Mid-infrared semiconductor lasers operating in the 2.0–5.0 μm spectral range play an important role for various applications, including trace-gas detection, biomedical analysis, and free-space optical communication. InP-based quantum-well (QW) and quantum-dash (Qdash) lasers are promising alternatives to conventional GaSb-based QW lasers because of their lower cost and mature fabrication infrastructure. However, they suffer from high threshold current density (Jth) and limited operation temperatures. InAs/InP quantum-dot (QD) lasers theoretically offer lower Jth owing to their three-dimensional carrier confinement. Nevertheless, achieving high-density, uniform InAs/InP QDs with sufficient gain for lasing over 2 μm remains a major challenge. Here, we report the first demonstration of mid-infrared InAs/InP QD lasers emitting beyond 2 μm. Five-stack InAs/In0.532Ga0.468As/InP QDs grown by molecular-beam epitaxy exhibit room-temperature photoluminescence at 2.04 μm. Edge-emitting lasers achieve lasing at 2.018 μm with a low Jth of 589 A cm−2 and a maximum operation temperature of 50 °C. Notably, the Jth per layer (118 A cm−2) is the lowest ever reported for room-temperature InP-based mid-infrared lasers, outperforming QW/Qdash counterparts. These results pave the way for a new class of low-cost, high-performance mid-infrared light sources using InAs/InP QDs, marking a notable step forward in the development of mid-infrared semiconductor lasers. Mid-infrared 2 μm InAs/InP quantum-dot lasers is first demonstrated, with a low threshold current density of 118 A cm−2 per layer and a maximum operating temperature of 50 °C.

We demonstrate a 1.55 mu m, 10 GHz passively mode-locked AlGaInAs/InP laser using a three-quantum-well (3QW) active region, a far-field reduction layer (FRL), and a semi-insulating (SI) InP substrate. This design minimizes self-phase modulation in long cavities, reduces beam divergence, and enables hybrid mode-locking. The device generates near-transform-limited 1.25 ps pulses with 5.63 ps rms timing jitter and reduced vertical divergence. This is the first demonstration of a 3QW passively mode-locked laser on SI InP with an FRL, providing a compact, low-noise, fibre-compatible optical clock source.

A preliminary exploration of symmetry breaking in the triangular lattice photonic crystal surface emitting laser (PCSEL) is outlined. This analysis shows that greater single mode stability and optical efficiencies can be achieved for asymmetric unit cell designs in triangular lattices. In particular, changing the fill factor alone, for void containing or all-semiconductor wedge unit cell devices can provide significant tunability of the radiation constant and gain discrimination. This opens up routes for the use of asymmetric unit cells in triangular lattices to enhance PCSEL performance.

We demonstrate a dual-wavelength DFB laser achieving stable sub-100 GHz dual-mode operation with >30 dB side-mode suppression ratio using a single injection current. Its simple, monolithic design offers intrinsic stability and straightforward fabrication for photonic microwave sources.

Semiconductors are extremely useful for temperature sensing owing to the strong temperature dependence of their optical and electronic properties. Silicon, the most widely used semiconductor, underpins modern electronics and is increasingly important in integrated photonics, offering a cost-effective platform for optical sensors. Silicon-based ring resonator (RR) temperature sensors operate via the temperature-dependent change in silicon's refractive index (dn/dT), which affects the optical modes in the ring. However, silicon has two main limitations: its indirect band gap makes it a poor light emitter, necessitating external light sources, and its thermal properties are fixed. In contrast, compound semiconductors, such as indium phosphide (InP), gallium arsenide (GaAs), gallium nitride (GaN) and indium arsenide (InAs), have direct band gaps, making them efficient light emitters as commonly used in light-emitting diodes and lasers. Their thermal properties can also be tailored through alloying. These features make them ideal for 'active resonator' temperature sensors with integrated light sources, allowing customization for various temperature ranges. This paper focuses on InP-based alloys, highlighting their fundamental properties and potential for integration into active quantum well-based heterostructures. These can be fabricated into micro-ring and other resonator designs. Integrating light sources within the sensor enhances both simplicity and functionality, paving the way for versatile temperature sensors suited to a wide range of applications. This article is part of the Theo Murphy meeting issue 'The redefined kelvin: progress and prospects'.

We present a four-wavelength mode-locked 1.55 μm DFB laser with a chirped four-phase-shifted sampled Bragg grating, delivering transform-limited pulses with high spectral purity—ideal for DWDM and multi-channel photonic applications.

