Compact infrared broadband source

Abstract

A device for the generation of supercontinuum in infrared fiber with a pump light comprising a microchip laser operating with a wavelength of 1.0 μm or greater that can be wavelength shifted though a nonlinear element to a wavelength beyond the two-photon absorption of the infrared fiber and launched into infrared fiber whereby the spectrum is broadened in the infrared fiber through various nonlinear processes to generate a supercontinuum within the mid-IR from 2 to 14 μm.

Claims

1. A device for generating a supercontinuum in an infrared fiber with a compact light source, comprising: a single pulsed microchip laser having an input laser bandwidth, wherein said laser operates with a wavelength of 1.0 micrometers or greater; a bulk nonlinear optical element operated without a cavity; and an infrared fiber transparent in the infrared; wherein light from said laser is wavelength shifted beyond the two-photon absorption of the infrared fiber through said bulk nonlinear optical element and launched into said infrared fiber to produce a broadband output having a bandwidth greater than said input laser bandwidth by at least 100% and an emission wavelength range from 2 to 14 micrometers.

2. The device of claim 1, wherein said laser comprises optically active elements Nd, Yb, Er, Dy, Pr, Sm, Eu, Ho, Tm, transition metal ions Cr or Fe, or any combination thereof.

3. The device of claim 1, wherein said bulk nonlinear optical element comprises periodically poled lithium niobate, periodically poled potassium titanyl phosphate, or periodically patterned gallium arsenide.

4. The device of claim 1, wherein said bulk nonlinear optical element comprises lithium triborate, beta barium borate, zinc germanium phosphide, potassium dihydrogen phosphate, silver thiogallate, silver selenogallate, gallium selenide, lithium indium sulfide, or lithium indium selenide.

5. The device of claim 1, wherein said bulk nonlinear optical element comprises a single-pass Raman converter.

6. The device of claim 1, wherein the infrared fiber is a chalcogenide glass fiber.

7. The device of claim 1, wherein the infrared fiber has an outer diameter in the range of 100 to 500 micrometers.

8. The device of claim 1, wherein the infrared fiber has a core size in the range of 1 to 100 micrometers in diameter.

9. The device of claim 1, wherein said laser has a pulse duration between 40 ps and 5 ns, and a repetition rate between 100 Hz and 100 MHz.

10. The device of claim 1, wherein the device weighs 20 kg or less, and wherein the device has dimensions of 20 cm×20 cm×20 cm or less.

11. The device of claim 1, wherein the infrared fiber has a core size in the range of 1 to 50 micrometers in diameter.

12. The device of claim 1, wherein said laser has a pulse duration between 100 ps and 2 ns, and a repetition rate between 10 kHz and 10 MHz.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic of compact supercontinuum device used for demonstration. L1-L4: collimation and focusing lenses. F: Long pass 2500 nm filter. Detector indicates the fiber coupled scanning monochromator. The band around the PPLN indicates a temperature controlled oven.

(2) FIG. 2 shows broadband light emission from chalcogenide fiber pumped with a compact microchip laser.

(3) FIG. 3 shows a side view of sample geometry to be used in a method for generating broadband light (not to scale) including (A) pump laser such as microchip laser (example sample dimensions 2 mm×2 mm, 1 mm long), (B) and (D) microlenses, (C) nonlinear element such as PPLN crystal (example dimensions 10 mm×1 mm, 20 mm long), and (E) infrared fiber such as AsSe fiber (example dimensions 10 um core, 200 um cladding, 2 m long).

(4) FIG. 4 shows a side view of sample geometry to be used in a method for generating broadband light (not to scale) showing pump, nonlinear element and fiber all mechanically bonded including (A) pump laser, (B) nonlinear element, and (C) infrared fiber.

(5) FIG. 5 shows a side view of sample geometry to be used in a method for generating broadband light (not to scale) having a low-power threshold broadband source module with pump laser (A) coupled into waveguide supported nonlinear element (B) such as waveguide PPLN chip, coupled to infrared fiber (C).

(6) FIG. 6 shows a side view of sample geometry to be used in a method for generating broadband light with optical parametric amplification as a main method for wavelength conversion in nonlinear element (not to scale) including (A) pump laser, (B) seed laser, (C) beam coupler, (D) nonlinear element, and (E) infrared fiber. Coupling between elements might require lenses depending on the beam sizes and geometries.

DETAILED DESCRIPTION OF THE INVENTION

(7) The present invention provides a method and device for generation of supercontinuum in infrared fiber with a pump light comprising a laser operating with wavelength of 1.5 μm or greater that can be wavelength shifted though a nonlinear element and launched into infrared fiber whereby the spectrum is broadened in the infrared fiber through various nonlinear processes to generate a supercontinuum within the mid-IR from 2 to 14 μm.

