Optical Networks - _3_5 Transmitters_39

An led is a forward biased pn junction in which the

Info icon This preview shows pages 12–14. Sign up to view the full content.

View Full Document Right Arrow Icon
An LED is a forward-biased pn -junction in which the recombination of the injected minority carriers (electrons in the p -type region and holes in the n -type region) by the spontaneous emission process produces light. (Unwanted nonradiative recombination is also possible and is an important factor affecting the performance of LEDs.) Because spontaneous emission occurs within the entire bandwidth of the gain medium (corresponding to all energy differences between the valence and conduction bands for an LED), the light output of an LED has a broad spectrum, unlike that of a laser. We can crudely think of an LED as a laser with facets that are not very reflective. Increasing the pump current simply increases the spontaneous emission, and there is no chance to build up stimulated emission due to the poor reflectivity of the facets. For this reason, LEDs are also not capable of producing high-output powers like lasers, and typical output powers are on the order of 20 dBm. They cannot be directly modulated (see Section 3.5.4) at data rates higher than a few hundred megabits per second. In some low-speed, low-budget applications, there is a requirement for a source with a narrow spectral width. DFB lasers provide narrow spectral widths but may be too expensive for these applications. In such cases, LED slicing provides a cheaper
Image of page 12

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
184 Components alternative. An LED slice is the output of a narrow passband optical filter placed in front of the LED. The optical filter selects a portion of the LED’s output. Different filters can be used to select (almost) nonoverlapping spectral slices of the LED output. Thus one LED can be shared by a number of users. We will see an application for this technique in Chapter 11. 3.5.3 Tunable Lasers Tunable lasers are highly desirable components for WDM networks for several rea- sons. Fixed-wavelength DFB lasers work very well for today’s applications. However, each wavelength requires a different, unique laser. This implies that in order to sup- ply a 100-channel WDM system, we need to stock 100 different laser types. The inventory and sparing issues associated with this are expensive and affect everybody from laser manufacturers to network operators. Laser manufacturers need to set up multiple production and test lines for each laser wavelength (or time-share the same production and test line but change the settings each time a different laser is made). Equipment suppliers need to stock these different lasers and keep inventories and spares for each wavelength. Finally, network operators need to stockpile spare wavelengths in the event transmitters fail in the field and need to be replaced. Having a tunable laser alleviates this problem dramatically. Tunable lasers are also one of the key enablers of reconfigurable optical networks.
Image of page 13
Image of page 14
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern