Abstract:
A device for creating an optic pulse with different wavelengths separated by time. A pump laser is configured to output energy to a dye cell which, responsive to the energy, outputs an optic pulse. Mirrors direct the optic pulse away from the dye cell towards a spectrograph. The spectrograph has an input and two or more outputs. The spectrograph receives and converts the optic pulse to a wavelength separated optic signal presented on the two or more outputs. A first optic cable has an input end and an output end. The input end receives a first output from the spectrograph. A second optic cable has an input end and an output end. The input end receives a second output from the spectrograph. The second optic cable is a different length than the first optic cable to establish a time shift between the signals exiting the first and second cable.

Description:
1. STATEMENT REGARDING FEDERAL RIGHTS 
       [0001]    This invention was made with government support under Contract No. DE-AC52-06NA25946 and was awarded by the U.S. Department of Energy, National Nuclear Security Administration. The government has certain rights in the invention. 
       2. PRIORITY CLAIM 
       [0002]    This application claims priority to and the benefit of U.S. Provisional Patent Application No. 62/236,096 filed on Oct. 1, 2015, the contents of which are incorporated by reference in its entirety herein. 
     
    
     3. FIELD OF THE INVENTION 
       [0003]    The invention relates to optic pulse generation and in particular to a method and apparatus for generating a wavelength dispersed pulse with wavelengths separated in time. 
       4. BACKGROUND 
       [0004]    When performing scientific research or material interaction studies, it is often beneficial or necessary to generate a chirped optical pulse. A chirped pulse may be used to record or illuminate an event which occurs during an ultra-short time scale. A chirped pulse is a pulse of energy, such as in the UV, visible, or near-infrared regions of the spectrum, with different wavelengths of the pulse which are dispersed, spread or chirped. However, numerous challenges exist in relation to prior art attempts to generate a chirped pulse. 
         [0005]    One prior art approach was using a long single fiber path to create dispersion, but this approach required large spools of fiber optic cable, which are heavy, expensive, space consuming, and very difficult to move. Another proposed solution is chirped fiber Bragg grating but this approach results in modulation on the group delay of CFBGs which leads to deviation of linearity of the dispersion. It is also known to use a rotating mirror/grating assembly, but this approach is slow and as a result does not meet the needs of applications requiring higher speed capabilities. 
         [0006]    Therefore, a need exists for an improved system to generate a chirped pulse and in particular a chirped or chirped-like pulse that is selectable in wavelength, spectral width, and temporal width. 
       SUMMARY 
       [0007]    This is a method for creating a chirped-like laser pulse that can be operated in the UV, visible, or near-infrared regions of the spectrum. The exemplary method of operation described below uses a pulsed broadband dye laser, wavelength dispersing element, and fiber-optic cables of varying lengths that terminate in a larger core recombining fiber or a simple connector to output a laser pulse that moves or is shifted in time and wavelength over the duration of the pulse. Total temporal and spectral pulse widths can be varied and made longer than what is available with prior art available chirped lasers. The device will have application anywhere a temporally longer chirped laser output might be desired or where a chirped output is needed in a specific spectral region. In one exemplary application, the purpose is to illuminate a rapidly (spatially) varying object, thus encoding temporal phenomena onto a wavelength for subsequent detection with compatible technology. 
         [0008]    To overcome the drawbacks of the prior art, a device for creating an optic pulse with different wavelengths separated by time is disclosed. In one embodiment, this device includes a pulsed pump laser configured to output energy and a dye cell configured to receive the energy from the pump laser and, responsive to the energy, output an optic pulse. One or more mirrors are configured to direct the optic pulse away from the dye container towards a spectrograph. The spectrograph has an input and two or more outputs and the spectrograph is configured to receive the optic pulse on the input and convert the optic pulse to a wavelength separated optic signal that is presented on the two or more outputs. Also part of this embodiment is a first optic cable having an input end and an output end with the input end configured to receive a first output from the spectrograph, and a second optic cable having an input end and an output end such that the input end is configured to receive a second output from the spectrograph. The second optic cable has a different length than the first optic cable which establishes a time shift between the signals first and second cable output. 
