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We demonstrate a novel digitally tunable optical filter, which is based on dense wavelength division multiplexed (DWDM) thin film filters and semiconductor optical amplifiers (SOA). It has the advantages of fast tuning speed, large tuning range, good temperature stability, simple control mechanism. It is also scalable without bringing additional insertion loss. The passband wavelengths are in consistency with those suggested by ITUT.
©2005 Optical Society of America
1. Introduction
Tunable optical filters have important application in fiber optic communications and other optical fields. Tuning ability is usually achieved by introducing extra phase shift via the electro optic, thermo optic, or piezoelectric effect, etc. Up to date, the most commonly used tunable filter structures are interferometric such as Fabry-Perot (FP) and Mach-Zehnder (MZ) interferometers[1]. They are usually poor in the performance of passband flatness and the crosstalk, and require robust electronics to stabilize the central wavelength. Hence, they are rarely practically used as switching elements in the dense wavelength division multiplexing (DWDM) optical networks.
In recent years, digitally tunable optical filter (DTOF) [2] has received increasingly attention. Structures based on the arrayed waveguide grating (AWG) [3,4,5] and On-Off[2] switches have been reported. DTOF (digitally tunable optical filter) based on AWG (arrayed waveguide grating) may require precision thermal control for AWG (arrayed waveguide grating) usually suffers the problem of temperature dependence. In this letter, we proposed a novel approach to realize the DTOF (digitally tunable optical filter) based on DWDM thin film filters (TTF) and semiconductor optical amplifier (SOA). Apart from the outstanding feature being consistent with the wavelength comb suggested by international telecommunication union-telecommunication (ITU-T), the proposed DTOF is very stable in temperature performance. Furthermore it requires less SOAs in comparison with the reported structures [4,5]
2. Filter configuration and description
Figure 1 shows the experimental configuration of the proposed DTOF (digitally tunable optical filter). It consists of eight DWDM TTFs (thin film filters), six SOAs, one 1×2 splitter and one 4×1 combiner. Each of the TTFs (thin film filters) is used as 1×2 or 2×2 wavelength selection element, as depicted in Fig. 2, depending on the position in the structure. For the 1×2 structure, we used a dual fiber collimator as one pair of input/output fiber ports and a single fiber collimator as the second output port. All the wavelengths from the input fiber will be reflected to the dual fiber collimator output, except for the wavelength which is equal to the center wavelength, λi, of the TTF (thin film filter). This wavelength will pass through the filter and arrive at the second output. In the 2×2 structure, we used anther single fiber collimator as the second input. The work principle of the 2×2 structure is the same as the 1×2 one. Because the TTF (thin film filter) only allows one wavelength to pass through, the wavelengths of the two input ports are arbitrary, i.e., independent of each other. In the DTOF (digitally tunable optical filter) we developed, TTFs (thin film filters) of λ2, λ4, λ6, λ8 are set as 2×2 wavelength selection element, TTFs (thin film filters) of λ3, λ5, are set as 1×2 wavelength selection element, and TTFs (thin film filters) of λ1, λ7 function as optical filters. The eight TTFs (thin film filters) are connected in the order as shown in Fig. 1.
The input light to the DTOF (digitally tunable optical filter) is divided into two portions via the splitter and is firstly “selected” by SOA11 or SOA12. For example, when SOA11 is turned on (SOA12 is off), all the wavelengths will arrive at the thin film filter with center wavelength of λ2 and only one wavelength, namely λ2, will pass through the filter, all the other wavelengths will be reflected and arrive at the subsequent TTFs (thin film filters). SOA2i (i=1,2,3,4) performs the second “selection” of wavelengths. In the above case, λ2 will arrive at the output through the 4×1 combiner if SOA21 is on. The control table is listed in Table 1. In the table, “1” means that the SOA is turned “ON” and “0” means that the SOA is turned “off”. It can be seen that this structure allows the center wavelength to be arbitrarily selected among the eight TTF (thin film filter) wavelengths. For example, the DTOF (digitally tunable optical filter) is set at λ1 with SOA12 and SOA21 “ON”. The routing path for λ1 is as follows: Input (multiple wavelengths)→SOA12(ON)→λ4(Reflection)→λ5(Reflection)→λ8(Reflection)→λ1(Transmission)→λ2(Reflection)→SOA21(ON)→Output.
