We propose a method to flatten Raman gain by using a polarization independent filter. It is formed by a fiber loop mirror contains a number of high birefringence fiber sections and polarization controller. The gain profile is observed for one and two sections of polarization maintaining fiber (PMF) with different lengths. By using single pump wavelength at 1455 nm and 5cm length of Hi-Bi FLM, we managed to flatten Raman gain to within ±0.5 dB over a bandwidth of 43 nm at the center wavelength of 1555 nm. By using two sections of Hi-Bi Fiber and longer PMF, which are 5 cm and 8.5 cm PMF in Hi-Bi FLM, we are able to flatten Raman gain to within ±0.5 dB over a bandwidth of 38 nm at the center wavelength of 1555 nm with lower gain suppression from its original gain profile.
© 2003 Optical Society of America
Raman Amplifier has been given much attention due to its broad bandwidth and flexible centre wavelength compared to EDFA. The Optical Signal to Noise Ratio (OSNR) is maximized when the signal variation in fiber is minimal for a fixed nonlinear weight. Therefore distributed Raman amplification is employed . One of the major issue in Raman Amplification for WDM is that its gain variation with respect to wavelength. A mathematical approach has been done in order to efficiently flatten Raman gain by using multi pump wavelengths . Furthermore, a broadband Raman amplifier in excess of 100nm bandwidth can be produced using high power pump with multiple wavelengths. This can be done by using 12-wavelength channels WDM Laser Diode pump unit . Raman gain can also be equalized by using two wavelengths high power pump in series with Long Period Fiber Grating (LPG) filter, which enables us to have 22.8 nm flattened gain .
Recently, there have been some investigations on the effect of polarization dependent gain (PDG) and loss (PDL) on all optical ultralong communication system. As Raman Amplification is polarization dependent phenomena [5,6], some studies have been done in order to minimize the polarization effect of Raman Amplifier, such as using depolarized pumping through polarization scrambling, or controlling the degree of polarization of the pump laser. In this paper, we report another method that can be used to flatten Raman gain with putting some considerations on the polarization effect. This is done by using optical filter, which has polarization independent property. A polarization independent optical filter has been proposed based on Sagnac Interferometer and the non-reciprocity of the birefringence to the polarization states . This interferometer has a strong noise rejection ability, which makes it widely used for signal stabilization in fiber optic sensors. The Sagnac interferometer is used to replace the polarizers in conventional birefringence filter . The general structure of Sagnac Birefringence Filter is shown in Fig. 1 below:
The coupler is a low loss 3dB coupler and the Sagnac loop contains an arbitrary chain of linear birefringence elements. If input polarization state is assumed to be: , according to the reciprocity of the Jones matrix, the output field will be given by: , where: Im (B) represents the coupling term (off diagonal) of the Jones matrix of the loop birefringence. The intensity transfer function is defined as: T=(Im(B))2 It is important to note that the intensity transfer function will be the same regardless of type of the input polarization state. The input polarization states are being preserved in this device, although it is birefringent. These phenomena show us that the birefringence filter is independent of the input polarization state and the output polarization state is a rotationally transformed version of the input state.
2. Experimental setup and measurements
Our method is to use a fiber loop mirror comprises high birefringence fiber and polarization controllers (PCs). In this experiment we used bi-directional pumping in order to suppress the noise and produce a better OSNR. Polarization controller has low insertion loss, which about 0.8dB. The experiment setup is shown in Fig. 2 below, where the input signal experiences Raman amplification through out 70km SMF and going through a high birefringence fiber loop mirror by a 50:50 coupler. A broadband source with wavelength range from 1520 to 1580 nm is injected in the end of one fiber as the input signal. The pump signal is injected by and goes through WDM coupler in both forward and backward propagating direction. Its wavelength is set to be 1455 nm with a total pump power of 1.2W. Signal is being transmitted through 70km SMF with an insertion loss of 0.22dB/km. The optical power of the broadband signal is measured by using an optical spectrum analyzer (OSA) with a resolution of 1nm. The optical isolator prevents the pump signal to enter the input source. A circulator is employed to extract the flattened signal coming from the fiber loop mirror and at the same time prevent this particular signal return back to the amplified transmission system.
The amplified Stokes signal is split by the 50:50 coupler into two counter-propagating beams. The output intensity of the fiber loop mirror is determined by the phase difference between beam interferences. As explained above that the filter has an important property that the transfer function is independent of the input polarization. Thus the output intensity is solely determined by the birefringence properties that form the loop. They are birefringence of the polarization maintaining fiber (beat length of PMF), lengths of PMF and polarization controller setting. By setting up a proper length of PMF and adjusting the polarization controller, the reflection spectrum of the mirror can be adjusted in such it compensates the Raman gain profile.
