Patent Application
20180159296
The present disclosure relates to high power lasers and, more particularly, to monolithic, integrated seed and high power pump sources for high power lasers.
Typical laser systems that use two gain mediums require pump sources for each medium. Additionally, typical laser systems discard any remaining pump energy that is reflected backward because it is not useful for use in subsequent gain stages. Conventional systems require additional components, greater space requirements, and are less efficient than the system of the present disclosure. Wherefore, it is an object of the present disclosure to overcome the above mentioned shortcomings and drawbacks associated with the conventional art.
One aspect of the present disclosure is an integrated seed and high power pump source, comprising a pump power oscillator comprising a first medium having a pump wavelength (λp), a leaky high reflector fiber Bragg grating (FBG), and a partially reflective FBG, where the pump power oscillator intentionally propagates a minor portion of pump wavelength (λp) radiation in the reverse direction through the leaky high reflector fiber Bragg grating (FBG) and propagates a major portion of pump wavelength (λp) radiation in the forward direction through the partially reflective FBG; a seed pre-amplifier comprising a second medium having a seed wavelength (λs) wherein the seed pre-amplifier is pumped by the reverse propagating pump wavelength (λp) radiation leaked backwards through a reflection of the leaky high reflector fiber Bragg grating (FBG); and a seed power amplifier comprising the second medium having the seed wavelength (λs) wherein the seed power amplifier is pumped by the forward propagating pump wavelength (λp) radiation through the transmission of the partially reflective FBG.
In one embodiment of the integrated seed and high power pump source is wherein the first medium is thulium-doped silica fiber and the second medium is holmium-doped silica fiber. In certain embodiments, the seed wavelength (λs) is about 2110 nm and the pump wavelength (λp) is about 1920 nm.
In another embodiment of the integrated seed and high power pump source the seed wavelength (λs) is outside a practical gain bandwidth of the first medium, but within the emission bandwidth of the first medium. In some cases, the pump wavelength (λp) lies in a spectrally overlapped region of the first medium and the second medium, such that there is significant gain in the first medium and significant absorption in the second medium.
In yet another embodiment of the integrated seed and high power pump source the seed pre-amplifier further comprises a seed diode and the pump power oscillator further comprises a pump/signal combiner. In some cases, the pump/signal combiner is pumped by a pump source having a pump source wavelength (λp2). In one example, the pump source wavelength (λp2) is about 795 nm.
Another aspect of the present disclosure is a method of pumping integrated amplifiers with a single pump source comprising providing a pump power oscillator comprising a first medium having a pump wavelength (λp), a leaky high reflector fiber Bragg grating (FBG), and a partially reflective FBG; providing a seed pre-amplifier comprising a second medium having a seed wavelength (λs); providing a seed power amplifier comprising the second medium having the seed wavelength (λs); propagating pump wavelength (λp) radiation from the pump power oscillator, in a forward direction, through the partially reflective FBG; pumping the seed power amplifier with the forward propagating pump wavelength (λp) radiation via the transmission of the partially reflective FBG; propagating pump wavelength (λp) radiation from the pump power oscillator, in a reverse direction, through the leaky high reflector FBG; and pumping the seed pre-amplifier with the reverse propagating pump wavelength (λp) radiation leaked backward via the reflection of the leaky high reflector FBG.
One embodiment of the method of pumping integrated amplifiers with a single pump source is wherein the first medium is thulium-doped silica fiber and the second medium is holmium-doped silica fiber. In one example, the seed wavelength (λs) is about 2110 nm and the pump wavelength (λp) is about 1920 nm.
Another embodiment of the method of pumping integrated amplifiers with a single pump source is wherein the seed wavelength (λs) is outside a practical gain bandwidth of the first medium, but within the emission bandwidth of the first medium. In some cases, the pump wavelength (λp) lies in a spectrally overlapped region of the first medium and the second medium, such that there is significant gain in the first medium and significant absorption in the second medium.
In another example of the method of pumping integrated amplifiers with a single pump source the seed pre-amplifier further comprises a seed diode and the pump power oscillator further comprises a pump signal combiner that is pumped by a pump source having a pump source wavelength (λp2). In yet another embodiment of the method, the pump source wavelength (λp2) is about 795 nm.
