Review of low timing jitter mode-locked fiber lasers and applications in dual-comb absolute distance measurement

Passively mode-locked fiber lasers emit femtosecond pulse trains with excellent short-term stability. The quantum-limited timing jitter of a free running femtosecond erbium-doped fiber laser working at room temperature is considerably below one femtosecond at high Fourier frequency. The ultrashort pulse train with ultralow timing jitter enables absolute time-of-flight measurements based on a dual-comb implementation, which is typically composed of a pair of optical frequency combs generated by femtosecond lasers. Dead-zone-free absolute distance measurement with sub-micrometer precision and kHz update rate has been routinely achieved with a dual-comb configuration, which is promising for a number of precision manufacturing applications, from large step-structure measurements prevalent in microelectronic profilometry to three coordinate measurements in large-scale aerospace manufacturing and shipbuilding. In this paper, we first review the sub-femtosecond precision timing jitter characterization methods and approaches for ultralow timing jitter mode-locked fiber laser design. Then, we provide an overview of the state-of-the-art dual-comb absolute ranging technology in terms of working principles, experimental implementations, and measurement precisions. Finally, we discuss the impact of quantum-limited timing jitter on the dual-comb ranging precision at a high update rate. The route to highprecision dual-comb range finder design based on ultralow jitter femtosecond fiber lasers is proposed.

Sensitivity Improvements for Picosecond Ultrasonic Thickness Measurements in Gold and Tungsten Nanoscale Films

Picosecond ultrasonics,as a nondestructive and noncontact method,can be employed for nanoscale metallic film thickness measurements.The sensitivity of the system,which determines the measurement precision and practicability of this technique,is often limited by the weak intensity of the ultrasonic signal.To solve this problem,we investigate the distinct mechanisms involved in picosecond ultrasonic thickness measurement for two types of metals,namely tungsten(W)and gold(Au).For thickness measurement in W films,theory and simulation show that optimizing the pump and probe laser wavelengths,which determine the intensity and shape of the ultrasonic signal,is critical to improving measurement sensitivity,while for Au film measurements,where acoustic-induced beam distortion is dominant,the signal intensity can be optimized by selecting an appropriate aperture size and sample position.The above approaches are validated in experiments.A dual-wavelength pump-probe system is constructed based on a passively mode-locked ytterbium-doped fiber laser.The smoothing method and multipeak Gaussian fitting are employed for the extraction of ultrasonic time-of-flight.Subnanometer measurement precision is achieved in a series of W and Au films with thicknesses of 43-750 nm.This work can be applied to various high-precision,noncontact measurements of metal film thickness in the semiconductor industry.

Sideband-free dispersion-managed Yb-doped mode-locked fiber laser with Gires–Tournois interferometer mirrors

High-order dispersion introduced by Gires–Tournois interferometer mirrors usually causes spectral sidebands in the nearzero dispersion region of mode-locked fiber lasers.Here,we demonstrate a sideband-free Yb-doped mode-locked fiber laser with dispersion-compensating Gires–Tournois interferometer mirrors.Both the simulation and the experiment demonstrate that the wavelength and energy of the sidebands can be tuned by changing the transmission coefficient of the output mirror,the pump power,and the ratio of the net cavity dispersion to the net third-order dispersion in the cavity.By optimizing these three parameters,the laser can generate a sideband-free,Gaussian-shaped spectrum with a 13.56-nm bandwidth at-0.0232 ps^(2)net cavity dispersion,which corresponds to a 153-fs pulse duration.

Large-scale absolute distance measurement with dual free-running all-polarization-maintaining femtosecond fiber lasers

We demonstrate a robust femtosecond LIDAR setup by using two free-running environmentally stable allpolarization-maintaining nonlinear amplified loop mirror mode-locked fiber lasers. Based on the asynchronous optical sampling method, a ranging accuracy of ±2 μm within 65 m has been achieved, as tested in an 80-m-long underground optical tunnel. Through the Kalman filter in real-time data processing, the measurement accuracy can be maintained at a 200 Hz update rate. This setup provides a practical tool for various large-scale industrial and astronomical ranging applications.

High pulse energy fiber/solid-slab hybrid picosecond pulse system for material processing on polycrystalline diamonds

We demonstrate an all polarization-maintaining(PM) fiber mode-locked laser seeded, hybrid fiber/solid-slab picosecond pulse laser system which outputs 40 μJ, 10 ps pulses at the central wavelength of 1064 nm. The beam quality factors M2 in the unstable and stable directions are 1.35 and 1.31, respectively. 15 μJ picosecond pulses at the central wavelength of 355 nm are generated through third harmonic generation(THG) by using two Li B3 O5(LBO) crystals, in order to get better processing efficiency on polycrystalline diamonds. The high pulse energy and beam quality of these ultraviolet(UV) picosecond pulses are confirmed by latter experiments of material processing on polycrystalline diamonds. This scheme which combines the advantages of the all PM fiber mode-locked laser and the solid-slab amplifier enables compact, robust and chirped pulse amplification-free amplification with high power picosecond pulses.

Fiber-delay-line-referenced optical frequency combs:three stabilization schemes

We demonstrate the stabilization of an optical frequency comb(OFC) using a segment of fiber delay line as a reference. A mode-locked Er-doped fiber laser is phase locked to a kilometer-long fiber delay line using three different schemes. The short-term stability of the comb modes in the OFC stabilized by these schemes is obviously enhanced, down to the 10;level at millisecond average time. Among these three schemes, phase locking two bunches of comb modes in the OFC to the same fiber delay line exhibits the lowest residual phase noise. Fiber-delay-line-referenced OFCs can provide reliable laser sources in precise metrology owing to the advances of low cost, compactness, and high integration.

