In this paper we present measured modulation responses on quantum-cascade lasers (QCL) up to 10 GHz. The obtained modulation response shows a flat response over the whole frequency range, proving the broadband capabilities of these devices. Even more striking is the absence of a strong resonance peak, which demonstrates the absence of relaxation oscillations, a feature which often limits the high speed performance of standard diode lasers. This property is quite attractive for use of these devices as high-speed data sources, particularly in applications where linearity is important. To address this possible application, the digital modulation properties of these devices were tested in a standard bit-error-rate-measurement at 2.5 GBit/s for cryogenically cooled QCL, showing the suitability of QCLs in digital as well as analog telecommunication application. In addition we present recent data showing bit error measurements and eye diagrams obtained for the pulsed mode operation of QCL near room-temperature.
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