Radial IR-GRIN lens prepared by multi-temperature fields manipulated gradient crystallization within chalcogenide glass.

Chalcogenide glass has achieved great success in manufacturing axial-type infrared gradient refractive index (IR-GRIN) lenses. However, studies on radial-type IR-GRIN lenses, which are more ideal for optical design, remain rare. The present study introduces what we believe to be a new method for preparing radial IR-GRIN lens by creating high refractive index (n) In2S3 nanocrystals within a 65GeS2-25In2S3-10CsCl (GIC, in molar percentage) glass matrix. Upon introduction of multi-temperature field manipulation, we have successfully achieved central crystallization and simultaneous gradient attenuation spreading toward the edge within GIC glass, providing a radial GRIN profile with Δn over 0.1 while maintaining excellent IR transparency. In addition, the optical and structural properties of the GIC GRIN samples were characterized. The relationship between Raman intensity and the n of glass ceramics at different heat treatment temperatures was investigated, thereby enabling the indirect confirmation of the presence of radial gradient crystallization within the prepared GIC GRIN samples through Raman intensity. Multiple experimental results have shown that this approach has excellent reproducibility and potential for large-scale productions.

Investigation of a twisting-fused side-pump coupler based on tellurite fiber.

Fiber side-pump couplers can enhance the output power of fiber laser due to their dependable and efficient operation and impressive power handling capability. We developed a tellurite fiber side-pump coupler by twisting and fusing a tapered pump fiber onto a target fiber. The effect of twisting parameters on coupling efficiency was comprehensively investigated through theoretical simulations and experiments. Experimental results exhibited an impressive coupling efficiency of 76.5% and a root mean square stability of 0.086% and 0.091% before and after one month, respectively, driven by an incident pump power of up to 4.2 W.

Compact and low-insertion-loss polarization beam-splitting multimode filter using pixelated waveguides.

A polarization beam-splitting multimode filter using pixelated waveguides has been presented and experimentally demonstrated in this paper. Finite difference time domain method and direct binary search optimization algorithm are employed to optimize pixelated waveguides to realize compact size, broad bandwidth, large extinction ratio, low insertion loss, and good polarization extinction ratio. Measurement results show that, in a wavelength range from 1520 to 1560 nm, for the fabricated device working at transverse-electric polarization, the measured insertion loss is less than 1.23 dB and extinction ratio is larger than 15.14 dB, while for transverse-magnetic polarization, the corresponding insertion loss lower than 0.74 dB and extinction ratio greater than 15.50 dB are realized. The measured polarization extinction ratio larger than 15.02 dB is achieved. The device's length is only 15.4 µm.

High-efficiency and broadband tunable Raman fiber laser in a chalcogenide fiber based on the Fresnel reflection.

A high-efficiency and broadband tunable chalcogenide fiber Raman laser with the Fabry-Perot (F-P) cavity formed by the Fresnel reflection was established. A maximum average power slope efficiency of around 43% and a maximum output peak power of about 2.9 W at 2148 nm were demonstrated by using a 2 µm nanosecond pump source. The laser shows a broadened pulse width of 674 ns and a broadband tunability of the central wavelength from 2100 to 2186 nm. The Raman Fabry-Perot cavity constituted by the Fresnel reflection from chalcogenide fiber endfaces can operate at any wavelength without the aid of any additional optical feedback element. This will facilitate the realization of fiber lasers with excellent performance and compact system, especially in the mid-infrared region.

Simulation of the generation conditions and influence parameters of a self-mode-locked erbium-doped fiber laser.

