Search

Two different types of bistability in proton-implanted GaAs quantum-well (QW) vertical-cavity surface-emitting lasers (VCSELs) have been observed. The first type of bistability has a small hysteresis width (/spl sim/50 /spl mu/A) in the light versus current and voltage versus current characteristics. Light-induced large negative differential resistance, random fluctuations, and self-pulsations are observed at the switching point. The emission patterns show that the bistability occurs at a spatially localized area under the output facet that covers only a small fraction of the N1S-/spl mu/m-diameter aperture. The bistability stems from spatially localized saturable absorption. The second type of bistability has a large hysteresis width (/spl sim/1 /spl mu/A) in the L-1 characteristics and is observed well above the threshold current. In this case, no observable bistable loop exists in the voltage versus current characteristics, and the bistability is associated with transverse mode-hopping.

The characteristics of gain-guided GaAs single-quantum-well laser arrays, using proton bombardment to confine the current flow to 11 stripes on 9-/spl mu/m centers, are described. The longitudinal-mode spectrum, the spectrally resolved far-field, and the near-field of this laser array were studied experimentally up to three times threshold current. The laser arrays are found to emit at /spl sim/10 longitudinal modes corresponding to the Fabry-Perot modes of the laser. The longitudinal spectrum is current dependent. Near threshold current, the spectrum consists of a number of longitudinal modes where only the fundamental mode of the array laser. At larger current, each of the longitudinal modes is broadened and the spectrum of each longitudinal mode consists of many individual emission lines of the array modes. For each longitudinal mode, the lowest order array mode emits at the longest wavelength and the highest order array mode emits at the shortest wavelength. The wavelength separation between neighboring array modes increases with increasing mode order. Fourteen array modes are found to emit simultaneously at three times threshold current. The experimental results are in good agreement with direct array-mode analysis based on the finite-difference method.

Single crystal quarter wave multilayer stacks consisting of alternating layers of GaAs and Al(0.3)Ga(0.7)As were grown on GaAs substrates by the molecular beam epitaxy method. This technique is capable of a very high degree of control of film thickness and layer reproducibility. The measured reflectivity spectrum is in good agreement with the reflectivity dispersion of GaAs and Alo.gGao.7As. calculated from the equivalent layer theory using the known index dispersion of GaAs and GaAs and Al(0.3)Ga(0.7)As.

We have utilized a new aluminum source, trimethylamine alane (TMAA), in the growth of graded‐index separate‐confinement heterostructure single quantum‐well GaAs/AlGaAs laser structures by low pressure (30 Torr) organometallic vapor‐phase epitaxy. We find lower carbon and oxygen incorporation in AlGaAs epilayers using TMAA since it does not contain a direct Al–C bond and it is not susceptible to the formation of volatile Al–O containing compounds. The oxygen and carbon concentrations were below the detection limits (< 5 × 1016 cm−3 and < 3 × 1016 cm−3, respectively) of the secondary ion mass spectrometry measurements. Broad‐area lasers with 10‐nm quantum wells and Al0.45Ga0.55As cladding layers exhibited threshold current densities of 140 A cm−2 for cavity lengths of 1 mm, internal quantum efficiencies of 81%, and intrinsic losses of 1.6 cm−1. These results demonstrate that extremely high‐quality AlGaAs and GaAs quantum wells can be grown with TMAA.

The authors describe InGaAsP-InP index guides strip buried heterostructure lasers (SBH) operating at 1.3 mu m with a 1.1- mu m guiding layer grown by a two-step atmospheric pressure metalorganic chemical vapor deposition (MOCVD) growth procedure. These lasers are compared with buried heterostructure lasers having similar guiding layers under the active layer but terminated at the edge of the active layer. SBH lasers with 0.15- mu m-thick active layer strips, 5- mu m wide, and guide layers varying from 0 to 0.7 mu m have threshold currents increasing from 34 to 59 mA, and nearly constant differential external quantum efficiencies of 0.2 mW/mA. The threshold current increases more rapidly with temperature with increasing guide layer thickness, with T/sub 0/ decreasing from 70 degrees C for lasers without a guide layer to 54.3 degrees C for lasers without a guide layer to 54.3 degrees C for lasers with 0.7- mu m-thick guide layers. Output powers of up to 30 mW/facet have been obtained from 254- mu m-long lasers and were found to be insensitive to guide layer thickness. >

The vertical cavity surface emitting laser is of growing interest because of unique advantages offered by the device geometry, including the potential for the integration of this laser structure with other devices for OEICs. Additional advantages include the potential for ultra-small active volume devices, light emission normal to the wafer surface, and large-area two-dimensional arrays. In this paper we present data on various heterostructures grown by metalorganic chemical vapor deposition directed towards the realization of a high-efficiency VCSEL in the lattice-matched InP-In 0.47 Ga 0.53 As system.

