What are Fiber Optic Transmitters and Receivers?

Fiber Optic Transmitters

An fiber optic transmitter converts electrical input signals into modulated light for transmission over a fiber optic cable. Depending on the nature of the signal, the resulting modulated light may be turned on and off or may be linearly varied in intensity between two predetermined levels. Figure (2) shows a representation of these two basic schemes.

The devices used as the light sources in optical transmitters are Light Emitting Diode (LED) and Laser Diodes. LEDs and Lasers are semiconductors mounted in a TO style can or microlens package that focus the beam of light right into the optical fiber connector. In the case of analog FM transmission, the Laser is “pigtailed” directly onto the surface of the emitter in order to reduce or eliminate back reflection and noise.

LEDs have a wide spectral frequency, are suited for large aperture multimode fibers and are used for short to moderate transmission distances. Lasers, on the other hand, feature a narrow band of wavelengths and can couple many times more power into the fiber than LEDs and therefore are useful in applications that require high speed, high bandwidth over long distances. Lasers are not stable over wide operating temperatures, however, with well-designed feedback circuitry, continuous stable output can be achieved.

Analog modulation takes a number of forms – see figure (3). The simplest is intensity modulation (IM) where the brightness of an LED is varied in direct step with the variations of the transmitted signal. In other methods, an RF carrier is first frequency modulated (FM) with another signal or, in some cases, several RF carriers are separately modulated, then are combined and transmitted as one complex waveform.

Fiber Optic Receivers

The fiber optic receiver converts modulated light coming from an optical fiber back into the original electronic signal applied to the transmitter.

The detector is a photodiode of either the PIN or the Avalanche type and is mounted in a similar package to the one used for the LED or Laser. Sensitivity of the receiver is specified as the minimum signal that it can receive (in dBm). Dynamic Range is the difference between the minimum and maximum acceptance levels. Receivers usually employ high gain internal amplifiers and require special circuitry to avoid saturation or distortion. When the optical dynamic range of the system is equal to the optical power budget, no saturation of the receiver can occur. Quality of signal transmission is equally good at short or long distances.

As in the case of fiber optc transmitters, fiber optic receivers are available in both analog and digital versions. Figure (4) is a functional diagram of a simple analog optical receiver.

Fiber Optic CCTV Transmission System Implementations

Fiber optics for CCTV applications are predominantly used in extended local installations linking cameras back to monitors with dedicated fibres for each link. A typical fiber optic transmission system layout is shown in Fig. 6.

This example illustrates the main features of any fibre optic system, which are as follows:

1. The fibre optic link and its associated terminal equipment fit between the camera and the associated monitor/controller and provide a transparent signal path i.e. the camera and controller do not know that the signals have been transmitted over fiber.

2. The camera output is a 1V peak to peak composite video signal.

3. Movable cameras have a telemetry receiver is mounted near to the camera movement mechanism. This telemetry receiver connects to the system controller to provide control of the camera pan/tilt and zoom PTZ functions.

4. At the control end of the link camera selection and movement is looked after by the system controller and video signal outputs from the controller are displayed on a local monitor(s).

5. Electrical to optical and optical to electrical converters provides the interfaces to the optical fibre transmission fibre.

6. At the camera end of the link the E/O converter is usually a single channel unit packaged in a small enclosure which can be conveniently mounted near to the camera or telemetry receiver. These E/O converters are not usually environmentally sealed and so need to be protected from the elements often mounting them in the telemetry receiver enclosure. In their most cost effective form a PTZ cameras E/O converter will use two multimode fibres to give a uni-directional video connection plus a bi-directional control data channel.

7. As an alternative these control and video link functions can be carried over a single fibre using optical transmission at two wavelengths, WDM – wavelength division multiplexing. These WDM links are more expensive than single wavelength links but they do save on fibre usage and they also can make the best use of a previously installed fibre infrastructure.

8. The E/O converter data interface must be compatible with that used by the system controller; these are often non-standard.

9. Fixed cameras can use a miniature E/O transmitter, which can connect directly to the camera BNC signal output. This link requires only one fibre.

10. The camera end E/O converter is connected to the transmission fibre through a patch box. This patch box provides a point of termination for the transmission cable and so prevents strain and wear and tear being placed on the transmission cable when installing, servicing or moving the terminal equipment. Optical connections between the E/O converter and the patch box are made with duplex patchleads (which are short fibre cable lengths terminated at each end with an optical connector). The patch box will only be a relatively small enclosure because it will only need to provide connectivity for a few fibre cores.

11. At the control room end of the link fibres from a large number of cameras will be concentrated. Equipment must therefore be packaged accordingly and most often this means the use of 19” rack mount units. E/O converters are manufactured in modular card format, which enables multiple video channels to be accommodated in a 19” cage. Typically one 3 U high rack can accept plug-in E/O converters for up to 30 video only channels or 10 video/data channels (or a mixture of both).

