Electromagnetically-induced transparency grid in acetylene-filled hollow-core PCF

Contaminant-free end-capped and single-mode acetylene photonic microcell for sub-Doppler spectroscopy

We report on the development of an acetylene-filled photonic microcell based on an assembly process that is contaminant free and requires no helium buffer gas nor gluing procedure. The microcell consists of a 7-m-long and 30 µm core-diameter inhibited-coupling guiding hollow-core photonic crystal fiber filled with acetylene gas at a pressure in the range of 80 µbar, sealed by capping its ends with fusion-collapsing a glass-tube sleeve, and mounted on FC connectors for integration. The microcell shows a robust single-mode behavior and a total insertion loss of ∼1.5dB. The spectroscopic merit of the formed microcell is tested by generating electromagnetic induced transparency and saturated absorption on R13 and P9 absorption lines, respectively. The sub-Doppler transparencies show a close to transit time limited linewidth of 17±3MHz. The latter was monitored for over 3 months. As a demonstration, the microcell was used to frequency stabilize a laser with fractional frequency instability improvement by a factor 50 at 100 s integration time compared to free running laser operation.

A Chip-Scale Optical Frequency Reference for the Telecommunication Band Based on Acetylene

Lasers precisely stabilized to known transitions between energy levels in simple, well-isolated quantum systems such as atoms and molecules are essential for a plethora of applications in metrology and optical communications. The implementation of such spectroscopic systems in a chip-scale format would allow to reduce cost dramatically and would open up new opportunities in both photonically integrated platforms and free-space applications such as lidar. Here the design, fabrication, and experimental characterization of a molecular cladded waveguide platform based on the integration of serpentine nanoscale photonic waveguides with a miniaturized acetylene chamber is presented. The goal of this platform is to enable cost-effective, miniaturized, and low power optical frequency references in the telecommunications C band. Finally, this platform is used to stabilize a 1.5 μm laser with a precision better than 400 kHz at 34 s. The molecular cladded waveguide platform introduced here could be integrated with components such as on-chip modulators, detectors, and other devices to form a complete on-chip laser stabilization system.

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.

Ultraslow weak-light solitons and their storage and retrieval in a kagome-structured hollow-core photonic crystal fiber

We investigate the formation and propagation of ultraslow weak-light solitons and their memory in the atomic gas filled in a kagome-structured hollow-core photonic crystal fiber (HC-PCF) via electromagnetically induced transparency (EIT). We show that, due to the strong light-atom coupling contributed by the transverse confinement of the HC-PCF, the EIT and hence the optical Kerr nonlinearity of the system can be largely enhanced, and hence optical solitons with very short formation distance, ultraslow propagation velocity, and extremely low generation power can be realized. We also show that the optical solitons obtained can not only be robust during propagation, but also be stored and retrieved with high efficiency through the switching off and on of a control laser field. The results reported herein are promising for practical applications of all-optical information processing and transmission via the ultraslow weak-light solitons and the kagome-structured HC-PCF.

Electromagnetically induced transparency and ultraslow optical solitons in a coherent atomic gas filled in a slot waveguide

We investigate the electromagnetically induced transparency (EIT) and nonlinear pulse propagation in a Λ-type three-level atomic gas filled in a slot waveguide, in which electric field is strongly confined inside the slot of the waveguide due to the discontinuity of dielectric constant. We find that EIT effect can be greatly enhanced due to the reduction of optical-field mode volume contributed by waveguide geometry. Comparing with the atomic gases in free space, the EIT transparency window in the slot waveguide system can be much wider and deeper, and the Kerr nonlinearity of probe laser field can be much stronger. We also prove that using slot waveguide ultraslow optical solitons can be produced efficiently with extremely low generation power.

In situ gas sensing using a remotely detectable probe with replaceable insert

We demonstrate a spectroscopic gas sensor using an optical fiber probe with a replaceable insert. The probe consists of a hollow-core photonic bandgap fiber (HC-PBGF) with a core diameter of 10.9 μm and a glass tube where a 2-μm hollow core fiber (HCF) with a gold coated end facet can be inserted. The HCF is designed to function as both a gate where gases can enter the HC-PBGF and a mirror reflecting the guided light back to the HC-PBGF. The opposite distal end of the probe is also designed to be able to regulate the gas pressure within the HC-PBGF for a high gas flow rate, while still transmitting the reflected light to the analysis instrument. The remote sensing probe, we believe, has much potential for detecting gases in hazardous environments.

Large-core acetylene-filled photonic microcells made by tapering a hollow-core photonic crystal fiber

We report on kagomé-lattice photonic microcells with low losses, large outer diameters, and large cores. The large (40-70microm) cores are accommodated by tapering the fibers and splicing the reduced ends to a single-mode fiber. We demonstrate the repeatability of this process and obtain splice losses of 0.6dB by optimizing the taper transition length. Narrow electromagnetically induced transparencies and saturable absorption are demonstrated in an acetylene-filled photonic microcell.

