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. alignment_laser_concept shows schematically how a HeNe laser beam can be matched to the x-ray beam properties.

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 alignment_laser_layout.

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 (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/e² x-ray beam sizes are d_horizontal = 3.5 mm and d_vertical = 2.4 mm. A typical 2 mW HeNe has a divergence of 1.6 mrad and a TEM₀₀ beam with a 1/e² output diameter of 0.63 mm. With lenses at the positions shown, the focal lengths become,

• Horizontal Telescope: L1 = -114 mm, L4 = +500 mm (f2/f1 = 4.375 ≈ 4.4)
• Vertical Telescope: L2 = -500 mm L3 = +800 mm (f2/*f1 = 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 _laser_layout. Optical and optomechanical components to condition and steer the beam into the beamline vacuum chamber are also located on the breadboard.

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 24V 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.