■••■••-■••■■MZIphotomixer

phasemodulator

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224 GHz

radiatednun wave

17th International Symposium on Space Terahertz Technology TH4-3

Photonic mm-Wave Local Oscillator

Robert Kimberk, Todd Hunter, C.-Y. Edward Tong, and Raymond Blundell

Abstract—A photonic millimeter wave local oscillatorcapable of producing two microwatts of radiated power at 224GHz has been developed. The device was tested in one antennaof Smithsonian Institution's Submillimeter Array and wasfound to produce stable phase on multiple baselines. Graphicaldata is presented of correlator output phase and amplitudestability. A description of the system is given in both open andclosed loop modes. A model is given which is used to predictthe operational behavior. A novel method is presented todetermine the safe operating point of the automated system.

Index Terms--ln terferom eter, photomixer, phasemodulation, phase locked loop, frequency multiplication,millimeter waves.

I. INTRODUCTION

A photonic local oscillator has been developed and usedto pump a SIS mixer at 224 GHz. The local oscillator wasinstalled in one antenna of the Submillimeter Array [1]. Asingle dish scan of Saturn was performed on February 242006. Observations of an ultra compact HII region and threequasars successfully produced stable fringes and amplitudeson a five element array. The test was performed on theevening of April 26 2006 UTC and the following day.

The configuration of the photonic lo not phase locked toan external reference signal (open loop) is shown in figure1. The optical path consists of a single 1550 nm diode laser,

diodelaser

YIG

fig.! open loop system

a lithium niobate optical phase modulator, a Mach Zehnderinterferometer and a photomixer whose output is connectedto a horn antenna. The photomixer was obtained from P.G.Huggard and B.N. Ellison of Rutherford Appleton Labs [2]in November of 2003. All optical devices and connectionsare polarization maintaining. The electrical path consists ofa YIG synthesizer, a frequency doubler, and a poweramplifier connected to the RF port of the phase modulator.The light from the laser is phase modulated at twice the YIGfrequency, then converted to an amplitude modulated signalby the Mach Zehnder interferometer (MZI). The photomixerresponds to the amplitude modulated signal and radiates that

Manuscript received May 10,2006.R. Kirnberk, T. Hunter, C.-Y. E. Tong and R. Blundell are with the

Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge,MA 02138 USA. (e-mail: etong@cfa.harvard.edu, andrblundell@cfa.harvard.edu ).

part of the am signal above the WR3 horn lower cutofffrequency. Figure 1 is a schematic of the open loop system.

II. THE OUTPUT SPECTRUM

The voltage output of the photomixer,V, can be describedby the following expression:

V 1+ Jp(2a1(p) cos((no)c / (om ) + mom° (1)

where n ranges from plus to minus infinity, V is theamplitude of the output signal, wc is the optical angularfrequency before modulation, and Wm is the modulationangular frequency. Jp(2aK p) is the Bessel function of thefirst kind with 2aK, as the argument. The quantity, aK, thepeak phase deviation of wc , is the product of the amplitudeof the modulation signal ,a, in volts and the optical phasemodulator response, K ,in radians / volt.An examination of expression (1) shows that:

a) The photomixer output contains harmonics of themodulation frequency Wm.

b) Given J, = -l n J„ and cos(0) cos( 0) when moc / Wm isan even multiple of 3t/2, odd harmonics cancel and onlyeven harmonics appear at one output of the MZI. The otheroutput of the MZI has the odd harmonics due to the extra at

phase shift at coc.

c) Given cos(0 +n/2) = —cos(-0 +n/2), when no), / co in is anodd multiple of n/2 then the even harmonics cancel and onlyodd harmonics appear at the output.

d) When ;Eck), / com meet the conditions of b) or c) theamplitudes of the harmonics are determined by 2aKp .Increasing the power into the optical phase modulator RFport will increase the number of harmonics.

e) For values of no), / (pm that do not meet the conditions of

b) or c) both odd and even harmonics exist at both MZIoutputs.

The frequency of the laser, co, , is controlled by the lasertemperature. The laser used, JDS CQF935/808, has awavelength temperature tunability of about 0.1 nm / °C,equivalent to about 7.85 * 10 10 rad /sec / °C at 1550 nm.Figure 4 shows the measured power of a selected harmonicas the laser temperature is varied. Note that the poweroutput completes a cycle in about 3° C. This corresponds toabout 23.55 * 10 11 rad / sec, about o)m . Note also that themaximum output power of the harmonic occurs at themiddle value of photomixer bias current.

Expression 1 assumes a photomixer with a flat responseover the output frequency. To estimate the true output power

206

Kp = radians / volt toc - aKpcom sin(Witt)phase modulator

a amplitude

laser

YIG

MZ I

n delayat corn

2aKpuom sin(wint) —

Ok - aKpokn sin(comt) +photo co. + aKpohn sin(wint)mixer —

E ,

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(I)1 0 1 5

Laser TEC Temperature (C)

-6

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17th International Symposium on Space Terahertz Technology TH4-3

the results of expression I need to be multiplied by thephotomixer's frequency response.

