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How to Select a Mirror
How to Select a Mirror Mirrors are an essential tool in any laser system, where they are used for beam steering and sampling, or for delivery of pump and alignment lasers. They find equal use in optical systems for general light routing, collection, and delivery. A high-performance mirror needs to have low scatter, high efficiency, and excellent durability, often in conjunction with high laser damage threshold. Choosing a mirror requires first identifying the wavelength and bandwidth required, at which point more than one type of coating may be offered. Each coating technology offers its own advantages and limitations as regards laser damage threshold, reflectivity, bandwidth, and transmitted wavefront error. CVI Laser Optics uses high quality N-BK7 substrates for mirrors designed for use at > 400 nm and Corning 7980 UV-grade fused silica for < 400 nm. All are precisionpolished to λ/10 surface figure prior to coating, with wedge ≤ 5 arc min, maximum diameter tolerance of + 0/- 0.25 mm, and thickness controlled to ± 0.25 mm or better. Surface quality is 10-5 for dielectric coated mirrors and 40-20 for metal coated mirrors, while clear aperture is a minimum of ≥85% of the central dimension for all mirrors. Bandwidth of operation Mirrors for laser applications vary in spectral range from dielectric coated laser line mirrors with only a few nanometers of bandwidth to broadband metal mirrors that reflect from visible wavelengths through to 20 μm. The bandwidth needed will depend on the emission wavelength(s) of the laser being used, as well as its stability or repeatability from unit to unit. Most excimer lasers emit at a single wavelength, while others like Nd:YLF and argon-ion lasers have multiple closely spaced emission wavelengths. Ultrashort pulsed lasers such as Ti:Sapphire emit a range of wavelengths simultaneously, and others like scanning dye lasers output a single wavelength at a time from within a wide tunable range. Dielectric coated mirrors have traditionally been used for narrowband mirrors, while metal coatings served broadband applications, albeit at lower reflectivity. Advances in coating technology in both areas have blurred these lines and increased the number of choices available for a given application. Coating types The simplest method of making a mirror is to deposit a metal coating on a glass substrate. Metal mirrors offer broadband spectral performance, are insensitive to angle and polarization, and are inexpensive to manufacture. Aluminum has good reflectivity at UV and visible wavelengths, while silver and gold are best for the visible through infrared, depending on wavelength. Durability can be an issue, however. Bare aluminum and silver are prone to oxidation, and all three can be easily scratched or damaged. This is addressed by our protected metal coatings, in which an overcoat composed of a single hard dielectric layer of half-wave optical thickness is applied to the metal coating to improve tarnish and abrasionresistance without significantly affecting the mirror’s optical properties. Another limitation of metal coatings is the dependence of their reflection spectrum on metal type, and the fact that reflectivity is lower overall than for a typical dielectric coated mirror. To mitigate this, we have developed enhanced metal coatings which use a thin dielectric film as overcoat to both protect the metal layer and increase the reflectance over a desired range of wavelengths or range of incidence angles. Even with these improvements, metal coated mirrors are limited in their power handling and can only be used for low power applications. All of our metal coated mirrors are inspected to 40-20 scratch and dig surface quality. Dielectric coatings employ quarter-wave thicknesses of alternately high and low refractive index materials applied to a substrate to form a multilayer stack. By choosing the coating materials and thicknesses carefully, the reflected wavefronts from each layer are made to interfere constructively to produce a highly reflective mirror. These coatings are remarkably hard, durable, and abrasion-resistant. Over a limited wavelength range, the reflectivity of a dielectric coating can easily be made to exceed the most efficient metallic coating. Furthermore, the coatings are effective for both s- and p-polarization