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Abstract
In this work, a high signal-noise ratio (SNR) dynamic biasing InGaAs/InAlAs avalanche photodiode (APD) is demonstrated experimentally and first applied in a laser radar system. Combining with the dynamic biasing technology, the APDs are operated in an unexploited voltage range between linear mode and Geiger mode, which, in this work, is defined as a transition zone. Surprisingly, it is found that the excess noise of dynamic biasing APDs decreases with the gain in this transition zone. As expected, the maximum useful gain is as high as 620 in the dynamic biasing mode, which shows a greater promotion than that of the DC biasing mode (17.5). Compared with the traditional DC biasing mode, the optimal SNR for dynamic biasing mode is improved by 14 dB without the degradation of response time as the peak optical power is 525 nW. Moreover, when SNR = 10, the peak optical power for the dynamic biasing mode is 43.4 nW, which shows a 57.5-fold (17.6 dB) reduction in comparison with the DC biasing mode (2495 nW). Therefore, we believe this new optical receiver will pave a new way in high-sensitivity and high-speed light detection.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Avalanche photodiodes (APDs) have been widely used as receivers in laser radar systems due to their high sensitivity [1,2]. However, the excess noise will increase with the gain (M), which limits the maximum useful M as well as the signal-noise ratio (SNR). Although the silicon-based APDs can be integrated with standard Si CMOS processes and have a low effective impact ionization ratio [3], they have poor performances in the eye-safety short-wave infrared (SWIR) waveband [4]. The InGaAs/InP and InGaAs/InAlAs APDs are commonly used in the SWIR waveband [5], however, they suffer from the high excess noise. For linear mode APDs operating in the SWIR waveband, the optimum gain is less than 50, more typically at M =10 to 20 [6]. After reaching the optimal value, the SNR will decrease due to the rapid increase of excess noise. As a result, it is hard to further improve the SNR for linear-mode operation APDs. To eliminate the impact of the excess noise, the APDs can be operated in Geiger mode as single photon avalanche diodes (SPADs) to acquire extremely high gain (M∼105) and even a single photon can be detected [5]. The laser radar systems with the SPADs show excellent performances for the detection of weak light, however, a long acquisition time is necessary to eliminate the impacts of dark counts and after-pulse [7,8]. We notice that there is an ignored voltage range from the voltage with the optimal SNR in linear mode to Geiger mode, which is defined as a transition zone in this work. The end of the linear mode is the voltage with the optimum SNR in the condition of static bias, and the beginning of the Geiger mode is the voltage at which the dark counts start to be generated. The transition zone is the gap from the end of the linear mode to the beginning of the Geiger mode and the breakdown voltage (Vbr) is included in the transition zone. Compared with linear mode, the transition zone with DC biasing mode has a larger gain as well as higher excess noise. However, the gain is not enough for detecting a single photon, which means the dark counts cannot be generated. Therefore, if there is one method that can maintain the high gain and restrict the excess noise in this transition zone, it will pave a new way in high-sensitivity and high-speed detection.
The excess noise is mainly generated in a long time tail of the avalanche process and a high-frequency field can quench the avalanche process before the establishment of the long time tail [9]. Therefore, a high-frequency biasing APD has the potential to reduce the excess noise and maintain the high gain in the transition zone. There are some outstanding works with dynamic biasing technology have been reported [10–16]. In [10,11], the dynamically biased APDs were proposed for improving the gain-bandwidth product and they proposed a new model to calculate the gain-bandwidth product and excess noise factor for the APDs with the dynamic biasing technology, which is different with the McIntyre’s formula for the static bias [17]. In [12], the sensitivity of dynamic biasing germanium APDs at 3-Gbps is improved by 4.2 dB in comparison with the DC biasing APDs. A 5.2 dB improvement in receiver sensitivity (25-Gbps) has also been demonstrated using a commercial InGaAs/InP APD with dynamic biasing technology [13]. Inspired by the above works, the APDs with the dynamic technology were introduced to the laser radar system in this work.
