Setup and characterisation of an autocorrelator for UV laser pulses

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Setup and characterisation of an autocorrelator for UV laser pulses

Henrik Schygulla

September 2019

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Abstract

Ultrashort laser pulses allow the study of non linear interactions between light and matter. Whilst the pulse length for most spectral wavelengths can be determined via second harmonic generation, this is not possible for high energy photons. The aim of this thesis is to build and characterise an autocorrelator receptive to ultraviolet light based on a two- photon absorption effect within sapphire. To help accomplish this, the pulse length of a near infrared laser was also determined though second harmonic generation. A silicon photodiode was used to measure the autocorrelation trace and subsequently a pulse length ofτNIR=1ps±0.06pscould accurately be calculated. Despite the UV detector displaying no clear quadratic response to an increased intensity, an autocorrelation trace could again be measured. The resulting pulse length amounts toτUV =6.5ps±0.8ps. Overall it has been shown that UV pulse lengths can be determined using a simplistic detector design based on a two-photon absorption effect within sapphire. Yet further research into this technology is necessary to assure a reproducible response of the detectors.

Zusammenfassung

Ultrakurze Laserpulse ermöglichen aufgrund ihrer hohen Intensität das Beobachten von nichtlinearen Interaktionen zwischen Licht und Materie. Während man die Pulslänge für viele Wellenlängen anhand des Phänomens der Frequenzverdopplung in einem geeigneten Kristall gut bestimmen kann, so ist dies für besonders hochenergetische Wellenlängen nicht mehr möglich. Das Ziel dieser Arbeit ist es daher, einen Autokorrelator zu verwirklichen, welcher empfänglich gegenüber ultraviolettem Licht ist. Der Detektor beruht auf einer in- duzierten Saphir-Leitfähigkeit mittels Zwei-Photonen-Absorption. Zusätzlich hierzu wird außerdem die Pulsdauer eines Lasers im nahen Infrarot-Bereich anhand der bereits erwähn- ten Frequenzverdopplung bestimmt. Mit Hilfe einer Silicium-Photodiode konnte diese Dauer sehr genau alsτNIR=1ps±0.06psermittelt werden. Auch wenn es nicht möglich war, für den UV-Detektor einen perfekt quadratischen Zusammenhang zwischen Ausschlag und Intensität nachzuweisen, so konnte dennoch eine Autokorrelationsspur gemessen werden. Die sich hieraus ergebende Pulsdauer beträgtτUV =6.5ps±0.8ps. Insgesamt konnte der Saphir basierte Detektor genutzt werden, um ultrakurze Pulsdauern im UV-Bereich zu messen. Allerdings ist die Methode noch nicht komplett ausgereift und es sind noch weitere Verbesserungen möglich, insbesondere im Bereich der Reproduzierbarkeit.

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List of Figures

1 A gaussian pulse and its autocorrelation trace . . . 3

2 Setup of the delay generator . . . 5

3 Setup of the autocorrelator . . . 7

4 Raw data of the signal on the oscilloscope . . . 10

5 Comparing the raw data for different bandwidth settings . . . 11

6 Comparing delay positions for 5Hz . . . 12

7 Calculating the calibration factor for 5Hz . . . 13

8 Comparing different delay positions for 10Hz . . . 14

10 Autocorrelation trace for a 5Hz rotation frequency,ω =1.21ps±0.01ps. . . . 15

11 Autocorrelation trace for a 10Hz rotation frequency,ω =1.2ps±0.01ps. . . . 16

12 Calculating the response of the detector . . . 20

13 Absorption coefficient for different peak intensities . . . 21

14 All the measured signals for different OPD after eliminating the offset . . . 23

15 Calculating the pulse length for an input power of 4.2mW . . . 23

16 Calculating the pulse length for an input power of 10mW . . . 24

17 Circuit drawing of the TIA . . . 29

18 Output power constant calculation . . . 29

19 Autocorrelation trace and FWHM of the NIR pulse used to create the UV pulses 30

List of Tables

1 List of Components . . . 9

3 Measuring the beam diameter with the knife-edge technique . . . 18

4 Measuring the response of the detector to an increase of the intensity . . . 22

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1 Introduction

Ever since its invention in 1960 the laser has developed into an important component of modern physics. This is because the coherent light source possesses unique attributes, which allow for its use in a variety of applications including communication, imaging and spectroscopy. In particular, the achievable high intensity is essential for the field of non-linear optics, which studies the non-linear interaction between light and matter. In this field pulsed laser light is also quite important, as it greatly increases the intensity of the laser beam.

The idea of pulsed light in contrast to a continuous laser beam, is to stop the emission of light in regular intervals and confine the emission in time. These pulses can then be characterised not only by their intensity and beam profile, but also their length. Subsequently, measuring that length is a necessity as well as the topic of this thesis. Because the length of these pulses can become as short as a few femtoseconds, traditional electronics are too slow to time resolve the process. The autocorrelation method solves this issue, by using the pulse itself to measure its duration.

Even though the implementation may vary, the aim of the intensity autocorrelation method is always to gain a signal which indirectly corresponds to the pulse length. This is done by splitting the initial pulse into two identical parts and then recombining them in a non linear material. The signal produced by the interaction of both pulses with the material then varies with the overlap of the two pulses in space and time and once both pulses have completely scanned one another, the autocorrelation trace can be used to calculate the original pulses length.

For many wavelengths including the visible and near infrared (NIR) light, a non linear crystal can be used as the non linear material to produce an autocorrelation signal due to second harmonic generation (SHG). However, these crystals are not light transmissive for ultraviolet (UV) light and thus no such signal can be generated for this specific wavelength spectrum.

The aim of this thesis is to build an autocorrelator for UV light by using the effect of induced conductivity via two-photon absorption within sapphire. The design of the detector unit will closely follow the design presented by K. Leedle et al. in 2017¹ and the results will be compared.

The project was separated into two tasks with the first one focusing on aligning the setup and measuring the pulse length of a NIR laser. The laser was used in combination with a non linear crystal, in order to obtain an autocorrelation trace by measuring the SHG signal. Once the pulse length had been determined, the second task was measuring the pulse length of a UV laser. The autocorrelation trace could be measured, by exchanging the detector unit in the existing setup for a sapphire based detector sensitive to two-photon absortion.

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2 Theoretical background

This section will provide a short introduction to the theoretical background of how to measure ultra short pulses. First the two photon interaction effect of SHG and photoconductivity will be described, followed by the method of autocorrelation itself. A segment on pulse length defini- tions will also be included and finally, the delay generator used in the setup will be depicted.

2.1 Two photon interaction

The fundamental insight of non-linear optics is, that the presumed linear interaction between matter and an electric field E is only an approximation. Instead of being linearly proportionate to E, the polarization density P is more closely described as a taylor expansion

~P= ε₀{χ⁽¹⁾E~¹+χ⁽²⁾~E²+χ⁽³⁾E~³+...} (1) whereχ is the electric susceptibility². For large electric fields the higher order terms are no longer negligible and their effects become observable.

