In 1983, at the time of the adoption of the present definition of the metre by the 17th General Conference on Weights and Measures (CGPM), the International Committee for Weights and Measures (CIPM) drew up recommendations for the practical realization of the definition. These have formerly been referred to as the mise en pratique of the definition of the metre. It was understood that the practical realization would, from time to time, be updated to take account of new measurements and improvements in techniques of laser stabilization. In 1992 the CIPM, acting on the advice of the then Consultative Committee for the Definition of the Metre (CCDM), adopted a revision of the mise en pratique in its Recommendation 3 (CI-1992). The text was published in Metrologia (1993/94 30 523–41). In 1997 a second revised version of the practical realization of the definition of the metre was adopted by the CIPM in its Recommendation 1 (CI-1997). The text was published in Metrologia (1999 36 211–44). In 2001, the CIPM again adopted a revised version of the practical realization of the metre and this time included recommended frequencies for other optical frequency standards. The text of this Recommendation (Recommendation 1 (CI-2002)) is given here∗. In addition, a revised list of recommended radiations, and an appendix containing the source data used in estimating the wavelengths, frequencies and uncertainties of the recommended radiations are given. In adopting the revised practical realization, the CIPM acknowledged the considerable effort put into its preparation by the Consultative Committee for Length (CCL) through its Working Group on the mise en

https://iopscience.iop.org/article/10.1088/0026-1394/40/2/316/pdf

Practical realization of the definition of the metre, including recommended radiations of other optical frequency standards (2001)

Laboratory optical atomic clocks achieve remarkable accuracy (now counted to 18 digits or more), opening possibilities for exploring fundamental physics and enabling new measurements. However, their size and the use of bulk components prevent them from being more widely adopted in applications that require precision timing. By leveraging silicon-chip photonics for integration and to reduce component size and complexity, we demonstrate a compact optical-clock architecture. Here a semiconductor laser is stabilized to an optical transition in a microfabricated rubidium vapor cell, and a pair of interlocked Kerr-microresonator frequency combs provide fully coherent optical division of the clock laser to generate an electronic 22 GHz clock signal with a fractional frequency instability of one part in 1013. These results demonstrate key concepts of how to use silicon-chip devices in future portable and ultraprecise optical clocks.

Architecture for the photonic integration of an optical atomic clock

Over the years, light scattering from acoustic waves has grown to be increasingly important in the fields of biology and medicine. This type of scattering, known as Brillouin scattering, has already seen a plethora of applications in fields such as physics. However, the potential for Brillouin scattering for medical imaging and diagnostics has only recently been considered. In this work, we summarize most of the applications of Brillouin scattering in biology to date, and some current work in our lab showing how Brillouin scattering is a worthy prospect for many emerging problems in biology and medical diagnostics.

Seeing cells in a new light: a renaissance of Brillouin spectroscopy

We demonstrate synchronization of laser-induced self-sustained vibrations of radio-frequency micromechanical resonators by applying a small pilot signal either as an inertial drive at the natural frequency of the resonator or by modulating the stiffness of the oscillator at double the natural frequency. By sweeping the pilot signal frequency, we demonstrate that the entrainment zone is hysteretic and can be as wide as 4% of the natural frequency of the resonator, 400 times the 1/Q∼10−4 half-width of the resonant peak. Possible applications are discussed based on the wide range of frequency tuning and the power gain provided by the large amplitude of self-oscillations (controlled by a small pilot signal).

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Frequency entrainment for micromechanical oscillator

Silicon photonics lacks a second-order nonlinear optical (χ(2)) response in general, because the typical constituent materials are centrosymmetric and lack inversion symmetry, which prohibits χ(2) nonlinear processes such as second-harmonic generation (SHG). Here, we realize high SHG efficiency in silicon photonics by combining a photoinduced effective χ(2) nonlinearity with resonant enhancement and perfect phase matching. We show a conversion efficiency of (2,500 ± 100)% W−1 that is two to four orders of magnitude larger than previous field-induced SHG works. In particular, our devices realize milliwatt-level SHG output powers with up to (22 ± 1)% power conversion efficiency. This demonstration is a breakthrough in realizing efficient χ(2) processes in silicon photonics, and paves the way for further integration of self-referenced frequency combs and optical frequency references. By combining a photoinduced effective χ(2) nonlinearity with resonant enhancement and perfect phase matching in a silicon nitride microring resonator, second-harmonic generation with milliwatt-level output powers with up to 22 ± 1% power conversion efficiency is demonstrated.

Efficient photoinduced second-harmonic generation in silicon nitride photonics

Absolute frequency measurement of a laser at 1556 nm locked to the 5S1/2−5D5/2 two-photon transition in 87Rb

Frequency stability of an optical frequency standard at 192.6 THz based on a two-photon transition of rubidium atoms

A 192.6-THz (1556 nm) distributed-feedback (DFB) laser is frequency-locked on a two-photon transition of rubidium at 385.3 THz (778 nm) using second-harmonic (SH) generation. With 43 mW at 1556 nm, we obtain a SH power of 15 /spl mu/W using a KNbO/sub 3/ crystal placed in a ring cavity. Optical feedback from this cavity is used to reduce the DFB laser linewidth to the 10-kHz level and control its frequency. The SH signal is used to injection-lock a 778-nm Fabry-Perot laser in order to increase the interrogation power. With this scheme, we observe two-photon transitions in rubidium and lock the 1556-nm laser frequency.

An absolute frequency reference at 192.6 THz (1556 nm) based on a two-photon absorption line of rubidium at 778 nm for WDM communication systems

Second-harmonic generation of a semiconductor distributed-feedback laser at 1560 nm is described. It is produced by use of a 4.8-mm-long KNbO(3) crystal at room temperature. As much as 2.2 nW of power at 780 nm is obtained for 11.3 mW of fundamental power incident upon the crystal. This signal is used to interrogate a component of the linear absorption profile of the D(2) line in (87)Rb (780.241 nm) and to produce an error signal used to frequency lock the 1560-nm distributed-feedback laser. Such a system can therefore be a candidate for establishing an absolute wavelength standard at 1560 nm.

Second-harmonic generation of a 1560-nm distributed-feedback laser by use of a KNbO(3) crystal for frequency locking to the (87)Rb D(2) line at 780 nm.

Second harmonic generation of a 192.1 THz semiconductor distributed feedback (DFB) laser is achieved using a KNbO/sub 3/ crystal in a resonant ring cavity. Optical feedback from this cavity is used to stabilize the laser frequency and reduce its linewidth. A second harmonic power of 5.5 /spl mu/W is generated with 38 mW incident on the cavity. We use the second harmonic signal to observe saturated absorption lines and orientation signals in rubidium vapor. Injection-locking of a 780 nm Fabry-Perot laser using the second harmonic signal is also demonstrated. With this scheme, we observe saturated absorption lines in rubidium.

M. Poulin

Papers

Absolute frequency measurement of a laser at 1556 nm locked to the 5S1/2−5D5/2 two-photon transition in 87Rb

Frequency stability of an optical frequency standard at 192.6 THz based on a two-photon transition of rubidium atoms

An absolute frequency reference at 192.6 THz (1556 nm) based on a two-photon absorption line of rubidium at 778 nm for WDM communication systems

Second-harmonic generation of a 1560-nm distributed-feedback laser by use of a KNbO(3) crystal for frequency locking to the (87)Rb D(2) line at 780 nm.

Progress in the realization of a frequency standard at 192.1 THz (1560.5 nm) using /sup 87/Rb D/sub 2/-line and second harmonic generation