CA1089067A - Electro-optical modulator/deflector - Google Patents
Electro-optical modulator/deflectorInfo
- Publication number
- CA1089067A CA1089067A CA257,719A CA257719A CA1089067A CA 1089067 A CA1089067 A CA 1089067A CA 257719 A CA257719 A CA 257719A CA 1089067 A CA1089067 A CA 1089067A
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- electro
- energy
- orders
- electrode pattern
- order
- 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.)
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- Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)
Abstract
ELECTRO-OPTICAL MODULATOR/DEFLECTOR
Abstract of the Disclosure An improved electro-optical modulator includes electro-optical material and an electrode pattern that is arranged in response to a voltage supply to provide an electric field in the material to influence the deflection of a light beam. The electrode pattern is non-symmetrical.
The beam thus can be selectively deflected in sense and in quantity to one or more of its higher energy orders according to the sense and value of the voltage supply. This is in contrast to prior art arrangements where usually only the first order is utilized, eg, in a cascadable electro-optic switch, so energy is uselessly dissipated in the second and higher orders.
Abstract of the Disclosure An improved electro-optical modulator includes electro-optical material and an electrode pattern that is arranged in response to a voltage supply to provide an electric field in the material to influence the deflection of a light beam. The electrode pattern is non-symmetrical.
The beam thus can be selectively deflected in sense and in quantity to one or more of its higher energy orders according to the sense and value of the voltage supply. This is in contrast to prior art arrangements where usually only the first order is utilized, eg, in a cascadable electro-optic switch, so energy is uselessly dissipated in the second and higher orders.
Description
~9~36'7 The invention relates to electro-optical modulator/
aeflectors.
Particular types of electro-optical modulators have been recently developed, such as, for example, described in Electronic Letters 9, 1973 pages 309 and 310, and in Proceedings of IEE Vol 119 No. 7, 1972, pages 807 to 814.
The operation of these modulators depends on the effect of applying a voltage to a symmetrical electrode pattern to induce periodic change of the refractive index in an electro-optical element. The resulting symmetrical periodic phasechange induced in the wave front of a light beam directed through or at the electro-optical element produces a far field pattern of the Raman-Nath form exhibiting symmetry about the zero energy order. The zero order of the output beam is then modulated by adjusting the applied voltage to the electrode pattern to alter the distribution of the light from the zero order into higher orders, and is often desired, to eliminate the zero orderO In many practical deflector configurations these modulators are inefficient as usually only the first order is utilized, in a cascadable electro-optic switch, for example, so that much energy is dissipated uselessly in the second and higher orders.
It is an object of the present invention to provide an improved electro-optical modulator.
In accordance with one aspect of this invention there is provided an electro-optica~ modulator comprising:
an electro-optical interaction medium, an electrode array formed on one surface of said medium and having its operative electrodes parallel to an incident light beam, ~ means for applying a ~oltage to said array, means for directing said light beam into said medium . - 2 -, -:
.
~)8~V67 such that the beam is totally internally reflected at said surface, said electrode array comprising a plurality of electrode pairs, each pair element separated by a distance which is less by several orders of magnitude than the distance between pairs, whereby, when a voltage is applied to said array, a diffractive grating is established within said medium causing diffraction of said light beam reflected from said surface.
Electro-optical modulator/deflectors according to the invention will now be described by way of invention with reference to the accompanying drawings in which:
Fig. 1 shows schematically and illustratively one type ;~
of modulator/deflector configuration;
Fig. 2 shows a conventional electrode pattern for use with modulators.
Fig. 3 shows a novel electrode pattern for use with modulator/deflectors;
Fig. 4 shows graphically the deflected beam energy distribution before and during the application of a selected voltage to the electrode pattern of Fig. 3;
~ ig. 5 shows schematically another type of modulator configuration having a symmetrical electrode pattern as earlier proposed; and Fig. 6 shows schematically a novel modulator/deflector of said another type of modulators.
'",'~.;
~ - 3 - ~
.
- . . .. ; ~ . .
6)fi7 Referring to Fig. 1, the modulator consists of electro optic material formed of a LiNbO3 xy cut crystal 10. The .
crystal 10 has three polished surfacesll, 12 and 13. The angles of surfaces 11 and 12 are arranged such that a col- -limated beam of light parallel to the plane of the surface 13 is deflected at the surfaces 11 and 12 to suffer total internal reElection at the surface 13. It will be appre-ciated that other than shown crystal shapes are possible to achieve the total internal reflection. ~owever, in the 10 form shown a crystal with overall dimensions of about 4 x 4 x 15 mm provides satisfactory operation.
According to earlier proposals an electrode pattern 14 is deposited on the surface 13 in an array as shown in Fig. :.
aeflectors.
Particular types of electro-optical modulators have been recently developed, such as, for example, described in Electronic Letters 9, 1973 pages 309 and 310, and in Proceedings of IEE Vol 119 No. 7, 1972, pages 807 to 814.
The operation of these modulators depends on the effect of applying a voltage to a symmetrical electrode pattern to induce periodic change of the refractive index in an electro-optical element. The resulting symmetrical periodic phasechange induced in the wave front of a light beam directed through or at the electro-optical element produces a far field pattern of the Raman-Nath form exhibiting symmetry about the zero energy order. The zero order of the output beam is then modulated by adjusting the applied voltage to the electrode pattern to alter the distribution of the light from the zero order into higher orders, and is often desired, to eliminate the zero orderO In many practical deflector configurations these modulators are inefficient as usually only the first order is utilized, in a cascadable electro-optic switch, for example, so that much energy is dissipated uselessly in the second and higher orders.
It is an object of the present invention to provide an improved electro-optical modulator.
In accordance with one aspect of this invention there is provided an electro-optica~ modulator comprising:
an electro-optical interaction medium, an electrode array formed on one surface of said medium and having its operative electrodes parallel to an incident light beam, ~ means for applying a ~oltage to said array, means for directing said light beam into said medium . - 2 -, -:
.
~)8~V67 such that the beam is totally internally reflected at said surface, said electrode array comprising a plurality of electrode pairs, each pair element separated by a distance which is less by several orders of magnitude than the distance between pairs, whereby, when a voltage is applied to said array, a diffractive grating is established within said medium causing diffraction of said light beam reflected from said surface.
Electro-optical modulator/deflectors according to the invention will now be described by way of invention with reference to the accompanying drawings in which:
Fig. 1 shows schematically and illustratively one type ;~
of modulator/deflector configuration;
Fig. 2 shows a conventional electrode pattern for use with modulators.
Fig. 3 shows a novel electrode pattern for use with modulator/deflectors;
Fig. 4 shows graphically the deflected beam energy distribution before and during the application of a selected voltage to the electrode pattern of Fig. 3;
~ ig. 5 shows schematically another type of modulator configuration having a symmetrical electrode pattern as earlier proposed; and Fig. 6 shows schematically a novel modulator/deflector of said another type of modulators.
'",'~.;
~ - 3 - ~
.
- . . .. ; ~ . .
6)fi7 Referring to Fig. 1, the modulator consists of electro optic material formed of a LiNbO3 xy cut crystal 10. The .
crystal 10 has three polished surfacesll, 12 and 13. The angles of surfaces 11 and 12 are arranged such that a col- -limated beam of light parallel to the plane of the surface 13 is deflected at the surfaces 11 and 12 to suffer total internal reElection at the surface 13. It will be appre-ciated that other than shown crystal shapes are possible to achieve the total internal reflection. ~owever, in the 10 form shown a crystal with overall dimensions of about 4 x 4 x 15 mm provides satisfactory operation.
According to earlier proposals an electrode pattern 14 is deposited on the surface 13 in an array as shown in Fig. :.
2 with the operative electrodes parallel to the incident light beam. A voltaga is applied in use to the electrode pattern and induces an electric fie.ld adjacent the surface 13 which .:.
alters the refractive index of the crystal. With the pattern shown, the modulator behaves in a similar manner to~.
a phase diffractian grating to alter the light output beam.
The output beam is diffracted into a.series of orders whose intensities vary with electrode voltage. For example, if a typical full modulation voltage of 70 volts is applied to the electrodes, the output light beam contains no zero order energy, the energy being transferred to other orders of energy. Thus, if these orders are stopped by suitable .
obstacles, the incident or original beam direction can be -seen to be modulated by the application of -the voltage.
.~
- 4 - .. : : .
:
?6t7 In Fig. 3,the electrode pattern comprises a plurality of pairs of electrodes, the fine pitch between electrodes ~ -~
forming the pairs being substantially less than the coarse pitch of the pairs. I-t is possible with this configuration to influence the beam such that a substantial majority of the beam is deflected into the first order~
In a typical situation in accordance with the present invention using the electrode pattern of Pig. 3 in the modulator/deflector, the deflected or output beam energy is shown in Fig. 4 for a selected voltage of 157 volts.
At zero applied volts, the input beam is not deflected so that the output beam energy distribution is as shown dotted in Fig. 4. If the applied voltaye is increased from the volts the distribution of energy changes from the dotted curve to arrive at the fulI line curve at or about ~157 volts.
Thus, at -~157 volts, in the configuration shown, a substan- -tial proportion ~in this case around 81%) of the beam energy is concentrated as first order energy.
At around ~314 volts the deflected beam energy concen-trates in the second order and it will be appreciated thatfor higher voltages higher energy order concentrations in the output beam could be achieved. In practice, the higher orders cannot normally be achieved effectively because electrical stress tends to cause breakdowns in the electro-optical material at voltages above about 250 volts in the arrangement described. In the other type of modulator/
deflector configuration to be described with reference to Fig. 6, higher orders are more readily achievable because the applied voltages can be substantially smaller for each energy order threshold.
:: :
, . ~:
Normally, as stated earlier, many practical devices rely for operation on only æero and first order output beams. If say a second or possibly a third order beam is desired, embodiments of this invention can be utilized. In a simple case, for example, the output beam energy can be concentrated in the first and second orders by selecting in Fig. 4 an applied voltage of around +235 volts. In that situation, the output beam energy is shared at least approximately equally between first order to second order, and zero and higher orders are at least almost eliminated.
Returning to Fig. 3, the precise dimensions for the electrodes will now be considered. The fine and coarse pitch of the electrodes depends on the electro-optical materials used and angle of incidence of the light ~angle ~ in Fig. 1) at the surface 13. In one configuration, a crystal formed of LiNbO3 ele¢tro- optical material is used. A thin layer of MgF2 of thickness of around the wavelength of the beam (0.6328 um) is deposited on the surface 13 of the crystal to prevent generation of a sub-diffraction pattern (as explained more fully J Physics D Applied Physics Vol. 7 1974 pages 2479 to 2482) by the electrodes in contact with the base of the crystal 10. The electrode pattern of Fig. 3 is then deposit-ed on the exposed MgF2 surface. The electrodes are 12 um wide and 3.5 mm long, the fine pitch between pair of elec-trodes is 45 um and the coarse pitch of the pairs i5 320 um.
When the angle ofoG (Fig. 1) is 85.25, an applied voltage of +I57 volts deflects 81% of the incident energy into the ~1 order ~see Fig. 4) as mentioned earlier.
For the same configuration, the application of -157 volts causes deflection of 81% of the ene~gy into the -1 order.
: ~', :, ::
' 0~,7 If the fine pitch is decreased, with the consequent increase in the coarse pitch, the magnitude of the voltage required to deflect the beam to the first order energy is appropriately reduced. However, there i5 a practical limi-tation, which depends on the electro-optical materials used, in that the fine pitch cannot be reduced too much otherwise a breakdown of the electrical field even at 157 volts in the region of the pairs of electrodes is likely to occur in the configuration described.
It will be noted that in this example the ratio of coarse pitch to fine pitch is approximately 7 to 1. General- , ly, we believe this or similar orders of ratio gi,ves the best results. The ratio depends upon the orientation property of the electro-optical material used and on the , "
angle of incidence ~ . The ratio relationship i5 however ;~
not a simple one and it is anticipated that although many similar ratios are operable, some experimentation is required to determine other optimum relationships.
It will be also noted that the operative magnitude of the applied voltage to deflect the beam into the first order, other orders or combinations of orders o energy is depen-dent on the angle ~ and the wavelength of the light used.
In Fig. 5 the other type of modulator is show~ and which is described more fully in Proceedings of IEE Vol. 119 No. 7, 1972 at pages 807 to 814. A symmetrical electrode pattern 20 is deposited on the broad face of a thin or , guiding electro-optical crystal 21. When a voltage is , "~
applied to the electrode pattern, the resulting periodic electrical field changes the retractive index of the crystal .
67 ~
for the input light beam which is polarised in a vertical direction. For particular electro-optical crystals, the light retains its polarisation while passing through the crystal 21. The light is diffracted as a result of the periodic field into first and higher orders, the plus and minus first orders being shown illustratively as the output ~;
beams in Fig. 5. In this arrangement although it is pos-sible to eliminate the zero order output beam by applying a voltage of say about 10 volts or above, second order and higher order output beams are also produced as a result of the applied voltage. In addition as shown both positive and negative energy orders are inherently produced.
In Fig. 6, a non-symmetrical electrode pattern 22 is provided. When a voltage of the polarity shown is applied ;
to electrode pattern 22, the output beam contains only positive order energy. Further, at around 10 volts, the majority of the output beam energy is first order eneryy.
Other orders of energy or combinations of orders Gf energy ;~
are provided by further increasing the voltage as discussed -20 earlier. If the polarity of the applied voltage is reversed -~
the beam is deflected into the negative energy orders.
The modulator/deflectors described are at least sub-- stantially insensitlve to temperature variation because a phase-modulated technique is used which is virtually inde~
pendent of temperature if temperature-independent electro :
optic coefficients are used, for example r22 in LiNbO3.
Suitable electro optic materials besides LiNbO3 include LiTaO3 B5N, ADP, XDP, KDXP, KDA and Ba2NaNb5O15.
'.-: ;'.' - 8 - ~
... - . "... .
6~
While a particular embodiments of the invention have been described above, it will be appreciated that various modifications may be made by one skilled in the art without departing from the scope of the invention as defined in the appended claims.
:
,':
' ~:
, _ g --:
:
alters the refractive index of the crystal. With the pattern shown, the modulator behaves in a similar manner to~.
a phase diffractian grating to alter the light output beam.
The output beam is diffracted into a.series of orders whose intensities vary with electrode voltage. For example, if a typical full modulation voltage of 70 volts is applied to the electrodes, the output light beam contains no zero order energy, the energy being transferred to other orders of energy. Thus, if these orders are stopped by suitable .
obstacles, the incident or original beam direction can be -seen to be modulated by the application of -the voltage.
.~
- 4 - .. : : .
:
?6t7 In Fig. 3,the electrode pattern comprises a plurality of pairs of electrodes, the fine pitch between electrodes ~ -~
forming the pairs being substantially less than the coarse pitch of the pairs. I-t is possible with this configuration to influence the beam such that a substantial majority of the beam is deflected into the first order~
In a typical situation in accordance with the present invention using the electrode pattern of Pig. 3 in the modulator/deflector, the deflected or output beam energy is shown in Fig. 4 for a selected voltage of 157 volts.
At zero applied volts, the input beam is not deflected so that the output beam energy distribution is as shown dotted in Fig. 4. If the applied voltaye is increased from the volts the distribution of energy changes from the dotted curve to arrive at the fulI line curve at or about ~157 volts.
Thus, at -~157 volts, in the configuration shown, a substan- -tial proportion ~in this case around 81%) of the beam energy is concentrated as first order energy.
At around ~314 volts the deflected beam energy concen-trates in the second order and it will be appreciated thatfor higher voltages higher energy order concentrations in the output beam could be achieved. In practice, the higher orders cannot normally be achieved effectively because electrical stress tends to cause breakdowns in the electro-optical material at voltages above about 250 volts in the arrangement described. In the other type of modulator/
deflector configuration to be described with reference to Fig. 6, higher orders are more readily achievable because the applied voltages can be substantially smaller for each energy order threshold.
:: :
, . ~:
Normally, as stated earlier, many practical devices rely for operation on only æero and first order output beams. If say a second or possibly a third order beam is desired, embodiments of this invention can be utilized. In a simple case, for example, the output beam energy can be concentrated in the first and second orders by selecting in Fig. 4 an applied voltage of around +235 volts. In that situation, the output beam energy is shared at least approximately equally between first order to second order, and zero and higher orders are at least almost eliminated.
Returning to Fig. 3, the precise dimensions for the electrodes will now be considered. The fine and coarse pitch of the electrodes depends on the electro-optical materials used and angle of incidence of the light ~angle ~ in Fig. 1) at the surface 13. In one configuration, a crystal formed of LiNbO3 ele¢tro- optical material is used. A thin layer of MgF2 of thickness of around the wavelength of the beam (0.6328 um) is deposited on the surface 13 of the crystal to prevent generation of a sub-diffraction pattern (as explained more fully J Physics D Applied Physics Vol. 7 1974 pages 2479 to 2482) by the electrodes in contact with the base of the crystal 10. The electrode pattern of Fig. 3 is then deposit-ed on the exposed MgF2 surface. The electrodes are 12 um wide and 3.5 mm long, the fine pitch between pair of elec-trodes is 45 um and the coarse pitch of the pairs i5 320 um.
When the angle ofoG (Fig. 1) is 85.25, an applied voltage of +I57 volts deflects 81% of the incident energy into the ~1 order ~see Fig. 4) as mentioned earlier.
For the same configuration, the application of -157 volts causes deflection of 81% of the ene~gy into the -1 order.
