Superimposed fast transient and permanent holographic gratings
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to laser-induced refractive index gratings, and more particularly pertains to superimposed fast transient and permanent holographic gratings, and techniques for producing such superimposed gratings. 
     2. Discussion of the Prior Art 
     Light scattering from laser-induced refractive index gratings in materials is the physical mechanism underlying many techniques for controlling light beams in devices useful in a variety of opto-electric technology applications. Examples include phase conjugators, holographic information storage, beam switching and amplification, and intracavity modulation of lasers. Currently known laser-induced grating devices are one of two types, materials producing permanent holographic gratings, and materials producing fast transient gratings. Permanent holographic gratings have high light scattering efficiencies but slow response times, while fast transient gratings have low scattering efficiencies but fas response times. 
     Moreover, Four Wave Mixing (FWM) processes are currently of significant interest in science and technology because of their importance in modern optical applications such as phase conjugation, and also because they provide a powerful spectroscopic tool for probing the properties of the interaction of light and matter. 
     The physical processes underlying the laser-induced gratings that give rise to FWM signals can be classified in two categories according to their decay times after the laser writing beams have been turned off. The first category is that of transient gratings with fast decay times, which includes thermal gratings, population gratings, and nonlinear mixing due to third order susceptibility. The second category is permanent gratings which remain in a relatively permanent state after the laser writing beams have been turned off. The most common cause of this type of grating is the photorefractive effect involving ionization of a defect, charge migration and charge trapping. Permanent gratings are also referred to as holographic gratings because of their potential use in holographic information storage applications. 
     However, the prior art generally discloses and teaches gratings which are only transient gratings, and gratings which are only permanent holographic gratings, with each having its own attendant advantages and disadvantages, and has not heretofore disclosed a device which simultaneously provides a superposition of both types of gratings. Moreover, the prior art has generally provided permanent holographic gratings in a variety of crystalline host materials such as lithium niobate, but has not heretofore produced permanent holographic gratings in a glass host, except amorphous semiconductor films called chalcogenide glasses. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a primary object of the present invention to provide superimposed fast transient and permanent holographic gratings having both high scattering efficiency and the capability of fast modulation response times. 
     A further object of the subject invention is the provision of superimposed fast transient and permanent holographic gratings which significantly enhance the signal strength normally available from transient gratings while still providing a fast response time therefrom. 
     An additional object of the subject invention is the production of holographic gratings which are erasable, and can also be produced to have transient modulation written therein. The resultant holographic gratings produced by the present invention are relatively permanent, but can be photo-erased by a single erasing laser beam or can be thermally erased by raising the temperature thereof. 
     In accordance with the teachings herein, the present invention provides a method of producing a superimposed fast transient grating on a permanent holographic grating which provides high scattering ability, fast modulation response times, and the capability of writing and erasing signals therein, in which a glass host is doped with trivalent rare earth ions, and is placed in the crossed field of two coherent radiation beams to resonantly pump the trivalent rare earth ions at a frequency in resonance with one of the absorption transitions thereof. 
     The present invention has been proven to provide an arrangement for producing fast transient gratings superimposed on permanent holographic gratings in several types of oxide glasses doped with rare earth ions, particularly Eu 3+   ions in lithium phosphate, lithium silicate, and pentaphosphate glasses. 
     The present invention is believed to operate by providing a glass host having high frequency vibrational modes coupled to the localized vibrational modes of the rare earth ions. In view thereof, the teachings of the present invention are believed to be applicable to a wide variety of different types of glass hosts doped with trivalent rare earth ions including praseodymium ( 59  Pr), neodymium ( 60  Nd), promethium ( 61  Pm), samarium ( 62  Sm), europium ( 63  Eu), terbium ( 65  Tb), dysprosium ( 66  Dy), holmium ( 67  HO), erbium ( 68  Er), and thulium ( 69  Tm). 
