Phase-coherent orthogonally polarized optical single sideband modulation with arbitrarily tunable optical carrier-to-sideband ratio

We propose and experimentally verify a novel approach to achieve phase-coherence orthogonally polarized optical single sideband (OSSB) modulation with a tunable optically carrier-to-sideband ratio (OCSR). In our scheme, the orthogonally polarized OSSB signal is achieved using a dual-polarization quadrature phase shift keying (DPQPSK) modulator without an optical band-pass filter (OBPF). Therefore, the proposed method is wavelength independent. The DP-QPSK modulator includes two parallel QPSK modulators locating on its two arms. The upper QPSK modulator of the DP-QPSK modulator is driven by two quadrature sinusoidal microwave signals and works at the frequency shifting condition whose bias voltages are optimized to suppress the optical. The lower QPSK modulator of that works at the maximum transmission point and the optical carrier is not modulated. The OCSR is continuously tunable by simply adjusting the bias voltages of the lower modulator. The frequency shifting optical signal from the upper QPSK modulator and the optical carrier from the lower QPSK modulator are combined together at the output of the DPQPSK modulator. The optical carrier and sideband are polarized orthogonally. The generated OSSB signals could be used to shift and code the phase of the microwave signal and generate ultra-wideband (UWB) microwave pulse. The proposed method is analyzed and experimental demonstrated. ©2016 Optical Society of America OCIS codes: (060.5625) Radio frequency photonics; (070.1170) Analog optical signal processing; (060.2310) Fiber optics; (060.2840) Heterodyne. References and links 1. E. D. Wooten, K. M. Kissa, A. Y. Yan, E. J. Murphy, D. A. Lafaw, P. F. Hallemeier, D. Maack, D. V. Attanasio, D. J. Fritz, G. J. Mcbrien, and D. E. Bossi, “A review of Lithium Niobate modulators for fiber-optic communications systems,” IEEE J. Quantum Electron. 6(1), 69–82 (2000). 2. U. Gliese, S. Norskov, and T. N. 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Introduction
Optical single sideband (OSSB) modulation has become an attractive technology for improving the performance of the fiber optical communication systems [1], long-distance radio-over-fiber (ROF) system [2,3], high-resolution optical vector network analyzer (OVNA) system [4], and microwave photonic systems [5].Compared with the optical double sideband (ODB) modulation, the OSSB modulation could enhance the spectral efficiency and alleviate dispersion-induced optical carrier power suppression.In the OVNA system, it provides a correspondence or intelligent one-to-one mapping between the electrical and optical domain to characterize the transmission response of the optical under-test-device (ODUT) [4].Various approaches have been reported to implement the OSSB modulation.One common method is based on the ODB modulation incorporating with an optical filter.
The optical filter could be a fiber Bragg grating (FBG) [6], steep-edge tunable optical bandpass filter (OBPF) [7], micro-ring resonator [4] to remove one sideband of the DSB signals.However, the approach depends on the wavelength of the optical carrier.Moreover, if an ODB signal undergoes through these optical filters, the optical filter induces the extra phase and amplitude response imposed on the residual sidebands and the optical carrier.The OSSB modulation can also be obtained based on electro-optical modulators combining with a Hilbert convertor.The key advantage of the approach is the OSSB modulation is independent on the wavelength of the optical carrier over the whole C wavelength band.A dual-electrode Mach-Zehnder modulator (DMZM) [4], a dual-parallel MZM (DPMZM) [8], an MZM cascaded with a phase modulator (PM) [9], two individual electro-absorption modulators [10], or two polarization modulators (PolM) [11] have been reported to achieve OSSB modulation, which are generally driven by two microwave signals with 90°phase difference.The OSSB modulation also could be achieved using phase modulation of semiconductor optical amplifier [12], a single bidirectional intensity modulator inside a Sagnac interferometer [13], or an electro-absorption modulated laser [14].The merit of these techniques lies in the fact that these schemes are not optical carrier wavelength dependent.However, the main limitation for the most of the previously demonstrated methods with or without an optical filter is the optical carrier and the corresponding first order sideband is not polarized orthogonally.Polarized