Patent Publication Number: US-2013250276-A1

Title: Laser Wind Velocimeter With Multiple Radiation Sources

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This patent application is a continuation of U.S. patent application Ser. No. 13/026,932, filed Feb. 14, 2011, now allowed, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     1. Field 
     This disclosure relates to the field of laser based velocity measurement of wind or solid objects. 
     2. Background Art 
     Conventional laser Doppler velocimeters (“LDVs”) transmit light to a target region (e.g., into the atmosphere) and receive a portion of that light after it has scattered or reflected from the target region or scatterers in the target region. This received light is processed by the LDV to obtain the Doppler frequency shift, f D . The LDV conveys the velocity of the target relative to the LDV, v, by the relationship v=(0.5)cf D /f t  where f t  is the frequency of the transmitted light, and c is the speed of light in the medium between the LDV and the target. 
     LDV&#39;s are have a wide range of applications including, but not limited to: blood-flow measurements, speed-limit enforcement, spaceship navigation, projectile tracking, and air-speed measurement. In the latter case the target consists of aerosols (resulting in Mie scattering), or the air molecules themselves (resulting in Rayleigh scattering). 
     Conventional LDVs for meteorological measurements or applications generally incorporate a single motorized telescope that takes measurements sequentially along different axes, or use three telescopes, switching from one to the next, and so on, to allow sequential measurements along the different axes. This approach is disadvantageous in that is does not enable simultaneous measurements of three components of velocity. 
     BRIEF SUMMARY 
     Therefore, what is needed is an LDV with more than one transceiver, and e.g., three or more transceivers, where all of the transceivers are transmitting a beam of light within a target region or regions substantially simultaneously. Using the plurality of transceivers each aimed at a different area of the target region allows for substantially simultaneous velocity measurements along a plurality of different axes, thus allowing for a multi-dimensional velocity determination. In one example, by simultaneously transmitting light to the different areas of the target regions, the accuracy of the readings is greatly improved while eliminating the need for any moving parts. In another example, the timeliness of the measurements is improved. What is also needed are improvements to LDVs that result in the use of eye-safe radiation sources, e.g., in the 1.4-1.6 micron range. 
     In one embodiment of the present invention, there is provided a laser velocimeter system comprising a coherent source configured to produce a coherent radiation beam, a modulator configured to receive the coherent radiation beam as input from the source and to produce a modulated radiation beam, one or more transceivers configured to receive the modulated radiation beam via a corresponding one or more optical fibers chosen from a first plurality of optical fibers, the one or more transceivers each configured to transmit the modulated radiation beam to a target region and to receive a reflected radiation signal from the target region, and, an optical mixer coupled to the one or more transceivers via a corresponding one or more optical fibers chosen from a second plurality of optical fibers, and coupled to the coherent source via one or more optical fibers chosen from a third plurality of optical fibers. The optical mixer is configured to receive the one or more reflected radiation signals from the corresponding one or more transceivers, receive one or more reference radiation beams from the coherent source, and determine, for each of the one or more transceivers, a corresponding one or more Doppler shifts based on the respective one or more reference beams and the corresponding one or more reflected radiation signals. 
     In another embodiment of the present invention, a method of determining a velocity of scatterers in a target region is provided comprising modulating a coherent radiation beam containing one or more frequencies, transmiting the modulated radiation to one or more target regions in the form of one or more radiation beams, receiving one or more reflected radiation signals from the target region, combining the received reflected radiation signals with one or more reference radiation beams, and, determining the velocity of scatterers in the target region based on the combined received and reference radiation. 
     Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. It is noted that the present invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
       The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the present invention and to enable a person skilled in the relevant art(s) to make and use the present invention. 
         FIG. 1  illustrates a laser Doppler velocimeter. 
         FIG. 2  illustrates an embodiment of a laser Doppler velocimeter with multiple transceivers. 
         FIG. 3  illustrates an embodiment of a radiation source module of the laser Doppler velocimeter. 
         FIG. 4  illustrates an embodiment of a transceiver module of the laser Doppler velocimeter. 
         FIG. 5  illustrates an embodiment of a receiver module of the laser Doppler velocimeter. 
         FIG. 6  illustrates a vector diagram of a motion compensation scheme for the laser Doppler velocimeter. 
         FIGS. 7 ,  8 , and  9 , illustrate various embodiments of laser Doppler velocimeters with multiple transceivers. 
         FIGS. 10 and 11  illustrate various embodiments of laser Doppler velocimeters with multiple transceivers and multiple radiation sources. 
     
    
    
     The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number. 
     DETAILED DESCRIPTION 
     This specification discloses one or more embodiments that incorporate the features of this invention. The disclosed embodiment(s) merely exemplify the present invention. The scope of the present invention is not limited to the disclosed embodiment(s). The present invention is defined by the claims appended hereto. 
     The embodiment(s) described, and references in the specification to “one embodiment”, “an embodiment”, “an example embodiment”, etc., indicate that the embodiment(s) described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. 
     Embodiments of the present invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. 
     Before describing such embodiments in more detail, however, it is instructive to present an example environment in which embodiments of the present invention may be implemented. 
     An example of an air speed LDV  10  is illustrated in  FIG. 1  and as disclosed in U.S. Pat. No. 5,272,513, the disclosure of which is incorporated herein by reference in its entirety. The LDV  10  includes a source  20  of coherent light which may, if desired, be polarized. The source  20  projects a first coherent beam of light  30  into a modulator  40 . The beam shaper  40  expands and collimates the beam  30  after which beam  30  enters a transceiver  60 . The transceiver  60  projects the beam  30  in nearly collimated form into the target region  45 . 
     The collimated beam strikes airborne scatterers (or air molecules) in the target region  45 , resulting in a back-reflected or backscattered beam  50 . A portion of the backscattered beam  50  is collected by the same transceiver  60  which transmitted the beam  30 , or to an adjacent receiver (not shown). The case where the same transceiver transmits and receives the light is known as a monostatic configuration, while the case of separate transmitters and receivers is known as a bistatic configuration. Monostatic configurations can only receive backscattered light. Bistatic configurations can be arranged to receive light that is substantially backscattered or at any other angle relative to the transmitted beam  30 . 
     The light  50  collected by transceiver  60  is then combined with a separate reference beam of light  70  in an optical mixer  80 . An ideal optical mixer combines the two beams in such a way that they have the same polarization and occupy the same space, and directs the result onto a photo detector with a bandwidth sufficient to detect the measured Doppler frequency shift. The photo detector produces an electrical current  85  which includes a component whose frequency is the mathematical difference between the frequency of the reference beam  70  and the backscattered beam  50 . The electrical current  85  is then analyzed by a signal processor  90  (e.g. electrical spectrum analyzer or a frequency counter) to determine the frequency difference and calculate the relative velocity component along the axis of the transceiver  60  between the LDV  10  and the target region  45 . 
