BLUE LASER OPERATING AT THE H-BETA FRAUNHOFER LINE

Abstract

The present invention provides a blue laser transmitter operating at the H-beta Fraunhofer line at 486.13 nm wavelength. The subject blue laser is based on pulsed lasing action in thulium doped into lutetium sesquioxide (Tm:Lu.sub.2O.sub.3). The laser wavelength is restricted by volume

Bragg grating to the vicinity of 1944 nm wavelength. The laser is operated with a q-switch to generate high-energy pulses within the nanosecond regime. The output at the 1944 nm wavelength is then frequency quadrupled in a single pass through non-linear crystals to a wavelength near the center of the H-beta Fraunhofer line. The operation at the 1944 nm wavelength in Tm:Lu.sub.2O.sub.3 is very efficient because this wavelength is located on a shoulder of a substantially broad emission peak at 1945 nm. In addition, at the 1944 nm wavelength, Tm:Lu.sub.2O.sub.3 has only a modest saturation fluence of about 15 J/cm.sup.2, which allows for efficient energy extraction.

Claims

1. A blue laser system comprising a laser source and a frequency quadrupling system (FQS); a) said laser oscillator including a pump module, a dichroic mirror, a laser gain material, a polarizer, a q-switch, a one-quarter wave plate, and a volume Bragg grating (VBG); b) said laser source being arranged as a laser oscillator; c) said laser gain element selected from the family consisting of Tm:Lu.sub.2O.sub.3, Tm:Y.sub.2O.sub.3, and Tm:Sc.sub.2O.sub.3; d) said VBG being arranged to restrict lasing in said laser oscillator to a wavelength of 1944.537±0.100 nm measured in air at standard conditions; and e) said FQS comprising a non-linear crystal wherein laser light resulting from said lasing at said 1944.537±0.100 nm arranged to be converted in said FQS to a wavelength in the vicinity of 486.13 nm measured in air at sea level and corresponding to the vicinity of the H-beta Fraunhofer line.

2. The blue laser system of claim 1, wherein said laser gain material is in a ceramic form.

3. The blue laser system of claim 1, wherein said laser source further includes a power amplifier.

4. The blue laser system of claim 1, further including a TEC in good thermal contact with said VBG.

5. The blue laser system of claim 4, wherein the temperature of said TEC is adjusted so that the center wavelength of said laser light wavelength in the vicinity of 486.13 nm is arranged to be within 0.025 nm of the Fraunhofer H-beta line center at 486.13 nm wavelength measured in air at sea level.

6. The blue laser system of claim 1, further including a pump module for generation of pump light; said module selected from the family consisting of laser diode, thulium fiber laser, and Raman-shifted erbium laser.

7. The blue laser system of claim 6, wherein said pump light in the vicinity of wavelengths selected from the family consisting of 796 nm, 1670 nm, and 1700-1900 nm.

8. The blue laser system of claim 1, wherein said volume Bragg grating is arranged to restrict said lasing to the wavelength band of <0.2 nm.

9. The blue laser system of claim 1, wherein said laser oscillator is operated with the q-switch to generate a train of 10-30 nanosecond pulses.

10. The blue laser system of claim 1, wherein said FQS comprises a non-linear crystal arranged to frequency double said output at said 1944 nm wavelength to the vicinity of 972 nm wavelength in a single pass through said crystal.

11. The blue laser system of claim 1, wherein said FQS comprises a non-linear crystal arranged to frequency double said 972 nm wavelength to the vicinity of 486 nm wavelength in a single pass through said crystal.

12. The blue laser system of claim 1, where the blue laser system further comprises a modulator wherein the modulator is provided with a continuous pulse stream and a coding stream and outputs a coded beam having modulated pulse stream.

13. A blue laser system comprising a laser source and a frequency quadrupling system (FQS); a) said laser oscillator including a pump module, a dichroic mirror, a laser gain material, a polarizer, a q-switch, a one-quarter wave plate, and a volume Bragg grating (VBG), a TEC, a power amplifier, and a pump; b) said laser source being arranged as a laser oscillator; c) said laser gain element selected from the family consisting of Tm:Lu.sub.2O.sub.3, Tm:Y.sub.2O.sub.3, and Tm:Sc.sub.2O.sub.3; d) said VBG being arranged to restrict lasing in said laser oscillator to a wavelength of 1944.537±0.100 nm measured in air at standard conditions; and e) said FQS comprising a first harmonic converter stage and a second harmonic converter stage wherein laser light resulting from said lasing at said 1944.537±0.100 nm arranged to be converted in said FQS to a wavelength in the vicinity of 486.13 nm measured in air at sea level and corresponding to the vicinity of the H-beta Fraunhofer line

