Abstract:
Along with several few-channel and low-density wavelength division multiplexer/de-multiplexer, a novel planar lightwave circuit (PLC) interleaver is invented to achieve high-density wavelength division multiplexing and de-multiplexing in a wavelength division multiplexing fiber communication system. The invention uses a PLC as its basic structure and applies the principle that the product of the number of output optical waveguides and the channel spacing is equal to the free spectral range of the spectrum of any output optical waveguide to make a compact PLC interleaver.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention relates to an interleaver structure and, in particular, to an interleaver used in a high-density wavelength division multiplexing fiber communication systems to achieve wavelength division multiplexing and de-multiplexing. 
     2. Related Art 
     The basic structure of a high-density wavelength division multiplexing optical fiber communication system is to divide mostly used transmission spectra in the optical fiber into several transmission channels according to the lightwave frequencies. Each channel uses its central frequency as its carrier frequency (such as ITU 100 GHz grid). The signals in all channels are merged together to be transmitted over one fiber so as to increase its transmission capacity. This is called the wavelength division multiplexing (WDM). Afterwards, the signals of all channels transmitted in the fiber are separated into different fibers to perform signal processing. This is called the wavelength division de-multiplexing (WDDM). Taking a wavelength division de-multiplexing system with a frequency interval of 50 GHz as an example (FIG.  1 ), there are optical signals satisfying the ITU grid 50 GHz×80 channels transmitting in the fiber. A 50 GHz×80 channels wavelength division de-multiplexer (DEMUX) can de-multiplex the signal light of all wavelength channels so that the signal of each channel is guided into a distinct fiber. 
     Alternatively, an interleaver can be equipped with two few-channel and low-density channel spacing multi-channel de-multiplexer to achieve the same object (FIG.  2 ). Similarly, optical signals satisfying the ITU Grid 50 GHz×80 channels are transmitted therein. In this case, a 50 GHz 1×2 interleaver can be used to perform the first stage de-multiplexing on the signal light. According to the functioning principles of interleavers, optical signals in 40 odd channels are output from one of the output terminal. Such optical signals satisfy the ITU Grid 100 GHz wavelength channel standard. Meanwhile, optical signals of the other 40 even channels are output from another output terminal. Such optical signals satisfy the ITU Grid 100 GHz wavelength channel standard with a channel offset of 50 GHz. Both of the optical signals can be de-multiplexed using 100 GHz multi-channel de-multiplexers (one of them having an offset of 50GHz) so that the optical signals of each channel are transmitted into distinct optical fibers. 
     Although the above two methods perform the same function, the later has a lower cost because the manufacturing of the 50 GHz×80 channels DEMUXs is much harder than that of the 100 GHz×40 channels DEMUXs. Also, the channel number of a single device is twice that of a 100GHz one, therefore the cost is often several times more expensive than that of two 100 GHz DEMUXs. Thus, using the structure in FIG. 2 costs much less than that of FIG. 1 even if an additional interleaver is included. 
     From the viewpoint of system upgrading, interleavers are indispensable. Suppose a 100 GHz×40 channels DEMUX transmission system needs to be upgraded into a 50 GHz×80 channels DEMUX system to double the transmission capacity. If one considers to upgrade it to the structure shown in FIG. 1, the original 100 GHz DEMUX has to be replaced by a 50 GHz×80 channels DEMUX. Besides the cost increases, the abandoned device is also a waste of cost. If one wants to upgrade it into the structure shown in FIG. 2, the 100 GHz DEMUX can be kept. One only needs to add another 100 GHz DEMUX (with an offset of 50GHz) and an interleaver. It does not increase or waste extra cost. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a planar lightwave circuit (PLC) interleaver that mainly utilizes a PLC structure. 
     Along with several few-channel and low-density wavelength division multiplexer/de-multiplexer, a novel planar lightwave circuit (PLC) interleaver is invented to achieve high-density wavelength division multiplexing and de-multiplexing in a wavelength division multiplexing fiber communication system. 
