21.4 kW peak power from a gigahertz multimode-diode-pumped solid-state laser with carrier envelope offset frequency detection

Abstract

Summary form only given. Typical frequency combs from modelocked lasers are either based on complex green-pumped Ti:sapphire oscillators or amplified fiber lasers. Ti:sapphire lasers deliver very short pulses and exhibit low noise levels, but they rely on multi-watt green pump lasers and complex Kerr lens modelocking (KLM). More convenient and robust diode-pumped fiber laser systems suffer from higher quantum noise and limited repetition rates. Excellent compact sources for frequency combs are ultrafast diode-pumped solid-state lasers (DPSSLs). They combine the favorable properties of cost-efficient diode-pumping, robust SESAM modelocking and an intrinsic low quantum noise level [1]. Furthermore, DPSSLs can achieve watt-level average powers without any amplification, even in combination with high gigahertz repetition rates. Frequency combs at gigahertz repetition rates offer increased power per mode and simpler access to individual optical lines. Applications in ultrafast data transmission, nonlinear spectroscopy and calibration of spectrometers greatly benefit from reliable high repetition rate frequency combs. For stabilization of a frequency comb the comb spacing (i.e. the pulse repetition frequency) and the comb offset (i.e. the carrier envelope offset (CEO) frequency) have to be measured and locked. Detecting the repetition rate is straightforward using a simple photodiode. However, measuring the CEO frequency is more challenging, especially at high repetition rates. The standard f-to-2f interferometer scheme for CEO detection is based on a coherent octave-spanning spectrum [2], which is a supercontinuum typically generated by launching ultrashort laser pulses with high peak power into a highly nonlinear photonic crystal fiber (PCF) [3].We investigated the feasibility for high repetition rate frequency comb generation from DPSSLs and report the highest average power, shortest pulse duration and therefore record high peak power from a DPSSL with gigahertz repetition rate. The soliton-modelocked laser is based on Yb:KGW (ytterbium-doped potassium gadolinium tungstate). Stable, fundamental cw modelocked operation at pulse repetition rates of 1.06 GHz was achieved with a semiconductor saturable absorber mirror (SESAM). Using a commercial fiber-coupled multimode diode laser as the pump source this compact diode-pumped solid-state laser reaches 3.33 W average output power with pulse durations of 121 fs at 1044 nm center wavelength (Fig. 1a, b). Therefore 21.4 kW peak power is directly obtained from this gigahertz DPSSL operating with an optical-to-optical efficiency of 26 %. We used this laser without additional pulse compression and explored the regime for stable CEO detection and self-referenceable gigahertz frequency comb generation. Launching only 901 mW of the total average power with a coupling efficiency of 70 % directly into a highly nonlinear photonic crystal fiber we generated an octavespanning spectrum and measured a CEO signal exceeding 30 dB S/N (Fig. 1c).