Abstract:
An industrially hardened terahertz electromagnetic transmitter and receiver module ( 29 ) is disclosed. The electromagnetic wave module has an optic ( 30 ) which relays an optical pulse from the delivery fiber ( 32 ) to the terahertz device. The relay optic ( 30 ) allows for a greatly reduced optical spot size as compared to the output of the optical fiber. Thus, the sensitivity of the overall system is enhanced by improving the efficiency of the terahertz device. The relay optic ( 30 ) allows the small spot of light to be aligned to the electromagnetic transmitter or receiver with sub-micron precision.

Description:
This application is a filing under 35 U.S.C. 371, which claims priority to International application Ser. No. PCT/US00/41172, filed Oct. 16, 2000, which claims the benefit of U.S. Provisional Application No. 60/159,358, filed Oct. 14, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a terahertz transmitter or receiver module More specifically, the present invention relates to a robust modularly packaged terahertz transmitter and receiver module. 
     BACKGROUND OF THE INVENTION 
     The present invention is concerned with the generation of terahertz electromagnetic radiation by a pulsed laser in a commercially packaged system In previous applications such as in a lab environment, a laser can be pointed directly through space at an optical switching element with negligible dispersive effects. To allow the commercial use of such a system the present invention must be industrially hardened and packaged. A laser pulse in a room environment may be deflected by objects or people and will suffer degradation from atmospheric effects, unacceptable conditions in an industrial environment. The laser must also be realigned constantly due to environmental effects on the material properties of the alignment mechanisms. By incorporating optical fiber cable and rugged packaging in the present invention, the laser light is given a predetermined path of travel and allows the present invention to be precisely aligned, ruggedly seated, and bundled into compact form. A ruggedly packaged, fiber-delivered, terahertz system allows people unfamiliar with the setup, alignment, or adjustment of ultrafast lasers, semiconductor physics, and optics, to use a time-domain terahertz system for experiments and applications outside the lab environment. 
     Another advantage of the fiber-delivered terahertz system is the ease with which the system can be reconfigured for use in either transmission or reflection experiments. Presently, this type of reconfiguration takes days. With the system of the present invention reconfiguration takes minutes. The terahertz transceivers, in particular, need to be built using advanced telecommunication packaging techniques in order to build these units with sufficient precision and maintain their ruggedness to such that they may be used in an industrial environment By directing short (&lt;1-ps) pulses of light to the substrate by using a fiber-delivery system, we allow for the terahertz transceivers to be freely positioned As discussed previously, present time-domain terahertz and frequency-domain terahertz systems are usable only in the research laboratory By using fiber optic packaging techniques, we are able to make these devices manufacturable and usable by people outside the research community. The basic concept compnses anchoring a fiber near the terahertz transmitter and/or receiver, giving the present invention a substantial advantage over previous free-space systems. 
     However, there are some drawbacks to simply butting the fiber up to the terahertz transmitter or receiver device First, the generated terahertz radiation couples into the high dielectric substrate material preferentially over air, thus improving the efficiency of the emitter if the fiber is butted up to the substrate, radiation would be coupled into the fiber, away from the emission aperture, reducing efficiency Also, the size of the beam of light emitting from the end of a single-mode fiber is about 5 μm or larger. This is too large to adequately generate or detect the terahertz radiation. 
     Another difficulty of present terahertz systems is the difficulty in aligning the optical axis (comprised of the optical fiber and the terahertz element) and the terahertz optics (comprised of the terahertz element and the attached hemispherical lens) The hemisphencal optic is either aplanatic or collimating as disclosed in U.S. Pat. No. 5,789,750, expressly incorporated by reference herein. It should be noted that this lens can be made from any number of materials that are effective at this wavelength regime Some examples are high-resistivity silicon (&gt;1 kΩ-cm), alumina, sapphire, or even polyethylene Furthermore, this lens can be anti-reflection coated to enhance terahertz output using a number of materials including parylene. 
     The new and improved system of the present invention solves these and other problems found in the prior art as will be illustrated and discussed hereinafter. 
     SUMMARY OF THE INVENTION 
     The present invention provides an intermediate or relay optic (GRIN or other focusing element coupled to the optical pulse delivery fiber) that allows for an adjustable optical spot size, which enhances the sensitivity of the overall system by improving the efficiency of the terahertz transmitter and the receiver. This spot of light must be aligned to the terahertz transmitter or receiver device with sub-micron precision. By using the relay optic we obtain a lever arm on this alignment, effectively increasing the accuracy by a factor proportion to the magnification of the relay optic. That is, the lens transforms movement of the optical fiber into a smaller movement of the focused optical spot. 