     The combined gratings are produced by crossing two coherent laser beams in the sample at room temperature or below, at a frequency in resonance with one of the rare earth absorption transitions. The superimposed gratings are produced while the radiation beams are on, and after the beams are cut off, the fast transient grating disappear, leaving only the permanent holographic grating in the glass host, with any information which is written therein. Thus, the present invention also provides permanent holographic gratings in glass hosts, which may or may not have information modulated therein. 
     The superposition of transient and holographic gratings in doped glasses is technically significant as it presents a new mechanism for producing a FWM signal in a solid, provides a new technique for investigating local structural properties of glasses, and also produces doped glasses useful in applications based on FWM, such as phase conjugation, beam switching, and optical storage. The technological applications are particularly significant because of the production of a holographic carrier grating that can be amplitude or frequency modulated by a transient grating. 
     Moreover, the present invention has applicability in many different technological areas. The production of rare earth doped fiber lasers and all-optical switches represent two devices which normally must be joined together for systems applications (causing coupling losses at the interface). The present invention should allow construction of a monolithic device incorporating a fiber laser and an optical switch in one piece of glass, thus eliminating interface coupling losses. 
     The present invention also has applications as an interference filter of gratings in fiber optics. The mechanism of forming the grating involves selectively exciting rare earth doping ions, thus allowing control over the written information, which provides a great variety of applications in optical information processing such as in communications, optical signal processing (computing), sensors, guidance, etc. In addition, the glass nature of this new material makes it directly compatible with optical fibers, allowing the construction of the interference filter in the optical fiber itself. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing objects and advantages of the present invention for superimposed fast transient and permanent holographic gratings may be more readily understood by one skilled in the art with reference being had to the following detailed description of several preferred embodiments thereof, taken in conjunction with the accompanying drawings wherein: 
     FIG. 1 illustrates an optical arrangement for producing superimposed fast transient and holographic gratings in a glass host doped with trivalent rare earth ions wherein the glass host is placed in the crossed field of two coherent laser beams; 
     FIG. 2 illustrates the time dependences of the buildup, transient decay, and erasure of FWM signals in EPP glass at 300 K, with total power of the laser write beams at 80 mW and the laser erasure beam power at 40 mW; 
     FIG. 3 illustrates the temperature dependence of the holographic grating erasure decay rate in EPP glass for an erase beam power of 40 mW; 
     FIG. 4 shows the dependence of the FWM scattering efficiency for EPP glass on the crossing angle of the write beams at 300 K, in which the triangles represent the scattering efficiency from the holographic grating and the circles represent the scattering efficiency from the transient grating; 
     FIG. 5 illustrates configuration coordinate diagrams for the energy levels of two Eu 3+   ions in EPP glass with two possible local configurations; the transitions depict schematically the generation of high energy phonons from radiationless relaxation of an excited Eu 3+  ion, the change in local configuration of a Eu 3+   ion in the ground state by absorption of high energy phonons, and the thermal and laser erasure of the new configuration; the solid line represents the potential curves under normal conditions, while the dashed curves represent the potential curves perturbed by the gradient produced by crossed laser beams; and 
     FIG. 6 illustrates schematically how the index of refraction grating depth is modulated by the transient grating superimposed on a permanent holographic grating 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Referring to the drawings in detail, the present invention concerns structures having superimposed transient and holographic gratings leading to a FWM signal with enhanced scattering efficiency and the capability of amplitude or frequency modulation. These results have been obtained on several different types of Eu 3+  -doped glass samples wherein the transient signal is associated with a population grating of the Eu 3+  ions and the holographic grating is attributed to local structural modifications in the glass hosts. 
     Three different types of oxide glasses doped with several mole per cent Eu 2  O 3  produced strong signals in different embodiments of the present invention. A first embodiment comprises EuP 5  O 14  (EPP) glass containing 16.7 mole % Eu 2  O 3 . 
