Characterisation, Measurement & Analysis
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Applications

  • Index Vs Temperature Option (dn/dT) for the Metricon Prism Coupler

    Option 2010-TC allows determination of temperature coefficient of refractive index by measuring index for thin films and bulk materials at temperatures as high as 200˚ C. Temperature coefficient data can also be used to calculate thermal expansion coefficients (Kang et al, Appl. Phys Lett. 81, 1438 (2002)).

    In the prism coupling technique, measurement of index at elevated temperature is complicated by the fact that the sample being measured is in intimate contact with the prism on the front side and the coupling mechanism on the back. If only the prism, the sample, or the coupling mechanism is heated, there will be significant temperature gradients across the prism/sample/coupling head interface, creating uncertainty in the temperature of the sample. In addition, since the refractive index of the prism also changes with temperature (and accurate knowledge of the prism index is required for accurate sample measurements) any gradient in the prism in the prism temperature will cause uncertainty in the prism index which will affect measurement accuracy.

    Metricon’s patented design for option 2010-TC overcomes these problems by employing three separate heating elements and two independent temperature controllers to heating both the prism and the coupling head/sample to the same temperature. The location of the heating elements is optimally located to minimize temperature gradients. One thermocouple in direct contact with the prism a few millimetres away from the contact point with the sample is used to control the temperature of the prism. Another thermocouple buried on the axis of the coupling head a distance of 2 mm from the back surface of the sample maintains the sample at the same temperature at the prism. Since the prism and the back surface of the sample are at the same temperature, temperature gradients are effectively eliminated.

    index
    Careful design in the location of thermal insulation also minimizes temperature gradient effects and allows the heating of only a very small mass. As a result, the total thermal load on the system is less than 20 watts and temperature of the rest of the system is unaffected so that option 2010-TC is fully compatible with all other Model 2010/M options (loss, additional lasers, detectors). Conversion from normal measurements to index vs temperature measurements can be accomplished in less than one minute and requires only the addition of a specially designed prism clamp.

    Best accuracy in measuring temperature coefficients is obtained by raising the sample to as high as temperature as possible (without undergoing glass transition temperatures) since this results in the largest index change compared to room temperature. However, raising the prism to high temperatures increases the risk of prism breakage due to stress on the prism caused by difference in thermal expansion coefficients of the prism and the mounting clamp. The mounting clamp for the prism for option 2010-TC has been carefully designed to minimize thermal stresses and can safely be taken to 200˚C.

    Finally, as mentioned earlier, accurate measurements of sample index requires accurate knowledge of prism index vs temperature. Temperature coefficient of index is provided with very prism ordered with this option. Once the temperature coefficient for the prism has been specified, the user only needs to enter measurement temperature to the control software and the refractive index of the prism will be correctly adjusted.

    Specifications

    • Temperature range: 25˚to 200˚C (typical 25˚ minimum ambient inside optics module due to laser heating).
    • Stabilization time: 3 minutes after 20 degree sample temperature change, 10 minutes after 50 degree temperature change.
    • Sample temperature uncertainty (estimated): less than 1˚C for temperatures up to 75˚C. Less than 2˚C for temperatures up to 200˚C.
    • Prism temperature uncertainty (estimated): less than 1˚C for temperatures up to 75˚C. Less than 2˚C for temperatures up to 200˚C.
    • Measurement procedure: User installs special prism clamp for thermal option (requires one minute), sets temperature in thermal controllers, and specifies measurement temperature in software. Measurement can typically be made after 3-10 minute temperature stabilization time.

    Metricon are represented in the UK/Ireland by Lambda Photometrics Ltd. With our long standing relationship and a demo system for sample evaluation or on-site demos, we are able to offer you the best advice on refractive index and thin film measurement. Please do contact us for support.

    W: www.lambdaphoto.co.uk

    E: contact@lambdaphoto.co.uk

    T: 01582 764334

  • Measuring Index/Birefringence of Bulk Polymers and Flexible Polymer Films using the Metricon Prism Coupler

    The Model 2010/M measures refractive index in machine (x), transverse (y), and perpendicular (z) directions for thin flexible polymer films and thicker bulk materials, permitting rapid and accurate determination of polymer density and crystallinity in approximately one minute. In-plane refractive index can also be measured along any arbitrary in-plane direction by a simple rotation of the sample about the coupling point. In some cases, information concerning index gradients in the material vs depth is obtainable, particularly in the case of high index surface skins. Measurement of coating thickness and index is also possible for coating thicknesses greater than approximately one micron. With a single prism, indices ranging from 1.3 to 2.0 are easily measured and measurements are rapid (typically 20 seconds), free of operator subjectivity, and do not require the use of index matching fluids. The spatial resolution of the measurement is approximately 1 mm, permitting determination of spatial differences in index/crystallinity.

    Index of polymer materials thicker than 10 microns (0.4 mil) is easily analyzed by the 2010’s bulk index measurement mode. In this mode, the 2010/M simply senses the critical angle at the interface between the prism and the material in contact with the prism:

    MI1

    If a material of index n is in contact with a prism of index np, as the sample and prism are rotated with respect to the stationary laser beam, light striking the base of the prism will be totally reflected to the system photodetector until the angle of incidence becomes less than the critical angle θc, where

    θc = arcsin(n/np) (1)

    The Model 2010/M determines the critical angle automatically and since np is well known, the film index is easily determined from equation (1).

