Non linear thin layer optical device

The invention relates to non-linear optical devices in which significant non-linear interactions are obtained by establishing a "quasi phase matching". The optical device according to the invention comprises on the surface of a substrate a periodic structure formed of zones in which the non-linear coefficient alternately assumes two values of opposite signal. The invention also relates to a process for producing this device. The invention allows generation of harmonics, frequency changing, parametric amplification or parametric oscillation.

This invention relates to non-linear thin-layer optical devices with which 
it is possible to obtain significant non-linear interactions, enabling a 
wave of different frequency to be generated from one or more incident 
waves. 
In order to obtain the best transfer of energy from the incident waves to 
the generated wave, the non-linear polarisation and the wave which is 
propagated freely at the interaction frequency must be in phase throughout 
the device. It is known that phase matching can be established in crystals 
having non-linear properties in guided or unguided optics. To this end, 
the propagation constant of the free wave must be equal to the sum of the 
propagation constants of the interacting waves. This result is obtained 
for example by acting on the polarisation directions of the waves, on the 
dimensions of the guide in guided optics or on the orientation of the 
crystals relative to the direction of propagation, thus utilising the 
anisotropy of these crystals. 
In numerous cases, it is impossible to fulfill the condition of phase 
matching. In cases where phase matching is possible, it is critically 
dependent upon the experimental conditions. It is for this reason that, in 
many cases, it is considered sufficient to establish a state of quasi 
phase matching by periodically reducing the accumulated phase difference 
by creating a network of suitable pitch consisting of zones for which the 
non-linear coefficient in the propagation direction alternately assumes 
two values of opposite sign. Although the results in regard to the 
effectiveness of conversion, i.e. in regard to the intensity of the 
generated wave relative to the excitation wave, are not as good as in the 
case of phase matching, they can nevertheless be interesting provided that 
the non-linear coefficient is high and that the number of zones is large. 
In the case of a bulk device, the network is formed by a stack of 
differently cut crystals, but unfortunately the requirements as to the 
precision of the elements in regard to their dimensions and alignment 
limit the choice of the materials and also the number of elements of the 
network. 
The present invention relates to a thin layer optical transmission device 
having a network-like structure and to a process for its production. The 
masking techniques commonly used in this field eliminate the problem of 
alignment and provide for high precision. In addition, the invention 
affords a possibility of electrical adjustment for adjusting the 
quasi-phase matching in accordance with the radiation using. Among other 
effects, the optical device allows generation of harmonics, frequency 
changing, parametric amplification and parametric oscillation. 
In accordance with the present invention, there is provided a non-linear 
optical device for generating a non-linear interaction between optical 
radiations propagated along a direction z in an homogeneous thin layer of 
ferro-electric material, said device comprising a thin layer made of 
ferro-electric domains and a first pair of electrodes deposited on said 
layer; said electrodes being arranged opposite one another and having a 
periodic shape delimiting a succession of odd and even zones of length L 
on said layer in said direction z, L being equal to an odd multiple of the 
coherence length of said interaction; the ferro-electric domains of the 
successive zones having been alternately polarised in opposite directions, 
so that the non-linear coefficient of said material alternately has 
opposite signs in the successive zones.

In the following description; we shall consider an optical 
guided-transmission device. However, the invention is also applicable to 
the case of non-guided propagation on the surface of a substrate. The 
description is concerned more especially with a device for generating 
harmonics from an incident luminous wave, although this does not exclude 
other potential applications utilising non-linear effects. It will first 
of all be assumed that the guide is monomode with respect to the incident 
radiation and the generated harmonic wave. 
FIG. 1 shows an optical wave guide 1 propagating a luminous wave 2 of 
wavelength .lambda.. The electrical field in the guide is expressed as 
follows: E.sub.w =A (x,y) [EXP i (wt-.beta..sub.w z)]e. A (x,y) represents 
the distribution of the amplitude of the field in the directions x and y 
perpendicular to the propagation direction z; .beta..sub.w is the guided 
propagation constant; w is the pulsation of the wave: w=2.pi.c/.lambda., c 
being the speed of light; e is the unit polarisation vector of the wave. 