Low‐frequency electrical current noise measurements are reported on double‐heterojunction AlGaAs/GaAs laser diodes fabricated on GaAs and on Si substrates. The noise spectra show a frequency dependence proportional with f−γ with γ close to unity. The spectral intensity is proportional to the current for smaller currents ( 1 mA). The diodes built on the GaAs substrate are 50 times less noisy than the ones built on the Si substrate. This effect is attributed to the fact that the density of dislocations at the Si interface is much larger than at the GaAs substrate/device interface.

The thermally induced biaxial stress in GaAs lasers grown on Si substrates is reduced by patterning the active region to an effective width of 10 μm and by introducing a compensating stress from a thermally deposited SiO2 layer. The largest stress compensation is obtained for oxide stripe lasers and results in a significant reduction in the degradation rate of the lasers.

Improved growth conditions by molecular‐beam epitaxy (MBE) and fabrication of state‐of‐the‐art AlGaAs/GaAs selectively doped heterostructure transistors (SDHTs) and ring oscillators on Si substrates are reported. In MBE growth, use of minimum As4:Ga flux ratio during initial nucleation combined with in situ thermal cycles gave a marked improvement in material quality. With this method, FWHM of x‐ray rocking curves was measured as low as 135 arcsec for a 3.5‐μm‐thick GaAs layer on Si. Although 3‐μm‐thick undoped GaAs buffer layers on p‐type Si substrates were fully depleted under a Schottky contact, a parallel n‐type conduction path confined in a thin region (

Two-dimensional arrays of 3*3 vertical cavity surface emitting GaAs-AlGaAs lasers with 7- mu m center spacing are described. The lasers were grown by molecular beam epitaxy and contain one grown GaAs-AlAs mirror under the active layer and a second, thermally deposited, high-reflectivity, SiO/sub 2/-Si mirror on top of the epitaxial layer. The array has a 295-K threshold of 90 mA, corresponding to 10 mA per laser. The individual lasers in the array have Gaussian beam profiles both spatially in the near field and angularly in the far field. When all the lasers in the array are operating in a coupled manner, the emission exhibits a lobed far-field pattern with a 5 degrees separation in the direction parallel to the contact edge and a 2.5 degrees separation perpendicular to the contact edge. >

We describe the implementation of a scanning Hall probe microscope of outstanding magnetic field sensitivity (∼0.1 G) and unprecedented spatial resolution (∼0.35 μm) to detect surface magnetic fields at close proximity to a sample. Our microscope combines the advantages of a submicron Hall probe fabricated on a GaAs/Al0.3Ga0.7As heterostructure chip and the scanning tunneling microscopy technique for precise positioning. We demonstrate its usefulness by imaging individual vortices in high Tc La1.85Sr0.15CuO4 films and superconducting networks, and magnetic bubble domains.

We report the growth and fabrication of InGaAs/GaAs strained quantum well (QW) lasers with a very low threshold current density, Jth, of

The threshold current density, internal quantum efficiency, internal loss, and emission wavelength of InGaAsP/InP multiquantum well lasers, grown by atmospheric pressure metalorganic chemical vapor deposition, were measured as a function of substrate orientation on and away from the (001) towards the nearest [011] and [111]A. The laser parameters are optimized for (001) oriented substrates. Substrate misorientations of more than 3° in either direction yield lasers with increased threshold current density and lower internal quantum efficiency. The internal loss increases and the surface morphology becomes visually poorer for substrate misorientations of 6°. The laser emission shifts to longer wavelength with increasing misorientation.