The generation conditions and influence parameters of self-mode-locked pulses in fiber lasers are theoretically studied. By establishing the simulation model of a self-mode-locked erbium-doped fiber laser (EDFL) with a high-concentration erbium-doped fiber-based saturable absorber (SA), the effect of gain saturation energy, orientation angles of the polarizer and analyzer with respect to the fast axis of the fiber, laser coupling output ratio, dispersion value and condition on the self-mode-locked pulse generation and performances are quantitatively analyzed. The result shows that a low laser coupling output ratio can help the formation of a self-mode-locked pulse. The anomalous dispersion self-mode-locked EDFL has a relative high tolerance for dispersion value change but requires high gain energy for mode-locked pulse generation. The normal dispersion one possesses a low mode-locked pulse formation threshold but is relative polarization sensitive. This study is of important reference significance for the investigation of mode-locked fiber lasers.

Solution-processed Er3+-Doped GeS2 chalcogenide glass films with NIR photoluminescence towards functional flexibility and integration.

Rare-earth doped chalcogenide films are major components in flexible and integrated photonic and optoelectronic devices for modern communication systems, metrology, and optical sensing. However, it is still challenging to develop a high concentration of rare-earth doping chalcogenide film with a smooth surface to realize efficient photoluminescence (PL). Here, we demonstrate that Er3+-doped GeS2 films are prepared by spin-coating based on a two-step dissolution process. Such a two-step process provides the high solubility of Er3+ in GeS2 films and exhibits efficient emission at ∼1.5 µm crossing the telecommunication C-band. The highest PL emission intensity is obtained in GeS2 films doped with 1.4 mol% of Er3+, and this PL in GeS2 films is reported for the first time. We propose adjustments of annealing parameters for improving the PL characteristics in such materials. Through the control precision of the heating rate and annealing temperature, the smooth surface of GeS2 films enables efficient photo-luminescence. This two-step dissolution-based strategy would pave a new path to design luminescent chalcogenide films for application in flexible and integrated optoelectronics and photonics.

Large mode-area all-solid anti-resonant fiber based on chalcogenide glass for mid-infrared transmission.

A large mode-area chalcogenide all-solid anti-resonant fiber has been designed and successfully prepared for the first time. The numerical results show that the high-order mode extinction ratio of the designed fiber can reach 6000, and the maximum mode-area is 1500 um2. The fiber possesses a calculated low bending loss of less than 10-2 dB/m as the bending radius is larger than 15 cm. In addition, there is a low normal dispersion of -3 ps/nm/km at 5 µm, which is beneficial for the transmission of high power mid-infrared laser. Finally, a completely structured all-solid fiber was prepared by the precision drilling and two-stage rod-in-tube methods. The fabricated fibers transmit in the mid-infrared spectral range from 4.5 to 7.5 µm with the lowest loss of 7 dB/m @ 4.8 µm. Modeling suggests that the theoretical loss of the optimized structure is consistent with that of the prepared structure in the long wavelength band.

Detection of copper ions in an aqueous solution by a dual-peak long period fiber grating functionalized with a polypyrrole-chitosan composite.

A dual-peak long period fiber grating (DP-LPFG) sensor functionalized by polypyrrole-chitosan composite was proposed for sensitive detection of Cu2+ ions in aqueous solution. The nitrogen atom on the polypyrrole ring and the amino group on the chitosan chain in the complex matrix can chelate the Cu2+ ions. Thus, the refractive index of the overlay changed and further modulated the transmission spectrum of DP-LPFG. After special design, the double peaks can move in opposite directions with the increase of Cu2+ ion concentration, thereby greatly improving detection sensitivity. The linear sensitivity of the fabricated sensor was measured to be 9.12 and 2.14 nm/ppm (0.61 and 0.14 nm/µM) for concentrations of 0.1-0.5 (1.5 µM-7.5 µM) and 0.5-2 ppm (7.5 µM-30 µM), respectively. In addition, the Langmuir isothermal model was used to evaluate the overall response of the sensor to Cu2+ ions quantitatively, and the detection limit was determined to be 0.05 ppb (0.75 nM). This ingenious sensor offers a new solution for sensitive detection of heavy metal ions in environmental water.

Chalcogenide GRIN glasses with high refractive index and large refractive index difference for LWIR imaging.