     FIG. 1 illustrates an optical arrangement for producing superimposed fast transient and holographic gratings in a glass host 10 doped with trivalent rare earth ions wherein the glass host is placed in the crossed field of two coherent laser beams. More specifically, FIG. 1 illustrates an arrangement for producing such superimposed gratings which can utilize a Spectra Physics cw argon laser 12 operating at 465.8 nm with a total power of 80 mW which is split into two beams 14, 16 which are weakly focused to diameters of about 1 mm and are crossed inside the sample 10 to form the gratings. A 10 mW He-Ne laser 18 can be used as the probe beam, and the diffracted signal beam can be detected by a Hamamatsu R1547 photomultiplier tube 20 after passing through a 0.25-m monochromator 22 to eliminate sample fluorescence. An EG&amp;G/PAR signal 24 averager can be used to process the signal before recordation by a strip chart recorder 26. 
     FIG. 2 illustrates the time dependences of the buildup, transient decay, and erasure of FWM signals in EPP glass at 300 K, with total power of the laser write beams at 80 mW and the laser erasure beam power at 40 mW. More specifically, FIG. 2 illustrates data obtained on EuP 5  O 14  glass demonstrating a typical buildup time for the signal of about 15 mins. using 80 mW of total laser power and a 2 mW He-Ne laser for the probe beam. This holographic grating is relatively permanent, but can also be erased. It can be photo-erased by a single laser beam or be thermally erased by raising the temperature well above room temperature. After writing the holographic grating, crossed laser beams in resonance with one of the rare earth absorption transitions will then produce a fast transient grating as shown. The grating decay time is the same as the fluorescence decay time of the rare earth ions. 
     In greater detail, FIG. 2 illustrates the time dependencies of the buildup and decay of the FWM signal in EuP 5  O 14  glass at room temperature. The time to reach the maximum FWM signal intensity is of the order of tens of minutes with the exact time depending on the laser power and wavelength. It is important that the laser wavelength be in resonance with a Eu 3+   absorption transition. This is consistent with the small values of the third order susceptibility tensor components measured by other prior art techniques, and demonstrates that a significant enhancement of the nonlinear optical properties of the material can be realized by resonantly pumping a rare earth ion absorption transition 
     When the writing beams are terminated, the FWM signal decays expontentially with a decay time of 2.6 ms, independent of the grating spacing. This is the same as the fluorescence decay time of 2.7 ms measured for the transitions from the  5  D 0  metastable state of Eu 3+   in this sample. However, the signal does not decay back to zero but rather to a constant level representing about 70% of the maximum signal. 
     The permanent FWM signal remains at the same high level for substantial periods of time, and has been verified experimentally for several days, and is believed substantially permanent. It can be erased by focusing a single laser beam on the same region of the sample in resonance with a Eu 3+   absorption transition. The erasure time is of the order of minutes, with the exact time depending on laser power and temperature and being independent of the writing beams crossing angle. 
     As illustrated by FIG. 2, the time dependence of the permanent signal erasure is highly nonexponential. However, a characteristic decay time equal to the e -1  value for the signal can be defined to describe the speed of the erasure under specific experimental conditions. 
     FIG. 3 illustrates the temperature dependence of the holographic grating erasure decay rate in EPP glass for an erase beam power of 40 mW. More specifically FIG. 3 illustrates the variation of the erasure decay rate versus temperature in the range 294 to 345 K. The erasure decay rate increases exponentially as the temperature is raised. This dependence can be described by an expression of the formula 
     
         K  EXP[-ΔE/k.sub.B T]                                (1) 
    
     where ΔE is the activation energy for the process and k B  is Boltzmann&#39;s constant. The slope of the curve in FIG. 3 gives an activation energy of 3,720 cm -1  for this case. Furthermore, the intensity of the FWM signal decreases as the temperature is raised above room temperature, and the permanent grating can be thermally erased by heating the sample to about 380° K. The temperature dependence of the scattering efficiency is consistent with a thermal activation energy of about 2,286 cm -1 . 
     FIG. 4 shows the dependence of FWM scattering efficiency for EPP glass on the crossing angle of the write beams at 300 K, in which the triangles represent the scattering efficiency from the holographic grating and the circles represent the scattering efficiency from the transient grating. The signal intensity, expressed in term of the scattering efficiency of the probe beam η, also varies with the crossing angle of the write beams as shown in FIG. 4. For both the permanent and transient signals, the scattering efficiency is largest at small crossing angles. This is typical behavior for FWM signals with the exact form of the curve depending on the coupling mechanism for the beam. The dashed lines in FIG. 4 represent best fits to the data points since the physical mechanism providing the beam coupling in this case is not well enough understood to allow true theoretical predictions to be developed. 
