Abstract:
An optical apparatus and associated method(s) that utilize zeroth-order feedback to provide precise positional information about optical components comprising the optical apparatus.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/538,523 filed Jan. 22, 2004. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates generally to the field of optical apparatus and in particular to an optical spectrometer and related method(s), suitable for use in single channel, or multiple-channel wavelength division multiplexed (WDM) communications systems or other optical systems.  
       BACKGROUND OF THE INVENTION  
       [0003]     Optical communication systems oftentimes use wavelength-division multiplexing to increase transmission capacity. More specifically, a plurality of optical signals, each having a different wavelength, are multiplexed together into a WDM signal. The WDM signal is transmitted over a transmission line, and then subsequently demultiplexed so that individual optical signals may be individually received.  
         [0004]     Successful implementation of high-speed WDM system depends upon the development of optical devices at reasonable cost. In particular, WDM systems, and other industries as well, require devices that provide the sorting and/or separation of wavelengths, for routing, measurement, or other purposes. One such device—an optical spectrometer and related method(s)—is the subject of the present invention.  
       SUMMARY OF THE INVENTION  
       [0005]     I have invented an optical spectrometer and associated method(s) that offers a number of advantages over existing optical spectrometers and method(s).  
         [0006]     Viewed from a first aspect, my invention is directed to an optical spectrometer method utilizing zeroth-order feedback to provide precise positional information.  
         [0007]     Viewed from another aspect my invention is directed to an optical spectrometer apparatus that accepts a plurality of input signals and provides a plurality of output signals, and may utilize my inventive zeroth-order feedback.  
         [0008]     Additional objects and advantages of my invention will be set forth in part in the description which follows, and, in part, will be apparent from the description or may be learned by practice of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0009]      FIG. 1  is a schematic representation of an optical arrangement according to the present invention;  
         [0010]      FIG. 2  is a schematic representation of an alternative embodiment of an optical arrangement according to the present invention;  
         [0011]      FIG. 3  is a schematic representation of an optical arrangement including position detection according to the present invention;  
         [0012]      FIG. 4 ( a - d ) shows two alternative patterns (a-b) and the resulting output (c-d) of intensity vs. time respectively;  
         [0013]      FIG. 5  is a schematic representation of an alternative arrangement of an optical arrangement including position detection and a single lens according to the present invention;  
         [0014]      FIG. 6  is a schematic representation of an optical arrangement including a retroreflector;  
         [0015]      FIG. 7  is a block diagram of a control system useful with and according to the present invention; and  
         [0016]      FIG. 8  is a schematic representation of alternative mirror-grating configurations useful with, and according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0017]     With reference now to  FIG. 1 , there is shown in schematic form an optical arrangement  100 , which exhibits our inventive teachings. More specifically, optical arrangement  100  depicts multiwavelength input signals carried on input fiber  110  being collimated by lens  120  and illuminating planar reflective diffraction grating  130  mounted on tip/tilt stage  140  capable of rotating the grating  130  about its X-axis or Y-axis. As can be appreciated, each wavelength of the multiwavelength input signals is diffracted into an angle corresponding to the wavelength.  
         [0018]     While the input signals are shown in  FIG. 1  being carried on input fiber  110 , it is understood that the input could be another type of optical waveguide, besides an optical fiber, or it could be a free-space aperture. Preferentially, the intensity profile of light in the input aperture is single-lobed, and the input numerical aperture is matched to the numerical aperture of the optics used in the optical apparatus.  
         [0019]     Diffracted signals  115  are focused by a second pass through lens  120  and are imaged onto a permanent spectral-plane spatial filter that passes a portion of the imaged spectrum and blocks the rest. As can be readily appreciated, a broad-spectrum multi-channel signal would be imaged into a continuous column of spots on the face of the permanent spectral-plane structure.  
         [0020]     Those portions of diffracted wavelength signal  115  which are imaged onto the opaque region and are absorbed or reflected, while the portions of wavelength signal  115  that are imaged onto the transparent aperture  170  are transmitted to illuminate an optical power detector  160  located immediately behind the aperture.  
