Results 2009

An Optical signal system has advantages in terms of the delay because it is independent on the wiring capacity. Optical functional active and passive devices on SOI platforms get much attention for photonic integrated circuit. III-V/Si Heterogeneous photonic integration devices, such as lasers and waveguide devices have been studied.

Results obtained in this research are as follows:

(1) Silicon based optical devices have a potential to realize ultracompact functional photonic integrated circuits. The development of active photonic devices such as lasers and optical amplifiers fabricated on silicon on insulator (SOI) platforms has attracted considerable attention. Injection type DFB lasers directly bonded on an SOI substrate were successfully realized for the first time. A threshold current as low as 104 mA was obtained under a room-temperature pulsed condition for the stripe width of 25 um and the cavity length of 1 mm.

(2) Ultra low-power consumption light source are essential to utilize the advantage of optical system in the short length optical interconnection. Toward low threshold and high efficiency performance, GaInAsP/InP membrane DFB laser with thin semiconductor core layer and low refractive index cladding layers has been studied. A lateral current injection (LCI) type laser composed of 400nm thin core layer including strain compensated five quantum wells, was demonstrated for electrically pumped operation of the membrane laser. Threshold current of 105 mA with the stripe width of 5.4 um and the cavity length of 1.47 mm were obtained.

(3) GaInAsP/InP LCI DFB laser on semi-insulating substrate was realized. A room temperature-pulsed operation was achieved for the cavity length of 300 um length. The threshold current of 27 mA and the threshold current density of 2.6 kA/cm2 were obtained. This device oscillated at an emission wavelength of 1540.7 nm and a side-mode suppression ratio (SMSR) of 35 dB at 2.0 I_th was obtained.

(4) Lower threshold and higher efficiency of GaInAsP/InP LCI FP laser on semi-insulating substrate was improved by reducing waveguide absorption loss and an amount of recombination in optical confinement layers. Threshold current of 11 mA (threshold current density was 900 A/cm2) and an external differential quantum efficiency of 33% were obtained for the device with 720 um-long cavity and 1.7 um wide stripe under RT-CW conditions.

(5) Dynamic characteristics of the LCI laser were investigated for future high performance membrane laser. The modulation efficiency of f_r was estimated to be 0.43 GHz/mA^1/2 from relative intensity noise (RIN) measurements. Eye opening up to 2.5 Gbps was confirmed at 37.5 mA bias and 0.8 Vpp modulation voltage with the PRBS word length of 2^7-1.

(6) Fundamental properties of a lateral junction type photodiode were investigated with LCI laser. The responsivity of 0.27 A/W, 3 dB bandwidth of 6 GHz and 7.5 GHz at a bias voltage of 0 V and -2 V, respectively, were obtained for the stripe width of 1.4 um and device length of 220um. An error free performance up to 6 Gbps at 0 V was confirmed.

(7) Wafer bonding technology was investigated for hybrid integration of optical devices based on III-V compounds on Si or silicon on insulator (SOI) substrates toward highly compact photonic- integrated circuits. A surface activated bonding method is expected to have the advantage of low-temperature process. Bonding strength of InP on Si (1.6 MPa) by the surface activated bonding technique was demonstrated. In addition, photoluminescence (PL) intensity from GaInAs/InP quantum-wells were investigated. The degradation of PL intensity after the bonding process was much larger than that after the plasma irradiation, and it was larger for the quantum-well close to the bonding interface.

(8) Loss reduction methods for silicon (Si) wire-waveguide on silicon-on-insulator substrate were investigated. Si waveguides with a Si core size of 200 × 440 nm² were fabricated with electron beam lithography and dry etching, using a double layer of electron beam (EB) resist mask, which consists of C₆₀ contained resist and conventional positive resist, to increase the selectivity between Si and the resist. An edge enhancement writing method during EB writing was also introduced to reduce the sidewall roughness of the waveguides to be 3.1 nm (3σ value). The transverse electric (TE) mode propagation loss measured at a wavelength of 1550 nm was 4.5 dB/cm, which is, to the best of our knowledge, the lowest value ever attained for a Si wire waveguide fabricated by the parallel plate reactive ion etching (RIE) method.

