Compact Robust Integrated PPM Laser Transceiver Chip Set with High Sensitivity, Efficiency, and Re-Configurability

There have been significant interests from NASA in integrated optical transceiver chips for near-earth space optical communications utilizing Pulse Position Modulation (PPM) scheme, especially those utilizing photonic integration on silicon with CMOS compatible technology. NASA’s desires include the realization of a PPM laser transmitter chip supporting data rates from at least 10Mbps to 1Gbps and a direct detection PPM receiver chip. These chips based on photonic integrated circuit (PIC) will be small in size and rugged. To address the abovementioned interests, our works focused on realizing integrated PPM transmitter and receiver chips based on a few key technologies we have, including heterogeneous integration involving bonding of an epitaxial layer of InP based materials onto silicon surface to provide the required optical gain for realizing lasers. Specifically, we planned to realize a pulse-position modulation (PPM) laser transmitter chip that would be capable of transmitting signals using 10 or more wavelength channels with around 25mW or higher per channel, individually modulated with bit rate from 10Mbps up to 1Gbps (per channel). This gives over 1Gbps total modulation rate (with 10 or more channels). The proposed research would enable us to realize innovative PPM transceiver chips with enhanced capabilities giving: (a) A transmitter chip that can send signals using 10 or more wavelength channels capable of around 25mW or higher per channel, and a matching receiver chip. When all the 10 channels are used concurrently, the total laser power can be increased to provide >200mW power when needed. This can help in emergency communication recovery, or increasing the communication distance. (b) Higher data rate than the 1Gbps PPM data rate targeted by NASA by having the capability to modulate from 10Mbps up to 1Gbps (per channel). With 10 or more wavelength channels, the total data rate can be 10 times higher. These extra data rate can be activated on demand using the same transmission beam. (c) Backup capability by providing multiple communication wavelength channels. (d) All these capabilities on a highly-integrated silicon-photonics chip. In comparison with the existing technology, the commercially available lasers currently are typically single wavelength. To have many wavelength channels, many lasers and a sizable wavelength multiplexer are needed, resulting in low robustness and large system size based on currently available components. Our integrated chip has a chip size smaller than 2mm x 2mm for the optical transceiver and would result in over 100x smaller foot print comparing to discrete-component based system provided by the present technology. The proposed works are meant to develop and demonstrate the key technologies that will make the above capabilities feasible. It is not intended to be exhaustive as that will require substantially larger efforts. Such integrated chips will provide an enabling technology that will enable space optical communication transceivers to be realized with higher robustness, enhanced capabilities, multiple functionalities, lower weight, and potentially lower costs. The technology will also benefit the photonic device and optical communication technology areas generally. Through the reaserch efforts, we have successfully shown the capability to realize transmitter chips with multiple lasing wavelength channels utilizing two approaches: (1) In the first approach, we realized a 4 wavelength-channel laser chip using an integrated diffraction grating as the wavelength selective element. The same diffraction grating is also used for combining the multiple wavelength channels into a single output waveguide; (2) In the second approach, 15 wavelength channels were realized by 15 integrated Distributed Bragg Reflector (DBR) based lasers that give 15 different lasing wavelengths. These lasers were designed so that the 15 wavelength channels span a wavelength range of 30nm with each laser capable of emitting about 22mW. On the laser modulation rate, we have demonstrated a modulation capability of over 140Mb/sec by direct laser current modulation. The modulation rate can be pushed higher in the future with further device optimization. Thus, we have shown the feasibility of realizing multiple-wavelength-channel transmitter chips with the capability of providing around 20mW per channel through two different schemes, with modulation capability of at least 140Mb/sec for each channel. The technology developed can be expanded in the future to provide more capabilities.