Chirped-frequency generation in a translated-grating-type frequency-shifted feedback laser

The properties of a frequency-shifted feedback (FSF) laser using a translated grating as the frequency-shifting element are investigated. FSF operation is attained by feedback of the first-order diffracted light from a grating coupler, which is translated in a direction perpendicular to the grating normal. A diode-pumped Nd:YVO(4) gain medium is used. Chirped-frequency components are periodically generated with a chirp rate of 1.8 THz / s. The unique spectral characteristics of this device are demonstrated with a Michelson interferometer and a heterodyne experiment.     

Spectral dynamics of an all solid-state frequency-shifted feedback laser

A frequency-shifted feedback (FSF) laser has multi frequency components with simultaneous linear chirping in its oscillation spectrum even though the gain medium is homogeneously broadened. The spectral intensity and its bandwidth were analyzed by the proposed spectral dynamics model based on the regenerative amplification of the continuously chirped comb components with phase shift according to each round trip in the cavity. The spectral intensity peaks at an instantaneous frequency which depends on the net gain in the cavity and the gain bandwidth of atomic transition. The oscillation bandwidth is defined as the product of the saturation-broadened bandwidth and the total resonant modes contributing to the FSF operation.     

Femtosecond Cr(4+):YAG laser with an L-fold cavity operating at a 1.2-GHz repetition rate

Pulses of 55-fs width and 1.2-GHz repetition rate at 1.52 mum are generated from an L-fold cavity Cr(4+):YAG laser by optimization of the lasing-beam mode inside the gain medium. The pulse width is comparable with that obtained from a standard Z-fold cavity. The pulses are characterized by a second-harmonic-generation frequency-resolved optical gating method. The pulse shape deviates from a sech(2) function because of its broad shoulders. With further miniaturization it should be possible to extend the Cr(4+):YAG mode-locked operation to multigigahertz rates.     

飞秒激光脉冲的高效率放大研究

 介绍了采用多通放大的飞秒脉冲掺钛蓝宝石激光放大系统,分析讨论了提高放大效率及缩短脉冲宽度的技术方法。在仅用能量290mJ的倍频Nd:YAG激光泵浦下,得到了输出峰值功率大于1.4TW、脉宽25fs、重复频率10Hz的超短超强激光脉冲。     

Broadly tunable femtosecond parametric oscillator using a photonic crystal fiber

Pulses as short as 460 fs and a tuning range as wide as 200 nm around 1 microm have been achieved from a photonic-crystal-fiber-based parametric oscillator. The ring cavity with only 65 cm of photonic crystal fiber is synchronously pumped with a tunable passively mode-locked Yb-doped fiber laser. Widely extended tunability is achieved by using the modulation instability gain in normal dispersion as the result of high-order dispersion in the photonic crystal fiber.     

Experimental characterization of an Yb-doped fiber ring laser with frequency-shifted feedback

We present experimental characteristics of an Yb³⁺-doped fiber ring laser operating with frequency-shifted feedback (FSF) through an acousto-optic modulator (AOM) and seeded by both a stationary continuous-wave (CW) laser and spontaneous emission. We show the spectrum and output characteristics for operations with several effective gain bandwidths, as established by Fabry-Perot etalons inside the cavity. Observation using a high finesse Fabry-Perot interferometer shows that, as expected from earlier work, although the spectrum of the FSF laser without seeding is continuous, when seeded by a CW-laser the spectrum consists of a comb of discrete modes, each offset from the seed by an integer number of AOM frequency shifts. The experimental results are in excellent quantitative agreement with the theory developed earlier [L. Yatsenko, B.W. Shore, K. Bergmann, Opt. Commun. 236 (2004) 183].     

Bandwidth-limited time-synchronized frequency tunable double pulse generation in a branched picosecond Nd: glass laser

Nearly bandwidth-limited, time-synchronized, frequency tunable picosecond pulses are generated in an active and passive mode-locked laser consisting of an Nd: silicate glass and an Nd: phosphate glass branch and a common antiresonant ring. An electro-optic Q-switch, an acousto-optic modulator and a saturable absorber in the antiresonant ring are responsible for mode locking and time synchronization. Three-plate birefringent filters in the gain branches are used for spectral narrowing (pulse duration broadening) and wavelength tuning. Pulses of 40 ps duration with a synchronization jitter of 10 ps are generated. A frequency tuning range from 0 to 350 cm^-1 is achieved.     

