The RF/photonic link is the basic element of microwave photonics. Previous RF photonic links often rely on optical intensity modulation. The inherent modulation nonlinearity leads to inadequate spurious free dynamic range (SFDR) for many sought-after microwave photonic applications in radar. A new phase-modulated (PM) link with an optical phase-locked loop (OPLL) demodulator could afford a promising solution for the SFDR. However, the efficacy of the PM link approach remained unsubstantiated as previous OPLL implementations had too restricted of a bandwidth for realistic applications. Here, we present a new OPLL photonic integrated circuit (PIC) chip that offered quadrupled bandwidth enhancement over the state of the art. The OPLL PIC represents the first OPLL linear optical phase demodulator with sufficient bandwidth for realistic microwave photonic applications. With help of the OPLL PIC, a PM RF/photonic link demonstrated a record-breaking SFDR of and a 3 dB SFDR bandwidth of . The combined SFDR and bandwidth performance amounts to around 1 order of magnitude improvement over the prior state of the art.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Microwave photonics relate to routing and processing RF/microwave signals in the optical domain, while exploiting the positive attributes of an optical fiber, including lightweight, wideband, and immunity to electromagnetic interference. Microwave photonics aim to execute a wide range of critical microwave functions [1–3]. However, its insertion in realistic systems, such as radar is impeded by the limited spurious free dynamic range (SFDR) of the RF/photonic link [4–6]. The restricted SFDR has always been one of the most challenging problems in the field of microwave photonics.
Current RF/photonic links primarily employ intensity modulation, which is highly nonlinear, resulting in inadequate SFDR. A plausible alternative is to apply a phase-modulated (PM) link approach [7,8] as depicted in Fig. 1. Unlike the intensity modulation, the optical phase modulation by a conventional modulator is highly linear. Thus, a larger SFDR is attainable if a linear phase demodulation can be realized. The PM link approach employs a virtual optical phase-locked loop (OPLL) that linearly demodulates the optical phase by tight phase tracking. The virtual OPLL presented in this work employs a pair of optical phase modulators and is different from a conventional OPLL that uses a tunable laser as a voltage-controlled oscillator (VCO). However, since the phase is the time integration of the frequency, in the frequency domain, the phase modulator pair can be treated as an equivalent VCO with a frequency-dependent scaling factor (). Thus, for simplicity, in the subsequent discussions, we just use the term “OPLL” to describe the phase demodulator.
Unlike conventional OPLLs that lock the DC optical phase offset, the OPLL in the PM link demodulates the optical phase by tight phase tracking. To attain the required tight phase tracking, the OPLL must have a large open loop gain over the entire wide RF instantaneous bandwidth (BW). Feedback stability requires the loop propagation delay be kept minimum (). The extreme loop delay curtailment necessitates the OPLL to employ an attenuation-counterpropagating (ACP) OPLL structure [9,10], which helps eliminating the phase delay of the in-loop optical modulators and photodetectors (PDs), and to be implemented as a photonic integrated circuit (PIC). Compared with the conventional direct-detection links, the challenge in the PM link is in its BW. Previous OPLL implementations [11–16] only demonstrated very restricted BW (). Thus, they failed to prove the efficacy of the PM link approach for realistic microwave photonic applications such as radar, which often calls for an BW of 500 MHz or wider.
In this paper, we report a new OPLL PIC that not only demonstrated record-breaking linearity but also quadrupled the OPLL BW over the current state of the art. The OPLL PIC represents the first OPLL demodulator suitable for insertion in realistic applications in radar. With the help of this OPLL PIC, a PM RF/photonic link exhibited SFDR and a 3 dB BW of 1 GHz. The link SFDR was only limited by the noise from an external optical amplifier.
The performance figure of merit (FoM) of the reported PM link with the new OPLL PIC, which is defined as the product between its SFDR in linear magnitude and its 3 dB BW, was found to be order of magnitude greater than the previous record. In addition, the reported link SFDR value also represents a new record among all RF photonic links reaching GHz range operation frequencies. Although in this report the OPLL operates in baseband, it is applicable to high RFs with the help of optical domain downconversion [17,18].
