Abstract:
A method for the high power generation and detection of terahertz radiation is presented. It comprises of an optical waveguide with a core, and a mostly hollow cladding or terahertz wave transparent material surrounding the core. The cladding region is a terahertz waveguide. A pump light source is coupled to the core to promote nonlinear optical process, such as Raman scattering, in the core which in turn leads to terahertz radiation being emanated or received through fiber cladding.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This patent application claims the benefit of provisional patent application Ser. No. 60/566,351 filed 2004 Apr. 30. 
     
    
     FEDERALLY SPONSORED RESEARCH  
       [0002]     Not applicable  
       SEQUENCE LISTING OR PROGRAM  
       [0003]     Not applicable  
       BACKGROUND OF THE INVENTION  
       [0004]     The invention generally relates to the generation of a coherent optical source having its center frequency in the terahertz (i.e., far infrared) band. More particularly, the present invention relates generally to generation and detection of terahertz radiation using stimulated process such as Raman scattering (SRS) in an optical waveguide such as a fiber.  
         [0005]     Terahertz (THz) radiations or T-rays represent the last bastions of relatively unexplored electromagnetic spectrum. Residing somewhere between microwave and infrared, T-rays could have frequencies anywhere from 0.1 to 20 THz. What makes T-rays special is the potential application of this radiation in many military, security, commercial, biomedical, pharmaceutical and, scientific research. Among the prospective benefits, better detection of concealed weapons, hidden explosives and land mines; improved medical imaging and more productive study of cell dynamics and genes; real-time “fingerprinting” of chemical and biological terror materials in envelopes, packages or air; better characterization of semiconductors; and widening the frequency bands available for wireless communication. On medical front it has been shown T-rays can be used as a far superior tool for early detection of breast lumps compared with today mammogram examinations. T-rays also can penetrate skin tissues for early detection of skin cancer before the actual appearance of the legions on the skin. Using powerful tomography algorithm T-rays can map a 3-D image of human body with much higher resolution than Nuclear Magnetic Resonance (NMR) for early detection of diseases throughout the body. Terahertz imaging could reveal interesting features of the many materials with distinct absorptive and dispersive properties in this spectral range, which corresponds revealingly with bio-molecular vibrations.  
         [0006]     Within the next decade, x-ray imaging systems will be replaced by imaging systems using terahertz frequency sources and detectors in areas such as medical, security and quality control applications. T-rays can penetrate most solid substance like x-rays. In contrast to x-rays, T-rays are non-ionizing, and thus are non-lethal and safer for imaging applications. Further, T-ray systems produce true high resolution images rather than shadowy images produced by x-ray systems.  
         [0007]     A heavy demand for terahertz technology also exists in the communications industry. Development of components necessary for a terahertz frequency heterodyne receiver will result in a dramatic increase in the available bandwidth in wavelength-division-multiplexed communications networks.  
         [0008]     However, while the benefits of having T-rays have been established for over a decade, having a compact, powerful and, coherent source of T-ray has eluded technologist so far. Needless to say whichever applications may ultimately materialize, many will require high-average-power broadband or narrowband tunable terahertz light sources. Currently, there are two basic methods for generating terahertz (THz) beams: First, using photoconductors and second, using nonlinear optical frequency conversion techniques. In the photoconductive approach, electrically biased high-speed photoconductors are used as transient current sources for radiating antennas. These include dipoles, resonant dipoles, transmission lines, tapered antennas, and large-aperture photoconducting antennas. See for example a paper by Shuji Matsuura et al. “Generation of coherent terahertz radiation by photomixing in dipole photoconductive antennas”, Appl. Phys. Lett. 70(5), pp. 559, 3 February 1997. In the nonlinear optical frequency conversion approach, second-order or higher-order nonlinear optical effects in unbiased materials are used. See for example, U.S. Pat. Nos. 6144679,6697186, and 5543960.  
         [0009]     Optical rectification by far is the most important of these nonlinear optics techniques. The optical rectification method requires no electrical bias and thus is simpler than the photoconductive approach. In this method, the nonlinear material is illuminated with ultrashort laser pulses of order of fs, causing a time-dependent polarization to be created in the material by way of the electro-optic effect. This induced polarization is proportional to the intensity of the excitation pulse, and produces radiation of electromagnetic waves having a terahertz bandwidth. With a suitable electro-optic material, the amplitude of the resulting terahertz field is controlled by the intensity of the optical excitation beam. In turn, this intensity is a result of the pulsewidth, energy, and spot size of the beam. The problem with this method is that at high optical intensities, the efficiency of the rectification mechanism may decrease due to competing nonlinear effects. The incident optical intensity at which these competing mechanisms will occur is material dependent.  
         [0010]     It has to be noted here that no matter which techniques is chosen the average terahertz power generated usually resides on order of microwatts even with many watts of average input pump power. These low powers obviously limit potential use of THz wave in many applications. In addition, most if not all the mentioned schemes require cryogenic cooling which further limits their use in commercial applications. In a recent paper by Carr et al. of Jefferson Lab a record average power level of 20 watts for terahertz pulses has been reported. Yet again, it has to be noted, the generated terahertz power is over a broad band and extracting monochromatic bands out of this still results in low powers. Further, the generated terahertz waves are by negative charge electrons achieving relativistic velocity via large and long electron linear accelerators. Such linear accelerators use large cryogenically cooled superconducting magnets. Moreover, the effect of coherent emission on the electron beam could result in a self-amplified instability, resulting in large pulse-to-pulse variations, thereby limiting the potential application of this technique. Therefore it is not very likely that this type of technology be reducible to commercially viable devices in foreseeable future.  
         [0011]     The present invention has numerous advantages over the prior art which can easily lead to commercial and military utilization. First the size of the present invention is small compare to aforementioned methods since there is no need for an electron linear accelerator or an ultra-short pulse source such as Ti-Sapphire laser. Second the generated terahertz average power may reside in hundreds of milli-watts as compared to microwatts for equivalent input pump powers. Third the entire radiation may easily be collected or focused due to its high temporal and spatial fidelity. Fourth the present invention can achieve broadband or tunable narrowband operation depending upon mode of operation. Fifth, in fiber versions of the invention, the radiation may be directly guided to remote and hard to reach areas. Sixth, with the present invention, there is no need for any cryogenic cooling schemes. Finally, the present invention may be used as a high speed terahertz receiver with bandwidth measured in many of GHz, limited only by its associated electronic components.  
       SUMMARY OF THE INVENTION  
       [0012]     It is accordingly a primary object of the present invention to provide a method and means of generating a high power wideband or narrowband coherent radiation in a terahertz region of the electromagnetic spectrum. It is also object of the present invention to provide a method and means of detecting radiation in a terahertz region of the electromagnetic spectrum.  
         [0013]     The applicant provides a terahertz generator/detector by utilizing optical nonlinear process such as Stimulated Raman Scattering (SRS) in an optical waveguide. Another possible nonlinear process to generate terahertz radiation is self phase modulation (SPM). Elementary excitations (for example phonons, polaritons, excitons, magnons, etc.) may be present in any waveguide core medium. The core may be made of soft glass such as phosphate or silicate, fused silica, fused silica doped with P, fused silica doped with semiconductor nano-crystals such as Si, GaAs, and CdSe. The Waveguide may also be a semiconductor such as GaAs, InP or multi quantum wells. Continuous Wave (CW) or pulsed pump light with frequency ωpump impinges upon such medium experience Raman spontaneous scattering and it is manifested as sidebands in its spectrum. In thermal equilibrium only lower frequency sideband ωs called stokes field is prominent. The Sidebands are shifted frequency photons generated by refractive index of the medium being modulated by optical phonons with vibration frequency ωtera. The frequency range of the vibrations (the width of the ωs sideband, Δωs, or equivalently the width of the Raman spectra) depends on the materials and doping concentrations and can be engineered to reside anywhere in 0.1 to 20 THz. For instance, with light elements (Boron, Lithium) vibration frequencies are in 0-20 THz region, intermediate elements (Rare earths, Ti, V, Mn, Cu) in 5-10 THz region, while heavy elements (Pb, Bi, W, Au, OS) in a few THz territory. As the intensity of the impinge pump light is allowed to increases, the power of the stoke frequency ωs grows with it and their difference or beat frequency [ωpump−ωs] can further drive the molecular oscillation coherently at frequency ωtera=[ωpump−ωs]. This in turn modulate the refractive index more strongly and reinforces the sideband frequency power even further which leads to what is known as Raman gain or Stimulated Raman Scattering (SRS) process. In polar media, a media consist of both positive and negative ions, the resulting increase in amplitude of phonon-polariton oscillations gained through SRS process generates radiation at terahertz frequency ωtera (similar to classical radiating dipole antenna). This terahertz generation indeed may occur in any optical waveguide with polar medium such as a fused silica or a GaAs semiconductor during SRS process. However, the generated radiation may quickly be reabsorbed if surrounding medium, for example the cladding, is not transparent to terahertz radiation. This is indeed the case for a conventional communication grade optical fiber for instance. It is the result of this re-absorption process that terahertz radiation is not generally observed in output of optical fibers or waveguides.  
