Erbium-doped Fluoride Research Articles

Fuseless side-pump combiner for efficient fluoride-based double-clad fiber pumping.

We report a novel technique for side-pumping fluoride-based double-clad fibers, allowing a record coupling efficiency of 93% and a maximum power handling near 100 W at 981 nm. Our simple technique is based on wrapping a silica taper around a fluoride fiber and, therefore, does not require any complex fusion between these two dissimilar fibers. Under passive cooling, pump combiners made of undoped and erbium-doped fluoride fibers were successfully operated during several hours at respective incident powers of 91 and 44 W. Heat management issues and active cooling strategies are also discussed. This innovative combiner is a keystone towards the development of compact and robust high-power mid-infrared fiber lasers and amplifiers.

Enhanced mid-infrared emission of non-oxide erbium doped fluorochloride glass

The fluorochloride glasses samples with good luminescent properties in the infrared region were successfully prepared by melt quenching method. The introduction of Cl− effectively improved the mid-infrared (MIR) luminescence of erbium-doped fluoride glass at 2.7 µm nearby. According to Judd-Ofelt theory, the increase in Ω2 after the introduction of Cl− indicates an increase in the asymmetry and covalence of the sample. X-ray diffraction (XRD), absorption spectrum, Raman spectroscopy, infrared Fourier (FTIR) and infrared fluorescence spectroscopy were specifically conducted. The emission cross section (1.04×10−20 cm2) and the absorption cross section (9.68×10−21 cm2) of the sample at 2.7 µm nearby were calculated by measurement. Therefore, non-oxide erbium-doped fluorochloride glass is a good luminescent material in the MIR band.

Broadband optical response of layered nickel ditelluride towards the mid-infrared regime

The broadband nonlinear optical material towards the mid-infrared spectral range is highly needed for civil and military applications. Here, we have investigated the nonlinear optical response of the nickel ditelluride (NiTe2) single crystal prepared by the chemical vapor deposition (CVD) method, and found that the layered NiTe2 exhibits attractive broadband nonlinear optical absorption performance towards the mid-infrared regime. We further explored the nonlinear optical response of the layered NiTe2 in the erbium-doped fluoride fiber laser, and have realized the stable Q-switched fiber laser with a pulse width of 708 ns and a repetition rate of 81 kHz around 2.8 µm wavelength. The experimental results may not only make inroads towards the understanding the nonlinear optical response of the topological materials, but also explore their broadband applications in mid-infrared photonics.

High infrared light yield of Erbium-doped fluoride crystals

The emission spectrum of Erbium-doped fluoride crystals excited by soft X-rays in the (400–1700) nm wavelength range is investigated between 3.2 K and 300 K. The largest component of the crystals emission is observed in the infrared range, which corresponds to a light yield of several tens of photons/keV. Such a large value is caused by energy transfer processes between the Erbium ions which convert the excitation of a dopant ion at high energies into the emission of multiple infrared photons. The present results show that this mechanism could be exploited to realize scintillators of high light yield.

Au nanocages saturable absorber for 3-µm mid-infrared pulsed fiber laser with a wide wavelength tuning range.

Au nanocages (Au-NCs) have attracted wide attention as low-dimensional materials with broadband absorption, ultrafast optical response, large third-order optical nonlinearity coefficient, and high photothermal stability and thermal tolerance. By employing Au-NCs as saturable absorbers, we demonstrate a widely tunable passively Q-switched erbium-doped fluoride fiber laser at the wavelength of 2.8 µm. When operates at 2778.0 nm, this laser delivers stable Q-switched pulses with a maximum average power of 584.6 mW at a pulse repetition rate of 80.6 kHz. The minimum pulse duration attained was 1.16 µs corresponding with the single pulse energy of 7.25 µJ. Our results present onefold increase in pulse energy over previously published values achieved from Au nanoparticles based 3-µm passively Q-switched fiber lasers. By introducing a plane ruled grating, a tuning rage of 57.0 nm from 2753.0 to 2810.0 nm is achieved, while maintaining stable Q-switched operation. To our knowledge, this is the first time to demonstrate that Au-NCs can realize mid-infrared pulsed laser. Our research results show that Au-NCs are promising broadband nonlinear modulators for mid-infrared pulse generation.

Stable 35-W Er: ZBLAN fiber laser with CaF2 end caps

We successfully demonstrate a 2838 nm erbium-doped fluoride fiber laser with an average output power of 35 W, and record an hour of continuous operation at 30 W. The temporal and spectral stability of the 2.8 μm laser represent a significant improvement over previous work. The success of this work can be largely attributed to the development of component technologies such as fiber Bragg grating fabrication and fine fusion splicing of fiber/fiber and a fiber endcap.

Theoretical simulations of the soliton self-frequency shift of mid-infrared femtosecond pulses in step-index tellurite optical fibers: broadband tunability and high efficiency

Broadband tunable ultrafast lasers using mid-infrared (MIR) fibers operating at a 3-5 µm atmospheric transmission window are attractive sources because of their numerous applications. Tellurite fibers possess the merits of large linear and nonlinear refractive indices, sufficient chemical stability, and wide transparency range up to ∼5 µm; also, they are highly suitable for high efficiency MIR ultrafast fiber laser sources based on soliton self-frequency shift (SSFS). We numerically simulate SSFS of MIR femtosecond pulses in step-index tellurite optical fibers. A femtosecond erbium-doped fluoride fiber laser at 3.5 µm is employed as the pump source. Parameters including the peak power of the input pulse and nonlinear fiber length are optimized for high efficiency broadband tunable MIR ultrafast laser performance. Our results show that a high-efficiency 3.5-6 µm wavelength-tunable femtosecond laser can be realized by employing SSFS in a 22-cm-long segment of tellurite step-index fiber pumped by femtosecond pulses with 10-300 kW peak powers at 3.5 µm. Ultra-high energy ratios of the most redshifted solitons to the input pulses of >50% are obtained across the 3.5-5 µm tuning range. The presented numerical study provides valuable guidance for SSFS of MIR femtosecond pulses in step-index tellurite fibers and is valuable for future high efficiency wavelength-tunable MIR ultrafast fiber laser development.

Dual-wavelength erbium-doped fluoride fiber laser.

The laser source with 3μm/2μm output wavelength has many application prospects in clinical medicine, photoelectric countermeasure, and scientific research measurement. An Er3+ doped ZBLAN fiber laser with output wavelength of 2.8μm and 1.6μm is experimentally studied. By setting the pump power to 5W, a continuous dual-wavelength output with a central wavelength of 2.803μm and 1.61μm is obtained and the corresponding maximum output power is 362.4mW and 108.6mW. The slope efficiency is 12.1% and 4.94% respectively. What's more, the slope efficiency is 12.1% and 4.94% respectively, and the fluctuation rates of peak power of the two wavelengths are 9.7% and 2.1% within 4h which indicate that the laser has relatively good stability.

3.42 µm lasing in heavily-erbium-doped fluoride fibers.

In this paper, we investigate laser emission at 3.4μm in heavily-erbium-doped fluoride fibers using dual-wavelength pumping. To this extent, a monolithic 7 mol% erbium-doped fluoride fiber laser bounded by intracore fiber Bragg gratings at 3.42 μm is used to demonstrate a record efficiency of 38.6 % with respect to the 1976 nm pump. Through numerical modeling, we show that similar laser performances at 3.4 μm can be expected in fluoride fibers with erbium concentrations ranging between 1 - 7 mol%, although power scaling should rely on lightly-doped fibers to mitigate the heat load. Moreover, this work studies transverse mode-beating of the 1976 nm core pump and its role in the generation of a periodic luminescent grating and in the trapping of excitation in the metastable energy levels of the erbium system. Finally, we also report on the bistability of the 3.42 μm output power of the 7 mol% erbium-doped fluoride fiber laser.

Gold nanostars as a Q-switcher for the mid-infrared erbium-doped fluoride fiber laser.

