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.

Visible femtosecond fiber laser

Femtosecond fiber lasers have revolutionized the industry of laser technology by providing ultrashort pulses of high brightness through compact, affordable, and reliable setups. In this work, we extend the scope of application of such sources by reporting, to our knowledge, the first femtosecond fiber laser operating in the visible spectrum. The passively mode-locked ring cavity is based on nonlinear polarization evolution in a single-mode Pr³⁺-doped fluoride fiber and runs in an all-normal dispersion regime. Compressed pulses at 635 nm have a duration of 168 fs, a peak power of 0.73 kW, and a repetition rate of 137 MHz.

Genetic relationships between weight loss in early lactation and daily milk production throughout the lactation in Holstein cows

After calving, high-yielding dairy cows mobilize body reserves for energy, sometimes to the detriment of health and fertility. This study aimed to estimate the genetic correlation between body weight loss until nadir and daily milk production (MY24) in first- (L1) and second-lactation (L2) Holstein cows. The data set included 859,020 MY24 records and 570,651 daily raw body weight (BWr) phenotypes from 3,989 L1 cows, and 665,361 MY24 records and 449,449 BWr phenotypes from 3,060 L2 cows, recorded on 36 French commercial farms equipped with milking robots that included an automatic weighing platform. To avoid any bias due to change in digestive content, BWr was adjusted for variations in feed intake, estimated from milk production and BWr. Adjusted body weight was denoted BW. The genetic parameters of BW and MY24 in L1 and L2 cows were estimated using a 4-trait random regression model. In this model, the random effects were fitted by second-order Legendre polynomials on a weekly basis from wk 1 to 44. Nadir of BW was found to be earlier than reported in the literature, at 29 d in milk, and BW loss from calving to nadir was also lower than generally assumed, close to 29 kg. To estimate genetic correlations between body weight loss and production, we defined BWL5 as the loss of weight between wk 1 and 5 after calving. Genetic correlations between BWL5 and MY24 ranged from -0.26 to 0.05 in L1 and from -0.11 to 0.10 in L2, according to days in milk. These moderate to low values suggest that it may be possible to select for milk production without increasing early body mobilization.

Enhancing Evanescent Wave Coupling of Near-Surface Waveguides with Plasmonic Nanoparticles

Evanescent field excitation is a powerful means to achieve a high surface-to-bulk signal ratio for bioimaging and sensing applications. However, standard evanescent wave techniques such as TIRF and SNOM require complex microscopy setups. Additionally, the precise positioning of the source relative to the analytes of interest is required, as the evanescent wave is critically distance-dependent. In this work, we present a detailed investigation of evanescent field excitation of near-surface waveguides written using femtosecond laser in glass. We studied the waveguide-to-surface distance and refractive index change to attain a high coupling efficiency between evanescent waves and organic fluorophores. First, our study demonstrated a reduction in sensing efficiency for waveguides written at their minimum distance to the surface without ablation as the refractive index contrast of the waveguide increased. While this result was anticipated, it had not been previously demonstrated in the literature. Moreover, we found that fluorescence excitation by waveguides can be enhanced using plasmonic silver nanoparticles. The nanoparticles were also organized in linear assemblies, perpendicular to the waveguide, with a wrinkled PDMS stamp technique, which resulted in an excitation enhancement of over 20 times compared to the setup without nanoparticles.

Toward low-loss mid-infrared Ga2O3-BaO-GeO2 optical fibers

The development of efficient and compact photonic systems in support of mid-infrared integrated optics is currently facing several challenges. To date, most mid-infrared glass-based devices are employing fluoride or chalcogenide glasses (FCGs). Although the commercialization of FCGs-based optical devices has rapidly grown during the last decade, their development is rather cumbersome due to either poor crystallization and hygroscopicity resilience or poor mechanical-thermal properties of the FCGs. To overcome these issues, the parallel development of heavy-metal oxide optical fiber from the barium-germanium-gallium oxide vitreous system (BGG) has revealed a promising alternative. However, over 30 years of fiber fabrication optimization, the final missing step of drawing BGG fibers with acceptable losses for meters-long active and passive optical devices had not yet been reached. In this article, we first identify the three most important factors that prevent the fabrication of low-loss BGG fibers i.e., surface quality, volumic striae and glass thermal-darkening. Each of the three factors is then addressed in setting up a protocol enabling the fabrication of low-loss optical fibers from gallium-rich BGG glass compositions. Accordingly, to the best of our knowledge, we report the lowest losses ever measured in a BGG glass fiber i.e., down to 200 dB km-1 at 1350 nm.

