OPTOELECTRONIC INTEGRATED CIRCUITS: Monolithic device monitors displacement

Dec. 1, 1996
Optoelectronic integrated circuits (OEICs) fabricated at the Paul Scherrer Institut (Zurich, Switzerland) using III-V semiconductors offer the possibility of monolithically integrating active and passive optical components.

Optoelectronic integrated circuits (OEICs) fabricated at the Paul Scherrer Institut (Zurich, Switzerland) using III-V semiconductors offer the possibility of monolithically integrating active and passive optical components. At the institute, Hans Zappe, his doctoral student Daniel Hofstetter, and their colleagues have developed a monolithically integrated optical displacement sensor fabricated in the gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs) material system.

Zappe`s group devised a specialized distributed-Bragg-reflector (DBR) laser-fabrication process that yields a single-chip device consisting of the laser, two photodetectors, two phase modulators, two Y-couplers, and two directional couplers. The displacement measurement chip was fabricated on a double-heterostructure layer sequence requiring only a single growth step. A vacancy-enhanced-disordering (VED) process was used to define absorbing photodetector and pump active regions as well as transparent waveguiding sections. Selective transparency was achieved in specific regions of the OEIC by quantum-well intermixing, which provides a controlled way of changing the shape of the well.

The output of the DBR laser is divided and fed into two nearly independent Michelson interferometers by a Y-coupler. A relative phase shift between the two reference arms is generated by phase modulators allowing the detection of two interference signals in phase quadrature and thus the changes in displacement direction. Interference signals could be measured at mirror distances of up to 20 cm with sub-100-nm resolution.

The layer structure used for these devices was grown by metal-organic vapor-phase epitaxy on a GaAs substrate and included an undoped 165-nm-thick Al0.3Ga0.7As waveguide core containing a single GaAs quantum well. This layer was sandwiched between a 1.1-µm-thick Al0.8Ga0.2As lower cladding layer (n-doped) and an 0.85-µm-thick Al0.8Ga0.2As upper cladding layer (p-doped). A 160-nm-thick highly p-doped GaAs cap layer completed the structure.

The DBR laser of the displacement sensor was operated continuous-wave at room temperature. Typical threshold currents were 30 mA, corresponding to a threshold current density of 2 kA/cm2. The emission wavelength was 822 nm, and the spectrum showed a side-mode-suppression ratio of approximately 25-30 dB. The discrete DBR lasers had an emission linewidth of about 500 kHz.

The reduction of optical crosstalk between these two elements was achieved by dry etching an isolation trench. This trench was filled with p-metallization layers to prevent the light from going directly from the laser into the photodetector. Despite the trench, optical crosstalk signals of 35 and 16 nA were seen. Future work will involve deepening the cut.

The end result was a double Michelson interferometer that allows for the determination of both magnitude and direction of a displacement, with 20-nm resolution. The detection of two 90° phase-shifted interferometer signals also resulted in an improved phase interpolation of f/20. The maximum measurement distance (at present about 0.2 m) is limited by the enhanced linewidth of the optical signal emitted from the sensor, about 800 MHz. Zappe says that despite the simple fabrication process, the integration of rather complex optical functions could be easily realized.

About the Author

Laurie Ann Peach | Assistant Editor, Technology

Laurie Ann Peach was Assistant Editor, Technology at Laser Focus World.

Sponsored Recommendations

Brain Computer Interface (BCI) electrode manufacturing

Jan. 31, 2025
Learn how an industry-leading Brain Computer Interface Electrode (BCI) manufacturer used precision laser micromachining to produce high-density neural microelectrode arrays.

Electro-Optic Sensor and System Performance Verification with Motion Systems

Jan. 31, 2025
To learn how to use motion control equipment for electro-optic sensor testing, click here to read our whitepaper!

How nanopositioning helped achieve fusion ignition

Jan. 31, 2025
In December 2022, the Lawrence Livermore National Laboratory's National Ignition Facility (NIF) achieved fusion ignition. Learn how Aerotech nanopositioning contributed to this...

Nanometer Scale Industrial Automation for Optical Device Manufacturing

Jan. 31, 2025
In optical device manufacturing, choosing automation technologies at the R&D level that are also suitable for production environments is critical to bringing new devices to market...

Voice your opinion!

To join the conversation, and become an exclusive member of Laser Focus World, create an account today!