     Ambiguities regarding whether the measured relative frequency is either positive or negative can be resolved by using the “in-phase and quadrature” detection method, as is known in the art. Another approach to resolving these ambiguities is to apply a stable, constant frequency shift either to the transmitted beam  30  or to the reference beam  70  (e.g. by using an acousto-optic cell). This creates an alternating current component in the electrical signal  85  with a frequency that is the sum of the constant frequency shift and the Doppler frequency shift, removing the directional ambiguity. An LDV wherein the frequency of the transmitted beam  30  and the frequency of the reference beam  70  are identical is said to use homodyne detection. Heterodyne detection is used when the frequencies of the transmitted beam  30  and reference beam  70  are different, 
     The reference beam  70  is selected to have a well-defined and stable optical frequency that bears a constant phase relationship with the transmitted beam  30 . This is known as coherence. The requirement for coherence is easily achieved by using a laser as the source  20  and tapping the source  20  to create the reference beam  70  by means of an optical splitter (not shown). 
     Source  20  can be either a CO 2 , Nd:YAG, or Argon Ion laser (preferably lasing in the fundamental transverse mode and in a single longitudinal mode). However, air-speed targets (aerosols and/or molecules) generate very weak return signals compared to solid objects. Thus air-speed LDV&#39;s incorporating these laser sources that work over a range of thousands or even tens of meters require large amounts of laser power and are thus too large, bulky, heavy, fragile and possibly dangerous to be used in many desirable applications like air-speed determination for helicopters. 
     However, source  20  can also be a lightweight, low-cost, highly efficient, rare-earth-doped glass fiber (referred to hereafter as a fiber laser). Fiber lasers have several enormous advantages over other laser. sources. Fiber lasers can be efficiently pumped by laser diodes whose emission wavelengths have been optimized for excitation of the rare-earth dopant. This makes the fiber lasers very energy efficient and compact, eliminating the need for cooling systems, flash lamps, and high current electrical sources. Moreover the glass fiber serves as a flexible waveguide for the light, eliminating the need for bulky optical components like mirrors and lenses that require rigid mechanical mounts in straight lines with stringent alignment tolerances. Fiber lasers are also more adaptable than solid-state lasers: the pulse repetition frequency (“PRF”) and pulse width in fiber lasers may be changed “on the fly,” while the PRF and pulse width in solid-state lasers are bound to narrow ranges or are even fixed. Source  20  can also be comprised of a laser diode coupled to an optical fiber. 
     Despite advances in conventional LDV&#39;s, improvements are still necessary. Sometimes it is desirable to locate the source laser  20  at a different, more accessible location than the transceiver  60 . For example, in a wind turbine generator (“WTG”) application the telescope can be located on the turbine, while its source laser and control electronics are best located in the nacelle or at the base of the tower that supports the WTG for ease of maintenance. In sailing applications the source is preferably located within the hull of the ship where it is protected from exposure to the elements. 
     These remote configurations can be made conveniently by using optical fiber to connect the source laser  20  and the transceiver  60 . Problems have occurred, however, in that the large optical power required for air speed measurements becomes limited by a non-linear effect that occurs in fiber optics known as stimulated Brillouin scattering (“SBS”). In fact, the longer a fiber optic is, the lower this limit becomes. The SBS power limit depends on other factors known to those skilled in the art, but it is a fundamental physical property of light traveling through transparent media and cannot be ignored. 
     Additional exemplary systems are taught in co-owned U.S. patent application Ser. No. 12/988,248 and PCT Appl. No. WO 2009/134221, which are both incorporated by reference herein in their entireties. 
     Embodiments of the present invention provide a velocimetry system for an LDV with no moving parts and which is lightweight enough to be used for many different applications which were, up to this point, not practical for LDVs. The disclosed LDV includes an active lasing medium, such as e.g., an erbium-doped glass fiber amplifier for generating and amplifying a beam of coherent optical energy and an optical system coupled to the beam for directing the beam a predetermined distance to a scatterer of radiant energy. The reflected beam is mixed with a reference portion of the beam for determining the velocity of the scatterer. 
     In using this device to measure wind velocity in the transceiver focal volume, the velocity component that is measured is that component along the axis of the transceiver. Therefore, for measurement of the “n” components of velocity, n independent measurements must be made along n non-collinear axes (where n is an integer). To accomplish this task n duplicate transceivers are disclosed, each carrying either a continuous wave (“CW”) beam or are simultaneously pulsed with a common seed laser source. Simultaneous pulsing and transmission through the n transceivers has the advantage that the velocity measurements each arise from the same moment in time, instead of from sequential moments in time. Thus, the resulting velocity determinations are more accurate as a result of simultaneous pulsing and transmission instead of sequential transmission. 
     By using optical fiber for both generation of the laser energy as well as wave guiding of the energy, the present disclosure provides a single, mechanically flexible conduit for light. This configuration allows the system to be more robust to vibration and temperature variation than a corresponding system comprising free space optical components. The only point at which light leaves the optical fiber system is for projection from the respective transceivers. Each of the optical fibers that transmits light is also the same fiber used to receive scattered light and thus the aerosol-scattered return beam is automatically aligned with the respective transceiver-fiber optic collection systems. 
     The use of fiber lasers such as e.g., erbium-doped optical fiber also has advantages in terms of the overall energy efficiency of the system. Because diode lasers are now available at the optimal pump wavelength of erbium doped glass, the erbium wave guide can be efficiently pumped by launching pump radiation down this wave guide. Thus, the system has greatly reduced cooling requirements and can operate off of a low voltage battery supply. 
     The disclosed velocimeter system is also eye-safe, light-weight, and easily scaled to high energy per pulse or CW operation. As described above, the velocimeter has “n” lines of sight. Thus, in order to determine an object&#39;s velocity or the wind velocity in one or more target regions, n transceivers are used, each simultaneously projecting a beam of light along a different axis. To determine three-dimensional velocity, as with wind velocity, three transceivers are used. To determine two- or one-dimensional velocity, e.g., for a car or boat moving on a plane or in a line, fewer transceivers may be used. The laser beams projected from the n transceivers are each pumped simultaneously and arise from a single laser source. The source may be co-located with the n transceivers, or may be located remotely with respect to the n transceivers. If the laser source is remotely located, fiber optic cables are used to carry the generated light beams to each transceivers. As described below in greater detail, a seed laser from the source is amplified and, if desired, pulsed and frequency offset, and then split into n source beams. The n source beams are each delivered to an amplifier assembly that is located within the n transceiver modules, where each of the n transceiver modules also includes an optical system such as a telescope. Amplification of the n source beams occurs at the transceiver modules, just before the n beams are transmitted through the optical system to one or more target regions. Thus, when the n source beams are conveyed through connecting fibers from the laser source to each of the n transceivers, the power of each of the source beams is low enough so as not to introduce non-linear behaviors from the optical fibers. Instead, power amplification occurs in the transceiver module, just before transmission from the optical system. Consequently, fiber non-linear effects are not introduced into the system. 
