Claims:1. An optical parametric oscillator (OPO) cavity comprising:

an input mirror being positioned at a first predefined angular position;
an output coupler mirror being positioned at a second predefined angular position; and
a reflector mirror; and
a nonlinear crystal positioned between the input mirror and the output coupler mirror, wherein a pump beam is coupled into the nonlinear crystal through the input mirror.
2. The OPO cavity as claimed in claim 3, wherein reflected pump beam, reflected signal beam and reflected idler beam travels collinearly in the cavity, which allows broad set of signal and idler wavelengths at output, when pump beam interacts with the different gratings in PPLN crystal.
3. The OPO cavity as claimed in claim 1, wherein two surfaces comprising a first surface and a second surface of the input mirror are optically coated at an angle of incidence 90-?i, and wherein the first surface is coated with anti-reflective (AR) coating for pump wavelength with coating performance (R)=0.5%, and the second surface is coated with AR coating for pump wavelength where R =0.5%, and highly reflective (HR) coating with R=90 % coating for signal band.
4. The OPO cavity as claimed in claim 1, where in two surfaces comprising a first surface and a second surface of the output coupler mirror are optically coated with at an angle of incidence 90-?c, wherein the first surface is coated with partially reflective coating with 40%= R= 85% for pump wavelength, and the second surface is coated with partially reflective coating with 60%= R=80% for signal band, and AR coating with R = 2 %for idler band.
5. The OPO cavity as claimed in claim 1, whereinthe reflector mirroris a universal reflector mirror, and wherein a reflection facing side of the reflector mirror is optically coated with high reflective coatingwith R=99% for pump wavelength, signal band and for idler band.
6. The OPO cavity as claimed in claim 1, whereinthe input mirror is a predetermined mirror which couples pumping light to the nonlinear crystal, wherein the input mirrorprovides necessary feedback for signal band oscillation in order to help the cavity to overcome the losses, and wherein the input mirror isolates the pumping wavelength and idler wavelengths and prevents unwanted coupling of the pump light and idler signal back into the pump source.
7. The OPO cavity as claimed in claim 1, wherein the reflector mirror is configured to simplify the broadband coating on the input mirror.
8. The OPO cavity as claimed in claim 1, wherein the tilt angle of out put coupler is90-?c that is defined by:

Sin (2?c)= (h+x)/Lb

whereh=PPLN Oven height
x =Desireddistance of beam path from the Oven
Lb=Ray path lengthof between output coupler and mirror.
, Description:TECHNICAL FIELD
[0001] The present disclosure relates, generally, to laser sources, and more particularly to a three mirrors Periodically Poled Lithium Niobate (PPLN) optical parametric oscillator cavity configuration for producing a broadband tunable laser source.

BACKGROUND
[0002] Background description includes information that may be useful in understanding the present invention.It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[0003] Frequency conversion is a very popular phenomenon to develop laser sources for wavelengths, for which the conventional laser sources are not available. There are many nonlinear crystals available, which are used for nonlinear frequency conversion e.g., KTP, KDP, LBO and BBO etc.
[0004] Still, many desirable frequency conversions/optical parametric oscillator (OPO)are limited by problems with conventional birefringent phase-matching techniques in available materials, such as Poynting-vector walk-off, low effective nonlinear coefficient and inconvenient phase-matching temperatures and angles.
[0005] The emergence of Periodically Poled Lithium Niobate (PPLN) has attracted renewed attention in this material. In PPLN, thelargest nonlinear index of Lithium niobate (LiNbO3) can be utilized. This allows efficient nonlinear conversionat modest power levels which are well below the damage threshold. PPLN is the first commercially available crystal in which efficient nonlinear conversion processes, such as harmonic generation or parametric interactions are not based on birefringence phase-matching but on a periodic structure engineered into the crystal, which is why it can simultaneously give n numbers of signal/idler pair , if n periodic structure engineered into the crystal.
[0006] There are many cavity configurations published and reported for PPLN optical parametric oscillators (OPO). Usually two mirror cavity configurations are the most common cavity configuration used OPO generation for coupling and feedback of idler and signal wavelengths. These cavities work well when OPO bandwidth is up to ~ 500nm range, either by moving the crystal or by tuning the temperature, and aim is to generate only a wavelength pair which lies close together in spectrum domain. When a wide range of spectrum (~3µm) generation is aimed, wideband coatings on input and output mirrors (anti-reflective (AR) coating for pump wavelength, highlyreflective (HR) coating for idler bandis required. For such system, it is difficult to get simultaneous HR coating for idlerband and highly transitive (HT) coating for pump wavelength at the inputmirror.While it is challenging to achieve these coatings, the material used for achieving the spectrum profile is also harmful to the environment.
[0007] There is always a trade-off between HT coating for pump beam and HR coatings for signal and idler band. If HT coating is required for pump beam, HR coatings for idler band has to be compromised (low reflectance). This can lead to the leakage of idler wavelength band back to the pumping source. This can potentially damage the source. Optical isolators cannot be used to isolate the idler signal as isolators have limited bandwidth few 100nm only, while pump beam and idler beam lies 1000nm apart in spectrum domain.
[0008] Similarly, If HT values for pump beam are reduced, efficiency of the system goes down, and higher pumping power is required to pump PPLN.
[0009] To overcome this problem, Bow tie and Z configuration cavities are used, which are four mirror cavities. Implementation on these cavities in product becomes difficult because of critical alignment of four mirrors.
[0010] Therefore, there is need in the art to provide a simple and efficient solution to overcome the foregoing limitations associated with the two mirror cavity and the four mirror cavity for the optical parametric oscillators

