The Optometronic 2000Super-resolution Refractive Index Profiling Introduction
The Nanonics Imaging Optometronic 2000 is a flexible, modular, passive-component test and measurement platform which allows for nanometric alignment with active feedback. It is applicable to both passive and active component characterization. The device under test (DUT) can be rapidly positioned and repositioned with nanometric precision on one of a multiple of channels of the DUT in a completely reconfigurable, stable (0.02 dB), and repeatable fashion during the test and measurement operation. The Optometronic 2000 provides modular solutions to critical problems in test and measurements. This application note describes the refractive index profiling module.
In the world of passive component photonic devices, waveguides stand in the center of a whole variety of components, from optical fibers to optical switches to arrayed waveguide optical multiplexers and demultiplexers. A crucial measurement of the properties of these waveguides is the distribution of the refractive index in the waveguide region. Errors in this refractive index cause errors in the guiding of light and in the distribution of the optical light field.
As part of the Optometronic 2000 system, Nanonics Imaging has introduced a module that provides a high-resolution solution to the problem of refractive index profiling (RIP). It combines the technology of aperture-based near-field scanning optical microscopy (NSOM) with lens-based optical differential interference contrast (DIC). This combination yields important advantages that permit high spatial and refractive-index resolution of exposed and embedded waveguides.
The Optometronic 2000 system can also provide modules for combining, with the RIP measurements, near-field and far-field super-resolution beam profiling of the most complicated waveguide structures. Presently, in collaboration with Agilent Technologies Inc., the Optometronic 2000 can provide previously unobtainable active feedback and reconfigurable test and measurements on multiple waveguides, arrayed waveguides and other photonic circuits. Thus, the Optometronic 2000 platform provides modules with a variety of light wave characterization solutions integrated with its singular RIP capabilities.
Optical Information with Reduced Effects of Diffraction
|
To reduce the effects of diffraction, near-field scanning optical microscopy (NSOM) is employed. This technique allows for the collection of optical data with additional information that is, in general, not available in RIP measurements.
|
|
 Figure 1. Near-field scanning optical microscopy (NSOM) plays a bridging role between conventional optical microscopy and atomic force microscopy (AFM). The additional on-line information added by NSOM to confocal DIC allows Nanonics to provide proprietary, super-resolution refractive index profiling measurements.
|
In the NSOM approach to optics, light is transmitted or collected through a sub-wavelength aperture. To form an image, either the aperture or the sample is scanned, and the light is transmitted or collected through the aperture at each position. The technique has achieved the highest optical resolution yet obtained, approximately 50 nm with visible light. This breaks by a full order of magnitude the resolution that can be obtained on most samples with far-field optics. In addition to this important resolution advantage of the method, a most significant aspect of this technique arises from the bridging role that it can play between the worlds of conventional optical microscopic techniques, such as DIC, on the one hand, and the rapidly growing synergistic imaging advantages of scanned probe microscopy techniques, such as atomic force microscopy (AFM), on the other (Figure 1).
|
|
The optical element of choice for near-field optical microscopy is a tapered, cantilevered optical fiber that is coated with metal to form a single sub-wavelength aperture at the tip of the fiber, as is shown in Figure 2. This device can act either as a point source of light or as a point collector of light.
|
|
Figure 2. The Nanonics tapered,
cantilevered optical fiber probe, with a
sub-wavelength aperture at its tip,
permits the combination of refractive index measurements with fine control of
the z-position of the collecting sub-
wavelength aperture.
|
The Optometronic 2000 platform employs an additional near-field optical channel to add information to lens-based, far-field optical confocal DIC. This combination permits unprecedented spatial and refractive index resolution.
|
General Description
The Optometronic 2000 system is shown in Figure 3 with a device under test (DUT) in place. In this case, the DUT is a multi-channel waveguide that can be held in a configuration that allows either the input or the output of the channels to be probed simultaneously by the optical fiber and lens. Alternately, the DUT can be placed as in Figure 4, where the channels of the waveguide are held in a horizontal position, perpendicular to the direction of light propagation.
 |
|
Figure 3. A device under test (DUT) in place in the Optometronic 2000 system.
