As a quasi-one-dimensional material that combines the excellent physical and chemical properties of crystals with the high specific surface area and optical waveguide characteristics of optical fibers， single-crystal fibers hold broad application prospects in fields such as high-power lasers， mid-infrared lasers， radiation detection， and high-temperature sensing. This paper provides a systematic review of the principles， technical characteristics， and research progress of the edge-defined film-fed growth method， the micro-pull-down method， the laser-heated pedestal growth method， and other novel growth techniques. The edge-defined film-fed growth method enables the high-throughput simultaneous growth of hundreds of optical fibers； the micro-pull-down method has been extended to eutectic， metallic， and spring-shaped irregular crystals； the laser-heated pedestal growth method has successfully produced ultra-fine single-crystal optical fibers over 50 m long with a diameter of only 16 μm； the multi-microchannel crucible method has enabled high-throughput fabrication of fluoride optical fibers； the aqueous solution method， meanwhile， provides a low-temperature growth pathway for organic and thermosensitive single-crystal optical fibers， among other applications. Finally， it summarizes the technical challenges and future directions for each method， offering a reference for the preparation and application of single-crystal fibers.

Single-crystal fiber materials combine the inherent advantages of single-crystal materials in terms of optical， scintillation， and multifunctional properties with the structural benefits of fiber configurations in miniaturization， flexible transmission， and device integration. They hold broad application prospects in fields of radiation detection， laser technology， and optoelectronic devices. In recent years， with the advancement of crystal growth technology， researches on the preparation methods and performance control of single-crystal fiber materials have being continuously deepened. This paper focuses on the research progress in the preparation and mechanical properties of single-crystal fiber materials. It systematically reviews the characteristics， scope of application， and current status of major fabrication processes， such as the laser-heated pedestal growth method， micro-pulling-down method， and Czochralski method. It also outlines the primary material systems for single-crystal fibers and， by using sapphire single-crystal optical fibers as an example， introduces their mechanical properties and defect mechanisms. Overall， the mechanical properties of single-crystal fibers are jointly determined by the intrinsic properties of the material and the preparation process， with defect control and stress regulation being the key to enhancing structural integrity and operational reliability. Future research should focus on controllable fabrication of high-quality， characterization of multiscale mechanical properties， and investigation of multi-field coupled service behavior to advance the development of single-crystal fibers toward high-performance and engineering applications.

Sapphire single-crystal fiber is an promising candidate material for optical sensing in extreme environments due to its excellent properties such as high-temperature resistance， corrosion resistance， radiation resistance and wide spectral transmission. However， most of the existing sapphire single-crystal fibers are non-cladding structures， which have problems such as weak mode confinement， high transmission loss and strong environmental sensitivity， which restrict their practical engineering applications. By constructing a low-refractive-index optical confinement structure outside or inside the sapphire single-crystal fiber， the cladding processing improves its transmission performance， improves the device level and expands the application scenarios. In this paper， the research progress in cladding technologies for sapphire single-crystal fiber is systematically reviewed. These technologies can be classified into three categories： external additive cladding techniques， internal modification cladding techniques， and micro-structured cladding techniques. The implementation principles， fabrication processes， representative studies， and performance characteristics of these technologies are summarized. The development status， technical features， and application potential of different cladding routes are also comparatively analyzed. On this basis， the development direction of sapphire single-crystal fiber cladding technology is prospected for extreme environment transmission and sensing applications.

All-solid-state single-frequency continuous-wave lasers are high-quality light sources for fundamental research and application fields， including quantum optics， high-precision measurement， and so on owing to their intrinsic advantages of narrow linewidth， excellent beam quality， high power stability and low noise. With the rapid development of science and technology and the increasing demands of applications， it has become particularly important to scale up the output power of lasers while maintaining their overall performance.

