IEEE Journal of Quantum Electronics

We report a novel approach for increasing the output power in passively mode locked semiconductor lasers. Our approach uses epitaxial structures with an optical trap in the bottom cladding that enlarges the vertical mode size to scale the pulse saturation energy. With this approach we demonstrate a very high peak power of 9.8 W per facet, at a repetition rate of 6.8 GHz and with pulse duration of 0.71 ps. In particular, we compare two GaAs/AlGaAs epilayer designs, a double quantum well design operating at 830 nm and a single quantum well design operating at 795 nm, with vertical mode sizes of 0.5 and 0.75 μm, respectively. We show that a larger mode size not only shifts the mode locking regime of operation towards higher powers, but also produces other improvements in respect of two main failure mechanisms that limit the output power: the catastrophic optical mirror damage and the catastrophic optical saturable absorber damage. For the 830 nm material structure, we also investigate the effect of non-absorbing mirrors on output power and mode locked operation of colliding pulse mode locked lasers.

We report a new deconvolution method for fluorescence lifetime imaging microscopy (FLIM) based on the Laguerre expansion technique. The performance of this method was tested on synthetic and real FLIM images. The following interesting properties of this technique were demonstrated. 1) The fluorescence intensity decay can be estimated simultaneously for all pixels, without a priori assumption of the decay functional form. 2) The computation speed is extremely fast, performing at least two orders of magnitude faster than current algorithms. 3) The estimated maps of Laguerre expansion coefficients provide a new domain for representing FLIM information. 4) The number of images required for the analysis is relatively small, allowing reduction of the acquisition time. These findings indicate that the developed Laguerre expansion technique for FLIM analysis represents a robust and extremely fast deconvolution method that enables practical applications of FLIM in medicine, biology, biochemistry, and chemistry.

The mode-locking of dissipative soliton fiber lasers using large mode area fiber supporting multiple transverse modes is studied experimentally and theoretically. The averaged mode-locking dynamics in a multi-mode fiber are studied using a distributed model. The co-propagation of multiple transverse modes is governed by a system of coupled Ginzburg-Landau equations. Simulations show that stable and robust mode-locked pulses can be produced. However, the mode-locking can be destabilized by excessive higher-order mode content. Experiments using large core step-index fiber, photonic crystal fiber, and chirally-coupled core fiber show that mode-locking can be significantly disturbed in the presence of higher-order modes, resulting in lower maximum single-pulse energies. In practice, spatial mode content must be carefully controlled to achieve full pulse energy scaling. This paper demonstrates that mode-locking performance is very sensitive to the presence of multiple waveguide modes when compared to systems such as amplifiers and continuous-wave lasers.

Intravascular photoacoustic (IVPA) imaging is a catheter-based, minimally invasive, imaging modality capable of providing high-resolution optical absorption map of the arterial wall. Integrated with intravascular ultrasound (IVUS) imaging, combined IVPA and IVUS imaging can be used to detect and characterize atherosclerotic plaques building up in the inner lining of an artery. In this paper, we present and discuss various representative applications of combined IVPA/IVUS imaging of atherosclerosis, including assessment of the composition of atherosclerotic plaques, imaging of macrophages within the plaques, and molecular imaging of biomarkers associated with formation and development of plaques. In addition, imaging of coronary artery stents using IVPA and IVUS imaging is demonstrated. Furthermore, the design of an integrated IVUS/IVPA imaging catheter needed for in vivo clinical applications is discussed.

Two types of spontaneous phase locking of the TEM 00q modes at 0.63μ have been observed for a mixed isotope tube (Ne²⁰, Ne²⁰). In both cases the mode locked laser output consisted of pulses which were less than 1 ns in duration. The pulse repetition frequencies were c/2L and c/L for the two types. Adjacent mode competition appears to play a role in determining which type occurs. The persistence of the self-locking was adversely affected by simultaneous 3.39μ oscillation. The mode power spectra for the self-locked configuration revealed a dip close to the center of the gain curve. The mode power spectra for the self-locked and the free-running situations were shifted to the high-frequency side of the gain profile, and for a mixed isotope tube this can be accounted for by an asymmetric gain curve. For a pure isotope tube (Ne²⁰) only one type of self-locking was observed, and the power spectra for the self-locked and free-running configurations were shifted to the low-frequency side of the gain curve. On the basis of the experiments performed, it proved possible to calculate the magnitude of the third-order nonlinear susceptibility of the active medium at 0.63μ which, it is believed, was responsible for the spontaneous mode locking. The possibility of employing nonlinear crystals as passive mode locking devices was examined and it was found that the effect required for locking for a piece of deuterated KDP 3 cm in length and placed within the optical cavity was ∼ six orders of magnitude too small.

