Optical Pulse Width Research Articles

Modeling of Short Optical Pulse Generation Using Electroabsorption Modulators

Ultra-short optical pulses synchronized to an electrical clock are required in ultra-high bit rate soliton transmission and optical signal processing systems. Use of sinusoidally-driven electroabsorption (EA) modulators is described to generate transform-limited optical pulses. We model pulse generation in sinusoidally-driven lumped-electorode EA modulators, defining a new device figure of merit. Generated optical pulse width is calculated as a function of extinction ratio slope for applied voltages 3dB bandwidth and sample length, and calculated results are compared with those reported for various modulator structures that include bulk modulators, InGaAs/InAlAs, and InGaAsP/InGaAsP multiple quantum well (MQW) modulators. This model is useful for designing and using EA modulators in optical short pulse applications.

FORTE observations of optical emissions from lightning: Optical properties and discrimination capability

Lightning type identification of temporally coincident optical‐VHF time series event pairs collected by the Fast On‐orbit Recording of Transient Events (FORTE) satellite's photodiode detector (PDD) and VHF instrument allows for the investigation of optical properties as a function of lightning type. General trends in the peak optical irradiance and characteristic pulse widths of PDD‐VHF coincident events are studied as a function of lightning type, using previously established techniques to identify lightning type based on VHF spectogram‐power time series. While lightning type cannot be identified from optical data alone, there are several notable features in the optical record. The distribution of observed characteristic widths of PDD events has a cutoff near 200 μs, which represents a lower limit on the combination of intrinsic optical emission time and pulse broadening due to photon scattering in the intervening clouds. Events with the highest peak optical irradiances observed at FORTE are typically positive initial return strokes. Also, the median value of peak optical irradiance for cloud‐to‐ground lightning events is more than double that for in‐cloud lightning.

A simple heterodyne frequency offset locking system for pulsed lasers

As a new approach the heterodyne frequency offset locking system which is applicable to a broad range of pulsed lasers is described. A 100 MHz wide-band and all-digital frequency discriminating technique is presented and an offset locking loop in which the frequency discriminator works accordingly is designed. The result of the experiment on offset locking between a CW CO 2 laser and an electro-optically Q-switched CO 2 laser with an optical pulse width 300 ns and its repetition rate 20 kHz showed that the beat frequency stability was kept within ±2 MHz .

Ultimate performances of an all-optical sampler based on ultrafast saturable absorbers created by heavy ion irradiation

We analyze the ultimate performances of an all-optical sampler based on ultrafast bulk GaAs saturable absorbers created by ion irradiation. The ion irradiation technique provides a unique way to adapt the sampler resolution to the optical pulse width since the absorber relaxation time can be adjusted over more than three decades with the irradiation dose. Subpicosecond time resolutions with pulse contrast of /spl sim/100 are achievable by using high irradiation doses. In standard conditions, the sampler noise performances are limited by the detector noise and sampling pulse amplitude fluctuations. Picosecond laser diode pulses with an average power of less than one microwatt are already measurable. The ultimate limit is imposed by the absorber luminescence shot noise.

Spatial Resolution Enhancement in BOTDR by Spectrum Separation Method

BOTDR (Brillouin optical-fiber time domain reflectometry) is a promising technique for measuring the strain distribution along an optical fiber which can be used for diagnosing civil structures. However, there is a trade-off between the spatial and the frequency resolutions, and its practical spatial resolution is limited to about 1 m; to date the application scope has been restricted. In order to enhance the spatial resolution without decreasing the measurement accuracy, we intentionally introduce two kinds of strain within the spatial resolution length determined by the optical pulse width, which results in Brillouin gain spectrum separation. In this paper, we describe the theoretical analysis of the spectrum separation method together with experimental verification, where a spatial resolution of 0.5 m is demonstrated. We further propose a new parabolic data fitting method especially effective in the spectrum separation method.

