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Abstract
An experimental comparison between individual and common wavelength-operation of a Y-branch distributed Bragg reflector (DBR) ridge waveguide (RW) laser at 785 nm with an electrically adjustable spectral distance is presented. The dual-wavelength Y-branch laser combines two laser cavities via a Y-section to a common output section. DBR gratings with different grating periods are associated with the two cavities, which set the emission wavelengths of the two branches. Implemented resistive heater elements allow separate wavelength tuning of the two branches, which can be operated individually for alternating emission wavelengths in applications such as differential absorption spectroscopy or shifted excitation Raman difference spectroscopy. Common wavelength operation simultaneously generates two emission lines suitable for the generation of THz radiation using difference frequency mixing. Hereby, the devices could potentially be used as single-chip light sources for a combination of Raman and THz applications. For the wavelength-operation comparison presented, the devices were operated up to optical output powers of about 105 and 185 mW in individual and common wavelength-operation mode, respectively. In individual operation mode, the devices show spectral single-mode emission over the whole operation range. In common operation mode, the spectral emission is predominantly single mode up to an optical output power of 65 mW. In both operation modes, mode hops typical for DBR lasers occur. At an optical output power of 50 mW, tuning of the spectral distance between the two wavelengths using the implemented resistor heaters is demonstrated. In both modes of wavelength operation, a flexible frequency difference between 0 and 0.8 THz (0 and 1.6 nm) with predominantly single-mode spectral emission is obtained.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. INTRODUCTION
Dual-wavelength diode-laser-based light sources have been presented for shifted excitation Raman difference spectroscopy (SERDS) or absorption spectroscopy. Both applications require an adjustable spectral distance between the two wavelengths according to the width of the spectral features under study.
In SERDS, the distance should be approximately the width of the targeted Raman lines [1–3]. The preferred difference between the two excitation wavelengths for measurements of solids and liquids can vary from about 3 to ${10}\;{{\rm cm}^{- 1}}$ (0.2 to 0.6 nm at 785 nm) [4] to ${160}\;{{\rm cm}^{- 1}}$ (10 nm at 785 nm) for the investigation of proteins [5]. 785 nm Y-branch DBR RW lasers emitting alternatingly at two slightly different wavelengths and a fixed spectral distance of about 0.6 nm were presented recently [6]. These devices reach an optical output power of about 215 mW. By implementing a common heater element next to both gratings, it was possible to adjust the spectral distance between the two emission wavelengths altogether up to 2 nm [7]. Therefore, these devices are well-suited for SERDS, as shown in [8,9].
In absorption spectroscopy, one approach is to use two wavelengths: one adjusted in the center of an absorption line (${\lambda _{\rm{on}}}$) and the other outside of it (${\lambda _{\rm{off}}}$). Here, the visible or near-infrared spectral region has the disadvantage in that only rather weak combination bands of interesting molecules can be used for the detection.
More promising would be the mid-infrared region with the fundamental ro-vibrational bands or even the rotational bands in the THz region, i.e., the frequency range below 3 THz. In this region, several gases have strong rotational lines suitable for the detection of the respective substances, whereas, in the mid-infrared nowadays, quantum cascade lasers (QCLs) working at room temperature are available, and the light sources for the THz range are even more challenging [10]. A potential pathway toward this region is the difference frequency generation of two simultaneously emitting wavelengths. Similar to the above-described Y-branch laser, dual-wavelength diode lasers in the near-infrared spectral range were presented for the generation of THz radiation [11–13]. Using a diode laser at 785 nm would require dual wavelength emission with spectral distances between 0.6 and 6.0 nm to cover the spectral range from about 0.3 up to 3 THz. The required antenna material for THz generation at that wavelength is available; recently, the generation of THz radiation and THz spectroscopy using a 785 nm Y-branch diode laser has been successfully demonstrated [14]. Moreover, 785 nm is one of the most common wavelengths used for Raman spectroscopy [15] and hence suitable for a combination of THz and Raman applications.
