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Optical Isolator: Application to Photonic Integrated Circuits

Optical Isolator: Application to Photonic Integrated Circuits. Tetsuya MIZUMOTO Dept. of Electrical and Electronic Eng. Tokyo Institute of Technology. Outline. Bulk optical isolator magneto-optic (Faraday) effect operation principle Waveguide optical isolator

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Optical Isolator: Application to Photonic Integrated Circuits

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  1. Optical Isolator:Application to Photonic Integrated Circuits Tetsuya MIZUMOTO Dept. of Electrical and Electronic Eng. Tokyo Institute of Technology IEEE Photonics Soc. distinguished lecture

  2. Outline • Bulk optical isolator • magneto-optic (Faraday) effect • operation principle • Waveguide optical isolator • TE-TM mode conversion isolator • nonreciprocal loss (active)isolator • nonreciprocal phase shift isolator • integration (direct bonding) • Non-magneto-optic approach IEEE Photonics Soc. distinguished lecture

  3. What happens? Isolator Photon injection  photon-generated carrier  disturbs carrier distribution (amplitude-noise)  carrier-induced index change (phase-noise) IEEE Photonics Soc. distinguished lecture

  4. Magneto-optic material Requirement - large magneto-optic (MO) effect --> 1-st order MO effect (Faraday rotation) - low optical absorption - temperature insensitive Rare earth iron garnet (R3Fe5O12) Y3Fe5O12 (YIG) --> (Y3-xBix)Fe5O12, (Y3-xCex)Fe5O12 enhancement of Faraday rotation IEEE Photonics Soc. distinguished lecture

  5. Characteristics of Y3-xCexFe5O12 (Ce:YIG) Spectra of optical absorption Spectra of Faraday coefficient M. Gomi, et al., J. Appl. Phys., 70(11), 7065-7067 (1991). IEEE Photonics Soc. distinguished lecture

  6. Bulk isolator Bulk isolator, in either beam interface or fiber interface, uses rotation of polarization. Basic configuration Namiki Input and output : same polarization IEEE Photonics Soc. distinguished lecture

  7. Bulk isolator birefringent plates polarization independent operation Fiber in-line isolator --> Walk-off FDK Isolation>35dB, IL<0.6dB Kyocera Isolation>30dB, IL<2.5dB T.Matsumoto (NTT), Trans. IECE, J62-C, 505-512 (1979). IEEE Photonics Soc. distinguished lecture

  8. TE-TM mode conversion type Cotton-Mouton Mode part Faraday selector part qm M Magnetooptic waveguide Translate Faraday isolator into waveguide one. TE-TM mode conversion Isolation:12.5 dB, l=1150 nm Length: 6.8 mm K. Ando, T. Okoshi and N. Koshizuka (present AIST), Appl. Phys. Lett., 53(1), 4 (1988). IEEE Photonics Soc. distinguished lecture

  9. TE-TM mode conversion rotates in a linearly polarized state Phase mismatched: Birefringence-free (phase matching) is essential to isolator operation. Faraday rotation in a birefringent medium Phase matched: d=bTE-bTM=0 IEEE Photonics Soc. distinguished lecture

  10. Waveguide isolators IEEE Photonics Soc. distinguished lecture

  11. Mode conversion: transversely leaky mode Nonreciprocal radiation (TM phase shift) Performance: - Isolation: 27 dB (l=1535 nm, L=4.1 mm) - wavelength sensitive (7 dB at l=1515 nm) T. Shintaku (NTT), Appl. Phys. Lett., 73(14), 1946 (1998). IEEE Photonics Soc. distinguished lecture

  12. Semi-leaky isolator: operation principle TE mode guided TM mode radiated LiNbO3 mode conversion  reciprocal Magneto-optic mode conversion  nonreciprocal (changes its sign for F/B) Anisotropy of LiNbO3  Semi-leaky waveguide Forward -k(Ce:YIG)+k(LiNbO3)=0 Backward k(Ce:YIG)+k(LiNbO3)≠0 unidirectional mode conversion Semi-leaky isolator is attractive; - relaxed fabrication tolerance - simple mono-section structure - easy control of magnetization - but, uniform and tight LiNbO3 / garnet contact is needed. S.Yamamoto, et al (Osaka U.), IEEE QE, 12, 764 (1976). direct bonding IEEE Photonics Soc. distinguished lecture

  13. Nonreciprocal loss (active) isolator Active group: U.Tokyo, AIST, Ghent U. Isolation: 14.7 dB/mm Insertion loss: 14.1 dB/mm (I=150 mA) H.Shimizu and Y.Nakano (U.Tokyo), JLT, 24, 38-43 (2006). IEEE Photonics Soc. distinguished lecture