         [0014]     Therefore, it is the object of this invention to provide a waveguide that its cladding is substantially transparent to terahertz radiation or devoid of materials to prevent terahertz re-absorption. Alternatively we can think of the cladding region as a waveguide itself for transport of terahertz radiation. To avoid confusion, we refer to the core as an optical waveguide while the cladding would be referred as a terahertz waveguide.  
         [0015]     One such optical waveguide is an air-clad fiber with a cow-web cladding structure. In this design the cladding is largely hollow or filled with air, thus transparent to terahertz wave, and the core is made with polar medium with high level of nonlinearity such as fused silica or soft glass to promote high degree of SRS. The core location could be centered or off-centered respect to mostly terahertz transparent cladding region. Any generated terahertz radiation mode from the core gains power as it travels along the core and would become bound and “gain guided” with its spatial extent emanating from the core into mostly hollow or terahertz transparent cladding. The materials surrounding the cladding may be made of polymers such as polyethylene to be transparent to waves in terahertz region. In this case the terahertz generated mode may be “index guided” as well as gain guided. Alternatively, the surface materials surrounding the cladding may be made to reflect waves in terahertz region for instance with metallic coatings. In this case the generated terahertz radiation mode would be “reflection guided” as well as gain guided. The terahertz radiation would then exit the fiber cladding at output end, the terahertz waveguide region. The typical length of mentioned fiber may be in neighborhood of few 100 meters to achieve good SRS. The core to cladding diameter ratio of such a fiber is proportional to ratio of optical to terahertz wavelengths.  
         [0016]     Alternatively, a semiconductor optical waveguide may consist of a core surrounded by a nearly hollow or terahertz transparent cladding region with typical length of 10&#39;s of millimeters, due to 1000-10000 greater non linearity and thus SRS magnitude. The materials surrounding the cladding may be made of polymers such as polyethylene to be transparent to waves in terahertz region. Alternatively, the surface materials surrounding the cladding may be made to reflect waves in terahertz region for instance with metallic coatings. Other options for core of the semiconductor waveguide are III-V semiconductors (such as GaAs, InP, InGaAs, InGaAsP, etc.) or II-VI semiconductors (such as CdS, CdSe, etc.) and multi quantum well structures made from III-V and II-VI semiconductor systems.  
         [0017]     Another structure of interest is a Polarization Maintaining (PM) waveguide. The fiber version may be realized for instance by an oval shape core for linear polarization maintaining. This allows only linear polarization mode to form provided pump light is also linearly polarized and it is coupled to either fast or slow axis of the oval shape core. In this case the generated terahertz radiation emanating from the hollow cladding is also linearly polarized. Alternatively a circularly polarization maintaining fiber type is also possible if the fiber pre-form is spun as the core is drawn during manufacturing process. The generated terahertz wave in this case is circularly polarized provided pump light is also circularly polarized as it is coupled to the fiber core.  
