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Laser Optics in Telecommunications: Understanding EM Waves and Pulses in Material Dispersion

Review EM waves in telecommunications, pulses in materials with dispersion, and real optical communication systems. Explore detector technology and laser principles. Learn how dispersion affects pulses and how to minimize dispersion problems in telecommunications.

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Laser Optics in Telecommunications: Understanding EM Waves and Pulses in Material Dispersion

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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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