reslab.htmlWDBNMSWDBlCq *7#7@06> > > > > >.f>>>> >>>`?x>? ??*?> ?????9?????? Photonics Research Laboratory

Photonics Research Lab

Electrical Engineering

Location:
041 Dreese Laboratory
Department of Electrical Engineering
phone: (614) 292-7434

Faculty:
Betty Lise Anderson
Graduate Students:
Craig Liddle
Dirk Schoellner
Carolyn Warnky

Current Projects

Optical True-time delay for Phased Antenna Arrays

Phased antenna arrays require true-time-delayed signals for beam steering and shaping without squint or bandwidth limitations. Such time delays are conventionally produced electronically, an approach that is expensive and bulky. We have developed a new device for producing the time delays necessary to drive a large array, but produce them optically.[1] Our approach uses a recirculating beam in a 5-mirror cavity based on the White cell. This appraoch is massively parallel and intrinsically robust. One device, using a single commercial spatial light modulator, could simulataneously produce hundreds of different delays for thousand of antenna elements. [2]

Spatial Coherence

It turns out that the spatial coherence function of a source such as a laser has some very interesting properties- for one thing, it is directly dependent on the spatial or transverse modes of the laser cavity. Until recently, little attention was paid to this because of the inherent difficulties in measuring spatial coherence accurately and easily. We have developed a technique to measure the spatial coherence using a fiber optic version of Young's interferometer- the fiber ends act as the slits.[3] With this method, we routinely measure the spatial coherence on low power beams as far as 3w away from beam center (w is the beam's spot size), and can determine the presence and relative strengths of higher order modes even when those strengths are small (a few percent), when other methods fail. [4]

Other uses of Spatial Coherence

We are currently investigating exploiting the spatial coherence of source as a communication channel. [5] By modulating the refractive index profile of an electro-optic waveguide, you could change a laser beams's spatial mode structure and therefore its spatial coherence. The spatial coherence detector is an electro-optic version of the twin-fiber interferometer. The spatial coherence of a course can be modulated independently of the intensity, thus giving a single beam an added depth of multiplexing. Applications include high-speed free-space optical links such as optical interconnections, and opticl data links with simultaneous telemetry transmission. This technique is surprisingly insensitive to misalignment errors. [6]

Fiber Optic Sensors

At OSU we have come up with a new approach to fiber optic microbend sensing which promises to greatly increase both sensitivity and reliability, not to mention design flexibility. In conventional microbend sensors, small closely spaced bends are introduced using a two deformers with teeth on them. The power through the fiber decreases as the bend amplitude is increased, because the perturbation causes coupling of energy from guided to radiated modes. We say, since the mode coupling is a resonant effect, highly dependent on the choice of the bend spacing, why not build a sensor based on varying the spatial bend frequency, rather than the amplitude? [7] Then you can run up and down the steep slope of the resonance peak. We have demonstrated such a device, and shown through computer modeling that one can control the sensitivity through the number of bends, the shape of the bends, and of course their amplitude. In conventional microbend sensors, it is difficult to use a large number of bends because the fiber must either stretch or slip in order to be bent, resulting in breakage. In our approach, the fiber is already bent, merely compresses like a spring, so the number of bends is essentially unlimited.

Microbend Mode Strippers

Speaking of microbend sensors, an OSU student (Zheng Qi) discovered a wierd problem with commercially available microbend mode strippers. These mode strippers consist of two plates with ridges, and when they are compressed together around a fiber, the higher-order modes are stripped off. Mode strippers are useful tools for a variety of fiber optic applications. The wierd problem is that the modes are only stripped off in the plane of the compression, so the beam pattern changes from a circular beam containing all sorts of modes, to an elliptical beam that has had only half of the higher modes removed, see figure below. [8] If the plane of the microbends is changed, the ellipse changes accordingly! It is interesting that these have been used for years and no one seems to have been aware of this problem. Those using the strippers have probably not been getting the mode pattern they expected!

Below are images of the output of a fiber. Upper left: unperturbed fiber. Upper right: microbends applied along one axis. Note that higher order modes are removed from the top and bottom but not from the sides. Lower left: the microbends are applied in the opposite plane. Lower right: Fiber is passed through two microbenders, one in each plane. Note overall intensity is less but mode distribution is even.

Misalignment losses in fiber joints

Other sensor work relates to coupling losses in fiber optic butt joints. When two fibers are aligned end to end, the amount of light coupled across the gap depends on the gap spacing, and any offsets between the two fibers. This loss can be exploited for sensing applications, but existing models do not accurately describe the loss, intended as they were, for worst case predictions of splice losses in the telecommunications industry. To properly account for realistic distributions of energy among the fiber modes, we have developed a model that allows an arbitrary distribution both spatially across the fiber face and angularly throughout its cone of emission. This technique points up interesting features in the loss curves for small separations not predicted by other models, but observed in the lab. [9]
Because fibers are appealing for sensing applications, particularly in adverse environments, we are also conducting radiation hardness tests of fiber sensor components. We are interested to see, for example, if there is a change in a laser diode's near and far field output patterns over long exposure to gamma radiation. [10]

