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Supercontinuum (SC) generation that leads to the emission of broadband radiation has been extensively studied. In particular, SC sources encompassing the wavelength range of 2–5 μm have attracted considerable interest in the last decade, and a continuous increase in the output power and spectrum width has been observed. To enable broadband and high-power SC generation, suitable nonlinear media combined with appropriate pump sources must be used, maintaining the output as spectrally flat. The emergence of specialty glass fibers, such as fluoroindate fibers, as well as the advances in fiber-based pulsed oscillators and amplifiers, have accelerated the development of high-power SC systems operating in the mid-infrared spectral band.
- supercontinuum generation
- mid-infrared
- fluoroindate fibers
Mid-infrared (mid-IR) supercontinuum (SC) generation has attracted considerable interest in recent years, owing to its advantageous applications in many important fields of science and industry. First, this spectral range covers the characteristic absorption lines of several versatile materials and important molecules (e.g., CO2, HCl, CH4, C2H6, O3, NO, and NO2) [1,2], leading to the adoption of mid-IR SC sources for imaging [3], spectroscopy [4,5,6], and remote sensing, including the remote detection of explosives and hazardous chemicals [7,8]. The improvements in these sources have also paved the way toward new possible applications in defense [3,9], utilizing directional infrared countermeasure (DIRCM) systems, where a collimated output beam can be used to blind an infrared detector of a heat-seeking missile [10,11,12]. Finally, their use in medicine (e.g., tissue ablation, coherent anti-Stokes Raman scattering (CARS) microscopy, and breath diagnostics) [13,14,15,16,17,18] as well as in the generation of few-cycle optical pulses in the mid-IR region [19,20,21] or for the stabilization of frequency comb lasers [22] are also worth mentioning. Some of the aforementioned applications require laser sources that deliver high-quality, high-power beams with a broadband and flat spectrum covering the entire 2–5 μm band. Therefore, research on such laser systems providing high spectral brightness, defined as the radiance per unit optical bandwidth, has been the primary focus in many laboratories.
Supercontinuum generation is a phenomenon, first described in the 1970s [23,24], in which high-intensity laser pulses launched into a nonlinear optical medium interact with it, leading to the emission of new light frequencies and, eventually, causing the output spectrum to be much broader than the spectrum of the pump radiation. The efficiency of this process depend on many factors, including the peak power of the irradiated pulses, nonlinearity, dispersion, transmission band, losses, and the interaction length of light with a nonlinear medium. Considering these issues, low-loss optical fibers are preferred for SC generation because they provide effective laser beam confinement in a small fiber core area over its entire length. A schematic of SC generation is shown in Figure 1.
Figure 1. Schematic setup for SC generation.
An SC is usually generated by pumping a nonlinear fiber with high-intensity femtosecond pulses delivered by mode-locked lasers. Spectral broadening of the propagation of light pulses is attributed to a combination of various third-order nonlinear effects, such as self-phase modulation (SPM), cross-phase modulation (XPM), four-wave mixing (FWM), dispersive-wave generation (DWG), and stimulated Raman scattering (SRS) [25]. Although this approach can lead to the generation of a wide continuum, the output power is usually limited to only the milliwatt level. To achieve higher output powers, laser systems that provide picosecond and nanosecond pulse trains with a high average output power must be employed.
The dynamics of process depends on the relationship between the duration of the pump pulse, the emission wavelength of the laser source, and the zero-dispersion wavelength (ZDW) of a nonlinear fiber [26]. When is pumped with femtosecond pulses in the anomalous dispersion region, such that the pump wavelength is longer than the ZDW of the nonlinear material, the spectrum broadening is determined mainly by soliton fission and soliton self-frequency shift (SSFS) [27,28]. By pumping an optical fiber with ultrashort pulses in the normal dispersion region, the SC generation mechanism is dominated by SPM. In the case of SC sources pumped with longer pulses (of the order of picoseconds and nanoseconds) at a wavelength corresponding to the anomalous dispersion regime, the emission of new frequencies is initialized with SPM and modulation instability (MI) [29], manifesting as a disintegration of the long pulses into a distributed spectrum of femtosecond sub-pulses during propagation in the fiber core. In the second step, the ultrafast sub-pulses are red-shifted toward the mid-IR region via soliton dynamics, such as soliton fission and soliton frequency shift, owing to the Raman effect [25,29]. The dominant physical mechanisms underlying SC generation in a fiber pumped with longer pulses in the normal dispersion region are essentially FWM and Raman scattering [29,30].
