Recent development of flat supercontinuum generation in specialty optical fibers

Supercontinuum (SC) generation has attracted a significant scientific interest in the past decades due to its promising applications covering the fields of metrology, spectroscopy, defense, as well as medical treatments. To date, researchers are devoted to improving the spectral width and flatness of SC generation by using specialty optical fibers. The flatness of the spectrum is of importance because it can improve the accuracy of measurement in practical applications. This paper summarizes the theory of SC, the state of the art of flat SC generation using optical fiber including photonic crystal fibers, soft glass fibers as well as germania-doped fibers, and suggests the future research direction of flat SC light source.

The effect of laser acupuncture on hypoxia tolerance and inflammation reaction in mice with optical fiber acupuncture needle intra body

Hypoxia is a disorder in which the tissues are not oxygenated adequately.The lack of oxygen may cause irreversible damage to the major organs such as the brain,heart and lungs.In severe cases,coma,seizures,and death may happen.In addition to the routine medication,acupuncture.treatment has been applied as a complementary treatment to counter hypoxia.In this paper,the optical fiber acupuncture needle was fabricated using the optical fiber imbedding into the metal capillary tube with needle tip,therefore made it easy to insert into the body for the laser treatment.The laser optical fiber needle treatment to the ST36 acupoint conducted with the laser.irradiation intra body.The normobaric hypoxia tolerance test results in mice show that the optical fber needle treatment to the ST36 acupoint with laser acupuncture appears to improve the tolerance to hypoxia.The mice treated with laser acupuncture expressed high level of IL-1βin serum.Our results suggest that laser optical fiber needle acupuncture may serve as a potential treatment for hypoxia.

Carbon-coated magnetic particles increase tissue temperatures after laser irradiation

Purpose:This work focused on the investigation the hyperthermia performance of the carboncoated magnetic particles(CCMPs)in laser-induced hyperthermia.Materials and methods:We prepared CCMPs using the organic carbonization method,and then characterized them with transmission electron microscopy(TEM),ultraviolet-visible(UV-Vis)spectrophotometry,vibrating sample magnetometer(VSM)and X-ray di®raction(XRD).In order to evaluate their performance in hyperthermia,the CCMPs were tested in laser-induced thermal therapy(LITT)experiments,in which we employed a fully distributedfiber Bragg grating(FBG)sensor to profile the tissue's dynamic temperature change under laser irradiation in real time.Results:The sizes of prepared CCMPs were about several micrometers,and the LITT results show that the tissue injected with the CCMPs absorbed more laser energy,and its temperature increased faster than the contrast tissue without CCMPs.Conclusions:The CCMPs may be of great help in hyperthermia applications.

Influence on Multimode Rectangular Optical Waveguide Propagation Loss by Surface Roughness

Optical scattering loss coefficient of muhimode rectangular waveguide is analyzed in this work. First, the effective refrac tive index and the mode field distribution of waveguide modes are obtained using the Marcatili method. The influence on scattering loss coefficient by waveguide surface roughness is then analyzed. Finally, the mode coupling efficiency for the SMFOpticalWaveguide （SOW） structure and MMFOptical Waveguide （MOW） structure are presented. The total scatter ing loss coefficient depends on modes scattering loss coeffi cients and the mode coupling efficiency between fiber and waveguide. The simulation results show that the total scatter ing loss coefficient for the MOW structure is affected more strongly by surface roughness than that for the SOW struc ture. The total scattering loss coefficient of waveguide decreas es from 3.97 x 10^-2 dB/cm to 2.96 x 10^-4 dB/cm for the SOW structure and from 5.24 - 10^-2 dB/cm to 4.7 x 10^-4 dB/ cm for the MOW structure when surface roughness is from 300nm to 20nm and waveguide length is 100cm.

