We design a narrow linewidth pulsed core-pumped double-cladding large mode area ytterbium-doped fiber amplifier (LMA-YDFA) operating at 1016 nm, and experimentally investigate the effects of different input signal laser power, YDF length, pump power and pumping mode on the output characteristics of the amplifier. The results show that the core pumped LMA-YDFA is feasible in power amplification, gain fiber shortening and ASE suppression. We compare the core pumped LMA-YDFA with the single mode ytterbium-doped fiber amplifier (SM-YDFA). The latter has the self-phase modulation (SPM) that the former does not have. The results show that the core pumped LMA-YDFA can better suppress the nonlinear effect and spectral broadening of the narrow linewidth pulse laser in the amplification process.
Supercontinuum (SC) generation in ytterbium-doped fiber amplifiers usually use single-wavelength laser seeds, which leads to the supercontinuum with strong Raman peak and unsatisfactory flatness. In this paper, a 1030 nm and 1064 nm dual-wavelength seeded ytterbium-doped fiber amplifier (YDFA) is demonstrated for the first time to generate a supercontinuum, the Raman peaks generated by 1030 nm and 1064 nm pulse lasers fill in the dips of each other, which is conducive to the flattening of the spectrum. The dual-wavelength seeded amplifier exhibits high spectral flatness with an 8 dB spectral bandwidth from 1 μm to 2 μm at the average output power of 1.92 W. The influence of the power ratio of 1030nm and 1064nm pulse laser seed and the impact of different inter-pulse time-delay of the two pulse lasers on the supercontinuum are investigated experimentally. The experimental results indicate that when the power ratio of 1030 nm and 1064 nm pulse lasers is 10:1, the flatness of supercontinuum has the best performance, which is determined by the absorption/radiation cross section of ytterbium ions, and the synchronous amplification of dual-wavelength pulsed seed lasers is more conductive to the broadening and flattening of the supercontinuum, as compared with the case of one seed laser. This work provides a new way to realize high flatness supercontinuum in YDFA.
Considering the difference in the divergence angle of different spectral components of supercontinuum laser, this paper researched the beam expansion effect of supercontinuum laser in atmospheric turbulence. The simulated laser transmission is carried out in the Von Karman model. The influence of turbulence of different intensities on the beam expansion is compared, the influence of turbulence on supercontinuum laser beam expansion with vertical, slant and horizontal transmission are also studied. The results show the beam expansion effect of strong turbulence on the short- wavelength part of supercontinuum laser is more obvious than that of moderate turbulence and weak turbulence. As the intensity of turbulence increases, the spot of the short-wavelength part increases rapidly first and then increases slowly and steadily, the spot of the long-wavelength part keeps almost unchanged at first, and also then increases slowly and steadily. The relationship between the elevation angle of laser beam emission and the beam width is calculated, and three cases are specifically analyzed. In the cases of vertical transmission and slant transmission, the beam width is related to the characteristics of the light source after collimation, which increases with the increase of the wavelength. When the laser is transmitted horizontally, the beam width has been greatly expanded, especially in the short-wavelength part. As the angle increases, the radius of the spot decreases rapidly first and then keeps almost unchanged. The radius of the spot of the short wavelength part reduces a larger proportion than that of the long-wavelength part.
We numerically calculated and analyzed the influence of a 7 × 1 fiber power combiner on supercontinuum (SC) laser beam quality in the range of 0.5 to 2.3 μm. The calculation results show the M2 oscillates within a range for a specific wavelength as the length of the output fiber changes. When the length of the output fiber is fixed, whether single input fiber or all fibers are injected with SC lasers, the M2 of the SC shows a nonlinear change and oscillates continuously throughout the entire spectrum, while it first decreases and then increases as the wavelength increases as a whole. Analysis of the numerical results shows that by changing the type of input fiber and the taper waist structure, the beam quality of both long-band and short-band parts of the SC laser can be improved. To verify theoretical calculations, the divergence angle of SC was measured as the beam quality after being transmitted by one 7 × 1 fiber power combiner in the spectral range from 0.5 to 2.3 μm. The divergence angle first decreases but then increases with a prolonging wavelength, and the curve of the experimental result renders a basic match with the calculation. To the best of our knowledge, currently no overall evaluation is reported on the beam quality of combined SC sources.
