2026 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 10, OCTOBER 2005 Uncooled DBR Laser Directly Modulated at 3.125 Gb/s as Athermal Transmitter for Low-Cost WDM Systems Yiran Liu, Student Member, IEEE, Andrew R. Davies, Jonathan D. Ingham, Member, IEEE, Richard V. Penty, Member, IEEE, and Ian H. White, Fellow, IEEE Abstract—An uncooled three-section tunable distributed Bragg reflector laser is demonstrated as an athermal transmitter for low-cost uncooled wavelength-division-multiplexing (WDM) sys- tems with tight channel spacing. A 0.02-nm thermal wavelength drift is achieved under continuous-wave operation up to 70 C. Dynamic sidemode suppression ratio of greater than 35 dB is consistently obtained under 3.125-Gb/s direct modulation over a 20 C–70 C temperature range, with wavelength variation of as low as 0.2 nm. This indicates that more than an order of magnitude reduction in coarse WDM channel spacing is possible using this source. Index Terms—Direct intensity modulation, optical fiber com- munications, semiconductor lasers, tunable lasers, wavelength- division-multiplexing (WDM) applications. I. INTRODUCTION D UE TO the current economic drive for low-cost compo- nents and solutions, coarse wavelength-division-multi- plexing (CWDM) technology is attracting increasing interest. The technology has major benefits as it employs uncooled laser transmitters, which have smaller footprints and lower power consumption. However, due to the thermal drift of such lasers being typically 0.1 nm/ C, CWDM usually specifies a 20-nm channel spacing to tolerate wavelength variation over an operating temperature range of least 70 C. Such channel spacing limits the number of channels available for a fiber link, and also prevents it from taking advantage of erbium-doped fiber amplification (EDFA). Therefore, athermal operation of wavelength-division-multiplexing (WDM) lasers, where the thermal drift of wavelength is suppressed, is of much interest, as it allows reduced channel spacing. Clearly any such source should operate under modulation. Although CWDM technology typically employs a 2.5-Gb/s channel data rate, the recent 10-Gb/s Ethernet 10 GBASE-LX4 [1] demands use of uncooled lasers on multiple wavelengths, with a higher data rate of 3.125 Gb/s for each wavelength channel. This presents a further challenge for athermal laser operation. We have previously reported athermal operation of a four- section sampled-grating distributed Bragg reflector (DBR) laser [2], with continuous-wave (CW) performance limited by tem- perature range and coarse wavelength control. We later reported Manuscript received April 26, 2005; revised June 22, 2005. The authors are with the Photonic Systems Group, Engineering Department, Cambridge University, Cambridge CB2 1PZ, U.K. (e-mail: yl250@cam.ac.uk). Digital Object Identifier 10.1109/LPT.2005.856367 Fig. 1. Demonstration of athermal operation principle. athermal performance based on a three-section DBR laser [3] by controlling the grating section bias only. In this letter, we report enhanced CW performance and, for the first time, dynamic operation of an uncooled DBR laser. Direct modulation at a data rate of 3.125 Gb/s is demonstrated with dynamic sidemode suppression ratio (DSMSR) consistently higher than 35 dB, over an extended temperature range of 10 C–70 C. Enhanced CW wavelength accuracy is achieved, with thermal drift of as low as 0.02 nm over 10 C–70 C temperature span. Furthermore, wavelength variation of 0.2 nm is demonstrated over the 20 C–70 C temperature range under direct modulation. II. PRINCIPLE OF OPERATION Fig. 1 illustrates experimentally the principle and basis of athermal operation. The emission wavelength of the DBR laser is recorded as the grating section bias current is swept from 0 to 75 mA. This is repeated over the temperature span from 10 C to 70 C, in 10 C steps, to demonstrate the thermal drift of the wavelength. As the dotted horizontal line in Fig. 1 indicates, it is then possible to restrict the wavelength to a fixed value by con- trolling the current into the grating section as the temperature increases. The device used in this letter is a three-section DBR laser, with the grating, phase, and gain sections being 600, 100, and 600 m long, respectively. The front facet has a 4% reflective coating and the rear facet is antireflection coated. An open-loop current tuning scheme is adopted while the laser emission is col- lected using a fiber lens and monitored by an optical spectrum analyzer (OSA). The peak wavelength and DSMSR are col- lected simultaneously. The operating temperature is monitored 1041-1135/$20.00 © 2005 IEEE