A Displacement Measuring Interferometer Based on a Frequency-Locked Laser Diode with High Modulation Frequency

Laser interferometers can achieve a nanometer-order uncertainty of measurements when their frequencies are locked to the reference frequencies of the atom or molecule transitions. There are three types of displacement-measuring interferometers: homodyne, heterodyne, and frequency modulation (FM) interferometers. Among these types of interferometer, the FM interferometer has many advantageous features. The interference signal is a series of time-dependent harmonics of modulation frequency, so the phase shift can be detected accurately using the synchronous detection method. Moreover, the FM interferometer is the most suitable for combination with a frequency-locked laser because both require frequency modulation. In previous research, low modulation frequencies at some tens of kHz have been used to lock the frequency of laser diodes (LDs). The low modulation frequency for the laser source means that the maximum measurement speed of the FM interferometers is limited. This paper proposes a novel contribution regarding the application of a high-frequency modulation for an LD to improve both the frequency stability of the laser source and the measurement speed of the FM interferometer. The frequency of the LD was locked to an I2 hyperfine component at 1 MHz modulation frequency. A high bandwidth lock-in amplifier was utilized to detect the saturated absorption signals of the I2 hyperfine structure and induce the signal to lock the frequency of the LD. The locked LD was then used for an FM displacement measuring interferometer. Moreover, a suitable modulation amplitude that affected the signal-to-noise ratio of both the I2 absorption signal and the harmonic intensity of the interference signal was determined. In order to verify the measurement resolution of the proposed interferometer, the displacement induced by a piezo electric actuator was concurrently measured by the interferometer and a capacitive sensor. The difference of the displacement results was less than 20 nm. To evaluate the measurement speed, the interferometer was used to measure the axial error of a high-speed spindle at 500 rpm. The main conclusion of this study is that a stable displacement interferometer with high accuracy and a high measurement speed can be achieved using an LD frequency locked to an I2 hyperfine transition at a high modulation frequency.

C+ distribution around S 1 in ρ Ophiuchi

We analyze a [C II] 158 μm map obtained with the L2 GREAT receiver on SOFIA of the reflection nebula illuminated by the early B star S 1 in the ρ Oph A cloud core. This data set has been complemented with maps of CO(3–2), 13CO(3–2), and C18O(3–2), observed as a part of the James Clerk Maxwell Telescope (JCMT) Gould Belt Survey, with archival HCO+(4–3) JCMT data, as well as with [O I] 63 and 145 μm imaging with Herschel/PACS. The [C II] emission is completely dominated by the strong emission from the photon dominated region (PDR) in the nebula surrounding S 1 expanding into the dense Oph A molecular cloud west and south of S 1. The [C II] emission is significantly blueshifted relative to the CO spectra and also relative to the systemic velocity, particularly in the northwestern part of the nebula. The [C II] lines are broader toward the center of the S 1 nebula and narrower toward the PDR shell. The [C II] lines are strongly self-absorbed over an extended region in the S 1 PDR. Based on the strength of the [13C II] F = 2–1 hyperfine component, [C II] is significantly optically thick over most of the nebula. CO and 13CO(3–2) spectra are strongly self-absorbed, while C18O(3–2) is single peaked and centered in the middle of the self-absorption. We have used a simple two-layer LTE model to characterize the background and foreground cloud contributing to the [C II] emission. From this analysis we estimated the extinction due to the foreground cloud to be ~9.9 mag, which is slightly less than the reddening estimated toward S 1. Since some of the hot gas in the PDR is not traced by low-J CO emission, this result appears quite plausible. Using a plane parallel PDR model with the observed [O I](145)/[C II] brightness ratio and an estimated FUV intensity of 3100–5000 G0 suggests that the density of the [C II] emitting gas is ~3–4 × 103 cm−3.

Concentration Measurements of Chlorine Atoms in A Plasma Reactor

Infrared absorption spectroscopy has been used to measure atomic chlorine concentrations over a range of plasma conditions in both Cl2 and CF3Cl discharges.These measurements were made utilizing the spin-orbit transitions in the ground state of atomic chlorine near 882 cm−1.The concentration studies were performed by passing light from a diode laser through a multi-pass (White) cell set in two opposed windows of a parallel plate plasma etching reactor.The plasma work was preceded by a laboratory measurement of the infrared absorption line strengths of the 2P1/2 ← 2P3/2 transition.This measurement was done in a known concentration of atomic chlorine produced in a low pressure discharge flow system by the reaction of Cl2 or HCl with excess fluorine atoms.These measurements resulted in an integrated line strength of 4.14 (±0.89) × 10−21 cm2-molecule−1-cm−1 for the strongest hyperfine component of the transition at 882.3626 cm−1.Measured atomic chlorine concentrations in Cl2 discharges varied between 0.2 and 8.0 × 1014 atoms/cm3, representing atomic chlorine fractions on the order of a few percent.The measured atomic chlorine concentrations increased approximately linearly with increasing power and pressure, and increased with increasing frequency above approximately 1 MHz.Below 1 MHz, the atomic chlorine concentration was relatively independent of frequency.