Re: 10 Gbit Ethernet PMD
Patrick Gilliland wrote:
> Rich, Ron, et. al.,
> No doubt there has been considerable development
> in the technology of multi-amplitude-signalling
> since '76. However, the fundamental and practical
> issues of laser physics trump some of the signal
> coding theories.
The flip side of the argument is that traditional optical communications links are
among the last holdouts to fall to modulation methods more efficient than binary
signaling. For all telecom and datacom applications to date there has been no
reason to deviate. Fiber (SMF) has unlimited bandwidth for all practical purposes.
However, 10 Gbps datacom applications require low cost, not just cost effective
optical communications links. This is the primary reason for investigating
alternative modulation methods.
I don't understand your comment about fundamental and practical issues of laser
physics. Semiconductor lasers exhibit excellent small-signal modulation
performance and are generally limited in practice to a bandwidth of 10 GHz because
of electrical parasitics. However, since semiconductor lasers are almost
exclusively employed in optical communications links and binary modulated,
small-signal performance is not applicable and only large signal performance is
relevant. Tradition certainly has its place, but...
> First, one must observe a population of laser diodes
> and their Power .vs. Current (P-I) plots. In doing so,
> it will be seen there are significant differences in the
> threshold current, slope efficiencies, linearity,
> saturated power, LED transition region, etc. This is
> why AM schemes in the past have always been baseband,
> bi-level modulation. It allows the designer to make a simple
> offset correction for the lasing threshold current. One can
> then pick two levels, a "1" and a "0" light level which can
> be set via controlling the peak modulation current.
A simple offset correction to set bias above threshold is generally chosen by a
designer of an optical transmitter employing binary signaling. Conversely, a
designer of an optical transmitter employing multilevel signaling may choose
instead to set laser bias around average optical power and then employ simple
optical intensity modulation via a modulation current to set n levels above and
below the bias point. For example, for PAM5, one could set 2 levels above and
below the bias point. The lowest level could correspond to a "0" light level for a
binary signaled system and the highest level could correspond to a "1".
> In the present GbE specification, there was not even a consensus
> to hold the mean power output of the laser transmitter tighter
> than a 6dB range. This does not bode well for any type of MAS
> laser transmitter. The case of an 850nm laser and mulimode fiber
> creates an additional degradation mechanism. VCSELs transmit an
> optical spatial mode which is dependent on forward current. At
> lower current levels near threshold ("0"), a smaller number of
> spatial modes are excited. At higher drive current levels ("1"),
> the laser may be fully moded and will begin to saturate in it's
> P-I performance. These modes may also be slow rise time modes or
> fast rise time modes depending on the polarization (see "High
> Speed Characteristics of VCSELs" by Jim Tatum, David Smith, Jim
> Guenter, and Ralph Johnson). The coupling lens will have some
> mode dependent characteristics as will the fiber.
I certainly don't want to give anyone the impression that I'm an optical expert,
and I fully understand that there is a significant amount of research to be done
in the area of multi-level optics. However, my gut tells me that multilevel optics
is an engineering exercise with no real "walls" or "cliffs" in sight. In addition,
tools such as closed-loop full-duplex feedback, virtually unlimited functionality
afforded by CMOS usage within a transceiver, modulation methods which reduce
direct modulation rates, etc. greatly simplify the engineering exercise. I warmly
welcome optical experts to join in with protocol and systems experts to arrive at
well engineered multilevel optical communications link.
> Therefore, the use of multimode lasers such as VCSELs becomes very
> difficult in a MAS encoding scheme. This is due to the differential
> delays experienced by the central axial modes and the higher order
> modes at full stimulation when coupled into a multimode fiber. This
> type of behavior also is well known in simple Fabry-Perot edge emitting
> lasers except for the polarization issue. I do not see a way to
> mitigate these characteristics of the multimode laser or inexpensive
> FP laser when coupled into multimode fiber using MAS modulation.
