Tunable External Cavity Research Articles

300-nm-thick, ultralow-loss silicon nitride photonic integrated circuits by 8-in. foundry production

Silicon nitride (Si3N4) photonic integrated circuits are rapidly developing in recent decades. The low loss of Si3N4 attracts significant attention and facilitates a wide range of applications in integrated photonics. In this work, we demonstrate the foundry fabrication of a 300-nm-thick 8-in. wafer-scale Si3N4 platform, with a microresonator intrinsic quality factor of up to 15×106, corresponding to an ultralow loss of 2.2 dB/m. Leveraging this platform, we develop a mature process design kit, achieving a single-mode waveguide propagation loss of less than 5 dB/m, an edge coupler loss of 1.3 dB, and an insertion loss of 0.07 dB for multimode interference couplers. Utilizing the processed Si3N4 chip, we realize a hybrid integrated tunable external cavity laser with a tuning range from 1534 to 1602 nm, a record-high side-mode suppression ratio of up to 76 dB, an optical power of 26 mW, and an intrinsic linewidth of down to 314 Hz. Our work lays a solid foundation for the further development of applications, including nonlinear optics, quantum optics, optical communications, and ranging.

The Effect of Lens Focal Length on the Output Characteristics of 1.55 μm Tunable External-Cavity Semiconductor Lasers

The Effect of Lens Focal Length on the Output Characteristics of 1.55 μm Tunable External-Cavity Semiconductor Lasers

Development of hybrid photonic integrated wavelength-tunable laser at 2 µm and its application to FMCW LiDAR

This paper reports on the experimental demonstration of a fully integrated frequency-modulated continuous-wave (FMCW) LiDAR sensing system, operating at 2.0 µm. It makes use of a widely tunable hybrid external cavity laser based on the combination of GaSb gain chip and silicon waveguide circuits. The single-frequency laser operation over the full spectral bandwidth of the gain chip is secured using a frequency-selective filter, consisting of two sequential microring resonators in a Vernier configuration. To increase the mode-hop free wavelength tuning range while preserving the linewidth of the laser, the heater of the phase section placed along the bus waveguide is synchronously controlled with two independent heaters placed on each microring resonator. This laser is then implemented for the development of an FMCW LiDAR, consisting of all-optical fiber-based two independent unbalanced Mach-Zehnder interferometers: k-space interferometer for the linearization of continuously swept laser frequency and main interferometer for the measurement of the distributed back-reflection over the distance. The optical frequency of the laser is continuously swept over a ∼100 GHz range (or Δλ=1.47 nm at the operating wavelength) at a modulation speed of 100 Hz. Using this wavelength tunable laser, a light detection and ranging system (LiDAR) is experimentally demonstrated, showing a very high axial resolution of 1.36 mm in air with an extremely high precision of ∼9 µm at a 100 Hz measurement rate.

Linewidth measurement of external cavity lasers

Narrowband laser sources are used in applications that require high-precision or stable optical frequency. Such applications include high-resolution spectroscopy, long-distance measurement, and coherent optical communication. The linewidth of a laser is a direct measure of the laser's stability; therefore, characterization of laser linewidth is essential. In practice, however, determining a laser's linewidth is not a trivial task and typically requires expensive equipment or a complex experimental arrangement. This paper presents a straightforward, low-cost method based on unbalanced interferometry, which allows us to determine the visibility of fringe patterns as a function of the optical path difference and, consequently, the linewidth of the laser. As a test laser, we use a tunable external cavity laser source at around 780 nm, where an interference filter is employed for wavelength selection. Data obtained by applying the interferometric technique to this laser and the analysis of these data, along with the resulting linewidth value, are presented. Given that the described measurement setup is inexpensive, straightforward, and pedagogically accessible, it is well-suited for an instructional physics laboratory experiment and will also be of interest to laboratory researchers.

Dynamic exploration of laser linewidth compression in resonant feedback external-cavity laser

