Electro-optical modulator that uses copper-tungsten electrodes for improved thermal stability

Abstract

Electro-optical modulator, comprising a single crystal element (12) exhibiting changes in internal birefringence in response to an applied stress, wherein the single crystal element (12) has opposing principal surfaces (12A, 12B) and exhibits a material-specific coefficient of thermal expansion, hereinafter referred to as CTE; and Electrodes (14, 16) formed from a CuW metal matrix composite and coupled to the opposite main surfaces of the single crystal element (12), wherein a W / Cu ratio of the CuW metal matrix composite is selected to create electrodes (14, 16) with a CTE that is substantially matched to the CTE of the single crystal element (12).

Description

Technical area

[0001] The present invention relates to high-performance electro-optical modulators and, more particularly, to the use of electrodes specifically designed to have a coefficient of thermal expansion (CTE) matched to the CTE of the modulator's crystal material. Background of the invention

[0002] An electro-optic modulator (EOM) is a device in which an electrical signal can be impressed upon an optical carrier. This electrical signal can be impressed upon the phase, frequency, amplitude, or polarization of the optical carrier, depending on the orientation of the enclosed electro-optic active material (crystal) and the system in which the modulator is used. The impressed electrical signal serves to modify the refractive index of the electro-optic active material, which in turn changes (i.e., "modulates") the optical carrier passing through the crystal. A polarization modulator can be configured as a voltage-controlled shutter capable of switching a laser source at rates from kHz to MHz with rise / fall times on the order of nanoseconds.EOMs used in such a configuration are susceptible to performance degradation due to thermal effects, including unwanted light leakage via stress birefringence, pointing shift, and thermal lensing.

[0003] One approach to addressing thermal issues associated with high-performance EOMs is to maintain the modulator package itself at a stable temperature, such as through the use of water cooling. However, water cooling introduces additional complexities in the package design and obviously requires additional facilities at the modulator installation site.

[0004] US 2016 / 0 246 080 A1 discloses an electro-optical modulator for high-voltage applications that exhibits reduced corona and arcing through the use of dielectric-coated electrodes in conjunction with a non-centrosymmetric crystal. The inclusion of an insulating coating (i.e., a dielectric material) on at least a portion of the electrodes reduces the possibility of arcing or corona without requiring any type of coating material to be applied directly to the crystal itself. Thus, the birefringence response of the crystal is not affected by this coated electrode configuration of the present invention. Brief description of the invention

[0005] The needs remaining in the prior art are solved by the present invention, which relates to the use of specialized electrodes, which are particularly designed to have a coefficient of thermal expansion (CTE) substantially adapted to the CTE of the crystal material of the modulator.

[0006] According to the principles of the present invention, a thermally stable EOM is provided by utilizing specialized copper-tungsten (CuW) metal matrix composite electrodes configured to have a specific W / Cu ratio such that the CTE of the electrodes is substantially matched to the CTE of the active crystal material. This contrasts with prior art configurations that utilize electrodes formed from either brass or a nickel-cobalt-iron alloy (sold under the tradename "Kovar"), neither material being particularly well-suited for CTE matching.

[0007] It is an aspect of the present invention that using electrodes with a CTE matched to the active crystal material largely eliminates (in fact, substantially eliminates) unwanted stress-induced birefringence in the crystal as the modulator temperature varies, since expansion / contraction of the electrodes with temperature is substantially equal to that experienced by the crystal. Minimizing this stress-based birefringence minimizes light leakage across the crystal, maintaining consistent switching behavior based only on the stress-induced birefringence related to the electrical modulation signal.

[0008] Furthermore, CuW metal matrix composite electrodes advantageously exhibit higher thermal conductivity than conventional electrodes formed from materials such as Kovar or brass. Consequently, the use of CuW metal matrix composite electrodes according to the principles of the present invention allows for the rapid dissipation of heat generated within the crystal, thus reducing temperature-induced changes in crystal behavior (e.g., pointing shift, thermal lensing).

