Abstract:
A dual-band optical filter is disclosed which has two transmission bands. The first band is relatively narrow (about 1 nm) and encompasses a wavelength of about 1550 nm, while the second band is relatively broad (greater than 50 nm) to transmit wavelengths in a range of 1300-1400 nm. Accordingly, the filter can be used in wavelength division multiplexed communication systems to simultaneously select a payload channel having a wavelength of approximately 1550 nm, and a 1310 nm service channel.

Description:
The present application claims the benefit of U.S. Provision Patent Application Serial No. 60/130,212, filed Apr. 20, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to optical interference filters and more particularly to a dual band optical interference filter capable of transmitting optical channels within a first and second passbands. 
     BACKGROUND OF THE INVENTION 
     Optical interference filters rely on principles of interference that modify the intensities of the reflected light incident on a surface. A familiar example of interference is the colors created when light reflects from a thin layer of oil floating on water. Briefly stated, by modifying the interface of a substance and its environment with a third material, reflectivity of the substance can be significantly altered. This principle is used in the fabrication of optical interference filters. These filters can be used as one of, or as the main filtering element in optical add/drop multiplexers employed in optical communication systems to select one or more channels from a transmission signal. 
     In its most simple form, an optical interference filter includes a cavity which is comprised of two partial reflectors (or mirrors) separated by a spacer. The number of spacers determines the number of cavities of the filter. Each partial reflector, also referred to as a quarter-wave stack, is typically constructed by depositing alternating layers of high and low refractive index dielectric materials upon a substrate where each layer has an optical thickness (defined as: physical thickness x refractive index) of a quarter wave (λ/4) (or odd multiple of a quarter wave) at the desired wavelength λ 0  of the filter. Exemplary high and low refractive index dielectric materials are TiO 2 , Ta 2 O 5  and SiO 2 , respectively. The spacer is typically a half-wave (or multiple half-wave) layer of low refractive index material (e.g., SiO 2 ). An interference filter has an associated transmission characteristic which is a function of the reflectance of the layers of high and low index materials associated with the stack. 
     In many applications, optical interference filters are constructed using multiple cavities. Typically, cavities are deposited on top of other cavities, with a quarter-wave layer of low index material therebetween. Multicavity filters produce transmission a spectra that are preferred in optical communication systems where steep slopes and square passbands are needed to select one or more optical channels. The larger the number of cavities employed, the steeper the slope of the transmission bandwidth associated with a particular filter. The transmission bandwidth of a multicavity filter is wider as compared with the transmission bandwidth associated with a single cavity filter. 
     FIG. 1 illustrates an exemplary transmission spectrum for a mirror comprising a plurality of high/low refractive index dielectric layers. The mirror exhibits high reflectivity at a stopband centered at λ 0  and rippled sidelobes including points A, B and C. 
     FIG. 2 is an exemplary transmission spectrum for a single cavity optical interference filter utilizing a pair of stacks each having the transmission spectrum shown in FIG.  1 . As can be seen in FIG. 2 the transmission response is acceptable at wavelength λ 0  (approximately 1550 nm). However, the response at wavelength λ 1  (approximately 1310 nm) falls on the sidelobe and/or within the ripple band of the transmission spectrum, thereby making transmission of a particular wavelength in this range unreliable. More specifically, the single cavity interference filter produces high transmittance at wavelengths referenced at points A and B, but also produces relatively low transmittance as referenced at point C. Thus, transmission at a first wavelength λ 0  may be reliable while transmission for wavelength λ 1  within the ripple band or sidelobe slope are subject to variations in the transmission characteristic. This is also true for wavelengths in the 1625 nm range. FIG. 2 demonstrates that interference filters typically provide a single reliable transmission band. 
     As noted above, optical systems can utilize one or more interference filters to select particular channels from a transmission signal. For example, a first filter may be used to select a pay-load channel associated with voice and/or data transmission in the 1.5 μm range and a second filter is used to select a service channel in the 1.3 μm or 1.6 μm range which carries system level and/or network monitoring information. The use of two separate filters, however, has several disadvantages. First, it increases overall system cost since it requires the manufacture and installation of two individual components. Secondly, optical networks typically have a predetermined loss budget, if exceeded, can compromise signal integrity. Each component, in this case an optical filter, contributes some loss to the overall network. By using two separate filters to select a payload channel and a service channel, each filter negatively impacts a network&#39;s loss budget. 
     Thus, there is a need for a filtering element used with optical communication systems capable of selecting a first and a second optical passbands. There is a further need to provide such a filtering element which reliably selects at least one wavelength corresponding to a payload channel as well as a wavelength corresponding to a service channel within an optical network. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Advantages of the present invention will be apparent from the following detailed description of the presently preferred embodiments thereof, which description should be considered in conjunction with the accompanying drawings in which: 
     FIG. 1 illustrate a transmission spectrum of a conventional mirror including a plurality of dielectric layers; 
     FIG. 2 illustrates a transmission spectrum of a single cavity filter including conventional mirrors; 
     FIG.  3 ( a ) illustrates a single-cavity interference filter consistent with the present invention; 
     FIG.  3 ( b ) illustrates a dual-cavity interference filter consistent with the present invention; 
     FIG.  3 ( c ) illustrates a three cavity interference filter consistent with the present invention; 
     FIG. 4 illustrate transmission characteristic of an exemplary triple cavity interference filter having a narrow transmission band at a wavelength around 1550 nm and a broad transmission band at a wavelength around 1310 nm; 
     FIG. 5 illustrates schematically a mirror having q dielectric layers of alternating high and low refractive indices; 
     FIG. 6 illustrates a transmission characteristic associated with the mirror shown in FIG. 5; 
     FIG. 7 illustrates the refractive index of each of the layers of an exemplary mirror utilizing the structure described in FIG. 5; 
     FIG. 8 illustrates a transmission characteristic of a three cavity filter consistent with the pent invention; 
     FIG. 9 a composite dielectric layer consistent with an aspect of the present invention; 
     FIGS.  10 ( a ) and  10 ( b ) illustrates a transmission characteristic and structure, respectively, of a two-material mirror consistent with an aspect of the present invention; and 
     FIG. 11 illustrates a transmission characteristic of a three-cavity filter using the mirror shown in FIG.  10 ( b ). 
    
    
     DETAILED DESCRIPTION 
     The interference filter in accordance with the present invention transmits a narrow (≈1 nm wide) wavelength band around a first center wavelength λ 0  (e.g., λ 0 ≈1550 nm) along with a broader (≈20 nm wide) wavelength band around a second center wavelength λ 1 , (e.g., λ 1 ≈1310 nm). This can best be explained with reference to a multilayer dielectric mirror with sufficiently high and broad transmission band around wavelength λ 1 . The high transmission peaks (referenced at points A and B in FIG. 1) are positioned within the passband of the multilayer dielectric mirror corresponding to wavelength λ 1  which is the center wavelength of the second broad band. If the first high transmission peak is chosen to coincide with λ 1  (i.e., peak referenced at A) the following equations can be used to determine the parameters of the basic multilayer dielectric mirror:                    n   H          (     λ   1     )           n   L          (     λ   1     )         =       1   +   r       1   -   r               (   1   )               r   =       1   -       (       sin        (       π   2            λ   0       λ   1         )         sin        (       π   2            (     q   -   1     )       (     q   +   1     )         )         )     2                 (   2   )                                
     where n H (λ 1 ) and n L  (λ 1 ) are high and low refractive indices at λ 1 ; r is the absolute value of the magnitude Fresnel reflection coefficient for the boundary between the high and low index layers; and q is the number of layers in the stack or mirror. Equations (1) and (2) can be satisfied, for example, with n H (λ 1 )≈2.05 (Ta 5 O 5 ), n L (λ 1 )≈1.44 (SiO 2 ), λ 0 =1550 nm, λ 1 =1310 nm, and q=13. In this manner, for a wavelength λ 0  (≈1550 nm), broad band transmission is achieved for a wavelength centered at λ 1  (≈1310 nm) as well as narrow band transmission at wavelength λ 0 . 
     Turning to the drawings in which like reference characters indicate the same or similar elements in each of the several views, FIG.  3 ( a ) schematically describes a single-cavity interference filter in accordance with the present invention comprising a spacer  30  interposed between a first and second mirrors  25  and  26 . FIG.  3 ( b ) illustrates a dual cavity interference filter  40  having a coupling layer  70  interposed between a first cavity  45  and a second cavity  75 . Coupling layer  70  can be, for example, a low index material having a quarter wave optical thickness. First cavity  45  includes mirrors  50  and  60  separated by spacer  55 . Second cavity  75  includes mirrors  80  and  90  separated by spacer  85 . Similarly, FIG.  3 ( c ) illustrates a triple cavity interference filter  100  having a first cavity  105 , a second cavity  110  and a third cavity  115 . First coupling layer  106  is positioned between first cavity  105  and second cavity  110 . Second coupling layer  117  is positioned between second cavity  110  and third cavity  115 . First cavity  105  comprises mirrors  102  and  103  separated by spacer  104 . Second cavity  110  includes mirrors  111  and  113  separated by spacer  112 . Third cavity  115  includes mirrors  118  and  120  separated by spacer  119 . 
     FIG. 4 illustrates a transmission characteristic of an exemplary triple cavity interference filter having a narrow transmission band at wavelength λ 0  (around 1550 nm) and a broad transmission band at wavelength λ 1  (around 1310 nm). 
     The interference filter described above has the properties which allow it to transmit both a narrow band centered at wavelength λ 0  (≈1550 nm) and a broad band at wavelength λ 1  (≈1310 nm). However, this embodiment transmits the bands for a particular wavelength λ 0 . If a different wavelength, for example λ 0 +Δ, is selected within the ITU channel grid and the gain band of an erbium doped fiber amplifier (1530-1570 nm), the broad transmission band centered around λ 1  would shift from 1310 nm proportionally with Δ, the difference between λ 0  and 1550 nm. The broad transmission band centered around λ 1  (for example, 1310 nm) can be maintained for different values of λ 0 , if the high transmission region in the pass band of the multilayer dielectric mirror is greater than 20 nm, which is sufficient to provide the dual band characteristics for a fixed pair of λ 0  and λ 1 . 
