Frequency modulation background signals from fiber-based electro optic modulators are caused by crosstalk

Abstract

Frequency modulated spectroscopy (FMS) performed by the use of fiber-coupled electro optic modulators (FC-EOMs) is often plagued by background signals that bring in noise and, by their temperature dependence, cause severe drifts. These signals cannot be zeroed out by the conventional technique of using a carefully adjusted polarizer that can be applied to free space electro optic modulators (EOMs). This can limit the use of FC-EOMs in high performance detection techniques. Here we provide an explanation to these background signals that is based upon crosstalk between various polarization directions of light in the fixed mounted polarization-maintaining fibers and the electro optic crystal. The description provides a basis for the previously demonstrated technique that utilizes an EOM regulated simultaneously by temperature and DC voltage to eliminate background signals from systems encompassing FC-EOMs.

Model for in-coupling of etalons into signal strengths extracted from spectral line shape fitting and methodology for predicting the optimum scanning range—demonstration of Doppler-broadened, noise-immune, cavity-enhanced optical heterodyne molecular spectroscopy down to 9 × 10^−14 cm^−1

“…The cause of this was traced to a structured background signal originating from the EOM. Although an EOM with a proton exchanged waveguide produces significantly fewer background signals than those made with titanium diffused waveguides [24], the background signals they give rise to are not zero. Since any background that is not infinitely stable will drift and bring in noise, the conditions for the EOM were stabilized.…”

“…1, is similar to that realized by Silander et al [16], although with a couple of alterations. In brief, the light from an Er-doped fiber laser (EDFL, Koheras Adjustik E15) was sent through a fiber coupled acousto-optic modulator (AOM, AA Opto-Electronic, MT110-IR25-3FIO), whose first-order output, shifted by 110 MHz, was fed to a fiber coupled electro optic modulator (EOM, General Photonics, LPM-001-15), the latter with a proton exchange waveguide to avoid RAM [24]. As is further described below, for some of the measurements, the temperature of the EOM was stabilized using a temperature controller (Thorlabs, TED200C).…”

Shot-noise-limited Doppler-broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry

“…This increases the white noise and brings in long-term drifts in the system [20]. There are two main causes of such signals; one is interference between various optical surfaces, usually termed etalons, while the other is residual amplitude modulation (RAM), which in particular can appear from EOMs [25,26]. Numerous efforts have been made to suppress these types of effects, e.g., antireflection coating, alignment of optical components, active feedback control of EOM [25], usage of proton-exchanged EOMs [20] and wedged crystal EOMs [27], application of etalon immune distance (EID) [28], and implementation of a differential NICE-OHMS strategy [24].…”

High-resolution trace gas detection by sub-Doppler noise-immune cavity-enhanced optical heterodyne molecular spectrometry: application to detection of acetylene in human breath

“…In short, the system is constructed based on an EDFL that operates at around 1534 nm, addressing an acetylene transition at 6518.4858 cm -1 with a line strength of 8.572×10 -21 cm -1 /(mol cm -2 ) and a dipole moment of 8.0 mD. The output of the laser is passed through a fiber-coupled acoustic optic modulator (f-AOM), a fiber-coupled polarizer (f-POL), and a fiber-coupled electro-optic modulator (f-EOM), the latter with a proton exchanged waveguide to minimize the generation of RAM [68] before it is emitted into free space by the use of a fiber-coupled collimator (f-C). The frequency of the light is controlled by both a PZT inside the laser and a radio frequency (RF) controlled f-AOM (by the use of a tunable VCO).…”