Optical control techniques for microwave phase locking applications

University College London

O

PHASE LOCKING APPLICATIONS

Ian David Blanchflower

1992

A thesis to the University of London for

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This thesis considers the use of optical control techniques for phase locking of microwave and millimetre wave oscillators. Optical control functions provide a convenient means of controlling high frequency circuits. With the increasing importance of optical techniques in signal distribution applications, there is further advantage to be gained from the use of optically controlled devices as optical-to- electrical transducers.

The optical phase locking systems considered are optical injection locking and optical frequency tuning within a phase locked loop. A third system, applying a combination of optical injection locking and optical frequency tuning within a phase locked loop, is considered in detail for the first time. General analytical models for the systems are derived and used to compare the operation. The analysis is effective in deriving new results for the effects of loop delay and differential phase in the novel optical injection locking/phase locked loop combination. Practical systems are demonstrated, employing an optically controlled 1.5 GHz MESFET oscillator. The results of optical frequency tuning and optical injection locking effects are presented, including a frequency tuning modulation response up to 100 MHz, and optical injection locking ranges up to 2 MHz. Measurements performed on the optical phase locking systems confirm the validity of the analysis.

The practical implementation of optical phase locking systems is discussed. The analysis shows that phase locked loop bandwidths in excess of 100 MHz are feasible with a moderate optical tuning sensitivity of 10 MHz/mW. The performance of practical systems, however, may suffer from limited tuning modulation bandwidths, frequency tuning range, and poor tuning linearity that are characteristic of some optically tuned oscillators. Optical injection locking is dependent on the operating conditions of the oscillator, and this restricts its application, and that of the combined injection locking/PLL system. Fractional locking bandwidths of up to 1% with a few mW of optical signal power are possible with present devices. Improvements in device design for optical access may allow bandwidths of several hundred MHz in millimetre wave oscillators.

Ac k n o w l e d g e m e n t

I would like to acknowledge the SERC for funding the work presented in the thesis and also Marconi Defence Systems Ltd., Stanmore for providing financial and industrial support.

To my wife, Damienne,

for her patience and understanding!

Ta b l eo f c o n t e n t s

List of illustrations List of tables Table of symbols References

Chapter 1: Introduction and overview

1.1 Introduction

1.2 Optical phase locking schemes

1.2.1 Phase lock loop schemes and injection locking 1.2.2 Optical phase and frequency control

1.3 Phase locked oscillator optical receivers 1.4 Contents

Chapter 2: Optical control of microwave and millimetre wave oscillators

2.1 Introduction

2.2 Direct optical control

2.2.1 Photoeffects and wavelength dependency 2.2.2 Gunn diode oscillators

2.2.2.1 Frequency tuning and amplitude switching 2 2 2 2 Trap effects

2.2.3 IMPATT diode oscillators 2.2.3.1 Theoretical estimate

2.2.3.2 Experimental frequency tuning 2.2.3.3 PM frequency response

2.2.3.4 Amplitude switching 2.2.3.5 Optical injection locking 2.2.4 MESFET oscillators

2.2.5.1 Frequency tuning

2.2.5.2 Optical injection locking 2.2.6 HBT oscillators

2.2.6.1 Frequency tuning 2.2.6.2 Optical injection locking 2.3 Indirect optical control

2.3.1 PIN diodes as optically controlled capacitors

2.3.1.1 Frequency tuning of a 25GHz HEMT oscillator 2.3.1.2 Frequency tuning of a 1.8 GHz DRO oscillator 2.3.2 Optically induced changes in conductivity

2.3.2.1 Optical frequency tuning 2.3.2.2 Amplitude switching 2.4 Sununary

Chapter 3: Oscillator phase locking by phase locked loop, injection locking and

injection phase locked loop

3.1 Introduction

3.2 Unified PLL models for system comparisons 3.2.1 Phase locked loops (PLL)

3.2.1.1 Non-linear dynamic equation 3.2.1.2 Small signal analysis

3.2.1.3 PLL terminology 3.2.2 Injection locked oscillators

3.2.2.1 PLL model for injection locking 3.2.3 Injection phase locked loops

3.2.3.1 Small signal analysis

3.2.4 System comparisons using unified PLL model 3.3 Operation in practical systems

3.3.1 Control loop stability

3.3.2.1 Stability in first order PLLs 3.3.1.2 Stability in second order PLLs 3.3.1.3 Stability in first order IPLLs 3.3.2.4 Stability in second order IPLLs

