Patent Application
20100269952
The disclosed system and method also related to systems and methods for filling microstructured or nanostructured optical fibers. The disclosed system and method relate to detection systems using absorption spectroscopy.
Absorption spectroscopy exploits the characteristic absorption of radiation that corresponds to the wavelengths associated with the excitation of atoms in the sample. The characteristic absorption spectra are usually well defined making the detection method highly selective. The sensitivity of wavelength selective spectroscopy is dependent on the path length of the absorption process in the sample, concentration of the sample, and the line strength of the absorbing species. Typical detection limits for biological and environmental regulatory atomic absorption spectroscopy methods range from a few tenths of a part per million down to a few tenths of a part per billion. Selectivity qualities can be compromised by spectral interference, i.e., the overlapping of absorption lines from different samples.
Gas absorption characteristics are typically modeled as being governed by Beer's Law, which defines the relationship between the measured intensity of peaks of the absorption spectra to the physical conditions of the gas sample.
Conventional methods of microstructured optical fiber absorption spectroscopy rely on diffusion to fill an optical fiber with the gas or sample being tested. The concentration gradient between the concentration of the filling gas outside of the fiber and the gas inside the fiber drives the diffusion of filling gas molecules into the air molecules. The rate of diffusion is controlled by a classical binary diffusion model where the binary diffusion coefficient between the filling gas and the air can be used to estimate the filling time along with the fiber length. However, these conventional filling methods result in response times (i.e., filling times) on the order of minutes for fibers having lengths greater than or equal to one meter in length. Consequently, these conventional methods are impractical for implementation in sensors for use in industrial process monitoring, intelligent buildings, or other application where fast response times are needed.
A delivery system for filling a microstructured or nanostructured optical fiber (MOF/NOF) includes an MOF/NOF having first and second ends. The first end of the MOF/NOF is disposed in a first chamber configured to have a first pressure, and the second end of the MOF/NOF is disposed in a second chamber configured to have a second pressure that is less than the first pressure. A material source introduces a material into the first chamber. A light source is oriented to emit light into one of the ends of the MOF/NOF. An optical detection system for monitoring filling of the MOF/NOF detects the light emitted by the light source from the other of the ends of the MOF/NOF.
In embodiments, the delivery system for MOF/NOF is configured as a sensor for analysis of the material introduced into the MOF/NOF.
An improved method and system for filling microstructured optical fibers (MOF) or nanostructured optical fibers (NOF), such as photonic bandgap (PBG) fibers, is provided. An improved fiber sensor, such as a photonic bandgap (PBG) fiber sensor, based on the improved filling technique is also described. The sensor has faster filling times compared to conventional PBG fiber sensors. In embodiments, the sensor includes a MOF or NOF fiber (e.g., a PBG fiber) having a first end disposed in a first chamber having a first pressure and a second end disposed in a second chamber having a second pressure. A gas (or other material for filling the fiber) is introduced into the first chamber, and a pressure differential resulting from the second pressure being less than the first pressure advantageously reduces the filling time of the PCB fiber with the gas compared to conventional diffusion methods. The pressure differential is sufficient to support forced convection, or put another way, pressure driven laminar convection. This forced convection can be modeled as a laminar flow. Light is emitted into an end of the optical fiber, and an optical detector receives light emitted from the light source at the other end of the fiber. The optical detector can be used to monitor the filling operation or a spectral analyzer can be provided for material analysis (e.g., material identification and concentration determination).
In an alternative embodiment, the light source need not be external to the gas housing 104. Moreover, the light source could be internal to the fiber 114. For example, the core of the fiber 114 could be provided with quantum dots which when energized emit light for transmission in the core of the fiber, similar to the operation of a florescent light.
Gas housings 104 and 118 may be any substantially air-tight housings defining internal chambers 106 and 116. In some embodiments, chamber 106 and 116 are disposed in a single housing and are separated by an airtight membrane or wall. A pump may be coupled to gas housing 104 and/or a vacuum may be coupled to gas housing 118 to create a pressure differential between first chamber 106 and second chamber 116. One skilled in the art will understand that other devices may be coupled to gas housings 104 and 118 to create a pressure differential between gas housings 104 and 118. In some embodiments, gas source 108 may be a pressurized gas source to provide the desired pressure differential between gas housings 104 and 118, which may limit the need for additional devices in the system for creating the pressure differential. Of course, any combination of a pressurized source, a pump and a vacuum could be used. While the gas source is illustrated as being in the first housing 104, this need not be the case. It should be understood that the gas can be introduced in the second gas housing 118 and flow away from the optical detection system toward the light source 102.
