Design and characterization of a self-matching photonic lantern for all few-mode fiber laser systems.

We model and demonstrate a self-matching photonic lantern (SMPL) device, which is designed to address the constraint of limited transverse modes generated by fiber lasers. The SMPL incorporates a FMF into the array at the input end of a traditional photonic lantern. The few-mode fiber at the output end is specifically configured to align with the few-mode fiber at the input, therefore named as SMPL. This paper details the design and fabrication of the SMPL device, validated by both simulation and experiment. The 980nm fundamental mode, injected via 980nm single-mode fibers, selectively excites corresponding higher-order modes at the few-mode port of the SMPL. Additionally, 1550nm fundamental and higher-order modes injected at the input end into the SMPL device demonstrates mode preservation and low-loss transmission characteristics. The SMPL is well-suited for developing a ring laser system, enabling selective excitation of 980nm pump light modes and facilitating closed-loop oscillation and transmission of 1550nm laser.

Measuring cell deformation by microfluidics.

Microfluidics is an increasingly popular method for studying cell deformation, with various applications in fields such as cell biology, biophysics, and medical research. Characterizing cell deformation offers insights into fundamental cell processes, such as migration, division, and signaling. This review summarizes recent advances in microfluidic techniques for measuring cellular deformation, including the different types of microfluidic devices and methods used to induce cell deformation. Recent applications of microfluidics-based approaches for studying cell deformation are highlighted. Compared to traditional methods, microfluidic chips can control the direction and velocity of cell flow by establishing microfluidic channels and microcolumn arrays, enabling the measurement of cell shape changes. Overall, microfluidics-based approaches provide a powerful platform for studying cell deformation. It is expected that future developments will lead to more intelligent and diverse microfluidic chips, further promoting the application of microfluidics-based methods in biomedical research, providing more effective tools for disease diagnosis, drug screening, and treatment.

A solid-state optical filter at 1530 nm based on Voigt anomalous dispersion effect in rare earth ion-doped crystal.

This paper proposes an ultra-narrow band solid state optical filter with Voigt anomalous dispersion at 1530 nm based on Er3+: LiYF4, sets a theoretical model for its realization of ultra-narrow band optical filtering, and performs simulations based on the model. The results show that the maxi-mum transmission of the filter is close to 80%, while the line-width is only in the order of 100 MHz, while its transmission peak can be tuned flexibly by adjusting the magnetic field. This filter has a natural advantage in space laser communications, which is another promising ultra-narrow band optical filter.

Mechanisms of Upconversion Luminescence of Er3+-Doped NaYF4 via 980 and 1530 nm Excitation.

To date, the mechanisms of Er3+ upconversion luminescence via 980 and 1530 nm excitation have been extensively investigated; however, based on discussions, they either suffer from the lack of convincing evidence or require elaborated and time-consuming numerical simulations. In this work, the steady-state and time-resolved upconversion luminescence data of Er3+-doped NaYF4 were measured; we therefore investigated the upconversion mechanisms of Er3+ on the basis of the spectroscopic observations and the simplified rate equation modeling. This work provides a relatively simple strategy to reveal the UCL mechanisms of Er3+ upon excitation with various wavelengths, which may also be used in other lanthanide ion-doped systems.

Auxiliary interferometer in an optoelectronic swept-frequency laser and its application to the measurement of the group refractive index.

An optoelectronic swept-frequency laser (SFL) is an optoelectronic feedback system that includes an auxiliary interferometer that can exert precise control over the optical frequency sweep. The arm-length difference (ALD) of the auxiliary interferometer directly affects the performance of the whole system. We established a theoretical model to choose the optimal ALD of an auxiliary interferometer in an optoelectronic SFL system using a frequency-modulated continuous-wave reflectometry experimental setup. The experimental results indicated that, based on our system, the optimal ALD was 7 m, which agreed with the theoretical analysis. As an example application, we implemented the proposed system for measurement of the group refractive index of a glass sample. A minimum measurement error of 0.12% was obtained with the ALD of 7 m.

On-line dynamic detection in the column chromatography separation based on an optical fiber surface plasmon resonance sensor.

