Crystalline Mirror Solutions


Ultralow Brownian noise
High Finesse > 200,000

View more information on the Thorlabs website!

High-performance mirrors with ultralow optical absorption and minimal Brownian noise are key ingredients for the construction of  optical atomic clocks, compact optical reference cavities and stabilized lasers, as well as the next generation of gravitational wave detectors. The high-quality-factor single-crystal coatings used in our xtal stable™ optics substantially reduce inherent thermo-mechanical fluctuations, enabling a significant improvement in the overall frequency stability of precision interferometers. We are extremely excited to have our mirrors employed in groundbreaking experiments and cutting edge commercial systems pushing the ultimate limits of laser linewidth and cavity noise performance.

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    Center wavelength

    900 nm

    2000 nm

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    Optical absorption

    < 1 ppm

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    Optical scatter

    < 5 ppm

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    Radius of curvature

    > 10 cm



Typically >99.99%, >99.999% achievable
Loss angle
<4 × 10⁻⁵ at 300 K, <5 × 10⁻⁶ at 10 K
Temperature range
Cryogenic, room temperature, and high temperature solutions available
Coating material
Single-crystal GaAs/AlGaAs
Coating surface quality
1 Å RMS micro-roughness
Substrate material
Typically fused silica, other materials available
Substrate diameter
0.5 - 1 inch (12.7 - 25.4 mm), other sizes available
Surface flatness
<0.10 wave P-V measured @ 633 nm
Similar to fused silica, cleaning instructions provided on request
Data Sheet
Click here to download the xtal stable specifications sheet

Cavity Assembly Service from CMS

CMS now offers optical contacting services for your complete reference cavity needs! Customers can supply their own spacer and ULE compensation rings, or optionally, provide specifications for procurement through our trusted network of vendors.

All completed Fabry-Perot cavities are tested in their final configuration, ensuring that customers receive a fully-functional reference cavity ready for immediate integration.

Contact CMS to discuss your requirements and how we can deliver the complete system to you.

Applications for xtal stable mirrors

Copyright: Courtesy of the Ye group and Brad Baxley/JILA

Precise Atomic Clocks

Low-temperature optical reference cavities using crystalline mirror technology are currently being tested and represent a new world-record in frequency stability. The expected stability surpasses the previously unattainable level of Δf/f below 10¹⁷ in one of second integration time and will establish a new milestone in the performance of optical atomic clocks.

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Optical Reference Cavities

Crystalline mirror technology allows for a significant reduction in the size of optical reference cavities, while simultaneously maintaining excellent noise performance. As a consequence, compact, space-based optical reference cavities are currently being developed. This serves as a primer for future space-based navigation systems using optical reference standards.


Gravitational Wave Detection

Crystalline coatings allow, for the first time, for the operation of laser-based interferometric gravitational wave detectors at the ultimate (standard) quantum limit. This goes hand-in-hand with a significant increase in the volume of observed space in a gravitational wave observatory. At this stage, large area-diameter crystalline coatings are being employed both at the German 10 meter prototype detector and the Australian 80 meter prototype detector, and further noise tests are underway at LIGO.

Relevant publications

  • Near-infrared scanning cavity ringdown for optical loss characterization of supermirrors

    G. W. Truong, G. Winkler, T. Zederbauer, D. Bachmann, P. Heu, D. Follman, M. E. White, O. H. Heckl, and G. D. Cole

    "Near-infrared scanning cavity ringdown for optical loss characterization of supermirrors", Optics Express, vol. 27, pp. 19141-19149, 8 July 2019.

    A cavity ringdown system for probing the spatial variation of optical loss across high-reflectivity mirrors is described. This system is employed to examine substrate-transferred crystalline supermirrors and to quantify the effect of manufacturing process imperfections. Excellent agreement is observed between the ringdown-generated spatial measurements and differential interference contrast microscopy images. A 2-mm diameter ringdown scan in the center of a crystalline supermirror reveals highly uniform coating properties with excess loss variations below 1 ppm.

  • Measurement of quantum back action in the audio band at room temperature

    J. Cripe, N. Aggarwal, R. Lanza, A. Libson, R. Singh, P. Heu, D. Follman, G. D. Cole, N. Mavalvala, and T. Corbitt

    "Measurement of quantum back action in the audio band at room temperature," Nature, online 25 March 2019.

