevée et une énergie élevée, en commençant par le développement du laser à 2050nm jusqu’à la conception et la mise en oeuvre de l’étireur, des amplificateurs et du compresseur d’impulsions.Dans la première section de la thèse, nous introduisons les principaux concepts de physique et les phénomènes nécessaires à la compréhension de la technique d’amplification par dérive de fréquence et au développement du laser à fibre à 2050nm. Cela inclut la dispersion, l'automodulation de phase et la diffusion Raman.Dans la deuxième section de la thèse, nous présentons le développement d’un laser entièrement fibré à maintien de polarisation et accordable sur plus de 170nm, de 1880nmà 2050nm, via le phénomene de décalage de fréquence solitonique (Raman soliton selffrequencyshift, SSFS). Notre systéme est basé exclusivement sur des fibres disponibles commercialement. Nous avons caractérisé le laser en termes de puissance, de spectre et de durée d’impulsion, et nous avons inclus une étape de post-compression qui repose sur des effets non linéaires pour atteindre une durée inférieure à 100 fs sur toute la plage d' accordabilité. Les simulations sur la post-compression de solitons montrent la polyvalence du laser, qui permet de personnaliser la durée de l’impulsion sur une plage spectrale donnée, ou alternativement à une longueur d’onde particulière.Dans la troisième section, nous avons testé le laser accordable dans une grande variété d’architectures d’étirement et de compression adaptées au CPA. Nous avons examiné les fibres et les réseaux de Bragg en volume étiré (CVBG) en tant que dispositifs d’étirement d’impulsions, ainsi que les paires de réseaux de diffraction et les CVBG en tant que compresseurs d’impulsions. Nous discutons la manière de dimensionner une paire d’étireur compresseuren tenant compte de la phase non linéaire et de l’effet de rétrécissement du gain qui se produit pendant l’amplification d’impulsions, ainsi que de l’évaluation des performances de l’étireur-compresseur. Deux systèmes laser d’amplification à dérive de fréquence ont été conçus et présentés, le premier visant des impulsions fs à large bande avec une puissance moyenne élevée, et le second système visant des impulsions ps à haute énergie. La technologie peu mature dans la région spectrale de 2μm et la faible disponibilité de dispositifs d’étirement adaptés entravent la compression d’impulsions à cette longueur d’onde.Dans la dernière section de la thèse, nous avons étudié les performances des fibres co-dopées Tm:Ho en configuration d’amplification. Nous discutons des principaux défis de ces fibres, notamment les effets de transfert d’énergie, la disponibilité des sources de pompage qui donnent lieu à deux schémas de pompage principaux : le pompage par diode et le pompage intra-bande, ainsi que les limitations en termes de taille de fibre. Nous avons testé les fibres dopées Tm:Ho, y compris les fibres à coeur large pour l’amplification d’impulsions à bande étroite et à large bande
The chirped pulse amplification (CPA) technique was developed to power scale the pulsesfrom mode-locked oscillators as the pulse energy was not sufficient to target applicationssuch as strong field physics and particle acceleration. Since its development in 1985 ithas been applied in a wide variety of commercially available laser systems and ultra-highpower laser facilities. The technique allows to circumvent the accumulation of non-linearphase which hampers pulse compression and allows to maintain the fluence of the pulsesbelow the laser induced damage thresholds (LIDT) of components. In this thesis wedevelop CPA laser systems operating at 2.05μm wavelengths with high average powerand high energy starting from the development of the seed laser up to the design andimplementation of the pulse stretcher, amplifiers and pulse compressor.In the first section of the thesis we introduce the physics background and phenomenarequired for understanding the chirped pulse amplification technique and the developmentof the seed laser. This includes dispersion, self-phase modulation and Raman scattering.In the second section of the thesis we present the development of an all-fiber polarizationmaintaining laser tunable over 170nm, from 1880nm up to 2050nm via Ramansoliton self-frequency shift (SSFS). The system is based on exclusively commercially availablestandard fibers. We have characterised the laser in terms of power, spectrum andpulse duration and we have included a post-compression stage that relies on non-lineareffects to reach the sub-100 fs duration across the whole tunability range. Simulations ofthe soliton post-compression shows the versatility of the laser which allows to customisethe pulse duration over a spectral range or for a particular wavelength. We believe thatthe laser is a versatile and robust alternative to Tm and Tm:Ho oscillators.In the third section we have tested the tunable laser in a wide variety of stretchingand compression architectures suitable for CPA. We have investigated fibers and chirpedvolume Bragg gratings (CVBG) as pulse stretching devices and grating pairs and CVBGsas pulse compressors. We discuss how to dimension a stretching-compressor pair takinginto account the non-linear phase and gain narrowing effect that takes place during pulseamplification and how to evaluate the stretching-compressor performance. Two differentchirped pulse amplification laser systems have been designed and presented, the first onetargets broadband fs pulses with high average power and the second system targets highenergetic ps pulses. The non-mature technology in the 2μm spectral region and the weakavailability of suitable stretching devices hinders pulse compression at this wavelength.In the last section of the thesis we investigated the performance of Tm:Ho co-dopedfibers in amplification configuration. We discuss the main challenges of these fibers includingthe cross-relaxation effects, the availability of pump sources which gives rise totwo main pumping schemes: diode pumping and in-band pumping and the limitations interms of fiber size. We tested Tm:Ho doped fibers, including LMA for narrowband andbroadband pulse amplification