Gradient refractive index (GRIN) materials utilize an internally tailored refractive index in combination with the designed curvature of the optical element surface, providing the optical designer with additional freedom for correcting chromatic and spherical aberrations. In this paper, new GRIN materials suitable for the second (3-5 µm) and third (8-12 µm) atmospheric windows were successfully developed by the thermal diffusion method based on Ge20As20Se60-xTex series high refractive index glasses, where the maximum refractive index difference (Δn) at 4 µm and 10.6 µm were 0.281 and 0.277, respectively. The diffusion characteristics and refractive index distribution of the GRIN glass were analyzed by Raman characterization. Furthermore, the performance of GRIN singlet and homogeneous singlet in the LWIR band (8 µm, 10.6 µm (primary wavelength), 12 µm) was compared, and the results showed that the GRIN singlet had better chromatic aberration correction and unique dispersion characteristics.

Low-loss chalcogenide microstructured optical fibers prepared by eliminating interfaces defects.

The loss of chalcogenide microstructured optical fibers (ChG-MOFs) is generally higher than that of step fibers, mainly due to the immature fiber preform preparation method and strong waveguide defect scattering. Chemical polishing is used to polish mechanically drilled preforms to prepare ChG-MOFs with low defect scattering. Firstly, the scattering loss caused by the defective layer of ChG-MOFs is studied theoretically and experimentally. Then, a single-mode photonic crystal fiber (PCF) was prepared to verify the effect of chemical polishing on reducing fiber loss. The experimental results show that the PCF average loss is reduced from more than 8 dB/m to less than 2 dB/m, and the minimum loss reaches 0.8 dB/m @ 2.7 µm. At the same time, the bending strength of the PCF after chemical polishing is also significantly improved.

Fabrication of a tellurite-fiber-based side-pump coupler based on the tapered-fused method.

In this study, (1 + 1) × 1 side-pump couplers made of tellurite fibers were fabricated and investigated. The whole optical design of the coupler was established on the basis of ray tracing models and validated by experimental results. By optimizing the preparation conditions and structural parameters, the tested component achieved a coupling efficiency of 67.52% and an insertion loss of 0.52 dB. To the best of our knowledge, this is the first time a tellurite-fiber-based side-pump coupler was developed. The fused coupler presented will simplify many mid-infrared fiber lasers or amplifier architectures.

Supercontinuum generation in As2S3 chalcogenide waveguide pumped by all-fiber structured dual-femtosecond solitons.

Supercontinuum sources with high compactness are essential for applications such as optical sensing, airborne detection and communication systems. In the past decades, the adoption of bulky optical parametric amplifier to pump various chalcogenide glass waveguides are widely reported for on-chip mid-infrared supercontinuum generation, but this usually leads to a large volume of the whole system, and is not practical. Therefore, integrating advanced femtosecond fiber lasers with optical waveguides using nano-fabrication technology are highly desired. However, the scarcity of compact pump sources and the dispersion-matched high-nonlinearity waveguide in short wavelength regions have hindered the advancement of integrated supercontinuum source performances in the near and mid-infrared region. In this study, we demonstrate a broadband supercontinuum source from As2S3 waveguide pumped by a compact dual-femtosecond solitons pulse source. The laser is completely fiber structured, and its wavelength can be readily tuned from 2 to 2.3 µm using Raman soliton self-frequency shift technology in a Tm3+-doped fiber amplifier. Furthermore, the As2S3 waveguide is designed with controllable dispersion and high nonlinearity for a broadband supercontinuum generation. These results will advance the development of on-chip supercontinuum sources based on chalcogenide waveguides.

Design and fabrication of large-mode-area multicore chalcogenide fiber with low bending loss.