These aspects of the disclosure are not meant to be exclusive and other features, aspects, and advantages of the present disclosure will be readily apparent to those of ordinary skill in the art when read in conjunction with the following description, appended claims, and accompanying drawings.
The foregoing and other objects, features, and advantages of the disclosure will be apparent from the following description of particular embodiments of the disclosure, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the disclosure.
The system of the present disclosure is a compact, robust method for combining and amplifying a dual wavelength source in a monolithic fiber amplifier. The system exploits the absorption and emission properties of two rare earth element doped fibers operating at similar wavelengths where a seed wavelength experiences modest gain (and minute absorption) in a first doped medium and a pump wavelength experiences significant gain in the first doped medium and significant absorption in a second medium. Additionally, the seed wavelength experiences significant gain in the second medium when pumped by the pump wavelength. This configuration allows for the generation of multiple wavelengths outside the effective gain region of a single medium, within a single gain aperture, without explicitly pumping two different lasers with two separate pump sources.
The need for the system of the present disclosure stems from the continuous drive to minimize laser complexity, reduce package volume and weight, and increase overall system efficiency. The present disclosure addresses these needs by exploiting the differences and commonalities of resonantly-pumped laser architectures.
Referring to
Referring to
In contrast, the system of the present disclosure simultaneously provides both pump and signal wavelengths out of the same aperture with the same beam prescriptions allowing for fewer optics and higher performance. See, for example,
One example of the system described herein uses the rare earth element dopants thulium and holmium in silica fibers. This system applies to any system sharing the above-referenced gain and absorption characteristics used to generate, transmit, and amplify two or more wavelengths. Another arrangement similar to the first example utilizes ytterbium (in place of thulium) and holmium where 1 um is the pump wavelength and 2.1 um is the seed wavelength.
Referring to
The pump power oscillator 2 in one example comprises a first medium 8 with a pump wavelength (λp), a leaky high reflector fiber Bragg grating (leaky HR FBG) 18, and a partially reflective fiber Bragg grating (PR FBG) 20. The first medium 8 is pumped by a series of pump diodes 16 at a pump source wavelength (λp2). The seed pre-amplifier 4 comprises a second medium 10 with a seed wavelength (λs) and in some cases also comprises a seed diode 12 or other seed source. The seed power amplifier 6 comprises the second medium 10′ with the seed wavelength (λs).
The pump power oscillator 2 comprises a first medium 8 that is pumped by a series of pump diodes 16 at a pump source wavelength (λp2). In certain embodiments, the pump source wavelength (λp2) is about 795 nm and generates radiation at the pump wavelength λp (e.g., 1920 nm). The pump diodes 16 provide radiation that passes through the pump/signal combiner 14, through the leaky high reflector FBG 18, the first medium 8, and the partially reflective FBG 20. In one embodiment, the first medium 8 is a thulium doped silica fiber (Tm:Silica). The pump power oscillator 2 is predominantly output-coupled in the forward direction according to the ratio of the reflectance/transmission of the leaky high reflector FBG 18 to the partially reflective FBG 20. A major portion of the pump wavelength (λp) radiation from the pump power oscillator 2 propagates in the forward direction through the partially reflective FBG 20 (as represented by the power Pf in
The seed diode 12 in one embodiment provides additional laser radiation with power Pseed at a seed wavelength (λs) that is coupled to a section of holmium doped fiber (Ho:Silica) 10 and acts as a seed pre-amplifier 4. The seed diode 12 can operate in continuous-wave, pulsed, or a hybrid mode. In certain embodiments, the seed wavelength (λs) is about 2110 nm. The seed pre-amplifier of the present disclosure is not limited to the use of a seed diode. In some embodiments, the seed diode can be replaced with a laser. Some possible lasers include, but are not limited to monolithic, FBG-based fiber oscillators, tunable fiber lasers, free-space hybrid fiber lasers, solid state lasers, gas lasers, chemical lasers, light emitting diodes, and the like.
As can be seen in
In certain embodiments of the present disclosure, the seed wavelength (λs) passes through the pump/signal combiner 14 into the pump power oscillator 2 where minimal gain is experienced. The pump power oscillator 2 is pumped by a pump source 16 having a pump source wavelength (λp2) as discussed above. In one example, the pump source wavelength (λp2) is about 795 nm and generates radiation at the pump wavelength λp (e.g., 1920 nm). This is predominantly output-coupled in the forward direction according to the ratio of the reflectance/transmission of the leaky high reflector FBG 18 and the partially reflective FBG 20 that compose the two feedback devices for the pump power oscillator 2.
At this point the seed and pump wavelengths are propagating collinearly in the core of the thulium fiber (i.e., the first medium). Both signals are coupled to the seed power amplifier 6 where the pump wavelength λp (e.g., 1920 nm) is absorbed and converted to the seed wavelength λs (e.g., 2110 nm) via excitation and emission within the holmium silica fiber (Ho:Silica) 10′. The output of the seed power amplifier is then terminated and both wavelengths (e.g., a seed wavelength (λs) of about 2110 nm and a pump wavelength (λp) of about 1920 nm) are emitted at the fiber output 22 with nearly identical beam propagation properties due to the absolute close proximity of each other in spectral space, common fiber exit criteria, and general Gaussian beam propagation. Some common fiber exit criteria include the core size, mode-field diameter, numerical aperture, and the like.
The relative powers attributed to the radiation at the pump wavelength (λp) throughout the fiber train are represented in a relative manner by the scale and direction of the top layer of arrows shown in
The relative powers attributed to the radiation at the seed wavelength (λs) throughout the fiber train are represented in a relative manner by the scale and direction of the lower layer of arrows as seen in
Referring to
In some embodiments, the leaky high reflector FBG will leak about 5% to about 50% with the exact amount determined by gain saturation characteristics of the seed pre-amplifier 4. The amount of power at the pump wavelength (λp) used to pump the seed pre-amplifier 4 depends on the total output power needed for amplifier saturation in the seed power amplifier 6. The system takes into account the minimal gain experienced in the pump power oscillator 2 and the seed provided to the seed pre-amplifier 4.
One embodiment of the present disclosure uses a seed source with a seed wavelength (λs) that is outside the practical gain bandwidth of a first medium 8. Here, the seed wavelength (λs) is equal to 2110 nm and the first medium 8 is thulium-doped silica (Tm:Silica). Though 2110 nm is within the emission bandwidth of Tm, the gain is extremely low; it is not practical to use Tm:Silica as a gain medium for 2110 nm for applications requiring high-efficiency amplification to high-power levels.
The pump wavelength (λp) is generated in the first medium 8, Tm:Silica, and lies in the spectrally overlapped region of the first medium 8 and a second medium 10, 10′, such that there is significant gain in the first medium 8 and significant absorption in the second medium 10, 10′. Here, the pump wavelength (λp) is about 1920 nm and the second medium is holmium-doped silica (Ho:Silica) 10, 10′.
Referring to
In certain embodiments of the present disclosure, the pump wavelength (λp) ranges from about 1800 nm to about 2050 nm. In certain embodiments of the present disclosure, the seed wavelength (λs) ranges from about 1940 nm to about 2200 nm. In certain embodiments of the present disclosure, the pump source wavelength (λp2) ranges from about 780 nm to about 800 nm.
As described herein, “fiber laser” is a monolithic fiber train with one explicit external pump source from a non-fiber source. The system of the present disclosure uses only one “pump” as seen for example in
There are many variations in laser architecture that may be used to exploit the described characteristics. The system design depends on the signal levels desired at the output aperture of the laser. In one such example, the seed diode that provides the initial radiation may be a laser of any architecture generating a desired wavelength. In other examples, the seed pre-amplifier may not be necessary. See, for example,
Alternately, the seed power amplifier may not be necessary. See, for example,
In some embodiments, the pump source for the pump power oscillator may be a fiber laser instead of diodes so long as it generates wavelengths that are efficiently absorbed by the second medium (e.g., Tm:Silica). For example, it is common to pump Tm:Silica in the 1500 nm region with an erbium silica fiber laser. It has also been demonstrated that Tm:Silica can be optically pumped in the 1100 nm region using a Raman-shifted ytterbium-doped fiber laser. It has been further shown that Tm:Silica can be pumped by another Tm:Silica fiber laser operating at a wavelength slightly shorter than the wavelength generated in the pump power oscillator. For example, a pump power oscillator 2 operating at a pump wavelength (λp) of about 1940 nm, is still highly absorbed by Ho:Silica, and can be pumped with a 1908 nm Tm:Silica fiber laser. Further, this disclosure considers all optical pump sources generating radiation at the above mentioned wavelengths (e.g., ˜1500 nm, ˜1100, nm, ˜1900 nm, ˜800 nm, and the like) regardless of the generation mechanism.
