Amorphous broadband polarizer with high extinction ratio and high power processing

Abstract

A polarizer design combines many ideal qualities of crystalline polarizers and many ideal qualities of beam splitting cubic polarizers manufactured with high power processing glass and thin film coating manufacturing processes. That is to say, the design has a wide optical bandwidth (delta lambda / lambda gt; 0.1) to achieve a high extinction ratio (gt; 100000: 1 Tp / Ts), with high power and pulse energy processing, and all four input / output ports are high optical quality surfaces (different from many crystalline polarizers), high transmissivity (Tpgt; 98%) and low and low bulk material dispersion (group velocity dispersion at 500, 1000 and 1500 nm, respectively, is 72, 21 and-22 fs2 / mm) in the film.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. Provisional Patent Application No. 63 / 567,344, filed on March 19, 2024. The disclosure of U.S. Provisional Patent Application No. 63 / 567,344 is incorporated herein by reference. Technical Field

[0003] The present invention relates generally to polarizers and, more particularly, to amorphous broadband polarizers having high extinction ratios and high power handling. Background Art

[0004] Many optical applications require polarizers with both a high extinction ratio and a wide bandwidth, such as optical devices for wide-bandwidth and / or tunable wavelength lasers or light sources, broadband optical isolators, fluorescence lifetime measurements, transient absorption spectroscopy, vibrational and Raman spectroscopy, and multiphoton microscopy. Typical polarizers used in these applications are made of crystalline materials, usually calcite, but also including yttrium orthovanadate, magnesium fluoride, quartz, and alpha-barium borate. Calcite (the most commonly used crystalline polarizer) is a mineral material whose material properties make it an excellent polarizer with a high extinction ratio, but it has disadvantages due to defects and cleavage planes in the natural crystal, which makes it difficult or impossible to polish the material to a high wavefront quality on the reflective output surface. In addition, impurities in natural calcite crystals can lead to reduced average or peak power handling capabilities, making the polarizer unsuitable for some high average power or high peak power applications.

[0005] In particular, high-power CW, nanosecond, picosecond, and femtosecond lasers used in laser material processing applications typically cannot use calcite polarizers, but instead rely on optically contacted polarizing beamsplitting (PBS) cubes made of high-power-processing synthetic materials such as fused silica and high-damage-threshold dielectric films. Typical PBS cubes have an internal angle of incidence (AOI) of approximately 45 degrees, which limits the bandwidth of high transmission and moderate extinction ratios to approximately Δλ / λ < 0.05 for commonly used thin-film materials (SiO2 low-index layers and HfO2, Ta2O5, or Nb2O5 for high-index layers, although other films can be used). Such polarizers are typically specified with extinction ratios of several hundred to several thousand to 1, which is good for power control but unsuitable for high-power isolators, which require extinction ratios > 100,000:1. Furthermore, these polarizers are typically specified at only one laser wavelength, rather than over a range of wavelengths, meaning that a polarizer specified at 1064 nm will not have optimal performance at 1030 nm (both are wavelengths commonly used in high-power laser systems) and have typical transmissions around Tp>95%.

[0006] Alternatively, wide bandwidth performance can be achieved by changing the PBS cube substrate material to a dense glass with a higher refractive index (e.g., N-SF1), which has a refractive index of approximately 1.6 to 1.8. The high-index substrate retains the 45-degree AOI inside the polarizing film while increasing the bandwidth relative to the fused silica substrate version. These types of polarizers achieve moderate extinction ratios (>1000:1 Tp / Ts) over a wide bandwidth with Δλ / λ>0.3, but typically have much lower power handling due to the nature of the substrate material, which has higher impurities, more bulk absorption, and a higher nonlinear refractive index. These high-index polarization beam splitter cubes are also bonded together with optical adhesive rather than optical contact, which has a lower damage threshold than the substrate or film material. In addition, high-index dense glass typically has an undetermined amount of internal birefringence, which can destroy the extinction ratio of the PBS, even if the polarizing coating can support a very high extinction ratio.

