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Abstract
We experimentally demonstrated an optical phase shifted quantizer using a cascade step-size MMI (CS-MMI), which was fabricated on a commercially available 220-nm SOI platform via multi-project wafer (MPW) process. An experimental setup was built to test the ability of the CS-MMI acting as a quantizer. The experimental results show that the proposed CS-MMI-based quantizer has an effective number of bit (ENOB) of 3.31bit, which is a little slighter than the ideal ENOB of 3.32bit. The operation range is 12 nm for ENOB≥3 bit. Moreover, the insertion loss of the CS-MMI is −1.26 dB at 1560 nm, the performance of the fabricated device agrees well with simulation results.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Analog-to-digital converter (ADC) plays a very important role in modern signal processing technology, such as high speed optical communication, advanced radar system, and medical imaging. Limited by the timing jitter and comparator bandwidth, conventional electrical ADC is difficult to meet the requirement of high resolution and ultra-bandwidth [1,2]. To overcome these bottlenecks, optical ADC which utilizing photonic technologies to sample and quantize the high speed analog signals, has attracted much attention in recent years. So far, lots of schemes of optical sampling and quantization are proposed to achieve the optical analog-to-digital conversion. Within them, optical quantizer, which is a key element in optical ADC, have been extensively studied to further develop the sampling-rate and resolution. Up to now, the high performance optical quantization schemes can be classified into two categories: optical amplitude quantization (OAQ) schemes [3–7], and phase-shifted optical quantization (PSOQ) schemes [8–13]. In OAQ schemes, high nonlinearities were used to transfer amplitude information of analog signal on wavelength of optical sampling pulses. For example, both [3] and [4] report a photonic quantizer realized by high nonlinear fibers, whereas the amplitude information of analog signal were converted into optical sampling pulses via SSFS (soliton self-frequency shift) effects. [5,6] demonstrate optical quantization schemes based on slicing the super continuum generated in the normal dispersion region of a highly nonlinear fiber. However, the OAQ schemes inevitably need high power (usually larger than 15 dBm) or/and long propagation distance (typically 1km), thus large footprint, to gain enough nonlinear effects. While PSOQ schemes are based on interference between two lights, one of which carries the amplitude information of analog signal in phase; by judging the power of the interference of the two lights, phase-shifted optical quantization can be realized. For example, [9] reported a PSOQ scheme using channel waveguide modulators with same half-wave voltages. [10] reported a novel photonic analog-to-digital conversion scheme using an array of Mach-Zehnder modulators (MZMs) with identical half-wave voltages but with different bias voltages. The authors in [10] further developed this scheme by adding electric circuit to realize linear combinations of the detected signals [11]. Reference [12] proposes a cascade PSOQ scheme to take full advantage of the phase space. Consequently, OADC schemes based on PSOQ utilize modulators to convert the amplitude of the signal into optical phase, and work in linear region, which means that the system is simple, stable and less power consumption. In [8], the proposed optical phase shifted quantizer uses only one modulator, which has the most compact footprint. Moreover, the quantization schemes based on PSOQ have the potential of small footprint and strong stability, which meets the requirement of on-chip integration.
In our previous work [8], we proposed a scheme of an optical quantizer based on CS-MMI with preliminary simulation analysis. Thereafter, we further investigated and optimized the device, and fabricated it on a commercially available SOI platform with a compact footprint of less than $7.5 \times 95\mu {m^2}$. In this paper, we report the experimental results of its application as an optical quantizer. The experimental results show that the optical quantizer based on our CS-MMI has an ENOB of 3.31 bit, which is only a little slighter than the idea ENOB of 3.32 bit. Measurement results show that the CS-MMI has insertion loss of 1.26 dB at 1560 nm. The operation bandwidth is 12 nm for ENOB≥3 bit, which we believe that is limited by the MMI1st, which can be further optimized.
2. Operation principle and device design
Detailed explanation of the ${\log _2}2N$-bit optical quantizer based on N-output CS-MMI is shown in [8]. During our optimization and device realization, we chose N=5. The schematic structure is shown in Fig. 1(a). The CS-MMI consists of two input ports (${E_P}$ and ${E_S}$), MMI1st, MMI2nd and 5 outputs (port1, port2, …, port5). Based on the theory in [8], the optical intensities (Ii, i=1, 2, …, 5) of the 5 outputs can be expressed as:
Fig. 1. (a) Structure, parameters, (b) cross section view, and (c) quantization curves of the ideal quantizer
The parameters of the CS-MMI are shown in Fig. 1(a), the device is designed on a SOI wafer with a 2µm-thick SiO2 cladding, a 220nm-thick top silicon layer and a 2µm-thick buried oxide layer, as shown in Fig. 1(b). After being carefully optimized, the final values of the main parameters are shown in Table 1.
