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We propose and demonstrate a photonic approach to a reconfigurable channelized radio frequency (RF) receiver for instantaneous RF spectrum monitoring and analysis. Our approach relies on the generation of high quality copies of the RF input by wavelength multicasting in a 2- pump self-seeded parametric mixer and the use of off-the-shelf filtering element such as Fabry-Perot etalon and wavelength division demultiplexers. The parametric channelizer scheme trades frequency non-degeneracy of the newly generated copies for ease of filtering design. Self seeding scheme employed to wavelength multicast the original RF signal to a large number of copies enables easy reconfigurability of the device by simple tuning of the three input waves, i.e. seed and pumps. Channelizer operation to up to 15GHz bandwidth and channel spacing of 500MHz is demonstrated. Reconfigurability is verified by tuning the receiver operating bandwidth and channel spacing.
©2011 Optical Society of America
1. Introduction
Channelized receivers are a specialized class of preprocessors which are critical to both commercial and defense applications [1-2]. Channelized receivers separate a wideband channel into a number of narrowband subrate channels (sub-bands) that can be processed in parallel. Such operation is similar to radio frequency (RF) spectrum characterization in that it provides spectral information. However, channelized receivers perform the operation in parallel for a near real time wideband spectral monitoring, in direct contrast with electronic spectrum analyzers. In the conventional approach, electrical domain channelizers are constructed using a bank of microwave filters or dielectric resonators and are often complex and bulky as they require a significant number of elements. The increasing physical bandwidth used in radar systems also limits the value of electrical channelizers. Recognizing these significant limitations photonic-assisted approaches have engendered a large interest. Indeed, photonic channelizers are especially versatile in that they are not only able to reduce size and complexity of channelizer device but can also analyze and process heterogeneous (amplitude/phase) signals, enable simultaneous monitoring of all sub-bands for full band analysis and allow for detection of frequency-hopping signals.
Photonic assisted channelized receivers relying on spectral splitting or optical domain filtering have been reported. Approaches based on Fabry-Perot etalons [3], diffraction gratings [4], array waveguide gratings [5], or all-optical mixing [6] were demonstrated but suffer from inherent insertion loss, distortions and amplification-induced noise. Fiber optical parametric devices are traditionally recognized as phase sensitive amplifiers and for their ability to perform noiseless signal regeneration. However, the versatility and complex nature of parametric devices, in their single or multiple-pump forms, have also established them as unique processing devices, capable of providing, wavelength conversion, signal conjugation and regeneration [7-8]. Recently, we have demonstrated that self-seeded parametric mixers, typically used for digital signaling [9], allow the possibility of linear multicast transfer characteristic essential for analog signal processing [10]. Relying on the ability to spectrally multicast an RF modulated signal in a low distortion manner, we present a parametric approach to photonic RF channelizers. The device copies the original signal to an array of arbitrarily spaced optical carriers by self-seeded multicasting and slices sub-bands out of each copy with a single periodic filter. By pairing the parametric channelized receiver with a photodetector array, single shot analysis of the microwave signal spectrum is achieved. A single resonant filter therefore channelizes the entire spectrum. A unique feature of the self seeded parametric mixer is that the frequency location of the copies is flexible and solely depends on the positioning of the input 2 pumps and signal. While small adjustments to copy positioning are particularly useful to compensate for filter fabrication tolerance, we will also show that these adjustments enable dynamic bandwidth and channel spacing reconfiguration of the channelized receiver.
2. Principle of operation
The schematic of a conventional photonic channelizer is shown in Fig. 1a . The RF signal has been modulated onto an optical carrier at f0 and is sent through a split-and-filter architecture. In order to separate the wideband RF spectrum into N narrower distinct sub-bands, the modulated signal is power split and sent through a bank of high-Q filters. This approach suffers from many drawbacks, the most crippling being high loss and stringent requirements on the filter design. Indeed, to synthesize a successful channelizer device capable of operating over a wide bandwidth, a large set of narrow yet precisely centered filters must be fabricated. The operating principle of the proposed parametric channelizer is illustrated in Fig. 1b. The bank of high-Q filter is replaced by a single periodic filter combined with wavelength multicasting of the original signal. The filter operates on all copies simultaneously for an effective fine filtering of the input RF signal. The bank of filters is effectively replaced by a “bank” of optical carriers all generated from a single input seed through parametric wavelength multicasting.