: ~', :, ::
' 0~,7 If the fine pitch is decreased, with the consequent increase in the coarse pitch, the magnitude of the voltage required to deflect the beam to the first order energy is appropriately reduced. However, there i5 a practical limi-tation, which depends on the electro-optical materials used, in that the fine pitch cannot be reduced too much otherwise a breakdown of the electrical field even at 157 volts in the region of the pairs of electrodes is likely to occur in the configuration described.
It will be noted that in this example the ratio of coarse pitch to fine pitch is approximately 7 to 1. General- , ly, we believe this or similar orders of ratio gi,ves the best results. The ratio depends upon the orientation property of the electro-optical material used and on the , "
angle of incidence ~ . The ratio relationship i5 however ;~
not a simple one and it is anticipated that although many similar ratios are operable, some experimentation is required to determine other optimum relationships.
It will be also noted that the operative magnitude of the applied voltage to deflect the beam into the first order, other orders or combinations of orders o energy is depen-dent on the angle ~ and the wavelength of the light used.
In Fig. 5 the other type of modulator is show~ and which is described more fully in Proceedings of IEE Vol. 119 No. 7, 1972 at pages 807 to 814. A symmetrical electrode pattern 20 is deposited on the broad face of a thin or , guiding electro-optical crystal 21. When a voltage is , "~
applied to the electrode pattern, the resulting periodic electrical field changes the retractive index of the crystal .
67 ~
for the input light beam which is polarised in a vertical direction. For particular electro-optical crystals, the light retains its polarisation while passing through the crystal 21. The light is diffracted as a result of the periodic field into first and higher orders, the plus and minus first orders being shown illustratively as the output ~;
beams in Fig. 5. In this arrangement although it is pos-sible to eliminate the zero order output beam by applying a voltage of say about 10 volts or above, second order and higher order output beams are also produced as a result of the applied voltage. In addition as shown both positive and negative energy orders are inherently produced.
In Fig. 6, a non-symmetrical electrode pattern 22 is provided. When a voltage of the polarity shown is applied ;
to electrode pattern 22, the output beam contains only positive order energy. Further, at around 10 volts, the majority of the output beam energy is first order eneryy.
Other orders of energy or combinations of orders Gf energy ;~
are provided by further increasing the voltage as discussed -20 earlier. If the polarity of the applied voltage is reversed -~
the beam is deflected into the negative energy orders.
The modulator/deflectors described are at least sub-- stantially insensitlve to temperature variation because a phase-modulated technique is used which is virtually inde~
pendent of temperature if temperature-independent electro :
optic coefficients are used, for example r22 in LiNbO3.
Suitable electro optic materials besides LiNbO3 include LiTaO3 B5N, ADP, XDP, KDXP, KDA and Ba2NaNb5O15.
'.-: ;'.' - 8 - ~
... - . "... .
6~
While a particular embodiments of the invention have been described above, it will be appreciated that various modifications may be made by one skilled in the art without departing from the scope of the invention as defined in the appended claims.
:
,':
' ~:
, _ g --:
:
Claims
1. An electro-optical modulator comprising:
an electro-optical interaction medium, an electrode array formed on one surface of said medium and having its operative electrodes parallel to an incident light beam, means for applying a voltage to said array, means for directing said light beam into said medium such that the beam is totally internally reflected at said surface, said electrode array comprising a plurality of electrode pairs, each pair element separated by a distance which is less by several orders of magnitude than the distance between pairs, whereby, when a voltage is applied to said array, a diffractive grating is established within said medium causing diffraction of said light beam reflected from said surface.
an electro-optical interaction medium, an electrode array formed on one surface of said medium and having its operative electrodes parallel to an incident light beam, means for applying a voltage to said array, means for directing said light beam into said medium such that the beam is totally internally reflected at said surface, said electrode array comprising a plurality of electrode pairs, each pair element separated by a distance which is less by several orders of magnitude than the distance between pairs, whereby, when a voltage is applied to said array, a diffractive grating is established within said medium causing diffraction of said light beam reflected from said surface.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB3459575 | 1975-08-20 | ||
| GB34595 | 1975-08-20 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1089067A true CA1089067A (en) | 1980-11-04 |
Family
ID=10367599
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA257,719A Expired CA1089067A (en) | 1975-08-20 | 1976-07-23 | Electro-optical modulator/deflector |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1089067A (en) |
-
1976
- 1976-07-23 CA CA257,719A patent/CA1089067A/en not_active Expired
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |