Uploaded July 2022 | Updated September 2026, 5 days ago
We demonstrate ‘tilt locking’ with performance near the LISA frequency stability requirement and explore thermal noise reduction by a factor of 1.6, using a combination of resonant higher order spatial modes.
The LISA mission will use laser interferometry to measure gravitational waves in space by taking precise measurements of distance. Laser light is a useful tool for ranging measurements as interferometric readout can be used to convert its relatively short and stable wavelength into an ultra-precise probe of strain. While lasers offer high fractional frequency stability, the required level of precision to measure gravitational waves in space demand that lasers be actively stabilised to avoid 'measuring stick’ errors. We explore the utility of using higher order spatial modes in laser frequency stabilisation techniques and discuss two specific methods currently being implemented at the Centre for Gravitational Astrophysics, an OzGrav node, at the Australian National University.
We demonstrate an implementation of “tilt locking”, a laser stabilisation technique where a laser is locked to an ultra-stable frequency reference (an optical cavity), and an error signal is generated by measuring the phase difference between a resonant fundamental cavity spatial mode and a higher order (TEM01) spatial mode produced by an intentional tilt of the input beam. This technique is an elegant approach and offers several benefits over more traditional locking techniques, such as Pound-Drever-Hall (PDH) locking, like the absence of radio frequency modulation and a simplified optical path. We demonstrate tilt-locking over LISA relevant timescales and present an amplitude spectral density over the LISA bandwidth, near to the LISA frequency stability requirement. We show that tilt locking has a similar performance to PDH locking and can approach fundamental shot noise and thermal noise limits, making it a potential candidate for space applications requiring high stability.
We present another use of higher order spatial modes in a proposed experiment to reduce the effect of thermal noise in optical cavity mirror coatings, that is currently a limiting noise in many metrology and fundamental physics applications. While current strategies to mitigate thermal noise have demonstrated promising improvements, some are challenged by the demanding environment of space, where size, weight and power requirements are carefully considered. We propose an optimised method for laser readout that offers a multiplying factor of improvement on current approaches to reduce thermal noise. We propose sampling the cavity with a fundamental mode and several higher order modes, where the resulting readout signals are combined in digital signal processing to synthesise a flattened beam profile on the cavity mirrors. By widening the effective spatial sampling within the cavity, the Brownian motion over the surface of the mirrors can be better averaged, lowering the effect of thermal noise by up to a factor of 1.6 with just three modes.
Utilising high order spatial modes in laser frequency stabilisation techniques – both for generating readout error signals or as a way of reducing thermal noise – offers opportunities for higher stability readout and lower noise with promising performance and implementations advantages for space-based laser instrumentation.
Author and Presenter: Namisha Chabbra
We demonstrate ‘tilt locking’ with performance near the LISA frequency stability requirement and explore thermal noise reduction by a factor of 1.6, using a combination of resonant higher order spatial modes.
The LISA mission will use laser interferometry to measure gravitational waves in space by taking precise measurements of distance. Laser light is a useful tool for ranging measurements as interferometric readout can be used to convert its relatively short and stable wavelength into an ultra-precise probe of strain. While lasers offer high fractional frequency stability, the required level of precision to measure gravitational waves in space demand that lasers be actively stabilised to avoid 'measuring stick’ errors. We explore the utility of using higher order spatial modes in laser frequency stabilisation techniques and discuss two specific methods currently being implemented at the Centre for Gravitational Astrophysics, an OzGrav node, at the Australian National University.
We demonstrate an implementation of “tilt locking”, a laser stabilisation technique where a laser is locked to an ultra-stable frequency reference (an optical cavity), and an error signal is generated by measuring the phase difference between a resonant fundamental cavity spatial mode and a higher order (TEM01) spatial mode produced by an intentional tilt of the input beam. This technique is an elegant approach and offers several benefits over more traditional locking techniques, such as Pound-Drever-Hall (PDH) locking, like the absence of radio frequency modulation and a simplified optical path. We demonstrate tilt-locking over LISA relevant timescales and present an amplitude spectral density over the LISA bandwidth, near to the LISA frequency stability requirement. We show that tilt locking has a similar performance to PDH locking and can approach fundamental shot noise and thermal noise limits, making it a potential candidate for space applications requiring high stability.
We present another use of higher order spatial modes in a proposed experiment to reduce the effect of thermal noise in optical cavity mirror coatings, that is currently a limiting noise in many metrology and fundamental physics applications. While current strategies to mitigate thermal noise have demonstrated promising improvements, some are challenged by the demanding environment of space, where size, weight and power requirements are carefully considered. We propose an optimised method for laser readout that offers a multiplying factor of improvement on current approaches to reduce thermal noise. We propose sampling the cavity with a fundamental mode and several higher order modes, where the resulting readout signals are combined in digital signal processing to synthesise a flattened beam profile on the cavity mirrors. By widening the effective spatial sampling within the cavity, the Brownian motion over the surface of the mirrors can be better averaged, lowering the effect of thermal noise by up to a factor of 1.6 with just three modes.
Utilising high order spatial modes in laser frequency stabilisation techniques – both for generating readout error signals or as a way of reducing thermal noise – offers opportunities for higher stability readout and lower noise with promising performance and implementations advantages for space-based laser instrumentation.
Author and Presenter: Namisha Chabbra
