# Widely tunable microwave phase shifter based on silicon-on-insulator dual-microring resonator.

**ABSTRACT** We propose and demonstrate tunable microwave phase shifters based on electrically tunable silicon-on-insulator microring resonators. The phase-shifting range and the RF-power variation are analyzed. A maximum phase-shifting range of 0-600 degrees is achieved by utilizing a dual-microring resonator. A quasi-linear phase shift of 360 degrees with RF-power variation lower than 2dB and a continuous 270 degrees phase shift without RF-power variation at a microwave frequency of 40GHz are also demonstrated.

**0**Bookmarks

**·**

**203**Views

- [Show abstract] [Hide abstract]

**ABSTRACT:**We introduce a new principle that enables separate control of the amplitude and phase of an optical carrier, simply by controlling the power of two stimulated Brillouin scattering (SBS) pumps. This technique is used to implement a microwave photonic phase shifter with record performance, which solves the bandwidth limitation of previous gain-transparent SBS-based phase shifters, while achieving unprecedented minimum power fluctuations, as a function of phase shift. We demonstrate 360° continuously tunable phase shift, with less than 0.25 dB output power fluctuations, over a frequency band from 1.5 to 31 GHz, limited only by the measurement equipment.Optics Letters 10/2014; 39(20). · 3.39 Impact Factor - SourceAvailable from: ieeexplore.ieee.org
##### Article: Integrated Optical Chemical Sensor Based on an SOI Ring Resonator Using Phase-Interrogation

Jin Liu, Xi Zhou, Zhi Qiao, Jianhao Zhang, Chenzhao Zhang, Tuowen Xiang, Lingling Shui, Yaocheng Shi, Liu Liu[Show abstract] [Hide abstract]

**ABSTRACT:**A phase-interrogation approach for the bulk refractive index sensing based on a silicon-on-insulator (SOI) ring resonator is introduced. The rapid phase variation around the resonance of the ring resonator is interrogated, and a single-sideband generation and coherent detection technology is adopted for the phase measurement. In the proposed approach, most of the intensity noise can be shielded, which leads to an ultra-stable reading for the phase signals. A sensitivity of $6times 10^{3} hbox{rad/RIU}$ and a detection limit of $2.5times 10^{-6} hbox{RIU}$ are demonstrated.IEEE Photonics Journal 10/2014; 6(5):1-7. · 2.33 Impact Factor - SourceAvailable from: Steve J Madden
##### Article: Tunable wideband microwave photonic phase shifter using on-chip stimulated Brillouin scattering

Mattia Pagani, David Marpaung, Duk-Yong Choi, Steve J. Madden, Barry Luther-Davies, Benjamin J. Eggleton[Show abstract] [Hide abstract]

**ABSTRACT:**We present the first microwave photonic phase shifter using stimulated Brillouin scattering (SBS) on-chip. The unique ability of SBS to generate both narrowband gain and loss resonances allows us to achieve low ±1.5 dB amplitude fluctuations, which is a record for integrated devices, along with 240° continuously tunable phase shift. Contrary to previous SBS-based approaches, the phase shift tuning mechanism relies on tuning the power, not the frequency, of two SBS pumps, making it more suited to on-chip implementations. We finally demonstrate that SBS pump depletion leads to amplitude response fluctuations, as well as increasing the insertion loss of the phase shifter. Advantageously, shorter integrated platforms possess higher pump depletion thresholds compared to long fibers, thus offering greater potential for reducing the insertion loss.Optics Express 11/2014; 22(23). · 3.53 Impact Factor

Page 1

Widely tunable microwave phase shifter based

on silicon-on-insulator dual-microring resonator

Minhao Pu,1* Liu Liu1, Weiqi Xue1, Yunhong Ding1,2, Haiyan Ou1,

Kresten Yvind1, and Jørn M. Hvam1

1DTU Fotonik, Department of Photonics Engineering, Technical University of Denmark,

Build. 343, DK-2800 Kongens Lyngby, Denmark

2Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology,

Wuhan, 430074, China

*mipu@fotonik.dtu.dk

Abstract: We propose and demonstrate tunable microwave phase shifters

based on electrically tunable silicon-on-insulator microring resonators. The

phase-shifting range and the RF-power variation are analyzed. A maximum

phase-shifting range of 0~600° is achieved by utilizing a dual-microring

resonator. A quasi-linear phase shift of 360° with RF-power variation lower

than 2dB and a continuous 270° phase shift without RF-power variation at a

microwave frequency of 40GHz are also demonstrated.

©2010 Optical Society of America

OCIS codes: (350.4010) Microwaves; (060.5625) Radio frequency photonics; (130.3120)

Integrated optics devices; (230.5750) Resonators.

