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We propose an in-line monitoring technique that uses 650 nm visible light for performing maintenance work on Fiber-to-the-home (FTTH) network quickly without the need for measuring skills or external devices. This technique is characterized by visible light (650 nm) generated by an SHG module from the 1.3 μm-band line signal. We fabricate a 1.3 μm-band quasi phase matched LiNbO3 (QPM-LN) module, and the measure the 650 nm second harmonic (SH) power to test the proposed short-pulse modulation method. The results confirm the feasibility of the short-pulse modulation method with different peak factors (PFs) (1.0-7.3). We also examine the effect of short-pulse modulation on system performance at the optical receiver by measuring the bit error rate (BER) of received data (1.25 Gb/s). The BER is basically unaffected by the PF (1.0-5.5). This means that the proposed technique has little influence on data reception as regards PF (1.0-5.5).
©2010 Optical Society of America
1. Introduction
FTTH is expanding as the dominant access network. Thus, FTTH maintenance by field engineers is increasing in importance. Major maintenance tasks for FTTH, which include opening to traffic, circuit transfer, and fault localization, demand fiber identification. The traditional technique is the non-destructive macro-bending method [1]. Two factors make fiber identification in FTTH a harder task than it is in general optical performance monitoring [2] for backbone networks or submarine systems. First is the extremely large number of FTTH subscribers. Second is the trend to using 1650 nm signaling over hole-assisted optical fiber (HAF) [3,4] because such fiber strongly confines the 1650 nm light to the fiber core. To overcome these problems, we need a simple but high intuitive method that does not need any particular skill or any external measuring device.
Visible light, which has shorter wavelength than that used for signaling, can be recognized by the field engineer with the naked eye. Accordingly, if we could force active fibers to emit visible light, the field engineer would be able to perform the maintenance tasks quickly without the need for a measuring device. However, finding a method by which FTTH can be forced to emit visible light without disturbing current services is a problem.
Converting some part of the communication band light into the visible band light is one way to solve this problem. In FTTH systems, such as single-star (SS) or passive optical networks (PON), 1.3 μm-band light is used as the upstream signal [5]. Fortunately, this means that we can obtain 650 nm visible (red) light from the upstream light by employing SHG. WE previously reported a visible light technique for monitoring optical fiber by a SHG device for optical access systems [6].
This paper introduces an in-line monitoring technique for FTTH that uses a QPM-LN module. The QPM-LN module is well known as a highly efficient SHG-based wavelength conversion device [7]; it is currently being actively researched for a wide variety of applications [8–10]. We fabricated a QPM-LN module that allows visible recognition of the optical transmission state without the need for an external measuring device. We also propose a technique for increasing the efficiency of SHG-based wavelength conversion by using short-pulse modulation. We measured the relationship between 650 nm SH power and the PF of the input 1.3 μm-band data signal (1.25 Gb/s). In addition, we assess the influence of short-pulse modulation on data reception by measuring the relationship between the BER and the PFs (1.0-5.5).
2. Principle of monitoring technique
2.1 System configuration
Figure 1 is a schematic representation of our proposed in-line monitoring technique for FTTH. The function of the visualizing unit (VU) is shown in Fig. 1. 1.3 μm-band light incident from an optical transmitter (Tx) is converted into 650 nm SH light in the VU. The generated 650 nm SH light is passed though a wavelength filter and then released from the VU. The field engineer merely needs to see the 650 nm SH light with the naked eye to check the line state. Each Tx can be identified by setting a unique blink pattern by setting the PFs appropriately. Most of the 1.3 μm-band signal passes through the VU and captured by the receiver (Rx) without any serious degradation in data reception quality.
2.2 Short-pulse modulation
We propose a technique for increasing the efficiency of SHG-based wavelength conversion by using short-pulse modulation. We use the non-linear power response of SH to control the visible light power [11–13]. Note that this technique uses only a small constant part of the 1.3 μm-band signal and so has little effect on optical data communication. Figures 2(a) and 2(b) show schematic representations of the quenching and lighting state of the VU, respectively. By using a non-return to zero (NRZ) signal, for example, data pulses are modulated into NRZ pulses (width T, intensity P(t)) in the quenching state as shown in Fig. 2(a). In contrast, data pulses are modulated into short pulses (width T/η, intensity ) in the lighting state as shown in Fig. 2(b). The VU converts part of the input instantaneous intensity of the 1.3μm-band data to the quenching and lighting state. Since this process has a square-law characteristic, the SH mean power obtained from the NRZ pulse (width T, intensity P(t)) can be written by using proportionality coefficient α as follows.
