WO2014057316A1

WO2014057316A1 - A method for generating or amplifying several wavelength laser radiation in an optical cavity, a laser source and a laser system

Info

Publication number: WO2014057316A1
Application number: PCT/IB2012/057370
Authority: WO
Inventors: Jonas JONUSKA
Original assignee: Integrated Optics, Uab
Priority date: 2012-10-10
Filing date: 2012-12-17
Publication date: 2014-04-17
Also published as: LT2012090A

Abstract

An object of the present invention is to provide a laser source capable of simultaneously generating several wavelength radiation at desired power ratio between each other. Said radiation of two or more wavelengths can be used for mixing of said wavelengths in a non-linear optical media in order to achieve different wavelength radiation than those amplified in the gain media. In the most preferred embodiment, a laser source comprises two or more reflective surfaces, arranged in an optical cavity (7), having essentially a single optical axis. Tuning of the laser is performed by moving or tilting the optical substrates comprising the reflective surfaces with respect to the axis of the cavity. As a result, desired ratio or proportions of average power are achieved for each of said wavelengths. Having the ability to change the power ratio is important for achieving simultaneous generation of several wavelengths in a single gain media, thus avoiding depletion of the exited state by the dominant wavelength.

Description

A METHOD FOR GENERATING OR AMPLIFYING SEVERAL WAVELENGTH LASER RADIATION IN AN OPTICAL CAVITY, A LASER SOURCE AND A LASER SYSTEM

FIELD OF INVENTION

This invention relates to lasers. More particularly it relates to laser sources capable emitting radiation of several wavelengths simultaneously or generating desired wavelengths by means of wave mixing in non-linear medium.

BACKGROUND OF INVENTION

Possibility of generating several wavelengths in a single laser device is of great interest and number of applications are available. Many bio-tech applications and tools are rather limited to the wavelengths currently available, thus some fluorescent dyes cannot be used or such parameters as absorption, distinction, Raman scattering or similar cannot be measured for wavelengths, which are not standard for diode pumped solid state lasers or laser diodes. Most popular designs of DPSS lasers feature 1064 nm, 1030 nm, 532 nm, 515 nm, which refer to fundamental and second harmonics of Neodymium or Ytterbium doped gain media, furthermore, third and higher harmonics are pretty common.

Widely tunable lasers, such as optical parametric amplifiers, generators and oscillators are suitable for most of spectroscopy needs and other applications, where variety of wavelengths are considered to be an advantage. However, such devices are extremely expensive and need significant amount of skills to operate.

Sum-frequency generation (SFG), difference frequency generation (DFG), four- wave mixing (FWM) lasers provide another alternative to demanding spectroscopy needs, but in order to achieve exotic wavelengths, complicated laser designs are employed, whereas several separate pump lasers are used to pump a non-linear crystal or complicated cavity designs are provided for effective simultaneous amplification and mixing of several wavelengths.

A US patent application No. US2009207868, published on Aug. 20, 2009 describes a tunable laser, which includes dispersion optics for separating generated laser pulses into first and second wavelength pulses directed along first and second optical paths. First and second reflective mirrors are disposed in the first and second optical paths, respectively. The laser's output mirror is partially reflective and partially transmissive with respect to the first wavelength and the second wavelength in accordance with provided criteria. A first resonator length is defined between the output mirror and the first mirror, while a second resonator length is defined between the output mirror and the second mirror. The second resonator length is a function of the first resonator length.

Another US patent No. 5.345.457 describes a dual-wavelength laser system with intracavity, sum-frequency mixing including a bifurcated resonant cavity having a first arm, a second arm and a common arm; a first laser element located in the first arm for providing a first input laser beam of a first wavelength; a second laser element located in the second arm for providing a second input laser beam of a second wavelength; a nonlinear-mixing element in the common arm; and a beam combining device for combining the first and second beams and submitting them to the nonlinear- mixing element for providing an output laser beam of a third wavelength whose energy is the sum of the energy of the input laser beams.

Other ways of achieving simplified laser cavities for SFG, DFG, FWM involve use of complex reflective coatings with different reflectivity for each of wavelengths to be amplified at desired ratio of average power. In such arrangement it is very difficult to achieve high luminous efficiency from the pump optical power to the output radiation. However a single laser diode can be used as a pump source.

