Sidelobe Suppression Ratio Research Articles

Silicon-based polarization-insensitive optical filter with dual-gratings.

A silicon-based polarization-insensitive optical filter is proposed and demonstrated. For the present on-chip polarization-insensitive optical filter, there is a dual-polarization mode (de)multiplexer, a TE-type multimode waveguide grating (MWG) with triangular corrugations and a TM-type MWG with rectangular corrugations. Here the triangular corrugations are introduced to reduce the undesired reflection and suppress the Fabry-Parot resonance. Furthermore, lateral-shift apodization is introduced for both two types of MWGs to suppress the sidelobes. For the fabricated device, the measured 3 dB-bandwidth is as large as ∼11 nm and the excess loss is ∼1.5 dB for both polarizations, while the sidelobe suppression ratios are 23 dB and 17 dB for TE and TM polarizations, respectively.

Experimental demonstration of blind equalization for OFDM/OQAM-VLC system

Orthogonal frequency-division multiplexing/offset quadrature amplitude modulation (OFDM/OQAM) in direct-detection system obtains many benefits such as free of cyclic prefix (CP) and high side-lobe suppression ratio in comparison with CP-OFDM signal. Unfortunately, for OFDM/OQAM signal, it needs an extra interference cancellation process to eliminate the imaginary intrinsic interference (IMI) when using one-tap frequency equalization known as interference approximation method (IAM). Blind equalization without a pilot overhead for OFDM/OQAM signal is proposed and experimentally demonstrated in visible light communication (VLC) system with limited modulation bandwidth. Experimental results show that blind equalization for OFDM/OQAM-VLC can effectively eliminate the IMI effect and can exhibit better symbol error rate on different subcarriers. Furthermore, compared with conventional IAM, it exhibits 3.2 and 2.7 dB overall improvement in Q2-factor using blind equalization based on the constant modulus algorithm and constant norm algorithm, respectively. In addition, the system can achieve a total data rate of 3.96 Gb / s over 5-m free space link with a bit error rate lower than hard-decision forward error correction limit of 3.8 × 10 − 3.

80 km Fading Free Phase-Sensitive Reflectometry Based on Multi-Carrier NLFM Pulse Without Distributed Amplification

Long range phase-sensitive optical time domain reflectometer (ϕ-OTDR) sensing mainly employs distributed amplification in the sensing fiber. It requires the light to be injected into both ends of the sensing fiber, which reduces the degree of freedom in embedding the fiber into structures. To overcome this problem, the key factors that affects the signal-to-noise ratio (SNR) in ϕ-OTDR system based on matched filter is analyzed thoroughly, and a single-ended long-range ϕ-OTDR that does not require distributed amplification is proposed. In this system, two key techniques are adopted for SNR improvement. To boost the pulse energy and suppress the self-phase modulation, the distortion of the amplified pulse is rectified by using iterative predistortion method; to mitigate the influence of the interference fading and the stimulated Brillouin backscattering, a three-carrier pulse is employed. In combination with the non-linear frequency modulation technique which yields a 42.7 dB side lobe suppression ratio, these approaches guarantee an achievable ϕ-OTDR of 80 km sensing range, 2.7 m spatial resolution, 49.6 dB dynamic range in the experiment. To the best of authors’ knowledge, this is the first time that a ϕ-OTDR without optical amplification in the sensing fiber is realized over such a long sensing range.

Silicon Multimode Waveguide Grating Filter at 2 μm

A silicon-based on-chip add-drop optical filter is realized for the wavelength-band of 2 μ m by integrating a multimode waveguide grating (MWG) and a pair of two-channel mode (de)multiplexers. The realized add-drop optical filter has low loss and high sidelobe suppression ratio in a broad band, achieved by optimizing the corrugation depth, the apodization strength, as well as the grating length. The measurement results agree very well with the theoretical simulation results. The demonstrated ∼100 μ m-long MWG has a large 0.5 dB bandwidth of 6∼26 nm and a sidelobe suppression ratio of 22∼27 dB. The thermal-tunability of the present optical filter is also demonstrated experimentally, and the measured tuning efficiency is about 0.16 nm/mW.

Hybrid phase-amplitude superoscillation element for nonscanning optical superresolution imaging.

