Abstract:
The specification describes an optical filter wherein long period gratings (LPGs) are used in a new configuration to provide in-line bandpass filtering of optical signals. In the basic embodiment, two dissimilar LPGs are installed in the optical fiber transmission line. The first LPG converts light, over a broad wavelength range, being transmitted in the fundamental, or LP 01 , mode of the optical fiber transmission line into light in a higher order mode (HOM) of the optical fiber. The mode-converted signal, with mode LP m,n,  is then coupled to a second waveguide, the second waveguide having transmission characteristics different from those of the first. The second waveguide supports propagation of light in a LP m,n  mode. The mode-converted signal is then transmitted through a second LPG where the signal over a selected narrow band of wavelengths that is accepted by the second LPG is converted back to a LP 01  mode. The selected narrow band in the LP 01  mode propagates efficiently over the remainder of the optical fiber transmission path.

Description:
RELATED APPLICATIONS  
       [0001]     This application claims the benefit of provisional application 60/301,164 filed Jun. 27, 2001, which is incorporated herein by reference. This application is a division of Ser. No. 10/167,684 filed Jun. 11, 2002. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to optical filters, and more particularly to optical bandpass filters with low loss.  
       BACKGROUND OF THE INVENTION  
       [0003]     Optical bandpass filers transmit light over a pre-determined band of wavelengths while rejecting, by absorption, radiation or scattering, all other wavelengths. Such filters are useful in laser cavities or optical communications systems. For example, they may be used to constrain the wavelength of operation of a laser, when deployed inside or outside the laser cavity. In optical communications systems, they can be used at the input of an optical receiver to separate unwanted light such as spontaneous emission noise outside the wavelength band of the signal. See D. M. Shamoon, J. M. H. Elmirghani, R. A. Cryan, “Characterisation of optically preamplified receivers with fibre Bragg grating optical fibers”, IEEE Colloquium on Optical Fiber Gratings, March 1996. Optical regenerators based on self-phase modulation require extracting a predetermined wavelength band from a broad spectrum of light.  
         [0004]     Several devices have been proposed and demonstrated to offer the functionality of bandpass filtering. Fiber Bragg gratings may be used in the reflection mode with a circulator, or in the transmission mode, to select a narrow wavelength band. See Xu, M. G.; Alavie, A. T.; Maaskant, R.; Ohn, M. M.; “Tunable fiber bandpass filter based on a linearly chirped fiber Bragg grating for wavelength demultiplexing”,  Electron Left.,  32, pp.1918-1919 (1996). Operation in the reflection mode requires addition of a circulator in the transmission line—this increases cost, loss and complexity for the device. A further drawback with Bragg gratings, used either in transmission or reflection, is that such filters can be highly dispersive, which gives rise to pulse shape distortions. See Lenz, G.; Eggleton, B. J.; Giles, C. R.; Madsen, C. K.; Slusher, R. E.; “Dispersive properties of optical filters for WDM systems”, IEEE Journal of Quantum Electronics, 34, pp.1390-1402, (1998). Alternatively, thin film dielectric filters may also be used as bandpass filters (see: U.S. Pat. No. 5,615,289), but in addition to their dispersive nature (similar to Bragg gratings) they suffer from the additional drawbacks of being free-space devices. Thus, light needs to be coupled into fiber pigtails for use in such fiber-optic systems. This increases loss, cost and complexity.  
         [0005]     An alternative technique for making bandpass filters uses two identical long-period fiber gratings (LPGs) that are spliced in series in the fiber-optic transmission line, with a core block between them. (See: U.S. Pat. No. 6,151,427) The first long period grating converts a narrow wavelength-band of core-mode light into a cladding mode, and the second identical grating couples the cladding-mode light back into the core mode. The core block between the two LPGs attenuates or scatters any light that was not converted into the cladding mode. There are drawbacks associated with this device—(1) the core block simultaneously attenuates light in all the modes, thus the device is inherently lossy. Consequently only higher-order cladding modes may be utilized, as lower order cladding modes will exhibit even higher loss; (2) the core block forms a discrete discontinuity in the fiber, which leads to undesired mode-coupling and inter-modal interference; (3) tuning such a filter requires simultaneously tuning both LPGs by identical amounts, as the filter operates properly only when both LPGs have identical spectra. In bandpass filters using acousto-optically generated or microbend-induced LPGs, there is an additional drawback: the device is inherently polarization sensitive.  
