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Abstract
A method for realizing a synchronized watt-level dual-wavelength vertical-external-cavity surface-emitting-laser (VECSEL) by using a tilted birefringence filter (BRF) is demonstrated. It is verified that by selecting suitable BRF material with different refractive index differences between extraordinary wave and ordinary wave, the dual-wavelength emission with a free spectrum range from sub-THz to tens of THz can be achieved. The output characteristics of such a dual-wavelength VECSEL are thoroughly investigated including its wavelength tunability and power difference. Finally, the intracavity optical parametric oscillator is applied to efficiently convert the dual-wavelength laser toward the mid-infrared region. The gain competition and longitudinal mode hopping performances for the multi-wavelength mid-infrared output are explored.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Dual-wavelength mid-infrared (mid-IR) laser source has special applications such as various gas detections [1,2]. However, generations of different wavelength regions as well as different free spectrum ranges (FSRs) are limited by the level transition of the laser gain medium. Vertical-external-cavity surface-emitting-laser (VECSEL) is a promising laser source thanks to its several desired characteristics [3,4]. The emission wavelength of the VECSEL can be flexibly designed from red to mid-infrared depending on its active layer [5–9]. Although the generation of VECSEL at the mid-IR region was still inefficient, such a wavelength range can be achieved by using a near-IR laser with the optical parametric oscillator (OPO) [10]. In addition, the broad gain bandwidth property of the VECSEL makes it an excellent source for mode-locked generation [11–14] as well as a tunable dual-wavelength source [15–24]. Synchronized dual-wavelength laser with proper free FSR can be applied in plenty of regions such as difference frequency generation (DFG) and terahertz (THz) technologies. It has been verified that a VECSEL is able to generate dual-wavelength emission with an FSR from sub-THz to tens of THz via multiple methods. However, these methods for generating dual-wavelength VECSEL typically require complicated designs such as multiple gain chips [17,18], separated resonators [19], or multiple active layer designs [20]. The complicated design is not suitable for further applications such as intracavity OPO where more complicated optics were required. As a result, it is of great interest to explore a simple method for obtaining dual-wavelength VECSELs.
An etalon is a common and widely utilized frequency control element in VECSEL for generating single- or dual-wavelength emissions [21,22]. However, the FSR of an etalon is limited by its thickness. In addition, the polarization state of the VECSEL cannot be controlled. Hence, an etalon is typically applied with other frequency control optics. Since the VECSEL emission FWHM was typically a few nanometers, it is very common to utilize a birefringence filter (BRF) to select the emission wavelength via rotation [3]. By tilting the BRF at Brewster angle, the VECSEL can further maintain linear polarized output while selecting the wavelength. By combining an etalon and a BRF, a linearly polarized dual-wavelength VECSEL has been demonstrated when the FSR of the etalon was smaller than the BRF transparent bandwidth [21]. However, we noticed the fact that BRF itself also has periodic transmission frequency peaks thanks to its wave retardation characteristics [23–26]. Although some of the former researches have demonstrated dual-wavelength near IR laser by using solid-state laser gain media [24–26], the applicable wavelength ranges cannot be flexibly adjusted. Furthermore, when the intracavity OPO method was applied to convert wavelength toward mid-IR regions, the strong coupling will lead to self-pulsing output [27,28]. Consequently, it is more suitable to utilize BRF in VECSEL to generate dual-wavelength emission and applied it for further mid-IR generation [10].
In this work, we thoroughly investigated the design criteria for using BRF for generating dual-wavelength VECSEL. In the past few years, the commonly used BRF material, Quartz, has been applied in broad-band solid-state lasers for obtaining dual-wavelength emissions [24–26]. The balanced peak intensity output for two colors can be achieved by adjusting the rotation angle of the BRF. It was further demonstrated that the FSR can be adjusted by changing the thickness of the BRF. However, we found that to cover the FSR to the sub-THz region, the required Quartz thickness will be significantly increased to a few centimeters. By analyzing the BRF characteristics, we proposed that selecting a proper BRF material can cover all capable FSR for the dual-wavelength output while maintaining the thickness. To be more specific, different FSR can be realized through the selection of birefringence material with proper refractive index difference between extraordinary wave and ordinary waves, ne-no. In our analysis, Quartz and KTP BRFs, with ne-no of 0.0087 [29] and 0.0918 [30], respectively, can result in FSR of one order difference whit similar thickness. Hence, the corresponding FSR when using KTP BRFs can be in the range of sub-THz while the thicknesses were barely a few millimeters.