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.

We demonstrate a compact monolithic mode-locked DFB laser with a waveguide Bragg grating, achieving stable three- and four-wavelength lasing at 57.4 GHz spacing. The device offers strong coherence and a high side-mode suppression ratio, suitable for dense wavelength division multiplexing and coherent communication applications. (c) 2025 The Author(s)

We demonstrate, for the first time to the best of our knowledge, a monolithic multi-wavelength mode-locked distributed feedback (DFB) laser based on waveguide Bragg grating microcavities, achieving simultaneous three-wavelength lasing near 1.55 μm within a single cavity. The device exhibits a uniform free spectral range of 0.46 nm (57.4 GHz), a side mode suppression ratio exceeding 30 dB, and near-transform-limited pulses (6.25 ps, time-bandwidth product = 0.359). The structure requires only one metalorganic vapor phase epitaxy growth and a single III-V material dry etching step, significantly simplifying fabrication and enhancing reproducibility. By halving and doubling the central cavity length, we also demonstrate dual- and six-wavelength operation with free spectral ranges of 0.75 nm and 0.27 nm, respectively highlighting the design versatility in tailoring the channel count and repetition frequency. This compact platform enables seamless photonic integration with semiconductor optical amplifiers, electroabsorption modulators, and other on-chip components, making it suitable for dense wavelength division multiplexing, coherent optical communications, and photonic sensing.

We demonstrate a novel 1550 nm AlGaInAs semiconductor laser with dual-mode operation; a mode-locked pulsed mode at 17.7 GHz with a timing jitter of 5.3 ps (10 kHz-100 MHz) and an RF linewidth of 22 kHz, and a single-wavelength continuous-wave mode with a 33 dB side-mode suppression ratio.

This paper presents a coupled-wave analysis of triangular-lattice photonic crystal surface emitting lasers (PCSELs) with transverse magnetic polarization. Six plane waves coupled by Bragg diffraction describe the two-dimensional optical coupling. Resonant mode frequencies are calculated for a lattice of circular holes at various fill factors and compared to the plane-wave expansion method. Analytical equations for coupling constants and mode frequencies are derived, and mode degeneracy as a function of fill factor is examined. Comparison to a square lattice TM mode PCSEL shows improved in-plane 2D coupling. The general equations for arbitrary unit cell dielectric functions are discussed, with predictions of the lasing mode supported by finite device calculations.

Current progress in the scaling of continuous wave optical output power and conversion efficiency of broad-area GaAs-based edge emitters, broad-area lasers (BALs), operating in the 900 & mldr;1000 nm wavelength range is presented. Device research and engineering efforts have ensured that BALs remain the most efficient of all light sources, so that in the past 10 years, power conversion efficiency at 20 W continuous wave (CW) output power from BA lasers with a 90 & mldr;100 mu m wide stripe has increased 1.5-fold to 57% (via epitaxial layer design developments), whilst peak CW power per single emitter has increased around 3-fold to 70 W (via scaling of device size), with further scaling underway, for example via use of multi-junction designs. However, the peak achievable CW power conversion efficiency and CW specific output power (defined here as peak output power from a 100 mu m stripe diode lasers with a single p-n junction) has changed remarkably little, remaining around 70% and 25 W, respectively, for the past decade. Fortunately, research to understand the limits to peak efficiency and specific output power has also shown progress. Specifically, recent studies indicate that spatial non-uniformity in optical field and temperature play a major role in limiting both power and conversion efficiency. Technological efforts motivated by these discoveries to flatten lateral and longitudinal temperature profiles have successfully increased both power and efficiency. In addition, epitaxial layer designs with very high modal gain successfully reduce threshold current and increase slope at 25 degrees C to values comparable to those observed at 200 K, offering a path toward the 80% conversion efficiency range currently seen only at these cryogenic temperatures. Overall, whilst operating efficiency and power continue to scale rapidly, a technological path for increased specific power and peak efficiency is also emerging.

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.

We present a new approach to quantum well laser design utilizing selectively-doped waveguides to enhance the efficiency and thermal stability of near- and mid-infrared lasers. For O-band lasers, we theoretically and experimentally assess the impact of this new approach on carrier recombination and Auger suppression.

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.