  14. Integration with active devices 4 dB isolation at l=1543.8 nm 4 dB 90mA 150mA 15OC active isolator 0.7 mm DFB LD 0.3 mm • compatible waveguide structure material & dimensions • nonreciprocal loss (active) excellent compatibility to active devices H. Shimizu and Y. Nakano (U.Tokyo), IEEE PTL, 19, 1973-1975 (2007). IEEE Photonics Soc. distinguished lecture

  15. Comparison: passive and active isolators IEEE Photonics Soc. distinguished lecture

  16. Waveguide isolator: nonreciprocal phase shift Interferometer type - Isolation: 19 dB (l=1540 nm, L=8.0 mm) J. Fujita, M. Levy and M. Osgood, Jr. (U.Columbia), Appl. Phys. Lett., 76(16), 2158 (2000). - Isolation: 25 dB (l=1600 nm, L=4.0 mm) Y. Shoji and T. Mizumoto (Tokyo Tech), Optics Express, 15, 13446 (2007). - wavelength insensitive designed to cover both 1.31/1.55 mm in a single chip Y. Shoji and T. Mizumoto (Tokyo Tech.), Optics Express, 15, 639 (2007). - polarization independent not by polarization diversity scheme Y. Shoji and T. Mizumoto (Tokyo Tech.) et al, JLT, 25(10), 3108-3113 (2007). IEEE Photonics Soc. distinguished lecture

  17. Interferometric isolator: operation principle Interferometric isolator • Single polarization operation → No need for phase matching → Fabrication tolerant • Simple in-plane magnetization IEEE Photonics Soc. distinguished lecture

  18. Nonreciprocal phase shift x z y 1st–order MO effect linear in b Nonreciprocal phase shift = (b+-b-) (m-1) IEEE Photonics Soc. distinguished lecture

  19. Nonreciprocal phase shift 2.0 l=1550nm SiO2 (n=1.45) TM0 mode d (CeY)3Fe5O12 SGGG (n=1.94) NPS/(p/2) [mm-1] 1.0 cutoff 0 0 0.2 0.4 0.6 0.8 1 Thickness of Ce:YIG guiding layer [mm] Nonreciprocal phase shift = (b+-b-) (m-1) IEEE Photonics Soc. distinguished lecture

  20. Interferometric isolator: calculated performance 0 0 10 0.1 20 Forward loss (dB) Backward loss (dB) 0.2 30 0.3 40 0.4 50 0.5 1.45 1.5 1.55 1.6 1.65 1.45 1.5 1.55 1.6 1.65 Wavelength (mm) Wavelength (mm) IEEE Photonics Soc. distinguished lecture

  21. Interferometric isolator: wideband operation q (forward) 2p q 3p/2 R q (backward) p Phase shift p Phase shift q (backward) q R q (backward) p/2 p/2 N q (forward) q (forward) N 0 l l 0 l l 0 0 -p/2 -p/2 q q (backward) (forward) N N Conventional design wideband design • dependences : MO effect waveguide dispersion Cancellation of wavelength dependences in backward propagation Y.Shoji and T.Mizumoto (Tokyo Tech.), Appl. Opt., 45, 7144 (2006). IEEE Photonics Soc. distinguished lecture

  22. Wideband design: experimental results 0 0 10 10 attenuation (dB) attenuation (dB) 20 20 forward 30 forward backward 30 backward 1500 1550 1600 1650 1500 1550 1600 1650 Wavelength (nm) Wavelength (nm) • measured with a reference of • straight waveguide (5ｄB loss) Conventional design Wideband design Larger isolation in wider wavelength range Y. Shoji and T. Mizumoto (Tokyo Tech.), Optics Express, 15, 13446 (2007). IEEE Photonics Soc. distinguished lecture

  23. Ultra-wideband design 0 Wideband design covers fully 1310 nm / 1550 nm bands and more. Isolation > 40 dB : @ 1260-1650 nm 10 1.55 mm Forward 1.55 mm Backward 20 1.31-1.55 mm Forward attenuation [dB] 1.31-1.55 mm Backward 30 40 50 1.3 1.4 1.5 1.6 Wavelength [mm] Y. Shoji and T. Mizumoto (Tokyo Tech.), Optics Express, 15, 639 (2007). IEEE Photonics Soc. distinguished lecture

