Tailoring of spatial coherence in a multimode fiber by selectively exciting groups of eigenmodes.

Control of the properties of speckle patterns produced by mutual interference of light waves is important for various applications of multimode optical fibers. It has been shown previously that a high signal-to-noise ratio in a multimode fiber can be achieved by preferential excitation of lower order spatial eigenmodes in optical fiber communication. Here we demonstrate that signal spatial coherence can be tailored by changing relative contributions of the lower and higher order multimode fiber eigenmodes for the research of speckle formation and spatial coherence. It is found that higher order spatial eigenmodes are more conducive to the final speckle formation. The minimum speckle contrast occurs in the lower order spatial eigenmodes dominated regime. This work paves the way for control and manipulation of the spatial coherence of light in a multimode fiber varying from partially coherent or totally incoherent light.

High-power low spatial coherence random fiber laser.

A high-power multi-transverse modes random fiber laser (RFL) is investigated by combining a master oscillator power-amplifier (MOPA) configuration with a segment of extra-large mode area step-index multimode fiber (MMF). Spatial coherence of the high-power multi-transverse modes RFL has been analyzed, which shows that speckle contrast is reduced dramatically with the output power increasing. In this way, considerably low speckle contrast of ~0.01 is achieved under high laser power of ~56 W, which are the records for multi-transverse modes RFLs in both spatial coherence and output power. This work paves a way to develop high-power RFLs with very low spatial coherence for wide-range speckle-free imaging and free-space communication applications.

Decoherence of fiber supercontinuum light source for speckle-free imaging.

Speckle-free imaging is attractive in laser-illuminated imaging systems. The evolutionary process of supercontinuum decoherence in extra-large mode area step-index multimode fiber is analyzed to provide high-quality broadband light source for speckle-free imaging. It is found that spectral bandwidth, number of spatial transverse modes, and decoherence among different modes all greatly contribute to speckle reduction. The combination of supercontinuum and extra-large mode area step-index multimode fiber can considerably increase the efficiency of decoherence process for speckle-free imaging. This work may enrich the research of speckle-free imaging and also provide guidance on speckle-free imaging using fiber-optics based light source.

Novel sensing concept based on optical Tamm plasmon.

This paper proposes a novel concept of refractive index sensing taking advantage of a high-refractive-index-contrast optical Tamm plasmon (OTP) structure, i.e., an air/dielectric alternate-layered distributed Bragg reflector (DBR) coated with metal. In the reflection spectrum of the structure, a dip related to the formation of OTP appears. The wavelength and reflectivity of this dip are sensitive to variation of ambient refractive index, which provides a potential way to realize refractive index sensing with a large measuring range and high sensitivity.

Ultra-long high-sensitivity Φ-OTDR for high spatial resolution intrusion detection of pipelines.

An ultra-long phase-sensitive optical time domain reflectometry (Φ-OTDR) that can achieve high-sensitivity intrusion detection over 131.5km fiber with high spatial resolution of 8m is presented, which is the longest Φ-OTDR reported to date, to the best of our knowledge. It is found that the combination of distributed Raman amplification with heterodyne detection can extend the sensing distance and enhances the sensitivity substantially, leading to the realization of ultra-long Φ-OTDR with high sensitivity and spatial resolution. Furthermore, the feasibility of applying such an ultra-long Φ-OTDR to pipeline security monitoring is demonstrated and the features of intrusion signal can be extracted with improved SNR by using the wavelet detrending/denoising method proposed.

Highly sensitive force sensor based on optical microfiber asymmetrical Fabry-Perot interferometer.

An asymmetrical Fabry-Perot interferometric (AFPI) force sensor is fabricated based on a narrowband reflection of low-reflectivity fiber Bragg grating (LR-FBG) and a broadband Fresnel reflection of the cleaved fiber end. The AFPI sensor includes a section of microfiber made by tapering and it achieves a force sensitivity of 0.221 pm/µN with a tapered microfiber of 40 mm length and 6.1 µm waist diameter. Compared with similar AFPI structure in 125 µm-diameter single mode fiber, the force sensitivity of the microfiber AFPI structure is greatly enhanced due to its smaller diameter and can be optimized for different force scales by controlling the diameter. The fabrication process of the AFPI sensor is simple and cost-effective. The AFPI sensor has better multiplexing capacity than conventional extrinsic fiber-optic Fabry-Perot sensors, while it also release the requirement on the wavelength matching of the FBG-pair-based FPI.

Coherently parallel fiber-optic distributed acoustic sensing using dual Kerr soliton microcombs.

Fiber-optic distributed acoustic sensing (DAS) has proven to be a revolutionary technology for the detection of seismic and acoustic waves with ultralarge scale and ultrahigh sensitivity, and is widely used in oil/gas industry and intrusion monitoring. Nowadays, the single-frequency laser source in DAS becomes one of the bottlenecks limiting its advance. Here, we report a dual-comb-based coherently parallel DAS concept, enabling linear superposition of sensing signals scaling with the comb-line number to result in unprecedented sensitivity enhancement, straightforward fading suppression, and high-power Brillouin-free transmission that can extend the detection distance considerably. Leveraging 10-line comb pairs, a world-class detection limit of 560 fε/√Hz@1 kHz with 5 m spatial resolution is achieved. Such a combination of dual-comb metrology and DAS technology may open an era of extremely sensitive DAS at the fε/√Hz level, leading to the creation of next-generation distributed geophones and sonars.

Graded-index fiber tip optical tweezers: numerical simulation and trapping experiment.

Optical fiber tweezers based on a graded-index multimode fiber (GIMMF) tip is proposed. Light propagation characteristics and gradient force distribution near the GIMMF tip are numerically investigated, which are further compared with that of optical fiber tips based on conventional single mode fibers. The simulated results indicated that by selecting optimal GIMMF length, the gradient force of the GIMMF tip tweezers is about 4 times higher than that of the SMF tip tweezers with a same shape. To prove the feasibility of such a new concept, optical trapping of yeast cells with a diameter of ~5 µm using the chemically-etched GIMMF tip is experimentally demonstrated and the trapping force is also calculated.

Random-lasing-based distributed fiber-optic amplification.

The gain and noise characteristics of distributed Raman amplification (DRA) based on random fiber laser (RFL) (including forward and backward random laser pumping) have been experimentally investigated through comparison with conventional bi-directional 1st-order and 2nd-order pumping. The results show that, the forward random laser pumping exhibits larger averaged gain and gain fluctuation while the backward random laser pumping has lower averaged gain and nonlinear impairment under the same signal input power and on-off gain. The effective noise figure (ENF) of the forward random laser pumping is lower than that of the bi-directional 1st-order pumping by ~2.3 dB, and lower than that of bi-directional 2nd-order pumping by ~1.3 dB at transparency transmission, respectively. The results also show that the spectra and power of RFL are uniquely insensitive to environmental temperature variation, unlike all the other lasers. Therefore, random-lasing-based distributed fiber-optic amplification could offer low-noise and stable DRA for long-distance transmission.

Distributed Raman amplification using ultra-long fiber laser with a ring cavity: characteristics and sensing application.

Distributed Raman amplification (DRA) based on ultra-long fiber laser (UL-FL) pumping with a ring cavity is promising for repeaterless transmission and sensing. In this work, the characteristics (including gain, nonlinear impairment and noise figure) for forward and backward pumping of the ring-cavity based DRA scheme are fully investigated. Furthermore, as a typical application of the proposed configuration, ultra-long-distance distributed sensing with Brillouin optical time-domain analysis (BOTDA) over 142.2 km fiber with 5m spatial resolution and ± 1.5 °C temperature uncertainty is achieved, without any repeater. The key point for the significant performance improvement is the system could offer both of uniform gain distribution and considerably suppressed pump-probe relative intensity noise (RIN) transfer, by optimized design of system structure and parameters.

Hybrid distributed Raman amplification combining random fiber laser based 2nd-order and low-noise LD based 1st-order pumping.

A configuration of hybrid distributed Raman amplification (H-DRA), that is formed by incorporating a random fiber laser (RFL) based 2nd-order pump and a low-noise laser-diode (LD) based 1st-order pump, is proposed in this paper. In comparison to conventional bi-directional 1st-order DRA, the effective noise figure (ENF) is found to be lower by amount of 0 to 4 dB due to the RFL-based 2nd-order pump, depending on the on-off gain, while the low-noise 1st-order Raman pump is used for compensating the worsened signal-to-noise ratio (SNR) in the vicinity towards the far end of the fiber and avoiding the potential nonlinear impact induced by excess injection of pump power and suppressing the pump-signal relative intensity noise (RIN) transfer. As a result, the gain distribution can be optimized along ultra-long fiber link, due to combination of the 2nd-order RFL and low-noise 1st-order pumping, making the transmission distance be extended significantly. We utilized such a configuration to achieve ultra-long-distance distributed sensing based on Brillouin optical time-domain analysis (BOTDA). A repeater-less sensing distance record of up to 154.4 km with 5 m spatial resolution and ~ ± 1.4 °C temperature uncertainty is successfully demonstrated.

All-optical bistable logic control based on coupled Tamm plasmons.

A method for realizing low-threshold all-optical bistable logic control is proposed based on Tamm plasmons (TPs), which are formed in an asymmetric dielectric Bragg reflector (DBR)-metal-DBR (ADMD) structure with a layer of Kerr medium embedded. The ADMD structure supports two TPs due to coupling of trapped modes at each metal-DBR interface, generating two dips in the structure's reflection spectrum. Thus, control (i.e., pump) and controlled (i.e., probe) light with wavelengths close to the two dips, respectively, can be imported. It is verified theoretically that, thanks to the enhanced Kerr nonlinearity related to excitation of high-quality TP, bistable switching at very low injection intensity can be initiated by strength or direction variation of the pump. Meanwhile, the probe changes correspondingly with the pump. Thus, all-optical bistable logic operation of the probe can be controlled by the pump.

Submonolayer biolasers for ultrasensitive biomarker detection.

Biomarker detection is key to identifying health risks. However, designing sensitive and single-use biosensors for early diagnosis remains a major challenge. Here, we report submonolayer lasers on optical fibers as ultrasensitive and disposable biosensors. Telecom optical fibers serve as distributed optical microcavities with high Q-factor, great repeatability, and ultralow cost, which enables whispering-gallery laser emission to detect biomarkers. It is found that the sensing performance strongly depends on the number of gain molecules. The submonolayer lasers obtained a six-order-of-magnitude improvement in the lower limit of detection (LOD) when compared to saturated monolayer lasers. We further achieve an ultrasensitive immunoassay for a Parkinson's disease biomarker, alpha-synuclein (α-syn), with a lower LOD of 0.32 pM in serum, which is three orders of magnitude lower than the α-syn concentration in the serum of Parkinson's disease patients. Our demonstration of submonolayer biolaser offers great potentials in high-throughput clinical diagnosis with ultimate sensitivity.

Low threshold 2nd-order random lasing of a fiber laser with a half-opened cavity.

In this paper, we reported the realization of 2nd-order random lasing in a half-opened fiber cavity, which is formed by a FBG with central wavelength at the 1st-order Raman Stokes wavelength and a single-mode fiber (SMF) performing as a random distributed feedback mirror. Using this proposed method, the threshold of 1st-order (2nd-order) random lasing is reduced to 0.7 (2.0) W, which is nearly 2 times lower than that observed in a completely-opened cavity.

Microbubble based fiber-optic Fabry-Perot interferometer formed by fusion splicing single-mode fibers for strain measurement.

We demonstrate an all-fiber optical Fabry-Perot interferometer (FPI) strain sensor whose cavity is a microscopic air bubble. The bubble is formed by fusion splicing together two sections of single-mode fibers (SMFs) with cleaved flat tip and arc fusion induced hemispherical tip, respectively. The fabricated interferometers are with bubble diameters of typically ~100 µm. Strain and temperature sensitivities of fabricated interferometers are studied experimentally; a strain sensitivity of over 4 Pm/µÎµ and a thermal sensitivity of less than 0.9 Pm/°C is obtained.

Highly sensitive refractive index sensor based on two cascaded special long-period fiber gratings with rotary refractive index modulation.