orthogonally OSSB modulation has important application in microwave photonic signal processing systems, such as microwave photonic phase shifter [15], complex coefficient microwave photonic filter [16], antenna array beam-forming [17], ultra-wideband (UWB) microwave signal generation with background-free [18].The polarized orthogonally OSSB signal is usually injected into the polarization dependent device.In this modulation format, the state of polarization (SOP) of the optical carrier and the correspondingly modulated sideband are orthogonal to undergo the polarization dependent effect.The orthogonal OSSB modulation has been realized using a differential group delay module [15], a MZM cascaded with a PolM [19], a PolM incorporating with an OBPF, an acoustic modulator, improved Sagnac-loop-based modulator [20].The key limitation for the acoustic modulator-based method is the low working bandwidth.The differential group delay modulebased scheme works usually at the single frequency case.The PolM-based polarized orthogonally OSSB modulation is a powerful method, but the OBPF have to be required to remove one of sidebands.For the MZM cascaded with a PolM, the accurate delay matching between the two microwave signals is strictly needed to obtain a wide bandwidth.The polarized orthogonally OSSB modulation has also been reported based on the stimulated Brillouin scattering (SBS) polarization pulling effect [21,22].However, the system is rather complicated to obtain the required orthogonal properties and has a poor stability.
For some realistic applications, it is very desirable that the polarized orthogonally OSSB modulation with a tunable optical carrier to sideband ratio (OCSR).However, the OCSR of the reported schemes is not almost changed.The ROF system often suffers from the lower receiver sensitivity at the remote end to degrade the bit error ratio (BER).The receiver sensitivity could be enhanced by simply optimizing the OCSR to improve the performance of the entire system [23][24][25].Up to now, several approaches have been verified to achieve the OSSB modulation with tunable OCSR based on an OSSB generator incorporating with a narrow band notch FBG or a polarization maintaining fiber Bragg grating (PM-FBG) [23] or a DPMZM combining with a 90-degree phase shifter [26] or optical injection-locked laser [27] or multimode interference coupler and three optical phase-modulator waveguides [28].However, the working bandwidth for the PM-FBG-based method is limited and the system has a poor long-term stability and is wavelength dependent.The SOP between the optical carrier and the corresponding sideband is not orthogonal for the DPMZM-based method.The three arms MZM are a special device and the scheme is rather complicated.The OSSB modulation with tunable OCSR is also demonstrated based on cascaded PolMs [19].The main limitation lies in the fact that the approach does not work at the wide bandwidth.Moreover, in the optical communication system, the BER performance is directly related with the OCSR [29,30].
In this paper, we propose and experimentally verify a novel and compact approach to achieve phase-coherence orthogonally polarized OSSB modulation with an optimal OCSR.The proposed method is based on a single modulator.In our scheme, the orthogonally polarized OSSB is firstly achieved by DP-QPSK modulator without an optical filter.Compared with [6][7][8][9][10], the proposed system is wavelength independent over the entire C wavelength band and has a polarization orthogonal property.The upper arm of the DP-QPSK modulator is driven by two quadrature sinusoidal microwave signals and works at the frequency shifting condition whose bias voltages are optimized to suppress the optical carrier.The microwave power and phase are optimized to achieve frequency shifting optical signals.
The lower arm of that works at the maximum transmission point.The optical carrier is not modulated in this arm and its power could be adjusted by tuning the bias voltages.Therefore, the OCSR is continuously tunable by simply adjusting the bias voltages of the lower arm modulator.Compared with the [19,23,27,28], the OCSR of the proposed method has a wider range, a better stability, and a smaller footprint.The frequency shifting optical signal from the upper modulator and the optical carrier from the lower modulator are combined together at the output of the DP-QPSK modulator with orthogonal SOP.The proposed method is analyzed and experimental demonstrated.The OSSB signal generator could be widely applied for the microwave photonic signal processing system.

