Chapter 6 Experimental results

Chapter 6

Experimental results

After having realized the simulation software we were ready for experimental measurements. Our aim was to obtain the NOLM input/output power characteristic, so we decided to use a pulse train (whose shape was almost triangular) as the input signal. As a result, for each pulse the instantaneous power flows almost linearly from zero to the peak power (for the rising edge) and vice-versa (for the falling edge), so the power characteristic’s shape can be initially intuited by observing the outgoing signal using an oscilloscope. In order to obtain an accurate result, input and output waveforms were saved and power characteristic was calculated using a spreadsheet.

Simulations revealed that a very high peak power could be needed to well resolve the NOLM characteristic. Using a high power Erbium-Doped Fiber Amplifier (EDFA) and low duty-cycle signals (with period T = 1μs and pulsewidth τ = 12ns) peak power as high as 50W could be reached.

6.1 50/50 NOLM exploiting Not-PM Fibers

Fig.6.2 Experimental Setup for Not-PM fibers.

Fig.6.2 shows the experimental setup used to characterize NOLMs based on Not-PM fibers. The electrical pulse train signal is generated by the agilent waveform generator. The tunable optical source generates a continuous wave at 1550nm which is then modulated by a Mach-Zehnder modulator thus generating an optical pulse train signal. Mach-Mach-Zehnder modulator needs a polarization controller in order to work in optimized conditions. The pulse train is then ready to be amplified by an EDFA and thanks to a low duty-cycle, peak power up to 50W could be reached. The pulse enters the NOLM and is split into two counterpropagating halves; we used a 50:50 fiber coupler and a variable attenuator for the internal unbalancing loss. The internal loop polarization controller is used to make the two signals be polarized on the same axis when reaching the coupler after a round trip. When pulse peak power is low nonlinear effects are negligible, so if the pulses are aligned their interference is completely destructive and the input signal is not transmitted out of the loop; on the contrary, if the signal halves have not the same polarization they don’t completely interfere and a fraction of input power is transmitted out of the NOLM. As a result, we can infer a procedure to optimize the polarization: at low input power, polarization must be controlled so that the output power is minimized, nearly cancelled.

6.1.1 OFS Highly Nonlinear Fiber

Ofs HNLF has a nonlinear coefficient γ = 10.5W-1

km-1 thanks to a Germanium-doped high-index core and a deeply depressed ring yielding an effective area around 11-12µm2. The spool we had at our disposal was 250m long.

Fig.6.3 Input Signal. Fig.6.4 Output signal.

By comparing Pin and Pout values we obtained our first experimental characteristic:

Fig.6.5 Experimental characteristic of a 50/50 NOLM with 250m HNLF.

The obtained characteristic substantially reflects what we expected, since it agrees with the simulated one:

Fig.6.6 Simulated results.

The NOLM we realized is suitable for pedestal suppression and noise reduction on space bits as no signal is present at the output for input power up to 1W. For input pulses with peak power around 6W even noise on marks is lightly reduced. In order to reach the same relative phase displacement with lower input power two approach would be possible. One is to increase the internal loop lumped loss; such a change reduces the counterpropagating wave’s phase shift thus increasing the relative one. The drawback is that the output power is reduced accordingly with the loss enhancement. The other way is to increase fiber length, for example doubling it should half the needed input power.

6.1.2 Bismuth-based Nonlinear Fiber

This particular fiber has an extremely high nonlinear coefficient γ = 1250W-1

km-1 thanks to the high nonlinearity of the glass material and the small effective core area. The fiber we used was only 1m long but it performed well allowing us to realize a more compact device. While for HNLF it was γL = 2.5W-1_{, in the present case 1m of Bi-NLF has γL = 1.25W}-1

so more power is needed to obtain the same behaviour, and we increased the internal loop loss to 5dB to enhance the phase displacement of the two counterpropagating beams. Simulation shows that under ideal conditions a peak power of about 9W is required to reach the first maximum on the characteristic:

Fig.6.7 Simulation of a Bi-NLF based NOLM

However, as we can see in Fig.6.8, fiber is fusion-spliced with standard single-mode fiber through UHNA (Ultra High Numerical Aperture) fiber; each splice introduces a loss of about 3dB and this deeply influences on NOLM’s behaviour.

