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A tunable, coherence-free, high-resolution microwave photonic bandpass filter, which is compatible to be inserted in a conventional fiber optic link, is presented. It is based on using two cross gain modulation based wavelength converters in a recursive loop. The double cross gain modulation technique solves the semiconductor optical amplifier facet reflection problem in the conventional recursive structure; hence the new microwave photonic signal processor has no coherent interference and no phase-induced intensity noise. It allows arbitrary narrow-linewidth telecommunication-type lasers to be used while enabling stable filter operation to be realized. The filter passband frequency can be tuned by using a wavelength tunable laser and a wavelength dependent time delay component. Experimental results demonstrate robust high-resolution bandpass filter operation with narrow-linewidth sources, no phase-induced intensity noise and a high signal-to-noise ratio performance. Tunable coherence-free operation of the high-resolution bandpass filter is also demonstrated.
©2012 Optical Society of America
1. Introduction
Photonic signal processing has attracted significant interest because of its high time-bandwidth product capabilities, its high sampling frequencies, electromagnetic interference immunity, and its ability to process high-speed signals directly within the optical fiber transport system [1].
Many photonic signal processing structures have been reported [2–15]. The conventional microwave photonic filters implemented using the delay line technique in the recursive structures [2,3] have the problems of coherent interference and phase-induced intensity noise (PIIN) [16]. The PIIN is the dominant noise source in the system, which significantly degrades the signal-to-noise ratio (SNR). We have reported several techniques, including the double-pass modulation technique [10], the Sagnac loop technique [11] and the delayed self-wavelength conversion technique [12], to overcome the coherent interference and the phase noise problems. However, they can only generate a limited number of taps, which limits the filter response resolution. A high-resolution, coherence-free and PIIN-free photonic filter can be realized using the frequency shifting technique [13] but this requires a large gain-bandwidth optical amplifier for the filter to operate at high frequencies. Furthermore, unlike the conventional recirculating delay line filter, the output of the frequency shifting recirculating delay line filter spreads over an optical frequency band with the bandwidth increases as the filter resolution and the filter operating frequency increase. This creates the difficulty of tuning the filter passband using the conventional wavelength tuning approach [3]. Recently, a tunable, high-resolution microwave photonic bandpass filter without PIIN has been demonstrated using stimulated Brillouin scattering (SBS) in an optical fiber [14]. However, it has high SBS noise [17], requires the modification of a fiber optic link configuration and has a fixed 3-dB bandwidth determined by the Brillouin bandwidth of around 20 MHz. Until now, there has been no report on a tunable, high-resolution, coherence-free microwave photonic filter that is compactible for insertion in a conventional fiber optic link without the need of modifying the link configuration.
The object of this paper is to present a new structure for realizing a high-resolution bandpass filter response without the coherent interference and phase noise limitations. It is based on using two cross gain modulation (XGM) based wavelength converters in a recursive structure. The double XGM based photonic signal processor can be operated at microwave frequencies with the bandwidth limited by the semiconductor optical amplifier (SOA) XGM bandwidth, which can be made large. Tuning the filter passband frequency can be realized by using a wavelength tunable laser and a wavelength dependent time delay component. The new signal processing structure is compatible for insertion in a conventional fiber optic link. Experimental results are presented that demonstrate a tunable, high-resolution, coherence-free and PIIN-free microwave photonic bandpass filter.
This paper is organized as follows. The new double XGM based microwave photonic signal processor is presented in Section 2. The problem of SOA facet reflection in the conventional recursive structure and the solution to overcome this problem are also described in this section. The analysis of the double XGM based microwave photonic filter transfer characteristic and the simulation results are presented in Section 3. Section 4 presents the experimental results that demonstrate a tunable, high-resolution, coherence-free and PIIN-free microwave photonic bandpass filter. Finally, conclusions are given in Section 5.
