An ultra-small silicon-based microring modulator and filter were proposed to generate and demodulate NRZ DPSK at 10 Gb/s. In this paper, we analyze performance dependencies of the modulator and demodulator under different operating conditions, such as variable laser linewidth, phase shift, demodulator offset and receiver bandwidth. Data quality of the microring-based DPSK transceiver can be optimized with eye-opening improvement of up to 7 dB. Transmission performance of the all-microring-based DPSK signal over a 70-km single mode fiber is compared to that of DPSK using a Mach-Zehnder modulator and a delay-line interferometer.
© 2008 Optical Society of America
Advanced data modulation formats have become important within the optical communications community, and of particular interest is differential-phase-shift-keying (DPSK) . When compared to on-off-keying, the DPSK data format exhibits better receiver sensitivity and tolerance to fiber nonlinearities. The DPSK signal, which carries information via the phase difference of adjacent symbols, is typically generated using either a phase modulator or a Mach-Zehnder modulator (MZM), while a delay-line interferometer (DLI) is used for demodulation and balanced detection. In general, these structures tend to be fairly large and difficult to fabricate into arrays.
Microring-resonator-based modulators and filters have attracted much attention in recent years, especially in silicon-based platforms [2–8]. Recently, we described a method for generating and demodulating DPSK signals operated at 10 Gb/s using microring structures . As compared to conventional MZMs and DLIs, the microring structures potentially require relatively small chip area and might be easier to fabricate into arrays for high-capacity transceivers. In that paper, a π phase shift from a single-waveguide ring resonator was induced to generate DPSK, and using a double-waveguide microring filter achieved demodulation of DPSK. In the demodulator, a band-pass port extracts duobinary signal while a notch port obtains alternate-mark inversion (AMI) signal. The conventional (de)modulators (MZM+DLI) and microring structures have different transfer functions when considering both phase and amplitude responses. These differences could result in important changes in system performance when employing the ring structures in DPSK links and networks.
In this paper, we analyze the performance of ring-based DPSK data and present a detailed characterization with varied laser linewidth, phase shift, demodulator offset, and receiver bandwidth. It is shown that the microring-based DPSK modulator suffers more from laser phase noise, compared to a MZM-based DPSK generator, while the microring based DPSK demodulator can be more tolerant to the frequency offset between demodulator and signal carrier. A careful optimization of these important parameters in microring-based DPSK (de)modulation may dramatically improve the system performance. Eye-opening is improved by up to 7 dB by optimizing bias condition, signal power and phase shift.
Typically, microring modulators are operated by shifting resonance peak of the ring. This shifting can be realized by varying carrier density and thus refractive index in the ring cavity, when a voltage is applied. The microring modulators are usually based on MOS capacitors [2, 3] or p-i-n diodes [4, 5]. Here a MOS-capacitor-based modulator is considered. In the optical domain, a microring in a single-waveguide configuration is designed to be over-coupled (i.e., waveguide-ring coupling > ring loss), causing a phase shift of 2π across resonance [10, 11]. As shown in Fig. 1(a), when the resonance peak is shifted, the continuous wave (CW) laser source can experience a phase shift of π across the center of the phase profile and have the same power in each bit duration. The NRZ DPSK data format is generated this way. It is important to note that there are intensity dips at bit transitions in the modulated signal , which are caused by the notched amplitude response of the ring resonator (see Fig. 1(a)). When the resonance profile is moved, the optical signal also experiences a fast phase shift, which gives rise to frequency chirp at the bit transitions.
Demodulation of DPSK signals is achieved using a double-waveguide microring filter, as shown in Fig. 1(b), in which one output port works as a band-pass filter to extract the duobinary signal from DPSK spectrum, and the other as a notch filter to produce the AMI signal . This microring filter functions as a discriminator to convert the phase-modulated signal to intensity-modulation for direct detection . The proposed demodulation scheme is not based on interference of the DPSK signal and its one-bit-time delayed version as in a DLI, and thus might be more tolerant to change in phase shift away from π in a DPSK signal.
