Mode-locked Femtosecond Laser Research Articles

Growth, spectroscopy and laser operation of Tm,Ho:GdScO3 perovskite crystal

We report on the growth, polarized spectroscopy and first laser operation of an orthorhombic (space group Pnma) Tm3+,Ho3+-codoped gadolinium orthoscandate (GdScO3) perovskite-type crystal. A single crystal of 3.76 at.% Tm, 0.35 at.% Ho:GdScO3 was grown by the Czochralski method. Its polarized absorption and fluorescence properties were studied revealing a broadband emission around 2 µm. The parameters of the Tm3+ ↔ Ho3+ energy transfer was quantified, P28 = 1.30 × 10−22 cm3µs-1, and P71 = 0.99 × 10−23 cm3µs-1, and the thermal equilibrium lifetime was measured to be 3.5 ms. The crystal-field splitting of Tm3+ and Ho3+ multiplets in Cs symmetry sites of the perovskite structure was determined by low-temperature spectroscopy and the mechanism of spectral line broadening is discussed. The continuous-wave Tm,Ho:GdScO3 laser generated 1.16 W at ∼2.1 µm with a slope efficiency of 50.5%, a laser threshold of 184 mW, a linear laser polarization ( E || c ) and a spatially single-mode output. The Tm,Ho:GdScO3 crystal is promising for broadly tunable and femtosecond mode-locked lasers emitting above 2 µm.

Stable Q-switched and femtosecond mode-locked erbium-doped fiber laser based on a CuSe nanosheets saturable absorber.

Stable Q-switched and femtosecond mode-locked erbium-doped fiber laser (EDFL) have been achieved using CuSe nanosheets as novel saturable absorber (SA), where the CuSe nanosheets were prepared by a hydrothermal method. The nonlinear optical properties of CuSe nanosheets were measured using an Z-scan setup, revealing nonlinear absorption coefficients of -3.67 ± 0.22 cm GW-1 at 1560 nm. The prepared CuSe nanosheets were mixed with polyvinyl alcohol (PVA) to obtain a CuSe-PVA SA with a modulation depth of 3.8 ± 0.13%, and it was utilized to realize a Q-switched EDFL, obtaining the narrowest pulse duration of 1.29 µs and the maximum output power of 5.96 mW, which corresponds to a pulse energy of up to 103.7 nJ. In addition, CuSe nanosheets were deposited on a D-shaped fiber (DSF) to fabricate a CuSe-DSF SA with a modulation depth of 5.6 ± 0.17%, and it was utilized to realize a mode-locked EDFL. The mode-locked EDFL demonstrated a low threshold of only 42 mW, a pulse duration of 740 fs, and a maximum output power of 9.7 mW. Meanwhile, it exhibited a high signal-to-noise ratio of 72 dB. To the best of our knowledge, this is the first time of CuSe nanosheets as SA in EDFL. The results demonstrate that CuSe nanosheets are a highly promising nonlinear optical material with great potential for applications in ultrafast photonics.

Design of an Optical Head with Two Phase-Shifted Interference Signals for Direction Detection of Small Displacement in an Absolute Surface Encoder

This paper presents the design and construction of a new optical head with two phase-shifted interference signals in an absolute surface encoder by using a mode-locked femtosecond laser. A series of discrete absolute positions of the scale grating is obtained from a series of peak wavelengths of the spectrum of the +1st- or -1st-order diffracted beam. The two beams at a specific wavelength λi interfere with each other to generate an incremental interference signal for high-resolution displacement measurement over a small interpolation range around the corresponding discrete absolute position xi. In the previous design of the optical head, the two beams were guided by optical fibers into a fiber coupler for the interference. This fiber optics design was simple and stable but could not identify the moving direction of small displacement within each interpolation range because only one interferential signal could be generated. The aim of this study is to develop a new design of the optical head, where two interference signals with a phase difference of π/2 are generated. For this purpose, free-space optics, instead of fiber optics, is adopted in the new optical head. Experiments are conducted to confirm the generation of the two phase-shifted interference signals. A Lissajous figure is plotted to verify the phase difference between the two signals.

