Optical filters based on integrated phase-shifted Bragg gratings (PSBGs) have attracted considerable interest because they feature a Lorentzian-shaped resonance, along with strong out-of-band rejection across a broad stopband region. The extent of the stopband can be conveniently adjusted by modifying the grating coupling coefficient [1]. Fabricating PSBGs on a thin-film lithium niobate (TFLN) platform enables the selection and modulation of specific wavelengths at ultra-fast speeds [2]–[3], offering substantial advantages in applications like datacom, sensing, and microwave-photonics systems [4]. In this abstract, a monolithic PSBG based on TFLN is proposed. The low waveguide propagation losses combined with the strong grating coupling coefficient of these devices, enable a narrow wavelength selection combined with a large stopband and effective electro-optic tunning capabilities.

We propose mode-locked distributed Bragg reflector (DBR) and distributed feedback (DFB) lasers based on multiple phase-shift gratings (MPSGs) for terahertz (THz) signal generation, with the latter incorporating an equivalent pi-phase shift. By integrating optimized MPSGs, we achieve multi-channel lasing with uniform reflectivity and dense channel spacing. Mode-locked DBR lasers operating at THz frequencies of 150 GHz, 400 GHz, 800 GHz, and 1.2 THz have been demonstrated, as confirmed by second harmonic generation measurements. Additionally, a 200 GHz mode-locked DFB laser was realized. Amplified by an erbium-doped fiber amplifier (EDFA), the mode-locked DFB laser output was injected into a photoconductive antenna (PCA) to generate THz signals, with the measured power reaching 19.6 mu W. These results highlight the potential of MPSG-based mode-locked lasers for compact and efficient THz generation systems.

This paper presents the design and modelling of a 3rd order gallium nitride (GaN) distributed feedback (DFB) laser for the cooling transition in Sr+ optical atomic clocks. A compact design of the laser source operating at 422 nm, with sufficiently narrow linewidth and high power output, is favoured to hit the required specifications for this application. These lasers provide a viable solution to bulky, benchtop lasers which are often frequency doubled to hit the required wavelengths, leading towards more compact systems.

We present the first, to our knowledge, demonstration of a 1550 nm multi-wavelength distributed feedback (MW-DFB) laser employing a third-order, four-phase-shifted sampled sidewall grating. By utilizing linearly chirped sampled gratings and incorporating multiple true π-phase shifts within a cavity, we achieved and experimentally validated a four-wavelength laser with a channel spacing of 0.4 nm. The device operates stably and uniformly across a wide range of injection currents from 280 mA to 350 mA. The average wavelength spacing was measured at 0.401 nm with a standard deviation of 0.0081 nm. Additionally, we demonstrated a 0.3 nm MW-DFB laser with a seven-channel output, achieving a wavelength spacing of 0.274 nm and a standard deviation of 0.0055 nm. This MW-DFB laser features a ridge waveguide with sidewall gratings, requiring only one metalorganic vapor-phase epitaxy (MOVPE) step and a single III-V material etching process. This streamlined fabrication approach simplifies device manufacturing and is well-suited for dense wavelength division multiplexing (DWDM) systems.

A monolithic optical injection-locked distributed-feedback (MOIL-DFB) laser is experimentally demonstrated, achieving a locking current range of 90–150 mA, a side-mode suppression ratio (SMSR) exceeding 45 dB, and a linewidth of 2.7 MHz.

Narrow-linewidth lasers are essential for coherent optical applications, including communications, metrology, and sensing. Although compact semiconductor lasers with narrow linewidths have been demonstrated, achieving high spectral purity generally necessitates passive external cavities based on photonic integrated circuits. This study presents a theoretical and experimental demonstration of a monolithic optical injection locking topological interface state extended (MOIL-TISE) laser. By monolithically integrating a TISE laser with a micro-ring resonator on an AlGaInAs multiple quantum-well platform, the proposed device achieves efficient photon injection and linewidth narrowing. Experimental characterization indicates stable single-mode operation over a wide injection current range (65 to 300 milliamperes), exhibiting a side-mode suppression ratio exceeding 50 decibels. The laser's Voigt linewidth was reduced from 2 megahertz to 4.2 kilohertz, with an intrinsic linewidth of 983 hertz extracted from power spectrum density, underscoring the MOIL-TISE laser's promise for coherent communications and modulation-free quantum key distribution applications.

