H. S. Sommers

Experimental properties of injection lasers: modal distribution of laser power

The paper reports extensive measurements and analysis in terms of physical parameters of the frequency and spatial distribution of the radiation from GaAs injection lasers operated at room temperature. Covered are most of the internal configurations found in modern structures. All cavities have similar lengths and widths, cleaved uncoated facets, and sawed sidewalls. The power ranges from threshold to full drive as set by the limit for facet damage. Qualitatively the beam and spectral widths grow almost discontinuously with current near threshold and more slowly at higher drive, with up to a thousand cavity modes sharing the power at full output. Quantitative analysis is enabled by a new model of a laser cavity with sawed sidewalls and by a new equation for the modal power distribution function, which has the form of a sum over the normal modes of the cavity. Summing gives new predictive expressions for the beam shape and for the dependence of half‐width on current, both of which are found to be obeyed by...

Experimental properties of injection lasers. V. Strong polarization

The spontaneous and coherent properties of a laser with strong TE polarization (E parallel to the junction) are measured and discussed in terms of the nonlinear theory of injection lasers. The laser cavity is 330 × 100 × 0.25 μm with a symmetrical double heterojunction, sawed sides, and cleaved facets. Its dispersion is that of the normal modes of a rectangular box. The current dependence of spontaneous emission indicates a modest increase in population inversion across the lasing region, the maximum overdrive papameter reaching only 0.4. The power dependence of the overdrive parameter deduced from the spontaneous emission agrees well with that from the lateral profile of the external beam. Within a rather large uncertainty from several factors, the calculated value of 25% for the extra prorated loss of the TM modes compared to TE modes arising from facet reflection accounts quantitatively for the persistence of complete polarization to maximum safe output. The critical power associated with the optical nonlinearity is around 5 mW per cavity mode, in agreement with other reported values, and is shown to be independent of facet area as predicted.

Experimental studies of injection lasers: spontaneous spectrum at room temperature

The current dependence of the spontaneous and coherent intensities and differential intensities from GaAs injection lasers operated at 300 K is reported. A compensated material with single heterojunction and a low‐doped double heterojunction unit were studied. While the spontaneous intensities rise smoothly with current at all power levels, the differential intensities show structure associated with onset of lasing. At threshold there is a sharp cusp, indicating a change in the internal dynamics. The data are found to be consistent with 100% radiative recombination but do not exclude nonradiative transitions. New features are negative differential intensity of lasing; a logarithmic increase of Fermi level with current in the lasing region; and marked differences of the spreading of the lasing spectrum and of the current saturation of the two lasers.

Experimental properties of injection lasers. IV. Modes of large cavity with sawed sides

The article is concerned with determination of the type of modes in wide injection lasers and in particular with identification of the normal modes in the profile of the beam in the plane of the junction. In a cavity 70 μm wide × 120 μm long, with cleaved facets and sawed sides, the modes are found to be those of a rectangular parallelopiped of the same geometry. The smooth lateral profile of the radiated beam is broken into a series of sharp lobes by inserting a low‐resolution high‐intensity spectrometer between angle scanner and detector. The spectrometer resolution of a fraction of the separation of successive longitudinal mode groups rejects most of the off‐axis modes. Longitudinal mode numbers are unambiguously assigned to the lobes in the beam profile, out to 70° from the facet normal, and the dependence of lobe position on longitudinal number identifies the geometry of the modes. Consideration of the lobe widths and irregularities reveals that the coherence width of the laser modes is essentially i...

Experimental and theoretical study of the spatial variation of junction voltage and current distribution in narrow stripe injection lasers

The spatial variation of the junction voltage and current distribution are deduced from measured profiles of the intensity of high‐frequency light across the emitting facet of stripe lasers. The dependence of voltage on current follows closely the current dependence deduced from study of the external beam. Below, threshold, the shape of the voltage profile is found to be practically independent of current. The change at threshold in the differential conductance of the junction has only a minor effect on the profile, although its effect is observable over the entire front of the coherent waves. However, there is no apparent modification of the profile connected with the structure of the coherent illumination of the facet. A new theory is developed of the current spreading in a layered structure which successfully describes the features of the measured distributions. Using only material parameters of the layers of the laser, it predicts to good approximation the observed current spreading. The modifications...