    The ability to measure refractive index in x, y, and z directions (see Fig .1) allows measurement of percent crystallinity since the average of the x, y and z indices is directly proportional to density/crystallinity (Robert J. Samuels, J. Appl. Pol. Sci., Vol. 26 (1981), p. 1383). Fig. 2 is an example of average index vs crystallinity for polypropylene from this reference and similar data exists for PET. Measurement of x, y, and z indices has also been used to study crystallinity, orientation, and tear properties in polyethylene (Krishnaswamy. et al., Polymer, vol. 41 (2000), p. 9205 and Fruitwala et al., 1994 ANJEC Proceedings, p. 2252). In addition, since the 2010’s index measurement probes primarily the first few microns at the surface of the material (in some cases, refractive index information can be obtained for deeper regions – see below) refractive index data can be obtained for both sides of the film and sample-side differences (e.g., preferential cooling rates) can be studied.

    Thickness and index can also be measured for very thin flexible polymer films as well as coatings or coextruded layers in the ~1-75 micron thickness range (see Fig. 3). The Model 2010 also offers unique capabilities for characterizing index gradients vs depth for the high index surface skins which often occur in polymer materials. If a high index region is present at the surface of a material, there may be two breaks in the intensity vs angle curve and it is often possible to measure the index at the surface as well as the bulk index. In some cases, individual waveguiding modes may exist in the high index skin, and it may even be possible to calculate an approximate thickness and index for the skin layer.

    To provide compatibility with Abbe refractometer measurements made at 589 nm, a 594 nm HeNe laser can easily be substituted for the 633 nm HeNe which is normally provided with the 2010/M.

    MI2
    Fig. 1. Measurement of x. y, and z indices for 2-liter PET beverage bottle: Refractive indices along axial (long axis), circumferential, and perpendicular plane directions for outer bottle surface.

    MI3

    Fig. 2. Volume fraction crystallinity (Vc) vs. average index for polypropylene

    MI4

    Fig. 3. Measurement of coating thickness and index on polyester sheet

     

    Metricon are represented in the UK/Ireland by Lambda Photometrics Ltd. With our long standing relationship and a demo system for sample evaluation or on-site demos, we are able to offer you the best advice on refractive index and thin film measurement. Please do contact us for support.

    W: www.lambdaphoto.co.uk

    E: contact@lambdaphoto.co.uk

    T: 01582 764334

  • High Accuracy Measurement Of Resist, Polyimide and Polymer Thin Films using the Metricon Prism Coupler

    The Model 2010/M’s ease in measuring thickness and index of relatively thick and optically absorbing films (including free-standing films) makes it an ideal tool for production or R&D measurements of photoresists, polyimides, or other polymers. In particular, the 2010/M’s routine ±.0005 index resolution (higher resolutions are available) makes possible routine monitoring of the overall processing consistency of these films with a speed, a simplicity, and a sensitivity which have not been possible before. With the TM option, both in-plane and perpendicular-to-plane indices can be measured, allowing simple determination of film birefringence. In addition, refractive index gradients with depth often found in films of this type can also be observed. Finally, with the VAMFO option, non-contact measurements of thickness only are possible on films as thin as two microns, or as thick as 100 microns.

    Virtually every stage in the processing of these films (hard or soft bake, optical exposure or polymerization, and development) results in changes to film thickness, refractive index, and dispersion (variation of index with wavelength). With its ability to resolve slight index changes, and to distinguish between subtle thickness and index changes, the Model 2010/M can effectively monitor process consistency (bake consistency, degree of polymerization, etc.) throughout the complex processing of these films.

    In comparison with ellipsometry, the 2010/M offers a routine ±.0005 refractive index resolution, a resolution which highlights processing inconsistencies which would otherwise go undetected with ellipsometry. Moreover, for many applications, index resolution as high as ±.0001 is achievable. In addition, typical resist, polyimide, or other polymer films fall into the thickness range from one to several microns, an optimum range for prism coupling measurements. This same thickness range presents difficulties for ellipsometry because of the cumulative effect of optical absorption across the full film thickness, and because of the ellipsometer’s requirement that film thickness be known in advance to half an ellipsometer period (i.e., ±75 to ±125 nm for typical films).

    In comparison with instruments based on spectrophotometry (simple interference vs. wavelength), the 2010/M offers the obvious advantage of independent measurement of both thickness and refractive index. Moreover, since spectrophotometric instruments must make assumptions about film dispersion (variation of film index with wavelength), they are calibrated only for fixed menus of certain film/substrate combinations in certain (and often restricted) thickness ranges. With the 2010/M, virtually any transparent or semi-transparent film from 0.4 to 15 microns (and up to 100 microns with the VAMFO option) on virtually any substrate material, can be measured.

    In a production setting, the Model 2010/M should be viewed not as yet another instrument for measuring film thickness, but as a powerful tool for monitoring overall processing consistency. As an example, the degree of solvent removal following bake, and thus bake consistency, can be monitored via small refractive index changes resolvable only with the prism coupling technique. If the 2010/M’s 20-second thickness-index measurement is simply substituted for an existing thickness-only measurement, sensitive monitoring of the overall consistency of resist/polyimide/polymer processing can be achieved without the addition of any process measurement overhead.

    In an R&D setting, the 2010/M’s ability to track often-subtle changes in film thickness and index during processing makes it the ideal tool for studying the fundamental behavior of these films. As an option, the system can even be provided with multiple wavelength light sources so film dispersion can be measured. The Model 2010/M’s data analysis software is completely general, ensuring that virtually any type of material, from the most mundane to the most exotic, is measurable.

    In general, measurements of films on transparent substrates or on transparent low-index underlying films are much easier to make with the prism coupling technique than with other instruments. The reason for this is that in prism coupling measurements, light is totally contained within the film being measured, and troublesome reflections from the back side of the transparent substrate or underlying film do not result. In addition, the 2010/M also offers major advantages for measurements of films over metal substrates or over underlying metal films, since the 2010/M is relatively insensitive to variability in the reflectivity/roughness of underlying metal layers which often lead to inaccuracies in other instruments.

