High Birefringence Photonic Crystal Fiber Research Articles

Design and study of high birefringent terahertz photonic crystal fiber with hybrid crystal lattice

In this paper, we propose a novel high birefringent terahertz (THz) photonic crystal fiber (PCF) with subwavelength circular air hole pairs in the core which are arranged as a hybrid crystal lattice structure. And its high mode birefringence is realized by reducing structure symmetry in core. A professional software COMSOL Multiphysics 4.0 is used for modeling, and the simulation results show that this kind of THz PCF exhibits a high birefringence on a level of 10-2and low confinement loss over a wide THz frequency range. Moreover, the birefringence or confinement loss can be controlled flexibly by adjusting some fiber parameters. Compared with a similar structure PCF for the optical communication, the THz PCF is easier to practically fabricate, owing to its large wavelength size.

큰 복굴절 특성을 가지는 광자결정 광섬유를 이용한 스트레인, 구부림 및 온도 특성

큰 복굴절 값을 갖는 광자결정광섬유는 실리카 단일 물질로 구성되어 온도 민감성이 매우 낮다. 이를 이용하여 온도변화에 영향을 받지 않는 스트레인 및 구부림 센서를 구현하였다. 이때, 스트레인 변화에 따른 민감도는 <TEX>$1.41\;pm/{\mu}{\varepsilon}$</TEX> 이고, 구부림에 대한 민감도는 타원코어 장축 방향과 단축 방향으로의 구부렸을 때 각각 <TEX>$0.93\;nm/m^{-1}$</TEX>, <TEX>$-1.6\;nm/m^{-1}$</TEX> 로 차이가 있었다. Highly-Birefringent Photonic Crystal Fiber (Hi-Bi PCF) is composed of a single material, silica, so that its temperature sensitivity is extremely low. Therefore, we propose afiber based Sagnac interferometer for measurement of strain and curvature independent of temperature variation. The sensitivities of strain and curvature (both axes) are measured to be <TEX>$1.41\;pm/{\mu}{\varepsilon}$</TEX> and <TEX>$0.93nm/m^{-1}$</TEX>(slow axis Y), <TEX>$-1.6\;nm/m^{-1}$</TEX>(fast axis X), respectively.

Design, Fabrication, and Sensor Applications of Photonic Crystal Fibers

Photonic crystal fibers (PCFs) with regularly placed air holes running along the fiber oer flexible design and unique transmission characteristics, like unlimited single-mode operation, that are hard to obtain in conventional optical fibers. In this paper, we will discuss the design, fabrication and sensor applications of photonic crystal fibers, particularly twin-core photonic crystal fibers (TCPCFs) and highly birefringent photonic crystal fibers (HB-PCFs). A numerical analysis of PCFs with various design was performed using a computer code developed with finite element method (FEM). Fabrication of PCFs is mainly based on the well-known stack-and-draw method. Controlled gas pressurization was employed during the fiber drawing process, and the microstructure of PCFs was well preserved by pressurizing the core and cladding regions independently. We will also describe the sensor applications of the fabricated PCFs for measuring the strain, bending, and transverse loads, e.g., TC-PCF-based in-line interferometers and HB-PCF-based birefringent interferometers. In the case of a TC-PCF-based in-line sensor, the small dierence in the eective indices of the two core modes leads to interference fringes, and the birefringence due to the twin cores results in polarization-dependent fringe spacing. A novel intensity-based bend sensor is demonstrated utilizing the bend-induced spatial fringe shift. With a HB-PCF inserted between two linear polarizers, the resulting interference pattern can be used to obtain the transverse load sensitivity for dierent angles of the applied load.

Hydrostatic pressure sensing with high birefringence photonic crystal fibers.

The effect of hydrostatic pressure on the waveguiding properties of high birefringence photonic crystal fibers (HiBi PCF) is evaluated both numerically and experimentally. A fiber design presenting form birefringence induced by two enlarged holes in the innermost ring defining the fiber core is investigated. Numerical results show that modal sensitivity to the applied pressure depends on the diameters of the holes, and can be tailored by independently varying the sizes of the large or small holes. Numerical and experimental results are compared showing excellent agreement. A hydrostatic pressure sensor is proposed and demonstrated using an in-fiber modal interferometer where the two orthogonally polarized modes of a HiBi PCF generate fringes over the optical spectrum of a broad band source. From the analysis of experimental results, it is concluded that, in principle, an operating limit of 92 MPa in pressure could be achieved with 0.0003% of full scale resolution.

Temperature-Insensitive Torsion Sensor With Enhanced Sensitivity by Use of a Highly Birefringent Photonic Crystal Fiber

We report on enhanced torsion sensitivity by using a highly birefringent photonic crystal fiber (HB-PCF)-based Sagnac interferometer. In order to increase the torsion sensitivity, we introduced an anisotropic microstructure into the cross section of an HB-PCF by enlarging the size of air holes of one row. This can result in a high birefringence of the order of 10-3 and low sensitivities to bending and temperature. The torsion sensitivity was measured to be high with ~0.06 nm/°.

