Frequency Combs: A Laser Chip for a New Generation of tiny Sensors

Good news! Amazing stuff!

"... A new ultra-energy-efficient tiny laser on a chip could enable powerful medical sensors to fit within a phone, new research finds.

The new device is a kind of frequency comb—a specialized laser that generates multiple wavelengths of light, each at a regular frequency interval. On a spectrogram it would look a bit like the teeth of a comb. In the roughly quarter century since they were first developed, these “rulers for light” have revolutionized many kinds of high-precision measurement, from timekeeping to molecular detection. In addition, each line of a comb can be isolated and have properties such as its amplitude modulated to carry data over fiber optics. ...

Now, scientists at Stanford University employed two different methods that previous work explored to create microchip-scale frequency combs. One strategy, called optical parametric oscillation, involves bouncing beams of laser light within a crystal, resulting in light organizing itself into pulses of coherent, stable waves. The other approach, known as phase modulation, sends laser light into a cavity and then applies radio-frequency signals to control the phase of the light, generating frequency repetitions for use in combs. However, both of these strategies come with drawbacks, such as energy inefficiency and a limited ability to adjust optical parameters. ...
To overcome these challenges, the scientists experimented with a material called thin-film lithium niobate, which has a number of advantages compared to silicon, the industry standard material. Two of these properties include how a broad range of light wavelengths can pass through it, and how it can allow light beams of different wavelengths to interact with each other to generate new wavelengths.

The new material accommodated both optical parametric amplification and phase modulation within a single cavity. The resulting “microcomb” is just 1 by 10 millimeters in size. Such a compact size suggests it could find use in personal devices the size of a phone or smaller, the researchers say. It could also be easily made at conventional microchip fabs, they add. ...

A High Efficiency, High Performance Frequency Comb

The new device’s efficiency at converting light pumped into the cavity into a comb exceeded 93 percent. It could generate 200 comb lines spaced about 5.8 gigahertz apart across more than 1 terahertz of frequencies. It proved highly tunable by simply adjusting the radio signal applied to it. ...

The scientists note that spacing between comb lines could reach 50 to 100 GHz and that the device could potentially work with blue to midinfrared light. This suggests that the microcomb could find use in applications such as medical diagnostics, fiber telecommunications, LIDAR, and spectroscopy. ..."

From the abstract:

"Optical frequency combs have revolutionized precision measurement, time-keeping and molecular spectroscopy. A substantial effort has developed around ‘microcombs’: integrating comb-generating technologies into compact photonic platforms. Current approaches for generating these microcombs involve either the electro-optic or Kerr mechanisms. Despite rapid progress, maintaining high efficiency and wide bandwidth remains challenging. Here we introduce a previously unknown class of microcomb—an integrated device that combines electro-optics and parametric amplification to yield a frequency-modulated optical parametric oscillator (FM-OPO). In contrast to the other solutions, it does not form pulses but maintains operational simplicity and highly efficient pump power use with an output resembling a frequency-modulated laser. We outline the working principles of our device and demonstrate it by fabricating the complete optical system in thin-film lithium niobate. We measure pump-to-comb internal conversion efficiency exceeding 93% (34% out-coupled) over a nearly flat-top spectral distribution spanning about 200 modes (over 1 THz). Compared with an electro-optic comb, the cavity dispersion rather than loss determines the FM-OPO bandwidth, enabling broadband combs with a smaller radio-frequency modulation power. The FM-OPO microcomb offers robust operational dynamics, high efficiency and broad bandwidth, promising compact precision tools for metrology, spectroscopy, telecommunications, sensing and computing."

Frequency Combs: A Laser Chip for a New Generation of Sensors - IEEE Spectrum The new frequency comb could make powerful sensors fit in a phone

‘Ruler for light’ could enable detailed measurement in personal devices Frequency combs have revolutionized precision measurement, but the bulky, power-hungry devices are limited to lab settings. A new efficient laser “microcomb” developed by Stanford researchers could bring that revolution to the handheld electronics realm.

Integrated frequency-modulated optical parametric oscillator (no  public access)

A microscope image showing a thin-film lithium niobate chip that contains eight of the new “FM-OPO” devices. One device has a footprint around 1×10 mm2 (highlighted here with a dashed rectangle).