Chalcogenide Glass Microfibers for Mid-Infrared Optics

Abstract

With diameters close to the wavelength of the guided light, optical microfibers (MFs) can guide light with tight optical confinement, strong evanescent fields and manageable waveguide dispersion and have been widely investigated in the past decades for a variety of applications. Compared to silica MFs, which are ideal for working in visible and near-infrared regions, chalcogenide glass (ChG) MFs are promising for mid-infrared (mid-IR) optics, owing to their easy fabrication, broad-band transparency and high nonlinearity, and have been attracting increasing attention in applications ranging from near-field coupling and molecular sensing to nonlinear optics. Here, we review this emerging field, mainly based on its progress in the last decade. Starting from the high-temperature taper drawing technique for MF fabrication, we introduce basic mid-IR waveguiding properties of typical ChG MFs made of As₂S₃ and As₂Se₃. Then, we focus on ChG-MF-based passive optical devices, including optical couplers, resonators and gratings and active and nonlinear applications of ChG MFs for mid-IR Raman lasers, frequency combs and supercontinuum (SC) generation. MF-based spectroscopy and chemical/biological sensors are also introduced. Finally, we conclude the review with a brief summary and an outlook on future challenges and opportunities of ChG MFs.

1. Introduction

In the past decades, optical microfibers (MFs) have brought wide opportunities in renewing and expanding fiber optics and technology at a wavelength scale [1,2,3,4,5]. Fabricated by a high-temperature taper-drawing technique [6], MFs exhibit excellent surface smoothness and diameter uniformity, which bestow them favorable features of low waveguiding loss [1,7], tight optical confinement, high fractional evanescent fields and large manageable waveguide dispersion [8], making them a versatile platform for both scientific research (e.g., optical nonlinearity [9,10,11] and atom optics [12,13,14]) and technological applications (e.g., optical sensors [15,16,17] and microlasers [18,19,20]). As one of the most widely studied optical materials, MFs based on silica have been employed for a variety of applications [3,4]; however, their uses in the mid-infrared (mid-IR, 2.5–20 μm) spectral range are restricted by strong absorption of silica glass [21]. In recent years, along with the rapid progress in mid-IR photonics, MFs for mid-IR photonics have attracted increasing interest [22,23].

Typically, mid-IR MFs are fabricated from a category of mid-IR-transparent materials, including oxide glasses (e.g., germanates [24], fluorotellurites [25] and tellurites [26]), fluoride glasses (ZBLAN) [27,28], chalcogenide glasses (ChGs, glasses containing one or more chalcogens: sulfur (S), selenium (Se) and tellurium (Te)) [29,30,31], as well as semiconductors (e.g., cadmium telluride (CdTe) and [32] silicon (Si) [33]). Among these materials, ChGs are mostly investigated for their special merits, including broadband intrinsic transparency (0.5–25 μm), high optical nonlinearity (about 100–1000 times larger than that of silica glass) and hospitality to rare-earth dopants [29,34,35]. The typical optical loss of ChG fibers in the mid-IR is around 0.1–10 dB/m [35], while the lowest loss, down to 0.012 dB/m at 3 μm, has been reported in multimode As₂S₃ fibers [36]. Additionally, photonic devices based on ChG MFs, including resonators [37,38], gratings [39,40,41] and sensors [42,43], have been investigated in the visible (VIS) or near-infrared (near-IR) region and have recently extended to the mid-IR region [44,45,46]. In this review, combined with the latest progress in ChG MFs, we first introduce the fabrication and optical waveguiding properties of ChG MFs. Secondly, we introduce ChG MF-based passive optical devices (e.g., couplers, resonators and gratings), as well as active and nonlinear applications of ChG MFs in the mid-IR region including Raman lasers, frequency combs and supercontinuum (SC) generation, followed by MF-based spectroscopy and chemical/biological sensors. Finally, we summarize this review and present a brief outlook into future challenges and opportunities in this field.

