A tunable Fabry-Perot (FP) filter made of silicon decreases dispersion in long-haul transmission systems that use DWDM transmitters. Tapio Niemi and others at the Helsinki University of Technology (Finland) developed a device that provides simultaneous spectral filtering and wavelength monitoring of a directly modulated distributed feedback (DFB) laser and is relatively simple to make.1
Directly modulated DFB lasers are attractive for long-haul DWDM networks because they are inexpensive. Two drawbacks of these lasers, however, are frequency chirp, which results in pulse broadening in single-mode fiber, and laser aging, which results in wavelength drift.
The frequency chirp can be controlled by temporal filtering by either fiber Bragg gratings or Fabry-Perot filters, but both gratings and commercial FP filters are fixed to a constant wavelength. Niemi's device is temperature-tunable.
The filter is made by depositing dielectric quarter-wave stacks on a double-sided polished silicon wafer. A ring-shaped thin-film resistor is deposited on one side, and this is used to temperature-tune the silicon. Because silicon's refractive index is dependent on the temperature, the optical thickness changes with temperature. A 10 x 5-mm filter chip is placed in the air gap of a fiberoptic beam expander. Before and after the chip, fiber couplers tap a small amount of the optical power in order to generate a feedback loop. By keeping track of the temperature of the chip (using a second thin-film resistor) while the chip is maintaining a steady transmission ratio, users can calculate the wavelength of the light. The total loss of the device including air gap, filter, and couplers is about 1.5 dB at the transmission maximum of the filter (see figure).
A test measurement with 350 km of standard single-mode fiber demonstrated that the spectral filtering improved the dispersion penalty from 2.4 to 0.9 dB. The wavelength could simultaneously be monitored in real time with an accuracy of a few picometers.
"This device is directly applicable to a large wavelength range," Niemi said. "For example, 1300- and 1550-nm wavelengths can be simultaneously managed." The dynamic wavelength-tuning range of the device is also very large. "We have made experiments where the center wavelength of the filter is continuously tuned over 30 nm," said Niemi. The standard silicon wafer makes manufacture of these devices cost-effective.
The researchers applied the device to monitoring multiple WDM channels and are designing a digital control circuit for the filter.2
"Our near-future plan is to apply [the device] for making a stable wavelength reference that operates over a huge optical band of 1100 to 1750 nm."
For more information, contact Tapio Niemi at firstname.lastname@example.org.
- T. Niemi et al., IEEE Phot. Tech. Lett. 13, 58 (January 2001).
- T. Niemi et al., Proc. Conf. on Lasers and Electro-Optics Europe 2000, CtuP4.