ANEXO B: Tabla asignación de valores a los controles

The Microstructured Fibre

Figure 6.9: SEM image of the cross section of the microstructured fibre used in the experiments.

The amplifying fibre used in the experiments is a hexagonally stacked pure silica small core microstructured fibre. A scanning electron microscope (SEM) image of the fibre cross section is shown in Fig. 6.9.1 _{From this image we could}

estimate d and Λ which in turn allowed us to calculate the nonlinearity and dispersion properties of the fibre. For reference, the relevant fibre parameters are listed in Table 6.2.

Parameter Value Hole diameterd 1.1µm Hole-to-hole spacingΛ 1.2µm Nonlinearity parameterγ 90×10−3_W−1_m−1 Dispersion parameterβ₂ 63×10−3ps2m−1 Lossα 0.2 dBm−1 LengthL 5 m

Table 6.2: Microstructured fibre parameters used in the experiments.

Because of the extremely small core size we expect the nonlinear effects in this fibre to be dramatically enhanced, and in particular we found it to exhibit some very unusual Raman properties. Specifically, one would expect that for

1_{A SEM is a microscope that uses electrons rather than light to form an image so that higher}

Chapter 6 Parabolic Evolution in Microstructured Fibre Raman Amplifiers 1530 1550 1570 1590 1610 1630 −16 −12 −8 −4 0 Wavelength (nm) Power (arb.)

Figure 6.10: Raman spectrum for the microstructured fibre of Fig. 6.9 taken at the time of the measurements.

a pure silica fibre the peak of the Raman gain spectrum should appear down- shifted from the pump by 13.2 THz so that for a 1.536µm pump we should choose a signal beam at 1.649µm. However, from Fig. 6.10 which shows the Raman spectrum of this fibre at the time of our measurements, clearly this is not the case as the peak gain is in fact at∼1.62µm. Furthermore, we also found that this Raman peak shifted in wavelength as the fibre was recleaved and the coupling was varied. This anomaly can possibly be attributed to the nonuni- formity of the fibre cross section and examples of alternative Raman spectra obtained in this fibre are shown in Fig. 6.11. Nevertheless, as a complete in- vestigation into the Raman properties of this fibre was beyond the scope of this thesis, we simply attributed this behaviour to the extreme nature of the fibre structure. In addition, it is worth noting that because of the small core

1550 1580 1610 1640 −30 −20 −10 0 Wavelength (nm) Power (dB) 1550 1600 1650 1700 −40 −30 −20 −10 0 Wavelength (nm)

Figure 6.11: Alternative Raman spectra for the microstructured fibre of Fig. 6.9 show- ing the wavelength dependence of the coupling.

Chapter 6 Parabolic Evolution in Microstructured Fibre Raman Amplifiers

size, the coupling efficiencies obtained during this experiment were typically of the order of 10%or less and were highly dependent on the wavelength of the coupled light.

The Signal Pulse Source

To ensure efficient parabolic pulse amplification in the microstructured fibre described above we required a picosecond pulse source at the peak of the Ra- man gain curve: ∼ 1.62µm. To this end, Dr M. O’Connor developed a syn- chronously pumped optical parametric oscillator (OPO) based on wavelength conversion in a periodically poled lithium niobate (LiNbO3) crystal (PPLN) (see Chapter 10 for a more complete description of PPLN). A detailed techni- cal description of a similar OPO has been published in Ref. [59].

A schematic diagram of the signal pulse source is given in Fig. 6.12. The pump source is a mode-locked Nd:YLF laser (Microlase DPM-1000-120), cou- pled with a Nd:YLF amplifier system, which operates at a wavelength ofλ = 1.047µmto produce4 ps(FWHM) hyperbolic secant pulses at a repetition rate of120 MHz. These pulses are injected into the OPO ring cavity where the wave- length conversion occurs in a19 mmlong PPLN crystal with a grating period of

30µm. The crystal was housed in an oven (dashed box) where the temperature could be adjusted to tune through the converted wavelengths and for signal

Figure 6.12: Experimental configuration of the OPO used to generate picosecond pulses at1.62µm.

Chapter 6 Parabolic Evolution in Microstructured Fibre Raman Amplifiers

and idler waves of λS = 1.62µm and λI = 2.96µm, the operating tempera-

ture was 135◦C. The cavity was surrounded by three highly reflecting (HR) mirrors and an output coupler (OC) with a 40% transmission fraction (at the signal wavelength) to produce∼550 mWof average power.

At this point we note that the source that will be used to pump the Raman in- teraction only operates at a repetition rate of the order of100 kHz(see following section). Thus if we were to consider launching the signal beam into the fibre at the maximum repetition rate of120 MHz, only 1out of∼1200pulses would be amplified. As slower detection devices [such as an optical spectrum anal- yser (OSA)] integrate the optical signals over a window in time, this averaging effect would make it almost impossible to detect the gain in a single amplified pulse. As a result, to reduce the number of pulses that enter the fibre stage an accousto-optic modulator (AOM) is used to chop the pulse train from the OPO into a series of pulse windows which occur at a repetition rate of 100 kHz. A schematic illustration of this can be seen in Fig. 6.13. An example of a typical window of pulses for an AOM trigger width of350 ns, obtained from a100 MHz

digital oscilloscope, is plotted in Fig. 6.14(a).2 _{Here the departure of the pulse}

window from the perfect rectangular function of Fig. 6.13 is due to the finite response time of the AOM and for window sizes less than ∼ 4µs leads to a reduction in the peak power of even the most central pulses. With the AOM window fully open (8.3µs) the maximum average signal power before the mi- crostructured fibre launch was30 mW.

Figure 6.13: Reduction of the signal pulse repetition rate for launch into the Raman amplifier system.

