Current dependence of performance of TES microcalorimeters and characteristics of excess noise

We are developing transition edge sensor (TES) microcalorimeter arrays for future Japanese astronomy missions. Although 6.6 eV energy resolution for 5.9 keV and 74 μs decay time are achieved, our energy resolution was limited by the excess noise of unknown origin. The dominant noise showed 1/R dependence and it only appeared when the current through the TES, I, is larger than 10 μA. This is explained if we assume a constant voltage noise source with a level of 2(4kT_CR_n)^1/2. We also investigated the influence of I on the TES performance and found the TES sensitivity is reduced when the ratio of current to the critical current I/I_C is large. It can explain the reduction of observed when operated without a magnetic shield. We also found a strong correlation between I_C and or the baseline width. Thus, we conclude that I_C is an essential parameter for the TES performance.     

Mode-hopping noise in index-guided semiconductor lasers and its reduction by saturable absorbers

Mode-hopping noise in index-guided semiconductor lasers is investigated. It is found that random switching between lasing modes and output power differences in those modes cause mode-hopping noise. An effective method to suppress such mode-hopping noise is proposed. High Te doping to an n-type GaAlAs cladding layer completely suppresses the noise. Te in GaAlAs forms a DX center that acts as a saturable absorber. This property stabilizes the laser mode and prevents mode competition. The minimum loss difference between lasing and nonlasing modes to suppress mode-hopping noise is also discussed.     

High-current resolution broad-band SQUID amplifier suitable for TEScalorimeter

A superconducting quantum interference device (SQUID) amplifier has been developed as a current detector with both high-current resolution and broad bandwidth for a transition edge sensor calorimeter. The amplifier is a two stage SQUID (TSS) that consists of an input-SQUID with a 38-turn input coil and a 100-serial SQUID array (100-SSA) output, and has been integrated on a 3 × 3 mm Si chip using Nb thin film fabrication technology. It is designed to increase the amplifier gain and maintain matching with the parameters of the calorimeter. To avoid flux trapping in the SSA, the washer coil of the dc-SQUIDs in the SSA was made with a narrow line width of 17.5 μm. We experimentally confirmed that the designed output voltage was achieved using a one-layer p-metal magnetic shield tube in the earth's magnetic field. The performance of the shielded TSS amplifier was evaluated in liquid helium. The TSS amplifier had a gain of 10 kV/A and an impedance of 0.07 Ω at 100 kHz. When a flux locked loop circuit was used to drive the amplifier, a current resolution of 1 pA/√Hz and a rise time of 1 μs were achieved     

An 8 mm length nonblocking 4×4 optical switch array

A novel single-mode-single-slip-structure S³ optical switch using carrier-induced refractive index change is proposed as a unit cell for a small polarization-independent nonblocking N×N optical switch array. Sixteen S³ optical switches have been integrated into a nonblocking 4×4 optical switch array on an InP substrate. The 8-mm-length InGaAsP/InP 4×4 optical array has shown satisfactory switching characteristics and is suitable for larger scale integration of optical switch arrays and also for integration with other active optical devices such as laser diodes     

Noise characteristics of high-frequency superposed laser diodes for optical disc systems

Excess noise peaks in an index-guided single-mode laser can be suppressed by superposing high-frequency current. By this method, index-guided lasers have stable noise characteristics and the noise level is about 7?12 dB lower than that in gain-guided multimode lasers at output power of about 3 to 5 mW.     

Ellipsometric evaluation of shape of corrugations for DFB-LD

The shape of corrugations for a DFB-LD is evaluated using ellipsometry. This technique provides a nondestructive method of evaluating corrugation shape. The corrugation on the surface of an InP substrate is assumed to be a thin film. The thickness and refractive index of this virtual thin film, measured by ellipsometry, provide information on corrugation depth and shape, respectively. With this method, it is possible to distinguish corrugations which have the same diffraction efficiency but different shapes     

High-speed scanning by dual feedback control in SNOM/AFM

We have developed a high-speed scanning near-field optical microscope (SNOM)/atomic force microscope (AFM) system including dual feedback controllers. The system includes an additional piezoelectric actuator with fast response in the z direction and a correction circuit to eliminate unnecessary components from the feedback signal. From the measurement of a patterned chromium layer of 2 x 2 microm2 checks on a quartz glass plate, we confirmed that our system had more effective feedback control and faster scanning than current SNOM/AFM systems that use only a piezo-tube. The scanning speed of the present system was estimated to be about five times faster than that of current SNOM/AFM systems.     

Ultralow chirp and high-speed 1.55 μm multiquantum wellλ/4-shifted DFB lasers

An ultra-low-chirp, high-speed, 1.55-μm, multi-quantum-well (MQW), λ/4-shifted, InGaAsP distributed-feedback (DFB) laser is demonstrated. A record chirp width of 0.4-0.5 nm (20-dB down full width) is achieved at 10-Gb/s direct modulation. This low-chirp single-longitudinal-mode operation is attained by an increase in the quantum size effect in the MQW active layer     

The POLARBEAR-2 Experiment

We present an overview of the design and development of the POLARBEAR-2 experiment. The POLARBEAR-2 experiment is a cosmic microwave background polarimetry experiment, which aims to characterize the small angular scale B-mode signal due to gravitational lensing and search for the large angular scale B-mode signal from inflationary gravitational waves. The experiment will have a 365 mm diameter multi-chroic focal plane filled with 7,588 polarization sensitive antenna-coupled Transition Edge Sensor bolometers and will observe at 95 and 150 GHz. The focal plane is cooled to 250 mK. The bolometers will be read-out by SQUIDs with \(32\times \) frequency domain multiplexing. The experiment will utilize high purity alumina lenses and thermal filters to achieve the required high optical throughput. A continuously rotating, cooled half-wave plate will be used to give stringent control over systematic errors. The experiment is designed to achieve a noise equivalent temperature of 5.7  \(\mu \) K \(\sqrt{s}\) , and this allows us to constrain the signal from the inflationary primordial gravitational corresponding to a tensor-to-scalar ratio of \(r = 0.01\) ( \(2\sigma \) ). POLARBEAR-2 will also be able to put a constraint on the sum of neutrino masses to 90 meV ( \(1\sigma \) ) with POLARBEAR-2 data alone and 65 meV ( \(1\sigma \) ) when combined with the Planck satellite. We plan to start observations in 2014 in the Atacama Desert in Chile.     

Suppression of leakage current in InGaAsP/InP buried heterostructure lasers by InAlAs strained current-blocking layers

InGaAsP/InP buried heterostructure (BH) lasers with InAlAs strained current-blocking layers are proposed. For use as a current-blocking material with a bandgap energy larger than that of InP, InAlAs is superior to InGaP. This is because a wide bandgap can be obtained in the InAlAs without large bandgap shrinkage caused by tensile strain. A two-dimensional simulation shows that the leakage current in BH lasers with thin pnpn blocking layers is suppressed up to 85 degrees C by employing 100 AA In/sub 1-x/Al/sub x/As(x>or=0.54) layers.<>