2. Double XGM based microwave photonic bandpass filter
The topology of the double XGM based microwave photonic signal processor is shown in Fig. 1 . It consists of an optical coupler connected in the way to form a recursive loop. The RF modulated optical signal with λ0 wavelength enters the loop and amplifies by an erbium-doped fiber amplifier (EDFA) before launching into the first XGM based wavelength converter, which is formed by a four-port optical circulator, a SOA, an optical isolator, a laser with λ1 wavelength and a fiber Bragg grating. The continuous wave light with λ1 wavelength from the laser is cross gain modulated in the SOA by the RF modulated optical signal, which enters the SOA in reverse direction [18,19]. The fiber Bragg grating with λ1 wavelength is used as an optical filter to filter out the optical signals and noise at all wavelengths except λ1. The cross gain modulated optical signal with λ1 wavelength after reflected by Grating 1 is then launched into the second XGM based wavelength converter, which has the same structure as the first XGM based wavelength converter. The laser in the second XGM based wavelength converter has λ2 wavelength. The continuous wave light with λ2 wavelength from the laser is cross gain modulated in the SOA by the RF modulated optical signal at λ1, which enters the SOA in reverse direction. The fiber Bragg grating with λ2 wavelength is used to filter out the optical signals and noise at all wavelengths except λ2. Half of the cross gain modulated optical signal with λ2 wavelength after reflected by Grating 2 is coupled out the recursive loop and is passed through an optical filter into the photodetector. The other half of the optical signal at λ2 is amplified by the EDFA and is launched into the first XGM based wavelength converter for wavelength conversion. The above process repeats continuously. The optical filter at the output of the recursive loop is used to filter out the RF modulated optical signal at λ0, i.e. to ensure only the cross gain modulated optical signal at λ2 into the photodetector.
Note that the structure shown in Fig. 1 has no closed optical loop. In other words, there is no optical signal circulating inside the recursive loop. Hence the structure shown in Fig. 1 does not generate any delayed optical signal. As such the double XGM based microwave photonic signal processor has no coherent interference and no phase-induced intensity noise. A narrow-linewidth telecommunication-type laser can be used as an optical source to obtain a stable output performance. On the other hand, the RF signal has a recursive loop through the SOA XGMs. Hence a high-resolution bandpass filter response can be obtained. The double XGM based microwave photonic bandpass filter shown in Fig. 1 can be inserted as a stand-alone processor unit at any location in a conventional fiber optic link, without any need to modify the link configuration.
It should be pointed out that a microwave photonic bandpass filter based on using a single XGM based wavelength converter in a recursive structure has been reported [15]. However, it suffers the SOA facet reflection problem. This is due to the semiconductor-air interface forms a facet at either end of the SOA. The facet acts as a mirror reflecting the optical signal. It can be seen from Fig. 2 that the cross gain modulated optical signal with λ1 wavelength is reflected by the rear facet of the SOA and is circulated inside the loop. This forms a closed optical loop. Hence, many unwanted delayed optical signals can be generated at the output causing the coherent interference and PIIN problems.
Fig. 2 Single cross gain modulation based microwave photonic bandpass filter with the unwanted optical signal path indicated by the dotted line.
The commercial nonlinear SOAs from the major SOA manufacturers such as Covega and CIP have low facet reflection of around −50 dB. Since the typical saturated gain of a commercial SOA is 10 dB, the double-pass SOA gain is 20 dB and hence the return loss of the SOA is around −30 dB when the SOA is operated in the saturation regime. It was found from the experiments that using either Covega SOA or CIP SOA to implement the single XGM based microwave photonic bandpass filter results in an unstable frequency response when narrow-linewidth telecommunication-type lasers are used. This is because experimental results show a low input optical signal power into the recursive loop and a high inside-loop EDFA gain are required to obtain a high-resolution bandpass filter response. The amplitude of the unwanted optical signal caused by SOA facet reflection is largely amplified when the gain of the EDFA inside the loop is high. This results in many unwanted delayed optical signals generated at the output into the photodetector causing coherent interference and PIIN. Therefore a coherence-free, PIIN-free, high-resolution bandpass filter response cannot be realized using the single XGM technique unless a SOA with no facet reflection is used. However, it does not exist. The experimental results in [15] show the PIIN generation in the single XGM based microwave photonic signal processor and an optical source with the linewidth larger than the filter frequency response free spectral range (FSR) is required to overcome the coherent interference problem.
