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Labor Division and Distributed Sensing in Swarm Systems
Zhiwen Liu, Gregory J. Steckman and Demetri Psaltis

Abstract. We demonstrate a holographic system which can record nanosecond events. Five frames of laser induced shock wave propagation were recorded using this apparatus with a time resolution of 5.9ns and frame interval of 12ns.

Summary. Fast events can be recorded in pulsed holograms. In order to resolve the individual holograms recorded with different pairs of pulses, spatial1, 2 or angle3, 4 multiplexing may be used. The method we describe uses the angular selectivity of thick holograms to resolve frames that are recorded with adjacent pulses. Two specially designed cavities are used to generate the signal and reference pulse trains. The advantage of our method is that the speed is limited by the pulse width of the laser instead of a scanning mechanism. The number of frames is limited by the dynamic range of the recording material, not its spatial extent.

Figure 1. Angular multiplexing

As shown in Fig.1a, a sequence of signal and reference pulses are incident on the holographic medium during the recording. The signal pulses all travel in the same direction while the reference beam direction changes from pulse to pulse in order to angularly multiplex holograms. After the recording, a CW laser at the same wavelength is used to read out individual frames. Depending on the incidence angle, different frames can be read out separately due to the angular selectivity of the thick hologram. In the experiments, both the signal and the reference pulse trains are generated by a single pulse from a frequency doubled Q-switched Nd:YAG laser (wavelength 532nm, pulse width 5.9 ns, energy per pulse 300 mJ and beam diameter 9mm). The cavity used to generate the reference pulses is shown in Fig.1b. The incident pulse is coupled into the cavity with a small mirror. The two lens form a 4-F imaging system. We break the symmetry of the cavity by slightly slanting the rear partial mirror so that after each round trip the pulse adjusts its direction slightly. The signal pulse train can be generated using the cavity shown in Fig.1c. The vertically polarized (perpendicular to the paper) incident pulse is coupled into the cavity using a polarizing beam-splitter. The pockels cell is timed to rotate the polarization of the pulse to horizontal direction (in the paper) after it first enters the cavity. It is turned off afterwards while the pulse travels back towards the opposite mirror. A l/4 wave plate is used to slightly rotate the polarization of the pulse and the induced vertical polarized component is coupled out of the cavity from the polarizing beam splitter. In both cases the pulse separation is controlled by the round trip time.

We generated five pairs of pulses using the above method. The diffraction efficiency of the pulsed holograms recorded these pairs of pulses is shown in Fig. 2a. The diffraction efficiency decays since there is less and less energy in the pulse train. The last hologram yields a diffraction efficiency of about 0.1% which is still well above the scattering noise level. The angular selectivity of the first hologram is shown in Fig.2b. Aprilis photo-polymer5 was used as the holographic recording medium.

Figure 2. Pulsed hologram

We used this appratus to record optical breakdown6, 7 events. We split the pulse from the laser and focused it on some sample. This pumping pulse can optically break down the object. Fig.3 shows the optical. breakdown on a PMMA sample. Frame A was recorded at about 1ns before the pumping pulse vanished. A,B,C,D and E are the successively recorded frames and the frame interval is about 12ns. F is the final direct image of the sample after the optical breakdown. The size of the image is 1.74mm X 1.09mm. The intensity of the pumping beam is about 1.6 X 1012W/cm2. Frame A shows the plasma created by the pumping pulse. The tail is likely due to the discharge in the air in front of the sample. In frame B, a shock wave is clearly seen. The average propagating speed of the shock wave between frame A and B is about 10 km/s and that between frame D and E is about 4 km/s.

In conclusion, we developed a hologarphic system to record fast events. The performance is comparable to the current state of the art of multi-camera system. Hologaphy has the advantage of recording phase and 3D information.

Figure 3. Optical break down in PMMA

References

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