A red-emitting tapered diode laser with a monolithically integrated distributed Bragg reflector grating is presented. The device is able to emit up to 1 W of spectrally stabilized optical output power at 5°C. Depending on the period of the tenth order surface grating the emission wavelengths of these devices from the same gain material are 635 nm, 637 nm, and 639 nm. The emission is as narrow as 9 pm (FWHM) at 637.6 nm. The lateral beam quality is M2(1/e2) = 1.2. Therefore, these devices simplify techniques such as wavelength multiplexing and fiber coupling dedicating them as light sources for µ-Raman spectroscopy, absolute distance interferometry, and holographic imaging.
©2012 Optical Society of America
Red-emitting diode lasers are used in a great variety of applications because of their inherent advantages such as high visibility, high efficiency, small size, robustness and ease of use.
But when applied in such fields as µ-Raman spectroscopy, (absolute distance) interferometry, pointing and range finding, laser metrology, and holography, red-emitting diode lasers have to meet additional requirements as a stable, narrow spectral linewidth at moderate output power and good beam quality. In the past, HeNe-lasers and red-emitting ECDLs  have been used. Recently, we reported on DBR-grating stabilized ridge waveguide lasers at 635 nm with an optical output power of nearly 100 mW adding longitudinal mode filtering to a single, monolithic device .
For applications which require an even higher output power and good beam quality, e.g. display applications, tapered diode lasers (TPLs) are key devices [3–6]. The TPL consists of a ridge waveguide (RW) section using index-guiding for the mode cleaning of the light and a tapered (TA) section for the amplification of the output power. But conventional TPLs lack the spectral selectivity, required e.g. for Raman spectroscopy, holography or wavelength multiplexing techniques. Prior to this, ECDL setups  and a monolithically integrated flared amplifier master oscillator power amplifier (MFA-MOPA)  were reported to achieve a suitable spectral mode filtering. The MFA-MOPA is a monolithic device which consists of two distributed Bragg reflector (DBR) grating sections serving as a back mirror and as an output coupler towards the tapered gain section. The grating structure was holographically defined, wet etched and overgrown by MOCVD. The device exhibits 250 mW of single spatial mode and a single frequency emission at 664 nm.
In this paper, we introduce DBR-TPLs emitting around 635 nm with a monolithically integrated DBR surface grating section acting as a highly reflective rear mirror, and a wavelength selective element offering high spectral radiance at pre-designed discrete wavelengths. We present details of the manufacturing of the DBR-TPL as well as the measured electro-optical properties. The characteristics of the DBR-TPL will be compared to a conventional Fabry-Perot (FP) resonator TPL with coated front and rear facets.
2. Epitaxial structure
The investigated devices of the DBR-TPL and the FP-TPL are from equal epitaxial structures, which were fabricated by the use of metal-organic vapor phase epitaxy in an Aixtron AIX 200/4 horizontal reactor (3 × 2-inch-configuration). A GaAs substrate with a 6° offcut towards (111)A-direction was used as basis for the vertical structure (Fig. 1 ) which is described in detail in Ref .
A 15 nm thick GaInP single quantum well is placed in the middle of an AlGaInP waveguide. The lower layer is n-doped with Si, the upper is p-doped with Zn. The n-doped cladding is made of AlInP and GaInP, whereas the p-cladding consist of AlGaAs. The p-type doping level is achieved by setting the epitaxial growth parameter resulting in an effective carbon doping. The layer structure was finalized by a highly doped GaAs capping layer.
FP-TPLs were processed from wafer A, an earlier growth run, whereas the DBR-TPLs are made from wafer B. To test the quality of the epitaxial material of wafers A and B, broad area lasers with 100 μm stripe width and a resonator length of 2000 μm were processed. The uncoated laser chips were characterized in pulsed mode (5 kHz, duty cycle 0.5% at 20°C). The characteristic data are summarized in Table 1 .
One observes that all parameters are reproduced within their respective uncertainty. The internal efficiency ηi and the internal loss αi are calculated using the dependence of the inverse differential efficiency on the resonator length. The modal gain coefficient Γg0 and the transparency current density jTR are determined using the logarithm of the normalized threshold current as a function of inverse resonator length assuming a logarithmic dependence of the current density on the modal gain. Temperature-dependent measurements of the power-current-characteristics provide a threshold-related characteristic temperature T0. Resonator length and temperature-dependent measurements are only available for wafer A.
