1. Introduction

Long-period gratings (LPGs) in a single-mode fiber (SMF) promote the coupling between the fundamental core mode and a forward propagating cladding mode, and vice-versa, at a resonant wavelength [1]. LPGs are produced by modulating the fiber refractive index with a period ranging from 100 µm to 1 mm. There are two types of refractive index modulation: permanent modulation, which includes irradiation with UV laser [2], CO$_2$ laser [3], femtosecond laser [4] and electric-arc discharge [5]; and temporary modulation, which can be achieved by pressing a periodically grooved plate onto an SMF [6] and by acousto-optic effect [7]. Due to the LPGs’ characteristics, several applications for optical communications have been proposed, such as band rejection filters [2], erbium-doped fiber amplifier gain equalizers [8], couplers, etc [9–11].

The first proposed LPG based couplers consisted of LPGs inscribed in photosensitive fibers using the UV irradiation technique [9,11]. These couplers provide wavelength-selective power transfer between two fibers, thus they were proposed for the development of low-cost add/drop components for wavelength-division multiplexing (WDM) [9,11]. It was also found that the coupling efficiency would improve for higher-order cladding modes with a proper LPG length and a surrounding refractive index higher than the refractive index of air [11]. In addition, the coupling efficiency can be further increased by introducing an offset distance between the LPGs, etching the fibers’ claddings [11], and adjusting the fibers strain [12]. LPGs based couplers using LPGs inscribed with CO$_2$ laser radiation were also proposed [10,13]. The couplers produced with standard SMFs present a coupling efficiency dependent on the fiber orientation [10], due to the asymmetric refractive index modulation associated with the fiber single-side exposure at a high dosage [10]. However, when using boron-doped fibers, a lower radiation dosage is needed for the LPG inscription, which may explain, in this case, the lower dependence of the coupling efficiency with the fiber orientation [10]. Another type of coupler employs microtapered LPGs inscribed with CO$_2$ laser irradiation in standard SMFs [13]. The resonant wavelength is shifted by twisting the fibers, which shows the tunability of the coupler [13]. Changing the strain applied to the fibers using piezoelectric ceramic (PZT) stretchers [14] or heating the LPGs with coil heaters [15] has also been proposed for tunability. Although LPG based couplers made from SMFs have been experimentally demonstrated, they may also be fabricated from multi-core fibers (MCFs). MCF transmission systems have been proposed as a solution to meet the ever-increasing demand for data [16]. Recently, an LPG based coupler for MCF transmission systems was numerically demonstrated [17]. This coupler can evenly distribute the light from an SMF to all cores of an MCF.

These previous works have addressed the potential of the LPG based couplers for the development of components for transmission systems, such as optical add/drop multiplexers, due to their coupling efficiency and wavelength-selectivity. It was already demonstrated that LPGs introduce low penalty in the signals transfer between cores of the same MCF, promoted by LPGs [5]. However, the reliability of the coupler in a transmission system and its impact on the signal modulation must be analyzed to show their suitability for integration in high-throughput SMF transmission systems.

In this work, we analyzed the impact of an LPG based coupler in a dual-polarization (DP) 400G coherent optical signal. For that, we first developed, optimized and thoroughly characterized a coupler using standard SMFs with LPGs inscribed with UV laser radiation. The coupling efficiency at C-band was optimized by adjusting the longitudinal distance between the LPGs. Finally, the device capability was evaluated by analyzing the bit-error-ratio (BER) along the C-band for 400G coherent optical signals.

2. Configuration and characterization of the LPG based coupler

The proposed configuration of the LPG based coupler consists of two standard SMFs with similar LPGs inscribed, parallel and close to each other, as displayed in Fig. 1. When light is launched into one SMF, its LPG promotes the coupling between the core mode and a certain cladding mode at the resonant wavelength. Meanwhile, the optical power at the cladding mode is transferred to the other SMF cladding by evanescent field through a similar cladding mode. Then, its LPG promotes the coupling of the excited cladding mode to the core mode at the resonant wavelength. This all without affecting the light at the wavelengths outside the resonance regions.