Because of this loss our first results showed only the first part of the characteristic since input power was not enough to induce a relative phase shift of π.

Fig.6.9 Input signal. Fig.6.10 Output signal.

Figs.6.9 and 6.10 show the effect of a NOLM used as a pulse compressor, with an output pulsewidth of 9ns while input was of 12ns. In order to obtain an higher peak power we further reduced duty-cycle by increasing the signal period.

Fig.6.13 Experimental Characteristic.

Fig.6.13 shows that internal loss was such that needed input power was twice than the expected one.

In spite of these problems our results with Bi-NLF were satisfactory and this fiber is promising for the implementation of more compact optical devices.

6.1.2.1 Implementation of a λ-converter exploiting Bi-NLF

As an application of Bi-NLF based NOLM we implemented a wavelength converter. The original signal is a 10 GHz pulse-train at λp = 1542 nm with a pulsewidth τ = 3 ps and period

T = 100ps. Such a signal can’t be viewed using a conventional oscilloscope because of the limitations of electronic devices. To overcome these limitations we used an autocorrelator: based on the Michelson interferometer, it provides at output port an electric signal proportional to the autocorrelation of the input signal.

Fig.6.14 Autocorrelation of the 3 ps input pulse.

A wavelength converter can be obtained by using a 50/50 NOLM in XPM configuration; the original signal is used as a control signal, thus modulating a continuous wave (cw) at λcw =

1560nm.

When the control signal is off only the cw propagates along the loop in clockwise and counterclockwise directions. The internal polarization controller must be tuned to align the two halves into the coupler to make the loop act as a mirror. The pump insertion coupler introduces a 3dB loss into the loop, thus unbalancing it. So cw power must be low enough to not induce SPM. When the control signal is on, the clockwise half experiences a XPM-induced phase shift:

p c P 2 L 2 φ = γ

While for the counterclockwise travelling half is:

p cc P 2 L 2 φ = γ

It is important to tune the pump polarization controller in order to maximize the nonlinear interactions between the control signal and the cw. As a result, the pump pulse make the two counterpropagating halves interfere constructively so a copy of the original pulse train is generated at λcw at NOLM output. A band-pass filter is required to suppress the signal at λp.

As seen before, the bismuth fiber we used suffered of loss on the internal splices; these losses were due to reflections introduced by the internal splices, so a fraction of cw is always present at the output port, thus generating a pedestal. Because of this fact, the autocorrelator was not able to distinguish the pulses, so we examined the output signal using an oscilloscope and an optical spectrum analyzer (OSA).

The oscilloscope can’t show the right shape and width of the pulses because of its limitations but it gives us useful indications: it shows the presence of the pulse train at λcw and the

presence of the pedestal. Also the OSA shows the modulated signal at λcw and the pump

signal at λp, which is still visible after the filter but whose power is 30dB lower than the signal

at λcw. The shape of the generated pulses is almost equal to the original pump pulse after

having travelled along the bismuth fiber. So we opened the loop and examined the effect of propagation along Bi-NLF on the 3 ps pulses.

Fig.6.17 Evaluation of the effects of propagation along Bi-NLF.

Fig.6.18 Autocorrelation of the input pulse. Fig.6.19 Autocorrelation of the output pulse.

Autocorrelation shows that the fiber introduces a light enlargement in the pulsewidth due to chromatic dispersion. The outgoing pulse is estimated to be 4 ps.

6.2 50/50 NOLM exploiting PM Fibers

Fig.6.20 Experimental Setup for PM fibers.

Fig.6.20 shows the experimental setup used to characterize a polarization maintaining fiber-based NOLM. Some changes were required in comparison with the previous one: fiber coupler has been replaced with a PM one, so internal loop polarization controller was removed since counterpropagating beams travel along fiber’s slow axis thus remaining always aligned. However signal coming out from EDFA has a random elliptic polarization, so we have to interface a Not-PM fiber with a PM fiber, where we want the signal to travel along the slow axis. To do that, we need a polarization controller and a Polarization Beam Splitter (PBS). Such a device splits a random-polarized beam into two beams and conveys them on the slow axis of two different PM fibers. So the polarization controller is used to regulate input signal polarization so that most power is conveyed on one output, which is then connected to the fiber coupler.

