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Optics Express

Optics Express

  • Editor: J. H. Eberly
  • Vol. 7, Iss. 8 — Oct. 9, 2000
  • pp: 280–284
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4×10 Gb/s terrestrial optical free space transmission over 1.2 km using an EDFA preamplifier with 100 GHz channel spacing

Dong-Yiel Song, Yoon-Suk Hurh, Jin-Woo Cho, Jung-Hwan Lim, Dong-Woo Lee, Jae-Seung Lee, and Youngchul Chung  »View Author Affiliations


Optics Express, Vol. 7, Issue 8, pp. 280-284 (2000)
http://dx.doi.org/10.1364/OE.7.000280


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Abstract

We demonstrate a transmission of 4×10 Gb/s WDM channels over 1.2 km of free space in 1.55-µm band. The transmitted beam is coupled into a single-mode fiber through a fiber-pigtailed collimator, which enables the use of standard 100-GHz channel spacing and an optical preamplifier at the receiver. All the received channels have Q values higher than 6.

© Optical Society of America

1. Introduction

Optical free space communication (OFSC) provides an attractive way of connecting network nodes and subscribers within limited times and budgets [1

1. T. H. Carbonneau and D. R. Wisely, “Opportunities and challenges for optical wireless; the competitive advantage of free-space telecommunications links in today’s crowded marketplace,” in Wireless Technologies and Systems: Millimeter-Wave and Optical, Paul Christopher, Leland Langston, and G. Stephen Mecherle, eds., Proc. SPIE3232, 119–128 (1998).

]. With the advent of erbium-doped fiber amplifiers (EDFAs), high capacity OFSCs in eye-safe 1.55-µm band become increasingly practical compared with conventional microwave communications and 0.8-µm band OFSCs. In 1994, a single 155 Mb/s OFSC channel transmission over 4 km was performed using a+14 dBm EDFA at the transmitter [2

2. D. R. Wisely, M. J. McCullagh, P. L. Eardley, P. P. Smyth, D. Luthra, E. C. De Miranda, and R. Cole, “4km terrestrial line-of-sight optical free space link operating at 155 Mbit/s,” G. Stephen Mecherle, ed., Proc. SPIE 2123, 108–119 (1994). [CrossRef]

]. Recently, a wavelength-division multiplexed (WDM) 4×2.5 Gb/s OFSC channel transmission has been performed over a 4.4 km span using two +33 dBm Er/Yb optical amplifiers at the transmitter with non-ITU 3.5 nm channel spacing [3

3. G. Nykolak, P. F. Szajowski, J. Jacques, H. M. Presby, J. A. Abate, G. E. Tourgee, and J. J. Auborn, “4×2.5 Gb/s 4.4 km WDM free-space optical link at 1550 nm,” in Proc. OFC ’99, paper PD11.

]. Also, a 16×2.5 Gb/s transmission over 4.4 km of free space [4

4. G. Nykolak, P.F. Szajowski, A. Cashion, H.M. Presby, G.E. Tourgee, and J.J. Auborn, “A 40Gb/s DWDM Free Space Optical Transmission Link Over 4.4 km”, in Free-Space Laser Communication Technologies XII, G. Stephen Mecherle, ed., Proc. SPIE3932, 16–20 (2000).

] and a 160 Gb/s single channel transmission over 200 m of free space [5

5. G. Nykolak, G. Raybon, B. Mikkelsen, B. Brown, P. F. Szajowski, J. J. Auborn, and H. M. Presby, “A 160 Gb/s Free Space Transmission Link”, in Conference on Lasers and Electro-Optics, OSA Technical Digest (Optical Society of America, Washington DC, 2000), CPD 15-1,2, pp. 29–30.

] have been reported. In [3

3. G. Nykolak, P. F. Szajowski, J. Jacques, H. M. Presby, J. A. Abate, G. E. Tourgee, and J. J. Auborn, “4×2.5 Gb/s 4.4 km WDM free-space optical link at 1550 nm,” in Proc. OFC ’99, paper PD11.

5

5. G. Nykolak, G. Raybon, B. Mikkelsen, B. Brown, P. F. Szajowski, J. J. Auborn, and H. M. Presby, “A 160 Gb/s Free Space Transmission Link”, in Conference on Lasers and Electro-Optics, OSA Technical Digest (Optical Society of America, Washington DC, 2000), CPD 15-1,2, pp. 29–30.

], the optical amplifier, based on conventional single-mode fiber (SMF) devices, was not used at the receiver.

In this letter, we demonstrate a 4×10 Gb/s OFSC channel transmission over 1.2 km of free space. In addition to the booster EDFA at the transmitter, an EDFA preamplifier is used at the receiver. We also compare the laser light and the incoherent amplified-spontaneous emission (ASE) light for OFSC applications.

2. Experiment

The experiment was conducted between the Kwangwoon University and a building 1.2 km apart from the university in the sunny afternoon of July 10, 1999. Before the experiment, there was a heavy rain in the morning. Thus, the transmission path was in a high humidity condition with haze.

