Rui Geng

Polarization Effects in Tilted Fiber Bragg Grating Refractometers

The polarization effects in tilted fiber Bragg grating (TFBG) refractometers are investigated theoretically. The polarization effects may degrade the sensor performance greatly, and should be considered to achieve high-precision surrounding refractive index (SRI) measurement. The possible paths to reduce the influence of polarization effects are discussed. For large SRI changes, the cutoff resonance wavelength can be used as sensing parameter to achieve a polarization insensitive SRI measurement with linear sensitivity of ~500 nm/u.r.i and SRI uncertainty of ~10-3 over SRI range of 0.1; for tiny SRI changes the measurement can be refined by monitoring proper individual resonance (close to the cutoff resonance wavelength) with accuracy approaching 2.5 × 10-5 over a SRI range of 0.001.

Single-mode operations in the large flattened mode optical fiber lasers and amplifiers

Transverse mode competitions are compared between the large mode area (LMA) fiber lasers with and without flattened fundamental modes (FMs). Numerical simulations show that the flattened-FM fiber lasers can support single-mode (SM) operation without employing other mode selection methods. This property is useful for designing LMA fiber lasers with SM operation. For the flattened-FM fiber amplifiers, the photonic crystal fiber (PCF) with a flattened FM is proposed to shape the Gaussian beam into a flat-top one, which is coupled into the input end of the active fiber as a seed. The SM operation based on this approach can be obtained.

Side pumping scheme with a dual-sided grating coupler

A robust dual-sided grating coupler with high coupling efficiency (CE) is proposed and rigorously analyzed for laser-diode (LD) bars side pumping double-clad fiber (DCF). It consists of a double-groove metal grating and a composite diffraction grating with dielectric–metaldielectric (DMD) sandwich structure placed behind the upside and downside of the inner-cladding of the DCF respectively. High coupling efficiencies are obtained when the incident pump light is TE-polarized (85.96%), TM-polarized (90.7%) and un-polarized (76.04%) respectively. It is also demonstrated that the coupler is tolerant to potential fabrication errors and variations of the incident angle and central pump wavelength.

Novel method to optimize the distributed pump powers in kilowatt ytterbium-doped double-clad fiber laser

Time independent rate equations for fiber laser are solved accurately with numerical method under multipoint boundary conditions. A novel method based on genetic algorithm is firstly proposed to optimize distributed pump powers and fiber segment lengths of kilowatt YDDC fiber laser. The uniformity of temperature distribution is improved by using optimized method. The calculated results show that the lower operating temperature and better uniformity can be obtained through increasing the number of segments at the cost of decreased output signal power.