Carbon nanodots derived from organo-templated zeolites for femtosecond mode-locked fiber laser

Carbon nanodots (CNDs) have been widely investigated due to their physicochemical and optical properties. However, the nonlinear saturable absorption property and its application in laser technology is rarely reported. Here, we report the CNDs as an ideal saturable absorber (SA) for ultra-stable femtosecond mode-locked fiber laser at a wavelength of 1.5 μm. The mono-sized CNDs of 2 nm are fabricated from a dipropylamine-templated zeolite precursor calcination with a temperature of 380 °C. The nonlinear saturable absorption property of the as-synthesized CNDs is investigated by using a balanced twin-detector measurement system. The modulation depth and saturable intensity are measured to be 33 % and 3.1 MW/cm2, respectively. By inputting the CNDs SA into the erbium-doped fiber laser (EDFL) ring cavity, a highly stable mode-locked EDFL with pulse width of 388 fs and single-to-noise ratio (SNR) of 84.4 dB can be obtained. In addition, stable soliton pairs with single-pulse width of 422 fs and SNR of 84.1 dB can also be achieved. These findings demonstrate that the carbon dots can be used as an ideal SA and has promising applications in ultrafast photonics.

Precise selection and amplification of a single comb line from femtosecond mode-locked lasers.

We propose and demonstrate a novel, to the best of our knowledge, scheme for extracting and amplifying a single comb line with a high signal-to-noise ratio (SNR) from a femtosecond mode-locked laser (FMLL). The scheme is realized based on the combination of pulse repetition-rate multiplication, optical injection locking, and the polarization-pulling-attributes of stimulated Brillouin scattering. The SNR of the selected and amplified comb line is more than 70 dB, and its frequency can be tuned at an arbitrary comb line position over the whole range of the FMML. The white frequency noise floor measured through a delayed self-heterodyne interferometry technique is around 80 Hz2/Hz. The scheme presented in this work has shown the potential for many applications such as ultra-precision metrology and spectroscopy.

589 nm yellow laser pumped Kerr-lens mode-locked Alexandrite laser producing sub-50 fs pulses.

We report a femtosecond Kerr-lens mode-locked (KLM) Alexandrite laser resonantly pumped by a 589 nm yellow laser. The 4 nJ pulses as short as 42 fs were obtained corresponding to a peak power of 100 kW. With the repetition rate of 104 MHz, the average power of 420 mW was attained. The time-bandwidth product of generated laser pulse was measured to be 0.324 with a beam quality factor of M2 ≤ 1.13. The exceptional performance of visible femtosecond laser may find potential applications in various fields.

Enhanced ultrafast dynamics and saturable absorption response of Nb2SiTe4/graphene heterostructure for femtosecond mode-locked bulk lasers

Two-dimensional (2D) heterostructure provides a promising opportunity to create new hybrid materials with unique tailored properties for novel electronic and photonic devices. Here, 2D Nb2SiTe4/Graphene (NST/G) heterostructure samples with different thicknesses of Nb2SiTe4 are fabricated by mechanical exfoliation and dry transfer methods. Their photocarrier dynamics and nonlinear optical responses are systematically investigated by pump–probe and Z-scan measurements. The results illustrate that Graphene acts as an electron transfer channel effectively shortening the photocarrier lifetime in NST. Moreover, the thinner of NST, the larger of the photocarrier recombination rate in NST/G heterostructure because of the high surface state density and strong electron–phonon coupling effect. Significantly, the 2D NST/G heterostructure with thinner NST also exhibits a larger nonlinear absorption coefficient and moderate saturable absorption parameters, which endow the high-performance passively mode-locked femtosecond laser. With 2 nm NST/G heterostructure as saturable absorber (SA), passively mode-locked Yb:KYW bulk laser is realized with the pulse width of 415 fs and output power of 498 mW. Our results pave a versatile way for designing desirable photonic devices and applications.

Improved peak-to-peak method for cavity length measurement of a Fabry-Perot etalon using a mode-locked femtosecond laser.

Differing from the conventional peak-to-peak method using two neighboring spectral peaks in the frequency-domain fringe spectrum of the spectral response of a Fabry-Perot etalon to a femtosecond laser, which contains N spectral peaks equally spaced with a spacing of the etalon free spectral range (FSR), the proposed method employs a pair of spectral peaks with a spacing of an integer multiple k (k ≫ 1) of FSR for measurement of the etalon cavity length d with a reduced measurement error. Under the constrain of the total N spectral peaks obtainable in the finite spectral range of the femtosecond laser, the optimized k is identified to be N∕2 in consideration of an averaging operation using N - k samples of d to achieve the minimum measurement error. The feasibility of the proposed method is demonstrated by experimental results with an uncertainty analysis based on "Guides to the Expression of Uncertainty in Measurement".

