| Active Device Applications |
The central role of photonics in the new millennium, which has often been predicted, is now becoming a reality. The Wavelength Division Multiplexing (WDM) revolution is upon us. Sprouting everywhere are the new technologies, companies, and devices that keep, in this revolutionary Internet era, information as photons for as many transfer operations as possible. The critical components that are poised to fuel the photonics explosion are already in view (see Figure 1).
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| Figure 1. Current Components in a WDM System | |
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Besides the devices currently being used in WDM systems, there are additional devices based on new technologies such as photonic band gap crystals, still developing in the wings, with their full impact still to be realized.
The further development of such devices with low loss, high throughput, and mass manufacture depends on characterizing, with ultra high resolution, the distribution of light in such structures in the near-field, at the output of the device, and the correlation of these microdistributions of light as the near-field gives way to the far-field. In addition, in many cases it is necessary to relate these microdistributions |
and nanodistributions of light to the exact topography, to the super-resolution material distribution (for example, dopant concentration), and to the exact spatial distribution of the electrical and thermal properties of the device. Near-field optics stands poised to fill all these needs for the developer and the manufacturer of components for WDM.
The Near-Field Optics Revolution
What is near-field optics, how has it proved itself capable of fulfilling the lofty requirements of this field and industry, and how can the researcher and manufacturer today rapidly acquire this great potential for device characterization and defect review? Near-field optical microscopy is one of the fastest growing imaging techniques. In this approach to optics [1, 2], light is transmitted through a sub-wavelength aperture and a sample is placed within the near field at a distance from the aperture that is a few times closer than the dimension of the wavelength of the light that is employed. To form an image, either the aperture or the sample is scanned, and the light is collected at each position of the aperture. The technique called near-field scanning optical microscopy (NSOM) has attained the highest optical resolution that has ever been achieved, approximately 50 nm with visible light. This breaks by an order of magnitude the actual resolution that can be obtained on most samples with visible light, and at the longer wavelengths normally associated with telecommunication components, the resolution improvement is even larger. In addition to this important resolution advantage of the method, a most significant aspect of the technique arises from the bridging role that it can play between optical imaging on the one hand and the rapidly growing imaging methodologies based on scanned probe microscopy (SPM) on the other (see Figure 2).
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Figure 2. Near-Field Optics (NSOM) plays a bridging role between conventional optical microscopy and atomic force microscopy
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In telecommunications, NSOM is developing into the only tool capable of looking at high-resolution distributions of light in optical and electro-optical devices with micro and nano dimensions. An indication of these developments is the investigations that have been performed on V-groove quantum |
wire lasers [3] and modulated,multiple quantum well communication lasers for communication applications [4]. The utility of NSOM in this area includes mode structure definition in optical fibers, other waveguides, and optical and electro-optical switches in which the mode characteristics are closely associated with every fraction of dB loss. There is little doubt that the application of NSOM in this critical role of imaging light distributions will increase in the Internet era, where the hottest components are optical and electro-optical devices such as, for example, fiber optic dense wavelength division multiplexing or WDM devices.
| From the modern inception of NSOM approximately 15 years ago [1, 2, 5], experimental difficulties have limited realizing the full potential of NSOM. These previous experimental constraints, principally the use of straight near-field optical elements based on the techniques introduced by Harootunian et al. [6], have limited the general applicability of the technique. Recently, there have been two major advances in NSOM instrumentation that resolve the experimental barriers to the effective implementation of NSOM. The first of these advances is the introduction of cantilevered near-field optical elements [7-9] (see Figure 3). This allows the combination of near-field optical measurements with the standard methods of atomic force microscopy. In such measurements, a simple diode laser is reflected off the cantilever onto a position-sensitive quadrant detector (see Figure 4). As the cantilevered tip moves in z as a result of alterations in the topography as the sample is scanned or as a result of alterations in the operation in the device, the signal on the |
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Figure 3. A coated, tapered, cantilevered near-field optical fiber element through which an argon ion laser at 488 nm is being transmitted. A fiber such as this can be used also to collect light from a laser or a fiber structure and to relate it to the topography from which the light emanates.