For the collimation and focusing of supercontinuum, the divergence angle is an important parameter. In order to obtain the far-field beam divergence angle corresponding to multiple wavelengths of the supercontinuum in the wide spectrum range of 0.5-2.3μm, two types of cameras are used to collect the far-field spot image. The range covers the supercontinuum that PCF can generate. The relationship between the beam quality/divergence angle and the wavelength is summarized. The experimental values are in good agreement with the theoretical values. To the best of our knowledge, it is the first report to measure the divergence angle of supercontinuum in such a wide range of wavelength. The divergence angle of the supercontinuum after transmission through a section of passive optical fiber with a core diameter of 10μm was also measured. The results show that the divergence angle of supercontinuum variation with wavelength exhibits different characteristics for the 10μm core fiber and the PCF.
Taking all the pulse instantaneous power into consideration, we investigate the pulse interference dynamics of dissipative soliton (DS) and dissipative soliton resonance (DSR) through a discrete model based on nonlinear optical loop mirror (NOLM). We numerically and experimentally acquire that the reflection and transmission spectra of the NOLM under DS and DSR regimes show significant wavelength selectivity which results from the intensity-dependent nonlinearity. The wavelength selectivity phenomenon may be helpful for in-depth understanding on different operation dynamics of saturable absorber in passive mode-locked lasers.
We numerically present the generation of four types of pulses using nonlinear Schrödinger equations in an all-normal-dispersion dual-pump mode-locked fiber laser based on a nonlinear amplifying loop mirror (NALM). Transition state (TS) between dissipative soliton (DS) and dissipative soliton resonance (DSR), typical DSR, DS, noise-like pulse (NLP), and nonmode-locked state are investigated in a two-dimensional pump power space. Different from previous research, we first find that the pulse peak power and width vary dependently and nonmonotonously under asymmetric coupling ratios in the NALM. Under different coupler ratios and amplifier powers, the shift and distortion of simulated NALM transmittivity curve which are caused by the pump power intercoupling and the amplifier saturation are demonstrated. The ratio of incident power and the saturable power of NALM (when the ratio is >2, 1 ∼2, ∼1, <1) directly determines the pulse states (NLP, DSR, unstable, DS, or TS) in the cavity. Overall, the utilization of NALM could effectively enlarge the saturable power as well as the transmittivity, which provide a road to increase the DSR peak power.
A high-energy, wavelength-tunable all-fiber picosecond MOPA laser is reported. The seed is provided by an active mode-locked oscillator which can be continuously tuned from 1030 nm to 1080 nm. The seed pulses with duration of 212 ps and average power of 3 mW are injected into a two-stage amplifier. The nonlinear effect in the main amplifier is alleviated by using a large mode area gain fiber, to avoid pulse distortion and spectral broadening. Over 10 W average output power ranged from 1030 nm to 1080 nm is achieved with the FWHM bandwidth and spectral signal-to-noise ratio of 0.2 nm and 30 dB, respectively. Measured pulse duration is less than 350 ps at repetition rate of ~536 kHz. The maximum peak power and single pulse energy reaches 54 kW and 18 μJ, respectively. The source is then used to pump a self-designed multi-core photonic crystal fiber (PCF) for supercontinuum generation. By tuning the wavelength of the laser to approach the zero-dispersion wavelength of the PCF, a broadband supercontinuum covering the wavelength range from shorter than 400 nm to longer than 2400 nm is achieved. The experimental results are in consistent with the theoretical analysis, which benefit from the wavelength continuously tunable property and the high peak power of the picosecond laser.
