``ALAS``: The Alignment Laser System
====================================

To facilitate proper alignment of the x-ray optics, apertures, drilled
mirrors, diagnostics and even the sample, an optical laser beam will
be injected somewhat upstream. Although it would be desirable to have
this laser beam propagate along a significant fraction of the x-ray
path, this is outweighed by the inconvenience of placing the laser in
the tunnel, as opposed to the experiment hutch. The solution is to
couple the alignment laser as far upstream as possible in the
experiment hutch.

To maximize the utility of the alignment laser, it should not only
propagates coaxially with the x-beam, but also mimic its profile and
divergence. This will allow us to adjust the KB mirrors to focus on
the sample. :numref:`alignment_laser_concept` shows schematically how
a HeNe laser beam can be matched to the x-ray beam properties.

.. _alignment_laser_concept:
.. figure:: alignment_laser_concept.pdf

    Conceptual arrangement of the optical alignment laser

    The concave lens of a Keplerian telescope expands the HeNe beam to
    the size of the x-ray at the in-coupling point, then the convex
    lens nearly recollimates the beam to match the x-ray divergence.
    The beam should then be focussed by the KB mirrors correctly,
    aiding in their alignment.

The x-ray beam is elliptical since the intermediate foci are offset
in the propagation direction. In order to match the x-ray mode, it is
necessary to use two orthogonal telescopes comprising cylindrical
lenses. A fairly compact optical bench should fit on a breadboard of
approximately 200 mm × 500 mm, as shown in
:numref:`alignment_laser_layout`.

.. _alignment_laser_layout:
.. figure:: alignment_laser_layout.pdf

    Optical bench for the alignment laser

    A nested pair of Keplarian telescopes expands and recollimates the
    HeNe beam to match the size, shape and divergence of the x-ray
    beam. The horizontal telescope comprises L1 and L4; the vertical,
    L2 and L3. The first two lenses are 100 mm and 150 mm from the
    laser output, respectively.

Optical
-------

Preliminary calculations of the optics indicate that it can be done
conveniently with a commercial HeNe laser. Taking a representative
x-ray photon energy of 500 eV (:numref:`XX`), XX fs pulse beam, we
find the horizontal and vertical beam divergence to be 80 µm and 85 µm
respectively. At the injection position, which is about 11.2 m
upstream of the sample, the 1/*e*\ :sup:`2` x-ray beam sizes are
*d*\ :sub:`horizontal` = 3.5 mm and *d*\ :sub:`vertical` = 2.4 mm. A typical 2 mW
HeNe has a divergence of 1.6 mrad and a TEM\ :sub:`00` beam with a 1/*e*\ :sup:`2`
output diameter of 0.63 mm. With lenses at the positions shown, the
focal lengths become,

* Horizontal Telescope: L1 = -114 mm, L4 = +500 mm (*f*\ 2/*f*\ 1 = 4.375 ≈ 4.4)
* Vertical Telescope: L2 = -500 mm L3 = +800 mm (*f2\ /*f*\ 1 = 1.6)

Mechanical
----------

The calculations assume the second lens collimates the beam since
the x-ray beam divergence is very small. The final matching is done by
translating the lenses slightly from their ideal positions using
motorized translation stages. Proper calibration during commissioning
should allow rapid setup to match the various x-ray beam properties.

The visible laser is situated on a breadboard mounted vertically on
the main support of the ALAS, as shown in :numref:`_laser_layout`. Optical and
optomechanical components to condition and steer the beam into the
beamline vacuum chamber are also located on the breadboard.

.. _laser_layout:
.. figure:: LaserLayout.svg

   Schematic of the ALAS optical layout as seen from the wall (south)
   side of the beamline. The visible laser beam is shown in pale blue
   and injected into the vacuum chamber at the upper right of the
   sketch.

An in-vacuum mirror directs the beam along the X-ray beam path. This
mirror can be lifted out of the beam path when not in use to allow the
X-ray beam to pass. Just upstream of the mirror is an imager upon
which the X-ray and visible laser beams can be overlapped spatially.
The imager camera is located on the same optical breadboard as the
laser.

The Laser source
----------------

The laser comprises an optical head and a controller. The controller
requires a 24\ V DC supply. The controller has an RS232 interface.
There is also a safety interlock input that will be connected to the
hutch laser safety system, not to Beckhoff. In normal use a shutter
will be used to block the beam instead of switching off the laser via
RS232.