(8) In one embodiment, a Nd:YAG microchip laser operating at 1.064 μm with pulse width of 700 ps and repetition rate of 20 kHz is used as a pump for optical parametric generation in a periodically poled lithium niobate crystal. The crystal period is set such that the 1.064 μm generates a 1.45 μm pulse and a 3.82 μm pulse in a single pass configuration (with no seed or cavity). The 3.82 μm pulse is coupled into a selenide-based optical fiber of approximately 2 m length, with an effective core diameter of 12 μm. The chalcogenide fiber broadens the light by various nonlinear phenomena to a bandwidth between 3.65 and 4.90 μm. FIG. 1 shows a schematic of the demonstration. FIG. 2 shows the spectrum of the generated supercontinuum.

(9) A sample schematic of a device based on the method described herein is shown in FIG. 3. A narrowband pump source from a microchip laser coupled into a nonlinear element through a lens. The pump light is converted to two (or more) wavelengths in a nonlinear element. The longer wavelength (say λ.sub.1) light is coupled to a non-linear fiber through another lens. The dispersion of the chalcogenide fiber can be normal, zero, or anomalous at the input λ.sub.1 wavelength, and the generated broadband light is emitted in the infrared within the range of 2 to 14 μm.

(10) A variation of the sample schematic in FIG. 3 is presented in FIG. 4. Here the microchip laser, nonlinear element and fiber are bonded together either mechanically or optically with no need of imaging optics.

(11) A variation of the sample schematic in FIG. 3 is presented in FIG. 5. Here the nonlinear element also allows for waveguiding of the light, and is aligned such that the output of the nonlinear element is coupled into an infrared fiber.

(12) A waveguiding nonlinear element use in an architecture such as that described in FIG. 5 or the one in FIG. 3, could comprise a fiber based Raman shifter or waveguide inscribed second order nonlinear element such as a waveguide based periodically poled lithium niobate chip. Note that neither of these elements requires that the dispersion of light at the input wavelength be anomalous and do not base their wavelength conversion on a physical mechanism called modulation instability. The use of a waveguide nonlinear element also allows for conversion of light at very low peak powers, with efficiency >5% possible even for light powers well below 500 W.

(13) The narrowband pump source can be a q-switched laser system, including microchip lasers with emission from 1 to 5 μm such as but not restricted to Nd:YAG lasers, Er:YAG lasers, Er:ZBLAN, as well as high power quantum cascade lasers (a wider list of materials has been presented in the background section). The nonlinear element need not be present if the pump source is sufficiently long wavelength as to avoid two-photon absorption in the fiber. Here the nonlinear fiber can comprise an appropriately transparent material such as fluoride, tellurite, germanate, halide or chalcogenide glass.

(14) Chalcogenide glass fibers suitable herein include fibers with outer diameter (O.D.) typically in the range of 50-1000 μm, and more typically in the range of 100-500 μm. Core size is in the range of 1-100 μm in diameter, but typically range from smaller cores of about 1 μm in diameter to larger cores of about 50 μm in diameter. Generally speaking, the smaller the core the higher the energy density and the broader the bandwidth, for a given power. In order to keep light within the core, its refractive index is kept higher than that of the clad.

(15) Examples of nonlinear elements supported by this method and device presented in this patent include nonlinear elements composed of a quasi-phase matched material such as periodically poled lithium niobate, periodically poled potassium titanyl phosphate, or periodically patterned gallium arsenide. Other nonlinear elements are also possible as those experienced in the field would know, as long as there is sufficient transmission at the pump wavelength through the nonlinear material. Examples are nonlinear crystals such as lithium triborate (LBO), beta barium borate (BBO), zinc germanium phosphide (ZGP), potassium dihydrogen phosphate (KDP), silver thiogallate (AGS), silver selenogallate (AGSe), gallium selenide (GaSe), lithium indium sulfide (LiInS2), lithium indium selenide (LISe). Additionally, the nonlinear element can be based on a Raman converter. The Raman converter can be in the form of a gas-cell, an optical fiber or crystal.

(16) A method where by frequency conversion prior to coupling into the fiber occurs through optical parametric generation, not requiring a set of mirror to form a cavity. A single pass configuration for the optical parametric generation is preferred. Alternatively, a method where frequency conversion prior to coupling into the fiber occurs through optical parametric amplification—where a seed is used to narrow the converted bandwidth, improve the mode or increase the power conversion. One arrangement for the case of optical parametric amplification is presented in FIG. 6. The power of the pump laser and a weaker seed laser are combined and coupled into a nonlinear element. The combination can occur through a dichroic mirror, a polarization element, evanescent coupler, interferometric waveguide combination or any other beam combiner. The polarization of the seed laser is selected to maximize the energy conversion from the pump laser to the new wavelength, and does not need to be the same as the pump. The effect of the polarization on the conversion efficiency should be clear for those knowledgeable in the art, and will depend on the nonlinear element architecture used.