An all-fiber frequency-shifted feedback laser for optical ranging; signal variation with distance

A recent paper [L.P. Yatsenko et al., Opt. Commun. 242 (2004) 581] provided a first-principles prediction for the optical ranging signals obtained when using a frequency-shifted feedback (FSF) laser system, seeded by a phase-modulated laser. Such a system has many useful advantages over other alternative FSF laser techniques. We report here experimental verification of that theory, specifically the variation of the amplitude modulation signal with both distance and modulation index of the seed laser. We describe the operation of an all-fiber FSF laser that uses an Er³⁺-doped active fiber as the gain medium. To improve the signal and minimize the noise we seed the FSF laser with a phase-modulated (PM) laser; the measurement of distance derives from a measurement of amplitude modulation within a narrow frequency interval. We demonstrate that the resulting system is capable of fast and precise measurements. With the bandwidth limitations of our current system we achieved an accuracy better than 0.1 mm. Although measurements based on interferometry offer the potential for much greater accuracy under carefully controlled conditions, the present method does not suffer from the presence of a material-dependent phase shift at the surface of the measured object.     

27Al field-swept and frequency-stepped NMR for sites with large quadrupole coupling constants

Spectra of nonspinning samples with large quadrupole coupling constants, 16-32 MHz, are acquired by frequency-stepping. A series of spin-echoes are acquired at arbitrary frequency increments, frequency-shifted in the time domain, and co-added as magnitude spectra. This procedure is derived from a method in use for field-swept NMR. The two methods are compared.     

Code-shifted reference transceiver for impulse radio ultra-wideband systems

In this paper, a novel type of transceiver for impulse radio ultra-wideband systems, the code-shifted reference transceiver, has been proposed to remove the delay element with an ultra wide bandwidth required by the transmitted reference transceiver. As compared to the frequency-shifted reference transceiver that removes the delay element through separating the reference and data pulse sequences with a group of analog carriers, the code-shifted reference transceiver separates the reference and data pulse sequences with a set of shifting and detection codes selected from Walsh codes. Therefore, the performance evaluations obtained from both theoretical analysis and computer simulations show that in conjunction with a much lower system complexity, the code-shifted reference transceiver can achieve a much better bit-to-error rate performance than the frequency-shifted reference transceiver can.     

Effects of asynchrony and ear of presentation on the pitch of mistuned partials in harmonic and frequency-shifted complex tones

When a partial of a periodic complex is mistuned, its change in pitch is greater than expected. Two experiments examined whether these partial-pitch shifts are related to the computation of global pitch. In experiment 1, stimuli were either harmonic or frequency-shifted (25% of F0) complexes. One partial was mistuned by +/- 4% and played with leading and lagging portions of 500 ms each, relative to the other components (1 s), in both monaural and dichotic contexts. Subjects indicated whether the mistuned partial was higher or lower in pitch when concurrent with the other components. Responses were positively correlated with the direction of mistuning in all conditions. In experiment 2, stimuli from each condition were compared with synchronous equivalents. Subjects matched a pure tone to the pitch of the mistuned partial (component 4). The results showed that partial-pitch shifts are not reduced in size by asynchrony. Similar asynchronies are known to produce a near-exclusion of a mistuned partial from the global-pitch computation. This mismatch indicates that global and partial pitch are derived from different processes. The similarity of the partial-pitch shifts observed for harmonic and frequency-shifted stimuli suggests that they arise from a grouping mechanism that is sensitive to spectral regularity.     

具有位移和绝对距离测量能力的回馈干涉系统

提出一种基于半导体激光器泵浦Nd∶YAG微片激光器的回馈干涉系统，适合于非配合目标的位移和绝对距离测量。该测量系统包括移频回馈干涉光学系统，基于泵浦调制的微片激光器调频系统和相敏检波信号处理系统3部分。介绍了移频回馈干涉的原理，给出了位移测量和绝对距离测量应用的原理和测量方法，分析了测量的分辨率。实际测量了发黑工件表面的位移和绝对距离。实验结果表明，该回馈干涉系统灵敏度高，对被测表面反射率的要求很低，兼有对非配合目标的高分辨率位移测量和绝对距离测量的能力。     

Classically determined frequency shift in the absorption coefficient

The large absorption coefficient of a dilute medium is frequency-shifted toward the blue relative to the dielectric constant. This shift is 3/4 as large as the red shift calculated by Lorentz for random or cubically symmetric placement.     