     The invention uses a PLC structure as its technical basis. Utilizing the principle that the product of the number of output optical waveguides and the channel spacing is equal to the free spectral range of any output optical waveguide spectrum, a PLC interleaver structure is designed to, along with several few-channel and low-density wavelength division multiplexer/de-multiplexer, construct a high-density WDM fiber communication system. 
     The disclosed PLC interleaver can be integrated with other PLC wavelength division multiplexers/de-multiplexers or other complicated PLC devices on the same chip through the PLC manufacturing techniques. The product does not only feature in small volume but is also convenient for mass production at a lower cost. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG. 1 shows a structure of a conventional WDM system with a channel spacing of 50 GHz; 
     FIG. 2 shows a structure of an interleaver accompanied with two few-channel and low-density channel spacing multi-channel de-multiplexing system in the prior art; 
     FIG. 3 shows a structure of the disclosed interleaver; 
     FIG. 4 illustrates a local structure of the invention, showing a free propagation region, locations of branches in an arrayed waveguide, and the relations between any two neighboring waveguide output terminals; 
     FIG. 5 is an output spectrum diagram of the interleaver, demonstrating one example of the invention; 
     FIGS. 6-8 show the output spectra of three interleavers with different geometries. 
     FIG. 9 shows a structure of a 50 GHz 1×4 interleaver of the invention; and 
     FIG. 10 is an output spectrum diagram of FIG.  9 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to FIG. 3, the disclosed PLC interleaver is a PLC device with an arrayed waveguide grating (AWG) structure. It has the typical wavelength division multiplexing (WDM) characteristics of the AWG. The interleaver contains: 
     an optical waveguide input terminal  10  that can connect to an input fiber transmitting input signals; 
     several optical waveguide output terminals  20  connecting to output fibers  21  transmitting output signals; 
     a free propagation region (FPR) that contains a first FPR  31   a  and a second FPR  31   b ; and 
     an arrayed waveguide (AW) with several branches  41  whose ends are located in the two FPRs  31   a ,  31   b  through which the branches  41  connect to the optical waveguide input terminal  10  and the optical waveguide output terminals  20 . 
     Both ends of each branch  41  of the arrayed waveguide  40  are located on the circles centered at the optical waveguide input terminal  10  and the optical waveguide output terminal  20 . The optical waveguide input terminal  10  and the optical waveguide output terminal  20  can be located either on the same side or opposite sides of the PLC interleaver. 
     When an input optical signal enters the optical waveguide input terminal  10  through an input fiber  11 , the optical signal spreads into the branches  41  of the arrayed waveguide  40  through the propagation of the first FPR  31   a . If adjacent branches are designed to have a fixed optical path difference, after the optical signals propagate through the array waveguide to the other end of the branches  41  and enter the optical waveguide output terminals  20  through the propagation of the second FPR  31   b , the wavefronts of the output optical signals are different corresponding to different optical paths. This effect makes optical signals with different wavelengths output through different optical waveguide output terminals  20 , providing necessary filtering effects on output optical waveguide spectra. This is the so-called wavelength division de-multiplexing (WDDM). This property is often used to make the multiplexer/de-multiplexer (MUX/DEMUX) in a WDM system. 
     Since the AWG uses discrete positions to process optical signals separately and then recombine them, the filtering character at the optical waveguide output terminals  20  gives a periodic curve in the spectrum. The period is called the free spectral range (FSR). Therefore, a main technique used in the invention is to take the AWG device as the basis. So that, through a proper design, the FSR is exactly equal to the product of the number of the optical waveguide output terminals  20  and the channel spacing. This renders the AWG device the properties of an interleaver. 
