Mode-locked Femtosecond Research Articles

Passively harmonic mode-locked GHz U-band femtosecond pulse generation based on soliton self-frequency shift.

We demonstrate a GHz U-band fiber laser harnessing soliton self-frequency shift (SSFS). The seed source is a passively harmonic mode-locked (HML) fiber laser based on carbon nanotubes (CNTs) polymer film. By adjusting the pump power and polarization controller (PC) appropriately, the repetition rate can be tuned up to 1.104 GHz with a side mode suppression ratio (SMSR) of 42.5 dB. The seed source with different repetition rates can be frequency shifted from C-band to U-band in a piece of dispersion-shifted fiber (DSF) after amplification. We selected three repetition rates (457.8 MHz, 700.1 MHz, and 1104.1 MHz) and systematically studied the impact of different repetition rates on SSFS. The broad tuning range of wavelengths covering the entire U-band can be obtained by adjusting the pump power of the main amplifier. To the best of our knowledge, the demonstrated laser is the first U-band femtosecond fiber laser based on passively HML with a repetition rate over 1 GHz.

Intra-cavity dark pulse generation through synchronized sum-frequency mixing

A Nd:YVO4 laser operating at 1064 nm generating a stable mode-locked train of 10 ps-long dark pulses with a 211 MHz repetition rate is presented. The mode-locking relies on a periodic loss modulation produced by intra-cavity sum-frequency mixing with a synchronous bright-pulse train from a mode-locked femtosecond Yb:KYW laser at 1040 nm. A modulation depth of 90% was achieved for the dark pulses, confirmed by cross-correlation measurements. The ultrafast loss modulation injects power into the Nd:YVO4 laser cavity modes beyond the laser gain bandwidth. At proper laser cavity length, the detuning interaction of these modes with the lasing modes leads to the generation of periodic ultrafast transients at frequencies above 1.5 THz.

2-GHz watt-level Kerr-lens mode-locked Yb:KGW laser.

We report on a 2-GHz high-power Kerr-lens mode-locked Yb:KGW laser pumped by a single-mode fiber laser. The output performance for two different output coupling rates was investigated. Stable bidirectional mode-locking operation at the repetition rate of 2.157 GHz was obtained with a 0.6% output coupler. The average output powers of bidirectional operation are 741 mW and 746 mW, with 123-fs and 126-fs pulse durations, respectively. By using a 1.6% output coupler, unidirectional mode-locking is achieved with 145-fs pulse duration and 1.7-W average output power, which, to the best of our knowledge, is the highest average power from Kerr-lens mode-locked GHz femtosecond oscillators.

A New Optical Angle Measurement Method Based on Second Harmonic Generation with a Mode-Locked Femtosecond Laser

This paper proposes a new optical angle measurement method based on second harmonic generation with a mode-locked femtosecond laser source by taking use of the unique characteristic of high peak power and high-intensity electric field of the femtosecond laser pulses. The angle is measured from the second harmonic wave intensity, which is a function of the angle between the laser beam axis and the optic axis of a nonlinear crystal attached to the target of interest. It is found that the beta barium borate (BBO) crystal is suitable as the nonlinear crystal for this purpose through theoretical simulation. As the first step of research, an experimental demonstration is also carried out in such a way that a change of the second harmonic wave intensity due to the angular displacement of BBO crystal is measured to verify the feasibility of the proposed principle of the angle measurement method.

Investigation and Improvement of Thermal Stability of a Chromatic Confocal Probe with a Mode-Locked Femtosecond Laser Source

An intentional investigation on the thermal stability of a mode-locked femtosecond laser chromatic confocal probe, which is a critical issue for the probe to be applied for long-term displacement measurement or surface profile measurement requiring long-time scanning, is carried out. At first, the thermal instability of the first prototype measurement setup is evaluated in experiments where the existence of a considerably large thermal instability is confirmed. Then the possible reasons for the thermal instability of the measurement setup are analyzed quantitatively, such as the thermal instability of the refractive index of the confocal lens and the thermal expansion of mechanical jigs employed in the probe. It is verified that most of the thermal instability of the measurement setup is caused by the thermal expansion of mechanical jigs in the probe. For the improvement of the thermal stability of the probe, it is necessary to employ a low thermal expansion material for the mechanical jigs in the measurement setup and to shorten the optical path length of the laser beam. Based on the analysis result, a second prototype probe is newly designed and constructed. The improved thermal stability of the second prototype probe is verified through theoretical calculations and experiments.

