Optical Networks - _3_5 Transmitters_39

By careful control over the cavity length we can make

Info icon This preview shows pages 16–19. Sign up to view the full content.

View Full Document Right Arrow Icon
mode. By careful control over the cavity length, we can make the wavelength spacing between the cavity modes equal to the WDM channel spacing. In order to obtain continuous tuning over the entire wavelength range, an ad- ditional third phase section can be added to the DBR, as shown in Figure 3.52(b). Injecting a third current I p into this section allows us to obtain control of the cavity mode spacing, independent of the other effects that are present in the laser. Recall from Section 3.3.5 that it is sufficient to vary the effective cavity length by half a
Image of page 16

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

View Full Document Right Arrow Icon
188 Components Gain region Grating I g I b I b I b Power Power Wavelength Wavelength Gain region Grating I g I b I b I , I p b I p Power Power Wavelength Wavelength I b Wavelength I b I p Wavelength Phase control Laser structure Cavity modes Bragg selection Tuning behavior (a) (b) Figure 3.52 Two- and three-section DBR lasers and their principle of wavelength se- lection. (a) Two-section DBR showing separate control of the gain and Bragg sections. (c) Three-section DBR, which adds an additional control for the cavity phase. wavelength (or equivalently, the phase by π ) in order to obtain tuning across an entire free spectral range. This is a small fraction of the overall cavity length and is easily achieved by current injection into the phase section. By carefully controlling I p to line up a cavity mode to correspond to the wavelength peak of the Bragg grating determined by I b , the wavelength can be tuned continuously over the tunable range. Two- and three-section DBRs capable of tuning over 32 channels in 50 GHz increments were demonstrated several years ago [KK90, Kam96] and are nearing commercial availability. Clearly, a major problem that needs to be solved is in the control of these lasers, which can be quite complicated. As the laser ages, or temperature changes, the control currents may need to be recalibrated; otherwise the laser could end up hopping to another wavelength. The hopping could happen back and forth rapidly, and could
Image of page 17
3.5 Transmitters 189 manifest itself as relative intensity noise (RIN) at the laser output. In a sense, we are eliminating the very fact that made DFB lasers so wavelength stable—a fixed grating. These problems are only compounded further in the more complex laser structures that we will discuss next. The DBRs that we have looked at so far are all limited to about a 10–15 nm tuning range by the 0.5–2% change in refractive index possible. Increasing the tuning range beyond this value requires a new bag of tricks. One trick makes the laser wavelength dependent on the difference between the refractive indices of two different regions. The overall variation possible is much higher than the variation of each of the individual regions. The so-called vertical grating-assisted coupler filter (VGF) lasers [AKB + 92, AI93] make use of this principle. The second trick is to make use of the Vernier effect, where we have two combs of wavelengths, each with slightly different wavelength spacing. The combination of the two combs yields another periodic comb with a much higher wavelength spacing between its peaks. Problem 3.28 explains
Image of page 18

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

View Full Document Right Arrow Icon
Image of page 19
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