[0009] The combination of reduced thermal stress and improved thermal conductivity results in an electro-optical modulator that is capable of operating over a wide temperature range with consistent results.

[0010] The CuW metal matrix composite electrodes can be mechanically clamped to the crystal or bonded to it using specific thin films that further improve thermal conductivity. The connection can take the form of a cold weld or a solder joint. In fact, one possible configuration might use one electrode mechanically clamped to the crystal and one electrode bonded to the crystal.

[0011] The active crystal material is generally defined as a single-crystal material whose crystal structure lacks inversion symmetry and exhibits electro-optical effects. Examples of such single-crystal materials useful in a modulator of the present invention include, but are not limited to, the following: CdTe, CdZnTe, ZnTe, and GaAs. According to the principles of the present invention, the specific formulation of the W / Cu ratio in the metal matrix composite is designed such that the CTE of the electrodes substantially matches that of the material selected for use as the crystal.

[0012] An exemplary embodiment of the present invention may take the form of an electro-optic modulator comprising a single-crystal element exhibiting changes in internal birefringence in response to an applied voltage (where the single-crystal element has opposite major surfaces and a material-specific coefficient of thermal expansion (CTE). The electro-optic modulator also includes a pair of electrodes formed from a CuW metal matrix composite and coupled to the opposite major surfaces of the single-crystal element. In accordance with the principles of the present invention, the electrodes are configured to have a W / Cu ratio such that the CTE of the CuW metal matrix composite electrodes substantially matches the CTE of the single-crystal element.

[0013] Other and further aspects and advantages of the present invention will become apparent in the course of the following discussion and with reference to the accompanying drawings. Short description of the drawings

[0014] Reference is now made to the drawings, Fig. 1 is a simplified block diagram of an exemplary electro-optic modulator constructed in accordance with the principles of the present invention; Fig. Figure 2 is a graph showing the CTE as a function of %W in a CuW metal matrix composite; Fig. 3 shows an embodiment of a combination of an exemplary crystal and a pair of specialized electrodes, wherein the electrodes are physically bonded to opposite major surfaces of the crystal; and Fig. Figure 4 shows another embodiment of this combination in which the specialized electrodes are mechanically attached (i.e., “clamped”) to the opposite major faces of the crystal. Detailed description

[0015] Reference in this specification to "an example" or "embodiment" means that a particular feature, structure, or characteristic described in connection with the example is included in at least one embodiment of the invention. Thus, appearances of the terms "example" or "embodiment" in various places throughout the specification do not necessarily refer to the same embodiment. Furthermore, as used herein, the terms "about," "substantially," and "mainly" mean that the recited characteristic (e.g., "CTE") need not be achieved exactly, but that deviations or variations, including, for example, tolerances, measurement errors, limitations on measurement accuracy, and the like, as known to those skilled in the art, may occur in amounts that do not preclude the effect that the recited characteristic was intended to provide.

[0016] Fig. 1 illustrates an exemplary electro-optic modulator 10 constructed in accordance with the present invention. The operation of the electro-optic modulator 10 relies on the ability to "modulate" the birefringence of an active single-crystal component 12 by applying an electric field potential ε to the crystal. A pair of electrodes 14, 16 are arranged to contact opposing major surfaces 12A, 12B of the crystal component 12. As also shown, the electrodes 14, 16 are coupled to an external electrical signal source 18 (i.e., a data signal for modulating the birefringence to impress the data signal on an optical carrier). In the presence of the electric field, internal birefringence is created, causing a rotation of the polarization direction of the crystal component 12.As a result of this polarization rotation, an optical signal propagating through the crystal component 12 undergoes a polarization transformation as it propagates from an input end face 12a to an output end face 12b of the crystal component 12.

