Optoelectronics Devices and Applications Part 9 docx

4.4 Two 90º-twist LC-SLMs coupled in parallel The considered above techniques for modulation of coherence and polarization are based on the use of 0º-twist LC-SLMs.. Measurements of coh

cross-( ) ( )( )

with elements ( )t x being the random (generally complex) functions of time It can be ij

readily shown (Shirai & Wolf, 2004) that the cross-spectral density matrix of the beam just behind the screen is given by the expression

x x x x and ( )Px P( )x This fact can be used to realize the modulation of coherence and polarization by means of a random polarization-dependent screen To minimize the light loss, the elements of the matrix ( )T x must be of the form

( ) exp ( )

ij

where ( ) x is some real random function To provide the desired statistical characteristics

of modulation, the function ( ) x has to be generated by computer The most appropriate candidate for physical realization of such a computer controled random phase screen is the LC-SLM

3 Elements of the theory and design of LC-SLMs

The LC represents an optically transparent material that has physical properties of both solids and liquids The molecules of such a material have an ellipsoidal form with a long axis about which there is circular symmetry in any transverse plane The spatial organization of these molecules defines the type of LC (Goodman, 1996) From the practical point of view, the most interesting type is so-called nematic LC, for which the molecules have a parallel orientation with randomly located centres within entire volume of the material Further we will consider the LCs exclusively of this type

Because of its geometrical structure the nematic LC exhibits anisotropic optical behaviour, possessing different refractive indices for light polarized in different directions From the optical point of view the nematic LC can be considered as an uniaxial crystal with ordinary

refraction index no along the short molecular axis and extraordinary refraction index ne

along the long molecular axis, so that it can be characterized by the so-called birefringence

where  is the wavelength of light and d is the thickness of LC layer

When LC material is placed in a container with two glass walls it receives the name of LC

cell The glass walls of the LC cell are linearly polished to provide the selected directions in

which the LC molecules are aligned at the boundary layers If the glass walls are polished in

different directions, then LC molecules inside the cell gradually rotate to match the

boundary conditions at the alignment layers, as illustrated in Fig 1 Such a LC cell received

the name of twisted LC cell The angle  between the directions of polishing is refered as

the twist angle

Ay

Ad

A z

Φ

Fig 1 Twist of LC molecules due to the boundary conditions at the alignment layers

According to (Yariv & Yeh, 1984), the amplitude transmittance of twisted LC cell with front

molecules aligned along x-axis is given by the Jones matrix

sin

sinsin

cos)exp(

)(LC

i

i i

)sin(

)sin(

)cos(

)(

Bellow we consider two important particular cases, namely when 0º and 90º In the first case we will reffer to the LC cell as 0º-twist LC cell and in the second case we will refer

0)2exp(

For 90º-twist LC cell Eq (13) takes the form

cos

sincos

sin2)exp(

LC

i

i i

direction, as is shown in Fig 2, it is possible to achieve the phase-only modulation (Lu & Saleh, 1990)

sincoscos

)(

As can be seen from this figure, starting from some critical value of the birefringence parameter  , approximately equal to 3 2, the amplitude transmittance t yx approachs to

unity while the phase transmittance rises linearly having a slope of approximately2 Thus,

when the birefringence parameter satisfies the condition3 2, the matrix given by Eq

(21) can be well approximated as

00



i

i.e the 90º-twist LC cell sandwiched between two crossed polarizers can be considered as

the phase-only modulator

Till now we assumed that parameter  characterizing the LC cell has a fixed value

Nevertheless, as well known (Lu & Saleh, 1990) , applying to the LC cell an electric field

normal to its surface, the birefringence parameter is no longer constant and changes in

accordance with

d n

n( ) ][

)(  e   o







where  is the tilt of the LC molecules with respect to the z axis caused by the electric field

Besides, the relationship between extraordinary refraction index and the molecular tilt can

be approximated as

o 2 o e

e( ) (n n)cos n

It has been shown (Lu & Saleh, 1990) that the dependence between tilt angle and applied

voltage has the form

c rms

c rms

2

,0

)

V V V

V V

where Vc is the threshold voltage, V0is the saturation voltage, and Vrmsis the effective

voltage Combining Eqs (27) – (29), it is possible to show that the birefringence parameter

 finds to be approximately proportional to the inverse value of the applied voltage

