CRO S S -REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of US Patent Application serial number 62/561,413, filed September 21, 2017, entitled "TUNABLE LIGHT SOURCE WITH BROADBAND OUTPUT," and US Patent Application serial number 15/809,624, filed November 10, 2017, entitled "TUNABLE LIGHT SOURCE WITH BROADBAND OUTPUT", which are incorporated by reference herein in their entireties.

FIELD OF THE INVENTION

The present invention generally relates to a light source providing tunable light of macroscopic power, and more particularly, is related to a light source which utilizes a broadband pump source, an optical parametric oscillator (OPO) and at least one additional nonlinear process for producing a tunable broadband emission of macroscopic power, especially at wavelengths less than 1.1 μm.

BACKGROUND

An optical parametric oscillator (OPO) is a light source emitting radiation with properties comparable to that of a laser. OPOs are nonlinear devices that split short wavelength pump photons into two longer wavelength photons, namely signal and idler photons. The wavelengths of the signal and idler photons are not independent from each other, but may be tuned in wavelength.

As shown by FIG. 1 , an OPO converts an input laser wave (the "pump") with frequency ω _p into two output waves of lower frequency ( ω _s , ω _i via second-order nonlinear optical interaction. The sum of the frequencies of the output waves is equal to the input wave frequency:ω _s + ω _i =ω _p . For historic reasons, the output wave with the higher frequency ω _s is called the signal, and the output wave with the lower frequency ω _i is called the idler. Because the OPO does not convert all the input energy into the signal and idler, a residual pump wave is also output.

OPOs need an optical resonator, but in contrast to lasers, OPOs are based on direct frequency conversion in a nonlinear crystal rather than from stimulated emission. OPOs exhibit a power threshold for an input light source (pump), below which there is negligible output power in the signal and idler bands.

OPOs include an optical resonator (cavity) and a nonlinear optical crystal. The optical cavity is an arrangement of mirrors that forms a resonator for light waves. Light confined in the cavity is reflected multiple times resulting in a multi-pass through the nonlinear crystal. The optical cavity serves to resonate at least one of the signal and idler waves. In the nonlinear optical crystal, the pump, signal and idler beams overlap.

While conventional lasers produce limited fixed wavelengths, OPOs may be desirable because the signal and idler wavelengths, which are determined by the conservation of energy and momentum (via phase matching), can be varied in wide ranges. Thus, it is possible to access wavelengths, for example in the mid-infrared, far-infrared or terahertz spectral region, which may be difficult to obtain from a laser. In addition, OPOs allow for wide wavelength tunability, for example, by changing the phase-matching condition. This makes OPOs a useful tool, for example, for laser spectroscopy. Utilizing additional nonlinear processes can further extend the range of accessible wavelengths (e.g. near-infrared, visible and/or ultraviolet spectral regions).

In addition, while prior light sources such as spectrally filtered plasma sources and supercontinuum white light lasers are available, these light sources suffer from poor photon (energy) efficiency (typically a few mW output power per nm). On the other hand, OPO/ nonlinear optics (NLO) technology may offer significantly higher energy efficiency with more narrow band output powers of greater than 10 mW. Thus, while supercontinuum and plasma sources produce a broad spectrum from which (for many applications requiring narrower bandwidth) parts are cut off, OPOs are capable of producing a tunable comparatively narrow band output (so there is no waste of power by filtering out). Therefore, there is a need in the industry to address one or more of the above mentioned shortcomings.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide a continuous wave (cw) light source based on OPO technology, plus further NLO processes. In particular, while typical cw OPO devices are used to generate high-power wavelength-tunable near-to mid-infrared (N IR to MIR) radiation of l ong coherence lengths, the present invention is capable of providing high power tunable wavelength emissions of short coherence length in the short NIR, VIS (visible) or even down to the UV (ultraviolet) region. In particular, the OPO module utilizes a broadband pump source and is configured to generate two waves: a resonant OPO wave that may have long-coherence length and a non-resonant wave of short coherence length widely adapting the band width of the pump source. At least one more intracavity nonlinear processes (e.g. sum frequency generation (SFG) or OPO) produces at least one more output-beam having a shorter wavelength than the pump- source and being broadband. If desired, the light source of the invention may utilize an optional second high power broadband pump source. The present light systems provide an emission spectrum that combines wavelength agility and high power, which will find use in a variety of applications including microscopy and the biotech industry.

