902
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 13, JULY 1, 2011
Enhanced Performance of Narrowband MillimeterWave Generation Using Shaped-Pulse-Excited Photonic Transmitters Jim-Wein Lin, H.-P. Chuang, F.-M. Kuo, Cheng-Han Lin, Tze-An Liu, Jin-Wei Shi, Chen-Bin Huang, and Ci-Ling Pan, Senior Member, IEEE
Abstract—We demonstrate a novel method to greatly enhance the narrowband millimeter-wave (MMW) power generation by use of a near-ballistic uni-traveling-carrier photodiode-based photonic transmitter (PT) excited by shaped optical pulses. The spectrum of the best tailored optical pulses is centered at 1545 nm and spread among 16 main frequency components (channels), each with a spectral width of 15.5 GHz and a spacing of 93 GHz. Compared with quasi-sinusoidal optical modulation, we achieve an enhancement in the spectral power density by 25 times. The power-enhanced narrowband MMW signal can be tuned over 80 GHz (60–142 GHz). In addition, we observe significant enhancement of MMW peak power with an increase of the reverse bias voltage. Index Terms—Millimeter wave (MMW), photonic transmitter (PT), pulse shaping, terahertz (THz).
I. INTRODUCTION
G
ENERATION of terahertz (THz) and millimeter-wave (MMW) electromagnetic pulses has attracted a lot of interest in the past decades due to the potential applications such as far-infrared spectroscopy, material inspection and THz imaging. Among these techniques, broadband THz radiation from photoconductive (PC) antennas based on THz time domain spectroscopy (THz-TDS) is promising for producing single-cycle THz pulses with bandwidth up to several THz. Apart from this common scheme, the narrowband pulses demonstrated in recent years is sometimes beneficial to researchers for selected applications [1], [2]. This approach relies on the quasi-sinusoidal optical modulation obtained by
beating two linearly chirped pulses with an optical delay line. Frequency tunability and enhanced spectral power density was demonstrated. More recently, optical pulse shaping techniques were employed to generate tunable THz radiation [3], [4]. This has led to greater control over the pulse duration and even arbitrary waveform synthesis. Recently, we have shown that the use of the photonic transmitters (PT) based on uni-traveling-carrier photodiode (UTC-PD) and near-ballistic UTC-PD (NBUTC-PD) [5] can provide higher output power than conventional PC antennas in the sub-THz regime. These superior features arise from its unique operation mode, where only electrons are the active carriers traveling through the junction depletion layer. The PTs are compatible with communication lasers ( m) and optical fiber technology. Furthermore, the required bias voltage of UTC-PD based PT (less than V) is much lower than that of the reported low-temperature-grown GaAs (LTG-GaAs) based PC (hundreds of V [4]) for even higher output power performance [6]. The frequency tunability from sub-THz to THz region has been achieved by tuning the device parameters [7]. In this letter we demonstrate power-enhanced narrowband MMW generation, for the first time to our knowledge, from NBUTC-PD based PT through excitation by shaped femtosecond pulses. As compared to previous work [4], our system has the advantages of much lower required bias voltage, lower optical power budget for the desired MMW output power, and is thus more suitable for the application to portable spectrometer and sensing instrument in sub-THz regime. II. DEVICE AND EXPERIMENTAL SETUP
Manuscript received January 20, 2011; revised March 17, 2011; accepted April 02, 2011. Date of publication April 19, 2011; date of current version June 08, 2011. This work was supported in part by the R.O.C. National Science Council under Grant NSC 98-2221-E-007-025-MY3 and Grant NSC 98-2221-E-007-026-MY3. J.-W. Lin, C.-H. Lin, and C.-L. Pan are with the Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan (e-mail: d907913@oz. nthu.edu.tw;
[email protected];
[email protected]). H.-P. Chuang and C.-B. Huang are with the Institute of Photonics Technologies, National Tsing Hua University, Hsinchu 30013, Taiwan (e-mail:
[email protected];