Table 1. Main parameters of quantizer
3. Fabrication and experiments
The CS-MMIs were then fabricated by the commercially available MPW service. The deep ultraviolet (UV) photolithography is used to define the pattern of the devices and the inductively coupled plasma (ICP) is used to etch the silicon core layer. Finally, a silica upper-cladding is deposited on the structure by a plasma enhanced chemical vapor deposition (PECVD) process.
3.1 Experiments of optical quantizer based on the CS-MMI
In order to use the CS-MMI as a quantizer, we designed a test structure to demonstrate optical quantization in the layout and sent to be fabricated as well. Figure 2(a) shows the optical micrograph of the test structure. The input light of 1560 nm was generated by a tunable laser and injected into the input port of the chip via a vertical grating coupler designed for TE mode, which was provided by the foundry with TM-isolation of over 23dB. After that, the light was split into two by a 1×2 MMI, one of which was treated as ${E_P}$ and linked to the upper port of the CS-MMI, while the other was treated as ${E_S}$ and was phase modulated by an electrical analogue signal, and then linked to the bottom port of CS-MMI. The outputs of the CS-MMI were measured by an optical power meter via other vertical grating couplers. Note that the purpose of this experiment is to measure the proof of concept of quantization curves of the CS-MMI, and limited by the MPW process (only providing passive process technology), therefore the electro-optical phase modulation was realized by using a heater, which was fabricated in the MPW process.
Fig. 2. (a) Optical micrograph of the test structure. (b) Detailed structure of the CS-MMI. (c) The relationship between the voltage and phase difference.
To measure the quantization curves, we gradually increased the voltage of the heater from 0V to 15V, and recorded the 5 outputs optical powers. The relationship between the voltage applied to the heater and the produced phase difference is shown in Fig. 2(c). The normalized output powers versus phase difference (i.e. voltage) for 5 outputs are shown in Fig. 3, the points are the normalized measurement results and the dash lines are the fit results. To calculate the ENOB from the quantization curves, we set the decision threshold at the middle of the transmission curves. In the Fig. 3(a), the gray line in the middle indicates the decision threshold, the red stars mark the ideal quantization level and the blue crosses mark the measured quantization level. Equation (2) was used to calculate the ENOB [14], where PFS is the maximum phase difference induced by the analog signal, ${\Delta _{step - i}}$ is the phase error for each quantization level. Thus, the measured ENOB of proposed quantizer was calculated be 3.31bit, which is a little smaller than the ideal ENOB of 3.32 bit, showing a very good performance.
The slightly difference between the experimental and the ideal ENOBs is caused by the phase error between the ideal and the measured results, as shown in the enlarged figures in Fig. 3(b) and Fig. 3(c) that the quantization levels with the maximum phase error (about 2.19 $^\circ$) and the minimum phase error (about 0.04 $^\circ$), respectively.
Fig. 3. (a) Normalized quantization curves. (b) Maximum phase error and (c) minimum phase error of the quantization level.
The insertion loss is −1.26dB, which is the insertion loss of the test structure minus the insertion loss of other reference structures (including input/output TE grating couplers and MZM structure).
Next, we investigated the operation bandwidth of the quantizer by measuring the ENOB under different wavelengths (from 1554 nm to 1570 nm). The results are shown in Fig. 4. Simulation results are also shown for comparison. We can see from Fig. 4 that the operation bandwidth is 12 nm if ENOB maintains larger than 3 bit. The quantization curves at the bottom show the phase error between measured and ideal quantization levels. As the ENOB drops, the phase error of each quantization levels becomes apparent.
Fig. 5. (a) Optical micrograph of the MMI2nd (b) Detailed structure of MMI2nd; Measurement results and simulation results with light injected from (c) middle port, (d) upper port, (e) bottom port.
From Fig. 4, we found that the operation bandwidth is small, both in experiment and simulation. To explore the reason of why the operation bandwidth is so small, we measured two parts of the CS-MMI, which are MMI2nd and MMI1st, respectively.
3.2 Second-step MMI
In the layout for MPW fabrication, both MMI2nd and MMI1st were fabricated separately. Figure 5(a) and Fig. 5(b) show the optical micrograph and the detailed structure of MMI2nd, respectively, where it has 3 input ports (${E_{P1}}^{\prime}$, ${E_S}^{\prime}$, ${E_{P2}}^{\prime}$, which are descripted in [8]) and 5 output ports (port1… port5).
