OPTICAL PARAMETRIC DEVICE BASED ON RANDOM PHASE MATCHING IN POLYCRYSTALLINE MEDIUM

Abstract

An optical parametric device (OPD), which is selected from an optical parametric oscillator (OPO) or optical parametric generator (OPG), is configured with a nonlinear optical element (NOE) which converts an incoupled pump radiation at first frequency into output signal and idler radiations at one second frequency or different second frequencies, which is/are lower than the first frequency, by utilizing nonlinear interaction via a random quasi-phase matching process (RQPM-NOE). The NOE is made from a nonlinear optical material selected from optical ceramics, polycrystals, micro and nanocrystals, colloids of micro and nanocrystals, and composites of micro and nanocrystals in polymer or glassy matrices. The nonlinear optical material is prepared by modifying a microstructure of the initial sample of the NOE such that an average grain size is of the order of a coherence length of the three-wave interaction which enables the highest parametric gain achievable via the RQPM process.

Claims

1. A nonlinear optical material for a nonlinear optical element (NOE) used in optical parametric device which converts an incoupled pump radiation into output signal and idler radiations utilizing a nonlinear interaction via RQPM process, the nonlinear optical material being selected from the group consisting of optical ceramics, polycrystals, micro and nanocrystals, colloids of micro and nanocrystals, and composites of micro and nanocrystals in polymer or glassy matrices.

2. The material of claim 1, wherein the nonlinear optical material is selected from II-VI semiconductors, or III-VI compounds with GaAs, GaP, and GaN belonging to group III, Si3N4 and SiC to group IV and Ga2O3 to group VI, wherein the II-VI semiconductor is selected from binary, ternary, or quaternary compounds including ZnSe, ZnS, ZnTe, ZnO, CdSe, ZnMgSe, CdMnTe, or CdZnTe.

3. The material of claim 1, wherein the nonlinear optical material is doped with ions of transition metals (TM) including Cr.sup.2+, Fe.sup.2+ or rare earth metals (RE) or co-doped with ions of TM and RE to provide spontaneous or stimulated seed for the signal, idler or signal and idler enabling decrease of a threshold luminance.

4. A nonlinear optical material for a nonlinear optical element (NOE) used in an optical parametric device for converting an incoupled pump radiation into output signal and idler radiations by utilizing a three wave nonlinear interaction via RQPM process, the nonlinear optical material being prepared by a method comprising modifying a microstructure of an initial sample of the NOE such that an average grain size is of the order of a coherence length of the three-wave interaction, thereby enabling the three wave nonlinear interaction with a highest parametric gain achievable via the RQPM process.

5. The material of claim 4, wherein the nonlinear material of the initial sample includes colloids, nanopowders, nanocrystals, optical ceramics, or polycrystals.

6. The material of claim 4, wherein the method includes a step of annealing of, hot pressing of or hot isotactic pressing of the initial sample, while controlling an environmental characteristic selected from the group consisting of medium surrounding the original sample, temperature, pressure, duration, dopants, and dopant concentration and a combination of these environmental characteristics,

7. A method of characterization nonlinear optical material for a nonlinear optical element (NOE) of an optical parametric device which converts an incoupled pump into output signal and idler radiations and which utilizes the nonlinear interaction via RQPM process, wherein a microstructure of the optical nonlinear material is configured such that an average grain size in the microstructure is of the order of a coherence length of three-wave interaction which enables a nonlinear interaction with a highest parametric gain that is achievable via the RQPM process, the method comprising the steps of: selectively coupling a pump radiation into the nonlinear optical material so as to enable a non-threshold nonlinear process in the NOE, wherein the non-threshold nonlinear process is selected from the group consisting of second harmonic generation, sum-frequency generation and difference-frequency generation; measuring a local distribution of the efficiency of parametric interaction in the RQPM-NOE, thereby mapping locations with highest efficiency of selective parametric interaction.

