Open Access
Add to BibSonomy
Abstract
In this paper, we propose a system for enhancing the RF output power of the photodetector, especially the power of fundamental tune and second-order harmonic, by feeding back part of the RF signal through an electrical feedback circuit. As a result of bias modulation and opto-electric mixing, the RF output power can be effectively enhanced. The structure of uni-traveling carrier photodetector (UTC-PD) is employed in this work. With the RF enhancement system, the power of fundamental tune and second-order harmonic improve by 6.4 dB and 9.9 dB respectively, under the condition of 26 dBm input optical power, 3 V bias voltage, and 14 GHz input optical signal. Further, it was observed that third-order harmonic appeared under the influence of this system.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Mobile communication data volumes are expanding to meet the demands for intelligent, high-speed, and low-latency services. Faced with the sharp increase in communications traffic, radio over fiber (RoF) system have attracted widespread attention [1–3]. As an indispensable component in RoF system for opto-electric conversion, high-speed and high-power photodetectors are crucial to achieving optical fiber links with high link gain, low noise figure and large spurious-free dynamic range [4–6].
With its separate absorption and collection layers, UTC-PD eliminates the limitations of hole transit time and weakens the trade-off between carrier transit time and RC time delay. Consequently, it is a suitable photodetector structure for RoF system because of its high-speed response and large saturation output photocurrent. In 2004, Wolfgang Schlaak's team achieved a UTC-PD with a 3 dB bandwidth of 100 GHz and a -7 dBm output power [7]. In 2018, Jesse S. Morgan et al. achieved a modified uni-traveling carrier photodetector (MUTC-PD) with a 3 dB bandwidth of 120 GHz and a diameter of 4 μm. Its RF output power at 160 GHz is -8.5 dBm [8]. In 2023, the BING XIONG team achieved a UTC-PD with a diameter of 3 μm and a 3 dB bandwidth of 230 GHz. The RF output power of the device at 220 GHz is -4.94 dBm [9]. It was also in this year that he demonstrated a waveguide terahertz (THz) photodetector capable of generating RF output power of 0.6 dBm at 240 GHz and 2.7 dBm at 280 GHz., respectively [10]. Obviously, through reduction in device size and optimization of device structure, UTC-PD's bandwidth has been continuously improving. The RF output power of the UTC-PD, however, is inversely proportional to its size. Therefore, it is necessary to develop a new method of increasing the RF output power of photodetectors to realize optical fiber links with high dynamic range and high gain.
In this paper, we propose a system for enhancing the RF output power of photodetector. This system loads part of the RF output signal to the bias end of the photodetector through the electrical feedback circuit, which introduce additional fundamental tune and high-order harmonic components by means of bias modulation [11] and opto-electric mixing [12–16]. The experiment results demonstrated that the output power of fundamental tune can be improved by up to 7.4 dB. The high-order harmonic components [17][18], which produced due to the nonlinearity of the photodetector are typically considered as interference components since their low output power. However, by implementing the electrical feedback system, the power of high-order harmonic, especially second-order harmonic, can be greatly improved, allowing them to be used as millimeter wave (MMW) or terahertz (THz) signals sources. Based on the results of the experiment, it has been demonstrated that it is possible to increase the output power of second-order harmonic by up to 12.2 dB. Furthermore, a significant third-order harmonic appeared under the influence of the system.
2. RF enhancement system
Figure 1 shows the common measurement setup of the photodetector’s RF output power. By heterodyning two tunable lasers with a wavelength near 1550 nm, a 100% modulated optical signal at ${f_{RF}}$ can be generated and amplified by an output power adjustable erbium-doped fiber amplifier (EDFA). Keysight 2636A source meter applied bias voltage through DC path of bias tee and monitored the photocurrent at the same time. The RF output of the device is displayed on the Keysight N9010A spectrum analyzer via bias tee.
In this work, the frequency of the optical signal ${f_{RF}}$ was set to 14 GHz so that even the third-order harmonic at 42 GHz can still be measured by our 44 GHz Keysight spectrum analyzer. Figure 2 shows the measured RF output power of the UTC-PD under different DC bias voltage. It can be observed that with increasing bias voltage, the RF output power of UTC-PD increases and eventually reaches a plateau under a fixed incident optical power. It reaches saturation around -3 V at an incident optical power of 23 dBm and the saturated output power is -10.36 dBm. The RF output power exhibits a saturation tendency at -4 V when the optical input power is 26 dBm and 29 dBm, which corresponds to -6.12 dBm and -2.56 dBm.
