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.

GaN laser diodes have the potential to be a key enabler for many quantum technologies, including quantum sensing, precision metrology, quantum communications and quantum computing, since the AlGaInN material system allows for laser diodes to be fabricated over a wide range of wavelengths from ultra-violet to visible. Furthermore, GaN allows the development of very high specification laser diode sources that are portable, robust and provide practical solutions that are otherwise unobtainable using more conventional laser sources. Novel applications for quantum technologies include GaN laser sources for cold-atom interferometry, such as next generation optical atomic clocks, quantum sensors and quantum metrology. Several approaches are taken to achieve the required linewidth, wavelength and power for cold-atom interferometry, including an extended cavity GaN laser diode (ECLD) system, and a distributed feedback ( DFB) GaN laser diode with side- wall etched nano-gratings. We report the development of a generic passive waveguide photonic integrated circuit (PICs) platform for quantum applications that covers all the key cold-atom wavelengths for quantum sensing, including the 369nm cooling transition for Yb+, 422nm for Sr + and 461nm for Sr neutral.

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.

The design and fabrication of a gallium nitride (GaN) based distributed feedback (DFB) laser diode (LD) with 3rd order sidewall gratings is demonstrated here. The aim is to provide a compact, narrow-linewidth light source for the cooling transition in strontium optical atomic clocks, which requires high accuracy in emission wavelength. These devices also have key roles to play in optical communications, sensing and medical applications.

Quantum technologies containing key GaN laser components will enable a new generation of precision sensors, optical atomic clocks and secure communication systems for many applications such as next generation navigation, gravity mapping and timing since the AlGaInN material system allows for laser diodes to be fabricated over a wide range of wavelengths from the U.V. to the visible. We report our latest results on a range of AlGaInN diode-lasers targeted to meet the linewidth, wavelength and power requirements suitable for quantum sensors such as optical clocks and cold-atom interferometry systems. This includes the [5s2S1/2-5p2P1/2] cooling transition in strontium+ ion optical clocks at 422 nm, the [5s21S0-5p1P1] cooling transition in neutral strontium clocks at 461 nm and the [5s2 s1/2 – 6p2P3/2] transition in rubidium at 420 nm. Several approaches are taken to achieve the required linewidth, wavelength and power, including an extended cavity laser diode (ECLD) system and an on-chip grating, distributed feedback (DFB) GaN laser diode.

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.

Quantum technologies containing key GaN laser components will enable a new generation of precision sensors, optical atomic clocks and secure communication systems for many applications such as next generation navigation, gravity mapping and timing since the AlGaInN material system allows for laser diodes to be fabricated over a wide range of wavelengths from the U.V. to the visible. We report our latest results on a range of AlGaInN diode-lasers targeted to meet the linewidth, wavelength and power requirements suitable for quantum sensors such as optical clocks and cold-atom interferometry systems. This includes the [5s2S1/2-5p2P1/2] cooling transition in strontium+ ion optical clocks at 422 nm, the [5s2 1S0-5p1P1] cooling transition in neutral strontium clocks at 461 nm and the [5s2 s1/2 – 6p2P3/2] transition in rubidium at 420 nm. Several approaches are taken to achieve the required linewidth, wavelength and power, including an Extended Cavity Laser Diode (ECLD) system and an on-chip grating, distributed feedback (DFB) GaN laser diode.

Miniaturization of laser sources is crucial to the translation of quantum technologies from the laboratory to the real world. Typically, the lasers required for cooling and trapping of atoms and ions make up a significant footprint of the measurement system. Increasing robustness and reliability whilst removing noise sources is a key challenge whilst reducing volume. Direct generation GaN based external cavity diode lasers offer lower SWaP-C compared to traditional frequency doubled alternatives. Butterfly packaged single frequency sources operation in the blue-UV allow numerous atomic transitions including Sr, Sr+, Yb, Yb+, Mg and Ca to be targeted.

GaN laser diodes have the potential to be a key enabler for many quantum technologies, including quantum sensing, optical atomic clocks and ion-trap & neutral atom quantum computing, since the AlGaInN material system allows for laser diodes to be fabricated over a wide range of wavelengths from ultra-violet to visible, allowing the development of very high specification laser diode sources that are portable, robust and provide practical solutions that are otherwise unobtainable using more conventional laser sources. We report our latest results on a range of AlGaInN diode-lasers targeted to meet optical atomic clock and quantum gravity sensor applications. This includes the [5s2S1/2-5p2P1/2] cooling transition in strontium+ ion optical clocks at 422 nm, the [5s21S0-5p1P1] cooling transition in neutral strontium clocks at 461 nm and the [5s2s1/2 – 6p2P3/2] transition in rubidium at 420 nm.

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.

