Double-pass Configuration Research Articles

60 nJ, 95 fs environmentally stable Er-doped fiber laser system

We demonstrate here an environmentally stable and extremely compactable Er-doped fiber laser system capable of delivering sub-100-fs temporal duration and tens of nanojoules at a repetition rate of 10 MHz. This laser source employs a semiconductor saturable absorber mirror mode-locked soliton laser to generate seed pulses. A single-mode-fiber amplifier and a double-cladding-fiber amplifier (both with double-pass configuration) are bridged by a divider and used to manage the dispersion map and boost the soliton pulses. By using 64 replicas, pulses with as high as 60 nJ energy within 95 fs duration are obtained at 10 MHz, corresponding to 600 kW peak power.

High-efficiency, high-gain, all-fibered double-pass configuration for low-duty-cycle temporally shaped nanosecond pulse amplification

Abstract A high-gain fiber amplifier at 1053 nm with a double-pass configuration is proposed. Compared to the Faraday-based double-pass structures, the pump efficiency is almost doubled, and the optical signal-to-noise ratio is improved as well. The effects of fiber length and the insertion loss of elements between two passes on the efficiency are studied experimentally, which is meaningful for the optimization of double-pass amplifiers. Using the fibered polarized beam splitters allow for the orthogonal propagation of forward and the backward signals on the same polarization-maintaining fiber, the polarization mode dispersion in the amplifier is hence totally avoided. With this structure, over 50-dB gain is obtained at pump power of 800 mW for a 1053-nm, 1-Hz nanosecond pulse.

Optical Ring Cavity Search for Axion Dark Matter.

We propose a novel experiment to search for axion dark matter that differentiates the phase velocities of the left- and right-handed polarized photons. Our optical cavity measures the difference of the resonant frequencies between two circular polarizations of the laser beam. The design of our cavity adopts a double-pass configuration to realize a null experiment and give a high common mode rejection of environmental disturbances. We estimate the potential sensitivity to the axion-photon coupling constant g_{aγ} for the axion mass m≲10^{-10} eV. In a low mass range m≲10^{-15} eV, we can achieve g_{aγ}≲3×10^{-16} GeV^{-1}, which is beyond the current bound by several orders of magnitude.

Drying behaviour of lemon balm leaves in an indirect double-pass packed bed forced convection solar dryer system

The thin-layer drying behaviour of lemon balm leaves in an indirect-mode solar dryer with forced convection was investigated. The solar air heater employed a novel counter flow double-pass packed-bed wire mesh layer configuration to enhance the energy gain of the heated air. The thermal performance of the solar dryer was experimentally evaluated under Babol (36.5387°N, 52.6765°E) prevailing weather conditions. The system was comprised of a double-pass packed bed of wire mesh air heater attached to a dryer cabinet. The collector thermal efficiency was determined under realistic conditions wherein solar irradiance changed from 600 to 900 W/m2, atmospheric temperature varied from 22 °C to 25 °C and the collector outlet temperature ranged from 38 °C to 68 °C. It was found that while increasing the flow rate from 0.006125 m3/s to 0.01734 m3/s improved the collector thermal efficiency by ~ 20%; further increasing the flow rate to 0.034378 m3/s had an adverse effect on the collector efficiency. Drying experiments were performed for lemon balm leaves with initial moisture content of 80% on wet basis to the final moisture content of 10%. Mathematical models were also tested to find the best thin layer model for describing the drying behaviour of lemon balm leaves.

Efficient second harmonic generation with compact design: double-pass and cavity configurations

We report on a detailed characterization of the conversion efficiency from 1064 nm to second harmonic continuous wave laser light with periodically poled potassium titanyl phosphate crystal, with the aim of building a very efficient pump for the squeezer of the Advanced Virgo gravitational wave interferometric detector. Firstly, we study the crystal in a double-pass configuration for light at both fundamental and harmonic frequencies. We discuss how the beam alignment and laser mode hopping affect the second harmonic generation (SHG) performance and we give a detailed map of the mode hopping phenomenon in our laser source. Secondly, we employ the same double-pass crystal to form a Fabry–Perot cavity that is resonant at the fundamental frequency. We then study the efficiency of the SHG cavity as a function of the crystal temperature and input power, and demonstrate a conversion efficiency as high as , the efficiency being limited only by the onset of subharmonic pumped parametric downconversion.

A Faraday-effect polarimeter for fast magnetic dynamics measurement on DIII-D.

