OPTOELECTRONIC INTEGRATED CIRCUITS: Monolithic device monitors displacement

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 Al_0.3Ga_0.7As waveguide core containing a single GaAs quantum well. This layer was sandwiched between a 1.1-µm-thick Al_0.8Ga_0.2As lower cladding layer (n-doped) and an 0.85-µm-thick Al_0.8Ga_0.2As 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.