Distributed Feedback Laser Diode Research Articles

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.

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

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

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.

Gain-switched pulse generation of 5.3 ps from 30 GHz-modulation-bandwidth 1270 nm DFB laser diode.

We generated gain-switched pulses via electrical pulse excitations in a 1270 nm distributed feedback (DFB) laser diode (LD) with a direct-modulation bandwidth of 30 GHz. The measurements revealed short-pulse widths of 5.3 and 8.8 ps with and without chirp compensation, via a single-mode optical fiber. The 5.3 ps pulses exhibited a spectral width of 0.40 nm (spectral bandwidth of 71 GHz), yielding a time-bandwidth product of 0.38. Although the gain-switched pulses in DFB LDs inherently contain linear and nonlinear chirp, optimized pumping conditions enable generation of nearly transform-limited ps pulses after linear chirp compensation.

2.3-µm single-frequency Tm: ZBLAN fiber amplifier with output power of 1.41 W.

We present here the first watt-level single-frequency thulium-doped ZBLAN fiber amplifier system operating at a wavelength of 2.3 µm. Continuous-wave output of up to 1.41 W was generated from a two-stage Tm: ZBLAN fiber amplifier with direct ground-state pumping at 793 nm. Seeded by a single-frequency distributed feedback diode laser at 2332 nm, the thulium-doped ZBLAN fiber amplifier emitted a laser with linewidth no more than 10 MHz at maximal output power. This study examines the impact of a 2.3-µm seed on the competitive laser transition of 2 µm. The findings indicate that direct pumping of a Tm fiber amplifier holds the potential for achieving higher power output within the 2.3-µm band.

Influence of sidewall grating etching depth on GaN-based distributed feedback laser diodes

Conventional sidewall gratings in GaN-based distributed feedback (DFB) laser diodes (LD) have a thick p-type layer, which may cause current spreading and carrier-induced anti-guiding effects, severely deteriorating the lasers performance. In this study, we report a novel fabrication technology to not only reduce the remaining p-type layer in the sidewall gratings but also realize close-coupled sidewall gratings. Afterwards, we further investigate the influence of the sidewall gratings etching depth on GaN-based DFB LDs. The results show an almost unchanged current injection efficiency, nearly coincided I–V curve and a near-field emission width for shallow etched structures, which indicate that the current spreading is neglectable in GaN-based ridge structure LDs. Based on this analysis, GaN-on-Si DFB LDs with an emission wavelength of 414 nm, full width at half maxima of 22 pm, and side mode suppression ratio of 19.1 dB were realized.

A Large-Scale Photonic CNN Based on Spike Coding and Temporal Integration

The real-time massive-data intelligent information processing tasks highlight a vital requirement for the novel, intelligent optimization hardware. Convolutional neural networks are highly capable of extracting the hierarchical feature map and enhancing recognition accuracy, with photonics-enabled implementations drawing considerable interest. Here, we propose a large-scale and reconfigurable photonic convolutional neural network (PCNN), based on a hardware-friendly distributed feedback laser diode (DFB-LD). Our approach applies biological time-to-first-spike coding to a DFB-LD neuron to perform a temporal convolutional operation (TCO) for image processing tasks. In PCNN, experimental results demonstrate that we successfully implement the TCO to extract the image features with convolutional kernels of size 11×11. Furthermore, we investigate the temporal pulse shaping of a DFB-LD neuron to build a densely-connected fully connected layer, which synaptic weights can be rapidly adjusted at a rate of 5 GHz, achieving full MNIST and Fashion-MNIST benchmark image classification tasks, with classification accuracies of 98.56% and 87.48%, respectively. This work highlights the potential of neuron-like optical analog computing platforms for real-time and more complex intelligent processing networks.

High‐pressure gas temperature sensing for exit plane of aero‐engine combustor using tunable diode laser absorption spectroscopy

AbstractWe have developed a sensor for the temperature measurement in the test section after the combustion chamber of an aero engine model by using direct absorption spectroscopy. Five distributed feedback diode lasers at near 1392, 1393, 1339, 1343, and 1469 nm are utilized as the light sources to detect H2O absorptions to improve the precision of temperature measurement combining with the Boltzmann plot method and the polynomial‐based spectral fitting model. The sensor shows a good performance in terms of its temperature measurement accuracy under high temperature of 673–1373 K and high pressure of 100–600 kPa condition in a tube furnace, which was lower than 4%. Moreover, three‐dimensional temperature distributions have been reconstructed via applying algebraic reconstruction technique at the test section after the combustion chamber of an aero engine model. The single‐point position temperature extracted from the algebraic reconstruction technique agrees well with that measured from the coherent anti‐stokes Raman scattering technique, where the relative error is within 5%. All the above results prove the reliability of our developed temperature sensor.

