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A novel photonic microwave notch filter with both negative and positive coefficients is proposed and demonstrated using a single optical source. These coefficients are generated in a highly nonlinear fiber by cross polarization modulation effect and guided through a high birefringence fiber, and finally detected. Due to the orthogonal polarity between the two coefficients, the proposed filter has a stable transfer response and no resonance peaks at baseband. The experimental results showed a stable notch filter characteristic with the free spectral range of 3.97 GHz over the range of 15 GHz.
©2006 Optical Society of America
1. Introduction
Photonic microwave filters (PMFs) have attracted great interest because they allow the processing of radio frequency signals in the optical domain with the advantages of wide bandwidth, immunity to electromagnetic interference, and low loss [1, 2].
Generally, PMFs have been implemented based on the concept of digital filter, in which the output is given by summing the delayed ones of the input signal with a coefficient assigned to each one. These PMFs are classified into two types: coherent filter and incoherent filter depending on their coherent characteristics. Coherent PMFs [3] use a single optical source and, therefore, the delayed one of the input signal has a deterministic optical phase relationship with the rest at the input of the photodetector providing that the time delay between the adjacent delayed signals is not larger than the coherence time. Thus, any slight change to the delayed signals is likely to affect the filter response because it uses optical interference. By contrast, when the time delay goes beyond the coherence time, the optical phase relationship between the adjacent delayed signals becomes completely random and, as a result, the filter operates in an incoherent regime. This filter is free from environmental perturbations and a stable filter response is obtainable. The only drawback is that the obtainable coefficient to each delayed signal is only positive in principle. This limits the possibilities of implementing various transfer functions, showing smooth transitions from the bandpass region to the band suppressed region, and also has a resonance peak at the baseband [1].
Various studies on the filters which are less sensitive as well as with negative coefficients have been reported [1]. One way to obtain a stable filter characteristic is to operate the filter in an incoherent regime by increasing the time delay over the coherence time. However, this reduces the achievable free spectral range (FSR). Recently, it has been reported that the PMFs using high birefringence (Hi-Bi) fiber could be free from the limitation imposed by optical coherence [4]. However, the filter supports only positive coefficients.
To implement negative coefficients in incoherent PMFs, various methods have also been presented, such as an optoelectronic approach using differential detection [2, 5], wavelength conversion using cross gain modulation in a semiconductor optical amplifier (SOA) [6], and complementary modulation using a dual output Mach-Zehnder modulator [7]. However, for differential detection, duplicated components are required for positive and negative coefficients. The wavelength conversion using SOAs is limited in the available bandwidth. The approach using a dual output Mach-Zehnder modulator requires a specially packaged modulator and its transfer response is also not free from the coherent length.
In this paper, we propose and demonstrate a novel photonic microwave notch filter with a negative coefficient together with a positive coefficient, based on cross polarization modulation (XPolM) in a highly nonlinear fiber. This fiber-based technique has quasi-unlimited bandwidth property because of the ultra fast frequency response of optical fiber [8]. Moreover, the positive and negative coefficients are orthogonal to each other and, as a result, stable and large FSR is obtainable regardless of the coherence length of the optical source.
2. Principle of operation
A schematic describing the basic principle of the operation is shown in Fig. 1. The inset shows direct-form realization of a digital filter system. The input signal is split into two, a0 and a1 And, a1 is delayed by the time delay (Td) and combined with a0. If a1 has an inverse form of a0, then a negative coefficient can be obtained. In our experiment, both positive and negative coefficients are generated inside a highly nonlinear fiber by using an XPolM effect. The modulated optical signal to be filtered is carried on the pump beam (λ1) and the probe beam (λ2) operates in continuous wave (CW) mode. These two beams are polarized at the fiber input with a 45° angle between their directions of polarization. When the pump beam is in ‘on’ state, the probe polarization is switched to x-axis from y-axis due to the pump-induced birefringence [9]. As a result, the inverted and non-inverted signals appear at y- and x-axis, respectively, and can be used as negative and positive coefficients.
Fig. 1. A schematic to describe the photonic microwave notch filter with both coefficients, Inset: direct-form realization of a digital filter system. HNLF: highly nonlinear fiber, BPF: optical bandpass filter, PSD: polarization selective device, PD: photodetector.
To implement a notch filter, each coefficient is separated by using a polarization selective device like a polarization beam splitter. One of the coefficients is optically delayed, and detected with a photodetector. Due to the orthogonal polarity between the two coefficients, the detected output does not include any optical interference components. The state of polarization of each coefficient is required to be preserved during the propagation until they are combined. In practice, it is not easy to preserve orthogonal property and, as a result, some noise components are included in the filter characteristic. To overcome this drawback, we used a high birefringence (Hi-Bi) fiber with fast and slow axes as a transmission medium, providing a time delay due to the difference in propagation velocity along the two axes. The time delay is controlled by the length of the Hi-Bi fiber, being associated with the FSR of the filter to be implemented. Furthermore, this scheme is more useful for a short optical delay (i.e., large FSR).