In this paper, we report a nonscanning optical superresolution imaging method based on a hybrid phase-amplitude superoscillation element. Using the Chebyshev polynomials as a basis set on the superoscillation waveform, the optimal combination of these, representing the optimal focal -spot in the local field of view, is found by genetic algorithm. Our numerical calculations demonstrate that a subwavelength focal spot with a full width at half-maximum as small as 253 nm is realized, which has more than 30 times improvement in sidelobe suppression ratio, and crucially, a greatly extended needle with continuously shrunken focal spot is yielded, which allows a large imaging tolerance in the axial displacement of the object. We then present our simulated results of the superresolution imaging on sparse point objects and continuous objects, where the practicality and effectiveness of this method are analyzed and discussed in detail.

Narrow-Band Add-Drop Filter Based on Phase-Modulated Grating-Assisted Contra-Directional Couplers

A narrow-band silicon photonic add-drop filter is demonstrated by using phase-modulated grating-assisted contra-directional couplers (contra-DCs) on the silicon-on-insulator platform. The apodization coupling is achieved by modulating the phase of one grating in a contra-DC, while keeping the other grating uniform. As the local effective index of the gratings remains almost unchanged, the designed apodized contra-DC has a large designing flexibility and a high sidelobe suppression ratio (SLSR). To obtain a narrow bandwidth, weak coupling is designed. Single-stage and dual-stage filtering are both studied. The experimental results show that single-stage filtering device has a 3-dB bandwidth of 0.9 nm and an SLSR of 18 dB. Dual-stage filtering device has a 3-dB bandwidth of 0.5 nm and an SLSR of 23 dB. Our devices are promising for on-chip applications that require narrow bandwidths such as wavelength-division-multiplexed channel filters.

Ultra-sensitive quasi-distributed temperature sensor based on an apodized fiber Bragg grating.

This work targets a remarkable quasi-distributed temperature sensor based on an apodized fiber Bragg grating. To achieve this, the mathematical formula for a proposed apodization function is carried out and tested. Then, an optimization parametric process required to achieve the remarkable accuracy that is based on coupled mode theory (CMT) is done. A detailed investigation for the side lobe analysis, which is a primary judgment factor, especially in quasi-distributed configuration, is investigated. A comparison between elite selection of apodization profiles (extracted from related literatures) and the proposed modified-Nuttal profile is carried out covering reflectivity peak, full width half maximum (FWHM), and side lobe analysis. The optimization process concludes that the proposed modified-Nuttal profile with a length (L) of 15mm and refractive index modulation amplitude (Δn) of 1.4×10-4 is the optimum choice for single-stage and quasi-distributed temperature sensor networks. At previous values, the proposed profile achieves an acceptable reflectivity peak of 10-0.426 dB, acceptable FWHM of 0.0808nm, lowest side lobe maximum (SL max) of 7.037×10-12 dB, lowest side lobe average (SL avg) of 3.883×10-12 dB, and lowest side lobe suppression ratio (SLSR) of 1.875×10-11 dB. These optimized characteristics lead to an accurate single-stage sensor with a temperature sensitivity of 0.0136nm/°C. For the quasi-distributed scenario, a noteworthy total isolation of 91dB is achieved without temperature, and an isolation of 4.83dB is achieved while applying temperature of 110°C for a five-stage temperature-sensing network. Further investigation is made proving that consistency in choosing the apodization profile in the quasi-distributed network is mandatory. If the consistency condition is violated, the proposed profile still survives with a casualty of side lobe level rise of -73.2070 dB when adding uniform apodization and -46.4823 dB when adding Gaussian apodization to the five-stage modified-Nuttall temperature-sensing network.

Apodized Spiral Bragg Grating Waveguides in Silicon-on-Insulator

We demonstrate an apodization technique by tapering the corrugations of spiral Bragg grating waveguides on the silicon-on-insulator platform, for the fundamental transverse magnetic mode. The transmission and reflection responses of uniform and apodized spiral Bragg grating waveguides are experimentally compared to show higher sidelobe suppression ratios by the proposed apodization scheme. We also present an apodized, period-chirped spiral Bragg grating waveguide, and the group delay of the proposed device has been measured; the results show an efficient suppression in the ripples of the group delay response.

