Mid-infrared tunable filter based on a femtosecond-written silica volume Bragg grating.

A silica volume Bragg grating (VBG) is used to filter the light of a mid-infrared (mid-IR) supercontinuum laser. The VBG with a 7 µm period was inscribed with 800 nm pulses with a 100 fs duration and the phase-mask technique over a glass thickness of 3 mm. Despite silica's absorption, the VBG allows obtaining a narrowband light source tunable from 2.9 to 4.2 µm with a full width at half maximum (FWHM) of 29 nm. This demonstrates the great potential of using femtosecond-written VBGs as highly tunable, yet selective, spectral filters in the mid-IR.

High-efficiency radiation-balanced Yb-doped silica fiber laser with 200-mW output.

The focus of this study was the development of a second generation of fiber lasers internally cooled by anti-Stokes fluorescence. The laser consisted of a length of a single-mode fiber spliced to fiber Bragg gratings to form the optical resonator. The fiber was single-moded at the pump (1040 nm) and signal (1064 nm) wavelengths. Its core was heavily doped with Yb, in the initial form of CaF2 nanoparticles, and co-doped with Al to reduce quenching and improve the cooling efficiency. After optimizing the fiber length (4.1 m) and output-coupler reflectivity (3.3%), the fiber laser exhibited a threshold of 160 mW, an optical efficiency of 56.8%, and a radiation-balanced output power (no net heat generation) of 192 mW. On all three metrics, this performance is significantly better than the only previously reported radiation-balanced fiber laser, which is even more meaningful given that the small size of the single-mode fiber core (7.8-µm diameter). At the maximum output power (∼2 W), the average fiber temperature was still barely above room temperature (428 mK). This work demonstrates that with anti-Stokes pumping, it is possible to induce significant gain and energy storage in a small-core Yb-doped fiber while keeping the fiber cool.

Towards real-time active imaging of greenhouse gases using tunable mid-infrared all-fiber lasers.

We report a tunable all-fiber laser emitting a maximum output power of 2.55 W around 3240 nm. The fiber laser cavity based on a fluoride fiber doped with dysprosium ions yields an efficiency of 42% according to the in-band launched pump power at 2825 nm. Due to a custom piezoelectric fiber Bragg grating (FBG) package, mechanical strains applied to the narrowband FBG used as the input cavity coupler allowed for fast tuning of the emission wavelength over a spectral range of 1.5 nm. This laser was deployed in the field in northern Québec (Canada) to assess its performances for remote sensing of methane in the presence of a significant amount of water vapor, i.e., over a hydroelectric reservoir. The preliminary results acquired during this field campaign confirm the great potential of the proposed approach for the development of a real-time active imaging system of greenhouse gases.

Demonstration of Type A volume Bragg gratings inscribed with a femtosecond Gaussian-Bessel laser beam.

To our knowledge, we report on the first demonstration of Type A VBGs inscribed in silver-containing phosphate glasses by femtosecond laser writing. The gratings are inscribed plane-by-plane by scanning the voxel of a 1030 nm Gaussian-Bessel inscription beam. This results in a refractive-index modification zone, induced by the appearance of silver clusters, extending over a much larger depth than those obtained with standard Gaussian beams. As a result, a high diffraction efficiency of 95% at 632.8 nm is demonstrated for a 2-µm period transmission grating with a 150-µm effective thickness indicating a strong refractive-index modulation of 1.78 × 10-3. Meanwhile, a refractive-index modulation of 1.37 × 10-3 was observed at a wavelength of 1.55 µm. Thus, this work opens the avenue for highly effective femtosecond-written VBGs suitable for industrial applications.

Femtosecond writing of intra-phase-mask volume Bragg gratings.

In this Letter, we report the first, to the best of our knowledge, femtosecond inscription of volume Bragg gratings (VBGs) directly inside phase-mask substrates. This approach showcases enhanced robustness as both the interference pattern generated by the phase mask and the writing medium are inherently bonded together. The technique is employed with 266-nm femtosecond pulses loosely focused by a 400-mm focal length cylindrical mirror inside fused-silica and fused-quartz phase-mask samples. Such a long focal length reduces the aberrations induced by the refractive-index mismatch at the air/glass interface which allows to inscribe a refractive-index modulation simultaneously over a glass depth reaching 1.5 mm. A decreasing gradient of the modulation amplitude from 5.9 × 10-4 at the surface to 1 × 10-5 at a 1.5-mm depth is observed. This technique has therefore the potential of increasing significantly the inscription depth of femtosecond-written VBGs.

Monolithic silica fiber laser operating at 585 nm.

We report, to the best of our knowledge, the first monolithic silica fiber laser operating in the visible. The laser cavity is based on a dysprosium-doped aluminosilicate fiber bounded by a pair of fiber Bragg gratings operating at 585 nm. The yellow laser signal reaches a record output power of 147 mW. Although the pump irradiation causes photodarkening, significant reduction of the photoinduced absorption losses is demonstrated via a photobleaching process with visible light.

Large area Bragg grating for pump recycling in cladding-pumped multicore erbium-doped fiber amplifiers.

We demonstrate for the first time that a Bragg grating can be written over a large area inside the cladding of a multicore erbium-doped fiber amplifier to increase the power conversion efficiency (PCE) by recycling the output pump power. Our results indicate that a Bragg grating covering ∼25% of the cladding area allows us to recycle 19% of the output pump power which leads to a relative increase of the PCE by 16% for an input pump power of 10.6 W in the specific case of an eight-core erbium-doped fiber with a length of 20.3 m and one core loaded with an input signal power of 1.5 dBm.

