Front Protection - Safran LRF 7047 Manual

LRF 7047- Integrator Manual

4.7 Front Protection

The LRF module is not gas tight. Therefore installing an LRF module into a host system requires a
protective environment with a window in front of the LRF module.
The laser diode emits laser light towards the objective lens. This lens sends out a mainly parallel beam,
with some angular spread called beam divergence.
Care has to be taken that there is no substantial absorption of the laser energy at 1'550 nm wavelength.
Standard optical quality glass should be used. The material of the window needs to be transparent for the
1'550 nm wavelength. If the 830 nm pointer is used, the glass has also to be transparent for this
wavelength (i.e. BK7 from Schott).
The antireflection coating of the window is recommended to be "broadband", in other words, effective for
visible light as well as for the laser wavelength. Choosing a narrow band must also allow the transmission
of temperature shifted laser light (e.g. 1'550 nm = 1'500 ...1'600 nm depending on temperature) and
optional pointer light (e.g. 790 nm ...870 nm).
If the coating reflects visible light, it will not be possible to see inside the LRF module, which is not a
problem, but it can appear like a mirror and reflect sunlight.
The coating should fulfill the following specifications*:
 visible light: R < 1 % for λ = 420 ... 650 nm, light angle 0°+/- 15° for both lenses.
If this is not fulfilled, you have an optical mirror. However, it is the decision of the system
integrator to specify transmission for visible light of the front windows.
 laser light: R < 0.5 % for λ = 1'500 ...1'600 nm, light angle 0°+/- 15° for both lenses.
If this is not fulfilled, the laser energy is reduced and internally reflected. This can cause
optical cross talk and lead to incorrect or no measurements.
Optional pointer light: R< 0.5 % for λ = 790...870 nm, light angle 0°+/-15° for transmitter lens

only.
If this is not fulfilled, the pointer energy is reduced and internally reflected. This reduces
the intensity of the pointer light and can even destroy the fiber laser source.
*Please refer to Appendix 9.3 Reference design for lenses as an example.
NOTICE
Coatings shall be applied to both surfaces, inner and outer
It is recommended to use two separate windows, one for the receiver channel and one for the transmitter
channel, to avoid optical cross-talk (scattered laser light from transmitter to receiver). Both windows
should be sufficiently flat (wedge error between surfaces of the same lens should be of max. 10 arcsec).
An uneven window will lead to a boresight issue, resulting in a reduction of achievable range
performance.
In addition, the optical Rx & Tx channels must be optically isolated from each other. This is also required
to prevent optical cross talk between both channels. This means, the area of Rx and Tx between the LRF
module's front lenses and the host system's front window must be isolated.
For optical isolation, the use of EPDM L 7510 material with shore hardness 50 +/-5 Shore A is
recommended.
914929_TML_LRF7047_en_Version B
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Public: 04.2019
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