Changes in output characteristics of broad-band superluminescent diodes in the process of long-term operation

Abstract

Methods and results of reliability tests of SQW broad-band superluminescent diodes (SLD-37 series), widely used in optical coherence tomography (OCT) are presented. Main attention was paid to the alteration of SLD spectral characteristics in the aging process. It is shown that the usage of these methods in the input control of processed epiwafers deployed in the production of active elements for SLD-modules ensures the estimation of their expected life time (MTTF).Broad-band SLDs are widely used as light sources in OCT [1]. For the ophthalmic OCT the most popular are SLDs of NIR range 800 -900 nm, based on QW heterostructures. Such lightsources of SLD-37 series are used worldwide in dozens of laboratories and companies that deal with research, development and production of new OCT systems, mainly for ophthalmic uses. Since their development, over 10000 SLD-37 modules have been fabricated and realized.These SLDs are based on SQW (GaAl)As/GaAs SC DH [2]. Their active channel is a traditional single transverse mode 4 flm-width ridge waveguide. Free-space output power amounts 5 -35 mW, depending on technical requirements. Density of light on the facets of the crystal amounts 0.12 -0.85 MW/cm 2 , or 1.2 -8.5 mW/flm respectively. These values are far below catastrophic optical damage threshold, which is about 3 -6 MW/cm 2 for these devices. Huge statistics shows that except for so-called "infant mortality", catastrophic degradation of such SLDs occurs very rarely. In contrast to laser diodes and SLDs based on bulk SC DH, spectrum shape of the concerned SLDs depends strongly on injection current and, moreover, can change significantly in long-term operation. To our knowledge, there are no works devoted to this aspect of broad-band SLDs reliability.The aim of this work was to research experimentally the changes of power and spectrum characteristics of SLD-37 devices, and to develop a method of estimating their lifetime.There is a lot of requirements for OCT SLDs. The injection current I is limited from above, output power P has both minimum and maximum, and so does the median 978-1-61284-813-6/11/$26.00 ©2011 IEEE 1 wavelength 'A.. Taking into account the double-humped spectrum shape, spectral flatness SF is limited also. Spectral ripple, or Fabry-Perot modulation depth, is also limited from above, while bandwidth 11'A. must exceed the minimum value.Specific challenges appear around 11'A. because of strong dependence of spectrum shape on injection current (see fig. 1a). If injections current are low enough, superluminescence is determined by transitions from fundamental state only, so the spectrum shape is bell-like. Short-wavelength second maximum, corresponding to transitions from 15t excited state, appears when we raise the current. When the current reaches some specific value 10, spectral maxima become equalized (l1S=O) and 11'A. reaches its maximum value. If the current is increased more, second maximum prevails. Fig. 1 b depicts the dependence of bandwidth on inequality of spectral maxima. It seems obvious that i...