Abstract:
A method and apparatus for correcting changes in the mean wavelength of a broadband optical source due to changes in temperature. An optical filter is placed so as to filter the output of source, the filter being selected so that, as a function of temperature, its center wavelength changes as fast or faster than the wavelength centroid of the source.

Description:
BACKGROUND OF THE INVENTION 
     Broadband optical sources have a wide variety of applications. However, the usefulness of any such source diminishes if its output spectrum changes with temperature. An example of an application prejudiced by temperature-induced wavelength drift is that of interferometric fiber optic gyroscopes, in which a change in mean wavelength changes the gyroscope&#39;s scaling factor. Although superfluroescent fiber-optical sources, most notably erbium doped superfluorescent fibers, have generally good wavelength stability, and are thus widely used in such gyroscopes, they are not immune from temperature-induced wavelength drift. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the invention is to compensate for temperature-induced changes in the mean wavelength of broadband optical sources. 
     Another object is to do the foregoing in optical sources useful with interferometric fiber optic gyroscopes. 
     Another object is to do the foregoing in actively doped optical fibers. 
     Another object is to do the foregoing in erbium doped superfluorescent optical fibers. 
     In accordance with these and other objects made apparent hereinafter, the invention concerns a optical system and method employing an optical source and an optical filter. The source&#39;s output has a wavelength centroid which changes with increasing temperature within a temperature range of interest. The filter is selected so that the wavelength specific magnitude of the rate of change with temperature of its center wavelength is greater than or equal to the wavelength specific magnitude of the rate of change with temperature of the source&#39;s wavelength centroid within the temperature range of interest. In so doing, the change in mean wavelength of the source due to temperature increase is offset by the change in mean wavelength of the filter. 
     These and other objects are further understood from the following detailed description of particular embodiments of the invention. It is understood, however, that the invention is capable of extended application beyond the precise details of these embodiments. Changes and modifications can be made to the embodiments that do not affect the spirit of the invention, nor exceed its scope, as expressed in the appended claims. The embodiments are described with particular reference to the accompanying drawings, wherein: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph illustrating power spectral density of an actively doped optical fiber. 
     FIG. 2 is a graph of the spectral response of an optical grating. 
     FIG. 3 is a graph illustrating the temperature dependence of the spectral response of FIG.  2 . 
     FIG. 4 is a graph illustrating the power spectral density of the optical fiber related to FIG.  1 . 
     FIG. 5 is a flow chart illustrating a scheme for selecting an optimal filter, according to the invention. 
     FIG. 6 is a schematic diagram of a system used for proof of principle of the invention. 
     FIG. 7 is a graph of data generated by the system of FIG.  6 . 
    
    
     DETAILED DESCRIPTION 
     With reference to the drawing figures, wherein like numbers indicate like parts throughout the several views, FIG. 1 shows the spectral density of an optical source, here the power density of a particular erbium doped superfluorescent fiber generated by testing the fiber in the laboratory, which is discussed in more detail in the Example, below. The solid curve in FIG. 1 is the fiber&#39;s spectrum at 25° C., the dotted curve is the spectrum at 72° C. From it, one can see that increased temperature increases output power at the short wavelength and long wavelength end of the spectrum (reference numbers  10  and  12 , respectively). The mean wavelength λ μ  of the spectrum is given by: 
     
       
         λ μ ={Σ i [λ i P(λ i )]}/{Σ i [P(λ i )]} 
       
     
     where i is an index for the wavelengths present in the spectrum, and P(λ i ) is the magnitude of the spectral density for the ith wavelength in the source&#39;s spectrum. This is also the formula for the centroid of P(λ) about the λ axis, and thus may be called the wavelength centroid, as well as the mean wavelength. Increases  10 ,  12  in the power density of FIG. 1 are disposed on opposite sides of what one would expect to be the mean wavelength of the density function, excursion  10  occurs at a low power portion of the curve, whereas excursion  12  occurs at the higher power end of the curve, and thus excursion  12  causes significantly more change in the wavelength centroid. In other words, the optical fiber whose response FIG. 1 shows will have an increasing mean wavelength λ μ  with increasing temperature. For the same reason, one can see by comparison of FIG.  1  and the above equation that filtering out a portion of the power density towards its long wavelength end will reduce the wavelength centroid. From this, it follows that any optical filter which has a temperature dependent center frequency within the bandwidth of optical fiber (about 1500 to 1600 nm in FIG.  1 ), and whose wavelength specific center frequency increases at a rate matches or exceeds the temperature-induced increase in the wavelength centroid of the fiber will offset to some useful degree the shift in mean wavelength. 
     FIGS. 2 and 3 illustrate performance of a long period optical grating, a preferred filter for use with the invention. By long period it is meant that the grating period is large with respect to anticipated system wavelengths such that the grating will reflect substantially no light. FIGS. 2 and 3, like FIG. 1, represent laboratory data taken on a specific grating, the details of which will also be discussed further in the Example below. However, the general form of the graphs in FIGS. 2-3 are generally exemplary for long period optical gratings. As seen in FIG. 2, the wavelength response of the grating within the bandwidth of the source&#39;s spectral density is that of a notch filter, with a center frequency  14  at about 1545 nm and a well-defined filter width, illustrated by notch width  16  at full wave half maximum. FIG. 3, a graph of the grating&#39;s center wavelength as a function of temperature, illustrates that the center frequency of the grating has a strong, positive going, temperature dependence, which moreover is substantially linear. Long period gratings are preferred filters for the invention because they have low back reflection. Also because the temperature of available gratings are several times larger than commonly used superfluorescent fiber sources (e.g. the erbium doped fiber of FIG.  1 ), the generally linear relationship between center frequency and temperature, coupled with the general insensitivity of filter widths to temperature, make such gratings relatively easy to design for any given application. 
     FIG. 4 is a graph, also of laboratory data discussed further in the Example, below, of the response of a composite system using the optical fiber whose spectrum is shown in FIG. 1, as filtered by the grating whose response is shown in FIGS. 2-3. Again, the solid line is data taken at 25° C. and the dotted line data taken at 72° C. As expected, the filter cuts a trough  18  out of the fiber&#39;s spectrum, reducing the total output of the fiber. However, the trough  22  at the higher temperature is upshifted in wavelength from the trough at  20 , cutting out a higher wavelength portion of the power density at the higher temperature, and thus offsetting temperature-induced increase in the mean wavelength of the power density. 
     FIG. 5 illustrates a scheme for systematically identifying the best filter for any given application. It is presupposed that one knows a priori the spectral density of the optical source over wavelengths of interest, e.g. by either generating a model, or by laboratory testing of the source. Given these data, one then performs three nested iterations  24 ,  26 ,  28  as follows: First select an initial filter width ( 24 ) and center wavelength ( 26 ), and an initial temperature ( 28 ). Using the selected filter parameters and the existing spectral density of the source, calculate the composite spectral density of the source plus filter ( 30 ) (i.e. such as is illustrated in FIG.  4 ). From this, use any known numerical technique to calculate the wavelength centroid ( 32 ). Iteratively repeat this over the temperature range of interest ( 33 ,  40 ,  28 ) to generate set of wavelength centroids, one each of which corresponds to one of the temperatures. Having done so, select a new center wavelength ( 26 ,  29 ,  38 ) and repeat the temperature iteration again, generating further centroids. Having repeated this for center wavelengths of interest, iteratively repeat the process for filter widths of interest ( 27 ,  36 ,  24 ). 
     At the end of this process, one will have generated a number of wavelength centroids corresponding to all combinations of filter width and center wavelength, as a function of varying temperature. One then uses the centroids to determine the pair of filter width and center wavelength which best stabilizes temperature-dependent centroid drift ( 34 ). To make this determination, one can use any known technique to test for optimal minimization, and example of which is simply to preselect a nominal value, e.g. corresponding to expected centroid value without filtering, subtract the nominal value from each centroid calculated for each temperature, and sum these differences for all centroids generated for each pair of filter width and center wavelength, i.e. summed over the temperature range of interest. The combination of filter width and center wavelength which produces the smallest sum would then be optimum ( 34 ). 
