Definir: Esclarecer el problema y el proceso

Due to the difficulties in obtaining significant signal encountered with the 1500 nm AOM it was decided to try the 750 nm AOM. This would maximise

Time (ps) 0 5 10 15 20 25 30 35 EOS signal (V) 7.4e-5 7.6e-5 7.8e-5 8.0e-5 8.2e-5 8.4e-5 8.6e-5 8.8e-5 9.0e-5 Time (ps) 0 5 10 15 20 25 30 35 EOS signal (V) 7.4e-5 7.6e-5 7.8e-5 8.0e-5 8.2e-5 8.4e-5 8.6e-5 8.8e-5 9.0e-5 Time (ps) 0 5 10 15 20 25 30 35 EOS signal (V) 7.6e-5 7.8e-5 8.0e-5 8.2e-5 8.4e-5 8.6e-5 8.8e-5 9.0e-5 Time (ps) 0 5 10 15 20 25 30 35 EOS signal (V) 7.2e-5 7.4e-5 7.6e-5 7.8e-5 8.0e-5 8.2e-5 8.4e-5 8.6e-5 8.8e-5 9.0e-5 (a) (b) (c) (d)

Figure 3.13: EOS measurements on the LT-GaAs device close (<50 µm) to the generation point of the QUIC signal. Note the candidate peak in each around the 13–15 ps point (circled).

the 1500 nm power available but of course reduce the 750 nm power. However, if successful there would be the option of attenuating the 1500 nm power to maximise the QUIC signal (c.f. Section 3.4.2).

The 1500 nm AOM was therefore removed from the 1500 nm arm of the QUIC system and the 750 nm AOM inserted in the 750 nm arm. The pulse paths were again realigned, first with each other (zero delay being found using the GaAsP photodiode as usual) and then realigned onto the photoconductive switch gap. Since the paths had changed as a result of the swap-over of AOMs the zero delay between the QUIC pulses and the probe pulses needed to be ascertained again, but as expected there was considerable

Time (ps) 0 5 10 15 20 25 30 EOS Signal (mV) 78 80 82 84 86

Figure 3.14: Average of results in Figure 3.13 a–d after shifting the timebases to overlap the candidate peaks.

difficulty in achieving this using an applied bias and an oscilloscope as the 750 nm signal was greatly reduced. Despite this, an approximation to the zero delay point was found and the EOS scan range increased to try and ensure the waveform of any QUIC electrical pulse was captured. The preliminary scans found no evidence of a signal, and further attempts were frustrated when it was found that the AOM had developed a fault and was failing to produce any modulated signal.

A second AOM was available and replaced the broken one in the 750 nm arm, requiring the beams to be realigned once more. Preliminary scans were carried out but the experimental work had to be stopped once again when the second AOM also failed. An investigation located the source of the fault to the power supply that supplied the 12 V required to power the AOM. It appeared that the power supply was occasionally spiking, sending excessive voltage to the connected AOM that damaged the internal circuitry. Unfor- tunately although the cause of the fault had been identified and the power supply could be replaced to prevent future occurrences, the lack of another

spare 750 nm modulator prevented this part of the experiment from being completed. The lack of conclusive results from the 750 nm measurements was frustrating, but this form of modulation should still be considered in any such future work.

3.6

Conclusions

A system was successfully designed and constructed with the purpose of gen- erating QUIC currents within a transmission line device mounted on semi- conductor. The creation of a QUIC current was demonstrated conclusively by measurements of the behaviour of charge accumulation across the de- vice for several different kinds of variation in the generating optical pulses, each measurement agreeing with the theory. An existing EOS system was then adapted and integrated with the QUIC system to enable time-resolved measurements of the electrical signals generated and a number of promis- ing measurements were made[29]. These experiments are highly important to this particular phenomenon and have, to current knowledge, never been carried out before. What has been highlighted is the difficulty of balancing the requirements of powering both a signal source and a probe using a single laser, as well as balancing the need for high frequency modulation with the consequent loss of power. Yet it must be realised that it is not as simple a matter as providing a separate laser source for the OPO and EOS system, since the problem is then introduced of needing to somehow synchronise both sources together and the inherent zero-jitter advantage of using a split source is inevitably lost.

