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Ch 39: Wave Functions and Uncertainty
Knight Calc - Physics for Scientists and Engineers 5th Edition
Knight Calc5th EditionPhysics for Scientists and EngineersISBN: 9780137344796Not the one you use?Change textbook
Chapter 39, Problem 19

A 1.5-μm-wavelength laser pulse is transmitted through a 2.0-GHz-bandwidth optical fiber. How many oscillations are in the shortest-duration laser pulse that can travel through the fiber?

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1
Determine the relationship between the bandwidth (Δf) and the time duration (Δt) of the pulse using the time-bandwidth product. For a Gaussian pulse, the time-bandwidth product is approximately Δt * Δf ≈ 0.44. Rearrange this to find the shortest pulse duration: Δt = 0.44 / Δf.
Substitute the given bandwidth of the optical fiber, Δf = 2.0 GHz (2.0 × 10⁹ Hz), into the equation for Δt to calculate the shortest pulse duration.
Calculate the period of one oscillation of the laser light using the relationship T = 1 / f, where f is the frequency of the laser. The frequency of the laser can be found using the speed of light equation: f = c / λ, where c = 3.0 × 10⁸ m/s (speed of light) and λ = 1.5 μm (1.5 × 10⁻⁶ m).
Substitute the calculated frequency of the laser into the equation T = 1 / f to find the period of one oscillation.
Determine the number of oscillations in the shortest-duration pulse by dividing the pulse duration (Δt) by the period of one oscillation (T): Number of oscillations = Δt / T.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Wavelength and Frequency

Wavelength and frequency are inversely related properties of waves. The wavelength (λ) is the distance between successive peaks of a wave, while frequency (f) is the number of oscillations per second, measured in hertz (Hz). For light, the speed of light (c) relates these two by the equation c = λf. Understanding this relationship is crucial for determining the number of oscillations in a laser pulse.
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Pulse Duration

Pulse duration refers to the time interval during which a pulse is active or has significant amplitude. In the context of laser pulses, shorter durations correspond to higher frequencies of oscillation. The duration of a pulse can be estimated using the bandwidth of the optical fiber, as a larger bandwidth allows for shorter pulse durations, which is essential for calculating the number of oscillations in the pulse.
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Bandwidth and Information Transmission

Bandwidth is the range of frequencies that a communication channel can transmit, measured in hertz. In optical fibers, a higher bandwidth allows for the transmission of more data and shorter pulse durations. The relationship between bandwidth and pulse duration is given by the time-bandwidth product, which indicates that a pulse's duration is inversely proportional to the bandwidth, thus affecting the number of oscillations in the pulse.
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