S-108.3110 OPTICAL COMMUNICATIONS

Erbium Doped Fiber Amplifier

EXPERIMENT 3: /

Erbium doped optical amplifier

Optical amplifiers

In telecommunications high speed of data transfer is desired. This is achieved by the transformation of an electrical signal into an optical signal, which is then launched into an optical fiber. At the other end of the optical fiber, the signal is restored in the electrical form. The advantage of using optical fibers is the very high speed of signal propagation (close to the velocity of light) and low losses (~0.22 dB/km). However, for very long distances (e.g. transatlantic) the fiber losses are enough that many amplifiers must be placed along the line (usually about one every 100km) to be able to carry a recoverable signal from one end to the other. In practice, loss limitations can be overcome by periodic regeneration of the optical signal using an electronic amplifier or an optical amplifier which amplify the optical signal directly. Owing to the simplicity and low-cost the optical amplifiers are in intensive use in the field of optical communication.

In modern optical communication, DWDM (DenseWavelengthDivision Multiplexing) - technique allows to feed the signals into a fiber at 4, 16 or even 128 different wavelengths per channel. Such a wide wavelength interval sets higher requirements for the bandwidth of the gain of an optical amplifier to be used: the bandwidth should be wide enough and, preferably, uniform.

Amongst the other fiber amplifiers, erbium-doped fiber amplifier (EDFA) is a suitable candidate for ligthwave system applications. EDFAs have gone from successful demonstration in the laboratory in 1987 to a mature product and standard industry practice now.

Erbium Doped Fiber Amplifiers (EDFAs) amplify optical signals using stimulated emission, just like in a laser diode, but stimulate the material with light instead of electricity. The amplifier itself emits radiatively in a signal band (usually about 1540nm) using excitation energy supplied to it by photons in a pump band (usually 980 or 1480nm) when stimulated by incoming photons in the signal band. Just like in a laser, the emitted photons then stimulate other emissions, so there is an exponential growth of photons. Supporting the amplifier is a pump laser, which supplies the amplifier's energy, a coupler, which combines the pump laser beams and the signal laser beam and puts them on a single fiber, and an optical filter, which removes the remaining traces of the pump beam so that it doesn't interfere with reception of the signal (Fig. 1).

Figure 1. Diagram of Erbium Doped Fiber Amplifier (EDFA) operation

The operation parameters of EDFA can be adjusted by changing the length, composition and refractive index profile of the fiber. Typically, the optical power level of the pump diode lasers is 10..100mW. The pumping can be realised in both directions and, even, simultaneously. Optical isolator blocks the back-reflection from the link and noisy propagating in the opposite direction to the signal. The back-reflection from the coupler of the optical fiber amplifier should be high enough to exceed the gain in order to avoid the lasing of the fiber cavity. The back-reflection can be observed as the noise level in the in the noise spectrum of the amplifier increases, which can occur at frequencies of the standing wave in the fiber.

Erbium has several important properties that make it an excellent choice for an optical amplifier. Erbium ions (Er3+) have quantum levels that allow them to be stimulated to emit in the 1540 nm band, which is the band that has the least power loss in most silica-based fibers. That gives them the ability to amplify signals in a band where high-quality amplifiers are most needed.

Energy levels of Er3+-ions in silica fibers are shown in Fig. 2. The energy level 4I15/2 corresponds to a ground state for laser transition. When the pump photon is absorbed Er-ion is excited to a level of 4I11/2 or 4I13/2, depending on the pump wavelength used. When erbium is excited by photons at 980 nm, it has a non-radiative decay to a state 4I13/2 where it can stay excited for relatively long periods of time (T1~10 ms). This property is extremely important, because the quantum efficiency of the device is dependent on how long it can stay in that excited state. If it relaxes too quickly, more photons are needed to keep it excited, meaning more input power is needed to make the amplifier work.

Erbium can also be excited by photons at 1480 nm (energy level 4I13/2). When excited that way, both the energy pumping process and the stimulated emission by the signal occur in the same wavelength and energy band.

Figure 2. Energy diagram of erbium ions in silica fibers.

EDFA pros…

  • gain is independent of the bit-rate and data type(digital data, video, sound,…)
  • wide bandwidth of the gain, which allows to amplify several wavelengths simultaneously → applicability for DWDM systems!
  • high gain and quantum efficiency
  • low noise and cross-talk
  • high level of saturation power(=output power)
  • polarization-independence
  • low losses at links

…and cons

  • Gain only around 1.55µm region
  • Relatively high price

Abbreviations and specific terms

DWDM / DenseWavelengthDivision Multiplexing
WDM / Wavelength Division Multiplexing
EDFA / Erbium Doped Fiber Amplifier
OSA / Optical Spectrum Analyzer (ANDO 6315)
ASE / Amplified Spontaneous Emission

Optical isolator

Device based on the rotation of the polarisation state of the beam by magnetic field works as an optical diode. Lets light though to only one direction.