The presence of a time-resolved QUIC signal is extremely encouraging, as is the size of the apparent pulse width. The issues are in the lack of power in the signal which are insufficient, it seems, to be of practical use in charac- terisation of high-speed instruments. It is important to realise that diverting power from the probing system is not enough, as a measurement of the pulse will still be required if the response of an instrument is to be characterised. The difficulties encountered when performing EOS suggests the best direc- tion for further work is primarily one of improving the power of the OPO, its

stability, maximising the available power for EOS and investigating methods to enhance the EOS signal-to-noise ratio at low powers.

Measurement of Ultrafast

Pulses

Once there are sources capable of producing optical pulses with durations measured in femtoseconds, there is a requirement for means of exploring and measuring the characteristics of such pulses at femtosecond timescales. How is it known that the pulse really is as short as it is thought to be, or even the shape that it should be? How is it known if it is transform-limited or frequency-chirped, if it is a well-behaved Gaussian or a distorted or even doubled pulse? It is true that the spectrum provides some information about the pulse, but without knowing the spectral phase there is no way of knowing the temporal shape of the pulse — a pulse that has travelled along a kilometre of optical fibre can have the same spectrum as one that has not. The most the spectrum alone will tell the researcher is that the pulse cannot be any shorter than the value set by Equation 2.3, but if there is a need to know more (and often there is) then what is required is some scheme for comprehensive temporal measurement of the femtosecond pulses. Yet measurement of an event requires a shorter event itself, so how is it possible to measure the shortest event available? The answer lies in using the pulse to measure itself.

Figure 4.1: A schematic for an intensity autocorrelation. The incoming pulse is split into two replicas one of which is time-delayed with respect to the other before both replicas are focused non-collinearly onto a nonlinear crystal. The resulting second-harmonic signal is measured using a photodetector. A slit prevents any second harmonic produced by each individual pulse reaching the detector.

4.1

Autocorrelation

For a long time, the (unsatisfactory) answer to the measurement of ultrafast pulses was to deduce this from an autocorrelation of the pulse. A typical autocorrelation set-up is shown in Figure 4.1. The pulse is split into two intensity replicas which are then overlapped in some medium (in this case a nonlinear crystal) to produce a signal that corresponds to the mutual corre- lation of the overlapped pulses.

By varying the time-delay between the pulses one serves to gate the other and the autocorrelation is produced. In the case where the signal is the second-harmonic of the two pulses and the pulse paths to the crystal are non-collinear as shown in the diagram, and making the assumption that the nonlinear medium responds instantly in time, the polarisation signal

resultant towards the detector will be

P(τ)∝E(t)E(t−τ) (4.1) where τ is the delay between the two pulses. On making the reasonable assumption that the electric field of this SHG signal is proportional to the polarisation, the intensity of the signal will be

I(τ)∝ |A(t)A(t−τ)| (4.2) where A(t) is the complex amplitude of the pulse. It is usually the case that the detector used to measure the resulting signal has a response much slower than the duration of the pulse being measured and so the signal measured will be an integration over time, this result being the intensity autocorrelation:

IAC(τ)∝ Z _+∞

−∞ I(t)I(t−τ)dt (4.3)

(This is also known as the background-free intensity autocorrelation, due to the lack of background signal from SHG of the individual pulses reaching the detector.) Since the intensityI(t) =|A(t)|2 , it is immediately apparent that the phase information of the pulse is lost. Also, on making the substitution

t0 _{=t−τ it is seen that} IAC(τ) = Z _+∞ −∞ I(t 0_+τ)I(t0_)dt0 _=I AC(−τ) (4.4)

and so the intensity autocorrelation is symmetric in time. Therefore, it fol- lows that symmetric and asymmetric pulses can yield the same autocorrela- tion, and more information about the pulse has been lost. Yet the intensity autocorrelation still provides some information, since if the pulse shape is al- ready known then deconvolution can be used to calculate the pulse duration. For some basic pulse shapes it is enough to divide the ∆τ of the autocorre- lation by a simple factor — by √2 for a Gaussian pulse, for example, or by 1.54 for a sech2 pulse.

