Apparatus and method for analog electronic fiber dispersion and bandwidth pre-compensation (EDPC) for use in 50 Gbps and greater pamn optical transceivers
Cost-effective high-data-rate optical data transceivers are presented, comprising an electronic analog transversal filter simultaneously providing one or more of bandwidth compensation and forward impairment compensations for the transmitted optical signal.
1. An optical transceiver module for n-level pulse-amplitude modulated (PAMn) optical symbols with n≥2, providing an interface between electronic data signals on a host and optical symbols transmitted through an optical-communication plant at a Baud rate, the transceiver comprising:
receiver section that comprises at least one photoreceiver to convert received optical intensity signals into analog electrical signals;
a PAMn digital signal processing (DSP) circuit providing at least logical interface to the electronic host data, forward error correction (FEC) capabilities, analog-to-digital conversion of the electrical signals from the photoreceiver(s), digital adaptive filtering of the converted received signal(s), and reconstruction of the filtered received signal(s) into data;
a transmitter section that comprises at least one laser and at least one interference modulator; and
an analog transversal filter circuit configured as an electronic dispersion pre compensator (EDPC) to filter PAMn transmission signals provided by the PAMn DSP based on anticipated direct detection of the PAMn transmission signals,
wherein the filtered signals emitted from the EDPC are connected to signal inputs of the interference modulator, wherein the EDPC effects a distortion pre-correction based on a predicted distortion for a direct detected signal.
2. The optical transceiver module of claim 1 wherein the EDPC filter circuit comprises a finite impulse response filter comprising at least three taps, amplifiers/attenuators adjusting the tap weight, and an electrical combiner to combine the signals from the taps.
3. The optical transceiver module of claim 2 wherein the at least three taps are 5 taps, 7 taps, or 9 taps.
4. The optical transceiver module of claim 2 wherein amplifiers/attenuators are field programmable.
5. The optical transceiver module of claim 2 wherein the EDPC further comprises an integrated driver for the output from the electrical combiner, the driver output power rating being sufficient to directly drive the interference modulator.
6. The optical transceiver module of claim 1 wherein the EDPC effects an approximate Hilbert transform on the analog signal.
7. The optical transceiver module of claim 1 wherein the predicted distortion corresponds to a multiple of half the maximum correction length provided by the receiver.
8. The optical transceiver module of claim 7 wherein the distortion pre-correction is selected from a pre-stored collection of tap weights wherein members of the collection represent different multiples of half the maximum.
9. The optical transceiver module of claim 7 wherein the EDPC also effects an approximate Hilbert transformation.
10. The optical transceiver module of claim 1 wherein the PAMn DSP is a PAM4 chip designed for a MSA standard of Table 1.
11. The optical transceiver module of claim 1 wherein the PAM optical signals convey (m≥1)*50 Gbps per wavelength⋅channel data streams and the module is functionally housed in a standard QSFP28, DD-QSFP, OSFP, CFP2, or CFP8 pluggable package.
12. The optical transceiver module of claim 1 wherein the interference modulator is a DDMZM and wherein two modulator signals are provided from the EDPC to the two DDMZM arms.
13. The optical transceiver module of claim 1 wherein the interference modulator is an IQ nested MZM and wherein four modulator signals are provided from the EDPC to the four MZM arms of the IQ nested MZM.
14. The optical transceiver module of claim 1 further comprising a fixed dispersion-compensation module to provide a range of applicable distances having a maximum distance longer than the span of the range.
15. The optical transceiver module of claim 1 wherein the EDPC performs an approximate Hilbert transform on the analog signal and distortion pre-correction based on a predicted distortion corresponding to a selected transmission length, wherein the EDPC outputs a signal proportional to m(t)+{circumflex over (m)}(t) and a signal proportional to m(t)+{circumflex over (m)}(t), and wherein the EDPC filter circuit comprises a finite impulse response filter comprising at least three taps, amplifiers/attenuators adjusting the tap weight, and an electrical combiner to combine the signals from the taps for each of the m(t)+{circumflex over (m)}(t) signal and the −m(t)+{circumflex over (m)}(t) signal.
16. The optical transceiver module of claim 1 wherein the EDPC provides for a maximum fiber length that is the sum of the EDPC pre-compensation length and the maximum compensation length of the PAMn DSP.
17. A method for determining the tap weights of an analog transversal filter circuit configured to condition an analog modulator signal from the PAMn processor to perform an approximate Hilbert transform, dispersion pre-compensation, or both to form a conditioned modulator signal, the method comprising:
iteratively correcting the tap weights, based on a weighted average of errors from a plurality of fiber lengths, involved in providing a modulated output optical signal to improve the dispersion for a composite range over the plurality of fiber lengths transmitting the optical signal for direct detection at a receiver wherein the procedure is performed based on a set of selected fiber lengths in the desired range specified as (L 1 , . . . , L N , N>1).
18. The method of claim 17 wherein the iterative corrections are performed through error vector magnitude measurements of an optical receiver positioned at the target distance of each fiber length.
19. The method of claim 18 wherein an optimization metric is evaluated by sending a signal for a test of the tap weights through a set of fibers with the selected lengths (L 1 , . . . , L N ).
20. The method of claim 18 wherein an optimization metric is evaluated with a programmable dispersion compensator based on test tap weights and the selected lengths (L 1 , . . . , L N ).
21. The method of claim 17 wherein tap-weight settings are determined for multiple distinct composite ranges of fiber lengths and said the settings for each length composite range are stored in an optical transceiver's digital memory as members of a collection for operative selection.