         [0018]     The two types of waveguide or fiber discussed above can also be made with more than one core such as a multi core structure. Each core may have the same or different materials each with different dopants embedded in them. Additionally, each core may have pump light with identical or different wavelengths resulting in single or multi-frequency terahertz radiation. The advantage of multi core structure is first, to increase the output power of the generated terahertz radiation and second, to increase the emitted bandwidth of the generated terahertz emanating from largely hollow cladding. This is important when an ultra wideband high power terahertz radiator is desired.  
         [0019]     To generate a narrow band monochromatic coherent terahertz radiation a second light source such as a laser, in addition to the pump source, may be coupled to the core from the same or opposite end of the waveguide. The frequency of this source may be selected to coincide with frequency ωs of generated shifted photons and may be tuned under the Raman gain spectra. The reason for narrowband tendency in this case stems from the fact that only shifted photons at specific frequency ωs, residing within Δωs width, are allowed to gain power by robbing more energy from the pumping light. As a result, the shifted photons with pre-select frequency ωs grow in intensity and lead to stronger pre-select beat frequency ωtera=[ωpump−ωs] which in turn drives the oscillating dipoles at frequency ωtera more forcefully to radiate narrow band higher power terahertz wave. Indeed this embodiment that radiates narrow band coherent terahertz wave may be interpreted as a terahertz fiber or waveguide laser. Another advantage of using the second optical source is to make the invention a tunable terahertz generator. By tuning or shifting the frequency of the second source ωs under the Raman gain spectral width Δωs, the resulting beat frequency, ωtera, also changes and forces the center of generated terahertz frequency to shift accordingly.  
         [0020]     Another way to generate a narrow band terahertz generation without the need for the second optical source is by a pair of Waveguide Bragg Gratings (WBG), one close to the input and other close at the output, along the core of the waveguide types discussed above. Such WBG can be imprinted or written directly in the core using well established techniques such as using appropriate phase masks and illuminating it with ultraviolet laser light. The center reflection frequency band of the grating is set at ωs, the frequency of stokes shifted photons in the medium. The reason for narrowband tendency in this case stems from the fact that only shifted photons at specific frequency ωs, residing within Δωs width, are allowed to reflected back and forth in the fiber core and robbing more energy from the pumping light. As a result, the shifted photons with pre-select frequency ωs grow in intensity and lead to stronger beat frequency ωtera which in turn drives the oscillating dipoles at pre-select frequency ωtera more forcefully to radiate narrow band higher power terahertz wave. Another advantage of adding WBG pair is to make the invention a tunable terahertz generator. By tuning or shifting the frequency of the pumping source, ωpump, or tuning the center frequency of the WBG, the resulting beat frequency, ωtera=[ωpump−ωs], also changes and forces the center frequency of generated terahertz to shift accordingly.  
         [0021]     The technique my also provide a multiple frequency line terahertz generator. If the intensity of the Raman shifted photons at frequency ωs=[ωpump−ωtera] becomes high enough, a secondary Raman process starts with generation of photons of frequency ωs−ωtera=[ωpump−2 ωtera] and accompanying coherent phonons. This secondary Raman process may be reinforced by addition of more pair of gratings at fiber ends as described above with center reflection frequency band of [ωpump−2 ωtera]. Therefore, a second pair of WBG, again one grating close to input and the other close to output end of the fiber or waveguide, may be placed along the core to promote the generation of secondary sidebands in the SRS spectrum. In general N pairs of gratings may be added with reflection band frequency set at [ωpump−N×ωtera] resulting in many sidebands but with frequency difference of ωtera between them. The beat frequency among any two adjacent sidebands would drive the oscillating dipoles with frequency ωtera to generate radiation in terahertz. The technique to generate multiple frequency line by SRS process by placing appropriate pairs of WBG is known as “Cascaded Raman” in the literature. See for example a paper by M. Dianov, et al, “Three-cascaded 1407-nm Raman laser based on phosphorus-doped silica fiber”, Optics Letters/Vol. 25, No. 6/pp. 402, Mar. 15, 2000. Therefore we have utilized the Cascaded Raman scheme in our favor to generate multiple frequency line terahertz radiation.  