2. OPLL CONCEPT
The challenge for the PM link is the OPLL linear phase demodulator. As shown in Fig. 1, an OPLL contains a pair of local phase modulators, a pair of balanced PDs, and feedback. An OPLL forces the locally generated optical phase to mirror the incoming optical phase via feedback. Thus, the differential voltage developed on the OPLL’s local phase modulators is a scaled replica of the link input RF voltage. Realistically, due to the finite open loop gain of the OPLL, there is a small phase tracking error between the incoming and the locally generated optical phases that causes nonlinear distortions in the OPLL output.
The phase demodulation linearity is best quantified by the third-order intermodulation intercept point (IP3) at its incoming optical phase. When ignoring nonlinearities inside the optical phase modulation, the IP3 at the input optical phase of the OPLL is given by 
With an ideal optical input, the noise performance to the OPLL demodulator is ultimately determined by the PD shot noise, which creates following equivalent input optical phase noise :
The shot-noise-limited SFDR of an OPLL is thus given by4)] defines the SFDR performance limit of the OPLL excluding impacts from optical sources. The actual SFDR of a PM link may be affected by other external optical noise sources, such as a laser or an optical amplifier.
3. OPLL PIC DESIGN
The design of the OPLL PIC is depicted in Fig. 2. The PIC consists of a pair of InP multiple quantum well (MQW) local phase modulator sections, a multimode interferometric (MMI) 3 dB optical coupler, and a pair of waveguide unitraveling carrier (UTC) PDs. The MQW phase modulator sections are 2.2 mm long each and have a 2.5 μm wide deeply etched optical ridge waveguide. The waveguide UTC PDs are both 200 μm long and 12 μm wide. Including the input and output waveguides, the total length of the 3 dB MMI coupler is 0.66 mm long. To introduce the desired RF loss for the ACP structure [9,10], the modulators employ a narrow lossy electrode ( wide) with series resistance.
The modulators, waveguide PDs, and 3 dB optical coupler share a common MQW optical waveguide, which contains 25 periods of quantum wells and barriers. The quantum confined stark effect (QCSE) inside the MQW waveguide facilitates highly efficient electro-optic modulation. We noticed that the phase modulation by the QCSE is not as linear as that of a modulator . This ultimately limited the shot-noise-limited SFDR of the OPLL. Inside the waveguide PDs, the optical field is evanescently coupled into an 80 nm -doped InGaAs light absorber layer. A three-step band-smooth layer is placed on each side of the light absorber to facilitate photocarrier transport through the heterointerfaces at the boundaries of the absorber. The ground electrodes of the modulators and PDs were tied to a common -doped InGaAsP contact layer. For simplicity, the signal electrodes of both the PD and the modulator were directly connected by feedback metal traces. Consequently, the modulators and PDs share identical DC bias voltage.
Different from earlier OPLL PICs , the waveguide UTC PD of the current PIC used an -doped () 200 nm thick InP collector layer. The unintentional-doped (UID) PD collector design of earlier PICs has led to severe OPLL performance penalties. We found that during the epitaxial layer growth, the -dopant (Zn) in the PD absorber layer and the -cladding layers can easily diffused into the collector, thus transforming the UID collector layer to a -doped layer. The unwanted -doping severely limited the detector’s BW and saturation photocurrent. The -doped collector design of the current OPLL PIC compensates the Zn diffusion, leading to significantly improved performance.
