Abstract:
An optical monitoring arrangement utilizes the properties of a blazed Bragg grating to redirect a portion of an optical signal out of the axial path and into a detecting device. A plurality of blazed Bragg gratings are utilized, each having unique properties, to increase the robustness of the monitor. In particular, by utilizing a plurality of N gratings, the bandwidth of the monitor may be increased N-fold (assuming no overlap in wavelength between gratings). Alternatively, an improvement in resolution can be obtained by utilizing a narrower bandwidth and measuring N times the number of raw data points within that bandwidth. A combination of increase in bandwidth and resolution may be obtained by a comprise between these two extremes. Chirped blazed gratings may also be employed.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to an optical monitor utilizing waveguide gratings to redirect certain wavelengths to a monitoring device and, more particularly, to using multiple blazed gratings to improve the resolution and performance of the monitoring device.  
         BACKGROUND OF THE INVENTION  
         [0002]    In multi-wavelength optical communication systems (usually referred to as “wavelength division multiplexed” or WDM systems), efficient wavelength-selective means for tapping electromagnetic radiation (to be referred to herein as “light”, regardless of wavelength) from an optical fiber could be advantageously used in a variety of functions, e.g., as a wavelength monitor, channel monitor, demultiplexer, amplifier monitor, or in a feedback loop with an optical amplifier.  
           [0003]    U.S. Pat. No. 5,061,032 issued to G. Meltz et al. on October, 1991, discloses an optical fiber tap that comprises a blazed, chirped refractive index grating selected to redirect light guided in the fiber such that it comes to a focus at a point outside of the fiber. As used throughout this discussion, the term “blazed” refers to a grating where the plane of the index perturbations (i.e., gratings) is not perpendicular to the propagation direction of the guided mode or modes within the fiber. A grating is “chirped” if the (optical) repeat distance Λ between the index perturbations is not constant as a function of the axial coordinate z of the fiber, i.e., if Λ=Λ(z). An exemplary dispersive optical waveguide tap including a blazed and chirped refractive index grating is disclosed in U.S. Pat. No. 5,832,156 issued to T. A. Strasser et al. on Nov. 3, 1998. In particular, Strasser et al. utilize a coupling means disposed contiguous with the optical fiber in the region of the grating to direct the tapped modes into an associated detector apparatus, such as an array of photodiodes. The blazed grating in the fiber also functions to angularly (θ) disperse the light such that different wavelengths can be imaged by the coupling means onto different detector elements within the array. Consequently, the electrical signals of the detector array map out the spectrum of light associated with the original signal propagating through the fiber.  
           [0004]    The optical performance and cost of various prior art optical monitors (as defined by parameters such as bandwidth, resolution and accuracy) are primarily determined by the grating/detector combination utilized. In many newer applications, such as measuring optical signal-to-noise ratios (OSNRs) of tightly-spaced dense WDM (“DWDM”) channels (e.g., 50 Ghz and closer spacings), extended dynamic range measurements, simultaneous monitoring of ultrawide (e.g., 60-80 nm) or multiple bands (e.g., C and L), as well as simultaneous monitoring of multiple network elements (e.g., amplifier and add/drop nodes), the combined cost/performance of the prior art devices does not meet the desired targets.  
           [0005]    Thus, a need remains in the art for an optical waveguide tap that remains cost-effective while able to provide increased bandwidth and/or resolution for new and emerging optical applications.  
         SUMMARY OF THE INVENTION  
         [0006]    The need remaining in the prior art is addressed by the present invention, which relates to an optical monitor utilizing waveguide gratings to redirect certain wavelengths to a monitoring device and, more particularly, to using multiple blazed gratings to improve the resolution and performance of the monitoring device.  
           [0007]    The present invention is embodied in an article comprising two distinct blazed Bragg gratings disposed side-by-side. For light incident from either output of an associated 1×2 optical switch, one or the other blazed Bragg grating serves to tap light out of the fiber and, ultimately, into an associated detector (photodiode array). In the monitoring arrangement of this particular embodiment of the present invention, only one blazed Bragg grating is “active” at a time. That is, light entering an input port of a 1×2 optical switch is directed exclusively to one of the two output ports (and thus into one of the pair of blazed Bragg gratings). In general, each grating may have a separate optical source associated therewith or, in yet another embodiment, a plurality of M optical signals may used, in a predetermined combination, with a plurality of N gratings.  
           [0008]    In accordance with the teachings of the present invention, the parameters of the two blazed Bragg gratings can be chosen such that, for example, they tap out light in different wavelength ranges, thus increasing the overall bandwidth of the monitor. Alternatively, an improvement in resolution can be obtained by utilizing a narrower bandwidth and measuring twice the number of raw data points within that bandwidth. A compromised embodiment of the present invention may improve both bandwidth and resolution by applying both techniques (i.e., by increasing, but not doubling, the bandwidth, an increased number of raw data points can still be collected).  
           [0009]    In its most general form, the monitor of the present invention may include N such blazed Bragg gratings, used with an associated 1×N switch (or a plurality of N off/on switches). In this case, an N-fold improvement in bandwidth/resolution can be obtained. When used in association with various other optical input elements (such as, for example, couplers or other pass through elements), any number of different optical signals may be used. In fact, the multiple grating monitor of the present invention may be used to study a variety of different optical signals associated with a given optical system.  
           [0010]    Other and further embodiments of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    Referring now to the drawings, where like numerals represent like parts in several views:  
         [0012]    [0012]FIG. 1 is a top view of a prior art blazed Bragg grating optical tap arrangement;  
         [0013]    [0013]FIG. 2 is a side view of the prior art arrangement of FIG. 1;  
         [0014]    [0014]FIG. 3 contains a side view of an exemplary multiple blazed Bragg grating optical tap arrangement formed in accordance with the present invention;  
         [0015]    [0015]FIG. 4 is a top view of the embodiment of FIG. 3, illustrating in particular the side-by-side disposition of the pair of blazed gratings as used in the present invention;  
         [0016]    [0016]FIG. 5 illustrates an alternative embodiment of the present invention, incorporating reflective elements in the arrangement;  
         [0017]    [0017]FIG. 6 is a side view of an embodiment of the present invention include N blazed gratings and an associated 1×N optical switch; and  
         [0018]    [0018]FIG. 7 is a side view of yet another embodiment of the present invention, where the 1×N optical switch of FIG. 6 has been replaced by a plurality of N on/off switches. 
     
    
     DETAILED DESCRIPTION  
       [0019]    In order to aid in the understanding of the present invention, a conventional, prior art optical tap monitor will be briefly described, where one such arrangement  10  is illustrated in a top view in FIG. 1 and a side view in FIG. 2. An optical waveguide  12  (in this case, an optical fiber—although any optical guiding medium may be used) includes a blazed Bragg grating  14 . To simplify the diagrams of FIGS. 1 and 2 (as well as all following illustrations), fiber  12  is illustrated by a single line. It is be understood that in actuality fiber  12  includes a core region (for light guiding) and a surrounding cladding layer, where blazed Bragg grating  14  is formed in the core in conventional fashion. By way of example, the fiber may comprise a conventional silica-base single mode fiber and the grating may be “written” (i.e., photolithographically etched) into the fiber using a phase mask. The grating period Λ and blaze θ (i.e., tilt with respect to the optical axis) are chosen to tap (hereinafter referred to as “redirect”, where the use of this term is understood as redirecting a portion of the optical signal power, anywhere from 1% to 99%, for example, as desired by the user) optical wavelengths within a desired bandwidth Δλ. Referring back to FIG. 1, an index-matched glass block  16  is disposed adjacent to fiber  12  and includes a lens element  18  for focusing the re-directed light signal onto a detector array  20 , where detector array may comprise an array of InGaAs photodiodes. Consequently, the electrical signals of the detector array map out the spectrum of light associated with the input signal. Conventional control electronics (not shown) can then be used to extract and analyze the detector array data. The specific data may then define the optical spectrum and the number of DWDM channels along with their wavelengths, powers, and signal-to-noise ratios (SNRs). As mentioned above, the prior art arrangement as depicted in FIGS. 1 and 2 is limited in terms of the bandwidth and resolution that may be achieved with the particular combination of blazed Bragg grating  14  and photodetector array  20 .  
         [0020]    The use of multiple blazed gratings, in accordance with the present invention, is considered to overcome these limitations of the prior art. FIG. 3 contains a side view of an exemplary optical monitor  30  formed in accordance with the present invention that utilizes a pair of blazed Bragg gratings  32  and  34  formed in an associated pair of optical fibers (or any other appropriate light guiding material)  33  and  35 , respectively, to improve the overall bandwidth and resolution of optical monitor  30 . A top view of optical monitor  30 , illustrating clearly the side-by-side disposition of blazed Bragg gratings  32  and  34 , is shown in FIG. 4. As with the prior art arrangements, gratings  32 , 34  are bonded to an index-matching glass block  36  including a lensed endface  38 . A photodetector array  40  is positioned so as to capture the optical signal redirected (“tapped”) by gratings  32 , 34 .  
         [0021]    In accordance with an exemplary embodiment of the present invention, first blazed Bragg grating  32  is formed to exhibit a first grating period Λ 1  and blaze angle θ 1  that will function to redirect a predetermined optical spectrum λ A -λ B  out of fiber  33  and toward detector array  40 . In particular, by controlling the angular displacement, the particular subset of photodiodes within array  40  that are illuminated by spectrum λ A -λ B  can be similarly controlled. Second blazed Bragg grating  34  is formed, in accordance with this embodiment of the present invention, to exhibit a second, different grating period Λ 2  and blaze angle θ 2  associated with the redirection of a different spectrum λ X -λ Y . The use of different blaze angles and grating periods function to change the spectral band imaged onto the detector array. Referring to FIGS. 3 and 4, a 1×2 optical switch  42  is shown as coupled to fibers  33  and  35 , where the state of switch  42  is used to control which grating is “active” at a time, since in this embodiment only one blazed Bragg grating is used at any given time. That is, light entering input port  44  of 1×2 optical switch  42  is directed exclusively to one of the two output ports  46  and  48  of switch  42 . Although not shown in the figures, optical switch  42  and photodetector array  40  are controlled by external electronics including a microprocessor for spectral calibration and various related calculations. It is to be understood that the arrangement of the present invention may use “unchirped” or “chirped” blazed gratings, as defined above, or any suitable combination of both “unchirped” and “chirped” blazed gratings.  
         [0022]    In general, the advantage of the arrangement of the present invention derives from the ability to control the parameters of gratings  32  and  34 . For example, the gratings can be designed such that they tap out light in different wavelength ranges, that is, with no spectral overlap between λ A -λ B  (denoted as range R AB ) and λ X -λ Y  (denoted as range R XY ). Consequently, switching 1×2 optical switch  42  between output ports  44  and  46  (equivalently, switching between gratings  32  and  34 ), results in spectra within the wavelength ranges R AB  and R XY , respectively, being imaged onto detector array  40 . Since the bandwidth, denoted B, of monitor  30  is proportional to the focal length ƒ, gratings  32 , 34  and the focusing optics (comprising glass block  36  and lens  38 ) can be chosen to provide improved performance in a number of different ways. For example, optical monitor  30  can be implemented so as to double the associated resolution of a given bandwidth. That is, using a factor of  2  longer focal length optics, each grating  32 , 34  can be used to image half of the desired bandwidth, yielding twice as many raw data points over the same wavelength range. Alternatively, the monitor&#39;s bandwidth may be doubled (for a given resolution) by implementing non-overlapping bands for each blazed Bragg grating. That is, by maintaining the same focal length optics, each grating  32  and  34  can be used to image distinct full bands onto photodetector array  40 , doubling the total wavelength range. It is to be understood that between these two extremes one may incorporate both increased resolution and increased bandwidth.  
         [0023]    Blazed Bragg gratings  32 , 34  may also be chosen such that they increase the total dynamic power range of monitor  30 . As is well known in the art, blazed gratings can be fabricated to tap from less then 1% to greater than 99% of the light out of the fiber. Consequently, two gratings of largely different strengths can be used in a device that is controlled to switch between the two depending on the input power level. In particular, for high input powers, the weaker grating would be used and, similarly, for low power input signals, the stronger grating would be employed, ensuring in each instance that a sufficient power optical signal will impinge monitor  40 . Advantageously, a significantly wider range of input powers (for example, up to 20 dB or more increase) could be measured with a single monitor  30 .  
         [0024]    The geometry of monitor  30  is considered to be exemplary only, there are various other arrangements, well-known in the art, that may utilize multiple blazed Bragg gratings in accordance with the teachings of the present invention. In particular, FIG. 5 illustrates a reflective geometry arrangement where lens  38  of FIGS. 3 and 4 is replaced by a concave mirror  50  that will reflect the tapped light back through glass block  36  and into (re-positioned) detector array  40 .  
