Tunable semiconductor lasers have been listed in numerous critical technology lists for future optical communication systems. Lasers with full band tuning ranges (C or L) allow reduction of the inventory cost and simplify deployment and operation of existing systems in addition to enabling wavelength agile networking concepts in future systems. Furthermore, monolithic integration of full band tunable lasers with modulators to form complete transmitters offers the most potential for reducing system size, weight, power consumption, and cost. This paper summarizes design, fabrication technology, and performance characteristics of widely tunable CW sources and transmitters based on chip scale integration of a Sampled Grating Distributed Bragg Reflector (SG DBR) laser with a Semiconductor Optical Amplifier (SOA) and Electroabsorption (EA) or Mach Zehnder (MZ) modulator. Widely tunable CW sources based on SG-DBR lasers exhibit high fiber coupled output power (20 mW CW) and side mode suppression ratio (>40 dB), low relative intensity noise (below -140 dB/Hz) and line width (<5 MHz) across a 40 nm C-band tuning range. Characteristics of EA-modulated optical transmitters include fiber-coupled time-averaged powers in excess of 5 dBm, RF extinction ratios > 10 dB, and error-free transmission over 350 km of standard fiber at 2.5 Gb/s across a 40 nm tuning range. Monolithic integration of widely tunable lasers with MZ modulators allow for further extension of bit rate (10 Gb/s and beyond) and transmission distances through precise control of the transient chirp of the transmitter. Systematic investigations of accelerated aging confirm that reliability of these widely-tunable transmitters is sufficient for system deployment.
Integration of active optical components typically serves five goals: enhanced performance, smaller space, lower power dissipation, higher reliability, and lower cost. We are manufacturing widely tunable laser diodes with an integrated high speed electro absorption modulator for metro and all-optical switching applications. The monolithic integration combines the functions of high power laser light generation, wavelength tuning over the entire C-band, and high speed signal modulation in a single chip. The laser section of the chip contains two sampled grating DBRs with a gain and a phase section between them. The emission wavelength is tuned by current injection into the waveguide layers of the DBR and phase sections. The laser light passes through an integrated optical amplifier before reaching the modulator section on the chip. The amplifier boosts the cw output power of
the laser and provides a convenient way of power leveling. The modulator is based on the Franz-Keldysh effect for a wide band of operation. The common waveguide through all sections minimizes optical coupling losses. The packaging of the monolithically integrated chip is much simpler compared to
a discrete or hybrid solution using a laser chip, an SOA, and an external modulator. Since only one optical fiber coupling is required, the overall packaging cost of the transmitter module is largely reduced. Error free transmission at 2.5Gbit/s over 200km of standard single mode fiber is obtained with less than 1dB of dispersion penalty.
While tunable lasers have been a focus of research and development efforts for over 10 years, they have only recently gained market acceptance in optical transport and networking. Tunable lasers offer many compelling advantages over fixed wavelength solutions in optical networks in that they reduce inventories, allow dynamic wavelength provisioning, and simplify network control software. More interesting, is that tunable lasers have been featured in optical network development efforts in every segment: access/enterprise, metropolitan, and long haul networks leading to a variety of desired specifications and approaches. In fact, the term 'tunable laser' has come to describe an increasingly broad range of technologies from monolithic semiconductor lasers, to MEMS (Micro-Electro-Mechanical Systems) based lasers and fiber lasers. This presentation will focus on monolithic, widely-tunable lasers which are promising candidates to satisfy the needs of all the market segments mentioned.