Silicon nitride has been extensively studied as high-refractive index material for distributed Bragg’s reflectors planned to be used in the 3rd generation of Gravitational Wave Detectors working at cryogenic conditions. The absence of mechanical loss of this material at cryogenic conditions and its high refractive index, make this material be considered one of the best options for the mirrors of the GWDs. The optimization of composition and structure of SiNx thin films to refine optical (refractive index, and optical absorption), and morphology (surface roughness, defects) have been carried out mainly using ion beam sputtering (IBS), plasma enhanced chemical vapor deposition (PECVD) and low-pressure CVD (LPCVD). This work reports the characterization of both silicon nitride (SiNx) and a new alternative silicon oxynitride (SiOxNy) thin film, deposited by ammonia free based PECVD. We measured and analyzed the composition of the films, as well as their stress, surface roughness, and optical constants, including refractive index and extinction coefficient at λ = 1550 nm. Under our deposition conditions, superior properties in terms of high thickness uniformity – free of cracks – at wafer scale, low compressive stress (range of kPa), low surface roughness (<1 nm), and high refractive index 2.2 were achieved in both materials, with pure composition lacking contaminants.
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 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 high precision quantum 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 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 high precision quantum 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 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 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.
Cavity-based single photon emission possesses a very high potential for future quantum networks and quantum communication systems. Fabry-Perot cavities especially, are a good candidate for these applications, thanks to a circular mode profile emission and low-lasing threshold. These properties are related to the small volume of the active region and the use of highly reflective Distributed Bragg mirrors (DBRs). The reflectance of the DBRs is related to the finesse of the cavity. In order to assure a strong coupling in the cavity, a high finesse is required and therefore a reflectivity value as high as 99.9999%. Achieving such a difficult goal faces many technical challenges and limiting parameters such as optical losses (scatter and absorption) and other limitations related to thin film coating technologies. The control of the mirror fabrication and losses will be addressed in this paper.
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.
consumption and financial burden of the multiple light sources required for such systems. The AlGaInN material system allows for single transverse mode laser diodes to be fabricated with optical powers up to 100 mW over a wide range from ~380 nm up to ~530 nm. By tuning the indium content and thickness of the GaInN quantum well, we have developed a range of AlGaInN diode-lasers targeted to meet the wavelength and power requirements suitable for optical clocks and atom interferometry systems.
One of the major limiting factors in nitride laser diode development has been the lack of a suitable low defectivity and uniform GaN substrate. Recently, single crystal growth of large area, very low dislocation-density and uniform GaN substrates are grown using a combination of high temperature and high pressure enabling a range of AlGaInN laser technology to be developed. This direct light generation at the required wavelength is crucial to reduce complexity and size of the overall system, and to ensure a high wall-plug efficiency that is critical for space and mobile applications.
We will present our development of GaN based, low SWaP, frequency-stabilised external-cavity seed and tapered amplifiers to operate at 461nm for first stage strontium cooling. This includes growth of custom optimised GaN epitaxy for operation at 461 nm, a robust ECDL geometry, a novel tapered amplifier design and important work in characterising the optical performance and minimising surface reflectivity to identify suitable working parameters.
Systems with the ability to observe and manipulate individual quantum states have been brought to applications that include among others satellite-free navigation and high-precision gravimetric sensing. Fundamentally, the applicability of quantum technology is limited by the complexity and financial burden of light sources required for such systems. These sources need to feature high optical power combined with compromised beam quality and frequency-stabilized narrow-linewidths. These parameters directly influence the performance of the quantum technology measurement system.
Semiconductor devices are able to provide high brightness over broad spectral regions through band-gap engineering. InGaN-based laser sources can be engineered to operate from 380nm to 530 nm. This aligns well with the transitions of atomic species such as strontium, magnesium and ytterbium. However, a challenge remains to offer the narrow-linewidths (<1 MHz) and the high powers (>100 mW) required for many of these applications.
We will present our development of GaN based narrow-linewidth seed and tapered amplifiers to operate at 461nm for first stage strontium cooling. This includes growth of custom optimised GaN epitaxy for operation at 461 nm, a robust ECDL geometry, a novel tapered amplifier design and important work in characterising and minimising the surface reflectivity to identify suitable working parameters. A comprehensive characterization of the device will be presented.
