Distributed Feedback Laser Diode Research Articles

Detuning dependence in current-light-output characteristics of GaN-based DFB laser diodes

Abstract We have developed GaN-based laterally coupled distributed-feedback laser diodes (LC DFB-LDs). We focused on the detuning between the DFB-oscillation wavelength and the gain-peak wavelength and systematically investigated the differences in the temperature dependence of the current-light-output (I -L) characteristics. As the detuning increased, the rate of temperature change in the threshold current decreased. Furthermore, the I -L characteristics of the DFB-LD with a detuning of +5.3 nm were nearly overlapped within the measured temperature range. Interestingly, the estimated characteristic temperature was negative (−499.4 K). The temperature coefficient of the gain-peak-wavelength shift increased linearly as the detuning increased. On the other hand, the temperature coefficient of the DFB-oscillation-wavelength shift showed a very small dependence on detuning and was 17 pm K−1. These results provide useful insights for designing light sources with very small temperature dependence or light sources that exhibit the best performance at specific temperatures.

Dual-Gas Sensor Employing Wavelength-Stabilized Tunable Diode Laser Absorption Spectroscopy and H-Infinity Filtering Algorithm.

A compact dual-gas sensor based on the two near-infrared distributed feedback diode lasers and a multipass cell has been established for the simultaneous measurement of methane (CH4) and acetylene (C2H2). The time division multiplexing calibration-free direct absorption spectroscopy is used to eliminate the cross interference in the application of multicomponent gas sensors. A wavelength stabilization technique based on the proportion integration differentiation feedback control is developed to suppress laser wavelength drift and an H-infinity (H∞) filter algorithm to reduce the system noise. The results show that the detection sensitivity of CH4 and C2H2 reaches 39.9 parts per billion (ppb) and 47.3 ppb in the optimal integration time of 556 s and 312 s, respectively. In addition, the 31 consecutive hours measured results of CH4 in outdoor ambient air show that the proposed detection technology is very suitable for high-precision in-situ measurement of trace gases.

An economical tunable diode laser spectrometer for fast-response measurements of water vapor in the atmospheric boundary layer

Abstract. Water vapor in the atmospheric boundary layer poses a significant measurement challenge, with abundances varying by an order of magnitude over short spatial and temporal scales. Herein, we describe the design and characterization of an economical and flexible open-path, fast-response instrument for measurements of water vapor. The in situ method of tunable diode laser absorption spectroscopy in the shortwave infrared was chosen based on a heritage with previous instruments developed in our laboratory and flown on research aircraft. The instrument is constructed from readily available components and based on low-cost distributed feedback laser diodes that enjoy widespread use for high-speed fiber-optic telecommunications. A pair of versatile, high-speed Advanced RISC Machine-based microcontrollers drive the laser and acquire and store data. High precision and reproducibility are obtained by tight temperature regulation of the laser with a miniature commercial proportional-integral controller. The instrument is powered by two rechargeable 3.6 V lithium-ion batteries, consumes 2 W of power, weighs under 1 kg, and is constructed from hardware costing less than USD 3000. The new tunable diode laser spectrometer (TDLS) agreed to within 2 % compared to a laboratory standard and displayed a precision of 10 ppm at a sample rate of 10 Hz. The new instrument is robust and simple to use, allowing users with little previous experience in laser spectroscopy to acquire high-quality, fast-response observations of water vapor for a variety of applications. These include frequent horizontal and vertical profiling by uncrewed aerial vehicles (UAVs); long-term eddy covariance measurements from fixed and portable flux towers; and routine measurements of humidity from weather stations in remote locations such as the polar ice caps, mountains, and glaciers.

A Rapidly Tunable Laser System for Measurements of NH2 at 597 nm Behind Reflected Shock Waves.