         [0006]     Thus, there exists a need for a bandpass filter that is an in-line fiber device, has low loss, is polarization insensitive, tunable, and simple to implement.  
       STATEMENT OF THE INVENTION  
       [0007]     According to the invention, LPGs are used in a new configuration to provide in-line band pass filtering of optical signals in an optical system. In a preferred case the optical system is an optical fiber system. In the basic filter embodiment of the invention, two dissimilar LPGs are installed in serially cascaded optical waveguides of the transmission line. In the first waveguide of the filter, the signal light is initially transmitted in the fundamental, or LP 01  mode. The first LPG converts the signal light, over a broad wavelength range, into a higher order mode (HOM) of the first optical waveguide. The conversion is achieved using a broadband LPG that provide strong mode conversion over a broad spectrum. See. S. Ramachandran, M. F. Yan, L. C. Cowsar, A. C. Carra, P. Wisk, R. G. Huff and D. Peckham, “Large bandwidth, highly efficient mode coupling using long-period gratings in dispersion tailored fibers, “ Optics Letters,  vol. 27, pp. 698-700 (2002); U. S. Pat. No. 6,084,996, both incorporated by reference herein. In the following description the broadband LPG is designated BB-LPG. The BB-LPG operates by exciting either a core guided HOM or a cladding guided HOM of the first optical waveguide. The mode-converted signal, with mode LP m,n , is then coupled to the second optical waveguide, the second waveguide having transmission characteristics different from those of the first optical waveguide. The second waveguide strongly couples to the LP m,n  mode, because the second LPG, which may be a conventional LPG, provides a strong narrow band coupling between LP m,n  and LP 01  modes of the second optical waveguide. The second LPG is referred to here as a narrow band LPG (NB-LPG).  
         [0008]     Light entering the dual LPG filter first encounters the BB-LPG where more than 99% of the signal is converted to a LP m,n  mode over a broad wavelength range. When the converted signal encounters the NB-LPG, the signal over a selected narrow band of wavelengths accepted by the NB-LPG is converted back to the LP 01  mode of the second waveguide. The selected narrow band in the LP 01  mode propagates efficiently over the remainder of the optical waveguide transmission path; i.e., the remainder of the second waveguide.  
         [0009]     The mode distribution profiles of the LP m,n  mode excited in the BB-LPG and the LP m,n  converted in the NB-LPG do not have to match, as long as the order of the modes (m and n) are the same. The BB-LPG and the NB-LPG may comprise separate waveguide sections, or may be formed in same length of waveguide. The waveguides are preferably optical fibers. If the LPGs comprise separate optical fibers spliced together the signal adiabatically couples at the splice since the mode shapes are the same. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0010]      FIG. 1  is a schematic diagram of the bandpass filter of the invention;  
         [0011]      FIG. 2  is a plot of grating period vs. wavelength showing the relationship for two LPGs in different optical fibers;  
         [0012]      FIG. 3  is a plot of intensity vs. wavelength showing the effective mode conversion properties for the BB-LPG and the NG-LPG;  
         [0013]      FIG. 4  is a plot similar to that of  FIG. 3  for the filter of  FIG. 1 , i.e. with the combined LPGs;  
         [0014]      FIG. 5  is a schematic diagram illustrating the use of the optical filter of the invention in a laser;  
         [0015]      FIG. 6  is a schematic diagram illustrating the use of the optical filter of the invention in a Raman laser;  
         [0016]      FIG. 7  is a schematic illustration of the use of the optical filter of the invention in an optical signal regenerator device; and  
         [0017]      FIG. 8  is a schematic representation of a system using the optical filter of the invention in an optical preamplifier for an optical receiver. 
     
    
     DETAILED DESCRIPTION  
       [0018]     Referring to  FIG. 1 , the arrangement shown represents the generic form of the bandpass filter of the invention. The filter comprises a first fiber section  11  with a BB-LPG  12 , a second fiber section  13 , with a NB-LPG  14 , and a splice  15  connecting the two fiber sections together. The splice  15  adiabatically converts the HOM of fiber  11  to the HOM of fiber  13 . The BB-LPG  12  has a grating period selected for broad-band mode conversion, and the NB-LPG  14  has a different grating period chosen for narrow band, i.e., bandpass, mode conversion. The choice of grating periods is illustrated in  FIG. 2 .  
