Chapter 5b

Optoelectronics Lecture notes

1. Heterostructure Laser Diodes Both carrier and photon confinements are readily achieved in modern laser by the use of heterostructure devices!

2. Heterostructure Laser Diodes Fig.7 shows a Double Heterostructure (DH) device based on two junctions between different semiconductor materials with different bandgaps . Semiconductors are AlGaAs with E g  2eV and GaAs with E g  1.4eV The p-GaAs region is a thin layer (0.1-0.2 m m) and constitutes the active layer in which lasing combination takes place Both p-GaAs and p-AlGaAs are heavily p-type doped and are degenerate with E F in the valence band.

3. Fig.7

4. Carrier confinement in DH When a sufficiently large forward bias is applied, E c of n- AlGaAs moves above E c of p- GaAs , which leads to a large injection of electrons in the CB of n- AlGaAs into p- GaAs as shown in Fig.7 . These electrons are confined to the CB of p- GaAs since there is a barrier D E c between p- GaAs and p- AlGaAs Since p- GaAs is a thin layer, the concentration of injected electrons can be increased quickly even with moderate increase in forward current . This effectively reduces the threshold current for population inversion or optical gain

5. Photon confinement in DH AlGaAs has a lower refractive index than that of GaAs Note: a wider bandgap semiconductor generally has a lower refractive index The change in the refractive index defines an optical dielectric waveguide that confines the photons to the active region of the optical cavity Thereby reduces photon losses & increases photon concentration It increases the rate of stimulated emissions Both carrier & photon confinement lead to reduction in the threshold current density.

6. The structure of DH A typical structure of a double heterostructure laser diode is similar to a double heterostructure LED . The doped layers are grown epitaxially on a crystalline substrate , which is n-GaAs It consists of the first layer substrate , n-AlGaAs, the active p-GaAs layer and the p-AlGaAs layer There is an additional p-GaAs layer called contacting layer , next to p-AlGaAs.

7. The structure of DH, cont The electrodes are attached to the GaAs rather than AlGaAs This allows better contacting and avoids Schottky junctions which limits the current The p- and n-AlGaAs layers provide carrier and optical confinement in the vertical direction by forming heterojunction with p-GaAs. The active layer is p-GaAs which means that the lasing emission will be in in the range 870-900nm depending on the doping level An important feature of this laser diode is the stripe geometry (stripe contact) on p-GaAs

8. Fig.8 : Heterojunction laser diode

9. Current Paths in DH Current density J from the stripe contact is not uniform laterally J is greatest along the central path, 1, and decreases away from path 1 towards 2 or 3 The current is confined within path 2 & 3 The current density paths through active region where J > J th as shown in Fig.8 defines the active region where the lasing emission emerges The width of the active region is defined by J from the stripe contact The optical gain is highest where the current density is greatest Such lasers are called gain guided .

10. Two advantages of using a stripe geometry The reduced contact area also reduces the threshold current I th . Typical stripe width may be as small as a few m m leading to typical threshold currents that may be tens of mA. The reduced emission area makes light coupling to optical fibers easier.

11. Improving rear facet reflectance The laser efficiency can be further improved by reducing the reflection losses from the rear crystal facet The refractive index of GaAs is ~3.7 , the reflectance is 0.33 By fabricating a dielectric mirror at the rear facet , which is mirror consisting of a number of quarter wavelength semiconductor layers of different refractive index It is possible to bring the reflectance close to unity and thereby improve the optical gain of the cavity

12. Lateral optical confinement The lateral optical confinement of photons to the active region is poor because there is no marked change in the refractive index laterally Laterally optical confinement can increase the rate of stimulated emissions It can be achieved by shaping the refractive index profile in the same way the vertical confinement

13. Fig.9 : Structure of heterostructure laser diode

14. Index Guided Fig.9 shows the active layer p- GaAs is bound both vertically and laterally by a lower index AlGaAs . It behaves as a dielectric waveguide and ensure the photons are confined to the optical gain region It is called buried double heterostructure laser diode Since the optical power is confined to the waveguide defined by the refractive index variation these diodes are called index guided