Unlike the previous works, the dynamic biasing InGaAs/InAlAs APDs are first used as the receivers in laser radar systems to improve the SNR in this work. As seen in Fig. 1, the operating voltage range is from Vl to Vg. Where Vl is the voltage with the optimum SNR in linear mode and Vg denotes the voltage that the dark counts start to be generated in Geiger mode. We defined this voltage range as the transition zone. Combining with the dynamic biasing technology supplied by the cosine wave, the InGaAs/InAlAs APDs are operated in this transition zone. In addition, the bias for the dynamic biasing mode is kept above breakdown voltage (Vbr) for a short period of time over time-varying biasing cycle (as shown in Fig. 1). As the increase of the peak-peak voltage of the cosine wave, the gain increases, however, the excess noise decreases. Therefore, a high SNR can be achieved. Compared with the optimum SNR in linear mode, the SNR in the transition zone has a significant improvement at the same peak optical power (Ppeak). As Ppeak = 525 nW, the optimal SNR for the linear mode and the dynamic biasing mode are 2 and 49.74, respectively. Moreover, the rise time (Tr) of the echo signal in dynamic biasing mode is not extended compared with DC biasing mode. Although the sensitivity for the new optical receiver is still lower than the SPADs, there are no dark counts and after-pulse effects. Therefore, the long acquisition time is not necessary anymore.
2. Experiment setup
The experimental setup is shown in Fig. 2(a), two synchronized electrical pulses are generated by the pulse generator, and one part is used to trigger the laser (1550 nm). The other is connected to the oscilloscope as a reference signal. The repeat frequency (ffre) and full width at half maximum (tw) of the electrical pulse are 10 MHz and 800 ps, respectively. The optical signal is attenuated by the variable optical attenuator (VOA) and then divided into two parts by the coupler. One part is monitored by an optical power meter and the other is expanded by a collimator. The optical signal reflected from the target A(B) is focused on the InGaAs/InAlAs APD through an aspherical lens. The cosine wave signal with a frequency of 1.25 GHz is generated by a signal generator, and the DC bias is supplied by the voltage source. Figure 2(b) shows the operating voltage range of APDs. The reverse voltage can be calculated using: Vbias = VDC +0.5VPPcos (ωt+φ). Where ω and φ denote the angular frequency and initial phase for the cosine wave signal, respectively. VPP is the peak-peak voltage of the cosine wave signal. VDC represents the DC bias. The peak of the laser pulse is corresponding to the peak of the cosine wave signal in the experiment by adjusting the phase of the cosine wave signal.
Fig. 2. (a) The experimental setup (b) The bias of the dynamic biasing APD (c) The schematic circuit diagram or the dynamic biasing optical receiver. (d) The diagram of the chip-on-board package
Figure 2(c) shows the circuits of the dynamic biasing technology. The low-pass filters (LPFs) are adopted in this receiver for eliminating the impacts of the cosine wave signal on the echo signal, and cutoff frequency of APDs with the LPFs is 800 MHz. When the optical signal is detected by the APDs, the avalanche signal is extracted by a resistance R0 (50 Ω) and it is further amplified by a low-noise amplifier (LNA) with a gain of 30 dB. Figure 2(d) shows the practical diagram of the circuits, an APD is bonded to a printed circuit board (PCB) with a chip-on-board (COB) package, which has the advantages of low cost and facilitating monolithic integration [18]. To achieve miniaturization, the ceramic LPFs are used in this work due to their smaller size in comparison with the microstrip LPFs [19]. For the InGaAs/InAlAs APD, a large active area of 200 µm (diameter) was fabricated for the laser-radar applications, and the widths of the multiplication layer and absorption layer are 200 nm and 2000nm, respectively. The optical signal is coupled to the APD through a single-mode fiber (SMF) in the SNR measurements. Although the test system is relatively complex, the pulse laser module and FPGA (Field Programmable Gate Array) can be used in a practical application to achieve the above measurement, which will simplify the system.