Finally, we are ready to define a LC-SLM as an electro-optical device composed by a large

number of LC cells (pixels) whose birefringence indices are controlled by the electrical

signals generated by computer and applied individually to each cell by means of an array of

electrodes The amplitude transmittance of 0º-twist LC-SLM or 90º-twist LC-SLM can be

described by Eqs (16) and (26), respectively, replacing parameter  by spatial function

We begin with the technique based on the use of a 0º-twist LC-SLM (Shirai & Wolf, 2004) It

is assumed that the incident light represents a linear polarized laser beam characterized by

the cross-spectral density matrix

2 2 2

2 1

sinsincos

sincoscos

4exp),

where E0 is the value of power spectrum at the beam centre, ε is the effective (rms) size of

the source, and  is the angle that the direction of polarization makes with the x axis It can

be readily verified that for such a beam (x1,x2)1 and P(x)1, i.e the beam described by

Eq (30) is completely coherent and completely (linearly) polarized

If the extraordinary axis of the LC is aligned along the y direction the transmittance of

0º-twist LC-SLM, in accordance with the previous section, is given by matrix

0

01

)(

1 1

LC

x x

where 0is a constant and (x)is a computer generated zero mean random variable which

is characterized by the Gaussian probability density

1)(

2exp)

()(

where  x1x2 and  is a positive constant characterizing correlation width of (x)

On substituting from Eqs (30) - (32) into Eq (10), one obtains

4exp),(



x x x

0

2 0

2

sin)]

()([expsin

cos)]

(exp[

)exp(

sincos)]

(exp[

)exp(

cos

x x x

x

i i

i

i i

(exp[

1

2exp1exp)()(exp)()(exp

Then, Eq (35) can be rewritten as

4exp),(



x x x

2 2

2 0

2 0

2

sin2

exp1expsincos2exp)exp(

sincos2exp)exp(

exp1exp

0cos

4exp)

,

2 2 2

2 2 2 2

2 1

2

sin2exp11)(

 x

Equations (42) and (43) show that the modulated beam is, in general, partially coherent and partially polarized The degree of polarization changes in the range from 1 to 0 with a proper choice of polarization angle of the incident beam The degree of coherence, for a fixed value of  can be varied by a proper choice of parameters  and  of the control signal (x), as it is shown in Fig 4

We would like to point out the following two shortcomings of the described technique Firstly, as can be seen from Eqs (42) and (43) this technique does not provide the independent modulation of the degree of coherence and the degree of polarization since both of them depend at the same time on the polarization angle  Secondly, as can be seen from Fig 4, this technique does not allow to obtain the values of the degree of coherence in a whole desired range from 1 to 0

Fig 4 Degree of coherence given by Eq (42) for  4 and different values of 

4.2 Two 0º-twist LC-SLMs coupled in series

To avoid the shortcomings mentioned above, the authors (Ostrovsky et al, 2009) proposed to use instead of a single 0º-twist LC-SLM the system of two 0º-twist LC-SLMs coupled in series as shown in Fig 5

Fig 5 System of two crossed 0º-twist LC-SLMs coupled in series The bold-faced arrows denote the extraordinary axis of liquid LC

The transmittance of the first SLM is just the same as in previous technique, while the

transmittance of the second one, whose extraordinary axis is aligned in the x direction, is

0)]

(2exp[

)

2 LC

x x

where birefringence 2(x) has the form

 ( )

2

1)

with 0 and (x)of the same meaning as stated in the context of Eq (32) The transmittance

of the system composed by two crossed 0º-twist LC-SLMs is given by matrix

0

0)]

(exp[

)exp(

)()()

x

x x

T x T x T

(



x x x

2 2

2

2 2

2

2 2

2 2

2 2

sin2

exp1expsin

cos2

exp1exp

sincos2

exp1expcos

2exp1exp

(47)

and then, using approximations (39) and (40),

.sin0

0cos2

exp4

exp),

2 2 2 2

2 1 2 1 2

 x

As can be seen from Eqs (49) and (50), the output degree of coherence in this case does not

depend on direction of the input polarization and changes in the whole desired range from

1 to 0

4.3 Two 0º-twist LC-SLMs coupled in parallel

The result resembling the one given above can be also obtained using the system of two

0º-twist LC-SLMs coupled in parallel Such a system has been described in (Shirai et al, 2005)

