Holography is a powerful technique of photography of a light field without a
lens for 3D imaging and display. Now, scientists at the Max-Planck Institute
of Quantum Optics are moving holography forward by implementing it with
optical frequency combs. Thousands of holograms over all colors of the
rainbow can be recorded. Via digital processing, each hologram provides a
three-dimensional image of the scene in which the focusing distance can be
chosen at will. Combining all these holograms renders the geometrical shape
of the three-dimensional object with high precision and no ambiguity. At the
same time, other diagnostics can be performed by the frequency combs: Here,
the scientists show molecule-selective imaging of a cloud of ammonia vapor.
Frequency combs go 3D
Reporting in Nature Photonics, an international team of scientists in the
group of Nathalie Picqué at the Max-Planck Institute of Quantum Optics (MPQ)
in Garching, Germany, demonstrates a new imaging technique with optical
frequency combs.
An optical frequency comb generator emits a regular train of short laser
pulses. The spectrum consists of a large number of precisely equally spaced
sharp spectral comb lines. Such frequency combs have made it possible to
count the wiggles of a light wave with high precision. Theodor Hänsch, head
of the Laser Spectroscopy Division at the MPQ, has shared the 2005 Nobel
Prize in physics for this invention. Later on, in the technique of
"dual-comb spectroscopy" developed at MPQ in the group of Nathalie Picqué,
all the spectral lines of a frequency comb were used to interrogate a sample
simultaneously over a broad spectral range, and the comb lines of a second
laser with slightly different spacing interfere on a fast photodetector for
read-out.
The new imaging method of hyperspectral digital holography extends the same
interference method to holographic imaging. "The setup appears deceptively
simple. It only uses two comb generators of slightly different pulse
repetition rates, a partly transmitting beam-splitting mirror, and a fast
digital camera sensor without lens," explains post-doctoral researcher
Edoardo Vicentini.
A 3D object is illuminated by one of the pulse trains, and the scattered
light is directed by the beam-splitter onto the camera sensor. The second
pulse train is directed onto the same sensor as a reference beam. The camera
registers a spatial interference pattern that changes with time, since the
two lasers emit their pulses with a varying time separation. A video
recording of such an interference pattern is shown in a supplemental video
available from Nature Photonics.
In traditional holography, a fine interference pattern is recorded on film,
and illumination of this hologram with a laser beam recreates the original
wavefronts from the object by optical diffraction. In digital holography,
the original scene is reconstructed by a computer program mimicking this
process. In one of the reported experiments, two coins at different
distances are used as objects. During digital reconstruction the focusing
distance can be changed so that either of the coins appears in focus while
the other appears blurred, as illustrated in the video.
"I was thrilled when I got a Matlab program to work, that could produce our
movie of reconstructed images rather quickly," says Theodor Hänsch.
"However, with a faster camera of megapixel resolution, the amount of
recorded data can become rather large so that data processing will become
more challenging."
Nathalie Picqué, pioneer of dual-comb spectroscopy, says, "Dual-comb
interferometers already produce breathtaking results in spectroscopy and in
ranging. The unique combination of broad spectral bandwidth, long temporal
coherence and multi-heterodyne read-out offers powerful new features to
holography. Our technique is likely to conquer new frontiers in scan-free
wavefront reconstruction and three-dimensional metrology. Further, it will
be exciting to explore its potential for microscopy of biological samples."
Reference:
Edoardo Vicentini et al, Dual-comb hyperspectral digital holography, Nature
Photonics (2021).
DOI: 10.1038/s41566-021-00892-x
Tags:
Physics