Chapter 5: Application of Laser Diode

In this chapter, applications of laser diodes are explained such as material processing, telecom, biology and so on. Learn about applications which you want to use laser diodes for and then select an adequate laser diode.

Table of Contents

5. Application of Laser Diode

5.1. Application to Fiber laser of laser diode

Fiber laser oscillator is composed of pump laser diode and active fiber as shown in figure.
Fiber laser amplifier is composed of signal laser, pump laser diode and active fiber as shown in figure.

Fig. 5.1.1. (a) Fiber laser oscillator and (b) Fiber laser amplifier

Laser diodes are used as pump laser and/or signal (seed) laser in fiber lasers.
Active fiber can be divided into two types single-clad fiber (SCF) and double-clad fiber (DCF).
Laser diode used in fiber lasers are shown in the table.

  • Laser diode used as pump-laser of single-clad fiber
  • Laser diode used as pump-laser of double-clad fiber
  • Laser diode used as seed-laser of fiber laser amplifier

Table 5.1.1. Pump-laser of single-clad fiber

Perpose Kind of fibers Doped rare earth Wavelength of laser diode(nm) Polarization maintaining Fiber Bragg grating Isolator Recommended manufacturers
Pumpimg Single clad Tm 793 Lumics
Yb or Er:Yb 976 3SP Group
OPTOENERGY
Er 976 3SP Group
OPTOENERGY
980 3SP Group
Fitel
Fitel
1480 Fitel
Fitel

Table 5.1.2. Pump-laser of double-clad fiber

Use Kind of fibers Doped rare earth Wavelength of laser diode(nm) > 100 W Volume Bragg grating Feedback protection Recommended manufacturers
Pumping Double clad Tm 793 DILAS
Lumics
Yb or Er:Yb 915 DILAS
Jenoptik
940 OPTOENERGY
976
Er 976 BWT Beijing
980

Table 5.1.3. Seed-laser of fiber laser amplifier

Use Kind of fibers Wavelength of laser diode(nm) Recommended manufacturer
Seed Tm Fiber Laser 2000 Elbana Photonics
Yb Fiber Laser 1064 3SP Group
Innolume
Lumics
SHIMADZU
QD Laser
Onefive
Er or Er:Yb Fiber Laser 1550 Onefive

Active fiber is the fiber that rare earth element such as Er, Yb, Nd, Pr, Tm and Ho is doped.
The wavelength of laser in laser diode varies depending on the semiconductor-composing element as shown in table. And rare-earth-doped fiber pumped in wavelength range of each laser diode is shown in table.
Laser diode can be used for broad wavelength range.
Laser diode is most important element to develop high-power fiber laser.

Table 5.1.4. Wavelength of each semiconductor-composing element and kinds of rare-earth-doped fiber to be pumped

semiconductor-composing element wavelength [μm] rare earth
InGaN/GaN 0.38-0.48 Pr
GaAs/AlGaAs 0.78-0.86 Nd, Tm
InGaAs/GaInAsP/GaInP 0.9-0.98 Yb, Er
GaInAsP/InP, AlGaInAs/InP 1.47-1.6 Er
InGaAsP 1.8-1.96 Ho

5.2. Application to DPSS laser of laser diode

Laser diodes are usually used as the pump-laser in the DPSS (diode pumped solid-state) laser.
Laser oscillator consists of a laser medium and its pump source.
As the pump source, flash lamps and laser diodes are usually used.

Flash lamp

Flash lamp is a cost effective source and it offers high output power. We must be careful a large heat load for the laser medium. This is because the flash lamp has a broad spectrum so that a large part of the optical energy transfers to heat energy.

