External Cavity Diode Laser Research Articles

Study on external cavity diode laser with a wide mode-hopping free tuning range

A wide mode-hopping free and narrow linewidth tunable laser diode source with a Littman-Metcalf arrangement based on a diffraction grating is presented. Several experiments are carried out to demonstrate the tuning characteristics and spectral linewidth of the proposed external-cavity diode laser source. A wide no mode-hopping continuous wavelength tuning range about 59.13 nm in ultra-C-band with a narrow spectral linewidth of less than 100 kHz is obtained. An optical signal to noise ratio of more than 65 dB and an output power more than 14.8 dBm over the whole tuning range can also be realized in a long-term free running condition. The proposed tunable laser allows simultaneously mode-hopping free and narrow linewidth tunable radiation, thus opening a door for practical application such as coherent detection.

Dark State Magnetometer Based on Enhanced Acousto-Optics Modulator

We report on detailed experimental setup and the results of an enhanced acousto-optics modulator (AOM) setup for observation of a dark state magnetometer. A Λ-type dark state based on D1 line of 87Rb:F=2→F′=1 Zeeman sub-levels with neon (Ne) buffer gas was created using a single laser and a scanning acousto-optics modulator. The technical challenges in using this method and how to overcome these difficulties are discussed, and we report on the observation of a dark state resonance with linewidth of 168 Hz and a detectable magnetic field of 9 nT. This method offers many advantages, including the creation of mutually coherent beams outside an external cavity diode laser (ECDL), where the beams are equally affected by external perturbations to the ECDL. Only factors related to the AOM dictates the difference between the two beams.

Intra-cavity frequency-doubled VECSEL system for narrow linewidth Rydberg EIT spectroscopy.

High-power, narrow-linewidth light sources in the visible and UV spectra are in growing demand, particularly as quantum information and sensing research proliferates. Vertical external-cavity surface-emitting lasers (VECSELs) with intra-cavity frequency conversion are emerging as an attractive platform to fill these needs. Using such a device, we demonstrate 3.5 MHz full-width half-maximum Rydberg-state spectroscopy via electromagnetically induced transparency (EIT). The laser's 690 mW of output power at a wavelength of 475 nm enables large Rabi frequencies and strong signal-to-noise ratio in shorter measurement times. In addition, we characterize the frequency stability of the VECSEL using the delayed self-heterodyne technique and direct comparison with a commercial external-cavity diode laser (ECDL). We measure the pre-doubled light's Lorentzian linewidth to be 2π × 5.3(2) kHz, and the total linewidth to be 2π × 23(2) kHz. These measurements provide evidence that intra-cavity frequency-doubled VECSELs can perform precision spectroscopy at and below the MHz level, and are a promising tool for contemporary, and future, quantum technologies.

Gallium-nitride-based interference-filter-stabilized external cavity diode laser with a surface-activated-bonded output coupler

We report on an interference-filter-stabilized external cavity diode laser (ECDL) using a gallium-nitride-based violet laser diode. Surface-activated-bonded glass substrates were employed as cat’s eye output couplers in order to suppress power degradation due to optical damage. From the results of a long-term frequency-stabilization test, mode-hop-free operation for about one week was demonstrated. After a continuous operation of about three weeks, the power degradation was less than 10%. The results show the potential of such interference-filter-stabilized ECDLs for practical and portable quantum technologies such as atomic clocks or atomic interferometers.

Absolute distance measurement using sinusoidal phase modulating frequency sweeping interferometry with a reference interferometer.

Frequency sweeping interferometry with reference interferometer based on sinusoidal phase modulating technique is proposed in this paper for absolute distance measurement. With the frequency of the external cavity diode laser (ECDL) swept continuously in sinusoidal, a HeNe laser was employed to monitor the drifts of the target and the reference length, and influences caused by drifts during the measurement were compensated in real time. Sinusoidal phase modulation with non-overlapping frequencies were applied to the two laser lights individually by two electro-optic modulators (EOM), and the interference phases corresponding to the two laser lights were extracted simultaneously using the phase generated carrier (PGC) demodulation based on frequency-division multiplex technique. Performance of the phase detection method has been verified by nanometer displacement measurements. Experimental results show that the measurement uncertainty can be considerably reduced by compensating the influences of drifts and by applying linear regression to get the ratio of interference phase changes between the measurement interferometer and the reference interferometer. Comparison of the absolute distance measurement with an incremental interferometer yields a measurement uncertainty of 10-5, which is in good agreement with the estimation of the measurement uncertainty.

