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Free space optical communication: laser sources, modulation schemes and
detection techniques
Conference Paper · February 2013
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Free Space Optical Communication:
Laser sources, Modulation Schemes and Detection
Techniques
Anshul Vats
Hemani Kaushal
V.K. Jain
Department of EE&CE
ITM University
Gurgaon (Haryana), India
u
Department of EE&CE
ITM University
Gurgaon (Haryana), India
Department of Electrical Engineering
Indian Institute of Technology
New Delhi, India
Abstract—Free space optical (FSO) communication is an
upgraded supplement to existing wireless technologies. FSO
technology leads vast modulation bandwidth, unlicensed
spectrum, cost effective deployment, quick redeploy and much
more. FSO systems are used to transmit and receive all kind of
data at high data rates (up to 100 Gbps). Today researchers are
preliminary focused to use the free space communication
systems to make their inter satellites links. This paper gives a
review on the laser diodes, modulation schemes and the
detection techniques which are deployed in the FSO systems.
Index Terms— Free space communication, Modulation schemes,
Laser diodes, Detectors.
I. INTRODUCTION
Laser communication or free space optical communication
is a former technology that involves information compressed
on to the optical light and transmitted through the space from
source to destination. FSO system applications use laser
diodes which produce narrow beam width optical signal. This
narrow beam width focuses a large amount of transmitted
optical power on to the receiver which gives the higher link
power efficiency. FSO systems enables 10 to100 times more
data to be transmitted/received with only utilizing 1% of
antenna aperture area when compared with RF antennas of
wireless communication [13]. It utilizes less power and mass,
provides a secure, jam free, unlimited bandwidth with no
regulation on the optical band/spectrum. It also provides
higher throughput levels with no interference from RF
frequencies. These systems have wide range of applications,
few of them are- connecting sites in an area, extending the
fibre optic cable network to the nearby buildings, in local
loop bypass, backhaul, disaster recovery, in last mile
applications, inter satellite links, links between spacecraft and
satellite and many more. Figure 1 shows the application
scenario of FSO technology. These systems operate very
much like a fibre optic connection which uses a fibre. The
main difference is that the attenuation from the cable is
known and can be controlled; But the FSO link uses
space/atmosphere as the media and the attenuation may vary
every second and is unknown. In FSO systems, laser diodes
are used to produce a signal in near infrared range.
Fig. 1. Free space optical communication applications
Here those laser diodes are used whose operating
wavelengths are 780 nm to 900 nm and 1500 nm to 1600 nm.
Figure 2 illustrate the optical link with different sub blocks
used at different stages of the link. In this block diagram, at
the transmitter end the laser diode produces a narrow beam
on which information signal is modulated and then
transmitted towards receiver [2]. Here the various modulation
schemes utilized are OOK, M-PPM and SIM. All these are
commonly used in FSO links. Now a days DPSK is also
getting popular. The received optical signal is filtered,
detected, and then demodulated. Commonly APD and PIN
detectors are used for detection. The corresponding detector
current is demodulated via corresponding demodulation
scheme to extract the original information [4]. Using FSO
system setup inter satellite link (ISL) can be set between two
satellites so as to decrease the set up cost, signal strength loss
due to hopping, scattering etc. The laser diodes, modulation
schemes and detection techniques are discussed in the
coming sections.
2.
Fabry Perot
1300/1550 nm
3.
Distributed
feedback
lasers
1300/1550 nm
Fig. 2. Block diagram of the FSO link
II. LASER DIODES
Laser diodes are used in free space optical communication
and inter satellite links over LEDs, because their beam does
not spread while covering longer distances also they have
higher frequencies which increases the modulation rate and
overall communication rates. They have faster rise and fall
times which enhances the switching speed and over all
throughput of the system.
Laser diodes which have operating wavelength centred at
800 nm and 1550 nm are generally preferable for FSO
systems and FSO-ISLs. It is so because we can design an eye
safe laser transmitter at 800 nm and 1550 nm wavelengths,
more over at 1550 nm window, allowable safe laser is fifty
times more as compare to that of 800 nm window. The factor
of fifty gives up to 17 db extra margin and make system to
propagate over longer distances and to aid higher data rates.
Prevalent Solid state laser is Nd: YAG (neodymium yttrium
aluminium garrnet) which operates at 1064 nm wavelength.
This laser is adequate to transmit immense amount of power
and is used in coherent systems with highly stable Nd: YAG
oscillator [18]. Table I shows the laser technologies which
are commonly used for FSO systems [3] and Table II gives
the compounds involved in the lasers diodes [11].
TABLE I. LASER SOURCES [3]
S.No.
Technology
type
Operating
wavelength
1.
VCSEL
~ 850 nm
Features
Lower power density
Cheap and readily
available.
No active cooling
High reliability
Output optical power: max
up to 20mWand typical
power: 6mW.
Low threshold and
operating current.
8.5Gbps data rate and
reliable up to 10Gbps.
