20 years of developments in optical frequency comb technology and applications

Tara Fortier; Esther Baumann

Source-linked AI summary

20 years of developments in optical frequency comb technology and applications

Tara Fortier, Esther Baumann

arXiv:1909.05384v1Sep 11, 2019physics.opticsphysics.app-ph

TL;DR

Optical frequency combs addressed the challenge of transferring and measuring optical frequencies for increasingly precise atomic clocks and spectroscopy. This review synthesizes two decades of comb source development, stabilization, nonlinear spectral extension, and applications. The resulting technologies support coherent optical–microwave conversion, broad spectroscopic coverage, and deployment across fundamental and applied measurements, while remaining constrained by application-specific complexity, efficiency, and detection limits.

Problem
Optical frequency measurement required simpler ways to connect high-stability references with large numbers of optical modes and broad spectral regions.
Method
The review gives a tutorial and historical synthesis of comb principles, source development, parameter stabilization, spectral extension, and applications.
Results
Optical frequency combs enabled direct phase-coherent optical–microwave conversion, broad spectroscopic coverage, and precision applications including clocks, spectroscopy, and astronomical calibration.
Takeaways & Limitations
Two decades of source development moved combs from laboratory precision tools toward compact, field-deployed systems across fundamental and applied measurement settings.
Takeaways & Limitations
Commercial adoption in LIDAR and FTIR replacement remains limited because comb cost and complexity still outweigh their precision, resolution, and acquisition benefits.

Abstract

from arXiv · show

Optical frequency combs were developed nearly two decades ago to support the world's most precise atomic clocks. 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 the terahertz to the extreme ultra-violet, has enabled precision measurement capabilities in both fundamental and applied contexts. This review takes a tutorial approach to illustrate how 20 years of source development and technology has facilitated the journey of optical frequency combs from the lab into the field.

1 Introduction

Optical frequency combs emerged to simplify precision optical-frequency measurement and connect high-stability references to broad, coherent optical spectra. Their versatility has driven applications spanning precision metrology, spectroscopy, and fundamental measurements.

Origins and motivation: Optical frequency combs replaced complex frequency chains with a single mode-locked laser for precision optical measurement.This simplification accelerated the development of optical atomic standards.
Core capabilities: A comb can synthesize 10^5 to 10^6 harmonically related optical modes from an electronic or optical reference with fidelity better than 1 part in 10^18.It also supports direct optical-to-microwave and microwave-to-optical conversion.
Spectral reach: Nonlinear ultrashort-pulse processes extend coherent frequency synthesis across the near-infrared, visible, extreme-ultraviolet, mid-infrared, and terahertz regions.Difference-frequency generation provides access to additional spectral domains.
Field development: The field has expanded rapidly, with more than 14,000 publications containing the exact phrase “optical frequency comb” over the past 20 years.The review presents a broad historical overview of this development.

2 What is an optical frequency comb and how does it work

An optical frequency comb is a phase-stabilized mode-locked laser whose coherent, equally spaced optical modes are determined by two microwave frequencies. Measuring and controlling the repetition rate and offset frequency enables precise optical–microwave conversion and broad metrology applications.

Comb structure: A phase-stabilized mode-locked laser generates hundreds of thousands to millions of harmonically related, mutually phase-coherent optical modes.These properties make the spectrum useful as a precision frequency ruler.
Applications: Controlling fr and f0 enables direct phase-coherent conversion between optical and microwave frequencies and supports a wide range of precision measurements.Early demonstrations included optical clocks, frequency-ratio measurements, fundamental-constant searches, spectroscopy, and precision electronic synthesis.
The comb equation: The comb equation νN = N · fr + f0 shows that every optical mode is defined by the repetition rate fr and offset frequency f0.The integer mode number N multiplies a microwave frequency into the optical domain.
The comb parameters: The repetition rate fr controls pulse-to-pulse timing and coarse spectral positioning, whereas f0 controls carrier phase and enables fine optical-frequency tuning.Without f0, an optical mode is known only to ±fr.
Parameter detection: Direct photodetection measures harmonics of fr but not f0 because the common offset cancels in differences between comb modes.The measured beat relation is νN − νM = (N − M) · fr.
Parameter detection: Self-referencing detects f0 by nonlinear comparison of frequency-doubled and fundamental comb light, requiring an optical octave of bandwidth.The detected relation is f0 = 2 · νN − ν2N.

