Abstract:
A multidimensional spectrometer encodes frequency information into laser pulses so that a frequency insensitive detector may be used to collect data for a multi-dimensional spectrograph only from intensity information and knowledge of a modulation providing the encoding. In one embodiment the frequency encoding may be done by a conventional interferometer greatly simplifying construction of the spectrometer.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under DK079895 awarded by the National Institutes of Health and 0350518 and 1013324 awarded by the National Science Foundation. The government has certain rights in the invention. 
    
    
     CROSS REFERENCE TO RELATED APPLICATION 
     Background of the Invention 
     The present invention relates to two-dimensional spectrometers. 
     Two-dimensional spectroscopy can reveal the interaction between coupled systems of atoms and/or molecules. In a 2D spectrum, electromagnetic transitions of atoms or molecules give rise to signals that lie on the spectrum diagonal, and if there are interactions between transitions within or among molecules, then cross peaks will also appear in the spectrum. Thus, the diagonal and cross peaks can be used to deduce molecular structures or identify mixtures of compounds, etc. Multidimensional spectra of 3D or higher provide additional information. 
     To collect a 2D spectrum, one needs to generate multiple light pulses to interrogate the sample. Some examples for two-dimensional spectroscopy are described, for example, in US patent application 2006/0063188 filed Sep. 15, 2005 and US patent application 2009/0161092 filed Dec. 21, 2007 both hereby incorporated by reference. 
     Existing multidimensional spectrometers require specialized and complex optical components and are correspondingly expensive and difficult to manufacture. 
     SUMMARY OF THE INVENTION 
     The present invention provides a simplified and robust multidimensional spectrometer that encodes frequency information into temporal characteristics of laser beams (changing spectrum or phase) traveling along two optical paths. This allows a multidimensional spectrograph to be generated with a simple and highly sensitive single channel detector without the need for frequency resolving the signal, which greatly simplifies the construction of the device. In one embodiment, this encoding may be done by common Michelson-type interferometers greatly simplifying construction of such spectrometers. 
     Specifically, the present invention provides multi-dimensional spectrometer having a first and second spectral modulator producing corresponding light pulses which may be directed through a sample volume to be received by an intensity sensitive detector. A controller communicates with the first and second spectral modulator to controllably modulate both the first and second light pulses so that a two dimensional spectrum of a sample in the sample volume may be generated solely from the intensity and modulation information. 
     It is thus one feature of a least one embodiment of the invention to permit multidimensional spectroscopy by using only two optical paths and without complex and expensive, frequency-discriminating detection such a spectrometer and an array detector. 
     The first and second spectral modulator may produce pairs of light pulses with first and second controllable time separations, the controller modulating the pulses by controllably altering the time between the pulses of each pulse pair. 
     It is thus a feature of a least one embodiment of the invention to provide simple mechanisms for synthesizing the required pulses and to control or characterize their properties, for example, by interference between two identical pulses. 
     The multi-dimensional spectrometer may include a processor receiving data from the detector and the first and second time separations, and applying a two-dimensional Fourier transform to the data as a function of the first and second time separations to provide a two-dimensional spectrograph of a sample in the sample volume. 
     It is thus a feature of a least one embodiment of the invention to permit rapid data acquisition by obtaining simultaneous absorption measurements at multiple frequencies that are later separated by a transformation such as the Fourier transform. 
     The multi-dimensional spectrometer may include a polarizer setting a polarization to a least one of the first and second light pulses. 
     It is thus a feature of a least one embodiment of the invention to permit polarization of the pulses to be controlled in certain experiment types. 
     The multi-dimensional spectrometer may include a phase modulator that alters the phase to a least one of the first and second light pulses. 
     It is thus a feature of a least one embodiment of the invention for the phases of the pulses to be controlled in certain experiment types. 
     At least one of the first and second spectral modulators may be an interferometer. 
     It is thus a feature of a least one embodiment of the invention to construct a multi-dimensional spectrometer from common optical components. 
     At least one of the first and second spectral modulators may provide a tracer beam and further include a fringe detector monitoring the tracer beam to deduce a change in the interferometer controlling the changes in time separation of the pulse pair formed by the interferometer. 
     It is thus a feature of a least one embodiment of the invention to permit rapid real time measurements with devices having mechanical element such as interferometer-type spectral modulators. 
     At least one of the first and second spectral modulators may provide an interference analyzer receiving a pair of light pulses of the spectral modulator to determine the absolute time separation of the pulses. 
