Optical waveguides – Optical fiber waveguide with cladding – Utilizing multiple core or cladding
Reexamination Certificate
2001-04-02
2003-05-13
Kim, Robert H. (Department: 2882)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing multiple core or cladding
C385S123000
Reexamination Certificate
active
06563995
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to optical wavelength filters, and in particular to passive and active filters using claddings with a depressed refractive index.
BACKGROUND OF THE INVENTION
Optical waveguides are designed to guide light of various modes and polarization states contained within a range of wavelengths in a controlled fashion. Single-mode optical fiber is the most common waveguide for long-distance delivery of light. Other waveguides, such as diffused waveguides, ion-exchanged waveguides, strip-loaded waveguides, planar waveguides, and polymer waveguides are commonly used for guiding light over short distances and especially for combining or separating light of different wavelengths, optical frequency mixing in nonlinear optical materials, modulating light and integrating many functions and operations into a small space.
In essence, a waveguide is a high refractive index material, usually referred to as core in optical fiber, immersed in a lower index material or structure, usually referred to as cladding, such that light injected into the high index material within an acceptance cone is generally confined to propagate through it. That is because at the interface between the high and low index materials the light undergoes total internal reflection (TIR) back into the high index material.
The advent of active fiber elements such as Er-doped amplifiers and fiber lasers has resulted in further developments in fiber claddings. For example, fibers with profiles designed for “cladding pumping” have been developed to stimulate lasing in the active core of single-mode fibers. Such fibers typically have a core, a multi-mode cladding layer and an outermost cladding layer. The pumping light is in-coupled into the multi-mode cladding layer and it crisscrosses the core as it propagates thus stimulating lasing. Unfortunately, once the fiber core is stimulated to emit at the desired emission wavelength, the core can also produce an unwanted signal at the so-called “Raman wavelength” due to stimulated Raman scattering. The Raman effect shifts the emission wavelength to the first Stokes wavelength of the fiber core and becomes dominant at high power levels. Hence, the output power of cladding pumped fiber lasers at the desired output wavelength is limited.
Similar problems occur when the active material in the medium has other emission wavelengths that are stimulated along with the desired emission wavelength. This problem is commonly referred to as amplified spontaneous emission (ASE) and may prevent the laser from operating at the desired wavelength by “drowning out” the desired frequency with much higher ASE intensity output at the undesired wavelengths. An example of this situation is encountered when working with neodymium (Nd) fiber lasers having a Nd-doped core surrounded by a double-cladding and using the
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transition of neodymium atoms to generate light of about 900 nm wavelength. Unfortunately, the 900 nm transition is three-level type and the neodymium atoms have a higher probability to emit light at roughly 1050-1070 nm where the transition is four-level type. The undesired light at 1050 nm easily dominates over the feeble 900 nm emission rendering the fiber laser or amplifier useless for most practical purposes.
Consequently, certain fiber devices such as fiber lasers and especially amplifiers, need to have a high loss at undesired long wavelengths. In the specific example of a Nd:glass laser or amplifier intended to emit or amplify light at 900 nm, it is necessary to obtain very high losses at 1050 nm. That is because the light at 900 nm is very feeble (it is 20 nm away from the peak of the emission at 880 nm), and the emission cross-section at 1050 nm is approximately 10 times the emission cross-section at 900 nm. Also, the light at 900 nm should be amplified, e.g., by 40 dB. Unfortunately, as the light at 900 nm is being amplified by 40 dB the light at 1050 nm is also amplified by 400 dB, thus completely dominating over the light at 900 nm. Clearly, light at 1050 nm needs to be attenuated by 400 dB or more to avoid this problem.
In addition, once about a kilowatt of power is being produced by the fiber laser or amplifier, even if it is just for a nanosecond or less, many nonlinear processes in addition to Raman scattering start interfering with generation of the useful light at 900 nm. These include phenomena such as Brillouin scattering, self-phase modulation as well as a host of other nonlinear optical mixing and conjugation processes via the third order nonlinear susceptibility &khgr;
(3)
of the fiber material. These effects limit the length of fiber which can be used in such fiber lasers or amplifiers. In fact, due to these effects the fiber length should be on the order of 1 m or even less. This means that light at 1050 has to be attenuated at an attenuation rate of about 400 dB/m.
In view of the above, it is clear that it would be desirable to selectively filter out unwanted wavelengths from a fiber laser or amplifier at a very high attenuation rate, e.g., 400 dB/m or more. This would be important for passive and active waveguides such as passive fibers, fiber lasers and fiber amplifiers.
In accordance with the prior art, waveguides are usually designed to prevent injected light from coupling out via mechanisms such as evanescent wave out-coupling (tunneling), scattering, bending losses and leaky-mode losses. A general study of these mechanisms can be found in the literature such as L. G. Cohen et al., “Radiating Leaky-Mode Losses in Single-Mode Lightguides with Depressed-Index Claddings”, IEEE Journal of Quantum Electronics, Vol. QE-18, No. 10, October 1982, pp. 1467-72. In this reference the authors describe the propagation of light in more complex lightguides with claddings having a variation in the refractive index also referred to as double-clad fibers. They teach that varying the cladding profile can improve various quality parameters of the guided modes while simultaneously maintaining low losses. Moreover, they observe that depressed-index claddings produce high losses to the fundamental mode at long wavelengths. Further, they determine that W-profile fibers with high index core, low index inner cladding and intermediate index outer cladding have a certain cutoff wavelength above which fundamental mode losses from the core escalate. These losses do not produce very high attenuation rates and, in fact, the authors study the guiding behavior of the fiber near this cutoff wavelength to suggest ways of reducing losses.
U.S. Pat. Nos. 5,892,615 and 6,118,575 teach the use of W-profile fibers similar to those described by L. G. Cohen, or QC fibers to suppress unwanted frequencies and thus achieve higher output power in a cladding pumped laser. Such fibers naturally leak light at long wavelengths, as discussed above, and are more sensitive to bending than other fibers. In fact, when bent the curvature spoils the W or QC fiber's ability to guide light by total internal reflection. The longer the wavelength, the deeper its evanescent field penetrates out of the core of the fiber, and the more likely the light at that wavelength will be lost from the core of the bent fiber. Hence, bending the fiber cuts off the unpreferred lower frequencies (longer wavelengths), such as the Raman scattered wavelengths, at rates of hundreds of dB per meter.
Unfortunately, the bending of profiled fibers is not a very controllable and reproducible manner of achieving well-defined cutoff losses. To achieve a particular curvature the fiber has to be bent, e.g., by winding it around a spool at just the right radius. Different fibers manufactured at different times exhibit variation in their refractive index profiles as well as core and cladding thicknesses. Therefore, the right radius of curvature for the fibers will differ from fiber to fiber. Hence, this approach to obtaining high attenuation rates is not practical in manufacturing.
The prior art also teaches waveguides with complex cladding structures rather than stress-
Arbore Mark A.
Kane Thomas J.
Keaton Gregory L.
Barber Therese
Kim Robert H.
Lightwave Electronics
Lumen Intellectual Property Services Inc.
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