## 2-Dimensional beamsteering using dispersive deflectors and wavelength tuning

Optics Express, Vol. 16, Issue 19, pp. 14617-14628 (2008)

http://dx.doi.org/10.1364/OE.16.014617

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### Abstract

We introduce a 2D beamscanner which is controlled by wavelength tuning. Two passive dispersive devices are aligned orthogonally to deflect the optical beam in two dimensions. We provide a proof of principle demonstration by combining an arrayed waveguide grating with a free space optical grating and using various input sources to characterize the beamscanner. This achieved a discrete 10.3° by 11° output field of view with attainable angles existing on an 8 by 6 grid of directions. The entire range was reached by scanning over a 40 nm wavelength range. We also analyze an improved system combining a virtually imaged phased array with a diffraction grating. This device is much more compact and produces a continuous output scan in one direction while being discrete in the other.

© 2008 Optical Society of America

## 1. Introduction

^{1}where large angle, active alignment is not required.

^{2}).

^{3}(MEMS). These mirrors operate considerably faster with multi-kHz sweeping speeds, but are limited to aperture sizes of a few millimeters. This trade-off between size and speed is a concern because a smaller aperture leads to a greater beam divergence which in turn decreases the range. Acousto-optic

^{4}deflectors are limited in their total angular range and electro-optic

^{5}crystals require kilovolt drive voltages to obtain an adequate angular range and aperture.

^{6}where the phase delay from each element in an array is given a phase delay such that emitting wavefronts match in the desired beam propagation direction. However, liquid crystals can only be tuned at kilohertz speeds.

## 2. 2-D Wavelength beam-scanning concept

_{m}and θ

_{i}the transmitted and incident angles respectively.

^{7}. This alone is insufficient for FSO pointing and tracking since objects exist in a 2-dimensional field of view. To reach the second dimension, we can include another diffraction grating that is oriented orthogonally to the first. This arrangement is shown in Fig. 1.

### 2.1 An AWG and a free space optical grating

_{awg}is the index of refraction of the waveguides and λ

_{m}is the central output wavelength within the m

^{th}diffraction order.

_{awg}, and the phase shift induced by the incremental length, ΔL, of successive waveguides.

_{N}, coming out of the waveguide array. The wavelength at channel N and order m is found by isolating λ

_{m}in Eq. (4) to arrive at Eq. (5).

_{o}, and that the output waveguides are centered such that θ

_{m}is zero at the middle of the output array. When these are assumed, the first term on the right hand side of Eq. (5) is equal to the wavelength hitting the middle of the array, λ

_{c}. These are found using Eq. (3). With these considerations, we now arrive at Eq. (6) where N is the output channel number and N

_{max}is the total number of channels.

_{v}) is re-imaged by the free-space grating demultiplexer before it is directed by the output lens (with f

_{o}focal length). The output angle is given by Eq. (8).

_{g}, tilted by θ

_{g}, at the Fourier plane and uses identical Fourier lenses with focal length f. The resulting output angle deflection caused by the diffraction grating is given by Eq. (9).

## 3. Experimental setup

^{th}diffraction order at 1550 nm. Since the AWG gives discrete outputs at specific wavelengths, the achievable scanning angles exist in a nearly rectangular array rather than a set of lines. The AWG channels were transferred into free space through a 635 µm pitch v-groove array. This led into a free space optical grating demultiplexer, taken from a Network Photonics WDM switch, consisting of a 100 mm focal length Fourier lens followed by a 3

^{rd}order, 300 lp/mm reflection grating (Newport catalog code 53-*-013R). The architecture is similar to that used in our prior work in developing a large channel wavelength demultiplexer

^{8}. Like the demultiplexer, the output from the two demultiplexers is a grid of single mode Gaussian spots where each one corresponds to a different wavelength. This is converted into a beam scanner with the addition of a gold mirror (to divert the light away from the components) and a Nachet Vision 8x, 25 mm focal length microscope objective to collimate and direct the beam.

