Note: Descriptions are shown in the official language in which they were submitted.
RCA 70,435
7 8 ~ ~ ~
1 Background of the Invention
This in~ention relates to light modulators and ',
; more particularly to electro-optical crystal modulators ,~
wherein the output light intensity is maintained constant at
a given value about which the modulation takes place. This ' ''
invention is particularly directed to a new control
system for an electro-opkic light moduLator which is
adapted to correct thermal effects causing variations
in the output light intensity.
It is conventional to employ electro,-optic
crystals for modulation of coherent laser radiation in, for
example, pulse`modulation communications systems. In
conventional apparatus, a beam of coherent light from a
, laser is pass~d through a polarizer to polarize the l1ght ,,
J 15 in a first plane. The polarize~ light is passed through an~1 , electro-optic crystal modulator which changes the insta~ta- ,
' neous orientation o the plane of polarization of the beam ~ ~'
., . . . ~
' ,in response to an electrlc field impr~ssed ~n the electro-
' optic crystal. The light transm1tted by the electro-optic
crystal modulator imping~s on an analyzer and is transmltted
' by~the analyzer to an extent which is a function o the ~ ;
,' ' angle formed by the polarization direction of the anal~zer ~ ~ ,
,',! ' and the instantaneous polarization direction~o~ the laser
~i beam lmpinging on the analyzer.
' A seriou6 problem occasioned by the use o~
electro-opt1c crystals as modulators is the
~ , variation in light output intensity caused by the change
,, in birefringence of the crystal as a function of
~ temperature. Here~ofore, this problem has lead to
; various types of oven and control systems to alleviate
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1 this problem. Qne system relies on the sensing of a
change of temperature at the crystal mount which is fed
back as a correction signal to the heater assembly
associated with the crystal mount. However, since
these temperature sensing devices cannol: sense the
crystal temperature, but only some temperature ~t a
point in close proximity, a continual drifting of the
birefringence and a resulting vari~tion in the depth
of modulation of the.transmitted light beam is experienced.
.Further, even if the crystal temperature were monitored
accurately, a problem arises from the fact that lt is
the temperature in a small center portion of the crystal
along the optical pakh which is important. For.example, -
in lasers of the CO2 type, the heating.effect of the.
laser beam itself can produce large temperature
:,
differentials along the cross-section of the electro~
~. optia crystal. ~ccordingly, sensing the temperature
-: : of the out`side surface of the crystal is of doubtful
~ value. In addition to the changes in birefringence
20 of the crystal modulator caused by the crystal temperature
variations, changes in temperature o any other
component which i5 part o the modulator assembly may. :
. also produce changes in the in~ensity of the output .
: ~ light beam. : . ~
:~ 25 ~ ther systems have been proposed.to correct for ~-
~l thermal affects in electro-optia modu1ators by controliing
the electric field bias applied to the modulator in
:~response to a feedback signal. These systems utilize d
low-frequenay probe signal which is superimposed on the
: ;~ modulator bias~and is detected by, fo~ example, a
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RCA 70,435
1 photodetector at the output of the laser modulator in order
to develop an error signal which is a function of the
deviation of the output from a maximum, or other desired
optimum output condition. The error signal is used to
change the electric field bias applied to the modulator
in order to restore the output to the maximum or desired
optimum condition. These systems are sometimes ad~quate
for low output level lasers and low modulation frequencies,
as the temperature changes encountered in these electro-
optic crystal modulator applications are relatively small.
However, in the case of a high power laser such as, for
example, a CO2 laser, the modulator absorbs a su~stantlal
amount of heat from the lascr beam itself. Additionally,
at high modulation frequencies the modulating RF
energy is also absorbed into the crystal material.
This results in substantial changes in temperature
which vary according to the type of modulation applied
ts the crystal modulatorO Accordingly, the birefri~gence ~ -~
of the crystals and therefore the optical path length
,
difference may change over many wàvelengths. The
voltages required to compensate for these changes
,.
ordinarily run into several tens of thousands of volts,
which may be well above the dielectric strength of the
:1 . . :
!'' electro-optic crystal material thereby resulting in a
j~ 25 breakdown of the crystal material.
