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Claims  |
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We claim:
1. A heterodyne Michelson interferometer for measuring a state of
polarization of radiation outgoing from a body, comprising:
a monochromatic light-beam source emitting a monochromatic light beam along
a path, said body being positioned along said path so that radiation in
the form of a light beam from said source is outgoing from said body;
a light-beam splitting and recombining device positioned along said path
downstream and provided with means for:
receiving said light beam from said body,
splitting the received light beam into a pair of partial beams,
sending said partial beams along two path branches, and
receiving reflected partial beams from said path branches and recombining
the reflected partial beam into a single beam;
respective mirrors positioned at ends of said path branches and
perpendicular to the partial beams sent therealong for forming said
reflected partial beams and reflecting them back to said light-beam
splitting and recombining device for recombination into said single beam;
means in one of said path branches for imparting to the partial beam sent
therealong a predetermined linear state of polarization, thereby forming
from the partial beam with said predetermined linear state of polarization
a reference beam, the other partial beam having said state of polarization
to be determined;
means for imparting a frequency shift to said other partial beam and
including an acousto-optical device driven by a radiofrequency signal and
receiving said other beam having said state of polarization to be
determined and emitting a first beam having the same frequency as the
other beam received by the acousto-optical device and a second beam whose
frequency differs from that of the first beam by a value equal to the
frequency of said radiofrequency signal, said acousto-optical device being
positioned so that at least said second beam is sent as said other partial
beam to the mirror perpendicular thereto, said acousto-optical device also
being positioned to be traversed by the second beam as reflected by the
last-mentioned mirror so as to emit a third and a fourth beam at least one
of which has a frequency different from that of said first beam and is
recombined into said single beam with said reference beam; and
polarization-analysis means for analyzing said single beam representing the
recombination of the reflected partial beams to generate electrical
signals representing beats between correspondingly polarized components of
the single beam and determining the state of polarization from intensities
and relative phase of said signals.
2. The interferometer defined in claim 1 wherein said body is a transparent
body.
3. The interferometer defined in claim 1 wherein said body is a reflective
body.
4. The interferometer defined in claim 1 wherein said acousto-optical
device is located in the path of the other partial beam outgoing from said
light-beam splitting and recombining device and is constructed and
arranged to recombine said reference beam and the beam recombined
therewith after the latter beam has undergone a frequency shift equal to
twice the frequency of the radiofrequency signal driving said
acousto-optical device.
5. The interferometer defined in claim 1 wherein light-beam splitting and
recombining device includes said means for imparting a frequency shift to
said other beam, the first and second beams emitted by said
acousto-optical device forming said one and said other partial beams,
respectively.
6. The interferometer defined in claim 5 wherein said acousto-optical
device is mounted so that said monochromatic beam from said source
impinges thereon at the Bragg angle to an optical longitudinal axis of
said acousto-optical device, said light-beam splitting and recombining
device recombining beams whose frequencies are respectively the sum and
the difference of the frequency of the monochromatic light beam from said
source and that of the radiofrequency signal driving said acousto-optical
device.
7. The interferometer defined in claim 5, further comprising respective
total-reflection prisms located along a path of one of said partial beams
between said acousto-optical device and a respective mirror, and along the
path of the recombined beam for respectively sending the respective beams
to the respective mirror and to said polarization-analyzing means for
increasing spatial beam separation.
8. The interferometer defined in claim 5, further comprising an iris in the
path of a beam emerging from said acousto-optical device and incident
thereon for effecting an alignment check of components of the
interferometer.
9. The interferometer defined in claim 5, further comprising an iris in the
path of a beam emerging from said acousto-optical device and incident
thereon for preventing a beam emerging from said acousto-optical device
from entering said source.
10. The interferometer defined in claim 1 wherein said body is a
single-mode glass fiber disposed between said source and said light-beam
splitting and recombining device.