Distributed feedback lasers, which feature rapid wavelength tunability, are not presently available in the yellow and orange spectral regions, impeding spectroscopic studies of short-lived species that absorb light in this range. To meet this need, a rapidly tunable laser system was constructed, characterized, and demonstrated for measurements of the NH2 radical at 597.4 nm. The system consisted of three main parts: (1) a distributed feedback diode laser operating at 1194.8 nm, (2) a fiber-coupled optical amplifier, and (3) a periodically poled lithium niobate (PPLN) waveguide for second-harmonic generation. A phase-matching optical frequency bandwidth of 118 GHz and a second-harmonic generation efficiency of 109%/W were determined for the PPLN waveguide, and the intensity and wavelength stability of the system were measured. The rapid-tuning capabilities of the laser system were characterized to explore its potential for use in scanned-direct absorption and wavelength modulation spectroscopy experiments. The feasibility of scanned-direct absorption up to a scan rate of 900 kHz and wavelength modulation spectroscopy at modulation frequencies up to 800 kHz were demonstrated. Finally, the system was deployed in a series of shock tube experiments in which the concentration of NH2 radicals was measured during the decomposition of NH3 behind reflected shock waves.

Frequency agile photonic integrated external cavity laser

Recent advances in the development of ultra-low loss silicon nitride integrated photonic circuits have heralded a new generation of integrated lasers capable of reaching fiber laser coherence. However, these devices are presently based on self-injection locking of distributed feedback laser diodes, increasing both the cost and requiring tuning of laser setpoints for their operation. In contrast, turn-key legacy laser systems use reflective semiconductor optical amplifiers (RSOAs). While this scheme has been utilized for integrated photonics-based lasers, so far, no cost-effective RSOA-based integrated lasers exist that are low noise and simultaneously feature fast, mode-hop-free, and linear frequency tuning as required for frequency modulated continuous wave (FMCW) LiDAR or for laser locking in frequency metrology. Here we overcome this challenge and demonstrate a RSOA-based, frequency agile integrated laser, that can be tuned with high speed, with high linearity at low power. This is achieved using monolithic integration of piezoelectrical actuators on ultra-low loss silicon nitride photonic integrated circuits in a Vernier filter-based laser scheme. The laser operates at 1550 nm, features a 6 mW output power and a 400 Hz intrinsic laser linewidth, and allows ultrafast wavelength switching within 7 ns rise time and 75 nW power consumption. In addition, we demonstrate the suitability for FMCW LiDAR by showing laser frequency tuning over 1.5 GHz at 100 kHz triangular chirp rate with a nonlinearity of 0.25% after linearization and use the source for measuring a target scene 10 m away with a 8.5 cm distance resolution.

Diode Laser Absorption Sensor for Ammonia Detection in a Shock Tube

With the improved availability and performance of low-cost near-infrared diode lasers, quantitative assessment of ammonia (NH3) as a carbon-free fuel has been made more accessible and affordable in both laboratory research and field applications. In the present study, an absorption sensor for NH3 measurement was developed by exploiting the affordability and flexibility of a near-infrared fiber-coupled distributed feedback diode laser. The first objective of the present study was to address the incomplete spectroscopic data for the absorption feature near 2.2 μm through the determination of line strengths and broadening coefficients with collisional partners including NH3, O2, N2, and Ar. Line shape characterization in both an absorption cell and a shock tube behind reflected shock waves was conducted to determine the temperature exponents of these broadening coefficients. The other objective was to demonstrate the capability of the sensor through NH3 detection in multicomponent mixtures and NH3 time-history measurements in the shock tube for investigating NH3 chemical kinetics at engine and gas-turbine-relevant conditions. The NH3 sensor developed in this study could serve as a cost-effective diagnostic tool for chemical kinetic measurements, combustion monitoring, and other applications where flexibility and robustness are prioritized.

