Name: microwave photonics systems based on whispering-gallery-mode resonators
Uploaded: 2013-08-05T12:00:00
Description: Watch this Scientific Journal Video about Microwave Photonics Systems Based on Whispering-gallery-mode Resonators at JoVE.com

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.</...

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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>
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 <td>Step motors 50 mm course</td>
 <td>Thorlabs</td>
 </tr>
 <tr>
 <td>3 axis nanostage</td>
 <td>Physik Instrumente</td>
 </tr>
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 <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 ...

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