3.1 Evolution of solid-state and fiber-based mode-locked combs

Solid-state and fiber-based mode-locked combs evolved toward broader spectral coverage, greater efficiency, and lower size, weight, and power. These developments now provide coherent access to wavelengths from the visible region into the mid-infrared and beyond while enabling compact fieldable systems.

Spectral coverage: Advances in solid-state and fiber technologies, combined with nonlinear fibers, provide near-continuous coherent spectroscopic coverage from 400 nm to approximately 4 µm.Short-pulsed systems are especially valuable for broad, bright coverage in the mid-infrared, terahertz, ultraviolet, and extreme-ultraviolet regions.
Performance and SWAP: Mode-locked comb design pursued higher performance and lower SWAP through improved stabilization, higher-bandwidth actuators, and common-mode measurement architectures.These approaches enabled optical-frequency synthesis fidelity at parts in 10^20.
Source evolution: Source architectures progressed from 80 MHz, 2 m Ti:Sapphire systems to gigahertz-rate, octave-spanning, fiberized, and monolithic palm-sized lasers.The latest systems include sub-100 fs Er/Yb:glass lasers.

3.2 Compact and chip-scale sources

Compact and chip-scale optical frequency comb sources include electro-optic, semiconductor, and micro-resonator platforms, each offering distinct combinations of tunability, spectral access, stabilization, size, and system complexity. These architectures are advancing integrated comb technology but still face limitations in optical efficiency, f0 access, noise, amplification, and integration.

Semiconductor lasers: Semiconductor sources include MIXSELs for high-power ultrashort pulses and QCLs for variable-spacing mid-infrared to terahertz generation.Optically pumped MIXSELs can produce sub-100 fs pulses and greater than 1 W, while QCLs offer variable mode-spacing from 5 to 50 GHz.
Electro-optic comb generators: Electro-optic combs provide wide-band, agile mode-spacing tuning and have enabled fully stabilized operation for astronomical spectrograph calibration.They also support pump wavelengths from 780 nm to 2 µm and require frequency stability at 1 part in 10^11 for the cited calibration application.
Current state-of-the-art in compact and chip-scale sources: Compact high-repetition-rate and semiconductor or electro-optic systems often require high-power amplification because their pulse energies can be more than 100 times lower than mode-locked lasers.Low output power, narrow optical bandwidth, and absent modelocking mechanisms contribute to the difficulty of continuum generation for f0 detection.
Current state-of-the-art in compact and chip-scale sources: QCL-based combs offer low SWAP and mid-infrared to terahertz access, but currently lack f0 access for precision operation; they have nevertheless become commercial dual-comb spectrometer platforms.Micro-resonator and semiconductor systems have not yet reached their full potential for compact precision comb generation.
Current state-of-the-art in compact and chip-scale sources: Micro-resonator combs can achieve controlled soliton formation and octave broadening for coherent f0 detection, but low optical efficiency and thermorefractive noise remain major constraints.Soliton formation requires significant pump detuning, producing optical efficiency below 3% and necessitating auxiliary amplification or helper lasers.
Current state-of-the-art in compact and chip-scale sources: Micro-resonator and semiconductor platforms are currently the only possible architectures for chip-based and integrated comb systems, despite incomplete optical integration.Future integration with CMOS-compatible waveguides, diode lasers, and miniature optical clocks could enable sub-watt optical and microwave synthesis systems.

4 Optical frequency comb applications

The review organizes optical frequency comb applications according to their stability requirements. It distinguishes high-precision frequency synthesis and measurement from applications with less stringent stability requirements.

Application categories: Optical frequency comb applications are grouped into two categories according to application stability requirements.The first category concerns high-precision frequency synthesis and measurement for comparing and disseminating atomic-clock signals.

4.1 Comb referencing, stabilization and performance

Optical frequency comb performance depends on how the comb parameters f0 and fr are referenced and stabilized. Wider separation between lock points improves frequency leverage, while offset-free or self-referenced schemes enable especially strong transfer of reference stability and noise.

