Addressing the requirements for RF photonics

Transcription

1 Invited Paper Addressing the requirements for F photonics George Brost AFL, 5 Electronic Pkwy, ome, NY 1441 brostg@rl.af.mil ABSAC his paper address the relationship between system requirements and device specifications and figures of merit for F photonic applications. 1. INOUCION Photonics has many attributes that makes it attractive for space-based platforms 1. hese include the size, weight, low loss, flexibility, and EI resistance of the optical fiber as well as the wideband capability of photonics. Some of the potential applications of microwave photonics in space-based platforms include F distribution links, antenna remoting, and true time delay. here remains however many challenges with respect to size weight, and power (SWaP) as well as the F performance requirements. he challenge to meeting these system requirements is, in the end, a challenge to improving the individual components. he starting point for discussing the insertion of photonics in space-based platforms is the point to point fiber optic link shown in Fig. 1. Here we show an externally modulated, direct detection link. o the basic link, various processing functions can be added, such as time delay, switching, or receiver pre-processing. From the F perspective, the performance impact of the photonic link appears very much like that of an amplifier, being characterized, in addition to frequency and bandwidth, by its gain (G), noise figure (NF), third order intercept point (IP3), as well as its power consumption. However, the photonic link is not developed as a complete subsystem, but rather the individual components are developed separately. In this paper we address the figures of merit which are used with these components, and in particular, the optical modulator. Laser F in in odulator Figure 1 Photonic Link. Processing P F out 1 Photonics for Space Environments IX, edited by Edward W. aylor, Proceedings of SPIE Vol (SPIE, Bellingham, WA, 004) X/04/$15 doi: /

2 5(357'&80(17$7,13$*( )UP$SSUYHG 0%1 7KHSXEOLFUHSUWLQJEXUGHQIUWKLVFOOHFWLQILQIUPWLQLVHVWLPWHGWYHUJHKXUSHUUHVSQVHLQFOXGLQJWKHWLPHIUUHYLHZLQJLQVWUXFWLQVVHUFKLQJH[LVWLQJGWVXUFHV JWKHULQJQGPLQWLQLQJWKHGWQHHGHGQGFPSOHWLQJQGUHYLHZLQJWKHFOOHFWLQILQIUPWLQ6HQGFPPHQWVUHJUGLQJWKLVEXUGHQHVWLPWHUQ\WKHUVSHFWIWKLVFOOHFWLQ I LQIUPWLQ LQFOXGLQJ VXJJHVWLQV IU UHGXFLQJ WKH EXUGHQ W 'HSUWPHQW I 'HIHQVH :VKLQJWQ +HGXUWHUV 6HUYLFHV 'LUHFWUWH IU,QIUPWLQ SHUWLQV QG 5HSUWV -HIIHUVQ'YLV+LJKZ\6XLWH$UOLQJWQ9$5HVSQGHQWVVKXOGEHZUHWKWQWZLWKVWQGLQJQ\WKHUSUYLVLQIOZQSHUVQVKOOEH VXEHFWWQ\SHQOW\IUILOLQJWFPSO\ZLWKFOOHFWLQILQIUPWLQLILWGHVQWGLVSO\FXUUHQWO\YOLG0%FQWUOQXPEHU 3/($6('175(7851<85)5077+($%9($''5(66 5(357'$7(''00<<<< 5(3577<3( '$7(6&9(5(')UP7 AUG 006 Conference Paper Postprint Aug 00 - Aug 004 7,7/($1'68%7,7/( &175$&7180%(5 AESSING HE EQUIEENS FO F PHOONICS In-House $87+56 George A. Brost 3(5)50,1*5*$1,=$7,11$0(6$1'$''5(66(6 AFL/SNP 5 Electronic Pky ome NY E*5$17180%(5 F35*5$0(/(0(17180%(5 604F G35-(&7180%(5 LINK H7$6.180%(5 N/A SN I:5.81,7180%(5 01 3(5)50,1*5*$1,=$7,1 5(357180%(5 AFL-SN-S-P ,1*01,75,1*$*(1&<1$0(6$1'$''5(66(6 AFL/SNP 5 Electronic Pky ome NY ',675,%87,1$9$,/$%,/,7<67$7(0(17 Approved for Public elease; distribution unlimited. SN / AFL/WS ,756$&51< ,7565( %(56 AFL-SN-S-P /(0(17$5<17(6 004 SPIE. his work has been published in the Photonics for Space Environments IX, Proceedings of SPIE VOL 5554 (004). he US Government has for itself and others acting on its behalf an unlimited, paid-up, nonexclusive, irrevocable worldwide license $%675$&7 his paper addresses the relationship between system requirements and device specifications and figures of merit for F Photonic applications. his paper has focused on the optical modulator, as it is the key component in achieving the required F performance within the constraints of space-based platforms. he modulator transfer function can be used to express the key parameters of the modulator, which are then used characterize the photonic link performance; G, NF, and IP3. odulation efficiency S is preferred over V because it accounts for the insertion loss. he input IP3 can also be calculated directly from the transfer function. 68%-(&77(506 F Photonics, optical modulator space-based platforms, photonic link 6(&85,7<&/$66,),&$7,1) 5(357 E$%675$&7 F7+,63$*( /,0,7$7,1) $%675$&7 Unclassified Unclassified Unclassified UL 180%(5 ) 3$*(6 10 1$0()5(6316,%/(3(561 Fazio Nash E7(/(3+1(180%(5,QFOXGHUHFGH 6WQGUG)UP5HY 3UHVFULEHGE\$16,6WG=

3 . odulation Efficiency. odulation efficiency is important to any photonic link, but it is particularly important to space-based platforms where total power is limited. It is a characteristic of the modulator. V π has been commonly used as figure of merit for external modulators, and has often been associated with modulation efficiency. Indeed, there has been a significant effort in recent years to reduce modulator V π. However, as we discuss below, V π is not the same as modulation efficiency. In this section we define the modulation efficiency and V π figures of merit for the external modulator. hese are based the relationship of the transfer function to the F gain. We define P in to be the optical power in the fiber which is input to the modulator, and P out to be the optical power coupled out of the modulator into the fiber. he optical power on the photodetector P, is P = η F P out, where η F is the optical loss in the fiber. We assume that P out is defined by the transfer function (V), such that P out = P in (V), where V is the applied voltage. Here the transfer function includes the input and output coupling losses as well as the modulator absorptive and bias losses. he C current in the photo detector is I = η P, where η is the photodetector responsivity. he F power depends upon the ac part of the photodetector current, which is given by I = [ P '( V ) η η ] V, (1) ac in b F F where d '( Vb ) = () dv V = V b is the slope of the transfer function at the bias voltage V b, and V F is the ac voltage applied to the modulator. he F gain, G, defined as the ratio of the F power out of the modulator to the F power into the modulator, can then be expressed as G = [ P ' η η ]. (3) in F where, and are the photodetector and modulator resistances. We define modulation efficiency S to be equal to the slope of the transfer curve S = '. (4) hen, the F gain is proportional to the square of the modulation efficiency. V π is a commonly used figure of merit for external modulators. For a ach-zehnder odulator (Z) the meaning of V π is well defined. It is the voltage which produces a 38 Proc. of SPIE Vol. 5554

4 π phase change between the two arms of the ach-zehnder interferometer. It is also the voltage that produces a maximum change in the output. he Z has a transfer function, given by 1 πv Z ( V ) = η [1 + cos( + ϕ)], (5) V where η is the modulator losses (coupling and absorptive) and ϕ is a phase angle. For a Z biased at quadrature the F gain of a photonic link can be expressed in terms of the photodector current I, G = I π V π π, (6) Alternately, the F gain can also be expressed in terms of the optical input power: G = Pinη mηη Fπ V π. (7) he V π figure of merit is specific to ach-zehnder modulators. However, for other types of external modulator, such as electro-absorption modulators (EA) and directional coupler modulators (C) it has been convenient to define an equivalent V π such that the expression for F gain is the same as that of the Z. his has led to two somewhat different definitions for V π equivalent, depending on which expression for the F gain is used. his V π equivalent can be expressed in terms of the transfer function. If equation (6) is used to express the F gain, then V π equivalent is defined as ( V ) V * π eq = π (8) ( V )' If equation (7) is used to express the F gain, then V π equivalent is defined as π 1 Vπ eq = (9) ' N where N is the normalized transfer function, given by N ( V ) =. (10) η m 3 Proc. of SPIE Vol

5 hen the gain can be expressed as G = I V π * πeq (11) or G = P η in mηη Fπ Vπ eq (1) It should be emphasized that V* πeq and V πeq are not interchangeable in equations (11) and (1). V* πeq and V πeq both use a normalized transfer function, but the normalization is defined in different ways. V* πeq is normalized with respect to the transmission at the bias voltage, while V πeq is normalized with respect to the transmission at 0 bias. he main issue addressed here is identification of the appropriate figure of merit to use for space based applications. V π has traditionally been used as the primary figure of merit for external modulators. For all other parameters remaining the same, a reduction in V π does correspond to improved modulator efficiency. However, V π is not the same as modulation efficiency. his is because V π is defined with respect to a normalized transfer function, and is therefore independent on the insertion loss of the modulator. For some analog applications power consumption is not a concern, and insertion loss can be overcome with increased optical power. For some high frequency applications drive voltage is a primary concern. For these applications V π may be the more relevant figure of merit for the modulator. For space-based applications power consumption is quite important, and the power budget allowed for the photonic link is generally limited. For such applications it is more desirable to have a single figure of merit for modulation efficiency that contains both V π and the insertion loss (η m ). he modulation efficiency S defined by equation (4) does just that. As seen by equation (3), it is the slope of the transfer function that expresses the modulator s effect on gain. hese points are illustrated in Figs he solid curves 1 and in Fig. correspond to two different transfer curves which have the same normalized transfer curve N, but different insertion loss. hese are cosine transfer curves of a Z with a V π = 1. Both would be characterized with the same value of V π, but the slope is reduced by the insertion loss, as indicated in Figure 3. In fact, it is the insertion loss that reduces the modulation efficiency, while the bias loss does not. Figure 4 shows the corresponding values of V* πeq and V πeq as a function of bias voltage. V* πeq and V πeq are both equal to V π = 1 at V = 0.5, which corresponds to the quadrature bias point, as the definitions for these FOs are defined relative to the Z biased at quadrature. However, the functional form is 40 Proc. of SPIE Vol

6 quite different. V* πeq goes to zero at the low bias point. However, the transmission and modulation efficiency (slope) also go to zero at the low bias. It would take an infinite amount of optical power to obtain the constant current I imposed by equation (11) N V Fig. ransfer curves 1 and, and normalized transfer curve N N ' V Fig. 3. Slopes of the transfer curves shown in Fig.. Fo(V) V πeq V* πeq V Fig 4. Figures of merit for the normalized transfer curve N. 5 Proc. of SPIE Vol

7 As illustrative examples consider the modulation efficiency of a LiNbO 3 Z with a 4V V π and insertion loss of 3 db (S = 0.). he polymer Z reported by Shi et al. 3 had a V π =0.8, but an insertion loss of over 13 db, with a corresponding modulation efficiency of S=0.1. Similarly, the EA modulator reported by Welstad et. al 4 had V πeq =1.1V, but with an insertion loss of over 7 db the modulation efficiency was only S= Noise Figure Noise figure is often more important of a concern for the system impact of the photonic link than the gain. While low gain can be compensated with post amplifiers, high link NF can put severe requirements on a low noise pre amplifier. Noise figure is defined in terms of the ratio of the SN in to the SN out, and for the photonic link can be expressed as 5 NF 1 Ns + N IN = 10 log( + + ) (13) G k G B where N S is the Shot noise which is proportional to I, and N IN is the IN noise which is proportional to I, k is Boltzmann s constant, and B is the temperature. Noise figure is reduced with increasing gain. With respect to the modulator, modulation efficiency is the appropriate figure of merit for NF, in the same way that it is for gain. In Figure 5 we plot NF as a function of modulation efficiency, for a photodetector current of 4 ma and a IN of -170 db/hz. For these parameters a modulation efficiency of 10 V -1 corresponds to a gain of about 1 db here are families of curves that depend on laser power and IN NF (db) S (1/V) Figure 5. Noise Figure vs modulation efficiency. 4 Proc. of SPIE Vol