In this paper, a comparison of individual and common wavelength-operation modes of Y-branch-DBR-RW diode lasers emitting around 785 nm will be presented. This comparison offers a more complete overview of the features of the Y-branch-DBR-RW diode laser. The devices shown have separate heater elements implemented close to the two DBR gratings providing separately controllable wavelengths of the two branches in individual and common wavelength-operation mode. Based on the measurement of the temperature dependence of the devices without heater operation, the thermal impact of the heater current on the gratings will be quantified. The investigations presented here cover a tuning of the spectral distance from 0 up to 0.8 THz. The electro-optical and spectral results for individual and common wavelength-operation mode will be discussed with respect to their application in Raman spectroscopy, especially SERDS and for the generation of THz radiation. The Y-branch lasers presented could be feasible as a single-chip solution for combined Raman and THz applications and could also be designed for other emission wavelengths.
2. LASER DESIGN
The vertical layer structure for the devices under investigation is identical to those already described in [16,17]. The diode lasers are based on a GaAsP single quantum well active layer embedded into 500 nm thick waveguide and 1000 nm thick cladding layers formed by AlGaAs with different aluminum content. The whole structure was grown using metal organic vapor phase epitaxy. The vertical far-field angle of the structure is 29°, measured at full-width at half-maximum. The structure has a high internal efficiency of ${\eta _i} = {0.93}$ and low internal losses of ${\alpha _i} = {0.91}\;{{\rm cm}^{- 1}}$. The material has a temperature stability with a characteristic temperature of the threshold current of ${T_0} = {140}\;{\rm K}$.
A top view photo of the dual-wavelength Y-branch laser is shown in Fig. 1. The device is segmented into four sections. The first section from the left in Fig. 1 contains the two DBR gratings with the associated separate heaters. The DBR sections have a length of ${L_{\rm{DBR}}} = {500}\;{\unicode{x00B5}{\rm m}}$ and were manufactured as tenth-order deeply etched Bragg gratings using electron beam lithography. Different grating periods around 1210 nm of the two DBR gratings are designed for dual-wavelength emission of about 782.4 and 783 nm. At the back facet, the width of each of the two gratings is 10 µm. The width of the gratings is tapered down to the RW width of 2.2 µm. The reflectivity of the gratings was estimated to be about 30%. Next to each grating, a resistor heater H1 and H2 is implemented. The resistor heaters are comprised of a thin titanium layer, a thin platinum layer, and a gold layer as its main component. The respective heater currents are ${I_{{\rm H}1}}$ and ${I_{{\rm H}2}}$.
Fig. 1. Schematic drawing of the electrical contact scheme of a Y-branch DBR RW-laser for individual and common wavelength operation overlaying a top-view photo of the device.
The Y-branch ridge waveguides are based on a sine-generated curve to form S-shaped bends according to the formulae given by Liu et al. [18]. As illustrated in Sumpf et al. [19], the curved waveguide has an overall length of 2000 µm, followed by a 500 µm long straight section as a common output. At the rear side of the laser, the lateral distance between the two DBR mirrors is 80 µm.
The lengths of the electric contacts differ from the waveguide layout and are depicted as pump sections, grounded section, and output section in Fig. 1. The pump sections of the branches L1 and L2 have a length of ${L_{{\rm L}1}} = {L_{{\rm L}2}} = {1320}\;{\unicode{x00B5}{\rm m}}$. The two branches can be operated individually with the respective currents ${I_{{\rm L}1}}$ and ${I_{{\rm L}2}}$ or commonly in the case of common wavelength-operation mode with the respective current ${I_{{\rm L}1 + {\rm L}2}}$. A short passive section of ${L_{\rm Y}} = {380}\;{\unicode{x00B5}{\rm m}}$ is grounded. A 690 µm long section is used as an output section controlled by the associated current ${I_{\rm{OUT}}}$.
After passivating the facets as described in [20], the front and rear facets were antireflection-coated to a reflectivity of ${R_f} = {5}\%$ and ${R_r} \approx {5} \times {{10}^{- 4}}$, respectively.
The devices were soldered p-side up on expansion-matched CuW heat spreaders using AuSn. These subassemblies were mounted on AlN base plates and these plates mounted on a copper-based conduction-cooled package (CCP) using PbSn. The footprint of the CCP is ${25}\;{\rm mm} \times {25}\;{\rm mm}$. Wire bonds were used to apply electrical contacts to the pump sections, grounded section, output section, and heaters on the p-side.
Two different wavelength-operation modes were investigated in this work. Addressing the potential application of SERDS, the two pump sections L1 and L2 were controlled individually (marked “individual operation” in Fig. 1). In the case of a potential light source for difference frequency generation, the two pump sections L1 and L2 were operated in parallel (marked “common operation” in Fig. 1).
3. ELECTRO-OPTICAL AND SPECTRAL CHARACTERIZATION
In this chapter, the electro-optical and spectral characteristics are compared for individual and common wavelength operation of the branches L1 and L2. First, the device properties without operating the heaters are discussed. Second, the spectral tuning is presented when using the heater elements in individual and common wavelength-operation mode.
A. Without Heater Operation
The power-voltage-current characteristics for a Y-branch DBR-RW-laser measured with individual and common excitation of L1 and L2 at a temperature of $T = {25}^\circ {\rm C}$ are shown in Fig. 2.
Fig. 2. Power-voltage-current characteristics for the individually and commonly operated branches L1 and L2 of a Y-branch DBR-RW-laser at $T = {25}^\circ {\rm C}$, ${I_{\rm{OUT}}} = {35}\;{\rm mA}$.
The output section was driven with an injection current of ${I_{\rm OUT}} = {35}\;{\rm mA}$, selected according to the best spectral performance for individual wavelength operation in previous investigations. The comparison of individual and common wavelength-operation mode was performed up to an optical output power of 100 mW, obtained at injection currents ${I_{\rm{pump}}} = {I_{{\rm L}1}} = {I_{{\rm L}2}}$ around 200 mA. As the injection current is split between the branches L1 and L2 in common operation mode, a maximal injection current of ${I_{\rm{pump}}} = {I_{{\rm L}1 + {\rm L}2}} = {400}\;{\rm mA}$ was chosen for common wavelength-operation.
For individual wavelength-operation, both branches show a comparable behavior (see red and blue curves in Fig. 2). Laser operation for L1 and L2 starts at injection currents ${I_{{\rm L}1}}$ and ${I_{{\rm L}2}}$ of about ${I_{\rm{start}}} = {25}\;{\rm mA}$. The start current differs from the usually defined threshold current for regular laser resonators without any absorbing or constantly driven sections within the resonator. Measured between 75 and 175 mA, a slope efficiency of about ${S_{\rm{individual}}} = {0.60}\;{\rm W/A}$ is determined. This is in good agreement with the devices presented in [7]. At an injection current of ${I_{{\rm L}1/{\rm L}2}} = {200}\;{\rm mA}$, output powers of ${P_{{\rm L}1}} = {110}\;{\rm mW}$ and ${P_{{\rm L}2}} = {105}\;{\rm mW}$ are measured. The maximum wall-plug efficiency for the branches ${\eta _{{\rm L}1/{\rm L}2}}$ amounts to about 0.29. Taking the constant injection current applied to the output section into account, the maximum efficiency in individual wavelength-operation mode is ${\eta _{{\rm L}1/{\rm L}2}} = {0.23}$.
In common wavelength-operation mode, the threshold current density for laser operation is the same as in individual wavelength operation. Hence, the start current should double in common wavelength-operation mode due to the twice-as-big pump area compared with individual wavelength operation. As expected, the start current in common wavelength-operation mode increases by a factor of 2 compared with individual wavelength-operation mode and amounts to ${I_{{\rm L}1 + {\rm L}2}} = {50}\;{\rm mA}$. The slope efficiency ${S_{\rm{common}}}$, measured between 75 and 175 mA, remains at 0.60 W/A. At ${I_{{\rm L}1 + {\rm L}2}} = {400}\;{\rm mA}$, an output power of ${P_{{\rm L}1 + {\rm L}2}} = {185}\;{\rm mW}$ is reached. The maximal wall-plug efficiency in common wavelength-operation mode is ${\eta _{{\rm L}1 + {\rm L}2}} = {0.24}$. Taking the constant current applied to the output section into account, the overall efficiency for common wavelength operation is ${\eta _{{\rm L}1 + {\rm L}2}} = {0.21}$. For common wavelength operation, the shift of the maximum wall-plug efficiency current to a larger injection current is expected due to the increased pump area.
The corresponding spectral characteristics for individual wavelength operation are given in Figs. 3 and 4. Please note that, for better visualization, only spectra above the start current are shown.
Fig. 3. Spectral behavior for the individually operated branch L1 of the Y-branch DBR-RW-laser from Fig. 2 at $T = {25}^\circ {\rm C}$, ${I_{\rm OUT}} = {35}\;{\rm mA}$.
Fig. 4. Spectral behavior for the individually operated branch L2 of the Y-branch DBR-RW-laser from Fig. 2 at $T = {25}^\circ {\rm C}$, ${I_{\rm{\rm OUT}}} = {35}\;{\rm mA}$.
It can be seen that the emission close to the start current shows spectral positions of ${\lambda _1} = {782.31}\;{\rm nm}$ and ${\lambda _2} = {782.91}\;{\rm nm}$ for L1 and L2, respectively. This results in a spectral distance of $\Delta {\lambda _{12,{\rm individual}}} = {0.60}\;{\rm nm}$. The measured side-mode suppression ratio (SMSR) close to the start current is about 20 dB. Along the operating range, the single-mode spectra shift to longer wavelengths. Mode hops occur, which are typical for the emission of DBR diode lasers (exemplarily highlighted in pink in Figs. 3 and 4). At a mode hop, the emission wavelength shifts to a shorter wavelength. The spectral distance covered by these mode hops is about 30 pm, which is in good agreement with the resonator length of $L = {3}\;{\rm mm}$. The spectra show narrow spectral widths of $\delta _{1/e2} \lt {25}\;{\rm pm}$, measured at the ${{1/e}^2}$ level and limited by the spectrometer (DEMON, LTB Berlin). Along the whole operating range, four mode hops are observed for L1 and two for L2. These result in a minor increase in spectral distance when increasing the injection current and herewith increasing the output power. At the maximum current ${I_{{\rm L}1/{\rm L}2}} = {200}\;{\rm mA}$, emission wavelengths of ${\lambda _1} = {782.38}\;{\rm nm}$ and ${\lambda _2} = {783.03}\;{\rm nm}$ are measured leading to a spectral distance of $\Delta {\lambda _{12,{\rm individual}}} = {0.65}\;{\rm nm}$. The SMSR is 30 dB at ${I_{{\rm L}1/{\rm L}2}} = {200}\;{\rm mA}$.
Fig. 5. Spectral behavior for the commonly operated branches L1 and L2 of the Y-branch DBR-RW-laser from Fig. 2 at $T = {25}^\circ {\rm C}$, ${I_{\rm OUT}} = {35}\;{\rm mA}$.