The above configuration can be expanded to larger size. The expansion will not bring additional insertion loss for every wavelength undergoes transmission or reflection at most five times (four reflections and one transmission), independent of the filter size (i.e., the number of wavelengths). Of course the TTFs (thin film filters) should be rearranged, i.e. configured as 1×2 or 2×2 structures according to the scale.
The number of SOAs to be used, which is important for cost consideration, is determined as follows. Suppose the number of wavelengths to be digitally tuned is N, where N=2n. The numbers of SOAs at the output side and the input are:
and,
respectively. The total number of SOAs is:
In comparison with the structure reported in Ref. [3] and Ref. [4], the proposed structure can save up 25% of SOAs for the same wavelength tuning range. The center wavelengths of the proposed DTOF are well in consistency with the ITU-T suggested ones. Furthermore it does not have the thermal stability problem as the arrayed waveguide grating does [6].
Table 1. The control table of the proposed DTOF
3. Performance measurement
Figure 3 shows the measured transmission spectrum of our DTOF (digitally tunable optical filter). The eight wavelengths range from 1552.54nm to 1558.14nm. Table 2 lists the insertion loss for each wavelength channel. The maximum difference of the insertion loss among all eight wavelength channels is 3.76dB. The drive current of each SOA during our test was set at 80mA for the “ON” state. The insertion loss difference can be compensated by optimizing the SOA drive current. We also measured the polarization dependent loss and the isolation. The polarization dependent loss is less than 0.4dB and the isolation is better than 40 dB. The relatively large polarization dependent loss is partially induced by SOA. Another contribution may arise from the angled incidence on TTF (thin film filters) for its 1×2 and 2×2 application, which may decrease via careful design. The tuning speed was tested and the result is shown in Fig. 4. The upper trace is the control signal to drive the SOA, and the lower trace is the optical response. It can be seen that the rise time and fall time of the optical response are 8.82 ns and 15.49 ns, respectively, and the delay time between the control signal and the optical response 31.07 ns. As is known, SOA has faster response than the measured results. This relatively larger rise/fall time and delay are mainly due to the poor response of the driving system which uses components at hand.
Table 2. The insertion loss for each wavelength channel
4. Conclusions and discussions
A novel structure of eight-channel digitally tunable optical filter based on thin film filters and semiconductor optical amplifiers has been proposed. The tuning speed is up to 8.82ns, the total insertion loss is between 2.14dB and 6.76dB. The isolation is over 40dB. The number of SOAs used in the proposed configuration is , where N is the number of tuned wavelengths. This means that the structure can save up 25% SOAs in comparison with the conventional digitally tunable optical filters. It can be expanded to large size with no additional insertion loss. Its wavelengths are well agreed with the defined ones by ITU-T and its temperature performance is stable. This kind of filter has potential application in wavelength selective switching based optical networks.
Acknowledgments
This work is partially supported by NSFC (ID: 90204006, 60377013), the Ministry of Education, China (ID:20030248035 and ID: 2003034258).
References and links
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4. M. Zirngibl, C.H. Joyner, and B. Glance, “Digitally tunable channel dropping filter/equalizer based on waveguide grating router and optical amplifier integration”, IEEE Photon. Technol. Lett. 6, 513–515 (1994). [CrossRef]
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6. Yoshinori Hibino, “Recent Advances in High-Density and Large-Scale AWG Multi/Demultiplexers With Higher Index-Contrast Silica-Based PLCs”, IEEE J. Sel. Top. Quantum Electron. 8, 1090–1101 (2002) [CrossRef]
