SPIE Proceedings [SPIE Photonics West '97 - San Jose, CA (Saturday 8 February 1997)] High-Speed...

nvited Paper

Opto-Electronic Oscillator and its Applicationsx. Steve Yao and Lute Maleki

Jet Propulsion LaboratoryCalifornia Institute of Technology

4800 Oak Grove Dr., Pasadena, CA 91109

ABSTRACTWe review the properties of a new class of microwave oscillators called opto-electronic oscillators (OEO). Wepresent theoretical and experimental results of a multi-loop technique for single mode selection. We thendescribe a new development called coupled OEO (COEO) in which the electrical oscillation is directly coupledwith the optical oscillation, producing an OEO that generates stable optical pulses and single mode microwaveoscillation simultaneously. Finally we discuss various applications of OEO.

Keywords: oscillator, opto-electronics, microwave, electro-optic modulator, laser, phase noise, optical pulses.

1. INTRODUCTIONTraditional microwave oscillators cannot meet all the requirements of photonic communication systems whichrequire high frequency and low phase noise signal generations. Because photonic systems involve signals inboth optical and electrical domains, an ideal signal source should be able to provide high frequency signals inboth optical and electrical domains. In addition, it should be possible to synchronize or control thesignal sourceby both electrical and optical references.

We have reported such a signal source14 that converts continuous lightenergy into stable and spectrally puremicrowave signals. This Opto-Electronic Oscillator, OEO, consists of a pump laser and a feedback circuitincluding an intensity modulator, an optical fiber delay line, a photodetector, an amplifier, and a filter, asshown in Fig. 1. Its oscillation frequency, limited only by the speed of the modulator, can beup to 75 GHz.5

2. PROPERTIES OF OEOOur studies2'3 have shown that the OEO has the following important properties:

1) The OEO has an electrical output port and an optical output port to provide a microwave signal and amodulated optical signal simultaneously, eliminating the costly and high loss electrical-to-optical andoptical-to-electrical conversions for the users.

SPIE Vol. 3038 • 0277-786X1971$10.00 97

Bias,

Fig. 1 Device description of the OEO

Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/02/2013 Terms of Use: http://spiedl.org/terms

98

2) In addition to have an electrical injection port for locking the OEO to a local reference signal, the OEO hasan optical injection port for injection locking the OEO to a remote reference signal, making it simple to integratethe OEO in a photonic communication system.

3) The OEO is a voltage controlled oscillator (VCO) whose oscillation frequency can be tune by applying avoltage to the bias port of the E/O modulator or to a PZT fiber stretcher.

4) No amplifier in the loop is needed if the optical power of the pump laser is high enough so that IPhR � V/iris satisfied, where 'ph the received photocurrent in the photodetector, R is the load resistance of thereceiver, and V7 is the half-wave voltage of the modulator. The elimination of the amplifier in the loopeliminates the amplifier noise, resulting in a more stable oscillator.

5) The oscillation frequency is f =(k+ 1 I 2)/i or f05 =k/r, depending on the bias, where k is an integer,representing different possible oscillating modes, 'r is the total group delay of the loop. For a Mach-Zehndermodulator based OEO, the oscillation amplitude is approximated as

vosc = — i/iGsI (1)

where I GsI is the open loop small signal gain of the OEO.

6) The phase noise spectrum is:

SRF(f ) =(ö/2)2 + (2)2(f' )2

for2irfT<<l (2)

where f' is the frequency offset from the oscillation frequency f0 and S is the noise to signal ratio of the OEOand is defined as:

3PNGA/osc 3= [4kBT(NF) + 2eIPhR + NRJNIhR]G/PQSC

(

In Eq. (3) P0 is the oscillation power of OEO and PN 5 the total noise density input to the oscillator whichequals to the sum of the thermal noise, the shot noise, and the laser's relative intensity noise (RIN) densities.In Eq. (3), kB IS the Boltzman constant, T is the ambient temperature, NF is the noise factor of the RFamplifier, e is the electron charge, and NRIN is the RIN noise of the pump laser.

