Multiple streams of high definition television (HDTV) and improved home-working infrastructure are currently driving
forces for potential fiber to the home (FTTH) customers . There is an interest to reduce the cost and physical size of
the FTTH equipment. The current fabrication methods have reached a cost minimum. We have addressed the costchallenge
by developing 1310/(1490)/1550nm bidirectional diplexers, by monolithic seamless integration of lasers,
photodiodes and wavelength division multiplexing (WDM) couplers into one single InP-based device. A 250nm wide
optical gain profile covers the spectrum from 1310 to 1550nm and is the principal building block. The device fabrication
is basically based on the established configuration of using split-contacts on continuos waveguides. Optical and electrical
cross-talks are further addressed by using a Y-configuration to physically separate the components from each other and
avoid inline configurations such as when the incoming signal travels through the laser component or vice versa. By the
eliminated butt-joint interfaces which can reflect light between components or be a current leakage path and by leaving
optically absorbing (unpumped active) material to surround the components to absorb spontaneous emission and nonintentional
reflections the devices are optically and electrically isolated from each other. Ridge waveguides (RWG) form
the waveguides and which also maintain the absorbing material between them. The WDM functionality is designed for a
large optical bandwidth complying with the wide spectral range in FTTH applications and also reducing the polarization
dependence of the WDM-coupler. Lasing is achieved by forming facet-free, λ/4-shifted, DFB (distributed feedback
laser) lasers emitting directly into the waveguide. The photodiodes are waveguide photo-diodes (WGPD). Our seamless
technology is also able to array the single channel diplexers to 4 to 12 channel diplexer arrays with 250μm fiber port
waveguide spacing to comply with fiber optic ribbons. This is an important feature in central office applications were
small physical space is important.
The laser diode technology, underpinning applications such as data storage, industrial lasers and optical telecommunications, still suffers from reliability and longevity limitations, especially in high power applications. A main problem for these lasers arises from facet oxidation, leading to increased absorption, power degradation and COMD device failure. Typically, high power devices initially show a low linear degradation and after some 100 hours, the degradation accelerates in a nonlinear fashion, indicating a degradation runaway condition. This article reports performance and reliability improvements that are based on a process which atomically seals surfaces and eliminates oxidation by forming stable nitrides on laser facets. The dangling bond terminating technology suppresses accelerated degradation associated with optical density and heat at laser facets. The dangling bond termination is demonstrated by improved COMD, decreased degradation at CW operation and a constant linear degradation rate at different QW temperature conditions (nonlinear degradation indicates advancement in the oxidation/optical absorption/facet heating/oxidation spiral). The technology is applicable to a range of material systems and has previously been demonstrated on InAlGaAs and InGaAs (increased COMD to >270 and 470mW/μm respectively). The devices with the typically lowest COMD levels (AlInGaAs) show a remarkably low linear degradation rate of <0.5%/kh during at CW life test operation at 90°C and a power level corresponding to 80W bar power. In addition to long term AlInGaAs laser life test results, this paper presents results on nitride facet passivation applied to 805nm InGaAsP devices, showing improved COMD to 400mW/μm and the initial CW life data confirms the general behavior of the previously life-tested InGaAs and InAlGaAs based devices.
The 40-year-old laser diode technology underpins applications such as data storage, industrial lasers and telecommunications but still suffers from reliability and longevity issues in high power applications, most notably in pumping of Nd:YVO4 and Nd:YAG lasers. Despite thermal advantages allowing expansion matched Au/Sn hard soldering, the main problem for InAlGaAs lasers is facet oxidation, which leads to increased absorption and COMD device failure. This article presents a novel process, which atomically seals the surface and eliminates oxidation by forming stable nitrides on the facet. Pulsed testing of 805 nm of Al>0.20InGaAs single mode devices with a protective nitride layer demonstrates stable median 1.3W COMD (30MW/cm2), after one hour of CW screening at 12.5mW/μm (50W bar power). A 200h burn-in at 12.5mW/μm (50W bar power) resulted in an initial power drop of 1-2% and a linear degradation rate of 0.1%/1000h, compared to an initial power drop of 5-18% and a degradation rate of 46%/1000h for lasers with only AR/HR-coatings. A subsequent 1000h life-test at 22.5mW/um (90W bar power) demonstrated a degradation rate of only 3%/1000h under stress test conditions due to p-side up mounting, 10°C higher ambient temperature and 57% higher operating current over typical high power bar operating power levels. The QW temperature was 53°C. No sudden device failures occurred.