Tunable GaN-Based Laser Diode

Transcription

1 Tunable GaN-Based Laser Diode Christoph Eichler We have investigated the wavelength tuning characteristics of a GaN-based cw-laser diode with an emission wavelength of 425 nm. Temperature dependent spectra have been measured and the shift of the longitudinal modes has been determined with a high resolution optical spectrum analyzer. By using the laser diode in a Littrow configuration [1] we achieved a tuning range of λ =4.37 nm. 1. Introduction Gallium-nitride based blue lasers are subject to intensive research because they are the key technology for the next generation of optical data storage [2]. Further areas of application include high resolution laser printers. Tunable blue lasers are interesting for example to analyze chemicals by their characteristic absorption spectrum. The emission wavelength of lasers can be tuned using different methods. One possibility is to change the operating temperature of the laser diode which results in a shift of the bandgap energy and a modification of the resonator length, thus affecting the emission wavelength. An other way is to apply an external, wavelength selective feedback to force a specific mode. In this work we first studied the temperature dependence of the emission wavelength and the resonator modes. In addition we operated the laser diode in a setup with wavelength selective feedback and investigated the tuning range and the side mode suppression ratio (SMSR) of this configuration. 2. Temperature Dependent Shift of the Emission Wavelength In order to determine the effect of the case temperature on the emission wavelength, we measured the spectrum of the laser at various temperatures in the range from 10 C to 30 C. The normalized spectra are shown in figure 1. The wavelength does not increase linearly with temperature, but shows mode hopping as can be seen in figure 1. This is caused by a shift of the gain curve with regard to the longitudinal modes of the resonator. Over the measured temperature range a shift of the emission maximum of 0.8 nm was observed (0.04 nm per Kelvin). This result is in good

2 2 Annual Report 2001, Optoelectronics Department, University of Ulm Fig. 1. Spectrum of the Nichia 425 nm laser at temperatures between 10 C and 30 C. Fig. 2. Wavelength shift of longitudinal modes of a Nichia 425 nm laser with increasing temperature. agreement with the equation of Varshni for the temperature dependence of the band gap E g (T ): E g (T ) = E 0 αt 2, (1) T + β where E 0 = ev, α = 8.32 ev/k and β = K [4, 5]. Using equation 1, an increase of the wavelength of nm is calculated for a change in temperature from 10 C to 30 C. The influence of the temperature on the Fabry-Perot resonator was examined by measuring the spectrum of the laser with a high resolution spectrum analyzer (Advantest Q8347, nm resolution) and tracking the wavelength shift of a single longitudinal mode with increasing temperature. Figure 2 shows that the mode shifts almost linearly to longer wavelengths with increasing temperature. A temperature coefficient dλ of nm/k dt

3 Tunable GaN-Based Laser Diode 3 is derived from the slope. First, heating of the device causes an expansion of the material and thus a physical extension of the resonator. Second, the refractive index of the material increases with higher temperatures, leading to an additional extension of the optical path inside the resonator. Assuming the change of the optical resonator length to be small and supposing a constant number of nodes of the standing wave, the position of the longitudinal modes in the spectrum will shift to longer wavelengths. 3. Laser in Littrow Configuration Another possibility to tune the emission wavelength is to use a selective feedback of the desired wavelength into the laser. Thus the gain at this wavelength is increased and the corresponding mode is preferred. Such a feedback can be realized using an external grating, which is described by the grating equation mλ = d(sin α + sin β), (2) where m is the order of diffraction, d is the grating constant, α is the angle of incidence, and β is the angle of the diffracted beam. For a given wavelength λ only those diffraction orders m are realizable where mλ/d < 2 is fulfilled. In a special case of equation 2 the light is diffracted back in the incidence direction, i.e. α = β. This arrangement is called Littrow configuration. The grating equation (2) is simplified to mλ = 2d sin α. (3) From this it follows that, after feeding back the diffraction order m = 1 into the laser, it is possible to tune the emission wavelength by rotating the grating which changes the angle of incidence α [1]. focussing lens fiber collimating lens laser on peltier-submount grating Fig. 3. Littrow-setup for tuning of the emission wavelength.

4 4 Annual Report 2001, Optoelectronics Department, University of Ulm Figure 3 shows a setup for a tunable laser based on the Littrow configuration. The position of the collimating lens and the rotation of the grating are controlled by Piezo actuators. This allows for optimum coupling into the laser and high resolution selection of the Littrow wavelength. The width of the tunable range of this configuration is limited by the shape and the width of the gain curve of the laser. To force a laser mode which is not in the maximum of the gain curve, the back diffracted power has to be large enough to reach the threshold condition of the desired mode. Otherwise the spectrum will be dominated by the mode which is emitted without grating. We used a grating with a groove spacing d = ( ) l mm = nm and coupled back the first order of diffraction m = 1 into the laser. The coupling was optimized by maximizing the output power, which is a result of a decrease of the threshold current by the back diffracted light. A reduction of the threshold current from 37.0 ma to 35.1 ma was evaluated. By coupling back a wavelength close to the emission maximum, the SMSR was increased to 24.7 db compared to 5.5 db of the laser without grating. This is due to the reduced resonator losses obtained by the grating feedback. Thereby the corresponding mode is enhanced and the other modes are suppressed. For modes with a larger distance from the maximum of the gain curve results a lower SMSR, because the losses compared to the wavelength without grating are larger (figure 4). The SMSR at the free running wavelength is only 3.4 db due to the spectral width of the diffracted light. Figure 5 shows the spectra of the laser diode for the shortest and the largest wavelength where stable operation has been achieved. The laser could be tuned from nm to nm. For wavelengths exceeding that range the emission wavelength of the laser without grating dominated the spectrum. As one can see in figure 5, at the edge of the tuning range the wavelength of the free running laser had already approximately the same intensity as the forced wavelength. Fig. 4. SMSR (side mode suppression ratio) of the Nichia 425 nm laser with grating. (gray: emission wavelength without grating)

5 Tunable GaN-Based Laser Diode 5 Fig. 5. The maximum tunable range of the Nichia 425 nm laser is 4.37 nm. 4. Conclusion We investigated the effect of the temperature on emission properties and measured an increase of the wavelength of 0.8 nm for a rise of case temperature from 10 C to 30 C. A temperature coefficient dλ = nm for the longitudinal modes was found. Applying dt K an external feedback an improvement of the SMSR of 19.2 db was achieved and the laser emission wavelength could be tuned over a range of 4.37 nm. References [1] C. Palmer, Diffraction Grating Handbook, Richardson Grating Laboratory, 4th edition, [2] Sony Deutschland GmbH, DVD Technologie, 2000 ( [3] S. Nakamura and G. Fasol, The blue laser diode: GaN based light emitters and lasers, Springer-Verlag, [4] S.C. Jain, M. Willander, J. Narayan, and R. Van Overstraeten, III-nitrides: Growth, characterization, and properties, J. Appl. Phys., vol. 87, no. 3, pp , [5] M. Leroux and B. Gil. Bandedge and optical functions of GaN, In J.H. Edgar, S. Strite, I. Akasaki, H. Amano, and C. Wetzel, editors, Gallium Nitride and Related Semiconductors. INSPEC, London, 1999.