Blue and Orange Two-Color CW Laser Based on Single-Pass Second-Harmonic and Sum-Frequency Generation in MgO : PPLN

We demonstrate a compact blue and orange-two color continuous wave laser source emitting at 487 nm and from 597.4 to 600.3 nm, respectively. The temperature tunable coherent orange radiation is achieved by frequency mixing 974 nm laser diode (LD) and a C-band amplified spontaneous emission laser source while the temperature insensitive blue radiation is generated by second-order quasi-phase-matching frequency doubling of 974 nm LD. We implement the simultaneous nonlinear processes in a single magnesium oxide doped periodically poled lithium niobate bulk crystal without the need of an aperiodic design.


Introduction
Visible lasers have become ubiquitous in our society for numerous applications ranging from scientific research to industry and entertainment.For instance, the 488 nm wavelength can be matched with hundreds of fluorophores specifically tailored to excite at, or close to, 488 nm in life science research applications [1].Similarly, orange lasers with wavelengths near 600 nm play a key role in biomedical applications [2], and laser projection displays [3].Nonlinear conversion processes such as second harmonic generation (SHG) and sum frequency generation (SFG) are equally efficient approaches for realizing such visible light sources with periodically poled ferroelectric crystals.On the other hand, quasi-phase-matching (QPM) technique in a nonlinear optical crystal like lithium niobate (LN) provides a fascinating route towards implementation of such laser sources due to its distinct advantages which include access to the largest nonlinear coefficient and high damage threshold [4].So far, the various approaches to implementing QPM range from uniform structures to Fibonacci optical superlattice [5], aperiodic [6], linearly chirped [7], apodized [8], and multi-segmented [9].The non-uniform structures are in general, more suitable for broadband wavelength conversion since they provide many spatial vectors compared to uniform structures which are suited to narrowband applications.This development has led to the simultaneous generation of multiple wavelengths in the visible range within the same crystal which has the advantages of high brightness and large depth of focus highly suitable for laser-based color displays.Various techniques to generate red (R), green (G), blue (B), and even yellow (Y) colors have been previously reported.These techniques include utilizing several crystals each for different nonlinear process [10], self-frequency doubling and self-frequency mixing in Nd 3+ -doped bulk aperiodically poled lithium niobate [11,12], and cascaded nonlinear interactions in aperiodically poled lithium tantalate (LT) [13,14] or stoichiometric LT [15,16].As an alternative to these approaches which make use of complicated domain distribution, it is desirable to continue to look for simple, compact, and efficient solutions utilizing single-pass simultaneous processes in uniformly poled structures.
Here we report the coincidental single-pass SFG and SHG phase-matching for the simultaneous generation of CW blue and orange light in a single magnesium oxide doped periodically poled lithium niobate (MgO:PPLN) crystal.The orange light is produced by frequency mixing 974 nm laser diode (LD) and a C-band amplified spontaneous emission (ASE) laser source (1525-1565 nm).The blue SHG light is generated by second-order QPM frequency doubling of 974 nm LD.We utilized the broadband nature of ASE for wavelength conversion into the visible range.Our device has the potential as two technologically interesting laser sources in MgO:PPLN in a simple and practical manner without the need for an aperiodic design.

PPLN Design and Fabrication
In order to realize multiple nonlinear processes in the same crystal, it is necessary to simultaneously fulfill their phase matching conditions.For SFG and SHG processes involving three interacting waves, a pump (ω 1 ), signal (ω 2 ), and an idler (ω 3 ), their respective wave vector mismatches due to material dispersion are [17] where k j = 2πn j /λ j is the wave vector at the corresponding frequency ω j , j = 1, 2, 3, λ j is the wavelength in vacuum, n j is the refractive index (extraordinary) at that wavelength that can be calculated using the Sellmeier equation [18].Λ SFG and Λ SHG are the QPM poling periods of SFG and SHG respectively given by 1 The two processes can occur simultaneously if Λ SFG = mΛ SHG where m is an integer.For a given material, Equations ( 3) and ( 4) can be satisfied for a specific pump and signal wavelengths and one periodicity.In this work, λ 1 is an LD source emitting 974 nm and λ 2 is a C-band ASE source at 1525-1565 nm.Using standard electric poling technique, we fabricated a single period QPM device on a 1 mm-thick z-cut MgO:LN crystal as reported in [19].The device length is 50 mm.To realize efficient SHG and SFG processes simultaneously, we are seeking the condition ∆k SFG = ∆k SHG = 0. Using SNLO (AS photonics) [20], we calculated the required periodicities as Λ SFG = 10.2 µm at 60 • C and Λ SHG = 5.1 µm at 110 • C based on Sellmeier equations with m = 2.We fabricated the PPLN to have a period of 10.2 µm for perfect quasi-phase-matched SFG and the second-order quasi-phase-matched SHG processes, and at the same time utilize the broadband nature of the ASE source.In this regard, we have coincidental phase matching for SHG of λ 1 but a decreased efficiency is expected because of the second-order phase matching [21].It is worth noting that, the SHG conversion efficiency can be enhanced by a factor of 4 theoretically by using first-order QPM period.The orange laser output is obtained by frequency mixing λ 1 and λ 2 whereas the blue laser is generated by frequency doubling of λ 1 .Theoretically, the maximum SHG power can be calculated using the relation 22] and the corresponding conversion efficiency is given as η = P SHG /P 2 1 .Similarly, the theoretical sum frequency output power can be estimated using the relation , and the corresponding conversion efficiency can be given as η = P SFG /P 1 P 2 , where c is the speed of light in vacuum, ε 0 is the permittivity in vacuum, and d e f f = 2d 33 /π is the effective nonlinear coefficient of MgO:PPLN.n SHG , n SFG , n 1 , and n 2 are refractive indices at λ SHG , λ SFG , λ 1 , and λ 2 , respectively, while ω f (~43 µm) is the confocal beam waist.Our design is well suited for Type-0 QPM interaction ( e + e → e ) in MgO:PPLN in order to take advantage of the largest nonlinear coefficient.We chose MgO:LN as the nonlinear material due to its low susceptibility to optical damage photo-refraction and visible induced infrared absorption [24].The poled crystal was then cleaned and wet etched in hydrofluoric acid (HF) to reveal the domain structure and the end faces were optically polished but have no anti-reflection coating.

Experimental Setup
The experimental setup for the single-pass simultaneous sum frequency generation of orange laser and second harmonic generation of blue laser is illustrated in Figure 1.As a pump (P 1 ), we used a laser diode (LD) fixed at 974 nm (λ 1 ) of 0.20 nm spectral width, whereas the signal (P 2 ) was a C-band (1525-1565 nm) ASE source (λ 2 ).A 50 mm long uniform MgO:PPLN was used for collinear single-pass SFG and SHG processes.The output of C-band ASE was combined with λ 1 inside a 50:50 wavelength division multiplexer (WDM) coupler and collimated with a C-lens before focusing at the center of the crystal using a coupling lens.We varied the temperature of the MgO:PPLN crystal to obtain the phase matching points for SFG and SHG.The temperature of the device was stabilized by a temperature controlled oven (CTL photonics) with an accuracy of ±0.1 • C. The output power and bandwidth were evaluated using a laser power meter (PM100D, Thorlabs, Newton, NJ, USA) and a fiber spectrometer (BIM-6001, Brolight, Hangzhou, China).

SFG and SHG Characterization
The maximum input powers measured before the crystal were P 1 = 229 mW and P 2 = 200 mW. Figure 2a shows the power of SHG light as a function of pump power, for an SHG wavelength of 487 nm.The LD power, P 1 was increased steadily to a maximum value of 229 mW by varying the operating current, and the ASE power P 2 was fixed at 200 mW (maximum).A maximum SHG power of 5.2 mW was measured at 487 nm corresponding to ~10%/W conversion efficiency, lower than the expected nonlinear conversion efficiency of ~29%/W.Figure 2b illustrates the SFG power as a function of the product of pump (P 1 ) and signal (P 2 ) powers incident on the crystal.The maximum sum-frequency generation of 9.3 mW at 598.8 nm is achieved, resulting in an overall conversion of ~20.3%/W, which is also lower than the expected conversion efficiency of ~42%/W.Possible reasons for low conversion efficiencies can be attributed to an inhomogeneous temperature distribution within the crystal and/or poling imperfections in the QPM structure.As for the low SHG conversion efficiency, one predominant reason is using the second-order QPM for the given domain period (10.2 µm), which can be improved by using the first-order QPM period (5.1 µm).The conversion efficiencies could be further improved by using a high fundamental power.The temperature of the MgO:PPLN was varied from 25 • C to 120 • C. Figure 3 shows the orange and blue laser intensities as a function of temperature.The SFG was always present with high intensity in the region 30-100 • C. We attributed this to the broadband nature of the ASE source used in this work.The drop in the SFG intensity below 30 • C and above 100 • C could be because of a phase-matching condition at λ 2 wavelength with low intensity.A further decrease in the SFG intensity at 110 • C may be attributed to a competition between the SFG and SHG processes.The high intensity of blue light could be a result of the QPM condition for SHG satisfied at 110 • C. When the temperature is set at 25 • C (Figure 4a, only SFG appears with relatively lower intensity.As the temperature is increased to 60 • C, the SFG intensity rises to almost twice as the intensity observed at room temperature as seen in Figure 4b.We attribute this to phase matching temperature of the SFG process being near 60 • C. There exists some competition between the SFG and SHG processes since both processes consume energy from the fundamental lasers [25,26].According to Figure 4c,d, we can see that SHG appears along with SFG and at 110 • C, the SHG intensity reaches its maximum.We estimated a reduction of SFG power by ~7% of its maximum (at 60 • C) when the SHG intensity is at its maximum.This phenomenon can be understood from the QPM theory that the relative intensities of nonlinear optical processes are dependent on the proximity of each process to perfect quasi-phase matching condition.The generation of blue laser is due to second-order SHG given that QPM even-order processes are possible since the average duty ratio may not be accurately 50:50 due to fabrication errors [27].When the temperature was increased beyond 110 • C, the intensity of both SHG and SFG decreased significantly as shown in Figure 3.This means that the device should be designed to operate below 110 • C in order to generate the blue and orange lasers within the same crystal.The inset in Figure 4d is a photo of the generated SHG and SFG beams.We also investigated the temperature dependence the central wavelength of SHG and SFG. Figure 5 shows the SHG and SFG spectra at 40 • C (black line), 60 • C (red line) and 110 • C (blue line) with a spectral resolution of ~0.7 nm.Whereas the central wavelength of SHG is not sensitive to the temperature variation (Figure 6), the central wavelength of SFG apparently changes by about 3 nm (597.4-600.3nm) from 25 • C to 110 • C.  The shift in the central wavelength of SFG is due to the broadband nature of ASE source and the change of SFG quasi-phase-matching condition.The latter comes explicitly from the temperature dependent refractive index of MgO:PPLN according to the Sellmeier equation.In particular, the phase-matching condition shifts to longer wavelengths with an increase in temperature.According to Figure 6, the experimental data for SFG are consistent with the theoretical values calculated by using the Sellmeier equation for the same temperature range [18], demonstrating that the MgO:PPLN crystal phase-matches over the entire 50 mm length.

Conclusions
We have demonstrated an approach to generate a dual-wavelength laser source based on single-pass second harmonic and sum frequency generation in a magnesium oxide doped, periodically poled lithium niobate crystal.A diode laser operating at 974 nm and a C-band ASE laser (1525-1565 nm) were used as the pump and signal respectively.Blue SHG light at 487 nm and orange SFG light tunable from 597.4 nm to 600.3 nm have been obtained by varying the temperature of the crystal from 25 • C to 110 • C. The measured central wavelength changes with a rate of ~0.04 nm/ • C, which can be understood from the temperature dependence of the refractive index of MgO:PPLN.We believe the dual-wavelength source will have potential applications such as in biomedicine and entertainment.

Figure 1 .
Figure 1.Schematic of the experimental setup.The beams are focused into single-pass MgO:PPLN to generate orange output by sum frequency generation (SFG) and blue output by second harmonic generation (SHG).LD: laser diode; ASE: amplified spontaneous emission; WDM: wave division multiplexer; C-lens: collimator lens; OSA: optical spectrum analyzer.

Figure 2 .
Figure 2. Measured (black squares) output power as a function of the fundamental power for (a) second harmonic generation (SHG), and (b) sum frequency generation (SFG).The solid curve in (a) corresponds to a quadratic fit to the SHG data using P SHG = η SHG P 2 1 and in (b) the solid curve is a quadratic fit for the SFG process using P SFG = η SFG P 1 P 2 , where η SHG = 10%/W and η SFG = 20.3%/Ware the SHG and SFG normalized conversion efficiencies determined from power measurements respectively.Other fit parameters are L = 50 mm (crystal length) and ω f = 43 µm.The error bars include the uncertainty in the output power measurements.

Figure 3 .
Figure 3. Intensity of SFG and SHG light as a function of temperature.

Figure 6 .
Figure 6.Dependence of central wavelengths of SHG (blue triangles) and SFG (black squares) on temperature.The red circles are calculated with the Sellmeier equation.