References and links

1. J. Capmany, and D. Novak, “Microwave photonics combines two worlds,” Nat. Photonics 1(6), 319–330 (2007).

2. S. Tonda-Goldstein, D. Dolfi, A. Monsterleet, S. Formont, J. Chazelas, and J. P. Huignard, “Optical signal

processing in radar systems,” IEEE Trans. Microw. Theory Tech. 54(2), 847–853 (2006).

3. J. Capmany, B. Ortega, D. Pastor, and S. Sales, “Discrete-time optical processing of microwave signals,” J.

Lightwave Technol. 23(2), 702–723 (2005).

4. M. Fisher, and S. Chuang, “A microwave photonic phase-shifter based on wavelength conversion in a DFB

laser,” IEEE Photon. Technol. Lett. 18(16), 1714–1716 (2006).

5. A. Loayssa, and F. J. Lahoz, “Broad-band RF photonic phase shifter based on stimulated Brillouin scattering and

single-sideband modulation,” IEEE Photon. Technol. Lett. 18(1), 208–210 (2006).

6. W. Xue, S. Sales, J. Capmany, and J. Mørk, “Microwave phase shifter with controllable power response based on

slow- and fast-light effects in semiconductor optical amplifiers,” Opt. Lett. 34(7), 929–931 (2009).

7. W. Xue, Y. Chen, F. Öhman, S. Sales, and J. Mørk, “Enhancing light slow-down in semiconductor optical

amplifiers by optical filtering,” Opt. Lett. 33(10), 1084–1086 (2008).

8. M. Povinelli, S. Johnson, and J. Joannopoulos, “Slow-light, band-edge waveguides for tunable time delays,” Opt.

Express 13(18), 7145–7159 (2005), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-13-18-7145.

9. L. Wei, W. Xue, Y. Chen, T. T. Alkeskjold, and A. Bjarklev, “Optically fed microwave true-time delay based on

a compact liquid-crystal photonic-bandgap-fiber device,” Opt. Lett. 34(18), 2757–2759 (2009).

10. Q. Chang, Q. Li, Z. Zhang, M. Qiu, T. Ye, and Y. Su, “A Tunable Broadband Photonic RF Phase Shifter Based

on a Silicon Microring Resonator,” IEEE Photon. Technol. Lett. 21(1), 60–62 (2009).

11. M. Pu, L. Liu, W. Xue, Y. Ding, L. H. Frandsen, H. Ou, K. Yvind, and J. M. Hvam, “Tunable Microwave Phase

Shifter Based on Silicon-on-Insulator Microring Resonator,” submitted to IEEE Photon. Technol. Lett.

12. J. Capmany, B. Ortega, and D. Pastor, “A Tutorial on Microwave Photonic Filters,” J. Lightwave Technol. 24(1),

201–229 (2006).

13. W. Xue, S. Sales, J. Mork, and J. Capmany, “Widely Tunable Microwave Photonic Notch Filter Based on Slow

and Fast Light Effects,” IEEE Photon. Technol. Lett. 21(3), 167–169 (2009).

14. J. Heebner, A. Vincent Wong, A. Schweinsberg, R. W. Boyd, and D. J. Jackson, “Optical transmission

characteristics of fiber ring resonators,” IEEE J. Quantum Electron. 40(6), 726–730 (2004).

15. T. Shoji, T. Tsuchizawa, T. Watanabe, K. Yamada, and H. Morita, “Low loss mode size converter from 0.3µm

square Si wire waveguides to single mode fibres,” Electron. Lett. 38(25), 1669–1670 (2002).

16. M. Pu, L. H. Frandsen, H. Ou, K. Yvind, and J. M. Hvam, “Low Insertion Loss SOI Microring Resonator

Integrated with Nano-Taper Couplers,” The Conference on Frontiers in Optics (FiO) 2009, FThE1 (2009).

1. Introduction

Microwave photonics has lately received increasing interests [1]. Microwave systems can

benefit from the characteristics of photonic semiconductor components such as compact size,

#122999 - $15.00 USD

(C) 2010 OSA

Received 25 Jan 2010; revised 26 Feb 2010; accepted 3 Mar 2010; published 11 Mar 2010

15 March 2010 / Vol. 18, No. 6 / OPTICS EXPRESS 6172

Page 2

large bandwidth, fast tunability, immunity to electromagnetic interference and low weight.