     It is thus a feature of a least one embodiment of the invention to permit the resulting spectrograms to be repeatably located in frequency space without corruption by mechanical inaccuracies of interferometer mechanisms. 
     The controller may execute a stored program to provide constant velocity scanning in the first and second controllable time separations while collecting data from the detector at sample times and marking the collected data at each sample time with a first and second time separation measured at each sample time. 
     It is thus a feature of a least one embodiment of the invention to permit rapid data acquisition in a mechanical system without requiring settling times normally required for precise mechanical control. 
     The constant velocity scanning may provide a scanning pattern selected from the group consisting of: substantially simultaneous variation of the first and second controllable time separations and substantially simultaneous variation of one of the first and second controllable time separations and intermittent variation of the other of the first and second controllable time separations. 
     It is thus a feature of a least one embodiment of the invention to permit flexible acquisition patterns that may be tailored to particular experimental requirements. 
     At least one of the first and second spectral modulators may be a pulse shaper that modulates the electric field spectral intensity, phase and/or polarization. 
     It is thus a feature of a least one embodiment of the invention to provide a multidimensional spectrometer that may flexibly employ alternative modulation systems. 
     The multi-dimensional spectrometer may include a beam splitter receiving a light pulse from a single laser source to provide a received light pulse to the first and second spectral modulator an optical delay element positioned in an optical path of at least one of the first and second spectral modulators. The controller may communicate with the optical delay element to control a time separation between the first and second light pulses. 
     It is thus a feature of a least one embodiment of the invention to permit the use of as few as one laser light source for generation of the probe and pump pulses. 
     At least one of the first and second spectral modulators may further control the time and phase envelope of the at least one of the first and second light pulses and wherein the controller further communicates with the modulator to alter the time and phase envelope of the at least one of the first and second light pulses. 
     It is thus a feature of a least one embodiment of the invention to provide a flexible control of other parameters of the probe and pump pulses. 
     The multi-dimensional spectrometer may further include at least one beam position detector and the controller further may operate to automatically align the beams with the sample volume by monitoring the beam position detector. 
     It is thus a feature of a least one embodiment of the invention provide a system with reduced optical paths that may be readily aligned automatically. 
     These particular features and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. The following description and figures illustrate a preferred embodiment of the invention. Such an embodiment does not necessarily represent the full scope of the invention, however. Furthermore, some embodiments may include only parts of a preferred embodiment. Therefore, reference must be made to the claims for interpreting the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a block diagram of the present invention employing two spectral modulators for the generation of laser pulses for two-dimensional spectroscopy; 
         FIG. 2  is a detailed block diagram of a first embodiment of the spectral modulators of  FIG. 1  employing Michelson-type interferometers and showing the optical path for a pulse and a local tracer laser beam and showing a fringe detector and phase detector used for characterization of the modulated pulses; 
         FIG. 3  is a time graph of an example pump pulse pair along the first optical path and an example probe pulse pair along the second optical path that might be produced by the present invention; 
         FIG. 4  is a detailed block diagram of the fringe detector of  FIG. 2  showing the extraction of quadrature waveforms using orthogonal polarizing analyzers; 
         FIG. 5  is a diagram of an interferogram produced by the phase detector of  FIG. 2  for deducing absolute delay and phase difference between the pulse pairs; 
         FIG. 6  is a flow chart showing high-speed constant velocity scanning and independent data acquisition possible with the present invention; 
         FIG. 7  is a diagram of a trajectory in time space for the collection of precursor spectrographic data per the present invention; 
         FIG. 8  is a block diagram of a computer suitable for use as a controller/processor per the present invention; 
         FIG. 9  is a detailed block diagram of a second embodiment of the spectral modulators of  FIG. 1  employing pulse shapers, e.g. with acousto-optic modulators, for producing controlled modulated pulses; and 
         FIG. 10  is a fragmentary view of an alternative embodiment of the spectral modulators of  FIG. 2  or  9  providing automatic beam alignment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , the present invention  10  uses a pulsed light source  12  producing a series of ultrashort laser pulses  14 . Example pulses may have a center frequency of 800 nm, a frequency FWHM of 28 nm, a temporal FWHM of 50 femtoseconds, and a repetition rate of 1 KHz. A pulsed light source  12  suitable for this purpose is described in Montgomery et al., 110 J. Phys. Chem. A, 6391-6394 (2006), incorporated herein by reference. The light pulse from the laser may be received by an optical parametric amplifier (not shown) to convert it into the mid infrared region to reduce the mechanical sensitivity of the optical system resulting from longer wavelengths. Generally, the output of the pulsed light source  12  will include multiple frequencies spanning a desired frequency range of the spectrogram to be produced. 
