Sensing systems for defense and security operate in evermore demanding environments, increasingly leaving the comfort
zone of fiber laser technology. Efficient and rugged laser sources are required that maintain a high level performance
under large temperature excursions and sizable vibrations. This paper first presents a sample of defense and security
sensing applications requiring laser sources with a narrow emission spectrum. Laser specifications of interest for defense
and security sensing applications are reviewed. The effect of the laser frequency noise in interferometric sensing systems
is discussed and techniques implemented to reduce phase noise while maintaining the relative intensity noise
performance of these sources are reviewed. Developments towards the size reduction of acoustically isolated narrow-linewidth
semiconductor lasers are presented. The performance of a narrow-linewidth semiconductor laser subjected to
vibrations is characterized. Simulation results of interferometric sensing systems are also presented, taking into account
both the intensity and phase noise of the laser.
Frequency noise reduction of semiconductor lasers using electrical feedback from an optical frequency
discriminator is an efficient and simple approach to realize narrow linewidth lasers. These lasers are of great
interest for applications such as LIDAR, RF photonics and interferometric sensing. In this paper, we review
the technological choices made by TeraXion for the realization of its Narrow Linewidth Laser modules. The
method enables to decrease the linewidth of DFB lasers from several hundreds of kHz to a few kHz. We
present the work in progress to integrate such system into a miniature package and to incorporate advanced
functionalities such as multi-laser phase locking.
Gain flatness of optical amplifiers over the communication bandwidth is a key requirement of high performance optical wavelength division multiplexing (WDM) communication systems. Most often, a gain flattening filter (GFF) with a spectral response matching the inverse gain profile is incorporated within the amplifier. The chirped fiber Bragg grating (CFBG) is an attractive technology to produce GFFs, especially in cases where very low error functions are required. Error functions smaller than or equal to ±0.1 dB for the full operating temperature range are now possible. Moreover, the systematic errors from cascaded filters are much smaller than for thin-film GFF, a factor of importance in a long chain of amplifiers. To achieve this performance level, the high-frequency ripples normally associated with CFBG-GFF have been reduced by combining state-of-the-art holographic phase masks and advanced UV-writing techniques. Lastly, to eliminate the residual low-frequency ripples and localized errors, we developed a laser annealing-trimming station. This fully automated station combines both the aging process and final trimming of the GFF refractive index profile to exactly match the required transmission spectra. The use of self-adjusting algorithms assures quick convergence of the error function within a very tight error band. The capital expenditure necessary to implement this new tool is small in relation to the gain in precision, reliability and manufacturing cycle time.
During the last five years, INO has conducted a program to develop high performance diode laser illuminator for ATV (active TV) system. Illuminator made using INO proprietary collimating method, which uses GRIN (mu) -lenses for the fast axis and plano-convex (mu) -lenses for the slow axis, were found to provide uniform and tightly collimated beam, diffraction-limited for the fast axis. Such collimated laser diode arrays were proven to efficiently illuminate non- cooperative scene at 1 km with only 6 W of average power.
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