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Significance
Modern optical communication systems are essential for high-speed, high-capacity data transmission, such as in internet networks, telecommunications, and data centers. However, they face significant challenges in terms of complexity and cost. One major limitation is the need for separate transmitters and receivers which increases the number of components required and the demand for complex synchronization mechanisms. Additionally, for bidirectional communication, conventional setups rely on doubling the number of transmitters and detectors which further amplify the overall system cost and complexity. Scientists have long sought to reduce these complexities and tried different ways to integrate the transmitting and receiving functions into a single component which will significantly decrease the optical hardware burden, improve efficiency, and reduce operational costs. However, achieving this while maintaining high data transfer rates and accuracy is not trivial and still a challenge. Indeed, the main obstacle is designing a system that can handle simultaneous bidirectional data transmission without compromising the integrity of the information or requiring complex external detection mechanisms. To this account, new study published in Physical Review Applied and conducted by Andreas Herdt, Markus Weidmann, and led by Professor Wolfgang Elsäßer from the Technische Universität Darmstadt in Germany alongside with Dr. Adonis Bogris from the University of West Attica, Athens in Greece and Richard Phelan from Eblana Photonics Ltd in Ireland, the researchers addressed these challenges by leveraging the phenomenon of mutual injection locking in discrete-mode diode lasers. They used compound laser states (CLS) and the laser-as-detector principle to create a simplified optical communication system that eliminated the need for external detectors and streamlined the whole process. The goal was to demonstrate that it is possible to establish a stable, high-performance communication channel with fewer optical components, which would not only reduce costs but also open doors for new applications in wavelength regions where conventional detectors are impractical.
The research team set up a system involving two 1550-nm discrete-mode diode lasers, which were connected through a polarization-maintaining optical fiber. That arrangement allowed for mutual coupling between the lasers, forming CLS and by this experimentally demonstrated the concept of a detector-free communication system with both lasers functioning as transceivers eliminating the need for external optical detectors. The lasers were tuned using current sources and their optical frequencies were precisely controlled to generate the desired CLS which acted as the carrier for the transmitted data.
The authors observed that when the two lasers were mutually coupled, their frequencies synchronized within a specific bandwidth known as the locking bandwidth. The degree of synchronization or mutual injection locking was a critical finding as it enabled the system to transmit data bidirectionally using the CLS as the information carrier. Moreover, they discovered that by varying the current to the lasers, they could effectively manipulate the detuning between the lasers, thereby encoding different bits of information. This frequency detuning resulted in measurable voltage changes at the terminal of each laser which were recorded by conventional voltmeters. The researchers used the laser-as-detector principle, wherein the terminal voltage of each laser changed in response to the alterations in the population inversion caused by the mutual coupling. Furthermore, the researchers were able to decode the transmitted data by comparing these voltage shifts to the uncoupled state of the lasers and successfully demonstrated that the system could reliably transfer data by observing that specific CLS corresponded to particular bit sequences. For instance, the researchers encoded and successfully transmitted the letters “TUD” from one laser to the other, and “hlo” in the reverse direction. The voltage changes at the terminals of the lasers provided a direct method for decoding the transmitted information. In addition to demonstrating reliable data transmission, the researchers tested the robustness of their system by transferring 5800 bits of random data over a period of 2.1 hours. They found that the system maintained a low bit-error rate of 7.4 x 10-3, which indicated that the mutually coupled lasers could maintain stable communication over extended periods without significant data loss. This robustness was attributed to the system’s ability to stay within the LBW, where the lasers remained locked to a common frequency. Furthermore, they discovered that the transmission speed could be increased by optimizing the control systems, although in the proof-of-concept experiment, the average bit rate was 1.3 bits per second. According to the authors, one of the most compelling findings was the system’s ability to detect disconnection events. During the experiments, the researchers observed that when a disconnection occurred between the lasers, the terminal voltages of both lasers remained unchanged for every transmitted bit. This statistical behavior provided a built-in method for detecting communication failures, adding another layer of reliability to the system. The experiments showed that, in cases of large data transfers, the system could automatically detect disconnections by monitoring shifts in the equilibrium between certain CLS.
In conclusion, Professor Wolfgang Elsäßer who is also Adjunct Professor at Trinity College Dublin in Ireland and who is associated with the Istituto di elettronica e di ingegneria dell’informazione e delle telecomunicazioni (IEIIT) del Consiglio Nazionale delle Richerche (CNR) at Politecnico di Torino in Italy and his colleagues from the research team developed an innovative approach to simplify optical communication systems and eliminated the need for external detectors, a truly fundamental shift that has the potential to reduce both the cost and complexity of such systems with a more efficient architecture for bidirectional communication with the integration of both transmitter and receiver functions within the same diode lasers. It has a significant improvement in transmission speed and data integrity, given the study’s demonstration of error-free data transfer with a low bit-error rate. This breakthrough could have wide-ranging applications specially in wavelength regions where conventional detectors are either pricy or unavailable such as the mid-infrared or terahertz range. Moreover, there is a potential to extend the utility of the new system to a variety of laser types beyond the 1550-nm discrete-mode lasers used in the study which can open the door to advanced communications in challenging environments such as free-space optical links or quantum cryptography. Additionally, the built-in ability to detect system disconnections through voltage measurements provide robustness to the communication process which is an important feature for real-time applications.
NOTES: The work had been performed in a research collaboration between the Semiconductor Optics group in the Institute of Applied Physics at Technische Universitaet Darmstadt (Germany), where Markus Weidemann performed a master thesis in Physics and Andreas Herdt a Ph. D. thesis in Physics conducted by Professor Dr. Wolfgang Elsäßer (email: [email protected]) who is also Adjunct Professor at Trinity College Dublin in Ireland and who is associated with the Istituto di Elettronica e di Ingegneria dell’Informazione e delle Telecomunicazioni (IEIIT) del Consiglio Nazionale delle Richerche (CNR) at Politecnico di Torino in Italy.
https://www.iap.tu-darmstadt.de/hlo/home_hlo/index.en.jsp
The 1550-nm emitting discrete-mode semiconductor laser devices lasers had been supplied to the project by Dr. Richard Phelan from Eblana Photonics Ltd. In Dublin (Ireland).
The modelling of the two coupled lasers and the discussions of the compound laser states had been performed in collaboration with Prof. Adonis Borgris from the Department of Informatics and Computer Engineering, University of West Attica in
Athens (Greece)
http://users.uniwa.gr/abogris/index.html
Top: the traditional, conventional approach based on two transmitter lasers and two receivers.
Bottom: The detector-free based approach realized by the research team relying on the laser-as-detector concept unifying receiver and transmitter laser in one single opto-electronic transceiver unit element.
Reference
Andreas Herdt, Markus Weidmann, Adonis Bogris, Richard Phelan, and Wolfgang Elsäßer. Detector-Free Fiber-Based Bidirectional Symmetric Communication Scheme Based on Compound States of Two Mutually Coupled Discrete-Mode Lasers. Phys. Rev. Applied 21, 014049 (2024)
