Demonstration of a violet-distributed feedback laser with fairly small temperature dependence in current-light characteristics

We have developed a GaN-based distributed feedback laser diode (DFB-LD) with the detuning of +5 nm to obtain a smaller temperature dependence of the threshold current. We found that the current-light characteristics almost overlapped up to 300 mW between 25 °C and 80 °C. The estimated characteristic temperature is about 2550 K. These indicate that our DFB-LD is promising for applications that require small temperature dependence in the output power and oscillation wavelength at constant operation current without precise temperature control.

U ntil now, 254 nm UV germicidal lamps have been widely used for sterilization purposes.However, when irradiating people, there was a risk of developing skin cancer and keratitis.[3][4][5][6] In addition, it was also verified to be effective against the new coronavirus. 2,4)This virus inactivation system that uses far UVC light can inactivate viruses without restricting people's activities in medical institutions, schools, public/commercial facilities, and food and beverage facilities, etc.It is attracting attention as a means of contributing to both the suppression of the pandemic and sustainable social activities.
Care222® is the world's first virus inactivation system that uses 222 nm ultraviolet light, and it is already being used in many situations. 7)The krypton chloride (KrCl) excimer lamp is used as a light source for this system.To further improve cost-effectiveness, module size, and power consumption, one possible method is to replace the KrCl excimer lamp with a semiconductor laser.10] However, it needs to shorten wavelength more for the safety of human exposure.Further breakthroughs are needed to develop a semiconductor laser that oscillates directly in 200-230 nm with sufficient output power.[13] Especially, the Cherenkov phase-matching which has the robustness to the wavelength variation of the pumping light source will lead to a cost-effective SHG module due not to the need for precise control of the case temperature. 13,14)In order to generate a second harmonic wave using Cherenkov phase-matching in a waveguide, such as investigated in Ref. 13, it will be desirable that the temperature dependences of the threshold current I th and output power are reduced as possible for easier temperature control and stable frequency conversion.
In this letter, we have developed a GaN-based DFB-LD with a plus detuning which is expected to reduce the temperature dependence of the I th and investigated the temperature dependence in the I-L characteristics.We found that the temperature dependence was fairly small between 25 °C and 80 °C for the DFB-LD with detuning of +5 nm.Furthermore, the characteristic temperature was about 2550 K, which was about 10 to 20 times larger than that of the GaN-based LDs ever reported. 28,30,31)igure 1 shows the schematic structure of our DFB-LD.This type of LD is a so-called laterally coupled (LC) DFB-LD. 19,22,24,26,27)The diffraction grating is formed by fabricating the periodic grooves at both sides of the ridge.[34][35] Although similar techniques may be applicable to the GaNbased DFB-LD when fabricating the diffraction grating, there are many technological barriers in terms of crystal regrowth.On the other hand, the LC DFB-LD does not require such a crystal-regrowth technique and is easier to fabricate, so we decided to apply this structure.The duty cycle is set to 0.8-0.9.This is because there is a concern that the smaller duty cycle will lead to a smaller coupling coefficient κ.The average refractive index of the diffraction grating layer is relatively lower when the duty cycle is small, and then the overlap between the diffraction grating layer and the lasing mode becomes small by being pushed down the lasing mode apart from the diffraction grating layer toward the n-side.The detuning ΔG, which is a parameter defined at 25 °C as a difference between the wavelength of the lasing mode λ DFB and that of the gain peak λ G just below the I th is set to be a plus since the temperature dependence in I th is generally expected to be reduced in such a detuning.