The cantilevered optical fiber and the lens of the DIC microscope can simultaneously monitor the DUT. The NSOM/AFM data from the fiber probe and the DIC measurement through the lens can be integrated to give super-resolution refractive index profiling.
The Optometronic 2000 can also profile the output of the most complicated waveguide structures. It can provide, with nanometer precision, reconfigurable Light Wave Measurements with active feedback of multiple or multi-channel waveguides. |
The cantilevered nature of the probe permits the technique of atomic force microscopy to be used to control the z-position of the near-field fiber optic element, which is also an atomic force probe, with single nanometer resolution. The method of z-position control of the probe is shown in Figure 5. As can be seen in this figure, a diode laser is reflected off the cantilever and onto a quadrant position sensitive detector (PSD).
As the topography or the position of the sample is altered, the signal at the PSD changes. The control system then alters the position of the nanometric resolution scanner to keep the sample-to-probe distance constant to within a nanometer or to record an image of the sample topography with nanometric resolution (Fig 5). |
Figure 4. Placement of a DUT on the Optometronic 2000 system allowing for refractive index profiling perpendicular to the direction of light propagation.
|
Figure 5. The z-position of the cantilevered optical fiber is sensed with
single nanometer precision using atomic force sensing.
Integrated DIC & NSOM
Confocal Advantage
The methodology of near-field optical microscopy, together with the on-line ability for AFM control, is combined with data that is obtained through the lens of the microscope, a confocal aperture placed after the lens and the Wollaston prism before the detector. In essence, this is a dual-channel system.
The first channel of optical data comes from the lens, while the second channel comes from the fiber optic element. The placement of confocal aperture after the lens of the microscope allows for an improvement in z-resolution by preventing out-of-focus optical rays collected by the lens from entering the detector.
For the refractive index measurements, the far-field optical channel contains a Wollaston prism that can split the rays of light into two beams with a separation delta-x (Fig. 6). These two beams have their phases altered as they reflect back to the lens and to the fiber probe. Without the fiber probe present, the configuration is a standard one for far-field DIC, except that the DIC is measured in a confocal arrangement where a confocal aperture has been placed before the detector, which is a point, rather than a multi-channel, detector. Thus, the first advantage of the Nanonics Optometronic 2000 Refractive Index Profiling System is the confocal nature of the measurement that allows for depth profiling of the waveguide. |
Figure 6. Wollaston Prism splitting rays of light into two beams with a separation delta-x.
|
NSOM/AFM Advantage
Added to this advantage is the near-field optical probe. This probe offers several novel additions to the DIC measurement.
First, at strategic locations in the image, the near-field optical probe can collect the DIC information before it is passed through the lens. The phase altered light beam that is collected by the near-field probe is analyzed relative to the phase of the sample illuminating laser beam (indicated in yellow in Figure 6). Such an addition allows one to correct the errors introduced by conventional lens-based, far-field imaging.
Second, the near-field probe has the additional advantage of allowing for extremely precise z-control and movement. In terms of z-control, the on-line AFM allows for on-line corrections of the z-position of the sample down to less than a nanometer. This is extremely important because uncontrolled sample motion causes phase noise.
Third, the near-field aperture can be moved away from contact with the coating on the waveguide. This movement, with a precise knowledge of z, allows for the re-measurement of the phase-altered beams through a medium of a known index of refraction (namely, air in this case) and with a distance known to within a nanometer. This permits a precise separation of the signal from any additional noise in the measurement and an accurate account of the surface refraction.
Thus, the presence of AFM/near-field optical probe significantly increases the accuracy of such measurements while allowing for constraints to be added for deconvoluting the effect of the lens at other points in the image that have only been measured with the lens.
RIP Results
An image of a multi-channel waveguide is shown in Figure 7.
The boxed area shows where the software in the overall scan of this waveguide indicated an error in the refractive index.
A DIC image of the boxed area of the waveguide is shown in Figure 8. This is a single plane at a particular z-value of the waveguide. A series of such images allows for a 3D refractive index profile of the waveguide to be determined.
3D Refractive Index Profile
The 3D refractive index profile of the DIC data obtained using the Optometronic 2000 is reproduced in Figure 9.
As can be seen in the images, there is a defect in this waveguide.