3.3.3 Differential delay

3.3.3.1 Differential delay effects on IPLL operation 3.3.4 Limits of PLL model for injection locking

3.3.5 Noise considerations 3.4 Summary and conclusion

Chapter 4: Characterisation of optical control effects in a MESFET oscillator

4.1 Introduction

4.2 Photocontrol mechanisms 4.2.1 Photoconductive effects 4.2.2 Photovoltaic effects

4.2.2.1 Photovoltaic gate biasing

4.2.2.2 Schottky junction layer charge redistribution

4.2.2.3 Photovoltage generation at the substrate/channel interface and back-gating

4.2.3 Other photoeffects 4.3 Experimental set-up

4.3.1 Oscillator design

4.3.2 Experimental arrangement 4.4 Photoeffects in the MESFET device

4.4.1 Gate photocurrent

4.4.2 Optical control of channel current 4.4.2.1 Forward bias characteristics

4.4.2.2 Comparison of Vgs and optical control 4.4.3 Discussion

4.5 Optical frequency tuning

4.5.1 Direct optical frequency tuning 4.5.1.1 Results

4.5.1.2 Discussion 4.5.2 Photovoltaic bias tuning

4.5.2.1 Vgs tuning

4.5.2.2 Photovoltaic tuning

4.5.2.3 Frequency response manipulation

4.6.1 Experimental procedure 4.6.2 Results

4.6.3 Discussion 4.7 Summary

Chapter 5: Experimental investigation of optical phase locking systems

5.1 Introduction

5.2 Experimental arrangement 5.2.1 Setup details

5.2.2 Loop design 5.3 Tracking performance

5.3.1 Static phase error from frequency detuning 5.3.2 Phase error transient response

5.3.2.1 Transient response of optical injection locking 5.3.2.2 Transient response of OPLL and OIPLL systems 5.4 Oscillator phase noise suppression

5.4.1 Measurement details 5.4.2 Results

5.4.3 Discussion

5.5 Stability and effects of delay within the PLL 5.5.1 PLL instability

5.5.2 OIPLL instability 5.6 Differential delay

5.6.1 Gain competition

5.6.2 Differential delay in second order type II OIPLLs 5.7 Summary

Chapter 6: Practical optical phase locking systems

6.1 Introduction

6.2 Optical frequency tuning in a phase locked loop 6.2.1 Effective oscillator tuning constant 6.2.2 Loop amplification and loop bandwidth

6.2.3 Practical system aspects

6.2.3.1 FM frequency response 6.2.3.2 Linearity

6.2.3.3 Tuning range 6.2.4 Summary and discussion 6.3 Optical injection locking

6.3.1 Optical injection locking sensitivity

6.3.2 Optical injection locking range variation with operating conditions 6.3.3 Summary and discussion

6.4 Optical injection phase locked loop 6.4.1 Loop gain

6.4.2 Practical system aspects

6.4.2.1 OIPLL natural frequency 6.4.2.2 Pull-in operation

6.4.2.3 Differential delay

6.4.2.4 AM and PM noise effects 6.4.3 Summary and discussion

6.5 Discussion

Chapter 7: Application of phase locked receivers to optical microwave signal distribution networks

7.1 Introduction

7.2 System calculations

7.2.1 Optical transmitter

7.2.2 Optical fibre network losses 7.2.3 Sensitivity

7.2.3.1 Sensitivity in direct detection receivers 7.2.3.2 Sensitivity in OPLL

7.2.3.5 Sensitivity in OIPLL 7.2.4 lin k budget

7.3 Practical aspects of direct detection receivers 7.3.1 Gain requirements in amplification chain 7.3.2 Phase accuracy in amplification chain 7.3.3 Monolithic technology

