Abstract:
A compact monostatic optical transmitter/receiver device simultaneously transmits an optical beam and collects returning light using a single lens or optical aperture. The system provides automatic alignment of the transmit and receive aperture and is compatible with fiber-coupled laser sources. Transmit light is emitted from a double-cladding fiber core while received light is coupled into the inner cladding of the same fiber. The transmit light propagating in the core and the received light propagating in the inner cladding are separated by the means of a diplexer comprised of a fused fiber coupler or a fiber-coupled micro-optic device.

Description:
GOVERNMENT INTEREST 
       [0001]    The invention described herein may be manufactured, used, sold, imported, and/or licensed by or for the Government of the United States of America. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to monostatic optical transmitter/receivers as they relate to laser detection. 
       BACKGROUND OF THE INVENTION 
       [0003]    A typical method of operation for Laser Detection and Ranging (LADAR) system or a Laser Rangefinder (LRF) is to emit a short (typically 1-10 ns), high peak power optical pulse in a narrow beam and detect its reflected return from a target. The time delay between emission and detection provides the range to the target, and knowledge of the beam direction provides target bearing information. The transmitted beam is collimated to provide low divergence to the target so that the incident spot is sufficiently small to provide required lateral resolution. 
         [0004]    In general, the target will reflectively scatter the incident beam into a large solid angle, so the amount of reflected power detected at the LADAR system will be proportional to the area of the receiver aperture. If the transmitted and received beams share a common aperture the system is described as monostatic. This type of system avoids misalignment and parallax problems common to LADARs and LRFs which have separate transmit and receive apertures (described as bistatic). It makes manufacturing of LIDARs and LRFs simpler since it eliminates the needed for time consuming precise alignment of the transmit and receive apertures required with bistatic systems. In monostatic operation, an optical means has to be provided for separating the transmitted and received beams before the aperture, so that the transmitted beam does not lose energy or degrade the operation of the photodetector, and the received intensity is directed primarily to the photodetector. In principle, this optical diplexing function might be as simple as a beam-splitter (which has excessive round-trip optical loss) or more complex (such as a non-reciprocal optical circulator). 
       SUMMARY OF THE INVENTION 
       [0005]    For the exemplary systems described herein, diplexers comprised of a fused fiber coupler or a fiber-coupled micro-lens structure are variously used to efficiently separate the transmitted and the received light. 
         [0006]    In one aspect, a monostatic laser system is disclosed, comprising a photodetector fiber-coupled to receive a received light; a light source fiber-coupled to transmit a transmitted light; a diplexer fiber coupled to the photodetector to separate the received light, fiber-coupled to the light source to separate the transmitted light, and fiber-coupled on an output side of the diplexer; and a transmit/receive lens having one side facing a flat-polished end of said fiber coupled on said output side of the diplexer. 
         [0007]    In another aspect, an optical diplexer is disclosed based on a fused fiber coupler to separate a received light propagating in a receive direction from a transmit light that is propagating in a transmit direction, and function as a fiber-based monostatic transmitter/receiver. Such a diplexer comprises a double-cladding fiber having a core for coupling at one end to an optical transmitter source; and a multimode-core fiber having a single cladding for coupling at one end to a receiver photodiode, the double-cladding fiber and the multimode-core fiber being fused together to be in optical contact with each other in a coupling region to allow optical power transfer between an inner cladding of the double-cladding fiber and the multimode-core fiber. An outer cladding of the double-cladding fiber and a jacket of the multimode-core fiber are stripped away in the coupling region to allow optical contact or fusion of the inner cladding of the double-cladding fiber and the multimode-core fiber. 
