We discuss high speed switching of lasing circular polarizations in VCSELs by optical spin injection. We conducted
polarization- and time-resolved measurements of two consecutive lasing outputs from a (110)-InGaAs/GaAs VCSEL at
77 K with different time delays between the two optical excitations for alternately up- and down-spin electrons. 1-GHz
switching of lasing circular polarizations has been demonstrated with taking advantage of the long electron spin
relaxation time τs
in (110)-QWs. Rate equation analysis closely reproduced the measured results and showed that
shortening the carrier lifetime τc while preserving the long τs is a straightforward solution for faster switching since the
residual unpolarized electrons limit the switching speed. Thus, we dry-etched the (110)-QWs into micro-posts to
introduce the surface non-radiative recombination using ECR-RIE, and investigated the τc and τs. Spin-polarized carriers
were optically excited in square posts with different sizes from 0.5 μm to 30 μm, and the time evolutions of two
orthogonal circular polarization components of photoluminescence were measured by a streak camera. The long τ
s (~1.3 ns) in the (110)-QW wafer is found to be preserved even when the sidewall boundaries with fast surface recombination
are introduced and the τc is drastically shortened. The same rate equation analysis indicated that spin-controlled VCSELs
with such (110)-QW micro-posts will exhibit faster switching thanks to the shortened τc and preserved long τs. In
particular, 20-GHz switching is expected with 0.5-μm posts, although the threshold pulse energy per unit area becomes
2.9 times larger than that for 1-GHz switching without post structure.
A novel photonic buffer memory with a shift register function is proposed. The buffer memory consists of a two-dimensional
array of polarization bistable vertical-cavity surface-emitting lasers (VCSELs), in which the bit state of the
optical signal, "0" or "1" is stored as a lasing linear polarization state of 0 or 90°. Input data stored as the polarization
states of the first VCSEL are transferred to the polarization states of the second VCSEL. In our experiment using 980
nm VCSELs, 10 Gbit/s optical buffering, 2-bit optical buffering, and a shift register function have been successfully
After a brief introduction of optical bistable operation of semiconductor active devices such as LDs and SOAs, recent progress in polarization bistable VCSELs and their applications for all-optical signal processing are presented. Applications include all-optical flip-flop operation with very low switching energy and high repetition rate, all-optical signal regeneration, and optical buffer memory.
A numerical model for the investigation of the ultrafast gain properties in asymmetrical multiple quantum-well semi-conductor optical amplifiers (AMQW SOAs) has been developed considering propagation of ultrashort optical pulses with different wavelengths. The dynamics of the number of carriers and carrier temperature are investigated for each quantum well. The results agree with the experimental results of pump probe measurements with different wavelengths. It is shown that gain recovery is slower for higher energy wells for pump signals of all wavelengths.
Modulators based on interband (IB) light absorption by intersubband (ISB) excitations in undoped quantum well structures (QWs) has the inherent advantage of ultrafast response without thermal dissipation at high bit rates. In this report we present an efficient scheme to achieve ultrafast modulation in the femtosecond regime using IB and ISB light pulses in a step-like type II semiconductor QW. The threshold control-light intensity for 100% modulation in the proposed structure is less than 1 pJ, which is at least an order of magnitude lower than in any excitonic optical switch proposed until now. The peak modulation efficiency in asymmetric QW's at 1 MW/cm2 is 40% which is twice than that estimated in symmetric QW's and can be enhanced to 100% at 10 MW/cm2. A modulation speed of 500 fs can be achieved without any serious degradation of the IB signal due to thermal dissipation. This is an important step towards the development of novel ultrafast optoelectronic devices based on the pulse shaping techniques.
We have demonstrated the highly nondegenerate four-wave mixing among sub-picosecond optical pulses in a 1.3 micrometers multiple-quantum well semiconductor optical amplifier (SOA). We could directly measure the four-wave mixing signal in the output spectrum by current pulse pumping of the device. We achieved a high conversion efficiency of over 10% at a frequency conversion range of less than 1 THz. The limitation of conversion efficiency due to the gain saturation of the SOA was effectively overcome by using the short optical pulses. Preliminary results of optical sampling with picosecond time resolution are also described.
We report a simple rate equation analysis taking into account the gain saturation to analyze the pitchfork bifurcation polarization bistability in laser diodes (LDs), as well as the rate equation analysis taking into account detunings of incident optical beam from the cavity resonant frequency of an LD. We show some of our experimental results displaying pitchfork bifurcation polarization bistability and the all-optical flip-flop operation in a vertical-cavity surface-emitting laser (VCSEL). We experimentally obtained the pitchfork bifurcation polarization bistability by using a trigger optical input in the VCSEL. We achieved the ultra- fast polarization bistable switching with a switching time of 7 ps by using picosecond optical trigger pulses.
We propose a new method for obtaining high-power short optical pulses without trailing pulses from an actively mode-locked laser diode. To quench the residual optical gain, a part of the output is reflected back into the laser diode with an orthogonal polarization and an appropriate time delay. The dynamics of the laser diode were numerically analyzed using traveling-wave rate equations. We show that the trailing pulses caused both by the excess gain and the residual reflection at the laser diode facet are drastically suppressed. We also report experimental results which demonstrate the effectiveness of the proposed method for the reduction of the trailing pulses caused by the excess gain using 1.55 micrometer InGaAsP laser diodes.
System synchronization is an important technology in constructing all-optical signal processing systems. To realize this, an all-optical timing extraction circuit is required, which recovers timing information from an incoming optical data stream, and produces an optical clock without intermediate electric stage. A potentially simple method of all-optical clock recovery uses self-pulsating laser diodes (SP-LDs), and many experimental studies have been reported so far. However, there is no report on theoretical works to our knowledge except the perturbation analysis by Lee and Shin. In this paper, we report numerical analysis of all- optical clock extraction using the SP-LD based on rate equations. The model used in this analysis is two-section SP-LD. The laser is divided into a gain region (region I) and a saturable absorber region (region II). The carrier lifetime of region II is much shorter than that of region I. THe photon density of injection signal is coherently coupled to the SP-LD light. To obtain the time response of the laser, we solved rate equations numerically. It is shown that we can extract clock pulses from signals which contain regulated digital patterns or a pseudo- random binary sequences (PRBS) data pattern. The results will explain the experimental results reported so far. Relative phase of the extracted clock with respect to the input signal varies when the power of the input optical signal fluctuates or a PRBS data pattern is used. However, this inherent effect can be minimized by using low input power level, although the locking range becomes narrow.