12. The fibre transmission cables are also handled in 19” rack enclosures because now we will be organising many fibre cores. These enclosures are called patch panels and they again provide a physical buffer between the transmission cable and the terminal equipment. Here the cable will be bought into the rear of the patch panel via a compression gland and the fibre cores will be broken out into the secondary coated cores. These cores will then be terminated with connectors, which are then connected into in-line adaptors mounted through the front bulkhead of the patch panel enclosure. This termination may either be carried out by the direct attachment of connectors to the fibre tails or factory terminated connectors tails will be spliced to the transmission fibre cores. If splices are used then the splice enclosures will be mounted in clips on the patch panel base. Patchleads then connect the patch panel bulkhead connections to the E/O converter optical connections. Copper leads then complete the connections to the system
controller and monitors.

As part of the cable installation the installer will have measured the installed cable loss, a function of position using a piece of test equipment called an OTDR (Optical Time Domain Reflectometer). This measurement serves to finger print the system and provides a point of reference for future system maintenance. It also provides the value of the end to end loss of each optical fibre used. The total loss must not exceed the optical margin specified by the equipment manufacturer, otherwise the transmitted picture quality may be impaired. In a correctly installed multimode system link lengths of 4 km for 850nm products and 8 km for 1300nm products are readily achieved.

VOSCOM’S VOS-1000 FIBER OPTIC TRANSMISSION SYSTEMS MAKES THE CONNECTION FOR CBS NEWS CREWS

For political conventions of news was live from Super Bowl VOSCOM’S VOS-1000 on the grounds of the fiber optic transmission systems, has contributed to CBS News Top Stories provide viewers at home. VOSCOM, a leading provider of fiber optic converter for audio and video routing solutions for the delivery of broadcasting and Pro A / V applications YOUR-winning CBS News in 1000 as part of their equipment for the transmission of video from multiple locations in the diffusion of various new programs.

“For the live broadcast of our regular news broadcasts in several places, the sand on the south beach, Sun Life asked Stadium during the Super Bowl, that the material must be very portable,” says Mel Olinsky, Director of the Office Operations, CBS News. “Working on-site, we also need to transport HD signals over long distances, which was impossible with copper. The VOS-1000 field optics fiber transmission provided that the all of our cable connectivity over a fiber strand multi without restrictions on duration. ”

CBS VOS-1000 widely used, several major events including political conventions and the last Super Bowl. During the week before Super Bowl VOSCOM Use VOS-1000 video transport for several new programs, including “The CBS Early Show,” CBS Weekend Evening News “and” Face the Nation “, all live in different places, including South Miami and Miami Beach Gardens Sun Life stage. For these shows, CBS News needed the ability to supervise both HD and SD video signals and wanted to and fro transportation from various locations in South Florida’s network OB truck, often parked near the place . A battery, bi-directional HD designed the fiber transmission system for field use and harsh environment applications, the VOS-1000 is the ideal portable solution for transmitting signals over distances ranging up three football fields away from turning over any local transport.

Frank Xu, Director General, VOSCOM, said: “The Place-ENG and production can be very hectic, especially in advance of important events.” He concluded: “The VOSCOM takes some of the stress of live shooting distance, as it is very easy, quite robust to any state in the field and transported extremely reliable. We are very pleased that our VOS-1000 plays a role, ensuring that emissions go up CBS News smoothly. “

CBS News continued to units VOS-1000 for remote broadcast. For more information on the VOS-1000, please visit www.voscom.com

CCTV PTZ Cameras Video Transmission over Fiber Optics Application

Single CCTV PTZ Camera Video Transmission over fiber optics, support 8-bit digitally encoded broadcast quality video, data and 10M/100M IP Ehternet over one multi-mode or single-mode optical fiber. The modules are directly compatible with NTSC, PAL, and SECAM camera systems and support RS-485, RS-232, and RS-422 data protocols. These Transmitter and Receiver are typically used in applications with PTZ cameras for security surveillance, CCTV, ITS, CIQ, etc.

Application:
Remote PTZ analog camera with fiber optical connection to be viewed on a video monitor.
EXAMPLE: Owner of building needs to view and control an PTZ Dome Camera from Monitor Center.

Solution:
Standard PTZ equipped CCTV camera is connected to the VOSCOM Fiber Optic Video & Data Transmitter using standard coax cable. The transmitter digitally compresses the signal for transmission across the Fiber Optical Cable.
At the Monitor Center, the receiver can receive the signal and the user can view the video image and control the camera movement using a standard Keyboard.
more information please find in the website: www.voscom.com

Notes:
1) VOSCOM Fiber Optic Transmitter can transmit 1~64 channels video signals and data support RS485, RS232, RS422. you can choose our fiber optic products according to your needs.
2) If you just need to control the PTZ cameras, one return data is enough, in theory, one channel data can control 1~128 PTZ cameras, the detail connection information you can refer to your PTZ camera’s manual.