EO/OE converters have their own calibration parameters that define the relationship between their electrical and optical phase planes. The calibration parameters for EO/OE converters are stored in EEPROM such that the system can automatically reach absolute calibration during link initialization.
Picosecond precise EO/OE determinations.
In this section a method is given with which photo-diode receivers can be calibrated with picosecond precision. This method can than be used to bootstrap the delay calibration of SFP receivers and transmitters. This is not an entirely straightforward task given the SFP electrical connectors. In order to facilitate this task a special Multi SFP crate was developed at Nikhef.
Theory
A Mach-Zehnder amplitude modulator (MZM) is used for the delay calibration. In this device an incident optical field is split and sent into two parallel arms of an optical waveguide interferometer. In one of the arms the optical field interacts with a radio frequency (RF) electric field, co-propagating with the optical field at approximately the same velocity, resulting into nearly instantaneous modulation of the optical field’s phase through the electro-optic (Pockels) effect. At the end of the interferometer the two optical waves are recombined, leading to amplitude modulation of the emanating optical field. Note that the MZM is essentially a four-port device with two optical and two electrical ports, corresponding to the ends of a bidirectional optical and a bidirectional electrical waveguide, respectively. A photo diode receiver detects the amplitude modulation of the optical wave and converts it back to the electrical domain. The nearly instantaneous interaction in the MZM from the electrical to the optical wave is used to accurately determine the time delay between the optical reference plane and the electrical reference plane of the opto-electrical receiver.
Figure 1: Set-up for the determination of the optical to electrical
delay ΔOE. The reference planes for the time delay determination are
indicated by the blue dashed lines.
Figure 1 shows the schematic of the set-up to determine the OE delay, ΔOE, of a receiver. The measurable delays D1..D4 between the various reference planes in this device are given by
D1 = ΔEA + ΔEB |
---|
D2 = ΔOA + ΔOB |
D3 = ΔEA + ΔOB + ΔEO + ΔOE |
D4 = ΔEB + ΔOA + ΔEO + ΔOE |
where ΔEA, ΔEB, ΔOA, ΔOB are delays from the four MZM ports to a virtual
reference plane R, and ΔEO, ΔOE are internal EO and OE delays of
respectively the MZM and opto-electrical receiver.
Combining the equztions for D1..D4 yields
ΔEO + ΔOE = (D3 + D4 - D1 -D2) / 2 |
---|
Assuming that de delays D1 to D4 can be measured, and under the assumption that ΔEO is very small and can be measured or estimated from the physical dimensions and composition of the MZM traveling wave device, ΔOE can be determined from this equation. Its uncertainty is determined by measurement uncertainties and the uncertainty in ΔEO. Once calibrated, the known delay ΔOE of the receiver can be used to determine an unknown EO delay ΔEOt of a transmitter, by direct EE delay measurement of ΔEOt + ΔOE.
Documentation and Tools
A detailed description of the experimental setup and meaurement results
can be found in the article below:
Measurement of optical to electrical and electrical to optical delays
with ps-level uncertainty
published in Opt. Express 26, 14650-14660 (2018)
Acknowledgement
Part of this work is funded by ASTERICS European Commission grant no
653477.
Support was received from NWO and STW.
Copyright
Copyright
Material in this section was copied and adjusted from arxiv.org/abs/1802.07567 and is available under the Creative Commons Attribution-ShareAlike 4.0 International License .
SFP calibration
The absolute calibration of SFPs using the method described above is work in progress and the authors hope that further methods will be documented here in the future, with the aim to come to a traceable calibration method.
Contacts
Peter Jansweijer, Henk Peek, Tjeerd Pinkert, Jeroen Koelemeij
Status
Date | Event |
---|---|
24-02-2016 | Start of project |
17-07-2017 | Made this sub sub-wiki page |
25-05-2018 | added theory overview and article |
26-05-2018 | create a slight reorganisation placing the overview and article in the bigger picture |
Last updated: 25 May 2018