The choice of a nonlinear material that can be considered for SC generation is determined by a number of parameters, such as the transmission band, nonlinearity, ability to handle high optical power, availability of materials of sufficient quality, and maturity of fiber manufacturing technology that is particularly important, considering practical aspects. To date, silica-based fibers are the predominant nonlinear media for SC generation, mainly owing to their high strength, low losses, high resistance to atmospheric conditions, and above all, their mature drawing technology. Such systems utilizing both passive and active silica fibers have been widely reported over the last decade (see [31–33] for examples). However, the longest emission wavelength that can be generated in this medium is ~2.8 μm, which results from the maximum phonon energy of silica glass (1100 cm−1) [34]. The use of silica glass fibers with high-GeO2-doped cores has extended the transmission beyond 3.4 μm [35,36]. However, to extend the spectrum toward the mid-IR further, the use of soft-glass fibers (with wider infrared transmission windows), such as fluoride [37–45], tellurite [46–48], and chalcogenide (ChG) (e.g., As2S3 and As2Se3) [49–55] fibers, is mandatory. Each of these media offers certain advantages and disadvantages, such as ease of manufacture, physical strength, wavelength transmission band, corrosion resistance, and thermal damage.
Fluorozirconate (ZBLAN) fibers have been used for SC generation with an average output power of several tens of watts; however, the output spectrum tends to be limited to ~4.7 μm on the long-wavelength side [38,39,41], which results from intrinsic fiber losses at longer wavelengths. Fluoroindate (InF3) fibers with lower phonon energy can guarantee an extended transmission window (to ~5.5 μm [56,57]) and the delivery of high average power output beams [58–61]. The recent progress in the drawing technology of single-mode tellurite fibers supporting light transmission in a band of ~0.4–5 μm allows considering them theoretically as an alternative to fluoride fibers [46,62,63]. Nevertheless, the current literature reports show that increased material absorption is related to water retention, and, consequently, a reduced damage threshold still hinders the scaling up of the SC power in this medium. Recently, an SC output power of 22.7Wgenerated from a fluotellurite fiber was also demonstrated [64], but in this case, the spectrum long-wavelength edge (LWE) was limited to 3.95 μm. For all aforementioned soft-glass fibers, the LWE of the spectrum is determined by the multiphonon absorption of the nonlinear medium. To achieve emission at wavelengths beyond this edge, short fiber lengths and pumping with high-intensity pulses are required. This approach is typically implemented in a laboratory environment.
Currently, the broadest SC spectra can be generated in ChG fibers with an extremely high nonlinear refractive index n2 that is approximately two orders of magnitude higher than that of fluoride fibers [65]. Although ChG fibers yield an extremely broad mid-IR SC spectrum, even exceeding 10 μm [49,50,66], the overwhelming number of demonstrations to date concern light emission at the milliwatt level. Nonetheless, recent reports on SC generation in As2S3 fibers show that output power scaling up to over 1W with an LWE over 6 μm is possible [67,68]. Further upscaling of average output SC power is also expected, particularly because the damage threshold for arsenic sulfide fibers is rather high, reaching ~2.9 GW/cm2 for 2 ns pulses at 1.9 μm [69] and over 1000 GW/cm2 for 150 fs pulses at 3 μm [70,71].
It is well-known that to achieve efficient spectrum broadening, particularly extending toward the mid-IR region, it is necessary to pump a nonlinear medium in the anomalous dispersion region, relatively close to its ZDW [25,30]. In the case of most available stepindex ChG fibers, the material ZDWs are usually located at wavelengths longer than 4.8 μm that are far away from the emission wavelengths of most available laser sources doped with Er3+, Tm3+, and Ho3+ ions. Consequently, such pumping does not offer optimum broadband SC emission and usually requires the use of complex and expensive optical parametric oscillators (OPOs). In the case of fluoride fibers, the choice of pump sources is wider. Both fluorozirconate and fluoroindate step-index fibers exhibit a ZDW within the wavelength range of ~1.5 to 2.1 μm, covered by the widely available and powerful pulsed laser systems. The advantage of InF3 fibers over ZBLANs is the wider transmission band that renders them ideal candidates for high-power SC generation in the 2–5 μm atmospheric window.