2. Fabrication

Excellent surface smoothness and geometric uniformity are regarded as essential guarantees for achieving low-loss optical waveguiding in MFs. Typically, for MFs made of glass, the high-temperature taper-drawing process is the best technique for fabricating high-quality MFs with extremely low surface roughness (e.g., <0.5 nm [47]), excellent diameter uniformity and circular cross-section [48]. Tapered fibers based on mid-IR materials such as tellurite glass and fluorotellurite glass have been successfully fabricated [25,26]. Besides, owing to the relatively low softening temperature (T_s, e.g., 180 °C for As₂S₃ glass) of ChGs, ChG MFs are typically fabricated from ChG fibers heated by an electric heater (several millimeters in heating zone length) at a temperature slightly higher than the T_s [31,49], as schematically illustrated in Figure 1a. When stretching the fiber bi-directionally at both sides, the fiber is elongated and forms a waist (i.e., the MF) with two transition tapers connected to the untapered fibers. Close-up scanning electron microscope (SEM) images of an as-fabricated ChG (As₂S₃ glass) MF are shown in the inset of Figure 1a, showing the excellent surface smoothness and diameter uniformity.

Usually, made from soft glass, as-fabricated ChG MF is relatively fragile and easily damaged by mechanical disturbance. To enhance the mechanical strength of ChG MF and reduce its sensitivity to the surrounding environment, in 2010, Baker et al. reported a kind of hybrid ChG-PMMA fiber taper consisting of an As₂Se₃ core and a PMMA cladding [31]. The PMMA cladding possesses a T_s compatible with As₂Se₃ glass and enables taper-drawing fabrication of an MF from the two materials at the same time. A 9.7-cm-length hybrid biconical As₂Se₃ fiber taper with a core diameter down to 0.8 μm and a PMMA cladding diameter of 2.4 μm was fabricated successfully. In 2011, they demonstrated a generalized heat-brush tapering approach, which allowed the ratio of the feed and drawing velocities changes within each tapering sweep [50]. The waist diameter of the as-fabricated As₂Se₃ fiber taper decreased linearly from 15 to 10 µm over a 2.0-centimeter length. In 2016, Li et al. fabricated polymer-cladded (PC, COP or PMMA) As₂Se₃ fiber tapers with a waist diameter down to 1.5 µm and a waist length of 10 cm [49], as illustrated in Figure 1b. Both ends of the ChG fiber tapers were polished and adhered to single-mode silica fibers using UV-cured epoxy for efficient light input and output. Owing to the large waveguide nonlinear parameter and engineerable chromatic dispersion of the MFs, nonlinear optical effects including self-phase modulation, four-wave mixing and Raman scattering with wavelength up to 2.2 µm could be achieved.

To ease the tapering system and obtain long MFs with uniform diameters with a large reduction ratio of fiber diameter between the preform fiber and the final MF, in 2020, Xie et al. demonstrated a two-step taper-drawing process [45]. As schematically shown in Figure 1c1, commercial As₂S₃ fiber was first heated and stretched bi-directionally with a low speed (0.4 mm·s⁻¹). Secondly, the pre-drawn MF with a diameter in several micrometers was stretched unidirectionally at a fast rate (6 mm·s⁻¹) at one end and a slow rate (0.4 mm·s⁻¹) at the other end to draw a long-waist MF with a uniform diameter. Biconical fiber tapers with a waist length longer than 5 cm and diameters of around 3 μm were easily fabricated for assembling mid-IR photonic devices. In addition, as-drawn As₂S₃ MF (3.5-μm diameter) could be assembled into a 62-μm diameter knot structure with an elastic strain of up to 5.6% (Figure 1c2), indicating the excellent structural uniformity and surface-defect-free condition of the MF.