     The pulses  14  may be received by a beam splitter  16  directing the pulses both to a first spectral modulator  18  and a second spectral modulator  20 . 
     In a first embodiment, one or both of the spectral modulators  18  and  20  may be a Michelson-type interferometer. In this case, the beam splitter  16  may also receive a continuous wave signal  22  from a conventional laser  24 , for example a helium-neon laser operating in the visible region. The continuous wave signal  22  is also directed to both of the spectral modulators  18  and  20  by action of the beam splitter  16 . 
     Referring now also to  FIG. 2  and  FIG. 3 , one embodiment of the first and second spectral modulators  18  and  20 , in the form of a Michelson interferometer, is shown. In the case of the first spectral modulator  18 , the interferometer may take a single pulse  14  and produce a modulated pulse  26 . When the spectral modulator is an interferometer of the Michelson type it is understood that the modulated pulse consists of a pair of pulses with a time separation of t 3 . The modulated pulse  26  is received by a focusing mirror  28  and directed through a sample chamber  30  or the like to illuminate a sample material  40  at a first time t 0 . It will be understood that other methods of directing the modulated pulse  26  through the sample chamber  30  may be used, for example, including one or more lenses. 
     In the case of the second spectral modulator  20 , the interferometer will likewise take a single pulse  14  and produce a modulated pulse  32  having a time separation of t 1  and also received by the focusing mirror  28  to be directed through the sample chamber  30  to arrive at the sample material  40  at time t 2  before the modulated pulse  26 . Alternatively, the two sets of pulses may also arrive in tandem or alternating with one another. This delay time t 2  is controlled by an optical delay element  34 , for example, a movable wedge or other similar delay element that may be static or preferably dynamic and controlled by an actuator  36  such as a motor and encoder system for type known in the art. 
     Either or both of the modulated pulses  26  and  32  may pass through other optical elements  41  including polarizers, lenses, intensity attenuators, and phase shifters and the like, which may be controlled manually or automatically by means of the controller  50  as will be described below. 
     The modulated pulse  32  may pass through the sample chamber  30  after interaction with a sample material  40  contained therein to be received by a light stop  37 . Emissions by the sample material  40 , caused by the combination of the modulated pulse  26  and the modulated pulse  32 , may pass out of the sample chamber  30  to be received by a detector  38  providing a measurement of light intensity or amplitude. The detector  38  may be a simple photo detector or may use a balanced detector system. Generally, the detector  38  need only provide sensitivity to light intensity and need not be able to discriminate among light frequencies. 
     A controller  50 , for example a computer to be described below, may provide bidirectional control signals  51  to and from the spectral modulators  18  and  20  controlling the relative delay changes between the modulated pulse-pair  26  (t 3 ) and between pulse-pair  32  (t 1 ) and reading fringe and phase information as will be described. Alternatively, when the spectral modulators are pulse-shapers, the controller will predetermine the properties of the modulated pulses. An additional control signal  53  may be provided to the actuator  36  to control the delay element  34  and thus the delay time t 2  and to the elements  41  to control polarization, intensity or the like for either or both of modulated pulses  26  and  32 . Data from the detector  38  may be collected by the controller  50 . The controller  50  may be connected to a graphic display terminal  52  or the like for displaying an output spectrogram or other data to a user input device  54  for entering data as may be required. Referring momentarily to  FIG. 9 , the controller  50  may also be used to alter the spatial or temporal overlap of the pulses  26 ,  32  in the sample chamber  30  by using, for example, a pin-hole detector  31  or similar target at the sample position and adjusting the optical path of the pulse is  26  and  32  by feedback using for example piezo crystals actuators  33  on the mirrors  28   a  and  28   b  and maximizing light received by the pin-hole detector  31 . 
     The pulsed light source  12  may produce a pulse trigger signal  60  that may be provided to the spectral modulators  18  and  20  for use as will be described below and to the detector  38 , for example for gating of the detector  38 . 