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quadrant detector is altered. The sensitivity of such a technique is atomic. In other words, a topographical alteration of single atoms is in principle capable of being detected.
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Figure 4. The method of control of the z position of the cantilevered tip. As the position of the tip is altered in z, the position of the reflected beam on the position-sensitive detector is altered, and this adjusts the position of the sample to keep the tip sample distance at the prescribed distance. |
The Advent of 3D Scanning Technology
The second development that is of importance to the telecommunications industry is the invention of 3D flat scanning technology [9,10]. This permits a uniquely flexible sample-handling system that readily allows integration into far-field optical measurements and permits waveguides, electro-optical switches, communication lasers, etc., to be appropriately scanned with the tip, either for the collection of light emitted from these structures or for the injection of light into these structures. These two developments, the cantilevered optical fiber tip and the 3D flat scanner, now bring this exciting new technology to aid the telecommunications industry in its quality control and in its development.
A near-field optical system based on these developments has been commercialized for general use by Nanonics Imaging Ltd. The Nanonics NSOM/AFM 100 (renamed MultiView 1000™), which is a general purpose system, can be used with any conventional optical microscope in the upright or inverted mode. The best choice for WDM component measurements is an upright microscope, such as a Zeiss Axiotech Vario, which provides rapid placement of the cantilevered fiber optic tip over the photonic component in which the light distribution is to be measured, while leaving the region below the sample completely free for electronic components, etc. It also permits injecting light with the subwavelength optical fiber tip into a waveguide structure, while collecting light after it has traversed the waveguide with a lens that can be placed below the structure. This is seen in Figure 5.
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Figure 5. A diagrammatic representation of the Nanonics MultiView 2000™ photonics defect review system. It permits not only collection of light from the surface of the structure using the cantilevered optical fiber, but also permits injection of light into structures such as a waveguide while simultaneously allowing collection of the guided light beam by a lens at the other end of the waveguide structure.
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An example of the type of results that can be obtained is the published data on a slab waveguide communication laser [4]. This is seen in Figure 6, where the predicted difference in the near-field and the far-field light distribution is highlighted. In essence, the axis of the ellipse, in going from the near-field to the far-field distribution, undergoes a ninety-degree rotation. The experimental results are shown in Figure 7. As can be clearly seen, the predicted rotation in the axis of the distribution of the light is measured for the first time using the Nanonics NSOM/AFM system by near contact with the laser surface using the AFM capability of the system and as the tip is raised in a controlled fashion into the far-field.
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Figure 6. A comparison of the predicted near-field and far-field light distribution from a slab waveguide communications laser.
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Figure 7. The experimentally determined far-field (A) and near-field (B) distribution of the slab waveguide laser shown in Figure 6.
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The profile of the laser emission can also be measured as a function of the current injected into this laser structure. These measurements are shown in Figure 8. In Figure 8A, the distribution of the spontaneous emission of the waveguide is monitored below the threshold current for laser action. In Figure 8B, the distribution of the emission is shown above the laser threshold. As can be seen, there are regions in the distribution where hot spots are present. This emission of the laser is compared with the AFM error signal. This signal is generated when the emission of the communication laser that is being imaged in Figure 8 is modulated. The tip can respond to this modulation, and the difference between the highest and lowest excursion of the tip for the two states of the electrical modulation of this semiconductor laser is called the error signal. As can be seen in Figure 8C, the error signal displays an inhomogeneity in the signal exactly where the emission has hot spots in the light distribution. One possible explanation for this behavior is that the error signal is diagnostic of the injection of charge into the laser; i.e, the cantilever deflects as the laser charge injection is modulated, and this error in the injection of the charge is potentially responsible for the error in the lasing action.
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Figure 8A. NSOM light distribution from the laser cavity in Figure 6 with an injected current that is below the threshold for lasing action.