Nonlinear dynamics of hybrid integrated laser based on self-seeding of a DFB-LD by using a Si3N4 microring resonator as filter feedback

In this work, we experimentally investigate the nonlinear dynamics of a hybrid integrated laser (HIL) composed of a distributed feedback laser diode (DFB-LD) and a tunable silicon-nitride photonic chip-based microring resonator (MRR), where the drop-port of MRR is connected to a Sagnac loop via a variable optical coupler (OC). Through varying the voltages loaded on the MRR (VMRR) and the variable OC (VOC), the central frequency and feedback coefficient of filter feedback can be adjusted, respectively. Via observing the optical power (time series), RF spectra, optical spectra of the HIL output, and calculating the Lyapunov exponent of the time series, diverse dynamical states including period one (P1), period two (P2), period four (P4), quasi-period (QP), chaos (CO) and self-injection locking (LO) can be distinguished. For different combinations of VMRR and VOC, the evolution routes for the nonlinear dynamical states of the HIL with the bias current of the DFB-LD (IDFB) are inspected and analyzed. By mapping the dynamical states in the parameter space of IDFB and VOC, the parameter regions for the HIL behaving different dynamical states are determined.

Submicron Embedded Air/GaN Diffraction Gratings for Photonic Applications

AbstractThe integration of photonic elements with nitride optoelectronic structures allows control of emitted light properties, which is advantageous for achieving, e.g., a single wavelength lasing. Positioning of the photonic structures on the top surface of GaN‐based devices is problematic, in particular, for deposition of a metal contact to p‐type top layer. In this work, custom‐shaped submicron air channels arranged periodically 150 nm below the sample surface, forming an air/GaN diffraction grating embedded within a volume of the structure is proposed and fabricated. The fabrication process includes selective area Si ion implantation, GaN regrowth using plasma‐assisted molecular beam epitaxy, ultra‐high‐pressure annealing for efficient electrical activation of implanted Si without diffusion, and electrochemical etching for the removal of selectively doped material. Embedded air/GaN diffraction gratings with periodicity of 460 and 631 nm are shown. Width of air channels ranges from 46 to 320 nm. Angle and polarization resolved reflectivity measurements combined with theoretical modeling confirm the designed optical performance of the embedded diffraction gratings in the GaN volume. The presented design and fabrication of custom‐shaped, fully integrated photonic structures buried below the surface paves the way for novel type constructions of optoelectronic devices, such as compact distributed feedback laser diodes

Strong Laser Emission Modulation by Coherent Perfect Absorption Inside Complex-Coupled Distributed Feedback Laser Diodes.

The proof-of-concept of the exploitation of Coherent Perfect Absorption (CPA) in electrically-injected distributed-feedback laser sources is reported. Capitalizing on the essence of CPA as "light extinction by light", an integrated laser-modulator scheme emerges. The key ingredient compared to conventional single-frequency laser diodes is a careful periodic in-phase modulation of both real and imaginary parts of the complex grating index profile that enables both single-frequency operation and 40dB line purity at the Bragg frequency. It is shown that this combination is most apt for the operation of CPA as a modulation mechanism that respects the laser spectral purity. The specific proof-of-concept is based on an ultra-short external cavity formed by a metallic micro-mirror, whose role is to generate the second beam of more conventional CPA interferometric approaches. The implemented complex-coupled grating is compatible with existing industrial technologies and promising for real-life laser source applications. Furthermore, the concept can be directly transferred to other material platforms and other wavelengths ranging from terahertz to ultraviolet.

High-power 940 nm DFB laser diode with large optical cavity.

High-power laser diodes with a stable wavelength and narrow linewidth are crucial for many applications. In this study, we introduce a first-order distributed feedback (DFB) grating into an asymmetric large-cavity laser diode operating around 940 nm. This design maintains high output power and offers a wide temperature locking range. The nearly sinusoidal shape first-order grating is fabricated by ultraviolet (UV) nanoimprint lithography, inductively coupled plasma (ICP) dry etching, and wet polishing. At a heat sink temperature of 25°C, the DFB laser diode, with a 200 µm stripe width and 4 mm cavity length, achieves a maximum output power of 24.8 W and a full width at half maximum (FWHM) of 0.4 nm under continuous-wave (CW) conditions. The maximum slope efficiency is calculated to be 1.04 W/A. At an output power of 10.7 W, the device reaches a peak wall-plug efficiency of 56%. Under quasi-continuous operation at 20 A, the laser output spectrum remains locked to the DFB grating over a temperature range from -10°C to 110°C, with a temperature coefficient of 0.062 nm/°C.