     With reference to the AWG device shown in FIG. 3, suppose the optical path difference between two branches  41  of the arrayed waveguide (AW)  40  in the AWG device is Δp, then the FSR can be obtained from the following equation: 
     
       
         Δ f   FSR   =c/Δp,   (1)  
       
     
     Where c is the speed of light in vacuum. The channel spacing depends upon the size of the second FPR  31   b  and the location of the output optical waveguide. Suppose the second FPR  31   b  has a refraction index of n FPR  and a radius of R. Denote the distance between any two branches  41  of the AW  40  in the second FPR  31   b  by d a , the distance between any two optical waveguide output terminals  20  by d o  (FIG.  4 ), and the central frequency of the optical signal by f c . Then the channel spacing Δf ch  is expressed by:                Δ                   f   ch       =           n   FPR     ·     d   a     ·     d   o           R   ·   Δ                   p       ·       f   c     .               (   2   )                                
     If it is used as an interleaver, then the following equation must hold: 
     
       
         Δ f   FSR   =N   out   ·Δf   ch,   (3)  
       
     
     Where N out  is the number of the optical waveguide output terminals  20  (i.e. the number of output optical waveguides). In other words, when the central wavelength λ c =c/f c , the channel spacing Δf ch , and the number of the optical waveguide output terminals  20  are known, one can use Eq. (3) to design the radius R of the FPR (both the first and the second FPRs have the same radius R), the distance d a  between any two branches of the AW 40, and the distance d o  between any two optical waveguide output terminals  20 . When the above requirements are satisfied, an interleaver with the desired optical output spectral properties can be constructed. 
     For example, FIG. 5 shows the spectra of PLC interleaver with two output waveguide terminals  20 , which is constructed using the above-mentioned techniques. The central wavelength is 1548.51 nm (193.6 THz), and the channel spacing is 0.4 nm (50 GHz). In the design, the FSR 0.8 nm (100 GHz) is exactly equal to the product of the number of the optical waveguide output terminals  20  (output optical waveguides) and the channel spacing. In the drawing, the solid curve and the dashed curve are the optical waveguide output spectra of the two optical waveguide output terminals  20 . The maximum of the solid curve matches the wavelength of the ITU 100 GHz DWDM Grid, while the maximum of the dashed curve matches the wavelength of the ITU 100 GHz Grid shifted by 50 GHz. Therefore, it satisfies the spectral properties of the desired interleaver. 
     FIGS. 6 through 8 are the output spectra of a 50 GHz 1×2 interleaver for other embodiment of the invention, where the solid and dashed curves represent the output spectra of two output optical waveguides, respectively. In FIGS. 6 through 8, the value of the distance d o  between any two adjacent optical waveguide output terminals  20  are different (d o  =10 μm in FIG. 6, d o =14 μm in FIG. 7, and d o =20 μm in FIG. 8) and the value of the radius R in the FPR  30  are changed correspondently, while keeping the ratio d o /R invariant. The central frequency and the channel spacing of the spectra do not vary, but the cross-talk between channels decrease as d o  increases (−18 dB, −36 dB, −50 dB, respectively). Therefore, the two design parameters do and R of the interleaver can be changed to obtain a desired cross-talk between channels. 
     Finally, it should be emphasized that the interleavers in the prior art are limited to the 1×2 type. However, using the techniques disclosed herein, one does not have such a limitation. Interleavers of the 1 ×N type can be designed, where N is the number of output optical waveguides (i.e. the number of the optical waveguide output terminals 20) and N can be greater than 2. As an example shown in FIG. 9, the 50 GHz 1×4 interleaver contains an optical waveguide input terminal  10  and four optical waveguide output terminals  20   a  through  20   d . According to the principle that the product of the output optical waveguide number and the channel spacing is equal to the FSR of the spectrum of any output optical waveguide, one can determine the structure and size of the interleaver. The spectra of the four output optical waveguides are indicated by the solid, dashed, dotted, and dash-dotted curves in FIG. 10, respectively. 
     While the invention has been described by way of example and in terms of the preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.