Our LC DFB-LD was fabricated as follows.Firstly, the InGaN-based LD structure was grown by MOCVD using commercially available c-plane GaN substrate.The InGaNdouble-quantum well (DQW) was applied as the active layer, and the peak wavelength of λ G was set to be approximately 410 nm.A 2 μm width ridge was then formed on the wafer using electron-beam (e-beam) lithography followed by a dryetching process.A regular third-order diffraction grating pattern was then formed at both sides of the ridge using ebeam lithography followed by a dry-etching process.In that lithography, the duty cycle was set to 0.8-0.9 as described above, and the period of the diffraction grating or pitch was set to have the detuning of ΔG ≅ +5 nm.Finally, the LD chips were fabricated through electrode processes on both the p-side and n-side.The cavity length and chip width of fabricated LDs were all 1000 μm and 200 μm, respectively.The front and rear facets were coated using dielectric films for anti-reflection (AR) and high reflection (HR) coatings with <1% and >95% reflectivities, respectively.Then, the fabricated chips were mounted on the AlN sub-mount and followed by being assembled on the TO-56 package.
Figure 2 shows the 45-degree-cross-sectional SEM images of the fabricated LD.As shown in the inset in Fig. 2 which shows the schematic of the observation direction, the bare chip was processed at a 45°angle to the ridge using a focused ion beam (FIB) etching system to make it easier to observe the diffraction-grating grooves and its cross section was observed.The depths of the diffraction-grating grooves were 52-92 nm, and the top-duty cycle, which means the duty cycle of the grating around the groove top, was 0.9 at both sides of the ridge.Note that the pitch Λ′ estimated from Fig. 2 and the actual pitch Λ is related to as Λ = 1/√2 × Λ′.
Figure 3 shows the temperature dependence of the I-L characteristics with ΔG ≅ +5 nm measured between 25 °C and 80 °C under continuous wave (CW) operation.The I-L characteristics above I th indicated linear characteristics without any kinks, which might mean that longitudinal and transverse single-mode operation was achieved within the operation current.As apparent from the figure, the I-L characteristics have almost overlapped each other to the extent that it is difficult to distinguish which straight line corresponds to which temperature condition.
Figure 4 shows the emission spectra measured with operation current I op ≅ 0.9I th at 25 °C.The λ G and λ DFB were at around 411.7 nm and 416.7 nm, respectively.This indicates that the detuning ΔG is +5 nm.Furthermore, the dip shown at around 416.5 nm would be the stop band, and its width was about 0.5 nm.This stop-band width is very large compared to that of GaN-based LDs reported. 18,24)This means that the coupling coefficient κ of our LD is very large as mentioned below.We will not discuss the estimation detail of the κ value of our LD here, because the stop-band width is affected by the facet phase as far as the reflectance of the facet coating is not zero, 36) therefore, we need to be careful when estimating the κ value using stop-band width obtained from the measurement.
Figure 5 shows the temperature dependences of the I th and slope efficiency (SE) which are obtained from Fig. 4. The characteristic temperature T 0 was estimated using th 0 The T 0 value of our laser was estimated to be 2550 K.This value is 10 to 20 times larger than that of GaN-based LDs reported. 28,30,31)Furthermore, SEs were estimated at around an output power of 250 mW were also almost independent of temperature and remained constant.
Figure 6 shows the emission spectra measured between 25 °C and 80 °C under CW operation, and the temperature dependences of the peak wavelength of the emission spectra.The longitudinal single-mode operation was confirmed up to 250 mW [Fig.6(a)].The estimated temperature coefficient of the wavelength shift was about 16 pm K −1 at each output power [Fig.6(b)].[19][20][21][22][23][24] The temperature dependence of the I-L characteristics is explained as follows.The λ DFB is also determined by not only the pitch but also the effective refractive index.Therefore, the λ DFB is increased with increasing temperature accompanied by temperature dependence in the effective refractive index.Similarly, the λ G is increased with increasing temperature accompanied by temperature dependence of the effective band-gap energy.The temperature dependence of the λ G is larger than that of the λ DFB for the GaN-based DFB-LD, as expected from the measurement result of the temperature coefficients of the oscillation wavelength of the DFB-LD and Fabry-Perot LD (FP-LD). 17)Generally, the smaller the absolute value of the difference between λ DFB and λ G , |λ DFB -λ G |, the better for laser oscillation.In other words, the I th is larger as |λ DFB -λ G | increases because it needs more injection current to reach threshold gain.However, in case of positive detuning such as ΔG ≅ +5 nm, as the λ G shifts toward a longer wavelength larger than the λ DFB with increasing temperature, |λ DFB -λ G | becomes smaller.As a result, the gain that increases with decreasing |λ DFB -λ G | compensates for the gain that decreases with increasing temperature.In other words, the proportion of the I th -increasing as the increasing temperature is suppressed due to decreasing |λ DFB -λ G |.In this way, the I-L characteristics should appear to be almost no temperature 052004-2 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd dependence and show a fairly large T 0 of 2550 K.With increasing ΔG further, such an LD should show smaller I th at higher temperatures compared with lower temperatures.Consequently, T 0 should be negative.We will report on ΔG dependence of the I-L characteristics and characteristic temperatures in the near future. 37)e above discussion is generally expected to occur with any DFB-LD in any wavelength band.However, it was more than we expected that not only I th but also SE was almost constant between 25 °C and 80 °C, and the I-L characteristics almost overlapped up to 300 mW.This interesting result must be due not only to very large ΔG, but also to achieving a fairly large coupling coefficient that oscillates even in very large ΔG.
At first sight, it seems that the value of ΔG ≅ +5 nm is not so large compared with that of other investigations on the InP-based DFB-LD.For InP-based DFB-LD, the result of the detuning of −25 nm was reported, for example, in Ref. 38.When discussing the physical effect of the detuning, however, it should be considered not as a wavelength difference which is often used for the useful index, but as an energy difference.This is because the gain can be thought of as a function that depends on energy, and then its width is also given in energy units.Therefore, the LD properties such as I-L characteristics are affected by the energy difference between DFB-lasing wavelength and gain-peak wavelength, rather than the wavelength difference itself which varies depending on the lasing-wavelength band.Based on this, ΔG ≅ +5 nm for the 410 nm band where GaN-based LD is used results in an energy difference of 36 meV, which is equivalent to ΔG ≅ +50 nm or +70 nm when converted to   The reason why the oscillation of the DFB-mode occurs steadily without that of the FP-mode or ASE even with such a fairly large energy difference should be, in the first place, that the gain region would be wider energetically for GaN-based materials or In-containing (Al, In, Ga)N alloy materials than other material systems such as In-based.The possible factor that widens the gain region would be due to spatial variation of the exciton energy in the active layers originating from both pronounced compositional inhomogeneity and intrinsic alloy broadening, 39) which was often considered to have a negative effect on optical gain properties for laser oscillation.
In addition, the reason why oscillation occurred even with the detuning with such a large energy difference was probably because a reasonably large coupling coefficient was obtained even with the laterally coupled diffraction grating described above (see Fig. 4 and its explanation), and the FP-mode must be well-suppressed by the present AR coating on the front facet.We considered that the above-mentioned characteristics acted positively, which led to the present interesting result.In summary, we investigated the temperature dependence in the I-L characteristics using the violet DFB-LD.We found that, for ΔG ≅ +5 nm, the temperature dependence in the I-L characteristics was fairly small, and the T 0 was estimated to be about 2550 K. Furthermore, it was more than expected that not only I th but also SE was almost constant within the measurement temperature range (25 °C to 80 °C), up to 300 mW.The temperature coefficient of the wavelength shift was about 16 pm K −1 .All these results indicate that our DFB-LD is promising for applications that require stable output power and oscillation wavelength in the wide temperature range without precise temperature control such as a light source for cost-effective and compact UV-C LD module based on the SHG using Cherenkov phase-matching.

Fig. 2 .
Fig. 2. 45-degree-cross-sectional view of SEM images.(a) One side of the ridge, and the inset figure shows the schematic of the observation direction.(b) Another side of the ridge viewed from the same direction.

Fig. 6 . 4 ©
Fig. 6.(a) Emission spectra under CW operation.(b) Temperature dependences of the peak wavelength at each output power.The dotted line is a guide to the eyes of the slope of 16 pm K −1 .