Advances in 1D topological photonic crystals have enabled robust light‐emitting devices through a topological interface state at the cavity center. In this study, a 1D TIS‐extended photonic crystal (1D‐TISE‐PhC) structure both theoretically and experimentally is demonstrated. a linearly dispersive, zero‐index 1D photonic crystal is integrated with a four‐phase shift (4PS) sampled grating so that photons propagate through the cavity without phase differences, enhancing robustness and extending the TIS. This extension yields a more uniform photon distribution along the laser cavity and mitigates spatial hole burning. This is fabricated and characterized a 1550 nm 1D‐TISE‐PhC semiconductor laser, achieving stable single‐mode operation from 60 to 420 mA, with a side‐mode suppression ratio of 50 dB. The device exhibited a linewidth narrowing effect, with the narrowest Lorentzian linewidth of 126 kHz and a typical linewidth of 150 kHz, nearly an order of magnitude lower than conventional distributed feedback Bragg lasers. Using reconstruction equivalent‐chirp technology with the 4PS grating allowed precise wavelength control in laser arrays, with a spacing of 0.796 nm ± 0.003 nm. This results confirm that the TIS remains intact in the TISE cavity, preserving topological protection and demonstrating improved design simplicity and fabrication tolerance for high‐power, narrow‐linewidth semiconductor lasers.

This work explores the design and fabrication of gallium nitride (GaN) lasers for use in optical atomic clocks. Software allows the laser material and grating structures to be modelled, and devices have been fabricated in the cleanroom based on this. Currently, many atomic clock systems use large, expensive lasers which are frequency doubled, and coupling requires lots of benchtop optical components. A distributed feedback (DFB) laser is one option to realise a compact source which can hit the specifications required for the atomic cooling process. Grating structures have been designed to produce single mode performance at key wavelengths in the blue part of the spectrum. Development of the etching process and metal deposition are being optimised to ensure the best quality lasers are produced. These devices can then be used for the cooling of atoms in optical atomic clocks, taking what is currently a lab scale technology to something which is much more compact. Single mode GaN lasers with high output power and narrow linewidth have been realised for this purpose. Other laser structures, such as surface emitting devices are also being considered. National Physical Laboratory are developing a compact ion trap and vibrationally insensitive cubic cavity which along with these compact lasers, will lead to a portable optical atomic clock.

Narrow linewidth lasers are indispensable for coherent optical systems, including communications, metrology, and sensing. Although compact semiconductor lasers with narrow linewidths and low noise have been demonstrated, their spectral purity typically relies on hybrid or heterogeneous external cavity feedback. Here, we present a theoretical and experimental demonstration of a heterogeneous free optical injection locking (HF OIL) semiconductor laser. By integrating a topological interface state extended (TISE) laser with a micro ring resonator (MRR) on an AlGaInAs multiple quantum well platform,we achieve monolithic photon injection and phase locking, thereby reducing the optical linewidth. We fabricated and characterized a 1550 nm sidewall HF OIL laser, achieving stable single mode operation over a broad current range (65 to 300 mA) and a side mode suppression ratio (SMSR) over 50 dB. Under injection locking, the devices Voigt fitted linewidth narrowed from over 1.7 MHz (free running) to 4.2 kHz, representing a three order of magnitude improvement over conventional distributed feedback lasers. The intrinsic linewidth of 1.4 kHz is measured by correlated delayed self-heterodyne frequency noise power spectrum density (FN PSD) method. Moreover, the HF OIL laser demonstrated high phase stability and the ability to transition from a random phased to a phase locked state. These results underscore the potential of HF-OIL lasers in advancing coherent optical communications and phase encoders in quantum key distribution (QKD) systems.