     The alignment problem found in the prior art is solved by the present invention, for example, by using mounting plates made of a maternal similar to the lens material. The terahertz element is mounted onto a window mounting plate using alignment marks (or fiducials) that are micro-fabricated onto the plate, and the relay optic and optical fiber are mounted to an optic mounting plate. Once assembled, both subassemblies can be aligned independent of the other. By carefully designing these various elements the entire system becomes much more manufacturable and rugged than previously obtainable. The use of the mounting plates also makes it easier to environmentally seal or hermetically seal the terahertz transmitter or receiver package. The mounting plates could be made of alumina a material compatible with such a process, while the lens could be made of any other material, and with any other optical design, that would be appropriate for the application at hand. 
     Moreover the present invention includes the use of a fiber to deliver short optical pulses to a terahertz transmitter or receiver More specifically, the invention uses a fiber, along with an intermediate optic, to deliver a focused beam of short (&lt;1-ps) optical pulses to a terahertz device. This device is the element containing the active area or volume in which the delivered light power either (1) interacts to produce out-going terahertz electromagnetic radiation, or (2) responds with in-coming terahertz radiation to produce an electrical signal or alter the optical beam in a measurable manner. In the first case, the device is a transmitter, and in the second, it is a receiver This terahertz device can be either a photoconductive element such as that disclosed in U.S. Pat. Nos. 5.729,017, 5,420,595 and 5,663,639 expressly incorporated by reference herein, or an electroptic or magneto-optic device such as those disclosed in U.S. Pat. Nos. 5,952,815 or 6,111,416 expressly incorporated by reference herein. 
     Further objects and advantages of the present invention will become apparent by reference to the following description of the preferred embodiment and appended drawings wherein like reference numbers reflect the same feature, element or component 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic overview of a terahertz electromagnetic radiation emission and detection system of the present invention; 
     FIG. 2 is an exploded isometric view of an embodiment of the terahertz transmitter and receiver module, in accordance with the present invention, 
     FIG. 3 a  is an assembled isometric view of an embodiment of the terahertz transmitter and receiver module, in accordance with the present invention; 
     FIG. 3 b  is an assembled isometric view of an alternate embodiment of the terahertz transmitter and receiver module, in accordance with the present invention; 
     FIG. 4 is a plan view of the terahertz transmitter and receiver module, in accordance with the present invention; 
     FIG. 5 is an isometric view of the photoconductive device, in accordance with the present invention; 
     FIG. 6 illustrates the mounting plate for the relay optic and optical fiber to be used in the preferred embodiment of the present invention; 
     FIG. 7 a  is a perspective view of the mounting plate for carrying the photoconductive device to be used in the preferred embodiment of the present invention; 
     FIG. 7 b  is a perspective view of the photoconductive device assembled to the mounting plate to be used in the preferred embodiment of the present invention; and 
     FIG. 8 is a diagrammatic overview of an alternate embodiment of the terahertz electromagnetic radiation emission and detection system of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a diagrammatic overview of one embodiment of an electromagnetic wave, such as a terahertz wave, generating and detecting system of the present invention. The system includes a pulsed Ti:sapphire laser  16  coupled by a fiber optic cable  18  to a terahertz transmitter  20 , optical delay  22 , terahertz optical system  27  and terahertz receiver  24 . Alternatively, any pulsed laser which is capable of producing an optical pulse of less than one picosecond in duration may be substituted for the pulsed Ti:sapphire laser  16  For example, the lasers described in U.S. Pat. No. 5,880,877 and lasers such as a Ti:Sapphire laser, a Cr:LiSAF laser, a Cr:LiSGAF laser, a Cr:LiSCAF laser, an Er-doped Fiber laser, an Yb-doped fiber laser and gain switched diode laser are appropriate substitutes for pulsed sapphire laser  16 . Moreover, the present invention is usable with a continuous wave source as presented in U.S. Pat. No. 5,663,639, expressly incorporated herein by reference. The terahertz transmitter  20  generates THz radiation that propagates through the first part of a terahertz optical system  27 , a sample  26 , a second part of a terahertz optical system  27  and is received by terahertz receiver  24  which outputs a signal proportional to the received THz radiation. The optical delay  22  determines which temporal portion of the signal is gated by the pulse at the terahertz receiver  24 . The optical delay is controlled by a computer/controller  28  that further receives the output signal of the terahertz receiver  24 . The terahertz optical system  27  can be of any kind described in U.S. Pat. No. 5,789,750 expressly incorporated herein by reference. 
