CA1057841A - Automatic optical bias control for light modulators - Google Patents
Automatic optical bias control for light modulatorsInfo
- Publication number
- CA1057841A CA1057841A CA250,978A CA250978A CA1057841A CA 1057841 A CA1057841 A CA 1057841A CA 250978 A CA250978 A CA 250978A CA 1057841 A CA1057841 A CA 1057841A
- Authority
- CA
- Canada
- Prior art keywords
- modulator
- output
- polarization
- crystal
- compensator
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour
- G02F1/0121—Operation of devices; Circuit arrangements, not otherwise provided for in this subclass
- G02F1/0123—Circuits for the control or stabilisation of the bias voltage, e.g. automatic bias control [ABC] feedback loops
Abstract
AUTOMATIC OPTICAL BIAS CONTROL FOR LIGHT MODULATORS
Abstract of the Disclosure In an electro-optic laser beam modulation system wherein the polarization modulation of the laser beam is converted into intensity modulation by an analyzer positioned in the path of the beam, a pair of photo-electric detectors detects a portion of the input and output beams. A divisor registers the ratio between the intensities detected. A compensator rotates the polarization of the output beam to establish a desired ratio, and a comparison system is connected to the divisor and the compensator to maintain the desired ratio.
Abstract of the Disclosure In an electro-optic laser beam modulation system wherein the polarization modulation of the laser beam is converted into intensity modulation by an analyzer positioned in the path of the beam, a pair of photo-electric detectors detects a portion of the input and output beams. A divisor registers the ratio between the intensities detected. A compensator rotates the polarization of the output beam to establish a desired ratio, and a comparison system is connected to the divisor and the compensator to maintain the desired ratio.
Description
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 ,. . .
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 ,. . .
-2- ~r~
:.
;,- . . . .. , ~ -, ., ~ ,.. . . . ...
. RCA 70,435 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 , ,. : ~
;: : . , :, . , : , , -. : ~ ; : -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.
:.
;,- . . . .. , ~ -, ., ~ ,.. . . . ...
. RCA 70,435 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 , ,. : ~
;: : . , :, . , : , , -. : ~ ; : -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 - . ' . ~S~
,~ ' . ~ .
RCA 70,435 7~
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 ; .
. ' RCA 70,435 ~'7~
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 ' ~7~
.: . : , .
.. . .
i'!: . . ~ .
RCA 70,435 l~S'~
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 ~ ~ :
.
. ,, , ' ' ............... ' ' ''~
3' ~ ' , :-RCA 70,435 ~S~
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 g_ ~
, ' ` , ' ~' RCA 70,435 ;~
S~7~
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 ~ --10--, , :, ~
- - : , . .. , , .. ,, , , :
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 - .
-.!
~C~ 70,~35 : .
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.
i, : . . ': . .
'' , ' ~, 'J ~:
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 - . ' . ~S~
,~ ' . ~ .
RCA 70,435 7~
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 ; .
. ' RCA 70,435 ~'7~
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 ' ~7~
.: . : , .
.. . .
i'!: . . ~ .
RCA 70,435 l~S'~
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 ~ ~ :
.
. ,, , ' ' ............... ' ' ''~
3' ~ ' , :-RCA 70,435 ~S~
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 g_ ~
, ' ` , ' ~' RCA 70,435 ;~
S~7~
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 ~ --10--, , :, ~
- - : , . .. , , .. ,, , , :
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 - .
-.!
~C~ 70,~35 : .
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.
i, : . . ': . .
'' , ' ~, 'J ~:
Claims (6)
- Claim 1 Continued.
means connected between said divisor means and said compensator means for operating said compensator means to maintain a constant ratio between said electrical signals detected by said detector means and consequently between the intensities of said input and output beams. - 2. An apparatus as recited in Claim 1, wherein both detector means are photo-electric detectors adapted to convert light intensities into electric voltages.
- 3. An apparatus as recited in Claim 2, wherein said compensator means comprises a heater assembly thermally coupled to said crystal modulator so as to change the thermal conditions of the latter according to the ratio of the light intensities detected by the detector means.
- 4. An apparatus as recited in Claim 2, wherein said compensator means comprises an optical device disposed in the path of the beam at the output of said modulator which is adapted to vary a degree of birefringence of the beam, and control means connected between said optical device and said divisor means for maintaining a desired ratio of the intensities detected by said detector means.
- 5. An apparatus as recited in Claim 4, wherein the optical device comprises a birefringent crystal and a heater assembly thermally coupled to said last-mentioned crystal whereby a change of temperature changes the degree of birefringence of the crystal.
- 6. An apparatus as recited in Claim 4, wherein the said optical device is a Babinet-Soleil compensator having a control knob wherein the position of said knob determines the degree of birefringence of said compensator, said control means comprising a motor mechanically coupled to said control knob to vary the position of said control knob to maintain said desired ratio.
1. An apparatus for providing modulation of light, of the type which comprises: a light source for providing a beam of coherent light radiation and polarized in a given direction; a polarization splitter positioned in the path of said beam; an electro-optic crystal modulator positioned in the path of said beam between said source and said polarization splitter for modulating the polarization of said beam about said given direction so that the polarization modulation of the beam is converted into an intensity modulated output beam, emerging from said polarization splitter, of a given intensity value about which said intensity modulation takes place; the improvement therein comprising:
a first light beam detector means positioned to detect the intensity of the light beam at the input of said crystal modulator and a second light beam detector means positioned to detect the light intensity of a polarization component of a said output beam, said detector means converting said respec-tive input and output beams into electrical signals, a divisor means for establishing the ratio of the electrical signals obtained by said first and second detector means, a variable compensator means for said modulator adapted to produce a rotation of the polarization of the bean at the output of said modulator and consequently vary the intensity of the beam emerging from said splitter,
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA250,978A CA1057841A (en) | 1976-04-26 | 1976-04-26 | Automatic optical bias control for light modulators |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA250,978A CA1057841A (en) | 1976-04-26 | 1976-04-26 | Automatic optical bias control for light modulators |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1057841A true CA1057841A (en) | 1979-07-03 |
Family
ID=4105773
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA250,978A Expired CA1057841A (en) | 1976-04-26 | 1976-04-26 | Automatic optical bias control for light modulators |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1057841A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5022034A (en) * | 1989-06-27 | 1991-06-04 | May A D | Laser device, including control of polarization mode |
-
1976
- 1976-04-26 CA CA250,978A patent/CA1057841A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5022034A (en) * | 1989-06-27 | 1991-06-04 | May A D | Laser device, including control of polarization mode |
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