3.6 m 4 m 6-m arm
Schreiber, a laser physicist at the Technical
University of Munich who led its design.
Schreiber later worked on ring lasers in
New Zealand, California, Germany, and Italy. “He is the lord of the rings,” says Jacopo
Belfi, a physicist at the National Institute
for Nuclear Physics in Pisa, Italy, who
works on GINGERino, a 3.6-meter square
ring laser that is a forerunner to GINGER,
a 6-meter, octahedral ring laser planned
for Italy’s Gran Sasso underground lab.
HAVING WON FUNDING from the European
Research Council, Igel offered Schreiber his
biggest challenge: designing ROMY. With
its 12-meter arms, ROMY is more sensitive
than previous ring lasers, capable of sensing
Earth’s spin to better than one part per billion.
And instead of one square ring, it has four
triangular ones. Three of them are required to
pin down rotations in any direction, and the
fourth adds redundancy. Construction began
in March 2016 and finished 6 months later.
Last month, engineers achieved first light
in all four rings at the same time—a sign
that the geometry of the tetrahedron is precise enough to keep all the lasers resonating
properly. “It’s everything or nothing,” Igel
says. “Every time the red [laser] light is visible, people are screaming, really excited.”
The team is now working on interfering the
lasers and measuring the Sagnac effect. They
hope to present their first proof-of-principle
measurements next week at a meeting of the
Ring of truth
Buried near Munich, Germany, is Rotational
Motions in Seismology (ROMY), a giant ring
laser. It will sense the rotation of Earth and tiny
wobbles of its spin axis—helping calibrate
GPS satellites. It also will detect twisting
motions from earthquakes, which researchers
have typically ignored.
Around the corner
Mirrors keep the lasers
circulating. Light leaking from
opposite beams is combined into
a signal that a photodetector
analyzes for clues to rotation.
Midway through the near-surface arm of
each ring, the steel tube shrinks to a small
glass capillary, where the laser is pumped.
The laser moving in the direction of spin or tilt has a
longer path, stretching its wavelengths. The opposite
laser is compressed. The resulting mismatch in
frequency, or beat, is proportional to the rotation rate.
A buried tetrahedron
Three rings are needed to
detect rotations in any direction.
A fourth ring adds redundancy.
Larger ring lasers are more sensitive, but also more susceptible
to environmental changes that cause measurement drift.