Mode-locked Femtosecond Laser Research Articles

Wavelength-Tunable Er$^{3+}$-Doped fs Mode-Locked Fiber Laser Using Short-Pass Edge Filters

We demonstrate a wavelength-tunable Er <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped femtosecond mode-locked soliton fiber laser by incorporating a short-pass edge filter into a ring cavity. When the filter is thermo-optically tuned toward shorter wavelengths to strongly attenuate the wavelengths longer than the cutoff, the constituting wavelengths of the mode-locked fiber laser move to shorter wavelengths and the laser center wavelength can be tuned over 1539.7 ~ 1560 nm. The cavity dispersion is preset in the deep soliton region to keep small fluctuations of laser pulsewidth when the laser wavelength is tuned.

Self-stabilization of a mode-locked femtosecond fiber laser using a photonic bandgap fiber

We demonstrate a self-stabilization mechanism of a semiconductor saturable absorber mode-locked linear-cavity Yb-doped fiber laser using an intracavity photonic bandgap fiber. This mechanism relies on the spectral shift of the laser pulses to a spectral range of higher anomalous dispersion and higher loss of the photonic bandgap fiber, as a reaction to the intracavity power buildup. This, in particular, results in a smaller cavity loss for the stably mode-locked laser, as opposed to the Q-switched mode-locking scenario. The laser provides stable 39-49 pJ pulses of around 230 fs duration at 29 MHz repetition rate.

Conversion of stability of femtosecond mode-locked laser to optical cavity length

In this contribution we propose a scheme for a generation of precise displacements through conversion of relative stability of components of a femtosecond laser into the length of a Fabry-Perot cavity. The spacing of mirrors of a Fabry-Perot interferometer represents a mechanical length standard referenced to stable optical frequency of a femtosecond mode-locked laser. With the help of a highly selective optical filter, it is possible to get only a few discrete spectral components. By tuning and locking the Fabry-Perot cavity to a selected single component it is possible to get a mechanical length standard with the uncertainty of the repetition frequency of the femtosecond laser. To verify the method, an auxiliary single-frequency laser is locked to the resonance mode of the cavity and simultaneously it is optically mixed with an independent optical frequency standard He-Ne-I2. The stability of the beat-frequency between these 2 lasers represents the stability of the Fabry-Perot cavity length. The stability recording evaluated through Allan variances for one hour of operation is presented. The pilot experimental setup is able to generate the length standard in the order of 0.01 nm for 20 min of integration time.

High-Speed High-Resolution Terahertz Spectrometers

We demonstrate and characterize both asynchronous optical sampling terahertz time-domain spectroscopy (AOS THz-TDS) and terahertz frequency comb spectroscopy (TFCS) as high-speed, high-resolution terahertz (THz) spectroscopy. Two mode-locked femtosecond (fs) lasers with slightly different repetition frequencies are used without a mechanical delay stage to generate and detect THz pulses, respectively. Repetition frequencies of the two fs lasers are stabilized by use of two phase-locked loops sharing the same reference oscillator, respectively. For AOS THz-TDS, the difference frequency between the repetition frequencies is optimized and a signal-to-noise ratio is measured as a function of a measurement time. Spectra of THz frequency comb and its individual modes are measured from TFCS. A spectral resolution of 100 MHz is obtained in the both types of spectroscopy.

Femtosecond mode-locked fiber laser employing a hollow optical fiber filled with carbon nanotube dispersion as saturable absorber

We propose a novel in-line saturable absorber incorporating a hollow optical fiber (HOF) filled with single-walled carbon nanotube (SWCNT) dispersion. The evanescent field of the propagating light in the ring core interacts with the SWCNT/polymer composite distributed over the whole length of the HOF. The proposed saturable absorber with all-fiber format offers the robust and long nonlinear interaction along the waveguide direction expecting the increase of the threshold for optical and thermal damages with simple fabrication process. Low concentration SWCNT/polymer composite exhibiting very broadband resonant absorption around 1.5 microm with low scattering loss is prepared and based on this, we successfully demonstrate the passively mode-locked fiber laser including the SWCNT-filled HOF where the spectral bandwidth and the pulse duration of the laser output are 5.5 nm and 490 fs, respectively, with a repetition rate of 18.5 MHz.

Ultrafast ranging lidar based on real-time Fourier transformation

Real-time Fourier-transformation-based ranging lidar using a mode-locked femtosecond fiber laser is demonstrated. The object signal and the reference signal are guided from a fiber Mach-Zehnder interferometer into a dispersive element. The two optical pulses extend and overlap with each other temporally, which yields a microwave pulse on the photodetector with its frequency proportional to the time delay between the two signals. The temporal interferograms are transformed from the time domain into the frequency domain using a time-to-frequency conversion function obtained in the calibration process. The Fourier transform is used in the data processing. A range resolution of 334 nm at a sampling rate of 48.6 MHz over a distance of 16 cm is demonstrated in the laboratory.

Properties and stability limits of an optimized mode-locked Yb-doped femtosecond fiber laser

Stable mode-locked operation in a simple normal-cavity-dispersion laser oscillator that consists of only Yb-doped fiber and saturable absorber is studied. The Yb-doped fiber design parameters: group velocity dispersion (GVD), nonlinearity coefficient, bandwidth (Yb-BW), length and gain are considered to be the controlling parameters of the laser cavity. The pulse characteristics such as the temporal width, spectrum and pulse energy as a function of these elements are reported here. A pulse spectrum transition from M-like to Pi-like and then to parabolic-like shape is observed with different values of the controlling parameters which are similar to that has been observed before in a solid-state laser. The stability limits in the domain of the Yb-BW and length are studied. The stability dependence on GVD, nonlinearity coefficient and gain of the Yb-doped fiber are elucidated. Moreover, the pulse instability dyn ics beyond stability limits are found to be similar to that reported before for similariton.

A passively mode-locked femtosecond soliton fiber laser at 15 μm with a CNT-doped polycarbonate saturable absorber

We report a new 1.5 mum femtosecond fiber soliton laser incorporating a polymer saturable absorber that consists of polycarbonate (PC) with carbon nanotubes (CNTs). We installed the polymer in a fiber laser with a cavity length of 5.1 m, in which an erbium-doped fiber was used as the gain medium. Stable passive mode-locking was achieved for a pump power of 303 mW at 39 MHz, and a pulse width of 115 fs with an average output power of 3.4 mW was obtained.

Quantum Improvement of Time Transfer between Remote Clocks

Exchanging light pulses to perform accurate space-time positioning is a paradigmatic issue of physics. It is ultimately limited by the quantum nature of light, which introduces fluctuations in the optical measurements and leads to the so-called standard quantum limit (SQL). We propose a new scheme combining homodyne detection and mode-locked femtosecond lasers that lead to a new SQL in time transfer, potentially reaching the yoctosecond range (10(-21)-10(-24) s). We demonstrate that this already very low SQL can be overcome using appropriately multimode squeezed light. Benefitting from the large number of photons and from the optimal choice of both the detection strategy and of the quantum resource, the proposed scheme represents a significant potential improvement in space-time positioning.

Dual femtosecond laser multiheterodyne optical coherence tomography

A time domain optical coherence tomography (OCT) system without moving parts is described, which is based on multiheterodyning utilizing two mode-locked femtosecond lasers. By synchronizing the two lasers to slightly different repetition rates and coupling to an interferometric OCT setup, we obtain amplitude-modulated beat signals representing the structure of the specimen under investigation. Our system is suitable for biological imaging as well as technical applications. We demonstrate high axial imaging depths of 150 mm with up to 5000 axial scans per second, achieving equivalent path scanning velocities of 750 m/s.

Continuous-wave tunable and femtosecond mode-locked laser operation of Yb:NaY(MoO_4)_2

Continuous-wave and mode-locked laser operations of Yb3+ in the disordered flux-grown NaY(MoO4)2 are demonstrated. The maximum slope efficiency achieved with Ti:sapphire and diode laser pumping is 63% and 38.6%, respectively. The continuous-wave tunability extends from 1005 to 1059 nm, and laser pulses with a duration of 91 fs at 1026 nm are obtained at a repetition rate of 90 MHz and an average power of 20 mW under Ti:sapphire laser pumping. Yb3+-doped NaY(MoO4)2 is locally disordered and exhibits the largest absorption and emission cross sections known so far for tetragonal double tungstates or molybdates.

Measurement of reactive atmospheric species by ultraviolet cavity-enhanced spectroscopy with a mode-locked femtosecond laser

We demonstrate the possibility of measuring parts in 10(12) by volume concentrations of radicals of high atmospheric interest, such as IO or BrO, as needed for monitoring these species in the environment. We apply cavity-enhanced absorption spectroscopy in the near UV range using a frequency-doubled Ti:Sa mode-locked femtosecond laser. Efficient broadband injection of a high-finesse cavity is obtained by matching this optical frequency-comb source to the comb of cavity transmission resonances. A grating spectrograph and a detector array disperse and detect the spectrum transmitted by the cavity carrying the absorption features of intracavity molecules. Spectra recorded over ~4 nm with 10 s averaging display a noise level of 8 x 10(-10)/cm.

Spectrally smoothed supercontinuum generation by using unstably mode-locked laser pulses

We report on a simple approach to obtain spectrally smoothed supercontinuum (SC) by using unstably mode-locked femtosecond laser pulses. The spectrum smoothing effect is due to the averaging of the supercontinua generated in a nonlinear photonic crystal fiber by modulated pulses. Theoretical analysis based upon solving the generalized nonlinear Schrödinger equation is presented, which qualitatively agrees well with the experimental observation. Finally, the application of the smoothed SC to broadband coherent anti-Stokes Raman scattering spectroscopy is demonstrated, which can result in a relatively smooth nonresonant background and suppress spurious peaks.

High repetition rate passively mode-locked erbium-doped fiber laser

We report a passively mode-locked high repetition rate erbium-doped femtosecond fiber laser via nonlinear polarization rotation, with a fundamental repetition rate of 101.94 MHz. The output power is 34 mW when pumped by a single mode fiber coupled laser diode at 370 mW. The spectral width is 25 nm, corresponding to a transform limited pulse width of 105 fs.

Diode-pumped Yb:KYW femtosecond laser frequency comb with stabilized carrier-envelope offset frequency

We describe the detection and stabilization of the carrier envelope offset (CEO) frequency of a diode-pumped Yb:KYW (ytterbium-doped potassium yttrium tungstate) femtosecond oscillator that is spectrally centered at 1033 nm. The system consists of a diode-pumped, passively mode-locked femtosecond laser that produces 290 fs pulses at a repetition rate of 160 MHz. These pulses are first amplified, spectrally broadened and temporally compressed to 80 fs, and then launched into microstructured fiber to produce an octave-spanning spectrum. An f-2f nonlinear interferometer is employed with the broadened spectrum to detect and stabilize the CEO frequency through feedback to the pump laser current. These results demonstrate that such a Yb-doped tungstate laser can provide an efficient, compact, high-repetition-rate optical frequency comb with coverage from 650–1450 nm.

Precise length etalon controlled by stabilized frequency comb

The progress in the field of optical frequency standards is oriented to femtosecond mode-locked lasers stabilized by technique of the optical frequency synthesis. Such a laser produces a number of coherent frequency components in certain interval of wavelengths. If we control the mode-locked laser by means of i.e. atomic clocks we ensure very stable frequency of these components. With respect to definition of SI unit ”one meter” on the basis of speed of light, the stabilized mode-locked laser can be used for implementation of this definition in a non-traditional way. In the work we present our proposal of a system, which converts excellent frequency stability of components generated by the mode-locked laser to a net of discrete absolute lengths represented by a distance of two mirrors of an optical resonator.

多脉冲振荡克尔透镜锁模飞秒激光器

Our experimental results show that the presence of a proper amount of negative group velocity dispersion is essential to multi-pulse operation of a Kerr-lens mode-locked femtosecond laser. We demonstrate that the pulse separations and the number of pulses contained within a cavity round trip are strongly dependent on the initial perturbations. The results allow us to get a better understanding on the influences of the convoluted self-phase modulation and intra-cavity dispersions on the stable multi-pulse oscillation in a Kerr-lens mode-locked femtosecond laser.

A wide-range optical frequency generator based on the frequency comb of a femtosecond laser

A precise way of optical frequency generation is demonstrated with direct use of the frequency comb of a mode-locked femtosecond laser. Only a single mode is extracted at a time on demand from the frequency comb through a composite filtering scheme and then amplified by means of optical injection locking with extremely low background noise. Generated output signals are found to preserve not only the narrow linewidths of the selected individual modes but also the absolute frequency positions of the original comb over a wide spectral range. These outstanding performances of optical frequency generation could find applications in high precision spectroscopy, frequency calibration, and length metrology.

Metrology under Control

AbstractMode‐locked femtosecond lasers enable ultra precise measurements based on fundamental constants. Although in terms of pulse properties the performance of the state‐of‐the‐art Ti:sapphire lasers are yet superior to alternative ultrafast laser technologies, it is the fiber laser technology that facilitates the development of breakthrough applications outside the laboratory environment. Reliability and robustness of fiber lasers are attractive features for industry, while the unique SWaP factor (size, weight and power) designates the fiber‐based optical frequency combs a promising enabling technology even for space science.

Femtosecond laser Fourier transform absorption spectroscopy

A femtosecond mode-locked laser is used for what is believed to be the first time as a broadband infrared source for high-resolution Fourier transform absorption spectroscopy. A demonstration is made with a Cr(4+):YAG laser. The entire nu(1)+nu(3) vibration-rotation band region of acetylene, observed after passing through a single-pass 80-cm-long cell, is simultaneously recorded between 1480 and 1600 nm, in 7.9 s with a signal-to-noise ratio equal to 1000. Two hot bands of the most abundant acetylene isotopologue and the nu(1)+nu(3) band of the (13)C(12)CH(2) are also present. Replacement of the usual conventional tungsten lamp by the bright laser source reduces by about a factor of 150 the recording time needed to get similar results. The noise equivalent absorption coefficient at 1 s averaging is equal to 7x10(-7) cm(-1)Hz(-1/2) per spectral element.