Junction Lasers Research Articles

Spatial, spectral, and temporal resolved measurements of semiconductor injection lasers - Part I. Homostructure lasers: Mode formation

Spatial, spectral, and temporal (SST) resolved measurements are carried out on homostructure junction lasers at room temperature. Two distinctly different mechanisms of transverse-mode formation are illustrated. One is attributed to anomalous interaction of spectral modes where their formations take place within a few nanoseconds of each other and is characterized by spectral separation of a few tenths of a nanometer with the eventual domination by the long wavelength mode/s. In the other, a second mode at shorter wavelength appears only above a current I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> . The delay of this second mode is well-fitted by the equation <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t_{d2} = \tau \ln I_{1}/(I - I_{0})</tex> with τ of the order of 27 ns. The separation of the two spectral modes increases during the pulse and finally settles to a magnitude an order higher than the first case. It is experimentally observed that a cavity-width reduction mechanism is present as the laser emission evolves during a pulse. Evidence of a non-linear gain versus injected carrier concentration due to the switching of power from one mode to another is discussed. The results have been explained in terms of changes in the cavity brought about by refractive index variation, gain distribution, or saturable absorption.

Changes in the optical properties of CW (AlGa)As junction lasers during accelerated aging

Changes occuring in selected optical properties of CW (AlGa)As double-heterostructure junction lasers during the initial stage of accelerated aging are reported. The characteristics selected for investigation are relevant to the operation of the laser in an optical communication system and include the current dependence of the lasing emission and the orthogonally polarized (nonlasing) emission, the symmetry of the emission with respect to the two output faces, the time stability of the light intensity, and the frequency of the internal laser resonance. In spite of the variability among the aging behaviors of individual lasers, characteristic changes did emerge as statistically significant. For example, a substantial decrease in the emission symmetry was commonly observed. A second important effect was the creation of a self-induced intensity pulsation, at a frequency typically between 300 and 600 MHz, in lasers which initially exhibited no intensity modulation. A statistical analysis of the data was made to correlate these instabilities with asymmetries in the lasing or nonlasing emission and/or changes in the emission symmetry. No evidence was found to support previously published conjectures that the pulsations are caused by the formation of dark-line defects or by axial nonuniformities in the pumping current.

Below-threshold waveguiding in a stripe-geometry junction laser

Direct measurements of the intensity profile along the junction plane of a cw stripe-geometry junction laser are reported for currents extending from well below to above threshold. We find that the spatial profile of the optical intensity emitted at the wavelength of one Fabry-Perot mode exhibits a pronounced narrowing as the current is increased toward threshold. The observed behavior is shown to be consistent with an intensity distribution consisting of a Gaussian mode with current-independent width superimposed on a background of spontaneous emission. This interpretation suggests that waveguiding exists along the junction plane at currents as low as 50% below threshold and is at most weakly dependent on current.

Saturation of the junction voltage in stripe-geometry (AlGa)As double-heterostructure junction lasers: A comment

Recently published data concerning a contemporary well-behaved cw stripe-geometry injection laser was accompanied by the assertion that above threshold both the junction voltage and the spontaneous emission exhibit ’’complete saturation’’. This paper shows by detailed analysis of the data that in fact the small incompleteness of the saturation agrees with the value predicted by the new nonlinear theory of injection lasers. The junction voltage, spontaneous emission, and mode powering of this cw laser are interconnected by the theory just as they were in the various types of pulsed lasers already described.

Observation of second derivatives of the electrical characteristics of double-heterostructure junction lasers

Measurements of the second current derivative of the external voltage d2V/dI2generated by stripe-geometry (AlGa)As junction lasers are reported and correlated with corresponding measurements of the first derivative dV/dI as a function of current. In the vicinity of the lasing threshold, a pronounced negative peak occurs in the second-derivative signal as a result of the voltage saturation induced by the stimulated emission. With further increases in current, the second derivative assumes nonzero positive values for lasers with incomplete saturation of the junction voltage, while complete saturation is characterized by a near-zero value for the derivative. A pronounced fine structure observed in the second derivative at currents above threshold indicates the presence of perturbations in the saturated state of a nominally well-behaved laser.

Nonlinearities in the emission characteristics of stripe-geometry (AlGa)As double-heterostructure junction lasers

Significant changes in the optical and electrical properties of stripe-geometry (AlGa)As double-heterostructure junction lasers have been observed to accompany a nonlinearity in the current dependence of the lasing emission. Over the nonlinear range, the optical field of the lasing emission shifts continuously with current toward one boundary of the active stripe. Simultaneously, effective gain saturation is lost and additional transverse modes are excited. The increasing gain is believed to occur primarily in spatial regions where the optical intensity is reduced as a result of the transverse motion of the field. Additional observations suggest that this transverse motion results from an interaction between the intense lasing field and the active medium. Consequently, gain depletion is discussed as a possible cause for the transverse instability. Implications of this model for a qualitative understanding of the improved output performance recently obtained from lasers with narrow stripes are also considered.

