1. Introduction

Photonic integrated circuits (PICs) have been a very active research area ever since the inception of integrated optics for the application of the wavelength division multiplexing (WDM) networks. One of the main size limitations to regular integrated optics based circuits is the weak optical confinement. This makes it very difficult to change the direction of optical waveguides in a very short distance with low loss. Photonic band gap based approaches offer promise of compact waveguide size that can be bent over very rapidly. However, wavelength dependence and the fabrication difficulty remain to be the challenges. Another approach is the total internal reflection (TIR) based mirrors [1]. Such mirrors can be combined with regular optical waveguides and can reduce the size of PICs drastically. The characteristics of the ring resonators are studied extensively in recent literature, and are considered to be one of the most promising components in future optical WDM system applications [2–6]. The biggest premise of these devices is their very small size compared to other wavelength selective devices. Large numbers of resonators can be integrated on a chip to increase functionality [7].

In this paper, we investigate the properties of the extremely small multimode-interference (MMI) coupled triangular ring resonator with TIR mirrors and the semiconductor optical amplifiers (SOAs). To the best of our knowledge, the fabrication and measurement of the extremely small MMI coupled triangular ring resonator with TIR mirrors and SOAs have not been performed. The process and the material used in the fabrication of these devices are compatible with conventional laser fabrication. Hence, these devices can easily be integrated with sources, detectors, modulators and other passive components. As such they have potential as optical switches and filters in PICs.

2. Basic operations

Figure 1 depicts the top schematic and the fabricated picture of the extremely small MMI coupled triangular ring resonator with TIR mirrors and SOAs. A standard ridge waveguide laser structure is used for the SOAs part. The SOA waveguide width is 3 µm. The triangular ring resonator is formed by employing the extremely small MMI coupler, TIR mirrors, and SOAs in a triangular geometry. To form a ring resonator, the first issue is that of coupling enough power per pass into the cavity. The easy integration of the triangular ring resonator can be achieved by using MMI couplers. Here, the compact ring resonator depends on the length of the MMI coupler. To make the MMI coupler as short as possible, MMI length can be reduced by reducing its width since an MMI length scales with the square of the width [8]. Therefore, to reduce the MMI width the input and output waveguide width and the gap between them should be made as small as possible. However, there are lithographic problems to get closer the gap of the two waveguides in the input and output ports of the MMI coupler. The length of the MMI also can be reduced significantly by deeply etching the MMI region [9]. Deep etching eliminates the need for precise etch depth control but sidewall roughness increases the propagation loss.

In the MMI design, we chose the extremely small MMI coupler, which couple the output power by mode interference into the triangular ring resonator using TIR mirrors. In this case, the input and output waveguides can simply be positioned at the left side of the MMI coupler without any need for further adjustment, because the MMI coupler and input/output waveguide width are 6 and 3 µm, respectively. Then, another input and output ports can be placed at the right side of the MMI coupler by using TIR mirrors. The incidence angle of the TIR mirrors positioned at the right side of the MMI coupler is 34°. This design scheme has a good advantage to remove the direct coupling between the two access waveguides, which has some problem when they get closer. The MMI length was determined to be 90 µm based on the finite-difference time-domain (FDTD) method. This resonator uses very short MMI couplers to couple about 50% of the incoming power into the resonator per pass. The MMI coupler used in the experiments was made by the conventional etching process without deep dry etching to reduce the MMI length. To the best of our knowledge, this length is among the shortest ever reported for this material system without deep dry etching the MMI region.

 

Fig. 1. Schematic diagram (left) and fabricated picture (right) of the MMI coupled triangular ring resonator with TIR mirrors and SOAs.

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Fig. 2. TIR mirrors loss as a function of the incident angle for both the TE and the TM polarization by using the FDTD method.

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There are significant advantages of ring resonators with TIR mirrors. First, the resonator itself is a low-loss optical guiding structure with high confinement. Second, the active and passive sections of the resonator have excellent overlap. Third, MMI sections can be designed independently out of passive waveguides with significant power coupling into the resonator. Except for the TIR mirrors, the optical mode is guided by regular semiconductor rib waveguides, which minimizes the effect of side wall roughness. The most important part of ring resonator with TIR mirrors is the realization of low loss and efficient turning mirrors. The TIR mirror loss as a function of the incident angle is calculated for both the transverse electric (TE) and the transverse magnetic (TM) polarization by using the FDTD method and is shown in Fig. 2. The parameters used here are a wavelength of 1.55 µm and the mirror block effective refractive index of 3.291. The critical angle for the TIR mirror with air interface is about 17.7°. The mirror loss is increased up to 1.6 dB for TE polarization and 5 dB for TM polarization at the incident angle of 18°. However, the mirror loss is less than 1 dB over the incident angle of 22° for both TE and TM polarizations. Therefore, the sharp incident angle of 22° was used in the TIR mirror inside the triangular ring resonator. The optical mode propagation at the TIR mirror is also shown in the inset of Fig. 2. The TIR mirror fabrication requires a deep etch to create TIR at the semiconductor air interface. In our previous work, we demonstrated very low loss TIR mirrors [10]. The SOAs in the resonator are used to compensate the internal waveguide and the mirror loss [11]. The waveguide length of two arms used within the triangular ring resonator is 120 µm and the length and width of the SOAs are 66 and 3 µm, respectively.

 

Fig. 3. Cross sectional profile of the active and the passive waveguide structures used in the experiments

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Figure 3 shows the cross sectional profile of the active and passive waveguide structures used in the experiments. The core of these waveguides is 0.35 µm thick InGaAsP with bandgap energy corresponding to 1.4 µm. The lower cladding is 1.8 µm thick n InP on n+ InP substrate. The upper cladding is 1.8 µm thick p InP followed by 0.1 µm p+ InGaAs cap layer. Active waveguide contains 0.146 µm thick multi quantum well (MQW) region on top of the core. MQW has 8 wells and 9 barriers. The well and barrier widths are 70 and 100 Å, respectively. The fabrication starts with the growth of the base material up to the position indicated by the horizontal arrow in Fig. 3. Then, active material is etched in areas where it is not desired. This is followed by the regrowth of a 1.8 µm thick p InP and a 0.1 µm p⁺ InGaAs cap layer. These waveguides are 3 µm wide and are etched 1.8 µm deep for single mode propagation. The rest of the fabrication involves well known dry etching and metallization steps. It should be noted that this approach requires only one regrowth and no wafer bonding for vertical coupling is needed. The required fabrication steps are exactly the same with laser fabrication except for one deep etching step to form the TIR mirrors. Therefore, these resonators can be directly integrated with tunable lasers, wavelength converters, and detectors.

3. Results and discussions

Filter characteristics are measured using another SOA integrated with the resonator as a broadband source [12]. This SOA is not shown in Fig. 1, but has exactly the same design as the SOAs inside the resonator. In the measurements, this SOA bias is kept fixed at 180 mA.

 

Fig. 4. Transmissions through the resonator at two different SOA currents when the resonator is excited with an integrated external SOA.

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Fig. 5. Details of the spectra around 1550 nm when the SOAs inside the filter is biased at 10 mA.

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Figure 4 shows the filtering characteristics of the resonator for two different bias currents of the SOAs inside the resonator. When there is no bias very little filtering is observed. This is due to strong absorption of the SOAs inside the resonator. At the longer wavelength, side absorption reduces and there is evidence of band stop filtering. However, band stops are not well developed and are very broad. This is due to excessive loss within the filter. When a bias current of 5 mA is applied to the SOAs inside the filter, first transparency is reached and then net gain is generated. This reduces the round trip loss inside the resonator and the extinction ratio increases while the bandwidth gets smaller. As a result, very well developed band stop characteristics are observed. The envelope of these characteristics is similar with the gain profiles of the outside SOA integrated with the resonator as a broadband source. Band stop filtering is clearly observed over 50 nm, which indicates that the TIR mirrors are broadband reflectors. Figure 5 shows the detail of this characteristic around 1550 nm with the bias current of 10 mA to the SOAs inside the filter. The slight variation of the extinction ratio between adjacent resonances is due to difficulty of locating the exact null position in the experiments. A free spectral range (FSR) of approximately 245 GHz is observed near 1550 nm. The on-off ratio is around 17 dB. The waveguide length used within the triangular ring resonator is 120 µm, which has a sharp angle with the TIR mirror. The incidence angle of the TIR mirror inside the triangular ring resonator is 22°.

4. Conclusions

We have investigated the properties of extremely small MMI coupled triangular ring resonator with the TIR mirrors and SOAs for the first time. The resulting FSR was approximately 2 nm near 1550 nm. In our previous work, we demonstrated the conventional resonator with the MMI length of 390 µm, which was observed with an on-off ratio of 8 dB [12]. For the devices reported here MMI length of 90 µm and an on-off ratio of 17 dB have been achieved, respectively. This length is among the shortest ever reported for this material system without deep dry etching the MMI region. The incidence angle of the TIR mirror inside the resonator is 22°. Such resonators can be scaled down to very small sizes since the round trip loss is dominated by the mirror loss and has very slight dependence on the total circumference of the cavity. Coupling in and out of such resonators is done using very compact MMI coupler. Since this coupling is lateral one can use the same material platform and process steps used for the fabrication of tunable lasers, SOAs and other active and passive devices. Furthermore, regular rib waveguides optimized for high gain can be used within the resonator. The only additional processing step is a deep etch to fabricate the TIR mirrors. Hence, such resonators can be directly integrated with other devices making compact and highly functional photonic integrated circuits possible. This integration is demonstrated by integrating an SOA within the cavity. Such SOA integration has the added advantage of tuning the resonator properties by changing the gain of the SOA. The triangular ring resonators with a sharp incidence angle are very attractive components due to their promise of compact size and high levels of integration to yield high functionality.