Reference schemes: Precise comb metrology requires knowledge and control of both f0 and fr, using microwave references, optical references, or a combination.The chosen reference and referencing frequency affect the stability and noise of the comb modes.
Stabilization methods: Full stabilization uses negative feedback to control fr and f0, whereas transfer-oscillator methods track their drift electronically to correct noise excursions.Passive approaches such as DFG can remove f0 without measuring or referencing it.
Offset-free combs: Passive difference frequency generation removes f0, producing an offset-free comb whose modes are integer multiples of fr.DFG requires large frequency separation and can produce low-power, narrow-bandwidth combs that need amplification and nonlinear broadening.
Performance: Comb-mode linewidth performance varies with the referencing scheme; schemes lacking direct access to f0 can use blue or red stabilization traces, while black and green traces require accessible or offset-free f0.Outside the lock points, the red-trace linewidth diverges with a slope set by reference noise and lock-point spacing.
Performance: Greater lock-point separation improves frequency leverage, making the self-referenced black trace the highest-performance scheme with lock points separated by hundreds of terahertz.Its stabilization constrains fr through an optical mode and f0 through a microwave reference near DC.
Performance: Green and black stabilization preserve the reference's fractional frequency stability across all comb modes, while self-referencing also enables optical frequency division to microwaves.An optical reference at 300 THz divided to 10 GHz can reduce optical phase noise in the microwave domain by 90 dB.

4.2 Combs for atomic clock comparisons

Optical frequency combs replaced frequency-multiplier chains for measuring and comparing optical clock frequencies by providing microwave-referenced access to comb modes and offset frequencies. They support rapid, high-precision relative comparisons and long-distance timing dissemination.

Frequency measurement: Optical frequency combs replaced frequency-multiplier chains by providing direct microwave-domain access to the comb offset frequency f0.The comb measures an unknown optical frequency as νopt = N·frep + f0 + ∆f.
Frequency measurement: Optical clock comparisons measure small offsets ∆f against microwave references, adding no measurable noise to exquisitely fine optical-clock transitions.For ∆f = 100 MHz, a 10^-12 microwave-reference error contributes only 3 parts in 10^19 fractionally on a 300 THz carrier.
Clock comparisons: Relative optical-clock comparisons avoid the absolute-frequency limitation imposed by microwave references and can reach 10^-16 uncertainty within seconds.Absolute measurements can require up to a month to approach 1 part in 10^16, whereas relative comparisons exploit higher short-term optical-clock resolution.
Clock comparisons: Frequency ratios eliminate the Hertz-definition limit because the ratio is unitless, enabling precision tests of relativity, fundamental-constant variation, and ultralight dark matter.The ratio uncertainty is limited by the optical transition frequencies alone, which can be controlled beyond 1 part in 10^-18.
Networks and dissemination: Optical fiber networks disseminate timing and compare clocks across cities and countries, reaching optical-clock comparisons at the mid-10^-17 level.These networks also support remote synchronization of facility lasers, accelerators, RF electronics, and ultrafast X-ray experiments within 30 femtoseconds.
Networks and dissemination: Two-way optical time/frequency transfer extends comparisons to locations without fiber, while reciprocal path fluctuations support syntonization and synchronization.Implemented over several kilometers, these links demonstrated timing errors as low as 7 as after 1 s of averaging.

4.3 Ultra low-noise microwave generation

Optical frequency division converts stabilized optical references into exceptionally stable, low-phase-noise microwave signals. The principal limitation is preserving optical pulse quality during photodetection, where distortion, saturation, and amplitude-to-phase conversion become critical.

Microwave synthesis: Optical frequency division derives microwave signals with stabilities better than 10^-15, exceeding room-temperature electronic oscillators by more than 100 times.Phase-stabilized OFC pulses are converted into electronic pulse trains for microwave generation.
Microwave synthesis: Optical frequency division also divides optical-reference noise, enabling a 10 GHz signal with phase noise of -100 dBc/Hz at 1 Hz.At 1 Hz offset, the spectral noise density is 10 orders of magnitude below the microwave carrier.
Practical limitations: Detector response limits, power handling, nonlinearities, and charge screening can distort high-energy pulses and convert optical amplitude noise into microwave phase noise.Low-pulse-energy OFCs with repetition rates above 1 GHz are preferred for the highest spectral purity.
Linear optical sampling: Dual-comb linear optical sampling circumvents photodetector response limits by converting optical cross-correlation into microwave interferograms sampled on megahertz timescales.The technique improves coarse ranging resolution from the millimeter to nanometer scale and supports remote clock transfer.

