Tunable lasers handbook phần 8 ppt

Improvements such as unstable resonators and graded reflectivity output mirrors have made pump lasers with good beam quality as well as high energy per pulse available.. However, as lase

through 22) ZnGeP, could tune over this range with a variation of about 4", the smallest angular range; CdSe would require about 14", the largest angular range AgGaS, does display an unusually flat tuning range about 4.2 ym Besides this the tuning curves are in general similar, except for the direction of the curvature

As such, selection of the best nonlinear crystal would probably be based on con- siderations other than the phase matching curves

9 PERFORMANCE

Optical parametric oscillators have developed from their initial stage where they were little more than a curiosity Initial performance was limited by lack of high optical quality nonlinear crystals nonlinear crystals with relatively small nonlinear coefficients and limited pump laser performance In addition, optical parametric oscillators were in competition with dye lasers in the visible and near infrared Pulsed dye lasers have an advantage because laser-pumped dye lasers do not necessarily require high beam quality from the pump laser In essence, dye lasers can serve as an optical integrator, converting a fixed-wavelength pump laser with relatively poor beam quality into a tunable laser with a better beam quality

In the face of these difficulties, optical parametric oscillators enjoyed limited com- mercial applications for a considerable time However, several increases in optical parametric oscillator technology have improved the viability of these devices

7 Optical Parametric Oscillators 335

1.064~rn Pump

\

Angle (degrees) FIGURE 16 Phase-matching curve for BBO for 0.537- and 1.064-pm pumps

Opticall quality of the nonlinear crystals has improved Optical quality improvements have occurred both in the form of decrcased absorption and decreased distortion For example, LiNbO, crystals were found to suffer from optically induced refractive index inhomogeneities It was found that, in part, these probllems could be traced to Fe impurities By decreasing the Fe impuri- ties, the susceptibility of optically induced refractive index inhomogeneities was decreased Similarly the short-wavelength absorption in AgGaSe, was corre- lated with a deficiency of Se By annealing these crystals in an atmosphere rich

in Se, the short-wavelength transmission of these crystals improved Initially some nonlinear crystals were deliberately doped with impurities to reduce growth time and therefore cost While some impurities are benign, others can cause unwanted absorption Increased absorption can limit the efficiency and average power limit mailable with a given nonlinear crystal In addition, some crystals tended to grow multidomain That is, not all of the nonlinear crystal was oriented in the same manner Multidomain crystals limit efficiency by limiting the effective length of the nonlinear crystal As growth technology improved, many of these problems were resolved

1 7 Phase-matching curve for AgGaS, for a 1.061-pn pump

Of perhaps more significance is the introduction of better nonlinear crystals particularly ones with a larger nonlinear coefficient Of particular note in the way of visible crystals are KTP, BBO, and LBO Crystals with nonlinear coeffi- cients as large as those available with these more recent crystals were not gener- ally available in the early developmental stages of optical parametric oscillators

In the infrared, AgGaSe, has developed to the point where it is presently com- mercially available for applications in the mid-infrared region Although this crystal has been known for some time, the availability and the absorption in the near-infrared region limited its utility In addition substantial progress has also been made with the commercialization of ZnGeP,

Pump lasers have also improved both in power and beam quality, a definite advantage when nonlinear optics are being used Improvements such as unstable resonators and graded reflectivity output mirrors have made pump lasers with good beam quality as well as high energy per pulse available The beam quality of pump lasers is often limited by thermal effects However, as laser diode array pumping of solid-state lasers becomes more common, the beam quality should improve even more since the thermal load on a laser diode array-pumped solid-state laser is less than a similar lamp-pumped solid-state laser at the same average output power In addition, injection seeding techniques have narrowed the linewidth of the pump

7 Optical Parametric OsciIIators 337

12.0 11.0 10.0

1 .o

Angle (degrees) FIGURE 1 8 Phase-matching c u n e for AgGaS, for a 2.10-pn pump

lasers Both increased beam quality and decreased linewidth can lead to an increased performance for the optical parametric oscillator