An external-cavity laser with resonant optical feedback is a common and mature method to achieve narrow linewidth lasers. Since the proposal of optical feedback in 1964, a series of studies on resonant optical feedback have been reported. Although longer feedback length leads to narrower linewidth, it also introduces coupling of environmental noise and causes instability in the laser cavity, which is detrimental to high-speed tuning of the laser wavelength. With such a challenge, current research on resonant optical feedback primarily focuses on the static results of laser linewidth compression. In order to obtain narrow linewidth high-speed tuned external-cavity lasers, further research on the laser dynamics is required to obtain dynamic tuning of narrow linewidth external-cavity lasers. In this work, we construct an external-cavity laser based on resonant optical feedback, and utilize a coherent detection method to measure the dynamic process of laser linewidth compression and wavelength tuning. Based on resonant optical feedback from the external cavity, the linewidth of the main-cavity laser can be compressed from near 100 kHz to the 100 Hz level, with the frequency noise compressed from the order of 104 to the 102 Hz2/Hz level. Since the wavelength switching process includes frequency shift process, linewidth compression process, and frequency stabilization process, etc., we first conduct a research on laser linewidth compression dynamics. In the results of linewidth compression dynamics, we discover that using a shorter fiber ring results in shorter linewidth compression time, which is advantageous for laser dynamic tuning. We subsequently carry out the laser wavelength switching process with a main-cavity switching time in milliseconds. In the wavelength switching process, while undergoing the frequency shift process of fast tuning transient laser frequency, the laser first undergoes the linewidth broadening process, then enters the main-cavity laser frequency stabilization process and undergoes the linewidth compression process at the same time. The experimental results show that the 100 m feedback fiber ring will not significantly increase the external-cavity laser wavelength switching time under the condition that linewidth compression is faster than main-cavity laser switching, while a longer fiber ring will introduce a strong noise. Our work experimentally demonstrates that shorter feedback ring lengths lead to faster compression, which provides valuable insights for the development of high-speed and high-precision tunable narrow linewidth external-cavity lasers.

Widely and fast tunable external cavity laser on the thin film lithium niobate platform

We develop an integrated external cavity laser with a broad wavelength tuning range and a fast switching speed on thin film lithium niobate (TFLN) platform. By butt coupling an InP-based optical gain chip with a TFLN external cavity chip, a hybrid lithium niobate/III-V tunable laser with a wavelength switching speed of 18 ns is achieved. Furthermore, using a cascaded external cavity filter composed of three micro-ring resonators (MRRs), the laser features an ultra-wide wavelength tuning range of 96 nm across the C + L + U bands, which is the greatest value known for tunable lasers integrated on TFLN. The maximum laser output power is 0.98 mW, with a measured side mode suppression ratio (SMSR) of better than 52 dB. The laser we developed is critical in applications such as optical sensors and wavelength routed optical switch.© 2023 xxxxxxxx. Hosting by Elsevier B.V. All rights reserved.

Hybrid integrated tunable external cavity laser with sub-10 Hz intrinsic linewidth

In this work, a hybrid integrated tunable external cavity laser (ECL) with a sub-10 Hz intrinsic linewidth is demonstrated. In this hybrid ECL, two Si3N4-based subwavelength hole defect assisted microring resonators, acting as high-Q laser reflectors, are butt-coupled with a chip-scale semiconductor optical amplifier at the two edges, respectively. The experimental results show that the hybrid ECL achieves a sub-10 Hz intrinsic linewidth across a wavelength tuning range from 1525 to 1565 nm, featuring a record lowest value of 6.06 Hz. The demonstrated hybrid ECL opens up opportunities toward the applications of high-speed optical communications and high-resolution light detection and ranging.

Design and Simulation of Silicon‐Based Tunable External Cavity Diode Lasers in the 1967-2033 nm Wavelength Range

Design and Simulation of Silicon‐Based Tunable External Cavity Diode Lasers in the 1967-2033 nm Wavelength Range

Study on the Technique for Wavelength Modulation Spectroscopy with Narrow Band Interference Filter Based External Cavity Diode Laser

In this study, an improved tunable external cavity diode laser (ECDL) is proposed. It is on the basis of narrow band interference filter, and its theoretical research is carried out as the light source of tunable diode laser absorption spectrum-wavelength modulation spectrum (TDLAS-WMS) system. The proposed model can use piezoelectric ceramic actuator to change and modulate its output central wavelength so that the laser central wavelength could periodically scan the absorption spectral band of the desired gas. Compared with the tunable monolithic diode laser, the narrow band interference filter based tunable ECDL might be able to accurately sense the type and concentration of a large variety of gases. Therefore, it can be used in the TDLAS-WMS system for high precision gas sensing and is expected to replace the tunable monolithic diode laser.

Hybrid integrated chip-scale laser systems

Photonic integrated circuits (PICs) allow for the rapid advancement of a wide range of optical devices on a compact platform, making them more useful and readily available in the commercial market. Various materials such as III–V semiconductors, silicon, silicon nitride, lithium niobate, and polymers are used to create PICs with certain unique properties. Hybrid integration can combine multiple material platforms via optical coupling and realize multi-functional PICs that overcome the limitations of a single material platform. This allows for a broad application base for hybrid integrated PICs, greatly enhancing their usability and practicality. In this paper, we will discuss the methodology and applications of hybrid integration for chip-scale laser systems, including narrow linewidth, widely tunable external cavity lasers, laser beam combining, integrated frequency combs, and integrated Pockels lasers.

External-cavity diode laser-based near-infrared broadband laser heterodyne radiometer for remote sensing of atmospheric CO2.