         [0021]     In operation, the tip/tilt stage  140  is controlled through electrical connections  145 . Rotating tip/tilt stage  140  around its X-axis will steer the dispersed spectral pattern illuminating the permanent spectral-plane structure to shift vertically, and modify the center wavelength of the signal entering detector  160 . In this exemplary embodiment shown in  FIG. 1 , the width of the aperture in the y-direction varies along the x-direction. Rotating stage  140  around its Y-axis will cause the dispersed spectral pattern illuminating the spectral-plane filter to shift laterally (as drawn, into and out of the page), and modify the wavelength bandwidth of the signal entering detector  160 . As can be appreciated, with this optical arrangement, the signal can be completely blocked from the photodetector by rotating the stage about the Y-axis so that the dispersed spectrum completely misses the aperture so that the photodetector dark current can be measured and subtracted from the signal current.  
         [0022]     With reference now to  FIG. 2 , there is shown an arrangement substantially similar to that shown in  FIG. 1 , in which a plurality of multiwavelength signals enter the system at input points arrayed along the x axis  210 [ 1 ] . . .  210 [N]. Each of these inputs is imaged on the spectral plane filter structure  250  as a separate wavelength-dispersed column along the y direction. With this arrangement, the length of the aperture  270  in the x direction may be chosen such that only one column illuminates the aperture at a time. By rotating the stage about the x axis, the spectrum of one input signal is measured. By rotating the stage about the y-axis, a different signal illuminates the aperture so that its spectrum can be measured.  
         [0023]     An alternative optical arrangement exhibiting additional aspects of my inventive concepts is shown schematically in  FIG. 3 . Shown therein is input  310 , lens  320 , grating  330  positioned on tip/tilt stage  340 , spectral plane device  360  having aperture  370 , and position detecting system including lens  380  and position detector  390 .  
         [0024]     With continued reference to  FIG. 3 , a multiwavelength input signal carried on input fiber  310  is collimated by lens  320  and illuminates planar reflective diffraction grating  330  mounted on tip/tilt stage  340  which, through its operation, rotates the grating about its X-axis or Y-axis, as desired. As described earlier, each wavelength signal beam is diffracted into an angle corresponding to its wavelength. Diffracted signals are focused by a second pass through lens  320  and are imaged onto a permanent spectral-plane device  360  having an exit slit aperture  370 .  
         [0025]     Provided that the system shown in the optical arrangement of  FIG. 3  is mechanically stable, the wavelength of light centered on the exit slit aperture  370  is uniquely determined by the angle θ( 347 ) of the grating  330 . A drive signal V d  (not shown), which may be applied to tip/tilt stage electrical connectors  345 , rotates the grating through angle θ according to a function θ(V d ). Upon calibration of the system, a correspondence can be determined between V d  and center wavelength λ(V d ) passing through the exit slit, so that the spectrum can be determined from a synchronous measurement of drive signal and photocurrent which may be measured at the exit slit  370 . Unfortunately, however, θ(V d ) may not be completely stable—it may depend on environmental factors or may drift over time due to aging processes. Moreover, the function may be frequency dependent. Thus, a separate means of measuring θ is helpful.  
         [0026]     As can be appreciated by those skilled in the art, the diffraction grating  330  disperses light into multiple orders. Additionally, the grating is designed to have high efficiency in one non-zeroth order, and this order is used for the spectral measurement. That fraction of the light diffracted into the zeroth-order may be used to measure the position of the stage.  
         [0027]     In  FIG. 3 , the zeroth order reflects off the grating and is collected by a lens  380 . As will be shown subsequently, the lens may advantageously be the same as that lens collecting the signal  320  or it may be a different lens. In this specific embodiment shown in  FIG. 3 , the zeroth-order-collecting lens  380  is physically distinct from the signal-collecting lens  320 .  
         [0028]     The collecting lens  320  images the zeroth order beam  395  onto a position detector  390 . It will be obvious to those skilled in the art that a curved mirror (not shown) could be used in place of the lens or the grating could have optical power to image the zeroth-order beam  395  onto the position detector  390 . Furthermore, and depending on the type of position detector used, the lens  380  may be omitted.  
         [0029]     For the zeroth-order beam, the angle of reflection from the grating is independent of wavelength. Consequently, all wavelengths comprising the zeroth-order beam  395  are imaged to a common spot on the position detector  390 . Thus, the position of the spot on the position detector  390  is uniquely determined by the position of the grating  330 . The position detector  390  may comprise a quad-cell or bi-cell detector or a “position-sensitive detector” (PSD). Alternatively, it may include a single photodetector that may be covered by a series of apertures.  