    Quantum mechanics places a fundamental limit on the precision of continuous measurements. The Heisenberg uncertainty principle dictates that as the precision of a measurement of an observable (for example, position) increases, back action creates increased uncertainty in the conjugate variable (for example, momentum). In interferometric gravitational-wave detectors, higher laser powers reduce the position uncertainty created by shot noise (the photon-counting error caused by the quantum nature of the laser) but necessarily do so at the expense of back action in the form of quantum radiation pressure noise (QRPN). Once at design sensitivity, the gravitational-wave detectors Advanced LIGO, VIRGO and KAGRA will be limited by QRPN at frequencies between 10 hertz and 100 hertz. There exist several proposals to improve the sensitivity of gravitational-wave detectors by mitigating QRPN, but until now no platform has allowed for experimental tests of these ideas. Here we present a broadband measurement of QRPN at room temperature at frequencies relevant to gravitational-wave detectors. The noise spectrum obtained shows effects due to QRPN between about 2 kilohertz and 100 kilohertz, and the measured magnitude of QRPN agrees with our model. We now have a testbed for studying techniques with which to mitigate quantum back action, such as variational readout and squeezed light injection, with the aim of improving the sensitivity of future gravitational-wave detectors.

  • Optical performance of large-area crystalline coatings

    M. Marchiò, R. Flaminio, L. Pinard, D. Forest, C. Deutsch, P. Heu, D. Follman, and G. D. Cole.

    "Optical performance of large-area crystalline coatings," Optics Express, vol. 26, no. 5, pp. 6114-6125, 5 March 2018.

    Given their excellent optical and mechanical properties, substrate-transferred crystalline coatings are an exciting alternative to amorphous multilayers for applications in precision interferometry. The high mechanical quality factor of these single-crystal interference coatings reduces the limiting thermal noise in precision optical instruments such as reference cavities for narrow-linewidth laser systems and interferometric gravitational wave detectors. In this manuscript, we explore the optical performance of GaAs/AlGaAs crystalline coatings transferred to 50.8-mm (2-inch) diameter fused silica and sapphire substrates. We present results for the transmission, scattering, absorption, and surface quality of these prototype samples including the defect density and micro-roughness. These novel coatings exhibit optical performance on par with state-of-the-art dielectric structures, encouraging further work focused on the fabrication of larger optics using this technique.

  • High-performance near- and mid-infrared crystalline coatings

    G. D. Cole, W. Zhang, B. J. Bjork, D. Follman, P. Heu, C. Deutsch, L. Sonderhouse, J. Robinson, C. Franz, A. Alexandrovski, M. Notcutt, O. H. Heckl, J. Ye, and M. Aspelmeyer

    "High-performance near- and mid-infrared crystalline coatings," Optica, Vol. 3, No. 6, 647-656, June 2016.

    Substrate-transferred crystalline coatings have recently emerged as a groundbreaking new concept in optical interference coatings. Building upon our initial demonstration of this technology, we have now realized significant improvements in the limiting optical performance of these novel single-crystal GaAs/Al𝑥Ga1−𝑥As multilayers. In the near-infrared (NIR), for coating center wavelengths spanning 1064–1560 nm, we have reduced the excess optical losses (scatter + absorption) to levels as low as 3 parts per million (ppm), enabling the realization of a cavity finesse exceeding 3×10^5 at the telecom-relevant wavelength range near 1550 nm. Moreover, we demonstrate the direct measurement of sub-ppm optical absorption at 1064 nm. Concurrently, we investigate the mid-infrared (MIR) properties of these coatings and observe exceptional performance for first attempts in this important wavelength region. Specifically, we verify excess losses at the hundred ppm level for wavelengths of 3300 and 3700 nm. Taken together, our NIR optical losses are now fully competitive with ion-beam sputtered multilayer coatings, while our first prototype MIR optics have already reached state-of-the-art performance levels for reflectors covering this portion of the fingerprint region for optical gas sensing. Mirrors fabricated with our crystalline coating technique exhibit the lowest mechanical loss, and thus the lowest Brownian noise, the highest thermal conductivity, and, potentially, the widest spectral coverage of any “supermirror” technology in a single material platform. Looking ahead, we see a bright future for crystalline coatings in applications requiring the ultimate levels of optical, thermal, and optomechanical performance.

