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Review of Scientific Instruments

Solid optical ring interferometer for high-throughput feedback-free spectral analysis and filtering

Benjamin Petrak, M. Peiris, Andreas Müller

Published Feb 1, 2015• 5 citations
Publisher

Abstract

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.

DOI
10.1063/1.4907403

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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.
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.
By applying a high-reflectivity dielectric coating on both sides of a commercial plano-convex lens, we produce a stable monolithic Fabry-Perot cavity suitable for use as a narrow band filter in quantum optics experiments. The resonant frequency is selected by means of thermal expansion. Owing to the long term mechanical stability, no optical locking techniques are required. We characterize the cavity performance as an optical filter, obtaining a 45dB suppression of unwanted modes while maintaining a transmission of 60%.
Quantum optics experiments frequently require the separation of single-photon-level signals from strong classical fields. In circumstances in which the signals are spectrally close, and one cannot make use of relatively simple separation methods based around differences in polarization state, optical mode, or beam direction, it is necessary to exploit the frequency difference itself as the means for signal separation. We have constructed and characterized an efficient and robust fiber-based filter, consisting of an all-fiber Fabry-Perot resonator, to achieve this goal. Our filter shows 31dB of suppression of unwanted signals and 76% transmission of the desired signal. The transmission of the filter was stabilized to within 2% of its maximum for over 35hours through simple temperature stabilization.
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.
We experimentally demonstrate a high-spectral-purity photon source by designing a dual-Mach–Zehnder-interferometer-coupled silicon ring resonator, wherein the linewidths of pump and signal (idler) resonances can be engineered independently. A high spectral purity of 95 % ± 1.5 % is obtained via a time-integrated g ( 2 ) correlation measurement, which exceeds the theoretical 93% bound of a traditional ring’s spontaneous four-wave-mixing photon source. This source also possesses high performance in other metrics including a measured coincidence of 9599 pairs/s and a preparation heralding efficiency of 52.4% at a relatively low pump power of 61 µW as well as high drop-to-through suppression of 20.2 dB. By overcoming the trade-off between spectral purity and brightness in the post-filtering way, such a method guarantees bright pure photons and will pave the way for development of on-chip quantum information processing with improved operation fidelity and efficiency.
Long-wavelength neutron interferometry using discrete optical elements is notoriously challenging due to stringent alignment and stability requirements. Here, we introduce a monolithic double-Laue neutron interferometer fabricated from a stack of commercial Bayfol HX photopolymer films. By recording holographic gratings simultaneously in multiple layers, we create a robust device that is inherently aligned, bypassing traditional stability problems. We demonstrate the device's function by observing the characteristic interference fringes in the diffracted intensity of both light and very cold neutrons. The interferometer is then used to perform in-situ characterization of the neutron beam's spectral profile, demonstrating its utility as a compact spectrometer. Our work establishes stacked holographic gratings as a simple, versatile, and powerful platform for matter-wave interferometry and metrology.