Course Title:Ultrafast Lasers

Course Level: From beginner to advanced 

Course Description:

his short course reviews the core physics and practical design rules of modelocked lasers with analytical models and explains why they enabled many of today’s key advances in ultrafast science. Following the time–frequency duality, we develop an intuitive and quantitative view of ultrashort pulse generation with modelocking. We cover both active modelocking (Siegman–Haus theory) and passive modelocking, including cases with and without dynamic gain saturation. A dedicated section addresses real-world stability issues: self-starting behavior, Q-switching instabilities, long open net-gain windows, and excess self-phase modulation (SPM), with practical mitigation strategies. Noise is explained with regards to power spectral densities. For passive modelocking, we compare fast and slow saturable absorbers, soliton modelocking, and introduce semiconductor saturable absorber mirror (SESAM) design principles that link absorber parameters to starting behavior, noise, and scaling limits. We then connect modelocked lasers to frequency combs, including carrier-envelope offset (CEO) phase measurement and stabilization. The course concludes with performance scaling trends (pulse duration, average power, repetition rate, and noise) and emerging opportunities such as dual-comb modelocking and applications in optical communications, positioned for participants attending Conference on Lasers and Electro-Optics (CLEO). 

Benefits and Learning Objectives: 

• Explain the physical motivation and basic principles of active and passive modelocking. 

• Use analytical models (including Siegman–Haus and soliton concepts) to understand parameter scaling and design trade-offs. 

• Design cavity, gain, dispersion, and nonlinear parameters to achieve robust self-starting, stable modelocking. 

• Specify and optimize saturable absorber parameters for stability and scalability, using SESAM design rules transferable to emerging absorber materials. 

• Diagnose and mitigate common instabilities (Q-switching, long net-gain window effects, excess SPM) and their practical limits. 

• Measure and interpret laser noise using power spectral densities, and connect noise metrics to system performance. 

• Relate modelocked lasers to frequency comb operation, including CEO concepts and stabilization requirements. 

• Identify the current performance frontier and evaluate new opportunities such as dual-comb modelocking and optical communication applications. 

Intended Audience:

This short course is intended for graduate students, researchers, and industry engineers who want a clear physical introduction to modelocking and frequency combs. It emphasizes core concepts, intuition, and practical design trade-offs rather than long derivations. Participants seeking step-by-step mathematical treatments can refer to my textbook Ultrafast Lasers (Springer, 2021). The course provides a foundation to scale ultrafast laser performance and to identify new regimes and applications. 

Instructor Biography: 

Professor Ursula Keller is Emeritus Professor of Physics at ETH Zurich (1993–2025) with over 30 years of teaching experience and author of the graduate textbook Ultrafast Lasers (Springer, 2021). She supervised 101 PhD students and published >500 papers. She pioneered key advances in ultrafast solid-state and semiconductor lasers, including SESAMs, single-cycle pulse generation, and frequency-comb stabilization. She co-founded several start-ups and has received major international honors, including the European inventor award, IEEE Edison Medal, OSA Ives Medal, SPIE Gold Medal, NAS and FRS membership.