Spectral characteristics of single-mode injection lasers: The power-gain curve from weak stimulation to full output

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Abstract
Experimental study of the mode powering in an assortment of single-mode lasers covering samples of most of the contemporary structures reveals a common property: the functional dependence on gain of the power in the strong mode. The high-power asymptote is a linear function of the gain, while the low-power asymptote of the light-emitting diode (LED) state is linear in the reciprocal of the departure of the gain from its threshold value. The entire dependence is expressed by a universal dimensionless function of the gain, which is independent of laser structure, and which describes both asymptotes, provides a reasonable connecting link, and gives an experimental fiduciary of threshold. The fiduciary is the current at the sharp maximum in the logarithmic derivative of mode power with respect to junction voltage. Exerimental study reveals that this fiduciary corresponds closely to the conventional extrapolated threshold, with the inflection point in the junction resistance, and with the peak in noise. The universal function yields the dependence of power in the dominant mode on junction voltage given by the steady-state solution of North’s P* theory. It is not consistent with the gain clamping at threshold that is described by familiar hole-burning theories of injection lasers, which limit the lasing region to far too small a range of gain. The same function is found to describe a published measurement of the transition of threshold of an He-Ne laser.