Skip to content

Notes on Laser Steering

One of the most critical step in the alignment procedure is the laser beam steering. We dedicate this page to specific recommendations and more detailed explanation on why it requires specific attention.

The overall steering system is shown in Figure 1 with different mirrors orientations. Note how changing the mirror orientation affects the angle at which the beam hits the sample but also at which height the beam intersects the lens.

Figure 1 - Two-mirror system to tune a beam

Hitting the lens in a non-axial way is generaly detrimental to image quality as shown in Figure 2. For tight spot size, and therefore better coupling efficiency, it is important to align the spectrometer in a way that is as close as possible to nominal, on-axis, case.

Figure 2 - Misaligning a single lens

In OpenRAMAN, this is achieved using the near and far adjusment of the steering mirrors as shown in Figure 3. Two frosted disks with alignment pinholes are coupled to the system to produce a reference mechanical axis.

Figure 3 - Beam tuning using frosted disk

This reference mechanical axis also matches the re-imaging lens that is in front of the slit, as shown in Figure 4. The long rods ensures that the mechanical axis of the re-imaging lens matches the mechanical axis of the sample holder lens. The alignement will never be perfect, but it is a large improvement over random placement of the components and will make a large difference in signal intensity when done properly.

Figure 4 - Mechanical Axis Reference

Although this procedure gives a relatively good alignment of the beam through the center of the different lenses, it is not enough to capture a signal on the spectrometerby itself - unless you are extremely lucky. That being said, it puts us very close to the final alignment.

To understand why this procedure is not sufficient to capture a maximized signal, think of the lens from the cuvette system and the lens from the re-imaging system as a 1:1 imaging system of the laser spot. The purpose of the final alignment is to bring the image of the laser/raman spot on the sample onto the center of the slit. But any misplacement of the slit or angular shifts between the two lenses will produce a shift of the laser spot image on the slit. Signal will be maximum only if you center perfectly the laser spot on the slit and is therefore more difficult with a narrow slit than with a larger one.

By making tiny lateral adjustments of the dichroic mirror, as shown in Figure 5, it is possible to move the laser spot lateraly and center it on the slit. As an order of magnitude, a 1° shift will displace the beam by more than 300 µm using a 19 mm achromat.

Figure 5 - Effect of tilting the dichroic mirror

The overall beam steering procedure is therefore threefold:

  1. Ensure a proper mechanical axis definition using 6" ER rods.
  2. Steer the beam using two frosted disk the best you can.
  3. Make tiny lateral adjustment using the dichroic mirror until the signal is maximized in the software.

Each part of the procedure is important and we strongly recommend that you revisit each steps when having problem catching the signal.