Photonic-chip-based all-optical ultra-wideband pulse generation via XPM and birefringence in a chalcogenide waveguide Kang Tan, 1,2 David Marpaung, 1,

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1 Photonic-chi-based all-otical ultra-wideband ulse generation via XPM and birefringence in a chalcogenide waveguide Kang Tan,,2 David Maraung,,* Ravi Pant, Feng Gao,,4 Enbang Li, Jian Wang, 2 Duk-Yong Choi, 3 Steve Madden, 3 Barry Luther-Davies, 3 Junqiang Sun, 2 and J. Eggleton Centre for Ultrahigh bandwidth Devices for Otical Systems (CUDOS), Institute of Photonics and Otical Science (IPOS), School of Physics, University of Sydney, New South Wales 26, Australia 2 Wuhan National Laboratory for Otoelectronics, School of Otical and Electronic Information, Huazhong University of Science and Technology, Wuhan 4374, China 3 CUDOS, Laser Physics Centre, Australian National University, Canberra, ACT 2, Australia 4 MOE Key Laboratory of Weak-Light Nonlinear Photonics, TEDA Alied Physics School and School of Physics, Nankai University, Tianjin 3457, China * d.maraung@hysics.usyd.edu.au Abstract: We reort a hotonic-chi-based scheme for all-otical ultrawideband (UWB) ulse generation using a novel all-otical differentiator that exloits cross-hase modulation and birefringence in an As 2 S 3 chalcogenide rib waveguide. Polarity-switchable UWB monocycles and doublets were simultaneously obtained with single otical carrier oeration. Moreover, transmission over 4-km fiber of the generated UWB doublets is demonstrated with good disersion tolerance. These results indicate that the roosed aroach has otential alications in multi-shae, multimodulation and long-distance UWB-over-fiber communication systems. 23 Otical Society of America OCIS codes: (6.6) Fiber otics and otical communications; (6.5625) Radio frequency hotonics; (7.7) Analog otical signal rocessing; (35.4) Microwaves; ( ) Ultra-wideband. References and links. G. R. Aiello and G. D. Rogerson, Ultra-wideband wireless systems, IEEE Microw. Mag. 4(2), (23). 2. J. Yao, F. Zeng, and Q. Wang, Photonic generation of ultrawideband signals, J. Lightwave Technol. 25(), (27). 3. S. Pan and J. Yao, UWB-over-fiber communications: Modulation and transmission, J. Lightwave Technol. 28(6), (2). 4. C. Wang, F. Zeng, and J. Yao, All-Fiber ultrawideband ulse generation based on sectral shaing and disersion-induced frequency-to-time conversion, IEEE Photon. Technol. Lett. 9(3), (27). 5. Q. Wang and J. Yao, UWB doublet generation using nonlinearly-biased electro-otic intensity modulator, Electron. Lett. 42(22), (26). 6. S. T. Abraha, C. M. Okonkwo, E. Tangdiongga, and A. M. J. Koonen, Power-efficient imulse radio ultrawideband ulse generator based on the linear sum of modified doublet ulses, Ot. Lett. 36(2), (2). 7. F. Zeng and J. Yao, Ultrawideband imulse radio signal generation using a high-seed electrootic hase modulator and a fiber-bragg-grating-based frequency discriminator, IEEE Photon. Technol. Lett. 8(9), (26). 8. J. Li, K. Xu, S. Fu, J. Wu, J. Lin, M. Tang, and P. Shum, Ultra-wideband ulse generation with flexible ulse shae and olarity control using a Sagnac-interferometer-based intensity modulator, Ot. Exress 5(26), (27). 9. E. Zhou, X. Xu, K.-S. Lui, and K. K. Y. Wong, A ower-efficient ultra-wideband ulse generator based on multile m-im conversions, IEEE Photon. Technol. Lett. 22(4), (2).. D. Maraung, L. Chevalier, M. Burla, and C. Roeloffzen, Imulse radio ultrawideband ulse shaer based on a rogrammable hotonic chi frequency discriminator, Ot. Exress 9(25), (2).. F. Zeng and J. Yao, An aroach to ultrawideband ulse generation and distribution over otical fiber, IEEE Photon. Technol. Lett. 8(7), (26). 2. Q. Wang and J. Yao, Switchable otical UWB monocycle and doublet generation using a reconfigurable hotonic microwave delay-line filter, Ot. Exress 5(22), (27). (C) 23 OSA 28 January 23 / Vol. 2, No. 2 / OPTICS EXPRESS 23