Fig. 1. Experimental setup. PC: polarization controller, EDFA: erbium-doped fiber amplifier, MOD: LiNbO3 modulator, FPF: Fabry-Perot filter, BF: optical band-pass filter, ATT: attenuator, Rx: optical receiver

We also transmitted only the 1555.7 nm laser channel. After then, we transmitted the incoherent ASE from an EDFA at the same bit rate and compared the results. In the latter case, the two receiver optical filters and EDFA3 were not used. The received powers were about –22 dBm with 2 dB power fluctuations in both light sources.

3. Results and Discussion

Figure 2 shows the received spectrum measured instantaneously for the 4×10 Gb/s transmission. The eye diagrams for the first 1555.7 nm channel before and after the transmission are shown in Fig. 3.

Fig. 2. Received spectrum.

Both eye diagrams are clearly open. Even under the rain, which occurred for a short time during the experiment, the eye performance was not degraded appreciably. The Q values and the channel power fluctuations before the PIN diode are shown in Table 1. All the received channels have Q values higher than 6. The Q value at the back-to-back operation is 10.17 for the first 1555.7 nm channel. It is worth to note that the channel fluctuations and Q values measured from the finite time interval are random variables. Although the Q value estimation does not give the BER directly, it gives an upper bound of BER even in non-Gaussian statistics especially in our experiment using the long random data pattern [7

7. N. S. Bergano, Undersea amplified lightwave systems design, in Optical Fiber Telecommunications IIIA, I. P. Kaminow and T. L. Koch, ed., pp. 317–318, (Academic Press, San Diego, C.A.1997).

].

Since the optical power level per channel before the optical preamplifier should be -30~-32 dBm per channel typically to obtain 10–12 BER, the power margin of this system is only a few dB. The power margin can be easily increased if we increase the booster amplifier output power and optimize the optics in the transmitter and the receiver telescopes.

The beam diameter at the receiver side was about 40 cm. Thus, the beam spreading loss after the transmission is 6 dB. The beam diameter at the transmitter side was about 25 cm implying 2 dB beam spreading loss even at the back-to back operation.

For the single channel transmissions, the Q values for the laser channel and for the ASE channel are very similar, 7.44 and 7.05, respectively. The laser channel loses its temporal coherence owing to the deformation of the wavefront during the transmission. Thus, the performance of the laser channel is degraded close to that of the incoherent channel in this case. The received spectrum of the incoherent channel is shown in Fig. 4.

Fig. 3. Eye diagrams for the first 1555.7nm channel (a) before and (b) after the transmission.

Table 1. Q values and channel power fluctuations before the PIN diode.

table-icon
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The short wavelength part is severely attenuated owing to the H2O components in the air. Thus, the longer wavelength ASE, for example, from the L-band EDFA will be more advantageous in incoherent OFSC systems.

Fig. 4. Received spectrum of the incoherent ASE channel.

In Figure 5, we have shown the distribution of the received intensity for the 1555.7-nm single laser channel transmission after turning off the modulation. Assuming a log-normal distribution, we find the scintillation index as 0.155 which is in the regime of weak fluctuations [8

8. L. Andrews and R. L. Phillips, Laser Beam Propagation through Random Media. Bellingham, Washington USA: SPIE Optical Engineering Press, pp. 112–113, (1998).

]. The scintillation index is defined as <I2><I>21, where I is the received intensity and <·> is the average of its argument.

With the SMF coupling at the receiver, the fiber-to-fiber loss is 25 dB in the back-to-back operation, 11 dB larger than the MMF coupling in [3

3. G. Nykolak, P. F. Szajowski, J. Jacques, H. M. Presby, J. A. Abate, G. E. Tourgee, and J. J. Auborn, “4×2.5 Gb/s 4.4 km WDM free-space optical link at 1550 nm,” in Proc. OFC ’99, paper PD11.

]. However, the SMF pigtail has higher quantum efficiency than the MMF pigtail in 10 Gb/s or faster photo-detectors. In addition, various conventional WDM devices can be used in the standard 100-GHz channel spacing or less.

Fig. 5. Distribution of the received intensity for the continuous-wave laser channel.

4. Conclusion

We have successfully transmitted 4×10 Gb/s WDM channels over 1.2 km of free space with the standard 100-GHz channel spacing. An EDFA preamplifier has been used at the receiver. We have also performed 10 Gb/s single channel transmissions using coherent and incoherent lights separately and shown that both have similar performances.

Acknowledgments:

This work was supported by grant No.2000-1-30200-004-3 from the Basic Research Program of the Korea Science & Engineering Foundation.

References and links

1.

T. H. Carbonneau and D. R. Wisely, “Opportunities and challenges for optical wireless; the competitive advantage of free-space telecommunications links in today’s crowded marketplace,” in Wireless Technologies and Systems: Millimeter-Wave and Optical, Paul Christopher, Leland Langston, and G. Stephen Mecherle, eds., Proc. SPIE3232, 119–128 (1998).

2.

D. R. Wisely, M. J. McCullagh, P. L. Eardley, P. P. Smyth, D. Luthra, E. C. De Miranda, and R. Cole, “4km terrestrial line-of-sight optical free space link operating at 155 Mbit/s,” G. Stephen Mecherle, ed., Proc. SPIE 2123, 108–119 (1994). [CrossRef]

3.

