Multi-band polarization switch based on magnetic fluid filled dual-core photonic crystal fiber

The research of high-performance polarization controllers is of great significance for expanding the application field of polarization optics. Here, a polarization switch is demonstrated by using a dual-core photonic crystal fiber(DCPCF)with four symmetrical air holes, placed above and below each core, filled with magnetic fluid(MF). The switch, which utilizes a magnetic field to change the coupling length ratio of the x and y polarization modes, enables dynamic tuning of the polarization state and extinction ratio. Numerical results show that when the working length is 36.638 mm, the magneto–optical polarization switch can operate in four communication bands, i.e., 1509 nm to 1520 nm, 1544 nm to1556 nm, 1578 nm to 1591 nm, and 1611 nm to 1624 nm. Moreover, the extinction ratio(ER) is greater than 20 d B in the fiber length range of 38.5 mm to 38.7 mm, indicating that the device has a good fault tolerance for the interception of the fiber length.

Design of supercontinuum generating photonic crystal fiber at 1.06, 1.31 and 1.55 µm wavelengths for medical imaging and optical transmission systems

We propose broad supercontinuum spectrum generating highly nonlinear photonic crystal fiber (HN-PCF) which can be used in ultrahigh- resolution optical coherence tomography and optical transmission systems. Using full vector finite difference method, we investigated the different properties of HN-PCF. Broadband su-percontinuum spectrum is numerically calculated by using nonlinear Schr?dinger equation. Investigation showed that it is possible to obtain longitudinal resolution in a biological tissue of 1.3 μm, 1.2 μm and 1.1 μm by using picosecond continuum light at center wavelengths of 1.06 μm, 1.31 μm and 1.55 μm, respectively.

Six-bit all-optical quantization using photonic crystal fiber with soliton self-frequency shift and pre-chirp spectral compression techniques

In this paper, we propose an optical quantization scheme for all-optical analog-to-digital conversion that facilitates photonics integration. A segment of 10-m photonic crystal fiber with a high nonlinear coefficient of 62.8 W-1/kin is utilized to realize large scale soliton self-frequency shift relevant to the power of the sampled optical signal. Furthermore, a 100-m dispersion-increasing fiber is used as the spectral compression module for further resolution enhancement. Simulation results show that 317-nm maximum wavelength shift is realized with 1550-nm initial wavelength and 6-bit quantization resolution is obtained with a subsequent spectral compression process.

Optimizational 6-bit all-optical quantization with soliton self-frequency shift and pre-chirp spectral compression techniques based on photonic crystal fiber

In this paper, we optimize a proposed all-optical quantization scheme based on soliton self-frequency shift(SSFS)and pre-chirp spectral compression techniques. A 10m-long high-nonlinear photonic crystal fiber(PCF) is used as an SSFS medium relevant to the power of the sampled optical pulses. Furthermore, a 10m-long dispersion flattened hybrid cladding hexagonal-octagonal PCF(6/8-PCF) is utilized as a spectral compression medium to further enhance the resolution. Simulation results show that 6-bit quantization resolution is still obtained when a 100m-long dispersion-increasing fiber(DIF)is replaced by a 6/8-PCF in spectral compression module.

Fiber optical parametric oscillator based on photonic crystal fiber pumped with all-normal-dispersion mode-locked Yb:fiber laser

We demonstrate a cost effective, linearly tunable fiber optical parametric oscillator based on a home-made photonic crystal fiber pumped with a mode-locked ytterbium-doped fiber laser, providing linely tuning ranges from 1018 nm to 1038 nm for the idler wavelength and from 1097 nm to 1117 nm for the signal wavelength by tuning the pump wavelength and the cavity length. In order to obtain the desired fiber with a zero dispersion wavelength around 1060 rim, eight sam- ples of photonic crystal fibers with gradually changed structural parameters are fabricated for the reason that it is difficult to accurately customize the structural dimensions during fabrication. We verify the usability of the fabricated fiber experimen- tally via optical parametric generation and conclude a successful procedure of design, fabirication, and verification. A seed source of home-made all-normal-dispersion mode-locked ytterbium-doped fiber laser with 38.57 ps pulsewidth around the 1064 nm wavelength is used to pump the fiber optical parametric oscillator. The wide picosecond pulse pump laser enables a larger walk-off tolerance between the pump light and the oscillating light as well as a longer photonic crystal fiber of 20 m superior to the femtosecond pulse lasers, resulting in a larger parametric amplification and a lower threshold pump power of 15.8 dBm of the fiber optical parametric oscillator.