Single- and dual-band dispersion compensation unit using apodized chirped fiber Bragg grating

In this study, a wideband dispersion compensation (WBDC) profile that effectively covers the entire C- and/or part of the L-band is designed and evaluated. Several apodizations with different apodization strengths applied to a chirp fiber Bragg grating (CFBG), different CFBG lengths (L), and different refractive index modulation amplitudes ( $$\Delta n$$ ) are investigated during the design and evaluation process. The design and optimization processes target parameters including a maximum full width at half maximum (FWHM), minimum group delay ripples (GDR) and an acceptable reflectivity and sidelobe suppression ratio (SLSR). A wavelength shift of no more than 2 nm is observed as a result of investigating the effect of temperature in the WBDC scenario. During single-stage operation, the results shows that a hyper-tanh with L = 15 cm and $$\Delta n$$ = 4e−4 is the optimum design choice that provides an FWHM of 36.9378 nm, a GDR of 0.85 ps, a reflectivity of $$-4.46706$$ dB and an SLSR of 42.08 dB. Optimization indicates that a tanh apodization with $$L =$$ 15 cm and $$\Delta n=$$ 4e−4 is the optimum choice for dual-stage operation that achieves an FWHM of 37.2244 nm, a GDR of 0.85 ps, a reflectivity of −5.36151 dB and an SLSR of 43.59 dB. Small variations in the SLSR level (e.g., 0.8 dB) are observed while investigating the effect of temperature on the dual-stage operation even in the worst-case operating scenario.

Spectral engineering of subwavelength-grating-based contradirectional couplers.

We tailor the spectral characteristics of silicon photonic contradirectional couplers (Contra-DCs), where the design of the coupler is based on placing a subwavelength grating (SWG) waveguide next to a strip waveguide. By tapering the gap distance between the SWG and strip waveguides, we demonstrate a compromise between sidelobe suppression and pass-band/stop-band extinction ratio such that the performance of the device as a potential optical (de)multiplexer is improved. The designs with different pass-band bandwidths of 12 nm, 9 nm, and 6 nm show 10 dB to 20 dB sidelobe suppression ratio and 15 dB to 35 dB extinction ratio. We also obtain a resonant transmission peak in the stop-band of the spectral response of the device by introducing a π phase shift into the gratings of the SWG waveguide. The resonant peak has 1 nm bandwidth and 7 dB extinction ratio, where the use of the SWG waveguide in the structure of such coupler allows the characteristics of the resonant peak to be highly sensitive to the cladding material, which is of strong desire in integrated sensing applications.

Amendment performance of an apodized tilted fiber Bragg grating for a quasi-distributed-based sensor.

In this work, the characteristics of reflectivity spectra produced inside a reflective-tilted fiber Bragg grating (R-TFBG) are investigated, seeking a remarkable performance that could be able to upgrade the sensitivity range for temperature-strain sensors of quasi-distribution type. We introduce an optimized performance through a comparative investigation among different evaluation parameters, such as core radius, tilt angle, and the elite selection of apodization profiles, in addition to the traditional parameters, such as grating length, L, and index modulation amplitude, Δn. Regarding the tilt angle, its increase affects the full width at half-maximum (FWHM) affirmatively, while having a negative impact on the maximum reflectivity. By controlling L and Δn, a compromised solution is achieved to retrieve the maximum reflectivity to be around 1.0. Regarding the sidelobes, the Kaiser profile is the best candidate that minimized the main sidelobe level (MSL) and raised the sidelobe suppression ratio (SLSR) at any tilt angle, while tanh apodization is the best choice from the perspective of raising the ramp down sidelobes asymptotic decay. The contrasts in the optimization process are examined through investigating the R-TFBG quasi-distributed sensors to be applied to a temperature-strain sensing system. The objective is to evaluate an assessment for the performance of a sensor system that extends the range and efficiency of temperature-strain ranges. Based on our analysis, the sensitivity range is upgraded for a temperature change to reach 179°C and for strain to be 3000 μϵ at a tilt angle of 10° with a FWHM of 0.063 nm, attenuation of -154 dB for an MSL of 75.5%, and an SLSR of -60 dB/nm.

