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 #4 is due today, HW #5 is assigned (due April 8) Final exam May 1 (Tentative)

3 Passive fiber components Fiber spicing and connectorization Directional couplers WDM couplers Isolators Tunable filters, resonators, AWG (homework)

4 Photonics crystal fibers Standard fiber PCF HC-PCF a PCF preform J. C. Knight, Photonic crystal fibers, Nature 424, (2003)

5 Limitation of standard fibers Loss: amplifiers every km limited by Rayleigh scattering cannot use exotic wavelengths like 10.6µm Nonlinearities: crosstalk, power limits (limited by mode area ~ single-mode, bending loss) also cannot be made with (very) large core for high power operation Radical modifications to dispersion, polarization effects? tunability is limited by low index contrast

6 Interesting breakthroughs [ figs courtesy Y. Fink et al., MIT ] [ R. F. Cregan et al., Science 285, 1537 (1999) ] white/grey = chalco/polymer silica 5µm 10.6µm (high-power CO 2 lasers) loss < 1 db/m (material loss ~ 10 4 db/m) [ Temelkuran et al., Nature 420, 650 (2002) ] 1.55µm loss ~ 13dB/km [ Smith, et al., Nature 424, 657 (2003) ] OFC 2004: 1.7dB/km BlazePhotonics

7 Interesting breakthroughs Endlessly single-mode [ T. A. Birks et al., Opt. Lett. 22, 961 (1997) ] Nonlinear fibers [ Wadsworth et al., JOSA B 19, 2148 (2002) ] polarization -maintaining [ K. Suzuki, Opt. Express 9, 676 (2001) ] low-contrast linear fiber (large area) [ J. C. Knight et al., Elec. Lett. 34, 1347 (1998) ]

8 Interesting Applications Dispersion compensation Pulse compression and deliver Supercontinuum generation Gas, liquid sensing Telecommunication?

9 Questions for thoughts Can you come up with a new design of optical fiber that would work much better than existing ones? New applications for optical fibers? Do optical fibers exist in nature?

10 Traditional optics Optical elements are used to split/combine, filter, focus, amplify, attenuate light Ti:sapphire laser

11 Fiberization in Optics Ti:sa femtosecond laser Femtosecond fiber laser

12 Passive fiber components Fiber coupler Variable fiber coupler WDM Isolator Attenuator Modulator Switches Pump/signal combiner Polarization splitter/combiner Collimator Fiber delay line Polarizer Tunable filter Circulator Faraday rotator mirror

13 (booster) amplifier transmission fiber dispersion compensation (in-line) amplifier transmission fiber dispersion compensation (pre-) amplifier WDM mux WDM demux EDFA EDFA EDFA l 1 l 2 Point-to-point WDM Transmission System - Building Blocks - transmitter terminal Tx transmission line point-to-point link section span amplifier span receiver terminal Rx l 1 l 2 l 3 SMF or NZDF SMF or NZDF l 3 l 4 DC DC l 4 l 5 l 5 l 6 l 6 l n Raman pump Raman pump l n

14 Erbium-doped fiber amplifier tap tap PD PD

15 Fiber laser Mode-locked ring fiber laser

16 Fiber cable construction Reinforcement needed to protect the fragile glass fiber

17 Fiber specs sheet

18 Fiber connectorization Fiber optics cable Fiber optics connectors Fusion splicer Before splicing After splicing

19 Fiber displacement Prof. Norwood

20 Longitudinal displacement Prof. Norwood

21 Angular deviation

22 Fiber connectors

23 Fiber connectors Insertion losses are generally <0.2 db typical <0.5 db max Connectors can have a flat polish or they may have an 8 o angle polish which reduces back reflections dramatically (< -60dB return loss) compared to flat polish (< -40dB return loss) FC/APC

24 Fiber connectors, FC/PC

25 Fiber splicing Fiber stripping Surface cleaning Fiber cleaving Fiber alignment Pre-fusion heating Fusing Splice evaluation Protection, strain relief Heat shrink sleeve

26 Fiber splicer

27 Fiber cleaver

28 Fiber stripper

29 Fusion splicing

30 Fiber optic couplers Optical couplers either split optical signals into multiple paths or combine multiple signals onto one path The number of input (N)/ output (M) ports, (i.e. N x M ) characterizes a coupler Fused couplers can be made in any configuration, but they commonly use multiples of two (2 x 2, 4 x 4, 8 x 8, etc.)

