OSA's Digital Library

Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 13, Iss. 25 — Dec. 12, 2005
  • pp: 10416–10423
« Show journal navigation

Wireless audio and burst communication link with directly modulated THz photoconductive antenna

Tze-An Liu, Gong-Ru Lin, Yung-Cheng Chang, and Ci-Ling Pan  »View Author Affiliations


Optics Express, Vol. 13, Issue 25, pp. 10416-10423 (2005)
http://dx.doi.org/10.1364/OPEX.13.010416


View Full Text Article

Acrobat PDF (331 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

We demonstrate transmission of audio and burst signals through a prototype THz analog communication link employing laser-gated low-temperature-grown GaAs dipole antenna as THz emitter and receiver. The transmission distance is about 100 cm. By using a direct voltage modulation format, we successfully demodulated a burst signal with a rising time of 41 μs. The corresponding modulating bandwidth achieved was 23 kHz in this first experiment. Noise analysis reveals a 10% power fluctuation in the received signal with on-off extinction ratio of greater than 1000. The transmission of a six-channel analog and burst audio signal with least distortion is also demonstrated. We further demonstrate the fidelity of the transmission of a melody through the THz link with and without any amplification.

© 2005 Optical Society of America

1. Introduction

2. Experimental methods

Fig. 1. Schematic diagram for experimental demonstration of the THz communication link: Ti: Sapphire: mode-locked Ti: Sapphire laser; A: amplifier; PC: personal computer

3. Results and discussions

Fig. 2. (a) Time traces of the input modulation voltage and corresponding decoded signal. (b) Frequency response of the THz transmission channel.

According to M. V. Exter et al. [7

7 . M. V. Exter and D. R. Grischkowsky , “ Characterization of and Optoelectronic Terahertz Beam System ,” IEEE Trans. Microwave Theor. Tech. 38 , 1684 – 1691 ( 1990 ). [CrossRef]

], the quantum noise and thermal radiation noise of photoconductive THz detectors are not significant, while Johnson noise (thermal noise due to photoexcited carriers) and laser shot noise are dominant. Johnson noise in the signal current is due to the photoexcited carriers, which is proportional to the inverse of the square root of the resistance, or the square root of the laser power [8

8 . M. Tani , K. Sakai , and H. Mimura , “ Ultrafast Photoconductive Detectors Based on Semi-Insulating GaAs and InP ,” Jpn. J. Appl. Phys. 36 , L1175 – L1178 ( 1997 ). [CrossRef]

]. Previously, we have shown that THz photoconductive detectors fabricated on LT-GaAs exhibited the lowest noise among similar devices [9

9 . T. -A. Liu , M. Tani , M. Nakajima , M. Hangyo , K. Sakai , S. Nakashima , and C. -L. Pan , “ Ultrabroadband terahertz field detection by proton-bombarded InP photoconductive antennas ,” Opt. Express. 12 , 2954 ( 2004 ), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-13-2954. [CrossRef] [PubMed]

].

In Fig. 3, we have plotted the time traces of (a) the square-wave modulation waveform biasing the transmitting antenna, (b) received digital signal from the Lock-in amplifier and (c) detector noise in the absence of THz transmission. The extinction ratio between on and off state of 1000 has been demonstrated. The modulation depth is nearly 100%.

Fig. 3. Time traces of (a) the square-wave modulation waveform, (b) received digital signal and (c) detector noise in the absence of THz transmission

Fig. 4. Time traces of (a) decoded audio signals at 5130 Hz (upper trace) and 513 Hz (lower trace) and (b) encoded (upper trace) and decoded 5 kbit/sec burst signal transmitted over the THz communication link.

In another experiment, a six-channel voice signal from the analog output of computer was first amplified by a low-power (average power ~ 2 W) computer speaker and used to bias the emitter antenna with peak-to-peak voltage of around 5 V. Figure 5 shows the (a) encoded and (b) decoded signals in the frequency domain. The audio signal can thus be reproduced with a quality comparable to that transmitted through commercial cellular phone networks with similar frequency spectral response.

Fig. 5. Spectra of (a) encoded and (b) decoded six-channel voice signals transmitted over the THz communication link.

We further demonstrate the fidelity of the transmission of a melody through the THz link. The music score is the School Song of National Chiao Tung University. The system is the same as that for one and six channel transmission experiments discussed above. The signal amplitudes were controlled to be lower than 8 volts. Figure 6(a) displays the whole time sequence (left hand side) and a sample spectrum (right hand side) of a portion of the encoded music score from computer speaker. Figures 6(b), (c) and (d) are the corresponding decoded audio signals from the receiving photoconductive antenna only, with current preamplifier set at a gain of 5×106 V/nA and 2×107 V/nA, respectively. A portion of the encoded and decoded music scores (from 17 to 31 seconds of the whole time sequence) for Figs. 6(a), (b), (c) and (d) can be played by downloading the corresponding multimedia files of the same name. In each of these files, the visual display shows spectra for the musical score in log (upper part of the video display of the multimedia file) and linear scales (lower part of the video display of the multimedia file) of frequency axis. Note that the fidelity of the transmitted music is already high for the system without the amplifier.