However， nonlinear effects become more and more severe with the increase of the output power for fiber lasers， and thermal effects are significantly enhanced under high pump power for rod crystal lasers. To address the limitations of fiber and rod crystal gain media， single-crystal fiber， a novel laser medium that combines the advantages of both， has become a more and more popular research hotspot in the field of all-solid-state lasers due to its excellent physicochemical properties， good thermal management characteristic， pump waveguide effect and high stimulated-Brillouin-scattering threshold. As is well known， scaling up the output power of the single-frequency continuous-wave laser is mainly implemented by means of a laser amplifier or a single resonator. This paper mainly focuses on the research progress of single-frequency continuous-wave lasers based on Nd∶YAG single-crystal fibers. A high-power linearly polarized single-frequency continuous-wave 1 064 nm laser based on the single-crystal fiber master oscillator power amplifier is presented， in which a 140 W low-noise single-frequency continuous-wave laser and an Nd∶YAG single-crystal fiber act as the seed laser and the laser medium of the master oscillator power amplifier， respectively. In order to obtain a high conversion efficiency， the mode-matching efficiency between the pump laser propagated with waveguide form and the freely propagated seed laser is optimized by considering the influence of the degradations of the polarization and the beam quality. When the incident powers of the pump and seed lasers are 262.6 and 126.3 W， respectively， the output power of the linearly polarized single-frequency laser reaches up-to 208 W. To the best of our knowledge， this is the highest output power based on an Nd∶YAG single-crystal fiber master oscillator power amplifier. In addition， a hybrid single-frequency continuous-wave 1 064 nm laser is presented， where both two Nd∶YVO₄ bulk crystals and two Nd∶YAG single-crystal fibers are employed as gain media. By combining the advantages of the good thermal management characteristic of the Nd∶YAG single-crystal fiber with the natural birefringence effect of the Nd∶YVO₄ bulk crystal， a stable single-frequency laser with high output power is achieved. Based on this scheme， a 120 W hybrid single-frequency continuous-wave 1 064 nm laser is obtained by employing Nd∶YAG single-crystal fibers as gain media in a single resonator.

The presented laser technologies provide a good reference for achieving high-power single-frequency continuous-wave laser output at different wavelengths based on single-crystal fibers， which will strongly promote the development of laser technology and related fundamental research and application fields.

Extreme environments， including aerospace， nuclear reactors， and deep-earth exploration， impose stringent requirements on reliable sensing technologies. Traditional silica-based optical fibers are prone to structural degradation under high temperatures and intense radiation. In contrast， single-crystal optical fibers， represented by sapphire and yttrium aluminum garnet （YAG）， have attracted attention as promising alternative media due to their excellent physical and chemical stability. This paper reviews the research progress of single-crystal optical fiber sensing technologies tailored for extreme environments. It summarizes the evolution of mainstream growth techniques for single-crystal optical fibers， such as laser-heated pedestal growth and the micro-pulling-down method， and discusses the improvements in mode control achieved through the development of cladding structures. Furthermore， it provides an in-depth analysis of the applications of in situ microstructure processing technologies—typified by femtosecond laser plane-by-plane inscription—in the fabrication of sensing components like high-quality fiber Bragg gratings. The paper also reviews practical monitoring applications of single-crystal optical fiber sensors under extreme operating conditions， including ultra-high temperatures， intense radiation， and high pressures. Finally， it outlines the future development trajectories of single-crystal optical fiber sensing technology， particularly focusing on material defect suppression， the preparation of specialized claddings， and anti-interference in signal demodulation.

Fiber Fabry-Perot （F-P） microcavity sensors garner widespread attention in the field of physical parameter sensing under high-temperature environments， owing to their advantages such as electromagnetic interference immunity， strong environmental adaptability， small size， and high precision. This paper reviews the research progress of sapphire fiber F-P microcavity sensors. The structural characteristics and high-temperature performance of intrinsic type， air-gap type， wafer type， thin-film deposition type， and composite cavity type microcavity sensors are sorted out， summarizing their stable measurement capability in high-temperature environments， with a maximum temperature measurement of up to 1 800 ℃. The current microcavity sensing technologies are confronted with key challenges， including long-term high-temperature stability， multi-parameter decoupling， system integration and large-scale preparation processes. Driven by new manufacturing technologies， sapphire fiber F-P microcavity sensors are moving towards miniaturization， batch fabrication， and multi-purpose applications.