     The placement of the power amplifier within the transceiver modules just before laser beam projection through a lens reduces the effect of nonlinear fiber behavior that is normally observed when there is a greater propagation distance between the power amplifier and the lens. In this way, the disclosed velocimeter is able to use a single seed laser and amplifier assembly that is remote from the power amplifier. The seed laser generates a beam that may be amplified, pulsed, and frequency shifted before the beam is split, if necessary, and directed to the remote power amplifiers. Power amplification only occurs just before transmission of the source beam through the lenses. Thus, as long as the amplified result is still within the linear operating region of the fiber to the remote amplifier, the disclosed velocimeter avoids the problems associated with non-linear fiber operation. 
     By using the disclosed velocimeter, object or wind velocities may be measured with a high degree of accuracy. Because the source laser is split into n beams, the measurements taken along all of the n axes are simultaneous. Additionally, splitting the source beam into n beams does not necessarily require that the source laser transmit a laser with n times the necessary transmit power, because each of the n beams are subsequently power amplified before transmission. The n beams may each be directed towards the same target region or may be directed to multiple target regions. A single beam may be used to simultaneously measure velocities at multiple points or span along a single axis. Additionally, the disclosed velocimeter has no moving parts, and is thus of reduced size and improved durability. As explained below, the disclosed velocimeter may be used with a platform motion sensing device such as e.g., an inertial measurement unit (“IMU”) or global positioning satellite (“GPS”) unit so that the motion of the velocimeter platform may be compensated during calculation of the measured velocities. Thus, because of the lightweight and non-bulky nature of the velocimeter, and because of the velocimeter&#39;s ability to compensate for platform motion, the disclosed LDV may be mounted on any moving platform (e.g., a helicopter, a boat, etc.) and still obtain highly accurate readings. 
       FIG. 2  is a block diagram illustrating an n-axis laser Doppler velocimeter system  100 . The system  100  includes a radiation source module  200 , n transceiver modules  300 , and an optical mixer  400 . Each of the modules are described in detail below. The radiation source module  200  generates n source beams  125  to the n transceiver modules  300 . The n transceiver modules  300  are for transmitting n beams of light  150  and receiving n scattered or reflected beams of light  160 . The transceiver modules  300  may be located in a physically separate location than the radiation source  200  and the optical mixer  400 . Alternatively, depending upon the application, all modules may be co-located. The radiation source module  200  also outputs a reference beam  255  to the optical mixer  400 . The optical mixer  400  combines the reference beam  255  with each of the scattered/reflected. beams  160  received by the n transceiver modules  300  that are passed on to the optical mixer  400  via optical fiber  405 . Doppler Shifts and hence, velocities, are calculated from the results of the combined signals. 
     The radiation source module  200  is illustrated in  FIG. 3 . The radiation source module  200  includes a laser source  210 , an optical amplifier (such as e.g., a fiber optic amplifier, illustrated a  330  in  FIG. 4 ) and an optical splitter  270 . The radiation source module  200  may also include an optical modulator  230  to provide a frequency shift (using e.g., an acousto-optic modulator), a polarization shift (using e.g. a Faraday rotator), or both, as well as to induce a temporal pulse shape (i.e. amplitude modulation). 
     Each of these components of the radiation source module  200  are coupled together and are described in greater detail below. 
     The laser source  210  and associated drivers and controllers provide the initial laser energy that may be feed into optical amplifier (see  FIG. 4 , feature  330 ). When the laser source output is combined with an amplifier, the result is a high power laser output. Typical laser sources  210  are small laser diodes (single-frequency or gain-switched), short-cavity fiber lasers, and miniature solid state lasers such as, for example, nonplanar ring oscillators (“NPROs”), or hybrid silicon lasers. The output from the seed laser source  210  is directed towards the optical modulator  230 , that may induce a frequency shift, a polarization shift, or both as well as provide a temporal amplitude modulation. A reference laser signal  255  is also output from the laser source  210 . 
     A frequency shifter (such as an acousto-optic modulator (“AOM”)) (as a possible component of the optical modulator  230 ) and associated RF drivers may provide a radio-frequency (“RF”) offset to the laser source output. This offset facilitates the later determination by a signal processor of the direction of any detected motion. The offset is provided by utilizing the acousto-optic effect, i.e., the modification of a refractive index by the oscillating mechanical pressure of a sound wave. In an AOM, the input laser beam is passed through a transparent crystal or glass. A piezoelectric transducer attached to the crystal is used to excite a high-frequency sound wave (with a frequency in the RF domain). The input light experiences Bragg diffraction at the periodic refractive index grating generated by the sound wave. The scattered beam has a slightly modified optical frequency (increased or decreased by the frequency of the sound wave). The frequency of the scattered beam can be controlled via the frequency of the sound wave, while the acoustic power is the control for the optical powers. In this way, a frequency shifter may be used to provide a frequency offset to the laser source output. An AOM may also be used as an optical modulator  230  to modulate laser signals from the source laser  210  in order to obtain pulsed LIDAR measurements. 
     Additional modulation of the seed laser output may be provided using an optical modulator  230  (such as e.g., semiconductor optical amplifier (“SOA”)). Although the SOA is not necessary for the system  100  to function, SOA-induced pulsing may be used to optimize the extinction ratio in the pulses. The SOA is capable of providing primary as well as secondary modulation of the seed laser source. The SOA may also be used to provide optical amplification to the laser source signal. The laser source  210  can also be modulated electronically. 
     An optical amplifier (feature  330  in  FIG. 4 ) can be either a semiconductor-based booster optical amplifier (“BOA”) or a fiber optic amplifier. The fiber optic amplifier includes a length of fiber doped by a rare earth element such as e.g., erbium (Er), erbuim-ytterbium (Er:Yb), etc. A single mode (“SM”) or multimode (“MM”) pump diode is used to excite the dopant material within the doped fiber. Optical signals from the SOA may be combined with the pump signals via a wavelength division multiplexer (“WDM”) or a tapered fiber bundle (“TFB”). In the optical amplifier  330 , the source light is amplified to a level below the power limit dictated by optical damage and nonlinear effects of the fiber. Amplifier spontaneous emission from the optical amplifier  330  is managed via the use of narrowband bulk filters or fiber Bragg grating (“FBG”) based filters. 
