Wavelength-independent coupler from fiber to an on-chip, demonstrated over an 85nm span Steven Wang, Tal Carmon, Eric Ostby and Kerry Vahala
Basics of coupling Importance of phase match ( λ ) 1 ( λ ) 2 ( λ) n 1 eff Efficient coupling ( λ ) 1 ( λ ) 2 Identical velocities Different velocities ( λ) n 1 eff Inefficient coupling 2 2 ( λ) = ( λ) = n eff 1 ( λ) Bent - Straight n eff 2 ( λ) ( λ ) 1 ( λ ) 2 Cavity Because of the asymmetrical geometry coupler is wavelength dependant
History Straight Bent fiber Asymmetrical Tapered fiber Fiber compatible, Wavelength dependant. Knight, Opt. Lett. 22, 1129-1131 (1997). Cai, PRL 85, 74-77 (2). Spillane, PRL 91, 4392 (23). Bent Corner Prism Almost symmetrical (Wavelength dependant) Tunable to a broad wavelength span Ulrich, JOSA 6 1337 (197) Sarid, APL 33 514 (1978) Shiller, Opt Lett 16 13 (1991) Ronald, Proc Ins Elect Eng Opt, 14, 177 (1993) Gorodetsky, Opt Comm,113 133 (1994) Need: 1) Wavelength independent 2) Fiber compatible
Motivation Straight Bent problematic Straight - Bent Power [AU] Signal Pump 2 3 4 5 6 7 Silica Transparency 1 12 14 16nm 18 2 Motivation: Open current technology of ultra-high Q cavities to be fiber accessible for applications in a regime spanning from the extreme UV to the IR band. 1μm 1μm One example: On-chip continuous visible emitter by third-harmonic generation. Need to couple in the IR pump (15 nm) while coupling out visible signal (5 nm) Carmon and Vahala, Visible continuous emission from a silica microphotonic device by third-harmonic harmonic generation Nature Physics (27), CLEO (26)
Can we combine symmetry with fiber coupler? Straight Bent fiber Past Bent Corner Today Bent Bent Tapered fiber Asymmetrical Fiber compatible, Wavelength dependant. Knight, Opt. Lett. 22, 1129-1131 (1997). Cai, PRL 85, 74-77 (2). Spillane, PRL 91, 4392 (23). Prism Almost symmetrical (Wavelength dependant) Tunable Ulrich, JOSA 6 1337 (197) Sarid, APL 33 514 (1978) Shiller, Opt Lett 16 13 (1991) Ronald, Proc Ins Elect Eng Opt, 14, 177 (1993) Gorodetsky, Opt Comm,113 133 (1994) Bent tapered fiber Symmetrical Wavelength independent Light velocity is the same for similar structures. Carmon et al, Submitted to Optics Express (27) This work
Bend-coupler fabrication Pull an optical fiber above a flame Push and twist Pull, anneal ~3 micron ~3 micron
Experimental Setup fiber Symmetrical coupling (wavelength independent) 5μm 4μm Diameter of a human hair
Experimental Results Variable coupling distance Bent - bent Operating along a 85nm span Visible wavelength is more than twice shorter than the IR Q for both colors > 2 millions (a) IR, 154.2nm Transmission [%] 1 8 6 4 2 1 Transmission [AU] FWHM =7.7MHz Q = 24 x 1 6 3 6 9 Detuning Frequency (MHz) 1 2 3 Detuning Wavelength (x1-12 m) (b) Visible, 682.1 Transmission [%] 8 6 4 2 Q = 23 x 1 6 1 2 Detuning Frequency (MHz) 1 2 3 4 5 Detuning Wavelength (x1-12 m) Transmission [AU] FWHM =17MHz
Experimental results ~1 μm 5μm Comparison with tapered fiber coupler Tapered coupler functions properly at one wavelength while at the other wavelength we could measure no coupling at all. (a) IR, 154.2nm Transmission [%] 1 8 6 4μm cavit y Bent coupler Transmission [AU] 4 FWHM Q = 2 =7.7MHz 24 x 1 6 3 6 9 Detuning Frequency (MHz) 1 2 3 Detuning Wavelength (x1-12 m) Transmission [%] Tapered fiber 1 8 6 4 2 1 2 3 4 5 Detuning wavelength (x1-11 ) 1 1 (b) Visible, 682.1 Transmission [%] 8 6 4 2 Q = 23 x 1 6 1 2 Detuning Frequency (MHz) 1 2 3 4 5 Detuning Wavelength (x1-12 m) Transmission [AU] FWHM =17MHz Transmission [%] 8 6 4 2 1 2 3 4 5 Detuning wavelength (x1-12 )
Experiment Fixed coupling distance 1 682.1 nm Simultaneous coupling of two wavelengths Transmission (%) 8 6 4 154.2 nm 2 1 2 3 Detuning Wavelength (x 1-11 m) Coupling distance scales with wavelength, this is why the short wavelength is under coupled while the long is over coupled
Advantages of bent-bent coupler (2-2nm, fiber compatible) mm length 1 μm length Evanescent wave All around <1 Deg Bent-bent: the mode is grazing the interface only in a short and azimuthally-selective region Bent-bent Bent-Straight Straight-Straight Good resistance for surface effects (e.g. roughness, contaminations) that degrade transmission Bent-bent configuration is Fat: Mechanically stable Thermally stable (Tapered fibers burn in vacuum) For a given velocity mismatch, the bent-bent configuration will acquire smaller phase-mismatch (better phase-match) Short coupling distance
Disadvantages of bent coupler Contains also high order modes Not a problem when coupling from fiber to. Light coupled out of the to high-order modes will be lost when entering the single mode fiber. Possible solution: Thin single-mode bent coupler (and thin ring )
Conclusion For circular Cavities the bent bent configuration Wavelength-independent (along an 85 nm span) Fiber compatible Facilitating applications spanning along the entire silica transparency span (2nm 2nm) The coupler mode is minimally grazing the surface Resistance for surface contaminations and roughness. Cross-section area is 25 times larger than Straight-straight coupler implies proportionally better: Mechanical strength Power handling capability For Photonic Crystals, thin films, waveguides etc : Free space Prism
www.its.caltech.edu/~tal/ vahala.caltech.edu/ Carmon et. et. al., al., Wavelength-independent coupler coupler from from fiber fiber to to an an on-chip on-chip,, demonstrated over over an an 85nm 85nm span. span. submitter to to Optics Optics Express (27) (27) The End
Over coupling Ideality better than 6% for the over-coupled regime and ideality better than 4% for the undercoupled region. The physical reason that ideality decreases with coupling distance is that high-order modes extend (evanescently) from the coupler to a greater distance which is longer than that of the fundamental mode. Transmission 1.8.6.4.2.5 1 1.5 2 2.5 Coupler to distance [μm] Carmon et. al., Wavelength-independent coupler from fiber to an on-chip, demonstrated over an 85nm span to Optics Express (27) S. M. Spillane, et. al., "Ideality in a fiber-taper-coupled microresonator system for application to quantum electrodynamics," Phys Rev Lett 91, 4392 (23).