Simultaneous transmission of time-frequency and data with co-amplification over urban fiber links

We demonstrate a simultaneous transmission of time-frequency and data over a 160-km urban business network in Shanghai.The signals are transmitted through a cascaded optical link consisting of 48 km and 32 km,which are connected by an optical relay.The metrological signals are inserted into the communication network using dense wavelength division multiplexing.The influence of the interference between different signals has been discussed.The experimental results demonstrate that the radio frequency(RF)instability can reach 2.1×10^(-14)at 1 s and 2.3×10^(-17)at 10,000 s,and the time interval transfer of one pulse per second(1 PPS)signal with less than 10 ps at 1 s is obtained.This work paves the way for the widespread dissemination of ultra-stable time and frequency signals over the communication networks.

Polarization and magneto-optical characteristics of Tb:YAG crystal-derived silica fiber via laser-heating drawing technique

Fiber quarter-wave plates and magneto-optical fibers are important components that greatly affect the sensitivity of fiberoptic current sensors.A Tb:YAG crystal-derived silica fiber(TYDSF)was fabricated using a CO_(2) laser-heating drawing technique.The linear birefringence of TYDSF was measured as 6.661×10^(-6)by a microscope birefringence measurement instrument.A fiber quarter-wave plate was fabricated by TYDSF at 1310 nm,which produced circularly polarized light with a polarization extinction ratio of 0.34 d B.Additionally,the linear birefringence of TYDSF was decreased by 22%by annealing at 750℃ for 7 h,and the Verdet constants of annealed TYDSF were measured to be 9.83,6.67,and 3.48 rad/(T·m)at 808,980,and 1310 nm,respectively.

Two-photon 3D printed spring-based Fabry-Pérot cavity resonator for acoustic wave detection and imaging

Optical fiber microresonators have attracted considerable interest for acoustic detection because of their compact size and high optical quality.Here,we have proposed,designed,and fabricated a spring-based Fabry-Pérot cavity microresonator for highly sensitive acoustic detection.We observed two resonator vibration modes:one relating to the spring vibration state and the other determined by the point-clamped circular plate vibration mode.We found that the vibration modes can be coupled and optimized by changing the structure size.The proposed resonator is directly 3D printed on an optical fiber tip through two-photon polymerization and is used for acoustic detection and imaging.The experiments show that the device exhibits a high sensitivity and low noise equivalent acoustic signal level of 2.39 mPa∕Hz^(1∕2)at 75 kHz that can detect weak acoustic waves,which can be used for underwater object imaging.The results demonstrate that the proposed work has great potential in acoustic detection and biomedical imaging applications.

Over 255 mW single-frequency fiber laser with high slope efficiency and power stability based on an ultrashort Yb-doped crystal-derived silica fiber

A single-frequency distributed Bragg reflector (DBR) fiber laser at 1030 nm has been demonstrated using a 0.7 cm long homemade Yb:YAG crystal-derived silica fiber (YCDSF).The absorption and emission cross sections of the YCDSF are 4.93×10^(-20)cm^(2) at 980 nm and 1.1×10^(-20)cm^(2)at 1030 nm.Using this gain fiber,an over 255 m W continuous-wave lasing in the constructed laser has been obtained at single transverse and longitudinal mode operation.The slope efficiency and the pump threshold of the fiber laser were up to ~35%and as low as 25 m W,respectively.The fiber laser also demonstrated an optical signal-to-noise ratio of 79 dB and a beam quality factor of 1.016 in two orthogonal directions.Its power fluctuation at 210.5 m W was less than 0.85%of the average power within 13 h.Moreover,the relative intensity noise and linewidth of the laser are also investigated at the different pump powers.The results indicate that the single-frequency DBR fiber laser has potential applications in high-quality seed sources and high-precision optical sensing.