A Faraday-effect-based radial-interferometer-polarimeter diagnostic has been developed to explore fast magnetic dynamics in high-performance DIII-D plasmas. The instrument measures radial magnetic field perturbations using three chords positioned near the magnetic axis. Newly developed solid-state sources operating at 650 GHz provide phase noise down to 0.01°/ and tunable bandwidth up to 10 MHz. Various systematic errors which can contaminate the polarimetric measurement have been investigated in detail. Distortion of circular polarization due to non-ideal optical components is calibrated using a rotating quarter wave plate technique. The impact of perpendicular magnetic field, i.e., the Cotton-Mouton effect, is evaluated. The error due to non-collinearity of probe beams is minimized to less than 0.5° for electron density up to 7 × 1019 m-3 by alignment optimization. Optical feedback, due to multiple reflections induced by the double-pass configuration, is identified and reduced. Coherent and broadband high-frequency magnetic fluctuations for DIII-D H-mode plasmas are observed.

Generating optical vortex with large topological charges by spiral phase plates in cascaded and double-pass configuration

Spiral phase plate (SPP), as a kind of optical component for generating optical vortex (OV), has the ability to convert the traditional non-singular beams into OV with spiral phase distribution. However, due to the demanding of machining accuracy, the production of high quality SPPs with large topological charges (TCs) is usually complicated and expensive. An approach to generate OV with large TCs based on low-order SPPs is presented. In the proposed scheme, the addition of TCs is realized by cascading multiple SPPs, and the double-pass configuration is employed to implement doubling of TCs. Based on the combination of cascaded and double-pass configuration, an OV with TCs as high as 28 is generated, using two SPPs of TCs 6 and 8, respectively. The method of alignment and the alignment deviation is discussed.

High-Gain Nanosecond Vortex Laser

We demonstrate a high-gain vortex power amplifier with the double-pass configuration based on the rod Nd:YAG crystal. In the experiments, a nanosecond vortex seed beam is converted from a conventional Q-switched Nd:YAG laser output with a spiral phase plate and then amplified with the Nd:YAG amplifier stage. A maximum amplification output energy up to 163.5 mJ is achieved at 20 Hz with a 3.2-ns pulse, corresponding to an amplification factor of 81.8. Further discussions are provided to find ways of increasing the power scaling.

Extended sub-Doppler resolution spectroscopy of the ν3 band of methane

We have performed sub-Doppler resolution rovibrational spectroscopy of the ν3 band of methane (12CH4) in the mid-infrared using an optical parametric oscillator in a double-pass configuration for saturated absorption. An optical frequency comb was used for absolute frequency calibration. We have measured 22 methane line centers ranging from the P(13) to R(10) transitions with a typical uncertainty of 0.40 MHz. These measurements improve previously known transition frequencies by at least an order of magnitude.

Simulation of NOM removal by capillary NF: A numerical method for full-scale plant design

The removal of natural organic matter (NOM) from boreal lake water by a novel capillary nanofiltration (NF) membrane was predicted using a computational fluid dynamics (CFD) modelling approach. The 2-dimensional axis-symmetric model was based on a 48 m3/day NF pilot plant operating in cross-flow mode on water containing 8 mg/L total organic carbon (TOC) at fluxes ranging from 10 to 25 L/m2/h and velocities ranging from 0.25 to 1.0 m/s. A “mass jump” source code developed using the solution diffusion model was used to simulate water flux and variations in NOM content as a function of axial and radial position in the capillary fibres. The model was validated within 3% inaccuracy using pilot data for filtrate TOC and UV254 absorbance and longitudinal pressure drop. The model was subsequently used to compare the effect of module length and number of stages on the design performance of a 110,000 m3/day NF plant. Simulations indicated that 1.5 m long modules operated in a double pass configuration removed 33% more NOM compared with 3.0 m long modules in a single pass. Moreover, the use of 1.5 m modules in the full-scale plant configured in a 10:5:3:2 four stage array achieved greater NOM removal than a 10:5:3 three stage at the same plant water recovery (90.5%) using lower recycle rates and lower net energy consumption. The paper demonstrates that the combination of experimental and numerical methods can be an effective tool for the design of nanofiltration plants for enhanced NOM removal.

