We describe the first experimental demonstration of a novel all-optical phase discrimination technique, which can separate the two orthogonal phase components of a signal onto different frequencies. This method exploits nonlinear mixing in a semiconductor optical amplifier (SOA) to separate a 10.65 Gbaud QPSK signal into two 10.65 Gb/s BPSK signals which are then demodulated using a delay interferometer (DI). Eye diagrams and spectral measurements verify correct operation and a conversion efficiency greater than 9 dB is observed on both output BPSK channels when compared with the input QPSK signal.
© 2013 Optical Society of America
As the demand for increased capacity in optical telecommunications networks grows, the use of higher order modulation formats has become more common. All-optical approaches to manipulation of such formats are necessarily more challenging than for return –to-zero-formats, since they must preserve the phase information in the original signals. We have previously simulated, using an experimentally verified model, a method of frequency converting the two orthogonal phase components of a signal . This paper experimentally demonstrates this novel all-optical phase discrimination scheme, which separates a QPSK signal into two BPSK signals at different frequencies which can then be demodulated using a simple delay interferometer (DI). This technique involves the use of nonlinear mixing in SOA to perform the phase discrimination. Using an SOA as the nonlinear mixing element has been extensively investigated [2–5] and has advantages when compared to other mixing elements such as nonlinear fiber. These advantages include providing gain, allowing for sub-milliwatt input powers, compact size, and ease of integration.
2. Principle of operation
The four-wave mixing (FWM) scheme used to produce two BPSK signals, I and Q, at separate frequencies is shown in Fig. 1. Four distinct pump lines are used, separated in frequency by 2Δf . The QPSK signal is placed Δf from both third and fourth pump lines. FWM in the SOA may be viewed as a two-stage process, in which beating between the QPSK signal and the pumps modulates the carrier density in the SOA. This carrier density fluctuation then in turn modulates the amplitude and phase of all signals passing through the SOA, including the other pumps. This modulation produces sidebands for each of the pumps at multiples of the beat frequency, the strongest of these being at the beat frequency Δf either side of each pump line. Since the QPSK signal is producing the beating the phase information encoded onto the carrier is also reproduced in the resulting sidebands. By correct choice of the relative phase and amplitude of the respective pumps and QPSK signal, interference between the various beat components, from the multiple pumps, will produce both the I (in-phase) and Q (quadrature) channels which make up the QPSK signal, with the I and Q channels being at different wavelengths. This technique is discussed in much more detail by R.P. Webb, M. Power and R.J. Manning .
3. Experimental Setup
Figure 2shows the experimental setup used in this system where initially a distributed feedback laser (DFB) with a linewidth of ~300 kHz was used in conjunction with two amplitude modulators each driven by a 42.6 GHz RF source to produce a spectral comb with seven lines which were spaced at the RF drive frequency. A more detailed explanation of this comb generation method can be found in . This comb was then injected into a four port wavelength selective switch (WSS) which was capable of controlling the individual amplitude and phase of each line of the comb. The WSS was used to produce four continuous wave (CW) pumps at the first output port (the upper port in Fig. 2), which were spaced 85.2 GHz apart. One further CW comb line, spectrally located equidistant between the two highest frequency pumps, was extracted to the second (lower) port of the WSS from the original comb.
This single CW comb line was modulated with a QPSK modulator driven by two 10.65 Gb/s 210-1 PRBS data RF signals to produce a 10.65 Gbaud signal. A dual output pattern generator which produced a PRBS and inverted PRBS was used as the source of these RF signals. These PRBS signals were also decorrelated by placing an extra length of coaxial cable in the path of the inverted PRBS signal. This gave rise to a ~40 bit offset between each pattern. The QPSK modulated signal was then passed through a phase modulator (φ), which was used to control the absolute phase of QPSK signal relative to the pumps. This allowed for any phase drift between the two paths to be manually corrected and to ensure correct phase discrimination, which is mentioned later in this paper. The four pumps and the QPSK signal were passed through separate polarisation controllers (PC) and recombined in a 3 dB splitter before they were passed through a polarization beam splitter (PBS). The two PCs and a power meter (PM) at the unused port of the PBS ensured both signals were co-polarised. This was important because the four-wave mixing (FWM) efficiency in the SOA is dependent on the relative polarisation of the signals [7, 8]. The combined signals were then injected into the SOA.
