20 years of developments in optical frequency comb technology and applications

Abstract

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 nearcontinuous spectroscopic coverage from microwave frequencies to the extreme ultraviolet , 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. T he 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 133 Cs 1. While 133 Cs 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 (see "Timing, synchronization, and atomic clock networks"). Optical signals, however, pose a significant measurement challenge because light frequencies oscillate 100,000 times faster than state-of-the-art digital electronics. Prior to 2000, the simplest method to access an optical frequency was via knowledge of the speed of light and measurement of its wavelength, accessible with relatively poor precision of parts in 10 7 using an optical wavemeter. 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 133 Cs primary frequency reference near 9.2 GHz, to the optical domain via a series of multiplied and phase-locked oscillators 2. The most complicated of these systems required up to 10 scientists, 20 different oscillators and 50 feedback loops to perform a single optical measurement 3. 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

Optical frequency metrology in the bending modes region

“…The limitations of laser technology have established Fourier-transform spectroscopy driven by incoherent light sources as a gold standard for the long-wavelength infrared (LWIR) region, yet not for optical metrology 14,15 and precision spectroscopy studies 16 because of the low resolution (0.0007 cm −1 or 21 MHz in the best cases 17 ) and the lack of absolute calibration for the frequency axis 18 . A powerful solution to both issues is direct comb spectroscopy 19,20 , which exploits millions of laser modes that compose the comb spectrum to directly probe molecular absorption over extremely large bands and with absolute frequency calibration 21 . Thanks to a time-domain counterpart formed by a coherent train of femtosecond laser pulse 20 , frequency combs are ideal sources for efficient nonlinear frequency conversion deep into the mid-infrared (MIR) 22 , also fostered by innovative solutions for laser gain media 23 , nonlinear materials 24 and frequency conversion schemes [25][26][27] .…”

“…A powerful solution to both issues is direct comb spectroscopy 19,20 , which exploits millions of laser modes that compose the comb spectrum to directly probe molecular absorption over extremely large bands and with absolute frequency calibration 21 . Thanks to a time-domain counterpart formed by a coherent train of femtosecond laser pulse 20 , frequency combs are ideal sources for efficient nonlinear frequency conversion deep into the mid-infrared (MIR) 22 , also fostered by innovative solutions for laser gain media 23 , nonlinear materials 24 and frequency conversion schemes [25][26][27] . However, it has only been recently that snapshots of entire bands have been acquired by a dual-comb approach 28 at high temporal and spectral resolution in the LWIR, from 6.7 to 16.7 µm (1500-600 cm −1 ) 29,30 .…”

Toward new frontiers for terahertz quantum cascade laser frequency combs

“…Traditionally generated by frequency-stabilized and controlled femtosecond mode-locked lasers, FCs quickly conquered the visible and near-infrared (IR) domain, where different generation techniques were successfully demonstrated, tested and commercialized, such as Ti:Sa [4], fiber-based lasers [5], optical microresonators [6], upconversion of low frequency sources [7] or downconversion of high-frequency combs [8]. High optical powers, outstanding long-term stability, broad spectral coverage and frequency tunability have been demonstrated so far [9]. Despite such impressive performances, most of these systems have a relatively large footprint, even in a fiber-based configuration.…”

Interrogating the Temporal Coherence of EUV Frequency Combs with Highly Charged Ions

“…The inverse of the coherence time τ c of such a FC determines the width of each comb tooth, and can be inferred by measuring the beating spectra between the corresponding pulse train and an independent ultrastable continuous-wave (cw) reference laser [3,4]. For optical FCs, coherence times longer than 1 s, or tooth widths narrower than 1 Hz, have been measured [3,4], which allows wide applications of FCs in high-precision spectroscopy [5,6], the search for exoplanets [7,8] and the construction of ultrastable optical atomic clocks [9].…”