A.M. Sergent

Measurement of the band gap of GexSi1−x/Si strained‐layer heterostructures

We have used photocurrent spectroscopy to measure the optical absorption spectra of coherently strained layers of GexSi1−x grown on 〈001〉 Si by molecular beam epitaxy. A dramatic lowering of the indirect band gap, relative to that of unstrained bulk Ge‐Si alloys, is observed. Our results for 0≤x≤0.7 are in remarkably good agreement with recent calculations of the effects of misfit strain on the band edges of coherently strained Ge‐Si heterostructures. At x=0.6, the gap is lower than that of pure Ge.

Mode-locked hybrid soliton pulse source with extremely wide operating frequency range

The authors report a mode-locked pulse source with extremely wide operating frequency range and very stable operation, through the use of a long, linearly chirped Bragg reflector as the output coupler integrated in a fiber external cavity. A 1.55 mu m strained MQW laser diode is used, with one facet high reflectivity (HR) coated for improved cavity Q, and the other antireflection (AR) coated to allow coupling to the external cavity and suppress Fabry-Perot modes. Near-transform-limited pulses are obtained over a frequency range of 700 MHz around a system operating frequency of 2.488 GHz, with pulsewidths of 50 ps, as required for a practical soliton transmission system.<<ETX>>

Measurement of heterojunction band offsets by admittance spectroscopy: InP/Ga0.47In0.53As

We discuss the use of admittance spectroscopy to measure the band offsets of semiconductor heterojunctions. By using this method to analyze the dynamic response of p‐n junctions containing lattice‐matched InP/Ga0.47In0.53As superlattices we can independently determine both the conduction‐ and valence‐band offsets for this materials system. We find that the sum of these offsets equals the known band‐gap difference between InP and Ga0.47In0.53As and that the ratio of the conduction‐band offset to the valence‐band offset is 42:58.

Improved CW operation of quantum cascade lasers with epitaxial-side heat-sinking

First results on the epilayer-side mounting of quantum cascade (QC) lasers are presented. Operated in continuous-wave (CW) mode, these lasers are superior to substrate-bonded devices. The maximum CW temperature is raised by 20 K (up to 175 K), and, at comparable heat sink temperatures, the performance with respect to threshold current, output power, and slope efficiency is greatly improved for the epilayer-side mounted devices. QC-laser-specific mounting procedures are discussed in this letter, such as the high reflectivity coating of the back-facet and the front-facet cleaving after mounting. Modeling of the temperature distribution inside the QC laser shows a strong temperature gradient within the active waveguide core, which partly explains the still low maximum CW operating temperatures.

Frequency response subtraction for simple measurement of intrinsic laser dynamic properties

The authors describe a new technique for extracting the intrinsic laser-diode dynamic properties accurately. This simple technique eliminates the need for accurate microwave calibration of the test equipment and problems of microwave reflections, nonideal frequency response of laser mount, and detector. The effect of the parasitic components of the laser diode are also eliminated from the results so that measurements of important dynamic properties of the laser can be found up to high frequencies (10-20 GHz) on standard laser diodes. The techinque being used to measure variations of resonance peak and damping factor at different bias levels for a standard bulk active region 1.3 mu m laser diode is shown.<<ETX>>

InGaAs/InP quantum well lasers with sub‐mA threshold current

We evaluate the effect of high‐reflectivity facet coatings on the threshold current of lattice matched and compressively strained InGaAs/InP quantum well lasers. A large decrease in the threshold current is observed in structures with low internal losses. Coated lasers exhibit threshold currents as low as 1.1 mA at 20 °C and 0.9 mA at 10 °C, down from ∼15 mA in as‐cleaved devices with cavity length of 200 μm. These changes are carefully modeled and the prospects for further reduction of the threshold current discussed.

Stable single mode hybrid laser with high power and narrow linewidth

We describe hybrid lasers combining a semiconductor gain section and fiber cavity with integrated chirped Bragg reflector. These devices have produced output powers of 27.5 mW in a narrow linewidth (400 KHz) stable single longitudinal mode. The use of a chirped reflector to stabilize the single mode output, and correct grating orientation are described. The laser output has a side‐mode suppression ratio of over 55 dB at 27.5 mW output, and relative intensity noise (RIN) below 160 dB/Hz.

Admittance spectroscopy measurement of band offsets in strained layers of InxGa1−xAs grown on InP

We report measurements of the conduction‐band offset in strained‐layer superlattices of InxGa1−xAs/InP. Admittance spectroscopy was used to measure the activation energy for thermionic emission of electrons over InP barriers in n‐type superlattices. Superlattice dimensions and x values were obtained from high‐resolution x‐ray diffraction and transmission electron microscopy studies. For x=0.37, 0.53, and 0.69, the values obtained for the conduction‐band offset are 175±25 meV, 210±20 meV, and 315±25 meV, respectively.

High temperature characteristics of InGaAsP/InP laser structures

We investigate the high temperature performance of conventional separate confinement and lattice matched and compressively strained multi‐quantum‐well InGaAsP lasers emitting at 1.3 μm. Low threshold buried heterostructure lasers operate reproducibly at temperatures as high as 130 °C. The rate of threshold change with temperature is described by T0∼45°–55° for both conventional and quantum well lasers. The rate of change is not influenced by any modifications in the active layer structure. In contrast, excellent correlation is observed between the active layer structure, parametrized as the threshold gain, and the peak cw operating temperature.

Semiconductor distributed feedback lasers with quantum well or superlattice gratings for index or gain‐coupled optical feedback

We introduce a semiconductor distributed feedback (DFB) in which the grating is fabricated out of quantum well (QW) or superlattice multilayers. This approach provides a very simple and effective scheme for achieving gain (loss)‐coupled DFB lasers. The present idea was successfully demonstrated with a 1.55‐μm wavelength 6‐QW In0.6Ga0.4As (5 nm)/InGaAsP (band‐gap wavelength=1.25 μm, 18.6 nm) separate confinement heterostructure DFB laser utilizing only a 2‐QW In0.62Ga0.38As (4 nm)/InP (9.3 nm) as the grating.