components, and can be designed for a wide range of angles of incidence. As AOI moves significantly away from the design angle, however, reflectance can be markedly reduced. CVI Laser Optics uses two primary technologies for dielectric coatings. Electron beam deposition utilizes an electron beam to vaporize the material to be deposited. When combined with careful control of the temperature and vacuum conditions, it creates uniform coatings with excellent optical characteristics, high laser damage threshold, and good reliability. Ion beam sputtering uses a very high kinetic energy ion beam to sputter target materials directly onto the substrate with a high level of accuracy and repeatability over numerous coating runs. It produces dense coating layers with almost no scatter or absorption, which minimizes spectral shift due to moisture absorption. In addition, the coating density and durability allows for high damage threshold and results in fewer pin-hole defects in the coated surface. This excellent film quality and uniformity results in environmentally stable optics with laser damage thresholds exceeding 40 J/cm2 pulsed at 1064 nm. When utilized by the best filter designers, IBS coating technology can also achieve superior optical performance, including enhanced reflectivity, minimal polarization dependence, reduced AOI dependence, and broader bandwidths. All of our electron beam and IBS coated mirrors are inspected to 10-5 scratch and dig surface quality. The coating technique alone does not determine performance. Control of the coating process is essential to achieving durable, high-reflectivity coatings. We use advanced production systems and methods to apply our coatings, and employ optical monitoring throughout the deposition process to check the intensity of reflected or transmitted light until a mirror coating is complete. All coating batches are rigorously tested and inspected to ensure consistent, high performance. Our state-of-theart deposition facilities are able to coat large volumes of standard catalog and custom optics, and we can also develop and evaluate new coatings for customers’ special requirements. In addition to our own range of N-BK7, fused silica, and Zerodur substrates, CVI Laser Optics’ coatings can be applied to customer-supplied substrates. Angle of incidence Metal coatings are, by nature, equally reflective at all angles of incidence. Dielectric coatings, however, owe their reflectivity to interference between the reflections from their many layers. As a dielectric coating is angled with respect to an incident beam, the effective thickness of the layers is altered and causes the spectrum of the coating to shift and change. As the angle of incidence increases from 0°, the spectrum of a typical dielectric coating shifts toward shorter wavelengths. Two distinct spectra also emerge, one for s-polarized light and one for p-polarized light. At larger angles, the spectrum becomes highly distorted, and the shift can be significantly different for s- and p-polarized light, depending on the mirror design. The spectral shift with angle can sometimes be used to tune a mirror to shorter wavelength, provided that the design allows and the change in AOI is kept relatively small. Using a single polarization also tends to reduce the distortion of the shifted spectrum. Mirrors used for routing are coated with designs optimized specifically for 45° AOI at a single laser wavelength, and are therefore used at peak reflectance wavelengths where polarization differences can be made negligible. Nonetheless, it can be important to consider the difference in reflection efficiency and spectral shape for different polarizations when operating at non-zero angles. It is also possible to very carefully design a dielectric mirror for which reflectivity remains high for a wide range of angles of incidence, from 0 - 45° or more. Our Semrock Ultrabroadband MaxMirror® (BBDM) and MAXBRIte™ Broadband Mirrors (MPQ) are good examples. Almost all of our other mirrors are offered in both 0° and 45° AOI designs to ensure optimal performance for most 2 | IDEX Optics & Photonics Marketplace www.marketplace.idexop.com applications, and custom mirrors can be made for other angles of incidence. Cone half angle (CHA) of the incident beam should be considered when working with dielectric mirrors. Spectral performance will vary over the range