We propose a self-optimization and auto-stabilization method for a 1-bit DMZI in DPSK transmission. Using the characteristics of eye patterns, the optical frequency transmittance of a 1-bit DMZI is thermally controlled to maximize the power difference between the constructive and destructive output ports. Unlike other techniques, this control method can be realized without additional components, making it simple and cost effective. Experimental results show that error-free performance is maintained when the carrier optical frequency variation is ~10% of the data rate.
© 2008 Optical Society of America
In differential phase shift key (DPSK) transmission, only the phase of the signal between 0 and π is modulated, without varying the signal intensity. Thus, DPSK has a higher resistance to optical fiber nonlinearity . Better receiver sensitivity is achieved by using a balanced receiver . These merits lead to wide research on DPSK in long-haul transmission areas [3, 4], though DPSK is more difficult to implement than general intensity modulation techniques.
In the DPSK receiver, a 1-bit delayed Mach-Zehnder interferometer (DMZI) plays an important role in converting the received phase-modulated data into intensity-modulated data. The 1-bit DMZI generates the intensity-modulated output signal constructively or destructively using the phase difference between two adjacent bits .
The 1-bit DMZI has an optical frequency transmittance curve which is dependent on its delay time. In WDM(Wavelength Division Multiplexing)-based DPSK transmission, the peak of the optical frequency transmittance curve in each receiver should be set to the selected frequency of each channel in order to achieve good transmission performance. Thus, when the system is started, each transmittance curve of the DPSK receiver must be manually set to the frequency of each channel. A self-optimization function for the 1-bit DMZI is needed to replace this troublesome manual set-up process.
During operation, frequency discordance between the peak of the transmittance curve and the received signal can occur if an optical frequency transmittance curve shifts or if the optical frequency of the received signal is unstable. Moreover, performance degradation due to this frequency discordance may occur even in the range of frequency stabilization which is required for a general WDM channel . Auto-stabilization of the 1-bit DMZI during operation is therefore necessary to maintain performance.
This paper proposes a simple and cost-effective method for self-optimization and auto-stabilization of a 1-bit DMZI in NRZ (Non Return to Zero)-DPSK. Section 2 presents measurements of frequency stabilization conditions for a 10 Gb/s DPSK signal, demonstrating the need for auto-stabilization of the 1-bit DMZI. In Section 3, the proposed method is explained. In DPSK modulation, it is impossible to control the 1-bit DMZI transmittance by monitoring only the power of each output port of the balanced receiver; additional components such as a carrier signal are therefore used to control the stabilization . This method inherently reduces the extinction ratio, degrading performance. In contrast, the proposed method controls the temperature of the 1-bit DMZI according to the power difference between two outputs of the balanced receiver without additional components, by using the characteristics of eye patterns. The experiment in Section 4 tests the performance of the proposed method for 10 Gb/s DPSK transmission. Results confirm that our proposed technique performs well for self-optimization and auto-stabilization of a 1-bit DMZI.
2. Optical frequency dependence of 1-bit DMZI
The DPSK receiver is composed of a balanced receiver and a 1-bit DMZI which converts the phase-modulated signal to an intensity-modulated signal, as shown in Fig. 1. The 1-bit DMZI has two output ports: a constructive port and a destructive port. The signals from the two output ports are logical inverses of each other.
Neglecting the insertion loss, the output of the constructive port is expressed as Eq. (1):
Therefore, the output intensity, I is:
where Td is the delay time (equal to one bit period), and ϕ(t) is the phase of the signal at time t. At the constructive port, the output is “1” when the phase difference between two adjacent input bits is zero, and the output is “0” when the phase difference is π.
Here, n is the effective refractive index, and Ld is the delay length (nLd=cTd). In Fig. 2, the FSR (Free Spectral Range) of the transmittance curve is inversely proportional to the delay time and directly proportional to the bit rate. The transmittance curves of the two ports cross each other by a half period. The optimum operation point is where the frequency of the input signal is located at the peak of the curve. As the optical frequency of the input signal deviates from the optimum operating point, the extinction ratio is reduced and signal distortion occurs. In a commercial DMZI made of SMF (Single Mode Fiber), the temperature dependence of the frequency variation is generally 1.25 GHz/°C. We can optimize the DMZI operation by controlling the temperature. The 1-bit DMZI performs TEC (Thermo Electric Cooler) temperature control, and a PZT (Piezoelectric Transducer) or heater is used on the delay path to maintain the frequency at the optimum operating point.
We measured the performance of a 1-bit DMZI by varying the optical frequency of the input signal. A tunable laser, an optical wavelength meter and a BERT (Bit Error Rate Tester) were used for the experiment. Figure 3(a) shows the power penalty for varying the optical frequency of the input signal when a 10 Gb/s DPSK signal is used. When the optical frequency is equal to the optimum (peak) point in Fig. 2, the eye opening is the largest, and the transmission performance is the best, as shown in Fig. 3(b). When the optical frequency of the input signal is changed by 400 MHz, the eye opening is reduced, and signal distortion occurs, as shown in Fig. 3(c). The experimental results confirm that the frequency deviation must be kept under 300 MHz in order to keep the power penalty below 0.5 dB. For intensity modulation, the required frequency stabilization range of the laser source is generally ~GHz. Thus, finer frequency stabilization of the optical source is required for DPSK than for intensity modulation. In practice, however, it is wasteful to make another optical transmitter with such tight frequency stabilization. Thus, automatic frequency tracking to the optimum operating point is required to maintain good performance for the 1-bit DMZI.