Fig. 6. (a) The whole time sequence (left hand side) and a sample spectrum (right hand side) of a portion of the encoded music score from computer speaker; (b), (c) and (d) are the corresponding decoded audio signals from the receiving PC antenna only (b), with current preamplifier set at a gain of 5×106 V/nA (c) and 2×107 V/nA (d), respectively. [Media 1] [Media 2] [Media 3] [Media 4]

Surprisingly, the signal quality for the case of detector only is still acceptable. The sound for amplified signal is, of course higher than that without amplification. The high frequency noise of the former, however, is also higher because of the amplifier. It cannot reveal high frequency components of the sound well, as opposed to the case of detector only. That is, the fidelity of sound for detector only is better than that using the amplifier. This suggests that the transmission of video signal should be feasible merely by improving the electronic interface.

Fig. 7. Noise spectra s detected by the receiving THz PC antenna only (a), with current preamplifier set at a gain of 5×106 V/nA (b) and 2×107 V/nA (c), respectively.

In the absence of the transmitted signal, the noise spectra as detected by the receiving THz photoconductive antenna only, with current preamplifier set at a gain of 5×106 V/nA and 2×107 V/nA are shown in Fig. 7(a), (b), and (c) respectively. The noise levels of the above cases are -100 dB, -80 dB and -70 dB with reference to the maximum input of the sound card, in that order. It also proves that the amplifier introduce noise with gain.

The effective range for free-space transmission of THz signals is also a concern. There are many water vapor absorption lines in the far infrared. Further, there is a general rise in absorption from sub-THz to ~ 1 THz [11

11 . R. A. Cheville , M. T. Reiten , R. McGowan , and D. R. Grischkowsky in D. Mittleman , Ed., Sensing with Terahertz Radiation ( Springer-Verlag, New York 2002 ), pp. 243

]. As a result, the absorption coefficient of water vapor at 1 THz, which does not correspond to a transition, is higher than that at certain sub-THz water absorption lines. Average attenuation over a range of frequencies is more relevant for our optically-gated THz link. Following Cheville et al. [11

11 . R. A. Cheville , M. T. Reiten , R. McGowan , and D. R. Grischkowsky in D. Mittleman , Ed., Sensing with Terahertz Radiation ( Springer-Verlag, New York 2002 ), pp. 243

], we estimated that the average absorption coefficient is around 10 km-1 for the frequency band of 0.1~0.5 THz Assuming a humidity of around 50% and neglecting other propagating losses such as diffraction and particle scattering, the maximum transmission distance for typical optically excited THz pulse is thus over 300 m.

4. Conclusions

Acknowledgments

The authors acknowledge the support from the Pursuit of Academic Excellence Program of the Ministry of Education and National Science Council of the Republic of China under various grants. We also acknowledge valuable suggestions and comments by Prof. Chi H. Lee, University of Maryland.

References and links

1 .

M. Z. Win and R. A. Scholtz , “ Impulse Radio: How it works ,” IEEE Commun. Lett. 2 , 36 – 38 ( 1998 ). [CrossRef]

2 .

S. Ramsey , E. Funk , and C. H. Lee , “ A Wireless Photoconductive Receiver Using Impulse Modulation and Direct Sequence Code Division ,” in The Int. Topical Meeting on Microwave Photonics’99, Technical Digest, 265 – 268 ( 1999 ).

3 .

E. Mueller and A. J. DeMaria , “ Broad bandwidth communication/data links using terahertz sources and Schottky diode modulators/detectors ,” Proc. SPIE , 5727 , 151 – 165 ( 2005 ). [CrossRef]

4 .

R. Piesiewicz , J. Jemai , M. Koch , and T. Kürner , “ THz channel characterization for future wireless gigabit indoor communication systems ,” Proc. SPIE , 5727 , 166 – 176 ( 2005 ). [CrossRef]

5 .

T. Kleine-Ostmann , K. Pierz , G. Hein , P. Dawson , and M. Koch , “ Audio signal transmission over THz communication channel using semiconductor modulator ,” Electron. Lett. 40 , 124 – 125 ( 2004 ). [CrossRef]

6 .

M. Tani , S. Matsuura , K. Sakai , and S. Nakashima , “ Emission characteristics of photoconductive antennas based on low-temperature-grown GaAs and semi-insulating GaAs ,” Appl. Opt. 36 , 7853 – 7859 ( 1997 ). [CrossRef]

7 .

M. V. Exter and D. R. Grischkowsky , “ Characterization of and Optoelectronic Terahertz Beam System ,” IEEE Trans. Microwave Theor. Tech. 38 , 1684 – 1691 ( 1990 ). [CrossRef]

8 .

