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Abstract
We demonstrate the third harmonic generation of a 1542-nm laser using a dual-pitch periodically poled lithium niobate waveguide with a conversion efficiency of 66%/W2. The generated 514-nm light is used for saturation spectroscopy of molecular iodine and laser frequency stabilization. The achieved laser frequency stability is 1.1×10−12 at an average time of 1 s, which is approximately one order of magnitude better than the acetylene-stabilized laser at 1542 nm. Uncertainty evaluation and absolute frequency measurement are also performed. The developed frequency-stabilized laser can be used as a reliable frequency reference at the telecom wavelength for various applications including optical frequency combs and precision interferometric measurement.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Frequency-stabilized lasers are of great interest, not only for application in the field of metrology, but also for high-resolution spectroscopy, optical communications, and for use in tests concerning fundamental physics [1,2]. Advances in optical communication systems that include dense wavelength-division multiplexing (DWDM) have motivated research on frequency-stabilized lasers in the 1.5 µm wavelength region. The International Committee for Weights and Measures (CIPM) recommended a value for the optical frequency at 1542 nm using the P(16) transition line in the ν1 + ν3 band of acetylene (13C2H2) with an uncertainty of 5 kHz (fractional uncertainty of 2.6×10−11) [3,4]. As the 1.5 µm laser light can be readily transferred through optical fiber networks, the optical frequency standards at this wavelength can easily be used at remote sites for applications such as optical clock comparisons [5], the Terrestrial Planet Finder Coronagraph project (TPF-C) [6], and the optical synchronization signal in the Atacama Large Millimeter Array (ALMA) [7]. Frequency-stabilized lasers at 1.5 µm are also used as a frequency reference for lidar measurements of atmospheric carbon dioxide to predict the future evolution of the climate [8]. Furthermore, frequency-stabilized lasers at 1.5 µm are important frequency references for optical frequency combs based on Er:fiber mode-locked lasers [9,10]. This is because: 1) frequency-stabilized lasers are much more stable in the short-term than the global positioning system (GPS) disciplined time base; 2) frequency-stabilized lasers can be used in cases where it is difficult to install GPS antenna and signal cables.
The CIPM recommended frequency of acetylene is based on a laser stabilized to an external acetylene cell within an enhancement cavity using the third harmonic detection technique [11]. This technique has been developed for the detection of weak saturated absorption lines of acetylene that are used in frequency-stabilized lasers [11,12]. Cavity-enhanced setups achieve high frequency stability and repeatability, but are complicated and often not reliable enough for practical use because the optical cavity can become unlocked when affected by external turbulence. However, the well-established telecom lasers combined with frequency-doubling technology provide a convenient solution for the development of frequency-stabilized lasers based on two-photon transitions [13] and the saturated absorption of the D2 line [14,15] in rubidium (Rb). In the former case, the Doppler-free, two-photon S–D transition at 778 nm has a relatively narrow linewidth of 500 kHz and a low sensitivity to the size and intensity of the laser beam. However, a magnetic shield is required for the setup of the stabilized laser in order to avoid shifts in the frequency due to the presence of magnetic fields. In the latter case, the Rb D2 line at 780 nm demonstrates a saturation intensity that is two to three orders of magnitude lower than that of acetylene and is easily used for laser frequency stabilization. However, the Doppler-free linewidth is at approximately 20 MHz, which is approximately 40 times larger than that of acetylene and the Rb two-photon transitions. By tripling the frequency of a telecom laser to the wavelengths associated with green light, iodine transitions with both a narrow linewidth and strong absorption can be accessed. The third harmonic generation (THG) of a 1.5 µm diode laser has been carried out using two periodically poled lithium niobate (PPLN) waveguide devices for the second harmonic generation (SHG) (1542 nm → 771 nm) and sum frequency generation (SFG) (1542 nm + 771 nm → 514 nm), respectively [16]. THG light has been used for saturation spectroscopy and laser frequency stabilization based on molecular iodine [17]. Although a good frequency stability was reported [17,18], no uncertainty evaluation or absolute frequency measurement has been disclosed. We also note that the iodine line observed in [17] is not that closest to the triple frequency of the CIPM recommended acetylene frequency.