The rapid development of aerospace， nuclear energy， and advanced manufacturing has increased the demand for reliable high-temperature sensing materials and technologies for extreme environments. Conventional silica fibers suffer from poor thermal stability at high temperatures， while sapphire fibers， although widely used， still face challenges in stable sensing above 1 800 ℃. Lutetium oxide （Lu₂O₃） single-crystal fiber （SCF） is a promising alternative because of its high melting point of approximately 2 490 ℃， excellent thermal stability， and favorable optical properties. It also retains the structural advantages of optical fibers， including a large aspect ratio， small diameter， and potential for device integration， making it attractive for high-temperature sensing beyond the temperature limits of sapphire fibers.

In this work， high-quality Lu₂O₃ SCFs with ［100］， ［110］， and ［111］ orientations were successfully grown by the laser-heated pedestal growth method. The fibers are uniform and transparent， with a diameter of 60 μm and a length of 1 m， and show no visible cracks or inclusions， indicating good crystal quality and stable growth. A third-order fiber Bragg grating （FBG） was then fabricated in the ［111］-oriented Lu₂O₃ SCF by femtosecond laser line-by-line scanning. The grating has a length of 4 mm and a period of 1.223 μm， and its reflection spectrum is centered at 1 549.47 nm with a full width at half maximum of 0.843 nm and a signal-to-noise ratio of 15.3 dB， confirming the formation of a well-defined grating structure. The temperature sensing performance of the fabricated Lu₂O₃ single-crystal fiber Bragg grating （LFBG） was systematically investigated over a wide temperature range from 20 ℃ to 1 600 ℃. The grating maintain a stable and repeatable spectral response during both heating and cooling， indicating excellent thermal stability of both the grating structure and the fiber material. As the temperature increase， the reflection peak shift monotonically toward longer wavelengths， with a maximum temperature sensitivity of 23.2 pm/℃ at 1 600 ℃.

These results demonstrate that LFBG can maintain stable optical response and reliable sensing performance at high temperatures. Owing to their high melting point， good optical transparency， and compatibility with miniaturized sensing devices， Lu₂O₃ SCF represents a promising platform for temperature sensing in extreme environments， including aerospace engines， nuclear energy systems， high-temperature furnaces， and advanced manufacturing equipment.

In this study， a Tm³⁺-doped CaGdAlO₄ （Tm∶CALGO） disordered single-crystal fiber with a doping concentration of 4% （molar fraction） was grown by the micro-pulling-down method. The crystal structure， optical properties， continuous-wave laser performance， and passively Q-switched pulsed laser characteristics integrated with BiOX （X=Cl， Br， I） layered crystal saturable absorbers were systematically investigated. The crystal belongs to the tetragonal system with the space group I4/mmm. X-ray diffraction and Raman spectroscopy show that the as-grown crystal exhibits a pure CALGO phase with good crystallinity. The absorption spectrum reveals a main absorption peak at 793 nm with a full width at half maximum of 19.4 nm， which is attributed to the inhomogeneous broadening induced by the disordered structure. The highest phonon energy measured by Raman spectroscopy is 614 cm^-1， which is lower than that of YAG crystal and beneficial for suppressing non-radiative relaxation. Fourier-transform infrared transmission spectroscopy shows that the crystal exhibits a transmittance exceeding 80% in the 2~6 μm mid-infrared region， with an infrared cut-off edge extending beyond 10 μm. In a straight cavity configuration， a maximum continuous-wave output power of 3.338 W is achieved from the Tm∶CALGO single-crystal fiber， with a slope efficiency of 27.3% and a central wavelength of 1 982.8 nm. When BiOX crystals were integrated into the laser cavity as saturable absorbers， stable passively Q-switched pulsed laser outputs were obtained under an output coupler transmittance of 5%. With BiOCl， BiOBr， and BiOI as saturable absorbers， the maximum single-pulse energies obtained are 18.564， 20.335， and 17.599 μJ， the shortest pulse widths are 420， 570， and 506 ns， and the highest peak powers are 44.199， 35.675， and 34.781 W， respectively. This work demonstrates the excellent performance of Tm∶CALGO disordered single-crystal fibers as gain media for 2 μm lasers and provides a feasibility validation for the integration of single-crystal fibers with novel saturable absorbers.

