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All-fiber wavemeter and Fourier-transform spectrometerMark Froggatt* and Tu ran Erdo gant

*NASALangley Research C enter , Ham pton, VA 23681t Th e Institute of Optics, University of Rochester, Rochester, NY 14627(Tel: (716) 275-7227; Fax: (71 6) 244-4936;e-mail: turan@optics.rochester.edu)Fiber phase gratings fabricated by ul travio le t id d i a t io n are known to couple light ou t the side of the

fiber [1,2]. Simply put, the angle of the out-coupled radiation, a, easured with respect to the Itorma1ofthe fiber axis, is related to the wavelength of l ight, A, and the per iod of the grating, A, bysins=n e .- A/A, where ne# is the effective refractive index of the f iber mode. Since the angle isdependent on wavelength, a s imple spectrometer can be made by directly measuring the out-coupledintensity vs. angle [3]. This can be accomplished by focusing the radiation onto a detector array.Wagener, et al, have demonstrated such a device with excellent performance using a chirped fibergrating to focus the light through an index-matched prism onto a line ar array [4].

In this paper we propose an d demonstrate an analogous but distinct all-fiber device in which the out-coupled radiation is measured directly on the cladding of th e fiber, making for a highly com pact, simpledevice. Rat her than measuring the far-field (focused) radiation, here we measure the interferencepattern formed between counter-propagating out-coupled beams. Mo nochrom atic l ight produces asinusoidal pattern, while polychromatic light produces th e Fourier transform of the spectrum. Here wedemonstrate this device, and describe how it can be used as a high-resolution wavemeter, a networkspectrum monitor (analogous to [4]), and a unique tool for i n situ analysis of the fabrication process orpost-fabrication testing of very long fiber Bragg gratings with demanding specifications (such as thoseused for dispersion compe nsation ).

Figure l(a) shows a diagram of the device inone possible configuration. He re it is assumedthat the counterpropagating light is produced byan in- line reflector whose reflectivity is chose n tooptimize the out-coupled signal at the expense oftransmitted loss (typically a few % is sufficient).A n air-gap reflector or a very short, broad-bandfiber grating may be used for th e reflector. If moresignal is desired with minimum added loss, aslightly less compact version is shown in Figurel (b ) , wherein two gratings and a loop are used.

,-CCDor

fiber grating reflector

cFigure 1: Two configurations for the Fiber GratingFourierTransform SDectrometer.

Th e light on the detecto r array is the superposition of the radiation from both gratings.T o demonstrate th is device, a fiber Bragg grating with a period of 533 nm (Bragg wavelength of 1545

nm) and a peak UV-induced index change of 1.4~10-3was used in the configuration of Fig. l(a). Thegrating had a gaussian profile with a FWHM of about 5 mm. A cleaved end-face of the fiber producedthe reflected wave. T h e inpu t consisted a tunable 780-n m external-cavity semiconductor laser (NewFocus), which coupled light ou t exactly normal to the fiber axis at a w avelength of 779.87 nm. The out-coupled light was measured using a standard, high-dynamic range (16-bit) CCD chip with a 9 pm pixelspacing. T h e chip was more th an a centim eter from the fiber since no special effort was made to removet h e protective window (which ended up being partly responsible for extraneous interference fringes).

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This setup was intended to demonstrate s imultaneously the abili ty of the device to function as awavemeter or spectrometer, and how a standard fiber grating designed for 1550-nm reflection may becharacterized using a 780 -nm source.

Figure 2(a) shows a se ction of each of four fringe patterns ob,tained by illuminating the device withfour different wavelengths. T h e wavelen gths are 776.72, 777.22, '777.72, an d 778.22 nm. No te tha t themere 4% reflection producing the counter-propagating wave yields a fringe visibility of about 38%.Figure 2(b) shows a plot of the average fringe intensity vs. position along th e array for each of the fourcases in Fig. 2(a).

