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. There are many types of laser-based displays. A projector with a single spatial light modulator is mainstream in the displays so far. This projector uses the laser light sources under pulse with duty of 20% to 50%. We improve a triple-emitter red color 638-nm high-power broad area laser diode (LD) for this application. Countermeasures are adopted to improve the output power characteristics and strengthen the facet to the fatal degradation. The improved triple emitter shows the output of 5.0 W at the injection current of 5 A under pulse with duty of 30%, case temperature of 25°C. The output is increased by 4.1% compared to our former one. The peak wall plug efficiency reaches to 41.6% under 25°C. The long-term aging test is also performed. Mean time to failure of the improved LD is estimated 72,000 h under the output of 3.5 W, pulse condition with duty of 30%. The value is ∼3.3 times longer than that of the former.
Laser based displays have gathered much attention because only the displays can express full color gamut of Ultra- HDTV, ITU-R BT.2020. There are many types of laser based displays. A projector with single spatial light modulator (SLM) is a main stream in the displays so far. This projector uses the laser light sources under pulse with duty of 20- 50 %. We improved a triple-emitter 638-nm high-power broad area (BA) laser diode (LD) for this application. Countermeasures were adopted to improve the output power characteristics and strengthen the facet to the fatal degradation. The improved triple emitter showed the output of 5.0 W at the injection current of 5 A under pulse with duty of 30%, case temperature of 25°C. The output was increased by 4.1% compared to the former. The peak wall plug efficiency reached to 41.5% under 25°C. The long term aging test was also performed. Mean time to failure of the improved LD was estimated 57,000 hours under the output of 3.5 W, pulse condition with duty of 30%. The value was approximately 2.6 times longer than that of the former.
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, such as 10 lm to 70K lm laser projectors and laser diode (LD) backlight liquid crystal display (LCD) TVs, have attracted much attention because of the large gamut, low power consumption, and so on. Laser light sources for the displays are operated mainly under CW, and requested to be highly reliable. In this work, we will present the latest reliability study on high-power 638 nm broad stripe (BS) LD with a window-mirror structure, which is formed by using Zn diffusion into a quantum well active layer. Although the LD showed no catastrophic optical mirror degradation (COMD) even above the output of 1.6 W initially, the LDs aged at output power around 1.0 to 1.5 W showed sudden degradation during 1,000 to 4,000 hours. The duration to the failure became shorter as the power increased. Electro luminescence (EL) imaging revealed that the root cause of the sudden degradation was COMD at the front facet even though the LD had a measure to COMD. The LDs aged at 0.42 W output showed no COMD up to 6500 hours with extremely stable operation. The result also revealed that the mean time to failure due to COMD was proportional to optical density to the power of -3.2. The LDs, which had 60% small power density compared to the former, showed stable one up to 4,000 hours without COMD at 1.25 W. It is clarified that maintaining a low optical output power density is essential to develop high-power and highly reliable red BS-LDs.
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.
Higher power laser diodes (LDs) with a wavelength of 637-639nm are strongly demanded as a light source of display
applications because luminosity factor of laser light is relatively high. In order to realize reliable high power operation,
we have optimized LD structure, focusing on improvement of power saturation and sudden degradation. As a result,
40μm-wide broad-area (BA) LDs with window-mirror structure have been designed. We fabricated two kinds of single
emitter LDs of 1.0mm cavity and 1.5mm cavity. The single LD is installed in conventional φ5.6 mm TO-CAN package.
The 1.0mm LD showed very high wall plug efficiency (WPE) of 33% at 25 ºC (23% at 45 ºC) in the power range of
around 300mW (30 lm). High output power of 600mW (60 lm) is realized by the 1.5mm LD. Both LDs have operated
for over 1,000 hours without any degradation. Estimated mean time to failure (MTTF) is 10,000 hours. In addition, we
fabricated an array LD consisting of 20 emitters (BA-LD structure), which shows reliable CW operation of 8W (at
junction temperature of 50 ºC) for 10,000 hours.
Short wavelength and highly efficient AlGaInP quantum-well laser diode is promising as a red light source for small
laser display application. Two kinds of the laser diodes are presented in this paper. A narrow ridge laser diode was
designed for single lateral mode. In addition, a broad area laser diode was optimized for the higher power operation. To
suppress a carrier leakage from an active layer, AlInP cladding layers were adopted to both of the lasers. Evaluation tests
of the fabricated lasers were performed under CW operation. The wavelength of the narrow ridge laser was 636.0 nm
under the condition of 25°C and 100 mW. Single lateral mode oscillation and the high wall plug efficiency of 29% were
obtained. The beam divergences were 16° and 8° in fast and slow axes, respectively. The broad area laser showed lasing
wavelength of 636.9 nm at 25°C for 200 mW output. The wall plug efficiency was 30% under this condition. Both of the
lasers showed both high luminance and high wall plug efficiency. These lasers are suitable for small laser display
A high-power and short-wavelength GaInP/AlGaInP quantum-well laser diode array was designed and fabricated.
Because a conduction band offset of this material system is small, a carrier leakage from an active layer is an important
limiting factor of the maximum light output. In this work, long cavity length of 1.5 mm, high front facet reflectivity of
18% and AlInP cladding layers were adopted to reduce the leakage. An evaluation test of the fabricated array was
performed under CW operation. At 15°C, high light output of 12W was obtained with injection current of 16A. The
lasing wavelength was 643.3 nm. Moreover, high wall-plug efficiency of 34% was achieved. These excellent
characteristics are considered to be due to the effective suppression of the carrier leakage.
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.
The 780nm 35mW I2SPB lasers(Inverted Inner Stripe Laser with a p-
GaAs Buffer Layer) with a thin active layer have been successflly
developed by employing MOCVD. Accelerated aging tests on the ISPB
lasers with the cavity length of 400iim at 60 °C, 30mW project
lifetimes (MTTF) greater than l0000hrs. The narrow standard
deviations of beam divergences parallel and perpendicullar to the
junction plane, respectively 0.18 and O.2Odeg, are achieved. The
small astigmatic distance less than 6iim and the high optical S/N ratio
more than 65dB (at f=2OkHz, BW=300Hz and P=3OmW) are obtained. These
exellent results indicate that the I2SPB lasers are available for the
Magneto-Optical Disk (MOD) applications.