Artist view of frequency comb and ring resonator

Russian physicists have developed a method for significantly narrowing the emission spectrum of an ordinary diode laser, via self-injection locking via feedback from a microring resonator. [Image: N.G. Pavlov et al./Nature Photonics]

In recent years, engineers have made tremendous progress in shrinking frequency combs—the hyper-precise “optical rulers” that have revolutionized laser spectroscopy—to work on the scale of CMOS chips. Yet those chip-scale combs have generally had to be pumped by powerful external, narrow-linewidth, single-frequency lasers that add bulk, complexity and cost.

Now a Russian research team has demonstrated a method for turning the output of a compact, cheap laser diode into a powerful narrow-linewidth laser source capable of driving on-chip frequency combs and other applications (Nat. Photon., doi: 10.1038/s41566-018-0277-2). To achieve that bit of legerdemain, the team cleverly used optical feedback from the same microresonator employed for frequency comb generation to lock the diode source’s smeared-out spectrum into a single, tight frequency peak.

The method, the researchers suggest, could open a route toward “the most compact and inexpensive highly coherent lasers, frequency comb sources, and comb-based devices for mass production.”

Frequency comb sources

Optical frequency combs are laser-created artificial spectra consisting of hundreds of thousands or even millions of discrete, equally shaped sharp spectral lines. These highly precise spectra have opened new approaches to molecular spectroscopy, an achievement recognized in the 2005 Nobel Prize in Physics awarded to frequency comb pioneers and OSA Honorary Members John Hall and Theodor Hänsch. More recently, so-called dual-comb spectroscopy approaches, which marry two frequency combs, have boosted the technique’s resolution, speed and sensitivity.

Early frequency combs commonly used femtosecond mode-locked lasers as the light source. In recent years, however, another kind of comb, the so-called Kerr comb or microcomb, has emerged that relies on the nonlinear Kerr effect in high-Q optical microring resonators. Researchers have even succeeded in fashioning on-chip dual-comb setups using such resonators, tied to an external laser source.

And there’s the rub. Because a microcomb must be pumped with a strong, single-frequency laser signal, with a linewidth comparable to the width of the microcavity resonance, these on-chip combs have needed to be connected to, and driven by, external lasers that can achieve the requisite power and narrow linewidth. That requirement limits the prospects for compact or wearable comb spectrometers for field-based chemical analysis, for example, and some other envisioned applications.

A self-locking solution

The team behind the new research, led by OSA member Michael Gorodetsky of the Russian Quantum Center (RQC) and the Lomonosov Moscow State University (MSU), wanted to see if a compact, cheap and not particularly narrow-linewidth laser diode could be converted into a narrow-linewidth source suitable for an on-chip microcomb. To do so, they decided to use the same microring resonator underlying the Kerr frequency comb as a source of optical feedback to force the diode laser into narrow-linewidth operation—a scheme called self-injection locking.

In the team’s setup, light from an inexpensive, 100-mW commercial laser diode—centered at telecom wavelengths and with a 10-nm spectral width—passes through a lens and is coupled via a glass prism to a magnesium-fluoride ring microresonator 5.5 mm in diameter. Radiation at the resonator’s whispering-gallery-mode frequency is resonantly backscattered and returned to the diode laser. The injection of low-noise, single-frequency backscattered laser power effectively reduces the phase and intensity noise, and locks the diode laser into operation at the injected frequency.

Laser and microcomb, from one setup

The research team found that it was able to use the setup to narrow the 10-nm spectral width of the original to a high-power transmission band only attometers (billionths of a nanometer) in spectral width. The researchers also found that, by carefully tuning the diode laser frequency, they could use the same system, without modification, to generate a 30-nm-wide soliton frequency comb, with a line spacing of 12.5 GHz, directly from the multi-frequency diode source.

The researchers believe that their system to create narrow-linewidth, compact lasers from cheap laser diodes could have a wealth of applications beyond frequency comb generation. For example, they note that such narrow-linewidth sources could prove useful in telecom, allowing a higher number of channels for increasingly bandwidth-constrained optical communication networks. And they see applications in wearable sensors, lidar for autonomous vehicles, and other areas. “The demand for such lasers,” team leader Gorodetsky noted in a press release accompanying the paper, “would be really high.”

In addition to RQC and MSU, the work included researchers from the Moscow Institute of Physics and Technology and the Samsung R&D Institute Russia.