Coherent light generators – Particular beam control device – Optical output stabilization
Reexamination Certificate
2000-04-27
2004-09-07
Scott, Jr., Leon (Department: 2828)
Coherent light generators
Particular beam control device
Optical output stabilization
C372S029016, C372S029023, C372S026000, C372S006000, C372S102000
Reexamination Certificate
active
06788718
ABSTRACT:
TECHNICAL FIELD AND PRIOR ART
The invention concerns the field of optical sources, and their use as components in ultrahigh frequency emitter arrays.
Ultrahigh frequency emitters making use of optical sources are used in the field of optical ultrahigh frequency telecommunications, as described in the document by G. Grosskopf entitled “Optical fibres deliver microwave broadcasts” published in O.L.E., Sep. 1996, p. 55-61.
One possible application of the invention is therefore “picocellular radiotelephony” with the use of the fiber telecommunication network as the channel for transporting information to the radiating points.
The generation of ultrahigh frequency signals by optical signal heterodyning requires light sources of the laser source type, shifted frequency-wise by the value of the ultrahigh frequency sought.
This frequency shift can be obtained in different ways:
a) with two lasers, there can be obtained, if they have slightly different optical cavity lengths, two emitters with slightly shifted wavelengths, and therefore with emission frequencies capable of differing by that of the ultrahigh frequency to be generated.
This principle of the optical transportation of ultrahigh frequency signals is shown in FIG.
1
.
Two optical sources
2
,
4
, emit respectively radiation at the frequency &OHgr; and &OHgr;+&ohgr; and are coupled to a mixer
6
(a photodiode) by means of optical fibers
8
,
10
. The part
12
, situated beyond the mixer, forms the antenna. A device
14
makes it possible also to slave the two lasers frequency-wise.
b) Within a single laser cavity, it is possible to make two modes of oscillation coexist simultaneously (with orthogonal, linear or circular polarization states), whose frequency difference can be adjusted to the value of the ultrahigh frequency sought. Such a system is for example described in the document by M. Brunel et al. entitled “Differential measurement of the coupling constant between laser eigenstates” published in Applied Physics Letters, vol. 70, no. 16, April 1997.
Other methods are known, like the use of an optical frequency shifter (typically, an acousto-optical modulator) for obtaining two sources shifted frequency-wise. This technique is not compatible with the frequencies sought (≈10 GHz).
All the known devices pose a problem of size.
Moreover, in the known devices, correction devices are necessary on account of the over-large spectral width of the optical signal.
Finally, laser diodes, used in the majority of cases, are in general modulated frequency-wise by their supply current, and therefore also, simultaneously, emission amplitude-wise.
Another example field of application of the invention is the radar field.
Modern radars use a so-called active antenna design in which the angular scanning function of the antenna is obtained not by rotation of the antenna itself but by that of its emission wave plane. The wave plane results from a phasing—in the direction sought—of elementary waves issuing from various radiating elements of the antenna. The phasing is generally obtained by the adjustment of delays on the transport routes of the different ultrahigh frequency signals.
FIG. 2
shows this principle schematically.
The radiation produced by an emitter
13
is divided into n beams each passing through means
15
,
17
,
19
,
21
for imposing a delay on them. The wave
23
emitted by the antennas
25
has its wave plane modified according to the different delays imposed.
The necessity of having a greater and greater number of radiating antenna elements in order to improve, notably, the angular resolution (2000 emitters are spoken of), and the need to provide an accurate management of the delays, lead to a complexity of the system which is difficult to control with conventional ultrahigh frequency techniques (sizes and weights incompatible with the requirement of certain devices, airborne devices notably).
DESCRIPTION OF THE INVENTION
The present invention concerns the use of components in an array, in planar and collective manufacturing technologies for implementing ultrahigh frequency emitters.
The invention therefore concerns an ultrahigh frequency emitting device having a number of lasers and N means making it possible to impose phase delays on the path of N laser beams, these means being implemented in an array or a bar.
The invention concerns in particular an ultrahigh frequency emitting device, having:
at least a first and a second laser, emitting at two different frequencies &ohgr;
1
and &ohgr;
2
,
means of slaving the first and the second laser frequency-wise,
a mosaic or a bar or an array of N elements (N≧2) placed on the path of the beam of the second laser, each element making it possible to impose a phase delay on the bear or the portion of beam which passes through it,
N means for mixing the beam emitted by the first laser and each of the N delayed beams, and for producing N signals of frequency &ohgr;
1
-&ohgr;
2
,
N antenna-forming means for emitting radiation at the frequency &ohgr;
1
-&ohgr;
2
,
The invention also concerns an ultrahigh frequency emitting device, having:
a plurality of N laser emitter pairs, implemented in a mosaic or an array or a bar, each laser emitter pair having a first and a second laser emitter emitting at a first and a second frequency &ohgr;
1
, &ohgr;
2
, which are different,
an array or a bar of N elements, each of them being placed on the path of the second laser emitter of one of said laser emitter pairs, and each element making it possible to impose a chase delay on the beam of said second laser emitter,
means of slaving each laser emitter pair, frequency-wise and phase-wise,
N means for mixing each of the beams emitted by the first emitters of the N laser emitter pairs with each of the beams emitted by the second emitters of the N laser emitter pairs and delayed by the elements making it possible to impose a phase delay, and for producing N signals at the frequency &ohgr;
1
-&ohgr;
2
,
N antenna-forming means for emitting radiation at the frequency &ohgr;
1
-&ohgr;
2
,
The invention is based on a principle of ultrahigh frequency electromagnetic wave generation (a frequency which may reach several hundred GHz) by means of the beating—the heterodyning—of at least 2 electromagnetic waves in the optical domain (of much higher frequencies, of the order of 10
14
Hz), generated by lasers. The use of elements in mosaics or arrays, for imposing phase delays, allows the implementation of a compact device.
The detection of frequency beats (a mixing function) is generally provided by a photodiode whose current is a non-linear function of the electromagnetic field.
One of the advantages of the invention is the possibility of “transporting” ultrahigh frequency signals with a low attenuation per unit length by virtue of an optical “carrier”. The attenuation per unit length in the fibers is in fact only of the order of 0.1 dB/km whereas it is 0.1 dB/m in an ultrahigh frequency conductor (coaxial).
According to another aspect, the laser sources can be microlasers or VCSELs (vertical cavity surface emitting lasers). These components are also compatible with a collective implementation, in the form for example of mosaics or arrays.
The device according to the invention then does not require any device for correcting the received signal.
This is because the chip laser sources (or microlasers) have a very small emission line width, of the order of a few hundred KHz, much lower than that of laser diodes (MHz) or that of VCSELs (also MHz).
Furthermore, chip lasers can be modulated (optical) emission frequency-wise with no “crossed amplitude modulation” (which is not the case for laser diodes which are in general modulated frequency-wise by their supply current, and therefore also emission amplitude-wise) This frequency modification is for example obtained by electro-optical type modulation of the optical length of the microlaser cavity.
The invention also concerns an ultrahigh frequency emitting device, having:
a plurality of N laser emitter pairs, implemented in a mosaic or an array or a b
Gidon Serge
Molva Engin
Thony Philippe
Commissariat a l'Energie Atomique
Jr. Leon Scott
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