  24. Photonic integrated circuit: device and material LD, SOA III-V semiconductor modulator, SW LiNbO3, III-V semiconductor l-MUX/DeMUX Silica Isolator Magneto-optic material • photonic integrated circuit • waveguide alignment  lithography process • materials  to be grown (deposited) on a common platform IEEE Photonics Soc. distinguished lecture

  25. Our approach: integration of isolator and LD Single polarization operation Direct bonding LD integrated with isolator Common semiconductor guiding layer (selective growth & mask process) • compatible waveguide structure material & dimensions H. Yokoi and T.Mizumoto (Tokyo Tech.), Electron. Lett., 33, 1787 (1997). IEEE Photonics Soc. distinguished lecture

  26. III-V waveguide isolator IEEE Photonics Soc. distinguished lecture

  27. Nonreciprocal phase shift x z y qF=-4500deg/cm linear in b Nonreciprocal phase shift = (b+-b-) (m-1) 1st–order MO effect IEEE Photonics Soc. distinguished lecture

  28. Bonding garnet on III-V garnet GaInAsP InP n(garnet) < n(III-V)  Evanescent field is to be used in MO garnet. direct bonding with no gap in-between • Challenging: epitaxial growth of III-V on garnet done by Dr. M. Razeghi (Thomson), JAP, 59, 2261 (1986) and Dr. J. Haisma (Philips), J. Cryst. Growth, 83, 466 (1987) IEEE Photonics Soc. distinguished lecture

  29. Surface activated bonding Surface activation in vacuum chamber IEEE Photonics Soc. distinguished lecture

  30. Direct bonding: garnet on GaInAsP/InP waveguide Ce:YIG / GaInAsP Ce:YIG GaInAsP Bonding strength Fracture in an InP substrate at a tensile > 0.5 MPa Low temperature heat treatment T.Mizumoto, et al, ECS Meeting, 1258 (2006). IEEE Photonics Soc. distinguished lecture

  31. Si-waveguide isolator L=364mm R=2.5mm MMI IEEE Photonics Soc. distinguished lecture

  32. Nonreciprocal phase shift in SOI WG TM mode External magnetic field Ex External magnetic field Ce:YIG Ce:YIG Si Si SiO2 SiO2 x y z Nonreciprocal phase shift (NPS):Db = b+ - b- Lp/2 (Min) ~300mm @0.2-mm-thick CeY2Fe5O12 (Ce:YIG)： QF = -4500 deg/cm H.Yokoi, et al (Tokyo Tech.)., Applied Optics, 42, 6605-6612 (2003) IEEE Photonics Soc. distinguished lecture

  33. Si-waveguide optical isolator SGGG Ce:YIG Ce:YIG Ce:YIG 10nm Si Si rib waveguide 300nm 2mm 300 Si SiO2 SOI 4.0mm H.Yokoi, et al (Tokyo Tech.)., Applied Optics, 42, 6605-6612 (2003) Bonding condition Anneal: 250 oC Press: 5 MPa, 1 hour Rib waveguide for reducing propagation loss (trial fabrication) IEEE Photonics Soc. distinguished lecture

  34. Measurement setup lens Sample TV monitor S IR camera CW N CCW ASE source S PMF PMF Optical switch TM mode Polarizer Spectrum Analyzer • 3-pole magnet --> anti-parallel magnetic field (S-N-S or N-S-N) • 2X2 optical SW --> reverses propagation direction (CWCCW) IEEE Photonics Soc. distinguished lecture

  35. First demonstration of Si-waveguide isolator Mag: N-S-N CCW -40 N-S-N -50 Transmittance (dB) -60 w/o H field CW S-N-S -70 1530 1540 1550 1560 1570 Wavelength (nm) Mag: S-N-S CW -40 -50 Transmittance (dB) Isolation: 21dB -60 CCW -70 1530 1540 1550 1560 1570 Wavelength (nm) • The interference reverses as the propagation direction is reversed. • The interference reverses as the magnetic field directions are reversed. First demonstration of Si waveguide isolator ! Y. Shoji, T. Mizumoto (Tokyo Tech), et al. APL, 92, 071117 (2008). IEEE Photonics Soc. distinguished lecture

  36. Si-waveguide isolator: insertion loss Ce:YIG upper clad (a) (b) -40 (c) transmittance (dB) -50 Single WG MZI -60 21dB Isolation 2.0 mm -70 4.0 mm 1530 1540 1550 1560 1570 wavelength (nm) (a) Coupling loss between fiber and waveguide x2 : 37 dB (b) Propagation loss : 4 dB Si waveguide (2.5 dB / 4 mm) + Absorption of Ce:YIG (0.2 dB) + reflection at bonding boundary (0.65 dB x2) (c) Excess loss of MZI : 4 dB Insertion loss of the isolator ((b)+(c)) : 8 dB IEEE Photonics Soc. distinguished lecture

