Showing papers in "IEEE Journal of Selected Topics in Quantum Electronics in 1996"

Semiconductor saturable absorber mirrors (SESAM's) for femtosecond to nanosecond pulse generation in solid-state lasers

Abstract: Intracavity semiconductor saturable absorber mirrors (SESAM's) offer unique and exciting possibilities for passively pulsed solid-state laser systems, extending from Q-switched pulses in the nanosecond and picosecond regime to mode-locked pulses from 10's of picoseconds to sub-10 fs. This paper reviews the design requirements of SESAM's for stable pulse generation in both the mode-locked and Q-switched regime. The combination of device structure and material parameters for SESAM's provide sufficient design freedom to choose key parameters such as recovery time, saturation intensity, and saturation fluence, in a compact structure with low insertion loss. We have been able to demonstrate, for example, passive modelocking (with no Q-switching) using an intracavity saturable absorber in solid-state lasers with long upper state lifetimes (e.g., 1-/spl mu/m neodymium transitions), Kerr lens modelocking assisted with pulsewidths as short as 6.5 fs from a Ti:sapphire laser-the shortest pulses ever produced directly out of a laser without any external pulse compression, and passive Q-switching with pulses as short as 56 ps-the shortest pulses ever produced directly from a Q-switched solid-state laser. Diode-pumping of such lasers is leading to practical, real-world ultrafast sources, and we will review results on diode-pumped Cr:LiSAF, Nd:glass, Yb:YAG, Nd:YAG, Nd:YLF, Nd:LSB, and Nd:YVO/sub 4/.

PHASAR-based WDM-devices: Principles, design and applications

Abstract: Wavelength multiplexers, demultiplexers and routers based on optical phased arrays play a key role in multiwavelength telecommunication links and networks. In this paper, a detailed description of phased-array operation and design is presented and an overview is given of the most important applications.

T-ray imaging

Abstract: The use of terahertz pulses for imaging has opened new possibilities for scientific and industrial applications in the terahertz frequency range. In this article, we describe the technology necessary to perform terahertz "T-ray" imaging, novel imaging techniques, and commercial applications of T-ray imaging.

A reliable method for extraction of material parameters in terahertz time-domain spectroscopy

Abstract: This paper introduces a novel method that allows fast and reliable extraction of material parameters in terahertz time-domain spectroscopy. This method could be applied for most materials and requires neither simplifying assumptions nor samples of different thickness for the extraction. The presented extraction procedure operates either on truncated terahertz signals when temporal windowing is possible, or on full ones otherwise. Some experimental examples covering all practical cases are given. In particular, the extraction procedure treats the tedious case of samples for which internal reflections of the terahertz pulse slightly overlap.

Construction of multiple-beam optical traps with nanometer-resolution position sensing

Abstract: We describe the design and construction of two different types of multiple-beam optical tweezers, each equipped with nanometer-resolution position detectors. Multiple optical traps can be created either by splitting a laser beam in two parts, based on its polarization, or time-sharing a single beam among several different locations. The advantages and disadvantages of optical tweezers based on either scheme are discussed, along with details of specific implementations. Various ways to detect microscopic movements of an optically trapped object are presented and compared, including designs that are relatively insensitive to absolute location of a trapped particle within the field of view. Two of many possible applications for such instruments are illustrated: the detection of molecular steps by kinesin motor molecules, and determinations of the stiffness of single microtubules.

Soliton mode-locking with saturable absorbers

Abstract: We investigate ultrashort pulse generation based on the fundamental soliton generation that is stabilized by a saturable absorber. The case of an absorber with a recovery time much longer than the pulsewidth of the generated soliton is investigated in detail. Based on soliton perturbation theory we derive equations for the soliton variables and the continuum generated in a mode-locked laser. Analytic criteria for the transition from stable to unstable soliton generation are derived. The results demonstrate the possibility of ultrashort pulse generation by a slow saturable absorber only. The theoretical results are compared with experiments. We generate pulses as short as 13 fs using only semiconductor saturable absorbers.

Three-dimensional computation of light scattering from cells

Abstract: Using the finite-difference time-domain method, three-dimensional scattering patterns are computed for cells containing multiple organelles. The scattering cross section and average cosine of the scattering angle are computed for cells as a function of volume fraction of melanin granules and mitochondria. Results show that small organelles play a significant role in light scattering from cells, and the volume fraction of organelles affects both the total amount of scattered light and the angular distribution of scattered light.