     FIGS. 2 through 4 illustrate embodiments of the terahertz transmitters and receivers  20  and  24  used in, for example, the system described above, in accordance with the present invention The component content and configuration of transmitters and receivers  20  and  24 , as will be described hereinafter and indicated by reference numeral  29 , is the same whether the device is used as an electromagnetic wave transmitter or receiver. 
     With specific reference to FIG.  2  and FIG. 5 a terahertz device  36  is mounted within device  29  for generating or detecting the electromagnetic radiation. The terahertz device  36  has a pair of electrodes  67  and  69  bonded to a low-temperature-grown Gallium Arsenide semiconductor substrate  68  or other suitable substrate material (as shown in FIG.  5 ). 
     With continuing reference to FIG. 2, device  29  further includes a relay optic  30 , such as a GRIN lens, which serves the dual purpose of making the device easier to manufacture and also helps focus the output of optical fiber  32  down to the optimal spot size. Furthermore, the relay optic  30  (or other intermediate optic) removes the fiber  32  from the immediate vicinity of the terahertz device  36 , which in the case of the transmitter, could cause the emitted terahertz radiation to couple into the fiber  32  rather than into the transmitter substrate. 
     An industrial hardened case or module  40  having a lid  41  seals the system to protect it from environmental variables and rough handling In one embodiment of the present invention, industrial hardened module  40  contains a dry inert gas such as nitrogen. Additionally, this module can be hermetically sealed to Bellcore standards. A plurality of electrical conductor pins  49  are bonded to electrically insulating bushings  52  which are pressed into and bonded to bushing apertures  54  in housing  40 . A fiber aperture  56  is is disposed in housing  40  and is configured to receive a ferrule  62  having fiber  32  bonded thereto. A plurality of mounting apertures  58  are also provided in housing  40  to mechanically secure device  29  to a mounting surface. Module body  40  may also be shaped to conform to standard parts shapes such as DIP or SOIC packages. 
     Further, FIG. 2 illustrates an optic mounting plate or launcher  42  that may be made from alumina or other suitable material, in accordance with the present invention. Plate  42  holds the optical relay  30 , fiber pillow block  47  and fiber  32  in place as well as providing electrical contacts for the device. Plate  42  is shown in further detail in FIG. 5 for use in the preferred embodiment. 
     A carrier or window  44  is also provided for ease of assembly of the terahertz device to the module (as will be described below). Window  44  can be easily fabricated using standard micro-fabrication techniques. By using this window  44 , which also can be silicon, or other compatible material, the assembly of device  29  is made much easier. Once this is done, the window  44  can be soldered or bonded to the module  40  A silicon, sapphire, alumina, or other style of terahertz lens  31  is mounted onto the back of window  44  for reducing the divergence of the electromagnetic wave radiation emanating from the terahertz device  36 . The lens  31  configuration is generally aplanatic. 
     A riser block  45  and a fiber pillow block  47  are provided to position the mounting plate  42  and the fiber  32  respectively to the appropriate height above a bottom inside surface of the module to insure optical fiber alignment with the relay optic and the terahertz device The riser block of course can be integrated into the bottom floor of the module thus, reducing component piece count. The fiber pillow block  47  is bonded to the mounting plate  42  using solder or epoxy This enables the fiber  32  to be manipulated until the teraherlz signal is optimized Solder or epoxy is then deposited onto the fiber pillow block to encase the fiber. The maternal is then set to affix the fiber  32  to the pillow block  47 . 
     FIG. 3 a  illustrates an embodiment of the present invention wherein fiber  32  is mounted remotely from relay optic  30 . 
     FIG. 3 b  illustrates an embodiment wherein the fiber  32  is integrated with relay optic  30  creating a fiber assembly. Fiber  32  may be bonded to relay optic  30  using solder, epoxy or other appropriate bonding agent. Assemblies of this kind can also be bought commercially from many vendors. The fiber assembly is then mounted to mounting plate  42 , preferably using solder. Notably, in this embodiment mounting plate  42 ′ does not include a longitudinal slot  70  as shown in the embodiment of FIG. 3 a  and in greater detail in FIG.  6 . Alignment of the fiber assembly is achieved by actively manipulating the entire assembly, not just the fiber as is the case in the previous embodiment. 
     Altematively, the present invention contemplates integrating the relay optic  30  into the optical fiber  32 . More specifically, the relay optic is formed out of the optical fiber material and configured to create a de-magnifying lens which would serve the same function as the relay optic. The lens must be configured such that an appropriate spot size is projected onto the terahertz device and wherein a minimum distance of {fraction (1/10)} of the longest wavelength present is maintained between the terahertz device and the integrated lens. 
