P. J. Anthony

Dispersion penalty for 1.3 mu m lightwave systems with multimode semiconductor lasers

The effect of fiber dispersion on the performance of lightwave systems is analyzed for the case where multimode semiconductor lasers operating near the zero-dispersion wavelength of the single-mode fiber are used as sources. Both the intersymbol interference and the mode-partition noise are considered in the discussion of dispersion-induced power penalties. The theory is in agreement with an experiment in which the bit error rate is measured for lasers at various bit rates. The tolerable limits on the deviation of the laser wavelength from the zero-dispersion wavelength are obtained for a 1.3- mu m system operating at 1.7 Gb/s. Monte Carlo simulations are used to predict the effect of mode-partition noise on the performance of such high-speed lightwave communication systems. >

Distributed feedback lasers with multiple phase‐shift regions

Phase‐shifted distributed‐feedback (PSDFB) lasers with multiple phase‐shift regions are proposed to reduce the effect of spatial hole burning in conventional PSDFB lasers where a single phase shift leads to a highly nonuniform axial distribution of the mode intensity. We analyze multiple phase‐shift PSDFB lasers by solving numerically the coupled‐wave equations. Our results show that the use of even two phase‐shift regions can substantially reduce the axial inhomogeneity of the mode intensity. Although the gain margin is generally reduced by the use of multiple phase shifts, it is still large enough for most practical purposes if the phase shifts are suitably optimized.

Effects of Ga(As,Sb) active layers and substrate dislocation density on the reliability of 0.87‐μm (Al,Ga)As lasers

Reliability data are presented for (Al,Ga)As double‐heterostructure lasers that emit light near 0.87 μm. Devices were grown with and without small additions of Sb to the active layer, with some devices grown on high dislocation density substrates. The reliability is more than an order of magnitude better for lasers with GaAs1−ySby active layers with y≊0.01 than for lasers with GaAs active layers. The rate of formation of dark line defects is reduced in the Ga(As,Sb) active layer lasers such that not all devices fail due to dark line defects. However, for Ga(As,Sb) active layer lasers grown on high dislocation density substrates, dark line defects formed very rapidly. An increase of roughly an order of magnitude in the substrate dislocation density resulted in a nearly three orders of magnitude decrease in the 70 °C lifetimes of Ga(As,Sb) active layer lasers.

Performance characteristics of a 1.5 mu m single-frequency semiconductor laser with an external waveguide Braff reflector

Single-frequency operation of 1.5- mu m semiconductor lasers was obtained by combining a regular Fabry-Perot laser to an external waveguide Bragg reflector. The laser is characterized by very pure single-frequency operation, 10-MHz linewidth, and greatly-reduced frequency chirp under direct modulation. The laser has been tested in 1.7-Gb/s transmission experiments over 82.5 km of fiber. >

Planar silica-glass optical waveguides with thermally induced lateral mode confinement

A new type of planar silica-glass optical waveguide is proposed utilizing the thermo-optic effect of glass. The vertical mode-field confinement is defined by the core slab sandwiched between the upper and lower claddings while the lateral confinement is achieved due to localized increase of refractive index induced by the thin-film heater placed on the upper cladding. An analytical expression of the steady-state temperature distribution across the waveguide cross-section is derived and the induced mode-field characteristics are analyzed by solving the vector-wave equation using the mode-matching method. Mode propagation losses due to the metallic heater and the leakage into the silicon substrate are both calculated. The bending loss of this thermally induced waveguide is also analyzed for future device implementation using this waveguide.

Full-wave analysis of coplanar waveguides for LiNbO/sub 3/ optical modulators by the mode-matching method considering nonideal conductors on etched buffer layers

Rigorous analysis of traveling-wave coplanar waveguide electrodes for LiNbO/sub 3/ optical modulator applications is presented by using an extended full-wave mode-matching method. The microwave propagation characteristics under the composite influence of substrate anisotropy, uniform or etched buffer layers, finite electrode thickness and conductivity, and metallization undercutting are accurately assessed by employing a network equivalent formulation. Variations of the coplanar waveguide microwave effective index and the characteristic impedance at low frequencies due to finite electrode conductivity are illustrated, and are important even though the mode is quasi-TEM in nature. The effect of etching the SiO/sub 2/ buffer layer is shown to be one possible method for lowering the microwave effective index while keeping the conductor loss at a fixed level.