     Once filtered, the amplified light is passed through an optical splitter  270 . The optical splitter  270  splits the light amongst the different transceiver modules  300 . As explained below, the light from the radiation source module  200  is transmitted to optical amplifiers  330  located within each individual transceiver module  300 . The use of an optical splitter instead of a switch or multiplexer allows the radiation source module  200  to be designed without any moving parts. In other words, no motors or switches need be used. 
     Light output from the optical splitter  270  and hence the radiation source module  200  is directed to the n transceiver modules  300  by way of n connecting fibers  125 . The connecting fibers  125  allow the radiation source module  200  to be remotely located (if desired) from the n transceiver modules  300 . As described above, the lasers carried by the connecting fiber bundle  125  are each at a sufficiently low power to avoid introducing the non-linear effects of the fiber. The fiber bundle  125  consists of multiple fibers of varying core sizes to carry different optical signals between the radiation source module  200  and the n transceiver modules  300 . These optical signals include the amplified source laser signal as well as a multimode pump laser signal from a pump laser  240  for the pumping of amplifiers at each of the n transceiver modules  300 . Furthermore, optical signals including optical monitor signals from the transceiver modules  300  are carried back to the radiation source module  200 . The optical monitor signals can trigger the shutdown of the radiation source module  200  in the event of a malfunction or error at the transceiver modules  300 . 
     One of the n transceiver modules  300  is illustrated in  FIG. 4 . Each of the transceiver modules  300  includes an optical amplifier  330  (such as a fiber optic amplifier), an optical switch  340  (such as e.g., a fiber optic circulator), and a transceiver lens  360  used to transmit and receiver optical signals from the target region  45  (of  FIG. 2 ). 
     Amplified source laser signals from the radiation source module  200  transmitted via optical fibers  125  to each of the transceiver modules  300  are further amplified within each of the transceiver modules  300  via the optical amplifier  330 . The optical amplifier  330  includes a rare earth doped fiber (such as e.g., Er:Yb double clad fiber). Pump light can be introduced into the rare earth doped fiber via a tapered fiber bundle (“TFB”) in a co-propagating or counter-propagating manner relative to the seed laser signal from the radiation source module  200 . The source laser signal is thus further amplified within the transceiver module  300 . The output of the optical amplifier  330  is then directed towards an optical switch  340  via TFBs or WDMs. 
     The optical switch  340  (such as e.g., a fiber optic circulator) allows a single lens  360  to be used to transmit and receive light, thus allowing the sensor to operate in a monostatic geometry. In the case where multiple lenses are used (at least one for transmitting a light beam and at least one for receiving a reflected light beam, e.g., a bistatic geometry), the optical switch  340  may not be necessary. The optical switch  340  may also be used in conjunction with an amplified spontaneous emission filter. Such a filter might be bulk optic or an FBG based filter. Such a filter may be installed to maintain laser eye safety, as necessary. It is often the case that these filters divert the amplified spontaneous emission (“ASE”) to another fiber optic. This diverted laser can be used to monitor the operation of the optical amplifier  330  to adjust the amplifier&#39;s power, or as a safety feature in remotely pumped applications. As a safety feature, a measurable drop in the diverted ASE could mean that the fiber cable has been severed and that the pump should be shutdown immediately. Alternatively, to reduce ASE in pulsed applications, the pump lasers themselves may be pulsed in synchronization. Pulsing the pump lasers also reduces power consumption, thus facilitating the use of battery operated systems. 
     Source light that reaches the transceiver lens  360  is projected onto a target object or region  45  (of  FIG. 2 ). Scattered or reflected light is returned to the transceiver module  300 . The transceiver lens  360  collects the scattered light back into the fiber. In the case of monostatic operation, the transceiver lens  360  focuses light back into the transmit fiber where the scattered light is separated out from the transmit beam by the optical switch  340 . Otherwise, for example, in the case of bistatic operation, the scattered light is focused into a different fiber. The collected scattered light is carried via fiber  405  to the receiving module  400  of  FIG. 2 . 
     The optical mixer  400  is explained in greater detail with reference to  FIG. 5 . The optical mixer  400  includes an optical coupler  420  (e.g. a fiber optic coupler) for combining the received signal  405  with the reference laser signal  255  into the same space (e.g., an output optical fiber). This combined signal  425  is then directed onto an electro-optic receiver  430  (e.g. a photodiode) that converts the mixed optical signal into an electrical signal. This signal is then digitized (via a digitizer  450 ) for convenient signal processing in order to extract the Doppler frequency shift (via a signal processor  440 ). If n transceiver modules  300  are used then the reference laser signal  255  must be split into n beams by splitter  410  for mixing with n optical mixers  400 . If n is large, then an optical amplifier may be required to boost the power of the reference beam  255  before splitting. 
     An optical coupler such as  420  (e.g., a 3 dB fiber optic coupler) generally produces two output beams  425 ,  426  of opposite phase. Beam  425  is the combined signal, as explained above. Beam  426  may also be used and applied to a second electro-optic receiver to create a balanced receiver, as described in U.S. Pat. No. 4,718,121, the disclosure of which is incorporated herein by reference. Balanced receivers are preferably used because they use all of the mixed signal, and result in the cancellation of intensity noise in the reference laser beam  255 . 
     Effective optical mixing also requires matching the polarizations of the received signal  405  and the reference laser signal  255 . Mitigating the loss of mixing efficiency due to uncontrolled polarization may require a more complicated optical mixing circuit than the one shown in  FIG. 5 , such as a polarization diversity receiver, described in U.S. Pat. No. 5,307,197, the disclosure of which is incorporated herein by reference. 
     The signal processor  440  receives the signal from the digitizer  450  and converts the signal into frequency space, calculates line-of-sight speeds from the Doppler shifts along each line-of-sight (i.e., from each of the n transceivers  300 ), and combines these speeds to determine a single velocity for the target object or region measured. Additionally, the signal processor  440  may use input from a motion sensor (preferably an attitude heading reference system or an IMU and a GPS or ground speed detection device) to determine if the platform upon which the transceivers  300  are mounted is moving. Any platform motion is detected and used to adjust or correct the measured velocity, as described in connection with  FIG. 6 . 
