Tunable wideband microwave photonic
phase shifter using on-chip stimulated
Brillouin scattering
Mattia Pagani,1,∗David Marpaung,1Duk-Yong Choi,2 Steve J. Madden,2Barry Luther-Davies,2and Benjamin J. Eggleton1 1Centre for Ultrahigh bandwidth Devices for Optical Systems (CUDOS), School of Physics,
University of Sydney, Australia
2CUDOS, Laser Physics Centre, Australian National University, Australia
∗mattia.pagani@sydney.edu.au
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.
© 2014 Optical Society of America
OCIS codes: (190.4390) Nonlinear optics, integrated optics; (290.5900) Scattering, stimulated Brillouin; (350.4010) Microwaves.
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1. Introduction
Radio frequency (RF) phase shifters are used to provide a tunable phase shift over wide fre-quency bands, and are a building block for phased-array antennas and RF communication sys-tems [1]. These applications demand certain stringent requirements; namely a 0–360◦tunable phase shift, flat over a frequency band spanning multiple GHz, while passing all inputs with no power variation (i.e., gain-transparent frequency response).
Microwave photonic (MWP) phase shifters exploit the huge bandwidth potential and re-configurability of photonics to overcome the shortcomings of electronic implementations [2]. Electro-optic modulators [3–7], stimulated Brillouin scattering [8, 9], cross-phase modulation [10], semiconductor optical amplifiers [11,12], and Bragg gratings [13–16] have all been shown capable of achieving tunable phase shifts over multi-GHz bands. More recently, the emergence of integrated microwave photonics (IMWP) has demonstrated that MWP signal processors can maintain high performance, with enhanced robustness, as well as achieving reductions in size, weight, cost, and power consumption [17]. IMWP phase shifters have been demonstrated using ring resonators [18, 19], as well as integrated waveguide Bragg gratings [20]. Numerical com-putations using silicon graphene waveguides have also shown very promising results [21], but
have yet to be confirmed in practice.
All of these IMWP phase shifters make use of optical carrier processing, in which a phase shift imparted on the optical carrier is converted by square-law photodetection to a phase shift in the electrical domain [22]. In order for optical carrier processing to work effectively, the optical response used to shift the phase of the carrier must be narrow so as to not distort the optical sideband; in addition it must not affect the amplitude of the carrier, to prevent power fluctuations at the output of the phase shifter. While achieving impressive results, all these prior techniques do not meet either of these requirements, resulting in suboptimal operation.
Alternatively, stimulated Brillouin scattering (SBS) is ideal for optical carrier processing due to its narrow linewidth [23]. More importantly, SBS interactions are capable of inducing both gain and loss resonances, which can be used to shift the phase of an optical carrier without changing its amplitude [8]. Traditionally constrained to long, bulky spools of fiber, the recent demonstration of significant SBS gain in an integrated platform [24] has opened the way for breakthroughs in the field of IMWP [25–27].
In this work, we exploit the high SBS gain coefficient of a 6.5 cm long As2S3rib waveg-uide [24], to demonstrate the first MWP phase shifter using on-chip SBS, depicted in Fig. 1. The technique presented here employs a principle similar to that introduced by Loayssa and Lahoz in [8], where two SBS pumps are used to shift the phase of the optical carrier. However, unlike the previous approach, tuning of the phase shift is achieved by controlling the power of the pump waves, as opposed to their frequency. This results in a more energy-efficient tun-ing mechanism, more suitable to chip implementation. We demonstrate 240◦of continuously tunable phase shift, over a 1–15 GHz frequency band, with low ±1.5 dB power fluctuations.
Additionally, we provide the first detailed analysis of the insertion loss, which is the main restriction to real-world applications. We show that SBS pump depletion leads to variations in the amplitude response of the phase shifter, as the phase shift is tuned. This is an undesirable effect, which sets a limit to the optical signal power into the waveguide. This limit is shown to be significantly higher for short chip implementations, compared to long fiber implementations, where SBS pump depletion occurs at a lower power threshold. Because of this, it is preferable to exploit SBS in an integrated platform. This striking property constitutes a promising step towards fully-integrated MWP signal processors.
RF output signal
RF frequency
Power
Laser Modulator Photodetector
RF input signal Optical frequency Carr ier Power Optical frequency Power RF frequency Power ChG chip SBS
Fig. 1. Structure of the SBS-based phase shifter. The SBS process takes place in the chalco-genide (ChG) waveguide, and is used to change the phase of the optical carrier.
