OEwaves has pioneered the technology of crystalline whispering gallery mode (WGM) optical microresonators, which are used in a variety of our products, including lasers. These mm-scale elements exhibit the highest quality factor (Q) and finesse, beyond what is possible with bulk or integrated optical resonators.  Q’s in excess of 1011 and finesse in excess of 107 have been demonstrated. OEwaves fabricates WGM resonators from a variety of crystals, including calcium fluoride, magnesium fluoride, lithium tantalate, quartz, silicon, and even diamond. The specific materials serve the specific applications utilizing linear and nonlinear properties of these resonators.  For example, magnesium and calcium fluoride can be used for generation of Kerr frequency combs, while lithium tantalate can be used in applications where a change the index of refraction with voltage is needed, such as for a modulator. Other applications, such as second harmonic generation are also supported by these crystalline resonators.  The high Q of the resonator leads to high sensitivity of the modulator and reduced “effective” V-pi.  

​

The advantage of a crystalline WGM resonator over other approaches, including resonators fabricated with epitaxial growth on chip, is that the ultra-high quality factor allows reduced power (a few mW) to excite linear and nonlinear functionalities.  The larger mode volumes of crystalline resonators also reduce the fundamental quantum noise in these structures, compared to ring resonators.  
 

The performance of a variety of systems such as communications and radar depend on the spectral purity of the wave that carriers the information signal. In systems where the carrier frequency is in the microwave, mm-wave, or sub-mm wave range, the process of frequency conversion allows translating the high frequency of the carrier and retrieve the signal at baseband or IF; or conversely, translating a low frequency to a larger frequency.  This is because the frequency of the carrier in these systems is derived from a high spectral purity reference, and the highest performing references such as quartz oscillators fundamentally have low frequency, in the 10’s to 100’s of MHz.  Several stages of up/down conversion, which include mixing, amplification and filtering, make frequency conversion possible. This is a cumbersome burden on the size, weight and power budget of the system, and the overall cost.  Furthermore, the multiplication of the low frequency reference to the desired microwave/mm-wave frequency by a factor of N multiplies the noise by a factor of 20 logN.

OEwaves has developed a single step dual conversion system based on its unique photonics technology.  In this approach, the information signal is modulated on an optical carrier, which is then mixed with an optical LO on a photodetector to up-convert the signal to a frequency determined by the difference frequency of the two optical sources. The mixing simply occurs at a photodiode, which is a quadrature detector and acts as a quadrature mixer. The bandwidth of the detector can be set to achieve the required high frequency, in the up-conversion process. Similarly, a received signal can be down-converted on a low bandwidth photodetector, which also serves to filter out the higher frequency mixing products. The spectral purity of this system at all frequencies is quite high, and at mm- and sub-mm wave frequencies exceeds conventional electronics approaches by as much as 30 dB and higher.

Benefits of this approach include a significantly simplified architecture that maintains the same level of low noise irrespective of the frequency of operation. This approach also helps to significantly reduce the size, weight and power consumption of the system.

The next step in increasing the functionality of the up/down conversion is realization of a photonic receiver is a full receiver/channelizer function.  In this approach optical filtering is added to select a specific channel where the signal is received and is subsequently fed to the digital system processing at the backend.  OEwaves has developed special optical filters that allow selecting a channel width based on the application of interest.  Combined with the high bandwidth of the photonic dual converter above, and the agility to select the band of interest, this approach to realization of a channelizer provides outstanding performance not matched by conventional electronics.  It is also particularly suitable for application in software defined radios