Optical Networks - _3_5 Transmitters_39

In order for lasing to occur we need to have an

Info icon This preview shows pages 19–22. Sign up to view the full content.

View Full Document Right Arrow Icon
In order for lasing to occur, we need to have an overlap between the two reflection peaks of the Bragg gratings and a cavity mode. Even though the tuning range of each reflection peak is limited to 10–15 nm, combining the two sets of reflection peaks results in a large tuning range. Just as with the two- and three-section DBR lasers, a separate phase section controls the cavity mode spacing to ensure continuous tuning. An additional complication with this approach is that because the reflection peaks taper off, the current in the gain region needs to be increased to compensate for the poorer reflectivity as the laser is tuned away from the primary Bragg reflection peak. Another way of getting the same effect is to use periodically chirped gratings instead of the gratings shown in Figure 3.54. This structure is called a super-structure grating DBR laser. The advantage of this structure is that the chirped gratings provide a highly reflective set of peaks over a wider wavelength range than the sampled grating structure.
Image of page 19

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

View Full Document Right Arrow Icon
3.5 Transmitters 191 Gain region I g I b 2 I b 1 I p Phase control Power Wavelength I b 1 Power Wavelength I b 2 Power Wavelength I , I , I p b b 1 2 Cavity modes Sampled grating 1 Sampled grating 2 Sampled grating, pitch L 2 Sampled grating, pitch L 1 l L 2 /(2 ) n eff 1 l L 2 /(2 ) n eff 2 Figure 3.54 A sampled grating DBR laser and its principle of wavelength selection. Grating-Coupled Sampled Reflector Laser The GCSR laser is a combination of a VGF and a sampled or super-structure grating, as shown in Figure 3.55. The VGF provides a wide tuning range, and the SSG grating provides high selectivity to eliminate side modes. In a sense, the VGF provides coarse tuning to select a wavelength band with multiple cavity modes in the band, and the SSG grating provides the wavelength selection within the band. Just as in the two- and three-section DBR lasers, an additional phase section provides the fine control over the cavity modes to provide continuous tuning within the band to suppress side modes. Laser Arrays Another way to obtain a tunable laser source is to use an array of wavelength- differentiated lasers and turn one of them on at any time. Arrays could also be used to replace individual light sources.
Image of page 20
192 Components Gain section Coupling section Phase section Sampled grating section I g I c I b I p Waveguide 1 Waveguide 2 Figure 3.55 A grating coupled sampled reflector laser. One approach is to fabricate an array of DFB lasers, each of them at a different wavelength. Combined with temperature tuning, we can use this method to obtain fairly continous tuning. A major problem with this approach is in the wavelength accuracy of the individual lasers in the array, making it difficult to obtain a comb of accurately spaced wavelengths out of the array. However, if only one laser is to be used at any given time, we can use temperature tuning to make up for this inaccuracy.
Image of page 21

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

View Full Document Right Arrow Icon
Image of page 22
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