-- Main.demar182 - 04 May 2015

Development of a Wavelength Tunable Laser Using External Optical Feedback

Luke A. DeMars & Erik L. Tilseth

University of Minnesota

Methods of Experimental Physics Spring 2015

Abstract

The goal of this project was to develop a tunable light source with a monochromatic output by coupling a superluminescent diode, with center wave length of 830 nm and bandwidth of 30 nm, and a diffraction grating to make an external optical feedback cavity. By developing this our group has effectively made multiple laser sources in one cavity. The results from the tunable light source showed a mode spacing of 0.35 nm measured over a range of 20 nm. From this measurement it was calculated that the cavity length of the SLD is 281.09 μm.

Introduction

Superluminescent diodes are a light emitting semiconductor device that has a large bandwidth and high output power as can be seen in figure 1 a) [1]. Due to the mixture of these properties an SLD is an ideal device for developing a tunable laser. In order for the SLD to produce highly coherent light similar to a laser source it had to be coupled to an external feedback cavity [1,2]. In the first stage of this project an external cavity was formed by coupling the SLD with a mirror. As shown in figure 1 b), the external cavity only allowed certain frequencies (modes) to oscillate. With this multimode light source our group characterized the SLD’s threshold current at which lasing occurs, the non-lasing and lasing spectrum, and the optimal amount of light to feedback into the cavity in order to minimize power loss. After the multimode laser was characterized, the mirror was then replaced with a diffraction grating so that single mode lasing could be achieved as seen in figure 1 c).

Figure 1, The progression of the SLD output throughout the project. a). SLD output without external feedback. b). SLD output with external feedback. c). Tunable laser output.

Theory

A superluminescent diode, shown in Figure 2, consists of a pn junction containing a waveguide [3]. When a potential difference is applied across the pn junction, holes from the p type material are injected into the n type material and electrons from the n type material are injected into the p type material. As a result, electrons and holes combine and in order to conserve energy and momentum light is generated by both spontaneous and stimulated emission.

Figure 2, A diagram of the SLD shows that when light is generated by injecting current into the SLD the light is dircted out of the partially transmitting facet. This structure suppresses multiple reflections with in the cavity, and as a result produces mostly spontaneous emssion.

In a semiconductor device, spontaneous emission occurs when an electron randomly falls from the conduction band into the valence band and combines with a hole. Where as stimulated emission occurs when a photon with energy equal to the bandgap energy of the SLD is incident on an electron in the conduction band. The electron is then excited down to the valence band and combines with a hole. Due to this excitation, two photons of the same momentum, energy, and phase of the incident photon are produced [4].

The SLD by itself produces mostly spontaneous emission due to the waveguide having a partially transmitting facet which suppresses stimulated emission. The spontaneous emission bandwidth is determined by the band structure of the SLD as can be seen in figure 3 [4]. As described by equation 1, the energy of the photon that can be produced via spontaneous emission is limited by the band gap energy, Eg, and the quasi fermi levels, Fc and Fv, of the SLD. The band gap energy is the minimum energy difference between the conduction and valence band, and the quasi fermi levels describe the chemical potential of the electrons and holes in the conduction band and valence band respectively.

(1)

Figure 3, A diagram of the energy vs. momentum band structure of the SLD. The energy levels of the band structure limits the energy of photon produced during spontaneous emission.

The moment at which the SLD switches from producing majority spontaneous emission to stimulated emission is known as lasing. By producing majority stimulated emission the bandwidth of the SLD will narrow and become laser like. Lasing is achieved in a semiconductor laser by running the device at threshold current. For the SLD though, in order to reach lasing an external optical feedback cavity had to be added to the system in order to overcome the suppression of stimulated emission from the partially transmitting facet of the SLD. By adding an external cavity two restrictions were placed on which modes will oscillate inside the SLD.

The first restriction is that only half wavelength, λ, integers can exist inside a cavity of length L as shown in equation 2. This results in a discrete value of mode oscillations possible, and the frequency spacing between the modes is described in equation 3 below where n is the index of refraction of the semiconductor and c is the speed of light.

(2)

(3)

The second restriction is that only modes that overcome the gain threshold will oscilate as shown in figure 4. The gain threshold is the total amount of loss in your resonator, and loss inside of the cavity is mostly due to absorption and scattering.

Figure 4, Only the modes above the gain threshold will oscillate. For example, in the diagram above only the three modes above the black line would be outputed by the SLD.

Experimental Apparatus: Multimode Laser

The experimental setup that was used to determine the SLD non-lasing/lasing spectrum, the optimal amount of light feedback, and threshold current spectrum is shown in Figure 5. Light left the SLD through a collimating lens, and then reflected off a gold mirror placed opposite to the SLD. The light reflected by the gold mirror was then directed back in to the SLD by adjusting the tilt of the mirror. A neutral density filter was placed in the external cavity for two reasons. The first reason was to reflect light out of the external cavity to analyze its spectrum with an Ocean Optics Spectrometer which has a resolution of 1.5 nm. Power measurements were also made with this apparatus by replacing the spectrometer with a power meter. The second reason for implementing the ND filter was to be able to control how much light was reflected back into the SLD. By controlling the amount of feedback the power lost was minimized and the output power of the multimode laser was maximized [2].

