KEYWORDS: Single mode fibers, Signal attenuation, Free space optics, Nonlinear optics, Signal processing, Signal detection, Beam path, Laser communication terminals, Telecommunications
Hollow-Core Photonic Crystal Fibers (HC-PCFs) represent an innovative technology that alters the cross section of an optical fiber, guiding light via its photonic bandgap rather than through traditional total internal reflection. This method minimizes the overlap between the optical mode and the silica core, significantly reducing non-linear issues encountered in high peak power modulation formats. These issues currently restrict the data rate, range, and modulation options in free-space laser communication links due to the high launch peak powers from optical terminals. Traditional attempts to lessen non-linear impairment constrain optical terminal’s fiber length. Such re-designs often leave critical components, like High Power Optical Amplifiers (HPOA), vulnerable to environmental factors, thereby decreasing system reliability. Moreover, they mandate an individual HPOA for each aperture, thus limiting system flexibility. Nonlinearities also limit the utilization of multiple wavelength channels, a technique that could otherwise improve communication link throughput. In this paper, we propose and investigate a solution to these challenges by replacing the Single-Mode Fiber (SMF) post-HPOA with HC-PCFs. Guiding high peak power light through a hollow-core fiber instead of an SMF mitigates nonlinearities. This decreases the system’s Bit Error Rate (BER) for a given optical power and enhances the overall system reach by 10 dB compared to an SMF system with nonlinear constraints. Additionally, we present an analysis of various commercial HC-PCFs, describe a splicing method along with insertion loss for each type of hollow core fiber, and report on an experiment conducted to quantify the improvement in laser communication links offered by HC-PCFs.
The leap to 100 Gbps data transmission rates has relied on coherent communication technology that use dual-polarization modulation formats. While several complex modulation formats use polarization to increase data rate, it can be an unwanted degree of freedom in free space links that baseline single-polarization modulation formats. In links that are signal-to-noise ratio (SNR) limited; have receivers with limited processing resources; or rely on polarization for duplex through a shared aperture; single polarization links may be preferable. Often times, a system of polarization-maintaining (PM) fibers and PM amplifiers preserve single-polarization signals from degradation as they propagate; however, these systems can be challenging to implement due to tight tolerances on components and PMfiber splices. In this paper we present a method for recovering single-polarization signals from arbitrary polarization received signals using integrated dual-polarization coherent receivers. This removes the reliance on PM fiber components while maintaining single polarization receiver performance. The algorithm uses the received signal on both polarization channels to reconstruct the initial single-polarization coherent waveform. This is accomplished by implementing a polarization rotation and polarizing filter in digital signal processing (DSP). A feature of this method is it combines the signal energy in each of the receiver’s polarization channels while rejecting the noise energy in the polarization that is orthogonal to the signal polarization. This preserves SNR while simplifying subsequent DSP steps by eliminating the unwanted polarization mode. Perhaps most importantly, our algorithm is deterministic and can be added to established DSP processes without requiring significant processing.
The development of space-based, free-space optical (FSO) communication systems is exciting for expanding internet connectivity worldwide. These systems will incorporate dense, low-earth constellations with short intersatellite links. Key to the performance of these satellite constellations are flexible architectures that support higher rates via complex modulation formats, with FSO data links varying between 10-100 Gb/s. However, prior efforts have designed custom modems optimized for each link, severely limiting their flexibility. An alternative is to leverage advances developed by the fiber telecom industry which offer high-rate high-sensitivity digital coherent communication systems while minimizing size, weight, and power (SWAP). These low-SWAP systems rely on commercially available microfabricated integrated coherent receivers (μICRs). Here we present data to help qualify a commercially available μICR for a space application; this data was collected through a series of environmental tests. This work thus expands the reach of coherent systems, allowing for the development of low-SWAP space-based FSO communication systems buttressed with commercially available μICRs.
We achieved the space qualification of the μICR by monitoring the component’s bandwidth and electrooptical (EO) transfer function as environmental testing conditions were varied. We selected these environmental conditions to simulate a low-Earth orbit. The environmental testing included: (i) irradiation using a cobalt-60 source up to a total ionizing dose of 100 kilorads, extending qualification to all of the commercial orbits; (ii) thermal cycling with survival temperatures ranging from -40 °C to 70 °C and operational temperatures varying between -5 °C and 65 °C with the part cycled between its survival temperature range twice and its operational range an additional ten times over a 7-day period; (iii) vibration testing to 28 GRMS for 180 seconds on each axis; (iv) shock to a maximum of 1201 g; and (v) thermal vacuum testing at ∼ 6.3 × 10−6 torr. We observed no degradation in device EO performance after environmental testing.
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