[0017] In the special configuration of Fig. 1, the crystal component 12 is positioned between a first polarizing element 20 and a second polarizing element 22, wherein the polarization state of the second element 22 is rotated by 90° relative to the first element 20 (in some cases, the second polarizing element 22 is referred to as the “analyzer”).

[0018] An input optical carrier beam first passes through the first polarizing element 20, creating a beam with a "known" polarization state. The polarized beam then passes through the crystal 12 and is rotated by a predetermined polarization angle proportional to the electric field ε created by the presence of a voltage across the electrodes 14, 16. The amplitude of the modulated optical signal emerging from the second polarizing element 22 is thus a function of the electric field applied to the crystal 12 (i.e., indicative of the orientation between the polarized input signal and the electric-field-induced polarization state of the crystal).

[0019] The arrangement of Fig. 1 thus functions as a voltage-controlled shutter capable of switching a laser at rates from kHz to MHz with rise / fall times on the order of nanoseconds. However, heat generation within crystal 12 is known to be problematic, potentially resulting in additional unwanted modulation of the propagating optical signal. Indeed, one source of temperature increase within crystal 12 is the propagation of the optical carrier beam itself, which inevitably leads to the absorption of optical energy within the crystalline material, thereby increasing its temperature.

[0020] An undesirable effect of increasing crystal temperature is "thermal lensing," a well-known effect in which those regions of the crystal experiencing a greater temperature change exhibit a greater refractive index change. The presence of a temperature gradient across the crystal results in the formation of a refractive index gradient, which adversely affects the optical beam propagating through the crystal. Furthermore, the presence of elevated temperatures is likely to introduce unwanted stress-induced birefringence within the crystal due to a CTE mismatch between the crystal and conventional materials used to form the electrodes.

[0021] Therefore, according to the principles of the present invention, the electrodes 14, 16 are formed from a specialized material having a CTE specifically designed to match the CTE of the crystal component 12. The specialized material preferably comprises a CuW metal matrix structure, wherein the ratio of W to Cu is controlled to provide CTE matching ("matching" in this case is defined as substantially the same value, within limits as mentioned above).

[0022] By adjusting the CTE, the possibility of temperature-related, undesirable stress-induced birefringence within crystal 12 (related to the stress at the interface between crystal 12 and electrodes 14, 16) is significantly reduced, if not essentially eliminated. As a result, the change in birefringence experienced by crystal 12 is controlled solely by applying (and switching) the voltage across electrodes 14, 16, thereby stabilizing modulator performance under high-power, high-temperature operating conditions.

[0023] Furthermore, CuW has been shown to have a thermal conductivity on the order of magnitude greater than that of conventional electrode materials (e.g., brass, Kovar). This relatively high thermal conductivity of CuW (greater than 100 W / mK compared to 17 W / mK thermal conductivity of Kovar) improves cooling of the modulator crystal, thereby reducing pointing shift and thermal lensing. Indeed, the use of specialized CuW metal matrix composite electrodes according to the principles of the present invention enables the operation of high-performance EOMs in a less exotic environment than water-cooled assemblies. However, it should be understood that the CTE-adapted electrode / crystal assembly of the present invention can still be used in a water-cooled modulator system.

[0024] In embodiments where the single crystal material selected for use is CdTe (with a CTE of 5.9* 10 -6 / K), CuW metal matrix composite electrodes designed to have a W / Cu ratio of 93 / 7 exhibit essentially the same CTE. In high-performance EOM embodiments using GaAs crystals (with a CTE of 5.6*10 -6 / K), using a W / Cu ratio of 95 / 5 results in a CuW metal matrix composite electrode with the same CTE value. Fig. Figure 2 is a graph illustrating the CTE of CuW as a function of the percentage of W in the metal matrix structure, which can therefore be used to determine a specific W / Cu ratio best suited for a particular crystal material. Since Cu and W are not mutually soluble, the electrode material is composed of finely dispersed copper and tungsten phases. In a mold, a selected volume of molten Cu is used to infiltrate the W matrix to yield a final product exhibiting the desired, specialized W / Cu ratio required for CTE matching to the crystal component of the modulator.