Generation of few-cycle laser pulses by coherent synthesis based on a femtosecond Yb-doped fiber laser amplification system

We demonstrate a coherent synthesis system based on femtosecond Yb-doped fiber laser technology. The output pulse of the amplification system is divided into two replicas and seeded into photonic crystal fibers of two parallel branches for nonlinear pulse compression. Because of the different nonlinear dynamics in the photonic crystal fibers, the compressed pulses show different spectra, which can be spliced to form a broad coherent spectrum. The integrated timing jitter between the pulses of two branches is less than one tenth of an optical cycle.By coherently synthesizing pulses from these two branches, 8 fs few-cycle pulses are produced.

Sub-30 fs Yb-fiber laser source based on a hybrid cascaded nonlinear compression approach

Ultrafast lasers with high repetition rate,high energy,and ultrashort pulse duration have enabled numerous applications in science and technology.One efficient route to generate such pulses is postcompression of high-power Yb-doped lasers.Here,we report on the generation of 24.5 fs pulses with an output energy of 1.6μJ and a repetition rate of 500 kHz.The pulses are obtained by using a hybrid cascaded nonlinear compression of the pulses delivered by a Yb-based fiber chirped pulse amplification(CPA)system.In the first stage,the initial 390 fs laser pulses are compressed to 100.7 fs based on spectral broadening in three fused silica plates.In the second stage,the pulses have been shortened to sub-30 fs by means of nonlinear compression in a hollow-core fiber.Overall,we could achieve∼16 times temporal shortening with the proposed approach.The results show that our system can effectively generate few-cycle pulses at a relatively high repetition rate and high energy,which can benefit future possible applications.

Nonlinearity optimization of dissipative-soliton fiber laser for generation of pulses with 350 kW peak power

We demonstrate a nonlinearity optimization method by altering distribution of passive fibers in a dissipative-soliton mode-locked fiber laser to level up output parameters. In the numerical simulation, we found that the passive fiber segment after gain fiber characterizes the highest average B-integral among fiber segments. By reducing the length of this fiber section and keeping the total passive fiber length as constant, the output pulse energy can be effectively scaled up while maintaining a short dechirped pulse duration, resulting in boosting peak power. With this method, 37-n J pulses are generated from a dissipative-soliton mode-locked cladding pumped ytterbium-doped single-mode fiber laser in the experiment. The pulse can be dechirped to 66 fs with 350 k W peak power. Moreover, the pulse pedestal is suppressed by a vector-dispersion compressor.

Passive optimization of pump noise transfer function by narrow band-pass filtering in femtosecond fiber lasers

Fluctuation of pump power is one of the major sources of temporal and intensity noise in femtosecond fiber lasers.In this work,the transfer functions between the relative intensity noise(RIN)of the pump laser diode(LD)and the output RIN,between the RIN of the pump LD and timing jitter of femtosecond fiber lasers are systematically studied.It is demonstrated,for the first time to our knowledge,that the amplitude of the pump RIN transfer function can be effectively decreased by an intra-cavity narrow band-pass filter.In particular,for normal-dispersion lasers,the 3-d B bandwidth of the transfer function can also be narrowed by two-thirds,with a steeper falling edge.Furthermore,with the narrow band-pass filtering,the transfer function is almost independent of the net intra-cavity dispersion due to amplifier similariton formation.The proposed scheme can effectively isolate the pump-induced noise without the need of complex active pump LD control and intra-cavity dispersion management,thus providing an easy way for practical high-power,high-stability femtosecond fiber laser design and related high-precision applications outside the laboratory.

Spectral filtering effect on multi-pulsing induced by chirped fiber Bragg grating in dispersion-managed mode-locked Yb-doped fiber lasers

We numerically and experimentally investigate the multi-pulsing mechanism in a dispersion-managed mode-locked Ybdoped fiber laser.Multi-pulsing occurs primarily owing to the inherent filtering effect of the chirped fiber Bragg grating.The spectral filtering effect restricts the spectral broadening induced by self-phase modulation and causes extra loss,leading to a decreased pump power threshold for the multi-pulsing state.Numerical simulations show that multi-pulsing emerges at a lower pump power when the spectral filter bandwidth becomes narrower.In the experiment,the spectral width increases as the net cavity dispersion approaches zero.Pulses with wider spectral widths experience more loss from the spectral filtering effect,leading to a decreased pump power threshold for multi-pulsing.Therefore,the net cavity dispersion also has an impact on the multi-pulsing threshold.Based on this conclusion,we devise a strategy to obtain single-pulsing operation with the shortest pulse width and the highest pulse energy.

xtraction of internal phase motions in femtosecond soliton molecules using an orbital-angular-momentum-resolved method

Internal motions in femtosecond soliton molecules provide insight into universal collective dynamics in various nonlinear systems.Here we introduce an orbital-angular-momentum(OAM)-resolved method that maps the relative phase motion within a femtosecond soliton molecule into the rotational movement of the interferometric beam profile of two optical vortices.By this means,long-term relative phase evolutions of doublet and triplet soliton molecules generated in an all-polarization-maintaining mode-locked Er-fiber laser are revealed.This simple and practical OAM-resolved method represents a promising way to directly visualize the complex phase dynamics in a diversity of multisoliton structures.