Microwave photonic phase shifter based on a nonreciprocal optical phase shifter inside a Sagnac interferometer

Optics Communications 324 (2014) 127–133

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Microwave photonic phase shifter based on a nonreciprocal optical phase shifter inside a Sagnac interferometer E.H.W. Chan n School of Electrical and Information Engineering and Institute of Photonics and Optical Science, University of Sydney, NSW 2006, Australia

art ic l e i nf o

a b s t r a c t

Article history: Received 17 December 2013 Received in revised form 18 February 2014 Accepted 11 March 2014 Available online 28 March 2014

A new microwave photonic phase shifter is presented. It is based on an optical single-sideband modulator and an optical phase shifter inside a Sagnac interferometer. It relies on the use of the novel nonreciprocal optical phase shifter to obtain an optical phase shift, which converts into an RF phase shift. The microwave photonic phase shifter has the advantages of realising a continuous 0–3601 phase shift on microwave signals using off-the-shelf components and only requiring a single control. It also has a compact structure and robust performance. Experimental results are presented, which demonstrate 0–3601 phase shift of microwave signals over a sub-octave frequency band. & 2014 Elsevier B.V. All rights reserved.

Keywords: Phased-array radar Phase shifter Sagnac interferometer Optical signal processing Microwave photonics

1. Introduction Photonic signal processing offers a new and powerful paradigm for processing high bandwidth signal. It eliminates the electromagnetic interference and overcomes the bottlenecks caused by limited sampling speeds in conventional electrical signal processors [1]. Many applications such as radars and phased array antennas require controlling the phase of a microwave signal. The current electrical phase shifters either have limited bandwidth, have limited phase shift range, or can only realise discrete phase shift. As such, research on microwave photonic phase shifters has been conducted over the past 30 years. Various architectures with different performances have been reported [2–17]. The technique [4–7] that relies on converting an optical phase shift into an RF phase shift is of interest because of its simplicity as only one control is needed. However, it requires coherent detection of two optical signals in a Mach Zehnder interferometer. Hence, the phase shifter needs to be implemented on an integrated optical structure in order to obtain a stable performance. Stimulated Brillouin scattering can be used to implement a microwave photonic phase shifter via the conversion of an optical phase shift into an RF phase shift without the need of using an integrated optical structure [13]. However, it requires a long length of fibre, which needs to be temperature controlled in order to obtain a stable performance, and requires two optical modulators, which increases the cost and the system complexity.

In this paper, we present for the first time that a microwave photonic phase shifter based on the optical-to-RF phase shift conversion technique implemented using a Sagnac interferometer. The new microwave photonic phase shifter has a compact structure and can be constructed using commercially available optical components while having a robust performance. The Sagnac interferometer based microwave photonic phase shifter is verified using VPItransmissionMaker photonic simulation software [18] and is demonstrated experimentally. Both the simulation and experimental results confirm continuous 0–3601 RF phase shift that can be obtained by controlling the nonreciprocal optical phase shift inside a Sagnac interferometer. This paper is organised as follows. The operation principle of the Sagnac interferometer based microwave photonic phase shifter is described in Section 2. The analysis and the simulation results of the phase shifter are presented in Section 3. This section also presents the Jones matrix analysis of the novel nonreciprocal optical phase shifter (NOPS) to realise the nonreciprocal optical phase shift for the light travelled in opposite direction inside the Sagnac interferometer. Experimental results for the microwave photonic phase shifter, which demonstrate a continuous 0–3601 phase shift on an RF signal over a sub-octave frequency band, are described in Section 4. Finally, conclusions are given in Section 5

2. Phase shifter topology and operation principle n

Tel.: þ 61 2 9351 4866; fax: þ 61 2 9351 3847. E-mail address: [email protected]

http://dx.doi.org/10.1016/j.optcom.2014.03.024 0030-4018/& 2014 Elsevier B.V. All rights reserved.

