Distributed Feedback Gratings Research Articles

Monolithic top emitting room temperature THz DFG QCL with index grating for wavelength selection

We present a high performance top emitting room temperature monolithic terahertz (THz) quantum cascade laser based on difference frequency generation (DFG) and a buried modulated distributed feedback grating. Using a semi-insulating InP:Fe substrate combined with an index-coupled grating could minimize the internal losses for the THz mode. The laser consists of two separate transversely superimposed gratings. The first grating provides selective feedback for the mid-infrared pump modes and is realized in an index-coupled DFG grating designed for single-mode operation for two distinct MIR wavelengths. The second metallic grating for THz extraction is carefully aligned to the spatial distribution of the MIR modes. The resulting device lases pulses up to 150 °C in the single mode. The maximum THz output power at RT is 100 μW measured at 3.37 THz. A THz wavelength tuning of around 0.065 nm/K was achieved through thermal tuning of the MIR pump lasers.

Design of 16-wavelength high-power heterogeneous III-V/Si laser arrays with varying stripe width and grating period

Multi-wavelength laser array is currently one of the most attractive technologies to provide high density links for advanced optical communications and computing. The conventional scheme of varying grating period is almost impossible to achieve 8-wavelength or more. And, the optical output power of heterogeneously integrated quantum dot laser can hardly meet the commercial demand. In this paper, we report a 16-wavelength high-power heterogeneously integrated quantum dot distributed feedback laser array. To the best of our knowledge, this is the first heterogeneously integrated quantum dot laser array that tunes the lasing wavelength by combining the scheme of varying the stripe width and grating period of laser units. The effective refractive index of gratings is calculated by eigenmode expansion method. When the grating period is fixed at 198 nm, and the stripe widths of laser units are 2.00 μm, 2.25 μm, 2.60 μm and 3.15 μm, respectively, the corresponding lasing wavelengths are 1307.94 nm, 1308.49 nm, 1309.06 nm and 1309.63 nm, respectively, and the wavelength spacing of the adjacent laser units is about 100 GHz. By additionally adjusting the grating period between 196 nm and 202 nm with 2 nm spacing of grating period, a 16-wavelength QD DFB laser array is designed with the wavelength range from 1294.73 nm to 1336.09 nm. In addition, asymmetric distributed feedback grating is adopted to improve the optical output power at the front-end of laser arrays. The maximum ratio of the output power at the both end of laser units is 19.6 when the grating duty cycles of the front-end and the back-end of the λ/4 phase-shift is 0.7 and 0.5, respectively, and the grating lengths is 250 μm and 450 μm, respectively. This work will promote the performance improvement of heterogeneously integrated quantum dot lasers and develop the quantum dot lasers towards multi-wavelength.

Millimeter-wave generation with a room-temperature nonlinear quantum cascade laser

Millimeter-wave difference frequency generation is reported for a dual-wavelength mid-infrared quantum cascade laser operating at room temperature. To overcome a low mid-infrared-to-terahertz conversion efficiency below 1 THz, a long-wavelength, high-performance mid-infrared quantum cascade laser structure with higher nonlinear susceptivity is adopted. By designing the efficient allocation of mid-infrared pumps to two sections of fabricated distributed feedback grating, a closely separated dual-wavelength (λ1 ∼ 13.53 μm and λ2 ∼ 13.39 μm) laser oscillation was obtained. Consequently, a millimeter-wave emission at a frequency of 231 GHz was successfully observed at room temperature.

Phase-locked single-mode terahertz quantum cascade lasers array

We demonstrated a scheme of phase-locked terahertz quantum cascade lasers (THz QCLs) array, with a single-mode pulse power of 108 mW at 13 K. The device utilizes a Talbot cavity to achieve phase locking among five ridge lasers with first-order buried distributed feedback (DFB) grating, resulting in nearly five times amplification of the single-mode power. Due to the optimum length of Talbot cavity depends on wavelength, the combination of Talbot cavity with the DFB grating leads to better power amplification than the combination with multimode Fabry−Perot (F−P) cavities. The Talbot cavity facet reflects light back to the ridge array direction and achieves self-imaging in the array, enabling phase-locked operation of ridges. We set the spacing between adjacent elements to be 220 μm, much larger than the free-space wavelength, ensuring the operation of the fundamental supermode throughout the laser's dynamic range and obtaining a high-brightness far-field distribution. This scheme provides a new approach for enhancing the single-mode power of THz QCLs.

