The increasing use of wave-division-multiplexing (WDM) optical communications is taxing the capability of conventional WDM optical monitors to faithfully measure the spectrum of transmitted light. This is key to meeting quality-of-service (QoS) requirements. Present instruments offer scan rates of 20 to 30 ms, limiting their application.
To answer this challenge, Anritsu's advanced microelectromechanical-system (MEMS) optical spectrum analyzer module provides dramatic performance, size, and price advantages over existing solutions. It's 90 mm wide by 40 mm long by 20 mm high, yet it can handle a 45-nm wide wavelength range from 1525 to 1570 nm with a fast scan rate of 2.6 ms (Fig. 1).
Even when data-processing time is included for the measured signal, measurement time is just 5.3 ms. The analyzer also features a 33-dB dynamic range, a wavelength resolution bandwidth of 0.1 nm, and picometer wavelength accuracy.
A CLOSER LOOK
The analyzer module consists of a collimator lens, a fiber Bragg diffraction grating, a Faraday rotator to erase the grating's polarization dependency, a silicon MEMS optical scanner, an etalon for measurement compensation, and photodiodes (Fig. 2).
Collimated input impinges on the grating where the light is diffracted. This first-order diffracted light strikes the MEMS scanner's mirror and is reflected back to the grating, where it is reflected again. The grating and optical scanner form a Littman optical filter. To remove polarization dependency from the grating, two orthogonally polarized optical signals are generated at every other scan. Their outputs are summed into the computer.
The 20- by 10- by 0.5-mm scanner is actuated electrostatically by mechanical resonance with a driving voltage of 300 V p-p at a 250-kHz resonance frequency.
A signal generator, a PC, and a 12-bit analog-to-digital converter (ADC) are used for measurements (Fig. 2, again). By using a higher-resolution ADC, a higher-accuracy signal can be measured. The wavelength of the input light reaching the photodiode changes constantly and cyclically according to the rotation angle of the MEMS scanner. Measuring the output voltage of the photodiode and using the rotation of the MEMS scanner allows the spectrum of the input WDM signal to be measured.
Anritsu believes there are lots of other potential applications besides WDM analysis. Researchers could use the module to accurately observe the strain caused by earthquakes. By measuring the tuning transition of light sources, high-speed failure could be detected in optical communications systems using Lambda routing. Distortion caused by fiber Bragg grating sensors could be measured too. Also, because the module is small and draws less power than existing solutions, it can be used during design development as well as in the field.
Although no specific price has been set, the module is expected to cost at least 50% less than existing instruments. Anritsu predicts product availability will be the end of this year.