Mode-locked Femtosecond Laser Research Articles

Femtosecond Laser-Based Microwave Signal Generation and Distribution

We review our most recent progress in microwave photonic applications of ultralow timing jitter femtosecond mode-locked fiber lasers. Sub-femtosecond timing jitter (integrated from 10 kHz to >10 MHz offset frequency) optical pulse trains can be generated from various types of mode-locked fiber lasers using dispersion engineering and intracavity filtering. To fully utilize such ultralow-jitter lasers for microwave photonic applications, we demonstrate a sub-femtosecond-resolution (−159 dBc/Hz phase noise floor) phase detection method between optical pulse trains and microwave signals using a Sagnac-fiber-loop-based device named the fiber-loop optical-microwave phase detector (FLOM-PD). Using ultralow-jitter mode-locked Er-fiber lasers and FLOM-PDs, we generate 10-GHz microwave signals with −142 dBc/Hz absolute single-sideband phase noise at 10-kHz offset frequency. When an all-fiber Michelson interferometer-based repetition-rate stabilization method is further employed, the phase noise is suppressed to −90 dBc/Hz at 10-Hz offset frequency, which results in 3-fs absolute rms timing jitter integrated from 10 Hz to 10 MHz offset frequency. Long-distance microwave phase transfer via optical fiber links is also demonstrated using the FLOM-PD as a means for stabilizing pulse time-of-flight in fiber transfer. Relative frequency instability of 6.5 × 10−19 is demonstrated for 2.856-GHz microwave signals transferred over a 2.3-km-long fiber link. We anticipate that the capability of generation, characterization, stabilization, and transfer of ultralow-noise microwave signals using ultralow-jitter femtosecond mode-locked lasers will find more applications in microwave photonics in the near future.

Femtosecond mode-locked erbium-doped fiber laser based on MoS2–PVA saturable absorber

We fabricate a free-standing few-layer molybdenum disulfide (MoS2)–polymer composite by liquid phase exfoliation of chemically pristine MoS2 crystals and use this to demonstrate a soliton mode-locked Erbium-doped fiber laser (EDFL). A stable self-started mode-locked soliton pulse is generated by fine-tuning the rotation of the polarization controller at a low threshold pump power of 25mW. Its solitonic behavior is verified by the presence of Kelly sidebands in the output spectrum. The central wavelength, pulse width, and repetition rate of the laser are 1573.7nm, 630fs, and 27.1MHz, respectively. The maximum pulse energy is 0.141nJ with peak power of 210W at pump power of 170mW. This result contributes to the growing body of work studying the nonlinear optical properties of transition metal dichalcogenides that present new opportunities for ultrafast photonic applications.

High power mode-locked rod-type fiber femtosecond laser with micro-joule energy

We report a high power all-normal-dispersion (ANDi) mode-locked laser based on nonlinear polarization evolution (NPE) technique using rod-type fiber with polarization maintaining (PM) characteristic. With 85μm gain core diameter, 31W of average power at repetition rates of 57.93MHz, which corresponds to the pulse energy of 0.53μJ, is demonstrated under a pump power of 93W. The pulse duration of 124fs after compressor is obtained at the central wavelength of 1033nm as well as the measured power jitter of 0.3% over a period of 2h.

Harmonically mode-locked femtosecond fiber laser using non-uniform, WS2-particle deposited side-polished fiber

We investigated the feasibility of using a WS2-deposited side-polished fiber as a harmonic mode-locker to produce a femtosecond fiber laser with a frequency of 1.51 GHz. Our work focuses on using a side-polished fiber platform with non-uniform WS2 particles prepared through liquid phase exfoliation method without centrifugation. Femtosecond optical pulses were generated from an all-fiberized erbium-doped fiber-based ring cavity by increasing the pump power to achieve a tunable pulse repetition rate from 14.57 MHz to 1.51 GHz (104th harmonic). The characteristics of the output pulse were systematically investigated to analyze the pulse repetition rate, harmonic order, average output power, pulse energy, and pulse width as a function of the pump power. The output performance of the laser was compared to that of a laser based on a microfiber-based WS2 film SA described in (Yan et al 2015 Opt. Mater. Express 5 479–89). This experimental demonstration reaffirms that a side-polished fiber is an effective platform to implement an ultrafast harmonic mode-locker, and non-uniform WS2 particles prepared via simple liquid phase exfoliation method without centrifugation provide a suitable saturable absorption response at 1.55 μm.

