P. Besomi

Frequency chirp under current modulaton in InGaAsP injection lasers

We have measured the frequency chirp in gain‐guided, weakly index‐guided, and strongly index‐guided InGaAsP lasers under direct‐current modulation. The measured chirp width is largest for gain‐guided and smallest for weakly index‐guided ridge waveguide lasers. The chirp width for 1.5‐μm InGaAsP lasers is about a factor of 2 larger than that for 1.3‐μm InGaAsP lasers of the same structure. The frequency chirping results from a modulaton of the carrier density which modulates the effective refractive index of the guided mode. The frequency chirping can introduce a limitation on the performance of single‐frequency injection laser sources for high bit‐rate digital transmission in fiber communication systems at 1.55 μm.

Enhanced indium phosphide substrate protection for liquid phase epitaxy growth of indium‐gallium‐arsenide‐phosphide double heterostructure lasers

Thermal degradation of indium phosphide (InP) single crystal substrates prior to liquid phase epitaxy growth has been virtually eliminated by using an improved protection technique. The phosphorus partial pressure provided by a Sn‐In‐P solution localized inside an external chamber surrounding the InP substrate prior to growth prevents thermal damage to the surface. Nomarski contrast photomicrographs, as well as photoluminescence and x‐ray diffractometric measurements indicate that InP substrates protected by this method suffer a negligible deterioration, in contrast to the results of the more commonly used cover‐wafer method.

Effect of photocarrier spreading on the photoluminescence of double heterostructure material

Lateral spreading of photoexcited carriers can suppress the photoluminescence signal from double heterostructure wafers containing a p‐n junction independently from saturable defects in the material. Experimental results are presented and a theory is derived to show that in any double heterostructure with a p‐n junction the photoluminescence is suppressed if the power of the excitation source does not exceed a threshold value. The threshold power has been calculated in terms of material parameters, in excellent agreement with experimental results. This effect is the basis of a novel technique used for a nondestructive optical determination of the p‐clad layer sheet conductance as well as junction misplacement in double heterostructures, both of which are important parameters for injection lasers.

Improved linearity and kink criteria for 1.3‐μm InGaAsP‐InP channeled substrate buried heterostructure lasers

Nonlinearities or kinks in the light‐current characteristics of channeled substrate buried heterostructure lasers are associated with higher order transverse mode transition and lateral mode movement. The active area has been reduced to stabilize the transverse mode. Fundamental mode operation up to high powers (24 mW/facet) has been obtained in devices with threshold current as low as 15 mA. Measurements and calculations are presented which show that nonlinearities occur at higher output power in lasers with reduced active area.

Optical properties of a 1.3-µm InGaAsP superluminescent diode

The optical properties of a 1.3-µm InGaAsP-InP buried heterostructure (BH) superluminescent diode (SLD) are described. The spectra of this device exhibit a large number of longitudinal modes. The light output at constant current from an SLD has a strong temperature dependence in the superluminescent region (high currents) and week temperature dependence in the spontaneous region (low currents). This severe temperature dependence will limit the systems application of this type of device unless thermoelectric cooling is used. The coupling efficiency (butt-coupling) into a 0.23-NA 50-µm-diameter graded index fiber is 26 percent. A model of the SLD including a temperature dependent nonradiative mechanism (same as in 1.3-µm InGaAsP lasers) suggests that the strong temperature dependence in the superluminescent region is a fundamental property of the device.

Fiber-dispersion and propagation-delay measurements with frequency- and amplitude-modulated cleaved-coupled-cavity semiconductor lasers.

By using combined frequency and amplitude modulation of a cleaved-coupled-cavity semiconductor laser together with a phase-sensitive detection scheme, we have measured the chromatic dispersion in a single-mode silica fiber. The method presented can measure dispersion down to 0.03 psec/nm km and, with a slight modification to the measurement routine, also measure the total propagation time with a 10(-7) accuracy.

Optical evaluation of indium gallium arsenide phosphide double-heterostructure material for injection lasers

Optical methods used for evaluation of InGaAsP double‐heterostructure (DH) material are described. The photoluminescence (PL) efficiency of the active layer in DH wafers and its spatial variation are shown to be correlated with the threshold current density of the broad area lasers processed from the corresponding wafers. The simultaneous measurement of the PL signal and the transmitted intensity of the excitation source through the active layer is a useful technique for monitoring imperfection in the active layer. The sheet conductivity of the epitaxial p layers and p‐n junction misplacement can be determined from the variation of the PL signal as a function of the power of the optical pump source. In addition, conventional spectrally and spatially resolved PL indicates the compositional homogeneity and the doping concentration of the active layer. In general, we find the 1.06‐μm yttrium aluminum garnet (YAG) laser to be a most convenient tool for evaluation of InGaAsP DH material.

High quality indium gallium arsenide phosphide double heterostructure material grown by the near equilibrium liquid‐phase‐epitaxy technique

A comparative study of the source seed and near equilibrium liquid‐phase epitaxy (LPE) growth techniques is described. The results of such important characteristics as broad area laser thresholds, quaternary active layer mismatch, and photoluminescence peak wavelength for 1.3‐μm InGaAsP double heterostructure material were compared. Double heterostructure material grown by the near equilibrium LPE growth technique exhibited a better overall quality. Moreover, run to run reproducibility of these same parameters, as well as their eventual intrawafer variability, were monitored over an extended period of time. Again, the near equilibrium LPE growth technique readily led to excellent quality double heterostructure material. Finally, these wafers were processed into oxide stripe gain guided lasers, having a high percentage of devices with acceptable electro‐optical characteristics.

Stripe geometry InP/InGaAsP lasers fabricated with deuteron bombardment

Stripe geometry gain-guided InP/InGaAsP lasers have reproducibly been fabricated with deuteron bombardment. Good electrical isolation was observed in 250-µm-long laser chips, and CW thresholds as low as 105 mA have been achieved.