~, . .
The present invention overcomes the disadvantages
of the prior art and proposes a system which further
simplifies and reduces the cost o that disclosecl in
.. . . . .
United States Patent Number 3,780,296 issued to t:he
present inventor ~n Deaember 18, 1973, and assigned ~o
RCA 70,435 ~
1~3S~
1 RCA Limited. The present system senses the average
intensity of both the input and output beams. The
ratio of the intensities is compared wit;h an adjustable
external component and the result is used to change
the temperature of the modulation crystal or of an
~ ~ :
external birefringent crystal compensator or external
... .
phase shiftiny device.
The present arrangement is particularly
suitable when the following condi~ions are met:
l. - A high carrier-to-noise ratio is always available
at the recelving end. In this condition, a slight
departure from the optimum conditions is rather
! inconsequential. This applies particularly to very
short communication links.
2. - Occasional infrequent readjustments of optical
b1as are acceptable. Such a frequency would be at the
rate of not more than once or twice a day.
3.~ - The system is expected to be at an acc6ssible~
location to allow the adjustments. ~ ;
2~ 4. - The u1timate~performance in carrier/noise is not
re~uired.
, .
S. - The cost must be relatively ~low and in particular
10w6r than the sy6tem disc1066d 1n the ~nited Statès~
~ ~ ~Patent Number 3,780,296.
;~ ~25 ~ Summary of _the Inv~ntion
The present invention relates to an electro~
optic modulator comprising a~light source for providing
an input beam of coherent potarized light radiation, an
elect~o-optic crystal modulator positioned in the path
~ of the 1ight beam, with an electr1c field applied to the
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RCA 70,435
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1 modulator to modulate the polarization state of the beam,
-~ and~a modulation splitter posit.ioned in the path of the
beam to convert the polarization modulation o~ the beam
into intensity modulation of the output beam emerging
from the modulation splitter. First and second
light intensity detectors are positioned to detect a .
portion of the input beam and of the modulated output
beam. The ratio between the input and the output
beams is obtained by a divisor means and i5 maintained
constant by a comparison means which controls a
compensator means, that is, rotates the polarization ;
of the output beam and consequently maintains the :
intensity of the output beam at a given value relative
to the input beam. . . . .. .
Brief Description of_the~ _ awin~
Fig.. l i9 a block diagram of a preferred . ~.
embodiment of a laser beam modulator in accordance wlth
the~present invention;
Figs. 2 & 3 are two block diagrams of a portion
.of the modulator shown in Fig. l comprising alternative
compensator means;
Figs. 4 & 5 are graphical representations ~ ~.
illustrating the relationship between the changes of
temperature of the modulator crystal and the variations
of the intensity of two output beams.
Detailed Descrlption -
: Referring now to Fig. 1, there is shown
generally at 10 an electro-optic crystal modulator
system in accordance with ~he present invention. 'rhe
lassr source 12 provides a coherent light radiatio.n which
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RCA 70,435
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1 passes through a polarizer 14 ~o give a linearly polarized
input be~m 16. The beam 16 passes through an'electro-
optic crystal modulator 18, such as a ~err cell, Faraday
modulator or a Pockel cell in which the light beam
undergoes polari~ation modulation in response to an ~'
electric field being applied to the modulator by an ~ :~
external source, not shown. The output beam at 16b
from modulator 18 is then transmitted through a
polarization splitter 22, which on the one hand converts ' ~ :
10 the polarization modulation 16b intQ intensity
~; .
modulation at 16c, and on the other hand, extracts the ' .~
polarization cornponent perpendicular' to the original . ~ .'.
polarization after modulation has taken place. This
extracted beam is shown as 23. The polarization splitter;
22:is a polarization prism such as~the known Glan Thompson'
prism. The polarlzation splitter 22~performs the double '
function which ~1s~, first to serve as an analyzer to ' :~
provide beam 16c,:and as a reflector to reflect the ~ ~ :
beam 23 which is complementary:to the beam 16c.
