Mode-Locked Laser Diodes and Their Monolithic Integration

Abstract

We describe mode locked laser diodes (MLLDs) operating from 40 GHz to >1 THz. Below 40 GHz, operation at the fundamental cavity frequency is appropriate, but at higher frequencies harmonic mode-locking (HML) is used. To increase output power and control nonlinear behavior, the gain needs to be modified, and an AlGaInAs/InP structure with a three-quantumwell active layer and passive far-field reduction layer is presented. In 40-GHz all-active MLLDs, this structure offers improvements in internal loss, slope efficiency, mode locking (ML) range, RF linewidth, timing jitter, far field pattern, and coupling efficiency to a single-mode fiber. By creating a transparent passive waveguide in part of the cavity, a 490-fs pulse width was achieved, the shortest reported from any quantum well MLLD. Several approaches are described for HML: colliding pulse ML operating up to 240 GHz, coupled cavity ML up to 160 GHz, and sampled Bragg grating constructions up to 1.28 THz. This latter approach offers stable HML over a wide range of bias conditions. Finally, we report a synchronized multicolor ML laser array, containing four 40-GHz lasers with designated wavelength registration for optical code division multiple access or optical time division multiple access applications.

1.55-μm AlGaInAs/InP Sampled Grating Laser Diodes for Mode Locking at Terahertz Frequencies

“…The SGDBR laser has been reported previously [4], [5] and, as shown in Fig. 1(a), consists of a reflecting facet, a saturable absorber (SA), a gain section and a SGDBR reflector.…”

“…The gratings of the C-SGDBR MLLD are etched into the side-walls of a ridge waveguide as shown in in Fig. 1(b) [4], [5]. The mode-locked frequency is set by the round trip time of one period of the sampled grating (P):…”

“…Λ represents the grating period and is 245 nm. The coupling coefficient of a uniform grating, κ 0 , was measured to be approximately 80 cm −1 when using a reactive-ion etching (RIE) machine with an optimized dry etch recipe [4], [5]. Fig.…”

“…These DBR lasers make use of conventional sampled-gratings (C-SGDBR), with grating sections spaced apart by sections of waveguide with no grating. Devices based on C-SGDBRs have been shown to offer far superior reproducibility, controllability, and a wider operation range than all other reported types of THz repetition frequency MLLDs [4], [5]. This class of device therefore represents a significant advancement in the HML field.…”

“…Mode-locking is a well-established route for generating short pulses, and the short cavity length of semiconductor lasers also offers high repetition frequencies (a 500 μm cavity corresponds to a roundtrip frequency of ∼80 GHz). A robust solution for generating Manuscript still higher frequency pulse streams is to utilize harmonic mode-locking (HML) techniques, with compound-cavity modelocking (CCM) [1], [2] and colliding pulse mode-locking (CPM) [3] widely reported (for a recent review, see [4]).…”