_{awg}, of 1.5 and a ΔL of 199 µm to have the AWG operate in the 194

^{th}diffraction order at 1550 nm. We also assumed that there is no apodization from any apertures which might have limited the field. The figure shows 6 columns and 8 rows of achievable directions. Each row corresponds to a single channel from the AWG, and the horizontal separation between the rows is created by dispersion from the diffraction grating. The 6 columns are a result of fitting 5 FSRs (~8nm for the AWG) in the total wavelength range.

## 4. Experimental results

^{8}suggests that the insertion losses are uniform across the wavelength tuning range with approximately 8 dB loss.

^{2}beam diameter which matches the expectations of a bare fiber output being collimated by the microscope objective. We note that this beam waist can be changed by selecting a different collimating lens.

^{8}, we showed that in these 2D dispersive systems, the crosstalk inherent in the AWG and the linewidth of the optical source determines the amount of crosstalk between neighboring wavelength channels. The prior results also show that there is virtually no (>30 dB) crosstalk between channels which are separated by one FSR. Using the devices in our setup as an example, the Agilent laser linewidth is 100 kHz which is negligible compared to the 100 GHz AWG channel pitch. This means that the crosstalk is determined by the crosstalk between AWG channels which we measured as greater than 35 dB. Figure 7(a) also shows no apparent leakage of the optical power into neighboring channels.

## 5. Beam scanner with VIPA

^{10}, is a glass slab that acts much like a highly dispersive diffraction grating. The VIPA operates on the principle of virtual images interfering with one another, as illustrated in Fig. 9. These virtual images are of a line source which is created by focusing a collimated beam with a cylindrical lens. This line source is focused at the entrance aperture of the glass slab which is tilted at a small angle. The back surface of the slab is nearly 100% reflective while the front slab has a graded reflectivity to allow output light. In this arrangement, the light will undergo multiple reflections and create a series of staggered virtual line sources. The light leaks out of the front surface into free-space where the cylindrical waves created by the virtual line sources interfere with each other. The graded reflectivity equalizes the leaked power of successive reflections. Ultimately, this resembles a diffraction grating operating with a large tilt and very high order.

_{v}is the thickness of the VIPA, n

_{v}is its index of refraction, θ

_{v}is the tilt angle of the VIPA and θ

_{o}is the output angle.

_{o}is shown explicitly in Eq. (11).

_{o}and will therefore keep the optical power in diffraction orders that fall within this angular range. In a beamscanner, especially one used for secure free-space optical communications, only one diffraction order should exist within the field of view for each wavelength. To ensure this, the optical path length of the VIPA, t

_{v}n

_{v}, inherits a maximum limit. When this optical path length is small, the value of the last factor in Eq. (11) is significantly different at adjacent diffraction orders. This difference should be large enough such that the adjacent diffraction orders fall outside the possible range of θ

_{o}and only one diffraction order exists for each wavelength. This requirement is shown in Eq. 12 where f

_{#}is the f-number of the cylindrical lens.

^{st}order. VIPAs typically have a 1.5 index, 1 mm thickness. Figure 11(a) shows the output if we use a typical VIPA tilted at 2.5° where the spots at 1553 nm are circled. This figure reveals that multiple diffraction orders are created. As mentioned earlier, this is because the optical path, t

_{v}n

_{v}of the VIPA is too large.

## 6. Conclusion and discussion

## Acknowledgments

## References and links

1. | G. Nykola, G. Raybon, B. Mikkelsen, B. Brown, P. F. Szajowski, J. J. Auborn, and H. M. Presby, “A 160 Gb/s free space transmission link” in |

2. | M. Cole and K. Kiasaleh, “Signal intensity estimators for free-space optical communications through turbulent atmosphere.” IEEE Photon. Technol. Lett. |

3. | L. Zhou, J. M. Kahn, and K. S. J. Pister, “Scanning micromirrors fabricated by an SOI/SOI wafer-bonding process,” J. Microelectromech. Syst. |