Distributed Vibration Sensor With Laser Phase-Noise Immunity by Phase-Extraction φ-OTDR

We have demonstrated a distributed vibration sensor based on phase-sensitive optical time-domain reflectometer (φ-OTDR) system exhibiting immunity to the laser phase noise. Two laser sources with different linewidth and phase noise levels are used in the φ-OTDR system, respectively. Based on the phase noise power spectrum density of both lasers, the laser phase is almost unchanged during an extremely short period of time, hence, the impact of phase noise can be suppressed effectively through phase difference between the Rayleigh scattered light from two adjacent sections of the fiber which define the gauge length. Based on the phase difference method, the external vibration can be located accurately at 41.01 km by the(φ-OTDR system incorporating these two lasers. Meanwhile, the average signal-to-noise ratio (SNR) of the retrieved vibration signal by using Laser I is found to be -37.7 dB, which is comparable to that of -37.5 dB by using Laser II although the linewidth and the phase noise level of the two lasers are distinct. The obtained results indicate that the phase difference method can enhance the performance of(φ-OTDR system with laser phase-noise immunity for distributed vibration sensing, showing potential application in oil-gas pipeline monitoring, perimeter security, and other fields.

Broadband Acoustic Vibration Sensor Based on Cladding-Mode Resonance of Double-Cladding Fiber

We have proposed and demonstrated a double-cladding fiber (DCF) with cladding?mode resonance property for broadband acoustic vibration sensing. Since the fundamental mode in the core waveguide is able to be coupled to LP05 mode in the tube waveguide once the phase-matching condition is fulfilled, the transmission spectrum can exhibit a dip with a large extinction ratio. An acoustic vibration could induce the wavelength shift of such transmission spectrum, so that the intensity variation at a wavelength near the dip is coded with the information of the acoustic vibration signal. By demodulating the response of intensity variation, the frequency of the applied acoustic vibration signal can be recovered. Such a DCF-based sensor with an intensity modulation could measure the acoustic vibration with a broadband frequency range from 1 Hz to 400 kHz and exhibits the maximum signal-to-noise ratio (SNR) of ?80.79 dB when the vibration frequency is 20 kHz. The obtained results show that the proposed DCF-based acoustic vibration sensor has a potential application in environmental assessment, structural damage detection, and health monitoring.

Black Phosphorus-Film With Drop-Casting Method for High-Energy Pulse Generation From Q-Switched Er-Doped Fiber Laser

We have reported high-energy pulse generation from a Q-switched Er-doped fiber laser based on black phosphorus (BP) with a drop-casting method. Poly dimethyl diallyl ammonium (PDDA) is employed to protect BP. A passive Q-switching operation is achieved by using the BP/polyvinyl alcohol (PVA) film as the saturable absorbers (SAs). When the pump power increases from 130.4mW to 378mW, the repetition rate increases from 13.33kHz to 26.6kHz, and the pulse duration decreases from 10.67p,s to 7.11 (is. The maximum pulse energy is 468.03 nJ, to the best of our knowledge, which is the highest pulse energy produced from Q-switched fiber laser based on BP-SA at 1.5 pm. The obtained high-energy pulse demonstrates that BP/PVA film fabricated by such a drop-casting method can provide an ideal SA for high pulse energy generation from fiber lasers, and it has a potential application of remote sensing.

Real-time observation of vortex mode switching in a narrow-linewidth mode-locked fiber laser

Temporal and spatial resonant modes are always possessed in physical systems with energy oscillation.In ultrafast fiber lasers,enormous progress has been made toward controlling the interactions of many longitudinal modes,which results in temporally mode-locked pulses.Recently,optical vortex beams have been extensively investigated due to their quantized orbital angular momentum,spatially donut-like intensity,and spiral phase front.In this paper,we have demonstrated the first to our knowledge observation of optical vortex mode switching and their corresponding pulse evolution dynamics in a narrow-linewidth mode-locked fiber laser.The spatial mode switching is achieved by incorporating a dual-resonant acousto-optic mode converter in the vortex mode-locked fiber laser.The vortex mode-switching dynamics have four stages,including quiet-down,relaxation oscillation,quasi mode-locking,and energy recovery prior to the stable mode-locking of another vortex mode.The evolution dynamics of the wavelength shifting during the switching process are observed via the time-stretch dispersion Fourier transform method.The spatial mode competition through optical nonlinearity induces energy fluctuation on the time scale of ultrashort pulses,which plays an essential role in the mode-switching dynamic process.The results have great implications in the study of spatial mode-locking mechanisms and ultrashort laser applications.