We normally tune the laser temperature so that the twooutputs of the MZI are purely even and odd harmonics ofthe modulation frequency. The two advantages of this modeare that the frequency separation of harmonics is greatest

Fig. 2. output as difference of phase modulatedlaser and delayed phase modulated laser

(two times the modulation frequency), and the power ismaximized for any harmonic.

Figure 2 shows the instantaneous angular frequency [3](d0/dt , where 0 phase) at key points along the signal path.Instantaneous frequency is a useful concept when dealingwith frequency or phase modulation. The MZI splits theincoming power, phase shifts one component, then sums thetwo components. The greatest frequency multiplicationoccurs when the modulation frequency is equal to one halfof the free spectral range (FSR) of the MZI. This conditionadds a it radian phase delay at the modulation frequency.The instantaneous difference frequency then becomes:2aKp cum sin(comt)= aK, wm sin(wmt) — ( wc + aKp wm sin(wmt

oic al(p (i)m sin(wmt) — ( tu - aKp com sin(wrnt )).

Fig.4 laser temperature vs single tone powerand photomixer bias current

The resulting instantaneous frequency, 2aKp um sin(wmt),describes a sinusoidal chirp. The maximum frequency of thechirp is equal to twice the product of the peak phasedeviation, 2a1( , and the modulation frequency, tom . As themodulation frequency is moved away from FSR/2 themaximum instantaneous frequency of the output is reduced.

III. CLOSED LOOP SYSTEM

In order to operate a radio interferometer, the localoscillators of all the receivers must be phase locked to acommon reference tone. The closed loop configuration ofthe photonic local oscillator, where the phase of the mmwave output is locked to a reference frequency, is shown infigure 3. A second photomixer is connected to the unusedMZI output. The output of the added photomixer isconnected to a harmonic mixer, which in turn is connectedto the IF port of the phase locked loop (PLL) through afrequency diplexer and IF amplifier. The PLL control outputis connected to the YIG fine tune input, and controls theYIG output phase. In this way the YIG is locked to a downconverted 112GHz, the third multiple of w m. The multiplesof the modulation frequency, wm, are all related with respectto phase. Locking to any multiple stabilizes the phase of allthe other multiples. Not shown in figure 3, is the 6 to 8 GHzlo pumping the harmonic mixer, the 109 MHz referenceconnected to the PLL, and the coarse tuning signal to theYIG

Iv. OPEN LOOP PHASE NOISE AND STABILITY

The open loop phase drift of the system was tested bypower splitting the output of a microwave synthesizer,modulating our system with one of the two identical outputsand using the other output to pump a conventional multiplierchain. The synthesizer's output frequency was 12.5 GHzand the final frequency in both legs was 75 GHz. Theoutputs of both the photonic and conventional multiplierswere down converted using mixers with a common localoscillator and the phase was compared with a vectorvoltmeter. Differential phase drifts of 5 degrees / hr wereobserved. The phase drift tracked the room temperature

diode phase photolaser modulator MZI -■-■••••• mixer

horn

224 GHzphotomixer

YIG

harmonic radiated

mixer inm-wave

phase lock loop diplexer

fig. 3 closed loop system

change. The differing lengths and temperature coefficientsof coaxial cable and optical fiber involved in this test mightexplain the observed phase drifts. One degree at 75GHz isequal to about 7.4vm in quartz fiber.

The phase noise of the output of the photonic localoscillator is dominated by the multiplied phase noise of themodulation source. We expect the system to act like aclassical multiplier, the output phase noise will be the phasenoise of modulation source plus 20 log M. M is equal to theratio output frequency / input frequency.

The open loop phase noise of a heterodyne two laser localoscillator is similar to the phase noise of two lasers. Incontrast the contribution of the single laser phase noise tothe open loop phase noise of our system is determined byphase noise of the laser times ( T / tc )2 ,where -rc is thecoherence time of the laser and -t is the path delay of theMZI. The value of ( t I te )2 is very small for a MZI with a

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17th International Symposium on Space Terahertz Technology TH4-3

free spectral range of 75 GHz (a delay of 1.3*10 -1 s) and alaser coherence time of 10-6s.

V. ASTRONOMICAL OBSERVATIONS

The photonic local oscillator was installed in antenna 6 ofthe Submillimeter Array on Mauna Kea, Hawaii, and tunedto 224 GHz. An observation of the quasar 3C273 wasobtained on the evening of April 26 2006 UTC. The outputof the correlator yielded stable phase and amplitudes foreach of the baselineswith the seven operating antennas. The next dayobservations of the ultra compact HII regionG138.295+1.555 [4], the quasar 3C84, and the quasar3C454.3 were obtained with a five element array. Figure 5shows the phase (dots) and amplitude (gray trace) of eachbaseline at the correlator output. Baselines that includeantenna 6 do not show a greater scatter of values thanbaselines that do not include antenna 6.Baseline lengths range from 16 m at the top to 69 m at thebottom of figure 5. Line spectra of 13CO2-1 and 12CO2-1were observed in the source G138.295+1.555. The 232.4GHz beacon attached to the Subaru building was observedas shown in figure 6. Figure 7 is a graph of the 12CO2-1line from G138.295+1.555 at the correlator ouput.