It is evident from Eq. (3) that the phase noise of the OEO decreases quadratically with the frequency offsetfrom the oscillation frequency. For a fixed frequency offset, the phase noise decreases quadratically with theloop delay time. For large enough loop delay and strong enough optical pump power, the phase noiseapproaches relative intensity level of the laser, below -140 dBc/Hz at 10 kHz for a Mach-Zehnder modulatorbased OEO.

7) The OEO's phase noise is independent of the oscillation frequency f0. This result is significant because itallows the generation of high frequency and low phase noise signals with the OEO. On the contrary, the phasenoise of a signal generated using frequency multiplying methods generally increases quadratically with thefrequency.


3. MULTI-LOOP OEO FOR SINGLE MODE SELECTIONIn the OEO, many modes can oscillate in general. Single mode operation of the oscillator is established byincluding a RF filter in the loop to allow only one mode to oscillate while suppress other oscillation modes.Because the mode spacing is the inverse of the delay time of the feedback loop, the bandwidth of the filter hasto be very narrow for long feedback loops. Unfortunately for a sufficiently long loop, it is impossible to find aRF filter that is narrow enough to sustain single mode operation. For example, for a loop length of 1 km, themode spacing is 200 kHz and the filter bandwidth has to be on the order of few hundreds kiloherz. Such anarrow bandwidth is extremely difficult to obtain for a filter centered at 10 GHz. In addition, the inclusion of anarrow filter in the loop scarifies the oscillator's tunability, making the oscillator not suitable as a frequencysynthesizer.

We present here a multi-loop technique that permits the OEO to operate in a single mode while having a longloop length, resulting in a reduced phase noise. It relaxes the requirement of bandwidth or eliminates the needof a RF filter in the loop, making the oscillator widely tunable. Furthermore, it reduces the oscillationthreshold of the OEO by as much as 6 dB, making it easier to realize an OEO without employing a RFamplifier.

Fig. 2 Two example configurations of the double loop OEO

To illustrate the operation principle of the multiloop technique, a double loop OEO is shown in Fig. 2. Unlike asingle loop OEO which only has one feedback loop, in a double loop OEO there are two feed back loops ofdifferent lengths. The open loop gain of each loop is less than unity, however the combined open loop gain ofboth loops is larger than unity. For a double loop OEO, the possible oscillation frequencies must add up inphase after each round trip around both loops:

f0=(k+1/2)/ri=(m+1/2)/2 forG(V0)<O, (4a)

f0 = k/r1 = m[r2 for G(V0) > 0, (4b)

where m is an integer, G(V0 ) is the open loop voltage gain, and and T2 are the ioop delays of loop 1 andloop 2 respectively. From Eq. (4a) and Eq. (4b) one can see that if loop 2 is n times shorter than loop 1 ( =nT2),then n must be an odd integer for the case that G(V0 ) <0 and an integer (even and odd) for the case thatG(V0) > 0 . The mode spacing is then dictates by the shorter loop: Af = l[r2 . On the other hand, the phasenoise of the oscillator is dictated by the longer loop, resulting in an oscillator having large mode spacing andlow phase noise.

The RF spectrum of the double loop OEO can be analyzed using the same quasi-linear theory developed for thesingle loop OEO.1 Similar to the single loop OEO, the recursive relation for the double loop oscillator can beexpressed as:

V(co)= (g1e'°' +g2e°2 )v1(w), (5)

where 1, (cv) is the complex amplitude of the circulating field after round trip, g1 is the complex gain of loop 1and g2 is the complex gain of loop 2. The total field of all circulating fields is thus:

99

RF out

a) RF coupler

Photodiodes

Photodiode2' 00

AmplilPhotodiodel

00tN)


gout () = g2e'2 )f (w) =_ (g1e'1± g2e2 )

(6)

The corresponding RF power P(w) I V0(a)I2/2R is therefore:

P(w)= 1V012/2R(7)

1+1g1 2 +1g212 +21g1 IIg2Icos[1 (co) — 2 (a)] — 2[1g1 Icos1 (o)+1g1 Icos1 (cv)]

where

i=1,2 (8)

In Eq. (8), is the phase factor of the complex gain g.