We have developed a frequency stabilization scheme for CO₂ lasers using only external modulation via an electrooptic modulator (EOM). One of the two laser sidebands which are generated by the EOM and frequency-modulated is set in resonance with a Fabry-Perot cavity, itself filled with OsO₄ as an absorber. The saturation signal of an OsO₄ line detected in transmission of the Fabry-Perot cavity is used for stabilization. We obtained a stability of 0.1 Hz (Δν/ν=3.5 10^-15) on a 100-s time scale, and a reproducibility up to 10 Hz with the strongest OsO₄ reference lines. These results largely improve the performance of our previous setup for which modulation was applied through piezoelectric transducers. Further, the stabilized laser is not frequency-modulated and is easily tunable

CsB₃O₅ has been found to be phase matchable for SFG down to 0.185 μm by mixing the fifth harmonic of a Nd:YAG laser at 1.0642 μm and the output of a KTP parametric oscillator pumped by the second harmonic of the same Nd:YAG laser at 20.0°C. Improved Sellmeier's equations and nonlinear optical constant of this crystal are reported

An extremely high output power has been obtained with a new structure laser named the buried twin-ridge substrate (BTRS) laser. The very thin active layer formed on a ridged substrate permitted high power output increasing the catastrophic damage level. The buried stripe formed with a blocking layer remarkably improved the current confinement lowering the threshold current. A multilayer coating technique was applied to both facets to increase the front facet output. Fundamental transverse mode is achieved at more than 100 mW in CW with an uncoated laser while the maximum output power attained is as high as 200 mW in CW operation with a multicoated laser.

Peak powers of 0.2 GW were obtained in 12-J pulses 60-ns full width at half maximum (FWHM) from a room-temperature atmospheric-pressure mixture of H 2 and F 2 initiated with an electron beam. The chemical efficiency was 0.25 percent and the electrical efficiency was ∼100 percent.

LiB₃O₅ has been found to be phase matchable for sum-frequency generation down to 0.2325 μm at 20.0°C by mixing the Nd:YAG laser wavelength at 1.0642 μm with the second harmonic of a visible dye laser. Sellmeier's equations, which are highly accurate from 0.22 to 1.32 μm, are reported. It is demonstrated that a much shorter wavelength of 0.218 μm could be generated in this compound by mixing the fourth harmonic of a Nd:YAG laser with the idler of the KNbO₃ parametric oscillator tuned to 1.208 μm

A report on the efficient generation of highly stable 1.7-W average power UV pulses at 0.266 μm in BeSO₄·4H₂O is presented. BeSO₄·4H₂O has been found to be a superior material for high peak-power and high average-power UV generation at 0.266 μm. A Gaussian-like beam having an average power of 1.7 W was generated without damage to the crystal at room temperature

InGaN-GaN represents an important heterostructure with applications in electronics and optoelectronics. It also offers a system where we can study the effects of interface roughness, alloy clustering, and the piezoelectric effect. In the paper, we examine how these factors influence the photoluminescence and excitation photoluminescence in InGaN-GaN quantum wells. We examine the Stokes shift as a function of the excitation level and doping and relate the values to the piezoelectric effect and disorder in the system. Detailed comparisons are made with experimental results

Avalanche multiplication and excess noise have been measured on a series of Al_xGa_1-xAs-GaAs and GaAs-Al_xGa_1-xAs (x=0.3,0.45, and 0.6) single heterojunction p⁺-i-n⁺ diodes. In some devices excess noise is lower than in equivalent homojunction devices with avalanche regions composed of either of the constituent materials, the heterojunction with x=0.3 showing the greatest improvement. Excess noise deteriorates with higher values of x because of the associated increase in hole ionization in the Al_xGa_1-xAs layer. It also depends critically upon the carrier injection conditions and Monte Carlo simulations show that this dependence results from the variation in the degree of noisy feedback processes on the position of the injected carriers.

Extreme ultraviolet lithography is a leading candidate for volume production of nanoelectronics at the 32-nm node and beyond. In order to ensure adequate maturity of the technology by the start date for the 32-nm node, advanced development tools are required today with numerical apertures of 0.25 or larger. In order to meet these development needs, a microexposure tool based on SEMATECH's 0.3-numerical aperture microfield optic has been developed and implemented at Lawrence Berkeley National Laboratory, Berkeley, CA. Here we describe the Berkeley exposure tool in detail, discuss its characterization, and summarize printing results obtained over the past year. Limited by the availability of ultrahigh resolution chemically amplified resists, present resolving capabilities limits are approximately 32 nm for equal lines and spaces and 28 nm for semi-isolated lines.