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.

Ultrafast laser writing of arbitrary long low-loss waveguides in optical fibers

We propose an innovative femtosecond laser writing approach, based on a reel-to-reel configuration, allowing the fabrication of arbitrary long optical waveguides in coreless optical fibers directly through the coating. We report few meters long waveguides operating in the near-infrared (near-IR) with propagation losses as low as 0.055 ± 0.004 dB/cm at 700 nm. The refractive index distribution is shown to be homogeneous with a quasi-circular cross section, its contrast being controllable via the writing velocity. Our work paves the way for the direct fabrication of complex arrangements of cores in standard and exotic optical fibers.

Modified astigmatic beam technique for laser writing

The ultrafast laser writing of optical waveguides and devices is increasingly ubiquitous among the photonics community, mostly for its flexibility and three-dimensional fabrication capability. The well-known astigmatic beam technique is the simplest method to inscribe near-circular cross-section waveguides. In this paper, we report on a significant enhancement to the widely used astigmatic beam technique that makes it more flexible and yields a more circular waveguide cross section. By simply superposing a long-focus lens before the laser inscription objective lens, we demonstrate that the normalized squared radial deviation from a perfectly circular waveguide cross section can be reduced to <4×10^-4, which is a significant improvement compared to >0.1 typically obtained using the standard astigmatic beam technique, or >0.7 with a Gaussian beam. The modified technique also makes it easy to use the full power delivered by the laser, which is not usually the case with the standard technique. A technique to optimize the waveguide shape prior to the inscription by in situ laser-induced plasma emission imaging is also discussed.

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.

Dysprosium-doped silica fiber as saturable absorber for mid-infrared pulsed all-fiber lasers

We report on a mid-infrared Q-switched erbium-doped all-fiber laser using a dysprosium-doped silica fiber as saturable absorber for the first time in this wavelength range. Moreover, we demonstrate the use of a highly reflective chirped fiber Bragg grating written in a silica fiber as the input coupler for such lasers. This Q-switched all-fiber laser generates a stable pulse train centered at 2798 nm with a maximum average power of 670 mW at a repetition rate of 140 kHz with a pulse duration of 240 ns and a pulse energy of 4.9 µJ.

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.

Dual stage fiber amplifier operating near 3 µm with milijoule-level, sub-ns pulses at 5 W

We report a 2800 nm Er³⁺-doped fluoride fiber amplifier that delivers 1 mJ pulses with an average power of 5 W and pulse duration of 1 ns at 5 kHz repetition rate. To the best of our knowledge, this is the highest pulse energy achieved from a fluoride-fiber-based system operating near 3 µm, and the W-level average power and short pulse lengths make the system a promising tool for biomaterials processing.

From near-UV to long-wave infrared waveguides inscribed in barium fluoride using a femtosecond laser

Depressed-cladding waveguides (DCWs) of various sizes were inscribed in barium fluoride, allowing single-mode operation in the entirety of its transmission window (λ=0.2-12µm). Using femtosecond laser pulses at 515 nm, type I laser modified tracks were overlapped to form circular waveguides, whose cross-sectional geometry and numerical aperture were tailored to accommodate 0.405, 2.85, and 10.6 µm light. The mode profile, propagation loss, refractive index profile, and numerical aperture of the optimized waveguides were analyzed and compared with theory. We particularly demonstrate the challenging inscription of a large DCW for single-mode operation at 10.6 µm with propagation loss of <0.63dB/cm.