3. Device fabrication
The design of the DBR-TPLs is shown in Fig. 2 . The AlGaAs-cladding of the used epitaxial structure gives the opportunity to use standard reactive ion etching (RIE) by Ar and Cl to carry out the structuring of both, the grating and the ridge waveguide (RW) of the DBR-TPL devices. Therefore, standard photolithography resist is usable as etch mask. This is beneficial in terms of mass- and low-costs-production.
The process starts with the definition of the DBR-sections of the DBR-TPLs by the use of an i-line stepper (NIKON NSR-2005i10C). Therefore, grating periods of Λ = 972 nm, 976 nm, and 980 nm were chosen to obtain tenth order surface gratings. The photomask gratings were transferred into the epitaxial structure by BCl3-Ar-RIE (see Fig. 1). Further details on the fabrication of the DBR surface gratings can be found in Ref . A high grating order of a DBR, such as the tenth order used here, requires a high duty cycle to provide sufficient reflectivity. The reflectivity of similar DBR-gratings were reported to be around RDBR ≈60% .
All further process steps defining RW- and TA-sections of the DBR- and FP-TPLs were carried out in an almost identical manner. The total length of our DBR-TPLs is 2.25 mm with a RW-section of 0.25 mm, and a DBR surface grating section of 0.5 mm. The FP-TPLs were 2 mm long in total with a 0.5 mm long RW-section. The width of the ridge is 5 µm for the DBR- and the TP-TPL, respectively. The TA-section had a full taper angle of 4° and a length of 1.5 mm for both types of devices. All facets were passivated and coated. The front facet reflectivity was chosen to be 1% for each. The rear facet reflectivity was manufactured to be > 90% for the FP-TPL and < 0.1% for the DBR-TPL. All investigated TPLs were mounted p-side down on a structured CVD-heat spreader and soldered to a conductively cooled package (CCP). This allowed for an optimum of heat removal from the laser chips and individual drive currents for the RW- and TA-sections.
4. Optical properties of the DBR-TPLs
In Fig. 3(a) the optical output power as a function of the current through the tapered section (ITA) of a DBR-TPL with Λ = 976 nm is compared to that of a FP-TPL. Both TPLs were operated with separate contacts for RW- and TA-section. The RW-current was optimized for both lasers to be 80 mA for the DBR-TPL and 20 mA for the FP-TPL. The temperature of the heat sink was stabilized with a Peltier element to 15°C. Figure 3(b) shows the power-current (P-I)-characteristic of the DBR-TPL for heat sink temperatures between 5°C and 30°C.
Figure 3(a) illustrates that the threshold current of the DBR-TPL is higher (ITA = 992 mA vs. 682 mA) than the one from the FP-TPL, and the slope is less steep (sDBR = 0.51 W/A; sFP = 0.82 W/A). Taking into account the similar characteristic data of structure A und B summarized in Table 1, this indicates a lower reflectivity of the DBR-grating compared to that of the rear coating of the FP-TPL. The threshold currents and the slopes were deducted only from a linear fit to the first part of the P-I-characteristics (violet lines: ITA, DBR-TPL: 1.05 to 1.7 A and ITA, FP: 0.75 to 1.4 A) because the curves of both TPLs deviate from the ideal for higher TA-currents at 15°C. The slope of the characteristic curve of the DBR-TPL decreases due to thermal roll-over. The thermal roll-over effect is reduced significantly when the heat sink temperature is lowered, allowing an output power of more than 1.1 W at 5°C (Fig. 3(b)).
The P-I-characteristics of the FP-TPL at ITA > 1.4 A show several kinks which can be attributed to sudden spectral shifts over several nanometer in the spectrum (shown later in Fig. 4 ). The P-I-characteristics of the DBR-TPL demonstrate power undulations with increasing taper current ITA. This behavior is caused by thermally and carrier induced refractive index shifts and is typical for DBR-laser devices .