[email protected]). F.-M. Kuo and J.-W. Shi are with the Department of Electrical Engineering, National Central University, Zhong-li 32001, Taiwan (e-mail: 975201125@cc. ncu.edu.tw;
[email protected]). T.-A. Liu is with the Center for Measurement Standards, Industrial Technology Research Institute, Hsinchu 30011, Taiwan (e-mail:
[email protected]. tw). 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.2011.2142301
Fig. 1(a) and (b) show the top-view of the MMW PT and a schematic diagram of our experimental setup for narrowband MMW generation, respectively. In the PT (see Fig. 1(a)), the NBUTC-PD with a 100 m active area is flip-chip bonded on an aluminum-nitride (AlN) substrate and monolithically integrated with a quasi-Yagi Antenna. The high saturation current-bandwidth product performance of the NBUTC-PD ensures high-power MMW generation from our PT. Details of these components and process of design can be found in our previous work [5]. For generating and characterizing the MMW waveform of this device, we use a THz-TDS system as shown in Fig. 1(b). A mode-locked Er:doped fiber laser fs input pulses at a repetition rate (MLFL) produces of 100 MHz around a wavelength of 1550 nm. Optical pulses from one branch of the fiber laser enter the pulse shaper from
1041-1135/$26.00 © 2011 IEEE
LIN et al.: ENHANCED PERFORMANCE OF NARROWBAND MMW GENERATION USING SHAPED-PULSE-EXCITED PTs
903
Fig. 2. (a) Intensity autocorrelation traces of the shaped pulses varying the slit numbers; and (b) corresponding THz-TDS results of generated MMW waveforms. Fig. 1. (a) Top-view of our MMW-PT device; and (b) the experimental setup of THz-TDS system for narrowband MMW generation and detection. PBS: polarization beam splitter. PC: polarization controller.
a circulator through a polarizing beam splitter. Individual frequency components of the input pulses are diffracted by an 1100 l/mm grating and collimated by a 500 mm focal length lens onto the back focal plane of the lens, where a 640-channel liquid crystal modulator (LCM, CRI SLM-640-D-NM) and a mirror are located [8]. The LCM functions as a reconfigurable slit mask that selects a number of specific wavelengths of a fixed spectral width. The frequency components selected are recombined and exit the pulse shaper through output port of the circulator. Insertion losses of the LCM and the whole pulse shaper are dB and 7.4 dB, respectively. After amplification in an erbium-doped fiber amplifier (EDFA), the shaped pulses are launched into the PT through a lensed-fiber and the photo-generated current is fed into the quasi-Yagi radiator in the PT to excite a WR-10 waveguide horn antenna for MMW generation. Two off-axis parabolic mirrors are used to collect and focus the MMW wave onto a broadband InGaAs PC dipole antenna. Femtosecond pulses from a branch of the MLFL was delayed and used to gate the antenna for sampling the optically generated MMW signal. In our experiments, the PT is reverse-biased between and V and modulated for lock-in detection. III. RESULTS AND DISCUSSION The spectra of the tailored optical pulses are centered at 1545 nm and spread among either 2, 4, 8 or 16 main frequency components (channels), The spectral width of each channel is 15.5 GHz and the spacing between two adjacent channels is 93 GHz (15.5 GHz 6, i.e., every 6th channel of the LCM is open). The frequency separation of 93 GHz is the same as the repetition rate of shaped pulse train. Fig. 2(a) shows the autocorrelation traces of different shaped optical pulses obtained by varying the number of effective slits in the LCM. The corresponding MMW waveforms generated with the PT excited by these shaped pulses are shown in Fig. 2(b). It can be seen that the full-width half-maximum (FWHM) duration of the shaped optical pulses is shortened from 6.5 ps to around 0.84 ps (Gaussian pulse shape assumed) when the slit number increases from two to sixteen. This is expected since the total spectral width is broadened with the increased number of channels. On the other hand, the corresponding waveforms shown
Fig. 3. Field spectra of the MMW radiation generated by (a) unshaped pulse and (b) 93-GHz pulse sequence with 0.84-ps pulsewidth.