In this design, we introduced a surface plasmon resonance (SPR) fiber-sensing probe into a column chromatography (CC) system to realize on-line dynamic detection in sample separation. The refractive index of the gel around the probe would be adjusted dynamically by the concentration change of the sample during CC separation. To demonstrate the separation and on-line detection process, bovine serum albumin (BSA) and riboflavin-5-phosphate sodium (FMN-Na) are chosen as the analytes in a Sephadex gel filtration chromatography system. The results show that the apparent reversible shift of the SPR spectrum can characterize the separation process. Specifically, the separated BSA with an outflow time of 8 min can cause a resonance wavelength shift of 15.5 nm, and the FMN-Na with an outflow time of 26 min can cause a shift of 8.4 nm. This on-line dynamic detection of SPR spectra has great potential to save time and simplify the analysis process compared to the complex thin layer chromatography detection steps in traditional manual CC.

Upconversion enhancement through a facile, ultrafast, and low-threshold laser annealing strategy.

Particle size significantly affects the brightness of luminescent nanocrystals. Herein we firstly adopt a 1530 nm CW laser as the optical heating source to increase the particle size of Er3+ heavily doped nanocrystals, leading to giant enhancement of the luminescent intensity. The advantages of this method are mainly feature along the facile route, with an ultrafast process, and low threshold of the laser power density. The detailed mechanisms of the laser annealing are carefully investigated. In addition, fluorescence intensity ratio behaviours using different emission bands are comparatively investigated.

Efficient nanoheater operated in a biological window for photo-hyperthermia therapy.

Remotely monitoring and regulating temperature in a small area are of vital importance for hyperthermia therapy. Herein, we report ~11 nm NaErF4 nanocrystal as the ultra-small nanoheater, which is highly safe for biological applications. Under 1530 nm photon excitation, upconversion intensity of NaErF4 is significantly enhanced as compared to the conventionally used 980 nm pumping source. Upconversion mechanisms are discussed on the basis of power dependence measurements. Importantly, light-to-heat transformation efficiency of NaErF4 through 1530 nm pumping is determined as high as 75%. Efficient NIR emission, centered at ~800 nm and thus within the biological window, is used for the temperature feedback. The potential applications of this highly efficient nanoheater for controlled photo-hyperthermia treatments are also demonstrated.

Highly efficient upconversion luminescence of Er heavily doped nanocrystals through 1530 nm excitation.

Improving luminescence efficiency is of vital importance for applications of rare-earth-doped upconversion materials. Herein, we present highly efficient upconversion nanocrystal, which is brighter than the state-of-the-art Er3+/Yb3+ co-doped core-shell material, through Er3+ heavily doping and 1530 nm excitation. Moreover, upconversion characteristics and mechanisms of Er3+ heavily doped core nanocrystals and their core-shell counterparts are investigated carefully.

Concentration dependent optical transition probabilities in ultra-small upconversion nanocrystals.

Transition probability is of vital importance for luminescence process, whereas the effects of doping concentration have not been explored in the Er3+:NaGdF4. In this work, we investigate the radiative transition probabilities of Er3+ highly doped NaGdF4 sub 10 nm nanocrystals using J-O theory. It is found that the transition probabilities vary with changing Er3+ concentration, especially altering the ratio of Er3+ 2H11/2 to 4S3/2 level, which is highly useful for optical thermometers as they are thermally coupled. To validate the concentration dependent transition probabilities, significant enhancements of upconversion luminescence are achieved by epitaxial growth of the inert shell, and thermal sensing behaviors are investigated using the improved samples.

Localized surface plasmon resonance sensing structure based on gold nanohole array on beveled fiber edge.

This paper proposes a simple, stable, sensitive, and angle-dependent localized surface plasmon resonance (LSPR) sensing structure based on multi-mode optical fiber. We adopted the template transfer method to integrate a nanohole array onto a fiber tip with beveled angle. Experimental results indicated that beveled angle structured probe sensor outperform the flat optical fiber tip structured LSPR sensor in our experiment. We tested the sensitivity and the figure of merit (FOM) of the probe beveled angle from 5°-22°, with refractive index ranging from 1.333-1.385, to find that sensitivity and FOM were optimal at fiber tip bevel angle of 7°, reaching 487 nm/RIU and 29 respectively.

Modified FIR thermometry for surface temperature sensing by using high power laser.

The FIR (fluorescence intensity ratio) technique for optical thermometry has attracted considerable attention over recent years due to its high sensitivity and high spatial resolution. However, it is thought that a heating effect induced by incident light may lead to temperature overestimations, which in turn impedes the reliability of this technique for applications which require high levels of accuracy. To further improve the FIR technique, this paper presents a modified calibration expression, which is suitable for surface temperature sensing, based on the temperature distribution (calculated through COMSOL software). In addition, this modified method is verified by the experimental data.