14. A method of generating narrow band coherent laser radiation with wavelength centered at 486.13 nm corresponding to the Fraunhofer H-beta line comprising the steps of a) Operating a laser source including a pump module, a Tm:Lu.sub.2O.sub.3 laser gain material (LGM), a q-switch, a volume Bragg grating (VBG); b) Temperature tuning the center wavelength of said VBG to four times the wavelength of said Fraunhofer H-beta line; c) Closing the q-switch shut; d) Pumping said LGM by said pump module; e) Storing laser energy in said LGM; f) Opening said q-switch; g) Extracting laser power at 1944 nm wavelength from said LGM through said VBG; h) Converting a portion of said laser power at 1944 nm wavelength to a first harmonic at 972 nm wavelength in said first non-linear crystal; i) Converting said first harmonic at 972 nm wavelength to a second harmonic at 486 nm wavelength in said second non-linear crystal; and j) Closing said q-switch shut.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG. 1 is schematic diagram showing a concept for remote detection of underwater objects with a blue laser remote sensing instrument.

[0024] FIG. 2 is schematic diagram showing a concept for underwater communication with a blue laser transmitter.

[0025] FIG. 3A is schematic diagram showing a concept for underwater communication with a blue laser transmitter where the transmitter is above the water surface and the receiver is submerged.

[0026] FIG. 3B is schematic diagram showing a concept for underwater communication with a blue laser transmitter where the receiver is above the water surface and the transmitter is submerged.

[0027] FIG. 4 is a plot of the minimum diffuse light attenuation coefficient of selected open-ocean water types that has been adapted from a publication by M. G. Solonenko and C. D. Mobley, “Inherent optical properties of Jerlov water types,” Applied Optics, 54, 17, 99, pages 5392-5401, 2015.

[0028] FIG. 5 is a close-up view of the solar irradiance spectrum near the H-beta line, which has been adopted from a publication by F. E. Hanson et al., “Feasibility of a 486 nm Fraunhofer laser source Based on a .sup.4F.sub.3/2.fwdarw..sup.4I.sub.9/2 neodymium laser,” Technical Report 1480, Naval Command, Control and Ocean Surveillance Center, 1992.

[0029] FIG. 6 is a plot of the spectral gain of Tm:Lu.sub.2O.sub.3 at selected levels of inversion (beta) and indicating the relative positions of the natural emission peak at 1945 nm and the targeted fundamental lasing wavelength of 1944 nm suitable for frequency quadrupling to the H-beta Fraunhofer line at 486 nm, which has been adopted from a doctoral dissertation thesis by P. Koopmann, “Thulium and Holmium-Doped Sesquioxides for 2 μm Lasers,” submitted in 2012.

[0030] FIG. 7 is a schematic diagram showing the blue laser transmitter of the subject invention.

[0031] FIG. 8 is a schematic diagram showing the blue laser transmitter of the subject invention used with a modulator suitable for communication.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] Selected embodiments of the present invention will now be explained with reference to drawings. In the drawings, identical components are provided with identical reference symbols in one or more of the figures. It will be apparent to those skilled in the art from this disclosure that the following descriptions of the embodiments of the present invention are merely exemplary in nature and are in no way intended to limit the invention, its application, or uses.

[0033] Referring now to FIG. 7, there is shown a blue laser assembly 100 in accordance with one preferred embodiment of the present invention. The blue laser assembly 100 comprises a laser source 150 and a frequency quadrupling system (FQS) 160. The laser source 150 further comprises a laser oscillator 172 consisting of a pump module 110, dichroic end mirror 112, Tm:Lu.sub.2O.sub.3 laser gain medium (LGM) 114, polarizer 116, q-switch 118, quarter wave plate 122, and volume brag grating (VBG) outcoupling mirror 126. In some variants of the subject invention, the laser source 150 may also include a power amplifier (PA) 128 that may be used to boost the energy of the output laser pulses before conveying them to the FQS 160. The dichroic end mirror 112 and the VBG mirror 126 will form a laser resonator.

[0034] The pump module 110 may be a laser diode generating pump light 130 at about 792 to 796 nm, or at about 1670 nm, or in the range of about 1600-1944 nm. As another alternative, the pump module 110 may be an erbium fiber laser Raman-shifted to deliver pump light at about 1670 nm as described by O. Antipov in “Highly efficient 2μm CW and Q-switched Tm3+:Lu2O3 ceramic lasers in-band pumped by a Raman-shifted erbium fiber laser at 1670 nm,” published in Optics Letters Vol. 41, No. 10/May 15, 2016. While this approach adds complexity, it offers improved lasing efficiency and reduced waste heat in the Tm:Lu.sub.2O.sub.3 LGM. As yet another alternative, the pump module 110 may be a thulium fiber laser generating a pump light in the range of about 1700-1900 nm. Improved lasing efficiency and reduced waste heat offers to offset the added complexity of this approach. The dichroic end mirror 112 is made of material with good transmission at the pump wavelength and it is coated for high reflectivity at the lasing wavelength around 1944 nm and for very low reflectivity/high transmission at the pump wavelength on the side facing the LGM 114 and for very low reflectivity/high transmission at the pump wavelength on the side facing the pump module 110. The dichroic end mirror 112 may have a convex surface on the side facing the LGM 114. The LGM 114 is made of Tm:Lu.sub.2O.sub.3 material, which may be in a single crystal or ceramic form. Preferred Tm doping levels are between 1 and 2% atomic. The LGM 114 may be configured as a rod, fiber, disk, thin slab, or a planar waveguide (PWG).

[0035] The q-switch 118 may be either acousto-optic, electro-optic, or a passive (saturable) light modulator. The VBG 126 is of the reflective type and it is preferably arranged to be in the range of about 30% reflective and 70% transmissive to 90% reflective and 10% transmissive at the center wavelength of 1944.537±0.100 nm (preferably 1944.537±0.050 nm) when measured in air at standard conditions. The VBG 126 preferably has a bandwidth of less than 0.3 nm full-width at half maximum (FWHM) (preferably less than 0.2 nm, and most preferably less than 0.1 nm). The VBG 126 is optionally mounted on and in good thermal communication with a TEC 124 (thermoelectric cooler). TEC devices are known to be capable of heating as well as cooling. Controlling the TEC temperature allows for precise temperature tuning of the VBG central wavelength. The PA 128 may be provided as needed to boost the energy of the output laser pulses of the oscillator 172 before conveying them to the FQS 160. The PA 128 uses its own Tm:Lu.sub.2O.sub.3 LGM that may be configured as a rod, fiber, planar waveguide (PWG), thin slab, or a disk. Further PA 128 may use its own pump 168.

[0036] The FQS 160 comprises a first harmonic converter stage 162 and a second harmonic converter stage 164. The first harmonic converter stage 162 uses a suitable nonlinear crystal (such as periodically-pooled lithium niobate (PPLN), potassium titanyl phosphate (KTP), and Lithium niobate (LiNbO3)) for conversion of the output beam 140 at 1944 nm wavelength into a first harmonic beam 166 at about 972 nm wavelength. The second harmonic converter stage 164 uses a suitable nonlinear crystal (such as periodically-pooled lithium niobate (PPLN), potassium niobate (KNbO3), barium borate (BBO), or lithium triborate LBO) for conversion of the first harmonic beam at about 972 nm wavelength into a second harmonic beam 170 at 486.13 nm wavelength when measured in air and closely corresponding to the center wavelength of the Fraunhofer H-beta line. Preferably the first harmonic converter stage 162 and the second harmonic converter stage 164 are arranged for efficient harmonic conversion in a single pass through their respective non-linear crystals, thus offering a simple layout. This is possible by providing the beam 140 with pulse energy, pulse length, and beam size to attain beam intensities at which single pass through the non-linear crystals converts much of the pulse energy into a harmonic.

[0037] In operation, the pump module 110 generates pump light 130 and injects it through the dichroic end mirror 112 into the LGM 114 where it is largely absorbed. Absorbed pump light pumps Tm ions into the upper laser level according to a known process. With the q-switch 118 being closed shut, there is absence of a suitable optical feedback, thus, laser energy may be momentarily stored in the LGM. At predetermined times, the q-switch 118 is arranged to open and pass therethrough 1944 nm light. This, in turn, allows the VBG 126 to receive optical radiation from the LGM 114 and reflect some of it back, thus providing optical feedback to the LGM. As a result, laser oscillations commence, and the energy stored in the LGM 114 is transferred into the intracavity beam 120. The intracavity oscillations may last for a few nanoseconds to about 100 nanoseconds (preferably 10 to 30 nanoseconds), which may significantly depend on the configuration of the laser resonator, especially the physical separation of the dichroic mirror 112 and the VBG outcoupling mirror 126. A portion of the intracavity beam 120 passes through the VBG 126 and forms the output laser beam 140. The output laser pulse may be on the order of a few nanoseconds to about 100 nanoseconds long but preferably it is 10 to 30 nanoseconds long. If necessary, the pulse energy of the output laser beam 140 may be further boosted in the PA 128 before being injected into the FQS 160. Pulsing of the laser may be repeated at suitable repetition rate. Typical pulse repetition rate may be as low as a single pulse, or few Hz, or as high as hundreds of kilohertz range.

[0038] In the FQS 160, the laser beam 140 at 1944 nm wavelength is fed into the first harmonic converter stage 162 and substantially converted to a first harmonic beam 166 at 972 nm wavelength in a single pass through the non-linear crystal. The unconverted portion of the fundamental beam at 1944 nm wavelength may be removed by a dichroic beam splitter (not shown). The first harmonic beam 166 at 972 nm wavelength is now fed to the second harmonic converter stage 164 and substantially converted to a second harmonic beam 170 at 486 nm wavelength in a single pass through the non-linear crystal which. The resulting second harmonic beam 170 may be provided to an application. The unconverted portion of the first harmonic beam at 972 nm wavelength may be removed by a dichroic beam splitter (not shown).

[0039] In some variants of the invention, the dichroic mirror may be omitted and the surface 182 of the LGM 114 facing the pump module 110 may be coated with a dichroic coating for high reflectivity at the lasing wavelength around 1944 nm and for very low reflectivity/high transmission at the pump wavelength.

[0040] Referring now to FIG. 8 there is shown a blue laser assembly 100 operating with a modulator 180 as a part of a transmitter that may be used for communication of information. In particular, the blue laser assembly 100 generates a second harmonic beam 170 at 486.13 nm wavelength comprising a continuous pulse stream 172, which are directed into the modulator 180. The modulator 180 is provided with a coding stream 176 and it accordingly modulates the beam 170 with the continuous pulse stream 172 into a coded beam 178 having a modulated pulse stream 174.

[0041] Another preferred embodiment of the present invention may use Tm:Y.sub.2O.sub.3 LGM in lieu of the Tm:Lu.sub.2O.sub.3. While Tm:Y.sub.2O.sub.3 is deemed to have a lower gain at 1944 nm than Tm:Lu.sub.2O.sub.3, it offers lower cost due to the higher abundance of the yttrium element (Y).

[0042] Yet another preferred embodiment of the present invention may use Tm:Sc.sub.2O.sub.3 LGM in lieu of the Tm:Lu.sub.2O.sub.3. Tm:Sc.sub.2O.sub.3 is deemed to have a lower gain at 1944 nm than Tm:Lu.sub.2O.sub.3.

[0043] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” and “includes” and/or “including” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

[0044] The terms of degree such as “substantially”, “about” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. For example, these terms can be construed as including a deviation of at least ±5% of the modified term if this deviation would not negate the meaning of the word it modifies.

[0045] The term “suitable,” as used herein, means having characteristics that are sufficient to produce a desired result. Suitability for the intended purpose can be determined by one of ordinary skill in the art using only routine experimentation.

[0046] Moreover, terms that are expressed as “means-plus function” in the claims should include any structure that can be utilized to carry out the function of that part of the present invention. In addition, the term “configured” as used herein to describe a component, section or part of a device includes hardware and/or software that is constructed and/or programmed to carry out the desired function.

[0047] The terms “Fraunhofer F line”, “Fraunhofer H-beta line”, “H-beta Fraunhofer line”, and “H-beta Fraunhofer dip” refer to the dip in solar radiation spectrum with its center known to be at 486.1342 nm wavelength when measured in air.

[0048] The terms “486 nm wavelength” and “486.13 nm wavelength” refer to the center wavelength of the Fraunhofer H-beta line. The terms “1944 nm wavelength” and “1944 nm wavelength” refer to the 1944.537 nm wavelength measured in air at standard condition, which is about four times the wavelength of the Fraunhofer H-beta line.

[0049] The term “standard conditions” means air at 100 kilopascals pressure, 20 degrees Celsius temperature, 50% relative humidity, and carbon dioxide content of 450 parts per million.

[0050] Different aspects of the invention may be combined in any suitable way.

[0051] While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the present invention as defined in the appended claims. Furthermore, the foregoing description of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the present invention as defined by the appended claims and their equivalents. Thus, the scope of the present invention is not limited to the disclosed embodiments.