Several different concepts are involved in the assessment of the performance

of an optical parametric oscillator including threshold, slope efficiency, total effi- ciency photon efficiency, and pump depletion Optical parametric oscillators can

be operated either in a cw or a pulsed mode Of the two modes of operation the pulsed mode is much more common since the operation of an optical parametric oscillator is enhanced by a high power density The threshold in the cwr mode is straightforward to define as the amount of pump power required to achieve opti- cal parametric oscillation In the pulsed mode the observable threshold, rather than the instantaneous threshold is usually quoted; however this is not alw ays made clear While slope efficiency is sometimes quoted, it could represent either the ratio of the increase in power at the output wavelength to the increase in power at the pump wavelength or the increase in power of both the signal and idler wavelengths to the increase in power at the pump wavelength In the pulsed mode it could be quoted at the instant of peak power or it could be quoted for the total output energy Although laser theory usually predicts a nearly linear increase

in the output with increases in the input optical parametric oscillator theory does not necessarily predict the same approximation However, in practice a linear

338 Norman P Barnes

12.0 11.0 10.0

FIGURE 1 9 Phase-matching curve for AgGaSe, for a 2.10-km pump

increase of the output with the input is often observed Total efficiency suffers from many of the same ambiguities as slope efficiency It could imply the output power or energy at one or both of the signal and idler wavelengths divided by the pump power or energy Photon efficiency normalizes the pump power and energy and the output power or energy by the energy of the pump and output photon, respectively Thus a unity photon efficiency would imply that the power

or energy efficiency would be in the ratio of the pump wavelength to the output wavelength Pump depletion usually compares the pump pulse transmitted through the optical parametric oscillator with and without oscillation occurring

As such, it is closest to the efficiency calculated using both the signal and idler

Continuous wave optical parametric oscillation was reported by using a Ba,NaNbjO,, crystal [51] It was pumped by a frequency-doubled Nd:YAG laser A threshold of 45 mW was observed when the wavelengths available

7 Optical Parametric OsciI/atois 33

FIGURE 20 Phase-matching curve for CdSe for a 2.10-ym pump

ranged from 0.98 to 1.16 pm With 0.3 W of pump power, the available power at both the signal and idler wavelengths was estimated at 0.003 W, yielding an effi- ciency of 0.01 Later by using a cw Ar ion laser for a pump laser, a threshold as low as 2.0 mW was achieved A power output of about 0.0015 W was achieved

at about 2.8 times threshold While a continuous pump was employed, the output consisted of a series of pulses with pulse lengths ranging from 0.1 to 1.0 ins in length [52]

More efficient operation in the near infrared was obtained by two researchers both using LiNbO, as the nonlinear crystal In one case a frequency- doubled Nd:glass laser was used as the pump source [53], and the other used a Q-switched Cr:A1,03 laser [54] In the first case; a threshold of z.bout 5.0 kW was required for -a 8.Q-mm crystal length At twice threshold, a peak output power of 1.8 kW was achieved yielding an efficiency of 0.18 In the second case

a threshold of 65 kW was achieved in a doubly resonant arrangement with a 9.35-mm crystal length With the doubly resonant arrangement, 0.22 of the peak pump poweir was converted to the signal at 1.04 pm On the other hand, with a singly resonant arrangement only 0.06 of the peak pump power was converted

to the signal Although the efficiencies reported in these experiments are impres- sive, the output energy of these devices is in the millijoule range or less

FIGURE 2 1 Phase-matching curve for ZnGeP, for a 2.10-pm pump

A device tunable across the visible region of the spectrum was produced by

using ADP as the nonlinear crystal [ S I A frequency-quadrupled Nd:YAG laser,

yielding about 1.0 mJ/pulse at 0.266 pm, was utilized as the pump Gains were

high enough with this configuration that external mirrors were not necessary to

obtain significant conversion With the 50-mm ADP crystal oriented normal to

the pump beam, an average power conversion of the pump to the outputs in the

visible region of the spectrum was as high as 0.25 Temperature tuning the crys-

tal from 50 to 105°C allowed the region from 0.42 to 0.73 pm to be covered

A cw optical parametric oscillator tunable in the red region of the spectrum,

from 0.680 to 0.705 pm, was demonstrated using an Ar ion laser operating at

0.5145 pm in conjunction with a 16.5-mm LiNbO, crystal [52] To avoid opti-

cally induced refractive index inhomogeneities, the crystal was operated at ele-

vated temperatures, nominally 240°C A threshold of 410 mW was possible At

2.8 times threshold, 1.5 mW of output power was available even though the out-

put mirror only had a transmission of approximately 0.0004

An optical parametric oscillator tunable in the mid-infrared region was

obtained by using a Nd:YAG laser directly as the pump and a LiNbO, crystal

[56] Operation in this region of the spectrum is more difficult because the gain

7 Optical Parametric Oscillators 341

coefficient is inversely proportional to the product of the signal and idler wave- lengths To help compensate for the low gain, a 50-mm-long crystal was used Using angle tuning the spectral range from 1.1 to 4.5 pm could be covered The threshold was 4.0 mJ when the oscillator was operating near 1.7 ym An energy conversion efficiency of 0.15 was reported