  • Coherent cancellation of photothermal noise in GaAs/Al0.92Ga0.08As Bragg mirrors

    T. Chalermsongsak, E. D. Hall, G. D. Cole, D. Follman, F. Seifert, K. Arai, E. K. Gustafson, J. R. Smith, M. Aspelmeyer, and R. X. Adhikari,

    "Coherent cancellation of photothermal noise in GaAs/Al0.92Ga0.08As Bragg mirrors," Metrologia, vol. 53, no. 2, pp. 860-868, 9 March 2016.

    Thermal noise is a limiting factor in many high-precision optical experiments. A search is underway for novel optical materials with reduced thermal noise. One such pair of materials, gallium arsenide and aluminum-alloyed gallium arsenide (collectively referred to as AlGaAs), shows promise for its low Brownian noise when compared to conventional materials such as silica and tantala. However, AlGaAs has the potential to produce a high level of thermo-optic noise. We have fabricated a set of AlGaAs crystalline coatings, transferred to fused silica substrates, whose layer structure has been optimized to reduce thermo-optic noise by inducing coherent cancellation of the thermoelastic and thermorefractive effects. By measuring the photothermal transfer function of these mirrors, we find evidence that this optimization has been successful.

  • Technology for the next gravitational wave detectors

    V. P. Mitrofanov, S. Chao, H.-W. Pan, L.-C. Kuo, G. Cole, J. Degallaix, and B. Willke

    "Technology for the next gravitational wave detectors," Science China Physics, Mechanics & Astronomy, vol. 58, 120404, December 2015.

    This paper reviews some of the key enabling technologies for advanced and future laser interferometer gravitational wave detectors, which must combine test masses with the lowest possible optical and acoustic losses, with high stability lasers and various techniques for suppressing noise. Sect. 1 of this paper presents a review of the acoustic properties of test masses. Sect. 2 reviews the technology of the amorphous dielectric coatings which are currently universally used for the mirrors in advanced laser interferometers, but for which lower acoustic loss would be very advantageous. In sect. 3 a new generation of crystalline optical coatings that offer a substantial reduction in thermal noise is reviewed. The optical properties of test masses are reviewed in sect. 4, with special focus on the properties of silicon, an important candidate material for future detectors. Sect. 5 of this paper presents the very low noise, high stability laser technology that underpins all advanced and next generation laser interferometers.

  • Mapping the optical absorption of a substrate-transferred crystalline AlGaAs coating at 1.5 µm

    J. Steinlechner, I. W. Martin, A. Bell, G. D. Cole, J. Hough, S. Penn, S. Rowan, and S. Steinlechner

    "Mapping the optical absorption of a substrate-transferred crystalline AlGaAs coating at 1.5 µm," Classical and Quantum Gravity, vol. 32, no. 10, 105008, 21 May 2015.

    The sensitivity of second and third generations of interferometric gravitational wave (GW) detectors will be limited by the thermal noise of the test-mass mirrors and highly reflective coatings. Recently developed crystalline coatings show a promising thermal noise reduction compared to presently used amorphous coatings. However, stringent requirements apply to the optical properties of the coatings as well. We have mapped the optical absorption of a crystalline AlGaAs coating that is optimized for high reflectivity for a wavelength of 1064 nm. The absorption was measured at 1530 nm, where the coating stack transmits approximately 70% of the laser light. The measured absorption was lower than (30.2 +/- 11.1) ppm which is equivalent to (3.6 +/- 1.3) ppm for a coating stack that is highly reflective at 1530 nm. While this is a very promising low absorption result for alternative low-loss coating materials, further work will be necessary to reach the requirements of < 1 ppm for future GW detectors.

  • Sensing Earth rotation with a helium-neon ring laser operating at 1.15 µm

    K. U. Schreiber, R. J. Thirkettle, R. B. Hurst, D. Follman, G. D. Cole, M. Aspelmeyer, and J.-P. R. Wells

    “Sensing Earth rotation with a helium-neon ring laser operating at 1.15 µm,” Optics Letters, vol. 40, no. 8, pp. 1705-1708, 15 April 2015.