8. The method for nonlinear optical material characterization of claim 7 further comprising using the identified locations with the highest efficiency of selective parametric interaction which serve as pumped regions inside the RQPM-NOE for the implementation of the OPD.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The above and other aspects and features of the invention are further discussed in detail and can be more apparent in light of the following drawings, in which:

[0020] FIG. 1 is the illustration of the energy conservation condition for the parametric frequency conversion in the NOE crystals;

[0021] FIG. 2 is the illustration of the momentum conservation or phase matching condition for the efficient parametric frequency conversion in the NOE crystals.

[0022] FIG. 3 is a general optical schematic of the inventive OPD.

[0023] FIGS. 4A-4C schematically illustrate various configurations of the inventive OPD.

[0024] FIG. 5 illustrates a grain size/annealing time dependence.

[0025] FIGS. 6A-6C illustrate different OPO/OPG schemes known to one of ordinary skill in the nonlinear optics.

[0026] FIG. 7 is a setup for the prepared NOE characterization via SHG.

[0027] FIG. 8 is a false-color map of SHG efficiency distribution in the sample characterized in the setup of FIG. 7.

SPECIFIC DESCRIPTION OF THE INVENTION

[0028] Reference will now be made in detail to the disclosed system. Wherever possible, same or similar reference numerals are used in the drawings and the description to refer to the same or like parts or steps. The drawings are in simplified form and are far from precise scale.

[0029] Referring to FIG. 3, an exemplary schematic of the inventive optical parametric device (OPD) 10 is based on random phase matching (RQPM) in disordered .sup.(2) polycrystalline materials such as ceramic-based nonlinear optical element NOE 25. The NOE 25 is configured to down-convert via RQPM process an incoupled pump radiation at first frequency .sub.1, which is generated by an optical pump 12, into output signal and idler radiations 26 at least one lower second frequency .sub.2. The OPD 10 may be configured as an optical parametric generator (OPG) or optical parametric oscillator (OPO).

[0030] The configuration of OPD 10, as shown in FIG. 3, represents the OPO which necessarily includes a resonant cavity or resonator 30. As shown, a four-mirror resonator includes optical components 14, 16, 18, and 24. The illustrated resonator is just one of multiple possible schemes of the resonator that can be used within the scope of the disclosure, as discussed below.

[0031] The OPO can be singly resonant when it is resonant at either the signal or the idler frequency. Alternatively, the OPO may be doubly resonant with both signal and idler radiations being simultaneously resonant. Still another possibility is a triply resonant OPO in which all three pump, signal and idler radiations are resonant at the same time.

[0032] The heart of OPD 10 is of course NOE 25. The material used for NOE 25 within the scope of this disclosure is selected from the group consisting of optical ceramics, polycrystals, micro and nanocrystals, colloids of micro and nanocrystals, and composites of micro and nanocrystals in polymer and glassy matrices. In particular the disordered .sup.(2) crystalline material is selected from II-VI semiconductors including binary, ternary, or quaternary compounds, such as ZnSe, ZnS, ZnTe, ZnO, CdSe, ZnMgSe, CdMnTe, or CdZnTe. It is also possible to use the disordered material selected from groups III-VI. The group III may include GaAs, GaP, GaN), group IV may be represented by Si3N4, SiC), while Group VI is represented by, e.g., Ga2O3.

[0033] The NOE 25 may be also doped with ions of transition metals (TM) including Cr.sup.2+, Fe.sup.2+ or all suitable rare earth (RE) metals, The NOE 25 co-doped with ions of TM and RE is another salient feature of the disclosure allowing the nonlinear optical element to simultaneously provide luminescence, laser amplification and parametric amplification. Selecting the dopants to provide luminescence at the desired frequencies of respective signal and idler radiations provides a seed for either the signal or idler decreasing the OPO threshold.

[0034] The key to success in achieving the OPO or OPG action in the disclosed OPD is preparing the crystalline material of NOE 25 which has a microstructure with the desired grain size. It has been discovered that for enabling the highest parametric gain via RQPM process the average grain size should be of the order of a coherence length of the three-wave interaction.