Whispering gallery mode temperature sensor of liquid microresonastor.

We propose and demonstrate a whispering gallery mode (WGM) resonance-based temperature sensor, where the microresonator is made of a DCM (2-[2-[4-(dimethylamino)phenyl] ethenyl]-6-methyl-4H-pyran-4-ylidene)-doped oil droplet (a liquid material) immersed in the water solution. The oil droplet is trapped, controlled, and located by a dual-fiber optical tweezers, which prevents the deformation of the liquid droplet. We excite the fluorescence and lasing in the oil droplet and measure the shifts of the resonance wavelength at different temperatures. The results show that the resonance wavelength redshifts when the temperature increases. The testing sensitivity is 0.377 nm/°C in the temperature range 25°C-45°C. The results of the photobleaching testing of the dye indicate that measured errors can be reduced by reducing the measured time. As far as we know, this is the first time a WGM temperature sensor with a liquid state microcavity has been proposed. Compared with the solid microresonator, the utilization of the liquid microresonator improves the thermal sensitivity and provides the possibility of sensing in liquid samples or integrating into the chemical analyzers and microfluidic systems.

Dual-truncated-cone structure for quasi-distributed multichannel fiber surface plasmon resonance sensor.

We propose and demonstrate an effective method to adjust the dynamic range of a fiber surface plasmon resonance (SPR) sensor by introducing a multimode fiber-sensing probe with a dual-truncated-cone (DTC) structure. When the grind angle of the DTC structure increases, the dynamic range redshifts. Based on this result, we fabricate a quasi-distributed two-channel multimode fiber SPR sensor by cascaded-connecting a DTC-sensing probe of 14° grind angle and a traditional transmitted multimode fiber (TMF)-sensing probe in the same fiber. The corresponding sensitivities of two sensing probes are 3423.08 nm/RIU and 2288.46 nm/RIU. By using this quasi-distributed multichannel fiber SPR-sensing approach, we may improve the detecting accuracy by extracting, calibrating, and compensating for the signals caused by the nonspecific bindings, other physical absorptions, and temperature changes in detecting samples, truly achieving dynamic detection in real-time. The excellence of this multichannel fiber SPR sensor is that the sensitivity of each subchannel-sensing probe stays unreduced after it is cascaded-connected in the main-channel fiber; the sensor is based on the multimode fiber, which is inexpensive, accessible, and convenient to be universalized in applications.

Single-fiber tweezers applied for dye lasing in a fluid droplet.

We report on the first demonstration of a single-fiber optical tweezer that is utilized to stabilize and control the liquid droplet for dye lasing. In order to trap a liquid droplet with a diameter of 15-30 µm, an annular core micro-structured optical fiber is adopted. By using wavelength division multiplexing technology, we couple a trapping light source (980 nm) and a pumping light source (532 nm) into the annular core of the fiber to realize the trapping, controlling, and pumping of the oil droplet. We show that the laser emission spectrum tunes along the same size as the oil droplet. The lasing threshold of the oil droplet with the diameter of 24 µm is 0.7 µJ. The presented fiber-based optical manipulation of liquid droplet micro-lasers can be easily combined with the micro-fluidic chip technology and also may extend the application of optical fiber tweezers for micro-droplet lasing technology in the biological field.

Distributed fiber surface plasmon resonance sensor based on the incident angle adjusting method.

We propose and demonstrate a distributed surface plasmon resonance (SPR) fiber sensor based on a novel, simple, and effective incident angle adjusting method. For normal fiber SPR sensors, it is hard to realize distributed sensing because it is hard to produce two dynamic ranges (resonance wavebands) with a great difference. The dynamic range depends on the incident angle, and therefore, we propose an incident angle adjusting method that is implemented by grinding an eccentric-core fiber to different angles, which helps to produce different SPR wavebands with great difference, thus realizing distributed sensing. In our two cascaded distributed configuration, with the refractive index range of 1.333-1.385, the fiber grind angles are 9° and 17°, the testing wavelength ranges are 613-760 nm and 745-944 nm, and the average testing sensitivities are 2826 nm/RIU and 4738 nm/RIU, respectively. Larger resonance wavelengths are associated with larger testing sensitivities. This distributed fiber sensor has important significance in the fields of multichannel liquid refractive indices and temperature self-reference measurements.

