TCOM 503 Fiber Optic Networks

1. TCOM 503Fiber Optic Networks Spring, 2007 Thomas B. Fowler, Sc.D. Senior Principal Engineer Mitretek Systems

2. Topics for TCOM 503 • Week 1: Overview of fiber optic communications • Week 2: Brief discussion of physics behind fiber optics • Week 3: Light sources for fiber optic networks • Week 4: Fiber optic components fabrication and use • Week 5: Fiber optic components, modulation of light • Week 6: Noise and detection • Week 7: Optical fiber fabrication and testing of components

3. New resources • Agilent Technologies (old HP test & measurement division) educator’s page: http://www.educatorscorner.com/tools/lectures/slides/ • Lots of great material on networks, instruments, basic electronics, RF networks, and optical networking

4. Study topics for final exam • Principles of fiber optic cable and devices (reflection, refraction, interference, diffraction) • Types of fiber optic cable • Types of distortion and other problems involved with optical fiber • Operation of LEDs and lasers • Operation of detectors • Operation of EDFAs • Resonant couplers/wavelength selective couplers & splitters • Other optical devices • Isolators - Fabry-Perot filters • GRINs - Dielectric filters • FBGs - Modulators & Modulation types

5. Fiber optic components • Diffraction gratings • Filters • Modulators • Switches • Repeaters

6. Diffraction gratings • Change angle of light as a function of its wavelength • Acts like a prism • Effectively does Fourier transform of light • Separates waveform in time domain into a number of waveforms in frequency domain • Used because of control they give over light • Can be readily fabricated using technology used to make other optical components

7. Types of diffraction gratings • Refractive • Light passes through material with grating etched on its surface • Typically glass or plastic • Commonly used in optics, but not generally employed in communications applications • Reflective • Light reflects off of surface with closely spaced lines • Generally fabricated in a medium, often along with other components • Can be formed in almost any material where optical properties can be varied in a regular way with period close to wavelength

8. Uses of diffraction gratings • Function as wavelength selective filters • Block or pass desired wavelengths • Enable combining or splitting optical signals • Used as reflectors in some devices

9. Principle of operation for diffraction gratings • Basic equation ml = gs ( sin q + sin fm ) • Where gs = grove spacing m = order of refracted ray (m = 0, 1, 2, 3…) l = wavelength of incident ray q = angle of incidence fm = angle of refraction

10. Operation of diffraction grating (continued) • If q = 0 we get old diffraction formula sin fm = ml/gs • For m = 0 get ordinary reflection (sin q = -sin fm) or q = -fm • Solving for sin fm we get sin fm = ml/gs – sin q • If gs >> ml, then we can solve for multiple values of fm, and as m becomes larger, fm becomes smaller: Source: Dutton

11. Operation of diffraction grating (continued) • By setting grove spacing gs ~ ml, solution only exists for m = 0 or 1 • Example: l = 1550 nm, q = 45o, gs = 1200 nm • Gives sin fm = 0.5846 for m = 1, fm = 35.8o • Sin fm = 1.87 for m = 2 (impossible) • Note that if we change l to 1560 nm, sin fm = 0.5929, fm = 36.4o • Enough that it would be easy to split the two

12. Shapes for diffraction gratings • No effect on angles, but does affect strengths of diffracted beams or “orders” • Blazed grating transfers large portion of power to first order • Operates only over restricted range of wavelengths Source: Dutton

13. Wavelength selection with diffraction gratings Source: Dutton

14. Wavelength selection (continued) • Devices tend to be costly because of very high precision required • Large number of closely spaced wavelengths can be separated

15. Examples of use of diffraction gratings Source: Anritsu

16. In-Fiber Bragg gratings (FBGs) • Extremely simple, low cost wavelength selective filter • Wide range of applications • Construction • Ordinary single mode fiber a few cm long • Grating formed by variation of RI of core lengthwise along fiber • Resonant wavelengths reflected back, others passed Source: Dutton

17. Construction of FBG • Only small variation in RI required: 0.0001 • Center wavelength given by l = 2 neffL Where l = center wavelength reflected back neff = average RI of material L = physical period of fiber grating Fig 168 Source: Advanced Optics Solutions Gmbh

18. Application of FBGs • Wavelength stable lasers: stabilize laser so that it produces a narrow band • See p. 161; use two FBGs and one Erbium-doped fiber • Dispersion compensation (requires Chirped gratings where spacing varies along length) • Allow existing optical networks designed for 1300 nm band to operate in 1550 nm band • Effectively shapes pulses • See p. 419 • Wavelength selection in WDM systems • Allow separating out particular wavelengths when used with circulators

19. Characteristics of FBGs • Center wavelength: wavelength at center of reflection band • Bandwidth: range of wavelengths reflected • Reflectance peak: fraction of incident light reflected back at center wavelength • Parameters which determine characteristics: • Grating period • Grating length • Modulation depth (grating strength, determined by RI) • RI contrast profile • Can vary over length of grating, called “apodization”

20. Reflection spectra .2 nm • Extra peaks caused by change in RI which looks like a mirror to wavelengths out of band • By tapering strength, can reduce peaks Low RI contrast High RI contrast After Apodization Source: Dutton

21. Spectra (continued) Source: Advanced Optics Solutions Gmbh

22. Spectra (continued) Source: Corning

23. Chirped FBGs • “Chirp” in optical context means some change in frequency • A chirped FBG is one in which period of grating changes • Leads to variation in response to wavelength (frequency) along length of grating • Can be done in 2 ways • Vary period of grating • Vary average RI of grating • Different wavelengths reflected from different parts of grating • Imposes wavelength dependent delay on signal Source: Dutton

