Abstract:
An optical domain spectrum analyzer/channelizer employs multicasting of an analog signal onto a wavelength division multiplexing grid, followed by spectral slicing using a periodic optical domain filter. This technique allows for a large number of high resolution channels. Wideband, 100% duty cycle, spectrum analysis or channelization is made possible permitting continuous time wideband spectral monitoring. The instantaneous bandwidth of the spectrum analyzer/channelizer is equal to the full radio frequency bandwidth of the analyzer/channelizer.

Description:
FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT 
     The United States Government has certain rights in this invention. Licensing inquiries may be directed to Office of Research and Technical Applications, Space and Naval Warfare Systems Center, Pacific, Code 72120, San Diego, Calif., 92152; telephone 619-553-2778; email: T2@spawar.navy.mil. Please reference Navy Case No. 101250. 
    
    
     BACKGROUND 
     Electrical and optical techniques have been applied to wideband radio frequency (RF) spectrum analysis/channelization. 
     A conventional electrical domain RF channelization technique employs a bank of narrowly spaced filters. For wideband applications where relatively high instantaneous bandwidth (IBW) is sought (i.e. a significant number of channels), designs become difficult to tune, bulky and expensive. An alternative electrical domain method of wideband channelization uses a high speed analog-to-digital converter (ADC) followed by a Fast Fourier Transform (FFT). 
     Though progress has been made in increasing ADC IBW, current ADC IBWs are inadequate to monitor the entire RF spectrum, where tens of gigahertz bandwidth is required. When the frequency range of interest is greater than the ADC IBW, the full frequency range can still be analyzed by appropriate splitting, filtering, amplifying, down conversion and digitization of the resulting multiple channels. This approach however suffers from increased size, weight, and power requirements. 
     Because of the limitations of the electrical domain methods, several optical domain RF spectrum analyzer/channelization approaches have been proposed. One highly studied approach employs an incoherent transversal filter. This design implements necessary negative filter coefficients, but this adds complexity to the design. There are also attendant performance degrading coherent effects that must be mitigated with this design. 
     The coherent transversal filter approach offers the possibility of excellent channelization performance and this approach is in fact commonly used in integrated devices for channel spacing relevant to wavelength division multiplexed optical networks, i.e., &gt;25 GHz. For channel spacing relevant to RF applications, however, the required delay times are longer and therefore the tuning and stability of devices using the coherent approach become increasingly difficult. 
     Other optical domain RF channelization approaches exist such as described in R. K. Mohan, et al., “Ultra-wideband spectral analysis using S2 technology” Journal of Luminescence 127, 116 (2007) and M. Stead, “Using Dispersion in a Fiber-Optic Loop to Perform Time Domain Analogue RF Signal Auto-Correlation,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2011), paper OThW3. 
     SUMMARY 
     An optical domain spectrum analyzer/channelizer employs multicasting of an analog signal onto a wavelength division multiplexing (WDM) grid, followed by spectral slicing using a periodic optical domain filter. This technique allows for a large number of high resolution channels. Wideband, 100% duty cycle, spectrum analysis or channelization is made possible permitting continuous time wideband spectral monitoring. The IBW of the spectrum analyzer/channelizer is equal to the full RF bandwidth of the analyzer/channelizer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a general approach to an analyzer/channelizer according to an exemplary embodiment as described herein. 
         FIG. 2  is a graph showing pump copies as produced via four-wave mixing in accordance with an embodiment shown herein. 
         FIG. 3  illustrates pump and signal replication according to an embodiment shown herein. 
         FIG. 4  illustrates a detailed approach to an analyzer/channelizer according to an exemplary embodiment according to the description herein. 
         FIG. 5  is a detailed description of a mixing function in accordance with an embodiment described herein. 
         FIG. 6  is a graph depicting experiment data from an analyzer/channelizer having 10 GHz to 14 Ghz and of 10 optical channels. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1 , there is a shown a general approach to an optical domain spectrum analyzer/channelizer system  10  according to the description herein. System  10  includes an operation that can be broken down into three stages including a processor front-end stage  12 , a mixing stage  14  and a processor back-end stage  16 . Theses stages will initially be described in terms of their processing functions and will be later be described by way of example. 
     Front-end stage  12  modulates an RF signal of interest  18  onto an optical signal laser carrier  20  via an electro-optical modulator  22  that is null-biased  24 , thereby generating RF modulated signal laser carrier signal  26 . Signal  26  is spectrally combined with two continuous wave (CW) amplified low noise laser inputs  30  and  42  (from source  34 ) in multiplexer  28  to thereby pump a parametric mixing process. 
     Mixing stage  14  performs a multicasting operation, creating a comb of copies of the pump lasers and of the signal laser.  FIG. 2  depicts experiment data of the optical spectrum of the pump laser comb of copies. Third-order nonlinear processes enable mixing between the two original pump lines and serves to create the pump laser comb of copies, with a period equal to the frequency differences between the two pump lines.  FIG. 3  illustrates how third-order nonlinear processes result in mixing of the signal laser with the pump lines, thereby creating a signal laser comb of copies. The signal laser comb position and the signal laser comb free spectral range (FSR) can be arbitrarily selected by tuning the frequency of the three laser sources. Third order nonlinear processes can be any one or a multitude of different phenomena such as four-wave mixing, self-phase modulation and cross phase modulation. 
