Optical Networks - _3_5 Transmitters_39

The reasoning for this condition is the same as that

Info icon This preview shows pages 4–7. Sign up to view the full content.

View Full Document Right Arrow Icon
an integral multiple of half the wavelength in the cavity. The reasoning for this condition is the same as that used for the Fabry-Perot cavity. This condition is called the Bragg condition and was discussed in Section 3.3.3. The Bragg condition will be satisfied for a number of wavelengths, but the strongest transmitted wave occurs for the wavelength for which the corrugation period is equal to half the wavelength, rather than some other integer multiple of it. Thus this wavelength gets preferentially amplified at the expense of the other wavelengths. By suitable design of the device, this effect can be used to suppress all other longitudinal modes so that
Image of page 4

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

View Full Document Right Arrow Icon
176 Components Gain medium Gain medium Corrugation Corrugation Corrugation (a) (b) Light emission Light emission Figure 3.44 The structure of (a) a DFB laser and (b) a DBR laser. In a DFB laser, the gain and wavelength selection are obtained in the same region, whereas in a DBR laser, the wavelength selection region is outside the gain region. the laser oscillates in a single-longitudinal mode whose wavelength is equal to twice the corrugation period. By varying the corrugation period at the time of fabrication, different operating wavelengths can be obtained. Any laser that uses a corrugated waveguide to achieve single-longitudinal mode operation can be termed a distributed-feedback laser. However, the acronym DFB laser is used only when the corrugation occurs within the gain region of the cavity, as shown in Figure 3.44(a). When the corrugation is outside the gain region, as in Figure 3.44(b), the laser is called a distributed Bragg reflector (DBR) laser. The main advantage of DBR lasers is that the gain region is decoupled from the wavelength selection region. Thus it is possible to control both regions independently. For exam- ple, by changing the refractive index of the wavelength selection region, the laser can be tuned to a different wavelength without affecting its other operating parameters. Indeed, this is how many of the tunable lasers that we will study in Section 3.5.3 are realized. DFB lasers are inherently more complex to fabricate than FP lasers and thus relatively more expensive. However, DFB lasers are required in almost all high-speed transmission systems today. FP lasers are used for shorter-distance data communica- tion applications. Reflections into a DFB laser cause its wavelength and power to fluctuate and are prevented by packaging the laser with an isolator in front of it. The laser is also usually packaged with a thermoelectric (TE) cooler and a photodetector attached to its rear facet. The TE cooler is necessary to maintain the laser at a constant operating temperature to prevent its wavelength from drifting. The temperature sensitivity of
Image of page 5
3.5 Transmitters 177 Gain cavity External cavity Figure 3.45 The structure of an external cavity laser.
Image of page 6

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

View Full Document Right Arrow Icon
Image of page 7
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