Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
1999-12-17
2002-04-16
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S245000, C359S246000, C359S250000, C359S251000
Reexamination Certificate
active
06373617
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a device able to process an optical signal. More particularly the present invention relates to a device able to process an optical signal by means of an optical control system and in a manner independent of the state of polarization of this optical signal.
Following the widespread development of optical-fibre transmission systems, there is an urgent need for devices which are able to perform, at high speed, different operations on the bits which constitute the digital information of an optical signal.
The present apparatus used for the processing of optical signals are inadequate for managing the increasingly higher transmission rates which are possible in optical-fibre transmission systems. In fact, these apparatus consist of digital electronic devices which are based typically on serial processing of the information or optical devices which, however, are controlled by electrical signals. All these devices have a limited band compared to the optical band available in optical-fibre transmission systems.
Moreover, the optical signals coming from an optical transmission line have a random, and therefore unpredictable, state of polarization. In fact, during their propagation along an optical transmission means, they are subject to a random disturbance of their state of polarization (SOP). More particularly, in an optical fibre, typically a single-mode optical fibre, the SOP of an optical signal is disturbed both on account of manufacturing defects (such as, for example, a not perfectly circular geometry of the core and/or lack of homogeneity of the material) and on account of the action of external agents (such as variations in the external temperature, displacements of the fibre, vibrations and stress). These disturbances are random and unpredictable. Even a relatively short fibre (for example with a length of one metre), if subjected to stress or to variations in temperature, may disturb significantly and in a random manner the SOP of the optical signal which is propagated there.
SUMMARY OF THE INVENTION
The inventors of the present invention have therefore considered the problem of fully exploiting the optical band available in optical-fibre transmission systems using an optical device which is able to process an optical signal in optical form by means of an optical control system and in a manner independent of its state of polarization. More particularly the inventors have considered the problem of fully exploiting the optical band available in optical-fibre transmission systems using an optical device comprising a crystal element possessing properties which are electro-optical (i.e. it becomes birefringent when subject to the action of an applied electric field and its refraction indices vary with a variation in intensity in said electric field) and photo-conductive (its refraction indices, assuming a constant applied electric field, may be further varied by the action of the light which photo-generates charge carriers which screen the action of the applied electric field).
According to its first aspect, the present invention therefore relates to an optical device comprising:
a) a first input for at least one first input optical signal having a random state of polarization;
b) a first optical element capable of dividing, in space, said at least one input optical signal into a pair of optical signals which are substantially parallel and have a predetermined state of polarization perpendicular to one another;
c) a first crystal element for propagation, in free space, of at least said pair of optical signals coming from said first optical element, said first crystal element being devoid of internal separation planes, having electro-optical properties and being associated with electrodes so as to apply a voltage in a direction substantially perpendicular to the direction of propagation, along said first crystal element, of said at least one pair of optical signals;
d) an optical control element for supplying at least one optical control beam, having a predetermined power, to said first crystal element, said at least one optical control beam being superimposed on said at least one pair of optical signals and having a direction of propagation which along said first crystal element is substantially perpendicular to said direction of application of said voltage, said at least one optical control beam being capable of causing to rotate through a predetermined angle the state of polarization of said at least one pair of optical signals in said first crystal element;
e) a second optical element capable of combining said at least one pair of optical signals coming from said first crystal element in a single output optical signal;
f) a first output for said output optical signal.
During the course of the present description and the claims:
the expression “optical control beam superimposed on an optical signal” is used to indicate an optical control beam which is superimposed on said optical signal in the whole region crossed by it in a crystal element devoid of internal separation planes;
the expression “optical control beam collinear with an optical signal” is used to indicate an optical control beam which is propagated in the same direction in which this optical signal is propagated;
the expression “a crystal element devoid of internal separation planes” is used to distinguish a single crystal from elements obtained from the combination of two or more crystals;
the expression “propagation in free space” is used to indicate all the modes of propagation of an optical signal not guided by a suitable waveguide such as, for example, an optical fibre.
Preferably, said optical control beam is also substantially collinear with said at least one pair of substantially parallel optical signals.
Typically, said at least one pair of optical signals lies in a plane parallel to one face of said first crystal element.
Advantageously, said input consists of an optical fibre.
Typically, said first optical element comprises a right-angled reflecting prism and a polarization separator.
Similarly, said output also advantageously consists of an optical fibre. Moreover, said second optical element typically also comprises a right-angled reflecting prism and a polarization separator.
According to a first embodiment, said optical device also comprises a second output and said second optical element also comprises a second right-angled reflecting prism.
According to a second embodiment, the optical device according to said first embodiment also comprises a second input for at least one second optical signal having a random state of polarization and said first optical element also comprises a second right-angled reflecting prism.
Said first and second optical signals, which have a random state of polarization, may each be superimposed on a respective optical control beam and be propagated in two different regions of said first crystal element or, alternatively, may be associated with the said optical control beam superimposed on both of them. In this latter case, said first and second optical signals are propagated preferably in the same region of said first crystal element.
A typical example of an optical element comprising two right-angled reflecting prisms and a polarization separator is described in the patent U.S. Pat. No. 5,305,136 included herein by way of reference.
According to a variant, said first and said second optical elements may each consist of a calcite prism, for example, a calcite prism manufactured by BERNHARD HALLE with a polarization insensitivity of about −40 dB.
Advantageously, a suitable optical collimator is associated with each of said inputs and said outputs. Preferably, said optical collimator consists of a “grin” type lens.
Preferably, said first crystal element consists of a single crystal of cadmium telluride doped with indium (CdTe:In).
A typical example of said first crystal element is described in the European patent application No. 97201874.1, filed in the name of the same Applicant, which is included herein b
Boffi Pierpaolo
Martinelli Mario
Piccinin Davide
Tonini Andrea
Epps Georgia
Needle & Rosenberg P.C.
Optical Technologies Italia SpA
Tra Tuyen
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