Y.C.K. acknowledges financial support from the European Research Council through the project NextPhase (ERC StG 278616). Authors also acknowledge support from the Centre National d'Etudes Spatiales (CNES, France) through the Project SHYRO (Action R&amp;T R-S10/LN-0001-004/DA: 10076201), from the ANR project ORA (BLAN 031202), and from the Region de Franche-Comte, France.

The protocol consists in 5 main stages: In the first one, the whispering-gallery-mode resonator is made. In order to control the progress of the polishing of the resonator, surface state measurements are carried out. In the third stage, we fabricate the tool that will launch light in the resonator. Once those two main tools are manufactured, we use them to visualize optical high-Q resonances. Finally, using a high-power input laser beam, the resonator behaves in a nonlinear fashion and Kerr combs are produced.
1. Polishing the Resonator
In this stage, an optical window crystalline resonator (MgF2 or CaF2, readily available from optical component retailers) is shaped and polished. This polishing procedure converts them into high-quality WGM resonators. The customized polishing tower is presented in Scheme 1.
<ol>
	<li>Glue the crystalline optical window on a stick that can be held by the air bearing spindle motor.</li>
	<li>Coat a V-shaped metallic guide with the appropriate polishing support tissue, and pour 10 &mu;m abrasive powders (aluminum oxide, diamond, or silicon carbide) mixed with water. Approach this guide to the spinning disk (approximately 5,000 rpm, 20 g pressure) and start grinding it. Depending on the material and the spinning speed, this process can last from 2 hr (for CaF2) to 4 hr (for MgF2). This polishing step will give its bi-convex shape to the resonator. At the end of this step the disk should have the shape presented in Scheme 2.</li>
	<li>The next step is generally called "grinding and polishing" procedure24. It typically consists in repeating the preceding step with abrasive particle of size 10 &mu;m, 3 &mu;m, 1 &mu;m, 250 nm, and finally 100 nm. The supporting tissue should be adapted to each particle size, being less stiff for smaller grains. To avoid scratches and stripes, a horizontal translation of the guide can be performed. At each step of successive grinding and polishing, the state of the surface should be improved.</li>
</ol>
2. Controlling the State of the Surface
<ol>
	<li>A visual control under an optical microscope is the first step for a surface state control: the disk is opaque to light in the first stages; however, after a successful 1 &mu;m particle polishing, the disk becomes transparent and its sides reflect light: the so-called optical polish has been reached and the quality factor of the resonator should be in the 105 - 106 range.</li>
	<li>For smaller abrasive, the eye is not able to evaluate the state of the surface, even using a standard microscope. At this point, an interferometric measurement of the surface state is needed. Use a microscope equipped with a Mirau interferometer objective lens and with a white light source. The image of the resonator interferes with a reference plane, thus revealing by a white light phase shifting processing tool the surface height independently at each point with a resolution of a fraction of the wavelength, namely a few nanometers. This measurement can also be used to evaluate the curvature of the disk25.</li>
	<li>By modifying the length between the sample and the objective, determine the optical phase of the resonator reflection and calculate the height variations of the surface. This can be automated thanks to a dedicated computer, and a map of the surface height is created, allowing for the determination of the roughness of the sample. Monitor the surface roughness as explained in Figure 1, and stop the grinding-polishing procedure when the interference fringes are the smoothest possible.</li>
</ol>
3. Drawing the Taper
To couple light in the resonator, a very small optical fiber is needed: its diameter should be around 3 &mu;m (about 20 times smaller than a human hair).
<ol>
	<li>Strip a standard single-mode silica fiber (SMF) off its plastic and polymer coating on an approximate 5 cm length. For monitoring purposes, the fiber should be connected to a laser source at its input, and a photodiode at its output.</li>
	<li>Fix each size of the uncoated section of the fiber to two computer-controlled high-resolution motors. Using the computer interface of the motors, configure them to move with a constantly accelerated motion, so that each side of the fiber will be pulled apart.</li>
	<li>Heat the uncoated fiber between the two fixation points with a blowtorch lamp for about 1 min before starting the stretching. The flame should be gentle in order not to blow the taper away once it is very small.</li>
	<li>Start the motors movement, and thus, the stretching of the fiber. Once the drawing is started, one can monitor the transmission of the taper using a laser source and a photodiode: interference patterns will appear during the process, their frequency will increase, and, finally, they will disappear for a waist diameter near 1 &mu;m. At this stage, the motor and the flame should be stopped simultaneously.</li>
</ol>
4. Coupling Light in the WGM Resonator
In this stage, the taper is used to couple light in the resonator and to observe the high-Q eigenmodes of the cavity, which are represented in Figure 2.
<ol>
	<li>Fix the resonator on a 3-axis Piezo-controlled translation stage. Approach it to the fiber taper at a distance of less than 1 &mu;m. The relative position of the fiber taper and the resonator is monitored, thanks to a microscope, and a mirror is used to control the vertical positioning and tilt angle.</li>
	<li>Connect the fiber taper to a visible laser diode: the resonator should be illuminated when the coupling is efficient, as shown in Figure 3.</li>
	<li>Connect the fiber taper to a mode-hop free laser with a narrow linewidth (narrower than the linewidth of the resonance) to one end, and a photodiode connected to an oscilloscope on the other end. The transmission response of the resonator can be obtained by scanning the wavelength at the input. Evaluate the quality factor of the resonator using the obtained transmission spectrum, by calculating the ratio between the resonance frequency of the modes and their linewidths (full width at half maximum).</li>
	<li>A more accurate measurement is performed with the "cavity-ring-down" experiment26, where the wavelength sweeping is fast enough to obtain interferences between the resonating light decaying in the resonator and the detuned light at a subsequent time. One can fine tune the positioning of the taper and the resonator to increase the coupling Q-factor and obtain the typical pattern shown in Figure 4. The associated fit curve gives the quality factor of the resonator.</li>
</ol>
5. Generating the Comb
In this last stage, a high power pump laser excites nonlinear effects in the resonator.
<ol>
	<li>Insert an optical amplifier between the tunable laser and the resonator.</li>
	<li>Thanks to the photodiode and oscilloscope, fine-tune the laser source so that the input wavelength is next to a resonance.</li>
	<li>Connect the output fiber to a high-resolution optical spectrum analyzer and increase the input power while slightly detuning the pump wavelength. New frequencies will appear on each side of the pump peak: this is a Kerr optical frequency comb.</li>
	<li>Switching back to the photodiode, we can observe the beatings between the different spectral modes created. Using a microwave band-pass filter, one can isolate a pure frequency in this electric signal with very low noise.</li>
</ol>

