MacV: Miniaturised atomic clocks using VCSEL pump sources

Coherent population trapping (CPT) based miniature atomic clocks require low power, single mode laser diodes that can be directly modulated at a few gigahertz. Vertical Cavity Surface Emitting Lasers (VCSELs) are ideal for this application primarily due to their very low power consumption, wide wavelength tuning coefficient, reduced sensitivity to optical feedback, extended device lifetime, and small device footprint. Commercially available VCSELs have linewidths of ~50-100 MHz, and while this can be a problem for many other laser spectroscopy applications, it does not substantially compromise the quality of a CPT resonance. Conversely, due to the circular beam profile, VCSELs are particularly susceptible to polarisation instabilities; however, there are several novel design modifications that can be implemented to address this issue. Currently, there are no UK sources or any supply chain of reliable and robust VCSELs for miniature atomic clocks and a very limited number of commercial manufacturers globally developing VCSELs at the opimium wavelength for the application (Cs D1 - 894nm). Our project will establish a UK strategic capability focussed on the development and volume production of VCSEL laser sources, tailored specifically for to support the adoption of miniaturised atomic clock applications.
This research team will focus on designing the epitaxial layers for operation at 894nm with maximum modulation speed, while maintaining a reasonable linewidth. It will contribute to the design of the fabricated structure to ensure a single polarisation.
It will also contribute to the characterisation of chip scale prototypes and test structures for fast feedback to the epitaxial growth and for evaluation of the designed structures at the chip level.

The findings of the QT roadmap indicate 'the market for quantum timing devices is expected to become a £100 million in 5-10 years, growing to a multi-£100M market in 10-20 years.' Other applications and spin-off opportunities include VCSELs at 894 nm for Cs-based magnetometers, which exploits the Faraday effect which, along with the CPT method, is shown be more effective at the D1 line of Caesium. We are conscious of the growing demand for high resolution miniature magnetometer sensors, particularly in medical and marine sensing sectors. Currently, this market is served by SQUID based sensors; however, these are considered too bulky and expensive, and is thus severely restricting the widespread adoption. It is anticipated that optical magnetometers will supersede SQUID based sensors within the next 5 years.
The primary exploitation route will be derived from direct sales by the consortiums industrial partners. CST will offer both chip scale and packaged (eg TO can format) VCSELs as a qualified foundry platform in line with its existing business model. This will lead to several types of revenue:
- generic product sales of VCSEL die and packaged devices to the fixed MacV specification;
- sale of custom parts derived from the MacV platform in collaboration with specific end users;
- foundry sales from VCSEL based wafer fabrication services for broader applications for high performance VCSELs.
CST currently manufactures a range of custom and generic laser diode solutions in volumes of 10k-1M per month, and so has the experience and infrastructure to develop and ramp to mid-high volumes. CSC will derive revenue from wafer epitaxial sales to CST, which usually amount to 25-30% of the chip manufacturing cost. In addition, the offering will compliment and reinforce the existing foundry wafer business of CSCs parent company IQE (currently ~ £10M pa related to VCSEL applications) by opening up new volume applications for VCSELs. NPL will derive sales by licencing the miniaturised atomic clock design to manufacturers, and provide measurement and calibration services to systems specialists and potential end-users such as e2v, Airbus, Plextek, MBDA, Raytheon, Jodrell Bank, Leonardo, BT, and DSTL. Furthermore, the opportunity will promote and further NPLs role in engaging and collaborating with its strategic industrial partners.
The consortiums value chain will be protected by the existing IP strategies of the partners- it is likely that chip fab and epitaxial process IP will be protected as proprietary know-how. Novel materials, chip geometry, aspects relating to CPT design and new single stage process IP will be formally protected by patent, the exact terms of which will be defined in our collaboration agreement. The complete value chain generated by the consortium will be highly exploitable given the output will be collectively optimised from material to sub-system. However, it should be emphasised that the partners will have the freedom to exploit 'spin-off' opportunities for individual components.
Dissemination will be delivered via marketing collateral of the industrial partners, and by wider participation in UK and international trade shows and workshops. In particular we will utilise CSC+IQEs Open Innovation programme to generate new applications and product development consortia. This typically delivers regional themed innovation boot camps and culminates in an annual open Innovation Conference in Cardiff. The will be selective opportunities for academic publications relating to design and characterisation of the MacV platform and sample devices will be freely available to the UK academic community via Cardiff Universities Manufacturing Hub initiative to promote new applications and systems R+D.