Theoretically, when the light is injected from the middle port, MMI2nd is actually a 1 ${\times}$ 5 splitter. When the light is injected from the side ports, the output of each ports have fixed transmission (−4.09dB, −4.95dB, −6.99dB, −10.97dB, −20.00dB) [15]. We measured the insertion losses of the 5 outputs with the light injected from each three ports and compared the measurement (Mea.) results with the simulation (Sim.) results, the results are shown in Figs. 5(c), 5(d), 5(e).
The measurement results are consistent with the simulation results, although the insertion losses have fluctuation, due to the noise and fabrication imperfections. From the results in Figs. 5(c)–5(e), we can see that the measurement results are steady over the wavelength from 1540 to 1580 nm, we believe that MMI2nd should not be the main reason causing narrow-bandwidth operation of the whole quantizer.
3.3 First-step MMI
Figures 6(a) and 6(b) show the optical micrograph and the detailed structure of MMI1st, respectively, which has two input ports (${E_P}$, ${E_S}$) and 3 output ports (${E_{P1}}^{\prime}$, ${E_S}^{\prime}$, ${E_{P2}}^{\prime}$).
Fig. 6. (a) Optical micrograph of the MMI1st (b) Detailed structure of MMI1st; Measurement and simulation results with light injected from (c) upper port, (d) bottom port.
Theoretically, when the light inject from the upper port, it is split into two and outputs from port1 and port3 with the same power and $\pi /2$ phase difference. When the light inject from the bottom port, it outputs from the port2 only. We measured the outputs of MMI1st with the light inject from the upper port and bottom port, respectively. The results are shown in Figs. 6(c) and (d), corresponding simulation results are shown as well. Note that the devices were optimized at wavelength of 1550 nm, however the measured optimal performance were obtain at wavelength of 1560 nm. The deviation of center wavelength between simulation and experiments is due to the inconsistency between simulation and reality, since the center wavelength is related to the size of MMI1st. In order to show the comparison clearly, the X-axis indicating wavelength uses relative values (wavelength shift from the center wavelengths, 1550 nm in simulation, 1560 nm in experiments) instead of absolute wavelength values.
We can see from Figs. 6(c) and 6(d) that the extinction ratio reaches the peak (over 39 dB for upper port incident and over 43dB for bottom port incident) at the center wavelength and decrease gradually as the wavelength change (about 10dB in the 12nm bandwidth), indicating that MMI1st is very sensitive to wavelength, and the trends of the transmission curves are consistent with the ENOB curves in Fig. 4. Therefore, comparing with the measurement results (Fig. 5) of MMI2nd, we believe that the reason causing the quantizer narrow operation bandwidth is the wavelength sensitivity of MMI1st.
To further verify this, we assumed an ideal MMI1st with broad bandwidth and high extinction ratio, maintained MMI2nd unchanged, and repeated the ENOB simulation. The results are shown in the black dotted line in Fig. 7, which has a very broad operation bandwidth (The ENOB is over 3.26-bit in the measured wavelength range), further confirms that the narrow-bandwidth operation of the quantizer is due to wavelength-sensitive feature of the MMI1st. Therefore, in order to enlarge the operation bandwidth of the quantizer, our future effort should be focus on alleviate the wavelength sensitivity of MMI1st.
Fig. 7. Comparison of ENOB with measurement, simulation result of fabricated quantizer and simulation result with MMI1st.
4. Conclusion
We experimentally demonstrated an optical quantizer based on a fabricated CS-MMI. An ENOB of 3.31 bit was obtained at a wavelength of 1560 nm which is close to the ideal ENOB of 3.32 bit. The operation bandwidth is 12 nm for ENOB≥3 bit which is caused by wavelength-dependent feature of the MMI1st of the CS-MMI. Our future work will focus on the improvement of the operation bandwidth of the CS-MMI by designing a wavelength-insensitivity MMI1st. The measurement results also show that the CS-MMI has insertion loss of 1.26 dB at 1560 nm. The footprint of the CS-MMI is less than $7.5 \times 95\mu {m^2}$, which is simple and stable for on-chip integration. The OADC based on the proposed device is expected to have significant applications in photonic integratable optical communication links, optical interconnection networks, and real-time processing systems.
Funding
National Natural Science Foundation of China (61875020, 61935003); National Key Research and Development Program of China (2019YFB1803601).
Disclosures
The authors declare no conflicts of interest.
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