Optical parametric oscillation further into the mid-infrared region was POS- sible by using a CdSe crystal Initially, a Nd:YAG laser operating at 1.83 pm was used as the pump [57] Later, a HF laser, operating around 2.87 ym was used for a pump [%I In the former case, threshold for a 21-mm crystal length was observed

to be between 0.55 and 0.77 liW A power conversion efficiency of 0.40 was inferred by measuring the depletion of the transmitted pump In the latter case, threshold for a 28-mm crystal length was found to be 2.25 kW At about twice threshold, a signal power of 0.8 kW was observed that indicated a power efficiency

of 0.15 By employing angle tuning, a signal was generated over the range from 4.3 to J.5 pm Corresponding to this the idler was tuned between S.l 10 8.3 pm Optical jparametric oscillator operation can be enhanced by utilizing a mode- locked pump [59] For one set of experiments, a mode-locked Nd:glass laser operating at- 1.058 ym was amplified to produce an output of 0.55 J By using an

342 Norman P Barnes

etalon in the Nd:glass laser resonator, the pulse length could change from 7 to 60

ps Using a KDP crystal, this produced about 0.15 J of second harmonic A

LiNbO, crystal with a length of 20 mm was utilized as the nonlinear crystal It was housed in an oven to allow temperature tuning With the optical parametric oscillator tuned to 0.72 ym an output of 6 mJ was achieved To utilize the peak power associated with the pump the length of the optical parametric oscillator had to be adjusted so that the circulating pulse was in synchronism with the inci- dent pump pulse train With a 7.0-ps pulse length a change in the length of the resonator in the range of 0.1 mm produced a factor of 10 change in the output energy In a different experiment a mode-locked Ho:YAG laser was used to pump a CdSe optical parametric oscillator [60] A similar enhancement in the conversion was effected by using the mode-locked pump pulse train

An attractive optical parametric oscillator for use in the mid-infrared region was demonstrated using AgGaSe, as the crystal Although CdSe could cover much of the mid infrared its limited birefringence limited its tuning capability However, much of the mid infrared could be covered using long-wavelength pump lasers including a 2.04-pm Ho:YLF [61] or a 1.73-pm Er:YLF [I71 laser Use of a 23-mm crystal length with the 1.73-ym pump resulted in a threshold of 3.6 mJ A slope efficiency measuring only the signal at 3.8 pm, of 0.31 at 1.5 times threshold was achieved simultaneously On the other hand, with the 2.05-

pm pump, a threshold of 4.0 mJ was achieved along with an energy conversion into both the signal and idler of 0.18

Substantial energy conversion has been demonstrated using BBO as the nonlinear conversion by two different groups Both groups used the third har- monic of a Nd:YAG as the pump In one case two opposed crystals, one 11.5

mm in length with the other 9.5 mm in length, were used to minimize birefrin- gence angle effects [62] Efficiency in this case is defined as the sum of the sig- nal and idler energy output divided by the incident pump energy Here signifi- cant saturation in the conversion efficiency was observed, nearly 0.32; that is, 7

mJ of output energy for 21 mJ of pump In the other case, a 10-mm crystal length yielded a quantum conversion efficiency as high as 0.57 at a signal wavelength of 0.49 pm by double passing the pump through the nonlinear crystal [63]

By simply using more energetic pump lasers more output energy can be obtained By using a Nd:YAG oscillator and amplifier, a pump energy of about 0.35 J/pulse could be obtained Using two opposed KTP crystals 10 mm in length for birefringence angle compensation a nearly degenerate optical para-

metric oscillator was demonstrated [ 6 3 ] Signal and idler wavelengths were I 98

and 2.31 ym, respectively The threshold for this arrangement was about 100 mJ and the slope efficiency was as high as 0.48 At the full input energy 0.115 J/pulse was produced Even higher energy per pulse could be obtained by simply scaling the device in cross section while retaining the same energy density

