• Keränen, Kimmo
• Korhonen, Pentti
• Mäkinen, Jukka-Tapani
• Ollila, Jyrki
• Karioja, Pentti
• Heikkinen, Veli
• Kautio, Kari

Abstract

A hermetic fiber pigtailed laser module utilizing passive devicealignment on a low-temperature cofired ceramics (LTCC) substrate isdemonstrated. The 3-D shape of the laminated and cofired ceramicsubstrate provides the necessary alignment structures, including groovesand cavities, for the laser-to-fiber coupling. When the laser diodechip and component tolerances are tight enough, the passive alignmentallows high coupling efficiency realizations of multimode fiberpigtailed laser modules. The ceramic substrate is intrinsically hermeticand it opens up the possibility to use the substrate as an integratedpart of the hermetic module package. In our concept hermetic sealing isproduced by utilizing a Kovar frame, which is soldered to an LTCCsubstrate. The Kovar frame has a hole for a fiber feed-through and ahermetic glass-metal seal between fiber and frame is processed using aglass preform. The module can be used as a transmitter in a laser pulsetime-of-flight distance sensor and in this application it can beoverdriven by a factor of 10. This means that the peak optical power inthe pulses can be several dozen watts. The laser chip allows this kindof overdriving, due to the fact that the duty factor in the operation isonly 0.0% at 2 kHz pulsing frequency, which leads to an average powerof several milliwatts. The simulated nominal coupling efficiency betweenthe 210 mum times 1 mum stripe laser and the 200/220 mum step indexfiber (NA = 0.22) was 0.65. The measured coupling efficiency of thehermetically sealed prototypes varied from 0.14 to 0.64, where theaverage was 0.39. A leak rate of 1 times 10^-7 . .. 8 times 10^-7 [atm times cm³/s]was measured in the helium leak tests of the final operationalprototypes, when the modules were tested according to MIL-STD-883Fmethod 1014.9 specification. The rather high leak rate is mainly due tothe helium absorbed by the fiber polymer buffer layer and rubber guardtube in the pressurization process. The leak rate for the dummy modulesusing a buffer stripped fiber without a rubber guard tube was 3 times 10^-9 . .. 1 times 10^-8 [atm times cm³/s]. The maximum allowed leak rate for this size of hermetic module is 1 times 10^-7 [atm times cm³/s]. The background helium level before and after the tests was less than 3 times 10^-10 [atm times cm³/s]. Measurements proved that the manufacturing procedure is capable of producing hermetic fiber pigtailed laser modules.

Topics

• impedance spectroscopy
• glass
• glass
• laser emission spectroscopy
• ceramic
• rubber