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What are Fiber Optic Transponders?

1. What is fiber optic transponder?


In optical fiber communications, a transponder is the element that sends and receives the optical signal from a fiber. A transponder is typically characterized by its data rate and the maximum distance the signal can travel.

>> The difference between a fiber optic transponder and transceiver

A transponder and transceiver are both functionally similar devices that convert a full-duplex electrical signal in a full-duplex optical signal. The difference between the two being that transceivers interface electrically with the host system using a serial interface, whereas transponders use a parallel interface to do so.

So transponders provide easier to handle lower-rate parallel signals, but are bulkier and consume more power than transceivers.

>> Major functions of a fiber optic transponder includes:

  • Electrical and optical signals conversions
  • Serialization and deserialization
  • Control and monitoring


2. Applications of fiber optic transponder

Multi-rate, bidirectional fiber transponders convert short-reach 10 Gb/s and 40 Gb/s optical signals to long-reach, single-mode dense wavelength division multiplexing (DWDM) optical interfaces.

The modules can be used to enable DWDM applications such as fiber relief, wavelength services, and Metro optical DWDM access overlay on existing optical infrastructure.

Supporting dense wavelength multiplexing schemes, fiber optic transponders can expand the useable bandwidth of a single optical fiber to over 300 Gb/s.

Transponders also provide a standard line interface for multiple protocols through replaceable 10G small form-factor pluggable (XFP) client-side optics.

The data rates and typical protocols transported include synchronous optical network/synchronous digital hierarchy (SONET/SDH) (OC-192 SR1), Gigabit Ethernet (10GBaseS and 10GBaseL), 10 G Fibre Channel (10 GFC) and SONET G.709 forward error correction (FEC)(10.709 Gb/s).

Fiber optic transponder modules can also support 3R operation (reshape, retime, regenerate) at supported rates.

Often, fiber optic transponders are used to for testing interoperability and compatibility. Typical tests and measurements include jitter performance, receiver sensitivity as a function of bit error rate (BER), and transmission performance based on path penalty.Some fiber optic transponders are also used to perform transmitter eye measurements.

>> Major Applications of fiber optic transponder

300-pin MSA fiber optic transponders can transparently carry a native 10G LAN PHY, SONET/SDH and Fibre Channel payload with a carrier grade DWDM Optical Transport Network (OTN) interface without the need for bandwidth limitation.

Transponders offer G.709 compliant Digital Wrapper, Enhanced Forward Error Correction (FEC) and Electrical Dispersion Compensation (EDC) for advanced optical performance and management functions superior to those found in DWDM Transponder systems.

They support full C or L band tunability and is designed to interoperate with any Open DWDM line system that support 50GHz spaced wavelengths per the ITU-T grid.

  • Enables reach extension on SONET, Storage Area Network (SAN), Gigabit Ethernet, and dispersion limited links
  • Wavelength services and Metro optical access overlay
  • Agile Optical Networks

>> Other Applications

1) Multimode to Single Mode Conversion

Some transponders can convert from multimode to single mode fiber, short reach to long reach lasers, and/or 850/1310nm to 1550nm wavelengths.  Each transponder module is protocol transparent and operates fully independent of the adjacent channels.


2) Redundant Fiber Path

Each transponder module can also include a redundant fiber path option for extra protection.  The redundant fiber option transmits the source signal over two different optical paths to two redundant receivers at the other end.

If the primary path is lost, the backup receiver is switched on. Because this is done electronically rather than mechanically, it is much faster and more reliable.


3) Repeater

As an optical repeater, some fiber optic transponders effectively extend an optical signal to cover the desired distance. With the Clock Recovery option, a degraded signal can be de-jittered and retransmitted to optimize signal quality.


4) Mode Conversion

Mode conversion is one of the quickest and simplest ways of extending multimode optical signals over greater distances on single mode fiber optics.

Note:  Most receivers are capable of receiving both multimode and single mode optical signals.



3. 10Gb/s transponder block diagram


Fiber optic transponders do the simple conversion from low-speed electrical signals to high-speed optical signals

These optical transceivers with built-in MUX/DEMUX come in a compact package with a multiplexing function converting 622Mbps low-speed electrical signals to a 10Gbps ultra-high-speed optical signal.

They can contribute to significantly smaller and cheaper optical interfaces in communications equipment and switches/routers.




4. How to select a transponder?

Selecting fiber optic transponders requires an understanding of jitter measurements and BER measurements.

>> Jitter measurement

There are three types of jitter measurement: jitter generation, jitter tolerance, and jitter transfer. Jitter analyzers are used with fiber optic transponders and test boards.

  • Jitter generation data includes current and maximum values for jitter peak – peak, jitter + peak, jitter – peak, and jitter RMS (root mean squared).
  • Jitter tolerance and jitter performance are scaled values.

The following figure shows an example setup for jitter measurements.


>> BER measurement

For BER measurements, test boards with fiber optic transponders are used with pulse pattern generators, error detectors, reference lasers, and reference receivers. Case temperature is an important variable.

The following figure shows an example setup for Bit Error Rate Measurement.



>> Transmitter eye measurement

For transmitter eye measurements, fiber optic transponders are used with pulse pattern generators, reference lasers, and high-speed oscilloscopes.

There are significant differences between the filtered eye and the unfiltered eye. Eye measurements vary by distance and, because of the error rate (ER), may require optical modulation amplitude (OMA) instead.

The following figure shows an example setup for transmitter eye measurement.


>> Path Penalty Measurement

For path-penalty testing, test boards with fiber optic transponders are used with pulse pattern generators, reference lasers, reference receivers, and fiber spools. Original chirp and optimal chirp are important parameters to consider.

The following figure shows an example setup for path penalty measurement.



5. 300-pin Transponder MSA



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