[0035] The material used for experiments on inventive OPD 25 is ZnSe. It is a II-VI semiconductor with a bandgap of 2.7 eV and a perfect candidate for nonlinear optics applications because of its outstanding transparency (0.55 to 20 m), relatively high 2-nd order nonlinearity (d14=20 pm/V), high optical damage threshold, and good mechanical properties. The QPM orientation-patterned ZnSe structures on patterned GaAs substrate are known to be used in SHG and DFC processes. But its use in other nonlinear frequency conversion processes, such as the parametric oscillation/generation, is not known to Applicants and can be explained, among others, by insufficient quality of the samples.

[0036] The example of successfully prepared RQPM ZnSe ceramic samples includes the following method. Eighteen (18) samples of ZnSe CVD grown by II-VI Inc. with a size of 1163 mm were individually sealed in quartz ampoules under 10.sup.5 Torr vacuum and annealed, by a pair of samples at time, in an oven at fixed temperature 900C. The annealing time varied from 6 to 10 days with a half a day interval. After annealing, the samples were removed from the ampoules and together with two untreated samples were chemically etched in hot 30% wt. NaOH solution at 94-96 C. for 30 min. The digital microscope images of the samples grain microstructure were analyzed in terms of the average grain size. It was revealed that the grain size of untreated samples was between 50 and 60 m. The annealed samples have shown an apparent trend of grain size increase to 100 m after 8 days of annealing, as shown in FIG. 5. The average grain size of the tested samples chosen for OPO experiments was close to the optimal of L.sub.coh100 m.

[0037] Alternatively, instead of annealing, a bot pressing step of the initial sample may be utilized by controlling an environmental characteristic selected from the group consisting of pressure in addition to gas atmosphere, temperature, time, dopants, and dopant concentration and a combination of these environmental characteristics which are also applicable to the annealing method.

[0038] Still another alternative method includes a hot isotactic pressing step of the initial sample while controlling an environmental characteristic selected from the group consisting of pressure, temperature, time, dopants and dopant concentration and a combination of these environmental characteristics. Also important for all of the above disclosed methods is to pay particular attention to whether any given process is implemented in the presence of air, in vacuum, or gaseous medium such as, e.g., H2 or Ar. The temperature range is defined between room and melting temperatures

[0039] FIGS. 4A-4C illustrate possible configurations of the inventive OPD. Specifically, OPD 10 of FIG. 4A, which is conceptually similar to that of FIG. 3, is configured with two separate resonators 38 and 36 of respective pump laser 12 and an OPO based on NOE 25, which in turn may be doped with light emitting ions. The resonators 38 and 36 each are defined between respective frequency discriminators with an output coupler 40 of pump resonator 38 being spaced apart from input coupler 42 of OPO resonator 36.

[0040] The OPD 10 of FIG. 4B features an alternative configuration of pump laser and OPO resonators. As shown, the laser medium of pump laser 12 and NOE 25 of the OPO are placed inside a resonant cavity 44 common to both mediums. The cavity is defined between high reflection frequency discriminator 46 and low reflectivity frequency discriminator 48 configured to provide circulation of pump, signal and idler radiations in the common resonant cavity.

[0041] FIG. 4C shows the inventive OPD which, like the scheme of FIG. 4B, has common cavity 44 for laser and OPO gain elements with frequency discriminators 46 and 48 defining the cavity therebetween. In contrast to FIG. 4B, a monolithic medium 50 is configured such that it simultaneously functions as a laser medium and nonlinear medium upon being doped with laser active optical centers.

[0042] FIGS. 6A-6C illustrate respective alternative configurations of the resonator provided with the inventive NOE 25. Referring specifically to FIG. 6A, assuming that short intense pump pulses, such as femtosecond (fs) pulses, are used, a single pass through NOE crystal 25 may be sufficient to convert the pump radiation into the signal and idler radiations. As well understood by one of ordinary skill worker in the nonlinear optics art, the illustrated scheme relates to an optical parametric generator (OPG). For pulses with smaller intensity, parametric frequency conversion is weaker which necessitates a resonator which in combination with NOE crystal 25 defines the OPO as shown in FIG. 6B. However, as opposed to the 4-mirror ring configuration of FIG. 3, the resonator of FIG. 6B includes only two mirrors: input coupler 32 and output coupler 34. The configuration of FIGS. 6A-6B have been disclosed in combination with pulsed optical pump 12. Alternatively, the pump may operate in a continuous wave (CW) regime. FIG. 6C illustrates CW pump 36 used in conjunction with a bowtie or ring resonator which has a four mirror OPO cavity with two flat and two concave mirrors.