Figure 2:patterns in part (a).(a ) Fringes measured at four different wavelengths; (b) average fringe intensity for each of the four

Although the fringes appear to be reasonably well defined, it is evident from the fringe intensity plotsin Fig. 2(b) tha t th e fidelity for this demonstration is not very good. Th is is due in part to the use of astandard, windowed CCD array positioned at some distance from the f iber, rather than contacting thearray to the fiber as shown i n Fig. l (a ).In theory, the spectrum I (A) is given by the magnitude of the Fourier transform of the measuredfringe intensity I( z ) minus the DC ontribution lo ,or

I I

where K, is the spatial frequency. T he transform may be readily com puted using a Fast-Fourier-Transform (FET) routine on a com puter, essentially in real time. For a fringe intensity pattern Z(z)consisting of N points (typically N is chosen to be 2m , where m is an integer), only the first N / 2 pointsare meaningful. The se correspond to spatial frequencies varying betw een 05 Kz I / p ,here p is thepitch of the detector array (p = 9 pm for the experiments here) . From this theory it can be shown thatthe range of measurable wavelengthsU about a nominal w avelength A is approxima tely given by

(2)For example, for p = 10 pm at 1550 nm, the range is AA - 42 nm. T he reso lu t ion of the measurablespectrum is determined by the length Np of the detector array (assuming the grating length exceeds th i s

A24neffPM G - .

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length). T h e minimum resolvable spatial frequency is Kz= 2n /Np , which enables us to approximatethe minim um resolvable wavelength in terval ail: a2 2 ~ a6 . =-.%?ffNP NSo, or example, an array with a pitc h of 10p m and 1024 pixels (1 cm long) has a resolution of 6il= 0.08n m a t 1550nm. For the 780-nm experiment demonstrated here, the theoretical range and resolution forp = 9 p m a n d N = 512 are AA - 12 nm and SA - 0.045 nm .

Taking the Fourier transform of the data sets in Fig. 2(b), we show he spectra for the tunable laser ateach of the four wavelengths shown in Fig. 3(a ). These may be compared to th e spectra measured on astandard Op tical Spectrum Analyzer (A ndo ) shown in Fig. 3(b) (with a 0.05 nm resolut ion). Th ewavelengths separated by 0.5 nm are clearly resolvable, although the spectra are somewhat noisy. Th is ismainly caused by extraneous interference fringes from the imperfect measurement system.

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oo+ . ,I775 776 777 778 779 780 775 776 777 778 779 780Wavelength (nm) Wavelength (nm)

Figure 3:Transform Spectrometer; (b) same spec tra measured on a standard op tical spectrum analyzer (linear scale).

(a) Spectra of tunable laser at four different wavelengths measured with the Fiber Grating FourierFor in situ or post-fabrication characterization of 1550-nm fiber Bragg gratings, the 780-nm source

wavelength is fixed. Th en th e opticalperid (ne#) of a short section of the grating may be monitoreddirectly as tha t section of th e grating is being fabricated. W e estimate th at standard (unb lazed) gratingswith a UV-induced index chang e as small as 5x10-5 may be monitored in this way.

In conclusion, we present a device concept that may be applied to an extremely compact wavemeteror spectrum analyzer for source stabilization or network monitoring, but w hich also forms th e basis for anearly ideal long fiber Bragg grating fabrication tool, in that it allows direct monitoring of the opticalperiod of the grating as it is fabricated.[l ] G. Meltz, W. Morey, and W. Glenn, In4iber Bragg grating tap, Paper TuG1, Opt. Fiber. Comm. Conf.(1990).[2] T. Erdogan and J. Sipe, Tilte d fiber phase gratings, . Opt. Soc.Am. A, 13,296 (1996).[3] P. St. J. Russell and R. U lrich, G rating-fiber coupler as a high-reso lution spectrometer, Opt. Lett., 10, 291(1985).[4] J.L. Wagener, T. A. Strasser, J. R. Pedrazzani, J. DeMarco, and D. J. DiGiovanni, Fiber grating opticalspectrum analyzer tap, Paper 448, Euro. Conf. onOpt. Comm. (1997).

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