     The high transmission band around wavelength λ 1  associated with the mirror shown in FIG. 1 can be broadened by collapsing the adjacent transmission peaks A and B and eliminating low transmission point C. This is achieved by depositing a dielectric material having a refractive index n H1 ≈1.55-1.58 for the third layer as well as for the q- 2  layer in the mirror. For example, FIG. 5 illustrates a mirror having q dielectric layers of alternating high (H) and low (L) refractive indices. The third layer and the q- 2  layer have an associated refractive index of ≈1.55-1.58. FIG. 6 illustrates a transmission characteristic associated with this mirror structure resulting in a broader transmission band around wavelength λ 1  (≈1310 nm). 
     FIG. 7 graphically illustrates the refractive index of each of  17  layers (q=17) of an exemplary mirror utilizing the structure described with reference to FIGS. 5 and 6. Ta 2 O 5  (n H ≈2.05) was selected as the high refractive index material for layers  1 ,  5 ,  7 ,  9 ,  11 ,  13  and  17 . SiO 2  (n H 1.44) was selected as the low refractive index material for layers  2 ,  4 ,  6 ,  8 ,  10 ,  12 ,  14  and  16 . Layers  3  and  15  have refractive index n H =1.58. Layers  3  and  15  may be deposited either by a properly ratioed co-deposition of high and low index materials, or by depositing materials having a refractive index of 1.58 (e.g., mullite, which is a mixture of 76-80% of Al 2 O 3  and 20-24% of SiO 2 ). 
     A three cavity interference filter having the structure described with reference to FIG.  3 ( c ) where each mirror ( 102 ,  103 ,  111 ,  113 ,  118  and  120 ) is formed using the structure described in FIGS. 5-7. A transmission characteristic associated with this three cavity filter is shown in FIG.  8 . As can be seen, the 100 nm broad transmission band around wavelength λ 1  (≈1310 nm) allows a choice of wavelengths λ 0  within the gain band of a typical erbium doped fiber amplifier (1520 nm-1570 nm) without sacrificing transmission around wavelength λ 1 . 
     The interference filter described above calls for the deposition of a third material having an intermediate refractive index value in the range of 1.55-1.58 with respect to the high and low refractive index materials forming each mirror. However, the introduction of this third material into the deposition process is less desirable from a manufacturing perspective. Accordingly, the third material having an intermediate refractive index used to form layers  3  and q- 2  (e.g. layers  3  and  15  referenced in FIG. 7) of an exemplary mirror can be formed by a symmetrical composite consisting of a layer of high index material (e.g., Ta 2 O 5 , n H≈ 2.05) clad by a pair of low index material layers (e.g., SiO 2 , n L ≈1.44) as schematically shown in FIG.  9 . This composite material has an optical thickness of one quarter wave at λ 0 , and its effective refractive index is 1.58. If δ H  represents the optical thickness (in quarter waves at λ 0 ) of the thin high index layer comprising the composite, the optical thicknesses δ L  of the low index material can be calculated as follows:                δ   L     =       1   π        a                   tan   (       2          n   H     ·     n   L             (       n   H   2     +     n   L   2       )          tan        (       π   ·     δ   H       2     )           )               (   3   )                                
     The resulting structure has an optical thickness of one quarter wave at λ 0  and allows the use of turning point monitoring for quarter waves during layer deposition. 
     FIGS.  10 ( a ) and  10 ( b ) show a transmission characteristic and the structure, respectively, of the two-material mirror with essentially identical characteristics to a three material mirror design (FIGS.  6 - 7 ). In this example, δ H =0.14 and δ L =0.4257. It should be noted, that the cladding layers of FIG. 9 with optical thicknesses of 0.4257 (λ/4) joining the one λ/4 thick low index material layers surrounding the original layer with intermediate refractive index n H1 =1.58, forms low index layers of 1.4257 λ/4 optical thickness at λ 0 . 
     FIG. 11 illustrates a transmission characteristic of a three cavity filter using the two-material mirror structure described above. As can be seen, the broad transmission band around wavelength , (≈1310 nm) allows a choice of wavelengths λ 0  within the gain band of a typical erbium doped fiber amplifier (1520 nm-1570 nm) without sacrificing transmission around wavelength λ 1 . 
     Consistent with a further aspect of the present invention, the third layer shown in FIG. 10 b  can be eliminated and the optical thickness of both the second and fourth layers can be increased to have an optical thickness of 1.5 times a quarter wavelength to form a single continuous layer having an optical thickness of ¾ a quarter wavelength. Likewise, the q- 2  layer can be eliminated and the optical thickness of the q- 1  and q- 3  layers can be increased to have an optical thickness of 1.5 times a quarter wavelength to form a single continuous layer also having an optical thickness of ¾ a quarter wavelength. 
     While the foregoing invention has been described in terms of the embodiments discussed above, numerous variations are possible. Accordingly, modifications and changes such as those suggested above, but not limited thereto, are considered to be within the scope of the following claims.