Tunable Vertical-cavity Surface-emitting Laser Research Articles

Wide‐Wavelength Tunable Mid‐Infrared Lasing Based on Black Arsenic Phosphorus

AbstractMid‐infrared (MIR) semiconductor laser sources are essential for applications in gas detection, infrared imaging, and high‐speed free space optical communications. At present, the dynamic wavelength tunability of MIR lasers based on lead salt, antimonide, and quantum cascade lasers is limited and their compact size as well as heterogeneous integration are still challenging. Two‐dimensional (2D) materials, used as optical gain materials, have the flexible tunability and compatible van der Waals integrations—providing many new possibilities for constructing MIR laser sources of large tunability and high integration. Here, wide‐wavelength tunable MIR vertical cavity surface emitting lasers (VCSELs) integrated on silicon substrates are realized based on 2D black arsenic phosphorus (b‐AsxP1−x). The emission wavelength of the optically pumped laser device is demonstrated to be tuned from 3.42 to 4.65 µm at room temperature, which could be controlled by adjusting the alloy composition and thickness of the gain media b‐AsxP1−x. The tunable MIR VCSEL device would pave the way for further exploration of 2D materials‐based IR lasers working as on‐chip light sources.

High-Speed Two-Dimensional Spectral-Scanning Coherent LiDAR System Based on Tunable VCSEL

Non-mechanical LiDAR systems have attracted extensive research interests due to their advantages of robustness and high beam-steering speed. Recently, spectral scanning has emerged as a promising solid-state beam-steering method, which combines dispersive elements with frequency-swept laser sources to perform ultrafast beam-steering. However, swept light sources used in LiDAR systems, such as semiconductor lasers and supercontinuum sources, are usually suffering from low sweep rate, bulky and high-cost that severely hinder the development of spectral-scanning LiDAR. Here, we propose a non-mechanical spectral-scanning frequency-modulated continuous-wave (FMCW) LiDAR system based on a simple compact tunable vertical-cavity surface-emitting laser (VCSEL). The fast-swept VCSEL together with wavelength division multiplexing (WDM) technology is employed for high-speed beam-steering. Millimeter-level ranging accuracy and a lateral resolution of 0.286° with an overall depth voxel acquisition rate of 128 kHz are achieved experimentally.

12 Gbit/s indoor optical wireless communication system with ultrafast beam-steering using tunable VCSEL.

Optical wireless communication (OWC) using line-of-sight connections has great application potential for indoor scenes due to its advantages of high data transmission speed and privacy. In our proposed system, we use infrared tunable vertical-cavity surface-emitting laser (VCSEL) as light source, array waveguide gratings (AWG) combined with 6 × 6 integrated optical fiber arrays as a router to realize ultrafast beam-steering and high capacity point-to-point data transmission, which makes the indoor OWC system compact, low-cost, and easy to install. The high tuning rate of VCSEL enables the channel switching to be completed within 1.7 µs. Based on the modulation format of non-return-to-zero on-off keying (NRZ-OOK), a data rate of 12 Gbit/s per channel can be realized with high sensitivity through 3.1 m free space when the detected optical power is low. The system is proved to be a flexible link with high-speed communication for mobile terminals in a limited space.

Indirect stress and air-cavity displacement measurement of MEMS tunable VCSELs via micro-Raman and micro-photoluminescence spectroscopy

We employ micro-Raman spectroscopy to optically infer the stress experienced by the legs of a bridge-type microelectromechanical systems (MEMS) used in high contrast gratings tunable vertical cavity surface emitting lasers (VCSELs). We then employ micro-photoluminescence (PL) spectroscopy to indirectly measure the air cavity displacement of the same MEMS structure. Results from micro-Raman showed that electrostatically actuating the MEMS with a DC bias configuration yields increasing residual stress on the endpoints of the MEMS with values reaching up to 0.8 GPa. We simulated a finite element model via Comsol Multiphysics which agrees with the trend we observed based on our micro-Raman data. Our micro-PL spectroscopy showed that change in the air cavity of the VCSEL structure resulted in a change in the full width of the PL peak emitted by the layer consisting of four pairs of distributed Bragg reflectors. The change in the full width of the PL peak was due to the change in the optical cavity induced by displacing the MEMS via externally applied bias and agrees with our transfer matrix convolution simulation. These optical characterization tools can be used for failure analysis, MEMS design improvements, and monitoring of MEMS tunable VCSEL devices for mass production and manufacturing.

