For better beam quality:
Gaussian mirrors

The beam quality of lasers can be significantly improved through the application of Gaussian output-couplers. Many other applications require the use of optics having defined local reflection values. An example of this is beam formation using soft apertures.

The high intensity of the laser beam requires the use of components with a high damage threshold. Dielectric coatings are best suited to meet these requirements.
LASER COMPONENTS specializes in the production of such customer-specific coatings.

Structure

The principle of operation is based on a Fabry-Perot etalon. Fig. 1 shows a cross section through a monochromatic graded-reflectivity coating. A so-called spacer layer S of varying thickness d is enclosed by two identical mirrors M. Where d is an even-numbered multiple of l/4, the system will be fully transparent for wavelength l, i.e. the reflection vanishes. On the other hand, if d is an odd-numbered multiple of l/4, reflectivity will be governed by the reflection of the mirrors M. Function d (x, y) thus determines local reflection. In the simplest cases this function can be rectangular or circular (see photos). LASER COMPONENTS offers both versions as standard. The production method used ensures exact symmetry for both geometric patterns. Other customer-specified reflection profiles can be provided on request.

The Fabry-Perot principle used in the structure ensures that the phase shift caused by the coating is minimised. Wave-front deformation of coherent radiation fields is kept at a minimum.

Characterization

LASER COMPONENTS offers graded-reflectivity coatings for many fields of application. Determining the optimum solution for a special application will, of course, require some information from the customer. The following paragraphs explain all the significant parameters involved. The discussion is limited to coatings with rectangularly or circularly symmetric profiles.

Values of Reflectivity Rcen, Rout

Two important parameters are the reflectivity values in the outer zone and in the centre zone, Rcen, Rout
(see Fig. 3).

LASER COMPONENTS defines all coatings with
Rcen > Rout as GAUSSIAN MIRRORS (GM); this includes the so-called super-gaussian mirrors with a Gaussian order higher than 2. It is generally assumed that Rout = 0 applies for GMs; however other values can be specified by the customer.

All coatings with Rcen < Rout are referred to as apodizing apertures or SOFT APERTURES (SA).

Laterale Dimension, w / wx, wy

Generally 1/e² is used; this applies both for circular as well as rectangular geometries. If the coating is circular, only one value, w, is required. For rectangular geometries two values, wx and wy, must be specified according to the coordinates. The following applies: the coordinate with the larger w is defined as the x-coordinate, i.e. wx > wy (Fig. 3).

Gaussian Order, n / nx, ny

Here too only one value, n, is required for circular symmetry. Two values, nx and ny, must be specified for rectangular profiles. Please note for rectangular geometry the larger lateral dimension wx always corresponds to a larger Gaussian order.
The following approximation applies:
nx/ny » wx/wy

Wavelength of Operation

Dielectric coatings having a defined reflectivity function
R (x, y) are generally monochromatic. It is thus essential to specify the desired wavelength.
LASER COMPONENTS currently offers Gaussian mirrors for 1064 nm. Other wavelengths will be offered in due course.

Laser Beam Parameters, E, f

Specifying the pulse energy, E, pulse duration, t, and repetition frequency, f, of the laser will ensure that the damage threshold of the components is sufficiently high for the application. A high damage threshold coating version will be provided for components used in high-power laser operation.

Ordering Information