Ultra-broadband polarization splitter based on graphene layer-filled dual-core photonic crystal fiber

An ultra-broadband polarization splitter based on graphene layer-filled dual-core photonic crystal fiber （GDC-PCF） that can work in a wavelength range from 1120 nm to 1730 nm is proposed in this paper. Through optimizing fiber configuration, the polarization splitter has an extinction ratio of-56.3 dB at 1.55 μm with a fiber length of 4.8 mm. Compared with the photonic crystal fiber reported splitters, to our knowledge, the GDC-PCF splitter with the extinction ratio below-20 dB has a super wide bandwidth of 610 nm. Due to the excellent splitting characteristics, the GDC-PCF will be used in coherent optical communication systems in a wavelength range from infrared to mid-infraed.

Mid-infrared supercontinuum and optical frequency comb generations in a multimode tellurite photonic crystal fiber

We numerically investigate the mid-infrared(MIR)supercontinuum(SC)and SC-based optical frequency comb(OFC)generations when the three optical modes(LP_(01),LP_(02),and LP_(12))are considered in a multimode tellurite photonic crystal fiber(MM-TPCF).The geometrical parameters of the MM-TPCF are optimized to support the multimode propagation and obtain the desired dispersion characteristics of the considered three optical modes.When the pump pulse with center wavelengthλ=2.5μm,width T=80 fs,and peak power P=18 kW is coupled into the anomalous dispersion region of the LP_(01),LP_(02),and LP_(12)modes of the MM-TPCF,the-40-dB bandwidth of the generated MIR SCs can be up to 2.56,1.39,and 1.12 octaves,respectively,along with good coherence.Moreover,the nonlinear dynamics of the generated SCs are analyzed.Finally,the MIR SCs-based OFCs are demonstrated when a train of 50 pulses at 1-GHz repetition rate is used as the pump source and launched into the MM-TPCF.

Design of a photonic crystal fiber polarization beam splitter with simple structure and ultra-wide bandwidth

A novel polarization beam splitter(PBS)based on dual-core photonic crystal fiber(DC-PCF)is proposed in this work.The proposed DC-PCF PBS contains two kinds of lattices and three kinds of air holes to form the asymmetrical elliptic dual-core structure.By using the full-vector finite element method,the propagation characteristics of the proposed DC-PCF PBS are investigated.The simulation results show that the bandwidth of the proposed DC-PCF PBS can reach to 340 nm,which covers the S+C+L+U communication bands,the shortest splitting length is 1.97 mm,and the maximum extinction ratio appears near wavelength 1550 nm.Moreover,the insertion loss of the proposed DC-PCF PBS is very low.It is believed that the proposed DC-PCF PBS has important applications in the field of all-optical communication and network.

Specialty optical fibers for advanced sensing applications

Optical fiber technology has changed the world by enabling extraordinary growth in world-wide communications and sensing.The rapid development and wide deployment of optical fiber sensors are driven by their excellent sensing performance with outstanding flexibility,functionality,and versatility.Notably,the research on specialty optical fibers is playing a critical role in enabling and proliferating the optical fiber sensing applications.This paper overviews recent developments in specialty optical fibers and their sensing applications.The specialty optical fibers are reviewed based on their innovations in special structures,special materials,and technologies to realize lab in/on a fiber.An overview of sensing applications in various fields is presented.The prospects and emerging research areas of specialty optical fibers are also discussed.

Supercontinuum generated in all-normal dispersion photonic crystal fibers with picosecond pump pulses

The supercontinuum （SC） generation in all-normal dispersion （ANDi） photonic-crystal fiber （PCF） pumped by high power picosecond pulses are investigated in this paper. Our results show that an octave SC may be achieved by pumping the ANDi PCF with picosecond pump pulses. However, the PCF length required may have to be lengthened to several tens of centimeters, which is much longer than that with femtosecond pump pulses. The relatively long PCF gives rise to much higher Raman gain and stronger Raman frequency shift compared to those with femtosecond pump pulses, which in turn not only cause a distorted temporal waveform and an un-flattened spectrum, but also severely degrade the coherence of the generated SC.

