Abstract:
There is provided a gas cell. The gas cell has an optical fiber that includes a first hole that contains a first reference gas that absorbs a first wavelength of light, and a second hole having a content selected from the group consisting of (a) a vacuum and (b) a second reference gas that absorbs a second wavelength of light.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application is a divisional of U.S. patent application Ser. No. 10/499,870, filed Sep. 17, 2004, which is a U.S. National Stage Entry of PCT/EP02/00487, filed Jan. 19, 2002. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to wavelength calibration.  
         [0003]     Currently, reference signals for wavelength calibration of instruments and systems used, e.g. in telecommunications, are obtained from optical absorption or emission lines of electronic or vibrational states of molecules, such as acetylene, HCN, or CO 2 , which are contained in conventional glass cells. Details are disclosed e.g. in U.S. Pat. No. 6,249,343, U.S. Pat. No. 5,450,193, and U.S. Pat. No. 5,521,703.  
       SUMMARY OF THE INVENTION  
       [0004]     It is an object of the invention to provide an improved wavelength calibration. The object is solved by the independent claims. Preferred embodiments are shown by the dependent claims.  
         [0005]     There is provided a gas cell. The gas cell has an optical fiber that includes a first hole that contains a first reference gas that absorbs a first wavelength of light, and a second hole having a content selected from the group consisting of (a) a vacuum and (b) a second reference gas that absorbs a second wavelength of light. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     Other objects and many of the attendant advantages of the present invention will be readily appreciated and become better understood by reference to the following detailed description when considering in connection with the accompanied drawings. Features that are substantially or functionally equal or similar will be referred to with the same reference sign(s).  
         [0007]      FIG. 1  shows a setup for providing a wavelength reference measurement according to the present invention.  
         [0008]      FIG. 2  illustrates, in cross sectional view, in principle an embodiment of the fiber  10  according to the present invention. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0009]     According to the present invention, an optical fiber is applied as a gas cell for wavelength calibration purposes. The optical fiber preferably comprises a hole or an arrangement of holes in or along the fiber, in which a sufficient part of the optical mode field distribution is localized. The hole or the arrangement of holes is filled with the gas for providing absorption lines for the wavelength calibration.  
         [0010]     Mode-guiding in the fiber can be achieved preferably in two ways: 
        An arrangement of holes acts as an effective medium with lower refractive index than other regions of the fiber, e.g., the solid glass core of the fiber. In this case, the mode is usually guided in the glass of the fiber core, and only a small portion of the field distribution is localized in the holes. However, an arrangement of regions (or “shells”) with different hole densities can also be applied which mimics a profile of the effective index of refraction analogous to that in a conventional optical fiber. In this case, the fraction of the mode density localized in the holes will be larger.     An arrangement of holes acts as a photonic crystal which has very high reflectivity for modes guided in the region surrounded by the photonic crystal region. This region can be a very large diameter “hollow core” which then guides most of the mode intensity.        
 
         [0013]     According to the invention, the holes in such fiber are filled with a defined gas or gas compound used as wavelength reference standard. The use of such fiber gas cells thus allows such fiber to enormously increase the interaction length of the light with the gas molecules compared to only a few cm in conventional gas cells. Therefore gases with rather low absorption, such as CO 2 , can be used. This is especially useful in the telecommunications L band.  
         [0014]     Further, the inventive fiber gas cells can be provided more compact, more flexible and better suited to fiber-optic instruments than the bulky cuvette-type conventional cells used today. Problems of pig-tailing and free-space connections across free path lengths of several cm can be significantly reduced.  
         [0015]     Additionally, the volume of toxic gases, e.g. HCN, required for some applications can be significantly smaller. This has benefits for manufacturers, operators, and environment. Finally, fiber gas cells can be provided cheaper than conventional ones. Only a few meters of fiber are needed at most.  
         [0016]     In a preferred embodiment for making the inventive fiber gas cells, air-filled hollow cores of “normal” photonic crystal fibers are filled with a desired gas or gas mixture. This can be achieved e.g. by pumping on one side and attaching a gas reservoir on the other side of the fiber. End pieces consisting of flat glass, microlenses as well as other optical, source or detection elements could be attached, for example by gluing or arc welding methods.  
         [0017]     Alternatively, small pieces of frozen gas crystals or small amounts of liquid gas can be inserted in the evacuated fiber that is then sealed. The fiber fills with gas as the crystals or the liquid evaporate.  
         [0018]     Since gas filling of holes with small diameters might suffer from the large resistance of the very narrow channels, the whole fiber growth process is preferably performed in another embodiment in an environment (e.g. under pressure) of the desired gas or gas mixture.  
         [0019]     In a preferred embodiment, the optical fiber is provided in accordance with a hollow-core fiber as disclosed by J. C. Knight et al., Optics Letters 21, 1547 (1996), a “holey” fiber as disclosed by M. Ibanescu et al., Science 289, 415 (2000), or a photonic crystal fiber as disclosed by J. Broeng et al., Danish Opt. Soc. News, p. 22, June 2000 or J. Broeng et al., J. Opt. A: Pure Appl. Opt. 1, 477 (1999) or R. F. Cregan et al., Science 285, 1537 (1999).  
