Physics PhD Thesis Defense by Ferda Canbaz





Title: Graphene Mode-locked and Kerr-lens Mode-locked Operations of Cr3+:LiSAF Lasers Near 850 nm and Tm3+:YLF Lasers Near 2300 nm

Speaker: Ferda Canbaz

Time: August 15, 2018, 10:00

Place: SCI 103

Koç University

Rumeli Feneri Yolu

Sariyer, Istanbul

Thesis Committee Members:

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

Prof. Dr. Alper Kiraz (Koç University)

Assoc. Prof. Sarper Özharar (Bahçeşehir University)

Assoc. Prof. Kaan Güven (Koç University)

Assoc. Prof. Ümit Demirbaş (Antalya Bilim University)


Femtosecond lasers operating in the near-infrared and mid-infrared regions can be used in many scientific and technological applications such as pumping of optical parametric oscillators to reach longer wavelengths, electronic/vibrational spectroscopy, and biomedical imaging. This thesis work particularly focuses on Cr3+:LiSAF and Tm3+:YLF lasers, which provide broadly tunable coherent emission near 850 nm and 2.3 µm. The broad tuning range of these lasers further makes it possible to generate femtosecond pulses. In addition, both gain media can be used to construct low-cost lasers by using low-power, widely available diode pump lasers. In recent years, graphene has attracted a great deal of attention as a passive mode locker for the generation of femtosecond pulses, due to its fast and broadband saturable absorption. In the experiments described below, both graphene as well as Kerr-lens mode locking were employed to produce ultrashort pulses with record performance from Cr3+:LiSAF and Tm3+:YLF lasers operating at 850 nm and 2.3 µm.

In the first part of the thesis, we report a femtosecond Cr3+:LiSAF laser, mode locked by using a monolayer graphene saturable absorber (GSA) for the first time. The tight-focusing resonator architecture made it possible to operate the Cr3+:LiSAF laser with only two 135-mW, 660-nm low-cost single-mode diode lasers. At a pump power of 270 mW, the laser produced nearly transform-limited 68-fs pulses at 850 nm with a pulse repetition rate of 135 MHz and an average output power of 11.5 mW. By carefully optimizing the resonator group delay dispersion, we further demonstrated the shortest pulses directly generated to date from a GSA mode-locked laser. In particular, with a pump power of 275 mW, the Cr3+:LiSAF laser produced as short as 19-fs, nearly transform-limited pulses with a repetition rate of 107 MHz and average output power of 8.5 mW. Once mode locking was initiated with the GSA, stable femtosecond pulses could be obtained. In addition, the femtosecond output of the laser could be tuned from 836 nm to 897 nm with pulse durations in the range of 80-190 fs. We further performed detailed mode locking initiation tests across the full cavity stability range of the laser to verify that pulse generation was indeed initiated by the GSA and not by Kerr lens mode locking.

In the next series of experiments, we developed a new source of mid-infrared femtosecond pulses, based on a Tm3+:YLF laser at 2303 nm. Two different techniques were employed to generate femtosecond pulses with the Tm3+:YLF laser. In the first experiment, an undoped ZnSe substrate was included in the resonator to provide enhanced nonlinear phase modulation during KLM operation. The Tm3+:YLF laser was end-pumped with a continuous-wave Ti3+:sapphire laser at 780 nm. With 880 mW of pump power, the KLM Tm3+:YLF laser generated 514-fs pulses at a pulse repetition rate of 41.5 MHz with an average power of 14.4 mW. In the second experiment, we developed, for the first time to our knowledge, a GSA mode-locked Tm3+:YLF laser operating near 2.3 µm. Since graphene introduces a constant loss of 2.3% per transit, the resonator was extended and double pumped to obtain sufficient intracavity pulse energy. GSA mode-locked Tm3+:YLF laser produced 1-ps pulses at a repetition rate of 17.2 MHz with 42 mW of average output power at 2303 nm.

The major contributions of this thesis work may be summarized as follows. We demonstrated, for the first time to our knowledge, graphene mode-locked operation in a Cr3+:LiSAF laser at 850 nm. We further generated the shortest pulses (19 fs) from a GSA mode-locked solid-state laser.  A systematic method was proposed and demonstrated to clearly identify the initiation mechanism of mode locking. Finally, Kerr-lens mode locked and GSA mode locked operations of a mid-infrared Tm3+:YLF laser were demonstrated for the first time. We foresee that the experiments described in this thesis can lead to the development of novel femtosecond laser sources in the near- and mid-infrared regions.