     Referring now to  FIG. 2 , each of the spectral modulators  18  and  20 , when implemented as interferometers, may receive the pulse  14  and continuous wave signal  22  along a common optical path that may be divided by beam splitter  62  to direct both the pulse  14  and continuous wave signal  22  to a movable reflector  64  controlled by actuator  66  and a stationary reflector  70 . The movable reflector  64  may be moved along an axis  68  of the received pulse  14  and continuous wave signal  22  by the actuator  66  which may, for example, be a motor and position encoder pair. It will be understood that the movable reflectors  64  alternatively may be operated without direct control by the controller  50 , for example, to free-run in a reciprocating pattern. This operation is possible because of the tagging of the data with accurate modulation information related to the position of the movable reflector  64  at the time of data sampling as will be described. 
     Reflected light from both the movable reflector  64  and the stationary reflector  70  may be received by a beam splitter  72  combining this light and directing it along an exiting optical path  74  and an internal optical path  76 . A difference in the optical paths (1) from the beam splitter  62  through the movable reflector  64  to the optical path  74  after beam splitter  72  and (2) from the beam splitter  62  through the stationary reflector  70  to the optical path  74  after the beam splitter  72  defines the time and phase separation of pulse pairs of either the modulated pulse  26  or the modulated pulse  32  depending on the particular spectral modulators  18  and  20 . In this way, the time values t 3  and/or t 1  may be controlled by movement of movable reflector  64  according to the general principle of the Michelson-type interferometer as will be understood in the art. 
     The internal optical path  76  provides the modulated pulse  26  (or modulated pulse  32 ) to a intensity detector  77 , such as an infrared pyroelectric detector, which produces an interferogram output  80  which will be used to determine the absolute delay and phase difference between the pulse-pairs of the modulated pulse  26  and those of modulated pulse  32  as will be described further below. 
     The continuous wave signal  22  follows the same optical paths described above with respect to pulse  14  also traveling along the internal optical path  76  after interference with itself producing interference pattern  87 . The interference pattern  87  presents a varying average intensity resulting from constructive and destructive interference between portions of the continuous wave signal  22 . As such, it measures the same paths producing the modulated pulse  26  and modulated pulse  32  and can be used to calibrate path length changes. The interference pattern  87  is measured by a detector  86  to provide a fringe count output  90  as will be described. A beam splitter  82  and appropriate filters (not shown) may use to separate the interferogram  87  from the modulated pulses  26  or  32 . Since interference pattern  87  is used only to measure delay changes, the continuous wave tracer beam and pulsed laser beam may also follow different but equivalent paths through the interferometer as will be understood in the art. 
     Referring still to  FIG. 2 , a wave plate  100  (for example, providing one quarter wavelength phase delay of the continuous wave signal  22 ) may be inserted in a portion of the optical path exclusive to stationary reflector  70  to provide a circular polarization of the continuous wave signal  22  from the stationary reflector  70 . When this signal combines with the continuous wave signal  22  from the movable reflector  64  the interference pattern  87  consists of a superposition of circular and linear polarization as indicated by arrows  101  in  FIG. 4 . Referring now also to  FIG. 4 , the interference pattern  87  may be analyzed at two orthogonal polarizations, for example, by using a polarization cube  102  to separate the interference pattern  87  into two paths with perpendicular polarizations associated with corresponding detectors  106  and  108 . As will be understood from this description, each detector  106  and  108  will produce an intensity signal  110  and  112  respectively that will be in quadrature phase. These quadrature intensity signals  110  and  112  allow a direction and magnitude of relative path length changes between the interferometer arms to be determined (by counting peaks or level crossings and analyzing the apparent relative order of the intensity signals  110  or  112  which will change according to directions of the delay change). From this information, a precise determination of the change of the time delays t 3  and t 1  for the modulated pulse  26  and modulated pulse  32  can be made. This fringe counting, which may be conducted by a program within the controller  50  or by an independent controller, provides a rapid indication of the change in these time values t 3  and t 1 , and each intensity measurement by detectors  38  and  77  can thus be located on a (two-dimensional) time grid as now described. 
     Asynchronously with the motions of the actuators  66 , per process block  125  (as will be described further below) the pulsed light source  12  may be fired periodically as indicated by process block  126  to produce a pulse  14 . At each time of firing, as indicated by the pulse trigger signal  60 , the controller  50  may record intensity information from the detector  38 , a point of the interferograms from detectors  77  and fringe count outputs  90  from fringe counters  86  (one for each interferometer). The fringe count outputs are used to tag measured signal intensities and interferogram points which will be appended to the collected data per process block  130 . 
