How to select the right saturable absorber mirror (SAM) ?
 
>  Order number SAM-l-A-x-t      (please click on the parameter to find the description)  
 
>  l - laser wavelength, for which the given absorptance A in the data sheet is valid. The SAM can be still used at slightly different wavelengths without significant change of parameters within a wavelength interval of about ± l/50. The change of reflectance R and absorptance A = 1-R with wavelength is shown in the data sheet.  
 
>  A - absorptance at low pulse fluence below the saturation. Because of the vanishing transmittance of the underlying Bragg mirror the absorptance can be deduced from the low intensity reflectance R simply by A = 1 - R.
  • For mode-locking of solid state lasers the common values are A = 0.7 … 2 %
  • For mode-locking of fiber lasers the common values are A = 20 … 40 %
  • Rule of thumb: absorptance A ~ T/2 (half of the transmittance T of the output coupler)
  • Design issues: The SAM structure can be resonant or anti-resonant for the laser wavelength. The anti-resonant SAM has a higher saturation fluence and damage threshold and a broader wavelength region with a similar absorption (and reflection) value. The resonant SAM has a lower saturation fluence and damage threshold and shows a small spectral bandwidth.
  
 
>  X - mounting of the SAM chip  
  Diode pumped solid state laser (DPSSL):  
  To remove the dissipated heat from the SAM chip with standard dimensions of 4mm x 4mm area und 0.4 mm thickness, the SAM must be glued or soldered on a copper heat sink. Soldering results in the best thermal conductance, which is recommended for mean optical output power > 1 W. We offer standard diameters of copper heat sinks with 12.7 mm (1/2"), 25.0 mm and 25.4 mm (1"). The 25.0 mm diameter heat sink is also available as water cooled version.
Besides the choice of the heat sink the position of the soldered or glued SAM on the heat sink is important. In some cases the mounting on the edge of the heat sink may be advantageously to get a low angle of incidence of the laser beam onto the SAM. 		 
  Order example: SAM-2000-4-25.4s-t , edge mounted,
Saturable absorber mirror for 2000 nm wavelength ± 30 nm,
4 % low intensity absorptance,
chip size 4mm x 4mm,
soldered on the edge of a copper heat sink with 25.4 mm diameter,
relaxation time constant t 		up
 
  Fiber laser:  
  The butt coupling of a small SAM chip with lateral dimensions of ~ 1.3mm x 1.3mm (without any glue in the optical path) is a good way to get a rigid construction.
Please note: Due to the change of the medium in front of the SAM during the fiber butt coupling from air (n = 1) to silica (n = 1.46) the reflectance R and the absorptance A = 1 -R changes. This change depends on the design of the SAM structure (resonant, antiresonant, dielectric cover) and is also accompanied with a slightly increase of the optical field strength on the SAM surface. 		 
 
  There are two possibilities to order SAMs for fiber butt coupling:  
  You can do it yourself, if you order a set of 5 small SAM chips with an area of 1.0mm x 1.0mm or 1.3mm x 1.3 mm.  
  Order example: SAM-2000-35-0-t, batch of 5 pieces 1mm x 1mm,
Saturable absorber mirror for 2000 nm wavelength ± 20 nm,
35 % low intensity absorptance,
batch of 5 unmounted chips with chip size 1mm x 1mm, 400 µm thick,
relaxation time constant t 		 
 
  You can order a fiber butt coupled SAM. In this case the SAM is mounted without any glue in the optical path onto the ceramic ferrule of a FC/PC connector. You have the choice of the singlemode fiber type SMF 28 and the connector type on the second fiber site. Our standard fiber length is 1m. The shortest fiber length is 25 cm.  
  Order example: SAM-2000-35-FC/PC-t, fiber SMF 28,
Saturable absorber mirror for 2000 nm wavelength ± 20 nm,
35 % low intensity absorptance,
mounted on a 1 m long singlemode fiber SMF 28 with FC/PC connector,
relaxation time constant t 		 
 
  Removal of the dissipated optical power from the fiber coupled SAM chip:
Some optical power is absorbed in the SAM chip and must be removed by heat conduction to the surroundings.
  • There is a heat conduction to the ceramic ferrule of the connector and also to the air.
  • We offer a passive heat sink for fiber coupled SAM, which is constructed for the FC/PC connector. It can be mounted onto a larger metal surface with screws.
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>  t - relaxation time constant  
  The SAM absorbs some photons of the laser pulse. The absorbed photon energy is assigned to electrons in the valence band of the semiconductor absorber, which are excited into the conduction band. As long many excited electrons from the valence band are in the conduction band, the absorber is bleached and has a reduced absorption. The excited electrons lost their excess energy after a mean relaxation time t due to different effects ( e. g. phonon or photon emission). This mean relaxation time depends on the defect density of the semiconductor absorber material.
From the energetic point of view the optimum relaxation time t is somewhat larger than the pulse width. In this case the optical power loss due to the absorption in the saturable absorber is minimal and a low long time degradation of the absorber can be expected.
On the other hand it can be advantageously in some cases to use a SAM with a shorter relaxation time than the pulse width to get a stable mode-locking regime. 		up