Optical differential low-noise receivers and related methods
Low-noise optical differential receivers are described. Such differential receivers may include a differential amplifier having first and second inputs and first and second outputs, and four photodetectors. A first and a second of such photodetectors are coupled to the first input of the differential amplifier, and a third and a fourth of such photodetectors are coupled to the second input of the differential amplifier. The anode of the first photodetector and the cathode of the second photodetector are coupled to the first input of the differential amplifier. The cathode of the third photodetector and the anode of the fourth photodetector are coupled to the second input of the differential amplifier. The optical receiver may involve two stages of signal subtraction, which may significantly increase noise immunity.
1. An optical receiver comprising:
a differential amplifier having first and second inputs and first and second outputs;
a first photodetector configured to produce a first photocurrent and a second photodetector configured to produce a second photocurrent, the first and second photodetectors being coupled to the first input of the differential amplifier, wherein an anode of the first photodetector is coupled to a cathode of the second photodetector thereby producing a first differential current based on a difference between the first photocurrent and the second photocurrent;
a third photodetector configured to produce a third photocurrent and a fourth photodetector configured to produce a fourth photocurrent, the third and fourth photodetectors being coupled to the second input of the differential amplifier, wherein a cathode of the third photodetector is coupled to an anode of the fourth photodetector thereby producing a second differential current based on a difference between the fourth photocurrent and the third photocurrent; and
a photonic circuit configured to provide:
a first optical signal to the first and third photodetectors, and
a second optical signal to the second and fourth photodetectors.
2. The optical receiver of claim 1 , wherein the anode of the first photodetector and the cathode of the second photodetector are coupled to the first input of the differential amplifier.
3. The optical receiver of claim 1 , wherein the cathode of the third photodetector and the anode of the fourth photodetector are coupled to the second input of the differential amplifier.
4. The optical receiver of claim 1 , wherein the first, second, third and fourth photodetectors are formed monolithically on a common substrate.
5. The optical receiver of claim 4 , wherein the common substrate comprises a silicon substrate.
6. The optical receiver of claim 4 , wherein the first, second, third and fourth photodetectors are disposed within an area of 0.1 mm 2 on the substrate.
7. The optical receiver of claim 1 , wherein the first, second, third and fourth photodetectors have equal responsivities.
8. The optical receiver of claim 1 , wherein the first, second, third and fourth photodetectors are photodiodes.
9. The optical receiver of claim 1 , further comprising an analog-to-digital converter coupled to the first and second outputs of the differential amplifier.
10. The optical receiver of claim 1 , wherein the photonic circuit is configured to generate the first and second optical signals by combining a modulated optical signal with a reference optical signal.
11. A method for receiving an input signal, the method comprising:
combining the input signal with a reference signal to obtain first and second optical signals;
producing first and second photocurrents by detecting the first optical signal with a first photodetector and with a second photodetector, respectively, and producing third and fourth photocurrents by detecting the second optical signal with a third photodetector and with a fourth photodetector, respectively; and
producing a pair of amplified differential voltages using first and second differential currents, the first differential current obtained based on a difference between the first photocurrent and the second photocurrent, and the second differential current obtained based on a difference between the fourth photocurrent and the third photocurrent.
12. The method of claim 11 , wherein combining the input signal with the reference signal comprises combining the input signal with the reference signal with a directional coupler.
13. The method of claim 11 , wherein producing the pair of amplified differential voltages comprises producing the pair of amplified differential voltages using the first and second differential currents with a differential operational amplifier.