1. Introduction

Recently there has been a lot of interest in advanced modulation formats to enhance both spectral efficiency and transmission robustness [1]. Homodyne binary or quadrature phase-shifted keying (PSK/QPSK) is one of the most promising candidates. Some homodyne receivers implemented by high-speed electronic circuits have been successfully demonstrated, such as a phase-diversity optical homodyne receiver assisted by high-speed digital signal processing (DSP) technology [2], and a synchronous PSK/QPSK detection scheme with analog or digital carrier recovery [3]. However, these schemes suffer from one or more of the following potential drawbacks: i) a complicated and costly design is required because of the need to provide a high-speed electrical pre-coder or digital post-processing; ii) an additional stable local-oscillator laser is required with matched wavelength and optical polarization/phase tracking; iii) real-time demodulation is difficult to implement for high-speed systems; and iv) there are stringent requirements for the linewidth of the lasers.

In this paper, we proposed and experimentally demonstrated a novel self-homodyne PSK/QPSK modulation technology based on periodically inserting pilot symbols in the data stream to provide an absolute phase reference [4] and demodulating the phase information using a simple half-symbol-delayed Mach-Zehnder delay interferometer (MZDI) in the receiver. Compared with other existing homodyne or conventional differential PSK/QPSK (DPSK/DQPSK) techniques [5,6], this scheme offers the following advantages: i) a high-speed, complicated differential pre-coder is not required in the transmitter, and a high-speed DSP electrical circuit or local oscillator is not required in the receiver to recover the carrier's phase; and ii) for the proposed QPSK scheme, simultaneous detection of in-phase (I) and quadrature (Q) is achieved by using only one delay-interferometer and following balanced photo-detector, which reduces the system complexity and implementation cost. Moreover, the linewidth requirements for the laser are relaxed by using an interferometer with only a half-symbol delay, which provides more stable performance under temperature variations and good tolerance against the demodulator phase error [7]. In this paper, dispersion tolerances of proposed QPSK and PSK signals were also experimentally investigated.

2. Operation Principle

The operation principle of the proposed self-homodyne PSK and QPSK systems is illustrated in Fig. 1. For the proposed homodyne PSK signal, in the transmitter, the light from a laser diode is phase-modulated by a return-to-zero radio-frequency (RZ-RF) signal with a duty cycle of 50%. As shown in Fig. 1, by leaving certain time-slots un-modulated, pilot symbols are periodically inserted in the data stream, which provides an absolute optical phase reference for the adjacent phase-modulated data slots. The phase information is, therefore, coded using an absolute phase referenced to the inserted pilot symbols, instead of using a relative phase referenced to the adjacent bits. At the receiver side, an interferometer with only a half-symbol delay performs self-homodyne direct detection. Because the phase demodulation is carried out based on the phase reference provided by the inserted unmodulated time slots rather than the relative phase reference provided by the adjacent bits, no additional high-speed differential pre-coder is required in the transmitter, and no digital signal post-processing or local oscillator need be employed in the receiver to recover the carrier’s phase. Note that, in PSK case, as the interference demodulation is carried out between the phase modulated slot and the adjacent two pilot slots, after the photo-detection, the data is recovered with NRZ data format, instead of RZ data format.

The operation principle of the proposed QPSK system is illustrated in Fig. 1(ii). Different from PSK system, after formatting as 50% RZ-RF signals, I and Q tributaries are employed in the phase modulation to achieve the quaternary phase states in the light. In the receiver side, if assuming +π/4 phase shift and half-symbol time delay are introduced for b-branch in the MZDI, as shown in Fig. 2, the relative phase shifts of the data slot respective to the pilot symbol slot are different for the two time slots within one bit period after the MZDI. The data slot is -π/4-phase-shifted respective to the pilot symbol slot at the first time slot, whereas +π/4-phase-shifted at the second time slot. Thus, after the interferometer, the detected data at two time slots correspond to I and Q components of QPSK, respectively. With the proposed inserted pilot symbol technology, a simultaneous detection of I and Q components of QPSK can be achieved. After the MZDI, an optical or electronic de-multiplexer can be employed to separate the I and Q components for each of the data channels. Here, an electronic de-multiplexer scheme is utilized in Fig. 1(ii). After photo-detection, the bit rate of the detected RF data is doubled, compared with that of driving RF signals at the transmitter.

 

Fig. 1. Operation principle of the proposed self-homodyne (i) PSK and (ii) QPSK systems.

Download Full Size | PDF

 

Fig. 2. Principle illustration of simultaneous detection of I and Q components of the proposed self-homodyne QPSK system.

Download Full Size | PDF

3. Experiment and results

 

Fig. 3. Experimental setup of the proposed 40-Gb/s self-homodyne QPSK system.

Download Full Size | PDF

To evaluate the performance of the proposed homodyne PSK/QPSK schemes, the performance was experimentally investigated for the homodyne PSK and QPSK systems with symbol rates of 20 Gbaud (20 Gb/s for PSK and 40 Gb/s for QPSK). A schematic diagram of the experimental setup for the 40-Gb/s homodyne QPSK system is shown in Fig. 3. Light at 1547 nm from the laser diode was fed into an integrated QPSK modulator, where 0-π and 0-π/2 phase modulations were applied with a dual-drive Mach-Zehnder modulator (MZM) and following phase modulator, respectively. In order to generate the RZ-RF signals to drive the phase modulator, a 10-Gb/s NRZ-RF PRBS data stream [e.g. CH1 shown in Fig. 4(a)] with a length of 2³¹-1 was first converted to a 50% RZ-RF signal (CH1) and multiplexed into 20-Gb/s RZ-RF signals by combining it with its 20-bit delayed version (CH2). Then, the generated 20-Gb/s RZ-RF signal [e.g. CH1+CH2 shown in Fig. 4(b)] and its 20-bit delayed version were used to drive the QPSK modulator. Note that, different from the scheme discussed in Ref. [8], complementary RZ-RF signals were employed to drive the two arms of the dual-drive MZM to achieve the phase modulation (0, π). Un-modulated pilot symbols were periodically inserted in the phase-modulated data stream by driving the RZ-RF signals for the phase modulation. Hence, a 40-Gb/s homodyne QPSK signal with inserted pilot symbols was successfully generated. In the 20-Gb/s homodyne PSK case, only one tributary 20-Gb/s RZ-RF signal was used to drive the 0-π phase modulator. Before being launched into the optical receiver, the signal was pre-amplified by an erbium-doped fiber amplifier (EDFA) and extracted by a 1-nm FWHM optical bandpass filter (OBF). In the receiving end, only one delay interferometer with a delay time of 25 ps, corresponding to the half-symbol period of 20-Gbaud systems, was employed for demodulating the I and Q tributaries. The interference between the phase-modulated signal and the un-modulated inserted pilot symbols is achieved. After the half- symbol-delayed MZDI, detected I and Q components for two channels are separated by using commercial electrical de-multiplexer.

For the proposed PSK system, because 50% RZ-RF signal (pulse-width: ~25ps) was employed in the phase modulation, after the 25-ps delay interferometer, 20-Gb/s NRZ data (e.g. CH1+CH2 shown in Fig. 4(c)) with a bit period of 50 ps was obtained. Then, the 20-Gb/s demodulated NRZ data was electrically de-multiplexed into two 10-Gb/s data streams for bit-error rate measurement (e.g. CH1 shown in Fig. 4(d)). To demonstrate the operating principle of the proposed PSK system, as shown in Fig. 4, a pattern ‘100111001011010001’ of CH1 is used as an example to illustrate the waveforms at different points in Fig. 1. The detected eye diagram is shown in Fig. 5(a).

For the proposed QPSK system, as discussed above, after the interference in half-symbol-delayed MZDI, I and Q components were simultaneous obtained in the detected 40-Gb/s data stream. The corresponding eye diagram is shown in Fig. 5(b). To separate the I and Q components for each of the data channels, an electronic de-multiplexer was employed. The I and Q components of each data channel, CH1 and CH2, were finally obtained at bit rate of 10 Gb/s for further BER measurement. The slight difference in the eye diagrams between the detected I and Q components is mainly attributed to the limited frequency response of the phase modulator employed for the (0, π/2) phase modulation. Better performance is expected if using an I-Q QPSK modulator [9].

 

Fig. 4. Measured waveforms for the 20-Gb/s self-homodyne PSK system at the corresponding points in Fig. 1: (a) input 10-Gb/s CH1, (b) input 20-Gb/s CH1+CH2, (c) detected 20-Gb/s CH1+CH2, (d) detected 10-Gb/s CH1.

Download Full Size | PDF

The measured BER characteristics of the two-channel 40-Gb/s QPSK and 20-Gb/s PSK signals are shown in Fig. 6. For both 40-Gb/s QPSK and 20-Gb/s PSK signals, a sensitivity difference of less than 0.3 dB was observed for the two de-multiplexed channels, CH1 and CH2. The receiver sensitivities at BER of 10^-9 for de-multiplexed CH1 and CH2 of 20-Gb/s PSK signal were -31.6 dBm and -31.8 dBm, respectively. For 40-Gb/s QPSK signal, after the electronic de-multiplexer, four 10-Gb/s data channels were obtained, which corresponds to I and Q components of CH1 and CH2, respectively, as illustrated in Fig. 5(b). For CH1, the receiver sensitivities of I and Q components were -28 dBm and -27.1 dBm, respectively. While for CH2, the receiver sensitivities of I and Q components were -28.3 dBm and -27 dBm, respectively. The optical spectra of the obtained 20-Gb/s PSK and 40-Gb/s QPSK are shown in Fig. 7.

 

Fig. 5. Eye diagrams of (a) 20-Gb/s PSK and (b) 40-Gb/s QPSK, where the I and Q components of two channels are indicated (10 ps/div).

Download Full Size | PDF

 

Fig. 6. BER performance measured at 10 Gb/s for the detected two channels of 40-Gb/s QPSK (I and Q components) and 20-Gb/s PSK systems.

Download Full Size | PDF

 

Fig. 7. Optical spectra of 20-Gb/s PSK and 40-Gb/s QPSK signals.

Download Full Size | PDF

To assess the dispersion tolerance of the proposed QPSK/PSK schemes, receiver sensitivities at a BER of 10^-9 were experimentally evaluated with respect to different lengths of dispersive fibers. As shown in Fig. 8, dispersion tolerances of about 83 ps/nm and 131 ps/nm with a power penalty of less than 2 dB were obtained for 40-Gb/s QPSK and 20-Gb/s PSK, respectively. The narrower tolerance for QPSK is attributable to the introduced additional phase modulation.

The proposed self-homodyne phase modulation scheme with inserted pilot symbols is scalable to higher bit-rate systems. Moreover, it can be applied to other multi-level phase modulation schemes. For example, when applying it to eight-phase-shifted keying (8PSK) system, it is possible to detect the corresponding three data tributaries using only two MZDIs with half symbol delay and +π/4 or -π/4 phase shift.

 

Fig. 8. Dispersion tolerance investigation of 20-Gb/s PSK and 40-Gb/s QPSK signals.

Download Full Size | PDF

4. Conclusion

We successfully demonstrated a novel homodyne PSK/QPSK system using RZ-RF driving phase modulation at the transmitter side and self-homodyne direct detection with half-symbol delay interferometer at the receiver side. Inserted pilot symbols carry absolute phase information, instead of relative phase information. As un-modulated time-slots are periodically embedded into the phase-modulated optical stream, a simple and more-stable half-symbol delay interferometer could be employed to recover the phase information at the receiver side. No complicated pre-coder, post-processor, or local oscillator is required in the system. Moreover, for the proposed QPSK system, simultaneous detection of I and Q components was successfully achieved just using one half-symbol-delayed MZDI and following balanced photo-detector, which significantly reduces the implementation cost and system complexity. Dispersion tolerances for PSK and QPSK were also investigated. The proposed scheme is scalable to higher bit-rate systems, and also can be applied to other multilevel phase-shifted keying systems, e.g. 8PSK.