Demonstration of real-time structured-light depth sensing based on a solid-state VCSEL beam scanner.

We demonstrated a real-time scanning structured-light depth sensing system based on a solid-state vertical cavity surface-emitting laser (VCSEL) beam scanner integrated with an electro-thermally tunable VCSEL. Through a swept voltage added to the tunable VCSEL, a field of view of 6°×12° could be scanned with a scanning speed of 100 kHz by the beam scanner. Adopting the beam scanner, the real-time depth image with a lateral resolution of 10,000 (20×500) was obtained by measuring a step target placed at 35cm. The frame rate could be >10Hz even if sunlight shot noise is artificially added to the experimental data. By using a higher-speed camera, a potential lateral resolution could be reached at 50,000 (100×500) with a frame rate of > 20Hz. By using flat optics, a compact scanning module offering line pattern with FoV of >40°×20° was also demonstrated. It could help to realize high-resolution and high-accuracy structured-light sensing with a compact module.

MEMS-VCSEL as a tunable light source for OCT imaging of long working distance

We present a wavelength tunable vertical-cavity surface-emitting laser (VCSEL) constructed by die-bonding a half-cavity InGaAs laser diode (LD) chip onto a silicon-on-insulator chip with a microelectromechanical system electrostatic diaphragm mirror that functions as a Fabry–Perot interferometer. As a result of the short cavity length, the integrated tunable LD has single-mode lasing characteristics with an extremely large coherence length of 150 m or more. The developed wavelength tunable LD is used to perform swept-source optical coherence tomography with a large scan depth, which is applicable to ophthalmic observation for the diagnosis of pathologic nearsightedness based on the measurement of the axial length of an eye.

Liquid crystal tunable vertical cavity surface emission laser with inner cavity sub-wavelength grating

With the rapid development of information technology, a wavelength-tunable vertical cavity surface emitting laser (VCSEL) is urgently needed as an optical signal source in dense wavelength division multiplexing (DWDM). Liquid crystal tunable VCSEL realized by utilizing the birefringence characteristics of liquid crystal has the advantages of stable polarization, high reliability, continuous wavelength tuning. In this paper, a liquid crystal tunable VCSEL structure based on intracavity sub wavelength grating is designed, and the influence of liquid crystal layer and sub wavelength grating on the wavelength tuning characteristics of VCSEL are analyzed and studied in depth. The results show that the thickness of the liquid crystal layer in the tunable VCSEL structure not only affects the wavelength tuning range, but also determines the mode hopping in the tuning process. In addition, an effective refractive index antireflection layer is formed by designing the subwavelength grating structure, and the refractive index difference between the liquid crystal layer and the semiconductor layer is optimized to further improve the wavelength tuning range and tuning efficiency. When the center wavelength is 980 nm, the tuning range is increased by 42%, reaching 41 nm, and the wavelength tuning efficiency is increased by 41%. It provides a new method of designing the VCSEL laser with high beam quality and continuous wavelength tuning.

Swept Source Lidar: simultaneous FMCW ranging and nonmechanical beam steering with a wideband swept source.

Light detection and ranging (lidar) has long been used in various applications. Solid-state beam steering mechanisms are needed for robust lidar systems. Here we propose and demonstrate a lidar scheme called "Swept Source Lidar" that allows us to perform frequency-modulated continuous-wave (FMCW) ranging and nonmechanical beam steering simultaneously. Wavelength dispersive elements provide angular beam steering, while a laser frequency is continuously swept by a wideband swept source over its whole tuning bandwidth. Employing a tunable vertical-cavity surface-emitting laser and a 1-axis mechanical beam scanner, three-dimensional point cloud data has been obtained. Swept Source Lidar systems can be flexibly combined with various beam steering elements to realize full solid-state FMCW lidar systems.

