Self-heterodyne interference spectroscopy using a comb generated by pseudo-random modulation

Abstract

We present an original instrument designed to accomplish high-speed spectroscopy of individual optical lines based on a frequency comb generated by pseudo-random phase modulation of a continuous-wave (CW) laser. This approach delivers efficient usage of the laser power as well as independent control over the spectral point spacing, bandwidth and central wavelength of the comb. The comb is mixed with a local oscillator generated from the same CW laser frequency-shifted by an acousto-optic modulator, enabling a self-heterodyne detection scheme. The current configuration offers a calibrated spectrum every 1.12 µs. We demonstrate the capabilities of the spectrometer by producing averaged, as well as time-resolved, spectra of the D1 transition of cesium with a 9.8-MHz point spacing, a 50-kHz resolution and a span of more than 3 GHz. The spectra obtained after 1 ms of averaging are fitted with complex Voigt profiles that return parameters in good agreement with expected values.

Real-Time Dynamic Atomic Spectroscopy Using Electro-Optic Frequency Combs

“…In the upper panel, we set the pedestal amplitude to zero and the residuals show significant deviation from the data near the center of the absorption peaks. The lower panel allows for both pedestals and spectral holes and shows fit residuals (in transmittance) with a standard deviation of 6 × 10 −3 : this level is consistent with the broadband noise of the detection process [40]. The pedestal is modeled with a Gaussian spectrum (see Sec.…”

“…It is based on a comblike probe generated from a single Ti: sapphire cw laser tuned near 895 nm. Light from the laser, emitting at the frequency ν c , is split into two paths: a fraction of the power is sent to an electro-optic modulator to generate a frequency comb by pseudorandom phase modulation [40]. A 2 9 − 1 ¼ 511 maximal length sequence is generated at 5 Gbits=s and delivered to the modulator to generate a comb with modes at frequencies of ν c AE nf r , where f r ¼ 5 GHz =511 ¼ 9.784 74 MHz is the repetition rate of the digital code.…”

“…More details together with the calibration procedure are described in Ref. [40]. An auxiliary pump-laser beam (2.2 mm diameter, 200 μW ∼ 2I sat ) propagates through the vapor cell, and it crosses the probe beam (2.2 mm diameter) with an approximately 2°angle in the middle of the cell.…”

“…We use an auxiliary laser that pumps the same manifold and observe rapid changes induced in the spectrum by making use of a recently demonstrated technique [40], based on a comb generated by the modulation of a continuous-wave (cw) laser [41][42][43][44]. The comb is directly mapped to the electrical domain without any spectral compression [40,45] in order to approach the ultimate Fourier limit. A tunable laser would need to scan at 3000 THz=s in order to make an equivalent measurement using a scanned approach.…”

“…In order to aid the reader, we first summarize the salient elements of the apparatus, although we refer the reader to Ref. [40] for full details. We then describe our new findings on the evolution of multiple narrow-band features during optical pumping and relaxation back to equilibrium.…”