     The results presented herein illustrate that resonant excitation of Eu 3+   ions in this glass host results in a transient population grating of the Eu 3+   ions in the  5  D 0  metastable state and a permanent grating associated with local structural modifications of the glass host. The FWM signal is proportional to the square of the effective electric field induced in the material by the crossed laser beams. This can be expressed as the sum of the induced fields due to the population grating and the host grating, E eff    E pop  +E host . Since the first term decays with the fluorescence decay time of the  5  D 0  level, τ, and the second term is a constant, the time dependence of the signal when the write beams are turned off is given by 
     
         I.sub.sig  E.sup.2.sub.pop e.sup.2t/τ +2E.sub.pop E.sub.host e.sup.t/τ +E.sup.2.sub.hos.                           (2) 
    
     For this case the first term is much smaller than the last two and can be neglected as a practical matter. Equation (2) then predicts a signal that decays with the Eu 3+   fluoresence decay time down to a constant value proportional to E 2  host which is precisely the observed dependence shown in FIG. 2. Note that in the absence of the permanent grating, the signal due to the transient population grating will decay as e 2t/ τ  as observed in other resonantly pumped doped materials. 
     An important consideration concerns the physical mechanism producing the laser induced modification of the glass host giving rise to the permanent grating. Laser-induced refractive index changes in glasses have been observed for different types of glasses under different experimental conditions, but the previous results do not explain the results described herein. Tests have demonstrated that no permanent grating is established unless the Eu 3+   ions are directly excited with two crossed laser beams. The results observed thus far on the properties of the FWM signal strength and erasure rate for different glass samples, temperatures, and excitation conditions tend to eliminate mechanisms such as photoionization, bond rearrangements associated with trapped exciton effects, and thermally activated conformation changes, and indicates more specifically the mechanism shown schematically by a configuration coordinate diagram in FIG. 5. FIG. 5 illustrates configuration coordinate diagrams for the energy levels of two Eu 3+   ions in EPP glass with two possible local configurations. The transitions depict schematically the generation of high energy phonons from radiationless relaxation of an excited Eu 3+   ion, the change in local configuration of an Eu 3+  ion in the ground state by absorption of high energy phonons, and the thermal and laser erasure of the new configuration. The solid line represents the potential curves under normal conditions, while the dashed curves represent the potential curves perturbed by the gradient produced by crossed laser beams. 
     The mechanism giving the transient grating in this material is the excited state population distribution of Eu 3+   ions which have a different absorption and dispersion properties in the excited state than in the ground state, which is equivalent to an exciton grating in other materials. The mechanism for the permanent grating in this material is shown in Figure 5. Glass hosts have different possible local configurations of the atoms surrounding the doping ion and they can have different values for the refractive index. The heat generated through phonon relaxation processes causes the glass surrounding the Eu 3+  ions to go into a different configuration which remains when the write beams are turned off until enough thermal or optical energy is added to the system to allow the configuration to return to its original state. An equivalent holographic grating can be established in other materials by photoionizing a defect and allowing the charge to be trapped on a different defect thus producing a local change in the refractive index. 
     It is assumed that the network forming and modifier ions of the glass host can arrange themselves in two possible configurations in the local environment of the Eu 3+   ions, with each configuration resulting in a different index of refraction for the material. This produces double minima potential wells for the electronic states of the Eu 3+   ions as shown in Figure 5. Tunneling can occur between the potential wells, but under normal conditions of optical excitation and decay, the ions remain in the configuration represented by the lower energy potential curves. However, in the presence of the sinusoidal pattern created by the crossed laser write beams, there is a tendency for some of the ions to assume the configuration represented by the higher energy potential curves. One cause of this could be the generation of localized, high energy vibrational modes through the radiationless relaxation of the Eu 3+   ions in both the excited and ground state manifolds. These processes occur through &#34;multiphonon&#34; emission processes, and in glasses each process can generate several high energy phonons, approximately 1000 cm -1 . Because these are generated as local modes around the Eu 3+   ions, there is a high level of nonthermalized vibrational energy around each ion. This local &#34;effective temperature&#34; is easily enough to overcome the potential barrier for crossing into the higher minimum potential well configuration. Raman spectra and resonant Raman spectra show strong coupling of the Eu 3+  ions to high frequency vibrational modes in the glasses in which holographic gratings are produced and not in the other glasses investigated. 
