Optical: systems and elements – Optical amplifier
Patent
1995-12-07
1998-11-10
Moskowitz, Nelson
Optical: systems and elements
Optical amplifier
385213, G02F 121, G02B 610
Patent
active
058352589
DESCRIPTION:
BRIEF SUMMARY
The field of the invention is that of optical systems requiring amplitude modulation, particularly at UHF rates.
Currently, one of the problems often encountered is that of the ratio between the electrical power PE, used to perform amplitude modulation, and the electrical power PS supplied by a photodetector capable of detecting the modulated optical wave. This aspect is particularly important in the UHF field, and, moreover, in this case, the modulation signal is of the analogue type which leads to modulation being performed with a modulation ratio less than 1, in order to preserve the quality of the signal. The power generated by the photodetector is then relatively low.
Conventionally, an optical amplifier can be employed before detection by a photodetector, in order to augment the optical signal. It is also possible to use a high-power laser source and an external modulator.
These techniques make it possible to augment the useful optical signal without to that end augmenting the UHF power necessary to generate it.
However, the optical amplifier and the photodetector run a high risk of saturating, by receiving not only the modulated optical power but also the continuous background resulting from a modulation ratio less than 1. The optical power is not used optimally, since it is present not only on the important signal (modulated signal) but also on a continuous background which is of no use to the application.
In order to resolve this problem of limitation of modulation ratio due to the presence of a continuous background, the present invention proposes a device for amplification of amplitude modulation ratio of an optical beam capable of eliminating the continuous background present in a modulated signal. To that end, the device according to the invention comprises means making it possible to make two optical waves, which are of the same amplitude with offset frequencies, interfere, and thereby to obtain a purely sinusoidal optical wave without continuous background.
More precisely, the subject of the present invention is a device for amplification of modulation ratio of a single-frequency incident optical wave, of frequency FL, comprising an optical amplifier AO, characterized in that it comprises, between the incident wave and the amplifier: receiving a part of the incident wave, one of the arms also being fed by an electrical device capable of creating a UHF wave at the frequency FM within the said arm, this interferometric device comprising means for generating, at the output of the two arms, two optical waves of the same amplitude, offset in frequency.
The modulation ratio amplification device, may advantageously comprise, in one of the arms, a frequency converter Tf, controlled by a UHF wave at the frequency FM, and supplying, as output, an optical wave at the frequency FL and an optical wave at the frequency FM+FL, respectively of amplitude a.sub.1 (FL) and a'.sub.1 (FM+FL), the other arm comprising a continuous phase shifter D.sub..phi., giving rise, as output, to an optical wave at the frequency FL of amplitude a.sub.2 (FL) such that, recombined at the output of the interferometric device, with the optical wave of amplitude a.sub.1 (FL), an optical wave of amplitude a'.sub.1 (FL) results therefrom.
The modulation ratio amplification device according to the invention may also comprise a Mach Zehnder-type interferometer, each of the arms including a phase shifter, one phase shifter being continuous and creating a phase shift of .pi., the other phase shifter creating a sinusoidal phase shift at UHF. At the output of the interferometric device, there is mainly interference between two optical waves of the same amplitude at frequencies respectively of FL-FM and FL+FM.
A further subject of the invention is a modulation ratio amplification device in which the interferometric device is a Sagnac microinterferometer into which a UHF phase shifter is introduced. The UHF phase shifter may be fed with UHF.sub.2.phi.FM. Two optical waves are also made to interfere, at the output of this interferometric dev
REFERENCES:
patent: 5074631 (1991-12-01), Hamano et al.
patent: 5144468 (1992-09-01), Weverka
patent: 5206674 (1993-04-01), Puech et al.
patent: 5243617 (1993-09-01), Pocholle et al.
patent: 5289309 (1994-02-01), Delacourt et al.
patent: 5416858 (1995-05-01), Riviere
Delacourt Dominique
Papuchon Michel
Vodjdani Nakita
"Thomson-CSF"
Moskowitz Nelson
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