The electrical field corresponding to a free wave travelling at the 
pulsation 2 w is expressed as follows: 
E.sub.2w =B (x,y) EXP i (2 wt-.beta..sub.2w z) s. B (x,y) represents the 
distribution of the amplitude of the field; .beta..sub.2w is the guided 
propagation constant of the free harmonic wave; s is the unit polarisation 
vector. The electrical field E.sub.w induces a polarisation of which the 
development to the second order may be expressed as follows: 
p=.chi..sup.(1) E.sub.w +.chi..sup.(2) E.sub.w E.sub.w ; .chi..sup.(1) is 
the linear susceptibility tensor and .chi..sup.(2) is the non-linear 
susceptibility tensor. The second order term P.sub.NL =.chi..sup.(2) 
E.sub.w E.sub.w translates the non-linear response of the medium to the 
applied field. In the interests of simplification, it will be assumed that 
the second order tensor .chi..sup.(2) may be reduced to a single 
coefficient .chi..sub.NL for a given direction of E.sub.w, so that: 
EQU P.sub.NL =.chi..sub.NL A.sup.2 (x,y)EXP i(2wt-2.beta..sub.w z). 
The interaction between the polarisation P.sub.NL and the free wave 
E.sub.2w produces the generation of an harmonic wave. This interaction 
depends essentially upon the phase shift between the two waves: 
(2.beta..sub.w -.beta..sub.2w)z. The calculations developed in the journal 
THOMSON-CSF, Vol. 6, No. 4 of December 1974, lead to the expression of the 
intensity I of the generated harmonic wave: 
##EQU1## 
where K is a constant which depends upon w, upon the effective indices of 
the guide for the fundamental wave: n(w) and the harmonic wave: n(2w) and 
upon the coefficient .chi..sub.NL ; I.sub.o is the intensity of the 
excitation wave; n(w) and n(2w) are respectively associated with 
.beta..sub.w and .beta..sub.2w by the relations: 
EQU .beta..sub.w =n(w)w/c and .beta..sub.2w =2n(2w)w/c. 
The variations of I with respect to the interaction length z are 
represented in the diagram of FIG. 2. When phase matching is established, 
i.e. 2.beta..sub.w =.beta..sub.2w, i.e. n(w)=n(2w), the curve C.sub.1 is 
obtained. The harmonic waves generated by the various points of the guide 
are always in phase and their respective energies are added together. When 
n(w).noteq.n(2w), the curve C.sub.2 is obtained. The intensity of the 
harmonic wave periodically passes through a maximum: for z=L.sub.c, 
z=3L.sub.c, . . . where L.sub.c is the length of coherence: L.sub.c 
=.pi./(2.beta..sub.w -.beta..sub.2w). Between two maxima, the intensity 
disappears because of an inverse transfer of energy. Since the required 
objective is the generation of harmonics with maximum effectiveness, if 
phase matching is impossible or difficult to obtain, it is possible to 
obtain a quasi-phase matching whereby it is possible, where z&gt;L.sub.c, to 
avoid energy being transferred to the fundamental wave. Where z=L.sub.c 
(or z=3L.sub.c . . . ) the polarisation P.sub.NL is in phase opposition 
relative to the wave E.sub.2w. By inverting the sign of the coefficient 
.chi..sub.NL, it is possible to produce a phase shift of .pi. and thus to 
reestablish the conditions under which energy is transferred to the 
harmonic wave. By periodically effecting this in version of sign by a 
network of which the pitch is equal to L.sub.c, the curve C.sub.3 is 
obtained. Since the value of I increases with the interaction length z, 
the desired values may be obtained with a sufficiently long guide. 
FIG. 3 shows the structure of an optical device according to the invention. 
The guide 1 situated on the surface of a substrate 3 is formed by a 
succession of zones of length L.sub.c or an odd multiple of L.sub.c, 
aligned in the direction of propagation of the light. In these zones, the 
non-linear coefficient .chi..sub.NL alternately assumes two opposite 
values .chi..sub.1 and .chi..sub.2. The incident radiation 2 is introduced 
into the guide by a coupling device 6. A wave 20 having two components of 
wavelength .lambda. and .lambda./2, the latter being the harmonic 
component, is collected at the output end. 
The successive steps of a process for producing the device of FIG. 3 are 
shown in FIG. 4. The described embodiment leads to the sign inversion: 
.chi..sub.1 =.chi., .chi..sub.2 =-.chi., but may readily be extended to 
the general case where .chi..sub.NL assumes two separate values of 
opposite sign. According to the invention, the sign inversion of 
.chi..sub.NL is obtained by using a ferro-electric material and inverting 
the polarisation of the ferro-electric domains. The first step shown at 
(a) and (b) concerns the production of the wave guide. FIG. 4(a) shows a 
substrate 3 made of a ferro-electric material which we shall assume to be 
lithium tantalate which is particularly advantageous because its 
non-linear coefficient along the axis z: .chi..sub.33 is very high (about 
20.10.sup.-12 m/V). The guide is obtained by localised metallic diffusion, 
for example of niobium Nb. A strip 4 of niobium corresponding to the 
location of the desired guide is formed by masking. The replacement of 
tantalum atoms by niobium atoms creates in the unmasked part a zone having 
an index higher than the index of the substrate. Diffusion is carried out 
at about 1000.degree. C., i.e. above the Curie temperature which is about 
700.degree. C. The guide 1 shown at (b) is obtained after diffusion. 
The second step shown at (c) consists in polarising the crystal, and more 
particularly the guide zone, so that all the ferro-electric domains are 
polarised in the same direction. This direction corresponds to the axis c 
of the crystal. An electrical voltage V.sub.o is applied between 
electrodes 7 and 8 attached to the substrate 3, creating a transverse 
electrical field. This step is carried out at a temperature slightly below 
the Curie temperature. The voltage V.sub.o must be sufficient for all the 
ferro-electric domains to be polarised in the same direction symbolised by 
the arrow x. The non-linear coefficients in the guide and, in particular, 
the coefficient .chi..sub.33 relative to an incident wave polarised 
parallel to the axis c of the crystal are constant. 
The third step is shown at (d). It involves the production of the network 
of FIG. 3. After having removed the electrodes 7 and 8, a set of 
electrodes 9 and 10 in the form of crenels, of which the spacing is equal 
to the length of coherence of the guide, is deposited on the substrate 3. 
The value of the length of coherence may be obtained either empirically or 
by calculation. It is approximately 5 .mu.m for the selected example. The 
distance between the electrodes is alternately equal to d, which may be 
selected equal to the width of the guide, and D which is considerably 
greater. A voltage V.sub.1 opposite in polarity to V.sub.o is applied 
between the electrodes 9 and 10 so that the electrical field V.sub.1 /d Is 
sufficient to invert the direction of polarisation of the domains, the 
field V.sub.1 /D being two weak to effect the inversion. After suppression 
of the voltage V.sub.1, there are obtained zones of length L.sub.c : I, 
II, III, . . . , in which the domains are alternately oriented in the 
direction x (II, IV, VI) and in the direction x' opposite to x (I, III, 
V), so that the coefficient .chi..sub.NL is alternately positive and 
negative, whilst retaining the same value .chi..sub.33. The electrodes are 
formed by masking processes similar to those used for the production of 
semi-conductors. The precision is about 0.1 .mu.m over a length of as much 
as 5 cm. The number of zones may thus be very considerable. Because of the 
difficulty to know the exact value of the coherence length, it may be 
preferable to form several sets of electrodes differing in their spacing 
and to determine which set produces the best quasi phase matching by 
measuring the harmonic power generated. In this case, too, the inversion 
of polarisation takes place at a temperature slightly below the Curie 
temperature. The temperature may be reduced providing the voltage V.sub.1 
is increased. The exact value of the voltage V.sub.1 is experimentally 
determined because, by means of optical processes using polarised light, 
it is possible to observe the inversion of the domain polarisations. 
