Multimode fiber laser gratings

Details Multimode fiber laser gratings Multimode fiber laser gratings

2001-09-27

2004-06-15

Ip, Paul (Department: 2828)

Coherent light generators

Optical fiber laser

C372S049010, C372S075000

Reexamination Certificate

active

06751241

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to gratings for active fibers, and in particular to a multimode grating for 3-level double-clad fiber and tapered fiber lasers.
2. Technical Background
Optical fiber is the favored transmission medium for telecommunications due to its high capacity and immunity to electrical noise. Silica optical fiber is relatively inexpensive, and when fabricated as a single transverse mode fiber can transmit signals in the 1550 nm band for many kilometers without amplification or regeneration. However, a need still exists for optical amplification in many fiber networks, either because of the great transmission distances involved, or the optical signal being split into many paths.
As illustrated schematically in
FIG. 1
, a conventional amplifier
10
is interposed between an input transmission fiber
12
and an output transmission fiber
14
. Erbium-doped fiber amplifiers (EDFAs) have been found quite effective in providing the required optical gain, as one example of the amplifier
10
. Another example of the amplifier
10
is a fiber with Raman gain. Both transmission fibers
12
,
14
need to be single-mode, because higher-order modes exhibit much greater dispersion (typically the limiting factor for the fiber transmission distance at high data rates). The EDFA
10
includes a length (on the order of tens of meters) of an erbium-doped silica fiber
16
, as is well known in the art. It is well known that an erbium optical fiber amplifier operating in its purely three-level mode is capable, when pumped at a wavelength of 980 nanometers (nm) of amplifying optical signals having a wavelength of 1550 nm. The doped fiber
16
should also be single-mode in order to maintain the transmission signal integrity. The doped fiber
16
is optically active due to the presence of Er
3+
ions or other rare-earth ions, which can be excited to higher electronic energy levels when the doped fiber
16
is pumped by a strong optical pump. Typically, an optical pump source
18
inputs the pump into the doped fiber
16
through a pump source fiber
20
coupled to either the undoped upstream fiber
12
or the doped fiber
16
through a wavelength-selective directional coupler
22
, but downstream coupling is also known. For efficient coupling into the single-mode Er-doped fiber
16
, the pump source fiber
20
should also be single-mode. An operative EDFA may contain some additional elements (such as an isolator or a gain-flattening filter), which are well known to the art but not relevant to the understanding of the background of the present invention.
Conventionally, one typical pump source
18
has been an edge-emitting semiconductor laser that includes a waveguide structure (in what is called a “stripe” structure) that can be aligned with the single-mode pump source fiber
20
to provide effective power coupling. However, this approach has failed to keep up with modern fiber transmission systems incorporating wavelength-division multiplexing (WDM). In one approach to WDM, a number of independent lasers inject separately modulated optical carrier signals of slightly different wavelengths into the transmission fiber
12
. The EDFA has sufficient bandwidth to amplify carrier signals within about a 40 nm bandwidth. A large number of multiplexed signals to be amplified require in aggregate a proportionately large amount of pump power. Over the past decade, the number of WDM channels preferably utilized in a standard network has increased from about four to current levels of forty or more, but at best the output power from a single-stripe laser source has only doubled.
In search for a higher powered laser source, the broad-area diode laser remains the most efficient and least expensive pump source. Recent progress in semiconductor laser technology has led to creation of a broad-area laser diodes with output powers of up to 16 W. Devices 100 &mgr;m wide with a slow-axis numerical aperture (NA) of less than 0.1 and output power of 4 Watts at 920 and 980 nm are now passing qualification testing for telecommunication applications. With proper coupling optics, the beam of such a laser diode can be focused into a spot as small as 30×5 &mgr;m with an NA of less than 0.35 in both transverse directions. The optical power density in such a spot is ~1.3 MW/cm
2
, which should be high enough to achieve transparency in 3-level laser systems.
One approach for utilizing inexpensive high-power broad-area pump lasers involves cladding-pumped, or double-clad fiber designs for the optical pump
18
. The advantages of cladding-pumped fiber lasers are well known. Such a device effectively serves as a brightness converter, converting a significant part of the multimode pump light into a single-mode output at a longer wavelength.
Cladding pumping can be employed to build a separate high-power single mode fiber pump laser. A source based on the pure three-level 978 nm Yb
+3
transition has long been suggested as a pump for EDFAs because this wavelength is close to the desired pumping wavelength of 980 nm. However, the cladding-pumped technique has been determined in practice to be ineffective for pumping pure three-level fiber lasers, such as the 980 nm transition of ytterbium, because of various fiber laser design parameters that have to be satisfied.
Practical double-clad amplifiers and lasers have been mostly limited to 4-level systems. Double-clad fiber lasers offer better performance for four-level lasing (where the lasing occurs in a transition between two excited states) than for the three-level one (where the lasing transition is between the excited and the ground state). For example, for the rare-earth ion, Ytterbium (Yb), the three-level transition is at 978 nm and competing higher-gain four-level transition is at about 1030-1100 nm.
In a double-clad laser, an outer cladding confines the pump light from a primary pump source in a large cross-sectional area multimode inner cladding. The much smaller cross-sectional area core is typically doped with at least one rare-earth ion, for example, neodymium or ytterbium, to provide lasing capability in a single-mode output signal. Typically, a neodymium-doped or ytterbium-doped double-clad fiber is pumped with one or several high-power broad-area diode lasers (at 800 nm or 915 nm) to produce a single transverse mode output (at the neodymium four-level transition of 1060 nm or the ytterbium four level transition of 1030-1120 nm, respectively). Thus, conventional double-clad arrangements facilitate pumping of the fiber using a multimode first cladding for accepting and transferring pump energy to a core along the length of the device. The double-clad laser output can be used to pump a cascaded Raman laser to convert the wavelength to around 1480 nm, which is suitable for pumping erbium. To date, a double-clad design by itself (that is, without an additional Raman converter) does not produce a sufficiently high output in any of the appropriate absorption bands for EDFAs or is not available commercially.
How much pump light can be coupled into a double-clad fiber inner cladding depends on the cladding size and NA. As is known, the “etendue” (numerical aperture multiplied by the aperture dimension or spot size) of the fiber should be equal to or greater than the etendue of the pump source for efficient coupling. The numerical aperture and spot size may be different in both axes so there may be an etendue in the x and y directions that must be maintained or exceeded.
Typically, a high numerical aperture NA
clad
, related to the difference in refractive index between the first and second cladding is desired. If there are two claddings instead of one, the index of the first cladding is n
clad,1
and the index of the second cladding is n
clad,2
such that NA
clad
=(n
clad,1
2
−n
clad,2
2
)
1/2
. In the well-known design, the first clad layer is made of glass and the second is made of plastic (fluorinated polymer) with a relatively low refractive index in order to increase

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