24. Chirped FBGs (continued) • Shorter wavelengths must travel farther before being reflected • Major use: equalizing response of older fiber networks (1300 nm) so that they can operate in 1550 nm band • Refers to dispersion compensation • Works because shorter wavelengths tend to be ahead of longer wavelengths in smeared (dispersed) pulse

25. Chirped FBGs (continued) • Characteristics • Need to be long for most applications • Require apodization in order to smooth response • Apodization=changing shape to smooth discontinuities • Ripple effectively adds 3 db to noise of signal • Response Source: Dutton After apodization No apodization

26. Chirped FBGs (continued) • Improved response with higher grating strength After apodization, 2x grating strength Source: Dutton After apodization No apodization

28. Source: http://www.laser2000.co.uk/pdfs/view2.pdf

29. Multiple FBGs • Possible to put multiple FBGs on a single fiber to achieve better results • Either sequentially or on top of one another

30. Use of FBGs to make bandpass filters • Use blazed (“slanted”) FBGs to reflect selected wavelength out of the fiber • Use of multiple sections allows creation of a bandpass filter • Each section tuned to different band of frequencies Source: Dutton

31. Phase-shifted FBG • By shifting phase in middle of FBG, a narrow transmission band can be created • Too small by itself, but multiple phase shifts can make a useable passband Source: Dutton

32. Long-period FBG • Grating period is hundreds or thousands of times the resonant wavelength • Power coupled forward rather than backward • In single mode fiber, no mode available in fiber for it to couple to • Couples into cladding, eventually dissipates • Effect is similar to blazed grating, where resonant wavelengths removed from system • Application is same: equalizing gain or response curves of devices such as EDFAs

33. Source: Fujikura

34. Waveguide Grating Routers (WGRs) • Also called Arrayed Waveguide Gratings (AWGs) • Use planar waveguide technology • Function is similar to Littrow grating • Utilize principle of constructive and destructive interference to separate wavelengths

35. Array Waveguide Grating (AWG) l l l l l l l l 1a 4b 3c 2d 1a 2a 3a 4a l l l l l l l l 1b 2b 3b 4b 2a 1b 4c 3d l l l l l l l l 1c 2c 3c 4c 3a 4d 1c 2b l l l l l l l l 1d 2d 3d 4d 4a 3b 2c 1d .. translate into .. .. columns Rows .. If only one input is used: wavelength demultiplexer! Source: Agilent

36. Operation of AWGs • Input power diffracts into separate waveguides • Waveguides have different lengths • Signals interfere in output free space coupler so that only one goes out on each output line Source: Dutton

37. Performance characteristics of AWGs Source: Dutton

38. Source: Lucent

40. Filters • Definition: a filter is a device that selects or passes a band of frequencies (wavelengths) • May have sharp or gradual cut-off characteristic • May exhibit “ringing” or other effects • Gratings act as filters in many applications • Filters much more important in WDM applications than in single-wavelength applications

41. Practical filters Bandwidth Source: Dutton

42. Filter characteristics • Center wavelength: mean wavelength between two band edges • Peak wavelength: wavelength at which attenuation is least (or greatest for band rejection filter) • Nominal wavelength: design wavelength • Bandwidth: distance between band edges where response is down a given amount, usually 3 db

43. l i-1 l i l i+1 Passband Crosstalk Crosstalk Filter Characteristics • Passband • Insertion loss • Ripple • Wavelengths(peak, center, edges) • Bandwidths (0.5 dB, 3 dB, ..) • Polarization dependence • Stopband • Crosstalk rejection • Bandwidths (20 dB, 40 dB, ..) Source: Agilent

44. Optical filters: Fabry-Perot (Etalon) • Essentially a resonator, like an organ pipe or a stringed instrument • Consists of cavity bounded on either end by a partially silvered mirror • If mirrors can be moved, called an “interferometer” • If mirrors fixed, called an “Etalon” Source: Dutton

45. Fabry-Perot filter operation • Light of a different frequency than the resonant frequency is mostly reflected, so very little enters chamber • What enters the chamber undergoes destructive interference • Light of resonant frequency which tries to reflect actually undergoes destructive interference from light already in chamber going out through mirror • Nearly all light of resonant frequency therefore enters • Can only exit through opposite mirror • Transmission characteristics depend on percent reflection of end mirrors

46. Fabry-Perot filter operation (continued) • Reflecting surfaces must be extremely flat • 1/100th of a wavelength • Very difficult to make • Mirrors typically made with 99% reflectivity

47. Fabry-Perot filter measures • Quality of filter or “goodness” of filter is called “finesse” • Energy stored in filter / energy passing through it • Analogous to “Q” in electrical theory • Higher reflectivity of mirrors => higher finesse • Resonant wavelengths given by l = 2Dn/m Where n = RI, D = distance between mirrors, m = 1,2,3…

48. Fabry-Perot filter measures (continued) • Distance between peaks in response curve called “Free Spectral Range” (FSR) FSR = l2/(2nD) where n = RI, D = distance between mirrors Finesse = FSR/FWHM = pR½ / (1-R) where FWHM = Full width half maximum, R = reflectance

49. Cascading of filters • Typically Fabry-Perot filters with different FSR are cascaded • To get rid of unneeded lobes • To sharpen response Source: Dutton

50. Tuning of Fabry-Perot filters • Can be made tunable by changing mirror spacing • Typically done with piezoelectric crystal attached to a mirror • High voltage (300-500 v) • Slow (1 ms) • Also can be made by putting liquid crystal material in gap • RI changes if current passed through • 30-40 nm • 10 microseconds