Figure 5, This is the apparatus that was used to characterize properties of the SLD. Note that gold mirrors are used because the SLD operates in IR, and gold will reflect most of this light.

Experimental Appartus: Tunable Laser

The apparatus used to study the wavelength tuning properties of the SLD is shown in figure 6. The gold mirror was replaced with a diffraction grating in order to tune the wavelength of the SLD output. The grating was set in the Littrow configuration, meaning the first order of the diffracted light was directed along the axis of the incident beam [5]. In order to reflect the first order back into the SLD the grating had to be at a particular angle called the blaze angle. The blaze angle, 28 degrees in this case, is determined by the structure of the diffraction grating as described in equation 4 and pictured in figure 7.The zeroth order of the diffracted light was then guided into a Ocean Optics spectrometer, with a resolution of 0.4 nm, using two gold mirrors. Note that a higher resolution spectrometer was needed in this case because the bandwidth of the tunable laser was smaller than that of the multimode laser.

Figure 6, A diagram of the tunable laser apparatus.

(4)

Figure 7, A diagram of the structure of a blaze grating. Blaze gratings are desinged to distribute the majority of light diffracted into the first and zeroth order.

Results and Analysis

The output spectrum without external feedback SLD is shown in figure 8. The results show that the non-lasing spectrum of the SLD has a center wavelength of approximately 830 nm, and a half power width of 18 nm. Also with this data set the band gap energy of the SLD was determined by finding the largest wavelength emitted by the SLD and then using equation 5 to determine the corresponding photon energy. The largest wavelength was measured by finding the point at which the spontaneous emission signal reached 3dB of attenuation in comparisson to the noise. The band gap energy of the SLD was found to be 1.44 eV. This band gap energy suggests that the semiconductor material is !GaAs which has a well known index of refraction of 3.5 [4].

(5)

Figure 8, The non-lasing spectrum of the SLD. The non-lasing spectrum of the SLD is characterized with a large bandwidth and a relatively low intensity.

When the external cavity was implemented, lasing occurred after some mirror adjustments. As can be seen in figure 9, this not only decreased the bandwidth of the output spectrum to 5 nm, but it also increased the intensity by a factor of ten.

Figure 9, Lasing Spectrum of the multimode laser. The bandwidth in this case decreased by more than a factor of three, and the relative intensity increased by a factor of ten.

By adjusting the the reflectance of the neutral density filter a peak in power output was achieved. Maximum power output occured when reflectance was at 12% as shown in figure 10. At this optimal amount of feedback, the power output was measured as a function of injection current with and without an external optical feedback. In figure 11, at approximately 89 mA the trend in power increase no longer has a negligible slope difference, and this was determined to be the threshold current at which lasing occurs. The maximum power output with an external cavity was approximately 3 mW.

Figure 10, By rotating the neutral density filter the amount of light reflected back into the SLD was controlled. The results showed that when 88% of the SLD's output is reflected back into the device by the feedback mirror then the power is maximized.

Figure 11, Once lasing occured at the threshold current the rate at which power changed increased significantly.

The results from the tunable laser are shown in figure 12. Using Ocean Optics software the mode spacing between adjacent peaks was measured to be 0.35 nm. Equation 6 was then used to convert mode spacing to hertz, where %$\overline{ \lambda }$% is the average wavelength of the spectrum and Δλ is the mode spacing. Equation 3 was used to find the cavity length of the SLD using the known mode spacing and the justified index of refraction. The cavity length was calculated to be 282.91 μm.

(6)

Figure 12, The output of the tunable laser was measured at 130 mA over a range of 14.23 nm. The mode spacing between adjacent peaks were measured to be 0.35 nm.

Conclusion

Our group characterized and developed a tunable laser by implementing external optical feedback using a diffraction grating. From our measurements of the mode spacing of the SLD the internal cavity length was determined to be 282.91 μm.

One improvement that could be made in the future would be to obtain a spectrometer with higher resolution that is hassle free to implement. As for appications, it would be interesting to use the tunable laser to do spectroscopy studies which is very wavelength dependent. This could be done by shining the tunable laser on a flourescent sample, and then determining which wavelength causes the largest excitation of photons.

Acknowledgements

Thanks to Kurt Wick and Prof. James Leger for their help and guidance.

References

[1] Sheu, F., & Luo, P. (2007). Development of a variable spectral-width, wavelength-tunable light source using a superluminescent diode with optical feedback. [[http://www.bibme.org/apa/journal-citation?new=true][American Journal]]of Physics, 769-769.

[2] Saleh, B., & Teich, M. (1991). Lasers. In Fundamentals of Photonics. New York: Wiley.

[3] E. F. Schubert, Light-Emitting Diodes, 2nd ed. Cambridge University Press, New York, 2006.

[4] A. Yariv, P. Yeh, Photonics: Optical Electronics in Modern Communications, 6th ed. !OxfordUniversity Press, New York, 2007.

[5] E. G. Loewen, E. Popov, Diffraction Gratings and Applications, 1st ed. Marcel Dekker, New York, 1997.