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.

Measurement of the mercury (6s6p)P13 -state lifetime in the frequency domain from integrated absorbance data

Quantitative UV laser absorption spectroscopy is implemented to measure the strength of the $(6{s}^{2})\phantom{\rule{0.28em}{0ex}}^{1}S_{0}\ensuremath{\rightarrow}(6s6p)^{3}P_{1}$ intercombination transition of Hg atoms near the wavelength of 253.7 nm and to retrieve the lifetime of the excited state. Tunable, single-mode, coherent radiation in the deep UV is produced through a double-stage second-harmonic generation process, starting from the output radiation of an external-cavity diode laser at 1014.8 nm. This latter is referenced to an optical frequency comb synthesizer. The measurement strategy exploits the relationship between the strength of an optical transition and the spontaneous emission Einstein coefficient. The complete uncertainty budget leads to an overall relative uncertainty of 0.6%, with a statistical contribution of 0.28 ns, which is the lowest ever obtained for the $(6s6p)\phantom{\rule{0.28em}{0ex}}^{3}P_{1}$ lifetime. The new value, amounting to $125.12\ifmmode\pm\else\textpm\fi{}0.79\phantom{\rule{0.28em}{0ex}}\mathrm{ns}$, compares well with most of the past determinations within two times the quoted uncertainties. Despite this, it does not agree with the weighted average of the 12 prior values, thus suggesting that a critical reassessment of the uncertainty of some of the past measurements is probably necessary.

Simultaneous distance and vibration mapping of FMCW-LiDAR with akinetic external cavity diode laser

In this study, the simultaneous distance and vibration mapping of frequency-modulated continuous-wave light detection and ranging (FMCW-LiDAR) is performed using an electro-optical external cavity diode laser (ECDL). Owing to the akinetic operation regime of the ECDL, a uniform (R2 = 1.0000) interference signal can be generated over a bandwidth of 1 GHz at an optical frequency sweep rate of 10 kHz without any postprocessing or system for linearization. Furthermore, the proposed ECDL exhibits a high optical frequency sweep rate of 200 kHz with a considerable linearity of R2 = 0.99975. Based on a point sensing experiment using the FMCW-LiDAR system, three measurands, namely, distance, vibration frequency, and vibration velocity change, are demonstrated with a highly linear relationship between the beat frequency and amplitude of each individual parameter. Furthermore, temporal variations in multiple targets with rapidly frequency-varying vibrations, such as those occurring in music or voice sounds, can be successfully acquired by tracking the temporal change in the beat frequency of each target. Moreover, three-dimensional (3D) mapping of distance, vibration frequency, and vibration velocity change is simultaneously implemented in the same 3D space. The results show that the proposed FMCW-LiDAR system can be used for the 3D visualized display of various measurands within the same space frame.

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.

Frequency Stability Transfer in Passive Mode-Locked Quantum-Dash Laser Diode Using Optical Injection Locking

In this paper, we present an experimental study of the metrological stabilization of a solid-state frequency comb for embedded metrology applications. The comb is a passively mode-locked laser diode based on InGaAs/InP Quantum-dash structure emitting optical lines into a 9 nm bandwidth centered at 1.55 $\mu$m with a repetition rate of 10.09 GHz. The frequency stabilization is achieved by optical injection locking of the comb with an external cavity laser diode referenced onto a metrological frequency standard. One observes the transfer of the spectral purity from the injection laser to the neighbouring modes of the injected one as well as the transfer of stability to the adjacent modes. The measurement of the long term stability highlights a frequency noise with random walk behavior specific of the passive mode locking process. Demonstration of sidebands of the injection laser at the repetition frequency of the comb also makes it possible to propose a transfer mechanism and to consider a complete stabilization of the frequency comb at a metrological stability level.

Hyperfine structure and absolute frequency of 127I2 transitions at 514 nm for wavelength standards at 1542 nm

The hyperfine structures of the P(57)45-0, P(91)48-0, and R(73)46-0 lines of molecular iodine ( 127 I 2 ) at 514 nm were studied using third-harmonic generation of a 1542 nm external-cavity diode laser and Doppler-free spectroscopy. The frequencies of the 1542 nm diode laser locked to the hyperfine transitions of these iodine lines are close to those of the P(16) transition in the ν 1 + ν 3 band of acetylene ( 13 C 2 H 2 ), which is used as a wavelength standard for telecom applications. The absolute frequencies of the observed 59 iodine hyperfine transitions were determined with an uncertainty of 5.4 kHz (fractional uncertainty of 9.3 × 10 − 12 ). Highly accurate hyperfine constants were obtained through fitting of the measured hyperfine splittings to a four-term Hamiltonian that includes electric quadrupole, spin–rotation, tensor spin–spin, and scalar spin–spin interactions with an uncertainty of a few kHz. The observed hyperfine transitions provide wavelength standards for telecom applications with various optical frequencies and reduced uncertainties compared with the acetylene wavelength standard.

Advances in wide-tuning and narrow-linewidth external-cavity diode lasers

The external-cavity diode laser is advantageous in terms of low noise, high side-mode suppression ratio, high temperature stability, simple structure, and low cost, which has been the preferred scheme to realize wide-tuning and narrow-linewidth characteristics. Thus, it has been widely used in optical communication, lidar, environmental monitoring, spectral analysis, optical coherence tomography, and other frontier fields. Herein, we introduce the technical scheme and review the development status of wide-tuning and narrow-linewidth external-cavity diode lasers in detail. Primarily, the structure, working principles, and performance characteristics of external-cavity diode lasers are deeply analyzed according to different structures. The structural characteristics, key technologies, optical performance and application fields of state-of-the-art studies in recent years are discussed. Finally, the challenges and potential development prospects of wide-tuning and narrow-linewidth external-cavity diode lasers are analyzed and discussed.

Advances in wide-tuning and narrow-linewidth external-cavity diode lasers

Advances in wide-tuning and narrow-linewidth external-cavity diode lasers

Frequency noise stabilisation of a 1550 nm external cavity diode laser with hybrid feedback for next generation gravitational wave interferometry.

Longer wavelength lasers will be needed for future gravitational wave detectors that use cryogenic cooling of silicon based test-mass optics. Diode lasers with a 1550 nm wavelength output are potential seed light sources for such a detector, however diode laser devices have a different spectral profile and higher frequency noise than the solid state lasers used in current detectors. We present a frequency stabilisation system for a 1550 nm external cavity diode laser capable of reducing the laser frequency noise to a level of 0.1HzHz up to 1 kHz with a unity gain frequency of 535 kHz using a hybrid analogue-digital servo with in-loop cancellation of resonant features. In addition, a method of high speed digital filter optimisation and automated design is demonstrated.

Wideband current modulation of diode lasers for frequency stabilization.

We present a current modulation technique for diode laser systems, which is specifically designed for high-bandwidth laser frequency stabilization and wideband frequency modulation with a flat transfer function. It consists of a dedicated current source and an impedance matching circuit both placed close to the laser diode. The transfer behavior of the system is analyzed under realistic conditions employing an external cavity diode laser (ECDL) system. We achieve a phase lag smaller than 90° up to 25 MHz and a gain flatness of ±3 dB in the frequency range of DC to 100 MHz, while the passive stability of the laser system is not impaired. The potential of the current modulation scheme is demonstrated in an optical phase-locked loop between two ECDL systems with a phase noise of 42 mradrms. The design files are available as an open-source project.