Mid-IR carbon dioxide (CO2) gas sensing is critical for monitoring in respiratory care, and is finding increasing importance in surgical anaesthetics where nitrous oxide (N2O) induced cross-talk is a major obstacle to accurate CO2 monitoring. In this work, a novel, solid state mid-IR photonics based CO2 gas sensor is described, and the role that 1- dimensional photonic crystals, often referred to as multilayer thin film optical coatings [1], play in boosting the sensor’s capability of gas discrimination is discussed. Filter performance in isolating CO2 IR absorption is tested on an optical filter test bed and a theoretical gas sensor model is developed, with the inclusion of a modelled multilayer optical filter to analyse the efficacy of optical filtering on eliminating N2O induced cross-talk for this particular gas sensor architecture. Future possible in-house optical filter fabrication techniques are discussed. As the actual gas sensor configuration is small, it would be challenging to manufacture a filter of the correct size; dismantling the sensor and mounting a new filter for different optical coating designs each time would prove to be laborious. For this reason, an optical filter testbed set-up is described and, using a commercial optical filter, it is demonstrated that cross-talk can be considerably reduced; cross-talk is minimal even for very high concentrations of N2O, which are unlikely to be encountered in exhaled surgical anaesthetic patient breath profiles. A completely new and versatile system for breath emulation is described and the capability it has for producing realistic human exhaled CO2 vs. time waveforms is shown. The cross-talk inducing effect that N2O has on realistic emulated CO2 vs. time waveforms as measured using the NDIR gas sensing technique is demonstrated and the effect that optical filtering will have on said cross-talk is discussed.
Active control of the spectral and temporal output characteristics of solid-state lasers through use of MEMS scanning micromirrors is presented. A side-pumped Nd:YAG laser with two intracavity scanning micromirrors, enabling Q-switching operation with controllable pulse duration and pulse-on-demand capabilities, is investigated. Changing the actuation signal of one micromirror allows a variation of the pulse duration between 370 ns and 1.06 μs at a pulse repetition frequency of 21.37 kHz and average output power of 50 mW. Pulse-on-demand lasing is enabled through actuation of the second micromirror. To our knowledge this is the first demonstration of the use of multiple intracavity MEMS devices as active tuning elements in a single solid-state laser cavity. Furthermore, we present the first demonstration of control over the output wavelength of a solid-state laser using a micromirror and a prism in an intracavity Littman configuration. A static tilt actuation of the micromirror resulted in tuning the output wavelength of an Yb:KGW laser from 1024 nm to 1031.5 nm, with FWHM bandwidths between 0.2 nm and 0.4 nm. These proof-of-principle demonstrations provide the first steps towards a miniaturized, flexible solid-state laser system with potential defense and industrial applications.
The Laser Induced Damage Threshold (LIDT) and material properties of various multi-layer amorphous dielectric optical coatings, including Nb2O5, Ta2O5, SiO2, TiO2, ZrO2, AlN, SiN, LiF and ZnSe, have been studied. The coatings were produced by ion assisted electron beam and thermal evaporation; and RF and DC magnetron sputtering at Helia Photonics Ltd, Livingston, UK. The coatings were characterized by optical absorption measurements at 1064 nm by Photothermal Common-path Interferometry (PCI). Surface roughness and damage pits were analyzed using atomic force microscopy. LIDT measurements were carried out at 1064 nm, with a pulse duration of 9.6 ns and repetition rate of 100 Hz, in both 1000-on-1 and 1-on-1 regimes. The relationship between optical absorption, LIDT and post-deposition heat-treatment is discussed, along with analysis of the surface morphology of the LIDT damage sites showing both coating and substrate failure.
Multiple individually-controllable Q-switched laser outputs from a single diode-pumped Nd:YAG module are presented by using an electrostatic MEMS scanning micromirror array as cavity end-mirror. The gold coated, 700 μm diameter and 25 μm thick, single-crystal silicon micromirrors possess resonant tilt frequencies of ~8 kHz with optical scan angles of up to 78°. Dual laser output resulting from the actuation of two neighboring mirrors was observed resulting in a combined average output power of 125 mW and pulse durations of 30 ns with resulting pulse energies of 7.9 μJ and 7.1 μJ. The output power was limited by thermal effects on the gold-coated mirror surface. Dielectric coatings with increased reflectivity and therefore lower thermal stresses are required to power-scale this technique. An initial SiO2/Nb2O5 test coating was applied to a multi-mirror array with individual optical scan angles of 14° at a resonant tilt frequency of 10.4 kHz. The use of this dielectric coated array inside a 3-mirror Nd:YAG laser cavity led to a single mirror output with average Q-switched output power of 750 mW and pulse durations of 295 ns resulting in pulse energies of 36 μJ.