As explained above, beam 16c varies in in~ensity .
~:
due in particular~to variatlons in temperature in the' ~. ;'
electro-optic modulator 18. In order to detect and . ' : .
~ automatically control these~variation5~0f intensity,~
6~ ight inte~sity~beam splitter 24 which is a partIy ' :
;; ~ 25 :reflective mirror, i's positloned acrosz the Lnput
: . beam 16 while polari~ation sp'litter 22 is positioned
: ~ acrosz the output beam 16b. Each of these splitterz 22
,' & 2~ reflect a portion of the beam through beams 23 & 25
:'which are respectfully detected by detectors'26 & 28.
30 ~ Deteotor 2~ senzes the input beam 16 while detèctor 26
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RCA 70,435
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1 senses the output beam 16b. These detectors may be photo- ;
electric detectors, photo-cells, P-I-N detectors or in ~ :
general any electro-magnetic radiation detectors which
converts the intensity of a beam into a voltage
S indication. A divisor 30 receives the voltage recorded
by both detectors 26 and 28 in order to establish a
ratio between the electrical signals received. Such ~`
a divisor may be a small integrated circuit computer : .
which is intended to perform a division. The.voltage . ~
Vd which is the resultant voltage of the divisor 30 is ~ ~:
. compared with an adjustable dc voltage 32 which is
Located at the input of an operational amplifier 34.
The output of the amplifier 34 is connected to an :
appropriate heater assembly 36 thermally coupled to
the modulator 18 so as to vary the thermal conditions ~ -
of the birefri:ngent crystal contained therein. A
heater assembly of few w`atts is usually sufficient ~ ~
' for this purpose. . : : .
The theory ~ehind this arrangement can be~
explained as follows. The intensity modulation I at the - : :~
': output 16c can be ~hown to be.related to.the phase
. : shift r between the two polarization components resolved -
.. .
~-~ along the electro-optlc axis~of ~he modulator crystal ~
by the expression: ! ' . . ~., '';.
~ .25 ~ I=K3IOsin2r/2; . (1) ~:
.~ Where Io is .the intensity.of the.source-, and
:. K3 is the constant~ :
The phase shift r is itself produced by and
: ~ ~
. is proportional to a voltaga applied to:the crystal : : ~:
of the modul3tor. Although the description of the ~ ~ :
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RCA 70,435
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1 electro-optic efect refers here for definiteness to a
Pockel-effect type of crystal, similar arguments would
also apply to other electro-optic effects such as the Kerr
or Faraday modulator. The formula (l) would, of course,
be different for these other cases. ~ut of the
polarization splitter 22, the intensity at 23 is given
by:
Ic=K2IoCs r/2 (2)
Where K2 is another constant.
Detector 26 ~onverts this intensity into a
voltage Vc proportional to this intensity, so that
c 4 c
Detector 28 monitors the yet unmodulated source
by detecting the beam 25, namely:
~ (4)
The output of the detestor 28 is then
b K5Ib (5) ~
The output of both d~tectors 26 & 28 are then
~ .
applied to an operational or digital divisor 30. The
~.
; 20 output voltage Vd is then given by the formula
Vd=~K6Cos r/2 ~ (6)
Where K6 is a constant.
From formula (6) it can be realized that Vd
is independent of the term representing the intensity
of the source, namely Io. Consequently, the fluctuations
in the source intensity will not a~fect the output voltage~ ;
-
The opera~ion of the system will now be
,
explained by referring to the graphical representations
shown i~ Figures 4 and 5. These figures represent the
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RCA 70,435 ;~
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1 va~iations of intensity (I) relative to the variations of
temperature (T). In particular, it is notecl that
; as the temperature of the modulator crystal changes,
the intensity of the output beam 16c varies in a ~ :
sinusoidal type curve as shown in Figure 4. The
corresponding output beam 23 varies as shown in Figure 5.
