In optical-fiber communications, power variations caused by propagation-path variations, as well as component losses or gains, put stringent requirements on a receiver's dynamic range.1,2 Therefore, optical-power equalization is needed to limit power variations to within a small range. Several opto-electronic and all-optical approaches have been proposed to achieve this using saturated semiconductor optical amplifiers (SOAs).3–5
We have demonstrated nonlinear polarization rotation in SOAs, which enables all-optical power equalization and reduces power fluctuations. This process operates similarly to Mach-Zehnder interference, but the role of the different arms is taken over by the transverse-electric and transverse-magnetic modes of the incoming coherent light.6,7 The dashed box in Figure 1 shows that our power equalizer consists of a SOA, two polarization controllers (PCs), an optical-bandpass filter (BPF), and a polarization beam splitter (PBS). The degraded signal is injected into the SOA through the first PC. The latter sets the input signal's state of polarization (SOP), so that optimum birefringence is achieved in the SOA. The output is then sent into a PBS, which is used as a polarizer and acts as a discriminator. PC2 introduces an additional phase between the coherent modes to adjust polarization discrimination.
Figure 1. Experimental setup for power equalization (PE). ATT: Attenuator. BERT: Bit-error-rate tester. BPF: Bandpass filter. CW: Continuous wave. EDFA: Erbium-doped fiber amplifier. ISO: Isolator. MOD: External modulator. PBS: Polarization beam splitter. PC: Polarization controller. SOA: Semiconductor optical amplifier.
We set up the system such that the data signal has a reverse gain, which helps to reduce power fluctuations. For example, when the signal suffers from power fluctuations in the high-level (‘1’) region, the additional birefringence in the SOA leads to rotation of the probe light's polarization, resulting in lower gain. As a consequence, the power output through the PBS remains unchanged and power equalization is realized.
We experimentally demonstrated the power equalizer using the setup in Figure 1. The continuous-wave light from the laser source was first externally modulated by a 10Gbit/s pseudo-random binary-sequence pattern of length 231−1. We used SOA1 to generate power fluctuations, which can be changed by adjusting the driving current. Increased power fluctuations lead to a degraded extinction ratio (ER) of the modulated signal. The power fluctuations are combined with the data signal by an optical coupler. Our experimental SOAs (Inphenix) had a fiber-to-fiber gain and a polarization differential loss of approximately 18 and 3dB, respectively. The PBS had a polarization ER of 22dB and thus acted as a discriminator. The BPFs were used to remove spontaneous noise.
Figure 2 shows the measured static transfer curve. When the input power is below −10dBm, the output power is limited to approximately −20dBm. When the input power is between 0 and 10dBm, the output power is roughly 6dBm. Fluctuations in both cases amount to approximately 2dBm. When the power is between −10 and −5dBm, the output power can be mostly suppressed.
Figure 2. Measured static output power as a function of input power. dBm: Power ratio in decibels (dB) of the measured power referenced to one milliwatt.
In addition, we demonstrated the power equalizer's dynamic performance. Figure 3 shows the output versus input ERs for a 10Gbit/s data signal. The ER of the output signal remains at a high level when that of the input signal varies between 2.5 and 9.5dB. The ER of the optical signal increases from 4.0 to 12.3dB, so that a significant improvement is achieved. We thus clearly demonstrated the capabilities of power equalization. Figure 4 presents bit-error-rate (BER) curves before and after power equalization, together with back-to-back measurements. The input signal has a poor ER of 4.0dB. We observe that 6dB penalty improvements are obtained at a BER of 10−9. The BER performance of the equalized signal is close to that of the original back-to-back modulated signal. We are currently investigating the use of our proposed power equalization method for higher-data-rate applications.
Figure 3. Measured output versus input extinction ratios (ERs).
Figure 4. Bit-error-rate measurements before and after PE. The input optical signal has a poor ER of 4.0dB.
This work was supported by the National Natural Science Foundation of China (grants 60736038, 60907008, and 60925019) and the Chinese Education Ministry's Funds for New Teachers (grants 200806141102 and 20090185120027).
Shang-Jian Zhang, Li-Gong Chen, Ya-Li Zhang, Kan Zhang, Shuang Liu, Yong-Zhi Liu, Yong Liu
University of Electronic Science and Technology of China
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