High continuous-wave power surface emitting terahertz lasers integrated with multimode interferometer

Distributed feedback (DFB) surface emitting semiconductor laser realizes power extraction by periodic photonic heterostructure. A direct method to increase the output power is scaling up the area of active region, however, the length of the device is restricted by the coupling strength of the DFB grating. Herein we develop a high-power single-mode surface emitting scheme by monolithically integrating second-order DFB terahertz quantum cascade laser and a multimode interferometer, which can significantly enhance the optical power by increasing photon density and altering field distribution in the grating region. Application of this concept leads to high continuous-wave (CW) surface emitting power of 55 mW, with single-lobed far-field pattern and single-mode operation at about 3.9 THz, which is among the best reported results. Such an integrated scheme can be combined with any one-dimensional photonic heterostructure, which has great potential in further improving the pulsed and CW output power of single-mode semiconductor lasers.

Design of enlarged-asymmetric-power single-mode silicon evanescent lasers with asymmetric distributed feedback gratings

Great achievements in the bonded III–V lasers have pushed the productization process of silicon photonic chips. In this work, we report the design of silicon evanescent lasers with asymmetric gratings, which can provide high output power and stable single-longitudinal-mode operation. The high coupling efficiency and low threshold current density are achieved by linearly expanding the width of rib waveguide from 0.4 µm to 2 µm through 150 µm taper coupler. By optimizing the λ/4 phase-shift position ratio from conventional 0.50 to 0.64, the ratio of output power at both sides of the bonded lasers (P2/P1) is improved from1.0 to 5.0, and the normalized grating coupling coefficient (κL) remains at 2.78. By optimizing the duty cycle at one side of λ/4 phase-shift from 0.5 to 0.8 and another side remains at 0.5, P2/P1 is 3.3, and κL lowers from 2.78 to 2.21. Besides, keeping the phase-shift position ratio at 0.64 while changing the duty cycle to 0.7, P2/P1 increases significantly to 7.5 with κL of 2.59. This work provides a step forward towards the development of high-power silicon-based light sources.

Dual optofluidic distributed feedback dye lasers for multiplexed biosensing applications

Integrated optofluidic devices have become subjects of high interest for rapid biosensor devices due to their unique ability to combine the fluidic processing of small volumes of microfluidics with the analysis capabilities of photonic structures. By integrating dynamically reconfigurable optofluidic lasers on-chip, complex coupling can be eliminated while further increasing the capabilities of sensors to detect an increasing number of target biomarkers. Here, we report a polydimethylsiloxane-based device with two on-chip fluidic distributed feedback (DFB) laser cavities that are integrated with an orthogonal analyte channel for multiplexed fluorescence excitation. One DFB grating is filled with 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran dissolved in dimethyl sulfoxide. The second grating is filled with rhodamine 6G dissolved in a diluted ethylene glycol solution. We present characterization of both lasers through analysis of the lasing spectra for spectral narrowing along with a power series to observe threshold behavior. We then demonstrate simultaneous detection of two different fluorescent microbeads as a proof of concept for scalable, single biomarker analysis using on-chip optofluidic lasers.

Dual-Color Lasers in Interlayer-Free Solution Processed Polymeric Bilayer Devices.

