Picosecond Pulse Generation Research Articles

Contributed Review: A new synchronized source solution for coherent Raman scattering microscopy

Based on vibrational spectroscopy, coherent Raman Scattering (CRS) microscopy allows label-free imaging of biological and chemical samples with endogenous image contrast. Two-color, synchronized picosecond pulses are typically used for high spectral resolution imaging, which in turn constitutes a dramatic laser source challenge for CRS microscopy. Recently, synchronized time-lens source, inspired from ultrafast optical signal processing, has emerged as a promising laser source solution and has found application in various modalities of CRS microscopy. Time-lens is based on space-time analogy, which uses a "lens" in the time domain to compress long optical pulses or even continuous waves to ultrashort pulses, mimicking a lens in the space domain. Phase and intensity modulators driven with electrical signals are used in the time-lens source for picosecond pulse generation. As a result, the time-lens source is highly versatile and naturally compatible with modulation capabilities. More importantly, if the electrical signals used to drive the time-lens source are derived from other laser sources, such as mode-locked lasers, then synchronization between them can be realized, underlying the physics of a synchronized time-lens source. In this paper, we review recent progress on the basic principle, design of the synchronized time-lens source, and its applications to CRS microscopy of both biological and chemical samples.

Generation regimes of bidirectional hybridly mode-locked ultrashort pulse erbium-doped all-fiber ring laser with a distributed polarizer.

We report on the stable picosecond and femtosecond pulse generation from the bidirectional erbium-doped all-fiber ring laser hybridly mode-locked with a coaction of a single-walled carbon nanotube-based saturable absorber and nonlinear polarization evolution that was introduced through the insertion of the short-segment polarizing fiber. Depending on the total intracavity dispersion value, the laser emits conservative solitons, transform-limited Gaussian pulses, or highly chirped stretched pulses with almost 20 nm wide parabolic spectrum in both clockwise (CW) and counterclockwise (CCW) directions of the ring. Owing to the polarizing action in the cavity, we have demonstrated for the first time, to the best of our knowledge, an efficient tuning of soliton pulse characteristics for both CW and CCW channels via an appropriate polarization control. We believe that the bidirectional laser presented may be highly promising for gyroscopic and other dual-channel applications.

A Novel Picosecond Pulse Generation Circuit Based on SRD and NLTL.

Because of the importance of ultra-wideband (UWB) radar in various applications, short pulse generation in UWB systems has attracted a lot of attention in recent years. In order to shorten the pulse, nonlinear transmission line (NLTL) is imported, which expands the application of step recovery diode (SRD) for pulse generation. Detailed analysis and equations for this SRD and NLTL-based pulse generation are provided and verified by simulation and experimental results. Factors that could cause pulse waveform distortions are also analyzed. The generator circuit presented in this paper generates 130ps and 3.3V pulse, which can be used in UWB radar systems that require sub-nanosecond pulses.

Generation of picosecond laser pulses at 1030 nm with gigahertz range continuously tunable repetition rate.

We report on a watt range laser system generating picosecond pulses using electro-optical modulation of a 1030 nm single frequency low noise laser diode. Its repetition rate is continuously tunable between 11 and 18 GHz. Over this range, output spectra and pulse characteristics are measured and compared with a numerical simulation. Finally, amplitude and residual phase noise measurements of the source are also presented.

High-Energy Picosecond Pulse Generation by Gain Switching in Asymmetric Waveguide Structure Multiple Quantum Well Lasers

A multiple quantum well laser diode utilizing an asymmetric waveguide structure with a large equivalent spot size of ~3 μm is shown to give high energy (~1 nJ) and short (~100 ps) isolated optical pulses when injected with <;10 A and ~1-ns current pulses realized with a MOS driver. The active dimensions of the laser diode are 30 μm (stripe width) and 3 mm (cavity length), and it works in a single transversal mode at a wavelength of ~0.8 μm. Detailed investigation of the laser behavior at elevated temperatures is conducted; it is shown that at high enough injection currents, lasers of the investigated type show low temperature sensitivity. Laser diodes of this type may find use in accurate and miniaturized laser radars utilizing single photon detection in the receiver.

Electron gun for diffraction experiments off controlled molecules

A dc electron gun, generating picosecond pulses with up to 8 × 106 electrons per pulse has been developed. Its applicability for future time-resolved diffraction experiments on state- and conformer-selected laser-aligned or oriented gaseous samples was characterized. The focusing electrodes were arranged in a velocity-map imaging spectrometer configuration. This allowed us to directly measure the spatial and velocity distributions of the electron pulses emitted from the cathode. The coherence length and pulse duration of the electron beam were characterized by these measurements combined with electron trajectory simulations. Electron diffraction data from a thin aluminum foil illustrated the coherence and resolution of the electron-gun setup.

A Novel Low-Ringing Monocycle Picosecond Pulse Generator Based on Step Recovery Diode.