    For measurements of index anisotropy (due to mechanical stretching, molecular orientation/polarizabilty, or electrical poling) the 2010/M is unsurpassed, provided rapid and simple measurements of index in x- y- and z-axes. Normally, the basic 2010/M measurement uses TE modes to measure refractive index in a direction parallel to the film plane along a line perpendicular to the plane of incidence. For films with in-plane index anisotropy (e.g., stretched films) simple rotation of the film sample about the measurement allows index measurement along any line lying in the plane of the sample. Finally, with the TM option, refractive index in the direction perpendicular to the film plane is easily measured.

     

    highaccuracy
    Measurement of In-plane (TE) and Perpendicular Plane (TM) Index for Thick Polyimide

    In many cases of practical interest, the 2010/M can currently provide semi-quantitative measurement of refractive index variation in films with depth, and development of new algorithms is in progress which should make this measurement truly quantitative. Please consult Metricon for details.

    For applications in which contact to the film is undesirable, the VAMFO option can provide routine non-contact thickness-only measurement of films in the thickness range of 2-100 microns in 20 seconds or less, with occasional refractive index monitoring provided by prism coupling measurements at appropriate sampling intervals. Like the prism coupling technique, VAMFO measurements are monochromatic, so that the inaccuracies of spectrophotometric techniques caused by film dispersion (index change vs. wavelength) are avoided. Changeover between prism coupling and VAMFO measurements is rapid (less than one minute). For more details, please request a data sheet on the VAMFO option, #2010/M-VO.

    Metricon are represented in the UK/Ireland by Lambda Photometrics Ltd. With our long standing relationship and a demo system for sample evaluation or on-site demos, we are able to offer you the best advice on refractive index and thin film measurement. Please do contact us for support.

    W: www.lambdaphoto.co.uk

    E: contact@lambdaphoto.co.uk

    T: 01582 764334

  • Spectroscopic Measurements of Index Vs Wavelength (Dispersion) using the Metricon Prism Coupler

    Multiple-wavelength operation is available for Metricon’s Model 2010/M to provide rapid and accurate curves of thin film and bulk material dispersion (index vs wavelength) similar to the results from a spectroscopic ellipsometer.

    If index is measured at three or more wavelengths a complete index vs wavelength (dispersion) curve can be easily generated in just a few seconds using the built in Cauchy fitting software embedded in the 2010/M’s control software:

    Spectro

    Fig. 1.  Cauchy dispersion curve of GaN on sapphire from measurements at 446, 532 and 633 nm

    The fit equation is provided, and for wavelength ranges in the visible, Vd and Ve Abbe numbers are calculated and displayed and the user can also select up to six default wavelengths for which index is automatically calculated (index is calculated at a single wavelength, 589 nm, in the above example).

    In addition, for materials with index 2.0 or less, Metricon’s new MetriconFit software allows extremely accurate determination of continuous index vs wavelength data from Model 2010/M measurements of index at just a few discrete wavelengths. For example, measurements at only three wavelengths, 405/473/635 nm or 635/980/1550 nm, can provide highly accurate dispersion curves over the 405-635 or 635-1550 nm ranges. For materials with index up to ~2.0, the worst case fitting error for these fits is less than .00006 (and for most materials and wavelengths fitting error will be much less than .00006). A fitting error of .00006 means that the fit adds no more than .00006 to the typical Model 2010/M measurement error of .0001-.0002 and, as a result, calculating index at any intermediate wavelength is only slightly less accurate than actually using a laser to measure the sample at that wavelength.

    spectro2Fig. 2. Maximum MetriconFit error vs sodium D line index for 244 tested glass and crystalline test materials.

    Fig. 2 above was generated by generating MetriconFit dispersion curves from refractive index values at 405, 473 and 633 nm for 244 test materials - the entire Schott and Ohara glass catalogs plus a few crystalline materials for which very accurate dispersion data is available. Index for all 244 materials was then calculated by MetriconFit at 5 nm intervals and compared to the original data and the worst case error is shown plotted against the sodium D line index of the test materials. Worst case fitting error generally increases as index of the test materials increases but is still less than .00006 for all materials. Data shown is for the 405/473/633 nm fit but results are similar for 633/980/1550 fit.

    MetriconFit software is also available to determine index over the 405-1000 nm range from index measurements at four wavelengths (405, 473, 633 and 980 nm), and over the 405-1550 nm range from index measurements at five wavelengths (405, 473, 633, 980 and 1550 nm).

    For those wishing to provide their own fitting software, measured index-wavelength pairs can also be easily copied from the Model 2010/M control software and pasted into a user’s own fitting software.

    For users wishing to use their own laser sources, the 2010/M is also available with a port for user-supplied lasers, and with suitable dichroic beamsplitters to combine beams onto a single path, a single port can be used with several wavelengths.

    Metricon are represented in the UK/Ireland by Lambda Photometrics Ltd. With our long standing relationship and a demo system for sample evaluation or on-site demos, we are able to offer you the best advice on refractive index and thin film measurement. Please do contact us for support.

    W: www.lambdaphoto.co.uk

    E: contact@lambdaphoto.co.uk

    T: 01582 764334

  • Characterization of SPR and Waveguide Structures for Sensor Applications using the Metricon Prism Coupler

    The Metricon Model 2010/M is an ideal tool for developing surface plasmon resonance (SPR) and waveguide structures for use as bio- or other sensors. Such sensor devices usually rely on measuring the angular shift of an SPR resonance or waveguide propagation mode caused by the adsorption of thin layers of target materials on the surface of the sensor.

