A high-sensitivity modem and high-dynamic range optical automatic gain controller (OAGC) have been developed to provide maximum link margin and to overcome the dynamic nature of free-space optical links. A sensitivity of −48.9 dBm (10 photons per bit) at 10 Gbps was achieved employing a return-to-zero differential phase shift keying based modem and a commercial Reed-Solomon forward error correction system. Low-noise optical gain was provided by an OAGC with a noise figure of 4.1 dB (including system required input loses) and a dynamic range of greater than 60 dB.
©2011 Optical Society of America
Airborne free-space optical communications (FSOC) links provide an appealing and complementary enhancement to current radio frequency (RF) systems because of their inherent benefits of high-bandwidth and directional communication. Although high-data rate FSOC systems are not feasible through clouds or thick fog due to pulse broadening and direct beam attenuation , employing them in a hybrid RF/optical link configuration can yield a system that can operate under most weather conditions and provide high-bandwidth, secure, jam-proof communications during optimal conditions [2,3].
For the FSOC link, beyond attenuation effects and line-of-sight limitations, performance is primarily driven by optical turbulence along the beam path [4,5]. This results in intensity fluctuations at the receive terminal where large power swings can occur in millisecond scales. The impact on channel performance is especially severe when faded below receiver sensitivity as intolerably large bit error rates are encountered. To mitigate channel fading and increase the probability of error-free communications, various terminal and modem architectures have previously been developed  with current efforts including adaptive optics (AO) terminals with single mode receivers [2,3,5], tip-tilt terminals coherent receivers [6–8] or with multimode receivers [9,10], and multi-aperture terminals with tip-tilt control and single-mode receivers [11,12].
These approaches have had varying degrees of success, largely driven by the amount of turbulence they operate over with satellite links encountering the most benign conditions and air-to-ground links the most challenging due to turbulence near the ground . For example, Smutny et al.  reported on an inter-satellite link utilizing tip-tilt terminals with coherent receivers operating at 5.6 Gbps across nearly 5000 km with a bit error rate (BER) below 10−9. However, when that same system was tested in the Canary Islands over a 142-km link with moderate turbulence (ro typically at about 5-6.5 cm) as characterized by the Fried parameter , 40-dB intensity fluctuations were reported as typical  and BER measurements varied in bursts between 10−4 to 10−6 as noted in Fig. 9 of . It must also be noted from that figure that a large percentage of the time the BER = 1 due to complete loss of data from link fades below receiver sensitivity. For a tip-tilt system with multimode receivers, Fletcher et al.  and Cunningham et al.  present 30-40 dB receive power in fiber (PIF) variations at the terminal output for a range of link distances (16-132 km) and configurations (air-to-air, air-to-ground, and ground-to-ground). Examining Fig. 7 in , PIF from a 132-km, air-to-air link is presented with about a 35-dB spread (ranging from −56 to −21 dBm). With the reported error-free sensitivity of −34 dBm for the 2.5 Gbps link, this result implies that approximately 60% of the PIF distribution is below sensitivity and thus leads to complete data loss during those fades. While atmospheric turbulence data was not available, it is assumed that all links were in the saturated regime given the consistency of the PIF distributions. For a multi-aperture system with tip-tilt control and single-mode receivers, Walther et al.  present PIF data that has about 50-dB fluctuations on a per channel basis for a 50-km, air-to-ground link. The system was estimated to have operated under conditions at 1.1x the commonly used Huffnagel-Valley (H-V) 5/7 atmospheric turbulence model, which produces a ground ro of 5 cm . In terms of data throughput performance, leveraging spatial diversity they report error-free data delivery using a 2.7-Gbps link at ranges of up to 30 km. At ranges beyond that, data throughput suffered with 10% loss reported at ranges of 56-59 km during evening runs under moderate turbulence estimated at 0.7x the H-V 5/7 model.