We numerically present the generation of five types of pulse states using nonlinear Schrödinger equations in an all-normal-dispersion (ANDi) mode-locked fiber laser based on a nonlinear amplifying loop mirror (NALM). Using a two-dimensional pump power space of the two amplifiers, we investigate the pulse characteristics among typical dissipative soliton resonance (DSR), dissipative soliton (DS), transition state (TS) between DS and DSR, noise-like pulse (NLP) and unstable pulsation state. Different from previous research results, we firstly find that asymmetric coupling ratio of the NALM causes dependent and non- monotonous variation of the pulse peak power and width of DSR. The location of incident power and the saturable power of NALM (when the former is 2 times higher, higher, equal, lower than the later) determine the pulse states (NLP, DSR, unstable, DS or TS) in the cavity. The pump-power intercoupling-induced frequency-selective NALM transmission is demonstrated by the simulated pulse evolution over round-trips.
The effectiveness and efficiency of laser processing depends on absorption characteristics of materials. A combination of a low-energy supercontinuum with a high-energy narrowband laser was proposed to form a potential new laser source for materials processing. The broadband spectrum of the supercontinuum is expected to slightly change the properties of target materials by materials absorption, and then the narrowband laser is able to process the materials effectively. This new laser source was realized by seed pulse-shaping. The seed pulses were formed by a high and short pulse with a low and long pulse, which were generated from an electrically modulated laser diode operating at 1064nm. After being amplified, the pulses went through a section of photonic crystal fiber. The average output power and spectrum could be widely tuned as the pulse durations, amplitudes and relative delay time varied.
We report the experimental results of ultraviolet-extended broadband supercontinuum (SC) generation in a carefully designed uniform seven-core photonic crystal fiber (PCF) pumped by Ti:sapphire femtosecond laser at 800 nm. Three different PCFs of various core diameters are fabricated to achieve group-velocity matching for ultraviolet components. A wide optical spectrum spanning down to 350 nm is obtained, which is the shortest wavelength SC generation in multicore PCF to date. High spectral flatness (10 dB) has been achieved in the entire visible window.
The visible supercontinuum (SC) sources has played an important role in biomedical applications. However, the small core size of photonic crystal fiber (PCF) restrict the development of high power SC by its small mode field area. In addition, the zero dispersion wavelength (ZDW) of the PCF with small core diameter is usually below 1 μm, which is far away from the 1.06 μm laser which is the most commonly used pump source. As the ZDW of PCF shifts away from the pump, the intensity of visible light decreases correspondingly. We promote a new technique to get an enhanced visible SC with high output power, which involves enhanced visible SC generation in a seven-core PCF pumped by a high power 1016 nm fiber laser. Muti-core PCFs offer a possibility of scaling up the mode field area to a large extent without remarkable change in dispersion properties, which show great potential in high power SC generation. Using a 1016 nm fiber laser as the pump makes the pump wavelength closer to the ZDW of PCF, which could raise the intensity of visible light. In this paper, we report an enhanced visible SC generation ranging from 400 nm to 2300 nm in a seven-core PCF pumped by a 1016 nm picosecond fiber laser. The visible light (400~800 nm) occupies 31% of the total SC power 24 W and the power of the visible light is about 7.4 W.
We report the results of our recent experimental investigation of the modulation frequency detuning effect on the output pulse dynamics in a pulse modulated actively mode-locked ytterbium doped fiber laser. The experimental study shows the existence of five different mode-locking states that mainly depend on the modulation frequency detuning, which are: (a) amplitude-even harmonic/fundamental mode-locking, (b) Q-switched harmonic/fundamental mode-locking, (c) sinusoidal wave modulation mode, (d) pulses bundle state, and (e) noise-like state. A detailed experimental characterization of the output pulses dynamics in each operating mode is presented.
A supercontinuum source with larger than 200 W average power covering 1 μm to beyond 1.7 μm is constructed in a
four-stage master-oscillator power-amplifier (MOPA) configuration. The process of power amplification and spectral
evolution during supercontinuum generation and the influence of pulse duration on this process are investigated. For the
signal light in normal dispersion region, Raman effect plays dominant role in the first phase and the power transfer from
amplified signal light to long wavelength region can be substantially achieved. When the spectrum is extended across the
zero dispersion wavelength, soliton effect becomes the main effect for spectral broadening and the power transfer
efficiency is lower to make the signal light a high peak. With similar average output power, narrower seed pulse leads to
higher peak power and can induce Stokes waves earlier which leaves longer fiber for soliton propagating. Thus the
spectrum can be shifted to longer wavelength. However, the signal light becomes a higher spike. In a word, narrower
seed pulse leads to higher signal peak and wider SC source.