Fig. 2. The output power of UTC-PD at beat frequency of 14 GHz and the input optical power is 23dBm, 26dBm, and 29dBm, respectively.
For an increase in the output power of a photodetector, it is typically necessary to have a thicker absorb layer or a larger active region. However, increasing the thickness of the absorption layer or the area of the active region will result in an unacceptable dark current and bandwidth drop, which in turn negatively affect the sensitivity of the photodetector. Therefore, we propose an electrical feedback system, as shown in Fig. 3, which can increase the RF output power of the photodetector without requiring any changes to its structure or size. Different from the normal RF measurement setup as depicted in Fig. 2, a two-end power splitter is used in this system to split the output signal of the UTC-PD into two paths, one for output testing, and the other for signal feedback through the electric feedback circuit.
As a result, part of the RF output signal will be present on the bias end as bias modulation signal. It can produce additional fundamental tune components by way of bias modulation. In addition to the fundamental tune component, the power of high-order harmonic component with frequency $n \times {f_{RF}}$ can also be increased significantly. Since the electrical feedback signal will be opto-electrically mixed with the input optical signal, producing additional high-order harmonic components.
Figure 4 illustrates the UTC-PD’s output power enhancement under the influence of our system with an incident optical power of 26 dBm, an input optical signal frequency of 14 GHz and a bias voltage of 3 V. The RF output power of the system increases by 6.4 dB for fundamental tune and by 9.9 dB for second-order harmonic. Furthermore, we observe that the UTC-PD produces a significant third-order harmonic component when used in combination with the RF enhancement system.
Fig. 4. Red line: RF output spectrogram of the normal system; Blue line: RF output spectrogram of the RF output enhancement system under the condition of 26dBm input optical power and 3 V bias voltage.
The epitaxial structure of UTC-PD used in this article is shown in Fig. 5(a). It mainly consists of a p-doped absorption layer, two space layers, a cliff layer, a collection layer, a heavily doped electron blocking layer and two highly doped electrode contact layers. Different from the PIN-PD, the electrons are the dominant carriers in UTC-PD to move across the depletion layer. The 22 nm spacers can reduce the conduction band discontinuity between the absorption and collection layer. The 10 nm heavily doped InP cliff layer is used to enhance the electric field at the absorption and collection layer interface. The thickness of the collection layer and the absorption layer are selected as 220 nm and 350 nm to balance the transit time and junction capacitance. The heavily doped InGaAsP layer is set to provide an electric barrier. The detailed design of the UTC-PD for high-speed response and low-bias operation can be found in our previous work [19].
Fig. 5. (a) The schematic of the cross-section of the UTC-PD. (b) Measured dark current versus the bias voltage of the two sizes of UTC-PDs.
Using a standard semiconductor photonic device manufacturing process, the UTC-PD is grown on the semi-insulated InP substrate and fabricated as a mesa structure. Figure 5(b) shows the measured dark current of the two sizes of UTC-PD. The dark current of the 24 μm-diameter UTC-PD is below 1 nA until the reverse bias voltage is up to −3 V. With the diameter of 34 μm, the dark current of the UTC-PD is around 1 nA, which indicates the devices have a good fabrication process with few defects. In this study, we used UTC-PD with a diameter of 38 nm.
3. Result and discussion
Figure 6 illustrates the RF output power of the UTC-PD under different dc bias voltages, based on optical power of 23 dBm, 26 dBm and 29 dBm respectively. The bias voltage varies from 0 V to 3.5 V. It can be seen that under different input optical powers, UTC-PD's power of fundamental tune and high-order harmonic achieve a prominent increase by the RF enhancement system. By applying the appropriate bias voltage, the power of fundamental tune can be increased by 5-6 dB. This is because that the voltage at the bias end will not only contain the DC bias, but also a radio frequency component at ${f_{RF}}$ as shown in Eq. (1) as a consequence of the electrical feedback signal.
where ${V_b}$ presents the total voltage of the bias end, ${V_{DC}}$ is the DC bias voltage, ${V_{{f_{RF}}}}$ presents the amplitude of the electrical feedback signal. The bias modulation signal, i.e., second term in Eq. (1), can affect the photocurrent by changing the concentration and velocity of carriers within the photodetector [17]. Therefore, UTC-PD will generate an additional output radio component with frequency at ${f_{RF}}$, resulting in an increase in the power of the fundamental tune.Moreover, based on Fig. 6, it is evident that the RF output enhancement system contributes significantly to the power of the high-order harmonic. For example, the output power of the second-order harmonic can be increased by 12.2 dB under the condition of an input optical power of 29dBm and a bias voltage of 3 V. The power improvement of the high-order harmonic mainly comes from the following two aspects.
