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Agenda

Agenda. Review from last class EM waves in a waveguide: telecommunications Pulses in materials with dispersion Real optical communications systems Detector technology. M.E.: 16.6 “Lasers” Siegman. Aside: Problem set 1 – due tomorrow. Office hour: 11:30-12:30 today

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Agenda

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  1. Agenda • Review from last class • EM waves in a waveguide: telecommunications • Pulses in materials with dispersion • Real optical communications systems • Detector technology M.E.: 16.6 “Lasers” Siegman Aside: Problem set 1 – due tomorrow. Office hour: 11:30-12:30 today I put up links to Melles-Griot catalog on website. Laser Optics – Phys460

  2. 1.Last class review dn/d0 !! But ALL materials have some dispersion: , where Ignore higher order terms Group velocity= Phase velocity= Laser Optics – Phys460

  3. 1. Review, “Superluminal light propagation” Real work: L. J. Wang, A. Kuzmich & A. Dogariu Nature406, 277 - 279 (20 July 2000); Why did they not use a ns duration pulse to observe the effect? Laser Optics – Phys460

  4. 2. Telecom: Pulses with dispersion Next term in taylor series: If k”0, pulse envelope changes shape as a function of z! Consider the pulse intensity: Rewrite specifically in terms of the pulse duration where t=tc is the peak of the Gaussian Pulses with greater bandwidth experience more temporal broadening Laser Optics – Phys460

  5. 2. Telecom: Pulse broadening but Define a characteristic length where pulse duration increases by: “Dispersion” length Initial pulse duration 10ps 200m 50fs 5mm Example: For standard glass (BK7) at 850nm Laser Optics – Phys460

  6. 2. Telecom: What does a broadened pulse look like? • Instantaneous frequency: • Example monochromatic wave: • Broadened Gaussian pulse: Careful about sign: NOTE Laser Optics – Phys460

  7. 2. Telecom, After propagation - chirped pulse Peak of envelope: z GVD -“group velocity dispersion” Laser Optics – Phys460

  8. 2. Telecom, Chirp: experimental demonstration http://www.firstpr.com.au/slinky/audio/ Laser Optics – Phys460

  9. Frequency vs. time for transit http://www.firstpr.com.au/slinky/audio/ Laser Optics – Phys460

  10. 2. Telecom, Example of pulse propagation 100d d NOTE CORRECTION Laser Optics – Phys460

  11. 2. Telecom, Pulse propagation summary • If n is constant for all freq, phase velocity =group velocity and no pulse spreading • If n has a linear dependence on , envelope moves at a different speed than crests, but no pulse spreading. • If n has a nonlinear dependence on  , pulse shape changes. Laser Optics – Phys460

  12. 2. Telecom, Maximum bit rate? Methods of reducing material dispersion problems: • minimize n dependence on  (dispersion shifted fibres) • Recompress pulses using material with opposite sign • Don’t use pulses that are too short INPUT Real world: wavelength division multiplexing Coarse WDM: 20nm spacing Dense WDM: 100GHz spacing nm Laser Optics – Phys460

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