Laser Diode Structures

Vertical cavity lasers are a hot topic in optoelectronics because of their many advantages over edge-emitting types. They are highly coherent, and surface emitting, which means no more unpleasant cleaving and great easing of alignment procedures, as well as the greater ease of integration with electronic devices on the same chip. We have designed a vertical cavity laser that is actually a ring laser (rather than a conventional Fabry-Perot laser), [11] which allows for coupling to adjacent devices as well as surface emission. This allows a small part of the beam from one laser to be injected into an adjacent laser, for injection locking. By arranging a string of these, one can build a phased-locked surface emitting array for high power applications.
Laser diode arrays can be either coherent (all elements in phase, for applications in which beam quality is an issue) or incoherent (to get huge amounts of power out of a laser without blasting the facets off). Most current vertical cavity arrays are incoherent because of the difficulty of phaselocking two of these adjacent devices. Edge-emitting phase-locked arrays are commercially available, and we have looked into the possibility of using these for holographic optical interconnections.[12] To write a hologram, one needs two (or more) beams that are mutually coherent, and this is usually done by using one laser and a beamsplitter to pick off some of the energy. The beamsplitter produces a second beam coherent with the first. Bulk optics are to be avoided, however, because of size and alignment problems, so we want to use the mutually coherent phase locked elements of a phased diode array as the sources in an optical interconnection. The individual elements can be selected using monolithically integrated electroabsorption modulators.

Optical Computing and Interconnections

Holography is only one approach to optical interconnections. Other approaches include using banks of optical switches (liquid crystal, acousto-optic, etc. to shuffle and exchange beams to get them going to the right final address. One difficulty has always been how to convey the data and the address simultaneously without slowing down the system. One approach, proposed by researchers at OSU, is to use the two polarizations of the light- one carries the address information, the other the data.[13] The switch arrays themselves also have a large number of design issues, particularly efficiency and crosstalk. One approach under development at OSU to improve efficiency is to take advantage of total internal reflection. By arranging optical switches on a rigid substrate in such that the input beams are internally reflected off the substrate walls between switches, we can get much higher efficiencies than those possible with earlier generation schemes.

Acousto-Optics

Actually, I had nothing to do with this- Yih-Tyng Wu, a grad student in Biomedical Engineering at OSU, discovered how to accurately account for the conservation of momentum in the acousto-optic interaction. Previous accounts have been approximate, but Yih-Tyng figured out how to make it precise, using relativity. [14] All because of a homework problem in EE 833!

References

1. B. L. Anderson, S. A. Collins, Jr., C. A. Klein, E. A. Beecher, S. B. Brown, "Photonically Produced True-Time Delays for Phased Antenna Arrays," Applied Optics, 36(32), pp. 8493-8503, 1997.
2. E. A. Beecher*, B. L. Anderson, S. A. Collins, Jr., "Optical true-time delay using a switched compound cell device: demonstration and characterization," submitted to Applied Optics July 7, 1997. 3. B. L. Anderson and P. L. Fuhr, " Twin-fiber interferometric method for measuring spatial coherence," Optical Engineering 32: 5, 926-932 (1993).
4. L. J. Pelz and B. L. Anderson, "Practical use of the spatial coherence function for determining laser transverse mode structure," Optical Engineering, 34(11), pp. 3323-3328, 1995.
5. B. L. Anderson, L. J. Pelz, "Spatial coherence modulation for free space communication,"Applied Optics , 34,( 32), pp. 7443-7450, 1995.
6. L. J. Pelz, B. L. Anderson, "Robustness of spatial coherence multiplexing under receiver mislaignment," to appear in Applied Optics, February 1998.
7. B. L. Anderson and J. A. Brosig, " New Approach to Microbending Fiber Optic Sensors: Varying the Spatial Frequency," too appear in the January issue of Optical Engineering : (1995).
8. B. L. Anderson and Zheng Qi, "On the use of microbend fiber optic mode strippers: a cautionary note," Optics and Photonics News, 6(11), November, 1995.
9. K. M. Taylor, B. L. Anderson, "Misalignment Losses In Fiber Optic Joints Due To Angular Misalignment For Arbitrary Energy Distribution," Optical Engineering, 34( 12), pp. 3471-3479, 1995.
10. M. C. Hastings, B. L. Anderson, B. C. Chiu*, D. E. Holcomb, Effects of gamma radiation on high-power laser diodes, IEEE Transactions on Nuclear Science, 43 (3), pp. 2141-2149, June 1996.
11. B. L. Anderson, " Vertical Cavity Ring Laser," IEEE Photonics Technology Letters 6: 3, 330-333 (1994).
12. B. L. Anderson, T. B. DeVore, and B. D. Clymer, " Use of laser-diode arrays in holographic interconnections," Applied Optics31: 35, 7411-7416 (1992).
13. D. C. Butzer, B. L. Anderson, and B. D. Clymer, " A highly efficient interconnection for use with a multistage optical switching network with orthogonally polarized data," Applied Optics,, 34(11), pp. 1788-1800, 1995. 14. Y. T. Wu, B. L. Anderson, "Spurious momentum mismatch introduced by an approximate model of acousto-optic interactions," Applied Physics Letters, 68(22), pp. 3066-3068, 1996.

Awards

Larry Pelz won the Alumni Research Award, May 1995.

Larry Pelz won the Best Paper in Engineering at the 1992 Graduate Research Forum at The Ohio State University for his paper entitled "Determination of the Transverse Mode Structure in Optoelectronic Devices Using Spatial Coherence Properties and a Novel Measurement Technique" .

BLA 1/2/98
utring;)VgetInputStreamspec regionMatchesLjava/lang/Exception;#(Ljava/net/URL;Ljava/lang/St45515277 @@?~*q<NOfghov7q>Y^} 8 cNl4W~)W!&&B** ++,-S-./a001tĿ!!!!!!!! ! ! !!!K1t2F334E525566E7a7f77777!!!!!  67l",6f6U"@71t7 888888?RHH(FG(HH(d'h=/R@H-:LaserWriter 8 New York444EeEP9 reslab.htmlBetty Lise AndersonBetty Lise Anderson