11. A heterodyne Michelson interferometer for measuring a state of
polarization of radiation outgoing from a body, comprising:
a monochromatic light-beam source emitting a monochromatic light beam along
a path, said body being positioned along said path so that radiation in
the form of a light beam from said source is outgoing from said body;
a light-beam splitting and recombining device positioned along said path
and provided with means for:
receiving said light beam from said body,
splitting the received light beam into a pair of partial beams,
sending said partial beams along two path branches,
receiving reflected partial beams from said path branches and recombining
the reflected partial beam into a single beam, and
imparting a frequency shift to said other partial beam;
respective mirrors positioned at ends of said path branches and
perpendicular to the partial beams sent therealong for forming said
reflected partial beams and reflecting them back to said light-beam
splitting and recombining device for recombination into said single beam;
means in one of said path branches for imparting to the partial beam sent
therealong a predetermined linear state of polarization, thereby forming
from the partial beam with said predetermined linear state of polarization
a reference beam, the other partial beam having said state of polarization
to be determined, said means for imparting a frequency shift to said other
partial beam including an acousto-optical device driven by a
radiofrequency signal and receiving said other beam having said state of
polarization to be determined and emitting a first beam having the same
frequency as the other beam received by the acousto-optical device and a
second beam whose frequency differs from that of the first beam by a value
equal to the frequency of said radiofrequency signal, said acousto-optical
device being positioned so that at least said second beam is sent as said
other partial beam to the mirror perpendicular thereto, said
acousto-optical device also being positioned to be traversed by the second
beam as reflected by the last-mentioned mirror so as to emit a third and a
fourth beam at least one of which has a frequency different from that of
said first beam and is recombined into said single beam with said
reference beam; and
polarization-analysis means for analyzing said single beam representing the
recombination of the reflected partial beams to generate electrical
signals representing beats between correspondingly polarized components of
the single beam and determining the state of polarization from intensities
and relative phase of said signals.
12. In a heterodyne Michelson interferometer for measuring the state of
polarization of a radiation outgoing from a transparent or reflecting
body, comprising;
a monochromatic light beam source (1) irradiating said body (4) so that a
light beam is outgoing from said body (4);
a light-beam splitting-recombining device (8; 21), which receives a light
beam (5) coming from the body (4), splits it into a pair of partial beams
(5a 5b; 5d, 5e), sends such partial beams (5a ; 5b; 5d, 5e) along two
branches of the interferometer ending at respective mirrors (9, 10; 23,
25) arranged perpendicularly to the direction of propagation of the
partial beams, and receives and recombines into a single beam the partial
beams reflected by such mirrors (9, 10; 23, 25);
means (13; 24) inserted in one of such branches to give the partial beam
sent along it a predetermined linear state of polarization, so as to
obtain a reference beam;
means (11, 12) for frequency-shifting the other partial beam, which
presents the state of polarization to be determined; and
means (15, 16, 17, 18, 19, 20) for analyzing in polarization the recombined
beam (14; 27), to generate electrical signals representing the beat
between the equally polarized components of the recombined beam and to
obtain the state of polarization from the intensities and the relative
phase of such electrical signals, the improvement wherein:
the frequency shifting means comprise an acousto-optic device (11, 21)
driven by a radiofrequency signal, which device receives a beam (5)
presenting the state of polarization to be determined, emits a first beam
having the same frequency as the received beam and a second beam whose
frequency differs from that of the received beam by a value equal to the
frequency of the driving signal, and sends at least the second beam
towards one of the mirrors (9, 23), the device (11; 21) being arranged so
as to be traversed again by the reflected beam and consequently to emit a
third and fourth beam of which one at least has frequency different from
that of the received beam and is combined with the reference beam.
13. The improvement defined in claim 12, wherein said acousto-optic device
(11) is inserted along the path of the second partial beam (5a ) outgoing
from the beam splitting device (8) and is arranged so as to recombine such
second partial beam with the reference beam after the former has undergone
a frequency shift equal to twice the frequency of the signal driving the
acousto-optic device (11).
14. The improvement defined in claim 12 wherein said acousto-optic device
(21) is inserted along the path of the beam (5) emitted by the source (1)
and forms the beam splitting-recombining device, the first and second
beams (5d, 5e) emitted by the acousto-optic device (21) forming said first
and second partial beams).
15. The improvement defined in claim 14 wherein said acousto-optic device
(21) is mounted so that the beam (5) coming from the source (1) impinges
thereon at the Bragg angle to its optical longitudinal axis, said device
(21) recombining beams whose frequency is the sum and respectively the
difference of the frequency of the beam emitted by the source and that of
the signal driving the acousto-optic device (21).