3. Results and discussion
We have used one section of Hi-Bi FLM, which comprises 1 section of PMF and 2 polarization controllers. The PMF we used has a nominal beat length of 3mm at the wavelength of 1550 nm. The PMF length was set to about 5cm to cover the Raman gain profile with a peak gain at 1555 nm. The insertion loss of the Hi-Bi FLM is about 2.7 dB. The high loss is primarily caused by coupling losses between PMF and ordinary fiber. Figure 3 shows the flattened Raman gain and its corresponding reflectivity spectrum. By adjusting the PC, we can shift the reflection spectrum of the filter in such it will have bottom notch at the highest peak of Raman gain, which is about 1555 nm in this case. By adjusting the polarization controller we can control the depth of the notch as well. The Polarization Mode Dispersion (PMD) caused by the high birefringence fibers in the fiber loop mirror is very small as its length is short. Below is the spectrum of the interferometers in the fiber loop mirror.
From 1525 nm up to 1575 nm range, the original Raman gain Profile has a peak gain of 16.4dB at 1555 nm. The gain fluctuation is from 11.15 dB up to 16.4 dB, which is about ±2.65 dB. By adjusting the polarization controller we managed to flatten the gain from 1525nm up to 1568 nm (43 nm) with gain fluctuation of ±0.5dB, which represents a significant improvement over the uncompensated gain variation in the same wavelength range. The average flattened gain is about 11.3 dB, which is 5 dB suppressed from the peak gain. However this gain value is still within reasonable level as the average gain of the non-flattened amplifier is about 14.5 dB. This shows that the flattened gain is averagely suppressed for about 3.2 dB.
Next, we used longer section of Hi-Bi Fiber with an 8.5 cm PMF. The result is shown in Fig. 4. It can be seen that the longer the PMF, the birefringence effect is getting stronger, which ultimately results in shorter period of the repetitive reflection spectrum. In this section we managed to flatten the gain from 1535 nm up to 1567 nm (32 nm) with gain fluctuation of ±0.5 dB. The average flattened gain is about 13dB, which is 4dB suppressed from the peak gain. This shows to us that by using longer Hi-Bi Fiber, we get a higher average gain but lower bandwidth of the flattened gain. We can further investigate for different length of PMF until we managed to get the reflectivity spectrum that precisely matches the Raman gain profile. For different beat lengths of PMF, the characterization of the reflectivity spectrum would be significantly different.
Moreover, we can use more sections of the birefringence fiber, which may give us more interesting phenomena. In our second setup, we used two sections of Hi-Bi Fiber with a 5 cm and an 8.5 cm length. Figure 5 shows the experimental setup for Raman gain flattening by using 2 sections of Hi-Bi FLM. Two PMFs and 3 PCs are employed in the fiber loop. The result is shown in Fig. 6. In this section we managed to flatten the gain from 1530 nm up to 1568 nm (38 nm) with gain fluctuation of ±0.5 dB. The average flattened gain is about 12.5 dB, which is 4dB suppressed from the peak gain.
This method enables us to have a good flattened gain with still maintaining lower suppression from the original gain profile. As a comparison, when we used a Hi-Bi fiber length of 5 cm, the reflection spectrum just matches to compensate the Raman peak at 1555 nm and we could obtain such a broad flattened gain. However, the drawback is that the gain suppression is relatively high. By using longer Hi-Bi fiber length, which is 8.5 cm in this case, the bandwidth of flattened gain is reduced, however we obtained lower gain suppression. A better way is to combine both Hi-Bi fiber length of 5 cm and 8.5 cm so that we could get a relatively wide flattened gain (with 38 nm bandwidth) and at the same time still maintain low gain suppression (4dB).
We have demonstrated a method to flatten Raman gain by using Hi-Bi fiber loop mirror. With a single section of 5cm length Hi-Bi fiber, we can manage to flatten the gain to within ±0.5dB over a bandwidth of 43 nm at the center wavelength of 1555 nm. The average flattened gain is about 11.3dB, which is 5dB suppressed from the peak gain. By using two sections of Hi-Bi fiber with a 5 cm and an 8 cm length, we can manage to get a flattened gain within ±0.5dB over a bandwidth of 38 nm from 1530 nm to 1568 nm with lower gain suppression. From these experimental results, we concluded that by using this method, we could control the gain profile of Raman Amplifier. By adjusting the Hi-Bi fiber length and polarization controller we would be able to obtain a reflection spectrum curve, which matches the gain profile of Raman Amplifier.
References and Links
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