A novel in-line polarization-dependent microfiber interferometer (PD-MFI) is proposed and experimentally demonstrated, which is tapered from a commercial polarization-maintaining fiber. Different from conventional MFIs, the transmission spectra of such MFIs are highly polarization-dependent, due to the mode-sensitive birefringence. The experimental results agree well with the theoretical predictions. Moreover, exploiting the polarization-dependent property of PD-MFIs, we demonstrate a simple and flexible scheme of generating polarity-switchable ultra-wideband pulses in the optical domain. Doublet pulses with a central frequency of 6.28 GHz and a 10-dB bandwidth of 7.86 GHz are obtained. Hence, with the advantages of being fiberized, simple fabrication and robustness, these PD-MFIs can be attractive elements in optical signal processing, optical sensing, optical fiber communication, and microwave photonics.
© 2013 OSA
In the past few years, optical microfibers have attracted intensive interest [1–3] due to advantages of ultralow loss, flexibility, strong field confinement and large evanescent fields. And microfiber-based devices have been applied in the areas of optical sensing [4–7], nonlinear optics , atom optics , micro- and nano-photonics [10–12] and optical signal processing [13–15]. Especially, in-line microfiber interferometers (MFIs) are widely studied since they offer numeric exciting properties, such as all fiber components, easy fabrication and robustness. The principle of MFI primarily stems from mode beating behaviors . Besides, MFIs have large evanescent waves and can act as platforms for the interaction between guided optical waves and surrounding medium. So far, they have been extensively employed for optical sensing [17–20] and fiber lasers [21, 22]. Li et al. have demonstrated that MFIs due to polarimetric interference can enable ultrahigh sensing of ambient refractive index change . Hence, techniques combining the mode beating and polarimetric interference may be potential for optical sensors.
On the other hand, the use of ultra-wideband (UWB) signals for wireless communication has attracted considerable attention in the past few years .However, due to the low power spectral density, UWB signals can only propagate within tens of meters in wireless communication systems. Distribution of UWB signals over fiber links, i.e., UWB-over-fiber technology, provides a promising solution . Consequently, the generation of UWB signals in the optical domain is highly demanded since no extra optical-to-electrical conversion is required. Various approaches of photonic generation of UWB signals have been reported . Specially, many schemes of generating UWB signals in the optical domain have been recently demonstrated based on nonlinear optical devices, such as semiconductor optical amplifiers (SOAs) [26–29], highly nonlinear fibers (HNLFs)  and nonlinear nanowires . However, these nonlinear optical devices as well as multiple laser diodes (LDs) result in high cost and increased system complexity. Hence, flexible and cost effective schemes of generating UWB signals based on passive optical devices are favored. Additionally, pulse shaping using interferometric systems has been widely investigated. Early in 2007, Yongwoo Park et al. reported picosecond pulse shaping based on temporal coherence synthesization . Later, reshaping of an ultrashort Gaussian-like pulse into a flat-top pulse was demonstrated using an integrated Mach-Zehnder interferometer (MZI) . However, all-fiberized devices can be more easily integrated in UWB-over-fiber systems.
In this paper, we demonstrate a novel in-line polarization-dependent microfiber interferometer (PD-MFI). The PD-MFI is fabricated from a commercial polarization-maintaining fiber with flame-heated tapering approach and has the characteristics of simple fabrication, immunity to electromagnetic interference and easy integration with fiber systems. It has comb-like transmission spectra because of the beating between different modes. These spectra are highly polarization-dependent and this property can be attributed to the mode-sensitive birefringence. The experimental results are in good accordance with the theoretical analysis. Besides, owing to vernier effect, the MFI transmission is polarization-independent for some specific wavelengths. Hence, by adjusting the working wavelength, polarization-dependent or -independent operations in the optical domain can be realized with a PD-MFI. Then, by exploiting the polarization-dependent characteristics of PD-MFIs, we implement the photonic generation of ultra-wideband (UWB) signals based on a PD-MFI, a phase modulator and a single laser diode. Polarity-switchable UWB doublet pulses with a central frequency of 6.28 GHz are successfully obtained with the method of phase-modulation-to-intensity-modulation (PM-IM) conversion. The 10-dB bandwidth of the doublets is up to 7.86 GHz, corresponding to a fraction bandwidth of 125%. Hence, the proposed PD-MFIs can be applied in microwave photonics, optical communication, optical signal processing and optical sensing.
