We present preliminary results relating to laser emission at 3:16 µm from a Dy fiber laser that is diode pumped at 800 nm. To allow strong diode pump absorption and to capture improved quantum efficiencies resulting from cross relaxation, the Dy:ZBLAN fiber was co-doped with Tm ions in a 10 to 1 concentration ratio to Dy. A resonant energy transfer from Tm to Dy provides an inversion on the 6H13/2 to 6H15/2 transition. Maximum output power of 5.5 mW at a slope efficiency of 1.3 % was produced from a highly non-optimal arrangement. System performance is bench marked against well established resonant pumping of the Dy upper state. Measurement of fluorescence lifetimes of both dopants allows for qualitative assessment of the energy transfer efficiency. A potentially detrimental energy transfer mechanism is identified and discussed.
In this paper, we report the first (to the best of our knowledge) GaN laser diode, emitting at 445 nm, pumped dysprosium (Dy) doped ZBLAN fiber laser for yellow emission using a simple setup. In our yellow laser experiment, we have used a commercially available Dy-doped ZBLAN fiber, which is originally designed for mid-infrared lasers demonstration and not optimized for visible laser design, as a laser active medium. For yellow (∼576 nm) lasing, we have exploited the 4F9/2 to 6H13/2 laser transition of a Dy ion, which is a quasi four level system. The performance of the yellow laser system is investigated by using two different Dy-doped fiber lengths (0.6 m and 5.95 m). The measured lasing thresholds are 7 mW and 28 mW for 0.6 m and 5.95 m of Dy-doped fiber, respectively. However, the maximum laser slope efficiency with respect to absorbed pump power is only 2.3% for 0.6 m of Dy-doped fiber. The laser slope efficiency decreases to 0.9% and the threshold increases to 28 mW for 5.95 m of Dy-doped fiber, which are result of fiber background loss at the signal wavelength. In addition, we have observed the pump excited state absorption at 445 nm pumping wavelength and estimated the pump ESA cross-section via numerical simulation.
Mid-infrared (mid-IR) fiber lasers that are based on dysprosium (Dy) as the active laser ion provide emission in the wavelength range between 2.6–3.4 μm and can thus bridge the spectral gap between holmium (Ho) and erbium (Er) based mid-IR lasers. Another distinct feature is the wide choice of pump wavelengths (1.1 μm, 1.3 μm, 1.7 μm, and 2.8 μm) that can be used. To date, pump wavelengths shorter than 1.1 μm have not been reported and all demonstrated pump wavelengths apart from in-band pumping suffer from pump excited state absorption (ESA). In this paper, we report new excitation wavelengths, 0.8 μm and 0.9 μm, for Dy-doped mid-IR fiber lasers. We have measured 18.5% and 23.7% slope efficiency (relative to launched pump power) for 0.8 μm and 0.9 μm pumping wavelengths, respectively. By comparing the residual pump power of experimental and numerical simulation data of a 0.5 m Dy-doped fiber, we have found that these new excitation wavelengths are free from pump ESA. Moreover, the high power laser diodes are commercially available at these new excitation wavelengths; therefore, the realization of a diode-pumped Dy-doped mid-infrared fiber laser might become feasible in the near future.
The yellow spectral region has applications in medicine, ophthalmology, acne treatment, and ytterbium based optical clock, which is recommended as the secondary representation of System International (SI) second. Lasing in this spectral region can be achieved by using sum frequency and second harmonic generation, which are complex methods. Here, we have reported the first (to the best of our knowledge) GaN laser diode pumped dysprosium (Dy) doped ZBLAN fibre laser for yellow lasing using a simple setup. Due to the four-level system, the laser oscillation threshold is low and about 7.0 mW for 0.6 m of Dy-doped ZBLAN fibre. However, the maximum slope efficiency is only 2.3% with the available 2% output coupler. In our investigation, we have identified two possible factors, fibre background loss and pump excited state absorption, for such a low slope efficiency.