         [0022]     Yet as mentioned previously, it is another goal of this invention to provide a method and means for detection of terahertz radiation. The basic waveguide or fiber structure that has just been discussed for generation of terahertz wave may be used in reverse, for detection of such waves.  
         [0023]     Again we assume the waveguide cladding is mostly hollow or made of material that is largely transparent to terahertz radiation. If waveguide or fiber core is being optically pumped, either co- or counter-propagating to the direction of incoming terahertz wave, some power is converted to frequency ωs. As incoming terahertz radiation at frequency ωtera is collected and focused into the hollow cladding, a portion of the wave is absorbed by oscillating dipoles of the core medium. Absorption causes the amplitude of oscillating dipoles to grow which in turn leads to modulation of the index of refraction of the material and leads to an increase in stokes component power. The increase in power of the sideband frequency ωs may be then used as a measure of terahertz wave presence. Due to sub-picosecond response of dipoles in core media, the detection scheme may be capable of ultra high speed operation with bandwidth of several terahertz.  
         [0024]     For practical considerations for reception, it is desirable to have a terahertz detection device with its intake free from any obstruction such as a pumping light source. It is therefore beneficial to have the pump light and incoming terahertz waves coupled in opposite side of the hollow waveguide. Therefore, one end of the waveguide may be utilized as intake to receive terahertz wave while the opposite end may be used for core pumping. On the pump side, we can use a wavelength division multiplexer (WDM) coupler to divert the shifted ωs photons to a power monitoring port. This power monitoring port may include an optical band-pass filter with centered band-pass frequency of ωs and an optical power meter (a low speed optical to electrical converter) or a high speed receiver for data receptions. In order to improve pump conversion efficiency, one may place a waveguide Bragg Grating (WBG) close to waveguide intake end, with center reflection frequency of ωpump. The pump light is then allowed to travel the length of the waveguide twice for more efficient pump conversion. Furthermore by addition of second Bragg Grating at the same intake end (next to first WBG) with center reflection frequency of ωs, one can collect more shifted photons hence improving the terahertz detection. This is due to recycling of stokes shifted photons at frequency ωs toward the WDM coupler and re-routing it to the receiver. However this improvement in detection efficiency may be accomplished at the expense of terahertz bandwidth response caused by round trip time delay of reflected stokes shifted photons.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]     Other objects, features, and advantages of the invention will be apparent from the following description taken together with the drawings in which:  
         [0026]      FIG. 1  illustrates a basic wideband terahertz generator waveguide.  
         [0027]      FIGS. 2   a  through  2   j  depict various preferable core/cladding structures of the terahertz generator waveguide.  
         [0028]      FIG. 3  illustrates a narrowband tunable terahertz generator with addition of a second optical light source to embodiment of  FIG. 1 .  
         [0029]      FIG. 4  depicts another possible narrowband terahertz generator waveguide utilizing a pair of WBG to embodiment of  FIG. 1 .  
         [0030]      FIG. 5  shows the terahertz generator waveguide with addition of a second pair of WBG to embodiment of  FIG. 4  for multiple frequency coherent terahertz generation.  
         [0031]      FIG. 6  shows a multi-core terahertz generator waveguide structure utilizing incoherent pumping scheme.  
         [0032]      FIG. 7  illustrates a multi-core terahertz generator waveguide structure utilizing coherent pumping scheme.  
         [0033]      FIG. 8  shows a basic wideband terahertz wave receiver waveguide.  
         [0034]      FIG. 9  illustrates a terahertz wave receiver waveguide with reduced pumping threshold as compared to  FIG. 8  receiver.  
         [0035]      FIG. 10  shows a terahertz wave receiver waveguide with enhanced terahertz detection as compared to  FIG. 8  receiver.  