Spontaneous power and the coherent state of injection lasers

A proposed relation between spontaneous and coherent properties of injection lasers is applied to several published results. On experimental and theoretical grounds it is shown that the parameter 1+X, the ratio of conventional gain coefficient to prorated loss for the strongest cavity mode, can be determined from the spontaneous emission. Over a limited range above unity, 1+X is the ratio of spontaneous intensity at the operating point to its value at threshold. This supplies a needed experimental connection between spontaneous studies at frequencies higher than the lasing band and the coherent properties. Published data of the current dependence of spontaneous emission are discussed in this framework, and the information needed to intercompare their internal state is pointed out. In general, fundamental differences in laser dynamics should be evidenced by radically different values of the critical parameter P* of the nonlinearity associated with the power dependence of X. All published values of P* are o...

Response of Photoconducting Imaging Devices with Floating Electrodes

A simplified charge control type of analysis is made of the response of photoconducting imaging devices with a floating electrode. The change of electrode potential with discharge of the device is found to have an importance not previously recognized. The results are relevant for a variety of electrostatic photographic devices and show the dependence of the performance on certain details of the image development process.The floating electrode limits the gain during exposure to light to one charge pair per quantum absorbed. For the Vidicon TV camera picture tube, a detailed study of the process of reading the stored information demonstrates that net gain exceeding unity can be achieved. The upper limit to the gain is the ratio of the time for the scanning beam to traverse a picture element to the transit time of a free carrier through the layer. This conclusion holds when the time required to trap a free carrier exceeds its recombination lifetime. None of the gain‐bandwidth relations yet deduced for photoc...

A comprehensive model of the injection laser: Formulation and test of the current dependence of spontaneous and coherent emission

Extrapolation of the conventional diode equation into the oscillating region gives formal expressions for the internal and external efficiencies in terms of waveguide parameters, threshold current, and the voltage derivative of laser power. The latter, which depends on band and laser theory, is examined for two models. Linear perturbation theory gives qualitative conclusions which are of little utility for actual devices. Combination of diode analysis with the band theory of Stern and the P* theory of North gives a comprehensive theory of the current dependence of the static properties of the laser. The assumptions of the extrapolation are discussed, and experimental examples are presented to illustrate the utility of the new diode analysis. The comprehensive theory is used to interpret various known properties of devices such as linearity of power with current, change of internal efficiency with waveguide thickness, change of junction resistance at threshold, and several scaling laws. The types of experi...

Experimental properties of injection lasers. VII. Narrow stripe lasers with rigid waveguide

The properties of a type of (AlGa)As double‐heterojunction laser with buried cavity are studied as a function of current and of delay after the start of a 10‐μs square current pulse. It is found that the waveguide walls are rigid, for the mode shapes, mode dispersion, threshold, efficiency, and power‐current characteristic are independent of current and delay. The spectra are also independent of delay except for a wavelength shift from the change of temperature. The modes are tightly confined with a half‐width of 2 μm of the fundamental mode at the facet. The two lowest spatial modes dominate the emission, which is almost completely contained in one spectral doublet. The power in each member of the doublet (single‐mode power) is linear in junction voltage from less than 100 μW to the full output of 3 mW, with slope giving a critical power P*≈10 mW as in wide lasers with rigid guides. The strong doublet is transferred to successively longer wavelength modes as the current or delay is increased. The transfe...

Experimental properties of injection lasers. III. Current dependence of polarization

The power distribution function which describes the shape and current dependence of the lateral beam is applied to a phenomenological model of weakly polarized injection lasers. It predicts that the polarization of coherent power should drop rapidly from an infinite value at threshold to near unity at high drive. Careful measurement of polarization of a laser representative of the model, with especial attention to rejection of the spontaneous emission, supports this conclusion. The form of the power distribution function used in the analysis implies that lasers with strong polarization at high drive have large threshold differences of the two polarizations, suggesting that present theories of the source of polarization are inadequate. The difficulty of interpreting the present results in terms of spatial hole burning or inhomogeneities is pointed out.