The Application of Fiber Bragg Grating Filter in Lidar

The Application of Fiber Bragg Grating Filter in Lidar a

Bo Liu*a, Fan Yia, Geng Yanga Laboratory of Middle and Upper Atmosphere, Wuhan University Abstract

Now a new technology has emerged called “Fiber Bragg Gratings” which allow very narrow bandwidth filters to be made in optical fibers. These fiber gratings have been applied to the communications industry at the 1.55 micron telecommunications wavelength, but there is evidence that the same technology will work at the 589nm wavelength of interest to Na Fluorescence Lidar. Because of its high reflectivity and narrow bandwidth, it can be used instead of present dielectric interference filters and Fabry-Perot etalon in the Lidar receiving system, which will drastically improve the optical efficiency and the signal-to-noise ratio of Lidar. It will also drastically simplify the Lidar receiver.

Key Words: Fiber Bragg Grating, Lidar, signal-to-noise ratio 1.

Introduction

1.1 Typical Lidar system A Lidar is similar to the more familiar radar, and can be thought of as laser radar. It transmits and receives electromagnetic radiation, but at a higher frequency. Lidars operate in the ultraviolet, visible and infrared region of the electromagnetic spectrum. At present, different types of Lidars are employed to obtain atmospheric measurements and use the data for studies of clouds, atmospheric composition, temperature, wind, ozone depletion,

Telescope

climate change and so on. A typical Lidar contains a transmitter, receiver and detection system. Its transmitter is a laser,

Beam

while its receiver is an optical telescope, and

expander

Lidars typically use extremely sensitive detectors

Laser

called photomultiplier tubes (PMT) or avalanche

Steering mirror

photodiode (APD) to detect the backscattered light. To enhance the signal-to-noise ratio, pick-up the feeble backscattered light by atmosphere from the background radiation, most of Lidars use interference filter and Fabry-Perot

Fiber optic Lens

1

etalon to isolate the background light . For

Digitizer

convenience, many Lidars focus the light

storage

Detector

received by the telescope on to a fiber and transmit it to detector. But to use interference

Data

Filter

filter or Fabry-Perot etalon, light must be coupled out of the fiber and made collimated by

Fig 1: A typical Lidar system with fiber

Optical Fibers and Passive Components, edited by Steven Shen, Shuisheng Jian, Katsunari Okamoto, Kenneth L. Walker, Proceedings of SPIE Vol. 5279 (SPIE, Bellingham, WA, 2004) · 0277-786X/04/$15 · doi: 10.1117/12.521465

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lens (Figure 1), which will not only make the system more complex and difficulty to adjust, but also bring additional attenuation to the signal light, so we think if the filter and fiber can be integrated. Fortunately, a new technology has emerged called “Fiber Bragg Gratings” which can fit us very well (Figure 2).It has been applied to the communications industry at the 1.55 micron telecommunications wavelength, but there is evidence that the same technology will work at the 589 Fiber

nm wavelength of interest to our Na Fluorescence Lidar 2. 1.2 Fiber Bragg grating and its fabrication

Optical circulator

Since the discovery of photosensitivity in optical fibers in 1978

Fiber Bragg filter

by CRC's Dr. Kenneth Hill 3, there has been a great interest in the fabrication of Bragg grating within the fiber optic core. The grating is based on the principle of Bragg reflection. When light propagates through periodically alternating regions of higher and

Detector

lower refractive index, it is partially reflected at each interface between those regions. If the spacing between those regions is

Fig.2: The detecting system with fiber

such that all the partial reflections add up in phase—when the

Bragg filter

round trip of the light between two reflections is an integral number of wavelengths—the total reflection can grow to nearly 100%, even if the individual reflections are very small. Of course, that condition will only hold for specific wavelengths. For all other wavelengths, the out-of-phase reflections end up canceling each other, resulting in high transmission. So it acts as a band rejection filter passing wavelengths that are not in resonance with the grating and strongly reflecting the wavelengths which satisfy the Bragg condition. The resonance wavelength of the Bragg grating is related to temperature and tension which can be used to tune its Bragg wavelength. To fabricate a fiber Bragg grating, manufacturers must permanently modify the refractive index of the fiber via the photosensitive effect. This is accomplished by exposing the optical fiber to ultraviolet (UV) light with a wavelength around 240 nm or less. Early demonstrations of Bragg grating fabrication used dual-beam interferometric approach 2, but the stability of the interference pattern could easily be compromised by mechanical vibrations. A more reliable method for volume manufacturing uses phase masks to contact print the gratings. A phase mask is itself a grating, etched in silica, which diffracts UV light at normal incidence into the +1 and –1 diffractive orders. These two orders interfere to create the desired interference pattern just behind the mask, which is where the fiber is placed. Typical exposure times vary from a few seconds to a few minutes, depending on the type and strength of grating 4.

