EditColliding-Pulse Modelocked Laser with Photonic Crystal Reflector
Introduction
A colliding pulse modelocked laser (CPM) generates picosecond optical pulses for the arrayed-waveguide grating (AWG) based spectral phase encoders to apply a phase code at the transmitter and remove the phase code at the receiver. Therefore, the transmitter CPM needs to provide a broad spectrum for spectral phase encoding and the receiver CPM needs to provide a short transform limited pulse for time grating. The focus of this summary is the novel CPM laser formed using deeply-etched mirrors (DEM) for integration with the SPECTS-OCDMA spectral-phase encoding operation and nonlinear-thresholder time-gating operation.
Integration with InP
Integration of active and passive O-CDMA components require a common waveguide structure for optical
amplification by semiconductor optical amplifiers (SOAs), optical modulation by electro-absorption (EA)
modulators, and phase shifters in the AWG based encoders and decoders. The EA modulators and SOAs are formed
in two growth steps. Fabrication starts with metal organic chemical vapor depositon (MOCVD) growth of the ndoped
InP wafer and semiconductor optical amplifiers (SOA) layers, consisting of seven 9 nm 1.58 Q InGaAsP
quantum wells (QW) and 5 nm 1.17 Q InGaAsP barriers designed for maximum gain at 1550 nm. A regrowth
deposites the EA QWs and a second regrowth the p-doped cladding layers. EA, SOA and passive areas are defined
by photolithography and wet etching. Dry-etching defines the waveguides. A Fe-InP layer is regrown for
simultaneous passivation of the waveguide sidewalls, electrical isolation, and planarization of the surface for
subsequent metallization: (AuGeTi\Au) on a highly doped InGaAs layer.
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Fig. 2 (a) shows the simulated reflectivity and transmission vs. the air gap width of a slab waveguide of
0.5 μm. Maximum reflectivity of 57% was calculated for a 450 nm gap rather than the ideal case (387 nm gap)
because of divergence of the optical mode in the gap. As the gap becomes larger, the transmission (blue) and the
power remaining in the waveguide (red) decreases due to mode mismatch of the diffracting wavefront in the air gap
and the waveguide. At larger gaps, the oscillations of reflection vs. gap width vanish and the reflectivity converges
towards the facet reflectivity (black line). DEM reflectivity is larger than FR even for variations of the gap size up to
400 nm. Fig. 2 (b) shows the simulated bandwidth of the DEM. The reflectivity of the DEM is essentially constant
across the operation bandwidth of the CPM (20 nm around 1550 nm). Deviations of the DEM sidewalls from
vertical cause the largest reduction of reflectivity due to increased scattering. According to FDTD simulations, at 14
degrees off vertical, the scattering loss reduces the reflectivity by 50%. Fig. 2 (c) shows the measured time domain
impulse response of the reflection of a DEM looking into the AR-coated facet, using an optical-vector network
analyzer (OVNA). Varying coupling loss between the lensed fiber and the waveguide make exact measurements of
the mirror reflectivity difficult. Including an estimation of the coupling loss between the lensed fiber and waveguide
(4-6dB per pass), allows for estimation of the reflectivity to be greater than -10 dB.
Modelocked Laser Fabrication
The active and passive O-CDMA components share a common wave guiding core structure of Q(1.15). Critical O-CDMA functions and components for integration include optical amplification by semiconductor optical amplifiers (SOAs), optical modulation by electro-absorption (EA) modulators, and phase shifting in the AWG based encoders and decoders. Fabrication starts with metal organic chemical vapor deposition (MOCVD) growth of the n-doped InP wafer and semiconductor optical amplifiers (SOA) layers, consisting of six 9 nm Q(1.55) quantum wells (MQW) and seven 5 nm Q(1.17) barriers designed for a PL 1545 nm. A regrowth deposits the EA MQWs and a second regrowth the p-doped cladding layers. EA, SOA and passive areas are defined by photolithography and wet etching. Dry-etching defines the waveguides. A Fe-InP layer is regrown for simultaneous passivation of the waveguide sidewalls, electrical isolation, and planarization of the surface for subsequent metallization: (Ti\Pt\Au) on a highly doped InGaAs layer. Refractive indices of the cladding and core are 3.3102 and 3.4511, respectively.
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Results
A 10 GHz RF drive signal from a microwave synthesizer in combination with a reverse voltage DC bias to the
absorber, synchronizes the CPM to an external reference. A comparison of the hybrid modelocking characteristics
of a CPM formed by DEM (DEM-CPM) as shown in Fig. 3(c,d,e) and a CPM formed by facets (FR-CPM) as shown
in Fig. 3(f,g,h) demonstrates the DEM effectiveness for integration in the O-CDMA platform. Both the DEM-CPM
and FR-CPM produce a stable, 100 ps period pulse train indicated by both the narrow spike in the RF spectra and
sampling oscilloscope traces. The timing jitter is estimated by integrating the single side band (SSB) phase noise
spectrum offset 20 kHz and 80 MHz to be 243 fs for the DEM-CPM compared to 283 fs the FR-CPM.