Microwave phase shifters are key components in many microwave applications, such as

phased-array antennas [2] and microwave filters [3]. So far, several schemes for phase shifting

have been reported [4–11]. For instance, a phase shift of 110° with 6dB RF power variation

was obtained by using a distributed-feedback (DFB) laser through wavelength conversion [4],

Based on stimulated Brillouin scattering (SBS) in an optical fiber, a phase shift of 360° was

achieved with less than 3dB RF power variation [5]. Two semiconductor optical amplifiers

(SOAs) were cascaded to realize a phase shifting range of 0~240° with controllable RF power

response based on slow-light effects [6,7]. A photonic-crystal fiber device was used as a phase

shifter with a tuning range of 0~70° caused by band-edge effects [8,9]. Recently, silicon-on-

insulator (SOI) microring resonators (MRRs) have also been used as phase shifters [10,11]. A

shifting range of 0-260° was obtained with thermo-optic tuning from a high-power control

light [10]. Previously, we also demonstrated an electrically tunable phase shifter based on one

MRR with a phase-shifting range of 0-336° in [11]. However, it is difficult to realize a full

360° phase shift by using a single MRR. This limits its practical applications in microwave

systems since many applications, such as microwave photonic filters [12,13], require phase

shifters with a full 360° tuning range. In addition, the RF power varies dramatically during the

phase shifting operation [11] which also hampers the applications.

In this paper, we propose and demonstrate a microwave phase shifters based on a dual-

microring resonator (DMRR), which employs two cascaded MRRs with independent,

electrically controllable, micro heaters. The RF phase-shifting range and the RF-power

variation are analyzed for the proposed phase shifter. A maximum phase-shifting range of

0~600° is achieved by utilizing a DMRR. A quasi-linear phase shift of 360° with ~2dB RF-

power variation and a continuous 270° phase shift without noticeable RF-power variation at a

microwave frequency of 40GHz are also demonstrated. These devices can be easily integrated

with photonic and electronic circuits.

2. Design

Fig. 1. Schematic layout of an RF phase shifter

Figure 1 shows the schematic layout of an RF phase shifter. The RF signal is imprinted on an

optical signal via an external modulator generating an optical signal composed of a strong

carrier at ω0 and two major sidebands, red-shifted and blue-shifted by the RF frequency ωrf. A

notch filter is used to filter out one of the sidebands and then the optical signal with two

frequency components is sent to the optical resonator (here an MRR). The optical field input

to the optical resonator can be expressed as

0010

( ) exp() exp[ ( ) ]

t

rf

E tAjtAj

ωωω

=++

(1)

where A0 and A1 are the amplitudes of the two frequency components of the optical signal.

The output field after the optical resonator can be therefore expressed as

000011

' exp[ (

01

'( )' exp( ) exp(

⋅

)) ] exp(

t

⋅

)

rf

E t A Ajtj A Ajj

ωθωωθ

=++

(2)

where A’0, A’1 and θ0, θ1 are the amplitude transmission gain and the induced optical phase

shift at the corresponding frequencies for the MRR, respectively. Finally, the output optical

#122999 - $15.00 USD

(C) 2010 OSA

Received 25 Jan 2010; revised 26 Feb 2010; accepted 3 Mar 2010; published 11 Mar 2010

15 March 2010 / Vol. 18, No. 6 / OPTICS EXPRESS 6173

Page 3

signal is detected by a photo detector (PD) and the RF component of the output signal from

PD is

0011

' cos[

01

( )

t

'( )]

AC rf

i A A A At

ωθθ

∝ ℜ+−

(3)

where ℜ is the responsivity of the PD. The phase of the output RF signal is therefore

determined by the optical phase difference θ0-θ1. And if the resonance of the optical resonator

can be tuned (between the two optical frequencies) to change the two optical phases, the phase

difference can be varied and then realize the RF phase shift.

2.1 Phase shifter based on a single microring resonator (MRR)

Fig. 2. Schematic of an all-pass single MRR.

Figure 2 shows the schematic drawing of an all-pass single MRR, where κ and a are the

amplitude coupling coefficient of the coupling region and roundtrip amplitude transmission

coefficient, respectively. The transmittance of the complex field at the through port can be

expressed as

1

j

out

j

in

E

rae

E

are

φ

φ

−

−

=

(4)

where φ is the roundtrip phase change of the ring, which is related to the optical frequency,

and r is the amplitude transmission coefficient of the coupling region which satisfies the

relation r2+κ2=1 for lossless coupling. The phase shift of the transmitted light can be derived

as [14]

11

sinsin

cos

ar

tantan

cos1

r

−

ar

−

ar

φφ

πφ

φφ

−−

Φ =+ ++

(5)

Figures 3(a) and 3(b) show the optical intensity transmission and phase shift at the through

port of the MRR with different amplitude coupling coefficients κ. Here, we assume the

diameter of the SOI MRR is 35µm and the effective group index of the SOI waveguide is 4.26

which correspond to a free spectral range (FSR) of 640GHz. The propagation loss of the SOI

waveguide composing the ring is assumed to be 2dB/cm. As shown in Fig. 3(b), the phases

experience monotonically a full 360° phase shift from negative to positive detuning with

different curve shapes. If an optical signal carrying a microwave signal with two frequency

components, i.e. a carrier frequency ω0 and one sideband frequency ω0 + ωrf, is input to the

MRR, the phase difference of the two frequency components can be changed in different ways

depending on the value of the coupling coefficient κ of the MRR. In other words, the RF

phase-shifting performance will be different for the MRRs with different coupling

coefficients. Figures 3(c) and 3(d) show the RF power and RF phase shift, respectively, for

the MRRs as a function of detuning of the resonance frequency (ωMRR) from the carrier

frequency (ω0) over an FSR tuning range. Here, we assume the RF frequency (ωrf) is 40GHz.

