Ultrashort Fiber Laser Research Articles

Multi-state broadband spectrum and ultra-short pulse spatiotemporal mode-locked Yb-doped multimode fiber laser

In the multimode fiber laser, a phenomenon of multi-state broadband spectrum and ultra-short pulse spatiotemporal mode-locking (STML) is obtained. The dissipative soliton, multiple pulses, harmonic pulse and bound soliton can be observed by rotating the polarization controllers(PCs). The spectral bandwidth of dissipative soliton is 19.04 nm at the central wavelength of 1039.40 nm, and the pulse width is 3.58 ps, acquiring the widest spectrum and the shortest pulse in an all-fiber dissipative soliton STML laser. As the STML state changes from dissipative soliton to bound soliton, spectral bandwidth decreases slightly. The spectral width and modulation period of the loose bound soliton with central wavelength of 1038.24 nm are 18.03 nm and 0.76 nm, respectively. We realize the conversion between the multiple states, which provides a scheme for the study of nonlinear soliton dynamics.

Femtosecond laser plane-by-plane inscription of ultra-short DBR fiber lasers for sensing applications.

We propose and demonstrate the inscription of ultra-short distributed Bragg reflector fiber lasers (DBR-FLs) in Er/Yb co-doped fiber (EYDF) using a femtosecond laser plane-by-plane (Pl-b-Pl) method. By integrating the spherical aberration (SA) with a laser 2D scanning process, a planar refractive index modification (RIM) region can be induced in the fiber core. Thanks to the Pl-b-Pl inscription, a high-quality fiber Bragg grating (FBG) in an EYDF is produced, exhibiting a grating strength exceeding 40 dB and an insertion loss of 0.1 dB. Subsequently, an ultra-short DBR-FL with an entire length of 7.3 mm is fabricated by the Pl-b-Pl inscription. The Pl-b-Pl ultra-short DBR-FL exhibits an improved slope efficiency of 0.7% compared with the DBR-FLs fabricated using another two direct-writing techniques, namely line-by-line (LbL) and point-by-point (PbP) methods. Furthermore, this ultra-short DBR-FL generates single-frequency and single-polarization radiation with a narrow linewidth (9.4 kHz) and a low relative intensity noise (-105.8 dB/Hz). Moreover, a wavelength-division-multiplexed array consisting of eight ultra-short DBR-FLs with distinct lasing wavelengths is successfully created. The Pl-b-Pl ultra-short DBR-FLs with excellent output performances offer significant potential for high-sensitivity sensing applications requiring high spatial resolution.

Optimally design of amplifier self-similar pulse in thulium-doped multimode fiber laser

High-energy ultrashort pulses fiber lasers could be widely used in scientific research and industrial applications. Researchers often use two methods to boost the pulse energy, one is to achieve self-similar pulse (SSP) output in a single mode fiber (SMF) laser, and the other is to use multimode fiber (MMF) instead of SMF. Here we combine the above two methods to optimally design the output characteristics of SSP in thulium-doped MMF lasers. The influence of different parameters, including spectral filter bandwidth, fiber length, and gain saturation energy, on the output characteristics of SSP have been analyzed, and an optimal SSP with pulse energy of 63.48 nJ, spectrum width of 52.53 nm, dechirped pulse peak power of 283.34 kW, and dechirped pulse width of 219.74 fs has been obtained.

Discriminating the existence regions and dynamics between quartic self-similar pulse and dissipative pure quartic soliton in ultrashort fiber laser

Recently, both quartic self-similar pulses (QSSPs) and dissipative pure-quartic-solitons (DPQSs) pulses have been theoretically demonstrated in mode-locked fiber laser with pure positive fourth-order dispersion (FOD). Here, we investigate the existence regions and dynamics of those two soliton pulses by numerically solving the cubic-quintic Ginzburg-Landau equation. We find the positive FOD fiber laser tends to emit QSSPs in the presence of high nonlinear gain, while it is apt to generate DPQS pulses in the case of low nonlinear gain. An unstable pulse region separates the QSSP region from the DPQS region. The characteristics of QSSP and DPQS pulses are dependent on the nonlinear gain and FOD. The QSSP can carry more energy than that of DPQS pulse, because the DPQS pulse with large energy could be unstable due to the excessive pulse pedestal. Our results are not only useful for understanding the dynamics of the DPQS pulse and QSSP in the cubic-quintic Ginzburg-Landau equation with pure FOD, but also provide a guideline for generating high-energy pulses from the positive FOD fiber laser.