In embodiments, fiber 114 is a MOF fiber having a hollow core or microstructured cladding or lattice, for example a photonic crystal fiber (PCF) or PBG fiber. The length of the fiber 114 may vary between lengths of a fraction of a meter (e.g., less than 0.5 meters) to lengths in excess of hundreds of meters. The air cavities of these fibers provide a strong interaction of the optical field and the delivered material with low losses due to bending and other attenuation mechanisms.
Gas source 108 may provide a variety of gases to gas housing 104 for undergoing spectral analysis. The gas provided by gas source 108 may be sealed within gas housings 104 and 118 by closing valves 112 and 124, which are configured to be opened and closed to provide a hermetic seal as well as gas exhaustion. As described above, pressurized gas sources may be used to reduce the number of components in the system.
In embodiments, the optical detection system/analyzer 120 is configured to monitor the filling of the optical fiber by detection of the light emitted from the fiber. When the system is configured as a material sensor, the spectrometer 120 is configured to perform absorption spectroscopy on light transmitted from light source 102 that exits second end 114b of PBG fiber 114. In one embodiment, spectrometer 120 is an optical spectrum analyzer (OSA) capable of detecting multiple-component gases based on the light transmitted by light source 102. The detection sensitivity of spectrometer 120 can be adjusted by varying the type of light source 102. For example, narrow bandwidth light sources with high intensities provide high signal-to-noise ratios for enhanced detection limits compared to broadband light sources. Broadband light sources are useful for systems that do not require narrow spectral linewidths or have high absorption coefficients.
The pressure difference between the chambers of the gas housings 104 and 118 increases the speed at which gas provided by gas source 108 fills PBG fiber 114, which results in shorter response times compared to conventional diffusion-based systems. Fill times of less than one minute have been achieved for PBG fibers 114 having lengths of less than one meter and pressure differentials of less than 15 psi through experimentation.
A first end 126a of single mode fiber 126 is butt coupled to the first end 114a of PBG fiber 114 in gas housing 104. The butt or direct coupling of single mode fiber 126 and PBG fiber 114 enables light waves transmitting along single mode fiber 126 to be coupled into PBG fiber 114 with minimal signal loss while enabling gas from gas source 108 to be received within PBG fiber 114. Gas source 108 is coupled to gas housing 114 through pipe 110 having valve 112 disposed along its length. A pressure transducer 128 is coupled between gas source 108 and valve 112, although one skilled in the art can appreciate that pressure transducer 128 can be located between valve 112 and gas housing 104 and/or provided to monitor the pressure of gas housing 118. Gas housings 104 and 118 define holes through which PBG fiber 114 extends such that the second end 114b is disposed within the second chamber 116. Second end 114b of PBG fiber 114 is butt coupled with first end 130a of multi-mode fiber 130, which is coupled to optical spectrum analyzer 120.
Light source 102 was configured to output light having a wavelength centered at a 1550 nm. The alignment conditions of the light source 102 and fibers 126, 114, and 130 were monitored using a camera system to image the hollow core region at the fiber output to verify that the light was launched into the hollow core PBG fiber 114 at the fiber input from single mode fiber 126.
Nitrogen was used to purge PBG fiber 114 of residual gases and to obtain a spectroscopic baseline reading at OSA 120 for comparative purposes. The gas sample was admitted into the first chamber 106 at a designated pressure that was monitored by pressure transducer 128. Pressures were measured above ambient pressure at room temperature. The sample gas flowed through PBG fiber 114, which had its second end 114b butt coupled to multi-mode fiber 130. The butt coupling of PBG fiber 114 and multi-mode fiber 130 enabled gas to exit PBG fiber 114 into chamber 116, which was vented to the atmosphere through valve 124. The transmitted output was measured by OSA 120 with a 0.2 nm resolution.
The spectral information was recorded at regular time intervals to monitor the filling dynamics of various lengths of PBG fiber 114. The sweep time, which was approximately one second, was constant for all measurements and was small compared to measurement intervals. Experiments were performed with C2H2 and CO2 gas samples provided by gas source 108. The concentrations of the gas for each gas source 108 used in the experiment were certified by the manufacturer. Fiber lengths of 0.3 m, 0.7 m, 1 m, 1.9 m, and 27 m were tested.
