Coherent light generators – Particular resonant cavity – Distributed feedback
Reexamination Certificate
1998-11-20
2001-04-10
Font, Frank G. (Department: 2877)
Coherent light generators
Particular resonant cavity
Distributed feedback
C372S006000, C372S032000, C372S092000, C372S091000, C372S022000, C372S064000, C372S102000, C372S038060, C372S020000, C372S050121, C372S108000, C372S026000, C359S326000, C359S330000, C359S341430, C356S460000, C356S464000
Reexamination Certificate
active
06215809
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to optical gain media coupled with optical gratings to provide for wavelength control of a laser source and, more particularly, relates to an apparatus for stabilization of laser sources with an optically coupled waveguide grating in close proximity to the laser source, which may be used as an optical power source for an optical solid state or fiber amplifier or laser.
BACKGROUND OF THE INVENTION
In U.S. Pat. Nos. 5,485,481 and 5,715,263, which are assigned to the assignee of this patent application and are incorporated herein by reference in their entirety, there is disclosed the utilization of a fiber grating coupled to a gain medium comprising a semiconductor laser source to control, stabilize and maintain stabilization of the operation of the laser within a given wavelength band. 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” or industry-standard enclosure that is sealed to hide the components inside the enclosure as well as to improve handling and environmental ruggedness. A few examples of industry-standard enclosures are those known in the industry as “P5”, “P6”, 14-pin butterfly packages, various coaxial packages such as a TO container and a variety of newer designs including so called MINI-DIL, MINI-SMD and MINI-PIN packages.
In employing a semiconductor laser source having a comparatively wide gain bandwidth, the laser source will tend to have multiple longitudinal modes. Due to changes in various operating conditions, such as changes in ambient temperature or operating current, the laser operation may readily jump from one longitudinal mode to another. Changes in light output intensity caused by the mode jumps are sufficient in magnitude to affect the performance of a fiber amplifier being pumped by such a source because these changes cause a corresponding jump in the reciprocal lifetime of the excited energy level of the amplifier active element, which is usually a rare earth material. This in turn causes noticeable jumps or modulation in the amplified light signal in the amplifier. These modulations cannot be tolerated in optical fiber telecommunication systems.
The foregoing mentioned patents address this problem by placing a fiber grating in the output of the laser source, spaced from the laser source, having a bandwidth sufficiently wide to cause the laser source to operate in the “coherence collapse” regime. In essence, the fiber grating is positioned a sufficient distance from the laser source and the small amount of light reflected back into the laser source can be characterized as a weak source of “noise” to the laser source. The operation of the laser source locks onto this noise-like feedback and maintains its operation within the wavelength bandwidth of the fiber grating. In this coherence collapse regime, the laser source is not allowed to lock onto any one wavelength or longitudinal mode but instead is induced to jump from one longitudinal mode to another. The bandwidth and spacing of the reflective grating is controlled to cause the laser source to jump between modes at a rate that is higher than the reciprocal lifetime of the amplifier active element. In a sense, the amplifier gain element acts as a low-pass filter to smooth the changes in light intensity caused by jumps between longitudinal modes of operation. As a result, mode transitions have little or no effect on the operation of the fiber amplifier.
The length of the pigtail fiber between the laser source output facet and the fiber rating represents an external cavity in addition to the laser source cavity. While coherence is maintained in the laser source cavity, no coherence is established in the external cavity because of its comparatively long length and the wide bandwidth of the grating. To provide reasonable assurance of stabilization in the coherence collapse regime, the length of the pigtail fiber between grating and laser output facet should be greater than the coherence length of the laser source. Also, it is generally preferred that the reflectivity level of the laser source output facet be higher than the reflectivity level of the grating; however, this is not an absolute requirement, depending upon the fiber grating bandwidth and the distance of the fiber grating from the laser source output facet.
The use of a reflector grating in this manner helps bring about coherence collapse because the optical feedback from the fiber grating acts as a perturbation of the coherent optical field formed in the laser source cavity. This perturbation acts to break the coherence of the laser operating mode, which is referred to as coherence collapse, and broadens the bandwidth of the laser emission by several orders of magnitude, resulting in multiple longitudinal mode operation of the laser source. The fiber grating effectively locks the laser source 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 significantly reduces the magnitude of mode-transition noise in the laser so that no single longitudinal mode produced by the laser source contains, for example, more than about 20% of the total optical power produced by the laser source. 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 operating 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.
An important aspect for achieving coherence collapse is that the fiber grating provides a sufficiently wide bandwidth, such as several GHz, so that no particular longitudinal mode dominates operation of the laser.
The distance between the laser source output facet and the reflective grating is an important consideration in achieving coherence collapse, as mentioned above. For many embodiments, 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 optical field inside the laser source cavity and coherent operation of the laser will result. Coherent emission is very useful for some applications but it is much less stable for the application of pumping solid state or fiber amplifiers and lasers because of the mode-transition noise that results when the laser operating characteristics change, such as may result from changes in ambient temperature or operating current. As a result, if the grating is too close to the laser source output facet, intermittent transitions between coherent and coherence collapse states of operation will cause power output fluctuations detrimental to the operation of such amplifiers and lasers.
To assist the maintenance of coherence collapse of the laser emission, the fiber grating should be located at a sufficient optical distance from the output facet of the laser source, which may be, for example, about 50 cm. to 100 cm. from the laser source output facet. This distance should be greater than the coherence length of the laser source so that optical feedback from the fiber grating remains incoherent, thus, helping ensure the laser consistently remains in a state of coherence collapse. The coherence length is related to the bandwidth of the fiber grating in that wider bandwidth gratings can be placed closer to the laser source but it is also related to other parameters such as the operating wavelength of the laser source and the fiber grating reflectivity and period. Beyond the coherence length, the phase of the optical feedback from the fib
Bortz Michael L.
DeAndrea John
Lang Robert J.
Ventrudo Brian F.
Waarts Robert G.
Flores Ruiz Delma R.
Font Frank G.
Gallagher & Lathrop
Lathrop David N.
SDL Inc.
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