If the gain of each ioop is less than unity, no oscillation may start independently in either loop. However, forthe frequency components satisfying Eq. (4), oscillation can start collectively in the two loops. When Eq. (4) issatisfied, we have

1(w)=2kir (9a)

2(w) = 2mr (9b)

1(w)—2(w)=2(k—m)c (9c)

Substitute Eq. (9) in Eq. (7) yields:

1v012/2RflW) 2 2 (10)l+1g11 +1g21 +2!g111g21—21g11—21g11

In order for the oscillation to start from noise, we must have:

1+1g112 +1g212 +21g111g21—21g11—21g11= 0 (11)

For 1g11=1g21, we obtain:

1g11=1g21=0.5 (12)

This is the oscillation threshold for the double loop OEO. If initially the small signal gain in each loop islarger than 0.5, the nonlinearity of the E/O modulator or the amplifier will bring the gain to 0.5 after theoscillation is started and stabilized.

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-140 -140 . -120

-150 iMMMM150\/ / ____-100 -50 0 50 100 -100 -50 0 50 100 -'lOO -50 0 50 100

Frequency (kHz) Frequency (kHz) Frequency (kHz)

Fig. 3 Calculated Spectra of a Double Loop OEO

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We used frequency discriminator method to measure the phase noise of the double ioop OEO and theexperimental setup is shown in Fig. 5. In the setup, the reference fiber has a length of 12.8 km, significantlylonger than the longer loop of the OEO. Both the loop fiber and the reference fiber are acoustically isolated ina box padded with lead-backed foam. Fig. 6 shows the measured phase noise as a function of offset frequency.It should be noticed that at 10 kHz from the carrier (5GHz), the phase noise of the double loop OEO is-l32dBc/Hz, more than 10 dB lower than the phase noise of a best quartz multiplied to 5 GHz. As a comparison,we measured the phase noise of a high performance HP frequency synthesizer (HP8672A) using the same theexperimental setup by disconnecting the 1 km ioop of the OEO at the RF input port to the modulator (after the-20 dB probe) and then connecting the output of the frequency synthesizer to the RE input port. The resultindicates that the synthesizer's phase noise at 10 kHz away from the 5GHz carrier is -100 dBc/Hz, more than30 dB higher than that of the double loop OEO. The fact that this phase noise level is consistent with thedata supplied by the manufacturer confirms the validity of the measurement setup. Fig. 6 also shows that theslope of the phase noise as a function of the frequency offset is 30 dB/decade, about 10 dB/decade higher thanpredicted3. This is an indication that a 1/f noise source is present in the loop and has to be removed for betternoise performance. This noise source may come from the RF amplifiers used in the loop or from the 1/f RINnoiseof the laser at close-to-DC frequencies.

Fig. 5 Experimental setup for measuring the phase noise of the double loop OEO. The thin line indicates theoptical fiber route and the thick line indicates the electrical RF route. The optical output from the OEOis delayed by a 12.8 km fiber and then converted to an electrical signal (LO) to mix with the electricalsignal (RF) directly coming out from the OEO. To prevent the LO feeding back to the oscillator, twoisolators with a total of 70 dB isolation was used.

-4

c' -6

-l0CZl2

-13

-8

102 3456

102 2 3456Frequency offset (Hz)

2 3 45610

102

Fig.6 Phase noise measurement of the double ioop OEO.

detector 1 km Fiber 12.8 km Fiber


4. COUPLED OPTO-ELECTRONIC OSCILLATORIn previously reported OEO, the optical oscillation of the pump laser is isolated from the electronic oscillation.In this section, we present a Coupled Opto-Electronic Oscillator (COEO) in which the laser oscillation isdirectly coupled with the electronic oscillation. Such a coupled oscillator easily accomplishes single modeselection even with an OEO of a very long loop, a task which is difficult to accomplish in an ordinary OEO. Inaddition, in a COEO a multimode laser is used to pump the electronic oscillation, making it more efficient.Finally, the COEO may provide a link between the optical and the microwave oscillations, and can be used togenerate stable optical pulses and microwave signals simultaneously.