Fig. 6. The power of fundamental tune and high-order harmonic under different dc bias voltages, based on optical power of (a) 23 dBm, (b) 26 dBm, and (c) 29 dBm.
Firstly, RF feedback signal will contain high-order harmonic with frequency $n \times {f_{RF}}$ under the influence of the nonlinearity of the photodetector. Therefore, the bias voltage in Eq. (1) can be rewritten as Eq. (2).
Secondly, the radio frequency component of the bias modulation signal can be opto-electronic mixed with the input optical signal to enhance the power of high-order harmonic. In Fig. 7, the measured amplitude of the fundamental tune is shown under different bias voltages to illustrate the principle of opto-electronic mixing. And the dashed lines are the fitted curves. It can be observed that when bias voltage is increased, the amplitude of the fundamental tune increases, then stabilizes at a certain level. Therefore, the RF output signal can be expressed as Eq. (3) under the influence of the bias modulation signal.
Fig. 7. Amplitude of the fundamental tune versus bias voltage at fRF = 14 GHz. The dashed lines are the fitted curves.
The enhancement effect on the power of the high-order harmonic components becomes more apparent as the optical power increases, as illustrated in Fig. 6. Particularly, Fig. 6(b) and Fig. 6(c) demonstrate that UTC-PD exhibits a third-order harmonic signal at optical input powers of 26dBm and 29dBm. It is due to the fact that the strength of the amplitude and the slope rate of the photocurrent is proportional to the input optical power as demonstrated in Fig. 7. Since under higher incident optical power, which produces more photocarriers, changes in bias voltage have a greater impact on carrier distribution and carrier velocity inside the photodetector.
The high-order harmonics generated by photodetectors in ordinary systems are weak, so they usually do not have a substantial application. It is interesting to note, however, that our electric feedback system can significantly enhance the power of these high-order harmonics as shown in Fig. 6. As a result, it is possible to use it as a high-frequency signal source by adding a filter to extract a certain high-order harmonic signal as illustrated in Fig. 8. For example, the electric feedback system will be able to generate a 100 GHz MMW carrier signal if we increase the frequency of the incident RF signal from 14 GHz to 50 GHz.
4. Conclusion
In summary, a system based on electrical feedback circuit is proposed for enhancing the output RF power of photodetectors. This system exports the RF output signal from the photodetector to a two-end power splitter, with one end for output testing, while the other for feeding back the RF signals. These electrical feedback signals effectively enhance the output power of the fundamental tune and high-order harmonics by means of bias modulation and opto-electronic mixing.
In this paper, we demonstrate the system is able to achieve a 5-6 dB enhancement in the output power of the fundamental tune component and a 9-12 dB increase in the power of the second-order harmonic. Furthermore, A third-order harmonic component will appear under the influence of this system when the incident optical power reaches 26dBm and 29dBm. Our next step will be to further optimize the structure of UTC-PD in order to achieve higher RF output enhancement effects, as well as to apply the system to the generation of millimeter wave and terahertz signals.
Funding
State Key Laboratory of Information Photonics and Optical Communications; National Key Research and Development Program of China (2018YFB2200104, 2018YFB2200803); National Natural Science Foundation of China (61574019, 61674018, 61674020, 61874147, 61904016).
Acknowledgments
The authors would like to thank Prof. Kai Liu, Prof. Xiaofen Duan, and Prof. Xiaoming Ren of Beijing University of Posts and Telecommunications for helpful discussion.
Disclosures
The authors declare no conflicts 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
References
1. A. M. Zin, M. S. Bongsu, S. M. Idrus, et al., “An overview of radio-over-fiber network technology,” in International Conference On Photonics2010 (2010).
