The low complexity, low size, weight and power Mid-Wavelength Infra-Red optical communications transceiver concept presented, realized and tested in the laboratory environment. Resilience to atmospheric impairments analyzed with simulated turbulence. Performance compared to typical telecom based Short Wavelength Infra-Red transceiver

Choa, Fow-Sen

Krainak, Michael

Luzhanskiy, Edward

Merritt, Scott

Yu, Anthony

NASA Technical Reports Server

Low size, weight and power concept for mid-wave 
infrared optical communication transceivers based on 
Quantum Cascade Lasers  
EDWARD LUZHANSKY,1,2,* FOW-SEN CHOA,1 SCOTT MERRITT2, ANTHONY YU2 AND 
MICHAEL KRAINAK2 
1 University of Maryland Baltimore County, 1000 Hiltop Circle, Baltimore, MD 210250 
2 NASA, Goddard Space Flight Center, 8800 Greenbelt Rd., Greenbelt, MD 20771 
* Corresponding author: luzhaed1@umbc.edu 
Received XX Month XXXX; revised XX Month, XXXX; accepted XX Month XXXX; posted XX Month XXXX (Doc. ID XXXXX); published XX Month XXXX 
The low complexity, low size, weight and power Mid-Wavelength Infra-Red optical communications transceiver concept presented, realized and 
tested in the laboratory environment. Resilience to atmospheric impairments analyzed with simulated turbulence. Performance compared to 
typical telecom based Short Wavelength Infra-Red transceiver. 
OCIS codes: (140.5965) Semiconductor lasers, quantum cascade; (010.1330) Atmospheric turbulence; (060.2605) Free-space optical 
communication 
http://dx.doi.org/10.1364/AO.99.099999 
1. INTRODUCTION 
Free Space Optical Communication (FSO) 
technology through multiple demonstrations has 
proved to be a viable solution for high data rate, 
always spectrum available and low probability of 
detect and intercept demanding applications.  
FSO community demonstrated solutions to all 
technical challenges including micro-radian level 
pointing and atmospheric turbulence correction [1]. 
The remaining critical hurdle to trigger wide spread 
of FSO systems in commercial and military 
applications is system’s cost effectiveness. To 
accomplish this FSO systems shall be simple and 
efficient. Elimination of complex and power 
“hungry” elements of the system will support an aim 
for low cost solutions.  
 Atmospheric attenuation and scintillation play 
significant role to the health of FSO links. Mid-
wavelength infrared (MWIR) region (3 um – 5um) 
has a superior transmission through atmosphere 
compared with that of Visible, Near-Infrared (NIR), 
Short-wavelength infrared (SWIR) and even Long 
Wavelength Infrared (LWIR) [2,3].  Compared with 
NIR and SWIR it has better penetration through fog 
and therefore shall enable better links availability 
for tropical and marine locations [4,5]. Moreover, 
atmospheric turbulence has lesser impact on MWIR 
links. Therefore Adaptive Optics is not required for 
most applications [6,7]. Also, diffraction limited 
MWIR systems requiring less sophisticated 
pointing stability sub-systems due to larger beam 
divergence. Lastly, MWIR terminals can transmit 
more power and stay below Maximum Permissible 
Exposure (MPE) threshold to avoid damaging 
biological effects to the eye or skin [8].   
 Since 1970s there was an aim to develop longer 
wavelength FSO systems. Those systems were 
based on bulky, high power consuming and 
expensive gas lasers and cryogenically cooled 
detectors [9]. The recent accelerated advances in 
Quantum Cascade Lasers (QCLs) as well as 
progress in mercury cadmium telluride (MCT)  
photo-diodes enable low Size, Weight and Power 
(SWaP) and therefore cost efficient optical systems 
operating in MWIR and LWIR spectrum regions 
[10].  Moreover, it is feasible to operate Quantum 
Cascade Lasers at room temperature without 
cooling [6,7]. 
https://ntrs.nasa.gov/search.jsp?R=20150021900 2019-08-31T05:19:37+00:00Z
 Pulse Position Modulation (PPM) is an efficient 
modulation format for “photon starved links”. With 
the high order M-ary format it is possible to reach 
high peak power at moderate average power levels 
[11]. PPM enables operation of high peak power 
QCLs at room temperature, therefore increasing 
wall-plug efficiency of communication system [12]. 
Our MWIR system operates with 32-ary PPM signals 
reaching sub-tenth percent duty cycle. An uncooled 
photoelectromagnetic Mercury Cadmium Telluride 
detector was used to complete the low SWAP 
communication transceiver model system.  
 In this paper we investigate MWIR communication 
concept which promises to eliminate SWaP driving 
thermal management and wavefront correcting 
system components. Performance of MWIR is also 
compared to traditional SWIR system in the 
presence of simulated atmospheric turbulence. 
Anticipated size, weight and power of FSO system is 
analyzed. Sample link analysis demonstrating 
feasibility of this concept for real applications is 
presented. 
 