     Although not all applications of the disclosed LDV  100  require platform motion compensation, the disclosed LDV  100  (or at least the transceiver module  300  of the LDV  100 ) is portable and may easily be located on a moving platform such as a boat, ground vehicle or aircraft. As discussed above, the LDV  100  directly measures the relative motion of air scatterers with respect to the transceiver module  300  by detecting the Doppler frequency shift. If the LDV  100  is fixed to the ground, then its measurement is the wind speed and direction. However, an LDV  100  undergoing linear motion measures the relative wind speed and direction. If the linear speed and direction of the moving platform is known, then the wind speed can be extracted from the relative wind measurement. Additionally, the LDV  100  may undergo both linear and rotational motion as encountered on floating platforms. The rotational motion introduces an additional frequency shift since the optical focal volumes are moving rapidly through the air. This frequency shifts yields a speed measurement that is not necessarily useful to (1) meteorologists since it does not represent wind or (2) navigators since it does not represent relative wind. This rotational component must be isolated and compensated for in order to report useful wind data. 
     Referring to  FIG. 6 , a vector diagram of a motion compensation scheme  600  for the disclosed LDV is depicted. Platform motion of platform  1  is composed of linear translations of the platform&#39;s center of mass  2  and rotations about the center of mass  2 . Mounted on the platform  1  is an LDV  100  with n transceiver modules  300 . At least one of the n transceiver modules  300  (e.g., the i th  transceiver module  300 ) is co-located with the LDV  100  on the platform  1 . The velocity of the i th  focal volume or target region  45  is given by Equation 1, below: 
       Eq. 1 
         {right arrow over (v)}   fi   ={right arrow over (v)}   c.m. +{right arrow over (ω)}×{right arrow over ( r )} i ,
 
     where {right arrow over (v)} c.m.  is the linear velocity of the center of mass  2  of the platform  1  (and thus the LDV  100 ), {right arrow over (ω)} is the angular velocity of the platform  1 , and {right arrow over (r)} i  is the displacement vector from the center of mass  2  of the platform  1  to the ith focal volume or target region  45 . The displacement vector is {right arrow over (r)} i ={right arrow over (r)} c.m. +{right arrow over (L)} i ,, where {right arrow over (r)} c.m. . is a vector from the center of mass  2  of the platform  1  to the transceiver modules  300  and {right arrow over (L)} i =f{circumflex over (L)} i  and is a vector from the ith transceiver module  300  to the ith focal volume or target region  45 . The magnitude factor f is either the focal length in a focused system or the range in a range-gated system. The Doppler frequency shift created by this velocity is proportional to its component (δ i ) along the laser line of sight {circumflex over (L)} i : The i th  Doppler frequency shift is equal to 2ε i /λ, where λ is the laser wavelength and: 
       Eq. 2 
       δ i ={right arrow over ( v )} fi ·{circumflex over ( L )} i ={right arrow over ( v )} c.m. ·{circumflex over ( L )} i +({right arrow over (ω)}×{right arrow over ( r )} i )·{circumflex over ( L )} i .
 
     The first term of Equation 2 (i.e., {right arrow over (v)} c.m. ·{circumflex over (L)} i ) is the desired shift due to the relative linear motion between the i th  target region  45  and the moving platform  1 . The second term of Equation 2 (i.e., {right arrow over (ω)}×{right arrow over (r)} i )·{circumflex over (L)}) represents the rotational motion and can be written as ({right arrow over (r)} c.m. ×{circumflex over (L)} i )·{right arrow over (ω)} using the rules of cross products with the fact that ({right arrow over (ω)}×{right arrow over (L)} i )·{circumflex over (L)} i =0. The procedure for motion compensation in a three-dimensional system is to measure the three taw Doppler shifts and the angular velocity with an IMU, then subtract off ({right arrow over (r)} c.m. ×{circumflex over (L)} i )·{right arrow over (ω)}. This corrected frequency shift is used to compute the three-dimensional relative wind at the i th  target region  45 . 
     The angular velocity and attitude (pitch/roll angle) of a moving platform may change rapidly with time. It is important to measure the Doppler shift in a short amount of time so as to allow an assumption that the state motion is frozen (thus allowing the assignment of one value of angular velocity and attitude to each measured Doppler frequency shift). Accordingly, the laser pulse repetition frequency (“PRF”) and the number of pulses N acc  are chosen so that the total time of data collection (i.e., N acc /PRF) is less than 200 milliseconds, for example. The angular velocity is measured before and after the N acc  pulses are collected and the average value is used in the compensation calculations for {right arrow over (ω)}. 
     Although LDV  100  has been described in reference to the system and module architectures depicted in  FIGS. 2-5 , these architectures are exemplary and are not intended to be limiting. For example,  FIG. 7  illustrates an additional LDV architecture in the form of LDV  700 , As in LDV  100  (of  FIG. 2 ), LDV  700  includes a source module  720 , a transceiver module  730  and a optical mixer  740 . However, in LDV  700 , the source module  720  does not include a splitter. Instead, radiation generated at the source module  720  is conveyed to the transceiver module  730 , where the generated radiation is amplified by amplifier  732  and then split via splitter  734  for use by the n transceivers  736  in the transceiver module  730 . In LDV  700 , only one remote amplifier  732  is used instead of n remote amplifiers. 
       FIG. 8  illustrates an additional LDV architecture in the form of LDV  800 . Here, LDV  800  includes a source module  820 , one or more transceiver modules  830  and an optical mixer  840 . The source module  820  does not include a splitter. Also, the transceiver modules  830  do not include amplifiers. Instead, an external amplifier  832  and splitter  834  are used. Radiation is generated at the source module  820  is conveyed to the remote amplifier  832  where it is amplified and then split via splitter  834  for delivery to the n transceiver modules  830 . As in LDV  700  (of  FIG. 7 ), only one remote amplifier  832  is used in LDV  800 . 
     The disclosed LDV embodiments have been explained in the context of fiber-optic-connected modules in a way that allows the transceiver modules  300 ,  730 , and  830  and optical amplifiers  330 ,  732 , and  832  to be remotely located from the radiation source modules  200 ,  720 , and  820 . The transceiver modules  300 ,  730 , and  830  need not include any electronics and can be purely optical modules. Motion compensation, laser sources, and signal processing occurs at the radiation source modules  200 ,  720 , and  820  and optical mixers  400 ,  740 , and  840 . Thus, the operation of the transceivers  300 ,  730 , and  830  is significantly improved due to less noise from the radiation source modules  200 ,  720 , and  820  and receiver modules  400 ,  740 , and  840 , greater mounting stability and easier maintenance. It is to be understood, however, that the foregoing descriptions of LDVs  100 ,  700 , and  800  are purely exemplary and are not intended to be limiting. 
       FIG. 9  illustrates a system  900 , according to an embodiment of the present invention. In one example, system  900  includes a radiation source  920 , a modulator  940 , a transceiver  960 , an optical mixer  980  and a signal processor  990 . These elements may operate similarly to analogous features discussed above. In one example, one or more of modulator  940 , transceiver  960 , and mixer  980  may include multiple elements, i.e., one or more modulators, one or more transceivers, and one or more mixers, discussed in detail below. 