         [0008]    Yet, in another aspect, a monostatic laser system is disclosed based on a fiber-coupled micro-lens structure. Such a monostatic laser systems comprises a multimode fiber with its lens face facing a fiber side of a receiver lens, and a receiver face of the multimode fiber facing a receiver photodiode; a transmitter fiber having a core with its lens face facing a fiber side of a transmitter lens, and a transmitter face of the transmitter fiber facing an optical transmitter source; a double-cladding fiber having a core with its lens face facing a fiber side of an output lens, and an output face of the double-cladding fiber facing an output side; and a diplexer based on an angled mirror configured in relation to said transmitter lens, receiver lens and output lens. Said transmitter lens and said output lens are arranged in an imaging configuration, where a near-field intensity distribution from the core of the transmitter fiber is first imaged by said transmitter lens at half-point between said transmitter lens and said output lens, where transmitted light from said optical transmitter source passes through a hole in said angled mirror. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Additional advantages and features will become apparent as the subject invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: 
           [0010]      FIG. 1  shows a block diagram of an exemplary monostatic Laser Detection and Ranging (LADAR) system or a Laser Rangefinder (LRF); 
           [0011]      FIG. 2  shows an exemplary optical receiver/transmitter aperture based on double-cladding fiber; 
           [0012]      FIG. 3   a  shows an exemplary diplexer configuration based on a fiber coupler; 
           [0013]      FIG. 3   b  shows an exemplary cross-sectional view of a coupling region of the exemplary diplexer configuration based on a fiber coupler; 
           [0014]      FIG. 4  shows another exemplary diplexer configuration based on a fiber coupler; and 
           [0015]      FIG. 5  shows an exemplary diplexer based on micro-lenses. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    A block diagram of an exemplary monostatic Laser Detection and Ranging (LADAR) system or a Laser Rangefinder (LRF) is shown in  FIG. 1 . As exemplified in  FIG. 1 , such an exemplary system  100  is comprised of a photodetector  120  for the received light, a light source  110  for the transmitted light, and a diplexer  130  to separate the received light from the transmitted light. 
         [0017]    As disclosed here, the detector  120 , light source  110  and diplexer  130  are fiber coupled. The light source  110  can be a fiber master-oscillator/power-amplifier (MOPA) device operating at near 1550 nm, or 1060 nm, seeded with pulse modulated laser diode. The fiber  140  on the output (right) side of the diplexer can have a double-cladding structure. As shown in  FIG. 1 , a flat-polished end of an exemplary double-cladding fiber (DCF)  140  faces a transmit/receive lens  150 . 
         [0018]      FIG. 2  shows such an exemplary fiber  240  on the output side of the diplexer having a double-cladding structure comprising an outer cladding  241 , an inner cladding  242  and a core  243 .  FIG. 2  also show the flat-polished end  244  of the double-cladding fiber (DCF)  240  facing the TX/RX lens  250 . 
         [0019]    An exemplary DCF  240  can be comprised of a small diameter core  243 , typically in the range of d cr =5-20 μm with a numerical aperture of NA core =0.08-0.16, and a large diameter d c , higher NA inner cladding  242 , typically d d =50-500 μm and NA clad =0.4-0.7 respectively. The large difference in the NAs of the core and the inner cladding has been used previously to implement a dual numerical aperture confocal bar-code scanner. 
         [0020]    Transmitter light emerges from the DCF core  243  and is collimated by the TX/RX lens  250 . The lens focal length f is chosen to give a desired collimated beam diameter and divergence angle. The lens NA is chosen to be equal to the NA of the DCF cladding (e.g.,  242 ), which is substantially larger than DCF core  243  NA, so that only the central portion of the lens  250  is used to collimate the transmit beam (e.g.,  260 ). For a specific lens focal length f the field of view (FOV) of the receiver (e.g.,  120 ) is given by the inner cladding diameter, FOV=fd c . 
         [0021]    Light reflected (e.g.,  270 ) from a remote diffused surface is collected by the lens (e.g.,  250 ) and focused onto the flat-polished DCF end (e.g.,  244 ). Rays focused by the lens (e.g.,  250 ) conic from the full lens aperture, and their incidence angles cover the full range of the lens NA. Of those rays that fall on the core region of the DCF only ones within the NA of the core are launched into the core (e.g.,  243 ), with the rest captured guided by the high inner DCF cladding (e.g.,  242 ). Because the inner core (e.g.,  243 ) and its cladding (e.g.,  242 ) are concentric, at the fiber tip they are automatically aligned with respect to the common monostatic TX/RX lens (e.g.,  250 ) for any position of the fiber tip (e.g.,  244 ) in the focal plane of the lens. 
         [0022]    The fraction of received light power (e.g.,  270 ) collected by the lens (e.g.,  250 ) that is coupled into the inner cladding (e.g.,  242 ) is given by: 
         [0000]      1−(NA core /NA clad ) 2  
 
         [0000]    For typical numerical aperture ratios described above, substantially all of the collected light is coupled into the inner cladding (e.g.,  242 ) and only a small fraction is coupled into the fiber core (e.g.,  243 ). For a typical inner cladding NA=0.47 and a core NA=0.12, about 93% of light collected by the TX/RX lens (e.g.,  250 ) can be estimated to couple into the inner cladding (e.g.,  242 ). 
         [0023]    Actually, the fraction of the light power coupled into the inner cladding (e.g.,  242 ) can be expected to be higher than that calculated from the NA ratio alone. Since collected light fills the entire lens aperture, for a lens with non-diffraction limited performance the focused spot diameter will be significantly larger than the fiber core. This “spill-over” effect reduces the fraction of received light that couples into the core, regardless of incidence angles of the focused rays. The lens surface shape can be intentionally designed so that it performs near the diffraction limit over a small fraction of its diameter but operates far from diffraction limit over the remaining surface. For example, a lens with a numerical aperture of 0.5 can be made to produce near-diffraction limited beam collimation for light emerging from DCF core with an NA=0.12, but produce a focused spot that is significantly larger than diffraction limit for collected light filling the entire lens surface corresponding to an NA of 0.5. 
         [0024]    An integral aspect of the disclosure is the diplexer (e.g.,  130 ). The variously disclosed diplexer performs the function of separating the received light (e.g.,  121 ) from the transmit light (e.g.,  111 ) that is propagating in the opposite direction. Ideally, the diplexer should introduce minimal loss for both the received light and transmitter light: its construction should be compact, insensitive to vibrations and temperature variations, and compatible with fiber coupled light sources and the double cladding fiber-based monostatic TX/RX shown in  FIG. 2 . 
         [0025]    The first exemplary diplexer configuration is shown in  FIG. 3   a .  FIG. 3   b  shows a cross-sectional view of an exemplary coupling region of a diplexer configuration based on such a fiber coupler. 
         [0026]    Such an exemplary all-fiber device can be made with two fiber types, one with a DCF structure (e.g.,  340 ) and the other with a single-cladding/multimode-core (MM) structure (e.g.,  330 ). The two fibers are fused together or are in optical contact with each other in order to allow optical power transfer between the inner cladding of the double-cladding fiber (e.g.,  340 ) and the MM fiber (e.g.,  330 ). The outer cladding of the DCF (e.g.,  340 ) and jacket of the MM fiber (e.g.,  330 ) are stripped away in the coupling region (e.g.,  331 ) to allow optical contacting or fusion of the inner cladding of the double-cladding fiber (e.g.,  340 ) and the MM fiber (e.g.,  330 ). The coupler (e.g.,  300 ) is fabricated so that strong optical coupling (e.g.,  331 ) is achieved between the inner cladding of the DCF (e.g.,  340 ) and the MM fiber (e.g.,  330 ), without causing significant perturbation of the signal light propagating in the DCF core (e.g.,  343 ). This assures that there is no significant light loss for the transmitter light coupled from the optical source (e.g.,  310 , such as a fiber MOPA) into the core (e.g.,  343 ) of the double-cladding fiber (e.g.,  340 ). The MM fiber output is coupled into the receiver photodiode (e.g.,  320 ). 
         [0027]    For a sufficiently long coupling length, the fraction of power propagating in the multimode DCF inner cladding (e.g.,  340 ) that cross-couples into the MM single cladding fiber (e.g.,  330 ) is given by, 
         [0000]      A M /(A DCF +A M ) 
         [0028]    where A DCF  is the cross-sectional area of the DCF inner cladding and A M  is the cross-section area of the MM fiber. For a 200 μm diameter of MM fiber and 100 μm DCF inner cladding diameter, this coupling fraction is 80%. While the 20% loss in the received signal is acceptably low for most LIDAR and LRF systems, a larger coupling fraction is often desirable and can be achieved by several techniques. For the first exemplary embodiment, the diameter of the MM fiber (e.g.,  330 ) can be increased, although this is not always desirable since the output of MM fiber output is coupled into a small diameter photodiode (e.g.,  320 ), with a typical active area diameter of 50-80 μm required for frequency a response of ˜1 GHz. For the second exemplary embodiment, the inner cladding of the DCF (e.g.,  340 ) can be reduced to achieve higher cross-coupling ratio, although this is also not always desirable since a smaller DCF reduces the FOV of the monostatic TX/RX, and also makes the fiber more difficult to work with. 