<ol>
	<li>Oraevsky, A. N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Whispering-gallery+waves.">Whispering-gallery waves.</a> Quantum Electronics. 32, 377-400 (2002).</li><li>Matsko, A. B., Ilchenko, V. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optical+Resonators+With+Whispering-Gallery+Modes-Part+I:+Basics.">Optical Resonators With Whispering-Gallery Modes-Part I: Basics.</a> IEEE J. Sel. Top. Quantum Electron. 12, 3-14 (2006).</li><li>Ilchenko, V. S., Matsko, A. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optical+Resonators+With+Whispering-Gallery+Modes-Part+II:+Applications.">Optical Resonators With Whispering-Gallery Modes-Part II: Applications.</a> IEEE J. Sel. Top. Quantum Electron. 12, 15-32 (2006).</li><li>Maker, A. J., Armani, A. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fabrication+of+Silica+Ultra+High+Quality+Factor+Microresonators.">Fabrication of Silica Ultra High Quality Factor Microresonators.</a> J. Vis. Exp. (65), e4164 (2012).</li><li>Savchenkov, A. A., Matsko, A. B., Ilchenko, V. S., Maleki, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optical+resonators+with+ten+million+finesse.">Optical resonators with ten million finesse.</a> Optics Express. 15, 6768-6773 (2007).</li><li>Sprenger, B., Schwefel, H. G. L., Lu, Z. H., Svitlov, S., Wang, L. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=CaF2+whispering-gallery-mode-resonator+stabilized-narrow-linewidth+laser.">CaF2 whispering-gallery-mode-resonator stabilized-narrow-linewidth laser.</a> Optics Letters. 35, 2870-2872 (2010).</li><li>Vahala, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Optical Microcavities. , (2004).</li><li>Matsko, A. B., Savchenkov, A. A., Yu, N., Maleki, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Whispering-gallery-mode+resonators+as+frequency+references.+I.+Fundamental+limitations.">Whispering-gallery-mode resonators as frequency references. I. Fundamental limitations.</a> J. Opt. Soc. Am. B. 24, 1324-1335 (2007).</li><li>Savchenkov, A. A., Matsko, A. B., Ilchenko, V. S., Yu, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Whispering-gallery+mode+resonators+as+frequency+references.+II.+Stabilization.">Whispering-gallery mode resonators as frequency references. II. Stabilization.</a> J. Opt. Soc. Am. B. 24, 2988-2997 (2007).</li><li>Chembo, Y. K., Baumgartel, L. M., Yu, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Toward+whispering-gallery+mode+disk+resonators+for+metrological+applications.">Toward whispering-gallery mode disk resonators for metrological applications.</a> SPIE Newsroom. , (2012).</li><li>Armani, D. K., Kippenberg, T. J., Spillane, S. M., Vahala, K. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ultra-high-Q+toroid+microcavity+on+a+chip.">Ultra-high-Q toroid microcavity on a chip.</a> Nature. 421, 925-929 (2003).</li><li>Hofer, J., Schliesser, A., Kippenberg, T. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cavity+optomechanics+with+ultrahigh-Q+crystalline+microresonators.">Cavity optomechanics with ultrahigh-Q crystalline microresonators.</a> Phys. Rev. A. 82, 031804 (2010).</li><li>F&#252;rst, J. U., Strekalov, D. V., Elser, D., Aiello, A., Andersen, U. L., Marquardt, C. h., Leuchs, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quantum+Light+from+a+Whispering-Gallery-Mode+Disk+Resonator.">Quantum Light from a Whispering-Gallery-Mode Disk Resonator.</a> Phys. Rev. Lett. 106, 113901-1-113901-4 (2011).</li><li>Del'Haye, P., Schliesser, A., Arcizet, O., Wilken, T., Holzwarth, R., Kippenberg, T. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optical+frequency+comb+generation+from+a+monolithic+microresonator.">Optical frequency comb generation from a monolithic microresonator.</a> Nature. 450, 1214-1217 (2007).</li><li>Kippenberg, T. J., Holzwarth, R., Diddams, S. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Microresonator-Based+Optical+Frequency+Combs.">Microresonator-Based Optical Frequency Combs.</a> Science. 322, 555-559 (2011).</li><li>Spillane, S. M., Kippenberg, T. J., Vahala, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ultralow-threshold+Raman+laser+using+a+spherical+dielectric+microcavity.">Ultralow-threshold Raman laser using a spherical dielectric microcavity.