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, <i>RF</i> 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.
A summary of current work involving the development of high performance, wavelength-tunable integrated optical transmitters for analog applications is given. The performance of sampled-grating DBR lasers integrated with an SOA and an electroabsorption or Mach-Zehnder modulator is evaluated in terms of E/O conversion efficiency, noise performance and dynamic range. Optimization options to maximize either gain, noise figure or spurious-free dynamic range in analog link applications are discussed. It is shown how the combination of chip-scale integration and the use of bulk waveguide Franz-Keldysh absorption allows coupling of a large optical power level into the electroabsorption modulator, and its effects on the modulation response and analog link performance. Link results on an integrated SGDBR-SOA-EAM device includes a sub-octave SFDR in the 125 to 127 dB/Hz<sup>4/5</sup> range and a broadband SFDR of 103-107 dB/Hz<sup>2/3</sup> limited by third order intermodulation products or 95-98 dB/Hz<sup>1/2</sup>, limited by second order intermodulation products, over a 1528 to 1573 nm wavelength range.
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.
Recently 850-nm wavelength has been established as the standard for local area interconnects and data-link modules using GaAs/AlGaAs vertical cavity lasers (VCLs) have become commercially available. However, the lowest threshold current (I<SUB>th</SUB>) up-to-date has been obtained from 980-nm VCLs using strained InGaAs quantum wells. In this presentation we report an ultralow CW, room temperature I<SUB>th</SUB> of 156 (mu) A from a 2.8 micrometers diameter VCL with three AlInGaAs quantum wells in the active region. The AlInGaAs/AlGaAs quantum well active region is used to achieve laser emission near 850 nm while maintaining the benefits of strain in lasers. Previous studies have shown that strained AlInGaAs/AlGaAs in-plane lasers exhibit the same suppression to the propagation of dark-line defects as strained InGaAs lasers. Here we have performed a preliminary burn-in study on our devices to study the reliability in AlInGaAs. AlGaAs VCLs for the first time. We found that devices showed no degradation in either output power or threshold current after 30 hours of on-wafer testing at a constant current density of 22 kA/cm<SUP>2</SUP> and junction temperature of 140 degrees C. We also measured devices at various stage temperatures and found that the lowest I<SUB>th</SUB>, 110 (mu) A for the 2.8 micrometers diameter VCL, occurs near 230 Kelvin, where the quantum well gain peak and the cavity mode are aligned. In addition, we examined the behavior of the external differential efficiency as a function of device size and found that due to a thicker oxide aperture than intended, optical scattering losses start to dominate for devices smaller than 4 micrometers diameter.
Recent results from the authors group are summarized as a general indicator of the current state of the art in vertical-cavity surface-emitting lasers (VCSELs). These include results from engineered-aperture VCSELs with high wall-plug efficiencies at low powers, high-efficiency bottom-emitting cryo-VCSELs with wavelengths < 900 nm, low-threshold AlGaInAs VCSELs emitting at 850 nm, and arrays of VCSELs used in parallel free-space links as well as WDM arrays butt-coupled to multimode fiber. Analysis indicates that size-dependent losses limit the scaling of VCSELs below 5 micrometers in diameter unless special engineered apertures and/or short cavities are used. The analysis also shows that lateral carrier confinement is necessary to obtain efficient devices below 2 micrometers in diameter.
In this paper, we measure the size dependent optical scattering and electrical losses in etched-post and dielectrically apertured vertical-cavity lasers (VCLs). We show that reduced optical scattering losses are responsible for the dramatic improvement in device scaling seen with the use of the oxide-defined apertures. Furthermore, we experimentally show how to reduce this optical scattering loss through the use of thin apertures. We find that the electrical losses (due to current leakage around the active region and carrier diffusion in the active region) in the structures are minimized by reducing the doping near the active region, minimizing the current leakage. Finally, based on the experimental results, theoretical design curves for VCL scaling are calculated.
The rapid pace of advances in vertical-cavity surface- emitting lasers (VCSELs) has continued over the past couple of years. The widespread use of dielectric apertures formed primarily by lateral oxidation has provided much lower cavity losses, and this has enables a large decrease in device threshold as well as an increase in efficiency. The lowest optical losses have been obtained with thin or tapered oxide apertures. Within the past year, new strained- layer materials such as AlGaInAs have been incorporated to extend the benefits of strain to the 850 nm wavelength range. A record threshold of 290 (mu) A at 840 nm has been obtained. Devices have been designed for ultra-wide operating temperature ranges by using gain from different quantum levels at different temperatures. Submilliamp thresholds from 77 K to 373 K were demonstrated. The inclusion of low-loss dielectric apertures in wafer-bonded 1.55 micrometer InP/GaAs has yielded VCSELs with submilliamp thresholds for the first time. In addition, there has been considerable effort in making VCSEL arrays for parallel or free-space interconnect applications. Multiple wavelength arrays for even denser interconnects or wavelength addressing schemes have also been explored. In this paper we review some of this recent progress and point out issues still inhibiting further advances.