7.4 Practical aspects of optically phase locked oscillator receivers 7.4.1 Systems

7.4.2 OPLL phase locked receivers 7.4.3 OIPLL phase locked receivers

7.4.4.1 Remote data mixing schemes 7.4.4.2 Phase priming

7.5 Summary

Chapter 8: Summaiy

8.1 Introduction

8.2 Optical phase locking systems

8.2 Use of optical control in phase locking systems 8.3 Phased array applications

Appendices

Appendix A Phase locked loop operation A/1 PLL definitions

A/2 Linear tracking A/3 Non linear tracking

A/3.1 Tracking limits: hold-in range A/3.2 Acquisition

A/4 Noise effects A/4.1 Phase noise

A/4.2 Amplitude modulation noise

Appendix B. Experimental system design

arrangement B/3 Loop design

B/3.1 Open loop gain B/3.2 Time constants

Appendix C Noise analysis of experimental OPLL C/1 Sources of noise

Appendix D. Signal-to-noise ratio in microwave direct detection PIN-FET receivers

D/1 Signal-to-noise expression D /2 Noise

D/3 Calculation

Lis t o f il l u s t r a t io n s

Fig. 1.1 Optical control of microwave oscillators

Fig. 1.2 Oscillator phase locking by phase locked loop employing optical frequency control

Fig. 1.3 Optical injection locking with closed loop control of static phase error

Fig. 1.4 Alternative methods of microwave modulated optical signal reception

Fig. 3.1 Schematic of oscillator phase locking systems

Fig. 3.2 Block diagram of phase locked loop

Fig. 3.3 Block diagram of injection locking and its equivalent phase locked loop representation

Fig. 3.4 Block diagram of injection phase locked loop assuming equivalent PLL representation of injection locking

Fig. 3.5 Simplified block diagram of injection phase locked loop used for small signal analysis

Fig. 3.6 Nyquist diagrams indicating stability in first order phase locked loops

Fig. 3.7 Nyquist diagrams for an ideal second order loop with varying the loop damping coefficient, ç

Fig. 3.8 Nyquist diagrams comparing the condition for stability in first and second order phase locked loops

Fig. 3.9 Nyquist diagrams for first order injection phase locked loops

Fig. 3.10 Nyquist diagrams for second order injection phase locked loops

Fig. 3.11 Nyquist diagram indicating the limit to the natural frequency in a second order injection phase locked loop

Fig. 3.12 Schematic of injection phase locked loop indicating the high frequency paths and their delay times

Fig. 3.13 Rationalisation of injection phase locked loop with differential delay

Fig. 3.14 Nyquist diagram showing the failure of the injection phase locked loop to meet the stability criterion when the differential phase is equal to n

Fig. 3.15 Injection phase locked loop showing the introduction of AM noise at the input to the phase locked loop component

Fig. 4.1 Cross-section view of a GaAs MESFET indicating the principal photoeffects

a load line construction for assessing the photovoltaic gate bias effect

Fig. 4.3 Detail of NEC 900000 MESFET

Fig. 4.4 MESFET small signal equivalent circuit and oscillator circuit diagram

Fig. 4.5 Experimental arrangement for the illumination experiments

Fig. 4.6 Laser power/bias current characteristics

Fig. 4.7 Gate circuit photocurrent variation with illumination power

Fig. 4.8 Ids/Vds characteristics with and without illumintion

Fig. 4.9(a) Drain-source current variation with gate-source potential

Fig. 4.9(b) Drain-source current variation with illumination power

Fig. 4.10 Frequency shift with illumination power

Fig. 4.11 Optical tuning responsivity variation with intensity modulation frequency

Fig. 4.12 Frequency tuning effect of gate-source bias potential

Fig. 4.13 Optically induced frequency shift due to photovoltaic gate bias (Rg = 10 kohms)

Fig. 4.14 Frequency response of overall tuning responsivity arising from photovoltaic gate bias and residual tuning effects

Fig. 4.15 Spectrum analyser display sequence showing the optical injection locking of the MESFET oscillator operating at 1.48 GHz. Injection locking range = 1.1 MHz

Fig. 4.16 Optical injection locking range variation with peak optical signal power

Fig. 5.1 Experimental arrangement for the OIPLL system

Fig. 5.2 Schematic of a phase locked loop using optical frequency tuning

Fig. 5.3 Static phase error variation with input signal frequency for the optically injection locked oscillator.