Photodiode

Semiconductor device, in which an absorbed photon creates electron-hole pair resulting in photocurrent, which can be measured.

Diode laser

Drive current creates population inversion inside a semiconductor material, which leads to lasing.

Cross-talk

Interaction between the signals propagating within the same component of communication system.

Booster amplifier

Optical amplifier with high saturation power, which is placed just after the transmitter to increase the transmitted power.

Pre-amplifier

Preamplifiers are used to increase the transmission distance by putting an amplifier just before the receiver to boost the received power and to minimise the noise amplification.

Monochromator

Diffraction-based device, in which the desired wavelength is selected by rotation of the grating.

List of equipment

  • 1X2 (50/50) coupler
  • 1X2 switch
  • 10/90 coupler
  • 1550 nm diode laser
  • Optical spectrum analyzer(OSA)
  • Erbium doped fiber amplifier

All components except and EDFA are described in the instruction of Experiment 4: Wavelength Division Multiplexing.

Erbium doped fiber amplifier

The principles of Erbium doped fiber amplifier to be used in this work are close to those of the described above. The device does not have power switch and, thus, it is on when the power cable is attached to the power source. In order to minimise the back-reflection from EDFA the ends of the input and output fibers of the amplifier are inclined and polished. Attaching the normal FC-connector to the amplifier damages the end of the fibers. Thus, the other equipment can only be linked to EDFA by using special adaptors. Output optical power from EDFA is ~10mW which can damage eye when looking directly into the beam.

Do not look into the optical output beams of the optical amplifier neither that of the diode lasers!

Principles

The aim of this work is to study the properties of Erbium-doped fiber amplifier. The gain, the noise and the noise figure are measured as a function of input optical power. The effect of so-called Gain competition observable in DWDM application is studied. Finally, EDFA output optical power is measured as a function of time using the modulated input optical power and the wide-band photodetector.

Measurement 1: Noise spectrum of optical fiber amplifier

In fiber amplifiers not only the input signal is amplified but also the light resulting from the photons of spontaneous emission from the excited energy levels is amplified. This light is added to the signal as noise. The spontaneously emitted photons can be amplified at the frequencies corresponding to the energy levels, which have the population inversion created by fiber amplifier pumping. By studying the noise spectrum the spectral shape of the gain can be obtained. By pumping the fiber amplifier along the same direction with the signal (co-propagating pump) and studying the noise spectrum, the unused amount of total pump power can be estimated. This effect is due to the saturation of population inversion and non-ideal absorption of pump signal in Erbium-doped fiber.

Figure 3. Measurement setup used for studying the noise spectrum of the fiber amplifier without input signal.

Extreme care has to be taken when handling optical fibers. Good approach to link the fibers is to push the fiber end so long than the leading stripe goes to the end of connector slit and, thereafter, by easy turning connect the fiber properly.

  1. Plug EDFA into the voltage source. As EDFA does not have special switch, it is now on. Two LEDs on the front panel show the operation regime. In the case of additional red LED lightning, EDFA operates incorrectly.
  2. Connect the output of EDFA to the input of OSA (Optical Input).
  3. Select from OSA functions SPAN settings START WL: 350 nm and STOP WL: 1600 nm .
  4. Select OSA resolution (SETUP-settings) 5 nm.
  5. Select from OSA functions LEVEL settings AUTO REF LEVEL on.
  6. Measure the optical output power from EDFA within selected wavelength region (SWEEP function, SINGLESWEEP setting). Repeat the measurement by pushing SINGLE SWEEP newly. It is reasonable to conduct all measurements twice until the settings are not changed.
  7. Select from OSA functions SPAN settings START WL and STOP WL the wavelength region to be more accurately studied.
  8. Select OSA resolution (SETUP-settings) 1 nm.
  9. Measure the optical output power from EDFA twice. Plot the final spectral curve (COPY). If the printer doesn’t work, sketch an image of the display to your measurement field book.

Measurement2: Gain, ASE noise level and Noise figure as functions of signal power

The most important parameter of the fiber amplifier is the gain. Normally, the gain is given in decibels (dB) as a relation to the input signal. The fiber amplifiers can be used as pre-amplifiers or boosters, sometimes as in-line amplifiers, as well.

The gain of the fiber amplifier is based on the stimulated emission resulting from the population inversion created by pumping. At low levels of pumping power, the population inversion decreases as the distance from the input end increases, and depends on the signal power. At high levels of signal power, the population inversion (and accordingly the gain) drops to zero near the output end of the fiber amplifier. This is a result of saturation of the gain. Output power of the fiber amplifier operating under such conditions consists of amplified signal and input power. Thus, the gain is inversely proportional to the input power.

