Optical: systems and elements – Optical frequency converter – Dielectric optical waveguide type
Patent
1996-08-21
1998-05-05
Lee, John D.
Optical: systems and elements
Optical frequency converter
Dielectric optical waveguide type
359326, 385122, G02F 139
Patent
active
057483620
DESCRIPTION:
BRIEF SUMMARY
The field of the invention is that of optical frequency converters for which the waves are confined in a guide produced within a substrate having 2nd-order non-linear optical properties.
The various operations capable of being performed in a converter are summarised in the following table in which the optical frequencies of the incident or emitted waves are respectively designated by .omega..sub.i or .omega..sub.e :
______________________________________ Frequency doubling .omega..sub.i + .omega..sub.i .fwdarw. .omega..sub.e
= 2.omega..sub.i
Parametric fluorescence
.omega..sub.i .fwdarw. .omega..sub.e1
+ .omega..sub.e2
Sum of frequencies .omega..sub.i1 + .omega..sub.i2 .fwdarw. .omega..sub.e
3
Difference of frequencies
.omega..sub.i1 - .omega..sub.i2 .varies.3 .omega..sub.
e3
______________________________________
In order to obtain a high conversion efficiency, it is necessary to ensure the following two conditions:
The first condition relates to phase matching along the interaction length between the incident non-linear polarization induced by the incident illumination or illuminations and the wave or waves that this polarization has given rise to.
The second condition relates to the spatial overlap of the guided modes of the incident waves and of those created within the non-linear medium.
This second condition is difficult to satisfy fully, especially if the first condition relating to phase matching is provided artificially by the periodic modulation of a parameter involved in the non-linear interaction to compensate for the difference in propagation constant .DELTA.k associated with the dispersion of the refractive index of the non-linear material in question.
This is because the techniques generally used for providing this periodic modulation and those used for producing the guide do not lead to an always satisfactory overlap of the guided modes in the region in which this periodic modulation is obtained, since inscribing a guide by a positive index difference, without adversely affecting the intrinsic non-linear parameter of the material, usually imposes small index differences which are not conducive to maximum confinement of the guided modes in the guide. Typically, the situation is as in the configuration illustrated in FIG. 1 in which the guided modes extend over a depth P into the substrate, whereas the periodic modulation is provided over a depth D, which is less than P.
In point of fact, to obtain the best possible conversion efficiency, it is important that there be a maximum overlap between the guided modes and the region in which the phase-matching condition is satisfied. If the guided modes propagate outside this region, part of the light energy is not used for interaction in the frequency-conversion phenomena.
Within the framework of a frequency doubler, it is usually sought to obtain overlap of the 1st-order (or fundamental) mode of the incident wave at .lambda.l and of the 1st-order mode of the wave created at 1/2.lambda..sub.1, being in the direction Oz (indicated in FIG. 1). This is because, in the lateral direction Oy, overlap with the non-linear grating causes no problem given that the latter may be significantly wider than that of the guide.
In the case of a frequency converter of an optical parametric oscillator type, it is preferably sought to obtain overlap of modes of different orders. This is because the overlap of 2nd-order modes of the wave emitted at .omega..sub.i with the overlap of the 1st-order modes of the waves greater at .omega..sub.e1 and .omega..sub.e2 is facilitated.
In order to remedy this insufficiency in spatial overlap of the guided waves in the region in which the phase-matching condition is satisfied, the invention proposes to use a layer of a material of refractive index N, deposited on the surface of the non-linear material in which the phase-matching condition has been produced. More specifically, the subject of the invention is a frequency converter comprising a guide of refractive index n.sub.g, supplied by one or more light waves
REFERENCES:
patent: 5155791 (1992-10-01), Hsiung
patent: 5339190 (1994-08-01), Yamamoto et al.
patent: 5424867 (1995-06-01), Nihei et al.
patent: 5619369 (1997-04-01), Yamamoto et al.
patent: 5631766 (1997-05-01), Venhuizen et al.
Delacourt Dominique
Papillon Dominique
Papuchon Michel
Truffer Jean-Patrick
"Thomson-CSF"
Lee John D.
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