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TUTORIAL: Wavelength Division Multiplexing and Directional Multiplexing

Fig 1: How WDM works

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Fig 2: Circulator

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Fig 3: Fiber coupler

Fig 4: 8-chnl CWDM

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Fig 5: Enclosed WDM

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Optical fibers, especially singlemode (SM) fibers, are among the most underused transmission media commercially available. For example, a single strand of single mode fiber can carry:

>2400 Gbps-DWDM (dense wavelength division multiplexing)-120 chnls
>640 Gbps-CWDM (course wdm and circulator)-32 channels
>320 Gbps-CWDM (course wavelength division multiplexing)-16 channels
>160 Gbps-CWDM (course wavelength division multiplexing)-8 channels
>80 Gbps-CWDM (course wavelength division multiplexing)-4 channels
>80 Gbps-WDM (wavelength division multiplexing+circulators)-4 channels
>40 Gbps-WDM (SM or MM) or circulator technology-2 channels
>20 Gbps-without optical multiplexing-1 channel

In practice, though, the actual bandwidth of a SM fiber carries is less than 1 Gbps in industrial applications, about 1% of its full potential. Undoubtedly, there is a huge untapped reserve for design engineers to take advantage of. We will focus on WDM, circulator, the combination, and CWDM in this tutorial as DWDM is conceptually similar to CWDM and is rarely required beyond long haul telecommunications.

WDM (wavelength division multiplexing-Fig 1)

There are two common technologies used to multiplex two wavelengths in one fiber: fused biconical tapered fiber (FBTF) and free space optics (FSO). FBTF type WDM costs less but offers limited optical performance (~17 dB isolation). FSO offers much higher isolation (>40 dB) and is widely used on both SM and MM fiber. Since it is not always easy to quantify the system requirement on isolation, it is safer to consider FSO only. However, if the quantity is large enough one might want to investigate FBTF devices to reduce cost. The general rule of thumb is: Isolation (in dB) > 3 dB + (optical budget - total loss).

The most common wavelengths for SM fiber based WDM are 1310 and 1550 nm. 850 and 1310 nm (typically LED light sources) are common with MM fibers. The diagram on the left shows the concept of standard WDM.

Circulator (directional multiplexing-Fig 2)

Circulator is the least understood among all multiplexing devices. Instead of multiplexing different wavelength, circulators multiplex data streams with the same wavelength in the opposite directions. This concept may be difficult to understand unless one thinks of a fiber as a waveguide instead of a conducting wire. Waves do not interfere with each other when traveling in the opposite directions within the same waveguide. Therefore, one won't be limited to unidirectional transmission where a different wavelength is not available. For example, a pair of two-fiber media converters equipped with 1550 nm lasers can be connected through one fiber by adding a circulator before each converter (see diagram on the left). Circulators are only available with SM fiber (1310 and 1550 nm). So a 3 dB coupler (Fig 3) is the only alternative in the case of MM fiber. In some SM applications, lower cost 3 dB fiber couplers can also be used as substitutes. The key is that the transmitter is not sensitive to reflection, which is almost always true with MM fiber based transmitters. One can always add optical isolators in SM fiber based systems to eliminate reflection.

WDM + Circulator (hybrid multiplexing)

When the two different devices are used together, one can establish two streams of bidirectional data through one SM fiber. This solution may appear intimidating. However, manufacturers such as Princetel can integrate the devices in a box leaving only the connector ports labeled with port numbers. That will greatly simplify the installation process and eliminate errors. If all possible, use angled connectors in this scheme to avoid crosstalk caused by reflection from non-angled connectors. It is hard to over emphasize the importance of low reflection in this approach. The most common choices are FC/APC, SC/APC, and LC/APC.


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CWDM (course wavelength division multiplexing-Fig 4)

If an application requires more than two bidirectional streams of data through one fiber, CWDM is the perfect solution. Like circulators, CWDM is a SM fiber based technology. The most popular technique is FSO. The dedicated spectral range is 1270 to 1610 nm with 20 nm spacing between channels. 8-channel CWDM across the longer band

(1470, 1490, 1510, 1530, 1550, 1570, 1590, 1610 nm)

is more popular than the one across the shorter band

(1270, 1290, 1310, 1330, 1350, 1370, 1390, 1410 nm).

Combining the two bands will result in total 16 channels. It is also possible, though rarely done, to combine CWDMs with circulators to double the capacity.

All multiplexing devices mentioned above are truly passive, requiring no electricity to operate. Used wisely, they can greatly enhance system capability without upgrading to higher fiber count. Fig 5 is an example of an enclosed WDM device available from Princetel. It is also apparent that SM fibers have much higher bandwidth potential for future upgrade. WDM (850/1310 nm) and couplers are the only technologies suitable for MM fiber.

In practice, CWDM based media converters are not always available. An active device called CWDM Transponder fills the gap by bridging multiple standard media converters and the desired CWDM wavelengths. In other word, CWDM Transponder allows the user to extend up to 16 applications over one pair of single mode fiber using WDM technology. Protocol independence and rate-agility allow CWDM Transponder to accommodate data rates greater than Gbps. Standard media converters from multiple vendors can be tied to the same transponder for single or dual fiber transmission.

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