HFTA-08.

0: Receivers and Transmitters in DWDM Systems

The rapidly growing internet traffic demands a near-continuous expansion of data-transmission

capacity. To avoid traffic jams on the data highways, network providers need a technique that
provides fast, flexible, and cost-effective bandwidth expansions. One such technique is the data-
transmission technology called Dense Wavelength Division Multiplexing (DWDM), which
augments network data throughput via the existing fiber infrastructure.

DWDM Technology
In conventional long-haul fiber-transmission systems, data is transmitted at a certain bit rate,
using (for low signal dispersion) a single wavelength from the second optical window (1300nm
range), or (for low signal attenuation) a single wavelength from the third optical window
(1500/1600nm range). To achieve higher transmission capacity, you can raise the bit rate based
on Time-Division Multiplexing (TDM), or install additional fiber cables in parallel with the
existing ones, or both.

The second approach requires expensive and time-consuming construction work, therefore
increasing the bit rate would seem to be the more cost-effective way to achieve higher bandwidth
within an existing fiber network. The absence of technologies like a mature and cost-effective
process for high-speed IC development, and the limitations of physical fiber media (like fiber-
polarization mode dispersion) do not allow the realization of practical commercial transmission
systems beyond 40Gbps. Upgrading a single fiber link from 2.5Gbps (for example) to 10Gbps
quadruples the bandwidth capacity, but a transmission technology called Dense Wavelength
Division Multiplexing (DWDM) can multiply the capacity by multiples as high as 160.

DWDM takes advantage of a physical phenomenon that allows multiple wavelengths of light to
travel simultaneously through a single-fiber cable. It allows multiple high-bit-rate signals to ride
through the fiber media together, each on a different color of light.

Another significant advantage of WDM transmission (compared with TDM long-haul trunks) is
bit-rate transparency as conferred by the purely optical functions that are mandatory in such
systems like optical multiplexers and demultipexers, optical line amplifiers (OLAs), and in
future, optical 3R regenerators for ultra-long-link distances. In principle, therefore, the link
includes no bit-rate-limiting elements that would require a change of optical line components to
achieve a higher bit rate.

HFTA-08.0 (04/04) page 1 of 9

Overview of DWDM system components

The basic elements of a DWDM transmission system are the optical multiplexer, the optical line
amplifier (OLA), and the optical demultiplexer (Figure 1).

1 1

OLA1 OLAn
DWDM DWDM
Transmitter DCF Receiver

n Optical MUX Optical DeMUX n

OLA : Optical Line Amplifier ( EDFA )

DCF : Dispersion Compensating Fiber

Figure 1. Example of a Dense Wavelength-Division Multiplex (DWDM) link.

An optical multiplexer combines all the received wavelengths of the L band (1530nm-1565nm)
and C band (1570nm-1620nm) into one wavelength-multiplexed light signal. Todays systems
achieve wavelength separations of 0.4nm or less, which allows about 160 potentially usable
wavelengths. The L- and C-band limitation is determined by the optical line amplifier, which is
able to amplify incoming light signals from the L- and C-bands only. Optical line amplifiers for
the 1300nm window are still under development.

One of the most widely used technologies for implementing the optical line amplifier is the
erbium-doped fiber amplifier (EDFA). An EDFA contains a pump laser working at 980nm or
1480nm, which raises electrons to a higher energy level. If light is received with wavelengths
within the L or C band, those electrons fall into a lower energy band after emitting photons with
the wavelength of the incoming light. The resulting light-domain amplification is independent of
the bit rate. Depending on the distance between optical mux and demux, several EDFAs can be
cascaded with a typical span of about 100km. This technique allows optical transmission links of
several hundred kilometers without need for an electronic signal regeneration.

A disadvantage of EDFAs is that some electrons at the higher energy level generate uncorrelated
optical noise by falling into the lower energy band spontaneously. Because DWDM links usually
contain a chain of OLAs, this optical noise is amplified in the following EDFAs, and the
resulting noise accumulation lowers the receivers signal to noise ratio (SNR) as compared to
systems without OLAs. Whats more, this optical noise is asymmetric because it affects logic
high levels more than the low levels.

At the receiver side, an optical demultiplexer converts the incoming wavelength-multiplexed

signal into the corresponding individual wavelengths launched at the transmitter side. This

HFTA-08.0 (04/04) page 2 of 9

demultiplexing function includes very narrow optical filters, for which smaller wavelength
separations require greater design effort. Apart from the basic system elements mentioned above,
a DWDM system may include other functionslike an optical booster after the optical mux,
dispersion compensation, or an optical preamplifier in front of the optical demuxwith the aim
of improving system performance and extending the link length.