Fig. 1 (a) Conventional split-and-filter photonic channelizer architecture requiring a bank of high-Q filters. (b) Copy-and-filter all parametric channelizer architecture relying on a single periodic filter.
The photonic channelized receiver architecture (Fig. 2a ) consists of a tunable seed laser at f0, an electro-optic modulator, a parametric wavelength multicaster driven by 2 pump waves and the combination of a Fabry-Perot etalon/wavelength splitter such as a dense wavelength division multiplexer (DWDM). The laser seed at frequency f0 is supplied to the electro-optical modulator. The RF signal detected by an antenna is directed to the modulator which modulates the signal seed, thus producing frequency sidebands on either side of the optical carrier frequency f0. The modulated seed is then sent to the parametric wavelength multicaster in the form of a 2-pump self seeded mixer [9]. Two original pumps efficiently mix in the nonlinear platform highly nonlinear fiber (HNLF) to engender high order pumps, referred to as self seeded pumps. When an input seed is coupled to the parametric mixer, first and higher order copies are generated. A total of k pumps (original and self seeded) can spawn N= 2k multicast copies. The N high quality copies are positioned at N distinct frequencies f1 to fN. The spacing between copies is predetermined by the frequency position of the two original pumps and the signal seed and can be easily tuned to match specific channelizer requirements. The combined multicast beam is coupled to an etalon with periodic transfer function providing filtering function at fixed optical frequencies. The etalon is characterized by its free spectral range (FSR), the spacing between two transmission peaks, and its finesse, the ratio between the FSR and 3dB width of the transmission peak (resolution). To achieve channelization, one design is to lock each wavelength copy at a fixed, incremented offset from an etalon transmission peak called carrier-to-peak (Fig. 2b). The parametric mixer design guarantees that the copy-to-copy frequency spacing is slightly detuned from the filter FSR. The spacing between copies, Δ, can be expressed as Eq. (1), where ‘a’ is an integer number. The detuning parameter strictly defines the spacing between the channelized receiver adjacent RF sub-bands, i.e. its spectral resolution. In practical terms, each copy is sliced at a specific sub-band region, defined by.
Fig. 2 (a) Schematic of the photonic channelized receiver based on parametric wavelength multicasting followed by etalon and DWDM filtering. (b) Illustration of the relationship between a periodic etalon transmission characteristic (cian) and equally spaced optical signals generated by wavelength multicasting at f1, and f2 resulting in pre-defined carrier-to-peak offset .
For example given an etalon with 25GHz FSR, 1GHz resolution channelization can be achieved by positioning multicast copies 499GHz apart for a=20 or by 124GHz for a=5. Figure 2b illustrates the relationship between the etalon FSR and detuned copy-to-copy spacing resulting in the carrier-to-peak offset. While is shown only for copy1 at f1, each copy has its own unique carrier-peak offset, i.e. is not a fixed value but varies from copy to copy. It is interesting to note that frequency channel allocation amongst copies can be varied and that Eq. (1) is one of many ways to achieve channelization. For our demonstration, copy spacing was set as given in Eq. (1).
Wavelength multicasting with a given Δ spacing, requires precise relative positioning of the pump and signal seed frequencies. Multicast copies are generated by four wave mixing and appear symmetrically around the pumps. Equation (2)a and b describe the pump separation () and the seed-pump separation () required for a Δ copy spacing, where fp1,2 are the pump 1 and 2 frequency, respectively, fseed, the seed frequency and fpx the frequency of the pump closest to the seed.