Effects of compression on soft tissue optical properties

Abstract: Tissue optical properties are necessary parameters for prescribing light dosimetry in photomedicine. In many diagnostic or therapeutic applications where optical fiber probes are used, pressure is often applied to the tissue to reduce index mismatch and increase light transmittance. In this paper, we have measured in vitro optical properties as a function of pressure with a visible-IR spectrophotometer. A spectral range of 400-1800 mm with a spectral resolution of 5 nm was used for all measurements. Skin specimens of a Hispanic donor and two Caucasian donors were obtained from the tissue bank. Bovine aorta and sclera, and porcine sclera came from a local slaughter house. Each specimen, sandwiched between microscope slides, was compressed by a spring-loaded apparatus. Then diffuse reflectance and transmittance of each sample were measured at no load and at approximately 0.1, 1, and 2 kgf/cm/sup 2/. Under compression, tissue thicknesses were reduced up to 78%. Generally speaking, the reflectance decreased while the overall transmittance increased under compression. The absorption and reduced scattering coefficients were calculated using the inverse adding doubling method. Compared with the no-load controls, there was an increase in absorption and scattering coefficients among most of the compressed specimens.

Scaling optoelectronic-VLSI circuits into the 21st century: a technology roadmap

Abstract: Technologies now exist for implementing dense surface-normal optical interconnections for silicon CMOS VLSI using hybrid integration techniques. The critical factors in determining the performance of the resulting photonic chip are the yield on the transceiver device arrays, the sensitivity and power dissipation of the receiver and transmitter circuits, and the total optical power budget available. The use of GaAs-AlGaAs multiple-quantum-well p-i-n diodes for on-chip detection and modulation is one effective means of implementing the optoelectronic transceivers. We discuss a potential roadmap for the scaling of this hybrid optoelectronic VLSI technology as CMOS linewidths shrink and the characteristics of the hybrid optoelectronic transceiver technology improve. An important general conclusion is that, unlike electrical interconnects, such dense optical interconnections directly to an electronic circuit will likely be able to scale in capacity to match the improved performance of future CMOS technology.

Plasma formation in water by picosecond and nanosecond Nd:YAG laser pulses. I. Optical breakdown at threshold and superthreshold irradiance

Abstract: We investigated plasma formation in distilled water by 30-ps and 6-ns Nd:YAG laser pulses of 1064-nm and 532-nm wavelength for focusing angles between 1.7° and 32°. We determined the optical breakdown thresholds and analyzed the plasma length achieved at superthreshold irradiance, The parameter range investigated covers the parameters used for intraocular laser surgery. The experimental results are compared to theoretical models for the calculation of breakdown thresholds and the description of plasma growth for superthreshold breakdown. We found that at λ=1064 nm the measured thresholds for both pulse durations coincide with the calculated thresholds for the generation of seed electrons by multiphoton ionization. The breakdown process is completed by avalanche ionization. The seed electron density required for breakdown is about 4×10/sup 9/ cm for the 6-ns pulses, and 1.4×10/sup 11/ cm/sup -3/ for the 30-ps pulses. No spot size dependence of the irradiance threshold for breakdown was observed. The average threshold is by a factor of 5.9 higher for 30-ps pulses (I/sub th/=4.5×1011 W/cm2) than for 6-ns pulses (Ith=0.76×10/sup 11/ W/cm/sup 2/). At angles below approximately 2°, the threshold is influenced by self-focusing effects. The breakdown thresholds at 532 nm are slightly lower than at 1064 nm. Here, multiphoton ionization contributes considerably to the generation of free electrons throughout the whole process of plasma formation. Our results for plasma formation at superthreshold energies support a "breakdown wave" mechanism of plasma growth. For picosecond pulses, the breakdown threshold can be considered to be time-invariant, but for nanosecond pulses there is probably a decrease of the threshold during the laser pulse which may be due to UV-radiation emitted from plasma created at the beginning of the pulse.

