Perform optical tests with electrical equipment
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 2/1/2004
A 40-Gbps optical modulator, which takes an optical "carrier" and modulates it based on an electrical RF input signal. The modulator converts carrier-wave laser light into optical pulses.
THE CHALLENGEMeasure the frequency response of modulated optical signals at frequencies up to 50 GHz. Lightwave component analyzers cost more than $500,000—too much for many small companies to afford. Engineers needed to find a lower-cost method of characterizing the modulator's output.
THE TOOLS- Anritsu 65-GHz vector network analyzer. www.us.anritsu.com.
- Cascade Microtech VNA calibration software. www.cascademicrotech.com.
- Centellax 43-Gbps lithium niobate modulator driver module. www.centellax.com.
- Discovery Semiconductors PIN diode photodetector. www.chipsat.com.
- JDS Uniphase erbium-doped fiber amplifier. www.jdsu.com.
- Santec tunable laser source. www.santec.com.
- Thorlabs polarization controller. www.thorlabs.com.
Optical components such as modulators must pass a bandwidth test or they will fail an eye-diagram test. Before performing eye-diagram measurements, engineers at JGKB Photonics (Vancouver, BC, Canada; www.jgkb.com) measure a component's bandwidth and return loss. "Our customers test optical components for their eye openings using a pattern generator and a sampling oscilloscope" says RF design engineer Mark Fairburn. "If a component lacks the necessary bandwidth, its eye will be unacceptably small." Fairburn measures the modulator's frequency response at frequencies up to 50 GHz to verify that the modulator's bandwidth exceeds 40 GHz.
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A vector network analyzer measures the bandwidth of an optical signal. |
Working for a small company, Fairburn and product-management director Hiroshi Kato couldn't afford to buy a lightwave component analyzer, an instrument that can cost more than $500,000. Instead, they spent about $90,000 on new and used equipment to make the measurements.
The test system includes a tunable laser source, an optical receiver, and a 65-GHz vector-network analyzer (VNA). (JGKB purchased the VNA on the used-equipment market.) The laser source supplies a lightwave "carrier" to the DUT. The VNA provides an RF signal to the modulator driver, which boosts the RF signal's amplitude. With that signal, the modulator modulates the light carrier from the laser source. A polarization controller converts the light from the transelectric (TE) mode to the transmagnetic (TM) mode so that the light properly enters the modulator's waveguide. After the light exits the modulator's waveguide, the controller converts the light back to the TE mode. A photodetector converts the modulated light carrier into an electrical signal that the VNA can measure.
Before it can accurately make the measurements, the test setup requires calibration. For that, Fairburn and Kato use VNA coaxial calibration standards and calibration software. They also perform a power leveling of the VNA to ensure a constant RF power across the frequency band (40 MHz to 50 GHz). The engineers assign a value of 0 dB to the power output at 40 MHz as a reference. Software then finds the frequency where the modulator's output amplitude falls to –3 dB.
RESULTSThe test system lets JGKB engineers make preliminary measurements on new modulator designs. If a DUT's output amplitude drops off too quickly as a function of frequency, then the engineers know that the device's eye-diagram openings will be too small and they need to take corrective action.
The system measures both the optical bandwidth of the modulator and its return loss. Insufficient bandwidth results in eye diagrams whose eye openings will appear too round at the corners because of amplitude clipping. Return-loss measurements tell the engineers if the modulator's signals overdrive or underdrive the modulator. Either of these conditions will produce unacceptable jitter in the modulator's eye diagrams, thus increasing bit-error rate.