7 O p t i c a l Parametric Oscillators 343

10 TUNING

Tuning of the opical parametric oscillator can be handled using the same techniques as described in the chapter on solid-state lasers (Chapter 6; see also Chapter 2) However, significant differences do exist that can be attributed to the difference in the operating principles of the two devices Some of these differ- ences are manifest in the coarse tuning available with phase matching of the optical parametric oscillator and in the time-varying instanteous gain, A hich has

to be taken into account if injection seeding is to be utilized However, because many of the tuning and line narrowing elements are discussed in Chapter 6, the5 will not be discussed here Rather, the tuning aspects unique to the optical para- metric oscillator will be emphasized

Coarse tuning of Lhe optical parametric oscillator can be accomplished using either angular or temperature tuning In fact any effect that causes a differential change in the refractive indices at the pump signal and idler wavelengths could

be used to effect tuning For example, tuning could be achieved using an applied pressure through the stress optic effect However, to date, only angular or tem- perature tuning has received wide application To calculate the tuning rate, the partial derivatives of the phase mismatch can be used According to a theorem in partial differential calculus

Using this relation, the tuning rate can be approximated by

for angular tuning and

for temperature tuning To evaluate the derivatives of A k with respect to the direc- tion of propagation and temperature the results of Sec 1 can be used Thus

in general Of course, the partial derivative lvith respect to angle for ordinary waves is zero in uniaxial crystals For temperature tuning

344 Norman P Barnes

Individual partial derivatives with respect to angle are evaluated in Section 4 Partial derivatives of the index of refraction with respect to temperature are listed for the more common crystal in Section 8 Thus, to determine the particu- lar wavelength that will be generated the phase-matching condition can be cal- culated as done for a variety of situations in Section 8 Tuning near the phase- matching condition can then be found by using the preceding equations Linewidth can be determined by using the approach also described in Section 4 Injection seeding of an optical parametric oscillator can be accomplished in much the same way as injection seeding of a solid-state laser Injection seeding has been demonstrated for several optical parametric oscillators operating in the visible and mid-infrared regions [65-671 However, there are several significant differences between seeding an optical parametric oscillator and injection seed- ing a solid-state laser [67] One of these differences occurs during the critical pulse evolution time interval During this phase of the development, not much energy is extracted However, the spectral properties of the output are deter- mined by the competition between the seeded and unseeded modes In a solid- state laser, the gain is nearly constant since the stored energy or the population inversion density is nearly constant In an optical parametric oscillator, the gain varies with the pump power Thus, for a pulsed pump, the gain varies with time Although this makes the description of the competition more complex, it does not prevent seeding A second difference is in the extraction of the energy In a solid-state laser, as the seeded mode extracts the energy stored in the upper laser level, it hinders the development of the unseeded mode by decreasing its gain However, in an optical parametric oscillator, there is no stored energy Thus for injection seeding to be highly successful the seeded pulse should continue to extract the energy from the pump pulse as fast as it arrives at the crystal A third difference exists in the saturation effect In a solid-state laser the laser pulse extracts the energy stored in the upper laser level to the point where the gain falls to zero However, in an optical parametric oscillator, the gain may not fall

to zero in the presence on the seeded pulse A nonzero gain allows the unseeded modes to continue to extract energy from the pump and thus decrease the effi- cacy of the seeding process

In doubly resonant optical parametric oscillators, spectral output of the device may be unstable due to an effect referred to as the cluster effect If both the signal and idler are resonant, oscillation can only occur at frequencies that satisfy both the conservation of energy and the resonance condition Because of these simulta- neous requirements, the frequencies that oscillate may not occur at the minimum phase mismatch as shown in Fig 23 By operating away from the point at mini- mum phase mismatch, the output can be significantly reduced Worse still, the

7 Opticat Parametric Oscillators 345

I -Increasing Signal Frequency- ;

-increasing Idler Frequency-

FIGURE 23 Cluster effects in doublJ resonant devices

closest set of frequencies that satisfies both the resonance condition and the con- servation of energy can vary on a shot-to-shot basis For example, the pump fre- quency may experience small variations caused by small variations in the level of excitation oE the pump laser A small variation in the pump frequency may cause a

much larger difference in the frequencies that satisfy both the conservation of energy and the resonance condition Due to instabilities associated with the cluster effect, the doubly resonant optical parametric oscillator is often avoided

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Tunable External-Cavity Semiconductor Lasers

Paul Zorabedian

Photonics Technology Depar-mient Hevvlett-Puckar-d Labor-utories Pulo Alto, Culqomiu

1, INTRODUCTION

1 l What Is an External-Cavity Laser?