Compact distributed fiber SPR sensor based on TDM and WDM technology.

By using a twin-core fiber (TCF), we propose and demonstrate a novel distributed SPR sensor, which employs both the time division multiplexing (TDM) technology and the wavelength division multiplexing (WDM) technology together. The proposed sensor has two sensing passages with four sensing channels (and there are two sensing channels in each sensing passage). We employ the TDM technology to realize the two passage distributed sensing, which are parallel-connection; and we employ the WDM technology to realize the distributed sensing of two channels in a sensing passage, which are series-connected. In order to realize the TDM technology, we employ a two-core fiber, which has two cores in a same cladding, being equal to dividing the traditional single core into two independent sensing zones; in order to realize the WDM technology, we employ a fiber end polishing-connecting method to adjust the resonance angle/wavelength to realize the dynamic range separation. This twin-passage four-channel twin-core fiber SPR sensor is suitable for applying in fields of the multi-channel liquid refractive index and temperature self-reference measurement.

Twin-core fiber SPR sensor.

We propose and demonstrate a novel surface plasmon resonance (SPR)-sensing approach by using the fundamental mode beam based on a twin-core fiber (TCF). Although normally in a fiber SPR sensor, a multimode fiber (MMF) has often been used to improve the coupling efficiency; for improving fiber SPR sensor sensitivity, single-mode beam is optimal. We provide a novel method to employ the single (fundamental)-mode beam to SPR sense based on the TCF. We grind the TCF tip to be a frustum wedge shape, and plate a 50-nm sensing gold film on the end face, and two 500-nm reflected gold films on the side faces of the wedge tip. By using this new configuration, we reduce the mode noise effectively and get a high testing sensitivity (the testing highest sensitivity reaches to 6463 nm/RIU). This SPR probe can be applied in a microfluidic chip and monitors the refractive index (RI) charges of the flow liquid in the microfluidic channel in real-time. The probe successfully monitors the refractive index of liquid ranged from 1.3333 to 1.3706, and the average sensitivity reaches to 5213 nm/RIU in the solution, which is much higher than most multimode SPR systems.

Micro particle launcher/cleaner based on optical trapping technology.

Efficient and controllable launching function of an optical tweezers is a challenging task. We present and demonstrate a novel single fiber optical tweezers which can trap and launch (clean) a target polystyrene (PS) microsphere (diameter~10µm) with independent control by using two wavelengths beams: 980nm and 1480nm. We employ 980nm laser beam to trap the target PS microsphere by molding the fiber tip into a special tapered-shape; and we employ 1480nm laser beam to launch the trapped PS microsphere with a certain velocity by using the thermophoresis force generated from the thermal effect due to the high absorption of the 1480nm laser beams in water. When the launching force is smaller than the trapping force, the PS microsphere will be trapped near the fiber tip, and the launching force will blow away other PS microspheres in the workspace realizing the cleaning function; When the launching force is larger than the trapping force, the trapped PS microsphere will be launched away from the fiber tip with a certain velocity and towards a certain direction, realizing the launching function. The launching velocity, acceleration and the distance can be measured by detecting the interference signals generated from the PS microsphere surface and the fiber tip end-face. This PS microsphere launching and cleaning functions expanded new features of single fiber optical tweezers, providing for the possibility of more practical applications in the micro manipulation research fields.

An in-fiber integrated optofluidic device based on an optical fiber with an inner core.

A new kind of optofluidic in-fiber integrated device based on a specially designed hollow optical fiber with an inner core is designed. The inlets and outlets are built by etching the surface of the optical fiber without damaging the inner core. A reaction region between the end of the fiber and a solid point obtained after melting is constructed. By injecting samples into the fiber, the liquids can form steady microflows and react in the region. Simultaneously, the emission from the chemiluminescence reaction can be detected from the remote end of the optical fiber through evanescent field coupling. The concentration of ascorbic acid (AA or vitamin C, Vc) is determined by the emission intensity of the reaction of Vc, H2O2, luminol, and K3Fe(CN)6 in the optical fiber. A linear sensing range of 0.1-3.0 mmol L(-1) for Vc is obtained. The emission intensity can be determined within 2 s at a total flow rate of 150 µL min(-1). Significantly, this work presents information for the in-fiber integrated optofluidic devices without spatial optical coupling.