OBJECTS OF THE PRESENT DISCLOSURE
[0011] Some of the objects of the present disclosure, which at least one embodiment herein satisfies are as listed herein below.
[0012] It is an object of the present disclosure to provide a simple and efficient solution which can overcome the limitations associated with two mirror cavity and the four mirror cavity for the optical parametric oscillators.
[0013] It is an object of the present disclosure to provide a three mirrors cavityconfiguration for producing a broadband tunable laser source.
[0014] It is an object of the present disclosure to provide a three mirrors Periodically Poled Lithium Niobate (PPLN) optical parametric oscillator cavityfor producing a broadbandlaser source.
[0015] It is an object of the present disclosure to provide a simple and efficient three mirrors cavityfor PPLN optical parametric oscillator for producing a broadband tunable laser source.
[0016] It is an object of the present disclosure to providean improved and cost effective broadband tunable laser source device.
[0017] These and other objects of the present invention will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings.

SUMMARY
[0018] The present disclosure relates, generally, to laser sources, and more particularly to a three mirrors Periodically Poled Lithium Niobate (PPLN) optical parametric oscillator cavityconfiguration for producing a broadband tunable laser source.
[0019] In an aspect, the present disclosure provides an improved broadband tunable laser source cavity, such as an optical parametric oscillator (OPO) cavity, including an input mirror being positioned at a first predefined angular position; an output coupler mirror being positioned at a second predefined angular position; and a reflector mirror. The broadband tunablelaser source device (hereinafter, also referred to as broadband tunable laser source) further includes a broadband nonlinear conversion crystal positioned between the input mirror and the output coupler mirror, wherein a pump beam is coupled into the broadband nonlinear conversion crystal through the input mirror.
[0020] In an embodiment, the broadband nonlinear conversion crystal is a Periodically Poled Lithium Niobate (PPLN), wherein broadband nonlinear conversion crystal is configured to generate signal and idler beam pair when the pump beam is focused on the broadband nonlinear conversion crystal.
[0021] In an embodiment, the broadband nonlinear conversion crystal includes different gratings.
[0022] In an embodiment,reflected pump beam, reflected signal beam and reflected idler beam travels collinearly in the cavity, which allows broad set of signal and idler wavelengths at output, when pump beam interacts with the different gratings in PPLN crystal.
[0023] In an embodiment,two surfaces comprising a first surface and a second surface of the input mirror are optically coated at an angle of incidence 90-?i, and wherein the first surface can becoated with anti-reflective (AR) coating for pump wavelength with coating performance (R) <0.5%, and the second surface can becoated with AR coating for pump wavelength where R <0.5%, and highly reflective (HR) coating with R>90 % coating for signal band.
[0024] In an embodiment,two surfaces comprising a first surface and a second surface of the output coupler mirror are optically coated with at an angle of incidence 90-?c, wherein the first surface can be coated with partially reflective coating with 40%= R= 85% for pump wavelength, and the second surface can becoated with partially reflective coating with 60%= R= 80% for signal band, and AR coating with R= 2 % for idler band.
[0025] In an embodiment, the reflector mirror can be a universal reflector mirror, and wherein a reflection facing side of the reflector mirrorcan be optically coated with high reflective coating with R=99% for pump wavelength, signal band and for idler band.
[0026] In an embodiment, the input mirror can be a predetermined mirror which couples pumping light to the broadband nonlinear conversion crystal. The input mirror may provide necessary feedback for signal band oscillation in order to help the cavity to overcome the losses.
[0027] In an embodiment, the input mirror isolates the pumping wavelength and idler wavelengths and prevents unwanted coupling of the pump light and idler signal back into the pump source.
[0028] In an embodiment, the reflector mirror can be configured to simplify the broadband coating on the input mirror, which is critical because of simultaneous transmission of pump wavelength and reflective broadband coatings for signal and idler wavelengths band.
[0029] In an embodiment, thethe tilt angle of output coupler is 90-?c that is defined by:

Sin (2?c)= (h+x)/Lb(1)

Whereh=PPLN Oven height
x =Desireddistance of beam path from the Oven
Lb=Ray path length of between output coupler and mirror
[0030] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.

BRIEF DESCRIPTION OF THE DRAWINGS
[0031] The accompanying drawings are included to provide a further understanding of the present disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure.The diagrams are for illustration only, which thus is not a limitation of the present disclosure.
[0032] FIG. 1 illustrates a three mirrors cavity for the proposed broadband tunable laser source, in accordance with embodiments of the present disclosure.
[0033] FIG. 2 illustrates a PPLN crystal with various gratings of the proposed broadband tunable laser source, in accordance with embodiments of the present disclosure.
[0034] FIG. 3 illustrates exemplary tuning curves of a mutigrating PPLN of the proposed broadband tunable laser source,in accordance with embodiments of the present disclosure.
[0035] FIG. 4 illustrates an exemplary representation of a three mirrors cavity of the proposed broadband tunable laser source,in accordance with embodiments of the present disclosure.
[0036] Persons skilled in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and may have not been drawn to scale. For example, the dimensions of some of the elements in the figure may be exaggerated relative to other elements to help to improve understanding of various exemplary embodiments of the present disclosure. Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features, and structures.

DETAILED DESCRIPTION
[0037] In the following description, numerous specific details are set forth in order to provide a thorough understanding of embodiments of the present invention. It will be apparent to one skilled in the art that embodiments of the present invention may be practiced without some of these specific details.
[0038] If the specification states a component or feature “may”, “can”, “could”, or “might” be included or have a characteristic, that particular component or feature is not required to be included or have the characteristic.
[0039] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.
[0040] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.
[0041] Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and/or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified thus fulfilling the written description of all groups used in the appended claims.
[0042] Exemplary embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. These embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of the invention to those of ordinary skill in the art. Moreover, all statements herein reciting embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future (i.e., any elements developed that perform the same function, regardless of structure).
[0043] Embodiments explained herein relate to to laser sources, and more particularly to a three mirrors Periodically Poled Lithium Niobate(PPLN) optical parametric oscillator cavityconfiguration for producing a broadband tunable laser source.
[0044] In an aspect, the present disclosure provides a three mirror cavity configuration for a broadband conversion source where tuning of wavelength is possible by moving a PPLN crystal OR by changing temperature of the PPLN crystal. The proposedcavity design of three mirrors as shown in FIG.1, wherein, there is no need to change mirrors for getting different bands of signal and idler wavelengths. Coating of the mirror 1and 2 do not have wideband coatings for a particular transmission or reflection, but works for signal band 1.2 µm to 2.1µm and idler band 2.1µm to 4.5µm
[0045] In an embodiment, proposed cavity design of three mirrors as shown in FIG.1, wherein, the design prevents coupling of idler wavelengths back into the source, which can potentially damage the source. Optical isolators cannot be used to isolate the idler signal as isolators have limited bandwidth of few 100nm only, while pump beam and idler beam lies 1000nm apart in spectrum domain.This proposed three mirrors cavity design provides greater tenability comparison to a conventional two mirror cavity as well as cost effective and robust compare to a conventional Bow tie and/or Z cavity configuration.
[0046] FIG. 1 illustrates a three mirrors cavity for the proposed broadband tunable laser apparatus,in accordance with embodiments of the present disclosure. The proposed broadband tunable laser source a cavity of three mirrors,such as an optical parametric oscillator (OPO) cavity, including an input mirror (hereinafter also referred to as input coupler) 1 being positioned at a first predefined angular position; an output coupler mirror (hereinafter, also referred to as output coupler) 2 being positioned at a second predefined angular position; and a reflector mirror 3. The broadband tunable cavity (hereinafter, also referred to as broadband tunable cavity) further includes a broadband nonlinear conversion crystal 4 that is crystal is a Periodically Poled Lithium Niobate (PPLN) positioned between the input mirror 1 and the output coupler mirror 2, wherein a pump beam is coupled into the broadband nonlinear conversion crystal (hereinafter, also referred to as PPLN or PPLN crystal) 4 through the input mirror 1. The PPNL crystal 4 is configured to generate signal and idler beam pair when the pump beam is focused on the broadband nonlinear conversion crystal.
[0047] PPLN 4 is a broadband nonlinear conversion crystal, in order to extract all signal and idler wavelengths, corresponding feedback cavity should also be broadband.
[0048] In an embodiment, various gratings can be engineered into the PPLN 4, which makes it a broadband wavelength conversion source.Each grating can generate one signal and one idler wavelength.
[0049] As per energy conservation law:
???? = ???? + ??i(2)
Where ?p, ????and ????, are the pump, signal and idler frequencies respectively.
[0050] Quasi-phase matching states which signal and idler pairings will be efficient. The quasi-phase matching condition is given by:
??? = ???? - ???? - ???? - 2??/?(3)
Where?is the grating periodicity andkxare thewave vectors of the corresponding pump, signal and idler waveand??? is The wave-vector mismatch.
[0051] If n gratings are fabricated into PPLN 4, 2n wavelengths can be generated, which makes PPLN 4 a broadband tunable source.
[0052] In order to extract signal and idler from the PPLN 4, feedback is provided by the cavity which includes the mirrors 1, 2 and 3.These mirrors can be specially coated for pump, signal and idler wavelengths in order to increase the gain of signal and idler wavelength.
[0053] As shown in the FIG.1, a focused pump beam is coupled into the PPLN 4, through the input mirror 1which has anti reflecting (AR) coating for pump wavelength, high reflectivity coating for signal band and may not have specified coating for idler band. The PPLN crystal 4 has various gratings which can generate idler between 1.2 µmto 2.1 µm and idler 2.1 µm to 4.5 µm. The coupling mirror 2can be partially reflective for signal band and AR coated for idler band. The reflector mirror 3 can be a universal reflector mirror which completes the ray diagram and helps in separating residual pump and idler.
[0054] In an embodiment, the PPLN 4 can be kept at an elevated temperature in order to avoid photo refractive damage of the crystal. Angles of mirrors 1 and 2 are so decided that it ray paths do not get obstructed by the PPLN Oven and the mirror 3 iskeptat 0 deg angle of incidence.
[0055] In an embodiment, when pump beam is focused on particular grating of the PPLN 4 one signal and idler beam pair can be generated. Signal beam can be partially reflected and partially transmitted by output coupler 2. The pump beam can be also partially transmitted and partially reflected by output coupler 2. Idler beam is mostly transmitted and residual idler is reflected through the output coupler 2. The reflected beams of the output coupler 2 are again reflected by the mirror 3 and guided towards the input coupler 1.
[0056] In an embodiment, as the input coupler 1 is highly transitive to pump beam, residual pump beam is dumped to an optical beam dump 5. Signal beam is totally reflected by the input coupler 1, towards the output coupler 2 again, which provides the necessary gain by providing feedback. Residual idler signal is mostly transmitted and partially reflected back to the mirror 2.
[0057] In an embodiment, two surfaces including a first surface and a second surface of the input mirror 1 can be optically coated at an angle of incidence 90-? i, and wherein the first surface can be coated with anti-reflective (AR) coating for pump wavelength with coating performance (R) =0.5%, and the second surface can be coated with AR coating for pump wavelength where R =0.5%, and highly reflective (HR) coating with R=90 % coating for signal band. The input mirror 1 may provide necessary feedback for signal band oscillation in order to help the cavity to overcome the losses.
[0058] In an embodiment, the input mirror 1 isolates the pumping wavelength and idler wavelengths and prevents unwanted coupling of the pump light and idler signal back into the pump source.
[0059] In an embodiment, two surfaces comprising a first surface and a second surface of the output coupler mirror 2 are optically coated with at an angle of incidence 90-?c, wherein the first surface can be coated with partially reflective coating with 40%= R= 85% for pump wavelength, and the second surface can be coated with partially reflective coating with 60%= R = 80% for signal band, and AR coating with R = 2 % for idler band.
[0060] In an embodiment, the reflector mirror 3 can be a universal reflector mirror.Areflection facing side of the reflector mirror3 can be optically coated with high reflective coating with R=99% for pump wavelength, signal band and for idler band.
[0061] In an embodiment, the reflector mirror 3 can be configured to simplify the broadband coating on the input mirror, which is critical because of simultaneous transmission of pump wavelength and reflective broadband coatings for signal and idler wavelengths band.
[0062] By using three mirror cavity configuration, residual pump radiation andidler wavelength band of the PPLN 4 are not coupled back to the pump source.
[0063] In an embodiment, the output coupler 2 can be a standard mirror which requires no special coating development. In an embodiment, the input mirror 1 can be a standard selective beam splitter.
[0064] In an embodiment, by using this proposed three mirror cavity configuration it is possible to develop a low cost broadband PPLN OPO source.
