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Abstract
Yellow-orange lights, valuable in photodynamic therapies, spectroscopy, and optogenetics, are limited by the narrow bandwidth and bulky setup via the conventional Raman or optical parametric oscillation processes. Moreover, flatness in the broad-band spectrum is also important for the aforementioned applications with extended functions. In this paper, by carefully designing grating-periods of a step-chirped PPMgLN ridge waveguide for sum frequency generation (SFG), we report a compact broad-band yellow-orange light with bandwidth of 5.6 nm and an un-reported flatness (<1.5 dB). Correspondingly, the optical conversion efficiency is 232.08%/W, which is the best SFG efficiency for PPMgLN at the yellow-orange region, to the best of our knowledge. The results could also be adopted for other broad-band SFG process toward the vis-infrared region in an integrated structure.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Yellow-orange laser, with wavelengths within 590∼600 nm gets a variety of applications in photodynamic therapies [1], flow cytometry [2], spectroscopy [3], astronomy [4], optogenetics and neuroscience [5]. However, several measures for it, such as Raman lasers [6–8], dye lasers [9,10] and optical parametric oscillation [11] have limitations in beam quality, system compactness, or lasing efficiencies. As well, there are still no commercial diode lasers with appropriate band gaps to cover this wavelength region [12]. Fortunately, sum frequency generation (SFG) basing on the nonlinear crystals [11,13–15], is an attractive way for the yellow orange lasers. Especially, taking advantage of the d33 coefficient of Lithium Niobate (LN), efficient SFG at 590∼610 nm can be supported in a periodically poled structure (PPLN). However, yellow orange laser with broad-spectrum and high-flatness is limited by the regular uniform PPLNs, owing to their narrow band-widths and sensitivities in temperature fluctuation. Hence, tailoring the poled period with multiple period [16], fan-out period [17], or step chirp (SC) [18] is of necessity for a compact broadband tunable device at desired wavelengths.
With a SC-PPLN, bandwidth of the SFG light can be extended by using the broad-band laser sources for frequency-mixing. Broadband yellow-orange laser, pumped with a C-band ASE laser source, was firstly verified by Choge [19]. Using a 20 mm period poled MgO doped LiNbO3 crystal (PPMgLN), the yellow-orange spectrum was extended from 593 nm to 599 nm with a maximum power of 4.3 mW [19], corresponding to the conversion efficiency of 4.5%/W by considering the incident power of the fundamental wave (FW). Increasing length of the SC-PPMgLN to 50 mm, the lasing power was increased to 7.1 mW with a conversion efficiency of 7.4%/W [20]. By placing the unified PPMgLN and SC-PPMgLN in cascade, maximum power of 10 mW was demonstrated. However, the conversion efficiency was limited at 10.4%/W. Moreover, it is hard to flatten the broadband spectrum owing to the tight-focused FW spot inside the bulk SC-PPMgLN. Recently, LN-based waveguides, characterized of high optical field confinement within a small space, received intensively researched in frequency optical comb, frequency stabilization, and integrated quantum chip [21–25]. Thus, it is of significance to utilize the waveguide-based SC-PPMgLN to evaluate the promotion of SFG efficiency for broadband yellow-orange laser generation and seeking the improvement in spectrum flatness.
In this paper, SC-PPMgLN-based ridge waveguide is successfully fabricated with the bonding and precision diamond cutting method. Using a broad-band ASE laser source covering the C-band, together with a fiber-couple single-mode LD at 975.6 nm, broad-band flat yellow-orange laser with a FWHM spanning from 594 nm to 601 nm was obtained, corresponding to a conversion efficiency of 232.08%/W. The proposed waveguide structure, having no rigorous requirement in the pump power can be widely integrated for quantum signal detection, medical sensing, or bio-imaging [26–28].
2. Device design
Sum frequency generation (SFG) for frequency is υSFG occurred while propagating two photons, at frequency υ1 and υ2, through a χ2 nonlinear material. This process complys with the conservation of energy [29], while the state of matter remains unchanged.
As well, conservation of the momentum (ΔK = 0) should be maintained for a high conversion efficiency. In the PPLN crystal, the phase matching condition can be satisfied with the assistance of the poling period, that is [30] :
Here, ni (i = 1, 2, SFG) denotes the effective refractive index for the guided wave υi. Hence, for Δk = 0, the poling period $\Lambda $ can be expressed as:
Fig. 1. (a) distribution of the polling period for the broad-band SFG process toward 592 nm to 610 nm with an ASE source at C-band and a 975.6 nm pump light. (b) evolution in effective indices and poling period with the SFG light and the C-band light.
Figure 2 depicts the designed SC-PPMgLN with sub grating periods varying from 9.9 to 10.25 µm, similar design as Ref. [31] for 1560 nm laser generation basing on the type II phase-matched parametric down-conversion, in which each part is with a 1 mm long grating with single period of Λi. Initializing Λ1 as 9.90 µm, and the increment Δ between successive sub-periods Λi as 0.01 µm, there are 35 segments in total, corresponding to a 35 mm long waveguide. Also, this design is very suitable for type-0 QPM interaction, as all three waves are polarized parallel to the c-axis of PPMgLN for utilizing the maximum nonlinear coefficient d33.
Figure 3 illustrates the flow chart for preparing the SC-PPMgLN. Firstly, the SC structure was fabricated in a 0.5 mm thick Z-cut MgLN crystal using the standard pulsed voltage poling technique. And then, the PPMgLN crystal was firmly bonded with a 0.5 mm thick lithium tantalite (LT) crystal through direct bonding with SiO2, which dominated by the van der Waals force. As well, very small amount of silicon-based material is coated as an adhesive layer to enhance the bonding force and decreasing the insertion loss.The LT crystal here is used as the substrate, whose refractive index is smaller than that of PPMgLN crystal. Via precisely thinning and polishing, PPMgLN thin-film layer with thickness of 11 µm was prepared. Finally, the ridge structure was formed by cutting with a high-precision diamond blade. Both ends of the designed waveguide was polished and antireflection coated to reduce the reflection of the lights for SFG. Via optical microscopy, the ridge waveguide was measured with a half-height width of 12.16 µm and a ridge height of 11.07 µm respectively (Fig. 4), corresponding to a base-angle θ of 74°. With these parameters, guide-wave distribution inside the SC-PPMgLN waveguide was simulated (Fig. 4(a)), where single TM mode with polarization parallel to the vertical direction, covering the wavelengths for SFG can be supported (Fig. 4(b)).
Fig. 4. Characterization of the prepared SC-PPMgLN: (a) typical single TM mode distribution at 595 nm with a (b) cross-section view, where the arrow line denotes polarization of the guided mode; (c) microscopy of the end surface of the waveguide.
3. Evaluating the SFG performance
Experimental setup for generating the broad band yellow orange laser is depicted in Fig. 5. Home-made ASE light source, covering the C-band from 1525 nm to 1565 nm was used as the signal source, which has the maximum output power of 5 mW and Flatness < 2 dB (Inset of Fig. 5).The ASE spectrum was measured with an optical spectrum analyzer (YOKOGAWA, AQ6370D) with a resolution of 0.1 nm. Fiber-coupled single-mode Laser diode (LD, type:LC96Z400, II-IV Inc.), with maximum power of 3 mW, peak wavelength of 975.6 nm and bandwidth of 2 nm was used as the pump source. The pump light and signal light were combined with a WDM before guided into the SC-PPMgLN for SFG. To meet the requirement in polarization for QPM, an in-line polarization controller was employed for optimizing the frequency-conversion efficiency. The SC-PPMgLN was mounted onto a TEC-controlled oven for controlling, where the working temperatures were ranging from 25 to 65 °C with an accuracy of 0.1 °C. Before extracting the SFG light from the residual pump and signal lights, all the lights were collimated by a plano-convex lens F1 with focus length of 10 mm and a band-pass filter (high transmittance at below 800 nm and high reflection at above 800 nm).
Fig. 5. Schematic diagram of the SFG process for broad-band yellow orange laser generation (Inset: lasing spectrum of the ASE source)
Setting the ASE power at its maximum value where incident power of 1.41 mW was detected, Fig. 6 shows the evolutions in the intensity of the SFG with pump power of the 976 nm LD. Conversion efficiency of the SFG process was calculated by η=Pout/(P1P2), where P1 denotes pump power, P2 the signal power, and Pout the SFG light. Under maximum diode pump power of 2 mW, maximum SFG power of 6.58 µW was obtained, corresponding to η of 232.08%/W (18.95%/W·cm2) and a slope efficiency of 0.35%. This power level is competent in integrated optics concerning the weak detections, such as medical sensing or quantum information [26–28]. This is the reported maximum conversion efficiency for a PPLN-based SFG process toward the yellow orange region. Owing to the well confined guide-waves inside a waveguide structure [21,22,32,33], characteristic weak conversion efficiency for the chirped PPMgLN (< 66%/W [19–20,24,28]) could be compensated, especially at low fundamental wave powers. According to the theoretical model for SFG [29,30], higher yellow-orange laser power could be improved by increasing power of the ASE source. In the future experiment, we try to build an amplification stage for the ASE laser and the technical problem will be how to keep the spectral flatness of the ASE source after amplification. Beam shape of the SFG in the far-field was detected by a CCD camera (Inset of Fig. 6), which is with trapezoid-shape as cross-section of the ridge waveguide (Fig. 4(c)).
Fig. 6. Evolutions in conversion efficiency and SFG power with the pump power, where power of the ASE source was set at its maximum value. (Inset: far-field beam shape of the yellow-orange light)
At the maximum LD pump power of 2 mW, evolution in the SFG spectrum with the working temperatures was measured and depicted in Fig. 7. The main lasing bands are spanning between 594 nm and 601 nm, which well matches the ASE band from 1525 nm to 1565 nm and the designed polling periods Λ of 9.90 µm and 10.25 µm (Fig. 1(b)). With the increased working temperature, the SFG light became broader with the enhanced shoulders at around 595 nm and 600 nm, which means the whole spectrum became broader and flat. We define flatness F of the SFG spectrum according to the average value Iave and the standard deviation ΔI of the intensity data within the FWHM range of each spectrum, that is F=ΔI/Iave. Figure 8 shows the changed peak wavelength, spectral bandwidth, and flatness F with the working temperatures from 35 °C to 55 °C. Obviously, red-shift in the SFG process can be verified by the changed peak wavelengths from 597.95 nm to 598.75 nm (Fig. 8(a)), which is attributed to the modified refractive index by heat [29]. The changed refractive index is more suitable for SFG toward longer wavelength. As well, bandwidth of the SFG light was extended with the increased temperature (Fig.8b). We attributed the decreased bandwidth at temperature above 50 °C to the deformation of the waveguide end-surface and resulted weak coupling efficiency, which could be compensated by slightly modifying the incident pump light.Furthermore, according to the temperature-dependent refractive index equation for the LN crystal [34,35], effective refractive indexes for the two fundamental waves and the SFG laser inside waveguide were recalculated and inserted into Eq.2 for calculating the peak wavelengths and SFG bandwidth in Fig.8a and 8b respectively. Here, the cut-off poling periods at 9.9 µm and 10.25 µm were used. To calculate the changed SFG peak wavelength, peak ASE wavelength at 1557.3 nm was selected, where the experimental data can be matched quite well (Fig. 8(a)). However, obvious deviation between the calculated and experimental SFG bandwidths occurs, although the two curves have similar evolution tendency. We mainly attribute this deviation to the poor conversion efficiency within the cut-off poling periods at temperatures away from 50 °C, which limits the real SFG bandwidth. At temperatures of 40 °C and 45°C, the SFG bandwidths are measured nearly the same owing to the unchanged cut-off wavelengths at the FWHM positions of each SFG spectrum (Fig. 7). However, obvious difference in the detail spectral structures at 40 °C and 45°C could be observed in Fig. 7, where the spectrum flatness tuned better with the increased working temperature. Best flatness with value of 0.14 was obtained under 52 °C. Correspondingly, the fluctuation in spectral intensity within the FWHM bandwidth was below 1.5 dB, lower than the ASE source (Inset of Fig. 5). This result is better than the previously bulk PPMgLN-based SFG processes [20]. Within a broad temperature range, the spectral bandwidths are maintained at above 5 nm, where maximum value of 5.6 nm, approaching the designed bandwidth ∼6 nm was obtained.
Fig. 8. Evolutions in (a) peak wavelength, (b)spectral bandwidth, and (c)flatness with the changed working temperatures from 35 °C to 55 °C.
4. Summary
In summary, SC-PPMgLN ridge waveguide, with sub periods from 9.9 µm to 10.25 µm was fabricated for broad-band SFG toward the yellow-orange region from 594 nm to 601 nm, where a C-band ASE source with spectral intensity fluctuation below 2 dB and a single-mode LD at 975.6 nm were applied. By carefully controlling the working temperature at 52 °C, maximum SFG power of 6.58 µW was obtained with a conversion efficiency of 232.08%/W, spectral bandwidth of 5.6 nm, and a spectral flatness of 0.143 (< 1.5 dB). The results could also be adopted for other broadband SFG process toward the visible and infrared regions for the scope of integrated optics.
Funding
the Self-deployment Project of Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China (2021ZZ104); National Natural Science Foundation of China (61875200, 61905246); National Defense Basic Scientific Research Program of China (JCKY2022210C007).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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