Ultra-short pulse compression at 1065 nm in nonlinear photonic crystal fiber

A pulse compressor has been designed using a 13 mm highly nonlinear photonic crystal fiber to compress pulses centered at 1065 nm from 28 fs to 1.8 fs with a compression factor of 16.2. This compression is achieved by using a high level of energy and generating different orders of solitons without resorting to large values of fiber's dispersion.     

Large-pitch kagome-structured hollow-core photonic crystal fiber

We report the fabrication and characterization of a new type of hollow-core photonic crystal fiber based on large-pitch (approximately 12 microm) kagome lattice cladding. The optical characteristics of the 19-cell, 7-cell, and single-cell core defect fibers include broad optical transmission bands covering the visible and near-IR parts of the spectrum with relatively low loss and low chromatic dispersion, no detectable surface modes and high confinement of light in the core. Various applications of such a novel fiber are also discussed, including gas sensing, quantum optics, and high harmonic generation.     

Femtosecond pulse compression in a hollow-core photonic bandgap fiber by tuning its cross section

We present a numerical study of soliton pulse compression in a seven-cell hollow-core photonic bandgap fiber. We analyze the enhancement of both the compression factor and the pulse shape quality of 360 nJ femtosecond pulses at the wavelength of 800 nm by tuning the cross section size of the fiber. We use the generalized non-linear Schrödinger equation in order to modeled the propagation of light pulses along the fiber. Our numerical results show that output compressed pulses can be obtained, in a propagation length of 31 cm, with a compression factor of 5.7 and pulse shape quality of 77% for a reduction of 4.5% of the cross section size of the fiber. The predicted compression factor is 3 times larger than that experimentally obtained in such propagation length of the pulse in a hollow-core photonic bandgap fiber.     

Design and fabrication of hollow-core photonic crystal fibers for high-power ultrashort pulse transportation and pulse compression

We report on the recent design and fabrication of kagome-type hollow-core photonic crystal fibers for the purpose of high-power ultrashort pulse transportation. The fabricated seven-cell three-ring hypocycloid-shaped large core fiber exhibits an up-to-date lowest attenuation (among all kagome fibers) of 40 dB/km over a broadband transmission centered at 1500?nm. We show that the large core size, low attenuation, broadband transmission, single-mode guidance, and low dispersion make it an ideal host for high-power laser beam transportation. By filling the fiber with helium gas, a 74?μJ, 850?fs, and 40?kHz repetition rate ultrashort pulse at 1550?nm has been faithfully delivered at the fiber output with little propagation pulse distortion. Compression of a 105?μJ laser pulse from 850?fs down to 300?fs has been achieved by operating the fiber in ambient air.     

Partially liquid-filled hollow-core photonic crystal fiber polarizer

A compact fiber polarizer is demonstrated by the filling of selected air holes of a hollow-core photonic crystal fiber (PCF) with a liquid. The liquid-filling results in an asymmetric waveguide structure, leading to a large polarization dependent loss. A 6 mm long ethanol-filled PCF exhibits a polarization extinction ratio of ～18 dB over a wavelength range from 1480 nm to 1600 nm.     

Selective mode excitation in hollow-core photonic crystal fiber

Modes are selectively excited by launching light through the cladding from the side into a hollow-core photonic crystal fiber. Measuring the total output power at the end of the fiber as a function of the angle of incidence of the exciting laser beam provides a powerful diagnostic for characterizing the cladding bandgap. Furthermore, various types of modes on either side of the bandgap are excited individually, and their nearfield images are obtained.     

