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Abstract
We demonstrate bandwidth measurement and high-speed data transmission of meter-scale connectorized ultra-flexible multimode waveguide links with a maximum length of 180 cm. The pulses propagating through the waveguides broadened linearly with the increase of the length from 20 cm to 240 cm and the estimated mode delay from the pulse broadening was 0.093 ps/cm. The corresponding waveguide bandwidth decreased inversely with the increase of waveguide length, leading to a bandwidth-length product of 42 GHz·m. Degradation in bandwidth due to the introduction of bending or twisting was small when the samples were bent with a bending radius as small as 1 mm for 3 turns or twisted for 4 full turns, respectively. Error-free transmission of 30 Gb/s non-return-to-zero (NRZ) signal was achieved with a record link length up to 140 cm to the best of our knowledge. Our results show that the demonstrated flexible waveguides have both excellent optical and mechanical properties and are ideal for high-speed optical interconnects application especially those have a strict requirement on flexibility.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In recent years, large-scale data centers and high-performance computers are increasingly limited by the interconnection capacity, which is due to the rapid increase of power consumption and bandwidth requirements driven by the strict reduction of the critical size of complementary metal-oxide-semiconductor (CMOS) according to Moore’s law [1–3]. Optical interconnects have attracted significant interest when operating at high data rate (>10 Gb/s). Compared with their electrical counterparts, they have advantages of high bandwidth, improved power efficiency, high interconnection density and anti-electromagnetic interference [4–7].
Multimode polymer waveguides are considered to be one of the promising transmission media for implementing high-speed board-level optical interconnects. They have good compatibilities with both printed circuit boards (PCBs) and fiber optics. Transmission loss of multimode polymer waveguides can be lower than 0.05 dB/cm at a wavelength of 850 nm [8–12]. They can be coupled to and from graded-index multimode fibers (GI-MMF) with low coupling loss. However, signal degradation due to their multimode nature leaves concerns on both maximum achievable transmission data rate and applicable distance. Board-level optical interconnection systems employing multimode polymer waveguides and vertical cavity surface emitting lasers (VCSELs) have been demonstrated [13,14]. The optical characteristics, bandwidth and high-speed data transmission performance of meter-scale spiral multimode polymer waveguides fabricated on rigid substrates have been investigated [15–21]. The loss and bandwidth performance can also be optimized using refractive-index engineering and launch conditioning [21]. By adjusting their refractive index profile, a bandwidth-length product of a GI polymer waveguide larger than 60 GHz·m has been achieved [19], and 56 Gb/s 4-level pulse amplitude modulation (PAM-4) data transmission has been demonstrated over a 1-meter-long multimode polymer waveguide [18,20].
In addition, highly flexible film-type polymer waveguides can withstand bending, twisting, and stretching. In terms of assembly, the mechanical flexibility of waveguide provides much more freedom for the design and formation of devices. In space constrained applications, characteristics of the thinness and flexibility of waveguides provide great advantages. Multiple sheets can be stacked for complex routing, resulting in improved channel density and enhanced bandwidth, which emerge as a potential alternative of fiber bundles [22,23]. Flexible polymer waveguide ribbons with connectors can be deployed as freely detachable and reconfigurable board-to-board or chip-to-chip in high-speed optical interconnection links [1,24]. These advantages of flexible polymer waveguides can be expected to extend photonics, which is traditionally limited to rigid and planar shape factors, to the three-dimensional flexible applications [25]. For waveguide on rigid substrate, bending is realized by UV lithography with a fixed pattern. However, the bending of flexible waveguide is mainly along the upper/lower surfaces, which is different from waveguide bending on rigid substrate along sidewall direction. It is known that the waveguide roughness of the upper/lower surfaces and sidewall is different. Both the differences in refractive indices distribution and roughness will lead to difference in bending properties [9]. In some practical applications with strict flexibility requirements, the deformation of flexible waveguide leaves concerns on their high-speed transmission performance.
