WIDE BANDWIDTH MICROWAVE PHASE DETECTION USING

WIDE BANDWIDTH MICROWAVE PHASE DETECTION USING OPTICAL INTENSITY MODULATORS. P.P Roberts1, G.E. Town2 and W.E. Wilson1. 1

CSIRO, Australia Telescope National Facility, PO Box 76, NSW, 1710, Australia. 2

Department of Electrical and Information Engineering, University of Sydney, NSW, 2006, Australia.

ABSTRACT A new architecture for wide bandwidth microwave and optical-microwave phase detection using cascaded optical intensity modulators and synchronous detection is presented. This architecture has the potential to operate to many tens of gigahertz using currently available components. An experimental setup and measured results which confirm the proposed architecture are given. The proposed architecture can also be used for wide bandwidth signal correlation. INTRODUCTION The ability to measure the phase difference between two signals is a fundamental requirement in many electronic and communication systems and various methods such as digital gates

and diode mixers are used to accomplish this, depending upon the frequency of the signals involved. Such phase detectors are used in many and varied applications ranging from phase locked loops to angle of arrival detectors [1]. By using the multiplier property of a cascade of two optical intensity modulators a phase detector with very wide bandwidth performance can be realised. Cascaded intensity modulators have been studied previously [2] in the context of wide bandwidth mixers for signal frequency translation but no attention seems to have been paid to the DC properties of this arrangement. A cascade of two intensity modulators, as depicted in Fig. 1, which are biased at their maximum linearity half intensity points then detected by a photodiode and time averaged, have a response for small signal inputs x(t) and y(t) given by

I det =

 π  η I0  π x( t )1 + y ( t ) 1 + 4  Vπ  Vπ 

,

x(t)

y(t)

VARIABLE DELAY

PHASE SWITCH

MODULATOR

MODULATOR

(1)

where Io is the incident optical intensity, Vπ is the half wave voltage of the modulators and η is the responsivity of the photodiode. Expanding this expression and using the fact that the AC coupled input signals are zero mean results in

LASER

+1/-1

I det =

η I0 η I0 π 2 + x( t ) y( t ) . 4 4 Vπ2

OSC

VOUT

(2)

There is a constant bias equal to one quarter of the detected incident optical intensity and a term proportional to the time averaged product of the input signals. For two CW inputs, with relative phase difference ϕ, this term is proportional to cos(ϕ) and so provides a direct measure of the phase difference. To maintain linearity the input signals must be small signal to drive the modulators only near the region of the bias point. Hence the term of interest is typically less than 10-3 of the constant bias term. In order to separate the wanted term from the bias term we use a synchronous detection scheme. One of the signals is passed through a phase switch, which alternately inverts the phase of the signal, and the sign of the detected optical intensity is synchronously switched before being passed to a low pass filter. In this way the constant term is averaged to zero with the desired term retained.

Figure 1. Phase detector experimental setup. RESULTS To demonstrate the phase detector architecture the experimental arrangement of Fig. 1 was constructed. A CW signal of 1.0 GHz was used and the output voltage as a function of delay in the variable delay line measured. The results are presented in Fig. 2, which show clearly the sinusoidal response over an approximately 180°phase range. The phase range was limited by the length of the variable delay line and the operating frequency of the optical modulators available to us.

400

200

0 output voltage (mV)

-200

-400

-600

-800

-1000 0

50

100

150

200

250 300 delay (ps)

350

400

450

500

Figure 2. Results of phase detection experiment. In the experimental setup the oscillator frequency was set to 100 Hz, which was locked to the 50 Hz mains frequency to reject mains pickup. The phase switch was implemented using a wide bandwidth double balanced mixer and taking the RF port as input, the LO port as output, and driving the IF port with a switching current source to alternately turn on opposite pairs of diodes in the mixer quad diode ring, and so reverse the direction of current flow in the output LO transformer. DISCUSSION Since optical intensity modulators with bandwidths of several tens of GHz are available commercially the arrangement in Fig. 1. can be used for very high frequency direct microwave phase detection. For instance, an angle of arrival detector as presented in [4] could

be implemented using the proposed phase detector without optical downconversion, or alternatively a hybrid all optical approach using a combination of both methods could be pursued. If the first intensity modulator in Fig. 1. is replaced with a general intensity modulated optical signal an optical microwave phase detection [3] can be performed. That is, the phase difference between an intensity modulated optical signal and a microwave signal can be measured. This situation could typically arise in an optical beam forming phased array antenna. For input signals which are broadband and stationary the time averaged product calculated by the arrangement of Fig 1. measures the correlation between the two input signals. This can be employed, for example, in a broad bandwidth interferometer for radio astronomy, which was the original motivation for this work. CONCLUSIONS A new method for wideband phase detection, optical-microwave phase detection and correlation has been presented. The method relies on the wideband multiplier property of cascaded optical intensity modulators in conjunction with a synchronous detection scheme to separate the small desired effect from a much larger background level. An experimental setup was constructed and

measured results presented which confirm the method. REFERENCES [1]

P.D. Biernacki, R. Madara, L.T. Nichols, A. Ward, and P.J. Matthews, “A Four Channel Angle of Arrival Detector Using Optical Downconversion” IEEE Int. Microwave Symp., June 1999, pp 885-888, vol 3.

[2]

G.K. Gopalakrishnan, W.K. Burns, and C.H. Bulmer, “Microwave-Optical Mixing in LiNbO3 Modulators” IEEE Trans. Microwave Theory and Tech., vol 41, No.12, Dec 1993, pp 23832391.

[3]

T. Berceli, “Optical-microwave phase detection” IEE Proceedings-J, vol. 139(4), Aug 1992, pp 296-300.

[4]

P.D. Biernacki, L.T. Nichols, D.G. Enders, K.J. Williams, and R.D. Esman, “A Two-Channel Optical Downconverter for Phase Detection” IEEE Trans. Microwave Theory Tech.,vol 46, No. 11, pp. 1784-1787, Nov., 1998.