3. Optical Waveguiding Properties

MFs offer a number of attractive optical properties including tight optical confinement, strong evanescent field and tailorable waveguide dispersion [4,8]. The special merits of ChGs bestow the ChG MFs more possibilities. In this section, we discuss the optical properties of As₂S₃ (transmission range: 0.7–6 μm [35]) and As₂Se₃ (transmission range: 1–10 μm [35]) MFs contributed to applications such as optical waveguiding and nonlinearity. We assumed that the MF has a circular cross-section and an infinite air cladding with a step-index profile, and only the fundamental mode (HE₁₁) was considered. The refractive indices of the As₂S₃ (As₂Se₃) glass around 1.55-μm and 4-μm wavelengths are 2.44 (2.73) and 2.41 (2.68), respectively [51,52]. The refractive index of As₂Se₃ glass around 8-μm wavelength is 2.67.

The effective refractive index (n_eff = β/k₀, β is the propagation constant of the guided mode, k₀ = 2π/λ is the wavenumber of the transmission wavelength λ), effective mode area (A_eff) and fractional evanescent fields (η) are essential waveguiding parameters for designing fiber-optic devices, including near-field couplers, gratings, resonators and sensors [31,53]. Figure 2a–c shows the calculated As₂S₃ and As₂Se₃ MF diameter-dependent n_eff, A_eff, and η in the mid-IR region, respectively. The results in the 1.55-μm wavelength (dash lines) are also shown for comparison. ChG MFs with diameters larger than the wavelengths of the guided light exhibit n_eff of HE₁₁ mode close to the refractive indices of the bulk materials (Figure 2a), which is independent on the guided wavelength. When the diameter reduces close to or smaller than the wavelength, the n_eff decreases dramatically, indicating more fractional evanescent fields exist around the MF (Figure 2b), beneficial for enhancing interactions between the mid-IR light and samples surrounding the MF in applications such as optical trapping, mid-IR spectroscopy and chemical/biological sensing related to molecular fingerprints [3,17,54]. Meanwhile, owing to the high nonlinear refractive index n₂ of As₂S₃ and As₂Se₃ materials and tight optical confinement (i.e., small A_eff) of the guided modes in MFs, these MFs usually have large effective nonlinearity γ (γ = k₀ n₂/A_eff) and relatively low-threshold nonlinear optical effects when their diameters are close to the wavelength of the guided light in the material (i.e., λ/n, λ and n are the wavelength in air and refractive index of material, respectively. See Figure 2c) [31,55].

4. Photonic Applications

4.1. Near-Field Optical Couplers

MF near-field couplers are simple fiber-optic components that can be used for efficient in/out-coupling with other optical structures such as micro/nano waveguides [48,56,57,58], whispering gallery mode (WGM) microcavities [59,60] and photonic crystal structures [61,62,63]. By precisely adjusting the coupling length or gap distance between the MF and the photonic structure, the phase-matching condition (and thus the coupling efficiency) can be precisely adjusted. MF near-field couplers possess the advantages of easy operation and small footprints.

With increasing demands of in/out coupler at mid-IR range in recent years, mid-IR MFs have been applied in the near-field optical coupling of a variety of photonic structures, including mid-IR waveguides and microcavities. For example, in 2018, to test the waveguiding properties of CdTe microwires (a quasi-one-dimensional single-crystal semiconductor) in the mid-IR region, Xin et al. used a 1.5-μm-diameter As₂S₃ MF as input coupler and a 5-μm-diameter As₂S₃ MF as output coupler for a 1.5-μm-diameter CdTe microwire (Figure 3a) [32]. The input MF diameter was carefully chosen to ensure single-mode operation within the 4.45–4.7 μm range. The relatively large effective refractive index contrast between the As₂S₃ MF and CdTe microwire can decrease the end-face reflection and achieve flat broadband coupling over the 4.45–4.7 μm and 8.36–8.6 μm range.

