Laser-based displays have attracted much attention because they are the only displays that can express the full color gamut of Ultra-High Definition Television (U-HDTV), International Telecommunication Union Radio Communication Sector Broadcasting Service (ITU-R BT).2020. We have developed 638-nm broad-area laser diode (LD) with 75-μm-wide dual emitters and achieved wall plug efficiency of over 40.5% at 25°C. But slow degradation is still a factor limiting the lifetime due to the requirement for a high temperature and high-power operation. We performed the long-term aging test of 638-nm LDs under the different conditions, including a high-power operation such as 1.3 to 2.5 W, and investigated the behavior of output power characteristics, especially the dependence of its output characteristics on temperature. After long-term aging, the threshold current of the LD increased and its slope efficiency (SE) decreased. The measured dependence of the output characteristics of the LD on temperature was converted into that on the junction temperature using the thermal resistance between the junction and package. The latter dependence showed an increase of threshold current and no change of SE. This result indicates that the slow degradation of the red broad-area LD was caused by the increase of the nonradiative recombination rate in the active layer during aging.
Laser based displays have gathered much attention because only the displays can express full color gamut of Ultra-HDTV, ITU-R BT.2020. One of the displays uses the lasers under pulse such as a single spatial light modulator (SLM) projector, and the other does ones under CW such as a multiple SLM projector and a liquid crystal display. Both types require high-power lasers because brightness is the most important factor in the market. We developed two types of 638-nm multi-emitter high-power BA-LDs assembled on Φ9.0-TO, that is, triple emitter for pulse and dual emitter for CW. The triple emitter LD emitted exceeding 6.0 W peak power under 25°C, frequency of 120 Hz, and duty of 30%. At high temperature, 55°C, the peak power was approximately 2.9W. The dual emitter emitted exceeding 3.0W under 25°C, CW. It emitted up to 1.7 W at 55°C. WPE of the dual emitter reached 40.5% at Tc of 25°C, which is the world highest in 638-nm LD under CW to the best of our knowledge, although that of the triple emitter was 38.1%. Both LDs may be suitable for laser based display applications.
Reliabilities of the 638-nm triple emitter broad area laser diode (BA-LD) with the window-mirror structure were studied. Methodology to estimate mean time to failure (MTTF) due to catastrophic optical mirror degradation (COMD) in reasonable aging duration was newly proposed. Power at which the LD failed due to COMD (PCOMD) was measured for the aged LDs under the several aging conditions. It was revealed that the PCOMD was proportional to logarithm of aging duration, and MTTF due to COMD (MTTF(COMD)) could be estimated by using this relation. MTTF(COMD) estimated by the methodology with the aging duration of approximately 2,000 hours was consistent with that estimated by the long term aging. By using this methodology, the MTTF of the BA-LD was estimated exceeding 100,000 hours under the output of 2.5 W, duty cycles of 30% .
Substantial limitation of output power in AlGaInP based red broad area (BA) laser diode (LD) originates from an electron
thermal overflow from an active layer to a p-cladding layer and fatal failure due to catastrophic optical mirror degradation
during the LD operation. New red BA-LD was designed and fabricated. The LD chip had triple emitters in one chip with
each stripe width of 60 um, and was assembled on Φ9.0 mm -TO package. The LD emitted exceeding 5.5 W at heat sink
temperature of 25 °C and 3.8W at 45 °C under pulsed operation with frequency of 120Hz and duty of 30%, although the
current product, which has a 40 um single emitter chip assembled on Φ5.6mm –TO, does 2.0 W at 25 °C. The lasing
wavelength at 25 °C and 2.5W output was 638.6 nm. The preliminary aging test under the condition with the operation
current of 3.56A, CW, auto-current-control mode (ACC), and the heat sink temperature of 20 °C (almost equal to the
output of 3.5 W) indicated that the MTTF due to COMD was longer than 6,600 hours under CW, 22,000 hours under the
pulse with duty of 30%.
Laser based displays, as pico to cinema laser projectors have gathered much attention because of wide gamut, low power consumption, and so on. Laser light sources for the displays are operated mainly in CW, and heat management is one of the big issues. Therefore, highly efficient operation is necessitated. Also the light sources for the displays are requested to be highly reliable. 638 nm broad stripe laser diode (LD) was newly developed for high efficiency and highly reliable operation. An AlGaInP/GaAs red LD suffers from low wall plug efficiency (WPE) due to electron overflow from an active layer to a p-cladding layer. Large optical confinement factor (Γ) design with AlInP cladding layers is adopted to improve the WPE. The design has a disadvantage for reliable operation because the large Γ causes high optical density and brings a catastrophic optical degradation (COD) at a front facet. To overcome the disadvantage, a window-mirror structure is also adopted in the LD. The LD shows WPE of 35% at 25°C, highest record in the world, and highly stable operation at 35°C, 550 mW up to 8,000 hours without any catastrophic optical degradation.
Wall plug efficiency (WPE) of 830 nm single mode LD is dramatically improved with AlGaInP material. Conventional
near infrared LDs based on AlGaAs have poor temperature characteristics due to small energy gap in conduction band
between a p-cladding layer and an active layer. An AlGaInP based LD is a most effective candidate for the excellent
characteristics because of the large gap. A high power 830 nm LD is newly designed based on AlGaInP. The LD shows
excellent temperature characteristics as To of 154 K, and its WPE is around 40% at 400mW, CW output, 60°C case
temperature. They also show very stable operation at the condition up to 1,100 hours.
