Abstract:
An archival waveguide memory device is provided and comprises a large number of elongated waveguides and a series of partially reflective elements distributed within each of the waveguides in accordance with data to be stored within the waveguides so as to act upon an input optical signal in each one of the waveguides and to generate a reflected output optical signal able to be uniquely correlated with the data. A method of recording data in a waveguide is also provided. A method of retrieving data stored in a waveguide is further provided and comprises injecting a pulsed optical signal in the waveguide, detecting a reflected output optical signal from the waveguide, and decoding a temporal variation in the reflected output optical signal to retrieve the data.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The application claims priority on US provisional patent application Ser. No. 60/331,390 as U.S. provisional patent application Ser. No. 60/331,389 both filed Nov. 15, 2001 by Applicant. This application is further related to published PCT patent application for SEGMENTED WAVEGUIDE ARRAY GRATING FILTERS, publication No. WO 03/042737 published on May 22, 2003 by Applicant and to U.S. patent application for SEGMENTED WAVEGUIDE ARRAY GRATING FILTERS, application Ser. No. 10/494,040 published as application no. 2004-0258358 on Dec. 23, 2004 by Applicant, all of which are hereby incorporated by reference. 

   FIELD OF THE INVENTION 
   The invention relates to an optical memory. More specifically, it relates to a segmented waveguide array gratings (SWAG-) based memory. 
   BACKGROUND OF THE INVENTION 
   Modern society has come to depend in an essential manner on valuable information stored in archives, hopefully securely. The physical security level of the traditional medium, paper, is however very low. Wear and tear, water, fire and chemical agents present in the air can destroy paper on a short time scale. With the birth of the computer age, two new means have been introduced for archival storage. One is the magnetic recording, the second one is the optical disk. 
   The security level of magnetic recording is lower than that of paper: valuable information can be erased in little time by mistake, mischief, malice, or unfortunate decisions. In addition a variety of physical causes can destroy magnetically recorded information. These include mechanical failures of the read-out equipment (e.g. head crashes), spontaneous magnetic domain reversals, heat, fire, and stray magnetic fields, including those produced by high-power electromagnetic pulses from lightning or hostile man-made devices. 
   The second widespread archival medium is the optical disk, i.e. the ubiquitous compact disk called “CD” for music and CD-ROM for data, and the increasingly popular dense video disk or “DVD”. The optical disk&#39;s physical security level is moderately high. Provided the disk is not exposed to excessive heat or light (e.g. direct sunlight), and provided the surfaces are protected from excessive scratching, the stored information can be guaranteed for a lifetime, in the 10- to 30-year range. That still leaves fire and excessive heat as potential agents undermining the physical security of current optical disks. 
   The need for ever larger data storage capacities has led the optical disk industry towards increasing both the surface density of information carrying pits and the number of layers hosting these information pits. The industry has progressed from the compact disk storing 650 MB on one 86-cm 2  layer (or 7.5 MB/cm 2 ), to the latest four-layer DVD which can store 17 GB over the same 86-cm 2  area of access to information. For the four-layer DVD this represents an access density of 0.2 GB/cm 2 . 
   Despite the impressive storage capacity of the latest DVD, the construction of large Petabyte-range archives, would require tens to hundreds of thousands of disks, and its operation would necessitate the use of cumbersome juke-box-type retrieval mechanisms for rapid remote access. Such mechanical handling of large numbers of disks would diminish the security and reliability of the archival memory. 
   In the on-going effort towards increasing the access density of information, the optical disk industry has gone from one layer in the CD, to two and four layers in the DVD, and towards even more layers in research carried out in a number of laboratories. 
   SUMMARY OF THE INVENTION 
   Accordingly, an object of the present invention is to provide a reliable archive memory. 
   According to a broad aspect of the present invention, there is provided an archival waveguide memory device comprising: a large number of elongated waveguides; and a series of partially reflective elements distributed within each of the waveguides in accordance with data to be stored within the waveguides so as to act upon an input optical signal in each one of the waveguides and to generate a reflected output optical signal able to be uniquely correlated with the data. 
   According to another broad aspect of the present invention, there is provided a method of recording data in a waveguide comprising: providing a waveguide; and creating a series of segments in the waveguide having predetermined effective indices of refraction (n eff ) with a distribution within the waveguide to act upon an input optical signal in the waveguide to generate a reflected output optical signal, the series being determined according to data to be stored in the waveguide. 
   According to a further aspect of the present invention, there is provided a method of retrieving data stored in a waveguide having a series of segments with predetermined effective indices of refraction (n eff ) with a distribution within the waveguide to provide a reflection of an optical signal in the waveguide, the series being determined according to the data stored in the waveguide, the method comprising: injecting a pulsed optical signal in the waveguide; detecting a reflected output optical signal from the waveguide; and decoding a temporal variation in the reflected output optical signal to retrieve the data in one or many bandwidth length. 
   According to another broad aspect of the present archival optical memory, bits of information are structurally stored inside bulk glass in the form of a matrix of segmented waveguide array gratings. Following the injection of an ultrashort laser pulse into one of the Swag (Segmented Waveguide Array Grating) waveguides, the information bits can be read out by means of pure time domain and/or spectrally resolved time domain reflectometry. Using glass as the preferred embodiment, the information could be stored reliably for millions of years. 