Here we propose a somewhat modified version of this technique

The technique is based on the use of two 0º-twist LC-SLMs with orthogonal orientations of

their extraordinary axes placed in the opposite arms of a Mach-Zehnder interferometer as it

is shown in Fig 7 The polarizing beam splitter at the interferometer input separates the

orthogonal beam components E x(x) and E y(x) so that each of them can be independently

Fig 6 Degree of coherence given by Eq (49) for  4 and different values of 

Fig 7 System of two crossed 0º-twist LC-SLMs coupled in parallel: PBS, polarizing beam

splitter; M mirror; BS beam splitter The bold-faced arrows and circled dots denote

polarization directions

modified by different LC-SLMs The modified beam components are superimposed in the

conventional beam splitter at the interferometer output Disregarding the negligible changes

of coherence and polarization properties of the electromagnetic field induced by the free

space propagation within the interferometer, one can represent the considering system as a

thin polarization-dependent screen with the transmittance given by matrix

0

0)]

(2exp[

)(

2

1

x

x x

1)( 0 1(2))

2 (

with 0 and 1(2)(x)of the same meaning as stated in the context of Eq (32) It is assumed

also that the variables 1(x) and 2(x) are generated by two different computers so that

they can be considered as statistically independent with the separable joint probability

) 1 ( 2 2 ) 2 (

1 ( ) ( )]} exp[

it may be readily shown that the cross-spectral density matrix W(x1,x2), the degree of

coherence (x1,x2) and the degree of polarization P(x), in this case, are just the same as

ones given by Eqs (48) – (50)

4.4 Two 90º-twist LC-SLMs coupled in parallel

The considered above techniques for modulation of coherence and polarization are based on

the use of 0º-twist LC-SLMs The consistence of these techniques is well grounded in theory

but no relevant experimental results have been yet reported This situation can be explained

by the lack at present of commercial 0º-twist LC-SLMs with the required characteristics

Here we propose an alternative technique which uses widely available commercial 90º-twist

LC-SLMs and, hence, can be easily realized in practice The proposed technique is sketched

schematically in Fig 8

In spite of its outward resemblance with the technique described in the previous subsection,

this technique has two essential distinctions Firstly, instead of 0º-twist LC-SLMs here the

orthogonally aligned 90º-twist LC-SLMs are used in the opposite arms of the Mach-Zehnder

interferometer Secondly, the conventional beam splitter at the output of interferometer is

replaced by the polarizing one As a result, taking into account that two polarizing beam

splitters coupled in series act as crossed polarizers, each arm of interferometer can be

considered as the system shown in Fig 2 with 10º and 290º In accordance with

Section 3 such a system realizes the phase-only modulation of the correspondent orthogonal

component of the incident beam

Fig 8 System of two crossed 90º-twist LC-SLMs coupled in parallel: PBS, polarizing beam

splitter; M, mirror The bold-faced arrows and circled dots denote polarization directions

Again disregarding the negligible changes of coherence and polarization properties of the electric field induced by the free space propagation within the interferometer, the system shown in Fig 8 can be considered as a thin polarization-dependent screen with transmittance given by matrix

(2exp[

)]

(2exp[

0)

2 2 2

0 2 1

cos00sin2

exp4

exp),

5 Measurements of coherence and polarization

In practice the efficiency of the techniques described in previous section can be verified by measuring the degree of coherence and the degree of polarization of modulated electromagnetic beam The main idea of such a measuring is well known (Wolf, 2007) and consists in the implementation of four two-pinhole Young’s experiments with different states of polarization of the radiation emerged from each pinhole Nevertheless, the practical realization of such an idea proves to be difficult in consequence of physical impossibility to insert the needed optical elements just behind the pinholes Here we present the technique for measuring the degree of coherence and the degree of polarization proposed by the authors (Ostrovsky et al, 2010), which permits to avoid this difficultness