We demonstrate the first stable mode-locking from an Er3+ doped fluoride fibre laser cavity using various novel two-dimensional saturable absorber materials such as PtSe2 and MXene operating near 2.8 μm wavelength to the best of our knowledge. The linear cavity includes a high reflective chirped fibre Bragg grating to provide wavelength stability. The observed mode-locked pulse train has a 30 MHz repetition rate and an average power of 223 mW. Our results demonstrate the feasibility of using the novel two-dimensional nanomaterials such as PtSe2 and MXene into the fibre laser cavity for the application in mid-infrared wavelength regime.
The performance of mid-infrared fiber lasers operating on the 3.5 μm transition in erbium has improved significantly since the first demonstration that dual wavelength pumping allowed efficient operation. In this contribution, we will discuss the progress of fiber lasers that operate on this transition with an emphasis on advances towards short pulse generation and wavelength agility. Mode-locked operation using saturable absorption is a robust means of achieving ultra-short pulse operation in the near infrared but achieving this in the mid-infrared has been elusive. We will also describe our characterization of the mid-infrared performance of graphene, a material which has been very successfully applied to mode-locked pulse generation in the near infrared.
Mode-locked fiber lasers are currently limited to sub-3-μm wavelengths, despite application-driven demand for longer wavelength mid-IR pulse sources. Erbium- and holmium-doped fluoride fiber lasers are emerging for 2.7-2.9 μm emission, yet further extending this coverage is challenging. Here, we propose a new approach using dysprosium-doped fiber with frequency shifted feedback (FSF), achieving 33 ps pulses with up to 2.7 nJ energy, tunable from 2.97 to 3.30 μm. Notably, this is the longest wavelength mode-locked fiber laser and the most broadly tunable pulsed fiber source to date. Simulations are also performed, offering insights into the dynamics of FSF pulse generation.
The development of new, compact mid-infrared light sources is critical to enable biomedical sensing applications in resource-limited environments. Here, we review progress in fiber-based mid-IR sources, which are ideally suited for clinical environments due to their compact size and waveguide format. We first discuss recent developments in mid-IR supercontinuum sources, which exploit nonlinear optic phenomena in highly nonlinear materials (pumped by ultrashort pulse lasers) to generate broadband spectra. An emerging alternative approach is then presented, based on broadly tunable mid-IR fiber lasers, using the promising dysprosium ion to achieve orders of magnitude higher spectral power density than typical supercontinua. By employing an acousto-optic tunable filter for wavelength tuning, an electronically controlled swept-wavelength mid-IR fiber laser is developed, which is applied for absorption spectroscopy of ammonia (NH3), an important biomarker, with 0.3 nm resolution and 40 ms acquisition time.
We explore the potential of a new mid-infrared laser transition in praseodymium-doped fluoride fiber for emission around 3.4 μm, which can be conveniently pumped by 0.975 μm diodes via ytterbium sensitizer co-doping. Optimal cavity designs are determined through spectroscopic measurements and numerical modeling, suggesting that practical diode-pumped watt-level mid-infrared fiber sources beyond 3 μm could be achieved.
Previously reported progress in 3 micron dysprosium doped ZBLAN fiber lasers achieved record conversion efficiency but was limited in tuneability due to the inband nature of the pumping scheme. Near infrared pumping has also been demonstrated but was limited in conversion efficiency due both to pump excited state absorption and large quantum defect. We address these limitations by employing a Raman fiber laser operating at 1700 nm as a pump source. Reduced quantum defect shows promise for efficiency gains while maintaining near infrared pumping and the increased gain bandwidth shows promise for pulsed operation.
We demonstrate a passively mode-locked holmium-praseodymium co-doped ring fiber laser that produces an estimated 950 fs pulsewidth and peak power of 4.3 kW at a pulse repetition rate of 74 MHz. The measured center wavelength was 2.86 µm which overlaps more strongly with liquid water whilst better avoiding atmospheric water vapor which overlaps more strongly with previously reported ultrafast Er3+ fiber lasers operating at 2.8. Thus the present system should display better long term stability compared to the Er3+-based system and at the same time, be a more practical tool for interaction with biological tissues.