After fine filtering, the output of the etalon is sent to a wavelength splitter, such as a DWDM to divide the common multicast beam into widely separated channels corresponding to copy frequencies. Each wavelength can then be individually detected. The additional filtering required to roughly separate the N optical carriers, imposes a limitation on copies frequency separation Δ. Indeed for channelization to be performed correctly Δ has to at least three times greater than the analyzed modulation frequency (ωRF). Taking into account required guard bands and DWDM roll-off, we set the limitation on Δ as stated by. While this requirement limits how close multicast copies can be generated, parametric wavelength multicasters have been shown to operate over very wide bandwidth [11] such that the operating range of the parametric channelizer can be substantial.
3. Experimental setup
The experimental setup is shown in Fig. 3 . The microwave signal was generated by an RF synthesizer and modulated onto the tunable signal seed by a phase modulator. The self seeded parametric multicaster was constructed from two tunable external cavity lasers (ECLs) (Pump1 and Pump2) which were amplified, filtered (BPF1 and BPF2), and combined by a wavelength division multiplexer (WDM). The modulated signal seed was then coupled with the amplified pumps and launched into a 100m long segment of highly nonlinear fiber (HNLF) characterized by a zero dispersion wavelength (ZDW) at 1554nm and a slope of 0.03 ps/nm2-km. The HNLF was spooled with a programmed periodically increasing longitudinal tensioning in order to shift the Brillouin frequency along the HNLF and thereby increase the stimulated Brillouin scattering (SBS) threshold. The threshold was increased by more than 6dB, eliminating the need for pump phase modulation [10]. The multicasting operation was monitored through a 1% tap while the remainder of the multicast beam was free-spaced coupled to a commercial off the shelf Fabry-Perot etalon with specified FSR of 25GHz and finesse of 100, i.e. 3dB bandwidth of 250MHz. The experimental characterization of the etalon is shown in the inset of Fig. 3. Amplified spontaneous emission (ASE) was injected in the etalon and resulting transmission features were observed on a high resolution optical spectrum analyzer (15MHz res). The measured FSR and finesse were 25GHz and 119, respectively. Lastly, in place of a WDM, partition of the generated multicast copies was performed by a tunable band pass filter with 0.25nm bandwidth. The filtered output was detected by a low speed photodiode and observed on an oscilloscope after RF amplification.
Fig. 3 Experimental setup. BPF: band pass filter. RF: radio frequency generator. MOD: modulator. HNLF: highly nonlinear fiber. WDM: wavelength division multiplexer. PD: photodiode. Insets: high resolution spectra of the etalon transmission features
4. Parametric channelizer operation
4.1 Optical performance of parametric multicaster
First the optical performance of the self-seeded parametric mixer was evaluated. The two pumps P1 and P2 were roughly positioned on either side of the HNLF ZDW to yield the best equalized response at the output of mixer. For compatibility with readily available WDMs and filters, the pumps were positioned in the vicinity of 1559nm and 1551nm, respectively. The pumps could be tuned over 0.5nm range imposed solely by the bandwidth of the ASE rejection band pass filters (BPF1 and BPF2). To illustrate the pump self-seeding process, the two pumps were injected in the HNLF without a signal seed. P1 and P2, carrying 550mW and 730mW of power respectively, efficiently mixed inside the HNLF to engender a comb of higher order light as shown in Fig. 4a . Pump depletion of 1.7dB and 2.7dB was measured on Pump1 and Pump2, respectively, resulting in high power transfer to the second order pumps P2’ and P1’. Less than 1.4dB ripple was achieved between the 1st and 2nd order pumps.
Fig. 4 Optical spectrum (0.1nm res) at the output of the multicasting mixer (a) pumps only illustrating self seeding of higher order pumps, (b) with signal for pump ON and OFF (high sensitivity). Self seeding resulted in the generation of strong higher order pumps (P2’ and P1’) and a large number of copies.