We demonstrated passively Q-switched mid-infrared (mid-IR) erbium-doped fiber lasers by using gold nanostars (GNSs) as the Q-switcher. The nonlinear optical responses of the GNSs synthesized via the seed-mediated method have been characterized via a Z-scan technique, and the modulation depth and saturation intensity of the GNSs are measured to be 25% and 15.75 kW/cm2, respectively. The Q-switched fiber laser can deliver a maximum average power of 454mW with corresponding pulse energy of 3.6μJ and pulse duration of 536ns at a repetition rate of 125kHz under the incident pump power 3.5W. To the best of our knowledge, this is the first Letter that reports that the GNSs can act as a Q-switcher for the mid-IR erbium-doped ZBLAN fiber lasers. This Letter can deepen the understanding of the nonlinear optical behavior of the gold nanomaterials and may make inroads for the excellent mid-infrared optoelectronic devices.

Towards power scaling of 28 μm fiber lasers

We report the demonstration of a 2824nm passively cooled erbium-doped fluoride fiber laser delivering a record average output power of 41.6W in continuous-wave operation. The splice-less cavity is based on intra-core fiber Bragg gratings written directly in the active erbium-doped fluoride fiber, which is bidirectionally pumped at 980nm to reduce heat load. To the best of our knowledge, this result is the highest average output power achieved with a mid-infrared fiber laser. The long-term performance of different protective endcaps is also investigated at high-power operation.

10 W-level gain-switched all-fiber laser at 2.8 μm.

We report a simply designed gain-switched all-fiber laser emitting a maximum average output power of 11.2W at 2.826µm. The corresponding extracted pulse energy is 80µJ at a pulse duration of 170ns. These performances significantly surpass previous gain-switched demonstrations and are close to the state-of-the-art Q-switched laser performances near 2.8µm, but with a much simpler and robust all-fiber design. The spliceless laser cavity is made of a heavily erbium-doped fluoride glass fiber and is bounded by fiber Bragg gratings written directly in the gain fiber through the protective polymer coating.

Mid-infrared supercontinuum generation in fluoride fiber amplifiers: current status and future perspectives

The quest for a compact and efficient broadband laser source able to probe the numerous fundamental molecular absorption lines in the mid-infrared (3–8 µm) for various applications has been going on for more than a decade. While robust commercial fiber-based supercontinuum (SC) systems have started to appear on the market, they still exhibit poor energy conversion into the mid-infrared (typically under 30%) and are generally not producing wavelengths exceeding 4.7 µm. Here, we present an overview of the results obtained from a novel approach to SC generation based on spectral broadening inside of an erbium-doped fluoride fiber amplifier seeded directly at 2.8 µm, allowing mid-infrared conversion efficiencies reaching up to 95% and spectral coverage approaching the transparency limit of ZrF4 (4.2 µm) and InF3 (5.5 µm) fibers. The general concept of the approach and the physical mechanisms involved are presented alongside the various configurations of the system to adjust the output characteristics in terms of spectral coverage and output power for different applications.

L-Band Flat-Gain Raman with Erbium-Doped Fluoride Hybrid Optical Amplifier for Superdense Wavelength Division Multiplexing System

To the best of our knowledge, we explore, for the very first time, the performance using Raman-EDFFA hybrid amplifier for super dense multiplexing system. We evaluate the system in terms of flat gain, noise figure, and good rating quality factor for L-band. A highest flat gain of 26.01 dB is observed over a wavelength range of 1,578 to 1,640 nm with a smaller variation of 1.5 dB without using any cost influence techniques. Further, we also observe that an input power level of −25 dB is quite suitable for acceptable amplification.

High average power and energy microsecond pulse generation from an erbium-doped fluoride fiber MOPA system.

We reported a high average power and energy microsecond pulse erbium-doped fluoride fiber MOPA system centered at 2786.8 nm. The master oscillator was a passively Q-switched erbium-doped fluoride fiber laser based on SESAM in a linear cavity. Then a one-stage erbium-doped fluoride fiber amplifier was used to boost its average output power to 4.2 W and pulse energy to 58.87 μJ. The pulse duration and repetition rate were 2.29 µs and 71.73 kHz, respectively. To the best of our knowledge, the achieved average output power and pulse energy are the recorded levels for the passively Q-switched fiber lasers at 3 μm wavelength region.