A microwave photonic phase shifter can be implemented by integrating an optical frequency shifter in parallel with an optical

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E.H.W. Chan / Optics Communications 324 (2014) 127–133

phase shifter as shown in Fig. 1 [5]. An RF phase shift can be obtained by controlling the phase of the optical carrier through the optical phase shifter. The same principle is used to realise an RF phase shift in the Sagnac interferometer based microwave photonic phase shifter. Fig. 2(a) shows the topology of the Sagnac interferometer based microwave photonic phase shifter. Continuous wave light from a laser source is fed into a polarisation maintaining optical coupler connected in a way to form a polarisation maintaining Sagnac interferometer. The light is split equally, so that half travels in the clockwise (CW) direction and the other half travels in the counter clockwise (CCW) direction. Inside the interferometer, there is a single-sideband (SSB) modulator, which is formed by a dual-drive Mach Zehnder modulator and a 901 hybrid coupler [19]. The SSB modulator is driven by an input RF signal and is biased at the quadrature point. Note that, due to the velocity mismatch effect in the LiNbO3 travelling wave electro-optic modulator, the modulation efficiency is low at high frequencies when the modulator operates in the reverse direction [20]. Hence, the CCW light passed

Input RF signal Optical frequency shifter

Continuous wave light

fc+fRF

RF modulated optical signal

Optical phase shifter fc Control voltage Fig. 1. A conventional microwave photonic phase shifter structure. fc and fRF are the continuous wave light carrier frequency and the input RF signal frequency respectively.

Input RF signal Bias 0° 90°

Γ dB isolation optical isolator

Dual-drive modulator CW light

Nonreciprocal optical phase shifter

CCW light

Laser Optical isolator

50:50 coupler

Photodetector

Input RF signal Backward modulated 0° 90° optical signal

Bias

Forward modulated optical signal

through the SSB modulator in opposite direction to the RF signal propagating inside the modulator consists of an optical carrier and a small-amplitude RF modulation sideband. The amplitude of this RF modulation sideband reduces as the frequency increases. At high frequencies (4 10 GHz), this sideband is negligible compared to the sideband carried by the light travelled in the CW direction inside the Sagnac interferometer. Hence one can think of the light passed through the SSB modulator in backward direction consists of an optical carrier only. On the other hand, the light passed through the SSB modulator in the forward direction consists of an optical carrier and an RF modulation sideband as shown in Fig. 2(b). It can be seen from Fig. 2 that if an optical filter is used after the modulator to filter out the optical carrier carried by the CW light then the optical components of the two counterpropagating light after travelling through the Sagnac interferometer are the same as that of the conventional microwave photonic phase shifter shown in Fig. 1. Hence an RF phase shift can be obtained by using a NOPS to control the phase of the optical carrier travelled in the CCW direction inside the Sagnac interferometer. An optical isolator with Γ dB isolation can be used as a unidirectional attenuator inside the Sagnac interferometer as shown in Fig. 2 (a) to suppress the unwanted optical carrier travelled in the CW direction. This makes the CW light carrier to be much smaller than the CCW light carrier. Therefore, after photodetection, the wanted output RF signal generated by the CCW light carrier beats with the CW light RF modulation sideband dominates the unwanted output RF signal generated by the beating between the carrier and the RF modulation sideband carried by the CW light. Note that the use of a Γ dB isolation optical isolator inside the Sagnac interferometer also reduces the amplitude of the RF modulation sideband carried by the CW light, which causes output RF signal power reduction. However, this problem can be overcome by using a high power laser source or using an erbium-doped fibre amplifier (EDFA) to increase the optical power into the photodetector. The CW and CCW light after travelling through the Sagnac interferometer recombine at the coupler and are then detected by the photodetector. Fig. 3 shows the topology of the novel NOPS used in the Sagnac interferometer to obtain the nonreciprocal optical phase shift. It consists of a variable waveplate in between two oppositely oriented 451 Faraday rotators connected in free space. The variable waveplate, which is normally referred to as a Babinet compensator, is adjusted to have a 451 device angle and its phase angle can be set between 01 and 3601. The NOPS does not change the light polarisation state but introduces different amounts of optical phase shift to the light passed through the NOPS in opposite direction. A nonreciprocal optical phase shift is obtained by controlling the phase angle of the variable waveplate. Due to the presence of the NOPS inside the Sagnac interferometer, the two counterpropagating light components have different optical phases. This optical phase difference is converted to an RF phase shift when the CCW light optical carrier beats with the RF modulation sideband carried by the CW light at the photodetector. Since the two counterpropagating lights travel in exactly the same path, there is no coherent interference problem. Hence the output of the Sagnac interferometer based microwave photonic phase shifter is stable and insensitive to environmental perturbations. An optical modulator inside a Sagnac interferometer has been used

Faraday rotator

Faraday rotator

fc

Dual-drive modulator

fc fc+fRF

Fig. 2. (a) Structure of the Sagnac interferometer based microwave photonic phase shifter. (b) Optical spectrum of the light passed through the single-sideband modulator in opposite direction. The bold line represents polarisation maintaining components.

45°

Variable waveplate

–45°

Nonreciprocal optical phase shifter Fig. 3. Structure of the nonreciprocal optical phase shifter. The Faraday rotators and the variable waveplate are connected in free space.

E.H.W. Chan / Optics Communications 324 (2014) 127–133

The performance of the NOPS can be obtained from the Jones matrix analysis [24]. The Jones matrix of a θ1 Faraday rotator is given by
 cos θ  sin θ J FR ðθÞ ¼ ð1Þ sin θ cos θ and the Jones matrix of a variable waveplate with the device angle of 451 is given by    # "  j sin ϕ2 cos ϕ2 ϕ ϕ J WP ðϕÞ ¼ ð2Þ cos 2  j sin 2 where ϕ is the phase angle of the variable waveplate. Therefore the Jones matrices of the NOPS for the light travelled in the forward and backward directions are given by J NPS;f or ðϕÞ ¼ J FR ð451ÞJ WP ðϕÞJ FR ð  451Þ

ð3Þ

J NPS;back ðϕÞ ¼ J FR ð 451ÞJ WP ðϕÞJ FR ð451Þ

ð4Þ

The Jones vectors of a linear polarised light travelling in parallel to the slow axis of the polarisation maintaining fibre passing through the NOPS in the forward and backward directions are given by #
 " 1 cos ϕ2  j sin ϕ2 ð5Þ J NPS;f or;out ðϕÞ ¼ J NPS;f or ðϕÞ ¼ 0 0 J NPS;back;out ðϕÞ ¼ J NPS;back ðϕÞ


 1 0

" ¼

cos ϕ2 þ j sin 0

ϕ 2

# ð6Þ

It can be seen from (5) and (6) that, regardless of the value of the variable waveplate phase angle ϕ, the light after passing through the NOPS remains linear polarised and travels in parallel to the slow axis of the polarisation maintaining fibre. In other words, the polarisation state of a linear polarised light travelled in parallel to the slow axis of the polarisation maintaining fibre passing through the NOPS in both forward and backward directions remains the same for different phase angles ϕ. The optical phase difference between the light passing through the NOPS in the forward and backward direction was simulated using (5) and (6) for different variable waveplate phase angles. The simulation results shown in Fig. 4 indicate that the phase difference between the two counterpropagating lights after passing through the NOPS equals to the negative of the variable waveplate phase angle. The CW light electric field at the output of the Sagnac interferometer based microwave photonic phase shifter is given by  i 1pffi pffiffiffiffihpffiffiffi π Ecwo ¼ lEin Γ 2 cos ωc t  þ βRF cos ðωc þ ωRF Þt ð7Þ 4 4 where l is the insertion loss of the components inside the Sagnac interferometer, which includes the SSB modulator loss, the optical isolator loss and the NOPS loss, Ein is the amplitude of the electric field at the Sagnac interferometer input, Γ is the isolation of the optical isolator, ωc is the angular optical carrier frequency, βRF ¼ πVRF/Vπ, VRF is the input RF voltage, Vπ is the modulator switching voltage and ωRF is the input RF signal angular frequency. The CCW light electric field at the output of the Sagnac interferometer based

Counterpropagating light optical phase difference (Degree)

3. Analysis and simulation results

0 – 50 – 100 – 150 – 200 – 250 – 300 – 350 0

50

100

150

200

250

300

350

Variable waveplate phase angle (Degree) Fig. 4. Phase difference between the two counterpropagating light after passing through the NOPS as a function of the variable waveplate phase angle.

– 10

CCW and CW light sideband amplitude ratio (dB)

in microwave photonic filtering [21], single-sideband suppressed carrier modulation [22] and rotation sensing [23], but not in microwave photonic phase shifting. This is the first report of a Sagnac interferometer based microwave photonic phase shifter.

129

– 15 – 20 – 25 – 30 – 35 10

15

20

25

30

35

40

Frequency (GHz) Fig. 5. CCW and CW light RF modulation sideband amplitude ratio versus the RF signal frequency when a 10 dB isolation optical isolator is used on the left hand side of the Sagnac interferometer.

microwave photonic phase shifter is given by
pffiffiffi

1pffi 3π Eccwo ¼ lEin 2 cos ωc t þ þΦ 4 4  sin ωRF τ þ βRF cos ððωc þ ωRF Þt þ π þ ΦÞ ωRF τ

ð8Þ

where τ¼ nmodL/c is the signal transit time in the travelling wave SSB modulator, nmod is the refractive index of the LiNbO3 waveguide in the SSB modulator, c is the speed of light in vacuum, L is the modulator electrode length and Φ is the optical phase difference between the two counterpropagating light introduced by the NOPS, which equals to the negative of the variable waveplate phase angle. Fig. 5 shows the amplitude ratio of the RF modulation sideband carried by the CCW and CW light inside the Sagnac interferometer as a function of frequency when a 10 dB isolation optical isolator is used inside the Sagnac interferometer. This shows the CCW light sideband is more than 20 dB below the CW light sideband at the frequency above 10 GHz. Hence the RF modulation sideband carried by the CCW light is neglected in the analysis and the CCW light electric field at the output of the Sagnac interferometer based microwave photonic phase shifter can be written as

1pffi pffiffiffi 3π Eccwo  lEin 2 cos ωc t þ þ Φ ð9Þ 4 4 The output optical power of the Sagnac interferometer based microwave photonic phase shifter is the square of the sum of the CW and CCW propagating signal output electric fields. It is

E.H.W. Chan / Optics Communications 324 (2014) 127–133

given by 2

P o ¼ jEcwo þEccwo j hpffiffiffiffipffiffiffi   1 π 2 cos ωc t  þ βRF cos ðωc þ ωRF Þt ¼ lE2in Γ 16 4

2 pffiffiffi 3π þ 2 cos ωc t þ þ Φ 4

ð10Þ

The output RF photocurrent can be obtained from (10) and can be written as qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  a  pffiffiffiffi 1 2 ð11Þ I RF ¼ ℜ lP in Γ βRF a2 þ b sin ωRF t þ tan  1 16 b where pffiffiffiffi a ¼ Γ  cos Φ  sin Φ

ð12Þ

pffiffiffiffi b ¼  Γ  sin Φ þ cos Φ

ð13Þ

25

25

20

20

15

15

10

10

5

5

0

0 0

5

10

15

Maximum change in output RF power (dB)

Output RF signal power reduction (dB)

ℜ is the photodiode responsivity and Pin ¼E2in is the optical power into the Sagnac interferometer. The last term in (11) is the phase of the output RF signal, which can be controlled by adjusting the nonreciprocal optical phase shift Φ. This shows the converison of the optical phase shift, introduced by the NOPS, to the RF phase shift. Note that both the output RF signal amplitude and phase in (11) are dependent on the optical isolator isolation Γ as well as the nonreciprocal optical phase shift Φ. The changes in the output RF signal amplitude during the phase shifting operation is caused by the carrier and the RF modulation sideband of the CW light which beat together generating an unwanted output RF signal that affects the wanted output RF signal formed by the carrier of the CCW light beats with the RF modulation sideband of the CW light at the photodetector. An optical isolator located on the left hand side of the interferometer attenuates both the carrier and the RF modulation sideband of the CW light, which makes the unwanted output RF signal power smaller than the wanted output RF signal power. This reduces changes in the output RF signal power during the phase shifting operation. However, this also reduces the output RF signal power. Fig. 6 shows the reduction and maximum change in output RF signal power of the Sagnac interferometer based microwave photonic phase shifter for 0–3601 phase shift as a function of the optical isolator isolation. It can be seen from the figure that an 11 dB isolation optical isolator is needed to ensure that the change in the output RF signal power is less than 5 dB during the phase shifting operation. This causes a 14.8 dB reduction in the output RF signal power compared to that without having an optical isolator

20

inside the Sagnac interferometer. The reduction in the output RF signal power can be compensated by using a high power laser source or using an EDFA to increase the optical power into the photodetector. Fig. 7 shows the phase of the RF signal at the output of the Sagnac interferometer based microwave photonic phase shifter for different settings of the variable waveplate phase angle when an 11 dB isolation optical isolator is used inside the Sagnac interferometer. This shows that a continuous 0–3601 RF phase shift can be achieved by controlling the phase angle of the variable waveplate in the NOPS to introduce a nonreciprocal optical phase shift in the two counterpropagating optical signals. The effect of the two counterpropagating RF modulation sideband amplitude ratio variation with frequency, as shown in Fig. 5, on the Sagnac interferometer based microwave photonic phase shifter amplitude and phase responses was investigated. This was done by including the small RF modulation sideband carried by the CCW light in the phase shifter analysis. It was found from the simulation that the counterpropagating RF modulation sideband amplitude ratio variation with frequency causes o0.2 dB ripple in the amplitude response and o 31 ripple in the phase response of the Sagnac interferometer based microwave photonic phase shifter for the RF signal frequency above 10 GHz. It should be pointed out that due to reflections and phase changes of the electrical signal propagating inside the electro-optic modulator, the modulator backward modulation response can be different to theory [25]. Because of this, the unwanted RF modulation sideband carried by the light propagated in CCW direction inside the Sagnac interferometer can be 10 dB higher than that obtained from the theoretical model, which cannot be neglected in the analysis. However, this problem can be solved by inserting a narrow bandwidth optical filter on the left hand side of the Sagnac interferometer as shown in Fig. 2(a). This optical filter, which operates in both directions, passes the CW and CCW light optical carrier but filters out the unwanted RF modulation sideband carried by the CCW light. It needs to have high return loss to avoid light reflections inside the Sagnac interferometer, which creates multiple optical paths causing the coherent interference problem. After examining various types of optical filters, a thin film optical filter is suitable for this application. An example of a bidirectional, narrow bandwidth, high return loss thin film optical filter is the flat-top optical comb filter from Optoplex [26]. The Sagnac interferometer based microwave photonic phase shifter was simulated using the VPItransmissionMaker to verify the RF phase shifting operation. The light travelled in the CW and CCW directions inside the Sagnac interferometer were represented by the light travelled in two unidirectional loops consisting of a quadrature biased SSB modulator. The SSB modulator inside the

300 250

RF phase shift (Degree)

130

200 150 100 50 0

Optical isolator isolation (dB) 11 dB

Fig. 6. Sagnac interferometer based microwave photoinc phase shifter output RF signal power reduction and the maximum changes in the output RF signal power for 0–3601 phase shift as a function of the optical isolator isolation.

– 50 0

50

100

150

200

250

300

Variable waveplate phase angle (Degree) Fig. 7. RF phase shift versus variable waveplate phase angle.