While a complete analysis of the merits and trade-offs of different FSOC terminal architectures is beyond the scope of this manuscript, from the modem perspective, the driving requirements are error-free dynamic range and receiver sensitivity. As just presented, regardless of the terminal beam control and fade mitigation approach, every modem architecture must have a high-dynamic range to deal with the large intensity swings experienced under strong (saturated) turbulence conditions. Additionally, a robust FSOC modem must maximize the link margin by having as low a receiver sensitivity as possible to minimize the probability of link outages from intensity fades and to extend the range of operation. In this regard, coherent receiver architectures can achieve the best theoretical sensitivities ; however, for 10-Gbps class receivers, optically pre-amplified, single-mode receiver architectures are the most practical as components are widely available and near-quantum limited sensitivities have been achieved. Uncoded sensitivities of 43 photons/bit (ppb) at 5 Gbps (−45.6 dBm) using an On-Off-Keyed (OOK) receiver , 52 ppb at 10.7 Gbps (−41.5 dBm) using a differential phase shift keying (DPSK) receiver , and 39 ppb at 42.7 Gbps (−36.9 dBm) using a return-to-zero DPSK (RZ-DPSK) receiver  have been demonstrated for fiber based systems with BERs = 10−9. For multimode systems, the best candidate receivers are avalanched photodiodes (APDs) typically having uncoded sensitivities of −35 dBm at 2.5 Gbps or −29 dBm at 10 Gbps (~1240 ppb) [17,18]. A significant drawback is that APDs are more susceptible to damage from high-receive powers than PIN-based receivers and they suffer from limited error-free dynamic ranges with the best reported at under 30 dB [19–21]. For long-range FSOC links, PIN-based receivers (without optical amplification) are generally not used due to their poor sensitivity.
In the approach detailed here, as part of the Defense Advanced Research Projects Agency (DARPA) Optical RF Communications Adjunct (ORCA) program, single-mode modems were developed to leverage on the technology base of the telecommunications industry, achieve high data rates of 10 Gbps and beyond with high-sensitivity, and operate in the 1550 nm band for reduced eye-safety concerns [2,3]. The single-mode optical modems interface to an AO terminal providing 35-Zernike components of compensation for improved turbulence compensation from the transmitting terminal and improved coupling into the output fiber from the receive terminal. Even with AO compensation, the PIF dynamics for this class of links can be very dynamic in nature. As an example, typical PIF data is presented in Fig. 1 illustrating about 40 dB swings that the modems must compensate. This data was taken at the ground receiver of an air-to-ground, 183-km link during the ORCA program testing in May 2009 under strong turbulence conditions estimated as 5x the H-V 5/7 model [2,22]. Figure 1 also presents the output (POF, power-out-of-fiber) from an OAGC system that optically compensates for the dynamics in the received signal (PIF) to provide a stabilized output for receiver protection while also providing low-noise amplification [1,2,4] for improved receiver sensitivity.
This paper details the development of a high-sensitivity modem and a high-dynamic range OAGC in support of the ORCA program. These systems are designed provide maximum link margin and overcome the dynamic nature of the targeted FSOC links to maximize the range of operation (i.e. link distances and configurations) where error-free communications can be established. The intent is to support the ORCA system with a sensitivity of better than −47 dBm so as to provide error-free communications up to the 85th percentile of turbulence estimated at 5x the H-V 5/7 model .
2. System description
2.1 Optical automatic gain controllers
The first function of the OAGC system is protection of the photodiodes and follow-on electronics from catastrophic damage caused by high receive optical powers at the output of the FSOC terminal. This condition can occur when closing short distance links or even on longer (>100 km) links during benign turbulence . Additionally, architectures employing fixed gain optical pre-amplifiers, such as erbium-doped fiber amplifiers (EDFAs), that can output power levels well above the damage thresholds of detectors in response to rapid power transients in a “Q-switch” effect  are especially susceptible. To prevent this, the OAGC optically amplifies or attenuates as necessary through a series of multiple gain stages as presented in Fig. 2 and discussed in  to output a constant power at a level of optimal performance for the detector. In essence, the time-variant optical input [I(t)] is translated into a constant amplitude output with a variable optical signal-to-noise ratio [OSNR(t)]. A sample data set demonstrating the OAGC maintaining its output (POF) constant while the input (PIF) varied during field operation is also presented in Fig. 1 . Note that this data used a first generation OAGC developed with JHU/APL Internal Research and Development funds.
