Photonic bandgap fiber enabled Raman detection of ...

Photonic bandgap fiber enabled Raman detection of nitrogen gas Rui Chen*a1, Peter J. Codellaa, Renato Guidaa, Anis Zribia**, Alexey Verta, Radislav Potyrailoa, Marko Ballerb a General Electric Global Research, One Research Circle, Niskayuna, NY, USA 12309; b General Electric Global Research, Freisinger Landstrasse 50, Munich, Germany 85748; ABSTRACT Raman detection of nitrogen gas is very difficult without a multi-pass arrangement and high laser power. Hollow-core photonic bandgap fibers (HC-PBF) provide an excellent means of concentrating light energy in a very small volume and long interaction path between gas and laser. One particular commercial fiber with a core diameter of 4.9 microns offers losses of about 1dB/m for wavelengths between 510 and 610 nm. If 514nm laser is used for excitation, the entire Raman spectrum up to above 3000 cm-1 will be contained within the transmission band of the fiber. A standard Raman microscope launches mW level 514nm laser light into the PBF and collects backscattered Raman signal exiting the fiber. The resulting spectra of nitrogen gas in air at ambient temperature and pressure exhibit a signal enhancement of about several thousand over what is attainable with the objective in air and no fiber. The design and fabrication of a flowthrough cell to hold and align the fiber end allowed the instrument calibration for varying concentrations of nitrogen. The enhancement was also found to be a function of fiber length. Due to the high achieved Raman signal, rotational spectral of nitrogen and oxygen were observed in the PBF for the first time to the best of our knowledge. Keywords: Raman, nitrogen gas, photonic bandgap fiber, enhancement

1. INTRODUCTION Raman spectroscopy is a scattering phenomenon where an incident photon exchanges energy with a molecular orbital or the electronic structure of a material. The exchange can either give energy to or remove energy from the molecule or structure, therefore generating Stokes or anti-Stokes shift. Raman spectroscopy has been applied in a lot of applications for liquid, solid and gas sensing albeit with different sensitivities, with or without enhancement means. The work reported in this paper is focused on evaluating the enhancement effect of a hollow-core photonic bandgap fiber (HCPBF) in gas detection. 1.1 Challenges for Raman gas sensing Raman scattering intensity is a function of the incident light power (I0), the wavelength (λ) of the incident light, the concentration of analyte material (c), and the scattering cross section of the analyte molecule (J). In addition, any experimental setup and/or sample have its own restrictions on the ability of the instrument to collect and analyze light. This factor is usually called the instrument factor (K). Therefore, a simplified equation for the observed Raman Scatter can be expressed as follows [1] (1) R = I0 c J(λ) K(λ) / λ4 Clearly, if the experimental conditions are controlled, Raman spectroscopy should be capable of quantitative analysis. Using the same sample form on the same instrument under the same experimental conditions results in two of the variables, K and λ, being constant. J is always constant for a specific molecule or material, the magnitude of which depends on the molecular electronic structure. For maximum Raman response, the electronic structure should lack a permanent dipole and be highly polarizable. Nitrogen gas is a model example of such molecule. However, Raman signal also depends on analyte concentration. The concentration factor is essentially 1 for a solid or liquid. For a gas sample, the factor is approximately 4.5 x 10-5. Therefore, Raman spectroscopy of a gas usually involves high laser power (hundreds of mW to several Watt) or a multi-pass arrangement where the illumination laser beam is focused on *[email protected]; phone 1 518 387-5194; fax 1 518 387-6972 ** Present address: United Technology Corporation, Hartford, CT, USA 06101 Photonic Microdevices/Microstructures for Sensing, edited by Hai Xiao, Xudong Fan, Anbo Wang, Proc. of SPIE Vol. 7322, 73220N · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.817823

