On the structure of waveguiding regions for high-order core modes of solid-core photonic-crystal fibers

Waveguiding properties of the higher-order modes in solid-core photonic-crystal fibers are studied numerically. The dispersion diagram is calculated for glass fibers with refractive indices of 1.611 with a hole-to-period ratio of 0.5 and 0.8. It is shown that, below the cutoff of the higher-order core modes, frequency regions exist within which these modes may propagate due to bandgaps in the cladding. In other words, the guiding effect in the solid-core fibers may result both from total internal reflection and from resonant reflection by the periodic cladding.     

Waveguide modes of electromagnetic radiation in hollow-core microstructure and photonic-crystal fibers

The properties of waveguide modes in hollow-core microstructure fibers with two-dimensionally periodic and aperiodic claddings are studied. Hollow fibers with a two-dimensionally periodic cladding support air-guided modes of electromagnetic radiation due to the high reflectivity of the cladding within photonic band gaps. Transmission spectra measured for such modes display isolated maxima, visualizing photonic band gaps of the cladding. The spectrum of modes guided by the fibers of this type can be tuned by changing cladding parameters. The possibility of designing hollow photonic-crystal fibers providing maximum transmission for radiation with a desirable wavelength is demonstrated. Fibers designed to transmit 532-, 633-, and 800-nm radiation have been fabricated and tested. The effect of cladding aperiodicity on the properties of modes guided in the hollow core of a microstructure fiber is examined. Hollow fibers with disordered photonic-crystal claddings are shown to guide localized modes of electromagnetic radiation. Hollow-core photonic-crystal fibers created and investigated in this paper offer new solutions for the transmission of ultrashort pulses of high-power laser radiation, improving the efficiency of nonlinear-optical processes, and fiber-optic delivery of high-fluence laser pulses in technological laser systems.     

Photonic-crystal fibers with a photonic band gap tunable within the range of 930–1030 nm

The physical principles of photonic-crystal fibers with a photonic band gap tunable in the visible and near-IR spectral ranges are demonstrated. Direct numerical integration of the Maxwell equations with the use of the finite-difference time-domain technique reveals the possibility of creating holey fibers with a photonic-crystal cladding whose photonic band gap lies within the frequency range characteristic of widespread solid-state femtosecond lasers. The fabrication of holey fibers with a pitch of the two-dimensional periodic structure of the cladding less than 500 nm allowed us to experimentally observe a photonic band gap in transmission spectra of holey fibers tunable within the range of 930–1030 nm. This photonic band gap is satisfactorily described within the framework of the proposed numerical approach based on the finite-difference time-domain method.     

Spectral broadening of femtosecond laser pulses in fibers with a photonic-crystal cladding

Changes in the spectra of femtosecond laser pulses propagating through fibers with a cladding having the structure of a two-dimensional photonic crystal are experimentally investigated. It is demonstrated that the waveguide properties of defect modes of photonic-crystal fibers provide an opportunity to considerably increase the efficiency of spectral broadening of short laser pulses as compared with conventional fibers.     

高阶非线性对双折射光纤中亮孤子成形的影响

本文采用拉格朗日近似方法讨论了高阶非线性对双折射光纤中亮孤子传输特性的影响,给出了计入五阶非线性时入射到双折射光纤中的光脉冲演化成束缚亮孤子对的阈值条件。研究结果表明,与不计入高阶非线性的情形相比,正的高阶非线性减小入射脉冲演化成束缚亮孤子对的振幅阈值;负的高阶非线性的存在,不仅提高了入射脉冲演化成束缚亮孤子对的振幅阈值,而且还存在着入射脉冲演化成束缚亮弧子对的振幅上界。     

高阶非线性对双折射光纤中亮孤子成形的影响

本文采用拉格朗日近似方法讨论了高阶非线性对双折射光纤中亮孤子传输特性的影响，给出了计入五阶非线性时入射到双折射光纤中的光脉冲演化成束缚亮孤子对的阈值条件。研究结果表明，与不计入高阶非线性的情形相比，正的高阶非线性减小入射脉冲演化成束缚亮孤子对的振幅阈值；负的高阶非线性的存在，不仅提高了入射脉冲演化成束缚亮孤子对的振幅阈值，而且还存在着入射脉冲演化成束缚亮弧子对的振幅上界。     