Device DescriptionWe first constructed a ring laser with a semiconductor optical amplifier customer-made by E-Tek DynamicsCorporation. The amplifier has a small signal gain around 15 dB peaked at 1298 nm and has a build in opticalisolator with an isolation of 30 dB. To construct the ring laser, a 3 dB optical coupler with an excess loss of 0.5dB was used. The final constructed ring laser is shown in Fig. 7a and the measured P-I (power vs. drivingcurrent) curve is shown in Fig. 7b. It is evident that the ring laser has a threshold of 50 mA and a slopeefficiency of 0.16 W/A. The output power reached 15 mW with a driving current of 253.6 mA.

50 100 150 200 250

Driving Current (mA)

Fig..7 a) The ring laser built with a semiconductor optical amplifer . The coupler has a coupling ratio of 52% fromport 2 to port 3 and a ratio of 48% from port 2 to port 4. b) The measure P-I curve of the ring laser..

The ring laser has many longitudinal modes with a mode spacing determined by the ioop (cavity) length of thering. The measured mode spacing is 23.3 MHz, corresponding to a loop length of 8.58 meters. When the SOAwas ordered, a RE port was built in so that we can modulate the gain of the amplifier.

With this ring laser, we constructed an Opto-Electronic Oscillator as shown in Fig. 8a. The output of the ringlaser (from port 2 of the coupler) is connected to a second coupler with a coupling ratio of 10%. 90% of the lightfrom the ring laser is detected by a photodetector and amplified by an RF amplifier. The amplified signalthen goes through a variable delay line, an RF bandpass filter, an RF variable attenuator, and finally ancoupler before being fed back to the RF modulation port of the SOA to form an opto-electronic feedback loop.Just like an OEO, when the gain of the feedback ioop is larger than one, an electro-optical oscillation willstart. The RF variable delay line is used to adjust the ioop length and the variable attenuator is used to adjustthe ioop gain. The RF coupler is used to pick out the oscillation signal.

The opto-electronic feedback ioop (more than 100 meters) is much longer than the ioop length of the ring laser,resulting in a corresponding mode spacing much smaller than the mode spacing of the ring laser, as shown in Fig.8b and 8c. The center frequency of the RF bandpass filter is chosen such that it is equal to a RF beat frequency ofdifferent modes of the ring laser, as shown in Fig. 8c. The bandwidth of the filter is chosen to be narrower thanthe spacing of the beat frequencies (equivalently the mode spacing of the ring laser). Within the pass band,there are many OEO modes compete to oscillate. However, the winner is the mode with a frequency closest to abeat frequency of the laser's longitudinal modes because only this OEO mode can then effectively mode lock thering laser. When the laser is mode locked, the beating between any two neighboring modes will add up in phase

1R23=52%R24=48%

Couplera)

output input

1 I I1z 0',—, .00

b°!1(

U) o.;- . 00() Q:0 . 0

p

0

RF input

103


(c)

{MTFM—0 Modes of the opto-electronic loop(_ L0 Modal beating spectrum of the laser

Fig. 8 a) The construction of a coupled opto-electronic oscillator. b) Possible oscillating modes defined by the opto-electronic ioop. c) Modal beating spectrum of the laser. An electrical filter with a bandwidth narrower than themode spacing of the laser is sufficient to ensure single mode oscillation of the OEO, despite of the much smallermode spacing of the opto-electronic loop.

with a frequency equal to the frequency of the oscillation mode of OEO and hence reinforce the OEO mode thatmode locks the ring laser. The mode spacing of the mode-locked laser equals to the oscillation frequency of theOEO and is a multiple of the natural mode spacing of the laser.

