Integrated Semiconductor Modelocked Lasers Hazara University Mansehra, KP, Pakistan Integrated Semiconductor Modelocked Lasers Dr Jehan Akbar

Outline Introduction Introduction to Semiconductor lasers Modes of a Laser Semiconductor mode locked lasers Wafer structure Modelocked lasers features & fabrication Devices structure Devices characterization High power modelocked lasers

Semiconductor lasers The Semiconductor Laser was Invented almost simultaneously by four groups in 1962. In 1972: Charles H. Henry invents the quantum well laser, which requires much less current to reach lasing threshold than conventional diode lasers and which is exceedingly more efficient Comparing to the other types of lasers, semiconductor lasers are attractive due to their compact size, direct electrical pumping, high efficiency and low cost. Semiconductor technology is easy to make and compatible with other electronic devices In September 1962 researchers from IBM, independently and almost simultaneously with researchers fromGeneral Electric and MIT's Lincoln Laboratory, demonstrated laser action in the semiconductor gallium arsenide Semiconductor lasers can emit light in a wide spectral range spanning from the near ultraviolet to the far infrared The most commonly used semiconductor laser material systems include GaAs/AlGaAs, InGaAsP/GaInAs/InP and InGaAs/AlGaInAs/InP

Semiconductor Lasers: Basics In semiconductor lasers, electrons and holes are injected into the active region through electrical pumping, which introduces population inversion and produces optical gain via stimulated emission. If the injected carrier density is large enough, the stimulated emission of the photons overcomes the losses and the laser achieves gain. Electrical Pumping Population inversion Stimulated emission Lasing action (Laser)

Mode locking Mode-locking is a technique used to generate coherent, high repetition rate and ultra short pulses by virtue of phase locking of the longitudinal modes inside a laser cavity λ Intensity λ Laser Output Spectrum Intensity

Schematic of a Modelocked Laser Frequency of the laser corresponds to the total length of the cavity: Practical constraints limit stable mode locked operation to 640 GHz Higher repetition frequencies are obtained by using Harmonic mode locking

Device Features Single mode operation: The ridge waveguide of the laser was optimized by beam propagation simulations for single mode Operation of the device. n-InP substrate MQW-GRINSCH Ti/Pt/Au SiO2 AlGaInAs dry etch stop layer

Experimental setup for output power measurements Ge PD Device Temperature controlled Copper mount

Output Power Measurements Current Voltage Average output power is more than 50 mW

Experimental setup for mode-locking characterisation

Mode Locking results SA 3V, Gain current 60mA AC Pulse train 25.3ps ∆t = 0.9ps AC Pulse train Isolated Pulse

Mode locking results: Cont; 3 dB BW 9.2 nm Optical spectrum The pulse width increases as the gain current is increased. This is due to the increase in the non-linear effects such as self phase modulations

Radio Frequency (RF) Measurements SA 3V, Gain current fixed at 60mA ∆ʋ = 130kHz RF spectrum (full span) RF spectrum (zoomed)

Far-field Measurement Results Farfield-3D view Farfield-2D view 3 QW Laser

Problems in MLLS Mode locking – optical pulse generation Noise in semiconductor mode locked lasers Simplified & inexpensive method for reducing phase noise in PMLLDs Pulse stabilisation and sub-picosecond jitter in a 40 GHz PMLLD Solution All-optical regenerative mode locking

Passively operating mode locked laser at 40 GHz Pulse width = 2.1ps

Noise in mode locked lasers Changes in amplitude and phase in the circulating field due to: Spontaneous emissions Thermal and other technical noise Resonator losses Phase fluctuations – random walk Linewidth enhancement factor – differential gain Schawlow–Townes equation for linewidth of laser is : where Toc denotes the output coupler transmission, ltot the total resonator losses (which may be larger than Toc), Trt the resonator round-trip time

Optical regenerative mode locking

40 GHz laser – jitter and linewidth reduction

Supermode noise

Supermode noise suppression technique

20 GHz Passively mode locked laser

Supermode noise suppression - results

Linewidth and phase noise reductions

Optical spectra and pulse width 3dB Bandwidth = 5 nm Δpw = 2 ps

Conclusions AlGaInAs/InP Mode-Locked Lasers operating at 40 GHz: Stable single mode output, Lower pulse widths and RF line-widths Wider range of stable mode locking Increased coupling efficiency with optical fibers due to lower divergence angles Regenerative Optical Mode-locking: Simplified & inexpensive method for reducing phase noise Pulse stabilisation and sub-picosecond jitter in a 40 GHz MLL Super-mode noise suppression > 40 dB using composite cavity loop Not limited by high frequency driving electronics (i.e. low noise terahertz lasers)

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