High-quality factor (Q, defined as the ratio of the resonant wavelength to the full width at half-maximum (FWHM)) resonators in the mid-IR are critical to promote the performances of mid-IR laser sources, cavity-based sensors and mid-IR frequency comb [66]. However, achieving ultra-high Q cavities and measuring their quality factors in the mid-IR remain challenging. In 2016, Lecaplain et al. developed an efficient coupling technique for WGM-based fluoride crystalline microresonators assisted by tapered ChG fibers (Figure 3b1) [64]. The ChG tapered fibers were pulled down to the 1-μm diameter (i.e., a ChG MF) to achieve phase matching with the fundamental WGMs of the resonator. By translating the microresonator along the taper relative to the waist, the phase-matching conditions were tuned precisely. Figure 3b2 shows the normalized transmission as a function of the coupling parameter. The data were consistent with the theoretical model ($K={\kappa }_{ex}/{\kappa }_0=\left(1\pm \sqrt{T}\right)/\left(1\mp \sqrt{T}\right)$, in which ${\kappa }_0$ is the intrinsic loss rate, ${\kappa }_{ex}$ is the coupling loss rate and T is the transmission). More recently, Xie et al. used As₂S₃ biconical tapered fibers for the first characterization of ChG microsphere resonators in the mid-IR region (Figure 3c) [65]. A 1.6-μm-diameter As₂S₃ fiber taper was used for near-field coupling with ChG microspheres with the coupling loss less than 3 dB over 4.465–4.705 μm.

4.2. MF-Based Resonators

The combination of low-loss waveguiding and high-efficiency evanescent coupling between adjacent MFs bestows a kind of competitive high-Q resonators in various structures (e.g., loop [67,68], knot [1,69], ring [70,71,72] and coil [73,74]), showing the desirable features of easy fabrication, fiber compatibility and resonance tunability. During the past two decades, silica MF-based resonators have attracted wide attention in the VIS and near-IR applications, including optical filters, sensors and lasers [57,74,75,76]. Additionally, there are several kinds of high-Q (10⁴–10⁵) resonators based on ChG MFs in the near-IR region, including SU8 polymer-embedded As₂S₃ MF knot resonators (MKRs) (Figure 4a1,a2) [38], self-touching As₂Se₃ MF loop resonators (MLRs) by thermally shaping As₂Se₃ glass into an MF and splicing to cleaved silica fiber tapers (Figure 4b1,b2) [37] and resonators based on photoinduced WGMs localized in the cross-section of the ChG MFs [77]. These studies are beneficial to the further exploitations of ChGs, but they were not extended to the mid-IR region.

In 2020, Xie et al. demonstrated the first ChG MF-based resonator in the mid-IR region [45]. As₂S₃ MFs with uniform diameters and long waists were fabricated via a two-step taper drawing process (Figure 1c1). The MF structure was immersed in a drop of ethyl alcohol to avoid possible surface damage and assembled to a knot with double ends naturally connected to fiber tapers. Figure 4c1 shows an 824-μm diameter MKR assembled from a 3.2-μm-diameter As₂S₃ MF. A Q factor of about 2.84 × 10⁴ was obtained at the wavelength of 4469.14 nm (Figure 4c2,c3). The free spectral range (FSR) of the MKR can not only be tuned by tightening the knot structure in liquid (Figure 4c4), but the resonance peaks can also be thermally tuned at a thermal tuning rate of 110 pm·°C⁻¹ (Figure 4c5). Furthermore, to increase the long-term stability of the MKR device and isolate it from contaminations, a 551-μm-diameter MKR was embedded in low-index polymer (PMMA) film while a Q factor of 1.1 × 10⁴ around 4.5-μm wavelength was retained.

As MF-based mid-IR resonators are essential for MF-based optical technology, they are expected to attract increasing attention in mid-IR optical applications, including mid-IR microlasers, narrow-wide filters, cavity-enhanced spectroscopy and the label-free detection of molecules [54,78].

4.3. MF Gratings

MF gratings have attracted intense attention owing to their miniaturized footprints, large fraction of evanescent field and high flexibility. Typical grating fabrication techniques include femtosecond (fs) laser inscription [79,80], ultraviolet laser irradiation [81] and focused ion beam milling [82,83]. So far, MF gratings have been employed for photonic sensors for measuring the refractive index, force and temperature, as well as for functionalized devices, including tunable filters and Fabry–Perot cavities [84,85,86,87].

