Single-excimer-pulse writing of fiber gratings by use of azero-order nulled phase mask: grating spectral
response and visualization of index perturbations
B. Malo, D. C. Johnson, F. Bilodeau, J. Albert, and K. 0. Hill
Communications Research Center, P.O. Box 11490, Station H, Ottawa, Ontario K2H 8S2, Canada
Received March 22, 1993
Optical fiber Bragg reflectors have been written by irradiating the fiber from the side through a phase mask with asingle pulse of high-power 249-nm excimer-laser light. Efficient tapping of light to the radiation modes has beenachieved for light at wavelengths shorter than the Bragg wavelength. The photoinduced periodic refractive-indexperturbations have been observed directly with an optical microscope and are shown to have the same period asthe phase mask and to be highly localized on one side, the irradiated side of the fiber core-cladding boundary.
Recently it has been demonstrated that fiber Bragggratings can be photoimprinted in the core of an op-tical fiber by irradiating the fiber from the side withultraviolet light that passes through a silica glassphase mask.1' 2 In comparison with earlier fiber grat-ing writing techniques,3-5 the use of a phase masksimplifies significantly the optical apparatus neededto write the Bragg reflectors. The mask is compact,is easy to align, has reduced sensitivity to mechan-ical vibrations, and does not noticeably degrade onexposure to high-power ultraviolet light. Further-more the temporal coherence requirements on theirradiating light are reduced, thereby permitting theuse of a low-cost KrF excimer laser as the writinglight source. The first application of KrF excimerlasers in fiber grating writing was by Hill et al.
5
to write point by point the index perturbations ofa fiber mode converter grating. The application ofthe KrF excimer laser to write Bragg reflectors withthe external holographic writing technique4 was notfeasible, because the coherence of this laser lightsource was too low. With a phase mask, however,the same low-coherence KrF excimer-laser source canbe used to write fiber Bragg reflectors, thus elim-inating the need for expensive, frequency-doubled,XeCl excimer-laser-pumped dye-laser systems or line-narrowed injection-locked KrF excimer systems.
These advantages of phase masks suggest theirapplication for writing gratings in fibers by use ofa single pulse from an KrF excimer laser. The mo-tivation for single-pulse writing of Bragg reflectors isthat the gratings can be photoimprinted in the opticalfiber as it is being drawn from a preform, leadingto low-cost devices. Such a process may also beuseful as a means of fiber marking for identificationpurposes.
The writing of a Bragg fiber grating by a singleexcimer light pulse has been demonstrated already.6'7The light source used in these experiments is a line-narrowed injection-locked KrF excimer laser, whichensures the spatial and temporal coherence neededin the external holographic writing technique.4 Inthis Letter we demonstrate the use of a phase mask
for writing fiber gratings with a single pulse froma Lumonix EX-510 KrF excimer laser, and we reporton the characteristics of the resulting photoimprintedfiber gratings.
The experimental setup used for writing fibergratings with a phase mask is described in Ref. 1.The phase mask used in this research is made fromhigh-quality fused-silica glass and has a square-wavesurface corrugation with a period A = 1060 nm.Ultraviolet light passing through the phase mask isdiffracted into several different orders. Our maskis designed to suppress the amount of light thatis diffracted into the zeroth-order beam (<5% at249 nm). Thus most of the diffracted light (-80%)is contained in the ± 1-order diffracted beams.
The essential difference between the experimentsreported in this Letter and those described in Ref. 1is that the fiber in the current work is exposedto a single excimer-laser pulse at a fluence levelof approximately 1 J/cm2 , whereas in Ref. 1 thegratings were fabricated by irradiating the opticalfiber with excimer-laser pulses at 50 pulses/s for5 to 20 min at fluence levels ranging from 100 to500 mJ/cm2 . With the low-fluence multiple-pulseexposure conditions, the photoimprinted Bragg grat-ings have a period A/2 and thus reflect light infirst order at a Bragg wavelength given by ABragg =
2neff (A/2), where neff is the effective refractive indexof light in the optical fiber mode at the Braggwavelength. In contrast, the single-pulse exposureconditions result in gratings with a period A and havesignificantly different reflection and transmissioncharacteristics. This result is consistent with therecent report by Reekie et al.8 that a differentmechanism for photosensitivity is occurring duringthe formation of gratings produced with a single high-power light pulse.
In this research, we photoimprint the Braggreflectors by placing the phase grating in closeproximity to a section of bare optical fiber. We useAndrew Corporation D-type polarization-maintainingfiber, which has a cutoff wavelength of 1200 nm, abeat length LB = 7 mm at 1300 nm, a core-cladding
Fig. 1. Transmission spectrum of a Bragg reflector witha peak reflectivity of 99.5%.
An = 0.031, and an elliptical core with dimensions2 -nm X 4 /,tm. The phase grating is placed suchthat the corrugations in the silica slab are adjacent tothe curved side of the D-fiber cladding with the longaxis of the corrugations oriented perpendicular to theaxis of the optical fiber. Light from the excimer laseris directed normal to the flat surface of the phasegrating and passes through. The diffracted beamsinterfere to form a fringe pattern that photoimprintsthe Bragg reflector. All gratings reported here havea length of 4 mm and are photoimprinted by a singlepulse of 249-nm light from the excimer laser havingan irradiation fluence of -1 J/cm2 on the cladding ofthe optical fiber.
