Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
1999-05-28
2001-08-28
Allen, Stephone B. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C359S297000
Reexamination Certificate
active
06281487
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a device for protecting an optical system against excess luminosity.
In the scope of the present invention, the term optical system is intended to mean any natural means, such as an eye, or artificial means, such as an optical sensor or a camera, capable of sensing a light beam.
2. Description of the Related Art
It is known that, in particular, the development of lasers both for civil applications and for military applications, and the diversity of existing types of lasers, in particular as regards wavelength, pulse duration, luminous energy, etc., make it necessary to use specific protection devices in order to provide effective protection of such an optical system.
Various solutions are known for providing such optical protection.
A first known solution relates to the use of protection filters, in particular:
filters with fixed rejection bands. However, it is necessary in this case to know the wavelengths liable to be encountered in the environment in question, and, further, such filters interfere with the operation of the protected optical sensors (coloration, weak transmission, etc.) and are generally ineffective against polychrommatic excess luminosity;
active filters triggered after the detection of excess luminosity. Such filters have response times that are generally long and are unsuitable for protection against pulsed laser beams; or
filters which are triggered by excess luminosity and whose operation is based on nonlinear optical effects. The latter filters do not guarantee protection over a wide spectral band compatible with the operating bands of many optical sensors.
This first known solution is therefore hardly satisfactory.
A second known solution consists in using an irreversible type of protection device, that is to say one whose optical properties are degraded irreversibly above a determined energy threshold of the incident light beam, so as to prevent the latter from then reaching the optical system to be protected. Such a solution is justified by the low probability of excess luminosity existing in a light beam observed by an optical system.
A solution of this type is described in U.S. Pat. No. 3,602,576 which discloses a protection device comprising:
means for forming, from an incident light beam, a first observation light beam which is transmitted to the optical system and a second destruction light beam specified below;
a photochemical shutter comprising a reflecting metallic layer and an explosive capable of being triggered by a light beam which has luminous intensity above a predefined value;
first optical means for guiding said observation light beam toward said optical system after reflection on the metallic layer of said photochemical shutter;
second optical means for guiding said destruction light beam toward said explosive of said photochemical shutter along a second optical path, the first optical path followed by said observation light beam to said photochemical shutter being longer than said second optical path.
Thus, when the incident light beam has excess luminosity corresponding, for example, to a laser pulse, this excess luminosity is encountered in the destruction beam which then triggers said explosive, thus destroying the photochemical shutter and preventing, after destruction, the observation beam from reaching the optical system. Further, since the optical path of the destruction beam is shorter than that of the observation beam (to the photochemical shutter), the triggering of the destruction of the shutter precedes the arrival of the corresponding observation beam on the latter.
However, because of the significant duration between the arrival of the destruction beam and the explosion itself, a solution of this type is quite ineffective for the applications envisaged in the present invention, in particular for protection against lasers of short pulse durations less than 1 millisecond, since the observation beam exhibiting the excess luminosity will in this case nevertheless still reach the optical system, and further, for long enough to damage or destroy it, before said explosion and the interruption to its path.
Further, the explosion of said photochemical shutter risks very greatly damaging the other elements of the protection device, and even said optical system.
This second solution is therefore not satisfactory either.
SUMMARY OF THE INVENTION
The object of the present invention is to overcome these drawbacks. It relates to a simple, effective and inexpensive device for protecting an optical system against excess luminosity of an incident light beam, and in particular a laser of short pulse duration.
To this end, according to the invention, said device of the type comprising:
means for forming first and second light beams from said incident light beam;
an optical reflection means;
first optical means for guiding said first light beam toward said optical system after reflection on said reflection means; and
second optical means for guiding said second light beam toward said reflection means along a second optical path, the first optical path followed by said first light beam to said reflection means being longer than said second optical path,
is noteworthy in that said optical reflection means is a mirror comprising a reflecting layer capable of being ablated at least locally when it is subjected to a luminous intensity higher than a predefined value, and in that said first and second optical means respectively focus said first and second light beams on the same focusing point of said reflecting layer of said mirror and form first and second paths having a length difference such that, when said incident light beam has excess luminosity, said second light beam initiates ablation of said reflecting layer at said focusing point, said ablation being continued and completed by the combined action of said first and second light beams.
Thus, by virtue of:
on the one hand, the use of said mirror, the reflecting layer of which can be ablated extremely rapidly, especially compared with the response time of the aforementioned photochemical shutter; and
on the other hand, the advocated destruction method combining the action of the two light beams, namely the initiation of the ablation by the second beam and the continuation and completion of said ablation by the two beams combined,
the problems with the time taken to activate the protection, which exists in the device disclosed by the aforementioned U.S. Pat. No. 3,602,576, are effectively remedied and particularly effective protection is thus obtained.
Further, since only ablation of the reflecting layer or of part of it is carried out, there is only an extremely small risk of damage to the other elements of said device. The latter can therefore be reused, either as it is or by replacing said mirror, or by moving it when the reflecting layer is ablated only locally.
It will further be noted that, in addition to being simple and inexpensive, the device according to the invention is effective over a wide spectral band, on account of the corresponding properties of the mirror used.
Although such a mirror is known, its use as such, that is to say without incorporating it in the device according to the invention, does not make it possible to obtain the results and the advantages which are obtained by the present invention, and it cannot be effective enough for the applications envisaged in the present invention. This is because, for identical focusing conditions, the energy threshold per unit area for ablating the metallic layer of the mirror is generally higher than the energy threshold per unit area capable of damaging the optical systems in question.
The present invention overcomes this latter drawback:
on the one hand, by combining the energy contributions of the two light beams; and
on the other hand, by focusing these light beams onto the same focusing point, throughout the optical field.
Consequently, the characteristics of the present invention make it possible both:
to reduce the response time so as
Fournier Gilles
Hubert-Habart Christophe
Aerospatiale - Societe Nationale Industrielle
Allen Stephone B.
Stevens Davis Miller & Mosher LLP
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