Figure 4.2: A schematic for an interferometric autocorrelation. The incoming pulse is split into two replicas one of which is time-delayed with respect to the other before both replicas are focused collinearly onto a nonlinear crystal. A filter blocks the fundamental signal and the second-harmonic is measured using a photodetector. This second harmonic will be a combination of the product of the two pulses and their individual second harmonics.

tic, the lack of phase and temporal shape information limits its utility and so other methods have been investigated to try to obtain more pulse infor- mation. One of the most important developments was the interferometric autocorrelation [30] (also known as the fringe-resolved autocorrelation, or FRAC). This differs from the normal intensity autocorrelation in that the two replica pulses overlap collinearly within the crystal, with the same po- larisation. The autocorrelation is then resolved interferometrically. This set-up is shown in Figure 4.2.

In this situation the polarisation signal incident on the detector will be given by

∝ (A(t)eiωt_eiφ_+A(t−τ)eiω(t−τ)_eiφ₎2 _(4.6)

(4.7) where φ is the phase of the carrier wave relative to the amplitude envelope and ω is the carrier frequency. As with the intensity autocorrelation, the slower detector integrates over time and this produces the interferometric autocorrelation equivalent to (4.3) I(τ)F RAC ∝ Z _+∞ −∞ ¯ ¯ ¯A2_(t)ei2ωt_+A2_(t−τ)ei2ω(t−τ)_{+ 2A(t)A(t−τ)e}iωt_eiω(t−τ)¯¯_¯2_dt (4.8) Expansion of (4.8) leads to four terms in the integral, which can be separated out into four integrals that combine to make the interferometric autocorre- lation

I(τ)F RAC =Iback(τ) +Iint(τ) +Iω(τ) +I2ω(τ) (4.9)

where Iback(τ) = Z _+∞ −∞ |A(t)| 4_+|A(t−τ)|4_dt = 2 Z _+∞ −∞ I 2_{(t)dt (4.10)} Iint(τ) = 4 Z _+∞ −∞ |A(t)| 2_|A(t−τ)|2_dt = 4 Z _+∞ −∞ I(t)I(t−τ)dt (4.11) Iω(τ) = 4 Z _+∞ −∞ < ³ (I(t) +I(t−τ))A∗_{(t)A(t−τ)e}iωτ´_{dt (4.12)} I2ω(τ) = 2 Z _+∞ −∞ < ³ A2_(t)(A∗_(t−τ))2_ei2ωτ´_{dt (4.13)}

The first term (4.10) is clearly a constant value, and is simply the back- ground signal of the autocorrelation. The second term (4.11) is the intensity autocorrelation (4.3) multiplied by a factor of 4. The last two terms (4.12) and (4.13) are the two coherence terms that oscillate with ω and 2ω, re- spectively. A typical interferometric autocorrelation is shown in Figure 4.3. Note again the same time-symmetry as for the intensity autocorrelation:

Time (fs) 0 200 400 600 Signal (V) -0.04 -0.03 -0.02 -0.01 0.00 0.01 0.02 0.03 0.04

Figure 4.3: Interferometric autocorrelation of a ’Mira’ Ti:S laser. Note the 8:1 ratio between the top-to-bottom distance and the bottom-to-baseline distance. This ratio provides a useful check that the autocorrelation has been made correctly.

IF RAC(τ) = IF RAC(−τ).

An important feature of the interferometric autocorrelation arises from the integral values at τ = 0

Iint(0) =Iω(0) =I2ω(0) =Iback(0) (4.14) Iback(0) = 2 Z _+∞ −∞ |A(t)| 2_{dt (4.15)} ThereforeIF RAC(0) = 8 R

|A(t)|4dt. Asτ → ±∞we also see thatIF RAC →0

and so there is an 8:1 ratio between the peak of the interferometric autocor- relation and the background level. This feature proves extremely useful for checking the alignment of an autocorrelation system, since an absence of this 8:1 ratio indicates an error somewhere.

Time (fs) 0 100 200 300 400 500 600 Signal (V) -0.40 -0.35 -0.30 -0.25 -0.20 -0.15 -0.10 -0.05 0.00

Figure 4.4: Interferometric autocorrelation of the ’Femtolaser’ Ti:S <12 fs laser. The extra-cavity dispersion compensation required for this laser is absent, resulting in highly chirped pulses. The effect of the chirp can be seen in the wings of the autocorrelation where the interference fringes have reduced, to the point of loss in places, and the envelope is approaching that of the intensity autocorrelation.

tion 1.1), since for a chirped pulse it can be shown [30] that destructive interference in the wings of the autocorrelation will cause the FRAC enve- lope to merge with the intensity autocorrelation envelope. Figure 4.4 shows an example measurement of a chirped pulse.