Optical combs yield extreme-accuracy gigahertz RF oscillator

It may seem at times that there is a divide between the optical/photonic domain and the RF one, with the terahertz zone between them as a demarcation. If you need to make a transition between the photonic and RF words, you use electrooptical devices such as LEDs and photodetectors of various types. Now, all or most optical systems are being used to perform functions in the optical band where electric comments can’t fulfill the needs, even pushing electronic approaches out of the picture.

In recent years, this divide has also been bridged by newer, advanced technologies such as integrated photonics where optical functions such as lasers, waveguides, tunable elements, filters, and splitters are fabricated on an optically friendly substrate such as lithium niobate (LiNbO₃). There are even on-chip integrated transceivers and interconnects such as the ones being developed by Ayar Labs. The capabilities of some of these single- or stacked-chip electro-optical devices are very impressive.

However, there is another way in which electronics and optics are working together with a synergistic outcome. The optical frequency comb (OFC), also called optical comb, was originally developed about 25 years ago—for which John Hall and Theodor Hänsch received the 2005 Nobel Prize in Physics—to count the cycles from optical atomic clocks and for precision laser-based spectroscopy.

It has since found many other uses, of course, as it offers outstanding phase stability at optical frequencies for tuning or as a local oscillator (LO). Some of the diverse applications include X-ray and attosecond pulse generation, trace gas sensing in the oil and gas industry, tests of fundamental physics with atomic clocks, long-range optical links, calibration of atomic spectrographs, precision time/frequency transfer over fiber and through free space, and precision ranging.

Use of optical components is not limited to the optical-only domain. In the last few years, researchers have devised ways to use the incredible precision of the OFC to generate highly stable RF carriers in the 10-GHz range. Phase jitter in the optical signal is actually reduced as part of the down-conversion process, so the RF local oscillator has better performance than its source comb.

This is not an intuitive down-conversion scheme (Figure 1).

Figure 1 Two semiconductor lasers are injection-locked to chip-based spiral resonators. The optical modes of the spiral resonators are aligned, using temperature control, to the modes of the high-finesse Fabry-Perot (F-P) cavity for Pound–Drever–Hall (PDH) locking (a). A microcomb is generated in a coupled dual-ring resonator and is heterodyned with the two stabilized lasers. The beat notes are mixed to produce an intermediate frequency, f_IF, which is phase-locked by feedback to the current supply of the microcomb seed laser (b). A modified uni-traveling carrier (MUTC) photodetector chip is used to convert the microcomb’s optical output to a 20-GHz microwave signal; a MUTC photodetector has response to hundreds of GHz (c). Source: Nature

But this simplified schematic diagram does not reveal the true complexity and sophistication of the approach, which is illustrated in Figure 2.

Figure 2 Two distributed-feedback (DFB) lasers at 1557.3 and 562.5 nm are self-injection-locked (SIL) to Si₃N₄ spiral resonators, amplified and locked to the same miniature F-P cavity. A 6-nm broad-frequency comb with an approximately 20 GHz repetition rate is generated in a coupled-ring resonator. The microcomb is seeded by an integrated DFB laser, which is self-injection-locked to the coupled-ring microresonator. The frequency comb passes through a notch filter to suppress the central line and is then amplified to 60 mW total optical power. The frequency comb is split to beat with each of the PDH-locked SIL continuous wave references. Two beat notes are amplified, filtered and then mixed to produce f_IF, which is phase-locked to a reference frequency. The feedback for microcomb stabilization is provided to the current supply of the microcomb seed laser. Lastly, part of the generated microcomb is detected in an MUTC detector to extract the low-noise 20-GHz RF signal. Source: Nature

At present, this is not implemented as a single-chip device or even as a system with just a few discrete optical components; many of the needed precision functions are only available on individual substrates. A complete high-performance system takes a rack-sized chassis fitting in a single-height bay.

However, there has been significant progress on putting multiple functional locks into single-chip substrate, so it wouldn’t be surprising to see a monolithic (or nearly so) device within a decade or perhaps just a few years.

What sort of performance can such a system deliver? There are lots of numbers and perspectives to consider, and testing these systems—at these levels of performance—to assess their capabilities is as much of a challenge as fabricating them. It’s the metrology dilemma: how do you test a precision device? And how do you validate the testing arrangement itself?

One test result indicates that for a 10-GHz carrier, the phase noise is −102 dBc/Hz at 100 Hz offset and decreases to −141 dBc/Hz at 10 kHz offset. Another characterization compares this performance to that of other available techniques (Figure 3).

Figure 3 The platforms are all scaled to 10-GHz carrier and categorized based on the integration capability of the microcomb generator and the reference laser source, excluding the interconnecting optical/electrical parts. Filled (blank) squares are based on the optical frequency division (OFD) standalone microcomb approach: 22-GHz silica microcomb (i); 5-GHz Si₃N₄ microcomb (ii); 10.8-GHz Si₃N₄ microcomb (iii) ; 22-GHz microcomb (iv); MgF₂ microcomb (v); 100-GHz Si₃N₄ microcomb (vi); 22-GHz fiber-stabilized SiO₂ microcomb (vii); MgF₂ microcomb (viii); 14-GHz MgF₂ microcomb pumped by an ultrastable laser (ix); and 14-GHz microcomb-based transfer oscillator (x). Source: Nature

There are many good online resources available that explain in detail the use of optical combs for RF-carrier generation. Among these are “Photonic chip-based low-noise microwave oscillator” (Nature); “Compact and ultrastable photonic microwave oscillator” (Optics Letters via ResearchGate); and “Photonic Microwave Sources Divide Noise and Shift Paradigms” (Photonics Spectra).

In some ways, it seems there’s a “frenemy” relationship between today’s advanced photonics and the conventional world of RF-based signal processing. But as has usually been the case, the best technology will win out, and it will borrow from and collaborate with others. Photonics and electronics each have their unique attributes and bring something to the party, while their integrated pairing will undoubtedly enable functions we can’t fully envision—at least not yet.

Bill Schweber is a degreed senior EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features. Prior to becoming an author and editor, he spent his entire hands-on career on the analog side by working on power supplies, sensors, signal conditioning, and wired and wireless communication links. His work experience includes many years at Analog Devices in applications and marketing.

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