Development of Semiconductor Laser for Optical Communication
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SPECIAL Development of Semiconductor Laser for Optical Communication Tsukuru KATSUYAMA The performance of the semiconductor laser has been dramatically improved by applying quantum well structure including strained layer superlattice and innovation of crystal growth techniques such as organometallic vapor phase epitaxy. The semiconductor laser used for optical communication came to be indispensable for our life as an optical component con - necting not only long-distance large-capacity trunk networks but also access networks. This paper describes the develop - ment of the semiconductor laser for optical communication focusing mainly on Sumitomo Electric’s R&D activities. With the progress of optical transmission technology, various kinds of semiconductor lasers have been developed for the application to wavelength division multiplexing, high speed, low power consumption, and photonic integration. Keywords: semiconductor laser, optical communication, quantum well 1. Introduction 2. Development of high performance FP laser The performance, functionality and productivity of 2-1 Materials and crystal growth techniques the semiconductor laser have been dramatically improved Wavelengths used for optical communication are since its invention in 1962. Today it came to be indispen - mainly 1.55 µm for long-distance transmission and 1.3 µm sable for our life as optical components connecting home for short- and mid distance transmission due to the min - and the Internet as well as long-distance large-capacity imum loss and minimum dispersion in optical fiber, re - trunk networks. spectively as shown in Fig. 1 . However, recently over 400 It may be said that the information revolution pulled nm band ranging from 1260 nm to 1675 nm came to be by the explosive spread of the Internet is originated from used by the introduction of the WDM technology men - three innovations from 1969 to 1970, the room tempera - tioned later. It is required that the band gap of the mate - ture continuous wave operation of the semiconductor rial composing semiconductor laser correspond to the laser, the invention of the low loss optical fiber and the oscillation wavelength of the laser and its lattice constant beginning of ARPAnet (Advanced Research Projects match to the lattice constant of the substrate to maintain Agency Network) experiment. In those days, we already high crystal quality. Typical compound semiconductors started research and development of optical fiber and that meet these conditions are GaInAsP, AlGaInAs grown compound semiconductor materials such as GaAs which on the InP substrate and GaInNAs, (Ga)InAs dots struc - was widely used as a substrate material of compound semi - ture grown on the GaAs substrate. Today, GaInAsP is conductor devices. We started the research and develop - widely used from view points of reliability of device and ment of compound semiconductor devices for optical handling in its process. communication from the middle of the 1980’s in order Liquid Phase Epitaxy (LPE) has been used for the to establish optical communication business vertically in - growth of high quality compound semiconductors. How - tegrating technology from materials, devices to systems. ever, molecular beam epitaxy (MBE) and Organometallic This paper describes the development of the semi - conductor laser for optical communication focusing mainly on Sumitomo Electric’s R&D activities with the InP substrate progress of transmission technology. By the beginning of GaInAsP 1990’s 1.3 µm Fabry-Perot (FP) lasers were developed for AlGaInAs the application to metro-access networks. In the 1990’s GaAs substrate practical use of wavelength division multiplexing (WDM) (Ga)InAs Quantum Dot GaInNAs(Sb) started and pumping lasers for fiber amplifiers and dis - dB/km s tributed feedback (DFB) lasers were developed for WDM s o 1.0 l 1260nm 1675 nm application. In the 2000’s with the recovery from the IT n O+E+S+C+L+U o i bubble burst, further improvements of modulation speed, s 415nm(55THz) s i 0.5 0.33dB/km m power consumption and functionality were progressed by s n a the development of new material, vertical cavity surface r 0.18dB/km T 0 emitting laser (VCSEL) and photonic integration. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 Wavelength (µm) Fig. 1. Fiber transmission loss and related compound semiconductors SEI TECHNICAL REVIEW · NUMBER 69 · OCTOBER 2009 · 13 Vapor Phase Epitaxy (OMVPE) have been developed rap - ing condition (6) . The effect of stain on the performance of idly from the beginning of the 1980’s. These crystal devices has been intensively studied and demonstrated growth technologies are suitable for mass production and such as the reduction of threshold current (7), (8), higher fre - enable to grow a very thin layer which is essential for re - quency characteristics and improved reliability (9)-(11) . alizing high performance devices. Especially, OMVPE We have applied strained quantum well structure to which is suitable for the synthesis of phosphorus com - visible laser (12)-(15) and pumping laser for optical fiber am - pounds and selective area growth has been used for the plifier (12), (16) for the first time and confirmed significant growth of semiconductor lasers. We established OMVPE improvement of device performance. The strained quan - crystal growth techniques as a core technology for the fab - tum well technology provided the tool to control effective rication of semiconductor devices in the late 1980’s. We mass in addition to quantum size effect and came to be a designed the reactor and the gas supply system of OMVPE core technology for creating high performance com - for realizing ultrafine structure with good uniformity. Ex - pound semiconductor devices by expanding the flexibility cellent uniformity over a 2-inch wafer with monolayer het - of the band engineering. ero-interface abruptness was obtained (1) -(3) . 2-3 Development of all OMVPE grown MQW FP laser 2-2 Development of quantum well structure We first developed 1.31 µm MQW FP laser for metro- The lattice constant and the band gap of a compound access network application. Figure 3 shows a schematic di - semiconductor can be controlled by changing the compo - agram of the laser. The active region of the laser consists sition of the compound. In the structure that two com - of GaInAsP MQW. After the first growth, the wafer was pound semiconductors with different band gaps pile up wet etched to form mesa stripe waveguide and p-InP and alternatively, electrons and holes are confined in the lower n-InP current blocking layers were selectively grown. band gap layers. The thickness of the lower band gap layer Then the upper cladding layer and contact layer were approaches to electron mean free pass (several 10 nm), grown to form planar buried heterostructure (PBH). Fi - energy levels of electron and hole are quantized. This nally, the trench was formed to reduce parasitic capaci - structure is called quantum well (QW) or multiple quan - tances and electrodes were made after thinning the tum well (MQW) when the structure consists of multiple substrate. It should be mentioned that OMVPE was em - layers. In the MQW structure, novel phenomenon which ployed for all crystal growth process to improve the uni - cannot be obtained in the bulk material appears. By ap - formity over a 2-inch wafer. The device showed excellent plying MQW structure to the active region of the laser, the high temperature characteristics with a maximum oper - performances of the laser such as threshold current, tem - ating temperature of 130 °C and good uniformity over the perature characteristics and modulation frequency were 2-inch wafer with standard deviation of threshold current significantly improved (4), (5) and MQW became an essential of 0.76 mA (17) . technology for realizing high performance lasers. Further improvement of laser performance was ex - pected by introducing a strain in the active region of the laser in addition to MQW technology. In general, the lat - tice constant must be matched to that of the substrate to p-electrode p-GaInAs contact maintain high crystal quality. However, in the MQW struc - p-InP clad ture, the strain caused by lattice mismatch can be accumu - n-InP block lated in the crystal by deforming the lattice without p-InP block generating misfit dislocation since the layer is very thin as Trench shown in Fig. 2 . Therefore, the flexibility of MQW design n-InP substrate will be improved by relaxing the restriction of lattice match - n-electrode GaInAsP MQW active region Electron GaInAsP well A A GaInAsP barrier B B B Active region Hole A A Fig. 3. FP laser structure Dislo- B B cation Substrate A Substrate A Substrate A This MQW laser manufacturing process innovation promoted the laser which had been treated like an art Dislocations are gene No dislocation is generated when B layer is craft to the mass production stage and contributed to the rated when B layer is thinner than the critical thickness thicker than the critical (B layer is under biaxial compressive strain) reduction of production costs and the improvement of thickness productivity. This wafer process has formed the backbone of our manufacturing technology of semiconductor lasers. Fig. 2. Strained quantum well structure 14 · Development of Semiconductor Laser for Optical Communication 3. Development of lasers for WDM application The high power laser module using this device shown in photo 1 was highly evaluated in the market. 3-1 Progress of WDM transmission technology For improving optical output power of the laser mod - WDM transmission is a method to transmit different ule, the polarization and the wavelength multiplexing wavelength optical signals in a single fiber. Therefore, by were also used. We fabricated fiber-grating laser modules applying WDM transmission technology, significant in - using 1.48 µm semiconductor optical amplifiers (SOAs) crease of the transmission capacity can be realized rela - and fiber gratings with oscillation wavelength ranging tively easily without installing new optical fibers.