1. Introduction

The lightwave synthesized frequency sweeper (LSFS) [1–4 ] is a unique optical source that generates a series of optical frequencies multiplexed in the time domain. It has been widely used in wavelength-division multiplexing (WDM) networks [5], optical coherence tomography (OCT) [6], medical imaging [7], dispersion measurement [8], and remote sensing of the atmosphere [9], etc.

A LSFS has mainly three parts: a frequency-stabilized master laser, an intensity modulator and an optical loop. The optical loop consists of an optical coupler, an erbium doped fiber amplifier (EDFA), a band-pass filter (BPF) and a frequency shifter. The frequency shifter shifts the frequency of the incident signal by a constant value each time when the signal passes through the optical loop. The EDFA compensates the signal power loss in the loop to increase the sweeping span of LSFS. Meanwhile, BFP is used to suppress the amplified spontaneous emission (ASE) noise introduced by EDFAs. But as the bandwidth of BFP approaches or exceeds the homogeneous broadening range (usually 6~7nm) of erbium doped fiber (EDF), the ASE noise accumulates with the circulation in the optical loop and may form new frequency peaks offset from the original signal frequency. The ASE noise not only consumes most of the saturated EDFA output power, but also causes mode competition and spectrum instability, so the BPF can only effectively suppress the accumulation of ASE noise within the homogeneous broadening range of EDF, which seriously limits the sweeping span and the number of LSFS’s output pulses. Therefore, synchronous filter whose center frequency tracks the frequency of incident signal was proposed and proved to be an effective solution to suppress ASE accumulation [10]. Up to now, the widest sweeping span reported is 1.2THz with a step as small as 120MHz [11]. However, the synchronous filter is based on mechanical scanning electronically controlled, which requires complicated system to tune the filter’s peak frequency quickly and precisely.

In this paper, we introduce a self-induced auto-tracking filter (SIATF), which is based on unpumped EDF. The standing waves in unpumped EDF induce spatial-hole-burning effect and lead to a self-induced fiber Bragg grating (FBG). The center frequency of this Bragg grating can automatically tracks the frequency of the incident optical signal. Therefore, it works as the SIAFT. By using SIATF, the output sweeping span of the LSFS breakthrough the limitation due to the homogeneous broadening of EDF. In our work, a sweeping span of 12.48nm within 3.5dB power change is obtained experimentally, which corresponds to 1.56THz sweeping span. In addition to the sweeping span, SIATF has also the advantages of simple structure and low cost.

2. Experimental setup and operation principle

Figure 1 shows the schematic layout of the experimental setup. Optical fibers are marked by solid lines and the electric wires are marked by dashed lines. The system consists of a distributed feed-back (DFB) laser with 3MHz linewidth ($\lambda \text{=}1546.96\text{nm}$in the experiment) as the master laser, a pulse generator, an optical coupler, two EDFAs, a tunable flat-top band-pass filter (FTBPF) with the bandwidth tunable from 0.1nm to 15nm, a single-sideband (SSB) modulator which is driven by radio-frequency (RF) source, an isolator (ISO), three polarization controllers (PC) and the SIATF. The length of the optical loop is 76.2m without the SIATF and 89.4m with the SIATF, respectively. The total optical loss involving SIATF is about 31dB. The output of DFB laser is internally modulated into a square pulse sequence by the pulse generator. The length and the period of modulated optical pulse sequence are ${\tau }_1$ and ${\tau }_2$ respectively. ${\tau }_1$ is set to 100ns. It is shorter than the round-trip time of the optical loop, which allows us to monitor the ASE noise power between pulses as shown in Fig. 1. Time between pulse trains is determined by ${\tau }_2$, which is 100μs in our experiments. ${\tau }_3$ is the time between pulses in a pulse train, which is the difference value between ${\tau }_1$ and the round-trip time of the optical loop. The EDFAs operate near saturation to compensate for the optical loss in optical loop. The PCs are used for controlling the polarization state of the optical signal. The SSB modulator acts as frequency shifter. The FTBPF limits the sweeping span and the ASE noise. The power of the light pulse from master laser is divided by a 50/50 fiber coupler. Half of the pulse power is directly sent to the output port as the original pulse. The other half is launched into the optical loop and the frequency is shifted by a fixed value after each circulation in the loop. The output pulses of LSFS direct into optical spectrum analyzer (OSA) and oscilloscope (OSC) for analyzing.

