Optical phase locked loop for transparent inter-satellite communications

Opical phase locked loop for ransparen iner-saellie communicaions F. Herzog 1, K. Kudielka 2,D.Erni 1 and W. Bächold 1 1 Communicaion Phoonics Group, Laboraory for Elecromagneic Fields and Microwave Elecronics, Swiss Federal Insiue of Technology Zurich, CH-8092 Zurich, Swizerland 2 Conraves Space AG, CH-8052 Zurich, Swizerland herzog@phoonics.ee.ehz.ch Absrac: A novel ype of opical phase locked loop (OPLL), opimized for homodyne iner-saellie communicaion, is presened. The loop employs a convenional 180 3 db opical hybrid and an AC-coupled balanced fron end. No residual carrier ransmission is required for phase locking. The loop acceps analog as well as digial daa and various modulaion formas. The only requiremen o he ransmied user signal is a consan envelope. Phase error exracion occurs hrough applying a small sinusoidal local oscillaor (LO) phase disurbance, while measuring is impac on he power of he baseband oupu signal. Firs experimenal resuls indicae a receiver sensiiviy of 36 phoons/bi (-55.7 dbm) for a BER of 10 9, when ransmiing a PRBS-31 signal a a daa rae of 400 Mbi/s. The sysem seup employs diode-pumped Nd:YAG lasers a a wavelengh of 1.06 µm. 2005 Opical Sociey of America OCIS codes: (060.1660) Coheren Communicaions; (060.2920) Homodyning; (350.6090) Space opics. References and links 1. L. G. Kazovsky and D. A. Alas, A 1320-nm Experimenal Opical Phase-Locked Loop: Performance Invesigaion and PSK Homodyne Experimens a 140 Mb/s and 2 Gb/s, J. Lighwave Technol. 8, 1414 1425 (1990). 2. D. F. Hornbachner, M. A. Schreiblehner, W. R. Leeb and A. L. Scholz, Experimenal deerminaion of power penaly conribuions in an opical Cosas-ype phase-locked loop receiver, in Free-Space Laser Communicaion Technologies IV, Proc. SPIE 1635, 10 18 (1992). 3. S. Norimasu, K. Iwashia and K. Noguchi, An 8 Gb/s QPSK Opical Homodyne Deecion Experimen Using Exernal-Caviy Laser Diodes, IEEE Phoon. Technol. Le. 4, 765 767 (1992). 4. B. Wandernoh: 20 Phoon/Bi 565 Mbi/s PSK Homodyne Receiver using Synchronisaion Bis, IEE Elecron. Le. 28, 387 388 (1992). 5. W. R. Leeb, Opical 90 Hybrid for Cosas-Type Receivers, IEE Elecron. Le. 26, 1431 1432 (1990). 6. J. R. Barry and E. A. Lee, Performance of Coheren Opical Receivers, in Proceedings of he IEEE (Insiue of Elecrical and Elecronics Engineers, New York, 1990), pp. 1369 1394. 1. Inroducion Several phase locking schemes for opical homodyne deecion a nanowa signal power levels are known, namely he balanced loop [1], he cosas or decision-driven loop [2, 3] and he SyncBi loop [4]. In he applicaion of broadband iner-saellie communicaion, all of hese loops have specific benefis and drawbacks. The balanced loop is a simple design wih a convenional 180 3 db hybrid in he receiver, bu i requires he ransmission of a residual carrier. The (C) 2005 OSA 16 May 2005 / Vol. 13, No. 10 / OPTICS EXPRESS 3816

residual carrier increases he average signal power, which will drive an average-power limied opical amplifier, siuaed on he ransmiing saellie, more likely ino sauraion. The cosas or decision-driven loop does no need a carrier signal for phase locking, due o he deecion of in-phase and quadraure signal componens. To perform his operaion, i requires an opical 90 hybrid wih a coupling facor differen from 3 db (e.g., 90% in-phase, 10% quadraure). Compared o a 180 3 db hybrid, he complexiy of such a device is significanly increased [5]. The SyncBi loop circumvens boh he 90 coupler and he residual carrier ransmission, a he cos of high speed signal processing elecronics on he ransmiing and receiving saellie. Furhermore, i is resriced o purely digial daa ransmission. This aricle proposes a new ype of OPLL, which was specifically designed for broadband iner-saellie communicaion a nanowa signal power levels. I employs a 180 3 db opical hybrid wih an AC-coupled balanced fron end. The ransmied daa can be any analog or digial signal wih a consan envelope. Due o he variey of acceped inpu signals, an opical link wih such a receiver can be considered as ransparen o he user. Since he OPLL exracs he phase error signal by synchronously demodulaing a deerminisic LO phase disurbance, i is called diher loop. 2. OPLL principle of operaion The proposed OPLL design, in conjuncion wih he experimenal sysem seup, is shown in Fig. 1. A sinusoidal signal (called he diher signal) of small ampliude and frequency ω d is applied o he LO fas (piezo) frequency uning inpu. The diher signal ampliude is chosen such ha i generaes a LO phase deviaion of approximaely ±10, and he diher frequency has o be much higher (e.g., by a facor of 10) han he naural frequency of he loop. The LO phase deviaion propagaes hrough he balanced receiver and can be measured a he sysem oupu as a flucuaion in signal power. This is done wih a power deecor, followed by a bandpass filer. The bandwidhs of he deecor and he filer are chosen such ha hey do no limi he loop dynamics. In a muliplier, he power flucuaion is synchronously demodulaed wih he diher signal. The 90 delay before he muliplier has o be inroduced because of he frequency-o-phase conversion in he LO. The low pass filered oupu signal of he muliplier is proporional o he phase error and can be used for LO phase conrol. Figure 2(a) shows a signal-space represenaion for BPSK ransmied daa a differen phase error condiions, wih magnified views in subsequen Figs. 2(b) 2(d). In Fig. 2(b), he phase error is zero, herefore he LO carrier harmonically oscillaes around he in-phase axis. The deeced signal power a he sysem oupu is maximum when he diher signal has a zero crossing, and drops o a cerain level a a minimum and maximum of he diher signal. Thus, he main componen of he power flucuaion has wice he frequency of he diher signal. The bea produc of hese wo signals afer he muliplier is far beyond he loop filer bandwidh (a ω d and 3ω d ), generaing a loop filer oupu of zero. The LO phase remains consan, and he sysem is in a sable operaing condiion for zero phase error. In Fig. 2(c), he signal-space diagram is drawn for a posiive phase error. The main componen of he power flucuaion has he same frequency, bu is 180 ou of phase o he diher signal. Muliplicaion yields negaive low frequency componens, resuling in a loop filer oupu which is less han zero. Consequenly, in case of a negaive phase error, shown in Fig. 2(d), he power flucuaion is equal in frequency and phase, and he loop filer feeds a posiive value ino he LO frequency conrol inpu. In boh cases, he phase error will decrease. I mus be poined ou ha he dihered LO phase reduces he receiver sensiiviy, e.g. a diher ampliude of ±10 leads o a power penaly of 0.14 db. Such a power penaly is presen in every OPLL ype, eiher as incomplee modulaion of he carrier (balanced loop), quadraure componen deecion (cosas or decision-driven loop), or an increased noise bandwidh of he receiver (C) 2005 OSA 16 May 2005 / Vol. 13, No. 10 / OPTICS EXPRESS 3817

RF in Nd:YAG Transmier Laser Modulaor Driver Mach-Zehnder Modulaor Bias Conrol (Carrier Suppression) 180 3dB Coupler } {{ } Transmier Power Monior 180 3dB Coupler Opical Aenuaor }{{} Free-space loss simulaion 180 3dB Coupler Nd:YAG Local Oscillaor Laser Slow (hermal) uning Loop Filer sin(ω d ) Acquisiion Logic / Loop Conrol 90 Delay RX Amplifier Sage Bandpass AGC RF ou Power Deecor } {{ } Homodyne receiver Fig. 1. Experimenal sysem seup wih proposed new OPLL (blue indicaes opical componens). Free space losses are simulaed wih an opical aenuaor. (C) 2005 OSA 16 May 2005 / Vol. 13, No. 10 / OPTICS EXPRESS 3818