Graded refractive index In0.2Ga0.8As/GaAs strained single quantum well ridge lasers, emitting at 0.98 μm, have been grown by molecular beam epitaxy. For pulsed excitation, laser operation has been obtained up to 180 °C for 1016‐μm‐long lasers. This record high operating temperature results from a high T0=160 °C characteristic of these long lasers. Anomalous threshold current dependences on temperature are found for shorter lasers and are attributed to band filling at high current densities and a shift of the emission from the n=1 quantum level to the n=2 level.

Optically pumped lasing has been observed up to 100 K in a Cd 0.2 Zn 0.8 Te ZnTe superlattice grown on a GaAs Si substrate. At 100 K, rapid degradation of the lasing signal is seen. This result is inferior to that previously reported for a similar structure grown on a GaAs bulk substrate, for which room-temperature lasing was seen with no degradation. X-ray rocking curve and morphology measurements suggest that the II–VI epilayer has limited ability to heal the extra defects that are present when a GaAs Si , rather than a bulk GaAs, substrate is used.

Data are presented demonstrating the optically coupled operation of a 3×3 two‐dimensional array of AlxGa1−xAs‐GaAs vertical‐cavity surface‐emitting lasers. Room‐temperature threshold current for the array is 90 mA, with the device geometry allowing for light emission from the epitaxial side of the device.

Data are presented demonstrating room‐temperature operation of AlxGa1−xAs‐GaAs vertical‐cavity surface‐emitting lasers grown on Si substrates. The device structures are grown using molecular beam epitaxy and a grown‐in quarter‐wave AlAs‐GaAs stack is used as the n‐side reflector and a nonalloyed Ag dot used as the p‐side reflector/contact. Pulsed threshold currents of ∼125 mA are obtained for a 15‐μm‐diam device.

Two dimensional 3x3 coupled arrays of GaAs/AlGaAs vertical cavity surface emitting lasers (VCSELs) with 7 pm center to center spacing are described [l]. The VCSELs emit normal to the epitaxial surface of the wafer and their compact size, simplicity of fabrication and low threshold per laser makes them ideal for the array geometry. Earlier VCSELs had larger spacings between lasers and were not coupled [WI. The lasers were grown by MBE and had the geometry shown in Fig. 1 with one grown quarter wave 23.5 GaAs/AlAs layer pair mirror under the 1 pm thick GaAs active layer and a thermally deposited high reflectivity four layer pair SiO2/Si mirror over the epitaxial layer. The top contact consisted of a 2 pm wide metal grid with a 3x3 array of 5x5 pm2 openings deposited using lift-off through the first Si02 layer of the mirror. Contact to the grid was made by a 400 pm2 Cr/Au pad. In this geometry the lasing regions are defined by the top mirror array. The current injected through the metal grid diffuses and spreads out under the mirrors resulting in a spatially dependent carrier density that exhibits a slightly lower carrier density under the center of the mirror. The lowest order transverse mode of a single laser is Gaussian and the spatially dependent refractive index due to the carrier density introduces a self focusing of the beam outside the laser cavity.

We show that if the active region of a quantum‐well (QW) laser is grown at a high temperature such that the sticking coefficients of group III species are less than unity, the emission wavelength λ and threshold current density Jth are sensitive to the spatial variations in substrate growth temperature Ts despite the time average flux variations of less than 1% across the surface of a rotating 7.5 cm diam substrate in our molecular‐beam epitaxy system. InGaAs strained QW lasers, in which the active region is grown at Ts < 580 °C at which In and Ga have unity sticking coefficients, have uniform Jth (212±4 A cm−2) and λ(989±1 nm) on a 5 cm diam substrate. These lasers were grown with In mounting. In contrast, GaAs QW lasers, which are grown at ∼700 °C with In mounting, have less uniform Jth (395±20 A cm−2) and λ(850±9 nm). This is the result of nonuniform substrate heating which is partly due to voids in the In back coating. This causes variations in GaAs growth rate because of the high temperature dependen...

We demonstrate the operation of a semiconductor laser pumped fiber Brillouin amplifier. An optical gain of 5.5 dB/mW of pump power is obtained. The 15-MHz intrinsic bandwidth of the amplifier was extended more than one order of magnitude by adding frequency modulation to the pump laser. Bit-error-rate receiver sensitivity measurements with an in-line Brillouin amplifier demonstrate a 16-dB improvement in the system gain at 10 Mbit/s and a 8.5-dB improvement at 90 Mbit/s. Noise calculations show that the amplifier has an excess noise factor of 50-500 depending on the amplifier length and pump power.