 

Fig. 1. Schematic diagram of the cross-profile and coupling region of the LPG based coupler.

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Both fibers are SMF ITU G. 652 from Cabelte. The fibers are kept at close distance between each other by using two ferrules with a mean bore diameter of ($258\,\pm \,2$) µm. The LPGs’ period is $\Lambda$ and they are longitudinally separated by an offset distance, $L_0$. The LPGs were fabricated using the point-by-point technique through UV laser radiation, being the SMFs previously hydrogen loaded to enhance their UV photosensitivity. The LPGs are identical, presenting the following parameters: period $\Lambda \,=\,500$ µm; length $L=1.85\,\rm {cm}$ (37$\Lambda$); exposure time of 30 s and laser energy of 2.5 mJ (Coherent, BraggStar Industrial LN). After inscription, the fibers were annealed during 24 h at 85 $^{\circ }$C to diffuse out the hydrogen of the SMF and stabilize the LPGs’ transmission spectra. The transmission spectra were characterized using a supercontinuum broadband source (Fianium Whitelase, SC-400-2) and an optical spectrum analyser (OSA) (Yokogawa, AQ6375), as all the spectra presented in this work. Figure 2(a) displays the transmission spectrum of both LPGs after annealing, which shows that the LPGs are similar and present three attenuation bands, namely at around 1435 nm, 1475 nm and 1555 nm. The goal of this work was to develop an LPG based coupler for fiber optic-communication systems that typically use the C-band, thus the coupler study was focused on the third resonant wavelength of the LPGs (1555 nm). However, the coupler was also characterized for the two main bands to analyze the LPG and the coupling behavior with different cladding modes. The experimental near-field modes of these two resonances were measured through a laser beam profiler (Duma Optronics, BeamOn IR1550), and can be seen in Fig. 2(b). The field distribution of the cladding mode at 1475 nm seems to correspond to an HE$_{13}$ optical mode and at 1555 nm to an HE$_{14}$ optical mode.

 

Fig. 2. (a) Transmission spectra of the inscribed LPGs. (b) Experimental near-field image of the cladding modes associated to the second and third attenuation bands (HE$_{13}$ and HE$_{14}$, respectively).

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The SMFs with the LPGs inscribed were integrated into the coupler’s experimental setup, as displayed in Fig. 3. Three ports were considered in the coupler: Port 1, where the light is launched into one SMF; Port 2, at the end of the same SMF; and Port 3, at the end of the other SMF. One SMF has one end glued to a fixed platform and the other end fixed to a manual linear stage, allowing to adjust the applied strain. The other SMF has one end fixed to a motorized linear stage (Newport, UTS150PP) and a free end attached to a weight of approximately 39 g, which keeps the fiber straight while the motorized linear stage changes the distance between LPGs.

 

Fig. 3. Diagram of the experimental setup of the LPG based coupler.

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To achieve the offset distance between LPGs that optimizes the coupling efficiency, we measured the output power at Port 3 normalized to the optical power launched into Port 1 for different values of $L_0$. We used two tunable external cavity lasers (ECL) emitting at the wavelengths of 1475 nm and 1555 nm, the ones in the resonance regions of the LPGs, and with an FWHM (full width at half maximum) of <30 MHz, much narrower than the one of the LPGs. $L_0$ ranges from $-20$ mm to 50 mm: being zero when one LPG starts right after the other, negative when the LPGs are overlapping by $|L_0|$ and positive when LPGs are separated by $L_0$. The power transfer at 1475 nm and 1555 nm was measured three times to calculate the average power transfer for different offset distances $L_0$. The results are presented in Fig. 4(a), a maximum power transfer of about $-4.3$ dB (37%) is achieved at 1475 nm for an offset distance of 45 mm, while at 1555 nm, the maximum power transfer of $-6.0$ dB (25%) is observed at an offset between 0 mm and 5 mm. This shows that the two bands have different coupling distances, which is due to the coupling to different cladding modes. An attenuation band associated with a higher-order mode may present a smaller coupling distance because the mode is less confined to the cladding region, so the optical mode HE$_{13}$ (1475 nm) presents the maximum coupling for a higher offset distance than the HE$_{14}$ (1555 nm).

