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Abstract
We present a frequency down conversion RF/photonic link with an attenuation-counter-propagating optical phase lock loop (ACP-OPLL) photonic integrated circuit (PIC) receiver. Frequency down conversion was accomplished by nonlinear optical phase modulation in a 2.5mm long transmitter (TX) multi-quantum-well (MQW) phase modulator. The down-conversion link demonstrated RF to IF conversion loss of ~9dB and spurious free dynamic range (SFDR) of ~118dB∙Hz2/3.
© 2017 Optical Society of America
1. Introduction
In a radar frontend, a RF/Photonic link can be used to connect antennas with the radar digital signal processor backend. The optical link offers the benefits of wide bandwidth, low transmission loss and immunity to electro-magnetic interference. Conventional direct detection (DD) RF/photonic links suffer from poor spurious free dynamic range (SFDR) due to the nonlinearities in optical intensity modulation [1–5]. For solution, a coherent phase modulated (PM) link with an attenuation-counter-propagating optical phase locked loop (ACP-OPLL) linear phase demodulator has been proposed. The PM link is capable of achieving orders of magnitude larger SFDR than that of the DD links [6]. On the other hand, a radar frontend also requires the antenna signal to be down-converted to the intermediate frequency (IF) or baseband, where it can be digitized. The frequency down conversion was often performed by electronic mixers. But electronic mixers have large spurious distortion and conversion loss. Several optical approaches have also been proposed, including dual MZ modulator [7], optical heterodyning [8,9], optical sampling [10,11], and nonlinear optical phase modulation inside a multiple quantum well (MQW) modulator [12–14]. Among these methods, the nonlinear optical phase modulation offers the best conversion efficiency.
In this paper, we present the first frequency down conversion RF photonic link with a discrete nonlinear transmitter (TX) optical phase modulator and a monolithically integrated ACP-OPLL phase demodulator as shown in Fig. 1. The third order intermodulation intercept point in the down-converted optical phase [15] was measured to be 6.4π. The measured SFDR is ~118dBHz2/3. To the best of our knowledge, this represents the best frequency down-conversion linearity performance among the down-conversion optical links. We noted that the design and fabrication of the ACP-OPLL PIC has been reported in Ref [16], which focused on straight RF signal transportation over an optical fiber. In contrast, this paper addressed a complete RF photonic frontend system, where information is not only transported through an optical fiber but also frequency down-converted with high dynamic range to allow backend signal processing.
2. Concept
In Fig. 1 the TX MQW phase modulator was slightly forward biased to realize strong nonlinear phase modulation. Thus, when both the RF and LO inputs are applied to the modulator, their mixing product will be generated in the optical phase due to the phase modulation nonlinearity. The mixing product is then retrieved by the ACP-OPLL. The nonlinear sources inside the down-conversion link include: the TX MQW phase modulator, the local (LO) MQW phase modulators inside the OPLL, and the OPLL homodyne phase detection. The TX MQW phase modulator is slightly forwarded biased, and its phase modulation response is approximated by an exponential function [13]. In presence of a two-tone RF input (with frequency of ωRF1 and ωRF2), it creates the following third order inter-modulation distortion tone (IMD3) at the ACP-OPLL output:
where ω1 and ω2 are the frequencies of the in-band distortion: ω1 = 2ωRF1-ωRF2-ωLO, ω2 = 2ωRF2-ωRF1-ωLO. ARF and ALO are the amplitudes of the RF and LO signals respectively. VT is the thermal voltage. η is a constant related to the TX MQW modulator forward current-voltage characteristics [14]. α is a coefficient related to the modulator bias voltage, quantum well design, and the modulator length. IPD is the photocurrent generated by each PD. G(ω) is the OPLL open loop gain given as:where β1(LO) is the linear phase modulation sensitivity of the LO phase modulator inside the OPLL, and Zload is the ACP-OPLL’s load impedance.The IMD3 tones caused by the LO phase modulator (IMD3LOPM) and homodyne phase detection (IMD3HPD) inside the ACP-OPLL were given by:
where β3(LO) is the third order nonlinear modulation sensitivity of the LO phase modulator inside the ACP-OPLL, and β1(TX) is the frequency down-conversion efficiency of the TX modulator and it can be approximated by [14]:The IMD3 tones of the LO MQW phase modulator and the homodyne phase detection can be used to cancel the IMD3 tone of the TX MQW phase modulator.
On the other hand, the link output noise floor can be given by:
where <δθTX >is the optical phase noise, τ is the delay mismatch between the two optical paths in Fig. 1, RIN is the laser relative intensity noise, Zload is the PD load impedance, ZTX is the TX modulator termination resistance, Γ is the disbalance of the balanced PDs inside the ACP-OPLL, GlinkV is the link voltage gain, <δIshot> is the PD shot noise current given by <δIshot>2 = 2IPD∙e.Due to the OPLL feedback, its output noise floor is suppressed by its open loop gain. With a perfect path match (i.e. τ = 0) and an ideally balanced PD (Γ = 0), the laser RIN and optical phase noise should not contribute to the system output noise. In this case, the link system should achieve photodetector shot noise limit with large photocurrent and sufficient link voltage gain. In photodetector shot noise limit, (5) can be simplified as:
The output voltage noise floor can be converted to an equivalent optical phase noise given by:The optical equivalent optical phase noise only depends on the photocurrent.3. Design
The down-conversion RF photonic link contains a TX MQW phase modulator and a monolithically integrated ACP-OPLL. The TX phase modulator employs a deep-ridge waveguide structure as shown in Fig. 2(a). The modulator is shown in Fig. 2(b).
