Dianyuan Fan

High-power and highly efficient operation of wavelength-tunable Raman fiber lasers based on volume Bragg gratings

Highly efficient and high-power operation of Raman fiber lasers in fixed-wavelength and wavelength-tunable cavity configurations based on a graded-index multimode fiber is reported. Fixed-wavelength and wavelength tunable operating regimes are achieved using volume Bragg gratings (VBGs) with center wavelengths of 1658 nm and 1750 nm, respectively. The fixed-wavelength laser yielded a maximum output power of 10.5 W at 1658.3 nm with a FWHM linewidth of ~0.1 nm for the launched pump power of 23.4 W, corresponding to a slope efficiency of 82.7% with respect to the launched pump power. The measured beam quality in the form of M² factor is ~1.35, corresponding to the fundamental mode of the fiber. For the wavelength-tunable Raman fiber laser, a wavelength tuning range of 37 nm from 1638.5 to 1675.1 nm is obtained with a maximum output power of 3.6 W at 1658.5 nm for the launched pump power of 13.0 W.

Absolute left-handed behaviors in a triangular elliptical-rod photonic crystal.

In this paper we theoretically study the left-handed behaviors in a two-dimensional triangular photonic crystal made of elliptical rods in air. An absolute left-handed region is found in the second photonic band by using the plane wave expansion method to analyze the photonic band structure and equifrequency contours. Typical left-handed behaviors such as negative refraction, flat superlensing and plano-concave lensing are demonstrated by the finite-difference time-domain simulations. These behaviors are also compared with the quasi-negative refraction and the resulted focusing effects in a square-lattice two-dimensional photonic crystal.

Field and dispersion properties of subwavelength-diameter hollow optical fiber

We have investigated the basic properties of subwavelength- diameter hollow optical fiber with exact solutions of Maxwells equations. The characteristics of modal field and waveguide dispersion have been studied. It shows that the subwavelength-diameter hollow optical fibers have interesting properties, such as enhanced evanescent field, local enhanced intensity in the hollow core and large waveguide dispersion that are very promising for many miniaturized high performance and novel photonic devices.

Transverse intensity distributions of a broadband laser modulated by a hard-edged aperture

By means of the Huygens-Fresnel diffraction integral, the field representation of a laser beam modulated by a hard-edged aperture is derived. The near-field and far-field transverse intensity distributions of the beams with different bandwidths are analyzed by using the representation. The numerical calculation results indicate that the amplitudes and numbers of the intensity spikes decrease with increasing bandwidth, and beam smoothing is achieved when the bandwidth takes a certain value in the near field. In the far field, the radius of the transverse intensity distribution decreases as the bandwidth increases, and the physical explanation of this fact is also given.

Widely tunable, narrow bandwidth polycrystalline ceramic Er:YAG laser with a volume Bragg grating.

We report on a tunable polycrystalline ceramic Er:YAG laser with a total tuning range of 23.1 nm by using a volume Bragg grating. The whole tuning range was composed of three parts, 1614.2 nm - 1621.5 nm, 1629.3 nm - 1635.1 nm and 1639.2 nm - 1649.2 nm. The bandwidth of the output spectrum (FWHM) of the tunable Er:YAG laser was narrowed down to <0.05 nm over the whole tuning range. To our knowledge, this is the widest tuning range so far in this material and it is the first time to report an Er:YAG laser operating at around 1630 nm. In the tuning range of 1629.3 nm- 1635.1 nm, the maximum output power of 1.4 W was obtained at 1633.0 nm with a slope efficiency of 20.9% with respect to the incident power.

Optical properties of a square-lattice photonic crystal within the partial bandgap

Optical properties of a two-dimensional square-lattice photonic crystal are systematically investigated within the partial bandgap through anisotropic characteristics analysis and numerical simulation of field pattern. Using the plane-wave expansion method and Hellmann-Feynman theorem, the relationships between the incident and refracted angles for both phase and group velocities are calculated to analyze light propagation from air to photonic crystals. Three kinds of flat slab focusing are summarized and demonstrated by numerical simulations using the multiple scattering method.

Conceptual design of a 10 PW class laser with a hybrid amplification chain.

We design a 10 PW class laser system with a hybrid amplification chain. The hybrid amplification chain with a total gain of 10(4) composed of Ti:chrysoberyl amplifiers and Ti:sapphire amplifiers. The ability of this hybrid amplifier chain to control gain narrowing and gain saturation is demonstrated by numerical simulations.

Electromagnetic fields and transmission properties in tapered hollow metallic waveguides

We analyze the electromagnetic spatital distributions and address an important issue of the transmission properties of spherical transverse-electric (TE) and transverse-magnetic (TM) eigenmodes within a tapered hollow metallic waveguide in detail. Explicit analytical expressions for the spatital distributions of electromagnetic field components, attenuation constant, phase constant and wave impedance are derived. Accurate eigenvalues obtained numerically are used to study the dependences of the transmission properties on the taper angle, the mode as well as the length of the waveguide. It is shown that all modes run continuously from a propagating through a transition to an evanescent region and the value of the attenuation increases as the distance from the cone vertex and the cone angle decrease. A strict distinction between pure propagating and pure evanescent modes cannot be achieved. One mode after the other reaches cutoff in the tapered hollow metallic waveguide as the distance from the cone vertex desreases.

Subwavelength imaging by a dielectric-tube photonic crystal

In contrast to previous two-dimensional coated photonic crystals, in this paper we propose a left-handed one that is made of dielectric tubes arranged in a close-packed hexagonal lattice. Without metallic cores, this structure is low-loss and convenient to fabricate. Negative refraction and its resulting focusing are investigated by dispersion characteristic analysis and numerical simulation of the field pattern. With proper modification at the interface, the image is improved. With better isotropy than that with noncircular rods, planoconcave lenses made by dielectric tubes focus a Gaussian beam exactly at R/|n−1|.

Moiré technique for spatial coherence measurements of soft-x-ray lasers

We present a novel moiré diagnostic that allows the time-averaged spatial coherence properties of soft-x-ray lasers to be measured. The technique is an extension of the theoretical work in a recent paper [J. Opt. Soc. Am. A 16, 323 (1999)] and is based on the concept of decompositions of the far-field divergence for partially coherent beams. An appealing feature of the approach is that the spatial coherence width at other planes can be evaluated from the measured one at a given plane on the assumption of shape-invariant beams.