Saturated absorption spectroscopy is performed on the acetylene nu(1) + nu(3) band near 1532 nm inside photonic bandgap fibers of small (approximately 10 microm) and large (approximately 20 microm) core diameters. The observed linewidths are narrower in the 20 microm fiber and vary from 20 to 40 MHz depending on pressure and power. Variations in the background light transmission, attributed by others to surface modes, are significantly reduced in the 20 microm fiber. The optimum signal for use as a frequency reference in a 0.8 m long, 20 microm diameter fiber is found to occur at about 0.5 torr for 30 mW of pump power. The saturation power is found by modeling the propagation and attenuation of light inside the fiber.
We designed and fabricated a functionally integrated optical waveguide illuminator specially for common-path digital holographic microscopy through random media. The waveguide illuminator creates two point sources with desired phase shifts, which are located close to one another so that the common-path condition of the object and reference illumination is satisfied. Thereby, the proposed device permits phase-shift digital holographic microscopy free from bulky optical elements such as a beam splitter, an objective lens, and a piezoelectric transducer for phase shifting. Using the proposed device, microscopic 3D imaging through a highly heterogeneous double-composite random medium was experimentally demonstrated by means of common-path phase-shift digital holography.
Saturated absorption spectroscopy signals inside a 20 µm diameter acetylene-filled photonic bandgap fiber are optimized for use as optical frequency references. Modeling of the light propagation along the fiber reveals a low saturation intensity.Acetylene is used extensively for frequency standards in the near-infrared [1][2][3][4]. Photonic bandgap (PBG) fiber allows light and gas to be confined to a small area over long interaction lengths, facilitating nonlinear interactions such as Raman scattering [5], electromagnetically-induced transparency [6] , and saturated absorption spectroscopy [7]. Here, we demonstrate saturated absorption inside PBG fibers with high signal-to-noise ratio, and measure a low saturation intensity ( ~35 mW). We also identify the optimum operation parameters for a given fiber length and diameter. In Fig. 1, the transmitted probe power versus frequency reveals saturated absorption features inside PBG fibers at a variety of pressures, for a pump power of 29 mW. -500 0 500 0.0 0.2 0.4 0.6 0.8 1.0 i 0.15 T ii 0.26 T iii 0.53 T iv 0.72 T v 2.25 T Fractional Transmission Frequency (MHz) i ii iii iv v ECDL EDFA EDFA 2x AOM 70% 30% PC PBG PBS PBS 2 λ 4 λ PD 10% 90% Iso. Diagnostics Diagnostics VC VC Fig. 1. (left) Fractional transmission versus optical frequency detuning at different gas pressures, for the 12 C 2 H 2 P (11) transition with a pump power of 29 mW incident on the fiber. The narrow saturated absorption feature is offset from zero frequency because there is a frequency different between the pump and probe beams, imposed by an acousto-optic modulator (AOM). (right) Schematic of the experimental setup.The experimental set-up used to realize the saturated absorption spectra is shown in Fig. 1. An extended cavity diode laser (ECDL) emitts ~5 mW, 10% of which is amplified by an IPG Photonics ® Erbium-doped fiber amplifier (EDFA) to up to 500 mW. Also shown are an isolator (Iso.), a polarization controller (PC), a polarizing beam splitter (PBS), the vacuum chambers (VC), and a photodetector (PD). The frequency diagnostics include a glass cell filled with 12 C 2 H 2 at 50 Torr, in order to locate the relevant transitions, and a Michelson interferometer, used to monitor the laser frequency as it is swept across the transition under study. The polarization is corrected using a half-wave and a quarter-wave retarder before the pump beam is separated from the probe with a PBS.The theory of saturated absorption spectroscopy is well-known in vapor cells [8]. Beer's law describes the transmission of light through a medium as I = I 0 e -αs(ν)l , where I 0 is the incident intensity, I is the transmitted intensity, and l is the length of the medium. From the transmitted intensity, we calculate l α s (ν), as shown in Fig. 2. then l α s (ν) is fit to an appropriate function, taking into account the Gaussian nature of the Doppler broadened signal and the Lorentzian saturation dip. Figure 3 shows the dependence of the Lorentzian full-width-half-maximum on optical power. Figure 3 also shows the frequency ...
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