Multiwavelength organic lasers have attracted considerable interest in recent years due to the cost efficiency, wide luminescence coverage, and simple processability of organics. In this work, by simply spin coating immiscible polymeric gain media in sequence, dual-wavelength (blue-green or blue-red) amplified spontaneous emission (ASE) was achieved in bilayer devices. The blue emission, water/alcohol-soluble conjugated polyelectrolyte, poly[(9,9-bis(3'-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]dibromide (PFN-Br), was used as the bottom layer. The commercially available nonpolar solvent soluble polymer poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) and its blend with poly(3-hexylthiophene) (P3HT) were used as the top active layers offering green and red emission, respectively. This novel compact configuration, without interlayers between the two active layers, offers potential for developing various applications. The carefully selected top and bottom layer polymers not only meet the conditions of immiscibility and different emission wavelength range but also have a common absorption band in UV, which allows simultaneous blue-green or blue-red dual-color ASE behaviors observed in the bilayer devices under the same 390 nm laser excitation. By introducing two-dimension (2D) square distributed feedback (DFB) gratings with different periods (300 nm for blue, 330 nm for green, and 390 nm for red) as cavities, single mode blue-green (Eth = 245 μJ cm-2) and blue-red (Eth = 189 μJ cm-2) lasers were achieved by focusing the excitation laser spot on different 2D DFB gratings area. Furthermore, we found it possible to gain sufficient light confinement for red emission along its diagonal direction (Λ ∼424 nm), whereas the 2D DFB gratings offer feedback for blue emission from the 300 nm period along the rectangle direction. Therefore, both blue and red lasers were eventually achieved in the same PFN-Br/F8BT:P3HT bilayer device on the single 2D DFB gratings with a period of 300 nm in this work.

High Q‐Factor and Low Threshold Continuous‐Wave Near‐Infrared Lasing with Quasi‐2D Perovskites

AbstractQuasi‐2D perovskites have shown great potential in achieving solution‐processed electrically pumped laser diodes due to their multiple‐quantum‐well structure, which induces a carrier cascade process that can significantly enhance population inversion. However, continuous‐wave (CW) optically pumped lasing has yet to be achieved with near‐infrared (NIR) quasi‐2D perovskites due to the challenges in obtaining high‐quality quasi‐2D films with suitable phase distribution and morphology. This study regulates the crystallization of a NIR quasi‐2D perovskite ((NMA)2FAn−1PbnI3n+1) using an 18‐crown‐6 additive, resulting in a compact and smooth film with a largely improved carrier cascade efficiency. The amplified spontaneous emission threshold of the film is reduced from 47.2 to 35.9 µJ cm−2. Furthermore, by combining the film with a high‐quality distributed feedback grating, this study successfully realizes a CW NIR laser of 809 nm at 110 K, with a high Q‐factor of 4794 and a low threshold of 911.6 W cm−2. These findings provide an important foundation for achieving electrically pumped laser diodes based on the unique quasi‐2D perovskites.

Multimode Lasing in All-Solution-Processed UV-Nanoimprinted Distributed Feedback MAPbI3 Perovskite Waveguides

In this work, we present an all-solution fabrication approach for external second-order 1D distributed feedback (DFB) gratings using soft UV-nanoimprint lithography (UV-NIL) above archetypical methylammonium lead iodide (MAPbI3) perovskite films. This high-throughput method can be carried out in an ambient environment and requires only slightly elevated temperatures as low as 70 °C, gentle imprint pressure, and the use of compatible UV-NIL resin. Under stripe-shaped optical excitation, we observe simultaneously occurring optical phenomena in our high-gain strong-scattering perovskite films, namely amplified spontaneous emission, random lasing, and 1D DFB lasing. In pursuit of distinguishing these mechanisms, we explore far-field emission patterns and output polarization. Additionally, the DFB lasing is hardly attenuated when a thin absorbing indium tin oxide (ITO) film, commonly used as an electrode in fully contacted electrical devices, is inserted between the perovskite film and the DFB grating. As a result, we reproducibly achieve single and multimode, low-threshold (below 100 μJ·cm–2), narrow linewidth (below 0.2 nm), and strongly polarized (extinction ratio above 50) optically pumped DFB lasing for MAPbI3 waveguides with and without an adjacent ITO layer. We believe that the proposed resonator integration approach can be extended toward complete electrically active devices, enabling an alternative integration scheme to achieve current-injection lasing.