Suppression of polarization instability in ultrashort Fabry–Pérot fiber laser

Due to the weak birefringence from the intracavity fiber, vector solitons are easily generated in GHz-fundamental-repetition-rate mode-locked fiber lasers, which can exhibit diverse time-varying polarization dynamics. These soliton dynamics can lead to polarization instability of the pulse train, which prevents industrial and scientific applications that require stable and uniform pulse trains. However, it is hard to suppress the polarization instability due to insufficient space for inserting traditional polarizers and difficulties in enhancing the fiber birefringence in the ultrashort fiber laser cavity. To this end, here we propose an alternative method that can effectively transform the vector solitons into scalar solitons by exploring gold nanorod (GNR) film as a polarizer in the laser cavity. First, the theoretical studies of polarization dynamics in ultrashort Fabry–Pérot (FP) fiber lasers with and without intracavity GNR film are conducted. The results indicate that the use of GNR film can significantly suppress the polarization instability and generate scalar solitons. Then, the large-scale preparation of GNRs with longitudinal surface plasmon resonance absorption peak of >1380 nm is realized by a two-step seed-mediated method, and the GNR film with an operation wavelength range covering the C + L band is fabricated by electrospinning. Finally, we apply the GNR film to a 2-cm-long FP fiber laser, and convert polarization rotation vector solitons to linearly polarized solitons (LPSs) at 1.5 μm. The polarization extinction ratio of the improved LPS pulse train is up to 33 dB.

High ytterbium concentration Yb/Al/P/Ce co-doped silica fiber for 1-µm ultra-short cavity fiber laser application.

We demonstrate a high ytterbium concentration Yb/Al/P/Ce co-doped silica fiber by conventional modified chemical vapor deposition (MCVD) technology and solution doping process. The fiber has a Yb concentration of about 2.5 wt%, and the corresponding core absorption coefficient is measured to be ∼1400 dB/m at 976 nm. The gain coefficient was measured to be approximately 1.0 dB/cm. It is found that the Yb/Al/P/Ce co-doped silica shows a lower photodarkening-induced equilibrium loss of 52 dB/m at 633 nm than the Yb/Al/P co-doped silica fiber of 117 dB/m. Using the heavily Yb3+-doped silica fiber, a compact and robust ultrashort cavity single-frequency fiber laser was achieved with a maximum output power of 75 mW and a linewidth of 14 kHz. Furthermore, a compact passively mode-locked fiber laser (MLFL) with a repetition rate of 1.23 GHz was also proposed using our developed Yb-doped fiber. The laser properties of the proposed lasers were systematically investigated, demonstrating the superior performance of this fiber in terms of photodarkening resistance and ultrashort-cavity laser application. Furthermore, utilizing an all-fiber structure based on silica-based fiber offers the significant advantage of high stability and reliability.

Coherent optical gyroscope on the basis of the bidirectional ultra-short pulse Erbium ring fiber laser

A gyroscopic effect has been studied in the bidirectional ultra-short pulse erbium-doped fiber ring laser hybridly mode-locked with a co-action of a Single-Walled Carbon Nanotube-based saturable absorber (SWCNT-SA) and Nonlinear Polarization Evolution introduced by means of the short-segment polarizing fiber. Owing to the Kerr nonlinearity contribution through self-phase modulation and self-steepening effects to the carrier-to-envelope phase slip of both clockwise (CW) and counterclockwise (CCW) solitons, wideband controllable tuning of a gyroscope bias point has been demonstrated by means of an appropriate adjustment of either intracavity polarization or pump power. Angular velocity detected ranges from 0.12 deg/s to 360 deg/s while the highest rotation sensitivity reaches 7 kHz/(deg/s) for 0.79 m2 single-coil ring gyroscope being in agreement with the calculated scale factor value. The gyroscope dead band with upper limit of ≈5.15 deg/s corresponding to the beat-note frequency of ≈15 kHz has been detected during clockwise rotation of the gyroscope. The highest gyroscope resolution of ≈0.01 deg/s=38 deg/h has been estimated on the basis of the bias point drift measurement.