     In one example, source  920  is coupled to optical mixers  986 - 1 - 1  to  986 - n - m  via respective paths  930 - 1 - 1  to  930 - n - m,  transceivers  960 - 1  to  960 - n  are coupled to optical mixers  980 - 1  to  980 - n  via respective paths  950 - 1  to  950 - n,  and optical mixers  980 - 1  to  980 - n  are coupled to signal processor  990  via respective paths  985 - 1  to  985 - n.    
     In one example, source  920  comprises a coherent radiation source  922 , e.g., as a laser. In an example, laser  922  can be a fiber optic laser. In another example, laser  922  can be a rare-earth-doped fiber laser. In another example, laser  922  can be an erbium-doped fiber laser. 
     In one example, modulator  940  includes one or more modulators  942 - 1  to  942 - n,  n being a positive integer. In one example, first modulator  942 - 1  can operate to introduce a temporal amplitude modulation. In an example, the temporal amplitude modulation induced by modulator  942 - 1  can be of the form of a pulse. In an example, the temporal amplitude modulation can be of the form of a square wave pulse. In an example, the temporal amplitude modulation can be of the form of a sequence of pulses. In an example, the temporal amplitude modulation can be of the form of a sequence of pulses each with fixed duration of a first time duration separated by a second time duration. In an example, the temporal modulation can be of the form of an arbitrary sequence of pulses of arbitrary shape and duration separated by arbitrary delays. In an example, the temporal amplitude modulation can be of the form of a sequence of square wave pulses. 
     In an example, modulator  942 - 1  can be a semiconductor optical amplifier (SOA). In another example, modulator  942 - 1  can operate to induce a frequency modulation so as to shift the frequency of the source radiation to a higher or lower frequency. In an example, modulator  942 - 1  can be an acousto-optic modulator (AOM). 
     In an example, modulator  942 - 2  can operate to introduce a polarization modulation. In an example, the polarization modulation can be a rotation of the linear polarization of the source radiation. In an example, the polarization modulation can be such as to change a linear polarization of the source radiation into elliptical polarization. In an example, the polarization modulation can change an elliptical polarization of the source radiation into a linear polarization. In an example, modulator  942 - 2  can be a birefringent crystal. In an example, modulator  942 - 2  can be coupled to a Faraday rotator  9460  In an example, modulator  942 - 2  can be any device known in the art that operates to introduce a polarization modulation to the source radiation. 
     In one example, the use of first and second modulators  942 - 1  and  942 - 2  in series allows for a pulse amplitude modulation, such as a smaller pulse window (shorter duration and amplitude) within a larger pulse. 
     In an example, modulator  940  may also contain one or more optical isolators  944 - m,  where only isolator  944 - 1  is shown in  FIG. 9 . Optical isolators can be used to ensure that light propagates only in one direction along an optical fiber just as a diode in an electrical circuit ensures that current only flows in one direction. 
     In an example, transceiver  960  includes one or more transceiver modules  960 - 1  to  960 - n.  Each transceiver module  960 - 1  can include a splitter  964 - 1 , one or more transceivers  966 - 1 - 1  to  966 - 1 - m,  m being a positive integer, and an optional delay  968 - 1 . Splitter  964 - 1  can be a 1×m splitter, splitting a beam received from modulator  940  into in beams, one for each transceiver  9664  to  966 - m.  Each of the transceivers  966 - 1 - 1  to  966 - 1 - m  can comprise similar features and function similarly to transceivers  300  as shown in  FIG. 4  and described above. 
     In one example, delays  968 - 1  to  968 - n  are used to adjust the relative phases of the radiation input to transceivers  966 - 1 - 1  to  966 - n - m  to account for differing path lengths between the various transceivers and source  920 . 
     In one example, optical mixer  980  includes one or more mixer modules  980 - 1  to  980 - n.  For example, corresponding transceiver modules  960 - 1  to  960 - n  are coupled. via respective paths  9504  to  950 - n  to corresponding optical mixers  980 - 1  to  980 - n.  In one example, each mixer Module  980 - 1  to  980 - n  includes an optional delay  982 - n  along path  930 - n  coupled to source  920 , a splitter  984 - n,  one or more mixers  986 - 1 - 1  to  986 - 1 - m,  and optional delays  988 - 1 - 1  to  988 - 1 - n  coupled along paths  950 - n  to respective transceivers  966 - 1 - 1  to  966 - 1 - m  in respective transceiver modules  960 - 1  to  960 - n.    
     In one example, delays  982 - 1  to  982 - n  can be used to adjust the relative phases of the radiation input to mixers  980 - 1  to  980 - n  to account for differing path lengths between the source and mixer modules  980 - 1  to  980 - n    
     In one example, delays  988 - 1 - 1  to  988 - n - m  can be used to adjust the relative phases of the radiation input to the various mixers  986 - 1 - 1  to  986 - n - m  from the respective transceivers  966 - 1 - 1  to  966 - n - m  to account for differing path lengths between the respective mixers and transceivers. 
     In one example, spliter  984 - 1  can split a beam from source  920  into m beams that travel to corresponding mixers  986 - 1 - 1  to  986 - 1 - m  along respective paths  930 - 1 - 1  to  930 - 1 - m.  As discussed above, the optical mixers can measure a Doppler shift associated with radiation received by each transceiver  960  or  966  reflected back from the target regions relative to that of the source  920 . Thus, the function of the beam splitters  984 - n  is to provide reference signals from the source  920  to each of the mixers  986  that are needed in order to compare with the reflected radiation signal so as to measure a Doppler shift. 
     In one example, signals from each of the mixers  980 - 1  to  980 - n  are received via paths  985 - 1  to  985 - n  at signal processor  990 . These signals can be the digitized form of the respective Doppler shifts calculated by the various mixers as described above with reference to  FIG. 5 . In an example, the signal processor  990  can calculate a velocity component associated with each transceiver  960  or  966 . 
       FIG. 10  illustrates a system  1000 , according to an embodiment of the present invention. In one example, system  1000  includes a radiation source  1020 , a modulator  1040 , a transceiver  1060 , an optical mixer  1080  and a signal processor  1090 . These elements may operate similarly to analogous features discussed above. In one example, one or more of modulator  1040 , transceiver  1060 , and mixer  1080  may include multiple elements, i.e., one or more modulators, one or more transceivers, and one or more mixers, discussed in detail below. 
