We present an investigation of the optical property of orbital angular momentum (OAM) for use in the detection of objects obscured by a turbid underwater channel. In our experiment, a target is illuminated by a Gaussian beam. An optical vortex is formed by passing the object-reflected and backscattered light through a diffractive spiral phase plate at the receiver, which allows for the spatial separation of coherent and non-coherent light. This provides a method for discriminating target from environment. Initial laboratory results show that the ballistic target return can be detected 2-3 orders of magnitude below the backscatter clutter level. Furthermore, the detection of this coherent component is accomplished with the use of a complicated optical heterodyning scheme. The results suggest new optical sensing techniques for underwater imaging or LIDAR.
In this effort, we report on the preservation of the spatial mode quality as composite vortex beams propagate through a flashlamp pumped amplifier system. Because of the spatially asymmetric nature of the transient thermal lensing, a laser beam propagating through this type of amplifier will be distorted. This makes an ideal environment to assess the mode integrity of propagating composite vortex beams. We demonstrate that a 3-lobe composite vortex beam can propagate under extreme transient thermal lensing and maintain the mode structure through the amplification process. Even though the actual amplification wasn’t the main thrust of this effort, we demonstrate gain greater than a factor of 4 for two different seed energy levels. Since the flashlamp pumped system is an extreme case, this result shows the potential for using concentric vortex beams in high power amplifiers and could open up new applications in propagation and sensing.
Underwater optical communication has recently become the topic of much investigation as the demands for underwater data transmission have rapidly grown in recent years. The need for reliable, high-speed, secure underwater communication has turned increasingly to blue-light optical solutions. The blue-green visible wavelength window provides an attractive solution to the problem of underwater data transmission thanks to its low attenuation, where traditional RF solutions used in free-space communications collapse. Beginning with GaN laser diodes as the optical source, this work explores the encoding and transmission of digital data across underwater environments of varying turbidities. Given the challenges present in an underwater environment, such as the mechanical and optical turbulences that make proper alignment difficult to maintain, it is desirable to achieve extremely high data rates in order to allow the time window of alignment between the transmitter and receiver to be as small as possible. In this paper, work is done to increase underwater data rates through the use of orbital angular momentum. Results are shown for a range of data rates across a variety of channel types ranging in turbidity from that of a clear ocean to a dirty harbor.
This work examines the propagation properties of two superimposed coherent orbital angular momentum (OAM) modes for use in underwater systems as an alternative to amplitude modulation. An OAM mode of <i>l</i>=+2 is interfered with OAM mode <i>l</i>=-1 from a λ = 540 nm laser source. These OAM modes are superimposed using a Mach-Zehnder (MZ) interferometer combined with diffractive optical elements. By manipulating the optical path length of one of the MZ legs, the interference of these beams can be temporally controlled. The spatial profile is maintained in a turbid environment up through 4.9 attenuation lengths for both cases.
Space division multiplexing of optical beams has recently been demonstrated for improving the bandwidth of optical communication links. This paper will explore the use of space division multiplexing utilizing blue lasers for potential undersea applications. Experimental results will be shown for optical vortices utilizing a range of charge numbers corresponding to various Orbital Angular Momentum states.