2.1.1 Second harmonic generation

SHG is an effect which occurs because of the second order term and describes the process of frequency doubling.³ With a sufficiently high photon intensity in a non-inversion symmetrical crystal a weak non-linear polarization can be induced at double the frequency P₂∝~E². Then the polarized material can radiate light with double the input frequency in accordance with Maxwell’s equations. Subsequently the emitted light will have half of the input wavelength.

2.2 Photoconductivity

Photon induced conductivity is closely related to the science of semiconductors. By absorbing the incoming light, the material can be excited and an electron is raised into a higher energy state. This then increases the number of free electrons and electron holes, which function as electric charge carriers. Consequently, the conductivity of the material increases and photon induced conductivity is achieved.

In an isolator this effect can only be observed if the photon energy is high enough to lift the electrons above the band gap to the next energy level. Within sapphire the inherent band gap amounts to roughly 8.9 eV⁴, which is why two photons in the UV region are required to excite the material.

2.3 Autocorrelation

Intensity autocorrelation measures the intensity of a SHG signal, which is created by overlap- ping two identical pulses in a non-linear crystal. If the pulses are overlapped at an angle to one another, three SHG signals can be observed. First, a signals from each of the two pulses is created and travels in the same direction as the respective input beam. The third signal can be observed once spacial and temporal overlap is achieved, as it uses one photon from each beam to induce a polarisation. The emitted beam therefore travels in the centre of the other two due to momentum conservation. The intensity of this centred signal can be described as a function of the delayτ between the two pulses with a maximum atτ=0⁵

I_Ac(τ) = Z +∞

−∞

I(t)I(t−τ) (2)

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The whole autocorrelation trace can be gained by changing the delay between the two pulses over the entire area of overlap. This is done in a multi shot setting, though it is sometimes possible to gain the trace with a single input pulse. The multi shot setting was chosen because it allows for a higher resolution and for the measurement to remain consistent between the NIR and UV wavelengths, since the UV detector cannot be used in a single shot setting.

With the autocorrelation trace, the original pulse length can be calculated, since mathematically every pulse shape has a definite, characteristic autocorrelation trace. Therefore it is possible to compute the original pulse, by approximating the shape of the input pulse. Although most pulses are not gaussian shaped and instead can be more closely described with asech² function⁵, for convenience reasons this is the shape approximated for them in this bachelor thesis. Since the characteristic trace for a gaussian pulse is gaussian shaped as well, it allows for easy calculations between the two. Such a pulse and the corresponding autocorrelation trace is shown below in Figure 1.

Figure 1: The relation between the full width half maximum (FWHM) for a gaussian pulse and its’ autocorrelation trace.⁶

The full width half maximum of a pulse is the width at which the intensity is at half of its maximum. For gaussian pulses, the relation between the autocorrelations FWHM and that of the original pulse can be described byτAutocorrelation=1.41τ_Pulse, which will be used to calculate the length.⁷

A helpful feature of the autocorrelation trace can already be derived from Equation (2) and the fact that two identical pulses are being used: the trace must be symmetrical. This can be used when assessing whether the setup is aligned and functioning correctly.

2.4 Gaussian pulses

As mentioned before, the FWHM value is used to define a pulse length and therefore it is convenient to calculate some factors needed in this thesis. Since the program Igor Pro 7 was used to analyse the given data and this program returns the(1/e)width when using a gaussian fit, the calculation will be about how to switch between these values. The function

f(x) =y₀+e⁻⁽

x−x0 width)²

(3) is used, with width²=2σ². To simplify the term,x₀=0 is assumed. Now if x=width, the function above returns the value (1/e). Instead the FWHM is wanted and the return should be 1/2 so in order to achieve this,x=ω must be multiplied with a factor f =p

2ln(2):

f(x) =y₀+e⁻

(ω·f)2

2σ2 =y₀+e^ln(¹²⁾ (4)

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This calculation states, that at a distance ofω· f from the peak, the intensity of the signal will have halved. Since the full width between both these distances is wanted, the FWHM of the autocorrelation trace can be gained by doubling this value: ω·2f =τ_{FW HM}.

Above the factor between the FWHM of a gaussian pulse and its autocorrelation signal is shown to be 1.41, thus the equation to gain the FWHM of the original pulse from the (1/e) width of the autocorrelation trace can be summarized as

τ_pulse(ω) =ω· 2f

1.41 (5)

2.5 Peak intensities

Another value needed in the analysis is the peak intensity of a laser pulse in the focus point of the lens, so some important formulas will now be displayed. First, the peak power of a pulse can be gained from the average energy of the pulse with

P_p=E_p τp

·0.94 (6)

whereτ_pis the FWHM and the factor 0.94 is inherent to a gaussian pulse form. [cite] To gain the peak intensity, this peak power is simply divided by the cross section of the beam and thus by the area:

I_p= 2P_p

πW₀² (7)

Here,W₀²is the radius of the laser beam profile.⁸

Next, the magnification factorMof a lens will also be mentioned. It is used when calculating the new beam radius after being focused in the focal point of a lensW₀_neu =M·W₀. It can be calculated with the focal length of the lens f and the rayleigh length of the laserz₀, as well as the distance from the focal pointz:

M= |_z−^f_f| q1+ (_z−^z⁰_f)²

= 1

q1+ (^z_f⁰)²

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The simplification can be done withz=0, which is the case when calculating the magnification factor for a collimated beam in the focal point of the lens.⁹

Finally, the formula to calculate the rayleigh length for any given gaussian beam is shown below. The rayleigh length being defined as the distance the beam travels after which the cross section will have doubled:

z₀=W₀²π

λ (9)

In this equationλ represents the wavelength.¹⁰

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2.6 Delay generator

The delay generator used in this setup for the NIR measurements consists of two parallel mirrors. These are connected by a board and can rotate around their shared centre. The path of the light then varies, depending on the position of the mirrors, as shown in Figure 2. The calculations presented in this section (2.6) closely follow the ones presented by Z. Yasa and N. Amer in 1981.¹¹

Figure 2: Visualisation of how the delay generator changes the optical path of the laser. The dotted line represents the setup after rotating an angleθ. R is the radius of the rotation shaft, d is the diameter of the mirrors, ψ is the incident angle of the beam on the mirrors andθ is the rotation angle.¹¹

The optical path difference (OPD) ∆l(θ) as a function of the rotation angle θ can be described as:

∆l_exact(θ) =4R[sin(θ)sin(2ψ)−(1−cos(θ))(1−cos(2ψ))] (10) For small angles (θ) it can be simplified into

∆l(θ) =4Rθ[sin(2ψ)−1

2θ(1−cos(2ψ))] (11)

and even further, if the non-linear term is neglected. This results in the equation:

∆l_approx(θ) =4Rθsin(2ψ) (12)

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It approximates that the OPD is linearly proportionate to the rotation angle and therefore with time, ifθ(t) =const. Should this be the case, then the measured pulse width can be converted to the actual pulse width with the help of a constant factor f.

