Step-tunable all-fiber laser apparatus and method for dense wavelength division multiplexed applications

#This invention relates to step-tunable fiber lasers and, more specifically, to step-tunable fiber lasers for dense wavelength division multiplexed (DWDM) applications. It also pertains to methods by which such lasers may be made to operate.

#A known method for increasing the capacity of a fiber optic communication system is wavelength division multiplexing, or WDM. WDM increases capacity by permitting the use of more than one optical carrier on a single fiber. WDM is accomplished by multiplexing the outputs of more than one laser onto the fiber. After transmission, at the receiving end, the multiple wavelengths are demultiplexed.

#The concepts behind WDM have been extended to use a set of closely-spaced wavelengths in the 1550 nm window. The International Telecommunication Union (ITU) has proposed the use of a grouping or grid of wavelengths in this window. The channels are anchored to a reference at 193.10 THz and equally spaced in frequency, the closely spaced grids having channels 100 GHz or 50 GHz apart. In the wavelength range between 1528.77 nm and 1560.61 nm, the ITU 100 GHz grid comprises 41 channels. This method of WDM is known as dense wavelength division multiplexing, or DWDM.

#FIG. 1 is a schematic diagram of a presently preferred embodiment of a step-tunable fiber-optic ring laser according to the present invention.

#FIG. 2 illustrates the spectral overlap of the passbands of the electrically tunable filter and the periodic filter according to the present invention.

#FIG. 3 illustrates the combined spectral response of the periodic and tunable filters according to the present invention.

#FIG. 1 is a schematic diagram of a presently preferred configuration for a step-tunable fiber-optic ring laser 10 according to the present invention. The configuration of FIG. 1 preferably includes several cavity or resonator elements, as described below. While for the sake of illustration a specific ordering of elements is shown for the ring configuration described immediately below, it will be readily apparent to one having ordinary skill in the art that cavity elements may be placed within the cavity according to different positions other than those shown and described.

#As shown, the arrangement of FIG. 1 includes a periodic filter 20. The periodic filter 20 is a filter which has high transmissivity for light with frequency within certain frequency bands, i.e., passbands, at approximately constant frequency spacing. In the preferred embodiment the periodic filter 20 has passbands spaced apart at a frequency spacing of 200 GHz or a sub-multiple of that spacing, such as 100 GHz, 50 GHz, 25 GHz, 12.5 GHz, or other. The periodic filter 20 limits the possible lasing wavelenghts of the step-tunable ring laser 10 by introducing lower optical loss at the passbands as compared to other wavelengths, with the effect of precluding laser action at the low transmissivity bands. In the preferred embodiment, the center wavelength of one, or more, of the passbands may be kept within certain values of a predetermined frequency so as to obtain a laser output with a wavelength according to International Telecommunications Union (ITU) frequency grid recommendations, or according to any other set of wavelengths. The solid line in FIG. 2 illustrates a calculated normalized transmission characteristic of one such periodic filter. For certain applications the periodic filter 20 may be tunable by electrical or mechanical means.

#The periodic filter 20 is optically coupled to an electrically tunable filter 30. Here and elsewhere, “optically coupled” means arranged so that optical radiation may pass from one to the other or vice versa, and possibly passing through one or more active or passive intermediate optical elements along the way. Also, in the figures it will be understood that the solid lines interconnecting the components are intended to depict optical paths. In the presented preferred embodiments the optical interconnecting path is defined with optical fiber. Also, in the figures it will be understood that the dashed lines interconnecting the components are intended to depict electrical connections.