Wide Tunable Spectrum and High Power Narrowed Linewidth Dual-Wavelength Broad Area Diode Laser

Abstract

We demonstrate a dual-wavelength broad-area diode laser system with narrow linewidth and wide spectral tunability using a composite external cavity comprising a volume Bragg grating and a Littrow-type transmission grating. One wavelength is stabilized at 780.25 nm with a linewidth of ~0.13 nm, while the other achieves a continuous tuning range of 772.24–786.43 nm with a linewidth of ~0.17 nm. The system exhibits a side-mode suppression ratio exceeding 20 dB across the entire tuning range. At a dual-wavelength separation of 4.29 nm, the total output power reaches 2.62 W. Additionally, we successfully validate the system’s potential for nonlinear optical applications.

1. Introduction

Dual-wavelength diode lasers have garnered significant attention for applications such as terahertz generation, Raman spectroscopy, alkali laser pumping, etc., [1,2,3]. Currently, two principal methodologies are employed to generate dual-wavelength emission from diode lasers. One approach uses monolithic devices with customized internal structures [4,5], while the other employs external-cavity feedback. The typical performance and configurations of different dual-wavelength laser schemes are compared in Table 1.

In contrast to the limited output power inherent in internal cavity designs, external cavity configurations are more suitable for achieving watt-level power output [6]. Recent research trends indicate a clear evolution in dual-wavelength laser architectures: early systems employing dual diffraction gratings are being superseded by simpler composite cavities incorporating only one VBG and one reflective grating. Nevertheless, a common drawback of reflective grating-based setups is their folded optical path, which considerably increases system complexity. Furthermore, in the current architecture, the polarization properties of the diode laser are still employed to adjust the intensities of the two wavelengths, which could, in turn, constrain future power scaling and broader application.

Table 1. Evolution of dual-wavelength laser diode technologies: A Summary of key parameters and configurations.

Year	Reference	Wavelength Tuning Range	Power	Configuration	Key Feature
2008	Kim et al. [7]	3 nm		Micro-Heater	Internal cavity
2016	Sumpf et al. [8]	2 nm	0.2 W	Bragg reflector gratings	Internal cavity
2009	Zolotovskaya et al. [9]	3 nm	1.7 W	Multiplexed Bragg mirror	External cavity
2011	Chi et al. [10,11]	6 nm	1.5 W	Double Littrow gratings	External-cavity tapered amplifier
2011	Liu et al. [12]	2 nm	1.5 W	The reflective grating	Based on two diode laser arrays and a common external cavity
2016	Zheng et al. [13]	23 nm	0.04 W	The volume Bragg grating (VBG) and the reflective grating	Based on a double external cavity superluminescent diode (SLD)
2018	Zheng et al. [14]	2 nm	0.4 W	The volume Bragg grating (VBG)	External-cavity broad-area feedback (DFB) laser with a single external-cavity configuration
2022	Lu et al. [15]	15 nm	>1.8 W	The VBG and the holographic reflective grating	Realization of dual-wavelength output power adjustment based on polarization

Table 1. Evolution of dual-wavelength laser diode technologies: A Summary of key parameters and configurations.

Year	Reference	Wavelength Tuning Range	Power	Configuration	Key Feature
2008	Kim et al. [7]	3 nm		Micro-Heater	Internal cavity
2016	Sumpf et al. [8]	2 nm	0.2 W	Bragg reflector gratings	Internal cavity
2009	Zolotovskaya et al. [9]	3 nm	1.7 W	Multiplexed Bragg mirror	External cavity
2011	Chi et al. [10,11]	6 nm	1.5 W	Double Littrow gratings	External-cavity tapered amplifier
2011	Liu et al. [12]	2 nm	1.5 W	The reflective grating	Based on two diode laser arrays and a common external cavity
2016	Zheng et al. [13]	23 nm	0.04 W	The volume Bragg grating (VBG) and the reflective grating	Based on a double external cavity superluminescent diode (SLD)
2018	Zheng et al. [14]	2 nm	0.4 W	The volume Bragg grating (VBG)	External-cavity broad-area feedback (DFB) laser with a single external-cavity configuration
2022	Lu et al. [15]	15 nm	>1.8 W	The VBG and the holographic reflective grating	Realization of dual-wavelength output power adjustment based on polarization

In this paper, we propose a compact dual-wavelength laser with a composite external cavity combining a Littrow-type transmission grating (TG) and a reflection volume Bragg grating (VBG) in a collinear path. One wavelength is stabilized at 780.25 nm by the VBG, while the TG enables tuning of the other wavelength from 772.24 nm to 786.43 nm. The system achieves a high side-mode suppression ratio (SMSR) of 20 dB and a maximum output power of 2.65 W at a 4.29 nm wavelength difference, with a power imbalance of ~4%. Its potential for nonlinear optics is demonstrated through four-wave mixing (FWM)-based blue light generation in rubidium vapor.

2. Experiment

Figure 1 illustrates the architectures of the dual-wavelength composite external cavity. The packaged 100 µm × 3 mm single-emitter broad-area diode laser was mounted on a water-cooled aluminum block maintained at 25 °C. The fast-axis divergence was collimated to 10 mrad using a standard fast-axis collimator lens with an effective focal length of 350 µm. The beam quality factor M² was measured to be 34 in the slow axis, which was significantly larger than that in the fast axis. The laser beam is further collimated in the slow and fast axis using two cylindrical lenses ($C{L}_{SA}$ f = 50 mm; $C{L}_{FA}$ f = 60 mm), whose size is 12 mm × 1.5 mm.

The VBG is one of the diffractive elements with a reflectivity of 14% and spectral selectivity of 0.1 nm at 780 nm. It is placed perpendicular to the incident path and is 80 mm away from the diode laser through a collimation beam region. The 0-th order diffracted light is the only selected spectral component that returns to the emitter along its original optical path, while other spectral components are completely transmitted. The feedback from the VBG locks one wavelength to its central diffraction spectral line. Therefore, the VBG and the diode emitter constitute the first external cavity.

The other diffractive element is a Littrow-type transmission grating whose parameters and principle are the same as those described in [16]. It is positioned at the self-collimation Littrow angle of 53.16° and features a groove density of 2052 grooves/mm with dimensions of 25 mm × 25 mm × 1.5 mm. The element is located 240 mm away from the VBG. When the beam passes through it, there are four types of diffracted light, including −1 order reflected (−1R), −1 order transmitted (1T), 0 order reflected (0R), and 0 order transmitted (0T) diffracted laser. The −1R laser is fed back to the emitter, so the TG and diode laser form the second external cavity. The 0T order diffracted laser as the output of the whole external cavity diode laser system. The 0R, −1T order diffracted lights are the cavity losses. By adjusting the rotation angle of the TG, the wavelength selection can be achieved. Meanwhile, two different external cavities are spatially overlapped and spectrally separated. The composite cavities can oscillate at two laser wavelengths, and their spectral separation would be governed by the spatial overlap of the two cavities.

3. Results and Discussion

The broad-area diode laser spectrum in the free-running case and locked case is shown in Figure 2. In the free-running case, the diode laser exhibits a rather broad spectrum (~3 nm, FWHM) centered at 778.39 nm with the driving current of 7 A. Under the dual-wavelength operation, the VBG stabilizes one wavelength at 780.25 nm with a linewidth of ~0.13 nm, while the other wavelength is controlled by the TG and exhibits a linewidth of ~0.17 nm. The spectral profile is measured by an optical spectra analyzer (OSA) with the resolution of 0.02 nm. We have obtained the wide central wavelength tunable range spanning from 772.24 nm to 786.43 nm by adjusting the rotation angle of the TG in this experiment, as illustrated in Figure 3. The maximum difference between the dual wavelengths is 7.97 nm. Additionally, a high side-mode suppression ratio (SMSR) of dual-wavelength is better than 20 dB in the entire tunable range except for the wavelength difference of 6.18 nm. This means this wavelength is far away from the gain spectral range of the diode laser, which led to the SMSR decrease.

Note that due to the relatively low groove densities of the employed transmission grating used in the experiment and the output beam size of the diode laser being 1.5 mm, the current theoretical linewidth narrowing limit is 0.15 nm. To achieve further linewidth reduction under a fixed incident angle, two strategies may be implemented: increasing the grooves of grating or enlarging the laser beam size. Given the practical requirements for compatibility with VBG and system integration, the former is the more viable approach. Numerical simulations indicate that with the transmission grating with a groove density exceeding 3000 grooves/mm under a consistent beam size of 1.5 mm could theoretically achieve a linewidth reduction to below 0.1 nm.

We used a 10 W power meter to measure the output power of the composite external cavity diode laser and the individual cavity, respectively, for the dual-wavelength laser system with the driving current of 7 A, as shown in Figure 4. The total output power of the laser system gradually decreases with the dual-wavelength difference increasing as the wavelength is locked. The total output power peaks at 2.62 W when the wavelength difference is 4.29 nm, and remains above 2.37 W across the tuning range. The low output power is caused by two wavelengths being locked by VBG and the transmission grating being far away from the diode gain spectrum. It is unfavorable to mode competition.

Meanwhile, the power of two wavelengths can be adjusted by regulating the incident angles of two gratings in this system. Not only is it beneficial for power balance fine tuning, but it is also more suitable for semiconductor arrays and stacked arrays. The corresponding output powers of the two wavelengths are almost consistent in Figure 4 calculated through the peak-area method. The power imbalance between wavelengths is less than 4%, attributed to inter-cavity mode competition. Additionally, the material of VBG is photo-thermal-refractive (PTR) glass, which is more sensitive to temperature. By adding a dedicated temperature controller to the VBG, it will effectively reduce thermal drift, thereby significantly improving the independence and accuracy of the tuning process, and opening up broader application prospects for this configuration [17].

4. Application

Based on the FWM reference [18,19,20], we have conceived a new idea for FWM production with the help of the dual-wavelength laser. To verify the possibility of the method, we further added a 150 mm focal length plano-convex lens and an ${}^{85}\mathit{Rb}$ vapor cell with a diameter of 25 mm and length of 80 mm behind the dual-wavelength laser architecture, as shown within the red box in Figure 1. The beam size changes to 3 mm × 1 mm through the laser lens. The dual-wavelength laser locked at 775.95 nm and 780.24 nm corresponded to the rubidium $5P_{3/2}\to 5D_{5/2}$ and $5S_{1/2}\to 5P_{3/2}$, respectively, with the drive current of 5.4 A. The total output power was measured to be 1.84 W and the power of two wavelengths was nearly equal. The dual-wavelength laser is used to pump the ${}^{85}\mathit{Rb}$ vapor cell when the controller temperature is 413 K and a dicrotic mirror (HR 98%@ 420 nm and HT 96%@ 780/795 nm) was used to separate the pump laser and FWM output.

Lastly, we obtained the blue fluorescence and the spectrum of FWM, as shown in Figure 5, measured by a spectrometer with a resolution and accuracy uncertainty of 0.1 nm. It can be clearly seen that a bright fluorescence appeared in the vapor cell. Meanwhile, there is a peak in the output FWM spectrum, whose wavelength is 420.16 nm (in Air). The above results demonstrate the system’s capability for high-efficiency nonlinear conversion, paving the way for blue laser development [21].

In addition, the dual-wavelength laser can be further implemented in photonic crystal dielectric material systems. Leveraging its unique wavelength synergy, it enhances the optical field control and response sensitivity of the material, thereby significantly improving the performance of sensors based on such materials. This results in notable optimizations in detection accuracy, stability, and multi-parameter identification capability [22,23,24].

5. Conclusions

In conclusion, we present a novel composite external-cavity architecture for high-power dual-wavelength laser operation. The design achieves simultaneously a narrow linewidth and broad tunability, with one wavelength stabilized at 780.25 nm by a volume Bragg grating (VBG) and the other tunable from 772.24 nm to 786.43 nm via a thermal gradient control, providing a tuning bandwidth of ~14.19 nm. The respective linewidths are measured as ~0.13 nm and ~0.17 nm, while the optical signal-to-noise ratio remains above 20 dB across the entire tuning range. The system delivers a maximum dual-wavelength output power of 2.62 W at a wavelength difference of 4.29 nm and maintains a power level above 2.37 W throughout the operational band. Furthermore, its successful application in rubidium-based four-wave mixing, generating 420 nm light, confirms its strong potential for high-power rare-gas lasers, photonics crystals and nonlinear frequency conversion. This integrated architecture combines key advantages such as narrow bandwidth stability, wide spectral tuning, polarization insensitivity, and ease of operation, and can be applied to diode laser bars and stacks, thereby further enhancing the output power of the laser. This scalability opens a promising route to developing advanced high-power, dual-wavelength laser systems for applications in nonlinear optics and precision spectroscopy.