We design a seven-core photonic crystal fiber with specifically designed dispersion and group velocity profile which is optimized for high-power visible supercontinuum (SC) generation pumped by ∼1-μm pulsed lasers. The fiber has both a large air-filling fraction and a large effective mode field area. Additionally, the in-phase supermode of this fiber exhibits an even field distribution after mode modification. The simulation results suggest that it has a great potential to generate a high-power SC extending to 400 nm, which is highly desirable in biological applications.
In this paper, we propose an optimized design of the picosecond MOPA fiber laser by using hybrid-doped Yb fibers. The
detailed implementation is that high-doped gain fiber with relatively small fiber core is adopted in the low-power
pre-amplifiers, thus the shortest fiber could be ensured and the thermal load is not so heavy. In the power amplifier stage,
the normal-doped large mode area gain fiber is utilized, then the thermal effects can be minimized and the output beam
quality could be ensured. The design simultaneously takes into consider the output beam quality, thermal load, nonlinear
phase shift, pulse peak power of the picosecond fiber laser. With using the existed Yb-doped fibers, we demonstrate a
high-power picosecond MOPA fiber laser with the proposed method. The average output power is up to 110 W, the total
optical conversion efficiency is 63.2%. No stimulated Raman scattering, amplified spontaneous emission and residual
pump light are observed in the output spectrum. The - 6 dB bandwidth of the output spectrum at the full output power is
~ 4 nm.
Self-organized coherent laser array seems to be a promising method for coherent combining fiber lasers. Phase
locking is realized by mutual energy injection, without any active phase stabilization, requirement of the fiber
length or the output power for individual lasers. In this paper, a novel self-organized coherent combination
configuration of an array of fiber lasers has been demonstrated. Mutual injection locking and coherent combining
of three and four individual erbium-doped fiber lasers have been reported.The fiber lasers are mutually injected
by couplers. Under free running state, the far field beam profile is a simple intensity superposition as incoherent
beams, under mutual injection locking, good coherence has been obtained in the far field of the fiber laser arrays.
This scheme is quite easy to implement and can be extended to combine more beams.
The influence of the delivery fibers, drawn by pump combiner or added initiatively, on stimulated
Brillouin scattering (SBS) is discussed, based on the rate-equations combining with SBS. The impacts
of pump schemes are simulated and the results indicate that the amplifier performance is considerable
undermine by the delivery fiber in co- and counter- pumped amplifiers. As a suitable pump scheme for
high power all-fiber amplifiers, we focus on the situation in the co-pumped amplifiers, in which the
lengths, Brillouin gain coefficient and core diameter of the delivery fiber make great impacts on the
performance of amplifiers. The impacts of the temperature gradients along the fiber, which can broaden
the SBS gain profile and thereby suppress SBS, on the amplifier performances are discussed. Suitable
suppression scheme is proposed to overcome the influences of delivery fiber on the amplifier
performance.
The phase information of the thin heated air cylinder transection was acquired with a self-collimated interferometer
and line CCD camera at the frame rate of 27kHz , based on the presumption of axial symmetry ,using addition algebraic
reconstruction algorithm , 2-dimension index-of-refraction distribution of the heated air in the transection was acquired
,and the 2-dimension temperature distribution was calculated, the temperature shift was observed, the central
temperature of the air cylinder changed between 330K and 430K. The results show the method could be used to the
measurement of near axial symmetry aero-optical medium.
Coherent combination of fiber lasers through mutual injection locking is demonstrated experimentally in this paper. By
moving the mutual injection couplers from the output port to the high reflection feedback port of the lasers, a modified
combining configuration is constructed with obviously enhanced slope efficiency as compared with the conventional
one. The laser efficiency increases from 29.7% to 37.8% by this modification. The corresponding maximum output
power enhancement of the combined laser is 26.6%. This modification increases not only the individual child laser
powers but also the combining efficiency. The physical connotation of the modification on the improvement of the laser
performance has been discussed.