 

Fig. 1 Schematic diagram of the experimental setup.

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The proposed SIATF is shown in the red dotted box in Fig. 1. It consists of a circulator, a piece of unpumped EDF and a fiber mirror reflector. A standing wave is formed in the EDF by interference between the input and the counter-propagating reflected lightwave. The spatial interference pattern leads to spatial-hole-burning effect and then induces a slight change $\Delta n$ in refractive index of the EDF. According to the standing wave theory, the spatial period of light intensity distribution in EDF is $\Lambda =\lambda /2n_{eff}$, where $\text{λ}$ is the wavelength of the incident signal and $n_{eff}$ is the effective refraction index of the EDF. Thus, a self-induced filter, whose central frequency equals to the incident signal frequency, is generated [12–14 ]. The full-width-half maximum (FWHM) bandwidth of the filter can be written as [15,16 ]:

This shows that the filter can automatically track the frequency of the incident optical signal. The bandwidth of this filter is in the order of megahertz, which can effectively suppress the ASE noise around the signal.

3. Experimental results and discussion

Firstly the FTBPF and SIATF were removed from the system shown in Fig. 1 and the value of frequency shift introduced by SSB modulator is set as 10GHz. The ASE of EDFAs without any filtering then was obtained and the output spectrum of the LSFS is shown in Fig. 2 .

 

Fig. 2 Output spectrum of the LSFS without any filter.

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As shown in Fig. 2, the original wavelength of the DFB laser is 1546.96nm. Since there is not any suppression on the ASE, the sweeping signal decreases rapidly in the next few circulations while the ASE noise accumulates quickly. Three unstable ASE peaks caused by homogeneous broadening of EDF are found at around 1553.5nm, 1560.1nm and 1567.4nm. The output spectrum is unstable and the offset of the first ASE peak from the original wavelength differs in a range of 6nm to 7nm, so the homogeneous broadening range of EDF is around 6nm to 7nm.

Then the FTBPF was inserted into the optical loop to suppress the ASE noise. When the bandwidth of FTBPF is set at 3.78nm, 5.40nm, 8.40nm and 14.62nm, the output spectrums of the LSFS without SIATF are shown in Figs. 3(a)-3(d) respectively.

 

Fig. 3 Output spectrum of the LSFS without SIATF.

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There are 44 pulse circulations within 1.2dB power change in Fig. 3(a) and 60 pulse circulations within 2dB power change in Fig. 3(b). The corresponding sweeping span are 3.52nmand 4.82nm. It shows that when the bandwidth of FTBPF is less than the homogeneous broadening range of EDF, the ASE noise is effectively suppressed. The output spectrum almost covers the whole bandwidth of FTBPF. The sweeping span broadens as the FTBPF’s bandwidth increases. When the bandwidth of FTBPF exceeds the homogeneous broadening range of EDF, the sweeping signals power decreases rapidly. There are only 30 pulse circulations within 3dB power change and 2 pulse circulations within 7dB power change, as shown in Figs. 3(c) and 3(d) respectively. In this case, the ASE noise consumes the power of saturated EDFA, causes mode competition and spectrum instability. The number of ASE peaks changes from 1 to 2, the power increase from −35.48dBm to −31.84dBm from Figs. 3(c) and 3(d). Therefore, we can draw a conclusion that the power and the number of ASE peaks increases along with the FTBPF’s bandwidth.

