Agile mode switching between LP01 mode and LP11 mode and power amplification in specific mode can be achieved at the same time by inserting an acoustically-induced fiber grating into a Raman fiber amplifier. The maximum signal light output power of the LP01 mode and LP11 mode is 8.51W and 7.98 W with the characteristic frequency of 769 kHz and 763 kHz, respectively. Moreover, the ratio of the two modes can be adjusted by modulating the frequency loaded on the acoustically-induced fiber grating. This work could provide an example of realizing a mode-switchable Raman fiber amplifier for practical application.
We propose an all-fiber configuration of tunable bandpass filter (TBPF), consisting of a mode selective coupler (MSC) and an acoustically induced fiber grating (AIFG). Either the MSC or the AIFG can act as high-order mode converters in a few-mode fiber. The working wavelengths of the MSC and the AIFG are chosen to achieve complete conversion cycle between the core-mode linearly polarized (LP01) mode and the LP11 mode. The tunability of the TBPF is achieved by changing the frequency of the radio frequency signal. An all-fiber tunable ring laser based on the TBPF is used to generate a stable continuous-wave laser with 3-dB bandwidth of around 0.8 nm. The tunable wavelength ranges from 1530 to 1580 nm. Furthermore, the TBPF is introduced into the cavity of a mode-locked fiber laser. Wavelength-tunable mode locking pulses are obtained at central wavelengths from 1530 to 1560 nm.
We research the refractive index (RI) sensing characteristic based on the bandpass spectrum caused by the self-imaging effect in the single-mode-multimode-single-mode (SMS) fiber structure theoretically and experimentally. A new selectable parameter, i.e., no-core fiber (NCF) length, is investigated for improving the sensitivity of the sensor. The results show that the sensor’s sensitivity will be enhanced by shortening the NCF length when the self-imaging number remains constant. Experimentally, a maximum sensitivity of 1923 nm/RIU (RI unit) has been achieved when the RI ranges from 1.334 to 1.434. This work demonstrates a method to improve the sensitivity of SMS-fiber-structure-based RI sensors featuring a low cost, compact size, low insert loss, and high sensitivity optical fiber RI sensor.
We present the numerical simulations of soliton spectral tunneling (SST) effect in multi-cladding single mode fibers
with three zero-dispersion wavelengths. Fiber geometries with appropriate refractive index difference and fiber radius
are selected to generate this unique dispersion property. The mechanism of multi-cladding fibers exhibiting multi zerodispersion
wavelengths is discussed. Simulation results clearly show SST effect in the proposed multi-cladding fiber.
The dependence of the output SST frequency on fiber geometry is investigated numerically and analytically. The
detailed studies present the phase-matching condition of dispersive pulses in different engineered dispersion curves of
multi-cladding fibers. The soliton numbers of input pulse show a significant role on the threshold length of SST effect.
We propose As2S3 slot waveguides, which have four zero-dispersion wavelengths and exhibit a flattened and low dispersion over a 1800-nm bandwidth from near-IR to mid-IR wavelength range. The dispersive wave generation
is investigated based on this kind of dispersive profile by launching femtosecond pulses into As2S3 waveguides. Detailed simulations under realistic conditions show that considerable amount of dispersive wave emission occur
in As2S3 waveguide and therefore can be desirable for optical communication and on-chip signal processing, such as frequency metrology, optical coherence tomography and microscopy.
Seeded second harmonic generation is investigated by including an in-phase second-harmonic pulse.
Soliton compression from a broader input pulse with low threshold pump intensity is numerically
demonstrated in collinear quasi-phase-matching grating. Through changing the group-velocity
mismatch and pump intensity, frequency shift of another input signal pulse can be obtained with pulse
duration of few cycles, which show potential tool in ultrafast pulse application and is useful for
generation of shorter pulses with clean temporal profiles.
A method to reconstruct a chirped quasi-phase-matching grating by using a discrete layer-peeling algorithm is
demonstrated. The coupling distribution and grating structure can be synthesized from the transmission spectrum.
Experimental verification shows that the method can reconstruct the grating distribution efficiently. This method can be
used an efficient approach of quality inspection on quasi-phase-matching grating.