Figure 3 shows the data of the photocurrent and dark current changing with the reverse voltage. The measured responsivity (M = 1) of this InGaAs/InAlAs APD is 0.9 A/W. The dark current at 90% of breakdown voltage is 300 nA.
Fig. 3. The data of dark current, photon current (1550 nm light), and gain of an InGaAs/InAlAs APD change with the reverse voltage.
The SNR can be calculated using [20]:
where Vs-peak is the peak voltage of the optical signal, VRMS represents the total root mean square (RMS) noise, which consists of excess noise (Vex), thermal noise (Vth), and amplifier noise (VA). The thermal noise and the amplifier noise are constant in the measurement. The Vs-peak and VRMS are measured with the oscilloscope. The measured gain of the dynamic biasing APDs can be calculated using: where Ppeak and Pm represent the peak power and mean power of the laser pulse. $\Re$ and MA are the responsivity for the APD and the gain of the amplifier, respectively.3. Results and discussion
As seen in Fig. 4(a), the SNR in DC biasing mode changes with the gain under 3.4 µW Ppeak. The Vth and VA were measured as 2.6 mV, which are the dominant source of the total noise as the gain is less than 17.5. The maximum SNR of 14.8 is achieved at approximately 49.8 V with a gain of 17.5. As the gain is greater than 17.5, the excess noise is the main source of the total noise, therefore, the SNR decreases with the excess noise. For the dynamic biasing mode, the SNR for different VPP was measured with the DC voltage of 49.8 V. From Fig. 4(b), it is found that the SNR of the dynamic biasing mode increases with the VPP. When VPP >4.27 V, the dark counts start to be generated, which will disturb the signals. Thus, the transition zone for the InGaAs/InAlAs APDs is from 49.8 V to 51.935 V. As a result, the optimum SNR of 78.6 is achieved with the 4.27 V VPP. In [21], it is reported that the excess noise decreases with the VPP for Si and GaAs in theory. As shown in Fig. 4(b), it is also found that the excess noise decreases with the VPP for dynamic biasing InGaAs/InAlAs APDs in the experiment. To our best knowledge, it is the first time to realize this phenomenon in an InGaAs/InAlAs APD. Moreover, the measured dark current also decrease with the VPP (as shown in the inset of Fig. 4(b)). The excess noise is mainly generated at a long tail of the avalanche process, which is caused by the feedback between the electrons and holes ionization process [9]. The high frequency bias can quench the avalanche process before the establishment of the long tail, therefore, resulting in the decrease of excess noise. These experimental results indicate that the dynamic biasing technology can quench the avalanche process effectively and the higher the VPP, the better the quenching effect.
Fig. 4. (a) The data of SNR change with the reverse voltage (DC bias) as the peak optical power is 3.4 µW (b) The SNR versus the peak-peak voltage (Vpp) of the cosine wave as the peak optical power is 1.32 µW (dynamic biasing mode, VDC = 49.8 V), and the measured dark current changes with the Vpp (inset)
As seen in Fig. 5(a), the SNR for dynamic biasing mode is larger than the DC biasing mode at the peak optical power range from 10.5 nW to 331.8 µW. As the SNR is 10, the Ppeak of the dynamic biasing mode is 43.4 nW, which shows a 57.5-fold reduction (17.6 dB) than that of the DC bias (2495 nW). As Ppeak =525 nW, the SNR for the dynamic biasing mode is 49.8, which has a 24.9-fold (14 dB) improvement in comparison with the DC biasing mode. As shown in Fig. 5(b), the maximum useful gain is 620 for dynamic biasing mode, which is greater than that in linear mode (17.5). To further improve the SNR, a trans-impedance amplifier (TIA) can be utilized to replace the 50 Ω resistor and low-noise amplifier in a practical receiver due to its high gain and low noise. It is also found in Fig. 5(b) that the gain for the DC biasing mode and dynamic biasing mode decrease with the Ppeak, which is ascribed to the linearity degradation for the APDs [22]. The holes will accumulate near the charge layer and the electric field generated by the accumulated holes will cancel out the electric field in the multiplication layer. Therefore, the gain will degrade. Moreover, this effect is more significant if the input optical power is higher.