Previously, ChG fiber gratings working in the mid-IR region [88] and ChG MF gratings working in the near-IR have been reported [40,89] (Figure 5a), while mid-IR MF gratings have been seldomly studied. In 2020, Cai et al. demonstrated the first mid-IR MF Bragg grating (mFBG) based on an As₂S₃ MF (Figure 5b1) [46]. The transmission spectrum shows a 15 dB depth around 4.5-micrometer wavelength (Figure 5b3), indicating a photo-induced refractive index change of 0.02. The dependence of the grating formation on the accumulated influence of exposure power density and time was also investigated. The interference pattern exposure technique can be applied to other photosensitive ChG MFs.

Compared with conventional long-period grating (LPG), MFs could increase the depth of the evanescent field into the ambient environment, which may be helpful to enhance the performance of MF-based optical sensors. In 2019, Wang et al. reported a temperature sensor based on long-period fiber gratings (LPFGs) inscribed on tapered multimode ChG fibers [90]. Simulation results show that the temperature sensitivity could be increased by reducing the waist diameter of tapered fiber. A resonant wavelength at 3 μm was obtained when the period of LPFGs was 176 μm and the maximum sensitivity of temperature could achieve 12.6 nm/°C at a 3-micrometer wavelength.

4.4. Raman Lasers

The Raman gain provided by silica fibers has led to Raman fiber lasers in the near-IR range for optical imaging and telecommunications-related applications [91]. The mid-IR Raman fiber lasers have been applied in LIDAR, pharmaceutical science and military applications [92,93]. The Raman gain coefficients of typical ChGs (e.g., 4.3~5.7 × 10⁻¹² m/W for As₂S₃ and 2~5 × 10⁻¹¹ m/W for As₂Se₃ at 1.5 μm [94]) are more than 900 times larger than that of silica, making them excellent candidates for the mid-IR Raman lasers. In 2017, Abdukerim et al. demonstrated an all-fiber Raman laser based on a PMMA-coated 10-centimeter As₃₈Se₆₂/As₃₈S₆₂ MF [95]. The Raman laser emitted at 2.025 μm with pump pulses at 1.938 μm and a threshold pump power of 4.6 W (Figure 6a). To generate a shower of Raman solitons at the same wavelength, in 2021, Guo et al. pumped a 1960-nanometer picosecond laser into a tapered fluorotellurite MF and demonstrated Raman solitons around 3 μm [96]. The group velocity dispersion was controlled by the diameter of the fluorotellurite MF, and all the generated solitons could be put at the same wavelength.

4.5. Frequency Conversion

Besides the large nonlinear optical coefficient and broad mid-IR transparency, ChGs are excellent candidates for mid-IR frequency conversion (e.g., optical parametric oscillation (OPO) and optical parametric amplification (OPA)) for their high laser damage threshold and moderate birefringence [100]. The efficiency of frequency conversion depends on phase matching, nonlinear gain and losses. The tapering of ChG fibers to increase nonlinearity while compensating the normal material dispersion has been demonstrated. In 2016, Abdukerim et al. reported the first fiber optical parametric oscillator (FOPO) using an As₂Se₃-COP MF at 2 μm based on four-wave mixing (FWM) [101]. The MF with a core diameter of 1.47 μm led to a waveguide nonlinearity of 24 W⁻¹m⁻¹ and a zero-dispersion wavelength of 1.875 μm. The FOPO had a pump threshold power of 5 W and covered a tunable wavelength range of 290 nm. In the FWM process, where pump and idler are far-detuned, the exact phase-matching wavelength is highly sensitive to chromatic dispersion and hard to control. In 2019, Alamgir et al. demonstrated the far-detuned wavelength converter by in-situ monitoring the output wavelength while tapering down the As₂Se₃ fiber [97]. As the tapering proceeded, the phase-matching condition was satisfied closer to the pump wavelength and the gain spectrum gradually shifted towards shorter wavelengths. The idlers were generated with a spectral range of 2.347–2.481 μm from a pump wavelength of 1.938 μm (Figure 6b).