Smartphone Screen Integrated Optical Breathalyzer

One third of fatal car accidents and so many tragedies are due to alcohol abuse. These sad numbers could be mitigated if everyone had access to a breathalyzer anytime and anywhere. Having a breathalyzer built into a phone or wearable technology could be the way to get around reluctance to carry a separate device. With this goal, we propose an inexpensive breathalyzer that could be integrated in the screens of mobile devices. Our technology is based on the evaporation rate of the fog produced by the breath on the phone screen, which increases with increasing breath alcohol content. The device simply uses a photodiode placed on the side of the screen to measure the signature of the scattered light intensity from the phone display that is guided through the stress layer of the Gorilla glass screen. A part of the display light is coupled to the stress layer via the evanescent field induced at the edge of the breath microdroplets. We demonstrate that the intensity signature measured at the detector can be linked to blood alcohol content. We fabricated a prototype in a smartphone case powered by the phone’s battery, controlled by an application installed on the smartphone, and tested it in real-world environments. Limitations and future work toward a fully operational device are discussed.

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.

Direct-laser-written integrated mid-IR directional couplers in a BGG glass

The development of coherent sources and other optical components for the mid-infrared has been hampered by the lack of sturdy materials that can withstand high power radiation or exposition to harsh environment. BGG glasses are robust materials transmitting over the 2.5-5 μm region. We report here the direct femtosecond laser fabrication of efficient directional couplers integrated in a BGG glass chip. The photonic components are characterized from 2.1 to 4.2 μm and compared to similar structures inscribed in silica glass samples. At 2.85 μm, a 99% relative cross transmission is reported in BGG glass. The experimental measurements are in good agreement with the coupled mode theory for wavelengths up to 3.5 μm.

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.

1.4 W in-band pumped Dy3+-doped gain-switched fiber laser at 3.24 µm

In this Letter, we report, to the best of our knowledge, the first demonstration of an in-band pumped gain-switched Dy³⁺-doped fiber laser operating at 3.24 µm. The monolithic cavity bounded by two fiber Bragg gratings was pumped by a gain-switched Er³⁺-doped fiber system. It produced stable nanosecond pulses in a single-pulse regime on its entire operating range from 20 kHz to 120 kHz. A record average power of 1.43 W was achieved for a repetition rate of 120 kHz, and a record pulse energy of 19.2 µJ was achieved at 60 kHz. These results represent a significant improvement in Dy³⁺-doped pulsed fiber laser performances and open the way to applications in the fields of remote sensing and material processing.

High resolution temperature sensor based on frequency beating between twin DFB fiber lasers

We present a high resolution temperature sensor using the beat frequency between the longitudinal modes of twin single-mode distributed feedback fiber lasers. The lasers are made by femtosecond inscription of π-shifted fiber Bragg gratings in a thulium-doped fiber. Combining the light from two single frequency fiber lasers on a photodetector produces a rf beat frequency signal which is dependent on temperature. Experimental results show a sensitivity of 1900 MHz/°C, leading to a precision of 0.0007 °C.

Nonlinear increase, invisibility, and sign inversion of a localized fs-laser-induced refractive index change in crystals and glasses

Multiphoton absorption via ultrafast laser focusing is the only technology that allows a three-dimensional structural modification of transparent materials. However, the magnitude of the refractive index change is rather limited, preventing the technology from being a tool of choice for the manufacture of compact photonic integrated circuits. We propose to address this issue by employing a femtosecond-laser-induced electronic band-gap shift (FLIBGS), which has an exponential impact on the refractive index change for propagating wavelengths approaching the material electronic resonance, as predicted by the Kramers-Kronig relations. Supported by theoretical calculations, based on a modified Sellmeier equation, the Tauc law, and waveguide bend loss calculations, we experimentally show that several applications could take advantage of this phenomenon. First, we demonstrate waveguide bends down to a submillimeter radius, which is of great interest for higher-density integration of fs-laser-written quantum and photonic circuits. We also demonstrate that the refractive index contrast can be switched from negative to positive, allowing direct waveguide inscription in crystals. Finally, the effect of the FLIBGS can compensate for the fs-laser-induced negative refractive index change, resulting in a zero refractive index change at specific wavelengths, paving the way for new invisibility applications.