Theory and principle
The schematic diagram of the proposed phase-coherence orthogonally polarized OSSB modulation with tunable OCSR signal generator is shown in Fig. 1, which consists of a laser diode (LD), a DP-QPSK modulator, two polarization controllers (PC1, PC2), a microwave source (MS), an electrical 90 degree hybrid coupler (HC) and an optical spectrum analyzer (OSA).The PC2 connected after the DP-QPSK modulator and a polarizer (Pol) are used to check the orthogonal polarization property of the generated OSSB signal.A narrow line-width optical signal is firstly injected into the DP-QPSK modulator which is driven by microwave signals.The DP-QPSK modulator is a commercially available integrated device including two QPSK modulators, a polarization beam splitter (PBS), a polarization beam combiner (PBC).The optical carrier is equally divided into two parts with orthogonal polarization after the PBS via the PC1.Each arm of the DP-QPSK has a QPSK modulator with identical optical length.The microwave signal emitted from the MS is injected into the HC and separated into two branches with identical power and phase difference of 90 degree.The QPSK modulator is structured as two MZMs (MZM1 and MZM2) setting in parallel and forming the third MZM (MZM3).A polarization beam combiner (PBC) multiplexes of the two outputs optical signals of the pair of QPSK modulator with orthogonal polarizations.In our scheme, the two MZMs of the upper QPSK modulator are driven by a cosine and a sine microwave signal with the same frequency.The relative optical phase between the two MZMs is set as π/2 by simply adjusting the bias voltages.The MZMs in the two arms of the QPSK modulator work at the minimum transmission point (MITP) by controlling the bias voltages of the MZMs.Based on the above setting, the frequency shifting optical signal could be obtained at the output of the upper QPSK modulator with frequency spacing of the drive microwave signal.The lower QPSK modulator of the DP-QPSK modulator is not modulated.With the DP-QPSK modulator, we can flexibly change the frequency interval between the optical carrier and the frequency shifting optical signal by changing the microwave frequency.Assuming each MZM with infinite optical extinction ratio (ER) and perfect power splitting ratio, the normalized optical field at the output of the upper QPSK modulator is given by where E in = exp(jω c t) is the normalized optical filed of the input signal, and ω c the angular frequency of the optical carrier.Applying the Jacobi-Anger Expansion [4], the Eq. ( 1) could be rewritten as follow: where J n (•) is the Bessel function of the first kind of order n.As can be seen from the Eq. ( 2), it is clearly observed that the QPSK modulator at the output of that has the spectral components with the frequency shifts of ω m , 3ω m , and 5ω m …A series of sidebands are generated locating at one side of the optical carrier due to the nonlinearity of the QPSK modulator.They will beat with each other as well as with the optical carrier in the PD, generating the fundamental frequency microwave signal and the undesired microwave harmonic components.The optical field at the output of the lower QPSK modulator could be represented as: Both the optical signals from the upper and lower QPSK modulators are combined in the PBC to generate a polarized orthogonally OSSB modulation optical signal.The optical field at the output of the DP-QPSK modulator along the X and Y principle axes of that could be written as: where φ 1 = φ 2 is the static phase shift induced by the direct current bias of the lower QPSK modulator, which can be expressed as φ 1 = φ 2 = πV/V π-bias .V π-bias is the static half-wave voltage of the MZMs.φ 2 is the static phase shift controlled by the bias voltage of the MZM3.
As can be seen from Eq. ( 4), the SOP between the optical carrier and the odd-order sidebands is orthogonal.Figure 2(a) shows the variations of J 1 (β), J 3 (β) and J 5 (β) versus β.As can be seen from Fig. 2(a), the amplitude of the frequency shifting optical signal could be tuning by changing the power of the drive microwave signal.Generally, the phase modulation index is less than 2π due to the limited microwave power of the drive microwave signal.For β = 1.839 rad, we have J 1 (β) = 1 which is the maximum value and J 3 (β)≈J 5 (β)≈0.Therefore, the undesired microwave harmonics in the electrical domain could be eliminated.In Fig. 2(b), the normalized intensity of the first-, third-, and fifth-order optical spectral components are shown, which are calculated by P 2n-1 = [J 2n-1 (β)] 2 .It is clearly shown that the sideband suppression ratio between the adjacently odd sidebands is determined by the phase modulation index of the upper modulator.As the phase modulation index decreases, the power ratio between P 1 and P 3 increases.As can be seen from Fig. 2(b), the power of P 1 reaches the maximum value and the P1/P3 = 8.5 dB is the minimum value when β = 2 rad.Therefore, there is a tradeoff between the power of the first-order optical signal and the sideband suppression ratio for the higher-order components.
The OCSR of the generated OSSB modulated signal could be continuously tuned by controlling the bias voltages of the lower QPSK modulator.We have the OCSR as follow: It is obviously observed that the OCSR of the generated OSSB signal depends on the phase modulation index of the upper QPSK modulator and the static phase shift of the lower QPSK modulator.When the phase modulation index is a fixed value, the OCSR is tuned by simply changing the bias voltages of the lower QPSK modulator.Figure 3 shows the simulated OCSR as a function of direct current bias voltages of the lower QPSK modulator when the phase modulation index of the upper QPSK modulator is fixed at 1.839-, 1.5-, 0.5-, 3-rad.The OCSR could theoretically be continuously tuned from 19   In order to obtain the modulated microwave signal, a PC2 and a Pol attached after the modulator are used to project the polarized orthogonally optical signals into a polarization direction.The OSSB modulation optical signal are then injected into the PD to recover the modulated microwave signal, the photocurrent is given by: where E out = cos(θ) × E x + sin(θ) × E y , θ is an angle between polarization direction of the PC2 and the Pol.As can be seen from Eq. ( 6), the photocurrent consists of ac terms with odd-order frequency components.For some applications, the higher order sidebands may lead to some extra errors, such as in OVNA systems and in microwave photonic phase shifting system.To obtain the phase shift microwave signal, the microwave signal higher than the first-order ones are suppressed by controlling the power of the microwave signal, only leaving the fundamental frequency microwave signal.In this case, Eq. ( 6) can by simplified as It is clearly indicated that the phase of the recovered microwave signal could be continuously changed when angle θ varies over the range from 0 to π.The proposed orthogonally polarized OSSB signal generator could be explored for the phase coding microwave signal generation, wideband phase shifting microwave signal, optically controlled phased array antenna, multitap microwave photonic filter with complex coefficient and so on.