3. 1-bit DMZI control method
For intensity-modulated signal, the transmittances of the outputs of the 1-bit DMZI are expressed as Eq. (3). The transmittance of each output port is dependent on the optical frequency of the input signal. For optimum operation, the signal frequency should be at the peak of the constructive curve. As the signal frequency deviates from the optimum point, the output power of the constructive port is reduced. Thus, we can simply control the optimum point by monitoring the output power.
However, for a phase-modulated data stream with equal populations of level “0” and level “1”, the average power at each output port of the 1-bit DMZI is constant with variation of signal frequency, as shown in Eq. (4).
Here, Pport is the power at an output port of the 1-bit DMZI, P0 is the power of level “0”, and P1 is the power of level “1”. In DPSK modulation, it is impossible to assess the quality of the 1-bit DMZI transmittance by monitoring the power of each output port. For this reason, previous research introduced a signal with a carrier component to control the stabilization by detecting the carrier power. This method is not useful, however, because it is influenced by the signal power variation, and it inherently reduces the extinction ratio.
We propose a simple control method for the 1-bit DMZI which uses characteristics of eye patterns. In NRZ-DPSK, the limited bandwidth of the modulator produces an asymmetric shape of signal level(“1” or “0”) in the eye pattern, as shown in Fig. 4(b). At the constructive output port, there is a DC component (generated by consecutive 0 or π) in the eye pattern at the “1” level but not a DC component (generated by alternating 0 and π) at the “0” level, because of transition time between 0 and π phase. When the “0” level is continuous, it jumps up at the bit transition region (the intersection of 0 and π phase). This pattern is also maintained at non-optimum operating points.
Because of the asymmetric DC component, the output power of each port is changed from the value in Eq. (4) by variation of the optical frequency as shown in Fig. 2. At the optimum point, the DC level of the constructive port is highest, and that of the destructive port is lowest. As the optical frequency of the input signal is changed, the DC level of the constructive port decreases, and the DC level of the destructive port increases, creating an inferior eye opening as shown in Fig. 4(c). The difference between the output power from the constructive and destructive ports is maximized at the optimum point, where the time delay of the 1-bit DMZI exactly matches the bit period of the DPSK-modulated signal. Thus, by observing the power difference, we can automatically stabilize the 1-bit DMZI at its optimum point through feedback control.
Figure 5 shows the control circuit used to stabilize the 1-bit DMZI. The optical frequency transmittance of the 1-bit DMZI can be controlled coarsely or finely by using a TEC or heater. The TEC controls the temperature of the DMZI body, and the heater finely controls the delay time. In the circuit, voltages V1 and V2 are proportional to the output power of the balanced receiver. The average output voltage of the low frequency analog calculation unit is proportional to V1/V2. In the feedback control part, the driven voltage of the heater is controlled by this voltage ratio, V1/V2.
As shown in the inset of Fig. 5, the voltage ratio V1/V2 changes in a periodic fashion when the input optical frequency is shifted or the DMZI temperature is varied. This effect is a result of the characteristic of eye pattern in NRZ-DPSK. The period is the FSR, 10GHz. Since the voltage difference is maximized at the optimum point, we can use it to automatically stabilize the 1-bit DMZI at its optimum point through feedback control.
This control procedure for the 1-bit DMZI involves locating and maintaining its optimum frequency point. At cold-start, the automatic search procedure for the optimum operation point is carried out in two stages. First, points with the highest power ratio between the two ports are located through coarse temperature tuning over a wide spectral range (~2 FSR). In the second fine search stage, the final optimum point is selected through fine temperature tuning over a range of ±10% of the middle point found during the coarse tuning stage. The final optimum point is then maintained by preserving the highest voltage ratio.
4. Experimental results
This section describes an experiment carried out to test the proposed optimization and stabilization procedure for a DPSK receiver. The transmitter was composed of a tunable laser and a LiNbO3 phase modulator, and the receiver was composed of a 1-bit DMZI and a balanced receiver. 231-1 PRBS data sequences were used for 10Gb/s data streams. The optical frequency of input signal was measured using an optical wavelength meter, and data was recorded into a computer through GPIB.
Figure 6 illustrates the control procedure. During the coarse search stage, we measured the voltage ratio and BER by increasing the driven voltage of the heater. Three peak points were identified in the coarse search. The middle peak point was selected as the midpoint of the optical frequency tuning range for the second search stage. During fine search, we measured the voltage ratio by finely tuning the driven voltage of the heater. The maximum voltage ratio was set as the optimum operating point. In the stabilization procedure, we controlled the driven voltage of heater to maintain the optimization by monitoring the voltage ratio.
To confirm the efficacy of this control method, we tested the stabilization under frequency variation. Figure 7 shows the BER without (Fig. 7(a) and with (Fig. 7(b)) stabilization control, along with the frequency shift. When stabilization control was not used, the BER increased for ~1GHz frequency shift. With stabilization control, on the other hand, the BER was kept in an error-free state by feedback control, as shown in Fig. 7(b). During this feedback control, the heater’s driving voltage was changed over time to maintain an error-free state as shown in Fig. 8. The heater’s driving voltage was changed about 0.15 V per 1 GHz frequency shift.
We have proposed and tested a self-optimization and auto-stabilization method for the 1-bit DMZI in NRZ-DPSK . Self-optimization can reduce the cumbersome procedure of manual 1-bit DMZI calibration for each receiver when the WDM-based DPSK system is first initiated. Thereafter, auto-stabilization produces stable performance during operation with carrier optical frequency variation of ~10% of the data rate. Unlike previous techniques which require additional components such as a carrier signal, this proposed control method is simple, cost-effective, and self-sufficient.
This study was financially supported by research fund of Chungnam National University in 2006.
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