16. The improvement defined in claim 14 wherein along the path of one of
the partial beams, between the acousto-optical device (21) and the mirror
(23), and along the path of the recombined beam, between the acousto-optic
device (21) and the means (15) for polarization analysis, total-reflection
prisms (22, 26) are located which send such beams towards the mirror (23)
and the means (15) for polarization analysis, respectively, and improve
the spatial separation between said beams.
17. The improvement defined in claim 14 wherein the acousto-optic device
(21) is arranged so as to generate, when traversed again by the partial
beams (5d, 5e) after reflection thereof by the mirrors (23, 25), beams
having the same frequency as the beam incident thereon and to reflect back
such beams along the same path as the incident beam, and in that an iris
(28) is provided to intercept such beams for a check of the alignment of
the interferometer components and/or to prevent such beams from reentering
the source (1).
18. The improvement defined in claim 1 which is used to measure the state
of polarization at the output of a single-mode optical fiber (4) arranged
between the source (1) and the beam splitting-recombining means (8; 21). |
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Claims |
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Description |
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FIELD OF THE INVENTION
Our present invention relates to polarization measurements and, more
particularly, to a Michelson interferometer for determining the state of
polarization of a radiation emerging from a transparent or reflecting
body.
BACKGROUND OF THE INVENTION
It is known that a body transmitting or reflecting a light radiation can
introduce variations in the state of polarization of the light radiation.
Knowledge of the state of polarization of the radiation emerging from the
body is of importance to completely characterize the body in respect of
its optical properties, and is essential when exploiting interference or
beats between radiations, since these phenomena occur only when the
radiations are equally or correspondingly polarized.
Applications include well-known applications of classical optics, optical
coherent or heterodyne telecommunications (based on beats) and optical
fiber sensors or gyroscopes, requiring the use of fibers maintaining a
determined state of polarization.
A polarized radiation can be characterized by electromagnetic field
components in a reference system with orthogonal axes x, y. Considering
the electrical field alone, the two components are given by:
Ex=a1 cos .omega.t Ey=a2 cos(.omega.t+.psi.) (1)
where a1, a2 are the amplitudes of the two components and .psi. is the
relative phase. To determine the state of polarization it is necessary to
measure the ratio a2/a1 between the two amplitudes and phase .psi., whose
sign defines the rotation direction on the polarization image, described
on plane Ex, Ey as t varies. From these two values further information can
be derived necessary to characterize the body under test, e.g.
polarization beat length, in case of single-mode optical fibers.
It is also to be noted that the state of polarization can vary in time. In
case of optical waveguides, this usually occurs owing to variable
mechanical and thermal stresses which modify their optical properties.
In order to determine time-varying polarization state, interferometric
techniques have also proved to be useful. An example based on a
Mach-Zehnder interferometer has been described by us in the article A
heterodyne Mach-Zehnder polarimeter for real-time polarization
measurement, Optics Communications, Vol. 54, No. 2, 15 May 1985, and in
the paper "A fast heterodyne interferometer for real-time fiber
polarimetry" presented at IOOC-ECOC '85, Venice, October 1985.
Yet this solution requires all the light beams inside the device to be
coplanar, to avoid systematic errors which depend also on the polarization
state to be determined and which hence cannot be eliminated by a simple
instrument calibration.
A Michelson interferometer is intrinsically free from these disadvantages,
since the light beam emitted from the source is split into two beams which
are back-reflected; such beams are obviously coplanar, and the distance
between the beam-splitter and the mirrors can be maintained very short.
An example of apparatus for measuring the state of polarization, based on a
Michelson interferometer, is described in Ellipsometry and polarized
light, by R.m.A. Azzam and N. M. Bashara, North-Holland Publishing
Company, 1977, pages 262-265, and in the paper Automated laser
interferometric ellipsometry and precision reflectometry, by H. F.
Hazebroek and W. M. Visser, Journal of Physics, Section E, Vol. 16, 1983,
pages 654-661.