Fig. 5.4 Static phase error variation with input signal frequency for (a) the optically injection locked oscillator, and (b) a second order optical injection phase locked loop

Fig. 5.5 Phase transient for the optically injection locked oscillator due to switched input frequency

Fig. 5.6 Injection locking acquisition phase transient

Fig. 5.7 Phase transients for the optical phase locked loop and the optical injection phase locked loops due to switched input frequency

Fig. 5.8 Phase noise suppression by phase locking systems

Fig. 5.9 Phase noise-to-carrier ratio for phase-locked oscillator output spectrum

Fig. 5.10 Experimental arrangement for the investigation of the effects of loop delay

Fig. 5.11 Phase noise peaking in optical phase locked loop output spectrum due to loop delay

Fig. 5.12 Phase transient with switched input signal frequency for the optical phase locked loop with loop delay causing marginal stability

Fig. 5.13 Phase noise peaking in the optical injection phase locked loop output spectrum due to delay in the phase locked loop component

Fig. 5.14 Phase transients with switched input signal frequency for the optical injection phase locked loop with PLL loop delay and varying loop natural frequency

Fig. 5.15 OIPLL lock-in range variation with differential path length

Fig. 5.16 Spectrum analyser display sequence showing the failure of the optical injection phase locked loop to lock when the injection locking and phase locked loop act in opposition

Fig. 6.1 Diagram showing the practical implementation of an OPLL

Fig. 6.2 Diagram showing the practical implementation of a heterodyne OPLL

Fig. 6.3 Optical injection locking ranges in practical microwave oscillators

Fig. 6.4 Optical injection locking ranges in practical oscillators calculated assuming an optical coupling efficiency of 50%

Fig. 6.5 Diagrams showing the practical implementation of an OIPLL

Fig. 7.1 1:M optical fibre distribution network using optically phase locked oscillators

Fig. 7.2 Maximum operating powers demonstrated in semiconductor amplifiers and oscillators

Fig. 7.3 Remote data mixing scheme for the transmission of RF signals by optical fibre

modulation noise at the input of a PLL

Fig. B1 Circuit diagram for activa loop filter and laser driver

Fig. C l Schematic of experimental PLL system indicating sources of noise within the system

Fig. C2 Equivalent PLL schematic used for noise analysis

Fig. D1 SNR for a microwave optical link as a function of mean optical signal power

Lis t OF TABLES

Table 2.1 Optical absorption coefficients and absorption edges in various semiconductors

Table 2.2 Summary of optical frequency tuning techniques

Table 2.3 Summary of optically controlled microwave switching techniques

Table 2.4 Summary of optical injection locking techniques

Table 3.1 Phase locking system operating parameters

Table 4.1 Comparison of calculated and measured optical tuning responsivity at various means powers

Table 5.1 Experimental system parameters

Table 5.2 OPLL system parameters for loop delay stability measurements

Table 5.3 OIPLL system parameters for loop delay stability measurements

Table 5.4 Theoretical and measured limits to natural frequency in delay limited OIPLL

Table 7.1 Fixed and variable fibre network losses

Table 7.2 lin k budget for optical distribution network with various optical/millimetre wave receivers

Table A1 Expression for the natural frequency and damping coefficient in second order loops

Table A2 Static phase error in phase locked loops under varying input signal conditions

Table B1 Practical system loop gain parameters

Table B2 Experimentally determined time constants and loop parameters

Table D1 Values used for estimate of link SNR in Fig. D1

Symbol Unit Description

k J/K Hofmann’s constant (= 1.39 x 10^^ x ^ ) h J s P l a ^ constant (= 6.026 x 10"^* J s) q C electronic charge (= 1.6 x 10"^® C)

T K temperature

W . W microwave oscillator output power W injected microwave power

Q oscillator external loaded quality factor *o(‘) rad phase of the oscillator

«i(0 rad phase of the injected signal

"o rad s’ ^ ang. freq. of oscillator "l rad s'^ ang. freq. of locking signal

“£ rad s~^ ang. freq. of free running oscillator

Aw rad s*^

F(s) tranfer function of loop filter K , V rad”^ phase detector constant K rad s~^ V”^ oscillator tuning constant

A amplification within loop

K s - ' (= AK^Kj) overall amplification within loop H(s) — closed loop transfer function

G(s) — open loop transfer function

“n rad s'^ natural frequency of loop

damping coefficient of loop AWl rad s’ ^ lock-in range

AWg rad s"^ hold-in range AWp rad s'^ pull-in range

' " i n j rad s'^ injection locking range

K . s-^ effective amplification within inj. lock, equiv. loop G i n j W — open loop transfer function of inj. lock, equiv. loop