</a> Nature. 415, 621-623 (2002).</li><li>Liang, W., Ilchenko, V. S., Savchenkov, A. A., Matsko, A. B., Seidel, D., Maleki, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Passively+Mode-Locked+Raman+Laser.">Passively Mode-Locked Raman Laser.</a> Phys. Rev. Lett. 154, 143903-1-143903-4 (2010).</li><li>Grudinin, I. S., Matsko, A., Maleki, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Brillouin+lasing+with+a+CaF2+whispering+gallery+mode+resonator.">Brillouin lasing with a CaF2 whispering gallery mode resonator.</a> Phys. Rev. Lett. 102, 043902-1-043902-4 (2009).</li><li>Werner, C. S., Beckmann, T., Buse, K., Breunig, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Blue-pumped+whispering+gallery+optical+parametric+oscillator.">Blue-pumped whispering gallery optical parametric oscillator.</a> Optics Letters. 37, 4224-4226 (2012).</li><li>Knight, J. C., Cheung, G., Jacques, F., Birks, T. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Phase-matched+excitation+of+whispering+gallery-mode+resonances+by+a+fiber+taper.">Phase-matched excitation of whispering gallery-mode resonances by a fiber taper.</a> Opt. Lett. 22, 1129-1131 (1997).</li><li>Chembo, Y. K., Yu, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Modal+expansion+approach+to+optical-frequency-comb+generation+with+monolithic+whispering-gallery-mode+resonators.">Modal expansion approach to optical-frequency-comb generation with monolithic whispering-gallery-mode resonators.</a> Phys. Rev. A. 82, 033801-1-033801-18 (2010).</li><li>Chembo, Y. K., Strekalov, D. V., Yu, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spectrum+and+Dynamics+of+Optical+Frequency+Combs+Generated+with+Monolithic+Whispering+Gallery+Mode+Resonators.">Spectrum and Dynamics of Optical Frequency Combs Generated with Monolithic Whispering Gallery Mode Resonators.</a> Phys. Rev. Lett. 104, 103902-1-103902-4 (2010).</li><li>Chembo, Y. K., Yu, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=On+the+generation+of+octave-spanning+optical+frequency+combs+using+monolithic+whispering-gallery-mode+microresonators.">On the generation of octave-spanning optical frequency combs using monolithic whispering-gallery-mode microresonators.</a> Opt. Lett. 35, 2696-2698 (2010).</li><li>Brown, N. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optical+fabrication.">Optical fabrication.</a> Report MISC 4476 1LLNL. , (1990).</li><li>Strekalov, D. V., Savchenkov, A. A., Matsko, A. B., Yu, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Efficient+upconversion+of+subterahertz+radiation+in+a+high-Q+whispering+gallery+resonator.">Efficient upconversion of subterahertz radiation in a high-Q whispering gallery resonator.</a> Optics Letters. 34, 713-715 (2009).</li><li>Dumeige, Y., Trebaol, S., Ghisa, L., Ngan Nguyen, T. K., Tavernier, H., Feron, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Determination+of+coupling+regime+of+high-Q+resonators+and+optical+gain+of+highly+selective+amplifiers.">Determination of coupling regime of high-Q resonators and optical gain of highly selective amplifiers.</a> J. Opt. Soc. Am. B. 12, 2073-2080 (2008).</li><li>Gorodetsky, M. L., Ilchenko, V. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optical+microsphere+resonators:+optimal+coupling+to+high-Q+whispering-gallery+modes.">Optical microsphere resonators: optimal coupling to high-Q whispering-gallery modes.</a> J. Opt. Soc. Am. B. 16, 147-154 (1999).</li><li>Ilchenko, V. S., Yao, X. S., Maleki, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Pigtailing+the+high-Q+microsphere+cavity:+a+simple+fiber+coupler+for+optical+whispering-gallery+modes.">Pigtailing the high-Q microsphere cavity: a simple fiber coupler for optical whispering-gallery modes.</a> Opt. Let. 24, 723-725 (1999).</li><li>Del'Haye, P., Arcizet, O., Schliesser, A., Holzwarth, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Kippenberg+T.J.+Full+stabilization+of+a+microresonator+frequency+comb.">Kippenberg T.J. Full stabilization of a microresonator frequency comb.</a> Phys. Rev. Let. 101, 053903 (2008).</li></ol>