Figure 5 shows the spectral behavior for common wavelength operation of both branches L1 and L2 above the start current. At a current of ${I_{{\rm L}1 + {\rm L}2}} = {60}\;{\rm mA}$, both branches are active, and the peak emission wavelengths are ${\lambda _1} = {782.32}\;{\rm nm}$ and ${\lambda _2} = {782.91}\;{\rm nm}$ for L1 and L2, respectively. This leads to a spectral distance of $\Delta {\lambda _{12}} = {0.59}\;{\rm nm}$, comparable with individual wavelength operation. At an injection current of ${I_{{\rm L}1 + {\rm L}2}} = {400}\;{\rm mA}$, the peak emission wavelengths are ${\lambda _1} = {782.48}\;{\rm nm}$ and ${\lambda _2} = {783.09}\;{\rm nm}$ for L1 and L2, leading to $\Delta {\lambda _{12}} = {0.61}\;{\rm nm}$. Up to an optical output power of ${P_{{\rm L}1 + {\rm L}2}} = {65}\;{\rm mW}$ (${I_{{\rm L}1 + {\rm L}2}} = {160}\;{\rm mA}$), the spectral emission of both branches is predominantly single mode, and the spectral width is comparable with the spectral width in individual wavelength-operation mode. As in individual wavelength-operation mode, spectral mode hops occur. The SMSR is about 15 dB at the start current and more than 25 dB at ${I_{{\rm L}1 + {\rm L}2}} = {160}\;{\rm mA}$. In contrast with individual wavelength operation, side modes appear for ${I_{{\rm L}1 + {\rm L}2}} \gt {160}\;{\rm mA}$. However, the multimode behavior differs for emission wavelengths ${\lambda _1}$ and ${\lambda _2}$. For ${\lambda _1}$, pronounced multimode behavior occurs for injection currents above 160 mA. The mode spacing amounts to about 30 pm, which again is in good agreement with the resonator length of 3 mm. For ${\lambda _2}$, the SMSR is at least 12 dB along the whole operating range. The shown difference in the spectral behavior for common wavelength-operation compared with individual wavelength-operation is suspected to be caused by the fact that both branches contribute to the resonator. In individual wavelength-operation mode, the non-active pump section is absorbing. Hence, all light coupled into the nonactive pump section does not affect the active resonator. In the case of common wavelength-operation mode, light from branch L1 can partly be coupled into branch L2 and vice versa. The resulting perturbation presumably causes the multimode spectral behavior. This work focuses on the comparison of the two types of wavelength-operation modes for the operating parameters, which proved to be sufficient for individual wavelength operation in the past. The influence of different section currents on the spectral behavior in common wavelength operation and its potential improvement will be investigated elsewhere.
For estimating the wavelength dependence on temperature, Fig. 6 shows the peak emission wavelength versus the heat sink temperature for both branches L1 and L2 operated individually and commonly. The injection currents through the pump sections and the output section are kept constant at ${I_{{\rm L}1/{\rm L}2}} = {150}\;{\rm mA}$ or ${I_{{\rm L}1 + {\rm L}2}} = {150}\;{\rm mA}$ and ${I_{\rm OUT}} = {35}\;{\rm mA}$, respectively. These currents are chosen, as they ensure laser wavelength-operation at all selected heat sink temperatures. The emission wavelengths of the device increase approximately linear with heat sink temperature. A tuning rate of about 0.05 nm/K can be determined from the slopes. This is in good agreement with [21].
Fig. 6. Emission wavelength versus temperature $T$ for individual and common wavelength-operation of L1 and L2 at constant injection currents of ${I_{{\rm L}1/{\rm L}2}} = {150}\;{\rm mA}$ or ${I_{{\rm L}1 + {\rm L}2}} = {150}\;{\rm mA}$ and ${I_{\rm OUT}} = {35}\;{\rm mA}$.
Fig. 7. Emission spectra for L1 when applying a heater current ${I_{{\rm H}1}}$ to heater H1 at ${P} = {50}\;{\rm mW}$.
B. Heater Operation
As mentioned, tuning of the spectral distance is needed for different applications. By applying current to one of the resistive heaters, the temperature is increased locally, and the emission wavelengths shift toward longer wavelengths. The following investigations are performed at an optical output power of 50 mW. At this output power, spectral single-mode emission in individual and common wavelength-operation mode without heater operation is observed.
Figures 7 and 8 show the emission wavelengths when applying a heater current ${I_{{\rm H}1}}$ to heater H1 in individual wavelength-operation mode. For better visualization, only heater currents above 100 mA are shown; below ${I_{{\rm H}1}} = {100}\;{\rm mA}$, no shift in wavelength occurs. The branches are operated individually at injection currents of ${I_{{\rm L}1}} = {I_{{\rm L}2}} = {90}\;{\rm mA}$ with ${I_{\rm{\rm OUT}}} = {35}\;{\rm mA}$ ($P = {50}\;{\rm mW}$). A heater current ${I_{{\rm H}1}}$ up to 650 mA is applied, which corresponds to a heating power ${P_{{\rm H}1}}$ of about 1 W and a resistance ${R_{{\rm H}1}}$ of about 2.2 Ω at ${I_{{\rm H}1}} = {650}\;{\rm mA}$. It can be seen that the wavelength emitted from branch L1 shifts 1.53 nm from 782.31 nm up to 783.84 nm. Using the above-mentioned tuning rate, a temperature increase in the grating section up to 56°C can be assumed. Due to the small lateral distance between the two DBR gratings of only 80 µm and the absence of any additional thermal isolation, a thermal crosstalk occurs, and the emission from L2 shifts 0.58 nm from 782.90 to 783.48 nm. This corresponds to a temperature increase up to 37°C. Since the DBR grating of the branch L1 is designed for the shorter wavelength, the spectral distance between the two emission wavelengths first decreases and shows 0 nm at ${I_{{\rm H}1}} = {540}\;{\rm mA}$. Afterward, the spectral distance increases again. The spectral emission is predominantly single-mode. Only for emission wavelength ${\lambda _1}$, side modes occur at sporadic heater currents.
Fig. 8. Emission spectra for L2 when applying a heater current ${I_{{\rm H}1}}$ to heater H1 at ${P} = {50}\;{\rm mW}$.
Figures 9 and 10 illustrate the emission wavelengths when applying a heater current ${I_{{\rm H}2}}$ of up to 650 mA to heater H2 in individual wavelength-operation mode. The heating power ${P_{{\rm H}2}}$ amounts to about 1 W, and the resistance ${R_{{\rm H}2}}$ to about 2.2 Ω at ${I_{{\rm H}2}} = {650}\;{\rm mA}$. As expected, the spectral distance between the two wavelengths increases by increasing the heater current. A maximum spectral distance of $\Delta {\lambda _{12}} = {1.66}\;{\rm nm}$ is reached at heater current ${I_{{\rm H}2}} = {650}\;{\rm mA}$. Similar to operating heater H1, the temperature at the directly heated DBR grating increases to 58°C and to 36°C for the other DBR grating when operating heater H2.
Fig. 9. Emission spectra for L1 in individual wavelength-operation mode when applying a heater current ${I_{{\rm H}2}}$ to heater H2 at ${P} = {50}\;{\rm mW}$.
Fig. 10. Emission spectra for L2 in individual wavelength-operation mode when applying a heater current ${I_{{\rm H}2}}$ to heater H2 at ${P} = {50}\;{\rm mW}$.
For both emission wavelengths, the spectral emission stays in single-mode along the operating range with one exception for emission wavelength ${\lambda _1}$ at ${I_{{\rm H}2}} = {380}\;{\rm mA}$. For branch L2 in individual wavelength-operation mode, the output power of the device decreases from 50 mW at about 782.9 nm down to about 44 mW at an emission wavelength of about 784.6 nm. This is due to the gain spectrum of the lasing material having a peak wavelength of about 770 nm and a width (FWHM) of about 40 nm.
In Figs. 11 and 12, the emission wavelengths for common wavelength operation of the two branches are depicted. Again, the selected optical output power is 50 mW, which is reached at an injection current of ${I_{{\rm L}1 + {\rm L}2}} = {130}\;{\rm mA}$ and ${I_{\rm{\rm OUT}}} = {35}\;{\rm mA}$. When using heater H1, the spectral distance between the two emission lines can be decreased. In Fig. 11, the emission spectra are plotted against the heater current ${I_{{\rm H}1}}$ from 100 to 575 mA. For ${I_{{\rm H}1}}\lt {500}\;{\rm mA}$, the emission is predominantly single-mode.
Fig. 11. Emission spectra in common wavelength-operation mode when applying a heater current ${I_{{\rm H}1}}$ to heater H1 at ${P} = {50}\;{\rm mW}$.
Above 500 mA, the emission shows multimode behavior until a single-mode spectrum emerges at ${I_{{\rm H}1}} = {555}\;{\rm mA}$ with only one measurable emission wavelength ($\Delta {\lambda _{12}} = {0}\;{\rm nm}$). Above 575 mA, the spectra show multimode behavior (not shown here to ensure visualization of the single-mode behavior at ${I_{{\rm H}1}} = {555}\;{\rm mA}$), and the spectral distance increases like in individual wavelength-operation mode when using heater H1.
Using heater H2 in common wavelength-operation mode (Fig. 12), the spectral distance increases with increasing heater current ${{I}_{{\rm H}2}}$ like in individual wavelength-operation mode. The emission is predominantly single mode, but side modes occur for both emission wavelengths at sporadic heater currents. The maximal spectral distance for common wavelength-operation mode is $\Delta {\lambda _{12}} = {1.64}\;{\rm nm}$ at ${{I}_{{\rm H}2}} = {650}\;{\rm mA}$, which is comparable with individual wavelength-operation mode.
For comparison of the heater-based spectral tuning for individual and common wavelength-operation, the wavenumber distance and frequency difference versus heater current are shown in Fig. 13.
The device offers flexible wavelength adjustment in both modes of wavelength operation. As expected, the quadratic dependence of spectral distance on heater current is the same for individual and common wavelength operation. For SERDS, a tunable wavenumber distance from ${0}\;{{\rm cm}^{- 1}}$ to ${27}\;{{\rm cm}^{- 1}}$ in individual wavelength operation is accessible. The common wavelength-operation mode can be used for generating an adjustable beat frequency in the range from 0 to 0.8 THz. A larger difference could be easily obtained by increasing the spectral distance between the processed DBR gratings. For the devices presented here, the spectral distance is expected to be further enlarged with heating powers extending this investigation.
Fig. 12. Emission spectra in common wavelength-operation mode when applying a heater current ${I_{{\rm H}2}}$ to heater H2 at ${P} = {50}\;{\rm mW}$.
Fig. 13. Spectral distance of the emission wavelengths versus heater currents ${I_{{\rm H}1/{\rm H}2}}$ for individual and common wavelength operation of L1 and L2 at ${P} = {50}\;{\rm mW}$.
4. SUMMARY
In this paper, individual and common wavelength operation of dual-wavelength Y-branch DBR lasers were compared. The devices offer an electrically adjustable spectral distance between the emission wavelengths from the two branches. For common wavelength operation, it is possible to adjust the wavelength distance from $\Delta {\lambda _{12}} = {0}\;{\rm nm}$ up to $\Delta {\lambda _{12}} = {1.64}\;{\rm nm}$. When exciting the branches individually, a wavelength distance up to $\Delta {\lambda _{12}} = {1.66}\;{\rm nm}$ can be achieved. The output power in these experiments amounts to 50 mW at a heat sink temperature of ${T} = {25}^\circ {\rm C}$. With respect to the potential application of these devices for SERDS, this corresponds to a wavenumber distance of about ${27}\;{{\rm cm}^{- 1}}$, which meets the requirements of several liquids, solid samples, and partly also biological material. Applying a heater current around the zero-distance region would, e.g., deliver a light source well-suited for the generation of THz radiation over a tuning range up to 0.8 THz. Hereby, the devices presented offer a single-chip solution for the potential combination of Raman spectroscopic and THz applications, e.g., for a combined substance and structural analysis. The concept is transferable to other emission wavelengths.
Funding
Horizon 2020 Framework Programme (667933-2); Deutsche Forschungsgemeinschaft (DFG SU 144/12-1); Bundesministerium für Bildung und Forschung (16FMD02).
Acknowledgment
The authors would like to thank all co-workers at the FBH for the support of this work.
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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