This paper presents a high-performance low-ringing ultra-wideband monocycle picosecond pulse generator, formed using a step recovery diode (SRD), simulated in ADS software and generated through experimentation. The pulse generator comprises three parts, a step recovery diode, a field-effect transistor and a Schottky diode, used to eliminate the positive and negative ringing of pulse. Simulated results validate the design. Measured results indicate an output waveform of 1.88 peak-to-peak amplitude and 307ps pulse duration with a minimal ringing of -22.5 dB, providing good symmetry and low level of ringing. A high degree of coordination between the simulated and measured results is achieved.

Plasmonic Dicke Effect in Ag-Nanoclusters-Doped Oxyfluoride Glasses

We report the observation of picosecond kinetics (ps) of the luminescence decay in Ag nanoclusters dispersed in oxyfluoride glass host. Calculated normal oscillator strengths for the Ag nanoclusters cannot account for such fast kinetics of luminescence, suggesting its cooperative origin. This confirmed by the concentration dependence of the intensity of picosecond component. Further analysis shows that cooperativity does not relate to conventional superradiance but is rather mediated by plasmons of dispersed Ag nanoparticles around which Ag nanoclusters coordinated. The plasmon Dicke effect may be applied for the generation of picosecond pulses across a wide range of wavelengths in these glasses.

Multiwavelength 25-GHz picosecond pulse generation with phase modulation and double-side Mamyshev reshaping.

We demonstrate a simple, robust, and cost-effective method of generating high-speed multiwavelength picosecond optical pulses. This method is based on chirp compression of phase-modulated light, followed by nonlinear pulse compression and reshaping with a double-side Mamyshev reshaper. We show that this method becomes power efficient when the repetition rate is increased to 25GHz. Wavelength-tunable optical pulses with a repetition rate of 25GHz and pulse width of ∼2 ps, which can be temporally multiplexed to 100GHz, are experimentally obtained over a large spectral range with moderate electrical and optical power. Simultaneous pulse generation on four wavelengths is demonstrated.

Modeling and analysis of polarization effects in Fourier domain mode-locked lasers.

We develop a theoretical model for Fourier domain mode-locked (FDML) lasers in a non-polarization-maintaining configuration, which is the most widely used type of FDML source. This theoretical approach is applied to analyze a widely wavelength-swept FDML setup, as used for picosecond pulse generation by temporal compression of the sweeps. We demonstrate that good agreement between simulation and experiment can only be obtained by including polarization effects due to fiber bending birefringence, polarization mode dispersion, and cross-phase modulation into the theoretical model. Notably, the polarization dynamics are shown to have a beneficial effect on the instantaneous linewidth, resulting in improved coherence and thus compressibility of the wavelength-swept FDML output.

Signal Detection and Noise Modeling of a 1-D Pulse-Based Ultra-Wideband Ranging System and Its Accuracy Assessment

In this paper, we estimate the achievable localization error in an ultra-wideband (UWB) precise positioning system. The system is based on the time difference of arrival of narrow pulses to a multitude of base-stations. A mathematical model and simulation platform are developed to describe the behavior of the UWB indoor localization system. Limits on the location accuracy as a function of the parameters of the UWB system are described. A discussion of the dominant errors is presented including picosecond pulse generator jitter, sampling clock jitter, sampling rate, and system additive white Gaussian noise. We show a simple method to calculate the total system jitter, and describe error biasing phenomenon as the tag moves, approaching one base-station and moving away from another. Design curves are provided to determine the required specifications of the various system components to achieve a certain positioning accuracy.

High-efficiency Watt-level picosecond pulse generation based on Yb:Gd3AlxGa5−xO12 crystal

A diode-pumped passively mode-locked Yb3+:Gd3Al0.5Ga4.5O12 (Yb:GAGG) laser has been investigated by using a semiconductor saturable absorber mirror (SESAM) for the first time. Pulses with duration of 1.6ps were produced at the central wavelength of 1027.8nm. At the absorbed pump power of 8.4W, the maximum average output power of 1.02W was obtained with the repetition rate of 45MHz. The corresponding single pulse energy and the peak power were calculated to be 22.7nJ and 14.2kW, respectively.

Picosecond pulse generation from a Yb:CaF2–YF3 mode-locked laser

We experimentally investigated a picosecond (ps) diode-pumped passively mode-locked (ML) laser operation of a 5 at.% Yb, 5 at.% Y:CaF2 crystal obtained by the vertical Bridgman method. Without extra negative dispersion elements, the ML laser emitted stable 1.3 ps pulses at a repetition rate of 60 MHz with 514 mW average output power under the absorbed pump power of 6.43 W. The continuous wave (CW) operation was also reported with the average output power of 1.48 W, corresponding to the optical slope efficiency of 31.3%.

On-chip picosecond pulse detection and generation using graphene photoconductive switches.

