Optical waveguides – Having particular optical characteristic modifying chemical...
Reexamination Certificate
2000-01-27
2003-03-04
Ullah, Akm E. (Department: 2874)
Optical waveguides
Having particular optical characteristic modifying chemical...
C372S068000, C385S132000, C501S045000, C252S30140P
Reexamination Certificate
active
06529675
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of optics and lasers, and more specifically to a method and apparatus of integrating one or more optical waveguides on a glass substrate and of forming lasers therein.
BACKGROUND OF THE INVENTION
The telecommunications industry commonly uses optical fibers to transmit large amounts of data in a short time. One common light source for optical-fiber communications systems is a laser formed using erbium-doped glass. One such system uses erbium-doped glass fibers to form a laser that emits at a wavelength of about 1.536 micrometer and is pumped by an infra-red source operating at a wavelength of about 0.98 micrometer. One method usable for forming waveguides in a glass substrate is described in U.S. Pat. No. 5,080,503 issued Jan. 14, 1992 to Najafi et al., which is, hereby incorporated by reference. A phosphate glass useful for lasers is described in U.S. Pat. No. 5,334,559 issued Aug. 2, 1994 to Hayden, which is hereby incorporated by reference. An integrated optic laser is described in U.S. Pat. No. 5,491,708 issued Feb. 13, 1996 to Malone et al., which is hereby incorporated by reference.
There is a need in the art for an integrated optical system, including one or more high-powered lasers along with routing and other components, that can be inexpensively mass-produced. The system should be highly reproducible, accurate, and stable.
SUMMARY OF THE INVENTION
The invention provides, among its embodiments, a system and method for forming an optical system including a high-powered laser on a glass substrate, and a resulting glass-substrate-based optical system and method for operating the optical system. The invention is further directed to methods for forming optical waveguides on glass substrates, including: forming substrates with multiple waveguides and including wherein at least two of the multiple waveguides have differing wavelengths.
Further embodiments of the invention provide a laser component that includes a glass substrate doped with a laser species and having one or more, preferably multiple, waveguides defined by channels within the substrate. (As used herein, a “channel within the substrate” is meant to broadly include any channel formed on or in the substrate, whether or not covered by another structure or layer of substrate.) Each substrate waveguide (or “channel”) is defined within the substrate as a region of increased index of refraction relative to the substrate. The glass substrate is doped with a laser species which can be optically pumped (preferably a rare-earth element such as Er, Yb, Nd, Ho, Tm, Sm, Tb, Dy or Pr or a combination of such elements such as Er and Yb) to form a laser medium which is capable of lasing at a plurality of frequencies. Mirrors or distributed Bragg reflection gratings may be located along the length of a waveguide for providing feedback to create a laser-resonator cavity. One or more of the mirrors or reflection gratings is made partially reflective for providing laser output.
The laser component may constitute a monolithic array of individual waveguides in which the waveguides of the array form laser resonator cavities with differing resonance characteristics (e.g., resonating at differing wavelengths). The component may thus be used as part of a laser system that outputs laser light at a plurality of selected wavelengths. In certain embodiments of the invention, the resonance characteristics of a waveguide cavity are varied by adjusting the width of the channel formed in the substrate which thereby changes the effective refractive index of the waveguide. The effective refractive index can be changed by modifying the diffusion conditions under which the waveguides are formed as described below. Changing the effective refractive index thus changes the effective length of the waveguide cavity which determines the wavelengths of the longitudinal modes supported by the cavity. In another embodiment, the resonance characteristics of the waveguide cavities are individually selected by varying the pitch of the reflection gratings used to define the cavities which, along with the effective refractive index for the propagated optical mode, determines the wavelengths of light reflected by the gratings. In still other embodiments, the location of the gratings on the waveguides is varied in order to select a laser-resonator cavity length that supports the desired wavelength of light.
In a preferred embodiment, the waveguide or multiple waveguides, optionally as part of a laser element, are constructed from a glass substrate which is a phosphate alkali glass doped with a rare-earth element such as Er or Yb/Er. In the case of Yb/Er doped glass, it is preferred for maximal lasing efficiency that the Yb/Er ratio is from approximately 1:1 to 8:1, particularly 3:1 to 8:1. This has been discovered as a result of investigation as to the optimal erbium and ytterbium rare earth ion concentrations in rare earth doped laser glasses, in particular as erbium and ytterbium 1.54 &mgr;m laser sources and amplifiers employed in the fields of telecommunication and data transmission. Prior to this investigation, erbium/ytterbium doped glasses for such applications were typically characterized by low erbium concentration (generally much less than 1 wt % Er
2
O
3
content) with a corresponding high ytterbium content (typically at a ion ratio of greater than 10 ytterbium ions for each erbium ion input to the glass); such glasses are disclosed in, for example, U.S. Pat. Nos. 5,334,559 and 5,491,708. Such high levels of Yb were initially expected to yield high output powers and high slope efficiency based on prior experience with silicate and phosphate glass formulations, see for example U.S. Pat. No. 4,962,067. The prior art taught that low erbium doping levels (at most 0.15 mole % Er
2
O
3
) were required to avoid self quenching effects, and that the ytterbium content be set as high as possible (basically input Yb
2
O
3
to near the solubility limit in the glass, at least 6 mole % Yb
2
O
3
) in order to optimize the amount of pumping light absorbed within the laser glass.
However, according to this invention, it has been discovered that glasses with a Yb/Er rare earth content of from about 1:1 to 8:1 (particularly the glass called NIST-IT or IOG-1, a sodium-aluminum-phosphate glass having a content in melt of 1.15 wt % Er
2
O
3
and 4.73 wt % Yb
2
O
3
) demonstrated a higher output power (up to 180 mW compared to a prior high of only 16 mW) and higher slope efficiency (of at least 28% compared to a prior high of only 27%).
The inventors first expected that the poor performance of the prior art glasses was attributed to, at least in part, residual hydroxyl groups left in the glass. Rare earth ions excited in the glass are known to exchange energy with hydroxyl vibrational overtones in the glass, effectively robbing the excited state ions of the stored energy otherwise used to produce amplified laser emission. To investigate this possibility a series of sodium-aluminum-phosphate glasses containing 0.5 wt % Er
2
O
3
and 8.94 wt % Yb
2
O
3
with different hydroxyl content were prepared. The residual hydroxyl content in these glasses is proportional to the measured infrared absorption at 3.0 um and is detailed in Table 1.
TABLE 1
Hydroxyl Content Investigation in
Sodium-Aluminum-Phosphate Glass
Absorption at
Melt ID
3.0 um [cm
−1
]
NIST-1L
0.72
NIST-1H
1.77
NIST-1J
6.02
The results of this work indicated that although residual hydroxyl content certainly was detrimental to laser performance, it could not alone account for the poor performance of devices fabricated in these three glasses. In fact, melt NIST-1H and NIST-1L were characterized by an absorption at 3.0 um below 2.0 cm
−1
, a level expected to be low enough not to significantly influence laser performance. In particular, the absorption level in melt NIST-1L was less than 1.0 cm
−1
, a threshold value below which the prior art has shown is not an issue in evaluating laser performance of a given laser glass, see Cook, L. M. e
Hayden Joseph S.
Veasey David L.
Millen White Zelano & Branigan P.C.
Song Sarah U
The United States of America as represented by the Secretary of
Ullah Akm E.
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