Effects of Deposition Pressure and Target-Substrate Distance on Growth of ZnO by Femtosecond Pulsed Laser Deposition

Undoped Zinc oxide (ZnO) was successfully deposited on silicon (100) substrate by femtosecond pulsed laser deposition. A mode-locked femtosecond laser operating at 790 nm wavelength, 100 fs pulse duration and 80 MHz repetition rate was used as an excitation source. The depositions were carried out at vacuum pressures of 10-10 mbar and oxygen background gas pressures of 10 -10 mbar. Energy dispersive spectroscopy of samples grown without oxygen background gas shows higher zinc composition on deposited material as compared to oxygen that leads to off stoichiometric ZnO films. Scanning electron microscopy (SEM) images shows that increasing oxygen gas pressure increased the particle size of the deposited ZnO. The material deposited at 2x10 mbar oxygen pressure revealed clustering of nanorods forming a flower-like structure that has an average length of 2700 nm and an average diameter of 450 nm. The X-ray diffraction spectra show c-axis orientation of the deposited ZnO with (002) and (110) reflection.

Cu-Sn-S plasmonic semiconductor nanocrystals for ultrafast photonics.

Here, we show that solution-processed Cu-Sn-S semiconductor nanocrystals (NCs) demonstrate a tunable localized surface plasmon resonance band in the near infrared region, where strong saturable absorption occurs. A saturable absorber based on these plasmonic NCs enables the construction of a stable mode-locked femtosecond fiber laser operating at the telecommunication band.

Optical waveform monitoring based on a free-running mode-locked femtosecond fibre laser and four-wave mixing in a highly nonlinear fibre

AbstractOptical sampling based on ultrafast optical nonlinearities is a useful technique to monitor the waveforms of ultrashort optical pulses. In this paper, we present a new implementation of optical waveform sampling systems by employing our newly constructed free–running mode–locked fibre laser with a tunable repetition rate and a low timing jitter, an all–optical waveform sampler with a highly nonlinear fibre (HNLF), and our developed computer algorithm for optical waveform display and measurement, respectively. Using a femtosecond fibre laser to generate the highly stable optical sampling pulses and exploiting the four–wave mixing effect in a 100m–long HNLF, we successfully demonstrate the all–optical waveform sampling of a 10GHz optical clock pulse sequence with a pulse width of 1.8 ps and a 80Gbit/s optical data signal, respectively. The experimental results show that waveforms of the tested optical pulse signals are accurately reproduced with a pulse width of 2.0 ps. This corresponds to a temporal resolution of 0.87 ps for optical waveform measurement. Moreover, the optical eye diagram of a 10Gbit/s optical data signal with a 1.8 ps pulse width is also accurately measured by employing our developed optical sampling system.

Diode-pumped Kerr-lens mode-locked femtosecond Yb:YAG ceramic laser**Project supported by the National Major Scientific Instrument Development Project of China (Grant No. 2012YQ120047), the National Natural Science Foundation of China (Grant No. 61205130), and the Fundamental Research Funds for the Central Universities, China (Grant No. JB140502).

We experimentally demonstrated a diode-pumped Kerr-lens mode-locked femtosecond laser based on an Yb:YAG ceramic. Stable laser pulses with 97-fs duration, 2.8-nJ pulse energy, and 320-mW average power were obtained. The femtosecond oscillator operated at a central wavelength of 1049 nm and a repetition rate of 115 MHz. To the best of our knowledge, this is the first demonstration of a Kerr-lens mode-locked operation in a diode-pumped Yb:YAG ceramic laser with sub-100 fs pulse duration.

Diode-pumped Kerr-lens mode-locked Yb:CaGdAlO4 laser with tunable wavelength

We experimentally demonstrated a wavelength tunable Kerr-lens mode-locked femtosecond laser based on an Yb:CaGdAlO4 (Yb:CGA) crystal. The Kerr-lens mode-locked wavelength tuning range was from 1043.5 to 1076 nm, as broad as 32.5 nm, by slightly tilting the end mirror. Pulses as short as 60 fs were generated at the central wavelength of 1043.8 nm with an average output power of 66 mW. By using an output coupler with 1.5% transmittance, the Kerr-lens mode-locked average output power reached 127 mW with a pulse duration of 81 fs at a central wavelength of 1049.5 nm.

Mode-locked femtosecond all-normal all-PM Yb-doped fiber laser at 1060 nm

We report an all-normal-dispersion, all-fibre, all-PM, laser operating at a central wavelength of 1060nm. The laser is mode-locked using a nonlinear amplifying loop mirror and generates linearly polarised pulses that can be compressed to 360fs. The laser is based on our earlier scheme operating at 1030nm [1] and we discuss the similarities and differences between the two configurations. We also present amplification up to an output power of 1W using a commercially built amplifier and show through numerical methods that this pulse may be recompressible to 1.65ps.

Generation of 33 fs pulses directly from a Kerr-lens mode-locked Yb:CaYAlO_4 laser

We experimentally demonstrated a diode-pumped Kerr-lens mode-locked femtosecond laser with a Yb:CaYAlO4 crystal as the gain medium. Pulse duration as short as 33 fs was obtained directly from the oscillator at a repetition rate of 115 MHz. The central wavelength was at 1059 nm with a spectral bandwidth of 49 nm. These are, to the best of our knowledge, the shortest pulses generated from a Yb:CaYAlO4 oscillator.

