Abstract:
Transient signal measurements are bandwidth limited by present digitizer technology. If a transient signal can be stored in a gain-clamped regenerative delay line, such a signal can be regeneratively sampled, resulting in about an order of magnitude increase in measurement bandwidth. The approach involves converting electrical signals to optical signals with high fidelity, injecting such signals into a fiber-optic delay line, and then sampling injected signals repetitively, with signal generation provided by an erbium-doped gain-clamped fiber amplifier. Moreover, signal regeneration can be either steady state (i.e., amplification on each pass) or switched (i.e., amplification after signal levels have dropped significantly).

Description:
RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/540,856, filed Jan. 30, 2004, entitled, “Electrical Transient Sampling System Using a Regenerative Fiber Optic Delay Line,” which is incorporated herein by this reference. 
     
    
       [0002]     The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory. 
     
    
     BACKGROUND OF THE INVENTION  
       [0003]     1. Field of the Invention  
         [0004]     The present invention relates to a measurement system. More particularly, the present invention relates to an electrical transient sampling system that utilizes a gain-clamped optical fiber recirculation loop to substantially reproduce one or more transient signals.  
         [0005]     2. Description of Related Art  
         [0006]     The Nyquist sampling theorem states that an input signal must be sampled at a rate greater than twice the highest frequency component contained in the signal of interest. A beneficial sampling rate is often from about 4 to about 10 times the input bandwidth of the digital scope. However, when a true single-shot transient is to be analyzed, sampling such a signal can be problematic.  
         [0007]     In general, there are two ways to get more samples on a single-shot transient waveform: 1) increase the sample rate, or 2) sample the waveform repetitively. The most obvious way to obtain more samples on the waveform is to increase the sample rate by using a faster analog-to-digital converter. However, the fastest available commercial sampling oscilloscopes have a resolution on the order of 35 ps, making such oscilloscopes somewhat undesirable for measuring single-shot transient signals of less than about 200 ps.  
         [0008]     The second way to get more samples on a single-shot transient waveform is to reproduce the waveform and repetitively sample the single-shot transient signal waveform reproductions. Samples from different reproductions are combined to reconstruct the waveform. The reproduced displayed waveform is therefore made up of many acquisitions of the signal, similar to that of an analog scope.  
         [0009]     Background information for reproducing a single-shot transient signal that includes a regenerative fiber loop, is described and claimed in U.S. Pat. No. 6,738,133 B1, entitled “Method and Apparatus For Measuring Single-Shot Transient Signals,” issued May 18, 2004 to Yin, including the following, “Methods, apparatus, and systems, including computer program products, implementing and using techniques for measuring multi-channel single-shot transient signals. A signal acquisition unit receives one or more single-shot pulses from a multi-channel source. An optical-fiber recirculating loop reproduces the one or more received single-shot optical pulses to form a first multi-channel pulse train for circulation in the recirculating loop, and a second multi-channel pulse train for display on a display device. The optical-fiber recirculating loop also optically amplifies the first circulating pulse train to compensate for signal losses and performs optical multi-channel noise filtration.” 
         [0010]     Accordingly, a need exists for methods and apparatus that can measure a single-shot transient signal. The present invention is directed to such a need.  
       SUMMARY OF THE INVENTION  
       [0011]     Accordingly, the present invention is directed to a sampling system, that utilizes techniques for measuring replicated single-shot transient signals. Such a system generally includes: a single-shot transient signal acquisition and modulation unit; a gain-clamped regenerative delay line configured to produce a plurality of pulse replicas of a desired transient single-shot signal; and a timing means adapted for sampling the pulse replicas so as to substantially resolve and thus reproduce the signal.  
         [0012]     Still another aspect of the present invention is directed to a sampling method that includes: providing a detected single-shot transient signal; regeneratively gain-clamp looping the detected single-shot transient signal to produce a plurality of pulse replicas of the transient single-shot signal; and sampling the pulse replicas to substantially reproduce a detected single-shot transient signal.  
         [0013]     Accordingly, the present invention provides optical arrangements and methods that include a gain-clamped regenerative loop configuration to reproduce detected high bandwidth transient pulses generated by events such as, but not limited to, impulse radar, pulsed nuclear magnetic resonance, and shock physics. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The accompanying drawings, which are incorporated into and constitute a part of the specification, illustrate specific embodiments of the invention and, together with the general description of the invention given above, and the detailed description of the specific embodiments, serve to explain the principles of the invention.  
