Orbital angular momentum of light has attracted great attention in recent years. Optical ferris wheel, composed of the superposition of different orbital-angular-momentum modes, further broadens the application of orbital angular momentum of light in many fields. Here, we experimentally demonstrate a simple way to produce optical ferris wheel with controllable orbital-angular-momentum modes using a mask of spiral arrays. We show that optical ferris wheel can be rotated by adjusting the spiral arrays, and the rotation angle is studied analytically and experimentally. Our technique shows the advantages of wide operation bandwidth and flexible manipulation and the simplicity of our method may find potential application in optical manipulation.

The current inverter is the core component of the helicopter transient electromagnetic (HTEM) detection system. It should meet the concerns of low loss, high power, and fast turn-OFF time. This article proposes a new circuit topology based on nine-level inverter technology to overcome the drawbacks of typical PWM (pulse width modulation) inverters, such as switching losses and harmonics. This circuit topology overcomes the shortcomings of the traditional single constant voltage clamp circuit in which the turn-OFF time is not adjustable. Using an inverter with the proposed topology is able to avoid the complex PWM control method and switching loss. In this way, the current rising edge and falling edge of this inverter are also improved effectively. The proposed inverter has adjustable turn-ON-time and turn-OFF time, which is significantly different from the conventional single-clamp inverter. Through subsequent experiments, the inverter proved to have the capability of generating trapezoidal current waveforms. Moreover, by modifying the FPGA (Field Programmable Gate Array) control program, three different turn-OFF times are achieved. The nine-level inverter has a peak current of 1.5 A with an adjustable turn-OFF time from 129 μs to 162 μs. Moreover, the switching frequency of the inverter is reduced from 10 kHz to below 100 Hz. The experimental results further demonstrate that it achieves lower switching losses and more flexible transmission. Our work in this article provides an efficient way to improve the performance of HTEM detection systems.