Silicon‐doping level dependent diffusion of Be in AlGaAs/GaAs quantum well lasers

We have investigated Be diffusion during molecular beam epitaxial growth of GaAs/AlGaAs graded index separate confinement heterostructure laser structures using secondary ion mass spectrometry (SIMS). For growth at 700 °C we find that Be from the p‐type AlGaAs cladding layer diffuses into the quantum well and beyond. As a result, the p‐n junction is displaced from the heterojunction. The extent of Be diffusion is found to depend on the dopants in the graded index (GRIN) regions adjoining the GaAs active layer. When the GRIN segments are left intentionally undoped, Be diffuses through the entire p‐side GRIN, the quantum well active and a significant portion of the n‐side GRIN. However, when the GRIN regions are doped, respectively, with Be and Si on the p and n sides, the displacement of the p‐n junction caused by Be diffusion is significantly reduced. Assuming that Be diffuses from a constant source at the surface into a n‐type layer as a singly charged interstitial donor, our analysis predicts that incre...

Temperature and excitation dependences of active layer photoluminescence in (Al,Ga)As laser heterostructures

The excitation and temperature dependences of the active layer photoluminescence intensity in (Al,Ga)As laser heterostructures in the range 75–300 K are described. Of the wafers investigated, in a majority of those grown by molecular beam epitaxy and in all the wafers grown by liquid phase epitaxy, the AlxGa1−xAs (0≤x≤0.08) active layer photoluminescence intensity at any temperature increased linearly with increasing excitation intensity at high power levels (≥100 W/cm2) but decreased very rapidly with decreasing excitation intensity at lower power levels indicative of a p‐n junction. Since the excitation intensity required to saturate the losses due to the p‐n junction, Psat, increased with increasing temperature, the photoluminescence intensity measured at power levels (typically 1–10 W/cm21) lower than Psat decreased by a factor of 100–1000 on increasing the temperature to 300 K. In contrast, for a few wafers grown by molecular beam epitaxy, the active layer intensity varied linearly with excitation at...

Reduced threshold current temperature dependence in double heterostructure lasers due to separate p-n and heterojunctions

The reduced and even negative temperature dependence of the lasing threshold current that has been observed around room temperature for some (Al, Ga)As double heterostructure lasers grown by molecular beam epitaxy (MBE) can be explained by a separation of the p-n junction from the active layer heterojunctions. This separation requires minority carrier diffusion into the active layer for lasing and also allows majority carrier current flow out of the active layer. The latter current does not contribute to lasing and adds to the current necessary to achieve lasing. First-order calculations of the temperature dependences of these effects are presented, showing good agreement with measured threshold current and junction voltage dependences. The separation of the p-n and heterojunctions in the lasers with a reduced temperature dependence is supported by secondary-ion mass spectroscopy, electron beam induced current, and photoluminescence results. Besides the negative temperature dependence of the threshold current for the anomalous devices, the same remote junction effect can explain variability in the threshold current temperature dependence of seemingly normal devices and wafer-to-wafer variability in room temperature threshold currents. An improved understanding of these effects can be important in the design of lasers and optoelectronic systems.

Oscillation frequency tuning characteristics of fiber‐extended‐cavity distributed‐feedback lasers

Stable narrow‐linewidth (<100 kHz) operation has been demonstrated for a fiber‐extended‐cavity distributed‐feedback (EC DFB) laser emitting at 1.55 μm wavelength. We describe the observed oscillation frequency tuning characteristics of the laser obtained by controlling the temperature and injection current. The effects of distributed grating reflection and continuous frequency tuning schemes are examined for both the EC DFB laser and for the case of an EC DBR (distributed Bragg reflector) laser.