D. A. Ackerman, J. E. Johnson, L. J-P. Ketelsen, J. M. Geary,* W. A. Asous,* F. S. Walters,* J. M. Freund,* M. S. Hybertsen, K. G. Glogovsky,* C. W. Lentz,* C. L. Reynolds,* R. B. Bylsma,* E. J. Dean,* L. E. Eng,* O. Qasaimeh,* R. Gupta,* F. Ganikhanov,* V. Veliadis,* S. Roycroft,* T. Pinnington* and G. Rao,*
Agere Systems, Murray Hill, NJ 07974 and *Breinigsville, PA 18031 USA
A wavelength selectable source within a transmitter for high-speed, dense wavelength division multiplexed systems must tune across a substantial number of channels while meeting rigorous system specifications in each channel. It must be cost competitive with and offer the same level of stability, reliability and ease of use as single wavelength counterparts. We examine a tunable source designed to meet the needs of high bit-rate DWDM systems for long-haul and metro applications.
Dense wavelength division multiplexed (WDM) systems operating over long distances at high bit rates require stable, powerful, low-chirp optical sources to transmit closely spaced channels. Similar sources are required for cost-sensitive intermediate distance WDM systems. In both types of multi-channel systems, tunable sources relieve burdensome inventory, ease sparing and enable flexible bandwidth provisioning. Rapid tuning promises increased flexibility in network architecture. Wavelength selectable sources are required to deliver advanced functionality while meeting in every channel the rigorous performance and reliability specifications imposed upon single wavelength sources.
Various approaches achieve wavelength selectable functionality. DFB laser arrays offer time-tested stability and reliability  at the risk of reduced array yield. Output power is sacrificed in array element combiners and subsequently boosted with integrated semiconductor optical amplifiers. Wavelength control of each element is simple, but thermal tuning is very slow. Edge-emitting distributed Bragg reflector (DBR) lasers and broadly tunable variants [2,3] offer compact size and attendant low cost at the expense of complicated wavelength and modal control. Rapid electronic tuning of DBR lasers approaches the rate needed for packet switching . Tunable VCSELs coupled with integrated moving mirrors offer compactness, mode-hop free operation and ease of stabilization [5,6]. Directly modulated, electronically pumped tunable VCSELs  suffer from low output power while optically pumped versions  require power-hungry pump lasers and external modulators. Various configurations of compact external cavity lasers [7,8] offer high performance but require complex engineered solutions to assembly and mechanical stability problems. Our approach has been to develop a highly integrated InP chip comprising a DBR laser coupled to a semiconductor optical amplifier (SOA), power monitor and (optional) electro-absorption (EA) modulator . Hardware for closed-loop feedback ensuring wavelength and modal stability is incorporated into a small form factor transmitter package . We report on a fully functional high bit rate EA-DBR version of this source capable of error-free standard-fiber transmission at 2.5Gb/s over 680km  and 10Gb/s over 80km  on each of twenty-five 50GHz-spaced channels as well as a CW version, intended for use with an external modulator, capable of producing 20mW fiber coupled output across thirty-two 50GHz-spaced channels.
A schematic representation of the EA-DBR laser is shown in Figure 1. A 420µm gain section, chosen to give 100GHz mode spacing, and 250mm DBR tuning section form a DBR laser that is coupled to a 450mm SOA, a short power monitor and an EA modulator of length 135µm for use at 10Gb/s. Trenched isolation regions separate the five sections of the 1700 x 500µm chip. The SOA compensates for on-state modulator loss and free carrier absorption in the tuning section. A 900µm SOA boosts output power in a CW design that omits the modulator.
Figure 1: Schematic of EA DBR laser
Figure 2: Tuning and SMSR characteristics
A dual waveguide integration scheme allows independent optimization of DBR laser, amplifier and modulator performance. The active-passive integration scheme , shown in Figure 1, consists of an upper waveguide that contains the multiple quantum well active layers and a lower waveguide that is thicker in the tuning section. Two short lateral tapers in the upper waveguide adiabatically transfer the guided mode between upper and lower waveguides. High modal gain in the multi-quantum well active sections and tight optical confinement in the bulk-active tuning section yield efficient tuning, low radiation loss and low optical back-reflection at the interfaces.
Low-threshold (~ 10mA) EA-DBR laser chips are mounted in a thermo-electrically cooled 21-pin butterfly telecom laser package, with coaxial 10Gb/s RF modulator input connector. Typical wavelength vs. tuning current characteristics of the DBR laser (see Figure 2) show 13-14 100GHz-spaced steps, each with SMSR > 40 dB, all for tuning currents <100mA. Total tuning range of ~11nm is achieved with modest ±10°C temperature tuning. Currently, for twenty-five or more 50GHz-spaced channels, the tuning steps are operated at two temperatures separated by ~ 4°C. Average modulated output power of ~ 2mW can be maintained over all channels of the EA-DBR while, without the modulator using the CW-DBR version, 20mW fiber coupled power is obtained in each of 32 channels spaced by 50GHz (see superimposed spectra of Figure 3.)
Figure 3: Superimposed single mode spectra
Tuning or frequency modulation (FM) efficiency is highest at low tuning currents where carrier density in the tuning section is most sensitive to changes in tuning current. FM efficiency as high as 50GHz/mA is observed at the lowest tuning step, decreasing with increasing tuning current. CW linewidth, measured as a function of tuning current, shows a Lorentzian lineshape broadened by Gaussian noise. Gaussian broadening is observed to scale with FM efficiency, decreasing with tuning current, as well as tuning section admittance, which increases with tuning current. Careful filtering must be used to reduce line-broadening ingress noise from reaching the tuning section. Typical total linewidths of less than 10MHz are recorded for EA-DBR lasers over the tuning range, with maximum linewidth occurring at intermediate values of tuning current ~ 1mA.
Back facet (EA)-DBR laser output is split between a Fabry-Perot etalon wavelength discriminator and back-facet monitor to provide closed-loop wavelength stability at ITU gridpoints via device temperature control. Simultaneous closed-loop control using the front tap monitor maintains constant fiber-coupled output power by adjusting SOA current.
Modal stability is essential for long-term reliability of any tunable laser that can hop between modes. By monitoring slope of the back-facet output power of the (EA)-DBR laser as a function of tuning current  and correcting tuning current to maintain a constant slope, an operating point is fixed relative to mode boundaries. Low-bandwidth mode-hop avoidance allows continuous wavelength stabilization at operating points optimized for performance. The compact discriminator used for both wavelength and modal stabilization is housed within the butterfly package along with the DBR laser and device-specific firmware providing the user with factory-set look-up tables for each channel.
Rapid tunability using broadly tunable  and EA-DBR [14,15] lasers has been demonstrated. Fully electronic tuning in DBR-based devices is essential for sub-second channel switching. However, wavelength and mode stabilization on a very short time scale (~ 1ms) have yet to be developed for any tunable source.
Aging in tunable lasers has been reported by a number of workers in the field . We assess long-term reliability by indirectly monitoring changes in non-radiative current used by the tuning section to produce a given Bragg wavelength shift . We observe aging-induced changes in tuning current that are linear in Bragg wavelength shift, which is, in turn, proportional to carrier density. From these results, we conclude that aging is due to changes in non-radiative recombination rate in the tuning section. In accord with literature results, we observe that wavelength aging can be saturated before devices are deployed, thereby ensuring long-term reliability when used in conjunction with predictive transmitter firmware .
The small-signal bandwidth of the 10Gb/s EA-modulator is 19.7GHz at a bias of Ð1.5V. RF extinction ratio is > 14dB for a 2.5Gb/s modulator for V,mod = 2.0Vp-p, and > 8.5dB for a 10Gb/s modulator for Vmod = 1.7Vp-p. Peak-to-peak device chirp, measured by time-resolved spectroscopy is typically <0.2Å, attributable to isolation trenching and good AR coatings on the chip as well as careful package design to minimize electrical cross-talk.
Transmission at 2.5Gb/s through 680km of standard fiber yielded a power penalty at 10-9 BER of 0.5dB for the worst of twenty channels with wavelength and mode stabilization loops engaged. The EA modulator was driven by a 223-1 PRBS signal with a 2.0Vpk-pk drive. No error floors were observed. The dispersion penalty is slightly higher for the low frequency channels because the higher FM efficiency of the tuning section at low tuning current makes it more sensitive to power supply noise and electrical crosstalk with the integrated modulator . At 10Gb/s, dispersion penalties less than 1.3dB without error floors were achieved using a 231-1 PRBS signal for 25 operating wavelengths of the device over a single 80 km span of standard fiber (see Figure 4.)
Figure 4:Bit error rates for 10Gb/s, 80km standard fider trnsmission experiment for each EA-DBR laser channel.
In summary, various tunable sources for WDM systems, including our EA-/CW-DBR laser, continue to evolve. Broad and rapid tuning, high power, stability and reliability and the compromises required to balance performance against manufacturability, ease of use, small footprint and, importantly, cost continue to challenge the semiconductor photonics industry.
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