Systems and techniques for generating cylindrical vector beams
A technique is described for generating a cylindrically polarized beam, e.g., a radially or azimuthally polarized beam. An input optical fiber is provided that supports signal propagation in a fundamental LP 01 mode. A mode converter device converts the fundamental LP 01 mode into a higher-order LP 11 mode output that includes a linear combination of modes, including cylindrically polarized TM 01 and TE 01 modes and mixed HE 21 (even) and HE 21 (odd) modes. The LP 11 mode output propagates through a connected phase-engineered fiber having a refractive index profile that includes a steep refractive index step proximate to a peak amplitude of a mode intensity profile of the LP 11 mode, such that at least one cylindrically polarized mode has an effective refractive index that is sufficiently separated from those of the mixed modes to allow for coupling to the at least one cylindrically polarized mode with minimal cross-coupling.
1. A method for generating a cylindrically polarized beam, comprising:
(a) providing an optical signal comprising a fundamental LP 01 mode;
(b) providing an input optical fiber that supports propagation of the optical signal in the LP 01 mode;
(c) using a mode converter device to convert the LP 01 mode input into a higher-order LP 11 mode output, the higher-order LP 11 mode comprising a linear combination of cylindrically polarized TM 01 and TE 01 modes, and mixed HE 21 (even) and HE 21 (odd) modes;
(d) causing the LP 11 mode output to propagate through a phase-engineered fiber coupled to the mode converter device, wherein the phase-engineered fiber has a refractive index profile that comprises a steep refractive index step proximate to a peak amplitude of a mode intensity profile of the LP 11 mode output, such that at least one of the cylindrically polarized modes has an effective refractive index sufficiently separated from effective refractive indices of the mixed modes;
(e) selecting the at least one separated cylindrically polarized mode; and
(f) providing the at least one separated cylindrically polarized mode as an output.
2. The method of claim 1 , wherein the optical signal is a coherent light beam.
3. The method of claim 1 , wherein the at least one separated cylindrically polarized mode is radially polarized TM 01 mode.
4. The method of claim 1 , wherein the at least one separated cylindrically polarized mode is azimuthally polarized TE 01 mode.
5. The method of claim 1 , the effective refractive index of the at least one separated cylindrically polarized mode is separated from the effective refractive indices of the mixed modes by a factor of at least 1×10 −4 .
6. The method of claim 1 , wherein the refractive index step is at least approximately 0.015.
7. The method of claim 6 , wherein the refractive index step comprises a transition region no greater than approximately 1.0 micrometer in radial width.
8. The method of claim 1 , wherein the mode converter device is a grating fabricated into a portion of the phase-engineered fiber.
9. The method of claim 8 , wherein the grating is a microbend grating.
10. A fiber-based module for converting a fundamental LP 01 mode of an optical signal into a cylindrically polarized mode, comprising:
a mode converter device for converting the fundamental LP 01 mode into a higher-order LP 11 mode output, the higher-order LP 11 mode comprising a linear combination of modes, including cylindrically polarized TM 01 and TE 01 modes, and mixed HE 21 (even) and HE 21 (odd) modes; and
a phase-engineered fiber coupled to the mode converter device through which the LP 11 mode output propagates, the fiber having a refractive index profile comprising a steep refractive index step proximate to a peak amplitude of a mode intensity profile of the LP 11 mode output, such that at least one of the cylindrically polarized modes has an effective refractive index sufficiently separated from effective refractive indices of the mixed modes to allow for coupling to the at least one cylindrically polarized mode with minimal coupling to the mixed modes.
11. The module of claim 10 , wherein the mode converter device is a grating fabricated into a portion of an input optical fiber.
12. The module of claim 11 , wherein the grating is a microbend grating.