         [0019]     With reference to  FIG. 2 , phase-matching relationships are shown for LPGs coupling the LP 01  mode to the LP 02  mode (for example). Curve  21  represents the phase-matching relationship of a fiber that yields a BB-LPG at about 1470 nm. A unique feature of the curve  21  is the turn-around point (TAP) in the phase matching relationship. At the TAP a vertical line  22  is tangent to the phase-matching curve  21 . If a grating period has the value corresponding to the tangent line  22 , a broad range of wavelengths approximately satisfy the phase-matching relationship. For wavelengths in this range, a BB-LPG results, because the wavelengths are at or close to resonance near the TAP. In this example, a BB-LPG with a period of  115  microns (line  22 ) yields a BB-LPG at about 1470 nm.  
         [0020]     Curve  23  corresponds to the phase-matching relationship of a LPG in a second fiber. From curve  23 , one sees that the LPG yields a NB-LPG for any wavelength greater than about  1300  nm. That is, for wavelengths greater than 1300 nm, a vertical line corresponding to a selected grating period is never tangent to the phase-matching curve  23 . Thus, there is not a broad range of wavelengths that approximately satisfy the phase-matching relationship. For example, line  24 , intersects the phase-matching relationship at only one wavelength.  
         [0021]     In this illustration, curve  23  represents a fiber with the same index profile as that represented by curve  21 , but the fiber associated with curve  23  is drawn to a diameter that is 80% of the original diameter of the fiber associated with curve  21 . Dimensionally scaling a fiber in this manner shifts the TAP of the fiber. Thus, a fiber that yields a BB-LPG when drawn to one diameter, yields a NB-LPG when drawn to a different diameter. Dimensionally scaling is one of several ways to shift or adjust the TAP of the fiber. The same objective may be realized by inducing a constant index change in the fiber, or by etching the outer diameter of the cladding. While the former technique will result in shifting the TAP of both a cladding mode as well as a core-guided mode, the latter will be useful when the HOM employed for the bandpass filter is a cladding mode.  
         [0022]     To illustrate the characteristics of the fiber sections  11  and  13  in the filter of  FIG. 1 , experimentally obtained spectra are shown in  FIG. 3 . The phase-matching curve  21  of  FIG. 2  for fiber  11  (outer diameter, OD=121 μm) has a TAP at 1540 nm. An LPG written at the corresponding grating period (112.5 μm) converts the incoming LP 01  mode into the LP 02  mode over the entire C-band. The length of this grating is 1 cm, with an index perturbation of 5×10 −3 . This illustrates that more than 99% (&gt;20 dB) of light is converted over a spectral range between 1527 nm and 1571 nm.  
         [0023]     More generally, the wavelength range of the BB-LPG may be controlled by suitably designing a fiber with different dispersion properties for the fundamental mode and the HOM.  
         [0024]     The bandwidth, Δλ, of the BB-LPG is given by: 
 
Δλ= A×λ   res   /{square root}{square root over (L×ΔD×)}c  
 
 where ΔD is the difference in dispersion between the two modes that are being coupled by the BB-LPG, L is the length of the grating, λ res  is the resonant wavelength (where maximum coupling occurs), c is the velocity of light in a vacuum, and A is a constant determined by the maximum coupling strength of the grating. Thus, a variety of broadband spectra may be obtained utilizing this concept. The wavelength range in a typical BB-LPG may range from 40 to 100 nm. 
 
         [0025]     Fiber section  13  is a few-mode fiber similar to fiber  11 , but is drawn to an OD of 112 μm. The spectrum of an NB-LPG in this fiber is shown by curve  32  in  FIG. 3 . This spectrum is for a 5.7 cm long uniform LPG with a grating period of 120 μm, and an index perturbation of 1×10 −4 . The 3 dB bandwidth is 7 nm and provides 99.6%. (24 dB) mode-conversion at the resonant wavelength of 1555 nm. The LPG spectra may be tailored in bandwidth by modifying the dispersive properties of the fiber, or the physical parameters of the grating.  