Detection of mercury (II) ions in water by polyelectrolyte–gold nanoparticles coated long period fiber grating sensor

This paper presents mercury (II) ions detection based on long period fiber grating (LPFG) sensor written on a single mode optical fiber by electrical arc induced technique that is suitable to be used for long term monitoring purpose. In the work, the LPFG was coated with both polyelectrolyte (PE) layers to enhance its sensitivity as well as a layer of gold nanoparticles (AuNP) for reaction to the mercury (II) ions. Experiments were conducted using double-pass configurations with mercury (II) ions concentrations varied between 0.5 ppm to 10 ppm. The results showed that the resonance wavelength of the PE–AuNP coated LPFG notch shifted to the longer wavelength, with a total shift of 1.34 nm and transmission power increment of 1.74 dBm over a period of 5 h. The results were then compared with uncoated as well as PE-only coated LPFGs, where no significant changes in resonance wavelength and transmission power were observed for these LPFGs. A novel PE–AuNP coated LPFG sensor that is suitable to be used for in-situ, long term and remote monitoring has been successfully demonstrated and tested for the detection of mercury (II) ions in water.

Mode-filtering technique based on all-fiber-based external cavity for fiber-based optical frequency comb.

We developed a mode-filtering technique based on the all-fiber-based external cavity for a fiber-based optical frequency comb for high repetition rate (frep) frequency comb, and the carrier envelope offset frequency (fceo) can be detected and stabilized and is robust to environmental fluctuations. To achieve multiplication of the frep with a high multiplication factor using the fiber ring cavity, a long fiber was developed to mitigate the physical limitation inhibiting the shortening of the cavity length. In this study, the length of the fiber cavity was set to 6.7 m (free spectral range = 44.7 MHz) as the fiber-based comb length was 6.1 m. We were able to demonstrate a multiplication factor of 11, i.e., frep increased from 48.7 MHz to 536.0 MHz with a side mode suppression ratio of about 25 dB using the double-pass configuration.

Effects of stray lights on Faraday rotation measurement for polarimeter-interferometer system on EAST.

A double-pass radially view 11 chords polarimeter-interferometer system has been operated on the experimental advanced superconducting tokamak and provides important current profile information for plasma control. Stray light originating from spurious reflections along the optical path (unwanted reflections from various optical components/mounts and transmissive optical elements such as windows, waveplates, and lens as well as the detectors) and also direct feedback from the retro-reflector used to realize the double-pass configuration can both contribute to contamination of the Faraday rotation measurement accuracy. Modulation of the Faraday rotation signal due to the interference from multiple reflections is observable when the interferometer phase (plasma density) varies with time. Direct reflection from the detector itself can be suppressed by employing an optical isolator consisting of a λ/4-waveplate and polarizer positioned in front of the mixer. A Faraday angle oscillation during the density ramping up (or down) can be reduced from 5°-10° to 1°-2° by eliminating reflections from the detector. Residual modulation arising from misalignment and stray light from other sources must be minimized to achieve accurate measurements of Faraday rotation.

Amplification of a radially polarised beam in an Yb:YAG thin-slab

The use of an Yb:YAG thin-slab architecture for amplification of a radially polarised beam at 1030 nm is investigated and shown to be a promising route for power scaling. The detrimental impact of the Gouy phase shift on radial polarisation purity is considered and a simple scheme for effective phase shift management to restore polarisation purity is presented. Preliminary experiments based on a double-pass amplifier configuration yielded an output beam with a high radial polarisation extinction ratio of 15 dB and no degradation in polarisation purity despite the non- axial symmetry of amplifier gain medium. At 50 W of launched pump power a small-signal gain of 7.5 dB was obtained for a 25 mW input, whilst 4.4 dB gain was obtained for a 1.45 W input. The prospects for further power scaling are discussed.

Transverse mode selection in a Nd-doped fiber amplifier at 910 nm.

Selective amplifications of LP01 fundamental mode and higher order modes LP11 and LP01 are demonstrated in a double-pass Nd-doped LMA fiber amplifier operating at 910 nm. A multimode core fiber Bragg grating is employed to select a single guided mode by simply adjusting the wavelength of the seed signal. Although the M2 parameter of the output beam from the amplifier was ~2.5 in a single-pass configuration, a double-pass configuration with LP01 mode selection reduces the value of the M2 parameter to 1.06 in spite of the multimode nature of the core (V~5). In addition, it is shown that this amplifier configuration permits to lower both the power saturation and the parasitic emission at 1060 nm, which consequently increase the pump-to-signal conversion efficiency at 910 nm.

A 526 mJ Subnanosecond Pulsed Hybrid-Pumped Nd:YAG Laser

A hybrid-pumped Nd:YAG pulse laser with a double-pass two-rod configuration is presented. The focal length of offset lens is particularly studied to compensate for the thermal lens effect and depolarization. For input pulse energy of 141 μJ with pulse duration of 754 ps, the pulse laser system delivers 526 mJ pulse energy and 728 ps pulse width output at 10 Hz with pulse profile shape preservation. The energy stability of the laser pulse is less than 3%, and the beam quality factor is less than 2.26.