The two-photon absorption spectrum of GaAs was measured at 300 and 4 \ifmmode^\circ\else\textdegree\fi{}K by monitoring the resulting band gap luminescence. For $2\ensuremath{\hbar}{\ensuremath{\omega}}_{1}$ more than 0.01 eV above the direct band gap ${E}_{g}$ the luminescence has an approximately ${(2\ensuremath{\hbar}{\ensuremath{\omega}}_{1}\ensuremath{-}{E}_{g})}^{\frac{3}{2}}$ dependence characteristic of the allowed-forbidden mechanism corresponding to the two-band model. A weaker spectral dependence is observed when $2\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{-}{E}_{g}l0.01$ eV. This is tentatively identified with a contribution from the allowed-allowed mechanism of the three-band model which has an $\ensuremath{\sim}{(2\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{-}{E}_{g})}^{\frac{1}{2}}$ dependence, and exciton contributions in the continuum. The transition to the $n=1$ exciton is weak, and is attributed to the allowed-allowed transition of the three-band model.

Using proton bombarded stripe geometry lasers which emit intense optical pulses, we have measured the width of the optical beam in the plane of the junction as a function of time during the pulse. The width of the beam is qualitatively proportional to the change in the carrier density. The width increases during the quiescent period between pulses where the carrier density increases by current injection and decreases during the emission of the pulse. For one laser studied in considerable detail, the full width at half intensity decreases from 9 μm at the start of the pulse to 7.2 μm at the end of the pulse. The reduction in the width results from the self-focusing of the beam. It is due to an increase in the refractive index and the decrease in the gain distributions near the center of the stripe. The reduction in the beamwidth concentrates the mode to a region of sufficiently higher average gain to compensate for the reduction in spatial gain distribution. The self-focusing acts to reduce the damping of the relaxation oscillations, and thus enhances the effect of other nonlinearities such as saturable absorption in causing pulsations. The thermal induced refractive index distribution across the stripe is shown to play a crucial role in the gain instability caused by self-focusing.

Quaternary InAs 1-x-y Sb x P y ) and ternary InAs 1-x Sb x have been grown on InAs substrates using liquid phase epitaxy. The quaternary layers are lattice matched to InAs for y = 2.2x , and have bandgaps greater than InAs. The ternary layers have lower bandgaps than InAs, but have larger lattice constants. Reasonable quality growth has been obtained up to x = 0.15 where the luminescence peak is shifted to ∼4 from 3.1 μm and the lattice mismatch is ∼0.01. Optically pumped laser emission at 3.1 μm has been observed from 77 to 100 °K using an InAs active layer with InAs 1-x-y Sb x P y clading layers. Laser emission at 3.9 μm has been observed from 77 to 125 °K using a InAs 1-x Sb x active layer, with x\sim 0.13 , and InAs cladding layers. Laser emission from intermediate ternary compositions has also been observed.

The characteristics of phase-coupled laser arrays fabricated using Schottky barriers on the p-contact to confine the current flow to 5 or 10 stripes on 9 μm centers are described. The gain guided laser spots are very equally spaced, and in some cases do not coincide exactly with the position or number of conducting stripes. The self adjusting nature of the active region, together with the phase shift of π between the neighboring N laser spots results in extremely smooth far-field patterns consisting of 2 narrow, intense outer lobes, and N-2 much weaker interior lobes. The mode structure closely approximates the higher order modes observed in wide stripe laser. The separation and width of the outer lobes is defined by diffraction theory, and determined by the separation and number of lasing spots. Deviations from diffraction theory are found in some cases. The contributions to the far field were studied by examining the magnified image of the exit facet as a narrow slit was passed across the far field. The phase contributions present in the far field were studied by introducing a relative phase shift between the two symmetric halves of the far field. The relative phase change modifies the interference pattern of the magnified near-field image, causing a spatial shift of the imaged spots in the plane of the junction.