     Variations within the process of FIG. 5 will, of course, occur to those skilled in the art. For example, iterations  24  and  26  may be employed in reverse order. Moreover, one need not iterate both filter width and center wavelength, if, for example other application specific constraints were to mandate a specific choice of either filter width or center wavelength. 
     The foregoing describes optical sources and filters whose respective wavelength centroids and center wavelengths increase with temperature. This, however, is merely exemplary. One can as readily practice the invention with sources and filters whose respective centroids and center wavelengths decrease with temperature. 
     EXAMPLE 
     FIG. 6 shows an experimental apparatus used to demonstrate proof of principle. A laser diode  44  pumps an erbium doped superfluorescent optical fiber  46  with pump light of 980 nm, which fiber  46  converts to light at 1550 nm. Wavelength division multiplexer  48  directs the pump input to fiber  46  while isolating the remainder of the apparatus from the 980 nm light, and directing the retransmitted 1550 nm light to long period grating  52 . Isolator  50  protect against any back reflection in the system, and optical spectrum analyzer  42  generates power from fiber  46 . 
     In the test apparatus, 9.9 m of single-mode fiber was used, doped to 500 ppm of erbium by weight. The end of the fiber  46  was terminated with an angle cleave and several loops of fiber around a mandrel (not shown) to reduce reflection. Pump  44  was a Seastar Optics diode laser operating in single mode, which produced roughly 70 mW of fiber-coupled power. Isolator  50  reduced backreflected 1550 nm light by 55 dB. For measurement of intrinsic temperature dependence of fiber  46 , grating  52  was removed. 
     In characterizing the performance of grating  52 , data down to 30 Db below spectrum peak were used. A long period grating was selected because it provides several decades of attenuation in a 10-20 nm band (full width, half maximum). To design the grating with the correct properties for temperature compensation, the effect of removing a 10 to 20 nm wide slice from fiber  46 &#39;s output spectrum was modeled, with a 0.05 nm/° C. temperature coefficient. In this manner, a center wavelength of between 1540 nm to 1547 nm was found to be ideal. To test this, a long period grating was produced by excimer-laser exposure of an H 2 -loaded fiber through a 275 μm period dielectric amplitude mask. The resultant grating had a center wavelength of 1545 nm, full wave half maximum width of 20 nm, a length of 2 cm, and 12 dB of maximum attenuation. The transmission spectrum of this grating is shown in FIG.  2 . The temperature slope of grating  52  (FIG. 3) was measured at 0.048 nm/C. Because grating  52 &#39;s spectrum is sensitive to fiber bends, grating  52  was mounted to a quartz microscope slide under a small amount of tension, and mechanically coupled to the output of fiber  46  on the 1550 nm side of coupler  48 . 
     To measure intrinsic temperature dependence of fiber  46 , the fiber was placed in an oven and heated to 72° C. The oven was then turned off and the mean wavelength periodically measured as the fiber cooled. Laser pump  44 , coupler  48 , and isolator  50  were held at room temperature during the measurement. FIG. 4 represents these data at two temperatures, 25° C. and 72° C. These data are presented in FIG. 7, in which, open circle points represent wavelength centroid of fiber  46  without filter  52 , and black square points represent wavelength centroid as compensated by filter grating  52 . The data of FIG. 7 shows a linear increase of the centroid with temperature for the uncompensated fiber, but a virtually flat response for the compensated fiber across the temperature range tested, about 25° C. to 72° C. 
     The invention has been described in what is considered to be the most practical and preferred embodiments. It is recognized, however, that obvious modifications to these embodiments may occur to those with skill in this art. Accordingly, the scope of the invention is to be discerned from reference to the appended claims, wherein: