An all-optical incoherent scheme for generation of binary phase-coded ultra-wideband (UWB) signals is proposed and experimentally demonstrated. The binary phase coding is performed based on all-optical phase modulation in a semiconductor optical amplifier (SOA) and phase modulation to intensity modulation (PM-IM) conversion in a fiber delay interferometer (DI) that serves as a multichannel frequency discriminator. By locating the phase-modulated light waves at the positive and negative slopes of the DI transmission spectra, binary phase encoded UWB codes (0 and π) are generated. We also experimentally demonstrate a bipolar UWB coding system with a code length of 4, operating at 1.25 Gb/s. And the decoding is analyzed as well. Our proposed system has potential application in future high-speed UWB impulse radio over optical fiber access networks.
©2011 Optical Society of America
Ultra-wideband (UWB) systems specified by the Federal Communications Commission (FCC) can provide large bandwidth and high data rate under unlicensed spectrum from 3.1 to 10.6GHz . However, UWB signals can only transmit for short distance due to the extremely low radiation power less than −41.3dBm/MHz. In order to increase the area of coverage, it is desirable that the local UWB environments would be integrated into wired or wireless wide-range networks with UWB-over-fiber technology [2,3]. Therefore photonic manipulation (including generation, transmission, modulation, and encoding) of UWB signals is very attractive since it can be easily incorporated into UWB-over-fiber networks and eventually simplify the entire network [4–14].
Generally, UWB signals can be modulated by on-off keying (OOK), pulse position modulation, or binary phase-shift keying (BPSK). However BPSK signals give extra 3 dB in signal-noise ratio for any given noise level, and are of special interest for UWB-over-fiber systems. To date, several schemes have been proposed for generating BPSK by using cascaded fiber Bragg gratings and multi-laser sources [13,15], optical bandpass filters , polarization maintaining fibers associated with a polarizer , and relaxation oscillations of an optically injected distributed feedback laser . Especially, Ref . presented a high-chip-count phase coding using only one laser source with low costs. These schemes showed promising applications in code division multiple access (CDMA) technology for multiple user communications. However, all the phase-coded UWB signals were controlled by electrical data patterns, not by optical pulses.
In this paper, we present an incoherent approach to all-optical generation of BPSK UWB signals. The binary phase coding is performed based on all-optical phase modulation in a semiconductor optical amplifier (SOA) and phase modulation to intensity modulation (PM-IM) conversion in a fiber delay interferometer (DI) that serves as a multichannel frequency discriminator. By locating multiple phase-modulated light waves at the opposite slopes of the DI transmission spectra, a monocycle sequence with opposite polarities would be generated. We also experimentally demonstrate a CDMA-UWB coding system with a code length of 4, operating at 1.25 Gb/s. And a decoding approach is proposed and demonstrated as well. Different from the schemes mentioned above, the optical CDMA coding is controlled by optical data streams rather than electrical patterns, revealing better compatibility to all-optical networks.
2. Operation principle
The schematic of the proposed CDMA-UWB encoding system is shown in Fig. 1 . The continuous waves (CW) from a laser array with four wavelengths (λ 1~λ 4) are combined by an arrayed waveguide grating (AWG) and injected into a nonlinear SOA. Another optical Gaussian signal with its wavelength λ s, as a control light, is fed into the SOA to arouse cross phase modulation (XPM). The SOA serves as an all-optical phase modulator. Then the phase-modulated light waves are sent to the DI with each wavelength being located at the positive or negative slope of the DI periodical spectra. The DI serves as a multichannel frequency discriminator to achieve PM-IM conversion. Consequently, a pair of AWGs is used to separate each wavelength, delay for each channel, and recombine each channel to form binary phase-coded pulses.