It is clear that the maximum RF phase-shifting range increases as the amplitude coupling

coefficient decreases which results in higher quality (Q)-factor of the MRR [see Fig. 3(d)].

The RF power variation also increases with lower coupling coefficient (higher Q-factor) for

the MRR as illustrated in Fig. 3(c). The RF phase-shifting range can be further increased,

though, for the larger amplitude coupling coefficients by increasing the RF frequency, as

shown in Fig. 3(e). The higher Q-factor MRR with lower coupling coefficient is always

#122999 - $15.00 USD

(C) 2010 OSA

Received 25 Jan 2010; revised 26 Feb 2010; accepted 3 Mar 2010; published 11 Mar 2010

15 March 2010 / Vol. 18, No. 6 / OPTICS EXPRESS 6174

Page 4

preferred if a large RF phase shifting range is expected. However, a full 360° RF phase shift is

still difficult to realize through a single MRR [Fig. 3(d)], although the optical phases for all

the MRRs experience a 360° change [Fig. 3(b)], and the increased RF power variation would

become unbearable when a higher Q-factor MRR is employed [Fig. 3(c)]. In our previous

experimental demonstration [11], a phase shifting range of 0~336° was achieved by using a

single MRR with Q-factor of 28,000. However, the RF-power variation amounted to 11dB.

-320 -240 -160 -800

80160 240320

0.4

0.6

0.8

1.0

-320 -240 -160 -800

80 160 240 320

0

120

240

360

-320 -240 -160 -800 80 160 240 320

-5

-4

-3

-2

-1

0

-320 -240 -160 -800 80160 240 320

0

120

240

360

0 80 160 240320400 480 560 640

0

120

240

360

κ κ κ κ=0.2

κ κ κ κ=0.32

κ κ κ κ=0.55

κ κ κ κ=0.71

κ κ κ κ=0.2

κ κ κ κ=0.32

κ κ κ κ=0.55

κ κ κ κ=0.71

κ κ κ κ=0.2

κ κ κ κ=0.32

κ κ κ κ=0.55

κ κ κ κ=0.71

κ κ κ κ=0.2

κ κ κ κ=0.32

κ κ κ κ=0.55

κ κ κ κ=0.71

(a)

κ κ κ κ=0.2

κ κ κ κ=0.32

κ κ κ κ=0.55

κ κ κ κ=0.71

Transmission (a.u.)

(b)

Phase shift (degree)

Detuning (GHz)Detuning (GHz)

Detuning (GHz)

(e)

Frequency (GHz)

Relative RF power (dB)

Detuning (GHz)

(c)

-20 -1001020

-5

-4

-3

-2

-1

0

RF phase shift (degree)

RF phase shift (degree)

(d)

-20 -10010 20

0

120

240

360

-20 -100 1020

0.3

0.6

0.9

-20 -1001020

0

120

240

360

Fig. 3. Optical intensity transmission (a) and phase shift (b) as a function of the detuning (ω-

ωMRR) at the through port for the MRRs with different coupling coefficients κ. RF power (c)

and RF phase shift (d) for the MRRs as a function of the detuning (ωMRR-ω0) at a RF

frequency of 40GHz with different coupling coefficients κ. All the insets are the zoomed view

for a detuning range from −20GHz to 20GHz. (e) The maximum RF phase shift versus the RF

frequency for the MRRs with different coupling coefficients κ.

2.2 Phase shifter based on dual-microring resonator (DMRR)

Fig. 4. Schematic of an all-pass DMRR.

Figure 4 shows the schematic drawing of an all-pass dual-microring resonator (DMRR). The

two cascaded rings are designed to have the same nominal geometries. Here, we also assume

that an optical signal with two frequency components (ω0 and ω0+ωrf) is injected to the

DMRR. The diameter of the two MRRs and the RF frequency are 35µm and 40GHz,

respectively. Figure 5 illustrates the optical intensity transmission, the RF power, the optical

#122999 - $15.00 USD

(C) 2010 OSA

Received 25 Jan 2010; revised 26 Feb 2010; accepted 3 Mar 2010; published 11 Mar 2010

15 March 2010 / Vol. 18, No. 6 / OPTICS EXPRESS 6175

Page 5

phase shift and the RF phase shift for the DMRR at the through port with different resonance

offsets (ωMRR2-ωMRR1) between the two MRRs. A total optical phase shift of 720° can be

achieved for the DMRR. As shown in Figs. 5(a) and 5(c), the transmission spectrum and the

phase curve can be altered by offsetting the resonances for the two MRRs. When the

resonance offset increases from 0 to 3GHz, the notch bandwidth increases, with a reduced

notch depth, and the bottom of the notch becomes flat. As the resonance offset increases

further, the notch splits into two notches while the phase curve acquires a step-like shape.