     FIG. 4 is a plan view of the fully assembled device  29 , in accordance with the present invention Additionally, the connection of electrical jumpers  59  between mounting plate  42  and pins  49  are shown. 
     Referring now to FIG. 6 mounting plate  42  is shown in greater detail, in accordance with the present invention. Mounting plate  42  includes a longitudinal slot  70  for orienting relay optic  30  properly thereon. A plurality of fiducials  72  bonded to mounting plate  42  aid in positioning relay optic  30  longitudinally along mounting plate  42 . Solder pads  74  provide a surface to bond or solder relay optic  30  to mounting plate  42 . A first pair of electrically conductive traces  76  is also provided to carry electrical energy between the terahertz device  36  and pins  49 . A second pair of electrically conductive traces  78  is provided to locate and attach fiber pillow block  47 . Additionally, these traces may also carry current to resistively heat the solder or epoxy on the top of fiber pillow block  47  for securing the fiber  32 . 
     FIG. 7 a  shows the terahertz device carrier or window  44  in further detail, in accordance with the present invention. Window  44  has a set of four fiducials  90  that are provided to aid in positioning the terahertz device on window  44 . Conductive traces  92  provide a path to conduct electrical energy between the antenna and pins  49  via electrical jumpers (as shown in FIG. 6 b ). Conductive traces  92  also act as fiducials to position mounting plate  42  adjacent window  44 . Further, a perimeter trace  94  enables window  44  to be soldered to a window aperture  55  on module  40  A pair of tabs  96  are disposed on widow  44  to aid in rotationally aligning window  44  on module  40  (shown in FIG.  2 ). 
     With specific reference to FIG. 7 b  photoconductive device  36  is shown assembled to window  44 . Further, each of the biasing electrodes  67  and  69  are electrically connected to conductive traces  92  via electrical jumpers  93  to communicate electrical energy between the photoconductive device and window  44 . 
     In a preferred embodiment of the present invention device  29  is assembled as described below. An electro-optic subassembly is formed by mounting the terahertz device  36  to the window  44 . The electro-optic subassembly may then be bonded to the module as previously described. An optical subassembly is then formed by mounting the relay optic  30  and fiber pillow block  47  to optic mounting plate  42 . Next, the riser block  45  is mounted to the bottom surface of the module  40 . The optical subassembly is then positioned adjacent window  44  and bonded to the riser block Lens  31  is then bonded to window  44 . The optical fiber  32  and ferrule  62  assembly is threaded through aperture  56 . By actively monitoring the terahertz radiation either emitted or detected by the device, the fiber  32  can then be aligned accurately to the terahertz device and then soldered or glued into place. Then the fiber is bonded to the ferrule for strain relief and to seal the module/fiber connection. Finally, lid  41  is welded to module  40  to create a hermetically sealed package. 
     FIG. 8 is a diagrammatic overview of another embodiment of the terahertz electromagnetic radiation emission and detection system of the present invention. An optical pulse source  150  generates a sub-picosecond laser pulse that is dispersed in a dispersion compensator  152 . The dispersion compensator can include any dispersion device such as disclosed in U.S. patent application No. 09/257,421, expressly incorporated by reference herein. The dispersed laser pulse travels through a fiber optic cable  154 , fiber splitter  156 , and delivery fibers  158  and  160  where it is dispersed opposite to that of the dispersion compensator. The dispersion compensator has an opposite canceling dispersion effect as compared with the entire length of optical fiber. The resultant compressed pulse traveling through delivery fiber  160 , is delivered to the THz transmitter device  164  and THz radiation is generated. The pulse also travels through an optical delay  162  en route to a THz receiving device  166  The resultant compressed optical pulse contacts the THz receiver and THz radiation is detected. The resultant output signal is amplified by amplifier  168  and output to a controller/computer  170 . This system conveys the light pulses used to generate the THz signal through fiber optic cables and packaged lens systems, making it rugged and substantially immune to exterior environmental conditions. 
     The modular packaging of a terahertz transmitter or receiver of the kind discussed in this application has never been done. Research labs have been limited to free-space optical beam coupled terahertz devices. This packaged, fiber-pigtailed module has produced the most rugged and manufacturable terahertz devices ever. 
     In as much as the foregoing disclosure is intended to enable one skilled in the pertinent art to practice the instant invention, it should not be construed to be limited thereby but should be construed to include such aforementioned obvious variations and be limited only by the spirit and scope of the following claims.