A Method for Expansion of Z-Directional Measurement Range in a Mode-Locked Femtosecond Laser Chromatic Confocal Probe

A method is proposed to expand the Z-directional measurement range of a fiber-based dual-detector chromatic confocal probe with a mode-locked femtosecond laser source. In the dual-detector chromatic confocal probe, the Z-directional displacement of a measurement target is derived from the peak wavelength in the normalized intensity ratio from the two light intensities obtained by the two identical fiber detectors. In this paper, a new method utilizing the main-lobe and side-lobes of axial responses acquired from both the normalized intensity ratio Ia and the invert normalized intensity ratio In, which is the inverse of Ia, is proposed to obtain the seamless relationship between the peak wavelength and the Z-directional displacement of a measurement target. Theoretical calculations and experimental investigation are carried out to demonstrate the feasibility of the proposed measurement range expansion method.

Few-layer Ti3C2Tx (T = O, OH, or F) saturable absorber for a femtosecond bulk laser.

Few-layered titanium carbide (Ti3C2Tx), a novel two-dimensional (2D) Van der Waals material in the MXene family, was fabricated with a liquid-phase method and applied as a saturable absorber for a continuous-wave mode-locked femtosecond bulk laser. Pulses as short as 316fs with a repetition rate of 64.06MHz and maximum output power of 0.77W were achieved at the central wavelength of 1053.2nm, demonstrating the first known, to the best of our knowledge, application of MXene in an all-solid-state laser. Considering the flexible band gap for different surface functional groups of Ti3C2Tx, these results may promote the development of ultrafast photonics and further applications of 2D optoelectronic layered materials in the infrared and mid-infrared regions.

Femtosecond Alexandrite laser passively mode-locked by an InP/InGaP quantum-dot saturable absorber

An Alexandrite laser passively mode-locked using an InP/InGaP quantum-dot semiconductor saturable absorber mirror (QD-SESAM) was demonstrated. The laser was pumped at 532nm and generated pulses as short as 380fs at 775nm with an average output power of 295mW. To the best of our knowledge, this is the first report on a passively mode-locked femtosecond Alexandrite laser using a SESAM in general and a QD-SESAM in particular.

Diode-pumped high-peak-power femtosecond Yb+3:(Gd0.1Y0.9)2SiO5 laser at 1058.5 nm

A diode-pumped continuous-wave mode-locked (CWML) femtosecond Yb+3:(Gd0.1Y0.9)2SiO5 (Yb:GYSO) laser at 1058.5 nm with a semiconductor saturable absorber mirror (SESAM) was successfully realized and demonstrated. Pulses with a duration of 186 fs were obtained at a pulse repetition rate of 80.1 MHz with a maximum average output power of 564 mW, corresponding to a maximum single pulse energy of 7.04 nJ and peak power of 37.9 kW. To the best of our knowledge, this is the highest peak power ever obtained from a mode-locked femtosecond Yb:GYSO laser.

Passively mode-locked soliton femtosecond pulses employing graphene saturable absorber

We demonstrate a passively mode-locked fiber laser incorporating graphene thin film (GTF) as saturable absorber (SA). The SA is fabricated by sandwiching the GTF between two single mode fiber ferrules through a fiber adaptor. The transmission loss at 1560nm and non-linear saturation absorption modulation depth for GTF-SA are 0.8dB and 2.90%, respectively. An erbium-doped fiber laser cavity is constructed to verify the functionality of GTF-SA and is designed to have net anomalous dispersion. It generates large spectral width of 4.99nm with pulse repetition rate of 9.655MHz and pulse width of 670fs. Net anomalous dispersion and time bandwidth product higher than the sech2 transform-limited pulse validate the experimental result. In short, we demonstrate high performance GTF-SA that is able to generate ultrafast pulse duration in femtosecond range effortlessly with simple and green SA fabrication procedures.

Tunable second harmonic generation from a Kerr-lens mode-locked Yb:YCa4O(BO3)3 femtosecond laser**Project supported by the National Major Scientific Instruments Development Project of China (Grant No. 2012YQ120047), the National Natural Science Foundation of China (Grant No. 61205130), and the Doctor Fund from Southwest University, China (Grant No. SWU110645).

We experimentally demonstrated a diode-pumped Kerr-lens mode-locked femtosecond (fs) laser with a self-frequency doubling Yb:YCa4O(BO3)3 crystal. Sub-40 fs laser pulses were directly generated from the oscillator without extracavity compression. The central wavelength was tunable from 1039 nm to 1049 nm with a typical bandwidth of 35 nm and an average output power of 53 mW. For the first time, a self-frequency doubled second harmonic green laser with tunable range from 519 nm to 525 nm was observed.

