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Experimental Implementation of a Fiber Noise Cancellation System for Slow Light Laser Locking

Alexander Bengtsson

Published Jan 1, 2017• 0 citations
OpenAlex

Abstract

An electromagnetic wave propagating through an optical fiber is sensitive to external disturbances, such as mechanical vibrations and temperature and pressure fluctuations, which add phase noise onto the wave. For high-precision frequency-based applications this phase noise has to be removed. By partially reflecting light from the remote end of the fiber back through the fiber, and comparing it to light that has not passed through the fiber, their beat note can be phase locked to a stable reference, using an acousto-optic modulator for phase noise compensation. This effectively cancels the phase noise at the remote end of the fiber. In this project, two such fiber noise cancellation systems have been successfully assembled. The fiber noise cancellation systems will be used for laser locking to a slow light crystal cavity. Using spectral hole burning, a very low group velocity can be obtained in rare-earth-ion-doped crystals. By depositing mirrors on such a crystal, it could act as a reference cavity with a very long effective length, but without the increased difficulty of isolating the cavity from mechanical vibrations associated with a physically long cavity. By locking a laser to a slow light crystal cavity, an improved stability could potentially be achieved. A possible setup for slow light crystal cavity laser locking experiments, where the fiber noise cancellation systems have been integrated, is also presented in this thesis.