Q s d () Fig. 2(c) I I Fig. 2(b) s pwd () Fig. 2(d) Mean value = 0 (a) Signal-space diagram. (b) Magnified view for zero phase error. s d () Mean value > 0 s pwd () s d () s pwd () Mean value < 0 (c) Magnified view for a posiive phase error. (d) Magnified view for a negaive phase error. Fig. 2. Signal-space diagram for BPSK ransmied daa a differen phase error condiions. I: in-phase axis, Q: quadraure axis, : ime, s d (): diher signal, s pwd (): bandpass filered power deecor oupu signal. (SyncBi loop). From is communicaion performance, he presened phase locking scheme is comparable o previous designs. Bu he diher loop reduces he echnological complexiy by using a 180 3 db opical hybrid and an AC-coupled balanced fron end. Furhermore, he carrier-suppressed ransmier oupu signal is minimized in average power and herefore well suied for opical amplificaion. 3. Experimenal resuls An experimenal sysem has been se up as shown in Fig. 1. Opical sources for he ransmier (TX) and LO are diode pumped Nd:YAG lasers emiing a 1064 nm. All opical componens are fiber coupled wih polarizaion mainaining fibers. All elecrical circuis, excep he fronend, are made of HiRel (miliary or space) componens. The feedback loop has hree poles and a naural frequency of 10 khz. The hird pole has been inroduced o suppress 1/ f frequency noise componens of he lasers. The diher frequency is 110 khz, and he diher ampliude is approximaely ±10. For sho noise limied operaion of he receiver, a LO power of 10 mw has been chosen. Addiional circuis of he sysem include a modulaor bias conrol loop o suppress he opical carrier, and a field-programmable gae array (FPGA) based logic ha hermally scans he LO frequency for iniial acquisiion. A single phase-locking even is shown in Fig. 3(a). On he lef side of he oscillogram ( < 500µs), TX and LO frequencies differ by more han 10 khz. Due o his, he acquisiion logic (C) 2005 OSA 16 May 2005 / Vol. 13, No. 10 / OPTICS EXPRESS 3819

keeps he sysem in open loop mode. The power deecor oupu signal (lower race in Fig. 3(a)) follows he envelope of he bea produc of he wo opical sidebands. When he acquisiion logic deecs an inermediae frequency of 10 khz or less ( = 500µs), he loop is closed and he LO frequency and phase is adjused wihou cycle slip (500µs < < 800µs). The deeced power in closed loop mode decreases in Fig. 3(a) due o AC-coupling of he according signal. 2 Sho noise limi BER, calculaed from SNR Measured BER 3 (a) Single phase-locking even. Channel 1: AC-coupled power deecor oupu signal. Channel 4: Loop conrol signal. log 10 (BER) 4 5 6 7 8 9 10 11 12 70 68 66 64 62 60 58 56 54 Received opical power, dbm (b) Bi error performance from a PRBS-31, 400 Mbi/s ransmission experimen, and calculaed from SNR measuremen. Inse: Eye diagram a -54 dbm opical inpu power. Fig. 3. Measuremen resuls. Figure 3(b) shows resuls of a BPSK communicaion es, ransmiing a PRBS-31 signal a a daa rae of 400 Mbi/s. For a BER of 10 9, an average opical power of -55.7 dbm, or 36 phoons/bi, is required. This is a power penaly of 6 db o he sho noise limi of 9 phoons/bi. The low quanum efficiency of he phoodiodes accouns for he main opical losses (2.5 db). Addiional losses are due o phase dihering, he residual phase error, and opical losses in couplers and connecors. To verify he resuls of he BER es, an analog ransmission experimen has been performed. A single 800 MHz carrier was ransmied o measure he signal-o-noise raio (SNR) a he sysem oupu. When assuming Gaussian disribued noise processes, he BER can be calculaed from SNR [6] using BER = 1 SNR 2 er f c( ), (1) 2 where er f c( ) is he complemenary error funcion. I can be seen in Fig. 3(b) ha he analog and digial ransmission ess agree very well. 4. Conclusion A new ype of OPLL is presened. I employs a convenional 180 3 db hybrid and an ACcoupled balanced fron end. The loop does no require he ransmission of a residual carrier for phase locking. Analog or digial daa and various modulaion formas are acceped. The only consrain o he user signal is a consan envelope. A firs digial communicaion es indicaed a receiver sensiiviy of 36 phoons/bi for a PRBS-31 signal a a daa rae of 400 Mbi/s (BER = 10 9 ). Due o is echnological simpliciy and olerance o various signal formas, he proposed OPLL is well suied for broadband iner-saellie communicaion. (C) 2005 OSA 16 May 2005 / Vol. 13, No. 10 / OPTICS EXPRESS 3820