1. Introduction

Spatially and spectrally bright visible light generation has attracted much attention because of the potential applications in many visible-based microscopy and spectroscopy systems. Supercontinuum (SC) generation in optical fibres is a useful technique for generating broadband single-mode light, however it is difficult to achieve short wavelengths unless the zero-dispersion-wavelength (ZDW) of fibres is shifted to visible region and a suitable pump source is found to meet the requirements of phase matching. In photonic crystal fibre (PCF) one can design a fibre with a ZDW at wavelengths from 500 nm upwards by adjusting the geometrical parameters (hole to hole pitch, Λ, and hole diameter to pitch ratio, d/Λ). Early experiments to generate visible continuum in optical fibres used femtosecond Ti:sapphire lasers (λ~800 nm) and small-core PCF [1] or tapered fibres [2] or used picosecond Kr⁺ (λ=647 nm) lasers [3]. SC spectra extended from 400 nm across the visible and into the infrared. Femtosecond and picosecond lasers are however relatively complex, and we demonstrated a more compact continuum source using a microchip passively Q-switched Nd:YAG laser at 1064 nm and a PCF [4]. With a single nonlinear fibre the shortest wavelengths in a continuum are rarely shorter than half the input pump wavelength. Technologically simple systems employing various Nd³⁺ lasers are therefore confined to continuum spectra at >500 nm. This is not sufficiently short in wavelength for many visible-based microscopy and spectroscopy systems. Frequency doubled Nd³⁺ lasers offer intense pump light in the centre of the visible, and would seem ideal candidates for visible SC generation. Unfortunately the fibre core diameter required to give ZDW close to 532 nm is <1 µm, and whilst we have demonstrated visible SC from a Q-switched 532 nm source in PCFs and tapered fibres with 0.5–0.9 µm core diameter [5], the structures are readily damaged by high energy ns pulses. An alternative approach has been to use both 1064 and 532 nm pump pulses combined, to yield continuum from a 2 µm core PCF [6]. Most recently a white-light continuum which extends to blue pumped by a single 1064 nm source, has been achieved first in multiple tapers(ns, Q-switched 1064 nm laser [7]), and also in multiple PCFs(ps, modelocked 1060 nm laser [8]), with sequentially decreasing ZDW. In both cases the first stage is conversion of the infrared pump light to a continuum spanning both sides of the pump wavelength and extending down to the region 700–800 nm. This light is then sent to a second stage PCF or taper, where the ZDW is around 700 or 800 nm and so the light at 700–800 nm present in the continuum is used as a pump to generate a visible continuum.

Here we also demonstrate bright single-mode visible continuum generation pumped by a simple compact 1064 nm Nd:YAG microchip laser in a two step process, but with a fundamental difference. The first step of the process is not continuum generation in a fibre with ZDW <1064 nm [4,7,8], but rather four-wave mixing in a PCF with ZDW >1064 nm. We have previously shown that such fibres can be designed to generate narrow-band radiation at wavelengths widely separated from the pump wavelength with high efficiency [4]. The narrow band pulses generated in this way are shown to be efficient for subsequent visible continuum generation. To maximize the power close to visible region in stage one the signal wavelength should be as short as possible, whilst maintaining efficient conversion from 1064 nm pump source. For the experiments described in this paper we have chosen for the first fibre a PCF properly designed to yield intense 742 nm pulses which are then fed into a second PCF with ZDW~700 nm which generates supercontinuum from this output. As with the technique of [8], two PCFs with very different core sizes are required to extend the continua to short wavelengths, and splicing is not a straightforward low-loss option for coupling. We have recently developed post-fabrication processing techniques [9] which enable the whole two-stage optical process to be carried out in a single monolithic fibre device. This has the additional advantage that the final output is not directly from the second, small core, fibre but is coupled back into an output pigtail of the first, 5 µm core diameter, endlessly single mode fibre which may be easily coupled into an application system. Our two-stage process is schematically shown in Fig. 1. The absence of a splice in this system ensures low coupling losses (<0.12 dB) and so maintains a high output brightness.

 

Fig. 1. Schematic for the two-stage visible generation process. 1064 nm laser light enters at X and is converted in the pigtail into 742 nm by four-wave mixing. The 742 nm light at Y is then the pump for continuum generation in the inflated and tapered section. The output continuum is measured in the output fibre at Z. Both input and output sections are endlessly single-mode fibre with a relatively large (5 µm diameter) core.