Ultrafast and wide tunable VCSEL using graphene passive cavity

A novel approach for achieving ultrafast and wide tunable vertical-cavity surface-emitting lasers (VCSELs) is presented. The design is based on incorporating multiple graphene flakes separated by SiO2 dielectric in a λ/2 cavity and placing multiquantum wells in one of the quarter wave layers of the bottom mirrors. The device concept enhances both the wavelength tuning range and tuning speed which is attractive for many photonic applications. By applying a gate voltage on the graphene flakes, the effective refractive index of the cavity can be changed with ultrahigh speed. We have shown that 2 graphene flakes inside a 5 µm-diameter passive cavity provides tuning speed as high as 32 GHz and wavelength tuning range of 3.8 nm. The tuning speed increases to 52 GHz when the device diameter is reduced to 3 µm. The effect of spacer thickness and the number of graphene flakes on the performance characteristics of the device is investigated. Our analysis shows that tuning speed as high as 2 GHz and wavelength tuning range of ~ 12 nm is obtained for VCSEL that incorporates 6 graphene flakes separated by 10 nm SiO2 spacers and biased in parallel. The tuning speed can be increased to ~ 16 GHz when the flakes are biased in series at the expense of higher gate voltage. A tradeoff between the wavelength tuning range and the tuning speed is obtained.

Polarization control and tuning efficiency of tunable vertical-cavity surface-emitting laser with internal-cavity sub-wavelength grating**Project supported by the Jilin Science and Technology Development Plan, China (Grant Nos. 20180519018JH and 20190302052GX), Jilin Education Department “135” Science and Technology, China (Grant No. JJKH20190543KJ), the National Natural Science Foundation of China (Grant No. 11474038), and the Key Project of Equipment Pre-Research Fund of China

We design an 850 nm tunable vertical-cavity surface-emitting laser (VCSEL) structure using an internal-cavity sub-wavelength grating. The use of such a tuning structure allows for wider wavelength tuning range and more stable single-polarization as compared to conventional tunable VCSELs. The features of the internal-cavity grating effect on the wavelength tuning and polarization characteristics of the tunable VCSEL are analyzed. The simulation results show that the largest wavelength tuning range achieves 44.2 nm, and the maximum orthogonal polarization suppression ratio (OPSR) is 33.4 dB (TE-type) and 38.7 dB (TM-type).

CW Operation of a Tunable 1550-nm VCSEL Integrating Liquid-Crystal Microcells

An InP-based Vertical-Cavity Surface-Emitting Laser (VCSEL) with a liquid crystal (LC) microcell monolithically integrated on its surface for spectral tuning is investigated. Unlike tunable VCSELs integrating a movable membrane, here the physical length of the cavity remains unchanged and only the voltage applied on the LC ensures a refractive index modification for a particular polarization emitted by the VCSEL. This tunable VCSEL operates in CW at room temperature and exhibits more than 23 nm wavelength tuning around $1.55~\mu \text{m}$ at a maximum applied voltage of 20 V. The measured laser threshold around 6.5 mW is still comparable to VCSEL without LC microcell, a clear indication that the optical losses related the LC are very low. On the other hand, for this first optically pumped device, the lasing characteristics suggest that the LC birefringence is lower than expected. To assess this hypothesis, thermo-optical simulations have been conducted.

Frequency-agile injection-seeded terahertz-wave parametric generation

An injection-seeded terahertz (THz)-wave parametric generator (is-TPG) enables access to low-frequency fingerprints of molecules in the THz frequency region. However, its conventional scan repetition rate (SRR) is limited below 1 Hz. Thus, in this Letter, we propose an electrically controlled tuning system for the is-TPG, which provides high-speed scanning and random hopping agility. We achieved rapid THz frequency scanning on a pulse-by-pulse basis by employing a gain-switched laser diode and a micro-electromechanical system tunable vertical-cavity surface-emitting laser as the pump and seed lasers. A THz spectrum was acquired with a 10 times higher SRR of 10 Hz for the 1.6–3 THz range with a sampling resolution of 4.6 GHz.