High-power femtosecond vortices generated from a Kerr-lens mode-locked solid-state Hermite-Gaussian oscillator.

We report the generation of high-order transverse modes from a Kerr-lens mode-locked femtosecond laser. Two different orders of Hermite-Gaussian modes were realized by non-collinear pumping, which were converted into the corresponding Laguerre-Gaussian vortex modes using a cylindrical lens mode converter. The mode-locked vortex beams, with an average power of 1.4 W and 0.8 W, contained pulses as short as 126 fs and 170 fs at the first and second Hermite-Gaussian mode orders, respectively. This work demonstrates the possibility of developing Kerr-lens mode-locked bulk lasers with various pure high-order modes and paves the way for generating ultrashort vortex beams.

All-polarization-maintaining mode-locked thulium-doped femtosecond laser at 1.7 µm

We demonstrate a 1.7 µm femtosecond Tm-doped fiber laser system featuring an all-polarization-maintaining architecture. The seed oscillator is mode-locked by carbon nanotubes and delivers stable pulse centered at 1787.6 nm. With two backward pumped amplifiers, the average power of the laser is amplified to ∼458 mW. Employing proper dispersion management in an all-fiber chirped pulse amplification scheme and the soliton compression effect, we obtained a femtosecond pulse of 206 fs with a pulse energy of 8.8 nJ at a repetition rate of ∼52 MHz. To the best of our knowledge, this is the first demonstration of 1.7 µm femtosecond laser based on a thulium-doped oscillator with all-polarization-maintaining architecture.

High-energy femtosecond pulse generation from a hybrid mode-locked large-mode-area Er:ZBLAN fiber laser.

We report a hybrid mode-locked fiber laser at 2.8 µm based on a large-mode-area Er:ZBLAN fiber. Reliable self-starting mode-locking is achieved via the combination of nonlinear polarization rotation and a semiconductor saturable absorber. Stable mode-locked pulses with a pulse energy of 9.4 nJ and a pulse duration of 325 fs are generated. To the best of our knowledge, this is the highest pulse energy directly generated from a femtosecond mode-locked fluoride fiber laser (MLFFL) to date. The measured M2 factors are below 1.13, indicating a nearly diffraction-limited beam quality. Demonstration of this laser provides a feasible scheme for the pulse energy scaling of mid-infrared MLFFLs. Moreover, a peculiar multi-soliton mode-locking state is also observed, in which the time interval between the solitons varies irregularly from tens of picoseconds to several nanoseconds.

Theoretical and experimental investigations of dispersion-managed, polarization-maintaining 1-GHz mode-locked fiber lasers.

High-repetition-rate (up to GHz) femtosecond mode-locked lasers have attracted significant attention in many applications, such as broadband spectroscopy, high-speed optical sampling, and so on. In this paper, the characteristics of dispersion-managed, polarization-maintaining (PM) 1-GHz mode-locked fiber lasers were investigated both experimentally and numerically. Three compact and robust 1-GHz fiber lasers operating at anomalous, normal, and near-zero dispersion regimes were demonstrated, respectively. The net dispersion of the linear cavity is adjusted by changing types of PM erbium-doped fibers (EDFs) and semiconductor saturable absorber mirrors (SESAMs) in the cavity. Moreover, the long-term stability of the three mode-locked fiber lasers is proved without external control. In order to better understand the mode-locking dynamics of lasers, a numerical model was constructed for analysis of the 1-GHz fiber laser. Pulse evolution simulations have been carried out for soliton, dissipative-soliton, and stretched-pulse mode-locking regimes under different net dispersion conditions. Experimental results are basically in agreement with the numerical simulations.

