X
By Topic

IEEE Quick Preview
  • Abstract
  • Authors
  • Figures
  • Multimedia
  • References
  • Cited By
  • Keywords

Midinfrared Interband Cascade Laser for Free Space Optical Communication

A free space optical (FSO) link utilizing midinfrared (mid-IR) interband cascade lasers has been demonstrated in the 3- to 5-??m atmospheric transmission window with data rates up to 70 Mb/s and bit-error rate (BER) less than 10-8. The performance of the mid-IR FSO link has been compared with the performance of a near-IR link under various fog conditions using an indoor communication testbed. These experiments demonstrated the lower attenuation and scintillation advantages of a mid-IR FSO link.

SECTION I

INTRODUCTION

INTERBAND cascade (IC) lasers [1] are novel Sb-based semiconductor lasers that have demonstrated a high performance operation in the 3- to 5-Formula$\mu{\hbox {m}}$ wavelength region, including high output power [2], continuous-wave (CW) operation at high temperatures [3], [4], and high-speed modulation [5]. This progress enables utilization of IC lasers as optical transmitters in free space optical (FSO) communication links operating in the 3- to 5-Formula$\mu{\hbox {m}}$ atmospheric transmission window. Due to the favorable atmospheric properties, midinfrared (mid-IR) atmospheric transmission windows occupying 3- to 5- or 8- to 12-Formula$\mu{\hbox {m}}$ spectral bands are of particular interest for realization of high-speed FSO data links. The molecular (Rayleigh) and particulate (Mie) scattering [6] as well as the wavefront propagation phase errors decrease with wavelength [7] and, therefore, the transmission loss of an optical link operating in the mid-IR would be significantly reduced compared to the loss of the optical link at 1.5 Formula$\mu{\hbox {m}}$. The lower loss advantage of FSO links in the 8- to 12-Formula$\mu{\hbox {m}}$ spectral window has been recently demonstrated for operation during a bad weather condition (e.g., fog) [8], [9]. In addition, the spectral radiance of the main sources of background radiation in the atmosphere (Sun, Earth, Moon, city lights, etc.) has a pronounced minimum around 3.5 Formula$\mu{\hbox {m}}$, and thus the background noise will be minimized. Yet, the progress in the development of mid-IR FSO links has been hindered by the lack of high-speed lasers, optical amplifiers, and detectors operating in the desirable wavelength regions. In this work, we have utilized recently developed high-speed IC lasers [5] to realize a mid-IR FSO link operating at data rates up to 70 Mb/s with bit-error rate (BER) less than Formula$10^{-8}$. These results are the first step in the development of a practical mid-IR FSO communication system and are important for the future improvement of mid-IR lasers and detectors.

SECTION II

EXPERIMENTAL RESULTS

A. Device Design and Fabrication

We evaluated the high-speed performance of our standard structures reported previously [5]. In short, these lasers have been grown in a solid source Veeco Applied-EPI Gen-III molecular beam epitaxy (MBE) system on undoped p-type GaSb substrates. After the epitaxial growths, parts of the wafer were processed into broad-area mesa stripe (100- or 150-Formula$\mu{\hbox {m}}$-wide) lasers by wet chemical etching and metal deposition. Two-millimeter-long lasers were mounted onto an oxygen-free copper holder and wire bonded to 50-Formula$\Omega$ radio-frequency (RF) microstrip line. Lasers were then mounted on the temperature-controlled cold finger of an optical cryostat that was modified to enable high-speed measurements up to 18 GHz. Initially, light–current–voltage (Formula$L\hbox{\ndash}I\hbox{\ndash V}$), spectral and high-speed characteristics of fabricated IC lasers have been evaluated. In the latter, the high-speed modulation was achieved by driving the laser with an RF signal from a low-phase-noise signal generator and the photocurrent signal was measured with a fast quantum-well infrared photodetector (QWIP) [10]. The high-speed response of the IC laser, which was calculated from the measured amplitude of the RF signal, shows that the modulation bandwidth of IC lasers used in this work exceeds 3.2 GHz.

