Chip-scale optical phased arrays for inter-spacecraft communications

Optical phased arrays borrow concepts from radar phased array science and technology to provide non-mechanical beam steering of electromagnetic radiation in the far field. Like radar phased arrays, this is achieved through controlling the relative phases of individual emitters on the device. However, since the device size scales with the wavelength of the electromagnetic signal, moving from radar to optical signals involves a reduction in size by more than 4 orders of magnitude. As a result, optical phased arrays can be created on a compact, chip-scale platform. This is particularly of interest for inter-spacecraft communications where high bandwidth optical signals can be communicated in free-space from one location to another. Providing this functionality with a low SWaP, chip-scale device is crucial for space applications. Recently, many chip-scale optical phased arrays have been developed to provide non-mechanical beam steering of light at optical frequencies, including many demonstrations at the telecommunications wavelength of 1550 nm. Here we will discuss the existing demonstrations as well as highlight the tradeoffs between different designs. We will highlight the importance of spacing the emitters at a technologically challenging pitch that is half the operational wavelength in order to avoid the many negative effects of grating lobes, including power loss, steering range limitation, and the opportunities they provide for eavesdropping.