High-power mid-infrared femtosecond fiber laser in the water vapor transmission window

The recent demonstrations of ultrafast mid-infrared fiber lasers emitting sub-picosecond pulses at 2.8 μm have created an exciting potential for a range of applications including mid-infrared frequency combs and materials processing. So far, this new class of laser has been based on the I11/24-I13/24 transition in erbium-doped fluoride fibers, which lies directly in a region of high water vapor absorption. This absorption has limited the achievable bandwidth, pulse duration, and peak power and poses a serious problem for transmission in free space. In this Letter, we present an ultrafast mid-infrared fiber laser that overcomes these limitations by using holmium as the gain medium. Holmium allows the central emission wavelength to shift to nearly 2.9 μm and avoid the strong water vapor lines. This laser, which represents the longest wavelength mode-locked fiber laser, emits 7.6 nJ pulses at 180 fs duration, with a record peak power of 37 kW. At this power level, the laser surpasses many commercial free-space OPA systems and becomes attractive for laser surgery of human tissue, for industrial materials modification, and for driving broadband coherent supercontinuum in the mid-infrared.

Watt-level fiber-based femtosecond laser source tunable from 2.8 to 3.6 μm.

The development of compact and reliable ultrafast sources operating in the mid-infrared region could lead to major advances in both fundamental and applied sciences. In this Letter, we report on a simple and efficient laser system based entirely on erbium-doped fluoride glass fibers that generates high-energy Raman soliton pulses tunable from 2.8 to 3.6 μm at a high average output power. Stable 160 fs pulses at 3.4 μm with a maximum energy of 37 nJ, a corresponding average output power above 2 W, and an estimated peak power above 200 kW are demonstrated. This tunable source promises direct applications in laser processing of polymers and biological materials.

Mid-IR supercontinuum from 2.4 to 5.4 μm in a low-loss fluoroindate fiber.

A mid-infrared supercontinuum extending up to 5.4 μm is generated in a low-loss fluoroindate fiber. It is pumped with an erbium-doped fluoride fiber amplifier seeded with 400 ps pulses at 2.75 μm. Both fibers are fusion spliced to increase the robustness and long-term stability of the system. With more than 82% of the total power beyond 3 μm, this approach is promising for efficient mid-IR light generation.

Millisecond Photon Lifetime in a Slow-Light Microcavity.

Optical microcavities with ultralong photon storage times are of central importance for integrated nanophotonics. To date, record quality (Q) factors up to 10^{11} have been measured in millimetric-size single-crystal whispering-gallery-mode (WGM) resonators, and 10^{10} in silica or glass microresonators. We show that, by introducing slow-light effects in an active WGM microresonator, it is possible to enhance the photon lifetime by several orders of magnitude, thus circumventing both fabrication imperfections and residual absorption. The slow-light effect is obtained from coherent population oscillations in an erbium-doped fluoride glass microsphere, producing strong dispersion of the WGM (group index n_{g}∼10^{6}). As a result, a photon lifetime up to 2.5ms at room temperature has been measured, corresponding to a Q factor of 3×10^{12} at 1530nm. This system could yield a new type of optical memory microarray with ultralong storage times.

High Power Cascaded Erbium Doped Fluoride Fiber Laser at Room Temperature

We demonstrate a high power diode-cladding-pumped cascaded erbium doped fluoride fiber laser at room temperature with $^{4}\text{I}_{\mathrm {11/2}}\to ^{4}\text{I}_{\mathrm {13/2}}$ transition at $\sim 2.8~\mu \text{m}$ and $^{4}\text{I}_{\mathrm {13/2}}\to ^{4}\text{I}_{\mathrm {15/2}}$ transition at $\sim 1.6~\mu \text{m}$ . By simultaneously theoretically optimizing erbium ions dopant concentration and fiber length, maximum output power of 15.2 W centered at 2.885 $\mu \text{m}$ at a slope efficiency of 26.7% was achieved experimentally after the onset of cascaded transition centered at 1.613 $\mu \text{m}$ . Maximum output power and slope efficiency of 1.613 $\mu \text{m}$ cascaded laser were 3.2 W and 7.1%, respectively. In our case, no cooling designs were employed on the fiber owing to the use of lowly erbium doped fiber and cascaded laser cavity that weakened the cascaded multiphonon relaxation processes while improving the optical-to-optical conversion efficiency. To the best of our knowledge, this is the first 10-W level fiber laser at 3- $\mu \text{m}$ waveband without any special active or passive cooling designs.