Uncooled laser operation over wide temperature ranges
Uncooled distributed-feedback (DFB) lasers are routinely used commercially in a wide range of applications. We extended their single-mode-operation temperature range and improved their performance through threshold reduction by increasing the output power and by decreasing the variation of laser parameters with temperature, particularly for high temperatures.
We optimized the laser-structure design and fabrication process. Strained multiple quantum wells were incorporated into the active layer. Wide well widths and large well numbers (5–10) were employed to obtain both high gain and high differential gain. Using a proprietary grating design we achieved stable single-mode operation and improved the single-mode yield. The grating strength was designed to deliver a relatively strong product of the coupling constant (κ) and the relevant cavity length (L), κL = 1.5–2.0. An anti-reflectivity/high-reflectivity (AR/HR) coating scheme was adopted to enhance laser front power and suppress Fabry-Perot lasing. AR coatings provide low enough reflectivity within a wide spectral bandwidth. Calculations and measurement results showed that our coating technology continuously provides the thin film with a reflectivity of less than 10−3 and a bandwidth in excess of 65nm.
To further ensure single-mode lasing at low temperatures, sufficient negative detuning of the DFB-lasing wavelength relative to the material-gain peak frequency was introduced at room temperature. Base-wafer construction and three-step regrowth were both carried out in a metal-organic chemical-vapor-deposition (MOCVD) reactor. We implemented standard high-frequency laser processes were then implemented. The devices were tested on the bar level for initial performance evaluation. Full-scale characterization was accomplished in a transmitter optical subassembly (TOSA).
We have thus far demonstrated wide-temperature-range operation of 1310nm DFB lasers with very low threshold and good linearity.1–4 They were originally aimed at targeting broadband cable-television return paths,1 but have since been expanded to also include gigabit passive optical-network (G-PON) lasers2 at operational temperatures from −70 to + 110°C. We recently also presented4 wide-temperature-range functionality (from −45 to +125°C) for 1490nm DFB lasers. Our latest laser systems2–4 operate at data rates of 2.5Gb/s or higher. Their power performance, threshold current, power/slop change with temperature, and tracking errors are all much improved compared to the early results.1
Figure 1 shows the optical-output/electrical-input curves as a function of temperature for our 1490nm TOSA DFB lasers. At room temperature the laser threshold is only 5mA. The fiber-coupled power exceeds 5.3dBm (3.4mW) at the threshold for a 20mA bias current. Even at +110°C the threshold is still relatively low at only 34mA. An optical power of 1.7mW can be coupled into a single-mode fiber at the threshold for a 20mA bias current. The laser operates in single mode from −45 to 125°C with a side-mode-suppression ratio (SMSR) greater than 40dB, as shown in Figure 2. For 1310nm DFB lasers the single-mode-operation temperature range is even wider: we confirmed a 180°C operational temperature span (from −70 to 110°C) with SMSR>35dB (see Figure 3).
We also demonstrated high reliability of uncooled DFB lasers at 1310 and 1490nm. A stress life test at 100°C (5mW) was performed over 10,000h. Wide-temperature-range operation at other wavelengths in the 1310 and 1550nm bands has also been achieved. These uncooled wide-temperature-range DFB lasers all exhibit performance characterized by high reliability, high power, high modulation speed, slow power variation with temperature, and small tracking errors. These attributes make them ideal for G-PON applications as key components of optical network units and optical line terminals. Further reduction of the costs associated with the construction of G-PON lasers using new designs and a novel fabrication process is currently being pursued.
Prior to becoming chairman and chief executive officer of Archcom Technology Inc., T. R. Chen was a professor at the University of Electronic Science and Technology in China and a visiting faculty member at the California Institute of Technology.
Nong Chen used to be a senior researcher at Pioneer's Research & Development Laboratories in Japan and senior research faculty at the University of Tokyo. He is currently senior scientist and project manager.
Wei Hsin was director of wafer fabrication at Finisar Corporation before joining Archcom in the same role.
Steven Chen is senior scientist and MOCVD manager.
