We report on supercontinuum generation in individual fibers of a commercial Schott imaging fiber taper. Supercontinuum spectrum covering a wavelength range from about 500 nm to 1 µm was obtained. Unlike conventional approaches which use either a single micro-structured photonic crystal fiber (PCF) or an individual fiber or PCF taper, the availability of many fibers in an imaging taper can open new possibilities to independently and controllably generate supercontinuum arrays.
© 2006 Optical Society of America
Supercontinuum , which not only covers a broad range of wavelengths but also has laserlike high spatial coherence, has found many important applications such as frequency metrology , optical communication , imaging [4, 5], and spectroscopy [6, 7] to name just a few. Optical fiber based supercontinuum generation has recently attracted a considerable amount of interest since it only requires laser pulses with moderate energy (e.g., femtosecond pulses with a few nano-joules directly from an oscillator). For instance, tapers of conventional single mode fibers have shown to be able to generate a supercontinuum covering a wavelength range from 370 nm to 1.5 µm . With the recent advances of micro-structured photonic crystal fiber technology [9–12], an octave spanning supercontinuum can be routinely generated by coupling short laser pulses into a short length (e.g., a few cm) of a highly nonlinear photonic crystal fiber. Since light can be tightly confined in the core of a microstructured fiber, its effective nonlinearity is greatly enhanced. In addition, the dispersion properties of the micro-structured fiber can be engineered by properly designing the fiber structure [13, 14], such as the air hole diameter and pitch. Nowadays, supercontinuum generation is typically obtained from individual fibers or tapers. As a result, it is still a challenge to obtain an array of supercontinuum sources with independent control of their spectra. Supercontinuum generation from a micro-structured fiber with multiple submicron cores was previously studied . Here we report on the generation of supercontinua which, unlike previous efforts, makes use of individual fibers in a commercial imaging fiber taper (Schott). Due to the many fibers available, it can be potentially used to generate supercontinuum arrays that can lead to new applications that would benefit from the simultaneous availability of many white light sources.
2. Experiments and results
Imaging fiber tapers are typically used to transfer and magnify or de-magnify images . An imaging taper itself consists of many individual fibers that are closely packed together. Figure 1 shows a portion of the microscope images of two representative Schott imaging tapers. Individual fibers can be easily observed (the brighter region corresponding to core). These fibers are made of Schott-24 glass. They have a typical size of 6-25 µm at the larger end  and the refractive index difference between the core (n~1.8) and the cladding (n~1.5) is quite large. As a result, these are multi-mode fibers. Another interesting feature to note is that the cross sections of the cores are not of circular shape. Instead, they have a rectangular corner.
We used a 60× objective lens (Newport, numerical aperture: 0.85) to couple femtosecond laser pulses (KM Labs, pulse energy ~4.5 nJ, pulse width ~60fs, center wavelength ~ 808nm) into a single fiber of an imaging taper (model 25970MU, 8 µm fiber pitch at the large end, magnification ratio: 3.125, length ~2.5 cm) from the larger end to generate supercontinuum. By adjusting the position of the taper, different far field supercontinuum patterns were observed as illustrated in Fig. 2. These pictures were captured at a few centimeters away from the taper output end. Similar supercontinuum pattern can sometimes exhibit dramatically different colors as can also be seen in Fig. 2. Patterns 1 and 2 (see Fig. 2) have an average power of 120 mW and 110 mW respectively (with an incoming laser average power of 370 mW) and are the two most intense patterns. We simulated the guided modes at different wavelengths by using a commercial software package (Photon Design, http://www.photond.com) which uses a fully vectorial mode finder to solve for 2D waveguide structures. In our simulation, the fiber cross section was approximated as a polygon and the guided mode solutions at different wavelengths were used to calculate the far field profiles. Many modes are supported and similarly shaped intensity distributions are found at different wavelengths. Some representative calculated far field profiles are shown in Fig. 3 where qualitative agreement with some of the observed patterns can be seen.
Next, the generated supercontinuum at the output of the taper (the smaller end) was collimated with an objective lens (Newport 60×, numerical aperture: 0.85) and sent to an optical spectrum analyzer (Ando AQ 6315E). Figure 4 shows the measured supercontinuum spectra of Pattern 1, 2, and 3 respectively. The measured spectra of the three patterns are quite different in the short wavelength part (500 nm–700 nm). They are all peaked around the pump wavelength near 800nm and extend to the visible and near infrared regimes. These spectra cover a wavelength range from about 500 nm to beyond 1 µm. We measured the spectra of the supercontinuum generated by the incoming laser beam at different average powers and the results are shown in Fig. 5. Figures 5(a) and 5(b) show the spectral-power dependence for pattern 1 and 3. For comparison, we also measured the spectrum of the continuum generated in a different imaging taper (model 2562E, 6 µm fiber pitch at the large end, magnification ratio: 1.33, length ~1.5cm) and the result is given in Fig. 5(c). In these figures, the horizontal axis shows the average incoming laser power while the vertical axis represents the wavelength. Each column of the images represents a spectrum measured at an input power specified by the horizontal axis. As the power of the incoming laser increases, the supercontinuum spectrum broadens accordingly as can be observed in the figures. These measured spectral signatures are consistent with the spectral broadening observed with a self-phase modulation (SPM) dominated process. We do not know the n2 value of the Schott-24 glass and the detailed dispersion property of individual fibers at the time of this work. Nevertheless, notice that at the smaller end of the taper the effective modal area can be as small as Aeff<10 µm2. For silica fibers with similar modal area, we estimate that a nonlinear coefficient γ>0.2 kW-1cm-1 at λ=800nm (γ=n2ω0/cAeff ) can be obtained. Large nonlinear phase shift would be obtained if a femtosecond laser pulse with a few tens of kW peak power propagates for just a few centimeters.
A unique feature of the imaging taper is that it consists of many closely packed individual fibers. In the following, we demonstrate the generation of two supercontinua from two different fibers of the same taper. The schematic diagram of the experimental setup is shown in Fig. 6(a). A femtosecond laser beam (Spectra-Physics Tsunami, pulse width ~100 fs, average power ~1.8 W, center wavelength 810nm, repetition rate 80MHz) was first divided into two beams by a beamsplitter and then steered by mirrors into a 60× objective lens (focal length 2.8 mm) at a separation angle of about 2°. The separation of the two focused beams on the taper input facet (larger end) is therefore estimated to be ~100 µm which is much larger than the fiber pitch (~8 µm). This ensures that the supercontinua are generated from two different fibers. Both beams have about the same average power (~700 mW) right before the objective lens. The generated supercontinuum patterns were imaged by another objective lens onto an observation screen located a few meters away. Figures 6(b1) and 6(b2) show the observed supercontinuum patterns generated by the two incident beams respectively. The two beams were alternatively blocked during the experiment. When both beams were let through the two supercontinuum patterns can be observed simultaneously as illustrated in Fig. 6(b3). Since they are generated from different fibers of the taper, the two supercontinuum patterns are separated spatially. Next, we used a grating to disperse the generated supercontinuum. As expected, two rainbow-lines were produced which is shown in Fig. 6(c). To further illustrate our experimental results, here we also include two movies showing the two supercontinuum patterns and their rainbow spectra.
These experiments clearly demonstrate the generation of two supercontinuum sources which can be independently controlled. Finally, an aesthetically pleasing image can be obtained by propagating the supercontinua through a diffractive optical element sample (Digital Optics Corporation). In this case, a colored image of an eye was produced [Fig. 6(d)]. The two bright spots in the projected image are due to the undiffracted portions of the two supercontinuum beams propagating along slightly different directions.
In summary, we have investigated supercontinuum generation in individual fibers of an imaging fiber taper. The measured spectra of the generated supercontinuum cover a wavelength range from about 500 nm to 1 µm. We have also demonstrated the generation of two independently controlled supercontinuum sources from different fibers of the same taper. Since commercially available ultrafast amplified systems can routinely produce femtosecond pulses with a few mJ per pulse, an array of supercontinua can be potentially generated in an imaging taper with appropriate coupling, for instance, through a microlens array. We should point out that one limitation of the current imaging fiber tapers is that they consist of multimode fibers. Single-mode fibers not only can produce a more uniform modal profile, but will also allow for a better control of the supercontinuum generation process. Such single-mode tapers can be potentially achieved by further reducing the core size or lowering the index contrast between core and cladding.
We thank James Triba and Schott Fiber Optics for kindly providing the imaging tapers used in the experiments. We also thank Dr. Qing Wang for kindly allowing us to use the microscope in his lab and Peng Li for obtaining the microscope images. This work is supported by the National Science Foundation (ZL and FGO) and the Lehigh/Penn State center for optical technologies (ZL).
References and links
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