The signal seed (S) was positioned in the 1+ band [12] near 1553nm for an optimized parametric response. The output of the wavelength multicasting mixer is shown in Fig. 4b for 11dBm of signal input power. Each self-seeded pump generated additional copies of the input signal. The efficiency of the copying process depends on the power of the self-seeded pump and the phase matching between the different waves. While self-seeded multicasting was achieved over a wide bandwidth, only equalized copies are considered for this demonstration. While known small gain variations amongst copies can be compensated for by the receiving electronics, large fluctuations will limit the channelized receiver operating dynamic range and induce large quality divergence between sub-bands. For our demonstration, we selected 10 copies labeled I10 through I1 which were generated with less than 7.3dB gain ripple with respect to the input signal. The quality of the copying, quantified by the optical signal to noise ratio (OSNR), measured with 0.1nm noise bandwidth, and the efficiency, given by the gain/conversion for all copies are summarized in Table 1 . The OSNR of the signal before multicasting was measured to be 49.6dB. The decrease in OSNR after multicasting operation is attributed in part to generated parametric noise and noise pedestal around the pumps due to imperfect filtering.
Table 1. Optical Performance of Self-seeded Multicasting
4.2 Parametric channelizer modes of operation
The proposed photonic channelized receiver based on parametric multicasting is highly reconfigurable and can be tuned to match given applications and specifications. The operating bandwidth (BW), i.e. the range of RF frequencies that can be channelized, as well as the channel spacing, or resolution, of the channelizer can be varied by simply tuning the frequencies of the three input waves (P1, P2 and S) whereas the periodic filtering element is left untouched. As the periodic response of the etalon remains fixed, tuning of the pumps and signal effectively changes the relative frequency position of adjacent etalon peaks to the generated copy frequency, thus dynamically varying the sub-band allocation of that specific copy. To illustrate the reconfigurability of the proposed channelized receiver, four modes of operation were selected and demonstrated. The parameters of all 4 modes are summarized in Table 2 . Mode 1 and 2 both have 1GHz channel spacing with operating BW of 1-10GHz to 3-12GHz, respectively. The operating BW is limited by the number of copies utilized by the channelizer, 10 for this demonstration, and can be increased by optimizing the parametric mixer to generate additional copies. The channel spacing is varied to 500MHz and 1.5GHz for Mode 3 and 4, respectively. Since the number of copies is fixed for this demonstration, reducing the channelizer resolution also leads to a reduced operating BW, while decreasing the resolution allows for monitoring of a wider RF band. However, by increasing the number of generated copies and using subsets of them, bandwidth and resolution can be decoupled.
Table 2. Parameters for Parametric Channelizer Modes of Operation
The frequencies of the pumps and signal were calculated following Eqs. (1) and 2 are summarized in Table 2. Note that the pump spacing 2Δ = 1THz - δchannel, i.e. a=20. All the waves were aligned to the etalon which had a measured transmission peak at 193.02495THz. Reconfiguration of the channelizer relies on tunable lasers with fine resolution in order to reach optimal positioning. The three ECLs were tunable with 12.5MHz resolution, which was sufficient to match the achievable positioning to the calculated target. Reconfiguring the channelizer between the 4 modes of operation required tuning of Pump1 and Pump2 by less than 2.22GHz and 4GHz, respectively. This relatively small tuning of the waves did not result in any significant optical multicasting performance fluctuations and all modes resulted in similar optical responses as the one shown in Fig. 4.
4.3 Channelized receiver performance
The channelized receiver was tested by sweeping the input signal frequency while the optical power of all sub-bands and corresponding detected voltage was measured at the output of the device. The channelizing operation was also monitored on the high resolution (15MHz) optical spectrum analyzer. To illustrate sub-band selection by the channelized receiver, the RF input was swept from 0 to 10GHz with 1GHz steps and the high resolution spectra were retrieved on all copies. Illustration of sub-band channelization at Copy I1, I6 and I10 is shown in Fig. 5(a-c) . Superimposed spectra as the RF input is changed confirmed the successful selection of the targeted sub-bands, 10GHz, 5GHz and 1GHz, respectively, while adjacent channels were rejected with a minimum of 20dB extinction ratio. However the strong optical carriers at 190.535THz, 193.03THz and 195.027THz are still detected due to the Lorentzian etalon shape (shown in Fig. 5b). This superfluous optical energy is even more pronounced as the selected sub-band frequency decreases. The non-optimal filter shape affects the dynamic range of the channelizer: lower frequencies, i.e. closer to the carrier, have a reduced dynamic range as weak sidebands will not be detected. However, the extraction of optical carrier is not required for the operation of the channelized receiver. The DC component can be further suppressed by null-biased amplitude modulation to produce double sideband suppressed-carrier spectrum. Loss and extinction ratio are shown in Fig. 5d where input and output spectra are compared at I5. Loss is dominated by free-space coupling to the periodic filter. Indeed coupling loss was measured to be 6.3dB. The overall loss at I5 is 2.4dB as the coupling loss is reduced by multicasting gain of 3.9dB. Adjacent channels at 5 and 7 GHz are rejected with close to 20dB extinction ratio, consistent with expected etalon characteristics.