Laser-heated pedestal growth （LHPG） method is a well-established， crucible-free technique for fabricating micrometer-scale single crystal optical fibers， in which a localized molten zone is generated by laser heating of a feed rod and directional crystal growth is realized through precisely controlled feeding and seed-crystal pulling. As a fibrous “quasi-one-dimensional” functional crystal， the single crystal fiber combines the large aspect ratio of conventional glass fibers with the superior physical and chemical properties of bulk crystals， making it highly attractive for high-power lasers， high-energy radiation detection， high-temperature sensing， and optical communication. Owing to its extremely high heating temperatures， freedom from crucible contamination， and rapid solidification rates， LHPG is widely regarded as the only viable route for growing single crystal fibers at the 100 μm diameter scale. However， the high-temperature molten zone is governed by strongly coupled multiphysical interactions—including heat conduction， thermal radiation， melt convection， phase change， and surface-tension-driven flow—whose pronounced nonlinearity， together with the inability of conventional experiments to observe internal thermal and flow fields in real time， leaves the mechanisms governing process stability and interface evolution insufficiently understood and limits further improvements in crystal quality and dimensional uniformity.

To address this gap， a two-dimensional axisymmetric multiphysics model was established on the COMSOL Multiphysics platform， coupling geometric-optics-based laser energy absorption， solid-liquid heat transfer， and fluid dynamics that incorporates the Marangoni effect； the molten-zone boundary was fitted from experimental images of stably growing sapphire fiber. Using a 1 mm diameter α-Al₂O₃ feed rod， the study systematically investigated the coupled influences of laser-power fluctuation mode， fiber diameter， Marangoni convection， and pulling speed on the thermo-fluid characteristics of the molten zone. The results show that the form of laser-power fluctuation （5% amplitude） strongly affects thermal stability： square-， sine-， and triangular-wave modulation produced molten-zone temperature swings of 88， 80， and 61 K， respectively， with the triangular wave generating the weakest thermal shock and the smoothest temperature response， identifying it as the preferred reference signal for power-stabilization loops. Increasing the fiber diameter from 200 μm to 300 μm lowered the axial temperature gradient and yielded a more uniform thermal distribution； in both cases the gradient displayed a characteristic edge-high， center-low pattern， peaking near the laser-irradiated region and the solid-liquid interface， which indicates that small-diameter growth is inherently more difficult to stabilize. With a bond number far below unity， Marangoni convection was confirmed as the dominant flow mechanism： introducing surface-tension gradients raised the maximum melt velocity from the order of 10^-5 m/s （buoyancy-driven natural convection alone） to 0.07~0.1 m/s and reconstructed the flow field from a single vortex into a complex double-vortex structure. Finally， whereas a baseline pulling speed of 1.28×10^-4 m/s preserved both thermal and flow stability， raising it by three orders of magnitude destroyed the coupled steady state， necessitated a doubling of laser power， severely distorted the temperature field， and produced jet-like high-speed flow conducive to growth interruption.

Overall， this study clarifies the strongly coupled thermo-fluid mechanisms governing the LHPG process and the regulatory roles of key processing parameters in molten-zone stability. The findings provide a comprehensive theoretical foundation for optimizing growth conditions and practical guidance for improving the quality， uniformity， and structural integrity of single crystal optical fibers， particularly sapphire and other advanced functional crystalline materials.