A bit-rate-transparent network (an all-optical network) requires, in addition to transparent

DWDM point-to-point connections, additional network elements like optical add and drop
multiplexers (OADMs) and optical cross connects (OXCs). Available prototypes can
demonstrate the feasibility of this purely optical functionality, but todays network equipment
(even those called OADM and OXC) contain mainly electronic rather than optical core functions.

Further, ultra-long-haul point-to-point connections may need (depending on line distance) an

electronic 3R regeneration in the absence of mature, purely optical replacements. The all-optical
network, therefore, is still several years away. Regardless of whether complete or partial all-
optical networks are available, though, a networks line terminations must still convert light into
electrical signals, because equipment beyond the optical world still relies on electron-based
communications.

The network terminations for a DWDM long-haul point-to-point transmission system can be
realized with a dedicated line-termination card or with a wavelength transponder. Line-
termination cards are used for new installations, where (for example) a central office (CO)
transmits to and receives directly from the DWDM link. A wavelength transponder, on the other
hand, is essential if a DWDM link must be connected to existing CO equipment that includes the
old noncolored optical-network interface. The following discussion, valid for line termination
cards as well as wavelength transponders, focuses on specific design challenges associated with
an O/E receiver and a transmitter in the DWDM fiber network.

DWDM Transmitter
Two features are important for a DWDM system. First, to reduce system cost, the link should be
as long as possible without the need for electronic signal regeneration. Second, the system should
provide highly reliable data transfers. To improve service quality and extend line distance, a
forward error correction (FEC) function can be introduced (see Figure 2).

For pure SDH/SONET data, spare bytes in the signals frame structure can implement the in-
band forward error-correction function. Bytes required for the FEC function are inserted into the
frame by the overhead-processing ASIC. For protocol-independent DWDM systems, an out-of-
band FEC must be applied, which increases the bit rate but also increases the efficiency with
respect to an in-band FEC. The Reed Solomon FEC algorithm defined in the ITU-T G.975
recommendation is one example of a possible out-of-band FEC implementation. To provide the
overhead necessary for the correction function, that algorithm increases the transmission bit rate
by 7%.

HFTA-08.0 (04/04) page 3 of 9

 Laser Shut Down Pilot Tone

2 Wire Digital Interface Power Set CW Laser Tx Power Mon

Bias max Set Driver End of Life Flag
DS3902/MAX5417
Bias Current Mon
System Reference Clock APC
Wave length locking Unit
VCO Mon Diode
Ref. CLK
Generator Retiming Enable
MAX3670

Data
FEC / 50
Digital EAM
Wrapper SERIALIZER DRIVER
Encoding MAX3941
CLK TEC

Pulse Width Control -5.2V

Mod Set TEC Driver /

2Wire Digital Interface Pre - Bias Set Controller
DS1847/8 MAX1978

Figure 2. Example of a 10Gbps DWDM transmitter.

Instead of a Reed Solomon FEC, the digital wrapper function defined in ITU-T G.709 is likely to
become the champion. The signal is wrapped regardless of bit rate and protocol by a super
frame that includes (in addition to bytes for the FEC function) the addressing bytes necessary
for signal routingtransmitting the payload to its destination. The digital wrapper functions
overhead increases the transmission bit rate by a certain percentage, which in turn depends on
which digital-wrapper concept chosen. Regardless of the selected out-of-band FEC/digital
wrapper methodology, an additional IC is needed to support the related algorithm, or that
function must be integrated into the transmitters overhead-processing ASIC.

The FEC or digital wrapper processing is performed on the transmission signals lower-speed
parallel data stream. Parallel data leaving this processing function, therefore, must be serialized
to form the high-speed transmission signal. That task requires a serializer with on-chip clock
synthesizer for generating the transmission clock.

For long distance trunks it is very important to launch a low-jitter signal, meaning that jitter
generated by the serializer should be as low as possible, as should jitter of the external reference
clock applied to the integrated clock synthesizer. In many cases the available system-reference
clock not only doesnt fulfill these jitter requirements, its frequency is also lower than that
required. Clock generators with external VCXO or VCSO are available to provide the necessary

HFTA-08.0 (04/04) page 4 of 9

low jitter-reference frequency, and fully integrated circuits with internal VCOs are being
developed to reduce space and cost.

