|
Monolithic wavelength stabilized diode lasers, e.g. distributed Bragg reflector (DBR) ridge waveguide (RW) lasers,
are well-suited light sources for compact and portable Raman spectroscopic systems. In the case of in situ and
outdoor investigations, the weak Raman lines are often superimposed by daylight, artificial light sources or
fluorescence signals from the samples under study. Among others, shifted excitation Raman difference spectroscopy
(SERDS) has been demonstrated as a powerful and easy-to-use technique to separate the Raman lines from
disturbing background signals. SERDS is based on subsequential excitation of the sample with two slightly shifted
wavelengths. The Raman lines follow the change in the excitation wavelength whereas the non-Raman signals
remain unchanged. For SERDS dual-wavelength light sources, e.g., mini-arrays containing two DBR-RW lasers, are
requested. Moreover, for portable Raman instruments such as handheld devices robust and reliable excitation light
sources with lifetimes > 1,000 h are preferred.
In this work, reliability investigations of dual-wavelength DBR-RW mini-arrays over a total test time of 5,000 h are
presented. Wavelength stabilization and narrowing of the spectral emission is realized by 10th-order DBR surface
gratings defined by i-line wafer stepper technology. The DBR-section has a length of 500 μm, the devices a total
length of 3 mm. The ridge waveguide has a stripe width of 2.2 μm. Maximum output powers up to 215 mW per
emitter were measured. Over the whole power range, 95 % of the emitted power is within a spectral width of 0.15 nm
(2.5 cm−1), which is smaller than the spectral width needed to resolve most Raman lines of solid and liquid samples.
In a step-stress test, the devices were tested at 50 mW, followed by 75 mW and finally at 100 mW per emitter.
Electro-optical and spectral measurements were performed before, during and after the test. All emitters under study
did not show any deterioration of their electro-optical and spectral properties demonstrating their applicability for
portable Raman systems.
|
