PETRA-III Laser-wire

Transport fiber description

The laser beam produced by the 4 stages fiber amplifier is coupled into a photonic crystal fiber (PCF) for transport to the interaction point (IP) in the accelerator beam pipe.

The PCF fiber (Manufacturer NKT, model LMA-25) has a core diameter of 25 \x{03bc}m and is endlessly single mode at the wavelength of 1064 nm. The principle on which this type of fibers guide light is different from standard fibers. The guiding in standard fibers is achieved by a refractive index difference between core and cladding in order to achieve total internal reflection for rays with angle lower than the maximum numerical aperture (NA) of the fiber. In the case of PCF fibers, the total internal reflection is maintened by a pattern of holes around the core that generate constructive interference in a fashion similar to a multilayer mirror. The fiber is in this case very sensitive to wavelength and the bending angle.

The achieved coupling efficiency into the transport fiber is approximately 52%, therefore the maximum laser average power at the IP is 800 mW (approximately 7 kW peak power).

Transport fiber Coupling

The coupling in the fiber is achieved by using a 3D translation stage with differential adjustments and positional resolution of 5 nm (Thorlabs model MAX311D/M ).

As monitoring the output from the transport fiber is not easy thing to do if light is not already coupled into the fiber (as the output of the transport fiber is in the tunnel and it is not accessible during normal accelerator operation), a series of expedients need to be used.

Firstly a standard webcam is positioned in front of the input face of the fiber, as the sensor is sensitive to the near infrared and can give a rough indication of whether the beam hits the center of the fiber and if it is near to the focal position. Second, a short sample of fiber (taken from the transport fiber) is placed in the connector holder. In this case is possible to check that the light is coupled by measuring with a sensitive powermeter or a CCD camera. This sample of fiber is then replaced by the transport fiber and the output power measured by a post-IP powermeter (described in the next section). If there is a slight different position between the two fibers, the post-IP measurement is suffiecent to be able to optimise the coupling.

Final focus assembly and measurements

The end of the optical fiber and a multi-element focusing lens with a focal length of 125 mm (CVI model LAP 125) are mounted on an assembly of two translation stages (Newport model UTS50CC). Thetranslation stages allow movement with a resolution of 1 μm along the vertical direction for scanning the e-beam and the longitudinal direction for positioning of the focal plane.

The final focus optical system is made by the fiber output and the focussing lens. The lens is set at a 2f distance from the fiber output face in order to reproduce the image of the beam with 1:1 magnification at the IP. The expected beam size for this configuration would therefore be approximately 25 μm diameter. The actual measurement of the laser spot size at the IP is about 35 μm diameter as shown in the picture below. The M2 = 1.1 and tehrefore it's very near to ideal gaussian beam.

Post IP diagnostic

After the laser pass through the interaction chamber, two different measurements are performed. The laser is steered from the output window of the vacuum chamber by a 10% beam sampler. 90% of the power is dumped behind this sampler. The 10% sample is then split again between a photodiode (10%) in order to measure the relative pulse peak power and a powermeter (90%) for average power measurements.

The measurement at the post-IP has been calibrated with the output from the fiber directly with a powermeter. The calibration factor is 13E3, so a measurement of 40 μW corresponds to an average power at the output of the fiber of 520 mW.