Prior art inventions provide capability of simultaneous generation of several wavelength radiation and mixing thereof. However simplified and cost effective optical designs for the same purpose are still missing.

Herein and further, expressions 'mixing' or 'wave mixing' refer to any of sum- frequency generation (SFG), difference frequency generation (DFG), four wave mixing (FWM) or similar non-linear processes and principles.

SUMMARY

An object of the present invention is to provide a laser source capable of simultaneously generating several wavelength radiation at desired power ratio between each other and/or mixing of said wavelengths in a non-linear optical medium in order to achieve different wavelength radiation than those amplified in the gain medium.

In the most preferred embodiment, the laser source comprises a single laser diode (1 ) for pumping of the gain medium (3), an end mirror (14), preferably formed on the end of the gain medium crystal, two or more reflective surfaces (8, 9, 10, 16, 17), coated on optical substrates (5, 6, 15). Said substrates are arranged essentially on the optical axis of a laser cavity and oriented essentially perpendicularly to the generated beam. Said two or more reflecting surfaces are arranged to form an optical resonator with the common end-mirror (14). Tuning of the laser is performed by adjusting said optical substrates with respect to the incident generated beam, thus achieving desired amplification conditions for each of the emission lines of the same gain medium or multiple gain media arranged essentially on a single optical axis of the cavity.

Furthermore, at least one pair of reflective surfaces (16, 17) can be formed on a dispersive substrate (15), such as wedge, prism, lens or similar. The amplification ratio between two wavelengths can be effectively changed by moving said dispersive element along the optical axis of the cavity or to the sides. In case of amplification of more than two wavelengths, additional optical elements having reflective surfaces can be arranged on the optical axis of the cavity.

Having an ability to change the power ratio is important for achieving simultaneous generation of several wavelengths by avoiding depletion of the exited state by the dominant wavelength. Controlled amplification ratio can later be efficiently used for wave mixing, i.e. sum frequency generation, difference frequency generation, four-wave mixing or similar.

DESCRIPTION OF DRAWINGS

In order to understand the invention better, and appreciate its practical applications, the following pictures are provided and referenced hereafter. Figures are given as examples only and in no way should limit the scope of the invention.

Figure 1 . illustrates an optical laser design, where several substrates with reflective coatings are used to form cavities for amplification of different wavelengths

simultaneously;

Figure 2. illustrates another optical laser design for amplification of three wavelengths, where one pair of reflective surfaces is formed on a dispersive element and third reflective surface is formed on a flat optical substrate. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

An object of this invention is a laser source, which can be arranged to radiate many different, traditional and exotic wavelengths one at a time or several simultaneously. Laser optical design is simplified to essentially a single-axis resonator (7) and different wavelengths are amplified as active medium-specific emission wavelengths or generated by means of second harmonic generation (SHG), sum- frequency generation (SFG), difference-frequency generation (DFG) or four-wave mixing (FWM). As a result, variety of output wavelengths can be obtained for a lasing medium (3), which features more than one characteristic emission line. For example, Nd:YAG lasing medium features 4 key emission lines, when pumped with 808 nm pump beam. The characteristic emission lines for Nd:YAG are 946 nm, 1064 nm, 1 123 nm and 1319 nm. Second harmonic generated from these characteristic emission lines would be 473 nm, 532 nm, 561 nm and 660 nm. However, most of these fundamental and second harmonic wavelengths, except 1064 nm and 532 nm are not easily amplified because of dominating 1064 nm radiation, which strongly depletes the excited state. Willing to amplify laser radiation for other, non-dominant emission lines, the cavity has to be optimized in such a way, that 1064 nm radiation would be suppressed and good amplification conditions are created for certain weaker emission line or lines.

Similarly, radiation of higher harmonics and emission lines occurring from wave mixing - all of them can be amplified individually or in groups if certain conditions are met to suppress some radiation and stimulate other radiation. In other words, means for changing the ratio for amplification/generation between each of the wavelengths is needed. Herein and further in this description, by saying amplification, we mean both or any of generation of laser radiation from quantum noise or amplification from a signal, which is already generated or seeded.

In the most preferred embodiment, the laser source comprises a pump source (1 ), beam shaping optics (2), a lasing medium (2) and two or more reflective surfaces (8, 9, 10), which are formed on separate optical substrates (5, 6) arranged in the optical cavity (7). The reflective surfaces (8, 9, 10) form multiple optical resonators with the end mirror (14), which is preferably formed on the first end of the gain medium crystal (3) or arranged in the cavity as a separate mirror (not indicated in the figures).