Laser diode

According to the development of high power, high efficiency, and long life laser diodes (LD’s) in 1980s, the technology of solid-state lasers pumped by laser diodes has been developed. Since the emission spectrum of LD’s is narrow, high absorption efficiency can be easily achieved. Furthermore, high-density excitation can be achieved because of its high coherence of the output beams.
DPSS lasers are excited in the way as shown in Fig. 5.2.1. Figure 5.2.1. (a) shows the LD side-pumped system and (b) shows the LD end-pumped system.
Q-switch laser or Mode-locked laser is achieved by inserting some optical elements in the laser cavity. For the high-power DPSS laser, cooling system using air or water is necessary.

5.2.1
Fig. 5.2.1. Excitation method for the solid-state laser.
(a) LD side-pumped system, (b) LD end-pumped system

5.3. Application to Material Processing of laser diode

The Properties of Laser Sources for Material Processing

The features and types of laser sources for material processing are shown in a following table.

Table 5.3.1. The features of laser sources for processing

Medium Wavelength
(nm)
Oscillation form Max output Efficiency Type of process
Gas CO2 10600 CW
Pulse
(1 Hz〜a few hundred kHz)
A few dozen
10 W (CW)
8 % Cuting
Microfabrication
TEA-CO2 Gas pressure dependence Pulse A few dozen W Cuting
Excimer ArF 193 Pulse
(1 Hz〜1 kHz)
a few hundred W Microfabrication
KrF 248
XeCl 308
Solid
(Crystal)
YAG 1030
1064
CW
Pulse
A few dozen kW
(Disk, CW)
〜20 % @Rod
〜25 % @Disk
Cuting
Welding
Carving
YVO4 1064 Pulse
(1 Hz〜a few hundred kHz)
A few kW Cuting
Ti:Sapphire 800 Ultrashort Pulse
(1 Hz〜100 MHz)
A few dozen W Microfabrication
Fiber Yb fiber 1064 CW
Q switch
Ultrashort Pulse
(A few dozen kHz〜100 MHz)
A few dozen kW (CW) 〜35 % Cuting
Welding
Marking
Microfabrication
Laser diode Semiconductor < 980 CW
Pulse (1 Hz〜1 kHz)
A few kW 〜60 % Cuting
Welding

CO2 laser
CO2 lasers are widely used in laser processing owing to its low cost and high beam quality. This laser will be continued to use in laser processing.

YAG laser
Although the beam quality of the LD pumped YAG laser is relatively low, this laser offers another advantages such as high efficiency of the high harmonic generation. Previously the lamp excited YAG laser are used, but now such lasers are not used owing to its low lasing efficiency and low beam quality.

Yb fiber laser
Yb fiber lasers are advantageous in the beam quality with respect to that of LD’s. However, the initial cost is higher than the LD’s.

Direct laser diode system
Direct laser diode system offers very compact and low cost. However the beam quality of laser diodes is intrinsically lower than fiber lasers.

Fig. 5.3.1. Comparison of lasers for material processing

Laser surface processing

Laser surface processing is one of the laser processing techniques. The laser beams are used for surface reforming through the laser irradiation to the material surface. The laser surface processing includes heating, melting, and evaporating process. The heating process deals with surface quenching and annealing. The melting process deals with alloying, build-up welding, and grazing. The evaluating process deals with impact effect in the material and drying process. In the surface processing, a CO2 laser, a Nd:YAG laser, a laser diode, and a fiber laser are used as for a laser source.
 

 

5.4. Application to Telecom of laser diode

5.4.1. History of Telecom

In 1970 both low loss (20 dB/km) of silica-based optical fiber and laser diode operation at room temperature was achieved this was the first step for practical use of telecom. Since then, laser diode development has been made for telecom to achieve higher-speed and longer-distance transmission.

In the early stage, laser diode with wavelength of 0.8 μm and multi-mode fiber were used for optical fiber telecommunication. After this, band frequency with low data transmission loss has been shifted to a longer wavelength of 1.3 μm or 1.55 μm band have started being used. Also, single mode fiber (SMF) has begun to be used in long-distance transmission in order to remove the influence of transmission-velocity difference in the transverse-mode.

