Fiber-integrated terahertz spectrometer driven by ultrafast ytterbium doped fiber laser

Keskin, Hakan
In this thesis, development of a Terahertz time domain spectrometer (THz-TDS) driven by an ultrafast Ytterbium (Yb) doped ber laser, whose repetition rate can be tuned, was investigated to show that it can be used in Optical Sampling by Cavity Tuning (OSCAT) technique which enables the fast acquisition of THz pulse pro les. Central wavelength of the Yb-doped laser developed for this study is 1031 nm. The output average power was measured to be 90 mW which corresponds to about two nanojoule per pulse at 51 MHz repetition rate, and the pulse duration was measured with an autocorrelator to be 80 fs. Average output power and pulse duration stability of Yb-doped ber laser oscillator was examined at di erent repetition rates in order to test convenience of system for THz measurements. By tuning the repetition rate at 51 MHz by 25 kHz, we can scan a 100 ps time-window for the THz pulse measurement. This corresponded to a 6 mm cavity length tuning which altered the repetition rate of oscillator from by 53 kHz. Afterwards, THz-TDS system was built to understand the dependence of THz pulse pro le on the repetition rate of the Yb-doped ber laser. In this system InGaAs based photoconductive antenna (PCA) was used for THz generation and 2 mm thick <110> orientation ZnTe crystal was used in the detection part. Due to the limitation of the balanced photodetector with respect to the central wavelength of the laser, only a bandwidth up to 0.5 THz with a signal to noise ratio(S/N) of 60 was observed. THz signals measured at di erent cavity lengths overlap in both time domain and magnitudes which mean that THz measurements were not a ected by tuning the repetition rate of Yb-doped ber laser. In order to achieve this result, source parameters such as pulse duration and power of Yb-doped ber laser should be constant between measurements while scanning cavity length. By comparing THz signals at di erent repetition rates, it was shown that repetition rate tuning of Yb-doped oscillator can be used in OSCAT technique. In other words, we have shown that a Yb-doped ber laser can be used to rapidly scan the THz pulse pro le for future measurements.