Generation of optical signals with RF components

Details Generation of optical signals with RF components Generation of optical signals with RF components

1996-01-16

1998-07-07

Pascal, Leslie

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

359162, 359177, H04B 1004

Patent

active

057777713

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a method of generating signals having optical carriers. It finds particular application in the generation of radio frequency (RF) modulations to be carried over optical media.
2. Related Art
The principle of modulating an optical beam by an information content is well established, and various methods are known of achieving this. Some arrangements involve controlling the light source (typically a laser) by varying its input bias voltage. Other arrangements use optical devices in the path of the generated beam (typically on an optical fibre) to interrupt the beam. A known optical device for this purpose is the Mach-Zehnder interferometer. The principle of this device is to split the optical beam into two paths, one or both of which passes through a medium wh refractive index varies as a function of the electric potential applied to it. By applying an electrical signal one or both paths, the difference between the path lengths of the two beam paths can be varied, such that the re-combined beams interfere constructively or destructively depending on the electrical fields applied. The intensity of the recombined beam therefore varies in response to the varying electrical input signal.
It is known for the modulation carried by an optical signal to include a carrier frequency in the radio frequency (RF) range. This principle, known as `Radio by Fibre,` allows a radio signal, including its RF carrier, to be generated at one location and transmitted over the air from another, remote, location. The signal is typically carried by an optical fibre between these locations. This allows the equipment at the point from which the signal is to be transmitted over the air to be kept very simple. In its simplest form it need consist only of a detector to convert the optical input into an electrical signal, and an antenna for transmitting the electrical signal over the air. This is particularly useful in situations where an antenna has to be located at a point difficult of access, such as a hilltop, because the complex equipment required to generate the radio frequency carrier in particular the local oscillator can be located at another more accessible location. Moreover, it is possible to achieve economies in a branched network, in which one signal is transmitted to several antenna sites, because only one local oscillator is required to generate the carrier to be transmitted by all the antennas.
The detectors for these arrangements are typically photodectors. These produce an electrical output which varies with the intensity of incident light. This electrical output therefore corresponds to the modulation, but without the optical carrier frequency.
Known optical systems suffer from a number of practical limitations, in particular in the accurate transmission of high radio frequencies (of the order of a few tens of GHz). As frequencies approach the millimeter waveband (tens of gigahertz) it becomes increasingly difficult to achieve direct modulation of the laser by applying a signal to the input bias voltage, because of inherent physical limitations of the laser devices themselves. Similar constraints apply to modulation devices such as the Mach-Zehnder interferometers discussed above, as the high frequencies necessitate very small dimensions which impose constraints which reduce their efficiency. Velocity matching between the electrical and optical signals also becomes harder to achieve and maintain.
There is an additional problem with the application of signals by means of a Mach-Zehnder optical modulator. As explained above, the principle of these modulators is that as the voltage applied to the electrical input of a Mach-Zehnder interferometer is increased, the difference in optical path length increases. This results in the two optical paths passing in and out of phase, so that the amount of light passing through the interferometer varies as a periodic function of the applied voltage, and not as a linear function. This non-linear respo

REFERENCES:
patent: 5107358 (1992-04-01), Hodgkinson et al.
patent: 5166821 (1992-11-01), Huber
patent: 5227908 (1993-07-01), Henmi
patent: 5315426 (1994-05-01), Aoki
patent: 5359450 (1994-10-01), Ramachandran et al.
patent: 5379309 (1995-01-01), Logan, Jr.
patent: 5473457 (1995-12-01), Ono
Weik, Communications Standard Dictionary, p. 1008, definition of "up converter", 1983.
O'Reilly et al "Optical Generation of Very Narrow Linewidth Millimetre Wave Signals", Electronics Letters, 3rd Dec. 1992, 28, pp. 327-328.
Polifko et al, "Millimeter-Wave Optical Transmission with Combination Narrowband EOM and Laser Modulation", Fourth Optoelectronics Conference (OEC '92) Technical Digest, Jul. 1992, pp. 250-251.

Affiliated with

Also associated with

Rating

Generation of optical signals with RF components does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Generation of optical signals with RF components, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Generation of optical signals with RF components will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-1212246