Wavelength-tunable organic semiconductor lasers based on elastic distributed feedback gratings

Wavelength-tunable organic semiconductor lasers based on mechanically stretchable polydimethylsiloxane (PDMS) gratings were developed. The intrinsic stretchability of PDMS was explored to modulate the period of the distributed feedback gratings for fine tuning the lasing wavelength. Notably, elastic lasers based on three typical light-emitting molecules show comparable lasing threshold values analogous to rigid devices and a continuous wavelength tunability of about 10 nm by mechanical stretching. In addition, the stretchability provides a simple solution for dynamically tuning the lasing wavelength in a spectral range that is challenging to achieve for inorganic counterparts. Our work has provided a simple and efficient method of fabricating tunable organic lasers that depend on stretchable distributed feedback gratings, demonstrating a significant step in the advancement of flexible organic optoelectronic devices.

Optically Excited Lasing in a Cavity‐Based, High‐Current‐Density Quantum Dot Electroluminescent Device (Adv. Mater. 9/2023)

Optically Excited Lasing In article number 2206613, Namyoung Ahn, Victor I. Klimov, and co-workers demonstrate optically excited lasing in a fully functional colloidal quantum dot electroluminescent device with an integrated optical resonator prepared as a distributed feedback grating, representing an important milestone toward solution-processable laser diodes. The colloidal quantum dots are cast from solution onto a grating and excited either electrically or optically to produce multicolor output. Image credit: Albin Guyot.

Room temperature continuous-wave operation of a dual-wavelength quantum cascade laser

We report on the design and fabrication of a dual-wavelength switchable quantum cascade laser (QCL) by optimizing the design of a homogeneous active region and combining superposed distributed feedback gratings. Coaxial, single-mode emissions at two different wavelengths were achieved only through adjusting the bias voltage. Room temperature continuous-wave operation with output powers of above 30 mW and 75 mW was realized for single-mode emission at 7.61 µm and 7.06 µm, respectively. The simplified fabrication process and easy wavelength control of our designed dual-wavelength QCL make it very attractive for developing miniature multi-species gas sensing systems.

Entropy transfer function measurement with tunable diode laser absorption spectroscopy

Tunable diode laser absorption spectroscopy (TDLAS) with wavelength modulation spectroscopy (WMS) has been widely applied to measure the gas temperature and species concentration in combustion environments. In this work, we show the capability of TDLAS in measuring the coherent temperature fluctuations of a turbulent swirled flame. Two distributed feedback grating (DFB) lasers are used to probe the H2O absorption transitions near 7185.59 cm−1 and 6806.03 cm−1. Using WMS, the 2f/1f absorption spectra are converted into temperature at a rate of 5 kHz. The quantity of interest that we measure is the Entropy Transfer Function (ETF) which, for low Mach numbers, relates the upstream acoustic perturbation to the temperature fluctuation downstream of the flame. The ETF is obtained by taking the ratio between the measured temperature fluctuation downstream of the flame, measured with TDLAS and the upstream acoustic perturbation input, induced by loudspeakers with forcing frequency ranging from 40 to 250 Hz, reconstructed with the multi-microphone method. We show that the TDLAS-WMS technique can measure relatively low coherent temperature fluctuations as low as 5 K. It is found that the gain of ETFs of the swirling flame does not decrease monotonically with respect to the forcing frequency and exhibits a minimum response at particular frequencies depending on the operating conditions. Moreover, the ETFs for different operating conditions collapse on each other when plotted against the Strouhal number. The similarity between the measured ETFs and Flame Transfer Functions (FTFs) indicates a strong link between flame heat release rate response and the temperature fluctuation downstream of the flame. Furthermore, the measurement series demonstrate the strong capability and suitability of the TDLAS-WMS technique for entropy waves measurements, leading to the discovery of peculiar entropy responses at high frequencies.