The modulation instability (MI) is another frequency conversion process where the low-amplitude noise is parametrically amplified on the pump signal together with the growth of symmetric sidebands on both sides of the pump. Although most of the spontaneous MI requires pumping in the anomalous dispersion regime, MI can also be pumped in a normal dispersion regime when the fiber possesses a higher-order group velocity dispersion profile. To prove the broadband frequency conversion due to normal dispersion MI, in 2014, Godin et al. demonstrated a parametric frequency conversion based on MI at 2 and 3.5 μm by pumping an As₂Se₃-polymer MF with an fs OPO at 2.63 μm [98]. The measured frequency shift of 30 THz was the largest reported, using normal dispersion pumped MI in a single-pass configuration (Figure 6c). To prove the potential for improving the technical specifications of mid-IR fiber wavelength converter in terms of tunability and conversion efficiency, in 2017, Li et al. generated tunable parametric sidebands via MI in a 10-centimeter long As₂Se₃-CYTOP tapered MF with a diameter of 1.625 μm (Figure 6d) [99]. The widely spaced Stokes and anti-Stokes bands were generated with a frequency shift as large as 49.3 THz, the largest reported in soft glass materials.

4.6. Supercontinuum Generation

Supercontinuum (SC) generation in optical fibers originates from the interaction between ultrashort laser pulses and fibers with high nonlinearity [107]. ChGs are regarded as good candidates for mid-IR SC generation with a large transparency window and comparatively high optical nonlinearity [22].

The uses of MFs reduce the power consumption and allow a shorter interaction length for SC generation. In 2013, Al-kadry et al. demonstrated SC generation from 1260 to 2200 nm using a 10-centimeter long As₂Se₃ MF [108]. The pump wavelength was self-frequency shifted to 1775 nm in the anomalous dispersion regime of the As₂Se₃ MF to avoid two photons’ absorption. In the same year, Rudy et al. reported a generated SC from 1 to 3.7 μm in a 2.1-millimeter length As₂S₃ MF with 300-pJ 2-nanometer ultrafast pump pulses [109]. The spectral bandwidth was limited by the absorption caused by third harmonic generation. In 2014, Al-kadry et al. further used pump pulses of 800 fs at a 1.94-μm wavelength to generate an SC covered bandwidth of two octaves from 1.1 to 4.4 μm [102]. The waveguide nonlinearity of a 1.6-micrometer diameter As₂Se₃ MF was as high as 32.2 W⁻¹m⁻¹_, resulting in a pumping power of only 500 pJ (Figure 7a). In 2017, Wang et al. investigated the dependence of the SC spectral behavior on the waist diameter and the transition region length of the tapered MF (Figure 7b) [103]. They reported a 1.4–7.2-micrometer SC generation with an average power of 1.06 mW from a 12-cm-length As-S tapered MF pumped at 3.25 μm.

Tapering the fiber is also an important method to obtain a highly coherent SC by designing an all-normal dispersion of fibers to suppress soliton effects including soliton fission and Raman soliton self-frequency shift. In 2019, Li et al. reported a 1.5–8.3 μm SC generation with a high coherence property from As-S tapered MF pumped at 3.75 μm (Figure 7c) [104]. They found that the coherence properties gradually become worse when the waist core diameters of tapered MFs decrease and a flat and near-zero dispersion is beneficial for high-coherence SC generation.

To meet the demanding specifications including robust and alignment-free sources of broadband SC, the multimaterial all-fiber-based SC systems have drawn much attention. In 2015, Sun et al. fabricated multimaterial As₂Se₃-As₂S₃ ChG MFs with a zero-dispersion wavelength less than 2 μm [110]. The SC generation from 1.4 μm to longer than 4.8 μm was demonstrated by pumping the MF with 100 fs pulses at 3.4 μm. In 2017, Hudson et al. reported a 2.4 octave-SC generation by pumping a robust polymer-protected As₂Se₃-As₂S₃ MF with 230 fs, 4.2 kW peak power pulses at 3 μm (Figure 7d) [105]. An average power spectral density of 0.003 mW/nm was achieved. Furthermore, significant power existed at various octave points throughout the SC, making the system a good candidate for an f-2f interferometer.

It is worth mentioning that another type of ChG MFs, tapered fibers with suspended-core geometry that operate in the anomalous dispersion regime, have been used to generate SC within a broad spectrum. In 2018, Anashkina et al. designed and developed As₃₉Se₆₁ suspended-core MFs for mid-IR SC generation. SC generation ranging from 1 to 10 μm was proposed with 150-fs 100-pJ pump pulses at 2 μm [111]. In 2020, Leonov et al. reported SC generation covering 1.4 to 4.2 μm in As₃₉S₆₁ suspended-core MFs pumped by a Cr:ZnSe laser. A maximum SC average power of ~35 mW was obtained with an optical efficiency of ~35% [112].

In addition, materials other than ChGs have also been investigated for SC generation. For example, compared to that of ChG, telluride glasses have shorter zero-dispersion wavelengths that are desirable for near-to-mid IR SC generation. In 2019, Saini et al. obtained an SC spectrum spanning 1.28–3.31 μm using a 3.2-centimeter-long tellurite tapered MF pumped with 200 fs laser pulse at 2.0 μm [26]. Compared to fluoride and chalcogenide fibers, fluorotellurite fibers based on TeO₂-BaF₂-Y₂O₃ have better chemical and thermal properties. In 2019, Li et al. demonstrated broadband SC generation from 0.6 to 5.4 μm in a fluorotellurite tapered MF pumped by a 2010 nm fs pulses [25]. Recently, polysilicon waveguides have emerged as an alternative platform as they are cheap and flexible to be incorporated into a wide range of geometries. In 2019, Ren et al. fabricated the tapered polycrystalline silicon core MFs (SCFs), and an SC spanning from 1.8 to 3.4 μm was obtained [33].

4.7. Frequency Combs

In the past years, frequency combs have been studied from the VIS and near-IR to the mid-IR regions. Mid-IR combs are of great use for molecular fingerprint spectroscopy and are typically produced by OPOs [113,114], difference frequency generation [115,116], microresonators [117,118] and fs laser generation [119]. Wide frequency combs spectra in mid-IR have also been demonstrated using PPLN-based OPOs pumped by fiber-laser-based near-IR combs [120]. The mid-IR MFs could further broaden the frequency combs by SC generation when the generated SC maintains coherence with the pump laser. In 2012, Marandi et al. used a subharmonic OPO and subsequent SC generation in the tapered MF to convert the pulses of a conventional 1.5-micrometer frequency comb source to mid-IR [106]. The spectrum of the MF output extended from 2.2 to 5 μm at 40 dB below the peak (Figure 7e1). The beat frequencies in Figure 7e2 verified that the coherence properties of the initial frequency combs presevered. It is worth mentioning that, to simplify the mid-IR frequency comb generation, in 2014, Lee et al. demonstrated mid-IR frequency comb generation using a hybrid ChG-silica nanospike waveguide [121]. The waveguide was pumped directly by a two-micrometer frequency comb and could minimize the unwanted dispersion of pump pulses. The waveguide was small and simple using free-space coupling into and out of the fiber without an intermediate OPO.

4.8. MF-Based Sensors

The optical fiber sensors operating in the mid-IR region where the vibrational and rotational states of molecules are located play an important role in biomedical [122], chemical [123] and environmental applications [124].