The RF power and RF phase shift behave in a similar way, as shown in Figs. 5(b) and 5(d).

Therefore, the resonance offset can be tuned to a desired value (e.g., 3GHz in this case) to

obtain a wide notch bandwidth, a decreased notch depth and a flattened notch bottom, and if

the RF phase shifter is operated within this flat regime, one can realize an RF phase shift in a

certain range with minimal RF power variation since the RF power follows the optical power.

The insets in Figs. 5(b) and 5(d) show the zoomed views for the RF phase shift and RF power

variation, respectively, in the detuning range from −1GHz to 4GHz for the DMRR with a

fixed resonance offset of 3GHz. One can find that a quasi-linear phase-shifting range of 360°

can be obtained with less than 2.4dB RF power variation. A tuning range of 90° with

negligible RF-power variation can also be found within the detuning regime from 0.95GHz to

1.92GHz. Compared with the phase shifter based on single MRRs [see insets in Figs. 3(c) and

3(d)], the DMRR-based phase shifter offers a larger phase shifting range, a more linear phase

shift of 360° together with a much lower RF power variation. The bandwidth of the flat

bottom regime can be further increased by a careful design of the Q-factor of each MRR and

an optimal resonance offset between the MRRs, resulting in an increase of the tuning range

with constant RF power.

-505 1015 20

0.0

0.2

0.4

0.6

0.8

1.0

-5051015 20

-10

-8

-6

-4

-2

0

-50510 15 20

0

120

240

360

480

600

720

-5 05 10 1520

0

120

240

360

480

600

720

0GHz

2GHz

3GHz

4GHz

9GHz

14GHz

0GHz

2GHz

3GHz

4GHz

9GHz

14GHz

(a)

Transmission (a.u.)

Detuning (GHz)

0GHz

2GHz

3GHz

4GHz

9GHz

14GHz

(c)

Relative RF power (dB)

Detuning (GHz)

0123

-8

-6

-4

-2

(b)

Phase shift (degree)

Detuning (GHz)

0GHz

2GHz

3GHz

4GHz

9GHz

14GHz

(d)

RF phase shift (degree)

Detuning (GHz)

0123

160

240

320

400

480

Fig. 5. Optical intensity transmission (a) and phase shift (c) as a function of the detuning (ω-

ωMRR1) for the DMRR with different resonance offsets (ωMRR2-ωMRR1). RF power (b) and RF

phase shift (d) as a function of the detuning (ωMRR1-ω0) at an RF frequency of 40GHz. Insets

are zoomed views for the DMRR with 3GHz resonance offset for a detuning range from

−1GHz to 4GHz. Here, κ2, a2 are always assumed to be 0.04 and 0.995, respectively.

In the above analysis, we fixed the resonance offset of the two MRRs. We can also

suppress the RF power variation over a larger tuning range by varying the resonance offset

during the phase-shifting operation since the two cascaded MRRs are designed to be tuned

independently. Figure 6(a) shows the RF phase shift and the RF power as a function of

detuning of resonance frequencies (ωMRR1 and ωMRR2) from the optical carrier frequency (ω0)

#122999 - $15.00 USD

(C) 2010 OSA

Received 25 Jan 2010; revised 26 Feb 2010; accepted 3 Mar 2010; published 11 Mar 2010

15 March 2010 / Vol. 18, No. 6 / OPTICS EXPRESS 6176

Page 6

for the DMRRs with power coupling coefficient κ2 = 0.04. One can find that there are regions

where the contour lines for RF phase shift (color-shaded contours) and RF power variation

(black-curve contours) are not parallel. Therefore, the RF phase shift and the RF power can be

tuned independently to some extent. If the tuning of the resonances of the two MRRs can be

controlled in such a way that one of the contour lines for RF power is followed, there would

be no RF power variation in the phase shifting process. For instance, if we follow the −6dB

power contour curve, a continuous phase shifting range of ~280° can be obtained. The

maximum continuous RF phase shifting range without power variation depends on the Q-

factor of each MRR. The performance of a phase shifter based on a DMRR with a higher

power coupling coefficient of 0.3 (lower Q-factor) is also illustrated in Fig. 6(b). Although the

operating RF power level becomes higher and less sensitive to the detuning of the resonances,

the maximum phase shifting range that can be achieved following one contour line is

decreased to ~190°. Figure 7 shows the maximum RF phase-shifting range without power

variation and the operating RF power level as a function of power coupling coefficient κ2 of

the DMRR. It is obvious that the phase shifter based on higher Q-factor DMRR (lower

coupling coefficient) offers larger tuning range without power variation at the expense of an

increased overall RF-power loss. Since the absolute operating RF power is of less importance

for such a microwave phase shifter, we conclude that a DMRR with high Q-factor is more

preferable in this case.