         [0025]    As mentioned above, the teachings of the present invention can be extended beyond the use of a pair of blazed Bragg gratings to an arrangement employing a plurality of N blazed Bragg gratings. FIG. 6 contains a side view of an N-element optical monitor  60  including a plurality of N blazed Bragg gratings  62   1 - 62   N , stacked side-by-side, each grating formed within an associated optical waveguide (for example, optical fiber)  64   1 - 64   N . The plurality of gratings  62   1 - 62   N  are disposed adjacent to a glass block  66  and associated lensing element  68  which then function to focus the tapped light onto a photodetector array  70 . In this particular embodiment, a 1×N optical switch  72  is used to control the particular grating that is “active” at a given time. Such an arrangement is particularly well-suited for DWDM applications that require the use of multiple, closely-spaced wavelengths.  
         [0026]    In an alternative embodiment, 1×N switch  72  can be replaced by a plurality of N on/off switches  74   1 - 74   N , as shown in FIG. 7. This arrangement is particularly well-suited for monitoring multiple inputs, such as, for example, a plurality of N separate network elements that can be monitored with a single device.  
       EXAMPLE  
       [0027]    We have experimentally demonstrated the multiple grating optical waveguide monitor as depicted in FIGS. 3 and 4. A pair of unchirped fiber blazed Bragg gratings  32  and  34 , with center wavelengths of 1547 nm and 1575 nm, respectively, were used as the dispersive elements. The blaze angles θ 1  and θ 2  were chosen to be equal at a value of 9°, such that the wavelength 1547 nm from grating  32  and wavelength 1575 nm from grating  34  emanated at the same angle relative to the fiber axes, namely 18°. The gratings were photolithographically written in a conventional single mode silica-based fiber using phase masks. The lengths of the gratings were approximately 10 mm. The strengths of the gratings were such that ˜20% of the single mode light at the center wavelengths were tapped out of the fibers. To achieve the fiber-to-block coupling, the gratings were bonded to a fused silica glass block (n=1.44 at 1550 nm) with an optically transparent, closely index-matching (n=1.56) adhesive. Glass block  36  was nominally 6 cm×3 cm×1 cm in dimension. A high-reflective dielectric (R&gt;99%) concave mirror with a focal length of 100 mm served to focus the outcoupled light from both gratings  32  and  34  onto a 256 element linear InGaAs detector array (it is to be understood that a two-dimensional array can be used as an alternative). Each detector element was 30 μm in width and 250 μm long. The array covered a 35 nm wide range of wavelengths with each detector element corresponding to a 0.14 nm range of light. The inputs of the two gratings were fusion spliced to the outputs of a 1×2 opto-mechanical switch  42 . A 5V signal applied to the electrical leads of the switch was used to toggle the switch back and forth between output ports  46  and  48 .  
         [0028]    Laser radiation from tunable Hewlett Packard and Photonetics external cavity lasers (ECLs) were multiplexed together and used as inputs to the device. FIG. 8 shows the spectral response of a dual grating monitor designed as described in this Example when there is no voltage applied across the optical switch leads. In this configuration, the light entering the switch is directed to output port  46  and, hence, through grating  32 . The corresponding wavelength range incident on detector array  40  is 1556-1591 nm. Similarly, FIG. 9 displays the response when a 5V bias is applied across the leads. Here, light is directed to output port  48  and thereafter through grating  34 , resulting in a measured spectrum from 1529-1564 nm.  
         [0029]    It is to be understood that there are many variations to the embodiments as discussed above that are considered to fall within the spirit and scope of the present invention. For example, the blazed gratings may comprise fiber gratings, channel or planar waveguide gratings or, in general, any suitable type of light guiding gratings. Geometries other than those illustrated in FIGS. 3 and 5 may be utilized and, moreover, used in combination with any suitable type of lensing arrangement capable of focusing the tapped light onto a detecting device, such as, for example, a spherical lens, cylindrical lens or any appropriate combination of such lenses. Indeed, the detecting device itself may take on any suitable arrangement, including a one-dimensional array of photodiodes, a two-dimensional array, each defined as a single “detecting device”, or alternatively, a multiple number of separate, smaller detecting devices, perhaps each device for focusing light from a separate grating in a one-to-one relationship. Alternatively, multiple detectors could be used to study the spectra from different gratings. The multiple grating optical waveguide monitor of the present invention may also be utilized as a bidirectional device, accepting optical input signals from either end of the fiber grating structure. All of these variations are considered to fall within the spirit and scope of the present invention as defined by the claims appended hereto.