Since the theoretical limit of transmission loss (<0.2 dB/km) for optical fibers was achieved in the 1.55 μm band, the band has been used for long-distance trunk. With the 1.55 μm band influence from optical-fiber dispersion needs to be considered. The spectrum of laser lights widen as modulation speeds up. Long-distance transmission causes the disorder of high-speed signal leading to limitation of transmission distance. In order to avoid the disorder distributed feedback (DFB) laser diode was studied and commercialized, which can achieve narrow spectrum regardless of modulation.

With the development of erbium-doped fiber amplifier (EDFA), the frequency change (chirp) due to high-speed modulation of laser diode had become the cause of limitation in the transmission distance. In order to avoid the problem the method of modulation by an external modulator was suggested. There are two types of external modulations: one which uses a LiNbO3 crystal and one which uses a compound semiconductor crystal. High-speed operation has been achieved by DFB laser integrated with electro-absorption (EA) modulator.

Optical fiber amplification technology was born in the late 1980s. It has made great revolution in telecom technology by achieving non-relay long-distance transmission. In optical fiber amplifiers there are the method of pumping doped rare-earth ion and the method of using Raman amplification. In these methods 0.98 μm or 1.4 μm band laser diode are used. It enables capturing the wavelength as the optical signal channel. The development of wavelength-tunable laser diode which changes the oscillation wavelength by about 30 nm has attracted attention.

5.4.2. Basic Structure

Briefly speaking, laser diodes are diodes which emit light when injected with a current. In telecom, 1.3 μm and 1.55 μm band laser diodes are used and the light emission can be obtained by injecting forward bias current to an active layer. By using the active layer that is sandwiched by crystals with a wider-band gap, career (electrons and holes) is confined into the active layer and the emission efficiency increases. Refractive index of the active layer is slightly higher than the surrounding semiconductor crystal and the waveguide is made along the active layer. In the case of a Fabry-Perot (FP) laser diode cleavage surface of the crystal becomes the mirror of the resonator. It causes stimulated emission in the active layer achieving laser oscillation. DFB laser has different structure from FP laser and the resonator structure is composed of diffraction grating engraved in vicinity of the active layer.

Optical signals are generated by modulation of the current magnitude injected in to the laser diode. This method is called direct modulation. Direct modulation method and external modulation method are shown in table 5.4.1. Integrated laser diode has both EA modulator and DFB laser diode on the same device that adopted external modulation. Optical signal from the DFB laser diode is guided into the EA modulator that can absorb the signal upon applying reverse bias voltage. Since career density fluctuation inside the active layer is smaller compared to direct modulation external modulation can suppress the chirp. Therefore, external modulation method can achieve high-speed modulation with low spectrum disorder caused by the dispersion.

Table 5.4.1. Comparison between direct modulation and external modulation

System Direct modulation External modulation
Laser Optical Modulator + Integrated Laser Optical Modulator + Laser
Structure Constitution Integrated Integrated Separate or Hybrid Integrated
Light source Longitudinal mode control DFB DFB DFB
Transverse mode control BH BH (ridge) BH
Active layer material InGaAsP-MQW InGaAsP-MQW InGaAsP
Modulator Modulation principle Quantum-confined Stark effect electro-absorption EO effect
MZ interference
Transverse mode control BH (ridge) Diffused waveguide, others
Material InGaAsP-MQW LN (GaAs)
Characteristics Modulation bandwidth [GHz] ~20 >40 >30
Chirp index 2~3 ~0.25 -1, 0, 1

Active layer structure
When laser diode was first commercialized thin-film crystal was used as an active layer. Recently, multiple quantum well (MQW) is used for the active layer. Quantum well means that crystal with is about 10 nm thick is sandwiched by crystals with wider band gap and multi quantum well creating a MQW. Threshold current density for oscillation of the quantum-well structure is only a fraction of other structures.