Due to cross phase modulation (XPM) effect in the SOA, the optical field of CW probe signals after modulation can be expressed as
The output signal of the linear filter in frequency domain is given by , where E(ω) is the Fourier transform of E(t). Applying the inverse Fourier transform, the output probe light in time domain can be described by
The detected optical power is given by. From Eq. (2), we can deduce the output power in the form with
The first term on the right-hand side (RHS) in Eq. (3) is a DC term. If the optical carrier is located at the filter’s linear slope region with a small phase-modulation index, the third term on the RHS of Eq. (3) has much larger magnitude than the second term, so the second term can be neglected. Then the detected optical power is simplified by
3. Experimental demonstration and discussion
Based on the theory of CDMA coding, the code length should be at least N to accommodate N users. As a demonstration of multi-user communications, an all-optical bipolar UWB encoding system with a code length of 4 is experimentally investigated. The experimental setup for CDMA-UWB encoding system is also shown in Fig. 1. Four tunable laser diodes (LD1~LD4) are used as the laser array, whose wavelengths are 1554.1nm (CH1), 1555.75nm (CH2), 1558.17nm (CH3), and 1559.56nm (CH4), respectively. The wavelengths can be fine tuning with a resolution of 1pm. This is important to be well aligned to the DI spectrum slopes. The optical Gaussian pulse train is generated from a Mach-Zehnder modulator (MZM) driven by a bit pattern generator (BPG). The signal wavelength is fixed at 1563.5 nm. The repetition rate and the pulsewidth of the Gaussian pulses are fixed at 1.25GHz and 60ps, respectively. A commercial SOA (manufactured by CIP) is used for nonlinear optical signal processing with low polarization dependence (<0.5 dB) and biased 240 mA. Its 1/e gain recovery time is about 25 ps under the operation condition. Attenuators (ATT) are used to adjust the optical powers of input signals. The fiber-based DI reveals a comb spectrum, whose frequency shift can be adjusted by a voltage controller. The free spectral range (FSR) of the fiber-based DI is 40GHz. By properly tuning the probe wavelengths or tuning the spectral shift of the DI, both positive monocycle and its inverted copy can be achievable according to the filtering conditions. A pair of AWGs is used to separate the four CW beams with proper time delay and then combine them again. Each channel of the AWG has a bandwidth of 0.72nm. The temporal waveforms, electrical spectra, and optical spectra are analyzed through a digital communications analyzer (Agilent DCA86100C), electrical spectrum analyzer (Anritsu MS2668C) and an optical spectral analyzer (Anritsu MS9710C), respectively. In order to reduce the detrimental cross gain modulation (XGM) in the SOA, the powers of the CW beams are set at 5.2 dBm to make the SOA deeply saturated, and the peak power of Gaussian pulse is set at 4 dBm, a relatively low level.
Bipolar encoding is implemented by locating the wavelength of each laser at the positive or negative slope of the DI spectrum, to generate a positive or a negative monocycle pulse, marked “+1” or “-1” in the code sequence. We select Walsh-Hadamard codes as the orthogonal codes in our experiment. As for a code length of 4, the four orthogonal codes are. These four codes can be expressed by the corresponding Gaussian monocycle sequences. Figure 2(a) shows the output spectra of the binary phase-coded UWB signals. The power transmission of the DI is also shown in dash line for comparison. One can see that the wavelengths of CH1 and CH2 are located at the DI’s negative slope, but opposite are CH3 and CH4. Therefore, the monocycle polarities of CH1 and CH2 are different from CH3 and CH4, as shown in Fig. 2(b).
It should be noted that since the UWB monocycle pulses are obtained by introducing certain chirps to the probe frequency, the output monocycle pulses do not meet the transform-limited condition. On the other hand, the DI has a FSR of 40GHz, so the frequency span between the maximum transmission and the minimum transmission in one period is 20 GHz. The maximum acceptable chirps (red chirps and blue chirps) on the probe carrier are within 10 GHz, otherwise the generated monocycle will be distorted in the chirp-to-intensity mapping. In fact, the value of chirps is controllable since it is variable with the injected current of the SOA, input powers of the probe light and the Gaussian pulse, etc.
