Fedor Karpushko 1, Mark Mackenzie 1, Paul White 1, Gerald Bonner 2, Alex Lagatsky 2, Bence Szutor 1, Jonathan Jones 3, Yeshpal Singh 3
1 UniKLasers Ltd
2 Fraunhofer Centre for Applied Photonics
3 University of Birmingham
Research on the development of DPSS lasers suitable for quantum technologies
applications at 698.45 nm and 780.24 nm targeting Strontium and Rubidium transitions.
Quantum technology based on lasers has potential uses in many existing and developing applications. Existing atomic clocks are used in GPS satellites and in the synchronization of systems across the world. Improved atomic clocks based on elements such as Strontium offer significant improvements in accuracy and could lead to completely new uses.
Detecting buried or hidden objects can currently be done using techniques such as radar, acoustics & spring-based detectors. This is useful for applications such as locating natural resources & infrastructure inspection. Quantum sensors based on cold atoms could lead to improved sensitivity. With our project partners we developed lasers at 698.45 & 780.24 nm targeting Strontium & Rubidium transition lines for use in quantum applications.
Miniaturisation of laser systems
In addition to stability, another key requirement for a laser system is its size. It becomes particularly important when these systems are utilised in real world environments. This could in turn lead to exposure to high levels of vibration, temperature fluctuation, dirt & humidity. Two projects were undertaken to see how small a single-frequency laser we could create at 698.45 and 780 nm. The first system was built with a footprint of 20x8x7 cm using blue diodes pumping Pr:YLF. As shown in Fig 1., temperature changes correlated with the laser’s output power. This implied a need to improve the locking scheme which was implemented in the subsequent systems.
Fig 1: 698.45 nm system. Left: laser exterior and Fabry Perot interferometer showing single frequency. Right: Frequency response to temperature.
For the second system, research was done on the optimal pump scheme. Shown is a compact setup with a 640 nm diode pumping Alexandrite operating at 765 nm. While this gave good power, it was decided to move to a DPSS 640 nm pump, which gives superior mode matching and improved transverse mode behaviour. Powers up to 200 mW were achieved, now at 780 nm, at multi-frequency. Results from both projects were used in the subsequent Gravity Pioneer project.
Fig 2: Initial 765 nm system using diode pump. Left: The cavity layout on a benchtop. Right: Laser slope efficiency.
Fig 3: 780 nm system using a DPSS 640 nm laser as the pump source. This showed improved stability as well as increased power.
Single frequency system
Quantum applications typically require very small laser linewidths with high absolute frequency stability in order to target the narrow absorption lines of ultra-cold atoms.
To achieve this we use a “Bragg Range Michelson Mode Selection” technique, a variation of tilt locking, to ensure the laser is single-frequency & to compensate for a drift in spectral position. There is a very faint transverse mode in the laser cavity in addition to the dominant TEM00 mode. By overlapping these on a split detector it is possible to detect changes in frequency due to the different phase behaviour of the two modes.
Long term stabilisation
The Gravity Pioneer project aims to create a quantum-based gravity sensor usable in real setting for detection of objects and inspection of infrastructure. For more details of the projects please visit www.gravity-pioneer.org.
Using results of our previous projects we constructed an improved 780.24 nm system with single frequency performance and high stability.
Despite containing effectively two laser systems, a diode pumped 640 nm laser,
which in turn pumped a 780.24 nm laser,
the footprint of the laser was maintained at 20x8x7 cm.
The system remained single frequency, without mode hops, over 20 hours with reduced reaction to thermal changes. As shown the stability is ± 30 mHz. The linewidth was measured to be 270 kHz.
The project is ongoing with the goal of improving system power, stability & linewidth.
Fig 4: Laser frequency over time measured using a WS7 wavemeter (HighFinesse). Long term frequency drift is attributed to temperature.
This joint poster on DPSS Lasers for Quantum Applications was originally presented at the OSA Laser Congress 2019 in Vienna. Our successful collaboration with research heavyweights Fraunhofer UK and the University of Birmingham on projects like PLAID and MINUSQULE has established three new product lines: Solo 780.24 QT Series, Solo 698.4 QT Series and Solo 689.4 QT Series, targeting rubidium and strontium transitions