Effects of spontaneous emission excited state lifetime on the output performance of quantum well lasers

In this paper, we have investigated the effect of spontaneous emission’s excited state lifetime on the output performance of the quantum well laser using quantum well transmission line modelling technique. It has been found that as the spontaneous emission lifetime increases the turn-on transient response of the quantum well laser is delayed, exhibiting the larger peak and steady state arrival times and the laser steady output has larger noise. Meanwhile, the increase in spontaneous emission lifetime leads to the larger peak and steady state arrival times of the carrier density variation during the turn-on process. Also, due to the increase of the spontaneous emission lifetime, the bandwidth and magnitude of the laser steady output spectra increase while the side lobe suppression ratio decreases and there exists a blue shift in the peak wavelength. These studies confirm that the quantum well laser with less spontaneous emission lifetime has more superior output performance (fast transient response, less noise and narrow bandwidth); they also provide an effective method to compare and predict the spontaneous emission lifetimes for different semiconductor optical materials.

Silicon photonic bandpass filter based on apodized subwavelength grating with high suppression ratio and short coupling length.

A compact silicon bandpass filter with high sidelobe suppression is proposed and experimentally demonstrated using an apodized subwavelength grating (SWG) coupler. The device is implemented by placing a SWG waveguide next to a strip waveguide, and apodization is employed with a Gaussian profile to taper the gap between the two waveguides. A high sidelobe suppression ratio of 27 dB can be obtained with a 3-dB bandwidth of 8.8 nm and an insertion loss of 2.5 dB. Owing to the large optical phase mismatch between the two waveguides and the presence of the SWG waveguide, the coupling length of the device is reduced to 100.3 μm. The experimental results validate our proposed apodized-SWG-based contradirectional coupler (contra-DC) as a promising device in suppressing out-of-band components in coarse wavelength division multiplexed (CWDM) optical communication systems.

A microwave photonic filter based on multi-wavelength fiber laser and infinite impulse response

A microwave photonic filter (MPF) based on multi-wavelength fiber laser and infinite impulse response (IIR) is proposed. The filter uses a multi-wavelength fiber laser as the light source, two sections of polarization maintaining fiber (PMF) and three polarization controllers (PCs) as the laser frequency selection device. By adjusting the PC to change the effective length of the PMF, the laser can obtain three wavelength spacings, which are 0.44 nm, 0.78 nm and 1.08 nm, respectively. And the corresponding free spectral ranges (FSRs) are 8.46 GHz, 4.66 GHz and 3.44 GHz, respectively. Thus changing the wavelength spacing of the laser can make the FSR variable. An IIR filter is introduced based on a finite impulse response (FIR) filter. Then the 3-dB bandwidth of the MPF is reduced, and the main side-lobe suppression ratio (MSSR) is increased. By adjusting the gain of the radio frequency (RF) signal amplifier, the frequency response of the filter can be enhanced.

Transmission line model for strained quantum well lasers including carrier transport and carrier heating effects.

This paper reports a new model for strained quantum well lasers, which are based on the quantum well transmission line modeling method where effects of both carrier transport and carrier heating have been included. We have applied this new model and studied the effect of carrier transport on the output waveform of a strained quantum well laser both in time and frequency domains. It has been found that the carrier transport increases the turn-on, turn-off delay times and damping of the quantum well laser transient response. Also, analysis in the frequency domain indicates that the carrier transport causes the output spectrum of the quantum well laser in steady state to exhibit a redshift which has a narrower bandwidth and lower magnitude. The simulation results of turning-on transients obtained by the proposed model are compared with those obtained by the rate equation laser model. The new model has also been used to study the effects of pump current spikes on the laser output waveforms properties, and it was found that the presence of current spikes causes (i) wavelength blueshift, (ii) larger bandwidth, and (iii) reduces the magnitude and decreases the side-lobe suppression ratio of the laser output spectrum. Analysis in both frequency and time domains confirms that the new proposed model can accurately predict the temporal and spectral behaviors of strained quantum well lasers.

Sidelobe suppression analysis of microwave photonic filter based on spectrum-shaped optical frequency combs

A finite impulse-response microwave photonic filter is typically achieved based on spectrum-shaped optical frequency combs and a dispersive element. We propose an analytical model to describe the amplitude responses of the sidelobes. The model shows that the sidelobe suppression ratio is limited by the spectrum structure of the optical combs. By taking Gaussian-profiled combs as an example, it is both theoretically and experimentally proved that the suppression ratio can be improved by optimizing the spectral power range, which is defined as the ratio of the maximum tap weight to the minimum tap weight.

Complex apodized Bragg grating filters without circulators in silicon-on-insulator.

