Crystalline resonators add properties to photonic devices
Mainstream broadband applications greater than 1GHz currently dominate radio frequency (RF) photonics, driven by expanding digital communications needs. However, narrowband applications represent important opportunities in analog communications, high-spectral purity RF and microwave signals, photonic front ends, and narrow passband (1–100MHz) RF signal processing. Our work is focused on using the advantages of ultra-high quality factor (Q) crystalline whispering-gallery-mode (WGM) resonators in such applications (see Figure 1), and we have made several recent advances in functional device integration.
Crystalline WGM resonators fabricated with electro-optic (EO) materials are particularly useful as narrowband modulators (see Figure 2). The extremely high intrinsic optical Q and small mode volume of WGMs provide for both narrow bandwidth and unprecedented effective interaction lengths of optical and RF fields. Modulators operating with saturation powers as low as −26dBm (corresponding to effective Vπ≃18mV) have been demonstrated in X- and Ka-band modulator prototypes at optoelectronic (OE) waves. Resonator-based modulators can also efficiently operate at any desired wavelength within the transparency window of the EO material. These devices play key roles in micro-optic implementation of microwave photonics functions.
We recently introduced a new class of WGM-based single sideband (SSB) modulators that results in a high RF return. They also exhibit center frequency tunability over a very wide range (exceeding an octave). As such, they represent a new class of highly efficient devices that simultaneously provide a narrow modulation bandwidth over a very wide RF range. These properties make them easier to use in current applications and enable new applications such as widely tunable RF photonic receivers and oscillators.
Optoelectronic oscillators (OEOs) are used to generate spectrally pure RF signals using photonics.1 A generic OEO includes a laser, an amplitude EO modulator (EOM), one or multiple optical delay lines, a fast photodiode, a narrowband RF filter, and an RF amplifier. The EOM modulates the continuous wave light emitted by the laser. The modulated signal passes the optical delay line(s) and is transformed into an RF signal with the photodiode. The RF signal is subsequently filtered, amplified, and fed back to the modulator. As a result, a closed active RF circuit is produced that will generate self-sustained oscillation when the RF amplification exceeds the circuit's integral loss. Some circuit elements can be merged or replaced. For instance, a directly modulatable laser and an optical bandpass filter can be used instead of the RF filter, but the oscillation principles remain intact. High-Q crystalline WGM resonators can be used to replace the EOM or for buffering the RF signal and cleaning the OEO's supermode spectrum.2 The tunable resonant SSB modulators enable tunable, compact OEOs.3 We have developed various types of ultra-compact OEOs based on high-Q WGM resonators. These oscillators operate in the X-, Ku-, and Ka-bands and are able to generate spectrally pure RF signals characterized with less than −120dBc/Hz phase noise at 100kHz from the carrier (see Figure 3).4 The phase noise floor (<−140dBc/Hz) is limited by the signal's shot noise received at the photodiode. We have demonstrated both tunable and fixed frequency oscillators.
Photonic RF receivers have large dynamic range and high sensitivity at high frequencies. These compact devices also use relatively little power for operation. Direct processing of high-frequency RF signals with conventional electronic approaches is hindered by the absence of efficient RF amplifiers, detectors, and methods for up and down conversion of the received signals. RF photonic receivers enable conversion of the RF signals to an intermediate frequency (IF) or to baseband, where subsequent efficient detection is feasible. We recently developed a Ka−band photonic receiver based on all-resonant interaction of light and RF radiation in solid-state WGM resonators5, 6 (see Figure 4). The core of the receiver, a mixer based on material nonlinearity of the EO WGM resonator, operates well for frequencies ranging from several GHz to 100GHz. The coherent photonic receiver can have a spurious free dynamic range exceeding 55dB and sensitivity of better than −100dBm in the 10MHz reception band. (The sensitivity does not degrade with increasing RF frequency.) It also enables separation of detecting and processing RF signals in space when combined with high-quality optical links used to transmit the up-converted RF signals.
Notch filters are used to reflect electromagnetic radiation within a selected spectral region while allowing high transmission outside of it. Tunable RF photonic filters benefit phase array and other kinds of radar. Our work led to a novel, highly efficient photonic WGM notch filter that is based on a dual-polarization interferometer with coincident optical paths in the arms. We developed a 10MHz filter with 5.5dB insertion loss and 45.5dB of rejection.7 The measured rejection value is limited by the finite (3kHz) line width of our laser.
Our work suggests that RF photonics can strongly benefit from the use of crystalline WGM resonators. Our research has already resulted in several novel WGM-based photonic devices with advanced functions. Future work in the field will focus on improved packaging of the resonators to produce devices that withstand severe environmental conditions. We expect that maturing fabrication, handling, and coupling techniques for resonators, together with adaptation of assembly methods and components from broadband photonics, will result in successful deployment of unique narrowband, high-sensitivity, and tunable functions of WGM resonators in compact manufacturable devices.
Andrey Matsko focuses on quantum and nonlinear optics. He has more than 100 journal publications and holds 11 US patents. He is a principal engineer and studies the properties and applications of WGM in dielectric optical resonators.
Anatoliy Savchenkov, principal engineer, is an expert on crystalline WGM optical resonator design and fabrication. He holds over 25 US patents/patent applications, has a track record of technological innovations, and numerous journal publications.
David J. Seidel, vice president of engineering and product development, was previously at Jet Propulsion Laboratory, where during his 18-year tenure he conducted basic research and system engineering, and served as a project technologist for the Laser Interferometer Space Antenna project.
Lute Maleki, company president and chief executive officer, is an inventor with more than 40 US patents and applications, including the opto-electronic oscillator, has authored and co-authored over 100 refereed publications and over 200 conference proceedings, and was awarded the distinguished IEEE I. I. Rabi Award and the NASA Exceptional Engineering Achievement Medal.