GaInAsP/InP strained-quantum-film, -wire, and -box lasers have been studied both theoretically and experimentally. A new type of distributed reflector (DR) laser, fabricated by the same fabrication processes as those of quantum-wire lasers and distributed feedback (DFB) lasers with wirelike active regions, has also been studied. Furthermore novel photonic devices, such as a left-handed light controlling device and a laser transistor have been studied both theoretically and experimentally.

Results obtained in this research are as follows:

(1) A Distributed reflector (DR) laser, which consists of the active DFB and passive DBR sections with a quantum-wire structure, was studied. DFB and DBR sections are integrated by using the energy blue shift due to the lateral quantum confinement effect. For a DR laser with low-threshold and high-efficiency operation, a high reflection DBR mirror is required. From the theoretical and experimental investigations of DBR reflectivity, a DBR section with the reflectivity of over 90% was confirmed. For further threshold current reduction, a DR laser with a phase-shifted DFB section was studied. Phase-shifted grating can be fabricated easily by changing the EB lithography patterns. From the theoretical analysis, it was found that threshold current can be reduced to half by adopting a, /8-shifted grating.
Experimentally, sub-mA threshold current (I_th) operation of a DR laser as low as 0.8 mA and an external differential quantum efficiency from the front facet (η_df) of 20% have been achieved under RT-CW conditions. In order to reduce waveguide loss and realize higher differential quantum efficiency, regrowth condition of an optical confinement layer (OCL) was modified. As a result, waveguide loss was reduced to 4 cm^-1, which is much less than the previously reported values of 6-7 cm^-1. From DR lasers fabricated with the new condition, a low I_th of 1.6 mA and a high η_df of 53% were obtained with the 215-μm-long DFB section and the 2.1-μm stripe width. η_df of 53% is at least twice that of the previously reported DR lasers. Then, the reduction of threshold current was achieved by phase-shift and shortened cavity length. For the DR laser with λ/16 phase shift, a DFB section length of 85 μm, and the stripe width of 1.5 μm, a threshold current as low as 0.9 mA and an ηdf of 48% were obtained The injection current for the 1-mW light output was 3.6 mA, which is, to the best of our knowledge, the lowest ever reported in all kinds of edge-emitting-type single mode laser.

(2) Semiconductors lasers with high optical feedback tolerance have attracted great interest in low-cost optical transmitters. Since optical isolators in a package module are costly and bulky, lasers operating without isolators are strongly demanded to reduce the module cost. Since DR lasers with wire-like active regions have strong index coupling and high longitudinal-mode stability, high feedback tolerance is expected. We experimentally investigated the optical-feedback tolerance of DR laser with wire-like active regions for the first time. Static and dynamic feedback tolerances were examined by relative intensity noise (RIN) measurements and by performing a bit-error-rate test, respectively. The critical feedback level was investigated by tracing RIN at the relaxation oscillation frequency of 8.4 GHz. A very high critical feedback level of 12.5 dB, which is 10 dB higher than conventional lasers, was obtained. And isolator-free 2.5 Gb/s 10 km transmissions was demonstrated under 13.5 dB optical back-reflection. A small power penalty of 2 dB against a 13.5 dB feedback level was achieved, while power penalties against 10 km SMF transmission were negligible below the critical feedback level.