[0007] Thus, there has long been a need for a polarizer design that combines many of the desirable qualities of crystalline polarizers with those of beamsplitting cube polarizers fabricated using high power processed glass and thin film coating manufacturing processes, but without the aforementioned disadvantages. Summary of the Invention

[0008] One embodiment of the present invention provides a polarizer design that combines many of the desirable qualities of a crystalline polarizer with those of a beamsplitting cube polarizer fabricated using high power processed glass and thin film coating fabrication processes. That is, the design achieves a high extinction ratio (>100,000:1 Tp / Ts) over a wide optical bandwidth (Δλ / λ>0.1), high power and pulse energy handling, high optical quality surfaces on all four input / output ports (unlike many crystalline polarizers), high transmittance (Tp>98%), and low dispersion in the thin films and low bulk material dispersion (group velocity dispersion of 72, 21, and –22 fs at 500, 1000, and 1500 nm, respectively). 2 / mm).

[0009] One embodiment of the present invention provides a polarizing beam splitter comprising a thin film polarizing coating positioned between a first optical contact glass sheet and a second optical contact glass sheet; wherein the thin film coating provides a polarization range of Δλ / λ>0.2 with an extinction ratio greater than 10,000:1, and a polarization range of Δλ / λ>0.1 with an extinction ratio greater than 100,000:1; and wherein the substrate glass is a low birefringence material configured to provide a peak extinction ratio of >100,000:1. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 A polarizer according to an embodiment of the present disclosure is shown. DETAILED DESCRIPTION

[0011] The description of the illustrative embodiments according to the principles of the present disclosure is intended to be read in conjunction with the accompanying drawings, which are considered part of the entire written description. In the description of the embodiments disclosed herein, any reference to direction or orientation is merely for the convenience of description and is not intended to limit the scope of the present disclosure in any way. Relevant terms, such as "lower", "upper", "horizontal", "vertical", "above", "below", "upward", "downward", "top" and "bottom" and their derivatives (e.g., "horizontally", "downwardly", "upwardly", etc.) should be interpreted as referring to the direction described subsequently or shown in the accompanying drawings in question. These relevant terms are only for ease of description and do not require the device to be constructed or operated in a specific direction unless explicitly stated. Terms such as "attachment", "attach", "connect", "couple", "interconnect" refer to a relationship in which structures are fixed or attached to each other directly or indirectly through an intermediate structure, as well as removable or rigid attachments or relationships, unless otherwise explicitly described. In addition, the features and benefits of the present disclosure are illustrated by reference to exemplary embodiments. Therefore, the present disclosure should not be expressly limited to exemplary embodiments showing some possible non-limiting combinations of features, which may exist alone or in other combinations of features; the scope of the present disclosure being defined by the appended claims.

[0012] This disclosure describes the best mode currently contemplated for carrying out the present disclosure. This description is not intended to be taken in a limiting sense, but rather provides examples presented for illustrative purposes only, with reference to the accompanying drawings, to inform those skilled in the art of the advantages and configurations of certain embodiments. Like reference numerals represent like or similar parts throughout the various views of the drawings.

[0013] It is important to note that the disclosed embodiments are merely examples of the many advantageous uses of the innovative teachings herein. In general, statements in this application specification do not necessarily limit any of the various claimed disclosures. Furthermore, some statements may apply to some inventive features but not to others. In general, unless otherwise indicated, singular elements may be plural and vice versa without loss of generality.

[0014] One embodiment of the present invention uses synthetic fused silica with a particularly low birefringence (<2 nm / cm in the preferred embodiment, although values ​​<5 nm / cm may be used depending on the size of the polarizer), combined with high damage threshold thin film coatings and a unique geometry to produce a polarization beam splitting optic with the following characteristics:

[0015] -Theoretical maximum extinction ratio Tp / Ts>1000000:1;

[0016] -Verified by measuring the extinction ratio Tp / Ts>100000:1;

[0017] -Extinction ratio of Δλ / λ~0.4 >10000:1 theoretical bandwidth;

[0018] -Extinction ratio of Δλ / λ~0.3 >100000:1 theoretical bandwidth;

[0019] -TP>98% for full bandwidth Δλ / λ~0.4;

[0020] - Polarization film group delay dispersion: <50fs 2 (over the entire bandwidth, for reflected S polarization and transmitted P polarization);

[0021] - Transmitted wavefront error <λ / 10 on all input / output surfaces at 633nm;

[0022] - Surface quality of all input / output surfaces is 20-10 (super-polishing can improve it to 10-5);

[0023] -Laser-induced damage threshold:

[0024] CW: >20,000W / cm CW at 1070nm (alternative unit: >5MW / cm 2 );

[0025] Pulse: >5J / cm at 1030nm, 100+ps, 10+kHz 2 .

[0026] Figure 1 A polarizer according to an embodiment of the present disclosure is shown. Polarizer 100 includes a first glass sheet 110, a second glass sheet 120, and a thin-film polarizing coating 130 applied to the interface between the first and second glass sheets, such that the two glass sheets are in optical contact. In one embodiment, the thin-film polarizing coating includes multiple layers of alternating high and low refractive indices. The first glass sheet includes an input / output surface (I / O 1) 111 and an input / output surface (I / O 2) 112, and the second glass sheet includes an input / output surface (I / O 3) 123 and an input / output surface (I / O 2) 124. Figure 1Figure 2 shows a polarizer geometry and beam examples according to one embodiment. The S and P polarization states refer to the vertical and horizontal directions in a typical laboratory frame. I / O 1 to I / O 4 are the input and / or output faces of the polarizer. In one embodiment, the input / output face (I / O 1) 111 is parallel to the input / output face (I / O 3) 123, and the input / output face (I / O 2) 112 is parallel to the input / output face (I / O 4) 123. Due to the symmetry of the design, the polarizer can be used in other orientations (i.e., rotated 180 degrees) or for bidirectional beams (e.g., as an output polarizer in an optical isolator), and all input / output faces of the device have high surface quality (i.e., 20-10 or 10-5 super polish).

[0027] In one embodiment of the present disclosure, Figure 1 Relaxation of the internal film incident angle θ in [ 1 ] from a fixed design angle of 45 degrees to a free parameter enables wide-bandwidth polarization extinction ratio performance for synthetic fused silica substrates (glass sheets). The incident angle θ is set to be close to or equal to the Brewster angle for the refractive index difference between the high-index layer and the low-index layer. Furthermore, the refractive index of the low-index layer is substantially similar to that of the substrate, creating a situation where there is almost no reflection from the film stack for the p-polarization state, and the film stack is designed to have very high reflection for the s-polarization state. This can be achieved with conventional quarter-wave stack reflectors or more complex film designs, such as chirped layers or numerically optimized designs.

[0028] One advantage of this broadband polarizer layout is the choice of bulk material. Bulk damage occurs inside the optical substrate when a short pulse with sufficient peak power (approximately the pulse energy divided by the pulse duration) collapses into optical filaments due to nonlinear processes in the material. This is in contrast to the dense flint glasses used in typical broadband polarizers (for N-SF10, n2 is 30×10 -16 cm 2 / W) compared to the lower nonlinear refractive index of fused silica (n2 is 3.2×10 -16 cm 2 The use of synthetic fused silica increases the power handling of the polarizer relative to high-refractive-index based polarizers due to the increased filamentation threshold.

[0029] The choice of optical material is also crucial for polarization extinction ratio and damage threshold performance, as not all synthetic fused silica materials possess low absorption and low intrinsic birefringence. High optical power handling requires low absorption (supporting <300 ppb metallic impurities and hydroxyl (OH) content over the appropriate operating wavelength range, with low OH levels for visible light and near- and short-wave infrared, and high OH levels for some UV applications) to prevent overheating and damage to the optics. Furthermore, nonuniform heating of the optics from an inhomogeneous laser beam can reduce the polarization extinction ratio through an increase in optical birefringence, which is caused by nonuniform stresses induced by thermal expansion of the optical substrate. The polarization extinction ratio also critically depends on the optical substrate itself not altering the polarization state of the incident light. Therefore, an optical substrate with low intrinsic birefringence (specifically <2 nm / cm) is desirable. Finally, the choice of optical material determines whether the two prisms comprising the polarizing beam splitter can be optically contacted or require adhesive. Adhesives, such as optical glue or epoxies, always compromise optical power handling because they have higher light absorption than most bulk glasses and can often contain contaminants from the manufacturing process that also increase absorption and reduce optical power handling.

[0030] Another benefit of this geometry is that all the input and output surfaces are close to perpendicular (zero degree) angle of incidence. This is advantageous for designing polarization-insensitive AR coatings over a wide operating bandwidth. For example, a 6-layer AR coating can easily achieve Ravg < 0.5% over the entire operating bandwidth of Δλ / λ ~ 0.4. The internal polarizer coating in optical contact can also be applied between two flat substrates without hexagonal geometry, which will maintain the main advantages of this embodiment (high extinction ratio, high power handling, wide bandwidth, low group delay dispersion and low wavefront distortion), with the disadvantage of having more complex AR coatings on the input and output surfaces to address non-perpendicular angles of incidence.

