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Investigations of the dynamics of the far-infrared p-Ge laser emission reveal strong periodic soliton-like intensity spikes with less than 100 ps duration. We interpret these spikes as self-mode-locking of p-Ge laser modes. The effect becomes more pronounced when a GaAs/ AlGaAs/InGaAs quantum well structure on a semi-insulating GaAs substrate is inserted into the laser cavity.

Operation of the far-infrared gallium-doped p-type germanium laser in the broadband range above 70 cm À1 , which requires relatively high magnetic fields, is demonstrated using novel permanent magnet assemblies. NdFeB and SmCo magnet assemblies were compared. The applied field from a NdFeB assembly was found to decrease strongly on cooling to the cryogenic temperature required for p-Ge laser operation, but that of a SmCo assembly changes insignificantly.

Abstract-The temporal dynamics, spectrum, and gain of the far-infrared p-Ge laser for composite cavities consisting of an active crystal and passive transparent elements have been studied with high temporal and spectral resolution. Results are relevant to improving the performance of mode-locked or tunable p-Ge lasers using intracavity modulators or wavelength selectors, respectively. It is shown that an interface between the active p-Ge crystal and a passive intracavity spacer causes partial frequency selection of the laser modes, characterized by a modulation of their relative intensities. Nevertheless, the longitudinal mode frequencies are determined by the entire optical length of the cavity and not by resonance frequencies of intracavity sub-components. Operation of the p-Ge laser with multiple interfaces between Ge, Si, and semi-insulating GaAs elements, or a gap, is demonstrated as a first step toward a p-Ge laser with an external quasioptical cavity and distributed active media.Index Terms-Laser modes, laser tuning, submillimeter wave lasers, submillimeter wave resonators, submillimeter wave spectroscopy, submillimeter wave technology.

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Active mode locking of the far-infrared p-Ge laser giving a train of 200 ps pulses is achieved via gain modulation by applying an rf electric field together with an additional bias at one end of the crystal parallel to the Voigt-configured magnetic field. Harmonic mode locking yields a train of pulse pairs with variable time separation from zero to half the roundtrip period, where pulse separation is electrically controlled by the external bias to the rf field.

A robust metal-free intracavity fixed-wavelength selector for the cryogenically cooled far-infrared p-Ge laser is demonstrated. The device is a back mirror consisting of a thin silicon etalon and dielectric SrTiO 3 flat. A laser line width of 0.2 cm Ϫ1 is achieved, which corresponds to an active cavity finesse of ϳ0.15. The wavelength position and spectral purity are maintained over a wide range of laser operating fields. Use of SrTiO 3 lowers the laser resonance line frequencies by ϳ1 cm Ϫ1 compared with expectations for metal mirrors. The effect is due to phase shift, which is determined from far-infrared reflectivity measurements of SrTiO 3. A p-Ge laser with such selector is free from danger of electrical breakdown and mirror oxidation during repeatable thermal cycling, which makes it more reliable than previous selection schemes for practical applications.

Abstract