Multicore fiber (MCF) has a larger mode-area (LMA) compared to traditional single-core fiber, making it easy to get a mode area of more than 3000 µm2 with an optimized MCF structure. Here, a fine-structured 19-core fiber based on chalcogenide glass was fabricated using a combined method involving extrusion, drilling, and rod-in-tube for the first time. The fiber has a minimum transmission loss of 1.8 dB/m at 6.7 µm. When the bending radius exceeds 6 cm, a low bending loss of about 0.6 dB appears, and the experimental data are in good agreement with the simulation results. In addition, the supermode characteristics of the 19-core fiber are analyzed from both perspectives of simulation and experiment, and these results are perfectly in good agreement. We believe it opens a new way to develop high-power and bend-resisting fiber with such kind of multicore structure.

Optimization of multispectral chalcogenide glass for large-size fabrication.

Chalcogenide glass has become one of the essential IR lens materials in passively athermalized long-wave IR devices. However, that there is no multispectral chalcogenide glass capable of large-size fabrication raises challenges to the development and popularization of multispectral imaging systems combining visible, near-IR, and mid-IR. In this work, we developed a novel chalcogenide glass capable of a record-big (Ø120 mm) fabrication through the compositional optimization of GeS2-Ga2S3-CsCl glass with introduction of Sb2S3. Its transmission window is characterized as ranging from 0.51 to 11.2 µm, which means it could be employed as a multispectral lens transmitting visible and IR signals in a co-aperture IR optical system. In addition, a method of three-stage thermal analysis is proposed to evaluate the glass-forming ability of chalcogenide glass through simulating the melt-quenching process of chalcogenide melt in a vacuum-sealed silica ampoule. This work not only shows an innovative multispectral chalcogenide glass with promising applications but also introduces a simple and convenient technique for screening chalcogenide glass with ultrahigh glass-forming ability capable of large-size fabrication.

High-Q lasing in Nd3+-doped phosphate glass microsphere resonators.

Nd3+-doped glasses are the most widely used laser gain media. However, Nd3+-doped non-silica microsphere lasers generally have lower quality (Q) factors due to the presence of non-radiative energy-loss impurities in traditional glass systems. In this work, we report the first, to the best of our knowledge, Nd3+-doped phosphate glass microsphere laser with the highest Q-factor of 1.54 × 106 among all Nd3+-doped non-silica glass microsphere lasers. Whispering gallery modes in the 1020-1120-nm band can be obtained for a typical microsphere with a diameter of 82.57 µm. When the pump power exceeds the threshold of 0.17 mW, single- and multi-mode microsphere lasing can be generated under 808-nm laser diode (LD) pumping. Typical Q-factors of the phosphate glass microspheres can reach 106, which is at least an order of magnitude higher than those of other Nd3+-doped non-silica glass microsphere lasers. The Nd3+-doped phosphate glass microsphere laser reported in this work can be considered as an active optical/photonic device with low pump thresholds.

Low-loss and broadband polarization-insensitive high-order mode pass filter based on photonic crystal and tapered coupler.

In this Letter, a polarization-insensitive high-order mode pass filter is presented, designed, and experimentally demonstrated. When TE0, TM0, TE1, and TM1 modes are injected into the input port, TM0 and TE0 modes are filtered, and TE1 and TM1 modes exit from the output port. To attain compactness, broad bandwidth, low insertion loss, excellent extinction ratio, and polarization-insensitive property, the finite difference time domain method and direct-binary-search or particle swarm optimization algorithm are employed for the optimization of structural parameters of the photonic crystal region and the coupling region in the tapered coupler. Measurement results reveal that, for the fabricated filter working at TE polarization, the extinction ratio and insertion loss are 20.42 and 0.32 dB at 1550 nm. In the case of TM polarization, the corresponding extinction ratio and insertion loss are 21.43 and 0.30 dB. Within a bandwidth from 1520 to 1590 nm, insertion loss smaller than 0.86 dB and extinction ratio larger than 16.80 dB are obtained for the fabricated filter working at TE polarization, while in the case of TM polarization, insertion loss lower than 0.79 dB and extinction ratio greater than 17.50 dB are realized.

Third-order optical nonlinearity of CsPb(Br/I)3 metal halide perovskites nano-crystals embedded chalcogenide glass.