2 3. M. Bolea, J. Mora, B. Ortega, and J. Camany, Otical UWB ulse generator using an N ta microwave hotonic filter and hase inversion adatable to different ulse modulation formats, Ot. Exress 7(7), (29). 4. J. Wang, Q. Sun, J. Sun, and W. Zhang, All-otical UWB ulse generation using sum-frequency generation in a PPLN waveguide, Ot. Exress 7(5), (29). 5. K. Tan, J. Shao, J. Sun, and J. Wang, Photonic ultra-wideband ulse generation, hybrid modulation and disersion-comensation-free transmission in multi-access communication systems, Ot. Exress 2(2), 84 2 (22). 6. Y. Yue, H. Huang, L. Zhang, J. Wang, J.-Y. Yang, O. F. Yilmaz, J. S. Levy, M. Lison, and A. E. Willner, UWB monocycle ulse generation using two-hoton absortion in a silicon waveguide, Ot. Lett. 37(4), (22). 7. D. Maraung, C. Roeloffzen, R. Heideman, A. Leinse, S. Sales, and J. Camany, Integrated microwave hotonics, arxiv:2.44 (22). 8. J. Camany, I. Gasulla, and S. Sales, Microwave hotonics: Harnessing slow light, Nat. Photonics 5(2), (2). 9. M. Mirshafiei, S. LaRochelle, and L. A. Rusch, Otical UWB waveform generation using a micro-ring resonator, IEEE Photon. Technol. Lett. 24(5), (22). 2. V. Ta eed, N. J. Baker, L. Fu, K. Finsterbusch, M. R. E. Lamont, D. J. Moss, H. C. Nguyen, B. J. Eggleton, D.- Y. Choi, S. Madden, and B. Luther-Davies, Ultrafast all-otical chalcogenide glass hotonic circuits, Ot. Exress 5(5), (27). 2. B. J. Eggleton, B. Luther-Davies, and K. Richardson, Chalcogenide hotonics, Nat. Photonics 5, 4 48 (2). 22. B. J. Eggleton, T. D. Vo, R. Pant, J. Schr, M. D. Pelusi, D. Yong Choi, S. J. Madden, and B. Luther-Davies, Photonic chi based ultrafast otical rocessing based on high nonlinearity disersion engineered chalcogenide waveguides, Laser & Photonics Reviews 6(), 97 4 (22). 23. R. Pant, C. G. Poulton, D.-Y. Choi, H. Mcfarlane, S. Hile, E. Li, L. Thevenaz, B. Luther-Davies, S. J. Madden, and B. J. Eggleton, On-chi stimulated Brillouin scattering, Ot. Exress 9(9), (2). 24. R. Pant, A. Byrnes, C. G. Poulton, E. Li, D.-Y. Choi, S. Madden, B. Luther-Davies, and B. J. Eggleton, Photonic-chi-based tunable slow and fast light via stimulated Brillouin scattering, Ot. Lett. 37(5), (22). 25. A. Byrnes, R. Pant, E. Li, D.-Y. Choi, C. G. Poulton, S. Fan, S. Madden, B. Luther-Davies, and B. J. Eggleton, Photonic chi based tunable and reconfigurable narrowband microwave hotonic filter using stimulated Brillouin scattering, Ot. Exress 2(7), (22). 26. M. Pelusi, F. Luan, T. D. Vo, M. R. E. Lamont, S. J. Madden, D. A. Bulla, D.-Y. Choi, B. Luther-Davies, and B. J. Eggleton, Photonic-chi-based radio-frequency sectrum analyser with terahertz bandwidth, Nat. Photonics 3(3), (29). 27. M. S. Rasras, D. M. Gill, S. S. Patel, K.-Y. Tu, Y.-K. Chen, A. E. White, A. T. S. Pomerene, D. N. Carothers, M. J. Grove, D. K. Saracin, J. Michel, M. A. Beals, and L. C. Kimerling, Demonstration of a fourth-order olezero otical filter integrated using CMOS rocesses, J. Lightwave Technol. 25(), (27). 28. D. Dai and J. E. Bowers, Novel concet for ultracomact olarization slitter-rotator based on silicon nanowires, Ot. Exress 9(), (2). 29. M. Burla, D. Maraung, L. Zhuang, C. Roeloffzen, M. R. Khan, A. Leinse, M. Hoekman, and R. Heideman, On-chi CMOS comatible reconfigurable otical delay line with searate carrier tuning for microwave hotonic signal rocessing, Ot. Exress 9(22), (2).. Introduction Ultrawideband (UWB) technology has attracted great interest for its large bandwidth and high data rates under unlicensed sectrum. However, the low ower sectral density dictated by the sectral mask secified by the Federal Communications Commission (FCC) limits the transmission distance of UWB signals to less than m []. By taking advantages of low transmission loss and extremely broad bandwidth offered by otical communication systems, UWB-over-fiber (UWBoF) technology has been roosed to remove this limitation. In this aroach, UWB signals are generated and distributed in the otical domain [2, 3]. Over the years, different techniques have been reorted for UWB ulse generation, including sectral shaing followed by frequency-to-time maing [4], nonlinear biasing of an electro-otic modulator [5, 6], hase-to-intensity modulation conversion [7 ], sectral filtering using microwave hotonics (MWP) filters [ 3], and otical nonlinearity [4 6]. These techniques have shown good results, but most of them are based on otical fibers, which can be bulky. To reach their full otential, UWBoF systems need to adot hotonic integrated circuit (PIC) technology. Recently, the emerging field of integrated MWP [7] where PICs are used to maniulate radiofrequency signals [8] has attracted interest for its distinct advantages in terms of size, weight, ower, and cost. In this context, several hotonic-chi-based solutions for UWB ulse (C) 23 OSA 28 January 23 / Vol. 2, No. 2 / OPTICS EXPRESS 24

3 generation have recently been resented, including silicon waveguide emloying two-hoton absortion [6] or ring resonators for ulse shaing [, 9]. Although imressive results have been achieved, these aroaches imose several limitations. For examle, the technique reorted in [6] can only synthesize simle monocycle ulses by combining ositive Gaussian ulses, i.e. the normal Gaussian ulses, and negative Gaussian ulses, i.e. the ulses with inverted shae of normal Gaussian ones, at two different wavelengths, while the thermootical tuning emloyed in [, 9] with reconfiguration time of about ms makes fast data modulation imossible. Chalcogenide (ChG) hotonic chis [2 25] have recently emerged as a romising latform for imlementing integrated MWP. The material ossesses excellent roerties such as large and ultrafast third-order otical nonlinearities, low two-hoton absortion and negligible free-carrier effects [2, 2]. These roerties guarantee instantaneous and ure resonse, ideal for all-otical nonlinear signal rocessing [22]. Additionally, the ChG hotonic chi also exhibits large stimulated Brillouin scattering gain coefficient [23], which has recently been used for integrated MWP signal rocessing in the form of tunable otical delay lines [24] and a reconfigurable singe band-ass microwave filter with tunable center frequency [25]. In this work, we reort a hotonic-chi-based scheme for UWB ulse generation that offers a wide variety of ulse shaes and the otential for a host of modulation formats. These features are achieved by using a novel all-otical differentiator based on a chalcogenide (ChG) hotonic chi. We combine the effects of cross-hase modulation (XPM) and birefringence in an As 2 S 3 rib waveguide to generate olarity-inverted UWB monocycles with single otical carrier from the inut Gaussian ulse train. The high Kerr-nonlinearity of ChG [2] in a chi latform enables efficient XPM in a short length of 7 cm. Furthermore, we also combine these monocycles with a roer time delay to generate olarity-switchable UWB doublets and demonstrate the transmission of these doublets over a 4-km fiber link with good disersion tolerance. Additionally, the scheme can be extended for high-seed modulation and hybrid modulation format [5]. Consequently, the roosed aroach has the otential for multi-shae, multi-modulation and disersion-comensation-free transmission UWBoF communication systems, reresenting a significant advance in the field of integrated MWP. 2. Princile of oeration Figure shows the schematic of the roosal. The dashed box highlights the novel all-otical differentiator, where the hase-modulated robe through on-chi XPM is differentiated at the outut via on-chi birefringence and mode interference. The details are as follows. Fig.. (a) Schematic diagram of the roosal for hotonic-chi-based UWB ulse generation. (b) TE and TM mode rofiles of As 2 S 3 rib waveguide. A Gaussian otical ulse train at s Ps t, is couled into an As 2 S 3 waveguide together with a continuous-wave (CW) robe at λ. Due to XPM, the robe is λ as um light, ( ) (C) 23 OSA 28 January 23 / Vol. 2, No. 2 / OPTICS EXPRESS 25