The first prototype of the quantum well laser diode using GaAlAs related materials was reported in year 1975. Since it was found that the laser diode has one-third of the current density threshold (0.25 kA/cm2) compared to DH laser much research has been performed. It started to be used for laser diodes for telecom after InGaAsP related quantum well structure was made by metal organic chemical vapor deposition.

Fig. 5.4.1. Active layer structure. (a) double hetero structure and (b) MQW.

Control of transverse mode
Control of transverse mode of laser diode has two types (Fig. 5.4.2):
Method-A:Gain-guiding type structure controlling the gain depending on the place.
Method-B:Refractive index-guiding type structure controlling by refractive index difference of the crystal
Method-A confines oscillation light by using the phenomenon where current density becomes high directly under the electrode to inject current. Method-B confines oscillation light by making waveguide through refractive index difference between different crystals. Since method-B can make stable waveguide even if career density decreases inside the active layer with increasing light density in the resonator. Because of this it can be used for many types of laser diodes including telecom.

In refractive index-guiding type structure, the type with a stripe active layer buried into a crystal is called buried hetero (BH) type. In InGaAsP related laser diode because threshold current value largely depends on the temperature, BH structure which conducts operating current to active layer are used in many cases. Active layer width is designed to meet cut-off condition of high-order mode. This is about 1~2.5 μm. Also, in some cases ridge structure is adopted. Ridge structure is used for high-speed device since the structure can be achieved low volume easily and can be easily used for laser structure even for InGaAlAs related crystals relatively susceptible to oxidation.

5.4.3

Fig. 5.4.2. Control of transverse mode of laser diodes.
(a) gain-guiding type structure and (b) refractive index-guiding type structure.

Control of longitudinal mode
In optical fibers propagation constant which is decided by glass material and structure has wavelength dependence. Therefore, if laser diode with multi mode is used as the source spectrum is disordered by dispersion characteristics and propagation distance is limited. In order to avoid the problem DFB laser diode is used. With DFB laser diode longitudinal mode can be selected by diffraction grating in active layer. Consequently, DFB laser diode can be operated with single-longitudinal mode even with career density fluctuation in active layer with high-speed modulation. To create a diffraction grating a two-beam interferometry by an Ar laser or electron-beam lithography is used.

In order to enhance the selectiveness of longitudinal mode, phase-shift DFB laser and gain-coupling DFB laser were developed. Both phase-shift and gain-coupling DFB laser achieve stable and single longitudinal mode operation by adjusting reflected-light phase inside the resonator. The phase shift DFB laser adjusts the light phase by reversing grating period of diffraction grating in the resonator. The gain coupling DFB laser adjusts the light phase by engraving diffraction grating directly in the active layer and giving the gain a cycle.

Since GaAsP related semiconductor crystal mainly used for telecom is not susceptible to oxidation. Diffraction grating can be engraved on or in the vicinity of the active layer. With distributed Bragg reflector (DBR) laser diode, longitudinal mode is selected also by diffraction grating as well as DFB laser diode. DBR laser diode has structure of integrated diffraction grating outside the active layer. It was originally developed as the light source for coherent telecom and now it is one of the types of tunable laser diodes.

5.4.3. Various Laser Diodes Used for Telecom

In the telecom field 1.3 μm and 1.55 μm band laser diodes are used as main light source since silica fiber lasers have less transmission loss in the band. Additionally, laser diode with different band are used for pumping source for optical amplification or for short-distance optical link. Here are introduced various laser diodes used for telecom. In table 5.4.2, laser-diode modules are introduced in each wavelength bands.

0.8 μm
This is used for datalink combined with multi-mode fiber. Particularly, it has started to be used as a light source for short-distance data transmission such as gigabit ethernet by using VCSEL (vertical cavity surface emitting laser).

0.98 μm
This is used to pump EDFA used for optical signal amplification in the 1.55 μm band. Light excitation in this band can make optical amplification with low noise factor. It is used for pre-amplification at a receiver or for forward pumping at repeaters.

1.3 μm
FL type is used as light source for FTTH (fiber to the home), for subscriber line under 150 Mbit/s, as light source for office line up to 2.5 Gbit/s or for datalink. DFB laser diode is used for branch line or datalink, or analog-modulated laser (used for cable television). It is also used for pulse-light source for OTDR (optical time domain reflectance) designed for maintenance and inspection of optical fiber transmission line.

1.4 μm
This is mainly used as pumping excitation light source for EDFA. Because it is a wavelength close to 1.55 μm, it has a high energy conversion efficiency. Compared to 0.98 μm band, it is suitable for higher output power and can be used for high-output EDFA by backward pumping. Also, it can be used as pumping light source for Raman amplifier, which achieves signal amplification by induced Raman scattering.

1.5 μm
Because it is in the low loss band of optical fiber it is used for long-distance light source such as trunk. Because this band is highly influenced by dispersion of the optical fiber, DFB lasers which work with single longitudinal mode are mainly used. Main use is for research and development. small-lot production.

Table 5.4.2. Various laser diodes for telecommunication

Wavelength[μm] Structure Material Substrate crystal Output[mW] Use
longitudinal mode transverse mode
0.85 VCSEL GaAlAs GaAs 1 data link (~10 Gbit/s/ch)
0.98 FP BH(ridge) InGaAs
/AlGaAs
~500 pumping source for EDFA (1.5 μm band)
1 pumping source for EDFA *(1.3 μm band)
1.06 pumping source for EDFA *(1.4 μm band)
1.31 VCSEL* GaInNAs 0.5 datalink (~2.5 Gbit/s/ch)
FP BH(ridge) InGaAsP
InGaAlAs
InP 5~15 FTTH (~155 Mbit/s), office line, datalink (~10 Gbit/s), PON (~155 Mbit/s)
DFB 5
30
300
branch line,datalink (10 Gbit/s)
CATV
OTDR (pulse)
1.48 FP ~500 pumping source for EDFA
1.4 pumping source for Raman amplifier
1.55
(1.47 ~ 1.61)
5 FTTH (~155 Mbit/s)
DFB 10
50
200
trunk,metro,datalink (2.5 ~ 10 Gbit/s)
light source for external modulation
OTDR (pulse)
DFB integrated with EA 5 trunk,metro,datalink (2.5 ~ 40 Gbit/s)
DFB array* 10 metro (tunable wavelength)
DBR* 10
MEMS-VCSEL* 1
OTDM*

*for research and development, and small-lot production.

5.5. Application to Biology of laser diode

Lasers are used for wide variety of medical applications such as surgical treatments, cosmetics, and eye cares as shown in table. Basic of the laser treatment is removal of unneeded tissues and diseased tissues through the evaporation and carbonization of tissues by the illumination of a laser beam.

A surgery using a laser scalpel is most typical application. Compared with a conventional scalpel, the laser scalpel is advantageous by following reasons: i) The cut tissue is coagulated by the photothermal effect so that the procedure of stopping the flow of blood is unnecessary. This is effective to shorten surgical time. ii) The laser scalpel is sanitary because the one cut tissues without direct contact. In cosmetic treatments, the laser is used for removing moles, freckles, spots, and hairs. Recently the laser surgery for eye care including LASIK becomes popular. Types of surgical lasers include argon lasers, Nd:YAG lasers, CO2 lasers, excimer lasers and laser diodes.

Note that the misusing of lasers causes loss of eyesight and further deterioration of diseased tissue. Thus, it is important to use lasers in an appropriate manner.

Laser diodes have been used in a variety of medical applications such as PDT, Surgery (Coagulation, hemostasis-incision endoscopic), Snoring treatment Pain healing and mitigation of indirect jaw disease, Laser anesthesia, Shaping of the peri-implant gingiva, Whitening, Diagnosis of bone marrow, Hyperesthesia, Bruises, stains, freckles up, Hair loss, Headache, Whipping, Frozen shoulder, Elbow pain, Chest pain, Low back pain, Various pain such as indirect pain, Nerve paralysis, Contusion, Abrasions, Burn, and Treatment of keloid.

Table 5.5.1. Lasers for medical applications

Diagnosis and treatment Used laser Use
Ophthalmology Ruby laser Laser photocoagulation, Retinal detachment, LASIK (myopia, astigmatism treatment)
Fiber Laser LASIK
Excimer laser (ArF) Cornea removal, Cornea-like cut, Laser photocoagulation, Retinal detachment, LASIK
Surgery CO2 laser Laser knife, Ccancer treatment, Hay fever, Allergic rhinitis (Nasal mucosa ablation)
Nd:YAG Allergic rhinitis, Endoscopic incision, hemostasis and coagulation. Snoring treatment
Ar laser Photodynamic therapy (Photodynamic treatment)
Laser diode
(InGaAsP)
PDT, Surgery (Endoscopic coagulation, hemostasis and incision endoscopic), Snoring treatment
Dentistry CO2 laser Laser sealant, Treatment of hypersensitivity, Root canal treatment, Pigmentation removal, Enamel acid resistance strengthening
Er:YAG laser Pyorrhea treatment, Pigmentation removal, Gum whitening, Cutting hard tissue (bones or teeth), Incision or excision of Gingiva or mucosa
Nd:YAG laser Pyorrhea treatment, Laser anesthesia, Gum whitening
Ar laser Whitening (Bleaching), Pigmentation removal, Pigmentation removal, Enamel acid resistance strengthening
HeNe laser Stomatitis treatment, Easing and treatment of pain caused by temporomandibular joint disease
Laser diode Easing and treatment of pain caused by temporomandibular joint disease、Laser anesthesia、Peri-implant gingiva shaping, Whitening, Bone marrow diagnosis, Hyperesthesia
Aesthetic Plastic
Dermatology
CO2 laser Bruises, Stains and Freckles removel
Er:YAG laser Scars, rosacea, tattoos, skin linear, lentigines, warts, epidermal nevus, seborrheic keratosis, senile pigment nevus, etc.
Nd:YAG laser Tattoo, blue nevus, nevus of Ota, keloid, acne, wrinkles
Alexandrite Laser Stains, Ota nevus, seborrheic keratosis, traumatic tattoo, tattoos, acne, hair removal
Dye Laser Hemangioma simplex (red bruises), stain, freckles removal
Ruby Laser Bruises, spots, freckle removal, mole, senile plaque, flat nevus, nevus of Ota
Laser diode Bruises, spots, freckle removal, hair removal
Orthopedics Laser diode
(GaAlAs)
Headache, whiplash, frozen shoulder, elbow pain, chest pain, back pain, joint pain, nerve paralysis, contusions, abrasions, burns, Keloid treatment

5.6. Application to Laser Display and Illumination of laser diode

Laser diodes are very useful as a high-output light source because it has a larger amount of light compared to LEDs, and are possible to increase the light flux with a small number of elements. Also lasers have a high directivity. For this reason the optical design to use it as a light source is easy. Therefore, if laser diode lights are designed properly, can be made lite and compact allowing mobility. There are high expectations in use in the future.

5.6.1 Laser diode used laser display

A technique of laser display using laser diodes as its laser source is receiving a lot of attention because of following two reasons: One is the energy efficiency. Compared with other lasers, laser diodes are superior in laser efficiency. Such property is suited to today’s environment-conscious society The other is compactness. The development of compact displays is also important as well as that of the large size display. Development of much smaller device is important for the creation of a ubiquitous network society.

Concrete examples of the laser display with laser diodes are explained following sections.

5.6.2 Laser projector with laser diodes

Typically, a projector is the device for the projection a light or an image on the screen. Recently, a projector includes the device to enlarging images, which are indicating on another small display devices. In this section, such device for enlarging the images by using lenses is also defined as the projector. Laser projectors include scanning type and spatially modulation type for the projection.