This is effectively a double loop OEO described above, except that here the second ioop is a pure optical cavityand is inherent in the pump laser. However, at outside it is just like a single loop OEO without any extracomponents. Because the optical cavity can be made very short, a laser mode spacing can be made much largerthan possible with an opto-electronic loop. The larger mode spacing ensures a large frequency tunability of theCOEO and may result in the elimination of the bulky RF filter used in an ordinary OEO.

Experimental ResultsIn the first experiment, a bandpass filter centered at 300 MHz with a bandwidth of 13 MHz was used in theopto-electronic loop. The mode beating spectrum of the mode locked laser is shown in Fig. 9a. It was measuredat the optical output port of the COEO with a lasertron photodetector with a bandwidth of 18 GHz and a HP8562A spectrum analyzer. The peaks of the RF spectrum results from the beating between the longitudinalmodes in the ring laser. The lowest frequency corresponds to the beat between any two neighboring modes andthe second lowest frequency corresponds to the beat between every other modes, and so on. It can be inferred fromthe spectrum that about 20 modes of the ring laser were mode locked. Fig. 9b shows the RF spectrum of theoscillation signal measured at the RF output port. A fairly clean signal at 288 MHz with a power of -30 dBm isevident. Taking the -35 dB coupling ratio of the RF coupler into account, the RF signal circulating in the ioop isabout 5 dBm, which is limited by the RF amplifier used.

39.5

-10.5287.5 288.0 288.5 289.0 .

Frequency (MHz) Time (2 ns/div)

Fig. 9 a) Mode beating spectrum ofthe COEO measured at the optical output port. b) The BY spectrumofthe COEO measured at the electrical output port. c) The time domain measurement ofthe COEOat the optical output port.

(b)

-4 Id=62.74 mA

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Frequency (GHz)

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104


The ring laser, as expected, was also automatically mode locked by this self-generated RF signal to produce atrain of short optical pulses, as shown in Fig. 9c. The pulse width is about 250 ps while the periodicity of thepulses is about 3.6 ns. The time domain data were taken with a HP CSA8O3 communication signal analyzer.

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IdIO267iiSp lOl&lzRBW: 100 Hz

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Frequency (MHz) Time (SOOps/div)a) The RF spectrum of the COEO at RF output port.. The laser is driven at a current of 102.67 mA andspan and the resolution bandwidth of the measurement are 10 kHz and 100 Hz respectively. b) Thetime domain measurement of the COEO at the optical output port. A train of short pulses are evident.

In another experiment, a filter centered at 800 MHz with a bandwidth of 40 MHz was used to replace the 300MHz filter. Now the COEO oscillates at about 800 MHz and the oscillating RF signal mode locks the ringlaser. The RF spectrum of the COEO is shown in Fig. lOa. It is evident from the figure that a signal with ahigh spectral purity was obtained with the COEO. The corresponding pulse train of the mode locked ring laseris shown in Fig. lOb. It can be seen that the pulse width is about 50 ps and periodicity of the pulse train is about1.2 ns. It is clear that the pulse width is greatly shortened with the increase of the oscillation frequency. Notethat due to the self-correcting mechanism of the coupled oscillation discussed below, the mode locked laser andthe OEO were both very stable -- they did not seem to change during many hours of operation.

Linkage between the Optical Frequency and the RI FrequencyWe believe that the absolute frequency of a laser mode and the generated RF frequency are related. A stableRF oscillation will stabilize every longitudinal mode of the laser and the stable longitudinal modes of thelaser make the oscillating RF signal more stable. To understand this point, let's examine the following idealcases.

Case 1, the longitudinal modes of the laser are perfectly stable. In this case the beat signals between any twoneighboring modes have exactly the same frequency and add up in phase. This beat signal provide a strong gainto an OEO oscillation of the same frequency. This OEO oscillation will remain dominate even when the opto-electronic loop length is changed to favor other oscillation frequency. As a result, the OEO oscillationfrequency is stabilized by the laser frequency against the loop length fluctuations of the opto-electronic loop.