3. Experiments
The experimental setup is shown in Fig. 2. A tunable laser source (TLS1) with the linewidth of 200 kHz was used as a probe beam. Its wavelength was 1545 nm and the optical power was amplified by an erbium-doped fiber amplifier (EDFA1) up to 12 dBm. For the pump beam, a TLS2 with the center wavelength of 1550 nm was used and modulated using a LiNbO3 electro-optic modulator (EOM) with a radio frequency (RF) signal from either network analyzer for frequency scanning, or a pulse pattern generator for data transmission. The modulated signal was amplified by an EDFA2 up to 20 dBm to make the inverted and non-inverted signals orthogonal to each other. The probe and pump beams were combined by a 3-dB optical coupler. Then, the polarizations of the probe and pump beams were adjusted by the polarization controllers, PC1 and PC3, to generate an XPolM effect, as described in Fig. 1. The combined beams were fed into a HNLF (OFS, Denmark) with a Kerr coefficient (γ) of 10.4 (W∙km)-1 and a length of 1.5-km. At the output of the HNLF, only the probe beam was selected by an optical bandpass filter with a 3-dB bandwidth of 0.2 nm. The PC4 was used to match the polarization state of the inverted and non-inverted probe beams into the slow and fast axes of the Hi-Bi fiber, respectively. Then, two probe beams were propagated through a 200-m Hi-Bi fiber (Fujikura SM15-PS-U25A, beat length=4.1-mm @ λ=1550nm). From the fiber length, the time delay was about 252 ps. This time delay is extremely small compared to the coherence time (5 μs) of the TLS1 itself. Finally, the output was detected by the photodetector.
Fig. 2. Experimental setup. TLS: tunable laser source, PC: polarization controller, AMP: broadband electrical amplifier, EOM: LiNbO3 electro-optic modulator, EDFA: erbium doped fiber amplifier, Hi-Bi: high birefringence fiber.
4. Results and discussion
The feasibility of generating both positive and negative coefficients from the proposed scheme was investigated by using a tunable linear polarizer after a Hi-Bi fiber. The signal which has 10-Gb/s data patterns of ‘10111100’ was employed and their final outputs were measured with a sampling oscilloscope. The upper section of Fig. 3 shows the non-inverted signal which has data patterns of ‘10111100’. By rotating the transmission axis of a tunable linear polarizer up to 90°, the inverted signal which has data patterns of ‘01000011’ was obtained as shown in the lower section of Fig. 3. As expected, the time delay between the two signals was observed to be about 252 ps due to birefringence of the Hi-Bi fiber. The amplitude of the signals was controlled by adjusting the intensity of the pump beam. It is confirmed that the signal carrying on the pump beam was copied to parallel and orthogonal polarization components of the probe beam as the complemented format at the same time. These signals are used as the positive and negative coefficients necessary for notch filter.
To measure the FSR of the filter, the RF signal with the electric power of 12 dBm as an input signal was swept from 40 MHz to 15 GHz and its transfer response was plotted in Fig. 4, which shows the typical characteristic of a notch filter with sharp dips. The measured FSR was 3.97 GHz, as expected from theoretical calculations with the optical time delay of 252 ps. Due to the frequency characteristics of the electrical amplifier (Agilent 83006A) and electrical cables (Sucoflex 102E) used, peak levels in the transfer response were decreased slightly with an increase in the scanning frequency. Also, with the help of the polarization preserving characteristics of the Hi-Bi fiber, the filter could be operated in a stable condition even though the time delay is very short compared to the coherence time of the optical source. Also, no resonance peaks at baseband have been observed in the filter.
Figure 5 shows the influence of the probe wavelength to FSR. When the probe wavelength was changed from 1525 nm to 1575 nm with the step of 10 nm, FSR was almost constant for the 200-m-long Hi-Bi fiber and showed a small fluctuation within the measurement error range. This is because the time delay between two propagation velocities along the axes is almost independent of the operating wavelength and can be ignored.
Finally, to investigate the linearity of the proposed photonic microwave filter, the spurious-free dynamic range (SFDR) measurement was carried out with two tone electrical signals of 3 (f1) and 3.01 GHz (f2). These signals were fed to the EOM which was biased at the half power (quadrature) point. After generating an XPolM effect, the converted electrical spectrum was measured using a spectrum analyzer (Anritsu MS2668C). During the SFDR measurement, a linear low-noise amplifier with 23-dB RF gain was used to enhance the noise floor of the spectrum analyzer.
The IMD3 signals appeared at the frequency of 3.02 GHz (2f2- f1) and 2.99 GHz (2f1- f2). As shown in Fig. 6, the fundamental and IMD3 signal powers had a slope of 1 and 3 with respect to the RF input power. The SFDR was 96.8 dB-Hz2/3, which meets the SFDR requirement for the real application of analog fiber-optic links [10].
Fiber-based nonlinear devices are still less stable because of their relatively long length and inherent characteristics. However, such instability is able to be alleviated by using special fibers such as a HNLF with an ultra high Kerr coefficient [11] or a temperature insensitive photonic crystal fiber [12].
5. Conclusion
We have proposed and demonstrated a novel photonic microwave notch filter with negative and positive coefficients. These coefficients were simultaneously obtained by cross polarization modulation in a highly nonlinear fiber. As a result, no resonance peaks were observed at baseband. Due to the orthogonal property between polarizations of the two coefficients, a stable filter with large FSR can be achieved independently of coherent length. From the measured frequency response of the filter, FSR was 3.97 GHz from the single source with the linewidth of 200 kHz over the frequency range of 15 GHz.
Acknowledgements
The authors would like to thank Dr. Chul Soo Park for his useful discussions. This work was supported by grant No. B1220-0601-0014 from the University Fundamental Research Program of the Ministry of Information & Communication in Republic of Korea.
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