Highly stable and efficient erbium-doped 2.8 microm all fiber laser

4.8-μm CO-filled hollow-core silica fiber light source

Mid-infrared (MIR) fiber lasers are important for a wide range of applications in sensing, spectroscopy, imaging, defense, and security. Some progress has been made in the research of MIR fiber lasers based on soft glass fibers, however, the emission range of rare-earth ions and the robustness of the host materials are still a major challenge for MIR fiber lasers. The large number of gases provide a variety of optical transitions in the MIR band. When combined with recent advances in low-loss hollow-core fiber (HCF), there is a great opportunity for gas-filled fiber lasers to further extend the radiation to the MIR region. Here, a 4.8-μm CO-filled silica-based HCF laser is reported for the first time. This is enabled by an in-house manufactured broadband low-loss HCF with a measured loss of 1.81 dB/m at 4.8 μm. A maximum MIR output power of 46 mW and a tuning range of 180 nm (from 4644 to 4824 nm) are obtained by using an advanced 2.33-μm narrow-linewidth fiber laser. This demonstration represents the longest-wavelength silica-based fiber laser to date, while the absorption loss of bulk silica at 4824 nm is up to 13, 000 dB/m. Further wavelength expansion could be achieved by changing the pump absorption line and optimizing the laser structure.

Growth, Spectroscopy, and Laser Performance of a 2.79 μm Er: YSGG Single Crystal Fibers

Single crystal fibers combine the great specific surface area of fibers and the single crystal property of the bulk crystal which shows great potential for a high-power laser. For an Er-doped crystal, due to the fluorescence quenching at the 3 μm wavelength, high Er doping is necessary to increase the fluorescent up-conversion for the breaking limitation. However, a high Er doping concentration must lead to high heat accumulation, resulting in poor laser performance. Compared with an Er-doped bulk crystal, Er-doped SCF has the great potential to remove the heat in the crystal, and it is easy to obtain a high power. In this paper, Er: Y3Sc2Ga3O12 (Er: YSGG) single crystals were successfully grown using the micro-pulling-down method (μ-PD). Owing to the stably grown interface, the diameter of the crystal is 2 mm with a length up to 80 mm. Then, the measurements of Laue spots and Er3+ distribution indicated that our crystals have a high quality. Based on the as-prepared Er: YSGG SCF, the continuous-wave (CW) laser operations at 2794 nm were realized. The maximum output was 166 mW with a slope efficiency of up to 10.99%. These results show that Er: YSGG SCF is a suitable material for future high-power 3 μm laser operation.

Efficient 2075-nm laser emission from Ho3+-doped fluorotellurite glass in a compact all-fiber structure

In this Letter, we report an Ho³⁺-doped fluorotellurite glass all-fiber laser at 2075 nm. The gain fiber is pumped in-band with a 1976-nm fiber laser and connected by fusion splicing. A high-quality fusion splicing point with a loss of < 0.1 dB was obtained by finely adjusting the splicing power and offset. In addition, by optimizing the writing parameters, a third-order fiber Bragg grating (FBG) with a reflectivity of 98% was achieved at 2075 nm using the femtosecond laser direct-writing method. Using the FBG as the laser cavity mirror and a relatively short 28-cm-long home-made Ho³⁺-doped fluorotellurite fiber as the laser medium, a laser with a maximum unsaturated output power of 7.33 W was obtained, and the corresponding slope efficiency was as high as 93.4%. The first, to the best of our knowledge, demonstration of the fluorotellurite glass all-fiber ∼2.1-µm laser presented in this work may pave the way for a high-power 2.1-µm fiber laser with a compact structure.

Mid-infrared laser operation of Er3+-doped BaF2 and (Sr,Ba)F2 crystals

We report on the first, to our knowledge, mid-infrared laser operation of two Er³⁺-doped barium-containing fluorite-type crystals, BaF₂ and (Sr,Ba)F₂, featuring a low-phonon energy behavior. A continuous wave 4.9 at.% Er:(Sr,Ba)F₂ laser generated 519 mW at 2.79 µm with a slope efficiency of 25.0% and a laser threshold of 27 mW. The vibronic and spectroscopic properties of these crystals are determined. The phonon energy of (Sr,Ba)F₂ is as low as 267 cm^-1. The Er³⁺ ions in this crystal feature a broadband emission owing to the ⁴I_11/2 → ⁴I_13/2 transition and a long luminescence lifetime of the ⁴I_11/2 level (10.6 ms) making this compound promising for low-threshold, broadly tunable, and pulsed 2.8-µm lasers.

Erbium-doped ZBLAN fiber laser pumped at 1.7 µm for emission at 2.8 µm

In this paper, a novel, to the best of our knowledge, efficient pump scheme for an erbium-doped fluoride fiber laser with emission at 2.8 µm in the mid-infrared region is proposed and demonstrated. A singular pump source at 1.7 µm is used to excite Er³⁺ ions from ground state ⁴I_15/2 to lower laser level ⁴I_13/2, and then further boost the ions to ⁴I_9/2, where a non-radiation transition occurs for the Er³⁺ ions to reach upper laser level ⁴I_11/2. This scheme can efficiently recycle ions on the lower laser level ⁴I_13/2 by excited-state absorption, therefore realizing population inversion and enhancing laser efficiency. In our demonstration, a 660-mW laser output at 2.8 µm is achieved from a 1.7-µm core-pumped erbium-doped fluoride fiber laser, where the slope efficiency versus launched pump power is 23.7%. The proposed innovative pump scheme shows great potential to realize high-power, high-efficiency erbium-doped fiber lasers at 2.8 µm.

Viscosity of fluoride glass fibers for fused component fabrication

Fluoride glasses show great promise for mid-IR fiber-based applications. Their brittleness and low glass transition temperature have thus far been obstacles towards obtaining low-loss fused components. Here, we suggest a simple method to measure glass viscosity over a range of process temperatures of interest for fused coupler fabrication. We achieved tapers of inverse taper ratio (ITR) 0.12 in multimode fluoroindate fibers. Tapers with loss <0.1dB at ITR 0.3 and no visible defects were fabricated with high repeatability. This work paves the way towards low-loss fused optical couplers in fluoride glass fiber.

Direct femtosecond laser inscription of an IR fluorotellurite fiber Bragg grating

This study proposes a novel, to the best of our knowledge, development of fluorotellurite glass fiber Bragg gratings (FBGs). Shell-like morphology was achieved using a single femtosecond laser pulse illuminated through the fiber's polymer coating. Different FBG fabrication methods and parameters were systematically studied to optimize performance. The fluorotellurite FBG exhibited a high sensitivity to writing laser power and reflectivity saturation effect in repetitive writing. A low-insertion-loss fluorotellurite FBG with a reflectivity of over 99% and bandwidth of less than 1 nm was successfully inscribed. The flexible inscription methods can write an FBG at any wavelength in the fluorotellurite glass transparent window, and are applicable to infrared fiber lasers or sensors.

Optimized laser-written ZBLAN fiber Bragg gratings with high reflectivity and low loss

We report the direct femtosecond laser inscription of type-I fiber Bragg gratings (FBGs) into the core of soft-glass ZBLAN fibers. We investigate and compare various fabrication methods such as single pass (line by line), double pass, and stacking (plane by plane) to create the highest reflectivity FBGs (99.98%) for mid-infrared (mid-IR) applications. In addition, we experimentally demonstrate how the parameters that influence the coupling coefficient, i.e., refractive index modulation and overlap factor, can be controlled in these gratings to specifically tailor the FBG properties. The performance of the direct-written type-I gratings after 6 h of annealing is further analyzed, and the reflectivity increases by approximately 10 dB. To the best of our knowledge, this is the first demonstration of temperature-stable mid-IR FBGs with highest coupling coefficient (464  m^-1) and lowest loss (<0.5  dB/cm) without the use of an expensive phase mask.

