# Tag Info

60

It seems like you're referring specifically to http://www.nature.com/nphoton/journal/v8/n11/full/nphoton.2014.243.html . It can be read here: https://www.researchgate.net/publication/269099858_Ultra-high-density_spatial_division_multiplexing_with_a_few-mode_multicore_fibre . In this case, it's slightly more complicated than "an optical signal". The link ...

51

This is where the measurement scientist has to go into full sceptical and investigative mode. First thing. Fibre, as a passive material, is lossy. It absorbs power. Therefore the power arriving at the end of a length of fibre will be less than was launched. Period. No arguments. We don't do over-unity here. So what causes your observations? Single mode, ...

43

Rather than worrying about a research paper that's pushing things to the limit first start by understanding the stuff sitting in front of you. How does an SATA 3 hard drive in a home computer put 6 Gbits/s down a serial link? The main processor isn't 6 GHz and the one in the hard drive certainly isn't so by your logic it shouldn't be possible. The answer ...

26

The other answers have suggested some ways your experiment might have gone wrong. Let me tell you how to do a fiber attenuation measurement correctly. The standard technique is called a cut-back measurement. This means you set up your source feeding a long piece of fiber (say, 10 m). You then direct the output of that fiber into a large-area detector (...

17

In addition to the answer of TimB, there is another advantage of this optical communication. With RCA, the two networks connected have to be referenced to each other. In the case of optical, there is galvanic isolation between the two. As a result, there might be less issues with ground loops, networks can remain isolated, etc. It also means that the ...

16

Neil_UK's answer is pretty much spot on, i.e. your measurements are broken. :-( The first and most obvious problem is in the lengths chosen, 1m and 30m: These are both well within the edge effect ranges, i.e. the quality of the fiber end connections will dominate any actual attenuation loss. In particular, good quality single mode fiber at 1300 nm can come ...

15

Ignoring the details of the specific transmission in question (which @alex.forencich has already discussed in considerable detail), it seems like it's probably useful to consider the more general case. Although this particular transmission hit 255 Tbps through the fiber, extremely fast fiber links are already in regular use. I'm not sure exactly how many ...

15

I want to ask, within the scope of digital audio transmission, it is their any observable or measurable differences between the two cables? Actually, yes. Isolation: Optical fiber isn't conductive, so it solves ground loops, hum/buzz issues and any is insensitive to RF interference. Coax can also be isolated with a transformer, however this adds to the ...

14

You are right, this is the case but fiber optics can still have problems that can be perceived as noise that lead to incorrect data: Intersymbol interference: This is a kind of noise because the previous symbol that was sent will interfere with the actual symbol that is being sent. Thus the previous symbol will act as noise. Well known techniques to help ...

8

First, when you talk about the "speed" of a signal in optical fiber, that's ambiguous. You should be clear about whether you're interested in the latency (the time it takes a signal to travel from one end of the fiber to the other) or the bit rate. In this case, it seems most likely you're interested in the latency, or propagation delay. In my opinion if ...

6

Your photo appears to be SFP/SFP+ transceivers. SFP isn't really a connector type, is a transceiver standard. The actual connector (on the switch side) is a board edge connector, the other side can be a wide variety of connectors. If it's the actual fibre connector you're talking about, probably LC connector, which is the optical connector used in just ...

6

One-way Ethernet cables won't work with Gigabit network equipment and later, because without a return path the autonegotiation sequence will never complete. You'll see a "Network cable unplugged" or an equivalent message on both devices if you try to use such a cable. Older Ethernet devices won't work with simple one-way cables either, but can be fooled to ...

5

Depending on the units of the loss coefficient $\alpha$, there are two ways to calculate optical loss in a fiber, or any other uniform medium. For $\alpha$ in units of [1/length], $$\frac{P}{P_0} = e^\left({-\alpha_{1/km} L}\right)$$ For $\alpha$ in units of [dB/length] $${P \over P_0} = 10^{-\alpha_{dB/km} L/10}$$ You can set these two ...

5

If you want "one way data direction", you have to do it at a higher level. Various things assume bi-directional communication at the low levels, even if app-level data is only flowing one way. For example, even if you send data in only one direction over a TCP connection, there will still be packets going back and forth in both directions. You can get ...