The nonlinear optical properties of emerging metal halide perovskites (MHP) materials are sufficiently intriguing that this topic has become the hotspots in the realm of material science. Hence, we investigate the third-order nonlinear optical properties of CsPbBrx/I3-x (x = 1, 2, 3) MHP nano-crystals (NCs) embedded chalcogenide glass (ChG) within a GeS2-Sb2S3 pseudo-binary system, by monitoring the composition, excitation wavelength and intensity dependencies via femtosecond Z-scan technique. We have found that the intrinsic large optical nonlinearity of ChG can be further enhanced because of the incorporation of MHP NCs, and that the optical nonlinearity of MHP-ChG containing pristine Br NCs is more pronounced compared to its counterparts with mixed Br/I NCs, due to a combination of multiple factors.

Mid-infrared biomimetic moth-eye-shaped polarization-maintaining and angle-insensitive metalens.

Metalenses can potentially reduce the size and complexity of existing cameras, displays, and other optical devices, owing to their capability of flexible manipulation of the polarization, amplitude, and phase of light. However, metalenses capable of maintaining polarization and broadband wavefront shaping under arbitrarily polarized excitation have not been studied. In this study, we present the first demonstration of a biomimetic moth-eye-shaped metalens for polarization-maintaining, broadband and angle-insensitive focusing under an arbitrarily polarized excitation in the mid-infrared waveband (3.1-8.0 µm). Modulation and focusing efficiencies of 92% and 90%, respectively, were achieved. Moreover, a bifocal moth-eye-shaped metalens operating at normal and oblique incidences was realized. Compared to previously reported metalenses, the one proposed in this study exhibited a better focusing under oblique incidence, ensuring light transmission as effectively as a traditional lens. This study paves the way for the development of polarization-maintaining, broadband, and angle-insensitive microscale optical devices and imaging systems.

Single-mode suspended large-core chalcohalide fiber with a low zero-dispersion wavelength for supercontinuum generation.

Chalcogenide glass possesses outstanding advantages, such as supercontinuum generation, but its nonlinear applications were limited by large zero-dispersion wavelength (ZDW). Traditional suspended-core fibers can shift the ZDW to near IR with a tiny core size of less than 5 µm but a large evanescent wave loss exists in these fibers. In this paper, we prepared a novel suspended-core fiber (SCF) based on chalcohalide glasses for the first time via the extrusion method, in which the ZDW of the fundamental mode in the fiber with a core size of larger than 30 µm was successfully shifted to 2.6 µm. We also calculated confinement loss (CL) of propagation modes and fundamental mode energy ratio in the fiber. We found that the minimum CL ratio of the high order modes (LP11) to the CL of the fundamental mode is 124, indicating that the single-mode operation condition is satisfied when the wavelength is more than 4.6 µm. The lowest transmission loss is 1.2 dB/m at 6.5 µm. An ultra-broad supercontinuum spectrum, covering from 1.6 to 12 µm was generated in this suspended-core fiber pumped by a 5 µm femtosecond laser. Such a wide SC in the chalcogenide SCF is due to the large core size. All these results demonstrate the potential to use the large core SCF in the application of a mid-IR laser.

Rapid and sensitive detection of ammonia in water by a long period fiber grating sensor coated with sol-gel silica.

A sensitive ammonia sensor based on long-period fiber grating (LPFG) is designed and manufactured for the detection of ammonia concentration in water. Femtosecond laser direct writing technology is used to write LPFGs on standard single-mode silica fiber. A thin layer doped with basic dyes is coated on the optical fiber for sensing by using the sol-gel method. The thicknesses of sol-gel layers, which play a key role in the sensitivity of the LPFG sensor, were carefully studied. Experimental results show that LPFG with a functional layer of ∼340 nm has the best sensing performance, and the detection limit is 0.08 ppm. The response time of the sensor is less than one minute, and the sensor has good repeatability with a short recovery time. Compared with other organic molecules and ions in water, the proposed LPFG sensor has not only good reusability, but also selectivity for the detection of ammonia.