6.2.1 OFS PM Highly Nonlinear Fiber

This fiber exhibits the same nonlinear properties of the Not-PM version. We had at our disposal two 250m reels and a 500m one. We carried out several tests using 500m and 1km configurations but this fiber performed worse than the Not-PM version; this is non properly imputable to the fiber itself but it is due to a 3dB loss each reel presented. So simulation performances couldn’t be equalled.

At first we tested a 500m spool:

Fig.6.21 Input signal. Fig.6.22 Output signal.

Figs.6.21 and 6.22 show NOLM behaviour for an input pulse of 12ns;

The characteristic shows that for an input peak power of 15W interference at the coupler is completely constructive while at 25W input signal is reflected back and output power is reduced to zero. Due to the above-mentioned losses, experimental results are worse than simulated ones, as we can see in Fig.6.24:

Fig.6.24 Simulated characteristic.

In order to reduce input needed power we added the two remaining 250m reels to reach 1km. Nevertheless this increased the total fiber loss so doubling fiber length didn’t allowed us to half the required power. We used different values of duty-cycle (4%, 2%, 1%) and our results are shown below:

Fig.6.27 Input signal with duty-cycle = 2%. Fig.6.28 Output signal with duty-cycle = 2%.

Fig.6.29 Input signal with duty-cycle = 1%. Fig.6.30 Output signal with duty-cycle = 1%.

Fig.6.31 Experimental characteristic for different values of duty-cycle.

All our tests agreed on the fact that 11W input peak power is needed for the counterpropagating beams to reach a relative phase displacement of π.

6.2.2 Photonic Crystal Fiber

As seen in chapter 5, Photonic Crystal Fiber exhibits a nonlinear coefficient γ = 40W-1

km-1 thanks to the presence of air-holes in the cladding. This leads to a huge index step because of the large difference in the refractive index n between air (n = 1) and silica (n = 1.45). The fiber we tested was 30m long and presented a 16dB/km attenuation. This implies a Leff of about 18m. Fig.6.32 shows the simulation results:

Fig.6.32 Simulated characteristic of a 30m PCF based NOLM.

In our experimental measurements we used a 18W pulse as input signal and we could reach the first maximum:

Fig.6.35 Experimental results.

Experimental results are in agreement with simulated ones: output is null for input power up to 2W and 15W are required to reach the first maximum.

6.3 90/10 NOLM exploiting OFS HLNF

After having tested different 50/50 coupler-based configurations, we decided to carry out some experiments using different splitting ratios. One possible application of 50/50 NOLMs is noise reduction on space bits, as such a device acts as a mirror for low power signals (like noisy space bits). In order to reduce noise on marks it would be necessary a device whose power characteristic presented a flattened region corresponding to the input peak power. Simulation software shows that such a behaviour can be obtained using a highly asymmetric nonlinear loop mirror, with a splitting ratio of 90/10:

Fig.6.36 Simulated characteristic of a 90/10 NOLM with 1km of HNLF.

Fig.6.36 simulates the behaviour of a 90/10 coupler-based NOLM using 1km of OFS Not-PM HNLF. For power up to 800mW input signal is linearly transmitted out from the loop, while for power between 800mW and 1.3W nonlinear phase shift creates a flattened region where an increment of input power is reflected back thus leaving output unchanged. Experimental measurements agreed with simulations, as we can see below:

Fig.6.37 Input signal. Fig.6.38 Output signal.

Fig.6.39 Experimental characteristic.

So if a noisy pulse train with a peak power around 1W enters such a NOLM, noise on marks is reduced while the other parts of the input signal are transmitted out unchanged.