The laser was constructed using a 1.2 m long double-clad fluoride fiber doped with Ho3+ and Pr3+ ions and arranged into a unidirectional ring resonator that was resistant to instabilities associated with back reflections. Two semiconductor 1150 nm laser diodes with the maximum combined output of 7.5 W were used to pump the fiber. Mode-locking was achieved using the combination of two techniques: sub-picosecond pulses were produced by nonlinear polarization evolution after longer pulses were initially obtained using an in-cavity GaAs saturable absorber having a modulation depth of 90% and a relaxation time of 10 ps. A standard arrangement employing two waveplates and an optical isolator was introduced into the resonator to carry out nonlinear polarization rotation. The average power of the mode-locked laser reached 350 mW after the 50% outcoupling mirror. The RF signal-to-noise ratio reached 67 dB for the first peak at the resolution bandwidth of 10 kHz.
Recent progress in our work on the development of three micron class dysprosium-doped ZBLAN fiber lasers will be presented. Of particular note is the achievement of 51% slope efficiency which to our knowledge represents a record for all 3 micron class fiber lasers. This result is obtained for an in-band pumping scheme which also allowed for demonstration of continuous tuning over a range of 400nm with an extreme emission wavelength of 3.35 microns.
We demonstrate a range of novel functions based on a high index doped silica glass CMOS compatible platform. This platform has promise for telecommunications and onchip WDM optical interconnects for computing.
We present progress on 3 laser systems operating near 3 μm: a continuous-wave (cw) widely tunable narrow linewidth
system, a high peak power Q-switched system, and a passively mode-locked system. The cw system emitted average
powers of < 1 W over a wavelength range of 130 nm, with a spectral width of < 1 nm. The Q-switched system produced
pulses with 33 ns and peak power of 576 W. The mode-locked system produced ~20 ps pulses at a repetition rate of 27
MHz.
Fiber lasers emitting in the 2 μm wavelength range doped with thulium ions can be used as highly efficient pump
sources for nonlinear converters to generate mid-infrared radiation. For spectroscopic purposes, illumination and
countermeasures, a broad mid-infrared emission spectrum is advantageous. This can be reached by supercontinuum
generation in fibers, e.g. fluoride fibers, which up to now has, however, only been presented with either low
average power, complex Raman-shifted 1.55 μm pump sources or multi-stage amplifier pump schemes. Here we
present recent results of a new actively-mode-locked single-oscillator scheme that can provide the high-repetition
rate sub-ns pump pulses needed for pumping supercontinuum generators. A thulium-doped silica fiber laser is
presented that provides > 11 W of average power CW-mode-locked pulses at 38 MHz repetition rate at ~ 38 ps
pulse width. Upgrading the setup to allow Q-switched mode-locked operation yields mode-locked 40 MHz pulses
arranged in 60 kHz bunched Q-switch envelopes and thus increases further the available peak power. In this
Q-switched mode-locked regime over 5 W of average power has been achieved.
The Tm-doped silicate glass fibre laser that operates in the 2 micron region of the spectrum is fast becoming a mature
technology with output powers already exceeding 1 kW. In this paper, I will review a number of current and future
experiments that involve lasers pumped with the output from Tm-doped silicate glass fibre lasers including linear
systems e.g., the optical excitation of rare earth ions and nonlinear systems e.g., Raman fibre lasers.
Improvement in hybrid vapour phase epitaxy growing techniques of quasi-phase-matched orientation-patterned GaAs
(OP-GaAs) allows larger sample thickness and permits efficient operation as a mid-infrared optical parametric oscillator
at Watt-level average output powers [1-3]. Especially its low absorption loss (- 0.01 cm-1), its laser damage threshold
comparable to ZGP (- 2 J/cm2) combined with a large nonlinear coefficient, a good thermal conductivity, excellent
mechanical properties, and a wide transparency range (0.9-17 μm) are suitable properties for efficient non-critical phase
matched OPOs. As there is no natural birefringence in GaAs, phase matching is independent of polarization and
propagation direction, offering the ability to pump OP-GaAs with a variety of polarization states. Thus, even unpolarized
or poorly polarized sources like simple fiber lasers have been efficiently used as pump sources [4-5].