Plasma Assisted Chemical Vapor Deposition (PACVD) boron phosphide (BP) has long been established in service on materials such as germanium and FLIR grade zinc sulfide as a protective coating. As airborne systems are required to fly at higher speeds either coatings must become more protective or substrates must become more durable. For MWIR only systems it is logical to use silicon as a window or dome material as the natural hardness of the substrate provides good resistance to particle and rain erosion. The optical transmittance of uncoated silicon is not particularly good (~53% over the 3-5micrometers for a 5mm substrate and normal incidence, Pol=R, room temperature). Applying low energy multilayers on each surface boosts the transparency but offers no resistance to harsh environmental conditions. Although generally the silicon substrate is unaffected due to its hardness, the coating is eroded on the external surface and the transparency drops. With the deposition of boron phosphide by the improved PACVD process the adhesion is so great that not only does the BP not get removed by the erosion, but it protects the substrate at higher speeds. This paper presents data collected by several methods relating to airborne erosion by solid impact of an IR coating/substrate system.
Using well proven boron phosphide (BP) technology, Thales Optronics (formally Barr & Stroud Ltd.) have expanded the range of IR materials successfully protected to include gallium arsenide. BP has already been used as part of a dual band coating for FLIR grade ZnS which performs well environmentally and is currently used on several prototype dome systems. Having a hardness lower than germanium, gallium arsenide is perhaps not the first choice for applications required to perform well in harsh conditions however there are some other useful properties, among these is the recently reported ability to create low resistivity material for stealth applications and low free-carrier absorption at elevated temperatures. This paper will look at some of the measured optical and physical characteristics of this new substrate/coating system including rain erosion tested by whirling arm and solid particle erosion. In addition some attention will be given to the actual vs. theory performances and envisaged practical applications.
With the advent of common aperture systems comes a requirement for substrates and coatings that are transparent in both the visible and IR bands. While there are many suitable bulk materials there are surprisingly few coatings available that offer both antireflecting properties and substrate protection. Materials that need little environmental protection tend to be costly to fabricate and machine while others are far too soft to be of any great use. It is for this reason that particular attention has been given to multispectral zinc sulfide which is a relatively cheap material and has good transparency both in the visible and the IR up to -13micrometers . Although it is a soft material (~150kg.mm-2) it may be protected by a range of coatings. This paper will look at two main materials, ZrN deposited by RF reactive sputtering and YF3 by ion assisted deposition (IAD) which when used in conjunction offer both increased durability to the substrate and good tri-color transmittance for practical window applications.
Diamond is an ultra-durable material with high thermal conductivity and good transmission in the visible, near IR and far IR wavebands. Advances in the performance of synthetic diamond made by chemical vapor deposition promise an expanding range of applications for the material. An example is in advanced airborne windows and domes for high- speed flight, either as a window or as a protective coating for other IR window materials. Diamond has sufficient durability to withstand high-speed impact by solid particles and raindrops and a high level of thermal conductivity to minimize the effect of thermal shock due to aerodynamic heating. However, diamond is subject to oxidation in air at temperatures greater than 750 degrees C. After only a few seconds exposure at such temperatures the diamond surface becomes severely etched, and the optical transmission is degraded. Very high-speed flight can lead to temperatures in excess of 800 degrees C. This means that, in high temperature applications, a coating is required which can protect the diamond surface from exposure to air. In addition the coating must have excellent adhesion and mechanical durability, and itself be resistant to impact. Using sputtered coatings based on aluminium nitride, we have demonstrated complete protection for extended exposures at temperatures up to 1000 degrees C. The coatings also have excellent mechanical durability as demonstrated by particle erosion test.
Boron phosphide (BP) films deposited by plasma assisted chemical vapor deposition are known to be very effective in protecting substrate materials from solid and liquid particle erosion encountered in high speed flight. This paper describes the result obtained by protecting the 3-5 micrometers band substrate material silicon and the soft 8-11.5 micrometers band substrate materials zinc selenide. In addition results are presented to show that germanium can be very effectively protected from the corrosive effects of salt water and salt fog.
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