[0025] As mentioned above, there are a variety of different configurations that can be used to couple the specialized CuW metal matrix electrodes to the electro-optic crystal material to form a thermally stable EOM according to the principles of the present invention. Fig. Figure 3 illustrates an exemplary arrangement in which the electrodes 14, 16 are directly bonded to the crystal 12 by a thin layer 30 of an electrically and thermally conductive adhesive. The adhesive is applied to the opposing major surfaces 12A, 12B of the crystal 12 to form thin layers 30, and the electrodes 14, 16 are then contacted with the adhesive. In other embodiments, suitable metal films can be applied to both the exposed major surfaces of the crystal 12 and the mating surfaces of the electrodes 14, 16. The metal film is used to create either a cold weld joint or a solder joint (depending on the composition of the metal film).

[0026] Fig. Figure 4 illustrates another possible arrangement for coupling the electrodes 14, 16 to the crystal 12. The crystal 12 is here mechanically clamped between the electrodes 14, 16. A pair of clamps 40, 42 is in Fig. 4, where the directional arrows represent the mechanical force (F) used to hold the clamps 40, 42 in place against the electrodes 14 and 16, respectively. The clamping also creates the required electrical signal path (i.e., creating the electric field across the crystal 12). Although not explicitly shown, it is also possible to use an arrangement in which an electrode is physically attached to the crystal 12 (as in Fig. 3) and the other electrode is mechanically pressed against the surface of the crystal 12 (as shown in Fig. 4) is held in position.

[0027] It will be apparent to those skilled in the art that various modifications and adaptations can be made to the present invention without departing from the spirit and scope thereof. Accordingly, the inventive scope should not be limited to the specific embodiments described herein. Rather, the scope of the invention should be defined by the following claims and their equivalents.

Claims

[1] Electro-optical modulator comprising a single crystal element (12) exhibiting changes in internal birefringence in response to an applied voltage, the single crystal element (12) having opposing major surfaces (12A, 12B) and having a material-specific coefficient of thermal expansion, referred to herein as CTE; and Electrodes (14, 16) formed from a CuW metal matrix composite material and coupled to the opposing major surfaces of the single crystal element (12), wherein a W / Cu ratio of the CuW metal matrix composite material is selected to provide electrodes (14, 16) having a CTE substantially matched to the CTE of the single crystal element (12). [2] An electro-optical modulator according to claim 1, wherein the electrodes (14, 16) are mechanically clamped to the opposite major surfaces (12A, 12B) of the single crystal element (12). [3] An electro-optical modulator according to claim 1, wherein the electrodes (14, 16) are connected to the opposite main surfaces (12A, 12B) of the single crystal element. [4] An electro-optical modulator according to claim 3, wherein a thin adhesive film is disposed between the opposite main surfaces (12A, 12B) of the single crystal element (12) and the electrodes (14, 16). [5] An electro-optical modulator according to claim 3, wherein a thin metal film is disposed between the opposite main surfaces (12A, 12B) of the single crystal element (12) and the electrodes (14, 16). [6] An electro-optical modulator according to any one of the preceding claims, wherein the single crystal element (12) is selected from the group consisting of: CdTe, CdZnTe, ZnTe and GaAs. [7] An electro-optical modulator according to claim 6, wherein the single crystal element (12) comprises CdTe having a material-based CTE of 5.9*10 -6 / K, and the W / Cu ratio is selected to be 93 / 7 to form CTE-matched CuW metal matrix composite electrodes (14, 16). [8] An electro-optical modulator according to claim 6, wherein the single crystal element (12) comprises GaAs having a material-based CTE of 5.6* 10 -6 / K, and the W / Cu ratio is selected to be 95 / 5 to form CTE-matched CuW metal matrix composite electrodes (14, 16).

Images