We propose a novel approach for the automated reproduction process of mode-locked pulse generation in a fiber laser, employing a statistical correlation analysis algorithm to execute rapid spectrum searching and matching. Our system incorporates a servo-based automatic control strategy to realize precise reconstruction of specific mode-locked states from white noise in accordance with desired spectral characteristics. The average reproduction time is 70 ms for a similarity, defined by the percentage error between targeted and matched spectra, of 98.96%. Consequently, our real-time reproduction algorithm offers a highly promising method for the automated manipulation of mode-locked pulse generation in laser industry.
Low coherent pulse bound states have drawn significant interests due to their distinctive structures and dynamic behaviors. Single- or dual-wavelength interference-induced noise-like soliton molecules (NLSMs) characterized by the crossing features of a broad autocorrelation pedestal of noise-like pulses (NLPs) and symmetrically distributed autocorrelation multi-spikes of soliton molecules (SMs) are experimentally achieved in a fiber laser with a dualcavity structure, respectively. The generation of NLSM owes to the intrapulse interference within an NLP or interpulse interference between two NLPs, exhibiting its coherence varying from weak to strong. Moreover, the coherent pulse spacing within NLSMs can change from 8.42 to 44.93 ps by altering length differences between two subcavities. Our work provides a novel approach for manipulating low coherent pulse generation.
We report the cascaded generation of mid-infrared Raman solitons in an Erbium-doped fiber laser (EDFL) with a tunable Fabry-Perót filter. The first- and second-order Raman solitons were experimentally achieved at 1856 nm and 2029.6 nm, respectively, under pumping by a 1530-nm continuous wave (CW) laser with a 3-dB bandwidth of 0.1 nm generated from the EDFL. Their 3-dB bandwidths were 13.4 nm and 2.4 nm, respectively. It was found that the balance of Raman gain induced by nonlinear effects broadening the laser spectrum and Fabry-Perót filtering narrowing the laser spectrum in the EDFL resulted in the cascaded generation of Raman solitons within the wavelength band from 1800 to 2100 nm. It is the first time for a mode-locked EDFL operating at the wavelength band of 2.0 μm. Our work has great potential application as a novel optical source from near- to mid-infrared regimes.
In this paper, we report two types of dual-wavelength pulse superposition states, i.e., heterogeneous and homogeneous pulse patterns, in a ring fiber laser mode-locked by nonlinear polarization rotation (NPR) with a Fabry–Perót (F–P) minicavity. The heterogeneous pulse pattern is characterized by the coexistence of the triangular spectrum and soliton-like spectrum with central wavelengths of 1558.3nm and 1565.4 nm, respectively. Meanwhile, the homogeneous pulse is formed by the superposition of two solitons whose central wavelengths are 1558.7 nm and 1565.6 nm, respectively. The switching between heterogeneous and homogeneous pulse patterns is realized through adjusting polarization controllers and the pump power. Theoretical analysis shows that the two-type pulse superposition state generation is due to two independent filtering effects in the fiber laser, which originate from NPR and F–P filtering effects, respectively. Our results will provide a new method to produce abundant dual-wavelength pulse patterns.
Fiber based optical coherence tomography (OCT), which is combined by OCT and fiber probe, presents advantages of high longitudinal resolution, flexibility and miniaturization structure. However, the depth of focus (DOF) and transverse resolution depended on OCT fiber probes are two constraining parameters. The realization of high transverse resolution imaging could result in the reduction in the effective imaging range. To address this issue, a focal length controllable fiber probe based on microcavity structure is proposed in this paper. A segment of alcohol-filled silica capillary is inserted between a single-mode fiber (SMF) and a section of gradient-index (GRIN) fiber and works as a beam expander. By adjusting the surrounding temperature, the expanding condition changes due to the alcohol refractive index is different, eventually the focal length of the probe can be tuned. Experimental results prove that the focal length can be tuned freely from ~1835 μm to ~1650 μm while the temperature of the alcohol is changed from 20°C to 60°C.
A Mach-Zehnder interferometer (MZI) based on core-offset splicing technique for sensing applications is demonstrated. The interferometer is composed of a section of single-mode fiber sandwiched by two core-offset spliced standard optical fibers. The refractive index (RI) of the external media around the middle section of the MZI could be detected by investigating the peak wavelength of the MZI spectrum. Experimental results show that the proposed sensor exhibits a sensitivity to external refractive index of 17361 nm per refractive index unit and the temperature sensitivity of only 33 pm/°C. The present device has great potential in biochemical and medical sensing due to the advantages including easy fabrication, high sensitivity and excellent compactness.
A fiber laser with a semiconductor saturable absorption mirror (SESAM) and a graphene-polymer composite (GPC) film is constructed for achieving different soliton bunching patterns. The SESAM is used as a mode locker for self-started pulse generation, while the GPC provides a nonsaturable absorption effect for achieving a bunching in the laser cavity. There are three extra temporal patterns observed in the experiments through adjusting polarization controllers. They are chaotic bunching, weak coherent bunching and modulated harmonic bunching. The experimental investigation shows that dynamic nonsaturable absorption effect of an absorber is beneficial for generating different bunching patterns. Our work can have a deeper understanding of the formation of the complex soliton bunching patterns in pulsed lasers.
