Optical RF Filter Wavelength Locking with Fixed Offset
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Solution Overview
Problem
Current RF filtering technologies using whispering gallery mode resonators struggle with wavelength locking between the optical source and resonators, leading to suppression of the RF sideband spectrum, and are complicated by multiple modulations that create background interference noise, making it difficult to effectively lock to the optical carrier in the presence of multiple sidebands.
Innovation Solution
A system and method for wavelength locking an optical RF filter to a laser with a fixed offset frequency, utilizing a first whispering gallery mode resonator filter to split the input signal into a peak detection path and a re-insertion path, and a second filter to select and filter spectral components, while maintaining the optical carrier frequency locked to the resonator passband, thereby isolating the carrier and creating an offset to allow the RF signal of interest to pass through.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the optical carrier frequency is locked to the resonator passband with zero spectral offset, then the carrier power is maximized, but the RF sideband spectrum is rejected along with the laser spectrum
Solution Approach 1:
The patent applies local quality by creating a frequency offset between the optical carrier and the resonator passband. This offset is specifically designed to be larger than the RF signal bandwidth but smaller than the resonator bandwidth, allowing the carrier to be locked for maximum power while the RF sidebands pass through the resonator. This localized frequency adjustment resolves the contradiction by differentiating the treatment of carrier and sideband frequencies.
2Adaptability or versatility
If multiple modulations are applied to the optical source spectrum, then frequency translation of the RF signal is accomplished, but background interference noise is created that interferes with locking to the optical carrier
Solution Approach 1:
The patent extracts the optical carrier signal from the complex spectrum containing multiple modulated sidebands by utilizing the frequency offset characteristic. The carrier, being offset from the resonator passband by a specific amount, can be isolated and locked to independently of the other sidebands. This extraction allows the system to maintain frequency translation capability while eliminating the interference noise that would otherwise prevent carrier locking.
3Reliability
If time delay elements are used to compensate for group delay of filtered spectral components, then wavelength locking can be achieved, but the system complexity increases and effective operation is limited to relatively short optical paths
Solution Approach 1:
The patent applies preliminary action by pre-establishing a fixed frequency offset between the optical carrier and the resonator passband before the signal enters the resonator. This preliminary frequency arrangement eliminates the need for subsequent time delay compensation elements, as the frequency offset inherently provides the necessary phase relationship for wavelength locking. This resolves the contradiction by achieving reliable locking without adding complex time delay elements or limiting the system to short optical paths.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enables precise locking and isolation of the optical carrier signal from interfering sidebands, maximizing carrier power and reducing background noise, allowing for effective filtering and wavelength locking without the need for time delay elements, even in complex optical links with multiple modulations.
Implementation Method 1
Some optical resonators can support resonator modes of light called whispering gallery modes, and thus, are often referred to as whispering gallery mode resonators. Whispering gallery modes occur when light having an evanescent wave component travels via internal reflection around the periphery of the optical resonator.
Implementation Method 2
The evanescent wave component extends beyond the optical resonator's outer surface and may be coupled into an adjacent optical coupler as long as the optical coupler is located within the extent of the evanescent wave
Implementation Method 3
An optical power detector receives the peak detection path signal and converts the peak detection path signal into an electrical control signal and aligns the optical carrier frequency to a resonance frequency of the first filter
Data Source
AI summary
An optical filter is disclosed, including a first optical filter adapted to receive a first optical signal including an optical carrier frequency and a plurality of interference signal components. The first filter produces an output signal at the optical carrier frequency and a reflection signal. The output signal is split into a peak detection path signal and a re-insertion path signal. An optical power detector converts the peak detection path signal into an electrical control signal and aligns the optical carrier frequency to a resonance frequency of the first filter to maximize the power of the optical carrier frequency. A second optical filter receives the reflection signal and selects at least one spectral component while rejecting other spectral components and outputs a filtered signal that carries the selected spectral component. A signal combiner receives and combines the filtered signal and the re-insertion path signal.