We report on spin polarization modulation characteristics of InAlGaAs vertical-cavity surface-emitting lasers (VCSELs) under the optical injection locking. The results measured with the optical spin modulation technique and simulated with the spin-flip model indicate that a weak optical injection locking condition is suitable to exploit a strong and high-frequency resonance response under the spin polarization modulation. We discuss an effect of polarization switching in VCSELs which can be a cause of variation of the spin polarization modulation response and injection locking condition. Our findings suggest a strategy of the optical injection locking for spin-controlled VCSELs.
We numerically investigate characteristics of spin-controlled vertical-cavity surface-emitting lasers (spin-VCSELs) under injection locking by using spin-flip rate equations. Generation of a modulation sideband whose phase is correlated with an injected light into a spin-VCSEL, is an attractive application of spin-VCSELs to frequency-shifted local oscillators in coherent optical communication systems. Our results indicate that the spin polarization modulation with a high degree of spin polarization is important for efficient generation of strong modulation sideband. Additionally, matching the modulation frequency and polarization mode splitting in the spin-VCSEL contribute to achieve efficient and ideal single sideband generation.
KEYWORDS: Modulation, Signal detection, Orthogonal frequency division multiplexing, Spin polarization, Signal generators, Radio over Fiber, Polarizers, Vertical cavity surface emitting lasers, Polarization
We investigate the applicability of spin-controlled vertical-cavity surface-emitting lasers (spin-VCSELs) for optical transmitters in analog radio-over-fiber systems by using spin-flip rate equations. In addition to the current modulation for generating orthogonal frequency division multiplexing (OFDM) signals, we use the spin polarization modulation to excite a high-frequency polarization oscillation corresponding to a millimeter-wave carrier frequency. The polarization oscillation is converted to intensity modulation by using a polarizer, and the millimeter-wave carrier and the OFDM signals can be combined. Our results indicate that OFDM signals with millimeter-wave carrier can be generated by using the spin- VCSEL, and bandwidth of the current modulation in the spin-VCSEL limits data rate of the OFDM signals.
We propose and numerically investigate a method for direct in-phase quadrature (IQ) modulation of spin-controlled vertical-cavity surface-emitting lasers (spin-VCSELs) for coherent communications. In particular, an injection-locked spin-VCSEL is investigated by using spin-flip rate equations for generating quadrature amplitude modulation (QAM) signals. A polarizer 45° tilted to the polarization axis of the injected external light is used to extract the in-phase and quadrature-phase components of the output light from the injection-locked spin-VCSEL. It is confirmed that QAM signals can be generated with the proposed method successfully, and long electron spin relaxation time and small linear birefringence in the spin-VCSEL are suitable for efficiently generating clear QAM constellations with a low error vector magnitude. A detuning condition for the injection locking of the spin-VCSEL is found to be important for obtaining wide modulation bandwidth of the direct IQ modulation.
High-speed vertical-cavity surface-emitting lasers (VCSELs) are essential for future data communications with high speed, low cost, and low power consumptions. For exceeding intrinsic modulation bandwidth of VCSELs limited by the relaxation oscillation frequency, manipulation of electron spin polarizations in VCSELs has been attracting attention in recent years. In this study, we theoretically and experimentally investigate modulation characteristics of 1.55-m VCSELs under the spin polarization modulation to obtaining tailored modulation characteristics suitable for high-speed data communications. Spin-flip rate equation analyses reveal that a short spin relaxation time is suitable for a flat modulation response under the spin polarization modulation, and a 100-Gb/s operation is expected in InAlGaAs quantum well (QW) VCSELs with a spin relaxation time of 10 ps, linear birefringence of 100 GHz, and dichroism of 50 GHz. A wide 3-dB bandwidth of 23 GHz determined by a frequency split between two orthogonal polarization modes (~19.4 GHz) is experimentally confirmed under optical spin polarization modulations by using a commercially-available InAlGaAs QW VCSEL whose relaxation oscillation frequency is ~3 GHz. These results support the idea that the ultrahigh- speed optical signal generation is available at the telecom wavelength of 1.55 m by applying the spin polarization modulation to VCSELs. Additionally, a modulation format conversion technique for output lights from the polarization modulation to phase modulation by using a polarizer is suggested and confirmed by the spin-flip rate equation analyses.
Phase noise of a single mode semiconductor laser is reduced drastically by introducing a newly proposed optical negative feedback scheme. Proof-of-concept experiment confirms that the spectral linewidth of a semiconductor laser can be reduced to 1/1,000 successfully by applying the scheme.
We propose a novel InP-based traveling-wave electrode Mach-Zehnder modulator. It has an n-i-n isotype heterostructure to reduce both electrical signal loss and the optical loss caused by the p- type cladding layer. This device provides a large modulation bandwidth of more than 40 GHz. We have also developed a compact
push-pull driven modulator module. We obtained error-free operation for a 40-Gbit/s NRZ signal in a push-pull configuration with a very low driving voltage of 1.3 Vpp. We also confirmed that the modulator has low chirp characteristics by demonstrating a 100-km SMF transmission with a penalty of less than 1.5 dB for a 10-Gbit/s NRZ signal.
A monolithically integrated opto-electronic device is proposed as a fast wavelength-switching light source. This tunable duplex integrated light source comprises two wavelength-tunable distributed Bragg reflector (DBR) laser diodes (LDs), two MQW-electro-absorption optical switches, a Y-shaped waveguide coupler, a MQW-electro-absorption modulator, and two thermal drift compensators (TDCs). The wavelength-switching time of the optical switches was estimated to be 60 ps including a 50-ps rise time for the electrical-pulse generator. The wavelength of a 10-Gbit/s NRZ-modulated optical signal can be switched without bit loss. The function of the TDCs is to keep the device-chip temperature constant. Thermal-transient- induced wavelength drift with a millisecond-order time constant, which has been reported for DBR-LDs, and thermal crosstalk between the tuning regions of the integrated LDs, which causes wavelength fluctuation, are effectively suppressed by thermal-drift-compensation operation using the TDCs.
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