Stable SOA-based multi-wavelength fiber ring laser using Sagnac loop mirror incorporating a high-birefringence photonic crystal fiber

Multi-wavelength fiber laser using semiconductor optical amplifier (SOA) and Sagnac loop mirror (SLM) incorporating a newly designed high-birefringence photonic crystal fiber (HB-PCF) is experimentally demonstrated. The modal birefringence of the fabricated HB-PCF is estimated to be 1.1 × 10−3. Mainly, by adjusting a polarization controller in the fiber ring laser, the proposed fiber laser can operate at six lasing wavelengths with 2.7 nm intervals, the signal-to-noise ratio (SNR) of around 30 dB. The output power stability is 0.8 dB. In addition, we obtained near-perfect temperature independence in our multi-wavelength fiber laser system. The temperature dependence of the SLM is around 3 pm/°C.

Intensity measurement based temperature-independent strain sensor using a highly birefringent photonic crystal fiber loop mirror

A fiber-optic strain sensor is demonstrated by using a short length of highly birefringent photonic crystal fiber (HiBi-PCF) as the sensing element inserted in a fiber loop mirror (FLM). Due to the ultralow thermal sensitivity of the HiBi-PCF, the proposed strain sensor is inherently insensitive to temperature. When a distributed-feedback (DFB) laser passes through the FLM, the output power is only affected by the transmission spectral change of the FLM caused by the strain applied on the HiBi-PCF. Based on intensity measurement, an optical power meter is adequate to deduce the strain information and an expensive optical spectrum analyzer (OSA) would not be needed.

Enhanced transverse load sensitivity by using a highly birefringent photonic crystal fiber with larger air holes on one axis

We report on a transverse load sensor with enhanced sensitivity through the use of a birefringent interferometer based on a highly birefringent photonic crystal fiber (HB-PCF). The transverse load sensitivity can be enhanced by using a fabricated HB-PCF having larger air holes on its fast axis. The transverse load sensitivity was measured to be high: approximately 2.17 nm/(N/cm). The temperature-induced undesirable effects can be ignored because transmission outputs of our HB-PCF were stable with the change of the temperature. The sensing probe can be compact because of its high birefringence with the order of 10(-3) and no bending loss.

Design of two kinds of dual-core high birefringence and high coupling degree photonic crystal fibers

We propose two kinds of dual-core high birefringence and high coupling degree photonic crystal fibers (DHBHCD-PCFs) in this paper. The characteristics of birefringence and coupling are studied by multipole method. Numerical results show that the birefringence and the coupling length reach an order of 10 − 2 and 10 − 5 m at 1.55 μm, respectively. It is found that the birefringence and the coupling intensity increase with the increase of air-filling fraction, which is different from other dual-core fibers. The DHBHCD-PCFs with high degree of polarization-maintaining and high coupling degree are helpful for manufacturing minitype photonic apparatus.

High energy and long pulse generation with high-birefringence photonic crystal fibre and laser-diode pumped regenerative amplifier

We report on the generation of a high energy and long pulse for pumping optical parametric chirped-pulse amplification (OPCPA) by a high-birefringence photonic crystal fibre (HB-PCF) and a laser-diode-pumped regenerative chirped pulse amplifier. Using the femtosecond pump pulse centred at 815 nm, a 1064 nm soliton pulse is produced in the HB-PCF. After injecting it into an Nd:YAG regenerative amplifier with the glass etalons, a narrow-band amplified pulse with an energy of ∼4 mJ and a duration of 235 ps is achieved at a repetition rate of 10 Hz, which is suitable for being used as a pump source in the 800 nm OPCPA system.

High birefringence photonic crystal fiber with low loss and a broad single-mode range

A new type of pseudo-panda photonic crystal fiber (PP-PCF) with high birefringence, low loss, and a broad single-mode range was proposed and analyzed by using the finite difference frequency domain (FDFD) method. The modal cut-off property of the PP-PCF is quantitatively described with normalized frequency ( V-parameter). The numerical simulation results showed that by choosing suitable relative structure parameters, the birefringence of the PP-PCF may arrive at the order of 10 −2 in a broad single-mode region (1100–1800 nm). It is also shown that low confinement losses and adjustable total dispersion can be achieved in a properly designed geometrical structure of the PP-PCF.

Equal channel spacing Sagnac filter using high-birefringent photonic crystal fibers

An equal channel spacing Sagnac filter is proposed using a high-birefringent photonic crystal fiber (PCF) with birefringence square to the operation wavelength. A PCF with four smaller central air holes surrounded by larger air holes is applied in a Sagnac filter with optimized parameters to achieve equal channel spacing of 0.8 nm. Given that the birefringence of this PCF is square to the operation wavelength, the channel spacing of the proposed filter changes by only 0.03 nm within a wavelength range from 1400 to 1650 nm; this is about one order of magnitude less than that constructed with conventional high-birefringent fibers.