(3) Developing new access network technologies to fulfill the demands for high bandwidth is important issue. A promising solution is using optical injection locking (OIL) techniques, which enables modulation bandwidth enhancement, relative intensity noise (RIN) reduction, and chirp-managed transmission with a simple combination of typical optical components. While several experimental studies of high bandwidth of OIL lasers have been reported, low-power consumption operations were limited to vertical-cavity surface-emitting lasers due to the high operation current of conventional DFB lasers. Because Distributed reflector (DR) lasers with wire-like active regions have a sub-mA range of threshold-current operation as well as high output power sufficient for the module standards, DR lasers are promising candidate for OIL access network systems. Now, modulation bandwidth enhancement of DR lasers with wirelike active regions has been demonstrated by optical injection locking. The small signal bandwidth was increased to >15-GHz at a 5 mA bias, which is 4-times smaller bias than that for conventional edge-emitting lasers. Direct modulation of DR lasers at 10 Gbps has also been performed with a bias current of 5 mA and voltage swing of 0.4 Vpp.

(4) Distributed reflector (DR) lasers with wirelike active regions are promising for low-power consuming high speed optical transmitters. To realize high speed direct modulation of DR lasers, a low parasitic capacitance structure using Benzocyclobtene was proposed and tested by GaInAsP/InP Fabry-Perot lasers. From S21 measurement by a network analyzer, a small signal bandwidth of 10 GHz was obtained, which is about 50% increase from the previous double-channel high-mesa structure. Also, an eye opening of 10 Gbps signals was obtained and the low parasitic characteristic was confirmed.

(5) Laser transistors (LTs), consisting of three electrical terminals for lasing operation, have potentials to meet the demands of high speed modulation beyond laser diodes (LDs) and multi-functions by utilizing the three terminal operation in optical systems. Different operation behaviors such as low modulation damping of LTs compared with LDs were reported in GaAs systems. However, the details of the operation principle had not been well understood yet. We simulated the small signal response of LTs by modifying rate equations of LDs considering collector current. In the case of common base configuration, the maximum modulation bandwidth was increased to >40 GHz due to lower damping effect whereas that of common emitter configuration showed no difference with LDs.

(6) We successfully simulated the large signal responses of LTs for the first time. LTs with AlGaInAs quantum wells (QWs) for the emission wavelength of 1.3-μm were analyzed. The rise time of the carrier density above quantum wells NV.S. is faster than that of the LD and this difference results in 8 ps faster response of S in this case. This can be explained by that the electrons in LTs are quickly supplied to the barrier region above QWs because carriers can be easily stolen from the current flow to the collector unlike the LD case where most of carriers recombine in the base layer. Therefore, small damping effect by carrier capture effect can be obtained in LTs and this results in higher modulation bandwidth than that for LDs. Eye diagrams were calculated for a 5QWs AlGaInAs LT modulated under 40 Gbps NRZ pseudorandom bit sequence (PRBS) with the word length of 2^7-1. Clear eye opening can be obtained for the LT whereas the eye opening for the LD was much narrow.

(7) The AlGaInAs/InP alloy system is very attractive for thermoelectric coolerless operation due to its good temperature characteristics. However, the oxidization of Al-containing layers prevents high quality crystal growth during the embedding growth, resulting in not only poor lasing characteristics but also poor reliability. Therefore, quantitative studies of the regrowth interface quality and lasing characteristics of BH lasers based on AlGaInAs/InP are meaningful. We reported the effects of the thermal cleaning to the regrowth interface quality by means of the surface recombination rate estimated from the spontaneous emission intensity dependence on the stripe width. The initial wafer of AlGaInAs/InP BH lasers consisted of an n-InP cladding layer, 1.4% compressively-strained (CS) Al_0.15Ga_0.12In_0.73As_0.27 five quantum-wells ( 5QWs, 5 nm thick for 1.3 um wavelength) with -0.7% tensile-strained (TS) Al_0.25Ga_0.32In_0.43As_0.57 barrier layers sandwiched by AlGaInAs graded-index separate-confinement-heterostructure (GRIN-SCH) layers, a p-InP layer and a GaInAs contact layer grown on an n-InP substrate by organo-metallic vapor-phase-epitaxy (OMVPE). Various widths mesa stripes (2, 3, 5, 7, 10, 20, and 50 μm) were formed by wet and dry etchings using a SiO₂ mask. Before the regrowth, wet cleaning processes, Br₂:CH₃OH = 1:40000 which cleans the surface and a mixture of H₂SO₄:H₂O₂:H₂O = 1:1:40 which cleans the Al containing region, and 1% BHF which removes the oxidized layer, were carried out. Then the wafer underwent a thermal cleaning process with a PH₃ atmosphere in the OMVPE reactor to expose fresh regrowth surface prior to the growth of current blocking layers. The reactor temperature was fixed at 450°C and the cleaning time was varied for 15, 30, and 60 min. The internal quantum efficiency η_i and the waveguide loss α_WG were estimated to be η_i = 69 % and α_WG = 8 cm^-1, η_i = 72 % and α_WG = 4 cm^-1, η_i = 81 % and α_WG = 7 cm^-1, for the cleaning time of 15, 30 and 60 min., respectively. It is noteworthy that the longer cleaning time lead to higher η_i, and η_d = 63 % was obtained for L = 500 μm., These results indicate that the thermal cleaning is effective for reduction of the sidewall recombination velocity of AlGaInAs/InP BH lasers.

