Multiplexed gratings in crystal lock wavelengths
Valerie C. Coffey
Wavelength-division multiplexing demands separate laser sources emitting different wavelengths, as well as multiplexing devices to couple all of the signals into a single fiber. To provide 40-channel or even 100-channel spacing in one fiber, each wavelength requires stabilization to reduce crosstalk and bit-error rate (BER). Physicists at the University of Arkansas (Fayetteville, AR) have discovered an effective way to simultaneously lock wavelengths of many lasers using multiplexed photorefractive gratings formed in a single crystal.
Wavelength drift in a laser diode is due to fluctuations in temperature or driving current. Relative peak-diffraction wavelengths of fixed gratings within a single crystal are immune to the fluctuations of ambient temperature, defining locking positions for different laser diodes. The wavelength of a laser is locked via the peak wavelength of each fixed grating through feedback circuits.
The experiment was composed of two gratings that were recorded in a lithium niobate (LiNbO3) crystal. Using angular multiplexing, the writing beams were incident on the a-face of the crystal and the angle was adjusted so that each hologram was Bragg-matched for counter-propagating reflection at a specific infrared wavelength incident on the c-face of the crystal (see figure). The dimensions of the crystal were 5 × 5 × 10 mm3, with the c-axis oriented along the 10-mm edge. The writing beams used an argon-ion laser with a wavelength of 488 mm. The peak wavelengths for counter-propagation reflections were designated at 1537 and 1566 nm. The technique can be extended to form multiple gratings within a single crystal, and to lock relative wavelengths of many laser diodes, for central wavelengths from 1.1 to 4.5 µm.
Scientists used two laser diodes (LD1 and LD2) to measure the short- and long-term spectral response of the two gratings. The two wavelengths were stable for over 300 minutes to within 1.3 and 2.4 pm. In another experiment, wavelengths were stabilized to better than 5 pm over 48 hours. The thermal drift of the stabilized wavelengths, approximately 0.6 pm, is much smaller than the bandwidths of the multiplexed channels (50 pm) for temperature changes within 0.1°C. The BER caused by the diffraction tails from other channels can be minimized using apodized gratings or narrow band- filters. After the first few channels, the BER is not sensitive to increase in channel number; therefore, this technique will be suitable for large channel-number applications. For more information, contact Min Xiao at firstname.lastname@example.org.
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2. G. A. Rakujic and V. Leyva, Opt. Lett. 18, 459 (1993).
Angular multiplexing of gratings in a photorefractive crystal requires a set of writing beams with angle q. Each channel is designed for counter-propagation reflection at a desired laser-diode wavelength.