Pulsed electrical discharges in pure nitrogen have produced laser oscillation on an ultraviolet band of the nitrogen triplet system not previously reported above liquid-nitrogen temperature.

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The glass development laser (GDL) single beam 0.35 μm laser irradiation facility is now operational at the Laboratory for Laser Energetics of the University of Rochester. The system is based on a frequency tripled Nd:phosphate glass laser and is capable of generating in excess of 200 GW in 100 ps and 54 J in 0.5 ns pulses at 0.35 μm. A discussion of the system design, performance, and applications is presented in this paper. Copyright © 1981 by The Institute of Electrical and Electronics Engineers, Inc.

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In this paper, the intervalence subband optical transitions in p-doped In_0.49Ga_0.51P-GaAs quantum well structures are theoretically investigated. The intervalence subband optical transitions are modelled by the multiband effective mass equations incorporating the unitary transformation numerical method. The present formalism is based on the k&oarr;·P&oarr; perturbation theory as done to date but contains two significant improvements: 1) a more efficient treatment of band structures, optical matrix elements, and absorption coefficients; and 2) the avoidance of zero-order Bloch function approximation for calculating the intervalence subband optical matrix elements and absorption spectra in favour of correcting the first-order perturbation theory in order to take the remote band effects into account. Both of the requirements, especially the latter, play a very important role in gaining qualitative insight and obtaining quantitative calculation of optical selection rules. A systematical study of the subband structures, intervalence subband optical matrix elements, and absorption spectra is made for p-doped In_0.49Ga _0.51P-GaAs quantum wells, and a design guideline for near 10 μm infrared absorption is also discussed

Error rate characteristics of various digital optical modulation-demodulation schemes are studied. The main concern is whether we can improve receiving power levels to achieve a prescribed error rate by employing a coherent optical transmission system in place of the presently available amplitude-shift-keyed (ASK) baseband direct detection system. The receiving power level reduction in various modulation-demodulation schemes is calculated by taking into account the optical carrier wavelength, data rate, photodetector performance, local oscillator power level, and number of levels in multilevel codes. The phase-shift-keyed (PSK) homodyne detection system requires the least receiving power. The improvement in the receiving power level compared to the conventional ASK baseband direct detection system is expected to be 16-22 dB at the carrier wavelength of lambda_{c} = 0.5-3 mu m, 31-36 dB at lambda_{c} = 3-5 mu m, and 35-40 dB at lambda_{c} = 5-10 mu m.

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The interaction between an intense focused beam of optical photons and matter in gaseous form at low pressure (10^-3torr) brings into play some strongly nonlinear processes. These multiphoton processes occur through the simultaneous absoprtion of several quanta by an atom that may be thus either excited or ionized. The orders of nonlinearity of the interaction of a multimode Q -switched laser beam with rare gas atoms were measured with laser intensities up to 10¹³W ċcm^-2at 1.06μ and up to 10¹²W ċcm^-2at 0.53μ. The energy of the number of quanta corresponding to the order of nonlinearity is always close to the energy of an atomic level. The results seem to emphasize the particularly important role performed by bound states during the ionization process. Thus a two-stage ionization process seems far more probable than a single direct transition between the ground state and the continuum spectrum. Experimental values of multiphoton ionization probabilities are also given after having precisely determined the spatiotemporal intensity distribution function.

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Using external second-harmonic generation (SHG) in Ba 2 NaNb 5 O 15 , a second-harmonic (SH) power of 550 mW, mode locked at 453 MHz has been demonstrated. Short- and long-term stabilities of better than ±5 percent peak-to-peak variation have been obtained at up to the 400- mW level. The maximum stable SH power was limited by absorption of the SH by the Ba 2 NaNb 5 O 15 SHG.

A report is presented on the parametric oscillation in LiB₃O₅ pumped by a frequency-doubled Nd:YAG laser under temperature-tuned type-2 noncritical phase-matching conditions. Although the conversion efficiencies were limited to less than 1% due to the short crystal length available, the data reported are encouraging. They appear to justify further study and development of this device as a narrow-linewidth, broadly tunable solid-state source

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KTP has been found to have the special phase-matching loci where the temperature-induced phase mismatch becomes zero for type-1 and type-2 SHG of the Nd:YAG laser frequency at 1.0642 μm. This was demonstrated for type-1 phase matching. In addition, the temperature derivatives of the refractive indexes and the absolute value of d ₃₃ are reported