(8) One promising way of creating novel optical-communication devices is controlling the permeability, as well as the permittivity, of materials that form the devices. This can be achieved using the concept of left-handed materials (LHMs), or meta-materials, which have attracted growing attention in recent years. To examine the feasibility of such LHM optical devices, we fabricated an InP-based optical waveguide device combined with LHM and demonstrated magnetic interaction with the metamaterial and light that travelled in the waveguide. The LHM consisted of an array of minute metal split-ring resonators (SRRs) attached on the waveguide. The resonance wavelength of the SRR was set to 1.5 μm. Transmittance of light in the device depended strongly on the polarization and wavelength of the light. This shows that the SRR array interacted with the magnetic field of the light and produced magnetic resonance at optical frequencies. Our result is useful to develop waveguide-based metamaterial devices for optical communication.

To realize the seamless integration of optical and wireless communication systems, a novel method of direct conversion from optical signal to THz signal which requires simple semiconductor structures has been proposed and demonstrated. The integration of optical and THz functions can be realized by adding THz converter consist from simple semiconductor structure into THz generator such as resonance tunneling diode (RTD).

Results obtained in this research are as follows:

(1) Optical circuits and optical networks are being installed in everywhere from long-haul to inter/intra chip networks due to the high capacity of their data rate. On the other hand, because of wide bandwidth capability and high directivity compared with microwaves used in conventional wireless communication systems, sub-THz and THz waves are expected to be crucial frequency-bands in next generation wireless communication systems. Therefore, it is very important to realize the direct signal conversion method between THz wave signals and optical signals. We proposed and realized a novel direct conversion method using photon-generated free-carriers. Free carriers which generated by photon absorption absorb THz waves due to the skin effect and free-carrier absorption. By changing light power irradiated into the semiconductor, THz power passing through the semiconductor can be changed. Using this phenomenon, intensity modulation of THz waves can be realized. Experimentally the intensity change of sub-THz waves by the intensity change of optical input was observed using GaInAs modulator on an InP substrate. Using 192 GHz waves, a modulation depth of 45% was observed at an optical input power of 114 mW, which is 1.5-fold greater than the modulation depth of 96 GHz waves.

(2) A novel method of signal media conversion from optical signal to Terahertz (THz) signal by the skin effect and the free-carrier absorption using photon generated free-carriers in semiconductor was demonstrated. The modulator 2-μm thickness intrinsic GaInAs grown on semi-insulating InP substrate and 1.55 μm wavelength light was used. Using 192 GHz continuous sub-THz wave and 1.55 μm optical signal, the modulation depth of 45% and the modulated speed up to 2 MHz were demonstrated. The low modulation speed was attributed to the large rise time of THz signal due to the carrier spreading in the GaInAs modulator. In order to confine the photon-generated carriers, the GaInAs modulator was etched and a 1-mm-diameter disk was formed. As a result the rise time of sub-THz signal was reduced from 600 ns to 200 ns.