     Referring again to  FIG. 1 , back-end processing stage  16  performs signal segregation for use in spectral analysis or channelization. A narrow-band periodic filter  36  slices a narrow band or frequency bin out of each copy of the RF modulated signal laser comb of copies. The output of mixer  14  is tuned so that the FSR of the signal comb differs from the period of the narrow-band periodic filter by the filter&#39;s passband width. Each slice is routed to a separate optical channel  38  by a wave division demultiplexer  40 . Element  42  is either a photodetector when system  10  is used as a spectrum analyzer, or is a signal combiner wherein the separate optical channel is heterodyned with an optical carrier so that information on each channel can be extracted. 
     The Vernier relationship between the array of multicast signals and the narrow band filter passband ensures that the RF selected spectral bins are adjacent and non-overlapping. The filter&#39;s pass band position can be tuned so that the longest wavelength channel contains the lowest RF frequency bin of interest. This enforces a one to one mapping of RF bins to detection elements  42 , channelizing the entire RF frequency range without repetition. 
     Referring now to  FIG. 4 , there is shown a specific embodiment of the analyzer/channelizer presented by way of example. 
     In  FIG. 4 , two pump lasers  44  and  46 , spaced by Δω p =200 GHz, are first appropriately polarization controlled ( 48 ,  50 ) and are then combined in multiplexer  52 . The output of the multiplexer is boosted in an optical amplifier  54  such as an Erbium doped fiber amplifier (EDFA) to reach a 1 Watt optical power at the mixer input as the system is set up to operate between about 1 and 2 Watts at the mixer. The boosted multiplexer  52  output  56  is then spectrally separated in demultiplexer  58  to filter amplified spontaneous emission noise added by optical amplifier  54 . 
     A separate signal laser  60  is input into an electro-optic modulator  62  which is null biased  64 . The RF content  66  to be analyzed is input into modulator  62 , creating sidebands which contain the RF information ( FIG. 4A ). The RF modulated signal laser signal  68  is appropriately polarized  70  and amplified  72  for further processing. 
     Pump laser inputs ( 74 ,  76 ) and the RF modulated signal laser input are combined in a multiplexer  80 , such as a commercial off-the-shelf (COTS) ITU grid. Note that the signal laser is spaced Δω p /4=50 GHz away from a pump copy ( FIG. 4B ). Prior to the signal and pumps being input into mixer  82 , polarization beam splitter  84  and polarization controller  86  condition front-end multiplexed output signal  88  so that the polarization of output signal  88  is collinear and aligned to the optimal polarization of the mixer. 
     Mixing stage  82  has, as its purpose, the function of making copies of the pumps and signal and in effect generates a pump laser comb of copies and a modulated signal laser comb of copies. The signal copy spacing is in this instance Δω p /2=100 GHz. An elaboration of this mixing stage will follow; however, at this juncture of the description, the further processing of the copied signals will be described. 
     The analyzer/channelizer processor back-end begins with a filter  90  designed to attenuate the pump laser comb of copies. Following filter  90  is a narrow-band periodic filter  92  such as a tunable Fabry-Perot etalon. Filter  92  is set up with a passband Δω 0 =250 MHz and a passband spacing of Δω f =Δω p /2+Δω 0 =100.25 GHz so as to filter out one designated RF bin per signal copy. The etalon filtered output is then de-multiplexed with a standard dense WDM  94  like WDM  80 . The output optical channels  96  are further processed in detector elements  98 , either to measure output power for spectral analyzing purposes (such as via photo detectors) or are heterodyned with an optical carrier signal so that information can be extracted from each channel as present (as a channelizer). Alternatively, information contained in a subcarrier can be extracted. This approach is possible when the subcarrier and its sidebands are fully contained within one narrowband filter passband. 
     With 72 signal copies and a 250 MHz passband for each copy, the example implementation described covers a total IBW of 18 GHZ. This 18 GHz band can be positioned, for example, to cover the 2-20 GHz frequency range. This technology can also be used to cover a 100 GHz IBW by generating 100 copies of the signal and filtering these copies with a 1 GHz periodic filter passband. If a smaller resolution bandwidth is desired, this technology can also be used to cover a smaller frequency range, for example, with 100 copies covering a 1 GHz IBW. Channels with a 10 MHz resolution can be obtained with an appropriate etalon. 
     Referring now to  FIG. 5 , there is shown a detailed description of mixer  82  of  FIG. 4 . 
     Higher-order mixing has been investigated before, but is deemed impractical so far to reach high-copy-count due to the excessively high power needed to spawn a large number of higher-order pumps. Further, the use of intense pumps can lead to severe noise generation through Raman/Brillouin scattering and parametric fluorescence, thereby degrading the signal and copy quality. 
     Recognizing the problem associated with higher-order pump generation, the mixing processing of this description is designed to reduce pump power. It does so by utilizing precise, staged engineering of the mixer dispersion profile. This design is to enable power-efficient broadband mixing without resorting to excessive counter-productive pump levels. 
     The underlying principle of the mixing process will be described by considering the pump-pump interaction only, though  FIG. 5  illustrates pump and signal generation (signal shown dotted in the illustration). The dispersion-engineered mixer can be segmented into two stages, an initial stage corresponding to a seeding/compression function and a secondary stage focused on multi-casting. 
     Two pump waves, for example 1555.6 nm at 29.6 dBm power and 1559 nm at 29 dBm power, respectively, are first launched into the initial stage, which includes a highly-nonlinear fiber (HNLF) and a dispersive linear fiber sections. The pump waves interact along the nonlinear fiber section via third order nonlinear processes, such as four-wave mixing (FWM) to generate new optical tones. This nonlinear generation process can also be viewed as a nonlinear phase rotation induced by the intensity undulation in the time domain, inherited from the coherent beating between the two pump seeds. When the resultant chirped optical field propagates in a dispersive medium (linear section) possessing positive chromatic dispersion, the sinusoidal intensity profile will be compressed, thereby forming short pulses with considerable enhancement in peak power. Enhancements in spectral span and peak power in this compression stage facilitate efficient generation of higher-order pump tones in the subsequent stage, as it has been shown that the strength of nonlinear interaction scales with the peak intensity and the spectral span of the optical field. 
     Following the pulse compression generated in the linear section of the initial mixing stage, an efficient higher-order mixing occurs in the secondary mixing stage. When the pulses from the initial stage propagate in HNLF of the secondary stage, the optical field will experience extensive spectral broadening due to third-order nonlinear processes, such as self-phase modulation. The spectral broadening of the pulse train is equivalent to the creation of a comb of optical tones, with the frequency spacing defined by the pump-pump frequency separation. In the presence of the signal, higher-order pump generation simultaneously leads to spawning of multiple frequency-non-degenerate signal copies, where each pump wave creates two copies through sideband generation processes. 
     The first section (Nonlinear Section 1) used was a 100-meter HNLF which gave the initial chirping of the optical field. The fiber was characterized by a (global) zero-dispersion wavelength (ZDW) of 1552 nm, a dispersion slope of 0.028 ps/nm/km and a nonlinear coefficient of 12 W km. A longitudinal differential stress was applied to raise the stimulated Brillouin scattering (SBS) threshold beyond 30 dBm, thus eliminating the need for pump phase dithering. Following the initial chirping, the output from the first nonlinear section underwent linear temporal compression in a standard single-mode fiber (SMF), shown as the “Linear Section:” in the  FIG. 5 . The total length of SMF, including the pigtails for interfacing the mixer stages, was 7 m. An exceedingly efficient higher-order mixing was then accomplished in the subsequent HNLF section (Nonlinear Section 2). 
     The pump power budget imposes requirements with respect to the mixer design. First, the employed pump power defines the pulse compressor design. In addition, the output of the initial stage then needs to be matched by the chromatic dispersion in the secondary stage to accomplish the large bandwidth and maintain spectral flatness of the mixing process. The 200-m dispersion-flattened HNLF used in the secondary stage was selected to satisfy these requirements: it possessed low dispersion (|D|&lt;1 ps/nm/km) over the 1500-1650-nm band, allowing for uniform and efficient mixing. Furthermore, the concave parabolic dispersion profile with very low peak dispersion (D&lt;0.3 ps/nm/km) helped quench the noise generation induced by the modulation instability (MI), and flattening the spectrum by shock formation in the normal dispersion regime. 
       FIG. 6  plots example data corresponding to a 12 channel spectrum analyzer covering the RF frequency range from 10 to 14 GHz utilizing the 193.05 to 194.15 THz optical band. 
     The relative frequencies of the pumps and signal laser as well as the bandwidth of the periodic optical filter define the IBW and RBW of this invention. By simply changing pump and signal frequencies, and by changing the bandwidth of the optical filter, this invention can be modified on the fly. 
     In some of the configurations of this invention when an optical amplifier (e.g., an EDFA) is used to boost the pump power to a required level for efficient four-photon mixing to occur, any amplified spontaneous transmission can be filtered before being transmitted into the nonlinear fiber. 
     The EOM used can be a phase modulator, an intensity modulator (e.g., a Mach Zehnder modulator), and the modulator can be biased at quadrature or it can be biased to suppress the optical carrier. Use of a phase modulator has the advantage of fixed optical power operation of the parametric mixer, while suppressed carrier operation has the advantage of relaxation of system filtering design. 
     While low-order mixing is performed in the initial mixing stage described herein and higher order mixing is performed in the secondary mixing stage described herein, additional mixing stages can be implemented where enhanced resolution is desired. Such additional mixing stages could be incorporated as linear section/nonlinear section combinations. 
     Obviously, many modifications and variations of the invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as has been specifically described.