Laser wavelength ~ 1550 nm
SAM order information:
Part-No description: SAM-λ-A-τ-x
• λ - laser wavelength
λ - 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 ± λ/50.
The change of reflectance R and absorptance A = 1-R with wavelength is shown in the data sheet.
• A - low intensity absorption
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.
• τ - absorber relaxation time
The SAM absorbs some photons from leading adge 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 τ 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 τ 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.
• x - mounting condition for SAM
Solid state laser:
To remove the dissipated heat from the SAM chip with standard dimensions of 4.0 mm x 4.0 mm area and 0.45 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 > 0.2 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.
The butt coupling of a small SAM chip with lateral dimensions of 1.3 mm x 1.3 mm (without any glue in the optical path)
is a good way to get a rigid construction.
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.
Removal of the dissipated optical power from fiber coupled SAM chip:
Some optical power is absorbed in the SAM chip and must be removed by heat conduction to the surroundings.
There are the following possibilities to order SAMs for fiber butt coupling:
You can do it yourself, if you order a set of 4 small SAM chips with an area of 1.0 mm x 1.0 mm or 1.3 mm x 1.3 mm.
Fiber mount for 1.3 mm x 1.3 mm SAM chips.
Can be used with any fiber with FC/PC connector.
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 single mode fiber type SMF28 or PM1550 and the connector type on the second fiber site. Our standard fiber length is 1m. The shortest fiber length is 15 cm.
Passive heat sink for fiber coupled SAM, which is constructed for an FC/PC connector. The PHS can be mounted onto a larger metal surface with screws.