In spite of the high degree of precision obtained by the electronic 
maskers, if the number of zones is very large (if L.sub.c =5 .mu.m, the 
number of zones may reach 10,000), the final error is in danger of being 
troublesome. In addition, the value of the coherence length may depend 
upon experimental conditions, particularly temperature, and may also 
depend upon the wavelength because of its relation with the effective 
indices n (w) and n(2w). It is therefore of advantage to be able to 
effect, for each use of the device, a fine adjustment of the coherence 
length without modifying the electrodes. 
FIG. 5 shows one example of the adjusting means. Use is made of the fact 
that ferroelectric materials have electro-optical properties. By applying 
a suitable electrical field to each zone, it is possible to modify the 
indices n (w) and n (2w) differently and hence to act on the coherence 
length. Since the polarisations of two adjacent zones are inverted, 
electrical fields of opposite values have to be applied to two adjacent 
zones in order to obtain a uniform modification of the indices throughout 
the guide. A new set of electrodes 11 and 12 having a shape complementary 
to that of the electrodes 9 and 10 is used for the zones II, IV, VI. The 
electrodes 11 and 12 are isolated from the preceding electrodes by a layer 
13 of dielectric material, for example silica. The electrodes 9 and 12 are 
electrically connected as are the electrodes 10 and 11. The electrodes 11 
and 12 are connected to a source of variable d.c. voltage V.sub.2. An 
electrical field V.sub.2 /d is thus obtained in the zones I, III, V, 
whilst an electrical field -V.sub.2 /d is obtained in the zones II, IV, 
VI, the fields V.sub.2 /D and -V.sub.2 /D being very weak. By measuring 
the intensity of the harmonic wave, V.sub.2 is acted on to obtain the 
maximum intensity. By virtue of this possibility of electrical adjustment, 
it is possible to obtain a harmonic power of greater than 10 mW for a 
guide 5 cm long and an incident power of 100 mW. The wavelength range for 
the incident light may extend from 0.6 .mu.m to several .mu.m. 
It has been assumed thus far that the guide is monomode with respect to the 
incident wave and the generated wave. Although it is possible to use a 
multimode guide, the interaction is far less strong in that case because 
the coherence length depends upon the mode and the quasi-phase matching 
can only be obtained for one mode for the incident wave and one mode for 
the generated wave. It is therefore preferable for the dimensions of the 
guide to allow the propagation of a single mode in the wavelength ranges 
in question. 
The device produced by the process described above has various 
applications. In addition to the generation of harmonics, it may be used 
in the production of a parametric amplifier. The guide is excited by two 
waves, a pumping wave of pulsation w.sub.p and a signal wave of pulsation 
w.sub.s. If the pitch of the network is equal to 
.pi./.beta.(w.sub.s)+.beta.(w.sub.p)-.beta.(w.sub.s), the pulsation of the 
generated wave is w.sub.s so that the signal wave is amplified to the 
exclusion of any wave having a pulsation different from w.sub.s. 
Similarly, a parametric oscillator may be produced by placing a 
network-like guide with a pitch corresponding to a pulsation w.sub.s 
between two mirrors highly reflective to a radiation of pulsation w.sub.s. 
Of all the waves spontaneously transmitted in the cavity thus formed, only 
the waves of pulsation w.sub.s are amplified and an oscillator with a 
wavelength electrically controllable by the voltage V.sub.2 is obtained.