Optical Switch Applications

Optical switches are an integral part in fiber optic transmission systems and contribute to the development of the “all-optical” network.  The AV market can adapt these telecom components to enhance the capabilities of the overall AV transport system.  There are two basic types of optical switches – the O-E-O (Optical to Electrical to Optical) and the O-O-O, or all-optical switch.  Each has its place in fiber optic systems with their unique features and capabilities.

The fiber industry’s first optical switch was a simple mechanical 1×1 on-off switch using a moving fiber typically activated by a small DC control voltage.  Since then a number of switch variations have evolved including the more popular and versatile 1×2 switch.  This switch is used as a building block to develop more complex switching systems for optical routing and broadcasting.

NxM Matrix Switch – One of the most common switches for the AV market is the OEO switch.  This switch uses standard telecom optical SFP (Small Form Pluggable) transceivers on the inputs to convert the incoming optical signal to its native electrical digital data stream.  Similar optical transceivers are used on each of the corresponding outputs to convert the switched electrical data stream back to an optical signal for further transmission.  The switch matrix is electrical and includes other functions such as reclocking and retiming to help clean up the signal and return it to its original condition, discounting any conversion-related anomalies.  Most of these switches are multi-rate and can support a wide variety of data rates from approx 5 Mbps up to 4.25 Gbps, and some even higher.  However, the reclocking functions are generally performed at very specific data rates, namely 270Mbps, 1.485Gbps, 2.97Gbps and 4.25Gbps.  Other data rates are simply passed through the switch and regenerated without any retiming, reclocking and reshaping of the electrical signal.

These OEO switches have several features worth noting including the following:

• Point-to-Multipoint (broadcasting) capability
• Fully non-blocking
• Uses Optical I/O (SFP devices) with electrical switch engine

–  Electrical “3R” functions at selected data rates (Recovery, reclocking and regeneration)

• Does not degrade optical signal strength
• Available up to 4.25Gbps (Fiber Channel) data rate
• Available in configurations up to 1024 x 1024

The all-optical switch was introduced in the telecom market some years ago and has evolved to encompass the AV market.  Products such as matrix and protection switches in addition to ROADM (Reconfigurable Optical Add Drop) and other function-specific optical switches are becoming staples in the AV market.  The key feature of an all-optical switch is, as the name implies, that the signal is kept in the optical domain throughout the switching and routing process.  This has clear advantages over the OEO switch in that the all-optical switch is data rate and protocol agnostic, in the AV market space.  That is, any of the standard AV signals (video, audio & data) can be passed through the switch in any format (analog or digital).  In addition, these switches can also send bi-directional data on the same port – something the OEO switch cannot do.  In other words, bi-directional data on a single fiber (one port) can be easily routed in all-optical switches.

Table 1 highlights some of the main characteristics of both the OEO and all-optical matrix switches.

Table 1 – Matrix Switch Characteristics
Electrical-to-Optical-to-Electrical (OEO)	All-Optical (OOO)
Converts signal to electrical for switching	Keeps signal in optical domain
Lossless switching – Retimes, reclocks & regenerates signals (at specific data rates)	Lossy switching – Optical insertion loss from input to ouput
Unidirectional switching	Bidirectional switching
Point-to-point & broadcast modes	Point-to-point & broadcast mode (using photonic multi-casting architecture)
Specific digital date rate formats	Data protocol & rate agnostic
Available up to 1024×1024 (and higher)	Max input/output typically 256×256

1xn Optical Switches – Many people consider fiber a simple replacement transport infrastructure to copper.  This represents only one aspect of the overall capabilities of a fiber system.   Once the signal is in an optical format a number of other switching and routing capabilities exist.  As an example a simple 1×4 or 1×8 optical switch provides routing and broadcast capabilities very easily and much less expensive than an OEO equivalent.  Just as with the all-optical matrix switch, these optical broadcast switches are signal type, data rate and format agnostic.  Any signal that can be transported over fiber can be routed through one of these switches.  An alternative to this is the 2×4 or 2×8 all-optical switch which provides additional capabilities.  Figures 1 & 2 show an example of such switch designs.

Fiber Protection Switch – This type of switch is ideal in mission critical applications where it’s important to ensure fiber continuity between the transmitter & receiver units.  At the transmit location the signal is split into two redundant signals and sent over two diverse optical paths.  This switch design accepts these two optical signals from the same transmitter via the different fiber paths and will monitor the optical activity on each fiber.  One path is set as the primary optical path and will automatically switch to the redundant fiber path if this primary path were to be interrupted or if the signal level falls below a pre-determined optical threshold.

Figure 3 illustrates how such a switch might be configured.  At the transmit location the optical signal is split and sent over two diverse paths to the fiber protection switch, normally located near the receiver location.  The optical signal from each fiber is monitored in the switch and is compared to preset thresholds.  Once the signal in the primary fiber path falls below this threshold, the switch will automatically select the output from the redundant path and send that signal to the receiver unit.  In addition, various alarms are generated to indicate a fiber has failed and the switch has activated.

As with the other all-optical switches, this switch is data rate/protocol agnostic and will monitor and switch any optical signal presented to it.  In addition, this switch design will also switch bi-directional information on the same fiber further increasing its flexibility and versatility for such applications as video conferencing, remote arraignment, video with PTZ control, HD video with audio talk-back, etc.

Reconfigurable Optical Add-Drop Module – One of the more unique optical routers/switches is the Reconfigurable Optical Add-Drop Module or ROADM.  This device combines the features of wavelength multiplexing and demultiplexing with switching capabilities.  In short, a ROADM can drop one or more wavelengths at a given location while the other wavelengths on the fiber continue down the fiber to the next location.  One such application example is with digital signage systems.  Since a number of wavelengths can be transmitted down one fiber, the signals associated with each of these wavelengths can be dropped at a particular digital sign location while the other wavelengths, and even the dropped wavelength, can be sent to all subsequent locations for similar wavelength/signal drops.  In this way messages unique to individual signs can be associated with that sign while other signs can receive their specific message over the same fiber.

Figure 4 shows such an example of how this may be implemented.  Associated with each digital sign (drop location) is a ROADM.  Each of these ROADMs can be remotely addressed to drop a specific wavelength (and associated signals).  This type of architecture provides exceptional flexibility to the fiber network.  As with other all-optical devices, the signal stays in its optical format thus preserving signal quality without the need to down convert to its electrical signal and then back to an optical format for further transmission.

The OEO switch has its place in the AV market and is seeing increased use.  As with any conversion process, there are always some artifacts or anomalies generated when converting from one format to another (optical to electrical and back) leading to increased errors and system degradation.  All-optical switches to not have such anomalies and maintain the signals high quality throughout the switching process.  They also have much higher potential since they are protocol, data rate and direction agnostic.  As shown in this article, the all-optical switches help define the advanced role of fiber in the AV market and further illustrate the fact that fiber is significantly more than just a replacement for copper.  Once the consultants better understand and accept the diverse capabilities of fiber, you will see increased and far-reaching AV applications using this technology and at a lower cost.

This article provides a high-level overview of some of the optical switch types and technologies available to system designers.  Using these and other optical switch products, many other AV applications can be created to provide you with increased system versatility and help move to the holy grail of fiber systems – the All-Optical Network.

Fiber Optics for Correctional Facilities

Fiberoptics plays an integral part in various aspects of the correctional market including:
- Remote visitation within the facility
- Remote arraignment in conjunction with associated courthouses
- Video & audio surveillance within the facility, exercise yards, perimeter security, etc.

Correctional facilities are well-suited for fiber as many of the cable runs can be quite long including both indoor and outside installations. Exercise yards afford an ideal location for fiberoptics, particularly in lightening prone areas. One of the key features of fiberoptics is its immunity to lightening. Reliable video surveillance is of paramount important in correctional facilities. In the presence of lightening, any camera located outdoors is a prime target. Without fiber, the lightening pulse can travel down the copper cable and damage the entire communications/surveillance center. Utilizing fiber for transporting the video from the CCTV to the communications center isolates any lightening-induced pulse to the camera only – thus protecting the communications center.
Fiber also easily allows the facility to transport many signals over an individual fiber. This affords the user the ability to consolidate the signals in a single or several locations and multiplex them over a very small number of fibers – reducing equipment and installation costs.
Remote visitation and arraignment are closely related in that they both allow long distance audio and video communications without jeopardizing the safety of the personnel. These systems also minimize the expenses associated with transporting inmates to locations common with either the visitor or the presiding judge as it would be in an arraignment case.
The quality of video over an IP network is dependent on several factors including the number of nodes and cameras on the network. Latency associated with PTZ control can become an issue in certain situations. However, real-time fiber systems provide live, full-resolution video without the quality constraints of an IP system. At the main security location, the video & audio can be recorded in real time leaving nothing to the imagination or legal interpretation. If ‘networking’ the video is important, the video can be converted to an IP signal after it is received and recorded at the headend location. In this type of ‘hybrid’ system architecture, the quality of the video is maintained as long as possible and practical before it is converted to a less than optimal IP format.
Since fiber is also ‘signal proof’ different quantities and types of signals can be easily multiplexed on an individual fiber. This offers the facility expansion capabilities not available in traditional copper solutions. Fiber’s advantages make it the obvious choice for correctional and courthouse system architectures.

Mecklenburg County Jail, Charlotte North Carolina

Meridian Technologies was chosen as the fiber optic equipment supplier used within the Mecklenburg County Jail facilities in Charlotte, NC for remote visitation within the facility. The systems incorporate bi-directional video & audio for remote visitation. There are presently six visitation stations located at the main entrance to the facility, each with its own video & audio communication channels. The video & audio signals are routed from the main PBX system to the equipment bay located in the access space above the inmate housing units. Each of the eight inmate housing unit has two such visitation stations for a total of sixteen channels of video & audio. There are two of these equipment bay locations, one for each of the four inmate housing units.

A combination of video & audio multiplexers was used to transmit these various signals to and from the visitors’ center to the associated inmate locations. All of the signals are transmitted over standard 62.5/125µm multimode fiber. Due to fiber count limitations, wavelength multiplexing was used to consolidate a number of the video & audio channels onto one fiber. Special care had to be taken in the system design to ensure that the multimode fiber’s bandwidth could support the optical transmission data rate of the equipment. As such, the shorter wavelength (850nm) having a lower overall bandwidth was used to transmit 8 audio channels while the longer wavelength (1300nm) with a high fiber bandwidth was used to transmit the 8 video channels. As a result, the system provided high-quality video and audio over the long distances required while keeping within the bandwidth constraints of the fiber and maintaining adequate optical loss budget.

Expansion capabilities have been provided so that this visitation center can be easily expanded with no impact on the existing fiber system while providing a cost-effective and efficient way to add more remote visitation locations as demand dictates.

The fiber transmission equipment incorporates a number of diagnostic features to afford fast and efficient troubleshooting of the entire system – from camera and microphone to the monitor and speaker. At each location, indicator lights provide indication of the operational status of each incoming signal as well as the condition of the fiber transmitter, receiver and fiber infrastructure.

Multimode Fiber Implications for the AV Market

Because of its history, ease of use and presumed cost efficiency, multimode fiber (MMF) is the most ubiquitous type of fiber used in the professional Audio/Video market.  Graded Index multimode fiber for applications such as communications, CCTV security, audio/video, broadcast, etc, comes in two different physical sizes – fibers with a 50µm or 62.5 µm core diameter.  The outside or cladding has an industry-standard diameter of 125 µm.  The standard representation of these fibers is 50/125 µm or 62.5/125 µm.  The first of these multimode fibers was the 50/125 µm fiber developed in the mid 1970’s.  Shortly afterward the 62.5/125 µm fiber was introduced.  At the time, it was thought that this fiber with the larger core diameter would be more beneficial because more light could be coupled into it from the optical emitter (LED) and that this larger core would be more tolerant of manufacturing tolerances of the fiber itself, connectors, splices, etc.  At that time it was hard to imagine that this large core fiber would have such significant bandwidth limitations in terms of transmission distance.  Europe standardized on the 50/125 µm fiber while North America held on to the 62.5/125 µm version of this multimode fiber.

Due to modulation limitations of the LED, a faster light source, the VCSEL (Vertical Cavity Surface Emitting Lasers) was developed.  Besides providing vastly improved bandwidth values, the spatial distribution of light from the VCSEL also provides for significantly increased coupling efficiency into both the 62.5 & 50 µm core multimode fibers.    There is still approximately a 3dB decrease in coupling efficiency between the 62.5 and 50 µm fibers but the limiting factor now is one of fiber bandwidth for these high data rate signals, not necessarily optical loss budget.

Figure 1 illustrates the basic difference between the core & cladding diameters of both the 50 & 62.5µm multimode fiber.

Introduced a number of years ago, a new version of the legacy 50/125µm is the laser-optimized fiber.  This fiber is designed to have significantly improved performance in only one wavelength range – 850nm.

Table 1 shows the difference in bandwidth and attenuation as a function of fiber core diameter and type of laser-optimized fiber.

Table 1 Multimode Fiber Key Specifications
Fiber Type	Fiber Designation	Nominal Attenuation (dB/Km)		Nominal Bandwidth (MHz-Km)
		850nm	1300nm	850nm	1300nm
62.5μm (Legacy)	OM1	3.5	1.5	160	500
50μm (Legacy)	OM2	2.5	0.8	500	500
50μm (Laser-Optimized)	OM3	2.5	0.7	2,000	500
50μm (Laser-Optimized)	OM4	2.5	0.7	4,000	500

(Note that these fibers are now designated with the OM1 through OM4 types as specified by the ISO (International Standards Organization) standard 11801).  The specifications shown above are typical and will vary somewhat between manufacturers.  The color codes of the fiber jacket (patch cord) are also defined to differentiate legacy multimode fiber from laser-optimized multimode fiber.  The traditional orange colored patch cord jacket material indicates that the fiber is legacy 62.5/125 or 50/125 µm fiber.  An aqua colored jacket indicates that the fiber inside is laser-optimized multimode fiber.

Since most optical transport systems will have an optical budget of 12-20 dB, depending on data rate, it is unlikely that any of these systems will be loss-limited.  In other words, it is more likely that the system will be signal-quality (bandwidth) limited instead of signal-quantity (attenuation) limited.  As such, it’s important to understand how the bandwidth of these fibers affects the maximum transmission distance of high-definition video signals.

As mentioned in a previous application note, the bandwidth of the fiber is inversely proportional to the distance.  For example, if the fiber has a bandwidth of 500 MHz-km, the bandwidth of the fiber at a distance of 1 km will be 500 MHz.  At 2 km, the bandwidth will have been reduced to 250 MHz and at 5 km, the end-to-end bandwidth of that same fiber will now be 100MHz.

Many of the multimode RGB, DVI and HDMI fiber transmission equipment operates in the 850nm wavelength region.  Using a typical CWDM approach of wavelength multiplexing the RGB colors along with the sync pulses, a typical wavelength will have a data rate in the region of 2 Gbps.  This, obviously, depends on the resolution, refresh rate, compression (if any) and will vary somewhat around this number.  One of the unique advantages of wavelength multiplexing in fiber is that each wavelength can utilize the fiber’s full bandwidth capacity.  In other words, if the fiber is capable of transmitting 1 Gbps of data over 1 Km, each wavelength on that fiber can transmit the same 1 Gbps data rate thereby significantly increasing the combined data rate of the fiber.

With this as a reference point, if we assume that each wavelength in a wavelength multiplexed DVI channel is transmitting at a maximum data rate of 2 Gbps, the maximum distance that the signal can be transmitted over multimode fiber can be approximated in the following Table 2:

Table 2 Multimode Fiber Distance @ 2Gbps
Wavelength (λ)	62.5µm (OM1)	50µm (OM2)	50μm (Laser-Optimized) (OM3)	50μm (Laser-Optimized) (OM4)
850nm	160m	500m	2000m	4000m
1300nm	500m	500m	500m	500m

These distances will vary with fiber manufacturer, data rate, video resolution and other extrinsic factors related to the signal transmission.  Notice that the laser-optimized fiber is only optimized in the 850nm region of wavelengths.  While laser-optimized fiber can significantly increase the maximum distance of the signal being transmitted, it’s important to know what wavelengths the transmitter is utilizing.  As the table indicates, using a laser-optimized fiber while transmitting at 1300nm will not help to increase the maximum distance.  However, using 850nm, the maximum distance can be significantly increased by over a factor of 10 just by changing the type of fiber.  Transmitting at 1.485 Gbps, a standard HDSDI signal will have similar distance constraints while a 3G video channel will be limited even further because of the higher signal data rate.

As the data rates of these signals continue to increase, it’s becoming more important to understand both the capabilities and limitations of the fiber infrastructure.  Each fiber type has its advantages and disadvantages and, used properly, can yield a high-performance, high-reliability system that can support emerging technology trends.  It’s incumbent upon all fiber system and equipment designers to understand and properly utilize the capabilities of these fibers.

Multimode & Singlemode Fiber Basics

Of all the differences between multimode & singlemode fiber, the most fundamental differences are the size of the fiber’s core and the associated attenuation or loss and bandwidth of the fiber.  The fiber itself consists of 3  basic portions – the core, cladding and buffer or coating.  The core is the most central portion of the fiber where the light travels.  There are 3 basic fiber core diameter sizes for fiber systems that transport traditional video, voice, and data signals.  Singlemode fiber has a core diameter of nominally 9μm while multimode fiber has either a 50μm or 62.5μm core diameter.  For the most common types of fibers, the cladding is always 125μm while the protective coating has a diameter of 250μm.  Other buffers and jacketing materials help build the fiber up to more practical and rugged cable structures.

The basic rule of thumb is that the smaller the core diameter, the higher the fiber’s bandwidth and the lower the attenuation (loss in dB per kilometer).  The fiber’s attenuation and bandwidth are also dependent on wavelength.  The table below illustrates the approximate attenuation of both multimode & singlemode fibers.

Fiber Type		Multimode	Multimode	Singlemode
Core diameter		50μm	62.5μm	8 – 10μm
Attenuation (dB/km)	850nm	2.5	3.5	N/A
	1300/1310nm	0.8	1.4	0.3
	1550nm	N/A	N/A	0.2

Depending on the data rate and distance of the fiber transport systems they will be either loss or bandwidth limited.  The fiber will either attenuate the optical signal to such a point where the receiver cannot reliably recover the information or the bandwidth of the fiber will distort the signal to where it cannot be recovered even though there is plenty of optical signal at the receiver.

The bandwidth limitations of MM fiber can severely limit the transmission distance of high bandwidth video signals.  Fiber attenuation is one of the basic characteristics of fiber that needs to be understood and taken into consideration when designing any fiber transmission system.

Purpose of the Blog

Good evening,

I’ve started this blog to provide various Tips & Techniques as well as applications for using fiber optics.  My background in fiber optics extends back to the mid ’70s when fiber was used for little more than decorative lamps.  Starting with simple, low speed data links, fiber transmission has extended and expanded to areas of communications never envisioned in its infancy.  This Blog will deal with various existing and emerging fiber optic technologies and how to utilize them in some of these new applications and markets.

As the demand for bandwidth continues to expand, fiber has become the ubiquitous solution for long distance, high speed video, voice and data transmission systems.

As this blog evolves, I hope that you will find it interesting and informative.  As a reader, if you have any fiber optic topics that you would like to be addressed, I’d like you to send your comments and I’ll see what I can do to respond to your inquiries.

Enjoy the blog,

Ed Miskovic