Table 1 shows comparison of characteristics of state-of-the-art multimode polymer waveguides. Various interconnection technologies based on flexible polymer waveguides [26–33] and opto-electronic subassemblies integrated with flexible waveguides have been developed [34–38]. Studies on the effects of bending and twisting on the loss, crosstalk and bandwidth performance have been investigated with waveguide length of 24 cm, and the 1 dB excess loss radius is found to be 6 mm [26] and similar studies have been reported in [27]. Moreover, a new design of flexible polymer multimode waveguides is proposed in order to achieve improved bending loss performance and an excess loss of 0.5 dB for a 3 mm radius is obtained experimentally [31]. Although the data transmission rate of 25 Gb/s per channel has been reported on flexible multimode waveguides [34–36], these demonstrations only involve straight waveguides with the length less than 20 cm. 40 Gb/s data transmission with a minimum bending radius of 4 mm is realized on a 1 m-long flexible spiral waveguide [28].
Table 1. Comparison of characteristics of state-of-the-art multimode polymer waveguidesa
In this paper, we demonstrate bandwidth measurement and high-speed data transmission of meter-scale connectorized ultra-flexible multimode waveguides links with a maximum length of 180 cm. We directly observed that the pulses propagating through the waveguides broadened linearly with the increase of waveguide length from 20 cm to 240 cm based on an optical sampling technique, and the resulting mode delay was 0.093 ps/cm. The corresponding waveguide bandwidth decreased inversely with the increase of waveguide length, leading to a bandwidth-length product of 42 GHz·m. The bandwidth degradation was trivial with the samples bent down to 1-mm radius for 3 turns or twisted up to 4 full turns, respectively, and the error-free data transmission of 30 Gb/s non-return-to-zero (NRZ) signal was achieved with a link length up to 140 cm. It is the longest flexible waveguide link in the reported bandwidth and transmission experiments under flexure, to the best of our knowledge, which guarantee a desirable margin in terms of both optical power and bandwidth in consideration of real applications. Our results show that the flexible multimode waveguides have both excellent optical and mechanical properties and are suitable for high-speed optical interconnects application especially those have a strict requirement on flexibility.
2. Flexible multimode polymer waveguides
The flexible polymer optical waveguides were fabricated using polynorbornene by a “Photo-addressing” technique proposed by Sumitomo Bakelite Co., Ltd. [39,40]. As shown in Fig. 1(a), three 20 cm-long waveguide ribbons with a width of 3 mm were fabricated under the same conditions. Fabrication processes and waveguide parameters in detail were reported in [11,41]. Each waveguide sample has 12 channels and the waveguide pitch is 250 µm to match the separation of the fiber ribbons as shown in Fig. 1(b). The dummy cores are used to suppress crosstalk between adjacent waveguide channels. The core size is about 44×46 µm2 as shown in Fig. 1(c). As shown in Fig. 1(d), they have a W-shaped index profile with a graded-index and a step-index profile in the horizontal and vertical direction, respectively. The refractive index of the core is 1.553 (n1) and that of the cladding are 1.535 (n2) and 1.517 (n3) in horizontal and vertical direction, respectively, at a wavelength of 546 nm. The specific index distribution and larger index difference along the bending direction improve the flexibility performance, which guarantee a realization of the ultra-low bending loss of the flexible waveguide. In order to minimize the coupling loss of polymer mechanically transferable (PMT, recommended by Standard IEC 62496-4-1:2019) connectors and ensure the stability in the following experiments, commercially-available mechanically transferable (MT) clamps were employed to secure the connection of the two PMT connectors as shown in Fig. 1(e). The samples can be bent down to 1-mm radius without any crack or delamination.
Fig. 1. (a) Flexible waveguide ribbons with MT connectors; (b) The waveguide pitch and (c) waveguide end facet; (d) Measured index profile of the waveguide; (e) Flexible waveguide connected with short fiber jumpers.