Finally, Fig. 6.14(b) shows an autocorrelation trace of the input pulses into the Raman amplifier system. Assuming a hyperbolic secant pulse profile, the au- tocorrelation width of 6.12 ps indicates a true input pulse width of ∼ 4.0 ps

(FWHM). The fine oscillatory structure on the envelope of the pulse is due to the chopping of the pulse train by the AOM.

2_{Although the pulse-to-pulse spacing is large enough to be resolved accurately, the small}

Chapter 6 Parabolic Evolution in Microstructured Fibre Raman Amplifiers −100 0 100 200 0 0.5 1.0 Time (ns) Power (arb.) 70 75 80 85 90 0 0.5 1.0 Delay (ps) Autocorrelation (arb.) (a) (b)

Figure 6.14: (a) Typical example of a window of signal pulses for input into the mi- crostructured fibre. (b) Autocorrelation trace of the input pulses into the microstruc- tured fibre.

The Pump Pulse Source

The pump pulse source was also developed within the ORC and a detailed technical description can be found in Ref. [60]. Fig. 6.15 shows a schematic diagram of the experimental setup. The laser is seeded by a simple laser diode providing1 mWCW at1.536µmwhich is externally modulated to produce5 ns

square pulses. To compensate for the losses resulting from the chopping of the beam, an amplifier is also included in this preliminary stage. These pulses are then passed into a two stage, high gain Er3+_{:doped fibre preamplifier before}

CW Laser

Diode EDFA EOM EDFA AOM EDFA AOM

Pulse Generator Modulation Stage Stage 1 Stage 2 Er-Yb LMA Stage 3

Figure 6.15: Experimental configuration of the high power pump pulse source to pro- duce pulses at1.536µm.

Chapter 6 Parabolic Evolution in Microstructured Fibre Raman Amplifiers

finally being launched into the large mode area (LMA) Er3+_–Yb3+_{:doped fibre} amplifier. The three amplifier stages were separated by two AOMs triggered to gate through the pulses whilst stopping the ASE from passing between ad- jacent stages. In addition, a1 nm bandpass filter was placed before the LMA amplifier stage to eliminate the small amount of ASE in the time slot of the pulse. With the pulse generator operating at a repetition rate of 100 kHzthis setup produced5 nspulses with peak powers of∼1 kW.

6.4.2 Experimental Setup

The complete experimental setup to generate parabolic pulses in a microstruc- tured fibre Raman amplifier is illustrated in Fig. 6.16. The AOM in the sig- nal setup was triggered off the pump source so that the signal pulses were launched into the microstructured fibre to overlap with the pump pulses. Po- larisation controllers (PC) were included in the pump and signal launch paths so that both beams could be launched onto a single polarisation axis. Typi- cally, the average pump power before the microstructured fibre launch was

250 mW. Coupling into the fibre was achieved using a 2.75 mm focal length lens and coupling efficiencies of ∼ 10% and ∼ 5% were usually obtained for the pump and signal beams, respectively.3 _{This resulted in a maximum pump}

peak power of 40 W and a maximum signal peak power of 4.5 W (when the AOM window was fully open: 8.3µs) in the microstructured fibre.

To verify that the pulses were in fact experiencing gain from the Raman in- teraction, before attempting to measure autocorrelation traces of the output pulses we first looked at the output from the fibre on the digital scope. As mentioned previously [Section 6.4.1], the small bandwidth of the scope causes an exaggeration of the observed pulse widths so that this is not a true represen- tation of the output pulses. However, it is clear from Fig. 6.17(a) that the signal pulse which is overlapped by the pump pulse is experiencing gain. In order to see the unamplified pulses on this scale, this image was taken for a relatively low average input pump power of 100mW (corresponding to a peak pump power in the fibre of ∼ 13W) and resulted in ∼ 8 dB of gain. Increasing the input pump power to 250 mW(∼ 40 Wof peak pump power in the fibre), the

Chapter 6 Parabolic Evolution in Microstructured Fibre Raman Amplifiers

Figure 6.16: Schematic diagram of the experimental setup for parabolic pulse gener- ation in a microstructured fibre Raman amplifier. WDM=wavelength division multi- plexer, MF=microstructured fibre and RM=removable mirror.

amplified pulse in Fig. 6.17(b) is now sufficiently large that the unamplified pulses are undetectable on this scale, and this corresponded to a total pulse gain of∼ 22 dB.

Once we were satisfied that the signal pulses were experiencing Raman gain we then aligned the output to the autocorrelator. This was done with the AOM window fully open so that the power was at a maximum. A removable mirror was placed just before the entrance to the autocorrelator so that the output could be redirected to an OSA and thus we could obtain measurements of the pulses in the temporal and spectral domains under the same conditions.

−100 0 100 0 0.5 1.0 Time (ns) Power (arb.) −20 0 20 40 60 0 0.5 1.0 Time (ns) (a) (b)

Figure 6.17: Oscilloscope traces showing signal pulses with: (a)8 dBand (b)22 dBof Raman gain.

Chapter 6 Parabolic Evolution in Microstructured Fibre Raman Amplifiers

We recall that as the defining features of a parabolic pulse are the steep edges of the temporal profile and its linear frequency chirp [Chapter 4], a thorough characterisation of a parabolic pulse requires the use of the FROG measure- ment technique which allows for the complete retrieval of both the intensity profile and the phase (as described in Section 5.3.4). However, at the time of these measurements a FROG device at 1.62µmwas not available. Thus in or- der to establish any parabolic nature of the amplified pulses it was important to consider both the temporal and spectral measurements where, in the time domain the steep parabolic edges will translate to an autocorrelation trace with steep edges, and in the frequency domain the strong chirp will result in a broad spectral width [Section 3.2].