2.6.1 Error discussion

Since the small angle approximation is used and the non-linear term is neglected, a discussion should be held about how accurate any measurement is. It is important to note that the scanable delay range is limited, because at an angle θ_max, the input beam will no longer hit the mirror and will therefore not be reflected anymore.

θ_max ' d R

sin(ψ)

(2−cos(2ψ)) (13)

The range optical path differences∆lcan then be calculated by substituting Equation (13) into Equation (12)

∆l' 4dsin(2ψ)sin(ψ)

(2−cos(2ψ)) (14)

This constant is heavily influenced by what the incident angle of the beam on the mirrors (ψ) is and forψ=π/4 it is maximised. In the setupd=2.5cm,R=3.75cmand presumablyψ≈π/4, so thatθmax≈13^◦and therefore the range optical path differences∆l/c≈3.5cm/c=116.6ps.

This value of±116psis the OPD which can be generated by the Delay generator, however for a large OPD, the error of the approximations becomes relevant. This error can be precisely calculated for different scan ranges by comparing the exact Equation (10) with Equation (11), which includes the approximations.

For example, if the error over a pulse length of 5psis to be calculated, then the angle of 5ps OPD must first be determined. ∆l_exact(0.015rad) =0.2cm≈6.6psOPD, to which the relative error of the approximation is

∆l_approx(0.015)−∆l_exact(0.015)

∆l_exact(0.015) =0.75% (15)

The error of 0.75% over an OPD of 6.6psis gained by using the small angle approximation and neglecting the non-linear term in the Equations above. Naturally, a larger delay will increase the influence of these approximations as well as the error, which is why special precautions should be taken to assureθ ≈θ₀.

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3 Experimental setup

The experiment was divided into two parts; to measure the pulse length of a near infrared laser and the pulse length of an UV laser. Whilst a large portion of the setup remained the same, the detector unit had the be exchanged between the two measurements.

3.1 Setup autocorrelator

The complete setup for both the NIR and the UV measurements can be seen below in Figure 3. The laser changes for NIR (1030nm, 20mW, 50MHz) and UV (257.5nm, 0.1mW - 15mW, 10Hz pulse train with 800µs at 1MHz).

Figure 3: Setup autocorrelator: BS is a beamsplitter, M1 M2 are plain UV enhanced mirrors, LDS and RDS are linear and rotating delay stages consisting of two UV enhanced mirrors each, Pm is a parabolic mirror, CB is a control box for RDS, VA is a low noise voltage amplifier, Osc is an oscilloscope. NIR: LBO crystal is 1mm thick, SPF is a shortpass filter for green light, SI is a silicon photodiode, PD is a photodetector consisting of Si and a transimedance amplifier.

UV: HV is a high voltage supply.

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The autocorrelator without the detector unit is an interferometer. The laser light enters the setup, hits a beamsplitter BS at a 45^◦angle and is divided into two parts. The transmitted light is then reflected into a retroreflector LDS by the mirror M1. There the beam gains a small offset and is directed back towards the mirror and to the beamsplitter. Additionally, the reflector is mounted onto a linear stage to be able to adjust the distance the beam travels.

The initially reflected light enters the rotating delay stage RDS, whose function has been described above. This delay stage consists of two parallel mirrors, which reflect the beam so that the input beam and the output beam are parallel with an offset. The output beam then hits the mirror M2 at a 90^◦ angle, returning the beam through the delay stage onto the beamsplitter as well.

Because both beams hit the beamsplitter again, they are split once more and two beams of equal intensity are reflected/transmitted into a parabolic mirror PM. Since they are parallel with an offset created by the retroreflector, they are now both focused into the focal point of the mirror, where the detector unit is placed.

The output signal from these units are then amplified by a low noise voltage amplifier VA before being transferred into an oscilloscope Osc. to measure the data.

3.1.1 Setup NIR

The NIR laser emits light at 1030nm with an average power of 20mW and a repetition rate of 50MHz. The beam enters the setup and hits a pellicle beamsplitter as described above.

The detector unit consists of a lithium triborate (LBO) crystal with a thickness of 1mm, which is placed in the focal point of the parabolic mirror. The offset causes both beams to be reflected at an angle, which helps to filter the unwanted, individual SHG signal from the combined SHG signal after passing through the crystal. However, only a fraction of the input beams will have halved their wavelength due to two photon interactions, so a filter is needed to separate the SHG signal from the remaining NIR wavelength. This is why a shortpass filter SPF, allowing only green light to pass, is placed behind the LBO. Now the three SHG signals can be measured, from which the centre one is of interest. The measurements are taken with a silicon photodetector PD consisting of a silicon photodiode and a transimpedance amplifier.

The detector has a variable gain setting to change the sensitivity by increasing the resistance within this transimpedance amplifier. However, increasing the sensitivity also decreases the bandwidth of this detector, so a setting was chosen where a bandwidth of 150KHz remained.

For these measurements the rotating delay stage was used to generate a delay. An external trigger was sent into the control box, which then synchronised the input trigger, the rotation of the stage and the output trigger. It also manages the scan range of the delay generator, which was not needed. The high repetition rate of the laser, the limited rotation frequency of the delay stage (10Hz) and the high bandwidth of the detector resulted in the whole autocorrelation trace being measured in a single rotation of the stage.

3.1.2 Setup UV

The UV laser emits light in a 10Hz burst mode setting with a burst length of 800µs and an interburst repetition rate of 1MHz. The wavelength is centred at 257.5nm, the fourth harmonic of 1030nm, and the energy is tunable between 0.1mW and 15mW with the help of filters. The light enters the setup and hits a polka dot beamsplitter and passes through the setup. Originally another pellicle beamsplitter was to be used, but unfortunately the high laser intensity dam- aged it and this backup beamsplitter had to be used. This also resulted in a typical polka dot interference shape, which did not seem to have an effect on the measurement.

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The detector unit now consists of a high voltage power supply HV, the sapphire based detector SD and a transimpedance amplifier TIA. To measure the conductivity a 400V voltage was applied to the detector. This detector functions as follows: a 25µm thin layer of sapphire is placed between two layers of conducting, p-doped silicon. On the upper side where the laser hits the sapphire, the silicon is cut sharply while the lower side is connected to the positive pole of the power supply. This way, an electron surplus is generated on the lower layer and a current will flow once sapphire becomes conductive. The laser pulses induce this conductivity, which is sensitive to the overlap of both pulses, as it is a two-photon absorption effect. To analyse the signal with an oscilloscope, the current must first be converted into a voltage with the help of the transimpedance amplifier. This amplifier has a rather large resistance of 1MΩ, as the current is expected to be very low.

Further information on the components used in both setups can be seen in Table 1 below.