We first measured the insertion loss of all 36 channels of 3 waveguide samples using the experimental setup as shown in Fig. 2. The light from an 850 nm broadband laser diode (3 dB bandwidth: 844 ± 14 nm) was butt-coupled into and out from the connectorized waveguides using a pair of 50 µm-core GI-MMF jumpers. The straight flexible waveguides without any deformation need to be ensured during the measurement. As shown in Fig. 3, the average insertion loss per centimeter of the three waveguides including the coupling loss are 0.083, 0.081, and 0.083 dB/cm. In addition, all standard deviations of insertion losses of 3 samples are less than 1%. The MT connection losses were estimated to be ∼0.4 dB/PMT by comparing the insertion losses of connected waveguide links with the different lengths.
3. Bandwidth studies
The bandwidth of flexible polymer waveguides was estimated using direct time-domain measurement based on an optical sampling technique [42]. The experimental setup for measuring waveguide bandwidth is shown in Fig. 4. Two mode-locked lasers (Menlo Systems LAC-1550) with the center wavelength of 1560 nm were employed as signal and local laser and their pulse widths are 100 fs and 800 fs, respectively. The repetition rates of both lasers are about 250 MHz, and there is a small frequency difference of about 0.3 kHz between them. Two periodically poled lithium niobate crystals (PPLN: CTL Photonics M-1940-1970-1) matched with each of the two lasers were used as frequency doubling crystals to produce ultrashort pulses at a wavelength of 780 nm. The ultrashort signal pulse was sent to the waveguides under test. The pulse propagated though the waveguide interfered with the ultrashort pulse generated by local laser at the multimode 3 dB coupler and then detected by a 75-MHz balanced photo-detectors (BPDs) (Thorlabs PDB450c). A low-pass filter (LPF) with the cut-off frequency of 81 MHz was applied to reduce the impact of high-frequency noise and the DC block was used to remove the DC bias after BPD. Finally, the signal was sampled by an oscilloscope and analyzed by a personal computer. The lengths of all pigtail fibers used in the experiment were minimized to guarantee a large detection bandwidth of the system.
Fig. 4. Experimental setup for waveguide bandwidth measurement of (a) back-to-back link and (b) waveguide link.
3.1 Bandwidth of straight waveguides
In multimode fiber, the pulse broadening caused by intermodal dispersion is proportional to the fiber length [43], which leads to the fact that the bandwidth-length product of the fiber is a constant. Based on the same mechanism, we measured the dispersion-induced pulse widths and calculated the bandwidth of flexible waveguides with different lengths. Variation in the lengths of flexible waveguide link were obtained by connecting a different number of waveguides using polymer MT connectors or MT-MMF ribbon jumpers. In order to minimize the experimental error, the lengths of the MT-MMF ribbon jumpers used are as short as 15 cm.
The collected data were first processed by digital band pass filtering in order to maximally reduce the impact of low and high frequency noise. The original pulses, envelopes and fitting curves of the back-to-back (B2B) link and flexible waveguide (WG) links with different waveguide lengths of 40, 80, 120, 160, and 240 cm are shown in Fig. 5. Gaussian curves were adopted to fit the envelopes of the original pulses of both B2B and with waveguides and the corresponding full-width-at-half-maximum (FWHM) represents the pulse width. As shown in Fig. 6(a), the pulse widths of waveguides with different lengths and the corresponding linear fitting curve are indicated. The FWHM of the B2B link is about 1 ps, which guarantee a large capacity of the measurement system. The slope of the fitting curve, which represents mode delay per unit length of flexible multimode waveguides, is 0.093 ps/cm and exhibits good linearity. The standard deviation of the slope of the fitting curve, which is also the experimental precision of mode delay, is 0.003 ps/cm. Because the Gaussian fitting process will inevitably result in the missing of a part of the dispersion information, the data points in the figure were averaged using the measured FWHM of four different channels for a certain length and the standard deviation can be reduced by increasing the number of the measured waveguide channels.
Fig. 5. Original pulses, envelopes and fitting curves of B2B link and flexible waveguide (WG) links with waveguide lengths of 40, 80, 120, 160, and 240 cm.