Optimization of the Core Compound for Ytterbium Ultra-Short Cavity Fiber Lasers

Highly ytterbium-, aluminum- and phosphorus-co-doped silica fibers with low optical losses were fabricated by the MCVD method, utilizing an all-gas-phase deposition technique. Optical and laser properties of the active fibers with a phosphosilicate and aluminophosphosilicate glass cores doped with 1.85 mol% and 1.27 mol% Yb2O3 were thoroughly investigated. With the help of hydrogen loading, it was possible to induce highly reflective Bragg grating in both fiber samples using the standard phase-mask technique and 193 nm-UV laser irradiation. The ultra-short (less than 2 cm long) Fabry–Perot laser cavities were fabricated by inscribing two fiber Bragg gratings (highly and partially reflective FBGs) directly in the core of the fiber samples. The highest pump-to-signal conversion efficiency of 47% was demonstrated in such laser configuration using phosphosilicate fiber. The reasons for the low efficiency of aluminophosphosilicate fiber are discussed.

High-energy 2.8-μm Ultrashort Pulses Generation in an Er:ZBLAN Fiber Amplifier

Compact, high-energy ultrashort pulsed fiber lasers in the 2.8 μm wavelength have attracted extensive interest in many scientific and industrial applications. Here, we demonstrated the generation of μJ-level ultrashort pulses from a quasi-all-fiberized Er:ZBLAN amplifier employing a 2.8-μm frequency-shifted soliton as a seed laser. With backward pumping, the pulse energy was enhanced to 0.282 μJ when the average output power was amplified to 1.023 W. Mechanisms for the generation and amplification of ultrashort pulses were theoretically revealed by numerical simulation, and further cascaded indium fluoride (InF3) fiber to obtain a 4.26-μm redshift soliton numerically. This compact fiber amplification system consisting of frequency-shifted-based seed pulses and fluoride fiber amplifiers will be of practical interest in applications.

Generation of a 59 fs pulse with a 550 nm spectral range in a mode-locked Er-doped single-mode fiber laser system

We demonstrated a compact broad spectrum ultrashort fiber laser system that employed a similariton laser as a seed and fiber-optic nonlinearities in an erbium-doped fiber amplifier to broaden the optical spectrum. The system generated a stable 59 fs ultrashort pulse with a spectral bandwidth beyond 550 nm (i.e. 1490–2030 nm). The mode-locked oscillator produced an 85.9 fs similariton pulse at the wavelength of 1553 nm. The maximum average output power of the laser system was 251 mW. This new type of standard single-mode fiber-format energetic ultrashort system was suitable as a source for biological tissue imaging, broad-band spectroscopy, and optical frequency combs.

Ultra-short DBR fiber laser with high-temperature resistance using tilted fiber Bragg grating output coupler.

We demonstrate a high-temperature resistant distributed Bragg reflector (DBR) fiber laser that is highly compact with an entire cavity length of 12 mm. A partial-reflection tilted fiber Bragg grating (PR-TFBG) is used as the laser output coupler whose reflectivity can be adjusted by changing the TFBG tilt angle. The laser cavity consists of two strong gratings including the PR-TFBG and a high-reflection fiber Bragg grating (HR-FBG), which is directly fabricated in an Er-doped fiber using a femtosecond laser and a phase mask. The thermal stability of the PR-TFBG and HR-FBG is experimentally investigated. After an annealing process, their remained gratings are stable at high temperatures and strong enough for laser oscillation. The laser is also annealed before its stability is tested. The results show that the laser can stably operate in single longitudinal mode with a signal-to-noise ratio better than 65 dB over the temperature range from room temperature to 550°C.