Fig. 5. (a) The data of SNR changes with the peak optical power. (b) The data of gain changes with the peak optical power.
Figure 6(a) shows the signal pulses for DC biasing mode and dynamic biasing mode at the same optical power. The signal voltage of the dynamic biasing mode is larger than the DC biasing mode. The rise time is 1.02 ns for dynamic biasing mode, which is consistent with the DC bias (1.01 ns). It means that the new optical receiver can effectively improve the SNR without the response-time degradation. In theory, the rise time can be expressed as [23]: Tr = 0.35/BW and BW denotes the system 3 dB bandwidth. It is noted that the rise time for the laser pulse is 441 ps. When BW is 350 MHz, the rise time is 1 ns. As a result, the expansion for the rise time is ascribed to the limited 3 dB bandwidth (350 MHz) of the oscilloscope. Figure 6(b) shows the rise time at different Ppeak. There is a small variation in the rise time with Ppeak, which is very beneficial for improving the precision with a leading-edge timing discriminator in laser radar systems [6].
Fig. 6. (a) The signal pulse of DC bias (VDC = 49.8 V) and dynamic biasing mode (VDC = 49.8 V, Vpp = 4.27 V) at the same optical power (b) The measured rise time (Tr) for dynamic biasing mode at different peak optical power.
Figure 7 (a) shows the measured distance (L) with a dynamic biasing APD at different Vs-peak with VDC = 49.8 V, VPP = 4.27 V. L is the distance between the target and the collimator (as shown in Fig. 2(a)). The distance L was measured using the time-of-flight method, which can be calculated using [24]:
where c is the velocity of light, and tf denotes the time of flight. σ is the single-shot precision. Figure 7(b) shows the theoretical σ at different Ppeak. The high SNR of the dynamic biasing APDs can improve the precision. In the experiment, the real L is 1.71 m and 1.77 m as the target is located at A and B respectively, and the separation (△L) between A and B is 6 cm. It is found that there is a ±1.5 cm error between the measured distance and the actual distance. The ±1.5 cm error is ascribed to the long sampling interval of the oscilloscope (50 ps). The 50 ps (1.5 cm) sampling interval will cause uncertainty in the echo signal.Fig. 7. (a) The measured distance at different peak voltage of the echo signals (VDC = 49.8 V, Vpp = 4.27 V). (b) The theoretical precision versus the peak optical power.
4. Conclusion
In this work, the dynamic biasing APDs were utilized as the receivers in the laser radar systems. The main contributions of this work include 1) it is the first time to report the excess noise decreasing with gain for the dynamic biasing InGaAs/InAlAs APDs experimentally; 2) it is proved that the APDs combined with the dynamic biasing technology have the advantage of high SNR without the degradation of response time, which is very beneficial for high-sensitivity and high-speed detection in a laser radar system; 3) the operating voltage range of the APDs is broadened and it will establish a “bridge” between linear mode and Geiger mode. Although the InGaAs/InAlAs APDs were used in this work, we believe that this scheme can be used with any type of APD such as Si APDs, InGaAs/InP APDs, Ge/Si APDs, etc. Moreover, the trans-impedance (TIA) can be used to replace the low-noise amplifier to achieve higher sensitivity in the future. We found that the rise time is insensitive to the peak optical power in the range from 10.5 nW to 331.8 µW, which has the great potential to achieve high-precision detection with a large dynamic range.
Funding
National Key Research and Development Program of China (2018YFB2200204); Science and Technology Project from Wuhan City (2020010601012162).
Disclosures
The authors declare no conflict of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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