High harmonic generation in a gas-filled hollow-core photonic crystal fiber

High harmonic generation (HHG) of intense infrared laser radiation (Ferray et al., J. Phys. B: At. Mol. Opt. Phys. 21:L31, 1988; McPherson et al., J. Opt. Soc. Am. B 4:595, 1987) enables coherent vacuum-UV (VUV) to soft-X-ray sources. In the usual setup, energetic femtosecond laser pulses are strongly focused into a gas jet, restricting the interaction length to the Rayleigh range of the focus. The average photon flux is limited by the low conversion efficiency and the low average power of the complex laser amplifier systems (Keller, Nature 424:831, 2003; Südmeyer et al., Nat. Photonics 2:599, 2008; Röser et al., Opt. Lett. 30:2754, 2005; Eidam et al., IEEE J. Sel. Top. Quantum Electron. 15:187, 2009) which typically operate at kilohertz repetition rates. This represents a severe limitation for many experiments using the harmonic radiation in fields such as metrology or high-resolution imaging. Driving HHG with novel high-power diode-pumped multi-megahertz laser systems has the potential to significantly increase the average photon flux. However, the higher average power comes at the expense of lower pulse energies because the repetition rate is increased by more than a thousand times, and efficient HHG is not possible in the usual geometry. So far, two promising techniques for HHG at lower pulse energies were developed: external build-up cavities (Gohle et al., Nature 436:234, 2005; Jones et al., Phys. Rev. Lett. 94:193, 2005) and resonant field enhancement in nanostructured targets (Kim et al., Nature 453:757, 2008). Here we present a third technique, which has advantages in terms of ease of HHG light extraction, transverse beam quality, and the possibility to substantially increase conversion efficiency by phase-matching (Paul et al., Nature 421:51, 2003; Ren et al., Opt. Express 16:17052, 2008; Serebryannikov et al., Phys. Rev. E (Stat. Nonlinear Soft Matter Phys.) 70:66611, 2004; Serebryannikov et al., Opt. Lett. 33:977, 2008; Zhang et al., Nat. Phys. 3:270, 2007). The interaction between the laser pulses and the gas occurs in a Kagome-type Hollow-Core Photonic Crystal Fiber (HC-PCF) (Benabid et al., Science 298:399, 2002), which reduces the detection threshold for HHG to only 200 nJ. This novel type of fiber guides nearly all of the light in the hollow core (Couny et al., Science 318:1118, 2007), preventing damage even at intensities required for HHG. Our fiber guided 30-fs pulses with a pulse energy of more than 10 μJ, which is more than five times higher than for any other photonic crystal fiber (Hensley et al., Conference on Lasers and Electro-Optics (CLEO), IEEE Press, New York, 2008).     

Enhanced compression of femtosecond pulse in hollow-core photonic bandgap fibers

Compression of chirped free femtosecond pulses in hollow-core photonic bandgap fibers is investigated numerically. The results show that intrapulse stimulated Raman scattering can improve the quality of the compressed pulse. Positive third-order dispersion is the main limitation on the compression of the femtosecond pulse. However, the combined effect of the intrapulse stimulated Raman scattering and the negative third-order dispersion can form still shorter pulses than is possible with intrapulse stimulated Raman scattering alone. We also investigate the influence of width and peak power of input pulse on pulse compression.     

Visible supercontinuum generation in tapered photonic crystal fiber pumped by picosecond pulse at 1064 nm

Short photonic crystal fibers (PCFs) with different tapered waist diameter are made to extend the continuum spectrum in the visible range. The diversification of output continuum spectrum with the diameter of the tapered waist is experimentally observed. An all fiber visible supercontinuum source with 1.88 W output is demonstrated in our experiments. To the best of our knowledge, it is the highest all fiber visible supercontinuum generation in tapered PCF, pumping by picosecond pulse at 1064 nm. The suitably designed short tapered PCF can extend the visible spectrum, while, how to preserve the tapered waist is crucial for the all fiber visible supercontinuum source in the practical applications.     

