We present current work developed at INO on phosphate glass optical fiber for laser and amplifier applications at 1.54
microns. Core and cladding glasses were fabricated by a multi-components melting process which gave an uniform
refractive index core profile. Rod-in-tube method under Argon atmosphere was used to fabricate optical fibers. The
effect of nitrogen atmosphere on hydroxyl groups OH- during glass melting was studied. The absorption coefficient
calculated at 3.42 μm was found to be lower than 0.5 cm-1 which corresponds to less than 70 ppm OH-. Absorption and
emission cross sections were calculated at 1534 nm. Fabrication process allowed us to decrease background losses of
core Er3+ - Yb3+ co-doped fiber between 0.02 and 0.04 dB/cm. Laser power was measured at 1563 nm and a 26% slope
efficiency was achieved with a 22 cm-long single-clad fiber co-doped with 1.1 wt% in Er3+ and 11.1 wt% in Yb3+. For
the same fiber, an internal gain was found to be 20 dB at 1536 nm for a 5-cm-long fiber.
This paper reports on a novel pair of microlens arrays (MLA's) for efficient coupling of the high aspect ratio optical beam emitted by high-power laser diode linear arrays (also referred to as laser diode bars) into the core of multimode optical fibers. These novel MLA's overcome the limitations observed when using high fill factors laser diode bars. The MLA designs are described. Results from modelling work show good coupling performances for laser diode bars with fill factors up to 75%. The technique for fabricating the complex surface profiles of the MLA's is discussed. Masters are first fabricated and MLA's are then replicated, so that volume production at low cost can be envisioned. The fabricated MLA's have been used for reshaping and fiber coupling the output of a 10-mm laser diode bar. An efficiency of 74% has been obtained when coupling into an optical fiber having a core diameter of 400 μm and a numerical aperture of 0.22.
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.
Adverse weather, lack of background illumination and interference from artificial light sources can considerably reduce the effectiveness of passive LLLTV systems. Viewing in such conditions can be considerably enhanced by the addition of a pulsed laser illuminator synchronized to a gated LLLTV. Since the system has its own light source, surveillance is possible in zero or ultra-low light conditions. Furthermore using the technique of range gating, the effects of backscattering that occur in poor visibility conditions can be greatly reduced. The high resolution and self-illumination capability of the system permits positive identification of targets that would be undetectable or unresolvable with FLIR type systems.
1.5 MW, < 10 nsec long gain-switched laser pulses were obtained from a Nd+3 laser-pumped Cr+4:YAG laser with an efficiency of 16%. Tuning was achieved from 1.44 to 1.51 micrometers . Repetition rate of 30 pps was demonstrated at approximately equals 10 mJ/pulse.
A high reflectivity, graded reflectivity mirror (GRM) design has been recently developed for use in low-gain lasers. A GRM with 63% central reflectivity was used as an output coupler in a low magnification (M equals 1.4) unstable resonator Nd:Glass laser to obtain a Q-switched energy of 2 J in a 25 ns pulse and over 3.7 J in fixed-Q operation. The beam has a diffraction limited divergence of 0.15 mrad. In an Alexandrite laser, a GRM having a central reflectivity of 78% was used in a M equals 1.2 unstable resonator. A fixed-Q output energy of over 500 mJ and a Q-switched energy of 400 mJ was obtained in a diffraction-limited beam. The second harmonic output was increased by a factor of ten from that obtained with a multimode stable resonator.