Coherent light generators – Particular pumping means – Pumping with optical or radiant energy
Reexamination Certificate
1998-07-02
2001-08-14
Arroyo, Teresa M. (Department: 2881)
Coherent light generators
Particular pumping means
Pumping with optical or radiant energy
Reexamination Certificate
active
06275516
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to fiber lasers. More particularly, the present invention relates to an article for detecting and, optionally, correcting power drift in the output of a diode array source.
BACKGROUND OF THE INVENTION
FIG. 1
depicts a simplified schematic of a fiber laser
100
. Fiber laser
100
includes a light pump
102
, an optical fiber
106
and two reflectors, which, in fiber laser
100
, are embodied as respective high-reflectivity and low-reflectivity mirrors
110
and
114
. Mirrors
110
and
114
are disposed adjacent to respective fiber end faces
108
and
112
, defining a laser cavity
104
therebetween. In other embodiments, the reflectors are implemented as gratings that are formed within the fiber
106
. Light pump
102
, advantageously a laser diode or diode array, launches light
103
(i.e., photons) into the laser cavity
104
. The pump photons stimulate the emission of photons in the fiber
106
providing lasing output
116
at a characteristic wavelength, as described further below.
FIG. 2
shows a double-clad fiber
206
suitable for use as fiber
106
of fiber laser
100
. Double-clad fiber
206
comprises single-mode fiber core
208
, a multi-mode first cladding
210
surrounding fiber core
208
, and a second cladding
212
surrounding first cladding
210
. Light from light pump
102
is launched into first cladding
210
. In some embodiments, light is launched into first cladding
210
at an end of fiber laser
100
, known as “end-pumping,” (see FIG.
1
). In other embodiments (not shown), light from pump
102
is launched into first cladding through a side of fiber laser
100
. In the embodiment depicted in
FIG. 2
, first cladding
210
has a rectangular cross section, wherein the ratio of the lengths of the long side to the short side is in the range from about 1.5/1 to about 10/1. In other embodiments, the first cladding has a “star” or “D-shaped” cross section. Typically, first cladding
210
comprises pure silica.
The geometry and refractive indices of fiber core
208
, first cladding
210
and second cladding
212
are such that a substantial amount of light launched into first cladding
210
is coupled into fiber core
208
. Such an arrangement is advantageous since light can be launched into multi-mode cladding, such as first cladding
210
, without the high tolerances typically required for launching light directly into a single-mode core, such as fiber core
208
.
Fiber core
208
, typically 4-8 microns in diameter and comprising silica, is doped with one or more ionized rare-earth element (“active lasing element”), such as Nd
3+
, Yb
3+
, Tm
3+
, and Er
3+
. The active lasing element absorbs photons that are coupled into fiber core
208
. Absorbing such photons increases the energy state of the active lasing element and causes “population inversion.” As electrons in the active lasing element decay to lower energy states, photons are emitted that have a wavelength characteristic of the particular lasing element. In some embodiments, co-dopants, used for modifying the refractive index of the fiber core, are used in conjunction with the active lasing elements.
Second cladding
212
substantially prevents light from leaking from first cladding
210
to the outside environment. Such containment is accomplished by ensuring that the index of refraction of second cladding
212
is significantly lower than that of first cladding
210
(typically about 1.38 vs. 1.465). Second cladding
212
, depicted in
FIG. 2
as having a circular cross section, is suitably formed from a polymer, such as a fluoropolymer, or a low-index glass.
FIG. 3
depicts additional details regarding the launching of light into fiber laser
100
. In one embodiment, light pump
102
launches light
302
into a short connector-free section (“a pigtail”) of multi-mode fiber
304
having multi-mode core
306
. Fiber pigtail
302
is spliced to high reflector grating
110
and aligned to launch light
302
into first cladding
210
.
It is desirable, if not critical, to be able to measure the power launched into a fiber laser in order to monitor and correct for drifts in the diode pump and to accurately test the laser output. Presently, no satisfactory method exists for performing such measurements. In one prior-art approach, a detector is located at the back facet of a diode. Such an approach does not, however, measure drifts in launched power. Rather, it measures the power generated at the source (i.e., the diode). Launched light cannot be measured in this manner because reflecting (back-propagating) the launched light for a measurement by the detector may damage the diode. Moreover, a multimode tap would be needed to route such back-propagating light to the detector. Such taps are not readily available. In another method, “integrating spheres” are used to detect light scattered from a fiber pigtail, such a fiber pigtail
302
. Such detected light is disadvantageously not a measure of launch power. Furthermore, such integrating spheres are bulky.
As such, the art would benefit from a method and apparatus for accurately measuring the power launched into a fiber laser.
SUMMARY OF THE INVENTION
In some embodiments, an improved fiber laser incorporating feedback control in accordance with an illustrative embodiment of the present invention is disclosed. In one embodiment, the fiber laser includes a light pump, such as a laser diode or diode array, that launches light into a multi-mode fiber pigtail. The fiber pigtail is split into a first and a second section that are separated by a gap. The second portion of the fiber pigtail is attached to a first reflector, which is a high-reflectivity mirror or fiber grating. A double-clad optical fiber is disposed between the first reflectors and a second, relatively low reflectivity reflector.
The fiber laser further includes a sampler that is operable to sample at least a portion of the launched light traveling across the gap between the first and second portions of the multi-mode fiber pigtail. The sampled portion represents a known fraction of the total light signal launched into the first portion of the fiber pigtail.
In some embodiments, the sampled light is reflected, via the sampler, to a photodetector. The photodetector converts the sampled light into a first electrical signal and delivers it to a processor. The processor is operable to compare a value of the first electrical signal (e.g., voltage, etc.) to a set-point value indicative of a desired value for the electrical signal. The desired value is representative of a desired power of the sampled portion of the launched light. To the extent that a difference exists between the set point value and the value of the first electrical signal, the desired optical power is not being launched into the fiber laser. If such a difference exists, the processor generates and sends a control signal to a controllable current source. The control signal is operable to increase or decrease the current delivered from controllable current source to the light pump. Responsive to the control signal, the controllable current source thus delivers an appropriately-adjusted current to the light pump. In that manner, drifts in pump output are detected and corrected, thereby providing stable laser operation.
In other embodiments, the sampled light is absorbed by the sampler and a change in a property of the sampler due to such absorbed light is measured. The change in the property is correlated to the power of the absorbed light. The absorbed power is compared, using a processor, to an expected value of absorbed power for the quantity of absorbed light. If a differential exists, the processor generates and sends a control signal to a controllable current source as for the previously-described embodiment. Responsive to the control signal, the controllable current source delivers an appropriately-adjusted current to the light pump.
REFERENCES:
patent: 4873690 (1989-10-01), Adams
patent: 4899053 (1990-02-01), Lai
patent: 5305330 (1994-04-01), Re
Arney Susanne
Kosinski Sandra Greenberg
LeGrange Jane Deborah
Agere Systems Optoelectronics Guardian Corp.
Arroyo Teresa M.
Breyer Wayne S.
DeMont Jason Paul
DeMont & Breyer LLC
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