Coherent light generators – Particular resonant cavity – Distributed feedback
Reexamination Certificate
1999-12-16
2001-05-29
Lee, John D. (Department: 2874)
Coherent light generators
Particular resonant cavity
Distributed feedback
C359S341430, C372S006000, C372S029020, C372S038020
Reexamination Certificate
active
06240119
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to optical gain media coupled with optical gratings to provide for wavelength control and, more particularly, to an apparatus for stabilization of laser sources with an optically coupled waveguide grating in close proximity to the laser source which may be employed as an optical power source for an optical solid state amplifier or laser.
In U.S. Pat. No. 5,485,481, assigned to the same assignee of this application, and which is incorporated herein by reference, there is disclosed the utilization of a optical waveguide grating in the form of a fiber grating coupled to a gain medium comprising a semiconductor laser source to control and maintain the operation of the laser at a given wavelength within the laser bandwidth of operation. The fiber coupled laser is sometimes referred to as pigtailed laser. The assembly of the laser source together with the coupling optics and optic fiber pigtail are provided in a customer-convenient pin-out package.
As discussed in the above mentioned patent, the behavior of a semiconductor laser source under conditions of optical feedback is complicated by the effect of the laser cavity itself defined by the end facets of the semiconductor chip. The reflectivity of the grating as well as its wavelength are selected such that the broadband feedback from the laser cavity is greater than the feedback from the fiber grating. This is accomplished by insuring the reflectivity level of the front emitting facet of the laser source, providing feedback into the laser cavity, is higher than the reflectivity level provided from the pigtail fiber grating, providing reflected feedback into the laser cavity. The reflectivity level of the front facet of the laser source is very low to begin with to permit emission of the laser beam output but sufficient to provide for lasing conditions to be maintained in the laser source. The reflectivity of the fiber grating may be equal to but does not exceed the front facet reflectivity level. Thus, as an example, the reflectivity level of the grating in the fiber may be 3% while the reflectivity level of the front facet of the laser source may be 4%. Under these circumstances, the feedback from the fiber grating acts as a perturbation of the coherent optical field formed in the diode laser cavity. This perturbation acts to break the coherence of the laser emission, which is referred to as coherence collapse, broadening the bandwidth of the laser emission by several orders of magnitude, resulting in multiple longitudinal mode operation of the laser source. The fiber Bragg grating effectively locks the laser cavity output to the fixed wavelength of the fiber grating and centers the external cavity multi-longitudinal modes around that wavelength. The presence of the multi-longitudinal modes reduces the magnitude of mode-hopping noise in the laser. In addition, the center wavelength of emission remains near the wavelength of maximum reflection from the fiber grating. The laser source is, thus, constrained to operate within the grating bandwidth so that large fluctuations in wavelength of the laser source, such as caused by changes in temperature or current, are eliminated. Additionally, the laser source is not perturbed by extraneous optical feedback from reflective components located beyond the fiber grating, provided the level of extraneous feedback is less than that provided by the fiber grating.
To ensure the maintenance of the coherence collapse of the laser emission, the fiber grating is located at a sufficient optical distance from the front facet of the laser source, which may be, for example, about 50 cm. to 70 cm. from the laser source output facet. This distance must be greater than or equal to the coherence length of the laser source under the prescribed conditions of optical feedback, so that optical reflective feedback from the fiber grating remains incoherent, thus assuring the laser remains in a state of coherence collapse. At this distance, the phase of the external feedback from the grating is uncorrelated to the phase of the laser source emission at substantially all levels of laser current and temperature operation.
On the other hand, if the grating is placed within a few centimeters or less of the laser source, then the feedback from the fiber grating may be coherent with the electric field inside the laser cavity, and very narrow linewidth operation of the laser will result. Such emission is very useful for some applications but is much less stable for the application of pumping optical solid state amplifiers and lasers because of the onset of laser cavity-mode transition noise when the laser operating characteristics change, such as may result from changes in operating temperature or pumping current. As a result, there are back-and-forth intermittent transitions from incoherent to coherent state operation of the laser that cause power output fluctuations detrimental to the operation of such amplifiers and lasers.
The necessity of locating the fiber grating at an optical distance from the laser diode that is equal to or greater than the coherence length of the laser may impose unacceptable packaging requirements on some applications because a large amount of fiber pigtail is required. On the other hand, if the pigtail is to be included in the same package that contains the laser source, this package must be large enough to contain the length of required pigtail fiber. This may be impractical. On the other hand, if a smaller package is used to contain the laser source, the need to locate the fiber grating a greater distance from the laser source will require the pigtail fiber to extend outside the package. In certain applications, this may expose the pigtail to potential damage as well as to temperature or pressure fluctuations sufficient to adversely affect operating characteristics of the laser source. Therefore, in cases where the laser source package is required to be compact particularly for use with an optical fiber amplifier or fiber laser, such as a rare-earth doped amplifier or laser, it is desirable to locate the fiber grating in the pigtail as close as possible to the laser source so that the pigtail may be included in the package. However, as indicated above, stable operation of the amplifier or laser will not be adequately maintained and the laser diode source, under these conditions, will intermittently shift between coherent to incoherent operation causing power glitches or power kinks in the amplifier/laser output with changes in laser operating temperature and laser diode modulated output.
Therefore, it is an object of this invention to effectively avoid the undesirable effects of intermittent coherent operation of the laser source when the optical grating is positioned within the coherence length of the laser source.
It is another object of this invention to affect the operating current for driving the laser source so as to stabilize the operation of the laser source for applied applications, such as optically pumping of fiber amplifiers.
SUMMARY OF THE INVENTION
According to this invention, apparatus for providing an improved stabilized laser source comprises a laser source having a light beam output and an electrical input. An optical waveguide having an input end is optically coupled to the laser source to receive the light beam. A reflector reflects a portion of the light beam output back into the laser source. The reflector is formed at a distance from the laser source to cause the laser source to switch intermittently between coherence and coherence collapse states of operation. Means are provided to the electrical input of a drive signal for the laser source with variations of amplitude to the electrical input to induce the laser source to repetitively switch operating states between coherence and coherence collapse. The optical waveguide provides the laser source light beam output to an optical gain medium having an excited state of operation of duration equal to a relaxation time wherein intervals between the variations of driv
Carothers, Jr. W. Douglas
Lee John D.
SDL Inc.
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