# Noise Analysis of Second-Harmonic Generation in Undoped and MgO-Doped Periodically Poled Lithium Niobate

Authors: Yong Wang; Jorge Fonseca-Campos; Wan-guo Liang; Chang-Qing Xu; Ignacio Vargas-Baca
Journal: Advances in OptoElectronics
 (2008)

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## Abstract

Noise characteristics of second-harmonic generation (SHG) in
periodically poled lithium niobate (PPLN) using the quasiphase
matching (QPM) technique are analyzed experimentally. In the
experiment, a0.78μm second-harmonic (SH) wave was generated when
a 1.56 μm fundamental wave passed through a PPLN crystal (bulk or
waveguide). The time-domain and frequency-domain noise
characteristics of the fundamental and SH waves were analyzed. By
using the pump-probe method, the noise characteristics of SHG were
further analyzed when a visible light (532 nm) and an infrared
light (1090 nm) copropagated with the fundamental light,
respectively. The noise characterizations were also investigated
at different temperatures. It is found that for the bulk and
waveguide PPLN crystals, the SH wave has a higher relative noise
level than the corresponding fundamental wave. For the same
fundamental wave, the SH wave has lower noise in a bulk crystal
than in a waveguide, and in MgO-doped PPLN than in undoped PPLN.
The 532 nm irradiation can lead to higher noise in PPLN than the
1090 nm irradiation. In addition, increasing temperature of device
can alleviate the problem of noise in conjunction with the
photorefractive effect incurred by the irradiation light. This is
more significant in undoped PPLN than in MgO-doped one.


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## Body

## 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 LiNbO3 crystal simply by choosing a suitable poling period in the QPM structure; the
largest nonlinear component (i.e., d33) 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 LiNbO3 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 0.78μm 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 Figure1, 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 17–19μm, 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
1.09μm, 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
Figure3. 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.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.
(a)
Bulk(b)
Bulk(c)
Waveguide(d)
WaveguideThe corresponding frequency
spectra in the frequency range of 0–2 kHz, obtained by using the fast Fourier
transform (FFT), are shown in Figure4. 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.Frequency spectra corresponding to the
fundamental and SH temporal output traces in
Figure3.
(a)
Bulk(b)
Bulk(c)
Waveguide(d)
WaveguideSimilarly for a 4.5 cm long
MgO-doped PPLN crystal at room temperature, the temporal fundamental and SH output
powers are depicted in
Figure5. 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.Power fluctuations of fundamental and SH
waves in MgO-doped PPLN
crystal.
(a)
Bulk(b)
Bulk(c)
Waveguide(d)
WaveguideFrequency spectra
corresponding to the fundamental and SH temporal output traces in
Figure5.
(a)
Bulk(b)
Bulk(c)
Waveguide(d)
WaveguideTo 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 Figures3 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.
RMS
Noise
(%)Undoped
PPLNMgO-doped
PPLNPumpSHPumpSHBulk0.761.20.51.0Waveguide2.04.71.32.9The 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 Figures3 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
Figure2.For the undoped PPLN waveguide, the
results of output SH power are shown in
Figure7, 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.(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.
(a)(b)The noise amplitudes and frequency
spectra of the SH wave with and without the 532 nm pump are compared in
Figure8. 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).(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).
(a)(b)(c)(d)For the MgO-doped PPLN waveguide,
Figure9 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.(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).
(a)(b)(c)(d)For
the undoped bulk PPLN, the output SH trace is shown in
Figure10 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.For undoped bulk
PPLN, comparison of SH outputs with and without 532 nm irradiation.
(a)
No irradiation(b)
100 mWFor the
MgO-doped bulk PPLN, the noise characteristics are quite similar for different
irradiation powers up to 1 W, as shown in
Figure11. 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.For MgO-doped bulk
PPLN, comparison of SH outputs with different irradiation powers of 532 nm
irradiation.
(a)
No irradiation(b)
100 mW(c)
1000 mWFrom
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
Figure12, 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.(a)
RMS noise of SHG and (b) intensity change of SHG versus 1090 nm pump power,
measured at different temperatures.
(a)(b)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.


## 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
Figure3. 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.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.
(a)
Bulk(b)
Bulk(c)
Waveguide(d)
WaveguideThe corresponding frequency
spectra in the frequency range of 0–2 kHz, obtained by using the fast Fourier
transform (FFT), are shown in Figure4. 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.Frequency spectra corresponding to the
fundamental and SH temporal output traces in
Figure3.
(a)
Bulk(b)
Bulk(c)
Waveguide(d)
WaveguideSimilarly for a 4.5 cm long
MgO-doped PPLN crystal at room temperature, the temporal fundamental and SH output
powers are depicted in
Figure5. 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.Power fluctuations of fundamental and SH
waves in MgO-doped PPLN
crystal.
(a)
Bulk(b)
Bulk(c)
Waveguide(d)
WaveguideFrequency spectra
corresponding to the fundamental and SH temporal output traces in
Figure5.
(a)
Bulk(b)
Bulk(c)
Waveguide(d)
WaveguideTo 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 Figures3 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.
RMS
Noise
(%)Undoped
PPLNMgO-doped
PPLNPumpSHPumpSHBulk0.761.20.51.0Waveguide2.04.71.32.9The 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 Figures3 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
Figure2.For the undoped PPLN waveguide, the
results of output SH power are shown in
Figure7, 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.(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.
(a)(b)The noise amplitudes and frequency
spectra of the SH wave with and without the 532 nm pump are compared in
Figure8. 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).(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).
(a)(b)(c)(d)For the MgO-doped PPLN waveguide,
Figure9 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.(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).
(a)(b)(c)(d)For
the undoped bulk PPLN, the output SH trace is shown in
Figure10 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.For undoped bulk
PPLN, comparison of SH outputs with and without 532 nm irradiation.
(a)
No irradiation(b)
100 mWFor the
MgO-doped bulk PPLN, the noise characteristics are quite similar for different
irradiation powers up to 1 W, as shown in
Figure11. 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.For MgO-doped bulk
PPLN, comparison of SH outputs with different irradiation powers of 532 nm
irradiation.
(a)
No irradiation(b)
100 mW(c)
1000 mWFrom
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
Figure12, 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.(a)
RMS noise of SHG and (b) intensity change of SHG versus 1090 nm pump power,
measured at different temperatures.
(a)(b)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.



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*Source: 428971-2008-10-30.xml*