Proc. of SPIE Vol. 7322 73220N-1

the sample volume from a variety of directions [2, 3]. Typically an enhancement of 10’s to 100’s is achieved with multipass approach. Increasing Raman light collection efficiency by specially designed optics [4] and utilizing higher gas pressure [5] also helped to raise detected Raman signals. 1.2 Challenges for nitrogen gas sensing Analytical methods for the measurement of nitrogen are scarce. Most methods for gaseous nitrogen are based on gas chromatography. Since nitrogen has no absorptions in the visible, near infrared, or infrared wavelengths, there are no methods based on light absorbance spectroscopy. Due to its ubiquitous occurrence in the atmosphere (essentially 78% nitrogen, 21% oxygen, 0.93% argon, 0.038% carbon dioxide, variable amounts of water, and trace other gases), measuring nitrogen has been largely overlooked. However, there are occasions where measurement of all inert gases including nitrogen plays an important role such as determining thermal conductivity of natural gas [5]. Raman Spectroscopy may be able to fill the measurement requirements for nitrogen if an appropriate experimental arrangement were available. Numerous publications have reported the benefits of increased sensitivity using hollow core waveguides for the measurement of Raman spectra. Some of these are Teflon® AF liquid-core waveguides with a refractive index lower than that of the clear liquid fill in the waveguide core [6]. This causes an internal reflection at the liquid/Teflon® AF interface therefore keeping both the illumination beam and the Raman scattered light within the liquid core of the waveguide. Enhancements up to about 400x have been reported. In some special cases, a glass capillary tube will work, if the solvent is chosen to have a lower refractive index than glass, total internal reflection will be achieved and a signal enhancement will be observed. Other designs have used multiple silver coated capillary tubes with 12-30 folds signal enhancements [7]. 1.3 HC-PBF for Raman gas sensing Building on the idea of a hollow-core waveguide, this study will focus on using hollow-core photonic bandgap fibers (PBF) to realize Raman detection of nitrogen gases. These fibers employ a central hollow core surrounded by a honeycomb structure (Figure 1A). Contrary to traditional fiber optics that relies on refractive index difference to guide light, PBFs guide light based on the band gap created by periodic structure of air holes [8]. More than 95% of the light is guided through the central core. Photonic bandgap fibers have very low bending loss and very low attenuation. Guiding light in the hollow core holds lot of applications that are not possible before. The high light intensity and long interaction lengths enable efficient nonlinear interactions between laser light and low-density media; based on PBFs, stimulated Raman scattering at 100 times lower pulse energy and supercontinuum source are two important fields [8, 9]. Besides the nonlinear effect, the long interaction lengths between gas and laser while keeping the laser bream tightly confined in a single mode is particularly attractive for Raman spectroscopy. The photon intensity inside the hollow core is very large due to the micron-size space. The dimensions of both the core and honeycomb can be customized to yield fiber specifically tuned to a particular wavelength. The tight confinement of excitation light and Raman light eliminate the challenges associated with multi-pass approach and signal collection. One particular commercial fiber with a core diameter of 4.9 microns offers very low losses of about 1dB/m for wavelengths between 510 and 610 nm (Crystal Fiber A/S, Denmark, Figure 1B). If an Argon laser emitting at 514nm is used for illumination, the resulting nitrogen Raman scattered light will be shifted to 584 nm (nitrogen Raman shift is at 2331 cm-1). Both the excitation laser light and shifted Raman light are within the optimal transmission range of the fiber. In the work reported in this paper, a standard Raman microscope launches mW level 514nm laser light into the PBF and collects Raman scattering from the same end. The resulting spectra of nitrogen gas in air at ambient temperature and pressure exhibit a signal enhancement of about several thousand over what is attainable with the objective in air and no fiber. The enhancement is great enough even to see the rotation-vibration lines. The design and fabrication of a flowthrough cell to hold and align the fiber end allowed the instrument calibration for varying concentrations of nitrogen. The enhancement was also found to be a function of fiber length. Similar results were also observed for oxygen gas. This HC-PBF enhanced approach has the potential of greatly enhancing the gas phase spectrum of nitrogen or any other contained gas. It was during the authors’ preparation of this manuscript that a recent publication on HC-PBF enhanced Raman scattering of natural gas came to our attention [10]. In that paper, the authors have measured Raman signal of natural gas through 1.5m length of PBF (forward scattering mode) and reported several hundred folds signal enhancement over a traditional free-space arrangement. Both works have demonstrated great potential for HC-PBF enhanced Raman sensing. In our work, due to the high achieved Raman signal, rotational spectral of nitrogen and oxygen were observed in the PBF for the first time to the best of our knowledge.