  37. Non-magneto-optic approach 1 k2 -k2 w2 0.8 W w1 0.6 k1 -k1 w (2pc/a) 0.4 0.2 0 -3 -2 -1 0 1 2 3 kz (2p/q) “Indirect photonic transition” Backward: Mode-1 (w1, -k1) is coupled to mode-2 (w2, -k2). (-k1 - q = -k2, w2-w1=W: phase-matched) --> transition mode-2 (w2, -k2) filtered out Forward: Mode-1 (w1, k1) is uncoupled to mode-2 (w2, k2). (k1 - q > k2, phase-mismatched) --> no transition Zongfu Yu and Shanhui Fan (Stanford), Nature Photonics, 3, 91-94 (2009). IEEE Photonics Soc. distinguished lecture

  38. Non-magneto-optic approach 0-th 1-st Traveling wave (dynamic) modulation Example (l=1550 nm): d/e=5x10-4, f=20 GHz w=0.27 mm, L=2.19 mm Backward: effective coupling e(z,t)=d cos(W t - (-q)z) -k1 - q = - k2 w2-w1=W Z. Yu and S. Fan (Stanford), Nature Photonics, 3, 91-94 (2009). IEEE Photonics Soc. distinguished lecture

  39. Summary Optical isolators for photonic integrated circuits ★ Mode conversion isolator requirement of phase matching  limited fabrication tolerances ★ Interferometric isolator single polarization operation  no need for phase matching ultra-broad band operation (1.31/1.55 mm in a single chip) integration with active devices  Ce:YIG/ III-V, Ce:YIG/ Si low-temperature direct bonding first demonstration of Si waveguide isolator  21 dB isolation ★ Non-magneto-optic approach attractive (less restricted by material), but still challenging IEEE Photonics Soc. distinguished lecture

  40. IEEE Photonics Soc. distinguished lecture

  41. IEEE Photonics Soc. distinguished lecture

  42. Semi-leaky isolator: performance External magnetic field (Electromagnetic Coil) Tunable laser l=1550 nm W=3 mm 20.2 dB PMF 4.5 mm Power meter 1.5 mm PMF Polarizer constant coupling loss (-15 dB/facet) Measured isolation : 20.2 dB / 1.5 mm=13.5 dB/mm T.Mizumoto et al, IEICE Trans, J89-C, 423 (2006). T.Mizumoto et al, OFC2007, OThU4 (2007). IEEE Photonics Soc. distinguished lecture

  43. Outline • Part-1: Bulk nonreciprocal devices • magneto-optic effect (Faraday rotation) • operation principle of isolators and circulators • Part-2: Waveguide isolators • operational principles, design and characterization • TE-TM mode conversion isolators • Nonreciprocal loss isolator • Interferometric isolator • Semi-leaky waveguide isolator • Part-3: Waveguide circulators • Part-4: Non-magneto-optic approach IEEE Photonics Soc. distinguished lecture

  44. Faraday effect Dielectric tensor Circular polarization CW: CCW: IEEE Photonics Soc. distinguished lecture

  45. Faraday effect Linearly polarized wave --> two circular polarized components CCW circular polarized CW circular polarized IEEE Photonics Soc. distinguished lecture

  46. Faraday effect Forward Backward Reversal of propagation direction Reversal of H-field IEEE Photonics Soc. distinguished lecture

  47. Waveguide Faraday rotator E. Pross, et al. (Philips) , APL, 52(9), 682 (1988). N. Sugimoto, et al. (NTT) , APL, 63(9), 2744 (1993). IEEE Photonics Soc. distinguished lecture

  48. Isolator #2 #1 Isolator - two-port device - includes loss mechanism non-unitary matrix --> lossy IEEE Photonics Soc. distinguished lecture

  49. Outline • Part-1: Bulk nonreciprocal devices • magneto-optic effect (Faraday rotation) • operation principle of isolators and circulators • Part-2: Waveguide isolators • operational principles, design and characterization • TE-TM mode conversion isolators • Nonreciprocal loss isolator • Interferometric isolator • Semi-leaky waveguide isolator • Part-3: Waveguide circulators • Part-4: Non-magneto-optic approach IEEE Photonics Soc. distinguished lecture

  50. Circulator #2 #1 #3 Circulator - many-port device - lossless device unitary matrix --> lossless IEEE Photonics Soc. distinguished lecture

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