Free-space optical communication using mid-infrared or ...cqd.ece.northwestern.edu/pubs/files/ac0c446ac12b69... · networks compared to traditional wired or fiber-optic networks include

Free-space optical communication using mid-infrared or solar-blind ultraviolet sources and detectors

Ryan McClintock, Abbas Haddadi, and Manijeh Razeghi*

Center for Quantum Devices, Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, IL USA 60208

ABSTRACT

Free-space optical communication is a promising solution to the “last mile” bottleneck of data networks. Conventional near infrared-based free-space optical communication systems suffer from atmospheric scattering losses and scintillation effects which limit the performance of the data links. Using mid-infrared, we reduce the scattering and thus can improve the quality of the data links and increase their range. Because of the low scattering, the data link cannot be intercepted without a complete or partial loss in power detected by the receiver. This type of communications provides ultra-high bandwidth and highly secure data transfer for both short and medium range data links. Quantum cascade lasers are one of the most promising sources for mid-wavelength infrared sources and Type-II superlattice photodetectors are strong candidates for detection in this regime.

The same way that that low scattering makes mid-wavelength infrared ideal for secure free space communications, high scattering can be used for secure short-range free-space optical communications. In the solar-blind ultraviolet (< 280 nm) light is strongly scattered and absorbed. This scattering makes possible non-line-of-sight free-space optical communications. The scattering and absorption also prevent remote eavesdropping. III-Nitride based LEDs and photodetectors are ideal for non-line-of-sight free-space optical communication.

Keywords: free-space, communications, infrared, Type-II, quantum cascade laser, ultraviolet, solar-blind, III-nitrides

1. INTRODUCTION Free-space optical communications (FSO) is the transmission of data, voice, video, etc. through the atmosphere via a modulated emitter, typically a light emitting diode (LED) or a laser, and received by a photodetector, typically a photodiode, avalanche photodiode, or a photo-multiplier tube (PMT) device. The advantages of free space optical networks compared to traditional wired or fiber-optic networks include the lack of a need to run copper or fiber-optic cable and the ability to much more rapidly set up and change ad-hoc networks as needed. In these regards, free space communications is similar to existing radio communications. However, being optical rather than radio-frequency (RF) based there are no licensing concerns. The optical portion of the electromagnetic spectrum is unlicensed; thus the link can be established without the need for any FCC (Federal Communications Commission) or governmental licensing, unlike microwave or other RF technologies, hastening the implementation time and ability to relocate the network when, and if, necessary. Free space optical communications also have the added benefit that unlike RF systems they are highly directional or otherwise limited in range. This provides a high degree of security from un-detected interception by a third party.

Commercially, free-space optical communications is an ideal solution to the “last mile” communications problem. Existing high-bandwidth wired and fiber-optic data networks terminate in communications hubs. Connecting individual buildings to these hubs via wired connections requires major infrastructure projects, often at prohibitively high costs. In contrast, free-space optical communications merely requires a clear line and the installation of a small transceiver at each end of the link. The same technology can also be used for intranets—many institutions in urban environments tend to have offices in close proximity to one another (i.e. universities, financial institutions, the media, consulting firms, etc.) but existing high-speed data networks are either non-existent or are susceptible to eavesdropping on shared networks. A dedicated, non-shared, optical link would provide the needed security and data rates for such a situation.

Invited Paper

Quantum Sensing and Nanophotonic Devices IX, edited by Manijeh Razeghi, Eric Tournie, Gail J. Brown, Proc. of SPIE Vol. 8268, 826810 · © 2012 SPIE · CCC code: 0277-786X/12/$18 · doi: 10.1117/12.913980

Proc. of SPIE Vol. 8268 826810-1

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For the military, data infrastructure plays a critical role in control and dissemination of information on the battle field. Satellite communications is high latency, and the available bandwidth is limited. The ability to rapidly build ad-hoc optical networks is highly needed. The use of a point-to-point optical link instead of an RF link avoids interception of communications and reduces the probability that the source and destination of the communications link can be externally detected. The design and implementation of an outpost to outpost communications link mirrors that of a commercial last-mile link. However, by adopting a non–line-of-sight approach to the optical link, free-space optical communications can avoid the restriction necessary to establish a line of sight. Non–line-of-sight (NLOS) free-space optical communications system can be used to replace squad radios. 1,2