Modeling the tapering effects of fabricated photonic crystal fibers and tailoring birefringence, dispersion, and supercontinuum generation properties

The effects of tapering fabricated air-silica photonic crystal fibers (PCFs) by tailoring the key modal and nonlinear properties of PCFs have been studied by analyzing the tapered structure using a finite difference mode calculation algorithm. The process of tapering is simulated through repeatedly redefining the geometry of the fiber cross section in a progressively tapered dimension preserving the shape. We tested the performance of the analysis by evaluating the modal characteristics, namely, the mode-effective area, birefringence, dispersion, nonlinearity, and supercontinuum properties of some well-known PCF examples under successive tapered conditions. Tapering, as an additional parameter, is seen to improve birefringence of a typical high-birefringence PCF by 1 order of magnitude. The analysis also estimates the extent of tapering that is required to achieve a target amount of evanescent field that has potential applications in an evanescent field sensor. Our investigation with tapered PCF structures includes tailoring dispersion properties and increasing nonlinearity, which leads to broader and lower threshold supercontinuum generation. The analysis should, therefore, be useful as a ready technique for taper analysis of any arbitrary structure PCF and also in PCF-preform (stacking structure) analysis, which can provide preestimates of properties in a targeted dimension of the final PCF before drawing.

Coupling characteristics of dual-core high birefringence photonic crystal fibers

A kind of dual-core high birefringence photonic crystal fibers （DHB-PCFs） has been proposed in this paper. The birefringence， coupling length and dispersion characteristics of DHB-PCFs have been studied by multipole method using mode coupling theory. It is found that the birefringence of DHB-PCF with pitch 1.2 μm and air-hole diameter 1.0 μm is 1.24×10-2 in optical communication band of 1.55 μm wavelength. Correspondingly， dual-core coupling lengths are 21.6 μm and 24.3 μm for x-polarized mode and y-polarized mode， respectively. The DHB-PCF with high degree of polarization and very short coupling length is useful for manufacturing minitype photonic apparatus.

High birefringence and low loss circular air-holes photonic crystal fiber using complex unit cells in cladding

We propose a high birefringence and low loss index-guiding photonic crystal fiber (PCF) using the complex unit cells in cladding by the finite-element method. Results show that the birefringence and confinement loss in such PCF fiber is determined not only by the whole cladding asymmetry but also the shape of the PCF core. The maximal modal birefringence and lowest confinement loss of our proposed structures at the excitation wavelength of λ=1550nm can be achieved at 8.7×10−3 and 5.27×10−5dB/km, respectively.

Active phase control and frequency chirp effects on supercontinuum generation in high birefringence photonic crystal fiber

An acoustic-optics programmable dispersive filter (AOPDF) was first employed to actively control the linearly polarized femtosecond pump pulse frequency chirp for supercontinuum (SC) generation in a high birefringence photonic crystal fiber (PCF). By accurately controlling the second order phase distortion and polarization direction of incident pulses, the output SC spectrum can be tuned to various spectral energy distributions and bandwidths. The pump pulse energy and bandwidth are preserved in our experiment. It is found that SC with broader bandwidth can be generated with positive chirped pump pulses except when the chirp value is larger than the optimal value, and the same optimal value exists for the pump pulses polarized along the two principal axes. With optimal positive chirp, more than 78% of the pump energy can be transferred to below 750nm. Otherwise, negative chirp will weaken the blue-shift broadening and the SC bandwidth.

Design and characteristics of a near-elliptic inner cladding High birefringent polarization-stable photonic crystal fiber

A new near-elliptic inner cladding (NEIC) structure of polarization-stable highly birefringent photonic crystal fiber (HB-PCF) is proposed and analyzed numerically by using the full-vector finite element method (FEM) under the condition of anisotropic perfectly matched layers. The result confirmed that the birefringence degradation of the proposed NEIC-PCF is less than 1.2×10-5 when the air hole diameter of the inner cladding along the short axis varies from 1.30μm to 1.18μm (varied by ～10%)，while the average birefringence is of the order of 10-3 at 1.55μm and the mode field diameters along long axis and short axis of the elliptic are 9.2μm and 2.4μm，respectively，which shows that the proposed PCF have excellent polarization stability.

高双折射光子晶体光纤特性分析

高双折射光子晶体光纤特性分析

Design of a new type high birefringence photonic crystal fiber

A High birefringence microstructure fiber with four big elliptical air holes is investigated by using the finite-difference frequency-domain (FDFD) method. Based on the numerical simulations, the influence of the air-hole size and pitch upon the mode birefringence is analyzed. The work may be helpful for the design and fabrication of high birefringence PCFs.

Correlation between the birefringence and the structural parameter in photonic crystal fiber

In order to simply design a highly birefringent photonic crystal fiber (HB-PCF), we numerically simulated the correlation between the birefringence and the structural parameter of photonic crystal fiber with square-lattice or triangle-lattice air-holes by using multipole method. It is shown that the phase birefringence B( λ) and the group birefringence G( λ) can be modulated by the structure parameter of normalized wavelength λ/ Λ and the relative air-hole size d/ Λ. Numerical results show very high phase and group birefringence of the order of 10 −2. The group birefringence becomes negative in the region where phase birefringence increases with an increase in normalized wavelength that does not appear in traditional highly birefringent fibers.