The technique consists in applying the Mach-Zehnder interferometer shown in Fig 9 This allows the physical insertion of the appropriate optical elements for simultaneous and independent transforming the orthogonal beam components The polarizers P1 and P2 serve

to cut off only one of the orthogonal field components, while the removable rotators R1 and R2 serve to produce the rotation of one of the transmitted field component through 90˚ (for such a purpose a suitably oriented half-wave plate can be used) The operation description

of the technique is given bellow

The determination of the elements W  ij of the matrix W(x1,x2) is realized by means of the following four experiments In the first experiment the polarizers P1 and P2 are aligned to

transmit only x components of the incident field without any subsequent rotation of the

plane of polarization In the second experiment P1 and P2 are aligned to transmit only y

components of the incident field again without any subsequent rotation of the plane of polarization In the third and the fourth experiments the polarizers P1 and P2 cut off the different orthogonal components of the incident field and the corresponding polarization rotator R1 or R2 serves to allow the interference of these components

Fig 9 System for measuring the statistical properties of modulated beam: BS, beam splitter;

M, mirror; TP, translating pinhole; P1, P2, polarizers; R1, R2; polarization rotators

The spectral density or power spectrum of the field observed at the output of the interference system in each experiment can be described by the spectral interference law, which under certain conditions can be written as (Wolf, 2007)

,),,(22

cos22

222)

(

0

y x j i ξ , ξ α x z kξ ξ

, ξ W ξ S ξ S x

where S and i S j are the power spectra of the field components in the pinhole position 2ξ/ ,

k is the wave number, z is the geometrical path between the pinhole plane and the 0

observation plane, and α ijargW ij. From the physical point of view, Eq (57) describes an

image with periodic structure, known as the interference fringe pattern The measure of the contrast of the interference fringes is the so-called visibility coefficient defined as

.)()(

)()()

min max

x S x S

x S x S V

ij ij

ij ij

222

1)

6 Experiments and results

To verfy the proposed technique in practice, we conducted some physical experiments The experimental setup used in experiments is sketched in Fig 10 The principal part of the experimental setup was composed of two Mach-Zehnder interferometers coupled by the common beam splitter The first interferometer realized the modulation of the incident beam

as it has been described in Subsection 4.4, while the second one served for measuring the degree of coherence and the degree of polarization of the modulated beam as it has been described in Section 5

As the primary source we used a highly coherent linearly polarized beam generated by

He-Ne laser (Spectra-Physics model 117A, λ=633 nm, output power 4.5 mW) which can be well described by the model given by Eq (30) The laser was mounted in a rotary stage that allowed changing the polarization angle  without any loss of light energy As the 90º-twist nematic LC-SLMs we used the computer controlled HoloEye LC2002 electro-optical modulators which have resolution of 800 × 600 pixels (32 µm square in size) and can display the control signal with 8 bit accuracy (256 gray levels) The control of LC-SLMs was realized independently by two computers using a specially designed program for generating the random signals which obey the desired Gaussian statistics To realize the measurements of the degree of coherence we used two pinholes with diameter of 200 µm mounted on motorized linear translation stages

Fig 10 Experimental setup: L, laser; BE, beam expander; ZL, zoom-lens; PD, photodiode; the other abbreviations are just the same as in Figs 8 and 9

We realized two sets of experiments In the first set we measured the degree of polarization for different values of the polarization angle of incident beam and for the fixed value ( 1 )

of parameter  of the control signal The results of these experiments are shown in Fig 11

In the second set we chosed  4 and measured the degree of coherence varying the parameters  and  of the control signal to ensure the chosen values of parameter  The results of these experiments are shown in Fig 12 As a whole, the results obtained in both sets of experiments are in a good accordance with theoretical predictions

Fig 11 Results of measuring the degree of polarization for  1

Fig 12 Results of measuring the degree of coherence for  4 and different values of parameter 

7 Conclusion

In this chapter the problem of modulating the coherence and polarization of optical beams has been considered It has been shown that the LC-SLM represents an ideal tool for practical realizing such a modulation We have analized the known techniques of optical modulation based on the use of 0º-twist LC-SLM and have proposed a new technique based

on the use of two twist LC-SLMs Because of the wide commercial availability of

90º-twist LC-SLMs the proposed technique proves to be the most attractive one The justifiability of this technique has been corroborated by the results of physical experiments

Lu, K & Saleh B E A (1990) Theory and design of the liquid crystal TV as an optical spatial

phase modulator Optical Engineering, Vol.29, No.3, (March 1990), pp (240-246)