Other systems, methods and features of the present invention will be or become apparent to one having ordinary skill in the art upon examining the following drawings and detailed description. It is intended that all such additional systems, methods, and features be included in this description, be within the scope of the present invention and protected by the accompanying claims.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings i llustrate embodiments of the invention and, together with the description, serve to explain the principals of the invention. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. In the drawings, each like component is referenced by a like numeral. For purposes of clarity, every component may not be labeled in every drawing. In the drawings:

FIG. 1 is a general schematic diagram of a prior art OPO.

FIG. 2 is a schematic diagram of an OPO module pumped by a high power pump source according to a first embodiment of the present invention.

FIG. 3 is a schematic diagram of an OPO module pumped by a high-power pump source formed by a low-power light-source and an optical amplifier according to a second embodiment of the present invention. FIG. 4 is a schematic diagram of an OPO module pumped by a high power pump source and a second light source according to a third embodiment of the present invention.

FIG. 5 is a schematic diagram of an OPO module pumped by a high power pump source and a second light source according to a fourth embodiment of the present invention.

FIG. 6 is a more detailed schematic diagram of an example for pump source that may be used to pump the OPO module. The pump-source produces a multimode (a) or broadband output (b).

FIG. 7 is a more detailed schematic diagram for an exemplary broadband pump source followed by a tunable spectral filter.

DETAILED DESCRIPTION OF THE INVENTION

The following definitions are useful for interpreting terms applied to features of the embodiments disclosed herein, and are meant only to define elements within the disclosure. No limitations on terms used within the claims are intended, or should be derived, thereby. Terms used within the appended claims should only be limited by their customary meaning within the applicable arts.

Normally a light-source having more than 1 longitudinal mode is called "multi- longitudinal mode". However, as used within this disclosure, "multi-longitudinal mode" refers to a number of longitudinal modes greater than 3. A number of modes of 2 or 3 may be called "few single-modes" in this context.

As used within this disclosure, "broadband" refers to a bandwidth greater than 300 GHz, no matter if formed by multiple longitudinal modes, single broadened line(s) or an arbitrary spectral distribution, and no matter if before or after longitudinal mode scrambling. In this context broadband, broad linewidth, broad bandwidth and broad spectral width describes the same phenomena.

As used within this disclosure, "longitudinal mode scrambling" refers to a method for fast frequency- tuning of modes, continuously or via hopping. In this context, reference to "fast" frequency tuning of modes means faster than can be timely resolved for an application. One example of a fast frequency tuning of modes is a repetition rate of greater than 100 Hz (however, this is merely an example, and it is to be understood that reference to fast frequency tuning of modes in the present invention is not limited only to values of greater than 100 Hz).

As used within this disclosure, OPO generally refers to a continuous wave OPO (cw- OPO), rather than a pulsed OPO. In general, "continuous wave" or "CW" refers to a laser that produces a continuous output beam, sometimes referred to as "free-running," as opposed to a q- switched, gain-switched or mode locked laser, which has a pulsed output beam.

As used within this disclosure, shorter NIR refers to NIR wavelengths less than 1.1 μm.

As used within this disclosure, "mirror" refers to an optical element having at least one reflective surface. The reflective surface may reflect light received from one direction, but transmit light received from other directions. The reflective surface may reflect some

wavelengths and transmit other wavelengths. Further the reflective surface may partially transmit and partially reflect some wavelengths.

As used within this disclosure, "macroscopic power" refers to power levels greater than lOmW. Of course, it is to be understood that power levels greater than 100mW or greater than 1 W are also considered macroscopic.

As used within this disclosure, an emission produced by at least one pump source and OPO module that "widely adapts the bandwidth" of the at least one pump source refers to any of the following, and is dependent upon the resonant OPO wave relative to the pump source: (1) in the case where the resonant OPO wave is much narrower than the pump source (which is most typical) and the pump-acceptance-bandwidth of the NLO-crystals is broader than the pump- bandwidth, then at least one emission has approximately the same bandwidth (measured in units of GHz) as the pump source, (2) in the case where the resonant OPO-wave is also broadband and the pump-acceptance-bandwidth of the NLO-crystals is broader than the pump-bandwidth, then at least one emission has even larger bandwidth (measured in units of GHz) than the pump source, and (3) in the case where the resonant OPO-wave is much narrower than the pump source and the pump-acceptance-bandwidth of the NLO-crystals is narrower than the pump- bandwidth, then the acceptance bandwidths of either or both NLO-crystals will set an upper limit to the emission bandwidths.

