Future generations of gravitational wave detectors are being designed to scan most of the visible universe and observe exotic sources of gravitational waves such as neutron stars, supernovae and so on. Future detectors like LIGO-Voyager, Cosmic Explorer will very likely use cryogenically used Silicon mirrors (to lower thermal noise and increased robustness to laser power induced distortions of test masses). This calls for development of new laser sources and with that new squeezed light sources.

2 𝜇m laser source: We are currently developing an External Cavity Diode Laser [1] at 2 𝜇m which is a simple, inexpensive laser source that has applications beyond gravitational wave interferometry for example in in medicine, gas sensing, remote sensing, precision metrology, optical telecommunications, defence and LIDAR.

External Cavity Diode Laser with a gain chip and a diffraction grating in Littrow configuration Credit: Disha Kapasi, CGA

Frequency noise performance of the External Cavity Diode Laser (lavender) compared to the Tm fiber laser (red) and a 1.55 µm reference (green) [1].

2 𝜇m squeezer source:

We have also developed a squeezed light source at 2 𝜇m [2] which has currently produced 3.9±0.2  dB of phase controlled squeezing from 2 kHz to 80 kHz and 14.2±0.3  dB of anti-squeezing relative to the shot noise level [3]. The new phase control scheme implemented here has allowed examination of noise behavior at frequencies below 1 kHz and indicated that squeezing below this frequency was limited by dark noise of the photodiodes themselves and scattered light. Future work is planned to push the levels of squeezed light generation to as low as 12 dB below the shot noise level as well as to lower frequencies.

2 μm squeezer experimental schematic. The setup is broken up into 5 stages. (A) The main 1984 nm (red) laser is frequency stabilised to a fibre Mach-Zender interferometer. (B) The frequency-doubled 992 nm pump field (green) is generated from the Second Harmonic Generator. (C) The 992 nm field was then used to pump and stabilise the Optical Parametric Oscillator (OPO) (D1). A fibre tap-off (blue) from the main laser is used to generate the coherent locking field (orange). This field is then injected through the OPO rear mirror and was phase locked to the OPO pump field . (D2) The OPO squeezed output was directed towards a balanced homodyne detector where the final measurement was made. The transmitted coherent locking field interferes with the homodyne LO and was used to stabilise the squeezing ellipse angle [Credit : Min Jet Yap, PhD thesis, 2020].

Plot of a zero span squeezing measurement at 50 kHz with magnitude referenced to shot noise. In this plot we first see a scan of squeezing phase showing the magnitude of squeezing and anti-squeezing before the scan is stopped and the control loop engaged [3].

[1] D. P. Kapasi, J. Eichholz, T. McRae, R. L. Ward, B. J. J. Slagmolen, S. Legge, K. S. Hardman, P. A. Altin, and D. E. McClelland, “Tunable narrow-linewidth laser at 2 μm wavelength for gravitational wave detector research,” Opt. Express 28, 3280-3288 (2020).

[2] G. L. Mansell, T. G. McRae, P. A. Altin,M. J. Yap, R. L.Ward, B. J. J. Slagmolen, D. A. Shaddock, and D. E. McClelland. Observation of squeezed light in the 2 μm region. Phys. Rev. Lett., 120:203603, May 2018.

[3] M. J. Yap, D. W. Gould, T. G. McRae, P. A. Altin, N. Kijbunchoo, G. L. Mansell, R. L. Ward, D. A. Shaddock, B. J. J. Slagmolen, and D. E. McClelland. Squeezed vacuum phase control at 2 μm. Optics Letters, 44(21):53865389, 2019.