         [0009]    The device may further comprise a coupler configured to combine the light signal from the output end of the first optic cable and the second optic cable to a single path. In addition, it is also contemplated that the device may further comprise one or more lenses configured to focus the energy from the pump laser and one or more lenses configured to focus the output of the dye cell. 
         [0010]    Also contemplated are additional optic cables having an input end and an output end such that the input end is configured to receive an additional output from the spectrograph and the additional optic cables have a different length than the first optic cable and the second optic cable. The output end of the first optic cable and the output end of the second optic cable may be contained in one or more SMA connectors or any other type of connector or fiber optic cable termination. It is contemplated that the device may further comprise a large core fiber arranged to accept the first output from the first optic cable and second output from the second optic cable. In one embodiment, the energy from the pump laser is a 10 ns pulse width at 532 nm wavelength. This device may further comprise a fiber core size larger than a slit width of the spectrograph. 
         [0011]    Also disclosed is a device for creating an optic pulse with different wavelengths separated by time. One embodiment of this device includes a pulse generator configured to output an optic pulse and a wavelength dispersive device having an input and two or more outputs. The wavelength dispersive device is configured to receive the optic pulse on the input and convert the optic pulse to at least one wavelength separated optic signal that is presented on the two or more outputs. Also part of this device is a first optic cable having a first optic cable input and a first optic cable output. The first optic cable input is configured to receive a first output from the wavelength dispersive device and present a first optic cable output signal from the first optic cable output. A second optic cable is provided with a second optic cable input and a second optic cable output. The second optic cable input is configured to receive a second output from the wavelength dispersive device and present a second optic cable output signal from the first optic cable output. The second optic cable having a different length than the first optic cable thereby causing a time shift in the second optic cable output signal as compared to the first optic cable output signal. In this embodiment an optional fiber core is provided and configured to combine the first optic cable output signal and the second optic cable output signal onto a single core. 
         [0012]    In one embodiment, this device further comprises one or more lenses configured to focus the energy from the pump laser and one or more lenses configured to focus the output of the dye cell. In one configuration, the second optic cable is 1.5 meters longer than the second optic cable. The energy from the pump laser may be a 10 ns pulse width at 532 nm wavelength. The disclosed device of this embodiment further comprising a fiber core size larger than a slit width of the spectrograph. In one configuration, the wavelength dispersive device is a spectrograph and the pulse generator generates a multiband multiple wavelength pulse. 
         [0013]    Also disclosed is a method for generating a chirped optic pulse comprising generating a laser pulse such that the laser pulse includes two or more wavelengths. This method includes receiving the laser pulse at a wavelength dispersive device. The wavelength dispersive device spreads the laser pulse into at least a first output having a first wavelength, and a second output having a second wavelength. Then, receiving the first optic output on a first optic path and receiving the second optic output on a second optic path. The second optic path has a different length than the first optic path. Outputting a first path output from the first optic path and a second path output from the second optic path so that the first path output and the second path output form a chirped optic pulse. 
         [0014]    In one embodiment, the step of generating a laser pulse comprises providing exciting energy to a dye cell and responsive to the exciting energy, the dye cell outputting the laser pulse. This method of operation may further comprise combining the first path output and the second path output onto a single fiber. In one embodiment, the wavelength dispersive device comprises a spectrograph. The first optic path and the second optic path may be formed from optic fibers. 
         [0015]    Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In the figures, like reference numerals designate corresponding parts throughout the different views. 
           [0017]      FIG. 1  illustrates one example embodiment of a chirped pulse generator. 
           [0018]      FIG. 2  is an operational flow diagram providing an example method of operation. 
           [0019]      FIG. 3  is a plot of the un-tuned dye laser showing 20 overlapped pulses. 
           [0020]      FIG. 4  is a plot of the output of the seven individual fibers in the fiber bundle, each of which are normalized to near 1.0 amplitude. 
           [0021]      FIGS. 5A and 5B  are a plot of the final system output as a function of time. 