The authors declare that they have no competing financial interests.

This protocol allows producing high-Q optical resonators, to couple light into them and trigger nonlinear phenomena for various microwave photonics applications.
The first step of rough grinding should give its shape to resonator. After an hour of grinding with the 10 &mu;m abrasive powder, one side of the rim of the resonator should be conveniently shaped (see Scheme 2). The following step will smooth the surface of the resonator and when reaching the stage of the 1 ...

Whispering gallery mode resonators are disks or spheres of micro- or millimetric radius1,2,3,4. Provided that the resonator is almost perfectly shaped (nanometer-size surface roughness), laser light can be trapped by total internal reflection within its eigenmodes, which are usually referred to as whispering-gallery modes (WGMs). Their free-spectral range (or intermodal frequency) can vary from GHz to THz depending on the resonator's radius, while their quality factor Q can be exceptionally high5, ranging from 107 - 1011. Due to their unique property of stockpiling and slowing down light, WGM optical resonators have been used to perform many optical signal processing tasks3: filtering, amplification, time-delaying, etc. With the continuous improvement of fabrication technologies, their unprecedented quality factors make them suitable for even more demanding application in metrology or quantum-based applications6-13.
In these ultra-high Q resonators, the small volume of confinement, high photon density, and long photon lifetime (proportional to Q) induce a very strong light-matter interaction, which may excite the various WGMs through various nonlinear effects, like Kerr, Raman, or Brillouin for example14-19. Using nonlinear phenomena in whispering gallery mode resonators was proposed as a promising paradigm shift for ultra-pure microwave and lightwave generation. The fact that this topic intersects so many areas of fundamental science and technology is a clear indicator of its very strong potential impact on a wide range of disciplines. In particular, aerospace and communication engineering technologies are currently in need of versatile microwave and lightwave signal with exceptional coherence. The WGM technology has several advantages over existing or other prospective methods: conceptual simplicity, higher robustness, smaller power consumption, longer lifetime, immunity to interferences, very compact volume, frequency versatility, easy chip integration, as well as a strong potential for integrating the mainstream of standard photonic components for both microwave and lightwave technologies.
In aerospace engineering, quartz oscillators are overwhelmingly dominant as key microwave sources for both navigation systems (planes, satellites, spacecrafts, etc.) and detection systems (radars, sensors, etc.). However, it is unanimously recognized today that frequency stability performance of quartz oscillators is reaching its floor, and will not improve significantly anymore. Along the same line, their frequency versatility is limited and will hardly allow for ultra-stable microwave generation beyond 40 GHz. Microwave photonic oscillators are expected to overcome these limitations. On the other hand, in communication engineering, microwave photonic oscillators are also expected to be key components in optical communication networks where they would perform the lightwave/microwave conversion with unprecedented efficiency. They are also compatible with the ongoing trend of compact full-optical components in lightwave technology, which enable ultra-fast processing [up/down conversion, (de)modulation, amplification, multiplexing, mixing, etc.] without the need to manipulate massive (and then, slow) electrons. This concept of compact photonic circuits where photons control photons via nonlinear media aims to circumvent the bottleneck originating from virtually unlimited optical bandwidth versus limited optoelectronic processing speed. Optical communications systems are also very demanding for ultra-low phase noise microwaves in order to satisfy both clocking (low phase noise is equivalent to low time-jitter) and bandwidth (bit-rates increase proportionally to the clock frequency) requirements. In fact, in high-speed communication networks, such ultra-stable oscillators are fundamental references for several purposes (local oscillator for up/down frequency conversion, network synchronization, carrier synthesis, etc.).
Nonlinear phenomena in WGM resonators also open new horizons of research for other applications, such as Raman and Brillouin lasers. More generally, these phenomena can be merged within the broader perspective of nonlinear phenomena in optical cavities and waveguides, and it is a fruitful paradigm for crystalline or silicon photonics. The strong confinement and very long lifetime of photons into the torus-like WGMs also offer an excellent test-bench to investigate fundamental issues in condensed matter and quantum physics. The race to ever increased accuracy in electromagnetic signals also contributes to answer quintessential questions in physics, related to relativity (tests for Lorentz invariance), or the measurement of fundamental physical constants and their possible variation with time.
In this article, the different steps required to obtain crystalline optical whispering-gallery-mode (WGM) resonators are described and their characterization is explained. Also presented is the protocol to obtain the high quality tapered fiber needed to couple laser light into these resonators. Finally, a flagship application of these resonators in the field of microwave photonics, namely ultra-stable microwave generation using Kerr combs, is presented and discussed.
In the first section, we detail the protocol followed to obtain ultra-high Q WGM resonators. Our method relies on a grind and polish approach, which is reminiscent to the standard techniques used to polish optical components such as lenses or telescope mirrors. The second section is devoted to the characterization of surface roughness. We use a non-contact white light interferometric profilometer to measure the surface roughness which leads to surface scattering-induced losses and thereby lower the Q factor performance. This step is an important experimental test to evaluate the quality of the polishing. The third section is concerned with the fabrication a tapered silica fiber with diameter in the micrometer range in order to launch light in the resonator. To reach such small diameters, we adopt the "flame-brushing" technique, using simultaneously computer-controlled motors to pull the fiber apart, and a blowtorch to heat the fiber area to be tapered20. In the fourth section, the resonator and the tapered fiber are approached to one another to visualize the resonance signal of the whispering gallery modes using a wavelength-scanning laser. We show in the fifth section how, by increasing the optical power in the resonator, we manage to trigger nonlinear phenomena until we observe the formation of Kerr optical frequency combs, with a spectrum made of equidistant spectral lines. As emphasized above, these Kerr comb spectra have exceptional characteristics that are suitable for several applications in both science and technology21-23. We will consider one of the most noteworthy applications of WGM resonators by demonstrating an optical multi-wavelength signal whose intermodal frequency is an ultra-stable microwave.