2. TRANSCEIVER CONCEPT 
Our communication transceiver concept is based on 
the premise of low complexity approach with 
minimum utilization of power “hungry” and “bulky” 
components. Elimination of the need for transceiver 
cooling and atmospheric correction (adaptive 
optics) enables efficient and low cost free space 
communication concept. 
A. Uncooled Quantum Cascade Laser 
Uncooled QCL installed in custom copper mount. 
Low duty cycle operation (for high order PPM) 
enables room temperature operation without 
cooling. The transmitter is a directly modulated 
Quantum Cascade Laser with distributed feedback 
(DFB) structure needed to achieve single mode 
operation. Because of QCL’s low alpha parameter, 
the adiabatic chirp caused by direct modulation is 
negligible. Due to QCL’s wide angle emission a large 
size hemispherical lens is used to capture the laser 
output.  
B. Photoelectromagnetic receiver 
Photoelecromagnetic effect enables the operation of 
Mercury Cadmium Telluride (MCT) based receiver 
in uncooled condition. With a D* of ~1e-9 [13] the 
required power to close the link is ~1 uW. 
 
Figure 1 Photoelectromagnetic Detector from Vigo System  
C. Size, Weight and Power 
It is expected that transceiver will weigh about 90 
grams, will be less than 60 cubic cm and draw less 
than 5 watts of power.  The breakdown of these 
parameters is given in the  
 Size  
(cm3) 
Weight 
(grams) 
Power 
(Watts) 
Transmitter 1 20 1.5 
Receiver 8 50 0.2 
Drive 
Electronics 
50 50 3 
Total 
Packaging 
59 90 4.7 
Table 1 Size, Weight and Power breakdown of MidIR Transceiver 
 
D. Expected Link Analysis 
To support low size and weight approach we‘ve 
assumed 1 cm diameter transmit and receive 
apertures for link analysis. Having small apertures 
also helps to simplify pointing system to tenth of 
milljradian stability. We assumed simple pointing 
system will be able keep pointing error penalties 
under 1 dB. For the link we’ve selected a typical 10 
km limit for horizontal terrestrial and marine 
applications where coherence length r0 parameter 
could be substantially low (3 cm in our case).  
We have chosen 5.1 W of peak power as it was 
demonstrated by Razeghi group [12] in 2014. 
Because of use of large area MWIR receivers we can 
claim fairly low fading losses (0.5 dB). Scintillation 
losses are low as well. Somewhat large transmit and 
receive coupling (static) and implementation losses 
associated with the aim for simple inexpensive 
transceiver solution.  
Our link analysis points to the “healthy” 3 dB margin 
on 10 km MidIR communication links through the 
“harsh” atmospheric link (Figure 2). 
 
Figure 2 Notional Link Analysis for Mid-IR Free Space Optical 
Communications link. Wavelength – 4.1 um, distance – 10 km,    
data rate – 100 Mbps, r0 ~ 3.0 cm 
3. EXPERIMENTAL SETUP 
A. Atmospheric Turbulence Simulator 
In our setup we simulated optical turbulence with a 
simple approach described by Sandrine Thomas 
[14]. Optical phase plate was prepared by coating a 
2-inch Si window with resin material and installed 
it into commercial rotary stage [15]. Resulting 
phase screen was characterized by measuring the 
value of r0 with the optical transfer function 
method similar to the one outlined in [16].  
 
 
Figure 3 Snapshot of QCL beam through prepared phase plate 
 
B. Measurement Concept 
A bonded QCL die was installed into custom copper 
mount with no cooling. QCL emission was captured 
with hemispherical lens and directed to photo-
detector. Turbulence simulator (rotating phase 
plate) was placed in the collimated space of the QCL 
beam in off-axis position. Perturbed by simulated 
turbulence the beam was focused on an uncooled 
photoelectromagnetic Mercury Cadmium Telluride 
detector from Vigo Systems (see Figure 4). 
SWIR system was comprised of fiber pigtailed 
telecom DFB laser and LiNbO3 modulator and pin 
detector. Optical collimators were used to collimate 
transmit beam and couple received beam into fiber 
(Figure 4). SWIR beam was perturbed with phase 
plate in the same fashion as MWIR beam. 
 