    A typical SPR sensor structure consists of a glass slide covered on one side with a thin layer of gold or silver. Kretschmann-type SPR measurements are easily performed on the Model 2010/M using the technique described by Kotchev et al (Colloid Polym. Sci. vol 281 (2003), 343-352). In this method, a small drop of index matching oil is placed on the surface of the 2010/M measuring prism to make contact to the uncoated side of the SPR sensor device (figure 1):

    Characterisation

    The 2010/M’s computer-driven rotary table varies the incident angle, and a sharp dip in the light reflected from the glass/metal film interface, caused by the SPR resonance, occurs (curve 1 in Figure 2). The SPR resonance angle depends on the laser wavelength and the metal film thickness and type, but the angle is also strongly affected by the presence of even extremely thin layers (1-10 nm) present on the metal surface (curve 2 in figure 2). This extreme sensitivity of the resonance angle to both the thickness and the refractive index of thin adsorbed layers makes the SPR structure an ideal sensor for both organic/biological and inorganic material types at the nanolayer level.

    Since SPR resonances often occur at effective indices below the normal range of most Metricon prisms, special index matched prisms are available for SPR applications on the 2010/M. In addition, it is usually easy to accommodate SPR devices which incorporate fluidic apparatus to allow liquids to come into contact with the SPR sensor – if we know in advance the design of your cell and other special requirements, we can usually customize the 2010/M system to suit your needs at little or no additional cost.

    Waveguide sensors work in a roughly similar way to SPR sensors and usually rely on the angular shift induced in mode propagation angles caused by thin adsorbed layers of the target molecules on the wave guide surface (see Bahsi et al, Sensors, vol. 9 (2009), 4890-4900). Detection of very low levels of DNA molecules has also been demonstrated by absorption into porous silicon wave guides (Rong et al, Appl. Phys. Lett., vol. 93 (2008), 161109).

    The following is a partial list of journal articles referencing Metricon equipment for biosensor applications:

    M-L Anne , J. Keirsse , V. Nazabal , K. Hyodo, S. Inoue, C. Boussard-Pledel, H Lhermite, J. Charrier, K. Yanakata, O. Loreal, J. Le Person, F. Colas, C. Compère and B. Bureau, “Chalcogenide Glass Optical Waveguides for Infrared Biosensing”, Sensors, vol. 9(9) (2009), pages 7398-7411.

    Z.B. Bah_i, A. Büyükaksoy, S.M. Ölmezcan, F. _im_ek, M.H. Aslan, A.Y. Oral, “A Novel Label-Free Optical Biosensor Using Synthetic Oligonucleotides fromE. coli O157:H7: Elementary Sensitivity Tests”, Sensors, vol. 9 (2009) pages 4890-4900.

    S. Grego, J. R. McDaniel, and B. R. Stoner, “Wavelength interrogation of grating-based optical biosensors in the input coupler configuration”, Sensors and Actuators B: Chemical, vol. 131, Issue 2, 14 May 2008, pages 347-355.

    Y. Jiao and S. M. Weiss, “Design parameters and sensitivity analysis of polymer-cladded porous silicon waveguides for small molecule detection”, Biosensors and Bioelectronics, vol. 25 Issue 6, 15 February 2010, pages 1535-1538.

    D. Lloyd, L. Hornak, S. Pathak, D. Morton, and I. Stevenson, “SPARROW: A New Thin Film Based Biosensor Chip Technology,” Vacuum Technology & Coating, September 2004, pages 48-57.

    A. K. Manocchi , P. Domachuk, F. G. Omenetto, H. Yi, “Facile fabrication of gelatin-based biopolymeric optical waveguides”, Biotechnology and Bioengineering, vol. 103, No. 4, July 1, 2009, pages 725-732.

    M. Nordstrom, D. A. Zauner, A. Boisen, J. Hubner, “Single-Mode Waveguides With SU-8 Polymer Core and Cladding for MOEMS Applications”, J. Lightwave Tech., vol. 25, issue 5 (2007) pages 1284-1289.

    S. T. Parker, P. Domachuk, J. Amsden , J. Bressner , J. A. Lewis, D. L. Kaplan, F. G. Omenetto, ” Biocompatible Silk Printed Optical Waveguides”, Adv. Materials, vol. 21, Issue 23, pages 2411-2415

    G. Rong, J. D. Ryckman, R. L. Mernaugh, and S. M. Weiss, “Label-free porous silicon membrane waveguide for DNA sensing”, Appl. Phys. Lett., 93, 161109. (2008).

    G. Rong, A. Najmaie, J. E. Sipe, S. M. Weiss, “Nanoscale porous silicon waveguide for label-free DNA sensing”, Biosensors and Bioelectronics, Vol. 23, Issue 10, 15 May 2008, Pages 1572-1576.

    A. Samoc, A. Miniewicz, M. Samoc, J. G. Grote, “Refractive-Index Anisotropy and Optical Dispersion in Films of Deoxyribonucleic Acid”, J. Appl. Pol. Sci., vol. 105, 236-245 (2007).

    B.Y. Shew, C.H. Kuo, Y.C. Huang, Y.H. Tsai , “UV-LIGA interferometer biosensor based on the SU-8 optical waveguide”, Sensors and Actuators A: Physical, Vol. 120, Issue 2, 17 May 2005, Pages 383-389.

    L. D. Williams, M. Okandon, and S. Blair, “Design and characterization of a microheater array device fabricated with SwIFT-LiteTM”, J. Micro/Nanolith. MEMS MOEMS Vol. 7, 043035 (Nov. 10, 2008).