In communication applications even more important than the gain is the noise figure. The noise figure of the fiber amplifier is defined as the ratio of signal-to-noise ratios before and after the amplifier:

The noise figure is higher than unity for all amplifiers. In fiber amplifiers, the noise is mostly due to the amplified spontaneous emission (ASE). In telecommunication systems, the noise caused by ASE at the signal wavelength is essential to know. The estimation of this can be obtained using current measurement set-up.

Figure 4. Spectral output of the fiber amplifier. The rough method for determination of the noise level due to ASE is shown.

Figure 5. Setup for measurements of EDFA properties as functions of signal power.

EDFA operating characteristics can be quantatively measured using measurement system depicted in Fig. 5. The components should be handled with extreme care when building the set-up. As a first step, the noise spectrum of EDFA is measured qualitatively as a function of signal power.

  1. Select the wavelength interval to be measured from 1450nm to 1570nm.
  1. Select OSA resolution (SETUP-settings) 1 nm.
  1. Connect the output of Laser 1 to the Fiber coupler.
  1. Connect the output branch of 90 % of the coupler to the input fiber 1 of 1X2 switch.
  1. Connect the output branch of 10 % of the coupler to the input of EDFA.
  1. Connect the output fiber of EDFA to the input fiber 2 of 1X2 switch.
  1. Connect the output of 1X2 switch to the input of OSA (Optical Input).
  1. Turn 1X2 switch to the position 2. Now OSA (Optical Input) receives the signal amplified by EDFA.
  1. Measure the spectrum.
  1. Remove from LEVEL function AUTO REF LEVEL status.
  1. Remove from CENTER function AUTOCENTER status.
  1. Select from SWEEP function REPEAT status. Now OSA measures continuously.
  1. Confirm that the temperature controller of Laser 1 is on and that the potentiometer reading is 10.0.
  1. Switch on the mains power supply of the rack.
  1. Switch on the power supply of laser 1.
  1. Increase the current of laser 1 slowly up to 45 mA. What changes do you see in the spectrum measured by OSA? Write your answer in paragraph 2.1 in measurement table.
  1. Stop the REPEAT SWEEP function of OSA with STOP button.
  1. Decrease the current of laser 1 slowly down to zero.

Next assignment is to examine the properties of EDFA near the signal wavelength.

  1. Set AUTO REF LEVEL from LEVEL menu.
  1. Set AUTOCENTER from CENTER menu.
  1. Switch the 1X2 switch to position 1 (up). The output of laser 1 should now be connected to EDFA.
  1. Because the current of laser 1 is zero, the input of OSA should, in theory contain nothing but noise. This is not the case. Why? Write your answer in paragraph 2.2 in measurement table
  1. Adjust the current of laser one up to 25 mA.
  1. With the help of OSA, determine the peak power of laser 1 with this current setting.
  1. From the spectrum measured by OSA, determine the noise level with the help of LINE MARKER in MARKER menu. LINE MARKER 3 and 4 are horizontal markers.
  1. Record the measurement into TRACE A by setting TRACE A - FIX and change ACTIVE into TRACE B. Make sure that the DISPLAY of TRACE B is on.
  1. Switch the 1X2 switch to position 2 (down). Now the output of EDFA is connected to OSA.
  1. Determine the peak (output)power of EDFA with this input power.
  1. Determine the noise level (ASE) by using LINE MARKER 4 from MARKER menu.
  1. Record the measurement by setting TRACE B - FIX.
  1. Repeat measurements 6-12 and fill in Table 2.3 in the measurement table
  1. Draw a sketch of the output power of EDFA (dBm), ASE-level (dBm), Gain (dB) and the noise figure (dB) as a function of input power (dBm) in paragraph 2.4 of measurement table

Remember to include the effect of the 10/90 fibre coupler to the input power of EDFA.

Measurement field book

Date:______

Group number:______

Names and student numbers:

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Measurement 1: Noise spectrum of optical fiber amplifier

Sketch the spectrum below

Name the spectral components. Can you tell anything about the structure of the EDFA by looking at the output spectrum? What wouldyou say is the possible wavelength region this EDFA could be used in telecommunications?

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Measurement2: Gain, ASE noise level and Noise figure as functions of signal power

2.1

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2.2

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2.3

Injection current / Laser power at the input of the amplifier / noise level at the input of the amplifier / Amplified signal / Gain / noise level at the output of the amplifier / Noise figure NF
mA / dBm / dBm / dBm / dB / dBm / dB
17
20
25
30
35
45

2.4

Gain, ASE level and NF vs. Launched power

Evaluatethe lab works!

Problems? Improvements?

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1