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Applied Optics

Narrow-bandwidth tunable optical filter stabilized by Newton’s rings fringe analysis

Raju Bhai KC, Edward B. Flagg

Published Mar 26, 2024• 2 citations
Publisher

Abstract

The resolution of grating spectrometers is insufficient to resolve many features present in the emission spectra of solid-state quantum emitters. Spectral resolution may be improved by inserting a Fabry-Perot interferometer (FPI) whose length is tuned by a piezoelectric actuator. Yet piezo creep and thermal fluctuations cause instability that makes this solution unsuitable for measurement times longer than tens of seconds. To overcome this challenge, we employ active feedback derived from the Newton's rings interference pattern formed by the reflection of a single-frequency laser from the FPI cavity. The resulting FPI transmission frequency is stable to within 50 MHz over several hours.

DOI
10.1364/ao.513725

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The resolution of grating spectrometers is insufficient to resolve many features present in the emission spectra of solid-state quantum emitters. Spectral resolution may be improved by inserting a Fabry-Perot interferometer (FPI) whose length is tuned by a piezoelectric actuator. Yet piezo creep and thermal fluctuations cause instability that makes this solution unsuitable for measurement times longer than tens of seconds. To overcome this challenge, we employ active feedback derived from the Newton's rings interference pattern formed by the reflection of a single-frequency laser from the FPI cavity. The resulting FPI transmission frequency is stable to within 50 MHz over several hours.
Single photons emitted from self-assembled quantum dots have been widely studied to use as a promising qubit for quantum information processing. Therefore, it is critical to fully understand the emission spectra from the quantum dot's excitation if we want to use a single photon as a quantum bit. It is almost impossible to produce rotationally symmetric quantum dots due to various growth conditions and restrictions. So the real quantum dots do not have a perfectly symmetric structure. A broken rotational symmetry causes an asymmetric exchange interaction between electron and hole, leading to a fine structure splitting between two excited states. The energy structure of a neutral InGaAs self-assembled quantum dot can be envisioned as a V-shaped energy diagram. A three-level V-system gives Mollow septuplets or Mollow quintuplets depending upon the frequency of the excitation laser. The main focus of this dissertation is to design a tunable optical filter to explore Mollow septuplets and Mollow quintuplets. A grating spectrometer cannot resolve an emission spectrum of a semiconductor quantum dot (QD) due to its limited resolution. Typical frequency separations between features in the emission spectrum of a typical InGaAs quantum dots are 2-10 GHz, while the best resolution available for a commercial spectrometer is about 9 GHz. The solution is to use a Fabry-Perot interferometer (FPI) as a tunable optical filter. This technique demands precise control of the distance between the two FPI mirrors. One way to control the length of the FPI cavity is to use an optical feedback method to lock the cavity using a costly tunable, stabilized reference laser. Here we adopt an alternative approach that uses an inexpensive, single-frequency laser and spatial interference patterns to lock different interferometer designs. We mechanically connect the FPI cavity to a secondary cavity called a Shearing Interferometer (SI) that is actively stabilized by a simple and inexpensive optical feedback method.
Optical Fabry-Perot cavities are crucial tools for metrology experiments, where they achieve extreme length stability and now can serve as highly proficient references for probing fine atomic transitions. For some atomic physics experiments however, tunability of the system to atomic transitions enables atom-light interactions. Achieving both frequency stability and tunability in a single cavity has remained a challenge, forcing metrology experiments exploiting atom-cavity interactions to rely on external active feedback systems to stabilize the length of the cavity. Here, we describe a piezoelectrically-tunable cavity with a cancellation of the coefficient of thermal expansion at around 5^\circ\mathrm{C} 5 ∘ C , achieving a fractional frequency instability of 4× 10^{-13} 4 × 10 − 13 level for 1 s integration time. This advance eliminates the need for external stabilization in many atom-cavity experiments, making this design ideal for applications such as ultra-stable superradiant lasers and other cavity quantum electrodynamics experiments.
We demonstrate a robust, stable and compact spectral filter for use in quantum optics experiments by implementing Fabry-Pérot resonators within optical fibre using pairs of fibre Bragg gratings. With only temperature stabilisation, these devices show less than 20 MHz frequency drift over 12 hours with a maximum suppression of roughly 35 dB and overall transmission of 75%. These characteristics make fibre Bragg grating Fabry-Pérot resonators ideal for quantum optics experiments where single photon-level signals must be efficiently filtered from large optical fields.
Here, we present the thermal tuning capability of an alignment-free, fiber-integrated Fabry-Pérot cavity. The two mirrors are made of fiber Bragg gratings that can be individually temperature stabilized and tuned. We show the temperature tuning of the resonance wavelength of the cavity without any degradation of the finesse and the tuning of the individual stop bands of the fiber Bragg gratings. This not only permits for the cavity's finesse to be optimized post-fabrication but also makes this cavity applicable as a narrowband filter with a FWHM spectral width of 0.07 ± 0.02 pm and a suppression of more than -15 dB that can be wavelength tuned. Further, in the field of quantum optics, where strong light-matter interactions are desirable, quantum emitters can be coupled to such a cavity and the cavity effect can be reversibly omitted and re-established. This is particularly useful when working with solid-state quantum emitters where such a reference measurement is often not possible once an emitter has been permanently deposited inside a cavity.
Fiber Fabry–Perot cavities, formed by micro-machined mirrors on the end-facets of optical fibers, are used in an increasing number of technical and scientific applications, where they typically require precise stabilization of their optical resonances. Here, we study two different approaches to construct fiber Fabry–Perot resonators and stabilize their length for experiments in cavity quantum electrodynamics with neutral atoms. A piezo-mechanically actuated cavity with feedback based on the Pound–Drever–Hall locking technique is compared to a novel rigid cavity design that makes use of the high passive stability of a monolithic cavity spacer and employs thermal self-locking and external temperature tuning. Furthermore, we present a general analysis of the mode matching problem in fiber Fabry–Perot cavities, which explains the asymmetry in their reflective line shapes and has important implications for the optimal alignment of the fiber resonators. Finally, we discuss the issue of fiber-generated background photons. We expect that our results contribute toward the integration of high-finesse fiber Fabry–Perot cavities into compact and robust quantum-enabled devices in the future.
We describe a simple and inexpensive optical ring interferometer for use in high-resolution spectral analysis and filtering. It consists of a solid cuboid, reflection-coated on two opposite sides, in which constructive interference occurs for waves in a rhombic trajectory. Due to its monolithic design, the interferometer's resonance frequencies are insensitive to environmental disturbances over time. Additional advantages are its simplicity of alignment, high-throughput, and feedback-free operation. If desired, it can be stabilized with a secondary laser without disturbance of the primary signal. We illustrate the use of the interferometer for the measurement of the spectral Mollow triplet from a quantum dot and characterize its long-term stability for filtering applications.