8 4. SF / IP3 he linearity of a photonic link is often defined in terms of the two-tone SF for which the third order intermodulation term is the dominant spur. his is the case for sub-octave bandwidths. he SF is equal to the SN at the input power at which the intermodulation spur is equal to the noise. he SF then can be expressed as 3 SF = [ IP NF 10log( BW )], (14) 3 where IP3 is the input IP3 F power in dbm and BW is the bandwidth. Here it is of course assumed that the intermodulation term has a third order dependence on the input power. In cascading F components it is the IP3, G, and NF that are used in determining the over all system characteristics. he issue addressed here is the definition an appropriate figure of merit for a modulator that expresses the linearity. We start with of a two-tone input : V t) = Vb + V [sin( ω t) + sin( ω )] (15) ( 0 1 t We expand the transfer function in a aylor series about the bias voltage V b: = ( Vb ) + n= 1 n n ( )( V V 1 ( V ) b ) n! (16) where n n d = n dv (17) V = V b We keep terms to 5 th order. hen the power out at the fundamental is given by ( V ) = Vo + Vo + Vo (18) 8 96 For suboctave operation, the dominate spurious signal is usually due to the intermodulation term at ω 1 -ω, or ω -ω 1. his can be expressed as 7 Proc. of SPIE Vol

9 I ( V ) = Vo + Vo (19) 8 19 If the intermodulation term is dominated by the third order term, then it is meaningful to define the input IP3 point, determined by keeping only the first terms in equations 18 and 19, setting 1 = I, and solving for V o. hen V IP3 is given by 1 V IP 3 = 8 (0) 3 IP3can then be determined from V IP3. his is attractive as a modulator parameter because the IP3 point of the photonic link depends only on the modulator ( and ) and is used to determine the SF. his assumes that the photo-diode is operated at an optical power sufficiently below saturation such that it does not limit the SF. It follows from equation (5) that for an Z π V IP 3 = 8 V. (1) π hus, reducing V π may increase the gain and lower the NF, but it can compromise the SF. his is seen in Figure 6, where we plot the SF for a Z as a function of V π, 115 SF (db-hz /3 ) Pl = 40 mw Pl = 10 mw V π (Volts) Figure 6. SF of a Z as a function of V π. 44 Proc. of SPIE Vol

10 for two different laser powers. Here we have assumed 3dB insertion loss and laser IN is -170 db/hz. his behavior in the SF can be seen by examination of equation (14). As V π decreases the input IP3 also decreases according to equation (1). From equation (13) and Figure 5, the noise figure also decreases with increasing gain associated with decreasing V π (increasing modulation efficiency). hus, with decreasing V π, NF saturates while the input IP3 continues to decrease, resulting in a reduced SF. Analysis of equation (0) suggests that the appropriate strategy for increasing IP3, and therefore the SF, would be to minimize the third order derivative of the transfer function. his is indeed the strategy employed for many so called linearized modulators. However, this approach is usually limited because the fifth order term in equation (19) then dominates. 5. SUAY his paper has focused on the optical modulator, as it is the key component in achieving the required F performance within the constraints of space-based platforms. he modulator transfer function can be used to express the key parameters of the modulator, which are then used characterize the photonic link performance; G, NF, and IP3. odulation efficiency S is preferred over V π because it accounts for the insertion loss. he input IP3 can also be calculated directly form the transfer function. 6. EFEENCES 1. Photonic Aspects of odern adar, Henry Zmuda and Edward N. oulghlian, Ed., Artech House, Boston K.K. Loi, J.H. Hodiak, X.B. ei, C.W. u, W.S.C. Chang,.. Nichols, L.J. Lembo, and J.C. Brock, Low loss 1.3µm QW Electroabsorption odulators for High Linearity Analog Optical Links, IEEE Photon. echnol. Lett. Vol 10, pp , Y. Shi, W. Lin,.J. Olson, J. H. Bechtel, H.Zhang, W.H. Steier, C. Zhang, and L.. alton, Electro-optic polymer modulators with 0.8 half-wave voltage, Appl Phys. Lett., vol 77, pp 1-3, Welstand,,.B., J. Zhu, C.K. Sun, A.. Clawson, P.K.L. Yu, and S.A. Pappert, Combined Fanz-Keldys and Quantum-Confined Stark Effect Wavequide odulator for Analog Signal ransmission, J. Lightwave echnol., vol 17, pp497-50, Cox, C, in F Photonic echnology in Optical Fiber Links, ed. W. Chang, Cambridge University Press (00). 9 Proc. of SPIE Vol