A tunable external-cavity laser (ECL) (Fig 1) comprises an optical gain medium (a laser diode with antireflection coatings on one or both facets), optics for coupling the output of the gain-medium waveguide to the free-space mode cf the externall cavity, one or more wavelength-selective filters, and one or more mirrors for defining an external feedback path possibly with a piezoelectric translator (PZT) for fine tuning The external cavity may also contain additional components such as polarization optics bearnsplitters, prisms telescopes etc

1.2 Why Apply External Feedback to Laser Diodes?

7.2 limitations of Diode lasers

Semiconductor Fabry-Perot diode lasers are compact and easy to use, but they suffer from a number o f performance limitations that are potentially serious

in many applications: Solitary laser diodes are often multimode, and they exhibit large linewidths due to a short photon cavity lifetime and strong coupling

Ticnnhie Larws Handbod

349

FIGURE 1 Generic tunable extended-cavity diode laser

between the phase and amplitude of the intracavity optical field Laser diodes are somewhat tunable by varying temperature or current but these methods are awk- ward and have limited ranges, which do not fully exploit the broad semiconduc- tor gain bandwidth

ECLs retain in large measure the compactness and ease of use of solitary cavity diode lasers and in addition provide a number of performance enhance- ments A typical semiconductor ECL has a volume of -1000 cm3 A properly designed ECL will operate on a single external-cavity longitudinal mode The density of accessible modes is increased by the ratio of the external to solitary cavity lengths Truly phase-continuous tuning without mode hops is also pos- sible The linewidth of ECLs is greatly reduced in comparison to solitary diode lasers because of the longer photon lifetime of an external cavity The use of an external filter allows tunability across the wide gain bandwidth of the semicon- ductor gain medium

7.2.3 Comparison with Other Types of Tunable lasers

Compared to other types of tunable lasers, external-cavity semiconductor lasers are compact, are easily pumped by direct injection current excitation, have high wallplug efficiency, are air cooled, and have long lifetimes However, their output power is generally lower (typically -1 to 10 mW, although up to 1 W has been reported)

8 Tunable External-Cavity Semiconductor Lasers 3

1.3 Brief History of ECL Development

Several papers on external cavity lasers appeared in the early 1970s Some of these authors recognized a number of the basic issues of concern to the present- day designer and user of ECLs In the late 1970s several papers were also pub- lished in the Soviet literature The paper by Fleming and Mooradian in 1981 is the earliest reference cited by many authors since they were the first to stu@ the spectral properties of ECLs in detail

Considerable work was done in the early to mid-1980s at British Telecom Research Laboratories, motivated by the prospect of using ECLs as transmitters and local oscillators in coherent optical communication systems In a similar vein, the mid- to late 1980s saw a great deal of work at AT&T Bell Laboratories Eventually, the telecommunication companies realized that distributed feedback lasers (DFBs) and distributed Bragg reflector lasers (DBRs) would better suit their needs The end of the 1980s and early 1990s saw growing interest in ECLs

as sources for spectroscopic work and in commercial fiber optic test equipment

7.4 Scope of ECL Discussion

This chapter considers lasers operating in the strong-external-feedback regime This generally requires devices with facets that have dielectric anti- reflection (AR) coatings or tilted-stripe devices where the light exits the facet at the Brewster angle

This chapter deals mainly ufith the design and continuous wave (cw) proper- ties of laser diodes coupled to free-space external cavities using bulk optical lenses, prisms, filters, and mirrors Some treatment of integrated optic external cavities is also given We exclude the treatment of the important rnonolithically tunable DFB and DBR lasers The rationale for this is that the design of these lasers is very specialized and their fabrication requires sophisticated equipment that necessarily limits the number of organizations that can produce them Broadband tuning of DFB lasers over ranges comparable to ECLs has been obtained [l] However the linewidths of these lasers are 2 to 3 orders of magni- tude broader than that obtainable with ECLs