    Metricon are represented in the UK/Ireland by Lambda Photometrics Ltd. With our long standing relationship and a demo system for sample evaluation or on-site demos, we are able to offer you the best advice on refractive index and thin film measurement. Please do contact us for support.

    W: www.lambdaphoto.co.uk

    E: contact@lambdaphoto.co.uk

    T: 01582 764334

  • Bulk Material or Thick Film Index/Birefringence Measurement using the Metricon Prism Coupler

    The Model 2010/M can operate as a fully-automated refractometer, providing high accuracy measurement of refractive index and index anisotropy for solid or liquid bulk materials and thick films in the index range 1.0 -2.6 without use of toxic or corrosive index matching fluids. This bulk material index measurement is not an option, but a standard feature provided with every system.

    Index of materials thicker than 10 microns (0.4 mil) is easily analyzed by the 2010/M’s bulk index measurement mode. In this mode, the 2010/M simply senses the critical angle at the interface between the prism and the material in contact with the prism:

    bulk1
    If a material of index n is in contact with a prism of index np, as the sample and prism are rotated with respect to the stationary laser beam, light striking the base of the prism will be totally reflected to the system photodetector until the angle of incidence becomes less than the critical angle, θc where:

    θc = arcsin(n/np) (1)

    The Model 2010/M determines the critical angle automatically and since np is well known, the film index is easily determined from equation (1).

    This same approach can also be used to measure refractive index for thick films. In the case of a film, the critical angle of the prism/film interface establishes an upper limit to the angle at which film propagation modes can occur. As a film with fixed index gets thicker, the angle at which the first propagation mode occurs asymptotically approaches the critical angle as defined by equation (1) where n now refers to the film index rather than to a bulk index. If we make the approximation that the angle of the first film mode equals the critical angle, the error in the measured index due to this approximation is less than .004 for a film thickness of 3 microns, less than .001 at 5 microns, and less than .0003 at 10 microns. Thus, this approach can be applied with good accuracy to measure the index of materials with thickness ranging from several microns on up to “bulk” thickness.

    For measurement of liquid samples, a cell to bring the liquid in direct contact with the prism is available.

    The bulk index approach can also be used to study index anisotropy in solid materials. In normal operation (Fig. 2a), the Model 2010/M uses a polarized laser with TE incidence, i.e., electric field vibrating transverse to the plane of incidence (out of the plane of Fig. 1 and in the plane of Fig. 2 in the vertical direction). Thus, if the material in Fig. 1 is simply manually rotated against the prism (Fig. 2b), the electric field vector can be made to assume any orientation within the plane defined by the surface of the material, and the index for electric field vibration along any axis lying parallel to the material surface can be measured.

    bulk2

    In addition, with option #2010-TM, which rotates the laser polarization 90 degrees and provides TM incidence (magnetic field transverse to, and electric field parallel to, the plane of incidence), index perpendicular to the material surface can be measured. (Fig. 2c). This is because the electric field vibrates perpendicular to the material surface at the critical angle since the refracted ray must propagate parallel to the surface at the critical angle.

    Operation: In the bulk index mode, the angle of incidence on the sample/prism interface is varied using a motor-driven rotary table until a sharp drop is detected on the detector. The system software then calculates and displays bulk index from equation (1).

    Accuracy and resolution: With the high resolution rotary table (a no-cost option), worst case index accuracy is ±.001 and resolution is ±.0001 for bulk materials and films thicker than 10 microns. Absolute accuracy can be improved to approximately ±.0001, however, if the user is willing to perform a simple calibration procedure (measurement of an absolute index standard) with each prism. Accuracy decreases with film thickness, with measured index systematically low by .001 at 5 microns, and by .003-.004 at 3 microns.

    Measurement time: Typical measurement time is 10-20 seconds.

    Index measuring range: Minimum and maximum index measurable varies with prism type selected: 1.00-1.80 (200-P-1), 1.20-2.00 (200-P-4) and 1.55-2.45 (200-P-2 prism). Special 200-P-2 prisms are available to extend measuring range up to approximately 2.60 for visible wavelengths and silicon prisms are available to extend the measuring range up to 3.35 for wavelengths above 1100 nm.

    Dispersion (index vs wavelength): Unlike most conventional refractometers, which are single wavelength (typically 589 nm), the 2010/M can be equipped with as many as five lasers, allowing easy measurement of dispersion across a wide wavelength range. After index is measured at three or more wavelengths, the Model 2010/M fitting software generates an extremely accurate curve of index vs wavelength in only a few seconds.

    bulk3

    Measurement of ordinary and extraordinary indices of bulk lithium niobate

    Metricon are represented in the UK/Ireland by Lambda Photometrics Ltd. With our long standing relationship and a demo system for sample evaluation or on-site demos, we are able to offer you the best advice on refractive index and thin film measurement. Please do contact us for support.

    W: www.lambdaphoto.co.uk

    E: contact@lambdaphoto.co.uk

    T: 01582 764334

  • Waveguide Loss Measurement with the Metricon Prism Coupler

    This option measures loss of optical wave guides by scanning a fibre optic probe and photodetector down the length of a propagating streak to measure the light intensity scattered from the surface of the guide. The assumption is that at every point on the propagating streak the light scattered from the surface and picked up by the fibre is proportional to the light which remains within the guide. The best exponential fit to the resulting intensity vs distance curve yields the loss in db/cm. This method offers the advantages of quickness (typical measurement time, including the exponential fit, is 2-3 minutes) and simplicity (absolutely no sample preparation beyond creation of the layers which form the guide is required).

