We demonstrate a continuously-variable bit-rate receiver from 10 to 40 Gbit/s for DPSK demodulation. Unlike previous DPSK demodulators, this receiver is also capable of passing intensity modulated waveforms without distortion. Degradations imposed by receiver imperfections are presented and compared with a traditional DPSK delay-line interferometer.
© 2008 Optical Society of America
Future optical networks may be required to accommodate many different types of data traffic. This traffic diversity might result in: (i) variable bit rates that depend on signal quality, (ii) different modulation formats for increased robustness to impairments and spectral efficiency, and (iii) user-requested variable quality-of-service (QoS). Importantly, dynamic data-line-rate management is common to present non-optical networks and might be desirable for a future optical network. A key element in a versatile system is a single receiver design that can be tuned to recover a continuously-variable bit rate for several different types of data modulation formats. Possible data formats that might be important to accommodate include: (i) differential phase-shift-keying (DPSK) modulation formats and (ii) intensity modulation formats (e.g., OOK, PPM, and duobinary). A single multi-purpose design would enable greater efficiency, flexibility and cost-effectiveness.
DPSK demodulators are typically designed for a fixed data rate, equipped with limited amounts of phase tuning. Several techniques for DPSK demodulation have been proposed, including: (i) a delay line interferometer (DLI) in a Mach-Zehnder configuration , (ii) injection locking of a semiconductor laser diode , (iii) the use of a bandpass filter acting as a frequency discriminator [3,4] and (iv) the use of a polarization maintaining fiber (PMF) or a birefringent fiber loop (BFL) with a fixed differential group delay (DGD) between the fast and slow axes to convert DPSK to polarization-shift-keying [5, 6, 7]. A rate adjustable DPSK scheme using multi-chip detection was demonstrated in , capable of operating at integer multiples of 200 Mbit/s, up to 11.8 Gbit/s. The demonstrations in [6, 7] do not possess continuous rate-tuning capability and the scheme in  requires rate-dependent electronics, it can only operate at integer multiples of a base rate, and it incurs a power penalty for the higher rates .
In order to realize a multi-format receiver design at minimal cost it is desirable for all types of modulation formats to be allowed to traverse identical paths within the receiver. Unfortunately, it is not possible to send intensity modulated signals through a fixed interferometer configuration without incurring a performance penalty and/or interference between adjacent bits. This feature limits a receiver employing a fixed interferometer to differential phase-based modulation formats.
Furthermore, it has been demonstrated that the optimal demodulator delay is not always 1-bit, but instead depends on system parameters such as residual chromatic dispersion and the optical filtering bandwidth [10, 11]. A rate tunable demodulator would allow fine tuning of the delay to optimize system performance in the presence of distortions. To date, no continuously rate-tunable receivers have been reported for the demodulation of a DQPSK waveform.
In this letter we present a receiver design capable of: (i) demodulating optical DPSK signals (both NRZ and RZ) at continuously tunable data rates and (ii) passing intensity modulated signals (e.g., OOK, PPM, duobinary) without penalty. In our scheme we split the signal into two orthogonal polarization states, delay one with respect to the other using a tunable DGD element and interfere the signals using a polarization beam splitter, in a fashion similar to that in . The device implements a Mach-Zehnder interferometer (MZI) in the polarization domain with the additional feature of continuous tuning capability, provided by a commercially-available tunable DGD element. Utilizing this technique we experimentally demonstrate demodulation and detection of optical NRZ-DPSK at data rates of 10, 20 and 40 Gbit/s with little penalty when compared to results using a commercially available ITF demodulator . We also demonstrate experimentally the ability to pass OOK modulation with little penalty after transmission through the demodulator at 10, 20 and 40 Gbit/s.
Expanding the above equation the two outputs of the polarization beam splitter (PBS) can be expressed as:
Note that E+45° is linearly polarized at +45 degrees, whereas E-45° is linearly polarized at - 45 degrees. When the input field is linearly polarized 45° relative to the principle states of polarization (PSP) of the birefringent element (i.e., Ex=Ey = 1 and Ψ = 0) and the output of the birefringent element is aligned 45° relative to the PBS, the two outputs of the PBS simplify to the following:
Observe that equations (4) and (5) are identical to those of a traditional time-based delayline- interferometer. Furthermore, by adjusting the birefringent delay T, the free-spectral-range (FSR) of the interferometer can be adjusted.
If instead the input field is aligned to be along one of the axes of the birefingent material (i.e., Ex = 1 and Ey = 0 or Ex = 0 and Ey = 1) and the output of the birefringent material is aligned to be along one of the axes of the PBS, the output field is unaffected. In such a configuration, the receiver is no longer an interferometer but is instead just a simple polarizer and can therefore pass intensity modulated signals unaffected, a feature not available with a traditional DLI.