Effect of narrow spectral filter position on the characteristics of active similariton mode-locked femtosecond fiber laser.

A significant change in active similariton characteristics, both numerically and experimentally, is observed as a function of the location of the lumped spectral filter. The closer the spectral filter is to the input of the Yb(3+)-doped fiber, the shorter the de-chirped pulse width. The peak power of the de-chirped pulse has its maximum value at a certain location of the spectral filter. Four different positions of the spectral filter inside the laser cavity have been theoretically studied and two of them have been verified experimentally.

Counting the cycles of light using a self-referenced optical microresonator

Phase coherently linking optical to radio frequencies with femtosecond mode-locked laser frequency combs enabled counting the cycles of light and is the basis of optical clocks, absolute frequency synthesis, tests of fundamental physics, and improved spectroscopy. Using an optical microresonator frequency comb to establish a coherent link between optical and microwave frequencies will extend optical frequency synthesis and measurements to areas requiring compact form factor, on chip integration and comb line spacing in the microwave regime, including coherent telecommunications, astrophysical spectrometer calibration or microwave photonics. Here we demonstrate a microwave to optical link with a microresonator. Using a temporal dissipative single soliton state in an ultra-high Q crystalline microresonator that is broadened in highly nonlinear fiber an optical frequency comb is generated that is self-referenced, allowing to phase coherently link a 190 THz optical carrier directly to a 14 GHz microwave frequency. Our work demonstrates precision optical frequency measurements can be realized with compact high Q microresonators.

Stretched cavity soliton in dispersion-managed Kerr resonators

Stretched cavity soliton (SCS) in dispersion-managed nonlinear resonators is numerically investigated. SCS is found to stretch and compress twice during a round-trip propagation inside the dispersion-managed resonator, exhibiting a pulse dynamics similar to dispersion-managed mode-locked femtosecond lasers. Even though the breathing ratio is relatively small, the characteristics of SCS are significantly modified by the pulse stretching dynamics in the resonator. The output pulse will have a flatter spectrum around the center frequency. However, the power for the comb lines at the wing of the spectrum decays faster than the conventional sech-shaped CS, making dispersion wave emission harder to be excited in dispersion-managed resonators. Furthermore, stretching of the pulse lowers the nonlinear phase shift and makes it more resistant towards breather soliton instability. When shortening the cavity length to the microresonator scale, we find that ultrashort pulses can be generated through dispersion management, even in a low $Q$-factor cavity.

Coherent pulse interrogation system for fiber Bragg grating sensing of strain and pressure in dynamic extremes of materials.

A 100 MHz fiber Bragg grating (FBG) interrogation system is described and applied to strain and pressure sensing. The approach relies on coherent pulse illumination of the FBG sensor with a broadband short pulse from a femtosecond modelocked erbium fiber laser. After interrogation of the FBG sensor, a long multi-kilometer run of single mode fiber is used for chromatic dispersion to temporally stretch the spectral components of the reflected pulse from the FBG sensor. Dynamic strain or pressure induced spectral shifts in the FBG sensor are detected as a pulsed time domain waveform shift after encoding by the chromatic dispersive line. Signals are recorded using a single 35 GHz photodetector and a 50 G Samples per second, 25 GHz bandwidth, digitizing oscilloscope. Application of this approach to high-speed strain sensing in magnetic materials in pulsed magnetic fields to ~150 T is demonstrated. The FBG wavelength shifts are used to study magnetic field driven magnetostriction effects in LaCoO3. A sub-microsecond temporal shift in the FBG sensor wavelength attached to the sample under first order phase change appears as a fractional length change (strain: ΔL/L<10-4) in the material. A second application used FBG sensing of pressure dynamics to nearly 2 GPa in the thermal ignition of the high explosive PBX-9501 is also demonstrated. Both applications demonstrate the use of this FBG interrogation system in dynamical extreme conditions that would otherwise not be possible using traditional FBG interrogation approaches that are deemed too slow to resolve such events.

Diode-pumped femtosecond mode-locked Nd, Y-codoped CaF2 laser

A passively mode-locked femtosecond laser based on an Nd, Y-codoped CaF2 disordered crystal was demonstrated. The Y3+-codoping in Nd : CaF2 markedly suppressed the quenching effect and improved the fluorescence quantum efficiency and emission spectra. With a fiber-coupled laser diode as the pump source, the continuous wave tuning range covering from 1042 to 1076 nm was realized, while the mode-locked operation generated 264 fs pulses with an average output power of 180 mW at a repetition rate of 85 MHz. The experimental results show that the Nd, Y-codoped CaF2 disordered crystal has potential in a new generation diode-pumped high repetition rate chirped pulse amplifier.