    WLM1

    Measuring Waveguide Loss

    Comparison to other methods: The optical fibre method is identical in concept to the CCD camera approach for measuring the decay of the propagating streak but the camera must have very uniform response sensitivity over the full array. With the scanning fibre method, only a small single-element silicon or InGaAs detector is used and spatial uniformity is not an issue. In addition, for loss measurements beyond 1100 nm high quality spatially uniform IR cameras are quite expensive.

    Various multiple coupling prism approaches have also been used to provide accurate loss results. In the typical two prism technique, one prism is used to couple light into the guide and another prism is moved down the guide to couple out and measure the light remaining within the guide at multiple points. With this technique it is, however, very difficult to ensure that the coupling efficiency into the guide at the first prism is not changed as the second movable prism is brought into and out of contact with the sample. Unless the coupling faces of the prisms lie precisely in the same planes and the samples themselves are extremely flat, mechanical stresses as the second prism is clamped to the sample can cause a change in the coupling efficiency or possibly complete loss of contact at the first prism. To overcome this problem, various three prism approaches have been proposed to compensate for coupling efficiency changes at the first prism. The main problem with these approaches is that the apparatus is mechanically quite complex and measurements are slow because of the need to couple and decouple the sample to the movable prism multiple times while monitoring that the coupling efficiency of the first prism does not change.

    Another technique involves using a single prism to couple light into a sample and then immersing the sample into a bath filled with index matching fluid to couple out the light remaining within the guide. This technique is limited to relatively low index guides since the fluid must be closely matched to the index of the guide and the maximum index available for matching fluids is limited. Higher index fluids, moreover, are costly and often caustic or even poisonous, and use of a variety of different fluids may be required if working with waveguides over a range of indices. In addition, waveguide materials, especially polymers, may react either obviously or more subtly when placed in contact with the matching fluids, leading to loss behavior anomalies. A final requirement is that the sample must usually be cut or broken into a long narrow piece because of bath size/shape limitations. Cutting or breaking samples invariably generates large numbers of particles which can settle onto the sample surface and, unless extreme care is taken, it is difficult to avoid some scratching of the sample surface. While normal measurements of index and thickness are relatively tolerant of surface scratches or small particles, surface scattering losses, which often dominate the loss behavior, are extremely sensitive to the introduction of particles and scratches.

    The scanning fiber method offers the advantages of mechanical and operational simplicity, speed, elimination of costly or toxic index matching fluids, and absolutely no sample preparation, but samples must exhibit some surface scattering loss to provide a means of sampling the light intensity within the guide. In our experience, this is rarely a significant limitation since even low loss waveguides (below a few tenths of a db/cm) usually provide more than enough scattered light to be measurable. A more important requirement, is that the scattering behavior of the sample surface be uniform for the propagation path over which loss is measured. If, for example, one part of the streak scatters more than another, the scattered light at each point is not an accurate representation of the light remaining within the guide and the resulting intensity vs distance curve may not shown an exponential shape. While the 2010/M system displays the optimum fitted exponential superimposed on the raw intensity vs distance curve (see sample measurements below), and it is easy for the user to recognize if the pattern deviates from exponential behavior, accuracy of the loss measurement is reduced when samples exhibit non-exponential patterns. However, since surface scattering is an important mechanism of waveguide loss (and very often the dominant mechanism) if scattering efficiency changes spatially over the surface of the sample, this really means that the loss itself (however it is measured) is changing spatially over the surface of the sample and there is no one single loss figure which is representative of the entire sample. So the presence of a non-exponential curve is an important piece of information in itself, alerting the user that the loss behavior itself is spatially dependent.

    Loss measurement range: The 2010/M’s loss measurement option works over the range from 15 db/cm to ≈ 0.1 db/cm (see sample measurements below). Minimum loss measurable depends on how much light is scattered from the guide and whether the loss profile exhibits significant non-exponential behavior due to spatial variation in the surface scattering efficiency (see discussion above). The best method of ensuring that the loss measurement option provides good characterization for a particular class of guides is to submit samples to Metricon for evaluation.

    Waveguide index range measurable: The option can be used over the full index range measurable with the 2010/M system (1.0 to 3.35, with appropriate prism).

    Repeatability: For guides which exhibit essentially exponential behavior, measurements taken at various points on the sample are usually repeatable to ± 5% (i.e., a 1 db/cm measurement will vary by ± 0.05 db/cm). For guides with significantly non-exponential decay, measurement of individual propagation paths are usually repeatable to ± 5% or less (i.e., a 1 db/cm measurement will vary by ± 0.05 db/cm) but results may vary by ± 20% depending on the exact path which the streak traces down the sample.

    Accuracy: The science of loss measurement is in its infancy and no standards are available which permit certification of absolute loss accuracy. However, observance of an essentially exponential loss decay profile is good evidence that the sample is conforming to theory and the loss measurement apparatus is measuring the decay profile accurately. In the absence of a universally accepted loss accuracy standards, we invite and encourage comparative measurements between our apparatus and other techniques.

    Wavelength measuring range: From 405 to 1064 nm with silicon detector (option 2010/M-WGL1) or 520 to 1600 nm with InGaAs detector (option 2010/M-WGL2).

    Sample size: While measurements can often be made for propagation distances of 25-30 mm from the coupling point, propagation distances of 40-50 mm are desirable, especially for low loss guides. Any sample shape can be accommodated (circular, square, long and narrow). The length of the profiling scan is user adjustable with a maximum scan length of 55 mm. The fiber position is driven by a linear stepper motor with a linear resolution of 20 steps/mm, resulting in typical intensity patterns of 200 to 1000 data points.