In the tested gas regimes, the filling rates were assumed to be governed by a classical laminar flow. However, the regimes were following unsteady fluid dynamics theory, in which the filling gas displaced the resident gas in a “piston-like” fashion. For example,
The second end 114b of PBG fiber 114 was exposed to chamber 116 within gas housing 118, which was opened to atmospheric pressure such that the outlet pressure, P2, was fixed. Due to the fact that PBG fiber 114 was connected to a fixed-pressure gas cylinder, i.e., gas source 108, the inlet pressure, P1, was also fixed throughout the experiment. The properties of the gases (acetylene (C2H2), nitrogen (N), carbon dioxide (CO2), and air (O2)), such as viscosity, density, etc, were assumed to be constant. The density gradient of the gas is not significant for the range of pressures or volume of gas used to fill the length, L, of PBG fiber 114. The gradient may, however, affect the spectroscopic linewidths, which were not analyzed with the current system due to the broadening effects of multiple phenomena. The diameter, D, of the hollow core 134 of PBG fiber 114 was 12.5 μm.
The case in which the viscosities of the filling gas and the initial gas are similar to one another, the filling time linearly depends on viscosity. Consequently, the higher the viscosity of the gases, then the filling time will be higher compared to gases with lower viscosities. Additionally, the characteristic time for filling the fiber has an inverse relationship to the pressure difference, ΔP, and to the square of the diameter, i.e., D2, of the PBG fiber 114 while having a direct relationship to the square of the length, L2, of the PBG fiber 114. Thus, the diameter of the fiber core 134 and the length of the PBG fiber 114 have the most significant affect on the filling time.
The time period required for a gas to fill the length of a fiber can be determined from the velocity relationship for the speed at which L1 (
As L1 increases, the boundary between the two gases A and B within fiber 114 is displaced such that the speed at which L1 is increasing approaches the rate of the delivered gas filling the PBG fiber 114
Spectroscopic data were collected for the different lengths of PBG fiber 114. The normalized absorption peaks of the characteristic spectra were monitored to observe the filling times, pressure broadening of peaks, and bend sensitivity of the gas-filled fiber 114, as expected.
Simultaneous detection of individual gases in a mixture using a fiber having a length of 27 m was spectroscopically confirmed. The measured spectra are consistent with existing reference spectra for sample gases. The gas mixture included 5% C2H2, 50% CO2, and a balance of nitrogen. The absorption line strengths of CO2 are significantly less (approximately three orders of magnitude less) than C2H2 in the spectral region of interest.
Not only can the two gases be identified spectroscopically, but the concentration of each gas can be estimated by comparing absorbance measurements to a calibrated reference. Data were fitted to an exponential curve to approximate the time constants for the filling rate. Complete filling was assumed when 95 percent of the reference light from light source 102 was absorbed (i.e., T=3τ). Fiber lengths of approximately 2 m exhibited filling times of approximately 84 seconds for pressures of 13.5 psi as shown in
The data also demonstrate that filling times increased with the length of PBG fiber 114, which identifies that the fibers were being filled with the sample gases. Each of the trials demonstrated that the filling rates are non-linearly dependent on fiber length.
The impact of pressure on the filling time is also demonstrated by the data. For example, the filing time constant can be reduced to τ=0.93 second at a pressure of 15.08 psi from τ=1.75 seconds at 10.28 psi for a fixed fiber length of 0.3 m as shown in
The pressure-driven gas-filing technique described herein advantageously provides faster filling times compared to conventional diffusion-based techniques. A reduction in the time scales for filling have observed to be three orders of magnitude smaller than the conventional diffusion-based filling methods. Accordingly, reduced filling times may be achieved that enable longer fiber lengths to be used in systems such as sensor systems with increased sensitivity. Additionally, the reduced filling times enable PCB sensors to be used in a wide variety of applications including sensors to detect gas in engines, detection of fires, analyzing breath of a person or animal, detection of substances in airport or other security systems, and detection of a composition of a fluid sample for biomedical processing.
It should be understood that while the filling system has been illustrated in connection with gas filling of a MOF/NOF, such as a PBG fiber, the pressure driven filling system described herein can be used for filling MOF/NOF fibers with gasses, liquids, solids (e.g., gels) or combinations thereof. Moreover, as described above, the pressure differential between the ends of the fiber should be sufficient to induce convective flow, it should be understood that a combination of convective and diffusive flow may also be utilized. For example, the flow may be characterized at the ends of the fibers or at other localized regions as diffusive.
Although the invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claims should be construed broadly, to include other variants and embodiments of the invention, which may be made by those skilled in the art without departing from the scope and range of equivalents of the invention.
This application claims priority to U.S. Provisional Patent Application No. 61/171,881 filed Apr. 23, 2009, the entirety of which is herein incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61171881 | Apr 2009 | US |
