Holographic technology is the use of interference and diffraction to record and reproduce objects in the real principles of three-dimensional image recording and reproduction technology. The first step is to use an object light wave interference principle recorded information, namely, the shoot: photo objects are formed in the laser beam diffuse type of thing; another part of the laser beam shines as a reference on the holographic film, and material superposed beam interference, the object light points on the phase and amplitude of changes in conversion intensity in space, thus using interference fringe contrast and the spacing between the object light of all of the information recorded.
Highly multiplexed graded-index polymer waveguide hologram for near-infrared eight-channel wavelength division demultiplexing Ray T. Chen, Huey Lu, Daniel Robinson, and Tomasz Jannson Physicai Optics Corporation, 2545 West 237th Street, Suite 3, Torrance, California 90505 (Received 19 April 1991; acceptedfor publication 7 June 1991) An eight-channel single-modewavelength division demultiplexer operating at 740, 750, 760, 770, 780, 790, 800, and 810 nm with diffraction angle varying from 16”to 44”and using a graded index (GRIN) polymer waveguideis reported for the first time. Diffraction efficiency up to 55% was measured.The wavelength spreading of the Ti:A1203 laser ( -4 nm, 3 dB bandwidth) causesan averagecrosstalk figure of - 15.8 dB. The beamwidth of the diffracted signal as a function of the input beamwidth, the grating interaction length, and the diffraction angle are considered.Occurrence of the maximum value is further discussed.A waveguide lens is neededto efficiently couple the diffracted light into an output fiber whenever the diffracted beam size is beyond the core diameter of the fiber involved. Wavelength division multiplexing (WDM) and de- and dry and wet processingconditions have to be standard- multiplexing (WDDM) devices have been under intensive ized to validate the design process. To fabricate a wave- researchfor the past 15 years. Many WDMs and WDDMs guide hologram with a desired grating spacing, a well-col- that use absorption and/or interference filters”* and dif- limated beam is introduced onto the waveguide emulsion fraction gratings3” have been reported. Wavelength divi- 4 containing the hologram.‘ The object and referencebeams sion multiplexing and demultiplexing is a promising tech- thus generated require a very short temporal coherence nique for both optical communication and sensingsystems. length of the laser beam. For each Qj, two rotational angles Multiplexing an array of signal carriers with different op- qj and wit which control the value of Aj and the diffraction tical frequenciesgreatly enhancesthe transmission capac- angle 0, respectively, are selectedfor the waveguide holo- ity and the application flexibility of an optical communi- gram such that cation system. The dispersion characteristic of the diffraction grating provides an opportunity to employ pj=COS- (12,/2Aj) ‘ (21 to WD(D)M devices for optical encoders718 detect both linear and rotational positions. We have developed four- channel visible (543, 594.1, 611.9, and 932.8 nm)’ and and five-channel near ir (730, 750, 780, 810, and 840 nm) lo single-mode wavelength division demultiplexers using a Wj=0*5(7T - cjli) (3) graded index (GRIN) polymer waveguidein conjunction with a highly multiplexed waveguidehologram. Due to the can be satisfied simultaneously. In Eq. (2), jlR is the re- index tunability of the polymer guide, the reported device cording wavelength The beam size of the collimated beam can be implemented on an array of substrates.“-13 is much larger than the interaction length (submillimeter), In this publication, we are reporting for the first time i.e., grating thickness (Fig. 1), of the waveguide. As a an eight-channel single-mode waveguide WDDM. The result, each waveguide hologram is formed by two plane center wavelengths of these channels are located at 740, waves. For a perfectly phase-matched,lossless, unslanted 750, 760, 770, 780, 790, 800, and 810 nm. To construct a transmission grating, the diffraction efficiency can be writ- highly multiplexed waveguidehologram, eight channelsin ten a? this case,the dispersion of the polymeric material was first determined within the wavelength of interest. The phase- matching condition associatedwith each grating and the (4) corresponding diffracted beam can be constructed after- wards. Note that the isotropic characteristic of the polymer thin film significantly easesthe fabrication of the associated where Anj and d are the associatedindex modulation and diffraction gratings. Anisotropic diffraction is eliminated in the interaction length, respectively, and g is a constant this case.For each fixed wavelength;lj, the associateddif- which varies between 0 and 1 depending on the