M. Tani , K. Sakai , and H. Mimura , “ Ultrafast Photoconductive Detectors Based on Semi-Insulating GaAs and InP ,” Jpn. J. Appl. Phys. 36 , L1175 – L1178 ( 1997 ). [CrossRef]

9 .

T. -A. Liu , M. Tani , M. Nakajima , M. Hangyo , K. Sakai , S. Nakashima , and C. -L. Pan , “ Ultrabroadband terahertz field detection by proton-bombarded InP photoconductive antennas ,” Opt. Express. 12 , 2954 ( 2004 ), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-13-2954. [CrossRef] [PubMed]

10 .

G. -R. Lin , Y. -C. Chang , T. -A. Liu , and C. -L. Pan , “ Piezoelectric-Transducer-Based Optoelectronic Frequency Synchronizer for Control of Pulse Delay in a Femtosecond Passively Mode-Locked Ti:Sapphire Laser ,” Appl. Opt. 42 , 2843 – 2848 ( 2003 ). [CrossRef]

11 .

R. A. Cheville , M. T. Reiten , R. McGowan , and D. R. Grischkowsky in D. Mittleman , Ed., Sensing with Terahertz Radiation ( Springer-Verlag, New York 2002 ), pp. 243

OCIS Codes
(060.4080) Fiber optics and optical communications : Modulation
(060.4510) Fiber optics and optical communications : Optical communications
(260.3090) Physical optics : Infrared, far
(320.7080) Ultrafast optics : Ultrafast devices
(320.7160) Ultrafast optics : Ultrafast technology

ToC Category:
Research Papers

Citation
Tze-An Liu, Gong-Ru Lin, Yung-Cheng Chang, and Ci-Ling Pan, "Wireless audio and burst communication link with directly modulated THz photoconductive antenna," Opt. Express 13, 10416-10423 (2005)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-13-25-10416


Sort:  Journal  |  Reset  

References

  1. M. Z. Win and R. A. Scholtz, "Impulse Radio: How it works," IEEE Commun. Lett. 2, 36-38 (1998). [CrossRef]
  2. S. Ramsey, E. Funk, and C. H. Lee, "A Wireless Photoconductive Receiver Using Impulse Modulation and Direct Sequence Code Division," in The Int. Topical Meeting on Microwave Photonics '99, Technical Digest, 265-268 (1999).
  3. E. Mueller and A. J. DeMaria, "Broad bandwidth communication/data links using terahertz sources and Schottky diode modulators/detectors," Proc. SPIE, 5727, 151-165 (2005). [CrossRef]
  4. R. Piesiewicz, J. Jemai, M. Koch and T. Kürner, "THz channel characterization for future wireless gigabit indoor communication systems," Proc. SPIE, 5727, 166-176 (2005). [CrossRef]
  5. T. Kleine-Ostmann, K. Pierz, G. Hein, P. Dawson and M. Koch, "Audio signal transmission over THz communication channel using semiconductor modulator," Electron. Lett. 40, 124-125 (2004). [CrossRef]
  6. M. Tani, S. Matsuura, K. Sakai, and S. Nakashima, "Emission characteristics of photoconductive antennas based on low-temperature-grown GaAs and semi-insulating GaAs," Appl. Opt. 36, 7853-7859 (1997). [CrossRef]
  7. M. V. Exter and D. R. Grischkowsky, "Characterization of and Optoelectronic Terahertz Beam System," IEEE Trans. Microwave Theor. Tech. 38, 1684-1691 (1990). [CrossRef]
  8. M. Tani, K. Sakai and H. Mimura, "Ultrafast Photoconductive Detectors Based on Semi-Insulating GaAs and InP," Jpn. J. Appl. Phys. 36, L1175-L1178 (1997). [CrossRef]
  9. T. -A. Liu, M. Tani, M. Nakajima, M. Hangyo, K. Sakai, S. Nakashima and C. -L. Pan, "Ultrabroadband terahertz field detection by proton-bombarded InP photoconductive antennas," Opt. Express 12, 2954 (2004), <a href="http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-13-2954">http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-13-2954</a>. [CrossRef] [PubMed]
  10. G. -R. Lin, Y. -C. Chang, T. -A. Liu, and C. -L. Pan, "Piezoelectric-Transducer-Based Optoelectronic Frequency Synchronizer for Control of Pulse Delay in a Femtosecond Passively Mode-Locked Ti:Sapphire Laser," Appl. Opt. 42, 2843-2848 (2003). [CrossRef]
  11. R. A. Cheville, M. T. Reiten, R. McGowan and D. R. Grischkowsky in D. Mittleman, Ed., Sensing with Terahertz Radiation (Springer-Verlag, New York 2002), pp. 243

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.

Supplementary Material


» Media 1: AVI (1165 KB)     
» Media 2: AVI (1141 KB)     
» Media 3: AVI (1157 KB)     
» Media 4: AVI (1178 KB)     

« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited