The invention relates to non-linear optical devices having a frequency-conversion function.
Non-linear optical effects involving frequency conversion (e.g. optical parametric amplification, frequency up/down-conversion etc) generally require phase-matching of at least two optical fields for efficient operation. For example in the case of second harmonic generation, the refractive index nω of a non-linear medium for a fundamental field of frequency ω is required to be substantially the same as the refractive index n2ω for the second harmonic field at a frequency 2ω so that that the phase velocities of the fundamental and second harmonic fields are substantially the same. In bulk optical systems, phase-matching is typically achieved by exploiting the birefringence of a non-linear optical crystal.
Confinement of the optical fields within a non-linear optical waveguide provides a longer interaction length and hence higher conversion efficiency than is the case when using a non-linear medium in bulk form. However in certain cases, for example where the non-linear optical waveguide is a semiconductor waveguide, birefringent phase-matching is not possible. Some of these cases are important; for example semiconductor waveguides are of interest in the area of integrated photonics and have the advantage that they may be fabricated using well-established semiconductor processing techniques.
The present invention provides a non-linear optical device for converting optical radiation of a first frequency into optical radiation of a second frequency, said device comprising a first optical waveguide having a waveguide core comprising optically non-linear material, and means for exciting a first transverse mode of said optical waveguide at the first frequency, wherein the optical waveguide is arranged to support a second transverse mode at the second frequency such that the phase velocities of the first and second transverse modes at the first and second frequencies respectively are substantially equal.
The invention thus provides a non-linear optical device in which phase-matching of optical radiation at first ω1 and second ω2 frequencies is achieved by arranging for these frequencies to propagate in certain transverse modes within a non-linear optical waveguide, the order of the transverse modes being such that their phase velocities are substantially the same as a result of balancing the effects of chromatic and modal dispersion. For example, if ω1>ω2 (as would be the case for parametric down-conversion for example) then a TE00 rectangular waveguide mode of frequency if ω1 would have a lower phase velocity than that of a TE00 mode of frequency ω2. However if the optical waveguide supports a transverse mode of an appropriate order at the frequency ω2 then modal dispersion can counteract chromatic dispersion to achieve phase-matching and hence efficient frequency-conversion.
The means for exciting the first transverse mode of the first optical waveguide may comprise a second optical waveguide coupled to the first optical waveguide and means for exciting the fundamental mode of the second optical waveguide at the first frequency, wherein the longitudinal axes of the first and second optical waveguides lie in the same plane and intersect at an angle which provides for fundamental mode radiation at the first frequency in the second optical waveguide to excite said first transverse mode of the first optical waveguide. The angle at which the longitudinal axes of the first and second waveguides intersect dictates the order of the first transverse mode which is excited in the first waveguide and may be chosen appropriately for a particular device providing conversion from one frequency to another.
The efficiency with which optical radiation of the first frequency may be coupled from the second waveguide into the first waveguide is around 50%. However, if the means for exciting the first transverse mode of the first optical waveguide further comprises a third optical waveguide coupled to the first optical waveguide and means for exciting the fundamental mode of the third optical waveguide at the first frequency, the longitudinal axes of the first and third optical waveguides lying in the same plane and intersecting at an angle which provides for fundamental mode radiation at the first frequency in the third optical waveguide to excite said first transverse mode of the first optical waveguide, the total efficiency with which light of the first frequency is coupled from the second and third waveguides into the first waveguide is significantly greater than 50%.
A convenient arrangement for exciting the fundamental modes of the second and third waveguides comprises a multimode interference (MMI) splitter. A single input at the first frequency (e.g. from a semiconductor laser) may be divided into first and second portions by the MMI splitter (e.g. a 1-to-2 way MMI splitter); the first and second portions output from the MMI splitter may then be used to excite the fundamental modes of the second and third waveguides.
MMI devices for lossless splitting of an optical input are described in detail elsewhere, for example European Patent 0 563 084.
A device of the invention could be a parametric frequency up- or down-converter, the first frequency being the pump frequency. Alternatively the device could be an optical parametric amplifier or it could provide second harmonic generation, the first frequency being the pump frequency.
The first optical waveguide may be a semiconductor optical waveguide, for example a waveguide of the GaAs/AlGaAs material system.
Embodiments of the invention are described below with reference to the accompanying drawings in which:
In other embodiments of the invention, the angle at which the longitudinal axes of the input waveguides intersect that of the frequency-converting waveguide may be arranged to achieve excitation of any desired transverse mode of the frequency-converting waveguide. The particular transverse mode excited is chosen such that the phase velocity of the mode at the input frequency is substantially equal to that of some other supported mode of the waveguide at a desired output frequency, thus achieving phase-matching by balancing the effects of chromatic and modal dispersion within the frequency-conversion waveguide.
The device 100 of
Number | Date | Country | Kind |
---|---|---|---|
0822356.2 | Dec 2008 | GB | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/GB2009/002745 | 11/25/2009 | WO | 00 | 5/24/2011 |