Detuning of MRR1 (GHz)

Detuning of MRR2 (GHz)

-8

-6

-4

-4

-4

-4

-4

-4

-4

-4

-2

-2

-2

-2

-2

-15-10-5051015

-15

-10

-5

0

5

10

15

0100200 300400500600

Detuning of MRR1 (GHz)

Detuning of MRR2 (GHz)

-0.8

-0.8

-0.8

-0.6

-0.6

-0.6

-0.6

-0.6

-0.4

-0.4

-0.4

-0.4

-0.4

-0.4

-0.2

-0.2

-60-40 -200 20

-60

-50

-40

-30

-20

-10

0

10

20

0 100200300

Deg.

Deg.

(a)(b)

Fig. 6. Contour plots of RF phase shift for a 40GHz signal (in degrees, color-shaded contours)

and RF power (in decibels, black-curve contours) as a function of the detuning of resonance

frequencies (ωMRR1 and ωMRR2) from the optical carrier frequency (ω0) for tdhe DMRRs with

power coupling coefficient κ2 of 0.04 (a) and 0.3 (b), respectively.

0,00,1 0,20,30,4 0,5

120

180

240

300

360

RF power (dB)

RF phase shift (degree)

Power coupling coefficient

RF phase

RF power

-30

-20

-10

0

Fig. 7. Maximum RF phase shift and RF power level for constant-power operation as a

function of the power coupling coefficient κ2.

#122999 - $15.00 USD

(C) 2010 OSA

Received 25 Jan 2010; revised 26 Feb 2010; accepted 3 Mar 2010; published 11 Mar 2010

15 March 2010 / Vol. 18, No. 6 / OPTICS EXPRESS 6177

Page 7

3. Fabrication

Figure 8(a) shows the schematic diagram of a tunable MRR. The tunable MRR was fabricated

in an SOI wafer with a top silicon thickness of 250nm and a 3-µm buried silicon dioxide.

Diluted (1:1 in anisole) electron-beam resist ZEP520A was spin-coated on the wafer to create

a ~110-nm thick masking layer. The microring structure was defined in the ZEP520A layer

with electron-beam lithography (JEOL JBX-9300FS). The patterns were subsequently

transferred to the top silicon layer with inductively coupled plasma reactive ion etching. Then

a 550-nm thick benzocyclobuten (BCB) top cladding was spin-coated and subsequently hard-

cured. After that, 400nm of ZEP520A resist and electron-beam lithography were employed

again to define the pattern of the micro heater. Evaporation and lift-off techniques were used

as the last steps to form 100-nm thick titanium heaters together with contact pads. Figure 8(b)

shows an optical microscope picture of the fabricated single MRR with micro heater. The

waveguide width is 450nm and the diameter of the microring is 35µm. The heater width in the

ring area is 1µm. At both ends of the device, the waveguide is tapered from 450nm to 4µm to

expand the guided mode for more efficient fiber-to-chip coupling. The insertion loss of the

device is ~15dB, where we estimate the fiber to waveguide coupling loss to account for

~14dB. This loss can be lowered down to ~2dB using suitable mode converters [15,16].

Fig. 8. (a) Schematic diagram of the tunable MRR with micro heater. (b) Top-view microscope

picture of the fabricated tunable MRR with micro heater.

To test the tunability of MRR by applying electrical current on micro heater, we use a

single MRR with a coupling gap (ring-to-waveguide gap) of 200nm between the ring and

straight waveguide. Figure 9(a) shows the measured transmission spectra for the MRR with

different applied electrical powers. The 3-dB bandwidth of the resonant notch is 0.1nm which

corresponds to a Q-factor of ~15,500. The resonance red-shifts linearly as the applied power

increases as shown in Fig. 9(b). An electrical power of ~40mW is required for the resonance

shifting range of a whole FSR, and the maximum achieved resonance shift is ~8nm with this

design.

1565 1566

Wavelength (nm)

15671568

-45

-40

-35

-30

-25

010 20304050 6070

0

2

4

6

8

Transmission (dBm)

1.3mW

6.5mW

13.0mW

25.5mW

39.8mW

increase power

(a)

Resonance shift (nm)

Applied power (mW)

Measured

Fitted

(b)

Fig. 9. (a) Measured transmission spectrum with different applied power on the micro heater

for the MRR. (b) Measured resonance shift versus the applied power on the micro heater.