<table border="1">
<tbody>
 <tr>
 <td> Material Name</td>
 <td>Company</td>
 </tr>
 <tr>
 <td>Step motors 50 mm course</td>
 <td>Thorlabs</td>
 </tr>
 <tr>
 <td>3 axis nanostage</td>
 <td>Physik Instrumente</td>
 </tr>
 <tr>
 <td>TUNICS tunable laser source</td>
 <td>Yenista</td>
 </tr>
 <tr>
 <td>Optical spectrum analyzer APEX</td>
 <td>APEX Technologies</td>
 </tr>
 </tbody>
</table>

microwave photonics systems based on whispering-gallery-mode resonators

Microwave photonics systems rely fundamentally on the interaction between microwave and optical signals. These systems are extremely promising for various areas of technology and applied science, such as aerospace and communication engineering, sensing, metrology, nonlinear photonics, and quantum optics. In this article, we present the principal techniques used in our lab to build microwave photonics systems based on ultra-high Q whispering gallery mode resonators. First detailed in this article is the protocol for resonator polishing, which is based on a grind-and-polish technique close to the ones used to polish optical components such as lenses or telescope mirrors. Then, a white light interferometric profilometer measures surface roughness, which is a key parameter to characterize the quality of the polishing. In order to launch light in the resonator, a tapered silica fiber with diameter in the micrometer range is used. To reach such small diameters, we adopt the "flame-brushing" technique, using simultaneously computer-controlled motors to pull the fiber apart, and a blowtorch to heat the fiber area to be tapered. The resonator and the tapered fiber are later approached to one another to visualize the resonance signal of the whispering gallery modes using a wavelength-scanning laser. By increasing the optical power in the resonator, nonlinear phenomena are triggered until the formation of a Kerr optical frequency comb is observed with a spectrum made of equidistant spectral lines. These Kerr comb spectra have exceptional characteristics that are suitable for several applications in science and technology. We consider the application related to ultra-stable microwave frequency synthesis and demonstrate the generation of a Kerr comb with GHz intermodal frequency.

This five-step protocol enables to obtain WGM resonators with very high quality factors for microwave photonic applications.
The first step aims to give to the resonator the desired shape, as represented on Scheme 2. The main difficulty here is to manufacture a disk whose rim is sharp enough so that can it strongly confine the trapped photons, without leading to structural fragility from a mechanical standpoint. This polishing tower also possesses remarkable versatility as it ...

Watch this Scientific Journal Video about Microwave Photonics Systems Based on Whispering-gallery-mode Resonators at JoVE.com

Microwave Photonics Systems Based on Whispering-gallery-mode Resonators

The customized techniques developed in our lab to build microwave photonics systems based on ultra-high Q whispering gallery mode resonators are presented. The protocols to obtain and characterize these resonators are detailed, and an explanation of some of their applications in microwave photonics is given.

Microwave photonics systems rely fundamentally on the interaction between microwave and optical signals. These systems are extremely promising for various ...

microwave-photonics-systems-based-on-whispering-gallery-mode

Research

JoVE Journal

Engineering

16.8K Views.  FEMTO-ST Institute. The customized techniques developed in our lab to build microwave photonics systems based on ultra-high Q whispering gallery mode resonators are presented. The protocols to obtain and characterize these resonators are detailed, and an explanation of some of their applications in microwave photonics is given.

Video: Microwave Photonics Systems Based on Whispering-gallery-mode Resonators

Development of Whispering Gallery Mode Polymeric Micro-optical Electric Field Sensors

Optical modes of dielectric micro-cavities have received significant attention in recent years for their potential in a broad range of applications. The optical modes are frequently referred to as "whispering gallery modes" (WGM) or "morphology dependent resonances" (MDR) and exhibit high optical quality factors. Some proposed applications of micro-cavity optical resonators are in spectroscopy1, micro-cavity laser technology2, optical communications3-6 as well as sensor technology. The WGM-based sensor applications include those in biology7, trace gas detection8, and impurity detection in liquids9. Mechanical sensors based on microsphere resonators have also been proposed, including those for force10,11, pressure12, acceleration13 and wall shear stress14. In the present, we demonstrate a WGM-based electric field sensor, which builds on our previous studies15,16. A candidate application of this sensor is in the detection of neuronal action potential.
The electric field sensor is based on polymeric multi-layered dielectric microspheres. The external electric field induces surface and body forces on the spheres (electrostriction effect) leading to elastic deformation. This change in the morphology of the spheres, leads to shifts in the WGM. The electric field-induced WGM shifts are interrogated by exciting the optical modes of the spheres by laser light. Light from a distributed feedback (DFB) laser (nominal wavelength of ~ 1.3 &mu;m) is side-coupled into the microspheres using a tapered section of a single mode optical fiber. The base material of the spheres is polydimethylsiloxane (PDMS). Three microsphere geometries are used: (1) PDMS sphere with a 60:1 volumetric ratio of base-to-curing agent mixture, (2) multi layer sphere with 60:1 PDMS core, in order to increase the dielectric constant of the sphere, a middle layer of 60:1 PDMS that is mixed with varying amounts (2% to 10% by volume) of barium titanate and an outer layer of 60:1 PDMS and (3) solid silica sphere coated with a thin layer of uncured PDMS base. In each type of sensor, laser light from the tapered fiber is coupled into the outermost layer that provides high optical quality factor WGM (Q ~ 106). The microspheres are poled for several hours at electric fields of ~ 1 MV/m to increase their sensitivity to electric field.

A high-sensitivity photonic micro sensor was developed for electric field detection. The sensor exploits the optical modes of a dielectric sphere. Changes in the external electric field perturb the sphere morphology leading to shifts in its optical modes. The electric field strength is measured by monitoring these optical shifts.

Optical modes of dielectric micro-cavities have received significant attention in recent years for their potential in a broad range of applications. The ...

Analysis of Contact Interfaces for Single GaN Nanowire Devices

Single GaN nanowire (NW) devices fabricated on SiO2 can exhibit a strong degradation after annealing due to the occurrence of void formation at the contact/SiO2 interface. This void formation can cause cracking and delamination of the metal film, which can increase the resistance or lead to a complete failure of the NW device. In order to address issues associated with void formation, a technique was developed that removes Ni/Au contact metal films from the substrates to allow for the examination and characterization of the contact/substrate and contact/NW interfaces of single GaN NW devices. This procedure determines the degree of adhesion of the contact films to the substrate and NWs and allows for the characterization of the morphology and composition of the contact interface with the substrate and nanowires. This technique is also useful for assessing the amount of residual contamination that remains from the NW suspension and from photolithographic processes on the NW-SiO2 surface prior to metal deposition. The detailed steps of this procedure are presented for the removal of annealed Ni/Au contacts to Mg-doped GaN NWs on a SiO2 substrate.

A technique was developed that removes Ni/Au contact metal films from their substrate to allow for the examination and characterization of the contact/substrate and contact/NW interfaces of single GaN nanowire devices.

Single GaN nanowire (NW) devices fabricated on SiO2 can exhibit  a strong degradation after annealing due to the  occurrence of void formation at the ...

Analyzing the Movement of the Nauplius 'Artemia salina' by Optical Tracking of Plasmonic Nanoparticles

We demonstrate how optical tweezers may provide a sensitive tool to analyze the fluidic vibrations generated by the movement of small aquatic organisms. A single gold nanoparticle held by an optical tweezer is used as a sensor to quantify the rhythmic motion of a Nauplius larva (Artemia salina) in a water sample. This is achieved by monitoring the time dependent displacement of the trapped nanoparticle as a consequence of the Nauplius activity. A Fourier analysis of the nanoparticle&#39;s position then yields a frequency spectrum that is characteristic to the motion of the observed species. This experiment demonstrates the capability of this method to measure and characterize the activity of small aquatic larvae without the requirement to observe them directly and to gain information about the position of the larvae with respect to the trapped particle. Overall, this approach could give an insight on the vitality of certain species found in an aquatic ecosystem and could expand the range of conventional methods for analyzing water samples.

Analyzing the Movement of the Nauplius &#039;Artemia salina&#039; by Optical Tracking of Plasmonic Nanoparticles

We use optical tracking of plasmonic nanoparticles to probe and characterize the frequency movements of aquatic organisms.

We demonstrate how optical tweezers may provide a sensitive tool to analyze the fluidic vibrations generated by the movement of small aquatic organisms. A ...

Selective Area Modification of Silicon Surface Wettability by Pulsed UV Laser Irradiation in Liquid Environment

The wettability of silicon (Si) is one of the important parameters in the technology of surface functionalization of this material and fabrication of biosensing devices. We report on a protocol of using KrF and ArF lasers irradiating Si (001) samples immersed in a liquid environment with low number of pulses and operating at moderately low pulse fluences to induce Si wettability modification. Wafers immersed for up to 4 hr in a 0.01% H2O2/H2O solution did not show measurable change in their initial contact angle (CA) ~75&#176;. However, the 500-pulse KrF and ArF lasers irradiation of such wafers in a microchamber filled with 0.01% H2O2/H2O solution at 250 and 65 mJ/cm2, respectively, has decreased the CA to near 15&#176;, indicating the formation of a superhydrophilic surface. The formation of OH-terminated Si (001), with no measurable change of the wafer&#8217;s surface morphology, has been confirmed by X-ray photoelectron spectroscopy and atomic force microscopy measurements. The selective area irradiated samples were then immersed in a biotin-conjugated fluorescein-stained nanospheres solution for 2 hr, resulting in a successful immobilization of the nanospheres in the non-irradiated area. This illustrates the potential of&#160;the method for selective area biofunctionalization and fabrication of advanced Si-based biosensing architectures. We also describe a similar protocol of irradiation of wafers immersed in methanol (CH3OH) using ArF laser operating at pulse fluence of 65 mJ/cm2 and in situ formation of a strongly hydrophobic surface of Si (001) with the CA of 103&#176;. The XPS results indicate ArF laser induced formation of Si&#8211;(OCH3)x compounds responsible for the observed hydrophobicity. However, no such compounds were found by XPS on the Si surface irradiated by KrF laser in methanol, demonstrating the inability of the KrF laser to photodissociate methanol and create -OCH3 radicals.

