Small sealed lasers can run the risk of suffering serious damage to the cavity if the laser beam is reflected from the process backwards into the laser. Whenever a reflective material is processed, the use of a reflection isolator should be considered. An optical isolator only works with linearly polarized light and in conjunction with a phase retarder (quarter wave) in the system. An additional cooling jacket is available where necessary depending on various conditions.
- Prevents damage to the laser caused by back reflections
- Optional water cooling jacket
||same as C-RI, with water jacket for use above 100 W
When laser cutting or welding reflective metals, problems can occur if CO2 laser energy reflected by the workpiece is transmitted back through the beam-delivery path and into the laser cavity. Back reflection can result in unwanted fluctuations of emitted power, or even burn-out of the cavity optics.
Because the workpiece will be in, or near, the focal plane of the focusing lens, a large fraction of any reflected energy is directed back towards the laser cavity in the form of a fairly parallel beam. There are optical techniques which enable this problem to be overcome. These techniques rely on the manipulation of the polarization state of the laser beam.
ULO Optics’ ‘HPA’-series devices are designed for use as polarization isolators for high power CO2 laser systems. (This is a distinctly different, alternative role to their use as attenuators of linearly polarized beams).
Polarization’ refers to the direction of the electric field of the laser radiation. This direction may be constant with time,in which case the polarization is linear. The electric field direction may be randomly (and rapidly) fluctuating with time…random polarization, or rotating in a predictable way, once each wavelength…circular polarization. (A fourth, and more general state of polarization, ‘elliptical’, occurs where the electric field direction is rotating, but the field strength is changing in magnitude during rotation). The beam emitted by a CO2 laser must be either linearly polarized, randomly polarized, or a mixture of the two. Most industrial CO2 lasers have a folded cavity. These lasers tend to emit beams which are linearly polarized. The direction of the linear polarization may be vertical or horizontal to the base of the laser. Some laser manufacturers (eg: Rofin-Sinar) arrange to have linear polarization at 45° to the laser base. Only in the case of emitted linearly polarized laser beams can polarization isolators be used to block the back-reflected beam.
Historical note, and phase retarders
In the 1970’s it was discovered that the efficiency of the laser cutting process was polarization-dependant. When the metal moved in the direction of the electric field, efficiency was high. When the metal moved in a direction at 90° to the direction of polarization, the efficiency was low. Circularly polarized radiation gave a compromise, such that cut speed and quality were equal in all directions. (The direction of the electric field is spinning at about 3×1013 times per second). A reflective optical component, called a phase-retarder, was developed to convert linear polarization into circular polarization. Linear polarization incident at 45° azimuth angle θ, and at incidence angle i = 45°, is ‘analysed’ into equal-magnitude ‘S’ and ‘P’ components(1). One component is retarded by one-quarter wave (90° retardation) relative to the other. Upon recombining these ‘components’ of the polarization the result is a reflected beam which is circularly polarized.
Solving the back-reflection problem
After reflection from the workpiece, the reflected beam is still circularly polarized. On back-reflection through the phaseretarder assembly, the beam is re-converted into a linearly polarized beam. The direction of linear polarization is now at 90° to the direction of polarization in the out-going beam.
There are two main methods of using the changed direction of linear polarization of the back-reflected beam in order to isolate it from the laser cavity:
(a) A component placed in the beam path (before the phase retarder), which strongly reflects the out-going beam and
strongly absorbs the returned beam.
(b) A system placed in the beam path (before the phase retarder), which strongly transmits the out-going beam, and reflects
the returned beam into a water-cooled beam dump.
Method (a) is embodied in special mirrors, sometimes called ‘ATFR’ mirrors(3). The unwanted energy is absorbed in the specially-coated surface layer of the mirror. As a result, such mirrors are believed to suffer severe limitations on the amount and duration of back-reflected power that they will withstand. Method (b) is promoted by ULO Optics, and embodied in the isolator devices. The ‘HPA’ isolators are based on the use of Brewster plates, which transmit up to 100% of the out-going laser power, and which reflect the unwanted returned beam into a specially designed, water-cooled, dump region internal to these devices. This design principle allows use at higher levels of back-reflected power, for extended duration.
Important note: Neither of these types of isolator actually include a phase-retarder. They are ‘isolators’ in the sense of isolating one state of linear polarization from the orthogonal state of linear polarization
Vee-configuration Brewster systems
The HPA isolators must be placed between the laser cavity and a phase retarder. The Brewster plates are arranged in a Vee configuration. This configuration allows for compensation of lateral and angular deviations of the beam passing through the plates. In the product versions with standard (uncoated) Brewster plates, two plated are required to obtain approx 90% efficiency of reflection of the unwanted laser power. The laser damage threshold of polished, uncoated ZnSe has been measured at 3000W/mm. Therefore, the damage threshold of the standard Brewster plates far exceeds any likely irradiation level. The laser damage threshold of enhanced-coated plates has not been measured, but is estimated at approx 300W/mm (ie:
300W per mm of beam diameter). The end-user, knowing his laser power and beam diameter, might estimate whether or not the HPA isolators will suit his
system. Laser power levels recommended as maximum values in subsequent data sections are not absolute values, but are based on these considerations:
(1) Estimated allowable wavefront distortions due to thermal-lensing.
(2) Reasonable values for rate of cooling water supply.
Important note: If an HPA device is used in the alternative role as an attenuator, then high levels of power may be dumped for prolonged periods of time. The user should take due note of the water-cooling requirements so as to avoid over-heating.
Advantages / disadvantages of enhanced Brewster plates
Enhanced Brewster plates have a much higher efficiency in blocking the back-reflected beam. (When an HPA device is used as an attenuator, the enhanced coating eliminates the possibility of very low power ‘sidelobes’ in the transmitted beam. These can occur due to multiple reflections in non-enhanced Brewster plates).
Enhanced plates are more expensive. The complex coating on one surface leads to an increased level of absorption, and therefore a reduction in the irradiation level at which wavefront distortion may become noticeable. The efficiency of enhanced plates is sensitive to misalignment of the beam.