Passively Mode-Locked Femtosecond Laser with Disordered Crystal Nd:CGA as Gain Medium**Supported by the National Key Basic Research Program of China under Grant No 2013CB922402, the International Joint Research Program, and the National Natural Science Foundation of China under Grant Nos 61210017 and 11434016.

We present a laser-diode-pumped passively mode-locked femtosecond disordered crystal laser by using Nd:CaGdAlO4 (Nd:CGA) as the gain medium. With a pair of SF6 prisms to control the dispersion compensation, laser pulses as short as 850 fs at 1079 nm are obtained with a repetition rate of 124.6 MHz. The measured threshold pump power is 1.45 W. A maximum average output power of 122 mW is obtained under the pump power of 5.9 W. These results show that Nd:CGA could be a promising laser medium for generating femtosecond ultrashort pulse at about 1 μm.

Graphene mode-locked femtosecond Cr2+:ZnS laser with ~300 nm tuning range.

Graphene has proved to be an excellent broadband saturable absorber for mode-locked operation of ultrafast lasers. However, for the mid-infrared (mid-IR) range where broadly tunable sources are in great needs, graphene-based broadly tunable ultrafast mid-IR lasers have not been demonstrated so far. Here, we report on passive mode-locking of a mid-IR Cr:ZnS laser by utilizing a transmission-type monolayer graphene saturable absorber and broad spectral tunability between 2120 nm and 2408 nm, which is the broadest tuning bandwidth ever reported for graphene mode-locked mid-IR solid-state lasers. The recovery time of the saturable absorber is measured to be ~2.4 ps by pump-probe technique at a wavelength of 2350 nm. Stably mode-locked Cr:ZnS laser delivers Fourier transform-limited 220-fs pulses with a pulse energy of up to 7.8 nJ.

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.

Kerr-Lens Mode-Locked Femtosecond Yb:GdYSiO5 Laser Directly Pumped by a Laser Diode

We demonstrate the first Kerr-lens mode-locked operation in a diode-pumped Yb:GdYSiO5 oscillator. Under a diode pump power of 5 W, 141 fs pulses with an average power of 237 mW were obtained at a repetition rate of 118 MHz. The central wavelength was at 1094 nm with a bandwidth of 10.1 nm. Shorter pulses were obtained by adjusting the cavity to operate at a shorter wavelength, resulting in 55 fs pulse duration at the central wavelength of 1054 nm with a bandwidth of 23.5 nm.

Graphene mode-locked Cr:LiSAF laser at 850 nm.

We report, for the first time to our knowledge, a mode-locked femtosecond Cr:LiSAF laser initiated with a high-quality monolayer graphene saturable absorber (GSA), synthesized by chemical-vapor deposition. The tight-focusing resonator architecture made it possible to operate the Cr:LiSAF laser with only two 135 mW, 660 nm low-cost single-mode diode lasers. At a pump power of 270 mW, the laser produced nearly transform-limited 68 fs pulses with an average power of 11.5 mW at 850 nm. The repetition rate was around 132 MHz, corresponding to a pulse energy and peak power of 86 pJ and 1.26 kW, respectively. Once mode locking was initiated with the GSA, stable, uninterrupted femtosecond pulse generation could be sustained for hours. The saturation fluence and the modulation depth of the GSA were further determined to be 28 μJ/cm2 and 0.62%, respectively.

基于氧化石墨烯与半导体可饱和吸收镜的锁模飞秒掺铒光纤激光器

基于氧化石墨烯与半导体可饱和吸收镜的锁模飞秒掺铒光纤激光器

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.

Optical frequency comb interference profilometry using compressive sensing

We describe a new optical system using an ultra-stable mode-locked frequency comb femtosecond laser and compressive sensing to measure an object's surface profile. The ultra-stable frequency comb laser was used to precisely measure an object with a large depth, over a wide dynamic range. The compressive sensing technique was able to obtain the spatial information of the object with two single-pixel fast photo-receivers, with no mechanical scanning and fewer measurements than the number of sampling points. An optical experiment was performed to verify the advantages of the proposed method.

Optical Profi lometer Using Single Pixel Camera and Radio Frequency Comb Generated by Ultrastable Mode-locked Femtosecond Laser

Optical Profi lometer Using Single Pixel Camera and Radio Frequency Comb Generated by Ultrastable Mode-locked Femtosecond Laser