Raman Nath Diffraction

For a given orientation, if the RF frequency is slightly different from that required to match the Bragg criterion, diffraction will still occur. However, the diffraction efficiency will drop. The situation is shown in the figure below, where the acoustic wave-vector, K, is longer than the ideal “Bragg” wave-vector, K 0.

  1. Raman Nath Diffraction Data
  2. Raman Nath Diffraction Pattern
  3. Raman Nath Diffraction Definition

The Raman-Nath regime is observed at relatively low acoustic frequencies f and a small acoustooptic interaction length l (typically, f diffraction takes place at an arbitrary incidence angle of light (Fig. The diffraction pattern can contain many diffraction orders with symmetrical distribution of light. In 1922, Indian physicist C. Raman published his work on the 'Molecular Diffraction of Light', the first of a series of investigations with his collaborators that ultimately led to his discovery (on 28 February 1928) of the radiation effect that bears his name.

Abstract

We propose and analyze an efficient scheme for the lopsided Raman-Nath diffraction of one-dimensional (1D) and two-dimensional (2D) atomic gratings with periodic parity-time (PT)-symmetric refractive index. The atomic grating is constructed by the cold-atomic vapor with two isotopes of rubidium, which is driven by weak probe field and space-dependent control field. Using experimentally achievable parameters, we identify the conditions under which PT-symmetric refractive index allows us to observe the lopsided Raman-Nath diffraction phenomenon and improve the diffraction efficiencies beyond what is achievable in a conventional atomic grating. The nontrivial atomic grating is a superposition of an amplitude grating and a phase grating. It is found that the lopsided Raman-Nath diffraction at the exceptional point (EP) of PT-symmetric grating originates from constructive and destructive interferences between the amplitude and phase gratings. Furthermore, we show that the PT-phase transition from unbroken to broken PT-symmetric regimes can modify the asymmetric distribution of the diffraction spectrum and that the diffraction efficiencies in the non-negative diffraction orders can be significantly enhanced when the atomic grating is pushed into a broken PT-symmetric phase. In addition, we also analyze the influence of the grating thickness on the diffraction spectrum. Our scheme may provide the possibility to design a gain-beam splitter with tunable splitting ratio and other optical components in integrated optics.

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  • Received 4 September 2017

DOI:https://doi.org/10.1103/PhysRevA.97.033819

©2018 American Physical Society

Physics Subject Headings (PhySH)

Raman What we claim is:

1. An optical wavelength-division demultiplexer comprising:

a planar optical waveguide having a core and cladding region and being made of a material that is transmissive to light, the core having a first refractive index, the core having an input end and an output end having a longitudinal light guiding path therebetween, sides of the core being bounded by light guiding regions having a second refractive index, the light guiding regions for substantially confining light directed into the input end to within the core so that it may propagate to the output end; and,

means within the slab waveguide for changing the effective refractive index of sub-regions within the slab waveguide, said means for substantially perturbing incident light, said means and a region of the slab waveguide adjacent said means together forming a transmissive grating for operating in a Raman-Nath regime, for passing light incident upon the grating at a first angle, at second diffraction angle, the diffraction angle being dependent upon the first angle, the wavelength of the light incident upon the grating and upon the refractive index difference between the waveguide and the sub-regions within the waveguide, the period of the grating being substantially within an order of magnitude of the wavelength of the light incident upon it.


Raman
2. An optical wavelength-division demultiplexer as defined in claim 1, wherein the means within the slab waveguide for changing the effective refractive index of sub-regions within the waveguide are within the core of the waveguide, said means and a region of the core adjacent said means together forming the transmissive grating.

3. A optical wavelength-division demultiplexer as defined in claim 2 wherein said grating substantially prevents some of the light incident upon the grating from passing therethrough.

4. An optical wavelength-division demultiplexer as defined in claim 1, wherein the means for changing the effective refractive index comprise a plurality of bores transverse to the axis of propagation of the light incident upon the grating within the optical waveguide.

5. An optical wavelength-division demultiplexer as defined in claim 2, wherein the bores extends through at least the some of the cladding and into the core of the optical waveguide.

6. An optical wavelength-division demultiplexer as defined in claim 2, wherein the plurality of bores are substantially parallel spaced voids within the core region of the optical waveguide.

7. An optical wavelength-division demultiplexer as defined in claim 4, wherein the walls of the bores are coated with a coating that will substantially prevent light from passing therethrough.

8. An optical wavelength-division demultiplexer as defined in claim 4, wherein the plurality of transverse bores are spaced along a line that is transverse to the longitudinal axis of the optical waveguide.

9. An optical wavelength-division demultiplexer as defined in claim 4, wherein the plurality of transverse bores are spaced in an are to substantially conform to an optical wavefront.

Raman Nath Diffraction 10. An optical wavelength-division demultiplexer as defined in claim 2, wherein the core of the waveguide is comprised of plastic material being substantially transparent to light.

11. An optical wavelength-division demultiplexer as defined in claim 2, wherein the waveguide is comprised of polyimide.

12. An optical wavelength-division demultiplexer as defined in claim 2, wherein the core of the waveguide is substantially comprised of SiG0.3, and wherein the sides which clad the core are made of Si.

Raman Nath Diffraction Data


13. An optical wavelength selective device as defined in claim 1 including means integral with the core for focusing an incoming beam of light onto the transmissive refraction grating.

14. An optical wavelength selective device as defined in claim 13 including means integral within the core, for focusing light that has been transmitted through the grating onto a plurality of spaced detectors.

Raman Nath Diffraction Pattern


15. An optical wavelength selective device as defined in claim 14 wherein said means for focusing light comprises two parabolic reflectors.

16. An optical wavelength selective device as defined in claim 1, wherein the number of perturbations exceed 100.

17. An optical wavelength selective device as defined in claim 1, wherein the perturbations comprise substantially equally spaced bores extending into the core, the number of bores being greater than 500.

18. An optical wavelength selective device as defined in claim 1, wherein the number of perturbations exceed 1000.

Raman Nath Diffraction 19. An optical wavelength selective device comprising:

a planar waveguide having a core and being made of a material that is transmissive to light and having a first refractive index, the core having an input end and an output end, the sides of the core being bounded by light guiding regions for substantially confining light directed into the input end to within the core so that it may propagate to the output end;

means within the core having a second refractive index that differs from the first refractive index, said means for substantially perturbing incident light launched into the waveguide and for causing diffraction of said light, said means and the region of the core adjacent said means forming a transmissive grating for demultiplexing a plurality of wavelengths of the incident light, the period of the grating being such that it is within an order of magnitude of at least one of the plurality of the wavelengths of light, wherein the core is comprised of a plurality of SiG0.3 quantum wells, well barriers of said wells, being comprised of Si, and wherein the number of perturbations exceed 1000.



20. An optical wavelength selective device as defined in claim 19, wherein the light guiding regions are comprised of Si.

Raman Nath Diffraction Definition



21. An optical wavelength-division demultiplexer as defined in claim 2, including a lens integrally formed within the optical waveguide.