A novel dispersion-compensation superstructure fiber Bragg grating (SFBG) based comb gain equalizer for fiber optical
parametric amplifier (FOPA) is proposed. It is characterized by its eight-channel 100GHz DWDM-channel separations
and 80 km dispersion compensation function. The object reflectivity spectra of gain equalizer are calculated in two parts
independently. Inner channel part for every channel is designed according to the gain spectrum of fiber OPA. Inter-channel
part is calculated with Gaussian hypergeometric function (-0.5dB bandwidth 0.1nm, -45dB bandwidth 0.3nm) to
obtain slower rising and descending edges than the simply zeroed case. Then the SFBG is designed with layer-peeling
inverse scattering technique. Simulation results show that this SFBG satisfies all requirements. Using this SFBG as
comb gain equalizer, the gain of fiber OPA is flattened in every channel to within ±0.4dB among 8 ITU-T regulated
channels, with 80 km dispersion compensated.
Widely tunable All-optical wavelength conversion between picoseconds pulses based on Four-wave mixing is proposed
and experimentally demonstrated in cascaded highly nonlinear fiber. The signal pulse with 40-GHz repetition rate and
1.57-ps pulse width is adopted. The converted idler wavelength can be tuned continuously from 1540.6 to 1594.8 nm as
the CW laser wavelength is changed from 1514.5 to 1565.7 nm. No obvious changes of the pulse shape and width, also
no chirp are observed in the converted idler pulse.
The evolution properties of the self-similar parabolic pulse(similariton) with higher-order effects in microstructured fiber
amplifier with normal group-velocity dispersion are investigated in this paper. The numerical results show that the
higher-order effects greatly distort similariton's waveform, frequency and the linearity of chirp. The drift of pulse center
increases with distance. The influence of different higher-order effects on similariton is analyzed. High quality
propagation of similariton can be attained by manipulating the geometrical parameters of the microstructured fiber
amplifier. These results are significant in the further study of similariton propagation in high-power ultrashort fiber
amplifier, laser and transmission system.
Vector soliton is obtained in erbium-doped fiber laser via nonlinear polarization rotation techniques. In experiment, we
observe the every 4- and 7-pulse sinusoidal peak modulation. Temporal pulse sinusoidal peak modulation owes to
evolution behavior of vector solitons in multiple polarization states. The polarizer in the laser modulates the mode-locked
pulses with different polarization states into periodical pulse train intensities modulation. Moreover, the
increasing pumping power lead to the appearance of the harmonic pulses and change the equivalent beat length to
accelerate the polarization rotation. When the laser cavity length is the n-th multiple ratios to the beat length to maintain
the mode-locking, the mode-locked vector soliton is in n-th multiple polarization states, exhibiting every n-pulse
sinusoidal peak modulation.
A novel superstructure fiber Bragg grating (SFBG) based comb gain equalizer for fiber optical parametric amplifier (OPA) is proposed. This gain equalizer is characterized by its comb reflectivity spectrum. It can have two functions at least, one is flattening the gain in multi ITU-T regulated channels, the other is it can be used as multi-channel isolation filters taking full advantage of its comb reflectivity spectrum. In addition it can be dispersionless or have dispersion compensation function. A one-bump fiber OPA, 30dB bandwidth >15nm, is designed with one of its peak gains (41dB) located at the center of work wavelengths in a 16-channel 100GHz spaced WDM optical transmission system according to corresponding Chinese criterion (1548.51nm~ 1560.61nm). The object reflectivity spectra of gain equalizer are calculated in two parts independently. Inner channel part for every channel is designed according to the gain spectrum of fiber OPA. As a comb filter, its inter-channel part reflectivities are the smaller the better. But inter-channel part reflectivities can not be simply zeroed, for which would result in the rising and descending edges of reflectivity spectrum being too sharp to be performed by any filter. To split the difference, inter-channel part reflectivities are calculated with Gaussian hypergeometric function (-0.5dB bandwidth 0.1nm, -45dB bandwidth 0.3nm) to obtain slower rising and descending edges than the simply zeroed case, which is very necessary and important to be sure the object reflectivity spectrum not only has sharp enough edges to take the shape of comb but also is physically realizable. Adopting channel-by-channel windowing method to apodize the object reflectivity spectrum, a superstructure fiber Bragg grating is designed with lay-peeling inverse scattering technique. At last, according to the coupling coefficient calculated, reflection spectrum and group time delay are analyzed based on the method of transformation matrix. Calculation results show that this SFBG satisfies all requirements. Further calculation results show that, using this SFBG as comb gain equalizer, the gain of fiber OPA is flattened in every channel to within ±0.4dB among 16 channels, with channel isolation high up to -35dB. It needs to point out that this method can also be used to design gain equalizer for other optical amplifiers.
A novel 16-channel comb filter based on superstructure fiber Bragg grating (SFBG) is proposed for the first time. It is characterized by single-grating structure, equal reflectivity peaks, narrow channel bandwidth, flat-top, high bandwidth utilization ratio, standard 50 GHz channel-spacing, and realizing dispersion compensation and wavelength filtration simultaneously. Based on ameliorated layer-peeling algorithm, that is adopting channel-by-channel windowing method rather than all-channel integral windowing method, this novel SFBG is synthesized successfully. According to the grating structure of the synthesized SFBG, reflectivity spectrum, group delay and group delay ripper are analyzed based on the method of transfer matrix. The results show this SFBG does an almost perfect job of reproducing the desired spectrum.
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