Cavity ring-down spectroscopy with a laser frequency stabilized and locked to a reference target gas absorption for drift-free accurate gas sensing measurements

A new gas sensor system with fast response and ultra-high sensitivity has been developed based on a combination of frequency modulation spectroscopy (FMS) and cavity ring-down spectroscopy (CRDS). The system consisted of two distributed feedback laser diodes (DFB-LDs) emitting at frequencies 6251.761 cm-1 (Laser-1) and 6257.762 cm-1 (Laser-2), respectively. A portion of Laser-1’s output was used by a frequency modulation spectroscopy technique to lock its frequency precisely at a CO2 absorption peak, while the rest of its output was coupled to an optical ring-down cavity, together with the Laser-2 output. The Laser-2 operated at a non-absorbing frequency for real-time correction of any baseline ring-down time drift caused by environmental changes (e.g., temperature, pressure). Laser frequency stabilization achieved a 5-fold improvement in CRDS detection sensitivity. This new system was able to make measurements at a data rate of 9 Hz. Based on Allan deviation analysis, the absorbance detection limit of the system was 4.4 × 10−11 cm-1 at an optimum averaging time of ∼5 s, whereas the time-normalized sensitivity at 1 s was 7.3 × 10−11 cm-1/Hz1/2. Measurements of atmospheric CO2 mole fraction were conducted and demonstrated its good performance and reliability. This sensor will be particularly suitable for making drift-free measurements over long periods, in the fields of environmental and industrial gas sensing.

Low-threshold all-optical nonlinear activation function based on injection locking in distributed feedback laser diodes.

We experimentally demonstrate an all-optical nonlinear activation unit based on the injection-locking effect of distributed feedback laser diodes (DFB-LDs). The nonlinear carrier dynamics in the unit generates a low-threshold nonlinear activation function with optimized operating conditions. The unit can operate at a low threshold of -15.86 dBm and a high speed of 1 GHz, making it competitive among existing optical nonlinear activation approaches. We apply the unit to a neural network task of solving the second-order ordinary differential equation. The fitting error is as low as 0.0034, verifying the feasibility of our optical nonlinear activation approach. Given that the large-scale fan-out of optical neural networks (ONNs) will significantly reduce the optical power in one channel, our low-threshold scheme is suitable for the development of high-throughput ONNs.

A fiber-solid hybrid 1319 nm single-frequency MOPA Laser for sodium Doppler lidar

The narrow linewidth nanosecond pulse 1319 nm laser used to generate 589 nm laser is the core technology of sodium Doppler lidar. Here, a high gain fiber-solid hybrid amplifier seeded by a 1319 nm distributed-feedback laser diode (DFB-LD) is proposed to achieve high-power amplification of small signal 1319 nm pulse laser. A pulsed 1319 nm laser with a repetition frequency of 500 Hz, a pulse duration of 500 ns, an energy of 10.3 mJ and a linewidth of 1.9 MHz was developed by utilizing a three-stage fiber coupled end-pumped amplifier and a two-stage Innoslab amplifier, and the beam quality factor M2 is less than 1.6, the gain of the solid amplification system exceeds 28 dB.

Hybrid integrated narrow-linewidth semiconductor lasers.

Integrated narrow-linewidth lasers are the key devices in compact coherent optical systems of metrology, sensing, and optical microwave generation. Here, we demonstrate a hybrid integrated laser based on an optical negative feedback scheme. The laser is composed of a commercial distributed feedback (DFB) laser diode and an on-chip micro-resonator with a Q-factor of 0.815 million. The feedback optical field is coupled back to the laser cavity through the back facet. Therefore, the laser can maintain the lasing efficiency of the DFB laser diode. The linewidth of the DFB laser diode is compressed from 2MHz to 6kHz, corresponding to the linewidth reduction factor of 25.2dB. The theoretical result shows that the laser performance still has a huge improvement margin through precise control of the detuning between laser frequency and the micro-resonator, as well as the phase delay of the feedback optical field. The hybrid narrow-linewidth laser diode has wide application prospects in coherent optical systems benefitting from the low cost and volume productivity.

