Xijia Gu

All-fiber multimode interference bandpass filter

A novel design for an all-fiber bandpass filter based on a multimode interference reimaging phenomenon is presented. The filter has achieved low insertion loss with adequate bandwidth and isolation for coarse wavelength-division multiplexing. The filter can easily be made with any central wavelength that is compatible with the single-mode fiber used for its construction. The measured filter performance matches the theoretical predictions well. The filter can have broad applications in fiber-optic telecommunications, spectroscopy, and sensing.

Demonstration of all-fiber WDM for multimode fiber local area networks

We have successfully demonstrated wavelength-division multiplexing in a conventional 62.5-/spl mu/m diameter graded index multimode fiber using a fiber Bragg grating written on a novel multicylindrical shell multimode fiber. The performance parameters of the device, such as insertion loss and interband and intraband isolations, are presented. The issues and improvements of the device are discussed.

Fiber loop ring-down spectroscopy with a long-period grating cavity

We have successfully demonstrated cavity ring-down spectroscopy in a fiber ring with a long-period grating cavity. The cavity couples >99% of light from core to cladding, interacts with the environment surrounding the fiber, and then couples unabsorbed light back into the fiber core. This design provides a relatively large evanescent wave absorption area while maintaining a very low loss for the fiber ring. The effectiveness of this cavity as a sensitive chemical sensor is also demonstrated.

All-Optical Demultiplexing of WLAN and Cellular CDMA Radio Signals

Subcarrier multiplexed transmission of multimedia radio signals over fiber is often done to deliver broadband services cost effectively. These signals need to be demultiplexed, preferably in the optical domain, to avoid loss and noise due to optical-to-electrical conversion. However, it is challenging to optically isolate signals at subgigahertz range due to the need for very narrow optical bandpass filters with high selectivity and low insertion loss and distortion. We developed such a novel subpicometer all-optical bandpass filter by creating a resonance cavity using two closely matched fiber Bragg gratings. This filter has a bandwidth of 120 MHz at -3 dB, 360 MHz at -10 dB, and 1.5 GHz at -20 dB. Experimental results show that this filter optically separates two RF signals spaced as close as 50 MHz without significant distortion. This paper analytically and experimentally investigates the scenario when this filter was used with 2.4-GHz (wireless local area network) and 900-MHz (cellular wireless) radio signals. The bit-error rate of the underlying baseband data is related to the linearity and isolation of the filter.

Centralized Peer-to-Peer Video Streaming Over Hybrid Wireless Network

Video streaming over wireless network has drawn a great interest. In traditional wireless local area networks (WLANs), when the number of the users and the number of the flows increases, the contention for the wireless channel will lead to packet loss and packet delay. In this paper, we propose a centralized peer-to-peer video streaming over hybrid wireless network to improve the performance of the video transport over wireless Internet. The base layer of the video is transported from the server via the WLAN mode, which benefits the centralized management of the video distribution, while the enhancement layers are delivered over the multiple paths via the ad hoc mode, which can reduce the congestion in the access point (AP). The simulation results show that our proposed scheme can achieve a better perceptual video quality compared to the WLAN deployment

Dual-wavelength Yb-doped fiber laser stabilized through four-wave mixing

We report the observation of four-wave mixing in a high-power dual-wavelength twin-core Yb-doped fiber laser. The four-wave mixing process stabilizes dual-wavelength operation. We obtain dual-wavelength operation at 1090 nm with a narrow linewidth (typically < 40 pm), signal-to-noise-ratio > 55 dB, total output power as high as 1.85 W, and a record narrow wavelength spacing of 0.12 nm.

Simple Reconfigurable Photonic Microwave Filter Using an Arrayed Waveguide Grating and Fiber Bragg Gratings

We demonstrate a photonic microwave finite impulse response filter with reconfiguration and tuning capabilities. The filter is based on spectral slicing of a broadband source and optical fiber as a wideband dispersive medium. An arrayed waveguide grating and fiber Bragg gratings are used to select and vary the weight of the optical taps to achieve reconfiguration and tuning

Fiber comb filters based on UV-writing Bragg gratings in graded-index multimode fibers.

We report a new kind of comb filters based on fiber Bragg gratings in graded-index multimode fibers. It produces two groups of spectra with a total of 36 reflection peaks that correspond to 18 principal modes and cross coupled modes. The mode indices and wavelength spacings have been investigated theoretically and experimentally. This kind of comb filters may be used to construct multi-wavelength light sources for sensing, optical communications, and instrumentations.

All-Fiber Laser Beam Shaping Using a Long-Period Grating

We demonstrate a simple all-fiber device that can transform a Gaussian-shaped laser beam into a uniform intensity beam. The device is based on the interaction of a core mode and a cladding mode partially coupled out by a long-period grating. We have achieved good beam uniformity (<4% intensity variation) and reduced beam divergence (1.5deg of half divergence angle) while keeping the insertion loss to 0.1 dB. The measured beam uniformity matches well with simulated results. The device can also change the beam shape from a Gaussian to a donut, to top hat, and back to a Gaussian, dynamically by tuning the laser wavelength.

The efficient light coupling in a twin-core fiber waveguide

A model is proposed to establish the relationship between the light coupling efficiency and fiber length for two contacted large fiber cores. The model correctly described the dependency of the coupled light on fiber length that was verified in the experiment with a twin-core fiber. This model can be used in fiber laser and erbium-doped fiber amplifier designs that employ twin-core or multicore pumping.