A semiconductor QD fiber amplifier (SQDFA) is proposed and theoretically researched. The fiber amplifier makes use
of a tapered twin SMF coupler and a PbS QD-film is coated on the surface of it. In the tapered region, QDs will be
excited by the evanescent wave of a pump and produce a gain on signal. The fiber coupler is properly designed to ensure
a desired power ratio of the output signal. Using a 1530-nm signal and a 980-nm pump, an optical gain of about 8 dB is
achieved with a gain efficiency of 3.48dB/cm. This SQDFA has unique advantages including suppression to the
amplified spontaneous emission (ASE), simple fabrication and lower insertion loss and thus has a great potential in fiber
In this paper, fiber Bragg grating (FBG) is used as a fully distributed sensor to monitor tissue dynamic temperature
changes during laser-induced interstitial thermotherapy (LITT). This work is mainly realized by the correlative single
particle (CSP) algorithm, which is a rapid algorithm for spectrum reconstruction. Experimental LITT treatment was set
up by using 532nm laser applicator on a piece of fresh liver tissue. In the experiments, the dynamic temperature profile
was successfully demodulated with a refreshing speed of 11 seconds. With the aid of dynamic feedback, the
thermotherapy boundary temperature was well controlled around 35°C during the treatment by adjusting the laser output
power in real-time. Therefore, with this method, it is promising to precisely control the tissue temperature in vivo and
improve the safety of the LITT remarkably.
A novel quantum dots(QDs) optical fiber amplifier was proposed and demonstrated. It was fabricated by dip-coating the
PbSe quantum dots doped sol onto the taper region of fiber coupler. The PbSe quantum dots was synthesized according
to a colloidal method. And a lower refractive index sol was also synthesized as the host of PbSe QDs. A standard single
mode fiber was used to make the fusion tapered fiber coupler which had double input and output ports. With the simple
structure, a signal and a pump can be injected into the amplifier and excite the PbSe QDs through evanescent wave. As
shown in the experimental results, the amplified light wave was observed at 1550nm wavelength with 980nm wavelength
LD as pump.
We theoretically propose soliton-like compression of femtosecond pulses in two-segmented quasi-phase-matching (QPM)
grating. By using group-velocity (GV) matching scheme with type-I (o+o:e) second harmonic generation (SHG), we
numerically show soliton-like compression of femtosecond pulses with higher quality factor, lower intensity threshold
and better spatial-temporal pattern in segmented QPM than periodic structure with the same total crystal length.
A fiber-optic refractive index (RI) sensor is proposed based on a double-cladding special fiber. The double-cladding fiber
(DCF) consists of core, inner cladding and outer cladding. And refractive index of core and outer cladding is higher than
that of inner cladding. Through evanescent wave coupling, cladding mode can be excited resonantly at phase-matched
wavelength. Because the cladding mode transmission is sensitive to ambient refractive index variation, the DCF can be
used to solution refractive index sensor. By splicing a section DCF into standard single mode fiber (SMF), SMF-DCFSMF
sensor was constructed and studied for refractive index sensing. A resonant wavelength shift of 65nm was
achieved by changing ambient refractive index within the range of 1~1.4525.
An in-fiber Michelson interferometer is proposed based on a double-cladding (DC) special fiber. With the DC special
fiber, light wave can be partially coupled into outer cladding. The in-fiber Michelson interferometer can be constructed
by splicing a length of DC fiber into standard single mode fiber (SMF). The interferometer is very sensitive to ambient
refractive index change because fiber cladding is as one of interference arms. A sensitivity of 36nm/RIU has been
achieved in the range of 1.33~1.40 in this work. The proposed technique has the dominant advantage of simple
fabrication process, which can be expected to have wide applications in biosensors and chemical sensors.
In this paper, we fabricated and demonstrated a temperature-insensitive fiber bending sensor based on a special optical
fiber with cladding mode resonance. The special fiber, with pure silica core and a fluorine-codoped inner cladding,
fabricated by using conventional MCVD technique exhibited strong cladding mode resonance. The bend curvature
dependence of resonant spectrum was investigated with high sensitivity to be -10.15nm•m-1 and was found to possess an
insensitivity to temperature. The proposed special fiber bending sensor is simple and low cost.
We experimentally demonstrate that adiabatic compression of femtosecond pulse can be achieved by employing the
management of quadratic cascading nonlinearity in quasi-phase-matching gratings. Cascading nonlinearity is not a
simple analogy with third-order optical nonlinearity in term of the engineering properties of the magnitude and focusing
(or defocusing) nonlinearity. Femtosecond pulse compression is investigated based on type-I (e: o + o) collinear QPM
geometry of aperiodically poled MgO-doped LiNbO3 (MgO: LN). Group-velocity-matching condition is chosen to
generate quadratic femtosecond soliton consisting of fundamental (FF) and second harmonic (SH) pulses. Adiabatic-like
compression process is observed in the length of 50 mm linearly chirped QPM. Cascading nonlinearity is local managed,
instead of dispersion management used in fiber adiabatic soliton compression. Quadratic soliton including FF and SH
pulses are obtained from the compression of 95 fs FF pulse in the initial experiments. Dependence on the phase
mismatch and group velocity mismatch, cascading nonlinearity has a flexible property and presents a new challenge for
exploring femtosecond pulse shaping and control. The demonstrated pulse compression and control based on cascading
nonlinearity is useful for generation of shorter pulses with clean temporal profiles, efficient femtosecond second
harmonic generation and group-velocity control.
Conventional electro-optic modulators have been always fabricated based on channel waveguide with complicated electrodes. In this paper, we propose a new type of modulator based on proton-exchanged planar lithium niobate waveguide. After proton exchange and annealing, the refractive index of planar lithium niobate waveguide is gradually changed with the depth. Two-prism coupling method is used to determine the effective index of the planar waveguide by measuring the coupling angle of m-lines. When the transverse electric field is applied along the Z axis of the X-cut proton-exchanged planar lithium niobate waveguide, the index distribution of the waveguide will be changed due to the electro-optic effect, and the coupling angle of the m-lines will be also changed accordingly. Based on this principle, a new type of amplitude modulator is proposed theoretically. The relationship between the driven voltage and position of two electrodes are theoretically investigated. The change of refractive index of the waveguide caused by the applied transverse electric field as a function of waveguide depth is also studied. Compared with other type of amplitude modulators, it shows some advantages such as low fabrication cost and the low driven voltage.
A new optimization method for the design of integrated band-pass traveling-wave modulator based on aperiodic domain-inverted grating is demonstrated. The frequency response of 10GHz and 20GHz (0.5dB) centered at 200GHz are presented. The physical mechanism for this kind of modulator is ascribed to the constructive interference effect of the overall domains. The sequences and the length of the domains are optimized to realize the pre-designed wide, flat frequency response over the required frequency range by use of the simulated annealing (SA) method.
The grating period of optimal all-optical converter based on periodically poled LiNbO3 (PPLN) waveguide dependent on the mode sizes and effective index have been theoretically investigated. Variational method of TM mode in an annealed, proton-exchanged waveguide is first discussed. The optimal grating periods are obtained, which are sensitive to the waveguide width and wavelength change of pump waves, and not to input signal waves. The grating periods in 1.5 um-band are consistent with previously published experimental results.
In this paper, we investigate the acceptance bandwidths due to different polarization of fundamental waves for type I quasi-phase-matched (QPM) second-harmonic-generation (SHG) in bulk periodically poled lithium niobate (PPLN) theoretically, which are determined by the departures from ideal quasi-phase matching in periodicity, wavelength and temperature. This analysis is useful for establishing fabrication tolerances for practical QPM bulk PPLN device. The numerical comparisons of acceptance bandwidths between type I QPM SHG Ez(omega )->Ez2(omega ) and Ey(omega )Ey(omega )->Ez2(omega )(oo->e) at 150 degree(s)C in bulk PPLN are first presented. The grating periods for two various polarization interactions are given. The result shows that the acceptance bandwidths can be significantly enhanced by the use of the perpendicular polarization. Furthermore, the limitation of input fundamental wavelength for Ey(omega )Ey(omega )->Ez2(omega )(oo->e) QPM SHG in bulk PPLN is discussed.
A first order quasi phase matched periodically poled LiNbO3 for SHG with 6.5 micrometers period, 10-mm length and 0.5-mm thickness has been successfully fabricated by applying an external electric field with an optimum high pulsec voltage generator. About 18-mW of cw 0.532 micrometers green light was obtained pumped by a 1.1 W of cw 1.064 μm Nd:YAG laser with end coupling, which corresponds to 1.6 percent power conversion efficiency. The SHG normalized conversion is about 1.5 percent cm-1W-1, which shows 79 percent of the ideal nonlinear coefficient.