Minglie Hu

Multiwatt octave-spanning supercontinuum generation in multicore photonic-crystal fiber

High-power supercontinuum spanning over more than an octave was generated using a high power femtosecond fiber laser amplifier and a multicore nonlinear photonic crystal fiber (PCF). Long multicore PCFs (as long as 20 m in our experiments) are shown to enable supercontinuum generation in an isolated fundamental supermode, with the manifold of other PCF modes suppressed due to the strong evanescent fields coupling between the cores, providing a robust 5.4 W coherent supercontinuum output with a high spatial and spectral quality within the range of wavelengths from 500 to 1700 nm.

Effective surface plasmon polaritons on the metal wire with arrays of subwavelength grooves

In this paper we explore the existence of electromagnetic surface bound modes on a perfect metal wire milled with arrays of subwavelength grooves. The surface modes are axially symmetric transverse magnetic (TM) waves and have the same polarization state with the dominant propagating surface plasmon polaritons on the real metal wires. The dispersion of the fundamental surface mode has close resemblance with the dispersion of the surface plasmon polaritons. Moreover, we note that for TM polarization this metallic structure can be equivalent to a dielectric coated metal wire with defined geometrical parameters and effective refractive index of the dielectric coating. This metallic structure is expected to have some potential applications in the optical research in microwave or THz region.

Environmentally Stable, High Pulse Energy Yb-Doped Large-Mode-Area Photonic Crystal Fiber Laser Operating in the Soliton-Like Regime

A high pulse energy passively mode-locking fiber laser operating in the soliton-like regime is demonstrated. The laser is based on a linear cavity design. A segment of Yb-doped single-polarization large-mode-area photonic crystal fiber serves as the gain medium, and the self-starting mode-locking is achieved by a high contrast semiconductor saturable absorber mirror. The laser directly generates 600-fs pulses with 900 mW of average power at a repetition rate of 47.3 MHz, corresponding to a single pulse energy of 19 nJ. Furthermore, this fiber laser is directly used for pumping ZnTe to generate broadband terahertz radiation, resulting in a compact terahertz source.

A hollow beam from a holey fiber

A high-quality spectrally isolated hollow beam is produced through a nonlinear-optical transformation of Ti: sapphire laser pulses in a higher order mode of a photonic-crystal fiber (PCF). Instead of a doughnut shape, typical of hollow beams produced by other methods, the far-field image of the hollow-beam PCF output features perfect sixth-order rotation symmetry, dictated by the symmetry of the PCF structure. The frequency of the PCF-generated hollow beam can be tuned by varying the input beam parameters, making a few-mode PCF a convenient and flexible tool for the guiding and trapping of atoms and creation of all-fiber optical tweezers.

Self-similar evolution in a short fiber amplifier through nonlinear pulse preshaping.

We report on a nonlinear preshaper that optimizes initial pulses for self-similar evolution in a next fiber amplifier. It consists of a pair of gratings and a segment of single-mode fiber (SMF). The grating pair provides negative chirp to make the pulses preshaped temporally and spectrally in the SMF. With this optimization, the self-similar amplification can be realized in a 2.2 m Yb-doped fiber in a large range of pump power. After amplification, the pulse can be dechirped to transform-limited pulses with ~60 fs pulse duration.

High pulse energy mode-locked multicore photonic crystal fiber laser.

A high pulse energy passively mode-locked fiber laser operating in the all-normal dispersion regime is demonstrated. The gain material is an Yb-doped multicore photonic crystal fiber with 18 cores in array-type geometry. Robust and self-starting mode locking is achieved using a fast semiconductor saturable absorber mirror. The laser generates 180 nJ chirped pulses at a 14.48 MHz repetition rate for an average power of 2.6 W. The 1.15 ps output pulses are compressed to 690 fs outside the cavity.

Tunable Bandpass Filter With Solid-Core Photonic Bandgap Fiber and Bragg Fiber

A tunable spectral bandpass filter which includes two fibers with different bend loss edges-a solid-core photonic bandgap fiber with short wavelength bend loss edge and a Bragg fiber with long wavelength bend loss edge-is presented in this letter. The central wavelength and the bandwidth can be widely tuned around 1100 nm by controlling the bend diameters of the two fibers respectively. It can be a low-loss in-line fiber device due to its all solid structure and standard splice connection.

Improved fully vectorial effective index method for photonic crystal fibers: evaluation and enhancement

In the effective index method, which models photonic crystal fibers by means of a step-index waveguide analogy, two critical parameters are the effective index of the cladding and the effective core radius. It has been shown that the use of an effective core radius as a function of the relative air hole diameter, or also of the relative wavelength, can improve the accuracy of this method. We show, by comparison with a rigorous finite-difference frequency-domain method, that the reported improved fully vectorial effective index methods have commonly adopted a radius of the equivalent circular unit cell, which does not give the best accurate effective cladding index as compared with the use of an equivalent circular unit cell having the same area as the hexagonal unit cell. Furthermore, by defining both the radius of the equivalent circular unit cell and the effective core radius as a function of the relative air hole diameter, and the relative wavelength, we believe that the fully vectorial effective index method can be further enhanced in terms of accuracy of both the effective cladding index and the modal index.

Generation of 150 MW, 110 fs pulses by phase-locked amplification in multicore photonic crystal fiber

We use seven-core Yb-doped large-mode-area (LMA) photonic crystal fiber (PCF) to demonstrate phase-locked amplification of 0.7 W, 1 MHz, 1.9 ps laser pulses delivered by an LMA-PCF-laser-LMA-PCF-preamplifier system. Compression of the 24 W output of the multicore LMA-PCF amplifier with a grating compressor yields 110 fs pulses with a peak power up to 150 MW.

Reduction of timing jitter and intensity noise in normal-dispersion passively mode-locked fiber lasers by narrow band-pass filtering.

Fiber lasers mode-locked with normal cavity dispersion have recently attracted great attention due to large output pulse energy and femtosecond pulse duration. Here we accurately characterized the timing jitter of normal-dispersion fiber lasers using a balanced cross-correlation method. The timing jitter characterization experiments show that the timing jitter of normal-dispersion mode-locked fiber lasers can be significantly reduced by using narrow band-pass filtering (e.g., 7-nm bandwidth filtering in this work). We further identify that the timing jitter of the fiber laser is confined in a limited range, which is almost independent of cavity dispersion map due to the amplifier-similariton formation by insertion of the narrow bandpass filter. The lowest observed timing jitter reaches 0.57 fs (rms) integrated from 10 kHz to 10 MHz Fourier frequency. The rms relative intensity noise (RIN) is also reduced from 0.37% to 0.02% (integrated from 1 kHz to 5 MHz Fourier frequency) by the insertion of narrow band-pass filter.