           [0022]      FIG. 6  is a plot of Spectral output as a function of time for a 4-channel system. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    To overcome the drawbacks of the prior art and to provide additional benefits, a Long-Pulse-Width, Variable-Wavelength “Chirped” Laser Pulse Generator is disclosed.  FIG. 1  illustrates one example embodiment of a chirped pulse generator. This is but one possible configuration and it is understood that one of ordinary skill in the art will arrive at different configurations without departing from the scope of the claims. In general, this system produces a chirped or chirped-like pulse that is selectable in one or more: wavelength, spectral width, and temporal width. The long-pulse-width variable-wavelength device built to successfully demonstrate the concept is shown in  FIG. 1 . 
         [0024]    As shown, the pulse generator  104  includes a pump laser  104  that generates a laser pulse  112 . In other embodiments, any type of laser or light source may be used. The output of the laser  104  is presented to and focused by a lens  116 , such as a cylindrical lens, to a dye cell  124 . The dye cell may be liquid filled, solid such as a piece of plastic impregnated with dye. The cylindrical lens is configured to focus the output of the pump laser to a line, such as a linear pattern. The dye solution  124  may comprise a solvent at the appropriate concentration for lasing and may be contained in a container, such as but not limited to a 1 cm by 1 cm by 5 cm quartz cuvette. The pump laser  108  and dye container  124  operate in connection, using the pump laser to focus an exciting energy pulse to the dye cell which activates the dye cell to generate a wideband pulse. In other embodiments, other devices may be used to activate a second laser (dye cell) instead of the pump laser, such as but not limited to flash lamps or electrical discharge devices. Thus, the pulse source is not limited a dye laser as the luminous source. It is contemplated that any other multiple or broad spectral band, short temporal pulse width laser or other spectral light source would also work. 
         [0025]    The dye cell emits a range of wavelength range, which may be referred to as broadband output. In other embodiments other devices may be used, such as a flash lamp dye laser, but it is preferred to have a broadband pulse source such as in one embodiment, 10 or 2 nanometers wide broadband pulse source. In other embodiments other pulse widths may be used. 
         [0026]    The dye is chosen to provide the desired output wavelength range, and the pump wavelength is chosen accordingly for efficient dye pumping. In this embodiment, dye laser operates in the un-tuned wide-spectral-line-width mode, but in other embodiments, other modes of operation may occur. 
         [0027]    Mirrors  120 A,  120 B are provided at the back of the laser cavity and the front to function as shown by reflecting the laser pulse to a spherical lens  128  that in turn focuses the light signal onto the slit of an imaging spectrograph  132 . 
         [0028]    As is understood, lasers need to have a cavity to oscillate in and an active medium, such as the dye in the dye cell  124 . In this embodiment the back mirror  120 A is close to the dye cell to provide feedback namely the light is reflected back into the dye cell, and overlaps light in the other direction toward the second mirror  120 B, which in turn directs the output to the lens  128 . The second lens  128  focuses the reflection from the mirror  120 B from a generally round shape to a line. In one embodiment, lens  128  is a concentrator. 
         [0029]    The imaging spectrograph  132  is a device configured to receive a broad or multiband input and output two or more optical outputs which are of different wavelength. In this embodiment, the spectrograph  132  has one input and seven outputs such that each output carries a different wavelength of light. In other embodiments a greater number of inputs may be used and a greater number or fewer number of outputs may be used. The input to the imaging spectrograph  132  comprises a multiband signal that includes two or more wavelengths and the output of the imaging spectrograph comprises the input with the wavelengths spread or separated in space. In other embodiments, a device besides a spectrograph or its components may be used such as any device that spreads the light input by wavelength. For example, a prism or prism effect device may be used, or grating with corrective optics, or anyway wavelength dispersive device. 
         [0030]    The output fibers  140  align with different outputs of the spectrograph  132  to present the wavelength spread input signal to the linear fiber array  140 . The fiber array is two or more lengths of fiber, each of which receive the optical output of the imaging spectrograph  132 . The linear fiber array positioned at the spectrograph output image plane is adjusted to center the light on the fiber array. At the other end of the fiber bundle, two or more of the fibers are terminated in an SMA connector (not shown) or any other type of connector or termination. A SMA connector  136  is a connector type that terminates an optic fiber  140 . The optic fibers  140 , or fiber extensions which connect to the fibers, are of varying lengths and are connected to each of the fibers such that at least one fiber along the linear array is a different length than at least one other fiber. In one embodiment, each fiber is 1.5 meters longer than the previous fiber length which in turn causes the travel time of light in each successive fiber to be roughly 7.5 ns longer than its previous neighbor. Adjusting or changing the length of the fiber results in corresponding changes in the time differences between the travel time of light within the fiber. In this embodiment, there are seven optic fibers. 
         [0031]    In this embodiment, the output end of these fibers  140  are bundled in a close-packed arrangement in a circular single SMA connector  150  as shown. The fibers  140  may also terminate in a line slit. The output of the fibers  140  from the SMA may be use directly to illuminate a scene of interest, with or without a lens such that each fiber is carrying a slightly different wavelength and due to the different lengths of the optic fibers  140 , each wavelength is delayed in time. The output from the bundle of optic fibers  140  can be used to directly illuminate the scene of interest through an optional iris  154 , with or without a lens  158 , or butt-coupled to a short length of large core (large enough to accept the outputs of all the fibers) fiber (not shown) to mix the outputs and transport the chirped beam. The single, final output is the desired spread chirped pulse. 
         [0032]    Although shown as a complete system, it is contemplated that different light sources may be used besides the pump laser and dye cell. Likewise, although shown with various cylindrical lenses, a greater number or fewer number of lenses may be used and lenses of other shapes may be used Likewise, devices other than an imaging spectrograph may be used where such devices may serve a generally similar function as the imaging spectrograph. In addition, the imaging spectrograph is typically formed from multiple parts and as such, each individual element may be used to perform a generally similar function. 
         [0033]    To verify the operational parameters and performance of the proposed design a system was created using the general specifications and items listed below. It should be understood that the specifications and items listed below should not be considered the only configuration and the claims are not limited to this configuration. In this example configuration a pump laser was used with a 10 ns pulse width FWHM and a 532 nm wavelength. Other lasers, pulse widths, and wavelengths can be used. The dye is Rhodamine 6G but in other embodiments other dyes could be used. The solvent is Ethanol but it is contemplated that other solvents can be used, depending on the dye. 
         [0034]    The dye laser outputs a 4.5 nm pulse width FWHM and the mode is un-tuned, wide-spectral-line-width. As with the other aspects, this may change across different embodiments. The imaging spectrometer is a Jarrell Ash Mark X with a slit set to 100 microns. For higher power capability, a cylindrical lens can be used at the input, then a relay lens arrangement that includes a spherical plus cylindrical lens must be used at the output to match that of the linear fiber array. The grating in this unit is 200-grooves/mm and the dispersion is 3 nm/mm at the image plane. Other gratings and/or other spectral dispersing instruments can be used to give more or less spectral spreading of the broadband dye laser pulse across the fiber array. 
         [0035]    In this embodiment the fiber array is a Thor Labs BFL200HS02 fiber array that includes seven 200-micron core fibers (other sizes or quantities of fibers may be used). The individual fiber output is 0.5 nm FWHM and the spectral range is nominally 4 nm for the seven fibers used in the disclosed embodiment. A pulse of  2  μJ/pulse for brightest fiber, dropping to about 20% of this value for the weakest fiber output. Each fiber length segment was 1.5 m longer than the previous segment. As discussed above, changing the length of the fiber segments will adjust the duration of the resulting chirped pulse and separation between wavelength. 
         [0036]      FIG. 2  is an operational flow diagram providing an example method of operation. This is but one possible method of operation to create a long chirped pulse based on multiple discrete pulses. At a step  208  a pump laser generates a light beam using a laser or other light source. In other embodiments, other energy sources and types of energy may be used. Next, at a step  212 , a lens or other device focuses the light beam to a line shape which is directed to a dye container containing dye, such as a laser dye cell. The dye may comprise any type dye suitable for generating a multiband (multiple wavelength) optic output in response to an exciting input. 
         [0037]    At a step  216  the system generates a laser light beam due to the excitation of the dye in the dye cell. The resulting multiband optic signal is directed to one or more lens using one or more mirrors to an imaging spectrograph. At a step  220  the spectrograph separates the received laser light beam in to two or more optic outputs separated or differentiated by wavelength. Thus, the multiband input to the spectrograph is separated or spread based into multiple outputs such that the output optic signals have different wavelengths. Next, at a step  228  the system receives the two or more wavelength specific optic outputs at two or more optic fibers such that the two or more optic fibers are of different length. 
         [0038]    At a step  236  the fiber optic cables output the two or more wavelength specific optic outputs from the optic fibers such that the wavelength specific optic outputs are also spaced in time due to different lengths of optic fibers. At a step  240 , the system may optionally combine the optic outputs from optic fibers into a fewer number of optic signals to create the chirped optic output. Then, at a step  246  the system may direct the chirped optic output to focus plane or any other application using one or more lenses. 
         [0039]      FIG. 3  is a plot of the un-tuned dye laser showing 20 overlapped pulses. In this plot, the horizontal axis  304  represents wavelength in nanometers (nm) and the vertical axis  308  represents relative intensity of the optic signal. The plots  312  represent numerous outputs of the dye laser, which is also the input to the spectrograph. These plots will vary with dye, solvent, concentration, pump power, and other factors. 
         [0040]      FIG. 4  is a plot of the output of the seven individual fibers in the fiber bundle, each of which are normalized to near 1.0 amplitude. In this plot, the horizontal axis  404  represents wavelength in nanometers (nm) and the vertical axis  408  represents relative intensity of the plotted optic signal. The various plots  412  are the outputs from the spectrograph and are separate by wavelength by the spectrograph. Small changes in the grating angle can be made to select the region of the broadband dye output that is launched into the fibers. 
         [0041]      FIG. 5A and 5B  is a plot of the final output as a function of time. In this plot, the horizontal axis  504  represents wavelength in nanometers (nm) and the vertical axis  508  represents relative intensity.  FIG. 5A  is a three dimensional plot while  FIG. 5B  is a two dimensional plot. In this plot, the wavelength ranges from approximately 587.5 nm to 591.5 nm. In other embodiments, other wavelength ranges may be used. The z axis represents time in 2 ns steps with a 4 ns gate. Same data is shown in  FIG. 5A  and  FIG. 5B , but each normalized to near full scale. Also, because of the 10 ns FWHM pump laser pulse width and the 4 ns detector gate width, outputs from the separate adjacent fibers are significantly mixed. In other embodiments, greater separation may be established. 
         [0042]    The graph shown in  FIG. 5B  is the same data set as that down in  FIG. 5A , but with selected frames (times) only, normalized to 1. The strongest channels are best attenuated somewhat to prevent the spectral “tails” of one channel from competing with the outputs of the weak channels. This results in normalizing all outputs toward 1 at the expense of photons. 
         [0043]    It is contemplated that further optimization may be made to create an optimized system. For example, high-power coatings and/or different optics could be used to enable higher pulse energy throughput. A pump laser with half the pulse length may be used to further separate the individual outputs with the existing fiber delays. Another contemplated improvement is a slightly larger space between adjacent fibers in the linear array to thereby decrease channel-to-channel spectral overlap. 
         [0044]    In general, for best performance, spectral channels should be reasonably well separated. In one embodiment this is achieved by using a fiber core size somewhat larger than the slit width. In addition, or instead of this option, it is possible to also use a spacer between each fiber in the linear array, assuming a spectrograph with reasonable resolution/dispersion), and the pump laser pulse length can be less than the fiber output temporal spacing delays. Such an output was created and achieved with the existing system by disconnecting fibers  2 ,  4 , and  6  as shown in the plot of  FIG. 6 . This resulted in a 4-channel system with the desired performance.  FIG. 6  is a plot of Spectral output as a function of time for a 4-channel system. In this plot, each frame is a 2 ns increment. The plots  616 ,  618 ,  620 ,  622  are shown in relation to three axis. In this plot the axis  604  represents wavelength in nanometers the axis  608  represents relative intensity, and the axis  612  represents time. The plot line  628  shown in a  FIG. 6  represents an instant in time for references purposes. 
         [0045]    While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of this invention. In addition, the various features, elements, and embodiments described herein may be claimed or combined in any combination or arrangement.