chap 8 quantum cascade laser-based free space optical communications

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© 2005SpringerScience-BusinessMediaInc.001 : 1O.l007/sI0297-005-0052-2Originally published in J. Opt. Fiber. Commun Rep. 2,279-292 (2005)

Quantum cascade laser-based free space optical communications

R. Martini and E.A. Whittaker

Department of Physics and Engineering PhysicsStevens Institute of TechnologyHoboken , NJ 07030Email: rmart ini@stevens .eduandewhit tak@stevens . edu

Abstract. The advent of the quantum cascade laser (QCL) with emission wavelengthsavailable in the infrared range from 3 JIm through more than 100 JIm opens up thepossibility of exploiting infrared atmospheric transparency windows for free space optical communications. In an effort to establi sh the efficacy of using directly modulatedQCLs for free space communications we have conducted a series of investigationsthat demon strate the potential advantages of this technology. In these experiments wefirst establish that the QCL has very high modulation bandwidth. We then implementa practical free space communications link that under conditions of atmospheric fog,

dust and other obscurants offers significant transparency advantage when comparedwith near infrared wavelength sources presently used in commercial free space opticalcommunications links.

1. Introduction

Free space optical communications links have recently received attention in the research and commercial development communities because of their potential to serveapplications requiring high bandwidth with relatively high security to eavesdroppingbut without the complexity of installing optical fiber. Commercially available systemshave largely been based on the adaptation of telecommunications components in thenear-infrared spectral region. However, the free space (atmospheric) optical channelhas characteristics that are quite different from fiber-based channels. Molecular absorption, light scattering from suspended particulates, fog, rain and even snow all conspireto make a more complex and less controlled transmission medium for free space optical transmission systems. The choice of source wavelength is thus a critical parameter



394 R. Martiniand E.A. Whittaker

in the design of such systems and it is natural to explore whether a significant advantage may be gained by employing longer wavelength systems. However, existing longwavelength laser systems do not enjoy the advantages of the typical semiconductorsystem employed in telecommunication system such as small size, potential for highbandwidth direct current modulation and high quantum efficiency. A possible solutionfor this niche has recently become available in the form of the quantum cascade laser(QCL) emitting in the middle infrared (8-10 j.tm). QCL's have established themselvesas versatile semiconductor light sources for the mid-infrared (IR) spectrum and beyond(). '" 3.5 - 24 j.tm) and are already widely used for spectroscopic applications [I].

We will discuss first the high-speed modulation properties of the QCL, addressthen a comparison of FSO experiments between a QCL and NIR laser based link incontrolled low visibility experiments and finally present a recent FSO application of aQCL based link used for transmission of satellite TV data streams.

2. High Frequency Analog and Digital Modulation

Based on a unipolar lasing mechanism, the QCL possesses unique high-frequencycharacteristics with theoretical bandwidths in excess of several hundred Gigahertz[2]. Several publications address this issue for applications like gain-switching [3],mode-locking [4], and high-speed modulation without relaxation oscillations [5]. Wereport in the following from measurements on the high-speed bandwidth of QCLs aswell as successful high-speed data transmission as reported in [6]. These experimentswere performed using experimental QCLs provided by Lucent Technologies, grownby molecular beam epitaxy in the GaInAslAlInAs material system, and based on the"3-well vertical" design of the active region [I] . The results presented herein referto 1.25-mm long, 4.5-j.tm-wide deep-etched ridge lasers with an emission wavelengthnear 8.1 j.tm. Tooptimize their high-frequency characteristics, the lasers were packagedand processed using a chalcogenide lateral waveguide as described in [3].