Laser dynamics of a 10 GHz 0.55 ps asynchronously harmonic modelocked Er-doped fiber soliton laser are investigated both theoretically and experimentally. Theoretical analyses based on the master equation model solved by the variational method have indicated that all the pulse parameters of the laser output will exhibit complicated slow periodic variations in the asynchronous soliton modelocking (ASM) mode. New experimental methods based on analyzing directly the RF spectra of the ASM laser output have been developed to accurately determine the sinusoidal variation of the pulse timing and the pulse center wavelength for the first time. It is found that the pulse center wavelength variation can be as large as 1 nm half-peak-to-peak and the pulse timing variation can be as large as 3 ps. The consistency among all the experimental data and theoretical prediction is carefully examined and the results indicate that the ASM pulse dynamics observed experimentally are in good agreement with those obtained from the theoretical analyses.

Low pressure organometallic vapor-phase epitaxial growth of Ga_0.5In_0.5P and (Al_xGa_1-x) _0.5In_0.5P is examined. Epitaxial layers of bulk materials are characterized using photoluminescence, electroreflectance, Raman scattering spectroscopy, and surface morphology studies to determine lattice match and optimum growth conditions. Lattice matching at the growth temperature produces featureless growth surfaces, while lattice matching at room temperatures results in minimum photoluminescence linewidth but cracked surfaces due to tensile strain during growth. Raman scattering spectra of the quaternary reveal a three mode structure, with spectral peaks due to GaP-like, InP-like, and AIP-like LO phonons. Heterostructures are investigated including quantum shifts from a series of superlattices. These materials are incorporated in double-heterostructure lasers and single-quantum-well laser with graded-index separate confinement heterostructure

The electron transport characteristics of five n-i-n diodes with (Al_xGa_1-x)_0.5In_0.5P intrinsic barrier regions of various aluminum composition x were determined from the measured I-V characteristics between 60 and 310 K. From these measurements, three different transport regimes were identified. Fowler-Nordheim tunneling was observed at temperatures below 215, 260, 110, 150, and 120 K for aluminum compositions of x=0.4, 0.5, 0.6, 0.7, and 1.0, respectively, with applied electric fields in excess of 5 MV/m. The temperature dependence of the Fowler-Nordheim tunneling currents is shown in AlGaInP for the first time with direct bandgap AlGaInP exhibiting a strong linear decrease in apparent barrier height with increasing temperature. The measured barrier height using the thermionic emission model yields values close to the expected conduction band offset between the GaInP spacer layers and the AlGaInP intrinsic barriers, as measured using high-pressure photoluminescence, and provides a novel technique for measuring the direct-indirect crossover composition in AlGaInP. It is shown that the lowest lying conduction band in AlGaInP is the dominant barrier to electron transport. This has important implications for the design of AlGaInP laser diodes

The properties of (AlGa)_0.5In_0.5P, strained Ga_xIn_1-xP/(AlGa)_0.5In_0.5P heterostructures, and single quantum well (QW) laser diodes with Al_0.5In_0.5P cladding layers, prepared by low pressure organometallic vapor phase epitaxy, are described. The influence of biaxial strain upon the relative positions of the valence band edges are examined by analyzing the polarized spontaneous emission. Laser diodes with wavelength 620<λ<690 nm are also fabricated, using active regions of biaxially strained GaInP or AlGaInP. At longer wavelengths, threshold current densities under 200 A/cm² and efficiencies greater than 80% result from a biaxially-compressed GaInP QW active region. Short wavelength AlGaInP laser performance is hindered by the poor electron confinement afforded by AlGaInP heterostructures. Despite the electron leakage problem, good 630-nm band performance, and extension into the 620-nm band, is achieved with strained, single QW active regions

Results of reliability studies of GaInP (0.6-0.7 μm); AlGaAs, InAlGaAs, and InGaAsP (0.81 μm); GaAs (0.86 μm); and InGaAs (0.9-1.1 μm) quantum-well laser diodes are summarized. Conclusions drawn from over one million cumulative lifetest hours and substantial electron-beam-induced-current (EBIC) failure analysis are presented. Improvements in laser reliability with the addition of indium (in both sudden failures and gradual degradation) and with the elimination of aluminium (in gradual degradation) have been observed. These trends, combined with the observed inconsequential effect of strain, provide guidelines for the design of highly reliable lasers

We have investigated the frequency reproducibility of a He-Ne laser (0.63μ). We used the dependence of absorption on frequency in the strong-field standing-wave limit to provide stabilization. An absorption cell with a pure neon discharge at T = 73deg C could be placed both inside and outside the resonator. In the former case, a magnetic field was used to influence the gain profile of the line in the He-Ne mixture in such a way as to increase the accuracy with which the center of the absorption line corresponded to the frequency for maximum power output of the laser oscillation. In the latter case, we used a single-mode laser to obtain saturation of absorption in the cell. The long-term reproducibility obtained was approx 10^{-9} . The short-term stability was much better. To obtain a very narrow dip in the center of the absorption line and consequently much higher stability (10^-13), we have investigated the vibration spectrum of absorption in the strong field of a standing wave in the CO 2 laser (10.6μ) and He-Ne-Ch 4 (3.39μ) systems.