G. Nykolak, P. F. Szajowski, J. Jacques, H. M. Presby, J. A. Abate, G. E. Tourgee, and J. J. Auborn, “4×2.5 Gb/s 4.4 km WDM free-space optical link at 1550 nm,” in Proc. OFC ’99, paper PD11.

4.

G. Nykolak, P.F. Szajowski, A. Cashion, H.M. Presby, G.E. Tourgee, and J.J. Auborn, “A 40Gb/s DWDM Free Space Optical Transmission Link Over 4.4 km”, in Free-Space Laser Communication Technologies XII, G. Stephen Mecherle, ed., Proc. SPIE3932, 16–20 (2000).

5.

G. Nykolak, G. Raybon, B. Mikkelsen, B. Brown, P. F. Szajowski, J. J. Auborn, and H. M. Presby, “A 160 Gb/s Free Space Transmission Link”, in Conference on Lasers and Electro-Optics, OSA Technical Digest (Optical Society of America, Washington DC, 2000), CPD 15-1,2, pp. 29–30.

6.

J. S. Lee, Y. C. Chung, and C. S. Shim, “Bandwidth optimization of a spectrum-sliced fiber amplifier light source using an angle-tuned Fabry-Perot filter and a double-stage structure,” IEEE Photon. Technol. Lett. , 10, pp. 197–1199, (1994).

7.

N. S. Bergano, Undersea amplified lightwave systems design, in Optical Fiber Telecommunications IIIA, I. P. Kaminow and T. L. Koch, ed., pp. 317–318, (Academic Press, San Diego, C.A.1997).

8.

L. Andrews and R. L. Phillips, Laser Beam Propagation through Random Media. Bellingham, Washington USA: SPIE Optical Engineering Press, pp. 112–113, (1998).

OCIS Codes
(060.2330) Fiber optics and optical communications : Fiber optics communications
(060.4510) Fiber optics and optical communications : Optical communications

ToC Category:
Research Papers

History
Original Manuscript: August 15, 2000
Published: October 9, 2000

Citation
Dong-Yiel Song, Yoon-Suk Hurh, Jin-woo Cho, Jung-Hwan Lim, Dong-Woo Lee, Jae-Seung Lee, and Youngchul Chung, "4 � 10 Gb/s terrestrial optical free space transmission over 1.2 km using an EDFA preamplifier with 100 GHz channel spacing," Opt. Express 7, 280-284 (2000)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-7-8-280


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References

  1. T. H. Carbonneau and D. R. Wisely, "Opportunities and challenges for optical wireless; the competitive advantage of free-space telecommunications links in today's crowded marketplace," in Wireless Technologies and Systems: Millimeter-Wave and Optical, Christopher. Paul, Langston. Leland, Mecherle. G. Stephen, eds., Proc. SPIE 3232, 119-128 (1998).
  2. D. R. Wisely, M. J. McCullagh, P. L. Eardley, P. P. Smyth, D. Luthra, E. C. De Miranda, and R. Cole, "4km terrestrial line-of-sight optical free space link operating at 155 Mbit/s," G. Stephen Mecherle, ed., Proc. SPIE 2123, 108-119 (1994). [CrossRef]
  3. G. Nykolak, P. F. Szajowski, J. Jacques, H. M. Presby, J. A. Abate, G. E. Tourgee, and J. J. Auborn, "4 x 2.5 Gb/s 4.4 km WDM free-space optical link at 1550 nm," in Proc. OFC '99, paper PD11.
  4. G. Nykolak, P.F. Szajowski, A. Cashion, H.M. Presby, G.E. Tourgee and J.J. Auborn, "A 40Gb/s DWDM Free Space Optical Transmission Link Over 4.4 km", in Free-Space Laser Communication Technologies XII, G. Stephen Mecherle, ed., Proc. SPIE 3932, 16-20 (2000).
  5. G. Nykolak, G. Raybon, B. Mikkelsen, B. Brown, P. F. Szajowski, J. J. Auborn and H. M. Presby, "A 160 Gb/s Free Space Transmission Link", in Conference on Lasers and Electro-Optics, OSA Technical Digest (Optical Society of America, Washington DC, 2000), CPD 15-1,2, pp. 29-30.
  6. J. S. Lee, Y. C. Chung, C. S. Shim, "Bandwidth optimization of a spectrum-sliced fiber amplifier light source using an angle-tuned Fabry-Perot filter and a double-stage structure," IEEE Photon. Technol. Lett., 10, pp. 197-1199, (1994).
  7. N. S. Bergano, Undersea amplified lightwave systems design, in Optical Fiber Telecommunications IIIA, I. P. Kaminow and T. L. Koch, ed., pp. 317-318, (Academic Press, San Diego, C.A.1997).
  8. L. Andrews, and R. L. Phillips, Laser Beam Propagation through Random Media. Bellingham, Washington USA: SPIE Optical Engineering Press, pp. 112-113, (1998).

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