In this paper, we theoretically and experimentally demonstrated the residual chromatic dispersion (CD) measuring of 10Gbit/s NRZ and 40Gbit/s NRZ/RZ links by using a novel single sideband (SSB) spectrum phase difference detection technology. This method can differentiate positive and negative residual CD of the fiber link, the measuring range is dependent on frequency difference (FD) of two local oscillator (LO) and the FD can be adjusted up to CD range. This method is independent on data rate for intensity modulation direct detection (IM/DD) links. In condition of FD of 4 GHz, the measuring range was around ± 1000 ps/nm and resolution was better than 5 ps/nm for 10G NRZ link.
©2011 Optical Society of America
The rapid increase of internet traffics has created the need to migrate the current predominately 10Gbit/s optical transmission systems to 40Gbit/s or 100Gbit/s systems. Chromatic dispersion is a well-known effect that arises from the frequency-dependent propagation speed in fiber, and is one of the main impairments that limit the performance of high-speed optical transmission systems. For high-speed transmission system, it is essential that dispersion be completely compensated. Moreover, in reconfigurable optical networks a given channel may accumulate different amounts of dispersion by traversing varying path lengths over fibers with time due to temperature. An effectively residual CD measuring method is very important for such high-speed transmission systems [1–3]. Many schemes for residual CD monitoring have been proposed and demonstrated [4–15]. Most of them need to add components into transmission system, such as RF modulated ASE noise, pilot tone or optical frequency modulated (FM) signal into the distributed feedback (DFB) laser [3–5]. However, additional pilot tone, ASE noise or optical FM signal would degrade the system performance, in addition, any change to the transmitter will increase the cost and complexity of the system so that it is inapplicable for upgrading current commercial communication system. Recently, a CD monitoring scheme was proposed that detecting the phase of recovered clock at receiver for DQPSK link , there was no additional monitoring signal added to the transmitter, however, clock recovery and high speed phase comparator are necessary, which also results in the increased system cost and complexity. Asynchronous sampling scheme, such as amplitude histogram based on overall power statistics distribution, can avoid clock recovery , and thus has inherent low cost, however, since different impairments can cause similar degradation in amplitude histogram, and it is difficult to distinguish them . Delay tap asynchronous sampling has been proposed for multi-parameter monitoring [8–10], and even applied to commercial WDM system, this method can resolve the power evolution within each bit, providing a direct measurement of waveform distortion without extraction clock, that can reflect the pulse shape information which has a strong relationship with the transmission impairments in fiber link [14–16].
In this paper, we proposed a novel CD measurement method using a novel single sideband (SSB) spectrum phase difference detection technology. This method can easily differentiate the positive and negative residual CD of the fiber link, the measuring range is dependent on frequency difference (FD) that can be adjusted for CD range. The method is independent on data rate for intensity modulation direct detection (IM/DD) links. we theoretically and experimentally demonstrated the residual chromatic dispersion measuring of 10Gbit/s NRZ and 40Gbit/s NRZ/RZ links. Comparing with many schemes of CD monitoring have been proposed, this method has follow advantages, it can operating in line to measure fiber CD values and sign without any configuration in transmitter, not needing high-speed components so that it’s cost effective, and this scheme is easily implemented by using integration circuit (IC) technique that provided optical performance monitoring (OPM) function.
2. Principle of CD measurement
The model of this technique is shown in Fig. 1 . This technique is operated in electronic domain mixing by using two orthogonal I-Q configuration parts.
The incoming signal was dual sideband (DSB) signal, a tunable band-pass optical filter (TOF) was used to select upper or lower sideband of the incoming signal, in condition of getting the upper sideband, and two spectrum of the upper sideband signal were respectively given by:
When , , and , , the detected signals , , , after LPFs are determined:
The relative phase of the carrier and each optical spectrum for the and ( and ) are given by:
When , the phase difference is independent on LO signal’s phase, and is given by:
The GVD is then given by:
Where is light speed in vacuum, is optical carrier wavelength, .
3. Experimental implementation
Experimental demonstration was performed using the setup of Fig. 2 . The transmitter comprises a tunable laser (TL) operating at C band with 10MHz linewidth, the wavelength was respectively 1552.524nm, 1552.123nm, 1551.721nm, 1551.319nm. the part of PRBS generating 10 Gbit/s and 40Gbit/s pseudorandom bit sequence (PRBS), and pulse generator generats 66% NRZ /RZpulse shape, the optical carrier was modulated with a 10G NRZ and 40GHz NRZ/RZ PRBS of length through Mach-Zehnder modulator (MZM). The fiber under test (FUT) comprises single mode fiber (G.652 SMF) and dispersion compensation fiber (DCF). An erbuim-doped fiber amplitier (EDFA) was used to compensate the fiber loss. At the receiver, a tunable optical bandpass filter with 3dB bandwith of 0.25 nm was used to eliminate the redundant amplified spontaneous emission (ASE) noise and to choose upper and lower signal band. The optical signal of upper or lower band was detected by analog detector with 3dB bandwith of 8 GHz, the output RF electrical signal was split into four channels. In first level mixing unit (MU), two channels of them were sent to mixing with 2GHz local oscillator 1 (LO 1), in second MU, other two channels were sent to mixing with 6GHz local oscillator 2 (LO 2) that was third harmonic generation (THG) of LO 1, in each MU, I channel LO signal has phase difference with Q channel LO signal by using 90°phase shifter.
In this experiment we used mixer that operated at frequency range of 1.7GHz~7GHz to mix RF with LO signal. The analog-digital-converters (ADC) with 14 bit-width and 50 MHz sample rate were used to sampling the output analog intermediate frequency (IF) signals and converting to digital signal, signal processing was performed in Field Programmable Gate Array (FPGA: Xilinx:XC4VLX15).
4. Experimental results
The positive chromatic dispersion was added to the signal using three spools of SMF of 20, 40, 60, km corresponding, respectively, round about 335, 670, 1005 ps/nm. The negative chromatic dispersion was added to the signal using three spools of DCF of 2.5, 4.8, 7.2 km corresponding about −350, −670, −1005 ps/nm, respectively. The OSNR was varied with a variable noise loading stage using an ASE source. The OSNR was maintained at level of 20.5dB for all CD measurements. The CD measurement was tested without introducing additional DGD.
In 10Gbit/s NRZ transmission links, original phase value of measurement showed in Fig. 3 , phase values were linear relation with CD values of fiber link, and when CD value was 0 ps/nm, measured phase values were about 0.62 radian for all experimental wavelength, the value was called zero dispersion phase difference (ZDPD), so we can introduced corrector method that subtracting ZDPD from all original phase values in data processing. The results that measuring CD values compared to actual CD values is shown in Fig. 4 . Figure 5 showed the error of measured results.
In 40Gbit/s NRZ transmission links, original phase value of measurement was shown in Fig. 6 , phase values were also linear relation with CD values of fiber link, and values of ZDPD were about −1.52 radian for all experimental wavelength. The result that measured CD values compare with actual CD values was shown in Fig. 7 . Figure 8 showed the error of measured results.
In 40Gbit/s RZ transmission links, original phase value of measurement was shown in Fig. 9 , phase values were also linear relation with CD values of fiber link, and ZDPD value was about 0.278 radian for all experimental wavelength. Figure 10 showed amended results. The result that measured CD values compared with actual CD values was shown in Fig. 11 .
5. Impact of PMD and OSNR on CD measurement
To investigate the impact of PMD and OSNR on the CD measurement by using this method, we carried out experiments for three formats links respectively. In experiments of researching PMD affect, the level of CD was maintained at a constant level of 0 ps/nm while varying the DGD from 0 to 200 ps and maintained the OSNR at 20.5dB, and in experiments of researching OSNR affect, the level of CD was maintained at a constant level of 0 ps/nm while varying the OSNR from 10 to 30 ps without the added PMD. Figure 12 shows the impact of PMD and OSNR on CD measurement for10G NRZ link. Figure 13 shows the impact of PMD and OSNR on CD measurement for 40G NRZ link, and the impact of PMD and OSNR on CD measurement for 40G RZ link is shown in Fig. 14 .
It’s obvious that results of CD measurement were affected by PMD and OSNR, while additional DGD beyonds 150ps, the impact of PMD on CD measurement is limited. The error of measurement is sensitive to OSNR of links, at the same level of OSNR and DGD, the measure precision of 10G link is better than 40G links.
We have demonstrated a CD measurement method using a novel single sideband (SSB) spectrum phase difference detection technology. This method doesn’t require high-speed components, and the 3-dB bandwidth of tunable optical filter (TOF) doesn’t require extreme narrow, general TOF that has 35GHz 3-dB bandwidth can satisfied CD measuring in our experiment, and the method is independent on data rate for OOK links, 10G NRZ and 40G NRZ/RZ transmission link CD measuring have been demonstrated that the method is fit for different data rate link. In experimental results, ZDPD value was not zero, the reason is that the original phase difference between and without CD effect has existed, and this value is dependent on data rate, we can determine the ZDPD value through index CD value calibration or arithmetic amending.
Digital coherent detection was widely applied in backbone transmission links that based on DPSK, QPSK, PDM-DQPSK modulation format, the proposed method is not suitable to these systems, but such as 10G NRZ, 40G NRZ/RZ still present in some backbone transmission links, besides, OOK format is widely used in access network, for example, 2.5G NRZ, 10G NRZ, and later the speed will increase to 40G bit/s with NRZ or RZ formats, in the case, communication quality greatly affected by the CD of fiber link, so the optical performance monitoring (OPM) for OOK links is also significant, and CD monitoring is critical item of OPM.
This method is practicable for optic-electronic integration chip that contains OPM function. Comparing with other CD monitoring methods, this method can obtains actual signed CD value of fiber link, the measuring range is tunable, and the cost is effective, lower consumption and fast response speed, in the later, we focus on data processing arithmetic to improve measurement precision and optimize system performance without increasing any hardware cost, and we would deeply study CD and PMD measurement based on the method that make it apply to OPM for other data rate and modulation, etc.40G NRZ-DPSK/RZ-DPSK, 100G RZ-PDM-DQPSK.
This work is supported by National Basic Research Program of China [2010CB328302]. The author acknowledges his fruitful interactions with Fengguang Luo, Deming Liu, Lu Shi, Ming Tian, all from Wuhan National Laboratory for Optoeletronics and National Engineering Laboratory for Next Generation Internet Acess System, School of Optoeletronics Science and Engineering, Huazhong University of Science and Technology.
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