We generate a flat, polarized and single mode supercontinuum (SC) spanning 450–1750 nm in a highly birefringent photonic crystal fibre (PCF) pumped by a 1064 nm microchip laser. More than 99% of the total power is kept in a single linear polarization. The measured power coupling penalty due to the elliptical core is less than 6% (0.25 dB). As one of its applications, we demonstrate tuneable visible/UV generation in the nonlinear crystal BIBO pumped by this polarized SC source. A tuneable range of 400–525 nm is obtained by critical phase matching in BIBO. We also show the results of visible/UV generation in BIBO pumped by the signal wavelength of polarized four-wave mixing (FWM) in PCF.
©2008 Optical Society of America
Supercontinuum (SC) generation in photonic crystal fibres (PCFs) has been investigated extensively, including using compact picosecond fibre lasers or nanosecond microchip lasers at 1064 nm [1–4]. Recently much attention has been paid to extending the spectrum to short wavelengths by the use of either multiple spliced PCFs with sequentially decreasing zerodispersion wavelengths (ZDWs) , a single piece of ZDW decreasing PCF  or an inflated and tapered monolithic PCF device . The continuum generated is generally unpolarized, as the fibre core is large (5 µm) and circular, whereas many applications require polarized SC output from a birefringent fibre. This is readily achieved for PCFs pumped at around 800 nm, where the small cores required to achieve the necessary dispersion at the pump wavelength acquire large birefringence for very little ellipticity [5–7]. For operation at 1064 nm birefringence can be introduced through the application of stress rods whilst maintaining a circular mode. However, stress is not stable at high temperature and may be annealed out. Here we present a highly birefringent PCF based on simple form birefringence [8, 9] introduced to the endlessly single mode (ESM) PCF fabrication technique . Birefringence is simply introduced during fibre drawing process and offers very flexible control. A potential disadvantage of using form birefringence is that the mode is slightly elliptical which causes loss when coupling to circular core fibres or circular laser beams, however we show that this loss is small.
One of the potential applications of this polarized SC source is tuneable visible or UV generation by nonlinear frequency upconversion in nonlinear crystals. BIBO [10, 11] is an attractive nonlinear material for frequency conversion in the visible and UV as it combines the advantages of both high UV transparency and enhanced nonlinearity: the optical transmission of BIBO extends from 2500 nm in the infrared down to 280 nm in the UV. As a biaxial crystal, it offers versatile phase matching properties. Since the introduction of BIBO, a number of frequency conversion experiments have been performed, including secondharmonic generation (SHG) of continuous-wave (CW) radiation at 1.06 µm , single-pass SHG of a pulsed laser at 1.06 µm , Q-switched SHG at 1.06 µm , frequency doubling of a CW Nd:YAG laser , tuneable single-pass SHG of a mode-locked Ti:sapphire laser [16–18], CW or Q-switched intra-cavity SHG at 1.34 µm , tuneable intra-cavity SHG of a CW Ti:sapphire laser , and optical parametric oscillators (OPOs)[21, 22]. All of these experiments directly used a high power laser as the pump source. To achieve tuneability, a complex tuneable laser system such as Ti:sapphire laser is normally needed [16–18, 20, 22]. The presence of a polarized SC source makes compact tuneable visible/UV generation possible. Unlike the single pump wavelength source in previous frequency conversion experiments, the pump source here is a continuum source, so sum frequency mixing (SFM) becomes possible and the large residual peak at the SC pump wavelength may enhance the process.
2. Polarized continuum generation
We fabricated the birefringent PCFs using a solid core PCF preform with uniform air hole diameters. During the fabrication process, we used differential pressure to enlarge two specific holes alongside the core to induce birefringence. Figure 1 gives the scanning electron microscope (SEM) images of fibre A at different scales. The measured d 2/d 1 ratio is 1.7, hole to hole pitch Λ=3.09 µm and hole-pitch ratio d 1/Λ=0.45. The two big holes make the core area slightly elliptical: the long axis to short axis length ratio is 1.25. It is the asymmetry that induces the birefringence. Unfortunately asymmetry of the core can also reduce the coupling efficiency to the circular pump laser mode or to other circular fibres. To check this we measured the butt-coupling efficiency between this fibre and a standard symmetrical PCF. The measured power penalty at 1550 nm was less than 6% (0.25 dB). The fibre attenuation was measured to be about 10 dB/km at 1064 nm by the cut-back technique. The calculated polarization beat length at 1064 nm for these fibre parameters is 8.6 mm.
