Robust High‑Power Single‑Mode 1550 nm Semiconductor Optical Amplifier for Free‑Space Optical Communications（1）
Abstract
Watt-level semiconductor optical amplifiers (SOAs) at 1550 nm are ideal alternatives to erbium-doped fiber amplifiers (EDFAs) for free-space optical (FSO) communications and various other applications. They offer higher efficiency, a more compact form factor, and lower cost while providing high-power diffraction-limited output. We demonstrate a single-mode fiber-coupled packaged SOA that delivers over 30 dBm (1.2 W) continuous-wave fiber output power at 1550 nm with a total gain of 16 dB, enabled by recent advances in tapered semiconductor amplifiers for diffraction-limited output. Preliminary communication measurements are presented, showing open eye diagrams at output powers above 1 W for 10 Gbps differential phase-shift keying (DPSK) modulation formats. Watt-level collimated and fiber-coupled SOAs are now in volume production and available to customers.
Keywords: high-power diode lasers; high beam quality; high-brightness light sources; free-space optical communication; 1550 nm; semiconductor optical amplifiers
1. Introduction
Point-to-point free-space optical links enable long-distance data communication between line-of-sight transceivers. Free-space optical (FSO) communication is of particular interest in aerospace and military applications, facilitating high-speed communication between spacecraft, aircraft, and ground transceivers over potentially extreme distances. These systems demand high-power optical amplifiers and high-brightness output.
Semiconductor diode lasers are widely used in high-power laser systems due to their ability to generate high output power with favorable electro-optic conversion efficiency. However, conventional high-power edge-emitting array lasers suffer from low brightness (low power per unit angle), meaning energy emitted from large-area emitters cannot be focused into small spots on distant targets. This limits the practical utility of high-power lasers.
Consequently, diode lasers are often used to pump other optically active gain media, such as fiber lasers or solid-state lasers. These lasers operate in a single spatial mode to deliver the required high-brightness output. In such systems, fiber lasers or solid-state lasers can be regarded as brightness converters, transforming high-efficiency diode pump beams into high-brightness beams. Nevertheless, this conversion reduces overall system efficiency, as both the electro-optic conversion efficiency of diode lasers and the optical-to-optical conversion efficiency of gain media pumped by diode lasers must be considered. Furthermore, these cascaded laser systems typically involve numerous components, each requiring thermal management, further degrading efficiency.
Driven by technological advancements, there is persistent motivation to advance systems toward lower cost, smaller size, lighter weight, and lower power consumption (c-SWAP). Improvements in c-SWAP benefit users as long as performance is not compromised.
An attractive alternative system architecture compared with conventional approaches is to eliminate brightness conversion components and use diodes directly in end applications. Such direct-diode solutions can be more compact, efficient, and cost-effective. However, realizing this approach requires progress in the brightness of high-power, high-brightness edge-emitting diodes and amplifiers.
Here, we report devices operating in the 15xx nm wavelength band with brightness exceeding previously reported performance [2]. These devices offer substantial advantages and enable viable direct-diode systems.
2. Operating Principle of Tapered Semiconductor Amplifiers
The semiconductor optical amplifiers presented herein are based on a tapered chip architecture, which enables high output power with near-diffraction-limited beam quality. The operating principle of the amplifier is described below.
The chip consists of two sections: a pre-amplifier (pre-amp) section and a tapered power amplifier (PA) section, as shown in Figure 1. Seed light is coupled into the pre-amplifier, which features a ridge waveguide structure with a single-spatial-mode geometry that acts as a spatial mode filter. Light output from this section is then injected into the power amplifier, an unetched mode expansion region where the fundamental Gaussian beam can freely expand via diffraction. A flared electrode for current injection is patterned in this region, with an angle matching the natural diffraction angle of the injected beam. This section efficiently amplifies the signal from the pre-amplifier.
As the beam expands freely, the cross-sectional area of the optical mode increases with position. This design serves two critical purposes: first, it keeps the peak intensity far lower than in conventional high-power single-mode ridge waveguide amplifiers, ensuring excellent reliability; second, it significantly reduces the thermal resistance of the device, enabling higher power extraction than traditional single-mode devices.
Although devices based on tapered or flared amplifier geometries have been fabricated and even commercialized over the past decades, they have long suffered from beam quality degradation at high operating currents [3–6]. We have identified the root cause of this beam quality degradation. By addressing this fundamental issue, we have eliminated this effect, improved device performance, and achieved high single-mode power output, representing a major breakthrough.
Beam quality degradation is caused by the mismatch between the optical mode profile and the current injection profile. The transverse cross-section of the optical mode propagating in the power amplifier region is approximately Gaussian, while the transverse cross-section of current injection usually exhibits a top-hat distribution. Consequently, carriers in the overlapping region are converted into photons more efficiently through radiative recombination than those in the periphery of the optical mode, resulting in higher carrier density and stronger gain saturation at the beam center. This gives rise to transverse non-uniformity of carrier density and induces a plasma lens effect in the optical mode, meaning that the refractive index of the optical mode shows continuous-wave (CW) or quasi-CW dependence on carrier density.
In addition, thermal gradients exist inside the device, with the central temperature typically higher than that at the edges, forming the well-known thermal lens effect [7–9]. The combined effects of the carrier lens and thermal lens degrade the beam quality of tapered lasers and amplifiers. Any increase in optical power in lateral modes beyond the fundamental mode will cause the output beam to deviate from the diffraction limit. As the power in these lateral modes rises under higher drive currents, the beam quality of tapered diode lasers and amplifiers degrades further.
We have adopted targeted engineering design and optimized the current injection profile to better match the injection profile with the fundamental optical mode, thereby improving the single-mode output power of tapered semiconductor optical amplifiers.
3. Results
Semiconductor optical amplifier (SOA) devices were fabricated on indium phosphide (InP) epitaxial wafers using standard nanofabrication processes, including contact metal deposition, dielectric deposition, reactive ion dry etching, substrate thinning, and metallization. After wafer fabrication, the wafer bars were cleaved, and anti-reflection (AR) dielectric coatings were deposited on both the front and rear facets. Individual SOA devices were assembled chip-on-submount with hard solder on expansion-matched submounts and integrated into a compact dual-port fiber-coupled package (see Figure 2). Lenses with customized optical elements were actively aligned and fixed inside the package at the input and output sides of the SOA chip. Both input and output fibers are polarization-maintaining (PM) single-mode fibers (FC/APC connectors, core diameter 8 μm).
3.1 Amplifier Performance Characterization under CW Seed Light
The packaged semiconductor optical amplifier (SOA) was characterized in continuous-wave (CW) mode at a room temperature of 20 °C, using a stable narrow-linewidth distributed feedback (DFB) laser as the seed source. The results are presented in Figure 2. With the seed power fixed at 50 mW, the output power and voltage of the SOA were plotted as functions of the SOA drive current. An output power exceeding 1.2 W was achieved at an SOA current of 7.5 A, corresponding to an electro-optic conversion efficiency of 14%. During the measurement, the collimated beam power was recorded as 1.8 W, yielding a fiber coupling efficiency of 65%. The operating point is indicated in the inset, which is a computer-aided design (CAD) rendering of the dual-port fiber-coupled package. The optical spectra of the DFB seed and the packaged SOA output are also plotted.
At an input power of approximately 40 mW and a fixed SOA current of 7.5 A, the output power, gain (in dB), and total gain of the packaged SOA were measured versus seed power. The packaged SOA provided a gain of 15 dB at an output power of 1.2 W. These results are expected to disrupt the 1550 nm market by enabling compact and efficient fiber-coupled SOA products.
