O-Band Fiber Amplifiers (PDFAs)

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Typical gain spectrum for the PDFA100, PDFA100P, and PDFA100X optical amplifiers. A complete set of performance graphs is available on the Graphs tab.

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Operating Principle of the PDFA

Features

• Operating Wavelength Range: 1280 - 1330 nm (O-Band)
• Single Mode (SM) or Polarization-Maintaining (PM) Versions (See Table to the Right):
  • Available as a Benchtop System (SM or PM) or Plug-In PXIe Module (SM)
• Minimal (<100 ns) Signal Latency
• Operates in Constant Current, Power, or Gain Modes
• Telecom Industry-Standard 980 nm Pump Modules
• No Cross-Gain Modulation, Pattern Dependence, or Modulation Format Dependence
• OEM and Customization Options Available; See the OEM Capabilities Tab or Contact Tech Sales for Details
• Optional Gain-Flattening Filter (SM Only) Sold Below

Applications

• 400G and 800G Transceiver Testing
• Overload Testing of O-Band Receivers
• O-Band CWDM Transceiver Testing
• Silicon Photonic Device Characterization in O-Band
• O-Band Signal Transmission for Data Communications

Thorlabs' O-Band Praseodymium-Doped Fiber Amplifiers (PDFAs) offer high output powers, high small signal gain, and low noise figures (see the Key System Specifications table to the right), making them ideal for use within optical networks as either a booster amplifier or preamplifier. The PDFAs are available in single mode (SM) or polarization-maintaining (PM) models. Two form-factors with identical performance are offered for the SM models: a benchtop system (Item # PDFA100) and a plug-in PXIe module (Item # PDFA100X) that is designed to be operated inside a standard PXIe chassis. The PM model is available as a benchtop system (Item # PDFA100P); please contact Tech Sales for applications requiring a PM PXIe module.

The SM amplifiers are polarization-insensitive, and the input and output fibers to the amplifier are standard single mode fiber (Item # SMF-28-J9). The PDFA100P amplifier is a polarization sensitive benchtop system with input and output isolators that block light polarized along the fast axis; only light that is linearly polarized along the slow axis is amplified. The input and output fibers of the PM amplifier are Fujikura SM13-PS-U25A fiber, and the connector keys are aligned to the slow axis of the fibers. Output light from the PM amplifier has a polarization extinction ratio of >20 dB.

For applications that require PDFAs with custom form factors, power consumption, or optical specifications, please contact Tech Sales. Thorlabs also offers Ytterbium-Doped Fiber Amplifiers (YDFAs) and Erbium-Doped Fiber Amplifiers (EDFAs), which operate in the 1025 - 1075 nm and 1530 - 1565 nm (C-band) wavelength ranges, respectively.

Operation Modes
The PDFAs can be operated in three modes: automatic current control (ACC), automatic power control (APC), and automatic gain control (AGC). ACC mode drives the pump current in the PDFA at a fixed current level, which is accessible through the front panel of the benchtop units or via a command through the computer connected to the chassis the PXIe module is plugged into. The user can vary the gain and output power level of the instrument by adjusting the pump current level. These O-band fiber amplifiers can also be operated in APC or AGC mode, which maintain fixed output power or fixed gain, respectively. Note that for the PDFA100 and PDFA100P amplifiers, these two operation modes are only accessible through a command-line interface; the units can be connected to a PC via the USB type B port on the back panel of the instruments.

The command line interface can also be used to enable or disable the PDFA100 and PDFA100P amplifiers, set the current level in the ACC mode, or read status indicators. The indicators available through the USB interface include temperature error, interlock status, and emission status. For more details on these features, please refer to the instrument manual.

Fluoride Fiber Amplifier Technology
As Praseodymium (Pr) ions in silica transition to a lower excited state nonradiatively, PDFAs cannot be built using silica fibers. A higher radiative transition efficiency can be obtained by choosing fluoride glass as the host for the Pr ions, and Thorlabs' PDFAs are designed and manufactured using proprietary fluoride fiber to achieve high gain and output power. Please see the PDF Emission tab for more information on light emission in Pr-doped fluoride fibers (PDF). These O-band fiber amplifiers incorporate a relatively short length of PDF, which results in a signal latency <100 ns.

