A simple, cost-effective and switchable five-wavelength fiber ring laser based on a chirped moiré fiber grating (CMFG)
and a wavelength-tunable Sagnac loop interferometer (FSI) filter is proposed and experimentally demonstrated. To serve
as wavelength selective element, the CMFG possesses excellent comb-like filtering characteristics including stable
wavelength interval and ultra-narrow passband, and its fabrication method is easy and flexible. The usage of the FSI
filter helps remove the high-cost optical circulator and improve the output performance of optical signal-to-noise ratio
from 50dB to 70dB. With this laser configuration, the output power fluctuation of each wavelength is less than 0.5 dB
within a one-hour period, and the output power of different channels is almost identical (difference less than 1dB) within
the tunable range.
A compact in-fiber Mach-Zehnder interferometer comb-filter is demonstrated by splicing a section of twin-core fiber
(TCF) between two single mode fibers (SMFs). The temperature and strain induced wavelength shifts of the interference
fringes are experimentally monitored. Redshift (i.e., wavelength shifts to the longer wavelength side) is observed with
sensitivity of about 0.037 nm/°C for increased temperature, whereas blueshift (i.e., wavelength shifts to the shorter
wavelength side) is observed with sensitivity of about 0.866 pm/με for applied strain changes. This device is relatively
simple to fabricate and expected to have applications in high temperature or strain fiber optic sensors and the multiwavelength
fiber lasers.
Switchable single-longitudinal-mode (SLM) dual-wavelength erbium-doped fiber laser at room temperature is demonstrated. One fiber Bragg grating (FBG) directly written in a polarization-maintaining and photosensitive erbium-doped fiber as the wavelength-selective component is used in a linear laser cavity. Because of the polarization hole burning enhanced by the polarization-maintaining FBG, the laser can be designed to operate in stable dual-wavelength or wavelength-switching modes with a wavelength spacing of 0.202 nm by adjusting a polarization controller. The stable SLM operation is guaranteed by a saturable absorber. The optical signal-to-noise ratio of the laser is >40 dB. The amplitude variation in nearly 1.5 h is <0.5 dB for both wavelengths.
Switchable dual-wavelength fiber lasers with orthogonal polarizations, based on the semiconductor optical amplifier at room temperature, are proposed. One polarization-maintaining fiber Bragg grating (PMFBG) is used as the wavelength-selective filter in three different laser configurations. Owing to the polarization-dependent loss of the PMFBG, the laser can be designed to operate in stable dual-wavelength or wavelength-switching modes with a wavelength spacing of 0.336 nm at room temperature by adjusting a polarization controller in each laser configuration. The amplitude variation over nearly half an hour is less than 0.1 dB for both wavelengths in each laser configuration, which is more stable than that of erbium-doped fiber lasers with similar configuration.
Dual-wavelength with orthogonal polarizations erbium-doped fiber ring laser at room temperature is proposed. One
polarization-maintaining fiber Bragg grating (PMFBG) in a Sagnac loop interferometer is used as the wavelength-selective
filter. Due to the polarization hole burning (PHB) enhanced by the PMFBG, the laser can operate in stable dual-wavelength
operation with wavelength spacing of 0.336 nm at room temperature by adjusting a polarization controller
(PC). The optical signal-to-noise ratio (OSNR) is over 52 dB. The amplitude variation in nearly one and half an hour is
less than 0.6 dB for both wavelengths.
The all-fiber, multi-cavity, Fabry-Perot passband filters based on fiber Bragg gratings, up to seven, are presented and
modeled. The general formulas of the transfer function for the multiple-cavity Fabry-Perot filters are derived with the
transfer matrix method. Transmission spectrum characteristics of the filters with different number of cavities are
simulated, analyzed and compared. Numerical results show that near-rectangular bandpass shape can be realized by
choosing the proper index modulation depths for every forming FBGs. And the simulations clearly demonstrate that the
more we increase the number of cavities the more the shape of the central transmission peak is getting rectangular.
A novel reflection-type filter composed of microring resonator array and MZI is presented and analyzed. Simulation
results show that the devices can be used as reflection-type filters for DWDM system or wavelength-selective reflectors
for fixed or tunable lasers by properly choosing the values of coupling ratios.
The coupled-mode equations corresponding to a novel complex long-period-grating-assisted coupler (LPGAC), which consists of both the periodic refractive index modulation and gain/loss perturbation, is introduced and the close-form analytical solution is obtained, for the first time to our knowledge. And a unique unidirectional and
nonreversible filtering characteristic is achieved by adjusting the gain/loss to match with the refractive index modulation. In addition, the impact of deviations in the grating profile is also evaluated, and the results show that the required device performance can be realized by controlling the amplitude and phase deviation <5%.
Chirped fiber Bragg gratings are supposed to be cascaded for multi-channel dispersion compensation in DWDM
systems. The interaction between them restricts their employment. Gaussian and Super Gaussian apodization are used to
reduce the out-of-band reflection so as to suppress the interaction of the cascaded gratings. The increase of the channel
spacing can also diminish the interaction.
We fabricated linearly chirped fiber gratings by using uniform phase mask instead of chirped mask. The chirp of the grating is realized by precisely setting the distance between the fiber and the phase mask at every point of the fiber. In experiments we derived linearly chirped fiber grating which has dispersion -1102ps/nm, time delay ripple is 17ps. And also the asymmetry high order apodization method is used successfully to reduce the time delay ripple. The experiment results consistent with the simulation results. We can fabricate gratings with different chirp extent use one uniform phase mask conveniently by only changing the curve function of the fiber.
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