High capacity coherent optical transceiver for short reach applications
An optical transceiver includes a transmitter including transmitter signal processing circuitry configured to receive a transmit signal and provide two drive voltage signals V 1 , V 2 to a modulator configured to modulate a laser based thereon; and a receiver including i) optical couplers configured to coherently combine received signals with a Local Oscillator (LO) formed by the laser and provide the combined signals to photodetectors for balanced detection, and ii) receiver signal processing circuitry configured to demodulate outputs from the balanced detection, wherein the receiver signal processing circuitry comprises an analog front-end and digital back-end.
1. An optical transceiver, comprising:
a transmitter comprising transmitter signal processing circuitry configured to receive a transmit signal and provide two drive voltage signals V 1 , V 2 to a modulator configured to modulate a laser based thereon; and
a receiver comprising i) optical couplers configured to coherently combine received signals with a Local Oscillator (LO) formed by the laser and provide the combined signals to photodetectors for balanced detection, and ii) receiver signal processing circuitry configured to demodulate outputs from the balanced detection, wherein the receiver signal processing circuitry comprises an analog front-end and digital back-end,
wherein the laser has a frequency offset based on signal bandwidth to provide a signal carrier for reuse as the LO.
2. The optical transceiver of claim 1 , wherein the modulator comprises a Dual-Drive Mach-Zehnder Modulator (DD-MZM) with peak to peak drive voltages of the drive voltage signals V 1 , V 2 sufficient to cover a full unity circle of an I/Q plane.
3. The optical transceiver of claim 2 , wherein optical output field transitions across a predefined radial bisection line of the I/Q plane is prevented by providing the drive voltage signals V 1 , V 2 to force null transmission, wherein the transmitter signal processing circuitry is configured to re-set the drive voltage signals V 1 , V 2 for the null transmission.
4. The optical transceiver of claim 3 , wherein the transmitter signal processing circuitry is configured to apply chromatic dispersion compensation on the transmit signal.
5. The optical transceiver of claim 1 , wherein the transmitter signal processing circuitry is configured to add pilot tones to the transmitted signal, outside of signal spectrum and on opposite sides of the signal spectrum from an adjacent optical transceiver.
6. The optical transceiver of claim 5 , wherein a first pilot tone of the pilot tones is at a fractional N/K of symbol clock outside of the signal spectrum, N and K are integers selected to place the pilot tones slightly outside of the signal spectrum, and a second pilot tone of the pilot tones, orthogonal to the first pilot tone, is at (N+1)/K of the symbol clock outside of the signal spectrum.
7. The optical transceiver of claim 1 , wherein the analog front-end comprises Transimpedance Amplifiers (TIAs) and analog Radio Frequency (RF) processing functions integrated into a same circuit.
8. The optical transceiver of claim 1 , wherein the analog front-end is configured to down-convert pilot tones on the received signals and directly synthesize a Radio Frequency (RF) LO signal for signal conversion to baseband with corresponding recovery of frequency and phase offsets.
9. The optical transceiver of claim 1 , wherein the analog front-end is configured to down-convert pilot tones on the received signals and directly synthesize a symbol clock signal.
10. The optical transceiver of claim 1 , wherein the analog front-end is configured to down-convert pilot tones on the received signals and directly synthesize a polarization deconvolution matrix.
11. The optical transceiver of claim 1 , wherein the analog front-end is configured to down-convert the received signals to baseband and extract I and Q components.
12. The optical transceiver of claim 1 , wherein the analog front-end comprises Finite Impulse Response (FIR) filters for partial compensation of chromatic dispersion compensation, in combination with the transmitter signal processing circuitry of an adjacent optical transceiver.
13. The optical transceiver of claim 12 , wherein unused FIR filter taps in the analog front-end are shut down to conserve power.
14. The optical transceiver of claim 1 , wherein an Analog to Digital Converter (ADC) stage is between the analog front-end and the digital back-end, wherein the ADC stage operates at a symbol rate.
15. The optical transceiver of claim 14 , wherein an Effective Number of Bits (ENOB) of the ADC stages is limited to provide hard decision slicing of a symbol constellation.
16. The optical transceiver of claim 1 , wherein the digital back-end comprises a Digital Signal Processor (DSP) configured to perform signal decoding and Forward Error Correction (FEC) decoding.
17. The optical transceiver of claim 1 , wherein the transmitter and the receiver are housed in a housing compliant to one of Quad Small Form-factor Pluggable (QSFP) Double Density (DD) and Octal Small Form-factor Pluggable (OSFP).
18. An optical transceiver, comprising:
a transmitter comprising transmitter signal processing circuitry configured to receive a transmit signal and provide two drive voltage signals V 1 , V 2 to a modulator configured to modulate a laser based thereon, wherein the transmitter signal processing circuitry is configured to add pilot tones to the transmitted signal, outside of signal spectrum and on opposite sides of the signal spectrum from an adjacent optical transceiver; and
a receiver comprising i) optical couplers configured to coherently combine received signals with a Local Oscillator (LO) formed by the laser and provide the combined signals to photodetectors for balanced detection, and ii) receiver signal processing circuitry configured to demodulate outputs from the balanced detection, wherein the receiver signal processing circuitry comprises an analog front-end and digital back-end.
19. The optical transceiver of claim 18 , wherein a first pilot tone of the pilot tones is at a fractional N/K of symbol clock outside of the signal spectrum, N and K are integers selected to place the pilot tones slightly outside of the signal spectrum, and a second pilot tone of the pilot tones, orthogonal to the first pilot tone, is at (N+1)/K of the symbol clock outside of the signal spectrum.
20. An optical transceiver, comprising:
a transmitter comprising transmitter signal processing circuitry configured to receive a transmit signal and provide two drive voltage signals V 1 , V 2 to a modulator configured to modulate a laser based thereon; and
a receiver comprising i) optical couplers configured to coherently combine received signals with a Local Oscillator (LO) formed by the laser and provide the combined signals to photodetectors for balanced detection, and ii) receiver signal processing circuitry configured to demodulate outputs from the balanced detection, wherein the receiver signal processing circuitry comprises an analog front-end and digital back-end,
wherein the analog front-end comprises Finite Impulse Response (FIR) filters for partial compensation of chromatic dispersion compensation, in combination with the transmitter signal processing circuitry of an adjacent optical transceiver.