Side-mode Suppression Research Articles

High-power, stable single-mode CW operation of 1550 nm wavelength InP-based photonic-crystal surface-emitting lasers

1550 nm wavelength photonic-crystal surface-emitting lasers (PCSELs) are attractive for optical communication and eye-safe sensing applications. In this study, we present InP-based PCSELs featuring a double-lattice photonic-crystal structure designed for high-power single-mode operation at a wavelength of 1550 nm. These PCSELs demonstrate output powers exceeding 300 mW under continuous-wave conditions at 25 °C. Additionally, highly stable single-mode oscillation with a side-mode suppression ratio of over 60 dB is verified at temperatures from 15 °C to 60 °C. Measurement and simulation of photonic band structures reveal the impacts of the threshold gain margin and optical coupling coefficient on the single-mode stability.

Phase-locked dual-frequency microwave signal generation in an optoelectronic oscillator based on frequency mixing mutual injection

An approach to generating stable phase-locked dual-frequency microwave signals is proposed and demonstrated based on a dual-passband optoelectronic oscillator (OEO). Mode gain competition is broken by employing frequency mixing mutual injection effect to realize phase locking between the two oscillation signals, which is achieved by applying a single-tone signal to a microwave mixer in the OEO cavity. In addition, a dual-loop configuration with balanced detection is utilized to ensure a high side mode suppression ratio (SMSR) and ultra-low phase noise, which also enhances the stability of the generated signal. In the experiment, a phase-locked dual-frequency microwave signal at 9.9982 GHz and 10.1155 GHz is generated by using the proposed OEO scheme. The SMSR and the phase noise are 75 dB and –141 dBc/Hz@10 kHz, respectively. Additionally, the Allan deviation of the generated signal is in the order of 10−11@1 s. These parameters are superior to those based on the same OEO but with a single-loop configuration, which are also compared in detail.

Improved optical performance in circular-grating distributed feedback nanoplasmonic lasers

Optical modes control has been driven from vertical cavity surface emitting lasers (VCSELs) applications with multi-mode operation such as optical fiber communication or optical sensing. Optical gain materials coupling with optical resonators provides an effective way to tuning the optical mode of VCSELs. In this paper, a kind of novel circular-grating distributed feedback (CGDFB) nanoplasmonic laser is designed by using a special semiconductor nanorods array VCSEL coupled optically with a second-order distributed feedback (DFB) circular grating. In the device, the mode field is cooperatively controlled by the confinement of surface plasmon around the nanorod cavity and the spatial tuning with the DFB circular grating. By optimizing the design on the device comprehensively, the CGDFB nanoplasmonic laser exhibits the optimum lasing performance with a single-longitudinal mode peak at 511.2 nm with a side-mode suppression ratio of 45 dB. The 3 dB line-width is compressed to 1.98 p.m. at a driven current of four times of the threshold current of 124 mA. A six-fold rotational symmetric far-field mode corresponding to the third-order Laguerre Gaussian azimuth mode is produced by the DFB circular grating. This work paves a fundamental way for the mode engineering design of novel nanolaser in the technical applications of optical communication, sensing and integrated photonics.

Narrow linewidth laser based on a sidewall grating active distributed Bragg reflector.

We demonstrated a narrow linewidth semiconductor laser based on a deep-etched sidewall grating active distributed Bragg reflector (SG-ADBR). The coupling coefficients and reflectance were numerically simulated for deep-etched fifth-order SG-ADBR, and a reflectance of 0.86 with a bandwidth of 1.04 nm was obtained by the finite element method for a 500-period SG-ADBR. Then the fifth-order SG-ADBR lasers were fabricated using projection i-line lithography processes. Single-mode lasing at 1537.9 nm was obtained with a high side-mode suppression ratio (SMSR) of 65 dB, and a continuous tuning range of 10.3 nm was verified with SMSRs greater than 53 dB. Furthermore, the frequency noise power spectral density was characterized, from which a Lorentzian linewidth of 288 kHz was obtained.