The microwave photonic bandpass filter implemented using the double XGM technique can overcome the SOA facet reflection problem. This is because, with refer to the structure shown in Fig. 1, the unwanted optical signal at λ1 generated by facet reflection in the second SOA passes through Grating 2 without being reflected. Similarly the unwanted optical signal at λ2 generated by facet reflection in the first SOA passes through Grating 1 without being reflected. Therefore the unwanted reflected optical signals generated by SOA facet reflections do not couple out the loop and detect by the photodetector. Hence there is no coherent interference and no PIIN in the double XGM based microwave photonic bandpass filter.
3. Analysis and simulation results
The output of the double XGM based microwave photonic bandpass filter is given by
where κ is the optical coupler coupling ratio and x’(n) is the recursive signal from the laser with λ2 wavelength, which can be expressed aswhere G is the optical gain in the recursive loop, which is dependent on the EDFA gain and the gains of the XGM based wavelength converters, l1 and l2 are the loss of the optical circulator 1 and 2, γxy is the XGM wavelength conversion efficiency for converting an RF signal from λx to λy in the XGM based wavelength converter, and R1 and R2 are the reflectivity of Grating 1 and 2. Therefore the double XGM based microwave photonic bandpass filter output can be written asThe transfer function of the double XGM based microwave photonic bandpass filter can be obtained by applying z-transfer to (3) and is given bywhere z = exp(j2πTf), f is the RF frequency, T = (nL)/c is the delay time corresponding to the RF signal recursive loop length L, n is the fiber refractive index and c is the speed of light. This shows that there is one pole at κGl1l2γ21γ12R1R2 and a fixed zero at the origin.Since the cross gain modulated optical signal has a 180° RF phase difference to the input modulated optical signal, the RF signal phase remains the same after double XGM. The frequency response of the double XGM based microwave photonic signal processor is shown in Fig. 3 . It has the same shape as the conventional amplified recirculating delay line (ARDL) bandpass filter frequency response [2]. The filter 3-dB bandwidth can be controlled by adjusting the EDFA gain. The passband sharpness or the Q factor, which is defined as the FSR of the filter frequency response to the filter 3-dB bandwidth, is dependent on the pole location. A high-resolution, narrow-passband filter response is obtained as the pole approaches the unit circle. This shows high reflectivity fiber Bragg gratings are needed. The losses in the optical circulators can be compensated by controlling the EDFA gain. The XGM based wavelength converters need to have a high XGM wavelength conversion efficiency. The XGM wavelength conversion efficiency γxy is determined by the RF modulated optical signal power (pump power) and the continuous wave light power (probe power) into the SOA, and the SOA injection current. A high XGM wavelength conversion efficiency can be achieved by using a high pump power to saturate the SOA and a low probe power [18]. A high SOA injection current is also used to maximize the XGM wavelength conversion efficiency.
Fig. 3 The frequency response of the double cross gain modulation based microwave photonic bandpass filter (κ = 0.5, G = 2.062, l1 = l2 = R1 = R2 = 1 and γ01 = γ12 = γ21 = 0.98).
The filter passband frequency is determined by the RF signal recursive loop length. Therefore tuning the filter passband frequency can simply be done by changing the RF signal recursive loop length, which can be realized by inserting a wavelength dependent time delay component inside the recursive loop after the first XGM based wavelength converter and using a wavelength tunable laser in the first XGM based wavelength converter. The double XGM based microwave photonic bandpass filter implemented based on the discrete time optical signal processing technique has multi-passbands in the frequency response as the conventional ARDL filter. The unwanted passbands will limit the filter operating frequency range. However, techniques such as the Vernier effect [20] have been reported to suppress the unwanted passbands. This will significantly increase the filter frequency response FSR. Another factor that limits the operating frequency range of the double XGM based microwave photonic bandpass filter is the SOA XGM bandwidth. However, it can be made large, e.g. a SOA with a 3-dB XGM bandwidth of 40 GHz has been demonstrated [21]. Since integrated waveguide optical circulators [22], optical isolators [23] and optical filters [24] have been reported, the wavelength converter can be integrated onto a single substrate to form a small and compact device to be used in practical applications.