ISSN 0091-3286

Ostrovsky A S (2006) Coherent Mode Representations in Optics, SPIE Press, ISBN

0-8194-6350-7, Bellingham WA, USA

Ostrovsky A S ; Martínez-Vara P ; Olvera-Santamaría M Á & Martínez-Niconoff G

(2009a) Vector coherence theory : An overview of basic concepts and definitions, In: Recent Research Developments in Optics, S G Pandalai, (113-132), Research

Singpost, ISBN 978-81-308-0370-8, Kerala, India

Ostrovsky A S.; Martínez-Niconoff G.; Arrizón V.; Martínez-Vara P.; Olvera-Santamaría M

Á & Rickenstorff C (2009b) Modulation of coherence and polarization using

liquid crystal spatial light modulators Optics Express, Vol.17, No.7, (March 2009),

pp (5257-5264) ISSN 1094-4087

Ostrovsky A S.; Rodríguez-Zurita G.; Meneses-Fabián C.; Olvera-Santamaría M Á &

Rickenstorff C (2010) Experimental generating the partially coherent and partially

polarized electromagnetic source Optics Express, Vol.18, No.12, (June 2010),

pp.(12864-12871) ISSN 1094-4087

Shirai T & Wolf E (2004) Coherence and polarization of electromagnetic beams modulated

by random phase screens and their changes on propagation in free space Journal of the Optical Society of America A, Vol.21, No.10, (October 2004), pp (1907-1916) ISSN

1084-7529

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Schell-model beams Journal of Optics A: Pure and Applied Optics, Vol.7, No.5, (March

2005), pp (232-237) ISSN 1464-4258

Wolf E (2007) Introduction to the Theory of Coherence and Polarization of Light, Cambridge

University Press, ISBN 9780521822114, Cambridge, UK

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spatial light phase modulation Optical Communications, Vol.115, No.1, (March

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Recent Developments in High Power Semiconductor Diode Lasers

Li Zhong and Xiaoyu Ma

National Engineering Research Center for Optoelectronic Devices,

Institute of Semiconductors, Chinese Academy of Sciences

Beijing China

1 Introduction

Due to a number of advantages of diode lasers, such as small size, light weight, high efficiency etc., it has been the focus of the laser field from the beginning of the birth and has been widely used in industrial, military, medical, communications and other fields Especially, to a great extent, a tremendous growth in the technology of solid-state lasers has been complemented by laser diode array designs for pumping such solid-state lasers Significant applications continue to exist at common solid state laser systems such as yttrium aluminum garnet doped with neodymium or ytterbium (Nd:YAG or Yb:YAG, respectively) requiring pump light in the 780 nm to 1000 nm range Driven by the increasing demands of high-performance high-power laser pumping source and direct industrial processing applications, tremendous breakthrough have been realized in the main optical-electronic performances of high power semiconductor diode lasers, such as ultra-high peak power, super-high electro-optical conversion efficiency, low beam divergence, high brightness, narrow spectrum linewidth, high operation temperature, high reliability, wavelength stabilization and fundamental transverse mode operation etc These achievements are attributed to a combination of the maturity of semiconductor material epitaxy, the optimization of the laser waveguide structure, the cavity surface-passivation technology as well as the high effective cooling and packaging technologies The Occident and Japan keep ahead in this field with several large corporations actively engaged in this market, for example, Coherent, IMC, SDL, OPC, HPD, Spectrum-Physics of the U.S., OSRAM, JOLD, Frauhorf of Germany, THALES of France, SANYO, SONY of Japan, and ATC of Russia etc The wavelength of these industrial products ranges from 630 nm to

1550 nm, and optical output power levels from several W to 10 kW class In China, prominent progresses have also been made at a rapid rate Advances in the design and manufacture of the bars, together with effective means of stacking and imaging monolithic semiconductor laser arrays (bars), have enabled the production of robust sources at market-competitive costs In particular, the diode lasers for these systems have to meet high demands in relation to efficiency, power, reliability and manufacturability, which following the desire for reducing the cost per watt and the cost per hour’s lifetime for the customer However, with the enhancement of the power and beam quality, a series of new practical problems arise in the aspect of engineering, such as high cost of the high-current and low-

voltage power supply and short life span of micro-channel heat sink cooling etc Gradually, single-emitter semiconductor laser devices and mini-bars with high power and high beam quality are becoming the mainstream research trend and replacing the traditional cm-bars