High-power soliton-induced supercontinuum generation and tunable sub-10-fs VUV pulses from kagome-lattice HC-PCFs

We theoretically study a novel approach for soliton-induced high-power supercontinuum generation by using kagome lattice HC-PCFs filled with a noble gas. Anomalous dispersion and broad-band low loss of these fibers enable the generation of two-octave broad spectra by fs pulses, with high coherence and high spectral peak power densities up to five orders of magnitude larger than in standard PCFs. In addition, up to 20% of the output radiation energy forms a narrow UV/VUV band, which can be tuned by controlling the pressure in the range from 350 nm to 120 nm. In the temporal domain this corresponds to sub-10-fs UV/VUV pulses with pulse energy of few tens of microJ, caused by the formation of a high-order soliton emitting non-solitonic radiation.

Non-resonant wavelength modulation saturation spectroscopy in acetylene-filled hollow-core photonic bandgap fibres applied to modulation-free laser diode stabilisation

In this paper the application of Wavelength Modulation (WM) techniques to non-resonant saturation spectroscopy in acetylene-filled Hollow-Core Photonic Bandgap Fibres (HC-PBFs) and modulation-free Laser Diode (LD) frequency stabilisation is investigated. In the first part WM techniques are applied to non-resonant pump-probe saturation of acetylene overtone rotational transitions in a HC-PBF. A high-power DFB chip-on-carrier mounted LD is used in conjunction with a tuneable External Cavity Laser (ECL) and the main saturation parameters are characterized. In the second part a novel feedback system to stabilize the DFB emission wavelength based on the WM saturation results is implemented. Modulation-free locking of the DFB laser frequency to the narrow linewidth saturation feature is achieved for both constant and variable LD temperatures.

Highly-efficient coupling of linearly- and radially-polarized femtosecond pulses in hollow-core photonic band-gap fibers

We demonstrate extremely efficient excitation of linearly-, radially-, and azimuthally-polarized modes in a hollow-core photonic band-gap fiber with femtosecond laser pulses. We achieve coupling efficiencies as high as 98% with linearly polarized input Gaussian beams and with high-power pulses we obtain peak intensities greater than 10(14) W/cm(2) inside and transmitted through the fiber. With radially polarized pulses, we achieve 91% total transmission through the fiber while maintaining the polarization state. Alternatively with azimuthally-polarized pulses, the mode is degraded in the fiber, and the pure polarization state is not maintained.

Double photonic bandgap hollow-core photonic crystal fiber

We report on the design, fabrication and characterization of hollow-core photonic crystal fiber with two robust bandgaps that bridge the benchmark laser wavelengths of 1064 nm and 1550 nm. The higher-order bandgap arises due to the extremely thin struts of the silica cladding and their fine-tuning relative to the apex size. The optimum strut thickness was found to be approximately one hundredth of the cladding pitch.

Square-lattice large-pitch hollow-core photonic crystal fiber

We report on the design, fabrication and characterization of silica square-lattice hollow core photonic crystal fibers optimized for low loss guidance over an extended frequency range in the mid-IR region of the optical spectrum. The fiber's linear optical properties include an ultra-low group velocity dispersion and a polarization cross-coupling as low as -13.4 dB over 10 m of fiber.

Generation of large alkali vapor densities inside bare hollow-core photonic band-gap fibers

We demonstrate the ability to generate extremely large rubidium densities in uncoated hollow-core photonic band-gap fibers using light-induced atomic desorption. Once the fiber is exposed to Rb vapor for 1-2 weeks, and this atomic source is removed, the fiber yields large desorbable densities for an extended period of time. We show that optical depths greater than e(-1200) can be created within seconds. Our observed Rb densities are several orders of magnitude larger than any previously reported to be generated optically, and allow for the demonstration of a relatively easy-to-use fiber-based vapor cell capable of producing large optical depths without the need for thermal tuning.

Dynamic control of higher-order modes in hollow-core photonic crystal fibers

We present a versatile method for selective mode coupling into higher-order modes of photonic crystal fibers, using holograms electronically generated by a spatial light modulator. The method enables non-mechanical and completely repeatable changes in the coupling conditions. We have excited higher order modes up to LP(31) in hollow-core photonic crystal fibers. The reproducibility of the coupling allows direct comparison of the losses of different guided modes in both hollow-core bandgap and kagome-lattice photonic crystal fibers. Our results are also relevant to applications in which the intensity distribution of the light inside the fiber is important, such as particle- or atom-guidance.

Quasi-phase-matched high harmonic generation in hollow core photonic crystal fibers

The potential of hollow core photonic crystal fiber as a nonlinear gas cell for efficient high harmonic generation is discussed. The feasibility of phase-matching this process by modulating the phase of ionization electrons using a counter-propagating laser field is shown. In this way, harmonics with energies of several hundreds of eV can be produced using fs-laser pump pulses of microJ energy.