Polarization-dependent efficient Cherenkov radiation at visible wavelengths in hollow-core photonic crystal fiber cladding

Efficient Cherenkov radiation (CR) is experimentally generated by a soliton self-frequency shift (SSFS) in a knot of hollow-core photonic crystal fiber (HC-PCF). When the angle of the half-wave plate is rotated from 0° to 45° , the Raman soliton shifts from 2227 to 2300 nm, the output power of the CR increases 8.15 times, and the maximum output power ratio of the CR at 556 nm to the residual pump is estimated to be 20:1. The width of the output optical spectrum at visible wavelengths broadens from 25 to 45 nm, and the conversion efficiency of the CR can be above 28%. Moreover, the influences of the pump polarization and wavelength on the CR are studied, and the corresponding nonlinear processes are discussed.     

In-line fiber-optic etalon formed by hollow-core photonic crystal fiber

A novel fiber-optic in-line etalon formed by splicing a section of hollow-core photonic crystal fiber (HCPCF) in between two single-mode fibers is proposed and demonstrated, for the first time to our knowledge. Such a HCPCF-based etalon acts as an excellent optical waveguide to form a Fabry-Perot interferometer and hence allows the cavity length to be as long as several centimeters with good visibility as the transmission loss of the HCPCF is much smaller than that of a hollow core fiber; this offers great potential to generate a practical dense fiber-optic sensor network with spatial frequency division-multiplexing. This novel etalon is demonstrated for strain measurement, and the experimental results show that a good visibility of 0.3 and a strain accuracy of better than +/- 5 microepsilon are achieved.     

Modeling photonic crystal fiber for efficient soliton pulse propagation at 850 nm

We numerically investigate the dynamics of soliton propagation at 850 nm in chloroform filled liquid core photonic crystal fiber (LCPCF) by using both finite element method (FEM) and split step Fourier method (SSFM). We propose a novel chloroform filled PCF structure that operates as a single mode at 850 nm featuring an enhanced dispersion and nonlinearity for efficient soliton propagation with low input pulse energy and low loss over small distances. We adopt the projection operator method (POM) to derive the pulse parameter equations which clearly describes the impact of fourth order dispersion on the pulse propagation in the proposed PCF. To analyse the quality of the pulse, we perform the stability analysis of pulse propagation numerically and compare our results of the newly designed chloroform filled PCF with that of standard silica PCF. From the stability analysis, we infer that the soliton pulse propagation in modified chloroform filled PCF is highly stable against the perturbation.     

Electromagnetically induced transparency in Rb-filled coated hollow-core photonic crystal fiber

We report the observation of lambda-configuration electromagnetically induced transparency as well as optical pumping in rubidium-filled kagome-structure hollow-coated-core photonic crystal fiber. We show that a polydimethylsiloxane coating of the fiber core reduces the linewidth of the transparency below that which could be expected for an uncoated fiber. The measured 6 MHz linewidth was dominated by optical broadening.     

Picosecond pulses compression at 1053-nm center wavelength by using a gas-filled hollow-core fiber compressor

We theoretically study the nonlinear compression of picosecond pulses with 10-m J of input energy at the 1053-nm center wavelength by using a one-meter-long gas-filled hollow-core fiber(HCF)compressor and considering the third-order dispersion(TOD)effect.It is found that when the input pulse is about 1 ps/10 m J,it can be compressed down to less than20 fs with a high transmission efficiency.The gas for optimal compression is krypton gas which is filled in a HCF with a 400-μm inner diameter.When the input pulse duration is increased to 5 ps,it can also be compressed down to less than 100 fs efficiently under proper conditions.The results show that the TOD effect has little impact on picosecond pulse compression and the HCF compressor can be applied on compressing picosecond pulses efficiently with a high compression ratio,which will benefit the research of high-field laser physics.     

