Publisher’s Note: This paper, originally published on 6 September 2019, was replaced with a corrected version on 10 October 2019. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.
We report on a monolithic indium phosphide photonic integrated transmitter capable of generating high-speed return-tozero differential phase shift keying (RZ-DPSK) data streams for space optical communications as high as 5 Gbps. The integrated transmitter includes a sampled grating distributed Bragg reflector laser continuously tunable over 30 nm in the C-band, a semiconductor optical amplifier for amplification, a Mach-Zehnder modulator for encoding phase-shift-keying data, and electro-absorption modulator for return-to-zero pulse carving. The transmitter is situated in a custom electronics test bed for biasing various PIC sections and driving the modulators. Furthermore, this transmitter can also be utilized for 10 Gbps DPSK or NRZ-OOK.
We present results of an indium phosphide (InP) monolithic photonic integrated circuit (PIC) transmitter suitable for space-optical communications up to 10 Gbps for NRZ-OOK and DPSK modulation and up to 5 Gbps RZ-DPSK modulation. The PIC includes an SG-DBR laser tunable across the entire telecommunications C-Band, a semi-conductor optical amplifier (SOA), a Mach-Zehnder modulator (MZM) for efficient encoding of phase information, and an electroabsorption modulator (EAM) which serves as an RZ pulse carver. The transmitter PIC is integrated in a testbed with a custom board that provides biasing and driving electronics. The commercial-off-the-shelf (COTS) differential driver generates an estimated 4.5-5 Vpp differential modulation voltage for the dual drive MZM. An identical driver was used for the EAM 50% RZ pulse carver but only a single output was used with the other output terminated with a 50Ω load. The SOA and laser gain sections were biased at 90 mA each. Clear eye openings were achieved for all modulation formats.
Links at 1 micron offer key advantages over longer wavelength links. Both ultra-stable, low noise Nd:YAG lasers and high power efficiency, temperature-stable GaAs lasers operate at wavelengths around 1 micron. These components are particularly beneficial for quantum optical systems and links which require stability over a wide range of temperatures, such as are required in avionics. However, a key component missing in these 1 micron photonic links is a high-power photodiode receiver with high linearity and high quantum efficiency. Freedom Photonics and the University of Virginia have collaborated to develop photodiodes which fill this need. The photodetectors are based on an optimized vertically illuminated modified uni-traveling carrier (MUTC) photodiode technology. We report devices with quantum efficiencies in excess of 80% at 1064 nm, with a 3-dB bandwidth of 28 GHz, for a 20µm diameter device. The same device size handles very high power, with a 1-dB compression of >16 dBm RF power at a 64-mA photocurrent. These photodiodes have a major impact on peak performance of a photonic link, supporting high link gain and large bandwidths. Additionally, the high linearity of these devices minimizes noise and signal distortion, maximizing spurious-free dynamic range (SFDR). These are the first photodiodes of this type which have been packaged and made commercially available for this target wavelength.
High frequency analog RF photonic links are desirable to reduce the size, weight and power of RF systems by offering the replacement of lossy, bulky coaxial RF cabling for lightweight, low loss and broadband optical fiber, particularly in applications such as avionics and naval RADAR systems, electronic warfare and distribution of low-jitter clocks or local oscillator signals. Freedom Photonics and the University of Virginia have developed high power, wide-bandwidth optical photodetectors operating in the 1550-nm wavelength range. These photodetectors are based on vertically illuminated modified uni-traveling carrier (MUTC) photodiode technology. The devices have been developed into fully packaged, fiber-pigtailed modules with optimization for high powers or high speeds. This paper will present the architecture and experimental results of our range of photodiodes. One family of devices focuses on high power applications. These include high-power photodiodes with 3-dB bandwidths of 25 GHz coupled with output powers in excess of 23 dBm, as well as 35 GHz photodiodes with output powers greater than 19 dBm. Another family of devices focuses on high speed applications, including photodiodes with 3-dB bandwidths of >65 GHz and >100 GHz. These photodiodes, used in a photonic link, have a major impact on peak performance. The high power-handling capability and high speeds of these devices support high link gain and large bandwidths, while the high linearity of these devices minimizes noise and signal distortion, maximizing spurious-free dynamic range (SFDR).
Deep space exploration will require laser communication systems optimized for cost, size, weight, and power. To improve these parameters, our group has been developing a photonic integrated circuit (PIC) based on indium phosphide for optical pulse position modulation (PPM). A field-programmable gate array (FPGA) was programmed to serve as a dedicated driver for the PIC. The FPGA is capable of generating 2-ary to 4096-ary PPM with a slot clock rate up to 700 MHz.
This work demonstrates the operation of a photonic integrated circuit transmitter for space optical communication utilizing an RZ-DPSK modulation format realized on an indium phosphide monolithic integration platform. It includes a widely tunable sampled grating distributed Bragg reflector laser, a semiconductor optical amplifier for amplification and burst mode operation, a dual drive Mach-Zehnder modulator (MZM) that efficiently encodes phase information, and an electro-absorption modulator RZ pulse carver. <p> </p>The laser tuning range is approximately 35 nm across the telecommunications C-band. The MZM DC extinction ratio exceeds 15 dB for a differential drive voltage of 6 V peak-to-peak. Clear eye diagrams were demonstrated at 3 Gbps for OOK modulation and 1 Gbps for RZ-DPSK modulation.
Atmospheric methane (CH4) is the second most important anthropogenic greenhouse gas with approximately 25 times the radiative forcing of carbon dioxide (CO2) per molecule. CH4 also contributes to pollution in the lower atmosphere through chemical reactions leading to ozone production. Recent developments of LIDAR measurement technology for CH4 have been previously reported by Goddard Space Flight Center (GSFC). In this paper, we report on a novel, high-performance tunable semiconductor laser technology developed by Freedom Photonics for the 1650nm wavelength range operation, and for LIDAR detection of CH4. Devices described are monolithic, with simple control, and compatible with low-cost fabrication techniques. We present 3 different types of tunable lasers implemented for this application.
High-performance photodetectors (HPPDs), with high output power and bandwidth, are needed for RF photonics links. Applications for these HPPDs range from high-power remote antennas, low-duty-cycle RF pulse generation, linear photonic links, high dynamic range optical systems, and radio-over-fiber (ROF). Freedom Photonics is a manufacturer of high-power photodetectors (HPPD) for the 1480 to 1620nm wavelength range, now being offered commercially. In 2016, Freedom has developed a HPPD for similar applications extending into the V-band. The basic device structure used for these photodetectors can achieve over 100-GHz bandwidths with slight variations. This work shows data for RF power and bandwidth performance for various size photodiodes, between 10 μm and 28 μm in diameter. Measurement data will be presented, which were collected at both assembly level and for fully packaged detectors. For detector devices with bandwidth performance over 50 GHz, the generated RF power achieved is expected to be over 15 dBm. This performance is exceptional considering the photodiode is fully integrated into a hermetic package designed for 65 GHz. Improvements in the coplanar waveguide (CPW) transmission line and flip-chip bonding design were integral in achieving the higher saturation at the higher bandwidth performance. Further development is required to achieve a >100 GHz packaged photodetector module.
RF photonic systems place extremely high demands on optical component performance. To achieve this, a low noise, high power optical source, a high power, linear and low Vπ optical modulator, sharp and uniform optical filters and high saturation power photodetectors are required. While some of these individual components exist, they have not, to date, been integrated in any currently existing monolithic or hybrid photonic integration platform. In this paper, recent advances in discrete component performance is presented, including optical sources, modulators and detectors. In addition, options for the integration of these components onto an integrated photonic platform is reviewed.
High power photodiodes are needed for a range of applications. The high available power conversion efficiency makes these ideal for antenna remoting applications, including high power, low duty-cycle RF pulse generation. The compact footprint and fiber optic input allow densely packed RF aperture arrays with low cross-talk for phased high directionality emitters. Other applications include linear RF photonic links and other high dynamic range optical systems. Freedom Photonics has developed packaged modified uni-traveling carrier (MUTC) photodetectors for high-power applications. Both single and balanced photodetector pairs are mounted on a ceramic carrier, and packaged in a compact module optimized for high power operation. Representative results include greater than 100 mA photocurrent, >100m W generated RF power and >20 GHz bandwidth. In this paper, we evaluate the saturation and bandwidth of these single ended and balanced photodetectors for detector diameter in the 16 μm to 34 μm range. Packaged performance is compared to chip performance. Further new development towards the realization of <100GHz packaged photodetector modules with optimized high power performance is described. Finally, incorporation of these photodetector structures in novel photonic integrated circuits (PICs) for high optical power application areas is outlined.