Effect of spatial walk-off on squeezing properties of quantum optical frequency combs

Quantum optical frequency combs are of great significance in the fields of quantum computing, quantum information, and high precision quantum measurement, which can be produced by using a degenerate type-I synchronously pumped optical parametric oscillator (SPOPO). When anisotropic crystal is used as a nonlinear medium in the SPOPO, the spatial walk-off effect will occur due to the birefringence effect, which cannot be ignored and will adversely affect the generation of squeezed state. In this work, we investigate the influence of spatial walk-off effect on the squeezing level of quantum optical frequency combs both theoretically and experimentally. A Ti∶sapphire mode-locked femtosecond pulsed laser which produces 130 fs pulse trains at 815 nm with a repetition rate of 76 MHz is utilized as a fundamental source. Its second harmonic at 407.5 nm is used to pump the collinear BiB<sub>3</sub>O<sub>6</sub> (BIBO) crystal for generating the squeezed vacuum frequency comb. It is indicated that as the crystal length increases, the area of interaction between pump light and signal light decreases gradually. Thus the enhancement of squeezing is eventually limited by the spatial walk-off effect. According to the simulations, the squeezing level reaches a maximum value when the crystal length is 1.49 mm. The quantum properties of squeezed vacuum optical frequency combs obtained for four crystal lengths (0.5, 1.0, 1.5 and 2.0 mm) are subsequently measured experimentally. When the length of BIBO is 1.5 mm, the maximum vacuum squeezing of (3.6±0.2) dB is obtained, which is (7.0±0.2) dB after being corrected for detection loss. The experimental results are consistent with the numerical simulations. This study demonstrates that the spatial walk-off effect in nonlinear crystal is a significant factor affecting the quantum optical frequency comb, and the theoretical model presented in this paper can be used to provide a guideline for optimizing the experimental implementation.

Fabry-Pérot angle sensor using a mode-locked femtosecond laser source.

An angle sensor based on multiple beam interference of a Fabry-Pérot etalon employing a mode-locked femtosecond laser as the measurement beam is proposed. Output angles are evaluated by using estimated local maxima within a measurement bandwidth of an output fringe spectrum detected by an optical spectrum analyzer. In the proposed method, fringe spectra produced by beams from transmittance and a reflectance side of the Fabry-Pérot etalon are detected individually, and intensities of two spectra are divided to increase the visibility by narrowing a spectrum width. Confirmation of an increase in the visibility is conducted by comparing full width half maximum values of spectra obtained by a constructed optical setup, and evaluation accuracies were compared by repeating measurements for 100 times. The output angle using estimated local maxima of the divided spectra is then evaluated to verify the feasibility of the proposed method. As a result, it is confirmed that the proposed method improves the accuracy of angle determination by one order of magnitude.

90-fs Yb-doped fiber laser using Gires-Tournois interferometers as dispersion compensation

This paper reports a Yb-doped mode-locked femtosecond fiber laser based on a nonlinear amplifying loop mirror (NALM). The improved NALM structure ensures the high repetition rate of 97 MHz and the maximum output power of 126 mW. The Gires-Tournois interferometers (GTI) are used as dispersion compensation devices in laser resonator. The pulse output with full width half maximum (FWHM) of 26 nm and a dechirped pulse duration of 90 fs is obtained. As far as we know, this is the relatively short pulse in an improved NALM mode-locked Yb-doped fiber laser.

785-nm Frequency Comb-Based Time-of-Flight Detection for 3D Surface Profilometry of Silicon Devices

Three-dimensional integrated circuits (3D-ICs) are becoming more significant in portable devices, autonomous vehicles, and data centers. As the demand for highly integrated and high-performance semiconductor devices grows, recent 3D integration technologies focus on lowering the size of the micro-structures on such devices for high density. In order to inspect the 3D semiconductor devices, it is critical to measure the heights, depths, and overall surface profiles of the micro-structures made with silicon materials. Here, we demonstrate precise surface imaging for silicon devices by using a femtosecond mode-locked laser centered at 785 nm wavelength and an electro-optic sampling-based time-of-flight detection method with sub-10-nanometer axial precision. We could successfully measure the surface profiles as well as the step heights of silicon wafer stacks and micro-scale structures on silicon substrates.