On the other hand, the reduced divergence angle accelerates the improvement of beam quality, which is directly reflected in the decrease of the fiber diameter and the increase of the output power for fiber-coupled diode laser module Here we review and discuss the state of the art of high power semiconductor diode lasers, including single emitters, bars, horizontal bar arrays and vertical bar stacks, with the typical data presented Several key technological problems concerning the improvements of diode lasers performance, the optimization of packaging architectures and the developments of high beam quality of diode lasers will be discussed in section 2~5, respectively In section 6, we conclude with some thoughts on the future study directions and the developing tendency for high power diode lasers

2 Status of high-power diode laser technology and characteristics

2.1 Laser diode chip technology

Over the recent years, high power diode lasers have seen a tremendous evolution in material epitaxial growth technology, epi-structure optimization technique, cavity surface-passivation technology etc Epitaxial structure is designed for a specific range of operation

to optimize a combination of optical, electrical and thermal performance, generally minimizing both operating voltage and internal loss to achieve high efficiency with long cavities for high-average-power and high-brightness applications The details of these structures, such as material compositions, layer thicknesses, asymmetric or symmetric waveguide structure design, and doping profiles are selected to ensure that manufacturability and reliability are not compromised Important developments in epitaxial growth technology include the reporting of low loss materials (about 1 cm–1 for AlGaAs for example), the development of the strained materials with attendant benefits on gain and bulk defect pinning, and the development of aluminum-free materials such as InGaAs and InGaAsP with the latter material having been reported to wavelengths below 800 nm A number of careful studies are being reported on filament formation and current crowding in semiconductor lasers and methods for avoiding their deleterious effects With the improvement of the high-quality, low defect density epitaxial growth technology of semiconductor materials, the resonator cavity length of the existing cm bar has been increased from 0.6 ~ 1.0 mm to 2.0 ~ 5.0 mm, making a significant increase of the output power The large cavity length ensured low thermal and electrical resistivities of the devices

by increasing their active area The cavity length is selected mainly depending on desired operation power and is optimized for best power conversion efficiency (PCE) at the given condition

In continuous wave (CW) operation the output power from high power laser bars usually is limited by the thermal load that the assembly may dissipate Failure modes, like wear out of the output power or bulk failures are critical in CW operation For quasi continuous wave (qCW) applications the reliable output power in general is not thermally limited The robustness of the output facet of the devices and the degradation of the assembly under the cyclic thermal load become the critical matter Special methods of facet design and treatment have been employed to increase the COMD power threshold and suppress its degradation over the operating life of the device, such as facet passivation with a dielectric layer,

regrown non-absorbing mirror (ReNAM), intermixed non-absorbing mirror (iNAM), high vacuum (UHV) cleaved facets etc (Yanson et al., 2011) NAM-based techniques (ReNAM and iNAM) require further development to achieve required reliability figures Dielectric passivation and cleave-in-a-vacuum techniques are found to be the two best performing facet engineering solutions At 980nm, dielectric facet passivation can be employed with a pre-clean cycle to deliver a device lifetime in excess of 3,000 hours at increasing current steps Vacuum cleaved emitters have delivered excellent reliability at 915nm, and can be expected to perform just as well at 925 and 980nm By preventing exposure of a freshly cleaved facet to oxygen, the formation of surface oxides and shallow levels is avoided without the need for ion plasma cleaning.(Tu et al., 1996) A capping layer, also deposited in a vacuum, seals the facet and stops the penetration of oxygen Single emitters fabricated with these two techniques are packaged into fiber-coupled modules with 10W output and 47% efficiency (Tu et al., 1996)

ultra-2.2 Far field divergence angle control

As the basic unit of the integration of semiconductor laser system, the performance of different structure and different types of semiconductor laser device directly contributes

to the development of semiconductor laser systems, one of the most important developments is the reduction of the beam divergence and the increase of the output power According to the definition of the beam quality, the beam divergence angle is proportional to the beam-parameter product (Q or BPP), which is a measure of the beam quality Therefore the beam quality is under the direct control of the far field divergence angle Overall, the waveguide structure of semiconductor lasers leads to a serious asymmetry far-field beam quality In the fast axis direction, the output beam can be considered to be fundamental mode, but the divergence angle is large The compression

of the fast axis divergence angle can effectively reduce the requirements for the fast axis collimator aperture While in the slow axis direction, the output beam is multi-mode and the beam quality is poor The beam quality can be directly improved by reducing the divergence angle in the slow axis direction, which is the research focus in the field of the high-beam quality semiconductor laser