Fluoroindate or indium fluoride (InF3) glass fibers have a maximum phonon energy of ~510 cm−1 [82,83] compared with ~579 cm−1 for fluorozirconate glasses [84,85], and thus, they can provide a transmission window with an LWE extended to ~5.5 μm. They also exhibit a higher damage threshold than ChG fibers [83]. Moreover, fluoroindates have a glass transition temperature (Tg) of ~300 °C [57,81]; that is, higher than those of ZBLANs (Tg ~ 260 °C [86,87]) and ChG glasses (Tg ~ 185 °C for As2S3; Tg ~ 178 °C for As2Se3) [88]. The refractive index of these glasses is within the range of 1.47–1.53 [89,90], comparable to that of silicates, indicating that the Fresnel loss is less than 4%. The nonlinear refractive index n2 has been recently determined to be 3.2–4.3 × 10–20 m2/W [91]; that is, 1.5× higher than that for ZBLAN fibers and an advantage in the context of SC generation.
Currently, InF3 glass can be drawn into high-quality optical fibers. There are three commercial manufacturers of single-mode fluoride fibers (Le Verre Fluoré, Thorlabs, and FiberLabs); however, only two offer single-mode InF3 fibers. Figure 2 shows the attenuation curves of InF3 fibers obtained from two different producers as a function of the wavelength. The commercially available fiber offered by Thorlabs exhibits losses < 0.2 dB/m for all wavelengths between ~1.6 and 4.6 µm with a minimum value of ~0.03 dB/m at ~3.6 µm. However, the attenuation of the custom-made fibers could be even lower. For instance, Le Verre Fluoré provides InF3 fibers with losses as low as 0.02 dB/m within the wavelength band of ~1.9–4.1 µm. For all the presented fibers, for wavelengths over 4.8 µm, the material losses significantly increase, reaching more than 0.6 dB/m at 5 µm. The main contribution to absorption is made by the OH− groups at a wavelength of ~2.9 µm [92]. Multiphonon absorption is predominant beyond 4 µm.
Figure 2. Attenuation spectra of selected single-mode fluoroindate fibers (data provided by manufacturers [93,94]).
Another issue addressed in fluoroindate fibers is their mechanical strength, which is much lower than that of silica fibers [89]. Their fragility results from extrinsic defects, such as microcrystals, microbubbles, and core–cladding interface imperfections [97]. However, in recent years, the strength of all fluoride fibers has been significantly improved, allowing the user to stripe, cleave, and even fusion-splice them [42]. Furthermore, to enhance their strength, the fibers can be coated with Kevlar or stainless-steel jackets and connected with different types of connectors (e.g., FC/PC, FC/APC, and SMA-905). Furthermore, InF3 glass fibers have enhanced stability in atmospheric moisture compared with ZBLANs, owing to the lack of the NaF component. Thus, they satisfy the requirements of many industrial applications.
The optimal pump source for SC generation is a mode-locked laser that emits the high peak-power pulses necessary to trigger the nonlinear processes responsible for continuum evolution. Such pulses can be generated directly by single oscillators as well as optical parametric generators (OPGs) and amplifiers. Therefore, it is not surprising that the first demonstration of SC generation in an InF3 fiber was reported using ultrashort laser pulses [101]. This approach exhibits immense potential for broadband mid-IR SC generation in fluoride fibers; however, owing to the pump laser sources used, the output SC power was limited to milliwatts. To achieve a higher output SC power, more powerful pump sources are required. One successful approach is to use a 1.55-μm nanosecond or sub-nanosecond seed laser in tandem with Er3+-doped (or/and Er3+:Yb3+-doped) fiber amplifiers and Tm3+-doped fiber amplifiers (TDFAs) as pump sources [33,59,105]. Such a laser system configuration is commonly known as a master oscillator power amplifier (MOPA) [106,107,108]. In this method, seed pulses (of picosecond or nanosecond duration and the desired wavelength) can be delivered by Q-switched/gain-switched oscillators (including fiber lasers) or pulsed semiconductor lasers. The optical pulses are subsequently amplified to the required peak power level in an appropriately designed one-stage or multi-stage fiber amplifier. Finally, the amplified pulses are launched into a nonlinear medium.
This pump setup enables scaling up the average output SC power linearly while maintaining a relatively constant spectral width. This can be achieved by increasing the repetition frequency of the pulses generated by the seed with a simultaneous increase in the average pump power that can be easily realized by providing a suitable gain in the final power amplifier of the MOPA system. Provided that the peak power of the pumping pulses is at the same constant level, the width of the generated SC spectrum should be kept constant. An exemplary setup for SC generation based on a MOPA is shown in Figure 3.
Figure 3. Schematic of the setup for SC generation based on a MOPA seeded with ~1.55 µm pulses. EDFA: erbium-doped fiber amplifier, EYDFA: erbium:ytterbium-doped fiber amplifier, SMF: single-mode (silica) fiber, TDFA: thulium-doped fiber amplifier.