Scanning type
Intensity modulated multi-color laser beams are combined collinearly. Then the beams are scanned on the screen to project an image. Projected image is recognized by using the residual image effect. Initially, a polygon mirror and a galvanometer mirror are combined and used for horizontal line scanning and vertical line scanning, respectively. But now, the angle of the mirror is mechanically controlled to wide-range scanning. Recently, micro electro mechanical systems (MEMS) are used. The scanning type laser projector can be compact and focus free because the projector lens is not needed.

Since high power laser diodes and the technology of MEMS are developed, the laser projector with scanning type recently attracts attention. There is a tradeoff between the resonant frequency and the scanning angle. To high speediness and wide scanning angle simultaneously, the development of MEMS is underway.

Example:
PicoP developed by Microvision is a scanning type laser projector for sale. Red and blue laser diodes and the SHG of the red laser diode are combined collinearly and are reflected by a MEMS scanner.

Two-dimensional spatial light modulator type
In this type, red, blue, and green laser beams are modulated by two-dimensional spatial light modulator (2D-SLM), and then these beams are combined and projected on a screen by a projection lens. There are three types of 2D-SLM: a liquid crystal display (LCD), a digital light processing (DLP), and liquid crystal on silicon (LCOS). Now, LCD and DLP are typically employed for sale.

In the LCD type, a laser beam transmits a transmission type LCD to adjust the brightness and illuminates the screen to image. In the DLP type, a laser beam illuminates on a digital micromirror device (DMD). By applying digital signal to MEMS, the laser beam is reflected to the screen for image projection. In the LCOS type, the laser beam is reflected by using LCOS for the projection.

The intensity of light in the 2D-SLM is inversely proportional to the product between the area of the light source and divergence angle. For this reason, the point source, such as a laser, is suited for the light source of the laser projector with the 2D-SLM.

Example:
Barco develops a high-intensity laser projector. Light Blue Optics develops a touch screen type laser projector, which includes an infrared sensor and a technology of holography to sense human motion.

5.6.3 Laser TV with rear-projection system

Many displays much as digital signage and flat-screen TV uses LCD backlight system and a rear projection system are widely used. Laser TVs have been developed with these systems. Principles and examples of rear-projection system using RGB3 color lasers and backlight system is stated here.

Rear-projection system uses RGB3 color laser as a back light, superimposing the image signal with a spatial light modulator, outputting the enlarged image and projecting it on to the screen from the back side of the screen by a projection lens.

There is a rear-projection system laser TV that Mitsubishi Electric has developed [1].

It replicates a wide color space with xvYCC standard using Mitsubishi Electric’s own color management technology ” Natural Color Matrix (NCM)”.

This is a system that controls the three laser diodes that make up the three primary colors of light (RGB) and the colors that are actually displayed to be faithful to the video signal. DMD is used as a spatial light modulator In 2008 the worlds first laser TV was sold in America. This TV used laser diodes for the red and blue, and used second-harmonic generation (SHG) for the green [2]. This DLP projection TV has the following features: Color reproducibility 2 times the normal LCD TV, power consumption significantly lower than LCD or plasma, thinner than the previous rear projection TVs due to the “super-wide-angle optical engine” that provides a wide projection angle.

Also, unlike SHG method, there is another method to get RGB 3 color through exciting fluorescent material by laser light. Prysm, Inc (U.S) has the technique to output the image through scanning coated RGB fluorescent material with modulating blue laser diode. This is to use greatly different optical method, but has the character not to make speckle noise.

Laser television with liquid crystal backlight system
Liquid crystal backlight system has following two methods:
(1) Edge light system to expose the light uniformly into liquid crystal through guiding the light from the side to light guide panel and using diffusion and reflection sheet.
(2) Direct under system to expose the light over all of the liquid crystal through LED array.

Display with liquid crystal backlight system can be used not only for Television, but also
for larger scale application, such as digital signage (digital signage). It is expected that high efficiency laser diode realizes large-sized screen and display with high-intensity, high-resolution, and energy saving.