         [0036]      FIG. 11  depicts a terahertz wave receiver waveguide with simultaneous reduced pumping threshold and enhanced detection as compared to  FIG. 8  receiver. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0037]      FIG. 1  constitutes the most basic terahertz wave generator. In  FIG. 1 , core  11  is an optical waveguide surrounded by a terahertz waveguide  12 , a substantially hollow cladding or terahertz transparent region. In addition, the cladding  12  may be surrounded by substantially terahertz transparent material  10 . The surface  8  surrounding cladding  12  may be reflective or transparent to terahertz radiation. Pump source  13  provides optical pumping to core  11  through optical connection  14 . Source  13  may be a laser. Further, source  13  may be a tunable laser source. Optical connection  14  may be an optical fiber with a core size compatible with core  11  for optimum coupling efficiency. Alternatively connection  14  may represent a free space focused light coupled into core  11  from pump source  13 . The shading  17  represent the increase in generated terahertz wave as light from source  13  is being converted to stokes shifted photons by the core medium  11 . Light  16  emerging from the core  11  includes the unconverted portion of the pump light  13  and generated stokes component. Insert  1  illustrates the spectrum of the pump light  13  at input as  13   i  before being coupled to core  11  through connection  14 . Insert  2  shows the output light spectrum  16  exiting core  11 . The spectrum at output reveals the unconverted portion of the pump light as  13   t  and stokes component or stimulated Raman gain spectra  2 A with its associated spectral width  2 B. Radiation  15  represents the generated wideband terahertz wave dictated by spectral width  2 B emerging from the substantially hollow or transparent cladding  12 , the terahertz waveguide output.  
         [0038]      FIG. 2  illustrates a cut away cross sectional view of several preferred core/cladding geometries of the waveguide. Without loss of generality, and in order to avoid clutter, not all figures have been numbered.  FIG. 2   a  shows core  11   a , the optical waveguide, at the center surrounded by mostly hollow cladding or terahertz transparent  12   a , the terahertz waveguide. The region  10   a  may be made of dielectric material that is transparent to terahertz radiation or the surface  8   a  that surrounds the cladding  12   a  may be coated to reflect terahertz radiation. Core  11   a  may be supported in a hollow cladding  12   a  structure by terahertz transparent support membranes  9   a . The total number or shape of supports is immaterial as long as they hold the core  11   a  in the cladding  12   a  successfully. For instance, in  FIG. 2   a  eight supports,  FIG. 2   c  two, and  FIG. 2   f  only one support is shown. In  FIGS. 2   b  and  2   d  the supports  9   b  and  9   d  holding cores  11   b  and  11   d  respectively, has a cow-web shape.  FIG. 2   d  shows core  11   d  with an oval shape to promote polarization maintaining operation.  FIGS. 2   g  and  2   h  show a multi core structure of the present invention.  FIG. 2   g  shows three close-to-center cores while  FIG. 2   h  shows eight off-centered cores. In  FIGS. 2   h  and  2   i  there are no supports for the off-centered cores since they are positioned on the rim surfaces of the cladding  12   h  and  12   i  respectively.  FIG. 2   j  illustrates a special case where the core is only surrounded by cladding  12   j . Cladding  12   j  is not hollow but made of substantially terahertz transparent media. Here the cladding  12   j  captures most of the terahertz wave.  
         [0039]      FIG. 3  depicts the embodiment of the  FIG. 1  but with addition of a second light source  6 . Light source  6  may be a laser and further it may be tunable. Source  6  and pump  13  are both coupled to core  11  through coupler  7  and optical connection  14 . Coupler  7  may be a fiber-optic or a free space combiner. In this scheme 50% of light from each source namely  13  and  6  are lost. To avoid this loss, coupler  7  may be a fiber-optic or free space wavelength division multiplexer (WDM) to combine both sources  13  and  6  together without much loss. Insert  3  shows the output spectrum of the light  16  exiting the core  11 . The spectrum reveals the unconverted portion of the pump light as  13   t  and generated stokes component or stimulated Raman gain spectra  3 A. Radiation  15  represents the generated narrowband coherent terahertz wave emerging from the mostly hollow or transparent cladding  12  output at frequency ωtera. As the center frequency of the source  6  depicted in insert  3  as  6   t  (or center frequency of pump light  13 ) is varied within the spectral width  3 B the center frequency of the generated terahertz wave  15  is also changes according to ωtera=[ωpump−ωs]. Therefore this embodiment is a tunable narrowband terahertz generator.  
