Junhua Ji

Intensity-modulated magnetic field sensor based on magnetic fluid and optical fiber gratings

An intensity-modulated magnetic field sensor based on magnetic fluid and optical fiber gratings is proposed and experimentally demonstrated. The sensor is formed by a tilted-fiber Bragg grating (TFBG) coated by magnetic fluid (MF) and cascaded by a chirped-fiber Bragg grating (CFBG). Transmission of the TFBG is modulated by refractive index of the MF, which is sensitive to external magnetic field. The CFBG is well designed to reflect a broadband of light spectrally located at the cladding mode resonances region of the TFBG. Therefore, reflected optical power is modulated twice by the magnetic field and measurement is realized in reflection manner.

100-W CW cladding-pumped Raman fiber laser at 1120 nm

We report a 100 W continuous-wave cladding-pumped fiber Raman laser operating at 1120 nm. The fiber Raman laser consists of an 85 m long germanium-doped double-clad fiber in a 4% - 100% linear cavity, which is end-pumped by a multimode ytterbium-doped fiber laser source at 1064 nm. The Raman laser has a slope efficiency of 71% with respect to launched pump power. The laser output M2 is measured to be ~ 1.6 at 80 W of output power.

Analysis of the Conversion to the First Stokes in Cladding-Pumped Fiber Raman Amplifiers

In this paper, we analyze the optical conversion efficiency of a multimode pump source into a diffraction-limited beam in cladding-pumped Raman fiber lasers and amplifiers. We theoretically derive a relation of the cladding/core area ratio for efficient energy transfer from the pump into the first Stokes while avoiding the second Stokes generation that impedes the conversion efficiency of the first Stokes. In addition, we investigate experimentally the limits on the conversion efficiency in a pulse-pumped double-clad Raman fiber. We obtained a peak power conversion efficiency into the first Stokes in excess of 75% and a pulse energy conversion of 60%. We found that spatial pump mode depletion is the main limiting factor in this fiber, with concentric cladding and core. We discuss various solutions to overcome these limitations and improve on the current results.

Ultra-Low Quantum-Defect Heating in Ytterbium-Doped Aluminosilicate Fibers

We theoretically investigate the quantum defect between pump and signal photons in ytterbium-doped fiber lasers and amplifiers, and find that this can be as low as 0.6%. We find that the lowest quantum defects can be achieved with a low area ratio between the pump and signal waveguide of a double-clad fiber, and with high-brightness pumping in the core being an ultimate approach. The change in achievable quantum defect is small over a relatively large range of pump wavelengths, but it is still necessary to optimize the wavelengths and match the fiber length to reach the smallest quantum defect.

Analysis of Spectral Bendloss Filtering in a Cladding-Pumped W-Type Fiber Raman Amplifier

We design a double-clad fiber with a W-type core for use in a cladding-pumped fiber Raman amplifier operating at the 1st-Stokes wavelength. Compared to a conventional step-index core, a W-type core allows for a larger inner cladding so that pump sources with relatively poor beam quality can be utilized. This is because parasitic Raman conversion to the 2nd Stokes can be suppressed by bending loss, while the sharp spectral characteristics of a W-type fiber allows the loss to be kept low at the 1st Stokes. Without bendloss filtering, the ratio between the inner-cladding area and the effective area of the Stokes wave is limited to approximately eight. With a bendloss-filtered W-type fiber, area ratios of 34 are possible for core diameters up to around 18 μm and inner-cladding diameters up to around 82 μm. We also discuss limitations on the brightness enhancement due to optical damage and pump-signal walk-off in the pulsed regime.

Minimize quantum-defect heating in thulium-doped silica fiber amplifiers by tandem-pumping

In this paper, we propose a simple model to investigate the quantum defect between pump and signal photons in thulium-doped fiber amplifiers. The achievable quantum defects are limited by several factors, i.e., pump and signal wavelengths, fiber length, area ratio between inner cladding and core, and gain and absorption at signal and pump wavelengths, respectively. Through the proposed model, we find that the quantum defect can reach as low as 1%-level. We also find that the smaller area ratio between inner cladding and core, the lower quantum defect can be obtained. The ultimate pumping approach will be core pumping. This can be realized through tandem pumping the thulium-doped fiber by the other fiber source at shorter wavelength with high brightness. Finally, it is necessary to optimize the pump and signal wavelengths and match the fiber length to obtain the lowest quantum defect.

Tandem-pumped ytterbium-doped aluminosilicate fiber amplifier with low quantum defect

We show theoretically that a quantum-defect below 1% is possible in tandem-pumped Yb-doped aluminosilicate fibers operating off the gain peak. Experimentally, we reach a quantum defect of 2% and a slope efficiency of 90% or more.

High-brightness 210 μJ pulsed Raman fiber source

We report a 210 muJ pulsed high-brightness fiber source based on a cladding-pumped Raman fiber. It delivers Stokes light at 1112 nm, of 500 ns duration, 420 W peak power, and M2 1.8.

Multi-trench fiber with four gaps for improved bend performance.

A modified multi-trench fiber (MTF) design with gaps to create leakage channels is proposed and investigated numerically using the scalar finite-difference beam-propagation method algorithm. Great potential in single-mode operation, mode area enlargement, and resistance to bending is demonstrated. A high loss ratio (>50) between high-order modes and the fundamental mode is possible over a wide range of high-index ring thickness, gap width, and bending orientation. This reduces the required fabrication accuracy. We obtain an effective area of 920  μm2 at a wavelength of 1050 nm and a 20 cm bend radius with a high loss ratio (>100). Our modified gap MTF can possibly be fabricated by drilling holes in an ordinary MTF and inserting rods into the holes.

Near-diffraction-limited supercontinuum generation in a cladding-pumped nonlinear fiber converter

Through nonlinear scattering including stimulated Raman scattering and beam cleanup, a pulsed multimode pump beam is converted into a nearly diffraction-limited supercontinuum extending from 1 to 2 µm in a passive cladding-pumped nonlinear fiber converter.