Fiber amplifier technology offers key advantages over semiconductor optical amplifiers (SOAs) when used for boosting communication signals. Distortion effects typically associated with the saturation mechanism and gain dynamics of SOAs do not exist in praseodymium-doped fiber amplifiers (PDFAs), leading to improved compatibility with wavelength division multiplexed systems, advanced modulation formats, and high data rates. In particular, undesirable distortion effects such as cross-gain modulation and pattern dependence are eliminated in the PDFA as a result of its high saturation energy and slow gain dynamics. The PDFA gain dynamics are independent of pulse shape or modulation format due to a long upper-state lifetime that leads to negligible cross-talk between adjacent channels. For a comparison between the resulting PDFA and SOA eye diagrams, please see the PDFA vs. SOA tab. Featuring a ripple-free, or flat, gain spectrum, the PDFA produces uniform gain over a number of channels, which eliminates channel mismatches at the end of a link. The resulting clean, stable output is ideal for improving the power budget in data center applications.

Reza Salem
BU Leader, Fiber Lasers

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Questions?

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As shown in the schematic above and to the right, the main components of a PDFA include a wavelength-division multiplexer (WDM), input and output isolators, and a PDF. The WDM is used to combine the 980 nm pump laser and O-band signal light before the PDF, while the isolators, which only allow light to travel in one direction, prevent lasing by minimizing back reflections. The input and output isolators also serve to prevent any spontaneous emission signal from entering the amplifier and the pump light from exiting the O-band optical amplifier, respectively.

Gain Flattening Filter
To flatten the gain spectrum of the PDFA100(X) fiber amplifier for applications that require equal gain across a wide spectral range, Thorlabs offers a gain flattening filter (sold separately below). The filter is offered in an SM model for compatibility with both SM O-band fiber amplifier form factors. 

OEM Capabilities
For OEM customers, the PDFA100X module can be adapted to fit conveniently into other platforms; the 100 mm x 164 mm x 38 mm amplifier module core sets the minimum dimensions required. The front panel can be customized or eliminated entirely to suit the user's needs. The rear panel is also configurable for the user's power and communication protocols. See the OEM Capabilities tab for more details.

Performance Graphs

Performance may vary from unit to unit; this data reflects the typical performance of our PDFAs, and is presented for reference only. The guaranteed specifications are shown in the Specs tab.

Wavelength Dependence

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The typical gain as a function of the wavelength. The blue-shaded region denotes the specified operating wavelength range. Data is measured with the pump level set at 100% (Item #s PDFA100 and PDFA100P) or 1200 mA (Item # PDFA100X).

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The typical noise figure as a function of the wavelength. The blue-shaded region denotes the specified operating wavelength range. Data is measured with the pump level set at 100% (Item #s PDFA100 and PDFA100P) or 1200 mA (Item # PDFA100X).

Input Power and Pump Level Dependence at 1310 nm

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The typical output power as a function of input power at 1310 nm. Data is measured with the pump level set at 100% (Item #s PDFA100 and PDFA100P) or 1200 mA (Item # PDFA100X).

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The typical noise figure as a function of input power at 1310 nm. Data is measured with the pump level set at 100% (Item #s PDFA100 and PDFA100P) or 1200 mA (Item # PDFA100X).

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The typical gain as a function of input power at 1310 nm. Data is measured with the pump level set at 100% (Item #s PDFA100 and PDFA100P) or 1200 mA (Item # PDFA100X).

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The typical gain as a function of pump level at 1310 nm. Please note that the pump current level for the PDFA100 or PDFA100P system is specified as a percentage (bottom axis) and the pump current level for the PDFA100X module is specified in mA (top axis). The maximum current in the PDFA100X module is 1200 mA, and all current values can be converted to percentage by normalizing the value to the 1200 mA maximum.

ASE Power Spectral Density Spectrum

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The typical amplified spontaneous emission (ASE) power spectral density spectrum. Total ASE power for a typical unit is about 2 mW. Data is measured with the pump level set at 100% (Item #s PDFA100 and PDFA100P) or 1200 mA (Item # PDFA100X).