A near-infrared broadband (1500-1640 nm) laser heterodyne radiometer (LHR) with a tunable external-cavity diode laser as the local oscillator is developed and the relative transmittance, which represents the absolute relationship between the measured spectral signals and the atmospheric transmittance, is derived. High-resolution (0.0087 cm-1) LHR spectra in the spectral region of 6248.5-6256 cm-1 were recorded for the observation of atmospheric CO2. Combined with the relative transmittance, the preprocessed measured LHR spectra, the optimal estimation method, and the Python scripts for computational atmospheric spectroscopy, the column-averaged dry-air mixing ratio of CO2 of 409.09 ± 8 ppmv in Dunkirk, France on February 23, 2019, was retrieved, which is consistent with GOSAT and TCCON data. The near-infrared external-cavity LHR demonstrated in the present work has a high potential for use in developing a robust, broadband, unattended, and all-fiber LHR for spacecraft and ground-based atmospheric sensing that offers more channel selection for inversion.

Fully Integrated Solid-State LiDAR Transmitter on a Multi-Layer Silicon-Nitride-on-Silicon Photonic Platform

We report a LiDAR transmitter incorporating both a hybrid-integrated tunable external cavity laser and a high-resolution 2-D optical phased array beam-steerer. Widely tunable single-mode lasing over a span of approximately 100 nm is achieved with >42 dB side-mode-suppression-ratio, 18 mW output power, and 2.8 kHz linewidth. Two-dimensional beam steering in a field-of-view of 140° × 16° exhibits a beam divergence of 0.051° × 0.016° measured in full width at half maximum. Leveraging the low propagation loss, negligible nonlinear loss, and low thermal sensitivity of silicon nitride, and combining the high mode confinement and efficient thermal tuning of silicon, the device affirms the feasibility of high-power on-chip lasing, power bottleneck elimination, and low on-chip insertion loss. It represents the first demonstration of a fully integrated LiDAR transmitter on the multi-layer silicon-nitride-on-silicon photonic platform, revealing the potential of complementary integration in the effort toward a LiDAR transmitter of sufficient optical power budget.

1.7 THz tuning range pivot-point-independent mode-hop-free external cavity diode laser.

A novel mode-hop-free (MHF) tunable external cavity diode laser (ECDL) is demonstrated without mechanical pivot-point tuning. By corotating a periscope with an etalon and a narrow band pass (NBP) filter inside an external cavity, the cavity single longitudinal mode selection can be maintained, and continuous tuning can be achieved with optimal synchronization. A MHF continuous tuning range of 1.7 THz has been achieved with a semiconductor gain chip at the wavelength of 652 ± 2 nm experimentally, and the theoretical tuning range can reach over 4.8 THz. The laser linewidth is estimated to be less than 1 MHz (FWHM) by a scanning Fabry-Perot (F-P) and a Michelson interferometer. 1 mW output power with variation of less than 10% in the tuning region of roughly 1.7 THz has been demonstrated.

High Power 970 nm Semiconductor Laser With A Tunable Grating External Cavity

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Dual-Beam Photothermal Spectroscopy Employing a Mach-Zehnder Interferometer and an External Cavity Quantum Cascade Laser for Detection of Water Traces in Organic Solvents.

We report on a mid-infrared (mid-IR) photothermal spectrometer for liquid-phase samples for the detection of water in organic solvents, such as ethanol or chloroform, and in complex mixtures, such as jet fuel. The spectrometer is based on a Mach-Zehnder interferometer (MZI) employing a He-Ne laser, a mini-flow cell with two embedded channels placed in the interferometer's arms, and a tunable external cavity quantum cascade laser (EC-QCL) for selective analyte excitation in a collinear arrangement. In this study, the bending vibration of water in the spectral range 1565-1725 cm-1 is targeted. The interferometer is locked to its quadrature point (QP) for most stable and automated operation. It provides a linear response with respect to the water content in the studied solvents and photothermal analyte spectra, which are in good agreement with FTIR absorbance spectra. The method is calibrated and validated against coulometric Karl Fischer (KF) titration, showing comparable performance and sensitivity. Limits of detection (LODs) for water detection in the single-digit ppm range were obtained for chloroform and jet fuel due to their low background absorption, whereas lower sensitivity has been observed for water detection in ethanol due to pronounced background absorption from the solvent. In contrast to KF titration, which requires toxic reagents and produces waste, the developed method works reagent-free. It can be applied in an online format in the chemical industry as well as for fuel quality control, being industrial applications where traces of water need to be accurately determined, preferably in real-time. It thus holds great promise as a green alternative to the offline KF titration method, which is the current standard method for this application.

Development of a Near-Infrared Photoacoustic System for Selective, Fast, and Fully Automatized Detection of Isotopically Labeled Ammonia.