in Fig. 2(b) exhibit a fast oscillation period with a slow-varying envelope. Each constitutes a quasi-sinusoidal MMW with an oscillating frequency of 93 GHz. In this work, the optical power is adjusted to maintain the average output photocurrent of the NBUTC-PD at 50 A under the reverse bias voltage of V. This ensures that the PT operates in the linear region. As shown in Fig. 2(b), the peak amplitude of the MMW waveform obtained by using 16 channels or 0.84 ps burst increases by times compared with that of two-channel (quasi-sinusoidal modulation) or 6.5 ps-wide optical pulse. The corresponding spectral power density at 93 GHz is enhanced by times. That is, the spectral power density of MMW can be greatly enhanced by use of a burst signal instead of quasi-sinusoidal optical modulation. In our previous work [9], we have shown theoretically that the power enhancement of generated MMW by an optical pulse comprised of N comb lines with equal amplitudes over the sinusoidal (two-channel) case is given by . The data of the present work is consistent with the theory. In Fig. 3, we show the Fourier-transformed spectra of THz-TDS data for the MMW signal generated by the unshaped pulse train (dotted line) and 93 GHz pulse sequence with 0.84 ps pulsewidth (solid line), respectively. The photocurrent of the PT in this experiment was adjusted such that it is a constant around 200 A. It can be seen that a field enhancement of 3.5 dB (7 dB for spectral power density) at 93 GHz is achieved by using the 0.84 ps burst signal. Here we define the bandwidth of device as its MMW field drops to around dB ( dB for MMW power), as shown by the dashed lines in Fig. 3. The dotted curve represents frequency response of the PT, with a bandwidth of GHz (60–142 GHz). A high-frequency roll-off is observed beyond 140 GHz. This
904
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 13, JULY 1, 2011
characteristics indicate the potential power enhancement with our device using even higher bias voltages. IV. CONCLUSION
Fig. 4. Photocurrent dependent peak THz fields under the optical excitations of different (a) shaped pulses, and (b) reverse biases.
may be attributed to the limited optical-to-electrical (O-E) bandwidth of the NBUTC-PD and integrated antenna [10]. The narrower bandwidth of GHz for shaped-pulse excitation suggests the feasibility of the present approach. Fig. 4(a) is a plot of MMW intensity as a function of photocurrent under the optical excitation of different shaped pulses. The reverse bias voltage is fixed at V. Clearly, the value of the saturated photocurrent decreases as the number of channels increases. This phenomenon of saturation originates from the high-peak values of the photocurrent, which results in strong space-charge screening (SCS) effect [11] and limits the peak out power from NBUTC-PD. Nonetheless, the highest MMW spectral power density is achieved for the 16-channel case if the photocurrent is kept below 100 A. The observed trend is consistent with previous reports of shaped pulse excitation of PC antennas [3], in which the authors ascribe it to the saturation mechanism that limits the THz amplitude in the case of high-power single-pulse excitation. According to the injected optical waveform, the maximum peak output photocurrent of NBUTC-PD is estimated to be mA, and the corresponding MMW peak field intensity can reach V cm, which is comparable to that of [4]. Fig. 4(b) shows the photocurrent dependent THz field under different reverse bias voltage for the 16-channel case. It is obvious that the value for saturated photocurrent increase with the absolute value of the reverse bias voltage. Such effect can be understood by considering that a higher electric field is utilized for compensating the SCS effect. It should be noted that there is an optimal reverse bias at V when the photocurrent is below 100 A. Higher reverse bias voltage ( V) leads to reduced MMW signal. The behavior is similar to the trend reported previously [9]. The phenomenon was attributed to the existence of an optimal electric field in the collector layer of the PD that is able to sustain the overshoot drift-velocity of electrons. Once the photocurrent is increased over the saturation value, however, we observe higher peak MMW signal with the PD biased at V due to the reduced SCS effect. A factor-ofpower enhancement is obtained with an operating photocurrent of 200 A, as shown in Fig. 4(b). The bias-dependent output
We have developed a method of narrowband MMW generation by use of a NBUTC-PD based photonic transmitter and shaped femtosecond optical pulses. The spectral power density of narrowband MMW is found to be greatly enhanced by exciting with a shaped pulse instead of quasi-sinusoidal optical modulation. A factor-ofpower enhancement has been obtained. The tuning range of generated MMW signal is limited by the available bandwidth (e.g., 82 GHz: 60~142 GHz) of our device. Compared with previous work, the approach reported here has advantages of lower required bias voltage, lower optical power budget for the desired MMW output power and compatibility with fiber optical technology. REFERENCES [1] A. S. Weling and D. H. Auston, “Novel sources and detectors for coherent tunable narrow-band terahertz in free space,” J. Opt. Soc. Amer. B, vol. 13, pp. 2783–2791, 1996. [2] J. Y. Lu, L. J. Chen, T. F. Kao, H. H. Chang, H. W. Chen, A. S. Liu, Y. C. Chen, R. B. Wu, W. S. Liu, J. I. Chyi, and C. K. Sun, “Terahertz microchip for illicit drug detection,” IEEE Photon. Technol. Lett., vol. 18, no. 21, pp. 2254–2256, Nov. 1, 2006. [3] Y. Q. Liu, S. G. Park, and A. M. Weiner, “Enhancement of narrowband terahertz radiation from photoconducting antenna by optical pulse shaping,” Opt. Lett., vol. 21, pp. 1762–1764, 1996. [4] J. Y. Sohn, Y. H. Ahn, D. J. Park, E. Oh, and D. S. Kim, “Tunable terahertz generation using femtosecond pulse shaping,” Appl. Phys. Lett., vol. 81, pp. 13–15, 2002. [5] H.-J. Tsai, N.-W. Chen, F.-M. Kuo, and J.-W. Shi, “Front-end design of W-band integrated photonic transmitter with wide optical-to-electrical bandwidth for wireless-over-fiber applications,” in Proc. IEEE MTT IMS 2010, Anaheim, CA, May 2010, pp. 740–743. [6] H. Ito, S. Kodama, Y. Muramoto, T. Furuta, T. Nagatsuma, and T. Ishibashi, “High-speed and high-output InP–InGaAs unitraveling-carrier photodiodes,” IEEE J. Sel. Topics Quantum Electron., vol. 10, no. 4, pp. 709–727, Jul./Aug. 2004. [7] T. F. Kuo, H. H. Chang, L. J. Chen, J. Y. Lu, A. S. Liu, Y. C. Yu, R. B. Wu, W. S. Liu, J. I. Chyi, and C. K. Sun, “Frequency tunability of terahertz photonic transmitters,” Appl. Phys. Lett., vol. 88, p. 093501, 2006. [8] H. P. Chuang and C. B. Huang, “Generation and delivery of 1-ps optical pulses with ultrahigh repetition-rates over 25-km single mode fiber by a spectral line-by-line pulse shaper,” Opt. Express, vol. 18, pp. 24003–24011, 2010. [9] F.-M. Kuo, J.-W. Shi, H.-C. Chiang, H.-P. Chuang, H.-K. Chiou, C.-L. Pan, N.-W. Chen, H.-J. Tsai, and C.-B. Huang, “Spectral power enhancement in a 100-GHz photonic millimeter-wave generator enabled by spectral line-by-line pulse shaping,” IEEE Photon. J., vol. 2, no. 5, pp. 719–727, Oct. 2010. [10] F.-M. Kuo, J.-W. Shi, N.-W. Chen, C.-B. Huang, H.-P. Chuang, H.-J. Tsai, and C.-L. Pan, “20-Gb/s error-free wireless transmission using ultra-wideband photonic transmitter-mixer excited with remote distributed optical pulse train,” in Proc. OFC 2011, Los Angeles, CA, Mar. 2011, Paper OWT5. [11] P.-L. Liu, K.-J. Williams, M. Y. Frankel, and R. D. Esman, “Saturation characteristics of fast photodetectors,” IEEE Trans. Microw. Theory Tech., vol. 47, no. 7, pt. 2, pp. 1297–1303, Jul. 1999.