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Figure 1 (A) Scanning electron microscope image of a hollow-core photonic band gap fiber. The core of the fiber is 4.9 μm. (B) Typical attenuation spectrum for photonic bandgap fiber (HC-580-01). Both from website of Crystal Fiber A/S, Denmark (Reprinted by permission of Crystal Fiber A/S, Denmark).

2. EXPERIMENTAL 2.1 Fiber preparation Photonic bandgap fiber (HC-580-01, Crystal Fiber A/S, Denmark) was ordered through Thorlabs (Newton, NJ). Fiber preparation (stripping and cleaving) is extremely important for gas sensing using Raman detection. It was found that a heated fiber stripper (Amherst FiberOptics) produces less fiber breakage than a mechanical stripper. After the fiber was stripped, it was necessary to cleave the fiber to yield a pristine end with little or no damage to the photonic structure. A commercial diamond cleaver (Alcoa Fujikura) yielded satisfactory result. After preparation, fiber was imaged under microscope to ensure no damage to the photonic structure and proper measures were taken to prevent dust contamination. 2.2 Fiber mount and gas detection In the experiments with PBFs, several experimental arrangements were used to mount the fiber at the focus of the microscope. The fiber was first held in a chuck on the microscope stage, but the only gas available for test was air. A more versatile gas cell was further designed and manufactured in-house (a schematic drawing shown in Figure 2A). The cell was machined from a standard Ultra-Torr® fitting (Swagelok company, Cajon Division, Macedonia, Ohio). The resealable fitting at the bottom is the typical Ultra-Torr® stainless steel fitting; the other end is machined to hold a capillary or a fiber chuck in place. The cap on top has gas flow vents on both sides and an anti-reflective glass window at the top. As desired, different gases were purged through the cell or vacuum was applied to draw gas through. Microscope stage plate and tilting stage (Figure 2B) allows easy adjustment of alignment position and angle.


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2.3 Raman measurements Raman spectra were obtained on an Aramis Raman Microscope System (Horiba Jobin Yvon, Edison, NJ). The system was configured with 4 different laser lines (488, 514, 633, & 785nm) that can be selected to illuminate a micro or macro sample. Each line has its own rejection filter and alignment optics to assure a common focal point (Figure 3). All results reported in this paper were based on 514nm laser excitation and backscattered configuration. The microscope attached to the system has a computer-controlled stage. Light dispersion is provided by a 460 mm spectrograph with a turret accommodating 4 gratings (1800, 1200, 600, & 300 lines/mm). The 1800 line/mm grating was used for these measurements. The system incorporated a thermoelectrically cooled CCD detector. Holographic filter Spectrograph

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Figure 3 Simplified Optical pathway for Raman detection of nitrogen gas using photonic bandgap fiber.

3. RESULTS AND DISCUSSION 3.1 Optimal alignment of HC-PBF and atmospheric nitrogen measurement Alignment of excitation laser and hollow core of PBF is crucial for detecting Raman signals. Custom designed gas flow cell and computer controlled stage in Raman microscope allowed optimal alignment with µm resolution. With a 10X objective having a numerical aperture close to that of the fiber, the excitation laser light was focused to a spot size of about 10µm in diameter. The power at the focus spot was measured to be about 13mW. Raman spectra taken along a grid (preset at one µm per step) reconstructed an intensity map based on Raman peak; the highest Raman signal was from a narrow area of about 4µm in diameter (Figure 4A). This is consistent with the size of hollow core of this particular fiber, demonstrating the high photon intensity plays an important role in Raman signal enhancement. The honeycomb region also showed enhanced Raman signal but not as high as the central core.