Femtosecond inscription of large-area fiber Bragg gratings for high-power cladding pump reflection.

A new, to the best of our knowledge, method for inscribing fiber Bragg gratings inside a fiber's cladding based on the motorized rotation of the fiber is reported. By minimizing the aberrations induced by the fiber curvature on the femtosecond writing beam, this technique based on a phase mask allows to cover large transverse areas of a standard high-power fiber's cladding. With this approach, a first-order Bragg grating was inscribed in the pure-silica inner cladding of a 20/400-µm fiber. It was then implemented as a pump reflector at the end of a 36-m-long Yb-doped fiber laser reaching 600 W of output power, confirming the power handling capabilities of such a component. Comparison of the laser performances with and without the pump reflector showcases its great potential for increasing pump absorption inside cladding-pumped fiber lasers, which paves the way for significantly reducing their active fiber length.

Recent developments in lanthanide-doped mid-infrared fluoride fiber lasers [Invited].

Mid-infrared fiber sources, emitting between 2.5 µm and 5.0 µm, are interesting for their great potential in several application fields such as material processing, biomedicine, remote sensing and infrared countermeasures due to their high-power, their diffraction-limited beam quality as well as their robust monolithic architecture. In this review, we will focus on the recent progress in continuous wave and pulsed mid-infrared fiber lasers and the components that bring these laser sources closer to a field deployment as well as in industrial systems. Accordingly, we will briefly illustrate the potential of such mid-infrared fiber lasers through a few selected applications.

Wavelength stabilization of high-power laser diodes using Bragg gratings inscribed in their highly multimode fiber pigtails.

Efficient wavelength stabilization of an off-the-shelf high-power laser diode operating at 976 nm is demonstrated by using a highly multimode fiber Bragg grating (FBG). This first-order grating is inscribed with 400 nm femtosecond pulses inside the 200 µm/0.22 NA pure silica core of the diode's fiber pigtail. The FBG reduces the wavelength thermal drift of the 70 W diode by a factor of 12 while also reducing its emission linewidth by a factor of 2.8. At maximum output power, a power penalty of only 6% is measured. This promising approach offers a robust and compact scheme to stabilize the spectrum of high-power laser diodes that are particularly useful for fiber-laser pumping.

15†W monolithic fiber laser at 3.55†µm.

We report a dual-wavelength-pumped all-fiber continuous-wave (CW) laser operating at 3.55 µm that reached an output power of 14.9 W, which is, to the best of our knowledge, a record. The laser cavity, made of an erbium-doped fluoride fiber and bounded by two fiber Bragg gratings (FBGs), operates at an overall optical efficiency of 17.2% and a slope efficiency of 51.3% with respect to the 1976 nm launched pump power. The all-fiber design of the cavity not only allows for significant power scaling of the laser output, but also improves its long-term stability at high output power. The cavity design was set according to a numerical optimization that showed very good agreement with the experimental results.

2.3 W monolithic fiber laser operating in the visible.

We report, to the best of our knowledge, the first monolithic visible fiber laser pumped by a pigtailed diode. The robust cavity design proposed is based on a highly reflective fiber Bragg grating spliced to a double-clad praseodymium-doped fiber. The laser signal generated at 635.5 nm is single-mode, has a FWHM bandwidth of 0.16 nm, and reaches a maximum cw output power of 2.3 W. This demonstration breaks ground for the development of reliable high-power visible fiber lasers.

Femtosecond-written volume Bragg gratings in fluoride glasses.

We report on what we believe are the first volume Bragg gratings written inside bulk multicomponent fluoride glasses. The gratings inscribed with tightly focused infrared (IR)-femtosecond pulses in combination with the phase-mask technique exhibit refractive index modulations of up to 5×10-4 with reflectivities up to 90% at a wavelength near 2.8 µm. Such highly compact and narrowband filters could have a significant impact on numerous high-end applications from the UV to the mid-IR.

Erbium-doped aluminophosphosilicate all-fiber laser operating at 1584 nm.

We report on an ytterbium-free erbium-doped aluminophosphosilicate all-fiber laser, producing an output power of 25 W at a wavelength of 1584 nm with a slope efficiency of 30% with respect to the 976 nm absorbed pump power. The simple cavity design proposed takes advantage of fiber Bragg gratings written directly in the gain fiber. The single-mode erbium-doped aluminophosphosilicate fiber was fabricated in-house and was doped with 0.06 mol.% of Er2O3, 1.77 mol.% of Al2O3 and 1.04 mol.% of P2O5. The incorporation of aluminium and phosphorus into the fiber core allowed for an increased concentration of erbium without inducing significant clustering, while keeping the numerical aperture low to ensure a single-mode laser operation.

All-fiber laser pump reflector based on a femtosecond-written inner cladding Bragg grating.

We report on the writing of chirped cladding Bragg gratings using 400 nm femtosecond pulses and the phase-mask inscription technique in the pure silica inner cladding of the gain fiber. The usefulness of such a fiber component is demonstrated as a pump reflector in a cladding-pumped all-fiber laser. A reflectivity of 53% for the residual pump power guided in the highly multimode 125 µm fiber's cladding is achieved, allowing for significant pump recycling in the fiber laser cavity which therefore optimizes the overall laser performances. As proof of concept, adding this pump reflector at the output of a 25 W erbium-doped fiber laser operating near 1.6 µm increased its slope efficiency according to the launched pump power from 17% to 23%, with the beneficial effect of reducing by 25% the optimal gain fiber length. This new, to the best of our knowledge, fiber laser component would have a great impact on the efficiency and cost of high-power cladding-pumped fiber laser systems.