Emission Spectra of the typical DH laser

Transcription

1 Emission Spectra of the typical DH laser Emission spectra of a perfect laser above the threshold, the laser may approach near-perfect monochromatic emission with a spectra width in the order of 1 to 10 Å. High-resolution emission spectra (of a typical stripe-geometry DH laser) Sub-peaks, which are evenly spaced with a separation of = 7.5 Å, appear in the spectra belonging to the longitudinal modes. Because of these longitudinal modes, the stripe geometry laser is not a spectrally pure light source for optical communication.

2 Lasing condition: Laser Resonant Frequencies exp( j2 L) 1 2 L 2m, m 1,2,3,... 2 n Assuming the resonant frequency of the mth mode is: n = refractive index m mc m 1,2,3,... 2Ln m m 1 2 c Ln 2 2Ln

3 Spectrum from a laser Diode ( 0) g( ) g(0) exp : spectral width 2 2

4 Characteristics of the DH laser Threshold current density vs. active layer thickness The threshold current density decreases with decreasing d, reaches a minimum, and then increases. The increase of J th at very narrow active thickness is caused by poor optical confinement. Output power vs. diode current The light-current characteristics is quite linear above threshold. Temperature dependence The threshold current increases exponentially with temperature J th ~ exp [ T/T 0 ]

5 Examples of Nanostructured Semiconductor Lasers Quantum well lasers Advanced lasers: Quantum dot lasers Next time: Advanced lasers QCLs Single mode lasers DFB lasers VCSELs

6 Evolution of the threshold current of the semiconductor lasers

7 Quantum Well Laser Constant 2D density of states means a large concentration of electrons can easily occur at E1 (and holes at the minimum valence band energy) Population inversion occurs quickly without the need for a large current to bring a large number of electrons Benefits: Threshold current reduced, linewidth is narrower

8 Multiple Quantum Well (MQW) Laser Several single quantum wells are coupled into a multiple quantum well (MQW) structure. The significantly reduced temperature sensitivity of MQW lasers has been related to the staircase density of states distribution and the distributed electron and photon distributions of the active region. The optical confinement helps to contain the otherwise large losses from a narrow active region, leading to low threshold currents.

9 Bandgap engineering: Visible-UV-IR range

10 Red QW Laser Diode Diagram of red GaInP DQW laser Diagram showing the alloy composition through the layer structure of a two-well, separate confinement (Al y Ga 1 y )In 1 x P quantum well laser. The vertical distance axis is not to scale: the wells are each about 6.5 nm wide, the y=0.5 waveguide core is about 200 nm thick, and the cladding layers are each about 1 μm thick.

11 Violet QW Laser Diode Diagram of deep violet InGaN DQW laser structures From: Performance enhancement of deep violet indium gallium nitride double quantum well lasers using delta barrier close to electron blocking layer, J. Nanophoton. 2012;6(1): doi: /1.jnp

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13 Modes: longitudinal and transverse Longitudinal modes Transverse modes

14 Laser waveguides design for transverse confinement Vertical confinement Lateral confinement Gain-guided Index guided: ridges, ribs Buried heterostructure lasers

15 Graded Index Separate Confinement Heterostructure (GRINSCH) Laser GRaded INdex Separate Confinement Heterostructure (GRINSCH) Laser A narrower carrier confinement region (d) of high recombination is separated from a wider optical waveguide region Optical confinement can be optimized without affecting the carrier confinement GRINSCH-SQW and GRINSCH-MQW The threshold current for a GRINSCH is much lower than that of a DH laser Vertical confinement

16 Lateral confinement Efficient operation of a laser diode requires reducing the # of lateral modes, stabilizing the gain for lateral modes as well as lowering the threshold current. These are met by structures that confine the optical wave, carrier concentration and current flow in the lateral direction. Important types of laser diodes are: gain-guided, positive index guided, and negative index guided.

17 Gain guided: optical gain is highest where current density is greatest Stripe contact increases current density in the active region. The widths of the active region or the optical gain region is defined by current density from the stripe Stripe electrode Cleaved reflecting surface W L Oxide insulator p-gaas (Contacting layer) p-al x Ga 1-x As (Confining layer) p-gaas (Active layer) n-al x Ga 1-x As (Confining layer) n-gaas (Substrate) Current paths Substrate Electrode Substrate Elliptical laser beam Cleaved reflecting surface Active region where J > J th. (Emission region) Schematic illustration of the the structure of a double heterojunction stripe contact laser diode 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

18 Ridge laser

19 Index guided: optical power confined to waveguide Oxide insulation p + -AlGaAs (Contacting layer) p -AlGaAs (Confining layer) n- AlGaAs p -GaAs (Active layer) n -AlGaAs (Confining layer) n -GaAs (Substrate) Electrode Schematic illustration of the cross sectional structure of a buried heterostructure laser diode S.O. Kasap, Optoelectronics (Prentice Hall) Active layer is surrounded by lower index AlGaAs and behaves like a dielectric waveguide Ensures that photons are confined to the active or optical gain region Increases rate of stimulated emission

20 Buried heterostructure laser

21 Laser Diodes (temperature characteristics) The output characteristics of an LD are sensitive to temperature. =>As temperature increases threshold current increases exponentially. Output spectrum also changes. A single mode LD will mode hop (jump to a different mode) at certain temperatures. This results in a change of laser oscillation wavelength. increases slowly due to small change in refractive index and cavity length.