A 1.16-W superfluorescent fiber source (SFS) centered at 1561 nm with a 3-dB bandwidth of 8 nm has been achieved, under the pumping of a 3.56-W 976-nm laser diode array. The optical conversion efficiency reaches 32%. The source is constructed in a dual-stage configuration. The first stage is an ASE seed source with output power about 30 mW in the C band. The second stage is a backward-pumped high-power erbium-ytterbium-codoped double-clad fiber amplifier. An interesting phenomenon has been observed: low-power ASE seed source causes laser oscillation in the SFS, yet a relatively high-power ASE seed source prevents the SFS from lasing. The Rayleigh backscattering and the saturation effect of the amplifier are considered to explain the phenomenon.
The cavity configurations of erbium-ytterbium co-doped fiber ring lasers (EYDFL) have been experimentally investigated. Additional attention has been paid to the mode competition effect of the laser. It is demonstrated that even in a traveling wave cavity, mode competition occurs when the cavity configuration or the output splitting ratio are incorrectly chosen. By employing the proper cavity configuration and an optimal output splitting ratio, an extremely stable ring cavity EYDFL with fine-shaped laser spectrum is obtained at 1565.8nm. Output power of 1.07 W is achieved under 3.5W 976nm pump power, with an optical conversion efficiency of 30.6%.
A bidirectionally pumped spectrum-presliced multiwavelength fiber source is presented. Two 980-nm laser diodes are engaged to pump a section of erbium-doped fiber (EDF) in contrary directions. A double-pass Mach-Zehnder comb filter is utilized to filter and reflect the amplified spontaneous emission at one port of the EDF. An optical isolator is employed to guide out the multiwavelength outputs at the other end. The extinction ratio of the source is larger than 24.5 dB over the whole C band, and almost reaches 29 dB at 1530 nm. The integrated power of each channel is 0.58 mW at 1550 nm at 112.3-mW total pump power. The channel spacing and bandwidth are 0.81 and 0.30 nm, respectively. The ratio of the forward pump power to the total pump power is best set between 0.3 and 0.4, to obtain large extinction ratio and high output power simultaneously. The source is expected to have applications in wavelength-division-multiplexing local access networks and integrated optics.
With a published model that describes a nested fiber Raman cavity using FBGs as reflectors, we have made numerical simulations for the comparison between the performance of P-doped and Ge-doped fiber. Although the former fibers are the standard choice for fiber Raman lasers due to the large Raman gain, the latter can be also of interest because they present a large Raman shift so that the configuration of the Raman fiber laser can be simplified. We have considered a two-step Raman laser using a P-doped fiber and a six-step Raman laser using a Ge-doped fiber, both pumped by 1060 nm and emitting at 1480 nm. The effects of the Raman fiber length, output coupler reflectivity and splice loss upon the behavior of both lasers are studied. Simulation results show that the P-doped fiber laser requires a longer fiber length while the Ge-doped fiber laser requires a higher output mirror reflectivity, and the RFL using Ge-doped fiber is more sensitive to the splice loss because of its large Stokes wave number. Finally, by comparison in the optimum configuration, we find that the P-doped fiber laser shows better output characteristics than Ge-doped fiber laser.
A bidirectionally pumped spectrum pre-sliced multi-wavelength fiber source is presented in the paper. Two 980nm laser diodes (LDs) are engaged to provide the pump power. An optimized double pass Mach-Zehnder interferometer with extinction ratio larger than 41dB is utilized as the comb filter. The extinction ratio of the fiber source is larger than 24.5dB in the whole C band, and almost reaches 29dB at 1530nm. Integral power of each channel is 0.58mW at 1550nm under 112.3mW total pump power. Channel spacing and bandwidth are 0.81nm and 0.30nm, respectively.
In this paper a large mode area Yb3+-doped double cladding photonic crystal fiber laser is reported. The laser's output power reaches as high as 4.3W. The slope efficiency and the maximum optical-to-optical efficiency of laser output are 69.4% and 59.7%, respectively, with respect to absorbed pump power. Single transverse mode operation is obtained at central wavelength of 1072.3nm and the measured mode distribution agrees with the simulation by using scalar beam propagation method.
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