(17) A method whereby the pump laser is selected to maximize the power transfer to a Raman line or a cascade of Raman lines in an infrared fiber. The nonlinear element used after the pump would generate two new wavelengths with the energy spacing being close to or within the Raman gain of the fiber. One such embodiment would include the use of a pump laser and nonlinear element to generate such that the power carried at the pump wavelength λ.sub.p would be efficiently converted to two new wavelengths λ.sub.1 and λ.sub.2 where the spacing of the wavelengths would be chosen such that (λ.sub.1).sup.−1+(λ.sub.2).sup.−1=Λ±dΛ, where Λ is the Raman gain peak for the infrared fiber (in inverse wavenumbers) and dΛ represents the bandwidth of the gain. For example for As.sub.2S.sub.3, Λ would be 340 cm.sup.−1 and dΛ would be 60 cm.sup.−1. In this embodiment of the method, the power carried by both λ.sub.1 and λ.sub.2 would be coupled into the infrared fiber. Nonlinear propagation in the fiber together with Raman gain would lead to broadening of the pump colors from the two narrow bands around λ.sub.1 and λ.sub.2 to a broadband source. The Raman process would be efficiently excited as λ.sub.2 would seed Raman scattering from λ.sub.1.

(18) The following examples illustrate these embodiments.

Example 1

(19) An Er:YAG microchip laser operating with a pulse width of greater than 10 ps and less than 2 ns and wavelength around 2.8 μm converted in a periodically poled gallium arsenide crystal to generate a long wavelength laser pulse at 5 um. The pulses are launched into a solid core clad As—Se fiber where a supercontinuum is generated from 2 to 14 μm through a combination of Raman conversion and self phase modulation.

Example 2

(20) The system described in Example 1 where no nonlinear element is used and the laser is coupled directly into a chalcogenide fiber such as As.sub.2S.sub.3 fiber. The pulses are launched into a solid core clad As—S fiber where a supercontinuum is generated from ˜2.8 to 6 μm through a combination of Raman conversion and self-phase modulation.

Example 3

(21) A quantum cascade laser with peak power levels exceeding 1 W or average power continuous wave power exceeding 10 mW is coupled directly into a photonic crystal fiber of AsSe. The effective diameter of the AsSe fiber is designed to magnify the nonlinear parameter and reduce the required power level for supercontinuum generation to below 10 W.

Example 4

(22) A device whereas the pump laser is transition metal doped chalcogenide crystal (or ceramic) q-switched laser such as Cr:ZnSe, operating around 2.4 μm with pulse width below 2 ns. A nonlinear crystal element such as ZGP which converts the wavelength of the pump into the mid-infrared or far-infrared and is coupled into a solid core AsSe fiber.

Example 5

(23) A system composed of a compact laser pump source, a nonlinear element and an infrared fiber. The laser pump source (either a microchip laser based on one of the previously described compositions or a quantum cascade laser with a peak power greater than 10 W) operating at a repetition rate from CW to MHz, a nonlinear element for shifting the pump power to a wavelength which will not have significant two-photon absorption in a fiber. Example of such elements are quasi-phase matched crystals as those previously described, Raman shifters, and nonlinear crystals such as those previously described. An infrared fiber (such as those of material composition previously described) wherein the incident wavelength propagates in normal or anomalous regime, and broadens to at least 100% of the incident bandwidth.

Example 6

(24) A system composed of a compact laser pump source and an infrared fiber. The laser pump source (either a microchip laser based on one of the previously described compositions or a quantum cascade laser with a peak power greater than 10 W) operating at a repetition rate from CW to MHz, without a nonlinear element for shifting the pump as the pump wavelength already did not display two-photon absorption in the fiber. The infrared fiber (such as those of material composition previously described) wherein the incident wavelength propagates in normal or anomalous regime, and broadens to at least 100% of the incident bandwidth.

(25) The above descriptions are those of the preferred embodiments of the invention. Various modifications and variations are possible in light of the above teachings without departing from the spirit and broader aspects of the invention. It is therefore to be understood that the claimed invention may be practiced otherwise than as specifically described. Any references to claim elements in the singular, for example, using the articles “a,” “an,” “the,” or “said,” is not to be construed as limiting the element to the singular.