Monolithic beam combined quantum cascade laser arrays with integrated arrayed waveguide gratings.

Quantum cascade lasers (QCLs) are ubiquitous mid-infrared sources owing to their flexible designs and compact footprints. Manufacturing multiwavelength QCL chips with high power levels and good beam quality is highly desirable for many applications. In this study, we demonstrate an λ ∼ 4.9 µm monolithic, wavelength beam-combined (WBC) infrared laser source by integrating on a single chip array of five QCL gain sections with an arrayed waveguide grating (AWG). Optical feedback from the cleaved facets enables lasing, whereas the integrated AWG locks the emission spectrum of each gain section to its corresponding input channel wavelength and spatially combines their signals into a single-output waveguide. Our chip features high peak power from the common aperture exceeding 0.6 W for each input channel, with a side-mode suppression ratio (SMSR) of over 27 dB when operated in pulsed mode. Our active/passive integration approach allows for a seamless transition from the QCL ridges to the AWG without requiring regrowth or evanescent coupling schemes, leading to a robust design. These results pave the way for the development of highly compact mid-IR sources suitable for applications such as hyperspectral imaging.

1.55 µm wavelength band photonic crystal surface emitting laser with n-side photonic crystal and operation at up to 85 °C.

We describe the structure, fabrication, and measured performance of a 1543 nm wavelength photonic crystal surface emitting laser. An asymmetric double lattice design was used to achieve single mode lasing with side mode suppression ratios >40 dB. The photonic crystal was formed using encapsulated air holes in an n-doped InGaAsP layer with an InGaAlAs active layer then grown above it. In this way a laser with a low series resistance of 0.32 Ω capable of pulsed output powers of 171 mW at 25 °C and 40 mW at 85 °C was demonstrated.

Continuous ultra-wideband signal regeneration in random optoelectronic oscillators through injection locking.

By using an external injection locking method, for what we believe to be the first time, we experimentally demonstrate continuous ultra-wideband signal regeneration in random optoelectronic oscillators, achieving more adaptable signal processing capabilities than self-oscillation methods. Supported by theoretical analysis and experimental evidences, this system can regenerate any signal with sufficient gain in a random-feedback cavity, independent of cavity filters. Remarkably, enhanced phase noise performance with over 35.2 dB side mode suppression and a phase noise better than -86 dBc@1 kHz at higher injecting powers are demonstrated. Additionally, we successfully process complex multi-frequency communication signals, indicating potential applications in radar, remote sensing, and data communications.

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

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

Slotted surface gratings fabricated by selective area growth of the p-InP cladding layer for BH lasers.

In this paper, we present a novel, to our knowledge, method for the fabrication of slotted surface gratings for buried heterostructure (BH) lasers. In the device fabrication process, SiO2 strips needed for InP current blocking layer growth are reused for the formation of slot grating pattern masks. In the following growth of the p-InP cladding layer, because the slot areas are covered by SiO2, the InP material is grown selectively in only the areas outside the slot areas, forming slots of the surface gratings in the p-InP layer at the same time as the cladding layer growth. Single longitude mode BH lasers having slotted surface gratings have been fabricated successfully, and the spectra show higher than 40 dB side mode suppression ratio (SMSR). The adoption of the method helps to simply the device fabrication and thus lower the device fabrication cost notably.

The study on application of high-order tilted asymmetric Bragg gratings in quantum cascade lasers

High-order gratings eliminate the need for high-resolution lithography in the definition process. However, they are not widely adopted due to inability to provide sufficient coupling coefficients in long-wave mid-infrared quantum cascade lasers (QCLs). Fifth order tilted asymmetric gratings (TAS-gratings) distributed feedback (DFB) QCLs with stable single longitudinal mode are firstly reported in this paper. TAS-gratings exhibit longitudinal mode selection capability across a broad range of asymmetric distances around 1.18 μm and duty cycles around 0.5, which is attributed to their larger effective duty cycles and phase differences caused by an 18° tilt angle of the grating in the middle and two asymmetrical distributions of gratings on both sides. Further experimental results indicate that DFB QCLs with TAS-gratings exhibit stable excitation of single longitudinal modes within the duty cycle range of 0.48–0.54 due to the large coupling coefficients, the temperature drift coefficient of 0.29 nm/K of TAS-grating device is reduced by 64% compared with symmetric gratings (S-gratings) devices, the maximum side-mode suppression ratio (SMSR) of the devices exceeds 25 dB. TAS-gratings offer greater process tolerance, design flexibility and lower preparation difficulty. This study provides a solution to achieve sufficiently large coupling coefficients for high-order gratings applied to long-wave mid-infrared QCLs.

InGaN‐Based Whispering Gallery Mode Laser with Lateral Nanoporous Distributed Bragg Reflector Exhibits Superior Mode Selectivity

AbstractWhispering gallery mode (WGM) laser with single‐mode is important for optoelectronic applications because of the higher stability during the light signal transmission. But till now, for InGaN‐based WGM laser, most reports of lasing behavior are multimode lasing. To achieve single‐mode WGM laser, an InGaN‐based micro‐ring laser is proposed and demonstrated with lateral nanoporous distributed Bragg reflector (DBR), where the high‐order lasing modes are strongly suppressed. The side mode suppression ratio (SMSR) is more than 9 times larger than that of the micro‐disk laser with the same radius. Combined with FDTD simulation, the results show that the enhancements are originated from the suppression of the high‐order resonant mode because of the inner boundary of micro‐ring. This work provides a new solution to achieve stable single‐mode WGM laser for future photonic integration.

High-beam-quality low-resistance vertical-cavity surface-emitting laser array with graphene electrode.

Thermal crosstalk and current crowding effects are pressing issues that significantly impact the beam quality and efficiency of vertical-cavity surface-emitting laser (VCSEL) arrays. In this paper, by taking advantage of the excellent current transmission characteristics of graphene, what we believe to be a novel VCSEL array based on graphene electrode is designed to realize vertical current injections. The series resistance and self-heating of arrays are reduced by controlling the transport direction of the current, effectively suppressing the thermal crosstalk effect. Furthermore, high array beam quality is obtained by optimizing the current density distribution in active regions. Ultimately, the high-power quasi-single mode emission of VCSEL arrays is achieved by introducing graphene electrodes (Gr-VCSEL array) designs. Compared to traditional VCSEL arrays, the 10 × 10 Gr-VCSEL array demonstrates a 41% reduction in series resistance, a side mode suppression ratio of 32 dB, and a divergence angle around 12 °. This structure simultaneously achieves quasi-single mode emission and effectively suppresses the thermal crosstalk effect, providing a new method for the development of high-beam quality VCSEL arrays.

Parity time symmetry all-optical microwave oscillator using a polarization dependent Sagnac loop

An all-optical microwave oscillator based on parity-time symmetry was proposed and experimentally demonstrated. It mainly consists of a big feedback loop, an active small loop and a polarization-dependent Sagnac loop. The microwave seed signal originates from the beating between the continuous wave and the Stokes wave. The function of microwave envelope detection and feedback modulation are implemented through cross gain modulation of two semiconductor optical amplifiers. By employing the parity time symmetry loops and the Vernier effect, the single mode selectivity capability of the oscillator has been enhanced, and a single mode microwave signal with high quality can be generated. In the experiment, a highly quality microwave signal with a frequency of 10.71 GHz was generated. The single side band phase noise can reach −94.1 dBc/Hz@10 kHz and the side mode suppression ratio of the generated microwave signal reaches 41 dB. Compared to the previously reported all-photonic microwave oscillator, the single side band phase noise is improved by 7 dB at a frequency offset of 10 kHz, and by 20 dB at a frequency offset of 100 Hz. The results indicate that our proposed scheme significantly improves the quality of the generated microwave signals.

Non-uniform distributed silicon optical phased array for high directionality and a wide steering range.

A non-uniform distributed silicon optical phased array (OPA) is proposed and numerically demonstrated to realize high directionality and a wide range for beam steering. The OPA is composed of grating antennas with dual-layer corrugations along silicon strip waveguides, which can achieve a high directionality of 0.96 and a small divergence angle of 0.084°. To reduce the crosstalk between adjacent antennas and realize a wide steering range, the genetic algorithm is improved and utilized to arrange the locations of grating antennas. As a proof of concept, a 32-channel non-uniform distributed OPA is designed and thoroughly optimized. The simulation results successfully demonstrate a two-dimensional wide steering range of 70∘×18.7∘ with a side-mode suppression ratio (SMSR) over 10dB.

Study on Linewidth and Phase Noise Characteristics of a Narrow Linewidth External Cavity Diode Laser.

In the field of inter-satellite laser communication, achieving high-quality communication and compensating for the Doppler frequency shift caused by relative motion necessitate lasers with narrow linewidths, low phase noise, and the ability to achieve mode-hop-free tuning within a specific range. To this end, this paper investigates a novel external cavity diode laser (ECDL) with a frequency-selective F-P etalon structure, leveraging the external cavity F-P etalon structure in conjunction with an auxiliary filter to achieve single longitudinal mode selection. The laser undergoes linewidth testing using a delayed self-heterodyne beating method, followed by the testing of its phase noise and frequency noise characteristics using a noise analyzer, yielding beat spectra and noise power spectral density profiles. Furthermore, the paper introduces an innovative bidirectional temperature-scanning laser method to achieve optimal laser-operating point selection and mode-hop-free tuning. The experimental results showcase that the single longitudinal mode spectral side-mode suppression ratio (SMSR) is around 70 dB, and the output power exceeds 10 mW. Enhancing the precision of the F-P etalon leads to a more pronounced suppression of low-frequency phase noise, reducing the Lorentzian linewidth from the initial 10 kHz level to a remarkable 5 kHz level. The bidirectional temperature-scanning laser method not only allows for the selection of the optimal operating point but also enables mode-hop-free tuning within 160 pm.

Widely tunable single-mode interband cascade lasers based on V-coupled cavities and dependence on design parameters

We report an investigation of V-coupled cavity interband cascade (IC) lasers (ICLs) emitting in the 3-μm wavelength range, employing various waveguide structures and coupler sizes. Type-II ICL devices with double-ridge waveguides exhibited wide tuning ranges exceeding 153 nm. Type-I ICL devices with deep-etched waveguides achieved single-mode emission with wavelength tunable over 100 nm at relatively high temperatures up to 250 K. All devices exhibited a side-mode suppression ratio higher than 30 dB. By comparing the performance of all devices with different sizes and configurations, a good tolerance against the structural parameter variations of the V-coupled cavity laser (VCCL) design is demonstrated, validating the advantages of the VCCL to achieve single-mode emission with wide tunability.

Integrated self-injection-locked narrow linewidth laser based on thin-film lithium niobate.

Narrow linewidth lasers have a wide range of applications in the fields of coherent optical communications, atomic clocks, and measurement. Lithium niobate material possesses excellent electro-optic and thermo-optic properties, making it an ideal photonic integration platform for a new generation. The light source is a crucial element in large-scale photonic integration. Therefore, it is essential to develop integrated narrow linewidth lasers based on low-loss LNOI. This study is based on the multimode race-track type add-drop microring resonator with multimode interferometric coupler (MMRA-MRR) of the DFB laser self-injection-locked, to achieve the narrowing of linewidth to the laser. The microring external cavity was used to narrow the linewidth of the laser to 2.5 kHz. The output power of the laser is 3.18 mW, and the side-mode suppression ratio is 60 dB. This paper presents an integrated low-noise, narrow-linewidth laser based on thin-film lithium niobate material for the communication band. This is significant for achieving all-optical device on-chip integration of lithium niobate material in the future. It has great potential for use in high-speed coherent optical communication.

Ultrabroadband vernier cascaded microring filters based on dispersive Si3N4 waveguides

A viable candidate for use in silicon photonics and microwave photonics is the hybrid external cavity laser (ECL) chip, which offers a high extinction ratio, ultrabroadband mode-hopping-free tuning range, and small linewidth. It requires a photonic filter device with an ultrabroadband operating bandwidth and adjustable frequency selection capability. The Vernier cascaded microring filter is a prevailing filter technique that usually ignores chromatic dispersion and will result in noticeable frequency variations, particularly over a large frequency range. Based on dispersive Si3N4 waveguides, we develop ultrabroadband Vernier cascaded microring filters and examine the impact of chromatic dispersion. For the same waveguide geometry, the filter’s effective free spectral range (FSR) varies by more than 400 GHz with and without the dispersion. Furthermore, these Vernier filters, which are made of anomalous and normal dispersive waveguides respectively, exhibit mode hopping at the opposite frequency side. It leads to a sudden mode number leap and, consequently, a diversified dispersion condition for the convoluted filtering frequency. We show that this phenomenon is caused by the interplay between half of the FSR difference, and the accumulated frequency difference caused by the chromatic dispersion. Finally, the use of thermal-optical tuning enables accurate frequency tuning. Our findings offer a valuable resource for the engineering of hybrid ECLs at the chip scale.

Optofluidic tunable filters using ionic liquid electrolyte capacitors.

Tunable optical filter is a basic component for most optical systems. This study reports a unique design of Fabry-Pérot (FP) tunable filter by using an ionic liquid solution. The tunable filter consists of two neighboring regions: capacitor region and FP region. The former is in the form of electrolyte capacitor and the latter remains transparent as an FP cavity for light transmission. When the capacitor region is applied with a bias voltage, it attracts the ions from the FP region and thus reduces the ion concentration of the FP region, resulting in a change of the refractive index and eventually a shift of transmission peak of the FP cavity. Among four electrolyte solutions studied, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6) exhibits the best overall performance, such as low insertion loss (3.2 dB), large side mode suppression ratio (23 dB) and high stability (drift <0.2 nm). Additionally, a wavelength tuning of 0.17 nm/V is achieved over 0-17 V, providing a tunable range of 3 nm. This device features low bias voltage, no mechanical movement, easy fabrication and seamless integration with microfluidics systems, and may find potential applications in spectral analyzers and lab-on-a-chip biosensing systems.

Highly-reflective facet-coated multi-wavelength DFB laser array with exact wavelength spacings.

A distributed feedback (DFB) laser array of twenty wavelengths with highly reflective and anti-reflective (HR-AR) coated facets is both theoretically analyzed and experimentally validated. While the HR facet coating enhances high wall-plug efficiency, it inadvertently introduces a random facet grating phase, thereby compromising the lasing wavelength's predictability and the stability of the single-longitudinal-mode (SLM). In this study, two key advancements are introduced: first, the precisely spaced wavelength is achieved with an error of within ±0.2 nm using the reconstruction-equivalent-chirp (REC) technique; second, the random grating phase on the HR-coated facet is compensated by a controllable distributed phase shift through a two-section laser structure. The SLM stability can be improved while the wavelength can be continuously tuned to the standard wavelength grid. The overall chip size is compact with an area of 4000 × 500 µm2. The proposed laser array has a light power intensity above 13 dBm per wavelength, a high side mode suppression ratio above 50 dB, and low relative intensity noise under -160 dB/Hz. These attributes make it apt for deployment in DWDM-based optical communication systems and as a light source for optical I/O.