We describe 1 Gsymbol/s, 64 and 128 coherent quadrature amplitude modulation (QAM) transmissions over 150 km, in which we employ a frequency-stabilized C2H2 fiber laser, an optical phase-looked loop (OPLL), and a heterodyne detection circuit.
© 2008 Optical Society of America
Improving the spectral efficiency of WDM systems is very important if we are to increase the total capacity of optical transmission systems. Recently multi-level phase-shift keying (PSK) or a combination of PSK and amplitude-shift keying (ASK) have become attractive candidates for this purpose because multi bit information can be transmitted by 1 symbol data [1–4]. S. Tsukamoto et al. showed that a spectral efficiency of 2.5 bit/s/Hz can be obtained by using a polarization-multiplexed quadrature phase-shift keying (QPSK) signal . S. Hayase et al. proposed an 8-state (3 bit) per symbol optical modulation/demodulation scheme with a combination of QPSK and binary ASK . N. Kikuchi et al. also demonstrated 32-state (5 bit) per symbol optical transmission with quadrature ASK and 8-differential PSK . K. Kikuchi demonstrated an 8-PSK system with a phase-diversity optical homodyne receiver .
Coherent quadrature amplitude modulation (QAM) is also an alternative way of increasing the spectral efficiency in optical communication . QAM is a modulation format that combines two carriers whose amplitudes are modulated independently with the same optical frequency and whose phases are shifted by 90 degrees with respect to each other. These carriers are called in-phase carriers (I) and quadrature-phase carriers (Q). The QAM can assign a 2N-state by using I and Q, which is called 2N QAM. Here, N bits can be transmitted by 1 symbol data [6, 7].
In this paper, we describe in detail QAM coherent optical transmission achieved by using a heterodyne detection circuit with a frequency-stabilized fiber laser and an optical phase-locked loop (OPLL). Since the output beam of our frequency-stabilized laser has no frequency modulation, it is very useful as a light source for coherent communication and precise optical measurement. The frequency stability of the laser is as high as 1.3×10-11 for an integration time of 1 s and the linewidth is as narrow as 4 kHz. Using this coherent laser source, we have succeeded in 1 Gsymbol/s, 64 and 128 QAM coherent transmissions over 150 km.
2. QAM coherent optical transmission system
Figure 1 shows a block diagram of a QAM coherent optical transmission system. The QAM signal can be easily generated by adopting a complex MZ modulator, which consists of two MZ modulators and their combiner (coupler) with a 90 deg phase difference . This modulator is called an IQ modulator. A transmitted QAM signal and a local oscillator (LO) signal are heterodyne-detected with a PD. Then, the optical QAM signal is converted to an intermediate frequency (IF) signal. The IF signal data is first A/D converted and accumulated in a digital signal processor (DSP). All digital signals are demodulated by software in the DSP. That is, this transmission system operates in an off-line condition.
A coherent optical transmission at an optical carrier frequency requires a stable and fixed optical frequency difference between the transmitter and the LO if we are to obtain a stable IF signal. Therefore, a frequency-stabilized laser is indispensable for this kind of modulation scheme. Here, a C2H2 frequency-stabilized fiber laser is employed as the transmitter. In addition, an OPLL technique [9, 10] using a high-speed free running laser as an LO is also very important as regards the automatic frequency control of the IF carrier. In this section, we describe these key components of the QAM coherent transmission.
2.1 C2H2 Frequency-stabilized erbium-doped fiber ring laser
Figure 2 shows the configuration of a 13C2H2 frequency-stabilized erbium-doped fiber ring laser  used as a transmitter. The laser has two main parts. One is a tunable, polarization-maintained single-frequency fiber ring laser. The other is a laser frequency stabilization unit. The fiber laser is composed of a 1.48 µm InGaAsP LD, a polarization-maintained (PM) EDF, a wavelength-division multiplexing (WDM) coupler, an output coupler, a PM optical circulator, and a 1.5 GHz ultra-narrow PM fiber Bragg grating (FBG) filter . The FBG filter makes it possible to realize single-frequency operation by selecting only one longitudinal mode among many oscillation modes. The laser has two kinds of frequency controllers. One is a drum-type PZT with EDF wound around it. The other is a multi-layer PZT (MLP) on which an FBG is laid. When these controllers operate synchronously with a phase sensitive detection circuit, the laser frequency is continuously tuned over 2 GHz without mode hopping.