Metrology of laser-guided particles in air-filled hollow-core photonic crystal fiber

Micrometer-sized particles are trapped in front of an air-filled hollow-core photonic crystal fiber using a novel dual-beam trap. A backward guided mode produces a divergent beam that diffracts out of the core, and simultaneously a focused laser beam launches a forward-propagating mode into the core. By changing the backward/forward power balance, a trapped particle can be selectively launched into the hollow core. Once inside, particles can be optically propelled along several meters of fiber with mobilities as high as 19 cm·s(-1) W(-1) (precisely measured using in-fiber Doppler velocimetry). The results are in excellent agreement with theory. The system allows determination of fiber loss as well as the mass density and refractive index of single particles.     

基于光子晶体光纤光脉冲压缩的理论研究

从理论上研究了单根光子晶体光纤的光脉冲压缩,利用较短的具有高非线性系数的光子晶体光纤可以实现较高质量的光脉冲压缩。针对单根光子晶体光纤压缩光脉冲的限制,提出了一种基于光子晶体光纤非线性环路镜中交叉相位调制效应的脉冲压缩方法。理论研究表明,这种脉冲压缩方式可以压缩自身功率较低的信号脉冲,相比于单根光子晶体光纤的方法,可以得到更高压缩质量的脉冲,通过合理选择控制光脉冲的参数,可以有效地抑制基座。     

Distortion-free delivery of nanojoule femtosecond pulses from a Ti:sapphire laser through a hollow-core photonic crystal fiber

We demonstrate propagation of femtosecond pulses in the 800-nm range through a hollow-core photonic crystal fiber with preserved temporal and spectral profiles for pulse energies up to 4.6 nJ. Without the use of a prechirping unit, 170-fs pulses were transmitted essentially undistorted at 812 nm, near the zero-dispersion wavelength. Because of the air guidance of pulses, intensity-dependent nonlinear effects were minimal, with only 15% pulse broadening occurring at 350-mW average output power. This fiber thus is excellently suited for applications that require single-mode delivery of high-energy ultrashort pulses to the fiber output face such as, for example, miniaturized multiphoton microscopes.     

Femtosecond parabolic pulse nonlinear compression with gas-filled hollow-core fiber

We study theoretically the spectral intensity evolutions of the femtosecond Gaussian and parabolic pulses with different initial pulse energies and compare the nonlinear compressions of these pulses based on a meter-long hollow-core fiber filled with neon for different initial pulse durations. The pulses are first coupled into gas-filled hollow-core fiber for spectrum broadening, then compressed by the optimal chirp compensation. The parabolic pulse possesses a shorter pulse duration, larger peak power, and cleaner wings than Gaussian pulse. The properties are useful for compressing the pulses and thus generating the high-energy, short-duration pulses.     

Low loss broadband transmission in hypocycloid-core Kagome hollow-core photonic crystal fiber

We report on the fabrication of a seven-cell-core and three-ring-cladding large-pitch Kagome-lattice hollow-core photonic crystal fiber (HC-PCF) with a hypocycloid-shaped core structure. We demonstrate experimentally and theoretically that the design of this core shape enhances the coupling inhibition between the core and cladding modes and offers optical attenuation with a baseline of ～180?dB/km over a transmission bandwidth larger than 200?THz. This loss figure rivals the state-of-the-art photonic bandgap HC-PCF while offering an approximately three times larger bandwidth and larger mode areas. Also, it beats the conventional circular-core-shaped Kagome HC-PCF in terms of the loss. The development of this novel (to our knowledge) HC-PCF has potential for a number of applications in which the combination of a large optical bandwidth and a low loss is a prerequisite.     

Improved air-silica photonic crystal with a triangular airhole arrangement for hollow-core photonic bandgap fiber design

We propose an improved photonic crystal (PC) cladding design for existing air-guiding photonic bandgap (PBG) fibers whose cladding airholes are arranged in a triangular lattice pattern. By increasing the sizes of concentrated silica regions in the cladding PC, we can have a larger degree of freedom in controlling the cladding bandgap regions. We predict that a fiber made from this type of cladding would perform better in terms of the PBG-guiding wavelength range, radiation loss owing to finite cladding size, and the ability to avoid surface mode problems.