Distortion-Free Amplification

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50 Gbaud/s Eye Diagram for a Commerically Available SOA with Input Power of -3 dBm and Output Power of 10 dBm

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50 Gbaud/s Eye Diagram for the PDFA100 Fiber Amplifier with Input Power of -3 dBm and Output Power of 10 dBm

One advantage of Praseodymium-Doped Fiber Amplifiers (PDFAs), in comparison to semiconductor optical amplifiers (SOAs), is their slow gain dynamics. Communication signals include pulse sequences with random patterns. In the simple case of an on-off-keying (OOK) and non-return-to-zero (NRZ) format, the pulses have identical amplitudes but their width can vary from one bit period (20 ps for 50 Gb/s data rate) to many bit periods, depending on the type of data being transmitted. As a result, if the amplifier gain saturation has a time constant comparable to the bit period, the gain of the amplifier exhibits pattern dependence. This effect can result in severely distorted optical signals when the amplifier is driven into saturation. SOAs typically have carrier lifetimes on the order of tens of picoseconds, and pattern dependence is observed at standard communication data rates when they are driven into saturation. In contrast, PDFAs have an upper-state lifetime in the millisecond range, which makes them immune to this pattern dependence. This distinguishing characteristic between SOAs and fiber amplifiers has been previously studied for OOK with return-to-zero (RZ) and NRZ modulation formats¹. Signal distortion becomes a more severe issue in modern modulation formats, such as 4-level pulse amplitude modulation (PAM4).

The two diagrams shown above are measured by amplifying a 50 GBaud/s PAM4 signal with a 2⁷-1 pseudo-random bit sequence (PRBS) using both a Thorlabs' PDFA100 amplifier (left) and a commercially available SOA (right). The input power from both amplifiers is set at -3 dBm, output power from both amplifiers is set to 10 dBm, and the output signal from each amplifier is attenuated and sent into a high-speed receiver. A comparison of the two diagrams illustrates that the PDFA produces an open eye diagram with minimal distortion, while the SOA saturation dynamics cause significant distortion to the eye diagram. These eye diagrams were measured using Thorlabs' MX65E-1310 65 GHz Linear Reference Transmitter and RXM40AF Ultrafast Receiver. The PDFA100X module will provide the same optical performance as the PDFA100 benchtop unit.

Footnotes

1. R. C. Schimmel, R. J.W. Jonker, P. K. van Bennekom, Giok-Djan Khoe, and H. de Waardt, "Experimental assessment of 1.3-µm telecommunication systems incorporating a praseodymium-doped fiber amplifier (PDFA)," Proc. SPIE 4833, Applications of Photonic Technology 5, 2003.

The Praseodymium-Doped Fluoride Fiber Amplifier (PDFA) achieves signal gain through stimulated emission, which is when an incident photon causes an excited-state ion to relax to a lower energy state by emitting a photon. For this fiber amplifier, silica cannot be used as the host material because the excited state decays to the ground state nonradiatively via phonon relaxation. Materials with lower phonon energies, such as fluorides or chalcogenide glasses, are more suitable hosts. Thorlabs uses proprietary fluoride fiber doped with the rare-earth metal praseodymium to generate radiative emission and achieve signal gain.

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Schematics showing Pr³⁺ ion excitation and emission processes. ³H₄, ³H₅, and ¹G₄ are the ground, intermediate, and excited energy levels, respectively.

Stimulated Emission in the PDFA
The excitation and relaxation process in the Pr³⁺ ion behaves like a three-level system involving the ground state ³H₄, metastable state ¹G₄, and intermediate state ³H₅. In particular, when a ground state Pr³⁺ ion is pumped with a laser, the ion is excited to a metastable state with a long lifetime on the order of hundreds of microseconds. This long lifetime allows for population inversion, which means that the number of ions in the metastable state builds up and becomes larger than the number of ions in the ground state. The ion sits in the metastable state until a signal photon, with an energy matching the energy difference between the excited state and intermediate state, causes the ion to relax down to its ground state by photon emission. For the Pr³⁺ ion, this energy corresponds to a wavelength around 1310 nm. The ion then fully decays to the ground state through nonradiative phonon relaxation. Absorption and stimulated emission, as well as the relevant energy states, are illustrated in the first two panels of the schematic to the right.

