High-power linewidth-narrowed applications of laser diode arrays demand high beam quality in the fast, or vertical, axis. This requires very high fast-axis collimation (FAC) quality with sub-mrad angular errors, especially where laser diode bars are wavelength-locked by a volume Bragg grating (VBG) to achieve high pumping efficiency in solid-state and fiber lasers. The micron-scale height deviation of emitters in a bar against the FAC lens causes the so-called smile effect with variable beam pointing errors and wavelength locking degradation.<p> </p>We report a bar smile imaging setup allowing FAC-free smile measurement in both QCW and CW modes. By Gaussian beam simulation, we establish optimum smile imaging conditions to obtain high resolution and accuracy with well-resolved emitter images.<p> </p>We then investigate the changes in the smile shape and magnitude under thermal stresses such as variable duty cycles in QCW mode and, ultimately, CW operation. Our smile measurement setup provides useful insights into the smile behavior and correlation between the bar collimation in QCW mode and operating conditions under CW pumping. With relaxed alignment tolerances afforded by our measurement setup, we can screen bars for smile compliance and potential VBG lockability prior to assembly, with benefits in both lower manufacturing costs and higher yield.
Fiber-coupled laser diode modules employ power scaling of single emitters for fiber laser pumping. To this end, techniques such as geometrical, spectral and polarization beam combining (PBC) are used. For PBC, linear polarization with high degree of purity is important, as any non-perfectly polarized light leads to losses and heating. Furthermore, PBC is typically performed in a collimated portion of the beams, which also cancels the angular dependence of the PBC element, e.g., beam-splitter. However, we discovered that single emitters have variable degrees of polarization, which depends both on the operating current and far-field divergence. We present data to show angle-resolved polarization measurements that correlate with the ignition of high-order modes in the slow-axis emission of the emitter. We demonstrate that the ultimate laser brightness includes not only the standard parameters such as power, emitting area and beam divergence, but also the degree of polarization (DoP), which is a strong function of the latter. Improved slow-axis divergence, therefore, contributes not only to high brightness but also high beam combining efficiency through polarization.
In our research, we have theoretically studied a device that can serve as a modulating retroreflector (MRR) for applications in the visible spectrum. The device is comprised of a nanocomposite of a ferroelectric thin-film embedded with noble metal nanoshells. In comparison to the nanospheres, the nanoshells provide more flexibility in the design of the device. This MRR can be used in asymmetric communication links as an optical transceiver for mobile devices. The main conclusion from our study is that a nanocomposite-based MRR can save power, complexity, dimensions, and weight in comparison to standard communication links. This fact is very important for mobile platforms.
Underwater optical wireless communication is an emerging technology, which can provide high data rate. High data rate communication is required for applications such as underwater imaging, networks of sensors and swarms of underwater vehicles. These applications pursue an affordable light source, which can be obtained by light emitting diodes (LED). LEDs offer solutions characterized by low cost, high efficiency, reliability and compactness based on off-the-shelf components such as blue and green light emitting diodes. In this paper we present our recent theoretical and experimental results in this field.
Visible light communication (VLC) is a new emerging technology that uses standard visible light to transmit broadband data streams in addition to illumination. In our research we have theoretically studied an innovative device that can serve as a modulating retro-reflector (MRR) for VLC applications. The device comprises of a nanocoposite of ferroelectric thin-film embedded with noble metal nano-shells. In comparison to the nano-spheres, the nano-shells provide more flexibility in the design of the device. This MRR can be used in asymmetric communication links as an optical transceiver for mobile devices. The main conclusion from our study is that a nanocomposite based MRR can save power, complexity, dimensions and weight in comparison to standard communication links. this fact is very important for mobile platforms.
Modulating retro-reflectors (MRR) are beneficial for asymmetric free-space optics communication links. An MRR includes an optical retro-reflector and an electro-optic shutter. The main advantage of an MRR configuration is that it shifts most of the power, weight, and pointing requirements onto one end of the link. In this study an innovative device comprising of nanoparticle-embedded ferroelectric thin film is used as an MRR. The new modulator is mounted in front of a passive retro-reflector. In our study we calculated the link budget for lunar exploration scenario. The scenario includes a base station that communicates with several robots or astronauts. In our simulations, the base station illuminates a robot with a continuous-wave beam, i.e. an interrogating beam. The un-modulated beam strikes the MRR, which is located on the robot, and is passively reflected back to the base station carrying the data that has been modulated onto it by the MRR. In this scenario a robot and a base-station are 4km apart, with a clear line of sight. In addition, the innovative MRR is capable of achieving 12dB contrast ratio. Under these assumptions and using the nanoparticle-embedded ferroelectric MRR we calculated the required transmission power for a given bit-rate and BER.
We present an innovative method for modulating light using a ferroelectric thin-film embedded with metal nanoparticles.
Due to the electro-optic effect in ferroelectric PZT, changes in refractive index can be controlled by an external electric
field. Consequently, the local surface plasmon resonance of embedded noble metal nanoparticles changes with the
media's refractive index. As a result, their optical extinction cross-section is shifted and light passing through the film
could be controlled. In other words, an external electric field could modulate light. Using Mie theory for spherical
particles, we were able to approximate the metallic nanoparticle's diameter that generates the maximum optical contrast,
at a given wavelength. In addition, to establish an accurate model, we considered the impact on plasmon resonance
resulting from deformation of the nanoparticles. The deformation is caused by the piezoelectric property of the
ferroelectric host material. We assumed 20 nm diameter Au or Ag nanoparticles embedded in a 1 μm thick PZT film.
Simulations showed that these particles can reach an optical contrast of up to 12 dB, in the visible spectrum. In addition, deformation of particles had negligible impact on the shift in resonance frequency compared to the change in PZT refractive index. In this study we have shown that a nanocomposite comprising of nanoparticles embedded PZT thin film can perform as an optical modulator. This modulator will be able to achieve a high contrast with low power