Hongpu Li

Advances in the Design and Fabrication of High-Channel-Count Fiber Bragg Gratings

In this paper, we review our recent developments in the design and fabrication techniques for high-channel-count fiber Bragg gratings (FBGs). We have presented a theory for the phase-only sampled FBG and demonstrated that a sampled FBG of N channels would require radicN/etamiddot times higher maximum reflective index modulation than that of the single-channel FBG. We experimentally demonstrate a 45- and 81-channel linearly chirped FBG for nearly whole C-band dispersion compensation, which is fabricated with a novel diffraction precompensated phase mask. The grating specifications obtained agree well with the theoretical design. We have presented a novel method for multichannel FBG design, which enables us to design any kind of multichannel FBGs, where the spectrum response of each channel could be either identical or nonidentical. Particularly, the nine-channel nonlinearly chirped FBG, which is used as a simultaneous dispersion and dispersion slope compensator, has been demonstrated.

Phased-only sampled fiber Bragg gratings for high-channel-count chromatic dispersion compensation

Binary and multilevel phase-only sampling functions are proposed for the sampled fiber Bragg gratings (FBGs) with high channel count, which require significantly less refractive-index modulation than that does the sampled grating with amplitude sampling. The design using the new simulated quenching optimization with temperature rescaling results in high channel uniformity and minimum energy in the out-of-band channels. The technique can be applied to the sampled FBGs with very high channel count. A five-channel nonlinearly chirped multilevel phase-only sampled FBG for tunable chromatic dispersion compensation is demonstrated.

Tunable chromatic dispersion compensation in 40-Gb/s systems using nonlinearly chirped fiber Bragg gratings

Chromatic dispersion management and tunable compensation are essential features of 40-Gb/s wavelength-division-multiplexed (WDM) systems. In this paper, we demonstrate both single and multi channel 40-Gb/s tunable dispersion compensation using nonlinearly chirped FBGs (NC-FBGs). For single channel compensation, we show that the NC-FBG can be tuned over a moderate range (/spl sim/400 ps/nm) with tolerable third-order dispersion (<200 ps/nm/sup 2/) within the channels data bandwidth (intrachannel third-order dispersion). For multichannel systems, we demonstrate 4/spl times/40-Gb/s tunable dispersion compensation using sampled NC-FBGs in two configurations. First, we show that a single, sampled NC-FBG with fairly low intrachannel third-order dispersion induces negligible penalty on all four channels. This solution has a limited dispersion tuning range because of the deleterious intrachannel third-order dispersion. We show a moderate tuning range from -300 ps/nm to -700 ps/nm. Second, we demonstrate that two inverse, concatenated, sampled NC-FBGs can cancel the high deleterious intrachannel third-order dispersion, thus extending the dispersion tuning range. This solution provides both positive and negative dispersion values by stretching the two NC-FBGs separately. A tuning range of -300 ps/nm to +300 ps/nm with zero intrachannel third-order dispersion is shown.

Optical MEMS pressure sensor based on Fabry-Perot interferometry

By employing the surface and bulk micro-electro-mechanical system (MEMS) techniques, we design and demonstrate a simple and miniature optical Fabry-Perot interferometric pressure sensor, where the loaded pressure is gauged by measuring the spectrum shift of the reflected optical signal. From the simulation results based on a multiple cavities interference model, we find that the response range and sensitivity of this pressure sensor can be simply altered by adjusting the size of sensing area. The experimental results show that high linear response in the range of 0.2-1.0 Mpa and a reasonable sensitivity of 10.07 nm/MPa (spectrum shift/pressure) have been obtained for this sensor.

Brillouin-gain coefficients of chalcogenide glasses

We derived the relation between a figure of merit, M1, for the diffraction efficiency of acousto-optical devices and the Brillouin-gain coefficient, gB. We then estimated the values of gB and the phonon lifetime for typical As2S3 and As2Se3 chalcogenide glasses from known acousto-optical parameters. We found that the magnitude of gB for these two glasses is greater than 20 times that of fused silica.

Dynamic Behavior of Stimulated Brillouin Scattering in a Single-Mode Optical Fiber

We theoretically and experimentally investigate the transient stimulated Brillouin scattering (SBS) in a single-mode fiber. The experiment was conducted using a single-mode pulsed YAG laser and demonstrates that SBS can occur even in the nanosecond region. Due to the transient nature of SBS, pulse narrowing of the transmitted pulse is observed. The experimental results agree with the theory based on the coupled-amplitude equations for the pump, Stokes, and acoustic waves. The steady-state solution of SBS is obtained when the pulse width of the incident pulse is one 100-fold greater than the round-trip time within the fiber. We show that the instability of SBS depends on the magnitude of the nonlinear refractive index of the fiber. SBS instability behavior appears at different pump power levels when the nonlinear refractive index is an order of magnitude larger than that of the fused-silica fiber.

Multi-channel notch filter based on a phase-shifted phase-only sampled fiber Bragg graing

We propose a novel method for the implementation of a tunable multi-channel notch filter based on a thermally induced phase-shift phase-only sampled fiber Bragg grating (FBG). The proposed method is numerically and experimentally demonstrated. A 51-channel notch filter with a bandwidth (FWHM) of 0.026 nm and a tuning range of 0.6 nm has been achieved. This proposed technique offers the potential applications to the multiwavelength fiber laser and multi-channel all optical logic devices.

Advanced design of a multichannel fiber Bragg grating based on a layer-peeling method

A multichannel fiber Bragg grating (FBG) design based on a discrete layer-peeling method is described. This novel method enables us to design any kind of multichannel FBG in which the spectrum responses of the channels can be either identical or nonidentical. In particular, a nine-channel dispersion-free FBG and a nine-channel nonlinearly chirped FBG used simultaneously as chromatic dispersion and dispersion slope compensators are described. Unlike the general multichannel FBG designed by a sampling method, these two gratings have ideal flat-topped profiles in both the transmission and the reflection spectra. By optimally detuning the relative phases for the multiple-spectrum channels with an iterative layer-peeling method, we can make maximum use of length-limited photosensitive fiber. Moreover, we show numerically that one can reduce or eliminate the oscillation that inherently exists in the index-change envelope of our multichannel FBG by accepting a decreased extinction ratio of the in-band signal to the out-of-band noises.

Analysis of optical instability in a double-coupler nonlinear fiber ring resonator

Instability in a double-coupler nonlinear fiber ring resonator has been investigated by using a linear stability analysis. Numerical results show that although Ikeda instability appears in the positive-branch regions, it hardly interrupts bistable device application since the threshold power for instability is one or two orders of magnitude larger than the switching power for optical bistability. By the transient analysis when an optical pulse with a finite pulse duration is incident, we moreover confirm that period-doubling oscillations, being one of characteristics of Ikeda instability, take place in the unstable state.

Multiwavelength fiber laser based on the utilization of a phase-shifted phase-only sampled fiber Bragg grating.

A semiconductor-optical-amplifier-based fiber laser incorporating with a phase-shifted phase-only sampled fiber Bragg grating (FBG) is proposed and experimentally demonstrated. Thanks to the intrinsic property of the phase-shifted phase-only sampled FBG, the proposed approach has an unique advantage, enabling one to tune and switch all the wavelength lasing simultaneously. The obtained fiber laser has a stable output with more than 30 wavelengths lasing. The 3 dB linewidth is approximately 13 pm, the signal-to-noise ratio is larger than 50 dB, and the nonuniformity of the output is smaller than 7.4 dB.