Design of Photonic Crystal Fiber Capable of Carrying Multiple Orbital Angular Momentum Modes Transmission

For the traditional photonic crystal fibers with circular air holes, rectangular air holes are added to the fiber cladding. The periodic arrangement of the inner rectangular air holes allows the fiber structure to better match the annular mode field distribution of the vortex beam. The fiber structure was analyzed and calculated by COMSOL Multiphysics 5.4 finite element software, and the characteristics of fiber were analyzed, such as the dispersion, confinement loss, effective mode area and nonlinear coefficient. The results reveal that the photonic crystal fiber structure capable of carrying 50 orbital angular momentum (OAM) modes at the wavelength of 1.15 to 2.0 μm (850 nm). The effective refractive index difference Δneff between vector modes can reach 1 × 10-3, and larger difference can effectively separate the vector modes and improve the transmission performance of OAM modes. Moreover, the fiber has good performance, such as flat dispersion distribution of the low-order modes, low confinement loss below 10-9 dB·m-1, large effective mode field area and small nonlinear coefficient in the 850 nm wavelength range. Therefore, this fiber structure can be applied to the high-capacity communication system of fiber multiplexing OAM. In addition, the good characteristics of this fiber structure are of great significance for the transmission of vortex beam in fiber.

Gamma-radiation effects in pure-silica-core photonic crystal fiber

We investigated the steady state gamma-ray radiation response of pure-silica-core photonic crystal fibers（PSC-PCFs）under an accumulated dose of 500 Gy and a dose rate of 2.38 Gy/min. The radiation-induced attenuation（RIA） spectra in the near-infrared region from 800 nm to 1700 nm were obtained. We find that the RIA at 1550 nm is related with hydroxyl（OH^-） absorption defects in addition to the identified self-trapped hole（STH） defects. Moreover, it is proposed and demonstrated that reduced OH^-absorption defects can decrease the RIA at 1550 nm. The RIA at 1550 nm has effectively declined from 27.7 d B/km to 3.0 dB/km through fabrication improvement. Preliminary explanations based on the unique fabrication processes were given to interpret the RIA characteristics of PSC-PCFs. The results show that the PSC-PCFs,which offer great advantages over conventional fibers, are promising and applicable to fiber sensors in harsh environments.

Dispersion Properties in Total Internal Reflective Photonic Crystal Fiber

The dispersion properties in the short wavelength region of total internal reflective photonic crystal fiber have been studied by using the models of the equivalent twin waveguide soliton coupling,effective refractive index,effective normalized frequency and dispersion management solitons.It is shown that the dispersion in the cladding waveguide of the total internal reflective photonic crystal fiber is a positive dispersion,and the dispersion of its core waveguide is a negative dispersion.The method of the compensated probing laser diffraction by the phase hole induced by the stationary pumping laser in the cladding waveguide enables the average dispersion value of the total internal reflective photonic crystal fiber to be close to zero and the zero dispersion point to shift to the short wavelength region.

New High Nonlinear and Dispersion Flattened Photonic Crystal Fiber

A new high nonlinear dispersion flattened photonic crystal fiber is proposed. This fiber has three-fold symmetry core. The doped region in the core and the big air-holes in the 1-st ring can make high nonlinearity in the PCF. And the small air-holes in the 1-st ring and the radial increasing diameters air-holes rings in cladding can be used to turn the dispersion properties of the PCF. We can achieve the optimized optical properties by carefully selecting the PCF's structure parameters. A PCF with flattened dispersion is obtained. The dispersion is within ±0.8 ps·nm-1·km-1 from 1.50 μm to 1.62 μm. The nonlinear coefficient is about 12.645 6 W-1·km-1, the fundamental mode area is about 10.257 9 μm2 and the birefringence is about 3.086 96×10-5 at 1.55 μm. This work may be useful for effective design and fabrication of dispersion flattened photonic crystal with high nonlinearities.