2. V. Sharma, A. Singh, and A. K. Sharma, “Challenges to radio over fiber (RoF) technology and its mitigation schemes – A review,” Optik 123(4), 338–342 (2012). [CrossRef]
3. A. Ng’oma, “Radio-over-Fiber Techniques for Millimeter Wave Wireless Applications,” in International Topical Meeting on Microwave Photonics (2015), pp. 1–6.
4. C. Wei, X. Xie, Z. Wang, et al., “150 GHz High-Power Photodiode by Flip-Chip Bonding,” J. Lightwave Technol. 1–7 (2023).
5. T. Nagatsuma and H. Ito, “High-Power RF Uni-Traveling-Carrier Photodiodes (UTC-PDs) and Their Applications” (InTech, 2011).
6. A. Beling, X. Xie, and J. C. Campbell, “High-power, high-linearity photodiodes,” Optica 3(3), 328 (2016). [CrossRef]
7. H.-G. Bach, A. Beling, G. G. Mekonnen, et al., “InP-Based Waveguide-Integrated Photodetector With 100-GHz Bandwidth,” IEEE J. Select. Topics Quantum Electron. 10(4), 668–672 (2004). [CrossRef]
8. J. S. Morgan, K. Sun, Q. Li, et al., “High-Power Flip-Chip Bonded Modified Uni-Traveling Carrier Photodiodes with −2.6 dBm RF Output Power at 160 GHz,” in Photonics Conference (IEEE, 2018).
9. Y. Tian, B. Xiong, C. Sun, et al., “Ultrafast MUTC photodiodes over 200 GHz with high saturation power,” Opt. Express 31(15), 23790 (2023). [CrossRef]
10. M. Grzeslo, S. Dülme, S. Clochiatti, et al., “High saturation photocurrent THz waveguide-type MUTC-photodiodes reaching mW output power within the WR3.4 band,” Opt. Express 31(4), 6484 (2023). [CrossRef]
11. X. Xiao-hui, H. Yong-qing, D. Xiao-feng, et al., “Theoretical Study of Photodiode Bias Modulation Technology,” acta photonica sinica 48(5), 523002 (2019). [CrossRef]
12. A. W. Mohammad, H. Shams, K. Balakier, et al., “5 Gbps wireless transmission link with an optically pumped uni-traveling carrier photodiode mixer at the receiver,” Opt. Express 26(3), 2884 (2018). [CrossRef]
13. D. Yang, Y. Huang, and M. Zhu, “Study of the Optoelectronic Mixer Based on Modified Uni-Traveling Carrier Photodetector for Efficient Harmonic Mixing,” IEEE Trans. Electron Devices 70(7), 3661–3667 (2023). [CrossRef]
14. A. W. Mohammad, H. Shams, C.-P. Liu, et al., “60-GHz Transmission Link Using Uni-Traveling Carrier Photodiodes at the Transmitter and the Receiver,” J. Lightwave Technol. 36(19), 4507–4513 (2018). [CrossRef]
15. E. Rouvalis, M. J. Fice, C. C. Renaud, et al., “Optoelectronic detection of millimetre-wave signals with travelling-wave uni-travelling carrier photodiodes,” Opt. Express 19(3), 2079 (2011). [CrossRef]
16. E. Rouvalis, M. J. Fice, C. C. Renaud, et al., “Millimeter-Wave Optoelectronic Mixers Based on Uni-Traveling Carrier Photodiodes,” IEEE Trans. Microwave Theory Techn. 60(3), 686–691 (2012). [CrossRef]
17. G. Zhou and P. Runge, “Nonlinearities of High-Speed p-i-n Photodiodes and MUTC Photodiodes,” IEEE Trans. Microwave Theory Techn. 65(6), 2063–2072 (2017). [CrossRef]
18. Y. Hu, B. S. Marks, C. R. Menyuk, et al., “Modeling Sources of Nonlinearity in a Simple p-i-n Photodetector,” J. Lightwave Technol. 32(20), 3710–3720 (2014). [CrossRef]
19. T. Liu, Y. Huang, K. Liu, et al., “High-Speed Characteristics of Uni-Traveling-Carrier Photodiode Under Bias-Free Operation,” IEEE Photon. Technol. Lett. 31(19), 1553–1556 (2019). [CrossRef]