         [0029]    A third exemplary embodiment is shown in  FIG. 4 . In order to increase the cross-coupling fraction, the exemplary configuration shown in  FIG. 4  uses a modified coupler structure that circumvents these limitations. As in the previously described coupler, the coupler  400  in  FIG. 4  can be comprised of a DCF fiber (e.g.,  440 ) that is fused to, or is in optical contact with, a single cladding MM fiber (e.g.,  430 ). In this exemplary modified coupler, however, the DCF (e.g.,  440 ) is gradually tapered down to a sufficiently small diameter to achieve a desired cross-coupling ratio. Outside the cross-coupling section, the DCF (e.g.,  440 ) can have a larger diameter for easy handling and to maintain large FOV for the receiver (e.g.,  420 ). The coupler ( 400 ) in  FIG. 4  can have distinct sections, e.g., 4 sections as labeled A,B,C,D. As exemplified, coupling between fibers occurs in sections A-C. Section A can have an un-tapered length of DCF (e.g.,  440 ) in order to allow high NA rays in the DCF inner cladding to couple into the MM fiber (e.g.,  430 ) before reaching the down-taper. This is required since the down-taper increases the NA of the inner cladding light by the taper ratio, or the ratio of inner cladding diameter at the wide end of the taper to that at the narrow end. Section A assures that light propagating through the DCF taper does not exceed the NA of the DCF inner cladding. The length of coupling section B has a DCF down-taper, and section C also has a down-tapered DCF to achieve maximum DCF-MM fiber cross-coupling allowed by the fiber cross-sectional area ratio. In section D the down-tapered DCF is separated from the MM fiber before reaching the up-taper. This coupler feature is required to prevent light that is already in the MM fiber (e.g.,  430 ) from coupling back into the DCF (e.g.,  440 ). 
         [0030]    To avoid losses for the DCF core (e.g.,  443 ) in the coupler  400  shown in  FIG. 4 , the taper fraction has to be sufficiently small so that all modes that are launched into it from the light source (e.g.,  410 ) remain well confined in the core (e.g.,  443 ). For a typical single mode core with a typical diameter of 10 μm and NA=0.12, this means that the taper ratio should be below approximately 2.0. 
         [0031]    Another exemplary embodiment of a diplexer, as shown in  FIG. 5 , uses micro-lenses. In this exemplary arrangement the transmitter light emerging from the fiber core (e.g.,  513 ) is coupled into the core (e.g.,  543 ) of a DCF fiber (e.g.,  540 ) using a pair of lenses  1  and  2 . The lenses can be arranged in an imaging configuration, where the near-field intensity distribution from the core (e.g.,  513 ) of the fiber connected to the source (e.g.,  510 ) is first imaged by lens  1  at half-point between the two lenses, where it passes through a hole in an angled mirror (e.g.;  530 ). The transmitted light is then imaged onto the face (e.g.,  544 ) of the DCF  540  by lens  2 . If the cores (e.g.,  513  &amp;  543 ) of the two fibers are identical, then nearly complete (discounting Fresnel losses) core-to-core coupling efficiency can be achieved with such 1:1 imaging magnification. For dissimilar core diameters other magnification factors can be used to match the mode field diameters of the two fiber cores. 
         [0032]    The received light emerging from the inner cladding (e.g.,  542 ) of the DCF is imaged onto the face of a MM photo-receiver fiber (e.g.,  521 ) by the lens pair  2 ,  3 . An image of the near-field distribution of the DCF is imaged onto the angled mirror (e.g.,  530 ) by lens  2 , and after reflection is re-imaged onto the face (e.g.,  523 ) of the MM fiber by lens  3 . The MM fiber diameter, DCF inner cladding diameter and magnification factor of the lens pair  2 ,  3  are chosen so that the image of the DCF inner cladding matches the MM fiber diameter. A small fraction of the received light is lost because it falls on the hole in the diplexer mirror (e.g.,  530 ). This fraction is given by the ratio between the core and inner cladding areas A core /(A DCF ), which for a 10 μm core and a 100 μm cladding corresponds to a loss of only 1%. 
         [0033]    It is obvious that many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as described.