Reference will now be made in detai l to embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.

In general, embodiments of the present invention include devices and methods for producing tunable light of macroscopic power with a short coherence length (preferably less than 0.5mm) in the shorter NIR, VIS and or UV. In particular, the embodiments produce such light by using a broadband pump source together with OPO technology in combination with additional nonlinear processes. This is in contrast with prior OPOs, in which typical cw OPO devices are used to generate macroscopic-power wavelength-tunable near-to mid-infrared (NIR to MIR) radiation of long coherence lengths. In some embodiments, the present invention may achieve the desired emission by pumping the OPO module using a broadband pump (for example, a broadband pump in total being greater than 300 GHz broad). The light source of the invention may optionally utilize a second OPO pump source having a wavelength differing from that of the first pump source.

According to an embodiment of the invention, the OPO mod ule is pumped above the OPO-osciUation threshold. For example, the OPO module may be pumped up to about 2.5 times the OPO-oscillation threshold. Such pump power will likely result in a single-longitudinal mode resonant OPO wave. According other embodiments, the OPO may be pumped in excess of 2.5 times the OPO-oscillation threshold, for example, in excess of about 3 times the OPO-oscillation threshold, in excess of about 3.5 times, in excess of about 4 times, in excess of about 4.5 times, and even in excess of about 5 times the OPO-oscillation threshold. In embodiments utilizing a high power pump source which delivers a pump-power above about 2.5 times OPO-oscillation threshold, multi-longitudinal mode operation of the resonant OPO wave is likely achieved. As a non-limiting example, assuming an exemplary OPO oscillation threshold of approximately 2W, then a high-power pump source delivering between 2 and 5 Watt may keep the resonant wave being single-longitudinal mode, whereas for high-power pump levels greater than 5 Watts the resonant-wave may start being multi-longitudinal-mode and/or broadened.

Referring now to the various figures of the drawing wherein like reference characters refer to like parts, there is shown in FIG. 2 one embodiment of the light source in accordance with the present invention. As shown, a high power pump source 110 pumps the OPO module 150 with a power level above the OPO oscillation threshold, which in turn produces an emission spectrum. According to various embodiments, the high power pump source 110 may be in the form of a common high-power laser or diode, or an amplified diode or laser. According to an exemplary embodiment, the high power pump source 1 10 is a broadband source (e.g. in total being greater than 300 GHz broad). As depicted in FIG. 2, the OPO module 150 includes an optical resonator 152 resonant at least for one of the waves generated inside an NLO-crystal 154 (which may be configured for OPO-process) and at least one NLO-crystal 156 (which may be configured for SFG- or OPO- process). Both the NLO-crystals 154, 156 are provided with sufficient pump-acceptance bandwidth, in particular by utilizing sufficiently short NLO-crystals plus optionally a special shaped Gain-curve (for example, by using a ferroelectrically poled OPO crystal with multi- grating or chirped-grating along the beam path). This may enable control of a detailed shape of the emission spectrum. For example, in a crystal with chirped poling, the poling period may slightly vary along the beam-propagation resulting in a broadened gain curve and pump acceptance bandwidth for an OPO. Using a crystal with multi-grating along the beam path produces a super-positioning of gain-curves related to the different poling sections. For example, one crystal may contain several parallel chirped or multi-grating areas for wider wavelength- tuning. In some embodiments, a chirped-fanout grating may be used, having a gradual variation of poling period length along and perpendicular to the beam propagation direction.

According to one embodiment, the light source of the present invention is configured such that the emission spectrum of the OPO module 150 may be spectrally in a single- longitudinal mode in one part (for example, if the resonant wave is single-longitudinal mode), while other parts of the OPO module 150 emission spectmm widely adapt the bandwidth of the high power pump source 110. The latter refers to the non-resonant OPO wave and nonlinear processes involving the pump-wave. In addition, intracavity nonlinear processes involving the resonant OPO-wave are very efficient, because of the high power of the resonant OPO-wave. For example, for some OPOs the power-level of a resonant OPO wave may exceed 10W, 100W or even 1000W. Nonlinear processes involving a broadband pump source enable broadband output generation. In particular, by incorporating additional nonlinear processes, the range of accessible wavelengths (e.g. near-infrared, visible and/or ultraviolet spectral regions) can be extended. Therefore, intracavity SFG of the pump and the resonant OPO-wave is one preferred process. The output of this process may again be used for SFG with the resonant OPO-wave.