Electromagnetically induced transparency in Rb-filled coated hollow-core photonic crystal fiber

We report the observation of lambda-configuration electromagnetically induced transparency as well as optical pumping in rubidium-filled kagome-structure hollow-coated-core photonic crystal fiber. We show that a polydimethylsiloxane coating of the fiber core reduces the linewidth of the transparency below that which could be expected for an uncoated fiber. The measured 6 MHz linewidth was dominated by optical broadening.

Large-pitch kagome-structured hollow-core photonic crystal fiber

We report the fabrication and characterization of a new type of hollow-core photonic crystal fiber based on large-pitch (approximately 12 microm) kagome lattice cladding. The optical characteristics of the 19-cell, 7-cell, and single-cell core defect fibers include broad optical transmission bands covering the visible and near-IR parts of the spectrum with relatively low loss and low chromatic dispersion, no detectable surface modes and high confinement of light in the core. Various applications of such a novel fiber are also discussed, including gas sensing, quantum optics, and high harmonic generation.

Hollow-core photonic bandgap fibre: new light guidance for new science and technology

We review the progress made on the fabrication and applications of hollow-core photonic crystal fibres (HC-PCFs). The mechanism of the light guidance in these fibres is described along with their dispersion properties. We review the HC-PCF fabrication, the different results achieved in the fields of laser-induced particle guidance, low-threshold stimulated Raman scattering in hydrogen (vibrational and rotational), laser frequency metrology and quantum optics. Finally, we show the different new prospects opened up by these fibres.

Low optical insertion-loss and vacuum-pressure all-fiber acetylene cell based on hollow-core photonic crystal fiber

We report a novel and easy-to-implement hollow-core photonic crystal fiber cell fabrication technique based on helium diffusion through silica. The formed gas cells combine low optical insertion loss (1.8 dB) and vacuum acetylene pressure (microbar regime). The estimates of the final gas pressure, using both Voigt interpolation and electromagnetically induced transparency, show a good match with the initial fitting pressure.

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.

Low-light-level optical interactions with rubidium vapor in a photonic band-gap fiber

We show that rubidium vapor can be produced within the core of a photonic band-gap fiber yielding an optical depth in excess of 2,000. Our technique for producing the vapor is based on coating the inner walls of the fiber core with organosilane and using light-induced atomic desorption to release Rb atoms into the core. As an initial demonstration of the potential of this system for supporting ultralow-level nonlinear optical interactions, we perform electromagnetically induced transparency with control-field powers in the nanowatt regime, which represents more than a 1,000-fold reduction from the power required for bulk, focused geometries.

Measurement of the electronic transition dipole moment by Autler-Townes splitting: Comparison of three- and four-level excitation schemes for the Na2AΣu+1−XΣg+1 system

We present a fundamentally new approach for measuring the transition dipole moment of molecular transitions, which combines the benefits of quantum interference effects, such as the Autler-Townes splitting, with the familiar R-centroid approximation. This method is superior to other experimental methods for determining the absolute value of the R-dependent electronic transition dipole moment function mu(e)(R), since it requires only an accurate measurement of the coupling laser electric field amplitude and the determination of the Rabi frequency from an Autler-Townes split fluorescence spectral line. We illustrate this method by measuring the transition dipole moment matrix element for the Na2 A 1Sigma(u)+ (v' = 25, J' = 20e)-X 1Sigma(g)+ (v" = 38, J" = 21e) rovibronic transition and compare our experimental results with our ab initio calculations. We have compared the three-level (cascade) and four-level (extended Lambda) excitation schemes and found that the latter is preferable in this case for two reasons. First, this excitation scheme takes advantage of the fact that the coupling field lower level is outside the thermal population range. As a result vibrational levels with larger wave function amplitudes at the outer turning point of vibration lead to larger transition dipole moment matrix elements and Rabi frequencies than those accessible from the equilibrium internuclear distance of the thermal population distribution. Second, the coupling laser can be "tuned" to different rovibronic transitions in order to determine the internuclear distance dependence of the electronic transition dipole moment function in the region of the R-centroid of each coupling laser transition. Thus the internuclear distance dependence of the transition moment function mu(e)(R) can be determined at several very different values of the R centroid. The measured transition dipole moment matrix element for the Na2 A 1Sigma(u)+ (v' = 25, J' = 20e)-X 1Sigma(g)+ (v" = 38, J" = 21e) transition is 5.5+/-0.2 D compared to our ab initio value of 5.9 D. By using the R-centroid approximation for this transition the corresponding experimental electronic transition dipole moment is 9.72 D at Rc = 4.81 A, in good agreement with our ab initio value of 10.55 D.