Determination of the lasing threshold in stripe-geometry double-heterostructure junction lasers

Measurements of the noise fluctuations in the light intensity emitted by stripe-geometry junction lasers are used to relate the threshold for amplitude-stabilized oscillation to the behavior of the first and second derivatives of the diode’s electrical characteristics. The amplitude stabilization is observed to begin at a current above the point where the second-derivative signal attains its maximum value but below the current at which the junction voltage is fully saturated. Consequently, the electrical derivatives do not, in general, sense the exact threshold current but only provides bounds on its value.

Derivative measurements of the current-voltage characteristics of double-heterostructure injection lasers

Measurements of the electrical characteristics of double-heterostructure (DH) stripe-geometry AlGaAs junction lasers are reported. We show that the electrical characteristics of these lasers are adequately represented by a resistor in series with a p-n junction characterized by a classical diode equation, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I = I_{s} [\exp (q V_{j}/\etakT) - 1]</tex> . Devices have been measured with almost ideal junction characteristics with both the saturation current, I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</inf> , and the exponential parameter, η, constant, while in other diodes both η and I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</inf> exhibit a strong current dependence. Measurement of the first derivative <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dV/dI</tex> and the product <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">IdV/dI</tex> is shown to be an especially useful technique for electrical characterization, yielding accurate and direct determination of the series resistance, the exponential junction parameter, and the laser threshold current. The observation of lasing threshold from an electrical measurement, instead of from the more usual optical measurement, is discussed in terms of the saturation behavior of the junction voltage at and above threshold.

Electron-hole recombination in nitrogen-doped direct-band-gap GaAs1−xPx

Optical phase-shift lifetime measurements (77 °K) on N-free and N-doped direct-band-gap GaAs1−xPx are studied to determine the effect of the N isoelectronic trap on the lifetime spectrum τ (λ). An analysis of the recombination processes in direct-gap crystal is used to provide estimates for the N-trap carrier capture time and capture cross section, which for the NX state is σt≳10−13 cm2. It is shown that the minority-carrier lifetime is decreased by the addition of N doping. This effect, which is attributed to nonradiative recombination paths fundamental to the N trap, is thought to be the reason for the increased laser thresholds of N-doped In1−yGayP1−zAsz/GaAs1−xPx junction lasers as compared with similar N-free devices.

Saturation of the junction voltage in stripe-geometry (AlGa)As double-heterostructure junction lasers

Saturation of the junction voltage has been observed to occur at the onset of lasing in cw stripe-geometry (AlGa)As double-heterostructure junction lasers. Simultaneous measurements of the nonlasing but amplified spontaneous emission confirm that saturation of the optical gain and spontaneous emission accompany the voltage saturation as expected. With well-behaved lasers the observed saturation is maintained over currents to at least 50% above threshold. In other devices, a loss of saturation is commonly observed to occur simultaneously with a nonlinearity in the current dependence of the lasing emission.

Characteristics of RF injection locking of self-pulsing in an AlGaAs DH junction laser

The minimum RF power required for injection locking of the light pulsations in the output of a CW room-temperature AlGaAs double-heterostructure (DH) junction laser was found to depend exponentially on ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f-f_{r}</tex> ), where <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</tex> is the RF injection frequency and f <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</inf> is the self-pulsing frequency, and also, to depend inversely on the self-pulsing strength of the laser. Nearly 100-percent modulation depth is achievable at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f=f_{r}</tex> , and slight reduction of modulation depth occurs when <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f \gg f_{r}</tex> .

Analysis of intensity fluctuations in a junction laser with low quantum-efficiency regions

Fluctuation spectra due to quantum shot noise in the output from a junction laser are calculated using rate equations for a non-uniform double-sectioned laser with a low quantum-efficiency region. The pumping-rate dependence of the fluctuation spectra for the non-uniform model is found to be roughly the same as that for the ordinary uniform model. The degradation process is then simulated using the uniform and nonuniform models, and it is shown that qualitatively opposite changes, decay or growth of resonance phenomena, are ex- pected in the fluctuations as the degradation proceeds uniformly or nonuniformly.

Theory on long time delays, Q-switching and related phenomena in GaAs junction lasers

We present a theory that explains long time delays. Q-switching and related phenomena in GaAs junction lasers, based only on processes known to occur in these lasers.