Experiment and simulation
The proposed scheme was experimentally demonstrated based on the setup shown in Fig. 1.A narrow linewidth LD (Koheras AdjustiK E15) with linewidth of less than 0.1 kHz is employed to provide the optical carrier with the wavelength and the power of ~1550.116nm and 13.49 dBm, respectively.The optical signal is then injected into the DP-QPSK modulator (LAAM000), which has an optical bandwidth of 23 GHz and drive voltage at RF port of 3.5 V and at DC ports of 16 V and a DC extinction ratio for sub MZM of more than 20 dB.The optical power is equally divided along the X-axes and the Y-axes.In our experiment, the light-wave at X-axes is frequency shifted while that at Y-axis is not modulated.A PC1 (Thorlabs Inc.) is inserted between the LD and the modulator to reduce the polarization dependence loss and split the optical power into two identical parts.The sinusoidal microwave signal emitted from the MS driven to the upper arm of the DP-QPSK modulator was pre-amplified by an electrical amplifier (EA) which has a 3-dB bandwidth from 40 kHz to 38 GHz, a gain of 26 dB, and a maximum output power of 22 dBm.The amplified microwave signal is divided into two paths by a broadband electrical 90 degree HC with an operational bandwidth of 1-18 GHz.The electrical length difference of the two paths should be controlled less than 0.3 mm and the power of them should be adjusted by an attenuator  First of all, we tuned the bias voltages of the nested MZM and child MZM of the upper QPSK modulator to eliminate the optical carrier.We define the bias voltages as X1, X2, X3, Y1, Y2, and Y3, respectively, which subscript 1, 2 and 3 denote the MZM1, MZM2, and parent MZM of the upper and lower QPSK modulator.We set the DC bias voltages of the DP-QPSK modulator as X1 = 9.01 V, X2 = 2.37 V, X3 = 4.67 V, Y1 = 3.71 V, Y2 = 2.01 V, and Y3 = 0 V to suppress the optical carrier.These bias voltages could be controlled using an optoelectronic feedback control system by monitoring the optical power variation to avoid bias drift.Figure 4 shows the optical spectra at the output of the DP-QPSK modulator.Figure 4(a) shows the optical carrier could be eliminated more than 48 dB by adjusting these bias voltages.The second step was to tune the bias voltages of the lower QPSK modulator to let the optical carrier go through.Moreover, the microwave signal was fed into the upper QPSK modulator operating at the frequency shifted case.Therefore, the OSSB modulated signal was achieved.Figure 4(b) shows the obtained OSSB modulated signal when the driven microwave signal worked at 15 GHz.As can see from Fig. 4(b), it is clearly shown that the OCSR (the power ratio between the optical carrier and the + 1 order sideband) is more than 26 dB.Moreover, the amplitude of the undesired −1-st order sideband is suppressed more than 48 dB as compared with that of the optical carrier.Since the modulation condition is a small signal modulation, the amplitude of the higher order sidebands could be neglected.It is also worth noting that when the microwave signal was sent into the modulator, the optical carrier kept constant.Therefore, the DC bias voltages of the modulator are not affected by the microwave signal.Since frequency dependent components are not used, the OSSB modulation could work over a wide optical wavelength range.The proposed method overcomes the key limitations of the [6,7].For some applications, it is desirable that the OSSB generator could work at widely operational bandwidth.To do so, a sinusoidal microwave signal at a frequency from 8-to 15-GHz was applied to the upper modulator of the DP-QPSK modulator.The microwave power was manually adjusted to be ~-2 dBm, which will vary with the frequency of the microwave signal due to the drawback of the MS.The measured optical spectra at the output of the DP-QPSK modulator are shown in Fig. 5.A series of sidebands at the different frequency are generated around the optical carrier.The OSSB modulated signals are successfully obtained shown in Fig. 5  uneven phase response of the HC.It is also worth noting that the OSSB modulation also could be obtained over the drive microwave frequency range from 1-to 7-GHz.The lowest observed bandwidth of the OSSB generator is not only limited by the working bandwidth of the HC but also restricted by the resolution of the OSA.Therefore, the proposed method could operate in a wide electrically frequency range and effectively avoid the drawback of [6,7,15,21,22].Moreover, we measured the magnitude and phase response of the HC over 1-18 GHz shown in Fig. 6(a).As can be seen from Fig. 6(a), we could observe that its two output signals have almost identical magnitudes and a 90 degree phase difference.As described in Section 2, the OCSR of the OSSB modulation could be continuously tuned by simply controlling the bias voltages (Y1, Y2, Y3) of the lower QPSK modulator while the bias voltages of the upper QPSK modulator are unchanged.It is worth noting that the optical carrier at the X-axes polarization is suppressed.However, the suppressed optical carrier cannot be directly observed in the experiment because the optical carrier at the Y-axes polarization overlaps with the suppressed optical carrier at