In the external frequency stabilization unit, we employed a phase sensitive detection circuit with an LN frequency modulator, a 13C2H2 (3 Torr) cell, a photo detector (PD), a double balanced mixer (DBM), an electrical amplifier, and a low pass filter (LPF). The feedback circuit consisted of proportional and integral (PI) controls. With these setups, we can detect frequency deviations of the fiber laser from the center frequency of the P(10) linear absorption line, which has a center wavelength of 1538.8 nm and a spectral width of 500 MHz. The DBM generates a voltage error signal that is proportional to the frequency deviation, and the error signal is fed back to the PZT to control the laser frequency.
The laser output power was 4.5 mW for a pump power of 200 mW. The frequency stability of the laser estimated from the square root of the Allan variance  was 1.3×10-11 (2.6 kHz) for an integration time of 1 s. The linewidth measured with the delayed self-heterodyne detection method  was approximately 4 kHz.
2.2 OPLL for coherent transmission
Figure 3 shows our experimental setup for OPLL. This system is composed of an LO, a PD, a DBM, a synthesizer, and two feedback circuits. A 13C2H2 frequency-stabilized fiber ring laser is used as a transmitter, and a free running fiber laser is used as an LO whose configuration is almost the same as that of the transmitter except that an LN modulator was used for the high-speed tracking of the IF signal. The LO linewidth is also approximately 4 kHz. The signal from the transmitter is heterodyne-detected with the LO signal. The phase of the beat signal (IF signal: fIF=|ftrans-fLO|) is compared with the phase of the reference signal from the synthesizer (fsyn) by the DBM and the difference between them is fed back to the LO through the feedback circuits. The phase noise of the OPLL is mainly dominated by the loop bandwidth. The bandwidth of the fiber laser shown in Fig. 2 was determined by the response characteristic of the PZT, and therefore we improved the LO response time by using an LN modulator with an FM bandwidth of up to 1 GHz. We also used a PZT tuner to compensate for slow frequency drifts caused by a temperature fluctuation. These two feedback circuits have loop-filters with different bandwidths. One is a broadband filter (~1 MHz) for fast frequency tuning with the LN modulator, the other is a narrowband filter (~10 kHz) for slow frequency tuning with the PZT. The extracted IF signal is described in detail in section 3.1.
2.3 Digital signal processor
Figure 4 shows our experimental setup for the DSP. The IF signal data is first A/D converted and accumulated in a high-speed digital scope, whose sampling frequency, bandwidth, and vertical resolution are 40 Gsample/s, 12 GHz, and 8 bit, respectively. Then I and Q data are demodulated using a piece of software by multiplying synchronous cosine and sine functions, respectively, onto I+Q data. Finally the demodulated data is converted into binary data in the software decoder. Here the center frequency of the IF signal is determined by the operating frequency of the synthesizer used in the OPLL circuit. In this off-line system, we send the frequency information to the DSP, and a clock signal used for the IQ demodulation is generated by software processing.
3. QAM coherent optical transmission results
3.1 1 Gsymbol/s, 64 QAM (6 Gbit/s) coherent transmission
Figure 5 shows our experimental setup for a 1 Gsymbol/s, 64 QAM coherent optical transmission over 150 km . A coherent signal from a frequency-stabilized fiber laser passes through an EDFA and is then divided and input into two arms. One part is coupled to a QAM modulator. The other is coupled to an optical frequency shifter, which feeds a frequency up-shift of 2.5 GHz to the original optical carrier. Then the frequency-shifted beam is used as a pilot tone that tracks the optical phase of an LO under OPLL operation. We set the power of the pilot tone at the minimum level that could achieve stable phase locking in the OPLL circuit. The pilot tone power was 4 dB lower than the QAM signal power. The polarization of the pilot tone was set so that it was orthogonal to that of the QAM signal. Therefore, no additional optical bandwidth for the pilot tone is needed for signal detection with a polarization diversity receiver. I and Q data are simultaneously fed from arbitrary waveform generators (AWGs) to the QAM modulator. Here, the pattern length of the QAM signals is 4096 (27×32) symbols. After optical amplification with an EDFA, a 64 QAM signal combines with the pilot tone and these signals are coupled to a 150 km-long transmission fiber (DSF 75 km×2 spans) with a coupled power of -5 dBm. After the transmission, the QAM signal is heterodyne-detected with an LO signal (fLO) whose phase is locked to the pilot tone (fOFS). The phase of the beat signal (|fOFS-fLO|) is compared with the phase of the reference signal from the synthesizer (fsyn) by using a DBM, and the phase difference is converted to a voltage error signal and fed back to the LO through the feedback circuits. After passing through a polarization controller (PC4), a polarizer (Pol) and a 2 GHz FBG optical filter , the transmitted QAM signal and the LO signal are heterodyne-detected with a PD. The power level of the LO coupled to the PD is -1 dBm. Then, the optical QAM signal is converted to an IF signal and demodulated by the DSP. Here it takes about 1 s for the DSP processing.
