Generation of radio frequency modulated optical radiation

Details Generation of radio frequency modulated optical radiation Generation of radio frequency modulated optical radiation

1996-09-30

1999-06-29

Negash, Knife-Michael

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

359145, 359181, H04B 1004

Patent

active

059176369

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a method of generating modulated optical signals, and to a source of modulated optical radiation, and in particular to the use of a semiconductor electroabsorption modulator in a radio over fibre communication system.
2. Related Art
Demand for broadband telecommunication services to the home and to small business is expected to increase dramatically over the next decade. Optical fibre and millimeter-wave radio are both individually capable of supporting the large bandwidth requirements associated with these services. Providing optical fibre cabling direct to the home or business is one way of providing high capacity, but for operational reasons this is not always an appropriate solution. Alternatively millimeter-wave radio systems, such as the RACE Mobile Broadband system or radio LANs, are flexible and offer the advantage of expedient provision.
In recent years a hybrid of these two technologies has been developed, and is termed radio over fibre. Radio over fibre systems utilise optical fibre transmission to deliver radio signals directly to a point of free space radiation, usually an antenna site. Dependent on the application of the radio over fibre system, the radio signals may be of VHF, UHF, microwave or millimeter frequency. In general, a radio over fibre communications system will comprise a first, or central site, where an optical signal having a radio frequency carrier is generated, a second, or remote site, linked to the first site by an optical fibre, the second site having a transmitting radio antenna, and a third site having a receiving radio antenna. Thus,, data encoded on the optical carrier at the first site is optically transmitted, via the optical fibre, to the remote site, transmitted as a free space radio signal from the remote site to the receiving radio antenna at the first site, and demodulated. By delivering the radio signal optically, via the optical fibre link, it is unnecessary to generate a high frequency radio carrier at the remote site, which is usually not easily accessible, and not in a benign environment. Optical fibre is an ideal transmission medium for this purpose due its low loss, high frequency, and wide band capability. The principle advantage of radio over fibre systems is their ability to concentrate most of the expensive, high frequency equipment at a centralised location, allowing the remaining equipment at the remote site to be simple, small size, lightweight, and low power consumption. This results in straight forward installation, low maintenance, and a range of simplified options for electrical power provision. The centralisation of high frequency equipment also increases operational flexibility and the potential for frequency reuse or sharing between a number of users. Furthermore, the radiation frequency can be controlled centrally, in an environment shielded from severe climatic variations, and therefore can be extremely stable.
A key issue for radio over fibre communications systems will be the efficient generation of radio frequency modulated optical signals, since in order to benefit from the principle advantage of radio over fibre systems, the radio frequency carrier, modulated onto the optical signal, must be suitable for direct retransmission by a transmitting radio antenna, without upconversion. While techniques and components exist for generating modulated optical signals at lower radio frequencies, e.g. 1 GHz to 15 or even 20 GHz, as the required radio frequency modulated optical signal increases in frequency it is increasingly difficult to generate these optical signals efficiently, and cost effectively. Higher frequency radio over fibre communication systems are particularly attractive for a number of applications such as Multipoint Video Distribution Services (MVDS), and Mobile Broadband Systems (MBS). Frequency bands in the millimeter-wave range have been allocated in Europe to both these applications at 40 to 42 GHz and 62 to 66 GHz respectively. The advant

REFERENCES:
patent: 5434693 (1995-07-01), Tanaka et al.
Ogawa, "Microwave and Millimetre Wave Fibre Optic technologies for Subcarrier Transmission Systems", IEICE Transactions On Communications, vol. E76B, No. 9, Sep. 1993, Tokyo JP, pp. 1078-1090.
McQuate, "Calibration of Lightwave Detectors to 50 GHz", Technologies For Optical Fiber Communications, Jan. 25,1994, Los Angeles, pp. 336-344.
Polifko et al, "Millimeter Wave Optical Transmission With Combination Narrowband EOM and Laser Modulation", Fourth Optoelectronics Conference (OEC '92) Technical Digest, Jul. 1992, Tokyo, JP, pp. 250-251.
Yu et al., "Design and Fabrication of InGaAsP/InP Waveguide Modulations for Microwave Applications" SPIE, vol. 1703 (1992), pp. 304-312.

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