High-power supercontinuum generation in the mid-infrared pumped by a soliton self-frequency shifted source

We report the demonstration of a fiber-based supercontinuum source delivering up to 825 mW of average output power between 2.5 and 5.0 µm generated in all-normal dispersion regime. The pumping source consists of an amplified ultrafast Er^3+:ZrF₄ fiber laser providing high peak power femtosecond pulses at 3.6 µm with an average output power exceeding the watt-level. These pulses are spectrally broadened through self-phase modulation using commercial chalcogenide-based step-index fibers. Al₂O₃ anti-reflection coatings were sputtered on chalcogenide fiber tips to increase the launching efficiency from 54% to 82%, making this record output power possible, and thus confirming that such coatings can support watt-level pumping with intense femtosecond pulses. To the best of our knowledge, this result represents the highest average output power ever achieved from a As₂Se₃-based mid-IR supercontinuum source with the potential of a high degree of coherence.

Femtosecond laser writing of near-surface waveguides for refractive-index sensing

Femtosecond laser writing of optical waveguides and components in glasses has been a remarkably growing research field during the last two decades. However, such laser- inscribed optical components were mostly written within the volume of the glass due to the unavoidable ablation that arises when the focal spot is approaching the glass surface. This has generally limited the interaction of light with the surrounding medium thus preventing sensing functionality. In this paper, we present the inscription of surface and near-surface silver based waveguides in a silver containing glass with no need for additional processing as it is the case for standard type I waveguides. In addition, an ultra-sensitive refractive index sensor in a 1 cm glass chip is obtained based on near-surface waveguides interacting with liquid droplets acting as top-layer on the glass surface. Remarkably, the device exhibits a novel double-wing feature that sharpens the response and enhances its sensitivity. Our results highlight the advantages of silver based waveguides paving the way towards further surface based sensors in fibers.

Direct Inscription of on-surface waveguides in polymers using a mid-ir fiber laser

A detailed study of photo-inscribed optical waveguides in PMMA and polycarbonate using a mid-IR laser is presented. The wavelength of the laser is tuned near the absorption peaks of stretching C-H molecular bonds and the focused beam is scanned onto the surface of planar polymer samples. For the first time, we report the formation of optical waveguides in both polymers through resonant absorption of the laser beam. The optical properties of the waveguides were thoroughly assessed. An elliptic Gaussian mode is guided at the surface of both polymers. Insertion losses of 3.1 dB for a 30 mm long on-surface waveguide inscribed in PMMA were recorded. Such waveguides can interact with the external medium through evanescent coupling. As a proof of concept, the surface waveguides are used as highly sensitive refractometric sensors. An attenuation dynamical range of 35 dB was obtained for a liquid that matches the index of the PMMA substrate. Our results pave the way for large scale manufacturing of low cost biocompatible photonic devices.

Endcapping of high-power 3 µm fiber lasers

Fiber tip photodegradation through OH diffusion currently limits the long term operation of high-power fiber lasers and amplifiers operating near 3 µm. To address this issue, we investigate the resistance to OH diffusion of fluoride and oxide endcaps manufactured out of ZrF$_4$₄, AlF$_3$₃, GeO$_2$₂, SiO$_2$₂ and Al$_2$₂O$_3$₃ fibers. To this extent, the endcaps are spliced at the output of a 20 W continuous-wave fiber laser operating at 2.8 µm and their degradation over a 100 h time period is monitored. While the fluoride-based endcaps underwent failure during the first 10 h, their oxide counterparts survived the experiment, although showcasing degradation which was reflected as an increase of the endface temperature over time. To overcome this issue, we propose a novel method to completely suppress OH diffusion which consists in sputtering a nanoscopic diffusion barrier film made of silicon nitride (Si$_3$₃N$_4$₄) on the output face of the endcap. The effectiveness of the approach is validated on Al$_2$₂O$_3$₃, ZrF$_4$₄ and AlF$_3$₃ endcaps which show no sign of degradation after being used for more than a 100 h at the output of a 3 µm high-power fiber laser.