Plane-by-plane femtosecond laser inscription of first-order fiber Bragg gratings in fluoride glass fiber for in situ monitoring of lasing evolution

We report the femtosecond laser inscription of fiber Bragg gratings (FBGs) in an Er-doped fluoride glass fiber used for lasing at a mid-infrared wavelength of 2.8 µm. The lasing evolution is discussed in terms of the FBG reflectivity, wavelength transition to the Bragg wavelength, and output power of the mid-infrared fiber laser. A first-order and short (2.5-mm-long) Bragg grating showed a reflectivity of 97%, because of a laser-induced index modulation of 1.1 × 10^-3. This modulation was sufficient to saturate this system's output power. The laser oscillator is designed to lase in the atmospheric window of 2799-2800 nm slope. Further, this oscillator's efficiency is as high as 29.1% for the launched pump power over the range of 0.4-4.6 W and at a lasing wavelength of 2799.7 nm. This oscillator also exhibited a FWHM bandwidth of 0.12 nm.

Direct inscription of Bragg gratings into coated fluoride fibers for widely tunable and robust mid-infrared lasers

We report the development of a widely tunable all-fiber mid-infrared laser system based on a mechanically robust fiber Bragg grating (FBG) which was inscribed through the polymer coating of a Ho³⁺-Pr³⁺ co-doped double clad ZBLAN fluoride fiber by focusing femtosecond laser pulses into the core of the fiber without the use of a phase mask. By applying mechanical tension and compression to the FBG while pumping the fiber with an 1150 nm laser diode, a continuous wave (CW) all-fiber laser with a tuning range of 37 nm, centered at 2870 nm, was demonstrated with up to 0.29 W output power. These results pave the way for the realization of compact and robust mid-infrared fiber laser systems for real-world applications in spectroscopy and medicine.

Flexible femtosecond inscription of fiber Bragg gratings by an optimized deformable mirror

The period of fiber Bragg gratings is adapted by shaping the wavefronts of ultrashort laser pulses applied in a phase mask inscription technique. A specially designed deformable mirror, based on a dielectric substrate to withstand high peak powers, is utilized to deform the wavefront. A shift of about 11 nm is demonstrated for a Bragg wavelength around 1550 nm.

Fiber Bragg grating stabilization of a passively mode-locked 2.8 μm Er³⁺: fluoride glass fiber laser

We report the first stable mode-locking from an Er3+: fluoride glass fiber laser linear cavity operating near 3 μm to the best of our knowledge. The linear cavity includes a saturable absorber mirror and a fiber Bragg grating to provide a controlled and wavelength selective feedback. The pulse train has a 51.75 MHz repetition rate, an estimated 60 ps pulse duration, and an average power of 440 mW. The stable and self-starting mode-locking regime is confirmed by RF spectral measurements and is maintained over several hours.

Highly Er(3+)-doped ZrF4-based fluoride glasses for 2.7 μm laser materials

A new type of fluoride glasses with high erbium-doping concentration (up to 6 mol. % Er(3+)) is investigated. The intensive 2.7 μm fluorescence is demonstrated with minimized concentration quenching. The intensity parameters and radiative properties are determined from the absorption spectrum based on the Judd-Ofelt theory. The prepared Er(3+)-doped ZBYA glass possesses high predicted spontaneous-transition probability (28.92 s(-1)) and large calculated emission cross section (9.8×10(-21) cm(2)). All these results indicate that this Er(3+)-doped ZrF(4)-based fluoride glass has potential applications in 2.7 μm laser materials.

Understanding the fiber tip thermal runaway present in 3 µm fluoride glass fiber lasers

When the tip of a fluoride glass fiber is exposed to ambient air, water vapor reacts with the glass constituents, increasing the OH contaminants at the surface. These OH impurities then diffuse inside the glass according to Fick's laws. Laser radiation at around 3 µm is strongly absorbed by the OH contaminants, causing local heating of the fiber tip resulting in an increase of the diffusion process which ultimately leads to fiber tip destruction. We accurately model this phenomenon by combining the diffusion theory with a basic thermal equation. Experimental measurements are in agreement with the model predictions for a good range of operating conditions.

