Robert Byer at OSA’s Laser Congress in Nagoya, Japan.
A few days after the most recent announcement from the LIGO-Virgo collaboration, and a day before the announcement of the 2017 Nobel Prize in Physics (which, it’s been speculated, will honor the LIGO team’s discovery of gravitational waves), OSA 1994 President Robert Byer offered a behind-the-scenes look at the development of the solid-state lasers that make LIGO tick.
Byer, who spoke in a plenary address at the OSA Laser Congress in Nagoya, Japan, looked back in the talk to LIGO’s earliest beginnings in the 1970s, and described some of the challenges in developing the quantum-noise-limited systems crucial for reaching the LIGO instruments’ exquisite design sensitivities. And he dropped a broad hint at the end that, Nobel Prize or no, LIGO’s next announcement would be a big one.
A long history
Byer began by noting that he was going to take the audience back to “a time before many of you were born.” Specifically, he harked back to the year 1972, when the LIGO project was hatched in an all-night discussion between Rainer Weiss of the Massachusetts Institute of Technology (MIT) and Kip Thorne at the California Institute of Technology (Caltech).
It was not until 1988, however, that the idea of a project to build two vast interferometers to catch gravitational waves started to emerge as a realistic possibility, in a meeting in Boston with the U.S. National Science Foundation (NSF). And at that meeting, the lasers that would be used to drive the interferometer were still an open question.
“The going-in assumption, said Byer was that the laser for LIGO would be a 10-W argon-ion laser with 40 KW of electricity demand”—a laser that, he noted, likely wouldn’t be able to run continuously for more than 1,000 hours at a time. “We suggested instead that LIGO look at diode-pumped solid-state lasers.”
Developing the first LIGO lasers
Byer’s team, which was tasked with developing those lasers for LIGO, was, at the time, working on another project, coherent laser radar. He pointed out that the needs for that project—a coherent local oscillator, a power amplifier, and a heterodyne detector—were similar to the LIGO system’s requirements. But LIGO had an additional, stiffer spec: the performance needed to be quantum-noise-limited. “That’s a constraint that we usually don’t worry about in designing laser systems,” he said.
For the first LIGO facility, slated to go live in the year 2000, Byer’s team settled on a master-oscillator power amplifier (MOPA) architecture, with a solid-state nonplanar ring oscillator—a “ring of neodymium YAG”—for the seed laser. “This little chip of YAG,” he said, “became the front end of the LIGO instrument.” One big hurdle to getting the chip LIGO-ready, however, was determining whether the system was indeed quantum-noise-limited—and, in fact, Byer noted that it wasn’t even known at that point whether the MOPA architecture would allow the quantum-noise limit to be measured at all.
Hammering down the noise
Further experiments showed that the new “little chip of YAG” could indeed function at the first-generation LIGO’s demanding specifications, and the team, working with the start-up firm Lightwave Electronics, was able to deliver seven solid-state lasers to LIGO in 1995.
When LIGO first started trial operations in May 2001, Byer says, “we had a lot worse signal than we thought we would have.” But the team was able to hammer down noise levels and boost sensitivity year by year until, by the time of the instruments first live operating run in 2005-2007, the instrument’s original operating specs for had been met. “That was absolutely critical,” according to Byer, “because the NSF had agreed that if we met the requirements for … the first-generation LIGO, they would consider funding an advanced LIGO interferometer, with an order-of-magnitude increase in sensitivity.”
On to Advanced LIGO
The laser efforts of Byer’s team at Stanford University also figured into the second-generation LIGO, which experienced such spectacular success with its initial detection of gravitational waves in late 2015. While there was initially a competition for the laser architecture for Advanced LIGO among several groups, the MOPA architecture from Byer’s lab ultimately won out, and a nonplanar ring oscillator is “where the light starts” for Advanced LIGO as well.
The laser system for the second-gen LIGO, Byer explained, consisted of a 35-W preamplifier followed by an injection amplifier to boost the laser power to 180 W. But, he said, it turns out that the new LIGO can’t actually use 180 W of laser power. The reason, discovered by Amber Bollington, relates to the mind-bending sensitivity of the Advanced LIGO instrument.
“As you turn up the power, the absorbed light on the mirror gives you a slight dimple on the mirror,” said Byer. That tiny dimple, in turn, leads to the appearance of higher-order modes in the interferometer, which in turn creates an instability in which “the interferometer will dump all of its power from the fundamental mode into the higher-order mode,” and which grows over time. “LIGO ran into this instability, as we predicted it would,” Byer said. “So we don’t put more than 30 W into LIGO right now.”
Big mid-October announcement?
LIGO has clearly come a long way since the glimmer of the initial idea in the early 1970s; indeed, the persistence and ultimate success of the project in the subsequent 45 years has made it the poster child for long-term scientific and funding commitment. In a nod to that long history, Byer noted that the seven solid-state lasers created for the first-generation LIGO are still present as “backup lasers” in the Advanced LIGO facility.
And Byer hinted that the coming month may see still more impressive results in the quest for gravitational waves and “multimessenger astronomy.” Specifically, Byer offered a quick teaser of an upcoming news conference that the LIGO team will hold on 16 October to announce a new result. While Byer couldn’t reveal the content, what he did say suggested that his particular presser will be a don’t-miss event. “I’m willing to admit,” Byer said, concluding his talk, “that what they will announce will be one of the most important events in the history of astrophysics.”