Yet in another embodiment, at least one pair of reflective surfaces (16, 17) are formed on both sides of a dispersive element (15). Such arrangement causes first wavelength radiation to be reflected from the first reflective surface (16) of the dispersive element (15) and second wavelength radiation to be reflected from the second reflective surface (17) of the dispersive element (15). As a result, walk-off losses appear for each wavelength differently, i.e. different amplification/generation conditions are created for each of said wavelengths. The amplification/generation ratio is adjusted by tilting the dispersive element (15) with respect to the cavity axis and/or by moving it along the cavity axis and/or to the sides. As a result, one dominant wavelength radiation can be suppressed and another can have favorable conditions to be amplified. The shorter is the distance from the end mirror (14) to the dispersive element (15), the lower are the induced walk-off losses.

Said dispersive element (15) can be a wedged substrate, a prism, a lens, a grade-index plate or similar. In case of the wedged substrate or the prism, change of the amplification ratio occurs when the element is moved along the cavity axis. While in case of the lens or section of a lens, tuning is performed by moving the dispersive element (15) to the sides, i.e. curvature of one of the reflective surfaces changes with lateral position of the lens with respect to the incident beam (1 1 ).

Many different combinations of flat, curved or inclined reflective surfaces are possible when utilizing this invention. Reflective surfaces can be separated with a gap of free space or transparent medium, such as glass, silica or similar. The forms of reflective surface and the medium between them determines, how said reflective substrates (5, 6, 15) can be adjusted to create certain amplification conditions for each wavelength radiation. Most preferably the reflective surfaces (8, 9, 10, 16, 17) are coated with high reflectivity (HR) coatings or partially transmissive coatings for one of the wavelengths to be amplified and are transmissive to the other wavelengths. The optical laser design is not limited to two or three different wavelengths - depending on the gain medium, any number of specific emission lines can be generated/amplified by adding additional reflective surfaces to the cavity. Furthermore wavelengths achieved by non-linear processes can also be circulated in the cavity and used for further wave mixing with other wavelength radiation in non-linear medium (4).

It should be appreciated, that a person skilled in the art can use this technique in various ways in order to set desired ratio of amplification between several wavelengths. Application of different reflective and antireflection coatings to the surfaces of the dispersive element is a common skill and knowledge of a laser engineer, thus this invention is not limited to certain geometry of the dispersive element (15) or substrates (5, 6) as well as coatings applied thereto (16, 17, 8, 9).

Yet in another embodiment, two or more different gain medium elements (3) are arranged on the optical axis and two or more of the characteristic wavelengths (at least one wavelength from each gain medium) are selected and the cavity (7) is optimized for amplification of the selected wavelength radiation at desired power levels. In such arrangement it might be necessary to provide few different pump sources (1 ) for pumping of each of said gain medium crystals, as different materials require different wavelengths of the pump source.

Yet in another embodiment, an optical element having χ⁽²⁾ non-linearity (4) is arranged in the cavity to provide frequency doubling of the fundamental wavelengths, sum-frequency generation or difference-frequency generation.

Yet in another embodiment, an optical element having χ⁽³⁾ non-linearity (4) is arranged in the cavity to provide four-wave mixing or parametric amplification/oscillation/generation.

By saying dispersive element we mean any optical element, which causes different wavelength (or frequency) radiation to travel in different paths due to refraction on a surface of the optical element, according to Snell's law or due to refraction inside material because of change of optical properties throughout the aperture or transverse dimensions of the optical element.

As an example of this invention, we provide a description of achieving yellow- orange or 589 nm wavelength radiation by using the technique described above. 589 nm radiation is achieved by sum-frequency generation process, where two infrared wavelengths, which correspond to emission lines of a neodymium doped crystal are summed in a non-linear medium, such as BBO, LBO, KDP or other.

