Monolithic Compound-Semiconductor Photonic Integrated Circuits (PICs)

Technical Overview

Integration of multiple photonic components on a common substrate is highly desired not only to reduce the cost, size, weight and power (SWaP) of photonic subsystems, but also to improve the optical system performance through reduction of parasitic losses and physical delay lengths. The most basic photonic integrated circuit (PIC) for an optical communication link would include lasers, modulators, filters, amplifiers, and photodetectors. 

All of these components can be integrated together on a compound semiconductor substrate (e.g., GaAs, InP, or GaN), but doing so requires locally modifying the character of the semiconductor to suit the requirements of each component. Most commonly, this local modification involves altering the semiconductor bandgap so that light propagation through the material experiences either amplification, absorption, or lossless transmission. This alteration can be done with either selective removal and replacement of semiconductor through etch and regrowth techniques, or by modifying the existing semiconductor material through quantum-well intermixing (QWI) and/or a combination of both techniques.

Program Goals

  • Identify and define critical systems applications enabled by monolithic integration of compound-semiconductor components
  • Develop novel photonic components, integration techniques, and PIC subsystems in a number of compound-semiconductor material systems needed to meet application requirements or realize new concepts
  • Provide the government with access to compound-semiconductor PIC design, fabrication, and packaging resources

Research Highlights

Recently, monolithic compound-semiconductor PIC development at Lincoln Laboratory has focused on integrating the slab-coupled optical waveguide (SCOW) gain medium with other photonic components and waveguides. Examples of this PIC development include the following:

Schematic illustration of a mode-locked SCOWL laserFigure 1. Schematic illustration of a mode-locked SCOWL laser, image of one section of the ML-SCOWL die, and absorption measurements of the quantum-well intermixed bandgap for each section of the device. (Click on image to see larger version.)

  • Mode-locked lasers provide sources of optical pulse trains having picosecond pulse widths and femtosecond timing jitter. Applications enabled by these sources include photonics analog-to-digital converters (ADCs), direct microwave-frequency down-conversion, and wavelength-division multiplexed (WDM) on-chip interconnects. We have applied monolithic integration to realize a high-power, low-jitter mode-locked SCOW laser (ML-SCOWL) laser, which utilizes quantum-well intermixing to realize transparent and absorbing sections within the laser cavity to improve mode-locked laser performance (Figure 1). The bandgaps of the amplifier, electroabsorption modulator, and saturable absorber sections in the ML-SCOWL were tuned using QWI to optimize their function within the device. This optimization resulted in a reduction of the timing jitter of the ML-SCOWL from 1.2 ps to 0.2 ps.

  • The SCOW gain-medium's properties of high output power and low noise are uniquely enabling for compact optical transmitters for free-space optical communications and high-performance microwave photonic subsystems. For example, integrating a high-power SCOW amplifier (SCOWA) with a previously demonstrated high-performance InP-based electrorefractive modulator (Vπ-L ~ 0.9 V-cm) would realize an extremely compact and high-performance optical transmitter (Figure. 2). Modulator implementation in InP-based materials provides performance comparable or better than to LiNbO3-based modulators and can be integrated with 1.5 µm SCOW lasers and amplifiers in a monolithic PIC.
Figure 2. Image of the InP-based low-Vp electrorefractive modulator and measured performance showing Vπ = 0.9V-cm. Such a device could be integrated with SCOWL lasers and improve upon conventional modulator-transmitters within a smaller form factor. (Click on image to see larger version.)

 

*gallium arsenide (GaAs), indium phosphide (InP), gallium nitride (GaN)

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