Time-Gating for 7 Frames with a Digital Micromirror Device

Introduction  

The paper titled "Diffraction-gated real-time ultrahigh-speed mapping photography" ([2]) presents a method for capturing seven distinct frames at an ultra-fast speed.

Method  

The study employs a Digital Micromirror Device (DMD), a MEMS-based technology featuring an array of tiny mirrors that can toggle between two orientations at high speeds. Placed in the back focal plane of an imaging system, the DMD produces seven diffraction orders, essentially creating seven copies of the input image.

All mirrors in the DMD array tilt synchronously. During transitions from an "all-off" to an "all-on" state, this synchronous tilt yields a dynamic phase profile. This results in the diffraction envelope sweeping through the seven diffraction orders.

Equation 1 outlines the DMD's diffraction pattern:




This equation describes the intensity of a point target observed in the intermediate image plane. Two sinc functions modulate the diffraction orders: one depends on the width of individual mirrors and the other on the width of the device. The time-dependent element here is the instantaneous tilt angle θb of the mirrors.

Note: A potential error could be the cos(mπ) term located outside the sum over m.

Performance metrics indicate a frame interval of 0.21 us ± 0.03 us, corresponding to 4.8 million frames per second.

Applications  

While capturing just seven consecutive frames may seem limited, there are intriguing applications. Given the short integration time, the method requires either high light power or multiple cycles of the DMD mirrors. Potential applications include:

• Analyzing the ablation patterns of pulsed lasers
• Investigating the spatial distribution of light modes in a moving multi-mode fiber
• Studying transient changes in photonic integrated chips, such as superluminescent diodes or lasers with modulated gain sections
  

References  

- [1] AJD-4500 DMD Controller https://ajile.ca/ajd-4500/
- [2] Optica Paper https://opg.optica.org/optica/fulltext.cfm?uri=optica-10-9-1223&id=538060
- [3] Hacker News Discussion https://news.ycombinator.com/item?id=37523314