Plasma mediated ablation of biological tissues with nanosecond-to-femtosecond laser pulses: relative role of linear and nonlinear absorption

Abstract: Plasma mediated ablation of collagen gels and porcine cornea was studied at various laser pulse durations in the range of 1 ns-300 fs at 1053-nm wavelength. It was found that pulsed laser ablation of transparent and weakly absorbing gels is always mediated by plasma. On the other hand, ablation of strongly absorbing tissues is mediated by plasma in the ultrashort-pulse range only. Ablation threshold along with plasma optical breakdown threshold decreases with increasing tissue absorbance for subnanosecond pulses. In contrast, the ablation threshold was found to be practically independent of tissue linear absorption for femtosecond laser pulses. The mechanism of optical breakdown at the tissue surface was theoretically investigated. In the nanosecond range of laser pulse duration, optical breakdown proceeds via avalanche ionization initiated by heating of electrons contributed by strongly absorbing impurities at the tissue surface. In the ultrashortpulse range, optical breakdown is initiated by multiphoton ionization of the irradiated medium (six photons in case of tissue irradiated at 1053-nm wavelength), and is less sensitive to linear absorption. High-quality ablation craters with no thermal or mechanical damage to surrounding material were obtained with subpicosecond laser pulses. Experimental results suggest that subpicosecond plasma mediated ablation can be employed as a tool for precise laser microsurgery of various tissues.

Mode-locking ultrafast solid-state lasers with saturable Bragg reflectors

Abstract: We discuss mode-locking of low-gain solid-state lasers using a semiconductor saturable Bragg reflector structure. This recently developed low-loss mode-locking device consists of a single quantum well which acts as a saturable absorber incorporated into a high-reflectivity Bragg mirror. Highly stable mode-locking in solid-state lasers results from an ultrafast transient reflectivity in the device that is caused by saturation of the excitonic absorption in near-resonant conditions.

Ultrashort pulse lasers for hard tissue ablation

Abstract: To date, lasers have not succeeded in replacing mechanical tools in many hard tissue applications. Slow material removal rates and unacceptable collateral damage has prevented such a successful transition. Ultrashort pulses (<10 ps) have been shown to generate little thermal or mechanical damage. Recent developments now enable such short-pulse/high-energy laser systems to operate at high pulse repetition rates (PRRs). Using proper operating parameters, ultrashort pulse lasers (USPLs) could exceed the performance of conventional tissue processing tools and yield significant material volume removal while maintaining their minimal collateral damage advantages. As such, for the first time, USPLs offer real possibility for practical replacement of the air-turbine dental drill or other mechanical means for cutting hard tissues. In this study, the subpicosecond interaction regime was investigated and compared to nanosecond ablation by using a Titanium:Sapphire Chirped Pulse Amplifier (CPA) system with 1.05-/spl mu/m pulses of variable duration. Both 350-fs and 1-ns pulse regimes were studied. Ablation rates (ARs), ablation efficiency, and surface characteristics revealed through electron micrographs were investigated. The study characterized the interaction with a variety of hard tissue types including nail, midear bone, dentin, and enamel. With 350-fs pulses, tissue type comparison showed a remarkably similar pattern of ablation rate and surface characteristics. Negligible collateral damage and highly efficient per-pulse ablation were observed in this pulse regime. These observations should be contrasted with the 1-ns pulse ablation characteristics where strong dependence on tissue type was demonstrated and ablation efficiency was approximately an order of magnitude smaller. With efficient interaction which minimizes collateral damage, and with both cost and size of ultrashort pulse systems decreasing, the implications of this study are far-reaching for the efficient use of USPLs in several fields of medicine that currently apply traditional surgical methods.

Lanthanide-based resonance energy transfer

Abstract: Fluorescence resonance energy transfer is a powerful tool for studying nanometer-scale distances in biological macromolecules under physiological conditions. Using luminescent lanthanides instead of conventional fluorophores as donor molecules in energy transfer measurements offers many technical advantages and opens up a wide-range of new applications. Lanthanide photophysics and the instrumentation underlying these advantages are discussed. One new application, the study of conformational changes in the large protein complex actomyosin, which is responsible for muscle contraction and subcellular movement in many eucaryotes, is briefly discussed.