Bragg gratings operating in reflection are versatile filters that are an important building block of photonic circuits but, so far, their use has been limited due to the absence of CMOS compatible integrated circulators. In this paper, we propose to introduce two identical Bragg gratings in the arms of a Mach-Zehnder interferometer built with multimode interference 2 x 2 couplers to provide a reflective filter without circulator. We show that this structure has unique properties that significantly reduce phase noise distortions, avoid the need for thermal phase tuning, and make it compatible with complex apodization functions implemented through superposition apodization. We experimentally demonstrate several Bragg grating filters with high quality reflection spectra. For example, we successfully fabricated a 4 nm dispersion-less square-shaped filter having a sidelobe suppression ratio better than 15 dB and an in-band phase response with a group delay standard deviation of 2.0 ps. This result will enable the fabrication of grating based narrowband reflective filters having sharp spectral responses, which represents a major improvement in the filtering capability of the silicon platform.

Sidelobe Suppression Enhancement of Radiofrequency Photonic Filters via Time-to-frequency Mapping

We present a multi-tap microwave photonic filter with high selectivity through applying time-to-frequency mapping and optical frequency comb shaping techniques. When arranged in the time-to-frequency mapping stage, by a Fourier transform, the deviation of the optical taps to the target profile is significantly reduced while maintaining the apodization profile, resulting in high sidelobe suppression in the filters. By applying a simple time-to-frequency mapping stage to the conventional optical frequency combs, we demonstrate a substantially enhanced (>10dB) sidelobe suppression, resulting in filter lineshapes exhibiting a significantly high (>40dB) main lobe to sidelobe suppression ratio. These results highlight the potential of the technique for implementation in various passband filters with high sidelobe suppression.

Transmission of 100-Gb/s DDO-OFDM/OQAM over 320-km SSMF with a single photodiode.

In this paper, we propose a novel approach to simultaneously receive multi-band 100-Gb/s direct-detection optical signal with only one polarization and one conventional 40-GHz photodiode. The modulation format of orthogonal frequency-division multiplexing based on offset quadrature amplitude modulation (OFDM/OQAM) is selected to provide signal spectrum with high side-lobe suppression ratio, which can effectively reduce the electrical sub-band frequency interference. The whole 100-Gb/s OFDM/OQAM signal is comprised of 6 sub-bands with 16- and 32-QAM formats loading. Only one guard band is required to accommodate the overlapped 6-band signal-to-signal beat interference (SSBI). The receiver bandwidth is mainly limited by the digital storage oscilloscope (DSO) of 33 GHz. The transmission distance over standard single mode fiber (SSMF) is up to 320 km.

Performance optimization of apodized FBG-based temperature sensors in single and quasi-distributed DWDM systems with new and different apodization profiles

In this work, different FBG temperature sensors are designed and evaluated with various apodization profiles. Evaluation is done under a wide range of controlling design parameters like sensor length and refractive index modulation amplitude, targeting a remarkable temperature sensing performance. New judgment techniques are introduced such as apodization window roll-off rate, asymptotic sidelobe (SL) decay level, number of SLs, and average SL level (SLav). Evaluation techniques like reflectivity, Full width at Half Maximum (FWHM), and Sidelobe Suppression Ratio (SLSR) are also used. A “New” apodization function is proposed, which achieves better performance like asymptotic decay of 18.4 dB/nm, high SLSR of 60 dB, high channel isolation of 57.9 dB, and narrow FWHM less than 0.15 nm. For a single accurate temperature sensor measurement in extensive noisy environment, optimum results are obtained by the Nuttall apodization profile and the new apodization function, which have remarkable SLSR. For a quasi-distributed FBG temperature sensor the Barthann and the new apodization profiles obtain optimum results. Barthann achieves a high asymptotic decay of 40 dB/nm, a narrow FWHM (less than 25 GHZ), a very low SLav of −45.3 dB, high isolation of 44.6 dB, and a high SLSR of 35 dB. The new apodization function achieves narrow FWHM of 0.177 nm, very low SL of −60.1, very low SLav of −63.6 dB, and very high SLSR of −57.7 dB. A study is performed on including an unapodized sensor among apodized sensors in a quasi-distributed sensing system. Finally, an isolation examination is performed on all the discussed apodizations and a linear relation between temperature and the Bragg wavelength shift is observed experimentally and matched with the simulated results.