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High‐frequency modulation and suppression of chirp in semiconductor lasers

Vera B. Gorfinkel, Serge Luryi

1993
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Applied Physics Letters
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We propose a new method for modulating laser radiation by controlling simultaneously the pumping current and the optical gain in the active region. The latter can be independently varied by modulating the effective carrier temperature. The method allows to eliminate the relaxation oscillations and enhance the modulation frequency to 50 GHz. It also allows to suppress the wavelength chirping in optical communication systems operating at pulse repetition rates of 10 Gb/s. The common method of
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... lating the laser radiation amplitude by varying the pumping current suffers from two drawbacks. First, it is limited to relatively low frequencies ( f 5 10 GHz). Second, for f ,> 1 GHz, it is plagued by oscillations in the wavelength of the dominant mode (chirp). Both of these problems arise from the relaxation oscillations due to an intrinsic resonance in the nonlinear laser system (the electron-photon resonance). An alternative principle for modulating the laser output is to directly control by external means the gain coefficient go of the active medium, e.g., by varying the effective carrier temperature T, in the laser active region. Two highfrequency electron-heating mechanisms have been considered: (i) driving an electric current through the active region' and (ii) inducing intersubband absorption in quantum wells.' High-frequency modulation of T, by several tens of degrees has been demonstrated experimentally.3 Although this method in principle allows a faster laser modulation, by itself it eliminates neither the relaxation oscillations nor the frequency chirp. The new approach, proposed in this work, allows an enhancement of the coding frequency, suppressing the chirp at the same time. The key idea consists in a coherent combination of two independent means of controlling the output radiation: the pumping current I and the effective carrier temperature Tee We consider a semiconductor laser, subject to a timedependent pumping current I(t) and an external electronheating power P(t) . The nonlinear system can be described by standard rate equations4 for the carrier density n and the photon density S per unit volume as follows: dn z=J-Sg-Bn2, d.S x=(rg--7ph*9S+BBn29 coupled with an energy balance equation for T, dTe T,-T x=P( t) ---: .

doi:10.1063/1.109198
fatcat:4q3eapuvi5bxncgeq5keulghdq