Long period fiber gratings for the mitigation of parasitic laser effects in mid-infrared fiber amplifiers

A concept to mitigate parasitic lasing in mid-IR fiber amplifiers using a single long period fiber grating is shown. Using tightly confined ultrashort laser pulses at 800 nm, a grating was directly inscribed into the core of an erbium doped fluoride glass fiber showing a strong attenuation down to -27 dB at desired wavelength. The concept reveals great potential to improve the average output power and attainable spectral range of low repetition rate in-amplifier supercontinuum generation.

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.

10-W-level monolithic dysprosium-doped fiber laser at 3.24 μm

We report, to the best of our knowledge, the first entirely monolithic dysprosium (Dy)-doped fluoride fiber laser operating in the mid-IR region. The system delivers 10.1 W at 3.24 μm in continuous operation, a record for fiber oscillators in this range of wavelengths. The Dy³⁺ fiber is pumped in-band using an erbium-doped fiber laser at 2.83 μm made in-house and connected through a fusion splice. Two fiber Bragg gratings directly written in the Dy-doped fiber form the 3.24 μm laser cavity to provide a spectrally controlled laser output. This substantial increase of output power in the 3.0-3.3 μm spectral range could open new possibilities for applications in spectroscopy and advanced manufacturing.

Ultrafast Dy3+:fluoride fiber laser beyond 3 μm

We report a passively mode-locked Dy³⁺:fluoride fiber laser emitting around 3.1 μm based on the nonlinear polarization evolution technique in a ring configuration, using in-band pumping at 2.8 μm. Transform-limited and self-starting mode-locked pulses as short as 828 fs with a center wavelength around 3.1 μm and repetition rates up to 60 MHz are obtained. In the single-pulse regime, a maximum average output power of 204 mW is measured, corresponding to a peak power of 4.2 kW and a pulse energy of 4.8 nJ. This first demonstration, to the best of our knowledge, of a femtosecond mode-locked fiber laser emitting directly beyond 3 μm paves the way for frequency comb synthesis in the molecular fingerprint region.

Femtosecond laser inscription of depressed cladding single-mode mid-infrared waveguides in sapphire

Mid-infrared optical waveguides were inscribed in sapphire with femtosecond pulses at 515 nm. We show that such pulses induce a smooth negative refractive index change allowing for the inscription of a depressed cladding waveguide by closely overlapping the corresponding type I modification traces. The resulting structure consists of a highly symmetrical, uniform, and homogeneous waveguide. The size and numerical aperture of the waveguides were tailored to achieve efficient transmission in the mid-infrared. Single mode operation at a wavelength of 2850 nm and propagation loss of <0.37  dB/cm are reported for a 33 mm long depressed cladding waveguide. Thermal annealing was performed, and the refractive index contrast was still preserved to 50% (i.e., Δn=∼2.5×10^-3) up to 1400°C.

Towards power scaling of 2.8 μm fiber lasers

We report the demonstration of a 2824 nm passively cooled erbium-doped fluoride fiber laser delivering a record average output power of 41.6 W 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 980 nm 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 11.2 W at 2.826 µm. The corresponding extracted pulse energy is 80 µJ at a pulse duration of 170 ns. These performances significantly surpass previous gain-switched demonstrations and are close to the state-of-the-art Q-switched laser performances near 2.8 µm, 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.

