| MadSci Network: Engineering |
Greetings: Any type of information that can be carried by radio waves can also be carried by laser light beams. Both radio waves and laser beams are electromagnetic waves. This information is called modulation and it includes amplitude modulation (AM), frequency modulation (FM), phase modulation (PM), television (TV) and all forms of digital modulation. In fact, because of light's great capacity to carry information, the entire radio spectrum including all AM radio stations, FM radio stations, and hundreds of the television stations can be placed on a single laser beam. This is currently being accomplished in fiber optic Cable TV systems that use modulated laser diodes and fiber optic cables to distribute signals to homes. I recently answered a similar question about how to transmit sound on a laser beam and you can first read this information in the Mad Science archives at: http ://www.madsci.org/posts/archives/aug98/897352142.Eg.r.html As I discussed in the audio answer there are two methods to modulate signals on a laser (or LED) beam, Direct Modulation and External Modulation. External modulators cost several thousand dollars and they are used in fiber optic cable TV systems in which hundreds of TV signals are transmitted. Direct modulation can be used for your application and the cost will be determined by how handy you are with electronic circuitry. There are a number of methods by which direct modulation of light can be performed. Better quality video cameras, tape recorders and TV sets provide two options for transmitting audio and video signals between them (where you can insert your optical signal in place of the cable). They provide base band video and audio channels directly and require two cables between suitable TV (display) receiver sets and the signal generator. Or the video and/or audio signals can also modulate the combined video and audio signals on a Channel 3 or 4 radio frequency (RF) carrier so that the signals can be played on a conventional TV set tuned to channel 3 or 4. This circuitry is incorporated in most video recorders. Depending on your ability to work with electronics components, you have two primary options to transmit and receive video on laser light; 1) you can directly modulate a suitably biased laser diode with video or audio as amplitude modulation (AM) or 2) you can modulate a suitably biased laser diode with a channel 3 or 4 RF sub carrier with NTSC (the standard TV signal format in the USA) video and audio and modulate the NTSC signal onto a laser carrier beam. The problem that you point out about normalizing the video signal is technically called "DC Restoration" and most TV sets have a DC restorer circuit built in them. Thus method 2, the RF subcarrier method above takes care of the problem within the TV set with a DC restorer. In method 1, direct video modulation, it is less complex to modulate on a laser diode; however, if need DC restoration, you will have to construct a circuit within the optical receiver. In the past many low cost TV receivers did not have DC restoration circuits and the only time the restoration problem is noticeable in the TV picture is usually in dark, night time scenes. I would recommend that you start with a simple audio modulated optical link demonstration kit from Edmund Scientific Corp.( We used Edmund optical components in our first laser links that transmitted video signals 18 miles during the 1960s): http://www.edmundscientific.com The Edmund fiber optic communication designers kits use very low cost light emitting diodes (LEDs) which you can experiment with before you use more expensive laser diodes as optical sources. You will probably burn several LEDs out before obtaining proper bis level circuits (e.g. 50% average light power output). You can also use a low cost laser diode pointer as a light source if you are handy with constructing modulation electronics. Your most dificult problem will be to improve the bandwidth of the audio transmitter and receiver circuitry to operate with one or two Megahertz of bandwidth for analog video signals and most OPAMPS can easily due this. The same LED circuitry can then be adapted to drive a laser diode. I would disregard the DC restoration problem for it is a minor issue compared to getting sufficient bandwidth for video signals in the light transmitter and receiver circuitry. I would also suggest that you use base band video instead of the channel 3/4 signals for they require signals operating at VHF frequencies. Also; many electronics stores such as Radio Shack carry several experimenters handbooks which have optical links and they can aid you with the selection of OPAMP circuits for biasing and driving LED modulators and constructing optical receivers. Best regards, Your Mad Scientist Adrian Popa
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