4. | V. Nikulin, R. Khandekar, and J. Sofka, “Performance of a laser communication system with acousto-optic tracking: An experimental study,” Proc. SPIE |

5. | A. Yariv and P. Yeh, |

6. | B. Winker, M. Mahajan, and M. Hunwardsen, “Liquid crystal beam directors for airborne free-space optical communications,” in |

7. | I. Filinski and T. Skettrup, “Fast dispersive beam deflectors and modulators,” IEEE J. Quantum Electron. |

8. | T. K. Chan, J. Karp, R. Jiang, N. Alic, S. Radik, C. F. Marki, and J. E. Ford, “1092 channel 2-D array demultiplexer for ultralarge data bandwidth,” J. Lightwave Technol. |

9. | J. E. Simsarian, A. Bhardwaj, J. Gripp, K. Sherman, Y. Su, C. Webb, L. Zhang, and M. Zirngibl,, “Fast switching characteristics of a widely tunable laser transmitter,” IEEE Photon. Technol. Lett. |

10. | M. Shirasaki, “Virtually imaged phased array,” Fujitsu Sci. Tech. J. |

**OCIS Codes**

(050.1970) Diffraction and gratings : Diffractive optics

(080.1238) Geometric optics : Array waveguide devices

(060.2605) Fiber optics and optical communications : Free-space optical communication

**ToC Category:**

Diffraction and Gratings

**History**

Original Manuscript: May 15, 2008

Revised Manuscript: July 10, 2008

Manuscript Accepted: August 5, 2008

Published: September 3, 2008

**Citation**

Trevor Chan, Evgeny Myslivets, and Joseph E. Ford, "2-Dimensional beamsteering using dispersive deflectors and wavelength tuning," Opt. Express **16**, 14617-14628 (2008)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-19-14617

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### References

- G. Nykola, G. Raybon, B. Mikkelsen, B. Brown, P. F. Szajowski, J. J. Auborn, and H. M. Presby, "A 160 Gb/s free space transmission link" in Proceedings of Conference on Lasers and Electro-optics, (Washington, D.C., 2000), pp. 687-688.
- M. Cole and K. Kiasaleh, "Signal intensity estimators for free-space optical communications through turbulent atmosphere." IEEE Photon. Technol. Lett. 16, 2395-2397 (2004). [CrossRef]
- L. Zhou, J. M. Kahn, and K. S. J. Pister, "Scanning micromirrors fabricated by an SOI/SOI wafer-bonding process," J. Microelectromech. Syst. 15, 24-32 (2006). [CrossRef]
- V. Nikulin, R. Khandekar, and J. Sofka, "Performance of a laser communication system with acousto-optic tracking: An experimental study," Proc. SPIE 6105, 61050C, (2006). [CrossRef]
- A. Yariv and P. Yeh, Optical Waves in Crystals (Wiley, Hoboken, 2003), Chap. 8.
- B. Winker, M. Mahajan, and M. Hunwardsen, "Liquid crystal beam directors for airborne free-space optical communications," in 2004 IEEE Aerospace Conference Proceedings, (Big Sky. Montana, March 2004).
- I. Filinski and T. Skettrup, "Fast dispersive beam deflectors and modulators," IEEE J. Quantum Electron. 18, 1059-1062 (1982). [CrossRef]
- T. K. Chan, J. Karp, R. Jiang, N. Alic, S. Radik, C. F. Marki, and J. E. Ford, "1092 channel 2-D array demultiplexer for ultralarge data bandwidth," J. Lightwave Technol. 25, 719-725 (2007). [CrossRef]
- J. E. Simsarian, A. Bhardwaj, J. Gripp, K. Sherman, Y. Su, C. Webb, L. Zhang, and M. Zirngibl, "Fast switching characteristics of a widely tunable laser transmitter," IEEE Photon. Technol. Lett. 15, 1038-1040 (2003). [CrossRef]
- M. Shirasaki, "Virtually imaged phased array," Fujitsu Sci. Tech. J. 35, 113-125 (1999).

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