Comparative study of quantitative phase imaging techniques for refractometry of optical waveguides

A comparative study of quantitative phase imaging techniques for refractometry of optical waveguides is presented. Three techniques were examined: a method based on the transport-of-intensity equation, quadri-wave lateral shearing interferometry and digital holographic microscopy. The refractive index profile of a SMF-28 optical fiber was thoroughly characterized and served as a gold standard to assess the accuracy and precision of the phase imaging methods. Optical waveguides were inscribed in an Eagle2000 glass chip using a femtosecond laser and used to evaluate the sensitivity limit of these phase imaging approaches. It is shown that all three techniques provide accurate, repeatable and sensitive refractive index measurements. Using these phase imaging methods, we report a comprehensive map of the photosensitivity to femtosecond pulses of Eagle2000 glass. Finally, the reported data suggests that the phase imaging techniques are suited to be used as precise and non-destructive refractive index shift measuring tools to study and control the inscription process of optical waveguides.

High-energy picosecond pulses from a 2850 nm fiber amplifier

We report the demonstration of a 2850 nm diode-pumped Ho³⁺, Pr³⁺ co-doped fluoride fiber amplifier that delivers pulses with an average power of 2.45 W, 122 μJ energy, and 500 ps duration at a repetition rate of 20 kHz. To the best of our knowledge, the average power and pulse energy are the highest to be obtained from a sub-nanosecond fiber source operating in the 3 μm spectral region. The amplifier is seeded by an optical parametric generation source and is pumped around 915 nm using widely available InGaAs laser diodes.

Infrared supercontinuum generated in concatenated InF3 and As2Se3 fibers

We report on infrared supercontinuum (SC) generation through subsequent nonlinear propagation in concatenated step-index fluoride and As₂Se₃ fiber. These fibers were pumped by an all-fiber laser source based on an erbium amplifier followed by a thulium power amplifier. ZBLAN and InF₃ fibers were compared for the concatenated scheme. The broadest SC produced was achieved by optimizing the length of the InF₃ fiber. This arrangement allowed the generation of 200 mW infrared SC with high spectral flatness and spanning from 1.4 μm to 6.4 μm.

Femtosecond-written long-period gratings in fluoride fibers

Long-period gratings induced in fluoride glass fibers using femtosecond laser pulses at 800 nm are, to the best of our knowledge, demonstrated for the first time. By means of tightly confined ultrashort laser pulses, smooth periodic lines of refractive index changes are induced along the fiber core. Taking advantage of heat accumulation effects in the focal volume, attenuation peaks down to -24  dB, with sharp and predictable spectral resonances, were obtained. Thermal annealing of the grating up to 250°C yielded a significant reduction of the induced refractive index change. The gratings could find applications in various integrated mid-infrared optical devices, such as optical notch filters in fiber amplifiers.

Gain-switched fiber laser at 3.55 μm

We report, to the best of our knowledge, the first demonstration of a gain-switched fiber laser operating near 3.5 μm. A dual-wavelength pumping scheme consisting of a 1976 nm Q-switched fiber system and a continuous-wave 976 nm laser diode were used to gain-switch a monolithic erbium-doped fluorozirconate fiber laser cavity at 3.552 μm. Stable pulses were produced for repetition rates ranging between 15 and 20 kHz, and a record peak power of 204 W was achieved at 15 kHz. A quenching phenomenon was also observed at 15 kHz for 1976 nm pulse energies beyond 180 μJ.

Aperiodic nature of nanograting inscribed by femtosecond pulses

A systematic study was performed, both experimentally and theoretically, to investigate the structural periodicity of nanogratings inscribed by focused femtosecond pulses on the surface of dielectrics. The results surprisingly show that although nanogratings generally appear periodic they are in fact intrinsically aperiodic. In the perpendicular writing scheme, the groove spacing gradually decreases from the middle part towards both sides. In the parallel writing scheme, the groove spacing varies quasiperiodically and the variation differs with respect to pulse-to-pulse spacing. Constant groove spacing was obtained only for a particular pulse-to-pulse spacing. These Gaussian-apodized and quasiperiodic variations are found to be intrinsic. They arise from the fact that the grooves are created, depending on the writing scheme, either by a series of local-lobes with different amplitudes and material feedbacks or by a repeated generated leading side-lobe with similar but not identical amplitudes and material feedbacks. The production of each single grooves result from a nonlinear and localized process from which arises its aperiodic nature. All these intrinsically Gaussian-apodized and quasiperiodic variations can be well interpreted based on the incubation-based nanoplasmonic model.

