Wavelength tunable reflector

• US 20020131694A1
• Filed: 09/04/2001
• Published: 09/19/2002
• Est. Priority Date: 03/19/2001
• Status: Abandoned Application

• Current Assignee: BTI Photonics Inc.
• Original Assignee: BTI Photonics Inc.
• Inventors: Li, Feng, Kazakevitch, Anna, Tam, Robin, Fuh, Andy, So, Vincent
• Application Number:
  US09/944,381
• Publication Number:
  US 20020131694A1
• Time in Patent Office: Days
• US Class Current: 385/27
• CPC Class Codes: G02B 6/02061 Grating external to the fibre and in contact with the fibre, e.g. evanescently coupled, gratings applied to the fibre end mechanically induced in the fibre G02B6/02071, surface relief on the fibre G02B6/02066
• CPC Class Codes: G02B 6/266 the optical element being an attenuator
• CPC Class Codes: G02B 6/2932 comprising a directional router, e.g. directional coupler, circulator
• CPC Class Codes: G02B 6/29322 Diffractive elements of the tunable type G02B6/02195 takes precedence; optical modulation devices based on a change of the optical properties of the medium G02F1/00
• CPC Class Codes: G02B 6/29394 Compensating wavelength dispersion G02B6/29376 takes precedence; dispersion compensated optical transmission systems H04B10/2507
• CPC Class Codes: G02F 1/0115 in optical fibres
• CPC Class Codes: G02F 2201/307 Reflective grating, i.e. Bragg grating

Abstract

A novel wavelength tunable reflector and method are demonstrated. The wavelength tunable reflector comprises a birefringent material that may be a cholesteric liquid crystal with a helical structure that forms a phase grating. An optical transmission medium (OTM) is juxtaposed to the birefringent material such that a component of an evanescent field of an optical signal propagating through the OTM is perturbed by the phase grating. The period of the grating is tuned in at least one portion of the birefringent material by applying AC voltages across respective portion(s) of the birefringent material such that one or more channel of the optical signal, each having a specific center wavelength, are reflected. The wavelength tunable reflector is adapted for other applications such as wavelength selective add/drop multiplexers, wavelength selective variable optical attenuators, broadband spectrum equalization filters, gain flattening filters for optical amplifiers and re-configurable dispersion compensators.

Claims

1. A wavelength tunable reflector comprising:

an optical transmission medium (OTM); and

a birefringent material juxtaposed to a portion of the OTM so as to allow an evansecent field of an optical signal to interact with the birefringent material, the birefringent material having a tunable periodic variation in a refractive index in at least one portion of the birefringent material, the periodic variation forming a respective grating within each said at least one portion of the birefringent material.

2. A wavelength tunable reflector according to claim 1 wherein each grating has a period Λ

_i substantially satisfying a Bragg condition λ

_i=2n_effΛ

_i where n_eff is an effective refractive index, and each grating is adapted to cause at least partial reflection of any portion of the optical signal having a center wavelength λ

_i.

3. A wavelength tunable reflector according to claim 1 wherein the OTM comprises a core with a refractive index, n_core, and wherein the birefringent material has an extraordinary refractive index, n_e, and an ordinary refractive index, n_o, wherein a larger one of n_o and n_e is approximately equal to, but slightly less than n_core.

4. A wavelength tunable reflector according to claim 1 wherein the birefringent material is a liquid crystal.

5. A wavelength tunable reflector according to claim 1 wherein each grating is a phase grating.

6. A wavelength tunable reflector according to claim 1 wherein each phase grating has a tunable period.

7. A wavelength tunable reflector according to claim 5 wherein the birefringent material is a cholesteric liquid crystal with a helical structure.

8. A wavelength tunable reflector according to claim 7 wherein the helical structure has a helical axis which is perpendicular to a surface of the cholesteric liquid crystal that is adjacent to the portion of the OTM and wherein for each said at least one portion of the birefringent material a voltage applied across the cholesteric liquid crystal causes the helical axis to re-orient itself parallel to the OTM resulting in the helical structure forming the respective phase grating.

9. A wavelength tunable reflector according to claim 8 wherein a pitch length, p, of the helical structure forming the respective phase grating is dependent on the magnitude of the voltage applied.

10. A wavelength tunable reflector according to claim 8 wherein the cholesteric liquid crystal further comprises a dye adapted to absorb light and change a period of the phase grating.

11. A wavelength tunable reflector according to claim 1 comprising a plurality of electrodes adapted to allow application of a respective voltage across each said at least one portion of the birefringent material so as to tune the respective grating.

12. A wavelength tunable reflector according to claim 11 wherein the electrodes are thinner than the skin depth over which light can penetrate the electrodes allowing the evanescent field of the optical signal propagating through the OTM to interact with the birefringent material.