B. FSO Link

In the optical link experiment, the IC laser was modulated with a pseudorandom bit sequence (PRBS) signal from a bit-error-rate transmitter (BERT) and a bias tee was used to combine the high-speed modulation and laser dc bias current. The laser operated at Formula$T=77\ \hbox{K}$ under applied dc bias of about Formula$I=100\ {\hbox {mA}}$ with output power of Formula$\sim$10 mW at 3.0 Formula$\mu{\hbox {m}}$ [5] and the laser emission was collimated by a ZnSe lens. After a free space transmission of about 1 m, the optical signal was collected by a second lens and focused on a high-speed mercury–cadmium–telluride (MCT) detector manufactured by Vigo. Compared to QWIP [10] used in the initial high-speed laser characterization experiments, Vigo MCT detectors have much better sensitivity and higher operational temperature reachable with thermoelectric coolers that make them more practical for use in FSO experiments, yet their useful bandwidth was limited to about 70 MHz. After the detection, the high-frequency photocurrent signal was fed into a high-speed amplifier followed by a second BER receiver or high-speed oscilloscope after the bias tee.

Fig. 1 shows the results of an indoor FSO transmission experiment for data transmission rates from 10 to 70 Mb/s. The open eye diagrams acquired for different data transmission rates are clearly seen in this graph. At data rates of 30 and 50 Mb/s, the observed large size of eye opening, small distortion, and time variation of zero crossing demonstrate the high performance of the FSO link based on the developed IC lasers. The degradation of the open eye diagrams at data rates below 10 Mb/ was attributed to the laser heating/cooling during long on/off times and the upper limit of data transmission rate of about 70 Mb/s was set by the Vigo MCT detector bandwidth. These limitations to achievable data transmission rates can be overcome by using IC lasers with better thermal properties such as narrow waveguide ridge devices and by utilizing detectors with higher bandwidth.

Figure 1
Fig. 1. Open eye diagram at different data transmission rates of mid-IR FSO link. The lower traces correspond to the RF input and the upper that detected by the VIGO detector. The horizontal scale is 100 nS at the 10 Mb/s graph and 20 nS for all other graphs.

The performance of FSO links for different attenuations of the optical signal was also studied and BERs were measured using the BER receiver. Fig. 2 shows the open eye diagrams acquired for data transmission at a rate of 30 Mb/s for different attenuations of FSO link and the measured BER versus optical attenuation, showing exponential dependence at low BER. This graph demonstrates that an FSO link utilizing developed IC lasers can operate with BER less than Formula$10^{-8}$ that was the sampling time limit in this experiment, with attenuation up to 14 dB. The link BER, in fact, was limited by the receiver sensitivity where, for example, 18-dB attenuation corresponded to a received power of about 50 Formula$\mu\hbox{W}$. With the availability of more sensitive and high-bandwidth detectors [11], [12], our estimate suggests that the transmitters could support much higher photon efficiency optical links with attenuation exceeding 60 dB.

Figure 2
Fig. 2. Open eye diagrams of data transmission at the rate of 30 Mb/s for different attenuations of the FSO link. Insert (bottom-left) BER of FSO optical link operating at 30 MB/s versus attenuation. Dashed line is exponential fit to the data for attenuation ranging from 14 to 15 dB.

C. FSO Link at 1.5 Formula$\mu{\hbox {m}}$ versus 3.0 Formula$\mu{\hbox {m}}$

The indoor experimental test-bed was used to study the effects of fog on the performance of the demonstrated mid-IR FSO link and for comparison between mid-IR and 1.5-Formula$\mu{\hbox {m}}$ FSO links. The mid-IR FSO link utilized the developed IC laser, and a commercial telecom laser and detector were used for the 1.5-Formula$\mu{\hbox {m}}$ link. In this test, the collimated optical beams from both lasers were combined by a beam splitter, and then both signals passed through the fog so the mid-IR and near-IR FSO link shared the same optical path through the fog. After completing the assembly, both mid-IR and near-IR FSO links were tested and BERs versus attenuation for both links were measured (not shown).

Next, properties of the FSO link were evaluated in clear air and in the presence of “dry” and “wet” fogs, where the former was created by the flow of liquid nitrogen Formula$(\hbox{LN}_{2})$ in the air and the latter by a commercial fog generator. For the three different “wet” fogs generated (Fogs 1, 2, and 3), the fog density and the water particle size was minimal in “Fog 1,” and both density and particle size were increased in the “Fog 2” and “Fog 3” conditions. Based on the fog generator specifications, the average size of the water particles was estimated to increase from 7 to 15 Formula$\mu{\hbox {m}}$ between Fogs 1 and 3 but that was not verified experimentally.