 

Fig. 4. (a) Average output power at Port 3 for different offset distances, $L_0$, at $\lambda \,=\,1475$ nm and $\lambda \,=\,1555$ nm. (b) Transmission spectrum of the coupler at Port 2 and Port 3 when $L_0\,=\,0$ mm.

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For further analysis of the power transfer, the offset distance was optimized and set to $L_0$ = 0 mm, the minimum distance needed to achieve higher power transfer between the fibers at 1555 nm, because we intend to study the coupler performance at the C-band. The light was launched into Port 1 and the spectra were measured at Port 2 and Port 3, the results are displayed in Fig. 4(b). These results show that the light launched into Port 1 is transferred to Port 3 at the resonant wavelengths of the LPGs. The spectrum at Port 3 shows two peaks of power transfer, which corresponds to the two stronger attenuation bands of the LPG spectrum, while the optical power outside the resonance regions presents only the insertion loss between $-1$ dB and $-2$ dB. The FWHM of the attenuation band is smaller than the coupling band for both wavelengths. The first coupling band has its maximum power at around 1475 nm with about $-10$ dB (10%) and an FWHM of 13 nm. For the 1475 nm wavelength, the optical power that reached the end of the input SMF (Port 2) was $-23$ dB ($<1$%), having an FWHM of 2 nm. The second coupling band has its maximum power at around 1555 nm with about $-6$ dB (25%) and an FWHM of 15 nm. For the 1555 nm wavelength, the optical power that reached the end of the input SMF (Port 2) was $-11$ dB (8%), having an FWHM of 10 nm.

As is well known, LPGs are temperature sensitive, so we studied its impact on the performance of the coupler. For that, a hot plate (IKA, C-MAG HS 7) was placed below the coupler, and the temperature was varied in the range of 25 $^\circ$C to 40 $^\circ$C in steps of 5 $^\circ$C. For each step, the transmission spectra at Port 2 and Port 3 were measured, and the results are displayed in Fig. 5.

 

Fig. 5. Transmission spectra at (a) Port 2 and (b) Port 3, for different temperatures.

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In Fig. 5(a), we observe the behavior of the attenuation bands of the LPG 1 from 25 $^\circ$C to 40 $^\circ$C: the attenuation band at 1475 nm increases by 5 dB while at 1555 nm decreases by 3 dB. Furthermore, the resonant wavelength shift is lower than 1 nm for both dips. As the two bands are associated to different core to cladding mode couplings, their behavior should be also different. Nevertheless, the power transfer (Fig. 5(b)) decreases with temperature at both wavelengths. At 1475 nm, the power transfer decreases by 6 dB from 25 $^\circ$C to 40 $^\circ$C. At 1555 nm, the power transfer rapidly decreases by 6 dB at 30 $^\circ$C and then stabilizes for higher temperatures. The change in the transmission spectrum of the LPG alone does not explain the change in the power transfer: at 1475 nm the attenuation increases, but the power transfer also decreases; and at 1555 nm the decrease of the power transfer is not at the same rate as the decrease of the corresponding attenuation. Thus, temperature may also impact the coupling between the cladding modes of the fibers (promoted by evanescent field) probably by changing the optimal $L_0$. Taking into account these results, the coupler should be under temperature control.