The ACP-OPLL phase demodulator is shown in Fig. 3. It consists of a pair of push-pull MQW phase modulators, a multi-mode interference (MMI) coupler and a pair of balanced uni-travelling-carrier (UTC) waveguide photodetectors (PD). The optical components share a common MQW optical waveguide. In the PD section, light is evanescently coupled to the photodetector absorber. The bandwidth of the ACP-OPLL is ~300MHz.
Both the link TX phase modulator and the ACP-OPLL PIC were fabricated using Harvard CNS facilities [16].
4. Measurements
The ACP-OPLL’s phase demodulation sensitivity was first measured. The ACP-OPLL was placed inside an optical link with a commercial transmitter LiNbO3 phase modulator that has Vπ of 3.5V. By comparing the input voltage to the LiNbO3 modulator and the ACP-OPLL output, we determined that ACP-OPLL phase demodulation sensitivity is 0.24V/rad.
Next, the MQW TX phase modulator and the ACP-OPLL were applied to form a frequency down-conversion RF photonic link as shown in in Fig. 4. To enhance its open loop gain, a 250Ω serial resistance was inserted between the ACP-OPLL PIC output and the spectrum analyzer. Thereby, its output impedance was elevated to 300Ω. In order to overcome environmental perturbations, the slow-varying portion of the ACP-OPLL’s output was extracted through the DC port of an RF bias-Tee and fed back to a piezo-electric fiber-optic line stretcher to correct the slow-varying phase error. This ensures long-term stable phase locking.
Figure 5(a) shows a sample of the captured output 50MHz IF spectra when a 4GHz −1dBm two-tone RF input was applied to the TX modulator. The bias voltage of the ACP-OPLL and the TX modulator were −5.6V and 0.8V, respectively. The captured IF power was −18dBm. The actual IF power at the ACP-OPLL output as −10dBm due to the 250ohm resistor between the 50ohm load and the ACP-OPLL. Thus, we determine the RF to IF conversion loss was 9dB.
Fig. 5 Link down conversion measurement results, when PD photocurrent was 23.7mA per PD. (a) A sample of the link output. (b) OIP3 directly at the ACP-OPLL output.
The intermodulation intercept point (IP3) directly at the ACP-OPLL output (with 300ohm load) was found to be 19dBm (see Fig. 5(b)). Therefore, the ACP-OPLL output rms voltage at the third order intercept point () was 4.79V. From this, we determine the down-conversion link’s IP3 in the optical phase (i.e. phase IP3) to be 6.4π since the ACP-OPLL phase demodulation sensitivity was measured to be 0.24V/rad. We noted that the phase IP3 is defined as the rms magnitude of the input optical phase deviation, where the magnitude of the third-order nonlinear intermodulation distortion tones (IMD3) equals that of the fundamental tones in the link output.
The link demonstrated significant linearity improvement comparing to the previously reported link with a 1mm long MQW phase modulator and a discrete OPLL [12], where the phase IP3 is ~2.1π. This is due to the use of a longer TX phase modulator (2.5 mm), and the nonlinearity cancellation between the TX phase modulator and the monolithic ACP-OPLL PIC phase demodulator.
The link output noise floor was also measured. The captured noise power by the spectrum analyzer was −166.5 dBm/Hz. Due to the 250ohm resistor, the noise floor directly at the ACP-OPLL output should be −158.5dBm/Hz. From the noise floor and the IP3 measurements, the measured link SFDR was found to be 118.3dB∙Hz2/3.
The link conversion loss and SFDR were measured for different RF frequencies. The results are summarized in Fig. 6. The operating RF frequencies of the down-conversion link are limited by the parasitic capacitance of the TX modulator electrode. The IF bandwidth is limited by the minority carrier lifetime in the MQW region of the TX phase modulator [13]. Table 1 compares the frequency down conversion performance of this work with other reported optical domain frequency down-conversion approaches and a state-of-the-art electronic mixer. This work demonstrated superior SFDR performance.
Table 1. Comparison with reported optical and electrical frequency down-conversion approaches.
5. Summary
In this paper, we present the first frequency down-conversion RF photonic link employing a TX MQW phase modulator and a monolithically integrated ACP-OPLL phase demodulator PIC. With the help of a 2.5 mm long TX phase modulator and nonlinearity cancellation between the ACP-OPLL and the TX phase modulator, the link demonstrated excellent RF to IF conversion linearity. A down-conversion phase IP3 of 6.4π and SFDR of ~118dB∙Hz2/3 were observed. To our best knowledge, this presents the best performance among reported optical domain frequency down-conversion approaches.
Funding
Air Force Office of Scientific Research (FA9550-12-1-0194).
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