Hollow-core photonic crystal fiber-optic probes for Raman spectroscopy

We have implemented a new Raman fiber-optic probe design based on a hollow-core photonic-crystal excitation fiber surrounded by silica-core collection fibers. The photonic-crystal fiber offers low attenuation at the pump radiation wavelength, mechanical flexibility, high radiation stability, and low background noise. Because the excitation beam is transmitted through air inside the hollow-core fiber, silica Raman scattering is much reduced, improving the quality of the spectra obtained using probes of this design. Preliminary results show that the new probe design decreases the Raman background from the silica by approximately an order of magnitude compared to solid-core silica Raman probes.     

Enhanced four-wave mixing in a hollow-core photonic-crystal fiber

Hollow-core photonic-crystal fibers are shown to substantially enhance four-wave mixing (FWM) of laser pulses in a gas filling the fiber core. Picosecond pulses of Nd:YAG fundamental radiation and its second harmonic are used to generate a signal at the frequency of the third harmonic by the FWM process 3omega = 2omega + 2omega - omega. The efficiency achieved for this process in a 9-cm-long, 13-microm-hollow-core-diameter photonic-crystal fiber, designed to simultaneously transmit a two-color pump and the FWM signal, is shown to be approximately 800 times higher than the maximum FWM efficiency attainable with the same laser pulses in the tight-focusing regime.     

Analysis of the temperature dependence of the spectral characteristics for the hollow-core photonic-crystal fibers

The temperature dependence of the spectral characteristics of monocapillaries made of S87-2 glass and filled with air and ethanol is experimentally and theoretically studied in temperature intervals 23–90°C and 23–40°C, respectively. The same measurements are performed using the photonic-crystal fibers made of AR-Glass (Schott). The transmission spectra of the air-filled fibers are slightly transformed when the temperature is varied in the above interval. It is demonstrated that the transmission peak is red-shifted and the shift is proportional to the temperature when the cavities are filled with ethanol. It is also demonstrated that the temperature dependence of the shape of the transmission spectra is predominantly determined by the parameters of the medium that fills the hollow channel rather than the fiber material. The temperature sensitivities of the photonic-crystal fiber filled with ethanol and a monocapillary are 1.25 and 0.40 nm/°C, respectively.     

Phase-matched four-wave mixing of isolated waveguide modes of high-intensity femtosecond pulses in a hollow photonic-crystal fiber

Hollow-core photonic-crystal fibers with a special dispersion profile are shown to allow phase-matched nonlinear optical interactions of isolated air-guided modes of high-intensity femtosecond laser pulses confined in the hollow fiber core. We present theoretical and experimental studies of the four-wave mixing of fundamental and second-harmonic pulses of a Cr:forsterite laser with an initial pulse duration of about 50 fs and an intensity on the order of 10¹⁴ W/cm² in waveguide modes of a hollow photonic-crystal fiber with a core diameter of about 13μm.     

Solitons evolving toward few-and single-cycle pulses in photonic-crystal fibers

Numerical analysis of soliton dynamics in highly nonlinear photonic-crystal fibers reveals intriguing scenarios enabling efficient pulse compression down to single-cycle field waveforms. We derive simple analytical expressions relating the pulse width and the energy of ultrashort pulses generated through such a field-evolution dynamics to the fiber dispersion and nonlinearity.     

Kuznetsov–Ma soliton and Akhmediev breather of higher-order nonlinear Schrdinger equation

In terms of Darboux transformation, we have exactly solved the higher-order nonlinear Schrdinger equation that describes the propagation of ultrashort optical pulses in optical fibers. We discuss the modulation instability(MI) process in detail and find that the higher-order term has no effect on the MI condition. Under different conditions, we obtain Kuznetsov–Ma soliton and Akhmediev breather solutions of higher-order nonlinear Schrdinger equation. The former describes the propagation of a bright pulse on a continuous wave background in the presence of higher-order effects and the soliton's peak position is shifted owing to the presence of a nonvanishing background, while the latter implies the modulation instability process that can be used in practice to produce a train of ultrashort optical soliton pulses.     

Waveguide modes of hollow photonic-crystal fibers

Waveguide modes of microstructure fibers with a hollow core and a two-dimensionally periodic cladding are studied experimentally and theoretically. The spectrum of modes guided in the hollow core of these fibers displays isolated maxima, indicating that waveguiding is supported due to the high reflectivity of the fiber cladding within photonic band gaps. The main properties of the spectrum of modes guided in a hollow core of a photonic-crystal fiber and radiation intensity distribution in these modes are qualitatively explained in terms of the model of a periodic coaxial waveguide.     

Integral equations for photonic-crystal fibers

The integral and integro-differential equations for photonic-crystal waveguides (fibers) with both infinite and finite quasi-periodic dielectric coatings have been obtained, with previous consideration of the equations for 2D periodic photonic crystals with magnetodielectric and metallic inclusions. The corresponding numerical results are presented.     

Four-wave mixing in hollow photonic-crystal fibers

A radical enhancement of four-wave mixing in hollow-core photonic-crystal fibers is experimentally demonstrated. An enhancement ratio of about 800 relative to the regime of tight focusing is achieved for the four-wave mixing process 3ω=2ω+2ω?2ω, where ω and 2ω are the frequencies of fundamental radiation and the second harmonic of picosecond Nd:YAG laser pulses.     

Femtosecond supercontinuum characteristics control

Supercontinuum generation in the spectral range 530–1100 nm in series of photonic-crystal fibers pumped by femtosecond Ti:S laser Mira Oprima 900-F is achieved. The evolution of spectral characteristics of femtosecond supercontinuum is proofed to be dependent on pump pulse wavelength and power radiation. Polarization characteristics of supercontinuum spectral components are analyzed. We demonstrated experimentally the possibility of control of femtosecond supercontinuum generation.     

Propagation characteristics of a segmented cladding fiber

We propose a novel optical fiber design that consists of a uniform core and a segmented cladding formed by alternate regions of high and low refractive indices in the azimuthal direction. The structure is analyzed by use of the radial effective-index method, and the propagation characteristics of the structure are studied. The fiber has a highly dispersive cladding and shows characteristics similar to those of photonic-crystal fibers and holey fibers. The novel fiber offers the possibility of single-mode operation over a wide range of wavelengths with a large core diameter.     

Pump-probe nonlinear absorption spectroscopy of molecular aggregates using chirped frequency-shifted light pulses from a photonic-crystal fiber

Photonic-crystal fibers provide efficient nonlinear-optical transformations of femtosecond Cr: forsterite laser pulses, delivering linearly chirped frequency-shifted broadband light pulses optimized for pump-probe nonlinear absorption spectroscopy of molecular aggregates. The blue-shifted output of a photonic-crystal fiber with a spectrum stretching from 530 to 680 nm is used to probe one-and two-exciton bands of thiacarbocyanine J aggregates in a polymer film excited by femtosecond second-harmonic pulses of the Cr: forsterite laser.     

Broadband dynamic phase matching of high-order harmonic generation by a high-peak-power soliton pump field in a gas-filled hollow photonic-crystal fiber

Hollow-core photonic-crystal fibers are shown to enable dynamically phase-matched high-order harmonic generation by a gigawatt soliton pump field. With a careful design of the waveguide structure and an appropriate choice of input-pulse and gas parameters, a remarkably broadband phase matching can be achieved for a soliton pump field and a large group of optical harmonics in the soft-x-ray-extreme-ultraviolet spectral range.     

Wide-field-of-view narrow-band spectral filters based on photonic crystal nanocavities

We describe a novel approach to implementing wide-field-of-view narrow-band spectral filters, using an array of resonant nanocavities consisting of periodic defects in a two-dimensional three-material photonic-crystal nanostructure. We analyze the transmissivity of this type of filter for a range of wavelengths and in-plane incidence angles as a function of the defect's refractive index, the number of layers in the photonic-crystal reflectors, and the period of the defects and find that this structure diminishes the angular sensitivity of the resonance condition relative to that of a standard multilayer filter.