Design and characterization of traveling-wave electrooptic terahertz sensors

Abstract: We discuss the design considerations of traveling-wave electrooptic terahertz sensors, wherein we analyze limitations in frequency response due to velocity-mismatch in the sensor crystal. In zincblende electrooptic materials, optical group velocities are found to be able to match the phase velocity of low frequency terahertz wave at specific optical center frequencies. We observed excellent velocity-matching in ZnTe crystals within the tuning range of Ti:sapphire lasers. Terahertz pulses as short as 200 fs (full-width-at-half-maximum) have been measured using the ultra-broad-band radiation generated via terahertz optical rectification.

Nonlinear finite-element analysis of the role of dynamic changes in blood perfusion and optical properties in laser coagulation of tissue

Abstract: A nonlinear finite-element program was developed to simulate the dynamic evolution of coagulation in tissue considering temperature and damage dependence of both the optical properties and blood perfusion rate. These dynamic parameters were derived based on the Arrhenius rate process formulation of thermal damage and kinetics of vasodilation. Using this nonlinear model, we found that the region of increased blood flow that formed at the periphery of the coagulation region significantly reduces the heat penetration. Moreover, increased scattering in the near-surface region prevents light penetration into the deeper region. Therefore, if the dynamic parameters are ignored, a relatively significant overestimation of the temperature rise occurs in a deeper area resulting in an overestimation in predicted depth of coagulation. Mathematical modeling techniques that simulate laser coagulation may not provide reliable information unless they take into account these dynamic parameters.

Structural phase diagram of H x /Li 1-x NbO 3 waveguides: The correlation between optical and structural properties

Abstract: We show that proton exchanged Hx/Li1-xNbO3 single-crystalline solid solutions exhibit very complex structural chemistry, which is different from those known for powders. Seven crystallographic phases have been identified in Hx/Li1-xNbO3 layers. Correlation between the crystal structure and the ordinary and extraordinary refractive indices has been experimentally determined which allows us to explain some of the observed optical phenomena and to predict the characteristics of the great variety of proton exchanged waveguides.

Femtosecond self- and cross-phase modulation in semiconductor laser amplifiers

Abstract: We present detailed derivation of our new model for femtosecond pulse amplification in semiconductor laser amplifiers. The various dynamic nonlinear terms of gain compression and associated self-phase modulation are derived semiphenomenologically, and are discussed physically. Included are the effects of carrier depletion, carrier heating and spectral hole-burning, as well as linear and two photon absorption and the instantaneous nonlinear index. Additionally, we account for dynamically changing gain curvature and slope. We apply the theory to strong signal cross-phase-cross-gain modulation experiments with /spl sim/500 fs pulses in a broad area GaAs amplifier and show that the model accurately describes the observed complex phenomena. We also present experimental results on single beam strong signal amplification in two different quantum-well amplifiers using 150-200 fs duration pulses. For such pulse lengths, carrier heating becomes an integrating nonlinearity and its self-phase modulation is similar to that due to carrier depletion. Additionally, since the pulse spectrum is broad, the gain slope and curvature shift and narrow it. The resultant spectral distortions are very different than observed (and modeled) earlier for the /spl sim/500 fs pulses. The model is again able to correctly describe the evolution of these ultrashort pulses, indicating that it remains valid, even though pulse durations approach the intraband relaxation time.

Measurement of 10-fs laser pulses

Abstract: We report full characterization of the intensity and phase of /spl sim/10-fs optical pulses using second-harmonic-generation frequency-resolved-optical-gating (SHG FROG). We summarize the subtleties in such measurements, compare these measurements with predicted pulse shapes, and describe the implications of these measurements for the creation of even shorter pulses. We also discuss the problem of validating these measurements. Previous measurements of such short pulses using techniques such as autocorrelation have been difficult to validate because at best incomplete information is obtained and internal self-consistency checks are lacking. FROG measurements of these pulses, in contrast, can be validated, for several reasons. First, the complete pulse-shape information provided by FROG allows significantly better comparison of experimental data with theoretical models than do measurements of the autocorrelation trace of a pulse. Second, there exist internal self-consistency checks in FROG that are not present in other pulse-measurement techniques. Indeed, we show how to correct a FROG trace with systematic error using one of these checks.