Existing free-space communications systems are primarily based on near-infrared wavelengths. However, as discussed above going to shorter or longer wavelengths. The technology of high-performance mid-infrared sources and detectors has grown steadily over the past few decades. One of the major limitations at these wavelengths has been the need to cool the laser and/or source; however recent innovations have lead to high performance uncooled mid-IR sources and detectors that not make mid-IR free space communications feasible. Similarly, tremendous recent advances in the area of solar blind sources and detectors have made the idea of compact portable UV communications technology a near reality.

2. CURRENT TECHNOLOGY The current state of the art in free-space optical communications is based around un-cooled near-infrared sources and detectors. Such sources and detectors are readily available at low costs due to the use of near-infrared for commercial fiber-optic communications. Realizing the benefits and advantages of a free-space communications technology, industry has jumped at the opportunity to provide free space optical communications. Commercial turn-key communications systems are available from a number of different manufacturers. These systems typically use high power LEDs or laser diodes operating in the near-infrared range, ~0.8 to 1.55 μm, with output powers of ~100 mW. In applications requiring higher transmitter output powers, such as long distance links (greater than 3000 m), higher powers of multiple lower power emitters are used in unison to overcome atmospheric attenuation and scattering.

Within the near-infrared spectral range an additional concern is the eye safety of the laser. Although very high power 780 to 910 nm diode laser are readily available this portion of the near-infrared wavelength range poses an extreme eye hazard due to the cornea’s ability to transmit at these wavelengths and focus IR radiation on the eye causing damage to the retina. In contrast 1.55 μm is absorbed by the cornea reducing the potential for eye damage; this is the wavelength range already being used for fiber optic communications, and thus has seen a tremendous investment in research and development. The same diode laser sources used for fiber optic communications can be adopted for free space communications. As such, more recent commercial systems have adopted 1.55 μm semiconductor laser didoes as the near-infrared source.

On the receiver side, typically a silicon p-i-n photodiode or avalanche photodiode (APD) is used to achieve the required sensitivity and signal to noise ratios. Usual detector sensitivities, in the near-infrared wavelength range are approximately −45 to −10 dBm for APDs and –35 to +20 dBm for p-i-n photodiodes, depending upon the operational conditions (i.e. detector temperature, operating bandwidth, bias voltage, etc.).

Table 1 shows typical ranges and bandwidths for commercially available free space communications systems operating at 1.55 μm emission wavelength.

Transmission

Standard Data Rate (Mbit/s)

Clear Air Range (m)

Extreme RainRange (m)

Laser Power (mW)

OC-1/STM-0 51 50 to 3850 50 to 1800 100 OC-3/STM-1 155 50 to 4450 100 to 2000 320

OC-12/STM-4 622 100 to 3800 100 to 1720 280 Gig-E 1064 100 to 3600 100 to 1720 280

Table 1. Typical ranges and bandwidths for commercially available free space communications systems operating in the near infrared (1.55 μm).



Systems with even further operating ranges are possible by increasing the laser emitter power and significantly increasing the size of the receiving optics. However, there are a number of major limitations to the existing technology.

3. CHALLENGES WITH EXISTING TECHNOLOGY Companies desiring to become involved in this emerging field used existing telecommunications lasers and detectors for quicker integration and time to market. While providing a viable alternative for secure, freely-configurable, high-speed communications, fundamental physical properties limit the performance and practicality of current commercially available systems operating at these wavelengths.

The range of free space communications systems is limited by the atmospheric attenuation of the transmitted laser power. Below a finite power level, the receiving detector is unable to adequately process the received radiation under a specified bit error rate (BER). For a useful data link the BER should be 10-9 or better, which corresponds to a detector signal to noise ratio (S/N) of 6. Commercial, near-infrared systems overcome this limitation by increasing the output power of the emitted signal. An alternative solution is to choose an alternative wavelength where atmospheric attenuation is reduced and longer ranges can be achieved for a given output power.

