IST IP NOBEL "Next generation Optical network for Broadband European Leadership"

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1 DBR Tunable Lasers A variation of the DFB laser is the distributed Bragg reflector (DBR) laser. It operates in a similar manner except that the grating, instead of being etched into the gain medium, is positioned outside the active region of the cavity (Figure 6-5), also simplifying the epitaxial process. Lasing occurs between two grating mirrors or between a grating mirror and a cleaved facet of the semiconductor. Bragg reflector HR coating Active section AR coating Figure 6-5 In a DBR laser the grating is contained in a separate section Tunable DBR lasers (Figure 6-6) are made up of a gain section, a mirror (grating) section (for coarse tuning), and a phase section, the last of which creates an adjustable phase shift between the gain material and the reflector (to align cavity mode with the reflection peak, for fine tuning). Tuning is accomplished by injecting current into the phase and mirror sections, which changes the carrier density in those sections, thereby changing their refractive index (temperature can also be used to control refractive index changes, with lower tuning speed). Thus, at least three control parameters have to be managed, increasing the complexity of the system; moreover, the refractive index to current relation changes with time (due to p n junctions degradation, a certain current correspond to a smaller carrier density). Bragg reflector Phase section HR coating Active section AR coating Figure 6-6 Tunable DBR laser general structure The tuning range in a standard DBR laser seldom exceeds about 10 nm and it will not be further covered in this document. Wider tuning can be achieved adding other sections besides gain and phase sections and the various possible solutions are described in related paragraphs below. Being based on electrical effects, tuning speed is much faster than DFB, while optical output power of DBR is generally lower than for DFB lasers. Laser Array Tunable Lasers The DFB thermal tuning range can be expanded by having an array of lasers of different wavelengths integrated on the same chip. DFB selectable arrays operate selecting the DFB array element for coarse tuning and then exploiting temperature tuning for fine cavity mode tuning. Page 19 of 143

2 Common approaches to implement the coarse selection in DFB array are based on integrated on-chip combiners or on off-chip MEMS deflectors able to route the proper beam on the laser output. The advantages of the on-chip combiner approach are mainly the reliability and spectral characteristics that are basically the same as fixed wavelength sources. Disadvantages are relevant to the trade-off between power and tuning range (sometimes a SOA is added to counterbalance the combiner losses, that increment with the number of lasers), reduced yield and large real estate requirements. MEMS-based devices can improve optical output power and decrease chip size, but introduce an element that can significantly affect reliability. SANTUR (DFB array) Santur approach is based on a DFB array and an external MEMS tilt mirror is used to select the appropriate DFB 3. Figure 6-7 depicts the structure of a device able to tune over 33 nm in C-band. The DFB array chip contains 12 lasers with a wavelength spacing of 2.8nm. The mirror (only a small deflection is needed since the array is closely spaced) is placed at the focal plane of the collimating lens, and thus corrects for the spatial variation of the generated beam and delivers to the fiber nearly the full output power of the laser. The package is simplified (low cost passive alignment) since fine alignment is done electronically with the tilt mirror. Figure 6-7 Schematic of tunable laser package. The MEMS tilt mirror in the focal plane of the collimating lens selects one laser from the DFB array and allows for electronic fine tuning of alignment. 3 The module can provide 20mW power over the tuning range (Figure 6-8 shows superimposed spectra at 50GHz ITU channels). Tuning time is typically a second between channels, with the value limited by the TEC cooling capacity. The wavelength is locked on to the ITU grid with the accuracy of the external 50GHz spaced wavelength locker. Page 20 of 143

3 Figure 6-8 Superimposed spectra of module of 84 x 50GHz ITU channels at 13dBm or 20mW. 3 Control electronic manages three control loops. MEMs voltages are set to their calibrated values, but are continually optimized to maximize the fiber-coupled power (as measured by the wavelength locker, that also controls the temperature to stay on the grid), in case adjusting laser current to equilibrate the optical power. The feedback loop maintains the optimum alignment and compensates for possible mechanical drift or creep. During a wavelength-switching event, the MEMs mirror moves to an extreme position to blank the output by about 50dB while the new temperature and current values stabilize (thus acting as a built-in shutter/voa). The MEMs is then unblanked and the locker can provide fine wavelength control. The MEMs mirror can also be detuned during normal operation to provide an additional VOA functionality. The MEMs mirror is fabricated using bulk silicon micromachining and measures about 1.5 mm on a side and is coated with gold for high reflectivity at 1.55mm. The electrostatic mirror deflection is obtained by applying a voltage between the mirror surface and one of the pads beneath the mirror. Shock and vibration have no effect on the wavelength stability of the laser, but only cause minor amplitude modulation of fiber coupled power. The chip size is similar to that of fixed wavelength DFBs, and it contains no additional processing steps (just different masks that result in 12 lasers rather than one). The lasers share the same gain medium, and only vary in grating pitch, which is defined through direct write e-beam lithography. The total chip size is 1 mm long, while the laser ridges are only 500 microns 4. Page 21 of 143

4 Figure 6-9 Fully processed laser chip. 4 Further improvements have been studied in order to have a less expensive and more compact solution, substituting the external wavelength locker with an integrated wavelength locker and a quad detector for C-band operation at a channel spacing of 25GHz 5. As represented in Figure 6-10 beamsplitters are used to pick off part of the collimated beam and to direct a portion of it through a solid etalon to a standard photodetector, while the remainder is transmitted to a quadrant-type photodetector (used as a measure of the wavelength error, adjusted changing the DFB array temperature). The quadrant detector is used to measure the position of the beam reflected from the mirror by determining the relative power incident on each of the four quadrants. The position signal is then used by the control electronics to fine-tune the mirror position. The etalon is held at a constant temperature during operation using a separate TEC with respect to the DFB array; the possibility to adjust the etalon temperature also allows the etalon to be passively placed during assembly. Figure 6-10 Schematic of tunable laser package. 5 Figure 6-11 shows the frequency error and the deviation of the output power from the power set point that results when the module was switched randomly among 16 channels 1000 times. Frequency accuracy of +/ - 1.0GHz and power stability of +/-0.5dBm are routinely achieved. Page 22 of 143

5 Figure 6-11 Measured frequency error (left) and output power error (right) when the module was randomly switched 1000 times among different channels in the C-band. 5 An integrated solution including a 10 Gb/s LiNbO3 modulator and a liquid-crystalbased VOA (Figure 6-12) has been presented 6. VOA enables the output powers of the individual transmitters to be varied in order to change the spectral power density for gain tilt control of the EDFA. The dynamic range of modulated output power is typically 15 db. This VOA, in conjunction with the modulator, can be used for output shuttering while the laser is tuned. It can also be used to slowly bring up or down the transmitter so the network does not experience sudden changes in the spectral power density. Figure 6-12 Structure of an integrated transmitter. 6 Page 23 of 143