polariza- fraction angle 0j is given by tion of the incident beam.l2 The sinusoidal nature of the device requires precisecontrol of 0j, An, and d in order to ej= 2 sin - ’( Aj/2N&jAj). (1) generatea highly multiplexed hologram with uniform fan- out intensity. d is controlled by the lithographic process Here Aj is the grating spacing and Nes. is the effective and An is manipulated through exposure dosage and wet index of the polymer waveguideat waveleigth AP To pre- and dry processing parameters.To introduce the desired cisely control the Bragg diffraction angle 0,+ N,,(n) has to index modulation, the exposuretime tj needed for the fib be measuredbefore hologram formation. Coating thickness hologram should satisfy the following equation15 1144 Appt. Phys. Lett. 59 (lo), 2 September 1991 0003-6951/91/351144-03$02.00 @ 1991 American Institute of Physics 1144 Downloaded 15 Nov 2002 to 184.108.40.206. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp M”k@e Wavelenpfh Incident Beam Graded l&x Polymer Mcm~t~d~re Waveguide FIG. 1. Reconstruction of the waveguide WDDM device using a Ti:A1,03 FIG. 2. Mode dots from the output prism coupler (Fig. 2) of the 8- laser. channel WDDM. swings from 0”to 90”. The maximal value of 500 ,um and 13~ ,=-$Ln[ -hnj+ (an,,,- :z:Ani)]/(Anm=- 6 occurs, whenI - i’ ‘ Ani) (5) i=l It is obvious from Fig. 3 and Eq. (6) that the maxi- where p is the sensitivity constant for the emulsion, E is the mum S value shifts to larger diffraction anglesas the inter- exposureintensity of the laser beam, Ani is the index mod- action length increases.In order to pigtail with an output ulation for the ith exposure and Anmaxis the maximum fiber, the spot size should be well matched with the core index modulation for the holographic material. Note that diameter of the fiber. It is obvious from the preceding dis- the exposure dosage needed to generate a fixed value of cussion that there is a tradeoff betweenthe channel density index modulation increasesas the number of holograms to and the value of 6 (d, s1, ej> over which a number of be multiplexed increases.This is due to the fixed value of diffracted lights with different wavelengths can be effi- An max and the linear responseof the film. Equation (5) has ciently coupled into the fiber array. With a 6 value larger been experimentally confirmed and further results will be than the core diameter of the output fiber, a focusing lens presented in the future. For the present WDDM device, the gratings were designed to operate at the diffraction angles of 16”, 20”, 24, 28”, 32”, 36”, 40”, and 44”, to selec- tively disperse signals at the center wavelengths of 740, 750,760, 770, 780, 790, 800, and 810 nm, respectively. For each /l,) the corresponding ~j and Wj were selectedto gen- erate a waveguide transmission hologram with the desired Aj and 19,. Figure 1 shows the reconstruction of the wave- guide WDDM device using Ti:AlzOs laser light as the in- put signal. The interaction length of the multiplexed wave- guide hologram is 0.33 mm. The mode dots coupled out of the prism coupler are shown in Fig. 2 with the correspond- ing wavelengths as indicated. The observation of these clean mode dots verified the quality of the polymer wave- guide. A propagation loss of less than 0.5 dB/cm has been routinely achieved in a Class 100 clean room environment. As was previously reported, the channel density13of the WDDM device is a function of Ani, d, and 0) The corre- lation of these parameterscan also be observedin Eq. (4). A higher index modulation and longer interaction length provide us with a narrower FMHW (full width at half maximum) diffraction spreadingand, thus, higher channel density. With an incident beamwidth of 0, the beamwidth S of the associateddiffracted beam is a function of a, d, and 0) The computer-simulated results with R = 100 pm FIG. 3. Diffracted beamwidth as a function of grating interaction length are further illustrated in Fig. 3 where d variesfrom 10 to angle pm. anddiffraction with n = 100 1145 Appl. Phys. Lett., Vol. 59, No. 10, 2 September 1991 Chen et al. 1145 Downloaded 15 Nov 2002 to 220.127.116.11. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp 580- WDDM (740, 750, 760, 770, 780, 790, 800, and 810 nm) z with channel separationof4” in spaceand 10 nm in wave- 8 0 & 500- length has been demonstratedwith an averagecrosstalk 9 k figure of - 15.8 dB. The spreadingof the Ti:Al& laser s 42.0- ,o 0 0 0 0 0 turned out to be the major source of the crosstalk from 6 adjacentchannels.Theoretically, WDDM channelspacing 8 34.0- 0 0 as small as 1 nm is plausible under the current design F criterion. To understand the effect of finite beam size and E - E 26.0 interaction length, the diffracted beam spot size was also considered.The variation of beam spot size as a function of diffraction angle and grating interaction length with 100 Wavelength(nm) ,um input beamwidth is presentedhere. A diffraction spot whose beamwidth is larger than that of its output fiber FIG. 4. Diffraction efficiency of an I-channel WDDM. (The 33% chan- requires the implementation of a waveguidelens array to nel represents- 50 m W ditfracted power). enhancethe coupling efficiency. Further results on wave- guide lens arrays will be presentedin a future publication. the The device reported here not only enhances band- array is needed to provide high coupling efficiency. The width of optical interconnectsbut also provides us with a locally sensitized holographic emulsion can be employed new avenuethrough which dispersion-sensitive optical sen- for this purpose.Both holographic lensesproducedby two- can sors, such as optical encoders,7 be realized.The GRIN beaminterferenceand contact printing lensesproducedby property of the polymer waveguideallows us to implement optical lithography based on the waveguide holographic the reported device on an array of substrates.Such univer- material reported here can be fabricated. Recently, poly- sality greatly enhancesthe application scenariosin which mer waveguidelenseson Si and GaAs have been success- fully demonstrated. Further results will be presentedin a ” the conventional guided wave devicescan be used. future publication. The noise of the fan-out channelsof the This researchproject is sponsoredby SDIO. Helpful WDDM device is mainly from the crosstalk of the signals discussionswith Bill Martin from Army SDC and encour- from adjacent channels. An average crosstalk figure of agement from Carl Nelson of SDIG are gratefully ac- - 15.8 dB was measuredwith diffraction efficiency from knowledged. 33% to 52% among these output channels (Fig. 4). The spectral width of the Ti:A1z03 laser from spectrophysics was also measured.A - 3 dB bandwidth of -4 nm was found. The results suggestthat the W D M devicesof better than 4 nm wavelengthseparationcannot be experimentally I. ‘ Bennion, D. C. Reid, C. J. Rowe, and W. J. Stewart, Electron. Lett. 22, 341 (1986). realized without significant channel crosstalk. Theoreti- zW. V. Sorin, P. Zorahedian, and S. A. Newton, IEEE J. Lightwave cally, our device structure is capable of operating at a Technol. U-5, 1199 (1987). channel-to-channelspacing as small as 1 nm under the 3K. Kobayashi and M. Seki, IEEE J. Quantum Electron. QE-16, 11 current design when a DFB laser diode is employed.The (1980). - 15.8 dB crosstalk is primarily due to the wavelength 4S. Ura, M. Morisawa, T. Suhara,and H. Nishihara, Appl. Opt. 29, 1369 (1990). spreadingof the Ti:Al,Os laser rather than the waveguide Y. ‘ Fujii, J. Minowa, and Y. Yamada, IEEE J. Lightwave Technol. deviceitself. LT.2, 731 ( 1984). As far as the throughput intensity is concerned, the T. ‘ Von Lingelsheim, IEEE Proc. 131, 290 ( 1984). 33% diffraction efficiency representsan output power as R. ‘ T. Chen, M. R. Wang, G. Sonek, and T. Jannson,Opt. Eng. 30,622 (1991). high as 50 m W . For a communicationsysteminvolving the sD. W. Seal, Final Report to NASA, Cont. No. NAS3-25345, 1989. reported WDDM device, the system power budget will be R. 9MM. Wang, R. T. Chen, G. Sonek,and T. Jannson,Opt. Lett. 15,363 determined by laser power, modulation speed, bit error (1990). rate, and detector sensitivity. Employing a p-i-n FET as the “M . R . Wang, G. Sonek, R. T. Chen, and T. Jannson, IEEE Photon, Technol. Lett. 3, 36 ( 1991). demodulation scheme,theoretically we can utilize a -0.5 R. *‘ T. Chen, W. Phillips, T. Jannson,and D. Pelka, Opt. Lett. 14, 892 m W semiconductor laser to obtain 1 Gbit/s communica- (1989). tion with a 21.5 dB signal-to-noiseratio. The abovepower “R. T. Chen, M. R. Wang, and T. Jannson, Appl. Phys. Lett. 56, 709 budget assumes50% diffraction efficiency, 1 dB waveguide (1990). propagation loss, 3 dB waveguidecoupling loss, 2 dB ho- 13R.T. Chen, Proc. SPIE 1374,20 ( 1990). 14R.T. Chen, M. R. Wang, F. Lin, and T. Jannson, Proc. SPIE 1213,27 logram excessloss, 4 dB fiber propagation loss, 5 dB sys- (1991). tem power margin and room temperature operation con- 5R. ‘ T. Chen, M. R. Wang, and T. Jannson, AppI. Phys. Lett. 57, 2071 duction with an amplifier noise figure equal to 4. The ( 1990). current design allows us to provide 60-channelmultiplex- eR. ‘ T. Chen, H. Lu, and T. Jannson, Topical Meeting on Gradient-Index Optical Systems,Technical Digest Series (Optical Society of America, ibility with the maximum value of index modulation set at Washington, DC 1991), PD2-1. 0.1. We report, for the first time, a single-mode,GRIN- “R. T. Chen, Final Report to Army Harry Diamond Lab, Cont. No. polymer-basedwaveguide WDDM device. Eight-channel DAAL02.91-C-0034, 1991. 1146 Appl. Phys. Lett., Vol. 59, No. 10, 2 September 1991 Chen et al. 1146 Downloaded 15 Nov 2002 to 18.104.22.168. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp
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