1. Introduction

Over the last decade, fiber-to-the-x (FTTx) infrastructure based on time division multiplexing passive optical network (TDM-PON) have been developed and deployed in North America and Asia to provide Internet services for higher bandwidth and better quality against traditional cable-based networks. In order to deliver multi-play services in a PON system, numerous optical transceivers are commonly needed to process signals among three wavelength channels: 1310 nm for uploading, 1490 nm for downloading, and 1550 nm for video broadcasting1, and their cost dominate the overall capital expenditure. Thus, the development of low-cost optical transceivers using planar lightwave circuit (PLC) technology based on polymeric materials can offer advantages of easy fabrication and integration, cost-efficient packaging, and mass scale production. The key component in our proposed optical transceiver for TDM-PON is a multi-wavelength multiplexer based on the principle of multimode interferometer (MMI) that has excellent features, such as low polarization dependence, ease of fabrication, and compactness. The self-imaging or extraneous self-imaging effects of a simple MMI structure were reported to separate different wavelengths in a common length of recombination^2,3. However, it is difficult to separate two closely spaced wavelengths in a short propagation distance within an MMI that usually requires the length of tens of recombination lengths. Besides, to attain a common length for splitting more than two wavelengths in a single MMI structure can be a design challenge.

The ever-increasing demand for high bandwidth, video-centric network services has attracted intensive research for the components and systems of 10-Gbps symmetric TDM-PONs. Although the corresponding standard is still under discussion, it is believed that adding one additional wavelength channel centered at 1590 nm is necessary, and a turn-key design for a MMI based multi-wavelength multiplexer compatible with current and future PON transceivers is highly desirable for the convergence of traditional 1-Gb/s and next-generation 10-Gb/s TDM-PONs. Therefore, in this paper, we proposed and designed a compact four-wavelength multiplexer based on a new concept of cascaded step-size MMI (CSS-MMI) for 1G/10G hybrid TDM-PON applications. By combining two types of proposed CCS-MMI wavelength splitters, we realize a compact, four-wavelength multiplexer in only one or two recombination lengths (less than 1.5 cm). It is so far the shortest MMI device that can perform four-wavelength mux/demux function. In addition, the design of proposed MMI device is based on polymeric materials using existing low-cost production and packaging methods.

2. Filtering effects of cascaded step-size MMIs

 

Fig. 1. (a). Schematic diagram of a MMI filter and (b) simulation result of its 3dB passband width versus the ratio of the width of the MMI region to the width of input/output waveguide. The central wavelength is set to be 1500nm.

Download Full Size | PDF

A typical waveguide multimode interferometer (MMI) exhibits multiple propagation waveguide modes interfering with each other in the multimode region. Propagating along certain distance L_r, the propagation modes will be interfered and recombined back to its mirrored image for arbitrary input images⁴. Assuming n _r is the effective index and W _e is the effective width of the fundamental mode, the recombination length L_r is a function of effective width and wavelength:

When both input and output waveguides are connected at the corners of corresponding MMI devices as shown in Fig. 1(a), the combined structure will behave as a bandpass filter. Because of the wavelength dependency of the recombination length, waves of the specific wavelength can be recombined and filtered into the output waveguide at the given recombination length. Such basic MMI filtering structures have already been utilized in some cases⁵. Figure 1(b) shows the passband width in wavelength of the MMI filter as a function of the ratio of the width of the MMI region to that of the input and output waveguides at 1500 nm, where the size of input and output waveguides are set as 5 µm. Ideally, the passband bandwidth can be as narrow as possible when the ratio is increasing; however, it will increase the length of the MMI as depicted in Eq. (1). Moreover, it also implies larger coupling loss when the propagation mode difference between input waveguide and MMI region is larger. These two factors are correspondingly translated into higher insertion loss, as a tradeoff in the MMI filter design.

 

Fig. 2. Schematic structures of the proposed (a) type-I and (b) type-II CSS-MMI 1×2 wavelength splitters

Download Full Size | PDF

Normally, finding the least common multiple (LCM) of recombination lengths of different wavelength channels in MMI structure is utilized when designing a wavelength splitter^2,3,5,6, which may become more difficult when more channels are considered. Unlike the traditional approach, a new MMI design concept by using cascaded structure with step-size geometry is first proposed here to effectively shrink the total length of a wavelength splitter to only one or two times of its recombination length.

When optical beam propagates through MMI region along z-axis, the power distribution can be depicted by the superposition of its propagation modes as:

φ _s is the propagation mode of order s, c_s is the superposition coefficients, and β is the propagation constant determined by its mode number and the recombination length of the MMI region. When two MMI with similar widths are connected, and most of the optical power is coupled into the succeeding MMI with negligible loss, the optical power can be expressed by the superposition of the second MMI propagation modes φ_v’ with approximately the same superposition coefficients as in the first MMI region. At position of z, it can be rewritten as:

, where φ_v’ is the v _th propagation mode of the second MMI, and W₁, W₂ are the widths of the connected MMI respectively. L₁ is the length of the first MMI region. If the total phase difference is mπ, while m is an integer, it will be recombined to its original image. From Eq. (4), the proposed cascaded step-size MMI (CSS-MMI) 1×2 wavelength splitter can be categorized into two types. First of all, assume that the long wavelength is λ _a with recombination length L _ra(W), and the short wavelength is λ _b with recombination length L _rb(W). For type-I CSS-MMI 1×2 wavelength splitter as shown in Fig. 2(a), the short/long wavelength channel will be filtered out at the first/second recombination length, and the corresponding length of each MMI region of the type-I wavelength splitter is designed as:

The design parameter ε is the length adjustment, which can be obtained from the first equation in Eq. (5). When these two designed lengths are put into Eq. (4) for λ _b at z=L ₁, we will get its first mirrored recombined image with phase shift v ( v+2 ) π for each mode. At z=L ₂ for λ _a, we have 2v ( v+2) π phase shift and obtain its self-imaging. For type-II wavelength splitter sketched in Fig. 2(b), the long/short wavelength channel will be filtered out at the first/second output waveguide at their respective recombination length, and the corresponding length of each MMI region is

When these two designed lengths are put into Eq. (4) for λ _a at z=L ₁ or λ _b at z=L ₂, we will obtain both mirrored images with phase shift v ( v+2 ) π, but at different location. To confirm the wavelength demultiplexing effect, we use 2D beam propagation method (BPM) by Optiwave to simulate the optical output intensity. The core material is set to fit polymeric materials with index of 1.54, and the cladding material is set to be siloxane-like polymer with index of 1.49. In our design, both types of CSS-MMI 1×2 wavelength splitters have common parameters: the widths of the input waveguide, first MMI region, second MMI region, and output waveguide from left to right are 5 µm, 40 µm, 32 µm, and 5 µm respectively. The type-I CSS-MMI wavelength splitter exhibits the insertion loss of 0.2 dB and 0.7 dB at 1490 nm and 1550 nm, respectively. In addition, although the type-II CSS-MMI wavelength splitter has shorter length in total, it usually suffers more insertion loss and lower extinction ratio (ER). Insertion loss of type-II CSS-MMI wavelength splitter are 1.4 dB and 0.3 dB for 1490 nm and 1550 nm channels, respectively. The simulated field propagation patterns are shown in Fig. 3 and Fig. 4.

 

Fig. 3. BPM simulation result for the proposed type-I CSS-MMI 1×2 wavelength splitter with amplitude distribution of optical wave at (a) 1.49µm and (b) 1.55µm, respectively.

Download Full Size | PDF

 

Fig. 4. BPM simulation result for the proposed type-II CSS-MMI 1×2 wavelength splitter with amplitude distribution of optical wave at (a) 1.49µm and (b) 1.55µm, respectively.

Download Full Size | PDF

3. Compact four-wavelength multiplexer for 1G/10G coexisting TDM-PON

To implement a 1G/10G hybrid TDM-PON system, an optical multiplexer that can accommodate four wavelength channels: 1310 nm, 1490 nm, 1550 nm, and 1590 nm is essential. However, the separation of two closely spaced channels: 1550 nm and 1590 nm will become a major issue, which usually requires a total length of tens of recombination length in traditional single MMI demultiplexer. Otherwise, based on the new concept of MMI wavelength splitting mechanism, we designed a compact four-wavelength multiplexer of the proposed type-I and type-II CSS-MMI λ-splitters. The layout of the proposed four-wavelength multiplexer is depicted in the Fig 5. The widths of input and output waveguides are set to be 5µm. Since the 1310nm wavelength band is further away from the other three bandwidths, the requirement of two multiplexing stages were necessary. In the first stage, marked as MUX1, 1310 nm is filtered out by the type-I CSS-MMI λ-splitter. In order to increase the ratio of the width of MMI region and input waveguide of the next multiplexer, the other three wavelengths are passing through a gradually tapered waveguide (from 13 µm to 12 µm) into the second multiplexing stage MUX2. In the second stage, the downstream wavelength 1490 nm is filtered out first by type-I CSS-MMI λ-splitter, and then two channels with wavelengths 1550 nm and 1590 nm are directly split by type-II CSS-MMI λ-splitter.

In the 2D BPM simulation, TE polarization optical input with four different wavelengths is first set from the leftmost input waveguide, and separated into four output waveguides. Channels 1310 nm/1490 nm/1550 nm/1590 nm at the outputs have insertion loss of 0.18 dB/1.21 dB/1.34 dB/1.02 dB respectively. The extinction ratios for different wavelengths are ranging between 11.71 dB to 32.07 dB. The simulated beam amplitude propagation is shown in the Fig 6. The relative output optical spectra of the proposed CSS-MMI four-wavelength multiplexer are shown in Fig. 7, whose broadband characteristics exhibit its high tolerance to wavelength shifting in real network deployment.

 

Fig. 5. The layout design of 4-wavelength multiplexer based on two kinds of MMI wavelength splitter.

Download Full Size | PDF

 

Fig. 6. The BPM simulation of the amplitude of four wavelength channels in the proposed multiplexer. The total dimension of the simulated window is 125µm×17,500µm.

Download Full Size | PDF

 

Fig. 7. Relative output optical spectra of the proposed CSS-MMI four-wavelength multiplexer.

Download Full Size | PDF

5. Conclusions

For the first time, a polymeric four-wavelength multiplexer based on cascaded step-size multimode interferometer (CSS-MMI) for the convergence of current 1-Gbps and next-generation 10-Gbps TDM-PON is proposed and designed. Unlike the commonly used conventional wavelength multiplexing mechanism in MMI that has to match the beat length of several wavelengths simultaneously, the length of our proposed CSS-MMI is simply designated by the width of the MMI. Using the newly developed concept of CSS-MMI, wavelength channels can be separated with much shorter length than that in the conventional scheme. By combining two simple CSS-MMI 1×2 wavelength multiplexer, four channels 1310 nm/1490 nm/1550 nm/1590 nm are split in the total length of less than 1.5 centimeter. All parameters in the proposed design method are suitable for fabrication of polymeric or glass materials. The insertion losses of three downstream channels of 1490 nm, 1550 nm, and 1590 nm are 1.21 dB, 1.34 dB, and 1.02 dB, respectively and the insertion loss of the upstream channel 1310 nm is only 0.18 dB. The extinction ratios for different wavelengths are ranging between 11.71 dB to 32.07 dB, which can be further improved if we connect another simple MMI filter at the output waveguide.