Free-space Elements Research Articles

Distributed active phase-locking of an all-fiber structured laser array by a stochastic parallel gradient descent (SPGD) algorithm.

Coherent beam combining (CBC) of fiber laser array is a promising way to achieve high output power. Phase control is one key point to implement CBC. Appropriate feedback structures should be established to achieve phase control. Most feedback structures of CBC are established after the lasers emit to free space and consist of a set of lenses or mirrors. Those optical elements in free space may hinder array size and integration. In this paper, we demonstrated an all-fiber structured CBC method with distributed phase-locking. By adding an all-fiber measurement loop beside the main laser chain, the phase of main laser chain is appended to the measuring loop. Phases of each main laser chain are locked indirectly though the measurement loops by using stochastic parallel gradient descent (SPGD) algorithm. The principle of distributed phase-locking is also illustrated. Corresponding simulations are carried out and two-channel fiber lasers are coherently combined by this method. The experimental results show that the structure can achieve phase-locking effectively. Stable and distinct interference fringe is observed. Additionally, the structure proposed in this paper is straightforwardly building and expanding.

Cross Dipole Antenna Based Glass Substrate for Ambient Energy Harvesting

In this paper, a crossed dipole antenna based on Pyrex Glass substrate is proposed for dual bands, GSM 900 MHz and Wi-Fi 2.4 GHz. Each dipole is placed on a side of the substrate with the antennas oriented perpendicular to each other. Both antennas participate in an omnidirectional radiation pattern in perpendicular planes to form a radiation pattern similar to the isotropic. The realised gain for each dipole and the total radiation efficiency are 1.46 dB and 88% respectively. The glass substrate provides a reduction in the antenna size of about 36% compared with typical individual elements in free space.

Portable polarimetric fiber stress sensor system for visco-elastic and biomimetic material analysis

Non-destructive materials characterization methods have significantly changed our fundamental understanding of material behavior and have enabled predictive models to be developed. However, the majority of these efforts have focused on crystalline and metallic materials, and transitioning to biomaterials, such as tissue samples, is non-trivial, as there are strict sample handling requirements and environmental controls which prevent the use of conventional equipment. Additionally, the samples are smaller and more complex in composition. Therefore, more advanced sample analysis methods capable of operating in these environments are needed. In the present work, we demonstrate an all-fiber-based material analysis system based on optical polarimetry. Unlike previous polarimetric systems which relied on free-space components, our method combines an in-line polarizer, polarization-maintaining fiber, and a polarimeter to measure the arbitrary polarization state of the output, eliminating all free-space elements. Additionally, we develop a more generalized theoretical analysis which allows more information about the polarization state to be obtained via the polarimeter. We experimentally verify our system using a series of elastomer samples made from polydimethylsiloxane (PDMS), a commonly used biomimetic material. By adjusting the base:curing agent ratio of the PDMS, we controllably tune the Young's modulus of the samples to span over an order of magnitude. The measured results are in good agreement with those obtained using a conventional load-frame system. Our fiber-based polarimetric stress sensor shows promise for use as a simple research tool that is portable and suitable for a wide variety of applications.

Environmentally stable all-PM all-fiber giant chirp oscillator

We report on an environmentally stable giant chirp oscillator operating at 1030 nm. Thanks to the use of a nonlinear amplifying loop mirror as the mode-locker, we are able to extract pulse energies in excess of 10 nJ from a robust all-PM cavity with no free-space elements. Extensive numerical simulations reveal that the output oscillator energy and duration can simply be up-scaled through the lengthening of the cavity with suitably positioned single-mode fiber. Experimentally, using different cavity lengths we have achieved environmentally stable mode-locking at 10, 3.7 and 1.7 MHz with corresponding pulse energies of 2.3, 10 and 16 nJ. In all cases external grating-pair compression below 400 fs has been demonstrated.

Monolithic Highly Stable Yb-Doped Femtosecond Fiber Lasers for Applications in Practical Biophotonics

Operational and environmental stability of ultrafast laser systems is critical for their applications in practical biophotonics. Mode-locked fiber lasers show great promise in applications such as supercontinuum sources or multiphoton microscopy systems. Recently, substantial progress has been made in the development of all-fiber nonlinear-optical laser control schemes, which resulted in the demonstration of highly stable monolithic, i.e., not containing any free-space elements, lasers with direct fiber-end delivery of femtosecond pulses. This paper provides an overview of the progress in the development of such all-fiber mode-locked lasers based on Yb-fiber as gain medium, operating at the wavelength around 1 μm, and delivering femtosecond pulses reaching tens of nanojoules of energy.

Optimal Characteristics of an Arbitrary Receive Antenna

Several fundamental questions about the operation of receiving antennas are addressed, such as ldquoWhy does a receiving antenna scatter an incident field?rdquo and ldquoUnder what conditions does a receive antenna capture all of the available incident power?rdquo A new method is described by which the received power can be maximized for an arbitrary receiving antenna. The technique is first illustrated for two-dimensional infinite receiving arrays of electric and/or magnetic dipole elements, which result in simple plane waves for the scattered (re-radiated) fields. Optimal results (for maximum received power) are derived for several cases, and it is established that half the available incident power may be received by an array of electric (or magnetic) elements in free space, and that all available incident power may be received by an array that combines electric and magnetic elements, or one that incorporates a ground plane. Next, an arbitrary finite three-dimensional antenna enclosed by a mathematical spherical surface is treated using spherical vector wave functions. It is shown that half the available incident power can be received by such an antenna consisting of either TM or TE only elements, while all available incident power can be received when both TM and TE elements are used. It is also shown that the absorption efficiency for any optimal arbitrary antenna is 50%.

A fast MoM solution for large arrays: Green's function interpolation with FFT

A new type of fast method of moments (MoM) solution scheme for large arrays is developed using standard basis functions. Both fill and solve times are improved with respect to standard MoM solvers. The efficiency of the method relies on approximating the Green's function as a sum of separable interpolation functions defined on a relatively sparse uniform grid, along with use of the fast Fourier transform. The method permits the analysis of arrays with arbitrary contours and/or missing elements. Preliminary results show the effectiveness of the method for planar array elements in free space.