Doctor of Philosophy
Department of Physics
Brian R. Washburn
Mode-locked lasers that produce short optical pulses in the mid-infrared wavelength region have been sought out for a wide range of applications such as free space communication, molecular spectroscopy, medical diagnostics, and remote sensing. Here, a thulium and holmium (Tm/Ho) co-doped fiber laser that mode-locks in both the solitonic and stretched-pulse regimes is used to produce ultra-short pulses in the 2 [mu]m region. Nonlinear polarization rotation technique is used where fiber nonlinearity is responsible to mode-lock the laser. The anomalous group velocity dispersion of both the single mode and gain fibers used limit the laser operation in the solitonic regime where spectral bandwidth is 10 nm and hence the pulse duration is limited to 996 fs. In order to increase the spectral bandwidth and hence get the shorter pulses the anomalous dispersion of these fibers has to compensate using normal group velocity dispersion fiber in the laser cavity. High numerical aperture fibers, which have normal group velocity dispersion around 2 [mu]m due to its large and positive waveguide dispersion, can be used to compensate the anomalous dispersion of the gain and single mode fibers. We used a high numerical aperture fiber called UHNA4 in the laser cavity in order to compensate the anomalous dispersion of other fibers and mode-locked the laser in stretched pulse regime. The spectral bandwidth of the laser increased to 31 nm with corresponding pulse duration of 450 fs measured from the interferometric autocorrelation. The laser dynamics of the Tm/Ho co-doped fiber laser is also studied while going from the stretched-pulse to solitonic regime by fiber cut-back measurements of normal dispersion fiber. It was clearly observed that both the spectral bandwidth and the pulse duration changed significantly going from one region to the other.

Mode-locked fibre lasers have applications in research, defence, biological science, manufacturing and spectroscopy. The aim of this research was to investigate auto- matic control of a passive mode-locking technique known as non-linear polarisation rotation (NLPR) that exploits polarisation differences across a pulse’s profile that manifest due to different intensities. This research seeks to focus on how NLPR laser sources can be made stable, especially at longer wavelengths (such as 2 µm). Three different fibre lasers were developed to investigate different aspects of NLPR. An erbium mode-locked source was used to explore measurement techniques and competing operating regimes. The operating regimes were controlled using a manual polarisation controller. Characterisation revealed that the source produced 570 fs pulses with a central wavelength of 1567 nm, repetition rate of 17 MHz and pulse energy of 3 nJ. A thulium NLPR source was developed to further explore possible automation metrics and characterise a range of additional undesired operating regimes. Automation metrics tested to determine if the laser was continuously mode-locked included discontinuities in Stokes parameters, two-photon absorption signals and amplitude modulation across the radio-frequency spectrum of the laser. Characterisation revealed that the source produced 1.15 ps pulses with a central wavelength of 1990 nm, repetition rate of 21 MHz and pulse energy of 5 nJ. Lastly, a holmium NLPR source with electronic control of the intra-cavity polarisation state was built. To the best of our knowledge, this is the first diode-pumped NLPR mode-locked holmium source in the 2µm waveband. An automation mechanism was devised and implemented. Characterisation revealed that the source produced 770 fs pulses with a central wavelength of 2061 nm, a repetition rate of 17 MHz and pulse energy of 0.41 nJ.
Thesis (M.Phil) -- University of Adelaide, School of Physical Sciences : Physics, 2020

This thesis describes the development of microstructured rare-earth-doped silica fibres for high power lasers. Motivated by the demand for increasingly higher power fibre lasers, this research aims to overcome the power scaling limits from non-linear effects in optical fibres. The project focussed on the demonstration of the first air-clad holmium-doped silica fibre laser. Novel fabrication techniques were employed to directly drill air-claddings in preforms produced with modified chemical vapour deposition. Key limitations for increasing single-mode laser output powers are the onset of non-linear effects, the coupling of pump power from high-power low-brightness sources, and poor lasing efficiency. Holmium doped fibre lasers were identified as a promising pathway for overcoming these power scaling limits due to their potential for high efficiencies and long wavelength emission in silica. Low loss high numerical aperture air-clad fibres were used to address the limited pump coupling, which is a primary restriction for power scaling of holmium-doped fibre lasers. Low dopant concentration fibres were fabricated to prevent clustering of holmium ions, which can limit the efficiency of holmium lasers. To compensate for the reduced pump absorption, small cladding to core area ratios were used. Coupling ability of the reduced pump cladding area was achieved with the low-loss high numerical aperture air-claddings. Multiple cladding geometries were fabricated and characterised to show their suitability for fibre laser operation. The highest reported numerical aperture for holmium-doped double-clad silica fibre was achieved. Air-clad holmium-doped fibre lasers, pumped with in-house developed thulium lasers, were demonstrated. Over 10 W of output power at 2.1 μm was achieved with high beam quality. Mechanical, thermal and laser transition modelling was performed to analyse the lasing results. This revealed that the reduction of fibre length through changes in cladding geometry or increased dopant concentration are critical for efficient high-power double-clad holmium doped fibre lasers. Air-clad holmium-doped fibre lasers are an attractive pathway for increasing high-power fibre laser output powers. These findings show potential for compact infra-red laser sources, short fibre length laser applications, and are a key step in the development of efficient high-power directly-diode pumped holmium-doped fibre lasers.
Thesis (Ph.D.) -- University of Adelaide, School of Physical Sciences, 2016.

