Abstract

Frequency comb technology has revolutionized many aspects of spectroscopic measurements, most recently exemplified by the introduction of multi-heterodyne spectroscopy, which is now replacing traditional mechanically scanning Fourier transform spectrographs, providing orders of magnitude improvements in scan rate and resolution [1,2]. However, a power scalable and compact, optically integratable platform for multi-heterodyne spectroscopy is still missing. Here, we demonstrate Fourier transform spectroscopy (FTS) based on a pair of low noise, GHz repetition rate, Yb fiber frequency combs. We present a proof of concept all-fiber system and further contract all amplification and spectral broadening stages into a single all-fiber propagation path. The experimental setup comprising two near identical Yb fiber frequency combs, multiplexed with a fiber coupler, is shown in Fig.1 (a). The low noise, ultra-compact oscillator assemblies generate pulses with an output power of 100 mW at 1 GHz repetition rate compressible to 120 fs [3]. In the reference arm, one of the comb lines from each oscillator was phase locked to a single-frequency Nd:YAG NPRO by controlling the oscillator cavity length. Additionally, the difference frequency of the repetition rates (set to values between 1 Hz and 100 kHz) was locked at 2.5 mHz RMS, using feedback control of one pump diodes current. The electronic locks ensure an accurate data interpretation without the need for optical referencing [4]. In the signal arm, cross phase modulation induced signal corruption is prevented by recording interferograms with pulses separated by a third of the cavity round trip time realized with an imbalanced Mach-Zehnder interferometer. This scheme enables power scaling in a cladding-pumped fiber amplifier to W-level and frequency conversion for FTS measurements outside the bandwidth of our oscillators.

© 2009 IEEE