Robust High‑Power Single‑Mode 1550 nm Semiconductor Optical Amplifier for Free‑Space Optical Communications（2）
3.2 Amplifier Characterization for Data Communications
To evaluate the performance of semiconductor optical amplifiers (SOAs) in high-speed data communication applications with amplitude-modulated (AM) input signals, the setup illustrated in Figure 3 was adopted. A pulse pattern generator (PPG) generated a pseudo-random binary sequence (PRBS) with a pattern length of using non-return-to-zero on-off keying (NRZ, OOK). This sequence drove a modulator equipped with an integrated radio frequency (RF) driver and modulated a stable distributed feedback (DFB) laser source. The signal was modulated at a rate of 2.5 Gbps, and the seed light was amplified by a two-stage commercial off-the-shelf (COTS) erbium-doped fiber amplifier (EDFA) for benchmarking against the packaged SOA in the various experimental configurations described below.
The output of the amplifier was attenuated, converted into an electrical signal by an integrated photodetector, and analyzed using a digital communication analyzer (DCA). Eye diagrams were displayed by triggering on the clock output from a digital oscilloscope.
The left side of Figure 4 shows the directly measured eye diagram of the 2.5 Gbps electrical bitstream output from the pulse pattern generator (PPG). The right side presents the eye diagram after single-stage amplification by an erbium-doped fiber amplifier (EDFA), with average input and output powers of 10 mW and 400 mW, respectively. The eye diagram at the EDFA output is open, with distinct high and low levels and clear bit transitions, which is characteristic of amplitude-modulated (AM) data communication outputs using EDFAs.
An optical spectral filter was inserted after the EDFA, and the packaged semiconductor optical amplifier (SOA) was employed as the second-stage amplifier. The average output power of the EDFA was reduced so that 35 mW of average input power was coupled into the SOA. At a drive current of 5 A, the SOA delivered an output power of approximately 550 mW. The eye diagram for this setup is shown on the left side of Figure 5, where significant power overshoot can be clearly observed during bit rising-edge transitions. This overshoot is attributed to pattern dependence induced by carrier lifetime, an effect that occurs when the SOA gain exceeds the saturation level of the amplifier.
On the right side of Figure 5, the SOA was used as a single-stage amplifier with an average input seed power of 5 mW and an average output power of 300 mW, where pattern effects remain evident. Beyond these preliminary measurements, further research is needed to confirm the practicality of the packaged SOA in data communication applications.
Differential phase-shift keying (DPSK) is another common data communication format, in which the optical path length is modulated at high speed without altering the amplitude of the data signal; instead, data bits are encoded in the relative phase of the output signal.
In the experimental setup shown in Figure 6 below, the amplitude modulator was replaced by a lithium niobate (LiNbO₃) modulator driven by the pulse pattern generator (PPG). In addition, a 10 Gbps asymmetric Mach–Zehnder interferometer (AMZI) was placed after the attenuation stage. The AMZI operates by interfering each bit with the preceding one, thereby converting 10 Gbps phase data into amplitude-modulated data. This results in constructive interference (for 0–1 or 1–0 bit transitions, corresponding to a logic 1 output) or destructive interference (for 0–0 or 1–1 bit transitions, corresponding to a logic 0 output).
The amplitude-modulated output signal from the AMZI was then measured by a digital communication analyzer (DCA) and displayed as an eye diagram.
The left panel of Figure 7 shows the direct measurement of the 10 Gbps electrical eye diagram at the output of the pulse pattern generator (PPG). Bit transitions appear relatively rounded because the data rate is close to the upper limit of the PPG’s capability.
The eye diagram after single-stage amplification of the 10 Gbps output from the asymmetric Mach–Zehnder interferometer (AMZI) by a commercial off-the-shelf erbium-doped fiber amplifier (COTS EDFA) is shown on the right side of Figure 7. Bit transitions are shifted here, and the absence of consecutive 0-level bits in the second arm of the AMZI (not shown) results in an inverted mirror image of the eye diagram. Balanced photodetectors can be used to improve the eye diagram under this experimental setup.
Next, an optical spectral filter was added at the output of the erbium-doped fiber amplifier (EDFA), followed by the packaged semiconductor optical amplifier (SOA) for two-stage amplification. The resulting eye diagram is shown on the left side of Figure 8. In this configuration, the total input power to the EDFA was 6.3 mW, and the SOA delivered an output power of 550 mW at a drive current of 5 A. The eye diagram is qualitatively similar to that obtained with the EDFA alone (right side of Figure 7), but shows a clearly open eye and significant pattern-dependent power overshoot.
When the SOA drive current was increased to 7 A, the output power exceeded 1 W (right side of Figure 8). Some additional noise appeared in the high-level bits, but the eye diagram remained clearly distinguishable. These preliminary results demonstrate that tapered SOAs exhibit strong potential for high-speed data communications at watt-level power using the differential phase-shift keying (DPSK) format, and are promising candidates to replace one or more stages in cascaded fiber amplifiers.
4. Conclusions
Free-space optical (FSO) communication systems equipped with semiconductor optical amplifiers (SOAs) are expected to be more lightweight, compact, and feature higher system efficiency, lower complexity, and reduced cost compared to systems relying solely on fiber-based signal amplification. Our recent advances in high-brightness semiconductor optical amplifiers have enabled direct-diode solutions that were previously unfeasible. Preliminary measurement results demonstrate the application potential of watt-level differential phase-shift keying (DPSK) signals at 10 Gbps using 1550 nm tapered SOAs. In particular, smaller and higher-power FSO transceivers can bring significant advantages to spaceborne and airborne FSO communication systems.
