IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 12, JUNE 15, 2012
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Si/Ge Avalanche Photodiodes-Based Electrical Comb-Line Generators and Photoreceivers for Very-Fast Impulse Radio Wireless Linking Jin-Wei Shi, Feng-Ming Kuo, Daoxin Dai, and John E. Bowers
Abstract— We demonstrate the novel operation of a Si/Ge-based avalanche photodiode (APD) for the direct generation of ultra-wideband (UWB) frequency comb lines for impulse radio (IR) wireless communication. By applying a dc bias (Vbias ) over the breakdown voltage (Vbr ) of the APD, the device can exhibit a significant resonant frequency without any optical signal illumination, and function as an impact ionization avalanche transit time diode-based oscillator. Under an additional electrical intermediate frequency (IF) injection, several frequency comb lines, with a spacing equal to the IF frequency, can be derived from the second harmonic of the oscillating frequency. By mixing the output pulse train from the APD-based UWB generator with the data signal from another Si/Ge APD (on the same chip) operated in the linear mode (Vbias < Vbr ), which can perform high-sensitivity optical data detection, error-free IR wirelesslinking with data rate as high as 3.0 Gbit/s has been successfully achieved. Index Terms— Avalanche photodiodes radio (IR), ultra-wideband (UWB).
(APD),
impulse
I. I NTRODUCTION
T
HERE has been a great scarcity in the spectra available for wideband wireless communication created in recent years partly due to the tremendous increase in bandwidth requirements and partly due to the fact that most useful wireless communication channels are narrow and only located at the frequency range from 0.3 to 3.5 GHz [1]. An ultra-wideband (UWB) or impulse radio (IR) communication schemes [2]–[6] is one attractive solution to rescue us from this scarcity and realize Gbit/sec wireless data transmission [4]–[6]. However, the short electrical pulse tends to suffer from a large propagation loss in free-space and limited wireless network coverage. In order to overcome this problem, the photonic technique has been used to generate and transmit
Manuscript received February 10, 2012; revised March 21, 2012; accepted April 10, 2012. Date of publication April 27, 2012; date of current version May 14, 2012. This work was supported in part by the National Science Council of Taiwan under Grant NSC-100-2918-I-008-004 and in part by the DARPA MTO PICO Project. J.-W. Shi and F.-M. Kuo are with the Department of Electrical Engineering, National Central University, Taoyuan 320, Taiwan, and also with the Department of Electrical and Computer Engineering, University of California, Santa Barbara, CA 93106 USA (e-mail:
[email protected];
[email protected]). D. Dai and J. E. Bowers are with the Department of Electrical and Computer Engineering, University of California, Santa Barbara, CA 93106 USA (e-mail:
[email protected];
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2012.2195304
optical UWB pulse trains with special pulse-shape and high repetition rate [3]–[6] to further increase the coverage of the network. Nevertheless, such a photonic UWB or IR system is usually more bulky and complex than that of the electrical integrated circuits (ICs) based UWB signal generator. In this letter, we demonstrate a novel Si/Ge avalanche photodiode (APD) based electrical comb line generator and its application for IR wireless communication. By biasing a Si/Ge APD above its breakdown voltage (>Vbr ) and under dark operation (i.e., without any light injection), this device exhibits strong nonlinearity [7]–[10] and can generate high repetition rate UWB pulses under additional electrical intermediate frequency (IF) signal injection. The complex optical setup in reported photonic UWB or IR system can thus be eliminated [3]–[6]. The generated UWB pulse train is mixed with the data stream generated from the other neighboring Si/Ge APD (on the same chip) operating in the linear regime (∼0.9 Vbr ) for high-sensitivity 1.55 µm light detection. By use of such novel scheme, we successfully demonstrated IR wireless transmission with record high error-free data rate as 3.0 Gbps among the reported photonic IR systems [4]–[6]. II. M EASUREMENT S ETUP Figure 1 shows a conceptual diagram of our demonstrated device for the application in a photonic IR wireless communication system. In order to generate the UWB electrical comb-lines, our Si/Ge based APD must be biased above its breakdown voltage [7]–[10] and it would exhibit a significant resonant effect in the measured optical-toelectrical (O-E) response [7]–[10]. This phenomenon leads to the ultra-high gain-bandwidth products performance [8], [10]. However, the huge leakage current operation (∼mA) is always accompanied with such operation mode [7]–[10], which restricts its application of high-sensitivity light detection [8]. The resonant frequency of the Si/Ge APD used in this letter is at around 5 GHz [7], which is slightly increased with an increase in reverse bias voltage [7]–[10]. In order to generate frequency comb-lines, we must inject the electrical IF signal into our device under resonant operation in order to “deviate” from the original oscillation frequency [12]. Due to the strong device nonlinearity during oscillation, a large number of frequency comb lines with the spacing equal to the injected IF frequency, as will be discussed latter. For details about the structure of our device please refer to our previous work [7]; details of the measurement setup are
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Fig. 1. Schematic representation of the experimental setup for the electrical comb-line generation and IR wireless communication. LNA: low-noise amplifier. LA: limiting amplifier. MOD: electro-optic modulator. LPF: low-pass filter. PC: polarization controller.