   Throughout the present specification, the segmented waveguide array gratings will be referred to by the acronym and word “Swag”. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects and advantages of the present invention will become better understood with regard to the following description and accompanying drawings wherein: 
       FIG. 1  shows the proposed optical archival memory as a matrix of segmented optical waveguides; 
       FIG. 2   a  illustrates a plot of n eff  versus optical frequency for a 1.1-micron aluminosilicate core diameter imbedded in silica and  FIG. 2   b  illustrates a plot of n eff  versus optical frequency for a 0.9-micron aluminosilicate core diameter imbedded in silica; 
       FIG. 3  provides a schematic diagram for a device for reading-out the information by time domain reflectometry; 
       FIG. 4  presents an example of a segmented waveguide with its interfaces and the reflective light pulses that come from this waveguide; 
       FIG. 5  shows that a light pulse can be described in first approximation as the superposition of 12 Fourier-transform limited pulses; 
       FIG. 6  depicts a read-out device for reading through time and spectral domain reflectometry; 
       FIG. 7  explains the contribution of interfaces for each sampling window; 
       FIG. 8  illustrates the “peel-off” method for analyzing read-out signal by examining the situation for the first sampling; 
       FIG. 9  illustrates the “peel-off” method for analyzing read-out signal by examining the situation for the second sampling; 
       FIG. 10  shows a memory layer for an alternative approach and geometry for the archival optical memory where complex Swag structures are built in; and 
       FIG. 11  depicts the optical memory block and its read-out device. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A new approach to optical archival storage which features thousands of information-carrying layers and which could therefore increase dramatically the access density of information is described herein. The proposed optical archival memory is a matrix of segmented optical waveguides, where each 10-micron long segment constitutes the core of an optical waveguide and stores a bit of information by virtue of its diameter being either “thin”, i.e. 0.9 micron, or “thick”, i.e. 1.1 microns in the example presented below (see  FIG. 1 ). Pure time domain and spectrally resolved time domain reflectometry are used to read out the bits (see  FIGS. 2–9 ). 
   In the example presented in  FIG. 1  the segment cores are made of aluminosilicate which has a refractive index of 1.6. The surrounding silica, which has a refractive index of 1.444 at 1550 nm, constitutes the cladding of the segmented waveguides. The matrix of segmented waveguides can take the form of a thick optical disk or of a thick block of glass. Although glass is the preferred material to be employed, it is clear that other materials, notably crystalline silicon, could be used to implement the same idea. If silicon, which has a refractive index of nearly 3.42 at 1550 nm, were used for the core of the segmented waveguides, either silica or air could serve as the cladding. In this case the very large “delta-n”, i.e. the very large difference in core and cladding refractive indexes would result in very thin cores and it would allow waveguide separations on the order of one micron. 
   When constructed in the form of a thick optical disk, the information can be read out by mechanical and opto-electronic means, some aspects of which are similar to those in use with current optical disks (see  FIGS. 2 and 5 ). 
   An example of the parameters that are proposed is the following: read-out carried out with light at wavelengths in the 1520–1610 nm band; a 3500- to 17000-layer optical disk incorporating segmented waveguides forming a matrix on 4.4-micron distant centers;
         glass layers in the 2–10 micron thickness range making up a disk of 3.5 cm total thickness. In this case the surface density of access to stored information would be in the range 2–10 GB/cm 2 , i.e. 10 to 50 times more than with 4-surface DVDs. A 3.5-cm thick optical disk of 86-cm 2  access area would store 170–850 GB of information. The surface density of access to stored information for the Swag-based archival memory should be 10 to 100 times more than for current two-surface optical disks.       

   The Swag-based archival optical memory thus presents two fundamental advantages:
         1—the stored information is protected inside bulk glass from all the above-mentioned agents;   2—the very large surface density of access to information significantly reduces the number of disks required for an archive and would thus make it economically feasible for each disk, or each group of adjacent disks, to permanently have its own reading head, thereby avoiding cumbersome juke-box-type retrieval mechanisms and therefore allowing short access times.       

   Both of these features would lead to an archival memory presenting a high degree of physical security, reliability and unlimited readability. 
   In order to facilitate the description of the Swag-based optical archival memory, two specific examples will be used: the thick optical glass disk and the glass block. It will be clear to those skilled in the art that many variations of the basic design are possible. 
   1. The Thick Optical Disk 
   The thick optical disk  100  form of the archival optical memory will first be described. The example chosen is shown in  FIG. 1 . The 3.5-cm thick optical disk  100  is made up of some 3500 layers of glass  103 , each one of which is 10 microns thick and structurally stores binary information in the form of “thin” (binary “0”)  108  and “thick” (binary “1”)  109  cylindrical segments (or micro-columns) of higher index glass, for example aluminosilicate, embedded in the surrounding glass. With an index of refraction of approximately 1.6 the aluminosilicate columns  105  constitute the cores of segmented optical waveguides capable of transmitting light vertically down through the optical disk. The silica  106  in which the columns are imbedded has a refractive index of 1.444 at 1550 nm and it constitutes the cladding of the segmented waveguides. The thin  108  and thick  109  column diameters are chosen so that the segmented waveguide  105  operates in the single mode regime for light in the 1520–1620 nanometer band with a propagating mode diameter which is very close to the minimum value. For the example considered below this minimum beam diameter is close to 1.2 micron (full width at half maximum amplitude for the electric field profile in the transverse direction) and it occurs when the aluminosilicate core  105  has a diameter close to 1.0 micron. Consider the case where thin micro-columns  108  have a diameter of 0.9 micron, while thick micro-columns  109  have a diameter of 1.1 microns. The mode diameter for these two core diameter values is very nearly the same and is close to 1.25 microns. In the example given above the waveguide core separation  107  was quoted as 4.4 microns center to center. The coupling that occurs between waveguides  105  imposes a minimum separation. The coupling is diminished when waveguides  105  are dissimilar, which would be the case here since the waveguides  105  are made up of different sequences of thin  108  and thick  109  segments. To combat the coupling the waveguide separation  107  might have to be increased beyond 4.4 microns. Another way to combat coupling is to specify slightly different core diameters on alternate waveguides in the 2-D matrix. 