Figure 8B. NSOM light distribution from the laser cavity in Figure 6 with an injected current that is above the threshold for lasing action.
Figure 8C. Injected charge distribution measured using the simultaneous atomic force capabilities with an injected current above the threshold for laser action.
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V-Groove Lasers
Another example of light distributions from waveguide structures is a V-groove laser that acts as a quantum wire (QWR). This structure also has waveguide regions with higher dimensionality, including a region which acts as a quantum well (QW), a vertical quantum well (VQW), or a slab waveguide. This is shown in Figure 9A, where an electron micrograph of the laser facet is shown, and a diagrammatic representation is shown in Figure 9B. The resulting light distributions from many of these regions are shown in Figure 10, with the light distribution in the region of the QWR shown in Figure 11A. The simulation of this light distribution is shown in Figure 11B. Figure 12 shows the alteration in this laser's mode structure with a 0.8nm alteration in its wavelength.
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Figure 9. An electron micrograph of the V-Groove laser structure (A) and a diagrammatic representation of the structure (B).
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Figure 10. The light distribution from different regions of the laser structure obtained using the Nanonics NSOM/AFM V-Groove system.
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Figure 11 A. Experimental light distribution of the quantum wire region of the V-Groove laser structure obtained using a Nanonics NSOM/AFM system, compared to a theoretical simulation of the light distribution (B).
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Figure 12. Alteration in the mode structure with the wavelength of emission from a V-Groove quantum wire laser system, obtained using the Nanonics NSOM/AFM system.
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The AFM capability of the system can be used to monitor the exact topography of the laser structure in relation to the emission of light. There is a 150-nm offset between the geometrical and optical center when Figures 13A and C are compared. These figures were recorded simultaneously, while Figure 13B was recorded immediately following the images in A and C in order to show that the shift in Figures 13A and C was not caused by drift in the system.
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| Figures 13A and C. Light distribution and topography of the laser structure measured simultaneously with the Nanonics NSOM/AFM System. Figure 13B was recorded immediately after these images in order to show that there was no drift in the image. |
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Figure 14. An AFM sensing nanopipette-based thermocouple.
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Other simultaneous data that can be obtained with this unique optical profiler is the distribution of dopant concentration, which can be monitored by measuring simultaneously the capacitance and the light distribution. Such measurements have the potential to correlate the doping of the semiconductor with the light output from the semiconductor laser structure.
Finally, various measurements can also be performed on fibers, fiber amplifiers, fiber couplers, and electro-optical and optical switches. In the images below (Figure 16), we see the distribution of light from a single-mode 488-nm fiber for different injection conditions of the light into the fiber. Evanescent fields in fibers (Figure 17) and waveguides (Figure 18) and the light distribution in such couplers as star couplers (Figure 19)[11] are all important parameters in WDM component characterization.
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Figure 15. The NSOM light distribution (A) and the thermal distribution (B) obtained with the Nanonics NSOM/AFM system.
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Figure 16. The near-field distribution of light at 488 nm of a single-mode fiber obtained by the Nanonics NSOM/AFM ConfocalTM.
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Figure 17. Near-field optical images of the evanescent field for TE and TM polarization of a semiconductor waveguide. From Applied Physics Letters Vol. 73, 1035-7 (1998)
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Figure 18. 10 nm deep topographic alteration correlated with light leakage from a waveguide that corresponds to a ~0.05 dB guided power loss. From Applied Physics Letters Vol. 73, 1035-7 (1998)
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Figure 19. Near-field optical image of the star coupler section of a phasar device. From Applied Physics Letters Vol. 73, 1035-7 (1998)
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In essence, any component manufacturer or developer in this photonics era is in need of the Nanonics NSOM/AFM system to characterize his/her structure, both from a quality-control perspective and from the perspective of a developer who needs to rapidly improve the optical characteristics of the device in question. As workers in this field learn more about near-field optics and the Naonics NSOM/AFM system, such measurements will become the standard for all investigators and developers in the field.
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