Monolithically integrated multimode interference coupler-based master oscillator power amplifier with dual-wavelength emission around 830 nm

A monolithically integrated dual-wavelength multimode interference coupler-based master oscillator power amplifier is presented. It consists of two shallowly etched, laterally separated ridge waveguide laser cavities as master oscillators with individual distributed Bragg reflector gratings as cavity mirrors. A deeply etched coupling section containing S-bend shaped waveguides and a multimode interference coupler is used to couple the laser emission of the master oscillators into a shallowly etched single waveguide serving as power amplifier. Changing the etch depth for the coupling section enables a compact device layout. In addition, increased radiation angles of modes not coupled into the power amplifier help to suppress beam steering, otherwise indicated by laterally separated far-field intensity distributions. The device provides 0.5 W of dual-wavelength emission around 830 nm in individual and common operation. As designed, both emission wavelengths are separated by 0.5 nm with spectral widths below 20 pm, limited by the spectral resolution of the spectrometer. Both peak wavelengths remain within spectral windows of 50 pm within the available power range. This enables full flexibility selecting operating points for applications such as shifted excitation Raman difference spectroscopy and the generation of THz emission by photomixing. The emission wavelengths can additionally be non-continuously tuned by applying a heater current to resistors implemented next to the distributed Bragg reflector gratings. As an example, selected spectral distances of 0.5 nm, 1.0 nm, 1.5 nm, and 2.0 nm are demonstrated. Near field widths of 5 μm and far field angles of 17° result in beam propagation ratios of 1.4 (1/e2) in all operation modes and enable easy beam shaping or optical single-mode fiber coupling.


Introduction
Dual-wavelength diode lasers are requested light sources for spectroscopic applications.In Raman spectroscopy, an alternating operation of both wavelengths enables to separate Raman signals from disturbances such as ambient light, fluorescence or fixed pattern noise, by using shifted excitation Raman difference spectroscopy (SERDS) [1].A light source suitable for Raman spectroscopy and SERDS with short measurement times, low target concentrations, or weak Raman scatterers, should ideally provide dual-wavelength laser emission with optical output powers of several hundred milliwatts.The required spectral distance and individual spectral widths should match the signal bandwidths of the targets under study.Spectral distances between both wavelengths of 3 cm −1 -10 cm −1 (0.2 nm -0.7 nm at 830 nm) for most solid and liquid samples and individual spectral widths < 1 cm −1 for the majority of samples are typically sufficient to resolve the Raman signals [2].Constant emission wavelengths within the available power range enable full flexibility in selecting operating points without requiring spectral recalibrations during measurements.
In THz spectroscopy, simultaneous dual-wavelength laser emission is used to generate THz radiation by photomixing [3].Here, an adjustable spectral distance between both laser emission wavelengths allows for frequency tuning in the application.A near-diffraction limited spatial beam quality additionally enables easy beam shaping and efficient single-mode fiber coupling, typically used in such THz setups.
Corresponding dual-wavelength Y-branch diode lasers suitable for both applications have been reported at different wavelengths [4][5][6].At 785 nm, an emission wavelength well-established for Raman spectroscopy, Y-branch distributed Bragg reflector (DBR) ridge waveguide (RW) diode lasers with up to 200 mW of optical output power have been demonstrated [7].They have been applied in Raman field experiments outside of a traditional laboratory environment [8] as well as in a proof-of-principle application study for the generation of THz radiation within a range of 0.1 THz -0.3 THz [9].The lasers were based on two DBR RW laser cavities coupled into a common output waveguide using a Y-branch coupling section with S-bend shaped waveguides.Applying a heater current to resistors implemented next to the DBR gratings enabled continuous tuning of the spectral distance between both laser emission wavelengths by Joule heating from 0.0 nm to 1.7 nm [10].
Regarding their spatial emission characteristics, these Y-branch diode lasers showed a power dependent lateral separation between the far field intensity distributions at both wavelengths [7], impeding efficient singlemode fiber coupling.As an alternative, monolithically integrated 785 nm dual-wavelength master oscillator power amplifiers (MOPA) with multimode interference (MMI) couplers have been introduced [11].The devices provided narrowband dual-wavelength emission with output powers of 0.3 W and beam steering, as observed for Y-branch lasers, was successfully prevented.
In this work, a monolithically integrated MMI coupler-based DBR MOPA emitting around 830 nm is presented.This includes the realization of a suitable ternary AlGaAs vertical layer structure with a quarternary quantum well and the design of a corresponding lateral-longitudinal device layout for processing.The MMI-MOPA is characterized in individual and common dual-wavelength operation.The wavelength is an established wavelength in Raman spectroscopy to reduce background disturbances by fluorescence and has been applied for analyzing biological targets [12][13][14].In contest to previous results, it additionally enables generating continuous wave THz radiation using cost-effective low-temperature grown gallium arsenide antennas optimized for photomixing at 830 nm [15,16].First, the vertical layer structure and lateral-longitudinal device layout are discussed.Afterwards, the measured electro-optical and spectral characteristics are provided.For the first time, the latter includes demonstrations of non-continuously tuned spectral distances for such devices by applying heater currents to the implemented resistors next to the DBR gratings.Lastly, the spatial beam characteristics are presented.

MMI coupler-based dual-wavelength MOPA
The vertical layer structure of the 830 nm device under study consists of an Al 0.45 Ga 0.55 As n-cladding, a 600 μm Al 0.40 Ga 0.60 As n-confinement, a 6 nm thick InGaAsP quantum well as active layer, a 500 μm Al 0.40 Ga 0.60 As p-confinement and an Al 0.45 Ga 0.55 As p-cladding (figure 1).
Uncoated and unmounted 100 μm wide broad area lasers with different resonator lengths were characterized in pulsed mode (5 kHz, 1 μs).A measured emission wavelength of 826 nm for 1 mm long devices meets the anticipated spectral range.For these devices, laser operation starts at about 300 mA and a slope efficiency of 0.6 W A −1 is measured.The differential efficiency is η d = 0.8.With a characteristic temperature of the threshold current of T 0 = 91 K, the devices show a good temperature stability.From measurements of slope efficiencies as a function of cavity length a high internal efficiency of η i = 0.9 and low internal losses of α i = 0.8 cm −1 are obtained.Measurements of threshold current densities as a function of the inverse cavity length result in a modal gain coefficient of Γg 0 = 11 cm −1 and a transparency current density of j tr = 100 A cm −2 .The measured vertical far field angle is 27°at full width at half maximum (FWHM).Based on these material parameters, the layer structure was found suitable for processing 830 nm devices.
Figure 2 shows the lateral-longitudinal layout (a) and the electrical contact pattern (b) of a monolithically integrated MMI coupler-based DBR MOPA.The MOPA has a total length of 6 mm.Two RW laser cavities with DBR gratings at both sides are operated as master oscillators (MO).Their lengths are L MO = 3 mm.Both waveguides are laterally separated by 80 μm and have ridge widths of 3 μm.Grating periods of about 1235 nm and 1236 nm for the 10th order DBR gratings are realized for dual-wavelength operation around 830 nm with a spectral distance of 0.5 nm (7.3 cm −1 , 0.2 THz).The lengths are L DBR-R = 0.75 mm for the gratings at the rear side (DBR-R) and L DBR-F = 0.25 mm for the gratings at the front side (DBR-F), respectively.The short length is expected to reduce the diffraction efficiency of the front side gratings by at least 50% compared to the rear side gratings.For higher diffraction efficiencies, the ridge widths at the rear side gratings increase from 3 μm to 10 μm towards the rear facet.Resistors implemented next to each grating enable applying heater currents for wavelength tuning by Joule heating.
A deeply etched coupling section containing S-bend shaped waveguides and a MMI coupler is used to couple the laser emission of the MOs into a single shallowly etched waveguide operated as power amplifier (PA).For the process, an etch depth difference of about 700 nm is intended.According to simulations (FIMMPROP, Photon Design Ltd.), the change in etch depth increases the radiation angle of modes not overlapping with the guided modes in the amplifier waveguide [11].A combination with a sufficiently long waveguide prevents the radiated light from reaching the front facet and therefore helps to avoid far-field modulations causing beam steering.In comparison to previously reported Y-branch diode lasers, the larger etch depth in the coupling section additionally enables more compact S-bend waveguides.For the current device, their lengths have been reduced from 2000 μm [7] to 430 μm.The lateral waveguide distance at the left side of the MMI coupler could therefore also be reduced to 3 μm, while still avoiding waveguide crosstalk.Because the design length of the coupler changes proportionally with the square of the MMI region width [17], the short waveguide distance at the coupler enabled compact coupler dimensions of L MMI x W MMI = 370 μm x 13 μm.Low-loss transitions between shallowly and deeply etched device sections are obtained by 100 μm long adiabatic waveguide converters, resulting in a total coupling section length of 1 mm.
The single waveguide operated as PA has a total length of L PA = 2 mm.After 1 mm towards the front facet, the waveguide is bent by 3°with respect to the longitudinal axis to reduce feedback effects from the front facet.
Separate electrical contacts are realized to control all device sections individually.This includes operations of the MOs (I 1 , I 2 ), the S-bend waveguides (I S-bend ), the MMI coupler (I MMI ), and the PA (I PA ).Separate heater currents can be applied to the resistors implemented next to each grating (I Heat1-R , I Heat1-F , I Heat2-R , I Heat2-F ).
The facets are anti-reflection (AR) coated with reflectivities < 1 × 10 −4 and the device is mounted p-side up on a CuW heat spreader on AlN on a 25 mm × 25 mm conduction cooled package.

Experimental results
For characterization of the continuous wave operation, the MOPA is placed on a Peltier-cooled heat sink that is set to a temperature of 25 °C.Similar to previous measurements for devices at 785 nm [11], the injection currents I S-bend and I MMI are set to 20 mA to obtain transparency.
Figure 3 shows the optical output power of the MOPA as a function of the individual injection currents I 1 (a) and I 2 (b).The latter are increased to 0.5 A in 10 mA steps while the injection current to the PA is kept constant at I PA = 0.5 A.
At both wavelengths, laser operation is obtained at injection currents of about 20 mA.As expected for MOPAs, the measured curves show decreasing slope efficiencies at higher injection currents as a result of amplifier saturation.Based on a formula for MOPA saturation powers [18], the saturation injection currents I Sat as a function of the MO injection current I i (i = 1, 2) can be calculated by Dashed lines in figure 3 show the corresponding numerical fits.Considering P 0 ≈ 0.19 W and P max ≈ 0.50 W for both wavelengths, saturation injection currents of I Sat ≈ 0.16 A are obtained.The coefficients of determination for the fits are R 2 98.3% and increase to R 2 99.1% when neglecting the mode hop related dips in the power current curves.In order to estimate the corresponding saturation power, cleaved and AR coated MOs have been measured.Assuming coupling efficiencies of 50% into the PA [11], a saturation power of about 40 mW can be expected.At 0.5 A injected into the MOs, optical output powers of P MOPA ≈ 0.49 W are obtained.
The optical output power of the MOPA as a function of the injection current I PA in individual operation (a) and common operation (b) is provided in figure 4. The injection current is increased to 0.5 A in 10 mA steps.For individual operation, L 1 and L 2 are operated separately at constant injection currents of 0.5 A. These injection currents exceed the determined saturation injection currents but were selected due to superior spectral characteristics, providing constant narrowband laser emission as discussed below.Without amplification, the obtained output power is 58 mW.As expected from the results in figure 3, output powers of P MOPA ≈ 0.49 W are obtained at injection currents of I PA = 0.5 A. In common operation, both MOs are operated simultaneously at a constant total injection current of 1.0 A. Here, 114 mW are emitted from the device without amplification.As expected, this is about twice the output power obtained in individual operation.At 0.5 A applied to the PA, the optical output power reaches 546 mW.This is close to the powers obtained in individual operation and indicates the effect of amplifier saturation.
Figure 5 shows the corresponding emission spectra, measured with a double Echelle monochromator with a spectral resolution of 11 pm at 830 nm (DEMON, LTB Lasertechnik Berlin GmbH).All peak intensities of the spectra are individually normalized to 1.For better visualization, the plot for individual operation in figure 5(a) shows the spectral characteristics obtained at both wavelengths.Here, narrowband dual-wavelength laser emission is obtained within the entire power range.At 0.4 A, a single mode hop of about 30 pm, matching the free spectral range expected for 3 mm long resonators, is observed for L 2 .Both emission wavelengths remain within spectral windows of 50 pm (0.7 cm −1 ).This enables full flexibility in selecting operating points for applications.As designed, a spectral distance of about 0.5 nm (7.3 cm −1 , 0.2 THz) between both laser emission wavelengths is obtained and emission wavelengths of 829.57nm (λ 1 ) and 830.04 nm (λ 2 ) are measured at I PA = 0.5 A. The spectral widths are below 20 pm (FWHM), limited by the spectral resolution of the spectrometer.Similar results are obtained in common operation shown in figure 5(b).Again, a spectral distance of about 0.5 nm is measured and emission wavelengths of 829.70 nm (λ 1 ) and 830.24 nm (λ 2 ) are measured at 0.5 A. All spectral widths remain at about 20 pm (FWHM).A < 0.2 nm emission wavelength shift in comparison to individual operation can be explained by simultaneous operation of both MOs and increased Joule heating within the device.Within the entire power range, both wavelengths remain within spectral windows of 30 pm (0.4 cm −1 ).
In comparison to previously reported Y-branch diode lasers, the resistors next to the DBR gratings enable discrete wavelength tuning by Joule heating.Figure 6 shows selected spectral distances of 0.5 nm, 1.0 nm, 1.5 nm, and 2.0 nm obtained for the MOPA in individual operation (a) and common operation (b) at 0.1 W. For this experiment, a heater current I Heat2-R is solely applied to the resistor implemented next to the rear side grating of L 2 .In individual operation, a heater current of 540 mA changes the corresponding emission wavelength λ 2 from 829.91 nm to 832.07 nm.Due to a thermal crosstalk, slight wavelength changes are also observed for λ 1 .
Here, emission wavelengths between 829.44 nm and 830.10 nm are measured.The previously discussed spectral  widths of 20 pm (FWHM) are maintained during these measurements.A similar spectral behavior is obtained in common operation.Here, a heater current of 540 mA changes λ 2 from 830.00 nm to 832.10 nm.The thermal crosstalk changes λ 1 from 829.48 nm to 830.12 nm.The demonstrated spectral distances result in spans of 7.3 cm −1 − 29.0 cm −1 and 0.2 THz -0.9 THz available for applications.
In order to evaluate the spatial beam characteristics, figure 7 shows the normalized lateral near field and far field intensity distributions at 0.4 W in individual operation (a) and common operation (b).All profiles have been measured according to the method of the moving slit (ISO Standard 11146) and the optical alignment remained unchanged while switching between operation modes.The near field intensity distributions show widths of 5 μm at the 1/e 2 level.Slight asymmetries on the lower right flank of the beam profiles originate from the single bent waveguide serving as PA.Measured far field angles of 17°at the 1/e 2 level result in beam propagation ratios of M 2  1/e 2 = 1.4.Applying the second moment criteria increases the beam propagation ratios  to M 2 2.Mom = 1.8.Overlapping far field distributions in individual operation beam profiles indicate the absence of beam steering, which enables efficient single-mode fiber coupling for applications.

Summary
A monolithically integrated dual-wavelength MMI coupler-based DBR MOPA was presented.The device provides 0.5 W and emission wavelengths around 830 nm, separated by 0.5 nm.The spectral widths are below 20 pm.Both peak wavelengths remain within spectral windows of 50 pm along the available power range.This allows for full flexibility in selecting operating points for spectroscopic applications such as Raman spectroscopy, shifted excitation Raman difference spectroscopy or the generation of THz emission by photomixing.Comparing the current results at 830 nm to previous achievements at 785 nm proves a successful concept transfer to a ternary AlGaAs vertical layer structure with a quarternary quantum well.In addition, the spectral distance between both wavelengths can be non-continuously tuned by applying a heater current to resistors implemented next to the distributed Bragg reflector gratings.As an example, spectral distances of 0.5 nm, 1.0 nm, 1.5 nm, and 2.0 nm were demonstrated.Near field widths of 5 μm and far field angles of 17°result in beam propagation ratios of 1.4 (1/e 2 ) in all operation modes and enable easy beam shaping or optical singlemode fiber coupling.

Figure 1 .
Figure 1.Schematic illustration of the vertical layer structure.

Figure 2 .
Figure 2. Schematic top view of the lateral-longitudinal device layout (a) and corresponding electrical contacts (b).Illustrations not to scale.

Figure 3 .
Figure 3. Optical output power of the MOPA as a function of the injection currents I 1 (a) and I 2 (b) in individual operation (I S-bend , I MMI = 20 mA, I PA = 0.5 A).Dashed lines indicate numerical fits to determine the amplifier saturation currents.

Figure 4 .
Figure 4. Output power of the MOPA as a function of the injection current I PA in individual operation (a) and common operation (b) (I S-bend , I MMI = 20 mA, I 1 , I 2 = 0.5 A).

Figure 5 .
Figure 5. Emission spectra of the MOPA as a function of the injection current I PA in individual operation (a) and common operation (b) (I S-bend , I MMI = 20 mA, I 1 , I 2 = 0.5 A).

Figure 6 .
Figure 6.Normalized emission spectra showing selected spectral distances between both laser wavelengths obtained for the MOPA by applying a heater current I Heat2-R to the rear side grating of L 2 in individual operation (a) and common operation (b) at P MOPA = 0.1 W.

Figure 7 .
Figure 7. Lateral near field intensity distributions (top) and far field intensity distributions (bottom) of the MOPA at P MOPA = 0.4 W in individual operation (a) and common operation (b).