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Abstract

An alternative concept for mode coupling for the generation of ultra-short laser pulses will be presented. Based on the Kerr effect in thin-film systems, the Kerr band switch in a dielectric layer system implements an ultra-fast and loss-free switch concept at low cost, which covers almost the entire spectral range. This concept has high damage thresholds and, compared to semiconductor absorbers, absorbs only very small amounts of radiation energy and thus heats up only slightly.

 

Challenge

Ultra-short pulse lasers (USPL) have become increasingly important in recent years. Especially in medical, industrial or life science applications they are gaining more and more importance. Compared to laser systems with longer pulse duration, they minimize the thermal damage of the material during processing by the fast energy input, which is shorter than the thermal diffusion of the energy through the material to be processed. Therefore, they enable significantly higher precision than would be the case if longer pulse duration were used. This minimizes thermal damage zones and enables the production of precise structures on a nanometer scale. Nevertheless, the application field of USP lasers is often limited by their complex structure and the associated high price. In addition, the currently available USP laser sources can often only generate wavelengths in the infrared spectral range. The general laser principle for generating short laser pulses is based on mode coupling, which can be achieved with a variety of technologies and components. In USP systems commercially available today, so-called SESAMs (Semiconductor-Saturable-Absorber Mirror) are generally used for mode coupling. These switches are based on the saturable absorption of a semiconductor layer, which in most cases consists of InGaAs mixtures and is therefore limited to narrow spectral ranges due to the material-specific band edges. In addition, the switching of the SESAM is based on the absorption of part of the laser light, which leads to undesired heating of the component at high power levels.

Our Solution

The innovative Kerr band switch implements an alternative mode coupling concept based on the exploitation of the Kerr effect in thin film systems. The Kerr band switch consists of a dielectric layer system in which one or more Kerr-active layers are embedded. When exposed to high light intensities, these layers change their refractive index slightly, which influences the transfer behavior of the component. The Kerr-Band switch thus enables ultra-fast, loss-free switching of the resonator quality. The dielectric materials used to implement the Kerr-Band switch also have significantly higher damage thresholds than those of semiconductor materials. In addition, the materials absorb only very little radiant energy and thus lead to very little heating. Furthermore, switching wavelengths can be realized in a wide range from UV to MIR. In addition, the materials are transparent from the ultraviolet to the infrared spectral range and allow a cheaper production compared to the epitaxially grown semiconductor mirrors. With the help of modern thin-film technology, it is also possible to manufacture components cost-effectively and, above all, flexibly on curved substrates.
 

Figure 1

Figure 1: Schematic representation of a design form of the Kerr band switch layer system having, along the optical axis 7, successive layers 2 to 5, wherein a sudden change in refractive index 6 is formed in each case between two directly successive layers. In the region of the reflectors, the distances of the refractive index changes (RI) are equal to a quarter of the design wavelength Lambda. The regions between two reflectors are designed as resonators, here the thickness (OT) of the optical layers (4 and 5) is Lambda 1/2 or a multiple of Lambda 1/2. The resonators differ in the Kerr activity of their material and/or in the light intensity resulting in their regions, so that clearly different Kerr effects result in them.

Advantages

  • Extremely fast switching characteristics
  • low absorption of radiant energy (low heating)
  • high damage threshold, no degradation (long service life)
  • low manufacturing costs
  • simple construction, no complicated adjustment of apertures
  • Application in the entire spectral range from laser sources (UV to MIR)
  • not limited to plano optics, curved surfaces also possible

Applications

  • fast optical switch
  • Replacement for Semiconductor Saturable Absorber Mirrors (SESAM)
  • Pulse generation from USP
  • Mode coupling switching component
  • super fast power limiter
  • Circuit breakers in MOPA systems
  • Pulse characterization / shaping
  • Optical transistor

Development Status

Material studies were started to reduce the switching threshold in the required power range. Within the framework of a promoted further development, market-ready prototypes are to be developed.

Patent Status

Granted German Patent: DE102017129069B3
An internationale patent application has been filed.

Applicant:
    Laser Zentrum Hannover e.V.

Contact

Dr. Ireneusz Iwanowski
Patent Manager (Physics, Technology and Software)
E-Mail: iiwanowski(at)sciencebridge.de
Tel.: +49 (0) 551 30 724 153
Reference: CPA-2060-LZH
www.sciencebridge.de

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