The combination of a diode-pumped Nd:YVO4 picosecond laser source with tapered fibers allows the realization of a compact white light source with an average output power of up to 5.65 W. We generated supercontinua with a throughput of ~50% over a spectral range of up to 1140 nm by using a series of tapered fibers that were spliced together.
©2005 Optical Society of America
The increasing demand for white laser light, covering a large spectral range from the infrared to the UV, is drawing current research to the development of compact and inexpensive sources . For the realization of such a white light source, a highly nonlinear medium and a compact pump laser have to be combined. Up to now, the state of the art method in many laboratories around the world for generating a supercontinuum is the combination of a microstructured fiber with a standard femtosecond oscillator system . Such a system has the disadvantage of being complex and expensive. In addition, it provides only low output powers which limit the white light power to several hundred milliwatts. Recently, it was also possible to generate continua with pico- and nanosecond pulses in combination with microstructured and tapered fibers [3–5]. The coupling into a microstructured fiber is however highly critical and often suffers from thermal instability due to its small core diameter. The nanosecond experiment with tapered fibers in  generated only a few mW of output power. In this paper, we present the realization of a compact, stable, and inexpensive white light source using self-made tapered fibers [6,7] with a large input core diameter of 8.2 µm that delivers multi-Watt average powers.
Our approach uses passively mode-locked Nd:YVO4 pump lasers  developed by JENOPTIK Laser, Optik, Systeme GmbH, Germany, with tapered fibers to generate multi-Watt supercontinua. We used two different pump systems. Laser A has a pulse duration of 8 ps and a repetition rate of 85 MHz at 1064 nm center wavelength. This amplified system with a size of 60×30 cm2 can generate up to 30 W of laser light. The even more compact laser B has a size of 39×25 cm2 and differs in a larger repetition rate of 120 MHz, a slightly longer pulse duration of 9 ps, and an average output power of up to 6 W likewise at 1064 nm center wavelength.
White light laser sources have possible applications in a wide market, e.g., optical coherence tomography, frequency metrology, measurement technology, display systems, or in spectroscopy and microscopy. However, the generally expensive femtosecond laser system and sometimes the lack of power limit the number of applications. For example, optical 3D metrology of several feet large artefacts or supercontinuum generation as a source for selectable narrowband wavelengths often requires multi-Watt white light power. Our aim was therefore to reduce the cost and size of the laser system in combination with an enhancement of the average white light output power. The compactness and inexpensiveness arises from the fact that both oscillator types are directly diode-pumped and their cavities are folded several times. Together with the high output powers that these oscillators provide, they are the optimum choice to achieve our goal. In each case, spectrally-broad, spatially single mode, and stable supercontinua were generated — well suited for all the above mentioned applications. Figure 1 outlines the parameters of the two lasers and gives a rough draft of our set-up.
2. Experimental data
In the experiment we coupled the picosecond pulses (beam waist diameter 300 µm, divergence 4.5 mrad) into the tapered fiber with an objective (Linos 038722, NA: 0.3, focal length at 1150 nm: 15.56 mm) mounted on an xyz-stage. The distance between laser and input coupling was about 1.5 m. With both laser versions, a Faraday isolator (EOT 4I 1064) had to be utilized to prevent back reflections from the fiber into the laser. For the fabrication of the tapered fibers, we used single-mode Corning SMF 28 quartz fibers and drew them in a home-built fiber drawing rig . For drawing, the fibers were heated over a moving propane-butane-oxygen flame with a temperature close to the melting point of quartz. The drawn fibers consist of a taper region where the outer diameter decreases from 125 µm to a few micrometers over a distance of about 15 mm. This is followed by a waist region with a diameter set to a value between one to about five micrometers, followed by another taper region. Variation of the drawing velocity allows the control of the waist diameter in a very reproducible way. The measured waist diameter uncertainty is about 10% . All spectra in this article were taken with an Ando AQ 6315A optical spectrum analyzer.
2.1 Results with an 8 ps 30 W Nd:YVO4 amplifier
In order to obtain anomalous dispersion close to the zero dispersion wavelength (see Fig. 2(b)) and to simultaneously achieve a high nonlinearity, fibers with diameters around 2–4 µm showed the best performance at 1064 nm pump wavelength. Figure 2 displays four output spectra of a tapered fiber behind laser A as a function of input power in a logarithmic diagram. The tapered fiber had a waist diameter of 3.3 µm and a waist length of 90 mm. It is worth mentioning that even with 11 W input power, a throughput of more than 50% was obtained, resulting in a maximum output power of 5.65 W.
The broadening process starts with the generation of a Stokes and an anti-Stokes component around the pump (green line). The symmetric arrangement of further frequency pairs demonstrates that cascaded four-wave mixing  is responsible for the progressing spectral expansion (blue line). By increasing the input power, the threshold for Raman amplification  is exceeded. This results at 5.65 W output power in a supercontinuum of up to 260 nm width (all spectral widths are measured -20 dB below the peak). A further increase of the pump power broadened the spectrum even further, but ended finally with the destruction of the fiber. Hence, increasing the laser power is not an appropriate approach for further spectral broadening. Therefore, our next attempt was the lengthening of the fiber waist. This leads especially in the pico- and nanosecond regime to a stronger spectral broadening due to an enhancement of the nonlinear interaction length [1,3]. Compared to photonic crystal fibers (PCF), an increase of the waist length of a tapered fiber is more complicated, as a larger travel amplitute of the burner is needed. Therefore, the size of the drawing rig has to be adjusted. On the other hand, a longer waist corresponds automatically to a longer protective housing, making the tapered fiber less manageable. Thus, our approach was the manufacturing of a “double-taper”, consisting of two tapered divisions with 90 mm length each within a standard telecommunication fiber SMF28. The output spectrum as a function of the input power for such a constellation is shown in Fig. 3. It is striking that significant broadening already starts at lower input powers. Especially on the infrared side the spectrum extents further to about 1550 nm due to enhanced Raman scattering, although the output power is - compared to the maximum output power in Fig. 2 - explicitly lower. The dip at 1400 nm is due to the OH absorption band in the fiber that coupled the light into the spectrometer.
For a further improvement of the spectral broadening, a “triple taper” with three divisions of 3 µm waist diameter and a waist length of 90 mm, respectively, was utilized. Figure 4 shows the output spectra as a function of input power. In fact, with an output power of 4.2 W, the generated supercontinuum covers almost two octaves and the broadening also extends to the visible, leading to a spectrum ranging from 460 nm up to 1600 nm. We assume that the blue components are due to non-solitonic radiation from soliton splitting  and subsequent four-wave mixing. The condition for soliton formation, namely simultaneous anomalous GVD at the pump wavelength of 1064 nm together with self-phase modulation, is fulfilled for all used waist diameters in our experiments (see Fig. 2(b)). Surprisingly, the output efficiency was still more than 35%, demonstrating that multiple taper series will not decrease the throughput considerably. This is likely due to the fact that the first taper acts as a mode filter, shaping the spatial mode in such a way that it travels nearly unattenuated through the following tapers and waists.
2.2 Results with a 9 ps 6 W Nd:YVO4 oscillator
Furthermore, the compact 120 MHz oscillator (laser B) with a pulse duration of 9 ps was tested for the generation of white light in combination with tapered fibers. Due to the higher repetition rate together with the smaller maximum output power of about 4.7 W after the Faraday isolator, a spectral width like the one shown in Fig. 3 could not be expected. Nevertheless, by splicing tapered fibers together to increase the interaction length, a substantial broadening was achieved.
Figure 5 depicts the output spectra as a function of the number of tapered fibers spliced together. The output power drops slightly due to losses at the splices. The narrow output spectrum of a single 3.1 µm thick and 90 mm long waist can be increased to a width of 350 nm simply by splicing two additional fibers with a similar waist diameter and waist length in front of the single waist fiber.
It is an interesting challenge to investigate the question which waist diameter configuration leads to the broadest supercontinuum using laser B. The question is motivated by the fact that different fiber waist diameters possess different zero GVD wavelengths and therefore different anomalous dispersion around the pump. In fact, thinner waists result in smaller zero GVD wavelengths and hence larger anomalous GVD at 1064 nm (see Fig. 2(b)). Our experience showed that these different waist diameters would produce quite different spectra. Therefore, a very broad spectrum might be generated by splicing together a series of tapered fibers with different waist diameters. At the same time, we need to check whether a substantial broadening is not also achievable simply by choosing an “ideal” waist thickness and increasing the interaction length as much as possible. Accordingly, we prepared three sets of samples, each consisting of three tapered fibers with similar waist diameters and 90 mm waist length each that were spliced together. Afterwards these fibers were cut apart and a series of fibers with mixed diameters (2.1, 3.0, and 3.8 µm) was spliced together. Figure 6 shows the resulting spectra.
It is clearly visible that the broadest spectra were achieved with a triple taper consisting of three fibers with approximatly 2 µm waist thickness. This spectrum has a nice Gaussian shape and ranges from 1000 to 1350 nm. Increasing the diameter of the triple taper to about 3 µm decreases the width of the spectrum especially in the infrared to a total of 200 nm. The almost 4 µm thick triple taper generates nearly no continuum at all. The mixed waist thickness triple taper generates a continuum with a width of 250 nm — which is about 100 nm less than the 2 µm triple taper. This leads to the conclusion that mixing the diameter of the triple tapers leads to an averaging of the corresponding spectra of the homogeneously thick triple tapers. The fact that the spectra tend to spread out much stronger to the infrared points to the Raman effect as the dominant generation mechanism. As the Raman effect is only sensitive to the peak intensity in the waist, the fibers with the smallest diameters therefore generate the broadest spectra. The mixture of waist diameters leads to an averaging of the peak intensity, and hence an average broadening, as observed in the spectrum associated with the (2.1+3.0+3.8) µm waists.
The combination of compact high power picosecond laser sources with tapered fibers is well suitable for the generation of stable multi-Watt supercontinua. We utilized two types of laser sources differing in power and repetition rate. Laser A with up to 350 nJ peak power generated up to 5.65 W white light. Increasing the interaction length by splicing several tapered fibers together broadened the spectrum to up to 1140 nm. The even more compact laser B was capable for generating an up to 350 nm broad continuum by pumping it into an appropriately spliced configuration of tapered fibers. Finally, we discussed the influence of different waist diameters within one series of tapered fibers on width and shape of the generated supercontinuum spectra.
The authors thank Rui Zhang for calculating the GVD diagram. Harald Giessen and Jörn Teipel thank the DFG (FOR557) and BMBF (FKZ 13N8340) for support.
References and links
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