FTU Diagnostic System Based on THz Time-domain Spectroscopy

We present the state of development of a THz-based Time-Domain Spectroscopy (TDS) system for FTU Tokamak. TDS is ideal to measure plasma characteristics that are non-uniform and/or evolve during the discharge: THz pulses (0.1-2 THz) produced with femtosecond mode-locked lasers (790nm) conveniently span the spectrum above and below the plasma frequency and, thus, can be used as very sensitive and versatile probes of widely varying plasma parameters. This work is based on an existing collaboration between ENEA Frascati and the Photonics Group at Clarendon Laboratory, Oxford University, where a THz-TDS experimental apparatus has been assembled and characterised. The spectral response of diagnostic-relevant components and materials has been tested. The instrument is now being equipped with fiber-optics to guide the laser beam to the THz Emitter and Receiver to ease access through the Tokamak ports. Broadband THz pulse propagation in plasma has been simulated using a simple, yet effective Fourier Transform-based model. In the Tokamak environment, THz TDS can be used in conjunction with other diagnostic techniques, such as, interferometry and ECE, for the simultaneous measurement of dispersion and absorption to launch a bridge between major areas of research in the science landscape.

Pulse Energy Optimization in Multipass-Cavity Mode-Locked Femtosecond Lasers

We describe a comprehensive model, which accounts for reflection and clipping losses of notched multipass-cavity (MPC) mode-locked lasers and determines the optimum q-preserving MPC configuration, which maximizes the output pulse energy for a given pump power. In order to make realistic predictions, an initial experimental MPC resonator with negligible clipping losses was built and its power performance was measured in order to determine the values of the fixed model parameters such as the round-trip loss of the short cavity and reflection coefficient of the MPC mirrors. Then, by using these parameters and by taking into account all possible reflection and clipping loss mechanisms, the output pulse energy for various q-preserving configurations was calculated. The simulation results indicated that, a mode-locked Ti 3+ :Sapphire laser based on the optimum MPC configuration should produce 30 nJ of pulse energy with a 3% output coupler at the pump power of 2 W. The experimental setup constructed at the optimum MPC configuration generated nearly transform-limited 74 fs pulses with 33 nJ of output pulse energy, in very good agreement with the model predictions. We believe that the model and the experimental procedure described in this paper should prove very useful in the practical design and optimization of energy-efficient MPC mode-locked lasers.

Correlation between lamina cribrosa tilt angles, myopia and glaucoma using OCT with a wide bandwidth femtosecond mode-locked laser.

PurposeTo measure horizontal and vertical lamina cribrosa (LC) tilt angles and investigate associated factors using prototype optical coherence tomography (OCT) with a broad wavelength laser light source.DesignCross sectional study.MethodsTwenty-eight no glaucoma eyes (from 15 subjects) and 25 glaucoma eyes (from 14 patients) were enrolled. A total of 300 optic nerve head B-scans were obtained in 10 µm steps and the inner edge of Bruch's membrane opening (BMO) was identified as the reference plane. The vertical and horizontal angles between BMO line and approximate the best-fitting line for the surface of the LC were measured and potential associated factors were estimated with univariate and multivariate logistic regression analyses.ResultsThe median (interquartile range) horizontal and vertical tilt angles were 7.10 (2.43–11.45) degrees and 4.15 (2.60–6.85) degrees in eyes without glaucoma and 8.50 (4.40–14.10) degrees and 9.30 (6.90–14.15) degrees in glaucoma eyes, respectively. The refractive errors had a statistically significant association with horizontal LC tilt angles (coefficients, −1.53 per diopter) and glaucoma had a significant correlation with vertical tilt angles (coefficients, 6.56) using multiple logistic regression analysis (p<0.001).ConclusionsOCT allowed evaluation of the internal tilting of the LC compared with the BMO. The horizontal internal LC tilt angle was correlated with refractive errors, corresponding to myopic physiological changes, and vertical internal LC tilt was correlated with glaucoma, corresponding to glaucomatous pathological changes. These parameters have important implications for investigation of the correlation between myopia, glaucoma and LC morphological features.

Development of variable pulse width mode-locked femtosecond laser by applying coherent coupling technology.

A novel femtosecond mode-locked variable-pulse-width laser system is developed based on coherent coupling technology. The laser system is composed of two fiber ring lasers that share an output coupler. Output longitudinal modes are emitted by the corresponding longitudinal modes of the laser modules. In this study, one longitudinal-mode spacing is changed arbitrarily by tuning the cavity length to control the output longitudinal modes. In the experiment, the laser system generates about 150fs pulse width. The coupling result, which is the output pulse width, is changed by a length deviation of about 500μm. As a result, the laser successful widens the pulse width to 1.5ps at maximum and the tuning pulse width with 10fs resolution at minimum.