Wavelength, modal and power stabilization of tunable electro-absorption modulated, distributed Bragg reflector lasers

Abstract

Telephone +I-908-582-6389, Fax +1-908-582-2202 Optical sources for dense wavelength division multiplexed (DWDM) systems must produce constant wavelength [l] and power output over a system lifetime despite aging-induced changes that affect both wavelength [2] and power. Tunable sources, such as distributed Bragg reflector (DBR) lasers, require additional modal stabilization to prevent aging-induced failure due to mode hopping [3]. We demonstrate a practical scheme for simultaneously stabilizing the wavelength, mode and power of a tunable DBR laser which was then observed to operate stabilized and error-free within a transmitter at 2.5Gb/s over 680km of non-dispersion shifted fiber [4]. Feedback is proven robust against sudden and large perturbations.A five-section electro-absorption modulated (EA) DBR laser serves as an electrically tunable source coupled to three closed feedback loops, which independently provide robust modal, wavelength and power stabilization (see Fig. 1.) The EADBR laser produces a -1550nm, OdBm, 2.5Gb/s, low chirp data stream in any of 20 contiguous 5OGHz-spaced ITU channels. The EADBR laser consists of a highly reflective facet coated 410pm gain section, a 300pm Bragg reflector tuning section, a 600pm optical amplifier section, a short p-i-n power tap for monitoring the amplifier output and a 250pm EA modulator with diffractive window and anti-reflection coated facet. EADBR laser modes are separated by lOOGHz so that temperature tuning is required to access channels on a 5OGHz plan. Tuning range exceeds 8nm.Output from the highly reflective (HR) gain section facet is coupled to a wavelength discriminator consisting of a collimating lens, a beam splitter, a fused silica etalon, a pair of short p-i-n photodetectors and transimpedance amplifiers. The etalon has a lOOGHz free spectral range and coatings that provide a low finesse of -5. The beam splitter directs a portion of the collimated EADBR laser output through the etalon onto one photodetector while the remainder of the of the output strikes a second photodetector, serving to normalize the wavelength dependent photodetector signal. Wavelength stability is achieved by finely adjusting the EADBR laser temperature to maintain a constant ratio of wavelength dependent to wavelength independent signals. The etalon is arranged so that each ITU channel coincides with the steep flank of the etalon transmission characteristic. Neighboring 5OGHz channels use etalon transmission flanks of opposite sign slope, requiring reversal of the sense of the wavelength feedback loop.High gain in the wavelength feedback loop ensures short-term stability (-days) of *4pm in wavelength. Aging of the tuning section of DBR lasers can result in mode hops, despite wavelength stabilization [3]. Modal stability is ensured by fixing the operating point of the EADBR laser relative to the tuning current boundaries of a given longitudinal mode. This method [3] uses a small, low frequency dither current atop the DC tuning bias current to produce a small modulation in gain sec...

Wavelength control of tunable dense wavelength-division multiplexing sources by use of a Fabry–Perot etalon and a semiconductor optoelectronic diode

“…3 This change may give rise to mode hopping and incomplete tuning of a TLS. 4 The most popular components for wavelength control of DWDM lasers are Fabry-Perot ͑FP͒ etalons because they are stable and compact. [3][4][5] However, a FP etalon whose free spectral range ͑FSR͒ is usually chosen to be the same as the channel spacing cannot distinguish among different channels because it has a periodic wavelength.…”