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After the laser was characterized a synchronously pumped femtosecond OPO generating pulses in the infrared region was constructed. The experiments presented in this thesis were done by employing ring-geometry type OPOs. In this section I now present the design and the characterisation of the synchronously pumped OPO.

3.3.1 Introduction

Synchronously pumped OPOs have the same cavity length as their pump laser. When the OPO cavity length is changed, the OPO output wavelength changes due to the intracavity dispersion.

The OPO maintains synchronism with the pump laser but is oscillating at a slightly different wavelength [12].

3.3.2 PPKTP crystal design

We chose to build a ring type OPO cavity based on a periodically poled potassium titanyl phosphate (PPKTP) crystal. Since the Ti:sapphire pump pulses were broadband, we aimed to generate broadband signal and idler pulses too. The use of the quasi phase matching (QPM) technique allowed us more flexibility in the output wavelengths than would be possible using bi-refringent phasematching. The most common nonlinear crystals used for QPM are periodically poled lithium niobate (PPLN) and periodically poled potassium titanyl phosphate (PPKTP).

In comparison, both are transparent from 330 nm to 4.5 µm, the target range for our OPO frequency comb. PPLN has a greater effective nonlinear coefficient than PPKTP, but PPKTP has a nonlinear gain comparable to PPLN once the refractive index contribution to the figure of merit is taken into consideration. PPKTP can operate at room temperature without pho-torefractive damage [13] and has 3-4 times lower second-order dispersion (GDD) at the same wavelength in comparison with PPLN (see Chapter 4), so pulse broadening and pulse walk-off effects are reduced. The net GDD of an OPO must be compensated for the simultaneous ge-neration of broadband signal and idler pulses. Most of the dispersion is due to the nonlinear crystal. Our PPKTP crystal was 1.2 mm in length. While a longer crystal length may increase the gain and therefore might produce more power, it may not ultimately be efficient due to group delay walk-off. Using too long a crystal length will reduce the bandwidth of the generated pulses since the parametric gain bandwidth depends inversely on crystal length. All of these factors were considered when selecting the crystal length for the OPO.

Our goal was to generate a broadly tunable frequency comb in the mid-IR region from 1-4 µm covered by the signal and idler pulses. Since our pump laser central wavelength was fixed at 800 nm, the signal and idler wavelengths could be calculated using

1 Signal frequency combs spanning from 1-1.6 µm correspond to an idler wavelength shift from 1.6-4µm. A phasematching simulation for PPKTP was done in order to determine the approp-riate QPM grating period. In this way the grating periods necessary to cover a broad range of the signal wavelengths can be determined. The signal and therefore the idler wavelengths generated in an OPO depend on the grating period Λ. Our PPKTP crystal had multiple gra-tings which allowed the tunability of the central signal wavelength by moving the crystal in the vertical direction. In total 10 different grating periods were implemented from 25.4-27.25 µm. A crystal length of 1 mm was used for the parametric generation of a signal field which could efficiently generate signal wavelengths from 1-1.6 µm (see Figure 7). Here we consider a practical pump pulse has 32 nm bandwidth from 784-816 nm. Figure 8 shows that the pha-sematching is efficient for a signal wavelengths from 1.05-1.6 µm when the grating period is 26.75 µm. Grating periods from 25.4-27.25 µm can be adjusted in order to accurately shift the central signal wavelength according to the central pump wavelength (800 nm). The central

signal wavelength could be tuned with a stage or PZT across a broad range of wavelengths.

The broadband tunability of the phasematching condition is determined by the pump pulse bandwidth and the grating period. If the pump pulse is femtosecond, it allows broad signal tu-nability by using the same grating period when the crystal is short. The grating period change is not necessary which is not as critical for tuning, unlike the case for ps or ns pump pulses.

Figure 7. Phasematching condition for a 1 mm length PPKTP crystal with a grating period of 26.75 µm.

The detailed PPKTP crystal design which included OPO, SHG, blank and SFM sections is presented in Figure 8. The PPKTP crystal was used for a number of different experiments which will be presented in later chapters. The crystal was AR coated for the wavelengths from 750-850 nm (pump) and from 980-1620 nm for the signal. The multiple grating periods made it possible to generate signal pulses in a broad range of wavelengths from 1-1.6 µm. The resulting idler field was generated from 1.6-4 µm. Since our experiments involved locking the f_CEO of the idler or signal pulses, the additional gratings for pump+idler SFM and second-harmonic (SHG) of the signal pulses were implemented so as to provide strong visible outputs which were necessary in order to obtain a low-noise f_CEO heterodyne beat.

The crystal was mounted in an aluminium holder with an adapter. The mount ensured maxi-mum space for the beam going through the crystal which was attached to an optical mount (see Figure 9). The vertical and horizontal tilting was important for ensuring that the pump light from the crystal surface could be reflected back to the Ti:sapphire laser. For the purpose of alignment the reflected light from the PPKTP crystal had much lower power than the pump beam and was followed around the OPO cavity in the opposite direction. The alignment of

Figure 8. (a) The general structure of a grating section of the PPKTP crystal. It contains OPO, SHG, blank and SFM sections. The section structure is the same for all 10 grating periods from 25.4-27.25 µm; (b) The grating periods of the PPKTP.

Figure 9. PPKTP crystal holder: the crystal was glued on an aluminium plate which was attached to a Newport optical mount by using an adapter. The mirror mount could be moved by an x-y-z translation stage.

the reflected beam ensured that the phasematching condition was satisfied as we expected from calculations. On other hand, the light must not be exactly reflected back to the Ti:sapphire crystal, since it can disturb and stop the modelocking regime of the laser. The translation stage which held the mirror and the crystal was used to change the grating periods without re-aligning the OPO cavity. Nevertheless, since the OPO was tuned across a broad range of the wavelengths, the alignment had to be repeated for each oscillating wavelength especially if it was changed from 1.5-1.1 µm.