4 hase modulated with a Gaussian rofile, ( t ) ϕ, which is roortional to Ps () t. Prior to couling to the waveguide, the olarization of the robe light is aligned at an angle of 45 to the x-axis of the chi, thereby equally exciting TE and TM modes. The olarization of the signal ulses Ps ( t ) is adjusted to comensate olarization-deendent nonlinear coefficient of these two modes such that the total hase modulation indices for both modes are the same. However, due to birefringence in the waveguide, the TE and TM modes exerience a mutual time delay after 7-cm roagation. According to the simulation shown in Fig. (b), the two modes have different mode rofiles: the TM mode has a larger otical field in the cladding and the substrate than the TE mode, and thus has a lower effective refractive index. These rovide different grou refractive indices for achieving the mutual time delay, τ, which is estimated to be ~2.87 s. While in most cases the imact of on-chi birefringence is undesirable, the mutual delay between these modes is crucial to obtain the all-otical differentiation in roosed scheme. The outut from the chi is then assed to an otical band-ass filter (BPF) for removing the unwanted signal ulses. A olarization controller (PC) is then used to rotate the olarization of the robe and add a hase difference ϕ between the TE and TM modes. Then the robe is fed into a olarization beam slitter (PBS) to achieve interference of the light at two olarizations, of which the two outut orts rovide signals exressed by E ( t) iω cos t e θ e Ee E ( t) e sinθ + e.5αtm L+ i ϕ( t) + ϕ.5αte L+ i ϕ( t τ) ωτ.5αtm L+ i ϕ( t) + ϕ.5αte L+ i ϕ( t τ) ωτ sinθ, cosθ where α TE and α TM are the losses of TE and TM modes resectively, L is the length of ChG rib waveguide, E and ω are the electric field amlitude and otical carrier angular frequency of robe light resectively, and θ is the rotation angle comared to rincial axis of PBS, as shown in the lower inset of Fig. (a). When detected by hoto-detector (PD), the robe at the two PBS oututs converts to hotocurrents with a.c. terms exressed as π sin ϕ() t ϕ( t τ) + ϕ + ωτ + io () t 2 ex.5( αte αtm ) L sin ( 2 θ). io () t + (2) π sin ϕ() t ϕ( t τ) + ϕ + ωτ + 2 By adjusting PC such that θ =± 45 o and ϕ + ωτ+ π 2 = N π (N should be an integer) and considering that ϕ( t) ϕ( t τ ) () t () t is small enough due to small τ, Eq. (2) can be simlified to [ s() s( )] i sin o ϕ t ϕ t τ ϕ t ϕ t τ P t P t τ. (3) i ± sin ϕ t ϕ t τ ± ϕ t ϕ t τ ± P t P t τ o s s ϕ ( ), and hence of the inut signal ulses, ( ) According to Eq. (3), one can conclude that, for sufficiently smallτ, the outut intensity from PBS can be aroximated as the temoral differentiation of the hase variation of the robe, t P t. Thereby, we have constructed an allotical differentiator by exloiting XPM and birefringence in single ChG waveguide. For a Gaussian otical ulse train inut, the oututs of the PBS constitute Gaussian monocycles, which are the first-order temoral derivatives of the Gaussian ulses. Note that the generated monocycles are olarity inverted, as exected from Eq. (3). At each outut ort, the olarity of the monocycles can be easily changed by tuning the PC to shift either N between odd and s () (C) 23 OSA 28 January 23 / Vol. 2, No. 2 / OPTICS EXPRESS 26

