The need for better radar in World War II drove the development of radio frequency control, and its miniaturization in subsequent decades revolutionized a host of military and consumer applications. The generation of accurate signal frequencies from a single reference oscillator called microwave frequency synthesis, brought about many advanced technologies now critical to the military, such as wireless communications, radar, electronic warfare, atomic sensors and precise timing.
Much like how radio frequency synthesis allows for the precise targeting of a specific radio signal, optical frequency synthesis would allow light to be generated on demand at exact wavelengths with incredible precision. And just like how radio frequency synthesis now has wide ranging applications in today’s consumer products (from GPS systems to smartphones and TV remote controls), optical frequency synthesis will have a significant impact across consumer, scientific and military applications.
However, absolute frequency (or color) of light from a laser is difficult to set with precision, and laser frequencies tend to drift. The development of the “optical frequency comb” garnered a Nobel Prize in 2005, and enabled the demonstration of the first optical frequency synthesizer. Self-referenced frequency combs generated from femtosecond pulses in highly-nonlinear fibers have led to optical synthesizers with record breaking stability and accuracy. These systems, analogous to their radio-frequency counterparts, allow light to be generated on demand at exact wavelengths with errors of less than 10-15 or one-part-per-quadrillion.
Optical frequency combs were developed nearly two decades ago to support the world’s most precise atomic clocks. The optical frequency comb (OFC) was originally developed to count the cycles from optical atomic clocks. Atoms make ideal frequency references because they are identical, and hence reproducible, with discrete and well-defined energy levels that are dominated by strong internal forces that naturally isolate them from external perturbations. Consequently, in 1967 the international standard unit of time, the SI second was redefined as 9,192,631,770 oscillations between two hyper-fine states in 133Cs1. While 133Cs microwave clocks provide an astounding 16 digits in frequency/time accuracy, clocks based on optical transitions in atoms are being explored as alternative references because higher transition frequencies permit greater than a 100 times improvement in time/frequency resolution.
Optical signals, however, pose a significant measurement challenge because light frequencies oscillate 100,000 times faster than state-of-the-art digital electronics. For precision measurements seeking resolutions better than that offered by wavelength standards, large-scale frequency chains were used to connect the microwave definition of the Hertz, provided by the 133Cs primary frequency reference near 9.2 GHz, to the optical domain via a series of multiplied and phase-locked oscillators.
Because of the complexity, frequency multiplication chains yielded one to two precision optical frequency measurements per year. In 2000, the realization of the OFC allowed for the replacement of these complex frequency chains with a single mode-locked laser (MLL), enabling vast simplification to precision optical measurement and rapid progress and development into optical atomic standards. Acting as precision optical synthesizers, frequency combs enable the precise transfer of phase and frequency information from a high-stability reference to hundreds of thousands of tones in the optical domain. This versatility, coupled with near-continuous spectroscopic coverage from microwave frequencies to the extreme ultra-violet, has enabled precision measurement capabilities in both fundamental and applied contexts.
Frequency combs—on-chip integration on track
Prof. Kippenberg’s lab showed in 2007 that optical frequency combs could be created using tiny devices called “optical microresonators”: microscopic ring-shaped structures made from very fine silicon nitride measuring a few millimeters to a few tens of microns in diameter. These structures can trap a continuous laser light and convert it into ultra-short pulses – solitons – thanks to the special nonlinear properties of the device. The solitons travel around the microresonator 200 billion times per second and the pulsed output from the microresonator creates the optical frequency comb.
Researchers at EPFL, in a project led by Victor Brasch and Erwan Lucas, created what is called a “self-referenced optical frequency comb”. This is essentially a series of densely-spaced spectral lines whose spacing is identical and known. Because they are so well defined, optical frequency combs can be used as a “ruler” for measuring the frequency – or color – of any laser beam. By comparing an unknown color to this ruler, it is possible to calculate its frequency.
However, this implies a critical step called “self-referencing”, a method that exactly determines the position of each individual tick of the frequency ruler, but demands a very long ruler – a broad spectral range, as scientists say – which is challenging to obtain. The technology is amenable to integration with both photonic elements and silicon microchips. Establishing devices that provide a RF to optical link on a chip may catalyze a wide variety of applications such as integrated, atomic clocks and on-chip, and could contribute to making optical frequency metrology ubiquitous.