The principle of fiber evanescent wave spectroscopy (FEWS) is based on the interaction between evanescent fields and chemical or biological specimens to be investigated, showing desirable advantages of analyzing samples in situ in real time. In past decades, many FEWS experiments have been applied in different areas including the detection of contaminants in water [128], metabolic profiling of chronic diseases [129] and monitoring of chemical processes [130]. The characterization of human tissue based on mid-IR FEWS is an important method for early and rapid diagnosis of diseases including tumors or cancers [131]. Typically, the fiber diameter used for FEWS is much larger than the working wavelength, leaving much space for sensitivity enhancement.

Owing to the characteristic fingerprints of a wide range of gases (e.g., CO, CO₂, NO and CH₄) falling in the mid-IR region, MF-based gas sensors have been proposed based on mid-IR absorption spectroscopy in the last decade [132]. In 2019, Huang et al. demonstrated a nonlinear gas sensor based on third harmonic generation (THG) in cascaded ChG MFs for detecting CH₄ (Figure 8a1) [125]. Mid-IR fingerprint light was absorbed in the first section of MF and the rest transmission signal was converted into a near-IR signal after THG at the second section. The proposed mid-IR ChG MF CH₄ sensor had a theoretical detection limit as low as 7.4 × 10⁻⁸ (Figure 8a2). Another configuration combines graphene with MFs, which can provide high-sensitivity optical gas sensing from permittivity change of the graphene. In 2019, Wang et al. reported a mid-IR gas sensor based on the graphene Bragg grating integrated with the Si slot MF (Figure 8b1) [126]. The NO₂ concentration could be determined from the 3 dB bandwidth of the Bragg grating’s reflection band (Figure 8b2). A detection limit below 1 ppm and a sensitivity of 1.02 nm/ppm were achieved.

The optimization of the tapered transition structure of the fiber sensor is essential to obtain high-efficiency excitation of the evanescent fields to further enhance the sensor sensitivity. In 2021, Wang et al. reported a geometrical optimization of Ge-Sb-Se tapered MF for concentration sensing of methanol (Figure 8c1,c2) [127]. The highest sensitivity of 0.0120 a.u./% was obtained from the MF with a down-taper transition length of 5.4 mm and an up-taper transition length of 7.9 mm. Meanwhile, Wang et al. from the same group investigated the dependence of the sensitivity (of aqueous ethanol solution) on the MF parameters (e.g., diameter and length) [133] and predicted that the sensitivity could be improved by decreasing the MF diameter and increasing the length of the taper waist.

5. Discussion

In this article, a comprehensive survey of recent works on ChG MFs for the mid-IR range is presented, where the fabrication methods and the optical waveguiding properties (i.e., n_eff, A_eff and η) of ChG MFs are discussed. So far, high-quality ChG MFs (e.g., As₂S₃ and As₂Se₃ MFs) have been fabricated by a high-temperature taper drawing process and have shown great promise for broadband low-loss optical waveguiding in the mid-IR region. Based on these MFs, a new category of compact and fiber-compatible photonic devices has been developed for passive, active and nonlinear applications in the mid-IR region. These initial results have unambiguously suggested the great opportunities of using the ChG MF as a versatile platform for a miniaturized fiber-based platform for mid-IR optics.

6. Conclusions

Looking into the future, there are obvious challenges and opportunities for ChG MFs. First, compared with silica glass, ChGs possess a much higher refractive index and optical nonlinearity that are advantageous for tighter optical confinement and lower threshold for nonlinear optical effects but exhibit much lower stability against optical, thermal, chemical and mechanical damages. Therefore, better package or protection techniques/protocols should be developed for real applications of ChG MFs, and more research efforts could be made on the material aspects. Secondly, compared to those for the VIS and near-IR spectral ranges, fiber-compatible devices for mid-IR, ranging from light sources, couplers, polarizers and filters to amplifiers, are typically not mature or optimized, leaving both challenges and opportunities in the future study of mid-IR MFs. Finally, other potentials, such as MF-incorporated plasmonic and atom optics that have been quite successful in the VIS/near-IR region, have not yet been equivalently investigated in mid-IR, which could be the new driving force in this field.