the X-axes polarization of the DP-QPSK modulator.Figure 6(b) shows the optical spectra of the OSSB modulation with continuously tunable OCSR when the frequency of the drive microwave signal is 15 GHz.It is clearly shows that the power of the optical carrier could be continuously tuned over range of 40 dB while the power of the desired sideband keeps unchanged.In [19,27], the power of the sidebands varies with the power of the optical carrier which is undesirable in some applications.The corresponding OCSR could be tuned over range from −15 dB to 25 dB.In order to demonstrate the orthogonally polarized property of the OSSB generator, the PC2 and Pol are used to connect after the DP-QPSK modulator.In our experiment, an independent PBS (New Port Inc.) with an extinction ratio of more than 30 dB takes the place of the Pol.The optical spectra were recorded shown in Fig. 7 when the polarization angle between the polarization direction of optical carrier and that of the X-axes of PBS was changed from 0-to 90 degree.Figure 7(a) indicates the polarization direction of optical carrier is perfectly aligned X-axes of PBS.It can be seen from Fig. 7(a) that the desired sideband could be suppressed more than 20 dB when polarization angle is 0 degree.Meanwhile, the power of the optical carrier is attenuated about 4 dB which is attributed to the insertion loss of the PC2 and PBS. Figure 7(a)-(b) illustrate that there is an absolute orthogonality between the optical carrier and the corresponding sideband.In addition, in order to further confirm the orthogonal property of that, a polarization analyzer (PA, PAX5700) was used to measure the state of polarization (SOP) of the optical signal at the output of the DP-QPSK modulator.The SOPs of the OSSB modulated signals on the equatorial plane of the Poincare sphere.Two points in the Poincare sphere mark the SOP of the optical carrier or the optical sideband.In the measurement, the PC2 and PBS are also required.In general, the corresponding Stokes parameters and the degree of polarization are also measured.The Stokes parameters and the degree of polarization (DOP) of the optical carrier are [-0.9998,−0.0123, 0.0138] and 99.102%, respectively.The Stokes parameters and the DOP of the modulated sideband are [0.9997,−0.0206, 0.0092] and 99.542%, respectively.A 90 degree detuned SOP between the optical carrier and sideband is obtained which is corresponded to 180 degree on the Poincare sphere.It is evident that a perfectly orthogonal OSSB modulation is achieved.
The optical signal at the output of the DP-QPSK modulator was directly sent to the PD.The electrical spectrum captured by an electrical spectrum analyzer (ESA) is shown in Fig. 8(a) when the orthogonal polarized optical signal is fed into the PD.A pure microwave signal at frequency of 15 GHz is recovered due to the frequency beating between the optical carrier and the sideband with the help of a Pol.When the Pol is removed, the corresponding microwave signal also disappears.The orthogonal performance is again demonstrated because the electrical power is suppressed more than 36 dB.Furthermore, to clearly show the improvement of the receiver sensitivity, the microwave power at the different OCSR is recorded by the ESA (Advantest Inc.).It has been proved that the optimum OCSR in the ROF system is 0 dB for improving the transmission performance [25].The fundamental reason is the PD usually has a saturated input optical power.Under keeping the average optical power constant, the microwave signal at frequency of 15 GHz was measured with the assistance of an erbium-doped fiber amplifier (EDFA) with tunable output optical power.The generated microwave signal was measured shown in Fig. 8(b).It is obviously observed that when the OCSR is tuned, the power of the recovered microwave signal is changed.By changing the OCSR we found the maximum microwave output power.The power reaches the maximum value when the OCSR = 0 dB.The main advantage of the proposed method lies in the fact that there are no wavelength dependent devices and a larger tuning range, which implies that an additional device attached after the modulator usually required in other approaches [6,7] is no longer needed in our case.In the following part, we will show the scheme is capable of shifting the phase of the drive microwave signal.First, the system was reconfigured to measure the phase of the microwave signal after the microwave photonic processing system.In this case, an electrical vector network analyzer (EVNA) was introduced into the system.The microwave signal emitted from the EVNA (Agilent 8722 ET) at frequency of 15 GHz was applied to the electrical port of the DP-QPSK modulator.As discussed in Section 2, the undesired sidebands will be generated when the power of the microwave signal was too large.However, the beating between the higher bandwidth will not recognized since the EVNA only detect the fundamental frequency signal.Figure 9 shows the phase shift of the microwave signal at the frequency of 15 GHz by adjusting the PC2 over sweeping time from 0 to 120 s.It is clearly shown that no evident phase variation is found for different phase shifts which imply an excellent stability of the proposed system.