Environmentally stable Er-fiber mode-locked pulse generation and amplification by spectrally filtered and phase-biased nonlinear amplifying long-loop mirror

We report on environmentally stable long-cavity ultrashort erbium-doped fiber lasers, which self-start mode-locking at quite low thresholds by using spectrally filtered and phase-biased nonlinear amplifying long-loop mirrors. By employing 100-m polarization-maintaining fiber (PMF) in the nonlinear loop, the fundamental repetition rate reaches 1.84 MHz and no practical limitation is found to further decrease the repetition rate. The filter used in the long loop not only suppresses Kelly sidebands of the solitons, but also eliminates the amplified spontaneous emission which exists widely in low-repetition-rate ultrafast fiber lasers. The bandwidth of the filter is optimized by using a numerical model. The laser emits approximately 3-ps pulses with an energy of 17.4 pJ, which is further boosted to $1.5~\unicode[STIX]{x03BC}\text{J}$ by using a fiber amplifier.

Design of Ultra Large Normal Dispersion ZBLAN Photonic Crystal Fiber and Its Application in Mid-IR Ultra Short Fiber Lasers

A ZBLAN photonic crystal fiber (PCF) with normal dispersion is theoretically designed and investigated. The designed PCF can provide a large normal dispersion coefficient and a low confinement loss in a broad wavelength range of 1.8-3.6 μm. Especially, the PCF exhibits an ultra large normal dispersion value of -351.3 ps/km/nm and a small confinement loss of 0.05 dB/m at 2.9 μ m. By using the designed PCF as a dispersion compensator in a mode-locked ZBLAN fiber laser, stretched pulse with compressed pulse duration of 1.56 ps is numerically achieved.

超短脉冲光纤激光相干合成(特邀)

超短脉冲光纤激光相干合成(特邀)

Demonstration of arbitrary temporal shaping of picosecond pulses in a radially polarized Yb-fiber MOPA with > 10 W average power.

High precision surface processing has an unmet demand for picosecond pulses with arbitrary temporal profiles in radial polarization states and at high average powers. Here, simultaneous spatial and arbitrary temporal shaping of chirped 10 - 100 picoseconds pulses is demonstrated with an Yb-doped fiber laser system generating an output power of more than 10 W at 40 MHz repetition frequency. The closed-loop control algorithm carves the pulses using a commercial, rugged, and fiberized optical pulse shaper placed at the front end of the system and uses feedback from the output pulse shapes for optimization. Arbitrary complex temporal profiles were demonstrated using a dispersive Fourier transform based technique and limits set by the system were investigated. Pulse shaping in the spatial domain was accomplished using an S-waveplate, fabricated in-house, to change the linearly polarized fundamental mode into a doughnut mode with radial polarization. This was amplified in a final-stage few-mode large-mode area fiber amplifier. Placing both temporal and spatial shaping elements before the power-amplifier avoids complex and potentially lossy conversion of the spatial mode profile at the output and provides an efficient route for power-scaling. The use of properly oriented quarter- and half-wave plates, which have both low loss and high power handling capability, enabled the output to be set to pure radial or azimuthal polarization states. Using commercial off-the-shelf components, our technique is able to immediately enhance the versatility of ultrashort fiber laser systems for high precision material processing and other industrial applications.

Type IIa Bragg grating based ultra-short DBR fiber laser with high temperature resistance.

We report on the fabrication of a thermally resistant ultra-short distributed Bragg reflector (DBR) fiber laser based on the photo inscription of two wavelength-matched type IIa gratings in a thin-core Er-doped fiber. With continuous UV exposure, each Bragg reflector initially grows as a type I grating, followed by decay in strength, and then re-grows as a type IIa grating with enhanced thermal resistance. The DBR laser, with an entire length of 13 mm, can stably operate at 600°C with single longitude mode, which provides potential applications in high temperature environments.