Because the serializers output stage is not able to drive an optical transmitter, a driver function
is needed. That function adds jitter, unfortunately, so a retiming flip-flop should be integrated
into the drivers input stage to minimize data jitter. Usually the serial clock from the serializer is
applied to this retiming function, but a non-ideal interconnect between serializer output and
driver-retiming input can degrade the clock signal, which can also degrade the transmit signals
jitter performance. The retiming function should therefore be optional.

Another function useful for integration with the driver is pulse-width correction, which
introduces a pre-distortion for compensating the non-symmetrical rise and fall transitions in an
optical component.

Finally, the serial signal must be converted to an optical signal of dedicated wavelength. For
handling up to 160 different wavelengths, the wavelength separation must be no greater than
0.4nm. That calls for an optical source with highly accurate wavelength-stability control, very
narrow spectral line widths, and low chirp (the phenomenon of spectral line hopping due to high-
speed modulation). Instead of direct-modulated laser diodes, electro-absorption modulators
(EAM) or Mach Zehnder modulators (MZ) in combination with CW lasers fulfill the above
requirements for long distance transmission.

Housed in modules, these transmitters contain a Peltier element for adjusting to specific
wavelengths by setting the temperature, a laser diode that emits continuous light (CW laser
diode, DFB type), and a high-speed voltage-driven modulator. The Peltier element (a thermo-
electric cooler, or TEC) requires a driver circuit able to handle several amperes for setting the
CW laser diode to a specific temperature-related wavelength. To keep an adjusted wavelength
constant, temperature must be precisely controlled by the TEC controller circuit.

The TEC controller circuit can be space consuming if all functions must be realized with discrete
components like power FETs and operational amplifiers. Fortunately, space-saving and fully
integrated TEC drivers with on-chip power FETs and control loops are available to support
space-sensitive module integrations and applications with a multi-channel network interface. In
addition, a wavelength-locking function is needed for DWDM systems whose wavelength
separation is 0.4nm or less, and (depending on the system setup) for 0.8nm separation as well.
An etalon-based control unit (Fabry-Perot filter) can keep the wavelength within the tolerance
window, with the help of the TEC driver/controller function.

Another important transmitter parameter is the initial user-defined optical-transmit power, which
the CW laser must maintain despite aging and variations of temperature. The slope of a CW
lasers characteristic curve degrades with time and increasing temperature, so the lasers driver
circuit must set and maintain an average optical transmit power. That power level can be ensured
by an automatic power-control loop that compares the received photocurrent detected by the CW
lasers monitor diode (proportional to optical output power) with an initial defined reference
value corresponding to the desired optical output power. In addition, the driver should include an

HFTA-08.0 (04/04) page 5 of 9

alarm flag indicating the lasers end of life, a shutdown function for laser safety, a monitor output
for the CW lasers bias current, a limit setting for the maximum laser bias current, and an optical
average power monitor. Further, a low-speed pilot tone is useful for amplitude modulating the
optical output signal. That feature enables (for example) channel identification in DWDM
systems.

A modulator driver rather than a direct modulated laser driver should be used to drive an EAM or
MZ device, because optical modulators (unlike laser diodes) are usually matched to an
impedance of 50. The modulator driver should therefore be optimized for 50 loads, and
should deliver a modulation voltage rather than a current. EAM devices require a maximum
modulation voltage of ~3V, and MZ types need up to 7V. MZ modulators provide the narrowest
spectral line widths, but require a relatively high modulation voltage, and are more expensive
than EAM types. MZ modulators are therefore used in applications that involve ultra-long-haul
distances.

Both types require a dc pre-bias of the modulation voltage to optimize the optical modulators
chirp effect. Modulator drivers with internal pre-bias require just one interconnect between the
driver output and modulator. That feature allows space-saving module integration, and reduces
production effort by eliminating the external inductor usually required for setting up a bias-T
network.

DWDM Receiver
Because the optical signal for a DWDM receiver is perturbed by nonsymmetrical optical noise
(as explained above) in addition to the fiber attenuation and dispersion that affects conventional
TDM receivers, the DWDM receiver carries a greater burden. To increase the receivers input
sensitivity, its first element is usually an avalanche photodiode (APD), which multiplies electrons
via a voltage-controlled avalanche breakdown during the conversion of photons into electrons. In
order to achieve the multiplying effect, the APD must be reverse-biased (depending on type) up
to 90V.

The reverse bias for an APD must be tightly controlled to keep its multiplication factor (the gain
factor M) constant over temperature. This requires a low noise, low ripple, and highly accurate
voltage supply, which should derive the APDs high reverse-bias voltage from the boards
available supply voltage (3.3V or 5V).