Conclusion
We have theoretically analyzed and experimentally demonstrated a photonic approach to generate phase-coherence orthogonally polarized OSSB modulation signal with a tunable OCSR using a conventional DP-QPSK modulator without an optical filter.Compared with the other methods, the proposed method is wavelength independent, more flexible, wider bandwidth, larger dynamic range for OCSR, and more stable.The upper arm of the DP-QPSK modulator works at the frequency shifting condition, the lower arm of that works at the maximum transmission point and the optical carrier is not modulated.The OCSR is continuously tunable by simply controlling the bias voltages of the lower modulator.Compared with the [19,23,27,28], the OCSR of the proposed method has a wider range, a better stability.The footprint of the proposed OSSB signal generator is much smaller than alternative techniques.The frequency shifting optical signal from the upper arm and the optical carrier from the lower arm are combined together at the output of the DP-QPSK modulator with orthogonal SOP.In addition, the polarization orthogonal property is verified based on cascading PC and Pol.Therefore, the optical carrier and sideband are polarized orthogonally.The proposed method could be used to maximize the output RF power by controlling OCSR under keeping average optical power constant.The generated OSSB signals were used to shift the phase of the recovered microwave signal at frequency of 15 GHz.The OSSB signal generator could be widely applied for the microwave photonic signal processing system.