Table 1: List of Components

Name Description Manufacturer Part Number

NIR BS Pellicle Beamsplitter Thorlabs BP245B3

UV BS Polka Dot Beamsplitter Thorlabs BPD254-FS

M1 UV-Enhanced Aluminium Mirror Thorlabs PF10-03-F01

M2 UV-Enhanced Aluminium Mirror Thorlabs PF20-03-F01

PM Off-Axis Parabolic Mirror Thorlabs MPD019-F01

LDS Hollow Roof Prism Mirror Thorlabs HRS1015-F01

RDS Optical Delay Generator Femtochrome FR-203/130psec*

SPF Short Pass Filter Thorlabs FES0600

PD Si Switchable Gain Detector Thorlabs PDA36A

HV High Voltage Power Supply Tektronix SPEC-2290-5B

SD Sapphire Based UV Detector Home-built -

TIA Transimpedance Amplifier** Home-built -

VA Low Noise Voltage Preamplifier Stanford Research Systems SR560

OSC. Digital Serial Analyzer Tektronix DSA70804

* The unit was a special delivery from Femtochrome based on their optical delay generator but with UV-enhanced mirrors.

**The circuit diagram can be seen in appendix I, Figure 17

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4 NIR

The following part will focus on the analysis of the NIR light and the calculation of its pulse length. The execution chapter will give a quick overview on how the data was collected and then the following sections will analyse the obtained data.

4.1 Experiment

To measure the NIR pulse length the rotating delay device was set to two different rotation frequencies, 5Hz and 10Hz. As said before, a single rotation of the delay stage could measure the entire autocorrelation trace. The oscilloscope then averaged 16 of these traces to reduce background noise and flux within the trace to gain a clean signal. These signals are then saved.

A calibration of the device is needed, as the rotation frequency and other values influenced the signal on the oscilloscope. In order to calibrate the device the linear delay stage was moved a total of 5mm in small steps and the change on the oscilloscope was noted. With this data a calibration constant could be calculated and compared to its theoretical value. The calibration was done for both frequencies, even though one measurement would have been sufficient, as one can be calculated from the other. This was done to be able to compare the measured pulse lengths independently of one another, so any error within the analysis can be spotted more easily.

4.2 Analysis

An unedited autocorrelation trace from the oscilloscope can be seen in Figure 3. The y-axis shows the voltage measured by the oscilloscope, after the signal has been amplified. The x-axis shows the time stamp from the oscilloscope.

Figure 4: Raw data of the signal on the oscilloscope.

To better compare different measurements with one another the intensity has been nor- malised and the pulse centred at zero on the x-axis. Now the intensity is in arbitrary units.

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With the help of the gaussian fit the symmetry of the trace can be checked. As mentioned above, the trace must always be symmetrical, if the signal is displayed correctly. Figure 5, compares the same trace measured with two different bandwidth settings of the detector and immediately the difference in size is apparent. For a bandwidth of 5KHz the signal is broader, as the detector is unable to correctly resolve the process. The trace is also not symmetrical, which can be seen by closely comparing the plotted gauss fit with the autocorrelation trace. For a setting of 150KHz however, the trace is much narrower and the rotation of the delay generator can be measured more accurately. For this reason, a bandwidth of 150KHz was chosen for all future measurements.

Figure 5: Comparing the raw data for different bandwidth settings on the detector.

4.2.1 Calibration

The autocorrelation method shown in Equation (2)I_AC(τ)is a function of the delay between the two pulses. From this one can calculate the pulse length, therefore the objective of this analysis is to plot the intensity of the signal against the delay. But because the delay is generated by the delay generator and does not correspond to the time shown on the oscilloscope, a calibration of the device is necessary.

A theoretical value for the calibration factor can be calculated from Equation (5), which shows the optical path difference as a function of the rotation angle. For a constant rotation frequency of 5Hz the factor is a constant that amounts to:

∆l(θ) =4Rθsin(2ψ) ≈471cm/s (16)

WithR=3.75cm,θ =5Hz∗2π andψ =π/4.

Every second the delay generator creates an OPD of 471cm. With the speed of lightcthis can be written as ^∆l(θ_c ⁾ =15.7ps/ms. This means that if the device is rotating with a constant frequency of 5Hz, then every 1msan OPD of 15.7psis created. Consequently, 1msdelay shown on the oscilloscope corresponds to an OPD of 15.7ps.

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Since this value changes linearly with the rotation frequency it can be adjusted for any other frequency by multiplying it with a factor f = ^f_f²

1 with f₁being the original frequency (5Hz) and f₂ being the new frequency. For the second frequency of 10Hz used in the experiment the new calibration factor should therefore theoretically be

∆l(θ) =10Hz

5Hz ·15.7ps=31.4ps/ms (17)

4.2.2 Calibration for 5Hz

To measure the calibration factor seven autocorrelation traces were taken with additional path differences set between the measurements. This was done with the micrometer screw of the linear delay stage. As one pulse is being shifted in time, a different rotating angleθ now corresponds to the overlap position. This of course leads to a shift of the autocorrelation trace on the oscilloscope, because it is being triggered toθ =θ0. In Figure 6 all measured autocorrelation traces are plotted against the time measured with the oscilloscope and the shift can be observed.

It is noticeable that the peak intensity of the signal declines the further away the signal is from the initial signal at timet=0. A possible explanation for this observation is that the cor- relator is not perfectly aligned and that by moving the linear delay stage, the spatial overlap at the LBO crystal gets worse.

Another observation worth mentioning is that the delay is only changed in one direction.

This is again due to the linear stage loosing the spatial alignment if it moves in the opposite direction, as there was a slightly bent screw within the stage.

Figure 6: Autocorrelation traces with different delay settings for the linear delay stage To gain the calibration factor between the created delay and the OPD, the delay measured on the oscilloscope will now be plotted against the OPD set by the linear delay stage. For this, each of the traces from Figure 6 has been fitted with a gaussian function and their peak positions have been noted. Table 2 shows this data while Figure 7 shows the corresponding plot.

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Table 2: Data for delay on the oscilloscope and the corresponding delay of the linear delay stage. Data taken from Figure 4

Delay on Oscilloscope [ms] Linear Delay Stage [mm]

0 0

-0.375 -0.899

-0.71 -1.699

-1.06 -2.499

-1.41 -3.299

-1.76 -4.099

-2.17 -4.994

The linear fit onto these data points then calculates the value of the calibration factor in [2mm/ms]. The factor 2 stems from the setup of this autocorrelator, since moving the linear delay stage by any distance increases the created OPD by double that distance. With the speed of lightcthis value can then be converted into the comparable units [ps/ms].

Figure 7: Calculating the calibration factor, plot of data in Table 2.

The slope of the linear fit amounts tob=2.318^2mm_ms and following the considerations above, the calibration factorc_5Hzcan now be calculated with

c_5Hz= 2b

c =15.45ps/ms (18)

The value of 15.45ps/ms±0.01ps/ms fits the theoretical worth of 15.7ps/ms calculated for this device and the small difference that remains can be explained with the formula using slightly inaccurate values. The error attached to the measurement is the statistical error given by the fit function.

4.2.3 Calibration for 10Hz

The 10Hz rotation frequency calibration factor is calculated in an identical manner as the 5Hz calibration factor. First, all of the 7 measurements are plotted against the delay measured on the oscilloscope in Figure 8.