On the other hand, the SH mean power obtained from a short pulse (width T/η, intensity ) can be written as follows
where η is a peak factor. PF indicates the ratio between the maximum and mean powers. Equation (2) shows that is η times more efficient than despite the same total input power. Thus the conversion efficiency of the short pulses is η times greater than the conversion efficiency of the NRZ pulses. If we set the intensity of the SH power properly for against the threshold of the spectral sensitivity of the human eye, we can realize control of the lighting state of 650 nm SH light by using short-pulse modulation. In Fig. 2(c), the timing of the short-pulse modulation can be dynamically controlled so as to set specific data (user, service, etc). Therefore, converted visible light enters a blinking state that corresponds to the desired information. The field engineer can detect that the line is active as well the desired information. In Rx, 1.3 μm-band data, in both states, are converted into an electrical signal and the high frequency component is removed with a low-pass filter (LPF). As a result, the received data characteristic is basically unaffected by the short-pulse modulation. Therefore, this technique has little effect on data reception.3. Experimental setup
Figure 3(a) shows the experimental setup used to test the proposed method. We fabricated a QPM-LN module for this experiment as shown in Fig. 3(b). In this module, a 48 mm long QPM-LN waveguide is coupled to an optical fiber by a lens as shown in Fig. 3(c). We also examined the effect of short-pulse modulation on system performance. In this experiment, the short pulse was emulated by two-stage modulation. A 1.3 μm continuous wave (CW) light was modulated with a 1.25 Gb/s 231-1 pseudorandom bit sequence (PRBS) by using an LN-Mach-Zehnder modulator (LN-MZM). It was then modulated by short-pulse 1.25 Gb/s signals with a width of 100-800 psec that were synchronized to the PRBS. Three PF values (1.0, 2.4, 7.3) were set at the Tx output are shown in Fig. 4 . In the VU, the polarization state of the 1.3 μm-band signal were adjusted with a polarization controller (PC), and input into a QPM-LN module with constant average power (−10 dBm). The converted 650 nm SH light was divided by dichroic mirror, and emitted from the outlet port. Emitted 650 nm light was detected by a power meter that measured the average power. In this measurement process, we used a dark room to remove the visible light noise. The remaining unconverted 1.3 μm-band data passed through the QPM-LN module to the Rx with total insertion loss of 3.5 dB.The Rx converted 1.3 μm-band data from optical to electrical signals with an optical to electrical converter (O/E). The signals were then electrically filtered by with an LPF with 938 Hz cutoff frequency (), and detected with a sampling oscilloscope (OSC).
4. Results and discussion
Figure 4 shows the measured relationship between the PF of the input 1.3 μm-band data and the average power of the 650 nm SH light. This result shows that the 650 nm SH power increased linearly with the PF. Next, to confirm the difference in the visibility of the 650 nm SH light between the lighting and the quenching state, we took photographs at the PF 7.3 and PF 1.0 as shown in Figs. 5(a) and 5(b). Figures 5(a) and 5(b) show that we can recognize the lighting and quenching states with the naked eye. These results confirm the feasibility of controlling SHG-based infrared-to-visible wavelength conversion by short-pulse modulation with different PFs (1.0-7.3).
This also means that we can realize the blinking state of visible light without changing the mean power of the 1.3 μm transmission data, if the timing of short-pulse modulation is controlled dynamically on an appropriate time scale.
Figure 6 shows the relationship between the PFs of the 1.3 μm input data and the BER measured at the Rx. This figure also shows eye patterns for each PF (1.0-5.5). We obtained a clear eye pattern for each PF. It approached an RZ pulse at high PF values. This suggests that signals with high PF have a high-frequency component that was not filtered out in the experimental setup. We also confirmed a characteristic improvement in BER with an increase in PF. This suggests that RZ pulse amplitude is larger than NRZ pulse amplitude, if the input powers are the same. This means that changing the PF value (1.0-5.7) in the proposed technique has little impact on data reception.
5. Conclusion
We proposed a monitoring technique based on a 1.3 μm-band QPM-LN module that allows the field engineer to visually recognize the transmission state in FTTH circuits. We also proposed a technique for increasing the efficiency of SHG-based wavelength conversion and the generation of unique visible light pulse patterns by using short-pulse modulation. Our experimental results confirmed excellent control efficiency at the SH power of 650 nm by using a 1.25 Gb/s 231-1 PRBS with PFs (1.0-7.3). Photographs confirmed that the lighting and quenching states could be discriminated. We also measured the BER and confirmed that system performance was basically constant regardless of the PF values (1.0-5.7).
References and links
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