The Uncertainty Evaluation of Determining the Trace H2S in SF6/N2 Insulating Gas Mixture by Cavity Ring-Down Spectroscopy

Abstract H2S is one of the most important characteristic components of sulfur hexafluoride gas insulated electrical equipment. The detection of trace H2S is of great significance for early fault diagnosis of such equipment. Based on the 1578 nm wavelength distributed feedback diode laser (DFB-DL), cavity ring-down spectroscopy (CRDS) can effectively measure the concentration of trace H2S in SF6/N2 gas mixtures. Starting from the principle of methodology, this article establishes a mathematical model for evaluating the measurement uncertainty of CRDS and analyzes its sources of uncertainty: gas constant R1 , temperature T, Avogadro constant NA , gas pressure p in the optical cavity, volume V of the optical cavity, speed c of light, the absorption cross-section parameter σ(ν) of H2S molecules in the laser frequency ν, the ring-down time τ with absorption sample H2S and the ring-down time τ0 without absorption sample. After evaluating the uncertainty components, it was found that the main component was σ(ν) and the secondary components were τ and τ0. The H2S content in the 80% SF6-20% N2 mixed gas measured is (7.64 ± 0.12) × 10−6 mol/mol.

Blue GaN-based DFB laser diode with sub-MHz linewidth.

Distributed feedback laser diodes (DFBs) serve as simple, compact, narrow-band light sources supporting a wide range of photonic applications. Typical linewidths are on the order of sub-MHz for free-running III-V DFBs at infrared wavelengths, but linewidths of short-wavelength GaN-based DFBs are considerably worse or unreported. Here, we present a free-running InGaN DFB operating at 443 nm with an intrinsic linewidth of 685 kHz at a continuous wave output power of 40 mW. This performance is achieved using a first-order embedded hydrogen silsesquioxane (HSQ) surface grating. The frequency noise is measured using a cross-correlated self-heterodyne frequency discriminator, and two estimations of integrated linewidth are evaluated using 1/π integration and β-separation line integration methods.

High-power distributed feedback laser diode arrays with narrow spectral width over a wide temperature range

High-power semiconductor lasers with stabilized wavelengths are recognized as exemplary pumping sources for solid-state lasers. This study introduces distributed feedback (DFB) laser diode arrays designed to maintain an extensive temperature locking range. We report experimentally on high-power 808 nm DFB laser diode arrays. The first-order sinusoidal grating was fabricated using nanoimprint lithography, succeeded by inductively coupled plasma (ICP) dry etching and subsequent wet polishing. These 808 nm DFB laser diode arrays have demonstrated a measured output power of 134 W under a pulsed current of 150 A, with the heat sink temperature maintained at 25°C. The slope efficiency was determined to be 1.1 W/A. At a current of 150 A, the laser operated with a narrow spectral width over a wide temperature range, extending from −30 to 90°C, with a temperature drift coefficient of 0.0595 nm/K.