The second function of the OAGC is to provide low-noise optical amplification for improved receiver sensitivity. This aspect of performance is characterized by the noise figure (NF) and gain metrics. Although the theoretical NF limit for an optical amplifier is 3 dB without input losses , for practical systems that include taps to measure the input signal, isolators to remove backward-going signals at the input, and filters to discriminate between the receive and transmit wavelengths, 4.0-4.5 dB is a practical limit. Because the system NF is set by the first amplifier [13,23], this stage is designed for low NF and only has a moderate net gain of about 18-22 dB. Additionally, it is designed to gain clamp at input levels above −25 dBm to aid in maintaining the desired system output. The second stage incorporates a variable optical attenuator (VOA) with a response time of less than 4 µs to dynamically respond to input power fluctuations by adjusting the system gain. The third stage handles output regulation and is designed to handle higher output power with an adjustable gain of 20-30 dB. For the overall system gain metric, the effective OAGC gain is dependent on the pre-set target output level. For nominal target levels of −5 dBm and sensitivities with forward error correction (FEC) of better than −48 dBm at 10 Gbps, the OAGC’s net gain needs to exceed 40 dB.
The third critical function of the OAGC is to reduce bit errors arising from power fluctuations at the receiver that cause timing jitter in the digital eye. By maintaining a constant output, power transients are prevented from coupling through the follow-on electronics and degrading bit error rate (BER) performance in the FSOC link .
2.2 Optical modem
The ORCA optical modem interfaces between a network router’s short-reach transceiver and the free-space link via the FSOC terminal [2,3]. For outgoing data over the link, the incoming client data is recovered and encoded with a commercial off-the-shelf (COTS) AMCC Reed-Solomon (RS) FEC running a proprietary enhanced FEC (EFEC) that adds a 7% data rate overhead and provides more than 8 dB of gain  (Note: FEC gain is referenced to the BER = 10−12 point).
This digital signal is then encoded onto the appropriate C band (1530 to 1560 nm) channel with the chosen modulation format. For this effort, a return-to-zero differential phase shift keying (RZ-DPSK) waveform was implemented for the FSOC side transceiver. The RZ-DPSK transmitter architecture is presented in Fig. 3(a) . This approach is based on introducing phase changes (0 or pi) in the optical signal between adjacent bits and is of interest because of the improved receiver sensitivity that can be achieved (~3 dB) over OOK systems when balanced detection is employed . The RZ aspect of the format is used to improve receiver sensitivity by an additional 1 to 2 dB  by increasing the bit peak power through the average power limited launch EDFA  and to help with clock recovery since Non-Return-to-Zero formats have weak clock tones. After modulation, the signal is routed to a high-power EDFA and then to the FSOC terminal for transmission over the free-space channel.
On the receive side of the FSOC link, the dynamic signal is optically conditioned by the OAGC as previously described and filtered with a COTS 100-GHz wavelength division multiplexing (WDM) filter before being directed to the receiver. The DPSK receiver architecture prototyped in this effort is illustrated Fig. 3(b). A one bit delay Mach-Zehnder interferometer optically demodulates the phase encoded data through a bit differential approach, where the previous bit acts as a phase reference to the current bit for determining phase changes . The output is intensity modulated signals that are detected by a balanced receiver and conditioned by the integrated transimpedance amplifier (TIA) and limiting amplifier (LA) to output a clean digital signal.
3. System performance
The combination of a high-sensitivity transceiver and the low-noise OAGC results in a significantly improved system sensitivity that translates into additional link budget margin for the FSOC link as compared to previous related efforts, mainly the Integrated RF/Optical Networked Tactical Targeting Networking Technologies (IRON-T2) and the ORCA Phase I flight tests [2,3,5].
The characterization setup for the OAGC and long-haul DPSK transceiver is presented in Fig. 4 . Pseudo-random data from a pattern generator at 9.953 Gbps was encoded with the RS EFEC (255,239) to 10.7 Gbps and then transferred onto the optical carrier using the RZ-DPSK transmitter. FSOC channel characteristics were emulated with a link simulator consisting of a booster EDFA operating in saturation and a fast VOA that can be driven by a personal computer (PC), a signal generator, or manually. The simulator output served as the OAGC input (also referred to as PIF).
The OAGC normalized residual PIF fluctuations by optically amplifying or attenuating as necessary to provide a fixed output signal of −7 dBm after the WDM drop filter (50/100 GHz). The OAGC output (POF), was directed to the DPSK receiver to extract the digital data from the phase encoded optical signal. The data was then decoded by the FEC system, clock and data were recovered, and data transfer performance was finally tested by a BER tester.
