Bi-frequency operation in a membrane external-cavity surface-emitting laser

PLOS ONE RESEARCH ARTICLE Bi-frequency operation in a membrane external-cavity surface-emitting laser Jake Daykin ID1*, Jonathan R. C. Woods2, Roman Bek3, Michael Jetter ID4, Peter Michler4, Ben Mills5, Peter Horak5, James S. Wilkinson5, Vasilis Apostolopoulos1 1 School of Physics and Astronomy, University of Southampton, Southampton, Hampshire, United Kingdom, 2 Aquark Technologies, Romsey, Hampshire, United Kingdom, 3 Twenty-One Semiconductors GmbH, Neckartenzlingen, Germany, 4 Institute for Semiconductor Optics and Functional Interfaces, University of Stuttgart, Stuttgart, Germany, 5 Optoelectronics Research Centre, University of Southampton, Southampton, Hampshire, United Kingdom a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 OPEN ACCESS Citation: Daykin J, Woods JRC, Bek R, Jetter M, Michler P, Mills B, et al. (2023) Bi-frequency operation in a membrane external-cavity surfaceemitting laser. PLoS ONE 18(7): e0289223. https:// doi.org/10.1371/journal.pone.0289223 Editor: Xuejian Wu, Rutgers University Newark, UNITED STATES Received: May 26, 2023 * Abstract We report on the achievement of continuous wave bi-frequency operation in a membrane external-cavity surface-emitting laser (MECSEL), which is optically pumped with up to 4 W of 808 nm pump light. The presence of spatially specific loss of the intra-cavity high reflectivity mirror allows loss to be controlled on certain transverse cavity modes. The regions of spatially specific loss are defined through the removal of Bragg layers from the surface of the cavity high reflectivity mirror in the form of crosshair patterns with undamaged central regions, which are created using a laser ablation system incorporating a digital micromirror device (DMD). By aligning the laser cavity mode with the geometric centre of the loss patterns, the laser simultaneously operated on two Hermite-Gaussian spatial modes: the fundamental HG00 and the higher order HG11 mode. We demonstrate bi-frequency operation over a range of pump powers and sizes of spatial loss features, with a wavelength separation of approximately 5 nm centred at 1005 nm. Accepted: July 13, 2023 Published: July 27, 2023 Peer Review History: PLOS recognizes the benefits of transparency in the peer review process; therefore, we enable the publication of all of the content of peer review and author responses alongside final, published articles. The editorial history of this article is available here: https://doi.org/10.1371/journal.pone.0289223 Copyright: © 2023 Daykin et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All data supporting this study is openly available from the University of Southampton repository in Ref 37. doi: 10.5258/ SOTON/D2628. 1 Introduction Coherent and tunable continuous wave (CW) laser sources that operate simultaneously on two discrete optical frequencies see potential applications in a variety of fields, such as spectroscopy [1], communications [2, 3], metrology [4, 5], and THz generation [6–8]. Generating bi-frequency operation in semiconductor quantum well lasers has been an area of active research in recent years [9–14]. Previously bi-frequency operation in single cavity Vertical External-Cavity Surface-Emitting Lasers (VECSELs) has been demonstrated through the use of techniques such as: the inclusion of an intra-cavity birefringent filter [15, 16], the deposition of a loss inducing mask onto, or through the machining of, the VECSEL gain structure [7, 8]. In [14] we utilise a spatial loss inducing, laser ablated region on the surface of a high reflectivity cavity mirror to force bi-frequency operation. Analysis and optimisation of noise in bi-frequency VECSELs has been performed in [8, 12, 15, 17]. Bi-frequency VECSELs have also been used as coherent and tunable THz sources through the utilisation of the beat note between the two modes [7, 8]. PLOS ONE | https://doi.org/10.1371/journal.pone.0289223 July 27, 2023 1 / 13 PLOS ONE Funding: VA would like to thank the Engineering and Physical Sciences Research Council for the grant EP/T001046/1 titled "UK National Quantum Technology Hub in Sensing and Timing". https:// www.ukri.org/councils/epsrc/ The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist. Bi-frequency operation in a MECSEL Membrane External-Cavity Surface-Emitting Lasers (MECSELs) are optically pumped semiconductor quantum well lasers based on VECSEL technology, but lacking the intra-cavity distributed Bragg reflector (DBR), that were first demonstrated by Yang et al. in 2015 [18]. MECSELs also share many advantages present in VECSELs, such as narrow emission linewidth due to its class A dynamics and low Schawlow-Townes limit [9, 19–21] and potential to generate high quality, diffraction limited beams with M2 < 1.05 [22, 23]. The main advantages of MECSELs are: better heat extraction and faster fabrication, both of these advantages stem from the fact that MECSELs are much thinner structures than VECSELs. Here we combine a cavity geometry needed for bi-frequency operation with a MECSEL gain chip. We utilise spatial loss inducing, laser ablated masks on the surface of a high reflectivity cavity mirror. The laser ablation machining of the mirror was performed using a Digital Micromirror Device (DMD) [14]. The spatially dependent loss present in the MECSEL cavity allows for the loss of particular transverse cavity modes to be controlled, and thus promotes the oscillation of two specific spatial modes. Additionally, due to the fact that both modes share the same external cavity and because there is an overlap between the modes in the gain region, the noise in the system is common to both modes and coherence can be enforced between them [7, 15]. Using a MECSEL instead of a VECSEL gain chip gives us the following notable advantages: (a) MECSELs can be fabricated usually 10 times faster than VECSELs, due to the lack of an intra-cavity DBR. (b) The whole emission bandwidth of the active region can be used, again, due to the lack of the DBR. (c) Less complexity and greater flexibility in material combinations due to no longer needing to lattice-match the active region to the DBR, leading to the potential for new wavelengths. (d) The lack of an intra-cavity DBR in MECSELs allows for improved thermal management, and thus higher efficiencies and output powers [24–26]. These advantages give promise for future rapid prototyping of bi-frequency sources with higher output powers and with wider wavelength selection and tunability [27]. 2 Experimental methods 2.1 MECSEL gain sample The approximately 1 μm thick gain structure is fabricated using Metal (...truncated)