Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
1999-04-05
2001-01-30
Moskowitz, Nelson (Department: 3662)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C359S341430, C372S006000, C385S010000, C385S127000
Reexamination Certificate
active
06181465
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to fiber lasers and, more particularly, to high-power fiber lasers.
BACKGROUND OF THE INVENTION
Diode lasers are often used to pump erbium single-mode fiber amplifiers; however, a single diode laser typically generates only a relatively small amount of pumping power. Consequently, an array of diode lasers, or a laser bar, is conventionally used to generate a pump power level which is relatively large when compared with the pump power level provided by a single diode laser. The output beam produced by an array of laser diodes is highly multi-mode and thus not suitable for launching directly into a single mode fiber core. Therefore, in order to effectively couple a diode laser array to a single mode fiber core, a technique commonly referred to as “cladding pumping” is used. In a cladding pump technique, a single-mode core is surrounded by a multi-mode cladding layer which, in turn, is surrounded by an outermost cladding layer. A relatively high-power multi-mode pumping signal launched into the cladding from a diode array is substantially confined and guided within the multi-mode cladding layer. As the pumping energy propagates along the multi-mode cladding layer criss-crossing the doped fiber core, the energy is absorbed by the single-mode core. The absorbed multi-mode power is converted into a single-mode laser emission within the core. For many applications, this is an effective technique for supplying a relatively high-power pumping signal to a single-mode fiber laser.
However, one factor which limits the output power characteristic of a cladding pumped fiber laser is the conversion of a portion of the laser output signal from the desired lasing frequency to an unwanted so-called “Raman frequency.” This conversion occurs by a process known as stimulated Raman scattering which shifts the desired output frequency to the first Stokes frequency of the fiber core. In optical fibers manufactured from silica, the first Stokes frequency corresponds to a wavelength of approximately 450 cm
−1
or about 60 nanometers (nm) in the region of 1100 nm. Raman scattering is power-related and becomes significant when power levels increase above a threshold. The power limitation imposed by Raman scattering is more severe in pulsed fiber laser systems. With the effective area of typical cladding-pumped fibers and fiber lengths of about 50 meters, Raman scattering becomes significant at output power levels typically in the range of a few tens of watts. When Raman shifting occurs, the shifted wavelengths are also amplified in the laser oscillator thereby diverting the pumping energy also to the Raman-shifted wavelength output. Output at the desired lasing wavelength is therefore effectively limited.
A similar problem occurs when the laser gain at a desired lasing wavelength is less than the laser gain at another wavelength. In this case, amplified spontaneous emission (ASE) may occur at the other wavelength and prevent the laser from operating at the desired wavelength or require an increase in the pumping intensity in order to generate a desired power output. For example, it may be desirable to operate a neodymium (Nd) fiber laser comprised of a Nd-doped double clad fiber at a laser wavelength of 940 nm. However, neodynium is a three-level system at 940 nm and the amplifier gain of the fiber is usually higher at 1060 nm where Nd is a four-level system. Consequently, operation at a wavelength of 940 nm requires a relatively high pump intensity due to the presence of the competing energy level system.
It is therefore desirable to control the output power characteristics of cladding pumped fiber lasers.
It is further desirable to suppress unwanted wavelengths in cladding pumped fiber lasers.
It is further desirable to reduce the effective pumping of a shifted wavelength output due to Raman scattering.
SUMMARY OF THE INVENTION
In accordance with the present invention, the optical fiber employed as the active amplifying medium in a fiber laser is arranged to have a high insertion loss at the undesired frequency, while retaining a low insertion loss at the desired lasing frequency. In one embodiment, loss at an undesired frequency is introduced by using an optical fiber which has multiple claddings with an index profile which may include an elevated index region located in close spatial proximity to the core, specifically, within the evanescent coupling region of the core. This raised index section produces a distributed loss at the undesired frequency which is several orders of magnitude higher than the loss in an unmodified fiber, but no, or minimal, loss is introduced at the desired lasing frequency. This approach effectively raises the threshold at which, for example, Raman scattering occurs in the fiber and, therefore, results in a frequency-selective fiber. When such a fiber is used as the active medium in a laser, the output power can be considerably higher before Raman shifting occurs.
In another embodiment, an absorbing layer is placed around the core region. The absorbing layer is chosen to have a sharp absorption edge so that it absorbs highly at the undesired wavelength, but minimally at the desired lasing wavelength. Such a structure also produces a fiber with a distributed loss at the undesired wavelength.
In still another embodiment, the optical fiber is constructed with a core with long period gratings formed therein. The gratings are fabricated with a periodicity selected to provide a relatively high insertion loss at the undesired frequency while simultaneously providing a relatively low insertion loss at the lasing frequency. The long period gratings act as discrete loss elements to undesired wavelength signals propagating in the optical fiber. In some applications, it may be preferable to construct the fiber core with a region having a plurality of long period gratings formed along the fiber length. This approach can be realized in conventional optical fibers by conventional doping techniques.
In accordance with yet another embodiment, a bend loss technique is used to suppress the undesired wavelengths. For example, bending loss can be used to suppress amplified spontaneous emission (ASE) at an unwanted, longer wavelength due to a competing atomic energy level system in a fiber laser. The technique includes continuously bending the fiber with a selected bend radius and pumping the fiber with a pump signal. When the bend radius is selected properly, light at the unwanted wavelength radiates out of the core. The gain of the fiber at the unwanted wavelength is inhibited to prevent the occurrence of ASE at the unwanted wavelength, allowing the fiber to operate as a laser in the desired lasing wavelength.
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Luis Zenteno; High-Power Double-Clad Fiber Lasers; Journal of Lightwave Techn
Archambault Jean-Luc
Grubb Stephen G.
Waarts Robert G.
Zanoni Raymond
Kudirka & Jobse LLP
Moskowitz Nelson
SDL Inc.
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