We also do not explicitly consider vertical-cavity surface-emitting diode lasers (VCSELs) By their structure these lasers are well suited to Isw-cost, high- density uses in computer networks, but their short active regions provide low gain and require very high cavity Q to achieve oscillation At present, vertical- cavity lasers are limited to those materials systems that can be grown on GaAs substrates This has restricted the spectral coverage to wavelengths below 1 pm

So far the goal of the few published external-feedback studies on VCSELs is from the point of view of their applications to optical signal processing and optical communications They have comparable feedback sensitivity [2] and behave in agreement with theory developed for edge-emitting laser diodes [3]

352 Paul Zorabedian

To the best of my knowledge no work has been published so far with the intent

of achieving tunability because their low gain will not support much insertion loss for external cavity components However if VCSELs continue to grow in importance as some predict greater adaptation to their use in external cavities may follow

2 SEMICONDUCTOR OPTICAL GAIN MEDIA

2.1 Laser Diode Basics

A semiconductor laser diode (Fig 1) serves as the gain medium of an ECL The laser diode is a semiconductor device about 250 to 500 pm long by about 60

ym thick mounted on a copper or ceramic heat sink Current is injected through

a top ohmic contact Photons are generated and guided by the epitaxial layers of the structure The thin layer in which electrons and holes recombine to produce light is called the acth?e region Stimulated emission in the active region forms the basis for laser action driven by optical feedback from the facets or from the external cavity We start by reviewing some of the basic properties of laser diodes, which are important for the design of ECLs

2.2 Light Output versus Current Curve

The light output versus current (L-Z) curve (Fig 2 ) is characterized by the threshold current Z,, and the quantum efficiency q Saturation at high current is caused by ohmic heating and Auger recombination The linear portion of the L-Z

curve is explained by the laser diode gain model

2.3 Gain Model

2.3 7 Gain

The optical gain g varies nearly linearly with injected carrier density N :

where o is the differential gain cross section and NT is the carrier density

required for transparency

2.3.2 loss

The active region contains optical losses such as free-cmier absorption, scattering, and other possible effects These factors make up the active-region

8 Tunable External-Cavity Semiconductor lasers 353

I t h I

FIGURE 2 Schematic light output versus current curve

internal loss given by al,, The cleaved-facet ends of the active region constitute

a mirror loss amil given by

where Lint is the physical length of the internal cavity bounded by the facets with power reflectances R f l and Rf2 The Fresnel reflectance of a bare facet is

where 17 = 3.5 is the semiconductor index of refraction

2.3.3 Confinement Factor

gain The factor l- is called the confinement factor

Only a fraction r of the optical field lies within the active region and sees its

The threshold condition requires the optical field to be periodic with respect

to one round-trip of the diode cavity This leads to magnitude and phase condi- tions on the optical field The magnitude part of the threshold condition requires the gain g,, to be equal to the total round-trip loss:

354 Paul Zorabedian

Below threshold, the carrier density is proportional to the injection current Once the laser diode begins to oscillate the carrier density is clamped at the threshold value given by

The threshold current It,, is given by

where q is the electronic charge is the volume of the active region, and t c is the carrier lifetime Above threshold, the relation between output power Po,, and injection current Z is given by

The differential quantum efficiency q,,, is given by

2.4 Spectral Properties of Output

2.4.7 Diode Laser Axial Modes

The phase part of the threshold condition specifies the axial modes of the diode laser The frequencies V and wavelengths hq of the Fabry-Perot modes of the solitary diode laser are given by

where q is an integer c is the velocity of light, ne% is the index of refraction, and Lint is the physical length of the active region The frequency spacing between diode laser axial modes is thus given by

8 Tunable External-Cavity Semiconductor Lasers 355

Assuming neff = 3.5 and Lint = 250-500 pm, we find Avint = 85 to 170 GHz Many Fabry-Perot diode lasers, especially long-wavelength InGaAsP lasers, will oscillate in several axial modes simultaneously in the absence of a wave- length-selective element in the cavity

Schawlow-Townes foimula [5]:

The linewidth of a solitary single-mode laser diode is given by the modified

where u, is the group velocity, n for AlGaAs and InGaAsP lasers is about 2.6

and 1.6.;espectively, and a is the linewidth broadening factor

2.4.3 Linewidth Broadening factor

SP

The semiconductor index of refraction consists of real and imaginary parts

ti = 11' + in" (12)

The real and imaginary parts are strongly coupled compared to other laser gain

media The strength of this coupling is characterized by the line*i*idth hr-ocrdeiz-

increases with higher injection current [ 7 ] The degree of dependence of the C!

parameter on device geometry depends on the type of active region [SI For index-guided lasers (see discussion later), the a parameter is not strongly depen- dent on device geometry; that is, it is close to the value for bulk material For gain-guided and quantum-well laser diodes a may be geometry dependent and differ from the bulk value

curve It is very undesirable to use a laser diode that emits in a higher order transverse mode as a gain medium in an ECL because this may degrade the cou- pling efficiency and the wavelength resolution of the cavity

The near-field radiation emitted from a diode facet is a few-micron spot

somewhat elongated parallel to the p-iz junction Ideally this spot is a Gaussian

beam waist at the facet surface with planar wavefronts in both the parallel and perpendicular directions The far field is a highly divergent beam characterized

by full width at half-maximum (FWHM) angles for the directions parallel and perpendicular to the junction (Fig 3)

2.5.3 Astigmatism

In some laser diodes the facet spot has a planar wavefront perpendicular to the junction but it has convex curvature in the direction parallel to the junction Thus the parallel rays appear to diverge from a point inside the laser (Fig 4) This condition is known as astigmatism and it depends on the waveguiding structure used in the laser diode (discussed later) Even a few microns of astig- matism is undesirable, and astigmatic laser diodes should be considered unsuit- able for use as external cavity gain media

Laser diodes have modes that are polarized parallel to junction (TE) and perpendicular to the junction (TM) TE modes are usually more strongly guided

8 Tunable External-Cavity Semiconductor Losers 357

and thus see lower internal losses Laser diodes usually have TE polarization ratios of a1 least 100: 1 when biased well above threshold

2.6 Transverse Device Structures

The coupling between the active region and the external cavity occurs at the plane where the facet intersects the active region To design efficient coupling optics for this interface, it is useful to have a rudimentary understanding of the mechanisms by which carrier confinement and optical waveguiding are achieved

in the diode laser

358 Paul Zorabedian

Modern diode lasers are double heterostructures in the vertical direction

A thin active layer is sandwiched between top and bottom cladding layers the top layer being y-type and the bottom !?-type The active layer is composed of

a different semiconductor material having a lower band gap and consequently

a slightly larger index of refraction than the p and 17 cladding layers that lie above and below it The layers are comprised of various binary compounds and their associated lattice-matched ternary or quaternary alloys The relative position of the materials in the sandwich depends on whether the band gap of the binary is larger or smaller than that of the alloy For example in the case of

a GaAs/GaAlAs laser, the active layer is composed of GaAs and the cladding layers are composed of GaAlAs In the case of an InP/GaInAsP laser, on the other hand, the active layer is GaInAsP and the cladding layers are InP In a double-heterostructure device, the carriers are vertically confined by potential barriers and the photons are vertically confined by the refractive index gradi- ents of the slab waveguide formed by the cladding and active layers The active layer thickness in conventional lasers is -0.1 pm, while in quantum-well lasers the active layer thickness is about an order of magnitude thinner-about

10 nm

thickness of their active regions [9]

Laser diodes can be subdivided into two main categories according to the

2.6.2.1 Bulk Active Region

Conventional lasers have active regions that are about -0.1 pm thick At this magnitude, the carriers in the active region material exhibit the same properties

as in bulk material The active regions of conventional laser diodes are grown either by liquid-phase epitaxy (LPE) or vapor-phase epitaxy, which is also known as metalorganic chemical vapor deposition (MOCVD) Conventional growth methods are the most amenable to low-cost, high-volume production

2.6.2.2 Quantum-Well Active Region

When the thickness of the active region is reduced by about an order of magnitude to -10 nm, the carriers exhibit properties that differ from the bulk

because of quantum confinement Such devices are called quaiiturn-$%>ell laser diodes Quantum-well active regions can be grown by MOCVD or by molecu- lar-beam epitaxy (MBE) When used as gain media in ECLs, quantum-well lasers have advantages in terms of lower threshold current and increased tuning range