Fig. 3 .
Fig. 3.The relationship between the OCSR and the bias voltage of the QPSK modulator is shown while fixing β at 1.839-, 1.5-, 0.5-, 3-rad of the upper QPSK modulator.

(
ATT) to obtain the identical power.The obtained two microwave signals are explored into the two RF ports of the upper QPSK modulator, respectively.By tuning the bias voltages of the lower QPSK modulator, the OCSR of the orthogonally polarized OSSB modulated signal could be tuned over 40 dB range.Then, another PC and a Pol are connected after the DP-QPSK modulator to verify the polarization orthogonal property.The optical spectrum at the output of the DP-QPSK modulator and the Pol was observed by an OSA (Advantest Q8384) with a resolution bandwidth of 0.01 nm.

Fig. 4 .
Fig. 4. Measured optical spectrum at the output of the DP-QPSK modulator (a) the optical carrier suppression; (b) the OSSB modulation when the frequency of the microwave signal drive to the upper QPSK modulator is 15 GHz. (a).

Figure 5 (Received 9
b) exhibits the corresponding suppression ratio between the + 1st order sideband and the −1st order sideband.As can be seen from Fig.5(b), the suppression ratios are not constant which are attributed to the uneven output power of the MS and the #251381 Oct 2015; revised 22 Nov 2015; accepted 22 Dec 2015; published 7 Jan 2016 (C) 2016 OSA

Fig. 5 .
Fig. 5. (a) Measured optical spectra at the output of the DP-QPSK modulator over wide frequency range; (b) the corresponding OSSB suppression ratio.

Fig. 6 .
Fig. 6.Measured (a) magnitude and phase response of the HC; (b) optical spectra at the output of the DP-QPSK modulator with tunable OCSR.

Fig. 7 .
Fig. 7. (a) Measured optical spectra when the polarization direction of the optical carrier is in accordance with the X-axes of PBS; (b) measured optical spectra when the polarization direction is orthogonal.

Fig. 8 .
Fig. 8. (a) Electrical spectra of the microwave signal at frequency of 15 GHz when the SOP between the optical carrier and the corresponding sideband is parallel (in blue line) or orthogonal (in red line); (b) the electrical spectra of the microwave signal when the OCSR is tuned.

Fig. 9 .
Fig. 9.The phase shift of the microwave signal at the frequency of 15 GHz over sweeping time of 120s.