Mastering Lateral Radiation Losses in Tunable VCSELs

This paper deals with the loss mechanisms in tunable vertical-cavity surface-emitting lasers (VCSELs). The strong increase of the threshold gain at the tuning range edges motivates the investigation of the degradation mechanisms presented in this paper. A campaign of simulations performed with our in-house VCSEL ELectroMagnetic (VELM) simulation code, combined with a novel approach based on the study of the Poynting vector, allowed to identify significant lateral radiation losses. The proposal of technology-affordable countermeasures rests on the proper understanding of these phenomena.

Resonant-antiresonant coupled cavity VCSELs.

The wavelength tuning range of a tunable vertical-cavity surface-emitting laser (VCSEL) is strongly influenced by the design of the interface between the semiconductor cavity and the air cavity. A simplified model is used to investigate the origin of the dramatic differences in free spectral range (FSR) and tuning slope observed in semiconductor cavity dominant, extended cavity, and air cavity dominant VCSELs. The differences arise from the positioning of the resonant and antiresonant wavelengths of the semiconductor cavity with respect to the center wavelength. The air cavity dominant design is realized by designing an antiresonant semiconductor cavity, resulting in a larger tuning slope near the center of the tuning range and a wider FSR toward the edges of the tuning range. The findings from the simplified model are confirmed with the simulation of a full VCSEL structure. Using an air cavity dominant design, an electrically pumped laser with a tuning range of 68.38 nm centered at 1056.7 nm at a 550 kHz sweep rate is demonstrated with continuous wave emission at room temperature. This epitaxial design rule can be used to increase the tuning range of tunable VCSELs, making them more applicable in swept-source optical coherence tomography and frequency-modulated continuous-wave LIDAR systems.

Frequency domain diffuse optical spectroscopy with a near-infrared tunable vertical cavity surface emitting laser.

We present an approach for performing frequency domain diffuse optical spectroscopy (fd-DOS) utilizing a near-infrared tunable vertical cavity surface emitting laser (VCSEL) that enables high spectral resolution optical sensing in a miniature format. The tunable VCSEL, designed specifically for deep tissue imaging and sensing, utilizes an electrothermally tunable microelectromechanical systems topside mirror to tune the laser cavity resonance. At room temperature, the laser is tunable across 14nm from 769 to 782nm with single mode CW output and a peak output power of 1.3mW. We show that the tunable VCSEL is suitable for use in fd-DOS by measuring the optical properties of a tissue-simulating phantom over the tunable range. Optical properties were recovered within 0.0006mm-1 (absorption) and 0.09mm-1 (reduced scattering) compared to a broadband fd-DOS reference system. Our results indicate that tunable VCSELs may be an attractive choice to enable high spectral resolution optical sensing in a wearable format.

Electrically tunable VCSEL with intra-cavity liquid crystal: Design, optimization, and analysis of polarization- and mode-stability

Tunable vertical-cavity surface-emitting lasers (VCSEL) provide broad wavelength tuning range at very high modulation speed, but usually employ micro electromechanical systems, which are sensitive to vibrations. In this paper we propose a tunable 1550 nm VCSEL design with enclosed intra-cavity liquid-crystal (LC) cell. The laser is electrically pumped and the wavelength selectivity is obtained by varying LC’s refractive index through electro-optical tuning of the LC director.By performing one-dimensional transfer-matrix optical calculations, supplemented with a fully-vectorial three-dimensional self-consistent modeling of thermal, electrical and optical phenomena, we optimize the design and show the dependency of the emitted wavelength on LC cell thickness and the LC director orientation. The analysis is concluded with a possibility of achieving 68.5 nm tuning range in a continuous-wave operation regime and without polarization or transverse mode switching.