[0065] FIG. 2 illustrates a PPLN crystal 4 with various gratings of the proposed broadbandtunable laser source,in accordance with embodiments of the present disclosure. In an embodiment, there can be 10 pooled regions of various periods along the length of the crystal 4, which are separated by unpooled region of 0.1 mm.
[0066] FIG. 3 illustrates exemplary tuning curves of a mutigrating PPLN of the proposed broadband tunable laser source,in accordance with embodiments of the present disclosure. As shown in FIG. 3,at a given temperature signal and idler wavelengths, can be tuned by translating various gratings available in the crystal OR by keeping the grating constant and varying the temperature of the crystal 4.
[0067] FIG. 4 illustrates an exemplary representation of a three mirrors cavity of the proposed broadbandtunable laser source,in accordance with embodiments of the present disclosure. The tilt angle ofoutput coupler is90-?c , as shown in Fig 4is defined by:
Sin (2?c)= (h+x)/Lb
Whereh=PPLN Oven height
x =Desireddistance of beam path from the Oven
Lb=Ray pathlengthof between output coupler and mirror 3
[0068] Thus, it will be appreciated by those of ordinary skill in the art that the diagrams, schematics, illustrations, and the like represent conceptual views or processes illustrating systems and methods embodying this invention. The functions of the various elements shown in the figures may be provided through the use of dedicated hardware as well as hardware capable of executing associated software. Similarly, any switches shown in the figures are conceptual only. Their function may be carried out through the operation of program logic, through dedicated logic, through the interaction of program control and dedicated logic, or even manually, the particular technique being selectable by the entity implementing this invention. Those of ordinary skill in the art further understand that the exemplary hardware, software, processes, methods, and/or operating systems described herein are for illustrative purposes and, thus, are not intended to be limited to any particular named.
[0069] While embodiments of the present invention have been illustrated and described, it will be clear that the invention is not limited to these embodiments only. Numerous modifications, changes, variations, substitutions, and equivalents will be apparent to those skilled in the art, without departing from the spirit and scope of the invention, as described in the claim.
[0070] In the foregoing description, numerous details are set forth. It will be apparent, however, to one of ordinary skill in the art having the benefit of this disclosure, that the present invention may be practiced without these specific details. In some instances, well-known structures and devices are shown in block diagram form, rather than in detail, to avoid obscuring the present invention.
[0071] As used herein, and unless the context dictates otherwise, the term "coupled to" is intended to include both direct coupling (in which two elements that are coupled to each other contact each other)and indirect coupling (in which at least one additional element is located between the two elements). Therefore, the terms "coupled to" and "coupled with" are used synonymously. Within the context of this document terms "coupled to" and "coupled with" are also used euphemistically to mean “communicatively coupled with” over a network, where two or more devices are able to exchange data with each other over the network, possibly via one or more intermediary device.
[0072] It should be apparent to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein. The inventive subject matter, therefore, is not to be restricted except in the spirit of the appended claims. Moreover, in interpreting both the specification and the claims, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms “comprises” and “comprising” should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced. Where the specification claims refers to at least one of something selected from the group consisting of A, B, C …. N, the text should be interpreted as requiring only one element from the group, not A plus N, or B plus N, etc.
[0073] While the foregoing describes various embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. The scope of the invention is determined by the claims that follow. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.

ADVANTAGES OF THE PRESENT DISCLOSURE
[0074] The present disclosure provides a simple and efficient solution which can overcome the limitations associated with two mirror cavity and the four mirror cavity for the optical parametric oscillators.
[0075] The present disclosure provides a three mirrors cavityconfiguration for producing a broad and tunable laser source.
[0076] The present disclosure provides a three mirrors Periodically Poled Lithium Niobate (PPLN) optical parametric oscillator cavity for producing a broadband tunable laser source.
[0077] The present disclosure provides a simple and efficient three mirrors cavityfor PPLN optical parametric oscillator for producing a broadband tunable laser source.
[0078] The present disclosure provides an improved and cost effective broadband tunable laser source device.