5 even or θ between 45 and 45. Additionally, by combining the ositive and negative monocycles with an aroriate time delay, olarity-inverted doublets can also be generated. 3. Exeriments and discussions Figure 2 deicts the exerimental setu. The signal ulses were generated from a modelocked laser (Calmar FPL-3CFF) at 542. nm with a reetition rate of MHz and ulse width around 4 fs. In order to match the FCC mask, we use disersion comensating fiber (DCF) to broaden the ulses to a width of ~8 s. As shown in Fig. 3(g), the broadened ulses maintained a Gaussian-like shae, to which the certain frequency chir of generated ulses from mode-locked laser contributed. The CW robe at 552. nm was from a tunable laser source (TLS, Photonetics 3642HE57). After amlification by erbium-doed fiber amlifiers (EDFAs) and olarization adjustments, both signal and robe were injected into a 7-cm long ChG waveguide with a cross-section of 4 µm 85 nm and a high nonlinear coefficient of ~556 W km. The waveguide was disersion-engineered and had a low roagation loss of less than.2 db/cm [26]. The light was couled in and out of the chi using lensed fibers. Two 99: coulers were laced at the inut and outut of the chi to monitor the chi insertion loss, which amounted to ~9 db. The chi outut was routed to a BPF for removing the um light, which was followed by a PC and a PBS. Two 5:5 coulers were laced at the PBS oututs. For each couler, one of the oututs was directly routed to a PD. These oututs contained two olarity-inverted monocycles and are marked as otical oututs and 3 in Fig. 2. In order to generate a doublet from these monocycles, the other oututs of the coulers were delayed using a air of variable otical delay lines (VODLs) and combined in another 5:5 couler. This is marked as otical outut 2 in Fig. 2. To imlement UWBoF transmission, the generated doublets then roagated over 4-km of single-mode fiber (SMF). The temoral waveforms and corresonding electrical sectra of the UWB ulses from each outut were monitored by a digital communication analyzer (DCA, Hewlett Packard 8348A) and an electrical sectrum analyzer (ESA, Agilent E4448A), resectively. Fig. 2. The exerimental setu of the roosal for hotonic-chi-based UWB ulse generation with switchable ulse shae and olarity. The measured waveforms and electrical sectra of the generated UWB ulses are shown in Figs. 3(a)-3(f). From oututs and 3, olarity-inverted monocycles were generated, as shown with blue traces in Fig. 3(a) for ort and Fig. 3(c) for ort 3. At each ort, the olarity of the monocycles can easily be changed by reconfiguring the PC, as deicted with red dashed traces in Fig. 3(a) for ort and Fig. 3(c) for ort 3. As exected, the electrical sectra of monocycles contained relatively large lower-frequency comonents, as shown in Figs. 3(d) and 3(f). For this reason, it is desirable to use high-order derivatives of Gaussian ulses in UWBoF systems [2], for examle doublets. The generated waveforms of olarity-switchable doublets obtained from outut ort 2 are shown in Fig. 3(b). As exlained earlier, olarity inversion is obtained by adjusting the PC. As exected, the electrical sectra of the UWB doublets shown in Fig. 3(e) featured a higher central frequency relative to the monocycles, and thereby fitted the FCC mask (green dashed line in Fig. 3) much better than the monocycles. Moreover, the lower-frequency comonents (C) 23 OSA 28 January 23 / Vol. 2, No. 2 / OPTICS EXPRESS 27

6 can be further suressed by exloiting well-designed antennas with a desirable frequency resonse curve ossessing a flat to in the UWB band and a dee notch in the lowerfrequency region, which erforms like a band-ass filter [3]. The temoral waveform and electrical sectrum of the generated doublets after roagating through 4-km fiber without disersion comensation are shown in Figs. 3(h) and 3(i), resectively. As evident from these figures, the doublet accumulates only a small amount of distortion in its temoral waveform and has almost the same electrical sectrum, indicating that the UWB ulses have a good tolerance to disersion over fiber and are suitable for long-distance transmission in UWBoF communication systems. We believe that this comes from the fact that the hase