According to another embodiment, the light source is configured such that the emission spectrum of the OPO module 150 is all multi-longitudinal mode. For example, where the resonant wave is multi-longitudinal mode (for example, pump power much greater than 2.5 times the OPO-threshold), then the OPO module 150 may be configured to provide an emission spectrum where all output waves are multi-longitudinal mode.

As further shown, an optional first spectral filter 130 may be disposed between the high power pump source 1 10 and the OPO module 150, to filter parts of the spectrum from the high power pump source 1 10 and/or to change the spectral width of the high power pump source 1 10 output.

The OPO module 150 then outputs the light to an optional second spectral filter 160 disposed at an output side of the OPO module .150. The second spectral filter 160 may be used to fi lter parts of the OPO-output spectrum or change the spectral widths of the OPO-output. The light source thus provides macroscopic power tunable wavelength and tunable bandwidth emissions of short coherence.

FIG. 3 depicts another embodiment of the light source in accordance with the present invention. This embodiment is similar to that set out in FIG. 2, but adds a broadband optical amplifier 240 which is disposed between light source 210 (or the optional first spectral filter 130) and the OPO module 150 and amplifies the low power output of a light source 210 (which may be, for example, a laser, diode, lamp or super-continuum source having, for example, a power- level below lOOmW or below lOmW or even much lower) to a power level sufficient to overcome the OPO-threshold, The high-power output of the amplifier pumps the OPO module 150, which in turn produces a macroscopic power tunable wavelength broadband emission spectrum.

Similar to the embodiment described in connection with FIG. 2, the OPO module 150 of FIG. 3 includes an optical resonator 152 resonant at least for one of the waves generated inside the NLO-crystals 154, 156. The crystals 154, 156 are provided with sufficiently large phase- matching bandwidth. The OPO module 150 then outputs the light to an optional second spectral filter 160 which may be used to filter parts of the OPO-/SFG-output spectrum or change the spectral widths of the OPO-/S FG -outputs .

FIG. 4 depicts another embodiment of the light source in accordance with the present invention. This embodiment is similar to that set out in FIG. 2, with an additional light source 320. As discussed in connection with FIG. 2, a high power pump source 310 (e.g., a common high-power near infrared (NIR) laser or an amplified diode or laser) pumps the OPO module 150, which in turn produces a macroscopic power tunable wavelength broadband emission spectrum.

As depicted in FIG. 4, the OPO module 150 includes an optical resonator 152 resonant for at least one of the waves generated inside an NLO-crystal 154 (which may be configured for one OPO-process) and at least one NLO-crystal .156 (which may be configured for SFG- or another OPO process). Both the NLO-crystals 154, 156 are provided with sufficient broad phase- matching bandwidth, in particular by utilizing sufficiently short NLO-crystals.

In the embodiment depicted in FIG. 4, the high power pump source 310 provides an emission directly to the OPO module 150 without an intermediate spectral filter. However, it is to be understood that a spectral filter (not shown) may be disposed between the high power pump source 310 and the OPO module 150 if desired so as to filter parts of the spectrum from the high power pump source 310 or to change the spectral width of the high power pump source 310 output. As shown in FIG. 4, an optional spectra! filter 330 may be disposed between the additional light source 320 and the OPO module 150 so as to filter parts of the spectrum from the additional light source 320 or to change the spectral width of the additional light source 320 output.

As further depicted in FIG. 4, the additional light source 320 emission may pass only through the NLO-crystal 156. As such, the additional light source 320 may be one used in SFG generation. The OPO module 150 then outputs the light to an optional spectral filter 160 which may be used to filter parts of the OPO-/SFG-output spectrum or change the spectral widths of the OPCWSFG-output.

According to various embodiments, the OPO module may comprise two OPO processes. This is described in co-pending US Patent Application No.: 15/646,434 (the disclosure of this application being hereby incorporated by reference in its entirety). By providing two OPO processes in the OPO module, broadband non-resonant OPO- waves emissions may be provided when a broadband pump-source is used.

FIG. 5 shows a very advanced embodiment of an OPO module which utilizes two OPO processes (OPO-1 and OPO-2). Both OPO crystals are pumped by respective high-power broadband pump sources PI and P2. According to some embodiments, pump source P2 may be the SHG of pump source P I . Both OPO-1 and OPO-2 have a common resonator (such as a bow- tie ring-resonator). The resonant waves of OPO-1 (=S1) and OPO-2 (=12) are substantially equal. A third NLO-process (SFG-1) utilizes the sum- frequency generation of PI and SI. A fourth NLO-process (SFG-2) may utilize the sum- frequency generation of SFG-1 and SI . The following output emissions are expected to be broadband: I I , S2, SFG-1 and SFG-2.

