Contents lists available at ScienceDirect Optik journal homepage: www.elsevier.com/locate/ijleo Original research article Influence of exterior acoustic noise on narrow linewidth laser measurements using self-homodyne optical fiber interferometer Flavio J. Galdieri, Tiago Sutili*, Nikolai Melnikoff, Aldário C. Bordonalli, Evandro Conforti Optical Communications and Microwave Research Laboratory (LAPCOM), Department of Communications (DECOM), School of Electrical and Computer Engineering (FEEC), University of Campinas (UNICAMP), Campinas, SP, Brazil ARTICLE INFO Keywords: Acoustic noise Homodyne detection Laser linewidth broadening Mach-Zehnder interferometer Optical fiber Optical sensing ABSTRACT Spectral linewidth measurements of a HeNe laser were obtained by using a self-homodyne optical fiber Mach-Zehnder interferometer located in a vacuum controlled chamber. The data showed an apparent broadening of the highly-coherent laser linewidth from few hertz up to kilohertz with the change in the air chamber pressure, evincing an influence of external acousto-optic effects on the interferometer optical fiber. To support the results, a theoretical harmonic acoustic analysis was developed and then related to the correspondent interferometer phase noise spectrum measurements. Lorentzian approximations of the experimental laser apparent linewidth suggest that the interferometer was working in a quasi-coherent regime for all tested chamber air pressure. 1. Introduction Highly coherent lightwave signals in fiber optics, including outside sound influence [1,2], are relevant for applications such as optical coherent communications [3], vibrometry [4,5], frequency standard references [6,7], electro-optical devices characterization [8–10], and sensing [11,12] including ultra-stable narrow-linewidth lasers [13]. Recently, reports of high-Q resonators [14,15], and its integration as filters in the optical external cavity of semiconductor lasers, allowed the development of widely tunable optical sources (from 1500 nm up to 1580 nm) with low intrinsic linewidths (below 300 Hz) [16], including recent sub-hertz linewidth Brillouin laser with integrated Si 3 N 4 waveguide filter [17]. Devices with narrow linewidths are crucial to the continuous evolution of high precision optical sensing [18]. In addition, the improvement of highly coherent optical sources (linewidths up to a few tens of hertz) and the development of new linewidth measurement techniques yield to applications including quantum optical sensors [19], cold atom clocks [20,21], imaging applications [22], viruses and nanoparticles detection [23], photoacoustic spectroscopy for trace gas sensing [24], and crustal deformation measurement [25]. In this sense, it is crucial to fully understand the influence of en- vironmental factors on the optical carrier induced phase noise and linewidth broadening. A typical way to estimate the degree of coherence of a light source is by means of its linewidth [26] and one of often-used techniques to measure it is via a delayed self-homodyne optical fiber unbalanced Mach-Zehnder interferometer (UMZI) [27]. As the degree of laser coherence increases (resulting in a narrower linewidth), this technique requires an UMZI with a long time delay unbalance, resulting in an arm much longer than the other. It is very conceivable that, for highly coherent sources, like semiconductor or gas lasers, the susceptibility of the optical fiber to external factors (such as temperature variation and mechanical vibrations) can https://doi.org/10.1016/j.ijleo.2019.164101 Received 4 May 2018; Received in revised form 30 November 2019; Accepted 19 December 2019 ⁎ Corresponding author. E-mail address: tiagosutili@gmail.com (T. Sutili). Optik - International Journal for Light and Electron Optics 204 (2020) 164101 0030-4026/ © 2020 Elsevier GmbH. All rights reserved. T