Ultralow-jitter passive timing stabilization of a mode-locked Er-doped fiber laser by injection of an optical pulse train

The pulse timing of a mode-locked Er-doped fiber laser was stabilized to a reference pulse train from a Cr:forsterite mode-locked laser by all-optical passive synchronization scheme. The reference pulses were injected into a ring cavity of the fiber laser by using a 1.3-1.5 mum wavelength-division multiplexer. The spectral shift induced by cross-phase modulation between copropagating two-color pulses realizes self-synchronization due to intracavity group-delay dispersion. The rms integration of timing jitter between the fiber laser pulse and the reference pulse was 3.7 fs in a Fourier frequency range from 1 Hz to 100 kHz.     

Phase-locked ultrashort pulse trains at separate and independently tunable wavelengths

The generation and characterization of femtosecond pulses at three independently tunable visible wavelengths is reported. Selected spectral portions of a common continuum generated in sapphire are amplified in noncollinear optical parametric amplifiers. The phase relation of the pulse trains is analyzed with a nonlinear interferometer based on coherent anti-Stokes Raman scattering and is found to be locked to better than 250 mrad rms. Small spectral shifts of the pulses lead to interference behavior that is consistent with 1 kHz frequency combs.     

High-conversion-efficiency and tunable phase-stabilized infrared parametric laser source

High-conversion-efficiency and tunable self-phase-stabilized infrared laser pulses have been generated from a two-stage optical parametric amplifier. With a 1 kHz repetition rate 800 nm pump source, the output idler pulses are tunable from 1.2 to 2.4 μm, and the maximum output average power of the idler pulses is >2 W with the total 7.4 W pump power, and the maximum parametric conversion efficiency in the final optical parametric amplifier is near 60%. Due to the differential frequency process, the output idler pulses is self-phase-stabilized, the phase fluctuation can reach 0.374 rad (rms).     

Timing-jitter reduction in a fiber laser mode locked by an input bit stream

We present analytical and numerical results for the amount of timing-jitter reduction obtained in an all-optical clock-recovery device in which a data stream is used to mode lock a fiber laser through cross-phase modulation. This situation is analyzed when the laser pulses are close to optical solitons, so the effect of the data stream on the laser pulses is predicted from soliton perturbation theory. In our numerical simulations, both periodic input and pseudorandom input data streams are considered. The results show that for both types of data the rms timing jitter in the recovered clock is reduced significantly from that present in the input data.     

Quantitative measures of air-gun pulses recorded on sperm whales (Physeter macrocephalus) using acoustic tags during controlled exposure experiments

The widespread use of powerful, low-frequency air-gun pulses for seismic seabed exploration has raised concern about their potential negative effects on marine wildlife. Here, we quantify the sound exposure levels recorded on acoustic tags attached to eight sperm whales at ranges between 1.4 and 12.6 km from controlled air-gun array sources operated in the Gulf of Mexico. Due to multipath propagation, the animals were exposed to multiple sound pulses during each firing of the array with received levels of analyzed pulses falling between 131-167 dB re. 1 microPa (pp) [111-147 dB re. 1 microPa (rms) and 100-135 dB re. 1 microPa2 s] after compensation for hearing sensitivity using the M-weighting. Received levels varied widely with range and depth of the exposed animal precluding reliable estimation of exposure zones based on simple geometric spreading laws. When whales were close to the surface, the first arrivals of air-gun pulses contained most energy between 0.3 and 3 kHz, a frequency range well beyond the normal frequencies of interest in seismic exploration. Therefore air-gun arrays can generate significant sound energy at frequencies many octaves higher than the frequencies of interest for seismic exploration, which increases concern of the potential impact on odontocetes with poor low frequency hearing.     

Tunable phase-stabilized infrared parametric laser source

This paper reports that the tunable self-phase-stabilized infrared laser pulses have been generated from a two-stage optical parametric amplifier. With an 800 nm pump source, the output idler pulses are tunable from 1.3 μm to 2.3 μm, and the maximum output energy of the idler pulses is higher than 1 mJ at 1.6 μm by using 6 mJ pump laser. A carrier-envelope phase fluctuation of ～ 0.15 rad (rms) for the idler pulses is measured for longer than one hour by using a home build f-to-2f interferometer.     

Description of pulse propagation in a dispersive medium by use of a pulse quality factor

We investigated how the duration of short laser pulses evolves in a dispersive material, using rms widths and a propagation law based on a pulse quality factor. Experiments were carried out with femtosecond pulses (10 to 25fs at the temporal waist) propagating in bulk fused silica. Excellent agreement was found between theory and experiment. This approach does not require complete characterization of laser pulses and eliminates the need for any assumption regarding the interpretation of autocorrelation traces. The method is of general validity, and it can be applied to pulses of arbitrary shape.     

双光路测量红外飞秒激光脉冲的载波包络相位稳定性

描述了利用双光路自参考技术测量红外飞秒脉冲载波包络相移的方法,并通过建立的红外飞秒脉冲载波包络相移测量装置,实验测量了自主搭建的可调谐光学参量放大系统输出的红外飞秒激光脉冲的载波包络相移.对于1.6 μm的激光脉冲,测量得到在100 s内其相位抖动为115 mrad（rms）.实验结果表明双光路法具有易于调节、测量方便、应用性强等优点. 关键词： 飞秒激光测量 自参考技术 双光路 载波包络相位     

Timing noise measurement of 320 GHz optical pulses using an improved optoelectronic harmonic mixer

An improved optoelectronic harmonic mixer (OEHM) has been employed for measuring the timing noise of 320 GHz optical pulses that are generated from a 160 GHz mode-locked laser diode by the temporal Talbot effect. The OEHM makes use of a low-drive voltage LiNbO3 modulator and a W-band unitraveling carrier photodiode for converting the 320 GHz pulse intensity into a low-frequency electrical signal. The time domain demodulation technique has been used for the precise evaluation of phase noise power spectral density. The rms timing jitter has been estimated to be 311 fs for the 10 Hz-18.6 MHz bandwidth.     

Frequency Lock of an Optically Pumped FIR Ring Laser at 803 and 1626 GHz

We report an automatic frequency control (AFC) for an optically pumped far infrared (FIR) ring laser applicable for high resolution THz sideband spectroscopy by mixing a fraction of the laser power and a harmonic of a phase-locked synthesizer on a planar Schottky diode. We achieve a relative frequency accuracy of about 0.5 kHz rms at 803 GHz ( ¹⁵ NH ₃ ) and about 1 kHz rms at 1626.6 GHz (CH ₂ F ₂ ) over hours of lock time. The absolute frequency accuracy is estimated to be about 5 kHz at 1626.6 GHz.     

Femtosecond laser pulse energy self-stabilization

A photoconductive switch driving a Pockels cell ensures energy self-stabilization of ultra-fast laser pulses from single shot to hundreds of kHz. In this kind of stabilization, the pulse corrects itself. In a first experiment, the self-stabilization of a 20-Hz, 800-nm femtosecond Chirped Pulse Amplification laser shows a reduction of the energy fluctuations from 7% rms to 0.64% rms. PACS 42.60.Lh; 42.60.Mi; 42.65.Re     

High-energy picosecond Nd:YVO4 slab amplifier for OPCPA pumping

We demonstrate 12-ps pulses with up to 0.6-mJ pulse energy at repetition rates of 50 kHz and 100 kHz from a Nd:YVO₄ slab amplifier built in a simple four-pass configuration. Excellent noise performance with pulse energy fluctuations below 0.8% rms has been achieved by using 10-μJ seed pulses from a highly stable industrial laser system and moderate gain (30–46) in the slab amplifier.     

1-mJ, sub-5-fs carrier–envelope phase-locked pulses

We report the routine generation of sub-5-fs laser pulses with 1-mJ energy and stable carrier–envelope phase at 1-kHz repetition rate, obtained by compressing the multi-mJ output from a phase-locked Ti:sapphire amplifier in a rare-gas-filled hollow fiber. The dual-stage amplifier features a hybrid transmission grating/chirped mirror compressor providing 2.2-mJ, 26-fs pulses at 1 kHz with standard phase deviation of 190 mrad rms. We demonstrate hour-long phase stability without feedback control of grating position or rigorous control of the laser environment, simply by using small pulse stretching factors in the amplifier, which minimize the beam pathway in the compressor. The amplifier also integrates a versatile AOPDF (acousto-optic programmable dispersive filter) for closed-loop spectral phase optimization. The various factors influencing the overall phase stability of the system are discussed in detail. Using the optimized output, 1-mJ, 4.5-fs pulses are generated by seeding the neon gas filled hollow fiber with a circularly polarized input beam. A standard phase deviation of 230 mrad after the HCF is obtained by direct f-to-2f detection and slow-loop feedback to the oscillator locking electronics without any additional spectral broadening.     

Generation of 0.1 THz coherent synchrotron radiation with compact S-band linac at AIST

The 0.1 THz coherent synchrotron radiation (CSR) was successfully generated in the 90° bending magnet of the compact S-band linac with the achromatic arc section using the ultra-short electron bunch which has the energy of 40 MeV, the bunch charge of about 1nc and the bunch length less than 1 ps (rms). The electron bunch compression of 1 nC electron bunch was achieved less than 1 ps (rms) by controlling the Q-magnets in the achromatic arc section as the bunch length was measured by the rms bunch length monitor.     

Multi-mJ carrier envelope phase stabilized few-cycle pulses generated by a tabletop laser system

A compact system for the generation of few-cycle multi-mJ Carrier Envelope Phase (CEP) stabilized pulses is presented. At the output 1.9?mJ, 5.7?fs pulses were achieved after hollow fiber compression (HFC) of 5?mJ, 25?fs circularly-polarized pulses from a Ti:sapphire multipass chirped pulse amplifier (CPA). Polarization control of the generated pulses was done using all reflective phase retarders which can be nearly arbitrarily scaled for increasing energies. The CEP noise from the amplifier system is shown to be 190?mrad rms over a period of more than 7?hours. The full system, i.e., oscillator, amplifier, CEP stabilization, and HFC is compact enough to fit on a standard optical table.     

Noise characterization of an all-solid-state mirror-dispersion-controlled 10-fs Ti:sapphire laser

We characterized the phase and amplitude noise of a mirror-dispersion-controlled 10-fs Ti:sapphire laser pumped by a frequency-doubled cw diode-pumped Nd:YVO₄ laser and compared with these of the Ti:sapphire laser pumped by an Ar-ion laser. The rms timing jitters and rms amplitude noise for the all-solid-state and Ar-ion laser pumped Ti:sapphire lasers are calculated to be 0.31 ps rms and 0.71 ps rms and 0.15% rms and 0.32% rms, in the frequency range from 20 kHz to 400 kHz, respectively. The phase and amplitude noise characteristics of the Ti:sapphire laser were greatly improved by using the diode-pumped solid state laser as a pump source.     

Characteristics of biosonar signals from the northern bottlenose whale, Hyperoodon ampullatus

The biosonar pulses from free-ranging northern bottlenose whales (Hyperoodon ampullatus) were recorded with a linear hydrophone array. Signals fulfilling criteria for being recorded close to the acoustic axis of the animal (a total of 10 clicks) had a frequency upsweep from 20 to 55 kHz and durations of 207 to 377 μs (measured as the time interval containing 95% of the signal energy). The source level of these signals, denoted pulses, was 175-202 dB re 1 μPa rms at 1 m. The pulses had a directionality index of at least 18 dB. Interpulse intervals ranged from 73 to 949 ms (N?=?856). Signals of higher repetition rates had interclick intervals of 5.8-13.1 ms (two sequences, made up of 59 and 410 clicks, respectively). These signals, denoted clicks, had a shorter duration (43-200 μs) and did not have the frequency upsweep characterizing the pulses of low repetition rates. The data show that the northern bottlenose whale emits signals similar to three other species of beaked whale. These signals are distinct from the three other types of biosonar signals of toothed whales. It remains unclear why the signals show this grouping, and what consequences it has on echolocation performance.     

A comparison of 2D and 3D PIV measurements in an oblique jet

A stereo PIV (SPIV) acquisition and analysis system was developed to measure three velocity components in planar flow fields. The analysis software is based on a third order mapping function method. The system was calibrated by imaging a square grid in three measurement planes with two Kodak Megaplus cameras oriented at 30 to the bisector between them. The camera images were dewarped into real coordinates by employing a set of transform matrices computed for each calibration plane. Bias and rms errors were determined by comparing displacements measured directly with displacements estimated from the dewarping and recombination algorithm. The bias errors in the directions parallel with the measurement plane were negligible while the bias in thez direction was about 0.6 pixel. The rms errors, 0.2–0.3 pixels, were largest in thez direction. These errors were thought to result from limitations in the calibration method. The SPIV system was tested in a two-dimensional oblique jet with Reynolds number of 1800. The three dimensional results were taken in a vertical (x, y) plane parallel with the jet span. The SPIV results were compared with LDV data and two-dimensional PIV data obtained in a vertical (y, z) plane of the same jet. The SPIV measurements yielded accurate values for the in-plane mean and rms velocity components. The measured out-of-plane mean component was underestimated due to the bias error mentioned above. The rms component was accurate in part of the field but overestimated in another part due to local variations in rms error. It is expected that in the future, the out-of-plane errors can be minimized by improving the calibration and transformation procedures.     

高亮度虚火花电子束发射度的测量

介绍了对一台脉冲线加速器驱动虚火花放电装置产的电子束发射度的测量工作。在十隙虚火花放电室中育以１５Ｐａ的氮气，产生能量为约２００ｋｅＶ，束流２０００Ａ，直径为１ｍｍ和高亮度电子束。在距阳极５ｃｍ处测得电子束的均方根发射度εｒｍｓ≈４８ｍｍ．ｍｒａｄ，规一化发射率εｎ≈４７ｍｍ．ｍｒａｄ。     

Backside etching of fused silica with Nd:YAG laser

The laser-induced backside etching of fused silica with gallium as highly absorbing backside absorber using pulsed infrared Nd:YAG laser radiation is demonstrated for the first time. The influence of the laser fluence, the pulse number, and the pulse length on the etch rate and the etched surface topography was studied. The comparable high threshold fluences of about 3 and 7 J/cm² for 18 and 73 ns pulses, respectively, are caused by the high reflectivity of the fused silica-gallium interface and the high thermal conductivity of gallium. For the 18 and 73 ns long pulses the etch rate rises almost linearly with the laser fluence and reaches a value of 350 and 300 nm/pulse at a laser fluence of about 12 and 28 J/cm², respectively. Incubation processes are almost absent because etching is already observed with the first laser pulse at all etch conditions and the etch rate is constant up to 30 pulses.The etched grooves are Gaussian-curved and show well-defined edges and a smooth bottom. The roughness measured by interference microscopy was 1.5 nm rms at an etch depth of 0.6 μm. The laser-induced backside etching with gallium is a promising approach for the industrial application of the backside etching technique with IR Nd:YAG laser.