Fig. 5 Superimposed high resolution spectra (15MHz res) after etalon illustrating channelization with 1GHz resolution: (a) 10GHz sub-band monitoring at Copy I1, (b) 5GHz sub-band monitoring at Copy I6, and (c) 1GHz sub-band monitoring at Copy I10, (d) Input and output spectra of the channelized received for Copy I5.
The full characterization for modes 1 and 2 is illustrated in Fig. 6 . The normalized detected voltage on all 10 outputs is plotted as a function of input signal frequency. Clear discrimination between the targeted sub-band and non-targeted sub-bands was recorded at the photodiode for all outputs. The operating bandwidth was successfully changed from 1 to 10GHz to 3-12GHz operating bandwidth (Fig. 6a and b, respectively) by tuning pumps and seed. As mentioned previously, the strong presence of the carrier at low input frequency results in high background level as seen on copy 10 in Fig. 6a.
Fig. 6 Normalized detected voltage on each multicast copy as a function of input RF frequency. (a) Mode1 of operation: 1 to 10GHz operating range with 1GHz channel spacing. (b) Mode2 of operation: 3 to 12 GHz operating range with 1GHz channel spacing
The channelized receiver performance for the last two modes of operation was measured in a similar fashion and the results are plotted in Fig. 7a and b , for 500MHz and 1.5GHz channel spacing, respectively. Identical performance was retrieved with a clear sub-band selection and rejection on a 500MHz or 1.5GHz channel grid, as expected from the input wavelength map. Adjacent channel rejection of at least 15dB for 500MHz spacing and 24dB for 1.5GHz spacing were observed, in direct agreement with the measured etalon transmission characterization. Secondary peaks can be seen in Fig. 7b at copy1, 2, 3 and 4. This unwanted double sub-band detection resulted from combining a 25GHz FSR etalon and double sideband modulation. Indeed, while a 15GHz sub-band is aligned to targeted etalon line, the 10GHz sub-band from the opposite sideband will also be aligned to the etalon transmission line. Rejection of this secondary sub-band can be increased by tuning of the 0.25nm band pass filter. However, larger FSR periodic filter would entirely eliminate double sub-band detection and enable monitoring of wider bandwidth. Overall, each individual sub-band was successfully detected at its assigned copy, while non-matched sub-bands induced close to zero power changes at the detector resulting in true recognition of the input RF signal.
Fig. 7 Normalized detected voltage on each multicast copy as a function of input RF frequency. (a) Mode3 of operation: 3 to 7.5GHz operating range with 500MHz channel spacing. (b) Mode4 of operation: 1.5 to 15 GHz operating range with 1.5GHz channel spacing
4.4. Operating range and tolerance
Adjacent channel rejection quality, determined from the extinction ratio is solely determined by the transmission characteristics of the etalon filters. While the experiment used an off-the-shelf (Lorentzian passband) etalon, an optimized filter should have sharp edges to minimize channel crosstalk and flattened passband to not only reduce amplitude noise induced by laser instabilities but also to increase the channelizer tolerance to pump positioning. The effect of channel leakage is illustrated in Fig. 8 where the power difference measured when monitored sub-band is turned OFF and ON while all other non-matched sub-bands are present is plotted for all channelizer outputs. When all sub-bands are present (i.e. worst case), the total power leakage due to the non-optimized filter shape can be significant. As the monitored sub-band frequency decreases below 7GHz, crosstalk is artificially dominated by the superfluous optical carrier energy. Despite non-optimized etalon shape, more than 12dB, 11dB and 7dB of contrast can be achieved for 1.5GHz, 1GHz and 500MHz channel spacing, respectively.