To maintain constant gain in an APD, it can be temperature-controlled with a Peltier element, or

its reverse bias can be changed as a function of temperature. The second approach is usually
more cost effective. An available low-noise bias supply for APDs (an IC) is highly accurate,
produces voltages up to 90V, and includes features such as current limiting for APD protection,
an avalanche indicator flag, and an optional DAC for setting the reverse bias.

System management requires detection of the received signals average power. That can be
accomplished right after the APD, in the first pre-amplifier stage (the transimpedance amplifier,
or TIA), but the TIAs part-to-part tolerance excludes this approach as the most accurate way to
measure receive power. A better way is to detect the average photocurrent directly, from the

HFTA-08.0 (04/04) page 6 of 9

photo detectors bias-voltage source. A small current-monitor IC is available for PIN diodes and
APDs, which provides a current or voltage output proportional to the average photocurrent. That
product allows accurate detection even for photocurrents below 1uA.

After devising circuitry for the receiver diode, the designer must deal with optical noise launched
by the OLAs. Being asymmetric, that noise has a higher noise floor on logic 1 than on logic 0,
which reduces the BER significantly in a traditional receiver. As a result, the receiver chains
clock and data recovery (CDR) decision circuit (which distinguishes between logic 1 and 0 by
performing a time-and-amplitude decision on the incoming signal) must have the capability to
adjust the threshold level of its decision voltage before the amplitude decision is made. That
threshold adjustment shifts the amplitude-decision level from the middle of the signal-eye
opening towards logic 0, thereby achieving a symmetrical eye opening relative to the decision
level.

For a successful implementation of this BER optimization, the incoming signal should not be
distorted by electronic functions in front of the CDR. It is therefore essential that the signal-to-
noise ratio undergo minimum change between the APD and the decision function. As a
consequence, the preamplifier that converts APD current to voltage must implement linear signal
amplification over the entire dynamic range, and the following post-amplifier must add further
linear amplification without clipping. To facilitate adjustment of the voltage decision threshold,
the task can be accomplished by a linear, automatic gain control circuit (AGC) that provides a
constant voltage swing at the CDR input over the receivers full dynamic range.

The adjustment can be performed manually, deriving the decision threshold level from
experience, or via an automatic control loop that measures the BER. The manual adjustment is
cost effective for low bit rates (to 2.7Gbps), but for bit rates of 10Gbps and higher an automatic
BER optimization should be considered, as a consequence of the bit-rate-dependent lower margin
of the signal eye opening. If an FEC or digital wrapper decoding function is implemented on the
receiver board right after the CDR and deserializer, the actual receiver BER can be derived from
this function, which counts the corrected errors on the received signal. Such error-counter
information can then be used as a criterion for the feedback loop that controls the automatic
threshold-level adjustment (Figure 3a).

HFTA-08.0 (04/04) page 7 of 9

 3.3V

APD Supply APD Reverse

MAX1932 Bias Control

Current
Monitor Rx Power Mon
MAX4007
LOS LOL

FEC /
APD Post CDR / Digital
Amplifier De Wrapper
AGC Serializer Decoding
MAX3861 MAX3882
Pre-Amplifier
(Transimpedance)
Error count output

Voltage Level Decision Threshold

adjust Control

Figure 3a. Example of a 2.5Gbps DWDM receiver with linear preamplifier and AGC

An alternative method of adjusting the threshold level is to control the dc voltage at the
preamplifier output. As in the previous method, this entails linear amplification over the
dynamic input range of the preamplifier, plus an adaptive, automatic control of threshold level.
Because the output amplitude of the preamplifier is not constant, there is no alternative to
automatic threshold-level control, which receives its feedback from the FEC or digital-wrapper
error-counter output.

An advantage of controlling the threshold at the preamplifier output is that a simple limiting
amplifier can be used instead of an AGC function. An amplitude-decision circuit after the
preamplifier, like a limiting amplifier, is acceptable because the threshold level for the amplitude
decision is defined at the preamplifier output (Figure 3b).

HFTA-08.0 (04/04) page 8 of 9

3.3V

APD
APD Reverse
Supply
Bias Control
MAX1932

Current
Monitor Rx Power Mon
MAX4007
LOS LOL

FEC /
Limiting CDR / Digital
APD Amplifier De-Serializer Wrapper
MAX3971A Decoding
Pre- Amplifier
( Transimpedance )
Error count output

Adaptive Voltage Level Decision

Threshold adjust Control

Figure 3b. Example of a 10Gbps DWDM receiver with linear preamplifier and limiting
amplifier.

This article first appeared in Communication Systems Design, June, 2003.

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