Systematic characterization of a 1550 nm microelectromechanical (MEMS)-tunable vertical-cavity surface-emitting laser (VCSEL) with 7.92 THz tuning range for terahertz photomixing systems

Continuous-wave (CW) terahertz (THz) photomixing requires compact, widely tunable, mode-hop-free driving lasers. We present a single-mode microelectromechanical system (MEMS)-tunable vertical-cavity surface-emitting laser (VCSEL) featuring an electrothermal tuning range of 64 nm (7.92 THz) that exceeds the tuning range of commercially available distributed-feedback laser (DFB) diodes (∼4.8 nm) by a factor of about 13. We first review the underlying theory and perform a systematic characterization of the MEMS-VCSEL, with particular focus on the parameters relevant for THz photomixing. These parameters include mode-hop-free CW tuning with a side-mode-suppression-ratio >50 dB, a linewidth as narrow as 46.1 MHz, and wavelength and polarization stability. We conclude with a demonstration of a CW THz photomixing setup by subjecting the MEMS-VCSEL to optical beating with a DFB diode driving commercial photomixers. The achievable THz bandwidth is limited only by the employed photomixers. Once improved photomixers become available, electrothermally actuated MEMS-VCSELs should allow for a tuning range covering almost the whole THz domain with a single system.

Wavelength-Swept VCSELs

The ability to continuously tune the wavelength of a laser is of critical importance and is a fundamental building block for many optical systems, including wavelength division multiplexed optical fiber communication systems. Vertical cavity surface emitting laser (VCSEL) structure offers a unique advantage to engineer the lasing wavelength because of its ultrashort cavity length, inherently supporting a single Fabry–Perot mode. Hence, a continuous change in VCSEL cavity thickness leads to a continuous sweep of VCSEL wavelength. Wavelength-tunable VCSELs have been a subject of intense interest for the last two decades. Incorporating part or entire top mirror of a VCSEL in an optical microelectromechanical structure (MEMS), continuous wavelength sweeps have been reported. The monolithic integration brings together the best of both technologies and leads to an unprecedented performance in the continuously swept wavelength range. In addition, with the advances of ultrathin high contrast gratings and metastructures (HCG/HCM), the wavelength tuning speed of MEMS-VCSELs has been increased to 1–10 MHz range. Such lasers, now referred as wavelength-swept lasers, are enabling new applications in optical coherent tomography and light detection and ranging systems. In this paper, we review various structures and recent progress of MEMS-VCSELs. We summarize some of the early breakthroughs in designs and properties of micromechanical tunable VCSELs, including advances in HCG/HCM-based MEMS-VCSELs emitting at 850, 1060, and 1550 nm wavelength regimes. In addition, we report a brand new design leading to a record high Δλ/λo = 6.9% tuning ratio with 600 kHz speed at center wavelength of 1060 nm.

A signal impairment-aware scheme for next-gen flexible spectrum in 10 Gb/s VCSEL metro-access optical fibre networks

We experimentally characterized and optimized link performance of a multi-node flexible spectrum network. The cheap and low power consuming and wavelength tunable vertical cavity surface-emitting lasers (VCSELs) have been used to create colourless and wavelength selective flexible transmitters to enable up to 10Gb/s per channel/wavelength signals. With a −3.8dBm VCSEL optical power and chromatic dispersion mitigation, a 25km multi-node network has been experimentally demonstrated with a transmission penalty range of 0.2–3.2dB. Moreover, an impairment-aware mathematical model has been created to predict signal continuity for crosstalk and dispersion penalties for a hop-to-hop signal transmission and a bit error-rate (BER) evaluation at the 10-9 threshold level. This study is vital for creating routing and network algorithms for the next-generation optical fibre networks.

High-Contrast Gratings Performance Issues in Tunable VCSELs

Tunable vertical-cavity surface-emitting lasers (VCSELs) are becoming key components in several sensing applications, where single-mode, single polarization light is required. In such devices, high-contrast gratings (HCGs) are very promising mirrors, because of their high reflectivity, strong polarization discrimination, and low-volume occupation. However, their introduction might lead to unexpected increases in the threshold gain, destroying the tuning performance. In this paper, these phenomena are investigated and explained in detail. Although VCSEL modes are computed by means of our full 3-D vectorial code, a Gaussian beam model is used to obtain deeper insight into the HCG reflectivity mechanism in the presence of a finite beam.