OFCs were developed by drawing on single-frequency laser stabilization techniques and applying them to mode-locked (pulsed) laser systems. The result was a system that could synthesize 105–106 harmonically related optical modes from either an electronic or optical reference with a fidelity better than 1 part in 1018.
More importantly, OFCs enabled the direct conversion of optical-to-microwave frequencies and vice versa, enabling the extraction of microwave timing signals from optical atomic clocks. Beyond their application to precision optical metrology, OFCs were quickly recognized for their versatility as high-fidelity optical frequency converters and as sources of precisely timed ultra-short pulses. More broadly, by taking advantage of the nonlinearities possible with the ultra-short pulses, OFCs enable synthesis over broad spectral regions including the near-infrared, the visible domain and as far as the extreme ultraviolet (XUV). Generation of difference frequencies within the optical spectrum also allows for high-fidelity frequency transfer to the mid-infrared, terahertz, and microwave domains.
OFCs quickly found application to a multitude of diverse optical, atomic, molecular, and solid-state systems, including X-ray and attosecond pulse generation, coherent control in field-dependent processes, molecular fingerprinting, trace gas sensing in the oil and gas industry, tests of fundamental physics with atomic clocks, calibration of atomic spectrographs, precision time/frequency transfer over fiber and free-space11, arbitrary waveform measurements for optical communication, and precision ranging. To support this broad application space, OFCs have seen rapid changes in laser development to enable coverage at different spectral regions, varying frequency resolutions, and to enable the development of systems that offer lower size, weight and power (SWAP)
Ultralow-Loss Integrated Photonics for Lidar
Researchers at École Polytechnique Fédérale de Lausanne (EPFL) and Purdue University demonstrated a hybrid approach to on-chip acousto-optic modulation by combining piezoelectric aluminium nitride technology with ultralow-loss silicon nitride integrated photonics. The hybrid circuit allows wideband actuation on photonic waveguides with ultralow electrical power. A key feature is that it maintains the ultralow loss of silicon nitride circuits. Silicon nitride has emerged as a leading material for chip-scale, microresonator-based optical frequency combs (microcombs).
The researchers integrated microelectromechanical systems (MEMS) transducers made of aluminum nitride with a silicon photonic wafer to modulate a soliton microcomb at high frequencies ranging from megahertz to gigahertz. They fabricated piezoelectric aluminium nitride actuators on top of the silicon nitride photonic circuits and applied a voltage signal to them. The signal induced bulk acoustic waves electromechanically. The acoustic waves modulated the microcomb generated in the silicon nitride circuits. By monolithically integrating aluminium nitride actuators on ultralow-loss silicon nitride photonic circuits, the researchers demonstrated voltage-controlled soliton initiation, tuning, and stabilization with megahertz bandwidth.
Purdue researcher Hao Tian built the MEMS transducers and integrated them with a silicon nitride photonics wafer developed at EPFL. The researchers demonstrated two independent applications using the hybrid system. First, they showed optimization of a microcomb-based massively parallel coherent lidar. This approach could provide a route to chip-based lidar engines driven by CMOS microelectronic circuits. Second, they built magnet-free optical isolators by spatiotemporal modulation of a silicon nitride microresonator. “The tight vertical confinement of the bulk acoustic waves prevents cross-talk and allows for close placement of the actuators, which is challenging to achieve in p-i-n silicon modulators,” Tian said.
The circuit was manufactured using CMOS-compatible foundry processes. The fabrication processes were integrated, which could make the technology more viable commercially. The MEMS transducers were fabricated on top of the silicon nitride photonics wafer with minimal processing, the researchers said. “This achievement represents a new milestone for the microcomb technology, bridging integrated photonics, microelectromechanical systems engineering, and nonlinear optics,” EPFL researcher Junqiu Liu said. “By harnessing piezoelectric and bulk acousto-optic interactions, it enables on-chip optical modulation with unprecedented speed and ultralow power consumption.”
The new technology could provide the impetus to develop microcomb applications for power-critical systems in space, data centers, and portable atomic clocks, for example, or in extreme environments such as cryogenic temperatures. “As yet unforeseen applications will follow up across multiple communities,” EPFL professor Tobias Kippenberg said. “It’s been shown time and again that hybrid systems can obtain advantages and functionality beyond those attained with individual constituents.”
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