1. Introduction

In a color display system, red, green, and blue (RGB) are the three primary additive colors to form a full color map. A RGB laser is considered as an important light source for the next generation display system, since it can cover a wider color range than the other known light sources. Furthermore, a RGB laser could be the only light source to provide a collimated, high-power light beam for a long-distance, large-scale projector system.

There are usually two scanning scheme for a laser display device, one using MEMS deflectors and the other using LCD pixels. Despite several pulsed RGB laser sources have been demonstrated recently [1–5], the high peak power of those pulsed lasers could cause laser damage to the miniature deflectors, pixels, or a viewer’s eyes. The kHz pulse rate of some Q-switched RGB lasers is also too low for a laser-display system adopting the MEMS scanning scheme. Therefore, in view of the device lifetime, laser safety, and system cost, a CW RGB laser is still the preferred source for laser display applications.

A conventional laser usually emits radiation at a fixed wavelength, whereas a coherent radiation source based on nonlinear frequency mixing of lasers can be broadly tunable in its wavelength. Utilizing techniques of nonlinear frequency conversion, a high power and tunable RGB laser can be implemented from the well developed solid-state laser technology. The wavelengths of the three primary colors can therefore be optimized to cover the largest area in the color map for a laser display system. A wavelength tunable visible laser is not only useful for a laser-display system but also attractive to the photodynamic therapy (PDT) [6]. In PDT clinical treatment, several useful photosensitizers have been identified with their activation wavelengths located between the red and blue color range [7].

A CW, singly resonant optical parametric oscillator (SRO) using periodically poled lithium niobate (PPLN) as its parametric gain medium has been demonstrated with high efficiency over a broad tuning range in the mid-infrared (mid-IR) [8]. Bosenberg et al. [9] further integrated a sum frequency generator (SFG) cascaded to the parametric gain medium in a SRO and generated a CW, 2.5-W laser at 629 nm with more than 20% conversion efficiency from a CW Nd:YAG pump laser. As will be shown below, by taking advantage of the high intracavity signal power, a blue laser radiation can be further obtained by mixing the red and the resonated IR signal wave near ~1.5 μm in an additional SFG. By doubling the frequency of the remaining pump power at the output of the SRO, all the RGB colors can be efficiently obtained from such a SRO configuration. With the rapid development of fiber lasers [10], one can easily find high-power, diffraction-limited pump sources near ~1 μm, such as the Yb-fiber laser, in the market. In this paper, we report a successful experimental demonstration of a CW RGB SRO pumped by an economical multimode Yb-fiber laser. The multimode nature of the pump laser provides a broad temperature acceptance to the nonlinear crystals and makes the RGB laser system fairly insensitive to temperature.

2. Experiment setup

The schematic of our RGB SRO is showed in Fig. 1 . The SRO consists of four curved mirrors with a 100 mm radius of curvature arranged in a bow-tie configuration. In this work, we used 5-mol.%-doped MgO:PPLN crystals (made by HC Photonics Inc.) as the material for all the nonlinear wavelength converters. The use of the MgO-doped PPLN crystals, instead of congruent ones, is to avoid the photorefractive damage at visible wavelengths and the laser instability at the infrared wavelengths [11].

 

Fig. 1 Schematic of the RGB SRO pumped by a multimode Yb-fiber laser at 1064 nm. The bow-tie ring cavity consists of four curved mirrors with 100 mm radius of curvature highly reflecting at the resonant wavelength, 1.56 μm. The red SFG performs the sum frequency generation, 1/1064 nm + 1/1560 nm → 1/633 nm, to generate the red laser radiation. The blue SFG performs the process 1/633 nm + 1/1560 nm → 1/450 nm to generate the blue laser radiation. The residual pump laser is frequency doubled to the green laser radiation in a SHG outside the ring cavity. (SFG: sum frequency generator, SHG: second harmonic generator)

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The OPO PPLN crystal is 50 mm in its length and 30.4 μm in its period, phase matched to the 1^st-order parametric mixing of 1/1064 nm→1/1562 nm + 1/3337 nm at 90 °C. The red-SFG crystal is 10 mm in its length and 11.8 μm in its period, phase matched to the 1^st-order sum frequency process of 1/1064 nm + 1/1562 nm → 1/633 nm at 92 °C. The blue-SFG crystal is 10 mm in its length, phase matched to the sum frequency process of 1/633 nm + 1/1562 nm → 1/450 nm at 75 °C. Here, we first adopted a 3rd-order grating with a period of 14.1 μm for the blue SFG PPLN to ease the fabrication of the crystal and later replaced it with a 1st-order one with a period of 4.7 μm to show the scalability of the blue-laser power for our system. The two end faces of all the PPLN crystals were optically polished and coated with 3, 1.5, 1, 0.25, and 14% reflectance at the blue, red, pump, signal, and idler wavelengths, respectively. To independently tune the red and blue wavelengths, these MgO:PPLN crystals were installed in different ovens with ±0.1 °C temperature resolution. One can certainly make a monolithic crystal for the red and blue SFGs in a single oven, when there is a need for mass production in the future. The residual pump power was converted into green laser radiation in a single-pass second harmonic generator (SHG). The SHG PPLN crystal is 5 mm in its length and 6.5 μm in its period, phase matched to the 1st-order second harmonic process of 1/1064 nm + 1/1064 nm → 1/532 nm at 82°C. The chosen RGB wavelengths of our system covers 35% more area on the CIE 1931 standard chromaticity diagram than that covered by a typical National Television Standards Committee (NTSC) Primaries, R(0.67,0.33) G(0.21, 0.71) B(0.14, 0.08) [12].

To obtain high intracavity power and better nonlinear conversion efficiency at the red and blue wavelengths, four cavity mirrors all have high reflectance (>99.8%) at the signal wavelength. The input mirror, M₁, has reflectance of 1, 99.8, and ~4% at the pump, signal and idler wavelengths, respectively. Mirror M₄, made of fused silica, has reflectance of 1.0, 99.9, and ~2% at the pump, signal and idler wavelengths, respectively. We can monitor the mid-IR idler output power through the fused silica mirror. To deflect the pump laser into the OPO crystal and couple out the red power, the remaining two mirrors, M₂ and M₃, are both optically coated with reflectance of 99.8, 99.8, 4.0, and ~5% at the pump, signal, idler, and red wavelengths, respectively. The M₁ and M₂ mirrors are separated by 100 mm, and the M₃ and M₄ mirrors are separated by 140 mm. The total cavity length of the ring SRO is 500 mm. The symmetric mirror arrangement forms focal points at the center of the OPO crystal and near the center of the two SFG crystals, as shown in Fig. 2 . The blue curve represents the mode radius in free space; the red and brown curves represent the mode radius in the PPLN crystals. The waist radius of the two focal points is about 80 μm.

 

Fig. 2 Cavity-mode radius of the resonated IR signal wave versus position in the SRO cavity. Due to symmetry, the four curved cavity mirrors form focal points at the center of the OPO crystal and near the center of the two SFG crystals. The blue solid curve represents the mode radius in free space; the red and brown curves represent the mode radius in the SFG and the OPO crystals, respectively.

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The pump laser is a linearly polarized Yb-fiber laser (IPG YLM-25-LP) at 1064 nm, producing a maximum CW power of 25 W in a 1-nm (265 GHz) linewidth. The pump beam was polarized along the crystallographic z direction of the MgO:PPLN crystal and mode-matched to the SRO cavity by using a 150-mm focal-length lens. The pump beam enters the SRO cavity at the M₁ mirror, traverses the SFG crystals, and reflects from the M₂ and M₃ mirrors to pump the OPO crystal. The residual pump beam exits at the M₄ mirror. We employed a dichroic mirror to separate the idler and pump waves and used an additional 75 mm focal-length lens to refocus the pump beam to the center of the SHG crystal for green laser generation.

3. Result and discussion

We first optimize the red laser generation by using the red-SFG crystal alone in the SRO cavity. Figure 3 shows the measured red and idler laser powers versus the pump power. As soon as the pump power overcomes the cavity threshold (3W), the red and idler powers increase monotonically with the pump power. At 25-W pump power, 5.2-W red laser power at 633 nm was emitted through mirror M₂ and 1.9-W idler power at ~3.34 μm was emitted through mirror M₄. About 20% of the pump power at 1064 nm was converted to the red-laser power. The inset shows that the temperature bandwidth (full width at half maximum, FWHM) of the red-SFG is about 11 °C. We measured the temperature bandwidth by monitoring the red-laser output power at several different temperatures in the red SFG crystal, while fixing the OPO crystal temperature at 92°C. At 25-W pump power, the linewidth of the red laser was measured to be 0.25 nm (187 GHz), which is consistent with the value in a previous report using the same broad-band pump laser [13]. Owing to the broad linewidth of the pump laser, it does not require a highly stabilized crystal oven to obtain a stable output power for the red laser.

 

Fig. 3 Measured output powers of the red and idler lasers as a function of the pump power with no blue SFG in the SRO cavity. The oscillation threshold of the SRO is 3 W. At 25 W pump power, we obtained 5.2-W red laser power. The inset shows a temperature acceptance bandwidth of 11°C for the red laser at 25-W pump power.

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After optimizing the red laser performance, we first cascaded the 3rd-order blue SFG to the red SFG. At the oven temperature of 75 °C, we observed blue laser exiting mirror M₂. Figure 4 shows the measured red and blue versus pump power. As soon as the pump power exceeds the cavity threshold (4 W), the red and idler laser powers increase steadily. At 25-W pump power, 3.9-W red laser power at 633 nm (solid red dots) and 57 mW blue laser power at 450 nm (solid blue squares) were emitted through mirror M₂. At the same time, we measured 1.1 W idler powers at 3.34 μm through mirror M₄. We monitored the blue laser power as a function of the blue SFG temperature to deduce a temperature bandwidth of 4°C (FWHM), while keeping the OPO and the red-SFG crystal temperatures at 92 and 91 °C, respectively. At the maximum pump power of 25 W, the blue laser linewidth was measured to be 0.13 nm (193 GHz), which is approximately the same as the red laser linewidth. The inset shows the color images of the red and blue lasers taken by a CCD camera after we separated them in a Pellin-Broca prism. The images clearly show TEM₀₀ modes for the red and blue lasers. In order to show the scalability of the blue laser power, we further replaced the 3rd-order blue SFG PPLN with a 1st-order one of the same length. As shown in Fig. 4, the blue laser power (open blue squares) increases to a maximum value of 490 mW at 25-W pump power. As expected, the 490-mW blue-laser power is nearly 9 times that generated from the 3rd-order blue SFG. The corresponding red-laser power versus pump power is also plotted in the same figure with open red dots. At 25-W pump power, the ratio of the blue to the red laser power is 16%.

 

Fig. 4 Measured red and blue laser powers versus pump power from the SRO. The oscillation threshold is 4 W. With the 3rd-order blue SFG PPLN, 3.9-W (solid red dots) and 57-mW (solid blue squares) powers were generated at the red and blue wavelengths, respectively, at 25-W pump power. With the 1st-order blue SFG PPLN, 3.0-W (open red dots) and 490-mW (open blue squares) powers were generated at the red and blue wavelengths, respectively, at 25-W pump power. The inset is a color image of the red and blue lasers taken by a CCD camera, showing a TEM₀₀ mode profile for both red and blue lasers.

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After the pump wave exits the M₄ mirror, we refocused the residual pump wave to a ~25 μm beam radius at the center of the green SHG crystal. In Fig. 5 , 456-mW green power at 532 nm was obtained at 8.6 W residual pump power, corresponding to 1.24%/W/cm conversion efficiency. No photorefractive effect was observed in the green laser beam. The inset shows the green laser in operation. It can be shown from the CIE chromaticity diagram that, with reference to a typical 6,500K projector light source, the calculated balance power for our RGB laser is R:G:B = 1.217:1:0.712. Hence, given the 456 mW green-laser power, the optimal red and blue powers are 0.555 and 0.324 W, respectively. Since the generated red and blue laser powers from our SRO are higher, one could in principle scale up the green laser power by replacing the 0.5 cm long extracavity SHG PPLN with a longer one without affecting the output power of the red and blue lasers.

 

Fig. 5 Measured green-laser power versus residual pump power in the SHG. At 8.6-W pump power, 456 mW of green laser power was generated, corresponding to 1.24%/W/cm conversion efficiency. The inset shows the green laser in operation.

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As pointed out previously, the broad pump linewidth gives a fairly large temperature acceptance for the RGB laser system (11°C and 4°C for the red and blue lasers, respectively). Figure 6 shows measured rms power fluctuations of 5.8 and 7.6% for the red and blue laser powers, respectively, at 25-W pump power over a ~15-min duration. According to the specification of our pump laser, the pump power fluctuation is in the range of ± 2%. By using the slopes of the curves in Fig. 4, one can deduce that the ±2% pump power fluctuation accounts for about one-third and one-fifth the measured 5.8 and 7.6% power fluctuations in the red and blue laser powers, respectively. We suspect that some minor photorefractive effect could be responsible for the additional power fluctuations in the output laser powers.

 

Fig. 6 Measured power fluctuations in the red (left) and blue (right) lasers. The rms power variations are 5.8 and 7.6% for the red and blue lasers, respectively.

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4. Conclusion

We have successfully demonstrated a CW RGB laser by using a SRO installed with two intracavity SFGs and one extracavity SHG. All the wavelength converters were made of MgO:PPLN crystals. The pump laser is an Yb-fiber laser at 1064 nm. The red, green, and blue lasers are produced by summing the frequencies of the pump and signal lasers, doubling the frequency of the residual pump laser, and summing the frequencies of the red and signal lasers, respectively. At 25-W pump power, 3.9, 0.456, and 0.49 W powers at 633, 532, and 450 nm, respectively, were generated from the CW RGB SRO. The two separated SFGs offer independent wavelength tuning to the red and blue colors of the laser. The extracavity SHG offers another independent adjustment to the green-laser power without affecting the output power of the red and blue lasers. It is worth noting that, in our experiment, no saturation was observed for the RGB output powers at 25-W pump power, indicating the potential of further scaling up the output power of the RGB SRO. As a proof-of-principle experiment for the scalability, we show nearly 9 fold increase on the blue laser power when replacing a 3^rd-order blue SFG PPLN with a 1st-order one of the same length in the SRO. The broad pump linewidth is not necessarily a disadvantage in the overall laser efficiency. The different spectral components of the pump laser can contribute to different wavelength conversion processes, resulting in a broad temperature bandwidth for the whole system. In addition, the broad pump and output laser spectra could help reduce the speckle problem for a laser-display system. Therefore, our multi-mode pump laser is an economical choice to implement such a RGB laser source.