3.1. Absorption and Scattering Atmospheric attenuation arises due to absorption and scattering effects. Attenuation via absorption occurs from molecules suspended in the atmosphere which contain vibrational and rotational modes consistent with the wavelength of propagating light. Carbon dioxide (CO2) and water (H2O) are the main absorbing molecules present in most environments. Moving from the NIR towards higher wavelengths in the MWIR or towards lower wavelengths in the ultraviolet (UV) does not offer any appreciable improvement in terms of reduced molecular absorption as long as we stay within the main atmospheric transmission windows.

However, scattering is a physically selective process where relative dimensions between particle size and wavelength dictate the degree of scattering and overall attenuation. The three main atmospheric scattering mechanisms are Rayleigh, Mie, and non-selective. Rayleigh scattering occurs when light is incident upon particles with radii much less than the wavelength of radiation. The Rayleigh scattering cross-section can be expressed as:

Equation 1

where f is the oscillator strength, q is the electron charge, λ0 is the molecular natural wavelength, ε0 is the permittivity of free space, m is the electron rest mass, c is the speed of light, and λ is the wavelength of incident radiation. One observes that there is a very strong dependence of the Rayleigh scattering cross-section, and thus the resulting attenuation, on wavelength. This causes Rayleigh scattering to dominate in the visible and ultraviolet; however it is negligible compared to other scattering mechanisms in the near-infrared and mid-infrared and can usually be ignored at those wavelengths.

In the infrared, Mie scattering is considered to be the main source of scattering attenuation, and it dominates infrared propagation through the atmosphere. Mie scattering is caused by water molecules and aerosol particles, which are always present in the atmosphere and have radii comparable to the wavelength of the scattered light. The theory behind Mie scattering is relatively complex, but at the same time, well understood. The theory includes the particle size, shape, dielectric constant, and absorptivity. Where the uncertainty arises is in calculating reliable scattering coefficients because of the widely variable distribution of particle sizes and concentrations depending upon the operating environment. However, an empirical relationship has been developed3 and is used readily in specifying FSO systems and results in reasonable numbers for the scattering coefficient. The relation is expressed as:

Equation 2



where β is the Mie scattering coefficient, and C1 and δ are constants which depend upon aerosol concentration, size, and corresponding distributions. A relationship for δ was empirically developed and is expressed as: :

Equation 3

where V is the visibility in kilometers. δ has been shown to vary from approximately 1.3 to 1.6 depending upon the visibility conditions. Therefore, even in the absence of the strong wavelength dependence of Rayleigh scattering, there are absorption related advantages to operating in the mid-wavelength infrared compared to the near infrared.

In Figure 1 the Rayleigh scattering, Mie scattering, and the composite scattering coefficients are plotted as a function of wavelength. In order to calculate the scattering coefficients for Mie and Rayleigh scattering, values for the empirical variables in equations (1) and (2) were chosen such that the calculated values were reasonably close to what could be expected for an actual system.

Figure 1. Approximate Rayleigh (---) and Mie (···) scattering effect coefficients are plotted as a function of wavelength.

From Figure 1, it becomes clear that systems utilizing longer operating wavelengths can take advantage of reduced scattering coefficients for increased system performance. Moving from 800 nm to 1.55 μm there is a dramatic reduction in scattering based upon the coefficients chosen above due to the reduction of Rayleigh scattering. Moving from 1.55 μm to 5 μm, or longer, we see a further reduction. However, as noted above the slope of the Mei scattering curve (plotted on a log scale) is highly dependent on the visibility. Unlike in an optical fiber where the absorption and scattering are fixed, the visibility and thus scattering of free space are affected by the weather. Moving to longer wavelengths has a dramatic effect on the performance of the system during ‘extreme rain’ type conditions.

From Figure 1, it also becomes clear that systems utilizing shorter wavelengths will see dramatically more scattering. In the ultraviolet, Rayleigh scattering dominates, and we see relatively little contribution from Mie, or aerosol scattering. This makes ultraviolet a poor choice for long distance line-of-sight communications. However, this large fixed scattering is ideal for non-line of sight communications (NLOS). In NLOS communications we can point both the detector and the emitter at the sky, and use the Rayleigh scattered light to communicate even if there are obstacles preventing a direct line of sight. This allows for new approaches to setting up free-space optical communications networks.