Figure 1 shows the transmission response of a4-mm-long Bragg reflector written in the D-type fiber.For wavelengths longer than 1535 nm, the fiber has100% transmission. At 1535 nm, a sharp dip intransmission occurs as a result of light reflectionby the Bragg reflector (peak reflectivity 99.5% atABragg = 1535 nm). The FWHM of the reflection peakis 3.5 nm, which is much broader than the spectralwidth of 0.5 nm expected for a uniform Bragg gratingof length 4 mm. For wavelengths shorter than1535 nm, the single-pulse Bragg reflector transmitsonly 20% of the light in the measured range. TheBragg grating acts as an efficient tap (80%) forcoupling out of the optical fiber, light at wavelengthsshorter than the Bragg wavelength. The reflectionand transmission spectra obtained for this photoim-printed Bragg reflector are typical of Bragg gratingsthat do not extend uniformly across the core of theoptical fiber. Similar spectra have been obtainedfor Bragg gratings that are formed by etching asurface relief grating on the core-cladding boundaryof the optical fiber.9 The light lost (i.e., tapped out intransmission by the Bragg reflector) for wavelengthsshorter than the Bragg resonance is attributed tothe grating's providing phase-matched coupling of thereflected light into the cladding and radiation modesof the fiber.
The transmission response of the photoimprintedBragg reflector is sensitive to the fluence level ofthe UV light incidence upon the optical fiber. Pho-toimprinted gratings are not detected (reflectivity<5%) in fibers irradiated with a single 249-nm lightpulse having a fluence level 20% below 1 J/cm 2 .Figure 2 shows the response curve of a Bragg reflectorthat was photoimprinted with a fluence level that is
slightly lower (amount of decrease was not accuratelymeasured) than the fluence level used in fabricat-ing the Bragg reflector whose spectrum is shownin Fig. 1. The reduction in fluence level decreasesboth the Bragg peak reflectivity (-80% at ABragg =1534.2 nm) and the spectral width (FWHM = 1.0 nm).The light loss at shorter wavelengths is not presentin the spectral region shown in Fig. 2 but is shown inFig. 3, which is a transmission spectrum of the samephotoimprinted Bragg reflector over a larger spectralrange. The radiative loss extends from -1500 nm tobeyond 600 nm and increases from 20% at 1500 nmto over 80% at 600 nm.
A new feature in the transmission spectrum shownin Fig. 3 is the sequence of sharp transmission dipsthat occur at 1535, 1030, 770, and 620 nm. The lightat these wavelengths is not radiated but is reflectedback into the bound modes of the fiber. Since agrating with period A/2 cannot efficiently reflect lightat wavelengths of 1030 and 620 nm, we attributeall the reflections to a photoimprinted grating thathas a period A, equal to the period of the phasemask. Assuming the existence of this grating, thecondition for resonance is expressed as mAresonance =3070, where m = 1, 2, 3, 4, 5... is the order ofthe reflection. The constant 3070 is obtained bytaking the reflection at 1535 nm to be a second-orderreflection (m = 2) from the grating. The effective
Z 0.3
2-J
a:Z 0.2U)'1-
LdI arkB.-'-I
U,za:F- 0
1525 1530 1535 1540
WAVELENGTH (nm)
Fig. 2. Transmission spectrum of a Bragg reflector witha peak reflectivity of 80% in spectral region 1525 to1545 nm.
-a:zL9'-iU)
M
LiiI-'-I
U,za:M
100
ea
60
40
20
0 L_600 000 1000 1200 1400 1600
WAVELENGTH (nm)
Fig. 3. Transmission spectrum of the Bragg reflector asin Fig. 2 extended over the spectral range 600 to 1600 nmbut with a lower resolution than in Fig. 2.
Fig. 4. Photographic image of photoinduced perturba-tions (dark parallel fringes) as seen through an opticalmicroscope. Spacing between lines in image correspondsto the 1060-nm period of the phase grating.
index neff for light at 1535 nm is then given byneff = 1535/A = 1.44. Assuming that the light at theother resonant wavelengths has the same effectiverefractive index as at the 1535-nm wavelength andtaking m = 1, 2, 3, 4, 5,..., we obtain the seriesAresonance = 3070, 1535, 1023, 768, 614 nm, whichcorresponds closely with observation, except that theresonance at 3070 nm is not observable because ofthe high absorption of glass at this wavelength.
Using an optical microscope, we have been able toverify directly the existence of the A period gratingby observing the image of the photoinduced per-turbation produced by a single pulse of high in-tensity 249-nm light in the Andrew D-fiber. Theimage (see the photograph in Fig. 4) appears likea fringe pattern with the 1060-nm period of thephase mask. The high contrast in the fringe patternsuggests that a large photoinduced index changeis obtained. The irregularity in the borders of theperturbation pattern indicates that diffusion or melt-ing, and consequently a high temperature, obtainsduring the photoimprinting process. We have alsoobserved the photoinduced perturbations from theside, i.e., normal to the irradiating direction. The
photoinduced perturbations are highly localized onthe core-cladding boundary and do not extend acrossthe core of the fiber. The fact that we do not observea photoinduced perturbation with a period of A/2does not preclude the existence of a weak gratingwith this period since the spatial resolution of ourmicroscope viewing system is near its limit.
In summary, we have used a single high-powerlight pulse from an excimer laser in conjunction witha phase mask to photoimprint Bragg reflectors inoptical fibers. The transmission and reflection char-acteristics of the photoimprinted gratings are signif-icantly different from those photoimprinted at lowerintensities. The Bragg reflector functions as an effi-cient optical tap for light at wavelengths shorter thanthe Bragg wavelength. The photoinduced perturba-tions are localized on the core-cladding boundaryand do not extend across the core of the optical fiber.These results corroborate those in Ref. 9, where adifferent grating formation process is postulated tooccur at the high intensities used in producing grat-ings in the single high-power light-pulse regime.
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8. L. Reekie, J.-L. Archambault, and P. St. J. Russell,in Optical Fiber Communications Conference, Vol. 4of 1993 OSA Technical Digest Series (Optical Societyof America, Washington, D.C., 1993), paper PD-14,pp. 60-63.
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