3. Experimental results
We verify the proposed variable-rate, multi-format receiver design through detection of 10, 20 and 40 Gb/s NRZ-OOK and NRZ-DPSK data signals. The experimental setup is shown in Fig. 2. For detection of DPSK signals the input polarization is aligned 45° relative to the PSPs of the birefringent material. Alignment of the input polarization was performed with the assistance of a commercially-available polarization stabilizer by Adaptif. The birefringent material in this case was a commercially-available differential group delay (DGD) device by JDSU with tuning capability from 0 to 200 ps, with resolution of 5 fs. The DGD was set equal to the bit period (e.g. 25 ps for 40 Gb/s). This allows the receiver to be tuned to detect DPSK signals of rate 5 Gb/s and higher with extremely high resolution. The output of the DGD element was manually aligned to the PBS using a polarization controller (PC). In practice the PBS could be integrated with the DGD element and permanently aligned using polarization maintaining fiber (PMF).
The tunable demodulator is essentially a Mach-Zehnder interferometer operating in the polarization domain. We tested the spectral response with a polarized broadband noise source launched at the input to the demodulator. Shown in Figs. 3(a)–3(c) are the measured transmission responses for DGD values of 100, 50 and 25 ps, respectively. The expected free-spectral-ranges (FSR) of 10, 20 and 40 GHz were observed. The FSR was found to have an adequate extinction ratio (>20 dB) for DPSK demodulation , an indication that adequate splitting and beating of the polarization states is occurring in the demodulator.
We experimentally measured bit-error-rate (BER) performance at 10, 20 and 40 Gbit/s, as shown in Fig. 4 for NRZ-DPSK and Fig. 5 for NRZ-OOK. In order to isolate the penalty of our receiver from the penalty inherent in the setup we also took reference BER measurements. For OOK reference measurements we bypassed the demodulator completely. For DPSK reference measurements, commercially-available ITF DLIs were used. A 20 Gb/s DLI was not available, therefore reference results are not shown in Fig. 4 for 20 Gb/s DPSK. The pre-amplified receiver had a front-end EDFA with a noise figure of ~5 dB which degraded the bit error rate results. Additional penalty was also observed at 40G due to non-ideal electronics in both the transmitter and receiver.
As illustrated in Fig. 4(a), we observed a power penalty of <0.2 dB at 10 Gb/s and ~1 dB at 40 Gb/s for detection of DPSK signals when compared to detection using a traditional DLI. We attribute these penalties to non-ideal adjustment of the polarization before and after the DGD elements. Another source of degradation in our setup and a current limitation to a variable line-rate system is non-optimal optical and electrical filtering. If optical and electrical filtering remains fixed as the line-rate varies the filtering can not be simultaneously optimized for all expected data rates. Our setup included an optical filter which was approximately 0.5 nm in bandwidth, which is just over 2 times the 40 Git/s signal, but is around 8 times the 10 Gbit/s signal. This issue could be alleviated through the use of a switching bank of optical/electrical filters or through the use of tunable bandwidth filters.
As illustrated in Fig. 4(b), we observed power penalties of 0.2, 0.7 and 1.1 dB for detection of OOK signals at rates of 10, 20 and 40 Gb/s, respectively when compared to detection without the demodulator. Similar to the DPSK results, we attribute this penalty to slight misalignment of the polarization. Misalignment of the input polarization to the DGD results in a nonzero value in the orthogonal polarization and eventual interference after the PBS. Polarization misalignment relative to the PBS results in loss of power at the balanced detector.
4. Results and discussion
Shown in Fig. 5(a) is the power penalty associated with misalignment of the input state of polarization relative to the principal states of polarization of the birefringent element. For equal splitting between polarization states and maximum interferometer extinction ratio, the input to the DGD element should be aligned 45° relative to the PSPs. Misalignment results in unequal splitting between the two polarization states and is analogous to unequal power splitting in a traditional time-based interferometer. As shown in Fig. 5(a) a penalty of approximately 1 dB occurs for a misalignment of 13 degrees.
Similarly, misalignment to the axes of the PBS results in unequal combining of the two polarization states, reducing the extinction ratio of the interferometer. This is analogous to unequal combining in the second coupler of a traditional DLI. For optimal detection the state of polarization should be aligned 45° relative to the axes of the PBS. As shown in Fig. 5(b) a penalty of ~1dB is incurred for misalignment of +/−15°.
Shown in Fig. 5(c) is the tolerance to misalignment of the DGD relative to the bit rate. This penalty is directly analogous to non-optimal delay in a traditional delay-line based interferometer. With no distortions present the DGD is set equal to the bit rate. As discussed in  the optimal DGD value changes in the presence of distortions motivating the use of a tunable demodulator. As shown below a 1 dB penalty is incurred for a bit-rate to DGD mismatch of approximately 10 percent. This agrees well with previously reported results for a traditional DLI .
In this paper we demonstrate a continuously rate-tunable receiver design capable of demodulating DPSK transmission over a wide range through the use of a commercially-available tunable DGD element. Furthermore, this receiver has the added quality that it can be tuned to pass any type of intensity modulated waveform with very little degradation. DPSK demodulation was performed at rates of 10, 20 and 40-Gb/s with little power penalty when compared to demodulation using a commercially-available delay-line interferometer.
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