The paper discuss the best OP-GaAs OPO results achieved, to our knowledge, using a Q-switched 2.09 μm Ho:YAG
laser as pump source as well as results obtained with an OP-GaAs OPO directly pumped by a 2.09 μm Q-switched
Tm,Ho:silica fiber laser. With a 2.09 μm Q-switched Ho:YAG fiber laser pump source up to 2.9 W of average output
power was achieved at 20 kHz repetition rate, 3.9 W at 40 kHz and 4.9 W at 50 kHz. With a 2.09 μm Q-switched
Tm3+,Ho3+:silica fiber laser pump source, up to 2.2 W of average output power was achieved at 40 kHz repetition rate,
1.9 W at 60 kHz and 1.3 W at 75 kHz in the mid-infrared range.
Silicate glass based fiber lasers emitting in the shortwave infrared spectrum, i.e., 1.4 μm to 3 μm are based on the
phonon-terminated transitions of Er, Tm and Ho. In this review I will examine the pump, fiber and dopant configurations
that have been tested in order to maximize the efficiency and output power produced from these transitions and I will
highlight some of the issues that may need to be addressed in order to further scale the output power.
Gallium arsenide combines a large nonlinear coefficient, a good thermal conductivity, excellent mechanical properties,
and a wide transparency range (0.9-17μm). Improvement in hybrid vapour phase epitaxy growing techniques of quasiphase-
matched orientation-patterned GaAs (OP-GaAs) allows larger sample thickness and permits efficient operation as
a mid-infrared optical parametric oscillator at Watt-level average output powers.
Especially its low absorption loss (~; 0.01 cm-1), its laser damage threshold comparable to ZGP (~ 2 J/cm2) are suitable
properties for efficient non-critical phase matched OPOs.
As there is no natural birefringence in GaAs, phase matching is independent of polarization and propagation direction,
offering the ability to pump OP-GaAs with a variety of polarization states. Thus, even unpolarized or poorly polarized
sources like simple fiber lasers have been efficiently used as pump sources.
The paper will discuss recent results obtained with an OP-GaAs OPO directly pumped by a 2.09 μm Q-switched
Tm,Ho:silica fiber laser and a study on polarization effects using a Q-switched 2.09 μm Ho:YAG laser as the pump.
With a 2.09 μm Q-switched Tm,Ho:silica fiber laser pump source, up to 2.2 W of average output power was achieved at
40 kHz repetition rate, 1.9 W at 60 kHz and 1.3 W at 75 kHz in the mid-infrared range.
We show evidence of rapid photo-erasure of ultra stable Type-II fibre Bragg gratings written with a femtosecond laser
into the active medium when employed as high reflectors in >1kW medium power Yb3+-doped Q-switched fibre laser
cavities.
We demonstrate the first operation and preliminary characterization of a dual core double-clad fiber laser where the laser
operates in a coaxially doped thulium outer core region and in-turn resonantly pumps a laser with a holmium doped inner
core. The fiber laser is 790nm pumped producing up to 1.5 W near 2.1μm.
A distributed feedback laser was fabricated in Er3+-doped photonic crystal fibre. Single mode lasing is obtained with
<10kHz linewidth. The output was amplified to 12mW using a commercial erbium doped fibre amplifier (EDFA).
KEYWORDS: Silica, Fiber lasers, Ions, Absorption, Short wave infrared radiation, Semiconductor lasers, Diodes, High power lasers, Objectives, Laser development
I review our recent work in the area of high power 2 μm silica fibre laser development particularly in the area of highly
efficient Tm3+-doped silica and Ho3+-doped silica fibre lasers that are excited with diode lasers operating at 1150 nm.
A well-known side-effect from fibre Bragg grating UV-fabrication is short wavelength attenuation, where irradiation
with laser light, usually in the UV, generates both defect-induced absorption and scattering. These losses are especially
problematic for high power optical fibre lasers operating at shorter wavelengths where resonant assisted coupling into the
glass matrix through the rare earth ions can take place (e.g. Yb3+). In this, work we present a study of the relative
magnitude of short wavelength attenuation in gratings written by the point-by-point method using a Ti-sapphire
femtosecond laser operating at 800 nm. Such gratings are very stable and have been used as the feedback elements in
fibre lasers with powers exceeding 100 W. We show that the scattering properties responsible for the attenuation are
analogous to those associated with type II gratings written with UV lasers.