         [0015]      FIG. 1  shows a simplified diagram of a regenerative gain-clamped transient single-shot sampling system.  
         [0016]      FIG. 2  illustrates a plurality of reproduced pulses generated by the system of the present invention.  
         [0017]      FIG. 3  shows another example embodiment of the regenerative gain-clamped delay line of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     Referring now to the drawings, specific embodiments of the invention are shown. The detailed description of the specific embodiments, together with the general description of the invention, serves to explain the principles of the invention.  
         [0000]     General Description  
         [0019]     The present invention provides a measurement system and method that includes a regenerative gain-clamped delay line for temporally measuring the shape of single-shot transient signals. A beneficial way of reproducing such signals is to use the optical portion of the electromagnetic spectrum in a converted electrical to optical signal gain-clamped fiber loop geometry to enable the reproduction of pulse shapes having a frequency of less than about 100 GHz with a resolution of down to about 4 ps.  
         [0020]     A novel aspect of the present invention is the utilization of a doped fiber amplifier, such as, for example, an Erbium Doped Fiber Amplifier (EDFA) having a gain-clamped feedback loop. Such an arrangement entails an all-optical feedback lasing signal within a secondary loop, sustained by the amplifier itself, which clamps the average inversion and thus the gain for predetermined wavelengths to a desired level. By using such a design in the present invention, replica gain-clamped pulses can be surprisingly maintained with a signal to noise ratio of better than 10/1 for often up to about 1000 pulses within a primary optical loop so as to substantially reproduce a detected single-shot transient signal having a frequency of less than about 100 GHz with a resolution of down to about 4 ps.  
         [0000]     Specific Description  
         [0021]     Turning now to the drawings, a diagram that illustrates an exemplary embodiment of a system constructed in accordance with the present invention is shown in  FIG. 1 . The system, designated generally by the reference numeral  10 , and capable of being designed as a portable compact apparatus, generally includes a signal acquisition and modulation unit  1  (shown within a dashed rectangle), a gain-clamped regenerative delay line  12  (again shown within a dashed rectangle), and a timing means  47  (also shown within a dashed rectangle) for reproducing a detected single-shot transient signal.  
         [0022]     Signal acquisition and modulation unit  1  is arranged to receive a single-pulse transient signal  6  and tracks such a received signal  6  by the induced modulation of an electromagnetic radiation beam traversing through an integrated-waveguide modulator  4 , such as, for example, a LiNbO Mach-Zehnder modulator. The radiation source itself is often designed to be a laser  2  arranged to output up to about  60 mW of optical power and capable of outputting a wavelength range between about 1310 nm and about 1550 nm, more often however, the output is designed about the low loss 1550 nm window for optical fibers. A beneficial arrangement is for laser  2  to be a narrow linewidth (e.g., 1 MHz) laser source, such as, a Distributed Feedback laser (DFB), having a limited chromatic dispersion-induced signal distortion of about 0.14 ps/km. Although a DFB is often a beneficial arrangement, other radiation sources, e.g., tunable single-longitudinal output optical sources, such as, but not limited to, Distributed Bragg Reflectors, Sampled Grating DBRs, Grating-assisted Co-directional Couplers with Sampled Reflectors, and Vertical Cavity Surface Emitting Lasers capable of operating within the designed parameters may also be utilized when operating within the scope and spirit of the present invention.  
         [0023]     Turning back to  FIG. 1  so as to describe the method and system of the invention, such a modulated output by signal acquisition and modulation unit  1 , which is indicative of a detected single-pulse transient signal, is received by one or more optical elements, such as a polarizer  8  configured to restrict the vibration orientation of the output of laser  2 . An optical coupler  14 , such as, a 3 dB optical tap coupler, is configured to receive and direct such polarized components to gain-clamped regenerative delay line  12  (along a path denoted by the letter A).  
         [0024]     Gain-clamped regenerative delay line  12  of the present invention generally includes a recirculating delay loop  18  and an amplifier  26 , such as a Raman amplifier, more often a fiber amplifier, such as, an Erbium Doped Fiber Amplifier (EDFA) with a bandwidth of 5 tereahertz, and a noise figure of 3.5 dB. In addition regenerative delay line  12  includes a feedback loop (denoted by the letter F and shown as a dashed arrow path) for looping out and back to amplifier  26  a predetermined spectral bandwidth within the gain spectrum of amplifier  26  so as to deplete excited state ions and clamp the gain for a desired spectral bandwidth received from laser  2  (e.g., about 1550 nm).  