The biaxial tensile stress of 2.65 kbar in as‐grown GaAs/Si is reduced by post‐growth patterning of the GaAs and the reduction in stress is dependent on the pattern size and shape. For stripe patterns less than 15 μm wide the stress becomes largely uniaxial with stress relief normal to the stripe direction. Rectangular patterns exhibited stress relief in orthogonal directions, and have the lowest stress in the narrow direction of the rectangle. A 9×12 μm2 rectangle exhibited an average stress of 0.5 kbar.

Intense visible emission in the red, green, and blue has been observed from infrared to visible multiphoton Y1−xErxF3 upconversion phosphors pumped with 1.5‐μm radiation from InGaAsP‐laser sources. With 0.01

UHV‐deposited amorphous silicon was recrystallized on (100) single‐crystal substrates using pulsed Nd : YAG laser irradiation. Prior to deposition, substrates were prepared with either atomically clean surfaces, residual argon ion sputter damage, or from one to three monolayers of residual oxide. Epitaxial regrowth of bulk crystalline quality occurred regardless of substrate preparation. Annealing energy densities were significantly higher, however, for substrates retaining residual impurities. Certain layers were doped with Ga. At moderate doping levels recrystallization behavior was unaffected. At a 2% doping level, the annealing energy threshold dropped by one‐half and Ga atoms were displaced toward the epitaxial surface. It is shown that the decrease in energy threshold is due to an impurity‐induced increase in light absorption within the amorphous layer. Layers were crystallized with substitutional Ga concentrations of up to 10 times the equilibrium solid solubility limit.

It is shown that the well‐known damped relaxation oscillation resonance in the small signal equations for the dynamic photon and carrier densities becomes more highly damped when the photon density is axially inhomogeneous. An antireflective coating with R=0.02 on one laser facet reduced the resonance in the light output from a 1.3‐μm Vee‐groove laser yielding a flat response with a 3‐dB bandwidth of 5 GHz. We also show that the wavelength modulation or chirp, due to the carrier‐dependent contribution of the refractive index, is largely eliminated.

Double heterostructure lasers consisting of GaSb active layers with Al 0.4 Ga 0.6 Sb/ cladding layers were grown by molecular beam hetero-epitaxy on Si substrates. The intrinsic ∼ 12 percent lattice mismatch between the GaSb and the Si at the growth temperature is largely taken up by a GaSb/AlSb superlattice. The GaSb layer is under severe dilatory strain at room temperature due to the larger thermal contraction of the GaSb layer relative to that of Si from the growth temperature and resulting in shrinkage of the GaSb bandgap. We have observed optically pumped laser emission ranging from 1.63 μm at 85 K to 1.83 μm at 350 K. The exponential dependence of the threshold pump power and relative quantum efficiency exhibits a change in slope at ∼ 250 K. The exponential threshold-temperature dependence at 80 and 300 K are T_{0} = 158 and 100 K, respectively, and are higher than previously reported for GaSb lasers. At 300 K the threshold corresponds to an equivalent current density of 12 kA/cm2.

The transmitted and reflected second harmonic generation is computed for a parallel-plate Fabry-Perot structure including all the effects of interference and multiple reflections. As a function of wavelength, the spectra consist of rapid Fabry-Perot oscillations superimposed on the more slowly varying coherence-length fringes. The harmonic generation from a 97.5-μm-thick GaAs plate, obtained using a fundamental near 2μm from a wavelength-tunable parametric oscillator, is in good agreement with the theoretical calculations.

Liquid‐phase epitaxy has been used to grow four‐layer GaAs/AlGaAs double heterostructures on Si substrates that had been precoated with GaAs by molecular‐beam epitaxy and coating the remaining exposed parts of the Si wafer with SiO2. Direct growth of GaAs on Si by liquid‐phase epitaxy is not feasible because of the dissolution of the Si substrate by the Ga melt. As with the other growth techniques, photoluminescence from the GaAs cap layer and electroluminescence from the 0.2‐μm‐thick GaAs active layer indicate the luminescence peaks are shifted to lower energy as a result of the tensile strain in the plane of the layers that is caused by the different thermal contraction of the GaAS and Si layers as the wafer is cooled from the growth temperature. Optically pumped stimulated emission is observed from the cap layer at 80 K. The efficiency of the electroluminescence from the active layer was low, indicating the p‐n junction was displaced from the active layer due to the accidental excess doping resulting f...