To model the behavior of microring-based silicon modulators, an ideal 10 Gb/s NRZ drive voltage is sent through a low-pass five-pole Bessel filter, with drive pulses that have rise and fall times of ~10 ps. Variation in carrier density is simulated as a charging process following the applied voltage . The carrier transit time, defined as the duration required for carrier density to increase from 10% to 90% of its peak value, is considered to be 23 ps here . The refractive index is calculated from the carrier density. The ring resonator has a radius of 5 µm. A set of dynamics equations given in Ref. 13 is numerically solved to obtain the modulated signals, in which photon lifetime of ring resonators can be considered.
3. DPSK data quality dependencies in a microring-based transmitter
We first examine the dependence of signal quality on laser linewidth with a Lorentzian shaped spectrum. A drive voltage of 1 volt causes a phase shift of π, corresponding to a cavity Q-factor of 16000. It is noted that large linewidth may significantly degrade the DPSK signal waveform, as illustrated in Fig. 2. Signal distortion is quite severe for a laser with a 3-dB linewidth of 10 MHz. In contrast, signal quality is similar when the laser linewidth is 300 kHz and 10 kHz. This waveform distortion is attributed to the fact that the phase noise contained in the laser linewidth is greatly enhanced in the microring-based resonant structure . When light stored in the resonator is coupled out, it would beat with light traveling along the waveguide, which converts the phase noise to amplitude noise .
Figure 3 shows eye-opening penalty (EOP) of the microring-modulated DPSK as a function of the laser linewidth from 30 kHz to 30 MHz, for different cavity Q-factors ranging from 10000 to 22000. Drive voltage is modified according to a different cavity Q-factor to keep the phase shift fixed at π. Transit time is set to be 23 ps, and a DLI is used as a demodulator. These results are also compared to those obtained using a MZM-based modulator. We note that a high-Q ring modulator suffers from the linewidth problem much more than a low-Q resonator or a MZM does, and EOP caused by the laser phase noise increases approximately exponentially with laser linewidth and can be up to 6 dB. It is very important to carefully deal with this issue because, once the signal is degraded by the laser phase noise, one could not improve the detection bit-error-rate (BER) by simply adding an optical filter or increasing the received power in the receiver. Thus design guidance is highly needed. In particular, it is found out that a microring-based DPSK modulator with Q of 10000 works well with a laser of <10 MHz linewidth, in which EOP is <1 dB, and the required drive voltage is 1.88 volt.
We also examine the effect of drive voltage on the DPSK signal quality, which can change output power and phase shift in the modulated signal. The laser linewidth problem can be partially solved when the microring modulator is driven harder. In this way, the required drive voltage is higher, and the resonance shift becomes larger, which causes a higher transmittivity through the ring resonator and more phase shift. We choose cavity Q-factor to be 22000, since a high-Q resonator suffers more from laser phase noise. Drive voltage is increased from 0.77 to 2.2 volt, and correspondingly the induced phase shift is changed from exactly π to 1.41π. As shown in Fig. 4(a), signal power is increased almost linearly with induced phase shift. We note from Fig. 4(b) that over-driving of the microring modulator improves eye-opening dramatically by up to 7 dB for microring-based demodulation as well as by 5 dB for DLI-based one, with 3-MHz laser linewidth. It is important to mention that the DLI demodulator is based on interference of light waves and requires an accurate π phase shift. When the phase shift is changed, the DLI demodulator would have a relatively low extinction ratio that discounts the benefit from the signal power increment. But, a varied phase shift does not change the spectral distribution of duobinary and AMI signals in DPSK signal spectrum. Thus the microring demodulator operating in the frequency domain relaxes the requirement for an exact π phase shift. The over-driving of the modulator results in a 7-dB eye-opening improvement, which could compensate for the EOP caused by the laser linewidth problem. It should be noted that, for telecom applications, the phase shift in DPSK signals is desired to be exactly (or close to) π, which makes the signal more tolerant to phase noise induced by in-line amplifiers . In this case, one may not want to over-drive the modulator too much. But for short-reach or on-chip interconnects , system performance would benefit from overdriving the microring modulator.
4. DPSK data quality dependencies in a microring-based receiver
In the receiver, the quality of the demodulated DPSK signal at 10 Gb/s is examined when a frequency offset between the demodulator resonance frequency and the signal carrier is increased from 0 to 3 GHz. Here, we use a MZM as a DPSK modulator and choose the demodulator bandwidth of 8.7 GHz, corresponding to cavity Q-factor of 22000, which has been optimized in Ref. 9. As shown in Fig. 5(a), the microring-based DPSK demodulator is much more tolerant to demodulator frequency offset than a DLI, because DLI demodulation scheme is based on interference of optical waves and relies on accurate phase control that is reflected as frequency shift of DLI transfer function. According to the periodic property of DLI transfer function, the demodulated DPSK signal from a DLI would be completely distorted with ¼ bit-rate offset (that is, 2.5 GHz) and become inverted with the ½ bit-rate offset. In contrast, microring demodulator works in frequency domain and has a huge free spectrum range (e.g., more than ten nanometers for a ring radius of 1.5 µm ) and a relatively stable performance with much smaller EOP, compared to a DLI.
We also note from Fig. 5 that eye-opening can be improved by optimizing the receiver bandwidth, in which the PIN photodetector bandwidth varies from 0.5 to 2.3 times the bit-rate, and the demodulator bandwidth is 8.7 GHz. Eye-opening improvement is increased by >1 dB as the normalized receiver bandwidth by bit-rate is changed from 0.7 to 1.5. As shown in the eye-diagrams in Fig. 5(b), the eye-opening improvement comes mainly from better detection of demodulated AMI signal that contains more high frequency components.
5. Performance comparison in single-mode fiber transmission
Overall performance of the proposed microring-based DPSK modulator and demodulator in fiber transmission is compared to conventional scheme (MZM+DLI) in terms of power penalty, as shown in Fig. 6. Laser linewidth is 100 kHz, and transit time in the modulator is set to be 16 ps. Q-factors in the modulator and demodulator are chosen to be 10000 and 22000, respectively. The bias condition in the microring modulator is controlled to generate a phase shift of π. Receiver bandwidth is 17 GHz for the all-ring-based DPSK and 7 GHz for MZM+DLI. The power penalty is measured at 10-9 BER, in a pre-amplified receiver configuration. Over a 70-km-long single-mode fiber without dispersion compensation, the all-ring- based technique exhibits negative power penalty within 30-km transmission possibly due to frequency chirp  and becomes worse than the conventional scheme after 50 km.
6. Brief discussion
For on-chip applications, we note that an absence of ASE noise induced by optical amplifiers makes the performance of two types of DPSK links closer to each other, since DLI-based demodulation plus a balanced detection would reduce ASE-related signal distortion, which means, for telecom applications, the microring demodulator is a little disadvantageous in this sense. We believe the proposed microring-based DPSK generation works in principle for p-i-n diodes.
We have analyzed signal quality dependencies of integrated ultra-small silicon microring-based DPSK modulation and demodulation on laser linewidth, drive voltage, demodulator offset and receiver bandwidth. It is shown that the microring-based DPSK modulator is more sensitive to laser phase noise, while the microring-based DPSK demodulator can be more tolerant to a change in phase shift away from π and frequency offset between the demodulator center wavelength and the signal carrier. Data quality of the microring-based DPSK can be optimized with an eye-opening improvement of up to 7 dB. We also compared the data transmission performance of the microring-based DPSK over 70-km single mode fiber to that in a conventional DPSK link consisting of MZM and DLI, which shows that lower power penalty is obtained in a distance of <50 km using the microring-based scheme.
The authors would thank Dr. Cary Gunn, Rohan Kekatpure, Dr. Stefan Preble and Dr. Hidehisa Tazawa for their helpful discussions. This work is sponsored by Army Nanophotonics program and HP Laboratories.
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