Low Noise, High Saturation Power Semiconductor Optical Amplifiers (SOAs)

OBJECTIVE: To develop semiconductor optical amplifiers (SOA), with performances that match or exceed those of Erbium-doped fiber amplifiers, for potential applications in photonic integrated circuits and photonic systems. DESCRIPTION: The Erbium-doped fiber amplifier (EDFA) [1] has been the workhorse in commercial (e.g., long-haul fiber-optic communication systems) and defense (e.g., high power lasers) applications for more than three decades. The reasons for the success of EDFAs are low noise figure, large gain bandwidth, high saturation power, and low polarization dependence. The EDFA is typically pumped by either a 980 nm or a 1480 nm semiconductor laser diode, which already contains an optical amplifier, namely, the semiconductor optical amplifier (SOA). For the last two decades, the photonics industry has tried, in vain, to displace EDFAs by SOAs to reduce the cost as well as SWaP (size, weight and power). One of the main drawbacks of the SOA, in comparison with the EDFA, is its noise figure. Commercial SOA initially had noise figures above 6 dB due to coupling losses from the SOA waveguide to the optical fibers [2]. Recent efforts in reducing the coupling loss has results in SOA noise figures approaching 5 dB [3]. However, EDFAs can routinely maintain noise figures around 4 dB. The disadvantage in noise figure for SOAs is due to the fundamental noise processes of the semiconductor gain medium, and therefore, requires novel solutions to overcome. Another issue with SOAs is the low saturation power. Compared to the EDFA, the active region of the SOA has a cross-sectional area on the order of micron squared whereas the active region of the EDFA has a cross-sectional area on the order of 100 micron squared. To date, commercial SOAs can offer saturation power around 15 dBm, much lower than EDFAs [4]. The third disadvantage of the SOA is its polarization-dependent gain since, naturally, the quantum-well active region favors the transverse-electric mode in the amplification processes [5]. These three disadvantages, however, have not stopped the photonic industry's pursuits of eventually replacing EDFAs by SOAs. One such recent example is the use of broadband SOAs for digital coherent optical communication, in which one broadband SOA can replace three EDFAs in the S, C, and L band, respectively [6]. Nevertheless, the fundamental disadvantages of the SOA clearly hindered such efforts. For example, the polarization-dependent gain was reduced by using the complicated polarization-diversity configuration, which significantly increase the complexity of the device [6]. The relatively high noise figure also prevented the SOAs from being used for long-haul transmission. If the drawbacks in noise figure, saturation power and polarization sensitivity can be overcome, SOAs can have a tremendous impact on defense and commercial applications. Low-noise, high-power SOAs are extremely desirable in photonic integrated circuits such as RF photonic phased arrays for radars, microwave photonic filters and processors for electronic warfare, as well as high-power direct laser diodes. The purpose of the STTR program is to support the development of novel SOAs that can replace EDFAs at least in certain applications scenarios. PHASE I: To design, fabricate and characterize an SOA with optical gain of up to 20 dB or more with noise figure below 5 dB, saturation power above 20 dBm in the C band. In addition, provide a design that will further reduce the noise figure to below 4.5 dB and polarization-dependent gain to below 2 dB, and increase saturation power to above 27 dBm in the C+L band. Specifically, to formulate novel SOA device structures to reduce the noise figure and increase the bandwidth and saturation power. PHASE II: To design, fabricate and characterize an SOA with optical gain of up to 20 dB or more with noise figure below 4.5 dB, polarization dependence below 2 dB, saturation power above 27 dBm in the C+L band. Demonstrate the application of SOA for analog/digital photonic links and photonic signal processors. In addition, provide a design that will further reduce the noise figure to below 4.0 dB, and increase saturation power to above 30 dBm in the S+C+L band, with low polarization-dependent gain. Production-scale costs of the SOA should be studied to show viability for reasonable cost reduction at manufacturing volumes. Motivation for phase III follow-on investment should be made evident. PHASE III DUAL USE APPLICATIONS: Pursuit system-level defense and commercial applications based upon the SOAs developed in phase II. Clearly identify the advantages of the novel SOAs over EDFAs and other existing SOA structures. The photonic system employing multiple SOAs developed in this program should be integrated at a military installation or on a military platform in potential applications scenarios including but not limited to communications, electronic warfare, sensing and directed energy. Dual-use applications of the SOA in digital coherent fiber-optic communication systems and high-power laser systems are encouraged. REFERENCES: 1. Emmanuel Desurvire, Erbium-Doped Fiber Amplifiers: Principles and Applications, Wiley, New York 1994; 2. Niloy K Dutta and Qiang Wang, Semiconductor Optical Amplifiers (Second Edition), World Scientific, 2013; 3. J. R. Kim et al., "Spot-size converter integrated polarization insensitive semiconductor optical amplifiers," in IEEE Photonics Technology Letters, vol. 11, no. 8, pp. 967-969, Aug. 1999, doi: 10.1109/68.775315; 4. G.P. Agrawal, Fiber-optic Communications Systems, (Wiley, 2010); 5. M. A. Newkirk et al., "1.5 um multiquantum-well semiconductor optical amplifier with tensile and compressively strained wells for polarization-independent gain," in IEEE Photonics Technology Letters, vol. 5, no. 4, pp. 406-408, April 1993, doi: 10.1109/68.212680; 6. Jeremie Renaudier, Amirhossein Ghazisaeidi, "Scaling Capacity Growth of Fiber-Optic Transmission Systems Using 100+nm Ultra-Wideband Semiconductor Optical Amplifiers", Lightwave Technology Journal of, vol. 37, no. 8, pp. 1831-1838, 2019 KEYWORDS: Optical communications, high power laser, Semiconductor Optical Amplifier (SOA), erbium doped fiber amplifier (EDFA), laser diodes