With the development of high-power solid-state lasers and fiber lasers， the demand for optical isolators in the visible to near-infrared region is gradually increasing， necessitating the exploration of novel magneto-optical crystals to meet development requirements. In this paper， high-quality dysprosium aluminum garnet （Dy₃Al₅O₁₂， DyAG） single-crystal fibers were grown using the laser-heated pedestal growth method. The growth process of the crystals was investigated， and their fundamental physical properties were studied. XRD patterns show that， the crystal structure of DyAG fiber is consistent with that of conventional garnets， belonging to the cubic crystal system with space group of ${Ia} \overline{3} d$ and lattice parameters of a=b=c=11.974 Å. The crystal samples exhibit narrow diffraction peak full width at half maximum， indicating excellent crystalline quality. The electronic structure of DyAG fiber was calculated using first-principles method and the results reveal a relatively wide bandgap of 4.311 eV， with a refractive index maintained at approximately 1.93 for wavelengths greater than 500 nm. Further characterization and analysis of the crystal's optical properties reveal its excellent light transmittance in the wavelength range from 475 nm to 700 nm， achieving a transmittance of 77.5% which is nearly reaching the theoretical value of 80.8%. This makes it promising for application in magneto-optical devices operating within this spectral band.

In order to meet the demand for high-temperature monitoring in extreme high-temperature environments， this paper employs a femtosecond laser line-by-line scanning technique to inscribe high-quality gratings in magnesium aluminate （MgAl₂O₄） single-crystal fibers （MSCF） oriented along the ［100］， ［110］， and ［111］ crystallographic directions， respectively. The temperature response characteristics of these gratings were experimentally analyzed over the temperature range from room temperature （20 ℃） to 1 600 ℃. The results indicate that gratings with different crystal orientations exhibit excellent thermal stability and cyclic repeatability， and their central wavelengths show a regular redshift trend with increasing temperature. Benefiting from the intrinsic isotropy of the cubic crystal system， the gratings with three different crystal orientations show similar sensitivities （~39 pm/℃） in the high-temperature region. Through a comparative analysis with the sensing characteristics of sapphire fiber Bragg gratings （SFBGs）， this study further validates the potential advantages of MSCF in high-temperature environments， demonstrating the application potential of MSCF in ultra-high temperature extreme environment sensing scenarios.

Non-contact micro-displacement measurement is important for monitoring critical components in high-temperature harsh environments. Optical fiber sensors are attractive for such applications because of their compact size， electromagnetic immunity， and remote interrogation capability. Reflective Fabry-Pérot sensors with an external air cavity are particularly suitable for non-contact displacement measurement because the variation in the distance between the fiber end face and an external reflector can be converted into a change in the air-cavity length， which can be demodulated from the reflection spectrum. However， conventional cladding-free sapphire fibers support multimode transmission， which may induce mode coupling， spectral fluctuations， and unstable Fabry-Pérot interference signals. In this work， a reflective external Fabry-Pérot air-cavity micro-displacement sensor based on a femtosecond-laser-written single-mode guiding structure in sapphire fiber was proposed and fabricated. The external Fabry-Pérot air cavity was formed between the sapphire fiber end face and a gold-coated mirror. To improve the transmission characteristics of the sapphire fiber， refractive-index modification was introduced inside the fiber by femtosecond laser direct writing. A depressed-cladding structure was constructed in which the laser-modified peripheral region acted as an effective low-index cladding， while the unmodified central region served as the guiding core. With this design， single-mode transmission over a length of 4 cm was achieved， and multimode-transmission-induced interference in the sapphire fiber was effectively suppressed. The resulting clearer interference fringes provided a stable spectral basis for displacement demodulation. The displacement response of the sensor was experimentally investigated under different displacement step sizes. For the 5 μm step test， the cavity length was demodulated by calculating the average free spectral range of the reflection spectrum. The demodulated cavity length exhibited excellent linearity with the applied displacement， with a fitted slope of 1.000 0 μm/μm and a coefficient of determination R² of 0.999 97. These results indicate that the applied displacement can be accurately converted into the variation of the Fabry-Pérot air-cavity length. To further evaluate the response under smaller displacement variations， displacement tests with step sizes of 500， 100 and 50 nm were carried out. In these tests， a selected interference dip was tracked， and the displacement-wavelength response relationship was established. The displacement-wavelength sensitivities are obtained as 3.558 3， 3.368 5 and 3.037 6 nm/μm for the 500， 100 and 50 nm step tests， respectively. These results demonstrate that the proposed sensor provides reliable response and high-resolution tracking capability for displacement steps down to 50 nm. The main innovation of this work is the integration of a femtosecond-laser-written single-mode guiding structure in sapphire fiber with a reflective external Fabry-Pérot air cavity for non-contact micro-displacement measurement. The depressed-cladding structure improves the transmission state of the sapphire fiber and reduces the influence of multimode interference on the reflection spectrum， thereby enhancing the reliability of displacement demodulation. Benefiting from the excellent high-temperature stability of sapphire fiber， the proposed sensing structure shows potential for non-contact micro-displacement measurement in high-temperature environments. This study provides a useful reference for the development of single-mode sapphire-fiber-based displacement sensors for harsh-environment applications.