Let's suppose that it is decided to stabilize
the optical bias at point 40 in the graph of Figure 4. .
The portion of the output beam at 23 for that desirecl
condition, is represented at 42 in Figure 5. This intensity
produces a certain value at the output of the
, detector 26 which ls, for instance, Vcl. The cor~espond-
i . ing voltage out of the divisor 30 is determined as Vdi.
If the adjustable dc voltage Eb is set so that it
will be equal to Vdl, the system will stabilize
tself at point 40 provided proper aare is taken that ` :~
the feedback at the output of the operational amplifier ~ `~
is of the proper polarity. This system becomes a closed- .
.~ loop system which will hold the optical bias at the
. 20 :proper operating point without being affected by the~
.
fluctuation in the laser output Io.
If another desired~operating point~is situated
:.J ~ :
'3 at 4~ in graph of Figure 4 so as to produce, in beam 23,
~i an intensit~ close to a maximum or a minimum, it is
desirable to obtain, instead, a signal of 90 out
.~
Y of phase to Ic. This can be obtained readily by placing
;~ : a beam splitter 21 ahead of the polarization separator
. or splitter 22 and passing the output of the splitter
21 through a quarter wave plate - polarizer combination
', . 37. The beam 38 is now 90 out of phase with the output
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RCA 70,435
1 beam 16c and is shown at position 46 in Figure 5. When
such a correction is required, the output beam 38 instead
of 23 is sensed by the ~etector 26.
The polarization splitter 22 may be replaced,
in certain conditions, by an analyzer and a partly
reflective mirror. With this arrangement, the analyzer
eliminates one polarization component and uses an
inclined mirror to reflect a portion of the output beam
in the direction of the detector 26.
The presen~ system comprises many advantages.
It constitutes a closed-loop type of operation. The
information detected is indepqndent of the variation
in the intensity of the laser source. The present
system does not require a pilot signal as known in the
prlor art and consequently no potential interference~
exists with communication signals. Furthermore, it
allows the uAe of external compensation but above
all, it is a very simple system which is not expensive.
However, the system should not be expected to correct
;~ 20 errors caused by transmission variation within the
modulator nor correct for detectivity changes in
detactors.
From the foregoing, it can be seen that the
invention is a system which uses the error signal obtained
from the ratio betweèn the source output and the
modulator output to correct for polarization rotation
changes in the modulator by using temperature faedback of
~- the modulating crys~al. It is also possible to ùse an
~ axternal compensator such as shown in Figures 2 and 3.
1 30 ~ As seen ln Figure 2, between source~12~and polarization
.`, - 1 1 - .
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1 splitter 22, a birefringent material and preferably a ~.
bir~fringent crystal 50 may be introduced in the path ~::
of the main beam in order to receive the information ~1
provided by the operating amplifier 34. In this
; 5 arrangement, the electro-optic modulator 18 is not
; thermally controlled but only the crystal 50 is provided
with a heater assembly 52 for adiusting the thermal
conditions affecting the light beam ~etween the two
splitters 22 and 24. : ~:
Another example of an external compensator ~;
consists of an optical compensator such as a Babinet-
Solail compensator. The structure and operation of
. this compensator has been explained in detail in the
.j . , ' ' ::' '.
above-identifled United States Patent Number 3,780,296.
: 15 The desire-d compensation for the change in birefringence
- .
i9 obtained by applying eleatri~al signals to the .
! : input of a s~tepping motor which controls a knob
actuating the wedge members of the compen~ator 54 ~hown
¦ in Figure 3.
.l. 20 Compensators of the type di9closed provide . . ~ ~ .
.1 the proper optical blas and stabilize lt with time, .
!j independently of the chang s in temperature of the
~; crystal or other birefringent material in the modulator
'~ as~embly.
The optlcal modulator i5 of the type which has
~: ~ a modulation ou~put substantia1ly proportional to the :
ource input level and ln which the output varies as a
: smooth function of the.temperature with no discontinuit~es
~i~ in the function over the operating~region.
- 30
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