[0043] The scope of the disclosure is not limited to the above disclosed resonator configurations. For example, various types of the micro-resonator, i.e., resonators fully integrated within NOE 25, are well known to one of ordinary skill in the nonlinear optics are part of the disclosure.

[0044] Common to all of the above-disclosed configurations of the resonator, some of input or output or both input and output mirrors can be dichroic. The optical elements providing input of pump radiation into the cavity of the OPO or directly into the OPG as well as optical couplers operative to remove signal and idler (and remaining pump) radiation from the resonator/OPG do not have to necessarily be mirrors. As shown in FIG. 3, a wedge 22 located within the resonator may function as a coupler (not shown) or decoupler that may or may not be configured with AR coatings or mirror coatings.

[0045] Returning to FIG. 3, pump 12 is another salient feature of the disclosure. As mentioned above, it may operate in both CW and ns and sub-ns pulsed regimes. It has been discovered that for short (sub-100-fs) pump pulses with high peak power the optimal length of the crystal is typically small (sub-mm), while the coherence length of three-wave mixing can be relatively large (100 m in the mid-IR spectral range) Hence, a reduction factor of RQPM process, L/L.sub.coh, is not that large.

[0046] In light of this discovery, the last decade saw a rapid development of ultrafast lasers based on TM-doped II-VI semiconductors. Among these fs and ps lasers, based on ZnS and ZnSe doped with Cr.sup.2+ ions (wavelength range 2-3 m) are most frequently used. The advantages of Cr:ZnS/ZnSe materials include a very broad gain bandwidth that allows producing short (down to few optical cycles) pulses, the absence of excited state absorption, close to 100% quantum efficiency of fluorescence at room temperature, and convenient pumping by erbium and thulium doped fiber lasers with a conversion efficiency in excess of 60%. Currently Cr:ZnS/ZnSe lasers can produce more than 7 W of the average power in the mode-locked regime, and more than 1 GW of peak power in the regime of chirped-pulse amplification. These lasers are also found to be very suitable for pumping of mid-IR OPOs based on GaAs. Using this type of lasers within the scope of the disclosure allows the use of the same materials for NOE 25 of OPD 10 and pump laser 12. Accordingly all of the above disclosed materials used for NOE 25 are suitable for the pump laser.

[0047] An ultrafast laser can be used to generate an optical frequency comb. As known, the shorter the laser pulses, the broader the range of frequencies in the comb. The mode-locked lasers can be configured to emit femtosecond pulses lasting quadrillionths of a second, or millionths of a billionth of a second. The resulting comb may span several hundred thousand uniformly spaced apart frequencies, or teeth, enabling flexible and accurate measurements of wide-ranging or widely varied phenomena.

[0048] The OPD 10 is synchronously pumped, i.e., pumping of OPD 10 with pulses is in synchronism with the pump-emitted pulses. Synchronization requires matching the pump pulse repetition rate and the round-trip frequency of the OPO, The match may be monitored and controlled by a variety of means including, for instance, a mechanical control mechanism, advantageously the latter includes a servo system. The latter, in case of measure mismatch, sends a control signal to an actuator operative to affect one of optical components of the resonator and thus adjust the round trip time. FIG. 3 illustrates a piezo element 24 operatively coupled to a servo system 52 and mounted on HR mirror 20 which, for example, is gold coated.

[0049] FIG. 7 illustrates a setup 55 implementing a method of characterization of the microstructure of prepared sample 25 characterized from the viewpoint of SHG (4.7->2.35m) conversion efficiency using a nanosecond 4.7-m laser source. The rationale behind the use of SHG for evaluating purposes is that it is an inverse process with respect to OPO and hence SHG efficiency is a direct indicator of the OPO gain that can be achieved. Another advantage of the use of an inverse SHG process is an absence of a threshold for the nonlinear frequency conversion. This threshold-less process greatly simplifiers the characterization of the microstructure. Alternatively, instead of the SHG, SFG or DFG process may be implemented for the same purposes.

