DARPA CS-WDM



DARPA CS-WDM

Quarterly Report

Award Number N66001-02-C-8026

“On-Chip Integration of Advanced Wavelength and Switching Functions for Wavelength-Agile Analog/Digital Optical Networks”

Report Date

March 31, 2003

Principal Investigator

Daniel J. Blumenthal

Room 5163, Engineering I

University of California

Santa Barbara, CA 93106

Tel: (805) 893-4168

Fax: (805) 893-5705

Email: danb@ece.ucsb.edu

Report Distribution

cdrl@spawar.navy.mil

Hodiak@spawar.navy.mil

jshah@darpa.mil

alex@ece.ucsb.edu

Table of Contents

1. Executive Summary 2

2. Program Management Plan 2

2.1. Program Plan Progress Summary 2

2.2. All Optical Tunable Wavelength Converter 4

2.2.1. Integrated Sampled-Grating DBR tunable laser and the Mach Zehnder WC 4

2.2.2. Performance optimization of AOTWCs 6

2.3. Filter/Mux/Router 8

2.4. OEOIC-WC 9

2.4.1. Directly Modulated SGDBR Wavelength Converters 9

2.4.2. Integrated External Modulator Wavelength Converters 11

2.5. Wafer Processing Subcontract (Agility Communications) 12

3. Publications 12

4. Programmatic Interactions and Collaborations 13

4.1. Test Facility: MIT Lincoln Labs 13

4.2. Architecture Study: MIT 13

5. Patents 13

6. Appendix A: Program Management Plan for Baseline Period 14

7. Appendix B: Budget Summary for FY02 15

Executive Summary

The scope of this program is to develop and demonstrate chip-scale integrated wavelength conversion and routing functions on a common substrate. The wavelength converters and routers developed will be photonic integrated circuits on InP substrates. The wavelength converters and routers will be compatible with both analog and digital transmission requirements. Issues involved in integration will be studied. Three classes of device will be investigated. The program is organized and managed by sub-area according to these three classes of device:

• Sub-area I (AOWC): In-plane SOA interferometric wavelength converter.

• Sub-area II (Filter/Mux/Router): Wavelength router and multiplexers based on filter and interferometric technologies.

• Sub-area III (OEIC-WC): Optoelectronic integrated wavelength converter with monitoring function capabilities.

In this quarter, the following significant achievements were made:

1. Increased tuning range of integrated laser/AOWC to 40 nm.

2. Designed and fabricated mask for new TIR mirror based ring resonator filter.

3. Fabricated first generation directly modulated OEIC wavelength converter.

Program Management Plan

A program management plan for the baseline period is included as an Appendix in the form of Microsoft Project Gantt Chart identifying major tasks, milestones of the major tasks and their completion dates. A graphical representation of the budget for the first spending increment (FY02) is also included as an appendix to this quarterly report to the distribution list in the form of an excel graph showing the (1) monthly planned, committed and most recent actual spending, (2) cumulative spending (planned, committed and actual), and (3) cumulative total of money received from the government.

1 Program Plan Progress Summary

Table 1 summarizes progress in each research sub-area with respect to the project plan shown in Appendix A.

Table 1. Summary of status of tasks and milestones.

|Task/Milestone |Description |Due Date |Status |

|Task: AOWC |Development of growth and processing technologies to fabricate |1/31/03 |Completed 12/5/02|

| |integrated MZI-WC with tunable laser using offset quantum well | | |

| |design for active/passive waveguide fabrication | | |

|Milestone: AOWC |Report details of integration process and process |2/3/03 |Completed 2/3/03 |

| |characterization | | |

|Milestone: AOWC |Mask fabrication, device processing and mounting |4/21/03 |Completed 3/15/03|

|Milestone: AOWC |Digital and analog characterization |9/1/03 |In Progress |

|Task: InP Ring Resonator |Development of growth and processing technologies to fabricate |9/20/02 |Completed 12/1/02|

| |disk resonator filter compatible with in-Plane SOA-WC | | |

|Task: InP Ring Resonator |Mask fabrication, device processing, mounting and AR coating |12/13/02 |Prototype |

| | | |Completed |

| | | |12/13/02 |

|Task: InP Ring Resonator |Fabricate and test small resonators |3/25/03 |Partially |

| | | |completed, filter|

| | | |redesigned based |

| | | |on results |

|Task: InP TIR Mirror |Development of growth and processing technologies to fabricate |9/20/02 |FIB Prototype |

| |strongly guiding waveguides and mirrors | |Completed 12/1/02|

|Task: InP TIR Mirror |Mask fabrication, device processing |1/31/03 |FIB Prototype |

| | | |Completed 12/1/02|

|Task: InP TIR Mirror |Fabricate and test TIR mirrors |3/31/03 |Fabrication |

| | | |complete, testing|

| | | |underway |

|Task: Directly Modulated |Develop growth and processing technologies to fabricate |12/13/02 |Completed 11/2/02|

|OEOIC-WC |integrated PICs with tunable laser | | |

|Task: Directly Modulated |Device fabrication and basic characterization of directly |5/2/03 |In progress |

|OEOIC-WC |modulated OEIC wavelength converter | | |

|Task: Design PD/EAM with no |Study and assess alternative SOA and detector designs for the |12/13/02 |Completed |

|Interface Electronics |directly modulated | |12/10/02 |

|Task: Design PD/EAM with no |Study and evaluate saturable absorber and gain levered direct |12/31/02 |10/15/02 |

|Interface Electronics |modulated devices | | |

|Task: Design PD/EAM with no |Design and simulate various embodiments of PD/EAM modulated |8/29/03 |In progress |

|Interface Electronics |wavelength converters | | |

2 All Optical Tunable Wavelength Converter

1 Integrated Sampled-Grating DBR tunable laser and the Mach Zehnder wavelength converter

We have developed a robust fabrication process in the UCSB cleanroom. We have established a set of verification tests to quantify and check all the critical steps in the fabrication process. The process uses the most automated and up to date equipment available in our cleanroom which should yield very reproducible fabrication results and reduce the fabrication turn-around time. We have performed basic laser material characterization in order to verify the process and assess its quality and compatibility with the platform epi/regrowth process. The AR coating design has been optimized.

The first batch of wavelength converters integrated with the widely tunable lasers was successfully fabricated as shown in Figure 1. Currently we have three different WC designs that are operational (both static and dynamic operation). All of the devices use a common surface ridge technology and have a common tunable laser design and different interferometer structures (short multimode interference splitter (MMI) – S-bend Mach-Zehnder Interferometer (MZI) (AOWC), simple MMI-MZI (TAOMI) and folded MMI-MZI (TAOMI II)).

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Figure 1 - The first generation of the integrated sampled grating DBR laser and a MZI wavelength converter

The fabricated devices operate in the L-band region, with around 20nm of tuning range (1561nm to 1580 nm). The measured output power is currently –2dBm and is largely limited by thermal crosstalk between the active region of the laser and the input semiconductor optical amplifier SOA used to decrease the insertion loss. These issues are being addressed in the next generation WC design. The insertion loss is currently on the order of 15 dB and will also be improved in the next design phase.

Both static and dynamic characterizations are in progress. To date, the best static electrical extinction measured is 28 dB (Figure 2). Error free operation was achieved at bit rates of 2.5GB/s and 5 GB/s and example eye diagrams are shown in Figures 3 and 4. Carrier dynamics in the SOA of the MZI currently limit the speed of operation to 5 GB/s.

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Figure 2 - Electrical control of the wavelength converter – static electrical extinction curve

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Figure 3 – Measured eye diagram for the wavelength converted signal at 2.5 GB/s

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Figure 4 – Measured eye diagram for the wavelength converted signal at 5 GB/s

Preliminary bit error rate measurements were conducted at 2.5 GB/s with no observed error floor and a power penalty of 2.6 dB. These results, shown in Figure 5, are for wavelength conversion from two different input wavelengths to one output wavelength for the TAOMI design.

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Figure 5. Measured BER curves for TAOMI device design

Our goal for the immediate future is to finish the characterization of the devices fabricated – determine bit error rates for different combinations of input and output wavelength over the entire tuning range of the current L-Band devices. We will then o design and implement improved devices that have reduced insertion losses (by decreasing passive waveguide losses and increasing the output power of the laser) as well as increased maximum operating speed and operation in the C-Band.

2 Performance optimization of AOTWCs

In this part of the project our goal is to optimize WC performance using numerical tools to accurately predict the static and dynamic behavior based on a particular WC design. Over the past quarter we have moved further towards being able to fully simulate the wavelength converters.

We have implemented a Quantum Well gain model to predict the material optical properties used in the integrated platform. Our model is capable is of accurately computing the Spontaneous Emission rate in the material. Over the past quarter we have extended the capabilities of the model to compute anomalous dispersion (change in index due to change in gain) through the Kramers-Kronig relations. We are now working to determine the required parameters for dynamically simulating the cross-gain and cross phase modulation (XGM\XPM) effects in our devices.

We have implemented a dynamic rate equation based model for Semiconductor Optical Amplifiers (SOA) in order to model the behavior with digitally modulated signals. The model has been extended to account for the bidirectional nature of ASE in the SOA structures which can have significant effects on the gain and bandwidth of these amplifiers. The model has been used to study carrier lifetimes in different active waveguide structures (Figure 6) and improve our understanding on how different factors such as waveguide layer structure, optical mode power etc. influence it and hence the speed of operation for these devices.

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Figure 6 - Example simulation of optical power and carrier lifetime as a function of position with an SOA

Figure 7 shows NRZ bits after propagating through the device along with the wavelength converted data under inverting operation. The slower rise-time of the edge due to lower carrier lifetime during input 0’s is clearly seen in the simulation as is also seen in actual experiments. We are currently working on validating our model parameters using gain and index measurements on SOAs that we fabricated.

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Figure 7. Example input and output bit patterns from WC simulation tool

3 Filter/Mux/Router

During the last quarter we have completed process development and fabrication of ring resonators. Based on these preliminary ring resonator results we have modified the original design and fabricated a new resonator design that employs turning mirrors. Figure 8 shows the schematic of the filter with turning mirrors. In this case a square guiding structure coupled with total internal reflection turning mirrors forms the resonator cavity. There are significant advantages to this design. First the coupling between the passive input output waveguide and the resonator cavity is through a regular coupler. This should increase the amount of power coupling significantly. Second, the resonator cavity itself is a low-loss optical guiding structure with high confinement. Third, the active section of the cavity enables excellent overlap of the propagating wave and the gain region improving pump efficiency. The optical mode is confined inside the resonator cavity, minimizing the effect of sidewall roughness. Also there is no etching through the active region, hence gain should be obtained at a lower current density. This design is fully compatible with the laser and wavelength converter and integration is much easier. But for this design to be successful, the quality of the mirrors needs to be extremely high. We have completed the mirror design associated process development. Figure 9 shows a deep etch inside a turning mirror. Test of these filters is currently underway.

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Figure 8. Newly designed resonator based on turning mirrors.

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Figure 9. Deep etch inside a turning mirror for filter in Figure 8.

4 OEOIC-WC

1 Directly Modulated SGDBR Wavelength Converters

In this quarter, the first generation directly modulated OEIC wavelength converters, introduced in the last quarter has been characterized. The devices has been AR-coated, mounted and wire-bound to an AlN-carrier substrate for heat-sinking and proper RF monitor probing. Figure 1 shows the resulting optical AC gain performance. The gain peaks at about –10dB, and the bandwidth is above 3GHz at higher bias currents.

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Figure 1: Small signal optical AC gain as a function of frequency for different gain section bias currents and conversion from 1547nm to 1545 nm.

The spurious-free dynamic range (SFDR) of the device has also been characterized. The SFDR was measured at 0.5 GHz using two tones separated by 1 MHz. To reduce any intermixing components between the two tones at the input, the two tones were modulated to two separate optical sources, coupled together and tuned to a wavelength offset of 60GHz around 1555 nm, well above the detector bandwidth to avoid coherence effects. Figure 2 shows the measured SFDR: 92.3dBHz2/3, at 55mA bias to the gain section, while converting from 1555nm to 1545 nm. Figure 3 shows the SFDR-performance for different gain section bias currents.

The wavelength tuning range of the SGDBR laser was compromised due to a defective mirror design. Nevertheless, 2.5 Gb/s wavelength conversion were conducted from 1547nm to an output wavelength of 1545nm, 1550nm, 1552nm, and 1560nm. In all cases, error-free conversion was obtained. Figure 4 shows the detected eye diagram at 1545nm. Signal monitoring was achieved by connecting the gain section to a 50( serial resistance and an RF coplanar line for RF probing. Error-free signal monitor at 2.5 Gb/s was obtained. Figure 5 shows the detected monitor eye diagram. Future plans include further characterization of the signal monitor performance.

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Figure 2: Measured SFDR for conversion from 1555nm to 1545nm at 55mA gain section bias.

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Figure 3: Measured SFDR as a function of gain section bias

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Figure 4: Detected and filtered eye-diagram at 2.5 Gb/s for conversion from 1547nm to 1545nm.

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Figure 5: Detected and filtered eye-diagram at 2.5 Gb/s for monitor signal taken over the directly-modulated SGDBR gain section.

In summary, the first generation OEIC directly modulated embodiment provides about the expected level of performance for the quality of the devices fabricated. Simple improvements in contact resistance should result in the ability to significantly increase the preamplifier gain to provide more photocurrent and reduce overall insertion loss from the -10 dB level observed to near zero net insertion loss. The SFDR and modulation bandwidth should also improve directly as the improvement in laser output which will result from the corrected mirror design as well as the improved contacts.

The design of the first integrated externally modulated OEIC-WC has been completed and fabrication begun. Results from both Mach-Zehender and EAM external modulators are expected within the next month. Simulations suggest that improved speed and SFDR should result.

2 Integrated External Modulator Wavelength Converters

A first generation mask has been designed and is in house where wavelength conversion is achieved using the signal from a photodetector to drive an external integrated Electro absorption modulator (EAM). Processing is currently underway and should be completed within the next several weeks. In addition, a dual EAM scheme is being examined for increased linearity and better chirp performance, and a Mach Zehnder based device is under examination for low drive voltage, more efficient power handling capability, and chirp design.

5 Wafer Processing Subcontract (Agility Communications)

The wafer delivery, regrowth and AR coating schedule is on target per project requirements and subcontract. The planned and actual EPI wafer delivery schedule is shown in Figure 14.

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Figure 14. Subcontract EPI wafer delivery schedule (actual and planned)

Publications

The following publications have resulted to-date from the research funded under this program.

1] “Monolithically Integrated Mach-Zehnder Interferometer Wavelength Converter and Widely-Tunable Laser in InP,” Milan L. Mašanović, Vikrant Lal, Jonathon S. Barton, Erik J. Skogen, Daniel J. Blumenthal, Larry A. Coldren, submitted to IEEE Photonics Technology Letters, 2003.

2] “Widely-Tunable Chip-Scale Transmitters and Wavelength Converters,” Larry A. Coldren , IPR 2003, Invited talk

3] “InP Laterally Tapered Wide-bandwidth Optical Power Splitter,” Xuejin Yan, Marcelo Davanco, Milan Masanovic, Wenbin Zhao, Daniel J. Blumenthal, to be presented at CLEO, Maryland. June, 2003.

4] “Demonstration of Monolithically-Integrated InP Widely-Tunable Laser and SOA-MZI Wavelength Converter,” Milan L. Mašanović, Erik J. Skogen, Jonathon S. Barton, Vikrant Lal, Daniel J. Blumenthal and Larry A. Coldren To be presented at the Fifteenth International Conference on Indium Phosphide and Related Materials, May 12 - 16, Santa Barbara, CA, 2003.

5] “Multimode Interference-Based 2-Stage 1x2 Light Splitter for Compact Photonic Integrated Circuits,” Milan L. Mašanović, Erik J. Skogen, Jonathon S. Barton, Joseph Sullivan, Daniel J. Blumenthal and Larry A. Coldren, To Appear in IEEE Photonics Technology Letters, May 2003.

6] “Cascaded Multimode Interference-Based 1x2 Light Splitter for Photonic Integrated Circuits,” M. Masanovic, E. Skogen, Barton, J. Sullivan, D. J. Blumenthal, and L. Coldren, Topical Meeting on Integrated Photonics Research (IPR), Vancouver, Canada, Paper IThA5, Jul 14-17, 2002.

7] “Integrated Devices for Wavelength-Agile All-Optical Networks,” D. J. Blumenthal, Topical Meeting on Integrated Photonics Research (IPR), Vancouver, Canada, Paper IWB1, July 14-17, 2002 (Plenary Paper)

Programmatic Interactions and Collaborations

1 Test Facility: MIT Lincoln Labs

Visit to MIT-LL is scheduled for May 2003 to test first generation optical WCs. Two UCSB students will visit with devices for 7 days and help get up and running at the test facility.

2 Architecture Study: MIT

• Meeting with Vincent Chan at UCSB is scheduled for April 17th.

Patents

None to date.

Appendix A: Program Management Plan for Baseline Period

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Appendix B: Budget Summary for FY02

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