Passively cascade-pulsed erbium ZBLAN all-fiber laser

We propose and numerically demonstrate a cascade pulsing mechanism in a CW-pumped Er³⁺:ZBLAN all-fiber laser system. In the design, the laser was pumped at 980 nm and passively Q-switched at 1.6 μm. The Q-switched resonance reduced the population on ⁴/13/2 of the erbium gain fiber, thereby creating a population inversion between the levels of 4/11/2 and ⁴/13/2, and instantly inducing an intense gain-switched pulse at 2.7 μm. Sequential 2.7-μm single-mode pulsing with a pulse energy of 170 μJ and a peak power of 6 kW was achieved with an absorbed pump power of 0.65 W.

12 W Q-switched Er:ZBLAN fiber laser at 2.8 μm

A diode-pumped, actively Q-switched 2.8 μm fiber laser oscillator with an average output power of more than 12 W has been realized through the use of a 35 μm core erbium-doped ZBLAN fiber and an acousto-optic modulator; to our knowledge, this is the first 3 μm pulsed fiber laser in the 10 W class. Pulse energy up to 100 μJ and pulse duration down to 90 ns, corresponding to a peak power of 0.9 kW, were achieved at a repetition rate of 120 kHz.

High-power pulsed diode-pumped Er:ZBLAN fiber laser

We report on the operation and performance of a gain-switched Er:ZBLAN fiber laser based on an active pulsed diode pump system. The produced laser pulses offer high peak powers while retaining the high average powers and efficiency of the cw regime. The measured pulse duration was about 300 ns and nearly independent of the pump repetition frequency. The maximum obtained 68 W of peak power is the highest reported, to our knowledge, for diode-pumped Er:ZBLAN fiber lasers, and the 2 W of average power at the repetition frequency of 100 kHz is 2 orders of magnitude higher than previously reported average power in a pulsed regime. The obtained slope efficiency was 34%.

20 W passively cooled single-mode all-fiber laser at 2.8 μm

A maximum output power of 20.6 W at 2.825 μm from an erbium-doped all-fiber laser is reported, which we believe is the highest output power for this laser transition in single-mode operation. The slope efficiency of the passively cooled laser was up to 35.4% with respect to the absorbed pump power. Accounting for an estimated round-trip intracavity loss of 1.3 dB, we calculated a theoretical conversion efficiency of 39.5%, which is 15% higher than the Stokes efficiency of 34.3%. We believe this is the first experimental confirmation of the predicted pump energy recycling for this fiber laser. The narrow laser linewidth varied from 0.09 to 0.16 nm from low to maximum output power.

Stable 10 W Er:ZBLAN fiber laser operating at 2.71-2.88 μm

We have developed a diode-pumped tunable 3 μm fiber laser with a cw output power of the order of 10 W with the use of an erbium-doped ZBLAN fiber. A tunability range of 110 nm (2770 to 2880 nm) with an output power between 8 and 11 W was demonstrated. As the pump power was increased, the center of the wavelength range was shifted toward longer wavelengths, but the width of the wavelength range was largely unaffected. The total tunability range for various pump power levels was 170 nm (2710 to 2880 nm). To our knowledge, this is the highest performance (output power and tunability) obtained from a tunable 3 μm fiber laser.

Erbium-doped all-fiber laser at 2.94 microm

We report what we believe is the first demonstration of laser emission at 2.94 microm in an erbium-doped fluoride fiber laser. The low-loss all-fiber Fabry-Perot laser cavity was formed by two fiber Bragg gratings of 90% and 15% reflectivities in a 6.6 m, 7 mol.% Er-doped double-clad fiber. A maximum cw output power of 5.2 W was measured, which is to our knowledge the highest reported to date for a diode-pumped laser at this wavelength. A coreless endcap was fused at the output fiber end to prevent its deterioration at high output powers. Our results, including the slope efficiency of 26.6% with respect to launched pump power, suggest that erbium could be a better alternative than holmium in the search for a replacement for the flashlamp-pumped Er:YAG at 2.94 microm.