Applications are- optical
fibre communications,
computer mice, gas
sensing, optical clocks,
thresholdless lasers.
4.
Solid state
lasers
1064 nm
50 times higher power
density.
Long life
Low eye safety criteria.
Output Optical power:
20mW-100mW and typical
power: 28mW.
±0.03db CW power
stability.
Insensitive to back
reflection & stabilised for
short & long term
application.
Narrow spectral resolution.
Internal digital modulation.
Upto 40Gbps data rate
Applications are- in
dichroic filters, add-drop
multiplexers with banks of
miniature tuned fused silica
or diamonds, optical
wavemeter, laser resonator,
laser absorption
spectrometry techniques, in
gravitational wave
detection.
Compatible with EFDA
Higher data rates upto
40Gbps.
Small temperature
dependence.
Complex fabrication
Narrow emission linewidth
of < 1nm.
Provide superior
longitudinal mode
discrimination over Fabry
perot.
Output optical
power: >20mW and typical
power; 1-2Wwhen
combined with EFDA.
± 0.01db CW power
stability
Applications are- DWDM,
CATV and long haul
communication.
High power in infrared
spectrum.
Small gain bandwidth of
the order of 1 nm or less.
Very good coherence and
suitable for homodyne
systems.
Natural birefringent.
Laser gain is strongly
polarization dependent.
Applications areophthalmology to
correct posterior capsular
opacification, flow
visualization techniques in
fluid dynamics, soft
tissue surgeries , laser
designators and laser
rangefinders, cavity ringdown spectroscopy, laser
pumping, laser induced
break-down spectroscopy.
TABLE II. COMPOUNDS USED IN LASER DIODES [11]
S.No.
Operating Wavelength
Compound(s)
1.
620-895 nm
Ga(1-x)Al(x)As
2.
904 nm
GaAs
3.
1100-1650 nm
In(1-x)Ga(x)As(y)P(1-y)
4.
1550 nm
In(0.58)Ga(0.42)As(0.9)P(0.1)
5.
1604 nm
Nd3+:Y3Al5O12 ; Nd3+:YVO4 ;
Nd3+YLiF4
Longer distance FSO systems require high speed
modulation, low power consumption, smaller footprint
operate on wide range of temperatures without degradation,
MTBF which exceeds 10 years. To meet all these necessities
manufactures uses VCSEL laser for short wavelength range
and DFB/Fabry perot laser for longer wavelength ranges.
Other laser diodes generally are not suited for FSO
applications systems.
III. MODULATION SCHEMES
Different modulation schemes exists which are well suited
for the free space optical communication. Most commonly
used modulation techniques are on-off keying (OOK), pulse
position modulation (PPM), differential pulse shift keying
(DPSK), differential quadrature pulse shift keying (DQPSK)
and subcarrier intensity modulation (SIM). OOK is the
simplest modulation scheme (because of its design and
implementation) in which transmitter is ON only for the
binary bit ‘1’ and OFF for bit ‘0’. OOK modulation scheme
is widely commercially available for the applications of FSO
systems [12]. Here non-linearity of components is not an
issue but it requires adaptive threshold when dealing with
fading channels. Besides its advantages, OOK is not an
optimal modulation scheme for the channels under turbulence
conditions as turbulence directly affects the signal intensity.
It can be sub optimal scheme if a fixed threshold is decided
[11].
Another modulation scheme is M-ary pulse position
modulation (PPM). It is well suited for direct detection of
optical signal transmitted through wireless space. PPM offers
a great advantage of eliminating the decision threshold
dependencies on the input power. Therefore it is a power
efficient modulation. The main limitation is that it requires
more bandwidth than that of OOK [6]. PPM needs a complex
transmitter and receiver designs because of high
synchronisation needed between them. If we keep on
increasing M, time slots during which an optical pulse takes
place decreases. This tends to increase the information
transmitted per signal. Thus higher transmission efficiency,
but also increases the required bandwidth by M/log2M times;
which reduce the band utilization efficiency. In M-PPM more
difficulties occurred in the recovering the symbol timing
reference [6].
DPSK scheme encodes the data bits on its phase, can
extenuate serious effects of scintillation to some extent.
DPSK has benefit over OOK, that it has ~3db lower optical
SNR needed to obtain a given BER if a balanced receiver is
used [6]. For OOK quantum limit for an optically preamplified receiver is 41 photons/bit, this reduces to 22
photons/bit with a balanced detector. It gives higher data
rates over PPM and OOK with increase in complexity in
receiver. In DPSK, bandwidth decreases linearly with
decrease in data rates thus it is not suitable for the lower data
rates. Also its receiver require single mode optical signal free
from phase noise which decreases the collection efficiency of
the signal. Due to all these limitations, the use of DPSK
modulation scheme in the turbulence free links such as
between satellites or air to satellite is limited [6].
When we compare the binary modulation schemes like
OOK, DPSK with the DQPSK scheme, it doubles the
spectral efficiency by making advantage of two signal
quadrature of an optical carrier signal [5, 6].