Fig. 6. (a) Pulse widths of dispersion pulses and (b) the measured bandwidth of different waveguide lengths and the corresponding fitting curves. The bandwidth was obtained by subtracting the Fourier transform of envelops of the input pulse and the output pulse.
The measured bandwidth of flexible waveguides with different lengths is shown in Fig. 6(b). The estimated 3 dB bandwidth was obtained by subtracting the Fourier transform of envelops of the input pulse (B2B) and the output pulse. The pulse broadening caused by differential mode delay increased with the increase of waveguide length, and the measured bandwidth is inversely proportional to the waveguide length. The corresponding bandwidth-length product of the flexible multimode waveguide, is 42 GHz·m, which is the averaged results of 4 channels.
3.2 Bandwidth of bent or twisted waveguides
The bandwidth of flexible polymer waveguide link under bending or twisting conditions was investigated. The pictures of flexible polymer waveguide under 1 mm-bending radius with 3 turns or 4 full (360°) twist turns are shown in Fig. 7. The bending and twisting result in the increasing of mode coupling, which may affect bandwidth performance of flexible waveguides. Both ends of waveguides were fixed and the waveguide links were kept as straight as possible.
Fig. 7. Flexible polymer waveguide (a) under 1 mm-bending radius for 3 turns and (b) under 4 full (360°) twists.
The measured bandwidth of 20 cm-long waveguides under different bending radii or different twist turns are shown in Fig. 8. All bends were wrapped three turns around a cylindrical mandrel with a designated radius. The locations of bending were all kept in the middle along the waveguides. The data points in the figures were averaged by the measured bandwidth of six different channels. The measured bandwidth of straight waveguide (201 GHz) is also indicated for comparison. There is no obvious bandwidth degradation when the waveguides were bent down to 1 mm-radius for 3 turns. Degradation in bandwidth due to the introduction of twisting was small when the samples were twisted up to 4 full turns.
Fig. 8. Measured bandwidth of the 20 cm-long waveguides with (a) waveguide bending and (b) waveguide twisting.
4. High-speed transmission performance
In order to evaluate the practical high-speed signal transmission capability of the flexible multimode polymer waveguides, NRZ transmission at a data rate of 30 Gb/s was conducted. The experimental setup for the test is shown in Fig. 9. Electrical NRZ signal with a pseudo-random binary sequence (PRBS) length of 29−1 at a data rate of 30 Gb/s was generated from a pulse pattern generator (PPG, KEYSIGHT N4951B). Light from the DBR-LD at 850 nm was coupled to the intensity modulator with a bandwidth of 25 GHz, modulated by the NRZ signal. A polarization controller (PC) was employed in order to obtain the maximum output optical power. The modulated light signal was launched into the waveguides by MT-MMF ribbon jumpers. A multimode variable optical attenuator (VOA) was used for optical power adjustment and the light from the VOA was detected using a fiber-coupled photodiode (PD) (New Focus 1484-A-50) with a bandwidth of 22 GHz. After being amplified by a 50 GHz radio frequency (RF) amplifier (SHF S807), the electrical signal from the PD was transmitted to the BERT (KEYSIGHT M8046A) or a wide-bandwidth digital sampling oscilloscope (KEYSIGHT DCA-X 86100D) for analyzing respective bit error rate (BER) performance and to record the eye diagrams of the received signal, respectively.
4.1 High-speed performance of straight waveguides
The high-speed transmission performance of straight waveguide was investigated. Received eye diagrams of the B2B link and straight waveguide links with different lengths are shown in Fig. 10(a). The received optical power (Pout) is noted as well as the voltage and time scale of the recorded waveforms. Open eye diagrams were obtained for the B2B link and all waveguide with different lengths. Reduction in eye opening can be observed due to additional dispersion and noise when the waveguide was inserted into the link. Although this phenomenon is more obvious with the increase of waveguide length, an open eye diagrams were still observed with the waveguide length reaching 180 cm.
Fig. 10. (a) 30 Gb/s eye diagrams of straight waveguide links and (b) BER curves of waveguide links with different lengths.