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However, because the rotation frequency is higher than before, the OPD generated in one second is higher compared to the 5Hz value. This also means that the OPD generated permsis higher and therefore, when creating the same OPD with the linear delay stage, the traces should display a smaller delay on the oscilloscope.

Figure 8: Autocorrelation traces with different delay settings of the linear delay stage at a 10Hz rotation frequency

As expected, the change on the oscilloscope is only half as large as for the 5Hz measurement.

From this Figure 8 the peak positions of the traces can once again be determined and then be plotted against the OPD set by the linear delay stage. This is shown in Figure 9 and the linear fit function calculates a slope ofb=4.64^2mm_ms .

c_10Hz= 2b

c =30.94ps/ms (19)

Figure 9: Calculating the calibration factor for 10 Hz rotation frequency

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This factor of 30.94ps/ms±0.2ps/msis once again close to the calculated value of 31.4ps/ms.

It fits the previously determined value of 15.45psfor the 5Hz rotation frequency even better, because it is almost the exact double of this number. As explained before, the factor between these two calibration constants should be f =2 which is almost perfectly the case. The con- sistency between the calibration factors supports the argument made above, that the difference from the theoretical value stems from the mathematical equation using inaccurate values.

With the calibration for both rotation frequencies complete, the pulse length will be calculated for both 10Hz and 5Hz pulses next.

4.3 Pulse length

By multiplying the x-axis and the calibration factor the axis can be rescaled to display the optical path delay. Now the goal of plotting the intensity against the delay is complete and the input pulse length can be calculated from the width of the gaussian fit applied to the autocorrelation trace with Equation (5) as displayed in the theoretical background.

τ_pulse(ω) =ω· 2f

1.41 (5)

Figure 10: Autocorrelation trace for a 5Hz rotation frequency,ω =1.21ps±0.01ps.

For a rotation frequency of 5 Hz, the autocorrelation trace is depicted in Figure 10. The width of the gauss fitω =1.21ps±0.01psamouts to an input pulse length of

τ_pulse_5Hz(ω) =1.21ps· 2f

1.41 =1.01ps ±0.01ps (20)

Figure 11 shows the 10Hz autocorrelation trace and hereω=1.2ps±0.01ps, therefore τ_pulse_10Hz(ω) =1.22ps· 2f

1.41=1ps ±0.01ps (21)

The stated errors are deduced from the fit uncertainty.

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Figure 11: Autocorrelation trace for a 10Hz rotation frequency,ω =1.2ps±0.01ps.

4.4 Error discussion

The given error for both pulse lengths is derived from the uncertainty of the gauss fit. However there are more factors to consider, namely the calibration factor and the influence of the delay generator. With the relative error of the calibration factor being<1%, it is important to have a good approximation of the second influence. To calculate the error over the total scan range it is presumed thatθ₀=0 can be determined with an uncertainty of±3mm OPD≈1.15^◦. Then, because the calibration created an OPD of 10mm, the total scan range used will amount to 1.3cm. Now the delay generator’s uncertainty for this range can be determined with Equation (8): For the angle 0.1rad≈5.8^◦the path difference is 1.32cmand the error 5.4%.

In comparison with the other influences, the error of the delay device is dominant and a quadratic addition of the relative errors gives the relative error of the final result:

5Hz pulse: 1.01ps±0.05ps 10Hz pulse: 1ps±0.06ps

These two results are almost identical and lie within the error of the other measurement. There- fore the pulse length can be assumed to be≈1pson average.

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5 UV

The next part will focus on analysing the UV pulses. First, a short introduction will be given on how these measurements were taken, followed by the analysis.

5.1 Experiment

Apart from the wavelength, the main difference between measuring the NIR pulse length and the UV one is that the laser now operates in a 10Hz burst setting. Because of this the autocorrelation trace could no longer be measured in a single rotation of the delay generator, but instead multiple rotations are needed. In theory, the delay generator could be triggered to the 10Hz bursts and then generate a different delay for each burst to gain the trace, but unfortunately this did not work due to trigger difficulties. Instead the oscilloscope was triggered to the bursts and the linear delay stage was used to create the delay between measurements. This made the cali- brating of the delay generator obsolete, but measuring a trace quite lengthy and time consuming.

Before determining the pulse length, the device must first be characterised. This is done by taking a non-linearity measurement to calculate how well the detector responds to an increase of intensity. The measurement itself was achieved by blocking off one arm of the autocorrelator and measuring the increase of the signal whilst varying the input power. The presumed quadratic response can then be calculated and compared to the existing results. Once the temporal overlap is found, the autocorrelation trace can then be measured as described above.

5.2 Characterisation

The first step to characterising the detector is to measure the non-linear response. The data for this is shown in the first two columns of Table 4 which features the input optical power before the beam enters the setup as well as the voltage measured by the detector after being amplified. While the optical power is displayed in [mW], the unit of the voltage is [Vms], because the integral of the signal was evaluated instead of the peak current. This means the presented value corresponds to the whole current which flowed instead of just the peak current. To compare these measurements with the ones presented byK. Leedle et al.¹, these units need to be converted to matching ones. The goal is to plot the peak photon intensity at the detector in [GW/cm²] against the average current which managed to flow because of the induced conductivity in [A].

For this a number of calculations are needed, which will be done next. First the beam diameter will be determined using the knife-edge technique and afterwards the peak intensity equivalent of 1mW input optical power will be calculated. Finally the measured signal on the oscilloscope will be used to calculate and display the current which passed the detector.

5.2.1 Beam diameter

The knife-edge technique is a fast approach to measure a beam diameter. A knifes edge is slowly inserted into the beams path while the output intensity is measured. The angle between the movement of the knife and the propagation direction of the beam is usually 90^◦, so that no additional calculation is needed when determining the diameter. The position of the knife is noted when the intensity on the detector drops to 90% and 10% of the original intensity. The difference between these values is then defined as ∆r≈2W₀. Table 3 presents the three measurements taken to determine the beam diameter.

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Table 3: Measuring the beam diameter with the knife-edge technique Distance 90% Intensity [mm] Distance 10% Intensity [mm] ∆r[mm]

19.25 20.6 1.35

19.23 20.58 1.35

19.3 20.59 1.29

From this the the beam diameter is calculated to be 2W₀=1.33mm±0.05mmand now the peak intensity in the focal point can be calculated next.

5.2.2 Peak intensity

The powermeter measures the input optical power in [mW]. This means that in order to calculate the average energy per pulse, this input power must be divided by the number of bursts per second and by the number of pulses per burst:

E_p= E_B

#pulses = P

#busts/s·#pulses = 1mW

10s⁻¹·800 = 1

8·10⁻⁶J (22)

With the average energy per pulse, the peak power per pulse can now be calculated with Equa- tion (6):

P_p= E_p

τ_p ·0.94=

1

8·10⁻⁶J

6.5ps ·0.94≈18.08·10³W (23) Note, that the pulse length is the one calculated in section 5.3. Now the peak intensity according to Equation (7) amounts to:

I_p= 2P_p

πW₀² (7)

However, because the intensity in the focal point of the detector is of interest,W₀ needs to be adjusted accordingly. This is done with Equations (8) and (9) by determining the magnification factorM and the rayleigh lengthz₀.