of the chired inut ulse is not transferred to the UWB waveform because XPM deends only on the intensity of the inut ulses. This roerty combined with single carrier oeration roosed in this scheme leads to a better disersion tolerance than reviously reorted chi-based techniques [4, 6] Time (s) -4 5 Time (s) -4 5 Time (s) -4 Power (dbm) Power (dbm) Power (dbm) Frequency (GHz) Exerimental Result Gaussian Fitting 5 5 Frequency (GHz) Power (dbm) 5 5 Frequency (GHz) Time (s) 5 Time (s) 5 5 Frequency (GHz) Fig. 3. (a), (b), (c) Temoral waveforms and (d), (e), (f) corresonding electrical sectra of generated UWB monocycles and doublets measured at outut ort, 2 and 3, resectively. The blue curves in (a), (b), and (c) deict the waveforms of obtained UWB ulses, while the red dashed curves show the inverted ulses after adjusting PC before PBS to shift either N between odd and even or θ between 45 and 45. (g) The signal ulses measured at the outut of DCF, which were then couled with CW robe and injected into ChG chi. (h) Temoral waveforms and (i) corresonding electrical sectrum of negative doublet after roagating over 4-km SMF link. The FCC sectrum masks are added in green dashed lines. The novelty of this scheme lies in the use of on-chi XPM and on-chi birefringence to induce a time delay between the TE and TM modes. Thus, it is useful to study the role of the on-chi time delay, τ, in the generation of the monocycle ulses. We simulated and analyzed the imact of the variation in this time delay on the roerties of the outut UWB ulses, as shown in Fig. 4. When the time delay changes from less than s to ~2 s, the generated (C) 23 OSA 28 January 23 / Vol. 2, No. 2 / OPTICS EXPRESS 28

7 UWB ulses maintain nearly the same monocycle shae and their eak ower increases almost linearly. However, when τ further increases, the ulse shae becomes slightly distorted and tends to searate into a ositive Gaussian ulse followed by a negative one, as shown in the uer right inset of Fig. 4. As indicated in Fig. 4, the time delay of 2.87 s rovided by 7-cm ChG waveguide is located in the roer region for monocycle generation. Fig. 4. Simulation results of eak ower of generated monocycles as a function of mutual grou time delay τ between TE and TM modes. The insets from bottom to to are the ulse shaes with τ of.2, 2 and 8 s, resectively. The green block shows the roer region for monocycle generation. 4. Potential modulation techniques for multi-access UWBoF systems For a ractical UWBoF communication system, the information must be encoded, which is done by using different ulse modulation schemes such as ulse amlitude modulation (PAM), binary hase-shift keying (BPSK), ulse shae modulation (PSM), and ulse osition modulation (PPM) [3]. As mentioned earlier, the roosed scheme has the advantage of flexibly imlementing multi-modulation formats. In this section, we analyze the otential of imlementing PAM, BPSK, PSM, PPM, and the hybrid modulation of these four [5] in the hotonic-chi-based UWB generator. Note that the hybrid modulation scheme is articularly interesting to increase the transmission bit rate even further [5]. (C) 23 OSA 28 January 23 / Vol. 2, No. 2 / OPTICS EXPRESS 29

8 Fig. 5. Proosal for imlementing PAM, BPSK, PPM, PSM and hybrid modulation format of the four into revious exerimental setu Figure 5 shows the extension of roosed scheme to accommodate multile modulation formats. The additional hardware includes four bit-attern generators (BPGs) for the data generation, two electro-otic modulators that include one Mach Zehnder modulator (MZM) after the DCF and one olarization modulator (PolM) before PBS, two otical switches at the three outut orts. Using this scheme, PAM can be achieved by modulating the ower of the signal ulses via the MZM. Considering the cross-hase modulation index is roortional to the otical ower of those signal ulses, the