Optical waveguides – With optical coupler – Plural
Reexamination Certificate
1999-06-18
2001-01-16
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
With optical coupler
Plural
C385S001000, C385S004000, C385S031000, C385S043000, C385S045000, C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06175672
ABSTRACT:
BACKGROUND
The present invention relates generally to transmissive RF links, and more particularly, to an improved RF wide bandwidth low loss (even gain) high performance low noise fiber optic link that can be compactly and reliably packaged.
The prior art this invention replaces relates to current commercial off-the-shelf (COTS) RF fiber optic links and current narrow bandwidth links. One disadvantage of current RF fiber optic links is that they do not meet many of the demanding RF performance parameters needed in advanced RF systems. The present invention achieves a significant leap forward in overcoming prior RF fiber optic link deficiencies.
It would therefore be an improvement in the RF systems art to have an RF wide bandwidth, low loss, high performance, and low noise optical transmissive link. It would also be an improvement in the RF systems art to have an RF wide bandwidth, low loss, high performance, and low noise optical fiber link.
SUMMARY OF THE INVENTION
The present invention provides for a high performance low noise wide RF bandwidth fiber optic link that achieves a significant improvement in key RF link parameters. The fiber optic link has a very wide bandwidth from 1 to 18 GHz and higher using currently available technology. The key link parameters addressed by improvements provided by the present invention are RF insertion loss (gain), signal-to-noise ratio, noise figure, spur free dynamic range, and close-in and far out relative phase noise.
The RF fiber optic link selectively employs a number of techniques to improve various link parameters. The link may be structured to have a high power light source, such as a laser, that provides light output having a high level of optical power. A feedback circuit may be disposed around the light source that reduces relative-intensity-noise levels produced by the light source at low frequencies. A modulator is used to modulate the light output of the light source. Preferably, a dual output modulator is used to provide two amplitude modulated optical signals whose respective outputs have RF modulation on the light output which is “effectively” 180 degrees out of phase. An optical fiber may transmit the modulated optical signal(s). A photodetector without a load resistor directly on its output that is operable at the high level of optical power receives the modulated light and recovers the RF signal. A dual balanced photodetector is preferably used in conjunction with the dual output modulator. An RF component, such as an amplifier, can be coupled to an output and integrated with a photodetector and thus provides a load resistance for the link for wide bandwidth operation.
Projected near-term key link parameters are a frequency bandwidth of 1 to 18 GHz, a link gain of at least 0 dB, a signal-to-noise ratio of 165 dBc/Hz, a noise figure of 18 dB, a spur free dynamic range of 117 dBc/Hz and a relative phase noise at 10 kHz of −135 dBc/Hz relative to an RF carrier at 10 GHz. These are achieved without the use of RF amplifiers in the input or output paths of the link. Further out in time, better link performance will be achieved as component technology improves. For comparison, current typical commercial off the shelf (COTS) link parameters without RF amplifiers exhibit alink frequency bandwidth of 1 to 18 GHz, a gain of about −45 dB, a signal-to-noise ratio of about150 dBc/Hz, a noise figure of about45 dB, and a spur free dynamic range of about 100 dBc/Hz, and close-in (phase) noise (at 10 kHz) of about −120 dBc/Hz.
The present invention combines several techniques for improving link performance using the latest improved link components that can now be fabricated in a laboratory. The techniques preferably include the use of: (1) laser noise reduction using a balanced photodiode detector pair, (2) a feedback circuit around the laser to reduce the significant relative-intensity-noise levels at low frequencies (<1 MHz), (3) high power handling capability photodiode detectors, (4) a high power CW semiconductor laser, (5) a dual output external modulator, and (6) a photodiode detector without a conventional 50 ohm output load resistor.
All of these techniques have been demonstrated independently but have never been put togehter to achieve a lossless high performance RF wide bandwidth low noise fiber optic link. Also, some of the techniques are known to a very limited technical community and some of the improvements in component technology have not generally been believed to be feasible.
The present invention provides a fiber optic link that can meet the demanding parameters of the latest state of the art RF systems and bring with it all the many advantages provided by fiber optics which include wide RF bandwidth, low loss in fiber, light weight, small size, no electromagnetic interference (EMI), remoting and monolithic integration. Previously, the link parameters restricted the practical use of RF fiber optic links to limited specific areas of use in RF system a (such as long delay lines). The present invention opens the use of RF fiber optics to most RF system applications and thereby provides for significant link cost reductions obtained by large usage.
One way to define the link in the present invention is as follows. Take a typical RF wide bandwidth amplifier with a gain of about 25 dB (typical for radar) that might be used in a high performance radar system front end and measure its performance parameters of bandwidth, gain, signal-to-noise ratio, noise figure, spur free dynamic range, relative close-in and far out phase noise. Now place the amplifier at the input to the fiber optic link described herein and measure the same parameters as listed above for the combination of amplifier and link. The combination will have substantially the same performance parameters as the amplifier as measured above.
The present fiber optic link may be used in current and future radar, electronic warfare and communication applications. The fiber optic link may be used in both military and commercial RF systems for ground, air, sea and space based applications.
REFERENCES:
patent: 5473459 (1995-12-01), Davis
patent: 5933265 (1995-12-01), Nagarajan
Hayes Robert R.
Krikorian Kapriel V.
Loo Robert Y.
Newberg Irwin L.
Ng Willie W.
Alkov Leonard A.
Lenzen, Jr. Glenn H.
Palmer Phan T. H.
Raytheon Company
LandOfFree
RF wide bandwidth lossless high performance low noise... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with RF wide bandwidth lossless high performance low noise..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and RF wide bandwidth lossless high performance low noise... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2515512