Monolithically Integrated Tunable Three- Section All-Active DBR Laser Diodes With 30 nm Tuning Range

We present a monolithically integrated three-section all-active AlGaInAs/InP tunable distributed Bragg reflector (DBR) ridge waveguide laser diode that uses thermal tuning mechanism and mode-hopping tuning principle to achieve wide-range wavelength tuning. It is similar to the traditional three-section DBR laser structure, including a gain section, a phase section, and a DBR grating section. The difference is that the phase section and the DBR section incorporate the same active quantum well structure as the gain section. In this case, weak effective refractive index changes of the three active sections affected by the current injection will occur since the carrier density is clamped by the threshold gain condition of the DBR laser diode. Despite this limitation, a noncontinuous tuning up to ∼30 nm (1564–1594 nm) can be observed using only current injection at a constant temperature of 25 °C. Significant thermal effects occurred during the tuning process. In addition, a discontinuity is introduced in the tuning because it does not contain a passive region for phase adjustment. However, the device still provides a stable single longitudinal mode output with nine channels, a wavelength spacing of about 4 nm, and a side mode suppression ratio (SMSR) of more than 30 dB. Furthermore, the DBR laser diode can be easily fabricated utilizing the conventional distributed feedback laser diode fabrication process without secondary epitaxy and heater metal. The all-active DBR laser diode will be expected to be applied in the coarse wavelength division multiplexing (CWDM) systems with the advantage of low cost.

Two-Way 5G NR Fiber-Wireless Systems Using Single-Carrier Optical Modulation for Downstream and Phase Modulation Scheme for Upstream

Fifth-generation (5G) millimeter-wave (MMW)/sub-THz signals over a two-way fiber-wireless system using single-carrier optical modulation for downstream and phase modulation (PM) scheme for upstream are proposed and implemented for the first time. Compared to multi-carrier optical modulation, using single-carrier optical modulation in downlink is attractive because it can suppress RF power fading due to fiber dispersion and interference caused by optical beating from multiple carriers. Furthermore, compared to intensity modulation scheme, using PM scheme with remotely injection-locked distributed feedback laser diode in uplink is attractive because it enables high robustness to noise and distortion. Through 40-km single-mode fiber, 500-m free-space optical, and 2-m/1-m/0.5-m (downstream)/4-m (upstream) RF wireless cascaded-medium, 5G new radio (NR) MMW/sub-THz signals are transported with low bit error rates and clear constellations in downlink/uplink performance. Such a newly-built two-way 5G NR fiber-wireless system shows promise to perform 5G NR evolution in MMW and sub-THz frequencies, which has significant impacts on fiber-wireless integration.

Wavelength-modulated photoacoustic spectroscopy sensor for multi-gas measurement of acetone, methane, and water vapor based on a differential acoustic resonator

Precise assessment of breath acetone and methane is significant to the medical diagnosis process. A multi-gas sensing system based on wavelength-modulated differential photoacoustic spectroscopy was developed for simultaneous measurement of acetone and methane. A distributed feedback diode laser emitting in the range from 3363 to 3371 nm was employed to scan the absorption lines of acetone and methane. The cross sensitivities in terms of spectral interference among acetone, methane, and water vapor are effectively eliminated by using a linear combination method of reference spectra for accurately determining the concentration of acetone and methane. The positive effect of water vapor on photoacoustic signal resulting from the light absorption of acetone and methane was precisely evaluated. To improve the instrument performance, a differential PA cell companying with a differential amplifier circuit is experimentally demonstrated in providing a better performance of noise suppression compared with a single acoustic resonator. With a low detection limit down to 0.43 ppm and 12 ppb (integration time of 10 s) for acetone and methane, the sensor shows a great potential for medical diagnosis in simultaneous measurement of acetone and methane.

Wavelength-Tunable Narrow-Linewidth Laser Diode Based on Self-Injection Locking with a High-Q Lithium Niobate Microring Resonator.

We demonstrate a narrow linewidth 980 nm laser by self-injection locking of an electrically pumped distributed-feedback (DFB) laser diode to a high quality (Q) factor (>105) lithium niobate (LN) microring resonator. The lithium niobate microring resonator is fabricated by photolithography-assisted chemo-mechanical etching (PLACE) technique, and the Q factor of lithium niobate microring is measured as high as 6.91 × 105. The linewidth of the multimode 980 nm laser diode, which is ~2 nm measured from its output end, is narrowed down to 35 pm with a single-mode characteristic after coupling with the high-Q LN microring resonator. The output power of the narrow-linewidth microlaser is about 4.27 mW, and the wavelength tuning range reaches 2.57 nm. This work explores a hybrid integrated narrow linewidth 980 nm laser that has potential applications in high-efficient pump laser, optical tweezers, quantum information, as well as chip-based precision spectroscopy and metrology.