Log Book Entry No.106

Days, Calendar Created: 20 Jul 2016 11:33:11 Days, Calendar Last Revised: 21 Jul 2016 16:58:50 Person JosephBayley Text Alignment and focus of camera and optical fiber

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Alignment of fiber horizontally and vertically

  • SBIG camera for spectrometer:
    • Exposure: 0.1s
    • Temp: 10 deg
    • Subframes
  • FireWire Camera:
    • records video at 30fps
    • Exposure: 1/500 s
  • Approximate focus was first found. set at 255 mm
  • The mirror was adjusted horizontally such that the laser moved horizontally in the camera frame.
    • Images were then taken of both the spectral line of the laser and the image on the FireWire camera as it was scanned, 40 images were taken.
    • The maximum intensity of the beam and its position were recorded on the FireWire camera and the summed intensity was recorded for the spectrum with removal of backgorund.
    • The intensity on the spectral line was then plotted against the position on the FireWire camera, this gave a plot shown below.

  • The peak position on the above plot is then used to set the position of the beam on the camera, which is approximately at a position of 200 pixels.
  • Once the position has been set the process was repeated scanning this time vertically, this gave a plot as shown below.
  • The best position for the beam to be on the camera for max intensity at this stage is then at pixel values [200,216].

Focussing the beam

  • Once the beam has been set to the best position as found earlier the camera and optical fiber were adjusted between distances of 220mm-290mm in 2mm intervals to find the best focus.
  • Here the same measurements as before were made. The intensity of the spectral line and the maximum intensity of the FireWire camera are plotted against focal distance.
  • The plot of both the camera and optical fiber is shown below.


  • With this image the approximate best focus was set at 250mm

Repeat of alignment

  • The alignment process was then repeated as before, this gave the below plots.


  • These gave maxima at a pixel position of [201,217], The image below shows the position of the laser of the camera at this point:

Horizontal Scans at different Focal lengths

  • For 6 different focal lengths, 240,244,248,252,256,260, a horizontal scan of the laser was recored on the camera and the spectrometer. This range was chosen as in the above focus plots, this is where the intensity flattens.
  • For each of these the position of the laser of the FireWire camera was plotted against the total intensity of the fibers.
  • The aim was that the focal length which gives the smallest width of a spectral line is the correct focus.
  • The plot below shows the intensity plot for each of the focal lengths, and the widths of the line.

  • This shows the widths does not change much in that range, this could be due to the fact that the focus is smaller than the size of the optical fiber, therefore, the integrated amplitude will remain the same in this range.
  • Therefore, by selecting a single fiber to integrate the intensity over, it should be possible to find focus.
  • The below plot shows the same results for a single fiber
  • This shows that the width of the line drops in the range of 252-256 mm .

  • by fitting a Gaussian function to both the image on the firewire camera and the plots above, the widths could be plotted against the focal length for both.

  • Each of these were then fit with a hyperbola function,
 \begin{equation} f(x; \mu,a,b) = a\sqrt{1+((x-\mu)^{2} / b^{2})} \end{equation}<br />

  • This gave parameters of,
parameter FireWire Fiber
mean 254.81.2 255.21.1
a 4.280.92 6.121.25
b 4.721.4 4.431.2

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Subject Alignment and focus of camera and optical fiber
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Topic revision: r4 - 21 Jul 2016 - JosephBayley

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