A dual-wavelength passively mode-locked Yb-doped fiber ring laser with an all-fiber format Sagnac loop as the spectral filters in the all-normal dispersion (ANDi) regime is reported demonstrated. By adjusting the polarization states in the cavity, both the output pulses duration and the central wavelength of mode-locked laser can be changed. The central wavelength of laser output is solely determined by the central wavelength of the channels of the Sagnac loop. It provides a new possibility to achieve an all-fiber format, multiwavelength and mode-locked laser output with the ANDi cavity in a simple and stable manner.
We report on multiple solitons generation in an all-normal-dispersion (ANDi) passively mode-locked
Yb-doped fiber ring laser with a cascaded long-period fiber grating as a spectral filter. Several different characteristic
pulse modes have been experimental observed from the same ring laser by adjusting the polarization controllers,
including uniform multiple solitons and nonuniform pulse-trains. In the uniform multiple solitons, the separation
between the adjacent solitons is independent of the number of solitons, which is fixed at 33.6 ns. In the nonuniform
pulse-train, multiple solitons up to 8 have been observed coexisting in the cavity, and the time distribution of the solitons
is fixed at 21.1 ns, 12.5 ns and 8.6 ns, respectively. The observed results of multiple solitons output will not only have
the potential applications in molecular detection, remote sensing and laser communication, but also be helpful to
understand the mechanism of multi-solitons generation in the fiber laser system.
We report a high-sensitivity refractive index sensor based on microfiber long period gratings (MLPGs) fabricated by a CO2 laser line-by-line inscription. The microfiber was fabricated by using hydrofluoric acid (HF) to etch a conventional single mode fiber (SMF). The MLPGs were fabricated by periodically tapering the microfiber with CO2-laser pulse line by line. We showed that a 45-period long period grating with about 15 dB attenuation dip can be realized in the microfiber with a diameter of ~32μm. The MLPGs were found to have a very high sensitivity to external refractive index, in which the sensitivity of the MLPGs can be 4 times than that of a normal long period grating (LPG).
A new method of using a cascaded long-period fiber grating (CA-LPFG) as the spectral filter in the ytterbium-doped
mode locking in the normal dispersive regime are proposed. The CA-LPFG was fabricated in a single mode fiber at
wavelength of 1064nm using a CO2 laser based system. A stable mode-locking laser output at wavelength of 1034nm
was observed and measured in the normal dispersive regime. In comparison with the conventional interference filters or
fiber Bragg grating filters, the CA-LPFB can provide not only an all-fiber low-loss but also a transmission mode.
In this paper, we demonstrate that a simple and practical phase-shifted fiber Bragg grating (PSFBG) operated in
reflection can provide the required spectral response for implementing an all-optical Hilbert transformer (HT), including
both integer and fractional orders. The PSFBG consists of two concatenated identical uniform FBGs with a phase shift
between them. It can be proved that the phase shift of the FBG and the apodizing profile of the refractive index
modulation determine the order of the transform. The device shows a good accuracy in calculating the Hilbert transform
of the complex field of an arbitrary input optical waveforms when compared with the theoretical results.
Cascaded chirped and phase-only sampled fiber Bragg gratings (CC-PSFBGs) are proposed for high efficient
multi-channel comb filters in wavelength-division multiplexing (WDM) systems. A CC-PSFBG consists of replicated
sections of chirped and phase-only sampled FBGs, in which the Talbot conditions for each section of phase-only
sampled FBGs are satisfied. Under the condition of phase matching among the cascaded sections, an arbitrarily tunable
channel spacing (tunable free spectral range, FSR) with both a controllable wavelength range and sufficient in-band
energy efficiency can be implemented. It offers a useful tool for improving the design flexibility and the energy
efficiency of conventional Talbot effect based multichannel comb filters.
Sampled and linearly chirped fiber Bragg gratings provide multiple wavelength responses and linear group delays
(constant dispersions) within each of the wavelength channels. We show that the sampled and chirped fiber Bragg
gratings can be used to perform multiwavelength signal processing. In particular, we demonstrate, by numerical
simulation, their use for performing real-time Fourier transform (RTFT) and for pulse repetition rate multiplication
(PRRM) simultaneously over multiple wavelength channels. To present how the sampled fiber Bragg gratings perform
the multichannel optical signal processing, a 9-channel sampled fiber grating with 100GHz channel spacing was
designed and the effect of ripples in both amplitude and the group delay channel on the performance of the signal
processing was examined and discussed.
In this work, we demonstrate that the spectral self-imaging phenomenon is not restricted to phase-only sampled and
linearly chirped fiber Bragg gratings(LCFBGs), rather they can be implemented in phase-only sampled fiber Bragg
gratings (FBGs) with an arbitrarily chirped grating period. We derive the general conditions at which spectral Talbot
phenomenon, namely, integer and fractional spectral self-imaging occurs. The multiplication of spectral channels,
respectively in the linear, square and cubic chirp coefficient conditions, and/or together, are implemented and observed
in phase-only sampled FBGs with both integer and fractional types using the transfer matrix method.
The spectral self-imaging phenomenon is observed in binary phase-only sampled fiber Bragg gratings (SFBGs) using
numerical simulations. Integer and fractional Talbot effects are obtained under conditions of Talbot effect. The
characteristics of the Talbot spectrum in terms of channel spacing and the group delay from the phase-only sampled
FBGs are discussed.
We present an analytical expression for sampled fiber Bragg gratings (SFBGs) with arbitrary chirps in sampling function
or grating period or combination of both. The relationship among the wavelength of each channel, the chirp coefficient
of the sampling and the grating period, and the total length of the grating is explicitly given. Specifically, the chirped
sampling function is first expanded into a new function using Fourier theory; the equivalent local Bragg period is then
obtained to derive the expression of the reflection peak wavelength. The overall wavelength position is obtained by
summation of both contributions from sampling chirp and the grating chirp. The calculated results based on the analytical
expression are examined with the conventional numerical results, which are found to be in excellent agreement.
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