For free-space optical communication links where platform size, weight, and power (SWaP) tend to be at a premium, it is important to design transmitters and receivers that can operate as close as possible to the theoretical best performance. For variable duty-cycle, multi-rate optical communication systems, as demonstrated in the NASA LCRD program, finite extinction ratio can become a significant contributor to transmitter implementation loss when the duty cycle becomes low. The ability to measure waveform extinction ratio of a transmitter with high-fidelity is important for the optimization of these transmitter designs. We present a new technique to measure the extinction ratio of a variable duty-cycle transmitter using an intradyne coherent receiver to capture the full electric field of the transmitter. Average power within a burst of data and within the dead-time of the waveform are separately calculated by gating in the time domain and then by filtering in the frequency domain. Our results show that extinction ratios as high as 45 dB can be accurately measured using this technique. We discuss how to choose the optimal bandwidth for integrating power in the frequency domain. Finally, we show the effects of signal-to-noise ratio on the fidelity of the extinction ratio measurement.
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.
For some free-space laser communications (lasercom) links impacted by atmospheric-induced scintillation, the principle of reciprocity provides knowledge of a remote terminal's received intensity fluctuations based on measurements of the local terminal's received intensity fluctuations. We evaluate a reciprocity-enhanced technique using optical switching between multiple spatially-diverse transmit apertures to mitigate against atmospheric-induced scintillation. Experimental bit-error rate measurements are presented to quantify the expected performance of this approach in comparison to standard static diversity approaches. We investigate system constraints of this reciprocity-enhanced transmitter diversity approach, and identify candidate optical switches and digital logic for implementation.
We report on a 1550-nm matched filter based on a pair of fiber Bragg gratings (FBGs) that is actively stabilized
over temperature. The filter is constructed of a cascaded pair of athermally-packaged FBGs. The tandem FBG
pair produces an aggregate 3-dB bandwidth of 3.9-GHz that is closely matched to a return-to-zero, 2.880-GHz
differential-phase-shift-keyed optical waveform.
The FBGs comprising the filter are controlled in wavelength using a custom-designed, pulse-width modulation
(PWM) heater controller. The controllers allow tuning of the FBGs over temperature to compensate and cancel
out native temperature dependence of the athermal FBG (AFBG) package. Two heaters are bonded to each
FBG device, one on each end. One heater is a static offset that biases the FBG wavelength positively. The second
heater is a PWM controller that actively moves the FBG wavelength negatively. A temperature sensor measures
the FBGs' temperature, and a feed-forward control loop adjusts the PWM signal to hold the wavelength within
a desired range.
This stabilization technique reduces the device's native temperature dependence from approximately 0.65
pm/°C to 0.06 pm/°C, improving the temperature stability by tenfold, while retaining some control for poten-
tial long-term drifts. The technique demonstrates that the FBGs can be held to ±1.5 pm (±188 MHz) of the
target wavelength over a 0 to +50°C temperature range. The temperature-stabilized FBGs are integrated into
a low-noise, optical pre-amplifier that operates over a wide temperature range for a laser communication system.
We report a single-polarization, optical low-noise pre-amplfier (SP-OLNA) that enhances the receiver sensitivity of heavily-coded 1.55-μm optical communication links. At channel bit-error ratios of approximately 10%, the erbium-doped SP-OLNA provides an approximately 1.0-dB receiver sensitivity enhancement over a conventional two-polarization pre-amplfier.
The SP-OLNA includes three gain stages, each followed by narrow-band athermal fiber Bragg gratings. This cascaded fiter is matched to a return-to-zero, 2.88-Gb/s, variable burst-mode, differential phase shift keying (DPSK) waveform. The SP-OLNA enhancement of approximately 1.0 dB is demonstrated over a range of data rates, from the full 2.88-Gb/s (non-burst) data rate, down to a 1/40th burst rate (72 Mb/s).
The SP-OLNA'sfirst stage of ampli_cation is a single-polarization gain block constructed from polarization-maintaining (PM) fiber components, PM erbium gain fiber, and a PM integrated pump coupler and polarizer. This first stage sets the SP-OLNA's noise figure, measured at 3.4 dB. Two subsequent non-PM gain stages allow the SP-OLNA to provide an overall gain of 78 dB to drive a DPSK demodulator receiver. This receiver is comprised of a delay-line interferometer and balanced photo-receiver. The SP-OLNA is packaged into a compact, 5"x7"x1.6" volume, which includes an electronic digital interface to control and monitor pump lasers, optical switches, and power monitors.
Mobile free-space laser communication systems must reconcile the requirements of low size, weight, and power with the ability to both survive and operate in harsh thermal and mechanical environments. In order to minimize the aperture size and amplifier power requirements of such systems, communication links must exhibit performance near theoretical limits. Such performance requires laser transmitters and receiver filters and interferometers to maintain frequency accuracy to within a couple hundred MHz of the design frequency. We demonstrate an approach to achieving high frequency stability over wide temperature ranges by using conventional DFB lasers, tuned with TEC and current settings, referenced to an HCN molecular frequency standard. A HCN cell absorption line is scanned across the TEC set-point to adjust the DFB laser frequency. Once the center of the line is determined, the TEC set-point is offset as required to obtain frequency agility. To obtain large frequency offsets from an HCN absorption line, as well as continuous laser source operation, a second laser is offset from the reference laser and the resulting beat tone is detected in a photoreceiver and set to the desired offset using a digital frequency-locked loop. Using this arrangement we have demonstrated frequency accuracy and stability of better than 8 MHz RMS over an operational temperature range of 0ºC to 50º C, with operation within minutes following 8 hour soaks at -40º C and 70º C.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.