Semiconductor saturable-absorber mirror assisted Kerr-lens mode-locked Ti:sapphire laser producing pulses in the two-cycle regime

Pulses of sub-6-fs duration have been obtained from a Kerr-lens mode-locked Ti:sapphire laser at a repetition rate of 100 MHz and an average power of 300 mW. Fitting an ideal sech(2) to the autocorrelation data yields a 4.8-fs pulse duration, whereas reconstruction of the pulse amplitude profile gives 5.8 fs. The pulse spectrum covers wavelengths from above 950 nm to below 630 nm, extending into the yellow beyond the gain bandwidth of Ti:sapphire. This improvement in bandwidth has been made possible by three key ingredients: carefully designed spectral shaping of the output coupling, better suppression of the dispersion oscillation of the double-chirped mirrors, and a novel broadband semiconductor saturable-absorber mirror.     

On the theory of frequency-shifted secondary emission of light-harvesting molecular systems

The expressions are obtained for the intensity of the frequency-shifted secondary emission of a chromophore playing the role of a reaction center in the simplest model three-chromophore molecular “lightharvesting” antenna, which is constructed and oriented in space so that the incident photons coherently excite two of its chromophore pigments. The quantum-field formalism was used, which takes into account the generalized (quantum-electrodynamic) dipole-dipole, as well as radiative and nonradiative dissipative interactions between pigments and the reaction center of the antenna. The special features of the excitation spectrum of the Raman scattering line and the frequency-shifted fluorescence spectrum of the reaction center of the molecular antenna under study are discussed. A comparison of the expressions obtained for the excitation and fluorescence spectra and with the corresponding expressions obtained for a bichromophore molecular system, which differs from a three-chromophore antenna by the absence of one of the pigments, revealed the properties of the mechanism of action of light-harvesting molecular antennas that have not been found earlier. In particular, it is shown that “the light-harvesting” caused by the collective dissipative interactions of pigments with the reaction center of the antenna can substantially exceed a sum of contributions from separate pigments.     

Mechanism and elimination of spectrum filtering in a high power mode-locked fibre laser witha free output coupler

The filtering mechanism of a free output coupler mode-locked laser based on large-mode-area photonic-crystal fibre is analysed. A filtering-soliton mode-locked laser with 495~fs pulse width and 21 nJ pulse energy is achieved. Another novel cavity configuration is established to eliminate the filtering effect. Pulses, each 457~fs in width and 16.5 nJ in energy, are obtained in a soliton-like regime. Pulses, each 387~fs in width and 15.8 nJ in energy, are also generated in a stretched pulse regime and could be dechirped to 119~fs externally to the cavity.     

Generation,stabilization, and amplification of subpicosecond pulses

A sync-pumped cw dye laser system has been used to produce subpicosecond pulses. Pulses as short as 0.7 ps, assuming a single-sided exponential pulse shape, were observed. A set of experiments was performed to investigate the origin and effects of noise in the sync-pumped system. A digital and an analog feedback loop have been designed to optimize the pulse width. The noise has been lowered by 10 dB for frequencies up to 10 kHz; long-term drift is also controlled by this method. A four-stage dye laser amplifier, pumped by a Nd:YAG laser which operates at a 10-Hz repetition rate, is synchronized electronically to the dye-laser picosecond pulses. A gain of 3×10⁶ has been achieved.This work was supported by the Joint Services Electronics Program     

Use of saturable absorber dyes for self-starting operation of a self-mode-locked Ti:Al2O3 laser

Extremely stable and self-starting operation of a self-mode-locked Ti:Al₂O₃ laser can be achieved by incorporating a jet of dilute saturable absorber in the laser resonator. We have demonstrated a laser which generates pulses as short as 47 fs, with 700 mW average power near 800 nm when pumped by 7 W from an all-lines argon laser. A mixture of saturable absorber dyes starts the mode-locking process but does not restrict the tuning range of the laser. Pulses less than 100 fs in duration are generated at wavelengths between 700 nm and 900 nm with two sets of optics. Pulses of 100 fs from this laser have been compressed to 26 fs with standard fibre-prism techniques.     

Subnanosecond amplified spontaneous emission pulses by a nitrogen pumped dye laser

Pulses of 100 ps duration and peak power up to 100 kW are obtained with a dye laser pumped by an atmospheric pressure nitrogen laser of 0.5 ns duration. The shortening of the dye laser pulses is attributed to amplified spontaneous emission.     

Nonstationary chaotic oscillations in lasers with frequency-shifted feedback

The nonlinear dynamics of lasers with frequency-shifted delayed feedback are investigated. Resonant excitation of sustained relaxation oscillations by harmonic resonance is demonstrated. Self-induced switching between sustained relaxation oscillation and spiking oscillation is observed as the feedback coefficient is increased. Observed instabilities are well reproduced by numerical simulations of proposed model equations. A statistical analysis of this switching phenomenon is carried out numerically, and the results indicate that an inverse-power relation with the feedback coefficient determines the periods over which the system dwells in its relaxation-oscillation state.