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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 2 — Jan. 17, 2011
  • pp: 717–726
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Etched beam splitters in InP/InGaAsP

Erik J. Norberg, John S. Parker, Steven C. Nicholes, Byungchae Kim, Uppiliappan Krishnamachari, and Larry A. Coldren  »View Author Affiliations


Optics Express, Vol. 19, Issue 2, pp. 717-726 (2011)
http://dx.doi.org/10.1364/OE.19.000717


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Abstract

An etched beam splitter (EBS) photonic coupler based on frustrated total internal reflection (FTIR) is designed, fabricated and characterized in the InP/InGaAsP material system. The EBS offers an ultra compact footprint (8x11 μm) and a complete range of bar/cross coupling ratio designs. A novel pre-etching process is developed to achieve sufficient depth of the etched coupling gaps. Fabricated EBS couplers demonstrate insertion loss between 1 and 2.6 dB with transmission (cross-coupling) 10%. The results show excellent agreement with 3D finite-difference time-domain (FDTD) modeling. The coupling of EBS has weak wavelength dependence in the C-band, making it suitable for wavelength division multiplexing (WDM) or other wide bandwidth applications. Finally, the EBS is integrated with active semiconductor optical amplifier (SOA) and phase-modulator components; using a flattened ring resonator structure, a channelizing filter tunable in both amplitude and center frequency is demonstrated, as well as an EBS coupled ring laser.

© 2011 OSA

1. Introduction

The optical coupler is a key component for photonic integration. Desirably the coupler should have: low loss, variable bar/cross splitting ratio designs, a compact footprint and be easily integrated with other photonic components on-chip. Traditional coupler designs are the y-branch, multi-mode interference (MMI), and directional coupler. For many applications a small foot-print directly translates into higher performance; this includes: short optical delays for linear RF-receivers [1

1. C.-H. Chen, J. Klamkin, S. C. Nicholes, L. A. Johansson, J. E. Bowers, and L. A. Coldren, “Compact beam splitters with deep gratings for miniature photonic integrated circuits: design and implementation aspects,” Appl. Opt. 48(25), F68–F75 (2009). [CrossRef] [PubMed]

], large free-spectral range (FSR) of micro resonators used as add/drop filters in wavelength division multiplexing (WDM) applications [2

2. R. Grover, V. Van, T. A. Ibrahim, P. P. Absil, L. C. Calhoun, F. G. Johnson, J. V. Hryniewicz, and P.-T. Ho, “Parallel-Cascaded Semiconductor Microring Resonators for High-Order and Wide-FSR Filters,” J. Lightwave Technol. 20(5), 872–877 (2002). [CrossRef]

], and high sensitivity for micro-rings used in biosensing [3

3. A. Yalcin, K. C. Popat, J. C. Aldridge, T. A. Desai, J. Hryniewicz, N. Chbouki, B. E. Little, V. Oliver King, Van, D. Sai Chu, M. Gill, M. S. Anthes-Washburn, Unlu, and B. B. Goldberg, “Optical sensing of biomolecules using microring resonators,” IEEE J. Sel. Top. Quantum Electron. 12(1), 148–155 (2006). [CrossRef]

]. The y-branch and MMI coupler is typically limited in compactness, where the smallest MMI coupler to date is 20 μm long, but also very sensitive to fabrication variations [4

4. Y. Ma, S. Park, L. Wang, and S. T. Ho, “Ultracompact multimode interference 3-dB coupler with strong lateral confinement by deep dry etching,” IEEE Photon. Technol. Lett. 12(5), 492–494 (2000). [CrossRef]

]. Thus, for compact integration, the directional couplers are most commonly utilized [2

2. R. Grover, V. Van, T. A. Ibrahim, P. P. Absil, L. C. Calhoun, F. G. Johnson, J. V. Hryniewicz, and P.-T. Ho, “Parallel-Cascaded Semiconductor Microring Resonators for High-Order and Wide-FSR Filters,” J. Lightwave Technol. 20(5), 872–877 (2002). [CrossRef]

,5

5. Y. Shi, S. He, and S. Anand, “Ultracompact directional couplers realized in InP by utilizing feature size dependent etching,” Opt. Lett. 33(17), 1927–1929 (2008). [CrossRef] [PubMed]

]. However, when the length of the direction coupler is decreased the coupling gap must be made extremely small in order to obtain any significant coupling. For lateral directional couplers this implies the use of electron beam lithography (EBL) and very small processing tolerances. While the coupling gap can be controlled precisely in vertical directional couplers, this design has the disadvantage of multiple waveguide stacks, thus demanding complicated fabrication often with additional material re-growths [6

6. S. J. Choi, K. Djordjev, S. J. Choi, P. D. Dapkus, W. Lin, G. Griffel, R. Menna, and J. Connolly, “Microring resonator vertically coupled to buried heterostructure bus waveguides,” IEEE Photon. Technol. Lett. 16(3), 828–830 (2004). [CrossRef]

]. Ultimately, for compact coupler designs, the cross coupling of direction couplers is still limited to a few percent. On the contrary, the etched beam splitter (EBS) has been proposed and investigated as an ultra compact coupler that offers a complete range of power splitting ratios independent of size [7

7. J. S. Osinski, C. E. Zah, R. Bhat, R. J. Contolini, E. D. Beebe, T. P. Lee, K. D. Cummings, and L. R. Harriott, “Miniature integrated optical Beam-Splitter in AlGaAs/GaAs ridge waveguides,” Electron. Lett. 23(21), 1156–1158 (1987). [CrossRef]

10

10. N. R. Huntoon, M. P. Christensen, D. L. MacFarlane, G. A. Evans, and C. S. Yeh, “Integrated photonic coupler based on frustrated total internal reflection,” Appl. Opt. 47(30), 5682–5690 (2008). [CrossRef] [PubMed]

]. The EBS is the monolithic version of a conventional bulk prism beam splitter, with the coupling mechanism based on the same physical phenomena of evanescent coupling through frustrated total internal reflection (FTIR) [11

11. S. Zhu, A. W. Yu, D. Hawley, and R. Roy, “Frustrated total internal reflection: a demonstration and review,” Am. J. Phys. 54(7), 601–607 (1986). [CrossRef]

]. The first EBS was realized over two decades ago [7

7. J. S. Osinski, C. E. Zah, R. Bhat, R. J. Contolini, E. D. Beebe, T. P. Lee, K. D. Cummings, and L. R. Harriott, “Miniature integrated optical Beam-Splitter in AlGaAs/GaAs ridge waveguides,” Electron. Lett. 23(21), 1156–1158 (1987). [CrossRef]

], but with very limited success due to immature fabrication technology. More recently, the EBS has demonstrated better performance with silicon-on-insulator (SOI) [12

12. Y. Qian, J. Song, S. Kim, and G. P. Nordin, “Compact 90 ° trench-based splitter for silicon-on-insulator rib waveguides,” Opt. Express 15(25), 16712–16718 (2007). [CrossRef] [PubMed]

], AlGaAs [13

13. B. Kim and N. Dagli, “Submicron Etched Beam Splitters Based on Total Internal Reflection in GaAs–AlGaAs Waveguides,” J. Lightwave Technol. 28(13), 1938–1943 (2010). [CrossRef]

] and polymer based waveguide platforms [8

8. L. Li, G. P. Nordin, J. M. English, and J. Jiang, “Small-area bends and beamsplitters for lowindex-contrast waveguides,” Opt. Express 11(3), 282–290 (2003). [CrossRef] [PubMed]

]. We have extended to the InP/InGaAsP material system in order to integrate the EBS coupler with an active gain platform [14

14. E. Norberg, J. Parker, U. Krishnamachari, R. Guzzon, and L. Coldren, InGaAsP/InP based Flattened Ring Resonators iwth Etched Beam Splitters,” in Conference on Integrated Photonics and Nanophotonics Research and Applications Nanophotonics, Technical Digest (CD) (Optical Society of America, 2009), paper IWB.

,15

15. U. Krishnamachari, S. Ristic, C.-H. Chen, L. Johansson, A. Ramaswamy, J. Klamkin, E. Norberg, J. Bowers, and L. Coldren, “InP/InGaAsP-Based Integrated 3-dB Trench Couplers for Ultra-Compact Coherent Receivers,” IEEE Photon. Technol. Lett. (to be published).

]. Here, we report on the modeling, fabrication and characterization of such EBS couplers in InP/InGaAsP. Furthermore, taking advantage of the active gain and phase modulation provided by the InP/InGaAsP material system, we have integrated the EBS in a novel flattened ring resonator structure to create a channelizing filter tunable in both center wavelength and extinction ratio.

2. EBS design and modeling

The general design of the EBS is two intersecting waveguides with a narrow lower index gap at the crossing point, making it a symmetric 2x2 coupler, as shown in Fig. 1(a)
Fig. 1 (a) Schematic of the EBS coupler illustrating the basic design parameters, incident angle (Θi) of the input and output waveguides, EBS air gap for evanescent coupling and the Goos-Hanchen shift (GHS) between input and output waveguides. (b) FDTD simulation showing the magnetic field intensity|Hy| . (c) Deeply etched InP/InGaAsP waveguide, with the calculated fundamental mode superimposed.
. For total internal reflection (TIR) to take place, the incident angle of the waveguides (Θi) needs to be greater than the critical angle (Θc). The power coupling across the EBS gap is facilitated by the evanescent wave present during TIR. In this work we used a 300 nm thick InGaAsP 1.3Q waveguide layer surrounded by InP cladding. For the lateral optical confinement we utilize air cladding by etching through the waveguide layer, as shown Fig. 1(c). For this waveguide design and using an air filled EBS gap, Θc is approx. 18°. The deeply etched waveguides provide strong lateral optical confinement and robust fabrication in any crystallographic direction due to the sole use of dry etching. For smaller resonator sizes with reduced ring radii, the strong optical confinement is crucial to avoid excess radiation loss from waveguide bends.

3. EBS fabrication

The fabricated EBS coupler is shown in Fig. 4
Fig. 4 SEM image of the EBS coupler. The lighter region shows where the SiNx insulation layer was selectively removed to achieve a semiconductor-air interface in the EBS gap. The highlighted EBS gap demonstrates anisotropic and smooth sidewalls and a sufficient etch through the waveguide layer.
, the EBS gaps demonstrate smooth anisotropic sidewalls with a satisfactory etch depth even for the narrowest gaps. However, all EBS gaps are widened by about 70 nm in the ICP process. This is attributed to minor mask degradation together with a slow lateral etch rate.

For integration of the EBS coupler with standard InP/InGaAsP PICs, there is only a single added process step, besides the pre-thinning process described above. The nitride layer used for isolating metal contacts from the substrate needs to be selectively removed inside of the EBS gap in order to achieve FTIR. This was done by using a 7 µm thick resist (SPR220-7) to cover deep waveguides and metal contacts. Then selectively opening a region around the EBS coupler and using a high pressure (300 torr) CF4 plasma etch to isotropically remove SiNx around and inside the EBS gap, see Fig. 4.

In this work we incorporated: active gain, phase modulation and low loss passive waveguides, by utilizing an active/passive InP/InGaAsP offset quantum wells (OQW) integration platform [21

21. E. J. Skogen, J. W. Raring, G. B. Morrison, C. S. Wang, V. Lal, M. L. Masanovic, and L. A. Coldren, “Monolithically integrated active components: a quantum-well intermixing approach,” IEEE J. Sel. Top. Quantum Electron. 11(2), 343–355 (2005). [CrossRef]

]. Passive waveguide sections and phase modulator regions are here defined by selectively etching away the quantum wells. This is followed by a single blanket re-growth to provide p-cladding everywhere.

3. EBS characterization

The experimentally measured coupling values and insertion loss of the EBS matches very well with the 3D-FDTD simulations, as demonstrated in Fig. 6(a,b,c)
Fig. 6 (a),(b) EBS power coupling and (c) insertion loss as a function of gap width for incident angles (Θi) of 24° and 26° at 1550 nm, for measured EBS coupler designs (squares) and 3D-FDTD simulations (dashed lines). (d) Measured power coupling as a function of wavelength for the 80/20 (bar/cross) EBS design.
. The largest transmission (cross) coupling design was the Θi = 24° with 0.42 μm air gap, which demonstrates an relative bar/cross coupling ratio of 80/20 and an overall insertion loss of 2.6 dB. The lowest insertion loss coupler design was the Θi = 26° with 1.1 μm air gap. This design demonstrates only 1.0 dB insertion loss, but has at the same time a very unequal bar/cross coupling ratio of 99.5/0.5. For WDM or other applications where a wide range of bandwidth is utilized, it is important that the coupling ratio and insertion loss does not change significantly with wavelength. We confirmed that the EBS coupler has a relatively weak wavelength dependence, as shown in Fig. 6(d). The reflection (bar) and transmission (cross) component for this 80/20 coupler varies only by <0.6 dB and <1.0 dB respectively over a 25 nm wavelength span in the C-band.

4. EBS couplers integrated with PICs

In order to further demonstrate the integration of the EBS coupler with the InP/InGaAsP integration platform, we designed and fabricated novel flattened ring resonator devices [13

13. B. Kim and N. Dagli, “Submicron Etched Beam Splitters Based on Total Internal Reflection in GaAs–AlGaAs Waveguides,” J. Lightwave Technol. 28(13), 1938–1943 (2010). [CrossRef]

]. Figure 7
Fig. 7 SEM image of a PIC filter device that utilize the flattened ring resonator design with EBS couplers. Highlighted waveguides shows the flattened ring and illustrates the reduction in resonator length versus a circular resonator design. Port 1 and 2 shows the input and output waveguide ports for the measured filter responses.
highlights the flattened ring design and how it offers a larger bending radius for a fixed resonator delay compared to a conventional circular design. An increased bending radius translates into a lower roundtrip loss, through reduced scattering and radiation loss. This becomes especially advantageous for micro-ring resonators where the bending radius needs to be very small. The resonator length (L) is defined by the incident angle (Θi) (in degrees) at the EBS coupler together with the bend radius (R) of the waveguides:L=ΘiRπ/45. The fabricated device in Fig. 7 utilizes the flattened ring resonator with a Θi = 24° / 0.5 μm EBS coupler design and a bending radius of 500 μm, giving a resonator circumference of 838 μm. The resonator incorporates a 375 μm SOA and two phase modulators (PM). In addition to the flattened resonator, the device also includes a Mach-Zehnder interferometer (MZI). Such a device could be used as the basic building block for higher order lattice filters [22

22. E. Norberg, R. S. Guzzon, S. C. Nicholes, J. S. Parker, and L. A. Coldren, “Programmable Photonic Lattice Filters in InGaAsP–InP,” IEEE Photon. Technol. Lett. 22(2), 109–111 (2010). [CrossRef]

].

Here we use the single resonator to demonstrate a tunable channelizing filter device. The filter function is measured by sweeping the wavelength of an external tunable laser and locking in the signal using an on-chip reversed biased phase modulator after the resonator. Figure 8
Fig. 8 Measured filter responses of the flattened ring resonator. (a) Tuning of pole-magnitude (G) by changing bias on the SOA (solid), with ideal simulated S21 filter function superimposed dashed). (b) Tuning of the filter center frequency by current injection in the phase modulators (PM).
shows the measured filter responses. Adjusting the current to the SOA the pole-magnitude (G) of the filter response is tuned, this is demonstrated in Fig. 8(a) for G=0.5 and 0.75 for SOA biases of 15 and 20 mA respectively. The filter function fits well with the theoretical S21 parameter of an ideal resonator shown in the inset of Fig. 8(a). By utilizing the phase modulators, the filter can easily be shifted in wavelength, as verified in Fig. 8(b). Hence, using the SOA and PMs together the resonator poles can be placed anywhere in the complex plane. This demonstrates the basis of a very versatile channelizing filter. For better stopband rejections and flat-topped passbands, several stages should be cascaded to produce higher order filters [22

22. E. Norberg, R. S. Guzzon, S. C. Nicholes, J. S. Parker, and L. A. Coldren, “Programmable Photonic Lattice Filters in InGaAsP–InP,” IEEE Photon. Technol. Lett. 22(2), 109–111 (2010). [CrossRef]

]. For this application, the small cross-coupling provided by the EBS is ideal, as it allows for high finesse filters with large stopband rejections [23

23. C. K. Madsen, and J. H. Zhao, Optical Filter Design and Analysis: A Signal Processing Approach, (Whiley-Interscience 1999), Chap. 6.

]

The flattened ring resonator can naturally also works as a laser if the SOA is biased at higher currents. The CW threshold for this device is 23 mA with single mode lasing and a side mode suppression ratio (SMSR) of ~32 dB demonstrated in Fig. 9
Fig. 9 Light-current characteristic of the flattned ring resonator demonstrating onset of lasing at ~23 mA, inset shows the lasing spectrum at 55mA, single mode operation with ~32 dB of SMSR.
.

5. Conclusion

Acknowledgements

This work was supported by DARPA under the PhASER program, a portion of this work was done in the UCSB nanofabrication facility, part of the NSF funded NNIN network.

References and links

1.

C.-H. Chen, J. Klamkin, S. C. Nicholes, L. A. Johansson, J. E. Bowers, and L. A. Coldren, “Compact beam splitters with deep gratings for miniature photonic integrated circuits: design and implementation aspects,” Appl. Opt. 48(25), F68–F75 (2009). [CrossRef] [PubMed]

2.

R. Grover, V. Van, T. A. Ibrahim, P. P. Absil, L. C. Calhoun, F. G. Johnson, J. V. Hryniewicz, and P.-T. Ho, “Parallel-Cascaded Semiconductor Microring Resonators for High-Order and Wide-FSR Filters,” J. Lightwave Technol. 20(5), 872–877 (2002). [CrossRef]

3.

A. Yalcin, K. C. Popat, J. C. Aldridge, T. A. Desai, J. Hryniewicz, N. Chbouki, B. E. Little, V. Oliver King, Van, D. Sai Chu, M. Gill, M. S. Anthes-Washburn, Unlu, and B. B. Goldberg, “Optical sensing of biomolecules using microring resonators,” IEEE J. Sel. Top. Quantum Electron. 12(1), 148–155 (2006). [CrossRef]

4.

Y. Ma, S. Park, L. Wang, and S. T. Ho, “Ultracompact multimode interference 3-dB coupler with strong lateral confinement by deep dry etching,” IEEE Photon. Technol. Lett. 12(5), 492–494 (2000). [CrossRef]

5.

Y. Shi, S. He, and S. Anand, “Ultracompact directional couplers realized in InP by utilizing feature size dependent etching,” Opt. Lett. 33(17), 1927–1929 (2008). [CrossRef] [PubMed]

6.

S. J. Choi, K. Djordjev, S. J. Choi, P. D. Dapkus, W. Lin, G. Griffel, R. Menna, and J. Connolly, “Microring resonator vertically coupled to buried heterostructure bus waveguides,” IEEE Photon. Technol. Lett. 16(3), 828–830 (2004). [CrossRef]

7.

J. S. Osinski, C. E. Zah, R. Bhat, R. J. Contolini, E. D. Beebe, T. P. Lee, K. D. Cummings, and L. R. Harriott, “Miniature integrated optical Beam-Splitter in AlGaAs/GaAs ridge waveguides,” Electron. Lett. 23(21), 1156–1158 (1987). [CrossRef]

8.

L. Li, G. P. Nordin, J. M. English, and J. Jiang, “Small-area bends and beamsplitters for lowindex-contrast waveguides,” Opt. Express 11(3), 282–290 (2003). [CrossRef] [PubMed]

9.

B. Kim, and N. Dagli, “Compact Micro Resonators with Etched Beam Splitters and Total Internal Reflection Mirrors,” in Integrated Photonics Research and Applications/Nanophotonics, Technical Digest (CD) (Optical Society of America, 2006), paper IWB.

10.

N. R. Huntoon, M. P. Christensen, D. L. MacFarlane, G. A. Evans, and C. S. Yeh, “Integrated photonic coupler based on frustrated total internal reflection,” Appl. Opt. 47(30), 5682–5690 (2008). [CrossRef] [PubMed]

11.

S. Zhu, A. W. Yu, D. Hawley, and R. Roy, “Frustrated total internal reflection: a demonstration and review,” Am. J. Phys. 54(7), 601–607 (1986). [CrossRef]

12.

Y. Qian, J. Song, S. Kim, and G. P. Nordin, “Compact 90 ° trench-based splitter for silicon-on-insulator rib waveguides,” Opt. Express 15(25), 16712–16718 (2007). [CrossRef] [PubMed]

13.

B. Kim and N. Dagli, “Submicron Etched Beam Splitters Based on Total Internal Reflection in GaAs–AlGaAs Waveguides,” J. Lightwave Technol. 28(13), 1938–1943 (2010). [CrossRef]

14.

E. Norberg, J. Parker, U. Krishnamachari, R. Guzzon, and L. Coldren, InGaAsP/InP based Flattened Ring Resonators iwth Etched Beam Splitters,” in Conference on Integrated Photonics and Nanophotonics Research and Applications Nanophotonics, Technical Digest (CD) (Optical Society of America, 2009), paper IWB.

15.

U. Krishnamachari, S. Ristic, C.-H. Chen, L. Johansson, A. Ramaswamy, J. Klamkin, E. Norberg, J. Bowers, and L. Coldren, “InP/InGaAsP-Based Integrated 3-dB Trench Couplers for Ultra-Compact Coherent Receivers,” IEEE Photon. Technol. Lett. (to be published).

16.

R. H. Renard, “Total reflection: a new evaluation of the Goos–Hänchen shift,” J. Opt. Soc. Am. 54(10), 1190–1197 (1964). [CrossRef]

17.

J. Parker, E. Norberg, R. Guzzon, S. Nicholes, and L. Coldren, “High verticality InP/InGaAsP etching in Cl2/H2/Ar ICP for photonic integrated circuits,” J. Vac. Sci. Technol. B (to be published).

18.

S. Bouchoule, G. Patriarche, S. Guilet, L. Gatilova, L. Largeau, and P. Chabert, “Sidewall Passivation Assisted by a Silicon Coverplate during Cl2-H2 HBr Inductively Coupled Plasma Etching of InP for Photonic Devices,” J. Vac. Sci. Technol. B 26(2), 666–675 (2008). [CrossRef]

19.

S. Rommel, J.-H. Jang, W. Lu, G. Cueva, L. Zhou, I. Adesida, G. Pajer, R. Whaley, A. Lepore, Z. Schellanbarger, and J. H. Abeles, “Effect of H2 on the etch profile of InP/InGaAsP alloys in Cl2/Ar/H2 inductively coupled plasma reactive ion etching chemistriesfor photonic device fabrication,” J. Vac. Sci. Technol. B 20(4), 1327–1330 (2002). [CrossRef]

20.

E. Norberg, R. Guzzon, and L. Coldren, “Programmable Photonic Filters Fabricated with Deeply Etched Waveguides,” in Proceedings of IEEE Conference on Indium Phosphide and Related Materials, (IEEE Photonics Society, Newport beach, CA, 2009), pp. 163–166.

21.

E. J. Skogen, J. W. Raring, G. B. Morrison, C. S. Wang, V. Lal, M. L. Masanovic, and L. A. Coldren, “Monolithically integrated active components: a quantum-well intermixing approach,” IEEE J. Sel. Top. Quantum Electron. 11(2), 343–355 (2005). [CrossRef]

22.

E. Norberg, R. S. Guzzon, S. C. Nicholes, J. S. Parker, and L. A. Coldren, “Programmable Photonic Lattice Filters in InGaAsP–InP,” IEEE Photon. Technol. Lett. 22(2), 109–111 (2010). [CrossRef]

23.

C. K. Madsen, and J. H. Zhao, Optical Filter Design and Analysis: A Signal Processing Approach, (Whiley-Interscience 1999), Chap. 6.

OCIS Codes
(130.0130) Integrated optics : Integrated optics
(230.1360) Optical devices : Beam splitters
(230.5750) Optical devices : Resonators
(250.5300) Optoelectronics : Photonic integrated circuits

ToC Category:
Integrated Optics

History
Original Manuscript: October 26, 2010
Revised Manuscript: December 17, 2010
Manuscript Accepted: December 22, 2010
Published: January 5, 2011

Citation
Erik J. Norberg, John S. Parker, Steven C. Nicholes, Byungchae Kim, Uppiliappan Krishnamachari, and Larry A. Coldren, "Etched beam splitters in InP/InGaAsP," Opt. Express 19, 717-726 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-2-717


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References

  1. C.-H. Chen, J. Klamkin, S. C. Nicholes, L. A. Johansson, J. E. Bowers, and L. A. Coldren, “Compact beam splitters with deep gratings for miniature photonic integrated circuits: design and implementation aspects,” Appl. Opt. 48(25), F68–F75 (2009). [CrossRef] [PubMed]
  2. R. Grover, V. Van, T. A. Ibrahim, P. P. Absil, L. C. Calhoun, F. G. Johnson, J. V. Hryniewicz, and P.-T. Ho, “Parallel-Cascaded Semiconductor Microring Resonators for High-Order and Wide-FSR Filters,” J. Lightwave Technol. 20(5), 872–877 (2002). [CrossRef]
  3. A. Yalcin, K. C. Popat, J. C. Aldridge, T. A. Desai, J. Hryniewicz, N. Chbouki, B. E. Little, V. Oliver King, Van, D. Sai Chu, M. Gill, M. S. Anthes-Washburn, Unlu, and B. B. Goldberg, “Optical sensing of biomolecules using microring resonators,” IEEE J. Sel. Top. Quantum Electron. 12(1), 148–155 (2006). [CrossRef]
  4. Y. Ma, S. Park, L. Wang, and S. T. Ho, “Ultracompact multimode interference 3-dB coupler with strong lateral confinement by deep dry etching,” IEEE Photon. Technol. Lett. 12(5), 492–494 (2000). [CrossRef]
  5. Y. Shi, S. He, and S. Anand, “Ultracompact directional couplers realized in InP by utilizing feature size dependent etching,” Opt. Lett. 33(17), 1927–1929 (2008). [CrossRef] [PubMed]
  6. S. J. Choi, K. Djordjev, S. J. Choi, P. D. Dapkus, W. Lin, G. Griffel, R. Menna, and J. Connolly, “Microring resonator vertically coupled to buried heterostructure bus waveguides,” IEEE Photon. Technol. Lett. 16(3), 828–830 (2004). [CrossRef]
  7. J. S. Osinski, C. E. Zah, R. Bhat, R. J. Contolini, E. D. Beebe, T. P. Lee, K. D. Cummings, and L. R. Harriott, “Miniature integrated optical Beam-Splitter in AlGaAs/GaAs ridge waveguides,” Electron. Lett. 23(21), 1156–1158 (1987). [CrossRef]
  8. L. Li, G. P. Nordin, J. M. English, and J. Jiang, “Small-area bends and beamsplitters for lowindex-contrast waveguides,” Opt. Express 11(3), 282–290 (2003). [CrossRef] [PubMed]
  9. B. Kim, and N. Dagli, “Compact Micro Resonators with Etched Beam Splitters and Total Internal Reflection Mirrors,” in Integrated Photonics Research and Applications/Nanophotonics, Technical Digest (CD) (Optical Society of America, 2006), paper IWB.
  10. N. R. Huntoon, M. P. Christensen, D. L. MacFarlane, G. A. Evans, and C. S. Yeh, “Integrated photonic coupler based on frustrated total internal reflection,” Appl. Opt. 47(30), 5682–5690 (2008). [CrossRef] [PubMed]
  11. S. Zhu, A. W. Yu, D. Hawley, and R. Roy, “Frustrated total internal reflection: a demonstration and review,” Am. J. Phys. 54(7), 601–607 (1986). [CrossRef]
  12. Y. Qian, J. Song, S. Kim, and G. P. Nordin, “Compact 90 ° trench-based splitter for silicon-on-insulator rib waveguides,” Opt. Express 15(25), 16712–16718 (2007). [CrossRef] [PubMed]
  13. B. Kim and N. Dagli, “Submicron Etched Beam Splitters Based on Total Internal Reflection in GaAs–AlGaAs Waveguides,” J. Lightwave Technol. 28(13), 1938–1943 (2010). [CrossRef]
  14. E. Norberg, J. Parker, U. Krishnamachari, R. Guzzon, and L. Coldren, InGaAsP/InP based Flattened Ring Resonators iwth Etched Beam Splitters,” in Conference on Integrated Photonics and Nanophotonics Research and Applications Nanophotonics, Technical Digest (CD) (Optical Society of America, 2009), paper IWB.
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