         [0040]      FIG. 4  illustrates another embodiment of the present invention. A pair of WBG  20  with center reflection frequency of ωs has been added close to intake and outtake of core  11  of  FIG. 1  respectively. The shifted photons are then allowed to re-circulate in the core  11  as indicated by arrows  22  and  23 . Insert  4  shows the output spectrum of the light  16  exiting the core  11 . The spectrum reveals the unconverted portion of the pump light as  13   t , stokes component or stimulated Raman gain spectra  4 A, Raman gain spectral width  4 B, and center reflection frequency  20   t  of the WBG pair  20 . Radiation  15  represents the generated narrowband coherent terahertz wave emerging from the mostly hollow or transparent cladding  12  output at frequency ωtera. As the center frequency of the pump source  13  (or center reflection frequency of WBG pair  20 ) is varied, the center frequency of the generated terahertz wave  15  is also changes according to ωtera=[ωpump−ωs]. The degree of tuning is dictated by the Raman gain width  4 B. Therefore this embodiment is also a tunable narrowband terahertz generator.  
         [0041]      FIG. 5  shows a second pair of gratings  25  has been added to embodiment previously depicted in  FIG. 4 . The center reflection frequency of the new WBG pair  25  is set at [ωs−ωtera]. Insert  5  shows the output spectrum of the light exiting core  11 . As it is clear the secondary stokes component  5 A with its spectral width  5 B is also present as compared with insert  4 . The arrows  28  and  29  indicate the second order generated shifted photons  25   t  at frequency [ωs−ωtera] are allowed to re-circulate in core  11 . This promotes a coherent multiple narrowband frequency terahertz operation of ωtera and 2ωtera. Radiation  15  represents the generated terahertz wave emerging from the mostly hollow or terahertz transparent cladding  12  at waveguide output. As the center frequency of the pump source  13  (or center reflection frequency of WBG pair  20  or  25 ) is varied the center frequency of the generated terahertz wave  15  is also changes. Again the degree of tuning is dictated by the spectral width  5 B (or  4 B). Therefore this embodiment is a tunable multi frequency line coherent terahertz generator.  
         [0042]      FIG. 6  shows a multi-core embodiment of the present invention. Fiber or waveguide  10  is shown with three cores  11 A,  11 B and  11 C. Each core is being optically pumped with three different laser sources  13   a ,  13   b  and  13   c  through connections  14   a ,  14   b  and  14   c . Again  14   a ,  14   b  and  14   c  may represent fiber to core  11 A,  11 B and  11 C connections respectively. Alternatively  14   a ,  14   b  and  14   c  may represent free space focused lights from sources  13   a ,  13   b  and  13   c  being coupled to cores  11 A,  11 B and  11 C respectively. The pump sources  13   a ,  13   b  and  13   c  may have the same or be tuned at different frequency. In this embodiment the terahertz generated wave contribution from each core  11 A,  11 B and  11 C can add up incoherently. This arrangement is an example of incoherent pumping. Although not shown in the  FIG. 6  each core  11 A,  11 B and  11 C may have pair of WBG, close to intake and outtake, as it was discussed in  FIGS. 4 and 5  for single or multi line frequency terahertz generation. Radiation  15  represents the generated terahertz wave emerging from the mostly hollow cladding  12  output.  
         [0043]      FIG. 7  shows a multi-core structure with pump sources that are coherently locked in frequency respect to each other. In this embodiment the terahertz generated wave contribution from each core  11 A,  11 B and  11 C may add up coherently. This is an example of multi-core structure with coherent pumping. The laser light from pump source  13  is being fed to three optical amplifiers  13   a ,  13   b  and  13   c  through splitter  30 . Splitter  30  may be a 1×3 fiber-optic splitter/coupler. Although not shown, the splitter  30  may also represent a 1×N splitter/coupler, in case of a multi-core waveguide  10  structure with N cores where N is any integer larger than 1. The light from each amplifier enters the corresponding cores  11 A,  11 B and  11 C through connections  14   a ,  14   b  and  14   c  respectively. The amplifiers could be a semiconductor or fiber type amplifier. Furthermore, to increase the pump power,  13   a ,  13   b  and  13   c  may be double-clad high power fiber amplifiers. Each core  11 A,  11 B and  11 C may have one or more pair of WBG as it was discussed in  FIGS. 4 and 5  for single or multi frequency line terahertz generation. Radiation  15  represents the generated terahertz wave emerging from the mostly hollow cladding  12  at output. As the center frequency of the pump source  13  (or center reflection frequency of WBG) is varied the center frequency of the generated terahertz wave  15  is also changes. Therefore this embodiment is a tunable terahertz generator.  