Relative second-harmonic power generated in a non-linear crystal has been measured for a multimode He-Ne 0.63-μ laser source operating unlocked and in two different types of spontaneous locking. For the same average laser power the harmonic power was 2.15 times larger for c/2L pulsing than for c/L pulsing, in good agreement with a predicted value of 2.18. The harmonic enhancement for c/2L pulsing compared with unlocked operation was 3.31, considerably lower than the analytic estimate of 4.2, indicating that significant interaction was present between the modes of the unlocked laser.

Nonlinear optical properties of HgS have been of interest lately. The electrooptic effect has been measured in a natural crystal and values for the two independent coefficients are reported here at wavelengths of 0.63 and 3.39 microns. The results agree well with predictions of purely electronic theories, but the wavelength dependence indicates an appreciable contribution of ionic motion to the optical polarizability.

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The efficient generation of the second-harmonic radiation of an Nd:YAG laser operating at 1.318 μ has been achieved in a deuterated cesium dihydrogen arsenate (CD*A) crystal. An overall peak power conversion efficiency of 40 percent was obtained for the experimental conditions described.

GaInAsP/AlGaAs lasers emitting at 0.67 μm in wavelength were fabricated on n-type (100) GaAs substrates by liquid phase epitaxy. The laser had a GaInAsP active layer clad by wide-gap Al0.7Ga0.3As layers. The fabrication conditions and lasing characteristics were discussed in detail at the first time. In order to obtain a low threshold laser wafer, it was required that the GaInAsP active layer be grown at 790C matching the lattice parameter with AIGaAs cladding layers, as well as adjusting the acceptor concentration in the AlGaAs cladding layer to be optimum. The mean threshold current density, Jth under the room-temperature pulsed operation was 5.6 kA/cm2 for the active layer 0.26 μm thick. From theoretical and experimental dependence of the Jth on the active layer thickness d, minimum Jth of 1.9 kA/cm2 was expected at d = 0.05 μm. The maximum light emission power and differential quantum efficiency was 30 mW/facet and 46 percent, respectively, for the 8 μm oxide stripe laser. The temperature characteristics of threshold and lasing wavelength were observed. Copyright © 1987 by The Institute of Electrical and Electronics Engineers, Inc.

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Fluorine atom laser oscillation has been observed at 0.7311 μ. The line was excited by pulsed electric discharge in mixed helium-fluorine gases at 5 torr and was accompanied by stronger, previously reported fluorine atom lines at 0.7039 and 0.7129 μ.

The threshold-current variation with temperature has been measured for Ga 1-x Al x As double-heterostructure (DH) lasers with AlAs mole fraction in the active layer x of 0.08 and 0.2, and with several heterojunction step heights Deltax . The threshold-temperature coefficient J th (350 K)/J th (300 K), which generally increases with decreasing Deltax , is found to be larger for x = 0.2 than that for x = 0.08 at the same value of Deltax , and also to be larger for the lasers with smaller effective electron diffusion length in the P cladding layer, in the case of x = 0.2 . These characteristics are well explained by a model of carrier leakage due to unconfined carriers in the active layer. It is confirmed by a good fit of the experimental results with the calculated values that the electron leakage in the Gamma conduction band of the P cladding layer dominates for x leq 0.1 , but the hole leakage in the N cladding layer increases with x and becomes comparable in magnitude with the electron leakage at x sim 0.2 .

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The authors present results obtained with a newly developed technology of growth of 〈100〉-oriented InGaAsP-GaAs structures, with their active region composition corresponding to lasing at λ=0.8 μm. The structures used are described, and the output loss dependence of the threshold current densities and differential efficiency for broad-area contact diodes based on these structures are discussed. The light-current characteristics show that such diodes with a 100-μm-wide stripe can produce continuous wave (CW) optical power in excess of 5 W. As shown by measurements of the local temperature rise near the active region, the rate of temperature increase above the lasing threshold is determined by the diode efficiency. No failures which could be attributed to a catastrophic growth of dark line defects have been observed to occur in these diodes. Lifetime tests on the laser diodes and studies of defect formation in the active region of the laser structures under optical pumping are discussed