Figure 4B showed a backscattered Raman signal of nitrogen gas from the HC-PBF based system with Raman shift at 2331 cm-1. Here the PBF was only a few centimeters long. This signal for nitrogen gas in ambient air (79% N2) at 1 atmosphere was found to be thousand fold over the free space signal attainable with the objective in air and no fiber present. -1.0

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Figure 4 (A) Plot of Raman intensity map. (B) Nitrogen spectrum at the center of intensity map. 10X objective was used and the focus point is about 10µm in size.

It was observed that the longer the PBF, the bigger the enhancement signal (more details in section 3.2). With a 1-meter long fiber, the enhancement was so large that the rotation-vibration lines were observable (Figure 5A). Similar enhancement effect was also observed for oxygen gas in ambient air (Figure 5B). The rotational spectra would usually require very high laser powers and high gas pressures [2]. The ratio of nitrogen/oxygen Stokes signal is close to 2.7, which is consistent with difference in Raman cross section between oxygen and nitrogen and the difference in Stokes wavelength.

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Figure 5 Raman spectra of nitrogen (A) and oxygen (B) taken for ambient air. Rotational spectral bands are clearly visible in Raman spectra of nitrogen and oxygen.


3.2 Photonic fiber length effect

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The Raman enhancement effect from HC-PBF was found to be a function of fiber length. In order to study the fiber length effect, a nearly 1-meter long fiber was used at first after optimal alignment. The other end of the PBF that was not facing laser light was cut off at 10 cm intervals. After each cutting, alignment was repeated and the maximum Raman signal was recorded. As is shown in Figure 6, the Raman signal gradually increases with increasing fiber length then levels off. The plotted line is the best fit according to expected functional form for analyte-filled waveguide Raman scattering: IR=Kxexp(-αx), where IR is the Raman intensity, K is a constant proportional to the intensity of the excitation radiation, the Raman cross-section and coupling efficiency of both incident laser excitation and Raman light, x is the physical length of the waveguide, α is the attenuation coefficient of the waveguide with the assumption that α is the same for both laser and Raman signal [6, 11]. The parameters extracted from the best fit is K=18 and α=0.54 m-1 with error mean square (MSE) = 0.00036. The value of α is within the range of reported attenuation coefficient for the photonic fiber (Figure 1B).

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Figure 6 Plot of normalized nitrogen Raman signal vs. fiber length. Measured gas is ambient air.

3.3 Nitrogen calibration and detection response time The design and fabrication of a flow cell to hold and align the fiber end allowed the instrument calibration from pure nitrogen to pure argon. Figure 7A shows a dynamic signal change with flow cell flushed with varying concentrations of nitrogen. The tested PBF fiber length is 59 cm and observed response time is within 100-200 second range. The selection of fiber length is a balance between the maximum signal intensity and required detector response, depending on final application. Higher gas pressure will also help speed up the gas exchange rate. There was a slight difference in calibration when plotting the Raman signal vs. concentration during loading and unloading phase (Figure 7B), the observed hysteresis was only 4%.


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Figure 7 (A) Dynamic Raman signal changes with air, then transition to 10% N2, 5% N2, 1% N2 then back to 5%, 10% N2, and finally air. PBF length = 59 cm, gas flow rate in gas flow cell = 500 ml/min. A very small house vacuum was applied to help draw air. (B) Calibration plot of Raman signal as a function of nitrogen concentration.

3.4 Conclusion With the use of hollow-core photonic bandgap fiber, the Raman spectra of nitrogen gas in air at ambient temperature and pressure exhibit a signal enhancement of about several thousand over what is attainable with the objective in air and no fiber. Due to the high achieved Raman signal, rotational spectral of nitrogen and oxygen were observed in the PBF for the first time to the best of our knowledge. The enhancement was found to be a function of fiber length. Similar results were also observed for oxygen. This has the potential of greatly enhancing the gas phase spectrum of nitrogen or any other contained gas. It can be envisioned that lower detection limit could be achieved with high gas pressure and higher laser power.

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