GpllW — open loop transfer function of PLL component of

IPLL

^BW s time constant for bandwidth limiting component in PLL

s total propogation delay within PLL

R(s) additional filtering response within loop (other than F(s))

rad phase error at oscillator

*pdW rad phase error at phase detector

^diff rad static differential phase error rad phase noise spectral density

P o l W incident optical power

P os W optical power at optical source

cm^/V s electron mobility cm ^/V s hole mobility

Jo A m”^ junction reverse saturation current density Jph A m~^ photocurrent density

♦p V hole quasi Fermi energy level

(continued)

Symbol Unit Description

Pn

^Pn

V V junction photovoltage arising from illumination m

W ' l / 2

modulation index of optical signal 7 optical injection locking coefficient

«„(s) rad s '^ W ^ optical tuning sensitivity of optically tuned oscillator

a optical attenuation

K.

Rl Ohms load resistance

F amplifier noise figure

B Hz bandwidth

n coupling effidency at photodiode of photoreceiver

V Hz frequency of photon

N V^Hz'^ noise voltage spectral density

♦n(0

‘„’(0

C W carrier power

SNR^ input signal to noise ratio RIN dBH z'^ relative intensity noise

MTR mixer noise temperature ratio

rad^ phase jitter variance

rad^ Hz"^ mean square phase jitter spectral density

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Chapter 1

In t r o d u c t io n a n d o v e r v ie w

1.1 I n t r o d u c t i o n

Optical control techniques are potentially useful for the control of microwave and millimetre wave circuits. The optical interface allows the control signal to be introduced simply and conveniently without parasitic effects and problems of signal isolation associated with electronic control. In addition, with the increasing importance of optical techniques for signal distribution, there are considerable attractions in employing optically controlled components to interface between electrical and optical domains.

Photoeffects have been studied in most microwave components used today. The basic process is one of photon absorption and consequent free carrier generation in the regions of the semiconductor exposed to the illumination. Optical control occurs when the photosensitive component is incorporated into a circuit. A number of optical control functions have been demonstrated. These include amplitude modulation in microwave amplifiers and oscillators, frequency modulation and injection locking in microwave oscillators, phase shifting, opto­ electronic mixing, and various switching functions in microwave transmission lines.

Impetus for research into optical control in recent years has come from the development of phased array antennas and the use of optical fibre microwave signal distribution networks. Modem phased array antennas are complicated microwave structures with complex and demanding signal distribution requirements. The distribution role is well suited to optical fibres which are relatively small and lightweight compared to conventional microwave waveguide or coaxial transmission lines, and offer immunity from electromagnetic interference [1][2][3]. In recent years two arrays using microwave optical fibre distribution networks have been demonstrated [4][5].

Concepts for optical fibre networks have developed from the simple distribution role to optical fibre processing techniques. Optical fibres can be used in true-time- delay beamforming networks [6][7][8] which match the time-of-arrival of the beam at a defined angle. Parallel processing has also been proposed as a means of calculating the amplitude and phase weights at the elements [9]. Simple experimental schemes have been demonstrated [10].

reduction in the number of components at each element. The simplification and cost reduction to the overall array with element numbers of several hundreds or thousands can be substantial.

Baseband optical control of gain and phase [11][12] has been studied specifically towards the development of optically controlled microwave monolithic integrated circuits. However there is most benefit in the reception of the microwave modulated optical signals which are difficult to process electrically. Work has been performed on microwave opto-electronic mixing [13] [14] [15] to enable the direct use of optical microwave local oscillator signals. Another technique, which is the subject of the present work, is the use of optically phase locked microwave oscillators for microwave optical signal reception [16][17]. Optical injection locking has attracted most interest, however other methods of optical phase locking include phase locked loops employing optical frequency modulation [18], and a novel method of closed loop optical phase and frequency control [19][20][21]. These systems will be introduced in more detail below.

1 .2 Op t ic a l PHASE LOCKING SCHEMES

Optical control techniques can be usefully applied to phase locking of microwave oscillators in situations in which electrical control is difficult. The use of optical control signal offers infinite isolation of the oscillator circuit and therefore avoids all parasitic effects from bias supplies and electrical contacts associated with the control signal introduction. Consequently optical control will be a strong candidate for very high frequency applications at which these effects are important.

There are three basic functions available for the optical control of solid state oscillators which provide the building blocks for optical phase locking schemes. These are illustrated in Fig. 1.1.

a) Amplitude switching. Under certain conditions the illumination can enhance or quench the oscillation.