One application of the system of this disclosure is to use the system as an integrated seed and pump source to injection seed a solid-state laser. The system disclosed herein decreases the complexity of a laser system when coupling both pump power and seed power into an oscillator. One can use a common set of optics to couple both the seed and the pump power into the solid state cavity as they are both emitted from the same aperture and demonstrate nearly identical Gaussian beam propagation properties. Properties such as polarization (or the lack thereof) can also be exploited to manipulate the coupling dynamics between the integrated seed/pump source and the solid-state laser to be pumped.
Another application exploiting the common-aperture aspect for the disclosure is a solid-state amplifier. Using the system of the present disclosure, one may place a solid state crystal immediately at the output of the laser, resulting in a very compact architecture due to the absence of beam steering/manipulating optics. For example, a seed wavelength (λs) of about 2090 nm and a pump wavelength (λp) of about 1933 nm can be generated using the principles of the present disclosure. Then, they can be coupled to holmium yttrium-aluminum garnet (Ho:YAG) amplifiers. Minimal beam coupling optics may be used to condition the beams for the intensities required to achieve efficient laser/amplifier operation.
The system of the present disclosure is especially useful for systems requiring peak or average power levels or spectral and/or brightness requirements that cannot be satisfied by fiber alone due to practical limitations or physics-based limitations such as detrimental fiber non-linearities. Some of these fiber non-linearities include stimulated Raman scattering, stimulated Brillouin scattering, four-wave mixing, cross-phase modulation, and the like.
The exploitation of a single pump power oscillator to pump integrated amplifiers in both directions (forward and backward) while passing the seed wavelength (λs) through the fiber train with virtually no loss and minimal amplification is distinctive. The pump wavelength (λp) can be exploited for pumping various laser mediums internal and external to the fiber laser.
The system of the present disclosure is also unique in the fact that the system does not use two non-sensitized fibers (for simplicity and practicality) to perform amplification/generation based upon the energy provided by a single pump source having a pump source wavelength (e.g. λp2). Here, the system generates two wavelengths outside the effective gain bandwidth of a single gain medium without using separate pump sources for each sensitized fiber. The seed pre-amplifier and the seed power amplifier do not have explicit pump sources pumped by an external pump source like a second fiber laser coupled into the fiber train as would be the case in conventional systems.
The system of the present disclosure provides a simple, highly integrated dual wavelength seed and pump source that is pumped by a single pump source. By using a leaky power oscillator to intentionally distribute pump wavelength radiation in both directions, the system amplifies a seed wavelength in a secondary doped medium without explicitly pumping that gain medium with a secondary external source. The system requires only one external pump source to excite two gain mediums. Typical lasers that use two gain mediums require pump sources for each medium.
Referring to
The system of the present disclosure has many applications. Two prominent applications of the system of the present disclosure are medical therapy lasers prone to damage due to high peak powers, and remotely-coupled non-linear converters where the peak powers required are too high for transmission in appreciable lengths of fiber. Another application is for Lidar/Ladar applications requiring compact, efficient, and portable sources.
While the principles of the disclosure have been described herein, it is to be understood by those skilled in the art that this description is made only by way of example and not as a limitation as to the scope of the disclosure. Other embodiments are contemplated within the scope of the present disclosure in addition to the exemplary embodiments shown and described herein. Modifications and substitutions by one of ordinary skill in the art are considered to be within the scope of the present disclosure.
