OPTI510R: Photonics. Khanh Kieu College of Optical Sciences, University of Arizona Meinel building R.626

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1 OPTI510R: Photonics Khanh Kieu College of Optical Sciences, University of Arizona Meinel building R.626

2 Announcements Homework #3 is due today No class Monday, Feb 26 Pre-record lecture this Friday at 2PM Mid-term exam will be on Feb 28 th (open books/notes)

3 Planar waveguides Waveguide fabrication Coupling light into planar waveguides Waveguide loss Example of waveguides and performance Example of applications Numerical simulation tools

$commonly used element: Titanium (Ti) Diffusion of Ti atoms within LiNbO3 crystal increases refractive index and forms the core region.$

4 LiNbO 3 waveguide LiNbO 3 waveguides do not require an epitaxial growth A popular technique employs diffusion of metals into a LiNbO3 substrate, resulting in a low-loss waveguide The most commonly used element: Titanium (Ti) Diffusion of Ti atoms within LiNbO3 crystal increases refractive index and forms the core region. Surface flatness critical to ensure a uniform waveguide

5 LiNbO 3 waveguide Electrodes fabricated directly on the surface of wafer (or on an optically transparent buffer layer An adhesion layer (typically Ti) first deposited to ensure that metal sticks to LiNbO 3 Photolithography used to define the electrode pattern

6 Silica glass waveguides Silica layers deposited on top of a Si substrate Employs the technology developed for integrated circuits Fabricated using flame hydrolysis with reactive ion etching Two silica layers are first deposited using flame hydrolysis Top layer converted to core by doping it with germania Both layers solidified by heating at 1300C (consolidation process) Photolithography used to etch patterns on the core layer Entire structure covered with a cladding formed using flame hydrolysis

7 Silica glass waveguides R. Norwood

8 Silicon-on-insulator waveguides Core waveguide layer is made of Si (n 1 = 3.45) A silica layer under the core layer is used for lower cladding Air on top acts as the top cladding layer Tightly confined waveguide mode because of large index difference Silica layer formed by implanting oxygen, followed with annealing R. Norwood

9 Polymer waveguides R. Norwood

10 Direct laser writing Top View Maskless Waveguide symmetry is uniform and highly reproducible Fast waveguide fabrication No clean-room environment required Design flexibility Side View R. Norwood lab Femtosecond laser writing

Direct laser writing A new, high-speed technique to fabricate low loss EO polymer waveguide using all-reflective multiphoton Top View imaging and direct laser

11 Direct laser writing A new, high-speed technique to fabricate low loss EO polymer waveguide using all-reflective multiphoton Top View imaging and direct laser writing system has been developed provides for positioning EO polymer waveguides at arbitrary positions on nanophotonic chip with sub-micron resolution Side View

thickness is 2.5μm - Δn 0.07 was achieved and measured on a test sample.

12 Direct laser writing A 4μm wide waveguide was fabricated by laser raster scan and stitching lithography method with 300μm 300μm frame size Top View - The required index contrast is achieved by exposing each point with 3kW peak power laser pulses - Film thickness is 2.5μm - Δn 0.07 was achieved and measured on a test sample. The cross-section is shown in (a) the output mode in (b). - (a): EO-P: EO polymer, SG: solgel, UB: unbleached, B: bleached. The top view of the waveguide is shown in (c) Side View C )

13 Integrated circuits fabrication

14 Integrated circuits fabrication

15 Integrated circuits fabrication

16 Coupling light into a waveguide The optical field in a waveguide is a superposition of modes: The input field can be also written as a superposition of modes: Where:

Coupling light into a waveguide Coupling light into a waveguide can be challenging: The waveguide is typically very small (a few microns) which requires high

17 Coupling light into a waveguide Coupling light into a waveguide can be challenging: The waveguide is typically very small (a few microns) which requires high precision alignment stages Need to consider mode matching for efficient coupling Need to prepare waveguide end-face Optics Express, Vol. 23, Issue 3, pp (2015)

18 Example 1. NA matching: the NA of the input beam has to be smaller than the NA of the waveguide 2. Beam size matching: The laser spot size has to be smaller than the mode size of the waveguide

$Prism coupling Through the use of a high refractive index prism,$ waveguide mode at discreet angles The air gap must be very small

19 Prism coupling Through the use of a high refractive index prism, the z component of the incident wavevector can be matched to a waveguide mode at discreet angles The air gap must be very small (on the order of a wavelength) in order for efficient coupling to occur

$polymer film on silicon Thin film refractive index and thicknesses are determined with high$

20 Prism coupler 2 d m sin c tan sin sin Transcendental equation Mode pattern for a polyimide polymer film on silicon Thin film refractive index and thicknesses are determined with high accuracy

21 Prism coupler 1550nm Photo Detector Coupling Head 1300nm 830nm Sample Prism 632.8nm

22 Coupling using a grating How to coupler light into a nano-silicon waveguide? is the period of the grating

23 Coupling using tapering How to coupler light from a nano-silicon waveguide to optical fiber? Silicon reverse taper from Silicon photonics wire waveguides