Sapphire-derived fibers （SDFs） have attracted increasing attention as promising optical fiber platforms for sensing applications in extreme environments， owing to the excellent thermal stability and chemical inertness inherited from sapphire-based core materials. Nevertheless， the controllable fabrication of SDFs with designed core diameters and stable optical performance remains a key challenge for their practical use in high-temperature sensing. Most previous studies have focused on material characterization or device-level demonstrations， whereas the relationship among preform design， drawing parameters， core-diameter controllability， and transmission quality has not been sufficiently clarified. In this work， we demonstrate the controllable fabrication of SDFs using millimeter-scale sapphire rods as core preforms in combination with the molten core method （MCM）. The aim is to realize stable continuous drawing of SDFs with tunable core diameters and to verify their feasibility for high-temperature optical sensing.During fabrication， a sapphire rod was used as the core material and was thermally processed within a silica cladding during high-temperature fiber drawing. By adjusting the drawing parameters， SDFs with different core diameters were obtained while maintaining a standard outer cladding diameter of 125 μm. Three representative SDF samples were selected for systematic characterization， with core/cladding diameter configurations of 21.0/125 μm， 18.1/125 μm， and 14.1/125 μm for samples Ⅰ， Ⅱ， and Ⅲ， respectively. The results demonstrate that the MCM-based drawing process enables effective regulation of the SDF core geometry， providing a feasible route for preparing SDFs with controllable structural parameters. Optical transmission measurements show that the as-drawn SDFs exhibit a low propagation loss of 1.08 dB/m in the near-infrared wavelength region， indicating that favorable optical quality can be maintained during the drawing of sapphire-derived core fibers.To further evaluate the sensing potential of the fabricated SDFs， a Fabry-Pérot interferometer （FPI） high-temperature sensor was constructed using the SDF with a 14.1 μm core diameter. The high-temperature sensing performance of the device was investigated over a wide temperature range from 100 ℃ to 1 100 ℃. The experimental results show that the SDF-based FPI sensor achieves a temperature sensitivity of 16.8 pm/℃ and exhibits good linear response characteristics， with a coefficient of determination （R²） higher than 0.997. In addition， the sensor maintains good repeatability during heating and cooling cycles， confirming the stability of the SDF-based interferometric structure under the present high-temperature testing conditions.This study demonstrates that the combination of millimeter-scale sapphire core preforms and the MCM drawing process provides an effective approach for the controllable fabrication of SDFs with different core diameters. The results clarify the influence of preform design and drawing-parameter regulation on the geometrical structure and optical transmission properties of SDFs. More importantly， the successful demonstration of an SDF-based high-temperature FPI sensor verifies the feasibility of the fabricated fibers for optical sensing in high-temperature environments. This work therefore provides both a material-processing basis and a device-level validation for the development of SDF-based high-temperature fiber sensors.