CHENG Yu

(Optoelectronics Research Centre,University of Southam pton,Southam pton,SO17 1BJ,UK)

1 Introduction

Display technologies play an important role in human′s life because it is a very importantway that human beings acquire information. Infor mation is thought as the most significant feature of the 21st century.Cathode Ray Tube technology(CRT)is used to be the most popular way that infor mation is conveyed.But now new technologies,such as Plasma Display Panels(PDP),Liquid Crystal Displays(LCD),Organic Light Emitting Diode(OLED)and Digital Light Processing(DLP)et al.,play main roles in our daily life.The truth to be told,each technology has its advantages and disadvantages.Moreover,current and up-and-coming inventions will make some fantastic electronic products in our daily life.

The display technologies have been motivated by the High-definition Television(HDTV)or“hivision”market all over the world.Also the need of large screen desktop display encourages the scientists to make more researches on the materials,electrons,optics,and methodset al. In this paper,some current technologies will be overviewed and several new technologies for display have been discussed too.Low cost,ultra-bright,large-viewingangle,large screen,flexible,and more colourful will be the features of tomorrow′s display applications.

2 Current display technologies

2.1 LCD display

Liquid crystals were first researched in 1888 by Friedrich Reinitzer[1],an Austrian botanist.Light is sent through the twisted liquid crystal structure curls following the molecular arrangement,and the light propagation can be modulated according to the changes of the orientation of the liquid crystals.That is why liquid crystal can be acted as a light modulator for display. LCD displays can be divided into passive matrix LCDsand activematrix LCDs(AMLCD)according to the circuits that are responsible for activating pixels.AMLCD is a very popular technology for large-size display.A typical AMLCD structure[2]is shown in Fig.1.

Fig.1 Structure ofAM driving display and the circuit.

Fluorescent lamps are widely used asLCD light sources for the reasons of low cost and high efficiency,but the lifetime is only 4 000-6 000 h[3].Nowadays,LED seems to have better perfor mance when it is used as the light source for an LCD backlight because it delivers displays that are brighter,more colourful and robust than ever before.LCD display technology can be used both for panels and projectors,but the LCD projector uses UHP lamps rather than fluorescent lamps.However,mercury(Hg)is not a green material and has the potential problem of damage to health.And a light source for the LCD technology need to be polarization to improve the efficiency.The light emitted from lamps or LED is unpolarized.The simple and efficient polarization conversion scheme using induced Surfaceplasmon-Polariton(SPP)on metallic gratings is a good way to improve the efficiencywithout increasing the manufacture cost if silver-or aluminium-coated gratings are built on the surface of the reflector[4].Compared with traditional CRT technology,the LCD display technology has advantages of cost and perfor mance that the size is relatively s mall and more compact,hence it is suitable for flexible display requirements. Further more,LCD display technology requires no high-voltage power.Due to the reasons mentioned above,LCD display technologies are widely used.But it may have some disadvantages,for example,with the increase of the LCD projector brightness,the lamp with more power is needed.But the lamp life t ime decreaseswith the increase of the output power.Consumers may need to change the lamps by themselves which make the product inconvenient.

2.2 PDP display

The color PDP as shown in Fig.2 was invented at the University of Illinois in 1946 by Prof.Bitzer and Prof.Slottow.The operating principle of PDP is that a visible light emitting phosphor is excited by the ultraviolet ray generated by the gas discharge.The inner space of the PDP panel is divided into numerous local cells. Each pixel has its address electrodes,display electrodes,RGB phosphors,xenon and neon gasmixture[5,6].

Fig.2 Simplified structure of the PDP with three electrodes.

Owning to the novel operating principle,each pixel generates its own light and as a result,the viewing angle is large,and the image quality is superior.The cell structure of the PDP can be built bigger and biggerwithout changing any optical properties of the emission,therefore the PDP has the advantage to build ultra-large panel display.Comparingwith OLED and LCD,PDP is ultra bright,so it has advantages in large screen size,wide viewing angle,long life,high contrast ratio,and its thinness makes PDP more suitable for a big-size display.

Professional manufacture,shipment and installation are needed for PDP due to the fragility of the plasma screen,and the increase of power consuming is its another disadvantage.As a results,the PDP may not be suitable for small size display.

2.3 OLED display

It is well known that OLED has many advantages,such as thinness display,lightweight,display under low temperature,high brightness and wide-viewing angle,therefore OLED attractsmuch research interest all over the world.Other advantages such as fast response and significantly lower cost make them a potential candidate for the display in the furture.OLEDs are thin-film multi-layer devices consisting of cover glass,electrode(-transparent-cathode),function layer,electrode(transparent anode),and substrate(glass).A typical structure ofOLED is shown in Fig.3.

Fig.3 Scheme of the typical structure ofOLED.

Although OLEDs have many advantages in flexible display,they still have some problems. For example,red and green OLED films have longer lifetimes(46 000-230 000 h),while blue OLEDs currently have much shorter lifetime(up to around 14 000 h).The manufacturing processes are expensive,and water can easily damage OLEDs[7].

2.4 DLP D isplay

DLP offers the best picture in the four kinds of displays.The DLP optical system uses a digital micro mirror device(DMD)developed by Texas Instrument Co.,and the DMD is a semiconductor light s witch where fine drivingmirrors are integrated.In theDLP display system,light is transmitted through a colour wheel before the light is incident to the DMD.The brightness of each pixel is determined by the time for which the light source illuminates the pixel,and the process is realized through Pulse W idth Modulation(PWM).The advantage of the DLP display technology is ultra clear picture and“hi-vision”for large screen display[8].DLP display technology also relys on light source such asUHP lamps or lasers.