We report on the use of graphene for room temperature on-chip detection and generation of pulsed terahertz (THz) frequency radiation, exploiting the fast carrier dynamics of light-generated hot carriers, and compare our results with conventional low-temperature-grown gallium arsenide (LT-GaAs) photoconductive (PC) switches. Coupling of picosecond-duration pulses from a biased graphene PC switch into Goubau line waveguides is also demonstrated. A Drude transport model based on the transient photoconductance of graphene is used to describe the mechanism for both detection and generation of THz radiation.

Active mode-locking of mid-infrared quantum cascade lasers with short gain recovery time.

We investigate the dynamics of actively modulated mid-infrared quantum cascade lasers (QCLs) using space- and time-domain simulations of coupled density matrix and Maxwell equations with resonant tunneling current taken into account. We show that it is possible to achieve active mode locking and stable generation of picosecond pulses in high performance QCLs with a vertical laser transition and a short gain recovery time by bias modulation of a short section of a monolithic Fabry-Perot cavity. In fact, active mode locking in QCLs with a short gain recovery time turns out to be more robust to the variation of parameters as compared to previously studied lasers with a long gain recovery time. We investigate the effects of spatial hole burning and phase locking on the laser output.

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We report on a diode-pumped passively mode-locked picosecond Tm:CaGdAlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> (Tm:CALGO) laser using a semiconductor saturable absorber mirror as the mode locker. With Tm:CALGO disordered crystal as gain medium, the mode-locked laser generated pulses with pulse duration of 27 ps, repetition rate of 128.6 MHz, and maximum average power of 330 mW at 1949.5 nm. To the best of our knowledge, this is the first demonstration of a mode-locked Tm:CALGO disordered crystal laser. The continuous-wave (CW) laser performance of Tm:CALGO was also investigated, and the maximum CW power was 730 mW. Our research results show that Tm:CALGO crystal is an excellent bulk medium for picosecond pulse generation at 2-μm wavelength.

Deep-red semiconductor monolithic mode-locked lasers

A deep-red semiconductor monolithic mode-locked laser is demonstrated. Multi-section laser diodes based on an AlGaAs multi-quantum-well structure were passively mode-locked, enabling the generation of picosecond optical pulses at 752 nm, at pulse repetition rates of 19.37 GHz. An investigation of the dependence of the pulse duration as a function of reverse bias revealed a predominantly exponential decay trend of the pulse duration, varying from 10.5 ps down to 3.5 ps, which can be associated with the concomitant reduction of absorption recovery time with increasing applied field. A 30-MHz-tunability of the pulse repetition rate with bias conditions is also reported. The demonstration of such a compact, efficient and versatile ultrafast laser in this spectral region paves the way for its deployment in a wide range of applications such as biomedical microscopy, pulsed terahertz generation as well as microwave and millimeter-wave generation, with further impact on sensing, imaging and optical communications.

Compact, high-pulse-energy, high-power, picosecond master oscillator power amplifier.

We report a compact, stable, gain-switched-diode-seeded master oscillator power amplifier (MOPA), employing direct amplification via conventional Yb(3+)-doped fibers, to generate picosecond pulses with energy of 17.7 μJ and 97-W average output power (excluding amplified spontaneous emission) at 5.47-MHz repetition frequency in a diffraction-limited and single-polarization beam. A maximum peak power of 197 kW is demonstrated. Such a high-energy, high-power, MHz, picosecond MOPA is of great interest for high-throughput material processing. With 13.8-μJ pulse energy confined in the 0.87-nm 3-dB spectral bandwidth, this MOPA is also a promising source for nonlinear frequency conversion to generate high-energy pulses in other spectral regions. We have explored the pulse energy scaling until the stimulated Raman Scattering (SRS) becomes significant (i.e. spectral peak intensity exceeds 1% of that of the signal).

A 1.33 µm picosecond pulse generator based on semiconductor disk mode-locked laser and bismuth fiber amplifier.

We demonstrate that a combination of ultrafast wafer bonded semiconductor disk laser and a bismuth-doped fiber amplifier provides an attractive design for high power 1.33 µm tandem hybrid systems. Over 0.5 W of average output power was achieved at a repetition rate of 827 MHz that corresponds to a pulse energy of 0.62 nJ.

PICOSECOND OPTICAL PULSE GENERATION USING CASCADED MACH-ZEHNDER ELECTRO-OPTIC LIGHT INTENSITY MODULATORS

In this paper, the author proposes a technique for the generation of picosecond optical pulses using cascaded Mach-Zehnder electrooptic light intensity modulators. Each individual Mach-Zehnder modulator is driven by the same microwave signal. By providing proper dc bias to the modulators, the repetition frequency of the generated optical pulse can be made equal to twice the frequency of the applied microwave drive signal. By tuning the microwave drive frequency, the repetition frequency of the generated optical pulse can be varied. The normalized optical pulse intensity and pulse width of the generated pulse have been calculated. The variation of the pulse width with different system parameters is also presented. Typical pulse width of 2.52 picosecond can be achieved using a cascade of four modulators driven by 60 GHz mm-wave signal.