Experimentally, we successfully demonstrated the synchronized dual-wavelength VECSEL with sub-THz FSRs by using various KTP BRFs. The total linearly polarized output power can easily achieve watt level. The output characteristics of the dual-wavelength VECSEL were fully investigated, including adjusting FSR, central wavelength, and intensity ratio. We further explored that with thicker KTP BRF, where the FSR became smaller, triple wavelength peaks can be induced at high pump power. We expected that further decreasing the FSR of BRF can generate multiple frequency outputs. Finally, the dual-wavelength near-IR VECSEL was applied with OPO to generate mid-IR emission. To improve overall efficiency, intracavity OPO is applied and some interesting spectral performances were explored. When strong depletion existed, the pump wave wavelength will have hole burning effect and hop to the next transmittance peak according to the BRF. Hence the overall bandwidth of the multiple-frequency output will become much larger. In our setup, up to five frequency peaks with equal spacing can be observed. In addition, the corresponding idler mid-IR output will also directly become multi-wavelength emission because of the quasi-phase-matching shared signal oscillation [31]. With further control methods, this phenomenon might be particularly useful when multiple frequency peaks with larger bandwidths were required, such as generating mode-locked pulse.
2. Experimental setup and analysis
Figure 1(a) illustrates the experimental setup of the dual-wavelength VECSEL. The pump source was a fiber-coupled laser diode at 808 nm with a maximum output power of 15 W. The fiber core diameter was 200 µm and the numerical aperture was approximately 0.22. A lens pair with two focus lenses was utilized to refocus the pump beam waist onto the gain chip with a ratio of 1. We utilized such pump diameter to avoid thermal rollover for the VECSEL according to previous experience [10]. The pump module was placed with an angle of approximately 30° and hence the pump spot was slightly elliptical. The laser resonator is a simple linear resonator with a gain chip, a BRF, and an output coupler (OC). The VECSEL gain chip consists of 12 InGaAs quantum wells resonant periodic gain structure operated at near 1060 nm and 28.5 pairs of GaAs/AlAs distributed Bragg reflector (DBR) for high reflection (HR, R > 99.9%) at 808 nm and 1060 nm [10]. The gain chip was flip-chip bonded onto a diamond heat spreader and the diamond was bonded onto a cooper water cooling holder. The upper layer of the gain chip has an anti-reflection (AR, R < 0.1%) coating at 808 nm and 1060 nm. Figure 1(b) shows the fluorescence of the gain chip at low pump power, which reveals that the gain bandwidth was up to 40 nm. The optical spectrum was measured by a grating optical spectrum analyzer with a minimum resolution of 0.01 nm (Ando AQ-6371b). The OC was a concave partial reflection mirror with a reflectivity of 98% near 1060 nm.
Fig. 1. (a) The experimental setup of the VECSEL with BRF and (b) the fluorescence spectrum of the gain chip.
The analysis of the BRF has been widely discovered in the past decade. The transmission central wavelength, λ, of the BRF can be shown as the following equation [23]:
It has been proven that by changing the thickness, d, of the BRF, Δλ can be adjusted [24–26]. However, when we required Δλ to decrease to a certain level, the BRF d must be significantly enlarged. Consequently, it is more suitable to change the BRF material with a different ne-no instead of changing d. Figure 2(a) illustrates Δλ with respect to ne-no at a fixed BRF d of 3 mm. We denoted some common nonlinear materials in the figure. We can find that the most common BRF material, Quartz, is suitable for Δλ range of tens of nanometers. For Δλ of nearly 5 nm, which corresponded to sub-THz FSR for 1-µm near-IR laser, KTP or BBO crystals will be more suitable. Further decreasing Δλ can be realized by using YVO4 crystal. Since the cost of the KTP crystal is more efficient than BBO and YVO4 crystals, we compared the difference between using Quartz and KTP BRFs. Notice that the KTP crystal was cut along the y-axis so that ne-no can be maximized [30]. In other words, the z-cut KTP crystal with ne-no of 0.0076 can cover the wavelength spacing to 40 nm, which was similar to that of Quartz crystal. Figure 2(b) depicts the wavelength spacing for Quartz and y-cut KTP crystals. Here, the utilized parameters were λ=1060 nm, no,Quartz = 1.5342, ne,Quartz = 1.5429, φB,Quartz = 57.1° [29], no,KTP = 1.7380, ne,KTP = 1.8298, and φB,KTP = 61.3° [30]. For Δλ = 5 nm, d of Quartz BRF must be up to 21.3 mm while the KTP BRF only required 2.1 mm. The analysis supports the fact that for realizing different Δλ orders, it will be more feasible to select a proper material first. To verify the wavelength selectivity of different KTP BRF dimensions more specifically, we also calculated the transmission, T, with respect to wavelength, which can be expressed as [23]:
From Eq. (2), we knew that as α approaches 0° or 90°, Δλ has maximum or minimum values respectively for a BRF with fixed d. Since we cannot operate at exactly 0° or 90°, we plot the transmission spectrum for α = 5° and 85° with d = 2 mm in Fig. 2(c), where the peak transmission separations, which was Δλ, was different. In addition, when d became 3 mm and 4 mm, Δλ became narrower. These parameters will be applied in the following experiment for further verification. It is worth mentioning that the transmission difference is simply 2% to 4%. Because the VECSEL is a low-gain material, such a small difference is able to suppress the laser output [3,4]. To enlarge the transmission difference, we can rotate α to near 45° where the modulation depth will be 100%.
Fig. 2. Theoretical analysis of wavelength spacing versus (a) ne-no and (b) BRF thickness for Quartz and KTP BRFs as well as the transmission spectra for (c) d = 2 mm KTP BRF operated at α = 5° and 85° and (d) d = 3 mm and 4 mm KTP BRFs operated at α = 85°.
3. Experimental results for dual-wavelength VECSEL with KTP BRF
According to Eq. (1), the transmitted wavelength can be adjusted by rotating the BRF angle, α. We utilized a y-cut KTP BRF with a thickness of 2 mm in the experiment first. The KTP BRF was placed at the Brewster angle of approximately 61°, which will be further optimized according to the laser power. The angle between the optical axis and the BRF surface was zero so Eq. (1) to (3) were valid [23]. Figure 3(a) depicts the optical spectra of the VECSEL with respect to the KTP BRF rotation angle at a pump power of 9.6 W. Because the rotation angle was sensitive, a high-precision mirror mount with <0.15° resolution was utilized for the optimization of the KTP BRF. However, we cannot precisely measure the angle of the mirror mount. Hence, we utilized the calculated rotation angle in Fig. 3(a), which suggested that the angle resolution was better than 0.15°. It was found that only when the emission peak was located in a wavelength range from 1057.5 nm to 1059.5 nm, the laser can generate single wavelength emission, which was the same as typical operation with a Quartz BRF. At other rotation angles, dual-wavelength emission was observed with different power ratios for two wavelengths. During the rotation, only one rotation angle leads to the balanced output peak intensity for the dual-wavelength pair. This result can be explained by the combined effects of the VECSEL gain profile and BRF transparent peaks. Since the VECSEL gain profile was not a top hat distribution, the balanced peak intensity was achieved only when two of the BRF transparent peaks match equal gain levels of the VECSEL. After one cycle of rotation, the laser wavelength keeps varying as shown in Fig. 3(a) but with slightly different Δλ. Δλ shown in Fig. 3(a) was approximately 4.6 nm. The maximum and minimum Δλ were achieved when α approaching 0° and 90°, corresponded to 5.8 nm and 4.6 nm, respectively. The optical spectra for these two rotation angles were shown in Fig. 3(b). We then applied KTP BRFs with different d to achieve Δλ adjustment. Figure 3(c) depicts the optical spectra when utilizing KTP BRFs with thicknesses of 3 mm and 4 mm, corresponding to wavelength spacing of 3.1 nm and 2.3 nm while α rotated from 90°. The Δλ measured in Fig. 3 were in good agreement with the calculated results in Fig. 2(c) and 2(d). The experimental result confirmed the feasibility of adjusting VECSEL dual-wavelength FSR by changing the thickness of BRF. In this experiment, the FSR tuning range was from 0.61 to 1.23 THz.
Fig. 3. The optical spectra of the VECSEL (a) when rotating 2-mm KTP BRF, (b) when operated at the balanced peak intensity at rotation angles of 0° and 90° for 2-mm KTP, and (c) when using 3-mm and 4-mm KTP BRFs at the balanced peak intensity.
After verifying the wavelength tunability of BRFs, we measured the output power of the dual-wavelength VECSEL. Figure 4(a) illustrates total output powers without BRF and with 2-mm BRF at balanced peak intensity output. When the BRF thickness increased to 4 mm, the output power will decrease by approximately 20%. We believed it came from the larger FSR that corresponds to the smaller photoluminescence intensity shown in Fig. 1(b). Without BRF, the threshold pump power was 1.06 W. It was observed that the VECSEL thermal rollover pump power was approximately 14 W. At the pump power of 13.6 W, the output power was approximately 4.07 W with a slope efficiency of 33.5%. After inserting BRF, the output power decreased to 3.53 W with a conversion efficiency of 25.9%. We did not measure the threshold pump power for the dual-wavelength emission because at low pump power, the VECSEL cannot maintain dual-wavelength output. It was found that at each pump power, the wavelength pair with balanced peak intensity cannot be adjusted. Since VECSEL emission wavelength will redshift when pump power, or chip temperature, increases, the dual-wavelength wavelength pair will also redshift. Figure 4(b) shows the wavelength pair of VECSEL at different pump power. We can clearly observe that the corresponding central wavelength will shift from 1055 nm to 1062 nm as the pump power increases. Consequently, after selecting the BRF, the emission wavelength of the dual-wavelength VECSEL can still be slightly adjusted. Notice that at each pump power, the BRF rotation angle must be further carefully optimized for balanced peak intensity output. It is observed that when different thickness BRF was applied, the output power will slightly vary. However, at the high pump power region, approximately larger than 10-W input, the VECSEL with 4-mm KTP BRF will generate multi-frequency output because the optical gain was high enough. Figure 4(c) shows the optical spectra for multi-frequency outputs. It might be capable to maintain dual-wavelength emission when an output coupler with lower reflectivity was utilized to suppress laser gain. However, it will require further exploration with various output coupler specifications.
Fig. 4. (a) Output powers versus input pump power for VECSEL, the optical spectra of (b) the wavelength red-shift property by using 2-mm KTP BRF at different pump powers, and (c) the multi-wavelength emission with 4-mm KTP BRF at high power.
Finally, we measured the stability of the dual-wavelength laser. Figure 5(a) and 5(b) illustrate the separated power stability with 2-mm KTP BRF. We separated two wavelength emissions by using a transmission grating. The temporal data was recorded by two fast InGaAs photodetectors (EOT ET-3010) with a high-speed digital oscilloscope (Lecroy, SDA6000A, 6 GHz bandwidth). From Fig. 5(a) and 5(b), we did not observe significant power instability or complementary between two wavelengths with an observation scale from nanosecond to millisecond. It is worth mentioning that the dual-wavelength VECSEL stability in such a linear resonator has been found with a temporal anticorrelated amplitude phase [15,16], which can be further stabilized with a feedback resonator. Since we did not perform the RIN spectrum measurement in this research because of the lack of equipment, we can only conclude that a simple method for generating linearly polarized tunable two colors output is provided. A further stabilizing method might be required when applying such a technique to high-stability THz applications. Nevertheless, we do verify the laser performances by applying the dual-wavelength VECSEL with OPO in the following.
Fig. 5. The temporal stability of the dual-wavelength VECSEL with time division of (a) 20 ns and (b) 2 ms.