Ultrafast fiber lasers based on self-similar pulse evolution: a review of current progress

Self-similar fiber oscillators are a relatively new class of mode-locked lasers. In these lasers, the self-similar evolution of a chirped parabolic pulse in normally-dispersive passive, active, or dispersion-decreasing fiber (DDF) is critical. In active (gain) fiber and DDF, the novel role of local nonlinear attraction makes the oscillators fundamentally different from any mode-locked lasers considered previously. In order to reconcile the spectral and temporal expansion of a pulse in the self-similar segment with the self-consistency required by a laser cavity’s periodic boundary condition, several techniques have been applied. The result is a diverse range of fiber oscillators which demonstrate the exciting new design possibilities based on the self-similar model. Here, we review recent progress on self-similar oscillators both in passive and active fiber, and extensions of self-similar evolution for surpassing the limits of rare-earth gain media. We discuss some key remaining research questions and important future directions. Self-similar oscillators are capable of exceptional performance among ultrashort pulsed fiber lasers, and may be of key interest in the development of future ultrashort pulsed fiber lasers for medical imaging applications, as well as for low-noise fiber-based frequency combs. Their uniqueness among mode-locked lasers motivates study into their properties and behaviors and raises questions about how to understand mode-locked lasers more generally.

Development of Ultrafast Laser Oscillators Based on Thulium-Doped ZBLAN Fibers

Fluoride glass known as ZBLAN is well known for high transparency in the infrared region and is a promising candidate for the host material for infrared lasers. Furthermore, low material dispersion of ZBLAN glasses around 2-μm wavelength region suggests the usefulness of ZBLAN fibers for developing ultrafast fiber lasers. Based on these considerations, we have conducted a detailed study of ultrashort fiber laser oscillators based on thulium-doped ZBLAN fibers. Two types of diode-pumped, mode-locked oscillators are demonstrated using single-mode ZBLAN fibers and double-clad ZBLAN fibers. The oscillator with single-mode ZBLAN fibers generate stable output pulses at 67.5-MHz repetition rate with the average output power of 19 mW. The broad spectra spreading over 300 nm enabled the generation of pulses as short as 45 fs in 2-μm region. The oscillator with double-clad ZBLAN fibers was built with a multimode laser diode as the pump source and was successfully mode-locked at the pump power of 1.04 W. The output pulses with the spectrum extending over 150 nm are compressed to 90 fs.

Preparation and spectral characteristics of Bi-doped double cladding fiber

Bi-doped double cladding fiber preform is prepared by modified chemical vapor deposition and solution doping process. The absorption spectrum of preform and the near-infrared (NIR) luminescence spectrum of fiber are experimentally investigated. The luminescence spectrum of the fiber shows a peak emission band at 1140 nm with a full-width at half-maximum (FWHM) of 130 nm under 976 nm LD exciting. Under 808 or 793 nm LD exciting, an ultra-broad NIR emission with an FWHM of more than 250 nm is observed in the Bi-doped fiber, and the range of luminescence spectrum is from 1000 to 1700 nm. After the heat treatment of the Bi-doped fiber preform slice under 900 ℃ for 1 h, under 808 nm LD exciting, the fluorescence intensity is about 4 times higher than that of the perform slice without heat treatment. The results indicate that the Bi-doped double cladding fiber is a promising gain medium used for ultrashort fiber lasers and tunable fiber lasers.

A 400-kHz High-Accuracy Laser Telemeter for Distributed Measurements of 3-D Profiles

A distributed telemeter is herein presented, which has specifically been designed for the high-speed high-resolution monitoring of trains in railway portals but can be adapted to a number of industrial and environmental applications. The system uses a single transmitter/receiver unit to feed, using optical fiber pairs, a number of measuring stations. It makes use of an ultrashort fiber laser operating at 400 kHz or, alternatively, a fiber-coupled diode laser and presents in-house developed solutions to obtain a 40-dB dynamic range, thus allowing to monitor targets within large distance/angle/color intervals. The system has been developed, tested, and installed on a train-monitoring portal in the Roma-Napoli railway, in the framework of an effort to increase the safety of trains entering tunnels in the Italian railway system.