F. R. Nash

Improved light‐output linearity in stripe‐geometry double‐heterostructure (Al,Ga)As lasers

Nonlinearities in the optical‐power‐output–versus–current characteristics of (Al,Ga)As stripe‐geometry double‐heterostructure lasers are found to be associated with excessive spectral broadening, asymmetries in the outputs from the two mirrors, and spatial movements of the lasing mode within the width of the stripe. Manifestations of these nonlinearities are severely deleterious in a variety of optical fiber communication applications. It is shown that the impact of these defects may be significantly reduced, with little lasing‐threshold‐current penalty, by fabricating devices with narrower stripe widths.

Semiconductor laser damage due to human-body-model electrostatic discharge

Various types of InP-based semiconductor lasers, Fabry–Perot (FP), and distributed feedback (DFB), in different wavelength regions of 1.3, 1.48, and 1.55 μm have been subjected to human-body-model electrostatic discharge (ESD) testing. The reverse V-I characteristics of these diode lasers were found to be generally most sensitive in detecting ESD damage than the forward characteristics (e.g., threshold current) of the laser. The laser ESD failure voltages were much lower for the reverse than the forward polarity and DFB lasers were found to be more vulnerable to ESD than FP lasers. The failure mechanism was found to be due to localized melting—a thermal effect—in both polarities of ESD testing. We also report the study of the latent ESD effects on the long-term aging rates of semiconductor lasers.

GaAs laser reliability and protective facet coatings

We have investigated the effects of Al2O3 facet coatings on the reliability of GaAs‐ (Al,Ga)As double‐heterostructure stripe‐geometry (12 μ) lasers operated cw in a 70 °C dry‐nitrogen ambient with power outputs equal to 5±2 mW/facet. No improvement was seen in the median lifetime of coated devices (some of which also had their sidewalls etched to remove saw‐cut damaged material) in either a nominally short‐lived class (≲100 h, 70 °C) or a long‐lived class (≳1000 h, 70 °C). Corroborating observations of the luminescence originating in the active layer and viewed through a partially contacted n side showed that commonly seen defects within the stripe, thought to be among the causes of laser degradation, were not eliminated by the presence of facet coatings. These defects are (i) a structureless ’’darkening’’ which, when it occurs, always does so at one or both mirrors, and (ii) the previously identified dark‐line defect (DLD) which often, but not always, occurs at a mirror. The DLD is associated more with t...

Observations of spontaneous phase locking of TEM 00q modes at 0.063µ

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.

Broadband optical coupling modulation

A KD*P internal coupling modulator has been operated in a high-power single-frequency 0.63-micron He-Ne laser similar to that described earlier by Smith. It is shown that distortion in the output due to resonant buildup of energy in the passive cavity modes when the modulation frequency is equal to c/2L can be reduced to an acceptable level. The modulator bandwidth was greater than 700 MHz, and the maximum optical output power was 3.6 mW. For an optical output power of 0.5 mW, the performance index was estimated to be 0.4 mW of RF driving power for each MHz of bandwidth.

Stabilization of aging‐induced self‐pulsations and the elimination of an initial temporally saturable mode of degradation in (Al,Ga)As lasers by means of facet coatings

We demonstrate that half‐wavelength facet coatings of Al2O3 can arrest the growth of pulsations in the output intensity of (Al,Ga)As double‐heterostructure lasers during cw aging in a 70 °C dry nitrogen ambient while also eliminating an initial mode of degradation. These concurrent effects of the coatings suggest a common origin for the aging‐induced pulsations and the initial degradation and also support a recently proposed model for the origins of pulsations.

Purging: a reliability assurance technique for semiconductor lasers utilizing a purging process

Prior to packaging, semiconductor lasers are purged by being subjected first to high temperature and high current simultaneously so as to suppress stimulated emission and stress the shunt paths which allow leakage current to flow around the active region. A prudent, but nonessential, second step is to lower the temperature and/or current so that the lasers emit stimulated emission (preferably strongly, near the peak output power), thereby stressing the active region. Lasers subjected to such a purge exhibit stabilized degradation rates in short times (of the order of a few hours) and provide a robust population which meets the performance criteria of long lifetime systems.

Reduced temperature dependence of threshold of (Al,Ga)As lasers grown by molecular beam epitaxy

We report pulsed‐current thresholds for stripe‐geometry lasers fabricated from some ternary active layer molecular beam epitaxially grown wafers, which are nearly temperature independent over a ≊100 °C range. Lasers from one such wafer, pulse‐operated over the practically important temperature range about room temperature, exhibit only a 3‐mA threshold decrease in going from −10 to +20 °C and only a 3‐mA increase in going from 20 to 50 °C. Fitting the data over small temperature intervals using the commonly observed exponential function for the temperature dependence of threshold (I = I0 exp(T/T0)) results in negative, infinite, and large positive values (⩾300K) for T0, as the temperature is increased from −10 to 75 °C. Lasers with such a range of threshold insensitivity may reduce pattern‐dependent effects and may have significant practical implications for the simplification of the feedback circuitry used to maintain the pre‐bias operating current in laser transmitters. Data examined over the temperatur...

Improvements of the electro‐optic properties of (Al,Ga)As lasers by means of reduced reflection dielectric facet coatings

We demonstrate that reduced reflectivity facet coatings of Al2O3 on both mirrors can increase the output power levels at which ’’kinks’’ in the light‐current characteristics of proton‐delineated 12‐μm stripe‐geometry (Al,Ga)As lasers occur without causing a significant softening of the ’’turn‐on.’’ The dielectric coating is intermediate between half‐ and quarter‐wavelength thicknesses. The advantage of this solution to the kink problem is that it can restore to usefulness lasers which had been previously rejected solely because their kinks occurred just at the power output desired for optical communication application. Although the spontaneous emission output just below threshold is increased somewhat, operation with an extinction ratio equal to 15:1 and reasonable limits on the amplitude of the modulating current is still quite possible. It is also found that reduced reflection coatings can decrease the depth of modulation of self‐induced pulsations and decrease the effects of ’’light jumps.’’ The benefi...

Stabilization of aging-induced self-pulsations and the elimination of an initial saturable mode of degradation in (Al,Ga)As lasers by means of facet coatings

We demonstrate that half-wavelength facet coatings of Al2O3can arrest the growth of pulsations in the output-intensity of (Al,Ga)As double heterostructure lasers during cw aging in a 70°C dry nitrogen ambient while also eliminating an initial saturable mode of degradation. These concurrent effects of the coatings suggest a common origin for the aging-induced pulsations and the initial degradation and also support a recently proposed model for the origin of pulsations based upon nonradiative surface recombination.