Photonic filtering of electrical signals
A filter and a method of filtering a high frequency electrical signal using photonic components is disclosed. The filter has a serially fiber-coupled laser source, a modulator, a filter, and a photodetector. The electrical signal is applied to the modulator. The modulated light propagates through the filter which is constructed to pass not only a modulated sideband, but also at least a fraction of light at the carrier frequency of the laser. The photodetector detects a signal at the beat frequency between the carrier and sideband signals, after both signals have propagated through the filter. As a result, a separate optical branch for light at the carrier frequency is not required, which considerably simplifies the filter construction and makes it more stable and reliable.
1. A filter for filtering an electrical signal, comprising:
an optical modulator for receiving light at a carrier frequency and for modulating light at a frequency of the electrical signal to obtain light at first and second frequencies;
an optical filter for receiving the modulated light and for selecting light at a first frequency to propagate therethrough with at least a fraction of light at the carrier frequency, while suppressing light at the second frequency, the optical filter comprising a dual-band optical filter having a first passband for light at the first frequency and a second passband for light at the carrier frequency; and
a photodetector for receiving light filtered by the optical filter and for detecting a signal at a beat frequency between the first frequency and the carrier frequency,
wherein the detected signal at the beat frequency comprises an output signal of the filter;
wherein the dual-band optical filter comprise first and second optical sub-filters connected in series, wherein the first and the second optical sub-filters each have and second passbands,
wherein the passband of the dual-band optical filter comprises an overlap region between the first passbands of the first and the second optical sub-filters;
wherein the second passband of the dual-band optical filter comprises an overlap region between the second passbands of the first and the second optical sub-filters; and
wherein a central frequency and/or a bandwidth of the first passband of the dual-band optical filter is tunable by adjusting a central frequency of the first passband of the first optical sub-filter, of the second optical sub-filter, or of both first and the second optical sub-filters,
whereby the filter for filtering the electrical signal is tunable in frequency and/or bandwidth.
2. The filter of claim 1 , further comprising a light source for providing light at the carrier frequency.
3. The filter of claim 2 , wherein the light source is a laser.
4. The filter of claim 1 , wherein the first optical sub-filter comprises a first fiber Bragg grating having a transmission band corresponding to the first passband of the first optical sub-filter.
5. The filter of claim 4 , wherein the second optical sub-filter further comprises a second fiber Bragg grating coupled to the first fiber Bragg grating, the second fiber Bragg grating having a transmission band corresponding to the first passband of the second optical sub-filter.
6. The filter of claim 5 , wherein the first fiber Bragg grating is tunable, whereby the bandwidth of the first passband of the dual-band filter is tunable.
7. The filter of claim 4 , wherein the first fiber Bragg grating is tunable, whereby the central frequency of the first passband of the dual-band optical filter is tunable.
8. The filter of claim 1 , wherein the optical filter includes a notch filter for attenuating light at a third frequency different from the first frequency and second frequencies.
9. The filter of claim 1 , wherein the optical modulator and the optical filter are coupled by an optical waveguide.
10. The filter of claim 2 , wherein the light source, the optical modulator, the filter, and the photodetector are coupled by an optical waveguide.
11. The filter of claim 10 , wherein the optical waveguide is an optical fiber.