Recent advances in thulium and holmium doped fibers are reviewed with focus on the design of highly efficient cladding-pumped holmium fibers and on the effect of temperature-dependent spectroscopic parameters on high-power thulium fiber laser operation.

The unique features of fiber lasers have stimulated interest in this new class of lasers. Rare-earth-doped fluorozirconate fibers are often superior to their silica-based counterparts owing to their multiphonon spectrum. This permits radiative transitions from levels that suffer nonradiative de-excitation in silica glasses. The potential number of lasingwave-lengths in fluoride glasses is thus large. This topic is still being studied, and many fundamental phenomena remain to be clarified. Major advances in the field have been recently published: new lasing wavelengths in erbium, holmium, thulium, neodymium, samarium, and praseodymium; Q-switched operation; diode-laser pumping; and cw-tunable operation. The upconversion laser is one of the most promising devices, and it has already been demonstrated with thulium at 480 nm and with holmium at 550 nm, for which a slope efficiency of 36% and an extracted power of more than 10 mW have been obtained at room temperature.

Currently, various types of pulsed lasers are used for ablation of tissue. Owing to their small penetration depth, mid-infrared and ultraviolet lasers are attractive candidates for precise tissue cutting. Furthermore, both the mid-infrared and ultraviolet laser pulses can be delivered via flexible fibers, which allows ablation of tissue in a water environment and in the human body. The mid-infrared holmium (wavelength λ = 2.09 μm) and thulium (λ = 2.01 μm) laser ablation is based on the absorption and subsequent vaporization of tissue water. As a result, with each laser pulse, a rapidly expanding and imploding vapor bubble (lifetime τ ≈ 500 μs, diameter ∅ up to 3 mm) is formed.1

Fraunhofer ILT develops low-linewidth, high-stability ytterbium-, thulium- and holmium-doped fiber amplifiers e.g. for pre studies of gravitational wave detectors in the context of the LISA and the ETEST project. Here, we give an overview on our 1 µm and 2 µm fiber amplifier development for use in space and on earth.

Laser emission at 2.1 μm from holmium-doped YAG at liquid-nitrogen temperatures was first reported in 1965.1 Recent performance improvements at room temperature using chromium and thulium sensitization have renewed interest in the practical application of this laser material. The potential efficiency, energy storage, and atmospheric and silica fiber transmission properties of this laser, combined with its eye-safe wavelength, have encouraged research into military, medical, materials-processing, and remote-sensing applications. As an aid to the design of Cr-Tm-Ho:YAG laser systems, we have measured the basic laser parameters and spectral and thermal characteristics of a free-running, room-temperature, flashlamp-pumped Cr-Tm-Ho:YAG system. Birefringent tuning of this system revealed at least nine distinct low-threshold transitions. Simple analysis of the performance data yields the basic parameters of transition energy levels, emission, and absorption cross sections, saturation fluences, storage lifetime, and heat deposition. The dynamics of the chromium- and thulium-sensitized holmium laser are complicated and are not yet fully understood. Our progress toward understanding and modeling the temperature, concentration, and excitation dependence of the pumping efficiency and lifetime of the holmium laser system will also be reported.

According to the absorption properties of tissue, a laser for microsurgery should emit radiation of 2.9 µm wavelength. This wavelength is absorbed in tissue within 1 µm and leads to evaporation with a minimum thermal damage in the walls of the incision. If monomode emission is used, the laser light can be focused to a very small spot. The resulting thin coagulation layer, however, cannot stop bleeding from larger vessels. For haemostasis a wavelength of 2 µm with a penetration depth of several hundred µm is preferable. For application in surgery therefore both wavelength 2 µm and 3 µm have to be available. These wavelengths can be generated in fibre lasers doped with Erbium for 3µm emission [1] or Thulium and Holmium for 2 µm emission [2]. Monomode fibre lasers are optically excited with a pump beam that has to be launched into the small rare earth- doped core of the fibre with a diameter of typically less than 10 µm. In view of this small target also the pump laser has to be operated in low transverse mode order or even in fundamental mode. For such a pump laser the available power is limited to about 1 Watt. In our experiments an output power of 158 mW has been reached for ZBLAN:Er3+ fibres [1] and 30 mW for silica:Tm:Ho fibres. These values were only limited by the available pump power. The output power of CW monomode fiber lasers, can be enhanced using a double clad fibre structure [3]. Double clad fibres can be excited with powerful multimode laserdiode arrays. An output power of several Watts has already been demonstrated in a Nd3+ doped double clad fibre [4,5].