     A single laser beam can create this local heating effect which enhances tunnelling between the potential wells of the two possible configurations. However, relaxation to thermal equilibrium will still result in predominant occupancy of the lower minimum potential well. Crossed laser beams create a population gradient of the high energy phonons causing a gradient in the local &#34;effective temperature&#34;. Describing the motion involved in the structural rearrangement of the atoms as an ionic conductivity process, the tendency of the vibrating atoms will be to diffuse away from the peak of the temperature gradient. This gives a directional bias to the hopping of the atoms between sites of different configurations, which can be schematically represented by a change from the solid to the dashed line potential curves shown in FIG. 5. Although this effect may be quite small, over the several minute time period of the grating buildup, it leads to an increased occupancy of the higher minimum potential well configuration than present under normal conditions. When the crossed laser beams are turned off, the excess population of the higher potential configuration will remain. 
     In the model described by FIG. 5, erasure occurs thermally by activation over the potential barrier of about 2,286 cm -1  in the ground state. Since higher multiplets of the  7  F term are located within this energy above the  7  F O  ground state, they may play some role in the thermal erasure process. Erasure with a single laser beam occurs with an activation energy of 3,720 cm -1 . It is difficult to associate this with a single potential barrier in FIG. 5 since it will involve the potential barriers in the  7  F O  ground state, the  5  D 2  state excited by the laser, and the  5  D 0  state level populated through radiationless relaxation of the rare earth ions in the excited state. Typical relaxation dynamics would result in the energy barrier in the  5  D 0  level making the dominant contribution to the thermal enhancement of the laser erasure, but contributors from thermal processes in other states cannot be ignored. 
     FIG. 6 shows schematically how the index of refraction grating depth is modulated by the transient grating superimposed on the holographic grating. The efficiency of four-wave mixing of laser beams depends on the square of the grating modulation depth. Thus by this technique we have produced an amplitude modulated signal beam riding on a strong carrier beam. As seen in FIG. 2, the holographic grating greatly enhances the signal strength normally available from the transient grating while the transient signal still provides a fast response time. 
     The mechanism described hereinabove provides a logical and viable explanation of the data presently available on laser-induced holographic gratings in doped glasses. 
     This observation of the superposition of transient and holographic gratings in doped glasses is important for several reasons: it represents the first observation of a new mechanism for producing a FWM signal in a solid; it provides a new experimental technique for investigating local structural properties of glasses; and it illustrates that doped glasses can be useful in applications based on FWM such as phase conjugation, beam switching, or optical storage. The technological applications are particularly significant because of the production of a holographic &#34;carrier&#34; grating that can be amplitude modulated by a transient grating. 
     The results of the present invention have been produced in two different types of phosphate and one silicate glass, all doped with Eu 3+ . The results have not been produced in Eu-doped borate, germanate, and fluoride glasses which have been tested, and the reason for this is not clear at the present time. 
     The unique method for producing these gratings is through resonant excitation of the rare earth ions. Although the exact mechanism for producing the refractive index change on the atomic scale is still not fully understood, it is believed to be associated with the generation of local modes of vibration through radiationless relaxation of the excited rare earth ions. These vibrational modes can cause local structural changes and experimental evidence supports this model. 
     While several embodiments and variations of the present invention for superimposed fast transient and permanent holographic gratings, and techniques for their production, are described in detail herein, it should be apparent that the disclosure and teachings of the present invention will suggest many alternative designs to those skilled in the art.