Ultrafast laser sources operating in the mid-infrared (mid-IR) region, which contains the characteristic fingerprint spectra of many important molecules and transparent windows of atmosphere, are of significant importance in a variety of applications. Over the past decade, a significant progress has been made in the development of inexpensive, compact, high-efficiency mid-IR ultrafast mode-locked lasers in the picosecond and femtosecond domains that cover the 2.0 μm–3.5 μm spectral region. These achievements open new opportunities for applications in areas such as molecular spectroscopy, frequency metrology, material processing, and medical diagnostics and treatment. In this review, starting with the introduction of mid-IR mode-locking techniques, we mainly summarize and review the recent progress of mid-IR mode-locked laser sources, including Tm3+-, Ho3+-, and Tm3+/Ho3+-doped all-solid-state and fiber lasers for the 2.0 μm spectral region, Cr2+:ZnSe and Cr2+:ZnS lasers for the 2.4 μm region, and Er3+-, Ho3+/Pr3+-, and Dy3+-doped fluoride fiber lasers for the 2.8 μm–3.5 μm region. Then, some emerging and representative applications of mid-IR ultrafast mode-locked laser sources are presented and illustrated. Finally, outlooks and challenges for future development of ultrafast mid-IR laser sources are discussed and analyzed. The development of ultrafast mid-IR laser sources, together with the ongoing progress in related application technologies, will create new avenues of research and expand unexplored applications in scientific research, industry, and other fields.

Review of mid-infrared mode-locked laser sources in the 2.0 μm–3.5 μm spectral region

Recent progress in the development of high power fibre lasers will be reviewed, and the prospects for scaling output powers to well beyond the hundred watt level, whilst maintaining diffraction-limited beam quality will be discussed

High power fibre lasers

Detection and analysis of volatile compounds in exhaled breath represents an attractive tool for monitoring the metabolic status of a patient and disease diagnosis, since it is non-invasive and fast. Numerous studies have already demonstrated the benefit of breath analysis in clinical settings/applications and encouraged multidisciplinary research to reveal new insights regarding the origins, pathways, and pathophysiological roles of breath components. Many breath analysis methods are currently available to help explore these directions, ranging from mass spectrometry to laser-based spectroscopy and sensor arrays. This review presents an update of the current status of optical methods, using near and mid-infrared sources, for clinical breath gas analysis over the last decade and describes recent technological developments and their applications. The review includes: tunable diode laser absorption spectroscopy, cavity ring-down spectroscopy, integrated cavity output spectroscopy, cavity-enhanced absorption spectroscopy, photoacoustic spectroscopy, quartz-enhanced photoacoustic spectroscopy, and optical frequency comb spectroscopy. A SWOT analysis (strengths, weaknesses, opportunities, and threats) is presented that describes the laser-based techniques within the clinical framework of breath research and their appealing features for clinical use.

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Laser spectroscopy for breath analysis: towards clinical implementation

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.

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10-W-level monolithic dysprosium-doped fiber laser at 324 μm

The mid-infrared (MIR) represents a large portion of the electromagnetic spectrum that is progressively being exploited for an enormous number of applications. Thermal imaging cameras, dental and skin resurfacing lasers, and narcotics detectors at airports are all mainstream examples involving the MIR, but potential applications of MIR technologies are much larger. Accessing the unique opportunities afforded by the MIR is critically dependent on the specific characteristics of MIR emitting sources that become available. In this review, we survey an important enabling technology to the opening up of MIR science and applications, namely that driven by fiber-based sources of coherent MIR radiation. In this review paper, we describe many of the key advances in the innovation and development of such sources over the past few decades and discuss many of the underlying science and technology issues that have resulted in specific recent source achievements, especially in light of new applications enabled by these new source capabilities. We also discuss a few specific anticipated future needs and some potentially disruptive approaches to future MIR fiber source development.

Fiber-based sources of coherent MIR radiation: key advances and future prospects (invited).

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.

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

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

15 W monolithic fiber laser at 3.55 µm

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.

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

We report on a splice-free erbium-doped all-fiber laser emitting over 20 W at a wavelength of 1610 nm, with a slope efficiency of 19.6 % and an overall efficiency of 18.3% with respect to the launched pump power at 976 nm. The simple cavity design takes advantage of fiber Bragg gratings written directly in the gain fiber through the polymer coating and clad-pumping from a single commercial pump diode to largely simplify the assembling process, making this cavity ideal for large-scale commercial deployment. Two single-mode and singly erbium-doped silica fibers were fabricated in-house: the first to assess the effects of a high erbium concentration (0.36 mol.% Er2O3), yielding a low efficiency of 2.5 % with respect to launched pump power, and the second to achieve the improved result mentioned above (0.03 mol.% Er2O3). Numerical simulations show the link between the performance of each cavity and ion pair-induced quenching.

Pascal Paradis

Papers

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

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

15 W monolithic fiber laser at 3.55 µm

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

20 W splice-free erbium-doped all-fiber laser operating at 1610 nm.