4.2 Spectral behavior
The spectral behavior of the DBR-TPL has been recorded with a double Echelle monochromator which has a resolution of λ/105 (λ = 635 nm, ΔλEchelle ≈6.5 pm). The spectra of the FP-TPL were recorded with a miniaturized grating spectrometer with a resolution of 0.1 nm.
The color contour plots in Fig. 4 show the spectral behavior of the DBR- and FP-TPLs over the current of the TA-section at 15°C for a fixed current through the RW-section (DBR: IRW = 80 mA; FP: IRW = 20 mA). While the FP-TPL exhibits serveral mode jumps over a wide spectral range (Fig. 4(a)), the spectral emission of three DBR-TPLs with grating periods of Λ = 972; 976; 980 nm is stabilized by the DBR-grating (Fig. 4(b)).
The color contour plot in the inset in the right of Fig. 4(b) shows the spectral behavior of a DBR-TPL with a grating period of Λ = 976 nm. The device displays a general red shift of λDBR = Δλ/ΔITA ≈91 pm/A (linear fit) that is attributed to a cross-heating of the DBR-section from the TA-section. Additionally, one can observe a fast shift of the emission wavelength by λf ≈299 pm/A (linear fit). Mode hops of the lasing spectra reoccur; they are of the same origin as the oscillations in the P-I-characteristic. The distance of the mode hops (Δλ = 18 pm) corresponds to the cavity resonance between front facet and DBR-grating. For a detailed description of the nature of the individual characteristics of the spectrum of DBR-TPL we refer to .
Figure 5 compares the spectral emission of the developed DBR-TPLs (grating period Λ = 972; 976; 980 nm) to that of a FP-TPL at 15°C at a fixed TA-currents. (ITA, DBR = 2.2 A; ITA, FP = 1.5 A).
Because of its unstable resonator geometry, the FP-TPL shows a broad spectral emission (ΔλFP-TPL = 364 pm, FWHM), while the three DBR-TPLs with the same broad gain spectrum from a comparable epitaxial structure show a narrow emission on three discrete emission wavelengths: 635.6 nm, 637.6 nm, and 639.6 nm. The inset in Fig. 5 is a magnification of the DBR-TPL emitting at 637.58 nm with a FWHM of 9 pm, limited by the resolution of the spectrometer.
4.3 Beam quality
TPLs are key devices for applications that require high power densities in almost diffraction limited beams e.g. for coupling into single mode or low mode number fibers. To assess the beam quality we used the method of the moving slit. We investigated the intensity profiles of the near field (Fig. 6(a) ), beam waist (Fig. 6(b)), and far field (Fig. 6(c)) of a DBR-TPL with Λ = 976 nm (λ = 637.2 nm) at a heat sink temperature of 5°C to determine the beam quality factor M2 according to the 1/e2-level and second order moments criteria (σ) (ISO11146).
At an output power of 1 W a beam quality factor of M2(1/e2) = 1.2 and M2(σ) = 4.6, respectively, is determined. Beam quality factors for further operating points (OP) – i.e. power levels at a certain heat sink temperature – are displayed in Table 2 . The power level is given as the optical output power in the dominant central lobe (CL) of the beam waist. It provides an estimate for the usable diffraction-limited power e.g. for single-mode fiber coupling.
The fraction of the central lobe of the (DBR-)TPLs with a full taper angle of 4° exceeds 80% for selected OPs. The beam quality factors according to the 1/e2-level are approximately 2 for the FP-TPL, whereas the beam quality factor of the DBR-TPL is almost diffraction limited. When examining the M2-criterion according to second order moments, which gives more weight to intensity further away from the centre, the M2(σ)-value of the DBR-TPL remains almost constant at 4.7 (OP 1.1 to 1.2). Here, the M2(σ) of the FP-TPL deteriorates from 5.1 to 8.4 (OP 2.1 to 2.2) for an increased pumping current of the TA-section. To explain this, the intensity distribution of the DBR- and FP-TPL at the beam waist position for increasing pumping current of the TA-section (T = 15°C) are shown in Fig. 7 .
Figure 7(a) shows that the DBR-TPL features small side lobes next to the central lobe at any TA-current, but the side lobes do not increase significantly with ITA. In contrast to this the FP-TPL does not exhibit significant side lobes at low power, but at higher TA-current the fraction of the intensity outside of the central lobe increases strongly (Fig. 7(b)). To quantify the changes, the intensity distributions at the beam waist position are evaluated according to the 1/e2-level and second order moments criteria (σ) and are shown in Fig. 8 as a function of the TA-current.
The diagram in Fig. 8(a) makes it apparent that the beam waist diameter of the DBR-TPL varies only little, whereas the beam waist of the FP-TPL increases with TA-current, in particular according to the second order moments criteria (Fig. 8(b)).
As the lateral beam quality is mainly determined by the index guiding of the RW-section, which has a 5 µm wide ridge for the DBR- as well as the FP-TPL, this improvement in beam quality may result from the fact that for the DBR-TPL the rear-side reflector behind the RW-section is realized by the narrow DBR surface grating with a width of 5 µm. Thus, only the fundamental mode is reflected properly. All higher modes exit the laser’s rear facet due to the anti-reflection coating. In contrast to this, the FP-TPL has a high reflective coating across the whole width of the rear facet and no modes are favored. Hence, higher order lateral modes may propagate more freely in the FP-TPL and are suppressed in the DBR-TPL.
5. Spectral radiance
Based on the power-current-characteristics (ch. 4.1), the spectral behavior (ch. 4.2), and the beam quality (ch. 4.3) it is possible to calculate the radiance β, the luminance L, and the spectral radiance βλ .
The radiance is defined as where we choose P corresponding to the power in CL. The beam quality factors according to the 1/e2-criterion given in Table 2 are used for the lateral direction M||2 (i.e. parallel to the plane of the epitaxial layers). The beam propagation in the vertical direction is determined by the epitaxial structure. Therefore, it is identical for DBR- and FP-TPLs. M⊥2 is derived from a caustic measurement by a CCD camera, where M⊥2 was 1.02. The visibility is the responsivity of the human eye according to CIE1931 . The visibility multiplied by the radiance results in luminance (), giving a measure for the light density perceived by a human eye. To calculate the spectral radiance, the FWHM of the spectrum Δλ given in Fig. 5 is used (). Table 3 shows the values for the radiance (or brightness) β, the luminance L, and the spectral radiance βλ of the DBR- and FP-TPLs at the OPs introduced in Table 2.
Table 3 points out that the radiance of the DBR-TPL surpasses the one of the FP-TPL already at 15°C, which becomes even more pronounced when cooled to 5°C. Here, a radiance of β = 168 MW/cm2/sr is reached. The internal grating also reduces the wavelength shift with current, which is beneficial for the visibility: The visibility of the FP-TPL degrades by about 10% between the two OPs (2.1 and 2.2), limiting the increase in luminance. The main advantage of the grating is the narrow spectral emission, leading to an improved spectral radiance of βλ = 18.6 GW/cm2/sr/nm of the DBR-TPL, which is almost a hundred times the value of the FP-TPL.
6. Summary, conclusion and outlook
In summary, we developed red-emitting TPLs with monolithic tenth order DBR surface gratings by standard i-line lithography and dry-etching techniques. These DBR-TPLs are able to emit up to 1 W optical output power at 5°C and 637 nm. In comparison with a standard FP-TPL the DBR-TPL has a narrowed spectral linewidth of less than 9 pm and an improved beam quality of M2(1/e2)horz. = 1.2. Because of the spectrally stabilized, high power emission in conjunction with the good beam quality a record spectral radiance of βλ = 18.6 GW/cm2/sr/nm could be demonstrated.
This dedicates these novel red-emitting DBR-TPLs for a large variety of applications like holographic imaging, µ-Raman spectroscopy or absolute distance interferometry. Furthermore, it potentially simplifies techniques such as fiber coupling, due to the improved beam quality over FP-TPLs, or wavelength multiplexing, to generate high radiance beams with low speckle contrast for flying-spot display applications.
In the future, we would like to further enhance the radiance of our DBR-TPLs by optimizing the temperature behavior, the grating reflectivity and the geometry of the TA-section, leading to a reduced laser threshold and reaching even higher output powers.
We would like to thank the German Federal Ministry of Education and Research (BMBF) for funding this work in the initiative “InnoProfile” (Grant No. 03IP613).
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