4. OPLL PIC FABRICATION
The OPLL PIC was cofabricated along with discrete modulator and PD test structures. The fabrication process is illustrated in Fig. 3. The fabrication starts with an InP epitaxial base wafer grown by molecular beam epitaxy. The epitaxial layer structure contains both the InGaAsP MQW waveguide and the UTC PD layers. First, by wet etching, the UTC PD layers were removed everywhere except for in the PD region. Next, the device -cladding layers were regrown by metal-organic chemical vapor deposition (MOCVD). Then, the deep-ridge optical waveguides were formed using the induction coupled plasma relative ion etching, which etched through the MQW waveguide layers and stopped in the -doped InP buffer layer underneath the waveguide. Next, a -mesa was formed by wet etch to expose an -doped InGaAsP contact layer. The -contact metal stack (Ni/Ge/Au/Ge/Au/Ni/Au, 5/14.5/25.5/14.5/25.5/20/100 nm) was then deposited using thermal evaporation and annealed to form the -ohmic contact. Next, to provide the electrical isolation and reduce the optical loss, the unwanted InGaAs -contact layers on the top of the passive optical waveguide were selectively removed by wet etch. Then, proton implantation was performed on the same passive waveguide regions to further improve electrical isolation and reduce optical loss. Next, to reduce the electrode parasitic capacitance, an -mesa is formed by selectively removing the unwanted -doped InP and InGaAsP contact layers. Then, a -contact metal stack (Ti/Pt/Au, 20 nm/40 nm/600 nm) was deposited directly on the top surface of the modulator and PD’s ridge waveguides with high precision, and the -ohmic contact was formed after annealing. Next, to further reduce the parasitic capacitance, a thick (850 nm) low-stress dielectric film stack of alternating and films was deposited by plasma-enhanced chemical vapor deposition. Finally, vias were opened on the thick dielectric film stack to expose the and the metals (see Fig. 4), and a thick interconnect metal (Ti/Au, 20 nm/1500 nm) was then deposited. In postprocession, the fabricated wafer was lapped to 120 μm thick, and the OPLL PIC was then scribed and mounted on the an AlN (see Fig. 5) ceramic carrier.
5. MEASUREMENTS AND DISCUSSION
First, with help from the cofabricated PD and modulator test structures, the FoM of the OPLL’s phase modulator was measured to be when biased at 7 V; the 3 dB BW of the OPLL’s waveguide PD was found to be ; the balance of the OPLL’s 3 dB coupler was determined to be better than 0.3 dB.
To determine the RC time-limited BW of the OPLL, the RF reflection coefficient of the OPLL PIC output was measured using a vector network analyzer (see Fig. 6). The OPLL can be approximated by a lumped-element circuit model as depicted in Fig. 6(b), where Rs is the series contact resistance, R_load is the load impedance of the OPLL output termination (), I_PD is the generated photocurrent, and C_mod and C_PD are the capacitances of the modulator and PD, respectively. By parameter extrapolation of the measured RF reflection [see Fig. 6(a)], the parameters for the lumped-element circuit was found to be7. In this setup, the CW output of a 1.55 μm narrow-linewidth fiber laser was amplified by a high-power erbium doped fiber amplifier (EDFA). To mitigate the strong amplified spontaneous emission (ASE) noise of the EDFA, the EDFA output was filtered by a narrowband fiber Bragg grating (FGB) with 2 nm BW and then split into two paths by a 3 dB optical coupler. For link balancing, which is needed for noise mitigation, each path contains a commercial transmitter (TX) phase modulator with a of 6 V. The PM optical signals were launched into each input waveguide of the OPLL PIC by a lensed fiber (with a spot size of 2.5 μm). All optical components in the setup are polarization maintaining. The differential outputs of the two PDs were directly extracted using a GSSG dual-output air coplanar RF probe. The RF outputs passed through RF bias tees before being combined (i.e., subtracted) by a 180 deg RF hybrid. The output of the hybrid was captured by an RF spectra analyzer. Thus, both PDs of the OPLL PIC saw 50 ohm RF load. We noticed that due to improved PIC design and fabrication, the new OPLL PIC can achieve sufficient open loop gain with only 50 ohm load impedance, while previous OPLL PICs [12–16] demanded much elevated load impedance ().
The DC bias to the OPLL PIC was applied to the DC port of two bias tees via two 100 ohm resistors (see Fig. 7). The slow-varying voltage drops on the two resistors were subtracted and then fed back to a piezoelectric (PZ) fiber-optic line stretcher through a feedback control circuit. The slow feedback is necessary for stable phase locking of the PM link as it compensates random phase fluctuations induced by environmental temperature and acoustic perturbations.