I review our recent work in the area of high power 2 μm silica fibre laser development particularly in the area of highly
efficient Tm3+-doped silica and Ho3+-doped silica fibre lasers that are excited with diode lasers operating at 1150 nm.
We review our recent work in the area of high power 2 &mgr;m and 3 &mgr;m fiber laser development. We present the recent
results of our highly efficient directly diode-pumped Tm3+-doped silica and Ho3+-doped silica fiber lasers, passively
switched 2&mgr;m fiber lasers, and discrete wavelength generation. We also present results from our recent Raman fiber
laser experiments using chalcogenide fibers.
Wavelengths around 1.15 μm, 1.3 μm and 1.7 μm can be used to pump Dy-doped ZBLAN fibre in order to generate ~3
μm with high efficiency. Previously the generation of 2.9 μm from the Dy-ZBLAN fibre was demonstrated by pumping
with 1.1 μm Yb-silica fibre laser sources. The laser slope efficiency and lasing threshold demonstrated was about ~5%
and ~1.78 W. In this investigation, the longer wavelength absorption band (6H9/2 , 6F11/2) centred at 1.3 μm of Dy3+-doped
ZBLAN is utilised and the lasing transition around ~3 μm takes places from 6H13/2 → 6H15/2. With this pumping scheme
the Stokes' efficiency is expected to be up to ~45%. A quasi-continuous wave Dy3+-ZBLAN fibre laser pumped by a
~1.3 μm Nd:YAG laser and operating at 2.96 μm with a bandwidth (FWHM) of ~14 nm has been demonstrated. For a
60cm fibre length, a threshold of 0.5W and a slope efficiency of ~20% with respect to the absorbed pump power was
observed. The overall pump absorption in the fibre was around 84%. The cavity reflectivities at 2.9 μm were 99% and
50%. The demonstrated slope efficiency was 45% of the Stokes' limit. The slope efficiency was around four times
higher and the threshold around 3.6 times lower than the previous performance demonstrated by using the 1.1 μm Yb
fibre laser pumping scheme. The higher performance achieved compared to the 1.1 μm pump scheme is due to the higher
Stokes' limit, lower pump ESA losses and higher cavity reflectivity. About 590 cm-1 Raman Stokes shift has also
detected by using 514.5 nm and 488 nm Ar ion laser as excitation pump sources.
We report power-scaling of an ytterbium-sensitized thulium-doped silica fiber laser generating up to 75 W of output power in the 2 μm wavelength range when cladding-pumped by a 975 nm diode stack. The slope efficiency is 32% with respect to launched pump power and the beam quality factor (M2) is 1.3. We also investigate the characteristics of this fiber in a tunable laser configuration, operating at ~10 W of output power with the tuning range extended from 2000 to 2080 nm at a launched pump power of 40 W.
Using Tm3+-doped double-clad silica fibre we have produced high power, high efficiency 2μm lasers. To date we have achieved a 59% slope efficiency relative to launched pump power using single end pumping and double passing the pump light. By pumping the fibre laser from both ends, we achieved up to 118W peak output power with 54% slope efficiency relative to launched power at 25% duty cycle. The quantum efficiency of this laser was 120% relative to launched pump power, which we attribute to a cross-relaxation process in Tm3+ (3H4,3H6→3F4,3F4). We have also demonstrated fixed wavelength operation of the laser near 1.9μm by using fibre Bragg gratings.
Recent progress in fiber laser technology has provided lasers operating at mid-infrared wavelengths, at power levels and temporal regimes previously inaccessible to bulk crystal lasers. This paper reports on the development of new continuous wave fiber lasers; a Tm-silica fiber laser operating at 1.98 micrometers , an Er,Pr:ZBLAN fiber laser at 2.78 micrometers and a Yb:Er-silica fiber laser at 1.5 micrometers , and pulsed fiber laser sources; gain-switched and Q-switched Tm- silica systems, specifically targeting applications in medicine. The first studies on the interaction of high- power, continuous wave mid-infrared fiber laser light and soft biological tissues are presented, demonstrating the ability of 2 and 3 micrometers fiber lasers to remove soft tissue cleanly, and with minimum collateral damage.