         [0030]     Elements of such a position detector are shown in  FIG. 4 . With reference to that figure, a single photodetector, for example, is positioned behind an opaque screen  4   a  having a series of transparent slits arranged in a periodic pattern. As a grating, such as that shown in  FIG. 3  by reference numeral  330 , is tilted around the x-axis, the zeroth-order spot travels horizontally to the right across the slits on the opaque screen  4   a , thereby producing a series of peaks that register in the photocurrent I position  in a manner such as that shown in  FIG. 4   c.    
         [0031]     Concurrently, and as a result of the grating being tilted, the dispersed spectrum passes across the single exit slit such as that shown prior in  FIG. 3  as reference numeral  370  and now in  FIG. 4   b . In this example, and as noted before, the signal comprises 3 wavelength signals. The relative power of each signal is indicated by the relative powers of the peaks 1, 2, and 3 in the plot of I signal  v. time that is shown in  FIG. 4   d.    
         [0032]     Through calibration, the correspondence between the temporal registration of the position signal and the peak transmittance of each signal wavelength onto the signal detector can be determined. This correspondence is only unique over a wavelength range Δλ that is scanned during the time it takes the zeroth-order spot in  4   a  to traverse one period of the position detector screen. If the uncertainty in Δ(V d ) is less than Δλ, then advantageously, the spectrum may be uniquely determined according to our inventive teachings by synchronous measurement of I position , I signal , and V d .  
         [0033]     With these inventive teachings in place, alternative embodiments may now be shown. One such alternative embodiment is shown in  FIG. 5 , which shows an optical arrangement where a grating is fixed, and a tip-tilt mirror in positioned close proximity to it. Additionally, the two lenses utilized in the example shown earlier with  FIG. 3 , is now reduced to a single lens.  
         [0034]     With specific reference now to that  FIG. 5 , a multiwavelength signal emerges from optical input  510 , which may include an optical fiber or other suitable structures, and is collimated by lens  520  which further directs the signal to mirror  543  that is positioned on tip/tilt stage  540 , which is controllably tiltable by applying appropriate signals to electrical contacts  545 . Signals reflected from the mirror  543  are directed to a fixed grating  530 .  
         [0035]     As noted in my discussion of the two-lens configuration of  FIG. 3 , the diffraction grating  530  disperses light into multiple orders and the grating is designed to have high efficiency in one non-zeroth order, and this high-efficiency non-zero order is used for the spectral measurement. That fraction of the light diffracted into the zeroth-order may advantageously be used to measure the position of the stage.  
         [0036]     In this example shown in  FIG. 5 , the zeroth-order diffracted beam  595  passes through and is focused by the same lens  520  as the multi-wavelength signal emerging from optical input  510 . More specifically, the zeroth-order diffracted beam  595  is focused by the lens  520  onto a position detector  590 . Simultaneously, the non-zeroth order is diffracted onto the mirror  543 , where it is further reflected through lens  520  and focused onto slit  570  within spectral plane structure  560 . Such a compact structure, can advantageously offer numerous design alternatives.  
         [0037]      FIG. 6  shows an exemplary double-pass monochrometer constructed according to my inventive teachings. Similar to that shown in  FIG. 1 , and with reference to  FIG. 6, 600 , multiwavelength input signals carried on input fiber  610 , and are collimated by lens  620  such that they illuminate planar reflective diffraction grating  630  mounted on tip/tilt stage  640  capable of rotating the grating  630  about its X-axis or Y-axis.  
         [0038]     The diffracted signals are focused by a second pass through lens  620  at the mid point of folded retroreflector  605 . The retroreflector  605  reflects the signals back through lens  620 , which collimates the beam and directs it onto diffraction grating  630  for the second time. The signals are further diffracted by grating  630  and imaged by lens  620  onto a permanent spectral-plane spatial filter  650 , having aperture  670 . Those portions of the diffracted wavelength signal that are imaged onto the aperture  670  are transmitted to illuminate an optical power detector  660  or other device located immediately behind the aperture.  