We have developed a series of dual-wavelength DFB lasers (DWLs) based on different sidewall grating designs. These include DWLs utilizing uniform Bragg gratings (UBG), conventional sampled Bragg gratings (C-SBG), two-phase shifted sampled Bragg gratings (2PS-SBG), and four-phase shifted sampled Bragg gratings (4PS-SBG) for THz signal generation. Additionally, we have explored the use of 4PS-SBG combined with equivalent chirp technology, lateral modulation of the grating coupling coefficient kappa, and four-phase-shifted sampled Moire gratings (4PS-SMG) for millimeter-wave (MMW) signal generation. All the DWLs were fabricated in the AlGaInAs/InP system operating around 1550 nm. For DWLs using UBG, even employing e-beam lithography at its resolution limit of 0.5 nm, the smallest frequency separation is constrained to approximately 400 GHz. C-SBG designs allow precise control of the frequency separation, down to 1.1 GHz, but the effective grating coupling coefficient is only 1/pi of that of UBG. Utilizing 2PS-SBG technology can automatically produce DWLs while maintaining the same sampling periods on both sides of the ridge waveguide. The 4PS-SBG demonstrates a higher effective kappa (approximately 0.9x that of UBG) compared to the 2PS-SBG (approximately 0.64x that of UBG). To ensure single longitudinal mode operation and mitigate longitudinal mode competition, an equivalent pi phase shift (EPS) is inserted at 1/3 of the DFB cavity length on one side, and another EPS is placed at 2/3 of the DFB cavity length on the opposite side. Using equivalent chirp methodology, the two peaks of the photon distributions of the two lasing modes are separated, significantly reducing the overlap region and ensuring stable dual-wavelength operation. Moreover, the wavelength separation can be adjusted by changing the chirp rate. Lateral modulation of the grating coefficient kappa allows tuning of the dual-wavelength separation by adjusting the DWL cavity length and the kappa value. We show that 4PS-SMG exhibits perfect apodization with a cosine profile and two pi phase shifts in the cavity, eliminating the need for intentional insertion of two pi phase shifts in the DWL cavity to achieve dual-wavelength operation. All the aforementioned DWLs serve as compact pumping sources for generating THz/MMW signals.

We report on the key design factors for the development of Type-II 'W'-lasers for O-band (1260-1360 nm) applications. We investigate the effects of InGaAs and GaAsSb quantum well composition and thicknesses on the emission wavelength and recombination efficiency as well as of (Al, Ga) As barriers on optimum electrical and optical confinement. Photoluminescence (PL) tests structures and full device structures were fabricated and characterised. 1.25 mu m emitting lasers were demonstrated with a threshold current density and Jth values of 480 +/- 10 A cm-2 at 290 K, whereas 1.3 mu m lasers showed an increased Jth value of 5.5-7 kA cm-2 at 290 K. The PL test structures exhibited a similar trend with decreasing intensity with increasing wavelength. Gain measurements of the 1.3 mu m device demonstrate reasonably low optical losses of 10-15 cm-1 and a threshold modal gain of approximate to 25 cm-1.

report, for the first time, a monolithic multi-wavelength passively mode-locked distributed feedback (DFB) laser operating simultaneously at four wavelengths near 1.55 mu m. The device incorporates two chirped sampled Bragg grating (SBG) designs within a single cavity: 1) chirped conventional SBG (C-SBG) and 2) chirped four-phase-shifted SBG (4PS-SBG). Both configurations achieve uniform 0.96 nm (similar to 120 GHz) wavelength spacing using a single DFB section electrode and monolithically integrated saturable absorber (SA) for synchronized passive mode-locking. This shared-cavity architecture ensures intrinsic stability of the frequency comb against environmental perturbations, as all longitudinal modes experience identical phase variations. The lasers exhibit high spectral purity with side-mode suppression ratios (SMSR) >20 dB, with the 4PS-SBG design offering wider bias current operation. Pulse characteristics include near-transform-limited performance for both designs: C-SBG yields 2.84 ps pulses (time-bandwidth product [TBP] = 0.334), while 4PS-SBG generates 2.79 ps pulses (TBP = 0.337). Fabrication employs a simplified ridge waveguide sidewall grating approach requiring only one metalorganic vapor phase epitaxy (MOVPE) step and a single III-V etch process, enhancing manufacturability. We further demonstrate the design's versatility by extending operation to six wavelengths using the 4PS-SBG structure. This integrated platform shows strong potential for dense wavelength division multiplexing (DWDM), coherent communications, and photonic sensing applications requiring compact, environmentally stable multi-wavelength sources.