         [0026]     The bandwidth, Δλ, of the NB-LPG is given by: 
 
Δλ= A×λ   res   /L×Δn   g  
 
 where Δn g  is the difference in the group indices between the two modes that are being coupled by the NB-LPG,-and the rest of the terms are as defined earlier. In addition to control over bandwidth, chirping the period of the grating, or apodizing the index perturbations of the gratings, can yield a multitude of spectral-shapes (e.g. rectangular, Gaussian, etc.). 
 
         [0027]     With reference to the elements of  FIG. 1 , the device functions as follows.  
         [0028]     Essentially all light in an entire communications band passing through BB-LPG  12  is converted to the LP 02  mode. As the LP 02  mode light traverses splice  15 , that splices fiber section  11  to fiber section  13 , the LP 02  mode light of fiber  11  is adiabatically converted into the LP 02  mode of fiber  13 . Then, the NB-LPG  14  selects a desired narrow portion of the spectrum and converts that back into the LP 01  mode. This results in a bandpass filter, with properties shown by curve  35  in  FIG. 4 .  
         [0029]     The device illustrated in  FIG. 1  provides a general platform for building band-selection as opposed to band-rejection filters with LPGs. The first mode-converter, BB-LPG  12 , only serves as a device that provides a spatially modified input for NB-LPG  14 , and does not define the spectral characteristics of the bandpass filter. The spectrum of the filter is defined by the inverted spectrum of the NB-LPG  14 . Further, instead of only one NB-LPG  14 , several NB-LPG may be added in series. Alternatively, the NB-LPG may be designed to possess multiple narrowband resonances. This enables the prospect of spectral shaping in the band-pass configuration by varying the spectral properties of one or more NB-LPG  14 .  
         [0030]     The device just described has an advantage over conventional filters using LPGs in that no core-blocking element is used. This avoids the loss inherent in devices with that configuration. For the purpose of defining this distinction, the transmission path between the BB-LPG and the NB-LPG functions adiabatically. Furthermore, the transmission path coupling the LPGs does not include an active attenuation element, i.e. an element that has an intended and deliberate function of attenuating light. It is not intended that a conventional splice, which is designed for minimum light attenuation as is not, in this context, an active attenuation element.  
         [0031]     The splice element  15  in  FIG. 1  adiabatically transforms the HOM of fiber  11  (after traversing the NB-LPG) into a HOM of fiber  13 . This may be achieved by inducing a heat profile along the splice and/or tapering one fiber with respect to the other to ensure that the two HOMs couple efficiently. In the embodiment shown in  FIG. 1 , the two fibers  11  and  13  are shown with a physical splice joining them, i.e. fibers  11  and  13  are separate fibers. The physical splice  15 , and the attendant loss in that splice, may be avoided by making a single fiber with two distinct transmission characteristics. For example, the diameter of the fiber may be made to vary longitudinally along the fiber length, i.e. a first section with a first diameter, and a second section with a second diameter. Alternatively, the cladding may be selectively modified from one portion of a single fiber to another. Therefore, the functional operation of splice  15  is the important aspect, and may be defined as a means for effecting a change from a HOM with one characteristic to a corresponding HOM with a different characteristic.  
         [0032]     Since uniform LPGs are not dispersive filters, the bandpass filters described here are not dispersive.  
         [0033]     The filter of the invention may also be tuned by inducing a shift in the phase-matching curve for the NB-LPG (curve  23  of  FIG. 2 ). Alternatively, doping the cladding of the fiber, or coating the outside cladding of the fiber with an electro-optic or non-linear optic material allows electrical or optical control of the resonant wavelength of the bandpass filter.  
         [0034]     The bandpass filter of the invention may be used in a variety of systems. For example, a variety of laser devices can be tuned using the bandpass filter of the invention as an intracavity element. In a conventional laser cavity, defined by two narrowband reflectors, one high reflector and one weak reflector (output mirror), the two narrow band reflectors are replaced by two broadband reflectors (one high and one weak) with the bandpass filter of the invention in the cavity. The lasing wavelength may be adjusted by tuning the filter. Multiple lasing wavelengths can be produced by having one or more NB-LPGS in the bandpass filter that have multiple resonances.  
         [0035]     An illustration of this generalized form of laser is shown in  FIG. 5 , where the gain fiber is shown at  38 , high reflector at  39 , weak reflector at  41 , input signal at  43 , laser pump at  44 , and WDM for combining signal and pump at  45 . The bandpass filter of the invention, shown generally ar  46 , and comprising BB-LPG  47 , NB-LPG  48 , and splice  49 , is placed inside the laser cavity as shown.  