Case 2, the OEO oscillation is perfectly stable. With this OEO oscillation signal driving the multimode laser,the mode spacing will be fixed by the OEO signal. One may argue in principle that the absolute frequencies ofall the mode can be shifted together without changing the mode spacing. However, we can not see anypractical mechanism that allow this to happen. This is because both the absolute frequency of each laser modem ( fm mc I nL , where n is the refractive index and L is the round trip laser cavity length and m is an integer)and the laser mode spacing ( 4f = cI nL) of the neighboring modes are inversely proportional to the effectivelaser cavity length. One cannot change the absolute frequency of each mode without changing the modespacing. Therefore, when the mode spacing is stabilized by the OEO oscillation, the absolute frequency of eachlaser modes is also stabilized. Taking the derivative of the mode frequency and the mode spacing we obtain:

dfm m(c I nL)[d(nL) I nL] = —mAf[d(nL) I nL] (13)

d(Df) = —(c I nL)[d(nL) I nL] =—Af[d(nL)I nL] (14)

dfm md(Af) = (fm ' Af)d(4f) (15)

105

E-u

C

Fig. 10


From Eq. (15) one can see that any mode spacing fluctuation will cause a larger mode frequency fluctuation witha multiplication factor of fm " Af. This is the exactly the same relation as in frequency multiplication of anyscheme.

Note that Eq. (15) is simple but extremely important because it established a link between the stability of anRF oscillation and an optical oscillation.

However, the dispersion of the laser material in the laser cavity may limit the effectiveness of the mutualstabilization of the optical and opto-electronic oscillations. Because of the dispersion, mode spacing willchange as a function of mode frequency. Therefore the maximum number of modes that can be mode locked willbe determined by the dispersion. It is possible there are several groups of modes in the laser cavity, with modesin each group are phase locked. However, the phase relations among the groups are random or partiallyrandom and may add noise to the OEO and the optical oscillation. Therefore, dispersion compensation in thelaser cavity may needed.

In summary, we demonstrated a Coupled Opto-Electronic Oscillator (COEO) in which the laser oscillation isdirectly coupled with the electronic oscillation. Such a coupled oscillator easily accomplishes single modeselection even with a very long opto-electronic loop, a task which is difficult to accomplish in an ordinaryOEO. In addition, in a COEO a multimode laser is used to pump the electronic oscillation, making it moreefficient. Directly using a multimode diode laser of a simple FP cavity, very compact and efficient COEO canbe constructed to a generate stable high frequency (>50 GHz) RF oscillation and stable, high repetition rate,and ultrashort optical pulses simultaneously. Finally, the COEO can provide a link between the optical andthe microwave oscillations, and can be used to generate stable optical pulses and microwave signalssimultaneously.

5. APPLICATIONSVoltage Controlled Oscillator.6 As mentioned earlier, the OEO is a special VCO with optical as well aselectrical output. Therefore it can perform all functions that a VCO is capable of for microwave photonicsystems.

Photonic Signal Mixing.6 The OEO can also be used for photonic signal up/down conversion. For such anapplication, a stable optical RF LO, or a modulated optical signal at a RF frequency, is required. The OEO canaccomplish just that, since one of its outputs gives the RF oscillation in optical domain.

Carrier Distribution6. Because the OEO can be injection locked by a remote optical signal, it can be used forhigh frequency RF carrier regeneration, amplification, and distribution. Such a capability is important inlarge microwave photonic systems.

Frequency multiplication.6 The injection locking property of the OEO can also be used for high gain frequencymultiplication. We used subharmonic injection locking technique and demonstrated phase-locking theoscillator operating at 300 MHz to a 100 MHz reference of 4 dBm. The output of the oscillator is 15 dBm,resulting in a gain of 11 dB and frequency multiplication factor of 3. We will also discuss frequencymultiplication using laser diodes nonlinearity. In this scheme, the OEO is tuned to operate at a nominalfrequency close to the nth harmonic of the reference signal driving the laser diode. Upon the injection of thelaser's output, the OEO will be locked to the nth harmonic. This scheme offers remote frequency multiplicationcapability and may be useful for many microwave photonic systems.