Recent advancements have broadened the application of photon filters based on Bragg gratings within optical communication networks and optical input/output interfaces. Traditional gratings, however, suffer from a fixed refractive index modulation distribution once manufactured, constraining their adaptability and flexibility. This study introduces a reconfigurable multi-channel photon filter on a silicon nitride on insulator platform. The filter incorporates an equivalent linearly chirped four-phase-shifted sampled Bragg grating with micro-heaters to enable thermo-optic tuning, facilitating programmable control over transmission spectral features. Experimental outcomes indicate the filter’s capability to seamlessly transition among single, dual, and quad-band configurations, as well as a band-stop mode, with independent tuning of each band. Moreover, optical frequency division multiplexing experiments using a 50 GHz semiconductor mode-locked laser have affirmed the filter’s tunability. In quad-band mode, band separations of 50, 100, and 150 GHz are achievable; in dual and single-band modes, band intervals extend from 150 to 250 GHz, allowing for precise single-wavelength selection. Featuring high tunability, minimal insertion losses, and superior signal side-mode suppression ratio, this filter structure supports the integration of programmable photonic devices into space optical communications, photonic integrated networks, and elastic optical networks.

Lasers with high-frequency spectral purity are critical for numerous applications, including sensing and spectroscopy [1] and microwave photonic devices [2]. Prior research has demonstrated that coupling a distributed feedback (DFB) semiconductor laser with a high-Q external optical cavity can create a self-injection-locked (SIL) system. However, effective hybrid SOI- or $\text{Si}_{3}\mathrm{N}_{4}$ -based SIL and external cavity laser (ECL) designs require highly efficient coupling components between the 111- V and silicon platforms. In addition, heterogeneous integration complicates the testing of individual components before their integration into complex systems. This requires stringent process control to ensure high yield, while also increasing fabrication complexity and cost.

Recent advances in topological one-dimensional photonic crystal concepts have enabled the development of robust light-emitting devices by incorporating a topological interface state (TIS) at the cavity center. In this study, we theoretically and experimentally demonstrate a one-dimensional TIS-extended photonic crystal (1D-TISE-PC) structure. By integrating a linearly dispersive zero-index one-dimensional photonic crystal structure with a four-phase shift sampled grating, photons propagate along the cavity without phase differences, enhancing the robustness to material variations and extending the TIS. Our findings indicate that extending the TIS promotes a more uniform photon distribution along the laser cavity and mitigates the spatial hole burning (SHB) effect. We fabricated and characterized a 1550 nm sidewall 1D-TISE-PC semiconductor laser, achieving stable single-mode operation across a wide current range from 60 to 420 mA, with a side-mode suppression ratio of 50 dB. The 1D-TISE-PC structure exhibited a linewidth narrowing effect to approximately 150 kHz Lorentzian linewidth. Utilizing reconstruction equivalent-chirp technology for the 4PS sampled grating enabled precise wavelength control in 1D-TISE-PC laser arrays, achieving a wavelength spacing of 0.796 nm +- 0.003 nm. We show that the TIS still exists in the TISE cavity and topological protection is preserved. Its mode extension characteristics mitigate the SHB so narrows the linewidth. We argue that the design simplicity and improvement of the fabrication tolerance make this architecture suitable for high-power and narrow-linewidth semiconductor lasers development.

A 780 nm distributed feedback (DFB) laser employing a four-phase shift sample grating and lateral modulated four-phase-shifted (LM-4PS) grating was successfully showcased. The LM-4PS effectively suppressed the longitudinal spatial hole-burning (SHB) effect, ensuring stable single longitudinal mode operation over an extensive range of operating currents.