MHzLED source for nanosecond fluorescence sensing

An inexpensive and portable pulsed light emitting diode (LED) source with variable repetition rate up to 10 MHz suitablefor time-resolved fluorescence sensing in medical,environmental and industrial applications is described. Thepocket-sized battery-driven source is pulsed by signals from asimple logic circuit, giving measured optical pulse widths of ~1.9 ns at 525 nm as recorded using time-correlatedsingle-photon counting. Pulses are presented for three LEDswith peak emissions at 525, 560 and 590 nm. Themeasurement performance is illustrated by the fluorescencelifetime and anisotropy decay of rhodamine 6G in various solvents and a fluorescence resonance energy transfer (FRET)sensor for detecting metal ions in water which uses a newmeasurand, namely the site distribution function ρ(r) ofmetal ion acceptors accumulating in an anionic porous polymerwith respect to the donor fluorophores. The ρ(r)dependence reveals a minimum fluorophore-metal ion separationof ~7 Å which explains the selectivity of such FRETsensors in discriminating against other quenching mechanisms.

Mars Orbiter Laser Altimeter: Experiment summary after the first year of global mapping of Mars

The Mars Orbiter Laser Altimeter (MOLA), an instrument on the Mars Global Surveyor spacecraft, has measured the topography, surface roughness, and 1.064‐μm reflectivity of Mars and the heights of volatile and dust clouds. This paper discusses the function of the MOLA instrument and the acquisition, processing, and correction of observations to produce global data sets. The altimeter measurements have been converted to both gridded and spherical harmonic models for the topography and shape of Mars that have vertical and radial accuracies of ∼1 m with respect to the planet's center of mass. The current global topographic grid has a resolution of 1/64° in latitude × 1/32° in longitude (1×2 km2at the equator). Reconstruction of the locations of incident laser pulses on the Martian surface appears to be at the 100‐m spatial accuracy level and results in 2 orders of magnitude improvement in the global geodetic grid of Mars. Global maps of optical pulse width indicative of 100‐m‐scale surface roughness and 1.064‐μm reflectivity with an accuracy of 5% have also been obtained.

Multiphoton confocal microscopy using a femtosecond Cr:Forsterite laser

With its output wavelength covering the infrared penetrating window of most biological tissues at 1,200-1,250 nm, the femtosecond Cr:forsterite laser shows high potential to serve as an excellent excitation source for the multiphoton fluorescence microscope. Its high output power, short optical pulse width, high stability, and low dispersion in fibers make it a perfect replacement for the currently widely used Ti:sapphire laser. In this paper, we study the capability of using a femtosecond Cr:forsterite laser in multiphoton scanning microscopy. We have performed the multiphoton excited photoluminescence spectrum measurement on several commonly used bioprobes using the 1,230 nm femtosecond pulses from a Cr:forsterite laser. Efficient fluorescence can be easily observed in these bioprobes through two-photon or three-photon excitation processes. These results will assist in the selection of dichroic beam splitter and band pass filters in a multiphoton microscopic system. We have also performed the autofluorescence spectrum measurement from chlorophylls in live leaves of the plant Arabidopsis thaliana excited by 1,230 nm femtosecond pulses from the Cr:forsterite laser. Bright luminescence from chlorophyll, centered at 673 and 728 nm, respectively, can be easily observed. Taking advantage of the bright two-photon photoluminescence from chlorophyll, we demonstrated the two-photon scanning paradermal and cross-sectional images of palisade mesophyll cells in live leaves of Arabidopsis thaliana.

Transient sum-frequency mixing with temporally tailored laser pulses

We discuss the possibility of using temporally tailored pump laser pulses to control the temporal width and shape of optical pulses generated in a process of transient sum-frequency mixing in crystals with second-order nonlinearity. Specific calculations performed in a model crystal in the case of fifth-harmonic generation are presented.

Optical pulse characteristics of sonoluminescence at low acoustic drive levels.

From a nonaqueous alkali-metal salt solution, it is possible to observe sonoluminescence (SL) at low acoustic drive levels with the ratio of the acoustic pressure amplitude to the ambient pressure being about 1. In this case, the emission has a narrowband spectral content and consists of a few flashes of light from a levitated gas bubble going through an unstable motion. A systematic statistical study of the optical pulse characteristics of this form of SL is reported here. The results support our earlier findings [Phys. Rev. E 58, R2713 (1998)], but in addition we have clearly established a variation in the optical pulse duration with certain physical parameters such as the gas thermal conductivity. Quantitatively, the SL optical pulse width is observed to vary from 10 ns to 165 ns with the most probable value being 82 ns, for experiments with krypton-saturated sodium salt ethylene glycol solution. With argon, the variation is similar to that of krypton but the most probable value is reduced to 62 ns. The range is significantly smaller with helium, being from 22 ns to 65 ns with the most probable value also being reduced to 42 ns. The observed large variation, for example with krypton, under otherwise fixed controllable experimental parameters indicates that it is an inherent property of the observed SL process, which is transient in nature. It is this feature that necessitated our statistical study. Numerical simulations of the SL process using the bubble dynamics approach of Kamath, Prosperetti, and Egolfopoulos [J. Acoust. Soc. Am. 94, 248 (1993)] suggest that a key uncontrolled parameter, namely the initial bubble radius, may be responsible for the observations. In spite of the fact that certain parameters in the numerical computations have to be fixed from a best fit to one set of experimental data, the observed overall experimental trends of optical pulse characteristics are predicted reasonably well.