The inset of Fig . I shows the experimental setup used to measure the highfrequency modulation response of the specified QCL. The device was biased above

its cw threshold, and modulated by the output of a synthesized signal generator withfrequencies ranging from 0.1 to 10 GHz at a level of 0 dBm. The light output wastransmitted over a distance of I m and detected with a GaAslAIGaAs quantum-wellinfrared photodetector (QWIP) [9], packaged for high-speed operation. The resultingphotocurrent was amplified and fed into a microwave spectrum analyzer, where themodulation amplitude was measured. The main part of Fig. I shows a typical frequency response of this device for a de current of 300 rnA and a laser temperatureof 20 K. The data were normalized to the frequency response of the detector [9] toreflect the modulation response of the QCLs. Aside from a low-frequency shoulder

around 2 GHz, which we ascribe to residual parasitic effects , the modulation responseremains relatively flat up to roughly 7 GHz, which is adequate for high speed broadband transmission. As discussed in [4], the observed modulation response is still fullyparasitics-limited, and nearly independent of the dc current.

To demonstrate the high-speed data transmistion capability of the QCL, the devicewas modulated with a non-return-to-zero (NRZ) pseudo-random bit stream (PRBS)from a bit error rate (BER) test system. This test pattern is the most common testsystem to monitor the performance of standard network systems. The quality of the



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communication link is thereby measuredusing an eye-diagram test as well as in thebit error rate (BER) measurement. In an eye-diagram measurement the clarity of adecision process between a logical "0" and "I" is determined as "openness" of theeye, which is created by averaging over multiple transmitted bit patterns, with therecordingoscilloscopesynchronized with the transmission clockrate. Figure2 showsa typical eye diagram observed using the same setup as mentioned above for a 2 3 1 _

I bit long pattern (amplitude2 Y, corresponding to 10 dBm rf power)at a data rateof2.5 Gbit/s for a heat-sinktemperature of 20 K and a de currentof 250 mAoIn general,a clear and open eye is observedfor heat sink temperatures up to 85 K, demonstratingthat thereceiving systemcan distinguishbetweena logical"0" or" I". However, some

deviation in the average "I " and "0" levels can be observed, which we attribute toheating and cooling of the laser due to preceding long "on" and "off'l-times. Thiseffect can be prevented using a return-to-zero (RZ) modulation, which is especiallysuitable at higher data rates, and at operating temperatures of the laser close to itsmaximumcw temperature.

We also performed BER measurements on the QCL data transmission system.In these tests the average percentageof incorrectlytransmitted bits ("0" identified as"I" and vice versa)withina standard transmission session is evaluated. Typical highperformance networksystems mustachievea minimum 10- 9 BER, which allowsfor

error-freedatacommunications utilizingenhanced forward errorcorrectionalgorithms.Naturallypreferableare lowerBERas theyenhancethestabilityof thecommunicationlink. Our QCL transmission link showederror-freedata transmission with a BER <10 - 1 2 observedfortemperaturesupto85Kandovertheentirerangeofdccurrenttested(175to 300 mA).Toquantifythe power margins for theseerror-freedata transmission,a variablebeam attenuator was included in the beam path. The threshold value for10 - 9 BER transmission was reached around3 dB attenuation, whichcorrespondstoa received modulation power of 500 JLW ( - 3 dBm). Similar results were obtained



396 R. Martini and E.A. Whittaker

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for attenuation of the electrical modulation amplitude. This relatively high value incomparison to fiber optic communication systems (1.3 or 1.55 /-Lm) is attributed tothe lower quantum efficiency and higher noise equivalent power (-20 dBm) of theQWIP detector as well as to the previously discussed deviation in detected digital level.Nonetheless, these measurements prove the ability of a QCL based FSO link to servein high bandwidth digital networks.

2.1. Stability fo r NIR-Laser and a QCL-Based FSO Link Under Strong Scattering

Aside from the modulation speed, other parameters have to be taken into account forthe design of a QCL-based FSO link. Design improvements in the basic technology areresulting in steady improvements in optical power, room temperature operation andmaximum wavelength of QCLs but whether these improvements will make a QCLbased long-wavelength free space links a logical choice for commercial development isstill not clear. Additional factors that must be considered include receiver performanceas well as the specific application environment.

A first step in accessing these parameters was made in recent experiments [9],which analyzed the transmission characteristics of three commonly used telecommunications wavelengths and the QCL using standard commercial photodetectors withoutevaluating the performance of the receivers themselves.

3. Experimental Apparatus

To better understand the wavelength dependence of a free space link we conducted asystematic study to compare systems using three wavelengths used by commerciallyavailable free space links (0.85, 1.3 and 1.55 /-Lm) with a quantum cascade laser basedlink operating at 8 /-Lm. The measurements were performed within a wind tunnel facility



New optical device technologies for ultrafast OTDM systems 397

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Fig. 3. Arrangementof laser source, detector and wind tunnel for the transmission experiments.The optical path was approximately 65 ft.

that can simulatespecificweatherconditionsand allowfordetailedcomparisonof thefour wavelengths. The 76-foot-longPacific NorthwestNational Laboratory's (PNNL)AerosolWindTunnel Research Facilityallows us to inject particleswith well definedsize distributions into the beam path and then to measurepower transmission and thebit-error-rate for each of the four wavelength lasers.

Figure3 showsschematically the setup used for these measurements. The PNNLwind tunnel was modified by adding optical windows at either end of the 76-footlong test leg of the tunnel.The three NIR wavelengths passedthrough two BK7 glasswindowsand the QCL beam was directed through an Amtir-I window. The four different optical links were sent through the 65-foot-long wind tunnel and detected byindependentphotodetectors (ThorlabsPDA-400) as well as a liquid-N, cooled MCTdetector. All lasers were operated in cw-mode above threshold and were electricallymodulatedaround 25 kH z . The modulation amplitude of the detected radiation wasextractedusing lock-indetection. All four transmitted signals were recordedusing anelectronicstripchart recorderimplementedusingLabView® on a personalcomputer.As theintensityvariedoverseveral ordersof magnitude, thescalesettings on the lockin amplifiers were changedseveral times. Rawdata were recordedand then processedby factoring in the scale changes and referencing transmitted intensity to initial intensity to produce the smoothlyvarying plots shown in this report. Wealso analyzedthe transmittedintensitiesto generatean approximate measureof signal-to-noise ratio(SNR) by calculating the rms fluctuation in intensity divided into the rms magnitudeof the intensity.

The wind tunnel was equipped withan injectionapparatus for water fog, oil fog

and dust particles.A samplingapparatuswas used to monitorthe density and in somecases, theparticlemass/sizedistributionof scatterersin thetunnel. Duringa particularmeasurement, the air velocitywouldbe set at a fixed value,typicallya fewmeterspersecond. The particle injectionapparatuswould then be activated. The particledensitythen gradually increased with time over the course of 30-60 minutes and the laserintensitieswererecorded.The particlemeasuring apparatusgenerallyshoweda linearincreasein particledensityas a function of time andthus thestripchart recordsof laser




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intensityas a function of timearealso uncalibrated measurements oflaser intensityasafunctionof particledensity.In a typicaldata run, wewouldactivatetheparticleinjectorand run themeasurementuntil thetransmittedintensityof themost severelyattenuatedbeam dropped below our detection limits. Wethen turnedoff the particleinjector andsimultaneously activateda ventilator, allowingthe particledensity to diminishand thetransmittedintensities to return to their unattenuatedlevels. Such a data run wouldlastapproximatelyone hour.

The particlesize distributiondepends on the type of particle and particledensity.A laser scattering apparatus measured the size distributionof the oil vapor and dustparticles. Some typical plots of size distributions for these two types of particles areshown in Fig. 4. For dust we obtained a broadband size distributionpeaking at '" 1.1jLm and ranging from 800 nm to 5.5 jLm, whereas the oil vapor distribution peakedaround 850 nm with a range from 600 nm to beyond 6 jLm. With increasing densitythe size distributionseemed to be constant for dust particle, whereas for oil vapor thesmaller particle concentration rises first strongly, with a reverting effect for higherconcentration.

