Variable Spectral Width Wavelength Tunable Light Source using a Superluminescent Diode

Maxwell Miller & David X. Moua

University of Minnesota

Methods of Experimental Physics Spring 2016

Abstract

We've built a wavelength tunable laser using an Superluminscent Diode as our light source. To produce an external feedback cavity which is required for lasing in our SLD, we have placed a diffraction grating which also allows us to select a wavelength within our range. Our laser has 27 distinct monochromatic and coherent wavelengths inside a range between 834 - 843 nm. The mode spacing between each wavelength is Δλ = 0.342nm. Our laser's central wavelength is 𝜆̅ = 838.8nm. Using these calculations, we were able to determine that the cavity length of the SLD is L = 294μm.

Introduction

Many lasers lase primarily at one distinct wavelength, and it can be difficult to get them to lase at other wavelengths. The goal for our project was to produce a laser that is capable of tuning it's wavelength over a range. We selected an SLD because it emits a broadband spectrum with high optical power. The SLD has an anti-reflection coating that prevents internal feedback. To make a laser out of the SLD, an external optical feedback is used to cause stimulated emission to occur which lowers the spectral-width. The spectral-width can vary based on the optical feedback ratio and the wavelength can be selected and monochromatic from the use of a diffraction grating and reflective mirror. Our experiment will have two parts, the first is to get the SLD to lase to test the power of the laser and the second part is to allow for selectable wavelengths.

Theory

A SuperLuminescent Diode (SLD) is comprised of a pn junction with a waveguide. The waveguide simply restricts the expansion of the wave to 2 dimensions. When a potential difference is applied across the pn junction, holes from the p type material are injected into the n type material which causes electrons from the n type material to be injected into the p type material [1]. Throughout this process, electrons and holes combine to conserve momentum and energy, and light is generated by process of spontaneous and stimulated emission. Figure (1) below represents the interior of the lasing cavity of a typical SLD.

Figure (1): This shows the interior of the SLD. The partially transmitting front facet of the SLD prevents lasing. Original Figure.

Spontaneous Emission is when electrons in higher energy state drop to lower energy state and releases photons at random intervals. This photons have energy equal to the difference in energy levels the electrons dropped from. Stimulated Emission occurs when photons pass near electrons in excited state causing the electrons to drop in energy and release photons identical to the incident photons with the same energy, frequency (wavelengths), and phase. Stimulated Emission is described below, in Figure (2).

Figure (2): Stimulated Emission releases two photons; The feedback photon and an identical photon are emitted and this results a gain in intensity. Taken from [2].

For lasing to occur, Stimulated Emission needs to dominate the Spontaneous Emission and this is achieved through population inversion. Population inversion is when there are more electrons at higher energy state than lower energy state. Population inversion sets the threshold current for lasing and the injection current fed to the SLD should reach this threshold current for lasing. The threshold current is where the longitudinal modes lases and are our selectable wavelengths. The higher the stimulated emission from increased injection current, the more selectable longitudinal modes. The mode spacing between the longitudinal modes' peaks are used to determine the SLD's internal cavity which is the Fabry-Perot cavity length that determines the boundary conditions for wavelengths selectable in our laser as show in equation (1).

𝑛𝜆/2 = 𝐿 (1)

The cavity length is calculated with equation (2) and (3) [3].

Δ𝑣 = c/(2nL) ⁽²⁾

Δ𝑣 is the frequency mode spacing and C is the speed of light. n is the index of refraction and it's 3.5 as given by our source [4]. L is the Fabry-Perot cavity length.

Δ𝑣 = 𝑐Δ𝜆/𝜆̅ 2 (3)

Δ𝜆 is the mode spacing between the longitudinal modes' peaks adjacent to one another. 𝜆̅ is the average center wavelengths of the longitudinal modes.

Experimental Setup

The first experimental setup is shown below in figure (3). The external feedback for lasing is a gold mirror and the neutral density filter is angled at 45 degrees to reflect parts of the laser for measurement by our Ocean Optic spectrometer which has a resolution of 0.34 nm. The reflected incident light is adjusted back into the SLD by the fine tuning of the gold mirror.

Figure (3): The injection current is controlled by the current source and the temperature regulator keeps our data consistent by keeping the SLD at fix temperature. Original Figure.

The second experimental setup is shown below in figure (4). The neutral density filter is replaced with a diffraction grating to keep the feedback for lasing and allow us to select monochromatic and coherent wavelengths of the laser. The diffraction grating used in the second experiment set up utilizes the Littrow configuration, explained in Figure (5) below. The Littrow configuration feedback the first diffraction order to the SLD and the other diffraction order is angled by a gold mirror to the Ocean Optic spectrometer to detect the Longitudinal Modes. When the diffraction grating is in this configuration, the equation for the groove pattern effect reduces to equation (4):

Figure(4): The blaze angle is adjusted by fine tuning of the diffraction grating to select wavelengths to feed into SLD for stimulated emission of monochromatic and coherent wavelengths. Original Figure.

Figure (5): The Littrow configuration transforms (a) to (b) by setting the incident angle and the first order diffraction angle equal to each other. Taken from [5]

𝑚𝜆𝐵=2𝑑𝑠𝑖𝑛(𝜃𝐵) (4)

where d is the distance between grooves, 𝜆𝐵 is the wavelength, 𝜃𝐵 is the blaze angle, and m is the diffraction order [5].

Results

The Injection Current vs Intensity of feedback and no feedback added to the SLD is measured and figure (6) shows the results. The threshold current from the results is 90mA where the effect of the laser starts to take place.

Figure (6): The laser's power (mW) at full injection current we could use is 10 times the power of a non-lasing SLD. Original Figure.

The second part of our experiment yields the results shown in figure (7). The diffraction grating allows us to observe 27 longitudinal modes with mode spacing of Δλ = 0.342 nm. The longitudinal modes and the gain envelope are analyzed in Origin 8.6. The Longitudinal modes follow a Lorentzian profile and the gain envelope of all the modes follows a Gaussian profile.

Figure (7): The Gain Envelope that covers the longitudinal modes covers the range of longitudinal modes. Our center wavelength given by the gain envelope is 𝜆̅ = 838.8nm. Original Figure.

L = 𝜆̅ 2/(2ncΔ𝜆) (5)

From the mode spacing given by the longitudinal modes and with equation (5), our SLD cavity length is L = 294 μm.

Conclusion

The goal of our experiment to build a wavelength tunable laser using an SLD was successful. We obtained a 21.5mW laser with a usable power of 2.1mW. Our laser has a central wavelength of 𝜆̅ = 838.8nm and an operating wavelength range between 834 – 842nm. We measured the mode spacing between the longitudinal peaks to be constant, and Δλ = 0.342nm. Using these calculations, we were able to obtain a cavity length inside the SLD of L = 294μm.

References

[1] Nave, Carl Rod. "The P-N Junction." Hyper Physics. 1999. Web. 03 Mar. 2016

[2] © User: V1adis1av / Wikipedia Commons / CC-BY-SA-3.0

[3] Saleh, Bahaa E. A., and Malvin Carl. Teich. Fundamentals of Photonics. New York: Wiley, 1991. Print.

[4] Sheu, F., & Luo, P. (2007). Development of a variable spectral-width, wavelength-tunable light source using a superluminescent diode with optical feedback. American Journal of Physics, 769-769.

[5] Richardson Gratings. "Determination of the Blaze Wavelength." Richardson Gratings. Newport, 1999. Web. 01 Apr. 2016. <http://www.gratinglab.com/Information/Technical_Notes/TechNote11.aspx>.