Finally the SIATF is inserted into the optical loop. The output spectrums of LSFS with SIATF are shown in Figs. 4(a)-4(d) . The same bandwidths of FTBPF are used.

 

Fig. 4 Output spectrum of the LSFS with SIATF under the same polarization state.

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When the bandwidth of FTBPF is less than the homogeneous broadening range of EDF, there are 44 pulse circulations within 1dB power change and 64 pulse circulations within 1.5dB power change, as shown respectively in Figs. 4(a) and 4(b). Compared with Figs. 3(a) and 3(b), the flatness of output spectrum in Figs. 4(a) and 4(b) is better. When the bandwidth of FTBPF exceeds the homogeneous broadening range of EDF, there are 102 pulse circulations within 3dB and 156 pulse circulations within 3.5dB, as shown in Figs. 4(c) and 4(d) respectively. No ASE noise peak is found in Fig. 4. Moreover, the sweeping span covers almost the whole bandwidth of FTBPF. The flatness of the output spectrum is slightly reduced in Fig. 4(d), which is primarily caused by the uneven gain spectrum of the EDFAs. The largest sweeping span of LSFS is restricted by the bandwidth of FTBPF. In our experiment the maximum span is 12.48nm, corresponding to 1.56THz.

Since the scanning time of OSA is 10s with 0.03nm resolution in our experiments, the spectrum results represent an average about 10⁵ pulse trains. Considering the averaging might conceal some faster dynamic features in the output of the LSFS, the temporal domain measurements are obtained by using a photo detector (with 3GHz bandwidth) and an oscilloscope. When the bandwidth of FTBPF is set at 5.40nm and 8.40nm, the temporal domain results of LSFS with and without SIATF are shown in Figs. 5(a)-5(d) respectively as examples.

 

Fig. 5 The temporal domain measurements of LSFS output.

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Figure 5 shows that the pulse number within the same range of power change in temporal domain equals to the spectrum results shown in Figs. 3 and 4 . It also shows that the noise in temporal domain is reduced by using the SIATF. To further analyze the suppressing effect of ASE noise, we defined a factor F = ASE noise power/total power. The factor F within sweeping span is obtained by analyzing the data in OSC and plotted versus the circulation number in Fig. 6 .

 

Fig. 6 Factor F as a function of the circulation number with or without SIATF.

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Figure 6(a) shows that when the bandwidth of FTBPF is less than the homogeneous broadening range of EDF, the factor F with SIATF is obviously smaller than that without SIATF. When the bandwidth of FTBPF is 3.78nm, the mean of factor F with SIATF is 7.03, while without SIATF is 11.68. When the bandwidth of FTBPF is 5.40nm, the mean of factor F with SIATF is 9.89, while without SIATF is 19.83. It proves that SIATF suppresses the ASE noise. On the other hand, Fig. 6(b) shows the results when the bandwidth of FTBPF exceeds the homogeneous broadening range of EDF. When the bandwidth of FTBPF is 8.40nm and LSFS doesn’t contain SIATF, the factor F decreases from 17.85 to 13.68 in the few firstly circulations. This is because the input original optical signal consumes the power of saturated EDFA. Then factor F increases rapidly to 29.11 in the next 9 circulations, this is due to the accumulation of ASE noise. The factor F with SIATF is smaller than that without SIATF. As the bandwidth of FTBPF is 8.40nm or 14.62nm, the mean of factor F is 11.18 or 13.39 respectively. From Fig. 6, it can be seen that the factor F keeps relatively stable.

4. Conclusion

The standing waves in unpumped EDF induce spatial-hole-burning effect, equivalent to introducing a Bragg grating. This Bragg grating works as a SIAFT, tracking the frequency of the incident optical signal automatically. We demonstrated that sweeping span of LSFS is not limited by homogeneous broadening range of EDF and almost covers the bandwidth of FTBPF by using the SIATF. A sweeping span of 12.48nm within 3.5dB power change was obtained in the experiment, which corresponds to a sweeping span of 1.56THz.