         [0039]     As can be seen from this arrangement, after one pass through the system, the signal is retro-reflected, through the action of retroreflector  605  about the axis perpendicular to the direction of dispersion so that the spectrum is inverted. The second pass through the system further disperses the signal so that the resolution of the system is effectively doubled. Furthermore, a spatial filter (not shown) may be inserted at the retroreflector  605  to filter out all but a band of wavelengths, thereby reducing the background light produced by omnidirectional scattering off the grating  630  in the second pass. The retroreflector  605  itself will perform spatial filtering if its clear aperture  606  is smaller than the length of the dispersed spectrum in the spectral plane.  
         [0040]     With these inventive structures and methods in place, one can quickly appreciate the benefits that various modifications or particular implementations of my teachings will produce. In particular, and with reference now to  FIG. 2 , one will recall the optical apparatus  200  in which a plurality of multiwavelength signals enter the system at input points arrayed along the x axis  210 [ 1 ] . . .  210 [N]. Each of these inputs is imaged on the spectral plane filter structure  250  as a separate wavelength-dispersed column along the y axis. With this arrangement, the length of the aperture  270  in the x direction may be chosen such that only one column illuminates the aperture at a time. By rotating the stage about the x axis, the spectrum of one input signal is measured. By rotating the stage about the y-axis, a different signal illuminates the aperture so that its spectrum can be measured.  
         [0041]     With that reference to that  FIG. 2  in mind and with reference now to  FIG. 7 , the tip/tilt stage  240  may be periodically modulated in angle about the y-axis such that the optical signal spectrum received from one of the inputs  210 [ 1 ] . . .  210 [N], and imaged on the spectral plane filter  250  periodically moves on and off the aperture  270 , thereby “chopping” the imaged signal. Chopping, in combination with synchronous detection using a lock-in amplifier is but one technique for improving the signal-to-noise ratio in measurements systems. In particular, the dark current of the photodetector  260  and the 1/f noise of the amplifier can be reduced this way.  
         [0042]     A simplified block diagram of such a detection scheme is shown in  FIG. 7 . Specifically, optical spectrometer  710 , utilizes functional input  730  to drive y-axis of the tip/tilt stage (not shown), produces I signal  which is supplied as input to transimpedance amplifier  720 , the output of which subsequently drives lock-in amplifier  740  in conjunction with functional input  730  to produce output signal  750  having the more desirable noise characteristics.  
         [0043]     With reference to  FIG. 8 , there is shown illustrative alternative arrangements for grating/mirrors as may be used with my invention. In  FIG. 8 ( a ), the input signal  810  is diffracted from stationary reflective diffraction grating  820  and illuminates mirror  830  that is positioned on tip/tilt stage  840  such that it is controllably tiltable by applying appropriate control signals through contacts  850 . Mirror  830 , is oriented such that a reflected signal is incident on reflective diffraction grating  820  where it is diffracted a second time. As can be appreciated, this configuration provides approximately twice the change in output angle as a function of input wavelength (spectral dispersion) as compared with configurations not exhibiting this “double diffraction”.  
         [0044]     Additionally, with reference to  FIG. 8 ( a ), shown therein are both zeroth-order  812  and non-zeroth-order  814  signals (in this example they are labeled as “1 st  order”) reflected from the surface of the grating  820 . As noted prior, the zeroth-order signal  812  is used to determine/measure positional information about the movable element (in this instance, the mirror  830 ). The non-zeroth-order signal  814 , is directed to an output which may be an optically useful aperture or other output such as a slit, whether uniform, tapered, stepped or varying, depending upon the particular nomenclature used.  
         [0045]      FIG. 8 ( b ) is an additional variation in which input signal  815  is diffracted in passing through transmissive diffraction grating  825  then is incident upon first surface mirror  835  also mounted on tip/tilt stage  845 . The reflected signal is diffracted again by a second pass through the transmissive grating  825  thereby producing output, first-order signal  819 . Similar to that shown for the discussion of  FIG. 8 ( a ), in this  FIG. 8 ( b ) both zeroth-order  817  and non-zeroth-order  819  signals are shown. As before, the zeroth-order  817  signals are used to determine positional information about the movable element (in this example, the mirror  835 ) and the non-zeroth-order  819  signals are directed to a suitable optical output. Lastly, while the non-zeroth-order  819  signals are shown labeled as “1 st  order”, our invention is not so limited and other order signals may be directed to output(s) as well.  
         [0046]     Of course, it will be understood by those skilled in the art that the foregoing is merely illustrative of the principles of this invention, and that various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention. Accordingly, my invention is to be limited only by the scope of the claims attached hereto.