A PPLN waveguide is a very attractive device used in various applications because of its high nonlinear conversion efficiency and capacity for accepting a fiber pig-tailing input. We also note that thin-film lithium niobate photonic waveguides have been recently found to have very high nonlinear conversion efficiency [19–21]. A high conversion efficiency and high damage threshold (∼W level) have been obtained using a PPLN ridge waveguide [22,23]. A SHG light of 25 mW was generated at 399 nm towards the short wavelengths at visible, by using a PPLN waveguide where a 798-nm fundamental power of 380 mW was coupled to the waveguide [24]. Recently, a dual-pitch PPLN waveguide, consisting of two monolithically integrated segments with different quasi-phase matching (QPM) pitch sizes, has been used to generate third-harmonic light for the 2f-to-3f self-referencing interferometer of an optical frequency comb [25].
In this study, we demonstrate the THG of a 1.5 µm laser using a dual-pitch PPLN waveguide with a chirped QPM pitch for the SHG stage. We confirm that the designed chirped QPM pitch is effective for overlapping the phase matching curves of the SHG and SFG for efficient THG with only a single temperature control required for the waveguide. A maximum THG power of 38.3 mW was obtained using an input power of 390 mW. The THG light generated at 514 nm was used to perform saturation spectroscopy and observe the hyperfine structure of the R(73)46-0 transition of molecular iodine with no enhancement cavity, near the third harmonic of the P(16) transition in the ν1 + ν3 band of 13C2H2 (see Fig. 1). The R(73)46-0 transition is different from the R(34)44-0 transition studied in Ref. [17] and the P(13)43-0 transition recommended by the CIPM at 515 nm, which are 125 GHz higher and 620 GHz lower, respectively 26, compared to the absolute frequency of the present transition. The frequency of the 1.5 µm laser was stabilized using the observed iodine transition and a frequency stability of 1.1 × 10−12 was obtained at an average of 1 s, which is approximately one order of magnitude better than that of the acetylene-stabilized laser [27]. The absolute frequency of the a1 component of the R(73)46-0 transition was also determined at 583 109 956 557 kHz ± 6 kHz, following the evaluation of frequency uncertainty. The frequency-stabilized laser developed via this method is an excellent optical frequency reference which is applicable for a wide range applications using the three-color outputs at 1542, 771, and 514 nm.
Fig. 1. Frequency atlas of the I2 absorption lines near the third harmonic frequency of the P(16) transition in the ν1 + ν3 band of 13C2H2 at 514 nm. The relative intensity of the iodine lines is taken from [26]. The intensity of the 13C2H2 transition is much weaker and is not on the same scale compared to the iodine lines.
2. Generation of the 514-nm light using a dual-pitch PPLN waveguide
The dual-pitch PPLN waveguide chip used for THG from 1542 nm to 514 nm consists of six PPLN waveguides (WG1-WG6) (NTT Electronics Corporation). Each waveguide has a combination of QPM pitches at both the first stage (SHG stage) and the second stage (SFG stage) for the generation of the third harmonic, as seen in Table 1. Figure 2 is an illustration of one PPLN waveguide. We note that the first stage has a chirped QPM pitch of either 18.175-18.250 µm (Chirp 1) or 18.225-18.300 µm (Chirp 2). In the first stage, second harmonic 1542-nm light is generated (1542 nm → 771 nm) with a relatively broad temperature acceptance owing to the chirped QPM pitch. The generated SHG light is then converted into 514-nm light in the second stage via SFG (1542 nm + 771 nm → 514 nm).
Fig. 2. Illustration of a dual-pitch PPLN waveguide for the third harmonic generation from 1542 nm to 514 nm (one example of the six waveguides). SHG: Second harmonic generation. SFG: Sum frequency generation.
Table 1. Quasi-phase matching (QPM) pitches of the first and second stages in each waveguide.
Each PPLN waveguide was 30 mm (first stage: 20 mm, second stage: 10 mm) × 12 µm × 11 µm (length × width × thickness). Both ends of the waveguide were coated with materials that are anti-reflective at wavelengths of 1542, 771, and 514 nm. The six PPLN waveguides were mounted on a metal carrer and its temperature was controlled using a Peltier cooler. The temperature was therefore the same for the first and second stages of PPLN.
Two kinds of lasers were utilized for the 1542-nm light source in order to evaluate the performance of the dual-pitch waveguides. One was a planar-waveguide external cavity diode laser with an output power of 10 mW and a laser linewidth of ∼2 kHz. The current and temperature of the diode laser were used as a fast and slow frequency control port, respectively. The other laser used was a single-frequency distributed feedback (DFB) fiber laser with an output power of 30 mW and a laser linewidth of < 1 kHz. The two frequency actuators, Piezo and temperature, were utilized by the fiber laser for rapid fine tuning and slow dynamic tuning, respectively. The output power was delivered from both lasers via a polarization maintaining fiber and amplified using a polarization maintaining erbium-doped fiber amplifier (EDFA). The output from the EDFA was then butt-coupled [28] into the waveguides. The output light from the waveguides contained the fundamental (1542 nm), SHG (771 nm), and THG (514 nm) light. Each light was carefully separated using several dichroic mirrors and the light power was measured using an optical power meter. Both the diode and fiber lasers were used for evaluating the waveguides, including both phase matching and output power.
Figure 3 shows the phase matching curves of the dual-pitch waveguides for both the SHG and SFG processes. The output power of the EDFA was approximately 100 mW. The phase matching curves of the SHG process were observed by injecting 1542-nm light into the waveguides from the opposite side (the second stage side), meaning that only SHG processes could occur in the waveguides. The phase matching curves of the SFG process were observed via the injection of 1542-nm light from the first stage side. As shown in Figs. 3(a) and 3(b), the full widths at half maximum of the phase matching curves for the SHG process using the Chirp 1 and Chirp 2 QPM pitches in the first stage were 12°C and 13°C, respectively. The phase matching curve of the SHG process shown in Fig. 3(a) was expected from the estimation. The QPM grating was not apodized, but linearly chirped with a limited range. The difference of the SHG phase matching curves in Figs. 3(a) and 3(b) could be caused by fabrication inhomogeneities. On the other hand, the full width at half maximum of the SFG phase matching curves was approximately 2°C. This corresponds to a wavelength acceptance of approximately 0.2 nm [29]. The large acceptance bandwidth of the SHG process compared to that of the SFG process enables the realization of same phase matching temperatures for SHG and SFG. Of the six waveguides, the same phase matching temperatures were realized for WG1, WG5, and WG6, leading to efficient wavelength conversions at 38.3°C, 32.4°C, and 24.6°C, respectively.
Fig. 3. SHG (771 nm) and SFG (514 nm) output power as a function of the temperature of dual-pitch waveguides when the QPM pitch of the first stage is Chirp 1 (a) and Chirp 2 (b).
Figure 4 shows the THG power at 514 nm (P3ω) as a function of the coupled fundamental power at 1542 nm (Pω). The solid circles represent the measured P3ω when WG1 was used. The waveguide temperature was set at 38.3°C. The P3ω obtained was 1.72 mW when the coupled Pω was set at 140 mW. The coupling efficiency of Pω was 70%. This includes not only the fundamental mode but also higher-order spatial modes. The normalized conversion efficiency (P3ω/Pω3) was calculated at approximately 66 %/W2
3. Saturation spectroscopy and frequency stabilization
3.1 Experimental setup
Figure 5 shows a schematic diagram of the experimental setup for the Doppler-free spectroscopy of molecular iodine and frequency stabilization. As more intensity noise was observed in the output of the fiber laser than the diode laser and it was found that this noise could affect the laser frequency (see Sect. 4), the diode laser was used here as a light source for the fundamental light at 1542 nm. The output of the diode laser was then amplified using an EDFA with an output power of approximately 400 mW.
Fig. 5. Schematic diagram of the experimental setup for Doppler-free spectroscopy and laser frequency stabilization based on the modulation transfer technique. EDFA: Erbium-doped fiber amplifier, PPLN-WG: Periodically polled lithium niobate waveguide, DM: Dichroic mirror, HWP: Half-wave plate, PBS: Polarizing beam splitter, AOM: Acousto-optic modulator, EOM: Electro-optic modulator, GTP: Glan–Thompson prism, PD: Photodetector, LO: Local oscillator, DBM: Double-balanced mixer, UTC (NMIJ): Coordinated Universal Time at the National Metrology Institute of Japan. The optical and electrical paths are shown as solid and dashed lines, respectively.
The main power from the EDFA was used for THG using the dual-pitch waveguides. Based on the results in Section 2, a packaged THG waveguide device with a fiber pig-tail was manufactured by the NTT Electronics Corporation. The output of the THG device is free space and contains not only the THG light but also the fundamental and the SHG lights. The 514-nm THG light is separated from the fundamental and SHG lights using a dichroic mirror, and then sent to an iodine spectrometer. A maximum output power of 38.3 mW at 514 nm was obtained when the input power of the THG device was 390 mW.
Doppler-free spectroscopy of molecular iodine was carried out using the modulation transfer technique [30–32]. The 514-nm light was separated into pump and probe beams using a half-wave plate and a polarizing beam splitter. The pump beam was then frequency-shifted by 80 MHz using an acousto-optic modulator (AOM) and phase-modulated using an electro-optic modulator (EOM) at a modulation frequency of 345 kHz. The pump and probe beams were overlapped within a 30-cm iodine cell. When saturation occurred, the sidebands on the pump beam were transferred to the probe beam by a nonlinear resonant four-wave mixing process [30,31]. The probe beam was then separated from the pump beam with a Glan-Thompson prism and detected by a photodetector. The modulation transfer signal of the spectral lines was obtained by demodulating the signal from the detector. The demodulated signal was monitored using an oscilloscope. This signal was then fed back into the diode laser through a servo system when frequency stabilization had been carried out. A fiber coupler (99:1) was introduced after the EDFA to separate 1 % of the EDFA output power for a beat measurement with a frequency comb referenced to the Coordinated Universal Time at the National Metrology Institute of Japan (UTC(NMIJ)). The beat frequency was monitored using a spectrum analyzer and measured using a dead-time free frequency counter (Pendulum CNT-90).
3.2 Saturation spectroscopy of molecular iodine
Figure 6 shows the observed modulation transfer signal of the R(73)46-0 transition. Laser frequency scanning was performed by tuning the injection current of the diode laser. The pump and probe beam power was 5.0 mW and 0.5 mW, respectively. The diameter of the pump and probe beams was approximately 1.5 mm. The cold-finger temperature of the iodine cell was held at − 5 °C, corresponding to an iodine pressure of 2.5 Pa. The temperature of the cell body matched the temperature of the controlled room at 23 °C. The detailed hyperfine structure of this transition was then observed for the first time, to the best of our knowledge. For the odd J number of the ground state (J = 73), the rovibrational energy level is split into 21 sublevels, resulting in 21 hyperfine components for the R(73)46-0 transition. As shown in Fig. 6, the a8 and a9 components overlap, as do the a11 and a12 components. The signal-to-noise ratio (S/N) of the a1 component was approximately 102 with a cutoff frequency of 1 kHz of the lowpass filter after demodulation.
Fig. 6. Doppler-free spectra of the R(73)46-0 transition of molecular iodine observed with a 1-kHz cutoff frequency of the lowpass filter after demodulation.
3.3 Frequency stabilization
Laser frequency stabilization was performed using the a1 component of the R(73)46-0 transition. Figure 7 shows the Allan standard deviation calculated from the measured beat frequency between the frequency-stabilized laser and the frequency comb. The Allan standard deviation was 1.1 × 10−12 at an average of 1 s, improving to 9 × 10−14 after an average of 600 s. The Allan standard deviation appears to reach the flicker of the system at an average of > 10 s. For comparison, the frequency stability of the UTC(NMIJ) is also shown in Fig. 7 as a black dotted line. Since the stability of the UTC(NMIJ) is better than that of the developed iodine-stabilized laser, the Allan standard deviation observed indicates the stability of the laser. The black dashed line in Fig. 7 shows the frequency stability of an acetylene-stabilized laser [27]. Compared to typical acetylene-stabilized lasers, the iodine-stabilized laser developed reached a stability that was superior by approximately one order of magnitude over a short time, and approximately a factor of 5 over longer time periods.
Fig. 7. The Allan standard deviation calculated from the measured beat frequency between the iodine-stabilized laser and the frequency comb. The black dotted (dashed) black line shows the frequency stability of the UTC(NMIJ) (an acetylene-stabilized laser).
3.4 Uncertainty evaluation and absolute frequency measurement
Figure 8 shows the result of the measurement for the absolute frequency of the iodine-stabilized laser based on the a1 component of the R(73)46-0 transition. Thirteen frequency measurements were performed over one week. Each measurement in Fig. 8 was calculated from more than 1000 beat frequency data measured with a gate time of 1 s. The uncertainty bar was given by the Allan standard deviation for the longest average time. The averaged frequency of the thirteen frequencies measured in Fig. 8 was 583 109 956 557 kHz. The standard deviation of the thirteen measurements was 1.2 kHz, which indicates the repeatability of the frequency-stabilized laser.
Fig. 8. Absolute frequency and repeatability of the iodine-stabilized laser. The solid red line indicates the average frequency of the thirteen measurements.
To evaluate the uncertainty of the measured absolute frequency, several frequency shifts of the iodine-stabilized laser were investigated (Table 2). Figure 9(a) shows the measured pressure shift of the laser locked to the a1 component of the R(73)46-0 transition. The measured slope of the pressure shift was −5.1 kHz/Pa. This value is larger than the reported value of − 2.98 kHz/Pa for an iodine-stabilized Nd:YAG laser [33]. The uncertainty of the temperature of the solid-state iodine crystal in the cold finger, which determines the uncertainty of the iodine pressure, is estimated to be < 0.5 K. This corresponds to an uncertainty of < 0.2 Pa of the pressure and results in a frequency uncertainty of < 1.1 kHz. Figure 9(b) shows the measured frequency shift of the frequency-stabilized laser as a function of pump power. The measured slope of the power shift was −3.6 kHz/mW. This is greater than the reported value of −149 Hz/mW in [33]. The uncertainty in the determination of the laser power is estimated to be < 10 %, which results in a frequency uncertainty of < 1.8 kHz. Figure 9(c) shows the measured frequency shift due to the adjustment in phase modulation between the modulation applied to the pump beam and a local oscillation port of the demodulation device. Phase 0 ° in Fig. 9(c) was defined as the phase in which the modulation transfer signal is at a maximum. The measured slope of 260 Hz/degree is larger than that reported by [33]. The uncertainty of the phase adjustment was set at < 10 degrees. This corresponds to a frequency uncertainty of < 2.6 kHz. The possible shift in frequency caused by contamination in the iodine cell should also be taken into consideration. Previous studies [32] have shown that this effect can add an uncertainty of 5 kHz to measurement results. We use the standard deviation from thirteen measurements (1.2 kHz) as the statistical uncertainty of measurement in this study. The total uncertainty is then calculated at 6 kHz, corresponding to a relative uncertainty of 1.0 × 10−11. The absolute frequency of the a1 component of the R(73)46-0 transition with the experimental parameters used, pump beam power = 5.0 mW, a beam diameter ∼ 1.5 mm, and cell cold finger temperature = −5°C (iodine pressure = 2.5 Pa), was determined as 583 109 956 557 kHz ± 6 kHz.
Fig. 9. Measured frequency shifts of the iodine-stabilized laser at 514 nm locked on the a1 component of the R(73) 46-0 transition: (a) pressure shift, (b) power shift, and (c) shift due to the adjustment in phase modulation.
Table 2. The most significant contributions to the estimated frequency uncertainty of the a1 component of the R(73)46-0 transition.
4. Discussion and conclusion
The temperature acceptance bandwidth of approximately 2.3°C seen in the dual-pitch waveguide device, which is basically limited by the second stage, allows for relatively easy temperature control of the device. If a chirp is not introduced in the first stage, the phase matching of the THG process is difficult to achieve, or may have a much narrower temperature acceptance bandwidth due to the manufacturing error of the pitches. A narrower temperature acceptance bandwidth could induce an instability of the intensity of the THG process. On the other hand, by adapting the chirped QPM pitch, the conversion efficiency of the SHG is decreased compared to that of a single QPM pitch crystal of the same length. However, considering the coupling loss between the two waveguides, the total conversion efficiency of the dual-pitch waveguide is higher than that of the two individual waveguides used in the previous study [16]. A dual-pitch PPLN waveguide with a chirped QPM pitch in the first stage is therefore confirmed to be a good strategy for the nonlinear wavelength conversion of THG and should be applicable to other wavelengths.
The frequency stability of 1.1 × 10−12 at 1 s obtained in the present experiment is comparable to the result of 4.8 × 10−14 at 1 s which was obtained at a similar wavelength using a narrow linewidth diode laser [17]. The main difference between the two experiments was the length of the interaction between the laser and the iodine, which was 0.3 m in the present study and 1.2 m in [17]. This should lead to a fourfold improvement of the results in [17], if all other conditions are the same. The iodine pressure and modulation frequency were 2.5 Pa (0.79 Pa) and 345 kHz (220 kHz), respectively, in the present study (in [17]). The higher pressure of the iodine in the present study increases the signal associated with absorption, but also causes pressure broadening at the spectral linewidth. For the iodine line at 514 nm, which has a natural linewidth of 50 kHz [34] (a factor of 5 narrower than an iodine line at 532 nm), it is therefore a good strategy to use a long iodine cell with a low iodine pressure, and a low modulation frequency to meet the requirements for the narrow spectral linewidth realized under these conditions. We note that different frequency counters and Allan deviation calculation algorisms used in different experiments may result in different frequency stabilities.
As mentioned in Section 3.1, relatively large intensity noise was detected in the output power of the fiber laser. Figure 10 shows the relative intensity noise (RIN) of the diode and fiber lasers. Compared to the RIN of the diode laser, the RIN of the fiber laser has two characteristic peaks at around 800 Hz and 260 kHz. The peak around 260 kHz is close to the modulation frequency of the modulation transfer spectroscopy and may affect the result of the laser frequency stabilization. A laser linewidth broadening of up to ∼ 1 MHz was observed in the output power of the EDFA after the fiber laser. This may have been induced by the intensity noise through an intensity-to-frequency conversion process in the EDFA. In applications where intensity and frequency noise are not critical, the fiber laser should still serve as a good light source due to its relatively high output power (30 mW) and larger possible range for wavelength tuning than that of the diode laser (1542.1 to 1542.6 nm). The wavelength tuning range corresponds to a frequency tuning of 62.8 GHz at 1542 nm and 188.5 GHz at 514 nm. Based on our calculation, the phase matching of the THG process in the dual-pitch waveguide for the whole frequency tuning range of the fiber laser can be achieved by detuning the waveguide temperature by approximately 5 K. There are tens of iodine lines within the tuning range of 188.5 GHz at 514 nm [26]. Detailed studies on the hyperfine structure of these iodine lines should provide a better understanding of the rotational and vibrational characteristics of the iodine hyperfine constants [35–38].
Fig. 10. Relative intensity noise of the diode and fiber lasers used in the present experiment as a function of Fourier frequency.
The iodine-stabilized laser developed has 3 output wavelengths at 1542, 771, and 514 nm, covering a wide range of frequencies. The output at 1542 nm is detected using an InGaAs photodiode, while the outputs at 771 and 514 nm are detected using a Si photodiode. The 1542-nm output light is only approximately 600 MHz from the acetylene-stabilized laser light in frequency, and can therefore be utilized in almost all applications that use an acetylene-stabilized laser. In addition, the developed frequency-stabilized laser has the advantages of 1) high reliability for long term operation 2) a modulation-free laser output, due to the exclusion of the optical cavity usually used in acetylene-stabilized lasers. One example of an application that uses a frequency-stabilized laser at the telecom wavelength is an astro-comb [39], based on a mode-locked erbium-fiber laser. We have developed a first-generation astro-comb, which was installed at the Okayama astrophysical observatory (OAO) in Japan [40]. The frequency-stabilized laser developed in this study is to be used in a new second-generation astro-comb for the OAO and other observatories. The 771-nm and 514-nm light can be used for precision interferometric measurements. For example, together with a compact frequency-stabilized laser at 531 nm [41,42], the 771-nm and 514-nm outputs provide an excellent solution for the excess fraction method [43] commonly used for gauge block measurements.
In conclusion, an iodine-stabilized laser at the telecom wavelength was developed using a dual-pitch PPLN waveguide. The dual-pitch waveguide, with a chirped QPM pitch in the first stage, demonstrated both a large acceptance bandwidth and a high conversion efficiency for the THG process from 1542 nm to 514 nm. The iodine-stabilized laser has been proved to have a frequency stability better than acetylene-stabilized lasers and is reliable for long-term operation. The developed frequency-stabilized laser has three outputs at different wavelengths and exhibits a good frequency performance for use in various applications.
Funding
Japan Society for the Promotion of Science (KAKENHI 18H01898, KAKENHI JP18H03886); Japan Science and Technology Agency Exploratory Research for Advanced Technology (JPMJER1304).
Acknowledgments
We thank Y. Nishida and H. Miyazawa from NTT Electronics Corporation for the helpful discussion concerning the PPLN waveguide. We dedicate this article to Atsushi Onae for his useful ideas related to this research.
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