The first WSS was used to control the relative phase and amplitude of each pump and QPSK signal to ensure maximum phase discrimination at the output of the SOA. The output from the SOA was then passed through a 1-bit delay interferometer, with a free spectral range of 10.65 GHz, to demodulate both BPSK signals simultaneously. A second WSS was used to filter out both the I and Q channels, which were observed simultaneously on a Digital Sampling Oscilloscope (DSO) with a sampling rate of 10 MS/s. Both channels were optically path-length matched between the output of the WSS and the two input ports of the DSO. This was to ensure the relative delay between both channels was maintained.
Initially, to verify the correct operation of this experimental setup and to optimize the system for maximum phase discrimination, both RF signals driving the QPSK modulator were disabled resulting in the CW signal passing through the QPSK modulator without incurring any modulation. The phase modulator (φ) was then driven with a low frequency (~600 Hz) sinusoidal signal through 2π radians to produce a low frequency phase modulated signal. This phase modulated signal was then combined and co-polarized with the four pumps as mentioned previously before being injected in the SOA. At the output of the second WSS two low frequency photo receivers were used to observe the resulting I and Q channels, without the use of a DI. The output of the two photo receivers was read into a computer using two analog to digital converters (ADC), and a Matlab program was then used to optimize the relative amplitude and phase of each of the four pumps and the low frequency phase modulated signal to produce the maximum phase discrimination. Figure 3shows the resulting I and Q channels of a sinusoidally phase modulated signal clearly displaying the two orthogonal phase components. Extinction ratios greater than 14 dB were measured for both channels.
Once optimization had been completed the low frequency RF signal was disconnected and the RF signals driving the QPSK modulator were turned on. Figure 4shows the input and output spectrum of signals entering and leaving the SOA.
A fiber-to-fiber conversion efficiency of 9.3 dB and 9.1 dB was measured for the I and Q channels respectively when compared with the QPSK input power. The output pattern of the demodulated BPSK signals was recorded simultaneously on a DSO, and their relative decorrelation was compared to the two 210-1 bit PRBS input patterns which drove the QPSK modulator. The expected decorrelation was observed between the two signals, ensuring correct system operation. As a further check to ensure that frequency conversion of the two orthogonal channels was occurring, the phase of the QPSK signal was changed by pi/2 radians with respect to the pumps. This resulted in an interchange of the PRBS signals observed on the I and Q channels, whilst maintaining the same decorrelation. Figure 5shows the two inverted PRBS patterns of the I and Q channels recorded on the DSO. The non-inverted PRBS pattern underwent a logical inversion due to the XOR operation of the delay interferometer which inverts the PRBS but not the inverted PRBS data.
Eye diagrams were also recorded and are shown in Fig. 6. Due to a slow phase drift of the QPSK signal relative to the four pumps, measuring the eye diagrams directly was not possible. Because of this the eye diagrams were post-processed offline from 200 ns long patterns of the I and Q channels saved on the DSO. Clear and open eyes, with extinction ratios of ~20 dB and ~14 dB, were observed for the I and Q channels. A manual phase controller for the QPSK signal was created by connecting a variable voltage source to the phase modulator located after the QPSK modulator. This was able to adjust for large phase drifts between the pumps and the QPSK signal, however, due to the random slow phase drift caused by vibrations and thermal fluctuations in both paths further optimization of the pump amplitudes and phases was prohibited. The degradation of the eyes is believed to be a result of this non-optimal phase discrimination. A phase stabilization system similar to  or  could be implemented to counteract this.
This paper has described the first experimental demonstration of a phase discrimination technique which uses FWM in SOA to separate the two orthogonal phase components of QPSK signal into two BPSK signals. These BPSK signals were then demodulated using a DI and clear and open eye diagrams were observed for the I and Q channels respectively Extinction ratios of ~20 dB and ~14 dB were measured for the I and Q channels respectively. Conversion efficiencies of 9.3 and 9.1 dB were also observed relative to the input QPSK signal at the output of the SOA, showing one of the advantages of using SOA as a mixing element.
This work was funded by Science Foundation Irelandgrant 06/IN/I969and CTVR CSETaward 10/CE/I1853.
References and links
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