> Despite the many advantages of bandwidth reduction which we all agree
> would be desirable, I do not see it as a practical solution on multi-
> mode fiber.
You're over my head with the above explanation. If you can, please translate it
into an effect which affects the multi-level eye.
> I believe Ron has made a significant point with regard to
> real world transmitter Signal to Noise Ratios (SNR).
> My initial estimates for transmitter S/N ratio show something
> on the order of a 7dB degradation for a PAM5 system. If the
> transmitter S/N is sufficient, the dominant factors could be
> elsewhere in the link. However, this is not usually the case
> with a multimode VCSEL. Transmitter SNRs of 13dB-15dB and lower
> are common with many real world VCSEL multimode devices. If MAS
> were to be workable I would certainly suggest single mode fiber and
> Distributed Feedback Lasers (DFBs) as a starting point.
My calculations for SNR reduction due to PAM5 are 6 dB electrical and 4.1 dB
optical. Please explain the difference.
PAM5 is a modulation method which is independent of laser type. It's clear that a
"clean" multilevel system can be constructed with DFB/DBR lasers and a matching
single mode fiber. However, I see no reason whatsoever to exclude any other
reasonable combination of laser and fiber. Reasonable means good small-signal
performance, reasonably low noise, etc.
> MAS laser encoding does sound like an interesting research program
> which may bear some fruit and I would be willing to contribute in
> any small way I might to developing the concept further. Perhaps what
> is needed is a working demonstration of the technology. Any volunteers?
Anyone with bare bones lab equipment and a high-speed Arbitrary Waveform Generator
can put together a working MAS prototype system and experiment with various lasers
and fibers. I don't know of an AWG which can operate at 5 Gbaud, but you can get a
pretty good idea of Gbaud range multilevel operation with this setup. I don't want
to plug a specific vendors product over this reflector so please do your own
search for "Arbitrary Waveform Generator" on the web.
> Pat Gilliland
> At 11:19 AM 11/17/99 -0800, you wrote:
> >Ron, Pat,
> >Virtually all communications links have made great strides to ensure
> reliable high
> >speed information over all media including optics, copper and wireless. If
> >optical systems are in their infancy with respect to cost effectively
> transport huge
> >amount of data reliably. The principal thrust behind multilevel optics is
> to cut the
> >cost of an optical transport which has very high data rate requirements.
> The IEEE
> >802.3 HSSG has now heard of at lease 4 general options to go this:
> >1) Reduce to cost of serial binary signaled systems by several orders of
> >(e.g. 1 bit/baud such as OC-192);
> >2) Demultiplex the data by a factor of n for transport across n individual
> >links (e.g. parallel optics);
> >3) Demultiplex the data by a factor of n for transport across n
> wavelengths using n
> >O/E sets and a single optical links (e.g. WWDM);
> >4) Encode the data into multiple bits/baud or multiple subcarriers for
> >across a single serial link (e.g. PAM, QAM, FSK, etc.).
> >I have proposed what I believe to be the simplest multilevel modulation
> scheme for
> >the 10 Gbps data transport. The tradeoffs are simple: Cut the line rate by
> 1/2 and
> >use 5 levels. Of course this results in SNR penalty of 4.1 dB optical, but
> >penalty can be easily recovered by other tradeoffs in a multilevel optical
> >including intelligent closed-loop full-duplex link design, Forward Error
> >FEC code, careful engineering to minimize laser noise (e.g. RIN, etc.).
> All of these
> >techniques are well known and/or straightforward and used in most existing
> >communications links.
> >I'd like to get a bit more specific as to exactly where the "walls" are
> with respect
> >to a multi-level optical link. I do see plenty of hurdles, but I also see
> >potential benefits from overcoming those hurdles and the means to do it. I'll
> >intersperse some more comments in the notes below.
Richard Taborek Sr. 1441 Walnut Dr. Campbell, CA 95008 USA
Tel: 408-370-9233 Cell: 408-832-3957 Fax: 408-374-3645