We describe the results of preliminary studies on the interaction of continuous wave (cw), mid-infrared fiber laser light and soft biological tissues. An Er, Pr:ZBLAN fiber laser operating at a wavelength of 2.71 micrometer was used at 800 mW output power, a Tm-doped silica fiber laser at 1.98 micrometer provided up to 5 W output power, and a Yb:Er-doped silica fiber laser at 1.5 micrometer was used at 800 mW output power. Surface changes in tissue samples are described qualitatively and quantitatively, ablation velocity in tissue is measured, where observed, at the 800 mW power level, and sample sections are described with reference to histologically recognizable markers of thermal damage. The basic science described prepares the way toward ultimate clinical evaluations and applications of these new laser sources.
We present results from a theoretical model that has been developed to simulate the 3-micrometer laser transition in Er3+ doped Y3Al5O12 (YAG), Y2Sc2Ga3O12 (YSGG), LiYF4 (YLF) and BaY2F8 (BaYF) host crystals. The rate equations for the lowest seven energy levels of Er3+ were solved numerically and laser action was simulated under cw, gain-switched (pulse pumped) and Q-switched operation with optical pumping at wavelengths of 975 nm and 795 nm. The relative performance of each laser crystal was compared under identical pumping and cavity conditions to establish the optimum crystal host, doping concentration and pump wavelength for each mode of operation. Some unexpected saturation effects were investigated that could limit the maximum practical pump fluence used for high energy Q-switched systems. We investigate possible additional multi-ion energy transfer processes that may cause the decrease in efficiency that is observed experimentally at high Er3+ ion concentrations. In addition, lower laser level deactivation by co-doping with Pr3+ in BaYF was simulated and compared with singly doped Er:BaYF for a range of Er3+ and Pr3+ concentrations. It was found that co-doping was not as effective as the cooperative upconversion process present in singly doped Er3+ crystals for efficient laser operation.
Theoretical modeling of high-power thulium-sensitized holmium fluorozirconate (Tm:Ho:ZBLAN) glass fiber lasers operating on the 2 micrometer transition in holmium is presented. High output power in single-mode can be achieved by pumping the inner cladding of the double clad fiber with a single-mode core with high-power multimode cw diode lasers. The possibility of higher output powers with the double-clad fiber laser (or DCFL) arrangement over the core-pumped fiber laser means that these particular lasers are well suited to a number of medical applications which require output of high power and good beam quality. In a preliminary step to characterize and optimize these devices, a computer model has been developed to analyze the performance of the device when the power coupling coefficients and doping concentrations are varied. It is shown that the form and hence values of the power coupling coefficients which relate to the transfer of power to and from the ensemble of nonabsorbing modes to the ensemble absorbing modes which propagate within the inner cladding and core of the fiber respectively are paramount to the operation of the DCFL. In addition, for a specific doping level the choice of pump wavelength can be made on the grounds of existing core-pumped fiber laser technology.
Theoretical modeling of high-power thulium-sensitized holmium fluorozirconate glass (Tm:Ho:ZBLAN) fiber lasers operating on the 2 micrometer transition in holmium is presented. High output power in single-mode is achieved by pumping the inner cladding of double clad fiber with single-mode core with high-power multimode cw diode lasers. The higher output powers possible with the double-cladded fiber laser (or DCFL) over the core- pumped fiber laser means that these particular lasers are suited to a number of medical applications which require output of high power and low divergence. In a preliminary step to optimize these devices, a computer model has been developed to analyze the performance of the device when the length of the fiber configuration and doping are varied. It is shown that the form of the coupling coefficient which relates the transfer of power from nonabsorbing modes to absorbing modes which propagate within the inner cladding and core of the fiber is paramount to the operation of the DCFL.
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