None.
1. Field of the Invention
Embodiments of the invention are most generally related to the field of fiber lasers. More particularly, embodiments of the invention are directed to a dual-single-frequency fiber laser and a method for generating a dual-single-frequency fiber laser emission.
2. Background Discussion
Fiber lasers have garnered attention as alternatives to solid-state and semiconductor lasers because of their advantages of, e.g., high reliability, thermal management, scalable output power, high beam quality, narrow bandwidth, and low noise floor. ‘Dual wavelength’ fiber lasers are attractive for applications in ranging, communications, and interferometers. They have been reported, e.g., with a high-birefringence fiber Bragg grating (FBG) in a ring cavity, a high-birefringence FBG in a linear cavity, a multimode FBG in a linear cavity, self-seeded multimode Fabry-Perot (FP) laser diodes, dual-FBGs with a circulator in a ring cavity, multiple bandpass filters in a ring cavity, and FBGs with multiple phase shifts in linear or ring cavities. The reported dual-wavelength lasers, however, typically operate in a multimode (and, therefore, multiple frequency) regime at each of the dual-wavelengths.
In view of the foregoing considerations and others that are appreciated by persons skilled in the art, the inventors have recognized a need for a ‘dual-single-frequency’ fiber laser, especially one that could be assembled and operated with relatively inexpensive and non customized components, a method for generating a dual-single-frequency fiber laser emission, especially a tunable dual-single-frequency fiber laser emission, and the benefits and advantages associated therewith.
An embodiment of the invention is directed to a dual-single-frequency fiber laser. The laser has a linear cavity including an active optical fiber lasing medium of length, L, extending between a first (input) end and a second (output) end of the active fiber, and an appropriate reflector coupled to the active fiber at the respective ends thereof to form the linear lasing cavity. The reflectors are distributed Bragg reflectors (DBRs), at least one of which is a polarization-maintaining DBR (PM-DBR). The laser also includes an appropriate pump source (or sources) having an output coupled into the laser cavity (fiber core or cladding, as appropriate). The length of the active fiber medium (and thus the laser cavity) is advantageously relatively short; in any case about 10 centimeters (cm) or less. In an aspect, L is substantially 3 cm or less, and in a particular aspect, 1≦L≦2 cm. In an exemplary aspect, L=1.5 cm. It will be appreciated by one skilled in the art that as L decreases, the rare earth doping concentration must increase; however, when a maximum doping concentration is not sufficient, the fiber length may need to be increased. According to a desirable aspect, L is decreased to the extent possible such that the gain/length product of the active medium is sufficient to reach a lasing threshold. In a particular aspect, the other DBR is a single-mode DBR (SM-DBR). According to a particularly advantageous aspect, the DBRs are fiber Bragg gratings (FBGs). In an illustrative aspect, a PM-FBG is fusion spliced to the output end of the active fiber and a SM-FBG is fusion spliced to the input end of the active fiber to form the linear cavity. According to various aspects, at least one of the PM-DBR and the DBR has a reflectance value, R, equal to or greater than 90%; the PM-DBR has a reflectance bandwidth (FWHM) less than or substantially equal to 0.1 nanometer; and the PM-DBR has a birefringence value sufficient to create a center-to-center peak spacing greater than or substantially equal to 0.2 nanometer. In alternative aspects, the DBRs may be thin film stacks deposited on the fiber ends as known in the art. In an exemplary aspect, the active fiber medium is a high concentration ytterbium doped silica glass fiber. In alternative aspects, the fiber may be doped with other typical rare earth materials including, but not necessarily limited to, erbium, holmium, thulium, praseodymium, neodymium; and the fiber may be a fluoride-based material, a phosphate-based material, or other optical waveguide material as appropriately known by a person skilled in the art. The output of the fiber laser will consist of two, spaced-apart wavelengths (λ1, λ2), wherein the two laser outputs are each single-mode, single frequency, orthogonally polarized outputs. According to a further advantageous aspect, all of the components of the embodied fiber laser described herein may be commercially available “off the shelf” components, thus benefiting assembly time, reliability, vendor selection, cost efficiency, and others considerations that will be appreciated by those skilled in the art.
Another embodiment according to the invention is directed to a method for generating a dual-single-frequency laser emission from a fiber laser. The method involves the steps of providing a linear cavity fiber laser according to one or more of the aspects described immediately above; adjusting the pump power output as a control mechanism to generate the desired dual-single-frequency laser emission and, if necessary or desirable, thermally adjusting one or both of the distributed cavity reflectors such that the ratio of the reflectance amplitude of the SM-DBR at a first wavelength of interest, RS(λ1), divided by the reflectance amplitude of the SM-DBR at a second wavelength of interest, RS(λ2), is sufficient to yield lasing in dual frequency operation with the desired ratio of power between the two frequencies; and, providing means for detecting the generation of a dual-single-frequency laser emission from the fiber laser. According to an exemplary aspect, the ratio RS(λ1)/RS(λ2) may be between 0.8 to 1.2 to obtain substantially equivalent powers in each of the dual wavelengths.
The foregoing and other objects, features, and advantages of embodiments of the present invention will be apparent from the following detailed description of the preferred embodiments, which makes reference to several drawing figures.
The terminology “dual-single-frequency” as used herein in relation to apparatus and method embodiments of the invention shall be understood to refer to a spectrum consisting of two separated laser emission spectra centered at wavelengths λ1, λ2, respectively, wherein the bandwidth of each emission spectrum encompasses essentially a single frequency (and thus represents single mode emission of each spectrum), further wherein each respective dual-single-frequency emission has a different relative polarization (e.g., orthogonal relationship).