 
Figure 4 Measurement Setup 
PPM signals were generated by Arbitrary Waveform 
Generator. 32-ary PPM modulation format was 
chosen to keep QCL duty cycle operation at few 
percent, therefore minimizing QCL thermal effects 
and enabling room temperature operation without 
Tx laser power 7.1 dBW
Static Tx optical losses -1.5 dB
Tx Pointing losses -1.0 dB
Tx antenna gain 86.1 dBi
Isotropic loss -102.0 dB/m2
Atmospheric loss -0.3 dB
Scintillation loss 0.5 dB
Rx aperture area -41.0 dB/m2
Static Rx optical losses -1.5 dB
Dynamic Fading losses -0.5 dB
RX Comm implementation loss -2.6 dB
Received Power -56.7 dBW
Receiver Sensitivity -60.0 dBW
MARGIN 3.3 dB
LINK SUMMARY
Transmitter
Receiver
Range
Q
C
L
Mid IR
Photo-detector
SWIR
RX
Face 
Plate
Rotary
Stage
SWIR
TX
Oscilloscope
Arbitrary 
Waveform
Generator
Pulse 
Generator
Current 
Probe
Fiber 
Collimator
trg data
word
SWIR 
Driver
Data
Storage
QCL
Hemi-
sphere
SWIR
Transmitter
SWIR
Receiver
Photoelectro-
magnetic
Detector
Folding
Mirror
any cooling. Due to bandwidth limitation of pulse 
generator slot rate was chosen at 20 MHz.  
Data was captured at various SNR levels with and 
without rotating phase plates. 
About 200 PPM words were used for each data set. 
Synchronized drive and PPM word boundaries 
signals were also recorded to support post 
processing analysis. All data was captured and 
initially processed with Labview (Figure 5). 
 
Figure 5 Labview GUI for capturing and cross-correlating 32-ary 
PPM data 
C. Measurement Results and comparison to SWIR links 
Each data sample was cross-correlated (Figure 5) 
and aligned with drive signal. Subsequently, words 
boundaries signal was aligned to data.  
By aligning signal data with drive signal we were 
able to accurately locate slots and words positions. 
Next, samples within each slot were summed. The 
slot with largest summed value was declared a 
“winner” and word data was decoded. This result 
was compared to drive signal’s data and word was 
defined in error or not based on whether data 
winning slot was matching that from the 
corresponding drive signal word. Word Error Rate 
(WER) was calculated from a complete set of co-
aligned words (between 90 and 150 depending on 
cross-correlation off-set). Most of WER values were 
above 5e-3 due to the limited sample data range. 
 
4. MEASUREMENTS RESULTS 
Figure 6 demonstrates measurement results for 
both MWIR (dash) and SWIR (dot) links [16]. The 
relative WER is plotted against phase plate’s 
rotation speed. As can be seen, MWIR links 
demonstrate significantly smaller dependence on 
presence of turbulence.  SWIR test results show 
clear deterioration of WER with increased 
turbulence speed, simulating increased Greenwood 
frequency.    
 
Figure 6 Effect of phase plate rotation speeds on WER                     
of MWIR and SWIR links 
With hard decoding Bit Error Rate will be equal to 
Word Error Rate. But, BER can be improved with 
soft decoding. We used Gagliardi’s [17] concept 
𝑩𝑬𝑹𝒔𝒐𝒇𝒕 =
𝑴
𝟐
𝑸√𝑺𝑵𝑹 =
𝐌
𝟐(𝐌 − 𝟏)
𝑾𝑬𝑹 
, where M is the PPM order. 
 Jitter is one of the key “culprits” impacting health of 
PPM communication links. And, due to the nature of 
our concept (low cost concept) it is expected that 
jitter would be substantial. It was previously shown 
[18] that utilization of Gray coding minimizing 
susceptibility of PPM links to poor jitter. Gray 
codding is a reflected binary code allowing only one 
bit change between consecutive values. As such it is 
allowing to spread errors for the effective error 
correction. However, investigation of the test data 
with Gray coding demonstrated no change in system 
performance (Figure 7) comparing to binary coding. 
As such, we conclude that jitter impact was 
negligible in our experiments.  
0
2
4
6
8
0.1 1 10 100 1000 10000
R
el
at
iv
e 
W
ER
Rotation Speed, pps
Effect of Turbulence on WER
- - - MWIR
 SWIR
 
Figure 7 Gray decoding of MWIR BER results versus Soft Decoding 
 
5. CONCLUSIONS 
Low cost MWIR transceiver concept was developed, 
tested and analyzed against comparable SWIR 
system. As predicted, MWIR system demonstrated 
superior tolerance to simulated atmospheric 
turbulence. Anticipated MWIR system performance 
on notional 10km free space link with sub-optimal 
atmospheric conditions was analyzed. Expected 
MWIR transceiver size, weight and power were 
presented as well. 
 
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1.E-3
1.E-2
1.E-1
1.E-3 1.E-2 1.E-1
G
ra
y 
D
ec
o
d
in
g 
B
ER
Binary Decoding BER
MWIR Gray Decoding Versus Binary Decoding


Low Size, Weight and Power Concept for Mid-Wave Infrared Optical Communication Transceivers Based on Quantum Cascade Lasers

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