CHAPTER 1

(a)

%

(b)

Pop

t

(c)

Sp

0

t

0 op

k fm

m

Fig 1.1 Optical control of microwave oscillators: (a) optical switching

(b) optical frequency tuning, (c) optical injection locking,

c) Optical injection locking. Modulation of the illumination results in modulation of the photoeffects operating within the device. Intensity modulation at a frequency close to the free running oscillation frequency or a subharmonic can achieve optical injection locking - that is the locking of the oscillator frequency to that of the microwave intensity modulated optical signal.

1.2.1 Phase lock loop schemes and ii^ection locking

An established technique for phase locking of oscillators at all frequencies is by phase locked loop (PLL). This is basically a feedback control loop which controls the frequency of an oscillator such that its output phase tracks the phase of an injected signal. Optical control of the microwave oscillator can be substituted for the electronic frequency control. Additional components are required, firstly to generate the optical controlling signal, for example a laser or light emitting diode (LED), and secondly to optically couple the light into the oscillator. The proposed loop is shown in Fig 1.2. Other components of the loop are unchanged. The advantages rest entirely in the relative merits of optical tuning compared to electrical frequency control. Important parameters requiring consideration are the maximum optical tuning range, the optical tuning modulation bandwidth, tuning sensitivity and linearity; all of which impact on the performance of the phase locking system. Phase locked loops using optical frequency modulation have not received much attention, and this is reflected in the current lack of information referring to the characterisation of optical frequency tuning effects.

Optical injection locking offers another method of phase locking by purely optical means. This approach differs fundamentally from the PLL above in that locking occurs to an optical signal. Therefore it is more useful for applications in which the locking signal received is in the optical domain already. The locking is a fundamental optical control function and therefore no further components are necessary. Compared to the PLL approach, locking is achieved conveniently and simply.

1.2.2 Optical phase and frequency control

CHAPTER 1

injected signal. The phase error is dependent on the ratio of the frequency difference between the injected signal and the free running oscillation frequency, and the injection locking range. Following Adler [22] it can be expressed in the form;

1 [ü>£ - wJ

L

J

- sin -I 1.1

where is the static phase error, and u>^ are the angular frequencies of the oscillator and injected signal, and is the optical injection locking range.

Frequency control of the oscillator is achieved by changing the injected signal frequency. Phase control is achieved by using the optical frequency tuning sensitivity to tune the oscillator free running frequency and therefore change the static phase error according to Equation 1.1. Phase control by this method has been demonstrated in the optical phase modulation at 500 kHz with 78.8° peak- peak phase variation of a 7.2 GHz MESFET oscillator [23]. The oscillator phase was controlled by varying the mean intensity of the optical injection locking signal.

The system can be converted to operate with closed loop control of the phase by sensing the phase error and feeding back to the frequency tuned oscillator Fig 1.3. The resulting system can be considered from two perspectives. From the point of view of injection locking, the feedback path, which operates as a phase locked loop, stabilises the system by reducing drift in the oscillator free running frequency. It also enables the injection locked oscillator to track frequency variations in the injected signal.

An alternative perspective considers the system as fundamentally a phase locked loop employing optical frequency modulation but with an additional link resulting in simultaneous injection locking. The injection locking effect provides a wideband gain in addition to the gain of the PLL and therefore can be used in the system design to reduce the requirement for the latter.

phase loo p d e te c to r filter in put

F(s)

optically tu n e d

laser lens oscillator o u tp u t

Fig 1.2 Oscillator phase locking by phase locked loop employing optical frequency control.

microwave m odulated optical signal

beam

optically tu n ed oscillator splitter

lens _{o u tp u t}

>

-laser

loop filter F(s)

p h o to d e te c to r phase detector

CHAPTER 1 7

1 3 P h a s e l o c k e d o s c i l l a t o r o p t i c a l r e c e i v e r s

Optically phase locked microwave oscillators can be used for the reception of microwave intensity modulated optical signals. Conventional reception methods employ photodiodes to convert to the electrical domain and subsequent electronic amplification. The phase locked oscillator acts as a photoreceiver with high narrowband gain and high output power. It therefore not only replaces the photodiode but also a large number of amplification stages, Fig 1.4.

As discussed above, an important application for optically phase locked oscillator receivers is in phased array antennas. Phased array antenna design has tended towards so-called active systems in which active solid state devices, located at the elements in the array aperture, provide the power to the transmit beam and amplification to the receive signals prior to summation. The microwave signals required to correlate the phase of the signals at the elements across the aperture can be distributed by optical fibre network. Therefore it is necessary to receive the microwave optical signals with high phase accuracy at each element of the array. The simplification offered to microwave optical signal reception by direct optical phase locking is highly desirable to the phased array anteima as this has the potential to substantially reduce system costs.

Conceptually optical injection locking is the simplest method of phase locking a microwave oscillator to an optical signal. Consequently this has attracted most attention. Direct optical injection locking in microwave oscillators has been demonstrated in IMPATT [28], MESFET [29], HEMT [30] HBT oscillators [31]. The simultaneous direct optical injection locking of two MESFET oscillators also has been demonstrated [16]. Indirect injection locking has been studied for the reception role [17]. However, indirect injection locking is performed by conversion from an optical to an electrical signal by photodiode and then electrical injection into the oscillator circuit. As such injection locking is not optical - the locked oscillator is used simply as a very high gain narrowband amplifier.

(a)

microwave m odulated optical signal

optically tu n ed oscillator

< s >

o u tp u t

— ►

(b)

microwave m odulated

optical signal o u tp u t

PIN diode and low noise front-end amplifier

cascaded gain stages

Fig 1.4 Alternative methods of microwave modulated optical signal reception: (a) by optically phase locked oscillator; (b) by photodetection

CHAPTER 1 9

Closed loop systems such as the phase locked loop, and the combined optical injection locking and phase locked loop system, have the ability to reduce the static phase error to zero. The application of conventional phase locked loops to phased arrays and, in particular, the use of phase locked solid-state microwave sources to obtain high power gain with few active devices was considered by Austin [32]. Wideband loops for high speed acquisition intended for pulsed operation in phased arrays also have been developed [33]. As a result, the optical techniques and closed loop optical phase locking schemes discussed above are appropriate for optical signal reception.

1.4 C o n t e n t s

The present work sets out to consider the three optical phase locking systems from both an analytical and an experimental perspective. The results enable a comparative assessment of the practical application of the three phase locking systems. The application of the phase locking systems to the reception of microwave optical signals, specifically in phased array antennas, will be considered and compared to the alternative conventional techniques.

Chapter 2 reviews the techniques for optical control of microwave oscillator circuits. The principal mechanisms causing the optical control effects are explained and the relative sensitivity of the optical control effects to the control signal magnitude discussed. The scope of the review covers the optical control functions of amplitude modulation, frequency modulation and injection locking. Oscillator control by both direct and indirect optical control are discussed. Direct optical control is performed by direct illumination of the active device in the oscillator circuit: examples of control in both two and three-terminal devices are described. Indirect control is achieved by incorporating a discrete photosensitive component into the oscillator circuit.

Chapter 3 presents an analytical model for the three phase locking systems introduced in Section 1.2. The analysis is presented in general terms and therefore is appropriate for systems employing both optical and electrical control techniques. The model is based on small signal phase locked loop analysis and enables direct comparison of the systems and also a convenient means of system design. In addition practical differences which distinguish the three systems are discussed.

phase lock loop. Chapter 4 concentrates on characterisation of the photoeffect and optical control functions. TTie results of the characterisation are compared to other published examples. Effort is made to identify the principal mechanisms in action, particularly in affecting the oscillator frequency control.

Chapter 5 describes experiments investigating the optical phase locking systems. The systems are assembled using the details of the control function characterisation in Chapter 4. The results firstly confirm the validity of the analytical model of Chapter 3 and secondly demonstrate practical effects predicted involving delay effects and stability conditions.

Chapters 6 and 7 consider the practicalities of optical phase locking systems. Chapter 6 considers the three systems separately, drawing comparisons where possible. For each a quantitative estimate, based on the review and experimental results, is made considering the available performance under ideal conditions. Then limitations that are likely in practice are discussed qualitatively.

Chapter 7 assesses the use of optically phased locked oscillators as receivers in optical fibre distribution networks specifically appropriated for the phased array antenna application. Phase locked oscillator receivers are considered for the reception of narrow bandwidth high frequency phase reference signals. A link budget calculation is made estimating the maximum number of oscillators that can be phase locked by the various phase locking systems and by a single optical signal transmitter via an optical fibre distribution network. The results are compared to the performance of a conventional receiver using photodetection at a photodiode. The practical aspects of the optically phase locked oscillators and the conventional receivers based on a photodiode and amplifier cascade are then discussed.

