Method for optical modulation at periodic optical structure band edges

Abstract

In the band edge region, transmission and reflection of an optical wave can be made very sensitive to the change in radiation wavelength, a change in the modulator material refractive index, and/or a change in the material absorption. Controlling these parameters with the increased level of sensitivity is provided by modulation using the band edge region. A preferred embodiment method of the invention uses a periodic optical structure (i.e., a grating) on top of an optical waveguide structure. The combination of the periodic structure and the optical waveguide is designed so that the reflection and/or transmission of the guided wave have broad pass bands with narrow transition bands. The optical structure is exposed to an incident laser radiation with wavelength in one of the transition bands. Modulation of the incident laser radiation is controlled by the change in refractive index or absorption in the optical guided wave structure produced by the modulation voltage. The incident radiation is in transition band, instead of the center portion of the pass band. A preferred method for designing a suitable periodic optical structure having broad pass bands with narrow transition bands uses a high coupling coefficient between the periodic structure and the optical waveguide mode. Increasing the product of the coupling coefficient and the length of the grating narrows the transition region band and increases sensitivity of the modulation.

1. A method for optical modulation, the method comprising steps of:
 employing an optical modulator including a waveguide and a grating upon the waveguide, said optical modulator having its band edge region matched to an optical wavelength of interest for modulation; and  
 controlling one of the transmission and reflection of an optical wave from said optical modulator near the band edge region.  
 
     
     
       2. The method according to  claim 1 , wherein said step of controlling comprises monitoring and controlling a wavelength of incident radiation used to stimulate the transmission and reflection from said optical modulator. 
     
     
       3. The method according to  claim 1 , wherein said step of controlling comprises changing a material refractive index of the optical modulator. 
     
     
       4. The method according to  claim 1 , wherein said step of controlling comprises changing a material absorption index of the optical modulator. 
     
     
       5. A method for optical modulation using a periodic optical structure having a waveguide and a grating structure with broad pass bands and narrow transition bands, the method comprising steps of:
 exposing the periodic optical structure to an incident laser wavelength in the middle of one of the transition bands or at the beginning of one of the transition bands;  
 controlling modulation of the incident laser at a given wavelength by applying a modulation voltage to the periodic optical structure.  
 
     
     
       6. The method for optical modulation according to  claim 5 , wherein the periodic optical structure comprises a one-dimensional optical grating coupled to a parallel optical waveguide. 
     
     
       7. The method according to  claim 6 , wherein the optical grating has a coupling coefficient and length to create wide pass bands with rapid transitions from pass to stop bands as a function of effective index of the waveguide. 
     
     
       8. The method according to  claim 5 , wherein the waveguide comprises a LiNbO 3  waveguide and the grating is formed upon a SiO x  or polymer buffer layer. 
     
     
       9. A method for designing a periodic optical structure having broad pass bands with narrow transition bands, the optical structure being suited for optical modulation at a band edge of the narrow transition bands, the optical structure including a waveguide and an optical grating, the method comprising steps of
 choosing a high coupling coefficient for the waveguide material;  
 choosing a bias voltage applied to the optical waveguide and a grating periodicity so that its Bragg wavelength is mismatched with the laser wavelength, and selecting a grating length to enhance modulation efficiency.  
 
     
     
       10. The method according to  claim 9 , wherein said step of selecting comprises optimizing the grating length to achieve a high on/off modulation ratio with a smallest change of effective index by the signal voltage. 
     
     
       11. A method, comprising:
   providing light to an optical modulator having a first transition band between a pass band and a stop band;        controlling transmission of said light in or near the first transition band.     
     
     
       12. The method of  claim 11 , wherein said modulator includes a periodic optical structure coupled to an optical waveguide. 
     
     
       13. The method of  claim 11 , wherein said controlling includes controlling a wavelength of said light. 
     
     
       14. The method of  claim 11 , wherein said controlling includes changing a refractive index of said modulator. 
     
     
       15. The method of  claim 11 , wherein said controlling includes changing an absorption of said modulator. 
     
     
       16. A method, comprising:
   providing light to an optical modulator having a first transition band between a pass band and a stop band;        controlling reflection of said light in or near the first transition band.     
     
     
       17. The method of  claim 16 , wherein said modulator includes a periodic optical structure coupled to an optical waveguide. 
     
     
       18. The method of  claim 16 , wherein said controlling includes controlling a wavelength of said light. 
     
     
       19. The method of  claim 16 , wherein said controlling includes changing a refractive index of said modulator. 
     
     
       20. The method of  claim 16 , wherein said controlling includes changing an absorption of said modulator. 
     
     
       21. A method, comprising:
   providing light having a first wavelength to an apparatus that includes a periodic optical structure and an optical waveguide, wherein said apparatus has a first transition band between a pass band and a stop band, and wherein said first wavelength is in the beginning or the middle of said first transition band;        applying a voltage to the apparatus in order to modulate said light within said first wavelength.     
     
     
       22. The method of  claim 21 , wherein said applied voltage changes a refractive index of said apparatus. 
     
     
       23. The method of  claim 21 , wherein said applied voltage changes an absorption of said apparatus. 
     
     
       24. The method of  claim 21 , wherein said periodic optical structure is a grating, and wherein said light is incident laser radiation. 
     
     
       25. An optical modulation system comprising:
   an optical waveguide;        a periodic optical structure coupled to said optical waveguide to provide an optical modulator having a first transition band for incident light, the first transition band being between a pass band and a stop band;        said optical modulator being responsive to voltage for controlling transmission of the incident light for said optical modulator in or near said first transition band.     
     
     
       26. The optical modulation system of  claim 25 , wherein said voltage controls a wavelength of said incident light. 
     
     
       27. The optical modulation system of  claim 25 , wherein said voltage changes a refractive index of said optical modulator. 
     
     
       28. The optical modulation system of  claim 25 , wherein said voltage changes an absorption of said optical modulator. 
     
     
       29. The optical modulation system of  claim 25 , further comprising:
   a source for said voltage.     
     
     
       30. The optical modulation system of  claim 25 , wherein said periodic optical structure comprises a grating.