Spectroscopic Study of Terahertz Generation in Mid-Infrared Quantum Cascade Lasers.

Terahertz quantum cascade laser sources based on intra-cavity difference-frequency generation are currently the only room-temperature mass-producible diode-laser-like emitters of coherent 1-6 THz radiation. Device performance has improved dramatically over the past few years to reach milliwatt-level power output and broad tuning from 1.2 to 5.9 THz, all at room-temperature. Terahertz output in these sources originates from intersubband optical nonlinearity in the laser active region. Here we report the first comprehensive spectroscopic study of the optical nonlinearity and investigate its dependence on the mid-infrared pump frequencies. Our work shows that the terahertz generation efficiency can vary by a factor of 2 or greater depending on the spectral position of the mid-infrared pumps for a fixed THz difference-frequency. We have also measured for the first time the linewidth for transitions between the lower quantum cascade laser states, which is critical for determining terahertz nonlinearity and predicting optical loss in quantum cascade laser waveguides.

Characterization of energy-efficient and colorless ONUs for future TWDM-PONs.

The Full Services Access Network group has recently selected the time and wavelength division multiplexed passive optical network (TWDM-PON) as the base technology solution for next-generation PON stage-2 (NG-PON2). Meeting the core requirements of NG-PON2 necessitates the following additional features in the transceivers of the optical network unit (ONU) that is located at subscriber premises: (a) legacy system compliant; (b) wavelength tunable; (c) cost-efficient; and (d) energy-efficient. To address these features, we investigate the properties of short-cavity vertical-cavity surface-emitting lasers (SC-VCSELs) for implementation as colorless ONU transmitters in future TWDM-PONs. Specifically, we investigate the tunability and transmission performance of the SC-VCSEL across the C-minus wavelength band for legacy system compliance. We report on error-free transmission across a 800 GHz tuning range with a potential aggregate upstream capacity of 80 Gbps over a system reach of 40 km and with a split ratio of 1:128 per wavelength channel. Results were achieved without dispersion compensation and electronic equalization. We also evaluate the energy efficiency of the SC-VCSEL in active, doze, and sleep mode. When in active mode, the SC-VCSEL transmitter block consumes 91.7% less power than a distributed feedback (DFB) laser transmitter block. When transitioning between doze and active modes, the transmitter block has a short settling time of only 205 ns, thus increasing the power-saving duration and consequently reducing the overall power consumption of the ONU. Through numerical analysis, evaluation of the energy-savings of the SC-VCSEL ONU over the DFB ONU under various modes of operation, demonstrates up to 84% of energy-savings. The capacity, tuning range, split ratio, system reach, and energy-savings arising from SC-VCSEL ONU implementation as reported in this work, exceed the minimum requirements of NG-PON2 for future TWDM-PON deployments.

Broadly tunable terahertz generation in mid-infrared quantum cascade lasers.

Room temperature, broadly tunable, electrically pumped semiconductor sources in the terahertz spectral range, similar in operation simplicity to diode lasers, are highly desired for applications. An emerging technology in this area are sources based on intracavity difference-frequency generation in dual-wavelength mid-infrared quantum cascade lasers. Here we report terahertz quantum cascade laser sources based on an optimized non-collinear Cherenkov difference-frequency generation scheme that demonstrates dramatic improvements in performance. Devices emitting at 4 THz display a mid-infrared-to-terahertz conversion efficiency in excess of 0.6 mW W(-2) and provide nearly 0.12 mW of peak power output. Devices emitting at 2 and 3 THz fabricated on the same chip display 0.09 and 0.4 mW W(-2) conversion efficiencies at room temperature, respectively. High terahertz-generation efficiency and relaxed phase-matching conditions offered by the Cherenkov scheme allowed us to demonstrate, for the first time, an external-cavity terahertz quantum cascade laser source tunable between 1.70 and 5.25 THz.

Energy-efficiency of optical network units with vertical-cavity surface-emitting lasers.