2. Principle of operation
The application of SBS as a gain-transparent, phase shifting mechanism was introduced by Loayssa and Lahoz [8]. The technique relies on counterpropagating a single sideband (SSB) modulated signal with two pump waves. The frequencies of the two pumps are symmetrically
up and down-shifted from that of the carrier by Ω, in the vicinity of the Brillouin frequency shift ΩB, as shown in Fig. 2. Pump 1 then acts as an SBS Stokes wave, and induces a loss resonance on the carrier, while Pump 2 acts as an SBS pump wave, inducing a gain resonance on the carrier. Pump 2 Pump 1 Pump 2 Pump 1 Sideb and Carr ier Optical signal spectrum SBS amplitude response SBS phase response
Fig. 2. Operating principle for the SBS-based MWP phase shifter, as presented in [8]. (a) The gain and loss resonances from an SBS Stokes (Pump 1) and pump (Pump 2) wave cancel out, while (b) their phase contributions add up, at the carrier frequency.
The evolution of the carrier power, Pc, and phase θc, together with the power of Pump 1, Pp1, and Pump 2, Pp2, can be obtained through manipulation of the steady-state SBS coupled mode equations [28], and is described by
dPp1 dz = g0 Aao Γ2B Γ2B+ 4(ΩB− Ω)2 Pp1Pc− αPp1 (1) dPp2 dz = − g0 Aao Γ2B Γ2B+ 4(ΩB− Ω)2 Pp2Pc− αPp2 (2) dPc dz = g0 Aao Γ2B Γ2B+ 4(ΩB− Ω)2 (Pp1− Pp2)Pc+ αPc (3) dθc dz = − g0ΓB Aao ΩB− Ω Γ2B+ 4(ΩB− Ω)2 (Pp1+ Pp2). (4)
Here g0is Brillouin gain coefficient, Aaois the acousto-optic effective area, ΓBis the Brillouin linewidth, and α is the loss coefficient of the medium. In the above expressions, the carrier is assumed to be travelling in the −z-direction, opposite to Pump 1 and 2, which travel along the +z-direction.
It is clear that if the powers of Pump 1 and 2 are equal throughout the whole SBS medium, i.e.,
Pp1(z) = Pp2(z) (5)
the first term on the right-hand side of Eq. (3) vanishes, and the SBS amplitude contributions cancel out at the carrier frequency. This amplitude response cancellation, depicted in Fig. 2(a), implies that the carrier power is not affected by the SBS process. Equation (5) requires that both Pump 1 and 2 be launched into the medium with the same power, i.e., Pp1(0) = Pp2(0), and that they evolve in the same manner as they propagate along the medium. This last statement is
satisfied when the first term on the right-hand side of Eqs. (1) and (2) vanishes such that dPp1
dz ≈
dPp2
dz ≈ −αPp1. (6)
This equation represents the undepleted pump condition [29]. This is a key requirement for phase shifter operation because it states that the amplitude response of the phase shifter does not fluctuate as the phase shift is tuned.
Pump Frequency Carr ier (a) Pump Power Carr ier (b)
Fig. 3. Phase shift tuning mechanism using (a) technique reported in [8]; (b) current energy efficient technique.
The total phase shift φ experienced by the carrier is the sum of the phase responses due to the two SBS interactions, as shown in Fig. 2(b), and can be found by integrating Eq. (4) over the length L of the SBS medium. In [8], tuning of the phase shift in the range ±180◦was achieved by simultaneously varying the frequency of Pump 1 and 2, as shown in in Fig. 3(a), while their input power remained fixed at
Pp1(0) = Pp2(0) = 2πAao
g0Leff
(7) where Leff=α1(1 − e
−αL) is the effective length of the medium. This has the effect of changing the value of Ω such that the optical carrier accesses a different point of the SBS phase response. It is clear that this method is highly inefficient since even small phase shifts require high SBS pump powers.
A more energy-efficient method for tuning the phase shift is to exploit the maximum phase shift provided by the SBS phase response, and is illustrated in Fig. 3(b). By fixing the frequen-cies of Pump 1 and 2, such that
Ω = ΩB± ΓB
2 (8)
the carrier always has access to the maximum (or minimum) phase shift, for a given power of Pump 1 and 2. The phase shift is then tuned by simultaneously varying the powers of Pump 1 and 2:
Pp1(0) = Pp2(0) = 2Aao g0Leff
|φ | . (9)
Using this new technique, initially suggested in [8], the amount of pump power becomes pro-portional to the amount of desired phase shift, making it less power-consumptive, and alleviat-ing the burden on the SBS medium.
3. Results and discussion 3.1. Phase shifter operation
The experimental setup used to implement the SBS-based phase shifter is shown in Fig. 4. A semiconductor laser diode was biased for continuous wave operation with 1550 nm wavelength, and 20 dBm of output power; the laser output was then split equally between two branches.
With reference to Fig. 4, the upper branch was used to generate the SSB-modulated carrier. A swept-frequency RF signal was generated using a vector network analyser (VNA), and fed to a dual-parallel Mach-Zehnder modulator (DPMZM) through a 90◦hybrid coupler. The DPMZM was biased for SSB operation, and the resulting optical signal was amplified using an erbium-doped fiber amplifier (EDFA1), before being coupled to a 6.5 cm long As2S3rib waveguide, through lensed fibers. The waveguide had a cross section of 0.85 × 4 µm, a mode area of 2.3 µm2, and a high SBS gain coefficient (g0∼ 0.74 × 10−9 m/W) [24]. The total waveguide insertion loss was 10 dB, comprising of 4 dB/facet coupling loss, and 0.3 dB/cm propagation loss. DPMZM 50:50 MZM EDFA1 EDFA2 VNA 90° 0° SG PC PC PC 90°ehybrid coupler ChG chip PC
Fig. 4. Experimental setup for the SBS-based MWP phase shifter. PC: polarisation con-troller; MZM: Mach-Zehnder modulator; DPMZM: dual-parallel MZM; SG: signal gen-erator; EDFA: erbium-doped fiber amplifier; ChG: chalcogenide; VNA: vector network analyser.
With reference to Fig. 4, the lower branch of the setup was used to generate Pump 1 and 2, as shown in Fig. 2. This was achieved by biasing a Mach-Zehnder modulator (MZM) for intensity modulation, with suppressed carrier. The modulation process generated two sidebands with frequency ωc±Ω, where ωcwas the frequency of the laser output, while Ω was the frequency of the RF signal being fed to the MZM by a signal generator (SG). This pump field was amplified by an EDFA and coupled to the chip, where it counterpropagated with the SSB-modulated carrier.
The SBS interaction between Pump 1 and 2 and the carrier resulted in a phase shift on the carrier. The frequency of the SG was set to 7.637 GHz and 7.695 GHz, for negative and positive phase shifts, respectively. As explained in Sec. 2, this resulted in the largest, most efficient phase shift for a given power of Pump 1 and 2. The frequency response for the phase shifter is shown in Fig. 5. The phase shift could be continuously tuned over a 240◦range, simply by varying the power of Pump 1 and 2. The maximum SBS pump power on the facet of the waveguide was 31 dBm (28 dBm for both Pump 1 and 2), corresponding to a ±120◦phase shift. Tuning of the phase shift resulted in ±1.5 dB fluctuations in the magnitude response. This was partly due to the reflection of the residual carrier of the SBS pump field, and partly due to the polarisation pulling effect of SBS on the signal carrier [30], which increased the polarisation-dependent losses along the waveguide.
The measurements were taken over a frequency span ranging from 1 to 15 GHz. The lower frequency limit was set by the 90◦hybrid coupler used for feeding the RF signal to the DPMZM. The upper frequency limit was equal to twice the Brillouin frequency shift ΩBof the medium
2 4 6 8 10 12 14 −4 −2 0 2 4 Frequency (GHz) Magnitude resp onse (dB) (a) 2 4 6 8 10 12 14 −100 −50 0 50 100 Frequency (GHz) Phase resp onse (degrees) (b)
Fig. 5. Measured (normalized) frequency response for the SBS-based MWP phase shifter.
(∼ 7.6 GHz in chalcogenide). The reason for this is that when the frequency of the sideband of the SSB signal is 2ΩBaway from the carrier, the SBS process acts on both the carrier and the sideband, resulting in a resonance in the magnitude response of the phase shifter [9].
3.2. Insertion loss analysis
Minimisation of the insertion loss is a design challenge which must be addressed, in order to enhance the performance and signal-to-noise ratio of the phase shifter. Following optimisation of the system, the most effective way to reduce the insertion loss is to increase the optical power into the modulator since, when using external modulation, the link gain is proportional to the square of the optical input power [31]. However, the carrier power must remain low enough to satisfy the condition in Eq. (6), such that pump depletion does not occur, and the phase shifter maintains a constant amplitude response for different phase shifts.