Direct laser writing of a new type of waveguides in silver containing glasses

Direct laser writing in glasses is a growing field of research in photonics since it provides a robust and efficient way to directly address 3D material structuring. Generally, direct laser writing in glasses induces physical modifications such as refractive index changes that have been classified under three different types (Type I, II & III). In a silver-containing zinc phosphate glass, direct laser writing additionally proceeds via the formation of silver clusters at the periphery of the interaction voxel. In this paper, we introduce a novel type of refractive index modification based on the creation of the photo-induced silver clusters allowing the inscription of a new type of optical waveguides. Various waveguides as well as a 50-50 beam splitter were written inside bulk glasses and characterized. The waveguiding properties observed in the bulk of such silver-containing glass samples were further transposed to ribbon shaped fibers made of the same material. Our results pave the way for the fabrication of 3D integrated circuits and fiber sensors with original fluorescent, nonlinear optical and plasmonic properties. The universality of these new findings should further extend in any silver-containing glasses that show similar laser-induced behavior in terms of silver cluster production.

5.6 W monolithic fiber laser at 3.55 μm

We report, to the best of our knowledge, the first monolithic erbium-doped fluorozirconate fiber laser bounded by two fiber Bragg gratings (FBGs) operating at 3.55 μm. Its output power and total optical efficiency are 5.6 W and 26.4% respectively, the highest ever achieved at this wavelength from a fiber laser. The monolithic design of the cavity also increases its stability and prevents fiber tip damage which has limited prior demonstrations to a maximum output power of 1.5 W. This Letter also studies the performances of the laser cavity for various output FBG reflectivities and presents numerical modeling results exhibiting remarkable agreement with experimental results.

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-infrared nonlinear absorption in As2S3 chalcogenide glass

We report mid-infrared (MIR) nonlinear absorption in As₂S₃ glasses which results from two-photon excitation of valence electron to the Urbach extension followed by strong linear absorption of excited states. The measured MIR nonlinear absorption can be 3 to 4 orders of magnitude stronger than the two-photon absorption in the near-infrared for similar laser intensities and does not result from contaminants, but it is intrinsic to As₂S₃ glasses. As₂S₃ fibers are widely used to generate supercontinuum by pumping them with high peak power laser pulses. For a 100 kilowatt peak power MIR soliton propagating in single mode As₂S₃ fiber, the nonlinear absorption can be of similar magnitude than the fiber background loss. Finally, for laser peak power around 1 MW, the MIR nonlinear absorption can be ~2 orders of magnitude larger than the fiber background loss in single mode As₂S₃ fiber.

Compact 3-8 μm supercontinuum generation in a low-loss As2Se3 step-index fiber

A mid-infrared supercontinuum source spanning from 3 to 8 μm is demonstrated using a low-loss As₂Se₃ commercial step-index fiber. A maximum average output power of 1.5 mW is obtained at a low repetition rate of 2 kHz. Thanks to the low NA step-index fiber, the output is single mode for wavelengths above ∼5  μm. The pump source consists of an erbium-doped ZrF₄-based in-amplifier supercontinuum source spanning from 3 to 4.2 μm. The effects of both the pump power and As₂Se₃ fiber length on the output characteristics are studied. To the best of our knowledge, this is the first compact supercontinuum source ever reported to reach 8 μm in a standard step-index fiber.

Femtosecond laser direct inscription of surface skimming waveguides in bulk glass

We present a detailed study of waveguide inscription near the surface of bulk glass using a femtosecond laser. Three silicate glasses used extensively as hosts for photo-induced photonic devices were examined. Our results show that near-surface waveguides generally present a low-index contrast, as the pulse energy damage threshold decreases sharply at close proximity to the surface. We devised a novel method to allow the formation of optical waveguides that exhibit a high-index contrast up to the surface of any transparent material. As a proof of concept, the inscription of near-surface single-mode waveguides operating at a wavelength of 405 nm is demonstrated.