3. Integration of the LPG based coupler in a coherent optical transmission system

The coupler optimized for the C-band, i.e., with a chosen $L_0$ of 0 mm, was integrated into a 400G coherent optical transmission system and the impact of the coupler on the transmission performance was evaluated. For that, a 400 Gb/s DP-16 QAM (quadrature amplitude modulation) signal was launched into Port 1 and analyzed at both output ports. Figure 6 displays the experimental setup. An electrical 400ZR-like signal with 60 Gbaud and 16 QAM mapping was pulsed-shaped with a root-raised cosine filter with 0.2 roll-off. The signal was uploaded to a 120 GSa/s arbitrary waveform generator (AWG), with an analog bandwidth of approximately 45 GHz (Keysight M8194A). We used a dual-polarization IQ modulator (bandwidth of 35 GHz) to modulate the signal into the optical carrier, which is given by a nano-integrated tunable laser assembly (nano-ITLA). To evaluate the bandwidth of the coupler, the emitting frequency was changed from 191.65 THz ($\sim$1565 nm) to 195.85 THz ($\sim$1530 nm) in steps of 0.20 THz. The signal was launched into Port 1, after being amplified by a fixed-power erbium-doped fiber amplifier (EDFA) so that the maximum power arriving at the coherent receiver was $-5$ dBm. The coherent receiver had a 40 GHz bandwidth. We used 4 real-time oscilloscopes (RTO), with 200 GSa/s sample rate and 70 GHz bandwidth (Tektronix DPO77002SX-R3), to sample and digitize the electrical I and Q components. From this, and in the offline-domain, we performed typical digital signal processing (DSP) techniques, obtaining symbol demaping and BER of the signal [18] at Port 2 and Port 3. Figure 7 displays the BER for both possible outputs.

 

Fig. 6. Experimental setup of the coherent optical transmission system with the proposed LPG based coupler.

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Fig. 7. Bit-error-ratio (blue line) and transmission (orange line) of the coupler at (a) Port 2 and (b) Port 3 at different wavelengths. The dashed lines are the OIF limit (dark) and the maximum bit-error-ratio obtained for back-to-back measurements (grey). The red and green circle are (c) examples of received constellation for both polarizations, at around 1539 nm (BER higher than OIF limit) and 1555 nm (BER lower than OIF limit), respectively. The blue points correspond to the recovered signal and the red points to the received errors.

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BER has to be lower than $1.25\times 10^{-2}$ (OIF, Optical Internetworking Forum, standardization for 400ZR transceivers [19]) to enable error free transmission after proper forward error correction (FEC) techniques. We can compare the received constellations in Fig. 7(c) and observe that when the BER is higher than the OIF limit, the received constellation presents more errors. The BER at Port 2, although higher than the back-to-back BER from 1548 nm to 1564 nm (attenuation band), is always lower than the OIF limit; thus the signal detected directly in the input fiber is not lost. At Port 3, we were able to recover the signal with BER similar to back-to-back performance in the range within the coupling band (1545 nm to 1564 nm). This shows that an LPG based coupler itself does not affect the modulation of the signal when it travels from one core to the other. Thus, this type of coupler could be used for coupling purposes in high-capacity coherent transmission systems.

4. Conclusion

An LPG based coupler was developed, characterized and integrated into a 400G coherent optical transmission system to evaluate its impact on the signals. The coupler, which consists of two SMFs with similar LPGs, presents two peaks of power transfer between fibers at around 1475 nm and 1555 nm, which correspond to the LPGs’ resonant wavelengths. The coupler was optimized by adjusting the longitudinal distance between the LPGs, achieving a coupling efficiency of about $-6$ dB (25%) at around 1555 nm. The efficiency of the coupler can be further enhanced by using LPGs with a stronger attenuation band at the C-band, changing the excited cladding modes to higher order modes, which can be achieved with a different LPG period, and/or by using a proper surrounding refractive index [11]. The impact of temperature on the power transfer was also analyzed, and it was shown that the coupling reduced significantly for higher temperatures, thus the coupler should be under temperature control.

After optimization, the impact of the coupler on 400G coherent optical signals along the C-band was evaluated. The results showed no penalty imposed over the optical signals at the wavelengths within the coupling band. Thus, the coupler itself does not affect the modulation of the signal. This work shows that LPG based couplers can be integrated into high-capacity transmission systems.