Semiconductor saturable-absorber mirrors (SESAMs) are a type of

saturable absorber Saturable absorption is a property of materials where the absorption of light decreases with increasing light intensity. Most materials show some saturable absorption, but often only at very high optical intensities (close to the optical damage ...

used in

mode locking Mode locking is a technique in optics by which a laser can be made to produce pulses of light of extremely short duration, on the order of picoseconds (10−12 s) or femtoseconds (10−15 s). A laser operated in this way is sometimes r ...

lasers. Semiconductor saturable absorbers were used for laser mode-locking as early as 1974 when p-type

germanium Germanium is a chemical element with the symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid in the carbon group that is chemically similar to its group neighbors s ...

was used to mode lock a laser which generated pulses of around 500 picoseconds. Modern SESAMs are III-V semiconductor single quantum well (SQW) or multiple

quantum well A quantum well is a potential well with only discrete energy values. The classic model used to demonstrate a quantum well is to confine particles, which were initially free to move in three dimensions, to two dimensions, by forcing them to occupy ...

s grown on semiconductor distributed

Bragg reflector A distributed Bragg reflector (DBR) is a reflector used in waveguides, such as optical fibers. It is a structure formed from multiple layers of alternating materials with varying refractive index, or by periodic variation of some characteristi ...

s (DBRs). They were initially used in a Resonant Pulse Modelocking (RPM) scheme as starting mechanisms for Ti:Sapphire lasers which employed KLM as a fast saturable absorber. RPM is another coupled-cavity mode-locking technique. Different from APM lasers which employ non-resonant Kerr-type phase nonlinearity for pulse shortening, RPM employs the amplitude nonlinearity provided by the resonant band filling effects of semiconductors. SESAMs were soon developed into intracavity saturable absorber devices because of more inherent simplicity with this structure. Since then, the use of SESAMs has enabled the pulse durations, average powers, pulse energies and repetition rates of ultra-fast solid-state lasers to be improved by several orders of magnitude. Average power of 60W and repetition rate up to 160 GHz were obtained. By using SESAM-assisted KLM, sub-six-femtosecond pulses directly from a Ti: Sapphire oscillator were achieved.

Ursula Keller Ursula Keller (born 21 June 1959) is a Swiss physicist. She has been a physics professor at the ETH Zurich, Switzerland since 2003 with a speciality in ultra-fast laser technology, an inventor and the winner of the 2018 European Inventor Award by ...

invented and demonstrated the semiconductor saturable absorber mirror (SESAM) which demonstrated the first passively mode-locked diode-pumped solid-state laser in 1992. "For almost two decades since then, her group at ETH Zurich has continued to define and push the frontier in ultrafast

solid-state laser A solid-state laser is a laser that uses a gain medium that is a solid, rather than a liquid as in dye lasers or a gas as in gas lasers. Semiconductor-based lasers are also in the solid state, but are generally considered as a separate class from ...

s both with detailed theoretical models and with world-leading experimental results, demonstrating orders of magnitude improvement in key features such as pulse duration, energy, and repetition rate. She also helped to spearhead industrial transfer of this technology. Today most ultrashort lasers are based on SESAM modelocking, with important industrial applications ranging from optical communication, precision measurements, microscopy, ophthalmology, and micromachining." A major advantage SESAMs have over other saturable absorber techniques is that absorber parameters can be easily controlled over a wide range of values. For example, saturation fluence can be controlled by varying the reflectivity of the top reflector while modulation depth and recovery time can be tailored by changing the low temperature growing conditions for the absorber layers. This freedom of design has further extended the application of SESAMs into modelocking of fiber lasers where a relatively high modulation depth is needed to ensure self-starting and operation stability. Fiber lasers working at 1 µm and 1.5 µm were successfully demonstrated.L.M. Zhao et al.
"Polarization rotation locking of vector solitons in a fiber ring laser"
, ''Optics Express'', 16,10053–10058 (2008).

References

{{reflist Nonlinear optics Optical devices