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2. Experiment

In the experiment, the pump laser is a Nd:YAG Q-switched microchip laser running at wavelength λ=1064 nm and emitting 0.6 ns (FWHM) pulses with 15 kW peak power at a 7.2 kHz repetition rate (Teem Photonics, NP-10820). After passing through a variable attenuator, which is made from a rotating half-wave plate followed by a polarizing beam splitter, these pulses are focused into a several meter long PCF structure as depicted in Fig. 1 and Fig. 2. A ×30 anti-reflection coated aspheric singlet lens is used to optimize the coupling. Up to 40 mW of power could be coupled into the fibre. The beam exiting in the device is then sent to an optical spectrum analyzer (OSA) or a power meter.

 

Fig. 2. Schematic of the inflated and tapered section (top) and scanning electron micrographs (SEMs) for device-A (bottom): (a) initial PCF-A before processing, (b) inflated fibre, (c) taper waist. All of the SEM images are to the same scale.

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The fibre used in our experiment is an endlessly single mode PCF, PCF-A, with a core diameter close to 5 µm and small holes [Fig. 2(a)]. This has a ZDW at about 1103 nm and gives strong FWM gain at 742 nm by the processes described in [4], allowing >35 % conversion of the 1064 nm pump light over a 3 m long fibre (Fig. 4(a)-blue trace). The idler wavelength, calculated to be at 1880 nm, is out of the measurement range of the spectrometer and so could not be observed. This fibre with signal wavelength at 742 nm was chosen for the signal wavelength being close to visible region whilst maintaining high conversion efficiency from pump.

In previous studies it has been well established that small-core PCF is ideal for SC generation from pulsed sources at wavelengths from 600 to 800 nm [1,2,3,9]. To take advantage of SC generation in PCF pumped by our wavelength converted output from PCF-A at 742 nm, we need to couple light from PCF-A into a structure with a small core and high air-filling fraction to achieve a ZDW at ~700 nm. We fabricated a device with very similar properties to a standard small-core PCF in the end section of PCF-A with very low loss in a tapering process recently developed by us [9]. At first pressure is applied to the air holes in a length of PCF-A with dry nitrogen at 7–10 bar. The fibre is then heated and stretched in a flame. The pressure makes the holes expand [Fig. 2(b)], whilst the stretching reduces the transverse dimensions of the fibre. The result, shown in Fig. 2(c), is that the core diameter is reduced from 5 µm to 1.7 µm, whilst the air-filling fraction is increased. Thus the fibre in the waist region of the inflated section is equivalent to a bulk small-core PCF, with a ZDW at around 700 nm. All transitions are made gradual so that the insertion loss of the entire device is just 0.12 dB at 1550 nm. The waist length is 120 mm (Fig. 1).

With the variable attenuator technique, we can study the continuum development as the launched power is increased. We measured the evolution of the spectra with respect to output power and input pigtail length (l, Fig. 1) by cutting back the fibre at the input end. The spectra for the device with l=7, 5, 3 and 2 m are illustrated in Fig. 3, where the x-axis is the output average power, the y-axis is wavelength and the colour indicates the power level for each wavelength component in the continuum spectra. In order to compare the different pigtail lengths on an equal footing, we need to present the results for each length for the same coupled input power. To achieve this, in these experiments we used the same technique to couple the pump laser into the input pigtail for each device and adjusted the alignment for maximum output power, thus the highest output power attained with each different input pigtail length will correspond to the same maximum coupled pump power. So by setting the minimum of the x-axis to zero and the maximum to the measured maximum output power in each case we should be able to ensure that the scale of all graphs is the same in terms of coupled input power. From the spectra, we can see that 3 m long pigtail is the optimum one for maximum visible continuum intensity, although the difference between 5 m, 3 m and 2 m is not great. The presence of an optimum pigtail length can be understood from the fact that the longer the pigtail is, the higher conversion efficiency from 1064 nm pump source to 742 nm but also the stronger Raman effect decreasing the peak power of 742 nm.

 

Fig. 3. Output power spectra (point Z of Fig. 1) for (a) 7 m, (b) 5 m, (c) 3 m, (d) 2 m PCF-A + inflated and tapered section (device-A). False-colour intensity scales in dBm/5nm are the same for all plots.

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The spectrum from the optimum pigtail length of 3 m is given in Fig. 4 (blue traces). The spectrum after 3 m of un-modified fibre is shown in Fig. 4(a), which demonstrates the spectrum incident on the inflated and tapered section at point Y in Fig. 1. The final output spectrum with 30 mW of average output power at point Z of Fig. 1 is shown in Fig. 4(b). It is obvious that SC generation taking place in the device fills almost the whole visible region and the gap between 742 nm and 1064 nm as well. In principle, this process is very similar to that reported in Ref. [4]. The difference is the shorter pump wavelength, shorter fibre ZDW, lower pump power and shorter length of fibre. It is the high nonlinearity resulting from the much smaller core size that overcomes the lower pump power and shorter fibre length and delivers good performance in visible SC generation. Raman Scattering will also act on 742 nm and 1064 nm pump wavelengths in the inflated and tapered section, which makes additional contribution to the gap filling between the two peaks and the long wavelength extension to 1350 nm.

 

Fig. 4. Spectra from (a) 3 m bare PCF-A (blue) and B (red), equivalent to point Y on Fig. 1, with average output power 37 mW, (b) device-A(blue) and device-B(red), point Z on Fig. 1, with average output power 30 mW. Note that peaks in the region 350–550 nm in (a) are artefacts of the spectrometer used.

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In comparison with Ref. [8], our device yields an intense peak at 742 nm instead of a continuum in the first stage, which means much more pump power could be provided to the second stage to enhance the visible continuum generation. With a single FWM process, the maximum conversion to the signal wavelength is governed by the partition of the energy of two pump photons at 1064 nm into a signal photon at 742 nm and an idler photon at 1880 nm. This gives a limit on the efficiency of conversion to 742 nm of 72 % (we achieve 35 % conversion in practice, half of this quantum limit). When the first stage is continuum generation, the pump light is spread out over a broad spectral range. Neglecting Raman contributions, half of the total energy will be at wavelengths shorter than the initial pump. Even if we assume that the energy of this short-wavelength portion is evenly distributed between 700 nm and 1064 nm, only about 30 % will lie in the useful range, 700–800 nm, for the second stage of continuum generation. This is just 15 % of the total pump power.

To illustrate the difference between a continuum and a FWM peak in the first stage, we made a very similar device to device-A with 3 m length of pigtail in another PCF, (PCF-B, hole-to-hole pitch and core diameter similar to PCF-A, but with slightly larger holes, ZDW at 1040 nm) and an identical inflated section, forming device-B. A 3 m length of PCF-B can generate a continuum spanning from 600 to beyond 1700 nm when pumped by the 1064 nm microchip laser. After interacting with the inflated section, the continuum extends to the visible region, but the power level is much lower than that of the device in PCF-A (device-A) because of lower pump power at 700–800 nm. The spectra for 3 m length of bare PCF-B and PCF-B + inflated section (device-B) are plotted in Fig. 4(red traces), which have the same average power level as PCF-A and device-A (Fig. 4, blue traces). From the spectra, we can see that device-B converted more power to long wavelengths (1300–1600 nm) than device-A, and device-A gives much more brightness in the visible. In order to see the continuum evolution clearly, we also plot the output power spectrum of device-B and put it together with that of device-A in Fig. 5. Not only does Fig. 5(b) indicate much lower power level in the visible range and higher power level in the infrared region, but also it shows the rapid extension of the spectrum of device-B into the visible as soon as the continuum extends to wavelengths close to the ZDW of the inflated section. The contrast with Fig. 5(a) is striking, with the direct FWM from 1064 nm to 742 nm occurring efficiently at low power and providing high pump intensity for the visible SC generation. It should be noted that the performance of devices with SC first then ZDW~700 nm (like device-B) can be improved by increasing the pump power, or the length of the second stage as in Ref. [8] where meter-length fibres are used. The efficiency of our technique is shown by the excellent performance of device-A, with a short length and at low power.

 

Fig. 5. Output power spectra for l=3 m for (a) device-A, (b) device-B. False-colour intensity scales in dBm/5nm are the same for both plots. Black solid lines are the ZDW of initial fibres (ZDW A, ZDW B) and black dashed lines are the ZDW of inflated sections (ZDW I).

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3. Conclusion

In conclusion, we have demonstrated a compact system which can give a bright single-mode visible light source (~-20 dBm/nm) with a compact and low-cost microchip laser and a single fibre device. The device is convenient to fabricate and both of the ends are 5 µm core PCFs which are compatible with conventional fibre optical components and offer good coupling efficiency in practical applications. Enhanced visible continuum brightness will widen potential applications in biomedical imaging and visible-based microscopy and spectroscopy systems.