A scalable laser system at 461 nm for laser cooling and trapping of Sr atoms

The realization of compact high-power narrow-linewidth laser sources in the visible range has been a long-standing interest for the atomic physics community, especially in the field of laser cooling and trapping of alkaline-earth and alkaline-earth-like atoms. We report a simple master-slave injection-locked laser system capable of generating 500 mW at 460.86 nm with a linewidth of 1.6 MHz to realize magneto-optical trapping of strontium atoms on the 5s^{2} ^{1}mathrm {S}_{0}-5s5p ^{1}mathrm {P}_{1} transition. The master laser used is an external cavity diode laser built with an anti-reflection coated laser diode in a Littrow configuration and the slave laser is built with a recently launched single mode high power GaN laser diode. We characterized the frequency noise, injection stability, spectral purity and relative intensity noise of the laser system. The reported laser system is used routinely to produce 10^7 strontium atoms at a temperature of 10 mK in a magneto-optical trap. The laser system offers a prominent alternative to the existing frequency-doubled laser systems in terms of cost, design flexibility, electrical power consumption, ease of scaling, intensity stability, and frequency tunability. The proposed system is also an effective solution for space-based experiments where a laser system’s size, weight, and electrical power consumption are crucial design parameters.

Spectrum-locked high-power volume Bragg grating external-cavity diode laser

Spectrum-locked high-power diode lasers are extremely sought after in many fields. Here, we establish the volume Bragg grating external-cavity diode laser model and investigate the impact of facet reflectivity on the output spectrum. The simulation results emphasize that optimizing reflectivity can realize better spectrum-locking and side-mode suppression ratio. To prove the feasibility, we built a diode laser array with 20 single emitters. Three different facet reflectivity are prepared, 0.1%, 0.5%, and 0.8%. As a result, the output spectrums of the array are improved obviously with the decrease in reflectivity. In the case of 0.1%, the side-mode is well suppressed under injection current up to 16 A, and the calculated current-spectrum shift is around 0.07 nm A−1. After beam combining and fiber coupling, an output power of 243.4 W and a brightness of about 11.2 MW cm−2-str is attained.

High-power external cavity diode laser with narrow linewidth emission and adjustable polarization state at 445 nm

A high-power external cavity diode laser (ECDL) system at 445 nm is proposed that has narrow-band linewidth emission and adjustable polarization state and can be used for nonlinear frequency conversion in deep ultraviolet spectrum. By introducing a half-wave plate to control the polarization state of the laser source, the first-order diffraction (R1st) efficiency is changed. The relationship between the linewidth and the first-order diffraction efficiency is discussed through simulation and experiment. In addition, the tunable ranges of the ECDL are measured. Finally, a continuous-wave output with the spectral width of 72 pm at 445 nm and a maximum optical output power of 1.52 W can be obtained when R1st is 25%, of which the tunable range has a minimum of 2.3 nm.

Emission Spectrum of an External Cavity Diode Laser under the Combined Action of Microwave and VHF Modulation of the Injection Current

Emission Spectrum of an External Cavity Diode Laser under the Combined Action of Microwave and VHF Modulation of the Injection Current

External-cavity laser diode using acousto-optic deflector as tunable grating.

In this study, a novel, to the best of our knowledge, external-cavity laser diode (ECLD) without a diffraction grating is proposed and demonstrated. In the proposed configuration, an acousto-optic deflector (AOD) acts not only as a deflector but also as a diffraction grating. Thus, the AOD functions as a wavelength-selective device, which helps improve the overall performance of the ECLD. In fact, the proposed configuration realizes a wide range of wavelength scanning with a simple configuration, highly efficient optical feedback, and a steady optical resonator with a constant cavity length. We confirm that the wavelengths scanned with the proposed ECLD agree well with theoretical calculations. The scanning range and maximum frequency response reached approximately 60 nm and 50 kHz, respectively. Moreover, reproducible measurements of the three-dimensional thickness distribution of a thin glass plate indicates that the proposed ECLD can be used for optical coherence tomography imaging systems.