   The top surface of the disk  100  is covered by an antireflection (AR) coating layer  101  which also provides protection for the glass disk  100 . Underneath the AR coating  101  a first thin segment 120 microns in length  102  serves as a buffer zone to separate distinctly in time the reflections from the AR coating  101  and the first reflecting interface  120  in  FIG. 4 . 
   The substrate  104  is an optically flat silica glass disk upon which the stack of layers  103 , the layer containing the buffer waveguides  102  and  101  are formed through deposition or other techniques. 
   As light propagates down the segmented waveguide, a reflection occurs at each interface that lies between a thin  108  and a thick  109  segment. As shown in  FIG. 2  this is due to the fact that the effective refractive index n eff  of the fundamental mode propagating down the waveguide changes abruptly (i.e. in less than one tenth of the wavelength of the read-out light pulse) in going from a thin  108  to a thick  109  segment.  FIG. 2   a  illustrates a plot of n eff  versus optical frequency for a 1.1-micron aluminosilicate core diameter  108  imbedded in silica and  FIG. 2   b  illustrates a plot of n eff  versus optical frequency for a 0.9-micron aluminosilicate core diameter  109  imbedded in silica. This discontinuity reflects light back into the waveguide, which is desired, but it also scatters (or couples) a small amount of light into radiative modes, which is not desired but which can be coped with. The loss caused by this scattering can be minimized by designing the thin  108  and thick  109  diameters of the waveguide segment cores (the micro-columns) to be smaller and larger, respectively, than the core diameter giving the minimum mode diameter, in such a way that the mode diameter is substantially the same in the thin  108  and thick  109  segments. This way the mismatch in going from a thin  108  to a thick  109  waveguide segment (and vice versa) will result in a substantial reflection coefficient, as desired, while minimizing the coupling to radiative modes, the undesired loss mechanism. 
   At a wavelength of 1550 nm the single interface reflectivity coefficient for the electric field amplitude in going from a thin  108  to a thick  109  segment is given by EQ. 1:
 
 r=[n   eff (thick)− n   eff (thin)]/[ n   eff (thick)+ n   eff (thin)]  (EQ. 1)
 
   This field amplitude reflection coefficient r has a value close to 0.60×10 −2  for the case when light goes from a 0.9-micron thin segment  108  into a 1.1-micron thick segment  109  of aluminosilicate embedded in silica. The power (or light intensity) reflection coefficient for a single interface is r 2  and is on the order of 0.36×10 −4 . Note that the sign of r in EQ. 1 becomes negative when the light goes from a thick to a thin segment. A thin-to-thick transition will be referred to below as an “up-transition” with positive r, and to a thick-to-thin transition as a “down-transition” with r negative. 
   Because r is small, in the discussions that follow regarding the light signals reflected back by the segmented waveguides  105 , the phenomenon of multiple reflections can be safely ignored. Compared to the directly reflected field amplitude, the three-times reflected light signal (which also comes back towards the input end) is down in strength by several orders of magnitude and it has therefore a negligible effect on the measured reflected light signal. 
   The attenuation suffered during propagation by the input read-out light and by the reflected light signal could be compensated for to a certain extent by doping the aluminosilicate core with erbium, and by optically pumping it. In other wavelength ranges other optically amplifying elements could also be used, like praseodymium for instance in the 1300 nm band. 
   Reading out the Information 
   As in the conventional optical disk, laser light is brought in from above and focused onto the disk&#39;s surface in such a way that the light is coupled with very high efficiency into a given segmented waveguide  105 . The incident read-out light is in the form of ultrashort pulses  110  which are either 50 or 500 fs and which are spaced 400 ps in time in the present example featuring a 3.5-cm thick optical disk  100 . Each incident ultrashort light pulse  110  gives rise to a great many reflected pulses originating at the numerous interfaces between thin  108  and thick  109  waveguide segments (see  FIG. 1 ). By temporally and spectrally analyzing this light one can recover the precise sequence of binary 0s (thin micro-columns  108 ) and 1s (thick micro-columns  109 ) structurally stored in the glass. What follows is a discussion of the read-out techniques. 
   a) Read-out by Time Domain Reflectometry. 
   The conceptually easiest way to read out the information in the present example is shown in  FIGS. 3 and 4 . One injects into a segmented waveguide  105  a 50-femtosecond pulse  110  from a laser such as a mode-locked erbium-doped fiber laser. The numerous interfaces between thin and thick segments in the segmented waveguide reflect a series of ultrashort light pulses back towards the input face of the disk. For interfaces sandwiching a thin 10-micron segment as is the case for interfaces  121  and  122  illustrated in  FIG. 4 , the two reflections  221  and  222  are spaced in time by about 102.2 fs at 1550 nm. For interfaces sandwiching a thick 10-micron segment as is the case for interfaces  120  and  121  illustrated in  FIG. 4 , the two reflections  220  and  221  are spaced in time by about 105.2 fs at 1550 nm. For the first layers in the optical disk  100  the reflected light echoes are thus well resolved in time and can be measured by a sampling technique using a suitably fast light gate  117 . 