A length, L, of highly ytterbium-doped SM single clad silica glass fiber 102 (referred to herein as the active fiber medium) having an absorption rate of 1700 dB/m at 976 nm is spliced between two fiber Bragg gratings (FBGs) 108, 110. At least one of the FBGs is a polarization-maintaining FBG (PM-FBG) (O/E Land Inc., QC Canada). In the exemplary embodiment L=1.5 cm as measured between a first, input end 104 of the active fiber and a second, output end 106 of the active fiber. The laser emission output direction of the device 100 is indicated by the arrow 111. As shown, a single-mode FBG (SM-FBG) 108 is fusion spliced to the input end 104 of the active fiber and a PM-FBG 106 is fusion spliced to the output end 106 of the active fiber. The SM-FBG 108 has a center wavelength of 1029.3 nm and a 3 dB bandwidth of 0.46 nm with a peak reflectivity of 99%. At least one of the FBGs should have a peak reflectivity equal to or greater than 90%. Each of the FBGs has a grating section length of about 3 mm. Thus the exemplary linear laser cavity formed by the active fiber medium and the two end reflectors has a total cavity length of about 2.1 cm. A wavelength-division multiplexer 112 is used to couple 976 nm single-spatial-mode pump light 114 from a pump laser 116 into the core of the active fiber medium.
It is to be appreciated that, according to an embodiment of the invention, it is intended that the laser output is dual-single-frequency, as that term is described hereinabove. In view thereof, the component arrangements and specification parameters described in regard to the exemplary device shown in
What is particularly important as well as being particularly desirable, is that the active fiber medium length, L, be relatively short and, in any case, less than substantially 10 cm. More advantageously, L≦3 cm. One skilled in the art will appreciate that the limiting condition is that the gain/length product of the active medium be sufficient to achieve lasing threshold within the constraint of L≦10 cm. Furthermore, the PM-FBG need not be located at the output end (vs. the input end) of the cavity, nor must the SM-FBG be a ‘single-mode’ FBG, as long as at least one of the FBGs 108, 110 is a PM-FBG.
The gain competition between polarizations at these two wavelengths determines the spectral properties of the laser. In ytterbium-doped fiber lasers, the ytterbium can be treated as a special homogenous broadening medium and thus permits only a single lasing mode. In a linear cavity, however, a standing wave will be formed between the two reflectors and thus spatial-hole burning (SHB) occurs. Additionally, polarization-hole burning (PHB) is similar to SHB in the sense that different polarizations will extract different gains from the active medium and, thus, affect lasers with birefringent components. Furthermore, gain saturation enhances the dual-frequency lasing through the modal competition process. Generally, the combined effects of SHB, PHB, gain saturation, thermal effects, and nonlinearities determine the modal behaviors of the fiber lasers.
An output power of 43 mW was achieved with the fiber laser setup depicted in
The single-mode (SM) operation at each lasing wavelength λ1, λ2 was verified with a Fabry-Perot spectrometer (FP,
The relative intensity noise (RIN) was measured using an electrical spectrum analyzer (ESA,
The polarization states of the exemplary dual-single-frequency fiber-laser output were measured with a quarter-wave plate and a polarizer. Each frequency exhibited a single polarization with a polarization excitation ratio of >20 dB. The two polarizations states are orthogonal, as expected from the PM-FBG. Since the FBG spectra were aligned at room temperature, dual-wavelength operation with two orthogonal polarizations could be achieved independent of ambient temperature. This may not generally be true if a temperature controller was necessary to align the two FBG spectra. Differential output peak powers could be generated by tuning the temperature of the FBGs differently. As the overlapping of the FBG spectra is changed by thermal tuning, the round-trip gain of the laser at two lasing frequencies will be changed and differential output peak power can be generated.
The exemplary dual-single-frequency laser demonstrated stable operation under perturbations of pump power. In the working regime, where the output power was on the order of 43 mW, the measured ratio of peak power at each wavelength changed with the pump current as 0.02 dB/mA. Therefore, a 1% change in pump power would lead to a 5% change in relative peak power. Practically, pump-power can be suitably controlled with commercial diode laser drivers to better than 0.01%, which would provide less than a 0.05% relative peak-power variation between the two fiber laser wavelengths. Thus the embodied dual-single-frequency fiber laser generated a highly stable output.
According to various aspects of the apparatus and method embodiments described herein, operating characteristics of the dual-single-frequency fiber laser may be customized by appropriate design and/or selection of the PM-FBG. Wavelength spacing between the dual lasing wavelengths can be controlled via the birefringence parameters of the PM-FBG. Laser emission wavelengths may be controlled as a function of the period of the both FBG's. Output power of the dual-single-frequency fiber laser may be scaled upward by optimizing the PM-FBG reflectance and via pump power adjustment.
Relaxation-oscillation effects (noise peaks) may be reduced by using, e.g., a negative-feedback circuit on the pump laser. The use of a polarization-filtering component in regard to the orthogonal polarizations of the dual emission will further enable the dual-single-frequency fiber laser to work in a single-polarization-single-frequency regime.
The foregoing description of the embodiments of the invention have been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.
This invention was made with government support under Cooperative Agreement No. DE-FC52-92SF19460 sponsored in part by the U.S. Department of Energy Office of Inertial Confinement Fusion. The government has certain rights in the invention.