Abstract
We report a harmonically mode-locked vertical external cavity surface emitting laser (VECSEL) producing 400 fs pulses at a repetition frequency of 175 GHz with an average output power of 300 mW. Harmonic mode-locking was established using a 300 µm thick intracavity single crystal diamond heat spreader in thermal contact with the front surface of the gain sample using liquid capillary bonding. The repetition frequency was set by the diamond microcavity and stable harmonic mode locking was achieved when the laser cavity length was tuned so that the laser operated on the 117th harmonic of the fundamental cavity. When an etalon placed intracavity next to the gain sample, but not in thermal contact was used pulse groups were observed. These contained 300 fs pulses with a spacing of 5.9 ps. We conclude that to achieve stable harmonic mode locking at repetition frequencies in the 100s of GHz range in a VECSEL there is a threshold pulse energy above which harmonic mode locking is achieved and below which groups of pulses are observed.
Highlights
Over the past 20 years since the introduction of femtosecond mode-locked solid-state lasers the parameter space covered by such lasers has expanded significantly
When the vertical external cavity surface emitting laser (VECSEL) was used with the 500 μm sapphire placed in contact with, but not liquid capillary bonded to, the gain sample the laser operated with a maximum output power of 300 mW before thermal rollover was observed
Thermal rollover was observed above this pump power, which was caused by the non-optimal thermal contact between the diamond and the heat sink
Summary
Over the past 20 years since the introduction of femtosecond mode-locked solid-state lasers the parameter space covered by such lasers has expanded significantly. A relatively recent advance has been to push the pulse repetition frequency up towards tens of GHz or more, aiming at applications such as optical clocking and sampling, arbitrary waveform generation, large-mode-spaced continuum generation and high bandwidth telecoms. Mode-locked solid state lasers have to date achieved fundamental repetition frequencies up to 160 GHz with picosecond pulses [1] and 10 GHz with femtosecond pulses [2]. The key challenge is to avoid the Q-switched mode locking regime that appears when the intracavity pulse energy is too small to saturate the absorber adequately. Mode-locked edgeemitting diode lasers are by contrast intrinsically suited to high repetition frequency operation, typically in the range 20-100 GHz [3]. The average power from these semiconductor pulse sources, is typically only a few mW
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