BER measurements were also carried out on all optical links and the obtained BER curves as a function of the average optical power are shown in Fig. 10(b). Error-free (BER < 10−10) data transmission was achieved over the waveguide links with lengths of 60 cm, 120 cm, and 140 cm. The power penalties for a BER of 10−9 are found to be about 0.6 dB, 1.3 dB, and 1.3 dB for waveguide length of 60 cm, 120 cm, and 140 cm, respectively. The insertion losses of straight waveguides are 3.55 dB, 8.39 dB, and 11.1 dB for the corresponding waveguide length as shown in Table 2. Taking into consideration the maximum achievable output power of the modulator of 4 dBm, the averaged received power of error-free (BER = 10−10) data transmission, and the additional 1 dB losses in the links such as connecting loss, the power margin can be estimated as shown in Table 2. For a 180 cm-long waveguide link, the minimum achievable BER is 3.6×10−8 at the average receive power of -10.5 dBm due to the insertion loss of 13.54 dB. It is also well below the 7% forward error correction (BER = 3.8−3) limitation. It is worth noting that the high-speed performance of the passive component is related to its insertion loss and frequency response. Because the bandwidth-length product of the flexible waveguide is 42 GHz·m, it is the insertion loss that becoming a more critical index to limit the high-speed transmission performance of the 180 cm-long flexible waveguide link.
Table 2. Insertion loss and power margin of straight waveguides
4.2 High-speed performance of bent or twisted waveguides
In order to confirm the effects of bending and twisting of flexible waveguides on high-speed transmission performance, transmission experiments at data rates of 30 Gb/s on 140 cm-long flexible waveguides under flexure or twisted conditions were conducted. Table 3 shows experimental conditions of the 140 cm-long waveguides with different radii. The excess loss was obtained by subtracting the insertion loss of bending waveguides and straight waveguides. Open eye diagrams were obtained for all the waveguides with different bending radii as shown in Fig. 11(a). Reduction in eye opening due to the insertion of waveguide links can be observed. However, the shapes of the eye diagrams were almost unchanged with the decrease of the waveguide radii, even to a bending radius of 1 mm. BER curves for the 140 cm-long waveguide links with different bending radii also show that there is no serious degradation of transmission performance as shown in Fig. 11(b).
Fig. 11. (a) 30 Gb/s eye diagrams and (b) BER curves of 140 cm-long straight waveguide links under different bending radii.
Table 3. Excess bending loss and power margin of waveguides
Table 4 shows excess twisting loss and power margin of the 140 cm-long waveguides with different twisting turns. Received eye diagrams for the back-to-back link and 140 cm-long waveguides with different twisting turns are shown in Fig. 12(a). The shapes of the eye diagrams were almost unchanged with the increase of twisting turns. BER curves also show that there is no serious degradation of transmission performance with the waveguide twisted up to 4 full turns as shown in Fig. 12(b).
Fig. 12. (a) 30 Gb/s eye diagrams and (b) BER curves of 140 cm-long waveguide links under different twisting conditions.
Table 4. Excess twist loss and power margin of waveguides
5. Conclusion
We demonstrate bandwidth measurement and high-speed data transmission of meter-scale MT-connectorized flexible multimode waveguide links with a maximum length of 180 cm. We directly observed that the pulses broadened linearly with the increase of waveguide length from 20 cm to 240 cm based on an optical sampling technique, and resulting in a mode delay of 0.093 ps/cm. The corresponding waveguide bandwidth decreased inversely with the increase of waveguide length, leading to a bandwidth-length product of 42 GHz·m. No obvious transmission impairments were observed when the waveguides were bent with a bending radius as small as 1 mm for 3 turns or twisted for 4 full turns. The error-free data transmission of 30 Gb/s NRZ signal was achieved with a record link length up to 140 cm. Our results show that the flexible multimode waveguides have both excellent optical and mechanical properties and they are suitable for high-speed optical interconnects application especially those have a strict requirement on flexibility.
Funding
National Key Research and Development Program of China (2019YFB1802900); The Major Key Project of PCL (PCL2021A14).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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