We report a novel ultrafast red-green-blue (RGB) laser source based on second harmonic generation from a two zero dispersion wavelength (TZDW) fiber continuum source. The TZDW fiber source consists of a custom-built Yb:fiber amplifier and a commercially available TZDW photonic crystal fiber (PCF) which enables low noise and efficient frequency conversion from the 1035 nm pump source to two spectrally localized pulses centered at 850 nm and 1260 nm with 39.6% and 33.7% power efficiencies. With angularly multiplexed simultaneous phase matching, we achieve mW average power of red, green and blue pulses at 630 nm, 517 nm and 426 nm from single pass second harmonic generation. With broad RGB bandwidths of 7.4 nm, 3.2 nm and 5.2 nm, the source is inherently speckle-free while maintaining an excellent color rendering capability with higher than 99.7% excitation purity of the RGB color primaries, leading to the coverage of 192% NTSC color gamut (CIE 1976). The reported source features a simple system geometry; its potential in power scaling is discussed with currently available technologies.
© 2015 Optical Society of America
Fiber supercontinuum sources employing nonlinear frequency conversion in photonic crystal fibers (PCF) have been studied extensively in the past decade, leading to a number of novel applications [1, 2]. With technological advances in high power Yb:fiber lasers, such PCF supercontinuum can be integrated with the fiber seeding laser to provide an all-fiber broadband light source, with beam quality, bandwidth and brightness far exceeding those of conventional sources. Certain applications such as laser projection displays , ultra-realistic imaging  and flow cytometry  require visible sources with high spectral density at discrete wavelengths of red, green and blue. Due to the specific spectral requirements, such sources have been typically obtained by frequency conversion from one or two near infrared sources with a series of cascaded nonlinear optical processes such as second harmonic generation (SHG), sum frequency modulation (SFM), optical parametric generation (OPG) and optical parametric amplification (OPA) . Despite the power scaling potential, the system arrangements can be quite complex, while cascaded nonlinear processes can lead to undesirable system characteristics such as increased noise and extreme environmental sensitivity. A fiber continuum type of source could in principle provide improved solutions to these problems which would pave the way towards large scale applications in industrial environments. Due to the symmetry in spectral broadening, efficient generation of localized white light fiber supercontinuum is typically realized using green short laser pulses. This however imposes a stringent requirement on the fiber dispersion profile since its zero dispersion wavelength (ZDW) needs to be tailored to the visible [7–9]. To convert to discrete RGB wavelengths, both fiber dispersion profile and pump source parameters need to be carefully engineered so that the pulse broadening is dominated by four wave mixing to excite two distinct spectral sidebands with the required spectral spacing on both sides of the green input. So far it has only been demonstrated employing wave-guiding in the secondary cores of a photonic band-gap fiber pumped by 80-ps green pulses . Besides the lack of commercial availability, the difficulty in light coupling and possible facet damage due to tight focusing limits its power scalability.
Many commercially available PCFs are designed with dispersion profiles matching the Yb:fiber laser wavelength to facilitate in-line connection. Among the various fiber designs, PCF with two ZDWs (TZDW) offers a unique route to efficient energy transfer from the pump wavelength to two narrow-band continuum bands beyond either side of the ZDWs [11–15], which is ideal for applications requiring high spectral intensities at certain wavelengths of interest. In previous work, we have employed such TZDW PCF to generate a 1.26 μm pulse for mid-IR difference frequency generation [15, 16]. The fiber also supports simultaneous generation of an 850 nm pulse, so that the dual ultrashort spectrally localized pulses are ideal for frequency doubling to the visible.
In this manuscript, we propose and demonstrate a novel approach to an ultrafast RGB laser source operating at 426 nm, 517 nm and 630 nm based on single pass second harmonic generation from a 1035 nm Yb:fiber laser and the efficiently excited Stokes and anti-Stokes dual pulses from a commercially available TZDW PCF. For display applications, the RGB source covers 192% NTSC color gamut (CIE 1976) and is inherently speckle-free due to its short coherence length. The reported fiber based source features a very simple geometry. Its potential in power scaling is discussed using currently available technologies.