Observation of distributed feedback lasing in silicon nanocrystals under electrical pumping

Electrically pumped distributed feedback (DFB) lasing in silicon nanocrystals (SiNCs) has been observed. The DFB SiNC laser was made based on a SiNC light-emitting diode (LED) by incorporating a second-order DFB grating. The light emission covered the visible regime. Optical gains in the order of 102 cm−1 were found in the light emitting regime. The DFB grating was so designed and fabricated that lasing wavelength in the red regime was selected. Signals emitting from the surface of the DFB SiNC laser were recorded. The laser was pumped by a pulsed direct-current power supply at room temperature. Four typical criteria of lasing were identified, i.e., 1) threshold behavior of the electroluminescence (EL) intensity versus pumping power density, 2) evident spectral narrowing of the EL spectrum, 3) well-recognized degree of polarization, and 4) small spread of light emitting angle. The latter three were for the pumping power density beyond the threshold one.

Markedly Improved Performance of Optically Pumped Organic Lasers with Two-Dimensional Distributed-Feedback Gratings

Continuous-wave (CW) lasing is still difficult to realize in organic laser dyes, one reason being the thermal degradation caused by intense photoexcitation. A decrease in laser threshold suppresses...

Distributed feedback laser with methylammonium lead bromide embedded in channel-type waveguides

Distributed feedback (DFB) lasing is achieved with CH3NH3PbBr3 perovskite embedded in channel-type waveguides. By using a focused ion beam, DFB gratings are engraved within the bottom surface of channel-type waveguides etched on a SiO2/Si wafer. The oriented CH3NH3PbBr3 crystallized inside the waveguide with the DFB gratings shows second-order DFB lasing mode in lateral and normal directions above a threshold excitation density of 47 μJ cm−2 which is one order of magnitude lower than that in the non-DFB laser. The splitting DFB peaks are well reproduced by finite-differential time-domain calculation. The peaks change to a power-broadened amplified spontaneous emission as the excitation fluence is increased far above the threshold.

Terahertz quantum cascade lasers with sampled lateral gratings for single mode operation.

In this paper, we presented single mode terahertz quantum cascade lasers (THz QCLs) with sampled lateral grating emitting approximately 3.4 THz. Due to strong mode selection, the implementation of sampled lateral grating on THz QCL ridges can result in stable single longitudinal mode emission with a side-mode suppression ratio larger than 20 dB. The measured peak power of the grating laser is improved by about 11.8% compared to the power of devices with uniform distributed feedback gratings. Furthermore, the far-field pattern of the presented device is uninfluenced by grating structures.

Ultralow power consumption of a quantum cascade laser operating in continuous-wave mode at room temperature.

We report an ultralow power consumption of a quantum cascade laser (QCL) emitting at λ ∼ 4.6 µm operating in continuous-wave mode at room temperature. The ultralow power consumption is achieved by using a high gain active region and shortening the device size. For the device with a 0.5-mm-long cavity and 3.2-µm-wide ridge, the threshold power consumption is as low as 0.26 W with an optical output power of 12.6 mW at 10 °C in continuous-wave mode, which represents the world's most advanced level. Furthermore, the threshold power consumption varies linearly with the operating temperature, where the linear change rate of 2.3 mW/K from 10 to 40 °C is low. As a result, the devices also show low threshold power consumption values of 0.33 W even at 40 °C in continuous-wave mode with an optical output power of 6.1 mW. In addition, the lasers can maintain a single-mode operation due to the short cavity length even if no distributed feedback grating is applied.

Distributed-feedback blue laser diode utilizing a tunnel junction grown by plasma-assisted molecular beam epitaxy

In this paper, we demonstrate a novel approach utilizing tunnel junction (TJ) to realize GaN-based distributed feedback (DFB) laser diodes (LDs). Thanks to the use of the TJ the top metal contact is moved to the side of the ridge and the DFB grating is placed directly on top of the ridge. The high refractive index contrast between air and GaN, together with the high overlap of optical mode with the grating, provides a high coupling coefficient. The demonstrated DFB LD operates at λ=450.15 nm with a side mode suppression ratio higher than 35dB. The results are compared to a standard Fabry-Perot LD.