Fig. 8 Contrast ratio for target ON/ OFF as a function of targeted sub-band for the three channel spacing
Of importance is the dynamic range performance. Figure 9 shows the detected sub-band voltage of I3 as a function of the modulator drive power. This range of input RF power resulted in measured optical sideband power at the photodiode of −40 dBm to −18 dBm. According to Fig. 9, the dynamic range of the channelized receiver is at least 20 dB. The range can be improved by careful use of RF and/or optical amplification, by carrier suppressed modulation to remove the flattening of the curve at low drive powers, and by filter optimization. Reduction of noise generation during multicasting by using high OSNR pumps and tailored ASE rejection filters will also enable a significant increase in the channelizer dynamic range to levels compatible with monitoring applications. To assess the effect of parametric multicasting, the channelized receiver response (Output) was compared to the seed response, bypassing multicasting. Very similar responses were measured with a small deviation observed at low drive power attributed to parametric generated noise, once again confirming the excellent performance of parametric multicaster on analog signals.
Finally, the channelized receiver tolerance to pumps miss-positioning was explored. The precise frequency positioning of the pumps is necessary to ensure that idler generation is well locked to the etalon transmission function. As multicasting relies on higher order light generation by four wave mixing, detuning of the pumps from the optimal position will result in increased detuning as the idler order grows [13]. Misalignment of the idlers with respect to the etalon transmission peak will result in sub-band loss and increased crosstalk. Figure 10a illustrates the relationship between pump detuning and idler maximum induced loss, when a single pump is misaligned or both in a co- or counter- fashion. The graph shows that pump stability is important to avoid PM-to-AM transfer and that detuning of 26MHz can lead to a 3dB power fluctuation. The effect of pump positioning was observed experimentally as shown in Fig. 10b. Data set 1 shows the measured idler detuning on all channels when the pumps were perfectly positioned. Data set 2 shows the detuning in the case when a single pump was misaligned by ~12.5MHz. As expected, higher order idlers experienced an increased frequency shift and an additional 0.5dB loss. The standard wavelength resolution of the ECLs were however sufficient to align all wavelengths to reach maximum performance (Fig. 9b, data set 1). While the long term stability of pumps and seed can be addressed by feedback control, the optimization of the filter element would significantly relax the stability constraints, as previously mentioned.
Fig. 10 (a) Induced idler loss due to pumps miss-positioning. (b) Experimental observation of idler detuning and corresponding loss for 2 sets of data.
5. Conclusion
We demonstrated a photonic approach for reconfigurable channelized receiver. Parametric process provides the means to generate wavelength degenerate copies of the RF modulated optical seed by self-seeded four wave mixing, eliminating the need for banks of high-Q filters. By relying on the ability to parametrically generate high fidelity wavelength copies of analog signals, a single resonant filter can be used to channelize the entire spectrum. We have shown that despite non-optimized shape of the off-the-shelf Fabry-Perot etalon used, each individual sub-band obtained at the output of the channelized receiver was successfully detected with less than 1.8 dB of cumulative loss. The reconfigurability of the proposed architecture was verified by tuning of driving pumps. Control over the input frequency map enable dynamic channelized receiver reconfiguration of channel spacing and/or operating bandwidth with successful detection of 1-10GHz and up to 15GHz RF input with 1.5GHz to 500MHz scale channelization. The proposed architecture can be scaled to a large number of channels to achieve analysis of arbitrarily wide channel and is only limited by the performance of parametric mixer device.
Acknowledgement
The authors acknowledge Sumitomo Electric for providing the highly nonlinear fiber used in the experiment.
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