350

E.H.W. Chan / Optics Communications 324 (2014) 127–133

CW and CCW circulation loops were driven by a 10 GHz RF signal with the modualtion index of 0.05 and 0.025  0.05 respectively where 0.025 represented the modulation index reduction at 10 GHz frequency caused by the CCW light travelled in opposite direction to the RF signal propagating inside the SSB modulator in the actual structure. An 11 dB attenuation optical attenuator was placed inside the CW circulation loop representing the 11 dB isolation optical isolator in the actual structure. The NOPS was implemented by a waveplate between two oppositely oriented 451 Faraday rotators. Fig. 8 shows the optical spectrum of the light after travelling through the CW and CCW circulation loops before recombining at the polarisation maintaining coupler. Due to the use of an 11 dB isolation optical isolator inisde the Sagnac interferometer, the optical carrier travelled in the CW direction is 11 dB below the optical carrier travelled in the CCW direction. Due to the velocity mismatch effect causing reduction in the backward modulation index, the RF modulation sideband carried by the CCW light is 22 dB below the RF modulation sideband carried by the CW light. The input RF signal together with the output RF signals of the Sagnac interferometer based microwave photonic phase shifter for different settings of the phase angle of the variable waveplate are shown in Fig. 9. A variable waveplate with a 01 phase angle results in a –451 phase shift of the input RF signal, which agrees with the analytical simulation result shown in Fig. 7. It can be seen from the figure that changing the phase angle of the variable waveplate results in shifting the phase of the RF signal. A continuous 0–3601 RF signal phase shift can be obtained. The VPI simulation results show the output RF signal power changes by around 5 dB during the phase shifting operation. The Sagnac interferometer based microwave photonic phase shifter has a simple structure as it only requires a single laser

Fig. 8. Optical spectrum of the (a) CW and (b) CCW light after travelled through the Sagnac interferometer.

131

source, a single modulator and a single photodetector. A 0–3601 RF signal phase shift can be obtained by a single control, i.e. the phase angle of the variable waveplate in the NOPS. The variable waveplate and the Faraday rotators required to implement the NOPS as well as the SSB modulator are commercially available. The Sagnac interferometer based microwave photonic phase shifter can be operated over a wide bandwidth. The upper operating frequency of the phase shifter is limited by the 3 dB bandwidth of the SSB modulator, which can have 40 GHz bandwidth. The lower operating frequency of the phase shifter is dependent on the backward modulation response of the SSB modulator where the velocity mismatch effect is high so that the RF modulation sideband carried by the CCW light can be neglected. Using a narrow bandwidth optical filter to further suppress the unwanted RF modulation sideband can extend the lower operating frequency of the phase shifter to few GHz.

4. Experimental results Due to the lack of a NOPS, the structure of the Sagnac interferometer based microwave photonic phase shifter is modified as shown in Fig. 10 for experimental demonstration. The optical source was a tunable external cavity laser with a narrow linewidth of 100 kHz. The laser wavelength was 1550 nm. A normal 50% coupling ratio optical coupler was used to construct the Sagnac interferometer. A 16.7 GHz bandwidth dual-parallel Mach Zehnder modulator (DPMZM) (Sumitomo) was inserted into the Sagnac interferometer. The modulator bias voltages were adjusted to obtain SSB modulation. The backward modulation response of the DPMZM was measured and found that it was around 20 dB below the forward modulation response for the RF modulation signal frequency above 10 GHz. With the inclusion of a 10 dB isolation optical isolator at the modulator input, the CCW and CW light RF modulation sideband amplitude ratio is only around  10 dB, which is 10 dB different to the theoretical prediction shown in Fig. 5. This results in a non-negligible sideband carried by the CCW light, which will cause ripples in the phase and amplitude responses of the Sagnac interferometer based phase shifter like the microwave photonic notch filter implemented using the Sagnac interferometer structure [27]. A narrow bandwidth optical filter used for filtering out this unwanted RF modulation sideband was unavailable during the experiment. Hence the DPMZM was located close to the centre of the Sagnac interferometer to increase the ripple free spectral range to tens of GHz so that the frequency response at the Sagnac interferometer output was relatively flat over a wide frequency range. The polarisation controller inside the Sagnac interferometer was used to introduce the nonreciprocal optical phase shift [27]. However, unlike the NOPS implemented using a variable waveplate and two Faraday rotators, adjusting the polarisation controller could also produce a net transformation of the polarisation state. Incorrect polarisation controller adjustment will lead to a polarisation dependent loss and ripples in the microwave photonic phase shifter amplitude and phase responses. Hence the use of polarisation controller, instead of polarisation maintaining components and a NOPS, is not recommended for practical implementation due to the requirement of a complex and careful control on the polarisation controller. Nevertheless, the fact that using a polarisation controller to introduce an optical phase shift inside the Sagnac interferometer is useful to test the operation of the microwave photonic phase shifter. The DPMZM had a built-in polariser at the input optical port, which blocked the light travelled in the fast axis of the polarisation maintaining fibre. The polarisation controller in front of the Sagnac interferometer

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E.H.W. Chan / Optics Communications 324 (2014) 127–133

Fig. 9. (a) Input waveform into the Sagnac loop based microwave photonic phase shifter, and output waveforms for (b) 01, (c) 901, (d) 1801 and (e) 2701 phase angle of the variable waveplate.

90°

0° DC DPMZM Polarisation controller

DC DC

Optical filter Laser

EDFA 50:50 coupler

Photodetector Network analyser

Fig. 10. Experimental setup of the Sagnac interferometer based microwave photonic phase shifter.

was adjusted so that the polarisation of the CW light entering the modulator was slightly away from the slow axis of the polarisation maintaining fibre, which introduced loss in the CW light. This

effect was equivalent to inserting an optical isolator on the left hand side of the interferometer to attenuate the CW light. An EDFA connected after the Sagnac interferometer was used to compensate for the system loss to ensure high power optical signal into the photodetector. This was followed by a 0.8 nm bandwidth optical filter at 1550 nm wavelength for suppressing the amplified spontaneous emission noise. The output optical signal was detected by a 50 GHz bandwidth photodetector whose output was connected to a network analyser to display the amplitude and phase responses. The measured amplitude and phase responses of the Sagnac interferometer based microwave photonic phase shifter for different polarisation controller settings are shown in Fig. 11. The responses were stable. Continuous 0–3601 RF phase shift over a sub-octave frequency range from 8 GHz to 16 GHz was obtained. Around 5 dB change in the amplitude response level was observed during the RF phase shifting operation. The ripples in the amplitude and phase responses were due to slight changes in the modulator response when the light with different polarisation states into the DPMZM. This can be avoided by using polarisation

E.H.W. Chan / Optics Communications 324 (2014) 127–133

inside the NOPS to introduce an optical phase difference between the two counterpropagating light inside the Saganc interferometer, which converts into an RF phase shift after photodetection. The Sagnac interferometer based microwave photonic phase shifter can be constructed using off-the-shelf components and requires only a single control. The RF phase shifting operation has been verified using VPItransmissionMaker photonic simulation software as well as experiments. Experimental results demonstrated 0–3601 RF phase shift over a sub-octave frequency band of 8 GHz to 16 GHz with around 5 dB changes in the output RF signal amplitude.

225 180 135

Phase (Degree)

133

90 45 0 -45 -90 -135 -180

Acknowledgements

-225 8

10

12

14

16

This work was supported by the Australian Research Council.

Frequency (GHz)

References 30 25 20

Amplitude (dB)

15 10 5 0 -5 -10 -15 -20 -25 -30 8

10

12

14

16

Frequency (GHz) Fig. 11. Measured (a) phase and (b) amplitude responses of the Sagnac interferometer based microwave photonic phase shifter for different polarisation controller settings.

maintaining components in the setup and using a NOPS to realise the phase shifting operation. 5. Conclusion A new microwave photonic phase shifter structure has been presented. It is based on a SSB modulator and a NOPS inside a Sagnac interferometer. Continuous 0–3601 RF phase shift can be obtained by controlling the phase angle of the variable waveplate

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