         [0044]     In  FIG. 8 , Core  35 , an optical waveguide, surrounded by a terahertz waveguide  32  that is a substantially hollow cladding or substantially terahertz transparent region constitutes the most basic terahertz receiver. In addition, the cladding  32  may be surrounded by substantially terahertz transparent material  33 . The surface  30  surrounding cladding  32  may be reflective or transparent to terahertz radiation. Incoming terahertz radiation  31  that is to be detected is focused at waveguide  33  intake into cladding  32 . At waveguide  33  outtake, pump light  34  is coupled to core  35  through connections  42  and  40  of WDM device  36 . Pump light  34  may be a laser. Additionally pump  34  may be a fiber pigtailed device with its core size compatible with core  35  for best fiber to waveguide light coupling efficiency. WDM device  36  may be a fiber optic coupler with its fiber core size also compatible to both core  35  and pump source  34  for best coupling efficiency. WDM coupler  36  connects pump light  34  with frequency ωpump through the first port connection  42 , to core  35  through its second port, connection  40 , while directs any light with center frequency ωs to power meter  37  through its third port, connection  41 . Alternatively if pump source  34  is not a fiber pigtailed device then the light from the source can be focused into core  35  through WDM device  36 . WDM device in this case may be a bulk optics element such as a fabry-perot or an interference filter. Again the device  36  allows the light from pump source  34  with frequency ωpump to pass through while it reflects any light with frequency ωs to power meter or receiver  37 . Receiver  37  is an optical to electrical converter detector. Through SRS process pump  34  causes stokes shifted frequency photons with frequency ωs to be generated which then may propagate in co or counter pump direction respect to pump  34  in core  35 . As terahertz radiation  31  propagates inside cladding  32  it is partially absorbed by core  35  which in turn steals more power from the pump  34  and add it to stokes shifted photons with frequency ωs. This causes an increase in power level of stokes shifted photons. This increase is monitored at outtake by power meter  37  through connection  41  as detection or presence of terahertz wave  31 .  
         [0045]      FIG. 9  shows another improved receiver structure of  FIG. 8 . By placing a WBG  38  with center reflection frequency ωpump in core  35 , at waveguide intake, we may re-circulate the pump  34  photons depicted by the arrow  43  for better pumping conversion efficiency. This may reduce the requirement for pumping the core  35  with high power source for SRS generation.  
         [0046]      FIG. 10  shows yet another improved receiver structure of  FIG. 8 . By placing a WBG  39 , in core  35 , at waveguide intake and with center reflection frequency ωs, the generated co pump propagating stokes shifted photons may also be re-routed back, depicted by the arrow  44 , to outtake and to power meter  37  via WDM coupler  36 . Therefore both the power of generated co and counter propagating shifted photons with frequency ωs may be detected by power meter  37 . This would enhance the terahertz detection threshold.  
         [0047]      FIG. 11  shows a more improved receiver structure respect to  FIG. 10 . By placing both the WBG  38  and WBG  39  in core  35 , at waveguide intake and with center reflection frequency ωpump and ωs respectively, the generated co pump propagating stokes shifted photons may also be re-routed back, depicted by the arrow  43 , to outtake and to power meter  37  via WDM coupler  36 . Therefore both the power of generated co and counter propagating shifted photons with frequency ωs may be detected by power meter  37 . WBG  39  would enhance the terahertz detection threshold as explained previously for  FIG. 10  structure. Further WBG  38  may reduce the requirement for pumping the core  35  with high power source for SRS generation as explained previously for  FIG. 9  embodiment.