In one exemplary embodiment (see Fig. 1 ), 1064 nm and 1319 nm emission lines are amplified simultaneously and form the common beam (1 1 ). 1064 nm radiation is reflected from the first reflective surface (8) and the 1319 nm radiation is reflected from the second reflective surface (9). Optical substrates (8, 9) are positioned with respect to the incident beams (1 1 , 12) in such manner that the dominant emission line of 1064 nm would experience walk-off losses, when reflected from the first reflective surface (8), and would be amplified to a limited level, while the much weaker emission line of 1319 nm would be amplified to the best effort by aligning the second reflective surface (9) essentially perpendicularly to the second beam (12). The sum-frequency for the indicated emission lines corresponds to 589 nm wavelength, which is also referred to as a yellow-orange radiation. Similarly, 607 nm, 551 nm, 546 nm, 513 nm and 501 nm radiation can be achieved by summing any 2 of 4 characteristic emission lines of Nd:YAG lasing medium. By contraries, in a difference frequency generation process, wavelengths of far- and mid-infrared could be generated. For the same Nd:YAG lasing medium, the resulting wavelenghts of DFG are 5504 nm, 3345 nm, 8530 nm, 6002 nm, 7557 nm and 20252 nm. Setting a good power ratio between two beams of different wavelengths is very important for achieving good efficiency of the SFG or DFG processes.

Different wavelength sets can be calculated for any lasing medium having several characteristic emission lines. Lasing media, such as Nd:YAG, Nd:YLF, Nd:YAP, Nd:LSB, Nd:GLASS, Ti:Sapphire, Er:YAG and many more can be used to gain benefit from this invention and a person skilled in the art should be able to readily use those materials using the principles described herein in order to implement this invention.

This invention should not be limited to certain gain medium or combination thereof. Both, several wavelengths from a single gain medium or several wavelength radiation from a combination of two or more gain medium crystals, are applicable and provide wide capabilities of generating exotic wavelengths.

Other non-linear processes, such as generation of third, fourth and higher harmonics are essentially specific cases of sum-frequency generation, therefore it will be not analyzed herein in detail. For a person skilled in the art it should be obvious, how radiation of several different wavelengths, with a controlled power ratio, could be used to generate other wavelength radiation whether inside the cavity (7) or outside.

Claims

1. A method of simultaneous generating and/or amplifying two or more wavelength radiation in a resonator having lasing media positioned on a single optical axis, c h a r a c t e r i z e d in that amplification conditions for each of said wavelength are changed by tuning two or more reflective surfaces (8, 9, 10, 16, 17) arranged essentially on the optical axis of the optical cavity, said reflective surfaces are essentially perpendicular or slightly inclined with respect to the incident beam (11, 12, 13).

2. The method according to claim 1, c h a r a c t e r i z e d in that one or more of the reflective surfaces is/are essentially flat.

3. The method according to claim 1, c h a r a c t e r i z e d in that one or more of the reflective surfaces is/are curved.

4. The method according to one of claims 1 to 3, c h a r a c t e r i z e d in that each of said reflective surfaces (8, 9, 10, 16, 17) are arranged on separate optical components, such as optical substrates.

5. The method according to one of claims 1 to 3, c h a r a c t e r i z e d in that at least one pair of the reflective surfaces (16, 17) is arranged on both sides of a single optical component, wherein the optical component is a dispersive optical element (15).

6. The method according to claim 5, c h a r a c t e r i z e d in that said dispersive optical element (15) is one of prism, wedged substrate, lens, portion of a lens or grade index-plate.

7. A laser source, comprising at least a pump source, gain medium, and first reflective surface (14), c h a r a c t e r i z e d in that two or more resonators are formed having essentially the same optical axis, whereas two or more separately tunable reflective surfaces are arranged in the cavity for changing amplification conditions for radiation of two or more of gain media-specific emission wavelengths.

8. The laser source according to the claim 7, c h a r a c t e r i z e d in that at least one pair of the reflective surfaces (16, 17) is arranged on both sides of a single optical component, wherein the optical component is a dispersive optical element (15).

9. The laser source according one of the claims 8 to 9, c h a r a c t e r i z e d in that a non-liner medium (4) is arranged in the optical cavity (7) to provide one or more of frequency doubling, four-wave mixing, sum-frequency generation, difference-frequency generation.

10. A laser system comprising at least a DPSS laser source, c h a r a c t e r i z e d in that said DPSS laser source features optical layout as described in one of the claims 7 to 9.

11. The laser system according to claim 10, c h a r a c t e r i z e d in that the system is arranged as one of a medical laser, a material processing workstation or a spectroscopy system.