Differences between single and double-pass schlieren imaging on diesel vapor spray characteristics

The use of schlieren imaging at high acquisition rate has been adopted as a standard optical technique for the analysis of vaporizing diesel sprays under engine-like conditions. A single-pass schlieren arrangement is typically used for the study of axially drilled single-orifice nozzles, as vessels with multiple optical accesses regularly allow line of sight visualization. Contrarily, for multi-spray nozzles, measurements are commonly performed through a single optical access, in which case a double-pass arrangement is employed. As a consequence, the light beams pass through the test section twice, increasing the optical sensitivity of the schlieren setup. However, the influence this has on the macroscopic spray characteristics is still unclear. The scope of this study is to analyze the differences in vapor phase penetration and spreading angle measured for the same injection event, through high-speed imaging, for both single and double-pass schlieren configurations. Experiments were carried out with a three hole nozzle with a nominal orifice diameter of 90μm, named Spray B from the Engine Combustion Network, using commercially available diesel fuel and in non-reactive conditions. The impact of different injection pressures, chamber temperatures and densities on the spray captured by each setup was assessed. On the results, vapor phase penetration and spreading angle followed the expected trend found in the literature, for the different boundary conditions tested. Comparing the optical setups, vapor phase penetration and spreading angle results obtained with the double-pass arrangement were marginally higher than those from the single-pass. The deviation was observed throughout all tested conditions. For spray tip penetration, although the discrepancy was approximately constant for different injection pressures and chamber temperature, it increased with increasing density. These results highlight the importance of a proper understanding regarding the limitations of optical diagnostics, in particular for results used in calibration of computational models.

A compact, flexible fiber-optic Surface Plasmon Resonance sensor with changeable sensor chips

We propose and demonstrate the concept of a novel compact, flexible fiber optic Surface Plasmon Resonance (SPR) sensor based on a double-pass Kretschmann-type configuration, where the SPR sensor chip can be replaced for various sensing applications. Simulation and experimental results demonstrate that the proposed fiber-optic SPR structure has a sensitivity to salt concentration of around 4.8μW/ppt.

Performance comparison of enhanced Erbium–Zirconia–Yttria–Aluminum co-doped conventional erbium-doped fiber amplifiers

A new Erbium–Zirconia–Yttria–Aluminum co-doped fiber amplifier (Zr-EDFA) with improved gain and noise figure characteristics is successfully demonstrated using a newly developed Erbium–Zirconia–Yttria–Aluminum Co-Doped fiber (Zr-EDF) as the gain medium. The fiber consists of Er2O3-doped ZrO2 rich nano-crystalline particles and thus it has a high erbium doping concentration with absorption pump power around 80.0dB/m at 980nm. Compared to the conventional silica based Erbium-doped fiber amplifier (Si-EDFA), the gain of the proposed Zr-EDFA is more efficient especially at longer wavelength region. At input signal of −10dBm, a flat gain of 19.5dB is obtained for the single-pass Zr-EDFA with gain variation of less than 1dB within a wavelength region from 1530 to 1570nm. In double-pass configuration, the measured gain varies from 17.5dB to 21.3dB within the wavelength region from 1525 to 1565nm, where the noise figure is maintained at below 10dB. The double-pass Zr-EDFA gain is also maintained above 12dB within a wavelength region from 1520 to 1600nm. These results indicate that the Zr-EDFA can achieve even better flat-gain value and bandwidth as well as lower noise figure than the conventional Si-EDFA

Tunable transportable spectroradiometer based on an acousto-optical tunable filter: Development and optical performance

We describe a high-performance, transportable, versatile spectroradiometer based on an acousto-optical tunable filter (AOTF). The instrument was developed for temperature metrology, namely, to determine the thermodynamic temperature of black bodies above the Ag freezing point (961.78 °C). Its main design feature is the attenuation of the diffraction side lobes (and, thus, out-of-band stray light) thanks to the use of a double-pass configuration. The radiofrequency tuning of the AOTF allows continuous, fine, and rapid wavelength control over a wide spectral range (650 nm-1000 nm). The instrument tunability can be easily calibrated with an Ar spectral lamp with reproducibility within 10 pm over one week. The instrument was characterised in terms of relative signal stability (few 10-4) and wavelength stability (1 pm) over several hours. The spectral responsivity of the instrument was calibrated with two complementary methods: tuning of the wavelength of the optical source or tuning the radiofrequency of the AOTF. Besides the application for thermodynamic temperature determination at the lowest uncertainty level, this instrument can also be used for multispectral non-contact thermometry of processed materials of non-grey and non-unitary emissivity (in the glass or metallurgical industries).