Comb Frequency and Pulse Generation.6 The OEO can also be used to generate frequency combs and square pulse.For this application, the OEO is chosen to operate with multimodes. A sinusoidal signal with a frequencyequal to the mode spacing or a multiple of mode spacing is injected into the oscillator. Just like laser mode-locking, this injected signal will force all modes to oscillate in phase. Consequently, we obtain a comb offrequencies that are in phase. In the time domain, the output signal is square pulses.

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Clock and carrier recovery.5 The same injection locking property of the OEO can also be used for clock andcarrier recovery. We have demonstrated clock recovery at 100 Mb/s and 5Gb/s, and obtained excellent results.Data rates up to 75 Gb/s can also be recovered using the injection locking technique with an OEO operating at 75GHz. Another important feature of the OEO technique is that the clock can be recovered directly from data justout of a fiber optic transmission line, without the need of optical to electrical conversion. In addition, therecovered clock signal has both optical and electrical forms and is easy to interface with a fiber opticcommunication system.

Similar to clock recovery, a carrier buried in noise can also be recovered5 by the OEO. We have alsodemonstrated the recovery of carrier from noise and increased carrier to noise ratio by 50 dB.

Generation of low jitter optical pulses. The newly developed COEO can be used to generate very low jitteroptical pulses. Although in our first demonstration the pulse width is 50 ps, we anticipate much shorter pulsescan be generated using the same principle. Such a stable and low jitter optical pulse source is very important inadvance digital communication systems.

6. SUMMARYWe reviewed important properties of Opto-Electronic Oscillators (OEO) that can simultaneously generatemicrowave and modulated optical signal. We then described a multiloop technique for single mode selectionand presented a double loop OEO operating at 5 GHz with a phase noise 10 kHz away from the carrier morethan 10 dB better than the best quartz oscillator (assuming the low frequency of the quartz is multiplied to5GHz). We later introduced a new development called coupled OEO (COEO) in which the electricaloscillation is directly coupled with the optical oscillation, producing an OEO that generates stable opticalpulses and single mode microwave oscillation simultaneously. We demonstrated such a COEO with a ring laserand produced stable microwave oscillation at 800 MHz and optical pulses of 50 picoseconds. We anticipate thatstable millimeter wave oscillations and femotosecond optical pulses can be simultaneously generated with sucha technique. Finally we discussed some applications of OEO.

ACKNOWLEDGEMENTSThis work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contractwith the National Aeronautics and Space Administration. We thank G. Lutes for helpful discussions.

REFERENCES

1. X. S. Yao and L. Maleki, 'High frequency optical subcarrier generator," Electron. Lett. 30 (18), pplS25-1S26

(1994).2. X. S. Yao and L. Maleki, "Converting light into spectrally pure microwave oscillation," Opt. Lett. Vol. 21 (7),pp. 483-485 (1996).3. X. S. Yao and L. Maleki, "Optoelectronic microwave oscillator," J. Opt. Soc. Am. B, Vol. 13 (8), pp 1725-1735(1996).4. M. F. Lewis, "Novel RF oscillator using optical components," Electron. Lett. Vol. 28 (1), pp. 31-32 (1992).5. K. Noguchi, H. Miyazawa, and 0. Mitomi, "75 GHz broadband Ti:LiNbO3 optical modulator with ridgestructure," Electron. Lett. Vol. 30 (12), pp. 949-951 (1994).6. X. S. Yao and L. Maleki, "Optoelectronic microwave oscillator for photonic systems," IEEE J. QuantumEletron. Vol. 32 (7), pp1141-1149 (1996).7. X. S. Yao and G. Lutes, "A high-speed photonic clock and carrier recovery device," IEEE Photonics Technol.Lett., Vol. 8 (5), pp 688-690 (1996)

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