Based on the index of refraction profile, the software determines the electromagnetic field within the waveguide in 2D and 3D. These images are shown in Figure 10 and Figure 11. The transmission loss as a result of the refractive index error in this waveguide is 0.7 dB.
The on-line AFM/NSOM and the confocal nature of the DIC measurements performed with the Optometronic 2000 allow for a high, 3D spatial resolution of 0.2 micron for embedded waveguides (waveguides with coatings) and 0.05 microns for exposed waveguides. The very low noise of the measurements permits one to obtain very high accuracy in refractive index measurements. Thus, depending on the scattering of the waveguide, the refractive index can be measured with <10-4 accuracy and with close to 10-5 accuracy for waveguides that are highly transparent, such as optical fibers.
Rapid Image Acquisition Option
While sample scanning with the proprietary Nanonics 3D nanometric sample scanning stage is an excellent solution for research and development applications, this solution takes minutes to obtain the data.
However, the confocal aspect of the measurement also allows for very rapid accessing of the data with a confocal beam scanning system that can be added as needed. Such an addition allows the data to be obtained in less than 20 seconds. Although beam scanning introduces aberrations other than spherical aberrations into the system, the additional constraints added by the fiber probe that can be used in the deconvolution algorithm can reduce their effects.
In conclusion, the Optometronic 2000 system allows for rapid measurements that can be readily automated for production applications.
|
Figure 7. DIC Image of the waveguide. Blue box indicates region of high-resolution scan.
Figure 8. Waveguide with obvious defect (angle=0, blue bar is 7.3 microns).
Figure 9. Relative Refraction Index Δn(x,y) in defect region.
Figure 10. Electromagnetic field propagation within the waveguide in the defect region.
Figure 11. Intensity distribution within the waveguide defect region.
|
System Specifications
| On-line Optics |
Type
|
Far-field, Near-field and Confocal Optics |
Accessibility
|
Free optical access along all axes for far-field observation, including from the direction of the probe fiber and the DUT and all perpendicular directions to the probe fiber/DUT axis. |
Detectors
|
InGaAs detector for Telecom Wavelengths, Avalanche Photodiode (APD), Photomultiplier Tube (PMT) |
| Video System |
On Line Video with Sensitivity to 2 ΅, CCD |
| Operating Modes |
| Far-field Confocal & CCD Profiling |
Transmission, Reflection, Collection
|
| Near-fieldOptical Profiling |
Transmission, Reflection, Collection |
| Refractive-Index Profiling |
Transmission and Reflectio |
| Topographic Profiling |
Contact or Non-contact |
| Thermal and Resistance Profiling |
Contact |
| Active Feedback Mechanism |
Atomic Force Sensing
|
| Optical, Topographic, Thermal, and Resistance Profiling Resolution |
| Optical |
from 50 nm |
| Refractive Index Profiling |
from 50 nm, depending on sample |
| Refractive Index Resolution |
Approximately 10-5, depending on sample
|
| Topographic |
from 100nm |
| Thermal |
from 100nm |
| Resistance |
from 100nm |
| Sample Positioning - Rough |
| Scanner |
Inertial piezo motion with piezoelectric Flat Scanner (thickness 7 mm) |
| Range |
6mm |
| Accuracy |
1΅m |
| Nanometric Scanning and Positioning |
| Scanner |
Piezoelectric Flat Scanner (thickness 7 mm) |
| Nanometric Scan Range |
70 - 140 ΅m (z), 70 - 140 ΅m (x-y)
|
| Spatial resolution |
<1 nm (z), <5 nm (x-y) |
| Thermal & Resistance Imaging and Measurements |
| Temperature |
300°C or greater, depending on sample to be investigated |
Thermal Sensitivity
|
0.01°C
Measured Resistance Change per degree: 0.38 Ω/°C |
| Resistance Sensitivity |
0.001 Ω |
| Electronics and Software |
Control System
|
Nanonics/Topaz Controller.
Low noise control system for rough, fine scanning, and active feedback with nanometric resolution. |
Software
|
Quartz software for Nanonics/Topaz controller (Win 95/98 and NT).
Active feedback control software.
Real time image display, image acquisition (up to 8 channels) and analysis, 3D rendering, & ability to provide collages of multiple images.
Refractive index profiling deconvolution software integration of confocal channel with near-field/atomic force constraints |
|
|