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An electromagnetic wave propagating through an optical fiber is sensitive to external disturbances, such as mechanical vibrations and temperature and pressure fluctuations, which add phase noise onto the wave. For high-precision frequency-based applications this phase noise has to be removed. By partially reflecting light from the remote end of the fiber back through the fiber, and comparing it to light that has not passed through the fiber, their beat note can be phase locked to a stable reference, using an acousto-optic modulator for phase noise compensation. This effectively cancels the phase noise at the remote end of the fiber. In this project, two such fiber noise cancellation systems have been successfully assembled. The fiber noise cancellation systems will be used for laser locking to a slow light crystal cavity. Using spectral hole burning, a very low group velocity can be obtained in rare-earth-ion-doped crystals. By depositing mirrors on such a crystal, it could act as a reference cavity with a very long effective length, but without the increased difficulty of isolating the cavity from mechanical vibrations associated with a physically long cavity. By locking a laser to a slow light crystal cavity, an improved stability could potentially be achieved. A possible setup for slow light crystal cavity laser locking experiments, where the fiber noise cancellation systems have been integrated, is also presented in this thesis.
Recent developments in the field of optical clocks and quantum information have fueled the necessity for both fiber phase noise cancellation and laser stabilisation. The Quantum Information group at Lund University is conducting research on quantum computation experiments based on rare earth ion doped crystals and also application of slow light effects induced by spectral hole burning in these rare earth ion doped crystals. Slow light effects can significantly improve laser stabilisation by increasing effective cavity length without inducing mechanical noise. In this thesis a fiber phase noise cancellation setup was designed and built. The fiber stabilisation will be used in a setup where a laser is stabilised using a slow light crystal cavity as frequency reference and a setup was designed for this application. By partially reflecting back light through a fiber and by performing a phase measurement and through feedback control, the fiber phase noise can be effectively cancelled. A method for first creating a slow light crystal cavity and then locking the laser to one of the resonance peaks of the cavity was planned and designed incorporating two fiber locking setups for phase noise cancellation. Suitable components for building the system was investigated and put together. Fiber locking experiment was then carried out.
In this thesis the application of slow light effects to the area of laser stabilisation is investigated. The frequency stability of lasers locked to external cavities is limited by the stability of the modes in the reference cavity. By using dispersive slow light effects the effective length of a cavity can be increased by several orders of magnitude, without a proportional increase in the sensitivity to fluctuations in cavity length, thereby yielding a net increase in stability. The effects are achieved through spectral hole burning in cryogenically cooled rare-earth ion doped crystals. Two sets of experiments are carried out, the first focusing on hole burning and the second focusing laser locking. Linewidths and frequency drift of the cavity spectrum are studied. Feedback control using an error signal generated from the slow light cavity is demonstrated, but successfully locking the laser is still yet to be achieved.
We present a technique for the stable transfer of an optical frequency reference over a kilometer-scale optical fiber link. This technique implements phase measurements and laser feedback to cancel out the phase fluctuations that are introduced to an optical frequency standard as it passes through the fiber. We also present results for a bench top experiment, developed for the Advanced LIGO lock acquisition system, where this technique is implemented to phase-lock two Nd:YAG lasers, through a 4.6 km optical fiber. The resulting differential optical frequency noise reaches a level as low as 0.5 mHz/ radical Hz for Fourier frequencies between 5 Hz and 20 Hz, which is equal to a fractional frequency stability of 1.7 x 10(-18)/ radical Hz.
Comparison of ultra-stable frequency references located in geographically distributed areas require a transfer technique, which offers stability of 10−17at 1 s over tens of kilometer. Such remote frequency transfer will allow comparing the optical frequency references of same or different atomic species (through a frequency comb bridge) for test of relativistic geodesy [1] or time variation of fundamental constants [2], and will contribute towards the database of frequency ratio measurements-an essential requirement for the re-definition of the SI second based on optical clocks [3]. It will also facilitate in creating an optical to microwave frequency link that can be used by various SI standards laboratory requiring stable microwave sources. In this paper, I discuss the modulated optical carrier wave based ultra-stable frequency reference transfer technique, and present a study of a laser interferometer based active fiber noise cancellation system and its applications.
Lasers with high spectral purity are indispensable for optical clocks and coherent manipulation of atomic and molecular qubits for applications such as quantum computing and quantum simulation. Stabilisation of the laser to a reference can provide a narrow linewidth and high spectral purity. However, widely-used diode lasers exhibit fast phase noise that prevents high fidelity qubit manipulation. Here we demonstrate a self-injection locked diode laser system utilizing a medium finesse cavity. The cavity not only provides a stable resonance frequency, but at the same time acts as a low-pass filter for phase noise beyond the cavity linewidth of around 100 kHz, resulting in low phase noise from dc to the injection lock limit. We model the expected laser performance and benchmark it using a single trapped 40^{40}40Ca+^{+}+-ion as a spectrum analyser. We show that the fast phase noise of the laser at relevant Fourier frequencies of 100 kHz to >2 MHz is suppressed to a noise floor of between -110 dBc/Hz and -120 dBc/Hz, an improvement of 20 to 30 dB over state-of-the-art Pound-Drever-Hall-stabilized extended-cavity diode lasers. This strong suppression avoids incoherent (spurious) spin flips during manipulation of optical qubits and improves laser-driven gates in using diode lasers with applications in quantum logic spectroscopy, quantum simulation and quantum computation.
We present a technique for coherence transfer of laser light through a branching fiber link, where the optical phase noise induced by environmental perturbations via the fiber link is passively compensated by remote users without the requirements of any active servo components. At each remote site, an acousto-optic modulator (AOM) is simultaneously taken as a frequency distinguisher for distinguishing its unique frequency from other sites' and as an optical actuator for compensating the phase noise coming from the optical fiber. With this configuration, we incorporate a long outside loop path consisting of a fiber-pigtailed AOM into the loop, enabling the significant reduction of the outside loop phase noise in the passive way. To further address the residual out-of-loop phase noise coming from the interferometer and the two-way optical frequency comparison setup, we design a low-noise active temperature stabilization system. Measurements with a back-to-back system show that the stability in our stabilization system is 2×10-16at 1 s, reaching 2×10-20after 10,000s. Adopting these techniques, we demonstrate transfer of a laser light through a branching fiber network with 50 km and 145 km two fiber links. After being compensated for the 145 km fiber link, the relative frequency instability is 3.4×10-15at the 1 s averaging time and scales down to 3.7×10-19at the 10,000 s averaging time. This proposed technique is suitable for the simultaneous transfer of an optical signal to a number of independent users within a local area.