Hazara University Mansehra KP Pakistan Integrated Semiconductor Modelocked

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

Outline • 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 Ø The Semiconductor Laser was Invented almost simultaneously by four groups in 1962. Ø 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 Ø Comparing to the other types of lasers, semiconductor lasers are attractive due to their compact size, direct electrical pumping, high efficiency and low cost. ØSemiconductor technology is easy to make and compatible with other electronic devices Ø Semiconductor lasers can emit light in a wide spectral range spanning from the near ultraviolet to the far infrared ØThe most commonly used semiconductor laser material systems include Ga. As/Al. Ga. As, In. Ga. As. P/Ga. In. As/In. P and In. Ga. As/Al. Ga. In. As/In. P

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 Intensity short pulses by virtue of phase locking of the longitudinal modes inside a laser cavity λ Intensity Laser Output Spectrum λ

Schematic of a Modelocked Laser ØFrequency of the laser corresponds to the total length of the cavity: ØPractical constraints limit stable mode locked operation to 640 GHz ØHigher 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. Ti/Pt/Au Si. O 2 Al. Ga. In. As dry etch stop layer MQW-GRINSCH n-In. P substrate

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

Output Power Measurements Current Voltage Average output power is more than 50 m. W

Experimental setup for mode-locking characterisation

Mode Locking results SA 3 V, Gain current 60 m. A 25. 3 ps ∆t = 0. 9 ps AC Pulse train Isolated Pulse

Mode locking results: Cont; 3 d. B 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 3 V, Gain current fixed at 60 m. A ∆ʋ = 130 k. Hz RF spectrum (full span) RF spectrum (zoomed)

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

Problems in MLLS • Mode locking – optical pulse generation • Noise in semiconductor mode locked lasers Ø Simplified & inexpensive method for reducing phase noise in PMLLDs Ø 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. 1 ps

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 3 d. B Bandwidth = 5 nm Δpw = 2 ps

Conclusions Al. Ga. In. As/In. P 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 d. B using composite cavity loop • Not limited by high frequency driving electronics (i. e. low noise terahertz lasers)

Hazara University, Mansehra, KP, Pakistan Thanks a lot for your attention !