Nonlinear optical properties of tin telluride topological crystalline insulator at a telecommunication wavelength

Topological crystalline insulators (TCIs) have a new topological state where topological nature comes from crystal symmetries without necessitating spin-orbit coupling. Their superb electronic, magnetic, and superconducting properties have intensively been studied. Despite great potential, however, their nonlinear optical properties are rarely examined. For the first time, this study investigates the nonlinear refraction and absorption properties of SnTe TCIs using the Z-scan techniques. A high nonlinear refractive index of -0.111 ± 0.004 cm2/GW and a nonlinear absorption coefficient of -(2.02 ± 0.135)× 103 cm/GW are obtained from SnTe TCIs at the commercially important telecommunication wavelength, being superior and/or comparable to two dimensional (2D) materials and/or renowned photonic crystals. Ab-initio simulations show that SnTe TCIs are suitable for ultrabroad-band application and mechanically stable at high thermal conditions. Lastly, we demonstrate that 1550 nm mode-locked femtosecond lasers can readily be achieved by exploiting the conspicuous nonlinear optical absorption of SnTe TCIs without using any nano-engineered ones like quantum dots. This study strongly indicates a promising potential of SnTe TCIs in a wide range of nonlinear optics, opening an avenue toward advanced TCIs-based photonic technologies.

Frequency stabilization of multiple wavelength lasers based on a broadband spectrum

We report on frequency stabilization of multiple wavelength lasers operating at 1389 and 1695 nm simultaneously on a broadband spectrum. These lasers are implemented in ytterbium optical lattice clock experiments, which need to have a narrow enough linewidth and maintain high long-term frequency stability. A 1560 nm femtosecond mode-locked laser with a narrow mode spacing of 250 MHz is used as a master laser, which is referenced to a local ultrastable optical cavity with the instability better than 1 × 10−15 at 1 s averaging time. Through the combination of erbium-doped fiber amplifier and high nonlinear fiber, the spectral width of the maser laser is broadened from 10 nm to more than 300 nm. The range of the broadened spectrum can cover 1389 and 1695 nm. Meanwhile, the spectral intensity at the corresponding wavelength can ensure that the signal-to-noise ratio of the beat signals between the two lasers and the broadened spectrum is about 30 dB at a resolution bandwidth (RBW) of 100 kHz. After phase locking the 1389 and 1695 nm lasers on the broadband spectrum, the residual linewidths are obtained to be about 0.8 Hz at 1 Hz RBW, and the stabilities are 3.5 × 10−16 and 4.7 × 10−16 at 1 s averaging time respectively, improving about six orders of magnitude. Our result can be conducive to obtaining the stabilized laser sources for the atomic optical clock, and will be of great significance for simplifying and miniaturizing the optical clock system.

Switchable femtosecond and picosecond spatiotemporal mode-locked fiber laser based on NALM and multimode interference filtering effects

We report, for the first time to the best of our knowledge, the switchable generation of versatile femtosecond and picosecond spatiotemporal mode-locked (STML) pulses in an all-few-mode fiber figure-8 laser at 1.55 µm. Both in the femtosecond and picosecond pulse regimes, the laser has pulse-pattern-tunable function, namely, the pulse patterns can be tuned from single-pulse, multiple pulses to soliton molecules. Particularly, the femtosecond soliton molecules operate at the loosely bound states while the picosecond soliton molecules can operate both at the loosely and tightly bound states. The theoretical analyses are carried out to give the further insight in the switchable generation of the femtosecond and picosecond pulses. It is found that the filtering effect of the nonlinear amplifying loop mirror (NALM) plays the major role in the generation of the femtosecond pulses while the multimode interference filtering effect induced by the active few-mode fiber-passive few-mode fiber structure plays the role in the generation of the picosecond pulses. The obtained results contribute to further exploring the characteristics of the STML fiber laser and such a laser with tunable output of versatile femtosecond and picosecond pulses has potential applications in optical communications, laser processing and measuring.

A photonic integrated circuit-based erbium-doped amplifier.

Erbium-doped fiber amplifiers revolutionized long-haul optical communications and laser technology. Erbium ions could provide a basis for efficient optical amplification in photonic integrated circuits but their use remains impractical as a result of insufficient output power. We demonstrate a photonic integrated circuit-based erbium amplifier reaching 145 milliwatts of output power and more than 30 decibels of small-signal gain-on par with commercial fiber amplifiers and surpassing state-of-the-art III-V heterogeneously integrated semiconductor amplifiers. We apply ion implantation to ultralow-loss silicon nitride (Si3N4) photonic integrated circuits, which are able to increase the soliton microcomb output power by 100 times, achieving power requirements for low-noise photonic microwave generation and wavelength-division multiplexing optical communications. Endowing Si3N4 photonic integrated circuits with gain enables the miniaturization of various fiber-based devices such as high-pulse-energy femtosecond mode-locked lasers.