Dear Colleagues,
I have received a reason behind one negative vote: "I would like to see 850nm wavelengths included in the PMD, per Ramana’s proposal.” I appreciate the feedback. Please, send the responses to the reflector, so we can use it for consensus building in a common discussion.
Next I am going to elaborate my technical opinion about including 850nm in the PMD: - Reliability topic:
- With wavelength 980nm, we can use Ibias = 7 mA or even higher, e.g. 8 mA, w/o reliability limits in high temperature, so the implementation has headroom. This is important for a good standard.
- For using 850nm, in order to meet reliability requirements marginally, we have to limit the bias current to ~5 mA.
- The bias current limitation is necessary to meet the automotive mission profiles based on wear out reliability mechanisms, which is also the main cause today of field returns in data-center applications.
- The VCSEL manufacturing industry has been improved for low-defect substrates to reduce the random failures. Experience reported for 940nm devices:
- Experimental results of up to 50 Gb/s transmission at extreme temperatures have been reported for 850nm and 980nm VCSELs in deep detail. Transmission characteristics are different depending on the VCSEL wavelength:
- This bias current limitation produces several effects:
- With 5 mA, OMAvcsel is reduced about 1.5 dB for the same ER=4 dB (based on experimental data obtained at Tbs = 125 ºC)
- Additionally, with 5 mA in TX, the RX sensitivity OMApd is about 1 dB worse, so link budget is reduced 2.5 dB total. Numbers are valid for 25 and 50 Gb/s. For 10 Gb/s and below the RX sensitivity would be less affected.
- OMAvcsel may be compensated by extra ER. Ramana suggested 5 dB. My calculation is ER > 6 dB. For me this is unrealistic at 125ºC with the VCSELs I tested, specially at 50 Gb/s PAM4. Increasing ER would affect the RX sensitivity to be even worse (due to non linear distortion), specially at 50 Gb/s.
- Someone may use FFE and more DSP in RX side … but this has limitations, and power dissipation will increase.
- Link budget for 980nm was presented: https://www.ieee802.org/3/cz/public/3_aug_2021/perezaranda_3cz_01a_030821_link_budget_proposal.pdf
- Calculate yourself the effect of removing 2.5 dB.
- Therefore, IMO, in case of 802.3cz, it is clear that TX and RX characteristics depend on TX wavelength. However, I believe that in case of .3db this was not necessary the same, because there is no reliability constrains as in automotive application.
- In .3cz, the specifications should be different (different OMA in TX and RX, different ER, different TDECQ, etc) depending on the TX wavelength, so the PHY types would be different depending on the wavelength, 850 or 980nm. In other words, having 850 and 980 means having 2 different PMDs for solving the same problem. Similar thing happens for 1310nm: the TX and RX characteristics are different, so the PHYs and PMDs are different, despite the proposal for broadband receivers.
- In summary, in case of 802.3cz, if we accept different wavelengths, then it is equivalent to accept different PMDs for the same problem. Different set of specifications is equivalent to different PMDs / PHY types, which is against the CSD response. Therefore, IMO broadband receiver is NOT in the scope of our work, unless CSD responses are revised.
- Assembly topic:
- Should the wavelength be selected only based on reliability or are other parameters that shape the wavelength selection? — No, there are other considerations, which have been already analyzed:
- Pluggable and AOC devices are not valid for automotive: cost, lack of volume scalability, no standard processes, not automated assembly processes (it would not pass automotive qualification auditory), EMC constraints, power consumption, space requirements, etc.
- Automotive and data-center are very very very different applications.
- OEMs and Tier1s expect from PHY vendors single component PHY chips that are assembled in the ECU PCB using standard processes of pick and place and reflow soldering.
- For the optical PHYs, everything needs to be integrated in a single chip: electronics, photonics, optics, fiber alignment, etc. and this component needs to be assembled in a fully automated line. In this point, flip-chip assembly option is crucial because it allows to drastically reduce the assembly time for achieving a given chip positioning accuracy. Flip-chip is not a demand for signal integrity, but for assembly cost reduction. Nevertheless, flip-chip makes easier design for signal integrity as everyone knows (e.g. return loss in high speed lines).
- Flip-chip assembly requires of light emitters with back emission and photodetectors with back illumination. For doing that, substrate transparency is necessary. The industry has done several attempts for solving flip-chip with 850nm devices (e.g. substrate back etching), however it has not resulted in a suitable process in terms of cost and quality.
- Someone may think on 940nm as a valid option (see contribution), because reliability levels would be similar to 980nm. However, flip-chip assembly is a problem with 940nm due to lack of transparency in the photodetector substrate.