In the measurement, the optical power launched to each input waveguide of the OPLL was set to 200 mW, which is safe from PIC facet damage. The OPLL bias was set to . In this condition, each PD generated a photocurrent () of . The photocurrent caused the following equivalent input optical phase noise [Eq. (3)] due to the PD’s shot noise of8, when the input RF was centered at 500 MHz and the RF power was set to 14 dBm per tone. Since the of the TX modulator is 6 V, each 14 dBm RF input tone generated a phase modulation index of 0.83. The measured relative third-order intermodulation distortion level is . We noted that when compared with the distortion of a reference MZ intensity modulator with an identical modulation index [see Fig. 8(b)], the distortion level of the OPLL output is less.
The third-order intermodulation intercept point (IP3) of the link was measured, and the result was depicted in Fig. 9. The input IP3 (IIP3) was found to be 46 dBm, which translates to a 44.6 V rms voltage on the TX phase modulator. Since the TX modulator has a of 6 V, the measured IP3 at the input optical phase of the OPLL is4)] at 500 MHz was determined to be
To ascertain the SFDR of the entire optical link, the noise floor at the OPLL output was assessed using an RF spectrum analyzer, resulting in an output noise floor value of at 500 MHz (also see Fig. 9), which is 8 dB higher than the calculated PD shot noise. We found that in this measurement, the output noise floor was dominated by the ASE noise from the external optical amplifier. Ideally, the system noise floor should be limited by the shot noise with balanced photodetection. However, the PD pair in the PIC is not ideally balanced, especially for the large ASE noise generated by the high-power EDFA, which is of a wideband and has random polarization. Therefore, the ASE noise failed to be sufficiently suppressed by the balanced PD.
Separately, the measured OPLL’s output IP3 (OIP3) is 27 dBm (also see Fig. 9). From the OIP3 and the output noise floor measurement, the SFDR of the complete RF/photonic link was found to be . This represents a new SFDR record for RF photonic links at 500 MHz.
The frequency dependence of the shot-noise-limited SFDR and the overall link SFDR are depicted in Fig. 10, which also compares with the prior state of the art and the theoretical prediction.
As shown in Fig. 10(a), the shot-noise-limited SFDR of the OPLL PIC reported in this work remains for frequencies up to 500 MHz, and for frequencies extending to 2 GHz. Above 500 MHz band, the OPLL’s shot-noise-limited SFDR exceeds previous state of the art by at least 1 order of magnitude. The 3 dB BW of the OPLL’s shot-noise-limited SFDR was found to be , which is over 4 times larger than the best prior art. The BW performance is sufficient for realistic microwave photonic applications.
Furthermore, the shot-noise-limited SFDR is in decent agreement with the theoretical calculation. We attribute the discrepancy with the theory to the nonlinear distortions of the OPLL’s internal MQW phase modulators, which are not included in the theoretical calculations.
Figure 10(b) compares the measured SFDR of the entire link obtained from the measured link OIP3 and output noise floor. The link SFDR is at 500 MHz and remains up to 1 GHz. This amounts to a 5.1 dB improvement in the peak SFDR value and times of improvement in the 3 dB BW of the link SFDR. Thus, the FoM of the reported PM link was found to be 9 times (of magnitude) larger than that of the previously reported best state-of-the-art PM links [14,16].
Finally, as shown in Fig. 10(b), in the frequency range up to 2 GHz, the measured link SFDR performance represents a new record for all RF/photonic links.
The fact that this high-performance OPLL was implemented in a monolithic PIC using a well-defined fabrication process has major significance. It implies that it can be mass produced with consistency, resulting in enhanced reliability and greatly reduced cost. Although the reported OPLL PIC measurement is in the baseband, the OPLL PIC can also retrieve high RF modulations with the help of optical domain downconversion by sampling [17,18]. These results clearly establish the viability of integrating the PM RF/photonic link in radar and other related microwave photonic applications.
Furthermore, the shot-noise-limited SFDR of the reported InP-based OPLL PIC chip was found to be limited by the unwanted nonlinear optical phase modulation characteristics of the integrated quantum well optical phase modulators. Thus, other emerging linear integrated optical modulator technologies, such as thin-film modulators, should further elevate the performance of future OPLL PICs. This calls for a new heterogeneous cross-material photonic integration platform.
Air Force Office of Scientific Research (FA9550-12-1-0194).
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