Optical modulator links processors to sensors
DAVE WILSON
A new fiber technology developed in the telecommunications industry allows optical-fiber transmission techniques to solve remote data-acquisition problems cost-effectively. The technology, which has been developed successfully in a prototype test in Switzerland, serves industrial applications that require fast bidirectional links between computer controllers and remote sensors.
Industrial applications demand high transmission rates, and they are often deployed in harsh electromagnetic environments. A bidirectional optical-fiber solution to these problems can be found in an electrically controlled, reflective light-modulator component designated remo.
Developed by Ascom Tech Ltd., Bern, Switzerland, in collaboration with the Institute of Microtechnology at the University of Neuchatel, in Neuchatel, Switzerland, the remo device interfaces processors to remote sensors and subsequently bidirectionally links these sensors to the processors.
In a typical system, a modulated light signal from a central control unit is guided through an optical fiber to peripheral remo units (see Fig. 1). In the remo component terminating the optical fiber, a portion of the light is used for receiving the data. The other part of the light is remodulated and sent back upstream to the central unit through the same fiber.
This reflective modulation is performed by an optical resonator manufactured from a thin polysilicon membrane and an underlying silicon substrate. Electrical fields deform the membrane and control the interference of the reflective path.
Laser-diode sources
A key benefit of this approach is that laser-diode sources are used only in the central control unit. No laser source, and, consequently, no associated power supply, is needed at the peripheral end, since it is the light received from the central control unit that is reused to carry the data on the return path.
According to Ascom Tech researchers, this approach also significantly reduces power consumption in the peripheral units. This means that for certain applications, the transceiver stations can be powered using long-life batteries, making them independent of external supplies.
The bandwidth transmission rate in both directions can be full duplexed up to 2 Mbits/sec. The light signal in the optical fiber causes no crosstalk in adjacent cables and is not affected by electromagnetic interference. Attenuation is minimal.
In developing components that use reflective modulation techniques, two methods are usually considered: optoelectronic and micromechanical. The optoelectronic approach is based on an electro-optic effect where the refractive index of a material is varied by the application of an electric field. This approach, however, has its disadvantages. First, a relatively large optical coupling loss exists between the fiber and the integrated waveguide. Second, the assembly-tolerance requirements are high, which leads to expensive component costs.
For these reasons, the developers of the remo component rejected the optoelectronic approach in favor of electromechanical modulation. The component comprises an electrostatically tunable Fabry-Perot resonator, formed by a free-standing polysilicon membrane and an underlying polysilicon electrode. An electrical field across the gap between the membrane and the substrate deforms the membrane, resulting in a detuning of the optical cavity. Consequently, the reflectivity of the cavity changes with the applied voltage.
Optical-cavity reflection
Because of this change, the degree of reflection of the optical cavity can be switched in fractions of a microsecond between 100% and 0%. To achieve this reflection, only a small change of the charge on the small capacitance defined by the resonator structure is necessary. This results in low power requirements and good performance at high frequencies. Indeed, the remo module offers an upstream signaling capability in excess of 2 Mbits/sec and a receiving bandwidth of up to 1.2 GHz.
In the remo package, the modulator chip and the receiving photodiode for the downstream signal have been combined. The singlemode fiber pigtail is connected to the remo chip through a conical blind hole in the silicon substrate, designed to guarantee the required lifetime fit of the fiber.
The packaging also has to protect the sensitive membrane structure. This has been achieved by using the indium gallium arsenide (InGaAs) substrate of the photodiode to form a dome over the membrane. In addition, the packaging has to allow for connection of electrical signals and encapsulate the device for lifelong protection.
A diagram and a three-dimensional view of the remo chip with the photodiode show the light-modulating section on the opposite side of the chip with respect to the fiber connection (see Fig. 2). The assembly is also protected by another housing. This secondary packaging layer also contains the electrical pin-out for connection to the printed circuit board.
The remo device is shown in the photograph, complete with photodiode, electrical pin-out and optical interconnection to a singlemode fiber.
Conventional modulators are characterized by large optical losses. Depending on the technology, coupling losses from the fiber to the modulator and back range from -5 to -15 dB. The Ascom modules, however, are claimed by the company to have a loss of only -2.5 dB.
From an electrical point of view, there appear to be advantages with the remo device. Since it presents a high ohmic resistance of greater than 10 gigaohms and a small capacitance of less than 5 pico-Farads, sustaining the bias voltage and delivering the electrical modulation signal requires little electrical power. Battery-powered operation is, therefore, possible with the devices. This is in contrast to active light sources, which often consume electrical power in excess of 50 mW. q
Dave Wilson writes from London.
Bidirectional Transmission Connects Businesses to Public Data Highways
The first remo prototype, an electrically controlled reflective light modulator component developed by Ascom Tech Ltd., Bern, Switzerland, has been completed successfully. The prototype demonstrated that the remo concept works for bidirectional data transmission. The device interfaces processors to remote sensors and subsequently bidirectionally links them.
A product development project for Ascom Telecommunications Ltd., in Cardiff, UK, is expected to be completed by the end of 1996. Its first application is expected to be connecting businesses to public data highways. But with its low power consumption, the remolink also has great potential as a data link in the industrial sector.
Fiber-optic data links from central control units to commercial sensors could be built using the device, especially in hazardous environments or in areas with high levels of electromagnetic interference.