To state conveniently, we define the polarity of CH1 as “+1”, and CH3 as “-1”. By adjusting the proper time delay for each channel, different UWB codes can be obtained. For example, Figs. 3 (b)-(d) show the achieved UWB code C2-C4, respectively. To generate code C1, we need to fine tune the wavelengths of CH3 and CH4 to the negative slope of DI spectra. Then all monocycle pulses are “+1”, as shown in Fig. 3(a). The pulse duration of each code is about 600ps, so the modulation speed should be less than 1.67 Gb/s. Figure 4 shows the radio frequency spectra for the corresponding orthogonal UWB codes, which indicate that the generated signals in the frequency domain fit well within the FCC mask.
To demonstrate multi-user communications, an example is given by considering two users, User2 and User3, with code C2 and C3 as their signature codes. Two data sequences, D2 = ”011001” and D3 = ”010111”, are encoded by using the signature codes C2 and C3, respectively. The temporal waveforms of the two coded sequences (CS2 and CS3) are shown in Figs. 5(a) and 5(b), respectively. These data sequences can be designed and modified by the input Gaussian pulse sequences.
At the receiver, the UWB decoding can be achieved by correlation operation between the received data sequences and the signature codes that are pre-stored at the receiver. Since the encode signals are orthogonal, only a data sequence with the same signature code can be recovered through correlation. In a practical system, the correlation for UWB decoding should be implemented either in electrical domain [16,17] or in optical domain . The correlation in electrical domain means that both signals to be correlated are converted into electrical signals with a high speed photodelector, and then the electrical signals produce an intermediate difference frequency to achieve correlation output with electrical signals. The correlation in optical domain means that the correlation is performed in optical domain, without optical-to-electrical conversion. Due to the hardware restraint, we could not measure the real-time correlation either in electrical domain or optical domain. However, similarly to Ref. [13,14], the decoding process is done by calculating the correlation between the measured signal and the signature sequence. The measured signal is recorded from the digital communications analyzer with 1350 sampling points for duration of 5000 ps, and the signature sequence is the ideal monocycle sequence with the same code pattern in the experiment.
Figures 6 (a) and (c) show the correlation between CS2 and C2, and correlation between CS3 and C3, respectively. As we expected, the original data sequences D2 and D3 are successfully recovered. Figures 6(b) and (d) show the correlation between CS2 and C3, and correlation between CS3 and C2, respectively. One can see that the output amplitudes are trivial compared to that of Fig. 6(a). This proves that data sequence could not be recovered by different signature codes, and there is little interference between User2 and User3 due to the orthogonality of the codes. Therefore, our proposed scheme is feasible for multi-user communications.
The code length can be extended by using more wavelengths to improve the correlation results. However, to increase the number of tunable lasers is not feasible since it is high cost and bulky. A more effective approach is to explore incoherent multi-wavelength light sources, such as wideband amplified spontaneous emission (ASE) source.
We proposed an all-optical binary phase-coded UWB generation for multi-user communications using an SOA and a fiber-based DI. The SOA serves as an all-optical phase modulator, and the DI serves as a multichannel frequency discriminator. The UWB coding system is based on incoherent summation of bipolar Gaussian monocycle sequences, which are obtained by locating the wavelengths at the positive or negative slope of the DI transmission spectra to achieve PM-IM conversion. We experimentally demonstrate a CDMA-UWB coding system with a code length of 4, operating at 1.25 Gb/s. And a decoding approach is proposed and demonstrated as well. Especially, the optical CDMA coding is controlled by optical data streams rather that electrical patterns, revealing better compatibility to all-optical networks.
This work was partially supported by the National Basic Research Program of China (Grant No. 2011CB301704), the National Natural Science Foundation of China (Grant No. 60901006), and the Fundamental Research Funds for the Central Universities of China (HUST:No. 2010QN033).
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