     In the present invention this is supplemented with the periodic absolute calibration of the time grid as will now be described. 
     Referring now to  FIG. 5 , as noted above detector  77  measures an interferogram  116  as a function of t 3  or t 1  caused by the interference between the pulse-pair  26  or the pulse-pair  32  for spectral modulator  18  or  20 . A Fourier transform (FT)  118  of the interferogram  116 , for example, performed by a program in the controller  50  can be used to produce spectra  120  each having a frequency dependent amplitude value  122  and a frequency dependent phase  124  as will be understood in the art. For example, three different spectra  120   a ,  120   b  and  120   c  may be computed from the interferogram  116  by starting the Fourier transform at different sample points near the peak of the interferogram. When the starting sample point is near the absolute delay zero between the pulse-pair  26  (or pulse-pair  32 ) which will be near the peak of the interferogram  116 , the phase  124  of that spectrum  120   b  will be substantially constant as a function of frequency as opposed to rising (per spectrum  120   a , FT started at earlier sampling points) or falling (per spectrum  120   c , FT started at later sampling point). This rising or falling of the phase of the spectrum  120  follows from the fact that different frequencies of the pulse  14  will be in phase and out of phase at different times. The starting sample points thus determined for interferometers  18  and  20  will be used as the starting points for the Fourier transform  140  of the signal recorded by detector  38 . The two-dimensional spectrogram  142  is given by the real part of that two dimensional Fourier transform multiplied by the spectral phases  124  (one for each dimension t 1  and t 3 ) as will be understood in the art. 
     Referring now to  FIG. 6 , the ability to precisely calibrate the two-dimensional time grid allows the open loop control of the actuator  66  for smooth and continuous motion as indicated by process block  125  in which the actuators  66  may be operated without reference to a priori knowledge of the precise position of the movable reflectors  64  in the spectral modulators  18  and  20 . The control is sometimes termed “constant velocity scanning” and refers primarily to the minimization of changes in velocity and does not require strict constant velocities. This constant velocity scanning contemplates that the actuators  66  themselves may have closed loop position sensing using local encoders; however, this closed loop control will be generally insufficiently accurate to precisely characterize time delays. 
     Referring now to  FIG. 7 , the actuators  66  may be operated so that the sample points  132  collected at process block  128  are spread in two dimensions corresponding generally to the times t 1  and t 3  as will be referred to here as a phase space  134 . Note that the open loop control does not require that the sample points  132  be in specific locations within the phase space  134  so long as they are approximately evenly distributed within that space. A possible trajectory for the actuators  66  under the control of process block  125  in the phase space  134  may be a simple raster pattern  136 ; however, other scanning patterns may be adopted as may be suitable for the particular actuators  66 . In one embodiment, each line in the t 3  dimension may be scanned in the forward and reverse direction and the sample points  132  interpolated to common t 3  values and averaged. Identical interpolation and averaging is used for the simultaneously recorded intensities by detectors  77 , producing the two interferograms  116 . At any time, but most simply at the conclusion of the collection of data for a given phase space  134 , the delay element  34  maybe moved to provide a new phase space  134 ′ displaced by a dimension corresponding generally to time t 2 . In this way, three dimensions of data may be obtained. It will be appreciated that other trajectories through phase space  134  may be adopted including those which provide for simultaneous change of dimensions t 1  and t 3  for example moving on a diagonal or circular trajectory. 
     Referring again to  FIG. 6 , as indicated by process blocks  138  and  140 , once phase space  134  has been fully sampled to the resolution desired, grid calibration (as described above) and a two-dimensional Fourier transform along the perpendicular axes of t 1  and t 3  may be carried out on the averaged/interpolated data to extract the desired two-dimensional spectrogram  142 . This Fourier transformation into a spectrogram  142  is possible because the spectral modulators  18  and  20  modulate the intensity in the dimensions t 1  and t 3 . This modulation encodes intensity by frequency so that when transformed a spectrum is revealed. 