There is an ever-growing requirement for compact atomic devices, such as optical atomic clocks, taking them from a labscale technology to a more robust solution. Optical atomic clocks have made significant advances over the last few decades and represent the pinnacle of precision measurement technology. However, many systems make use of large, expensive lasers which are power hungry and often frequency doubled to hit key wavelengths or alternatively rely on vibration sensitive external cavity diode lasers (ECDL). New approaches and technologies are required such as working with ion-based optical clocks where small, robust ion traps can be realized with the ion cooling controlled using a distributed feedback (DFB) laser. A promising platform for an optical atomic clock is the strontium ion system due to its convenient wavelengths and simple level structure. Of the required lasers only the 422 nm cooling laser is not wellserved by existing technology. The National Physical Laboratory (NPL) are developing a compact ion trap physics package and vibrationally insensitive cubic cavity that will form the basis of the portable optical clock. DFB lasers have been realized at 422 nm with high output powers and narrow linewidths. Modelling of the device epitaxy and grating structure show how these devices can be improved further. Overall, this will significantly reduce the SWaP compared to current systems.

Distributed feedback (DFB) lasers based on gallium nitride (GaN) have been fabricated and characterised for use in optical wireless communications. These devices find applications in free space visible light communication (VLC), but also play a fundamental role in underwater and space environments due to the low loss transmission at blue wavelengths. Devices have shown Gbit/s performance making them ideal candidates for low-cost, high speed data transmission.

By using asymmetric twin-waveguide technology, a 1.55- $\mu \text{m}$ sidewall grating distributed feedback laser monolithically integrated with a passive waveguide crossing was for the first time demonstrated for an optical beam forming network, which needs only one metalorganic vapor-phase epitaxy step. The distributed feedback laser with uncoated facets presents a side-mode suppression ratio of >44 dB and a low 3-dB linewidth of 68 kHz. An elliptical parabolic taper waveguide crossing had a theoretical crosstalk suppression ratio of 40 dB, while the measured cross-talk suppression ratio was at least 20 dB.

An asymmetric twin-waveguide 1.55-μm DFB laser integrated with a passive waveguide crossing was fabricated for the optical beam forming network. A DFB laser with a side-mode suppression ratio of >45 dB and a waveguide crossing with a 16 dB crosstalk suppression ratio were obtained.

We report on GaN lasers with extremely narrow linewidth (~1MHz) at ‘magic wavelengths’ for cold-atom quantum sensors and optical atomic clocks, using extended cavity GaN laser diodes and DFB GaN laser diodes.

Gallium nitride (GaN) laser diodes (LDs) are considered for visible light communications (VLC) in free space, underwater, and in plastic optical fibers (POFs). A review of recent results is presented, showing high-frequency operation of AlGaInN laser diodes with data transmission rates up to 2.5 Gbit/s in free space and underwater and high bandwidths of up to 1.38 GHz through 10 m of plastic optical fiber. Distributed feedback (DFB) GaN LDs are fabricated to achieve single-frequency operation. We report on single-wavelength emissions of GaN DFB LDs with a side-mode suppression ratio (SMSR) in excess of 35 dB.

We present an electroabsorption modulated laser based on an identical epitaxial scheme, side-wall grating, on- chip microwave probe interface, and a new low-permittivity planarization method. The modulation speed is significantly increased by reducing the electrode capacitance by planarizing with a 5- $\mu \text{m}$ -thick HSQ layer. Furthermore, implementing the electrode with a direct ground-signal-ground probe interface provides a straightforward interconnection that obviates the need for an external circuit and bonding wires. The device operates at 1565 nm wavelength with stable single-mode lasing, no mode-hopping, and a side mode suppression ratio above 35 dB. An extinction ratio of 19.5 dB was recorded at the maximum modulator bias of −4 V. The electrical to optical power response of the modulated signal at–3-dBo demonstrated a 19 GHz bandwidth at an extinction ratio of 7 dB, which supports error-free data transmission up to 27 Gbit/s.