For the magnification in the focal point of the parabolic mirror the simplified form of Equa- tion (8) can be used, as the beam is assumed to be collimated and thereforez≈0.

M= 1

q1+ (^z_f⁰)²

= 1

q

1+ (_25.4mm^5.16m )²

≈0.0049 (24)

The value for the rayleigh length is calculated with z₀ = ^W

2 0π

λ ≈5.16m as all the values are already known. The resulting beam radius in the focal spot of the parabolic mirror is thus

W₀_neu=M·W₀≈3.26µm (25)

Now the peak intensity in the focal point can also be calculated:

I_p= 2Pp

πW₀² ≈1.17·10¹⁵W

m² =11.7·10¹⁰ W

cm² (26)

The equation above returns the peak intensity in the focal spot of the parabolic mirror for this particular laser and for a given input power. The last factor to consider is how much the input optical power decreases before the pulse reaches the detector.

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Due to spatial problems, the ’input power’ measurement was taken before the beam enters the setup and therefore a factor to compensate for the power loss due to the limited reflectivity of the mirrors and the beamsplitter must be calculated. If the reflectivitym_r of the mirrors as well as the transmissionbs_t and reflectionbs_r index of the beamsplitter is known, then theoretically this decreasedshould amount to:

d=m⁴_r·bs_t·m⁴_r·bs_r·m_r (27) As after the input power measurement, the beam is reflected by four mirrors before it enters the setup. Here it passes the beamsplitter, is reflected by four mirrors again before being reflected by the beamsplitter into the parabolic mirror and the detector.

The manufacturer lists a reflectance ofm_r=90% and a transmission ration of about 42.5%R : 47.5T for the beamsplitter, so this theoretical value should be aboutd≈0.078.

The experimental determination of this factor confirm thatd≈0.071±0.002, which will be used in future calculations. The graph to this measurement can be seen in appendix II, Figure 18.

To summarise, the peak intensity at the detector can be calculated from the input power before entering the setup with

I_p=0.071·11.7·10¹⁰ W

cm² ≈8.33GW

cm² (28)

5.2.3 Average current

The last calculation needed is how to gain the average currentI_Awhich managed to flow because of the induced conductivity. This can be be done with Ohm’s law

I_A=U

R (29)

whereU is the voltage measured at the oscilloscope andRthe resistance used within the transimpedance amplifier. Since a voltage amplifier was used as well the measured voltage also needs to be divided by the rate of amplification, which in this case wasx200.

I_A= U

200·R (30)

Finally, this total current needs to be divided by the length of the signal on the oscilloscope to gain the average current.

Table 4 lists the measured values for the input power and the signal on the oscilloscope in columns one and two while columns three and four contain the corresponding value in desirable units.

5.2.4 Non-linearity

After all these calculations, the non linearity can now be determined. As the name suggests, the assumed interaction is one of higher order so the formula to calculate the average current from the peak intensity should be of the following form

f(x) =axⁿ+b (31)

wherenis of interest. To calculate this value a logarithm must be multiplied to both sides and the resulting formula now looks like this

log(f(x)) =nlog(x) +log(c) (32)

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withlog(c) =log(a) +log(b). The equation now shows a linear function wherenis the slope, which can easily be gained by fitting the data.

Figure 12 shows the columns three and four from Table 4 being plotted on a log-log scale.

The data is fitted with a function identical to Equation (31) and a slope ofn=1.47±0.14 has been calculated. Although it is not a perfectly linear function in the log-log scale as expected, it does become much more linear for larger peak intensities. This is favourable, since the pulse length measurements were taken at a peak intensity of ≈2·35GW/cm²and≈2·82GW/cm² and thus are more comparable with one another. The factor two in these equations comes from the way this data was taken. As mentioned above, one arm was blocked off whilst varying the input power whereas in the pulse length measurement both arms needed to be open, doubling the peak intensity.

The comparison with the published results finds that the peak intensity used differs by about a factor of 10³, while the measured average current is of the same magnitude. Even thought the peak intensity in this thesis is higher, the calculated slope is considerably lower than the presumed and desired n=2. This will also have an effect on the pulse length measurement which will be discussed later.

Figure 12: Relation between the peak intensity at the detector and the measured photocurrent in a log-log scale.

5.2.5 Absorption coefficient

Another value introduced byK.Leedle et al.is the absorption coefficientβ which is defined as β = 4N_qW_phτp

ALF² =N_qW_phτpπw²₀

LE²_p (33)

where Nq is the number of electrons produced via two photon absorption,W_phis the UV photon energy, τ_p is the laser pulse duration, A is the laser spot cross-sectional area, L is the device layer thickness, F is the laser fluence¹,w₀is the beam radius andE_pis the energy per pulse. This

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coefficient is a characteristic of any specific detector and linked to the response shown above.

Since it is already known that the slope of the response changes for lower peak intensities, it is probably going to be reflected in the absorption coefficient as well.

As all the values needed to determine this coefficient can be obtained with the calculations shown before it will not be explicitly depicted. Insteadβ will be calculated for and plotted every data point in Table 4. Then the mean value can be calculated and compared to the published value for a similar wavelength. Figure 13 shows the resulting plot.

Figure 13: Absorption coefficient for different peak intensities

As expected the change in the slope can also be seen here. For higher peak intensities the coefficient as well as the slope of the response can be described as constant values.

The calculated average amounts toβ= (5.28±3.59)·10⁻⁴cm/GW which can be compared to the already published values. For a wavelenght of λ =266nm the paper gives a value of β ≈ 0.01cm/GW and once again it seems this measurement is off by a factor of 100. It is interesting to note that in the previous work the coefficients strongly decreases for wavelengths longer thanλ =278nmas the bandgap is too large for a two-photons absorption effect. Yet, at the very edge of this bandgap for 278nm, the givenβ is comparable to the one calculated in this thesis.

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Table 4: Measuring the response of the detector to an increase of the intensity Input Power Signal Peak Intensity Average current

[mW] [Vms] [GW/cm²] [pA]

1.7 17.6 14 74.7

1.8 18.4 17.8 78.4

2.6 24.9 21.4 106

2.85 27.5 23.5 116.8

3.7 38.8 30.5 165

3.9 37.4 32.1 159

4.3 44.8 35.4 190.4

5 44.5 41.2 189.3

5.4 56.2 44.5 239.1

6.3 64.8 51.9 275.8

7.4 73 61 310.6

7.5 81 61.8 344.5

8 97.3 65.9 413.9

8.5 93.4 70.4 397.1

9.5 97.5 78.3 414.9

9.9 126.8 81.6 539.2

10.8 122.2 89 519.8

11.2 142.2 98.3 604.5

11.8 140.9 97.2 599.2

11.9 155.8 98.1 662.4

12.7 162.7 104.7 692

5.3 Pulse length

Since a non-linear response of the detector has been observed, the pulse length can now be calculated from the autocorrelation traces. In the execution part it is mentioned that the rotating delay stage did not work for this laser system, due to a trigger problem. Instead the manual delay stage had to be used to adjust the OPD, which is why no calibration was necessary.

The first task for the construction of the autocorrelation trace from the individual measurements is to eliminate the y-axis offset, so that the signals are more comparable with one another.

This is done by fitting each measurement with a fit function and then adding a constant to the wave so thaty₀≈0. Figure 14 depicts all signals measured for different optical path differences with the offset already eliminated, so the differences between the signals only originates from the two-photon absorption effect. This difference is quantified by measuring the area above each signal to gain a value proportional to the total current which passed the detector. Similar to the non linearity measurement, the photocurrent can then be determined and plotted against the position of the delay stage.

With this, the task of plotting the current against the OPD is quickly done. The peak of the trace needs to be centred at x=0 to gain the delay in [mm] and then this delay needs to be multiplied with ²_c for it to show the OPD in [ps]. Again the factor 2 originates from the setup as described in the NIR part.

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Figure 14: All the measured signals for different OPD after eliminating the offset Figures 15 and 16 show the current of the signal plotted against the OPD, as well as the gaussian fit applied to these data points. The width calculated by the fitting function amounts to: ω4.2mW =3.71ps±0.39psandω10mW =4.09ps±0.24psand there is a noticeable disparity between the two values with the relative difference between the measurements being as high as 10%.

Figure 15: Calculating the pulse length for an input power of 4.2mW

The relative error to the individual measurement also differs. While the 4.2mW autocorrelation trace has a large relative error of 10.5%, the 10mW measurement has a smaller uncertainty of 5.8%. This can partially be explained by the fact that the 10mW gaussian fit had more data points in total, was therefore less noisy and could calculate the function with a higher accuracy.

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From these two values, the FWHM of the initial pulse can be calculated with Equation (5):

τ_pulse_4.2mW(ω) =3.71ps· 2f

1.41 =6.2ps±0.65ps (34)

τ_pulse_10mW(ω) =4.09ps· 2f

1.41 =6.83ps±0.4ps (35)

Again, the difference between these values is caused by mismatch of the initial gaussian widths.

Figure 16: Calculating the pulse length for an input power of 10mW

Even though no delay generator had been used in this experiment and the error of the gauss fit is thus the only major uncertainty to these calculated pulse lengths, a discussion on how accurate these values are should be held once more. This is due to the fact that the two values differ quite a lot from one another and because the non-linearity measurements suggests the response of the detector is not quadratic.

5.4 Error discussion

The biggest reason to distrust these measured values results from the non-linearity measurement shown in Figure 12. It has been mentioned how the traces were measured with a peak intensity high enough to assume a constant slope ofn=1.47. However, this value is far from a quadratic response and has an impact on the accuracy. This systematic uncertainty is comparable to the bandwidth problem discussed in the NIR part, as it changes the form of the measured autocorrelation trace but is hard to compensate mathematically when determining the input pulse length.

Here the effect will be considered by slightly increasing the error to the final result.

Even though the UV pulse length of this laser was previously unknown and and there is currently no comparable measurement, the calculated length does seem reasonable overall due to two factors. The first one being that both results are quite similar. Whilst there is 10% difference between them, this can easily be explained through a minor misalignment or statistical uncertainties. The error bar of the 4.2mW measurement even covers the result of the 10mW measurement. The second aspect which makes these results trustworthy is found when considering the origin of the UV pulses.

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The pulses are generated through fourth harmonic generation of a NIR laser at 1030nm and the pulse length of this laser is known to be τNIR =8.3ps¹. As the resulting UV pulse length is expected to be lower, this value functions as a upper limit to the measurement.

Considering the discussion above, the final results can be summarised as:

τ_UV =6.5ps±0.8ps

5.5 Discussion

This following part will discuss some possible improvements to the setup of the autcorrelator and to the UV detector itself. Gaining a quadratic detector response and automating the delay stage are the two biggest changes which would improve the accuracy and also enable a faster measurement. The delay problem can be solved through a comparatively easy modification, as there are numerous different delay generators which can be employed. Unfortunately, the resolution to the slope issue is more complicated, as it is a characteristic of the detector. Without modifying the detector itself, one could try to vary the high voltage being applied and check if the response differs. Otherwise a new one must be manufactured and tested, as there are no parts which can easily be fixed.

A useful change to the setup would be to replace the hollow roof prism mirror for a flat mirror. This is because gaining a signal in general was quite difficult, as it was only visible when temporal overlap has been achieved and the correct position on the detector had been found.

The correct position being that the beam hits the detector on the edge of the sharply cut, upper silicon layer whilst in the focal point of the parabolic mirror. The process would have been greatly simplified if there had been temporal overlap already, which is what the change would enable. A flat mirror would eliminate the offset created by the hollow roof mirror and cause both beams to be layered on top of one another before being focused in the detector. These beams can then be used to determine the temporal overlap by changing the delay between the pulses until a fringe pattern appears.

An interesting change would have been to use this detector with a different laser system and capitalise on its high sensitivity. It would have especially made the non linearity measurements more comparable to the published one, which would have allowed for a better discussion on the reproducibility of this detector. The bandwidth limit of this detector setup could also be determined if the repetition rate of the laser was lower. Then it would have been compelling to test whether this limit could be increased by modifying some components like the TIA.

One of the problems encountered when using this detector was its high sensitivity to vibrations and it is probably a direct consequence of the easy to fabricate aspect. Since the layers of sapphire and silicon are simply taped together, whenever a noise would occur or somebody would talk in the same laboratory as the detector, it would register a strong enough signal to mask the autocorrelation trace. This could be due to the vibrations in the surrounding air caus- ing the sapphire layer to shake. The evident solution would be to use some conductive glue instead of tape, however then a procedure to ensure no non-conductive area exists between the layers must be developed. Regardless of the solution, this is definitely an area the design could improve upon.

1For the measurement see appendix III, Figure 19

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6 Conclusion

The NIR part of the experiment was successfully completed with the autocorrelator being sensitive to light at 1030nm. Once a sufficient bandwidth setting had been chosen, the setup was calibrated for two different rotation frequencies of the delay generator. Even though these values of 15.45ps/msand 30.94ps/msslightly differ from their theoretical counterpart, the difference can easily be explained with small measurement inaccuracies.

Using the calibration constant, the OPD could then be calculated from the delay shown on the oscilloscope and consequently a pulse length could be determined. These lengths amount to 1±0.06psand 1.01±0.05psand are almost identical.

Overall, the setup enables a very precise measurement of pulse lengths in the NIR spectral wavelength and it especially benefited from the high repetition rate of the laser. The delay generator also proved to be a valuable addition to the setup, as it tremendously speeds up the measuring process and provided a large delay range to quickly find the temporal overlap. A better calibration of the delay generator around the area of overlap would have been able to reduce the non-linearity error even further to below 5%. Should a higher accuracy be needed than calculated in this thesis, then changes to this component are definitely possible. Apart from the faulty linear delay stage, the autocorrelator managed to complete the given task flawlessly and even though improvements to the setup are possible, they are not a necessity. This is due to the basic design and the method of autocorrelation in combination with SHG in general being a well understood phenomena. It has already been employed to measure ultra short light sources byH.P.Weberin 1967¹²and commercial NIR autocorrelators are readily available.

