Transform-limited Pulse Width Research Articles

Manipulating the Spectral-Temporal Characteristics of Multi-GHz Ultrashort Pulses by Leveraging the Nonlinear Phase Modulation and CW-Seeded Four-Wave Mixing

Ultrashort pulse fiber lasers with multi-GHz repetition rates have recently attracted significant attention in various fields. However, the parameter tunability of multi-GHz ultrashort pulse fiber lasers is inherently limited by their miniaturized design, particularly the central wavelength and transform-limited pulsewidth. To this end, here we propose a method for externally manipulating the spectral-temporal characteristics of multi-GHz ultrashort pulses by exploring continuous-wave (CW)-seeded four-wave mixing (FWM) and nonlinear phase modulation (NPM) driven by miniaturized mode-locked fiber laser with a multi-GHz fundamental repetition rate. We first conduct a theoretical investigation on the picosecond-pulse-driven CW-seeded FWM, and then study the NPM for manipulating the spectral-temporal performance of multi-GHz ultrashort pulses, i.e., associated with the CW-seeded FWM for pulsewidth tunability. Then, we perform experiments to validate the theoretical findings, and a pulsewidth tuning range of 598 fs – 5.3 ps is achieved. Furthermore, a continuous wavelength tuning range from 1520 nm to 1550 nm is obtained. The experimental results show good agreement with the numerical simulation results. These efforts can provide a promising route to realize high-quality multi-GHz ultrashort pulses with flexible tunability of wavelength and pulsewidth through manipulating the spectral-temporal characteristics.

All-fiber mode-locked gigahertz femtosecond laser at 1610 nm using a self-developed long-wavelength gain fiber.

We report a compact all-fiber passively mode-locked ultrafast laser with a fundamental repetition rate of 1.6 GHz that uses a self-developed long-wavelength active fiber, i.e., a fluoro-sulfo-phosphate-based Er3+/Yb3+ co-doped fiber (only 6.2 cm in length). This active fiber can provide a net gain coefficient of 0.6 dB/cm at 1610 nm. The high-repetition-rate all-fiber mode-locked laser operates at a low pump power of only approximately 90 mW. The mode-locked pulse train has a period of 625 ps and a 3 dB bandwidth of 7.0 nm, which can support a transform-limited pulse width of 390 fs.

Biocompatible diameter-oscillating fiber with microlens endface.

Optical fibers have been widely applied to life science, such as laser delivering, fluorescence collection, biosensing, bioimaging, etc. To resolve the challenges of advanced multiphoton biophotonic applications utilizing ultrashort laser pulses, here we report a flexible diameter-oscillating fiber (DOF) with microlens endface fabricated by using Polydimethylsiloxane (PDMS) elastomers. The diameter of the DOF is designed to longitudinally vary for providing accurate dispersion management, which is important for near-infrared multiphoton biophotonics that usually involves ultrashort laser pulses. The variation range and period of the DOF's diameter can be flexibly adjusted by controlling the parameters during the fabrication, such that dispersion curves with different oscillation landscapes can be obtained. The dispersion oscillating around the zero-dispersion baseline gives rise to a minimized net dispersion as the ultrashort laser pulse passes through the DOF - reducing the temporal broadening effect and resulting in transform-limited pulsewidth. In addition, the endface of the DOF is fabricated with a microlens, which is especially useful for laser scanning/focusing and fluorescence excitation. It is anticipated that this new biocompatible DOF is of great interest for biophotonic applications, particularly multiphoton microscopy deep inside biological tissues.

OPCPA investigation with control over the temporal shape of 1.2 ps pump pulses.

We report a study of a compact, scalable up to TW peak power OPCPA with ∼1.2 ps pump pulses delivered from a Yb:YAG laser. Passive synchronization was ensured by using a small portion of the energy to generate a stable supercontinuum in the YAG, and the rest was directed to pump the three OPCPA stages. The temporal shape of the pump pulse was controlled by the degree of depletion of fundamental radiation in a two-cascade second harmonic converter. Under optimal conditions, the energy of amplified pulses reached ∼2.1 mJ with the support of a spectral bandwidth sufficient for a transform-limited pulse width of 8.6 fs.

