Materials Science and Engineering PhD Thesis Defense by İsmail Yorulmaz

KOÇ UNIVERSITY

GRADUATE SCHOOL OF SCIENCES & ENGINEERING

MATERIALS SCIENCE AND ENGINEERING

PhD THESIS DEFENSE BY İSMAİL YORULMAZ

Title: Continuous-wave Diode Pumping and Pulsed Operation of Alexandrite Lasers near 760 nm and Tm³⁺:YLF Lasers near 2300 nm

Speaker: İsmail Yorulmaz

Time: December 18, 2017, 13:00

Place: CASE 136

Koç University

Rumeli Feneri Yolu

Sariyer, Istanbul

Thesis Committee Members:

Prof. Dr. Alphan Sennaroğlu (Advisor, Koc University)

Assoc. Prof. Menderes Işkın (Koç University)

Assoc. Prof. Uğur Ünal (Koç University)

Assoc. Prof. Sarper Özharar (Bahçeşehir University)

Prof. Dr. Gönül Eryürek (Istanbul Technical University)

Abstract:

Alexandrite (Cr³⁺:BeAl₂O₄) and Tm³⁺:YLF lasers are important, emerging solid-state lasers which generate coherent radiation in the near-infrared (700-820 nm) and mid-infrared (1800-2100 nm and 2200-2400 nm) regions of the electromagnetic spectrum, respectively. Both lasers have potential applications in diverse fields including biomedical imaging, surgery, ranging and spectroscopy.

The experimental studies presented in this thesis investigate both the continuous-wave diode pumping and pulsed operations of Alexandrite lasers near 760 nm and Tm³⁺:YLF lasers near 2300 nm. In both cases, low threshold continuous-wave diode-pumped operation was demonstrated. In the case of the Alexandrite laser, pulsed operation was achieved by using the method of self-Q-switching. Passive Q-switching was employed in the case of the Tm³⁺:YLF laser to generate pulses by using a Cr²⁺:ZnSe saturable absorber.

The first part of the thesis focuses on the investigation of the temperature-dependent spectroscopic properties of the Alexandrite crystal and diode-pumped operation of the Alexandrite laser. First, the emission intensity and the fluorescence lifetime of Alexandrite were shown to decrease with increasing crystal temperature. Second, the diode-pumped laser performance was investigated in detail. In the laser experiments, the maximum output power of 48 mW was obtained with a slope efficiency of 36% at the input diode pump power of 170 mW. The laser slope efficiency decreased from 36% to 12% as the temperature of the gain medium was increased from room temperature to 200°C. Self Q-switching with pulse widths in the range of 5-15 µs and repetition rates in the range of 10-35 kHz was further observed by slightly changing the curved mirror separation of the cavity.

In the second part of this thesis, the continuous-wave operation of a 2.3-µm Tm³⁺:YLF laser was investigated. First, the excitation spectrum of the Tm³⁺:YLF was measured by using a tunable, narrow-linewidth Ti³⁺:sapphire laser. Also, the average absorption cross-section of the 1.5 at. % Tm³⁺:YLF was determined to be 0.77×10^-20 cm² by using power-dependent and position-dependent absorption saturation data. A single-mode 120-mW diode laser was then used for pumping the Tm³⁺:YLF laser cavity at 792 nm. In this configuration, low threshold lasing could be achieved with as low as 25 mW of input pump power by using a 1% output coupler. The maximum output power of 10.5 mW was obtained at 2305 nm with a slope efficiency of 11.4%. Second, by using a 250-mW diode laser, the threshold pump power and slope efficiency were measured as a function of effective output coupling. The minimum threshold pump power of 4 mW was measured at 0% output coupling. Power efficiency measurements showed that the highest slope efficiency of 10% was obtained around 0.7% output coupling and that the slope efficiencies beyond this output coupling decreased monotonically. The stimulated emission cross-section at 2305 nm was determined from the laser threshold data as 0.55×10^-20 cm².

The third part of this thesis focuses on the pulsed operation of the Tm³⁺:YLF laser at 2.3 µm. To the best of our knowledge, passive Q-switching of a 2.3-µm Tm³⁺:YLF laser was demonstrated for the first time by using a Cr²⁺:ZnSe saturable absorber. The pulse durations and repetition frequencies of the passively Q-switched pulses were in the ranges of 1.2-1.4 µs and 0.3-2.1 kHz, respectively. By using the power-dependent repetition frequency data, the small-signal loss of the Cr²⁺:ZnSe saturable absorber was further determined. In addition to passive Q-switching of the 2.3-µm Tm³⁺:YLF laser, preliminary data on pulsing generated by using a semiconductor saturable absorber and a graphene saturable absorber are also presented. We foresee that both Alexandrite and 2.3-µm Tm³⁺:YLF lasers operated in continuous-wave or pulsed regimes will find numerous scientific and technological applications.