We report on a process of in situ alteration of HF treated Si (001) surface into a hydrophilic or hydrophobic state by irradiating samples in microfluidic chambers filled with&#160;H2O2/H2O solution (0.01%-0.5%) or methanol solutions using pulsed UV laser of a relative low pulse fluence.

The wettability of silicon (Si) is one of the important parameters in the technology of surface functionalization of this material and fabrication of ...

Writing Bragg Gratings in Multicore Fibers

Fiber Bragg gratings in multicore fibers can be used as compact and robust filters in astronomical and other research and commercial applications. Strong suppression at a single wavelength requires that all cores have matching transmission profiles. These gratings cannot be inscribed using the same method as for single-core fibers because the curved surface of the cladding acts as a lens, focusing the incoming UV laser beam and causing variations in exposure between cores. Therefore we use an additional optical element to ensure that the beam shape does not change while passing through the cross-section of the multicore fiber. This consists of a glass capillary tube which has been polished flat on one side, which is then placed over the section of the fiber to be inscribed. The laser beam enters the fiber through the flat surface of the capillary tube and hence maintains its original dimensions. This paper demonstrates the improvements in core-to-core uniformity for a 7-core fiber using this method. The technique can be generalized to larger multicore fibers.

We describe a technique for inscribing identical fiber Bragg gratings into each core of a multicore fiber. This is achieved by introducing an additional surface into the optical path to mitigate lensing by the curved surface of the fiber cladding.

Fiber Bragg gratings in multicore fibers can be used as compact and robust filters in astronomical and other research and commercial applications. Strong ...

Fabrication and Characterization of Superconducting Resonators

Superconducting microwave resonators are of interest for a wide range of applications, including for their use as microwave kinetic inductance detectors (MKIDs) for the detection of faint astrophysical signatures, as well as for quantum computing applications and materials characterization. In this paper, procedures are presented for the fabrication and characterization of thin-film superconducting microwave resonators. The fabrication methodology allows for the realization of superconducting transmission-line resonators with features on both sides of an atomically smooth single-crystal silicon dielectric. This work describes the procedure for the installation of resonator devices into a cryogenic microwave testbed and for cool-down below the superconducting transition temperature. The set-up of the cryogenic microwave testbed allows one to do careful measurements of the complex microwave transmission of these resonator devices, enabling the extraction of the properties of the superconducting lines and dielectric substrate (e.g., internal quality factors, loss and kinetic inductance fractions), which are important for device design and performance.

Superconducting microwave resonators are of interest for detection of light, quantum computing applications and materials characterization. This work presents a detailed procedure for fabrication and characterization of superconducting microwave resonator scattering parameters.

Superconducting microwave resonators are of interest for a wide range of applications, including for their use as microwave kinetic inductance detectors ...

Stimulated Stokes and Antistokes Raman Scattering in Microspherical Whispering Gallery Mode Resonators

Dielectric microspheres can confine light and sound for a length of time through high quality factor whispering gallery modes (WGM). Glass microspheres can be thought as a store of energy with a huge variety of applications: compact laser sources, highly sensitive biochemical sensors and nonlinear phenomena. A protocol for the fabrication of both the microspheres and coupling system is given. The couplers described here are tapered fibers. Efficient generation of nonlinear phenomena related to third order optical non-linear susceptibility &#935;(3) interactions in triply resonant silica microspheres is presented in this paper. The interactions here reported are: Stimulated Raman Scattering (SRS), and four wave mixing processes comprising Stimulated Anti-stokes Raman Scattering (SARS). A proof of the cavity-enhanced phenomenon is given by the lack of correlation among the pump, signal and idler: a resonant mode has to exist in order to obtain the pair of signal and idler. In the case of hyperparametric oscillations (four wave mixing and stimulated anti-stokes Raman scattering), the modes must fulfill the energy and momentum conservation and, last but not least, have a good spatial overlap.

Efficient generation of nonlinear phenomena related to third order optical non-linear susceptibility &#935;(3) interactions in triply resonant silica microspheres is presented in this paper. The interactions here reported are: Stimulated Raman Scattering (SRS), and four wave mixing processes comprising Stimulated Anti-stokes Raman Scattering (SARS).

Dielectric microspheres can confine light and sound for a length of time through high quality factor whispering gallery modes (WGM). Glass microspheres ...

Nanostructured Ag-zeolite Composites as Luminescence-based Humidity Sensors

Small silver clusters confined inside zeolite matrices have recently emerged as a novel type of highly luminescent materials. Their emission has high external quantum efficiencies (EQE) and spans the whole visible spectrum. It has been recently reported that the UV excited luminescence of partially Li-exchanged sodium Linde type A zeolites [LTA(Na)] containing luminescent silver clusters can be controlled by adjusting the water content of the zeolite. These samples showed a dynamic change in their emission color from blue to green and yellow upon an increase of the hydration level of the zeolite, showing the great potential that these materials can have as luminescence-based humidity sensors at the macro and micro scale. Here, we describe the detailed procedure to fabricate a humidity sensor prototype using silver-exchanged zeolite composites. The sensor is produced by suspending the luminescent Ag-zeolites in an aqueous solution of polyethylenimine (PEI) to subsequently deposit a film of the material onto a quartz plate. The coated plate is subjected to several hydration/dehydration cycles to show the functionality of the sensing film.

A protocol for the synthesis of moisture-responsive luminescent Ag-zeolite composites is described in this report.

Small silver clusters confined inside zeolite matrices have recently emerged as a novel type of highly luminescent materials. Their emission has high ...

Fabrication of Nanopillar-Based Split Ring Resonators for Displacement Current Mediated Resonances in Terahertz Metamaterials

Terahertz (THz) split ring resonator (SRR) metamaterials (MMs) has been studied for gas, chemical, and biomolecular sensing applications because the SRR is not affected by environmental characteristics such as the temperature and pressure surrounding the resonator. Electromagnetic radiation in THz frequencies is biocompatible, which is a critical condition especially for the application of the biomolecular sensing. However, the quality factor (Q-factor) and frequency responses of traditional thin-film based split ring resonator (SRR) MMs are very low, which limits their sensitivities and selectivity as sensors. In this work, novel nanopillar-based SRR MMs, utilizing displacement current, are designed to enhance the Q-factor up to 450, which is around 45 times higher than that of traditional thin-film-based MMs. In addition to the enhanced Q-factor, the nanopillar-based MMs induce a larger frequency shifts (17 times compared to the shift obtained by the traditional thin-film based MMs). Because of the significantly enhanced Q-factors and frequency shifts as well as the property of biocompatible radiation, the THz nanopillar-based SRR are ideal MMs for the development of biomolecular sensors with high sensitivity and selectivity without inducing damage or distortion to biomaterials. A novel fabrication process has been demonstrated to build the nanopillar-based SRRs for displacement current mediated THz MMs. A two-step gold (Au) electroplating process and an atomic layer deposition (ALD) process are used to create sub-10 nm scale gaps between Au nanopillars. Since the ALD process is a conformal coating process, a uniform aluminum oxide (Al2O3) layer with nanometer-scale thickness can be achieved. By sequentially electroplating another Au thin film to fill the spaces between Al2O3 and Au, a close-packed Au-Al2O3-Au structure with nano-scale Al2O3 gaps can be fabricated. The size of the nano-gaps can be well defined by precisely controlling the deposition cycles of the ALD process, which has an accuracy of 0.1 nm.

A protocol for the design and fabrication of a novel nanopillar-based split ring resonator (SRR) is presented.

Terahertz (THz) split ring resonator (SRR) metamaterials (MMs) has been studied for gas, chemical, and biomolecular sensing applications because the SRR ...

Scalable Quantum Integrated Circuits on Superconducting Two-Dimensional Electron Gas Platform

To form a coherent quantum transport in hybrid superconductor-semiconductor (S-Sm) junctions, the formation of a homogeneous and barrier-free interface between two different materials is necessary. The S-Sm junction with high interface transparency will then facilitate the observation of the induced hard superconducting gap, which is the key requirement to access the topological phases (TPs) and observation of exotic quasiparticles such as Majorana zero modes (MZM) in hybrid systems. A material platform that can support observation of TPs and allows the realization of complex and branched geometries is therefore highly demanding in quantum processing and computing science and technology. Here, we introduce a two-dimensional material system and study the proximity induced superconductivity in semiconducting two-dimensional electron gas (2DEG) that is the basis of a hybrid quantum integrated circuit (QIC). The 2DEG is a 30 nm thick In0.75Ga0.25As quantum well that is buried between two In0.75Al0.25As barriers in a heterostructure. Niobium (Nb) films are used as the superconducting electrodes to form Nb- In0.75Ga0.25As -Nb Josephson junctions (JJs) that are symmetric, planar and ballistic. Two different approaches were used to form the JJs and QICs. The long junctions were fabricated photolithographically, but e-beam lithography was used for short junctions&#8217; fabrication. The coherent quantum transport measurements as a function of temperature in the presence/absence of magnetic field B are discussed. In both device fabrication approaches, the proximity induced superconducting properties were observed in the In0.75Ga0.25As 2DEG. It was found that e-beam lithographically patterned JJs of shorter lengths result in observation of induced superconducting gap at much higher temperature ranges. The results that are reproducible and clean suggesting that the hybrid 2D JJs and QICs based on In0.75Ga0.25As quantum wells could be a promising material platform to realize the real complex and scalable electronic and photonic quantum circuitry and devices.

Quantum integrated circuits (QICs) consisting of array of planar and ballistic Josephson junctions (JJs) based on In0.75Ga0.25As two-dimensional electron gas (2DEG) is demonstrated. Two different methods for fabrication of the two-dimensional (2D) JJs and QICs are discussed followed by the demonstration of quantum transport measurements in sub-Kelvin temperatures.

To form a coherent quantum transport in hybrid superconductor-semiconductor (S-Sm) junctions, the formation of a homogeneous and barrier-free interface ...