    We report on the operation of a 2.56  m^2 helium–neon based ring laser interferometer at a wavelength of 1.152276 μm using crystalline coated intracavity supermirrors. This work represents the first implementation of crystalline coatings in an active laser system and expands the core application area of these low-thermal-noise cavity end mirrors to inertial sensing systems. Stable gyroscopic behavior can only be obtained with the addition of helium to the gain medium as this quenches the 1.152502 μm (2s4→2p7) transition of the neon doublet which otherwise gives rise to mode competition. For the first time at this wavelength, the ring laser is observed to readily unlock on the bias provided by the earth’s rotation alone, yielding a Sagnac frequency of approximately 59 Hz.

  • Tenfold reduction of Brownian noise in high-reflectivity optical coatings

    G. D. Cole, W. Zhang, M. J. Martin, J. Ye, and M. Aspelmeyer

    “Tenfold reduction of Brownian noise in high-reflectivity optical coatings,” Nature Photonics, vol. 7, no. 8, pp. 644-650, August 2013.

    Thermally induced fluctuations impose a fundamental limit on precision measurement. In optical interferometry, the current bounds of stability and sensitivity are dictated by the excess mechanical damping of the high-reflectivity coatings that comprise the cavity end mirrors. Over the last decade, the dissipation of these amorphous multilayer reflectors has at best been reduced by a factor of two. Here, we demonstrate a new paradigm in optical coating technology based on direct-bonded monocrystalline multilayers, which exhibit both intrinsically low mechanical loss and high optical quality. Employing these ‘crystalline coatings’ as end mirrors in a Fabry–Pérot cavity, we obtain a finesse of 150,000. More importantly, at room temperature, we observe a thermally limited noise floor consistent with a tenfold reduction in mechanical damping when compared with the best dielectric multilayers. These results pave the way for the next generation of ultra-sensitive interferometers, as well as for new levels of laser stability.

  • Cavity optomechanics with low-noise crystalline mirrors

    G. D. Cole

    "Cavity optomechanics with low-noise crystalline mirrors," SPIE Optics & Photonics, Optical Trapping and Optical Micromanipulation IX, San Diego, CA, USA, 8458-07, 12–16 August 2012.

    Cavity optomechanics is a rapidly evolving field operating at the intersection of solid-state physics and modern optics. The fundamental process at the heart of this interdisciplinary endeavor is the enhancement of radiation pressure within a high-finesse optical cavity. Isolating this weak interaction, i.e. the momentum transfer of photons onto the cavity boundaries, requires the development of mechanical resonators that simultaneously exhibit high reflectivity (requiring low absorption and scatter loss) and low mechanical dissipation. In a Fabry-Pérot implementation, this is realized by fabricating suspended micrometer-scale mechanical resonators directly from high-reflectivity multilayers. Thus, the properties of the mirror material—particularly the loss angle and optical absorption—drive the ultimate performance of the devices. Interestingly, similar requirements are found in a broad spectrum of applications, ranging from gravitational wave interferometers to stabilized lasers for optical atomic clocks. This overlap leads to an intimate link between advances in the seemingly disparate areas of macroscopic interferometry (e.g. precision measurement and spectroscopy) and micro- and nanoscale optomechanical systems. In this manuscript, I will outline the fascinating implications of cavity optomechanics and present proof-of-concept experiments including MHz-frequency resonators aimed at the demonstration of quantum states of mechanical systems, as well as low-frequency (<1 kHz) devices for the observation of quantum radiation pressure noise. Additionally, I will discuss off-shoot technologies developed in the course of this work, such as a numerical solver for the determination of support-mediated losses in mechanical resonators, as well as a new strategy for the realization of ultra-high-stability optical reference cavities based on transferred crystalline multilayers.

Please contact CMS for more information about all our products. For example, part number 1064-10-254-FS10PAR is a 1064 nm HR crystalline coating on super-polished fused silica with a 1″ diameter, 1/4″ thick, planar / planar fused silica substrate, with backside AR coating and full-face HR centered at 1064 nm. Contact us for more details!