The energy savings of 10 Gbps vertical-cavity surface-emitting lasers (VCSELs) for use in energy-efficient optical network units (ONUs) is critically examined in this work. We experimentally characterize and analytically show that the fast settling time and low power consumption during active and power-saving modes allow the VCSEL-ONU to achieve significant energy savings over the distributed feedback laser (DFB) based ONU. The power consumption per customer using VCSEL-ONUs and DFB-ONUs, is compared through an illustrative example of 10G-EPON for Video-on-Demand delivery. Using energy consumption models and numerical analyses in sleep and doze mode operations, we present an impact study of network and protocol parameters, e.g. polling cycle time, network load, and upstream access scheme used, on the achievable energy savings of VCSEL-ONUs over DFB-ONUs. Guidance on the specific power-saving mode to maximum energy savings throughout the day, is also presented.

Long distance single-mode fiber transmission of multimode VCSELs by injection locking.

We report greatly enhanced transmission distance of multimode vertical-cavity surface-emitting lasers (MM-VCSELs) over standard single-mode fiber using optical injection locking. Transmission distance as high as 90 km is achieved at 10 Gb/s by frequency chirp inversion and higher order transverse mode suppression.

Greatly increased fiber transmission distance with an optically injection-locked vertical-cavity surface-emitting laser.

We demonstrate single-mode fiber transmission distance enhancement up to 120 km of a directly-modulated injection-locked VCSEL modulated by a 10Gb/s NRZ signal. Injection locking induced data pattern inversion of the VCSEL causes adjustable chirp, which greatly extends reach. Both experiments and simulations are shown to explain this phenomenon.

Greatly enhanced modulation response of injection-locked multimode VCSELs.

We report greatly enhanced modulation response of multimode vertical-cavity surface-emitting lasers (MM-VCSELs), for the first time, using optical injection locking. A 3-dB bandwidth of 38 GHz and 54 GHz resonance frequency are achieved using a MM-VCSEL with a 3 GHz free-running bandwidth. We show that transverse mode selection can be attained with optical injection locking through frequency and spatial detuning.

Novel cascaded injection-locked 1.55-mum VCSELs with 66 GHz modulation bandwidth.

We demonstrate a novel cascaded configuration of optically injection-locked (COIL) VCSELs, which enables a wide and tailorable direct modulation bandwidth. Up to 66 GHz bandwidth is achieved using VCSELs with an original, free-running 10 GHz bandwidth. Different configurations of cascading are discussed in detail with the focus on optimizing the modulation bandwidth. We also discuss scaling capability of this technique to achieve tailorable modulation response.

Novel modulated-master injection-locked 1.55-microm VCSELs.

We report a novel optical injection-locking configuration using a directly-modulated master VCSEL to injection-lock a slave VCSEL. We demonstrate an interesting RF modulation response for the modulated master laser. An RF gain up to 30 dB is attained for frequencies greater than a certain critical frequency f(c), which is approximatelty 10 GHz here and increases with the injection power. In addition, an RF phase change of 2 pi is achieved above f(c) by varying the wavelength detuning. The slope of phase-frequency curve represents an equivalent slow or fast light attained through the slave laser. We incorporate an amplifier model to explain this novel modulated-master injection-locked VCSEL configuration. Simulations show good qualitative agreement with the experimental results.

Long-wavelength tunable vertical-cavity surface-emitting lasers and the influence of coupled cavities.

In this paper, we present an InP-based micromechanically tunable VCSEL emitting in the 1.55microm wavelength region with a 26nm tuning range. The laser is based on a two-chip concept, allowing for a separate optimization of the curved top mirror and the amplifying component. Current confinement is achieved by a buried tunnel junction. The design of the microcavity ensures fundamental mode operation with a side mode suppression ratio exceeding 49dB even for large apertures. Simulations indicate that the tuning range is limited by coupled cavity effects and reveal important design criteria like an upper boundary regarding the device thickness.