         [0020]     Other applicable fiber structures are disclosed e.g. in WO-A-0022466, WO-A-9964903, WO-A-9964904, U.S. Pat. No. 6,301,420, WO-A- 0142831 , WO-A-0065386, or WO-A-0016141.  
         [0021]     For providing a wavelength reference measurement, the inventive fiber filled with gas having known absorption wavelengths is preferably coupled to a wavelength source providing the stimulus for the gas-filled fiber. A wavelength response signal of the gas-filled fiber in response to the applied stimulus is detected and analyzed. Comparing the detected wavelength response signal with the known absorption wavelengths then allows calibrating the provided wavelength analysis using the known absorption wavelengths. Calibration schemes and setups as disclosed e.g. in the aforementioned U.S. Pat. No. 6,249,343, U.S. Pat. No. 5,450,193, and U.S. Pat. No. 5,521,703, as well as other known wavelength measurement, control and calibration techniques, can be applied accordingly.  
         [0022]     Further preferred embodiments are: 
        The individual holes of the fiber gas cell are not all uniformly filled with the same gas used for wavelength calibration. Other possibilities include: (1) Some of the holes are filled with the reference gas and some holes are under vacuum (“empty”); (2) some of the holes are filled with the reference gas and others are filled with another gas, e.g. air. The gas cell, however, should be provided in a way that interaction of the light with the reference gas is strong enough to allow for wavelength measurement.     Different holes of the fiber gas cell are filled with different reference gases, e.g., C 2 H 2  and CO 2  in one and the same fiber. This allows the simultaneous measurement of reference wavelengths in different spectral regions, according to the gases used, in a single fiber gas cell.     At least two fiber gas cells having a certain length and being filled with different reference gases, e.g., C 2 H 2  and CO 2 , are spliced together, thereby forming a new fiber gas cell having a greater length. This arrangement allows the simultaneous measurement of reference wavelengths in different spectral regions, according to the gases used, in a single fiber gas cell.     A fiber gas cell having at least one end piece consisting of a lens or another means to improve the coupling of this fiber gas cell to other fiber-optical components and systems. The at least one end is mechanically coupled or fusion spliced to the fiber gas cell.     Fiber gas cell in combination with an optical system, such as but not limited to a source or receiver of optical power, to perform wavelength reference measurements.     An integrated system of fiber gas cell with light source and/or detector mounted directly onto the fiber ends for easy incoupling and/or detection of optical power.     Fiber gas cell using the broadband light from the spontaneous emission (SSE) of a laser as input illumination. Such a unit may, e.g., replace the combination of light-emitting diode (LED) and conventional gas cell used for wavelength calibration of an optical spectrum analyzer (OSA), since the SSE could be obtained from a tunable laser that is oftentimes used together with an OSA. In an OSA using heterodyne technology, the SSE could also be obtained from a built-in laser source.        
 
         [0030]     In  FIG. 1 , a fiber  10  filled with a gas having known absorption wavelengths is coupled to a wavelength source (or pump)  20  providing a stimulus signal for the gas-filled fiber  10 , which comprises a first fiber  12  and a second fiber  13  each filled with a respective reference gas and first fiber  12  and second fiber  13  are spliced/coupled together. A reservoir  50  is coupled to source (or pump)  20 . A wavelength response signal of the gas-filled fiber  10  through a lens  11  in response to the applied stimulus is detected by a detector  30  and analyzed by an analyzing unit  40 . The analyzing unit  40  compares the detected wavelength response signal with the expected absorption wavelengths known for the gas in the fiber  10 . Differences between actually measured absorption wavelengths and the expected absorption wavelength then allow calibrating the provided wavelength analysis of the analyzing unit  40 .  
         [0031]      FIG. 2  illustrates in principle, in cross-sectional view, an applicable embodiment of the fiber  10 , also known as a glass cell  101 , as known from: J. Broeng et al., Danish Opt. Soc. News, p. 22, June 22. The regular pattern of circles  100  denotes holes filled with gas, with a first density  102  and a second density  103 . The large cross-sectional area  10  in the center of the figure, having exemplary hexagonal symmetry, represents the hollow core of the fiber  10  and is also filled with gas. The almost circular gray-scale image denotes the field distribution of the fundamental guided mode of the fiber that occurs mainly in the gas-filled region.  
         [0032]     Referring again to  FIG. 1 , as an alternative to the arrangement of fiber  10  being configured to include first fiber  12  and second fiber  13 , fiber  10  can be a single photonic crystal fiber having a hollow core (see  FIG. 2 , reference  110 ) running the length of fiber  10 , and having a reference gas therein. This alternative embodiment of fiber  10  also includes, at each end of fiber  10 , a lens. Such lenses are preferably flat glass, microlenses. The reference gas is pumped into the hollow core and the lenses are then attached, for example by gluing or arc welding. As a further alternative, small pieces of frozen gas crystals or small amounts of liquid gas can be inserted in the evacuated fiber that is then sealed. The fiber fills with gas as the crystals or the liquid evaporate.