13. A wavelength tunable reflector according to claim 11 wherein the electrodes are made of Indium Tin Oxide (ITO) allowing an evanescent field of the optical signal propagating through the OTM to interact with the birefringent material.

14. A wavelength tunable reflector according to claim 11 wherein the electrodes are arranged in lines of electrodes on opposing faces of the birefringent material.

15. A wavelength tunable reflector according to claim 11 wherein the electrodes are coated with polyimide and the electrodes on one of two opposing faces of the birefringent material are rubbed unidirectionally perpendicular to the OTM while the electrodes on another one of two opposing faces of the birefringent material are also rubbed unidirectionally in a parallel but opposite direction to provide an in-plane axis of molecular orientation of molecules of the birefringent material.

16. A wavelength tunable reflector according to claim 1 comprising a controller adapted to control the periodic variation of each grating.

17. A wavelength selective add/drop multiplexer (WSADM) comprising the wavelength tunable reflector of claim 1.

18. A WSADM comprising the wavelength tunable reflector of claim 2, the WSADM adapted to allow dropping, through reflection, of at least one channel of one or more channels associated with the optical signal and adapted to allow adding, to the optical signal, at least one channel other than a channel of the one or more channels which are not dropped.

19. A wavelength tunable reflector according to claim 1 adapted to function as a wavelength selective variable optical attenuator.

20. A wavelength tunable reflector according to claim 2 adapted to function as a wavelength selective variable optical attenuator wherein a length of the gratings is adjusted to control the extent to which the any portion of the optical signal is reflected, thereby controlling attenuation of an un-reflected portion of the optical signal.

21. A broadband spectrum equalization filter (BSEF) comprising the wavelength tunable reflector of claim 1.

22. A gain flattening filter (GFF) comprising the BSEF of claim 21 adapted to equalize a gain profile of an optical amplifier.

23. A re-configurable dispersion compensator (RDC) comprising the wavelength tunable reflector of claim 1.

24. An RDC comprising the wavelength tunable reflector of claim 2 wherein the gratings form a chirped grating, whereby Fourier components of a waveform that enter the RDC sequentially in time, due to dispersion effects, are reflected at different points along the RDC in manner that Fourier components of a reflected waveform exit the RDC approximately simultaneously.

25. An RDC comprising the wavelength tunable reflector of claim 2 wherein the gratings form a chirped grating, whereby Fourier components of a waveform that enter the RDC sequentially in time, due to dispersion effects, are reflected at different points along the RDC in manner that Fourier components of a reflected waveform exit sequentially in time in a manner to pre-compensate for anticipated dispersion effects.

27. A method according to claim 26 wherein the juxtaposing a portion of the OTM comprises removing a portion of a cladding of the OTM and juxtaposing a core of the OTM adjacent to the birefringent material.

28. A method according to claim 26 wherein tuning the tunable grating comprises matching, at periodic intervals, a greater one of an extraordinary index of refraction, n_e, and an ordinary index of refraction, n_o, of the birefringent material with an index of refraction, n_core, of a core of the OTM, thereby forming the tunable grating.

29. A method according to claim 26 wherein the tuning of the tunable grating comprises applying a voltage across the birefringent material so as to control a period of the periodic variation.

30. A method according to claim 28 comprising setting a period of the periodic intervals to reflect distinct ones of the channels of the optical signal.

31. A method according to claim 28 wherein the matching comprises heating the birefringent material through convection resulting in a change in period of the periodic intervals.

32. A method according to claim 28 wherein the matching comprises heating the birefringent material by irradiating the birefringent material.

33. A method according to claim 26 wherein said tuning the tunable grating comprises tuning a length and period of a helical structure of the birefringent material.

34. A method of designing a wavelength tunable reflector of claim 2, the method comprising;

selecting at least one of one or more channels of the optical signal for reflection, wherein each channel has an associated one of the center wavelengths, λ

_i;

selecting, for each one of the selected channels of the optical signal, a fraction of power of the selected channel that is to be reflected;

determining, for each one of the selected channels of the optical signal, a length of a respective one of said gratings required to reflect the selected fraction of power of the selected channel;

determining, for each one of the channels of the optical signal, the period, Λ

_i, of the respective grating.

35. A method according to claim 34 wherein the determining, for each one of the selected channels of the optical signal, a length of a respective grating comprises choosing a plurality of sets of electrodes across which a voltage is applied to reflect the respective channel of the optical signal.

26. A method of reflecting a channel of an optical signal having a plurality of channels of respective center wavelengths, the method comprising;

juxtaposing a portion of an optical transmission medium (OTM) to a birefringent material in a manner such that an evanescent field of the optical signal is coupled to the birefringent material, the birefringent material having a tunable periodic variation in a refractive index that forms a tunable grating that allows the channel of the optical signal propagating through the OTM to be reflected; and

tuning the tunable grating so as to control the extent to which the channel of the optical signal is reflected.