   With 50-fs read-out pulses  110  however, dispersion is large and must in general be compensated for. The initially 50-fs pulse  110  spreads out as it propagates along a segmented waveguide so that the interface reflections from the deepest layers in the disk  100  have been broadened out to about 2 or 3 ps (depending on the thin/thick segment sequence) as they exit the entrance surface. This leads to a considerable temporal overlap of the light pulses reflected from various interfaces, thereby greatly increasing the difficulty of identifying the thin-to-thick segment transitions. 
   To counter this pulse-broadening effect a variable dispersion compensator  114  in  FIG. 3  can be used to recompress the reflected pulses to nearly their original 50-fs duration. This dispersion compensator  114  would be adjusted according to the depth of the layers  103  that one wishes to read out. Alternatively, one could use several reading heads for the disk, each head incorporating a fixed dispersion compensator  114  meant for a certain range of layer depths in the optical disk  100 . 
   With the proper dispersion compensation in place the sequence of reflected light echoes looks like the one shown in  FIG. 4 . Each light pulse reveals a transition from a thin to a thick segment or from a thick to a thin segment. (In  FIG. 4  segments  102 ,  141  and  144  are thin, the others are thick). The absence of a pulse  223  signifies no change in segment diameter as at the reflecting interface  123 . 
   For every segmented waveguide  105  in  FIG. 1  the first reflecting interface is an up-transition from the thin input waveguide  102  to a thick 10-micron long segment  140  in  FIG. 4 . This positive reflection serves as a fixed temporal reference pulse  220  for reading out the information contained in the reflections from the stack of 3500 layers  103  shown in  FIG. 1 . Each one of 3500 information-carrying layers  103  is 10 microns in the light propagation direction in this first example. 
   An alternative way of implementing time domain reflectometry as a way of reading out the stored information would be to use pre-chirped laser pulses, a technique known in optical communications to combat the effect of dispersion. Here the incident read-out laser pulse  110  would be pre-chirped and of longer duration so that the optical carrier frequency decreases with time (down-chirp). The amount of chirp would be adjusted according to the depth of the reflecting interfaces that one wishes to read out and according to the dispersion encountered in the bit sequence of segments preceding it. Upon exiting the disk  100 , the reflected pulse  99  will have been recompressed by the waveguide&#39;s dispersion to a duration of about 50 fs, thus allowing it to be temporally distinguished from adjacent reflected pulses. 
   In the following, assume that the effect of dispersion has been reduced to a negligible level for the layers of interest through dispersion compensation or by choosing a waveguide core diameter and a read-out wavelength for which dispersion is very small. In the example given here read-out wavelengths that are deeper into the infrared (i.e. wavelengths longer than 1600 nm) will give rise to less dispersion because the chromatic dispersion due to the glass material will counter to a certain extent the waveguide dispersion, just as occurs nowadays in dispersion shifted fibers for optical communications in the 1550-nm band. 
   The light signal reflected from the 3.5-cm thick stack of 3500 layers  103  extends over a time duration which is slightly under 375 ps in the 1500–1600 nm band. In order to measure the time profile of the reflected light  99  with 50-fs resolution one can use an ultrafast light gate  117  driven by a gating light pulse which is a portion of the input 50-fs read-out pulse  110  and which is transmitted by beamsplitter  111 . This gating light pulse goes through the variable optical delay line  115  shown in  FIG. 3  and then enters the ultrafast light gate  117 . The value of the delay D is being ramped up at the rate of 50 fs in 400 ps, i.e. 25 ps in 200 nanoseconds. The ultrafast light gate  117 , where light is used to control light, could be one using optical frequency up-conversion in nonlinear crystals. In this case the 50-fs gating pulse interacts in the nonlinear crystal with the light signal to be read out and it converts a large fraction of its energy into second harmonic light in the 750–800 nm deep red portion of the spectrum. Filter  118  ensures that only some frequency light enters the photodiode. 
   The 50-fs sample of deep red light is then detected by a fast photodiode  119  which puts out a 5-ps electronic pulse whose amplitude is proportional to the energy in the 50-fs sample of deep red light. State-of-the-art electronics similar to what is now used for 40 GB/s optical communications  138  can then digitize the amplitudes of these 5-ps pulses which are coming out at a rate very close to once every 25 ps. At this rate 15 samples can be extracted from the 375-ps reflected light signal in one sampling round. This sampling is sparse so that a total of 25 ps/0.05 ps=500 sampling rounds will be necessary in order to completely sample the signal. 
   The complete sampling of the reflected light signal will be achieved thanks to the ramping up of the optical delay line  115 . With the delay line  115  set for an initial delay of D 0  in  FIG. 1 , in the first sampling round the signal is sampled at times t 0 , t 0 +25.003 ps, t 0 +50.006 ps, t 0 +75.009 ps, t 0 +100.012 ps, . . . , t 0 +375.047 ps. The 3 fs (or 0.003 ps) adding up at each step is due to the ramping up of delay D. 
   The second sampling round begins 400 ps later when the read-out laser sends in another 50-fs pulse  110 . This time the delay line has D=D 0 +50 fs, so that the sampling times are now: t 0 +50 fs, t 0 +50 fs+25.003 ps, t 0 +50 fs+50.006 ps, t 0 +50 fs+75.009 ps, t 0 +50 fs+100.012 ps, . . . , t 0 +50 fs+375.047 ps. 
