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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.

We report the fabrication and characterization of silicon carbide microdisks on top of silicon pillars suited for applications from near- to mid-infrared. We probe 10 μm diameter disks with different under-etching depths, from 4 μm down to 1.4 μm, fabricated by isotropic plasma etching and extract quality factors up to 8400 at telecom wavelength. Our geometry is suited to present high Q single-mode operation. We experimentally demonstrate high-order whispering-gallery mode suppression while preserving the fundamental gallery mode and investigate some requirements for nonlinear optics applications on this platform, specifically in terms of quality factor and dispersion for Kerr frequency comb generation.

We demonstrate an optimized in-band-pumped dysprosium fiber laser with 73% slope efficiency and 1.1 W output at 3.15 μm: the first watt-level fiber source in the 3.0–3.4 μm region and highest ever efficiency mid-IR fiber laser.

We demonstrate the generation of 70-fs pulses (∼7 optical-cycles) from a mid-infrared mode-locked Ho:ZBLAN fiber laser using nonlinear compression with step-index chalcogenide fiber. Experiments are supported by numerical modelling, confirming the scalability of this approach.

We report the fabrication of fiber Bragg gratings (FBGs) in passive as well as doped fibers utilizing the femtosecond laser line-by-line technique. Narrow-linewidth linear gratings as well as broadband chirped gratings were inscribed into silica- and fluoride-glass optical fibers. Polarization-dependent characteristics of non-tilted and of 45°-tilted gratings are analyzed and the possibility of direct inscription into coated (i.e. non-stripped) fibers is demonstrated.

Output pulses from an ultrafast holmium-praseodymium co-doped fiber ring laser are spectrally broadened in different highly nonlinear chalcogenide fibers. Moderate levels of broadening are achieved in step-index fibers and the effect is utilized for nonlinear pulse compression. Much larger broadening factors can be observed in suspended-core fibers and polymer-protected robust all-chalcogenide microwires. Under optimized conditions, an ultra-broadband supercontinuum spanning from 2 – 12 μm is generated.

Mode-locked fiber lasers emitting short pulses of light at wavelengths of 2 μm and longer are reviewed. Rare-earth doped silica and fluoride fiber lasers operating in the mode-locked regime in the mid-IR (2–5 μm) have attracted attention due to their usefulness to spectroscopy, nonlinear optics, laser surgery, remote sensing and ranging to name a few. While silica fiber lasers are fundamentally limited to emission wavelengths below 2.2 μm, fluoride fiber lasers can reach to nearly 4 μm. The relative infancy of fluoride fibers as compared to silica fibers means the field has work to do to translate the mode-locking techniques to systems beyond 2 μm. However, with the recent demonstration of a stable, mode-locked 3 μm fiber laser, the possibility of achieving high performance 3 μm class mode-locked fiber lasers looks promising.

We report on our research into fluoride fiber-based 3 µm class laser sources. Using holmium as the gain medium, record-high peak power levels are achieved in mode-locked laser systems that emit ultrashort pulses in the water vapor transmission window around 2.9 µm. Highly nonlinear and dispersion-engineered hybrid chalcogenide-polymer microwires are utilized to extend the frequency range of those sources further into the mid-infrared. In addition, dysprosium- doped fibers are employed to demonstrate continuous-wave lasers that are tunable between 3 and 3.5 µm.

Silicon carbide (SiC) is a well-known material in the field of high temperature and high voltage electronics thanks to a high thermal conductivity, high electric field breakdown strength and high maximum current density [1]. Simultaneously with a strong inertness and a low thermal expansion, this makes silicon carbide a good material for extreme condition sensing [2]. The fact that the cubic structure (3C) of SiC can be grown on silicon makes it compatible with most of the silicon technology. At the same time, silicon carbide exhibits some good optical properties particularly promising for non-linear photonics from the near to the mid infrared range [3-4]. Its high refractive index (n∼2.6 at λ = 1.5μm) allows a good light confinement. Studies on other crystallographic structures [5] suggest that 3C-SiC may exhibit good non-linear properties with a nonlinear index comparable to silicon and nonlinear loss — often detrimental to non-linear processes — virtually inexistent thanks to wide its bandgap [6]. In this work, we report the linear optical and thermal characterization of silicon carbide microdisks on a silicon substrate designed for non-linear operations from near to mid-infrared wavelength.

We demonstrate record performance in a mid-IR ultrafast fiber laser by using holmium instead of erbium as the active gain medium. The 2.9 μm laser emits 180 fs pulses with 37 kW peak power.

We demonstrate a holmium-doped fluoride fiber laser that emits 37 kW peak power pulses with 180 fs pulse durations at 3 µm. The 100 nm redshift of holmium gain relative to erbium allows for minimal interaction with atmospheric water absorption resulting in a significant increase in performance for 3 µm class ultrafast fiber lasers.

We develop a computational model of a holmium-based laser which produces robust mode-locking at 2.9 microns with peak powers in the tens of kilowatts, far exceeding current fiber laser performance at 1.5 microns.

I will review our work on nonlinear waveguides for supercontinuum generation in the mid-infrared focusing on the nonlinear properties and waveguide performance of chalcogenide, silicon and silicon-germanium devices

We demonstrate a compact silicon-on-sapphire (SOS) strip waveguide sensor for mid-IR absorption spectroscopy. This device can be used for gas and liquid sensing, especially to detect chemically similar molecules and precisely characterize extremely absorptive liquids that are difficult to detect by conventional infrared transmission techniques. We reliably measure concentrations up to 0.25% of heavy water (Dsub2/subO) in a Dsub2/subO-Hsub2/subO mixture at its maximum absorption band at around 4 μm. This complementary metal-oxide-semiconductor (CMOS) compatible SOS Dsub2/subO sensor is promising for applications such as measuring body fat content or detection of coolant leakage in nuclear reactors.

The mid-infrared (mid-IR) spectral region holds great promise for new laser-based sensing technologies, based on measuring strong mid-IR molecular absorption features. Practical applications have been limited to date, however, by current low-brightness broadband mid-IR light sources and slow acquisition-time detection systems. Here, we report a new approach by developing a swept-wavelength mid-infrared fiber laser, exploiting the broad emission of dysprosium and using an acousto-optic tunable filter to achieve electronically controlled swept-wavelength operation from 2.89 to 3.25 {\mu}m (3070-3460 cm^-1). Ammonia (NH3) absorption spectroscopy is demonstrated using this swept source with a simple room-temperature single-pixel detector, with 0.3 nm resolution and 40 ms acquisition time. This creates new opportunities for real-time high-sensitivity remote sensing using simple, compact mid-IR fiber-based technologies., Comment: Invited article for APL Photonics

We present fabrication, simulation and characterization of silicon carbide microdisk on silicon pillar aiming at non-linear operation from near-infrared to mid-infrared. We report experimental Q factor of 1800 at telecom wavelength.

A waveguide array fiber laser is demonstrated. An AlGaAs waveguide array is used as a saturable absorber to achieve passive mode-locking in an erbium-doped fiber laser. The output consists of a very regular train of pulses. Autocorrelations show that the pulses consist of noise bursts due to incomplete mode-locking.

In this work silicon pillar waveguides have been proposed to exploit the entire transparent window of silicon. These geometries posses a broad and at dispersion (from 2 to 6 μm) with four zero dispersion wavelengths. We calculate supercontinuum generation spanning over two octaves (2 to >8 μm) with long wavelengths interacting weakly with the lossy substrate. These structures have higher mode confinement in the silicon - away from the substrate, which makes them substrate independent and are promising for exploring new nonlinear phenomena and highly sensitive molecular sensing over the entire silicon's transparency range

We demonstrate an octave spanning, 1.9–6.2 μm supercontinuum generation in a low loss silicon on a sapphire (SOS) nanowire. The supercontinuum is achieved by pumping in the low-loss window of SOS near 3.7 μm.

We demonstrate a fiber laser emitting 497fs pulses near 3µm using a mid-IR transparent fluoride fiber doped with Erbium. The potential of this laser as a source for a mid-IR fiber-based frequency comb is explored.

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.

Broadband laser light sources in the mid-infrared region attract enormous interest due to the plethora of applications they are enabling, including multispecies trace gas detection, free-space communications, and infrared countermeasures. Key to the progress in supercontinuum generation has been the wide availability of fiber-based near-infrared and bulk-optic mid-infrared pump sources and suitably engineered nonlinear media capable of supporting high-brightness supercontinua. A large proportion of the system complexity relates to the pump source itself with free-space systems based on parametric conversion being the most common for the generation of long-wavelength supercontinua. In an effort to realize all fiber and all mid-infrared supercontinuum sources, we combine a recently developed 2.9 µm ultrafast fiber laser based on holmium with an environmentally stable, polymer-protected all-chalcogenide tapered fiber. By launching 230 fs, 4.2 kW peak power pulses into the As2Se3/As2S3 tapered fiber, we demonstrate a spectrum spanning from 1.8 to 9.5 µm at the −20 dB points with an average power of more than 30 mW. This >2 octave-spanning supercontinuum is a crucial step toward robust mid-infrared broadband sources required for future field-deployable instruments.

By dual pumping the upper and lower laser level of the 5I6 → 5I7 transition (2.9 µm) of Ho3+ we demonstrate a power-scalable fiber laser that can be modulated between continuous wave lasing and superluminesence.

The linear and nonlinear optical response of SiGe waveguides in the mid-infrared are experimentally measured. By cutback measurements we find the linear losses to be less than 1.5dB/cm between 3μm and 5μm, with a record low loss of 0.5dB/cm at a wavelength of 4.75μm. By launching picosecond pulses between 3.25μm and 4.75μm into the waveguides and measuring both their self-phase modulation and nonlinear transmission we find that nonlinear losses can be significant in this wavelength range due to free-carrier absorption induced by multi-photon absorption. This should be considered when engineering SiGe photonic devices for nonlinear applications in the mid-IR.

We present the first demonstration of a 2 μm Brillouin laser based on chalcogenide fiber. A record-low lasing threshold of 52 mW is achieved, which is about 10 times lower than the earlier reports.

We generate a 1.9-4.9 µm supercontinuum in dispersion engineered silicon waveguides with a sapphire substrate. The sapphire substrate allows for extension of the long wavelength edge into the mid-IR by ~1000 nm over silicon-on-silica waveguides.

A maximum output power of 3.4 W, generated at 20.9 % efficiency, was obtained from a Ho3+, Pr3+-co-doped ZBLAN fibre laser pumped at 1150 nm by a Raman fibre laser and pumped by a tunable Yb3+ fibre laser.

We report the demonstration of a single-longitudinal-mode fiber laser operating at 2914 nm, which exhibits a spectrometer-limited linewidth of

There are now many research groups employing fluoride fibre in fibre lasers designed for high power emission at emission wavelengths in the 3 μm region and beyond. To create efficient and therefore high power output from fluoride-based fibre lasers our research group has explored a number of options. In this presentation, I will discuss these methods and offer some general rules-of-thumb that are applicable to the use of fluoride fibre.

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.

We report the demonstration of a single longitudinal mode fiber laser operating at 2914 nm, that exhibits a spectrometer-limited linewidth of

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.

We present the experimental observation of multi-wavelength fiber Bragg gratings in As2Se3 fiber. The gratings are internally written via two-photon absorption of 1550 nm pump light and its first and second order Stokes waves generated by cascaded stimulated Brillouin scattering (SBS). We demonstrate a parameter regime that allows for 4 dB grating enhancement by suppression of SBS.

Recent experiments involving long-wavelength fiber lasers and nonlinear media are presented. An optical fiber taper made from Chalcogenide glass that produced an octave-spanning spectrum (1–2 μιη) at ultralow pump pulse energy (77 pj) is presented. To translate these tapered fiber devices to the mid-Infrared range, we have explored several novel fiber laser configurations. Here, we recent work using a Holmium-doped fluoride fiber laser operating at 2.86 μτα. A GaAs-based SESAM allows for passive mode-locking with 20 ps pulse durations. This system and its applications for nonlinear optics in the mid-infrared wavelength range are covered.

We present a new method to create a coupled waveguide array via tapering a seven-core telecommunications fiber. The fiber based waveguide array is demonstrated to exhibit the novel physics associated with coupled waveguide arrays, such as discrete diffraction and discrete self-focusing. The saturable absorber characteristics of the device are characterized and an autocorrelation measurement reveals significant single-pass pulse reshaping.

We present a method to create coupled waveguide arrays via tapering 7-core germanosilicate fiber. Using an ultrashort pulse laser system the device is shown to exhibit nonlinear waveguide array physics, including nonlinear optical pulse chopping.

We present a new method to create a coupled waveguide array via tapering a multi-core telecommunications fiber. This device exhibits the novel physics associated with coupled waveguide arrays: Discrete self-focusing and nonlinear pulse chopping.

We report record low loss silicon-on-sapphire nanowires for applications to mid infrared optics. We achieve propagation losses as low as 0.8 dB/cm at λ = 1550 nm, ~1.1 to 1.4 dB/cm at λ = 2080 nm and2dB/cm at λ = 5.18 μm.

The burgeoning field of mid-infrared photonics requires a wide range pump and probe sources in order to develop the integrated optical components that are required for applications in medicine, defence and astronomy. Shortwave infrared (SWIR) fibre lasers based on mature fluoride glass optical fibre technology offers high power radiation in the 2.7 μm to 3 μm region; a region that is not covered by the emission from quantum cascade lasers. The Er3+-doped ZBLAN fibre lasers operating on the 4| 11/2 → 4| 13/2 transition offers high power [1,2] and broad tunability [3–5] in a diode pumpable format with multiple Watt output recently produced between 2.71 μm and 2.88 μm [6].

We report spectral phase clamping of femtosecond pulses propagating in a waveguide array. The waveguide sets the spectral phase of the output pulses to a fixed value, regardless of the input chirp.

We observe nonlinear pulse reshaping of femtosecond pulses in a waveguide array owing to coupling between waveguides. Amplified pulses from a mode-locked fiber laser are coupled to an AlGaAs core waveguide array structure. The observed power-dependent pulse reshaping agrees with theory, including shortening of the pulse in the central waveguide.

An Yb-fiber-frequency-comb-laser producing 75fs-pulses at 136MHz-repetition-rate and >10W-average-power is locked to a high-finesse cavity. A record intra-cavity average-power of up to 1.3kW and a peak-intensity of up to 8*1013W/cm2were reached, allowing ionization of Xe.

To prepare for future experiments on coherent spectroscopy and quantum control in the VUV spectral regions, we have pursued direct frequency comb spectroscopy and generation of phase coherent VUV frequency combs. High-finesse, low-dispersion passive optical cavities are used to enhance the power of femtosecond pulses for extreme nonlinear optics and to increase detection sensitivity for spectroscopy.

Silicon has attracted great interest as a platform for both linear and nonlinear integrated photonics for over 15 years. While its primary applications have been in the telecom window (near 1.5 μm), the capability of exploiting its full transparency window to 8 μm in the mid-IR is highly attractive, since this will open it up to entirely new applications in fields such as spectroscopy, chemical and biological sensing, and free-space communications. However, while silicon-on-insulator has shown great promise just beyond the telecommunications window [to the shortwave IR band (2.5 μm)], its wavelength range has been limited to < 4 μm by absorption in the silica cladding layer. Here, we demonstrate octave-spanning supercontinuum generation in silicon, covering a continuous spectral range from 1.9 to beyond 6 μm in dispersion-engineered silicon-on-sapphire (SOS) nanowires. This represents both the widest spectrum and longest wavelength generated to date in any silicon platform, and establishes SOS as a promising new platform for integrated nonlinear photonics in the mid-IR.

We propose pillar integrated silicon waveguides to exploit the entire transparent window of silicon. These geometries posses a broad and flat dispersion (from 2 to 6 μm) with four zero dispersion wavelengths. We calculate supercontinuum generation spanning over two octaves (2 to >8 μm) with long wavelengths interacting weakly with the lossy substrate. These structures have higher mode confinement in the silicon - away from the substrate, which makes them substrate independent and are promising for exploring new nonlinear phenomena and highly sensitive molecular sensing over the entire silicon’s transparency range.

We demonstrate a multiwavelength laser at 2 µm based on a hybrid gain scheme consisting of a Brillouin gain medium and a thulium-doped fiber. The laser has switchable frequency spacing, corresponding to the single and double Brillouin frequency shifts. In the 20 dB bandwidth, seven lasing channels with a frequency spacing of 0.1 nm (7.62 GHz) and eleven channels with a double-spacing of 0.2 nm (15.24 GHz) are obtained. A wavelength tunability of 1.3 nm is realized for both laser configurations by shifting the pump wavelength. Strong four wave mixing is observed in the double-spacing laser resulting in an improved performance: larger number of channels and better temporal stability.

Compact optical frequency comb sources with gigahertz repetition rates are desirable for a number of important applications including arbitrary optical waveform generation and direct comb spectroscopy. We report the generation of phase-locked, gigahertz repetition rate optical frequency combs in a chalcogenide photonic chip. The combs are formed via the interplay of stimulated Brillouin scattering and Kerr-nonlinear four-wave mixing in an on-chip Fabry–Perot waveguide resonator incorporating a Bragg grating. Phase-locking of the comb is confirmed with real-time measurements, and a chirp of the comb repetition rate within the pump pulse was observed. These results represent a significant step towards the realization of integrated optical frequency comb sources with gigahertz repetition rates.

Using a 3.1-μm optical parametric chirped-pulse amplifier (OPCPA), we generate a supercontinuum in a step-index chalcogenide fiber that spans from 1.6 to 5.9 μm at the −20 dB points. The rugged step-index geometry allows for long-term operation, while the spectral bandwidth is limited by the transmission of the As2S3 fiber.