C hapter 2

O p t ic a lc o n t r o l o f m ic r o w a v ea n d m il l im e t r ew a v eo s c il l a t o r s

2.1 I n t r o d u c t i o n

Optical control is important in providing an alternative to conventional electrical techniques for the control of high frequency circuitiy. This chapter reviews the techniques for optical control of microwave and millimetre wave oscillator circuits, discussing the mechanisms for the optical control and the performance demonstrated.

The discussion below is divided into two sections, concerned with direct and indirect optical control techniques. Optical control of microwave and millimetre wave oscillators can be performed by direct illumination of the active device in the oscillator circuit. Photon absorption and free carrier generation within the device causes changes in the operation and therefore in the oscillation amplitude and frequency. Similar control effects can be achieved by illumination of other photosensitive components incorporated into the oscillator circuit. This latter method is referred to as indirect control since the active oscillator device is not directly affected.

Chapter 1 discussed the principle optical control functions available for the control of solid state oscillators. Interest is mainly with optical frequency modulation and optical injection locking for phase locking applications. However amplitude modulation is often associated with the illumination effects and so is also considered below.

2.2 D i r e c t OPTICAL CONTROL

A recent book [34] reviews direct optical control and most of the recent publications on the subject. Below, the main points are described with the objective of conveying some feel for the control sensitivity that has been demonstrated, and that which is likely in the future.

2^.1 PhotoefTects and wavelength dependency

Photoeffects in semiconductor devices depend on photon absorption and subsequent electron-hole pair generation. The requirement for an absorption event is that the photon energy must exceed that of the semiconductor bandgap. This introduces some wavelength dependency. The absorption can be characterised by the absorption coefficient a, which is used to express the decay in photon flux density with penetration depth, x:

p ( x ) - p ( 0 ) .e x p [ - a x ] 2 . 1

where p(x) is the optical flux density x. The absorption edge refers to the illumination wavelength corresponding to the band-gap energy. Illumination with a wavelength longer than the absorption edge value will not be absorbed.

Table 2.1 shows the absorption parameters in various semiconductors. For wavelengths of less than 900 nm, most show some absorption. The longer wavelengths at 1.3 pm and

Table 2.1(a)

a absorption coefficient (m*^)

semiconductor 826 nm 1.26 pm 1.55 pm

Si 1.9 X 10*

Ge 4.53 X 10* 8.20 X 10* 4.6 X 10* [351

InP 3.09X 10* —

GaAs 1.1 X 10* — —

Table 2.1(b)

composition Ga^^Ini _ ^ A S y P ^ _ y

X 0 0.16 0.27 0.40 0.47

y 0 0.33 0.60 0.85 1.0

absorption edge 300 K

(pm) [36]

0.92 1.12 1.30 1.55 1.65

CHAPTER 2 13

1.55 /iin are used in optical fibre transmission systems due to their low attenuation in optical fibre. However absorption at these wavelengths is limited to a few GaAs/InP alloys.

2J22 Gunn diode oscillators

Most work on the optical control of Gunn diode oscillators to date has been experimental, with a wide range of effects observed [37,38,39,40]. A theoretical framework describing the photoeffects in a Gunn diode is yet to be developed.

222.1 Frequency tuning and amplitude switching

Illumination was shown to enable oscillation in a normally-off GaAs Gunn diode [37]. The diode was biased just below threshold. On illumination, the optically generated plasma causes a local decrease in electric field strength and this enhances sufficiently the field strength elsewhere to trigger the dipole domain oscillation current. The low-field plasma region also acts as a virtual anode for the domains. The position of the optical focus was observed to determine the oscillation period allowing frequency tuning between 6.7 and 10 GHz. Pulsed illumination with 610 nm wavelength and pulse duration and energy of 3 ps and 1 p j (333 mW peak power) triggered bursts of oscillation from the diode. The burst duration increased with nsing optical pulse e n e r ^ ^ 4 ns duration with optical pulse energ»«iof osTpTanj r^ecr.vei^ . This effect is associated with the finite lifetime of the induced plasma.

2 2 2 2 Trap effects