The second-generation OAGC described here employs a digital controller with a response time of about 3 µs utilizing POF as the control signal in a feedback architecture. The system has a NF of 4.1 dB and is built in a Versa Module Eurocard (VME) form-factor to support future flight tests. The system NF is above the 3 dB quantum limit because of input losses from an input connector (0.07 dB), PIF tap (0.4 dB), input isolator (0.4 dB), and two fiber splices (0.2 dB total estimate).
The ORCA modem sensitivity metric is defined as the average optical power at the front-panel input of the OAGC where the BER = 10−9. The BER performance for the RZ-DPSK/OAGC system, with and without FEC, is presented in Fig. 5 along with the sensitivity of the fielded ORCA Phase I system for comparison. The prototype system achieves a sensitivity of −47.5 dBm, which is a greater than 7-dB improvement over the Phase I system. Note that the RZ-DPSK BER baselines were performed with the just described second-generation OAGC, whereas the ORCA Phase I baselines are with a first-generation, proof-of-concept OAGC and modem developed for the Air Force Research Laboratory’s Iron-T2 program [2,3,5]. The 7-dB improvement results from (1) the waveform change from OOK to DPSK of about 3 dB, (2) a reduced NF between the first and second generation OAGC of about 2 dB, and (3) the addition of RZ pulse shaping of about 2 dB.
While these sensitivity results include required input losses to the system, for comparison to the literature, the input losses ahead of the first optical amplifier within the OAGC must be excluded as is done in [14–16,23,26,27]. Using this approach, the sensitivity of the ORCA OAGC/modem prototype is −48.6 dBm (10.8 photons/bit). Tests of a straightforward upgrade using a COTS 50-GHz WDM filter instead of the 100-GHz filter improved sensitivity by 0.3 dB to give a sensitivity of −48.9 dBm (10.0 ppb). Adding single-polarization discrimination (to further reduce amplified spontaneous emission noise) has been shown to provide additional sensitivity gains of about 0.4 dB . Although this would result in a projected sensitivity of −49.3 dBm (9.2 ppb) for this system, the benefit comes at the cost of the additional complexity of tracking polarization in a practical FSOC link between mobile platforms; therefore, for this effort, polarization discrimination was not investigated.
In addition to providing low noise amplification for improved sensitivity, the new OAGC system maintains the target output level to within < 0.3 dB for front-panel inputs of −48 to + 13 dBm. In combination with the DPSK transceiver and FEC system, this provides the ORCA link an error-free dynamic range of greater than 60 dB to handle the dynamic nature of FSOC links.
To characterize the dynamic response of the OAGC, a normalized disturbance rejection (NDR) figure of merit was developed. The NDR is a measure of the ability of the system to maintain the output target level (POF) during variations at the input (PIF) as a function of frequency. The setup illustrated in Fig. 4 is used to characterize the NDR with a signal generator driving the VOA with a sinusoidal waveform. This results in the OAGC input receiving a sinusoidal swing on a dBm scale. The measured POF is then normalized to the input amplitude to produce the NDR as defined byFig. 6 . As an example, a disturbance 500 Hz is normalized down 0.8 dB. Considering that the characteristic frequency of the atmosphere, also known as the Greenwood frequency [4,28], is usually in the tens to hundreds of hertz, the results presented here show that the OAGC has more than sufficient bandwidth and dynamic range to remove the residual PIF variations that are observed FSOC links.
As part of DARPA’s ORCA program, the authors prototyped a high-sensitivity RZ-DPSK modem and high-dynamic range OAGC system that improves on the performance of previously fielded systems to provide maximum link margin and overcome the dynamic nature of FSOC links. A sensitivity of −48.9 dBm (10 ppb) at 10 Gbps was demonstrated in combination with a COTS RS EFEC. Low-noise optical gain was provided by an OAGC with an NF of 4.1 dB (including system required input loses) to provide an error-free dynamic range of greater than 60 dB. This results in an FSOC modem solution that is compatible with previously fielded FSOC systems [2,3,5] with a sensitivity among the best reported for a coded system [15,16,27] and with unprecedented dynamic range for a DPSK/FEC receiver implementation.
This work was supported in part by the U.S. Department of Defense (DoD) under Contract FA8650-08-C-7848. The views, opinions, and/or findings contained in this article/presentation are those of the author/presenter and should not be interpreted as representing the official views or policies, either expressed or implied, of DARPA or DoD. Distribution Statement A, Approved for Public Release, Distribution Unlimited.
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