5

In the Gigabit Ethernet world, the media access controller (MAC) communicates with the physical layer chip (PHY) through the Gigabit Medium-Independent Interface (GMII) The GMII is an 8-bit-wide interface carrying 1000 Mb/s. So its clock rate is 125 MHz. The Physical Coding Sublayer (PCS) within the PHY performs the 8b/10b encoding. So its output rate is ...

5

Fiber optic does not radiate electromagnetically, but more important is that is immune to electromagnetic interference that can cause data corruption on copper in extreme conditions. Such interference may come from the arcing of a switch being switched off under load, or can be generated by a motor under high load.

4

You could add additional modulation, it would keep the receiver devices ALC features happy. As you have at least 3 MBit/s data rate available you have quite a lot of headroom. You could use basic FSK modulation with two tones of say 250kHz and 1000 kHz. This would let you use a rather simple demodulator (pulse width comparator) and have less than 20% bit ...

4

Since that device includes a PHY for wired ethernet over twisted pair copper, chances are you can't directly attach it to anything but twisted pair copper ethernet. The datasheet lacks any reference to standards, so it's very likely it's been tuned to exactly and exclusively that purpose. Best thing you can do is build it for wired ethernet, than use one of ...

4

The short answer is that for the kind of work you're talking about, an oscilloscope probably isn't a particularly useful piece of equipment. An oscilloscope can be useful for things like designing/testing/measuring the design of a network adapter, to assure against doing things like running (what are supposed to be) separate lines too close to each other so ...

4

The problem here is not the feasibility of the modulation, but the ability for cables to carry high speed signals. Especially with long length, it is much easier (and economical) to carry a high speed signal on en optical fiber than on an electric cable.

4

Do we use a similar filter after the photodiode to get rid of the out-of-signal-space noise, before sampling? Generally, we don't have a separate filter device or circuit. We'd rather connect the photodiode directly to a trans-impedance amplifier (TIA) chip to avoid losses due to impedance matching (the photodiode produces a current signal so we'd rather it ...

3

The primary limitation of the signal bandwidth of optical fiber is dispersion. Dispersion, as the term is used in fiber optics, is when one component of the signal propagates faster than another component. This leads to narrow input pulses stretching in duration as they propagate along the fiber, causing the fiber to act as a low-pass filter on the signal. ...

3

To make proper comparisons between fibre and cable you have to consider the photodiode at the end of the fibre to be part of the fibre and this is the weak link in terms of noise. Typically the Hamamatsu S5973 photodiode produces a noise equivalent power (NEP) of $1.5 \times 10^{-15}$ watts per Hz and given that the device is good for 1 GHz the noise power ...

3

As the question is tagged with "optical-fiber", I assume you mean the damping of the fiber. In this context, both numbers cannot be equal because a damping greater than 0 dB would be equivalent to a damping factor greater than 1. A damping of 0.22 dB would mean that the input power is $10^{0.022} = 1.05$ times higher then the output power.

3

This is somewhat oversimplified, but it gets the basics right. Think of a beam of white light shining into a prism. The output will be a rainbow, which means that the different colors exit the prism at different angles. A prism will work just fine "in reverse". That is, if you take a series of lasers of different colors, place them where their colors ...

3

Yep, high speed photodiodes are the simplest way to do this. A receiver will usually consist of a photodiode and transimpedance amplifier (current in, voltage out). After that, it's just a high speed serial electrical signal, and that gets fed into clock data recovery circuitry, deserializers, etc. There can be components in front of the photodiode, though. ...

3

It is possible on 10BASE-T and 100BASE-TX, but not on 1000BASE-T because the latter uses bidirectional transmission on each pair. To enable such a mode, you need an MDIO/MDC (management) access to the PHY at least at the TX side of the one-way link, to configure it like the following: disable AUTONEG force 100BASE-TX (or 10BASE-T, but not 1000BASE-T) force ...

3

In this context, a span is the cable length between two amplifying stations.

3

SONET is kind of popular in the telecomms world, very different to ethernet, and kind of cool in its way. In reality many non ethernet uses exist, but we tend to try to use rates that are close enough to something used by either the phone company or the datacenter because economies of scale make optical modules for those line rates all kinds of cheap. ...

3

Looking at what's in the SFP modules, it may not be all that difficult to use them directly. Since the signals are AC coupled to LVDS, you'll need to communicate in a DC balanced protocol such as Manchester, which your chosen ARM may or may not support (my current favorite Microchip SAME70 does support it). And, of course, add LVDS receiver/transmitters. ...

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