The degradation of single-quantum-well graded-refractive-index GaAs lasers and light-emitting diodes grown on silicon substrates by metallo-organic vapor deposition is studied. The mechanism resulting in degradation appears to be identical with that observed in GaAs devices on GaAs substrates. However, the devices degrade very rapidly, even under pulsed operation, owing to the presence of a very high density of non-radiative regions which results from the dislocations caused by the 4.1% lattice mismatch between the GaAs and the silicon substrate. Continuous operation has been obtained up to 160 K. At higher temperatures the dark regions quench the emission 3–5 μs after the start of a current pulse. This prevents the continuous operation at room temperature of these lasers. During operation the dark regions grow rapidly in size by non-radiative electron-hole recombination in the presence of the large tensile stress in the epilayer. We estimate that it will be necessary to reduce the defect density to less than about 10 4 cm −2 and to prevent the growth of defects in the active layer in order to achieve reliable operation of light-emitting devices.

The bandwidth characteristics of single and multimode optical fibers have been investigated with single and multilongitudinal mode laser sources operating at 0.8, 1.3, and 1.5 μm. It is shown that single-mode fiber with a cutoff wavelength of 1.3 μm can support 1 Gb/s transmission over at least 7.5 km with a 0.8-μm laser source.

The pulsations in the laser emission from several lasers have been studied as a function of temperature, cavity length, external mirror illumination, and effect of mirror etching. Selected DH lasers with 12‐μm‐wide stripes delineated by proton bombardment and 380‐μm‐long cavities were used. The modulation depth of the pulsations at 300 °K increases approximately linearly with current above a threshold value and becomes fully modulated at high output powers. At intermediate temperatures, the modulation exhibits a similar initial slope but is not complete at the highest measured output. At low temperatures, well‐developed pulsations are not observed; the modulation reaches a maximum at a low amplitude level and then decreases. The frequency dependence of the pulsations indicates that when the modulation level is increasing, the frequency f is relatively constant, and above the maximum moduation f 2 increases approximately linearly with output power. The dependence on cavity length was studied using a series...

Gaussian shaped picosecond pulses as short as 62 ps have been obtained from an array of 10 phase-coupled multiquantum well GaAs lasers. Combined dc and 75 ps long current pulses from a comb generator were used. Single pulses were obtained with an average power of 50 mW at a 960 MHz repetition rate which corresponds to a peak power of 0.8 W. These pulse widths are in good agreement with a theory of pulse formation based on the carrier and photon density equations with the gain expanded in a power series in time to yield Gaussian pulses.

The band structure of artificial periodic crystals grown by molecular‐beam epitaxy and consisting of sequential AlAs and GaAs monolayers differs slightly from AlxGa1−xAs random alloys having a comparable average Al content. The measured direct band gaps are up to 110 meV higher than the alloys, and the indirect gaps up to 70 meV higher. The measured dispersion of the refractive index agrees best with the theoretical dispersion curve of an alloy composition having a higher Al content than in the monolayer. This is consistent with the higher band gap of the monolayer. The alloy and monolayer exhibit a birefringence resulting from strain‐induced anisotropy of the lower and upper cutoffs of the complex part of the dielectric constant. The strain results from the differential thermal contraction of the layers and the GaAs substrate on cooling from the growth temperature to room temperature. The birefringence of the monolayer sample contains an additional contribution resulting from the periodic structure.

The spectral and modulation characteristics of injection locked distributed feedback lasers operating at 1.5-μm wavelength have been investigated. The total elimination of excess chirp in directly modulated lasers by the injection locking technique has allowed us to perform transmission experiments over 82.5 km of single mode fibers at 2 Gbit/s without any measurable penalty related to dispersion. Device design parameters for efficient chirp suppression are also given.