6.4 All-Optical Regeneration

We have seen how to project and realize a NOLM able to clean noise on spaces (with a 50/50 coupler) or marks (with a 90/10 one). The obtained results can be exploited for 2R regeneration (Reamplification and Reshaping). As for space noise reduction we can consider the characteristic of a NOLM based on a 250m long Not-PM HNLF (see 6.1.1):

Fig.6.40 Experimental characteristic of a 50/50 NOLM with 250m HNLF.

This characteristic exhibits a flattened region for input power up to 1W, so if a noisy pulse train whose peak power is between 2W and 5W enters the NOLM, noise on spaces is reduced while the pulses are transmitted out unchanged (except for a reduction of their pulsewidth). If input peak power is about 4.5W, than the outgoing signal is a pulse train with a peak power around 1.2W, clean spaces and noisy marks. When such a pulse train enters a NOLM whose characteristic is like that in Fig.6.41:

Fig.6.41 Experimental characteristic of a 90/10 NOLM with 1km of HNLF.

then noisy marks fall into the flattened region and noise is strongly reduced. Such a double-NOLM structure presents a global characteristic with two flattened regions: for input power up to 1W output is null and for input located between 4W and 5W output power is around 160mW:

In order to better understand the behaviour and the possibilities of a two-NOLM-cascade we decided to implement another simulation software. It has been obtained by doubling the previous one, using first NOLM’s Pout as second NOLM’s Pin. We also introduced an

amplitude controller between the fiber loops which can act as an attenuator or an amplifier.

Fig.6.43 Simulation software for 2-NOLM-cascade.

Simulations showed us that the flattened region used to clean marks can be extended by exploiting better the first NOLM; in fact the region around the first maximum can be used to partly reduce noise on marks.

The first NOLM “compresses” input power oscillations located between 4.5W and 7W into output power oscillations located between 1.1W and 1.45W. As a result, our aim is now to project and realize a 90/10-coupler-based NOLM whose flattened region is located between these values. Such a behaviour can be obtained using 500m of HNLF:

Fig.6.45 Noise suppression in 90/10 NOLM.

In conclusion, we realized a device able to turn any input power located between 4.5W and 7W into an output power of about 250mW and to suppress any input power below 1W:

6.4.1 All-Optical Regeneration of a 10 Gb/s signal

In order to evaluate the potential of the 2R NOLM-based regenerator we carried out a qualitative analysis based on the eye-diagram of a 3ps 10 Gb/s signal.

Fig.6.47 Experimeental setup for 2R Regeneration of a 3 ps 10 Gb/s signal.

Pico-source generates a 3ps 10GHz pulse train which is modulated through a Mach-Zehnder modulator by a pattern generator, whose output signal is a pseudo-random binary sequence (PRBS); the pattern generator must be triggered by the source and an optical delay-line is required to synchronize the pulse-train with the PRBS. The obtained 10 Gb/s signal is then corrupted with “artificial” noise: an erbium amplifier with no input signal introduces amplified spontaneous emission (ASE). Fig.6.48 shows the spectrum after the introduction of ASE: the signal is located at 1551 nm while ASE is spread all around.

Fig.6.48 Signal spectrum after ASE insertion.

This spectrum analysis is fundamental to estimate the OSNR (Optical Signal to Noise Ratio) within the signal band: by interpolating the ASE spectrum we can estimate noise power at signal wavelength; Fig.6.48 shows that noise power is about -31dBm and signal power is about -16dBm, so OSNR is about 15dB. At this point a band-pass filter suppresses noise located out of signal band. Since the filter is narrower than the signal spectrum, the outgoing pulses are about 10 ps and even signal power has been reduced. As a result, signal’s power is too low to enter directly the regenerator, so it has to be preamplified and then filtered again. Fig.6.49 shows the eye diagram before entering the regenerator. The 2R regenerator includes an erbium amplifier which gives to the signal the required peak power; the first NOLM, realized with a 50:50 coupler, suppresses noise on space bits and partly reduces noise on marks (Fig.6.50). Then the second NOLM, with a 90:10 coupler, suppresses the remaining noise on marks (Fig.6.51). The variable attenuator located between the loops allows to optimize the second NOLM by making the pulse peak power fall exactly on the flattened region of its characteristic.