1.

Introduction

With the constant increase in traffic demand and the advent of reconfigurable optical networks, a tunable optical comb filter has been demonstrated to be useful in many different areas of optical communication^{1 2 3 4} Especially, an optical all-pass filter using a ring resonator is worth noticing. However, it is better for the channel spacing of the filter and dispersion to be controlled simultaneously by various conditions. In this work, we propose a new simple filter for multichannel dispersion compensation using a motorized polarization differential delay line (DDL). It is possible to change easily the channel spacing with simple control electronics. In addition, we can control the group-delay characteristics of the proposed device by changing the power-coupling ratio of the variable coupler.

2.

System Configuration

Figure 1(a) shows the schematic diagram of the proposed variable channel-spacing dispersion compensation filter. The loop mirror consists of a polarization controller (PC), a variable coupler (Fiberpro, TC1410, Daejeon, Korea), and a DDL (OZ Optics, DDL-650, Carp, Ontario, Canada). Control of the splitting ratio in the variable coupler is achieved by adjusting the relative lateral positions of the mated fibers with the help of micrometers. DDL is the device that splits the light within a fiber into orthogonal polarizations, and then actively varies the time that one polarization travels compared to the other polarization before combining the two polarizations together again. The PC in the loop acts as a polarization state rotator for both clockwise and counterclockwise beams. It changes a fast-axis beam to a slow-axis beam and vice versa. These two beams recombine and have phase differences proportional to the polarization differential delay. The wavelength spacing of the peaks is as follows²:

Fig. 1

(a) Basic configuration of variable dispersion equalizer and (b) measured spectrum of the filter output when Δt is changed. (Δt is 2.5, 10, and 20 ps for Δf=400, 100, and 50 GHz, respectively.)

The phase response of the output beam can be obtained from the following:

The predicted group delay results are compared with the measured data in Fig. 2 and good agreement is reported. Figure 2(a) shows the calculated relative group delay characteristic when Δt is varying as 5, 10, and 20 ps. Figure 2(b) shows the measured characteristics of the filter when κ is 0.6.

Fig. 2

(a) Calculated relative group delay of a basic configuration filter. (b) Measured relative group delay when the differential delay is 2.5, 5, and 10 ps and κ is 0.6. The dispersion is 514.8, 192.9, and 86.8 ps/nm, respectively.  

3.

Discussion and Conclusions

We propose a variable channel-spacing tunable filter for dispersion compensation and demonstrate its feasibility by experiment. In the proposed method, the channel spacing and the dispersion can be varied with the change of polarization delay and the coupling constant by a simple tuning method. We can vary the channel spacing nearly continuously with the use of DDL controlled by a computer. The tuning speed is mainly dependent on a control interface unit and the moving stage’s speed in a DDL. The maximum speed can be up to a few megahertz or more. The temperature stability of this system is very good compared to other systems using interferometry or polarization maintaining fibers. These features can make it more useful and compatible to various environments. And in the need of greater design flexibility, the architecture can be easily extended to higher orders by cascading many rings. Because the PC used in the loop does not change the polarization state completely, and the power-coupling coefficient in our variable coupler has some errors, there is difference between the calculated and measured results.