Resonant optical wave power control devices and methods

Details Resonant optical wave power control devices and methods Resonant optical wave power control devices and methods

1999-12-07

2003-10-14

Bovernick, Rodney (Department: 2874)

Optical waveguides

With optical coupler

Particular coupling function

C385S001000, C385S016000, C385S028000, C385S050000

Reexamination Certificate

active

06633696

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to optical wave power control devices and methods, and more particularly to systems, devices and methods for modulating and switching signals transmitted in optical waveguides.
BACKGROUND OF THE INVENTION
In the now rapidly expanding technology of fiber optics, a number of discrete devices and subsystems have been developed to modulate, or otherwise control, optical beams that are at specific wavelengths. The approaches heretofore used, however, have not fully overcome one or more problems inherent in the requirements imposed by modern systems. Present day communication systems increasingly use individual waveguide fibers to carry densely wavelength multiplexed optical beams, and modulate the beams at very high digital data rates or with wideband analog data, or both.
For example, it is known how to modulate the power of a monofrequency laser source, typically a semiconductor laser. Using such a source, one must accept a limited modulation bandwidth because of constraints on the rate at which the laser can be turned on and off. In addition, this type of modulation introduces chirping, or spreading of the bandwidth of the signal from the monofrequency laser, so that dispersion variations with wavelength in signals that are transmitted in optical fiber over a substantial distance place an inherent limit on that distance. This approach does have the advantage, as compared to some other systems, of modulating at the source, so that continuity in the optical fiber structure can be preserved. However, semiconductor lasers that are modulated must be coupled to optical waveguides by means which introduce problems with yield, reliability and cost. Consequently, the limitations mentioned above are such that long distance transmission systems tend to employ external modulators.
The two forms of external modulators that are currently employed are monolithic waveguide devices. A widely used lithium niobate modulator of this type is based on a Mach-Zehnder interferometer and is being employed in long distance transmission systems and other applications because it creates clean waveforms at the highest data rates and produces a minimal amount of chirping. As a monolithic waveguide device, it must be coupled at its input and output to an optical fiber, which requires costly packaging and assembly but even so introduces a substantial mismatch between the chip waveguide and the optical fiber waveguide, thus entailing losses in the range of about 5 db. Furthermore, it is polarization sensitive and must be actively temperature stabilized to compensate for the thermal drift characteristics of the interferometer
A second waveguide device, more recently introduced, is also a monolithic on-chip device using an electro-absorption effect. This modulator is fabricated integrally with a semiconductor laser, requiring sophisticated and costly fabrication technology that inevitably decreases the yield of the overall laser device. In addition, such a device is subject to chirping, which places a limitation on high (10 gigabit/sec and higher) modulation rates. The integral laser/modulator chip must be coupled to optical fiber—again adding cost to manufacturing.
There are a number of other patents of recent interest which disclose variants on the monolithic device structure, but all require a matching technique to be used to function with an optical fiber. Mention of signal modulation is made in at least two patents which often employ dielectric microcavities for recirculating electromagnetic wave energy at optical wavelengths. “Whispering gallery mode” (WGM) structures, which comprise microresonators of generally spherical, ring, or disc-like configuration, are of dielectric material, e.g. glass or silica. They are essentially totally reflective and support internal modes at frequencies determined by size and other factors, with very low losses, and therefore high Q. They are being investigated for use in a number of different optical configurations. U.S. Pat. No. 5,343,490 to McCall, for example, discloses a closed loop WGM system configured as a thin element, described as “an active material element of thickness characteristically of a maximum of a half wavelength . . . ” (Col; 1, lines 62-63). Disks are described that have thicknesses in the range of 1,000-1,500 Å and have at least one optically active layer, sandwiched between thicker barrier layers. The optically active material may be InGaAs and the barrier layers InGaAsP material, for example. Fabricated into a microcavity using photolithographic techniques, the structure is described as having multiple potential functions. These comprise optically pumped single quantum well to multiple quantum well structures and various two port and three port devices which may function as, for example, detectors, data amplifiers, and current meters. It is mentioned in passing, as at Col. 6, lines 3-23, that the output may be modulated or unmodulated, but apart from general statements (e.g., “delicate destructive phase interference” in terms of canceling an unmodulated output) there is no teaching as to how modulation, much less high speed modulation could be effected. Continued evolution of this approach may lead to practical modulators at some point in time, but even then would face the barriers presented by the need for matching to fiber waveguide structures, and in cost, and in performance specifications such as insertion loss.
A somewhat related approach is described in the “Photonic Wire Microcavity Light Emitting Devices” application of Ho, et al. in U.S. Pat. No. 5,878,070. The inventors also describe a WGM microcavity with a gain medium of InGaAs sandwiched between InGaAsP layers of submicron thickness, but closely surround a ring of this optically active structure with an arc of lower refractive index waveguide material in a general U-shape, the side arms of which may be tapered (FIG. 9). With this arrangement, there is resonant photon tunneling from the active material of the gain cavity to the output-coupled waveguide, which serves as the core of the structure. The possibility of modulation, by varying the pumping power of the active medium section, is also suggested, (Col. 15, lines 54-58) with no specific implementation being described. Since the concept is based upon a discrete and particular active waveguide core and an arc of low refractive wave index material serving as an output waveguide in close association to it, is evident that the same problems that are presented by the McCall disclosure are also present here.
In addition to the rapidly increasing use of fiber optic systems, there is constant evolution toward denser wavelength division multiplexing and higher data rates per channel. This in turn means that factors such as spectral bandwidth, frequency stability, compactness and reproducibility are of added importance, and place added requirements on any new approach.
An all fiber modulator, one that assures the continuity of the wave energy transmitted along an optical waveguide, will therefore be of substantial potential benefit, if it can be provided in a form that offers sufficient dynamic range, and minimizes insertion losses while being capable of handling high data rates. It is evident that such a device, if wavelength sensitive, can also be used as an on-off switch, or a switchable bandpass filter, where required for specific applications. Preferably, for complex switching and routing systems having many channels, units using the same concepts can be fabricated using microlithographic or micromachining techniques.
SUMMARY OF THE INVENTION
These and other objectives of the invention are met by a power transfer structure and modes of operation which variably attenuate (modulates) or completely block (switches off) the power propagated in a section of an optical waveguide. To this end a short section of an optical waveguide is modified to couple power into an adjacent high Q resonator microcavity in which wave energy of a resonant mode recirculates with power accumulation before return

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