   The 500 sampling rounds take a total time of 500×0.4 ns=200 ns. Let us assume that the rotational speed of the disk  100  is on the order of 0.5 m/s=0.5 nm/ns. Then in 200 ns the read-out light beam focus will move by 100 nm=0.1 micron in the transverse direction. With the timing chosen so that the read-out beam will be precisely centered on one of the segmented waveguides  105  during the 250th sampling round, the largest offset from perfect alignment will be 50 nm. Since the input beam spot size will be about 1.2 micron (or 0.85 micron diameter in terms of power density), this slight offset will affect only slightly the level of power injected into and collected from the segmented waveguide  105  being read out, and it can therefore be tolerated. 
   With 3500 bits read out in 200 nanoseconds the instantaneous reading rate is 17 GB/s. Since it will take another 8.8 microseconds for the reading head to come over the next segmented waveguide which is 4.4 microns away from its neighbor, the average bit read-out rate is therefore 3500/(8.8×10 −6 )=398 Mb/s. This read-out rate can of course be further increased through the use of multiple read-out heads. 
   b) Read-out Through Time and Spectral Domain Reflectometry. 
   When one carries out purely time domain reflectometry as described above, the instantaneous rate at which information-bearing light pulses are being reflected is almost 10 Tb/s. This extremely high rate is currently the maximum bit rate achieved in field trials of state-of-the-art optical communications systems. In these trials several hundred wavelengths are used simultaneously so that the bit rate on each individual channel operating on an assigned wavelength is limited to 10, 20 or 40 GB/s. One crucial advantage of coding information in many channels, each having a limited bandwidth (usually under 80 GHz in optical frequency), is that it makes it possible to cope adequately with fiber optic dispersion by almost completely eliminating its deleterious effect with the help of dispersion compensators. 
   Similar multiwavelength tactics can also be employed here in reading out the archival optical memory in order to minimize the pulse dispersion problem and to ease therefore the task of measuring the time profile of the reflected light signals. The basic idea explained in  FIG. 5  is that an incident 50-fs pulse  110 , whose spectral width is 70 nm at half maximum spectral power density, can be thought of, in first approximation, as being equivalent to a superposition of 12 Fourier-transform limited pulses  158 , each 500 fs in duration and 7 nm in width at half maximum spectral power density, the 12 spectral peaks being spread out in 7-nm increments over the 1530–1610 nm band. Accurate measurements of the reflected light over the 375-ps time domain for each one of the spectral components will in principle yield most of the information that the 50-fs pure time domain reflectometry, with dispersion compensation, would have yielded. Each one of the 500-fs pulses  158  has 10 times less spectral width so that temporal dispersion is about 200 fs for the deepest layers, thereby lengthening the corresponding reflected pulses to 540 fs, a tolerable amount. 
   One implementation of this idea for reading out the information stored in the segmented waveguide is illustrated in  FIG. 6 . The same 50-fs laser pulse  110  as earlier is used as an input to the memory through beamsplitter  111 . The 12 Fourier-transform equivalent read-out pulses are spectrally Fourier-transform limited Gaussian-shaped pulses which have a duration of 0.500 ps (or 500 fs) at half maximum instantaneous power. The latter is taken to be the square of the optical electric field averaged over one optical period. With a Gaussian time profile for the light pulse, its spectrum is also Gaussian-shaped and has a spectral width Δf=0.88 THz at half maximum spectral power density, which translates to Δλ=7 nm in terms of wavelength in the 1520–1610 nm band. One must note that the corresponding electric field amplitude envelope for the 0.5-ps light pulse is 0.707 ps at half maximum amplitude. On the return side electro-optically adjustable wavelength selective filter  159  lets only light in the appropriate 7-nm wide sub-band go through to the sampling gate. 
   A interference filter  160  in  FIG. 6  is designed to lengthen the 50-fs pulse into a 250-fs pulse that will be used for driving the sampling light gate, which again could be used on optical sum-frequency generation in a nonlinear crystal. Since the reflected light pulses from individual interfaces in the segmented waveguide are 500 fs in duration, or slightly more, the 250-fs time resolution will be sufficient in  FIG. 6 . 
   As mentioned earlier the envelope of the optical electric field of the 500-fs read-out pulses is 707 fs at half maximum amplitude, again assuming a Gaussian shape. For the purpose of simplifying the discussion here below it is assumed that this electric field envelope is essentially 1.47 ps at the base, i.e. the effect of its leading or trailing edges for times ahead or behind its peak by more than 0.735 ps will be neglected.  FIG. 8  illustrates the idealized truncated pulse shape used for discussion purposes. In  FIG. 8 , the time origin is taken to be the time at which the peak of light pulse  220  reflected from interface  120  would arrive if it were completely alone. The least finite value retained for the electric field amplitude envelope is 0.05 (the peak&#39;s field amplitude is normalized to unity) occurring at 0.735 ps before and after the peak of the reflected pulse. 
   The round-trip delay between two interfaces sandwiching a segment is either 102.2 or 105.2 fs for thin  108  and thick  109  segments respectively. Solely for the purpose of simplifying the description of the read-out procedure and analysis here below, the round-trip delay will be taken to be 105 fs regardless of the segment&#39;s identity, be it a binary bit “0” or “1”. It is understood that in practice the computer carrying out the analysis of the spectral/temporal data would take into account the precise actual delays which are segment-identity dependent. Note that the 0.05-amplitude points, which nominally terminate the electric field envelope of the light pulse at an interval of 0.735 ps from the peak, correspond to the delay between reflections originating from interfaces which are separated by 7 segments. 
   Filter  118  in  FIG. 6  ensures that only the sum-frequency light (in the deep red) enters the fast photodiode. The reflected light signal  99  is sampled this time with a 250-fs window and with a period close to 25 ps as can be done with 40 GB/s electronics  138 . The same sampling technique is used as was described above for the pure time domain reflectometry measurement. This time, with each sampling round the variable delay of the variable optical delay line  115  is increased by 250 fs instead of 50 fs above. The sampling rounds now number (25 ps)/(0.25 ps)=100 and they cover a total time interval of 40 ns. The time domain measurements are carried out for 500-fs read-out light pulses whose wavelengths are centered at 1530, 1537, 1544, 1551, 1558, 1565, 1572, 1579, 1586, 1593, 1600 and 1607 nm, all of which fall within the ranges of the C and L bands of erbium-based optical amplification. 
   Time domain measurements at each wavelength could be carried out sequentially with the one set-up shown in  FIG. 6 . After the 40-ns complete sampling interval the tunable filter  159  could be electro-optically tuned to a new wavelength within 10 ns, and a new complete sampling interval started again. Since the total 12-wavelength sampling would now take 12×50 ns=600 ns, the relative speed of the disk  100  and laser heads would need to be reduced to 0.16 m/s to keep the input light in focus long enough over the segmented waveguide  105 . 
   This slow-down of the disk&#39;s speed can be avoided by another approach, which is to reproduce the set-up of  FIG. 6  in each of 12 reading heads, each one having its tunable filter  159  set at one of the 12 wavelengths. These 12 reading heads would be positioned to focus read-out light along the same track on the disk  100 . All data pertaining to one segmented waveguide  105  would be collated and analyzed by a fast computer. In this case the disk speed could be increased to 2.5 m/s since each one of the 12 laser heads would carry out the complete sampling in only 40 ns. 
   Let us now analyze the temporal/spectral data obtained from the complete sampling of the reflected light on 12 different wavelengths. When considering a sample i* at an arbitrary time t*, and at 12 wavelengths, as many as 16 or 17 truncated pulses overlap within the 250-fs sampling window  167  in  FIG. 7 . This can be seen in  FIG. 7  by assuming, for instance, that a sampling window  167  coincides at time t* with the peak of a pulse reflected from interface i*. The leading edges of the potential reflections from the next 7 deeper interfaces will have a finite value at time t*. Note that 7×105 fs=735 fs, the time interval from the peak of the pulse to the truncated leading or lagging edge. The 250-fs duration of the sampling window  167  (assumed to have an ideal rectangular time profile) adds 125 fs to the sampling&#39;s reach in each sampling time direction, so that potentially 8 deeper interfaces can contribute to the field amplitude at time t* in addition to interface i*. The same applies to the lagging edges of light pulses potentially reflected from the shallower 8 interfaces. The result is that as many as 17 pulses can potentially add up to give the resulting electric field amplitude at time t*. When the center of the sampling window does not coincide with the peak of one of a reflected light pulses (taken alone) one can have that 16 interfaces can potentially contribute to the reflected signal sample at a given time t*. 
   When analyzing the data for time t* at the 12 wavelengths, it would be very time consuming to search among the 2 16  or 2 17 =65 536 to 131 072 possible permutations of thin  108  and thick  109  segments in order to match the computed 12-point spectrum with the measured data. This would probably give ambiguous results in view of the limited precision of the sampling technique applied to a signal subject to quantum noise. A better approach to analyzing the data is to sort of “peel off” the thin/thick segment sequence starting at the beginning of the time profile, as explained here below. 
   First, the situation for the first sampling will be examined. Referring to  FIG. 8 , the temporal sampling window  162  will be idealized by a rectangular gate 250 fs in duration and precisely centered on the time t 0 =−735 fs at which time the light gate receives the leading edge of light pulse  220 , which is the reference pulse, i.e. the first reflection from the reference up-transition labeled  120  in  FIG. 4 . At this precise time the leading edge of pulse  221  from the possible down-transition at interface  121  (there would not be any transition  121 , nor any pulse  221 , if segment  141  were thick) is partly captured by the late part of the first sampling window  162  from t 0 +105 fs to t 0 +125 fs. As the read-out wavelength changes from 1530 to 1607 the round-trip optical path length of the 10-micron long segment  102  changes by nearly one wave so that a possible down-transition  221  could be detected in the 12-point spectral profile. If present, pulse  221  will interfere constructively with pulse  220  (the reference pulse) at some wavelength λ c  in this 77-nm wide range. At another wavelength, displaced by about 38 nm from λ c i, the two pulses  220  and  221  will interfere destructively. This data can therefore begin to give an idea about segment  141  being thin (interference present) or thick (no reflection, no interference). But since the overlap of pulse  221  with the sampling window  161  is very short (20 fs), segment  141  might not be identified unambiguously at this point. 
   Let us now examine the second sampling window  163  in  FIG. 9  centered on time t 0 +250 fs=−485 fs. It can be seen from  FIG. 9  that two more pulses then come into play. Pulses  220 ,  221 ,  222 , will contribute strongly to the coherent sum of the optical electric fields within the sampling window, with pulse  223  making a minor contribution. As the read-out wavelength varies from 1530 to 1607 nm, the round-trip optical path length difference between pulse  220  and pulses  221 ,  222  and  223  will change by very nearly, one, two, and three waves, respectively. This will make for a richly modulated 12-point spectrum, where each interface contributes to a different periodicity in the spectral profile and can thus be identified. Disregarding at this time the possible (but minor) contribution from segment  143 , a computer can search to match this measured 12-point spectrum with the spectrum computationally obtained by coherently adding the contributions of pulses  220 ,  221  and  222  that the different permutations of segments  140  and  141  would give rise to. The number of possibilities is only 2 2 =4 (segment  140  is always 1), so that a good match can be found very quickly. It can be safely assumed that segments  140  and  141  can now be identified with certainty. The identity of segment  142  is left open at this point. 
   By the expression “coherently adding” it is meant adding phasors in the complex plane with the proper delays between pulses being taken into account. In other words, during the computational analysis, one adds up sinusoids modulated by the 707-fs wide envelopes shown in  FIG. 9 , with due attention being paid to the delays, precise to a fraction of a femtosecond, between the pulses reflected by the various interfaces. The precise values of the parameters used by the computer program in modeling the measured spectra can be obtained ahead of time by measuring the complete spectral/temporal profiles produced by known segment sequences using the actual read-out heads used in the archival memory. It is assumed that the light-gating sampling technique, be it by optical up-conversion or by some form of opto-electronic sampling, measures the amplitude of the total electric field envelope of the reflected light. 
   When considering the third sampling window  164  centered at t 0 +500 fs=− 235  fs, the following pulses will add up coherently at various times and make a major contribution to the resulting sample amplitude:  220 ,  221 ,  222 ,  223 . Pulse  224  will make a minor contribution. With the sequence now known for segments  140 ,  141  and  142  the computer search can disregard the contributions from segment  143  and restrict its search to a set of 2 2 =4 possibilities for segments  141  and  142 . The identities of segment  143  can be left open. 
   Examining now the fourth sampling window  165  centered at t 0  +750 fs=15 fs, pulses  220 ,  221 ,  222 ,  223 ,  224 ,  225 ,  226  and  227  now make a major contribution, while pulse  228  makes a minor contribution. The computer now has the sequence of segments  140 ,  141 ,  142 ,  143  and  144  in hand, so that searching for the identity of segments  145 ,  146 ,  147  (the contribution of segment  148  being ignored at this point) is carried out within a set of 2 3 =8 permutations of ones and zeros. Given the strength of pulses  225 ,  226  and  227  in the sampling window  165 , the computer&#39;s data fitting to the 12-point spectrum in this sampling window  165  will identify segments  145 ,  146  and  147  with a high degree of confidence. 
   When considering the fifth sampling window  166  centered at t 0 +1.0 ps=265 fs. Pulses  220 ,  221 ,  222 ,  223 ,  224 ,  225 ,  226 ,  227 ,  228  and  229  will make the dominant contribution, while pulse  230  will make a minor contribution at this time. The computer has in memory the segment sequence from  140  to  147 , and now searches among a set of 4 permutations for the identities of segments  148  and  149 , the contribution of pulse  230  being neglected and the identity of segment  150  being left open. 
   Considering the sixth sampling window  167  centered at t 0 +1.25 ps. Pulses  220 ,  221 ,  222 ,  223 ,  224 ,  225 ,  226 ,  227 ,  228 ,  229 ,  230  and  231  will make the dominant contribution, while pulses  232  and  233  will make a minor contribution at this time. The computer has in memory the segment sequence from  140  to  149 , and now searches among a set of 4 permutations for the identities of segments  150  and  151 , the identities of segments  152  and  153  being left open. 
   Considering the seventh sampling window  168  centered at t 0 +1.5 ps. Pulses  222 ,  223 ,  224 ,  225 ,  226 ,  227 ,  228 ,  229 ,  230 ,  231 ,  232 ,  233  and  234  will make the dominant contribution, while pulses  220 ,  221 , and  235  will make a minor contribution at this time. The computer has in memory the segment sequence from  140  to  151 , and now searches among a set of 8 permutations for the identities of segments  152 ,  153  and  154 , the identity of segment  155  being left open. 
   Considering the eighth sampling window  169  centered at t 0 +1.75=1.015 ps. Pulses  224 ,  225 ,  226 ,  227 ,  228 ,  229 ,  230 ,  231 ,  232 ,  233 ,  234 ,  235  and  236  will make the dominant contribution, while pulses  222 ,  223 , and  237  will make a minor contribution at this time. The computer has in memory the segment sequence from  140  to  154 , and now searches among a set of 4 permutations for the identities of segments  155 , and  156 , the identity of segment  157  being left open. 
   With the eighth sampling window  169  centered at 1.015 ps, the reflected pulses  220  and  221  have dropped out. At this time 16 interface reflections are potentially contributing to the observed amplitude of the total reflected electric field envelope. 
   As one goes into deeper layers with sampling time, upper layer reflections keep disappearing from view as new ones come up from the deeper layers. The fact that there are never more than 17 layers contributing to the reflected light, and moreover that only 2 or 3 new layers appear effectively at each sampling step, makes the read-out problem highly tractable through computational fitting of the measured spectral/temporal data with the calculated time profiles at all 12 wavelengths for the assumed segment sequence. 
   Various schemes are possible to ease the data reading. One could encode 25-bit data blocks into a sub-set of a 28-bit sequences for instance, taking advantage of the additional bits to eliminate some sequences that might be difficult to read out unambiguously. Also, following every 100 layers, a sequence of 4 thin segments  108  followed by a thick segment  109  could be used in the analysis to verify that the read-out procedure is not in error and to produce a reflected pulse  99  from the up-transition to restore a precise timing reference. 
   Fabrication 
   The thick optical disk  100  could be fabricated layer by layer by using modern chemical vapor deposition techniques followed by modern methods of nanofabrication. Following the deposition of an aluminosilicate layer, a planarization step (through chemical mechanical polishing for example) could take place, followed by the spinning of photoresist. A rastered laser beam with electro-optic control over its focused spot size could then print a thin/thick hole pattern in the photoresist. After developing and baking the photoresist, a reactive ion etching machine would transfer the information into arrays of thin  108  and thick  109  aluminosilicate cylindrical segments, or micro-columns. The silica cladding layer  106  can then be deposited, followed again by a planarizing step that would stop at the tops of the minicolumns. Other ways of accomplishing the same goal are within the capabilities of modern nanofabrication technology. For mass production of encyclopedias for instance, masks could be used as in standard chip manufacturing. 
   2. The Optical Memory Block 
     FIGS. 10 and 11  illustrate an alternative approach and geometry for the archival optical memory. Here nanolithography is used to fabricate layers  170  where complex Swag structures  171  are built in. An example is described below where the manufacturing would require great complexity and precision, but where the read-out would be greatly simplified. 
   With the Swag technology it is possible to produce gratings which are sparse and which can therefore be interleaved in a straightforward fashion with highly predictable properties. In the alternative approach a bit “1” for read-out at wavelength λ i  is represented by a quarter-wave stub Swag minigrating  171  designed to reflect light within a given sub-band of wavelengths centered at λ i , where i can run from 1 to 10, for example. A bit “0” at this read-out wavelength λ i , and at one of the positions on 100-micron centers, is represented by the absence of a Swag minigrating at that position. In  FIG. 10  the quarter-wave stub Swag minigratings  171  occupy 100 microns in length along the z axis and they are set 100 microns apart when they stand for two adjacent binary 1s. In  FIG. 10  three Swag minigratings are interleaved in order to encode three binary “1” bits for read-out at each of three wavelengths. 
   A tunable laser producing a 500-fs pulse is used for read-out (see  FIG. 11 ). The read-out pulses are passed through a 50—50 beamsplitter  111  and focused by lens  184  into one of the segmented waveguides. The reflected pulses  172  are about 1.05 ps apart. They could be sampled as described above. More than 10 Swag gratings  171  can be interleaved so that 10 different sequences of bits can be read out at each of 10 different wavelengths. The interleaving must be done in the way prescribed in the Swag provisional patent application, where the positioning of a given segment interface at position z* requires taking into precise numerical account all the segments that read-out light will have encountered by the time it reaches the interface at z*. 
   A big advantage of the simple quarter-wave Swag minigrating approach is that a read-out pulse at wavelength λ 7  for example, will suffer very little reflection loss from minigratings  171  made for the other wavelengths λ 1  to λ 6  and λ 8  to λ 10 . In addition the optical power at 980 nm and/or 1480 nm which could be used to pump the erbium doped aluminosilicate glass core, would be very little attenuated by the multiple gratings made for reflection within the erbium amplification band. 
   With this reduced loss at a given read-out wavelength, and with fairly uniform optical amplification over long lengths of segmented waveguides  105 , the latter could be made 35 cm long instead of 3.5 cm in the thick optical disk  100  above. This would lead to a tenfold increase in capacity per cm 2  of access area. The Swag minigratings  171  are ten times longer than the 10-micron layers in the thick optical disk  100 , but with the tenfold interleaving the linear bit density along the light propagation axis z remains the same as in the disk. The tenfold increase in waveguide length leads to a tenfold increase in total capacity, to 1.7 TB per 86-cm 2  access area, equivalent to 100 DVDs of 17 GB capacity. This increased waveguide length would necessitate the use of dispersion compensators to compensate for the longer path lengths. 
   Instead of using a tunable laser for producing the 500-fs read-out pulse, one could use ultrashort pulses 50-fs in duration as earlier described for read-out. Ten different bit streams on the ten different wavelengths mentioned earlier would emerge, would be reflected by beamsplitter  111  and would be collimated by lenses  185  into a prism (or diffraction grating) spectrometer arrangement  186  in  FIG. 11 . The reflected light signal would be measured and recorded simultaneously on all ten wavelengths by means of ten photonic (e.g. optical sum-frequency generation in a nonlinear crystal) or optoelectronic sampling gates and accompanying electronics  173  to  182  in  FIG. 11 . The sampling techniques described above would be applied in this case as well. 
   3. Combining Techniques 
   If a titanium-sapphire laser with a nearly 400-nm bandwidth were used, the temporal/spectral read-out technique described above could be combined with the 50-fs read-out technique in order to increase the storage capacity by going to thinner layers, possibly as thin as two microns or less. In other words, one would use the titanium-sapphire laser to produce 50-fs pulses on several wavelengths spanning its lasing range from 700 to 1100 nm. Dispersion compensation would have to be incorporated. Multiple reading heads with different heads compensated for various depths could be used to speed up the data gathering and analysis process. The larger capacity offered would justify the added read-out hardware. Total capacity might approach a level equivalent to 1000 DVDs for the same 86-cm 2  access area. 
   It will be understood that numerous modifications thereto will appear to those skilled in the art. Accordingly, the above description and accompanying drawings should be taken as illustrative of the invention and not in a limiting sense. It will further be understood that it is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be applied to the essential features herein before set forth, and as follows in the scope of the appended claims.