Fiber Bragg grating sensing system for temperature measurements based on optically injected DFB-LD with an OEO loop

A novel fiber Bragg grating (FBG) sensing system, based on an optically injected distributed feedback laser diode (DFB-LD) with an optoelectronic oscillating (OEO) loop, is proposed and experimentally demonstrated for temperature measurements with high and tunable sensitivity. The FBG sensor device works as an edge filter to adjust the optical power of the injected beam in response to temperature variations. The optically injected DFB-LD works at Period-one (P1) oscillating state, and the central wavelength of the oscillating mode of the DFB-LD can be tuned by the variable power of the injected beam. Furthermore, an OEO loop is implemented to improve the signal quality of the generated P1 microwave signal. Hence, the sensing parameter of temperature is converted to the frequency variation of the generated P1 microwave signal in the proposed sensing system. In the proof-of-concept experiment, a series of P1 microwave signals are generated while different temperatures are applied to the FBG sensor. The sensitivity of the proposed FBG sensing system for temperature measurements can be tuned from 0.44322 GHz/°C to 1.25952 GHz/°C. The stability and repeatability experiments are also performed, demonstrating the high measurement accuracy (0.0629°C) and low error of the system. The proposed FBG-based sensing and interrogation system exhibits high sensitivity, large tunability, good linearity, and flexible sensing generality.

Demonstration of a violet-distributed feedback laser with fairly small temperature dependence in current-light characteristics

We have developed a GaN-based distributed feedback laser diode (DFB-LD) with the detuning of +5 nm to obtain a smaller temperature dependence of the threshold current. We found that the current-light characteristics almost overlapped up to 300 mW between 25 °C and 80 °C. The estimated characteristic temperature is about 2550 K. These indicate that our DFB-LD is promising for applications that require small temperature dependence in the output power and oscillation wavelength at constant operation current without precise temperature control.

Photonic-assisted wideband microwave frequency measurement based on optical heterodyne detection

A photonic-assisted microwave frequency measurement (MFM) method based on optical heterodyne detection is proposed and experimentally demonstrated. In the proposed MFM system, a linearly chirped optical waveform (LCOW) from a three-electrode distributed Bragg reflector laser diode (DBR-LD) and a multi-wavelength signal from a Mach-Zehnder modulator (MZM), where the signal under test (SUT) is modulated on an optical carrier from a distributed feedback laser diode (DFB-LD), are heterodyne detected by the photodetector (PD). A bandpass filter then filters the detected signal, and the envelope is detected by an oscilloscope. Then, frequency-to-time mapping is realized, and the signal frequency is measured. Thanks to the fast tuning rate and large tuning range of the DBR-LD, the proposed MFM system has a high measurement speed and a broad instantaneous measurement bandwidth. In the experimental demonstration, a measurement error below 39.1 MHz is achieved at an instantaneous bandwidth of 20 GHz and a measurement speed of 1.12 GHz/µs. The MFM of a frequency-hopping signal is also experimentally demonstrated. The successful demonstration of the MFM system with a simple structure provides a new optical solution for realizing broadband and fast microwave frequency measurements.

O-band single-photon quantum key distributor master-to-slave injection-locked DFBLD pair

In comparison with the rapidly developing progress on the optical C-band (1525-1565 nm) master-to-slave injection-locked transmitter to perform the coherent single-photon quantum key distribution (QKD), the development on the O-band (1250-1350 nm) QKD transmitter is somewhat delayed as the commercially available wavelength-matched narrow-linewidth distributed feedback laser diode (DFBLD) pair is hardly accessible up to now. By using the DFBLDs with only sub-MHz linewidth and relatively deviated wavelength for the first time, this work demonstrates the optically differential-phase-shift-keying (DPS) QKD by an O-band master-to-slave injection-locked DFBLD pair. The master and slave DFBLDs with wavelength fluctuations of ±0.05% and ±0.2 pm are controlled by a thermo-electric cooler with a feedback gain of 100. The 1-bit delay interferometer (DI) under thermo-insulation maintains its visibility at >96% with dPo/Po and dPo/dt measured below ±0.1% and ±1 × 10−3 mW/s. By RZ-OOK modulating the master DFBLD with step-like power coding at 150 µW to induce π phase shift in the injection-locked slave DFBLD, the rising-/falling-edge DPS envelope distortion of the slave DFBLD diminishes by decreasing the bias current of the master DFBLD from 7Ith (35 mA) to 2Ith (10 mA). This phenomenon enables the 128-bit DPS-QKD transmission with a quantum bit-error rate (QBER) of 3.57% and a secure key rate of 3.524 kbit/s in the 6-km SMF link. The O-band injection-locked single-photon DPS-QKD bit-stream with a mean photon number of 0.2 #/bit minimizes its decoding QBER to 3.88% and 4.84% for 512-bit and 1024-bit, respectively.