We present a refractive index (RI) sensor based on a fiber Mach-Zehnder interferometer (MZI) formed by two cascaded special long-period fiber gratings (LPFGs) with rotary refractive index modulation (RLPFGs), in which the coupling occurred between the guided mode and the high-order asymmetric cladding mode. The experimental results show that the RI sensitivity of a refractometer with an interaction length of 40 mm is up to 58.8 nm/RI in the range of 1.3344 to 1.3637, which is 3.5 times higher than that of an MZI formed by two normal LPFGs. The temperature sensitivity for the same parameters of an RLPFG-MZI is about 0.03 nm/°C. Such a kind of high-sensitivity, easy-to-fabricate and simple-structure interferometer may find applications in the chemical or biochemical sensing fields.

Refractive index sensing based on Mach-Zehnder interferometer formed by three cascaded single-mode fiber tapers.

We report a highly sensitive refractive index (RI) sensor based on three cascaded single-mode fiber tapers, in which a weak taper is sandwiched between the two tapers to improve the sensitivity of the sensor. Experimental results show that the sensitivity of the device is 0.286 nm for a 0.01 RI change, which is about four times higher than that of the normal two-cascaded-taper-based Mach-Zehnder interferometer. In addition, the sensitivity of the device could be enhanced by tapering a longer and thinner middle weak taper. Such kinds of low-cost and highly sensitive fiber-optic RI sensors would find applications in chemical or biochemical sensing fields.

A sequentially bioconjugated optofluidic laser for wash-out-free and rapid biomolecular detection.

Microstructures can improve both sensitivity and assay time in heterogeneous assays (such as ELISA) for biochemical analysis; however, it remains a challenge to perform the essential wash process in those microstructure-based heterogeneous assays. Here, we propose a sequential bioconjugation protocol to solve this problem and demonstrate a new type of fiber optofluidic laser for biosensing. Except for acting as an optical microresonator and a microstructured substrate, the miniaturized hollow optical fiber (HOF) is used as a microfluidic channel for storing and transferring reagents thanks to its capability in length extension. Through the capillary action, different reagents were sequentially withdrawn into the fiber for specific binding and washing purposes. By using the sequentially bioconjugated FOFL, avidin molecules are detected based on competitive binding with a limit of detection of 9.5 pM, ranging from 10 pM to 100 nM. It is demonstrated that a short incubation time of 10 min is good enough to allow the biomolecules to conjugate on the inner surface of the HOF. Owing to its miniaturized size, only 589 nL of liquid is required for incubation, which reduces the sample consumption and cost for each test. This work provides a tool to exploit the potential of microstructured optical fibers in high-performance biosensing.

A ray-transfer-matrix model for hybrid fiber Fabry-Perot sensor based on graded-index multimode fiber.

A theoretical model based on the ray-transfer-matrix method is developed for explaining the principle of a graded-index multimode fiber (GI-MMF) based hybrid fiber Fabry-Perot (GI-FFP) sensor. It is verified by the numerical simulations and experimental results that the high fringe contrast of the reflective spectrum of the sensor is due to the periodic self-focusing effect of the GI-MMF. The influence of the GI-MMF length on the shape of reflective spectrum and corresponding maximum fringe contrast are investigated. Experimental results are in good agreement with the theory. A typical GI-FFP sensor is fabricated and its response to the external refractive index is measured with a maximum sensitivity of approximately 160 dB/RIU (Refractive Index Unit).

Refractive index measurement using photonic crystal fiber-based Fabry-Perot interferometer.

We have constructed a novel refractive index (RI) sensor based on a fiber optic Fabry-Perot interferometer (FPI) by splicing a section of hollow core fiber between a single-mode fiber and a photonic crystal fiber (PCF). Owing to the air holes in the cladding of the PCF, various substances, such as liquids and gases with different RI, can enter or leave the in-fiber air cavity, which makes the device usable as a refractometer. In this paper, the fiber optic FPI sensor has been used to monitor the RI changes of air with different pressures, and the experimental results show that such a sensor has an RI sensitivity of 805.1 microm/RIU, and hysteresis is not observed. Moreover, the easy fabrication method gives the in-fiber refractometer many potential applications in the sensing field.