Kimio Shigihara

Laser diodes in photon number squeezed state

A laser diode with an intrinsic layer as the space charge limited current region is expected to emit a low noise (less than the shot noise level) light. However, when one applies the intrinsic layer to the laser diode, severe difficulty is faced. Because the intrinsic layer has a very high resistivity, the applied voltage to operate the laser diode is too large and causes catastrophic damage to the laser diode. Here we propose novel laser diodes which emit a low noise light. The first is an AlGaAs laser diode having an undoped layer between the active layer and the cladding layer which acts as the space charge limited current region. Fano factor, F/sub m/, of this laser diode is 28% smaller than the shot noise level (standard quantum limit, F/sub m/=1) at 21 mA (output power, P/sub 0/=20 mW). The second one is an InGaAsP laser diode having two tunnel barrier layers whose bandgap energy is larger than that of the cladding layer. The region between the barriers acts as the space charge limited current region, Fano factor, F/sub m/ of this laser diode is 47% smaller than the shot noise level at 21 mA (P/sub 0/=10 mW). On the other hand, an AlGaAs laser diode with the two tunnel barrier layers has Fano factor, F/sub m/ which is 43% smaller than the shot noise level at 21 mA (P/sub 0/=20 mW). The calculated amplitude noise spectral densities of the latter two laser diodes are in good agreement with the calculated values from Langevin method. However, the calculated amplitude noise spectral density of the former laser diode does not agree with the calculated value from Langevin method. This disagreement is also discussed.

Lateral mode controlled wide-stripe-lasers by modal reflector

A modal reflector having a high reflective (about 40 percent) region with 24 microns, 48 microns or 96 microns width on the output facet, and applied to gain-guided wide (150 microns) stripe lasers, is described. The partially high reflective region composing the modal reflector consists of an Al2O3/a-Si multilayer whose thickness is determined so as to match in phase between a part of the output from the highly reflective region and remaining outupt from the low reflective ( about 1 percent) region in order to reduce side peaks (fringes) in far-field patterns. Single-lobed far-field patterns for lateral direction as narrow as from 0.88 deg to about 3.5 deg are observed at up to 0.4 W under CW condition.

Transverse Mode Controlled Wide-Single-Stripe Lasers By Loading Modal Filters

Two kinds of modal filter for high power diode lasers with a wide stripe have been examined. The first one is a partially narrow region formed in the active region. Flared-SBA(self-aligned bent active layer) laser in which the active layer flares the width from the narrow filter region to the wide output facet oscillates in a fundamental transverse mode under cw condition up to the output power of 64mW. The second one is a partial coating at the output facet of lasers. By applying it to an SBA laser whose stripe width is 150 μm, the laser oscillates in a single lobed far-field pattern up to 300mW, whose divergent angle is as narrow as 1.7° in FAHM(full angle at half maximum) under cw conditions.

Estimation of strain arising from the assembling process and influence of assembling materials on performance of laser diodes

A method for estimating strain in a semiconductor laser chip arising from bonding to foreign materials. The method is based on measurements of dependence of lasing wavelengths on duty ratios of pulsed operation with a constant peak current. The wavelength at zero duty ratio (λ0) is extrapolated from this dependence, from which the strain is estimated. Estimated strain from this method is in good agreement with result obtained from the change of polarization ratio, transverse‐magnetic to transverse‐electric light outputs. The strain arising from packaging affects not only lasing wavelength but also operating characteristics such as threshold current. Moreover, thermal resistance of laser diodes can be obtained from this dependence of lasing wavelengths on duty ratios.

Suppression of Lasing Wavelength Change of 980 nm Pump Laser Diodes for Metro Applications

High-power 980 nm laser diodes (LDs) are used as pumping sources of erbium-doped fiber amplifiers (EDFAs). Reductions of costs and power consumption in EDFAs are very important for access or metro applications. System construction without fiber Bragg gratings (FBGs) and thermal electric coolers (TECs) is strongly required. Therefore, high-power 980 nm LDs with small lasing wavelength changes under various operational conditions are highly desired in these systems. We proposed a new technique of wavelength change suppression using a facet coating for 980 nm LDs. Less than one-third reduction of lasing wavelength change was realized by this method. Moreover, the proposed LDs possess more than 350 mW output power even at 95°C and good reliability at 50°C with 700 mA automatic current control (ACC).

High power and highly reliable 1020 nm laser diodes for praseodymium doped fiber amplifier

Ridge waveguide laser diodes emitting at 1020 nm, which is a pumping wavelength for praseodymium doped fiber amplifiers, have been developed. In oder to achieve high output power and highly reliable operation, a window structure is introduced by utilizing Si+ ion implantation and annealing. As a result, high output power more than 520 mW without catastrophic optical damage and lifetime longer than 15000 hours under the condition of 50 Celsius, 150 mW have been realized.