shown in Figure 1. The main difference between our current setup and that described in our previous work [11] is the adoption of a broadband microwave diplexer (K&L Microwave WZ01-5000/26000-K/K/K), instead of using a microwave power divider, to inject and extract the IF and generated high-frequency (HF) comb-line signal. Such a diplexer has lower and upper passbands at dc to 5 GHz (IF port) and 10 to 42 GHz (HF port), respectively. In the next stage of our APD (comb-line generator), we use a radio-frequency (RF) amplifier (MITEQ AMF-6F-10002600-40-10P, 10-26 GHz) to pick up the desired frequency comb-lines output from HF port (as shown in Figure 1) of diplexer for IR short-pulse wireless data transmission. In contrast to a microwave power divider, the use of a microwave diplexer can further minimize the leakage of the injected IF signal to the RF amplifier. Such leakage would seriously distort the shape of the generated UWB pulse signal, especially when the frequency of the injected IF signal is increased and close to the lower edge of operating bandwidth of the RF amplifier (10 GHz) used. As shown in Figure 1, during wireless data transmission, we use another Si/Ge APD on the same chip, which is operated in the linear mode (Vbias < Vbr ), to detect the incoming optical data with reasonable sensitivity. An optical 1.5 or 3 Gbit/sec pseudo random binary sequence (PRBS) (215 −1) data stream with on-off keying (OOK) modulation format is generated by use of a 1.55 µm CW laser and an external optical modulator. The converted electrical data and UWB pulse signal from two different APDs is then fed into the IF and local oscillator (LO) ports of an additional double balanced mixer (MITEQ DB0440L W1). The output from the RF port of the mixer is further connected with amplifiers and an antenna for wireless data transmission. Compared with the reported photonic UWB or IR systems [3]–[6], our demonstrated comb-line generator operates under dark conditions (without any light signal injection) and has a much simpler architecture. By biasing the two Si/Ge APDs on the same chip to have different operation modes, the high-sensitivity O-E data detection, electrical
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Fig. 2. Generated electrical comb-line spectra from our device (after passing the amplifier and diplexer) under different IF at (a) 1.5 GHz and (b) 3.0 GHz; here the IF frequency power = 16 dBm and Vbias = −23.1V.
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short-pulse generation, and data up-conversion, can all be realized by use of a single chip with an additional RF mixer. Furthermore, there is a strong potential for monolithic integration of our proposed architecture in a single silicon chip [13]. III. M EASUREMENT R ESULTS Figures 2 (a) and (b) show the measured comb-line spectra output from the connected RF amplifier (as shown in Figure 1). The Si/Ge APD is biased at −23.1 V, under a fixed IF power (16 dBm) and different IF frequencies at 1.5 and 3 GHz, respectively. The measured Si/Ge APD has a 100 µm2 active area and a breakdown voltage at −23 V. As can be seen in both (a) and (b), the same and very broad band electrical comb-line spectra, covering from near 10 to around 30 GHz, is generated. The low-frequency component, which is below 10 GHz, is filtered out by the RF port of the diplexer (10 to 42 GHz) and the connected RF amplifier (10 to 26 GHz). In addition, the generated comb-line spacing corresponds to the frequency of the injected IF signal. In order to further boost the data rate in our wireless transmission system, a high-repetition-rate UWB pulse train with a short pulse-width is preferred. Figures 3 (a) and (b) show the corresponding pulse train waveform measured by use of a sampling scope under the same conditions as in Figure 2. As can be seen, the pulse-width in both (a) and (b) is the same as 50 ps, which is equal to the inverse of the total bandwidth of the comb-line spectra (10-30 GHz, ∼20 GHz), and the repetition rate is around 9 GHz. This number just corresponds to the second harmonic of the fundamental oscillation frequency of our device at around 5 GHz. Such a fundamental frequency has been filtered out by the stop-band in the HF port of the connected diplexer (dc to ∼10 GHz) whereby the second harmonic frequency (∼10 GHz) thus determines the repetition rate of the pulse. Furthermore, as
SHI et al.: Si/Ge AVALANCHE PHOTODIODES-BASED ELECTRICAL COMB-LINE GENERATORS AND PHOTORECEIVERS
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On the other hand, for the case of 3 Gbit/sec data transmission, there are several UWB pulses in a single bit and the measured eye-patterns thus become NRZ. To the best of author’s knowledge the achieved data rate (3.0 Gbit/sec) is the highest ever reported for the photonic UWB or IR system for error-free wireless linking [3]–[6], [11]. IV. C ONCLUSION
Error Free (a) 100 200 300 400 Di t Distance(cm) ( ) Eye-Height: 66mV; Eye-Width: 272ps
Fig. 4. (a) Measured BER at 1.5 and 3 Gb/s versus wireless transmission distance under different injected optical powers of PRBS data. (b) Measured corresponding error-free RZ (NRZ) eye-pattern at 1.5 (3) Gb/s.
By biasing the two Si/Ge APDs under breakdown and linear operation modes, both optical signal detection and UWB pulse generation can be realized on a single chip thereby eliminating the complex optical signal processing setups needed for traditional photonic IR schemes. By the use of such a novel module as IR transmitter, we successfully achieved 3 Gbit/sec error-free IR wireless data transmission. R EFERENCES
shown in Figure 3 (a), there is a low-frequency envelope in the pulse train with a frequency at 1.5 GHz, which is just the frequency of the injected IF signal. We can thus use the envelope with the high-frequency UWB carrier inside to realize return-to-zero (RZ) IR wireless data transmission. In this scheme, each pulse envelope represents a single bit. On the other hand, when the injected IF frequency reaches 3 GHz, as shown in Figure 3 (b), the low-frequency envelope of the UWB pulse train become insignificant. This can be attributed to the fact that the frequency of the injected IF signal (3 GHz) is higher than that of the maximum modulation speed (∼1.5 GHz) of our Si/Ge APD. Under strong (∼16 dBm) IF signal injection, the operation modes of our APD would be switched back/forth between Geiger (Vbias > Vbr ) and linear (Vbias < Vbr ) operation and the generated large leakage carriers (current) at Geiger-mode regime must be swept out the active layers of APD during switching. This phenomenon thus limits the maximum modulation speed and it is usually observed in the dynamic operation of single photon APD [14]. Nevertheless, we can use such a continuous UWB pulse train to realize non-return-to-zero (NRZ) wireless IR data transmission, with several pulses in a single bit [5]. To perform the experiments, we adopted two dual ridge horn antennas (Satimo, SH 2000), having a working frequency from 2 to 32 GHz. As shown in Figure 1, at the receiver end, the horn antenna is connected to a fast power detector (Herotek, DT302-TBD), a low noise amplifier (Miteq AM-1309, LNA), and a 50 GHz sampling scope to detect the envelope of the received pulse train. Figure 4 (a) shows the measured bit-error-rate (BER) at 1.5 and 3 Gbit/sec versus wireless transmission distance under different optical injected powers of the PRBS data stream onto the Si/Ge APD, which operated in the linear regime. As can be seen, the maximum distance for the error-free (BER< 1×10−9) 3 (1.5) Gbit/sec wireless data transmission is around 1.3 (1.8) meters under −3 dBm optical power injection. Figure 4 (b) shows the corresponding measured error-free eyepatterns. We can clearly see that for the case of 1.5 Gbit/sec data transmission, each UWB pulse envelope represents a single bit, which corresponds to the RZ eye-pattern during data transmission, as indicated in Figure 3.
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