    Measurement procedure: The rotary table is first positioned so that the desired mode is excited and a propagating streak is obtained (this is easily done even if the beam is invisible). The fiber is then driven by a stepper motor into close proximity to the prism (at the start of the streak). The fiber is then automatically scanned down the propagation path and the intensity vs distance profile is displayed on the PC monitor. With a mouse, the user then selects the parts of the pattern to be used in the loss calculation, avoiding obvious peaks caused by scratches, particles, or other surface imperfections. Loss in db/cm is then calculated automatically by a least squares fit to the intensity data and the resulting exponential fit is superimposed over the intensity profile. To improve the fit, the portion of the intensity pattern included in the calculation can then be refined or changed and the loss recalculated. At any point, loss intensity patterns and results can be saved to disk or printed out on the system printer. The entire process, from locating the mode to the final loss calculation, requires approximately 2-3 minutes.

    WLM2

    Moderate loss waveguide: Peak at end due to light emerging from end of guide.

    WLM3

    Low loss waveguide (peaks at left/center due to particles).Good fit to underlying exponential obtained by fitting to regions between peaks.

    WLM4

    Very low loss guide

    Metricon are represented in the UK/Ireland by Lambda Photometrics Ltd. With our long standing relationship and a demo system for sample evaluation or on-site demos, we are able to offer you the best advice on refractive index and thin film measurement. Please do contact us for support.

    W: www.lambdaphoto.co.uk

    E: contact@lambdaphoto.co.uk

    T: 01582 764334

  • Optical Waveguide Characterization with the Metricon Prism Coupler

    Metricon’s Model 2010/M system transforms prism coupling from an esoteric research technique into a routine laboratory analysis measurement. With the 2010/M, wave guide characterization data which typically requires a skilled professional an hour or more to collect with home made apparatus can now be obtained in 30 seconds or less in a format providing complete documentation of results and measurement conditions.

    For all types of guides, the Model 2010/M provides rapid measurement of mode indices, and for step index guides, calculation of guide thickness and refractive index and, in most cases, substrate or cladding index. In addition, with the standard TE-mode measurement, indices for both guides and substrate materials are measurable along any in-plane direction, while the TM option provides measurement in the perpendicular-plane direction (see sample measurement below). Finally, the mode pattern (plot of reflected intensity vs angle of incidence) can provide the skilled user with a wealth of insight into wave guide behaviour which is not available with the m-line observation technique commonly used.

    For graded index guides, an index vs depth calculation based on the method of Chiang (J. Lightwave Technology, LT-3, p. 385, April 1985) is provided (see sample lithium niobate wave guide calculation below). A mode modelling feature is also provided which permits calculation of mode angles and effective indices from single and dual film thickness and index values input by the user.

    Until recently, wave guide characterization applications have centred on measuring planar wave guides, but two recent papers (B. Chen et al, IEEE Phot. J., 4(5), 1553-1559 (October 2012) and K. S. Chiang et al, Opt. Engr., 47(3), 034601-1 to 034601-4 (March 2008)) have shown the feasibility of using the 2010/M to measure effective mode indices for arrays of stripe and channel wave guides.

    Major components of the Model 2010/M include: a coupling mechanism which brings the wave guide and the prism into intimate contact in a gentle and reproducible fashion; a laser and a step motor-driven rotary table to vary the angle of incidence of the beam on the prism; and a PC-based controller which employs pattern recognition software to determine effective mode indices (and resulting thickness and index) from the pattern of beam intensity reflected from the base of the prism vs. incident angle. All aspects of system operation, including table positioning and detection of modes, can be performed fully automatically or manually by the user. An option to measure wave guide loss is also available.

    Further features of interest for integrated optics work include:

    Operating wavelength: Although the standard system operates at 633 nm, most systems for integrated optics provide one or two additional beamlines so that multiple lasers can be used with the system. The Model 2010/M can be supplied with additional lasers in the 400-1600 nm range, or the system can be configured with a port for a user-supplied laser external to the system. For such multiple beamline systems, wavelength selection consists simply of opening and closing the appropriate mechanical beam blocks and changeover between wavelengths requires 10 seconds or less. Popular wavelengths for optical wave guide measurements include 830, 1310 and 1550 nm. In addition, we have configured systems with 405, 473, 532, 650, 780, 850, 980, and 1064 nm lasers.

    Resolution/accuracy: A high resolution rotary table with software-selectable angular resolution of 0.9 or 0.45 minutes is usually ordered as a no-cost option for wave guiding applications. For this table, effective index (β-value) resolution is .0002 (±.0001) with 0.9 minute operation and .0001 (±.00005), with 0.45 minute operation. Absolute accuracy of prism coupler measurements are often limited to the ±.0003 range by uncertainties in the measuring prism angle and index. For many applications, however, calibration standards are available which will permit absolute index accuracy comparable to the rotary table effective index resolution (±.00005).

    Coupling arrangement: The 2010/M’s coupling apparatus provides effective and reliable coupling for a broad variety of substrate sizes and thicknesses, permitting attainment of optimum coupling conditions in just a few seconds. Accurate and reproducible coupling pressure without sample rotation or prism damage is obtained with a pneumatic coupling cylinder. A conveniently mounted pressure regulator permits rapid and quantifiable adjustments in coupling force. Standard coupling geometry is a single-prism with a section of a spherical stainless ball to dimple a small portion of the sample against the coupling face of the prism. If desired, the coupling fixturing can be made modular to permit quick interchange of a variety of coupling arrangements on the rotary table.