Fig.5 phase and amplitude at correlator output. Thesources are 3C454.3, G138.295+1.555, and 3C84

VI. AUTOMATION

The photomixer bias current limit is 5mA, above whichthe photomixer will be damaged. The bias current is afunction of laser power and temperature as well asmodulation frequency. This fact makes it necessary to finda path through the various combinations of the variables thatdoes not destroy the photomixer. To explore the effect oftemperature on the modulated output power, we biased thelaser with 60 milliamps of current (about 1/8 the normaloperational current) and measured the photodiode current asa function of laser temperature at several modulationfrequencies. As seen in Figure 4, we find a sinusoidalbehavior of photodiode current with respect to laser

Beacon oaserved With the Photonic LO tuned to 227.20 GHz• — • --i- •

5.198 5.2I

Fig.6 Beacon on Subaru Building observed fromSMA antenna 6

temperature. Although the amplitude of the sinusoiddepends on modulation frequency, we discovered that thereexist "fixed point" temperatures where the photodiodecurrent is independent of modulation frequency.We next locked the photonic LO using a W-band harmonicmixer and a digital PLL. We measured the power of thelocked signal at the PLL IF monitor port as a function oflaser temperature. Figure 4 shows that the IF power is atmaximum when the laser is operated at the fixed pointtemperatures.To optimize the IF power, we can use the followingalgorithm. Change the modulation frequency by somesignificant amount (1 GHz) and observe the change in thephotomixer current. Adjust the laser temperature slightlyupward and change the modulation frequency back to theinitial value. If the photomixer current decreases, thencontinue adjusting the laser temperature upward until thephotornixer remains constant with respect to the modulationfrequency. Conversely, if the photomixer currentincreases, then adjust the laser temperature downwardinstead.

VII. CONCLUSION

We have demonstrated that a photonic local oscillator canbe sufficiently phase and amplitude stable to be used as partof a radio interferometer at mm wave lengths. We believethat an increase in the frequency of modulation will allowfor greater output frequencies as well as greater separationbetween the harmonics of the modulation frequency.

VIII. Acknowledgment

The authors want to thank Smithsonian Institution for itssupport. We want to thank Bob Wilson for early insight andfor his belief that the system would work when most wereskeptical. We want to thank Hugh Gibson for a numericalanalysis of the system with both the Mach Zehnder andFabry Perot interferometers, and Robert Christensen forinstrumental instruction and observational guidance atMauna Kea. As well we want to acknowledge those peoplethat developed the concept before us, 0.1. Kotov et al [5],E.J. Bochove et al [6] to name a few. A development of theideas in this paper in greater detail was published in an

208

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_

17th International Symposium on Space Terahertz Technology TH4-3

earlier ISST proceedings [7] and may be a useful REFERENCES

background.

Fig.7 12CO2-1 spectral line observed in sourceG138.295+1.555

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th

International Symposium on Space TerahertzTechnology, April 27-29, 2004, p. 3

[2] P.G. Huggard, B.N. Ellison, c`Efficient Generation of guidedMillimeter-Wave Power by Photomixing" IEEE PhotonicsTechnology Letters, Vol.14,N0.2, February 2002.F.G. Stremler"Introduction to Communication Systems" Addison-Wesley, thirdedition page 320

[3] F.G. Stremler, "Introduction to Communication Systems" Addison-Wesly, third edition, 1990, p. 320

[4] T. Hunter " A Submillimeter Imaging Survey of Ultracompact HI!Regions" Thesis, California Institute of Technology, Source DAI-B58/01 July 1997.

[5] 0.1. Kotov, L.B. Liokwnovich, V.M. Nikolaev, V.Yu. Petrun'kin, andZekhraui Buabid, "Conversion of phase modulation of light intointensity by means of an external fiber-optic interferometer" St.Petersburg State Technical University, Pis'ma Zh. Tekh. Fiz. 23, 9-16, May 26, 1997.

[6] E.J. Bochove, and E.M. de Carvalho, "Conversion of a widebandfrequency-modulated signal to amplitude modulation throughdispersion in an optical fiber" Optics Letters, Vol. 7, No. 3, March1982.

[7] R. Kimberk, C.E. Tong, H. Gibson, R.W. Wilson, R. Blundell, S.Paine, and T.R. Hunter, " An Interferometric Microwave CombGenerator"' Proceedings of the I 3 th International Symposium on SpaceTerahertz Technology, March 26-28, 2002.

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