Figure 6 shows the electrical (IF) spectrum of a beat signal between the pilot tone and LO signals under PLL operation. The beat frequency was set at 1.5 GHz. The linewidth of the spectrum was less than the 10 Hz frequency resolution of the electrical spectrum analyzer. The phase noise estimated by integrating the single sideband noise power spectrum was 6.1×10-3 rad, which is sufficiently small to achieve 64 QAM.
Figure 7 shows the electrical spectrum of the IF data signal. The inset figure shows the relationship between the optical frequency and the QAM data-modulated signal, pilot tone, and LO signal. To prevent the influence of extra beat signals on the demodulation of the IF signal, the polarizations of the data signal and pilot tone were set so that they were orthogonal to each other, and the pilot tone to the data processing arm was removed with a polarizer (Pol) before being combined with the LO signal as shown in Fig. 5. Here the demodulation bandwidth was set at 2 GHz in the optical domain by using a narrowband FBG filter.
Figure 8 shows the bit error rate (BER) characteristics for this experiment. The word length of the QAM signal was limited to 4096 symbols, which corresponds to a BER of up to 4×10-5. The power penalty in Fig. 8 was approximately 3 dB, and no error was observed for a word length of 4096 when the received power level was above -27 dBm. The back-to-back constellations and eye patterns for the 64 QAM signal are shown in Fig. 9. Figure 9 (a-1)–(a-3) show constellations for received power levels of -27, -30, and -33 dBm, respectively. Above -30 dBm, a clear constellation was obtained. Eye patterns for I and Q are shown in Fig. 9 (b-1)–(b-3) and (c-1)–(c-3), respectively. Figure 10 shows the constellations and eye patterns after a 150 km transmission. Figure 10(a)–(c) correspond to received power levels of -24, -27, and -30 dBm, respectively. When the received power was -30 dBm, as shown in (a-3)–(c-3), the constellation points overlapped each other and the eye opening was greatly reduced. These results indicate that above -27 dBm, 1 Gsymbol/s, 64 QAM (6 Gbit/s) data were successfully transmitted over 150 km in a 2 GHz optical bandwidth.
3.2 Polarization-multiplexed 1 Gsymbol/s, 64 QAM (12 Gbit/s) coherent optical transmission
Polarization-multiplexing is a simple way of increasing the spectral efficiency. By introducing polarization-multiplexing in a 1 Gsymbol/s 64 QAM transmission, we achieved a 12 Gbit/s data transmission over 150 km in an optical bandwidth of 2 GHz . Figure 11 shows the experimental setup. The polarization of the pilot signal is set so that it is the same as one of the polarization axes in the two QAM signals. In this polarization-multiplexed system, the pilot signal must be separated spectrally from the QAM signal in order to separate the pilot from the QAM signals at the receiver. Thus the frequency of the pilot signal is set so that it is 2.5 GHz from the center frequency of the QAM signal. After optical amplification with an EDFA, the two orthogonally polarized 64 QAM signals are combined with the pilot signal and these signals are coupled into a 150 km-long transmission fiber (DSF 75 km×2 spans) with a coupled power of -11 dBm. The coupled power is less than that used with single polarization to remove the cross phase modulation. After the transmission, the two QAM signals are heterodyne-detected with an LO signal whose phase is locked to the pilot signal. Since the LO polarization can be arbitrarily rotated with a polarization controller, two orthogonally polarized QAM signals can be independently detected. The power level of the LO coupled to the PD is increased to 1 dBm.