The influences of the different fog conditions on the FSO link were investigated by measuring the temporal fluctuations of the received optical power for both mid-IR and near-IR channels at the different fog conditions, as shown in Fig. 3. Initially, the power fluctuations were measured in the absence of any air flow (denoted as “Clear” in the figures), then in the presence of the fog created by Formula$\hbox{LN}_{2}$ (dry fog) and in the wet fog (Fogs 1, 2, and 3). Fig. 4 shows the attenuation of the average optical power (relative to the optical power measured in clear air) over the 1-m link for both channels at different fog conditions. This graph shows that optical power of near-IR link decreased faster than the power of the mid-IR signal for all fog conditions, and at the “Fog 3” conditions for example, the attenuation of 1.5-Formula$\mu{\hbox {m}}$ channel is 2 times stronger than that of the 3-Formula$\mu{\hbox {m}}$ channel for 1-m-long FSO link.

Figure 3
Fig. 3. Received optical power versus time for near-IR (top) and mid-IR (bottom) channels during clear and fog conditions. The increased optical power fluctuations in the fog are clearly visible on the graph. The average optical power in the fog corresponds to the Formula$\hbox{BER}=10^{-2}$ for each of the link. The optical power of the near-IR laser used in this experiment is much lower than the power of mid-IR laser due to a better performance of near-IR detector.
Figure 4
Fig. 4. Attenuation of the optical signal over 1 m FSO link at different dry (top) and wet (bottom) weather conditions.
Figure 5
Fig. 5. Probability distribution functions of the optical signal for the (a) near-IR channel, clear, (b) mid-IR channel, clear, (c) near-IR channel, dry fog (the solid line is a log normal fit to the data), and (d) mid-IR channel (the solid line is a normal fit to the data).

Fig. 5 shows the probability distribution function of the optical signals with and without fog for 1.5- and 3-Formula$\mu{\hbox {m}}$ channels. The optical power of the near-IR channel showed a normal distribution with no flow, but in the dry fog it exhibited a strongly log normal distribution typical of a turbulent atmosphere. The scintillation index, Formula$\sigma_{I}^{2}$ calculated from Formula$\sigma_{I}^{2}=\langle{I}^{2}\rangle/\langle{I}\rangle^{2}-1$, where Formula$I(t)$ is the measured signal strength, is large Formula$(\sigma_{I}^{2}=0.35)$ for the near-IR channel in the dry fog showing that near-IR channel is strongly affected by scintillation. In contrast, the mid-IR 3-Formula$\mu{\hbox {m}}$ data showed only normally distributed power in both scenarios (without and with fog from the Formula$\hbox{LN}_{2}$ flow), demonstrating the advantage of using 3-Formula$\mu{\hbox {m}}$ wavelength link for transmission under strongly scintillating effects.

In the wet fog, the scintillation indexes were found to be small indicating very weak turbulence. Also, the measured ratios of the scintillation index did not follow that predicted by theory [13] when comparing the two wavelengths. These results indicated that in wet fog conditions the scintillation was not dominated by the wavelength dependence but by the detection and/or transmission power variations. This was also confirmed by looking at the probability distribution functions that show a normal behavior for both no flow and under fog conditions for both wavelengths (not shown).

SECTION III

CONCLUSION

We have utilized the high-speed IC lasers to realize an FSO link operating in the 3- to 5-Formula$\mu{\hbox {m}}$ atmospheric transmission window and compared its performance with that of a near-IR link at different fog conditions using an indoor communications testbed. This work has validated the suitability of IC lasers as a mid-IR light source for high-speed FSO communications links and has demonstrated the advantages of mid-IR FSO link in fog due to its lower attenuation and scintillation.

ACKNOWLEDGMENT

The authors are grateful to H. Hemmati, S. Forouhar, S. D. Gunapala, M. Hermann, E. Kolawa, J. A. North, L. G. Gref, and R. T. Odle for their support and encouragement.

Footnotes

The research described in this letter was performed at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (NASA).

A. Soibel, M. W. Wright, W. H. Farr, S. A. Keo, and C. J. Hill are with Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 USA (e-mail: asoibel@jpl.nasa.gov; Malcolm.W.Wright@jpl.nasa.gov; William.H.Farr@jpl.nasa.gov; Sam.A.Keo@jpl.nasa.gov; cory.j.hill@jpl.nasa.gov).