3.2. Eye Safety As with all lasers in the visible region, eye safety must be kept in mind during the design process to keep humans safe from harmful laser radiation. The powers used for free space communications are generally not high enough to cause burns on skin or surrounding structures but for wavelengths less than 1.4 μm only a few milliwatts of laser radiation directed into the eye is enough to severely harm or blind someone since these wavelengths are focused by the cornea and can thus easily damaging the retina. Though the probability of a well-aligned laser beam entering a human eye is small, the liability is still present for lasers operating below 1.4 μm, which could pose a hazard in the event that any stray reflections arise. Light from ‘eye-safe’ lasers, however, is absorbed in cornea and the fluid within the eye before it is focused on the retina where it can cause permanent damage. Regardless, guaranteeing a perfectly eye-safe system versus one that is “mostly safe” has its benefits to the consumer. Mid- and long-wavelength infrared lasers, operate within the eye-safe range making them an ideal choice for a safer free-space communications system.

In the ultraviolet, typically diffuse sources are used rather than collimated laser beams. This significantly reduces the eye hazard even though these wavelengths are not generally seen a being ‘eye-safe’. However, similar to in the infrared, once we go below ~300 nm the human cornea begins to absorb heavily and prevents ultraviolet light from reaching the retina. When coupled with the fact that the ozone layer absorbs nearly 100% of the sun energy at wavelengths less than 280 nm this makes <280 nm ideal wavelength range for NLOS ultraviolet communications.

4. DEVELOPMENT OF A MID-IR FREE-SPACE COMMUNICATIONS SYSTEM At the Center for Quantum Devices we have already demonstrated both the mid-wavelength infrared emitters and the mid-wavelength infrared detectors that are required to take full advantage of mid-infrared free space communications.

4.1. High Power, Un-Cooled, Mid-Infrared Quantum Cascade Lasers Since the invention of Quantum Cascade Lasers (QCL),4 their small size, wide wavelength range (3-160 μm), and high operating temperatures have positioned them well for use in free space communications systems. The CQD has led the world in the development of 100% duty cycle (continuous-wave)5 QCLs with high power6 and high efficiency7, which is essential for free-space applications. CW powers over 2.4 W at 300 K have been reported6 for mid-infrared QCLs with record efficiencies of nearly 27% at room temperature7 (300 K) and 50% when cooled8. In addition, the CQD was the first group to demonstrate high power continuous wave operation of QCLs at temperatures as high as 90 °C, which exceeds the operating temperature requirements for existing telecom and military applications.9 Recent lifetime and reliability studies10,11 at the CQD have demonstrated quantum cascade lasers with 4.6 μm emission wavelengths operating continuously at high powers (>150 mW) for over 18,500 hours (over 2 years).

The QCL used in the transmitter module emits at a wavelength of 4.8 μm and produces over 200 mW of output power at room temperature at a DC drive current of 0.75 A and an operating voltage of 12 V. Active heatsinking is required to stabilize the laser wavelength and dissipate the ~9 W of heat generated during laser operation, which is accomplished using a thermoelectric cooler and forced air copper heatsink to keep the laser at room temperature. A ZnSe aspheric optic provides a collimated output beam ~3 mm in diameter. Modulation of the output beam is performed by biasing the laser below threshold and coupling in a modulated signal using a DC bias tee.

4.2. High Declivity, Un-Cooled, Type-II Photodetectors The CQD has also achieved a number of world’s firsts in demonstrating photodetectors based on type-II InAs/GaSb superlattice photodiodes, initially proposed by Nobel laureate Leo Esaki in 1977.12 Type-II strained layer superlattices (SLS) have been demonstrated to be a promising alternative to current state-of-the-art Mercury Cadmium Telluride (MCT) photodetector technology throughout the infrared spectrum. The Center for Quantum Devices was the first in the world to demonstrate imaging from a room temperature operating, mid-wavelength infrared focal plane array based on type-II SLS.13,14

The un-cooled detector we use is one of our typical Type-II InAs/GaSb superlattice photodiode designs with a cutoff wavelength of ~5 μm. Single element detectors fabricated from this material were thoroughly characterized at room temperature and exhibited a specific detectivity, D*, of ~1 × 109 cm⋅Hz1/2W−1. Operating at a bandwidth of 155 Mbps



and a bit error rate (BER) of 1 × 10−9 (S/N = 6) this corresponds to a sensitivity of -30 dBm. This non-optimized device compares quite favorably to avalanche photodiodes (APDs) operating at 0.91 - 1.55 μm with a sensitivity of −45 dBm. Typical performance characteristics for these photodiodes operating at room temperature are shown in Figure 2.

Figure 2. Device performance characteristics of room temperature operating MWIR Type-II InAs/GaSb superlattice photodiodes.

4.3. Type-II Detector Response Speed For an initial proof of concept of high speed operation we used a quantum cascade laser, operating at room temperature, as a high-speed source of IR radiation at λ ~ 5 μm to study the response time of the uncooled Type-II devices. The detector used had a cutoff wavelength of ~8 μm. Figure 3 shows the current of the QCL as well as the output of the preamplifier versus time. The inset shows the schematic diagram of the measurement setup. The laser threshold current marks the current above which the laser starts emitting. Considering the fall time of the pre-amplifier (~50 ns) and the fall time of the output signal of the pre-amplifier (~110 ns), the detector response time was about (1102–502)1/2 = ~100 ns using the sum-of-squares approach. Such a detector response time would support data transmission rates of up to 10 Mbps. It should be noted that this laser and detector setup was not optimized for high speed performance and parasitic delays are believed to be arise from this particular measurement setup when the modulation frequency increases above ~1 MHz. Additionally, this detector was optimized for room temperature operation at this far extent of the MWIR regime and a detector with a shorter cutoff wavelength, optimized for high-speed data transmission is expected to allow for significantly higher bandwidth.

Figure 3. The current of the QCL laser (right axis) and the amplified output of the detector (left axis) versus time. Inset shows the measurement setup. Note that the laser emits only when the current is above the threshold level (dashed line).



4.4. Prototype System Development Following this successful laboratory demonstration, the next step was to take the laser and detector off of the lab bench and test their robustness in a demonstration system. For simplicity of the prototype, passive cooling was chosen for the detector module, and the laser was thermally stabilized at 21 °C with a thermoelectric cooler. Using a high power, high efficiency, laser source this allows for the quasi continuous wave duty cycles necessary for communications. For the initial demonstration a simple unidirectional system was designed and fabricated, as pictured below in Figure 4.

Figure 4. Prototype modular mid-infrared free-space communication system developed at the Center for Quantum Devices setup for laboratory testing on a 3 meter-long optical rail. The system is comprised of a room temperature continuous wave QCL transmitting module (left), and an un-cooled Type-II photon detector module (right). A small communications and control module for each endpoint (front) allows for setting up the communications link and verifying status via the 16x2 liquid crystal character display.

This prototype featured active alignment and automatic beam finding routines, features typically found only in the most advanced commercial systems. The receiver prototype consists of a stepper motor driven X-Y stage controlled by an 8-bit microcontroller which also handles all of the multiplexing, signal demodulation, user interface, etc. The received laser signal is amplified by a trans-impedance amplifier (TIA) circuit with fixed gain. The modulated signal can be serially decoded and the received data is output to an LCD screen or a standard RS-232 interface. Rough system alignment is done by using a red diode laser on the transmitter side to aim at a target on the receiver. As long as the red laser spot is within a specified target envelope, the automatic beam finding algorithm programmed into the microcontroller will handle the fine alignment in order to locate and track the infrared communications laser. No collection optics have been integrated the receiver side yet, but because of the low beam divergence and high laser power optics are not needed to illuminate the 1 mm × 1 mm detector element. In practice, typically the beam is intentionally expanded or diverges naturally and this is taken advantage of to reduce the demands on the tracking algorithm and to accommodate for building sway due to the wind, vibration, or thermal expansion. The drawback is that the power throughput is significantly decreased as the beam diameter increases, decreasing the effective range of the channel. This broadening of the beam is somewhat avoided with the use of an actively pointed system which will self-align itself.