980-nm-pumped Er-doped LiNbO/sub 3/ waveguide amplifiers: a comparison with 1484-nm pumping

Abstract: We have investigated by numerical modeling the relative efficacy of 980-nm and 1484-nm-pumped guided-wave optical amplifiers in Er-doped LiNbO/sub 3/. The shorter wavelength produces a larger differential gain, but the waveguide amplifier suffers from the photorefractive effect if no precautions are taken. Model calculations show that an optical gain of 2.5 dB/cm can be achieved at 80-mW 980-nm pump power, compared to 1.3 dB/cm when pumped at 1484 nm. Experimental results using 80 mW 980 nm give a gain of /spl sim/2.2 dB/cm (minus propagation loss). This gain, however, is degraded by photorefractive damage, which we find is not only due to the 980-nm pump light, but also to the green up-converted light, the latter having the larger effect. Several techniques were shown to reduce the photorefractive damage. ZnO indiffused waveguides in MgO-doped LiNbO/sub 3/ substrates proves to be the most effective. In bulk-doped Er:Mg:LiNbO/sub 3/ samples with ZnO waveguides, an optical gain of 1.6 dB/cm (minus propagation loss) was observed with 54 mW of out-coupled pump power.

Terahertz waveform synthesis via optical pulse shaping

Abstract: We describe the principles of free-space terahertz waveform synthesis by using a programmable optical pulse shaper to drive a photoconducting dipole antenna We illustrate this technique using several experimental examples, including manipulation of the amplitude and the phase of ultrafast terahertz waveforms as well as generation of ultrafast bit sequences at terahertz frequencies We present a theory which accurately predicts the shapes of the terahertz waveforms produced in our experiments In addition to the controllability of terahertz radiation, we have shown that optical pulse shaping can be used to avoid saturation of the terahertz field at high-peak power and increase generation efficiencies for terahertz radiations at selected, narrow-band frequencies

Analysis of cavitation dynamics during pulsed laser tissue ablation by optical on-line monitoring

Abstract: Flashlamp pumped mid-IR laser systems emitting in the 2-3-/spl mu/m wavelength range are widely used for various medical applications, especially for tissue ablation. Explosive evaporation is inevitably associated with this process due to the short pulse durations of these laser systems and the high absorption of tissue and water in this spectral regime. Tissue displacement and dissection occur in liquid environment as a consequence of the induced cavitation. Depending on the application these processes might enhance the tissue ablation but can also cause adverse tissue effects. The ablation dynamics were investigated by evaluating the change in reflected probe-light intensity re-emitted from the application fiber tip. The ablated cavity and the signal was correlated to fast-flash photographs of the event. Based on this reflection signal a water/tissue discrimination system is introduced which can widely support medical laser applications. In laser sclerostomy ab externo, for example, this approach can be used as a feedback system to automatically control the ablation process. With such a system, adverse effects to adjacent tissue in the anterior chamber of the eye can be minimized.

Challenges in optically interconnecting electronics

Abstract: The challenges associated with the development of free-space optically interconnected electronics are discussed. These include finding an application for which optics offers an improved performance, developing optical transceivers, and developing suitable optics and mechanics. The main focus of the paper is the discussion of some novel approaches that smart pixels offer to increase alignment tolerance.

Er-doped integrated optical devices in LiNbO/sub 3/

Abstract: The state-of-the-art of Er-doped integrated optical devices in LiNbO/sub 3/ is reviewed starting with a brief discussion of the technology of Er-indiffusion. This technique yields high-quality waveguides and allows a selective surface doping necessary to develop optical circuits of higher complexity. Doped waveguides have been used as single- and double-pass optical amplifiers for the wavelength range 1530 nm

Semiconductor optical space switches

Abstract: This paper reviews the various optical space switch structures on III-V semiconductor material. Their characteristics are discussed in the context of optical transport and switching network applications exploiting wavelength division multiplexing.

Plasma formation in water by picosecond and nanosecond Nd:YAG laser pulses. II. Transmission, scattering, and reflection

Abstract: For pt.I see ibid., vol.2, no.4, p.847-60 (1996) We investigated the transmission, scattering, and reflection of plasmas produced in water by Nd:YAG laser pulses of 6-ns and 30-ps duration. The transmission measurements comprise a large energy range at wavelengths of 1064 and 532 nm and various focusing angles between 1.7/spl deg/ and 22/spl deg/. This parameter range covers the parameters used for intraocular microsurgery, but also allows one to assess the influence of self-focusing on plasma shielding, which is only relevant at small focusing angles. We found that most of the laser light is either absorbed or transmitted; scattering and reflection amount to only a few percent of the incident laser energy. The transmission is considerably higher for picosecond pulses than for nanosecond pulses, regardless of the focusing angle. The plasma transmission increases with decreasing focusing angle. Self-focusing, which occurs at focusing angles below 2/spl deg/, leads to a further increase of transmission. The experimental results were compared with the predictions of the moving breakdown distributed shielding model. Only partial agreement could be achieved, because the model assumes a spatially and temporally constant absorption coefficient within the plasma which is not realistic. The model can, however, be used to determine the average absorption coefficient. Fits of calculated transmission curves to the experimental data at /spl theta/=22/spl deg/ yielded 900 cm/sup -1//spl les//spl alpha//spl les/1800 cm/sup -1/ nanosecond plasmas and 360 cm/sup -1//spl les//spl alpha//spl les/570 cm/sup -1/ picosecond plasmas. The efficacy of plasma-mediated intraocular laser surgery is higher with 6-ns pulses than with 30-ps pulses, because with the nanosecond pulses nearly 50% of the laser pulse energy is absorbed already at threshold, whereas it is only 8% with the picosecond pulses.

Traveling-wave photodetector design and measurements

Abstract: A simple and accurate design method for traveling-wave photodetectors is presented and illustrated by example. Fully distributed, hybrid-coplanar, traveling-wave photodetectors are designed. Linear and nonlinear responses are measured, and linear responses are compared with design predictions. Record bandwidths of 190 GHz and record bandwidth-efficiency products of 84 GHz are measured. Behavior under high-power illumination is investigated and contrasted with lumped-element photodetectors. A phenomenological model, implying fundamental bandwidth-power limitations, is used to interpret the results.

Monolithic integration of multiple-quantum-well lasers and modulators for high-speed transmission

Abstract: Monolithically integrated single frequency lasers and electroabsorption modulators are attracting considerable interest as optical sources for long-haul and high-bit-rate fiber-optic links. Their frequency chirpless nature has indeed allowed nonreturn-to-zero (NRZ) transmission beyond the chromatic dispersion limit. They also offer a great potential as soliton pulse generators. This paper discusses the many integration schemes devised for their realization with particular emphasis on the identical active layer (IAL) approach. Recent progress in their high-speed applications is reported.

Variable group-delay dispersion equalizer using lattice-form programmable optical filter on planar lightwave circuit

Abstract: The authors report, in detail, an integrated-optic variable group-delay dispersion equalizer based on a lattice-form programmable optical filter. The variable dispersion equalizer consists of an alternating cascade of symmetrical and asymmetrical Mach-Zehnder interferometers. An equalizer with nine symmetrical and eight asymmetrical interferometers is fabricated on a planar lightwave circuit and its dispersion varied step by step from -681 to +786 ps/nm in the operational frequency range of 16.3 GHz. The effectiveness of the equalizer is shown by compensating the dispersion of three different fibers with a single equalizer. The performance of the equalizer is also evaluated and examined by numerical investigations.

A three-dimensional modular adaptable grid numerical model for light propagation during laser irradiation of skin tissue

Abstract: Information regarding energy deposition during laser irradiation of structurally complex biological tissue is needed to understand and improve the results of clinical procedures. A modular adaptive geometry numerical model capable of simulating the propagation of laser light in a wide variety of multiple component tissues has been developed and tested. A material grid array is generated by assigning a value representing a tissue type to each of a large number of small voxels. The grid array is used to indicate optical properties in an existing variable step size, weighted-photon Monte Carlo algorithm that has been modified to account for voxels-to-voxels changes in optical properties. To test the model, simple geometric shapes and optical low coherence reflectometry images of rat skin have been used to create material grids consisting of epidermis, dermis, and blood. The model assumes 1-J/cm/sup 2/ irradiation of the tissue samples with a 1.0-mm diameter uniform beam at 585 nm. Computed results show good quantitative and qualitative agreement with published data. Various effects due to shading and scattering, similar to those suggested in the literature, are noted. This model provides a way to achieve more realistic representation of anatomical geometry as compared to other models, and produces accurate results.