High-Speed High-Power Free-Space Optical Communication: Direct Modulation of Watt-Level Photonic Crystal Surface-Emitting Lasers

Background Introduction

As a vital light source for optical communication, semiconductor lasers are widely used due to their small size, low cost, long lifespan, and high efficiency. For example, vertical-cavity surface-emitting lasers (VCSELs) are ideal for short-distance optical interconnects in data centers due to their low power consumption and wideband direct modulation capability. Distributed feedback (DFB) lasers, on the other hand, are widely used in long-distance optical fiber communication due to their single-mode operation. Recently, free-space optical communication (FSO) using semiconductor lasers has garnered significant attention as it can achieve high-speed transmission over long distances without the need for optical fibers. FSO technology holds potential applications in backhaul and fronthaul networks beyond 5G and future 6G communications, satellite communications, and deep-space communications. In these applications, high power and narrow beamwidth of lasers are critically important. However, traditional semiconductor lasers like VCSELs and DFB lasers cannot meet the requirements for high power and high-speed operation on a single chip simultaneously.

The solution based on photonic crystal surface-emitting lasers (PCSELs) can address the above issues. PCSELs can achieve large-area single-mode operation by utilizing resonance points in the two-dimensional photonic crystal, thus simultaneously achieving high output power and narrow beamwidth. The high power and narrow beamwidth of such lasers make them highly promising for free-space optical communication, eliminating the need for complex fiber amplifiers and optical lens systems typically required by conventional light emitters.

Paper Source

This research was conducted by Ryohei Morita and colleagues from Kyoto University, Tohoku University, and the KDDI Research Institute, among other institutions. The study was published in the July 2024 issue of the journal Optica.

Research Details

Research Process

1. Design and Calculation of Frequency Response:

   • Device Design: Initially, the research team designed a PCSEL capable of realizing both high power and high-speed direct modulation. By calculating the intrinsic (optical) and parasitic (electrical) frequency responses of the watt-level PCSEL, the team demonstrated the feasibility of several GHz-level direct modulation in large-area laser regions.
   • Numerical Calculation: The team used three-dimensional coupled-wave theory and considered carrier and thermal effects to calculate the input current, output optical power, and corresponding frequency response characteristics.

2. Device Fabrication and Experimental Verification:

   • Device Fabrication: Based on the calculations, the team fabricated a PCSEL with a 500 µm diameter, achieving watt-level continuous wave (CW) operation and several GHz-level direct modulation.
   • Experimental Verification: The frequency response characteristics were experimentally verified, and free-space transmission experiments were conducted, achieving more than 10 Gbps high-speed transmission using the PCSEL without requiring transmitting lenses, simulating virtual transmission over a distance of five kilometers.

3. Frequency Response Details:

   • Electrical Characteristics: The frequency response of the PCSEL includes intrinsic and parasitic electrical characteristics. The intrinsic frequency response is determined by the relaxation oscillation characteristics of the laser, whereas the parasitic frequency response is determined by the RC circuit characteristics formed by wires and solder joints. Numerical calculations were made for both parts of the frequency response to optimize the design for high-frequency modulation.

Main Experimental Results

1. Achievement of Watt-Level Power and Frequency Response:

   • In experiments, the 500 µm diameter PCSEL achieved watt-level optical power output with 3 A current injection, along with a modulation bandwidth of over 3 GHz.
   • The far-field beam pattern tests showed that the PCSEL exhibited narrow beamwidth and single-mode operation characteristics, with minimal beam divergence, indicating excellent optical performance in free-space transmission.

2. High-Speed Optical Communication Experiments:

   • In free-space transmission experiments, the research team achieved high-speed transmission exceeding 10 Gbps with the directly modulated watt-level PCSEL, and modulated 64QAM signals for virtual long-distance transmission over five kilometers.
   • The experimental results demonstrated that the combination of high-speed modulation and high-power output enables efficient free-space optical communication without the need for additional amplifiers or lenses.

Conclusion and Value

This study demonstrated the excellent performance of photonic crystal surface-emitting lasers under conditions of high power and high-speed direct modulation through numerical calculations, device design, fabrication, and experimental verification. Specifically, the research team designed a PCSEL with watt-level output power and several GHz modulation bandwidth and experimentally validated its practical application value in free-space optical communication.

The high power and narrow beamwidth characteristics of PCSELs provide unique advantages for long-distance FSO communication. Additionally, the absence of complex optical systems and fiber amplifiers makes PCSELs have broad application prospects in optical communication. Furthermore, PCSELs’ potential for narrow linewidth operation due to their large cavity size also implies important applications in future coherent optical communication.

Research Highlights

1. Combination of High Power and High-Speed Modulation: This research showcased the superior performance of PCSELs under watt-level power and several GHz high-speed modulation, offering new possibilities for optical communication technology.
2. Simplified Optical Communication System: Achieved simpler and more efficient optical communication without complex fiber amplifiers and lens systems, enabling more compact and energy-efficient future optical communication devices.
3. Possibility of Long-Distance Transmission: Successfully simulated free-space transmission over more than five kilometers, demonstrating the feasibility and advantages of watt-level PCSELs in long-distance optical communication.

Other Valuable Information

The research suggests that further enlargement of the laser’s emission area, improvement of device structure, and enhanced cooling performance will likely achieve higher output power and broader modulation bandwidth. Additionally, due to the large cavity size of PCSELs, the study also holds potential for important applications in future coherent optical communication. This research not only demonstrated the excellent performance of PCSELs under high power and high-speed conditions but also pointed the way for more complex and long-distance optical communication technology in the future.