2. Experimental set-up
Experimental set-up of the RGB laser source is shown in Fig. 1(a). The pump source is a custom-built Yb:fiber chirped pulse amplifier (CPA) delivering 1.3 W, 1035 nm, 240 fs pulses at 40 MHz. Its configuration has been discussed in detail in . The amplifier output is split by a polarizing beam splitter (PBS) paired with a half wave plate (HWP), where a portion of power is coupled into 12 cm of TZDW PCF (1050-Zero-2, NKT photonics) employing an aspheric lens with 4.5 mm of focal length (Thorlabs). Coupling efficiency of 35% is typically achieved. The TZDW PCF has a parabolic dispersion profile with its ZDWs close to 954 nm and 1152 nm, as is shown in Fig. 1(b) . Having a small fiber core with 2.2 µm of diameter, the nominal fiber nonlinearity is as high as ~0.037 (W·m)−1. Power and spectrum of the laser pulses are measured by a thermopile detector (818P-001-12, Newport) and an OSA (86142A, Hewlett-Packard), respectively. The generated Stokes/anti-Stokes pulses are separated by a dichroic mirror (DMSP1000, Thorlabs), and another HWP is used to adjust their polarizations for subsequent second harmonic generation. With careful angular multiplexing in order to achieve simultaneous phase matching, the dual color IR pulses from the TZDW fiber source and the residual pump pulses that are split prior to fiber coupling are individually focused inside a 3 mm thick, type-I BBO crystal cut at θ = 19.8° (Castech) for second harmonic generation. Spectra of the visible pulses are measured by a fiber coupled spectrometer (USB2000, Ocean Optics).
3. The optimal TZDW fiber source for χ(2) frequency conversion
Given the PCF parameters, the important launching parameters of the TZDW fiber source such as dispersion length, nonlinear length and soliton order can be readily estimated from Eqs. (1)-(3):19].
Measured output spectra of the TZDW fiber source at varying coupled-in powers are shown in Fig. 2. The typical SPM-induced spectral broadening exhibiting dual spectral peaks is observed at ~12 mW of coupled-in power. With further power increase, the dual peaks undergo rapid spectral shifting until stabilizing within the two normal dispersion regions (NDR) which are beyond either side of the fiber ZDWs, leading to a dual-band continuum that consists of two localized spectral peaks on the anti-Stokes and Stokes sides. We attribute the peak at the pump wavelength to be the residual pump which is not converted due to the short fiber length. In contrast to PCF with single ZDW, formation of Raman soliton and its self-frequency shift is well suppressed in the TZDW fiber , while the spectral broadening is dominated by SPM at the initial stage of pulse evolution and is possibly assisted by four-wave mixing (FWM)  and Cherenkov radiation . Once reaching the fiber NDR, the dual peaks evolve on their own under the interplay of SPM and fiber normal dispersion, leading to the spectrally localized continua that quickly get stabilized. A more rigorous theoretical study is needed to fully understand the pulse evolution dynamics for the specific TZDW fiber we used, which is beyond the scope of this manuscript. For RGB generation, the optimum condition of our current TZDW fiber source is found when the anti-Stokes and Stokes peaks reach 850 nm and 1260 nm at a coupled-in power of 240 mW, with the primary peak bandwidths measured to be 23 nm and 26 nm, respectively. Additional pump power leads to broader continua bandwidths while the growth in peak intensities becomes not significant, which is of less help for χ(2) frequency conversion.
The unique capability of efficient power conversion from the pump wavelength to two localized spectral region of interests sets the TZDW fiber source apart from the conventional broadband fiber supercontinuum . In order to accurately determine the conversion efficiencies, we measure the power of the anti-Stokes/Stokes peaks by filtering them out using a 1000 nm short-wave pass filter (DMSP1000, Thorlabs) and a 1180 nm long-wavelength pass filter (DMLP1180, Thorlabs), respectively. Figure 3(a) plots their power dependence with respect to the coupled-in pump power. The pump wavelength is much closer to the cut-off wavelength of the short-wave pass filter (and in addition, the lower ZDW) as compared to that of the long-wave pass filter (and the longer ZDW), which leads to the difference in threshold pump powers of the anti-Stokes and Stokes peaks. Slope efficiencies of power transfer from the pump pulses to the dual continua after stabilization within the fiber normal dispersion regimes are quantified by applying linear data fits to the subset of data corresponding to 140 – 360 mW of pump power (shown in the same figure), which are calculated to be as high as 45.5% and 46.4% for the anti-Stokes and Stokes pulses, respectively. At the optimum coupled-in power of 240 mW, the continua power are measured to be 95 mW and 81 mW for the anti-Stokes/Stokes peaks, corresponding to absolute power conversion efficiencies of 39.6% and 33.7% (photon efficiency of 32.5% and 41%) from the 1035 nm pump source.
In contrast to the conventional broadband supercontinuum that suffers from excessive broadband noise seeded from the laser shot noise , the TZDW fiber continuum also exhibits low intensity noise [11–13] and excellent phase coherence  due to the deterministic nonlinear processes during the frequency conversion (dominated by SPM). Here, employing high speed photodiodes paired with a RF spectrum analyzer (Agilent 8564E), the broadband noise power of each filtered out pulses are measured following a method similar to , showing at least 80 dB of suppression from the carrier RF power.
The length of the TZDW fiber (~12 cm) is determined empirically and we observed that cutting back the fiber would increase the power of the anti-Stokes and Stokes pulses at the cost of more required pump power. With a short length of PCF employed, the reported TZDW fiber source preserves the femtosecond pulse widths, with directly measured autocorrelation FWHMs of 200 fs and 227 fs respectively for the anti-Stokes and Stokes pulses (shown in Fig. 3(b)), which eliminates the need for post-compression for any subsequent applications.
4. Characterization of the RGB laser source
Theoretically calculated internal phase matching angles for type-I SHG in BBO and the crystal dispersion along its ordinary axis  are plotted in Fig. 4(a), with the guidance of which we displace the folding mirrors accordingly for simultaneous phase matching of the 1260 nm/850 nm pulses and the 1035 nm pump pulses that get split prior to fiber coupling. On the same graph we show the experimentally determined phase matching angles; the offset of ~0.5° from theoretical values is believed to be the crystal orientation error from its nominal cut angle of 19.8°, which is within the manufacturing tolerance.
In Fig. 4(b) we show the wavelength dependence of group velocity mismatch (GVM) and spatial walk-off angle between the infrared and the frequency doubled pulses, showing increase in both temporal and spatial walk-offs with increasing optical frequencies. For the anti-Stokes input, mixed beams of 850 nm/425 nm pulses get temporally separated within 1 mm of interaction due to the high GVM of 166 fs/mm, while for the 1260 nm pulses the cross-seeding is expected to be fulfilled utilizing the entire crystal length. In the spatial domain, degradation in conversion efficiency due to tighter focusing counter-balances the efficiency enhancement from increased peak intensities, as is quantitatively described by the Boyd and Kleinman expression . For our RGB source, the optimal focusing conditions are experimentally determined among a range of lens selections and the optimal focusing spot size were estimated to be 26 µm and 40 µm, respectively for the anti-Stokes and Stokes pulses.
At the optimal focusing condition, powers of the red, green and blue pulses are measured to be 3 mW, 50 mW and 1.6 mW respectively. With the aid of a variable metallic attenuator, we plot in Fig. 4(c) the measured red and blue power with respect to the attenuated Stokes/anti-Stokes power, which shows a parabolic increase in frequency converted power that is typical for second harmonic generation at the un-depleted pump regime. In the inset of the same figure we plot the green SHG power with the pump power increased all the way to the full CPA capability of 1.3 W. It can be seen that while at ~100 mW pump power the green power is comparable to that of red and blue, with the fully operated 1.3 W source the converted green power is as high as 400 mW, suggesting that lack of SHG conversion efficiency due to inadequate peak power is the limiting factor for the current RGB source.
Figure 4(d) shows the measured RGB spectra which center at 630 nm, 517 nm and 426 nm. Spectral bandwidths of the visible pulses are 7.4 nm, 3.2 nm, and 5.2 nm respectively for the red, green and blue, which are significantly larger than those of the typical picosecond RGB laser sources. Given the spectral bandwidth, their coherence lengths are calculated to be 17.1 µm, 26.6 µm and 11.1 µm, respectively for the red, green and blue pulses. Due to the short coherence lengths, the reported RGB source is inherently speckle-free. On the flip side, however, the broad source bandwidths may lead to deviation of the RGB primaries from monochromatic laser lines, it thus becomes crucial to determine the excitation purity and color rendering capability of the reported source. Tristimulus values (X, Y, Z) of the RGB color primaries can be calculated based on trichromatic color space theory :
Being a uniform chromaticity diagram (UCS), the CIE 1976 color space exhibits excellent uniformity in perceptual color difference and is more accurate for color measurements. Nevertheless, with the traditional CIE 1931 diagram being widely adopted in the display community, especially among display and light source manufactures, we plot the RGB primaries and the associated color gamut on both diagrams to satisfy preferences of both schools, as is shown in Fig. 5. In the same diagrams we annotate the NTSC color gamut (representative of color gamut for high-end HDTVs) as the benchmark reference.
It can be seen that despite the broad RGB bandwidths, the RGB primaries exhibit excellent color saturation (calculated excitation purities >99.7%) with hardly observable deviations from the monochromatic lines, leading to the coverage of 149.6% and 192.8% NTSC color gamut on the CIE 1931 and CIE 1976 diagrams, respectively. Detailed colorimetry data including chromaticity coordinates, dominant wavelengths, excitation purities and percentages NTSC color gamut of the reported RGB source are listed in Table 1.
For large scale laser cinema projection applications, Watt-level average power and high wall-plug efficiency is required. To address the lack of SHG conversion efficiency, higher-order mode TZDW fiber with reduced nonlinearity could be employed to increase the limit in pulse energy . A more straightforward approach, however, is to scale up the source average power while increasing the repetition rate of the Yb:fiber oscillator [30, 31], and to employ quasi-phase-matching (QPM) crystals such as periodically poled lithium niobate (PPLN) to replace the currently employed less efficient regime of single pass critical phase matching in BBO where a ten-fold increase in SHG efficiency may be expected with our current peak power. System advantages such as low-cost and robustness could be achieved through the use of our fiber based approach with simple geometry. During the preparation of this manuscript, we noticed that quasi-phase-matching periodically poled silica fiber for in-line second harmonic generation has been most recently reported , so that all-fiber integration based on our proposed scheme of RGB generation can in principle be realized. A photograph of our running RGB source output is shown in Fig. 6.
In conclusion, we have demonstrated a novel approach to an ultrafast RGB laser source based on single-pass second harmonic generation from the output of a TZDW fiber source which sustains efficient and low noise frequency transfer to spectrally localized anti-Stokes and Stokes pulses at 850 nm and 1260 nm. The resultant RGB source is inherently speckle-free due to the short coherence lengths without sacrificing its excellent color rendering capabilities. The reported fiber based, single-pass scheme exhibits a very simple geometry compared to cascaded optical parametric amplifiers etc., and significant power scaling could be realized with currently available technologies.
The authors would like to thank Professor Jennifer Kruschwitz for her help in color science and Professor Govind Agrawal for his help in nonlinear fiber optics.
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