22 P o (mw) C 25 C 50 C I (ma) Output optical power vs. diode current as three different temperatures. The threshold current shifts to higher temperatures S.O. Kasap, Optoelectronics (Prentice Hall) o (nm) Single mode Mode hopping Case temperature ( C) Single mode (a) (b) (c) Case temperature ( C) Multimode Case temperature ( C) Peak wavelength vs. case temperature characteristics. (a) Mode hops in the output spectrum of a single mode LD. (b) Restricted mode hops and none over the temperature range of interest (20-40 C). (c) Output spectrum from a multimode LD S.O. Kasap, Optoelectronics (Prentice Hall)

23 Laser Diodes (temperature characteristics) Remedies if mode hopping is undesirable: 1. Adjust device structure. 2. Implement thermoelectric (TE) cooler. Gain guided LDs inherently have many modes therefore the wavelength vs. temperature behaviour tends to follow the bandgap (optical gain curve as opposed to the cavity properties).

24 Advanced semiconductor sources Quantum dot (QD) LEDs and lasers

25 Nanowire LED TRUE WHITE LED WITHOUT PHOSPHOR CONVERSION glō s RGB nanowire LEDs (nleds) are made using one material system with the active layers grown on the crystallographicallyfavorable non-polar m-plane. M-plane GaN

26 Evolution of the threshold current of the semiconductor lasers

27 0-D (Quantum dot): An artificial atom Areal density: E ( E) ( E Ei ) E i

28 Theoretical quantum dots (a) Structure of a 4nm-high, 10 nm-wide hexagonal GaN quantum dot embedded in AlN. (b) Profile of the conduction band edge. (c) Maps of the dot electron ground state, (d) Map of the first excited state.

29 In Stranki-Krastanov growth of QDs: strain-mediated intra- and inter-layer interactions between the QDs Aligned array of GaN QDs in AlN

30 QDL Predicted Advantages Wavelength of light determined by the energy levels not by bandgap energy: improved performance & increased flexibility to adjust the wavelength Maximum material gain and differential gain Small volume: low power high frequency operation large modulation bandwidth Superior temperature stability of I threshold I threshold (T) = I threshold (T ref ).exp ((T-(T ref ))/ (T 0 )) High T 0 decoupling electron-phonon interaction by increasing the intersubband separation. Undiminished room-temperature performance without external thermal stabilization Suppressed diffusion of non-equilibrium carriers Reduced leakage

31 QDL Basic characteristics An ideal QDL consists of a 3D-array of dots with equal size and shape Surrounded by a higher band-gap material confines the injected carriers. Embedded in an optical waveguide Consists lower and upper cladding layers (n-doped and p-doped shields)

32 Edge emitting QDL

33 QDL Application Requirements Same energy level Size, shape and alloy composition of QDs close to identical Inhomogeneous broadening eliminated real concentration of energy states obtained High density of interacting QDs Macroscopic physical parameter light output Reduction of non-radiative centers Problem for nanostructures made by high-energy beam patterning since damage occurs during fabrication Electrical control Electric field applied can change physical properties of QDs Carriers can be injected to create light emission

34 Comparison of QD Laser with QW laser

35 QD Laser vs. QW Laser Comparison of efficiency: QWL vs. QDL

36 Bottlenecks First, the lack of uniformity. Quantum Dots density is insufficient the lack of correlation between QDs FWHM = mev Single dot Ensemble of QDs

37 Breakthroughs Fujitsu Temperature Independent QD laser 2004 Fujitsu's quantum dot laser fires data at 25Gbps (2010) Temperature dependence of light-current characteristics

38 InP instead of GaAs Breakthroughs Can operate on ground state for much shorter cavity length High T0 is achieved First buried DFB DWELL operating at 10Gb/s in 1.55um range Surprising narrow linewidth-brings a good phase noise and timejitter when the laser is actively mode locked Alcatel Thales III V Laboratory, France 2006

39 High-Performance Quantum Dot Lasers and Integrated Optoelectronics on Si

40 QD Laser vs. QW Laser In order for QD lasers compete with QW lasers: A large array of QDs since their active volume is small An array with a narrow size distribution has to be produced to reduce inhomogeneous broadening Array has to be without defects may degrade the optical emission by providing alternate nonradiative defect channels The phonon bottleneck created by confinement limits the number of states that are efficiently coupled by phonons due to energy conservation Limits the relaxation of excited carriers into lasing states Causes degradation of stimulated emission Other mechanisms can be used to suppress that bottleneck effect (e.g. Auger interactions)

41 Future Directions Widening parameters range Further controlling the position and dot size Decouple the carrier capture from the escape procedure to using by Reduce inhomogeneous linewidth broadening Surface Preparation Technology Allowing the injection of cooled carriers Combination of QD lasers and QW lasers In term of Raised gain at the fundamental transition energy

42 Promising properties High speed quantum dot lasers Directly Modulated Quantum Dot Lasers Mode-Locked Quantum Dot Lasers InP Based Quantum Dot Lasers Advantages Datacom application Rate of 10Gb/s Short optical pulses Narrow spectral width Broad gain spectrum Very low α factor-low chirp Low emission wavelength Wide temperature range Used for data transmission

43 Promising properties High power Quantum Dot lasers QD lasers for Coolerless Pump Sources Single Mode Tapered Lasers Advantages Size reduced quantum dot Small wave length shift Temperature insensitivity