A saturable absorber is a material which has decreasing light absorption with increasing light intensity. Most materials show some saturable absorption, but often only at very high optical intensities. We need saturable absorbers which show this effect at intensities typical in solid-state laser cavities. The key parameters for a saturable absorber are its wavelength range (where it absorbs), its dynamic response (how fast it recovers) and its saturation intensity and fluence (at what intensity or pulse energy it saturates).

When the saturable absorber response is strong enough to "switch" off the laser before it reaches steady-state, the laser emits much longer pulses with larger energies at much lower frequencies, typically in the kilohertz range. For the long cavities typical of our mode-locked lasers, these "macro" Q-switched pulses consist of an underlying train of mode-locked pulses, resulting in a combined operating regime called Q-switched mode-locked. By shrinking the laser cavity, however, it is possible to produce single-frequency, Q-switched pulses with pulsewidths in the range of hundreds of picoseconds, repetition rates from 50 kHz to >1 MHz, and pulse energies in the microjoule range.

Saturable absorbers used in the past were typically organic dyes, which suffer from short lifetimes, toxicity, and complicated handling, limiting their application to mostly laboratory systems. Alternative solid-state saturable absorbers include crystals such as Cr:YAG, which typically operate for only limited ranges of wavelengths, recovery times and saturation levels. Semiconductor materials, however, can be designed to absorb over a broad range of wavelengths (from the visible to the mid-infrared). We can also control the absorption recovery time, saturation fluence and intensity through the growth parameters and device design. Being solid-state, they don't experience the degradation typical of dyes.

We integrate the semiconductor saturable absorber into a mirror structure, resulting in a device which reflects more light, the more intense the light is. We call this general class of semiconductor saturable absorber mirrors - SESAMs devices. Working with the Ultrafast Laser Physics Laboratory at ETH, headed by the inventor of the SESAM, Prof. Ursula Keller, we have significantly improved the device design, fabrication process and long-term device reliability. We have SESAM device designs that can cover wavelengths from <800 nm to >1600 nm, pulsewidths from femtoseconds to nanoseconds, power levels from milliwatts to >10 Watts (see the reference list for further technical details).

Our expertise in high-brightness diode-pumping is a key feature in our lasers. Passive modelocking works best when the laser is pumped many times over threshold. In addition, many of the laser materials suitable for ultrafast pulses have low small-signal gain. Because diode lasers do not emit ideal Gaussian beams, collecting their output and optimizing mode-matching into the laser crystal are key features of our designs. We have demonstrated significant performance milestones in diode-pumpingof Cr:LiSAF and Nd:glass lasers, for example. We also are leaders in timing stabilization of mode-locked lasers, and have the instrumentation to test and verify sub-picosecond synchronization. Please ask us about your custom requirements. We can suggest, advice and possibly propose solutions to special problems involving other wavelengths (frequency conversion), higher powers amplified systems, etc.