There are two display technologieswhich do not need any light source,i.e.OLED and PDP,because they can emit RGB light themselves.ButLCD and DLP display technologies require light source which is not perfect now.What′s more,LCD and DLP display technologies are main commercial products available in the market.Therefore there is an ungent need for the developing of some new light sources.

3 New technologies for future′s display technology

3.1 SHG RGB laser for display

Projection displays using laser have several advantages compared to techniques mentioned above.The biggest advantage is the large colour gamut which can be obtained.Other advantages are high contrast ratio and high brightness. It is also possible to obtain nice projection on curved surfaces due to the large focal depth of the laser beams.For low-power systems,like home theatres,the semiconductor technology is the most promising technique.Due to the high beam quality which can be obtained from the solid-state lasers,theywill probably be competitive to semiconductor lasers even in the future.But,as one could suspect,the output powers were very low;the conversion efficiencies from the input pump power to the red and green powerswere below 1%,and the conversion efficiency from the input pump power to the blue power was much smaller than 1%[9,10].

High power infrared emitting Optically Pumped SemiconductorDisk Lasers(OPSDL)with good efficiency and good beam quality are the basis for generating blue and green laser radiation by Second Harmonic Generation(SHG).Disk lasers based on these pump scheme achieve infrared output power of more than 1.5W.Experimentswith intracavity SHG are in an early stage,300 mW of frequency doubled output power has been demonstrated[11].

Another RGB-OPO light source is highly efficient as it derives all three wavelengths from one drive laser.Four pump laserswith the total power of 128 W are employed to generate an average power of 30 W,524 nm green pulse laser.Then this green laser is used as a pump to generate total 15 W RGB laser[12](shown in Fig.4).

Fig.4 Scheme of RGB-OPO light source.

Due to the rapid progress of N IR fiber lasers,especially Yb3+-doped fiber lasers,fiber lasers also play a main role in display technology.A mixture structure of RGB laser schematic diagram is shown in Fig.5.The researchers achieved low electric consumption and wide colour gamut by using the efficient and compact air-cooled green SHG laser unit and the new illumination optics for speckle noise reduction and low light power loss[13].

Fig.5 Mixture structure of RGB laser schematic diagram.

3.2 RGB generation by photon ic crystal fiber

Microstructure fiber is a research focus in all over the world.Novel idea and design method enable this fiber to have some special properties in optics.P.Horaket al.in ORC use a special designed PCF to generate RGB laser from one single fiber.The pump source in the experiment is a frequency-doubled Ybdoped fiberMOPA which generates 80 ps pulses at 530 nm with a repetition rate of 32MHz and up to 2 W output power.Fiber parameters ared/Λ～0.935 andΛ～2.5μm to 4.7μm.And a total of 360 mW of RGB laser are obtained from this fiber[14].

3.3 Visible fiber laser by downconversion

The first visible fiber operated at the wavelength of 650 nm with Sm3+doped silica optical fiber was reported in the University of Southampton in 1990.M.C.Farrieset al.had successfully achieved a Sm3+-doped silicate fiber laser which has a slope efficiency of 12.7% and output power more than 25 mW by argon laser pump.Recent research shows that argon laser pump samarium fiber laser can be long lifetime running[15,16]. The pump used is an Ar+laser,which is expensive and low electric-optical efficiency. The samarium haseven bigger absorption cross section at 405 nm than that of the 488 nm.The blue diode lasers are widely used in DVD writers,and they can act as a pump source for the samarium fiber laser aswell.Therefore it is reasonable to use blue diode laser instead of Ar+laser as a pump source.The absorption spectra of Sm3+-doped phosphor silicate fiber and the characterization of the diode laser pumped samarium fiber laser are shown from Fig.7 to Fig.9.

Fig.6 RGB laser generated from a PCF fiber.

Fig.7 Sm3+-doped phosphor silicate fiber absorption spectra.

Fig.8 Relaxation oscillation(the scale ofxis millisecond).

Fig.9 Laser spectra of the blue diode laser pumped Sm3+-doped phosphor silicate fiber.

Tb3+-doped fluoride fiber laser was reported that a visible fiber laser operated at 542.8 nm when it was pumped at the wavelength of 488 nm.The output power of the 542.8 nm was 0.28 mW and a slope efficiency of 4.1% was achieved[17]. Fiber lasers have properties as a merit for display,such as diffractive l imit beam quality and flexible delivery.Fiber laser may have as long lifetime as 30 000 h,which is the longest lifetime and less power consuming.Due to the advantage ofmodulation and power amplifier of fiber lasers,the RGB fiber laser may have more potential in the future′s display application.Althought novel lasers have some advantages compare to traditional light sources,they are needed to improve the efficiency to make their advantage to be a merit.

4 Conclusions

In summary,four current display technologies have been overviewed.And the advantages and disadvantages of each technology have been discussed.One can see that the technology developing trend shows that LCD and DLP technologies are the most popular technologies.And new light source technologiesmay continue to boost the fast development of the display technologies.W ith the development of fiber laser technology,more and more low cost fantastic fiber laser productswill be used for future′s display technologies.

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