24 Waveguide loss Scattering loss Absorption loss Bending loss

25 Absorption loss Electronic absorption given by Urbach tail usually negligible in the near IR ( ) Aexp[ ( 0) / kt ] Vibrational absorption high n states of anharmonic oscillator describing molecular vibrations dominant source of absorption loss in the near IR k mcmx E0 m m C X

26 Absorption loss OH-absorption peaks Loss in a silica optical fiber

27 Absorption coefficient (1/cm) Absorption loss Most polymers absorb in near IR (1300 nm nm) C-H and OH groups have overtones Replace hydrogen with fluorine to reduce overtones 350 k = Loss(dB/cm) = 4.34a 4.5 cm polished plug Loss(dB/cm) n Wavelength (nm) n 2 d 1550 Absorption loss in PMMA (polymer)

28 Absorption loss 0.3 cm disk - upper limit on loss at 1550 nm of 0.03 db/cm Wavelength (nm) Transmission spectrum of Teflon AF

29 Scattering loss Extrinsic inhomogeneities - bubbles - dust - cracks Intrinsic inhomogeneities - density fluctuations - compositional inhomogeneity - structural inhomogeneity

30 Scattering loss Classic Rayleigh scattering - density fluctuations - typically very small and only relevant for fibers 2 8 ( 1)( iso 3 kt n n scatter :isothermal compressivity 2 1) 2

31 Bending loss Bending alters the incident angle

32 Bending loss 5 0Bending Loss (db/cm) NA=0.3 NA= Bending Radius(mm) 1550 nm light bending loss in two optical waveguides 1) NA = 0.14, d = 4 m 2) NA = 0.3, d = 2.5 m

33 Example of waveguides and performance

Ultra low loss channel waveguide Scanning electron

cladding buffer polymer substrate core (4µm x 4µm) Chip

34 Ultra low loss channel waveguide Scanning electron micrograph (SEM) image of a channel waveguide Output to Fiber Input from Fiber Photo of light emitted from channel wg cladding buffer polymer substrate core (4µm x 4µm) Chip size: 1.5cm X 3cm Bending radius: 4mm WG length: 100cm WG cross-section: 4 m X 4 m Loss < 0.05 db/cm from 1300nm to 1600nm R. Norwood

35 Ultra low loss glass channel waveguide Scale bar: 2mm Waveguide loss: 0.08dB/m

36 Silicon waveguide from Silicon photonics wire waveguides

37 Silicon waveguide from Silicon photonics wire waveguides

38 Etchless silicon waveguide

39 Etchless silicon waveguide

40 Etchless silicon waveguide

41 Applications Light transport, delay, splitting Modulators Lasers Nonlinear devices Detectors

42 Silicon modulators

43 Silicon modulators

44 Nonlinear devices

45 Numerical simulation tools Planar waveguide devices can be modeled and simulated by a variety of numerical methods. Several commercial software packages are available that are also used for device design and photomask layout: - Optiwave Corporation ( - BeamProp from Rsoft Design Group ( - Fimmwave and Fimmprop from Photon Design ( Numerical channel waveguide mode solvers with different levels of accuracy and efficiency include: Finite Difference Method (FDM), Finite Element Method (FEM), etc. Full vectorial, semi-vectorial, scalar Beam propagation method (BPM)

High Dn strip loaded waveguides n 1 n 2 3 m 1 m 3 m 3 m n 2 = 1.4738 n 1 = 1.5336 n 3 = 1.

46 High Dn strip loaded waveguides n 1 n 2 3 m 1 m 3 m 3 m n 2 = n 1 = n 3 = Simulated high G active section mode C. T. DeRose, et. al. Optics Express 17, 3316 (2009).

47 Sol-gel WG coupling to SMF-28 SMF-28 mode Buried sol-gel WG mode

Fiber-to-waveguide coupling n 4 n 2 n 3 6 m n 1 n 2 3 m 1 m 5 m 2-3 m 3 m n 2 = 1.4738 n 4 = 1.

48 Fiber-to-waveguide coupling n 4 n 2 n 3 6 m n 1 n 2 3 m 1 m 5 m 2-3 m 3 m n 2 = n 4 = n 3 = n 1 = Simulated passive section mode with 0.5 db coupling loss (Optiwave)

49 Electro-optics modulator Movie from Optiwave:

50 Questions for Thoughts Top View Why do waveguides have modes? Can you come up with a better way to calculate the modes of a waveguide? Can you come up with a better way to make waveguides? Can you guide light with something other than the waveguides we discussed? Side View

Lecture: Integration of silicon photonics with electronics. Prepared by Jean-Marc FEDELI CEA-LETI

Lecture: Integration of silicon photonics with electronics Prepared by Jean-Marc FEDELI CEA-LETI Context The goal is to give optical functionalities to electronics integrated circuit (EIC) The objectives