These documents disclose an ellipsometer, i.e. a device for measuring the
polarization state of radiation reflected by the surface of a body. In
that ellipsometer, polarized radiation is split by a beam splitter into
two fractions. One fraction is sent towards the body under test and
reflected onto a mirror by which it is reflected back onto the body and
hence to the splitter; the other, acting as a reference beam, is sent to a
corner reflector and therefrom to the splitter. The corner reflector is
oscillated so as to change by Doppler effect the frequency of the beam
sent back towards the splitter in the reference branch. The two beams are
recombined by the splitter into a single beam containing both frequencies.
The components parallel and perpendicular to the incidence plane on the
body under test are separated and sent to different detectors. A
microprocessor obtains the required information from the intensities of
the beat signals supplied by the detectors.
A system of this kind has a number of disadvantages. More particularly, the
corner reflector position is critical, since it has to be chosen so as to
make reference beam coincide with one of the two reflector
self-polarizations, in order to maintain the reference beam polarization;
there are moving parts, which always entail reliability problems; the
system operates at low frequency (200 Hz) so that it does not allow
detection of polarization fluctuations which are very rapid.
OBJECT OF THE INVENTION
It is the object of our invention to provide an interferometer which
overcomes these drawbacks and which does not present moving parts and
operates at high frequencies (from some tens to some hundreds of MHz), so
that it can follow even very short fluctuations of the state of
polarization.
SUMMARY OF THE INVENTION
The present invention provides a Michelson interferometer for measuring the
polarization state of a light beam outgoing from a transparent or
reflecting body, which comprises:
a monochromatic light source
a light-beam splitting-recombining device, which receives a light beam
coming from the source, splits it into a pair of partial beams, sends said
partial beams along two branches of the interferometer, ending at
respective mirrors perpendicular to the propagation direction of the
partial beams, and receives and recombines into a single beam the partial
beams reflected by such mirrors;
means inserted in one of such branches and arranged to give the partial
beam sent along said branch a predetermined linear polarization state, so
as to obtain a reference beam;
means arranged to shift in frequency the other partial beam, which presents
the state of polarization to be determined; and
means arranged to analyze in polarization the recombined beam, to generate
electrical beams representing the beats among equally polarized components
of the recombined beam and to obtain the state of polarization from the
intensities and the relative phase of such electrical signals.
According to the invention, the frequency-shifting means comprises an
acousto-optic device driven by a radiofrequency signal, the device
receiving a beam having the same frequency as the beam generated by the
source, emitting a first beam having the same frequency as the received
beam and a second beam whose frequency differs from that of the received
beam by a value equal to the frequency of the driving signal, and sending
at least the second beam towards one of the mirrors. The acousto-optical
device is arranged so as to be traversed again by the reflected beam and
consequently to emit a third and fourth beams, at least one of said third
and fourth beams having different frequency from the received beam and
being recombined with the reference beam.
BRIEF DESCRIPTION OF THE DRAWING
The above objects, features and advantages of our invention will become
more readily apparent from the following description, reference being made
to the accompanying drawing in which:
FIG. 1 is a diagram of an interferometer according to a first embodiment of
the invention; and FIG. 2 is a diagram of a second embodiment of the
invention.
SPECIFIC DESCRIPTION
The drawing shows the application of the invention to the measurement of
the state of polarization at the output of a single-mode optical fiber
which is located between the source of the radiation used for the
measurement and the interferometer. This arrangement is the one that
better takes into account the fact that the fiber length may exceed source
coherence length, so that, if the fiber were to be inserted in an
interferometer branch, precise phase relations between the two radiations
to be recombined might no longer exist.
With reference to FIG. 1, a light-beam source 1, e.g. a semiconductor laser
without particular spectral requirements, emits a light beam which is
collimated and focussed at the entrance of a single-mode optical fiber 4
through a suitable optical system schematized by lenses 2, 3.
Beam 5, outgoing from fiber 4 and containing the information on the state
of polarization to be determined, is collimated by a further optical
system 6 and sent to a Michelson interferometer, denoted as a whole by 7.
The interferometer comprises a beam splitting-recombining device 8,
splitting beam 5 into a transmitted partial beam 5a and a reflected
partial beam 5b, and to branches, ending respectively at mirrors 9, 10.
