Optical Module Terms
- Transmission distance
- Maximum distance over which optical signals can transmit. Optical
signals sent from different types of sources can transmit over different
distances due to negative effects of optical fibers, such as dispersion
and attenuation.
- Interface rate
- Maximum rate of electrical signals that an optical device can
transmit without bit errors. Various interface rates are defined in
Ethernet standards, such as 125 Mbit/s, 1.25 Gbit/s, 10.3125 Gbit/s,
25.78125Gbit/s, and 41.25 Gbit/s.
- Encapsulation type
Appearance type of an optical module. Encapsulation types of
optical modules include SFP, eSFP, SFP+, XFP, QSFP+, SFP28, and QSFP28.
- SFP: small form-factor pluggable.
- eSFP: enhanced small form-factor pluggable. An eSFP module is
an SFP module that supports monitoring of voltage, temperature, bias
current, transmit optical power, and receive optical power. Because
all the SFP optical modules support these monitoring functions, eSFP
is also called SFP.
- SFP+: small form-factor pluggable plus, SFP with a higher rate.
SFP+ modules are more sensitive to electromagnetic interference (EMI)
because they have a higher rate. To reduce EMI, SFP+ modules have
more springs than SFP modules.
- XFP: 10GE optical module. X is the Roman numeral 10.
- QSFP+: Quad SFP+, four-channel SFP+.
- SFP28: with the same interface size as an SFP+ module. An SFP28
interface can use a 25 GE SFP28 optical module or 10GE SFP+ optical
module.
- QSFP28: with the same interface size as a QSFP+ module. A QSFP28
interface can use a 100GE QSFP28 optical module or a 40GE QSFP+ optical
module.
Wavelength division multiplexing modules differ from other
optical modules in center wavelengths. A common optical module has
a center wavelength of 850 nm, 1310 nm, or 1550 nm, whereas a wavelength
division multiplexing module transmits lights with different center
wavelengths. Wavelength division multiplexing modules are classified
into two types: coarse wavelength division multiplexing (CWDM) and
dense wavelength division multiplexing (DWDM). Within the same band,
DWDM modules are available in more types and use wavelength resources
more efficiently than CWDM modules. DWDM and CWDM modules allow lights
with different center wavelengths to be transmitted on one fiber without
interfering each other. Therefore, a passive multiplexer can be used
to combine the lights into one channel, which is then split into multiple
channels by a demultiplexer on the remote end. This reduces the optical
fibers required. DWDM and CWDM modules are used for long-distance
transmission.
The transmit power of a long-distance optical
module is often larger than its overload power. Therefore, when using
such optical modules, select optical fibers of an appropriate length
to ensure that the actual receive power is smaller than the overload
power. If the optical fibers connected to a long-distance optical
module are too short, use an optical attenuator to reduce the receive
power on the remote optical module. Otherwise, the remote optical
module may be burnt.
- Center wavelength
- Wavelength measured at the midpoint of the half-amplitude line
in the transmit spectrum.
- Fiber mode
- Mode of fibers defining based on core diameters and features of
optical fibers. Optical fibers are classified into single-mode fibers
and multi-mode fibers. Generally, multi-mode fibers have large core
diameters and severe dispersion, so they transmit optical signals
over short distances when working with multi-mode optical modules.
Single-mode fibers have small dispersion and can transmit optical
signals over long distances when working with single-mode optical
modules.
- Modal bandwidth
- Bandwidth measured at a point with transmit power several dB lower
than that of the point with the peak center wavelength. Modal bandwidth
reflects spectrum characteristics of an optical module.
- Fiber diameter
- Diameter of the core of a fiber. According to international standards
for optical fibers, the diameter of a multi-mode fiber is 62.5 um
or 50 um, and the diameter of a single-mode fiber is 9 um.
- Fiber class
- Optical signals with different wavelengths have their best working
windows in different optical fibers. To help efficiently adjust wavelengths
or dispersion features of optical fibers and change their refractive
indexes, the following classes are defined: multi-mode fiber (G.651),
common single-mode fiber (G.652), shifted dispersion fiber (G.653),
and non-zero shifted dispersion fiber (G.655). Multi-mode fiber (G.651)
and common single-mode fiber (G.652) are commonly used fiber classes.
- Connector type
- Type of the interface on an optical module to accommodate a fiber.
Commonly used connector types are LC (applicable to all the SFP, SFP+,
SFP28, and XFP modules) and MPO (applicable to some of QSFP+ and QSFP28
modules).
- Transmit optical power
- Output optical power of an optical module when it is working properly.
- Maximum receiver sensitivity
- Minimum average input optical power that the receiver of an optical
module can receive within a range of bit error rate (BER = 10-12).
- Overload optical power
- Maximum average input optical power that the receiver of an optical
module can receive within a range of bit error rate (BER = 10-12).
- Extinction ratio
- Minimum ratio of the average optical power with signals transmitted
against the average optical power without signals transmitted in complete
modulation mode. The extinction ratio indicates the capability of
an optical module to identify signal 0 and signal 1.
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