[0050] A typical result of SHG mapping is shown on a resulting false-color map of SHG efficiency distribution-histogram 65 shown in FIG. 8. The histogram 65 reveals a broad distribution related to random variation of the alignment and size of crystalline domains. As can be seen there are hot spots 70 which are lighter that are used for OPO amplification that surrounding spots. These hot spots 70 are further used to achieve the OPO oscillation when the characterized sample is placed in POD 10 of FIG. 3 and the coordinates of the desired locations are sent to a means operative to displace NOE 25 in multiple desired positions. Each of the positions corresponds to the one of the location identified as promising and characterized by this location located in the waist of the pump radiation. This process is automated and requires a computer with executable software. Once characterized and optimized, the NOE is ready to be incorporated in the inventive OPD.

[0051] A series of experiments was conducted using the setup of FIG. 3. The OPO was synchronously pumped by a Kerr-lens mode-locked 2.35-m Cr.sup.2+:ZnS laser 12, with 62-fs pulse duration, 650-mW average power, and 79-MHz repetition rate. The bow-tie ring OPO cavity was composed of an in-coupling dielectric mirror M1 14 with high transmission (>85%) for the 2.35-m pump and high reflection (>95%) for 3-8 m, two gold-coated parabolic mirrors M2 16 and M3 18 with 30 off axis angle and 30 mm apex radius, five gold-coated flat mirrors (for simplicity only M4 24 was shown in the setup figure, the other four gold mirrors were used for folding the cavity. An uncoated plane-parallel polished 1.5-mm-thick. ZnSe ceramic sample 25 was placed between the two parabolic mirrors 16 and 24 at Brewster's angle. A 0.3-mm-thick ZnSe wedge was used inside the cavity for variable outcoupling the OPO signal/idler waves.

[0052] The device was operating in a doubly-resonant frequency-divide-by-2 mode near degeneracy. In addition to lowering the pump threshold, this arrangement provides other advantages: (i) phase- and frequency-locking of the OPO output to the pulse train from the pump lasera prerequisite for creating precision mid-IR frequency combs, and (ii) the possibility of achieving extremely broadband spectrum due to negligible group velocity dispersion of ZnSe in the vicinity of 4.7-m subharmonic of the pump. The OPO action was achieved at 90 mW of average 2.35-m pump power. The output spectrum, measured with a monochromator and an MCT detector, spans 3-7.5 m (1330-3330 cm1) at 40 dB level, and is centered at 4.7-m degeneracy (FIG. 4). At the maximum pump, the OPO average power was 30 mW and the pump depletion was as high as 79%, which indicates that one can obtain considerable, approaching 100%, photon conversion efficiency.

[0053] Thus the disclosed device provides optical parametric oscillation that is based on random phase matching in a disordered .sup.(2) polycrystal, ZnSe ceramic. To the best of Applicants' knowledge this is (i) the first .sup.(2) OPO that utilizes ceramic material and (ii) the first OPO based on ZnSe. With ZnSe ceramic being an inexpensive alternative to QPM solutions based on orientation patterning, this device can be used as a prototype for broadly-tunable OPOs, as well as for generating multi-octave frequency combs.

[0054] Those skilled in the art will recognize or be able to ascertain using no more than routine experimentation many equivalents to the specific embodiments of the invention described herein. The disclosed schematics are representative of an OPO and the impetus for the presently disclosed structure lies in the use of the polycrystalline materials available to the inventors. It is therefore to be understood that the foregoing embodiments are presented by way of example only and that within the scope of the appended claims and equivalents thereto, the invention may be practiced otherwise than as specifically described. The present disclosure is directed to each individual feature, system, material and/or method described herein. In addition, any combination of two or more such features, systems, materials and/or methods, if such features, systems, materials and/or methods are not mutually inconsistent, is included within the scope of the present invention.