Subscriber intensity modulation is again one of FSO
modulation scheme. Alike PPM it would not require the
adaptive threshold (like in OOK) and not need much
bandwidth (like in PPM). SIM has a drawback that it suffers
from a high peak to average power ratio, thus giving poor
power efficiency. Also the non-linearity of the component is
a big issue when dealing with multiple subcarriers. One has
to choose the modulation schemes as per need of the
application with some trade off among described factors.
IV. OPTICAL DETECTION TECHNIQES
This section of paper tells about the photo detection
techniques and photo detectors which convert the received
optical signal into corresponding electrical signal for the
further signal processing unit or decision making unit of the
receiver. Photo detectors primarily extract the information
embedded on the optical carrier signal (it may be embedded
on frequency, phase or intensity of the optical signal). In FSO
systems, two common approaches are used for this purpose
and these are coherent and direct detection. Photo detectors
are the transducers which convert optical signal to the
corresponding electrical signal. Here avalanche photo diode
(APD) and PIN photo detectors are used for detection of the
information bits. Photo detectors should have high sensitivity
within its operational wavelengths; low noise levels and have
sufficient bandwidth to hold the needed data rates. Detectors
should have minimum effect on the response of detector due
to temperature fluctuations. Device should have long
operating life too [21].
PIN photo diodes have a P and N type semiconductor layer
separated by a very lightly n type doped intrinsic layer [15,
21]. The responsivity of PIN photodiode is always less than
unity. APD photodiode provides an inherent current gain
which increases the sensitivity of the detector because the
photocurrent is multiplied. Thus APD includes the gain or
multiplication factor in responsivity. Typical values for gain
are in the range of 50-300 [17, 21]. This implies responsivity
of APD is greater than unity. APD provides the higher
sensitivity as compared to PIN photodiode, but has
multiplication noise and is very much temperature sensitive
too [9, 21].
TABLE III. PHOTO DETECTOR’S MATERIAL AND CORRESPONDING
WAVELENGTH AND ENERGY GAP [15, 21]
S.No.
Material
Wavelength (nm)
1.
InGaAsP
1650-920
Energy gap (eV)
0.75-1.35
2.
InGaAs
1700
0.73
3.
4.
GaAs
Germanium
870
1600
1.424
0.775
5.
Silicon
1060
1.17
computational power. It can be implemented by using
Viterbi-decoder that is ‘practically optimum’ ML-detection
scheme to reduce computational complexity by surviving
path selections.
Before the transmission of the information, it is embedded
on the frequency, phase or intensity of the optical carrier
signal. This encoded signal is then fed onto the space
wireless channels to the receiver. At the receiver, depending
upon wether the local oscillator is used or not in the detection,
following detection techniques can be used.
• Direct Detection
• Coherent Detection
• Heterodyne Detection
• Homodyne Detection
• Maximum likelihood sequence Detection (MLSD)
• Iterative Detection
D. Iterative Detection
The iterative detection and decoding is performed by
iteratively passing soft (multi-bit) “a priori” information
between a detector and a decoder. The detector receives
modulation symbols, performs a detection function that is
complementary to the symbol mapping performed at the
transmitter, and provides soft-decision symbols for
transmitted coded bits. “Extrinsic information” in the softdecision symbols is then decoded by the decoder to provide
its extrinsic information, which comprises the “a priori”
information used by the detector in the detection process. The
detection and decoding may be iterated a number of times.
A. Direct Detection
In direct detection information is encoded with the
intensity variations. Here there is no need of local oscillator
for detection hence no synchronisation is needed between
receiver and transmitter. This is also called envelop detection
[17, 21].
V. CONCLUSION
This paper gives a brief review on various laser diodes
which can be utilized in the free space communication
system. These laser diodes can operate on different IR
spectrum windows such as 1550 nm, 1064 nm etc, to give
higher data rates and provide spectrum which is unlicensed
and without any regulations on it. So we can freely use the
band with higher capacities. They can be used in various
applications of FSO communication such as inter satellite
links, air borne to satellite link, last mile solution, LAN,
MAN and many more. Besides laser diodes modulation
schemes are also discussed which provide a brief idea about
modulation schemes suited for a particular FSO application.
Finally detectors and detection techniques are briefed which
are used in FSO application.
B. Coherent Detection
In coherent detection local oscillator is needed to obtain
optical signal operating at a particular wavelength. The
frequency and phase of local oscillator need not to be same as
that of received signal [19, 21].
1. Heterodyne Detection
In heterodyne detection the received signal is mixed with a
reference wave from local oscillator on the photo detector. It
is a relatively easy way of amplifying the photo current by
increasing the local oscillator power. This detection provides
the Improved SNR by increasing the local oscillator power.
IF frequency needed to monitor regularly so as to maintain
the IF centre frequency constant. Noise is also another
limitation which is contributed by shot noise, photo
detector’s noise and added by electronics. These are factors
which are the challenges while implementing the coherent
optical communication system [19, 21].
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