amlitude of the generated UWB ulses can be modified by continuously changing the driving voltage alied for MZM. In this way, the PAM with continuous tuning characteristics can be realized. There are two methods to imlement BPSK in this case. The first utilizes otical switch to alternatively choose the olarity inverted UWB ulses from the PBS oututs. Alternatively one can use the PolM for either alternatively setting N as odd or even or shifting θ between 45 and 45, according to Eq. (2). Moreover, by exloiting otical switch 2 for alternatively selecting the UWB ulse shaes between monocycle and doublet, PSM can be realized. Finally, PPM can be achieved by using otical switch to alternate UWB ulses with two orthogonal olarizations from outut ort and 3, resectively. In this case, a length of olarization maintaining fiber (PMF) is used to rovide a different grou delay for the two UWB ulses at the two olarizations. By roerly selecting the length of PMF, the mutual time delay between UWB ulses of two orthogonal olarizations can vary over a large range. Thus, by exloiting the MZM for PAM, PolM for BPSK, otical switch for PPM and otical switch 2 for PSM, it is ossible to imlement hybrid modulation of those four modulation formats to increase the transmission bit rate to at least four times the original, which is romising for future high-seed UWBoF communication [5]. In addition, the above concet can be further extended towards multi-user and multiaccess UWB communication systems. Considering that all the CW robes with same roer wavelength sacing will satisfy ϕ + ωτ + π 2 = N π and XPM can work in a large wavelength range, olarity-switchable UWB monocycles and doublets at multi-wavelengths are otential to obtain simly by couling more CW robes with roer wavelength sacing into the ChG chi and using wavelength-division demultilexer to searate them at the outut orts. Thus, the roosal is romising for wavelength division multilexing (WDM) communication alied to multi-user UWB systems, where different wavelengths can be used as different channels for different users. 5. Conclusion We have demonstrated a hotonic-chi-based scheme for all-otical UWB ulse generation by exloiting on-chi XPM and on-chi birefringence in single As 2 S 3 rib waveguide. By adjusting the PC and VODLs, both UWB monocycles and doublets with switchable olarity have been generated. Extension towards multi-wavelength UWB ulses with flexible ulse shae and olarity can be achieved by simly adding more CW robes with an aroriate wavelength sacing, which can be alied to multi-casting and multi-access UWB systems. The generated UWB ulses show good disersion tolerance because the frequency chir in the inut ulses is not transferred to the outut by XPM. The ossibility of simultaneously imlementing a number of modulation formats in this scheme has been analyzed. (C) 23 OSA 28 January 23 / Vol. 2, No. 2 / OPTICS EXPRESS 2

9 Finally, the scheme has the otential of on-chi and monolithic integration by integrating the discrete comonents like BPF [27], PBS [28], coulers and VODL [29] on a single chi. This will lead to advantages such as small size, lower loss, lighter weight and better tuning characteristics. Consequently, the roosed aroach has a high otential for multi-access, multi-modulation formats and PIC-based UWBoF communication systems. Acknowledgments This work was funded by the Australian Research Council (ARC) through its Discovery grant (DP96838), Federation Fellowshi (FF77656), Laureate Fellowshi (FL229), Center of Excellence CUDOS (Grant # CE8), Futures Fellowshi (FT853), the National Natural Science Foundation of China (Project number: , 6775 and ), and the Program for New Century Excellent Talents in University (NCET-- 82). All exeriments were erformed at the CUDOS laboratories at the University of Sydney. (C) 23 OSA 28 January 23 / Vol. 2, No. 2 / OPTICS EXPRESS 2