The research focus in the control of the fast axis divergence angle is how to balance the fast axis divergence angle and the electro-optical conversion efficiency Although a number of research institutions had press release of the continued access to fast axis divergence angle of only 3° and even 1°, but based on the consideration of the power, the electro-optical conversion efficiency and the cost, it is difficult to promote practical applications in the short term In the early year of 2010, the P Crump etc of German Ferdinand-Braun Institute has reported the fast axis divergence angle of 30° (95% of the energy range) obtained through the use of large optical cavity and low-limiting factors, meanwhile the electro-optical conversion efficiency of the device is 55%, which is the basic standards to practical devices The fast axis divergence angle of the current commercial high-power semiconductor laser devices are also dropped from the original of about 80° (95% of the energy range) to below 50°, which substantially lower the requirements for the numerical aperture of the collimator

In the slow-axis divergence angle control, recent studies have shown that, in addition to the device's own structure, the combination of the drive current density and the thermal effects of semiconductor lasers affect the slow axis divergence angle The slow axis divergence of a single emitter with long cavity length is of the most easy to control,

whereas in the array device, with the increase of the fill factor, the intensification of thermal cross-talk between the emitting elements will lead to the increase of the slow-axis divergence angle In the year of 2009, the Bookham company of Switzerland has successfully reduced the slow-axis divergence (95% of the energy range) of 9xx-nm 10

W commercial devices with 5 mm cavity length from 10o ~ 12o to about 7o In the same year, the Osram Company of German and the Coherent Company of the United States has reduced the slow-axis divergence of the array (95% of the energy range) to 7o level

2.3 High-temperature performance of laser bars and arrays

Since the performance of a diode laser is operating temperature dependent, high-power diode laser pump modules usually need a cooling system to control their operating temperature However, some diode laser applications require that high-power diode laser pump modules operate in a high temperature environment without any cooling In addition, the diode laser pump modules have to provide both high peak power and a nice pulse shape because certain energy in each pulse is required At such a high temperature, semiconductor quantum well gain drops significantly, and the carrier leakage and the Auger recombination rate increases Thus, the laser bar has a high threshold and low slope efficiency, resulting in very low power efficiency To reach certain power level at high temperature, the pump current has to be much higher than that at room temperature More waste heat is generated in the active region of the diode laser bar In addition, tens

of milliseconds pulse width with a few percent duty cycle forces the laser bar to operate in

a “CW” mode.(Ziegler et al., 2006; Puchert et al., 2000; Voss et al., 1996) The Lasertel Company has presented the development of high-temperature 8xx-nm diode laser bars for diode laser long-pulse (>10 milliseconds) pumping within a high-temperature (130 ºC) environment without any cooling.( Fan et al., 2011) The epi-structure is based on a large optical cavity separate confinement heterostructure with Al-free active region By adjusting Aluminum concentration in the AlGaInP barrier, introducing strain in quantum well (QW) and adjusting the width of QW, optimizing the strain and the width of quantum well, the gain is maximized, the loss and carrier leakage especially at high temperature is minimized and the optical confinement of the waveguide is also be improved Under the operation condition (130 ºC, 15 ms pulse width, 5 Hz frequency and 100-A current pulse), the high-temperature laser bars show robust and consistent performance, reaching 60 W (peak) power and having good pulse shape, as shown in Figure 1 The laser bars do not show any degradation after 310,000 15-millisecond current pulse shots They demonstrated over 40-millisecond long-pulse operation of the 8xx-nm

CS bars at 130 ºC and 100 A Regardless of the pulse shape, this laser bar can lase at extremely high temperature and output pulse can last for 8 ms/2ms at 170 ºC/180 ºC respectively, both driven by 60 A current pulses with 5-Hz frequency, 10 millisecond pulse width This is the highest operating temperature for a long-pulse 8xx-nm laser bar Figure 2 shows the high-temperature performance of the 3-bar stack array and its pulse shape at 130 ºC The peak power of the 3-bar array reaches 165 kW at 100A and 130 ºC, but the pulse shape is very sensitive to the current and the power of the array drops much faster than that of the CS bar, which may be attributed to the package difference between the CS bar and stack array