The paper is organized as follows. In Sec. II, the fabrication of PD-MFIs is introduced and PD-MFI transmission properties are investigated. In Sec. III, we implement the principle of PD-MFIs incorporating the mode birefringence of microfibers. Then, photonic generation of UWB pulses based on a PD-MFI is demonstrated in Sec. IV, as an application of PD-MFIs in optical signal processing. At last, our conclusions are given in Sec. V.
2. Fabrication and characteristics of in-line PD-MFIs
Commercial 125μm polarization-maintaining fibers (PMFs, YOFC PM1016-A) were used to fabricate in-line PD-MFIs and Fig. 1(a) shows the optical microscope (AxioLab A1, ZEISS) images of the PMF cross section. The in-line PD-MFIs were manufactured by tapering the PMFs into microfibers with improved flame-heated technique . A 2-mm-wide flame generated by the burning of butane was utilized as the heat source and had a temperature of ~950°C. The PMF acts as a preform for the fabrication of micrometer-diameter microfibers and its extremities were fixed onto stages connected to a computer. Via precisely controlling the moving speeds of both stages, uniform microfibers with diameters down to 1.5 μm and lengths up to 300 mm could be obtained. The generated MFIs were well pigtailed and Fig. 1(b) shows a typical micrograph of a transition of a tapered PMF. The inset depicts a uniform smooth microfiber with a diameter of 5.8 μm. The fiber geometry can be well preserved during the tapering process and as-fabricated microfibers are effectively birefringent .
Figure 2 shows typical measured normalized PD-MFI transmission spectra which were recorded by an optical spectrum analyzer (AQ6370B). The light intensity was normalized by the power at the highest peak of the transmission spectral curve. The blue line is the spectral curve for x-polarization and the green line is for y-polarization. Comb-like spectra are achieved, indicating that mode beating occurs. As can be seen in Fig. 2, the transmission spectra are highly polarization-dependent, which is distinct from those of conventional MFIs. This phenomenon is caused by the birefringence that is sensitive to guided modes. The principle of PD-MFIs will be carried out in the following section.
3. Principle of PD-MFIs
Firstly, it is necessary to investigate the mode properties of microfibers drawn from PMFs to get a better understanding of the principle of the in-line PD-MFIs. Figure 3(a) shows the geometrical diagram of fabricated microfibers. During the drawing process, the fiber geometry is retained. Since the original core of PMF is only about 8 μm, much smaller than the 125μm cladding, it can be neglected when PMFs are tapered into microfibers which have diameters of several micrometers. Consequently, the microfiber has a pair of low-index Boron-doped areas that are denoted in yellow in Fig. 3(a). Figures 3(b)-3(i) show the simulated intensity distribution of low-order guide modes (HE11, TE01, TM01 and HE12) using the software, COMSOL Multiphysics 4.2a. In the simulations, the radius of microfiber (R) was 2.5 μm, the radiuses of Boron-doped areas (r) were 1.45 μm and the distance (D) between the centers of Boron-doped areas was 2.2 μm. The wavelength was 1.55 μm. The refractive index (RI) of pure and Boron-doped silica was 1.444 and 1.434, repectively. Modes in Figs. 3(b)-3(e) are polarized in x-axis, with modes in Figs. 3(f)-3(i) polarized in y-axis. neff is the effective modal refractive index. As can be seen, degeneration of modes with the same mode number is slightly reduced and then mode birefringence is caused by the non-cylindrical distribution of material RI. With respect to HE11 mode, the birefringence, B = neff,x - neff,y, i. e., the neff difference of modes with x- and y- polarization states, is 6.6 x 10−5. For HE12 mode, B increases to −6.54 x 10−4. It is inspiring that B reaches up to 2.267 x 10−3 and −1.808 x 10−3 for TE01 and TM01 modes, respectively. Hence, the birefringence of the generated microfibers is highly mode-sensitive.
Secondly, the transmission spectra shown in Fig. 2 are nearly uniform comb and this suggests that two-mode beating is dominant. Traveling through the abrupt down taper (from the SMF to the MF), the PMF-core mode excites the fundamental and high-order modes of the MF. After the propagation, these modes are converted into the fundamental mode of PMF via the up taper (from the MF to the SMF) and mode recombination is realized. Consequently, mode interference is achieved . Assume that the length of microfiber is L. The lengths of the tapers are short enough to be neglected. The mode indices of the fundamental and higher-order modes are neff1 and neff2, respectively. The output fields Eout of PD-MFI for x- and y-polarizations can be described as
As shown in Fig. 2, transmission-extinction ratios for x- and y-polarizations are not very high, around −12 dB and −17 dB, respectively. This is due to the fact that the modes with the same polarization states have different powers. However, such fringe contrast can meet most of the demands for applications of signal processing and we focus on the polarization-dependent property of the device in this paper. For simplicity, it is assumed in the simulations that the two modes have equal powers. Hence, the power transmittances of PD-MFIs for x- and y-polarizations can be expressed by Tx = cos2(πΔnxL/λ) and Ty = cos2(πΔnyL/λ), where Δnx = neff1,x - neff2,x and Δny = neff1,y - neff2,y.Eq. (3), Δnx is not equal to Δny since the birefringence varies with modes. Then the free spectrum ranges (FSRs) for the two polarizations are different. Hence, it indicates that polarization-dependent operations at the same wavelength can be expected using this interferometer.
