THE DYNAMICS OF MODE-LOCKING

Kerr-lens mode locking (KLM) is the engine of ultrafast laser sources. A major research effort in our group is to explore the physics of mode locking. We manipulate and examine the Kerr-lens mechanism in various aspects with the aim to develop sources of ultrashort pulses and optical frequency combs with desirable optical properties that are otherwise unattainable: Specifically, we demonstrated robust and flexible control of the oscillation spectrum by shaping the gain of a KLM Ti:Sapphire laser, which steers the spectral mode-competition in a unique manner. By enhancing the nonlinearity in the laser cavity we could lower the mode-locking threshold drastically, even below the CW threshold, and explored its relevance for mode-locking with low intracavity pulse energy and high repetition rate. Recently, we examined the dynamical evolution of KLM with a complete numerical simulation of the laser operation., which led to a direct observation (in both simulation and experiment) of the nonlinear saturable loss mechanism that is responsible for the pulse formation [4] and we predicted and demonstrated a new effect in KLM, where the symmetry between the forward and backwards directions in a linear cavity is broken by the dual interaction with the Kerr-lens[3].

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Mode locking of semiconductor lasers with high pulse energy is a major applicative effort in my group. Relying on our expertise in laser physics and mode-locking we develop external cavity configurations, where short pulses of picosecond duration (6-100ps), record-high pulse energies (nano-Joule scale), and repetition rate in the range of 60-700MHz are generated from standard broad-area edge-emitter gain-chips that are driven by pulses of electrical current. This forms a breakthrough in mode-locking of semiconductor lasers, where pulse energies so far were in the few pJ range (or less) and multi-GHz repetition rates, limited primarily by the short excited-state lifetime in the semiconductor medium. A broad-area gain chip enables high pumping currents (>10A) and very high optical power, while the design of the external cavity enforces lasing with a single spatial mode and high-power efficiency (>75%, 2 watts,  demonstrated in CW [2]) with broad-area chips that are normally highly multimode. To mode-lock the laser we combine active and passive methods: Active mode-locking is invoked by pumping the chip with current pulses, that are synchronized with the repetition rate of the cavity (60-70MHz) and whose duration (few ns) matches the lifetime of the excited state in the semiconductor medium [in preparation]. This active gain switching generates optical pulses of 60-100ps duration with an energy of ~0.5nJ. For shorter pulses of 6-10ps duration [1], we employ passive mode-locking with an integral saturable absorber section on the gain chip that can be precisely reverse-biased for optimal pulse generation.