Reseach Progress of Photonic Crystal Fibers in China

Photonic Crystal Fibers have attracted worldwideinterest within the last decade due to their uniqueoptical properties and because they exhibit a muchhigher degree of design freedom compared to conventionaloptical fibers.In this article, the fabricationtechnologies of photonic crystal fibers and theirapplications at home and abroad were formulated atlength, especially in the following fields, such aslarge mode area active photonic crystal fibers andfiber lasers, birefringence fibers and sensors, highnonlinear photonic crystal fibers and frequencytransformation, dispersion compensation PCFs anddispersion compensation for telecommunicationsystems, and photonic band-gap fibers. Finally, accordingto the above analysis, the prospects anddeveloping trends of photonic crystal fibers in thefuture were presented.

Fiber Bragg Gratings in Small-Core Ge-Doped Photonic Crystal Fibers

This paper reports fiber Bragg gratings （FBGs） inscribed in a small-core Ge-doped photonic crystal fibers with a UV laser and a Talbot interferometer. The responses of such FBGs to temper- ature, strain, bending, and transverse-loading were systematically investigated. The Bragg wavelength of the FBGs shifts toward longer wavelengths with increasing temperature, tensile strain, and transverse-loading. The bending and transverse- loading properties of the FBGs are sensitive to the fiber orientations.

Different supercontinuum generation processes in photonic crystal fibers pumped with a 1064-nm picosecond pulse

Picosecond pulse pumped supercontinuum generation in photonic crystal fiber is investigated by performing a series of comparative experiments. The main purpose is to investigate the supercontinuum generation processes excited by a given pump source through the experimental study of some specific fibers. A 20-W all-fiber picosecond master oscillator-power amplifier （MOPA） laser is used to pump three different kinds of photonic crystal fibers for supercontinuum generation. Three diverse supercontinuum formation processes are observed to correspond to photonie crystal fibers with distinct dis- persion properties. The experimental results are consistent with the relevant theoretical results. Based on the above analyses, a watt-level broadband white light supercontinuum source spanning from 500 nm to beyond 1700 nm is demonstrated by using a picosecond fiber laser in combination with the matched photonic crystal fiber. The limitation of the group velocity matching curve of the photonic crystal fiber is also discussed in the paper.

High stability supercontinuum generation in lead silicate SF57 photonic crystal fibers

We report supercontinuum （SC） generation in a lead silicate SF57 photonic crystal fiber by using a 1550 nm pump source. The effective nonlinear coefficient of the SF57 fiber is simulated to be 111.5 W-1 .km- 1 at 1550 nm. The fiber also shows ultraflat dispersion from 1700 nm to 2100 nm. Our results reveal that with an increase of the average power of the incident pulse from 10 mW to 90 mW, the SC of the SF57 photonic crystal fiber is generated from 1300 nm to 1900 nm with high stability and without significant change in spectral broadening.

Dynamic Temperature Characteristics of Liquid Filled Photonic Crystal Fiber

The dynamic characteristics of high sensitivity temperature sensor are studied by using siphon method to fill the air hole near the core of the hollow photonic crystal fiber with Cargille matching liquid,and the two ends are fused with single-mode fiber in this work.We analyzed the working principle of filled photonic crystal fiber sensor by using the standard coupling mode theory of directional coupler.The coupling process was simulated by COMSOL software.When the photonic crystal fiber filled with 10 mm liquid was scanned by tunable laser,the temperature sensitivity was 7.50 nm /℃,the average temperature response time was 0.317 s,the average release time was 3.732 s,and the temperature variation linearity was 100%.The experimental results show that the liquid filled photonic crystal fiber has the advantages of high temperature sensitivity,fast response time and good linearity.

Propagation characteristics of photonic crystal fibers selectively filled with ionic liquid

We present a numerical and experimental study of the propagation characteristics of photonic crystal fibers(PCFs)selectively filled with ionic liquid(IL;1-butyl-3-methylimidazolium iodine).Three types of IL-filled PCF are investigated:one with all air holes filled,one with an IL-filled air hole in the second ring,and one with an IL-filled air hole in the third ring.The results show that the third type of IL-filled PCF is the most sensitive to temperature;the sensitivity of resonant dips between the LP01 and LP21 modes is−2.9 nm/XC.Moreover,the intensity of the resonant dips changes with the polarization angle of the light source;the sensitivity is−0.79 dB per unit polarization angle.Based on this property,IL-filled PCFs with different utilities can be realized by changing the filling position flexibly.Consequently,IL-filled PCFs can be used under flexible conditions and controllable temperatures to create a compact polarization-angle sensor.