         [0025]     Erbium-doped fiber amplifiers have an ultrafast (subfemtosecond) signal response, but also have a long excited state lifetime (10 ms). When a transient signal is injected into an EDFA, the excited state population is rapidly depleted. With the long excited state lifetime, recovery from this excited state depletion is slow, and the net result is the amplifier gain is greatly reduced. In transient sampling, this gain reduction results in signal reproductions with a rapidly decreasing amplitude. This reduced reproduction amplitude severely limits the number of samples and introduces significant signal distortion.  
         [0026]     We have surprisingly discovered that gain clamping the EDFA in a transient sampling apparatus, as disclosed herein, results in constant amplitude signal replicas having a signal to noise ratio of better than 10/1, which, when sampled, result in accurate signal reconstructions. Without gain clamping, signal reproduction accuracy is severely limited.  
         [0027]     Gain clamping is possible in an EDFA since the gain medium is homogeneously broadened, which means that excited state ions can participate in stimulated emission at different wavelengths. This is the key concept in gain clamping. The EDFA may be configured to lase on one wavelength, and amplify on another wavelength. In this configuration, excited state ions may be transferred from the lasing channel to the amplification channel instantaneously, compensating for changes in the amplifier gain due to the introduction of a fast transient signal pulse. The effect is to form an automatic gain control optical circuit.  
         [0028]     In  FIG. 3 , a train of reproduced signals in a gain clamped transient sampling device is shown. The original signal is pulse  92 , as shown in  FIG. 3 , and a plurality of all the other pulses  96  are replicas of this original signal. Note that the amplitude of the signal reproductions is constant. In practice, several thousand signal reproductions, often up to about 1000 pulses with usable amplitudes, i.e., having a signal to noise ratio of better than 10/1, and minimal distortions can be produced. This allows a much higher effective sampling rate than in transient sampling devices that do not incorporate a gain clamp.  
         [0029]     The signal is launched into gain-clamped regenerative delay line  12  (shown by path A) through, as one example configuration, a dispersion compensating (i.e., a fiber that includes positive dispersion (e.g., +18.5 ps/nm/km) and/or negative dispersion (e.g., −37 ps/nm/km)) low loss (i.e., about 0.2 dB) fiber arranged as recirculating delay loop  18 . Delay loop  18  can be configured prior to amplifier  26  or such a delay loop  18  can be configured to follow amplifier  26  with design details implemented (e.g., integrating positive and/or negative dispersion fiber into the geometry in either configuration) to compensate for residual chromatic dispersion innduced when using such optical techniques and configurations. The use of this type of dispersion compensation is advantageous for transient sampling. Conventional zero dispersion fiber has large nonlinear susceptibilities that degrade the operation of transient sampling devices. Non-zero dispersion shifter fibers have reduced parasitic nonlinearities, but the residual chromatic dispersion limits the number of samples in a transient sampling device. The use of spliced negative and positive dispersion fibers in the loop results in zero dispersion with very low parasitic nonlinearities due to the significant dispersion in each segment of the fiber.  
         [0030]     By arranging recirculating delay loop  18  to have predetermined lengths between about 5.5 km and 1.2 km, a time delay for pulses received from path B, as shown in  FIG. 1 , that traverses within such a loop, can enable state-of-the art sampling scope technologies having sampling rates between about 40 kHz and about 200 kHz to resolve single-shot transient pulses of less than about 100 GHz due to a plurality of generated and thus sampled replica pulses having a signal to noise ratio of better than 10/1.  
         [0031]     After traversing through recirculating delay loop  18 , such pulses can be directed to a second coupler  22 , (e.g., a  3  dB tap coupler) which then can direct the pulses to amplifier  26 . Amplifier  26 , as stated herein before, is often an EDFA, which includes a fiber whose core is uniformly doped with Erbium ions to produce a homogeneously broadened simple two-level system. It is to be appreciated that the present invention capitalizes on such a system by configuring a feedback loop to induce a process known to those skilled in the art as cross-gain modulation, i.e., by directing a feedback signal to amplifier  26  of one wavelength (e.g., 1532 nm) so as to influence the gain for a desired signal wavelength (e.g., 1550 nm).  
         [0032]     Generally, the feedback geometry, as shown in  FIG. 1 , can be arranged with an optical isolator  30  (to prevent reverse oscillations), a third optical coupler  34  to direct radiation along denoted paths F and/or B, a band-pass filter  36  for allowing only a predetermined feedback signal (e.g., 532 nm) to oscillate within feedback loop F, and a polarization controller  38  to restrict the polarization to a predetermined orientation so as to enable optical coupler  22  to direct the feedback signal in the polarization eigenstate of the loop, resulting in optimal amplitude for the signal reproductions.  
         [0033]     Specifically, the feedback mechanism is in a fiber-loop geometry so as to effectively produce a ring laser. Such an optical feedback arrangement causes instability in the loop and if the gain in the fiber amplifier is initially greater than the loop loss, the fiber loop path starts oscillating at a wavelength determined by, for example, in-line band-pass filter  36  centered at a desired wavelength, e.g., at 1532 nm. The flux within the loop for such a lasing wavelength increases until its gain equals the loop loss, thus fixing (i.e., clamping) a desired inversion in the Erbium core of amplifier  26  and thus the gain for a predetermined wavelength (e.g., 1550 nm). Although the gain in such an arrangement is fixed, an alternate desired inversion, and thus the gain can be changed (e.g., for 1550 nm) by configuring system  10  to produce a different feedback wavelength within the homogenously broadened gain spectrum of amplifier  26  or by designing for increased gain or losses within the loop geometry utilizing optical components or techniques known to those skilled in the art.  
         [0034]     Accordingly, a plurality of gain-clamped replica pulses of a detected. single-shot transient pulse is then directed along path B, as allowed by band-pass filter  40 . Polarization controller  44  is arranged to produce a predetermined polarization for such pulses so as to enable optical coupler  14  to direct a produced optically split pulse of each of the replicated pulses to timing means  47  along path C, as shown in  FIG. 1 , and to direct a produced optically split pulse of each of the replicated pulses to gain-clamped regenerative delay line  12  along path A, as shown in  FIG. 1 , to repeat the process of producing replicas of a desired signal while retaining operation within the polarization eigenstate of the loops.  
         [0035]     Timing means  47  (shown within a dashed rectangle) generally includes an optical receiver  50 , such as, but not limited to, a Shottky or pin photodiode, to detect replicated pulses, a state-of-the-art sampling scope  54 , and a Data Timing Generator  58  (shown having a received trigger pulse for timing purposes) to provide timing logic and reduce overall system jitter so as to optimize the number of pulses sampled by scope  54 .  
         [0036]     It is to be noted that although system  10 , as shown in  FIG. 1  has many types of undesired noise sources, such as, for example, scope noise (e.g., about 100 microwatts), modulator noise (e.g., about 50 microwatts), and EDFA noise of between about 25 to 900 microwatts. However, such noise is capable of being reduced in the present invention by replacing all fiber connections with fusion splices having, for example, a 0.3 dB loss, so that amplifier  26 , (i.e., EDFA), as shown in  FIG. 1 , does not have to run as hard and so as to reduce overall system noise.  
         [0037]      FIG. 3  shows another example embodiment of gain-clamped regenerative delay line  12 , as shown in  FIG. 1 , having common optical components, i.e., recirculating delay loop  18 , optical coupler  22 , amplifier  26 , optical isolator  30 , and polarization controllers  38  and  44  operating as described above. However, in the example embodiment of  FIG. 2 , an output of amplifier  26  through optical isolator  30  is directed via an optical circulator  32  to an optical fiber grating  39 , such as a Bragg grating, operating as a mirror for a predetermined wavelength (e.g., 532 nm). Upon reflection, optical circulator  32  can receive and direct such a predetermined wavelength to feedback loop F to fix the inversion of amplifier  26  as discussed above. Subsequently, gain clamped pulses having a desired wavelength (e.g., 1550 nm) are capable of being further directed by optical circulator  32  through fiber Bragg grating  39  (designed to be transmissive at such a wavelength) and along path B to be received by polarization controller  44  as discussed above in  FIG. 1 .  
         [0038]      FIG. 2  illustrates a plurality of gain-clamped pulse replicas of a single-shot transient pulse as produced by the system and methods of the present invention. As shown in  FIG. 3 , a first pulse  92  is substantially replicated by a subsequent pulse  96  and a plurality of later pulses  98 .  
         [0039]     Applicants are providing this description, which includes drawings and examples of specific embodiments, to give a broad representation of the invention. Various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this description and by practice of the invention. The scope of the invention is not intended to be limited to the particular forms disclosed and the invention covers all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the claims.