However, to measure UV pulses remains a more difficult task as most non-linear crystals are not usable in this spectral range. Different non-linear interactions are therefore needed to gain an autocorrelation trace of these pulses, some of which include sum-frequency or difference frequency generation with a reference pulse¹³ or using a two-photon induced photoacoustic signal in water¹⁴.

The design byLeedle et al.proposes to use a two-photon induced conductivity within sapphire as a non-linear interaction and is ”the first published pulse length measurement of a sub- 300 nm laser pulse in a nonamplified CW mode-locked laser with no reference pulse”.¹⁵ The defining features of this solution are the high sensitivity as well as the easy and fast fabrication, although it also has some drawbacks.

For the UV measurements the detector used clearly features a non-linear response to the photon intensity, which enables measuring the autocorrelation trace and subsequently the pulse length. This response is not quadratic but rather to the power of 1.47. Comparing these results to the ones measured by K. Leedle et al. shows that the slope is unusually low, but also that the peak intensity used to take these measurements was about a factor 1000 higher than what the authors used. On the other hand the calculated absorption coefficient is much lower than the corresponding published value, again by a factor of 100. Nonetheless, two pulse lengths for two different pulse energies of 6.83ps±0.4psand 6.2ps±0.65pswere determined. Even though these measurement feature a much larger error compared to the NIR results, the initial task of determining the UV pulse length could be completed. There are some aspects which can be improved upon, but overall the result is quite satisfying.

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Apart from the high sound susceptibility, the detector already meets some important aspects for a widespread use. The high sensitivity and the simplistic design at its foundation is supple- mented by the compact size and the easy handling. If the noise sensitivity could be reduced and the production be more streamlined to ensure accurately reproducible detectors, then it could potentially be relevant for commercial UV autocorrelators. Nonetheless, the device succeeds at being a fast and simple way to measure UV pulse lengths.

In conclusion, a NIR and a UV autocorrelator were successfully built in the span of this thesis. While the near infrared autocorrelator could accurately display the pulse length of the input laser, its ultraviolet counterpart managed to estimate the length with some uncertainty remaining. It was confirmed that a homebuilt UV-autocorrelator following the design byLeedle et al. can be used to measure pulse lengths.

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References

[1] Kenneth J. Leedle, Karel E. Urbanek, and Robert L. Byer. “Simple, picojoule-sensitive ultraviolet autocorrelator based on two-photon conductivity in Sapphire”. In: Applied Optics56.8 (2017), pp. 2226–2229. DOI:https: // doi. org/ 10. 1364/ AO. 56.

002226".

[2] Jean-Claude Diels and Wolfgang Rudolph.Ultrashort Laser Pulse Phenomena - Funda- mentals, Techniques, and Applications on a Femtosecond Time Scale. Second Edition.

Academic Press, Elsevier, 2006, p. 21.ISBN: 978-0-12-215493-5.

[3] J.C. Diels and W. Rudolph.Ultrashort Laser Pulse Phenomena, pp. 172–173.

[4] J. Olivier and R. Poirier. “Electronic Structure of Al₂O₃ from electronic energy loss spectroscopy”. In:Surface Science105 (1981), pp. 347–356.

[5] J.C. Diels and W. Rudolph.Ultrashort Laser Pulse Phenomena, pp. 458–459.

[6] Daniel M. Mittleman. “14. Measuring Ultrashort Laser Pulses I: Autocorrelation”. Uni- versity Lecture. 2015.

[7] J.C. Diels and W. Rudolph.Ultrashort Laser Pulse Phenomena, p. 477.

[8] Bahaa E. A. Saleh and Malvin Carl Teich.Fundamentals of Photonics. John Wiley and Sons, Inc., 1991, p. 85.ISBN: 0471839655.

[11] Zafer A. Yasa and Nabil M. Amer. “A Rapid-Scanning Autocorrelation Scheme For Continuous Monitoring Of Picosecond Laser Pulses”. In:Optics Communications36.5 (1981), pp. 406–408.

[12] H. P. Weber. “Method for Pulsewidth Measurement of Ultrashort Light Pulses Generated by Phase-Locked Lasers using Nonlinear Optics”. In: Journal of Applied Physics 97 (2009), pp. 759–763.

[13] E. Granados et al. “Asynchronous cross-correlation for weak ultrafast deep ultraviolet laser pulses”. In:Applied Physics B38.5 (1967), pp. 223–2234.

[14] H. Nishioka et al. “Single-shot UV autocorrelator that uses a two-photon-induced photoacoustic signal in water”. In:Optics Letters18.1 (1993), pp. 45–47.

[15] Kenneth J. Leedle, Karel E. Urbanek, and Robert L. Byer. “Simple, picojoule-sensitive ultraviolet autocorrelator based on two-photon conductivity in Sapphire”. In: Applied Optics 56.8 (2017), 2227 line 18. DOI: https : / / doi . org / 10 . 1364 / AO . 56 . 002226".

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I TIA circuit drawing

Figure 17: Circuit drawing of the TIA

II Output power constant

Figure 18: Calculating the transmission of the setup by measuring the ’input power’ before the beam enters the setup and the remaining ’output power’ at the detector

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III NIR pulse length

Figure 19: Autocorrelation trace and FWHM of the NIR pulse used to create the UV pulses

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IV Acknowledgments

I would like to use this opportunity and thank everybody who played a more or less active role in the creation of this bachelors thesis. This of course includes both my supervisors Prof. Dr.

Franz K¨artner and Dr. Bastian Manschwetus, without whom this project would not have been possible. Bastian in particular always managed to squeeze in a few minutes to discuss whatever problem I had encountered and his feedback greatly influenced the outcome of this thesis.

In no particular order I would also like to thank Ingmar Hartl, Chen Li and Sarper Salman for keeping track of my process, setting deadlines and for thoroughly helping me grasp the theoretical background. As Chen and Sarper previously worked on the UV autocorrelator as- signment, they were a great aid in and out of the lab.

Finally I would also like to express my gratitude towards the FS-LA group at DESY for enabling me to work on this project and all the colleagues and office co-workers who contributed to a great workplace. Also I would like to thank my family, my friends and Katja Mehrwald, for always managing to lift my spirits when I was down.

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V Statutory declaration

I hereby formally declare that I have written the enclosed bachelor thesis independently and have not used sources or means without declaration in the text. Any thoughts from others or literal quotations are clearly marked. This thesis was not used in the same or in a similar version to achieve an academic grading or is being published elsewhere. The submitted thesis is identical to the one handed in on the electronic storage device.

I further agree that this bachelor thesis will be published.

Place, Date Signature

Setup and characterisation of an autocorrelator for UV laser pulses