Figure 4 shows the simulated transmission spectra of PD-MFI. The blue line is the response for x-polarized lightwaves, with the red line for y-polarized lightwaves. In the simulations, L was 70 mm with neff1,x = 1.4. Δnx and Δny were set to be 0.14 and 0.13, respectively. This yielded a mode birefringence difference of 0.01. As can be seen in Fig. 4, for the 1546.9nm wavelength, x-polarized lightwave can be nearly fully transmitted through the PD-MFI while the y-polarized component is almost exhausted. The polarization-dependent property is in accordance with the experimental demonstrations. However, due to the nature of vernier effect, the polarization-dependent property of PD-MFI is related to the operation wavelength. As shown in Fig. 4, the transmission of PD-MFI is nearly polarization-independent for the wavelength of 1545.3 nm and 1548.6 nm. Hence, polarization-dependent or -independent optical operations can be implemented with the use of one PD-MFI by changing the working wavelengths.
4. Photonic generation of UWB pulses based on PD-MFIs
The PD-MFI provides an idea for polarization-selective optical signal processing. Based on this method, a couple of monocycle pulses with opposite polarities can be simultaneously obtained at the same wavelength using PM-IM conversion. By incoherently combining the monocycle pulses, polarity-switchable doublet pulses can be synthesized. In this approach, diverse UWB pulses can be generated in a simple system.
4.1 Principle of the scheme
Figure 5(a) shows the operation principle of UWB pulse generation based on a PD-MFI. Optical Gaussian phase-modulated signals with equal power on x- and y-polarizations are injected into the PD-MFI. λ0 is the carrier wavelength, with φ0 the phase of input phase-modulated signal. The detuning between the carrier wavelength and the PD-MFI is illustrated in Fig. 5(b). The blue solid and the green dotted lines denote the PD-MFI transmission spectra for x- and y-polarizations, respectively. The carrier wavelength is set at the intersection point of the spectral curves. At such a point, the slope of the spectral curve is positive for x-polarized lightwave while it is negative for lightwave on y-polarization. Hence, using the approach of PM-IM conversion , monocycle pulses with opposite polarities at the same wavelength are obtained on x- and y-polarizations after the PD-MFI, respectively. Then by recoupling the two pulses with an appropriate time delay τ, positive or negative doublet pulses can be generated.
Figure 6 shows the simulation results. In the simulations, the Gaussian phase-modulated signal has a 10-dB pulse-width of 100 ps, as shown in Fig. 6(a). The blue solid line and the red dotted line represent the power and the phase of signals, respectively. Figure 6(b) shows the temporal waveforms of output pulses with various τ. The blue dashed line corresponds to the waveform of τ = −125 ps. By changing τ to −25 ps, a negative UWB doublet pulse is achieved, represented by the green solid line. With τ = 0 ps, there are little “AC” components since the complementary monocycle pulses compensate for each other, shown by the shallow-green dashed line. When the time delay was adjusted to 25 ps, a positive UWB doublet pulse is obtained, as presented by the pink-dashed line. Electrical spectra of those waveforms are illustrated in Fig. 6(c). In practice, the Federal Communications Commission (FCC) regulates the electrical spectral range of UWB signals. The red dotted line is the FCC spectrum mask. The “DC” term of the electrical spectra of UWB signals were neglected to show the image of high-frequency components more clearly. It can be seen that the spectra of generated doublet pulses are within the FCC spectrum mask. Hence, the proposed method is flexible in generating diverse UWB pulses in the optical domain.
4.2 Experimental setup
In addition to the simulations, experiments were also carried out and the setup is schematically depicted in Fig. 7(a) . A CW lightwave from a tunable LD was fed to a phase modulator (PM) through a polarization-controller (PC). The PC was used to make the laser polarization state parallel to the principal axis of the PM. The PM was driven by a data sequence from a bit pattern generator (BPG). The data had a repetition rate of 20 gigabits per second with a fixed pattern of one “1” per 16 bits. The return-to-zero (RZ) electrical Gaussian pulse had a 10-dB pulse-width of about 100 ps and was amplified by a microwave amplifier (MA) before feeding to the PM. The optical phase-modulated signal was then applied to the PD-MFI through another PC. After the PM-IM conversion, the signals were divided into two taps by a polarization-beam splitter (PBS). An optical delay-line (ODL) was employed to control the relative time delay between the two taps that were further recombined by a polarization-beam coupler (PBC) to generate UWB doublet pulses. Amplified by an Erbium-doped fiber amplifier (EDFA) and attenuated by a tunable attenuator (ATT), the power of generated optical signals was then split by a polarization-independent optical coupler (OC) for efficient detection. The electrical spectrum and the waveform of the UWB pulses were measured by an electrical spectrum analyzer (ESA, MS2668C) and a digital communications analyzer (DCA, 86100C), respectively.
Figure 7(b) shows the measured transmission spectra of the PD-MFI. The blue and green lines represent the spectral curves for x- and y-polarizations, respectively. As can be seen in Fig. 7(b), the free spectrum ranges (FSRs) of both polarizations are slightly different: for x-polarization, FSR is 0.83 nm and for y-polarization, it is 0.77 nm. In this investigation, the carrier wavelength of 1560.48 nm (λ0 in Fig. 7(b)) was selected. The reason is that the large linear slopes of transmission spectra can strengthen the “AC” component of UWB monocycle pulses generated by the PM-IM conversion. Furthermore, at such an intersection point, the slopes have nearly equal absolute values and opposite signs. This is useful to achieve a pair of balanced polarity-reversed monocycle pulses for the synthesis of doublet pulses.
4.3 Experimental results
Figure 8(a) and 8(b) show the waveforms of generated negative and positive UWB monocycle pulses. The corresponding electrical spectra are shown in Fig. 8(e) and 8(f), respectively. The black dash-dotted and the red dotted lines are the fitted spectral envelope and the FCC spectrum mask. As can be seen in Fig. 8(e), negative UWB monocycle pulses (Ix1) with a central frequency (f0) of 3.64 GHz are obtained. The 10-dB bandwidth (10dB-BW) is up to 10.45 GHz. In addition, UWB monocycles pulses (Iy1) with opposite polarity are generated on y-polarization, as depicted in Fig. 8(b).
The waveforms of positive and negative UWB doublet pulses generated are presented by Fig. 8(c) and 8(d), respectively, and electrical spectra of these waveforms are shown by Fig. 8(g) and 8(h). The positive doublet signal has an f0 up to 6.36 GHz with a spectrum matching the FCC spectrum mask. The corresponding 10dB-BW is 7.86 GHz, leading to a fraction bandwidth ~125%. With respect to the negative doublet pulses, f0 is 5.00 GHz, relatively lower than that of the positive. The actual local spectral slope of this PD-MFI is slightly nonlinear, which results in that the peak of positive monocycle pulse is stronger than the dip. The resulted unbalanced monocycles bring down the peak frequency of the doublets since the doublet pulse is the superposition of monocycle ones. By optimizing the microfiber fabrication to get PD-MFIs with sign-reversed linear slopes of transmission spectra, balanced negative monocycle and doublet pulses can be obtained.
In summary, novel in-line PD-MFIs are proposed and experimentally demonstrated. Fabricated from a commercial polarization-maintaining fiber, the PD-MFIs have highly polarization-dependent transmission spectra. This is attributed to that as-fabricated microfibers are effectively birefringent and the birefringence is highly sensitive to guided modes. The experimental results are in consistent with the theoretical analysis. Besides, owing to the vernier effect, the transmission of the interferometer is polarization-independent for some specific wavelengths. Hence, polarization-dependent or -independent operations can be realized with a PD-MFI by adjusting the operating wavelength. Based on the polarization-dependent effect of PD-MFI transmission, a simple and flexible scheme of photonic generation of polarity-switchable UWB doublet pulses is proposed and experimentally implemented. The resulted doublet pulses have a central frequency of 6.28 GHz and a 10-dB bandwidth of 7.86 GHz. Hence, being advantageous for ease to fabrication and fiberized components, these PD-MFIs can find applications in a variety of areas including optical communication, optical signal processing and optical sensing.
The authors would like to show special thanks to Mr. Nai Peng for constructive discussions and providing the PMFs. This work is partially supported by the Nature Science Foundation for Distinguished Young Scholars of China (No. 61255501), the Program for New Century Excellent Talents in Ministry of Education of China (Grant No. NCET-11-0168), and the National Natural Science Foundation of China (Grant No. 60901006, and Grant No. 11174096).
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