    Bulk index measurements: For characterization of wave guide substrates or cladding, the Model 2010/M provides automated and precise measurements of index (including birefringence) for bulk materials or thick films. Accuracy and resolution of bulk index measurements are comparable to normal wave guide measurements (see above). For full details, please see "Bulk Material or Thick Film Index/Birefringence Measurement".

    Prisms: Metricon offers four standard coupling prisms spanning the effective index range from 1.0 to 2.65. Additional prisms are available to extend the index range up to 3.35. Prisms are supplied in convenient mounting clamps which permit interchange of prisms in less than 30 seconds (unmounted prisms can be supplied for special applications). Prisms with thin conductive coatings are available for surface plasmon or polymer poling measurements.

    TE/TM modes: Normal system operation is with TE polarized light. If TM operation is desired, an automated polarization rotator is available for each beamline to provide TM incidence at the prism. The rotator is automatically driven into the beamline whenever TM operation is selected, and the TM-modified mode equation is used for analysis.

    Compatibility with further data analysis: The system does complete calculation of mode indices as well as thickness and index of the guiding layer, but the software has been carefully designed to allow portability of data to user-supplied software. Effective mode indices, which form the basis of most subsequent wave guide calculations, are prominently displayed and may easily be input into user-supplied programs. In addition, the complete mode pattern (plot of reflected intensity vs angle) can be written to an ASCII data file for further analysis by other software.

    Loss measurement: A loss measurement option based on the scanning fibre approach (which samples light scattered from the surface of the guide) is available for planar waveguides with a top cladding of air. Waveguide losses ranging from 15 db/cm down to as low as 0.1 db/cm are measurable with this option. We welcome the opportunity to demonstrate the performance of this option with customer-provided samples. For additional information see application note “Wave guide Loss Measurement Option 2010/M-WGL1/2″.

    14224295615251422429650699

    Mode pattern for lithium niobate wave guide and resulting index gradient calculation

    Metricon are represented in the UK/Ireland by Lambda Photometrics Ltd. With our long standing relationship and a demo system for sample evaluation or on-site demos, we are able to offer you the best advice on refractive index and thin film measurement. Please do contact us for support.

    W: www.lambdaphoto.co.uk

    E: contact@lambdaphoto.co.uk

    T: 01582 764334

  • Sub-mm Internal Glass Structuring Using Femtosecond Lasers

    The short femtosecond pulse duration, pulse stability and excellent beam quality of the Onefive® Origami XP laser allow novel beam splitter structures to be created within the bulk of glass.

    Onefive GmbH – CH-8046 Zu¨rich – Switzerland

    The Origami XP is the first all-in-one, single-box, microjoule, femtosecond laser with the laser head, controller and air-cooling system all integrated into one box. A simple and compact chirped pulse amplification system is capable of >50 μJ pulse energy, 4 W average power and pulse duration below 400 fs.

    Femtosecond laser-based industrial production is proven to have several advantages for quality control, high-precision, and elimination of post production cleaning needs. For example, the short fs pulse duration (see Figure 9) and excellent beam quality (Figure 10) allow novel beam splitter structures to be created within the bulk of glass, as shown in Figure 11, with the modification dimensions  of the order 20µm, which is beyond the capabilities of longer pulsewidth lasers. The superb reliability and pulse-pulse stability of the Origami XP permits reproducible beam splitter arrays to be written in the glass.

    onefive1

    Figure 9 Clean, pedestal-free temporal pulse shape of the Origimai XP.

    onefive2

    Figure 10 Origami XP beam quality.

    onefive3Figure 11 1x2 and 2x4 beam splitter structures inscribed by the Origami XP within PMMA bilk acrylic glass (Images courtesy F. Chen, Shandong University, China).

    Founded in 2005, Onefive GmbH has introduced a novel generation of advanced laser modules. Onefive combines the compactness, ease of use and solidness of fiber lasers with the superior noise performance, pulse quality, power level and life-time of solid-state lasers while maintaining a competitive price.

  • High resolution imaging using LASOS LDM-XT laser diode modules

    The LASOS® LDM-XT laser series is the latest generation of laser diode modules that allows high resolution microscopy for both OEM and Laboratory applications.

     

    lasos1aFigure 5 LDM-XT laser diode module beam coupling options.

    LDM-XT can be delivered either (i) free-beam or (ii) by single mode fibre with direct modulation at frequencies up to 300 MHz. The LDM-XT are available in a wide range of wavelengths from 375 to 830 nm at power levels up to 250 mW. The LDM-XT have additional options such as reduced Spectral Width of <1 nm, circular output beam, enhanced beam quality (M2 value <1.2) and speckle homogenization (see Figure 6). The LASOS CommanderTM software offers PC based controls via USB interface.

    Lasos2

    Figure 6 SHINE option to reduce image noise.

    These advantages have led the LDM-XT to be the lasers of choice for high resolution and confocal microscopy with the image noise reduction (shown in Figure 6) allowing high resolution images such as Figure 7 to be produced.

    Lasos3

    Figure 7 Confocal microscope images of (a) a zebra fish embryo and (b) rat kidney using Lasos LDM-XT modules. Courtesy of Carl Zeiss.

    The LDM-XT products work seamlessly with the LASOS Diode-Pumped Solid-State (DPSS) products, with as many as 6 wavelengths combined into the integrated, multi-laser light MC6 series shown in Figure 8. Several lasers can be coupled into a single optical fibre with each laser modulated individually.

    Lasos4

    Figure 8 Multi wavelength modules and beam combiners available with free beam or fibre coupled lasers.

    LASOS is the European leader in manufacturing gas lasers and the world's largest OEM supplier for laser scanning microscopy. The company’s purpose-built facility is situated in Jena, Germany, a world centre for photonics innovation and expertise.

     

     

     

     

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