Figure 12 shows the BER characteristics of a polarization-multiplexed 64 QAM transmission. It is interesting to see that the parallel polarization between the QAM and the pilot signals has very little effect on the 150 km transmission. This is because the pilot signal power was set at the minimum level (-12.8 dBm) for stable OPLL. The power penalty for both polarization data was approximately 3 dB, but no error was observed up to 4×10-5 when the received power level was above -29 dBm. Figure 13(a)–(c) are the constellations and eye patterns for the back-to-back case and after 150 km transmissions with orthogonal and parallel polarizations, respectively. Clear constellations and eye patterns were obtained at a received power of -26 dBm for both polarization data. These results indicate that the data speed was successfully doubled by using a polarization-multiplexing technique.
3.3 Polarization-multiplexed 1 Gsymbol/s, 128 QAM (14 Gbit/s) coherent optical transmission with Nyquist filter
In a microwave system, the bandwidth reduction of a data signal plays an important role in increasing spectral efficiency. The Nyquist filter has been widely used in microwave transmission systems to reduce the signal bandwidth without intersymbol interference [17, 18]. Here, we employed a Nyquist filter in a transmitter. In addition, by improving the low-frequency flatness of the IQ modulator response, we increased the QAM data multiplication level from 64 to 128.
The experimental setup we used for a polarization-multiplexed 1 Gsymbol/s, 128 QAM coherent optical transmission over 160 km is the same as in Fig. 11 except that we replaced the 150 km DSFs with 160 km SMFs whose effective area is approximately twice of that of the DSFs. By using a raised-cosine Nyquist filter with a roll-off factor of 0.35, the bandwidth of the QAM signal was reduced from 2 GHz to less than 1.4 GHz. The two orthogonally polarized 128 QAM signals are combined with the pilot signal and these signals are coupled into a 160 km-long SMF with a coupled power of -6.5 dBm. By replacing DSFs with SMFs, the coupled power can be increased without causing nonlinear phase rotation in the transmission fiber .
Figure 14 shows the electrical spectrum of the IF data signal when the polarization of the QAM signal is orthogonal to that of the pilot signal. The inset figure shows the relationship between the optical frequencies of the QAM data-modulated signals, the pilot signal and the LO signal. Here the demodulation bandwidth was set at 1.4 GHz in the optical domain by using a narrowband FBG filter. Almost the same spectrum was obtained for the parallel data condition.
Figure 15 shows the BER characteristics of a polarization-multiplexed 128 QAM transmission. The power penalty was approximately 3 dB for both polarizations, but no error was observed up to 3.5×10-5 when the received power level was above -29.5 dBm. These results indicate that polarization-multiplexed 1 Gsymbol/s, 128 QAM data were successfully transmitted over 160 km at a received power level of more than -29.5 dBm. Figure 16 shows the back-to-back constellations and I and Q symbol eye patterns for a 128 QAM signal whose polarization is orthogonal to the pilot signal. Figure 16(a)–(c) correspond to received power levels of -29.5, -32.5, and -35.5 dBm, respectively. Above -32.5 dBm, we observed a clear constellation and a clear eye opening for I and Q, respectively. We obtained almost the same back-to-back performance for the other QAM signal whose polarization was parallel to the pilot signal. Figure 17 shows the constellations and eye patterns after a 160 km transmission, where the polarization of the QAM signal is orthogonal to the pilot signal. Figure 17(a)–(c) correspond to received power levels of -26.5, -29.5, and -32.5 dBm, respectively. When the received power was set at -29.5 dBm, the constellation points overlapped and the eye opening was reduced as shown in Fig. 17(b). The constellation and the eye opening were worse for a received power of -32.5 dBm. Almost the same data were obtained for the parallel data condition.
As a result, we achieved a 14 Gbit/s data transmission over 160 km in an optical bandwidth of 1.4 GHz. For multi-channel transmission, we need only one pilot tone signal to obtain stable IF signals when we adopt carrier signals whose phases are locked together. In this case, the spectral efficiency is approximated by dividing the bit rate of each channel by the channel spacing. However, a guard band may be needed to realize a WDM system because of the channel crosstalk caused by nonlinear interactions such as four-wave mixing.
We have described in detail QAM coherent optical transmission using heterodyne detection with a frequency-stabilized fiber laser and OPLL. The frequency stability of the laser was 1.3×10-11 and the linewidth was 4 kHz. Using the coherent laser and a newly developed OPLL, 1 Gsymbol/s, 64 and 128 QAM signals were successfully transmitted over a 150 km fiber. These results indicate that, with respect to coherency, we can handle optical beams in the same way as microwaves. Even better results may be expected than with conventional QAM wireless transmission or metallic cable transmission because optical fibers have wider bandwidths and no fading noise.
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