5.6.4 Head-up display with a laser

Head-up display is the device that the information is imaged on the windscreen of cars and airplanes. It offers high level of safety when people is steering vehicles because there is no movement of viewpoint to see meters or navigation systems. Further enhancement of the safety can be achieved by imaging the information on to the eyes. The laser contributes the improvement of the visibility of the projected images.

The technology of the head-up display also uses in medical treatments. In drawing blood samples, by projecting the map of blood tubes on a human body makes easy to draw the blood. In image diagnosis, by projecting the results of X-ray inspection and/or computed tomographic scanning on the patient body makes easy to give a diagnosis.

Example:
Microvision and Pioneer develops a laser-projected head-up display. This is achieved by applying the technology of RGB laser scanning, which was developed by Microvision, to the projector module.

5.6.5 Wearable display

The wearable display is the display device incorporated in clothing and accessories. The glasses and hats are typical applications of the wearable display. In the case of glasses, a projector and a screen are installed in the temple. The screen is a transparent flat plate, which locates in the view field. We can see the images through the projection on the screen. In the case of hats, the display is hanging down.

Example:
Brother industry have been developing the glasses-type retinal imaging display. Retinal imaging display images not on the screen but on the eye directly. This device uses eye-safe low-bright light to the projection. The light is scanned on the eye so fast that the image can be recognized as a view by means of the residual image effect. The light source equips RGB-color lasers to display full-color images.

5.7. Application to Plant Production of laser diode

5.7.1 The light sources for plant factories

Plant factory uses some kinds of lights, such as a high-pressure sodium-vapor lamp, a fluorescent light, a light-emitting diode (LED), a metal halide lamp, and an incandescent lamp.

High-pressure sodium-vapor lamp

Due to high luminous efficiency, low cost, and long life time, high-pressure sodium-vapor lamps are the main light source in the plant factory, which uses sunlight as a source. The drawback of this lamp is low luminous efficiency in the blue and red region, which needs in growing of the plant. This lamp is so heat hot that plants have to keep the distance to the lamp.

Fluorescent light

The fluorescent light is used in the perfectly controlled plant factory owing to the capability for emitting visible lights and its cost effectiveness. Furthermore, the low radiation of heat realizes multiple cultivation. The drawbacks are the low luminous efficiency of the red light and a short lifetime.

Light-emitting diode (LED)

The LED gives several merits. The lifetime is longer than that of the fluorescent light by factor 5. The LED can also operates pulse-mode. The radiation of heat is low. Furthermore, since the emission spectrum of the LED matches both the absorption peak of the chlorophyll and the response of the photomorphogenesis, it is promoted that the LED is used as a light source for perfectly controlled plant factories. However, the initial cost is high compared with other light source. The blue-LED is especially expensive and has large power consumption.

Laser diode

The merit of the laser diodes is the same as that of LED. Differing from LEDs, laser diodes has so high directivity that the effective illumination can be achieved by laser scanning. Furthermore, laser diodes can emit puled lights. For these reasons, the illumination by laser diodes can offer high energy consumption and efficiency of the growing of the plant. However, the laser diode is highly expensive compared with the LED.

Merit

A laser diode is a compact and high brightness (high efficiency) light source that has a directivity unlike a LED. Because a laser diode has high electrical to optical conversion efficiency leading to low calorific value, it can save the air conditioning electric bills of plant factories. It is also possible for it to pulse operate in a speed that no other light source can match (picosecond order), fostering efficiency when pulse driving is even higher. Furthermore, because it is compact and high in output power, it necessarily need to be used near the plants. It can incorporate a lot of sunlight when used as a solar-using light source mounted on the sealing.

Demerit

Because the waste heat is low temperature, it is not possible to utilize the waste heat in the winter, and the cost is significantly higher compared to LED. Also safety measures must be made to avoid the lasers from sinning in personnel’s eyes.