In the SC experiment, the pump laser was a Nd:YAG Q-switched microchip laser (Teem Photonics, model NP-10820) running at a wavelength of Λ=1064 nm and emitting 0.6 ns (FWHM) pulses with 15 kW peak power at a repetition rate of 7.2 kHz. A polarizer was used to purify the input polarization and a rotating zero-order half-wave plate was put between the polarizer and the fibre input end to adjust the input polarization. A ×40 anti-reflection coated aspheric singlet lens was used to optimize the coupling. The input coupling efficiency was as high as 60%.
Figure 2(a) shows the SC spectrum with an average power of 30 mW from a 6 m length of fibre A when the input polarization was along the slow axis of the fibre. It can be seen that the short wavelength edge is as short as 450 nm and the spectrum is flat over the whole bandwidth from 450 to 1750 nm except for the residual pump peak. We note that this has a shorter wavelength edge in the blue than Ref. , although not as short as for some recent structures [3, 4]. We also checked the effect of input polarization on SC generation. Figure 2(b) shows the spectra for different input polarizations with the same pump power. The spectra are similar except that the spectrum extends to shorter wavelengths when the input polarization is along the fast axis. We believe this small difference is because of the slightly different dispersion properties of the two polarization modes.
In this study, the most important parameter is the polarization property of the continuum source. We checked the polarization of the output light by a broadband birefringent crystal polarizer. We define the polarization extinction ratio (PER) as the power ratio P s/P f or P f/P s, where P s and P f are the measured power when the output polarizer’s axis is set along the slow and fast axes of the fibre respectively. The output PER was P s/P f=10 and P f/P s=40 when the input polarization was along the slow and fast axes (99% and 97.5% single polarization). This is much higher than the PER (less than 2) of a SC produced by a PCF without birefringence and means the output light is well linearly polarized for all the spectral components. The difference between the slow and fast axes derives from the intensity-induced nonlinear birefringence [6, 7]. When the input polarization is along the slow axis, the nonlinear birefringence enhances the linear birefringence; on the contrary, when the input polarization is along the fast axis, the nonlinear birefringence can result in the total cancellation of linear birefringence.
3. SFM and SHG in BIBO
As an application of the polarized SC source we have demonstrated visible/UV output using SFM and SHG in a second-order nonlinear crystal. For a particular nonlinear crystal and pump power, the main factors limiting the conversion efficiency are the acceptance angle and acceptance bandwidth of the crystal and the effective nonlinear interaction length. Both acceptance angle and bandwidth are determined by the crystal length and the effective nonlinear interaction length is determined by the crystal length and the Rayleigh length of the pump beam. Either a longer crystal and less focused pump beam or shorter crystal and tighter focused pump beam may give optimum efficiency. However, in many applications, the generated beam needs to be coupled back into fibres, which requires good beam quality. A beam which is too tightly focused will yield a big divergence angle which will make both measurement and collection difficult. We therefore prefer a relatively long crystal and weakly focused pump beam. Here we chose a 5 mm long BIBO crystal. In order to make full use of the crystal, the optimum Rayleigh length of the focused beam is 2.5 mm, which corresponds to a 20 µm beam waist diameter at a wavelength of ~1 µm. Thus we should choose a ×4 lens or lens combination to focus the beam coming from the 5 µm core diameter fibre. In practice, we tried with different lenses or lens combinations with various magnifications and ×4 lens combination showed the best performance.
3.1 Nonlinear conversion of continuum
The experimental configuration for nonlinear conversion is shown in Fig. 3(a). The continuum source setup was the same as the polarized SC generation setup, except that we used a shorter piece of fibre (0.6 m) because here we need maximum power in the wavelength range of 750–1000 nm rather than maximum bandwidth. We set the pump polarization along the slow axis of fibre because of the higher output PER. The spectrum taken at point O in Fig. 3(a) is plotted in Fig. 3(b). By use of the continuum source, we can realize a tuneable visible/UV source by just simply rotating the crystal to the critical phase matching angle. To take the advantage of the high nonlinearity of BIBO, the most interesting plane for nonlinear optical interactions is yz (ϕ=90°), which offers the highest effective nonlinearity, with a maximum effective nonlinear coefficient deff ~ 3.4 pm/V. Type I (e+e → o) phase matching is available for SFM and SHG in this plane for angles 90° < θ < 180° [11, 16]. The crystal was cut for type I phase matching in the yz plane at internal angle close to θ=155° at normal incidence, and the facets were anti-reflection coated for 850 nm and 425 nm. All experiments were performed at room temperature. We were able to achieve wavelength tuning from 400 to 525 nm, limited by the crystal aperture at larger angles.
In order to determine the nonlinear processes involved we measured the powers for some specific wavelengths in the continuum generated from a 0.6 m length of fibre A using 10 nm bandwidth interference band pass filters (about 45%–50% transmission). The measured powers are shown in table 1 together with the measured powers for corresponding upconverted wavelengths. We notice that the powers at fundamental wavelengths from 800 nm to 950 nm are quite similar, but the powers at upconverted wavelengths differ by a large amount. Moreover, the power at 475 nm is 120 µW which would correspond to 40% SHG conversion efficiency from 950 nm (taking the filter transmission efficiency at 950 nm into account). This would be a remarkable efficiency for such a low pump power. To understand this, we plot the spectra for two typical wavelengths, 425 nm and 475 nm in Fig. 4(a) and calculate the SFM tuning phase matching curves [type I, Fig. 4(b)] by using Sellmeier equations . From Fig. 4(b) we can see that by changing the crystal orientation 475 nm output can be generated by many different SFM processes as well as by SHG of 950 nm. In particular at an angle of about 162.1° (vertical red line), SFM 1064 nm + 860 nm → 475 nm will occur. This will be an efficient process as the residual pump intensity at 1064 nm is very strong and the 860 nm component is included in the continuum, and can explain the high output power at 475 nm. We have previously measured the residual pump in a similar continuum (but 15 m of fibre) to be around 10% of the total output power . Whilst this is a small fraction of the total power, it is concentrated in a narrow spectral range and is far stronger than any of the infrared powers measured in table 1. If we therefore propose that the crystal is placed at 162.1°, we should see strong 475 nm output, but Fig. 4(b) also shows that we should see other blue/visible wavelengths generated from other SFM/SHG processes at this angle: a wavelength slightly longer than 475 nm will be produced by SHG of a wavelength longer than 950 nm; many wavelengths shorter than 475 nm will be produced by SFM of one photon with a wavelength longer than 1064 nm and one shorter than 860 nm. As the input is a continuum spectrum all these processes will occur and we observe a spectrum with a peak at 475 nm, but a broad tail to shorter wavelengths [Fig. 4(a)]. There is a short wavelength limit to the tail as we can see from Fig. 4(b) that to generate 450 nm at an angle of 162.1° requires pump wavelengths less than 710 nm, which are not available in the continuum after the 715 nm long-pass filter. This type of spectrum is observed when generating visible wavelengths from 500 nm to 450 nm.
Compared with the spectrum at 475 nm, the spectrum at 425 nm is symmetrical [Fig. 4(a)]. We can see from the phase matching curve [Fig. 4(b)] that at 156.5° (vertical orange line), SFM 1064 nm+710 nm → 425 nm can take place. This will be weaker than the SFM process considered above generating 475 nm using 1064 nm, as the intensity at 710 nm in the continuum is low. There is now no tail to shorter wavelengths as that would require wavelengths shorter than 710 nm for SFM.
To generate 400 nm there is no SFM possible using the residual pump at 1064 nm, so this must be achieved by SHG of the 800 nm component of the continuum, with consequently very low observed output power.
From this discussion we can see that the nonlinear process contributing to frequency upconversion here is mainly SFM rather than SHG. Since the pump power is not very high the generated components have very low power, but the powers are still higher compared with what can be generated directly from SC (table 1). Also the use of continuum source brings some new features such as compact, convenient tuning. With the development of high power picosecond fibre lasers, there is a clear possibility for generating more power in the blue/UV.
3.2 Nonlinear conversion of discrete FWM wavelengths
As we discussed above, the visible/UV generation based on a continuum source is tuneable but the power is low. Another way to get a pump source for frequency upconversion is to fabricate a different PCF yielding distinct FWM peaks instead of a broad continuum . Here the fibre we were using is labelled as fibre B. Fibre B was made in the same way as fibre A, except that the pressure applied in the holes was slightly different so that d 1/Λ=0.29, Λ=2.95 µm and d 2/d 1=1.8. The pump source, at 1064 nm, is now in the normal dispersion regime of fibre B, so a FWM process will take place , in this case generating a signal wave at 834 nm and an idler at 1478 nm.
The conversion from the pump to the signal at 834 nm can be very efficient, leading to sufficient power at 834 nm to allow Raman conversion from this wavelength as it propagates further down the fibre. We chose an optimum fibre length of 1.2 m in order to balance the FWM efficiency and Raman effect. Like fibre A, fibre B is a highly birefringent fibre. The PER was 245 and 60 when the pump polarization was set along the slow and fast axes respectively, which corresponds to 99.6% and 98.4% of power kept in a single polarization. Again for efficient nonlinear upconversion, we set the pump polarization along the slow axis. The FWM spectrum is illustrated in Fig. 5(a). A signal at 834 nm and an idler at 1478 nm are generated. The power at 834 nm was measured to be 14 mW, which is 37% of the total output power 38 mW. We used the ×4 lens combination to focus the beam into 5 mm BIBO and rotated the crystal for phase matching. We got bright visible output at two different angles. The spectra are plotted in Fig. 5(b). The power measurements showed that the powers at 417 nm and 467 nm were 0.53 mW and 0.76 mW respectively. 417 nm comes from SHG of 834 nm and 467 nm comes from SFM between 834 nm and 1064 nm. The SHG efficiency is about 3.8%, which is comparable with previous results at similar pump power level . Also the spectra are symmetric because there is no additional contribution from other wavelengths. For the existing fibre and pump laser, the FWM peak is fixed. But for different fibres with slightly different dispersion properties, the FWM peaks are various. From this point of view, the SHG will be tuneable by use of different fibres in the first stage. We have fabricated PCFs with FWM peaks at around 700 nm, which means the short wavelength can be tuned to as short as 350 nm in this way. The operation is more complex, but the system is still compact and we get much higher power output than the continuum case.
According to Figs. 4(a) and 5(b), the short wavelengths are generated with no pedestal, down to the level of -35 to -45 dB. This is very useful in some applications such as fluorescence microscopy, where generated fluorescence has longer wavelength but is very weak. The clean spectrum with high extinction ratio makes it easier to distinguish the fluorescence from any background at the same wavelength emitted by the pump. This source is therefore potentially more useful than simply a filtered continuum as a tuneable pump for such applications.
For many applications, it is necessary to couple the generated short wavelength light to a single mode fibre. We therefore measured the coupling efficiency of the free space beam to a piece of ESM PCF. A power of 66 µW at 417 nm was coupled into a 3 m length of ESM PCF. The efficiency was 12.5%, which is not high because the beam is not round due to the spatial walk-off between the fundamental beam and SHG beam in the crystal [24, 25]. If a walk-off compensation technique using multiple crystals [24, 25] could be applied in this experiment, both the upconversion efficiency and the free space-fibre coupling efficiency would be enhanced.
In conclusion, we demonstrate a simple way to fabricate highly birefringent PCFs. Using this kind of birefringent PCF, we made a compact continuum source with more than 99% of power in a single linear polarization, whilst the modal asymmetry is sufficiently low that it yields less than 6% (0.25 dB) measured power coupling penalty with symmetrical PCF. The continuum source is flat over the entire bandwidth from 450 nm to 1750 nm.
By use of SC and FWM processes in the birefringent PCFs, we realized tuneable visible/UV generation by critically phase matched SFM and SHG in BIBO. The tuning range was from 400 nm to 525 nm. Further improvements including the use of high power picosecond fibre laser and walk-off compensation technique, are proposed to enhance this compact and convenient tuneable visible/UV source.
C. Xiong is supported by ORS scheme. W.J. Wadsworth is a Royal Society University Research Fellow. This project is partially supported by the EPSRC (Grant No. EP/D058074/1).
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