Monolithic frequency converter

Abstract

Tunable monolithic cavity-based frequency converter pumped by a single-frequency laser where cavity resonance(s) are achieved by independently changing the temperatures of different sections of the crystal, including the periodically poled section and one or more adjacent, non-poled regions. Having independent control of the phase matching temperature and the cavity resonance for a down-converted beam increases the efficiency.

Claims

1. A monolithic resonator based apparatus comprising: a resonator made of a single bulk non-linear crystal with an input facet and an output facet, wherein each facet has a partially reflective coating for at least one wavelength and one of a spherical polishing and a flat polishing, and the non-linear crystal has a periodically-poled active section and at least one non-poled side section located next to the input facet or the output facet, and means for controlling the temperature of the active section and the at least one side section, independently.

2. A monolithic resonator based apparatus according to claim 1 wherein the non-linear crystal comprises two non-poled side sections and wherein the means for controlling the temperatures are capable of independently controlling the temperatures of the active section and the two side sections independently.

3. A monolithic resonator based apparatus according to claim 1 wherein the means for controlling the temperature are thin layer resistors made of a conductor deposited on a support in contact with the non-linear crystal.

4. A monolithic resonator based apparatus according to claim 1 further comprising means for changing a pressure applied to the non-linear crystal, as a degree of freedom additional to temperatures of all the sections of the crystal used to fulfill the multiple resonance conditions of the cavity independently of the temperature of the active section.

5. A driving method for a monolithic resonator based apparatus according to claim 1 wherein phase matching is maintained by controlling the temperature (T.sup.c) of the active section of the non-linear crystal and cavity resonances are maintained by controlling the temperature of the at least one side section (T.sub.L, T.sub.R) of the crystal.

6. A use of the monolithic resonator based apparatus according to claim 1 as a multiple resonance parametric down-conversion device, a sum frequency generation device or a second harmonic generation device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) To complete the description and provide for better understanding of the invention, a set of drawings is provided. Said drawings illustrate a preferred embodiment of the invention, which should not be interpreted as restricting the scope of the invention, but only as an example of how the invention can be carried out.

(2) FIG. 1 shows a first embodiment of the monolithic frequency converter of the invention.

(3) FIG. 2 shows details of the crystal applied to the invention. In a particular 3-section embodiment, both faces of the same are spherically polished, whereas in 2-section geometry embodiments one surface is spherically polished while the second surface of the crystal can have planar as well as spherical polishing. Active sections of the crystals are marked by stripes, and are kept in phase matching temperature T.sub.C. Tuning sections (grey) are maintained at their respective temperatures T.sub.L and T.sub.R in the 3-section geometry, and T.sub.S in the 2-section geometry in order to ensure cavity resonance(s).

(4) FIG. 3 shows a detail of the crystal and the resistive heaters applied to it.

(5) FIG. 4 is a graph comparing the second harmonic generation efficiency when optimizing all the degrees of freedom independently, as in the invention, or using the temperature of the entire crystal as a degree of freedom to control resonance, with a resulting imperfect phase matching control, as in the prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(6) With reference to FIG. 1, the monolithic frequency converter of the invention in a first embodiment is a doubly resonant second harmonic generation device that has both faces of the crystal polished and coated so that a stable cavity is formed. The coating on the input (left) side of the crystal is partially transmissive for the fundamental and totally reflective for the second harmonic (SH) and the output side is partially transmissive for the SH and totally reflective for the fundamental beam. The elements of the particular embodiment depicted in FIG. 1 are marked as follows: 1. Upper support of a piezoelectric actuator 2. Lower support of the piezoelectric actuator, rigidly connected to the upper support 3. Piezoelectric actuator, preferably with a spherical end piece that ensures uniform distribution of pressure applied to the surface of the crystal, in order to reduce the possibility of breaking the same. 4. Upper polished glass plate, for the purpose of applying a uniform pressure to the entire upper surface of the crystal 5. Lower polished glass plate with resistive heaters and temperature sensors, shown in detail in FIG. 3. 6. Monolithic second harmonic generating crystal, with faces spherically polished and coated in the case of the 3-section embodiment. In the 2-section embodiments, one of the crystal faces can be planar. 7. Thin layer resistors made of indium tin oxide (ITO) deposited on the lower glass plate

(7) I.sub.L, I.sub.R and I.sub.C denominate currents flowing through the left, right and centre ITO heaters respectively and T.sub.L, T.sub.R and T.sub.C are the temperature sensor readings corresponding to the heaters. A person skilled in the art will recognize that the heaters can be made of a variety of materials, not necessarily ITO.