     In one example, source  1020  is coupled to optical mixers  1086 - 1 - 1  to  1086 - n - m    1080  via respective paths  1030 - 1 - 1  to  1030 - n - m,  transceivers  1060 - 1  to  1060 - n  are coupled to optical mixers  1080 - 1  to  1080 - n  via respective paths  1050 - 1  to  1050 - n,  and optical mixers  1080 - 1  to  1080 - n  are coupled to signal processor  1090  via respective paths  1085 - 1  to  1085 - n.    
     In one example, source  1020  comprises sources  1022 - 1  to  1022 - m,  each of which can be any source of coherent radiation, e.g., as a laser. In an example, each of sources  1022 - 1  to  1022 - m  can be a fiber optic laser. In another example, each of sources  1022 - 1  to  1022 - m  can be a rare-earth-doped fiber laser. In another example, each of sources  1022 - 1  to  1022 - m  can be an erbium-doped fiber laser. 
     In an example, each of sources  1022 - 1  to  1022 - m  can provide radiation at one or more different frequencies. In another example each of sources comprises m distinct frequencies. In an example, source  1020  further comprises a multiplexer (MUX)  1024 - 1 , e.g., a wave length division multiplexer, that combines beams from the various radiation sources into a single optical fiber  1026  that is output from source  1020  as input to a modulator  1040 . 
     In one example, modulator  1040  includes one or more modulators  1042 - 1  to  1042 - n.  In one example, first modulator  1042 - 1  can operate to introduce a temporal amplitude modulation. In an example, the temporal amplitude modulation induced by modulator  1042 - 1  can be of the form of a pulse. In an example, the temporal amplitude modulation can be of the form of a square wave pulse. In an example, the temporal amplitude modulation can be of the form of a sequence of pulses. In an example, the temporal amplitude modulation can be of the form of a sequence of pulses each with fixed duration of a first time duration separated by a second time duration. In an example, the temporal modulation can be of the form of an arbitrary sequence of pulses of arbitrary shape and duration separated by arbitrary delays. In an example, the temporal amplitude modulation can be of the form of a sequence of square wave pulses. 
     In an example, modulator  1042 - 1  can be a semiconductor optical amplifier (SOA). 
     In another example, modulator  1042 - 1  can operate to induce a frequency modulation so as to shift the frequency of the source radiation to a higher or lower frequency. In an example, modulator  1042 - 1  can be an acousto-optic modulator (AOM). 
     In an example, modulator  1042 - 2  can operate to introduce a polarization modulation. In an example, the polarization modulation can be a rotation of the linear polarization of the source radiation. In an example, the polarization modulation can be such as to change a linear polarization of the source radiation into elliptical polarization. In an example, the polarization modulation can change an elliptical polarization of the source radiation into a linear polarization. In an example, modulator  1042 - 2  can be a birefringent crystal. In an example, modulator  1042 - 2  can be coupled to a Faraday rotator  1046 . In an example, modulator  1042 - 2  can be any device known in the art that operates to introduce a polarization modulation to the source radiation. 
     In one example, the use of first and second modulators  1042 - 1  and  1042 - 2  in series allows for a pulse amplitude modulation such as a smaller pulse window (shorter duration and amplitude) within a larger pulse. 
     In an example, modulator  1040  may also contain one or more optical isolators  1044 - m.  Optical isolators  1044  can be used to ensure that light propagates only in one direction along an optical fiber, e.g., similar to a diode in an electrical circuit ensures that current only flows in one direction. 
     In an example, transceiver  1060  includes one or more transceiver modules  1060 - 1  to  1060 - n.  Each transceiver module  1060 - 1  can include a demultiplexer (DEMUX)  1064 - 1 , e.g., a wave length division DEMUX, one or more transceivers  1066 - 1 - 1  to  1066 - 1 - m,  and an optional delay  1068 - 1 . Demultiplexer  1064 - 1  can split a beam received from modulator  1040  into m beams, each having a separate frequency, one for each transceiver  1066 - 1  to  1066 - m.  Each of the transceivers  1066 - 1 - 1  to  1066 - 1 - m  can comprise similar features and function similarly to transceivers  300  as shown in  FIG. 4  and described above. 
     In one example, delays  1068 - 1  to  1068 - n  are used to adjust the relative phases of the radiation input to transceivers  1066 - 1 - 1  to  1066 - n - m  to account for differing path lengths between the various transceivers and the source  1020 . 
     In one example, optical mixer  1080  includes one or more mixer modules  1080 - 1  to  1080 - n.  For example, corresponding transceiver modules  1060 - 1  to  1060 - n  are coupled via respective paths  1050 - 1  to  1050 - n  to corresponding optical mixers  1080 - 1  to  1080 - n.  In one example, each mixer module  1080 - 1  to  1080 - n  includes an optional delay  1082  coupled to source  1020 , a demultiplexer (DEMUX)  1084 , e.g., a wave length division DEMUX, one or more mixers  1086 , and optional delays  1088  coupled to respective transceivers  1066 . 
     In one example, delays  1082 - 1  to  1082 - n  can be used to adjust the relative phases of the radiation input to mixer modules  1080 - 1  to  1080 - n  to account for differing path lengths between the source  1020  and mixer modules  1080 - 1  to  1080 - n    
     In one example, delays  1088 - 1 - 1  to  1088 - n - m  can be used to adjust the relative phases of the radiation input to the various mixers  1086 - 1 - 1  to  1086 - n - m  from the respective transceivers  1066 - 1 - 1  to  1066 - n - m  to account for differing path lengths between the respective mixers and transceivers. 
     In one example, wave length division demultiplexers  1084 - 1  to  1084 - n  can split a beam from source  1020  into m beams, each with a different frequency, that travel to corresponding mixers  1086 - 1 - 1  to  1086 - 1 - m.  As discussed above, the optical mixers measure a Doppler shift associated with radiation received by each transceiver reflected back from the target regions relative to the source. Thus, the function of the wavelength division demultiplexers  1084 - 1  to  1084 - n  can be to provide reference signals from the source  1020  to each of the mixers  1080 / 1086  that are needed in order to compare with the reflected radiation signal so as to measure a Doppler shift. 
     In one example, signals from each of the mixers  1080 - 1  to  1080 - n  are received via path  1085 - 1  to  1085 - n  at signal processor  1090 . These signals can be the digitized form of the respective Doppler shifts calculated by the various mixers as described above with reference to  FIG. 5 . In an example, the signal processor  1090  can calculate a velocity component associated with each transceiver. 
       FIG. 11  illustrates a system  1100 , according to an embodiment of the present invention. In one example, system  1100  includes a radiation source  1120 , a modulator  1140 , a transceiver  1160 , an optical mixer  1180  and a signal processor  1190 . These elements may operate similarly to analogous features discussed above. In one example, one or more of modulator  1140 , transceiver  1160 , and mixer  1180  may include multiple elements, i.e., one or more modulators, one or more transceivers, and one or more mixers, discussed in detail below. 
