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Method for forming spatially-varying distributed Bragg reflectors in optical media
| Details |
Inventors: Mizrahi, Victor; Mollenauer, Linn F.;
Assignee: AT&T Bell Laboratories (Murray Hill, NJ)
Primary Examiner: Ben; Loha
Assistant Examiner: Juba, Jr.; John
Attorney, Agent or Firm: Finston; Martin I.
A method for forming a grating in a photosensitive medium such as a photosensitive optical fiber. The method comprises impinging a pair of interfering, actinic beams onto the medium, and during the impinging step, advancing the illuminated portion of the interference pattern relative to the medium. The advancement is carded out without changing the phase, or registration, of the interference pattern. According to one embodiment of the invention, a grating having a spatially dependent period is produced by varying the wavelength or the intersection angle of the actinic beams during the advancement. According to a second embodiment of the invention, a grating having a spatially dependent refractive index perturbation is produced by varying the dose of actinic radiation received by the medium during the advancement. |
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DETAILED DESCRIPTION We have discovered a new method for making gratings, such as Bragg gratings, having spatially dependent periods.
In contrast to prior an methods, the period can be independently specified in different portions of the grating.
As a result, a broad range of functional forms can be specified for the spatial dependence.
In one embodiment, the invention involves a method for forming a grating along an axis, to be referred to as an "optical propagation axis", by exposing a photosensitive medium.
The direction of this axis is referred to herein as the "axial" direction.
The method includes the step of producing two collimated, non-collinear beams of electromagnetic radiation having an actinic wavelength .
lambda.
, i.
e.
, a wavelength capable of inducing refractive index changes in the medium.
The two beams are impinged on at least a portion of the medium at an intersection angle .
phi.
, such that a periodic interference pattern is created on the impinged portion.
The method further includes the step of advancing the illuminated portion of the interference pattern relative to the medium such that at least local coherence of the interference pattern is preserved.
The method further includes, during the advancing step, the step of changing the product ##EQU2## such that the interference pattern has a spatially varying period.
In a second embodiment of the invention, a Bragg grating is formed by producing interfering, actinic beams, impinging them on a photosensitive optical medium, and advancing the illuminated portion of the interference pattern relative to the medium, as recited above, resulting in formation of a refractive index perturbation in the medium.
The inventive method, in this embodiment, further includes, during the advancing step, varying the dose of actinic radiation received by the various points of the resulting refractive index perturbation.
As a result of this variation, the average amplitude of the perturbation is made to vary, in the axial direction, according to a predetermined pattern
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