Sonoluminescence: How bubbles glow

We review recent attempts to elucidate the phenomenon of sonoluminescence in terms of fundamental principles. We focus mainly on the processes which generate the light, but other relevant facts, such as the bubble dynamics, must also be considered for the understanding of the physics involved. Our emphasis is on single bubble sonoluminescence which in recent years has received much attention, but we also look at some of the excellent work on multiple bubble sonoluminescence and its spectral characteristics for clues. The weakly ionized gas models were recently studied most thoroughly and are remarkably successful when combined with a hydrodynamic bubble model, in terms of reproducing observed spectral shapes, intensities, optical pulse widths and the dependencies of these observables on the experimental parameters. Other radiation models, such as proton tunnelling radiation and the confined electron model, were not combined with hydrodynamic models and/or have freely adjustable parameters so that their relevance to sonoluminescence studies is at present less critically tested.

Dynamic characteristics of optical intersecting-waveguide modulators/switches with curved electrodes

The dynamic behavior of optical modulators/switches in the form of intersecting waveguides with curved electrodes is formulated. It is shown that the electrode curvature is very effective in achieving reflected pulses in good resemblance to the incident ones. Further, the speed of operation is found to be high and is shown to depend upon the commonly known capacitance of the device and the time required for refractive index variations to settle down; and also upon the relative pulsewidths of the optical and modulating signals and the extent of their synchronization. The intersection angle is found to influence the propagation delays of the spatial components, which are equalized by properly chosen structural parameters, to achieve unbroadened and undistorted output pulses. The flexibility in the bandwidth of even a fabricated device, through adjustment of the driving conditions, is demonstrated and the capability of such devices to handle a very wide range of optical pulsewidths is clarified.

Experiments and simulations of picosecond pulse switching and turn-on delay time in Y-Ba-Cu-O Josephson junctions

We report our studies on single-picosecond electrical pulse switching of YBa2Cu3O7-x (YBCO) grain-boundary bicrystal Josephson junctions. The test structures consisted of coplanar strip (CPS) transmission lines that were patterned in 50 nm thick YBCO films prepared on (100) MgO bicrystal substrates. Each CPS contained a 5 µm wide, 10 µm long microbridge and a 5 µm wide grain-boundary Josephson junction. A train of 100 fs wide optical pulses from a Ti:sapphire laser was used to excite the microbridge and generate ~2 ps wide electrical pulses, which were then applied to switch the junction. The junction response was studied at 20 K using a cryogenic electro-optic sampling system. We measured a 0.65 ps wide, single-flux-quantum pulse generated by the junction as well as a 0.7 ps junction turn-on delay time. Junction switching and turn-on delay time have been compared with numerical simulations.

Generation of 650-Femtosecond Optical Pulses from a Gain-Switched Laser Diode Using Second-Harmonic Generation

The frequency doubling and pulse compression of an optical pulse is discussed. The optical pulse was generated from a gain-switched laser diode, and its timing jitter was reduced to less than 210 fs by the CW-light injection technique. The pulse width and the center wavelength of the fundamental optical pulse were 4.8 ps and 1550 nm. By amplifying the fundamental pulse with two optical fiber amplifiers, the waveform of the pulse was distorted, and the pulse spectrum was spread by the Kerr effect in the amplifiers. In periodically poled lithium niobate, high-efficiency second-harmonic generation occurred with the high-peak power of the distorted fundamental pulse. The generated second-harmonic optical pulse width was 650 fs, and its waveform showed good agreement with the sech2 shape. This optical pulse is useful as a sampling pulse that can be used in electrical-optical or optical-optical sampling because its repetition rate can be easily varied.

Limit of optical pulsewidth in the gain-switched DFB semiconductor laser

We investigate the minimum optical pulsewidth that is obtainable in the gain-switched distributed-feedback semiconductor laser. The minimum obtainable optical pulsewidth is determined by either the period doubling or the saturation of modulation efficiency. The shift of the resonance frequency to the lower frequency side with increase of the modulation current brings about these phenomena. It is predicted theoretically that the minimum obtainable pulsewidth is about 0.2 times the period of the small-signal resonance. The minimum pulsewidth after compression with a dispersion compensating fiber is also discussed. Experimental measurements are in good agreement with the theory.

Spatial resolution enhancement of a Brillouin-distributed sensor using a novel signal processing method

It is known that the ultimate spatial resolution for a Brillouin-based sensor is limited by the lifetime of the phonons in the fiber that mediate the Brillouin loss process. At optical pulse widths less than 10 ns (corresponding to one meter spatial resolution) the Brillouin line width is considerably broadened, causing a severe penalty in resolving the Brillouin frequency shift. Around 5 ns the Brillouin line width is too broad to allow an accurate frequency determination. The fiber optics group at the University of New Brunswick, Canada, has recently developed an automated system for strain measurements in a distributed sensing system that uses a novel signal processing technique to measure strain at resolutions finer than the Brillouin line width limit. Strain has been resolved to 20 /spl mu//spl epsiv/ at 500 mm and to 40 /spl mu//spl epsiv/ at 250 mm.

Effect of Leakage Current on Pulse-Width Characteristic in Q-Switched Two-Section AlGaAs Multiple-Quantum-Well Semiconductor Lasers

We investigated the effect of leakage current on optical pulse width in Q-switched two-section AlGaAs multiple-quantum-well (MQW) lasers, employing numerical analysis using traveling-wave rate equations which include the effect of the carrier leakage. The analysis shows that the pulse width of Q-switched semiconductor lasers is broadened when carrier leakage current is present. This effect becomes significant and is one of the main reasons for the pulse-width broadening for short cavity. These numerical results agree with previously obtained experimental results. [1]

Mode-locked ring laser with output pulse width of 0.4 ps

The output pulse width of a mode-locked ring laser composed of an erbium-doped fiber amplifier, Mach-Zehnder optical modulator, and optical band-pass filter depends largely on the repetition frequency and the wavelength characteristics of these optical circuit elements. In previous experiments, the output pulse width was in the order of 5 ps at a repetition frequency of 5 GHz. The principal reason was that the narrow passage band of the optical circuit elements made it extremely difficult to generate an ultra-short optical pulse. Consequently, we examined how to narrow the optical pulse width by flattening the wavelength characteristics of these optical circuit elements. Furthermore, we drove the optical modulator in the cavity using a frequency multiplier to operate at an effectively higher frequency By widening the wavelength passage band of all the devices in the optical circuit, we achieved an output pulse width of 0.4 ps at a repetition frequency of 5 GHz; the pulse peak power was more than +23 dBm, and the time-bandwidth product was 0.34. We successfully tested an ultra-short optical pulse source with an output pulse width of 0.4 ps with no external pulse compression using a mode-locked ring laser.

Method of Ultrashort Optical Pulse Measurement.

This paper describes the principle of ultra-short optical pulse measurement including novel measuremet methods that have recently been developed. The SHG auto-correlation method is most widely used to estimate the short optical pulse width. However, two-photon absorption in LED's and photodiodes has been applied to generate the nonlinear correlation signal instead of SHG, and has revealed remarkable performance in the short pulse measurement. On the other hand, the cross-correlation method such as the optical sampling method has been introduced into the measurement of transient response of optical devices and the pulse pattern measurement in ultra-high speed optical transmission systems. The reconstruction of the waveform and phase from the measured correlation signal is also an important problem. This paper deals with the FROG (Frequency Resolved Optical Gating) developed for this purpose.

Passive modelocking in semiconductor lasers with monolithically integrated passive waveguides

A low-loss passive waveguide, fabricated by quantum well intermixing (QWI), has been integrated with an active region to form a modelocked semiconductor laser with an extended passive cavity (ECL). The 4 mm-long device has a threshold current of 18 mA and, modelocking at 9.3 GHz, generates 3.5 ps wide optical pulses with jitter levels as low as 9.3 ps (1 kHz–1 MHz). The variation of the modelocking frequency with the biasing conditions of such devices is also measured and a tuning range of ~1% of the modelocking frequency is observed.