The use of photonic integrated circuits and components in many areas across the general area of laser systems is increasing. Example applications of such systems include free space optical communication, remote standoff sensing, solid state and fiber laser pumping, LIDAR for autonomous vehicles, and atomic laser systems for position, navigation, and timing. In this talk we will review the design, performance, and robustness of Freedom Photonics high performance integrated photonic components for these applications and others, focusing in particular on recent advancements in our products at 780 nm, 1060 nm, 1310 nm, and at 1550 nm.
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.
NASA is working with US industry and academia to develop Photonic Integrated Circuits (PICs) for: (1) Sensors (2) Analog RF applications (3) Computing and free space communications. The PICs provide reduced size, weight, and power that is critical for space-based systems. We describe recent breakthrough 3D monolithic integration of photonic structures, particularly high-speed graphene-silicon devices on CMOS electronics to create CMOS-compatible highbandwidth transceivers for ultra-low power Terabit-scale optical communications. An integrated graphene electro-optic modulator has been demonstrated with a bandwidth of 30 GHz. Graphene microring modulators are especially attractive for dense wavelength division multiplexed (DWDM) systems. For space-based optical communication and ranging we have demonstrated generating a variable number of channels from a single laser using breadboard components, using a single-sideband carrier-suppressed (SSBCS) modulator driven by an externally-supplied RF tone (arbitrary RF frequency), a tunable optical bandpass filter, and an optical amplifier which are placed in a loop. We developed a Return--to-Zero (RZ) Differential Phase Shift Keying (DPSK) laser transmitter PIC using an InP technology platform that includes a tunable laser, a Semiconductor Optical Amplifier (SOA), high-speed Mach-Zehnder Modulator (MZM), and an electroabsorption (EAM) modulator. A Silicon Nitride (SiN) platform integrated photonic circuit suitable for a spectrally pure chip-scale tunable opto-electronic RF oscillator (OEO) that can operate as a flywheel in high precision optical clock modules, as well as radio astronomy, spectroscopy, and local oscillator in radar and communications systems is needed. We have demonstrated a low noise optical frequency combs generation from a small OEO prototypes containing very low loss (~1 dB) waveguide couplers of various shapes and sizes integrated with an ultrahigh-Q MgF2 resonators. An innovative miniaturized lab-on-a-chip device is being developed to directly monitor astronaut health during missions using ~3 drops of body fluid sample like blood, urine, and potentially other body fluids like saliva, sweat or tears. The first-generation system comprises a miniaturized biosensor based on PICs (including Vertical Cavity Surface Emitting Laser – VCSEL, photodetector and optical filters and biochemical assay that generates a fluorescent optical signal change in response to the target analyte.
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.
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.
We propose and simulate integrated optical devices for accelerating numerical linear algebra (NLA) calculations. Data is modulated on chirped optical pulses and these propagate through a multimode waveguide where speckle provides the random projections needed for NLA dimensionality reduction.
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.
Sampled Grating Distributed Bragg Reflector (SGDBR) monolithic tunable lasers are now entering the production
phase in telecommunications applications. These tunable lasers are unique in that they offer wide wavelength tuning
(1525 to 1565 nm), fast wavelength tuning (5 ns) and high speed amplitude modulation all on the same monolithic
chip1,2,3,4. This work studies the applicability of SGDBR monolithic tunable laser diodes for biomedical imaging using swept-wavelength or Fourier domain optical coherence tomography. This paper will present our work involved with utilizing the strengths (table 1) of this SGDBR laser class and mitigating the weaknesses (table 2) of this device for swept-wavelength imaging applications. The strengths of the laser are its small size (portable solutions), wide wavelength range (good distance resolution), fast switching speeds (improved update rates), wide choice of center wavelengths, and lower power consumption. The weaknesses being addressed are the complicated wavelength tuning mechanism (3 wavelength control currents), wider laser linewidth (10s of MHz), moderate output power (10mW ), and the need for improved laser packaging. This paper will highlight the source characterization results and discuss an initial measurement architecture utilizing the SGDBR measurement engine.
We demonstrate 10Gbit/s operation of two different types of monolithic photocurrent driven wavelength converters (PD-WC). These photonic integrated circuits use a Semiconductor Optical Amplifier (SOA)-PIN photodetector receiver to drive an Electro-absorption (EA), or Mach-Zehnder (MZ) modulator that is integrated with a SGDBR tunable laser. We demonstrate improvements in optical bandwidth, insertion losses, device gain, and modulation efficiency.
Wavelength converters are seen as important to the scalability, flexibility, and cost of future optical networks. These devices have opportunities for deployment in optical switches, routers and add/drop multiplexers. This talk will outline the latest results of monolithic and hybrid photocurrent-driven wavelength converters (PD-WC) based on either the direct modulation of a bipolar cascade SGDBR laser or by external modulation using an Electro-absorption (EA), or Mach-Zehnder (MZ) modulator using integration building blocks such as a semiconductor optical amplifiers (SOA), SGDBR lasers, PIN detectors and EA and MZ modulators. As the input and output waveguides are separate in this configuration of wavelength converter, an optical filter is not required to reject the input signal at the output which is desirable particularly with wavelength tunable applications where the response time of a filter could limit system performance.
In this work, we describe tunable wavelength converters based on a photodiode receiver integrated with a tunable laser transmitter. Devices are fabricated on a robust InP ridge/InGaAsP waveguide platform. The photodiode receiver consists of an integrated SOA pre-amplifier and a PIN diode to improve sensitivity. The laser transmitter consists of a 1550 nm widely tunable SGDBR laser modulated either directly or via an integrated modulator outside the laser cavity. An SOA post-amplifier provides high output power. The integrated device allows signal monitoring, transmits at 2.5 GB/s, and removes the requirements for filtering the input wavelength at the output. Integrating the SGDBR yields a compact wavelength agile source that requires only two fiber connections, and no off-chip high speed electrical connections. Analog and digital performance of directly and externally modulated wavelength converters is also described.
A summary of current work involving the development of high performance, wavelength-tunable integrated optical transmitters for analog applications is given. The performance of sampled-grating DBR lasers integrated with an SOA and an electroabsorption or Mach-Zehnder modulator is evaluated in terms of E/O conversion efficiency, noise performance and dynamic range. Optimization options to maximize either gain, noise figure or spurious-free dynamic range in analog link applications are discussed. It is shown how the combination of chip-scale integration and the use of bulk waveguide Franz-Keldysh absorption allows coupling of a large optical power level into the electroabsorption modulator, and its effects on the modulation response and analog link performance. Link results on an integrated SGDBR-SOA-EAM device includes a sub-octave SFDR in the 125 to 127 dB/Hz4/5 range and a broadband SFDR of 103-107 dB/Hz2/3 limited by third order intermodulation products or 95-98 dB/Hz1/2, limited by second order intermodulation products, over a 1528 to 1573 nm wavelength range.
The Sampled-Grating Distributed-Bragg-Reflector laser(SGDBR) provides wide tunability (>40nm), and high output power (>10mW). Driven by the demand for network reconfigurability and ease of implementation, the SGDBR has moved from the research lab to be commercially viable in the marketplace. The SGDBR is most often implemented using an offset-quantum well epitaxial structure in which the quantum wells are etched off in the passive sections. Alternatively, quantum well intermixing has been used recently to achieve the same goal - resulting in improved optical gain and the potential for multiple bandgaps along the device structure. These epitaxial "platforms" provide the basis for more exotic opto-electronic device functionality exhibiting low chirp for digital applications and enhanced linearity for analog applications. This talk will cover state-of-the-art opto-electronic devices based on the SGDBR platform including: integrated Mach-Zehnder modulators, and integrated electro-absorption modulators.