Recent progress of GaAs-on-Si technology is reviewed and the importance of initial nucleation is emphasized. Growth initiation with a gallium prelayer at a suitable low temperature combined with migration enhanced epitaxy and in-situ thermal annealing may be able to give substantially improved material quality. During the initial growth, use of the minimum necessary As4:Ga flux ration is found to be critical. Patterned growth or post-growth patterning releases tensile stress only if the growth edges are free. GaAs-on-Si was almost completely relaxed from stress by post-growth patterning to 5 μm × 5 μm size patterns. Steps on the surface owing to substrate misorientation do not affect the two-dimensional electron gas (2DEG) transport properties in AlGaAs/GaAs selectively doped heterojunction transistor (SDHT) structures. For a sheet density of 1012 cm−2, a 2DEG mobility greater than 50 000 cm2 V−1 s−1 at 77 K was obtained on silicon substrates, which was found to be adequate for the fabrication of the state-of-the-art devices. For 1 μm-gate-length SDHTs, maximum transconductances of 220 and 365 mS mm−1 were measured at 300 K and 77 K, respectively. A minimum propagation delay time of 28 ps stage−1 was measured at 300 K for direct coupled field effect transistor logic (DCFL) ring oscillators with a power dissipation of 1.1 mW stage−1. The propagation delay time reduced to 17.6 ps stage−1 at 77 K. These results are comparable to the SDHT technology on GaAs substrates.

The photodetection capabilities of 1.3- mu m channel substrate lasers are studied. The diodes can be used either as lasers or detectors in single-mode-fiber half-duplex communication links. As a detector, a responsivity of 0.26 A/W is obtained by efficiently coupling the input radiation into the facet using a lensed single-mode fiber. The spectral bandwidth extends from 1.1 to 1.37 mu m. The dark current at room temperature is 200 nA at 1-V bias and due in part to current leakage through the Fe-doped InP blocking layer. As lasers, the diodes have a frequency response of 4 GHz, and as detectors, the 3-dB bandwidth is 600 MHz. >

Raman spectra have been observed in AgGa${\mathrm{S}}_{2}$, AgGa${\mathrm{Se}}_{2}$, and CuGa${\mathrm{S}}_{2}$ using argon, krypton, and dye lasers. Of the 13 Raman-active vibrations, we have observed 12 of the modes of AgGa${\mathrm{S}}_{2}$, 9 modes of AgGa${\mathrm{Se}}_{2}$, and all the modes of CuGa${\mathrm{S}}_{2}$. The modes of AgGa${\mathrm{S}}_{2}$ which are also infrared active are in good agreement with the modes determined from a Kramers-Kronig analysis of the ir reflectivity. Agreement is also found with several of the transitions previously determined from the ir spectrum of CuGa${\mathrm{S}}_{2}$. The intensity of the ${A}_{1}$ mode of AgGa${\mathrm{S}}_{2}$ and CuGa${\mathrm{S}}_{2}$ dominates the other Raman lines when the laser excitation is well below the band gap. A resonant interference effect decreases the intensity of this mode as the band gap is approached.

Proton bombarded stripe geometry lasers have been actively mode locked in an external cavity consisting of a lens, a Fabry‐Perot etalon, and a mirror. Under optimized conditions, pulses as short as 5.3 psec have been observed from measurement of the second harmonic autocorrelation function. The dependence of the pulse width on external parameters was studied in detail. The pulse width t1/2 decreases with increasing direct current. The functional dependence of the pulse width on current indicates that a saturable absorption mechanism is present in the laser which causes significant pulse shortening. The pulse width decreases with increasing rf current. The pulse width is a sensitive function of the frequency detuning of the applied rf current. The shortest pulses are observed when the applied frequency is less than the round trip frequency of the pulse in the cavity. The frequency dependence of the pulse width was obtained using a long cavity with multiple pulses in the cavity. The shortest pulses occur between 0.9 and 1.4 GHz. The Fabry‐Perot etalon limits the emission to a 1.9‐A spectral band. At optimum mode locking the band is ∼30 A below the peak of the multimode emission obtained without the etalon. The shift of the gain curve to lower enery indicates that additional loss is introduced by the feedback from the external cavity. The experimental results are shown to be in good agreement with a theoretical description of mode locking.