(8) The doubly resonant second harmonic generator based on a Fabry-Perot cavity as the first embodiment of the invention needs the following resonance conditions to maximize its emission for a given wavelength: Red resonance Blue resonance Relative phase between the second harmonic fields created in consecutive passes through the active region of the crystal. This phase must be maintained for constructive interference.

(9) Additionally, in order to maintain phase-matching, the active section of the crystal must be kept at the phase matching temperature T.sub.C.

(10) In this embodiment, all three resonance conditions are controlled for an arbitrary wavelength. This is possible because the thermo-optical coefficient (derivative of the refractive index with respect to the temperature) has different values for the fundamental and second harmonic, so that T.sub.L and T.sub.R, the temperatures of the side sections of the crystal, serve as two independent degrees of freedom (DOF) that can tune the cavity so as to satisfy two out of the three resonance conditions for any arbitrary wavelength.

(11) The third resonance condition is met using the elastooptic effect, by stressing the entire crystal by means of an actuator, for example a piezoelectric actuator. These controls are compatible with maintaining the phase matching condition using T.sub.C, the temperature of the active section.

(12) FIG. 4, represents the dependence of the generated SH power on the fundamental power with all three degrees of freedom and phase matching optimized. Since this is a doubly resonant cavity, for it to work efficiently, one must maintain fundamental resonance, second harmonic resonance, and relative phase between second harmonic generated in two consecutive passes through the active region (in addition to the phase matching). This is achieved by using the two side section temperatures and the piezo voltage for the three resonance conditions while keeping the active region at the phase matching temperature. The dashed curve shows the same relationship of SH power to input fundamental power, with optimization of only the piezo voltage and the temperature of the entire crystal, thereby trying to achieve resonances at the cost of phase matching. This comparison shows that using full-crystal temperature as a degree of freedom to achieve cavity resonance(s) yields less conversion efficiency than does employing multiple independent temperature controls of the phase matching temperature.

(13) The invention, a monolithic resonant frequency converter, can also be used as a degenerate double resonance OPO with the same tuning method. An analogous tuning strategy can be employed in non-degenerate OPO scenarios as well, in order to maintain signal and idler resonance. Apart from that, single resonance monolithic devices (second harmonic generators and down-converters) can benefit from elements of the tuning method that relies on having different sections of the crystal at different temperatures, being able to independently control the single cavity resonance and phase matching condition.

(14) The following table illustrates the driving methods for both 3-section and 2-section embodiments of the invention. The coatings on the faces of the crystal decide whether the device works with single or double resonance. Abbreviations used in the table: SHGsecond harmonic generation, PDCparametric down-conversion, SFGsum frequency generation. Elastooptic effect refers to fulfilling one of the cavity degrees of freedom by applying pressure to the crystal, whereas wavelength tuning means that the wavelength of one of the beams involved in the nonlinear interaction is adjusted to a cavity resonance. The column describing advantages over prior art points out which devices would benefit from increased efficiency applying the independent control of temperatures of sections of the crystal, and also, the cases in which the advantage is the possibility of tuning of the emission wavelength within the cavity free spectral range.

(15) TABLE-US-00001 Advantage Number of degrees Controls for Controls for over prior of freedom Driving methods 3-section 2-section art 4 doubly resonant T.sub.C, T.sub.L, T.sub.R and not possible increased (phase matching, 2 SHG elastooptic efficiency cavity resonances, doubly resonant effect or and possible relative phase) degenerate PDC wavelength wavelength (Type I) tuning tunability (if wavelength not used as a control) 3 doubly resonant T.sub.C, T.sub.L and T.sub.R T.sub.C, T.sub.S and increased (phase matching, 2 nondegenerate PDC elastooptic efficiency cavity resonances) or SFG effect or and possible (Type I or II) wavelength wavelength tuning tunability (if wavelength not used as a control) 2 singly resonant SHG T.sub.C, T.sub.L and/or T.sub.C and T.sub.S wavelength (phase matching, 1 or SFG T.sub.R tunability cavity resonance) singly resonant degenerate PDC (Type I) singly resonant nondegenerate PDC (Type I or II)

(16) As it is used herein, the term comprises and derivations thereof (such as comprising, etc.) should not be understood in an excluding sense, that is, these terms should not be interpreted as excluding the possibility that what is described and defined may include further elements, steps, etc.

(17) On the other hand, the invention is obviously not limited to the specific embodiment(s) described herein, but also encompasses any variations that may be considered by any person skilled in the art (for example, as regards the choice of materials, dimensions, components, configuration, etc.) to be within the general scope of the invention as defined in the claims.