         [0036]     A specific form of laser, a cascaded Raman fiber laser, is especially adapted for use with the filter of the invention. These devices can be made to operate at multiple wavelengths by employing multiple sets of Bragg gratings that comprise narrowband high-reflectors and output couplers/weak reflectors. These pairs of Bragg gratings define laser cavities, and the resonant wavelength of the narrowband grating defines the lasing wavelength.  
         [0037]     An embodiment of this device is illustrated in  FIG. 6 . The Raman resonator  51  comprises fiber  52  and gratings  53 ,  54 . In the conventional device, the resonator is bounded on both sides by a narrow band grating. In this embodiment, the resonator is bounded by broadband high-reflector  48 , and weak reflector  49  gratings. Now, the resonant wavelength is controlled by a bandpass filter  56  of the invention. The bandpass filter  56  comprises, as described earlier, BB-LPG  57 , NB-LPG  58 , and splice  59 . Tuning is achieved by tuning the NB-LPG of the bandpass filter. Since LPGs (and therefore the LPG bandpass filters) are widely tunable by methods described earlier, a swept wavelength source, useful in Raman amplified optical communications systems, would be easily realizable. For more details on these systems, and for swept wavelength implementations in these systems, reference is made to co-pending application Ser. No. 10/098,200 filed Mar. 15, 2002, entitled “Wideband Raman Amplifiers”, which is incorporated herein by reference. More generally, multiple NB-LPG  58  may be added in series to provide simultaneous tunable operation of the device over several lasing wavelengths.  
         [0038]     The bandpass filters of the invention are also well adapted for use with self-phase modulation (SPM) based optical regenerators. These devices are relatively simple and typically comprise a non-linear fiber associated with a bandpass filter (see U.S. Pat. No. 6,141,129). The signal is launched into the non-linear fiber section, thereby inducing SPM in the signal. The effect is to broaden the spectrum of the portions of the signal that exceed a selected intensity value. The bandpass filter then selects a predetermined portion of the broadened spectrum. Ideally the bandpass filter for this application should be tunable to account for temporal changes in the spectrum. It is also desirable that the filter be dispersion-less. These desiderata are satisfied by the bandpass filter of the invention.  
         [0039]     A schematic representation of an optical regenerator using the bandpass filter of the invention is shown in  FIG. 7 . The non-linear fiber section is shown at  62 , and the bandpass filter, comprising BB-LPG  67 , NB-LPG  68 , and splice  69 , is shown generally at  66 .  
         [0040]     Pre-amplifiers for optical receivers employ noise filters for removing spontaneous emission noise from the received signal. This noise originates from amplifiers along the transmission path. The noise is typically over the entire spectral range including the signal wavelength, and substantial sidebands of the signal. This background noise degrades the signal-to-noise ratio of the signal to the receiver. A bandpass filter could selectively attenuate noise outside the signal wavelength, and therefore improve the optical signal-to-noise ratio, but would be especially effective if the bandpass filter were dispersion-less. That characteristics is provided by the bandpass filter of the invention.  
         [0041]     An optical pre-amplifier system as just described is shown in  FIG. 8 , where the optical amplifier  72  is followed by a bandpass filter, shown generally at  76 , comprising BB-LPG  77 , NB-LPG  78 , and splice  79 . The filtered signal is then introduced into a receiver,-represented in the figure by light sensitive diode  81 .  
         [0042]     The invention is described above using optical fibers for implementing the bandpass filter of the invention. Similar devices may be constructed using other forms of waveguides, for example planar waveguides in optical integrated circuit (OIC) devices.  
         [0043]     The long period gratings described here may be formed by various techniques. A common approach is to write the gratings into a Ge doped fiber using UV light. However, other methods may also be used. For example, microbend induced LPGs are suitable. These can be realized with acousto-optic gratings, arc-splicer induced periodic microbends, or by pressing the fiber between corrugated blocks that have the required grating periodicity.  
         [0044]     Various additional modifications of this invention will occur to those skilled in the art. All deviations from the specific teachings of this specification that basically rely on the principles and their equivalents through which the art has been advanced are properly considered within the scope of the invention as described and claimed.