#122999 - $15.00 USD

(C) 2010 OSA

Received 25 Jan 2010; revised 26 Feb 2010; accepted 3 Mar 2010; published 11 Mar 2010

15 March 2010 / Vol. 18, No. 6 / OPTICS EXPRESS 6178

Page 8

4. Experiment setup

The experimental setup used to measure the fabricated device is shown schematically in

Fig. 10(a). Light from a tunable laser source (TLS) was modulated through a Mach-Zehnder

modulator (MZM) by a microwave signal from the network analyzer. A fiber Bragg grating

(FBG) notch filter was used to filter out one sideband of the modulated signal. After that, the

optical signal, with the envelope modulated at the microwave frequency in the time domain

(i.e., with two peaks of the desired frequency spacing in the spectral domain [see Fig. 11 (b)])

was generated and sent into the fabricated sample as shown in Fig. 10(b). The coupling gaps

between the two microrings and the straight waveguide are 150nm which corresponds to a

power coupling coefficient of 0.16. The transmission spectrum can be altered by applying

current to one of the micro heaters (i.e., the micro heater for MRR1 [see Fig. 11(a)]). In the

measurement, we applied 0.8mW initially on MRR1 to get a suitable resonance offset between

the two MRRs. The polarization of the input light was adjusted to the quasi transverse-

electrical (TE) mode by a fiber polarization controller (PC). By adding extra power equally to

both micro heaters simultaneously, the resonance frequency of the DMRR can be tuned

together with respect to one of the peaks of the optical signal, as illustrated in Fig. 11(b), and

then the phase difference between the two peaks is changed. Amplified by an erbium-doped

fiber amplifier (EDFA), the output signal was detected by a high-speed photo detector (PD),

and converted to the microwave signal. The network analyzer was then used to extract the

information of phase and power variations of the microwave signal.

Fig. 10. (a) Experimental setup for phase-shift measurements. (b) Optical microscope picture of

the fabricated dual-microring resonator with micro heater.

#122999 - $15.00 USD

(C) 2010 OSA

Received 25 Jan 2010; revised 26 Feb 2010; accepted 3 Mar 2010; published 11 Mar 2010

15 March 2010 / Vol. 18, No. 6 / OPTICS EXPRESS 6179

Page 9

15381539 1540

1539 1540

-60

-50

-40

-30

-20

Transmission (dBm)

Transmission

Wavelength (nm)

2.5mW

1.6mW

0.8mW

0.1mW

(a)

40GHz modulated signal

Wavelength (nm)

0.0mW

1.6mW

3.4mW

5.1mW

Increase power

(b)

Fig. 11. (a) Measured transmission spectrum of the DMRR with different applied power on the

micro heater for MRR1. (b) Measured transmission spectrum of the DMRR with additional

applied power on both micro heaters (see the color curves) and the generated 40GHz

microwave signal with carrier wavelength of 1539nm (see the black curve). Here, 0.8mW

power is initially applied on micro heater for MRR1.

5. Experimental results

The measured RF phase shift and RF power variation for a 40GHz microwave carrier as a

function of the applied electrical power on both micro heaters are shown in Fig. 12(a). A

continuously tunable RF phase shift is demonstrated, and the maximum RF phase shift of

540° is achieved with the RF power variation of ~4dB. However, if the device is operated

within the gray region shown in Fig. 12(a), one can obtain not only a quasi-linear phase shift

of 360°, but also an RF power variation of only 2dB. In this case, the total required electrical

power for phase tuning is ~2mW. As compared to the single-MRR based device in our

previous demonstration [11], not only a larger and more linear RF phase shift is achieved, but

also the RF power variation is largely suppressed. Figure 12(b) also shows the measured result

for another DMRR with a smaller coupling gap of 100nm (larger power coupling coefficient

κ2 = 0.33) which corresponds to a lower Q-factor. For this measurement, the reference power

applied to the micro heater for the MRR1 was set to 0.5mW to get a suitable fixed resonance

offset between the MRRs. A phase shift of 390° is still achieved as shown in Fig. 12(b).

Although the maximum phase shift is reduced from 540° to 390° and the total required

heating power is increased to ~8.5mW, the total RF power variation in the whole tuning range

is only 1dB which is much smaller than that of the high-Q device. Figures 12(c) and 12(d)

show the measured maximum RF phase shift and maximum RF power drop, respectively, for

the DMRRs with different power coupling coefficients (different Q-factors). The DMRR with

higher Q-factor (smaller power coupling coefficient) provides larger RF phase shifting range

together with larger RF power variation as shown in Figs. 12(c) and 12(d), which complies

with the simulation results. The maximum phase shifting range of 0~600° was achieved by

using a DMRR with a coupling gap of 175nm which corresponds to a power coupling

coefficient of 0.11.