Additionally, in the pump depletion regime, the SBS interaction between Pump 2 and the carrier causes Pump 2 to deplete, resulting in a decrease in the gain resonance on the carrier. On the contrary, the SBS interaction between Pump 1 and the carrier causes Pump 1 to be amplified, corresponding to an increase in the loss resonance on the carrier, as can be shown with Eqs. (1) – (3). This means that in the SBS pump depletion regime, the amplitude contributions from Pump 1 and 2 on the carrier no longer cancel out, and the carrier experiences a net loss. Therefore, increasing the optical signal power into the waveguide increases the amount of pump depletion, resulting in a higher net loss on the carrier (i.e., carrier depletion), which ultimately limits the
0 5 10 15 20 −15 −10 −5 0 Carrier power (dBm) Carrier atten uation (dB) Chip implementation Fiber implementation Method [8] on chip (a) 0 5 10 15 20 −15 −10 −5 0 Carrier power (dBm) Carrier atten uation (dB) Chip implementation Fiber implementation Method [8] on chip (b)
Fig. 6. Carrier attenuation, as a function of carrier power, at the output of the SBS medium for (a) 0◦phase shift; (b) 180◦phase shift.
output power of the optical carrier.
Figure 6 simulates the effect of increasing the optical power into the waveguide. The corre-sponding increase in the carrier power is counteracted by SBS depletion of the optical carrier, which in turn leads to a higher degradation of the insertion loss of the phase shifter. The sim-ulations do not include the additional attenuation due to spontaneous Brillouin scattering, and therefore represent a best-case scenario. The insertion loss degradation is a function of the phase shift, simulated for the two extreme cases at 0◦ and 180◦. For a 6.5 cm long chalco-genide waveguide, it is clear that the current SBS phase shifter technique, as well as being more efficient, results in less carrier depletion, especially for low phase shifts, compared to the technique demonstrated in [8]. Furthermore, the simulations show that a 1 km spool of silica fiber is more susceptible to SBS carrier depletion, meaning that chip implementations allow operation at higher carrier powers, with more potential for reducing the insertion loss. We note that when the pump power launched into the medium is the same for both chip and fiber cases, the length of the fiber can be reduced to 71 m. In this instance, the depletion threshold for both fiber and chip implementations is the same. Nevertheless, on-chip operation is more attractive for enabling the realisation of multiple functionalities on the same device.
An experiment was performed to observe the effect of a high carrier power on the operation of the phase shifter. The carrier power was increased to 20 dBm on the facet of a 6.5 cm long As2S3waveguide. The pump spectrum after the SBS interaction was recorded and is shown in Fig. 7(a). It is clear that for a 20 dBm carrier, the powers of Pump 1 and 2 at the output of the waveguide are no longer equal, meaning that Eq. (6) is no longer satisfied, and according to the simulations in Fig. 6, the insertion loss of the phase shifter is expected to fluctuate for different phase shifts. In this region of operation, we recorded the magnitude response of the phase shifter, shown without normalization in Fig. 7(b). We then tuned the phase shift using both the technique presented in this work, and the technique introduced in [8]. As expected, this last technique suffers from a higher insertion loss, for high carrier powers. Alternately, the new technique for tuning the phase shift delays the insertion loss degradation to higher carrier powers, thus allowing a further increase in the link gain of the phase shifter, as well as achieving more efficient operation.
−0.1 0 0.1 −40 −20 0 Wavelength (nm) P o w er (dBm) No carrier power 10 dBm carrier power 20 dBm carrier power (a) 5 10 15 −50 −45 −40 −35 −30 Frequency (GHz) Magnitude resp onse (dB) Current technique - 0◦ Current technique - 80◦ Technique from [8] - 0◦ Technique from [8] - 80◦ (b)
Fig. 7. (a) Pump spectra after the SBS interaction, for different carrier powers, and normal-ized for a 1550 nm wavelength. (b) Phase shifter magnitude response using both the current technique, and the conventional technique reported in [8], with a 20 dBm carrier power.
4. Conclusion
We have provided the first detailed analysis and demonstration of an MWP phase shifter making use of SBS in an integrated platform. Using a 6.5 cm long As2S3chalcogenide rib waveguide, we achieved 240◦of continuously tunable phase shift over a 14 GHz frequency range. The con-ventional SBS phase shift tuning approach [8], was modified to exploit the maximum available phase shift, for a given pump power. This resulted in less strain on the SBS medium, making this tuning technique more suitable to chip implementations.
The unique capacity of SBS to simultaenously excite both gain and loss resonances, enabled low ±1.5 dB fluctuations in the magnitude response as a function of phase shift which, to the best of our knowledge, is a record for IMWP phase shifters. We have proven that these magnitude response fluctuations, together with the insertion loss of the phase shifter, are limited by SBS pump depletion, which causes reductions in the carrier power. This effect was shown to be more prominent in fiber than on chip, meaning that integration has the capacity for higher performance at high optical signal powers, and promises greater improvements in the insertion loss of the phase shifter, which is essential for practical applications.
Acknowledgments
This work was funded by the Australian Research Council (ARC) through its Centre of Ex-cellence CUDOS (Grant Number CE110001018), Laureate Fellowship (FL120100029), and Future Fellowship (FT110100853).