Different environmental and industrial technologies seek for fast and automatic ammonia detection systems, capable of the selective measurement of the concentration of its isotopes at sub-ppm levels, without any interference with the common contaminants. In this work, we report the quasi-simultaneous measurement of 14NH3 and 15NH3 concentrations based on a near-infrared diode laser-based photoacoustic system. Using a widely tunable external cavity diode laser, four nearby wavelengths within the range of 1531.3–1531.8 nm were optimal circumstances for sensitive detection, while avoiding interference with water vapor. Subsequently, a more robust distributed feedback diode laser was employed to tune the laser wavelength on the sub-second timescale by varying its driving current rather than using much slower temperature tuning. The detection limit of our system is 0.15 and 0.73 ppm for 14NH3 and 15NH3 (with an accuracy below 0.1%), respectively, and the response time is 3.5 s.

High bandwidth frequency modulation of an external cavity diode laser using an intracavity lithium niobate electro-optic modulator as output coupler

We present a novel approach to high bandwidth laser frequency modulation. A lithium niobate chip is used as an intracavity electro-optic modulator in a tunable cateye external cavity diode laser. The modulator is conveniently integrated with the cateye output coupler, providing a unique approach to high bandwidth frequency stabilization and linewidth narrowing. The intracavity modulator feedback was successfully operated below 1 V and achieved superior frequency noise suppression compared to conventional feedback through diode injection current modulation. A closed loop bandwidth of 1.8 MHz was demonstrated, and the laser linewidth reduced to around 1 Hz as measured by the heterodyne measurement.

Tunable Chaotic External-Cavity Semiconductor Laser With Time-Delay Signature Suppression Including a Broadband Chirped FBG

A wavelength tunable chaotic external cavity semiconductor laser (ECSL) composed by a distributed-feedback laser diode (DFB-LD) and a broadband chirped fiber Bragg grating (CFBG), is proposed and experimentally demonstrated for generating tunable chaos signals with time delay signature (TDS) suppression. The experimental results show that by using a CFBG with a 3-dB reflection bandwidth of approximately 30 nm and a low dispersion of approximately 1.25 ps/nm, the chaotic ECSL can be tuned over a wide wavelength range, and the generated chaos possesses a low TDS of about 0.04. With the increase of the bias current of the DFB-LD, the chaotic ECSL is tuned towards long-wavelength direction, the effective bandwidth of the chaotic signal is extended, and the TDS of the chaotic signal remains effectively suppressed. To highlight the advantages of the proposed scheme, relevant experiments for the DFB-LD under fiber mirror feedback and fiber Bragg grating feedback are also performed and compared.

Temperature-Stabilized and Widely Tunable Vertical External Cavity Surface-Emitting Laser With a Simple Line Cavity

We designed and demonstrated a temperature-stable, wide-tuned, high-power Vertical External Cavity Surface-emitting Laser (VECSEL) with a simple linear cavity. The quantum well is optimized by using the commercial PICS3D software to obtain wide gain and good thermal performance. The curve of BF has also been optimized, and a birefringent filter (BF) with a thickness of 1mm is selected as the tuning element. Then, we obtained a continuously adjustable range cover from 1044.5 nm to 1092.1 nm, 8 W of output power, and temperature-stable VECSEL, which verifies the feasibility of the design. Based on the experimental results, we concluded that the gain intensity corresponding to the tuning range should be at least greater than 163 e <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">26</sup> /cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> eV <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> before the growth process ensures a wide tuning range. For the first time, to the best of our knowledge, we designed and obtained the tuning result of VECSEL in the linear cavity, and this is of great significance to the design of VECSEL with wide-tuning, high-power, and good temperature stability.

Simultaneous detection of CF4 and SIF4 using external cavity quantum cascade laser absorption spectroscopy

Direct and quantitative access to plasma etch processes (like the concentration of precursors or of molecular etch products) plays a key role for the industrial production of semiconductor devices. In this paper, a laser absorption spectroscopy sensor based on a pulsed, broadband tunable external cavity quantum cascade laser (ECQCL) was developed for potential plasma diagnostic analysis. To demonstrate this detection technique, the typical precursor molecule CF4 and the etch product SiF4 were selected as analytic targets. Composition attribution and concentration analysis for different CF4 and SIF4 mixture samples were experimentally investigated by selecting proper spectral window and self-established signal processing algorithm. By using a direct absorption spectroscopy detection scheme with a short path absorption cell of 29.6 cm, Allan deviation analysis indicates that detection limits of 9.39 ppm for CF4 and 46.66 ppm for SiF4 were obtained with a 1 s integration time, which can be further improved to ~ 1.63 ppm and ~ 7.28 ppm by averaging up to 120 s, respectively.