[0031] In summary, the present disclosure simultaneously provides the performance indicators listed below, which are not achieved by any other commercial or research optical device known to those skilled in the art; therefore, a different and novel optical device is provided to the market.

[0032] -High CW damage threshold

[0033] Low absorption optical materials;

[0034] Optical contact between the two prisms.

[0035] - High pulse damage threshold through selection of optical materials with high threshold for filamentation;

[0036] -High polarization extinction ratio

[0037] Thin film design;

[0038] Optical geometry (in particular, θ = 45 degrees is not required);

[0039] Optical material selection with low intrinsic birefringence and low absorption.

[0040] -Broadband polarization extinction ratio

[0041] Thin film design;

[0042] Optical geometric design.

[0043] -Low group delay dispersion

[0044] Thin film design;

[0045] Low group velocity dispersion optical materials.

[0046] - Low wavefront distortion and high surface quality of 4 optical entrance / exit surfaces

[0047] Optical geometry design;

[0048] Optical material selection allows for high-quality polishing on all surfaces.

[0049] In addition to the inherently wide bandwidth of quarter-wave stacked polarizing films (inherently low group delay dispersion (GDD) over the reflection bandwidth), more sophisticated numerical optimization methods can be used to design central polarizing films with even wider bandwidths (over an optical octave) while maintaining an extinction ratio of 10,000:1 or 100,000:1. Films designed for such wide bandwidths typically do not have low GDD in reflection, but this is not a requirement for many applications that could benefit from such broadband polarizing beam splitters.

[0050] Although this disclosure provides some detailed and specific descriptions of several embodiments, it is not intended that it should be limited to any such details or embodiments or any particular embodiment. Instead, it should be interpreted as providing the broadest possible interpretation based on the relevant art to effectively cover the various embodiments herein. In addition, the foregoing describes various embodiments foreseen by the inventors and provides feasible illustrations. Nevertheless, those skilled in the art may make unforeseen modifications to the present invention, and such modifications may still be considered to be substantially equivalent to the present invention.

Claims

1. A polarizing beam splitter comprising a thin film polarizing coating between a first optically contacted substrate glass sheet and a second optically contacted substrate glass sheet; wherein the thin film coating provides an extinction ratio greater than 10,000:1 over a polarization range of Δλ / λ>0.2, and an extinction ratio greater than 100,000:1 over a polarization range of Δλ / λ>0.1; and The substrate glass is a low birefringence material configured to provide a peak extinction ratio of >100,000:

1.

2. The polarizer according to claim 1, wherein The substrate glass and thin film materials are selected as low loss dielectric materials to enable power handling of >20,000 W / cm CW at 1070 nm (alternative units: >5 MW / cm CW at 1070 nm) 2 ), and >5 J / cm at 1030 nm, 100+ ps, 10+ kHz 2 Pulsed laser damage threshold.

3. The polarizer according to claim 1, wherein The geometry of the polarizer is selected to provide an internal angle of incidence, AOI, close to Brewster's angle for the internal thin film coating material, and approximately zero degree angles of incidence and exit.

4. The polarizer according to claim 1, wherein The substrate glass is fused silica, and its internal birefringence is less than 5 nm / cm, metal impurities are less than 300 ppb, and hydroxyl group OH content is less than 20 ppm.

5. The polarizer according to claim 4, wherein The film consists of SiO2 as a low refractive index layer and HfO2, Ta2O5, Nb2O5 or TiO2 as a high refractive index layer.

6. The polarizer according to claim 2, wherein The geometry of the polarizer is selected to provide an internal angle of incidence, AOI, close to Brewster's angle for the internal thin film coating material, and approximately zero degree angles of incidence and exit.

7. The polarizer according to claim 2, wherein The substrate glass is fused silica, and its internal birefringence is less than 5 nm / cm, metal impurities are less than 300 ppb, and hydroxyl group OH content is less than 20 ppm.

8. The polarizer according to claim 7, wherein The film consists of SiO2 as a low refractive index layer and HfO2, Ta2O5, Nb2O5 or TiO2 as a high refractive index layer.

Images