R. Q. Yang is with School of Electrical and Computer Engineering, University of Oklahoma, Norman, OK 73019 USA (e-mail: rui.q.yang@ou.edu).

H. C. Liu is with Institute for Microstructural Sciences, National Research Council, Ottawa, Ontario, K1A 0R6, Canada (e-mail: h.c.liu@nrc-cnrc.gc.ca).

Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org.

References

1. R. Q. Yang

"Infrared laser based on intersubband transitions in quantum wells"

Superlattices and Microstructures, vol. 17, pp. 77-83, 1995

2. C. L. Canedy , W. W. Bewley , J. R. Lindle , C. S. Kim , M. Kim , I. Vurgaftman and J. R. Meyer

"High-power and high-efficiency midwave-infrared interband cascade lasers"

Appl. Phys. Lett., vol. 88, p. 161103, 2006

3. K. Mansour , Y. Qiu , C. J. Hill , A. Soibel and R. Q. Yang

"Mid-infrared interband cascade lasers at thermoelectric cooler temperatures"

Electron. Lett., vol. 42, no. 18, pp. 1034-1035, 2006

4. M. Kim , C. L. Canedy , W. W. Bewley , C. S. Kim , J. R. Lindle , J. Abell , I. Vurgaftman and J. R. Meyer

"Interband cascade laser emitting at $\lambda=3.75\ \mu{\hbox {m}}$ in continuous wave above room temperature"

Appl. Phys. Lett., vol. 92, p. 191110, 2008

5. A. Soibel , M. W. Wright , W. Farr , S. Keo , C. Hill , R. Q. Yang and H. C. Liu

"High speed operation of Interband cascade Lasers"

Electron. Lett., vol. 45, no. 5, pp. 264-265, 2009

6. E. D. Hinkley

Laser Monitoring of the Atmosphere

1976, Springler-Verlag

7. L. C. Andrews and R. L. Phillips

Laser Beam Propagation Through Random Media

2005, SPIE Press

8. R. Martini , C. G. Bethea , F. Capasso , C. Gmachl , R. Paiella , E. A. Whittaker , H. Y. Hwang , D. L. Sivco , J. N. Baillargeon and A. Y. Cho

"Free space optical transmission of multimedia satellite data streams using midinfrared quantum cascade lasers"

Electron. Lett., vol. 38, no. 4, pp. 181-183, 2002

9. S. Blaser , D. Hofstetter , M. Beck and J. Faist

"Free-space optical data link using Peltier-cooled quantum cascade laser"

Electron. Lett., vol. 37, no. 12, pp. 778-780, 2001

10. H. C. Liu , M. Buchanan and Z. R. Wasilewski

"Short wavelength 14 $\mu{\hbox {m}}$. infrared detectors using intersubband transitions in GaAs-based quantum wells"

J. Appl. Phys., vol. 83, no. 11, pp. 6178-6181, 1998

11. M. D. Petroff and M. G. Stapelbroek

"Photon counting solid-sate photomultiplier"

IEEE Trans. Nucl. Sci., vol. 36, no. 1, pp. 158-162, 1989

12. X. Jiang , M. A. Itzler , B. Nyman and K. Slomkowski M. A. Itzler and J. C. Campbell

"Negative feedback avalanche diodes for near-infrared single photon detection"

Advanced Photon Counting Techniques III, Proc. SPIE, vol. 7320, p. 732011, 2009

13. M. W. Wright , J. Roberts , W. Farr and K. Wilson

"Improved optical communication performance combining adaptive optics and pulse position modulation"

Opt. Eng., vol. 47, no. 1, p. 016003, 2008

Authors

No Photo Available

Alexander Soibel

No Bio Available
No Photo Available

Malcolm W. Wright

No Bio Available
No Photo Available

William H. Farr

No Bio Available
No Photo Available

Sam A. Keo

No Bio Available
No Photo Available

Cory J. Hill

No Bio Available
No Photo Available

Rui Q. Yang

No Bio Available
No Photo Available

H. C. Liu

No Bio Available

Cited By

Cited by IEEE

1. Assessment of PbSe Photoconductors for the Realization of Free-Space Midinfrared Optical Communication Links

Rivera, C., Alvarez, M.

Photonics Technology Letters, IEEE, vol. 24, no. 4, pp. 267-269, 2012

Corrections

None