Cable-Handling

The designer of the under sea telemetry system must be confident that all of the essential components making up the system are available and proven. These include cables, connectors/couplers, transmitters/receivers and cable-handling equipment. An essential part of the
handling gear is a slipring or rotary joint in the winch to permit continuous operation of the under water system regardless of how much cable has been deployed.

Once the decision has been made to use fiberoptics, it is desirable to maintain the advantages of fiber through the complete telemetry system. This requires a multi-pass fiber optic rotary joint (FORJ) such as the Focal Model 190, which has been in service for over 10 years. It is usually combined with electrical sliprings for power. Until recently this product has been suitable only for multi-mode fibers that systems using single-mode fiber were generally required to convert the optical signals to electrical within the winch drum. This method permits the use of an electrical slipring but introduces some risk of signal degradation. It also places electronic components in an un-friendly environment and a difficult location for servicing. Conclusion

The trade off between multi-mode and single-mode fiber optic telemetry is fairly complex. Multi-mode has the advantages of easier interconnection due to the larger core, lower cost components and generally adequate specifications for most under sea applications. Single-mode has the advantage of even greater bandwidth and range specifications, reduced attenuation and perhaps most importantly less micro-bending loss under pressure. Designers should work with suppliers of all sub-system components, including the rotary joints. The building blocks are in place to allow the adoption within confidence of either.

Fiber Optic Equipment Components

Lightsources

As stated earlier, the two light sources available are the semi-conductor LD and the LED. Both devices have small physical dimensions, which make them suitable for optical fiber transmission. As the term diode suggests, the LDs and LEDs are pn junctions. Instead of being made from doped single crystals, they now have exotic combinations of two or more single-crystal semi-conductor materials. These hetero junctions are consequently called hetero-structures. The fundamental difference between an LD and an LED is the fact that the light from an LED is produced by spontaneous emission, where as light from an LD is made by stimulated emission. This results in the laser having an out put that is coherent and therefore has a very narrow spectrum, where as an LED has an incoherent output and a wide spectrum. The selection of an LD or LED for an optical transmission system depends upon the following factors:

	Required output power
	Coupling efficiency
	Spectral width
	Type of modulation
	Linearity requirements
	Band width
	Cost

LED.

The semi-conductor LED can be used in the surface-emitting or edge-emitting mode depending upon the type of fabrication. The surface-emitting style has good temperature stability and low cost. However, the coupling efficiency into the fiber is limited by its wide active area. The light power coupled from a commercially available LED into a single-mode fiber is about 100uW. The light power output is incoherent (i.e., the output is spread over a wide spectrum of about 40nm). The operational bitrate is limited by the parasitic capacitance with in the LED.

The edge-emitting LED has improved performance compared to the surface-emitting type. The structure can achieve a higher coupling efficiency into a single-mode fiber, and the narrower active layer compared to the surface-emitting style has a smaller capacitance, which allows higher bitrate operation. The low cost and improved temperature characteristics of the edge-emitting LED compared to the LD have stimulated a lot of research to improve the devices so that they can be used for fiber- in - the -loop(FITL).

For an LED to achieve are a sonable transmission distance (more than 10km between regenerators) at high bitrates (more than 622Mb/s), single-mode fiber, operating at the zero dispersion wave length, must be used. Multi-mode fiber operation significantly reduces the bitrate-regenerator distance product.

Structure and emission modes of al ight emitting diode.

Laser Diode.

The LD has evolved extremely quickly over the past decade. The development of the LD is central to the present-day long-distance capability of optical telecommunications. When the current density within the active region of the diode reachs a certain level, the optical gain exceeds the channel losses and the light emission changes from spontaneous to stimulated (i.e.,lasing). the threshold current which this occurs is quite low in the double hetero-structure semiconductor lasers and is typically 5to15mA at25°C. This is a very undesirable characteristic because it means that the drive current must be increased as the temperature increases in order to maintain a constant output power. The internal power dissipation within the diode itself contributes to an increase in temperature, so a runaway situation can occur if some form of temperature control is not used. Also, aging deterioates the laser performance. Furthermore, the wave length of the optical output is also temperature dependent. To counteract these problems, it has been non-practice to mount the LD on a Peltier-effect thermoelectric cooler with a feedback circuit to stabilize the temperature, and another circuit is included to maintain a constant drive current.

Recentn advances in laser technology using indium phosphide( in P) have resutled in LDs that do not require cooling. This is significant, because coolers are not only expensive but also require considerable power. Cooler-free lasers are very attractive for undersea link applications. There are three major types of LD:

	Fabry-Perot
	Distributed feedback
	Distributed Bragg reflector

 

Summary of the LD and LED sources. The LED has the following advantages when compared to the LD:

	Higher reliability
	Simpler drive circuit
	Lower temperature sensitivity
	Immunity to reflected light
	Low cost

these characteristics make LEDs suitable for short-distance applications. They are particularly attractive for LANs and subscriber loops where economy is a very important factor.

Characteristics of the laser diode.

Optical fiber is gradually finding application in all aspects of telecommunication systems. The three broad categories, which relate to short, medium, and long distance, are referred to as:

1. Local loop (for subscribers) and LANs

2. Inter office (or interexchange) traffic
3. Longhaul ( inter city traffic)

The distance, capacity(bitrate), and topology are the primary factors that influence these systems designs and the associated economic viability of constructing and operating them. Also, the extent to which optical fiber is deployed depends on the cost relative to traditional copper-based cable. Optical fiber has already displaced copper for long-haul and inter office traffic, and it is only a matter of time before the local loop becomes fiber. This chapter addresses the various technical factors that need to be taken into account to realize each of the above three system categories.

First, the optical fiber transmission systems technology has been evolving along two different paths:

1. Intensity modulated systems

2. Coherent systems

There have been several generations of systems so far. The early systems, before about 1990, did not have the luxury of optical amplifiers, and unregenerated distance was a major cost concern because regenerators were expensive. Optical amplifiers radically changed the landscape of fiber systems, and have now shifted the focus of attention to capacity(that is, bitrate).

First generation systems were primarily limited in regenerators pacing and bitrate by high fiber loss and excessive chromatic dispersion in the fiber caused by the use of LEDs. The development of 1.3um wave length fiber systems produced the improved regenerator spacing and bitrates of the second-generation systems. These systems were still operating with multi-mode fibers, which limited the performance by interference between the propagating modes of the fiber (i.e.,modal dispersion). The move to single-mode fibers operating at 1.3um gave the third generation a very impressive regenerators pacing and bitrate performance. The fourth generation benefited from shifting the operating wave length to 1.55um, which offers the minimum achievable attenuation for silica fibers.

The fifth-generation (which appeared in the early 1990s) included optical amplifiers, and today these systems vastly increase the distance between regenerators. Notice the optical amplifier is an optical repeater, and not are generator that involves conversion to the electronic doma in for pulse reshaping and retiming. The sixth-generation is WDM, and these systems became practicable in the mid-1990s. Placing multiple OC-192 bits treamsat different wave lengths on to each fiber allows terabit-per-second through put.

So far, all of the systems described use intensity modulation of the optical transmitter. The seventh generation will use phase-modulation of the optical transmitter, and a coherent detection scheme, which improves the receiver sensitivity. The very narrow spectrum distributed-feedback diodes used in these systems will allow very high operating bitrates. In addition, the coherent optical communication systems will allow FDM, which will give an astounding increase in the channel capacity because of the more efficient use of the available optical band width. The case in favor of using the coherent optical system in future designs appears to be clear-cut, but cheaper WDM designs will provide enough capacity for the near term. Soliton transmission will be the eighth generation, and will no doubt provide enough capacity for very broadband ISDN systems.

Perhaps one important surprise over the past five years has been the pace at which electronics has progressed to enable higher bitrate generation and processing. Only a few years ago, a bitstream of 565Mb/s was considered to be state of the art, but 10-Gb/s bitstreams have appeared remarkably quickly. This design advance has taken considerable pressure off the more exotic and expensive technical solutions for high-capacity systems. In other words, the development and deployment of FDM and soliton-based systems is not a surgent as it was once thought.

In the design of practical systems, there are many variables to take into consideration. First, what is the nature of the interconnection? There are three categories of interconnection:

1.Point-to-point(link)

2.Point-to-multi-point(broadcast)

3.Multi-point-to-multi-point(network)

So far, the focus of attention has been on the point-to-point link, and it has been assumed that the ever- increasing hunger for more band-width inevitably leads to the need for coherent detection technology. Although coherent systems are technically more elegant than the structurally simpler direct-detection systems, cost does not yet favor the coherent technology. In fact, the case for employing direct-detection systems is still very strong. When monolithic photonic integrated circuits reach maturity, the cost of coherent detection systems should be low enough to make it the dominant technology for all of the above applications. Until that day, the presently cheaper direct-detection systems will no doubt continue to have wide spread use.

Eight generations of optical fiber
communication systems.

Intensity Modulated Systems

Most optical fiber communications systems presently use the intensity modulation technique. As already stated, this is simply a non- off transmission, whereby the light from the optical source produces 1s or 0s by the light being on or off, respectively. This can be done by directly switching the source on and off, or externally blocking the source to form 1s and 0s. If a laser diode has its current changed from zero to its operating value, the frequency of the output from the laser changes by a small amount. This chirping becomes a very significant problem for coherent detection systems using PM or FM.

 

<Prev>