We have fabricated tunnel-junction InGaN micro-LEDs using plasma-assisted molecular beam epitaxy technology, with top-down processing on GaN substrates. Devices have diameters between 5 µm and 100 µm. All of the devices emit light at 450 nm at a driving current density of about 10Acm-2. We demonstrate that within micro-LEDs ranging in size from 100 µm down to 5 µm, the properties of these devices, both electrical and optical, are fully scalable. That means we can reproduce all electro-optical characteristics using a single set of parameters. Most notably, we do not observe any enhancement of non-radiative recombination for the smallest devices. We assign this result to a modification of the fabrication process, i.e., replacement of deep dry etching by a tunnel junction for the current confinement. These devices show excellent thermal stability of their light emission characteristics, enabling operation at current densities up to 1kAcm-2.

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.

Quantum technologies containing key GaN laser components will enable a new generation of precision sensors, optical atomic clocks and secure communication systems for many applications such as next generation navigation, gravity mapping and timing since the AlGaInN material system allows for laser diodes to be fabricated over a wide range of wavelengths from the U. V. to the visible. We report our latest results on a range of A1GaInN diode-lasers targeted to meet the linewidth, wavelength and power requirements suitable for quantum sensors such as optical clocks and cold-atom interferometry systems. This includes the [5s(2)S(1/2)-5p(2)P(1/2)] cooling transition in strontium+ ion optical clocks at 422 nm, the [5s(2)(1) S-0-5p(1)P(1)] cooling transition in neutral strontium clocks at 461 nm and the [5s(2)s(1/2) - 6p(2)P(3/2)] transition in rubidium at 420 nm. Several approaches are taken to achieve the required linewidth, wavelength and power, including an extended cavity laser diode (ECLD) system and an on-chip grating, distributed feedback (DFB) GaN laser diode.

Gallium nitride (GaN) doped with germanium at a level of 1020 cm−3 is proposed as a viable material for cladding layers in blue- and green-emitting laser diodes. Spectral reflectometry and ellipsometry are used to provide evidence of a reduced index of refraction in such layers. The refractive-index contrast to undoped GaN is about 0.990, which is comparable to undoped aluminium gallium nitride (AlGaN) with an aluminium composition of 6%. Germanium-doped GaN layers are lattice-matched to native GaN substrates; therefore, they introduce no strain, cracks, and wafer bowing. Their use, in place of strained AlGaN layers, will enable significant improvements to the production process yield.

We report on the characterization and analysis of a GaN-based distributed feedback laser diode (DFB-LD) with 3rd-order laterally etched sidewall gratings centered at a wavelength of 420 nm. We also compare the device parameters with two commonly used Fabry-Perot (FP) devices operating at 450 nm and 520 nm. Intrinsic properties of the devices were extracted, including damping factor, carrier and photon lifetimes, modulation efficiency, differential gain, and parasitic capacitance. These parameters showed that the DFB exhibits a lower damping rate and parasitic capacitance while demonstrating a higher modulation efficiency, indicating that the DFB shows good potential for communications applications. Additionally, spectral linewidth of a GaN DFB is reported. To the authors' knowledge, this is the first demonstration of parameter extraction and spectral linewidth measurement for GaN-based DFB-LDs.

Quantum technologies containing key GaN laser components will enable a new generation of precision sensors, optical atomic clocks and secure communication systems for many applications such as next generation navigation, gravity mapping and timing since the AlGaInN material system allows for laser diodes to be fabricated over a wide range of wavelengths from the U.V. to the visible. We report our latest results on a range of AlGaInN diode-lasers targeted to meet the linewidth, wavelength and power requirements suitable for quantum sensors such as optical clocks and cold-atom interferometry systems. This includes the [5s2S1/2-5p2P1/2] cooling transition in strontium+ ion optical clocks at 422 nm, the [5s21S0-5p1P1] cooling transition in neutral strontium clocks at 461 nm and the [5s2s1/2 – 6p2P3/2] transition in rubidium at 420 nm. Several approaches are taken to achieve the required linewidth, wavelength and power, including an extended cavity laser diode (ECLD) system and an on-chip grating, distributed feedback (DFB) GaN laser diode.

Quantum technologies containing key GaN laser components will enable a new generation of precision sensors, optical atomic clocks and secure communication systems for many applications such as next generation navigation, gravity mapping and timing since the AlGaInN material system allows for laser diodes to be fabricated over a wide range of wavelengths from the u.v. to the visible. We report our latest results on a range of AlGaInN diode-lasers targeted to meet the linewidth, wavelength and power requirements suitable for quantum sensors such as optical clocks and cold-atom interferometry systems. This includes the [5s2S1/2-5p2P1/2] cooling transition in strontium+ ion optical clocks at 422 nm, the [5s21S0-5p1P1] cooling transition in neutral strontium clocks at 461 nm and the [5s2s1/2 – 6p2P3/2] transition in rubidium at 420 nm. Several approaches are taken to achieve the required linewidth, wavelength and power, including an extended cavity laser diode (ECLD) system and an on-chip grating, distributed feedback (DFB) GaN laser diode.

In this chapter, we recall briefly the history of nitride semiconductor laser diodes and the main technical and scientific challenges related to their development and future perspective. We describe in more detail the recently emerged distributed feedback (DFB) nitride laser diodes and their possible applications. We also address devices that are less known but closely related to laser diodes such as superluminescent diodes and optical amplifiers, the latter ones being components needed for advanced visible light photonic systems.