The longitudinal mode spectrum of stripe geometry double heterostructure (DH) lasers exhibits a spectral broadening of ∼ 0.8 A when the laser emits pulsations. The broadening results from a chirp to longer wavelengths which occurs during the ∼ 0.4 ns long pulse. The chirp is due to an increase of the refractive index resulting from the reduction in the carrier concentration during the pulse. Both plasma and band to band interactions affect the index change with the latter mechanism giving the dominant contribution. A comparison of the wavelength shift obtained during the pulse with the current dependent shift of the longitudinal modes at currents well below threshold confirms that the change in carrier concentration is responsible for the chirp.

Highly single longitudinal mode operation of 1.3 μm InGaAsP/InP buried crescent lasers has been obtained using a 60-600 \mu m long external cavity defined by a spherical mirror. The intensity of the neighboring modes was suppressed by a factor of 1200 for dc excitation and by a factor of 800 for combined dc and 250 MHz current modulation.

Active and passive mode locking of strip buried heterostructure lasers is observed using an external cavity consisting of a lens and a mirror. Using the second‐harmonic‐generation method to obtain the second‐order autocorrelation function we obtain Gaussian pulses as short as 35.7 psec (FWHM) at a 1.9‐GHz repetition rate. The optical spectrum of the mode locked laser consists of longitudinal modes of the composite cavity spaced by ∼0.05 A, whose amplitudes are modulated by the 2.09‐A‐mode spacing of the laser itself, and contained under a Gaussian envelope of 9.5‐A FWHM. Additional very sharp structure (∼1.6‐psec FWHM) in the autocorrelation is shown to result from the presence of several laser mode groups.

The performance characteristics of 1.5-μm external cavity semiconductor lasers have been investigated. Measurements of phase, frequency, and amplitude stability are presented together with the power and threshold characteristics. Amplitude shift keyed coherent receiver sensitivity measurements gave a receiver sensitivity of -55.7 dBm at 150 Mbit/s equivalent to 137 photon/bit as compared to previous amplitude shift keying results of 520 photon/bit and the shot-noise limit of 36 photon/bit.

The dispersion of the refractive index corresponding to the group velocity n*_{1} has been measured as a function of wavelength. It is obtained from the longitudinal mode spacing of GaAs buried heterostructure lasers at threshold. The dependence of n*_{1} on wavelength contains an approximately constant term due to the refractive index n 1 and a strongly dispersive component due to -\lambda (\partialn_{1}/ \partial\lambda) . For a given spectral bandwidth, the dispersion of n*_{1} causes a temporal broadening of a pulse as it passes through the medium. This dispersive effect is shown to contribute to the width of 0.65 ps long pulses obtained recently from mode locked semiconductor lasers. By reducing the length of the laser, the dispersive effect is reduced and it is suggested that pulses as short as 10-13s should be obtainable from such mode locked semiconductor lasers.

We report picosecond pulse generation in low threshold buried optical guide lasers using combined direct and microwave current excitation. The pulse widths were obtained as a function of direct current for several levels of RF excitation using lasers 125 and 380 μm long. The pulses have a Gaussian shape with full widths at half maximum intensity ranging from 19 to 57 ps. The pulse widths were obtained from the second harmonic autocorrelation. The experimental results are in reasonable agreement with the theory of short pulse formation in sinusoidally modulated lasers. The pulse width decreases with increasing dc and microwave current, and decreasing laser length. The shortest pulses were obtained with a 125μm long laser using 0.25 W RF at 1 GHz and 35 mA de bias. Multiple pulses are emitted at high dc excitation levels. Using the above laser at the indicated current levels the emission consists of a burst of ∼10 pulses which are separated by 30 ps, and has an approximately exponentially decaying amplitude. Saturable absorption was introduced in the lasers by degradation and results in shortening the pulses. A comparison is made of the pulse widths obtained for sinusoidal microwave current modulation and for pulsed excitation where the excitation is obtained from a step recovery diode. Reduced pulse widths are obtained for short current pulse excitation.

We describe InGaAsP inverted rib waveguide lasers emitting at 1.54 μm with pulsed current thresholds as low as 60 mA, external quantum efficiencies of up to 0.2 per facet, and emitting 16 mW in the lowest order. Pulsed operation to 110 °C with T0 values of 50–72 °C were obtained. For continuous operation at 15 °C up to 21 mW per facet was observed with external quantum efficiencies of 0.15 per facet and T0 up to 62 °C. The maximum continuous operating temperature was 55 °C and was determined by heating of the junction.