Fabrication and characterization of short acetylene-filled photonic microcells.

We have developed short (6-10 cm), connectorized acetylene-filled photonic microcells (PMCs) from photonic bandgap fibers that may replace near-IR frequency references for certain applications based on gas-filled glass cells. By using a tapering technique to seal the microcells, we were able to achieve a high transmission efficiency of 80% and moderate line center accuracy of 10 MHz (1σ). This approaches the National Institute of Standard Technology Standard Reference Material 2517a 10 MHz (2σ) accuracy. Using an earlier Q-tipping technique, 37% off-resonant transmission and 5 MHz accuracy were achieved in finding the line center, but a large 13% etalon-like effect appears on the wings of the optical depth. The etalon-like effect is reduced to less than 1% by using the tapering method. In both cases, the microcells could be connectorized, albeit with a reduction in off-resonant transmission efficiency, for integration into multimode fibers or free-space optical systems. Although contamination is introduced during both fabrication techniques, the P13 PMC line center shifts are small with respect to the sub-Doppler line center. This shows that the PMC can be used for moderate-accuracy frequency measurements. Finally, repeatable measurements show that PMCs are stable in terms of total pressure over approximately one year.

Near diffraction-limited performance of an OPA pumped acetylene-filled hollow-core fiber laser in the mid-IR.

We investigate the mid-IR laser beam characteristics from an acetylene-filled hollow-core optical fiber gas laser (HOFGLAS) system. The laser exhibits near-diffraction limited beam quality in the 3 µm region with M2 = 1.15 ± 0.02 measured at high pulse energy, and the highest mid-IR pulse energy from a HOFGLAS system of 1.4 µJ is reported. Furthermore, the effects of output saturation with pump pulse energy are reduced through the use of longer fibers with low loss. Finally, the slope efficiency is shown to be nearly independent of gas pressure over a wide range, which is encouraging for further output power increase.

Carrier-envelope offset frequency linewidth narrowing in a Cr:forsterite laser-based frequency comb.

Cr:forsterite laser-based frequency combs are useful for spectroscopic purposes in the near-IR wavelength region. However, self-referenced Cr:forsterite combs tend to exhibit wide carrier-envelope offset frequency (f0) linewidths, which result in broad comb teeth. This can be attributed to significant frequency noise across the comb's spectral bandwidth. We have stabilized a prism-based Cr:forsterite laser comb and observed narrowing of the f0 linewidth from ∼1.4 MHz down to ∼100 kHz by changing only the prism insertion, and to 23 kHz by inserting a knife edge into the intracavity beam while keeping the same prism insertion. The noise dynamics of the Cr:forsterite laser frequency comb are investigated with the goal of explaining this f0 narrowing phenomenon.

Direct fiber comb stabilization to a gas-filled hollow-core photonic crystal fiber.

We have isolated a single tooth from a fiber laser-based optical frequency comb for nonlinear spectroscopy and thereby directly referenced the comb. An 89 MHz erbium fiber laser frequency comb is directly stabilized to the P(23) (1539.43 nm) overtone transition of (12)C(2)H(2) inside a hollow-core photonic crystal fiber. To do this, a single comb tooth is isolated and amplified from 20 nW to 40 mW with sufficient fidelity to perform saturated absorption spectroscopy. The fractional stability of the comb, ~7 nm away from the stabilized tooth, is shown to be 6 × 10(-12) at 100 ms gate time, which is over an order of magnitude better than that of a comb referenced to a GPS-disciplined Rb oscillator.

Acetylene frequency references in gas-filled hollow optical fiber and photonic microcells.

Gas-filled hollow optical fiber references based on the P(13) transition of the ν1+ν3 band of 12C2H2 promise portability with moderate accuracy and stability. Previous realizations are corrected (<1σ) by using proper modeling of a shift due to line-shape. To improve portability, a sealed photonic microcell is characterized on the 12C2H2 ν1+ν3 P(23) transition with somewhat reduced accuracy and stability. Effects of the photonic crystal fiber, including surface modes, are explored. Both polarization-maintaining (PM) and non-PM 7-cell photonic bandgap fiber are shown to be unsuitable for kilohertz-level frequency references.