Triple-range adjustable plane-concave multipass gas cell for high-precision trace gas detection based on TDLAS

A triple-range adjustable plane-concave multipass gas cell (PC-MPGC) was developed for high-precision trace gas detection based on tunable diode laser absorption spectroscopy. Effective optical path lengths (EOPLs) of 64.5, 42.5, and 20.5 m could be easily achieved by adjusting the incident beam angle. The trace gas sensor system employed a 1.653 µm distributed feedback diode laser to target a strong methane (CH4) absorption line, and direct absorption spectroscopy (DAS) was used to evaluate the performance of the system with a 64.5 m EOPL. A Savitzky–Golay filtering algorithm was employed to denoise the measured signal, and a third harmonic signal was used for laser frequency locking. A signal-to-noise ratio (SNR) of 191 was achieved, which corresponds to a detection sensitivity of 0.78 ppmv. The system response time was obtained through measurement of CH4 at concentration of 0 ppmv and 20 ppmv at 298 K and 1 atm, and it was found to be 11 s. Kalman filtering was applied to improve the detection sensitivity of the gas sensor. The stability of the system was analyzed using Allan deviation, and a minimum detectable limit of 21 ppbv was obtained for an integration time of 580 s. In situ continuous monitoring of atmospheric CH4 characteristics was performed for 72-h. The development of a new type of PC-MPGC paves the way for the detection of multicomponent gases at different absorption path lengths.

Optical feedback noise-immune cavity-enhanced optical heterodyne molecular spectrometry for sub-Doppler-broadened detection of C2H2.

A novel, to the best of our knowledge, noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) has been developed, utilizing optical feedback for laser-to-cavity locking with a common distributed-feedback diode laser. The system incorporates active control of the feedback phase and feedforward control of the laser current, allowing for consecutive laser frequency detuning by scanning a piezoelectric transducer (PZT) attached to the cavity. To enhance the fidelity of the spectroscopic signal, wavelength-modulated (wm) NICE-OHMS is implemented. Benefiting from the optical feedback, a modulation frequency of 15 kHz is achieved, surpassing the frequencies typically used in traditional NICE-OHMS setups. Then, the sub-Doppler-broadened wm-NICE-OHMS signal of acetylene at 1.53 µm is observed. A seven-fold improvement in signal to noise ratio has been demonstrated compared to NICE-OHMS alone and a limit of detection of 6.1 × 10-10cm-1 is achieved.

High-speed parallel processing with photonic feedforward reservoir computing.

High-speed photonic reservoir computing (RC) has garnered significant interest in neuromorphic computing. However, existing reservoir layer (RL) architectures mostly rely on time-delayed feedback loops and use analog-to-digital converters for offline digital processing in the implementation of the readout layer, posing inherent limitations on their speed and capabilities. In this paper, we propose a non-feedback method that utilizes the pulse broadening effect induced by optical dispersion to implement a RL. By combining the multiplication of the modulator with the summation of the pulse temporal integration of the distributed feedback-laser diode, we successfully achieve the linear regression operation of the optoelectronic analog readout layer. Our proposed fully-analog feed-forward photonic RC (FF-PhRC) system is experimentally demonstrated to be effective in chaotic signal prediction, spoken digit recognition, and MNIST classification. Additionally, using wavelength-division multiplexing, our system manages to complete parallel tasks and improve processing capability up to 10 GHz per wavelength. The present work highlights the potential of FF-PhRC as a high-performance, high-speed computing tool for real-time neuromorphic computing.