of incident angles contained within the CHA, so a mirror may not meet specification when incident light exceeds the CHA for which the mirror was designed. This tends to be a greater concern for optics with sharp spectral profiles. In addition to highly reflective mirrors, we keep a wide range of uncoated mirror substrates in stock made from N-BK7, Corning 7980 UV-grade fused silica, and Zerodur. These include round, square, and rectangular substrates ranging from 10 – 152 mm in dimension, with flat, convex, and concave surface profiles. These substrates can be coated with any of our high reflectivity, metal and partial reflecting coatings. We also maintain a stock of high energy partial reflecting laser mirrors (PR1) with 10 – 99% reflectivity in a variety of popular laser wavelengths, and offer quick turn on semi-custom partial reflectors specified by the user for 30 – 99% reflectivity and wavelengths from 532 – 1550 nm. be lower for broadband dielectric mirrors. For example, our MAXBRIte™ flat mirrors (MPQ) have a λ/4 or better surface flatness while λ/10 is standard for our MaxMirror® ultrabroadband mirrors (BBDM) and tunable broadband mirrors (depending on substrate size and thickness). Surface quality is also somewhat dependent on coating. All of our dielectric mirrors are inspected to 10-5 scratch and dig, while 40-20 is more typical for our metal coated mirrors. One final item to consider in mirror selection is the degree of curvature of the substrate. Convex or concave substrates can impart subtle beam-shaping to correct collimation, or they can allow the mirror to perform dual function as a focusing element to minimize losses within an optical system. A number of our mirrors are offered from stock or semi-customized with radii of curvature up to 10 m. We also offer our partial reflecting mirrors with a slight 30 arc min wedge to reduce back reflections. Whatever your requirements may be, remember that our catalog and semi-custom product offerings are only the beginning. Our technical staff is on hand to assist you in selecting or creating the optimum mirror for your application. Laser damage threshold Coating type is the primary factor to consider when choosing a mirror based on laser damage threshold (LDT). Metal coatings have the lowest damage thresholds. Broadband dielectric coatings such as MAXBRIte™ are better, but single-wavelength or laser-line coatings are better still. If even higher damage thresholds are needed, ion beam sputtering creates dense, robust coatings with low scatter and absorption to achieve maximum LDT. Our IBS Nd:YAG laser mirrors (YxS) are rated to 40 J/cm2 pulsed at 1064 nm. The substrate material is also a factor; higher damage thresholds can be achieved using fused silica instead of BK7. Making the final decision Bandwidth, center wavelength, laser damage threshold and AOI will largely determine the best mirror for your application, but a few other factors may also influence the decision. Surface figure or flatness of the mirror will determine the distortion of your beam profile. This parameter is held tightly to λ/10 @ 633 nm before coating and often maintained through coating, but can IDEX Optics & Photonics Marketplace www.marketplace.idexop.com | 3 Selection Guide: 4 | IDEX Optics & Photonics Marketplace www.marketplace.idexop.com Product Code Description Wavelengths AOI Coating type AR1 Argon-Ion Laser Mirrors for 488-515 nm 488 - 515 nm 0° or 45° e-beam dielectric Additional features AR2 Argon-Ion Laser Mirrors for 458-529 nm 454 - 529 nm 0° or 45° e-beam dielectric AR3 Argon-Ion Laser Mirrors for 351-364 nm 351 - 364 nm 0° or 45° e-beam dielectric AR4 Argon-Ion Laser Mirrors for 244-257 nm 244 - 257 nm 0° or 45° e-beam dielectric ARF Excimer Laser Mirrors for 193 nm / ArF 193 nm 45° e-beam dielectric BBDM Semrock MaxMirror® Ultrabroadband Mirrors 350 - 1100 nm 0° to 45°, variable IBS dielectric ▪ High reflectivity at any AOI from 0° to 45° DPY Diode Pumped Resonator Mirrors 1064/808 nm 0° e-beam dielectric ▪ R > 99.5% @ 1064, T > 95% @ 808 nm ▪ For pump delivery to Nd:YAG cavity; concave optional DUVA Deep UV Aluminum Mirrors 193 - 1200 nm Any high density aluminum EAV Enhanced Aluminum Mirrors 450 -650 nm Any high density aluminum ▪ Protective dielectric coating optimized for visible FLM Fiber Laser Mirrors CWLs from 780 1900 nm 0° or 45° e-beam dielectric ▪ 1030, 1064, 1070, 