#122999 - $15.00 USD

(C) 2010 OSA

Received 25 Jan 2010; revised 26 Feb 2010; accepted 3 Mar 2010; published 11 Mar 2010

15 March 2010 / Vol. 18, No. 6 / OPTICS EXPRESS 6180

Page 10

1.6

Applied power on MRR1 (mW)

2.4 3.24.0

0

120

240

360

480

600

720

2.02.53.03.54.04.5

0

120

240

360

480

600

720

0.100.15 0.200.25 0.30 0.35

360

420

480

540

600

0.100.150.200.250.300.35

-5

-4

-3

-2

-1

RF Power

RF Phase

(c)

Applied power on MRR2 (mW)

0.81.6

RF Phase

RF Power

Relative RF Power (dB)

RF phase shift (degree)

Semi-linear

region

(a)

-8

-6

-4

-2

0

2.43.2

(d)

Applied power on MRR2 (mW)

2.0 2.5

Relative RF Power (dB)

RF phase shift (degree)

Applied power on MRR1 (mW)

(b)

Maximum RF power drop (dB)

-8

-6

-4

-2

0

RF Phase

RF Power

1.5 3.03.54.0

RF phase shift (degree)

Power coupling coefficient

Power coupling coefficient

Fig. 12. Measured RF phase shift and RF power versus the power applied to the two micro

heaters for the DMRR with a coupling gap of 150nm (a) and 100nm (b). Measured maximum

RF phase shift (c) and RF power drop (d) for the DMRRs with different power coupling

coefficients.

Figures 13(a) and 13(b) present the measured RF phase shift and RF power as a function

of the applied power on the micro heaters for MRR1 and MRR2 of the DMRRs with a

coupling gap of 150nm and 100nm, respectively. The dotted lines in both figures show the

phase shifting operations in the previous measurements with fixed resonance-offsets. One can

find that this is not the optimal way to suppress the RF power variation. The applied power on

the micro heaters for MRR1 and MRR2 can be adjusted independently along a power contour

line [i.e., the upper −2dB solid contour line in Fig. 13(a)] to realize the phase shifting. As

shown in Fig. 13(a), a phase-shifting range from 180° to 450° can be obtained in this way

without noticeable RF power variation for the DMRR. And if we can tolerate 1dB RF-power

variation, we can obtain a continuous full 360° phase shift when operation is performed within

the region defined by the two upper contour lines of −2dB and −3dB [Fig. 13(a)]. For the

DMRR with lower Q-factor (narrower coupling gap), the maximum phase shifting range

without power variation is smaller, just as the theoretical prediction mentioned in subsection

2.2.

#122999 - $15.00 USD

(C) 2010 OSA

Received 25 Jan 2010; revised 26 Feb 2010; accepted 3 Mar 2010; published 11 Mar 2010

15 March 2010 / Vol. 18, No. 6 / OPTICS EXPRESS 6181

Page 11

Applied power on MRR1 (mW)

Applied power on MRR2 (mW)

-1.2

-1

-0.8

-0.8

-0.6

-0.6

-0.6

-0.6 -0.6

-0.4

-0.6

-0.4

-0.4

-0.4

-0.2

-0.2

-0.2

01234

0

1

2

3

4

0100200 300

Applied power on MRR1 (mW)

Applied power on MRR2 (mW)

-4

-3

-3

-2

-3

-3

-2

-2

-2

-1

-1

-1

-1

-1

-1

-1

0

01234

0

0.5

1

1.5

2

2.5

3

3.5

4

0100 200 300400 500

(a)

Deg.

(b)

Deg.

Fig. 13. Contour plots of measured RF phase shift (in degrees, color-shaded contours) and RF

power (in decibels, black-curve contours) as a function of the powers applied to the two micro

heaters for the DMRR with a coupling gap of 150nm (a) and 100nm (b). The dotted lines

represent the RF phase shifting operations in Figs. 12(a) and 12(b), respectively.

6. Conclusion

In conclusion, we have proposed and demonstrated microwave phase shifters based on

electrically tunable SOI DMRRs which are composed of cascaded rings with independently

controllable micro heaters. For a fixed resonance offset operation, a maximum phase shifting

range of 0~600° was obtained and a quasi-linear 360° phase shift has been achieved at a

microwave frequency of 40GHz with RF power variation lower than 2dB. A phase shift of

390° has also been demonstrated with only 1dB RF power variation using a DMRR with

lower Q-factor. For variable resonance-offset operation, a continuous phase shift of 360°

(270°) with only 1dB (0dB) RF power variation has also been demonstrated. Compared with

the single-MRR device, the phase shifter based on a DMRR offers larger phase shifting range

and more controllable RF power variation. These two advantages make the proposed device

potentially useful in practical and versatile microwave applications.

Acknowledgment

This work was supported by the Danish Strategic Research Council and the EU FP7 via the

projects NANO·COM and GOSPEL, respectively.

#122999 - $15.00 USD

(C) 2010 OSA

Received 25 Jan 2010; revised 26 Feb 2010; accepted 3 Mar 2010; published 11 Mar 2010

15 March 2010 / Vol. 18, No. 6 / OPTICS EXPRESS 6182