1. Introduction

Among a variety of nonlinear optical processes, second-harmonic generation (SHG) is one of the most well-known wavelength-conversion schemes [1–3]. In order to enhance the conversion efficiency of SHG, the phase velocities of the interacting waves must be matched, which can be achieved, for example, by selecting appropriate polarization states and the incident angle of a birefringent crystal [1]. Another method to match the phase velocities of the interacting waves is the quasiphase matching (QPM) technique, in which the ferroelectric domains of a nonlinear crystal are inverted periodically, and thus the phases of the interacting waves are controlled in a coherence length to produce constructive interference between the generated waves in different regions of the nonlinear optical medium [2, 3]. There are several advantages of this technique over other phase matching techniques. In particular, any wavelength can be phase-matched in the transparent range of a LiNb crystal simply by choosing a suitable poling period in the QPM structure; the largest nonlinear component (i.e., ) can be obtained; the propagating waves can undergo the largest nonlinear interaction in the crystal, enhancing the conversion efficiency and offering the possibility of engineering the nonlinearity [2–5].

In such nonlinear SHG processes, it is well known that the conversion efficiency of SHG is proportional to the power of the fundamental wave [1–3]. Therefore, high-power light sources (up to ~200 mW) are required to achieve efficient conversions in many practical applications [6, 7]. It has been found that LiNb waveguides are more vulnerable to high-power irradiating light than bulk crystals, especially to lightwave with a shorter wavelength, due to the photorefractive effect (PRE) [7–12]. In addition, in both bulk and waveguide PPLN crystals, the phase-matching conditions are influenced by the temperature distribution along the optical path of the interacting wave; and high-power irradiation is apt to generate uneven temperature distribution [2, 7, 13]. As a result, the thermal effect is an important aspect to be taken into account in high-power applications. In many applications, such as lidar, remote sensing, spectroscopy, coherent communications, dense wavelength-division, and time-division multiplexing and demultiplexing, the noise characteristics of second-harmonic (SH) waves generated in nonlinear interactions are concerned since the noise in the SH wave can significantly impact the accuracy of measurement [2–5, 14, 15, 16]. As a result, it is important to investigate the noise characteristics in SHG processes under different conditions, such as power, material, temperature, and so forth. So far, few systematic investigations of noise characteristics under different operating conditions and comprehensive comparisons of noise in bulk and waveguide PPLN as well as in doped and undoped crystals have been reported in the literature to the best of our knowledge.

In this work, we experimentally investigated the noise characteristics of SHG in undoped and 5 mol % MgO-doped PPLN crystals. In the experiment, a 0.78 μm SH wave was generated when a 1.55 μm fundamental wave passed through a PPLN bulk or an annealed proton-exchanged waveguide. The fundamental and SH waves were then separated through a beam splitter and sent to two photodetectors, respectively. By analyzing the time-domain and frequency-domain characteristics of the fundamental and SH waves, we studied the noise characteristics of the fundamental and SH waves. Furthermore, we applied a visible (532 nm) and an infrared (1090 nm) irradiation wave to the crystals, respectively, and observed the change of noise characteristics in the case of apparent photorefractive effects. The results obtained in this work are helpful and provide some guides in design and applications of SHG in PPLN.

2. Experimental Setup

The experimental setup is schematically shown in Figure 1, including a tunable laser source (Agilent 8164A), a thermal-electrical controller (TEC), a polarization-maintaining erbium-doped fiber amplifier (EDFA, KEOPSYS), and a wavelength-selective beam splitter. The temperature of the PPLN crystal is controlled using the TEC. Two pieces of polarization-maintaining fibers were used to connect the tunable laser and the EDFA. The output wave from the EDFA passed through a narrow bandpass filter to eliminate amplified spontaneous emission (ASE) of the amplifier. Two focusing lenses were used to couple light into and out of the PPLN crystal. The maximal injected power of the fundamental wave into the PPLN crystal was 500 mW. The output fundamental and SH waves from the PPLN crystal were separated through the beam splitter.

Figure 1: Experimental setup of SHG. PM SMF: polarization maintaining single-mode fiber. TLS: tunable laser source.

The crystal poling periods are in the range of 17–19 μm, which ensures their QPM wavelengths of SHG locate in the wavelength range of the tunable laser. The QPM structure was poled by the electrostatic discharge method, while the waveguides were fabricated by using the proton exchange technique and they only support the transverse-magnetic (TM) modes [13, 14, 17]. For the fundamental wave propagating in the bulk PPLN crystals, the beam waist was focused to 30–40 μm in diameter.

We also studied the noise characteristics of PPLN with extra pump irradiation at wavelength of 0.532 and 1.09 μm, respectively, by using the pump-probe method. The influence of the pump irradiation on the SHG noise was investigated experimentally. The experimental setup is shown in Figure 2, where another two-beam splitters were inserted in the optical path of Figure 1, used to combine the fundamental wave (namely, probe) and the pump light into the PPLN crystal at its input end, and to separate the pump light from the fundamental and SHG waves at the output end of the crystal, respectively. The other components and their functions are the same as those in the previous setup. The 532 nm green light is generated from a CW intracavity frequency-doubled Nd:YAG laser (Coherent, Verdi), whose maximal output power is 2 W. The 1090 nm light is from a single-mode Yb-doped double-clad fiber laser, and its maximal output power used in the experiment is 1 W.

Figure 2: Experimental setup of SHG with a pump. PM SMF: polarization-maintaining single-mode fiber. TLS: tunable laser source.

3. Experimental Results

3.1. SHG without Other Irradiation

First, we tested SHG from an undoped PPLN crystal at room temperature. For the bulk and waveguide PPLN, the fundamental and SH output powers exhibit certain fluctuations over the time as shown in Figure 3. To facilitate the comparison of noises under different conditions, the output powers are all normalized, that is, their average powers are scaled to unity. We can see that both fundamental and SH waves fluctuate with the time but at different amplitudes. Also, we can see that in both bulk and waveguide PPLN, the fluctuation amplitude of the fundamental wave is lower than that of the SH wave, and for each wave, its noise is higher in the waveguide PPLN than in the bulk.

Figure 3: Power fluctuations of fundamental and SH waves in undoped PPLN crystal measured in the time domain. It is shown that the SH wave has higher peak-to-peak fluctuation than that of the fundamental wave; and for either wave, the waveguide leads to higher noise than the bulk crystal.

The corresponding frequency spectra in the frequency range of 0–2 kHz, obtained by using the fast Fourier transform (FFT), are shown in Figure 4. In the bulk and waveguide PPLN, the fundamental and SH waves exhibit different noise spectrum structures. In particular, the SH wave has more noise spectral components than the fundamental wave. As a result, the total noise power is higher in the SH wave than in the fundamental wave. It is known from the nonlinear interaction relationship of SHG that any instability of the fundamental wave can be enhanced in the SH wave.

Figure 4: Frequency spectra corresponding to the fundamental and SH temporal output traces in Figure 3.

Similarly for a 4.5 cm long MgO-doped PPLN crystal at room temperature, the temporal fundamental and SH output powers are depicted in Figure 5. The power fluctuations of the fundamental and SH waves in the MgO-doped PPLN crystal have the same trends as those in the undoped PPLN crystal (shown in Figure 3), but the fluctuation amplitude of each wave is lower in Figure 5 than its counterpart in Figure 3. This implies that SHG in the MgO-doped PPLN has lower noise than that in the undoped PPLN. The corresponding frequency spectra are shown in Figure 6.

Figure 5: Power fluctuations of fundamental and SH waves in MgO-doped PPLN crystal.

Figure 6: Frequency spectra corresponding to the fundamental and SH temporal output traces in Figure 5.

To quantitatively describe the noise amplitude, we adopt root-mean-square (RMS) value here. The RMS values of the fundamental and SH temporal output traces (shown in Figures 3 and 5) are calculated and compared in Table 1. We can see that the SH noise intensity is nearly two times higher than its fundamental wave, the noise intensity in the waveguide is three times that in the bulk, and the noise in the MgO-doped PPLN is about 50% lower than that in the undoped PPLN.

Table 1: RMS noise values of fundamental and SH waves in MgO-doped and undoped PPLN.

The noise of the fundamental wave mainly results from the following aspects. First, the input wave from the tunable laser and power amplifier has certain noise, which usually exhibits the 1/f noise in the low-frequency range, and the Gaussian white noise in high-frequency range. Second, the instability of the coupling between the fiber and device contributes to the low-frequency fluctuation of the output. Third, a change in the input polarization state of the device may change the output power. The second and third terms vary from time to time, and contribute some spikes in the noise spectrum (mainly in the low-frequency range). These are more significant in the waveguide device than in the bulk PPLN. In fact, the waveguide devices are more sensitive to optical and mechanical perturbations than the bulk devices. With these impacts, the SHG power is more unstable than the fundamental power as shown in Figures 3 and 5. Our experimental results are consistent with the previous observations [18, 19].

3.2. SHG with 532 nm Irradiation

We then investigated the noise characteristics of SHG in the MgO-doped and undoped PPLN crystals with the 532 nm irradiation. The experimental setup is shown in Figure 2.

For the undoped PPLN waveguide, the results of output SH power are shown in Figure 7, where the 532 nm pump irradiation (10 mW) is applied to the PPLN waveguide at the time of 2 seconds. We can see in Figure 7(a) that after turning on the 532-nm pump irradiation, the SH output power exhibits an abrupt oscillation and then decreases quickly. The temporal trace becomes noisier with the 532 nm exposure than the case without the exposure. In Figure 7(b), the initial evolution of the SH power under the 532 nm exposure is depicted. There is an apparent undershoot followed by an overshoot when the 532 nm pump is applied. Thereafter, the SH power shows significant fluctuations. These are related to the photorefractive effect, [7–12] which cannot only change the efficiency of SHG but also increase noise in the SH wave.

Figure 7: (a) Output SH power from undoped PPLN waveguide before and after 10 mW 532 nm irradiation, (b) detailed change of SH power near the switching point.

The noise amplitudes and frequency spectra of the SH wave with and without the 532 nm pump are compared in Figure 8. The RMS value of noise in the case of the 532 nm exposure is about 2.3 times as high as that in the absence of the 532 nm exposure. From the corresponding frequency spectra shown in Figures 8(c) and 8(d), we can see that the 532 nm pump irradiation can increase noises mainly at a low-frequency range (<400 Hz).

Figure 8: (a) (b) Noise characteristics of SHG in undoped PPLN waveguide with and without 532 nm irradiation. (c) (d) The normalized frequency spectra corresponding to the temporal traces in (a) and (b).

For the MgO-doped PPLN waveguide, Figure 9 shows the noise amplitudes and frequency spectra of the SH wave with and without the 532 nm irradiation. The 532 nm pump power is 30 mW. The RMS noise value of the SH wave is increased by a factor of 1.48 under the 532 nm irradiation.

Figure 9: (a) (b) Noise characteristics of SHG in MgO-doped PPLN waveguide with and without 532 nm irradiation. (c) (d) The normalized frequency spectra corresponding to the temporal traces in (a) and (b).

For the undoped bulk PPLN, the output SH trace is shown in Figure 10 with and without 100 mW 532 nm irradiation. There is no apparent change in noise amplitude. In fact, the ratio of the RMS values in these two cases is 1.02.

Figure 10: For undoped bulk PPLN, comparison of SH outputs with and without 532 nm irradiation.

For the MgO-doped bulk PPLN, the noise characteristics are quite similar for different irradiation powers up to 1 W, as shown in Figure 11. The RMS noise is increased by 2% and 7% under 0.1- and 1.0-W irradiation, respectively. In addition, the average output power of the SH wave has nearly no change under the 532 nm exposure, which implies a good performance of SHG in MgO-doped bulk PPLN for high-power applications.

Figure 11: For MgO-doped bulk PPLN, comparison of SH outputs with different irradiation powers of 532 nm irradiation.

From the above experimental results, we can see that the undoped PPLN waveguide performs worst under the 532 nm irradiation in terms of SHG conversion efficiency and noise, while the SHG in the MgO-doped bulk is less sensitive to the 532 nm irradiation. The noise increase of the SH wave under the irradiation is related to photorefractive effect, which can change the refractive index in the optical path of light propagation. Such a change in the refractive index is somehow nonuniform in the PPLN, and may vary with both time and position, which can lead to temporal and spatial variations in the phase matching condition of SHG. In addition, other accompanying effects, such as thermal effect, optical scattering, and two-photon absorption can also affect the SH wave in the time domain.

3.3. SHG with 1090 nm Irradiation

Next, we tested the SHG in the previous undoped bulk PPLN under the 1090 nm irradiation. The RMS noise of SHG and its intensity change versus 1090 nm pump power are shown in Figure 12, respectively, at different temperatures. We can see in Figure 12(a) that on the one hand, similar to the previous 532 nm irradiation, the noise increases with an increase of 1090 nm pump power at any temperature; on the other hand, with an increase of device temperature, the noise tends to decrease. However, the amplitude of noise reduction is maximum 0.2%, which is not significant. In Figure 12(b), with an increase of 1090 nm pump power, the increase of SHG intensity change is apparent. In particular, it is raised by four times when the applied pump power varies from 50 to 400 mW. In addition, an elevation of device temperature is helpful to alleviate the problems of noise and attenuation.

Figure 12: (a) RMS noise of SHG and (b) intensity change of SHG versus 1090 nm pump power, measured at different temperatures.

Similar results were observed for the MgO-doped bulk PPLN crystals. However, the influence of increased temperature is not as significant as that for the undoped PPLN crystals. In the waveguides, it is found that the noise increase and intensity variation are several times higher than those in the corresponding bulk crystals.

4. Conclusions

We have shown the noise characteristics of the SH waves in bulk and waveguide PPLN crystals. It is found that for the bulk and waveguide PPLN crystals, the noise or instability of the SH wave is higher than that of the fundamental wave. For the same fundamental wave, the SH wave tends to have lower noise in a bulk crystal than in a waveguide, and in MgO-doped PPLN than in undoped one. In particular, the corresponding RMS value of the noise amplitude in a waveguide can be two times higher than that in a bulk PPLN. In addition, the photorefractive effect incurred by the irradiation light can degrade the conversion performance in terms of SHG efficiency and noise intensity. In the pump-probe method with the pumping wavelengths of 532 and 1090 nm, both SHG noise and intensity vary with pump power. The 532 nm pump can bring a more significant influence than the 1090 nm one. In addition, increasing crystal temperature can alleviate the noise and absorption problems to some extent, which is more significant to undoped PPLN than to MgO-doped PPLN. The quantitative results obtained in this work provide some useful information for the applications of QPM PPLN devices for SHG.

Acknowledgments

The authors would like to thank the Ontario Photonics Consortium (OPC), the Materials and Manufacturing Ontario (MMO), the Photonics Research Ontario (PRO), the Natural Sciences and Engineering Council of Canada (NSERC), the Canada Foundation for Innovation (CFI) under the New Opportunities Program.