- Broadband receiver topic:
- Broadband receiver proposal has been presented twice:
- I will summarize reactions and requests (not satisfied by proponent) to the broadband receiver proposal already presented:
- There is no doubt about the existence of SWDM PD. However, are they suitable for 802.3cz (automotive)?
- SWDM PD broad spectral response is not just a matter of AR coating. It is also a matter of photons absorption, InGaAs doping, band-gap engineering, vertical structures, etc.
- What is the availability of such kind of photodetectors developed for SWDM and which kind of technology is used to overcome the limitations of conventional GaAs and InGaAs photodiodes
- How many providers are in the market with this kind of PD technology? — Name of the broad band PD providers was requested to be given to the TF
- Maturity level: this technology is very recent compared with InGaAs PDs. What is the experience of the industry about aging and PVT variations?
- Is this PD technology suitable for automotive application? — Internal structure, compressive strains, lattice mismatches, failure modes, operation temperature range, reliability testing, qualification
- What is the relative cost of SWDM PD with respect to widely available and mature InGaAs PDs?
- By specifying the receiver has to be sensible between 840 and 990 nm, it precludes flip-chip assembly option, which has been argued as advantageous for automotive application.
- Standard die and wire bond assembly is more expensive than flip-chip when positioning accuracy is required
- What is the value proposition of broad band PMD for automotive application?
- Validation, PVT characterization, qualification, PD wafer-sort, and PHY final-test are going to be more expensive, because reception along the full wavelength range have to be tested. Why more expensive devices, qualification and production tests would be good for automotive application?
- A solution based on OM3 + 980nm VCSEL have been demonstrated to be optimal in terms of technical and economical feasibility and complete. 980nm VCSELs are easier to manufacture, with lower relative cost and bigger number of suppliers (https://www.ieee802.org/3/cz/public/may_2021/king_3cz_01a_0521.pdf ). Why does P802.3cz need a broad band PMD?
- There is no 850nm backward compatibility requirements in automotive application. We are free to choose the right wavelength, specially because fiber link is limited to 40 meters. Why do we have to choose more than one wavelength?
- Do we think market segmentation originated by multi lambda transmitters is positive for automotive?
- Do we think OEM is going to be happy with the extra cost of supporting broad band PMD?
- Most important: Are we solving a real OEM necessity with broad band PMD proposal?
I am sorry for the long response, but wavelength is not an easy topic in this project. There are a lot of arguments that we, the proponents of PMD based on OM3 + 980nm VCSEL, have considered. I hope you appreciate the effort to put all this information together; I did it to make easier to follow arguments and relevant contributions.
Thank you and best regards,
Rubén Pérez-Aranda KDPOF
Dear Colleagues,
Today, in the 802.3cz Interim Meeting we have succeeded in the adoption of 4 baseline proposals (i.e. 50G PCS/PMA, EEE/LPI, Loopback modes, and BER test mode). However, the most important motion, the one to adopt a PMD failed and most of the negative voters did not provide any reason for explaining their vote, and what for me is more important, they did not indicate what should be changed in that PMD baseline proposal to be accepted by them.
Today we voted the only one PMD proposal presented until now that is technically complete, fulfills the 100% of the objectives and is 100% consistent with the CSD responses. The other PMD proposals (i.e. GIPOF and Si-Photonics) are not technically complete (at least today), and either do not fulfill none of the objectives or only some of them. I understand that someone can vote against option “A”, when he/she offers an option “B”, being both A and B complete and consistent with the objectives and CSD. However, I can't understand a negative vote against option A with nothing to offer (at least today) and with no arguments.
Which part of the motion text and referenced contributions should be changed, with proposed remedy, in order the motion being accepted? Should the PMD based on OM3 + 980nm VCSEL not be included in 802.3cz draft and the task force should wait until other PMD options are complete and meet (new) objectives? Why (technical reason)? Should we have also included the other PMD options in the motion, even if they are incomplete and inconsistent with objectives, going against CSD? Did the motion text or the referenced contribution prevent the adoption of future PMD options like GIPOF and Si-Photonics (provided the objectives and CSD are revised first)?
Therefore, I kindly ask the following individuals to provide arguments for supporting their negative vote against the motion #3 for PMD adoption of today. These arguments may be used to identify gaps in understanding the procedures and building consensus in the future.
- Tadashi Takahashi, Nitto Denko Corporation
- Kazuya Takayama, Nitto Denko Corporation
- Kenji Yonezawa, AGC
- Hidenari Hirase, AGC
- Yuji Watanabe, AGC
- Satoshi Takahashi, POF Promotion
- Ichiro Ogura, Petra
- Taiji Kondo, MegaChips
- Hideki Isono, Fujitsu Optical Components
- Kenneth Jackson, Sumitomo
Thank you and best regards,
Rubén Pérez-Aranda KDPOF
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