     Referring now to  FIG. 9 , in a second embodiment, one or both of the spectral modulators  18  and  20  may employ a pulse shaper instead of an interferometer for providing the necessary output pulses (here designated as modulated pulses  26 ′ and  32 ′). As is understood in the art, a pulse shaper  150  may generally receive the pulses  14  from the beam splitter  16  (shown in  FIG. 1 ) and direct them, for example, by a first mirror  152 , to a diffraction grating  154  or similar device such as a prism, to produce a spectrally dispersed beam  156 . The spectrally dispersed beam  156  may pass through a lens system  158  to a focal plane at which an acousto-optic or other type of modulator  160  is placed. The acousto-optic modulator  160  may receive a control signal  51  from the controller  50  to attenuate different portions of the spectrally dispersed beam  156  and hence to provide selective frequency attenuation or phase modulations of the spectrum of the spectrally dispersed beam  156 . The spectrally dispersed beam  156 , as modulate, is then reconstituted by lens system  162  and grating  164  to be directed by mirror  166  out of the spectral modulator  18  or  20  and ultimately toward the sample chamber  30  (shown in  FIG. 1 ) as modulated pulse  26 ′ (for the first spectral modulator  18 ) and modulated pulse  32 ′ (for the second spectral modulator  20 ). Generally, the time envelope of modulated pulses  26 ′ and  32 ′ need not be two separate pulses but will have a similar spectral and phase characteristic as if synthesized by the techniques described with respect to the interferometer of  FIG. 2 . It will be understood that in this case the absolute (two-dimensional) time grid (phase-space) or equivalent information needed to extract the 2D-spectrum is typically encoded by the pulse shaper and pre-determined by the controller and need not be determined independently as described for the interferometer above. 
     It will be also appreciated that not only the spectral characteristics and phase characteristics of the modulated pulses  26 ′ and  32 ′ and modulated pulses  26  and  32  may be flexibly modified in this manner, but also the polarization, envelope shape, intensity and the like. It will be understood that a variety of variations on the described embodiments are contemplated by the present invention. For example although it is desired to use a single pulsed light source  12 , two separate coordinated laser sources may also be used. Different interferometers such as a Mach Zehnder interferometer may be used. The delay mechanism of the delay element  34  maybe a variety of different types of time delay systems including wave plates, retro reflectors, movable mirrors, polarizers and the like. Modified schemes for the generation of the quadrature signals  110  and  112  from the tracer beam and variants of the fringe counting algorithm may be employed. A variety of different optical systems including different lens and mirror configurations may be used instead of a mirror  28  for focusing the pulses on the sample chamber  30 . Additional polarizers and the like may be used to manipulate the modulated pulse  26  and modulated pulse  32  to control their interaction with the sample. A conventional dispersion type spectrometer may also be used in conjunction with an array detector such as a CCD device instead of detector  38  eliminating the need for one interferometer and one dimension of Fourier transform. Beam splitters need not be conventional partially reflective plates but can be other functionally identical structures including wire grid polarizers or the like. 
     Referring now to  FIG. 8 , the controller  50  may be, for example, a standard computer providing a processor  200  communicating via an internal bus with the memory  202  and in interface  204 , the latter communicating with the terminal  52  and the user input device  54  as well as providing the control signals  51  to the spectral modulators  18  and  20  and the delay element  34  and receiving phase and fringe counting information in return. The memory  202  may include the programs  206  implementing the above-described programs with respect to  FIG. 6  including phase counting frequency analysis Fourier transforms and the like. 
     Certain terminology is used herein for purposes of reference only, and thus is not intended to be limiting. For example, terms such as “upper”, “lower”, “above”, and “below” refer to directions in the drawings to which reference is made. Terms such as “front”, “back”, “rear”, “bottom” and “side”, describe the orientation of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import. Similarly, the terms “first”, “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context. 
     When introducing elements or features of the present disclosure and the exemplary embodiments, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of such elements or features. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements or features other than those specifically noted. It is further to be understood that the method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
     References in the claims and specification to a first and second modulator should not be interpreted as limited to two devices in separate housings or to two devices having separate components but may be realized with any device, including a single device, operating to provide independent modulation of two different light beams according to independent modulation signals. Likewise, each of the first and second modulators may be composed of multiple modulators. 
     References to “a controller” and “a processor” can be understood to include one or more controllers or processors that can communicate in a stand-alone and/or a distributed environment(s), and can thus be configured to communicate via wired or wireless communications with other processors, where such one or more processor can be configured to operate on one or more processor-controlled devices that can be similar or different devices. Furthermore, references to memory, unless otherwise specified, can include one or more processor-readable and accessible memory elements and/or components that can be internal to the processor-controlled device, external to the processor-controlled device, and can be accessed via a wired or wireless network. 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. All of the publications described herein, including patents and non-patent publications, are hereby incorporated herein by reference in their entireties.