Low-frequency intensity fluctuations of c.w. d.h. GaAlAs-diode lasers

The optical-emission fluctuations and the current noise of c.w. d.h. GaAlAs junction lasers have been investigated for the first time in the low-frequency range. In both the optical noise spectra and the current noise spectra, a l/f behaviour has been observed. It was found that a strong crosscorrelation exists between optical-emission noise and forward-current noise. Finally, an optoelectronic feedback method that practically eliminated the relative excess light noise has been demonstrated.

Transverse mode control of junction lasers optically coupled to single-mode fibers

Preliminary experimental results are reported to demonstrate that fundamental transverse mode operation can be obtained from a multimode double-heterostructure (DH) junction laser while simultaneously coupling its output into a single-mode optical fiber. Mode selection has been achieved by three different methods: 1) incorporating the fiber into the resonant cavity; 2) using the reflections off the diode-fiber interface; and 3) by a combination of the aforementioned methods. In all approaches the diode-fiber interface effectively becomes a mirror with a reflectivity which is higher for the fundamental mode than for all other modes of the DH waveguide. By using the reflections off the diode-fiber interface, a peak pulsed power of 80 mW has been obtained from the uncoated end of a single-mode fiber butted against a P-p-n-N junction laser. This laser radiated a peak power of 165 mW in a first-order mode when operated without coupling to the fiber.

Depolarization of the lasing emission from CW double-heterostructure junction lasers

Measurements of the polarization characteristics of the optical emission from CW Al z Ga 1-z As-Al x Ga 1-x As double-hetero-structure (DH) junction lasers are reported. Spectral observations demonstrate that in many lasers the lasing emission is not linearly polarized as expected from the conventional electromagnetic (EM) theory of the laser. A small but measurable field component is observed orthogonal to the main emission. In some lasers, the optical field is shown to be elliptically polarized, while in other devices the polarization state is more complex. The observed depolarization of the lasing modes is attributed to a strain-induced anisotropy in the dielectric properties of the laser waveguide. The anisotropic configuration which leads to elliptically polarized light is analyzed in detail and is shown to be equal to a pure shear in a plane parallel to the junction.

A new technique for measuring the thermal impedance of junction lasers

A new method is presented to measure the temperature of the optical cavity of a junction laser as a function of the average electrical power supplied to the device. The technique relies upon a null measurement of the exact wavelength of a single Fabry-Perot mode and therefore does not require a preliminary calibration measurement. Because of the small linewidth of the mode, very small ( C) temperature differences are easily measured. The method is utilized to obtain the thermal impedance for several different types of bonded stripe-geometry junction lasers.

Degradation and intensity fluctuations in CW AlGaAs double-heterostructure junction lasers

The intensity fluctuations in the output of CW AlGaAs semiconductor lasers were measured as functions of pumping current and operating time. The magnitude of fluctuation decreases with increasing aging time. The shift after degradation in the resonant-peak frequency for the normalized pumping rate is small. An increase of the half-width of the resonance peak is also observed, together with an increase of the threshold current and a decrease of the external differential quantum efficiency. The fluctuation spectra due to the quantum shot noise were calculated using rate equations with parametric variation of the pumping rate and the internal quantum efficiency. From the qualitative agreement between calculated and experimental results, changes in the intensity fluctuations after degradation are considered to be partly due to a decrease of the internal quantum efficiency or gain.

Suppression of intensity self-pulsations in CW junction lasers by frequency-selective optical feedback

Suppression of the self-induced intensity pulsations from CW junction lasers has been achieved by operating the laser with frequency-selective optical feedback. The feedback is achieved by optically coupling a diffraction grating to a laser mirror. The feedback suppresses all longitudinal modes except one or two and thereby eliminates the mode interactions necessary for sustained optical pulsing. The experimental results directly support the previously proposed model for self-induced optical pulsing in semiconductor lasers.

Noise characteristics of stripe-geometry double-heterostructure junction lasers operating continuously - I. Intensity noise at room temperature

Measurements of the noise fluctuations present in the output intensity from stripe-geometry double-heterostructure junction lasers operating continuously at room temperature are reported. In some but not all the lasers studied, the low-frequency (50-MHz) fluctuations exhibit the quieting expected of an amplitude-stabilized oscillator operating above threshold. The intensity noise in these lasers becomes shot noise limited at currents about 10 percent above threshold, even when many longitudinal modes are oscillating. Additional measurements demonstrate explicitly the effective elimination of the wave-interaction, or excess, noise during the transit of the threshold region. However, in other lasers which are nominally similar, only a partial reduction of the excess noise occurs above threshold, resulting in a noise level which can be more than ten times the shot-noise limit. In addition, we find in all lasers studied thus far no quieting at high frequencies (4 GHz), in apparent contradiction to the behavior expected of a well-stabilized oscillator.