     In one example, source  1120  is coupled to optical mixer  1180  via path  1030 , transceiver  1060  is coupled to optical mixer  1180  via path  1150 , and optical mixer  1180  is coupled to signal processor  1190  via path  1185 . 
     In one example, source  1120  comprises sources  1122 - 1  to  1122 - m,  each of which can be any source of coherent radiation, e.g., as a laser. In an example, each of sources  1122 - 1  to  1122 - m  can be a fiber optic laser. In another example, each of sources  1122 - 1  to  1122 - m  can be a rare-earth-doped fiber laser. In another example, each of sources  1122 - 1  to  1122 - m  can be an erbium-doped fiber laser. 
     In an example, each of sources  1122 - 1  to  1122 - m  can provide radiation at one or more different frequencies. In another example each of sources comprises m distinct frequencies. In an example, source  1120  further comprises a multiplexer (MUX)  1124 , e.g., a wave length division MUX, that combines beams from the various radiation sources  1122  into a single optical fiber  1126  that is output from source  1120  as input to a modulator  1140 . 
     In one example, modulator  1140  includes one or more modulators  1142 - 1  to  1142 - n.  In one example, first modulator  1142 - 1  can operate to introduce a temporal amplitude modulation. In an example, the temporal amplitude modulation induced by modulator  1142 - 1  can be of the form of a pulse. In an example, the temporal amplitude modulation can be of the form of a square wave pulse. In an example, the temporal amplitude modulation can be of the form of a sequence of pulses. In an example, the temporal amplitude modulation can be of the form of a sequence of pulses each with fixed duration of a first time duration separated by a second time duration. In an example, the temporal modulation can be of the form of an arbitrary sequence of pulses of arbitrary shape and duration separated by arbitrary delays. In an example, the temporal amplitude modulation can be of the form of a sequence of square wave pulses. 
     In an example, modulator  1142 - 1  can be a semiconductor optical amplifier (SOA). In another example, modulator  1142 - 1  can operate to induce a frequency modulation so as to shift the frequency of the source radiation to a higher or lower frequency. In an example, modulator  1142 - 1  can be an acousto-optic modulator (AOM). 
     Iii an example, modulator  1142 - 2  can operate to introduce a polarization modulation. In an example, the polarization modulation can be a rotation of the linear polarization of the source radiation. In an example, the polarization modulation can be such as to change a linear polarization of the source radiation into elliptical polarization. In an example, the polarization modulation can change an elliptical polarization of the source radiation into a linear polarization. In an example, modulator  1142 - 2  can be a birefringent crystal. In an example, modulator  1142 - 2  can be coupled to a Faraday rotator  1146 . In an example, modulator  1142 - 2  can be any device known in the art that operates to introduce a polarization modulation to the source radiation. 
     In an example, modulator  1140  can operate so as to send only a single frequency at a time from the source  1120  to the transceiver  1160 . 
     In one example, the use of first and second modulators  1142 - 1  and  1142 - 2  in series allows for a pulse amplitude modulation such as a smaller pulse window (shorter duration and amplitude) within a larger pulse. 
     In an example, modulator  1140  may also contain one or more optical isolators  1144 - m.  Optical isolators  1144  can be used to ensure that light propagates only in one direction along an optical fiber, just as a diode in an electrical circuit ensures that current only flows in one direction. 
     In one example, each transceiver module  1160  can include a demultiplexer (DEMUX)  1164 , e.g., a wave length division DEMUX, transceivers  1166 - 1  to  1166 - m,  and an optional delay  1168 . Demultiplexer  1164  can split a beam received from modulator  1140  into m beams, each having a separate frequency, one for each transceiver  1166 - 1  to  1166 - m.  Each of the transceivers  1166 - 1  to  1166 - m  can comprise similar features and function similarly to transceivers  300  as shown in  FIG. 4  and described above. 
     In one example, delay  1168  is used to adjust the relative phases of the radiation inputt to transceivers  1166  to account for a differing path length between each transceiver  1160  and the source  1120 . 
     In an example, transceiver module  1160  is coupled via path  1150  to corresponding optical mixer  1180 . In one example, mixer module  1180  includes an optional delay  1182  coupled to source  1120 , a mixer  1186 , and an optional delay  1188  coupled to transceivers  1166 - 1  to  1166 - m.    
     In one example, delay  1182  can be used to adjust the relative phase of the radiation input to mixer  1180  to account for path length differences between the source and mixer module  1180  relative to the path length between the source and transceiver module  1160 . 
     In one example, delay  1188  can be used to adjust the relative phase of the radiation input to the mixer  1186  from the transceiver  1166  to account for path length differences between the respective mixers and transceivers  1166  relative to the path length bet weeh the source and mixer module  1180 . 
     As discussed above, the optical mixer  1180  can measure a Doppler shift associated with radiation received by the transceiver  1160  reflected back from the target regions relative to a reference signal from the source  1120  via path  1130  input to the mixer  1180 . 
     In one example, the signal output from the mixer  1180  is received via path  1185  at signal processor  1190 . This signal can be a digitized form of the respective Doppler shifts calculated by the various mixers as described above with reference to  FIG. 5 . In an example, the signal processor can calculate a velocity component associated with each transceiver. 
     As an exemplary application, the disclosed laser velocimeter systems may be used to determine the velocity of wind approaching a wind turbine so that the wind turbine can be dynamically controlled to more efficiently produce renewable energy. In such an application, it would be advantageous and feasible with the disclosed systems to measure wind velocities at a plurality of target regions at different ranges either simultaneously or at successive time intervals. 
     While specific embodiments of the present invention have been described above, it will be appreciated that the present invention may be practiced otherwise than as described. The description is not intended to limit the present invention. 
     It is to be appreciated that the Detailed Description section, and not the Summary and Abstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections may set forth one or more but not all exemplary embodiments of the present invention as contemplated by the inventor(s), and thus, are not intended to limit the present invention and the appended claims in any way. 
     The present invention has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The, boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. 
     The foregoing description of the specific embodiments will so fully reveal the general nature of the present invention that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the present invention. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by the skilled artisan in light of the teachings and guidance. 
     The breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. 
     The claims in the instant application are different than those of the parent application or other related applications. The Applicant therefore rescinds any disclaimer of claim scope made in the parent application or any predecessor application in relation to the instant application. The Examiner is therefore advised that any such previous disclaimer and the cited references that it was made to avoid, may need to be revisited. Further, the Examiner is also reminded that any disclaimer made in the instant application should not be read into or against the parent application.