1550 nm standard, others custom HC2 Helium Cadmium Laser Mirrors for 325 nm 325 nm 0° or 45° e-beam dielectric HM Nd:YAG 1064/532 nm Dual Wavelength Mirrors 1064/532 nm 0° or 45° e-beam dielectric HN Helium Neon Laser Mirrors for 633 nm 633 nm 0° or 45° e-beam dielectric KRF Excimer Laser Mirrors for 248nm / KrF 248 nm 0° or 45° e-beam dielectric LDM Laser Diode Mirrors 670, 780, 980, 1300, or 1550 nm 0° or 45° e-beam dielectric??? MPQ MAXBRIte™ Flat Mirrors "Bands from 245 - 850 nm" 0° to 45°, variable IBS dielectric ▪ High reflectivity at any AOI from 0° to 45° ▪ Bandwidths from 145 - 220 nm ▪ Round or square substrates, λ/4 surface flatness PAUV UV Enhanced Aluminum Mirrors 250 - 600 nm Any high density aluminum ▪ Protective dielectric coating optimized for UV PAV Protected Aluminum Mirrors 400 - 800 nm Any high density aluminum ▪ Protective dielectric coating optimized for visible & NIR PG Protected Gold Mirrors 650 - 20,000 nm Any gold ▪ Protective dielectric coating optimized for visible through IR PLFM Semrock® Ultrabroadband Femtosecond Mirrors 650 - 1100 nm 0° or 45° IBS dielectric ▪ Low GVD for 40 - 120 fs Ti:sapph laser use ▪ High reflectivity at 1064 & 532 nm IDEX Optics & Photonics Marketplace www.marketplace.idexop.com | 5 PM Plane Round Mirror Blanks See material Any none ▪ Wide range of diameters in N-BK7, fused silica, or Zerodur, others custom PR1 High Energy Partial Reflecting Laser Mirrors CWLs from 193 - 1064 nm 0° e-beam dielectric ▪ User-specified reflectivity, wavelength, curvature/ wedge ▪ AR-coated second surface to minimize loss PS Protected Silver Mirrors 400 - 20,000 nm Any silver ▪ Protective dielectric coating optimized for visible through IR RM Plane Rectangular Mirror Blanks See material Any none ▪ Range of dimensions in fused silica, others custom SMCC Concave Spherical Mirror Blanks See material Any none ▪ Wide range of radii in N-BK7 or fused silica, others custom SMCX Convex Spherical Mirror Blanks See material Any none ▪ Wide range of radii in N-BK7 or fused silica, others custom SQM Plane Square Mirror Blanks See material Any none ▪ Wide range of sizes in N-BK7 or fused silica, others custom TLM1 Tunable Laser Line Mirrors CWL's from 190 - 1550 nm 0° or 45° e-beam dielectric ▪ User-specified reflectivity, wavelength, curvature/ wedge ▪ Flat, wedged, or concave substrate option TLM2 Tunable Broadband Mirrors CWLs from 450 - 2100 nm 0° or 45° e-beam dielectric ▪ User-specified center wavelength ▪ Bandwidth of 90 - 250 nm @ 0°, depending on CWL TLMB High Energy Ti:Sapphire Mirrors 740 - 860 nm 0° or 45° ??? ▪ Ultralow GVD for femtosecond laser use TLMW Enhanced Ti:Sapphire Mirrors 720 - 900 nm 0° or 45° e-beam dielectric ▪ Low GVD for ≥ 15 fs Ti:sapph laser use VUVA Vacuum UV Aluminum Mirrors 157 - 190 nm Any high density aluminum XeCl Excimer Laser Mirrors for 308 nm / XeCl 308 nm 45° e-beam dielectric Y1 High Energy Nd:YAG Laser Mirrors for 1064 nm 1064 nm 0° or 45° e-beam dielectric Y2 High Energy Nd:YAG Laser Mirrors for 532 nm 532 nm 0° or 45° e-beam dielectric 6 | IDEX Optics & Photonics Marketplace www.marketplace.idexop.com Y3 High Energy Nd:YAG Laser Mirrors for 355 nm 355 nm 0° or 45° e-beam dielectric Y4 High Energy Nd:YAG Laser Mirrors for 266 nm 266 nm 0° or 45° e-beam dielectric Y5 High Energy Nd:YAG Laser Mirrors for 213 nm 213 nm 45° e-beam dielectric Y13 High Energy Nd:YAG Laser Mirrors for 1319 nm 1319 nm 45° e-beam dielectric Y1S Ion Beam Sputtered Nd:YAG Laser Mirrors for 1064 nm 1064 nm 0° or 45° IBS dielectric Y2S Ion Beam Sputtered Nd:YAG Laser Mirrors for 532 nm 532 nm 0° or 45° IBS dielectric Y3S Ion Beam Sputtered Nd:YAG Laser Mirrors for 355 nm 355 nm 0° or 45° IBS dielectric Y4S Ion Beam Sputtered Nd:YAG Laser Mirrors for 266 nm 266 nm 0° or 45° IBS dielectric YH Nd:YAG 1064/633 nm Dual Wavelength Mirrors 1064/633 nm 0° or 45° e-beam dielectric YL1 Nd:YLF Laser Mirrors for 1047-1053 nm 1047 - 1053 nm 0° or 45° e-beam dielectric YL2 Nd:YLF Laser Mirrors for 524-527 nm 524 - 527 nm 45° e-beam dielectric YL3 Nd:YLF Laser Mirrors for 349-351 nm 349 - 351 nm 45° e-beam dielectric YL4 Nd:YLF Laser Mirrors for 262-263 nm 262 -263 nm 45° e-beam dielectric YL5 Nd:YLF Laser Mirrors for 209-211 nm 209 - 211 nm 45° e-beam dielectric ▪ R > 99% @ 1064, R > 80% @ 633 nm ▪ For Nd:YAG alignment using a HeNe laser IDEX Optics & Photonics Marketplace www.marketplace.idexop.com | 7
