Method and apparatus for generating a sequence of optical wavelength bands

#This invention relates to applications of nonlinear effects to generate frequency-shifted optical signals. More particularly, the invention relates to optical comb generation.

#In a wavelength-division multiplexed (WDM) optical network, independent signals are modulated onto optical carriers in distinct wavelength channels, and concurrently injected in a common optical fiber. Typical wavelength channels of current interest are spaced apart by 25, 50, 75, or 100 GHz. In most current WDM networks, the optical carrier in each band is generated by a separate laser. Since there are typically several tens of wavelength channels, the optical sources for carrier generation contribute substantially to the overall cost of the network. Those skilled in the art have recognized the advantages of a multiple-wavelength optical source that does not require a separate laser for each wavelength channel.

#It is known that a laser beam at essentially a single wavelength can be arranged to interact with a frequency-shifting optical element of an appropriate kind to generate a beam containing a sequence, or “comb,” of distinct wavelengths. Such an arrangement is described, for example, in U.S. Pat. No. 5,101,291, which issued on Mar. 31, 1992 to R. M. Jopson (the '291 patent).

#FIG. 1 is a simplified schematic drawing of an optical system in which an incident light beam is passed through two frequency mirrors in succession.

#FIG. 2 is a graphical representation of the use of two frequency mirrors to generate a converging set of shifted frequencies that compose a frequency comb. Wavelength increases from left to right in the figure, and therefore frequency increases from right to left.

#FIG. 3 is a simplified schematic diagram of a system of frequency mirrors and optical filters able to generate frequency combs such as those illustrated in FIG. 2.

#As noted above, a frequency mirror is an optical element that has a reference optical frequency F. When a beam of light of frequency fs passes through a frequency mirror, at least some of the light in the output beam will have such a frequency fout in contraposition to F; that is, F will be at the midpoint between fs and fout. Equivalently, fout=2F−fs.

#FIG. 1 illustrates the result of passing an incident light beam through two frequency mirrors in succession. Frequency mirror 10, denominated “FM1” in the figure, has a reference frequency F1, and frequency mirror 20, denominated “FM2” in the figure, has a reference frequency F2. As indicated in the figure, the output light that has been shifted in frequency by both frequency mirrors has the frequency fs+2(F2−F1). That is, the output frequency has been shifted by twice the difference between the two reference frequencies. As will be explained in more detail below, the frequency-shifted output can be repeatedly cycled back through frequency mirrors 10 and 20 to generate a frequency “comb;” i.e., a uniformly spaced sequence of shifted output frequencies. In the comb, the spacing between successive frequencies is 2(F2−F1), i.e., twice the spacing between the pump frequencies.

#Several types of optical materials are known to exhibit the nonlinear optical behavior characteristic of frequency mirrors. A currently preferred material is periodically poled lithium niobate (PPLN). The electrical polarization within such material has a laminar structure, with regions of similar electrical polarization occupying planes oriented orthogonally to the input light beam. The repeat distance A of the periodic polarization is related to the reference frequency and its second harmonic (which has twice the reference frequency) and to the respective refractive indices nref and n2ref of the PPLN for light of those frequencies, according to: 1 Λ = 2 ⁢ π Δ ⁢ ⁢ β = 2 ⁢ π β 2 ⁢ ref - 2 ⁢ β ref = 2 ⁢ π 2 ⁢ π ⁢ ⁢ n 2 ⁢ ref λ 2 ⁢ ref - 4 ⁢ π ⁢ ⁢ n ref λ ref = λ ref 2 ⁢ ( n 2 ⁢ ref - n ref ) ,