2.

Design and calculation

2.1 Design of the Bragg grating and its numerical simulation In the uniform fiber Bragg grating, Bragg wavelength

λB

is given by simply equation:

λ B = 2 nΛ where n is the retraction index of the fiber core, and Λ is the period of the grating. By the couple-model theory (CMT) 5, the peak Bragg reflectivity can be estimated by equation:

R = Tanh 2 (K LB )

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where LB is the Bragg grating length, and K is the coupling coefficient, which can be given by K = π∆n

λB

The bandwidth of Bragg

grating ∆λ H is defined as full width half maximum (FWHM) by:

∆λ H  1  R λ B ±  = R(λ B ) 2  2  it can be calculated by the equation 6:

2∆λ H =

λ2B πnLB

(KLB )2 + π 2

Let the length of the Bragg grating filter be 4 mm, and Bragg wavelength of 589.158 nm, while and the fiber refraction index is 1.45,

Fig. 3 Reflectivity spectra of the fiber Bragg grating

then we can get index modulation ∆n of

filter with center wavelength on 589.158 nm

1.307 × 10 −4 , and the peak Bragg reflectivity is 98.5%, the bandwidth of the filter is about 40 pm (Figure 3). 2.2 Design of the couple lens Our Lidar’s receiver telescope is a Cassegrain telescope with effective aperture of 1 m, focal length of 8 m, and field of view is 0.1 mrad. To couple light from the telescope to the fiber we use a lens (Figure 4). With the fowling equations we can get the parameters of the couple Couple lens

lens: Telescope focal plane

fC Φ = F F D

1 1 1 + = e F + fC fC Fiber core

where F and D are focal length and effective aperture of the telescope,

fC

f C is the focal length of the

couple lens, and Φ F is the diameter of the fiber

e

Fig. 4 The couple lens configuration

core. When core diameter equate to 400 um, we get



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f C = 3.2 mm and e = 3.2 mm , it may be made up of not a single lens. Assuming the fiber with NA of 0.22, and the lens be AR coated, the couple efficiency can be no lesser than 90%. Suppose that insertion loss of the fiber optic circulator be 0.7 dB, then the receiving system efficiency will above 75%.

3.

Conclusion

We have designed a fiber Bragg grating as the narrow-band filter and its couple lens for our Na Fluorescence Lidar at the Na resonance wavelength 589.158 nm. By the numerical simulation we can see it has both very high efficiency and ultra narrow bandwidth, which can drastically improve the receiver signal-to-noise ratio of Lidar, even can make it to be operated on the daytime. Further more, it makes the Lidar receiving system simpler and easily to adjust, all of these performances are more excellent than the other filters used in Lidar at present. Because of these advantages it can almost be employed in all kinds of Lidars. We can believe that with the improvement of its fabrication technology and development of other correlative products, such as the fiber optic switch, fiber optic amplifier, it will bring a revolution to the Lidar receiving system.

Reference 1.

Li, Steven X., Bruce M. Gentry, C. Laurance Korb, Savyasachee Mathur, and Huailin Chen, “Capacitively stabilized etalon technology for spaceborne wind lidar application.” 19th International Laser Radar Conference, pp, 915-918, 1998.

2. G. Meltz, W. W. Morey, and W. H. Glenn, “Formation of Bragg gratings in optical fibers by a transverse holographic method.” Opt. Lett, 14, pp, 823—825, 1989, 3.

K. O. Hill, Y. Fujii, D. C. Johnsen, and B. S. Kawasaki, “Photosensitivity in optical fiber waveguides: Application to reflection filter fabrication.” Appl. Phys. Lett, 32, pp, 647—649, 1978,

4. K. O. Hill, G. Meltz, “Fiber Bragg Grating Technology: Fundamentals and Overview.” J. Lightwave Technol, 15, pp, 1263—1276, 1997, 5. T. Erdogan, “Fiber Grating Spectra” J. Lightwave Technol, 15, pp, 1277—1294, 1997 6. Fiber Bragg Grating, Academic Press, New York, 1999

* Contact [email protected]; phone 86 27 87686760-801; Laboratory of Middle and Upper Atmosphere, Wuhan University, Wuhan, China, 430079

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