Mid-infrared gas filled photonic crystal fiber laser based on population inversion.

We demonstrate for the first time an optically pumped gas laser based on population inversion using a hollow core photonic crystal fiber (HC-PCF). The HC-PCF filled with 12C2H2 gas is pumped with ~5 ns pulses at 1.52 µm and lases at 3.12 µm and 3.16 µm in the mid-infrared spectral region. The maximum measured laser pulse energy of ~6 nJ was obtained at a gas pressure of 7 torr with a fiber with 20 dB/m loss near the lasing wavelengths. While the measured slope efficiencies of this prototype did not exceed a few percent due mainly to linear losses of the fiber at the laser wavelengths, 25% slope efficiency and pulse energies of a few mJ are the predicted limits of this laser. Simulations of the laser's behavior agree qualitatively with experimental observations.

10 kHz accuracy of an optical frequency reference based on (12)C2H2-filled large-core kagome photonic crystal fibers.

Saturated absorption spectroscopy reveals the narrowest features so far in molecular gas-filled hollow-core photonic crystal fiber. The 48-68 mum core diameter of the kagome-structured fiber used here allows for 8 MHz full-width half-maximum sub-Doppler features, and its wavelength-insensitive transmission is suitable for high-accuracy frequency measurements. A fiber laser is locked to the (12)C2H2 nu(1); + nu(3) P(13) transition inside kagome fiber, and compared with frequency combs based on both a carbon nanotube fiber laser and a Cr:forsterite laser, each of which are referenced to a GPS-disciplined Rb oscillator. The absolute frequency of the measured line center agrees with those measured in power build-up cavities to within 9.3 kHz (1 sigma error), and the fractional frequency instability is less than 1.2 x 10(-11) at 1 s averaging time.

A phase-stabilized carbon nanotube fiber laser frequency comb.

A frequency comb generated by a 167 MHz repetition frequency erbium-doped fiber ring laser using a carbon nanotube saturable absorber is phase-stabilized for the first time. Measurements of the in-loop phase noise show an integrated phase error on the carrier envelope offset frequency of 0.35 radians. The carbon nanotube fiber laser comb is compared with a CW laser near 1533 nm stabilized to the nu(1) + nu(3) overtone transition in an acetylene-filled kagome photonic crystal fiber reference, while the CW laser is simultaneously compared to another frequency comb based on a Cr:Forsterite laser. These measurements demonstrate that the stability of a GPS-disciplined Rb clock is transferred to the comb, resulting in an upper limit on the locked comb's frequency instability of 1.2 x 10(-11) in 1 s, and a relative instability of <3 x 10(-12) in 1 s. The carbon nanotube laser frequency comb offers much promise as a robust and inexpensive all-fiber frequency comb with potential for scaling to higher repetition frequencies.

Stabilization of a self-referenced, prism-based, Cr:forsterite laser frequency comb using an intracavity prism.

The frequency comb from a prism-based Cr:forsterite laser has been frequency stabilized using intracavity prism insertion and pump power modulation. Absolute frequency measurements of a CW fiber laser stabilized to the P(13) transition of acetylene demonstrate a fractional instability of approximately 2 x 10(-11) at a 1 s gate time, limited by a commercial Global Positioning System (GPS)-disciplined rubidium oscillator. Additionally, absolute frequency measurements made simultaneously using a second frequency comb indicate relative instabilities of 3 x 10(-12) for both combs for a 1 s gate time. Estimations of the carrier-envelope offset frequency linewidth based on relative intensity noise and the response dynamics of the carrier-envelope offset to pump power changes confirm the observed linewidths.

Saturated absorption spectroscopy of acetylene gas inside large-core photonic bandgap fiber.

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.