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Laser safety
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High End Laser Made in Germany
Last update: 4/6/2010 >
Start > Laser Technology



Important Hints for Handling with Lasers in the LLLT (Low Level Laser Therapy):
In this place there are published important details for the laser technology in a generally intelligible way. Furthermore there are hints for everyday for safe handling with lasers, too.

Laser Technology:
Laser Chip Structure
Beam Divergence
Differential Efficiency
Temperature Characteristics
Laser Wavelength
Polarity of the Laser Radiation

Calculations:
Calculation of the Beam Shift by Plane-Parallel Pane/Plate:
Examples of the Calculation of Power- and Energy Values of Low Level Lasers
Example of the Calculation of the Therapy Duration at CW Lasers

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Laser chip structure:





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Beam divergence (Qh/Qv):
The laser radiation diverges as follows according to Fig. 1: The light distribution is into the X-(Qh) and y-axis (Qv) measured in terms of the PN-junction of the laser beam emitting from this according to the chip surface. The beam divergence angle refers to the light distribution within a tolerance range from at most 50% of the maximum intensity of the laser (Peak) and is defined as Qh (parallel) and Qv (vertical) > (Fig.2).



Abb.1: Chip Topologie of a Laser Diode





Fig. 2: Beam divergence: Full angle at half maximum

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Differential efficiency (SE):
This is the relation between the increase of the visual laser performance (Oscillating range) and the rising operating forward current of the laser. The amount is given by the identification line joint between optical power and forward current of the laser diode (Fig.3).




Fig. 3: Light output power with respect to forward current

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Temperature behavior (If/P)

The characteristics of a laser diode vary strongly with temperatur particularly at shorter wavelength < 800nm. As a compensation an APC control (Automatic Power Control) usually adds to the application. At this control the visual output power is kept constant by a corresponding follow-up control of the diode current independently of the temperature of the laser diode (feedback loop > monitor diode/forward current). In addition a sensor should supervise the rise of the temperature and switch the laser off at exceeding the maximum operating temperature. A possible fall of the laser temperature below the max. operation conditions of the laser diode (outside temperature etc.) also should be supervised at extreme requirements (Fig.4).




Fig. 4

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Laser wavelength (LP):
Laser diodes are classified strictly according to the following modes:
Single mode:
These are usually hand-picked laser diodes for the preferential application in LWL technology. Advantage is the high focus ability of 0.35 to 0.5 mm* mrad. The laser diode emits only one wavelength.
Multi mode:
The laser diode emits light with wavelengths of different intensity in the near spectral range of a specific color. The highest intensity of the laser of both modes is defined by the maximum spectral intensity of the wavelength (Fig.5).




Fig.5

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Polarity of the laser radiation:
The characteristics of red lasers can vary in respect to the polarity of the radiation sent out.
Conventional infrared lasers and lasers with wavelengths of 650 to 675 nm (red) oscillate mainly in the TE mode (polarity direction is parallell to the PN junction plane). Laser diodes with wavelengths of 635 nm (mostly with performances < 30 mW) oscillate in the TM mode mainly. In this case it has to be taken into account that visual elements which come to the application with this laser, must be adapted correspondingly.

TE mode (transversally electric):
The electrical field stands perpendiculary to the direction of propagation of the laser beam.
The magnetic field points in the direction of the wave spreading of the laser beam.

TM mode (transversally magnetic):
The magnetic field stands perpendiculary to the direction of propagation of the laser beam.
The electrical field points in the direction of the wave spreading of the laser beam.



Fig. 6: Polarity of Laser radiation

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Beam shift by plane-parallel pane/plate:



Ss =
Beam transfer perpendicular to the beam direction
Sp =
Beam shift parallel to the pane
d =
Thickness of the pane/plate
alpha =
Angle of incidence
n =
Index of refraction

Calculation of the beam transfer in the angle to the optical axis:


Calculation of the beam shift parallel to the pane/plate:

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Examples of the Calculation of Power- and Energy Values of Low Level Lasers

Example of the StyloBeam-801002 Pulsed Laser:

Constants:
Beam Spot Size of the Surface to be treated: 0.01cm² (1mm²)
Max. Therapy Frequency: 10.000Hz
Peak Power: 40W
Pulse Duration: 200ns

Average Power
= Peak Power x Pulse Duration x Frequency
= 40W x (200 x 10^-9 Sec.) x 10.000Hz
= 0.08W > 80mW
Power Density
= Average Power / Surface Area
= 0.1W / 0.01cm²
= 8W / cm²
Energy Density
= Power Density x Treatment Time
= 8W x 1 Second
= 8 Joule / cm²
Total Energy
= Average Power x Treatment Time
= 0.08W x 60 Seconds
= 4.8 Joule per minute

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Example of the Calculation of the Therapy Duration at CW Lasers:

Operation without Frequency:
Operation with Frequency:
t = Predefined Therapy Time in Seconds
e = Predefined Energy Value in Joule:
p = Predefined Output Power in W


t = e / p
t = e / p x 2
Example 1:
Predefined Energy Value: 3 Joule
Predefined Output Power: 60mW = 0.06W
Predifined Frequency: N1: Nogier 1 = 292Hz


T
= 3 / 0.06 x 2

= 100 Seconds


Example 2:

Predefined Energy Value:

3 Joule
Predefined Output Power: 150mW = 0.15W
Predifined Frequency: Nogier 0 = CW-Betrieb -> No Modulation


T
= 3 / 0.15

= 20 Seconds











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