Co-lasing in an electrically tunable erbium-doped fiber laser

Future fiber optic communications systems will utilize wavelength division mutliplexing as a means of transmitting more information over the same number of fibers. A single source capable of generating multiple frequencies simultaneously would be an attractive option for such systems. We report here on an all-fiber, dual-frequency, colasing, widely tunable laser source with the potential for multifrequency co-lasing operation. A dual-frequency colasing source could also be used for variable difference frequency generation. We recently reported an all-fiber, low-threshold, widely tunable single-frequency, erbium-doped fiber ring laser with a tandem Fabry-Perot filter,le3 having interesting noise and spectral properties. These lasers are stable single-frequency sources in the 1.55 pm telecommunications window. Similar devices have shown linewidths as narrow as 1.4 kHz.4 The dual-frequency, co-lasing, widely tunable laser source is constructed from essentially the same components as the single-frequency source. By using a single ring, however, a reduction in the number of components that would be needed to construct two separate, single-frequency sources is achieved. Two experimental configurations for obtaining colasing operation were investigated. They are shown in Figs. 1 (a) and 1 (b). In Fig. 1 (a) we see the single-gain module configuration. The gain module (G) is a commercial erbium-doped fiber amplifier consisting of approximately 20 m of fiber. The index-raising codopants included aluminum. The erbium-doped fiber amplifier was pumped with a 980-nm laser diode and provided up to 37.2 dB of small signal gain and 10.3 dBm of maximum output power at 1532 nm. We did not have quantitative measurements of the gain as a function of wavelength, however, it was possible to achieve lasing over the entire region accessible with the tuning filters. The isolators (ISO) had less than 1 dB of forward loss and provided up to 37 dB of peak isolation in the wavelength range of interest. The polarizer (POL) was a plasmon-wave-type device, pigtailed with polarization preserving fiber. It had less than 1 dB of loss in the allowed polarization and 24 dB of loss in the orthogonal polarization. The two polarization controllers (PC) consisted of three quarter-wave plates made by winding fiber around three disks which could be rotated independently of each other. They had less than 1 dB of loss. The two tuning filters were placed in the arms of a Mach-Zehnder interferometer created by two 3 dB fusedfiber couplers. A calculation discussed below showed that the minimum frequency separation between the co-lasing frequencies was limited by the bandwidth of the tuning filters. The tuning filters were broadband Micron Optics fiber Fabry-Perot filters (BB FFP) . One had a free spectra1 range (FSR) of 4020 GHz, a bandwidth of 26.1 GHz, and an insertion loss of less than 3 dB. The other had a FSR of 4700 GHz, a bandwidth of 38.2 GHz and an insertion loss of less than 2 dB. We estimate that the total cavity length for either one of the two wavelengths was approximately 40 m corresponding to a mode spacing of 5 MHz. The total minimum loss seen by one wavelength was about 12 dB Co-lasing was achieved by tuning the BB FFPs to the desired wavelengths and then using the polarization controllers in combination with the polarizer to balance the losses with the amplifier gain. Output from the laser was split by a 3 dB coupler. Half the output was sent to a spectrometer and the other half to a scanning Fabry-Perot interferometer (Newport Research Super-Cavity SR170 FSR 6 GHz, resolution 1 MHz). The supercavity Fabry-Perot interferometer showed that the configuration in Fig. 1 (a) operated with only one longitudinal mode excited at each wavelength (see Fig. 2). The smaller peaks in the picture are transverse modes of the supercavity Fabry-Perot. Figure 3 shows a plot of the