Asynchronous optical sampling THz spectroscopy system based on monolithically modelocked laser diodes

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Our fundamental goal in this continuation project remains the development and realization of an operating extremely compact THz Time-domain sampling measurement system. Our concept is based on monolithically modelocked diode lasers and asynchronous optical sampling. In our first approach, one laser diode in combination with a photoconductive antenna acts as THz sender, and a second identical laser diode is synchronized to the first but operates on a slightly different repetition frequency in order to sample the THz signal received at the receiver photoconductive antenna asynchronously. For an even more compact system, we aim to integrate sender and receiver laser on one chip and to address the photoconductive antennas with fibers. In our second and new approach, we will realize an ASOPS-system with only one monolithically modelocked laser diode. The repetition frequency of the laser diode will be slowly varied and the receiver will be gated with a signal that is delayed with respect to the sender. This principle (Optical Sampling by Cavity Tuning, OSCAT) shall be demonstrated here with compact diode lasers for the first time. In the first part of the project we have demonstrated the first diode laser based THz-ASOPS-system. After this first demonstration with compact modelocked external cavity laser diodes, a first system with monolithically modelocked diodes and synchronization electronics has been set up. In addition to the proof-of-principle, our studies have shown the major challenges of the concept which we plan to address in this continuation project. These challenges include the specifications of the used laser systems, in particular the output power, the pulse width of the emitted pulses, and he stability of the repetition frequency. For the output power, we plan a further increase to > 50 mW average power, in order to achieve a better signal to noise ratio for the THz signal. To optimize the pulse width of the pulses hitting the antennas, two steps are required: first, the gain material of the semiconductor has to support the generation of pulses with a sufficient spectral bandwidth, and second, the strongly chirped pulses emitted by the laser diodes have to be compressed. For that, we use optimized semiconductor material using chirped quantum wells for a large gain bandwidth and compress the pulses with fibers with tailored dispersion and length. We aim to achieve pulse widths below 500fs and a THz bandwidth above 1.5 THz. The stability of the repetition frequency is a further major challenge. We plan to realize pulse trains with temporal jitter below 500 fs by different approaches. First we will reduce the jitter in the current ASOPS approach by optimized electronics, optical feedback and integration of the two lasers on one chip. Second, we will analyze, as a new approach, Optical Sampling by Cavity Tuning (OSCAT) with only one monolithically modelocked diode laser with slow repetition frequency variation.