Glass interposer for short reach optical connectivity

We propose a glass interposer containing femtosecond laser-scribed waveguides to interconnect silicon photonic chips. The glass interposer has an insertion loss of about 1.5 dB/cm, and simplifies alignment of silicon photonic chips. Our experiment shows that the insertion loss for the grating coupler/inscribed glass interface was only 0.5 dB higher than the estimated coupling loss of grating coupler to SMF. The 3 dB coupling degradation occurs after 5 µm of in-plane displacement between the laser-inscribed waveguide and the grating coupler.

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.

Refractive index and dispersion control of ultrafast laser inscribed waveguides in gallium lanthanum sulphide for near and mid-infrared applications

The powerful ultrafast laser inscription technique is used to fabricate optical waveguides in gallium lanthanum sulphide substrates. For the first time the refractive index profile and the dispersion of such ultrafast laser inscribed waveguides are experimentally measured. In addition the Zero Dispersion Wavelength of both the waveguides and bulk substrate is experimentally determined. The Zero Dispersion Wavelength was determined to be between 3.66 and 3.71 μm for the waveguides and about 3.61 μm for the bulk. This work paves the way for realizing ultrafast laser inscribed waveguide devices in gallium lanthanum sulphide glasses for near and mid-IR applications.

Watt-level erbium-doped all-fiber laser at 3.44 μm

We demonstrate a 3.44 μm all-fiber laser emitting a maximum of 1.5 W at room temperature, the highest continuous power ever generated from a mid-IR fiber oscillator clearly beyond 3 μm. The laser operates on the 4F(9/2)→4I(9/2) transition of erbium-doped fluoride glasses and relies on a dual pumping scheme at 974 and 1976 nm. By combining a dichroic mirror deposited on the input fiber tip and a fiber Bragg grating as an output coupler, a stable laser emission is produced with a FWHM bandwidth of less than 0.6 nm. The laser cavity has an efficiency of 19% with respect to the launched pump power at 1976 nm and no saturation is observed provided 974 nm co-pumping is sufficient. The joint effect of the two pumps is also investigated.

In-amplifier mid-infrared supercontinuum generation

Mid-infrared supercontinuum (SC) generation from 2.6 to 4.1 μm is demonstrated in a single-mode erbium-doped fluoride glass fiber amplifier pumped at 976 nm and seeded by an optical parametric generation (OPG) source emitting 400 ps pulses at 2.75 μm. Up to 82% of the SC power is generated beyond 3 μm. This novel and simple in-amplifier SC generation approach is promising for the development of compact and efficient sources operating in the mid-infrared, especially in the 3-5 μm band.

30 W fluoride glass all-fiber laser at 2.94 μm

We report the demonstration of a 2938 nm erbium-doped fluoride glass fiber laser delivering a record output power of 30.5 W in continuous wave operation. The passively cooled all-fiber laser cavity based on intracore fiber Bragg gratings has an overall laser efficiency of 16% as a function of the launched pump power at 980 nm and a single-mode output beam quality of M²<1.2. This power scaling demonstration of a fiber laser operating near the vibrational resonance of water is likely to have a significant impact on several biomedical applications.

All-fiber DFB laser operating at 2.8 μm

Single-frequency laser emission from a distributed feedback all-fiber laser operating in the vicinity of 3 μm is demonstrated. The laser cavity was made of a 30 mm long π-phase-shifted fiber Bragg grating inscribed in a heavily erbium-doped fluoride fiber using infrared femtosecond pulses and the dithering phase-mask technique. A maximum CW output power of 12 mW was obtained at 2794.4 nm by using a multimode pumping scheme. The narrow linewidth was characterized to be lower than 20 kHz using a heterodyne technique. This achievement represents a significant step toward the development of active frequency references operating in the mid-infrared.