Beam fraction 5a sent along the first branch enters an acousto-optic device
11, mounted with its optical axis oriented at the optimal angle (i.e. the
Bragg angle) with respect to beam incidence direction and driven by a
suitable radiofrequency electrical signal (e.g. a 40 MHz signal).
Device 11, as known, lets through input beam 5a unchanged in frequency and
direction, and emits a second beam 5c, with frequency equal to the sum or
difference between the frequency of the optical input radiation and that
of the electrical driving signal. The direction of beam 5c is determined
by the Bragg diffraction law, i.e. it is such that deflection 5a-5c
undergone by the beam owing to acousto-optic interaction is equal to twice
the Bragg angle. By the arrangement shown in FIG. 1, the second beam 5c
has a frequency equal to the sum of said two frequencies. Beam 5a is
intercepted by a suitable absorbing screen 12, while beam 5c reaches
mirror 9, perpendicular to its propagation direction, and is hence
back-reflected towards acousto-optic device 11.
Device 11 shifts in frequency and deviates again the received beam,
operating now on beam 5c. The beam deviated and shifted twice in
frequency, outgoing from device 11, is exactly superimposed on incoming
beam 5a and arrives at beam splitting-recombining device 8. The outgoing
beam which propagates unchanged can be intercepted by a device analogous
to screen 12 or, by a suitable component arrangement, it can be let out
from the interferometer without affecting the measurement.
Beam fraction 5b launched into the second branch of interferometer 7 is
caused to pass through a device 13 giving the beam a well-defined state of
polarization; device 13 can be e.g. a Glan-Taylor prism arranged to
transmit the linear polarization component at 45.degree. C. alone. The
polarized beam outgoing from prism 13 impinges orthogonally onto mirror
10, is back-reflected, traverses prism 13 again, emerging still linearly
polarized at 45.degree., and arrives again at splitting-recombining device
8. This beam constitutes a reference beam.
Splitting-recombining device 8 forms a beam 14 comprising the transmitted
fraction of the reference beam and the reflected fraction of the
frequency-shifted beam. Beam 14 is sent to a polarization analyzing device
15, e.g. a second Glan-Taylor prism with axes coinciding with those of
splitting-recombining device 8. Beats among equally polarized components
of the two radiations of recombined beam 14 are present at the two outputs
of prism 15. These beats are detected by detectors 16, 17, whose output
signals are suitably amplified in amplifiers 18, 19, and are fed to
measurement and/or display devices 20 (e.g. a vector voltmeter and/or an
oscilloscope operating in x-y mode) allowing measurement and/or display of
ratio Ex/Ey and of phase difference .psi.. Suitable processing means, not
shown, will obtain the desired fiber characteristics from measurements of
two or more polarization states obtained under different conditions.
Detector signals have amplitude proportional to a1 and a2 and relative
phase .psi.. In fact, supposing for simplicity sake that the reflected and
transmitted beams outgoing from device 8 have equal intensities, the beam
arriving at device 8 after reflection onto mirror 8 will be characterized
by an electric field
Emx=k.Eox.exp [i(.omega.+2 .OMEGA.)t]Emy=k.Eoy.exp {i[.omega.+2
.OMEGA.)t+.psi.} (2)
where: Eox=h.a1 and Eoy=h.a2 are the intensities before the double passage
through device 11; k, h, are constants taking into account losses due to
the efficiency of said device and to the beam splitting in device 8,
respectively, and .OMEGA. is the frequency of the signal driving device
11.
The reference beam, linearly polarized at 45.degree., is characterized by
an electric field
##EQU1##
where Eo, .psi. R are given by
Eo=Eox.sup.2 +Eoy.sup.2 -2EoxEoy cos .psi.R=arctg [Eoy sin.psi./(Eox+Eoy
cos.psi.)] (4)
The two fields are superimposed at the output of device 8 giving rise to a
sum of the homonymous components (2), (3) along axes x, y. Prism 15 will
send the component polarized along axis x towards detector 16 and the
component polarized along axis y towards detector 17.