4.4 Calibration of astronomical spectrographs

Optical frequency combs calibrate astronomical spectrographs by providing equidistant, stable frequency markers that reveal instrument drift. Astro-comb implementations meet demanding spacing, bandwidth, flatness, and stability requirements for high-precision radial-velocity measurements.

Calibration principle: Astro-combs calibrate spectrograph drift by projecting exactly equidistant optical modes onto the instrument’s imaging plane.The mode positions improve the spectrograph’s long-term frequency accuracy.
Instrument requirements: Astro-comb specifications include mode spacings near 30 GHz, coverage from 380 nm to 2.4 µm, spectral flatness below 3 dB, and stability of 3 × 10^-11.The stability corresponds to a 1 cm/s radial Doppler drift.
Demonstrated performance: High-precision comb calibrations reached 2.5 cm/s at HARPS/FOCES and below 10 cm/s at HARPS-N and the Hobby–Eberly Telescope.Astro-combs had entered deployment at spectrographs in 2008 and were present at least seven telescopes worldwide.

4.5 Optical frequency combs beyond the laboratory

Optical frequency combs extend precision measurement beyond laboratory metrology through broadband spectroscopy and ranging. Dual-comb spectroscopy down-converts broad optical information into electronically processable radio-frequency signals, while deployment remains constrained by optical power, complexity, and cost.

Distance measurements and laser ranging: High cost and system complexity remain major drawbacks for direct comb-based ranging.Smaller electro-optic, micro-resonator, and integrated systems are being developed, but low power per mode can require retro-reflecting mirrors beyond about 1 m.
Direct comb-based spectroscopy: Frequency combs support spectroscopy through broad bandwidth, high spatial coherence, and high frequency resolution and accuracy across the terahertz-to-ultraviolet range.These properties enable detection of multiple molecular species, longer interrogation paths, and sensitive mode-resolved measurements.
Direct comb-based spectroscopy: A repetition-rate difference of approximately 1–10 kHz makes dual-comb interferograms repeat every 1/∆fr and maps optical spectral information into RF bandwidth no greater than fr/2.With fr approximately 100 MHz, the optical signal is effectively downsampled by about 100,000 for processing with standard electronics.
Direct comb-based spectroscopy: Dual-comb spectroscopy uses two phase-stabilized combs with slightly offset repetition rates to down-convert tens of terahertz of optical bandwidth into hundreds of megahertz in the RF domain.The method preserves comb stability, accuracy, and sample-encoded amplitude and phase information, allowing reconstruction of molecular absorption spectra.
Direct comb-based spectroscopy: Direct dual-comb spectroscopy avoids moving parts and complex diffraction optics while offering faster acquisition, higher resolution, SI-traceable accuracy, and no instrument lineshapes.Field deployment still faces low per-mode power and spectral-magnitude fluctuations, particularly for broad or cluttered atmospheric spectra.

5 Perspective and future outlook

Optical frequency combs have achieved their strongest commercial success in precision metrology, while adoption in LIDAR and FTIR has lagged because performance benefits do not yet offset cost and complexity. Their integration with transportable optical clocks is positioned to support remote clock comparisons and future time-standard applications.

Commercial adoption: Precision metrology is where optical frequency combs have seen the most commercial success, partly because laboratory users accept high performance costs.
Commercial adoption: Optical comb replacements for traditional LIDAR and FTIR systems have not yet gained commercial traction because their cost and complexity remain too high relative to their benefits.This trade-off is driving development of compact, lower-SWAP spectroscopic sources.
Future outlook: Optical atomic clock accuracy has improved by five orders of magnitude over 20 years and is now 100 times better than state-of-the-art primary 133Cs microwave standards.Replacing frequency multiplier chains with combs simplified precision optical measurement and supported new clock species.
Future outlook: Comb-integrated transportable optical clocks are expected to support remote comparisons between metrology laboratories and intercontinental time-frequency transfer.These applications could contribute to high-precision geoid mapping and tests of fundamental physics.