Note that the electronic states of the Pr³⁺ ion are energy bands and not discrete energy levels, and there is a range of wavelengths the emission processes occur over, i.e. the PDFA bandwidth. This is as a result of the Stark-splitting caused by the local electric fields from the surrounding host lattice acting on the Pr³⁺ ion, as well as site-to-site variation within the amorphous host.

Stimulated vs. Spontaneous Emission
Signal amplification occurs when the ion relaxes via stimulated emission because one signal photon in results in two signal photons out; this second photon has the same energy, phase, and direction as the incident signal photon (coherent radiation). If the excited ion is not triggered during the span of the metastable state lifetime, the ions relax on their own via spontaneous emission. As shown in the last panel of the schematic, these photons have random phase and direction. While most of the photons generated by spontaneous emission will leave through the sides of the fiber, some propagate through the fiber core and experience signal gain, resulting in noise called amplified spontaneous emission (ASE).

PDFA100 an PDFA100P Benchtop Systems Front and Back Panels

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Back Panel

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 Front Panel

PDFA100X Front Panel

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Components Shipped with the PDFA100 or PDFA100P Benchtop System

The PDFA100 and PDFA100P Amplifiers contain the following components:

• Praseodymium-Doped Fiber Amplifier in Benchtop Package
• Interlock-Shorting BNC Connector
• Region-Specific Power Cord
• USB-A-79 High Speed USB 2.0 Type-A to Type-B Cable, 79" (2 m) Long
• FBC250 Connector and Bulkhead Cleaner

The PDFA100X Amplifier contains the following components:

• Praseodymium-Doped Fiber Amplifier in PXIe-Compatible Module Package
• Key to Activate Module

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PDFA100X Standard Module Package

Reza Salem
BU Leader, Fiber Lasers

Need a Quote?

PDFA Module for OEM Applications

For OEM customers, the PDFA100X package can be adapted to fit conveniently into other platforms; the 100 mm x 164 mm x 38 mm amplifier module core sets the minimum dimensions required.

All PDFAs integrate telecom-grade 980 nm pump lasers for reliable long-term operation and maximal uptime.

The PDFA100X front panel can be customized or eliminated entirely to suit the user's needs. The rear panel is also configurable for the user's power and communication protocols.

The PDFA100X package is available for single mode operation. However, if your application requires a polarization-maintaining PXIe module, please contact Tech Sales to discuss this option.

Click the "Contact Me" button to the right to discuss the customization requirements for your OEM module.

Fluoride Glass Characteristics

Fluoride fibers, used throughout the PDFA, are ideal for transmission in the mid-IR wavelength range and have lower refractive indices and lower chromatic dispersion than other fibers that transmit in this range, such as chalcogenide glass fibers. The heavy glass matrix of fluoride glass avoids quenching certain transitions in rare-earth-doped glass, which underlie the gain mechanism of the PDFA.

Thorlabs' Fluoride Fiber Manufacturing

The fluoride fiber used in our PDFA is manufactured at our fiber draw facility, located in Newton, NJ. The vertically integrated facility is well-equipped for high-volume manufacturing and is capable of producing many kilometers of fiber per year, enabling production of hundreds of amplifiers in the same time frame. In addition, because the fiber stays within Thorlabs' facilities from start to finish, the manufacturing parameters can be adjusted to accommodate unique custom orders or R&D needs.

Thorlabs' stringent manufacturing processes yield low attenuation over the mid-IR range as well as an extremely low hydroxyl ion (OH) content. Our tight quality control allows us to mitigate scattering and point defects in the fiber and to eliminate micro-crystallization in the glass matrix, which would otherwise reduce the performance of the fiber amplifier.

Click here to learn more about our in-house fluoride fiber manufacturing.

Click to Enlarge A Glass Drop During the Fiber Draw Process

Click to Enlarge Rare-Earth-Doped Fluoride Glass Preforms