Mode-locked thulium doped fiber laser with zinc oxide saturable absorber for 2 μm operation

In this work, a 2.0-µm mode-locked thulium doped fiber laser is demonstrated using zinc oxide (ZnO) as the saturable absorber (SA) in a thulium doped fiber laser (TDFL). The ZnO nanorods was prepared by the facile hydrothermal method, and then embedded in polyethylene oxide composite film to form the SA. The laser is capable of generating soliton pulses at a central wavelength of 1945.45 nm with a 3-dB bandwidth of 3.47 nm. The pulse width is measured to be 1.395 ps, only slightly higher than the calculated transform-limited pulse width of 1.15 ps. The peak power of the generated mode-locked pulses is then calculated to be as high as 41.88 W, with a signal-to-noise ratio of 50.5 dB. The fabricated ZnO film in this work would allow the generation of mode-locked pulses, which could be useful for applications in spectroscopy and medicine due to the eye-safe and highly sensitive nature of the 2.0-µm region.

Active mode-locking via pump modulation in a Tm-doped fiber laser

We propose and experimentally realize a new class of actively mode-locking technique using pump modulation for rare-earth doped fiber lasers. A Tm-doped fiber laser at 2 μm is mode-locked using the proposed active mode-locking via pump modulation technique. Low-threshold continuous-wave mode-locking is achieved with a transform-limited pulse width of 4.4 ps, a spectral bandwidth of 0.9 nm, and a repetition rate of 12.9 MHz. Second-harmonic mode-locking is also demonstrated by simply driving the pump current at an appropriate frequency. More importantly, we believe that this technique can be applied to mode-lock other rare-earth doped fiber laser systems such as erbium- and ytterbium-doped fiber lasers.

Sub-50 fs pulses around 2070 nm from a synchronously-pumped, degenerate OPO

We report generation of 48 fs pulses at a center wavelength of 2070 nm using a degenerate optical parametric oscillator (OPO) synchronously-pumped with a commercially available 36-MHz, femtosecond, mode-locked, Yb-doped fiber laser. The spectral bandwidth of the output is ~137 nm, corresponding to a theoretical, transform-limited pulse width of 33 fs. The threshold of the OPO is less than 10 mW of average pump power. By tuning the cavity length, the output spectrum covers a spectral width of more than 400 nm, limited only by the bandwidth of the cavity mirrors.

Electronic coherences and vibrational wave-packets in single molecules studied with femtosecond phase-controlled spectroscopy

Employing femtosecond pulse-shaping techniques we investigate ultrafast, coherent and incoherent dynamics in single molecules at room temperature. In first experiments single molecules are excited into their purely electronic 0-0 transition by phase-locked double-pulse sequences with pulse durations of 75 fs and 20 nm spectral band width. Their femtosecond kinetics can then be understood in terms of a 2-level system and modelled with the optical Bloch equations. We find that we observe the coherence decay in single molecules, and the purely electronic dephasing times can be retrieved directly in the time domain. In addition, the Rabi-frequencies and thus the transition dipole moments of single molecules are determined from these data. Upon excitation of single molecules into a vibrational level of the electronically excited state also incoherent intra-molecular vibrational relaxation is recorded. Increasing the spectral band width of the excitation pulses to up to 120 nm (resulting in a transform-limited pulse width of 15 fs) coherent superpositions of excited state vibrational modes, i.e. vibrational wave packets, are excited. The wave-packet oscillations in the excited state potential energy surface are followed in time by a phase-controlled pump-probe scheme, which permits to record wave packet interference, and to determine the energies of vibrational modes and their coupling strengths to the electronic transition.

Passive mode locking of ceramic Nd: YAG using (7, 5) semiconducting single walled carbon nanotubes

Passive mode-locking of a ceramic Nd: YAG laser was demonstrated using highly purified single walled carbon nanotubes (SWCNTs) as the saturable absorber (SA). Poly [(9,9-dihexylfluorenyl-2,7-diyl)-co-(9,10-an-thracene)] or PFO was used to extract a purified sample of semiconducting SWCNTs that consisted of mainly (7, 5) nanotubes (∼80% of the SWCNT ensemble). The pulses had a near Fourier transform-limited pulse width of about 8.3 ps in a non-dispersion compensated setup. In addition, Z-scan investigations revealed that SWCNT SA had a saturation intensity of about 1.7 MW cm −2 with a modulation depth of about 6% and a non-saturable loss of about 5.5%.

A Wavelength-Switchable Passively Harmonically Mode-Locked Fiber Laser With Low Pumping Threshold Using Single-Walled Carbon Nanotubes

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We propose and demonstrate a wavelength-switchable passively harmonically mode-locked fiber laser with low pumping threshold using single-walled carbon nanotubes. When the pumping power of 980-nm laser diode is 60 mW, passive mode-locking (PML) of a soliton erbium-doped fiber laser at the 23rd harmonic of fundamental cavity frequency is experimentally observed at the central wavelength of 1530 nm. By only adjusting the polarization of laser in the cavity, another PML at the 13th harmonic is successfully achieved at 1554 nm. The supermode suppression of each wavelength is larger than 30 dB. The pulses at 1530 nm are characterized in detail with a repetition rate of 328.44 MHz, a transform-limited pulsewidth of 1.3 ps, and a timing jitter of 142.6 fs. </para>

Dynamics of high-power self-similar light pulses in a fiber laser with a carbon-nanotube saturable absorber

We present a numerical analysis of the dynamics of high-power self-similar light pulses in a fiber laser with a saturable absorber based on carbon nanotubes (CNTs). This analysis allows us to identify lasing regimes in a CNT-mode-locked ytterbium-fiber laser enabling the generation of self-similar linearly chirped light pulses with an energy up to 330 nJ, which can be compressed to a 210-fs transform-limited pulse width through linear chirp compensation in a pulse compressor. We show that, for typical parameters of a fiber laser, the bandwidth and the minimum transform-limited pulse width of a self-similar laser output are primarily limited by the small modulation depth of a CNT absorber.

Femtosecond mode-locked Erbium-doped fiber ring laser with intra-cavity loss controlled full L-band wavelength tunability

By using a tunable-ratio optical coupler (TROC) to adjust the wavelength dependent intra-cavity loss, a L-band mode-locked erbium-doped fiber-ring laser (ML-EDFL) is demonstrated for generating wavelength-tunable femtosecond pulses. The change of output coupling ratio introduces different intra-cavity loss and shifts the peak of mode-locked gain profile to provide continuous detuning on wavelength of the ML-EDFL. A maximum tuning range of about 40 nm (from 1565.1 to 1605.3 nm) by decreasing the output coupling ratio from 95% to 5% is obtained, corresponding to a wavelength tuning slope of 2.25 nm/dB. The ML-EDFL exhibits a super-mode suppressing ratio as high as 47 dB and a pulsewidth of <5 ps at repetition frequency of 1 GHz. Nearly transform-limited pulsewidth of 580 fs is generated by linear dispersion compressing the EDFL pulses with a 32.5m-long single-mode fiber under an output coupling ratio of 10%.

Simultaneous spatial and temporal focusing for axial scanning

We show theoretically and experimentally that simultaneous spatial and temporal focusing can scan the temporal focal plane axially by adjusting the group velocity dispersion in the excitation beam path. When the group velocity dispersion is small, the pulse width at the temporal focal plane is transform-limited, and the amount of shift depends linearly upon the dispersion. By adding a meter of large mode area fiber into the system, we demonstrate this axial scanning capability in a fiber delivery configuration. Because a transform-limited pulse width is automatically recovered at the temporal focal plane, simultaneous spatial and temporal focusing negates the need for any dispersion pre-compensation, further facilitating its integration into a fiber delivery system. A highly promising application for simultaneous spatial and temporal focusing is an axial scanning multiphoton fluorescence fiber probe without any moving parts at the distal end and without dispersion pre-compensation.

Compact fiber-delivered Cr:forsterite laser for nonlinear light microscopy

We demonstrate a compact and self-starting fiber-delivered femtosecond Cr:forsterite laser for nonlinear light microscopy. A semiconductor saturable absorber mirror provides the self-starting mechanism and maintains long-term stability in the laser cavity. Four double-chirped mirrors are employed to reduce the size of the cavity and to compensate for group velocity dispersion. Delivered by a large-mode-area photonic crystal fiber, the generated laser pulses can be compressed down to be with a nearly transform-limited pulse width with 2.2-nJ fiber-output pulse energy. Based on this fiber-delivered Cr:forsterite laser source, a compact and reliable two-photon fluorescence microscopy system can thus be realized.

Rational Harmonic Mode-Locking of Erbium-Doped Fiber Laser at 40 GHz Using a Loss-Modulated Fabry–PÉrot Laser Diode

Rational harmonic mode-locking of an Erbium-doped fiber ring laser (EDFL) at repetition frequency of 40 GHz is demonstrated by using a purely loss-modulated Fabry-Pe/spl acute/rot laser diode (FPLD) at 1 GHz. The FPLD is neither lasing nor gain-switching, which requires a threshold modulation power of 18 dBm to initiate harmonic mode-locking of the EDFL. After chirp compensation, the nearly transform-limited pulsewidth and spectral linewidth of 3 ps and 1.3 nm are obtained at repetition frequency of 40 GHz, which corresponds to a time-bandwidth product of 0.31. The EDFL gradually evolves from harmonic mode-locking to injection-locking mode as the FPLD changes from loss-modulation to gain-switching mode by increasing its dc driving current.

Widely tunable sub-30-fs pulses from a compact erbium-doped fiber source.

Coupling femtosecond light pulses from an all-fiber Er:laser system into a dispersion-shifted and highly non-linear fiber, we generate output spectra exhibiting two broadband and mutually coherent maxima. Depending on the chirp of the input pulse, the spectral separation is easily tunable over a wide range up to values exceeding 100 THz. In this way, the source provides access to an ultrabroadband wavelength interval from 1130 to 1950 nm. Because of soliton effects, the long-wave component exhibits a transform-limited pulse width of 40 fs directly after the nonlinear element. The high-frequency part propagating in the dispersive regime is recompressed to pulse durations as short as 24 fs with an optimized prism sequence.

Ultrafast pulse shaping: amplification and characterization

We demonstrate high-resolution amplified pulse shaping using an acousto-optic modulator (AOM) at a center-wavelength of 795nm. The output pulses have energy of 200mJ/pulse and a transform-limited pulsewidth of 150fs. A spectral modulation of over 40 features is achieved in a single pulse. We characterize the pulses using the STRUT (Spectrally and Temporally Resolved Upconversion Technique). Using predistortion techniques, we demonstrate that the pulses can be shaped in amplitude and phase. We create a complex pulse shape with hyperbolic secant amplitude and hyperbolic tangent frequency sweep, which is useful for applications in adiabatic rapid passage experiments.

Diffraction of an extremely short optical pulse

We provide a rigorous, closed-form mathematical model of pulsed diffraction in both time and frequency domains and interpret its anomalies. The customary continuous-wave (cw) approximation is corrected for the regime below 3 fs in pulse width, the present state of the art. The time-differentiated aspect of diffraction is linked with old theory and the conservation of energy. Convolution of the pulse with differentiated aperture edges produces traveling waves in the focal plane. Their collision near the focal point corresponds to the cw case. (This is generalized for a Gaussian beam.) Spectral sampling depicts a mode-locked laser of extremely broad spectrum, validating the nonintuitive phenomena. The square-modulus tool is validated for these pulses. Ultrashort pulses are related to data transmission rates above 100 THz. Diffraction anomalies cause confusion and loss of information in the sidelobes. Anomalous diffraction may provide new diagnostics. Diffracted energy (salient sidelobes versus continuum) can measure transform-limited pulse width.

Dynamics of a mode-locked low-pressure photolytic I* laser

A longitudinally pumped photolytic iodine laser has been reliably mode-locked over a broad range of iodide pressures (1-30 Torr). Average mode-locked pulse widths ranging from 1.4-2.5 ns were observed as a function of CF/sub 3/I iodide pressure and added buffer gas (Ar). Low-pressure data indicate an increased gain bandwidth as a result of an initial nonthermal I* atom distribution following rapid (<20 ns) photolysis of the parent molecule, CF/sub 3/I. Nearly 1.75 eV of excess pump energy is available and is deposited into the internal vibrational and rotational degrees of freedom of the large radical and into the translation of the nascent iodine atom, allowing for high iodine atom velocities in the frame of observation. A diode laser system tuned to the 3-4 transition of spin-orbit excited iodine atoms was used to observe the time-dependent behavior of line center gain along the optical axis. Preliminary data indicate an I* atom velocity relaxation on the order of 1 /spl mu/s for an iodide pressure of 1 Torr, and approximately 300 ns for a pressure of 10 Torr. At 5 Torr of CF/sub 3/I, the laser mode build up time is 200 ns, allowing access to the increased gain bandwidth and resulting in mode-locked pulse widths less than the predicted transform-limited pulse width for an inhomogeneous gas laser.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Generation of transform-limited 32-fs pulses from a self-mode-locked Ti:sapphire laser

We show that near-transform-limited 32-fs pulses can be generated directly from a self-mode-locked titanium-doped sapphire oscillator. The laser operates at a center wavelength of 813 nm with a bandwidth of 22 nm in a TEM00 mode. Correct choice of the glass for the intracavity prism pair results in improved compensation of group-velocity dispersion, which yields near-transform-limited output pulses. Compensation for group-velocity dispersion in the output coupler and autocorrelator optics is necessary in order to measure this transform-limited pulse width.