31 Coupler applications Uses Splitter: (50:50) Taps: (90:10) or (95:05) Combiners Couplers are key components in Optical amplifiers Fiber lasers Optical switches Mach Zehnder interferometers Fiber-to-the-home networks Optical fiber sensors

32 Single-mode coupler behavior 100% coupling The coupling is wavelength dependent. Coupling occurs when the two fibers cores are very close to each other. Small changes effect the coupling ratio WDM coupler 100 % of the 1.3 µm light couples to the core of fiber B, and then back to the core of fiber A to emerge at Port C 100% of the 1.55 µm light couples to the core of fiber B and emerges at Port D Simple coarse WDM filters can be made in this way

33 Single-mode coupler properties Couplers are made by tapering fibers down, thereby making the core very small, resulting in most of the light propagating in the "multimode' cladding in the taper region If the adiabatic coupling regions vary slowly enough, then there is very little loss as the light propagates across the biconical taper

34 D Fused biconic taper fabrication Fabrication of a biconical taper Heat fiber uniformly over a width w to the glass melting point, T m Stretch fiber a distance L on both sides of the heated region w Heat D 0 D L 0 L z Fiber taper (top) and standard fiber (bottom)

35 Fused biconic taper fabrication or Coupling ratio, excess loss, PDL Completely automated technology with high throughput

36 Theory for directional couplers Four-port devices (two input and two output ports) Output can be split in two different directions; hence the name directional couplers Can be fabricated using fibers or planar waveguides Two waveguides are identical in symmetric couplers Evanescent coupling of modes in two closely spaced waveguides Overlapping of modes in the central region leads to power transfer

37 Theory for directional couplers Coupled-mode theory commonly used for couplers Begin with the Helmholtz equation:, everywhere except in the region occupied by two cores Approximate solution: F m (x,y) corresponds to the mode supported by the each waveguide: A 1 and A 2 vary with z because of the mode overlap Credit: Agrawal

38 Coupled mode equations Coupled-mode theory deals with amplitudes A 1 and A 2 We substitute assumed solution in Helmholtz equation, multiply by F 1 or F 2, and integrate over x-y plane to obtain: The coupling coefficient is defined as: Modes are normalized such that: Credit: Agrawal

39 Time-domain coupled mode equations Expand in a Taylor series around the carrier frequency w 0 as: Replace while taking inverse Fourier transform : Credit: Agrawal

40 Time-domain coupled mode equations Credit: Agrawal

41 Time-domain coupled mode equations Even though A 2 = 0 at z = 0, some power is transferred to the second core as light propagates inside a coupler Power transfer follows a periodic pattern Maximum power transfer occurs for e z = m /2 Coupling length is defined as L c = /(2 e ) Credit: Agrawal

42 Symmetric couplers Credit: Agrawal

43 Symmetric couplers Credit: Agrawal

44 Coupler performance parameters (I) Coupling ratio or splitting ratio: CR = Power fromanysingleoutput Totalpower out toallports = P t P T-out æ P CR = -10log 2 ö 10 ç è P 1 + P 2 ø 2 x 2 case in db Excess loss: L e = P in P T-out æ P Le =10log in 10 ç è P 1 + P 2 ö ø 2 x 2 case in db

45 Coupler performance parameters (II) Insertion loss: L i = Power from any single output Power input Isolation or crosstalk: = P t P in æ L i = -10log 10 ç è In db P t P in ö ø Liso = Input power atoneport Reflected power back intoother input port æ L iso =10log 10 ç è P in P 3 ö ø In db

46 Fiber star coupler Combines power from N inputs and divided them between N outputs 1 P 1 1 N P N N Coupling ratio æ 1 CR = -10log 10 ç è N ö =10log 10 N ø Excess loss æ Le =10log ç 10 ç è å P in N P out,i i ö ø

47 Wavelength-dependent couplers Wavelength-division multiplexers (WDM) types: 3 port devices (4th port terminated) 1310 / 1550 nm ( classic WDM technology) 1480 / 1550 nm and 980 / 1550 nm for pumping optical amplifiers 1550 / 1625 nm for network monitoring Common l 1 l 2 Insertion and rejection: Low loss (< 1 db) for path wavelength High loss (20 to 50 db) for other wavelength

48 Wavelength-dependent couplers Fused biconic taper is made and monitored as it is being pulled When 1550nm is in the bar state and 1310nm is in the cross state, pulling is stopped - - a coarse WDM filter results

49 WDM couplers Fused coupler type WDM Low loss (<0.5dB) Small size (35x5.5mm) Low cost (~$200) Thin film type WDM

50 WDM couplers

51 Isolators Polarization sensitive isolator Polarization insensitive isolator Low loss (<0.5dB) Small size (35x5.5mm) Low cost (~$200)

52 Isolators

53 Questions for Thoughts What is the new fiber component that you think may be useful to have? Can we replace all traditional optics with fiber-based components? How can you turn your experimental setup into fiber-based? Where are fiber-based components made? How can you start a successful company providing fiber components and devices?