15. Gain-induced guide Positive-index guide Negative-index guide can emit >100mW strong instabilities highly astigmatic more stable structure central region has higher n all guided light is reflected at dielectric boundary more popular compared to negative-index guide more stable structure central region has lower n most of light refracted into surrounding material and lost

16. Types of laser diodes The laser diode heterostructure based on GaAs and AlGaAs are suitable for emission ~900nm For operation in the optical communication, wavelength of 1.3 and 1.55 mm, Typical heterostructure are based on InP (substrate) and quarternary alloys InGaAsP which has a greater refractive index. The composition of the InGaAsP alloy is adjusted to obtain the required bandgap for the active and confining layers

17. Laser mode The optical resonator is essentially Fabry-Perot cavity as shown in Fig.10 which can be assigned a length (L), width (W) and height (H). L determines the longitudinal mode separation W and H determine the transverse (lateral) modes If the transverse dimensions (W & H) are sufficiently small, only the lowest transverse mode, TEM 00 , will exit.

18. Fig.10 : Laser Cavity

19. Transverse Mode of Laser

20. Longitudinal Mode of Laser ( c ) Relative intensity dl m Optical Gain l Allowed Oscillations (Cavity Modes) dl m ( b ) L Stationary EM oscillations Mirror Mirror l l m ( l /2) = L ( a ) l o

21. Elementary Laser Diode Characteristics The output spectrum from a laser diode (LD) depends on two factors The nature of the optical resonator used to build the laser oscillations The optical gain curve (line-shape) of the active medium

22. Single Mode & Multimode The actual modes that exist in the output spectrum depend on the optical gain these modes will experience The spectrum of optical power vs wavelength is either multimode or single mode depending on Optical resonator structure and the pumping current level Fig.11 shows the output spectrum from an index guided LD at various output power levels The multimode spectrum at low output power becomes single mode at high output powers The output spectrum of most gain guided LDs tend to remain multimode even at high diode currents

23. Fig.11 : Laser Mode

24. Temperature Sensitive Fig.12 shows the changes in the optical power vs diode current characteristics with the case temperature. As the temperature increases, the threshold current increases steeply e.g. exponentially of the absolute temperature Output spectrum also changes with the temperature The peak emission wavelength of single mode exhibits “jump” at certain temperature. A jump corresponds to a mode hop in the output

25. Fig.12 : Threshold current shifts to higher temperature

26. Mode Hop At the new temperature , another mode fulfills the laser oscillation condition which means a discrete change in the laser oscillation wavelength Between mode hops , l o increases slowly with the temperature due to the slight increase in the refractive index n and cavity length with temperature If mode hop are undesirable, then the device structure must be such to keep the modes sufficiently separated . Highly stabilized LDs are usually marketed with thermoelectric coolers integrated into the diode package to control the device temperature.

27. Slope Efficiency Slope efficiency determines the optical power P o of the output coherent radiation in terms of the diode current above the threshold current I th . If I is the diode current, the slope efficiency h slope is W/A or W/mA Slope efficiency depends on the LD structure & the semiconductor packaging

28. Conversion Efficiency The conversion efficiency gauges the overall efficiency of the conversion from the input of electrical power to the output of optical power. It can be easily determined from the output power at the operating diode current and voltage. In some modern LDs, this may be as high as 30%-40%.

29. Example: Laser output wavelength variation Given that the refractive index n of GaAs has a temperature dependence d n/ dT  1.5  10 –4 K –1 estimate the change in the emitted wavelength 870 nm per degree change in the temperature between mode hops

30. Solution Consider a particular given mode with wavelength l m , m ( l m /2n) = L d l m / dT = d/ dT [2n L / m ]  2 L / m ( d n / dT ) Substituting for L / m in terms of l m , d l m / dT  l m /n ( d n / dT ) =1.5  10 –4 K –1  870nm /(3.7) =0.035nmK –1 Note that we used n for a passive cavity whereas n above should be the effective refractive index of the active cavity which will also depend on the optical gain of the medium , and hence its temperature dependence is likely to be somewhat higher than the d n / dt value we used