A train of seed pulses is first pre-amplified in an erbium-doped fiber amplifier (EDFA) and erbium:ytterbium-doped fiber amplifier (EYDFA). Subsequently, resulting from MI and Raman scattering, the pulses propagating in a short piece of single-mode standard silica fiber split into shorter sub-pulses and are red-shifted [109], leading to a spectrum spanning from ~1.4 to 2.4 µm. Finally, the spectral components from the ~1.9 to 2.1 µm range are amplified and undergo further SSFS to longer wavelengths in a single-stage or dual-stage TDFA that acts as a nonlinear and active medium. This mechanism is well-described in refs. [110,111,112]. The spectral components from ~1.5–1.85 μm are absorbed by the TDF that acts as an isolator, preventing back-reflection-induced damage to the EDFAs and EYDFA, while wavelengths longer than 1.85 μm are amplified and further red-shifted during propagation through the TDF. In the fluoride fiber, the initially broadened pump radiation is extended further into the mid-infrared region, reaching the LWE theoretically determined by material losses.
Broadband and high spectral flatness SC generation can also be achieved by using ps-scale optical pulses with a central wavelength of ~2 μm. Recently, semiconductor fiber-pigtailed lasers operating at approximately 2 μm have become available that have paved the way for the development of fiber-based MOPA systems. Also 2 µm Q-switched or gain-switched and mode-locked laser systems can be effective pump sources for fluoroindate fibers. The main achievements in this field are summarized in the chart shown in Figure 4, which illustrates the average SC output power increase over the past nine years.
Figure 4. Summary of the most important reports on SC generation (with an LWE of output spectrum beyond 4 μm) in fluoroindate fibers published in the last decade. The bandwidth in the diagram represents the full spectral range (* average output power not revealed; it is supposed to be <100 mW).
Mid-infrared supercontinuum sources have developed rapidly in the last 15 years. The emergence of specialty glass fibers, such as fluoroindate fibers, as well as the advances in fiber-based pulsed oscillators and amplifiers, have accelerated the development of high-power SC systems operating in the spectral range of 2–5 μm. In addition to the output power, the spectral width and flatness of the continuum are important parameters that determine the SC source performance and its usefulness. The first demonstration of SC generation in a fluoroindate fiber was reported by Theberge in 2013 [101]. Since then, much effort has been devoted to improving all the key output parameters of such sources. In less than a decade, the output SC power has been scaled up by more than three orders of magnitude (from less than 10 mW to more than 11 W). Furthermore, interesting pump schemes that enable watt-level SC generation have been proposed. As can be seen in Figure 14, remarkable progress has been made in scaling up the output SC power (illustrated by the increased number of reports as well as increasingly higher output powers), particularly during the last four years. A record time-averaged output power of 11.8 W with a spectrum spanning from ~1.9 to 4.9 µm has been demonstrated, which is certainly not the power limit of this technology. This can be scaled up further if improved thermal management and heat dissipation techniques are implemented. Another vital parameter of mid-IR SC generation is the power distribution toward the red wavelengths. Using optimized nonlinear InF3 fibers and applying suitable pump sources, it was possible to demonstrate an LWE of 5.42 μm for low-power systems (an output power of 8 mW) and 5.1 μm for high-power operation (an output power of 4.06 W). Furthermore, an excellent spectral flatness (5 dB@ 2.0–5.0 µm and 10 dB@ 1.96–4.97 µm) was also achieved. This progress cannot occur without suitable pump sources, particularly those allowing output SC power scaling up. In addition to conventional femtosecond ML laser systems, including optical parametric generation techniques, a spectrum of interesting laser system solutions that provide picosecond and nanosecond pulses has been proposed. It includes fiber-based MOPA systems seeded with semiconductor lasers operating at wavelengths of 1.55 and 2 µm, Q-switched and ML fiber single oscillators, 2-µm fast gain-switched and simultaneous ML fiber lasers and amplifiers, and fiber amplifiers directly seeded with picosecond ML lasers. The InF3 fibers with the lowest attenuation and an appropriate profile of the dispersion curve supported the most efficient red-shifting of the generated SC with an LWE of up to 5.42 µm. There is scope for further spectrum extension, but it would be rather difficult because of the high absorption losses over 5 μm resulting from the multiphonon absorption edge of fluoroindate glasses.
This entry is adapted from the peer-reviewed paper 10.3390/app12104927
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