Double and single side-band suppressed-carrier optical modulator implemented at 1320 nm using LiNbO 3 crystals and bulk optics.

Transcription

1 Double and single side-band suppressed-arrier optial odulator ipleented at 13 n using LiNbO 3 rystals and bulk optis. Azad Siahakoun 1 and Sergio Granieri Departent of Physis and Applied Optis, Rose-Hulan Institute of Tehnology, Terre Haute, Indiana Kenneth Johnson Naval Surfae Warfare Center, Crane Division, Crane, Indiana 475. ABSTRACT We perfor double sideband and single sideband suppressed-arrier odulators using a Mah- Zehnder interferoeter in free spae. Two bulk LiNbO 3 rystals are used to odulate the optial bea at 1319 n in both branhes of the interferoeter. We present experiental results of the optial spetru using heterodyne easureent tehnique. These results show axiu arrier and sideband suppression of 4 db and 1 db respetively for 1 GHz odulation frequeny. Keywords: Single side-band suppressed-arrier odulation, RF signal proessing, eletro-opti odulator 1- INTRODUCTION Conventional intensity odulation involves a large arrier oponent while oherent single and double sideband suppressed-arrier have no arrier oponent. The suppression of an optial arrier leads to iproveent of iportant harateristis of RF fiber opti link suh as, linear dynai range and noise figure 1. When a signal is sent over a fiber, hroati dispersion auses eah spetral oponent to suffer different phase shifts along the link. Thus, the power in the photodetetor is severely affeted produing degradation of the link perforane. These penalties an be avoided by eliination of one sideband. In this paper we present the design of an eletro-opti odulator based in a z-ut lithiu niobate rystal together with its easured perforane harateristis. We apply the developed eletro-opti odulator in two odulation arhitetures: double and single sideband suppressed-arrier odulation. In the first appliation a odulator is used as phase odulator inside a Mah-Zehnder interferoeter. In the seond approah two odulators are plaed between polarizers and are used as aplitude odulators. In setion, we experientally deonstrate a double sideband optial odulation with suppressed arrier. In setion 3 the suppression of one of the two odulation sidebands is disussed. Finally, details of the design and fabriation of the odulators are presented in setion 4. Conlusions are given in setion 5. - CARRIER SUPPRESSION The experiental setup for arrier suppression is shown in Figure 1. A tunable solid-state laser provides the optial power for the input arrier at 1319 n. A polarizing bea splitter (PBS) divides the input optial bea into the two ars of a Mah-Zehnder interferoeter. A eletro-optial odulator plaed in one ar of the interferoeter is used as phase odulator (see setion 4 for ore details). The optial field at eah branh just before the seond bea splitter (BS) ay be expressed by 1 Postal address: 55 Wabash Ave., P. O. Box 19, IN E-ail: Azad.Siahakoun@Rose-Hulan.edu

2 E1( t) = E1 os( ω t), (1a) E t) = E os( ω t + ), (1b) ( φ where E 1, E, and ω are the field aplitudes and the optial arrier frequeny respetively, and φ is the relative phase shift. A half-wave plate is plaed between the laser soure and the polarizing bea splitter. The relative intensity between the two beas is ontrolled by adjusting the angle of the slow axis of this plate. Figure 1: Experiental setup for arrier suppression A linearly polarized light is passing trough the rystal parallel to the prinipal axis X, this is, 45 respet to the horizontal diretion in the laboratory. Two half-wave plates provide the desired linear polarization diretion at the input and output of the odulator. Using Eqs. (1), the output field after the seond bea splitter an be written as ( ω t) + E os[ ω t + φ ' + φ os( ω )] = E1 os t () where φ is overall onstant phase shift between two ars, and φ os( ω t) the phase shift produed by the odulator with ω the angular frequeny of the applied RF signal. The odulation depth φ is related to the rystal paraeters through the half-wave voltage V π as: φ = πv / Vπ, where the half-wave voltage is given by λ d Vπ =, (3) 3 n r l being λ the wavelength of the light, while l and d are the rystal length and the separation between two eletrodes respetively. In order to find the ondition to suppress the arrier oponent we expand Eq. () in ters of Bessel funtion as E( t) = E 1 E os( ω t) + E J ( φ ) 1 + high order ters, 16 J ( φ ) os( ω t + φ' ) { sin[ ( ω + ω ) t + φ' ] + sin[ ( ω ω ) t + φ' ]} where J n () is the n-order Bessel funtion. The first two ters in this equation is the arrier oponent, and the seond ter is the first order side bands. As an be seen fro Eq. (4), if the onstant phase fator φ =±18, and the aplitudes of the both beas are related by E1 = E J ( φ ), the arrier oponent is suppressed. The phase shift fator φ is set by varying the path differene between the ars of the Mah- Zehnder. Experientally, the phase differene is adjusted by oving irror M 1 that is attahed to a PZT atuator. The heterodyne ethod is used to shift the optial spetru towards the eletrial-frequeny doain. A Lightwave 1 diode-puped solid-state laser with 1319 n wavelegth provides the optial power for the input arrier. The band of interest is seleted by ixing the optial output of the interferoeter with a loal osillator using a fiber oupler. A NLK1356STB single-ode seiondutor laser with W optial power is used as loal osillator. Using this onfiguration, the oupler output is deteted with a 1 + (4)

3 GHz Ortel 4515A fiberopti reeiver. The output spetral oponents are visualized with a Tektroni 4 RF spetru analyzer. In order to deonstrate the feasibility of the proposal, we perfor the arrier suppression by odulating the optial arrier with a sine wave at a frequeny of 1GHz and power level of 3 db. Figure shows the heterodyned optial spetrus entered at MHz. The ases (a) through () orrespond to different positions of the irror M 1. The axiu and iniu power in the arrier oponent is onsistent with φ =, ±18 respetively. The easured power values for the arrier in (a) and () are 18.8 db and 44.4 db respetively. As an be seen fro Figures (a) and (), the diret RF ter at 1GHz is attenuated as an be dedued fro Eq. (4). Figure : Heterodyned optial spetru for (a) φ =, (b) φ =± 9, and () φ =± SIDE-BAND SUPPRESSION In order to suppress one of the side bands we perfor an optial version of the Hartley irowave single side-band generator used in eletrial odulation. Two aplitude-odulated signal are obined with suitable phase shift fators in the optial arrier as well as in the odulating RF signal. The experiental setup is shown in Figure 3. The ain differene with the setup desribed in Figure is that a seond eletroopti odulator is plaed in the Mah-Zehnder interferoeter. The experiental setup is siilar to the one presented in the last setion, however, in the present ase, both eletro-opti odulators work as intensity odulators. The linear polarized light propagates though the rystal along the Z-axis with its plane of polarization initially inlined at 45 to the privileged diretions X and Y as is shown in Figure 5(a) (see setion 4 for ore details). The total retardation between the S-oponents along both axes after the appliation of the voltage V is: Γ = πv / V, with the half-wave voltage given by π d V = λ π 3 n r l. (5) A half wave plate, plaed after the polarizing bea splitter, is used to set properly the plane of linear polarization before eah rystal. Notie that the polarizer P, plaed after the seond bea splitter, transits only the X-oponent of the eletri field. Thus, the field aplitude in eah branh of the interferoeter is 16

4 π V = E sin os os φ t Vπ ( ω t) ( ω + ) where V os( ω t) is the sinusoidal RF applied to the odulator, E is the input field aplitude and φ is a onstant phase fator. The optial signal given by Eq. (6) is intrinsially lak of arrier oponent. The arrier suppression in eah ar of the interferoeter is strongly related with the extintion ratio of the polarizer and the quality of the linear polarization state obtained fro the PBS. The output field of the Mah- Zehnder an be written as π V π V = E ( ) + ( + Φ) sin os ω t os( ω t) sin os ω t os( ω t + φ) Vπ Vπ where Φ is the phase shift between both applied RF signals while φ is the phase differene between arriers. The output field of Eq. (7) an be expanded in ters of the Bessel funtions as (6) (7) π V = E J1 V + os { os [( ω + ω ) t] + os[ ( ω + ω ) t + φ + Φ] [( ω ω ) t] + os[ ( ω ω ) t + φ Φ]} + high order ters. π + + (8) Figure 3: Experiental setup for side band suppression As it an be noted fro Eq. (8), setting the phase differene between arriers φ, and between the eletrial signals Φ, as ± 9, one of the sidebands will be eliinated. The 9 phase shift between eletrial signals an be obtained by driving one odulator with the desired signal and the other with its Hilbert transfor. Analogy to the last setion, adjusting the position of irror M1 sets the phase differene between the arriers. The phase shift between the eletrial signals is ahieved by plaing the odulators in an asyetri path with respet to the bea splitter BS. The phase delay is set to ±9 only for the appropriate frequeny that satisfies π ω =, (9) l l 1 where l 1, l, are the distanes fro the odulators to BS, and is the speed of light. The lasers are set to present a beating frequeny of GHz. The heterodyned output spetru, showing a residual arrier oponent is shown in Figure 4. The bias voltage on the PZT driver is adjusted to suppress the lower sideband (ase 4(a)) and the upper sideband (ase 4(b)).

5 Figure 4: Heterodyne optial spetru for two different positions of the irror M 1 that orrespond to arrier phase differenes of: (a) φ=9, (b) φ= DESING AND CHARACTERIZATION OF ELECRO-OPTIC MODULATORS The oponent in whih our odulators are based on is a lithiu niobate (LiNbO 3 ) rystal. This rystal has trigonal 3 syetry. The saples are Z-ut with a pair of eletrodes sputtered both sides. These eletrodes are perpendiular to the X-axis as depited in Figure 5(a). The diensions of the saples along the X, Y, and Z diretions are 9, 9, and 5 respetively. In an anisotropi ediu the refrative index hanges with both the diretion of polarization and propagation of the light. This index hanges an be easily visualized by eans of the index ellipsoid 3. When an eletri field is applied aross the ediu the prinipal axes diretions are odified. Figure 5: (a) Sheati of the LiNbO 3 rystal showing the rotation of the prinipal axes under applied voltage. (b) Mathing board: the lengths s and d related with the stub are hosen to ahieve 5 Ω athing ipedane at 1 GHz. As an be seen fro Figure 5(a), new axes X and Y results fro a rotation of the axes X and Y about the Z- axis by an angle of 45 in the presene of an external eletri field along the X-axis. The refrative indies along the new prinipal axis X and Y are 1 3 ' ( E) = n n r E, (1a) 1 3 ' ( E) = n + n r E. (1b) n x 16 n y 16

6 The rystal with the attahed eletrodes has an ipedane with apaitive behavior when a RF signal is applied. In order to adjust the ipedane between the rystal and 5 Ω transission line a athing board is required. The athing is ahieved by a irostrip with a tuned stub ethed in a ooper board. Figure 5(b) shows the sheati of the board. The board aterial is RT/duroid oz. with a board height of.8in and a ooper thikness of 34 µ. The auray of the athing was deterined by alulating the voltage standing wave ratio (VSWR) for frequenies around 1 GHz. The VSWR is given by 1 + Γ VSWR =, (11) 1 Γ where Γ is the refletion oeffiient for the applied RF signal. To easure this oeffiient, an RF signal was applied to the board-rystal through an RF-irulator sweeping the frequeny around 1 GHz. The refleted signal out fro the irulator was opared with the input signal using a HP 87A Network Analyzer. Figure 6 shows the easured VSWR for frequenies fro.8 to 1.1 GHz. If the board-rystal ipedane athes exatly 5 Ω, the refleted signal and, onsequently, the refletion oeffiient should be zero. In this ideal ase the VSWR=1 as is stated in Eq. (11). The iniu of the urve of Figure 6 indiates.95 GHz as the frequeny for optiu ipedane athing. Figure 6: VSWR vs. frequeny for the rystal with the 5 Ω athing board. In order to test the eletro-opti odulators we plaed eah between two rossed polarizers. A S- polarized bea, inident to the rystal, is parallel to the Y-axis (see Figure 5(a)). Thus, an aplitude odulated output signal is obtained when a sinusoidal RF signal is applied to the eletrodes. To easure the d-v π paraeter, a d voltage is applied to the eletrodes while the output optial power is easured with a photodetetor. Figure 7(a) shows the transission of the aplitude odulator when a d voltage fro through.5 kv is applied. Fro this urve a value of. kv for the d-v π is alulated. The odulation effiieny is easured plaing a quarter wave plate before the rystal in order to work in the linear response region. The RF signal fro a Fluke 661A synthesized signal generator was sent to the odulator through a Mini-iruit LZY- aplifier with 44 db gain in the GHz. bandwidth. The output RF power was easured using a 1 GHz Ortel 4515A photodetetor and a Tektroni 71 RF spetru analyzer. The odulator is driven with a 1 GHz frequeny single-tone RF signal. In figure 7(b) the output RF power versus input power fro the signal generator fro db to 8 db is plotted. 5- CONCLUSIONS The design and fabriation of a narrow band eletro-opti odulator have been desribed. Several harateristis suh as ipedane athing, d half-wave voltage and odulation effiieny are easured. In addition, two different optial odulation tehniques have been ipleented using the usto eletroopti odulator. Double and single sideband suppressed-arrier optial odulators are ipleented using Mah-Zehnder interferoeter and bulk optis in free spae propagation. Suppression of the arrier by 5 db and 1 db for the sideband are ahieved.

7 Figure 7: (a) Transission versus DC voltage applied to the eletrodes. (b) Output RF power versus input power fro the signal generator. ACKNOWLEDGMENTS The authors would like to thank Dan Purdy of Offie of Naval Researh for his support of this projet under the ontrat nuber N REFERENCES 1. M. Farwell, W. Chang, and D. Huber, Inreased Linear Dynai Range by Low Biasing the Mah- Zehnder Modulator, IEEE Photonis. Tehnol. Lett., 5, , K. Yonenaga, and N. Takahio, A fiber hroati dispersion opensation tehnique with an optial SSB transission in optial hoodyne detetion systes, IEEE Photonis. Tehnol. Lett., 5, , A. Yariv, Optial Eletronis, Chapter 9, Holt, Rinehart and Winston, New York, 1985.