Wavelength-switchable vortex beams based on a polarization-dependent microknot resonator

We experimentally demonstrated a method of generating continuously wavelength-switchable optical vortex beams(OVBs) in an all-fiber laser. A polarization-dependent microknot resonator(MKR) functions as comb filter and accounts for the narrow linewidth(0.018 nm) of multiwavelength channels. The wavelength interval corresponds to the free spectral range of the MKR. We exploit a fused SMF–FMF(single mode fiber–few mode fiber) mode coupler to obtain broadband mode conversion and successfully achieve multiwavelength switchable OVBs. As far as we know, this is the first report about identical multiwavelength vortex beams with topological charges of 1. It has been verified that each channel of the vortex beams preserves the same orbital angular momentum(OAM) properties through their clear spiral interferograms. Multiwavelength vortex beams with identical OAM properties are desirable for multiplexing, exchanging, and routing to further improve the capacity of optical fiber transmission.

Highly efficient excitation of LP01 mode in ring-core fibers by tapering for optimizing OAM generation

We have demonstrated the highly efficient excitation of the linearly polarized mode(LP01)in ring-core fibers(RCFs)by tapering the spliced point between the RCF and the standard single-mode fiber(SMF)to optimize all-fiber orbital angular momentum(OAM)generation.The tapering technique has been investigated theoretically and experimentally.Before tapering,only 50%of light can be coupled from SMFs to RCFs.The modal interference spectrum with an extinction ratio(ER)of～9 dB is observed,showing that higher-order modes are excited in RCF.By tapering the spliced point,90%of light is coupled,and the ER is minimized to be～2 dB,indicating that the higher-order modes are effectively suppressed by tapering.Such tapered spliced points of RCF–SMF are further applied for all-fiber OAM generation.The efficiencies of OAM+1 and OAM?1 generation are found to be enhanced by approximately 11.66%and 12.41%,respectively,showing that the tapered spliced point of the RCF–SMF is a feasible way to optimize OAM generation.

High signal-to-noise ratio fiber laser at 1596 nm based on a Bi/Er/La co-doped silica fiber

We fabricate a high-performance Bi/Er/La co-doped silica fiber with a fluorescence intensity of-33.8 dBm and a gain coefficient of 1.9 dB/m.With the utilization of the fiber as a gain medium,a linear-cavity fiber laser has been constructed,which exhibits a signal-to-noise ratio of 74.9 dB at 1596 nm.It has been demonstrated that the fiber laser has a maximum output power of 107.4 mW,a slope efficiency of up to 17.0%,and a linewidth of less than 0.02 nm.Moreover,an all-fiber single-stage optical amplifier is built up for laser amplification,by which the amplified laser power is up to 410.0 mW with pump efficiency of 33.8%.The results indicate that the laser is capable of high signal-to-noise ratio and narrow linewidth,with potential applications for optical fiber sensing,biomedicine,precision measurement,and the pump source of the mid-infrared fiber lasers.

Spectrally Flat Supercontinuum Generation in a ZBLAN Fiber Pumped by Erbium-Doped Mode-Locked Fiber Laser

We have experimentally demonstrated the flat supercontinuum(SC)generation using a 10-m-long ZrF4-BaF2-LaF3-AlF3-NaF(ZBLAN)fluoride fiber pumped by an erbium-doped mode-locked fiber laser incorporating carbon-nanotube-based saturable absorbers.In order to improve the spectral flatness of SC,the standardized single-mode fiber with different lengths is connected to the output of the mode-locked fiber laser before the pulse amplification.The generated SC with ZBLAN fiber exhibits the best spectral flatness with fluctuation less than 1.29 dB over the wavelength of 1571.8 nm−1803.1 nm,showing potential applications in optical sensing.