In wavelength region of red color, luminous efficacy rapidly increases as wavelength shortens. In that sense, red laser
diode (LD) with shorter wavelength is required for display applications. Experimental results for short wavelength
limitation in AlGaInP LDs are shown and discussed in this paper. Broad area LDs with 625, 630, and 638 nm are
successfully fabricated. Operation current versus output power (P-I) characteristics and its temperature dependence of
625 nm LD are inferior to that of 630 and 638 nm ones. The main reason might be carrier leakage, and the results
indicate that additional countermeasures to carrier leakage should be adopted to realize a 625 nm LD with the same
temperature characteristics as 630 and 638 nm LDs. Conversion efficiencies from input electrical power to luminous
flux output of the LDs are also studied. 625 nm LD has low efficiency, though brightness of 625 nm light is 1.7 times of
638nm one with the same output power. And 630 nm LD shows better conversion efficiency at high luminous flux
region than 638 nm one, though the P-I characteristics of 630 nm is worse than that of 638 nm one. The tendency is
inverted at low flux region, indicating that the lasing wavelength of red LD for laser display should be chosen carefully.
A kink mechanism of a red (658nm) laser diode (LD) has been investigated in order to achieve a higher power operation of over 200mW. The experimental results indicate that a main origin of the kink generation is due to the deviation of the refractive index step caused by the local heat generation at the stripe region. To reduce the heat generation at the stripe region, an extension of a cavity length of the LD is applied. The LD with the cavity length of
1500μm realizes a kink-free 200mW operation even at 80°C. Also, this LD shows a reliable pulsed operation of 230mW at 75°C and 250mW at 70°C for over 1700 hours. This is the highest power operation among narrow stripe 658nm LDs.
As light sources of CD-R/RW and DVD-RAM/RW, highly efficient high-power 785 nm (AlGaAs) and 660 nm (AlGaInP) lasers are demonstrated, respectively. A real-refractive-index waveguide with small internal loss is applied to both the lasers in order to reduce the operating current by improvement of the external differential quantum efficiency. The mirror degradation level is increased by reduction of the optical power density and/or a non-optical-absorbing effect of the window-mirror. As a result, the 785 nm window- mirror with AlGaAs current blocking layer has showed stable transverse mode operation up to 250 mW (kink level: over 300 mW at CW) with the high slope efficiency of 1.1 W/A. Reliable 140 mW-CW and 180 mW-pulse operation has been realized at 70 - 75 degree(s)C. As for a 660 nm laser with the window-mirror, the operation current at 70 mW is reduced by 40% due to the high slope efficiency (1.08 W/A) resulting from the low-loss ridge-waveguide. The lateral mode is well stabilized up to 70 mW by the effect of the narrow ridge stripe formed by a dry etching technique. Reliable 70 degree(s)C, 70 mW pulse (duty cycle: 50%) operation with a low operating current of around 120 - 140 mA has been achieved. In addition, the lasers have operated for over 1000 hours even at 70 degree(s)C, 80 mW.
We have investigated dependence of the saturation output power on the wavelength and the cavity length of red laser diodes (LDs) with the wavelength range of 646 - 689 nm. In the 60 degree(s)C CW operation of 650 micrometers -long-cavity LDs, the saturation powers of the 659 nm-LD and the 687 nm-LD were 90 mW and 124 mW, respectively. As a result of extension of the cavity lengths from 650 micrometers to 900 micrometers , the saturation output powers of the 659-nm LD and the 687 nm-LD are increased to 121 mW and 165 mW, respectively. This improvement has led to the first realization of 1000-hour, 100 mW CW operation of the 659 nm-LDs. Also, the 900 micrometers - long 687 nm-LDs have shown the reliable 120 mW CW operation at 40 degree(s)C for 1800-hour, for the first time.
Kink-free, high coupled power of 148 mW into a single mode fiber has been realized by narrowing the vertical beam divergence in 0.98 micrometers laser diodes. It has been found that the high-refractive index GaAs layers have crucial influence on the far field patterns as well as lasing spectrum, since the GaAs is transparent to the emission wavelength of 0.98 micrometers .
Proc. SPIE. 1634, Laser Diode Technology and Applications IV
KEYWORDS: Semiconductor lasers, Etching, High power lasers, Semiconducting wafers, Laser applications, Metalorganic chemical vapor deposition, Cladding, Quantum wells, Technologies and applications, Active optics
An approach to mass-production of triple quantum well lasers with a buried-ridge, loss-guided inner-stripe structure is demonstrated, using a large-scale metalorganic chemical vapor deposition. The lasers obtained from nine epi-wafers grown at one time show the uniform characteristics. In regard to high-power characteristics, the fundamental transverse mode up to 100mW and the maximum output power of - 170mW are realized at room temperature. Even at 95 C, the light output power of 100mW is obtained. The lasers have been operating over 1000 hours without failure at 6O C, 50mW. To realize the further uniformity and reproducibility of the laser characteristics, we have Introduced a newly developed etching method with an etching stop layer in the ridge formation.