In our experiment, we measure the complex field at the output of multimode fibers using off-axis holography. We noticed some instabilities of the fringes as shown in Fig. 1.

 

stability without clamp small

Figure 1. Fringes recording without isolation.

After investigation, it turned out that the sources of the instabilities were vibrations coming from the FPGA board, that has a fan. Since the board is linked to the DMD chip via a rigid flat cable, those vibrations were directly transmitted to the DMD, resulting in angle fluctuations.

To get rid of these vibrations, a quick and dirty solution turned out to be quite effective; we simply clamped the cable using some foam to absorb the vibrations and isolate the DMD from the board. Fig 2. and Fig 3. shows how it looks like for a DMD working in the horizontal position and at 45 degrees.

DMD horizontal

Figure 2. Isolated setup for a DMD in the horizontal configuration.

The second setup has two cables, it is a bit more difficult to achieve good contact with both cables, especially at a 45 degrees angle. However, having a piece of foam on one side, as long as the cable firmly presses against the foam, is sufficient to isolate the DMD.

DMD 45

Figure 3. Isolated setup for a DMD in the 45 degrees configuration.

We measure again the fringes and observe the movie shown in Fig. 4. We see a drastic stability improvement.

stability with clamp small

Figure 4. Fringes recording after isolation.

To quantify the difference between the system with and without the isolation setup, we use off-axis holography to measure the evolution of the phase as a function of the time at one position. The graph in Fig 5. proves that we get rid of fast fluctuations, likely due to the fan, but also more slow variations, probably due to some tension of the rigid cable that rotates slightly the DMD in time.

stability comparison

Figure 5. Evolution of the phase of one point of the complex measured field as a function of time without (red) and with (blue) isolation.

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Comments   

 
#2 Sébastien Popoff 2019-11-27 10:41
Quoting Lim Tang:
hello Dr.Popoff,

Is the unit of Time second? Can you show the figure of correlation with time?

Thanks a lot.


Yes, it is in seconds, sorry for not showing it on the graph.

Here plotting the correlation is not so relevant as the only parameter fluctuating seems to be the phase. Better quantifying directly this parameter.
Quote
 
 
#1 Lim Tang 2019-11-27 05:04
hello Dr.Popoff,

Is the unit of Time second? Can you show the figure of correlation with time?

Thanks a lot.
Quote
 

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