4. Applications toward mid-IR
Here, we demonstrated the frequency conversion toward mid-IR to show the capable applications and to verify the quality of the dual-wavelength VECSEL. After obtaining dual-wavelength output, the laser can be directly applied with proper MgO:PPLN crystal and OPO setup for obtaining dual-wavelength mid-IR output. However, our current output power barely reached the threshold of the extracavity OPO conversion. As a result, we utilized the intracavity OPO method for more efficient outputs. It has been demonstrated that by using VECSEL as the pump wave material, the intracavity OPO conversion can generate actual continuous-wave output without a self-pulsing effect [10,27]. Figure 6(a) depicts the experimental setup of the intracavity OPO. A separator with >99% transmittance at 1064 nm and HR (R > 99.9%) coating at 1550 nm was applied to separate the pump wave and signal wave. The idler wave reflectivity of the separator was measured to be 35%. Two concave mirrors with a radius of curvature of 50 mm were applied as the OPO cavity mirrors. Both of them have HR (R > 99%) coating at 1064 nm and 1550 nm as well as HT (T > 95%) coating at 3300 nm. A MgO:PPLN crystal with an aperture of 1 × 1 mm and a length of 50 mm was used as the OPO crystal. The period of the periodic poling was 30.49 µm. Both of the end surfaces of the MgO:PPLN crystal were coated with broadband AR (R < 1%) coating at the pump wave, signal wave, and idler wave region. We placed the MgO:PPLN crystal onto a copper holder with an active temperature controller operated at room temperature. The cavity design of the OPO resonator can be found in Ref. 10. Figure 6(b) illustrates the idler wave output power from the share OC of the intracavity OPO with dual-wavelength output. We utilized an external filter to reflect the pump wave and signal wave leakage. Notice that approximately 0.35 times idler power can be observed at another arm of the OPO cavity due to the reflection of the separator. The diode pump threshold power for the dual-wavelength idler wave was approximately 2.68 W. At a maximum diode pump power of 13.6 W, the idler wave output power reached 153 mW, corresponding to a conversion efficiency of 1.13%. If considering approximately 50% loss from the filter and another arm, the total extraction efficiency can reach 2.26%.
Fig. 6. (a) The experimental setup and (b) the idler output power from share OC for the intracavity OPO VECSEL.
After measuring the power of the intracavity OPO, we revealed the wavelength competition for the dual-wavelength VECSEL. The optical spectrum for the OPO experiment was measured by a Fourier transform Michael interferometer optical spectrum (Thorlabs, OSA205C) which covered wavelength from near-IR to mid-IR. It has been demonstrated that by using such a quasi-phase-matching OPO, the dual-wavelength pump wave will contribute to the same signal wave oscillation and significantly decrease the required threshold for OPO [31]. Then a corresponding dual-wavelength idler wave should be easily generated. However, due to the fact that the VECSEL was a relatively low-gain material, in this experiment, we observed serious pump wave depletion and significant wavelength hopping after OPO was achieved. Since the BRF has a periodic transmitted spectrum, the pump wave depletion will cause the laser longitudinal mode directly hopped to the next transmitted peak and then multiple frequency peaks with equal spacing will be observed. In addition, because all of these pump wave peaks can share equal signal wave oscillation, corresponding multi-wavelength peaks of the idler wave can be directly generated. Figure 7(a) and 7(b) show the optical spectra of the pump wave without OPO operation at a pump power of 13.6 W by using 2-mm and 4-mm KTP BRFs, respectively, which was similar to that observed in Fig. 4. After careful alignment of the OPO OC, the idler wave can be obtained and the depleted pump wave optical spectra became multiple peaks as shown in Fig. 7(c) and 7(d). The final idler wave optical spectra can be found in Fig. 7(e) and 7(f), where the outer intensity of the frequency peaks can be optimized through the adjustment of BRF to become a symmetric near-normal distribution profile. It is worth mentioning that by using such pump wave depletion performance with BRF properties, the overall span bandwidth of the pump wave can be broadened from 2.3 nm to 9.2 nm. This frequency domain broadening behavior might be particularly useful for some applications, such as generating a narrower mode-locked laser if another mode control method was utilized. Finally, to obtain a dual-wavelength idler wave output, we have to optimize the 2-mm KTP BRF angle carefully. We found that when the idler wave became dual peaks rather than multiple peaks, the pump wave without OPO will be only single wavelength emission when we block the OPO OC at this moment. Figure 8 demonstrates the optical spectra for the pump wave with and without OPO operation, the shared signal wave, as well as the final dual-wavelength idler wave.
Fig. 7. Optical spectra for (a) and (b) pump wave without OPO, (c) and (d) pump wave with OPO, as well as (e) and (f) idler wave when utilizing 2-mm and 4-mm KTP BRFs.
Fig. 8. Optical spectra for (a) and (b) pump wave without and with OPO, (c) signal wave, and (d) idler wave when optimizing the intracavity OPO VECSEL to be dual-wavelength emission at idler wave.
It is worth mentioning that all of the laser peaks observed in this work should be in multi-longitudinal mode due to the lack of frequency filtering optics. Although additional etalon might help to maintain single longitudinal mode output for the pump wave, it can still suffer from the mode hopping effect after OPO was achieved and become the multi-longitudinal mode output eventually. On the other hand, if the signal wave was operated with single longitudinal mode output, the total conversion efficiency will decrease. More importantly, the signal wave oscillation might fail to meet the quasi-phase-matching condition for dual-wavelength shared oscillation. We depicted signal wave and idler wave wavelengths with respect to incident pump wave wavelength in Fig. 9 according to quasi-phase-matching conditions in MgO:PPLN crystal with our designed period of 30.49 µm [32]. It can be found that one signal wave wavelength only corresponds to one pump wave wavelength. However, since our signal wave was multi-longitudinal modes with a linewidth of approximately 1 nm, it corresponded to an acceptable pump wave wavelength bandwidth of nearly 15 nm with a central wavelength of 1060 nm. Hence, all of the pump wave wavelengths achieved in this work were in the acceptable bandwidth. In fact, as long as the signal wave linewidth became wider, the acceptable pump wave bandwidth can become larger. On the other hand, this method might not be able to generate single longitudinal mode idler wave pair. When a single longitudinal mode idler wave pair is required, a traditional method of using cascade PPLN or APPLN might be more appropriate [33].
Fig. 9. The OPO signal wave and idler wave wavelengths with respect to the pump wave wavelength when using a MgO:PPLN crystal with a period of 30.49 µm.
5. Conclusions
We have demonstrated a dual-wavelength VECSEL by using a single BRF. It has been analyzed that by selecting BRF material with suitable ne-no, the dual-wavelength FSR can be in a range from sub-THz to tens of THz while maintaining the thickness of the BRF. Theoretically, we showed the suitable BRF materials and capable FSRs for various conditions. Experimentally, dual-wavelength VECSEL with sub-THz FSR is achieved by using y-cut KTP BRFs. The laser performance including power level, wavelength adjusting, and FSR tunability were thoroughly investigated. We further observed the capability for generating multiple frequency outputs at high-power regions while FSR was small. Finally, we convert the wavelength toward mid-IR by using intracavity OPO. By careful optimization, the dual-wavelength mid-IR output can be obtained with large FSR BRF. However, because of the serious pump wave depletion while the operation of intracavity OPO, we further observed wavelength hopping when a small FSR BRF was used. Such an effect significantly increases overall wavelength bandwidth because the transparent peaks of the BRF determined the hopped wavelengths. At high-power regions, we can observe a maximum of five wavelength peaks with equal spacing generated from such a setup. Notice that only three peaks can be generated when operated with near-IR only. Furthermore, since the quasi-phase-matching OPO was utilized, all of these pump wave frequency peaks can share the same signal wave oscillation. And hence, the idler wave with multiple frequencies can be easily obtained. This interesting performance might be particularly useful for further experiments that require wavelength bandwidth expansion.
Funding
National Science and Technology Council (NSTC-111-2112-M-239-002-MY3).
Acknowledgments
The authors thank HC Photonics Corp. for supporting this experiment.
Disclosures
The authors declare no conflicts of interest
Data availability
Data underlying the results presented in this paper are available in Ref. [32].
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