F urther means for reducing coherence length may arise from features of the pump source or the spectral filter (potentially enabling a longitudinal mode-scrambling of the pump-beam) or a dither on the resonant OPO-wave mode(s). Such means are described in U.S. Provisional Patent Application No. 62/ 561,428 entitled "Light Source With Multi-Longitudinal Mode Continuous Wave Output Based On Multi-Mode Resonant OPO Technology", filed on

September 21, 2017 (the disclosure of this application being hereby incorporated by reference). For example, additional scramblmg means for scrambling the resonant wave(s) may be provided within the OPO module. Such scrambling may be achieved in some embodiments by fast variation of the resonator-length or fast dithering on wavelength selecting elements. Additional longitudinal mode scrambling may be taken into accoimt as well (e.g., fast dither cavity length or effective Etalon-thickness or the effective ferroelectric grating period length. The latter two may be done mechanically or electro-optically). This would enable longitudinal mode-scrambling of the resonant OPO-wave.

FIG. 6 shows one example of how the coherence of a pump-source may be reduced by combining the outputs of at least two sources (e.g., laser diode 1 and laser diode 2) forming one common pump source (e.g., pump source 110, 210, 320, etc.). FIG. 6a depicts how two slightly wavelength-separated modes may be added up. Additional fast wavelength modulations results in broadened output. In this scheme, for example, two potential ly identical laser diodes may be used and swept anti-phase in current. The sweeping of the current results in a wavelength- and power-sweep. In this example, the center- wavelengths of the two laser diodes may be slightly different. When well-synchronized, it is possible to maintain the power of the combined output nearly constant while the combined output bandwidth is broadened by longitudinal-mode scrambling. FIG. 6b displays a simple case where two already broadband sources are combined. In this configuration, the center wavelengths of the two laser diodes are slightly different. As a result, the combined output is broadened.

FIG. 7 shows an example of how a tunable filter 130 may be used to manipulate the output-spectrum of an extremely broadband pump-source (for example, pump source 1 10, 210, etc., which may, for example, comprise a LED, a superluminescence diode, a supercontinuum or other extremely broadband source). As depicted in FIG. 7a, the emission output-spectrum of the extremely broadband pump-source (e.g., pump source 1 10, 210, etc.) without the tunable filter 130 may be much broader than that of the emission output-spectrum when the output from the extremely broadband pump-source (e.g., pump source 1 10, 210, etc.) passes through the tunable filter 130 as depicted by FIG. 7b. In this example, a tunable bandpass or a combination of tunable long-pass and short-pass may be used to provide the manipulated output-spectrum.

As such, the present invention provides a light source which is capable of generating tunable light of macroscopic power with a short coherence length. In particular, a broadband pump-source may be utilized to pump an OPO module such that the OPO-module generates an output having reduced coherence length at the wavelength of the non-resonant OPO wave (as compared to conventional OPO). In particular, a pump-source with a high-power level may be utilized to pump an OPO such that the OPO generates two new waves, generally referred to as the signal and idler, as noted in the background section. The OPO cavity is resonant for at least one of the two new waves, with at least parts of the output of the light-source widely adapting the bandwidth of the high power pump source (preferably providing a broadband output). The OPO module may be provided with NLO crystals having a broad pump acceptance bandwidth and a broadened gain-curve (for example, by a chirped ferroelectric poling, or with multi gain peaks by using multi poling crystal chips). Additional intracavity NLO processes (for example, SFG with the resonant wave or a second QPO-process) may optionally result in efficient generation of different wavelengths and macroscopic power levels (typically greater than 10 mW, possibly exceeding 1 W). The system may optionally use a second high power broadband source for mixing with the high power resonant wave. If desired, a variable spectral filter may be placed between the diode and the amplifier to enable bandwidth on demand.

According to the present invention, any generation involving the resonant OPO-wave is very efficient due to the high intracavity resonant wave power. Beneficially, the light source of the present invention is capable of efficiently producing radiation in the shorter NIR, VIS or even UV wavelength ranges. Such a light system is capable of generating light suitable for a variety of applications including microscopy and biotech.

In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.