The signals outgoing from the detectors are proportional to the intensities
(i.e. the squares) of the detected field components. Consequently, once
eliminated the d.c. components of the currents outgoing from detectors 16,
17 by filtering in amplifiers 18, 19, the corresponding electrical signals
Sx, Sy sent to measurement and/or display devices 20 are proportional to
the beats between field homonymous components (2), (3). Sx, Sy are hence
oscillating electrical signals at a frequency equal to the difference
between the frequencies of the two beams, and will have respectively
intensity
Sx=K.Eo.Eox cos(2.OMEGA.t-.psi.R) Sy=K.Eo.Eoy cos(2.OMEGA.t-.psi.R+.psi.)
(5)
From these relations one can see immediately that the phase difference
between the two signals is actually .phi. and that, taking into account
the values of Eox, Eoy, their amplitudes are proportional to a1 and a2
respectively.
In the embodiment of FIG. 2, where the interferometer is denoted by 70,
beam 5 collimated by optical system 6 is directly sent to acousto-optic
device 21, having the tasks of both device 11 and splitting-recombining
device 8 of FIG. 1. Beam 5 enters device 21 at the Bragg angle to the
optical axis of the device. Nondeviated outgoing beam 5d is collected by a
total-reflection prism 22 and sent onto a mirror 23, orthogonal to the
propogation direction of the beam reflected by the prism. This reflected
beam is hence sent back towards prism 22 and device 21. Deviated and
frequency-shifted outgoing beam 5e is on the contrary send to Glan-Taylor
prism 24 and to mirror 25, which are identical to prism 13 and mirror 10
of FIG. 1, respectively, and is reflected towards device 21.
Prism 22 allows a fair spatial separation between beams 5d, 5e and an easy
location of prism 24 and mirror 25, notwithstanding the small angle
between the two beams: this contributes to maintaining the longitudinal
interferometer dimensions limited.
Device 21, owing to its arrangement, causes on beams reflected by mirrors
23, 25 a frequency shift analogous to that performed on incident beam 5.
Hence, beam 5d will give rise to a beam with frequency .omega.-.OMEGA.
(deviated) and a beam with frequency .omega.+.OMEGA. (non deviated; beam
5e (with frequency .omega.+.OMEGA.) will give rise to a beam with
frequency .omega.+.OMEGA. (non-deviated) and a beam with frequency
.omega.+.OMEGA.2 (deviated). The non-deviated beam deriving from beam 5e
and the deviated beam deriving from beam 5d are superimposed into a
recombined beam 27 and are sent, through a second total-reflection prism
26 acting like prism 22, to Glan-Taylor prism 15 for polarization analysis
and subsequent heterodyne detection through detectors 17, 18. The
non-deviated beam deriving from beam 5d and the deviated beam deriving
from beam 5e are superimposed upon beam 5 and can be exploited for an
alignment check, using e.g. an iris 28 inserted between optical system 6
and device 21. The same iris can be used to prevent the beam from
reentering the cavity of a semi-conductor laser, thereby changing the
coherence length thereof.
What has been stated above for the embodiment of FIG. 1 applies also to the
embodiment of FIG. 2, apart from the fact that the beat takes place
between optical frequencies .omega.+.OMEGA. and .omega.+.OMEGA., and not
between frequencies .omega. and .omega.+2.OMEGA..
The description above clearly shows that the present invention eliminates
the drawbacks of the known device. In fact there are no moving parts, and
hence mechanical devices designed to generate motion are no longer
necessary; the mirrors in both branches are simple plane mirrors and not
composite members like the corner reflector of the prior art, so that no
problem arises of critical orientation of the reflecting means with
respect to the interferometer plane; finally, with the usual frequencies
used to drive device 11 or 21, the electrical signal containing
polarization information has a frequency of at least some tens MHz, and
hence even very short variations in the state of polarization can be
observed.
It is evident that variations are possible without going out of the scope
of the invention. E.g., if the state of polarization to be determined is
imposed by a transparent body whose thickness is much smaller than source
coherence length or by a reflecting sample (as in ellipsometric
measurements), the body can be inserted inside the interferometer, in the
branch which does not contain prism 13 or 24. In the case described of the
measurements on a transmitted beam, the processings necessary to obtain
the optical characteristics of the body from the state of polarization
ought to be modified so as to take into account, in the formulations, the
double crossing of the body by the light beam. This presents no difficulty
for a skilled worker in the art.
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