Wavelength selective optical routers

Details Wavelength selective optical routers Wavelength selective optical routers

1998-12-15

2001-03-13

Palmer, Phan T. H. (Department: 2874)

Optical waveguides

With optical coupler

Input/output coupler

C385S024000, C385S016000, C385S043000, C359S199200, C359S199200

Reexamination Certificate

active

06201909

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the communication of signals via optical fibers, and particularly to systems for routing and switching optical signals based on wavelength selectivity. More particularly, the invention relates to optical devices and subsystems using wavelength selective optical couplers.
DESCRIPTION OF RELATED ART
Low loss, wavelength selective couplers are important components for optical fiber communication networks based on wavelength division multiplexing (WDM). WDM enables an individual optical fiber to transmit several channels simultaneously, the channels being distinguished by their center wavelengths. An objective is to provide a precise wavelength selective coupler that is readily manufactured and possesses high efficiency and low loss.
Optical fiber gratings reported in the prior art typically operate in reflection mode. To gain access to this reflected mode in a power efficient manner is difficult, because the wave is reflected backwards within the same fiber. A first method to access this reflected light is to insert a 3 dB coupler before the grating, which introduces a net 6 dB loss on the backwards reflected and out-coupled light. A second method is to insert an optical circulator before the grating to redirect the backwards propagating mode into another fiber. This circulator introduces an insertion loss of 1 dB or more and involves complicated bulk optic components. A method to combine the filtering function of a fiber grating with the splitting function of a coupler in a low loss and elegantly packaged manner would be highly desirable for WDM communication networks.
Another method well known in the prior art uses directional coupling to transfer energy from one waveguide to another by evanescent coupling (D. Marcuse, “Theory of Dielectric Waveguides,” Academic Press 1991 and A. Yariv, “Optical Electronics,” Saunders College Publishing, 1991). This evanescent coupling arises from the overlap of the exponential tails of the modes of two closely adjacent waveguides, and is the typical mode of operation for directional coupler based devices. In contrast, non-evanescent coupling occurs when the entire optical modes substantially overlap, as is the case when the two waveguides are merged into a single waveguide. Devices that rely on evanescent coupling (e.g., directional couplers) in contrast to non-evanescent coupling have inherently weaker interaction strengths.
One realization of a directional coupling based device uses gratings recorded in a coupler composed of two identical polished fibers placed longitudinally adjacent to one another (J. -L. Archambault et al., Optics Letters, Vol. 19, p.180 (1994). Since the two waveguides are identical in the coupling region, both waveguides possess the same propagation constant and energy is transferred between them. This results in poor isolation of the optical signals traveling through the two waveguides, because optical power leaks from one fiber to the other. Another device also based on evanescent coupling was patented by E. Snitzer, U.S. Pat. No. 5,459,801. The length of the coupling region for this device should be precisely equal to an even or odd multiple of the mode interaction length for the output light to emerge entirely in one of the two output ports. A precisely positioned Bragg grating is then UV recorded in the cores of the waist region.
An alternative grating assisted directional coupler design reported by R. Alferness et al., U.S. Pat. No. 4,737,007 and M. S. Whalen et al., Electronics Letters, Vol. 22, p. 681 (1986) uses locally dissimilar optical fibers. It also is based on evanescent coupling. A serious drawback of this device is that the wavelength for which light is backwards coupled into the adjacent fiber is very close to the wavelength for which light is backreflected within the original fiber (about 1 nm). This leads to undesirable pass-band characteristics that are ill suited for add/drop filter devices designed to add or drop only one wavelength. The separation between the backreflected and backwards coupled wavelengths is impractically small (<2 nm) for the all-fiber, grating assisted directional coupler approaches of the prior art.
The conventional grating assisted directional coupler suffers from both a relatively low coupling strength and small wavelength separation of back-reflected and backwards coupled light. These problems arise because the two coupled optical waveguides remain physically separate and the light remains guided primarily in the original cores. Only the evanescent tails of the modes in each of the two waveguides overlap, corresponding to evanescent coupling. Two locally dissimilar optical fibers can instead be fused and elongated locally to form a single merged waveguide core of much smaller diameter, forming a mode coupler. The resulting optical mode propagation characteristics are effectively those of a multimode silica core/air cladding waveguide. The two waveguides are merged such that the energy in the original optical modes of the separate waveguides interact in a substantially non-evanescent manner in the merged region. The index profile of the optical waveguide varies sufficiently slowly in the longitudinal direction such that light entering the adiabatic taper region in a single eigenmode of the waveguide evolves into a single local supermode upon propagating through the adiabatic transition region. By merging the waveguides into a single wave propagation region, the wavelength selective coupling achieved upon the subsequent recording of an index of refraction grating in the waist of the coupler can be substantially increased. This device is called a grating assisted mode coupler, and is described at length in U.S. Pat. No. 5,805,751.
Add/drop multiplexers and demultiplexers are important functional elements in fiberoptic networks implementing wavelength division multiplexing and wavelength selective routing. A programmable, wavelength selective optical router that can exchange channels between different optical fibers is a critical element of such a network. Furthermore, collector boxes that bundle and distribute, according to wavelength, large numbers of wavelength channels from one fiber to a number of fibers are necessary to efficiently route the signal to the desired destination. For example, a collector box may be configured to separate 32 wavelength channels traveling along a single fiber into 4 groups of 8 sequential channels traveling along 4 fibers. As such, a collector box is a specialized form of wavelength demultiplexer.
Wavelength selective routers of the prior art typically use combinations of optical switches and wavelength demultiplexers/multiplexers. The optical switches are typically mechanical type switches with a response time of 10 ms and an insertion loss of 0.5 dB. Faster electrooptic, acoustooptic or thermal switches can also be used, however, these devices usually exhibit insertion losses in excess of 3 dB. The wavelength demultiplexers/multiplexers typically use components such as arrayed waveguide gratings (AWG's), fiber gratings and circulators, or thin film interference filters. AWG's suffer from large insertion loss, typically >6 dB per device. Fiber gratings and circulators offer high isolation and low crosstalk at the expense of large insertion loss, typically 2 dB per channel. Thin film interference filters offer relatively low loss ( 1 dB), but do not offer the crosstalk (<−25 dB) and isolation (>30 dB) performance necessary for high channel density WDM (<200 GHz channel spacing).
A wide range of add/drop multiplexer and router technologies have been described in the prior art, but each approach suffers from relatively high insertion loss as the number of channels is scaled up. That is, loss does not increase gracefully as the number of channels increases. This additional loss necessitates further amplification of the optical signals passing through the router, increasing the cost and limiting their appeal in networks incorporating a high level of wavelength

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