4. Experimental results
Experiments were set up as shown in Fig. 4(a) to verify the proof of principle for the new structure. A tunable external cavity laser, which had a linewidth of 150 kHz and a wavelength of 1540 nm, was used as the optical source. The continuous wave light from the optical source was intensity modulated by a quadrature biased electro-optic modulator. The modulator output was connected to a recursive loop formed by a 50:50 optical coupler, an instrument EDFA and two XGM based wavelength converters. Nonlinear SOAs from Covega were used for XGM wavelength conversion. The laser in the first XGM based wavelength converter was a tunable external cavity laser, which had a linewidth of 150 kHz and a wavelength of 1552 nm. Due to the lack of two four-port optical circulators, a three-port optical circulator and an optical filter were used instead of a four-port circulator and a fiber Bragg grating in the first XGM based wavelength converter. The optical filter had a center wavelength of 1552 nm and a 3-dB bandwidth of around 4 nm, which filtered out the unwanted optical signals at 1540 nm and 1546.3 nm caused by the SOA facet reflection. The laser in the second XGM based wavelength converter was a tunable external cavity laser, which had a linewidth of 500 kHz and a wavelength of 1546.3 nm. The fiber Bragg grating had a center wavelength of 1546.3 nm, a 3-dB bandwidth of 1 nm and a reflectivity of > 99%. Since the SOAs had a polarization dependent gain of 1 dB, polarization controllers were used to control the polarization states of the optical signals into the SOAs. Polarization controllers can be avoided by using polarization maintaining components to construct the recursive loop. A 3-nm bandwidth optical filter with a 1546.3 nm center wavelength was connected at the output of the recursive loop. The cross gain modulated optical signal at 1546.3 nm was detected by a photodetector. The filter transfer characteristic was measured using a network analyzer.
Fig. 4 (a) Experimental setup of the double cross gain modulation based microwave photonic bandpass filter. (b) Wavelength dependent time delay component for implementing the tuning operation in the double cross gain modulation based microwave photonic bandpass filter.
The RF modulated optical signal powers (pump powers) into the two SOAs were set to around 10 dBm and the powers of the continuous wave light from the lasers inside the wavelength converters (probe powers) into the two SOAs were set to around 2 dBm. The SOAs were operated at high driving current in order to achieve a high XGM wavelength conversion efficiency and to amplify the optical signal so that high optical signal power of around 10 dBm was obtained at the output of the wavelength converter. Figure 5 shows the superposition of three double XGM based microwave photonic bandpass filter responses measured at different time instants. The filter response was stable even the laser linewidths were much smaller than the filter FSR. This demonstrated the filter had no coherent interference effect. Figure 5 also shows the simulated frequency response of the double XGM based microwave photonic bandpass filter using the transfer function given in (4). Excellent agreement between the measurement and theoretical prediction can be seen. The filter Q factor is 105, which is much higher than the Q factor of 12 obtained using the single XGM technique [15]. The improvement in the Q factor is due to optimization of the system parameters by controlling the EDFA gain and the XGM wavelength conversion efficiency so that the pole is closer to the unit circle. The maximum stopband rejection level of the double XGM based microwave photonic bandpass filter is 36 dB, which is much higher than 18 dB obtained using the single XGM technique [15]. Note that the XGM wavelength conversion efficiency reduces as the frequency increases, which results in the reduction of the filter Q factor. This effect can be compensated by using a higher EDFA gain to maintain the same pole location when the double XGM based microwave photonic bandpass filter is operating at high frequencies.
Fig. 5 The superposition of three double cross gain modulation based microwave photonic bandpass filter responses measured at different time instants (solid) and the simulated double cross gain modulation based microwave photonic bandpass filter response (dotted). (a) Wideband response and (b) detailed section of the response around the filter passband frequency.
A 5 dBm RF signal at the filter passband frequency was applied to the electro-optic modulator for the SNR measurement. The optical power into the photodetector was −6.4 dBm. The RF signal together with the noise spectrum at the output of the double XGM based microwave photonic bandpass filter were measured on a spectrum analyzer as shown in Fig. 6 . Since the filter does not generate any delayed optical signal, it has no phase-induced intensity noise. The signal-spontaneous beat noise generated by the SOA and the EDFA was the dominant noise component. The noise power at the filter passband was measured to be −60 dBm for 100 kHz spectrum analyzer resolution bandwidth. The output RF signal power was −12.2 dBm. Hence the SNR was 97.8 dB/Hz, which is close to the frequency shifting amplified recirculating delay line bandpass filter [13]. The high SNR demonstrated the filter had no phase noise. Note that the SOAs and the EDFA used in the experiment were not a low-noise design. Using optical amplifiers with lower noise figure can reduce the amount of signal-spontaneous beat noise at the output of the double XGM based microwave photonic bandpass filter, which further increases the SNR.
Fig. 6 Measured noise spectrum and output RF signal of the double cross gain modulation based microwave photonic bandpass filter.
In order to show the photonic bandpass filter implemented using the single XGM technique has the coherent interference problem, the second XGM based wavelength converter shown in Fig. 4(a) was replaced by a 1 m long normal single mode fiber. The center wavelength of the optical filter at the output was tuned to pass the cross gain modulated optical signal at 1552 nm. Figure 7 shows the superposition of three single XGM based microwave photonic bandpass filter responses measured at different time instants. The filter response was unstable. The filter passband had over 10 dB fluctuations. Even we turned on the tunable laser coherent control function to broaden the laser linewidths, around 3 dB fluctuations in the filter passband was observed. This clearly demonstrated the coherent interference effect presented in the single XGM based microwave photonic bandpass filter.
Fig. 7 The superposition of three single cross gain modulation based microwave photonic bandpass filter responses measured at different time instants. (a) Wideband response and (b) detailed section of the response around the filter passband frequency.
To demonstrate the tunability of the double XGM based microwave photonic bandpass filter, the optical filter inside the recursive loop in the structure shown in Fig. 4(a) was replaced by a wavelength dependent time delay component. The wavelength dependent time delay component, which had the structure shown in Fig. 4(b), was formed by connecting the same wavelength channels of a multiplexer and a demultiplexer to each other through a length of fiber. The multiplexer and the demultiplexer had 3 dB insertion loss and 100 GHz channel spacing. The channel path lengths were designed to be different to each other. The EDFA connected to the wavelength dependent time delay component was used to compensate for the insertion loss of the multiplexer and the demultiplexer. The RF signal recursive loop length can be adjusted by changing the wavelength of the tunable laser in the first XGM based wavelength converter. The wavelength dependent time delay component also had the function of filter out the unwanted optical signals at 1546.3 nm and 1540 nm generated by the facet reflection of the SOA in the first XGM based wavelength converter. Three measured bandpass filter responses for different laser wavelengths of 1548.1 nm, 1549.7 nm and 1551.3 nm are shown in Fig. 8 . It can be seen from the figure that the filter had a sharp passband with a Q of excess 100. The passband amplitude remained the same as the passband frequency tuned. This demonstrated the tunability of the high-resolution photonic bandpass filter operating at microwave frequencies, which is free of coherence and phase noise limitations.
Fig. 8 Measured bandpass filter responses of the tunable double cross gain modulation based microwave photonic signal processor for different laser wavelengths of 1548.1 nm (solid), 1549.7 nm (dashed) and 1551.3 nm (dotted).
5. Conclusion
A tunable, coherence-free, high-resolution microwave photonic bandpass filter has been presented. It is based on the new double XGM technique implemented by using two XGM based wavelength converters in a recursive loop. This technique completely eliminates optical signal circulation inside the recursive loop due to the parasitic reflection at the SOA facet. The double XGM based microwave photonic signal processor can generate a bandpass filter response with the same shape as the conventional ARDL bandpass filter. The signal processing structure can be inserted as a stand-alone unit into the conventional fiber optic link without the need of modifying the link configuration. A wavelength dependent time delay component can be inserted into the signal processor for obtaining tunable high-resolution bandpass filtering by enabling the time delay corresponding to the RF signal recursive loop length to be varied. The double XGM based microwave photonic bandpass filter has no coherent interference effects or phase-induced intensity noise generation, and narrow-linewidth telecommunication-type lasers can be used with stable and robust filter operation. Experimental results, have demonstrated high Q and high stopband rejection level bandpass filtering with a high SNR of 97.8 dB/Hz. Results also demonstrate a tunable, high-resolution microwave photonic bandpass filter without coherence and phase noise limitations.
Acknowledgments
This work was supported by the Australian Research Council.
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