5. DEVELOPMENT OF A UV NON-LINE OF SIGHT COMMUNICATIONS SYSTEM

The ultraviolet portion of the optical spectrum, especially the solar-blind part (λ < 280 nm), is a unique strategic window for a number of commercial and military applications. UV light emitted from the sun at wavelengths less than



U

280 nm is strongly absorbed by ozone15 in the upper atmosphere.16 This means than little or no solar radiation reaches the earth surface at wavelengths less than 280 nm. At the same time, unlike in the mid-wavelength infrared, there is also negligible passive background at these wavelengths. This means that a 280 nm detector, even if pointed at the sun, will not record any signal under nominal conditions. This allows for very sensitive detection of 280 nm light arising from a man-made source.

In addition to the naturally very-low background at wavelengths less than 280 nm, any 280 nm light that may be generated terrestrially is very strongly scattered as shown above in Figure 1. This limited propagation means that the source must be relatively close to the detector, and provides immunity to remote interference. At the same time, this strong scattering can be taken advantage of to provide ultraviolet non-line of sight (UV-NLOS) optical communication.

UV-NLOS communications is a secure means to send data using low-power UV sources and sensitive detectors, and relies on the strong back-scattering of UV and very low natural background. The basic operation of such a system is illustrated in Figure 5 below.2 The emitter is pointed towards the sky and the UV light is scattered back towards the earth; due to the short wavelength UV is scattered more strongly than other wavelengths. This makes the system ideal for use where a direct line-of-sight cannot be established, such as in dense terrain or in an Urban-canyon environment when presence of tall cement and iron buildings would make radio communications difficult.

UV LEDλ<280nm

UV Detectorλ<280nm

Rayleigh Scattering

10–250 m

Obstacles

Figure 5. Illustrations of the operation of secure UV-based NLOS communication system.

In addition, due to the strong extinction coefficient (high scattering and high absorption) of air in this range, it is difficult to detect these signals from a long distance. The UV signal is almost completely extinguished after a distance of several hundred meters making this a secure covert means of communication. This makes eavesdropping on the NLOS communication’s UV signal very difficult from any significant distance, particularly in the forward direction: this is in marked contrast to conventional RF which can travel thousands of miles or more. Therefore, a combination of UV LEDs and UV photodetectors can be used for short-range secure communications. This is particularly important in areas where there are no means of RF communications, or where RF communications are unreliable, give away your position, or are prone to eavesdropping.

UV optoelectronic devices based on III-nitride compound semiconductors are the most eligible candidates for such applications, mainly due to their small size, robustness, and potentially high efficiency. Both the UV detectors and the UV sources necessary for such a system can be readily realized in the III-Nitride material system making it an ideal



choice for the development of a compact secure portable communication system. In the past few years, a number of groups have demonstrated UV LEDs and photodetectors based on the III-nitride material system.

5.1. 280 nm UV Sources In order to achieve short wavelength emission, a high aluminum mole fraction is necessary, which makes the growth and doping of the material very difficult. Despite the problems associated with growth of high Al-content AlGaN films, the Center for Quantum Devices (CQD) has reported deep UV LEDs emitting at the critical wavelength of 280nm, as well as LEDs emitting at even shorter wavelengths of 265nm and even as short as 250nm (Figure 6).

200 250 300 350 400 450

EL In

tens

ity (a

.u.)

Wavelength (nm)

280 nm

200 250 300 350 400 450

EL

Inte

nsity

(a.u

.)

W ave length (nm )

265 nm

200 250 300 350 400 450

EL

Inte

nsity

(a.u

.)

W avelength (nm )

250 nm

Figure 6. Room temperature electroluminescence from deep-UV LEDs at 280nm, 265nm, and 250nm.

The active region of these devices consists of multiple AlGaN-AlGaN quantum wells capped with a high aluminum composition AlGaN:Mg current blocking layer on the p-side. This asymmetric design helps to compensate for the lower mobility of holes and yields improved performance. Low temperature quaternary AlInGaN:Si is used as the bottom conductive n-type contact layer. P-type AlGaN is used on top of the device and caped with a thin p-type GaN layer to help in the formation of ohmic contacts.

250 260 270 2800

1

2

3

4

5

6

Pulsed power of LEDsDrive current = 700 mA

Pow

er (m

W)

Wavelength (nm)

Figure 7. Pulsed power output from arrays of 4 deep UV LEDs as a function of the wavelength.

For the 280 nm devices, the dominate electroluminescence peak occurs at 280 nm, with a FWHM of ~10 nm. The pulsed power (200 ns pulses, at 200 Hz) of an array of four 300 μm x 300 μm devices reaches 5.6 mW.17 Similarly, the CQD was the first group to report deep UV LEDs at the critical wavelengths of 265 nm and 250nm, with powers of 5.3 mW and 2.7 mW, respectively (Figure 7).18

5.2. 280 nm UV Photodetectors By developing a novel atomic layer epitaxy technique the CQD was able to realize very high quality transparent AlN templates with an RMS roughness of only 1.3 Å for a 5 μm × 5 μm scan size, and X-ray diffraction with a FWHM of



only 62 arc-seconds for the symmetric (00.2) peak.19 By using a novel Al0.5Ga0.5N silicon-indium co-doped layer as the bottom contact carrier concentrations of n ~ −5×1018 cm−3 and mobilities of μ ~ 60 cm2/V·s are obtained. This allowed us to report the highest quantum efficiency back-illuminated AlGaN based Solar-Blind p-i-n photodetectors with a record peak responsivity of 150 mA/W at 280 nm, corresponding to a high external quantum efficiency of 68%, increasing to 74% under 5 volts reverse bias.20

220 240 260 280 300 320 340 360 380

10-3

1x10-2

1x10-1

100

101

102

250 260 270 280 290 3000

10203040506070 Un-Biased

External Q.E.68% @ 280 nm

Exte

rnal

Qua

ntum

Effi

cien

cy

Wavelength (nm)

Un-Biased Peak Responsivity150 mA/W @ 280 nm

Res

pons

ivity

(mA

/W)

Wavelength (nm)

200 225 250 275 300 325 350 375 400 425 450

10-6

1x10-5

1x10-4

10-3

10-2

10-1Un-BiasedPeak Responsivity95.7 ma/W @ 255 nm

Res

pons

ivity

(A/W

)

Wavelength

Figure 8.Left.)Responsivity vs. wavelength, showing a peak responsivity of 150 mA/W at a wavelength of 280 nm; this peak responsivity corresponds to an external quantum efficiency of 68% (inset). Right.) Responsivity vs. wavelength, showing a peak responsivity of 95.7 mA/W at a wavelength of 255 nm.

The CQD has also successfully demonstrated back-illuminated photodetectors with a peak response as short as 255 nm (Figure 8, right). This device shows an unbiased peak responsivity of 95.7 mA/W at 255 nm with a FWHM of ~7 nm, which corresponds to a value of 46.5% for the external quantum efficiency of the device. The absolute response drops three orders of magnitude from the peak into the near-UV region.19

6. CONCLUSION Free space communications is a proven technology for secure, high bandwidth communications that can be operated without a license, discretely, and with rapid setup and installation. For fundamentally physical reasons atmospheric attenuation of the beam in adverse conditions such as snow, rain, and fog is expected to decrease as one moves further into the infrared. This makes mid-infrared a better choice for next generation free-space optical communications systems. We have used our unique capabilities in the mid-infrared to demonstrate a prototype free-space communication system. At the same time, the higher scattering in the near-IR can become a benefit if we move all the way to the solar-blind ultraviolet. We have discussed an idea for a secure non-line of sight communications system operating at wavelengths below 280 nm.

ACKNOWLEDGEMENTS The authors would like to acknowledge Dr. Andrew Hood, and Dr. Allan Evans for their work fabricating the prototype mid-infrared free-space communication system electronics.

Proc. of SPIE Vol. 8268 826810-10


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