In the next two figures, the data obtained for scattering from water fog and oilfog are shown. In the left-hand side of Fig. 5 the water fog results show a dramaticdecreasein intensityfor the three NIRwavelengths, whereasthe right-handside showsthe extractedSNR ratio separately foreach of the communication links separately. Atthe highest concentration(at 15:52), the 1.3-and 1.55-jLm beamsexperiencedover40dB losses and the 0.85 jLm somewhatless.

However, the 8-jLm beamexperiencedonly3 dB of attenuation.The correspondingSNR charts showa similar resultwithnearlyconstant SNR for the MIR linkand drops



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in the signal down to (and below) 10 dB for the NIR links. Figure 6 shows the dataset for oil fog. The oil fog differs from the water vapor in that the O85-p,m beam wasattenuated more than the 1.3- and 1.55-p,m beams but as with the water vapor the 8p,m beam shows almost no loss under circumstances that completely attenuate the NIRbeams. The difference between the water and oil fog most likely reflects differences inparticle size distribution . Unfortunately, the laser scattering apparatus that we used tomeasure the particle distribution is unable to measure the water fog size distributiondirectly so a direct test of this assumption could not be made.

However, the slope of the transmission data obtained during the particle densityincrease phase allowed us to make a summary comparison of the attenuation efficacyof each of the scattering media as a function of the transmission wavelength. To de

termine the scattering efficiency for the different wavelengths the transmission lossesalong the length of the link were calculated assuming a scattering concentration of1000 scatterers/crrr'. The results of this analysis are shown in Fig. 7. Although thereare slight variations in scattering efficiency for the three NIR wavelengths, in all casesthe 8 p,m beam is almost unaffected by scattering conditions (0.06-0.4 dB), whereasthe NIR beams experience much stronger attenuation ranging from ", 2.5 dB for dustto 10dB for water, having at least one order of magnitude stronger losses. The case forwater droplets is thereby even more interesting, as it reveals again the different wave-



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.length dependent behavior compared to dust or oil scattering: Here the link operatingat 850 nm experienced less losses then the other NIR links-whereas for the otherscattering system the losses seemed to be qualitatively stronger for longer wavelength.We attribute this behavior to a significantly different particle size distribution for thecreated water fog.

In an additional set of experiments we evaluated the influence of the scatteringenvironment on digital communication properties by measuring bit error rates (BER)and eye diagrams for each of the four wavelengths while stepwise increasing the particle concentration. For this purpose the sinusoidal modulation was replaced by a PRBSdata stream at a data rate of 34 MBitls (corresponding to a DS-3 data transmissionrate) and the detectors of the NIR wavelength links were replaced by Thorlabs D400FCdetector with I GHz bandwidth (the MCT detector for the MIR link provided enoughbandwidth for the communication measurements). The digital data were then ampli

fied corresponding to automatic gain compensation in standardized communicationsystems and analyzed with a Hewlett-Packard 3784A BERT. For further evaluationcorresponding eye diagrams were recorded in parallel using a Tektronix storage oscilloscope.

For the NIR communication links the signal quality decreased drastically withscatter particle density, resulting in noisier eye diagrams. Utilizing high gain (factor of125) and based on the relatively high link margin (typically SNR > 30 dB in the start)



Newoptical devicetechnologies for ultrafast OTDM systems 401

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error-free data transmission could still be ensured until higher scattering densities,but typically could not be sustained at the highest densities. The upper part of Fig. 8shows typical eye diagrams obtained for the 1.3-p,m link under conditions of low oilconcentration (0.57 rug/em") and roughly 20x higher oil concentration (II mg/cm")with 125x amplification. The open eye at the low concentration has completely closedat the high concentration, and the signal has highly degraded from 1.5 V to 200 m V.On the other hand, data transmission in the MIR-link only slightly affected and noamplification was needed for the tested environments. The comparable eye diagramsfor the 8 p,m link, shown in the lower part of Fig. 8, shows almost no degradationand we still obtained error free transmission (BER > 10- 13

) at the highest oil vaporconcentration. We obtained similar results for each of the other scattering materials.

Within these experiments the performance of free space optical communicationlinks operating at three near-infrared wavelengths (0.85, 1.3 and 1.55 p,m) using commercially available telecommunications diode lasers and at 8 p,musing an experimentalquantum cascade laser produced at Lucent's Bell Laboratories were compared underlow visibility conditions. To simulate a variety of atmospheric conditions, the four

links were transmitted through the 65-foot-long measurement leg of the Pacific Northwest National Laboratory's (PNNL) Aerosol WindTunnel Research Facility.The windtunnel was selectively injected with water vapor, oil vapor, and dust particles . We thenmeasured the transmitted intensity of each of the links as the scattering density increased. We also recorded bit error rate and eye diagrams using a bit error rate testeroperating at 34 MHz. The results show that each of the NIR wavelengths becomestrongly attenuated at the highest concentration of scattering particles whereas the




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8-J.lm QCL does not experience significant attenuation . Likewise, the NIR links areunable to sustain error free transmission at the highest attenuation levels but the QCLlink remains-error free under the same particle density conditions.

3.1. Satellite 1VTransmission Using a QCL-Based FSO Link

The first two parts of this chapter addressed the two dominant requirements for QCLbased FSO links: high modulation bandwidth and high transmissivity in scatteringenvironments. In this final section we report on the application of a QCL-FSO link

for high-bandwidth satellite TV-data transmission as described in [12]. The upper partof Fig. 9 shows the optical setup of the transmission link. The same QCL's describedin the modulation experiment were employed in this setup. The QCL beam was collimated using an f/3 ZnSe-lens and then transmitted over an open-air 100m path to aretroreflector, mounted on another building of Bell Laboratories in Murray Hill, NewJersey. The reflected light was collected using an f /9 telescope with an aperture of76 mm and focused onto a high-speed liquid nitrogen cooled MCT detector (SagemHgCdTe 0 II) . To compare the effect of the longer wavelength on the link quality and



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stability, a second beam was included in the path, originating from a 0.85 J.Lm diodelaser (10 mW output power) and detected with a standard Si-detector. To ensure anidentical beam path and easy adjustment the optics for the outgoing beam and for thedetection were rigidly connected to the telescope.

The lower part of Fig. 9 shows the electrical setup of the QC-Iaser and of thedetection system. The signal was received from a satellite dish using a LNB (low noiseblock down converter)-module. This high-frequency signal (750 MHz-I 045 GHz) wascombined with a dc current to drive the QC-Iaser continuously above its threshold. Themodulated laser radiation was transmitted over the total distance of ",,200 m beforeit was detected. The de component was split off with a bias-Tee and used as monitorof the received laser intensity. The high- frequency part was amplified and fed into aspectrum analyzer as well as into a standard satellite set-top-box connected to a TVmonitor.

Under typical QC laser operating conditions (500 rnA dc current at a temperatureof 25 K) the link could be run continuously and stably for at least five hours. Owingto the beamsplitter and the multiple optical elements in the outgoing beam path only7.5 mW of the initial 25-mW output power were actually used for the transmission.About 10% of the original intensity (0.75 mW) could still be detected under goodweather conditions after the transmission and the collecting telescope optics. Thesimultaneously transmitted beam with a wavelength of 0.85 J.Lm had comparable losses,which are therefore attributed to beam spreading and losses in the optical elements.

A typical example of the transmitted data stream is shown in the upper part of Fig.10. The modulation in the frequency region from 900 MHz to 1045 GHz contains thedigitally encoded information (QPSK-code: quadrature phase shift keying) consistingof around 800 television channels and 100 radio channel s. The dashed line in Fig. 10



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represents the original signal received from the LNB, whereas the solid line showsa typical signal detected after the QC laser-link. Owing to the limited bandwidth ofthe detector, the channels in the higher frequency region were detected with a 10dBhigher loss, reducing the number of actually decodable channels to around 650. Thelink power margin is 7 dB corresponding to a received power of 0.125 mW, belowwhich the receiver becomes unstable.

To evaluate the advantage of the longer wavelength relative to the collinearlypropagating near-infrared beam (0.85 t-tm) the intensity of the latter was monitored

in parallel. For typical weather conditions including clear air, strong rain as well asa thunderstorm, no differences in sensitivity were observed. Nevertheless, a stronglypronounced deviation was observed during a dense fog situation, with nearly zerovisibility. Figure II shows the temporal evolution of the detected de intensities forboth laser links, starting at a very dense fog situation in the early morning of 8/14/0 I.As the fog lifted slowly around 3:15 AM. the QC-laser link regained transmissionmuch quicker than the near-IR link. The QC-Iaser link had reached transmission ofnearly 70% of its optimal value, when the intensity of the near-IR link was still belowthe detection limit. As a result around 4:00 AM, the mid-IR television link became

stable again, whereas the near-IR link was still unstable almost for another hour. Thesuperior performance of the QC-Iaser link compared to the near-IR link can readilybe understood from the wavelength dependence of Rayleigh- and Mie-scattering andrelates well to the aforementioned windtunnel measurement.

Following this measurement the conclusion has to be drawn, that reliable freespace optical communications should be possible using the QCL under conditions thatwould make communications impossible using shorter wavelengths. As detector and



New optical device technologies for ultrafast OTDM systems 405

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Fig. 11. Comparison of the received intensities of the mid-IR (.\ = 8.1 jlm) and the near-IR(.\ = 0.85 jlm) link as a function of time. Fog came up around 2AM and progressively dissipatedduring the measurement.

laser technology continue to improve in the mid-infrared spectral range, this result islikely to be a driver for commercial development of such links.

Acknowledgments

The work reported in this chapter represents the re sults of collaborative effort s with

the Bell Laboratories-Lucent Technologies Quantum Cascade Laser group (F. Capasso, C. Gmachl, R. Paiella, H.Y. Wang, D. L. Sivco, IN . Baillargeon, A.Y.Cho), theremote sensing group at Pacific Northwest National Laboratory (W .w . Harper, Y-F.Su, J.F. Shultz), and H. C. Liu from National Research Council of Canada and wegratefully acknowledge the contributions of these collaborators. Portions of this workwere supported by the United States Department of Energy under contract DE-FG0399NVl3656 and by the US Army/DARPA under contract DAADI9-00-C-0096. R.Martini acknowledges the support of Bell Laboratories-Lucent Technologies duringthe early portions of this research.

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I I . Liu, H.C., Lin, J., Buchanan, M., and Wa silewski, Z.R.,: High-Frequency Quantum-WellInfraredPhotodetectors Measured by Microwave-RectificationTechnique, IEEEJ. QuantumElectron., 32, 1024-1028(1996).

12. Martini,R.,Gmachl,C.,Falciglia, J., Curti,EG., Betheae.G ., Capasso,E, Whittaker, E.A.,Paiella, R., Tredicucci, A., Sivco, D.L., and Cho, A .Y., High-speed modulation and freespace optical audio/video transmission usingquantumcascade lasers,Electron. Lett., 37,111-112 (2001).

13. Blaser, S., Hofstetter, D., Beck,M., and Faist,J., Free-space opticaldata link usingPeltiercooledquantumcascade laser,Electron. Lett., 37, 778-780 (2001).

14. Martini, R., Gmachl, Paiella, R., Capasso, E, Whittaker, E.A., Liu, H.C., Hwang, H.Y.,Sivco, D.L., Baillargeon, J.N., and Cho, A.Y, High-speed digitaldata transmission usingmid-infrared QuantumCascade lasers, Electron. Lett., 37, 1290-1291 (200 I).

chap 8 quantum cascade laser-based free space optical communications

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